Green Nano Solution for Bioenergy Production Enhancement 9789811693557, 9789811693564

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Clean Energy Production Technologies Series Editors: Neha Srivastava · P. K. Mishra

Manish Srivastava Maqsood Ahmad Malik P. K. Mishra Editors

Green Nano Solution for Bioenergy Production Enhancement

Clean Energy Production Technologies Series Editors Neha Srivastava, Department of Chemical Engineering and Technology, IIT (BHU) Varanasi, Varanasi, Uttar Pradesh, India P. K. Mishra, Department of Chemical Engineering and Technology, IIT (BHU) Varanasi, Varanasi, Uttar Pradesh, India

The consumption of fossil fuels has been continuously increasing around the globe and simultaneously becoming the primary cause of global warming as well as environmental pollution. Due to limited life span of fossil fuels and limited alternate energy options, energy crises is important concern faced by the world. Amidst these complex environmental and economic scenarios, renewable energy alternates such as biodiesel, hydrogen, wind, solar and bioenergy sources, which can produce energy with zero carbon residue are emerging as excellent clean energy source. For maximizing the efficiency and productivity of clean fuels via green & renewable methods, it’s crucial to understand the configuration, sustainability and technoeconomic feasibility of these promising energy alternates. The book series presents a comprehensive coverage combining the domains of exploring clean sources of energy and ensuring its production in an economical as well as ecologically feasible fashion. Series involves renowned experts and academicians as volume-editors and authors, from all the regions of the world. Series brings forth latest research, approaches and perspectives on clean energy production from both developed and developing parts of world under one umbrella. It is curated and developed by authoritative institutions and experts to serves global readership on this theme.

Manish Srivastava • Maqsood Ahmad Malik • P. K. Mishra Editors

Green Nano Solution for Bioenergy Production Enhancement

Editors Manish Srivastava Dept of Chemical Engineering & Tech Banaras Hindu Univ, Indian Inst of Tech Lanka, Varanasi, Uttar Pradesh, India

Maqsood Ahmad Malik Department of Chemistry King Abdulaziz University Jeddah, Saudi Arabia

P. K. Mishra Dept of Chem Engineering and Technology Indian Institute of Technology BHU Varanasi, India

ISSN 2662-6861 ISSN 2662-687X (electronic) Clean Energy Production Technologies ISBN 978-981-16-9355-7 ISBN 978-981-16-9356-4 (eBook) https://doi.org/10.1007/978-981-16-9356-4 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Contents

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Biomass Based Materials for Green Route Production of Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Amit Kumar Tiwari, Nirupama, Amar Nath Mishra, Sunder Lal Pal, and Dan Bahadur Pal Green Synthesized Bimetallic Nanomaterials for Bioenergy Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. H. A. G. K. Perera, J. P. Usliyanage, U. A. D. Y. S. Perera, S. A. K. K. Samaraweera, and G. Thiripuranathar

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Green Synthesis of Metallic Nanoparticles for Biofuel Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ankush D. Sontakke, Piyal Mondal, and Mihir K. Purkait

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Recent Advances in Synthesis of Iron Nanoparticles Via Green Route and Their Application in Biofuel Production . . . . . . . . . . . . . Pranjal P. Das, Piyal Mondal, and Mihir K. Purkait

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Green Synthesized Carbon and Metallic Nanomaterials for Biofuel Production: Effect of Operating Parameters . . . . . . . . . 105 Prangan Duarah, Abhik Bhattacharjee, Piyal Mondal, and Mihir Kumar Purkait

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Biosynthesis of TiO2 Nanoparticles and Their Application as Catalyst in Biodiesel Production . . . . . . . . . . . . . . . . . . . . . . . . . 127 Sheela Chandren and Rosliana Rusli

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Phyco-Nanotechnology: An Emerging Nanomaterial Synthesis Method and Its Applicability in Biofuel Production . . . . . . . . . . . . 169 Gyanendra Tripathi, Aqsa Jamal, Tanya Jamal, Maryam Faiyaz, and Alvina Farooqui

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Fungi-Mediated Green Synthesis of Nanoparticles and Their Renewable Energy Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 Rani Padmini Velamakanni, Ragini Gothalwal, Rani Samyuktha Velamakanni, Sridhar Rao Ayinampudi, Priyanka Vuppugalla, and Ramchander Merugu

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Green Synthesis of Nanoparticles by Plants and Their Renewable Energy Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 Ramchander Merugu, Ragini Gothalwal, Rani Padmini Velamakanni, Rani Samyuktha Velamakanni, Kanchana Latha Chitturi, and Farheen Naz

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Recent Advances in Conversion of Agricultural Waste to Biofuel by Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245 Riti Thapar Kapoor and Mohd Rafatullah

Chapter 1

Biomass Based Materials for Green Route Production of Energy Amit Kumar Tiwari, Nirupama, Amar Nath Mishra, Sunder Lal Pal, and Dan Bahadur Pal

Abstract To fulfil the energy requirement the exploitation of fossil fuels is now converted as a continuous process that has led to the uncontrolled depletion of our natural resources. In view of the loss of natural resources and their preservation, there is a necessity for the up-gradation and distension of renewable sources of energy presently. Furthermore, the negative effects of the exploitation of natural resources and fossil fuels on the change of climate and environment, global warming, as well as overall pollution, etc. should be studied and taken care of concomitantly. The availability of plenty of biomass materials from different sources and its wonderful possibilities as a renewable reserve makes it the best choice for the conversion, production, and storage of energy. Various methods such as gasification, pyrolysis, etc. are useful thermal treatments for the generation of materials such as biochar, bio-oil, syngas, etc. from the biomass, these products are the perfect sources of green and clean energy. In the trans-esterification process, pyrolysis and production of syngas, biochar is widely used as the main catalyst. Biochar-based other materials are also used in the technological manufacturing of batteries, fuel cells, and super-capacitors. Therefore, biomass and its derived materials can help us in the generation of power, conversion and its storage. The better use of biomass material would also help us in the reduction of environmental pollution global warming and reduced exploitation of natural resources, including secure sustainable development and energy security. Keywords Carbon sequestration · Energy security · Global population · Global warming · Syngas · Technological manufacturing

A. K. Tiwari · Nirupama · A. N. Mishra · D. B. Pal (*) Department of Chemical Engineering, Birla Institute of Technology, Ranchi, Jharkhand, India S. L. Pal Department of Chemical Engineering, Maulana Azad National Institute of Technology, Bhopal, Madhya Pradesh, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Srivastava et al. (eds.), Green Nano Solution for Bioenergy Production Enhancement, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-16-9356-4_1

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1.1

A. K. Tiwari et al.

Introduction

The continuous rise in the human population is a big challenge for natural resources, it is noticed that in the previous two centuries, it has been increased around seven times. According to Van Bavel (2013), in the year 2011, the total population was approx 7 billion (70,000 Lakhs) and it is expected it will add around 2 billion (20,000 Lakhs) more in the coming 25 years. This population growth has created an extra burden on the global environment. Natural resources suffering from a high level of unexpected stress s, and are also exploited in an improper manner. Therefore, the conservation and maintenance of these resources are should be done on a priority basis; which is necessary for the sustenance of the life and planet (Cropper and Griffiths 1994). The continuous growth of the population is directly associated with industrial development and urbanization, due to these developments; the requirement of energy became a fundamental need of the society. Nowadays, over lifestyle is changed and even most of the peoples are the user of technology and modern equipment frequently; for which it is required to continuous production, storage, and distribution sufficient amount of energy to fulfil the energy requirement. According to Nakicenovic et al. (2007), the consumption of energy will increase by around 1.1% per year; by 2030, this consumption might be increase up to 7.5  1020 J. The huge demand, consumption, and requirements of energy are always responsible for the reduction of natural resources especially fossil fuels. Tergin (2006) also stated that the reduction in fossil fuel reserves (e.g. coal, oil, and gas) that are used for energy production, which is now a big challenge for energy security. In addition, when fossil fuels are burnt for different purposes they release a lot of harmful gases and unwanted particles in the air, which is responsible for generating air pollution and global warming due to which changes in climate can be noticed (Jain et al. 2019). Nowadays, the reduction of fossil fuels due to overutilization created the big problem of energy security which is directly associated with global geopolitical issues. Uncontrolled utilization of available fossil fuel reserves is responsible for the depletion and it is very important to reduce and preserve the depletion of fossil fuel resources. According to Lund (2007), proper utilization and preservation of these resources can help to develop a wide range of alternatives of environment-friendly and economical renewable energy. These alternatives are sunlight, air (wind), earth, water, and biomass, etc. Production of energy is not the only solution to the problem, we should also think about the storage of converted energy which will help us to improve the production, storage, and capability to produce and store energy. Liu et al. (2010) also suggested that a better result can be obtained by developing systems for the storage of energy from eco-friendly, economical, and efficient materials and distribute it so that it can be easily utilized for the said purpose. Biomass such as bio-oil, ash, and biochar, which are derived from carbon-rich materials, have huge potential for the production and storage of energy (Saleh et al. 2020). Various previous research reports are available on the generation and storage of energy that includes storage of hydrogen (Zhang et al. 2017), catalysts for the fuel cells, batteries electrodes, and super-capacitors

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(Su et al. 2013). Apart from this, activated carbons carbon and nanotubes have also been found to be strong materials for the storage of energy. These materials are useful for the production of nanomaterials, also utilized in different applications in medical sciences (Zhang and Zhao 2009). The production of energy from these materials requires additional processes that require extra cost investments due to which it is not too much encouraged. McCarl et al. (2012) also suggested that the production cost of commercially produced activated carbons is six-time greater than the biochar. Similar kind of information provided by Ahmad et al. (2012), because the biochar production can be done at low temperature and it does not require an activation process; therefore, additional energy is not needed which help to reduce the cost of biochar production. These cost benefits associated with biochar production are good examples of economical production processes, which could help in sustainable process development. As suggested by Shaikh et al. (2020), biochar having few extra advantages such as non-carbonized fraction, high cation exchange capacity, etc., and these properties of biochar helping it in the removal of metallic and organic contaminants. Due to these properties biochar is found suitable for the elimination of metallic contaminants from the contaminated samples (Mondal et al. 2018). As per the suggestion of Bhattacharya et al. (2013), biochar could also be used for the treatment of polluted soil, water bodies, and groundwater. Apart from that, soil fertility can also be enhanced by the application of biochar because it acts as a microbial growth promoter and provides food and condition for multiplication of microbes in the soil (Pietikainen et al. 2000; Lehmann et al. 2011), it helps to increase the water retention capacity of the soils (Yu et al. 2013). Ventura and Yue suggesting that biochar helps to reduce the discharge of nutrients from the soil and enhancing the nutritional status. Therefore, on the basis of the above findings, we can say that biomass is an uncomplicated, economic, and environment-free stuff, which can give better options for energy security. Biochar is a product that is obtained from different biomasses such as wasted wood, agricultural and kitchen waste, sludge, and algal materials, etc. after passing it through a specific process known as “pyrolysis” and it is a highly porous and carbonaceous material, (Nautiyal et al. 2016), these biomasses are abundantly available in earth (Bar-On et al. 2018). Biomass having wide-ranging applications due to its different remarkable physiochemical properties such as enhanced porosity, good surface area, high stability, appreciable water holding capacity, excellent carbon content, and cation exchange capacity (Zhang et al. 2013). According to Barkat et al. (2020), biochar having a good amount of nutritional components and has an alkaline nature. Nowadays biochar is more popularized for the treatment of polluted water and soil (Zhang et al. 2013), sequestering carbon substances (Manyà et al. 2018), production of energy (Titirici et al. 2012), waste treatment and management (Hossain 2016), and used as a material for the production of super-capacitors, batteries, and fuel cells in the energy sector (Cha et al. 2016). Presently, biotechnological research is also involved in energy production using the immobilization of enzymes (nanotechnology) that are too much costly activity which commonly used for biofuels production (Srivastava et al. 2019). Apart from that, several nano-biocatalysts such as nanofibers, nanoparticles, nanotubes, etc. have been shown the capacity to enhance

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the production of biofuels. Therefore, it can be expressed that the biomass waste materials that could be utilized for the production of biochar can be applied for the production and storage of energy in the recent future.

1.2

Biomass and Bio-Energy Parameters

All kinds of agricultural wastes and kitchen wastes can be consumed for the preparation of biochar, syngas, and bio-oil which further helps in reducing impact on the environment by reducing landfill areas, enhancing sanitation, proper waste management, and achieving multiple sustainable developments. However, inconsistency, processing cost, and regulatory restriction limits its utilities (Kuppusamy et al. 2016; Tripathi et al. 2016). Agricultural bio-wastes considered as a pocket of solar energy; as these captures solar energy and consume CO2. During the photosynthesis, carbon atoms are produced and stored in it. Upon pyrolysis, these stored carbon releases energy (Lian et al. 2011). For proper utilization of biomass, it must contain moisture less than 30%, this may be obtained using natural or air drying (Bryden and Hagge 2003). Thermal processes such as tor-refraction (low temperature pyrolysis), carbonization, and hydrothermal carbonization: Heat treatment of wet biomass at elevated pressure and temperature), gasification, combustion (heating in the presence of oxygen, less efficient methods), and pyrolysis is being efficiently used to convert biomass materials into products such as biofuels, bio-energy, etc. With the gasification process syngas produces in abundance while produces lower quality biochar and bio-oil. On the basis of heating rate pyrolysis can be slow or fast process. Slow pyrolysis process produces more char while fast pyrolysis produces high yield of bio-oil. Other factors on which pyrolysis of biomass depends are vapour residence time, high surface area, particle size, porosity, ash content, and heating techniques such as heating by electricity or burning process (Asensio et al. 2013; Lee et al. 2017; Wang et al. 2018; Liao and Thomas 2019). The biomass for the generation of bio-energy has been shown in Fig. 1.1.

1.3

Steps and Parameters in Bio-Energy Production

Biomass availability and its renewability makes it a sustainable source for the production of the energy. It can be utilized to produce of gaseous, liquid, or solid fuel using various methods, e.g. thermal techniques (Bridgwater and Peacocke 2000). Quality of the energy produce can be affected by the properties of the biomass. These properties include its structural constituents, moisture content, decomposition temperature, etc. By using biomass for production of energy is pollution free and also reduction of CO2 release into the atmosphere. Now the commercialization and automobile, a large amount of sulphur oxides, nitrogen oxides, and methane releases into the atmosphere can also be reduced with the

1 Biomass Based Materials for Green Route Production of Energy

Drying at 60° C Hot air Oven

Biomass

Biomass Dust

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Charring at 400° C Muffle Furnance

Biochar Powder Cooling at 30° C Room cooling

Fig. 1.1 Thermal process: biomass to biochar

utilization biomass for energy production. This reduction of methane and sulphur and nitrogen release in atmosphere could help to reduce the cost of pollution abatement measures. In addition, this helps in the reduction of the greenhouse gases and temperature increase. Hence, the agricultural waste material based fuels would, gives several of repayment (Guruviah et al. 2019). Now-a-days, biomass extensively used for the production of liquid fuel and syngas using pyrolysis techniques (Lehmann et al. 2006). Pyrolysis treatment with lower temperature, less vapour residence, and extended heating rate produces liquid fuel like ethanol. However, pyrolysis techniques with elevated temperature, extended gas residence, and slow rate of heating produces fuel in the form of gas which is known as syngas. This gas is a mixture of CO and H2 along with trace of CH4, H2O, and CO2. In addition to bio-oil and syngas, pyrolysis of biomass also produces huge amounts of heat which could be utilized in number of purposes in the industries such as steam generation (Baker et al. 2013). A wide variety of biomasses are available in abundance which may be a probable feedstock for the fabrication of biofuels. These biomasses include kitchen waste, municipal solid waste, and paper wastes (Berge et al. 2011). These materials are mainly composed of carbohydrates, proteins, lipids, cellulose, lignin, and hemicelluloses. Due to compositional difference in the biomass feedstock, produced bio-oil may have diverse constituents in different quantities. These constituents can be alcohols, acids, aldehydes, and derived products from the hemicellulose and lignin of the raw biomass. In comparison to remnant fuels, biofuels is recognized as a green energy source due to negligible release of the SO2 and NO2. Still, bio-oil utilization is lacking behind because of its change in physical and chemical parameters. Notwithstanding, there are a variety of technologies are developed for the production of biofuels and it is used which supported its utility in areas like transportation (Xiu and Shahbazi 2012). Different steps are involved in the heat treatment of biomass; in the thermal processing of biomass, in the first step the conversion of biomass into gaseous form

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is performed follow by a condensation process to convert gaseous into a liquid material i.e., oil. After passing the biomass form different stages, this bio-oil is used for the production of syngas. Viscosity of bio-oil is an important property which decides the flow-ability, the viscosity of bio-oil is generally depends on the type and properties of biomass, used and independent of the reactor type (Sundaram and Natarajan 2009). It is already reported that the biomass which is smaller in size is producing highly viscous fuel than the bigger size biomass (Park et al. 2004). Lu et al. (2008) stated that if the biomass is processed at low temperature, then the produced bio-oil will have more viscous. Compared to other process parameters, the heating rate has least effect on the viscosity of the oil during production. An electrostatic precipitation and condensation system was designed by Yin et al. (2013) to study the boosted heating rate and its effect on bio-oil production with no effect on the viscosity of the oil. The biomass which has high moisture content at initial stage of bio-oil production will produce bio-oil with elevated amount of moisture which is not a good characteristic of bio-oil (Wildschut et al. 2009). Therefore, it is required to eliminate the initial moisture from the waste material before further processing. The excess moisture can be removed by using various drying and dehydration processes such as sun-drying, hot air-oven drying, or air drying. pH of bio-oil is an important factor of quality of final product, the raising of pH can be minimized by the process of Dehydroxygenation can be done to increase the bio-oil quality (Guruviah et al. 2019). To enhance the production in prospect of efficiency and worth, chemical catalysts can be applied in the biofuels production process. In the production of energy and conversion of biomass into economical useful products temperature plays an important role. Pyrolysis is a temperature based process by which we can convert biomass into different type of products such as biochar, syngas, and bio-oil, etc. from biomass materials. The conversion process or pyrolysis depending on different treatment factors such as heating temperature, heating rate, and vapour residence, etc. Different chemical and physico-chemical processes occur during pyrolysis of materials; these processes are decomposition of biomass, formation of new functional groups, volatilization and devolatilization, etc. Biochar production requires slow pyrolysis process; whereas for the production of gaseous product like syngas requires comparatively higher temperature (Goyal et al. 2008) and best quality biofuels may be formed by fast pyrolysis process. Process parameters like high temperature, extended time of vapour and reduced temperature are the more efficient factors for increased bio-oil yield (Onay and Kockar 2003). Another pyrolysis process (Flash-pyrolysis) yields highly viscous bio-oil which has properties that are very similar to commercially available diesel. Chemical catalysts can be used in pyrolysis process for the production of biofuels through fast heat exchange. In the production of energy and conversion of biomass into economical useful products temperature plays an important role. Pyrolysis is a temperature based process by which we can convert biomass into different type of products such as biochar, syngas, and bio-oil, etc. from biomass materials. The conversion process or pyrolysis depending on different treatment factors such as heating temperature, heating rate, and vapour residence, etc. Different chemical and physico-chemical

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processes occur during pyrolysis of materials, these processes are decomposition of biomass, formation of new functional groups, volatilization chemical catalyst are required during thermal treatment of biomass materials because they plays an important role in the quality of obtained products and their formation. They also minimizing the solid product formation, due to which the maximum liquid and gaseous products can be achieved (Balat et al. 2009). Alkali metals, nickel-based catalysts are the widely used chemicals required for catalysing the gasification. Various nanocatalyst such as CaO, TiO2-ZnO, hydrotalcite, and Cs/Al/Fe3O4 are been utilized in thermal treatment of biomass during production of bio-energy (Madhuvilakku and Piraman 2013).

1.4 1.4.1

Green Route for Energy Production Biomass for Energy Production

The pyrolysis is the one of the best methods to extract useful energy from the waste biomass. Pyrolysis of biomass yields to provide various useful products such as energy rich bio-oil, syngas, and biochar. Biochar is a carbonaceous product which left behind after the pyrolysis process. Sometimes, energy released during pyrolysis of biomass is higher than the combustion process (Nanda et al. 2016). This is because efficiency of retaining the carbon in pyrolysis is very long. Peters et al. (2015) carried out life cycle assessment in order to define application of biochar for energy production. They observed that these biochar can be utilized in number of application such as adsorption of pollutants from soil and water enhancement, activated carbon for biomaterial production, and for sorption of toxins and drugs in pharmaceutical industry (Nanda et al. 2016). Biochar is also an important substitute for coal in power plant industries. This is because coal leads to the production of environmental pollutants during combustion as well as during its mining. Whereas utilization of biochar eliminates these problems. A good amount of gases like carbon monoxide and hydrogen and heat produced during the biochar formation, which are collected and used as fuel. Pyrolysis of biomass also decreases the release of air pollutants (Peters et al. 2015). In addition, biochar act as an admirable source of carbon sequestration. During pyrolysis of biomass, carbon content in it get stabilized. Carbon is dependent on to biochar. These biochar are highly stable with a mean residence time of about 1600 years (Singh et al. 2012). Liu et al. (2015) checked carbon contained in the biochar and found that yearly CO2 emissions is about 0.3 billion tons. Windeatt et al. (2014) reported carbon dioxide of about 0.50 billion tons could be sequestered every year if annually 339.4 tonnes of biochar is being produced. The surplus porosity and high surface area of biochar helps to capture carbon dioxide using physisorption, surface functional groups interaction, formation of polar bond, and Van der Waals interaction (Creamer et al. 2014). By conversion to fuel this carbon dioxide can be utilized to exact energy. This carbon dioxide can be

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used for conversion of various valuable products such as hydrocarbon, methanol, dimethyl ether, syngas, etc. (Centi and Perathoner 2009).

1.4.2

Bio-catalyst for Energy

Municipal waste biomass can be used for the generation of energy, i.e. biogas aerobically (Proll et al. 2007). Shen et al. (2015) reported anaerobic digestion of biomass generates mainly 50–70% methane and 30–50% carbon dioxide. In addition to methane and carbon dioxide it also generates significant quantity of NH3, H2S, N2, H2, and O2. However, excess NH3 generation is problem (Mumme et al. 2014). Luo et al. (2014) reported that the anaerobic digestion if done in extremely acidic conditions, it will have antagonistic effects on the microbial community. Biologically synthesized nanoparticles (NPs) show catalytic characteristics only due to the high surface area and volume. From leaf extract biomass for the production of palladium nanoparticles for the dye removal (Petla et al. 2012). The soluble bio-based products coated NPs efficiently adsorb dye from dye degradation. The outcome of pH on the dye degradation using NPs was also studied by conducting sorption experiments in the laboratories and it was observed and reported that at every increase in pH there was an increase in the % dye removal. In the soil, the use of biochar support microbial activity and reduces the generation of the NH3 and CO2 (Mumme et al. 2014). This is possible due to its alkaline nature. Also, alkaline metals such as K, Ca, and Mg helps to dissolve and improve biomass digestion (Shen et al. 2015). Shen et al. (2015) observed an increase in CH4 generation, during anaerobic digestion of biochar. It was being noticed that during the anaerobic digestion CO2 might be converted to bicarbonates and carbonates. This will further improve the digestion mechanism stability and maintains alkaline nature of the system (Stams and Plugge 2009). Luo et al. (2014) reported increased CH4 production, because of high electrical conductivity of biochar, which accentuates interspecies electron transfer.

1.4.3

Biomass for Trans-esterification

In the recent years, production of biodiesel gained considerable attention. This is because of the utilization waste biomass which are non-toxic, eco-friendly, biodegradable and reduces CO2 emissions (Shahzad et al. 2017).Trans-esterification process (involving chemical changes) can be accentuated by homogeneous or heterogeneous catalyst (Gardy et al. 2017). However, generally heterogeneous catalysts are preferred because they can be reused without neutralization, can be easily separated, low cost and non-polar nature favours (Li et al. 2014; Kastner et al. 2012). Heterogeneous catalysts are made through biochar sulphonation and are used for trans-esterification of fats (Dehkhoda et al. 2010). Sulphonated biochar can be

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excellent catalyst for trans-esterification. Also, it improves biodiesel production. Feedstock which contain higher amount of free fatty acid like cooking oil can improves the biofuels fabrication (Li et al. 2014).

1.4.4

Bio-catalyst for Pyrolysis

Plastics, which become the part of our live from small household thing to sophisticated equipment, are now made by the plastics. But these plastics have many disadvantages; one major disadvantage is that it takes millions of years for its degradation. Nowadays, only about 20% of the total plastic wastes are recycled (Miandad et al. 2017; Nizami et al. 2016). These plastic wastes can be used for the production energy using pyrolysis techniques as it contains carbon-hydrogen bonds (Miandad et al. 2016, 2019). Biochar can be used as catalysts which can helps to improve thermal process. Krerkkaiwan et al. (2015) reported that the biochar can be excellent catalyst for the degradation of the plastics in pyrolysis process. Biomass helps in degradation process due the occurrence of Si, Al, and aluminiumhydroxides in it. These biochars can also be utilized as a sorbent for the pollutant removal in the post-treatment process which helps in improving biofuels quality. During pyrolysis process, carcinogenic polyaromatic hydrocarbons are produced such as polyethylene. These carcinogens can be removed by biochar sorbents (Budhwani 2015). Also, biochar is having porous structure and high surface area which aids in the polyaromatic hydrocarbons elimination (Yargicoglu et al. 2015).

1.4.5

Biomass Gasification for Syngas

Gasification of biochar is slower than the gasification of biomass. Biomass gasification continues with about 80% removal of mass, which is mainly due to the volatilization of the gases (Di Blasi 2008). CO2 generated in the gasification acts as a gasifying agent. Furthermore, during biochar pyrolysis it produces tar which causes crack on the char. This crack might collect carbon causing slow gasification process (Nanda et al. 2016). Due to the presence of alkaline metals, the biochar reactivity improves during gasification process (Li et al. 2006). Alkaline metals such as Na, K, Ca, and Mg act as a catalyst in gasification process which causes either increase or decrease reactivity. Catalyst sorbs gasifying agents and increasing the active sites causing improvement in the gasification process. Another parameter that can affect the gasification process is the surface and structure variability (Asadullah et al. 2010). In the power plant, biochar is used as a gasifying agent which is a better substitute of coal. Gasification process if done without catalyst then produces loose biochar. Whereas, catalyst based gasification produce biochar with cross-linked structures (Li et al. 2006). During biochar gasification, it generates syngas, which

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can be utilized in the production of hydrogen, alkane, and alcohol (Rostrup-Nielsen 2001). Biochar catalyst can also accelerate the production fuels (Huber et al. 2006).

1.4.6

Biomass Materials for Fuel Cells

Biomass may be used in the fuel cell like PEMF cell, microbial fuel cells, and direct carbon fuel cells are very promising for the electrical energy production using H2 and CH3OH (liang et al. 2013). In carbon fuel cells, biochar can be directly used (Jafri et al. 2018). Because biochar properly interact with the molten carbonate electrolyte, it will help in biochar diffusivity for the reaction. In biochar, carbon oxidizes to its oxides which initiates electron transports. Transportation of electrons helps in the electricity production. Biochar have high electrical conductivity which favours its utility in the fuel cells. Furthermore, biochar’s disordered carbon structure improves the biochar reactivity (Cao et al. 2007). Although poor crystal structure of biochar inhibits the electrical conductivity and reactivity (Cherepy et al. 2005). Also, porosity and high surface area of biochar helps in improving reactivity and, thereby, improves fuel cell performance. Li et al. (2009) reported surface functional groups helps in chemisorbing oxygen molecules during fuel cell functioning which leads to the improvement in its performance. Li et al. (2006) reported the presence of magnesium oxides, calcium oxides, and iron oxides acts as a catalyst in the fuel cell, whereas aluminium oxides and silicon has an inhibitory effect. Thus, it is important to optimize the percentage of inorganic material and ash content in the biochar in order to better fuel cell efficiency. These MFCs are promising for non-chemical and sustainable technology. MFCs are the promising technology for the future and can be used in wastewater treatment, organic compound degradation, and bio-energy generation. In the recent years, when global warming and depletion of the fossil fuels increasing tremendously. These MFCs are also used in the power generation, bio-hydrogen production, water desalination processes, and other useful chemicals. MFCs transform pollutants and nutrients present in wastewater into electricity using microorganism (Liu et al. 2004). Energy generated by MFC is utilized for energy compensation during wastewater treatment. In MFC biomass is used as a cathodic catalyst due to its benefits such as good durability, low cost, and high reactivity. Huggins et al. (2014) found that the power generation by biochar addition is comparable to activated carbon. Other benefits of biochar which makes it preferable in MFCs are its high porosity and surplus surface functional groups. Doping with nitrogen and phosphorus to biochar will improve oxygen reduction catalysis. In the MFC for longer run, biochar cathode biofouling must be taken into consideration (Janicek et al. 2015). Biofilm growth that reduces biofouling and upsurge their operability must be disinfected.

1 Biomass Based Materials for Green Route Production of Energy

1.4.7

11

Biomass Derived Products for Batteries

In the recent years, Li and Na ion batteries are extensively used due to its efficiency and high energy density (Bachman et al. 2016). Porous biochar with active functional groups and surface area may be utilized in the batteries. These biochar acts as ion diffuser, which helps to improve electrolyte interface, that are very important for the conquer electrochemical result (Wang et al. 2016). The storage capacity of the batteries can be improved by increasing biochar’s porosity and surface area. Storage capacity can also be improved by the tuning the electrode conductivity. Nitrogen doped biochar improve the storage capacity by inserting chemically active defects, hence increases the active site as well as conductivity (Yan et al. 2017). The carbon’s electro-negativity can also be improved by nitrogen doping, which helps in facilitating the more active sites for ion storage. It also helps in improving the electrochemical stability and electronic conductivity of the biochar. Highly porous biochar helps in promoting creditable cycle performance. Nitrogen doping also helps in reducing inner resistance and enhancing specific capacity of biochar. Cobalt oxides, iron oxides, and manganese oxides can be added in biochar so that it can be used as an anode electrode in batteries (Wang et al. 2015). Zhu and coauthors also showed the biochar can be able to summarize S and polysulphides in the Li-ion batteries that improves the battery stability by suppressing the active materials. However, biochar is not able to prevent polysulphides diffusion from the cathodes that causes adverse effects. Metal doping with cerium oxides, lanthanum oxides, and magnesium oxides on the biochar improves the cycling stability as well as ability of the batteries.

1.5

Advanced Technologies for Energy Production

Sun et al. (2018) prepared graphene nanosheet with 3D vertically aligned arrays using waste biomass. Nanosheet was prepared using hydrothermal carbonization technique followed by potassium hydroxide chemical activation to spruce bark. They found the 3D interconnected structures. These 3D structures have high surface area and pore volume. When these nanosheets are used in super-capacitors it found out with high capacitance and energy density. Using garlic skin, Zhang et al. (2018) synthesized 3D hierarchical porous graphitic carbon materials. Garlic skin first carbonized and then chemically activated with potassium hydroxide. When these nanosheets utilized for super-capacitors to observe with superior electrochemical properties. Microspores which developed in the material help in improving capacitance performance, reducing the offered resistance, and energy storage efficiency. Peng et al. (2018) synthesized hierarchical high capacitance 3D porous carbon material using Moringa olifera and suggested to use as electrode materials in super-capacitors. Zhou et al. (2016) synthesized graphitic carbon nanosheets using biomass waste (wheat stalk) and suggested to use these materials in Li-ion batteries to provide quick transport of Li ions and electrons. They employed hydrothermal

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process followed by graphitization treatment for the synthesis of the material. Graphitization treatment helps to reduce the voltage hysteresis which shows a promising alternative for electrochemical storage. Yuan and coauthors also showed the carbon materials from peanut residue and suggested to use as anode materials to lithium-ion battery. They used carbonization along with chemical activation by potassium hydroxide. They observed graphene-like structures and high degree of graphitization in the developed material. This material revealed rapid transfer of electron and intercalation/deintercalation of lithium ions. Li et al. (2020) have done a comprehensive review on the porous material utilization with the special focus on the metal-organic for energy and storage, solar and electrochemical energy conversion, challenges, and opportunities for advanced energy technologies. Sun et al. (2020) also reviewed about the lithium-ion and lithium-ion batteries used for energy storage with special focus on electrode materials and electrolytes. Guo et al. (2020) prepared alloy-based catalysts with silicon oxide/nitrogen doped bamboo leaves material.

1.6

Summary

Increasing energy demand and fossil fuel depletion created a status quo of energy security, which must be taken care sustainably. Biomass waste which is available in abundance grasps huge potential for the energy production. Biomass is a renewable and sustainable source. Thermal techniques such as gasification and pyrolysis were used to extract energy from the biomass. There are various parameters such as pyrolysis temperature, biomass type, vapour residence time, and heating rate which must be optimized in order to get higher yield. Biomass have various advantages which includes high electrical conductivity, high porosity, large surface area, and low production cost makes which make it appropriate candidate for the energy conversion and storage devices. Furthermore, nanoparticles, hetero atoms, and metal oxides doping to biochar will improve its fitness in the energy conversion and storage devices. Biomass is also used as catalyst in various applications such as pyrolysis, energy production, and syngas generation. Acknowledgment The authors thankfully acknowledge Birla Institute of technology, Mesra, Ranchi, Jharkhand. DBP is thankful to NPIU (TEQIP-III), Govt. of India for the financial support and Co-PIs of the project.

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Chapter 2

Green Synthesized Bimetallic Nanomaterials for Bioenergy Applications D. H. A. G. K. Perera, J. P. Usliyanage, U. A. D. Y. S. Perera, S. A. K. K. Samaraweera, and G. Thiripuranathar

Abstract A novel class of materials, “bimetallic nanoparticles” (BNPs), for catalysis have intensively investigated by integrating two different metals, which offer synergistic or novel features surpassing that of monometallic nanoparticles (MNPs). The green route of synthesizing BNPs is attracted immensely as a substitute to vanquish the drawbacks in conventional physical and chemical routes as the green route appears eco-friendly, inexpensive, and less time-consuming to synthesize. Among them, employing plants toward the synthesis of BNPs is emerging as advantageous compared to microbes. The application of BNPs as catalysts is a widespread, providential area considering their larger surface area, and thus utilizing the catalytic properties in altering the biomass into biofuel is a potential area to achieve maximum economic and environmental benefits. Hence, this chapter is a comprehensive contribution of the green synthesis of BNPs, the parameters that affect the synthesis of BNPs, characterization, practical applicability, and their application in bioenergy production. Keywords Green synthesis · Bimetallic nanoparticles · Phytochemicals · Microbes · Catalyst · Bioenergy

Abbreviations Ag Al Al2O3 As ATP Au

Silver Aluminum Aluminum oxide Arsenic Adenosine triphosphate Gold

D. H. A. G. K. Perera · J. P. Usliyanage · U. A. D. Y. S. Perera · S. A. K. K. Samaraweera · G. Thiripuranathar (*) College of Chemical Sciences, Institute of Chemistry Ceylon, Welikada, Rajagiriya, Sri Lanka e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Srivastava et al. (eds.), Green Nano Solution for Bioenergy Production Enhancement, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-16-9356-4_2

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Bi BNCs BNMs BNPs C Cd Ce CeO2 CH4 Co CO2 Cu Cu(OH)2 Cu2O CuO DLS DMF DNA EDS Fe FTIR H2 HMF HRTEM Ir La2O3 MALDI-TOF MDT MFH Mg MNMs MRI Ni NMR NMs NPs Pb Pd PDI PET Pt Pt(acac)2 Ru

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Bismuth Bimetallic nanocomposites Bimetallic nanomaterials Bimetallic nanoparticles Carbon Cadmium Cerium Cerium oxide Methane Cobalt Carbon dioxide Copper Copper(II) hydroxide Copper(I) oxide Copper(II) oxide Dynamic light scattering 2, 5-Dimethylfuran Deoxyribonucleic acid Energy dispersive X-ray spectroscopy Iron Fourier-transform infrared spectroscopy Hydrogen 5-hydroxymethylfurfural High-resolution transmission electron microscopy Iridium Lanthanum oxide Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry Magnetic drug targetting Magnetic fluid hyperthermia Magnesium Monometallic nanomaterials Magnetic resonance imaging Nickel Nuclear magnetic resonance spectroscopy Nanomaterials Nanoparticles Lead Palladium Polydispersity index Positron emission tomography Platinum Platinum(II) acetylacetonate Ruthenium

2 Green Synthesized Bimetallic Nanomaterials for Bioenergy Applications

SEM SERS Si SiO2 Sn SPR STEM TEM TGA TiO2 UV UV-Vis VA XAS XCT XPS XRD Zn ZrO2

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Scanning electron microscopy Surface-enhanced Raman spectroscopy Silicon Silicon dioxide Tin Surface plasmon resonance Scanning transmission electron microscope Transmission electron microscopy Thermo gravimetric analysis Titanium dioxide Ultraviolet Ultraviolet-visible spectroscopy Vinyl acetate X-Ray absorption spectroscopy X-Ray computed tomography X-Ray photoelectron spectroscopy X-Ray diffraction Zinc Zirconium dioxide

Introduction

Nanotechnology has existed since the dawn of time, and has constantly been utilized by nature to produce molecular assemblies in the body, such as lipids, proteins, carbohydrates, and enzymes. However, the formal discovery of nanotechnology is largely credited to Dr. Richard Phillips Feynman, an American physicist, and Nobel Laureate (Subramani et al. 2019; Toumey 2009). Nanotechnology is the reconstruction of matter at the molecular and atomic levels, within the 1–100 nm size range (Subramani et al. 2019; Bhushan 2016). At the nanoscale, the matter has distinct properties than the matter at a larger scale, hence the principles of quantum chemistry nor classical physics hold true in this nanoscale domain (Klabunde 2001). When a material’s dimensions are reduced from the usual large scale, the characteristic properties remain constant initially, however, minor changes occur gradually, and finally, substantial changes occur in the characteristics properties as the size of the particle becomes less than 100 nm (Bhushan 2016). Nanometer-sized particles display peculiar features allowing nanotechnology to pervade essentially every industry and aspect of our lives, serving as the foundation for incredibly effective and affordable computers, novel technologies for diagnostic and therapeutic purposes that could help humans live longer and healthier (Tarafdar et al. 2013; Iqbal et al. 2012). The present nanotechnology research can be classified into three comprehensive categories: nanotools, nanomaterials (NMs), and nanodevices (Iqbal et al. 2012).

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Any substance containing structured components with at least one dimension of less than 100 nm is named as NMs, and can be categorized as zero-dimensional, one-dimensional, two-dimensional, or three-dimensional based on the dimesonality (Rao et al. 2005; Hong 2018). They occur in a variety of morphologies such as spherical, tubular, and irregular shapes and can be single, fused, aggregated, or agglomerated (Rao et al. 2005; Hong 2018; Kumar and Kumbhat 2016). Nanoparticles (NPs), nanotubes, nanowires, nanorods, nanofibers, nanocomposites, and nanostructured surfaces are all included in this category of materials (Rao et al. 2005). Although some NMs may be found in nature, they were most likely created in laboratories using engineering procedures to get the desired qualities for industrial applications. The engineered NMs are made in unique ways to make use of their diminutive size and unique features that are not found in their bulk equivalents (Hong 2018). The properties of NMs vary significantly from those of bulk materials due to (Subramani et al. 2019) higher surface area to volume ratio and (Toumey 2009) effects of quantum confinement, allowing for the development of unique magnetic, electrical, chemical, optical, thermal, and biological capabilities (Iqbal et al. 2012; Hong 2018; Khan et al. 2016). Bimetallic NPs (BNPs) are a novel type of NMs made up of two distinct metal elements (Duan et al. 2020). They have grabbed the curiosity of scientists throughout the world in recent years due to their novel traits as they exhibit a blend features obtained from both the metals (Liu et al. 2012). Furthermore, synergistic effects, such as bifunctional, electron, and lattice strain, significantly enhance the unique physical and chemical characteristics of BNPs, thus, guarantee the catalytic activity (Duan et al. 2020; Liu et al. 2012). Within bimetallic nanomaterial (BNMs), the presence of two separate metals induces electron transport and readjustment of charge, bringing a shift in the d-band center and the state density of metal atoms (Hwang et al. 2012). Besides, in core–shell BNPs, the lattice strain could produce comparable outcomes. Furthermore, combining two different metals in actual applications could not only give them bifunctional behavior but also give new qualities and capacities (Duan et al. 2020; Liu et al. 2012; Zhang et al. 2011). Moreover, the controllable incorporation of at least two useful components to create an adaptable composite with increased performance has demonstrated to be a suitable procedure to accomplish material multifunctionality and a bigger scope of uses in an assortment of disciplines. Hence, in recent years, the combination of BNPs with porous metal organic frameworks has procured a ton of consideration in heterogeneous catalysis (Duan et al. 2020). The synthesis of NMs can be carried out using two main processes, such as top-down and bottom-up approaches (Iqbal et al. 2012; Devatha and Thalla 2018). The top-down process begins with a solid mass and is broken down into smaller nanosized particles using any mechanical means and then stabilized to the desired size (Devatha and Thalla 2018). Mechanical milling, photolithography, anodization, and plasma etching are a few of the most regularly utilized top–down processes (Iqbal et al. 2012). However, this strategy makes achieving the desired size challenging due to the difficulty in maintaining the extreme reaction conditions such as high pressures, temperatures, and an excessive amount of energy needed for these

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processes to take place. The bottom-up process begins with atomic-scale material, and which is made into the desired nanoscale by the use of chemical methods such as hydrothermal, sol-gel, gas phase, and hydrolysis. Although these procedures produce NPs, they are much expensive and can result in the release of contaminants. Furthermore, it is challenging to manage the surface chemistry, size, and structure of NPs using these approaches. However, a bottom-up strategy to produce NPs is preferable since it starts with simpler molecules and progresses to clusters and finally NPs, giving more manipulation over the size and shape of the NPs made (Devatha and Thalla 2018). The reaction between the precursor material and reducing agents such as sodium borohydride and hydrazine hydrate is required for chemical NPs synthesis to occur. However, the majority of these reducing agents are toxic compounds, having negative consequences for individuals and the environment, and thus it has created the interest for scientists to study biological techniques, such as the improvement of simple, greener, and environmentally friendly reducing agents for NPs creation, culminating in the intersection of nanotechnology, and green chemistry. Scientists revealed the utilization of plant materials, bio- and agro waste, microbes such as bacteria, fungi, and algae, and biodegradable polymers to synthesize different NPs in a more environmentally friendly manner (Bolade et al. 2020). The green synthesis of BNPs from a variety of biological sources and their efficient use in bioenergetics through the production of biofuels is currently of greater importance. The current primary energy use is dominated by conventional fossil fuels such as coal, oil, and gas, resulting in long-term sustainability issues such as dwindling fossil fuel supplies, environmental consequences, and large price variations. The increasing greenhouse gas emissions, drastic climate changes, and high energy demand have prompted to seek for alternatives to fossil fuels (Khoo et al. 2020). Wind and solar power, biodiesel, and hydrogen are examples of sustainable energy sources that have the potential to supply worldwide energy requirements while also minimizing the effects of climate change (Munir et al. 2019). Among all, the most cost-effective alternative energy source is bioenergy and is represented as solid, liquid, or gaseous fuels derived from biological sources (Khoo et al. 2020). Biofuels, biogas, biodiesel, bio-oil, and vegetable oil are only a few examples of bioenergy sources. Biodiesel has risen to prominence among renewable energy sources as a cost-effective, technically feasible, and environmentally beneficial alternative natural fuel (Munir et al. 2019). Various feedstocks, such as non-edible and edible oils, can be used to make biodiesel and bioethanol from wheat, corn, and sugarcane, are a few examples of first-generation feedstock bioenergy (Khoo et al. 2020; Munir et al. 2019). Non-food feedstocks such as lignocellulosic and microalgae biomass are utilized to produce bioenergy in the second and third generations feedstock, respectively (Khoo et al. 2020). The use of NMs is one of the most important initiatives nowadays for improving and facilitating the biofuels manufacturing process from waste biomass. The advantages of using NMs will undoubtedly add importance to the biofuels synthesis procedures, allowing it to become more sustainable by lowering expenses and having a positive ecological impact (Srivastava et al. 2017). This chapter mainly focuses on the green

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synthesized BNMs, their different architectures, applications, and utilization as catalysts for bioenergy production.

2.2

Comparison Between Monometallic Nanoparticles (MNPs) and Bimetallic Nanoparticles (BNPs)

MNPs are made up of only one metal, and hence the properties of these NPs are determined by the metal atoms that make them up. They can be metallic, magnetic, or transition metal NPs, etc. contingent upon the type of metal atom present. Most of the metals can be converted to their NPs, for example, Cu, Al, Fe, Ag, Cd, Au, Zn, and Pb (Ealia and Saravanakumar 2017). Metallic NPs can be classified into monometallic, bimetallic, trimetallic (consist of three different metal atoms), and multi-metallic (consist more than three types of metals). Also, they can be synthesized in a variety of ways, however, the green approach is the widely studied method at present due to various advantages it possesses over other conventional methods as mentioned above. The improved physical and chemical characteristics of metallic NPs stimulated noteworthy interest in recent decades owing to the variety of applications as optical, catalytic, and antibacterial agents (Sharma et al. 2019). BNPs have sparked more attention scientifically and technologically than MNPs since the properties of BNPs are determined by the constituent metals and their nanometer size. These are made by combining distinct architectural structures of metallic NPs, and thereby they have the tendency to enhance the energy of the plasmon absorption band of metallic mixtures, and thus, providing a versatile biosensing tool. These characteristics may be contrasting from those of pure elemental NPs and may comprise of thermal, electrical, and catalytic properties that depends on the size of the NPs. The bimetallic nanocomposite (BNC) materials, which combine inorganic NPs with biopolymers, have gotten immense consideration from the scientific community due to their wide range of uses, including UV protection gels, barrier coatings that protect against corrosion and scratch-resistant materials. The value of BNCs stems from their multifunctionality, which allows for the creation of new combinations of features that are impossible to achieve with ordinary materials (Sharma et al. 2019).

2.3

Classification of Bimetallic Nanoparticles

The BNPs can be classified according to the structure into two types as segregated structures and mixed structures. The segregated structures are constitute of two different metal atoms with a shared surface separating the two metal atoms, while mixed structures are the homogeneous arrangements of two metal atoms in an ordered or random manner. According to the arrangement of atoms, the BNPs are

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divided into four categories as alloyed, intermetallic, subcluster, and core–shells (Journal et al. 2016). In an alloyed structure, the two different metals are arranged randomly, whereas in an intermetallic structure they are arranged in an orderly manner. A core–shell structure consists of two metal atoms in an alternative arrangement, while the subcluster structure is a structure where both the metal atoms are exposed to the environment. Basically, alloyed and intermetallic structures are categorized under mixed structures while core–shell and subcluster structures are classified under segregated structures (Srinoi et al. 2018).

2.4

Architectures of Bimetallic Nanoparticles

The BNPs have been synthesized in various architectural configurations such as the crown-jewel structure, hollow structure, core–shell structure, heterostructure, and alloyed structure as given below in Fig. 2.1 (Liu et al. 2012). By varying the thermodynamic and kinetic criterions, specific management of the shape, size, constitution, and arrangement of BNMs can be achieved. The crown-jewel structure consists of two types of metal atoms where catalytically strong and expensive metal atom individually occupies the center while the catalytically weak and inexpensive metal atoms surround it like a jewel in a crown (Fig. 2.1a) (Journal et al. 2016). The crown-jewel structures can be synthesized using the chemical vapor deposition method where the catalytically strong metal is atomized in the presence of an electron beam evaporator, followed by diffuses and deposits at different places on the inexpensive metal surface. This process is fulfilled by using an ultra-high vacuum. The solution state method is an alternative method used for crown-jewel structure synthesis. Since these catalytically strong metals can be observed on the exterior of the catalytically weak metal cluster, they exhibit a greater catalytic activity, for example, Pd-Cu, Au-Pd. The hollow structure has a unique structural arrangement (Fig. 2.1b), where it has a pore with increased exposure to the environment through the surface of metals, and thus, it exhibits multifunctional characteristics. Furthermore, various nanomaterials can be injected into this hollow to obtain different properties (Mazhar et al. 2017). The galvanic replacement reaction is a process to synthesize hollow structures where the two metal NPs get in contact with each other, and the NP with higher reduction potential replaces the metal NP with lower reduction potential. Moreover, by

Fig. 2.1 Architecture of BNPs: (a) Crown-jewel structure, (b) hollow structure, (c) core–shell structure, (d) alloyed structure, (e) heterostructure (red and yellow are two metal types)

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changing the chemical environment and the diffusion process, the direction of the reaction can be controlled. The shell structure can be modified for various functions. Furthermore, the BNP structures have catalytic activities contrasting to their solid equivalents due to low density and minimum amount of materials that are used for the synthesis (Liu et al. 2012), for example, Pd-Sn, Pt-Cu. In the core–shell structure arrangement, the active metal is present as the core metal and is covered and supported by a ring of another metal, which is called a shell (Fig. 2.1c). As these particles have core and shell, they have unique properties. In this arrangement, the catalytically active metal is in the shell, whereas the catalytically less active metal is in the core, therefore mostly, the core is wasted (Liu et al. 2012). This happens due to the reduction of the core atom first, and then the nucleation of shell atoms occurs. Here, the catalytic activity is improved due to the core metals, which lead to the electronic modification of the ring metals. The core– shell structures can be synthesized by one pot core reduction method, in which both the metal precursors have to be added simultaneously to form the core as one metal will reduce due to the electromotive force difference of the two metal ions. The pre-formed NP acts as the seed required for the nucleation of the second metal around the core. The radiation induced metal ion reduction in lower doses also helps in the incorporation of core–shell structures, for example, Au–Ag BNPs (Srinoi et al. 2018). Drug delivery and photothermal therapy are some areas where the core–shell structures are used due to their pores in the core and shell. The Au-Co core–shell BNPs have been used to one step seeding growth in a short time (Banadaki and Kajbafvala 2014), for example, Au-Pd, Pd-Pt. An alloyed structure is the arrangement of two types of metal atoms having a homogeneous distribution in a single particle (Fig. 2.1d). Alloying of two different metal NPs will induce the alterations in structure, improve physicochemical properties, including increasing solid solubility with decreasing particle size than bulk samples (Mazhar et al. 2017). Alloyed structures can be synthesized by using a wet chemical synthesis method under canvassed reaction kinetics and also Xu et al. reported that using the co-reduction method, which is the most popular method to synthesize alloyed nanostructures, for instance, Ni-Fe alloyed NPs (Singh et al. 2018). Furthermore, synthesis of alloyed BNPs should be carried out under steady state reaction kinetics. In the wet chemical synthesis, the two metal ions are thermodynamically preferred to nucleate and grow due to the reduction potentials, and if not it may end up forming core–shell or hollow structures, for instance, Au-Pt BNPs cause an osteopromotive activity in contact with human mesenchymal stem cells (Loza et al. 2020). Alloyed Au-Co BNPs show long-term stability toward hydrolytic dehydrogenation and have higher catalytic activity (Banadaki and Kajbafvala 2014) for example, Ni-Fe and Au-Pd. In the heterostructure, one metal atom forms branches from the other metal atom (Fig. 2.1e) and can be synthesized through a wet chemical process, where two types of metals will nucleate and then grow on their own due to the varying reduction potential. Heterogeneous seed growth is also a powerful method to synthesize heterogeneous BNPs, for example, Pt-Pd heterogeneous BNPs are synthesized by Pd NP seeds and Pt(acac)2 are reduced in a medium of organic compounds (Liu et al.

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2012). Lu et al. reported that of Pd-Pt bimetallic dendritic NPs are 2.5 times farther active when compared with the same Pt mass needed for the ORR in the Pt/C catalyst. In addition, Pt based BNPs with heterostructure have taken the interest among scientists due to their proton exchange membrane fuel cells (Liu et al. 2012) for example, Pd-Pt and Pd-Ag.

2.5

Green Synthesis of Bimetallic Nanoparticles

The synthesis of BNPs is possibly achieved either via concurrent reduction of two types of metals or by consecutive reduction of a single metal upon the nuclei of the other metal. By varying the alloy ratio of the formed BNPs, their properties can be modulated over a vast range due to their collaborative capability to merge the features of a pair of independent metals in a single system that results mostly in bringing about either core–shell or alloy structures of BNPs (Thakore et al. 2019; Elemike et al. 2019). The synthesis of NPs is usually achieved via physical, chemical, and biological methods (Fig. 2.2). Yet, there are several shortcomings associated with the physical methodologies, including the consumption of excessive and continuous energy to maintain high temperatures and pressures, time-consuming procedures, elevated parameters, and the need for highly sophisticated instrumentation for the processes to undergo, and hence, this methodology is comparatively expensive (Abdullaeva 2017). Among the pre-mentioned synthesis methods, the most widely used is the chemical synthesis route which possesses a multistep reaction. Besides, it requires metallic forerunners, stabilizing media, capping agents, together with chemical reductants (inorganic and organic both) as the main components. The complications in the chemical synthesis method include defective surface formation, poor

Fig. 2.2 Flowchart representation of synthesis methods of bimetallic nanoparticles

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Cost effec ve Phytochemicals as reducing and stabilizing agents

Non-toxic

Mild reac on condi ons

Low energy consump on

Single step reac on

Water as the solvent Eco-friendly

Fig. 2.3 Advantages of green synthesis of metal NPs than conventional chemical synthesis

production rate, high pressure, high energy requirement, and higher cost to implement extreme reaction conditions. Moreover, the release of hazardous by-products tangled on the surface of NPs emerges as a major drawback in this methodology as it eventually results in environmental pollution. Thus, the development of an ecologically safe, non-toxic, and thrifty synthesis mechanism in synthesizing NPs is mandatory. As a result, a biological synthesis method has come to the fore as a trailblazing technique in the manufacture of NPs as it has the benefit of scaling up for high yields (Husen and Siddiqi 2014; Sciences and Iqbal 2019). Moreover, the green synthesis is the most captivating and smartest substitute for the generation of NPs outmaneuvering chemical and physical synthesis methods as it offers more competitive advantages such as simplicity, safety, stability, dynamic nature, fewer chances of failure, facility, eco-friendliness, cost-effectiveness, ease of characterization, energy efficiency, rapidity, and one step synthetic process (Fig. 2.3) (Alijani et al. 2020; Gour and Jain 2019; Kamli et al. 2021; Malik and Madan 2020). Furthermore, they are non-toxic to humans and more potent against pathogens and eukaryotic microorganisms at very low concentrations with the slightest side effect (Malik and Madan 2020). Besides, the metal NPs require only an ambient temperature and neutral pH and which provides an outstanding conversion and regulation of the magnitude, shape, and stability of the NPs (Gupta et al. 2020), which ultimately results in the preparation of diminutive NPs.

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Green synthesis of NPs is also regarded as in vivo biogenesis of NPs as it uses living eukaryotic cells and life forms, more categorically, sentient verdure and animate biomass (Suvarna et al. 2020). Furthermore, it is achieved via a bottomup approach, where it incorporates natural resources, including plant extracts (biomass, vitamins, amino acids, and starch) and microorganisms (bacteria (Husen and Siddiqi 2014; Mohamed 2019; Xu et al. 2020), fungi (Castro-longoria et al. 2011), algae (Ashokkumar and Vijayaraghavan 2019; Mirzaei et al. 2021), viruses, and yeast), and fermented extracts and their derivatives (Alijani et al. 2020; Kamli et al. 2021; Akinsiku et al. 2018a; Al-Asfar et al. 2018). The metal NP formation by using microorganisms is mostly achieved via microbial metabolism, where not only metal ions are reduced by various reducing agents located in the cytoplasm and cell wall of microorganisms, but also heavy metals are detoxified either by intracellular or extracellular mechanisms. However, as accumulated particles through intracellular production are of narrow size distribution with less polydispersity, the BNPs produced extracellularly have more commercial applications in various fields. Fungi species are of particular interest to other microbes due to the presence of fungal mycelia mesh, and thereby, withstand flow pressure and agitation in bioreactors. In addition, due to the fastidious growth, easy handling, fabrication, and ability to produce more stable BNPs without molecular aggregations, fungi have become more serviceable even after prolonged storage and with improved durability. Thus, the relatedness between metals and microorganisms give out useful amounts of biologically active compounds in the synthesis of BNPs and hence function as a biofactory. Zhang et al. have researched Shewanella oneidensis MR-1, which is capable of reducing numerous metal anions and cations through the multi-heme c-type cytochromes in fermentative respiration process. In addition, surface enzymes, proteins, and reducing constituents of S. oneidensis MR-1 in the sight of electron contributors have reduced both Pd(II) and Pt(IV) ions intracellularly and extracellularly (Xu et al. 2020). In contrast, microbial-assisted green synthesis of Au/Ag BNPs from green seaweed extract (algae extract) (Ashokkumar and Vijayaraghavan 2019) has detailed, mainly ulvan species, a cell wall polysaccharide, and other polysaccharides that have contributed as reducing and stabilizing agents for NPs formation. However, engaging microbes in the synthesis of NPs have several obstructions, for instance, the complications occur when impositioning on a macro-scale, the need for micropropagation, the requirement of sterile conditions, and the incorporation of synthetic protocols and toxic chemicals (Malik and Madan 2020; Nasrollahzadeh et al. 2019). Though there are many types of biological sources that have been utilized for the synthesis of BNPs, as per the published literature, the benefits are greatly achieved with the usage of plant materials due to the effective and fascinating characteristics of the phytochemicals present in the plant material, which immensely enhance the economic and environmental benefits (Abdullaeva 2017; Unuofin et al. 2020). According to the preference of the metal NPs to be synthesized, the pre-mentioned type of biological extracts are mixed with an aqueous solution of metal precursor, followed by stirring, heating, or passing radiations to trigger the reactions (Suvarna et al. 2020).

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The plant-based synthesis of metal NPs includes several vascular plants and thallophytes (algae, bryophytes) and fragments of them (fruits, leaves, pulps, stems, peels, roots, tubers, shoots, seeds, flowers seed kernels, and barks). The secondary metabolites present in plants have well-defined optical properties and stability in aqueous solution, as they contribute as reducing and capping agents, thus eradicates the need for very reactive as well as toxic chemicals (certain organic solvents) (Kamli et al. 2021; Malik and Madan 2020; Al-Asfar et al. 2018). In addition to metabolic substances such as alkaloids, flavonoids, saponins, steroids, tannins, essential oils, proanthocyanidins, triterpenes, and nutritional compounds (proteins, fatty acids, sugars, and enzymes), functional groups such as aromatic amines, polyols, carboxylic acids, and phenolic acids also have enormous potential in both reducing metal ions rapidly and non-hazardously, and in stabilizing as-synthesized NPs which can resist aggregation in biological mediums which were in well-defined sizes and shapes (Husen and Siddiqi 2014; Malik and Madan 2020). Apart from flavonoids, triterpenoids, steroids, monoterpenoids, sesquiterpenoids, lignans, organic acids, nitrogen-containing compounds, hydroxycinnamic acids, and other compounds can be stated as the dominant compounds in a plant. The existence of elevated levels of phenolic content (typically in the flavonoid genealogy) in the desired plant nominates that plant as the choicest source for the biosynthesis of NPs with bio-reducing agents. Sharma et al. have commentated, the synthesis of Cu NPs has become extra burdensome than that of Ag NPs, as Cu NPs oxidize rapidly amidst the synthesis, as a result, it leads to the formation of subsidiary substances, namely CuO (Copper (II) oxide), Cu2O (Copper(I) oxide, and Cu(OH)2 (Copper(II) hydroxide) than that of monoatomic Cu metal. It further has explained that the synthesis of Ag/Cu bimetal could arrest the oxidation of Cu metal and assists for the purpose of sustaining the entity, allowing a yield of high purity (Sharma et al. 2021). Aqueous extracts of the golden rod plant, Solidago canadensis, contain flavonoid aglycons, glycosides, acetyl-glycosides, polyphenolic compounds (such as quercetin, chlorogenic acid), saponins, hydroxycinnamates, and several mineral elements which reduce the metal ions to their NPs in the synthesis of Au–Ag bimetallic alloy NPs and prevent clustering of the particles (Elemike et al. 2019; Suvarna et al. 2020). It is reported that even though the reduction related to Ag ions can be stimulated by several phytoconstituents, the use of an additional reducing agent, ascorbic acid, is necessary toward the reduction of Cu ions due to Cu2O origination, as the extract functions only as a trivial reducing agent. Furthermore, proteins, reducing sugars, phenolics, ascorbic acid, and carotenoids which are rich in Sapota latex have been used to synthesize Cu/Ag BNPs (Thakore et al. 2019). The biosynthesis of Ag/Pt BNPs is achieved from polyphenolic compounds in an ethanol extract of V. mespilifolia which act as metal chelators, and singlet and triplet oxygen quenchers through donating an electron (Unuofin et al. 2020). Thus the reducing ability of the plant relies solely upon types of metabolites present in its tissues and the reduction potential of metal ions is in charge of the amount of accumulation of NPs (Fig. 2.4) (Seetha et al. 2020).

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Fig. 2.4 Bimetallic nanoparticle synthesis from plants, characterization and evaluation of biological activity

Phytochemicals are capable of adsorbing on to the surface of metal NPs due to the correlation of π-electrons in the absence of possible powerful ligating agents (donation of electrons) and contribute toward reduction of the metal ions (Elemike et al. 2019; Malik and Madan 2020; Nasrollahzadeh et al. 2019). These phytochemicals show important surface functional effects in a consequential manner for different cell varieties that affect toward cell membrane functionality, which also leads to the loss of ATP production, DNA replication, unconstrained cell mobility, and cellular penetrability (Sharma et al. 2021). Moreover, the comparative increment in the rate of reaction and the release of almost negligible industrial waste has increased the vast study in this area and encouraged large scale production of NPs (Malik and Madan 2020). Right after the synthesis of BNPs, it is a must to contemplate on crystallinity, size distribution, morphology, aggregation, porosity, adsorption potential, hydrated surface analysis, zeta potential, orientation, and dispersion with regard to BNPs in accordance with unique spectroscopic tactics. The most commonly approved techniques for BNP characterization can be mentioned as UV-visible spectroscopy (UV-Vis), transmission electron microscopy(TEM), high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), thermo-gravimetric analysis (TGA), energy-dispersive X-ray spectroscopy (EDS), dynamic light scattering (DLS), zeta potential, surface-enhanced Raman spectroscopy (SERS), nuclear magnetic resonance spectroscopy (NMR), matrix-assisted laser desorption/ ionization time-of-flight mass spectrometry (MALDI-TOF), dual polarization

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interferometry, and several other techniques. Out of these, techniques, AFM, SEM, and TEM are straight techniques in which data is obtained through images at various resolutions. UV-Visible spectroscopy (UV-Vis) is a well-known preliminary detection technique in NP production where NPs with the energy absorption capacity in the UV-Visible range offer a characteristic absorbance band. The UV-Visible band gives shifts for different chemical compositions and the atomic ordering in BNPs due to the switch of precursors into the BNPs and their aggregation state (Suvarna et al. 2020). This technique is used to monitor on kinetics in bio-reduction reaction via surface plasmon resonance (SPR), and generally, spectrum of the corresponding BNPs is compared with that of the MNPs (Husen and Siddiqi 2014) to preliminarily confirm the formation of BNPs. However, the main drawback in the aforementioned technique can be stated as the polydispersity of the acquired NPs as well as the varietal morphology which emerges with certain bio-reducing agents (López-Ubaldo et al. 2020). The FTIR is an indirect technique for observing interactive functional groups of natural products implicated during the synthesis. The vibrational spectra of the stabilizing and reducing compounds that have been attached to the surface of the BNPs can be detected. It also justifies the fact that the phytoconstituents are essential in order to stabilize BNPs (Gour and Jain 2019; Mubayi et al. 2012; Hussain et al. 2016). Predominant electron microscopy techniques include TEM, HRTEM, and Scanning Transmission Electron Microscope (STEM). Among them, the TEM is a requisite instrument in the analysis of BNPs, where it provides information on topographical features and size distribution. The tiny BNPs and crystallographic arrangement of a sample can be visualized at an atomic scale through utilizing TEM. HRTEM caters to the marginal measurement which exposes the information about the area composed of a crystalline BNP (glide plane, screw axes, lattice fringe, lattice vacancies, and defects). In addition, in situ TEM observation analyzes the particle growth directly. The STEM instrument is more serviceable for studying BNPs with the metals having similar lattice spacing. Thus the metallic territories can be distinguished easily. When STEM instrument is aligned together with High-Angle Annular Dark-Field imaging or Z-contrast (two visualizing modes), it offers chemical and structural information from the internal structure of the sample. Contrary to the TEM micrographs, it is tough to identify the margins of a core inside a core–shell framework of BNPs. As a solution, Field Emission Scanning Electron Microscopy identifies whether the shell surface is in a polished nature or in a rugged nature due to its high magnification power (Marakatti and Peter 2018; Voyles et al. 2002). X-ray radiation differentiates metals which are nearby in the periodic table, hence it is peculiarly beneficent for the investigation of BNPs. The X-ray assisted spectroscopic techniques comprise XRD, XPS, and X-ray Absorption Spectroscopy (XAS). XRD distinguishes dissimilarities via crystal magnitude, phases, chemical arrangement, structure, lattice spacing, and crystallinity of the BNPs. Therefore, due to the superlattice structure of core–shell BNPs, a completely different XRD pattern with additional peaks is revealed. The XAS investigates the interior parts of the BNPs and

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any functional groups that are mounted on them. Since each atom’s X-ray absorption pattern is different, it can be utilized to pinpoint the presence of each type of element promptly. XPS is solely based on photoelectric energy which investigates the mechanism of reaction and the bonding features of different elements in magnetic BNPs. The energy of the incident X-ray has the ability to overtake the energy of a valence and core electrons, which can act as a fingerprint for each element. The peak area is the parameter that is applied to determine the composition of BNPs via EDS mapping (Suvarna et al. 2020; Ghosh Chaudhuri and Paria 2012). Atomic force microscopy facilitates the quantitative motives with regard to independent NPs and particle clusters (length, size, height, and width), surface texture, and morphology which also can be used either in a liquid or gaseous medium. Although characterization tools remain as a tough task as most of the techniques have reached their technical limits, with current development it has been possible to obtain information on the atomic structure, chemical composition, and reactivity of almost all BNPs.

2.6

Applications of Bimetallic Nanomaterials

The NMs have a wide scope of uses, including the usage in pharmaceuticals to nutraceuticals, cosmetics, food and beverages, agriculture, surface coatings, polymers, paints and inks, and many more (Ingale and Chaudhari 2013). The BNPs offer usage in a wider range of areas owing to the synergistic and composition effects introduced via the metal pair consisting of different metals, and thus it shows improved properties (Sharma et al. 2019). The metallic NPs, which exhibit magnetic behavior or not, are widely used in nanomedicine. The magnetic nature of NPs with regard to biomedical applications can be clarified through characteristics such as saturation magnetization, relaxation, and the magnetic NP size. Considering BNPs, they have greater magnetic moments, spin–spin relaxations, and mechanical rotations, unlike those exhibited by MNMs. It is also known that bimetallization causes the size of the constituent metals to be even smaller than that in MNPs, and with a decrement in size, superparamagnetism is introduced. These intensified characteristics define the use of BNMs in biosensing materials (Srinoi et al. 2018; Behera et al. 2020). Among the above, biocompatibility is an important factor to be considered for medical or therapeutic reasons. Even though antibiotics exhibit antibacterial activity, the use of NMs as antibacterial agents has emerged due to the limitations with the use of antibiotics over the years. Moreover, the ability of the metal which are incorporated in these BNPs aids to specifically distinguish the microbial cells from the normal cells and act accordingly on them, hence makes them attractive antimicrobial agents. Further, the BNPs inhibit bacterial growth and easily combine with other biological molecules of concern along with improved antimicrobial effects (Srinoi et al. 2018). It is reported that glutathione capped Ag–Au BNPs exhibit excellent antimicrobial activity against a known oral pathogen, Porphyromonas gingivalis W83,

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compared to corresponding monometallic (Holden et al. 2016). The BNPs not only exhibit antibacterial activity but also exhibit antifungal activity which can be evident by the published literature, Ag–Ni BNPs against Candida albicans and plantsynthesized Ag–Fe NPs against Candida auris (Kamli et al. 2021). The plant-based synthesized Au–Ag BNPs act as antibacterial, photoantibacterial as well as antitubercular (relatable as tuberculosis is caused by mycobacteria) agents. Furthermore, the agglomerated Au-Pt BNPs exhibit antimicrobial activity against Escherichia coli, whereas the Ag–Ni BNPs are found to be potent antibacterial agents against E. coli, therefore expected to be used as antibacterial coatings on surfaces (Ding et al. 2017; Singh et al. 2016; Britto Hurtado et al. 2020; Akinsiku et al. 2018b; Parimaladevi et al. 2018). The NMs used in diagnostic applications are applied in imaging (in bioimaging) and sensing (i.e. as biosensors) (Srinoi et al. 2018). Characteristics such as increased photostability, long fluorescence lifetime, increased signal-to-background noise ratio, and the ability for non-invasive and deep tissue penetration promote the use of NMs in bioimaging (Huang et al. 2014). Furthermore, due to the enhanced effects on surface-to-volume ratio, electrical conductivity, biocompatibility, and catalytic activity, nurture the use of NMs in biosensing (Agrawal and Srivastava 2020). The BNPs exhibit enhanced magnetic and optical properties along with the abovementioned properties, make them attractive for diagnostic applications. The Ag–Pt BNPs are used to decorate electrospun nanoporous carbon nanofibers for dopamine sensing, Cu-Au NPs supported on a glassy carbon electrode for glucose sensing, and Au-Pt nanochains used for carcinoembryonic antigen (a glycoprotein) detection are a few of the examples where BNPs have been used in sensing in various molecules (Huang et al. 2014; Emam 2019; Cao et al. 2013). The BNPs can be used as electrochemical sensors too, in where the Au-Pt bimetallic nano chains have been incorporated in an electrochemical immunosensor for the detection of carcinoembryonic antigen (Cao et al. 2013). The NMs are used in optical imaging, X-ray CT methods, radionuclide, and magnetic resonance imaging. The thiol-capped ultrasmall Au–Bi BNPs are used in X-ray for CT imaging to obtain high signal contrasts, where Zn-Au BNPs are used for the treatment of tumors along with PET-mediated neuro-molecular imaging, while superparamagnetic Fe-Pt and water-soluble Fe-Ni BNPs were utilized as excellent MRI contrast agents, and Fe-Pt BNPs used for dual-modal CT/MRI molecular imaging are a few of the instances which prove the beneficial application of BNPs (Chou et al. 2010; Fuchigami et al. 2012; Nugroho et al. 2016; Cho et al. 2014; Ashik et al. 2018). Apart from diagnostic applications, BNPs can also be utilized in therapeutic applications, thermal treatments, and drug delivery. The thermal treatments or thermotherapy with the involvement of NMs make their ability to withstand high temperatures. Moreover, the studies have proven that the BNPs incorporated thermotherapy is used as a potent anticancer agent. For instance, Fe-Pt BNPs that have been previously used in magnetic fluid hyperthermia (MFH), a selective anticancer treatment, have also shown a good photothermal transduction efficiency. When Fe-Pt BNPs are used as a medium for MFH, the cells associated with these

2 Green Synthesized Bimetallic Nanomaterials for Bioenergy Applications

35

Lipid membrane coaƟng BNP Large pores Aqueous anƟ-cancer drug MagneƟc capsule

Fig. 2.5 Magnetically guided anti-cancer drug delivery system with an ultrathin shell comprised of an orderly BNPs network structure and large pores within (the pores comprise of the aqueous anticancer drug)

BNPs will only undergo cell death when the specific cells are heated up. Furthermore, the high photothermal transduction efficiency of these BNPs makes them useful in photothermal cancer therapy, where effective and selective cancer treatment is expected (Chen et al. 2013). The drug delivery includes attachment of the therapeutic agent to a mediator, for instance, the agents that are difficult to be delivered by the usual catalog or to achieve greater bioavailability of the drug. For example, the Au–Ag luminescent bimetallic nanoclusters have been developed for therapeutic suicide gene delivery in HeLa cancer cells, where magnetic drug targeting (MDT) has been involved (Dutta et al. 2016). Therefore, the use of MDT is not only allowing for active targeting and specific drug delivery but also can be used for imaging (Sun et al. 2008). For example, the Fe-Pt BNPs used for MFH can also be used for MDT where Fe-Pt capsules were assembled among Fe-Pt BNPs which then served as an efficient drug delivery system (Fig. 2.5) (Chou et al. 2010; Fuchigami et al. 2012). The BNP network capsule is the magnetic carrier for targeted drug delivery, thereby making sure an efficient guidance to cancer cells (Fuchigami et al. 2012). Nanofabrication or nanomanufacturing is an area that makes use of the additional mechanical properties of BNPs. It is the creation of nanostructures of various dimensions for a range of applications. For example, Au–Ag, Au–Cu, and Au–Pd homogeneous alloy BNPs have been found to allow highly controlled fabrication of alloy nanostructures (Nugroho et al. 2016). In another instance, distinct asymmetric nanomushrooms, bimetallic core
shell nanorings, and split nanoring arrays have been nanofabricated using combined colloidal lithography and nanoscale electro-deposition processes. These nanofabricated materials have potentially been used as metamaterials, biosensor surfaces, and in high-density magnetic memory storage, respectively (Cho et al. 2014). Catalysts are incorporated in chemical reactions to achieve efficient and effective reactions with increased reaction kinetics, cheaper investments, and lesser waste products. The use of BNMs over MNMs has numerous benefits, including improved

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catalytic activity, selectivity, and stability (Ashik et al. 2018). Since a combination of metals exists in BNPs, its composition determines the synergistic effect, which determines the nature of the adsorption that occurs on their surfaces, and therefore, determines their catalytic behavior. One important area where catalysis involves is wastewater treatment, where the bioremediation and dehalogenation process occurs. However, the major drawbacks of MNPs, including instability, lesser efficiency, and pH imbalances, can be prevailed over by the involvement of BNPs (Scaria et al. 2020). There are three steps involves in wastewater treatment such as adsorption, oxidation, and reduction. Studies have proved the usage of Fe–Ce BNPs in the adsorption of As(V), Fe–Ni BNPs on the adsorption of specific phosphorus pesticides, and Fe-Cu BNPs in coal mine combined with a polysaccharide bioflocculant, where via catalysis adsorption of concerned contaminants has occurred (Scaria et al. 2020; Mansouriieh 2016; Dlamini et al. 2021). Moreover, in another study, it has been reported that BNPs produced by coupling Fe with another metal, such as Au, Cu, and Ni, are much more effective in biological wastewater treatment than with Fe NPs alone (Bensaida et al. 2021). The wastewater treatment through dehalogenation using BNPs has been studied, where biosupported bimetallic Pd-Au nanocatalysts effectively dechlorinated certain environmental contaminants, whereas starch stabilizes Fe-Pd BNPs degraded chlorinated hydrocarbons in water (Nishimura et al. 2014; Wang et al. 2014). These methods involve reduction, where the oxidation state of the halogen is decreased during the process. Apart from wastewater treatments, BNPs serve roles in other catalytic procedures as well including electrocatalysis and photo-catalysis. Electrochemical reactions involve electron transfers in chemical reactions where electrocatalysts are responsible for accelerating these reactions. BNPs are mostly serving in electrocatalysisrelated energy storage and in conversion applications, including metal–air batteries, fuel cells, and solar fuels. Therefore, they are not only responsible in accelerating electrochemical reactions, but also for green energy methodologies. Pt is one of the best electrocatalysts that gives some peculiarities, high cost, and the production of various bi-products during catalysis when used alone in the form of MNPs, hence hinder the adsorption processes that occur on their surface. Therefore, the hindrances that occurred with the MNPs can be avoided with the use of BMNPs. On the contrary, photo-catalysis is the acceleration of reactions involved with the conversion of light energy to chemical energy. Many bimetallic systems have been studied with a combination of TiO2 in order to achieve efficient photocatalytic activity. Apart from these findings, maximization of the catalytic effect of BNPs through various factors by varying the structure type, changing the mole ratio of the two metals for composition variations, or by varying the constituent metal sizes within the nano-range have also been experimented on. Based on the recognition and determination of how these factors affect the reactions of concern will help in coming up with optimal conditions used for reactions and thus leading to effective and efficient catalytic procedures adhered with the use of BNMs (Liu et al. 2012).

2 Green Synthesized Bimetallic Nanomaterials for Bioenergy Applications

2.7

37

Bimetallic Nanomaterial as Catalysts toward Bioenergy Production

A substance or material which exhibits catalytic characteristics in nanoscale is defined as a nanocatalyst. They can be classified into two categories dependent on whether the catalyst is present as the substrate in the same phase or not, as homogeneous catalysts and heterogeneous catalysts (Saoud 2018). As reported previously, the BNPs have a high surface-to-volume ratio, which improves the catalyst’s functioning by providing more surface area for reactants to react with. These bimetallic nanocatalysts have the potential to alleviate some of the most prevalent difficulties with heterogeneous catalysts, including resistance during mass transfer, time expenditure, rapid deactivation, and inefficiency. Hence, efforts to produce novel types of nanocatalysts have been stepped up in this area. According to most of the research studies, nanocatalysts boost yield efficiency at somewhat gentler working conditions when compared to bulk catalysts (Akia et al. 2014). Hence, these BNPs can be used as efficient catalysts for bioenergy production as reported in Table 2.1 (Alonso et al. 2012). As the world’s energy demand grows, so does the usage of fossil fuels. This source of energy is, however, limited and has environmental issues. Therefore, biomass resources have become a popular and viable energy source due to their abundance and diversity. Moreover, liquid and gaseous fuels have been produced as an outcome of biomass conversion to biofuel (Akia et al. 2014). Biodiesel has received immense attention among the various biofuels made from biomass due to its decomposable, renewable, and harmless nature (Saoud 2018; Banerjee et al. 2014). The biodiesel is produced via the transesterification process with the use of a homogeneous catalyst. However, on the other hand, homogeneous catalysts have numerous drawbacks, including high manufacturing costs due to multiple post-synthesis stages, the inability to reuse the catalyst, and undesirable soap formation (Lam et al. 2010). The heterogeneous catalyst is an option that reduces the costs of separation and purification, making it suitable for commercial manufacturing. Yet, heterogeneous catalysts have lower activity than homogeneous catalysts. As a result, the goal is to create a superior catalyst that overcomes the shortcomings of both homogeneous and heterogeneous catalysts. In heterogeneous catalysis, BNPs have demonstrated to be very useful. They have unique features, such as increased reactivity, since the core metal particle can change the lattice strain of the shell metal, causing a shift in the shell metal’s electronic band structure. It is reported that the Au–Ag core–shell NPs can be produced at room temperature and also exhibits catalytic activity on the generation of biodiesel from sunflower oil by transesterification (Banerjee et al. 2014). Deng et al. used a co-precipitation approach with urea as the precipitating agent, followed by microwave-hydrothermal treatment, and finally calcination to produce hydrotalcite-derived particles containing Mg-Al. These BNPs were employed as a catalyst for biodiesel generation (Fig. 2.6) starting with Jatropha oil following pretreatment due to their high basicity (Deng et al. 2011).

Average diameter of 7.5 nm

The diameter of BNPs in hollow carbon spheres was observed to be 3.6  0.7 nm Carbon-coated cu-co BNPs varied in size from 10 to 60 nm. The average particle size is about 30 nm –

Pd/Au

Pt/Co

Boerhavia diffusa leaf extract Wood sawdust

–

Co/Ni

Fe–Zn/Al2O3

Sawdust of pinewood

HMF derived via thermochemical hydrolysis of starch and cellulose

HMF, platform molecule derived from cellulose

The particle size progressively diminished from 8.4 to 7.3 nm with the increment of Ni molar proportion in the bimetallic catalysts Size was seen to be less than 10 nm

Ni/Cu

Pd/Ru

Cu/Co

Dehydration of fructose to HMF

–

Cu-Ru/C

HMF obtained by dehydration of monosaccharides HMF derived from cellulose

Boerhavia diffusa leaf extract

Sunflower oil

Biomass used as raw material Jatropha oil

Size was less than 10 nm

Particle size of the synthesized BNPs Size of 7.3 nm by calculation, but as indicated by AMF investigation, they formed a layered design with a width of 0.941 μm and thickness of 381 nm Average size of 28  2.3 nm was observed

Co/Ni

Au/Ag

BNPs as nanocatalyst Mg/Al

Table 2.1 Bimetallic nanocatalysts used for biofuel production

Fermentative process Pyrolysis catalytic steam reforming

Catalytic steam reforming

Catalytic hydrogenation of HMF

Transesterification of plant oil Fermentative process Vapor phase hydrogenolysis of HMF Catalytic hydrogenation of HMF Hydrogenolysis of HMF Hydrogenolysis of HMF

Biomass conversion process Transesterification of plant oil

Hydrogen

Hydrogen

Hydrogen

DMF liquid fuel

DMF liquid fuel DMF liquid fuel DMF liquid fuel

DMF liquid fuel

Bioethanol

Biodiesel

Biofuel produced Biodiesel

Kodhaiyolii et al. (2019) Chen et al. (2015)

GomezBolivar et al. (2019) Gai et al. (2019)

Banerjee et al. (2014) Kodhaiyolii et al. (2019) RománLeshkov et al. (2007) Nishimura et al. (2014) Wang et al. (2014) Chen et al. (2017)

Reference Deng et al. (2011)

38 D. H. A. G. K. Perera et al.

Ru promoted Fe– Ni/γ-Al2O3 Si-Ni0 NPs

Nicu/γ-Al2O3 NiFe/γ-Al2O3 Steam reforming

Hydrothermal gasification Biomass pyrolytic gasification

Bagasse

Algal biomass named as Enteromorpha intestinalis Rice husk

The active metal particle sizes diminished gradually from 4.1 to 3.2 nm when the content of ruthenium was enhanced The active metal particle sizes diminished progressively from 4.1 to 3.2 nm as the content of ruthenium was enhanced –

Steam gasification

Municipal solid wastes

Sizes were observed from 0.3 to 0.35 mm range

Syngas

Hydrogen

Hydrogen

Hydrogen

Shen et al. (2014)

Norouzi et al. (2017)

Gao et al. (2017) Jafarian et al. (2017)

2 Green Synthesized Bimetallic Nanomaterials for Bioenergy Applications 39

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D. H. A. G. K. Perera et al.

Fig. 2.6 A schematic representation on the production of biodiesel by transesterification

In spite of the fact that biodiesel and biohydrogen are viewed as the finest substitutes to fossil fuels, bioethanol is the most extensively utilized biofuel throughout the world (Simas-Rodrigues et al. 2015). Bioethanol is a fuel made from renewable feedstocks such as corn, wood, sugar beet, straw, and wheat and is made almost exclusively from food crops (Demirbas 2007). Kodhaiyolii et al. stated that the Co-Ni NPs prepared via green approach are capable of synchronous generation of biohydrogen and bioethanol. However, the Co-Ni NPS augmented fermentation system resulted in a high bioethanol output and poor biohydrogen production (Kodhaiyolii et al. 2019). Hydrogen has recently been regarded as a viable choice among various alternative nonpolluting energy sources due to two main reasons, (1) it is a clean fuel that produces water as a by-product of combustion, (2) due to its low density, and it possesses the highest energy-to-mass ratio of all known fuels. The biohydrogen technique is favorable as the substrates used are renewable carbon biomass. On the other hand, biohydrogen generation is fundamentally sensitive to environmental aspects such as temperature, pH, nutrition supply, and toxin presence, and is far less stable than traditional thermochemical processes. Chen et al. proposed the reduction of water with Fe NPs at room temperature to be a viable procedure for hydrogen gas production. The method is easy to set up, requires no energy input, provides a clean byproduct, and produces hydrogen at a consistent pace. The production rate of hydrogen can be enhanced by 2–39 times by doping noble metals like Pd into Fe NPs. These BNPs produces pure hydrogen, and are free of common contaminants, such as CO2 and CH4. which are found in bio and thermal processes

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41

(Chen et al. 2011). Gai et al. produced heterometallic Ni
Cu NPs, in the absence of any noble metal, upheld on hydrochar to enhance H2 production (Gai et al. 2019). To lessen reliance on fossil fuels, the use of biomass to synthesize liquid fuels is of immense interest. The synthesis of 2,5-dimethylfuran (DMF) has been recognized as a promising liquid fuel generated from biomass. Cu-Ru-carbon and Pd-carbon catalysts are the conventional catalyst systems for the manufacture of DMF by hydrogenolysis of 5-hydroxymethylfurfural (HMF), a chemical that may be generated straightforwardly from cellulose. However, various by-products are produced as a result of the varied functionalities of HMF, resulting in a low DMF yield, and higher purifying expenses. The use of bimetallic catalysts is one of the methods for increasing the DMF output as bimetallic systems have an extra degree of freedom to tweak the geometric and electrical structures which allows to fine-tune the catalytic operation. Wang et al. proposed a very appealing path to renewable energy by the synthesis of DMF using hollow carbon nanospheres with Pt–Co BNPs (Wang et al. 2014). Chen et al. stated the use of Cu-Co BNPs with carbon coatings to produce the biofuel DMF (Chen et al. 2017). CO2 hydrogenation is one of the methods that can be used to synthesize methanol. CO2 being a component contributing to air pollution, can now be utilized to be converted to methanol, involving a reaction that requires a low energy input (Menin 2021). Therefore, this process is related to reducing the carbon footprint. Methanol production through CO2 hydrogenation with the use of BNPs has been studied. For example, Pd-Zn core–shell nanocatalysts were used and evinced for their effectiveness and high selectivity for CO2 (Liao et al. 2017). It is noteworthy to know that bimetallic catalyst synthesis methods affect catalytic activity exhibited by them. Impregnation is the simplest method out of the many procedures, which involves mounting metal precursors on oxides resulting in supported catalysts. Meanwhile, co-impregnation does also produces bimetallic catalysts that give out enhanced synergistic effects between the two constituent metals involved. Homogenously-distributed catalysts or those with definite stoichiometry can be prepared by co-precipitation. It is proved through some studies that bimetallic catalysts prepared through the co-precipitation method is much more effective in its action than those made through impregnation. Sol-immobilization being another procedure used is observed to result in much efficient and thermosstable catalysts than those produced through impregnation (Lia and Tsang 2018). Apart from the above-mentioned uses of bimetallic catalysts for bioenergy production, mentioned in the table (Table 2.2) are a few more reactions catalyzed by BNPs.

2.8

Conclusion

Accounting for the prevailing situation of the overexploitation of fossil fuels, new safe energy sources have to be identified to satisfy the increasing energy demands. Among the various alternatives, biomass appears to be the finest option as it is

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D. H. A. G. K. Perera et al.

Table 2.2 Chemically-synthesized bimetallic nanocatalysts used for biofuel production

Nanoscale/nm 0.8042  0.0014

Catalyst synthesis method Impregnation

Bioenergy application Bio-H2 production

–

Impregnation

Bio-H2 production

Impregnation

Bio-H2 production

EspinosaMoreno et al. (2018)

Ni/Fe

47.0  0.1, 35.7  0.2, 39.2  0.1, 47.0  0.1, 29.8  0.2 6.5–6.8

Impregnation

Pd/Fe

1–3

Impregnation

Cu/Co

10–60

Impregnation

Bio-liquid fuel production Biomass to value-added chemicals conversion Recovery of renewable biomass.

Ru/TiO2 and Co/TiO2

27.9

Impregnation

Yu et al. (2015) Liao et al. (2014) Chen et al. (2017) Tolek et al. (2021)

Pt/Fe-SiO2 Pt/Ni-SiO2

15.8, 9.5

Wetness impregnation

Pd/Ni

2–5

Impregnationreduction

Biofuel production

Ru/Co-SiO2 or ZrO2 Pd/Ag

–

Biofuel production

2–4.4

Pd/Cu

4.72, 4.16, 5.4

Wet impregnation Coimpregnation Incipient wet impregnation

BNPs as nanocatalysts Pt/Co

Pt/C-OX (where ox ¼ Al2O3, SiO2, ZrO2, and TiO2) Ir/Ni-La2O3 or CeO2 and Ir/Cu-La2O3 or CeO2

Use of lignocellulose biomass for the synthesis of numerous chemicals and fuels Bio-oil production

Upgrading of bio-oil Oxygenate upgrading for biofuels

Reference Reynoso et al. (2021) Vikla et al. (2021)

Chen et al. (2018) Zhang et al. (2018) Shu et al. (2020) Yue et al. (2020)) Bathena et al. (2020)

renewable and environmentally friendly. It is evident from the literature, though there are numerous methods for converting biomass into biofuel, utilizing nanocatalysts is the most effective and efficient approach as they have a high conversion rate and produce biofuel with high yield compared to that of other conventional methods. It can be concluded by the preceding discussion that the BNPs exhibit superior characteristics to MNPs, and are thereby, used as nanocatalysts in various applications, including bioenergy production. The BNPs

2 Green Synthesized Bimetallic Nanomaterials for Bioenergy Applications

43

synthesized using the green synthetic strategies has emerged as an interesting field of study due to the bioactive components present in them having capping and stabilizing effects, limiting NPs growth, and agglomeration. Finally, safety evaluations, economic, and life cycle analysis of NMs in bioenergy production are crucial for offering insights and guidelines for future study.

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Chapter 3

Green Synthesis of Metallic Nanoparticles for Biofuel Production Ankush D. Sontakke, Piyal Mondal, and Mihir K. Purkait

Abstract In today’s world, energy is the most essential entity for the socioeconomical development of a nation. Presently, the energy requirements of the world are basically handled via conventional fossil fuel. However, depletion in fossil fuel reserves urges for alternative fuels, for instance, biofuels, which are sustainable, environmentally friendly, and renewable. Nanotechnology has a wide aspect for providing more significant solutions to the challenges related to biofuel production via altering the characteristics of feedstock and biocatalysts. A relevant issue of discussion is the applications of nanoparticles (NPs) and green synthesis methods for biofuel generation. Considering the adaptability of nanoparticles of different forms and morphologies in biofuel production, the present chapter discussed in detail about the significance of cost-effective green synthesis methodologies for NPs. The chapter also explores the significance of NPs, feedstocks, and challenges involved in biofuel production. The detailed information related to the green synthesis of metal oxide or metal-based NPs via plant extracts, microalgae, bacteria, and other non-biological feedstocks is also provided. Furthermore, the factors affecting the synthesis NPs along with various functionalization approaches are elaborated for improving the yield and effectiveness of NPs via green synthesis methods. The presented chapter illustrates the recent advancement and application of iron, copper and other green metallic catalyst in biofuel/biodiesel production. Furthermore, the scope over improvements has been discussed for enhancing the overall efficiency of metallic NPs. Keywords Green synthesis · Metallic nanoparticles · Biofuel feedstocks · Functionalization · Biofuel production

A. D. Sontakke · P. Mondal (*) · M. K. Purkait Department of Chemical Engineering, Indian Institute of Technology, Guwahati, Assam, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Srivastava et al. (eds.), Green Nano Solution for Bioenergy Production Enhancement, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-16-9356-4_3

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Introduction

Energy is an essential entity for human development and plays an important role in the socio-economical development of a nation. Over the last few decades, energy needs have been enhanced drastically with the increment in rapid industrialization and the human population. Presently, fossil fuels contribute most of the requirements related to energy production. The scarcity and exhaustion of fossil fuels with the unstable price of energy production has threatened the world community and is also motivated to look forward to cheaper and more sustainable energy resources. Meanwhile, energy production via fossil fuels leads to the emission of greenhouse gas (GHG). Consequently, the concentration of GHG in the environment has been increased due to massive energy requirements for industrial development and human activities. Considering the importance of clean and sustainable energy resources, and to alleviate these challenges, the world community has taken the initiative to develop alternative energy sources such as biofuel (Sekoai et al. 2019; Rai et al. 2016; Saravanan et al. 2018). Biofuels include biodiesel, biogas, biohydrogen, and bioethanol. The research related to biofuels is gaining significant attention for energy generation due to the abounded availability of cheaper and non-edible feedstocks and a cleaner and environmentally friendly source of energy (Sekoai et al. 2019). The biofuels are majorly classified into primary and secondary biofuels, where the primary class biofuels are generated from the forests, plants, crop residue, and animal waste. However, the secondary biofuels are generated from both microorganisms and biomass feedstock. These secondary biofuels are further categorized based on the feedstocks into the first, second, and third-generation groups of biofuel (Sekoai et al. 2019; Aditiya et al. 2016). First-generation biofuels are produced from corn, sugarcane, sunflower oil, and wheat-like food crops. At the same time, the second-generation biofuels involve non-edible feedstocks such as Jatropha, Karanja, grass, corncob, and wheat/rice-straw (Zhang et al. 2018; Leong et al. 2018). These second-generation biofuels provide additional advantages over energy production via resolving social challenges such as water scarcity, deforestation, and food security (Leong et al. 2018; Ahmed and Sarkar 2018). Meanwhile, the last category of secondary biofuels, third-generation biofuels, is produced via several microalgae species. The research related to third-generation biofuels has been boosted since microalgae can bloom under various growing conditions and able to bring forth the different varieties of biofuels such as biodiesel, biogas, and bio-hydrogen (Leong et al. 2018; Ghimire et al. 2017; Wicker et al. 2021). The new generation biofuels with several advantages over their utilization for cleaner energy production also possesses certain challenges such as production cost, yield, pre-treatment of feedstocks, and lack of infrastructure for meeting the world’s energy requirements. Therefore, further research efforts are required for more efficient and cost-effective production of biofuels and evaluation of novel technological advancements. Nanotechnology may offer a more significant solution over the challenges related to biofuel production via altering the characteristics of

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feedstock and biocatalysts. In recent, the potential applications of nanobiotechnology for biosensors and sustainable production of bioenergy have motivated researchers to work over the development of biocatalysts for biofuel production (Rai et al. 2016; Zore et al. 2021). The nanoparticles (NPs) with their unique thermal, electrochemical, and optical characteristics have shown viable solutions toward various environmental and economic issues. Nanoparticles covers various interdisciplinary research fields due to their versatile properties. In addition, the presence of nanoparticles has influenced commercial impact in the pharmaceutical, polymer, electronics, and bioenergy-related research fields. The ability of the NPs to persist in diverse morphologies has also extended its versatility in a real-world application. Similarly, nanoparticles with crystallinity, higher surface area, superior catalytic activity, adsorption capacity, energy storage ability, and stability are considered excellent materials for biofuels systems (Bidir et al. 2021; Eggert and Greaker 2014). For instance, it was observed that NPs like metallic NPs and nanotubes were able to improve the metabolic reactions involved in bioprocesses for biofuel production (Antunes et al. 2014). In addition, nano-droplets and nano-crystals are utilized as a nano-additive for efficient blending of biofuel within convention fossil fuels such as gasoline and diesel (Srivastava et al. 2021; Malik et al. 2014; Tripathi et al. 2018). The nanoparticles are mostly used as a catalytic agent during biofuel production, where they play a substantial role in the electron transfer while enhancing the anaerobic activity for third-generation biofuels. Meanwhile, the nanoparticles can be synthesized via metal and metal oxide by both chemical and physical methods. The chemical methods include extensive usage of harmful, toxic, and costly reagents such as hydrazine hydrates, sodium borohydrides, and hypophosphite, which causes environmental hazards. The green synthesis methodologies provide a timely solution regarding cost and environmental hazards. As per the principle of green chemistry, the cost-effective and eco-friendly methodology for the synthesis of NPs must include renewable energy sources such as plants, waste materials, and microbes. In addition, energy optimization and minimization of waste discharges during the synthesis of NPs are also considered as a core aspects of green chemistry or synthesis. The synthesis of nanoparticles via green synthesis methods provides several advantages: easy and safe handling, cost-effectiveness, and environmental friendliness. Also, the NP synthesis via green synthesis methods eliminates the use of toxic reducing agents and external stabilizing and capping agents, which ultimately lowers the production cost and environmental hazards (Srivastava et al. 2021; Mondal et al. 2020). The utilization of nanoparticle and green synthesis methods for biofuel production is a timely topic of discussion. The detailed discussion related to the application of green synthesized metallic NPs for the generation of biofuels like biodiesel, bio-hydrogen, and bioethanol is scant. Because of the flexibility of nanoparticles in various forms and morphologies in biofuel production, the importance of costeffective green synthesis techniques for NPs was explored in depth in the current chapter. The chapter also explores the significance of NPs, feedstocks, and challenges involved in biofuel production. The detailed information corresponding to the

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green synthesis of metal oxide or metal-based NPs via plant extracts, microalgae, bacteria, and other non-biological feedstocks is also illustrated. Furthermore, the factors affecting the synthesis NPs along with various functionalization approaches are also elaborated for improving the yield and effectiveness of NPs via green synthesis methods. The presented chapter illustrates the recent advancement and application of iron, copper, and other green metallic catalyst in biofuel/biodiesel production. Finally, the future prospects and suggestions are provided for the improvement of biofuel production processes using NPs.

3.2

Feedstocks and Challenges for Biofuel Production

The biofuels are termed as gaseous or liquid fuels produced from renewable resources such as biomass and can be used in addition or in place of conventional fossil fuels. The biomass is biodegradable product fractions obtained from main products, residues and waste of forest, and agriculture. The feedstocks are categorized as edible and non-edible resources. However, considering the rapid growth in the human population, the utilization of edible sources is possibly impractical. Therefore, considering the importance of renewable energy sources and food scarcity, the world community must recognize technological advancements, welldirected policies, and infrastructure developments for biofuel production. As per the Paris agreement, the challenges related to biofuel production, including deforestation, must be considered for environmental sustainability while lowering the dependency on fossil fuels. Sources of renewable energy, for instance, biofuels, wind energy, and solar energy, contribute around 19% of total energy consumption throughout the world, out of which biofuel shares only 0.8%, and does not meet the criteria related to sustainability by the Kyoto protocol 2020 (García 2016). The Indian government has proposed a mandatory blending of bioethanol up to 20% in petrol or diesel, and it is effective since 2008 (Saravanan et al. 2018; Ministry of New & Renewable Energy 2011). As stated before, biofuels are categorized by type of feedstocks. All three biofuel generations are distinguished not only based on raw materials but also by the production technology involved to improve sustainability. The first-generation biofuels are produced using edible oilseeds and grains via convention chemical methods. In contrast, second-generation biofuels are primarily generated using non-edible entities such as wheat straw, Jatropha, and Karanja. Third-generation biofuels are generated via different microbes such as algae and bacteria. The details related to feedstocks and challenges for biofuel production are discussed subsequently.

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Edible Feedstocks

The edible feedstocks for biofuel or biodiesel production main consist of the oilseeds and grains such as palm, rapeseed, soybean, peanut, cotton, sunflower, and rice bran oils. Due to the vast production, the rice bran and rapeseeds are appropriate and wellknown feedstocks in Asian and American nations, respectively. The important attribute toward the feedstock selections for biodiesel production includes the oil yield, cultivated lands, composition of fatty acids. These attributes are majorly related to the profitability of biofuel production along with the engine performance and emissions. The worldwide usage of edible oils for the generation of biodiesel is projected as >33% and is expected to increase in the future. Such a scenario has threatened the world community, as with an increase in the use of edible oil for biodiesel production will also increase the land and water requirements. Therefore, the research and development over biodiesel production by edible oil is mostly dedicated toward sustainability and economic competitiveness. Some significant research attributes are the development of a heterogeneous catalysis system using nanomaterials and the use of external stimulus methods for increasing the rate of transesterification reaction and valorization of all the by-products (Sekoai et al. 2019; García 2016). For bioethanol production, starch or sugar-based edible sources such as corn, sugarcane, wheat, barley, molasses, and grains are widely used as feedstocks. In India, sugarcane molasses and grains are used particularly for bioethanol production. However, it is noteworthy that the edible crops used for biofuel production may lead to food scarcity as well as the water crisis. Deforestation is also an important concern over the use of edible feedstock, as limited land for the cultivation of edible crops directs the nation to explore the forest area for achieving their feedstock requirements (García 2016; Srivastava et al. 2020). Currently, the global bioethanol production is mainly based on edible feedstocks; the global contribution toward bioethanol production and related feedstocks is reported in Table 3.1.

3.2.2

Non-edible Feedstocks

As stated, the usage of edible feedstock as well as oils for the production of biofuel generates rivalry within food markets and creates water scarcity. The key objective of the present scientific community must include an investigation overuse of low-priced, non-edible, more accessible, and sustainable feedstocks alongside heterogeneous catalysts. The non-edible sources include oleaginous plants such as Jatropha, neem, Karanja, castor, mahua, and rubber seeds. The waste cooking oil (WCO) also serves as the primary feedstock for second-generation biofuels production. The non-edible oil contains toxic constituents, which makes them unfit for consumption, and therefore lowers the competition for the food and the feedstock as well. The significant advantages of non-edible oil are that it can grow in a wasteland,

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Table 3.1 Global contribution toward bioethanol production (García 2016)

Country USA Brazil Europe China Canada Thailand Argentina India Rest of the world Total

Production (US gallons in millions) 14,300 6190 1445 635 510 310 160 155 865 24,570

Share (%) 58 25 6 3 2 1 1 1 3 100

Feedstocks Corn Sugarcane Corn, sugar beet, wheat, barley, and rye Wheat, corn, sweet sorghum stalks, tapioca, cassava, and corncob Wheat and corn Cassava, rice, sugarcane, and molasses Grain, molasses, and juice Grain, molasses, and sugar Several Corn, wheat, molasses, sugarcane, barley, sugar beet, rice, sorghum, rye, and cassava

lowers the dependency on water and farmland, low cost, and eludes deforestation (García 2016). The major issues for the utilization of non-edible oils as feedstock in biodiesel generation are higher free fatty acid (FFA) contents, harvesting, and collections from the different areas. The higher FFA content oil suffers from high viscosity and saponification problems; therefore, feedstock pre-treatment is required (Ambaye et al. 2021). However, the recent development in the catalytic process has catalytic cracking overcomes the viscosity issues. Muthukumaran and Saravanan (2015) had produced hydrocarbon fuel from Indian laurel oil via reviewing the required process parameters using catalytic cracking. The feed was cracked using raw fly ash in a fixed bed catalytic reactor. The fuel was blended with the diesel tasted for engine performance. A B25 standard had shown comparable brake thermal efficiency with lower emissions (Muthukumaran and Saravanan 2015). Similarly, various studies have been reported on the efficient utilization of WCO, Jatropha, Karanja neem, and other non-edible feedstocks for biodiesel production (Srivastava et al. 2020; Ambaye et al. 2021; Bundhoo and Mohee 2018). The thirdgeneration biofuels utilize algae as feedstocks for biofuel production. Algae can be of micro- or macro-type obtained from the cyanobacteria and seaweed. There are >800,000 species that currently subsist on the planet, which can be harvested and grown with minimum nutrients within a month. The production of algae in wastewater is much economical and provides several advantages. The area requirement for algae production is 100 times lesser than that of other edible and non-edible oilseeds. However, proper conditioning with an optimum temperature of 20–30
C is required for the better growth of algae (Menetrez 2012). The significant challenges for biofuel production include (1) availability of low-cost lignocellulosic biomass, (2) pre-treatment, (3) infrastructure and logistics for production, (4) enzyme hydrolysis, (5) energy-efficient fermentation, (6) instituting the biofuel standard, (7) ensuring the minimum hazard to the environment,

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(8) by-product utilization, and (9) distribution logistics (García 2016; Ambaye et al. 2021).

3.3

Fundamentals of Nanomaterials and its Significance in Biofuel Production

The engineered nanomaterials are synthesized by redesigning the bulk materials at the molecular level. The smaller size and novel physicochemical properties of nanomaterials provide various opportunities for real-world application. Some of the distinct characteristics of nanomaterials are smaller size, higher surface-tovolume ratio, functionalization ability, stability in aqueous media, and most importantly, the optical, electrical, and magnetic properties (Srivastava et al. 2021; Khoo et al. 2020). These characteristics make nanoparticles suitable for adsorption, catalytic systems, drug delivery, and bioenergy-related application. The nanostructured materials are classified by the number of dimensions which are 100 nm, such as zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and threedimensional (3D) nanomaterials. The common nanomaterials are fullerene, quantum dots, nanotubes, nanoscrolls, graphene, dendrimers, and nanosheets. Recently, organic and inorganic nanomaterials are widely explored for several real-world applications. The inorganic NPs have owned significant attention due to their inertness, functionalization ability, and higher stability (Lohse and Murphy 2012). Inorganic NPs such as quantum dots, graphene, carbon nanotubes (CNTs), magnetic metal, metal oxides, and gold have shown great applicability and potential for several applications such as photocatalysis, drug delivery, membranes fabrication, environmental remediation, bioenergy, and sensing (Khoo et al. 2020; Sontakke and Purkait 2020; Sontakke et al. 2021). The NPs have been successfully used for biofuel production using transesterification, pyrolysis, and gasification. Also, the characteristics of NPs and nanomaterials as catalysts have driven the development of more economical, durable, and efficient biofuel production. The metallic NPs such as TiO2, MgO2, Fe2O3, and CuO have been widely used for biodiesel production (Vijayalakshmi et al. 2019). As stated previously, these nanomaterials can be fabricated via various synthetic and green routes; however, the properties of nanomaterials are firmly dependent on the method of fabrication. The green synthesis methods are still in progress. The details related to the fabrication of metallic nanoparticles via green synthesis methods using different biomass, plants, microalgae, and bacteria are provided in the subsequent section.

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Green Synthesis of Metallic Nanoparticles

Engineered nanomaterials have raised the potential of nanotechnology and materials science in every aspect of today’s world. The laboratory-prepared nanomaterials to enforce their potential must be brought forward and commercialized in the market. To lower the negativity, toxicological effects and riskiness of nanotechnology, green nanotechnology has paved the way for the application of nanomaterials (Mondal et al. 2020). Researchers have extensively utilized strong bases for synthesizing metallic nanoparticles (MNPs) and metal oxide nanoparticles (MONPs) such as sodium hydroxide and sodium borohydride as reducing agents. Capping and stabilizing agents were introduced to coat the nanoparticles formed. Moreover, organic solvents were added to dissolve them, which are found to be highly toxic and harmful to the environment. For synthesizing nanoparticles following the bottomup path, harmful and offensive chemicals such as sodium borohydride and hydrazine are often utilized for reducing and capping purposes, along with toluene/chloroform as dissolving solvents which are volatile in nature (Bystrzejewska-Piotrowska et al. 2009). The advantage of such a process includes the effective production of pure products, whereas the high cost of manufacturing and the usage of toxic chemicals are the major disadvantages. Preparation of MNPs, and MONPs through the green synthesis technique is now being considered as one of the best alternatives to chemical synthesis where biological systems (plants, microorganisms, and animals) or their parts are being utilized (Srivastava et al. 2021; Murillo et al. 2019; Singh et al. 2021). The significant advantages, uses, and applications of green chemistry with metallic NPs are shown in Fig. 3.1.

3.4.1

Synthesis of Metal Oxide Nanoparticles Using Plant Extracts

Bioactive components such as polyphenols, alkaloids, phenolic acids, proteins, terpenoids, and sugar in plant extracts utilized to synthesize nanoparticles possess an essential role in reducing, nucleating, and stabilizing the synthesized nanoparticles (Ahmed et al. 2016; Castro et al. 2011). When the plant extract and the metallic ion solution are added immediately, the biochemical reduction of the salts initiates to occur, and the solution mixture color changes, indicating the formation of nanoparticles. The activation period is marked initially where the zerovalent states of metal ions are attained or being converted to, and simultaneously nucleation process occurs (Malik et al. 2014). Subsequent to this step, the growth phase of the nanoparticles occurs where the neighboring reduced particles agglomerate together to form a larger bulk shape, generally spherical, triangular, and rod-shaped, which are thermodynamically more stable in nature (Akhtar et al. 2013). The final stage comprises of the coating and stabilizing ability of the plant extract for the nanoparticles synthesized, which provide them a more stable shape

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Fig. 3.1 Schematic representation of green chemistry with metallic nanoparticles (El Shafey 2020)

and size (Dwivedi and Gopal 2010). Various controlling factors, including reaction temperature, pH of the reaction medium, time of reaction, metal salt concentration, and the reducing property of plant extract, play a vital role in the synthesis and stabilization of NPs along with the yield percentage. On behalf of the existence of phytochemicals with water-soluble in nature present in fungi and bacteria, the metal ion reduction process is thus slow in comparison to the plant extract due to the long incubation period (Stephen and Macnaughton 1999). The phytomining process has been a plant-based metal nanoparticle synthesis technique where plants capable of high metal accumulation are grown on soils rich in metal content, and then subsequently, the metals are reduced in the presence of the plant metabolites and nanoparticles are formed (Rai et al. 2008). Usually, such processes are considered to be time-consuming and relatively slow. In order to overcome such limitations, Imran and Rani (2016) recommended to utilize the whole plant biomass and plant extract for nanoparticle synthesis purposes. The formation of metallic nanoparticles was found to be favored in an alkaline pH medium (Imran and Rani 2016). Compared to bacteria and fungi, plant-based synthesis is more advantageous due to the lack of pathogenicity (Pantidos 2014). Among the metallic nanoparticles, zinc oxide and iron oxide nanoparticles have been studied extensively by researchers. Parthenium hysterophorus was utilized by Datta et al. (2017) for synthesizing zinc oxide NPs having an average particle diameter of 16–45 nm (Datta et al. 2017).

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Similarly, Sutradhar and Saha (2016) synthesized zinc oxide (ZnO) NPs utilizing tomatoes’ aqueous extract (Sutradhar and Saha 2016). Zinc oxide nanostructures were synthesized using Aloe vera leaf extract by Sangeetha et al. (2011). Soni and Prakash (2011) explained the alteration in pH of the reaction medium’s functionality of the secondary biomolecules in the plant extract (Soni and Prakash 2011). It also concluded that pH alteration plays a key role in nanoparticle synthesis, and moreover, an acidic environment is not so effective for nanoparticle synthesis.

3.4.2

Nanoparticles Synthesis Using Bacteria

Various commercial biotechnological applications utilize bacteria toward bioleaching, bioremediation, and genetic engineering (Gericke and Pinches 2006a). Bacteria have been considered as a vital source for reducing metal ions and synthesizing nanoparticles (Iravani 2014). Prokaryotic bacteria and actinomycetes have been widely utilized for preparing metal/metal oxide nanoparticles. Due to the easiness of bacterial manipulation, bacterial synthesis has gained wide popularity in nanoparticle preparation. For synthesizing metallic nanoparticles, mostly the bacterial species used are: Lactobacillus spp., Escherichia coli, Actinobacteria sp., Pseudomonas sp., Klebsiella pneumonia, Bacillus cereus, and Corynebacterium sp. (Sunkar and Nachiyar 2012) Intracellular or extracellular mechanisms of nanoparticles synthesis are commonly found in bacterial species. Ahmed et al. (2003) utilized Pseudomonas stutzeri AG259 bacterium for synthesizing Ag nanoparticles. In this reaction process, an electron is donated by the NADHdependent reductase enzyme, which gets oxidized to NAD+. The electron donated is then being utilized for reducing Ag into nanoparticles (Ahmad et al. 2003). Similarly, Husseiny et al. utilized Pseudomonas aeruginosa for extracellular synthesis of reducing Au ions to Au nanoparticles (Husseiny et al. 2007).

3.4.3

Nanoparticles Synthesis Using Microalgae

Algae are widely known as aquatic microorganisms, which are found to be adequate for accumulating heavy metals and synthesizing metallic nanoparticles biologically (Murillo et al. 2019; Yew et al. 2021). Unicellular algae Chlorella vulgaris effectively reduced tetrachloroaurate ions into algal-bound gold and was subsequently reduced further to form Au nanoparticles. The results obtained showed that various tetrahedral and decahedral-shaped nanoparticles were accumulated over the cell surfaces (Luangpipat et al. 2011). Similarly, C. vulgaris extract was utilized to synthesize Ag NPs, and the extract, which consisted of proteins, acted as both stabilizing and reducing agents (Xie et al. 2007). Moreover, marine algae Sargassum wightii was utilized by Govindaraju et al. for extracellular synthesis of Ag, Au, and Au/Ag bimetallic nanoparticles in the range of 8–12 nm (Govindaraju et al. 2009).

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Microalgae are photoautotrophic microorganisms, generally unicellular (or) multicellular in nature and vary from 1 to 100 nm. The CO2 capturing efficiency of such species is 50 times greater than plants (Amin 2009; Xu et al. 2006). Oleaginous microalgae feedstock has gained great attention as an alternative energy source for producing biodiesel. In comparison to others, microalgae possess high biomass yield and oil content, higher photosynthetic efficiency, and higher growth rate (Abbaszaadeh et al. 2012). The main components of microalgae are carbohydrates, lipids, and proteins. Triacylglycerides (TAGs) form the lipids’ main component present in the microalgal species, which plays a vital role in biodiesel production (Sangeetha et al. 2011). Microalgae based biofuel production has the following merits (Spolaore et al. 2006; Mata et al. 2012) (Fig. 3.2): • Utilization of carbon dioxide from the atmosphere by the microalgae through photosynthesis. • Higher growth rate, doubling up within 24 h. • Apart from biofuel production, it can also be utilized for treating municipality, industrial, and agricultural wastewater. • Value-added products extracted from microalgae can be utilized in various industries. • Bioethanol production, and utilization as fertilizer, can be carried out for residual algal biomass. • Strong genetic capability to grow in harsh conditions and also can be cultured in wastewater. • Due to its higher photosynthesis efficiency, a higher amount of lipids are deposited. • The dearth of rivalry with food crops on the land.

3.4.4

Non-biological Green Route for Synthesis of Nanoparticles

Selection of the green method for synthesizing NPs consists of the challenging part, such as optimizing the energy of the synthesis process and the constraints involved in the experiment. Various physical conventional methods with modifications for synthesizing nanoparticles have the advantages of less energy requirement, low generation of hazardous materials, ease of operation, and higher yield rate. Advance treatment methods such as ultrasonication, microwave treatment, surfactant-based flocculation, and several others have also been studied extensively in many works. The utilization of sound waves in producing energy which is being harvested in sonication technique has been a very reliable source for producing controlled nanoparticles. Apart from being a clean route, such a technique is also considered to be easy to operate and time-consuming. Ultrasounds and microwave heating techniques have effectively been utilized as energy carriers for easily synthesizing

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Microalgae

High Lipid Content

Extraction of lipid

Biochemical conversion

Microalgal Residue

Lipid

Low Lipid Content

Bio-gas

Bio-oil

Bio-char

Transesterification (Methanol and Catalyst)

Biofuel

Glycerol

Other Industrial applications

Fig. 3.2 Microalgae-based energy conversion processes. (Reproduced with permission from Vijayalakshmi et al. 2019)

nanoparticles of specific morphology by easily breaking the agglomerated bulk (Sontakke and Purkait 2020, 2021; Arpia et al. 2021). Pang et al. reported the utilization of the sonication method for co-precipitation synthesis of lanthanum and strontium conjugated manganese dioxide nanoparticles of 24 nm after thermal treatment. The sharp transition was observed in the magnetic property of the synthesized nanoparticles from paramagnetic to ferromagnetic behavior at 366 K (Pang et al. 2003). Moreover, Al-Kaysi et al. studied the crystalline formation of zwitterionic organic nanoparticles from the amorphous phase utilizing the sonication technique in the solution phase. The synthesized nanoparticles were stable crystalline in nature and had a disk-shaped morphology of 140 nm. The study concluded the effectiveness of the sonication technique for enhanced colloidal stability and providing definite morphology of the nanoparticles (Al-Kaysi et al. 2005).

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Factors Affecting for the Synthesis of Nanoparticles

During the synthesis of metallic NPs via green chemistry, various regulatory parameters come into play, which results in a reduction, nucleation, and nanoparticle growth and stabilizing coating formation. The following main controlling parameters are discussed briefly in the following section:

Influence of pH During nanoparticle formation, the pH of the reaction medium plays a key role in the shaping of NPs synthesis and agglomeration (Khan et al. 2019). From previous works, it was perceived that at lower pH conditions (acidic condition), more agglomerated and larger particles are produced, in comparison to higher pH value (Sathishkumar et al. 2010; Dubey et al. 2010). Avena sativa (Oat) biomass mediated synthesis of Au nanoparticles showed that at lower pH 2, narrow particle diameter range was obtained within 25–85 nm, whereas when the pH increased to 3–4, the particle size decreased to 5–20 nm range (Armendariz et al. 2004). The results obtained concluded that at a higher pH range, influential functional groups within the extract played a vital role in shaping the narrow particle size NPs compared to the higher pH.

Influence of Reactant Concentration The amount of polyphenolic content within the plant extract plays a vital role and significantly influences the formation of metallic NPs. Huang et al. concluded that variation in the content of Cinnamomum camphora (camphor) leaf extract enormously affects the shape and size of the synthesized metallic nanoparticles (Huang et al. 2007). Chandran et al. varied the content of Aloe vera leaf extract and synthesized Au nanoparticles which varied from triangular plate shape to spherical on (Chandran et al. 2006). The study concluded the important role of carbonyl groups in the plant extract toward shaping nanoparticle growth.

Influence of Reaction Time Ahmad et al., in their research study, concluded that reaction time plays an essential role for synthesizing Ananas comosus (Pineapple) extract-mediated spherical Ag nanoparticles of size 12 nm, with rapid color change within 2 min (Ahmad and Sharma 2012). Dwivedi and Gopal, in their research work, utilized Chenopodium album leaf extract to synthesize Au and Ag NPs. It was observed during the experiment that within 15 min of reaction time, the formation of nanoparticles

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occurred and continued till 2 h. Whereas on continuing after 2 h, the rate of formation of nanoparticles decreased (Dwivedi and Gopal 2010).

Influence of Reaction Temperature The reaction temperature is generally termed to be an important parameter for synthesis purposes; moreover, it plays a crucial role in plant extract-mediated synthesis for determining the shape, size, and yield of nanoparticles (Srivastava et al. 2021; Sathishkumar et al. 2010). Studies from previous research showed that Citrus sinensis (sweet orange) peel extract-mediated synthesis of Ag NPs at 25
C reaction temperature yields nanoparticles of 35 nm diameter. Moreover, ~10 nm average diameter nanoparticles were synthesized when the reaction temperature was enhanced to 60
C (Kaviya et al. 2011). Research work conducted by Gericke and Pinches shows the variation of nanoparticle shape and rate of formation with reaction temperature variation. Au nanoparticles with spherical structures were predominantly formed at a lower temperature, whereas rod and plate-like structures were formed at higher temperatures (Gericke and Pinches 2006b). An increase in temperature enhances the reaction process along with particle formation; with an increase in temperature, the average particle size reduces, and the conversion rate increases steadily.

3.5

Functionalization of Metallic Nanoparticles

The NPs exhibits exceptional chemical and physical properties; however, they do not show appropriate surface characteristics for specific applications. That creates an opportunity for functionalization with suitable functional groups to improve their surface characteristics. The recent advancement in the NPs with functionalization of metal and metal oxide NPs have shown significant developments for catalysis, bio-imaging, and other biomedical application (Neouze and Schubert 2008; Razzaque et al. 2016). Functionalization improves the stability of various inorganic metallic and magnetic NPs and avoids accumulation. In addition, functionalization helps to regulate the shape, size, and surface chemistry of nanoparticles. Another aspect of functionalization is improving the compatibility within two different phases, such as inorganic nano-fillers in membranes or organic polymers. The surface modification sidesteps the homogeneity issues of the two-phase and enhances the mechanical properties like composites. A simple approach of NPs functionalization by organic groups provides sufficient ability to avoid aggregation. The aggregation of NPs tends to a reduction in specific surface area as well as interfacial energy, which affects the reactivity of particles (Fig. 3.3) (El Shafey 2020; Neouze and Schubert 2008). In most stabilization methods, surfactants are used to functionalize NPs; however, these methods also produce the waste stream and lower the product yield. Therefore,

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Fig. 3.3 Aggregation of nanoparticles in the absence and presence of stabilizing agents (El Shafey 2020)

it is essential to develop environmentally friendly or bioconvenient stabilizers. At present various green stabilizing agents such as polyphenols, citric acid, polymer ligands, and enzymes have shown significant functionalization and stabilization ability (El Shafey 2020). The nanoparticles can be functionalized via amino-based, metal-based, and hydrophobic-based functional groups. Liu et al. (2012) synthesized hydrophobic silica-coated magnetic NPs via silication. The hydrophobic magnetic NPs (HMP) are further used for the immobilization of lipase. Resulted NPs were used for the transesterification of olive oil for biodiesel production. The functionalized NPs successfully yield 70% biodiesel with the rate of 43.5 g/L h (Liu et al. 2012). Similarly, Wang et al. (2009) prepared amino-functionalized Fe3O4 magnetic nanoparticles (MNP). The Porcine, pancreas, Candida rugosa, and Pseudomonas cepacia lipase were further immobilized over amino-functionalized MNP. The obtained NPs were exposed to enzymatic transesterification of soybean oil and successfully converted to biodiesel 67% (Wang et al. 2009). The functionalization of metallic NPs are mostly done by impregnation, crosslinking of functional group as well as co-precipitation, hydrothermal and sol-gel techniques. As discussed previously, the main objectives regarding functionalization of NPs are stabilization, avoiding NPs aggregation via impregnation of functional groups of specific surface characteristics, and avoiding the oxidation process of magnetic and metallic NPs.

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Application of Green Synthesized Nanoparticles for Biofuel Production

The biofuels can be done by thermochemical or biochemical hydrolysis of lignocellulosic biomass along with the transesterification and fermentation of edible, non-edible, and microalgae feedstocks. However, the execution of these processes suffers from technological challenges as well as production cost and product yield issues. The introduction of green synthesized or bioinspired nanoparticles as a catalyst may provide a potential approach toward lowering the cost and increasing the yield of biofuel. Recently, nanoparticles with their size, surface characteristics, and ability to enhance the reaction rates in bioprocesses are gaining significant interest for biofuel production. Previously several nanomaterials such as nanotubes and metallic NPs are extensively utilized for the production of biodiesel and bio-hydrogen. This section discussed in detail about various green synthesized metallic nanoparticles used for biofuel generation and production yields.

3.6.1

Iron-Based Nanoparticles

The employment of green synthesized iron-based NPs as a catalyst for biomass transesterification and fermentation significantly enhances biofuel production processes’ efficiency, thermochemical stability, and productivity (Sekoai et al. 2019). While, in the case of lignocellulosic biofuel, these iron-based NPs may stimulate the reaction steps in biofuel production involving pre-treatment of biomass and fermentation process. Nanomaterials enhance the yield of sugar during the pre-treatment of biomass and reduce the use of chemical reagents, which ultimately reduces the cost associated with the pre-treatment process (Arora et al. 2020). Recently, Khalid et al. (2019) studied the synergetic effect of alkali pre-treatment with different concentrations of iron magnetite NPs. The results showed a significant increase in methane yield (129%) and biogas (100%) by using 2% NaOH and 120 ppm magnetite NPs for pre-treatment. However, when the magnetite NPs (100 ppm) are used alone for the pre-treatment, the production of methane and biogas was found to be 33% and 37% higher, respectively (Khalid et al. 2019). Pre-treatment of biomass removes the lignin, and the fermentation is carried away by hydrolysis of cellulose into fermentable sugar via enzymatic or chemical routes (Arora et al. 2020). However, the enzymatic hydrolysis is executed by cellulase, and harsh operating conditions are required for the complete hydrolysis of pre-treated biomass. The use of iron-based NPs increases the stability of enzymes during the immobilization to a certain wide range of pH and temperature while enhancing the efficiency of the hydrolysis process (Srivastava et al. 2021). Recent research outputs had supported the finding related to the use of iron NPs for improving the stability of enzymes at higher temperature and an eclectic range of pH. Ladole et al. (2017) studied the effect of sonication treatment for cellulase immobilization using

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magnetic nanoparticles (MNPs). It was observed that the catalytic activity of cellulase@MNPs increased by 3.6 times, also improvement in the thermal and pH stability was observed. The optimal temperature was found as 50–60
C, which is about 10
C higher than free cellulase. Also, the enzymatic activity was improved slightly for all the pH as compared to free cellulase (Ladole et al. 2017). Similarly, Abraham et al. (2014) observed the increase in enzymatic ability of cellulase on hematite and ferrite NPs, also the thermal stability was improved to 80
C for 4 h while retaining 66% of their initial activity (Abraham et al. 2014). The utilization of iron-based nanoparticles also increases cellulase storage stability as well as re-usability. Selvam et al. (2016) reported improvement in thermal stability during immobilization of cellulase using magnetite NPs. It was found that the enzymatic activity of cellulase is retained 70% of its initial activity after ten cycles, which results in a reduction in overall cost. In addition, iron-based NPs also enhances the production of biofuel via thermochemical treatment. The implementation of metallic NPs lowers the mass-transfer resistance, tar production, and reaction time while increasing the reaction rate and conversion (Selvam et al. 2016). Shen et al. (2015) reported a decrease in tar by 92.3% during the pyrolysis via nickel-iron-based nanocatalyst. The yield of syngas was observed as 2.11 L/g at 800
C. Alongside bioethanol and syngas, the green synthesized iron-based nanoparticles are extensively utilized for biodiesel production (Shen et al. 2015). Alves et al. (2014) used iron-tin oxide (Fe-SnO) NPs as catalysts for biodiesel production from soybean oil. The transesterification reaction was carried out for 1 h at 200
C, and a yield of 84% was obtained. The nanocatalyst was further recovered magnetically and reused for 4 times. It was found that there is no loss in the catalyst activity. Similarly, oxides of iron-cadmium (Fe-CdO) was prepared for biodiesel production. The surface area of the Fe-CdO catalyst was observed to be double to that of Fe-SnO; however, the catalytic activity was found quite similar for the esterification process (Alves et al. 2014). Chen et al. produced the biodiesel using microalgae with the help of mixed metal oxide Fe3O4/ZnMg (Al)O. The nanocatalyst showed excellent magnetic properties with a larger surface area and successfully obtained biodiesel with 94% yield. The catalyst may be recovered and reused; however, the conversion was further decreased to 82% after seven cycles, which signifies a reduction in the activity after each cycle (Chen et al. 2018). The utilization of ironbased NPs for biofuel production has proven to be a suitable alternative in heterogeneous catalysis systems. However, further functional modification may enhance the productivity and cost of biofuels.

3.6.2

Manganese Based Nanoparticles

Manganese (Mn) as a transition metal and its oxide form manganese oxide (MnO) termed as good catalyst due to the higher porosity, existence in the different valence states of Mn ions such as Mn4+/Mn3+/Mn2+ (Duan et al. 2015). Based on the oxidation state, the Mn ions form Mn oxides and exist as MnO, MnO2, Mn2O3,

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and Mn3O4 forms (Sharma et al. 2016). The MnO2 is utilized as a catalyst in chemical reactions, mainly as a single MnO2, supported MnO2, and composite MnO2. Similar to other metal oxides, the phase structure and morphology of MnO2 affect their catalytic performance (Huang et al. 2019). The catalyst can be synthesized by various methodologies such as sonication, precipitation, sol-gel, and hydrothermal methods. However, the sonication process is termed green synthesis methodology, as it does not involve any harmful chemical reagents and can be performed at room temperature within a short interval (Stegarescu et al. 2020). Recently, Stegarescu et al. (2020) had biochemically synthesized the MnO2 nanoparticles using tarragon and rosemary oregano plant extracts. The nanoparticles are synthesized by reducing the KMnO4, where the plant exacts acts as a reducing agent, and the reduction was carried away from Mn7+ ions to Mn4+ in final product as MnO2. It was determined that the specific surface area of nanoparticles synthesized via biochemical methods was four times higher to that of chemical method. The MnO2 nanoparticles obtained from rosemary oregano has larger crystalline size of 3.4 nm and surface area 348 m2/g with pore volume 0.77 cm3/g. The nanoparticles were examined for biodiesel generation from seeds oil and grape residues via microwave-assisted transesterification process. Also, yeast (Saccharomyces cerevisiae) has been used as a catalyst for the comparing the catalytic activity of MnO2. The MnO2-oregano nanoparticles shown a best catalytic activity in transesterification reaction than yeast catalyst. The reaction rate of transesterification was found to be 3.5 times higher for MnO2-oregano (Stegarescu et al. 2020). Meanwhile, along with the activity of the catalyst, the methanol: oil molar ratio, reaction temperature, time is significant reaction characteristics of transesterification reaction and directly affects the conversion of biodiesel. Numerous studies have been reported to optimize reaction parameters for higher conversion of biodiesel (Jha and Sontakke 2018, 2020; Di Serio et al. 2005). The heterogeneous catalyst is extensively studied for the production of biodiesel to shorten the production process and reduce the environmental effects with production cost. Dias et al. (2013) had compared the catalytic performance of calcium manganese mixed oxide, Cao, and NaOH heterogeneous catalyst for biodiesel production. In addition, they have investigated the impact of catalysts and different raw materials on the quality and conversion of biodiesel. In the case of different catalysts, the product quality was nearly similar at about 92.5 wt.% for NaOH and calcium manganese mixed oxide; however, for CaO, it was slightly higher at about 93.8 wt.%. For different raw materials WCO, pork lard and the mixture of WCO (78 wt%) and lard, the transesterification reaction was a bit slower in the case of lard. Meanwhile, similar progress was observed for heterogeneous catalysts in the response, independent of raw materials; however, water content was improved to that of homogeneous process, which suggests the further requirement of purification steps (Dias et al. 2013). Cherian et al. (2015) prepared MnO2 nanoparticles via the co-precipitation method for bioethanol production via immobilization of cellulase. The cellulase was prepared from Aspergillus fumigatus and immobilized over MnO2 NPs. The cellulase bounded MnO2 NPs were characterized via SEM and FTIR for validating

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the immobilization. In addition, properties of cellulase after immobilization were investigated for thermal, operational, and pH stability. The particles size of MnO2 NPs was found as 76 nm, and it was found to be increased to 101 nm after immobilization. Also, the enzymes after immobilization showed better thermal stability at the optimum temperature of 70
C for 2 h and re-usability by retaining 60% of its initial activity after five cycles. The bioethanol yield was found to be 21.96 g/L for MnO2 NPs assisted cellulase immobilization (Cherian et al. 2015).

3.6.3

Other Green Nanocatalyst for Biofuel Production

The application of green synthesized iron oxide and manganese oxides NPs are extensively studied for biofuel production. However, there are some other metallic, and non-metallic NPs are also explored for biodiesel and bioethanol production, which are discussed below. Recently, Varghese et al. (2017) synthesized CuO NPs from the extract of Centella asiatica leaves and used coconut oil for biodiesel production. The aqueous extract was used as a bio-reducing agent for the reduction of copper acetate in CuO NPs. The obtained biocatalyst (1 wt%) was used for transesterification coconut oil with 3:1 methanol: oil wt.% at 60
C. It was found that the coconut oil was effectively transformed into fatty acid methyl esters (FAME) and confirmed by GC-MS analysis (Varghese et al. 2017). Sarno and Iuliano (2018) tested immobilized lipase for the production of biodiesel from the spent coffee ground. An active biocatalyst was prepared by sonicating Fe3O4/Au@OA and 1 g citric acid solution in methanol. The yield of biodiesel was found to be 51.7% in 3 h; however, it was reached to 100% in 24 h. Also, the catalyst was stable for 60 days and retained 90% of initial activity and also showed an excellent re-usability (Sarno and Iuliano 2018). Saranya and Ramachandra (2020) synthesized a biological catalyst from a Cladosporium tenuissimum fungal strain for biodiesel production using diatom Nitzschia punctata. The raw fungal lipase was precipitated via ammonium sulfate and purified further with the help of the Superdex 200 gel chromatographic system. The lipase was stable at temperature 60
C and pH 6. The biodiesel yield was observed to be 87.2  0.47%, higher than acid catalyst (83.2  0.35%) (Saranya and Ramachandra 2020). Vahid et al. (2018) prepared magnesium-based nanoparticles/catalysts using combustion and hybrid co-precipitation methods. The hybrid co-precipitation methods were performed via hydrothermal- co-precipitation and ultrasound-assisted co-precipitation. The catalyst obtained by hybrid co-precipitation has shown higher surface area but lower catalytic activity and poor biodiesel production performance than that of the simple combustion method. As the catalyst obtained from the simple combustion method, it can be established that the catalytic activity might be related to the higher porosity and crystallinity of the catalyst. The catalyst prepared by the combustion method has shown a conversion rate of 95.7% and better re-usability results for six cycles. The study implied that crystallinity, pore size, and catalyst

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morphology are perceptible characteristics in transesterification reactions (Vahid et al. 2018). In the other study, Farooq et al. (2016) evaluated the catalytic activity of γ-Al2O3–MgO supported solid catalyst of bi-functional behavior over instantaneous esterification and transesterification reaction for biodiesel production using WCO. The catalyst support was synthesized via the wet impregnation method with a different weight ratio of Mg as 5, 10, 15, and 20 wt.%, respectively. Furthermore, the support was modified by Zn, Sn, and Mn metal oxides after loading of Mo 5 wt.%. The obtained catalyst with bi-functional behavior shown superior performance for biodiesel production. It was observed that Mo–Mn/γ-Al2O3–15 wt% MgO had provided the highest yield of 91.4% compared to Mo–Sn/γ-Al2O3–15 wt% MgO or Mo–Zn/γ-Al2O3–15 wt.% MgO catalysts. The optimum reaction parameters were observed as 100
C, 4 h and molar ratio of methanol: oil as 27: 1.5 wt% (Farooq et al. 2016). Viola et al. (2012) examined CaO, K3PO4, and SrO, solid catalysts for biodiesel production from fried oils. The SrO and CaO catalyst were used in granules and powder form, the granules were employed in catalytic bed reactor, and the results of FAME yield were compared with the batch reactor. It was found that granules do not make any difference in the FAME yield for 3 h of reaction time. Also, there was not much difference in the yield of biodiesel from catalytic bed reactor and batch reactor after 3 h of reaction. The CaO, K3PO4, and SrO catalyst has resulted in 92%, 78%, and 86% yield of FAME at 65
C with 5 wt.% loading. The catalyst had been reused for transesterification without regeneration; the yield was reduced by 10–20% due to the loss of catalyst activity and efficiency (Viola et al. 2012). Similarly, various transition metals, metal oxide and mixed metal oxides are utilized as a catalyst for biofuel production (Sekoai et al. 2019; Arora et al. 2020; Chooi et al. 2021). However, further improvements related to catalytic activity and the cost of preparation are obligatory.

3.7

Conclusion and Future Prospective

The rapid industrial and population growth had tremendously enhanced the world’s energy needs, and depletion in fossil fuels with their environmental hazards made the world community to look forward for better alternatives. Biofuels are sustainable, environmentally friendly, and renewable alternative fuels toward the conventional fossil fuels. The biofuels, for instance, biodiesel, bio-hydrogen, and bioethanol, had shown their superiority in terms of resources, availability of feedstocks, energy efficiency, production cost and most importantly, sustainability and compatibility toward the environment. In order to accomplish the future energy requirements and overwhelming the technological and economic barriers in biofuel production, the integration of nanomaterials had provided numerous advantages to increase the quantity and quality of biofuels. It was observed that the metallic catalyst had been extensively used for the production of biofuel mostly, biodiesel from edible and non-edible sources. However, most of the synthesis processes for metallic

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nanoparticles involves harsh chemical, which is corrosive in nature and expensive too. Fortunately, green synthesis methodologies had provided a suitable alternative toward the profitable and eco-friendly synthesis of biofuels. Several bio-components and natural extracts such as plants, seeds, bacteria, and yeast have been effectively engaged for the synthesis of metallic nanoparticles. These extracts have successfully stabilized the NPs via controlling their shape, size, and structure. The functionalization ability of nanoparticles had provided additional benefits for biofuel production by incorporating specific surface characteristics and improving stability. This chapter was organized in order to encompass the cutting-edge research findings related to feedstocks, green synthesis routes for NPs using plant extract, microalgae, and bacteria. Herein, the factors affection synthesis of NPs such as temperature, pH are also discussed along with functionalization methodologies of NPs. The applications of green synthesized metallic NPs are explored with the help of recent and relative pieces of literature. The major challenges for the rapid production of biofuels to fulfill the energy requirements of the world were found to be feedstocks availability, pre-treatment, product yield and utilization of by-products for enhancing profitability. It was found that the edible oil feedstocks for biofuel production suffer from the water as well as food and land crisis. However, deforestation for crop production to meet the energy requirements is also a major concern from an environmental safety point of view. The future prospects for the research related to biofuel production include the development of new novel nanomaterials, energy-efficient and economical process developments, improvement in the yield of biofuel, and technological advancements pertaining to by-product utilization. The green synthesis of metallic NPs is still a largely unexplored area, and further research efforts must be made to improve the techno-economic perspectives of these novel materials.

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Chapter 4

Recent Advances in Synthesis of Iron Nanoparticles Via Green Route and Their Application in Biofuel Production Pranjal P. Das, Piyal Mondal, and Mihir K. Purkait

Abstract Due to the scarcity of alternative energy production routes apart from conventional fossil fuels, the researchers have concentrated on various renewable sources for energy production. Biofuel has been one of the recent attractions of the research field by many scientists. Various ways and techniques have been excavated by researchers to produce biofuels which are thought to be the next-generation alternative to the current conventional production method. In this chapter, the role of green synthesized iron nanoparticles has been discussed in producing biofuels. Iron being the most abundant metal on earth is very cheap and easily available, and along with its green synthesis techniques of nanoparticle preparation provides the chapter an in-depth detailed analysis of such nanoparticle preparation and its application toward biofuel production. The chapter discusses about the green synthesis mechanism, and operating parameters for controlling the shape and size of iron nanoparticles. Moreover, the role of iron nanoparticles in producing biofuels is discussed in detail along with its toxicology effects in the environment. Future scope of developments and study provides a way for advancements of such study for better effectiveness. Keywords Green synthesis · Iron nanoparticles · Biofuel production · Toxicology study

4.1

Introduction

Nanotechnology can be termed as the evolution of matter utilizing chemical, physical, as well as biological processes and techniques with or without the addition of other constituents to create substances with specific functionalities, improved properties, and specialized attributes that can be employed in versatile applications. P. P. Das · P. Mondal (*) · M. K. Purkait Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Srivastava et al. (eds.), Green Nano Solution for Bioenergy Production Enhancement, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-16-9356-4_4

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Recently, nanotechnology has achieved a significant thrust in the area of research and development. It has demonstrated a major influence on almost every sphere of medicine and engineering and is still an ongoing area of research for exploration in novel applications. Owing to its specialized, unique and exceptional characteristics at nanoscale, a wide range of applications, viz. bioremediation of different inorganic and organic contaminants, chemical sensors, biomedicine, catalysis, pharmaceuticals, food conservation, and optoelectronics has gained a considerable importance over the last few decades (Khan et al. 2019). A nanoparticle is defined as a minute particle consisting of at least one of its dimension/size not higher than 100 nm. As such, the developed nanoparticles are prominent from the bulk materials, as they demonstrate distinct optical, thermal, and electrochemical characteristics, along with a significantly high surface area to volume ratio. Such distinct characteristics of the nanoparticles are accountable for its widespread utilization in the domain of environmental remediation, biotechnology, electronics, defense, chemistry, energy, agriculture, and various other technological areas. Different conventional methods for the fabrication of nanoparticles consist of several drawbacks, viz. (a) physical methods for the fabrication of nanoparticles are very costly and time consuming, thereby leading to less feasibility of the process for industrial scale production; (b) chemical methods for nanoparticle synthesis such as sol serum technique and element lowering approach vigorously utilizes various hazardous and toxic materials (hydrazine hydrate, sodium borohydride and hypophosphite) which are harmful for the environment, also the synthesis route for the production rate of nanoparticle is considerably slow and the nanomaterials formed are usually smaller in size with irregular structure and increased toxicity (Stefaniuk et al. 2016; Goswami and Purkait 2014). The limitations of such traditional methods can be easily subdued by the use of sustainable and green synthesis route of nanoparticles. Therefore, developing a proficient, sustainable, and cost-effective green procedure for the fabrication of nanoparticles is the current need. Steady and well-functionalized nanoparticles are reported to be developed from biological sources, viz. plant extracts, animal tissues, actinomycetes, yeast, bacteria, fungi, algae, and other microbes under ambient physiochemical environment. Such microbes act as sustainable precursors and are considered as eco-friendly. Nevertheless, factors such as social adaptability, resource availability, as well as economic viability causes a considerable influence on the overall process sustainability. There are several benefits and salient features associated with the green synthesis techniques of nanoparticles such as environment friendly, moderately inexpensive, safe handling, non-requirement of synthetic reducing, and capping agents and formation of immensely stable product. Last but not the least, other benefit involves the utilization of active natural compounds (plant extracts) as capping and stabilizing agents for the preparation of small sized nanomaterials in large quantities (Gonzalez-Moragas et al. 2015). Due to its significant catalytic properties, nanomaterials can be effectively utilized in improving the production of biofuels, which is an alternate and sustainable option to considerably replace the fossil fuels along with its harmful ecological impacts. The production of biofuels can be assisted by the fermentation and bioconversion of cellulosic or organic substrate

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Fig. 4.1 Schematic diagram of various green synthesis techniques and possible application of iron nanoparticles

utilizing fermentative microbes and cellulolytic enzymes. Moreover, during the conversion process of biomass to biofuel generation, iron acts as a co-factor for enzymes, which is accountable for modifying the enzyme performance, thereby resulting in the improvement of biofuel production. Also, in different processes of biomass to biofuel generation, the presence of iron considerably affects the metabolic and enzymatic activity of microbes by successfully tailoring the rate of electron transfer (Kandel et al. 2014; Astolfi et al. 2019). Figure 4.1 depicts an overall schematic diagram of the present chapter. Given these facts, this chapter is emphasized on the exploration of different existing sustainable and green routes to fabricate iron-based nanomaterials and nanocomposites. Various green synthesis approaches of nanoparticles utilizing plants, microbes, biocompatible green reagents, and low-cost energy related techniques have been explored in detail. Also, the influence of different physicochemical factors on the green route synthesis approach has been discussed with the aim of achieving more economic and sustainable fabrication techniques. Finally, the environmental applicability of green synthesized iron-based nanomaterials in the production of biofuels and the influence of its fabrication methods have been discussed

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elaborately along with the prevailing disadvantages and possible solutions associated with it, which is very scant in the current literatures.

4.2

Green Synthesis of Iron-Based Nanoparticles

In accordance with the green chemistry principles, utilization of renewable energy, optimized consumption of conventional energy, and minimization of waste released into the environment should be the basic criteria for an efficient and sustainable methodology for fabrication of nanoparticles. As such, the conceptualization of green synthesis route must incorporate the use of microbes, plants, waste materials, and biopolymers for the active bio-component along with moderate heating conditions and benign solvents. In many studies, it has been reported that water can be utilized as a solvent with bioactive compounds, especially polyphenols (plant extracts) as stabilizing, reducing, and capping agents. Such bioactive polyphenols were derived from roots, leaves, fruits, seeds, gums, fruit peels, and even waste of different vegetations. Figure 4.2 illustrates the diagrammatic representation of different green synthesis steps involved in the preparation of iron-based nanoparticles.

Fig. 4.2 Diagrammatic representation of steps involved in the green synthesis of iron nanomaterial showing (a) extraction, (b) reduction, (c) stabilization of nanomaterial. (Obtained with permission from Srivastava et al. 2021)

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Most of the green synthesized nanoparticles are usually stored at a definite pH in the aqueous form to impart uniform particle size and stability. In addition, utilization of biocompatible reagents which imparts less reactivity also helps in significantly improving the stabilization of the nanoparticles (Bolade et al. 2020). The green synthesis routes discussed in this chapter for the fabrication of iron nanoparticles are as follows:

4.2.1

Microorganism-Based Synthesis

Investigation of microbes, viz. fungi and bacteria have been well established for the fabrication of iron nanoparticles, owing to the substantial amount of bioactive molecules present in it. Fabrication of nanoparticles utilizing microorganisms follows metabolic pathways, viz. extracellular and intracellular routes. Extracellular and intracellular proteins, lipids, enzymes along with the chelating action of DNA subunits considerably play an active participation role as reducing agents during the fabrication process. Such bioactive compounds consist of high reduction potential and have the ability to contribute H+ ions to bring down the oxidative condition of the metal ions from higher to lower state. Fabrication of iron-based nanoparticles like magnetite (Fe3O4) by intracellular pathway can be carried out by sulfate-reducing bacteria, viz. Desulfovibrio magneticus RS-1T, at an optimal pH of 7.0 and operating temperature of 30
C. This helps in producing magnetosomes which lowers the iron metal salt content used as a precursor, thereby resulting in the production of irregular magnetic nanoparticles (bullet shaped) with an average diameter of 90–100 nm (Sakaguchi et al. 2002). Iron nanoparticles have also been fabricated via utilization of bacteria such as Bacillus licheniformis (1.26 g) and Bacillus subtilis (1.62 g) by lowering the mixture of precursor salts (FeCl3 and FeCl2) in a ratio of 1:2.5 with the help of reducing agents, viz. ammonia and urease enzyme, followed by an incubation time of 96 h. This resulted in the development of magnetic nanoparticles with size ranging from 37 to 97 nm (Daneshvar and Hosseini 2018). Moreover, fabrication of iron nanomaterials having an average diameter of 10–24.6 nm was investigated by utilization of fungus via subjection of FeCl3 (precursor salt) to 103 M Aspergillus oryzae TFR9 fungal culture for 12 h under an operating temperature of 28
C. The fungal assisted iron nanomaterial synthesis route consists of three mechanisms, viz. electron shuttle quinones, nitrate reductase action, or combination of the both. It was reported that the utilization of fungus over bacteria is more effective for the preparation of iron nanomaterials because of its high tolerance and broad surface area, which is convenient for longer life span and binding of metals (Tarafdar and Raliya 2013). In addition, the fabrication of nanoparticles utilizing both bacteria and fungi is more preferable in the mesophilic temperature range, while a higher range of pH is desirable for the fabrication of nanoparticles having smaller particle sizes. Moreover, it was reported that during the utilization of bacteria, the synthesis of nanoparticles is rapid owing to its short incubation time, whereas during the fungi-assisted biosynthesis process, production of a large amount of nanoparticle

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is possible as a result of the stress tolerance ability in fungal strains. In addition, an investigation has been conducted on the production of iron nanomaterials via green route by utilizing algae as microbes. In a study carried out by Salem et al. (2019), Fe3O4 nanoparticles were fabricated by utilizing red (Pterocladia capillacea) and brown (Colpomenia sinuosa) seaweed aqueous extracts with size ranging from 16.85–22.47 and 11.24–33.71 nm, respectively. The fabrication process was conducted at ambient temperature and FeCl3 was employed as the precursor salt along with 1 g of biomass. Likewise, Chandran et al. (2016) also outlined the fabrication of iron nanoparticles under ambient operational conditions with size ranging from 40 to 50 nm, via utilization of brown seaweed Dictyota dichotoma. Nanoparticle synthesis via the use of various microorganisms is a bottom-up strategy. This approach significantly assists in the fabrication of iron-based nanoparticles under optimal experimental conditions, lowers the energy demand, generates less toxicity, and is sustainable and eco-friendly.

4.2.2

Plant-Mediated Synthesis

Utilization of plants as a natural source of bio-reductants during the green synthesis of nanomaterials has found significant importance over the last few decades. Considerable investigations have been performed on a variety of plant sources, viz. flowers, leaf extract, roots, stems, and fruits in order to achieve an eco-friendly fabrication route for iron-based nanoparticles. Extraction of plant metabolites is done by heating the plant part at a temperature range of 60–100
C in deionized water, followed by utilization of the extracted solution to reduce the metal precursor at ambient temperature for a specific incubation period. Fabrication of zero-valent iron nanoparticle (nZVI) was investigated by utilizing various plant source and metal precursors as reducing agents such as eucalyptus leaves, blue berry leaves, young mango leaves, shoots extract, pure tea polyphenol, extract of cherry, mulberry and oak leaves, fruit extract of Terminalia chebula, leaf extract of Urticadioica, Thymus vulgaris and Rosa damascene, neem leaf extract, green tea leaves, tea powder as well as vine and grape marc leaves. In addition, the fabrication of iron oxide nanomaterials has been investigated by utilizing tea leaf and eucalyptus leaf extracts, Glycosmis mauritiana leaf extract, Ocimum sanctum leaf extract, Passiflora tripartita fruit extract, and aloe Vera extract (Srivastava et al. 2021). Plant extracts contain substantial amount of bioactive components, viz. polysaccharides, polyphenols, alkaloids, terpenoids, saponins, flavonoids, proteins, and vitamins. Such bioactive molecules act as a major component during the decrease in metal ion concentration and are primarily accountable for the development of nanoparticles. These compounds also tend to alter the chemical kinetics, which stabilize the nanoparticles, thereby acting as a capping agent. In an investigation conducted by Lohrasbi et al. (2019), iron oxide nanoparticles having a particle size range of 4.6–30.6 nm were successfully fabricated under ambient experimental conditions by utilizing the aqueous Plantago leaf extracts as reducing agent. Similarly, Vitta

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et al. (2020) utilized Eucalyptus robusta extract to fabricate iron nanoparticles under an operating condition of 40
C and an incubation period of 30 min, and eventually yielded a particle size distribution of 200–300 nm. Moreover, Demirezen et al. (2019) fabricated iron oxyhydroxide/oxide with a particle size of 20 nm by utilizing bioactive extract of Ficus carica. In this process, FeCl3 solutions and dried fruit extracts were fused together in a volume ratio of 1:1, followed by constant shaking up to 2 h where change in the solution color was taken as an index for the development of nanoparticle. It can be inferred that plant extracts are favorable for the production of iron-based nanomaterials in many aspects, viz. highly reproducible, easy availability, nearly non-toxic, and highly efficient in natural bioactive compounds. As such, the fabrication of nanoparticles utilizing plant extracts is sustainable and eco-friendly and also assist in lowering the overall production cost. Furthermore, a comparative analysis study of nanoparticle synthesis via chemical and green synthesis route was investigated by Nadagouda et al. (2010). The study reported that zero-valent iron nanoparticles fabricated via green synthesis pathway by extracting a definite amount (2 g) of tea powder in 100 mL volume of heated water, was effectively utilized to lower 0.1N of ferric nitrate and thereby resulted in the formation of 40–50 nm particle size nanostructures. On the other hand, fabrication via chemical route was carried out by utilizing 0.1N NaBH4 solution at ambient temperature which yielded nanostructures with an average diameter of ~500 nm. Thus, it was concluded that factors, viz. easy and ambient operational condition, presence of significant bioactive compounds, nearly zero cost availability, and being renewable considerably makes the plant assisted preparation of iron-based nanoparticles sustainable and economical.

4.2.3

Biocompatible Reagents Based Synthesis

The utilization of simulated non-toxic biocompatible compounds for the stabilization and fabrication of magnetic nanomaterial polymer composites has gained significant interest. He and Zhao (2005) enhanced the stabilization and dispersion of iron nanomaterials by utilizing starch stabilized (20% amylose content) bimetallic Fe/Pd nanomaterials in an aqueous medium. Gao et al. (2008) utilized sodium alginate biopolymer to fabricate magnetic Fe3O4 nanoparticles through a redoxmediated hydrothermal process from precursors such as urea and ferric chloride hexahydrate. The nanoparticles formed were 27.2 nm in diameter with uniform spherical morphology. Myoglobin and hemoglobin are considered as naturally occurring biomolecules containing significant amount of iron. A study conducted by Sayyad et al. (2012) utilizes a single-phase reduction reaction system to fabricate stabilized iron nanoparticles at ambient temperature. The developed nanoparticles showed crystalline properties with a reduced particle size range of 2–5 nm. This approach is important for the fabrication of biocompatible nanoparticles needed for medical purposes. Moreover, wood derived sugars were used by Yan et al. (2015) to synthesize simulated iron nanoparticles encapsulated with carbon via hydrothermal

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carbonization approach. Spherical nanoparticles with an average particle size of 100–140 nm were developed by using an iron core of diameter 10–30 nm to catalytically produce liquid hydrocarbons from syngas. Also, Demir et al. (2014) investigated the influence of various saccharides, viz. maltose, lactose, fructose, mannose, and galactose on the fabrication of magnetite nanomaterials. Except fructose, all other saccharides showed twofold applications both as capping and reducing agents. However, owing to its non-reducing property fructose only behave as a capping agent. The prepared iron oxide nanoparticles had a particle size of 3.5–13 nm and its surface structure was attributed to a mixture of aggregated rods, spheres, and dendritic nanostructures. Besides, iron oxide nanoparticles prepared with maltose, mannose, and galactose showed superparamagnetic properties. As such, the saccharides assisted magnetite iron oxide nanoparticles can be utilized in biomedical imaging practice. Furthermore, cellulose is widely known as the basic structural constituent of the primary cell wall of all plants, different varieties of oomycetes, and algae. Therefore, in order to prepare stabilized iron-based nanoparticles, substances consisting of high cellulosic content can be utilized. Moreover, iron oxide nanorods were synthesized by López-Téllez et al. (2013) via extraction of cellulosic contents from ethanol cured powdered orange peel. The study showed the linkage amongst various functional groups of derived cellulosic contents and the reduced metal ions via weak Van der Waals and electrostatic forces. These type of interactions assist in stabilizing the nanoparticles after preparation. The developed nanorods showed the characteristics of an adsorbent (adsorption capacity of 7.44 mg1) during the reduction of hexavalent chromium. Another biocompatible reagent used for the synthesis of nanoparticles is polysaccharides which are termed as polymeric carbohydrates consisting of glycosidic linkages. Chang et al. (2011) fabricated superparamagnetic iron oxide nanomaterials utilizing various polysaccharides, viz. carboxymethyl cellulose sodium, agar, and soluble starch. The polysaccharides were found to improve the biodegradability and biocompatibility of the prepared iron oxide nanomaterials, besides behaving as both capping and stabilizing agents. The particle size of starch stabilized nanomaterials was reported as 10 nm, which was lower than both agar and CMC. Also, CMC and starch based nanomaterials had a saturation magnetization of 35.75 and 36.16 emu g1, respectively, while that of agar-based nanomaterial was 20.43 emu g1. The study also reported that the developed polysaccharide based iron oxide nanomaterials are coercive in nature and demonstrates an exceptionally small hysteresis loop.

4.2.4

Microwave-Assisted Synthesis

Microwave-assisted synthesis incorporates the use of electromagnetic irradiation via molecular and ionic conduction, thereby simultaneously involving both the reducing agents and the precursor solvents in the reaction. During the application of microwave irradiation, Łuczak et al. (2016) observed that heating at elevated temperature

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directly takes place well within the sample. As the energy demand is low, microwave irradiation imparts instant heating, thereby assisting in the greener preparation of nanomaterial. Such approach assists in the stable dispersion of nanomaterials, while nucleation accelerates. The important factors associated with the microwave irradiation of nanoparticles involve dispersion, size regulation, synthesis time, and crystallinity. In an investigation carried out by Kombaiah et al. (2017a), it was found that the microwave irradiation process was much more efficient and cost-effective with regard to other thermal processes. Also, Schneider et al. (2017) outlined the rapid production of alendronic acid capped magnetic nanomaterials via microwave mediated biosynthesis process. Uniform nanomaterials with an average particle size of 6.2 nm were effectively fabricated at an operating temperature of 200
C. Also, the precursor solution was prepared by mixing iron acetylacetonate with triethylene glycol and the reaction was carried out at inert environment. Due to properties such as high boiling point, non-toxicity, high viscosity, and low isoelectric point, triethylene glycol has been significantly used as a green component. However, the production of nanomaterials with low yield is one of the major concern for its industrial scale operation, owing to the restricted vessel dimension in microwave irradiation process. Gonzalez-Moragas et al. (2015) overcame such limitation by the utilization of a multimode microwave unit which substantially assisted in scaling up the preparation of multigram iron oxide nanomaterials. In this study, anhydrous benzyl alcohol and iron (III) acetylacetonate were mixed together to form a precursor solution, followed by the preparation of nanomaterials in a lab-scale set up. The developed nanomaterials have an average diameter of 3.8 nm. Also, an increment in the operating temperature from 25 to 180
C in 20 min with subsequent cooling of the process at 300 W, over a time span of 3 min was reported. Throughout the scale up process, the temperature was maintained at 200
C and an elevated microwave power of 500 W was applied under optimal operational conditions. It was further reported that extending the reaction time of the synthesis led to a tenfold increment in the scale up process. Moreover, by applying the scale up process, a yield of >80% was obtained. Likewise, Williams et al. (2016) carried out the fabrication of versatile magnetite nanomaterials infused on a polymer support at an optimum temperature of 150
C for 20 min via single step microwave irradiation. In the study, FeCl24H2O and FeCl36H2O acted as precursors which eventually led to the formation of high crystalline nanomaterials with improved aqueous stability. In addition, a dual stage microwave irradiation technique could also be applied for the preparation of maghemite and magnetite nanomaterials. In this approach, a mixture of iron (III) acetylacetonate and oleylamine was formed to act as the precursor solution and oleic acid was used as the stabilizing agent. When the proportion of stabilizers in the solution was varied during the preparation, the obtained nanomaterials had an average particle size of 5 nm. Oleic acid was then heated at 120
C and oleylamine at 185
C for 1 and 1.5 h, respectively, under constant stirring. It was also reported that amidst the different stabilizers used, oleic acid exhibited significant influence in reducing the agglomeration of the prepared nanoparticles. Furthermore, Kombaiah et al. (2017b) compared the microwave mediated biosynthesis of ceramic iron nanomaterials in which okra plant extracts were utilized as reducing agents, with

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Table 4.1 Preparation of iron-based nanoparticles through various green synthesis pathways (Modified with permission from Malik et al. 2014) Prepared nanoparticle specifications Species/sources/ stabilizers Types

Average size (nm)

Morphology

Fe3O4 Fe3O4 Magnetic Fe3O4

60–80 50–200 18

Spherical Spherical Cubical

Eichhornia crassipes Citrus aurantium

Fe-NP Fe-NP

>100 17–25

Seed Syzygium cumini Biocompatible reagents Proteins Hemoglobin and myoglobin Sugars Sucrose Fibers Pectin Microwave based synthesis Plants Okra plant extract as reductant Biomass Ethanol as dispersant

Fe3O4

9–20

Rod shaped Slightly elongated Spherical

Fe-NP

2–5

Aggregates

Fe3O4 Fe3O4

4–16 5–18

Spherical Cubical

CoFe2O4

5–15

Spherical

FeMoO4

5–20

Plants

ZnFe2O4

7.2–8.5

Flower shaped Agglomerates

Green synthesis route/ pathway Microorganisms Bacteria Fungi Algae

Bacillus subtilis Aspergillus sp. Sargassum muticum

Plants sources Leaf Fruit peel

Okra plant extract as reductant

other available conventional heating methods. Both the irradiation techniques obtained aggregated, spherical nanostructures of zinc and cobalt ferrite nanomaterials respectively. However, it was observed that improved magnetic and optical characteristics along with low average particle size oriented nanomaterials were fabricated via microwave mediated synthesis as compared to the conventional heating methods. Table 4.1 depicts the green synthesis of iron nanoparticles via microbes, plant extracts, biocompatible reagents, and microwave-assisted treatment.

4.3

Potential Mechanism of Iron-Based Nanoparticles Via Green Route

The preparation of nanoparticle via green synthesis pathway is a cost-effective and highly efficient process. At first, it is necessary to prepare an extracted solution in deionized water via boiling the biomolecules such as animal tissue, plant materials or

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Fig. 4.3 Mechanism of green synthesis of iron nanomaterial showing different phases (a) activation stage, (b) growth stage, (c) termination stage. (Obtained with permission from Srivastava et al. 2021)

biomass at elevated temperature with subsequent filtration. The extracted solution contains substantial amount of secondary metabolites which differ in nature and the category of biomolecules used primarily determines its concentration. The secondary metabolites, viz. nucleoproteins, flavonoids, amino acids, polyphenols, etc. behave as an adequate capping and reducing agent. They assist in preparing the corresponding metal nanoparticle by reducing the metal precursor salts incorporated to the extracted solution under ambient temperature and definite volume ratio (Wang et al. 2017). Figure 4.3 represents the mechanism for the preparation of iron nanomaterials via green route synthesis. The phenomenon of iron nanoparticle preparation can be classified under three stages: (1) activation stage, (2) growth stage, and (3) termination stage. Activation stage is the primary step during the preparation of nanoparticles in which the metal salt precursor gets ionized, followed by its dissociation to release the positively charged metal ions. At the same time, the negatively charged bioactive compounds, viz. flavonoids, enzymes, polyphenols, quinones, alkaloids, amino acids, proteins, and quinones gets attached to the positively charged metal ions which resulted in the formation of intermediate complexes. Such bioactive components lead to chelation which eventually resulted in the depletion of metal ions from its higher to lower state of oxidation with subsequent nucleation of the depleted metal atoms, thereby forming the required particle size nanomaterials. The reduction rate of metal ions

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is significantly based on the reduction potential and active metabolites concentration in the extracted solution. Amidst all the available metabolites, amino acids, viz. arginine, lysine, phenols, tryptophan, and tyrosine consist of high antioxidant property and are very powerful reducing agents, which have the ability to efficiently minimize higher metal ion concentration within a short span of time, thereby improving the reaction yield. Furthermore, operational variables such as metal salt precursor, temperature, incubation period, and solution pH also affect the dispersity, size, and nature of the developed nanoparticles (Srivastava et al. 2021). The activation stage is accompanied by growth stage, in which the depleted metal atoms upon nucleation integrates with one another to expand in arbitrary size, thereby creating metal nanoparticles with varying surface structure that includes pentagonal, cubical, hexagonal, and spherical based on the experimental variables, viz. incubation period, temperature, property of metal precursors, solution pH, availability, and classification of reducing metabolites. Nevertheless, owing to the extended nucleation, particle aggregation occurs which creates huge clusters with deformed morphology. Hence, in order to achieve particles with appropriate surface structure, the thermodynamic energy of the materials should be reduced so that the formation of agglomerates can be prevented and the developed nanoparticles become stabilized (Shamaila et al. 2016). The growth stage of nanoparticle fabrication is accompanied by the termination stage. The termination stage is distinguished by enhanced thermodynamic strength of the fabricated nanoparticles. Here, the nucleated particles are coated by either capping or surfactant agents to restrict the random growth of nanoparticles. The use of capping agents resulted in lowering the particle surface charge and restricts the nucleated ligands from additional growth, thereby restricting the particle accumulation. As such, capping agents assist in obtaining nanoparticles of appropriate morphology, which relies upon the reaction conditions (Stefaniuk et al. 2016). The conventional routes for nanoparticle fabrication utilizes chemicals in the form of capping agents for stabilization, which are very harmful ad toxic for the eco-system, whereas plant metabolites are used as capping agents throughout the green route synthesis of nanomaterials. Different plant metabolites, viz. nucleic acids, terpenoids, amino acids, flavonoids, lipids, citric acid, and carboxylates act as the capping agents by modifying the chemical kinetics, thereby resulting in stabilized nanoparticles. Therefore, kinetics and thermodynamics of these three stages altogether regulates the shape, dispersity, size, and structure of the formed nanoparticles (Belloni et al. 2020).

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Influence of Various Parameters on the Green Synthesized Iron Nanoparticles

Physicochemical variables, viz. stabilizing/capping agents, solution pH, and extraction temperature plays a very significant part in the fabrication process and regulates the growth mechanism of nanoparticles via green synthesis pathway. This section elaborately discusses about the influence of such parameters on shape, size, and dispersity of the produced nanomaterials during the green synthesis pathway.

4.4.1

Effect of Stabilizing/Capping Agents

During the preparation of nanoparticles, utilization of surfactants as stabilizing/ capping agents lowers agglomeration, increases dispersion, and enhances adsorption ability of the nanoparticles. For example, cationic surfactant such as cetyltrimethylammonium bromide (CTAB) has been extensively investigated for capping and stabilization of iron oxide nanomaterials along with its influence on phosphate reduction via green synthesis approach. In an investigation conducted by Gan et al. (2018), both uncapped and capped nanomaterials were synthesized by employing sodium acetate as precursors and eucalyptus leaf extracts were used as reductant with FeCl36H2O. It was reported that a phosphate reduction efficiency of 80% was achieved within an hour when uncapped nanomaterials were used during the process. However, a significant improvement in the reduction efficiency of phosphate (95%) was obtained within the same operating time, during the application of CTAB-capped nanomaterials having an optimal surfactant content of 0.4 mM. Similarly, Cao et al. (2016) examined the influence of adsorbent doses, initial phosphate content, pH, and temperature on phosphate sorption capacity by utilizing CTAB-stabilized iron oxide nanomaterials (spherical shaped) fabricated from the extract of Eucalyptus leaves. As the temperature increases, the adsorption capacity also increases, which in turn is responsible for an increase in the activation energy, thereby favoring an endothermic process. With an increase in the CTABbased adsorbent dosages, an initial improvement in the adsorption capacity was observed followed by a decreasing trend. It was also found that an increase in the initial phosphate content resulted in an improved absorption, prior to the achievement of highest sorption capacity at 60 mg/L. However, the sorption efficiency was found to get reduced at high alkaline pH of 11–13, owing to the presence of electrostatic repulsion between the active sites of iron oxide and the phosphate anions. The adsorption efficiency also did not show much improvement at lower pH of 1–9. In addition, an important contributor to phosphate adsorption over a broad range of pH could be the use of ligand exchange instead of electrostatic interaction.

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Effect of Solution pH

The solution pH plays a substantial part in regulating the yield, morphology, and activity of the prepared nanoparticles. It leads to the variation of charge on the secondary metabolites which influences its degradation, chelation power, and adsorption of metal ions, thereby assisting in the determination of yield and morphology of the nanoparticles. During the active and growth stage of nanoparticle fabrication, the solution pH also influences the nucleation rate of metal atoms and under acidic condition the particles gets accumulated because of over nucleation, which resulted in lowering of the process yield. Sundaram et al. (2012) utilized gram-positive bacterium, viz. Bacillus subtilis to examine the influence of solution pH on nucleation, throughout the fabrication of magnetite nanomaterials. It was reported that solution pH lower than 11 assist in proper nucleation of the magnetite nanomaterials. However, when the pH is above 11, it does not support the growth of magnetite nanomaterials. The size and dispersity of the fabricated nanomaterials are also affected by the solution pH. Moreover, the particle size was observed to be smaller and less distributed at high solution pH, owing to the availability of a significant amount of functional groups required for binding of metals. Moreover, Herrera-Becerra et al. (2008) reported the preparation of smaller size (1–4 nm) magnetite nanomaterials having less dispersion at a solution pH of 10, whereas an improved dispersity and high particle size of 1–10 nm was observed at lower pH. The stability of nanoparticles is also influenced by the solution pH and basic pH conditions assist in the formation of nanoparticles with higher yield. On the other hand, nanoparticles prepared at higher pH exhibited enhanced catalytic activity. Aromal and Philips (2012) investigated the catalytic activity of nanoparticles prepared at various pH levels and found that the nanoparticles prepared at higher pH of 6 and 7 required less time (7 min) for catalysis as compared to the time taken (294 min) for nanoparticle formation at lower pH of 4. The above studies indicate that the solution pH is a very crucial factor during the formation of nanoparticles. Growth/nucleation may be easily regulated by adjusting the pH, resulting in the nanoparticle formation of various shapes and sizes. Physical characteristics of the formed nanoparticles can be easily modified in this way, thereby allowing improved catalytic properties of iron nanoparticles for use in bioenergy applications.

4.4.3

Effect of Operating Temperature

The operating temperature upon which the plant metabolites are derived substantially influences the agglomeration, shape, size, dispersity, and yield of the produced nanoparticle by regulating the nucleation rate of reduced metal atoms. The metabolites extracted at higher temperature results in the production of nanomaterials with less aggregation, as compared to those extracted at lower or moderate temperature. This phenomenon was validated by a research conducted by Ochieng et al. (2015)

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which observed that preparation of metal oxide nanoparticles by extracted plant metabolites of Spathodea campanulata at 100
C (high temperature) resulted in the generation of fine nanomaterials of diameter 50 nm with low agglomeration, whereas nanomaterials prepared by plant metabolites derived at 37
C (low temperature) resulted in an average diameter of 20 nm with high agglomeration. The reason could be attributed to the improved extraction ability of plant metabolites at elevated temperature. Also, crystallinity of the nanoparticles can be increased by increasing the calcination temperature. However, the solution pH decreases at elevated temperature, which affects the overall process efficiency. Njagi et al. (2011) observed the solution pH to be 6.30, 6.12, and 5.86 throughout the formation of extracted solution from Sorghum bran aqueous extracts at a varied temperature range of 30–80
C, thereby indicating a lowered pH and elevated temperature. The reason may be due to an improvement in ion dissociation at elevated temperature. In addition, Gnanaprakash et al. (2007) investigated the influence of operating temperature (25 and 50
C) and solution pH (3 and 5.7) on the development of magnetite nanoparticles and reported that ferrihydrite and goethite are formed at a solution pH of 3 and 5.7, respectively. Also, an improvement in the operating temperature of iron salts from 30 to 60
C led to an increase in the goethite formation efficiency from 78% to 100%, respectively. As such, it may be inferred that the process temperature is one of the most significant parameters throughout the preparation of nanomaterials and its alteration considerably influences the development of suitable crystal phase, shape, size, and surface structure. The fluctuation in the physical characteristics of nanoparticles in turn affects the catalytic properties of the fabricated products.

4.5

Application of Iron Nanoparticles in Biofuel Generation

Energy is the most essential prerequisite for achieving a country’s development goals, as rising population and industrialization necessitate a massive demand for energy. Due to the exhaustion of fossil fuel supplies and its detrimental effects on living beings and eco-system, there is an ongoing search for a renewable, environmentally benign, and economically viable alternative source of energy. Biofuel is a viable replacement to fossil fuels amongst the prevailing renewable energy choices, owing to the fact that it is renewable, clean, emits no pollutants, eco-friendly, and sustainable. Some of the available biofuels are bio-methane, biohydrogen, bio-ethanol, biogas, bio-oil, bio-methanol, and bio-diesel (Hossain et al. 2019). The hydrolysis of lignocellulosic biomass is carried out either biochemically or thermochemically to produce biofuel. The thermochemical generation of biofuels consists of processes, viz. combustion, pyrolysis, and gasification. However, cellulase enzyme system plays a primary role in the hydrolysis of lignocellulosic or organic biomass during the biochemical production of biofuels. Incorporating green synthesized iron nanoparticles as catalyst into the biofuels production process may serve as an innovative way to reduce the costs associated with both biochemical and thermochemical approaches. Also, it is expected that a higher surface area to volume

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Fig. 4.4 Effect of iron nanomaterial in the various steps of biofuel production from lignocellulosic biomass, namely: (a) pretreatment, (b) enzymatic hydrolysis, (c) fermentation. (Obtained with permission from Srivastava et al. 2021)

ratio as well as separation based on superparamagnetic property under an extrinsic magnetic field will result in an increased loading capacity and decreased diffusion limitation. Addition of iron nanoparticles in the reaction medium of biomass for the generation of fermentative biofuels considerably enhances the sustainability of fermentative microbes, thermochemical stability of hydrolytic enzymes as well as fabrication capacity of nanoparticles, thereby achieving greater yield of fermentable sugars followed by significant generation of biofuels. Iron nanoparticles may impact several stages of the reaction medium in the lignocellulosic production process of biofuels, including the prior analysis of lignocellulosic biomass along with biofuel generation to enhance the process efficiency (Sekoai et al. 2019). Figure 4.4 portrays the generation of biofuel via various preparation steps from lignocellulosic biomass. Khalid et al. (2019) discovered that mixing 120 mg/L magnetite nanoparticle with 2% pretreatment NaOH led to 100 and 129% improvement in biogas and methane yields, respectively, as well as an increase in the overall energy of 3765 kJ was reported. Incorporation of 100 mg/L magnetite nanoparticles itself enhances the production of biogas and methane by 37% and 33%, respectively. In an investigation

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carried out by Ladole et al. (2017); the immobilized cellulase enzyme on magnetic nanomaterials resulted in an increased pH and thermal stability, as well as an improvement in thermal deactivation energy. As compared to free cellulase, an increment up to 3.6 times in the cellulase activity was observed during immobilization. Moreover, the denaturation energy needed for the breakdown of cellulase immobilized enzyme showed a considerable increase of 48.17 kJ/mol against the denaturation energy of 28.06 kJ/mol for free cellulase. Selvam et al. (2016) reported that immobilized cellulase enzyme on magnetite nanoparticles exhibited an optimal temperature of 55
C in comparison to free cellulase (50
C), apart from reserving 75% of its initial activity after 10 cycles which can be used even after several cycles of magnetic separation against free cellulase which reserved only 14% of its original activity upon incubation. This further lowers the catalyst cost, improves life span, increases the enzyme activity and enzyme reusability, thereby making the process sustainable and economically viable. Furthermore, incorporating iron nanoparticles as a catalyst can improve the generation of biohydrogen through fermentation reaction. The nitrogenase and hydrogenase enzymes catalyze the electron transfers in microorganisms that are engaged in hydrogen generation via fermentation reactions, as demonstrated in the given equation: 2H+ + 2e ¼ H2 (Mishra et al. 2019). The presence of iron, which serves as a co-factor and is predicted to influence the electron transfer process linking hydrogenase and ferrodoxin eventually affects the activity of both enzymes, thereby leading to the determination of their catalytic activity. The conventional method of fermentation may be substantially enhanced by the utilization of iron nanoparticles, owing to its capacity to catalyze ferredoxinoxido-reductase activity by binding onto the enzyme’s active site, hence increasing the efficiency of electron transfer rate due to quantum size effect and higher surface area. Production of biohydrogen via glucose metabolism consist of two different metabolic routes: the route for the production of formate hydrogen is catalyzed by formate hydrogen lyase as shown in Eq. (4.1) and the route for the production of nicotinamide adenine dinucleotide (NADH) based hydrogen is catalyzed by hydrogenase as shown in Eq. (4.2) (Lin et al. 2016). HCOOH

Formate hydrogen iyase

NADH þ Hþ

!

Hydrogenase

!

H2 þ CO2

ð4:1Þ

NADþ þ H2

ð4:2Þ

It was observed that incorporation of iron nanoparticles alters the metabolic route during the generation of biohydrogen and assist in greater H+ to H2 reduction, thereby supporting an improved acetate formation and reduced ethanol production. By utilizing ferrihydrite nanorods, Zhang et al. (2019) observed an enhanced production of fermentative biohydrogen. It was reported that a concentration of 100 mg/L ferrihydrite nanorods as compared to the control fermentative medium significantly enhanced the overall production of biohydrogen by 69%. Moreover, ferrihydrite nanorods showed higher stimulating activity to produce biohydrogen when compared to hematite and magnetite nanoparticles. The reason may be

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attributed to the increased hydrogenase and metabolic activity, high cell growth, improved glucose conversion capacity as well as enhanced buffer acidification efficiency due to the utilization of ferrihydrite nanorods. Similar observations were reported on improved production of biohydrogen from dry grass by incorporating a concentration of 400 mg/L nZVI at a solution pH 7 and fermenting temperature of 37
C, thereby leading to a biohydrogen gas yield of 65 mL/g dry grass along with a generation rate of 12 mL/h, which was found to be higher against the control fermentative medium by 73.1%. It was also reported that, the time taken for biohydrogen production by nZVI was only 4 h against the control fermentative medium which took 20 h to complete the reaction. Nevertheless, it has also been reported that, an improvement in the iron nanoparticle content beyond the optimal concentration reduces the hydrogenase enzyme activity, which eventually resulted in lowering the efficiency and yield of biohydrogen generation (Yang and Wang 2018). In a study carried out by Yin and Wang (2019), it was observed that FeO nanoparticle concentration of 200 mg/L significantly assist in improving the algal biohydrogen production by 6.5 times via dark fermentation. Also, it was found that utilization of FeO nanoparticle considerably enhances the microbial growth, increases the production of acid and other metabolites, and improves the biohydrogen generation efficiency by employing strains of genus Terrisporobacter species and Clostridium. Hence, iron-based nanoparticles greatly affect the generation of biofuel by lowering the degradation temperature of biomass, reducing the formation of tar as well as increasing the biofuel yield. Moreover, as iron is one of the crucial catalysts during biomass to biofuels generation process, its fabrication via green route synthesis utilizing various plant extracts and waste biomass may significantly assist in lowering the net price of the available biofuel generation processes, thereby making it sustainable, eco-friendly and more viable for practical utilizations. Table 4.2 represents the type of nanoparticles reported in various biofuel production processes (Malik et al. 2014; Nasr et al. 2015; Engliman et al. 2017; Dolly et al. 2015; Wang et al. 2011, 2016; Xiu et al. 2010; Yang et al. 2013; Raita et al. 2015; Tran et al. 2012).

4.6

Toxicity Study and Environmental Risk

Despite its numerous environmental utilizations, iron-based nanoparticles constitute a significant threat to the environment. Wastewater and other industrial wastes when improperly managed, possess unique environmental risks in both groundwater and soil. On the other hand, synthesized iron nanoparticles may be transferred from one medium to another for particular remedial purposes. Amidst all the iron nanoparticles, nZVI are regarded as the most reactive. It was found that when nZVI were utilized for the treatment of groundwater pollutants, they were transported through porous soil and can withstand transformation on interaction with the pollutants and the exposed environment. Due to its distinctive morphological characteristics such as self-assembly, surface energy, particle size, and

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Table 4.2 Effect of nanoparticles on the performance of biofuel production Biofuel Biohydrogen

Biogas

Bio-diesel

Nanomaterials Fe2O3 nanoparticles γ-Fe2O3 nanoparticles FeO nanoparticles

Feedstock/ substrate Distillery wastewater Mixed culture

Production rate 44.28 mL/g COD

References Malik et al. (2014) Nasr et al. (2015)

Fe nanoparticles

Growth medium

Fe2O3 nanoparticles FeO nanoparticles

Waste-activated sludge Growth medium

FeO nanoparticles Fe3O4 nanoparticles Fe3O4 nanoparticles Fe3O4/SiO2 composite

Glucose Soybean oil

104.75 mL/g COD 1.92 mol/mol glucose 3.1 mol/mol malate 212.43 mL/g VSS 217.16 mL/g VSS 135 mL/g VSS 88 mol/mol oil

Palm oil

97.2 mol/mol oil

Yang et al. (2013) Wang et al. (2011) Raita et al. (2015)

Olive oil

>90 mol/mol oil

Tran et al. (2012)

Growth medium

Engliman et al. (2017) Dolly et al. (2015) Wang et al. (2016) Xiu et al. (2010)

manufacturing process, the nano-sized iron particles react significantly with the enzymes of living organisms as well as the metals present in environment. Nevertheless, the utilization of toxic compound, viz. hydrazine as capping agents during the biosynthesized process of nanoparticles makes the approach a slight unsafe for the environment (Mondal et al. 2020). The toxic effects of manufactured iron nanoparticles on the soil microbial mass were reported by Vittori Antisari et al. (2013). In a similar investigation, Fajardo et al. (2013) examined the proteomic stress and transcriptional activities on the soil bacterium Bacillus cereus caused by nano zero-valent iron. Furthermore, Auffan et al. (2008) via experimental tests identified a direct link between the oxidation state and cytotoxicity of iron nanoparticles. Several iron oxides, viz. maghemite, magnetite, and nZVI were examined and compared for their cytotoxic effects on the gram-negative bacterium E. coli. Compared to various iron oxide nanoparticles, nZVI showed a greater cytotoxic effect, during the experiments conducted. As a result, the study concluded that the oxidation state of iron nanoparticles performed a very crucial role in terms of cytotoxicity. The production of oxidative stress via reactive oxygen species causes such phenomenon. The reactive oxygen species refers to the extremely unstable radicals viz. hydroxyl radicals and superoxide radicals that binds to the cell membrane and eventually impairs their cell function. In another work, Lee et al. (2008) observed that nZVI exhibits very high bactericidal activity under anaerobic circumstances. The study showed a linear relationship between the dosage of nZVI and E. coli log inactivation. The toxicity of nZVI is considerably decreased under aerobic circumstances due to the oxidation process, thereby resulting in the

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development of an oxide layer. Further studies inferred that ferrous ions discharged from nZVI under anaerobic circumstances were perceived to be more hazardous. As such, it was inferred that nZVI stimulates the physical rupture of cell membranes, which led to its physical damage, while the biocidal actions of ferrous ions considerably aided in cell inactivation. In several investigations, the cytotoxicity of iron oxide nanoparticles was shown to be unaffected by either capping or stabilizing agents. Baumann et al. (2014) examined the cytotoxic activity of four different iron nanoparticles coated with dextran, ascorbate, polyvinylpyrrolidone, and citrate. Several toxicity testings were carried out on Daphnia magna neonates (water flea). Iron nanoparticles coated with dextran and ascorbate exhibited the highest immobilization efficiency, whereas the nanoparticles coated with citrate showed lowest immobilization. Utilization of dextran, acerbate, and citrate coated iron nanoparticles in large doses showed incomplete ecdysis, however, polyvinylpyrrolidone coated iron nanoparticles exhibited a minimal detrimental effect. It was also observed that stabilizing forces governing the hydrodynamic diameter had no influence on toxicity, but variables such as release of ions and colloidal stability in daphnids generate reactive oxygen species. Blinova et al. (2017) studied the cytotoxicity of both bulk and nano-sized iron oxide nanoparticles on Lemna minor (duckweed) and Daphnia magna (water flea). Both iron oxide nanoparticles exhibited very little biological effects and were less toxic (EC50 < 100 ppm) in nature, however, there was a noticeable reduction in the quantity of Daphnia magna neonates hatched at magnetite contents of 10 and 100 ppm. To address these toxicological concerns, several investigations have been performed to develop and alter the fabrication of green nanoparticles, which have proven to be less harmful and environmental friendly. Markova et al. (2014) studied the influence of green synthesized iron nanomaterials on ecological microorganisms, viz. invertebrate organisms (Daphnia magna) and green alga (Pseudokirchneriella subcapitata). The study reported that iron nanomaterials prepared via green tea extracts showed a detrimental influence on the cytotoxicity tests conducted. Although several studies showed that synthesized iron nanoparticles via green route are biocompatible to living organisms, yet there is currently a scarcity of research on its environmental toxicological consequences.

4.7

Challenges and Future Perspectives

In several literatures, preparation of iron-based nanomaterials has been described using different biosynthetic and green routes, viz. plant extracts, microorganisms, and waste biomass. However, the lack of sufficient knowledge regarding the presence of individual bioactive groups during fabrication along with its action mechanism requires further practical and theoretical investigation. Plants, microbes, and waste biomass along with their distinct varieties, include a wide range of bioactive chemical compositions. These features have a lot of potential in nanoparticle fabrication and hence should be investigated. As a result, plants, microbes, and biomass having higher iron nanoparticle production efficiency should be determined and

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investigated for enhanced industrial scale production via green synthesis route. Moreover, the chemical composition of similar plant species acquired from various geographical locations may differ, resulting in varied findings under identical experimental conditions that should be explored in detail. A significant amount of work is also needed to reduce the high costs related to the growth and preservation of microbe cultures during the fabrication of iron nanoparticles. Another limitation of iron nanoparticle preparation via green route is the production of poly-dispersed nanoparticles; as a result, in comparison to the traditional chemical and physical approaches, the formation of shape and size controlled iron-based nanoparticles such as nanorods, nanoflower, nanosphere, and nanostar via green route remains extremely challenging. Such shape and size based physicochemical characteristics of the produced iron nanoparticles can considerably affect the catalytic features for bioenergy related applications. As such, significant attention should be paid to the shape and size controlled synthesis process using green route. Several literatures have reported on the laboratory scale production of iron-based nanoparticles utilizing plant extracts and biomass. However, no pilot scale production of iron-based nanoparticles has been documented yet. The prospects of iron nanoparticles and its broad range of applications in the sphere of energy, necessitate industrial scale manufacturing, which still remains a major problem. Utilization of waste organic matter and biomass to produce iron nanoparticles while also determining the optimum raw material for its production is a step toward waste management and may provide a chance to turn waste into wealth. Iron nanoparticles perform a substantial role in improving the yield and grade of biofuel generation, and possess a tremendous capability to meet the energy crisis requirement, while at the same time involving a high manufacturing cost. Moreover, research should be more focused on the utilization of agro-based waste during the preparation of green synthesized nanoparticles. Also, the fabrication of metal-metal nanocomposites via green synthesis approach should be more emphasized, in order to improve the efficacy in wastewater treatment, including applications for heavy metal removal. There should be a standard technique for fabrication to regulate the risk management, owing to the hazardous characteristics of the produced iron nanoparticles. Since diverse fabrication processes lead to different characteristic and property based nanoparticles, appropriate screening of nano waste prior to its disposal is often ignored. This leads to variability in research output as well as difficulties in anticipating human health issues and assessing environmental hazards. Although, the use of iron nanoparticles produced through green pathway for biofuel production is a novel and unique strategy which has the potential to lower the total synthesis costs, yet it requires further research in this direction.

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Conclusion

The utilization of plant extracts, microbes, and waste biomass during the preparation of iron-based nanomaterials is an emerging technique. It has been observed that various bioactive reagents present in the extracted solution, function as both capping and reducing agents at the same time, thereby decreasing the use of hazardous compounds required in chemical synthesis. Nevertheless, each approach has its own set of benefits and limitations. Microorganisms used for the fabrication of iron-based nanoparticles encounter problems, viz. sluggish production rate, possibility of infections, as well as high preparation and maintenance cost of cultures, thus plants and biomass based synthesis methods may be considered as preferable alternatives. Furthermore, being eco-friendly, non-hazardous, having simple and free accessibility to plant sources, and constant generation of bioactive chemicals during photosynthesis can considerably lower the net cost of nanoparticle fabrication, making it more useful and cost-effective. Nonetheless, organic wastes produced from various industrial sectors can be utilized as raw materials for the production of iron-based nanoparticles. In comparison to nanoparticle synthesis approach via chemical methods, one of the primary problems faced by biological routes is shape-controlled fabrication, which significantly impacts the catalytic performance of iron-based nanoparticles, and thus more study should be conducted in this area. In addition, the utilization of iron-based nanoparticles as a catalyst has the potential to revolutionize the bioenergy sector. It was observed that iron-based nanoparticles accelerate the synthesis of biofuels (e.g. bio-diesel, bio-oil, biogas and biohydrogen) and these nanoparticles produced through green synthesis pathway are expected to make the cumulative biofuel manufacturing process more cost-effective and economically feasible. However, further study is required to investigate the utilization of iron-based nanoparticles synthesized via green pathway and their applications in biofuels generation process sustainably and cost-effectively. Finally, the existing challenges and opportunities for green synthesized iron nanoparticles are explored. To conclude, several green synthesis approaches have been elaborated in this chapter, which will be useful in future research as assets to evaluate and achieve further advancements in developing improved iron-based nanoparticles, in order to deal with real life wastewater with very low risk of cytotoxic impact on the eco-system. This chapter may be helpful to the readers in obtaining a comprehensive understanding on the preparation of green synthesized iron nanoparticles and its astounding success in biofuel production applications.

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Aromal SA, Philip D (2012) Green synthesis of gold nanoparticles using Trigonella foenumgraecum and its size-dependent catalytic activity. Spectrochim Acta A Mol Biomol Spectrosc 97:1–5 Astolfi V, Astolfi AL, Mazutti MA, Rigo E, Di Luccio M et al (2019) Cellulolytic enzyme production from agricultural residues for biofuel purpose on circular economy approach. Bioprocess Biosyst Eng 42(5):677–685 Auffan M, Achouak W, Rose J, Roncato MA, Chaneac C et al (2008) Relation between the redox state of iron-based nanoparticles and their cytotoxicity toward Escherichia coli. Environ Sci Technol 42(17):6730–6735 Baumann J, Köser J, Arndt D, Filser J (2014) The coating makes the difference: acute effects of iron oxide nanoparticles on Daphnia magna. Sci Total Environ 484:176–184 Belloni J, Marignier JL, Mostafavi M (2020) Mechanisms of metal nanoparticles nucleation and growth studied by radiolysis. Radiat Phys Chem 169:107952 Blinova I, Kanarbik L, Irha N, Kahru A (2017) Ecotoxicity of nanosized magnetite to crustacean Daphnia magna and duckweed Lemna minor. Hydrobiologia 798(1):141–149 Bolade OP, Williams AB, Benson NU (2020) Green synthesis of iron-based nanomaterials for environmental remediation: a review. Environ Nanotechnol Monitor Manag 13:100279 Cao D, Jin X, Gan L, Wang T, Chen Z (2016) Removal of phosphate using iron oxide nanoparticles synthesized by eucalyptus leaf extract in the presence of CTAB surfactant. Chemosphere 159: 23–31 Chandran M, Yuvaraj D, Christudhas L, Ramesh KV (2016) Bio synthesis of iron nanoparticles using the brown seaweed, Dictyota dicotoma. Biotechnol Indian J 12(12):112 Chang PR, Yu J, Ma X, Anderson DP (2011) Polysaccharides as stabilizers for the synthesis of magnetic nanoparticles. Carbohydr Polym 83(2):640–644 Daneshvar M, Hosseini MR (2018) From the iron boring scraps to superparamagnetic nanoparticles through an aerobic biological route. J Hazard Mater 357:393–400 Demir AYŞE, Baykal A, Sözeri H (2014) Green synthesis of Fe3O4 nanoparticles by one-pot saccharide-assisted hydrothermal method. Turk J Chem 38(5):825–836 Demirezen DA, Yıldız YŞ, Yılmaz Ş, Yılmaz DD (2019) Green synthesis and characterization of iron oxide nanoparticles using Ficus carica (common fig) dried fruit extract. J Biosci Bioeng 127(2):241–245 Dolly S, Pandey A, Pandey BK, Gopal R (2015) Process parameter optimization and enhancement of photo-biohydrogen production by mixed culture of Rhodobacter sphaeroides NMBL-02 and Escherichia coli NMBL-04 using Fe-nanoparticle. Int J Hydrogen Energy 40(46):16010–16020 Engliman NS, Abdul PM, Wu SY, Jahim JM (2017) Influence of iron (II) oxide nanoparticle on biohydrogen production in thermophilic mixed fermentation. Int J Hydrogen Energy 42(45): 27482–27493 Fajardo C, Saccà ML, Martinez-Gomariz M, Costa G, Nande M, Martin M (2013) Transcriptional and proteomic stress responses of a soil bacterium Bacillus cereus to nanosized zero-valent iron (nZVI) particles. Chemosphere 93(6):1077–1083 Gan L, Lu Z, Cao D, Chen Z (2018) Effects of cetyltrimethylammonium bromide on the morphology of green synthesized Fe3O4 nanoparticles used to remove phosphate. Mater Sci Eng C 82: 41–45 Gao S, Shi Y, Zhang S, Jiang K, Yang S, Li Z, Takayama-Muromachi E (2008) Biopolymerassisted green synthesis of iron oxide nanoparticles and their magnetic properties. J Phys Chem C 112(28):10398–10401 Gnanaprakash G, Mahadevan S, Jayakumar T, Kalyanasundaram P, Philip J, Raj B (2007) Effect of initial pH and temperature of iron salt solutions on formation of magnetite nanoparticles. Mater Chem Phys 103(1):168–175 Gonzalez-Moragas L, Yu SM, Murillo-Cremaes N, Laromaine A, Roig A (2015) Scale-up synthesis of iron oxide nanoparticles by microwave-assisted thermal decomposition. Chem Eng J 281:87– 95

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Chapter 5

Green Synthesized Carbon and Metallic Nanomaterials for Biofuel Production: Effect of Operating Parameters Prangan Duarah, Abhik Bhattacharjee, Piyal Mondal, and Mihir Kumar Purkait

Abstract Fossil fuel use and consumption have grown proportionally as a result of growing anthropogenic activities, notably in industry and transportation, resulting in significant environmental concerns. Because of growing concerns about the depletion of fossil fuels and their catastrophic influence on the environment, biofuel production and research are gaining popularity. However, a variety of barriers impede the commercialization of biofuels, despite the benefits they provide. Nanotechnology has increased in popularity in recent decades as a result of its ability to use a wide range of nanoscale materials with diameters ranging from 1 to 100 nm. As a result, NPs are used in a variety of industries, ranging from agriculture to electronics. Nanoparticles are excellent candidates for enhanced biofuel processes because of their exceptional features. The present chapter addresses potential of nanoparticles for biofuel production. A comprehensive narration was provided to understand the influence of various parameters on the performance of nanoparticles. Furthermore, the development in the production of green nanoparticles is highlighted. Finally, several viewpoints and restrictions related to process scale-up are thoroughly discussed, with an eye toward future advances. Keywords Biofuel · Nanoparticles · Green synthesis · Biodiesel · Biogas

P. Duarah Center for the Environment, Indian Institute of Technology Guwahati, Guwahati, Assam, India A. Bhattacharjee · P. Mondal (*) Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati, Assam, India e-mail: [email protected] M. K. Purkait Center for the Environment, Indian Institute of Technology Guwahati, Guwahati, Assam, India Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati, Assam, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Srivastava et al. (eds.), Green Nano Solution for Bioenergy Production Enhancement, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-16-9356-4_5

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Introduction

The two primary issues confronting the world now are ecological degradation and the depletion of fossil fuel reserves. The extraction and large-scale usage of these fossil fuels reduce the ground-based sources of carbon. Therefore, in the current situation, research into alternative fuels is justifiable, as it promises overall progress in environmental improvement, sustainable energy supplies, and the creation of a sustainable fuel life cycle. Alternative fuels of bio-origin can be utilized to alleviate global petroleum issues (Haldar and Purkait 2020). An energy-rich compound produced by biological mechanisms or obtained from living creatures, such as algae, plants, and bacteria is often known as biofuel. A range of fuels from biomass resources may be created. These include liquid fuels such as ethanol and gasoline as well as biodiesel and Fischer–Tropsch diesel fuel. Biodiesel, which is non-toxic and biodegradable, is one of the most promising alternatives that has spurred research. Biodiesels have quite a lot of matches with petroleum products and may be employed in regular diesel engines without or few changes. Animal fats or vegetable oils are transformed into biodiesel by chemical or enzymatic transesterification reactions with monohydric alcohols to yield fatty acid methyl esters (FAME). As with biodiesel, bioethanol too has a substantial global market presence. Bioethanol is used to produce gasoline-ethanol mixes, such as E15 and E85, by partly replacing gasoline with ethanol. The selection of the feedstock for commercial ethanol production primely depends on the climatic condition of the producer’s region. For instance, sugarcane is the primary feedstock in tropical countries such as Brazil and Columbia, for gasoline-ethanol production. In other regions such as the USA, China, and the EU, corn is the predominant feedstock. However, in south Asian countries such as Malaysia and Indonesia, the feedstock for bioethanol production is palm tree (Cheng and Timilsina 2011). Biogas is another common kind of biofuel. In general, energy-efficient biogas maybe used to generate heat and electricity, as well as a gaseous automobile fuel. Biogas (such as biomethane) can also be used in lieu of natural gas as a raw material for the production of components and chemicals. Anaerobic digestion produces biogas, which has considerable advantages over conventional kinds of bioenergy generation (Ganzoury and Allam 2015). It has been identified among the most energy-efficient and ecologically friendly bioenergy production technologies. From a long-term viewpoint, biohydrogen stands out as a good option for fuel. Despite the benefits of biofuels, there are a number of obstacles that prevent their commercialization. The high energy-intensive procedure, high expense, and low product yield are some of the key constraints of biofuel manufacturing, according to the authors. These constraints are being addressed by a number of different techniques that include the inclusion of nanoparticles (NPs). The area of nanotechnology has grown in popularity over the last several decades due to its capacity to employ an extensive range of nanoscale materials with dimensions varying from 1 to 100 nm. As a result, NPs are utilized in a range of sectors, from agriculture to electronics. The application of nanotechnology in

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various distinct disciplines is mostly ascribed to its unique features, such as its morphological advantages, nanoscale size, and high reactivity. The exceptional surface-to-volume ratio of NPs offers a higher number of active sites, which are necessary for an inclusive range of chemical reactions and biological activities (Eskandarinezhad et al. 2021). Availability of the high active sites and flexibility to show diverse morphologies has extended their usage in a number of fields, including controlled drug delivery, bioimaging, water purification, and so on. Furthermore, nanoscale materials react with other molecules at a quicker pace than big particles. Nanoparticles also have additional advantageous properties, such as excellent catalytic activity, high crystallinity, chemical stability, and excellent adsorption capacity. Despite the several advantages, conventional NP preparation methods have several drawbacks, including the possibility of contamination due to precursor chemicals, high cost, high energy intensity, the usage of toxic solvents, the production of potentially harmful by-products, and the possibility of structural defects affecting nanoparticle surface properties. Furthermore, recent environmental concerns have pushed scientists to priorities the avoidance of dangerous, ecologically destructive substances, as well as the advancement of greener technologies. As a consequence of current environmental concerns, scientists have been interested in green approaches for synthesizing NPs that do not include hazardous chemicals as a by-product (Mondal et al. 2020). Nanoparticles are excellent candidates for enhanced biofuel processes because of their exceptional features. Their primary function is catalysis, in which they transport electrons, decrease inhibitory compounds and increase anaerobic consortium activity. The application of nanoparticles in biofuels is still in its early stages. As a result, the current chapter discusses the potential of nanoparticles for biofuel generation. In light of this, the potential of NPs was investigated by examining the variables influencing nanomaterial performance. Furthermore, the advancement in green nanoparticle production is emphasized. Finally, different perspectives and constraints associated with process scale-up are comprehensively described, with an eye toward future advancements.

5.2

A Brief Overview of Biofuels and It Types

As a term, bioenergy refers to secondary energy generated from biomass, which may be used to generate power or biomass briquettes fuels. According to their feedstock sources, biofuels are generally divided into four generations. However, classification techniques for biofuels are inconsistent and vary widely (Ullah et al. 2018). Because of the availability of raw materials and the relatively easy biodiesel production method, the manufacture of first-generation biodiesel from edible raw materials was very common at the start of the biodiesel era. Conventional biodiesel oil sources include rapeseed, castor, canola, soybeans, coconut, sunflower, palm, and corn. Currently, edible biomass accounts for over 90% of all biofuels. However, numerous drawbacks, such as high raw material costs, reliance on food prices, expropriation of

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agricultural land, deforestation, and the susceptibility of edible plants to ecological conditions, rendered the production of biodiesel from edible plants inefficient. Second-generation raw materials are a viable alternative to using edible plants for biodiesel extraction. Non-food-based biomass, such as stems, leaves, and husks left behind after crop cultivation is harvested, as well as switch grass, grass, jatropha, and a range of other non-food crops, are used to make second-generation biofuels. However, due to the complexity of the extraction technique, a third-generation biofuel has been developed. Third-generation microbial species include microalgae and bacteria that consist of different concentrations of triglyceride, carbohydrate, or protein content depending on their particular strains. These raw materials do not compete with edible food; they do not require arable or fertile land to grow; their growth rates are much quicker, and their availability is much higher than conventional feedstocks based on crops or grains; and the development of these biofuels has almost no negative environmental consequences. In addition, a number of distinct genetically modified microalgae are being produced that show great promise for biofuel generation. This type of biofuels generated by genetically modifying microalgae is referred to as “fourth-generation biofuel” (Shokravi et al. 2021).

5.2.1

Liquid Forms of Bioenergy

Because of its carbon-based structural element, which can be immediately converted into liquid fuel, bioenergy is currently considered as the most potential renewable energy source, contributing to the world’s long-term energy supply. Liquid biofuels derived from fossil fuels are available in a number of forms for use in various internal combustion engines. Biodiesel and hydrogenated vegetable oils (HVO) have recently emerged as viable alternatives to petroleum-derived goods (Davoodbasha et al. 2021). According to the International Renewable Energy Agency (IREA), liquid biofuels such as ethanol (both conventional and advanced forms) and biodiesel will account for 10% of transportation sector energy usage by 2030, up from 3% in 2017. Due to its outstanding features such as greater-octane number, sustainability, and extraordinary heat of vaporization, bioethanol is recognized as a good alternative to fossil fuels including gasoline and diesel (Sanusi et al. 2020). In 1896, Henry Ford demonstrated the first feasible car that operated on pure ethanol. This sped up production of the Ford Model T vehicle models in 1908, which could operate on ethanol or a gasoline-ethanol combination. Ethanol was widely utilized as a fuel in Europe and the USA until the 1900s. However, after WWI, demand for ethanol fell considerably since its extraction became costlier than that of petroleumbased fuels. Companies such as General Motors and DuPont, on the other hand, were interested in using ethanol as an anti-knocking agent and as a possible fossil fuel substitute. In the next 20 years, scientists predict that it will be used as the primary biofuel in the world’s transportation sectors. Recently built advanced flex-fuel hybrid cars have the capability of combining ethanol with gasoline or using ethanol

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in its pure form. As part of the second generation of biofuels, biobutanol is also being investigated. It is widely used as a solvent in cosmetics, hydraulic oils, detergents, antibiotics, and medicines, as well as an intermediary in the manufacture of methacrylate and butyl acrylate, among other products and applications. There is a wide range of medicines that are produced using it as a solvent. As a biofuel, butanol has only recently entered the market. According to several research, butanol is advantageous when combined with diesel and other fuels. It was determined that the performance and emissions of indirect injection engines were affected by different butanol-biodiesel compositions by Yilmaz et al. (2014). Compared to biodiesel, the blended fuel emits fewer nitrogen oxides, but more greenhouse gases and hydrocarbons. It is a blend of monoalkyl esters and biomass-derived long-chain fatty acids that is an exceptional source of energy for internal combustion engines. Ester or alcohol esters with C14 to C22 alkyl chain lengths are used in biodiesel. As a result of its chemical composition, biodiesel is also a viable alternative to conventional diesel fuel (Yilmaz et al. 2014).

5.2.2

Gaseous Forms of Bioenergy

A variety of gaseous biofuels are available for use in electricity production and transmission. These fuels are usually considered as sustainable energy systems due to their potential for reducing harmful emissions and their contribution to economic growth, among other reasons. A greater heating value of 141.8 MJ/kg makes biohydrogen one of the most feasible alternatives to traditional sources of energy. There are both renewable and non-renewable hydrocarbon resources that are used to produce this secondary energy. Energy provider biohydrogen is expected to play an important part in the future global energy industry. A renewable fuel, hydrogen generates just water throughout the energy generation process, with no CO2 emissions. For the creation of energy, it is commonly used in the form of fuel cells. Biomass gasification is definitely significant, but further study is needed. Biogas is a gaseous biofuel used in the energy sector. Microbes assisted in the natural breakdown of organic material under an anaerobic environment, resulting in the production of biogas. By converting organic waste, anaerobic digestion generates energy, heat, or power. Among the most attractive paths for renewable energy generation in recent years has been anerobic digestion (AD) of agricultural and industrial waste and leftovers, local organic waste, wastewater, and other sources (Ganzoury and Allam 2015). However, the economic and commercial viability of liquid and gaseous biofuels is uncertain. In the near future, however, intensive research and development will expand the possibilities for its use as a biofuel, resulting in a greener, cleaner world.

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Conventional Approaches to Generate Biofuels

Chemical, physicochemical, thermochemical, or biochemical reactions create biofuels from the plants, crops, residue or waste, fat, lipid, etc. of living organisms. It is crucial to note that biomass has a low bulk density (BD), which limits its usage for bioenergy generation that is both efficient and cost-effective. Briquette technology might be helpful in this circumstance since it transforms or densifies biomass mechanically. High-pressure compression is used to condense biomass waste into briquettes and pellets. In addition to these advantages, briquettes and pellets have demonstrated promising results when fed into boiler systems via automated means. Currently, other raw materials are being explored due to a lack of timber reserves (Lin and Lu 2021). Diverse thermochemical methods may be used to generate energy from lignocellulosic biomass, as well as to attain other products including syngas, bio-oil, and chemicals. Biomass feedstock is chemically converted or thermally cracked into syngas using a catalyst in an anoxic or oxygen-deficient reaction environment in thermochemical processes, for instance, pyrolysis or catalyst-cracking reactions, gasification, oxidation, etc. (Liu et al. 2017). In terms of biomass usage, combustion is a well-proven and established method. Burning biomass at higher temperatures (800–1000  C) with the optimum quantity of O2 is an exothermic, redox reaction process. At times, the temperature may exceed 1400  C. Over 90% of energy expenditure is attributed to biomass burning. As a result of this process, soot, dust, ash, NOx, CO2, and CO2 are produced. Charcoal, bio-oil, and fuel gas are produced by means of pyrolysis, the thermal decomposition of biomass by heat in an oxygenfree environment. In an oxygen-free environment, pyrolysis produces charcoal, bio-oil, as well as fuel gas. Processes of pyrolysis may be classified into three categories based on operating conditions: traditional pyrolysis (carbonization), fast pyrolysis, and flash pyrolysis. Volatiles from biomass partially vaporizes during the slow pyrolysis process, leaving 80% of the char remaining after the process is completed. Fast pyrolysis is carried out between 300 and 700  C using biomass particles smaller than 1 mm in size at a quicker heating rate of 10–200  C for a brief residence duration of 5–10 s without oxygen. Hundred to 10,000  C per second fast flash pyrolysis yields 80% more bio-oil than conventional pyrolysis. The pyrolytic product composition is governed by the heating rate, residence time, and temperature (Yang et al. 2019). Char yield is enhanced by operating conditions such as reduced temperature, moderate rate of heating, and lengthy residence time. Due to its profitable manufacturing process, enhanced energy efficiency, and ecologically sound alternative for petroleum products, py-oil is receiving more attention nowadays. A variety of chemicals and turbines may be powered by Py-oil, as well as cars, generators, and electric generators. However, thermal stability and corrosion are two of the most significant constraints. Hydrogenation and catalytic cracking (physical and chemical) methods are necessary to enhance py-oil output and characteristics (Dimitriadis et al. 2021). There are also two more common processes for converting biomass into biofuel: gasification and liquefaction. During partial oxidation with

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oxygen or steam, biomass is transformed into syngas at 800–1000  C. However, a dry-cleaning system is required before the usage of syngas in vehicles or other machines. Gasification combined cycle (IGCC) is an emerging technology that uses contemporary coal gasification, syngas, and steam turbines to enhance coal power stations’ efficiency and minimize toxic emissions, according to the International Energy Agency (Ren et al. 2021). At 250–350  C and a high H2 partial pressure (usually 100–200 bar), biomass is liquefied into liquid biofuel. One of the two main types of liquification is: solvent liquefaction (SL) is based on organic acids and hydrothermal liquefaction (HTL) on water. In order to turn wet biomass into crude bio-oil, it must be thermally depolymerized. It’s a synonym for hydrous pyrolysis, which is also used. In HTL, biomass biopolymeric structure is broken down into liquid components over a predetermined length of time to generate bio-oil. Liquification of biomass with organic acids is a promising method, where the solvent aids in the dissolving of biomass pieces for the effective conversion of biomass into bio-oil. This technique, however, has significant drawbacks, including a greater solvent to biomass ratio, which results in fewer molecules interacting and a reduction in components dissolving. It is possible that liquefaction will react similarly to pyrolysis if the ratio is quite high. As a result, a low ratio promotes the disintegration of the biomass component (Brindhadevi et al. 2021; Condeço et al. 2021). Anaerobic digestion (AD), fermentation, esterification, and photo-fermentation are the primary biochemical processes that convert biomass into biofuel. Figure 5.1 illustrates the key stages involved in the AD process for biogas production. Without oxygen, AD is triggered when large volumes of wet organic matter accumulated. As a result of the anaerobic bacteria’s digestion of organic material, CO2 and CH4 are generated. Biogas plants are one example of a regulated environment where anaerobic processes may occur. An airtight container termed as a digester is used to store organic waste, such as livestock manure, and microorganisms. Dependent on the waste feedstock and the system architecture, biogas is typically constituted of 55–75% pure CH4. Transesterification is the biochemical process through which biodiesel is produced. The fuel, which is a monoalkyl ester of long-chain fatty acids, is a perfect alternative for fossil fuels, notably diesel fuel. Triglycerides (oil, lipids, and free fatty acids) are converted to monoalkyl ester, which is biodiesel. The feedstock is mixed with an alcohol like methanol, a catalyst like liquid acid (H2SO4), and a base like NaOH or KOH. The reversible interaction between triglycerides and alcohol occurs. As a result, more methanol can be injected to speed up the transesterification reaction and assure full transformation (Ganzoury and Allam 2015).

5.4

Types of Nanomaterials Used in Biofuel Production

Recent advances in nanotechnology have made nanoparticles the subject of intense investigation. Researchers have been studying many forms of nanomaterials for decades, including carbon-based (e.g., CNT, carbon hollow tubes), metal, metal

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Fig. 5.1 Key process stages of anaerobic digestion process for biogas production

oxide, and metal-organic frameworks. As a result of its size (excellent aspect ratio and specific surface area), excellent electronic conductivity, and exceptional chemical and mechanical characteristics, carbon nanotubes (CNTs) are a breakthrough material in many applications including the production of biofuel. The fact that CNTs are similar in size to enzymes is only one of the many reasons why CNTs are a preferred biofuel cell material (Ben Tahar et al. 2018). Additionally, CNTs are widely explored as nano additives in biofuel production. Other carbon-based catalytic materials such as carbon nanofibers, graphene oxide, and biochar, demonstrated to have significant potentials in the field of biofuel generation. When it comes to the extraction of biofuel and use, various metal nanoparticles (such as Zn, Cu, and Ag), as well as metallic oxides and metallic combinations, are utilized as additive chemicals in order to improve the physiochemical characteristics of the biofuel as well as the performance of biofuel. Another type of the popular ecologically sound NPs is strong basic metal oxides such as strontium oxide (SrO), copper oxide (CuO), cerium oxides (CeO), nickel oxide (NiO), and calcium oxide (CaO) reinforced zirconium oxide (ZrO2) are extensively explored by researchers due to their high catalytic activity (Rezania et al. 2021). Magnetic nanoparticles (MNPs) have attracted the attention of researchers since they are advantageous for large-scale biodiesel synthesis because they can be efficiently removed from the process medium and recycled (Changmai et al. 2021). Solid acid NPs have also

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attained considerable attention as a means of synthesizing biodiesel from inexpensive and non-food feedstocks. Esterification and transesterification of triglycerides may be performed concurrently using solid acid catalysts without soap production or corrosion of reactors or quenching stages. Biodiesel may be produced continuously, allowing for further process intensification. The fact that these catalysts may be recovered and reused makes them ideal for industrial biodiesel production (Gardy et al. 2018). Nanoporous materials have advanced dramatically in recent years. Metal-organic frameworks (MOFs), which are comprised of inorganic (metallic) and organic molecules, have a large surface area with adequate active sites, and may be easily changed by post-synthetic alterations, are intriguing materials among nanoporous materials. In the realm of biofuel generation, MOF has lately gained a lot of interest (Haldar et al. 2020). The potential of various types of nanomaterials has been discussed in the following sections.

5.5

Potential of Nanomaterials for Biofuel Production

A number of constraints exist in the current biofuels manufacturing process, preventing its commercialization. These constraints must be addressed if biofuels are to be used as a realistic renewable energy source. Using NPs in biofuel production might be a game-changer. Nanomaterials’ promise in liquid and gaseous biofuel generation is the focus of this section.

5.5.1

Liquid Biofuel Production

Transesterification, pyrolysis, anaerobic digestion, and hydrogenation for the generation of fatty esters, hydrocarbons, alcohol, and other biofuel technologies are examples of effective nanotechnology usage in biofuel technologies. In recent years, new nanostructured forms such as multi-walled carbon nanotube (MWCNT), nanofibers, nanorod, and metal-organic frameworks (MOFs) have been investigated for biodiesel generation in addition to conventional nanomaterials such as nano silicon, magnetic NPs, nano metal, nano metal oxide NPs. The steps of the biodiesel manufacturing process are depicted in Fig. 5.2. In the presence of alcohols, biodiesel is predominantly composed via transesterification of triglycerides (generally vegetable oil) and/or esterification of free fatty acids (FFAs). Propanol, methanol, butanol, and ethanol are among the alcohols that can be employed for this purpose. Due to its low cost and wide availability, methanol is the most often used solvent. In the presence of ethanol/ methanol and base catalysts such as NaOH or KOH, the transesterification reaction occurs at temperatures ranging from 50 to 80  C. A variety of parameters influence the rate of reaction, including type and concentration of catalyst, the molar ratio of

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Fig. 5.2 Process flowchart of biodiesel production from biomass

alcohol/oil, reaction time, temperature, oil moisture content, and mixing (Al Hatrooshi et al. 2020). Heterogeneous catalysts suffer from mass transfer resistance, rapid deactivation, prolonged preparation, as well as inefficiency. These limitations may be solved by nanocatalysts with large specific areas, strong catalytic activity, and excellent rigidity. Madhuvilakku and Piraman (2013) developed heterogeneous TiO2–ZnO and ZnO nanocatalysts for palm oil transesterification and exploited their performance. Numerous variables were found to affect the catalytic efficiency, including catalyst concentration, methanol-to-oil molar ratio, reaction time as well as temperature. With a 5 h reaction period and a lower catalyst loading of 200 mg TiO2–ZnO nano catalyst at a 6:1 molar ratio of methanol to oil and 60  C, a 98% transesterification conversion and 92% yield were achieved (Madhuvilakku and Piraman 2013). Nano-oxides have been implicated in the synthesis of biodiesel in a number of investigations. By changing the hydrogenation and acidic sites in the

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catalyst, adding oxide will affect the behavior. In the metal oxide, electron acceptors and proton acceptors are represented by positive metal ions and negative oxygen ions, respectively. A methoxyl anion and a hydrogen cation are formed when the (OH) link breaks at these positions which provide an adsorption site for methanol. The methyl esters are produced when the methoxide anion created in the heterogeneous catalyst interacts with triglyceride. Using varied calcination temperatures, Davoodbasha et al. (2021) produced CaO nanocatalysts and investigated their influence on the total transesterification of algal oil at various concentrations. In the transesterification process, a catalyst concentration of 3 mg L 1 of CaO produced at 400  C was determined to be optimal (Davoodbasha et al. 2021). Researchers have also found that nano-MgO can enhance the transesterification of soybean oil with supercritical/subcritical methanol in another experiment. If only a small amount of MgO nanocatalyst (0.5 wt%) is applied, the soybean oil methyl ester conversion can be significantly accelerated (Wang and Yang 2007). As a result of its surface structure, MgO nanocatalyst is able to increase its basic affinity, allowing it to act as a base for CaO nanocatalyst, increasing the catalyst’s contact surface and resulting in a significant increase in transesterification reaction yields. Both nanocatalysts were reported to increase biodiesel production yield. Using 0.7 g of CaO and 0.5 g of MgO NPs, a maximum biodiesel production of 98.95% was attained at a methanolto-oil ratio of 7:1 (Tahvildari et al. 2015). NPs are also used to boost lipid synthesis. It was claimed that waste cooking oil yielded 96% FAME after 2.5 h of reaction at 90  C using a 3 wt% magnetic SO4/Fe-Al-TiO2 solid acid catalyst (Gardy et al. 2018). A major waste product of biodiesel manufacturing is crude glycerol. Using it as a substrate for algal lipid synthesis, the resulting lipid may be recycled as a feedstock for biodiesel production. Using MgSO4, Sarma et al. (2014) examined whether Chlorella vulgaris might produce more lipids on CG-based medium when supplemented with the prepared NPs. MgSO4 was added to the procedure to increase lipid synthesis by 185.29  4.53%. MgSO4 nanoparticles were also shown to enhance lipid synthesis by 118.23  5.67% when applied (Sarma et al. 2014). In a similar manner, Jeon et al. (2017) produced silica and methyl-functionalized silica nanoparticles (SiO2-CH3) to improve the growth of microalgae for lipid yield (Jeon et al. 2017). As a biodiesel catalyst, acid-functionalized magnetic nanoparticles were effectively employed by Wang et al. (2015). Chemically functionalized Fe/Fe3O4 magnetic NPs with silica coatings were produced and utilized in the transesterification of glyceryl trioleate (Wang et al. 2015). The use of carbonbased NPs in the transesterification process has also been reported in a number of publications. A single-walled porous carbon nanohorn catalyst (SWCNH) distributed with Fe/Fe2O3 NPs has been developed by Poonjarernsilp et al. (2015). Experiments have shown that the initial esterification of palmitic acid occurs with the SO3H-Fe/Fe2O3-SWCNH catalyst, while transesterification of tributyrin occurs after palmitic acid has been consumed to a significant extent. More than 90% of the methyl palmitate produced by esterification may be obtained within 3–7 h (Poonjarernsilp et al. 2015). In another work, the author used hydrothermal sulfonation at 200  C in an autoclave reactor to produce four carbon-based solid acid catalysts from SWCNHs, oxidized SWCNHs (ox-SWCNHs), carbon black, and

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activated carbon. Sulfonated SWCNHs (SO3H-SWCNHs) exhibited the highest acid density of the four sulfonated specimens, which is a desired characteristic for usage as a biodiesel synthesis catalyst, according to the results (Poonjarernsilp et al. 2014). Due to its economic and environmental benefits, bioethanol is one of the most widely utilized alternative fuels in the transportation industry. Ecologically and economically, lignocellulosic biomass should be used in large-scale bioethanol production since substrate compatibility is a key cost factor. The method of producing bioethanol from biomass is complicated. Biomass pretreatment, enzymatic hydrolysis, fermentation, and ethanol generation are the four key stages in the process. Bioethanol production begins with the removal of lignin, which is accomplished through pretreatment of the raw biomass. Lignocellulosic biomass has a complicated structure, with 35–50% cellulose and 20–35% hemicellulose. Lignin makes about 15–20% of the biomass. Cellulose and hemicellulose must first be broken down in order to obtain them. Second, during the pretreatment procedure, numerous inhibitors of Saccharomyces cerevisiae are produced. Carboxylic acids, phenolic, and furans compounds are examples of these. Moreover, plant-based materials include pollutants that interfere with S. cerevisiae’s metabolism and reduce bioethanol production. Several studies have indicated that the immobilization of enzymes can considerably improve this scenario. For instance, cellulase immobilized on MnO2 NPs enhances the enzymatic activity while preparing bioethanol from sugarcane and thereby increased bioethanol production (Cherian et al. 2015). Sanusi et al. (2020) improve the yield of bioethanol by using the NiO NPs as a nanocatalyst. NiO NPs have the ability to improve the bioactivity of ethanol-producing yeast by lowering the oxidation-reduction potential of the procedure, increasing buffering capacity, and lowering inhibitor concentration to a more favorable level (Sanusi et al. 2020). Several studies have indicated that the use of the rotating magnetic field (RMF) is one of the ways to enhance the yield of the bioethanol production process by fermentation process with the Saccharomyces cerevisiae yeast (Rakoczy et al. 2016). In view of that, Konopacka et al. (2019) modified yeast with Fe3O4 NPs and utilize it for the production of bioethanol by using a magnetically-assisted bioreactor equipped with an RMF generator. The addition of Fe3O4 NPs improves the mixing process that improves the production of bioethanol (Konopacka et al. 2019). The use of NP incorporated biocatalysts such as cellulase, β-glucosidase, laccase, xylanase, and cellobiose, increases the stability of enzymes as well as catalytic efficacy of the bioethanol production process. Using the incorporation of magnetic biocatalyst in this scenario offers additional advantages such as the easy recovery of the biocatalyst (Rai et al. 2019). Sanchez-Ramirez et al. (2017) immobilized Trichoderma reesei cellulase on chitosan-coated magnetic NPs which improve the initial enzyme retains about 37%. It has also a good yield during the hydrolysis of the Agave atrovirens leaves and can be reused up to four times after recovery. This can considerably reduce the consumption rate of the enzyme during the bioethanol production process (Sanchez-Ramirez et al. 2017). In addition to Clostridium ljungdahlii, researchers have shown that Eubacterium limosum and Clostridium aceticum are capable of producing alcohol from syngas. However, poor gas to liquid to mass transfer reduced the production efficiency of the

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fermentation process. Kim et.al (2016) synthesized methyl-functionalized cobalt ferrite–silica (CoFe2O4@SiO2–CH3) NPs to enhance the production efficiency of bioethanol production via syngas fermentation (using Clostridium ljungdahlii) route by improving the gas-liquid mass transfer rate. The prepared NPs enhance the Clostridium ljungdahlii growth through improving the mass transfer rate which considerably increases the ethanol production rate by 213.5% (Kim and Lee 2016). One of the most promising approaches to creating biobutanol from biomass sources is the acetone-butanol-ethanol (ABE) fermentation process. The primary disadvantages of this technique are excessive energy consumption and toxicity inhibition. For large-scale manufacturing and industrial use, in situ removal and concentration of biobutanol from its aqueous solution is critical. In this circumstance, a technique of pervaporation-based on membrane technology is seen as an appropriate way due to the separation efficiency, less energy-intensive, and high mechanical strength (Liu et al. 2014). The property of the membrane is generally recognized to rely on its material and structure. The design and manufacture of novel membrane materials and the membrane structure control are therefore key topics of study on pervaporation membranes. Researchers have prepared various nanomaterial-based membranes to achieve high separation efficiency during the process. For instance, Li et al. (2020) combined ZIF-8-modified graphene oxide (ZGO) with polyether block amide (PEBA) and deposited it on a ceramic tubular substrate to develop a composite membrane for biobutanol recovery. The butanol/ water combinations in which the composite membrane was produced demonstrated good stability over 100 h (Li et al. 2020).

5.5.2

Gaseous Biofuel Production

One of the most significant ways for converting organic waste into sustainable energy (such as biogas) is anerobic digestion (AD). AD with high biological transformation is a widely accepted method for various waste management, which may be employed to generate domestic power heating, and cooking, etc. AD performed by a consortium of microbes is a very slow process and influenced by several variables such as operating temperature, pH, C/N, HRT, etc. Some of the additional constraints connected with AD are: inadequacy of process stability, low loading rates, sluggish recovery following failure, and particular demands on waste composition (Yin et al. 2014). Several studies evaluated the impact of NPs on the performance of AD, either as a complement or as a waste material component. Mu et al. (2011) studied the influence of four different metal oxide NPs, i.e., TiO2, Al2O3, SiO2, and ZnO NP. It was observed that the only ZnO had an inhibitory effect upon CH4 production and the effects of ZnO depended upon the dosage. No CH4 was affected by low nano-ZnO (6 mg/g-TSS). However, CH4 was reduced by 22.8% and 81.1% compared to control with the addition of ZnO to 30 and 150 mg/g TSS. The mechanisms research indicated that Zn2+ released from ZnO is a major cause for their inhibition, and ZnO has been inhibited in the case of metabolism and critical

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activity of the enzyme. However, the addition of the other three NPs did not exhibit any effect on methane production (Mu et al. 2011). According to Su et al. (2013), AD with zero-valent iron (nZVI) improved biogas and CH4 output by 30.4% and 40.4%, respectively. Iron NPs may efficiently lower the H2S concentration and also enhance the generation of CH4 during anaerobic digestion (Su et al. 2013). Wang et al. (2016) have assessed the effect on biogas production utilizing waste-activated sludge by four NPs representatives (ZVI, Ag, Fe2O3, and MgO). Usage of ZVI (10 mg/g TSS) NPs and Fe2O3 (100 mg/gTSS) has improved the generation of biogas by 120% and 117%, compared with control. These researches shows a good influence on methanogenic archaea activity, with low concentration of ZVI and Fe2O3 NPs (Wang et al. 2016). Researchers also noted the increase in the population of the methanogens within the anerobic digester by adding NPs such as ZVI (Yang et al. 2013). Al-Ahmad et al. demonstrated positive impacts on AD and biogas production by utilizing SiO2 metal nano-substances. At 55  C, they examined the influence of metal NPs (Fe, Pt, Pd, Ni, Ag, Co, and Cu) encased in porous SiO2 on the AD. All the reported NPs exhibited substantial increases in methane generation (Al-Ahmad et al. 2014). One of the most challenging benefits is the production of biohydrogen utilizing dark fermentation technology. Dark fermentation is highly promising due to unique properties like a high production rate, environmental operating circumstances, and a varied substratum consumption like diverse organic wastes. Despite these benefits, this biohydrogen generation technique is affected by several limitations such as poor biohydrogen output, lack of high-efficiency microorganisms generating biohydrogen and the absence of adequate substrates, and inadequate use throughout the fermentation process (Srivastava et al. 2021). Using dairy effluent as a substrate, Gadhe et al. (2015) examined the influence of Fe2O3 and NiO NPs on fermentative hydrogen generation. They discovered that adding 50 mg/L of Ni NPs elevated hydrogen production by 24% when compared to the control (Gadhe et al. 2015). Rambabu et al. (2021) performed a series of experiments with Fe3O4 NPs and nanocomposite prepared with date seed activated carbon and Fe3O4 (Fe3O4/ DSAC). The increased production rate of hydrogen and metabolites in the presence of Fe3O4 and Fe3O4/DSAC nanocomposites was suggestive of increased cell growth and hydrogenase activity. For enhanced hydrogen production, the dosage variation effect study revealed an optimal NP concentration of 150 mg/L for both additions. For this optimal concentration, Fe3O4 NP enhanced cumulative biohydrogen production by 1.84 times, whereas Fe3O4/DSAC nanocomposites increased cumulative biohydrogen production by 2.05 times (Rambabu et al. 2021). In mesophilic environments (37  C), Yang and Wang (2018) carried out a dark fermentative biohydrogen process with grass. The usage of Fe0 NPs was found to promote the activity of major producers of biohydrogen. In addition, the micronutrient analysis has demonstrated a variation in the composition of bacteria from Enterobacter sp. to Clostridium sp. implying that the Fe0 NPs have caused a more effective biohydrogen production pathway that promotes the activities of major approaches like Clostridium (Yang and Wang 2018). Metallic NPs have also shown the potential in biohydrogen production. Beckers et al. carried out a number of experiments on

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fermentative glucose hydrogen generation utilizing metallic (Pd, Ag, and Cu) and metallic oxide (FexOy) NPs. These metal nanoparticles were immobilized on low 10–6 mol L 1 porous silica (SiO2). Comparatively, the biohydrogen yield and biohydrogen generation rates of FeO-enclosed microorganisms increased by 38% and 58%, respectively. This was attributed to an increase in the activity of hydrogenase and electron transfer during the dark fermentation phase (Beckers et al. 2013).

5.6

Green Synthesis Approaches for the Preparation of Nanomaterials

Conventional NP preparation methods have several drawbacks, including the potential for contamination due to precursor chemicals, high cost, high energy intensity, use of toxic solvents, production of potentially harmful by-products, and the potential for structural defects to affect surface properties of the NP. Moreover, the latest environmental concerns have prompted scientists to focus on avoiding hazardous, environmentally damaging chemicals, as well as to progress toward greener technology. Scientists have become interested in biological techniques for synthesizing NPs that do not include harmful compounds as a by-product as a result of environmental concerns in recent years (Aarthye and Sureshkumar 2021). Plant extracts, microbes, proteins, and biomolecules are used in the green synthesis of NPs. The inclusion of such biological products in the synthesis process reduces the use of hazardous chemicals and improves the biocompatibility of the produced NPs. These nanoparticles have shown good compatibility in a variety of applications, including the biofuel industry. Various NPs produced using a green synthesis method for use in biofuels are summarized. For instance, Duman et al. (2019) synthesized magnetic NPs from pomegranate peels using a low cost, easy, and ecologically sound procedure in order to valorize it as a catalyst in the biodiesel production process (Duman et al. 2019). Similarly, Yatish et al. (2020) utilized Ochrocarpus longifolius leaves extract as new fuel to synthesize calcium titanate NPs (CaTiO3 NPs). The obtained CaTiO3 NPs were utilized as a heterogeneous base catalyst in the synthesis of biodiesel from dairy waste scum oil (DWSO). With negligible yield loss, the CaTiO3 NPs exhibit high catalytic endurance for up to five cycles (Yatish et al. 2020). Din et al. (2020) prepared Ni-Fe2O4 NPs using plant extract of Syzygium aromaticum and analyzed its catalytic behavior in slow catalytic pyrolysis for biofuel production. In another study, it was shown that biochemically produced MnO2 nanoparticles displayed good catalytic activity in the microwave-aided transesterification of seed oil and grape waste to produce biofuel (Stegarescu et al. 2019). Nayak et al. (2018) biosynthesized Ag NPs using Santalum album leaf as a fuel. The addition of Ag NPs to biodiesel resulted in a substantial boost in brake thermal efficiency, with greater doses of Ag NPs further improving the engine’s performance (Nayak et al. 2018). In a recent study, Hassaan et al. (2021) observed

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enhancement in biogas production while using ZnO NPs extracted from durum wheat (Hassaan et al. 2021). In the literature, it is also shown that NPs may be synthesized by utilizing microbes, fungi, and bacteria. Mahmood et al. (2019) prepared cobalt oxide (Co3O4) and nickel oxide (NiO) nanoparticles by using bread fungus and coriander leaves at room temperature. Co3O4 and NiO nanoparticles with grain sizes of 14.43 and 18.15 nm, respectively, were confirmed by XRD patterns. A method that was more efficient and needed less time and temperature was achieved by using the prepared NPs as a catalyst for gasification and transesterification of kitchen trash. In addition to being ecologically clean and efficient, it generates a high-quality biodiesel. Gases such as methane were generated as by-products, along with char (Mahmood et al. 2019). To produce Ag NPs smaller than 140 nanometers, a costeffective technique was devised by utilizing two distinct bacteria, Bacillus amyloliquefaciens and Bacillus subtillis (Ghiuță et al. 2018). Shen et al. (2018) performed an investigation on synthesis of Au NPs from different microbes. Their method involved comparing the average size of the AuNPs prepared by utilizing three distinct cell-free extracts from bacteria, yeast, and a filamentous fungus. The results of this research indicate that the fungus performed better than other types of organisms (Shen et al. 2018).

5.7

Factor Affecting the Performance of Nanomaterials

The efficacy of NPs in biofuel manufacturing processes is attributed to a number of factors. These variables include the temperature and pressure during the synthesis procedure, the pH of the medium, and many more. Some of these conditions are addressed in this section.

5.7.1

Temperature

In the production of NPs, temperature is a key parameter. A metallic NP can be calcined at temperatures varying from 100 to 700  C, based on the process used to create it. Studies have reported that the mean of the NPs size increases with the increase of reaction temperature. Magnetic characteristics of NPs exhibit considerable reliance on particle size (Qu et al. 2006). Figure 5.3a exhibits the XRD patterns of CoFe2O4 NPs at various reaction temperatures. It can be observed from the Fig. 5.3 that, at 40  C, the average crystallite size increases to 47.4 nm, whereas at 60  C, it expands fast to 29.6 nm. Cobalt ferrite crystals develop best at reaction temperatures of 80  C and higher, according to the data. Furthermore, it was reported that CoFe2O4 NPs synthesized at different temperatures show distinct magnetization loops and coercivity, remanence ratios, as well as magnetization vary with particle size (Qu et al. 2006).

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Fig. 5.3 (a) CoFe2O4 nanoparticle XRD patterns produced at various temperatures. (Reproduced with permission from Qu et al. 2006), (b) dose–response graphs for methane generation during bulk CuO exposure, (c) dose–response curves for methane generation when exposed to CuO NPs. (Reproduced with permission from Luna-delRisco et al. 2011)

5.7.2

Pressure

During the production of NPs, pressure is also a variable that may be adjusted. To produce a certain size, shape, and aggregation of NPs, pressure is applied to the reaction media. There is evidence that size NP of increases under high pressure. For instance, pressure has a considerable influence on the size of magnetite NPs, according to Yazdani and Edrissi (2010). A high surface-to-volume ratio of NPs and several other physical characteristics of magnetite, including surface tension and supersaturation may be responsible for this phenomenon. During the experiment, the size of the particles increased from 8.3 to 16.8 nm at 25  C by raising the reaction pressure from 300 to 6000 mbar (Yazdani and Edrissi 2010). Similarly, Geissler et al. (2010) described the influence of the amount of hydrogen pressure used during

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the reduction process has a significant impact on the size of the produced Ni NPs. For subnanometer thin films, the NP size dramatically rises with an increase in hydrogen partial pressure during reduction treatment, according to the study (Geissler et al. 2010).

5.7.3

pH of the Medium

It has been demonstrated that the pH of the synthesis affects the effectiveness of metallic NPs. NPs become more stable at pH levels less than 7. Because of this, it is possible to regulate the size and shape of NPs by altering the pH throughout the synthesis method (Sekoai et al. 2019).

5.7.4

Morphological Characteristics

As part of biofuel manufacturing procedures, various NP sizes, ranging from 5 to 100 nm, have been described in the literature. Producing nanoparticles depends on several aspects, including particle size, and it is important to identify an ideal set of operating parameters for each operation. The dimensions and concentration of NP added throughout the manufacturing process are important elements to consider (Abdel-Fatah 2018). The influence of NP size on emission and performance parameters was studied by Devarajan et al. (2019). CuO NPs of 10 and 20 nm diameters were used in a biodiesel-powered diesel engine. The efficiency of the fuel was compared to that of normal diesel fuel. In the investigation, the NPs concentration is set at 100 ppm. Fuel consumption and utilization per unit power and time are described by the brake-specific fuel consumption (BSFC) parameter. In the study, the BSFC of the utilized biofuel with NP load was greater than that of the diesel. The low BSEC was owing to biofuels’ lower calorific value compared to diesel, which necessitated more fuel use to maintain constant power production. According to the researchers, adding 10 nm CuO NPs to the BD100 material reduced its BSFC by 1.3% and increased its break thermal efficiency (BTE) by 0.7%. A 20 nm CuO NP in biodiesel improves performance even more than a 10 nm NPs. The BD100 also reduces smoke, CO, and hydrocarbon emissions, while increasing nitrogen oxides (NOx) emissions. CuO at 20 nm improves BTE, lowers BSEF, and reduces emissions of the main pollutants (Devarajan et al. 2019). Similarly, during AD of cattle dung, the impacts of bulk and nano-sized CuO and ZnO NPs on biogas and CH4 generation was explored by Luna-delRisco et al. (2011). A batch of anaerobically digested slurry was inoculated with CuO and ZnO NPs and microparticles at 36  C for 14 days. Bulk ZnO, bulk CuO, and ZnO NPs were less hazardous to anaerobic bacteria as compared to CuO NPs. CuO NPs suppress methane generation at least ten times more than their bulk equivalent, as seen in Fig. 5.3b, c. A similar trend was observed for ZnO NPs (Luna-delRisco et al. 2011).

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Future Prospective

Despite being efficient, cheap, and mature, these technologies combined with nanotechnology are still mostly at the laboratory and pilot scale, but they are projected to offer a viable alternative to traditional systems when they are developed at the commercial scale. A key role for NPs can be played in improving the quality and quantity of biofuel manufacturing technologies. NPs have been shown to influence biofuel production in a variety of ways, according to a number of studies. Though different types of NPs impact the reaction process, their specific mechanisms are yet unclear. The basic mechanism of NPs in the reaction may also be understood by molecular research. The cost of NPs synthesis should also be considered in order to produce biofuels at a cost-effective rate. Green nanomaterial synthesis for production of biofuels may help reduce the biofuels cost in general.

5.9

Conclusions

Current biofuel production methods combined with nanotechnology provide a chemical reaction that is stable and tolerable, minimizing the harmful effects of various solvents and catalysts used in biofuel production, as well as reducing the cost of biofuel production. Most fuel characteristics can be improved by adding NPs. In contrast, traditional NP development techniques are expensive and create hazardous products, therefore it is imperative to decrease the environmental toxicity risks associated with diverse chemicals employed in physical and chemical processes. “Green synthesis” is one of the alternative methods for developing NPs. NPs produced from biosynthetic materials were thoroughly explored in this paper. Ag, Au, and ZnO NPs are commonly produced from plant-based resources to increase the characteristics of biofuels. According to the literature cited, NP source, pH, pressure, temperature, and shape all have a key influence in determining the improved NPs characteristics. It was observed that NPs have a lot of promise for biofuel generation. All features, kinds, and possibilities of nanoparticles are discussed in the chapter, allowing researchers to study and carry out their research on NPs for efficient biofuel production.

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Chapter 6

Biosynthesis of TiO2 Nanoparticles and Their Application as Catalyst in Biodiesel Production Sheela Chandren and Rosliana Rusli

Abstract Titanium dioxide nanoparticles (TiO2 NPs) are capturing great interest worldwide with vast applications in various areas. However, their conventional synthesis methods often pose a risk to humans and the environment due to the utilization of toxic chemicals and laborious procedures. Several works have documented the biosynthesis method of TiO2 NPs using plants and microorganisms as alternative routes and have been proven to overcome the drawbacks of conventional methods without compromising their excellent properties. Recent studies revealed that TiO2 NPs had been applied as heterogeneous catalysts and catalyst supports for biodiesel production, successfully overcoming the separation problem with homogeneous catalysts with comparable yield. This chapter reviews the various types of plants and microorganisms used for the synthesis of TiO2 NPs and their application in biodiesel production through transesterification, esterification, and simultaneous reactions. Keywords Titanium dioxide · Nanoparticles · Biosynthesis · Biodiesel · Heterogeneous catalyst

S. Chandren (*) Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, Johor Bahru, Johor, Malaysia Centre for Sustainable Nanomaterials, Ibnu Sina Institute for Scientific and Industrial Research, Universiti Teknologi Malaysia, Johor Bahru, Johor, Malaysia e-mail: [email protected] R. Rusli Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, Johor Bahru, Johor, Malaysia © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Srivastava et al. (eds.), Green Nano Solution for Bioenergy Production Enhancement, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-16-9356-4_6

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Introduction

In recent years, nanotechnology has emerged to be widely regarded as one of the most promising areas of material sciences. As increasing the surface-to-volume ratio of materials can drastically change and generate new physical and chemical properties, this field exclusively deals with manipulating materials at the nanoscale. Any materials within the nanoscale (1–100 nm) are called nanoparticles, grouped into two main classes of organic and inorganic nanoparticles. Nanoparticles based on organic compounds usually consist of chitosan, dendrimers, ferritins, micelles, and liposomes, while inorganic nanoparticles majorly focus on metals, semiconductors, and magnetic nanoparticles (Khalid et al. 2020). Out of the numerous developed metal nanoparticles, titanium dioxide nanoparticles (TiO2 NPs) are vastly used, where approximately 25% of the worldwide industrial produce contains TiO2 NPs. According to the nano-database, the applications of TiO2 NPs are broad, including in the manufacturing of paints, paper, plastics, inks, cosmetics, food additives, and many others (Irshad et al. 2021). TiO2 NPs are primarily used as white pigments in these products. Other than that, TiO2 NPs demonstrate unique optical, thermal, magnetic, and electric properties, which caused them to be utilized in many fields such as sensing (Gökdere et al. 2019), medical devices (Jafari et al. 2020), solar cell (Gnida et al. 2021), agriculture (Gohari et al. 2020), wastewater treatment (Emamverdian et al. 2021), construction (Diamantopoulos et al. 2020), photocatalyst (Negi et al. 2021), drug delivery system (Cui et al. 2021), and biodiesel production (Patil et al. 2020). In order to form any nanoparticles, two principal approaches can be used (Fig. 6.1). The first approach is a top-down approach where larger molecules will be reduced to nano-size molecules mainly through physical methods, for instance,

Fig. 6.1 Synthesis methods for nanoparticles

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mechanical/ball milling, lithography, pulse laser ablation, etching, sputtering, molecular beam epitaxy, and pulse wire discharge. Although this approach is rapid and straightforward, it unfortunately produces poor-quality nanoparticles with imperfect surface structure and many impurities. Therefore, researchers frequently employ another approach in order to obtain a higher purity and quality of nanoparticles. The other approach in producing nanoparticles is the bottom-up approach, in which smaller molecules or atoms are combined by a self-assembly process forming bigger molecules. This approach involves both chemical and biological methods. Titanium dioxide nanoparticles (TiO2 NPs) can be synthesized by the bottom-up approach using various chemical methods. The most commonly used chemical reduction (Andronic and Enesca 2020), sol-gel process (Mahy et al. 2021), chemical vapor deposition (Song et al. 2021), microwave (Xiang et al. 2021), and hydrothermal methods (Leong et al. 2021). Nonetheless, these methods involve the use of a large number of chemicals, and some of them require specialized instruments and tools due to the high pressure and energy requirements. Based on previous studies, the chemical reduction method resulted in nanoparticles of inferior quality with large particle size, non-uniform shape, less stable, and more agglomeration (Xu 2018). Needless to say, there is a need to develop an alternative way that is not only environmentally friendly but also cost-effective, simple, efficient, rapid, high yield, easily upscaled, and requiring only normal air pressure and temperature. Hence, the biosynthesis method using various plants and microorganisms is developed (Nadeem et al. 2018).

6.2

Biosynthesis of TiO2 NPs Using Plants

Biosynthesis of metal nanoparticles using plants has many advantages over microorganisms (Nabi et al. 2018). The former is more convenient, cost-effective, higher yield, safe to handle, requires a shorter synthesis period, and most importantly, does not require the sustainment of the cell culture. Additionally, this approach boasts the ability to add value and increases the usage of plant waste materials such as fruit peel, seed, and stem. As a matter of fact, plants contain an abundance of metabolites and phytochemicals, which are useful for the synthesis of nanoparticles, including flavones, terpenoids, aldehydes, amides, and ketones. Generally, any part of plants such as leaves, roots, flowers, and fruits can be utilized to synthesize the nanoparticles. Usually, the method to synthesize the TiO2 NPs starts with the combination of a titanium source with a suitable solvent. In this first step, a choice of various titanium precursors can be employed, such as a bulk particle of TiO2, titanium tetraisopropoxide (TTIP), titanyl hydroxide (TiO(OH)2), titanium tetrachloride (TiCl4), and titanium oxysulfate (TiOSO4). Typically, a solution is prepared by dissolving the titanium precursors in distilled water or ethanol. Then the plant extract is slowly added drop by drop to the solution while constantly stirred under moderate

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Fig. 6.2 Biosynthesis of nanoparticles using plant extract

temperature. Interestingly, the change in the color of the solution can be observed, signaling the successful synthesis of the amorphous TiO2 NPs. Finally, the amorphous TiO2 NPs are filtered before washing with the desired solvents in order to eliminate any excess plant extract. Subsequently, they are dried and calcined with a typical temperature of 500  C for a few hours to produce crystalline TiO2 NPs. Figure 6.2 depicts the general steps of biosynthesis of TiO2 NPs using plants. In plant-mediated biosynthesis of TiO2 NPs, the phytochemicals and metabolites produced by the plant serve the purpose of reducing the titanium salt, hydrolyzing the titanium(IV) (Ti4+), solubilizing, and then polymerization of the TiO2 NP intermediate compounds (Ovais et al. 2018). A complete summary of plant materials used in the biosynthesis of TiO2 NPs is given in Table 6.1. The first attempt to biosynthesize the TiO2 NPs was made by Sundrarajan and Gowri in 2011 using leaves from Nyctanthes arbor-tristis and TTIP as the titanium precursor. The X-ray diffraction (XRD) analysis showed that the TiO2 NPs possess an anatase phase with high purity and crystallinity. Moreover, the scanning electron microscopy (SEM) micrographs unveiled a uniform spherical shape of TiO2 NPs with a 100–150 nm size range. Koca and Duman (2018) synthesized the TiO2 NPs using Mentha aquatic leaf extract with TTIP as the precursor. The TiO2 NPs showed

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Table 6.1 Examples of plant extracts used to synthesize TiO2 NPs Name and part of plant Calotropis gigantea (flower) Piper betel (leaf)

Precursor TiO (OH)2

Calcined temperature,  C/time, h –

TTIP

400  C/3 h

Ocimum tenuiflorum (leaf) Moringa oleifera (leaf)

TTIP

400  C/3 h

TTIP

400  C/3 h

Coriandrum sativum (leaf)

TTIP

400  C/3 h

Prunus domestica L. (peel) Prunus persia L. (peel)

TiO2

400–500  C/ 4h

TiO2

400–500  C/ 4h

Actinidia deliciosa (peel) Annona squamosa (peel) Catharanthus roseus (leaf)

TiO2

400–500  C/ 4h

TiO (OH)2

–

TiO2

–

TTIP

500  C/2 h

TTIP

500  C/5 h

TiO2

–

TTIP

570  C/ 3 h

TiCl4

500  C/4 h

Mentha aquatica (leaf) Mentha arvensis (leaf) Sesbania grandiflora (leaf) Syzygium cumini (leaf) Aloe barbadensis (leaf)

Size, nm and morphology 160–220 nm, spherical and aggregated 6.6 nm, rounded shape and cluster 7.0 nm, rounded shape and cluster 6.6 nm, rounded shape and cluster 6.8 nm, rounded shape and cluster Overall size 200 nm, cylindrical shape Overall size 200 nm, cylindrical shape Overall size 200 nm, cylindrical shape 23 nm, polydisperse and spherical 65 nm, clustered and irregular shape 69 nm, spherical

Crystalline phase –

20–70 nm, spherical 43–56 nm, triangular, square and spherical 18 nm, spherical and aggregated 60–80 nm, irregular shape

Anatase

References Marimuthu et al. (2013) Pushpamalini et al. (2021)

Anatase

Anatase

Anatase

Anatase

Ajmal et al. (2019)

Anatase

Anatase

Rutile

Roopan et al. (2012)

Rutile and anatase

Velayutham et al. (2012)

Anatase

Koca and Duman (2018)

Anatase

Ahmad et al. (2020) Srinivasan et al. (2019)

Rutile and anatase Anatase

Sethy et al. (2020)

Anatase

Rao et al. (2015) (continued)

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Table 6.1 (continued)

Precursor TiCl4

Calcined temperature,  C/time, h –

Trigonella foenumgraecum (leaf) Nyctanthes arbor-tristis (leaf) Phyllanthus niruri (leaf)

TiOSO4

700  C/3 h

20–90 nm, spherical and aggregated

TTIP

500  C/3 h

100–150 nm, spherical

Anatase

Sundrarajan and Gowri (2011)

TTIP

350  C/4 h

Anatase

Shanavas et al. (2020)

Psidium guajava (leaf)

TiO (OH)2

–

Rutile and anatase

Santhoshkumar et al. (2014)

Glycyrrhiza glabra (root) Withania somnifera (root)

TTB

400  C/4 h

Anatase

TiO2

–

Madadi et al. (2020) Al-shabib et al. (2020)

Sonchus asper (leaf) Echinacea purpurea Kniphofia foliosa (root) Tamarindus indica (leaf)

TTIP

500  C/

TiO2

–

TTB

500  C/3.5 h

30–50 nm, spherical and agglomerated 32.58 nm, spherical shape and cluster 60–70 nm, nanosphere 50–90 nm, aggregates of spherical and square 9–22 nm, spherical 120 nm, spherical cluster –, spherical

TTIP

500  C/3 h

Name and part of plant Aloe barbadensis (leaf)

Size, nm and morphology 20 nm, spherical

20–40 nm, spherical and aggregated

Crystalline phase Mixture of rutile, anatase and brookite Anatase

Anatase

Brookite – Anatase Anatase

References Rajkumari et al. (2019)

Subhapriya and Gomathipriya (2018)

Babu et al. (2019) Dobrucka (2017) Bekele et al. (2020) Hiremath et al. (2018)

an average particle size of 69 nm, which is smaller than that reported by Sundarajan and Gowri. The SEM images of the nanoparticles showed that they were spherical in shape, and the XRD analysis proved that they were in the anatase phase. Further investigation by the zeta potential analyzer showed that the formed TiO2 NPs have high stability with a zeta potential of 37.6 mV. These interesting results were believed to be caused by the high content of total phenolic and flavonoid in Mentha aquatic leaves with values of 337  2.15 and 15.75  0.25 mg/g, respectively. Based on that data, Ahmad et al. (2020) tried a different type of extract from the Mentha genus: Mentha arvensis. The phytochemicals present in Mentha arvensis leaves hydrolyzed the Ti4+ ion to form TiO2 NPs that exhibited strong light absorption near 400 nm. In addition, these nanoparticle sizes are almost comparable when

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using Mentha aquatic leaf extract, which is found to be nearly in the range of 20–70 nm. Replicating the procedure and titanium precursor, Hiremath and co-workers (2018) utilized the Tamarindus indica leaves to synthesize the TiO2 NPs. Remarkably, this plant extract was able to produce smaller nanoparticles compared to the previously mentioned study, ranging from 20 to 40 nm. Confirmed by the SEM image, these anatase TiO2 NPs are spherical, with evident aggregation. In addition, the Brunauer-Emmett-Teller (BET) analysis shows that the synthesized nanoparticles have mono-sized pores of 6.3 nm with a specific surface area of 75.50 m2/g. Moreover, TiO2 NPs with an extensive specific surface area and pore diameter of 105 m2/g and 10.50 nm, respectively, were synthesized by Sethy et al. (2020). They utilized Syzygium cumini leaf extract as a capping agent. Unfortunately, the SEM image revealed that this source of capping agent was unable to produce a uniform spherical nanoparticle. Instead, the spherical nanoparticles were found to be aggregated and irregularly shaped with average particles diameter of 18 nm. This result was supported by its low zeta potential value of 18.7 mV, which implies the fair stability of the TiO2 NPs. In 2020, Shanavas et al. attempted to synthesize the TiO2 NPs using Phyllanthus niruri leaf extract with a reasonably low calcination temperature of around 350  C. The XRD analysis showed that they successfully synthesized the crystalline anatase phase of TiO2 NPs without any trace of other phases. However, due to the fairly low calcination temperature, biomolecules, such as proteins and terpenoids that are naturally present in the leaf extract was not completely removed and still envelop the surface of TiO2 NPs. This occurrence is evidenced by the presence of these biomolecules peaks in the Fourier transform infrared (FTIR) spectrum of the nanoparticles. The remaining biomolecules initiated agglomeration of the spherical TiO2 NPs to a size range of 30–50 nm. As many biomolecules are naturally charged, they tend to be attracted to each other due to the electrostatic forces. Alternatively, Pushpamalini et al. (2021) synthesized the TiO2 NPs with a heating treatment of 400  C by using four different plants, namely Piper betel, Ocimum tenuiflorum, Moringa oleifera, and Coriandum sativum. All of the plant extracts were evinced to form crystalline TiO2 NPs with anatase phase. Surprisingly, these plant extracts were able to produce considerably small nanoparticles with particle sizes of 6.6, 7.0, 6.6, and 6.8 nm for Piper betel, Ocimum tenuiflorum, Moringa oleifera, and Coriandum sativum, respectively, as shown in Fig. 6.3. According to their FTIR spectra, the biomolecule peaks below 1400 cm 1 appeared to be very low; hence, the removal of biomolecules at 400  C can be considered as nearly complete. This result coincided with their transmission electron microscopy (TEM) images, as the round-shaped nanoparticles were found to be slightly clustered. In 2019, a different method was reported by Babu et al. in synthesizing the TiO2 NPs using the sol-gel technique with Sonchus asper leaf extract and TTIP. As revealed by the XRD graph and SEM micrograph of the TiO2 NPs, this method was able to produce anatase phase TiO2 that were spherical in shape, with narrow size distribution and small size nanoparticles in the range of 9–15 nm. Utilizing the same method, Madadi et al. (2020) synthesized the TiO2 NPs. Instead of using TTIP

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Fig. 6.3 TEM images of TiO2 NPs prepared using (a) Piper betel, (b) Ocimum tenuiflorum, (c) Moringa oleifera, and (d) Coriandrum sativum. Reprinted from Pushpamalini et al. (2021) with permission from Elsevier

as the titanium precursor, tetra-n-butyl orthotitanate (TBT) was utilized along with the root extract of Glycyrrhiza glabra. However, as indicated by the TEM image, the TiO2 NPs were found to be slightly larger than the previous study, with a diameter range of between 60 and 70 nm. Ethanolic root extract of Kniphofia foliosa–mediated biosynthesis of TiO2 NPs with TBT as the titanium precursor was reported by Bekele et al. in 2020. In contrast to the method employed by Sundrarajan and Gowri (2011), a sodium hydroxide solution was used to precipitate the TiO2 NPs in place of heating. A point to note is that the study compared the characteristics of TiO2 NPs synthesized at different volume ratios of TBT to Kniphofia foliosa extract. With the use of an XRD analyzer,

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Fig. 6.4 XRD patterns of TiO2 NPs synthesized in (a) 1:2, (b) 1:1, and (c) 2:1 volume ratios of TBT to Kniphofia foliosa extract. (Reprinted from Bekele et al. 2020 with permission from Hindawi publishers)

it was found that the 1:1 and 2:1 samples formed a high crystalline anatase phase with sharp and intense peaks (Fig. 6.4). Dissimilar to this, the 2:1 composition ratio loses its crystalline phase and is believed to be due to the excess quantity of the plant extract, which exceeded the surface of the TiO2 NPs that can be coated. On the bright side, this excess amount of root extract contributed to a tremendous amount of capping agents that stabilized the TiO2 NPs and formed smaller TiO2 NPs with a mean particle size of 8.5 nm compared to the 10.2 nm formed by the 1:2 ratio. The FTIR analysis identified various phytochemicals within the root extract of Kniphofia foliosa, such as anthraquinone, knipholone anthrone, and chrysophanol. Subhapriya and Gomathipriya in 2018 used titanium dioxide sulfate (TiOSO4) salt as titanium precursor with the help of Trigonella foenum-graecum leaf extract to synthesize TiO2 NPs. Detailed characterization of the TiO2 NPs verified the formation of anatase phase with spherical shape particles of 20–90 nm in diameter. In addition, they demonstrated strong light absorption at the wavelength of 400 nm. Due to the high levels of vitamins, minerals, amino acids, fatty acids, terpenoids, and flavonoids in the Aloe barbadensis mill or popularly known as aloe vera, this plant was used to biosynthesize TiO2 NPs along with TiCl4 (Rao et al. 2015; Rajkumari et al. 2019). In these reports, the TiO2 NPs were successfully precipitated without a

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Fig. 6.5 (a) SEM image, (b) EDX graph, (c) TEM image, and (d) size histogram of biosynthesized TiO2 NPs. (Reprinted from Roopan et al. 2012 with permission from Elsevier)

heating process or sodium hydroxide solution. After the calcination process, Rao et al. reported a high crystalline anatase phase of TiO2 NPs with the size range of 60–80 nm. In addition, Rajkumar et al. reported that without the calcination process, the synthesized TiO2 NPs were formed as a combination of anatase, brookite, and rutile phase. The SEM images verified that the nanoparticles were spherical in shape and polydisperse in the range of 20–50 nm. Roopan et al. (2012) disclosed astonishing discoveries of pure rutile phase of TiO2 NPs without the calcination process using Annona squamosa peel extract and titanium oxyhydrate (TiO(OH)2) as a starting material. Specifically, Annona squamosa contains phytochemicals of bornyl acetate, borneal, spathulenol, verbenone, and germacrene D. As a matter of fact, TiO2 NPs can be formed through the dehydration of TiO(OH)2. Therefore, the hydroxyl groups within these phytochemicals were believed to serve as a catalyst to dehydrate the TiO(OH)2 and later stabilized the formed TiO2 NPs. In the TEM images below, the rutile TiO2 NPs appeared as polydisperse particles averaging at 23 nm in size (Fig. 6.5).

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On account of the above discovery, Marimuthu et al. (2013) also employed TiO (OH)2 as the titanium precursor and aided with Calotropis gigantea flower extract to synthesize the TiO2 NPs. However, the XRD results suggested that the formed TiO2 NPs were impure with the presence of unreacted TiO(OH)2 and crystallized bioorganic molecules on the surface of the nanoparticles. This result is consistent with the report by Santhoshkumar and co-workers (2014). By utilizing the leaves of Psidium guajava, the TiO(OH)2 were dehydrated and eventually formed a mixture of crystalline anatase and rutile phase TiO2 NPs. A highly crystalline and pure TiO2 NPs were indeed difficult to synthesize without a calcination process. By using the energy-dispersive X-ray (EDX) analysis, the surface of the nanoparticles was found to be tainted with carbon, oxygen, magnesium, and chlorine, indicating the existence of biomolecules from the leaf extract. As a result, the nanoparticles were quite polydispersed and aggregated with an average size of 32.58 nm. In 2012, Velayutham et al. reported the bioreduction of bulk TiO2 to TiO2 NPs using Catharanthus roseus leaf extract based on the method by Sundrarajan and Gowri (2011) without the calcination process. The bioreduced TiO2 NPs were found to have an irregular shape and mainly were aggregated with a broad particle size distribution from 25 to 110 nm. Moreover, the XRD analysis obviously depicted the mixture of anatase and rutile phase form in the TiO2 NPs. A similar result was also obtained when the bulk TiO2 was reduced by Sesbania grandiflora leaf extract (Srinivasan et al. 2019). In spite of that, the synthesized TiO2 NPs were found to have a narrow particle size distribution of 43–56 nm with a mixture of triangular, square, and spherical shapes. Sesbania grandiflora leaf extract contains various phytochemicals with some essential functional groups, including amide, aromatic amine, secondary alcohol, phosphine, and sulfonates. The presence of these functional groups prevented the TiO2 NPs from aggregation, thus stabilized them. In another study, Al-shabib et al. (2020) utilized Withania somnifera root extract and TiO2 as starting materials to synthesize the TiO2 NPs. In the absence of the calcination process, the metastable phase of TiO2 of anatase and brookite was observed. Withania somnifera root extract contains plenty of active compounds, including withanolides, flavonoids, amino acids, alkaloids, sitoindosides, and phenolic compounds. These compounds reduced the bulk TiO2 to TiO2 NPs and stabilized them, as proved by the blue shift toward the maximum absorption wavelength from 440 to 395 nm. Zeta potential measurement of the TiO2 NPs proved the excellent stability of the nanoparticles with the value of 24 mV. The TEM results showed the spherical and square shapes of TiO2 NPs ranging in size of 50–90 nm. In the interest to address and mitigate the presence of impurities in the previously synthesized TiO2 NPs, Ajmal et al. (2019) included a calcination process in their procedure in order to produce pure TiO2 NPs. The study uses bulk TiO2 with three different fruits peel agro-waste extracts, specifically plum, peach, and kiwi scientifically named as Prunus domestica L., Prunus Persia L., and Actinidia deliciosa, respectively. From XRD analysis, all of the synthesized TiO2 NPs were successfully formed as pure anatase phases without any other phase. On the other hand, the SEM

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images showed unsatisfactory results as all of the formed TiO2 NPs were cylindrical in shape with a mean size of 200 nm.

6.3

Biosynthesis of TiO2 NPs Using Microorganisms

Microorganisms, including fungi, yeast, and bacteria, are known to secrete various substances consisting of numerous types of proteins, polysaccharides, organic acids, and enzymes (Costa et al. 2018). Therefore, efficient hydrolyzation and bioreduction processes of metal ions to metal nanoparticles are achievable by using microorganisms (Mughal et al. 2021). Moreover, these bioactive substances in microorganisms are able to coat the nanoparticles and improve their stability. Compared to plants, the utilization of microorganisms in the biosynthesis of nanoparticles is more practical for large-scale synthesis as it is more resistant to agitation and pressure (Salem et al. 2021). Moreover, by adjusting the culture parameters, such as temperature, pH, time, and quantity of biomass, researchers can manipulate the metabolism of microorganisms in order to produce nanoparticles with the desired physical properties. Microorganism-mediated nanoparticle synthesis can be achieved by using either intracellular or extracellular methods (Bahrulolum et al. 2021). In the intracellular method, the metal precursor is added to the culture then internalized within the microbial cell, resulting in the formation of nanoparticles. As a result, the extraction of the formed nanoparticles is slightly complicated since it needs chemical treatment, centrifugation, and filtration steps in order to disrupt the cell and release the nanoparticles. In contrast, the extracellular method involves the formation of nanoparticles via the entrapment of ions on the microbial cell surface. The metal ions are subsequently reduced to nanoparticles by the enzymes secreted by the microbial cell. This method is widely applied by researchers as it does not require an additional process to release the nanoparticles from the cells. Table 6.2 lists the different microorganisms and enzymes that have been utilized and studied for the formation of TiO2 NPs.

6.3.1

Biosynthesis of TiO2 NPs Using Fungi and Yeast

Compared to bacteria, fungi, and yeast are more beneficial as they possess a high tolerance to metals, are easy to handle, isolate, culture, and maintain without the need for sophisticated instrumentation. Using the extracellular method, Bansal et al. (2005) synthesized TiO2 NPs by making use of Fusarium oxysporum fungus and dipotassium titanium hexafluoride (K2TiF6) as starting materials in the biosynthesis of TiO2 NPs. Through this method, a high yield of TiO2 NPs was able to be produced with an estimate of 0.756 mg per gram of Fusarium oxysporum fungus biomass. The formed TiO2 NPs were in the brookite and rutile phase with a spherical shape ranging in size from 6 to 13 nm. In 2012, Rajakumar et al. explored the production

Bacillus cereus Bacillus amyloliquefaciens Bacillus licheniformis

Yeast Saccharomyces cerevisiae Saccharomyces cerevisiae Bacteria Streptomyces sp. HC1 Bacillus thuringiensis

Type of microorganism Fungi Trichoderma citrinoviride Fusarium oxysporum Aspergillus flavus 10–400 nm, mix of various shape 6–13 nm, spherical 62–74 nm, spherical

450  C/2 h

–

–

Precursor

TTIP

K2TiF6

TiO2

3–12 nm, spherical

43–67 nm, spherical and agglomerated 33–44 nm, irregular spherical – – 69–140 nm, mix of various shape 22–97 nm, spherical 16 nm, spherical

–

–

450  C/3 h 800  C/3 h

1200  C/3 h – 500  C/3 h

–

TiO2 TiOSO4

Ti(OH)4

TiO (OH)2 TiO (OH)2

8–20 nm, spherical

–

TiO (OH)2 TiCl3

Size, nm, and morphology

Calcined temperature,  C/ time, h

Table 6.2 Examples of microbes and microbial enzymes used for the biosynthesis of TiO2 NPs

Anatase

Anatase Rutile and anatase Rutile Anatase Anatase

Anatase

Rutile and anatase Anatase

Rutile and anatase

Rutile and anatase Brookite

Crystalline phase

(continued)

Sunkar et al. (2014) Khan and fulekar (2016) Suriyaraj and Selvakumar (2014)

Jalali et al. (2020)

Ağçeli et al. (2020)

Peiris et al. (2018)

Jha et al. (2009)

Rajakumar et al. (2012)

Bansal et al. (2005)

Arya et al. (2020)

References

6 Biosynthesis of TiO2 Nanoparticles and Their Application as Catalyst in. . . 139

Silicatein

Urease

Lysosome

Halomonas elongata Propionibacterium jensenii Microbial enzyme Alpha amylase

Aeromonas hydrophila Lactobacillus crispatus Lactobacillus sp. Lactobacillus sp.

Type of microorganism Bacillus mycoides

Table 6.2 (continued)

PHF-Ti TiBALDH TiBALDH TiBALDH

TiO (OH)2

TiO2 TiO (OH)2 TiO (OH)2 TiO (OH)2

TiO (OH)2 TiO2

Precursor TiO (OH)2

– –

2 nm, nanocrystalline 50 nm, nanocrystalline

15–80 nm, spherical

300  C/1 h

427  C/ 827  C/

105 nm, spherical

–

2.8 nm, nanocrystalline

20–50 nm, spherical and irregular shape 15–35 nm, spherical

– –

–

71 nm, spherical

–

2 mg/mL: 30–70 nm, hexagonal and spherical 15 mg/mL: nanorings with 55 and 33 nm of outer and inner diameter and 13 nm shell thickness 100 nm to 1 μm, sphere 10–50 nm, interconnected network

28–54 nm, spherical and uneven nanoparticles

–

–

Size, nm, and morphology 40–60 nm, spherical

Calcined temperature,  C/ time, h –

Anatase Rutile

Anatase

Amorphous Amorphous

Anatase

Anatase

– Rutile and anatase Anatase

Anatase

Rutile

Crystalline phase –

Sumerel et al. (2003)

Johnson et al. (2012)

Luckarift et al. (2006)

Ahmad et al. (2015)

Babitha and korrapati (2013)

Taran et al. (2018)

Kannan et al. (2015) Jha et al. (2009)

References ÓrdenesAenishanslins et al. (2014) Jayaseelan et al. (2013) Salman et al. (2014)

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of TiO2 NPs by employing Aspergillus flavus mycelium and bulk TiO2. The XRD patterns indicated that the resulting TiO2 NPs were a mix of anatase and rutile phases, while the TEM micrographs revealed a spherical and hexagonal shape of TiO2 NPs, that is polydisperse with a size distribution in the 60  5 nm range. The surface morphology of the TiO2 NPs suggested that they were aggregated but not in direct contact with each other, revealing the stability of the TiO2 NPs. This result also indicated that the reduction process of TiO2 to TiO2 NPs happened at the surface. The immobilized Aspergillus flavus mycelia matted together and were assumed to bind TiO2 NPs more efficiently than the bioactive substances secreted by them. In 2020, Arya et al. described the biosynthesis of TiO2 NPs with the use of Trichoderma citrinoviride and TTIP. This extracellular method was conducted in an acidic condition and finished with a calcination process. The TiO2 NPs mainly were a combination of irregular, spherical, pentagonal, triangular, and rod-shaped nanoparticles with a broad size distribution of 10–400 nm. Through this method, the biosynthesized TiO2 NPs also possess high stability and high particle homogeneity with hydrodynamic size distribution and polydispersity index values of 29.5 mV and 0.327, respectively. Nonetheless, its XRD pattern showed several peaks verifying the presence of two or more phases of the nanoparticles, mainly the rutile and anatase phases. Jha et al. (2009) reported the use of Saccharomyces cerevisiae or more commonly known as baker’s yeast, with TiO(OH)2 as the titanium precursor to synthesize TiO2 NPs. Baker’s yeast is known to secrete hydroxyl/methoxy derivatives of quinones, benzoquinones, toluquinones, and also membrane-bound oxidoreductase compounds. These compounds played important roles in the synthesis of the TiO2 NPs. The TEM micrograph showed nanoparticles that are almost spherical in shape with a mean size that comes out to be 12.57  0.2 nm. The XRD profiles of the TiO2 NPs suggested the presence of both the rutile and anatase phases. In another study, Peiris et al. (2018) presented an intracellular method to synthesize TiO2 NPs using baker’s yeast and titanium trichloride (TiCl3). Impressively, this method produced a high-purity crystalline anatase phase of TiO2 NPs without a trace of rutile phase indicated from the XRD pattern. Figure 6.6 shows the SEM and TEM images of the TiO2 NPs along with the histogram of the particle size distribution. The SEM and TEM images highlight the spherical-shaped TiO2 NPs with rough surfaces and narrow size distribution in the range of 3.6–12.0 nm. Interestingly, the micrographs also revealed a thin lamellar structure within the spherical TiO2 NPs.

6.3.2

Biosynthesis of TiO2 NPs Using Bacteria

Bacteria have been proven to be capable of mobilizing and immobilizing various metals and, in several instances, producing metal nanoparticles by reducing metal ions. Though little information exists about the formation mechanism of the metal nanoparticles by bacteria, the optimization of the process can result in the desired

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Fig. 6.6 (a, b) TEM micrograph, (c) histogram, and (d) SEM image of TiO2 NPs synthesized using baker’s yeast. (Reprinted from Peiris et al. 2018 with permission from Elsevier)

morphology and well-defined metal nanoparticles (Mukherjee and Nethi 2019). A Lactobacillus crispatus bacterium has been used by Salman et al. (2014) to synthesize TiO2 NPs with the help of TiO2 precursor without calcination process. The synthesized TiO2 NPs were found as anatase phase, spherical and oval in shape, and having an average size of 70.98 nm. In another study, an intracellular method by the same type of bacteria and precursor produced irregular and spherical-shaped nanoparticles with an average size range from 20 to 50 nm (Kannan et al. 2015). However, this method is time-consuming since the oxidation process to form the TiO2 NPs took nearly 96 h to complete. Using TiO(OH)2 as the titanium precursor, Jha et al. successfully synthesized TiO2 NPs by the extracellular method with a shorter synthesis time within 48 h. The XRD analysis showed a mixture of crystalline rutile and anatase phases of TiO2 NPs. The nanoparticle size distribution was confirmed to vary from 8 to 35 nm, determined through the TEM image. Like most species of bacteria, Lactobacillus sp. has a negative electro-kinetic potential thus readily attracts the titanium cation. In addition,

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the mildly acidic pH and low oxidation-reduction potential (rH2) aids in activating the membrane-bound oxidoreductase enzyme and facilitates the formation of TiO2 NPs. In another research work by Órdenes-Aenishanslins et al. in 2014, a saprophyte gram-positive and nonpathogenic soil bacilli of Bacillus mycoides was employed to synthesize TiO2 NPs with TiO(OH)2 as the starting material. A TEM analysis of the resulting TiO2 NPs revealed spherical-shaped particles with a 40–60 nm size range. Moreover, the micrograph also showed that the TiO2 NPs were coated with an organic substance suggested as an extracellular matrix that was produced by the bacterium. This organic substance is postulated to participate in the biotransformation and help in the stabilizing and capping of the TiO2 NPs. Through FTIR analysis, the organic substance was subsequently verified to be peptides or carbohydrates. In other studies, Sunkar et al. (2014) and Suriyaraj and Selvakumar (2014) have synthesized pure crystalline anatase phase TiO2 NPs without a calcination process. Sunkar et al. utilized Bacillus cereus and TiO2 as starting materials to biosynthesize the TiO2 NPs. The anatase phase of TiO2 NPs particles has sizes ranging from 69 to 140 nm with a mix of more than two shapes. Similar to the works by Jha et al., Sunkar et al. also believed the formation of TiO2 NPs resulted from the negative electro-kinetic potential of the bacterium that attracts the titanium cation and hydrolysis of the titanium cation to TiO2 NPs were mediated by the extracellular proteins of Bacillus cereus. The FTIR analysis of the nanoparticles supported this statement as the appearance of amide I and amide II functional groups from the protein backbone. Suriyaraj and Selvakumar (2014) synthesized the TiO2 NPs by means of extremophilic and radiation-resistant Bacillus licheniformis and titanium hydroxide (Ti(OH)4). The nanoparticles were formed at an optimum pH of 4.5 and reaction time of 24 h without any heating treatment, exhibiting spherical shapes with an average size of 16.3  5.5 nm. The XRD patterns showed that this room temperature method was able to produce a highly pure crystalline anatase phase of TiO2 NPs and was comparable to the calcined TiO2 NPs at 600  C. The high-resolution TEM micrographs of the Bacillus licheniformis before and after exposure to the titanium precursor revealed that the nanoparticles were formed on the cell wall surface. This result was in line with the findings from the selected area electron diffraction (SAED) analysis. Since microorganisms produce extracellular enzymes with oxidase functions, it is believed to convert the Ti(OH)4 to TiO2 NPs. Hence, the extracellular oxidase type of enzyme from the Bacillus licheniformis and acidic pH are indispensable for synthesizing the highly pure anatase phase of TiO2 NPs at room temperature. Khan and Fulekar (2016) investigated the effect of pH and time on the formation of TiO2 NPs with Bacillus amyloliquefaciens bacterium and TiOSO4 as the titanium precursor. From their result, the maximum production efficiency of the TiO2 NPs (0.28 g TiO2 NPs/0.025 M TiOSO4) was at pH 5 with a reaction time of 16 h. Via the calcination process, the TiO2 NPs were discovered to be in the anatase phase with a size distribution of between 22.11 and 97.28 nm. Similar to the previously mentioned studies, several peaks in the FTIR spectrum of the nanoparticles indicated the

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existence of amide I and amide II bands, likely attributed to polysaccharides proteins. Further analysis of the Bacillus amyloliquefaciens culture supernatants using native-Polyacrylamide Gel Electrophoresis (PAGE) found a high activity of α-amylase with a value of 43.37 IU. As a result, the α-amylase enzyme in the Bacillus amyloliquefaciens culture supernatants was deduced to be in charge of the reduction of TiO2 to TiO2 NPs and stabilizing them. Jalali et al. (2020) investigated the effect of different calcination temperatures toward the crystalline phase of the biosynthesized TiO2 NPs, in which the TiO2 NPs were synthesized using Bacillus thuringiensis and TiO(OH)2 as the titanium precursor. From the XRD analysis, amorphous TiO2 NPs were converted to anatase phase when calcined at 450  C. By increasing the temperature to 800  C, some of the anatase phase nanoparticles were converted to rutile phase, and the conversion was only complete at 1200  C. The SEM images of the amorphous TiO2 NPs showed a cluster of spherical shape nanoparticles. When calcined at 450  C, the TiO2 NPs were irregular, oval, and spherical, with a few aggregates having sizes between 33 and44 nm. Further increment of the calcination temperature increased the particle size of TiO2 NPs with immense grain growth. In 2013, Jayaseelan et al. successfully synthesized rutile phase TiO2 NPs without calcination process by utilizing TiO(OH)2 as the titanium precursor and bacteria Aeromonas hydrophila. In their method, the culture solution and TiO(OH)2 were mixed and incubated at 30  C for 24 h. After 24 h, the TiO2 NPs were formed as a white precipitate. This simple method yielded a highly pure crystalline rutile phase of TiO2 NPs without impurity peaks from the anatase and brookite phase in their XRD pattern. In addition, the FESEM images showed the presence of spherical and uneven shaped TiO2 NPs with a mean particle size of between 28 and 54 nm. Initial analysis of the nanoparticles by FTIR spectroscopy indicated that this biosynthesis process involved phenols, alcohols, lactones, primary amine, and aliphatic amines from the Aeromonas hydrophila. In order to specify the active substance responsible for the process, a detailed analysis was conducted using gas chromatography-mass spectrometry (GC-MS). The Aeromonas hydrophila broth extract contained a high percentage of glycyl-L-proline amino acids followed by 1-Leucyl-D-leucine, glycylL-glutamic acid, and uric acid with values of 74.41%, 15.74%, 6.90%, and 2.95%, respectively. Therefore, glycyl-L-proline was suggested to act as the capping agent, thus plays a pivotal role in the biosynthesis process of the TiO2 NPs. The biosynthesis mechanism started with the protonation of TiO(OH)2 when its lone pair of electrons on the oxygen atom picked up hydrogen ions from the glycyl-L-proline (Fig. 6.7). Then the protonated TiO(OH)2 was hydrolyzed to give titanium(III) ion (Ti3+). Finally, the deprotonated glycyl-L-proline pulled off hydrogen from Ti3+, initiating the formation of TiO2 NPs. In addition, the amino acids were also suggested to be in charge of the stabilization of the synthesized TiO2 NPs. Taran et al. (2018) successfully synthesized the pure anatase crystalline phase of TiO2 NPs at 37  C using the bacteria Halomonas elongata and TiO(OH)2 as the titanium precursor. In their method, the Halomonas elongata supernatant was added with TiO(OH)2 and incubated at 37  C, resulting in a white precipitate of TiO2 NPs. Though this method is relatively simple, it was time-consuming as the biosynthesis

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Fig. 6.7 The formation mechanism of TiO2 NPs involving glycyl-L-proline. (Reprinted from Jayaseelan et al. 2013 with permission from Elsevier)

process took 96 h of incubation time in order to form the TiO2 NPs. The FTIR spectra of the nanoparticles showed the presence of several functional groups from the Halomonas elongate supernatant, including alkyl halide, alkene, alkyne, and alcohol. In addition, the synthesized anatase phase of TiO2 NPs was observed to be spherical in shape with a mean size of 104.63  27.75 nm. A probiotic of Propionibacterium jensenii from coal fly ash effluent and TiO (OH)2 as precursor was successfully exploited by Babitha and Korrapati (2013) to synthesize pure crystalline anatase phase of TiO2 NPs. The FESEM micrograph depicted nanoparticles with a size distribution from 15 to 80 nm having smooth spherical shapes. In another work, Ağçeli et al. (2020) efficiently synthesized the crystalline anatase phase of TiO2 NPs with a simple and expedient method using the supernatant of Streptomyces sp. HCl and TiO(OH)2. Similar to the technique by Jha et al. (2009), the mix solution at pH 6.5 was subjected to a steam bath for 30 min at 60  C resulting in the deposition of coalescent white clusters of TiO2 NPs. According to the XRD analysis, the synthesized TiO2 NPs were formed as crystalline anatase phases without considerable impurities. The structural morphology of the nanoparticles was spherical, with an average size of 43–67 nm, as indicated by SEM analysis.

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Biosynthesis of TiO2 NPs Using Enzymes

In the biosynthesis of metal NPs by microorganisms, the secreted enzymes play a critical role as reducing agents, stabilizers, and inductors. Therefore, certain specific enzymes have been explored by researchers for the synthesis of TiO2 NPs. Sumerel et al. in 2003 synthesized TiO2 NPs using silicatein enzyme and titanium(IV) bis (ammonium lactate)-dihydroxide (Ti-BALDH) as starting materials. Silicatein is usually found in the glassy skeletal elements of marine sponges. This enzyme was proven to be involved in hydrolyzation and polycondensation processes by acting as the catalyst and structure-directing agents in the conversion of silicon alkoxides to silica and silsesquioxanes. The serine-histidine pair in the silicatein enzyme serves as an active site in the biosynthesis process. Hydrogen bonding between the hydroxyls group of serine and histidine seemingly increased the hydrophilicity of the serine oxygen. Due to this, the serine oxygen attacked the silicon atom to form a transitory Si-O bond and finally hydrolyzed the silicon alkoxide to silica nanoparticles. By using Ti-BALDH, the silicatein enzyme also successfully produced amorphous/ nanocrystalline TiO2 NPs evidenced by the XRD analysis, and its formation mechanism was suggested to be similar to the silica nanoparticles. Controlled heating monitored by XRD spectroscopy showed that the conversion of amorphous TiO2 NPs to crystalline anatase phase occurred at 427  C, and further conversion to rutile phase happened when the temperature increased to 827  C. In addition, the TEM micrograph revealed an increase in the particle size from an average of 2–50 nm when the anatase phase was converted to the rutile phase. Luckarift et al. (2006) synthesized TiO2 NPs using lysosome enzymes with two different titanium precursors, namely Ti-BALDH and potassium hexafluorotitanate (PHF-Ti). Lysozyme has the capability to hydrolyze a particular peptidoglycan linkage in the cell wall of gram-positive bacterial cells, thus widely known as an antibacterial enzyme. Both of the starting materials produced an amorphous nanosphere of TiO2 NPs with an interconnected network between them, as indicated from their XRD patterns and SEM and TEM micrographs. However, a very noticeable difference in their particle size diameters was observed with Ti-BALDH as the titanium precursor produced much smaller particles than when using PHF-Ti with size ranges between 10 to 50 nm and 100 nm to 1 mm, respectively. In addition, energy-dispersive X-ray spectroscopy (EDS) analysis of the nanoparticles synthesized from PHF-Ti indicated the presence of titanium, oxygen, potassium, and phosphorous elements in the nanoparticles. Owing to this, heating treatment of the TiO NPs by calcination process undesirably converted them to phase-pure potassium titanyl phosphate. Moreover, Johnson et al. in 2012 analyzed the effect of the incubation temperature and reaction time on the crystalline phase and surface morphology of TiO2 NPs. In the study, TiO2 NPs were synthesized using Ti-BALDH as titanium precursor and urease enzyme with several incubation temperatures of 25, 40, and 60  C for 12, 24, and 36 h, respectively. According to the XRD patterns, all synthesized nanoparticles formed crystalline anatase phase regardless of the reaction parameters. Likewise, the

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changes in incubation temperature and reaction time were also shown to be inconsequential to the surface morphology of the TiO2 NPs with crystallite size found to be uniform around 2.8  0.34 nm. As lactic acid has been known to have an affinity toward the surface of TiO2, the hydrolyzed lactato ligand from the Ti-BALDH molecule was adsorbed on the newly formed TiO2 NPs, thus hindering the diffusion of growth units onto their surfaces. In addition, the growths of TiO2 NPs were also limited due to the slow growth kinetics since the reaction temperature is lower than under hydrothermal conditions. In 2015, Ahmad et al. probed the effect of alpha-amylase enzyme concentration toward the crystalline phase and surface morphology of the synthesized TiO2 NPs with TiO(OH)2 as the precursor. The alpha-amylase enzyme extracted from Aspergillus oryzae fungus contains large amounts of amino acids in which 21 residues are proline and 12 of them are exposed. As mentioned by Jayaseelan et al. in 2013, proline has the ability to hydrolyze TiO(OH)2 to give Ti3+ and eventually produce a highly pure crystalline anatase phase of TiO2 NPs. According to the XRD patterns of the synthesized TiO2 NPs using concentrations of 2–5 mg/mL of the alpha-amylase enzyme, both produced a similar XRD pattern which indicated highly pure crystalline anatase phase of TiO2 NPs without any significant peaks of impurity. In contrast, the surface morphology of the nanoparticles was remarkably affected by the alphaamylase enzyme concentration. With 2 mg/mL of the alpha-amylase enzyme, the grain size of the nanoparticles was found to vary from 30 to 70 nm with an average size of 50 nm, and this value decreased to 25 nm when increasing the concentration of alpha-amylase enzyme to 15 mg/mL. Moreover, at 2 mg/mL of the alpha-amylase enzyme, the TiO2 NPs were in a hexagonal and spherical shape. Interestingly, increasing the concentration of alpha-amylase enzyme to 15 mg/mL produced nanoparticles with the addition of nanorings. Further analysis of the nanorings using high-resolution TEM as shown in Fig. 6.8 confirmed the formation of the nanoring with the average ring outer, inner diameters, and the shell thickness being determined as 55, 33, and 13 nm, respectively. Based on the findings from this study, the surface morphology of nanoparticles can be adjusted using different concentrations of enzymes to attain the intended shape and size of the nanoparticles.

6.4

Application of TiO2 NPs in Biodiesel Production

Currently, fossil fuels such as natural gas, coal, and petroleum are the main contributors to the world’s total energy output. Starting with the industrial revolution, their consumption has since escalated, and this growing demand for fossil fuels is increasingly challenging to meet as many experts have predicted and warned about the depletion of these sources of energy in the near future. Apart from that, overconsumption of fossil fuels has been evidenced to cause devastating effects on human and environmental sustainability. Fossil fuels are known to release high concentrations of nitrogen oxides, carbon monoxide, carbon dioxide, sulfur dioxides, and particulate matter that inevitably cause global warming and several health

148 Fig. 6.8 TEM and HR-TEM micrograph of TiO2 NPs synthesized using alpha-amylase enzyme with a concentration of (a) 2 mg/ mL and (b, c) 15 mg/mL. (Reprinted from Ahmad et al. 2015 with permission from Elsevier)

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problems to humans (Lelieveld et al. 2019). Due to these alarming detriments, alternative energy sources are urgently required to wean off the over-reliance on fossil fuels and improve human health and environmental sustainability. Biodiesel is one of the most promising substitutes for fossil fuels that have garnered the interest of many researchers as it permits the replacement of fossil fuels in engines without necessitating modifications. The application of biodiesel has favorable effects as a renewable source of energy, such as decreasing air and water pollution, reducing greenhouse gas emissions, cost efficiency, and providing economic stability to countries that heavily depend on imported fossil fuels (Jeswani et al. 2020). This type of fuel comprises long-chain fatty acids, specifically monoalkyl esters produced directly or indirectly from biomass like animal fats, vegetable oils, and waste oil. Vegetable oils can be sorted into two types: edible and non-edible. Non-edible vegetable oils include karanja, neem, rubber, mahua, polanga, jatropha, sea mango, while examples of edible vegetable oils include peanut, coconut, sunflower, rapeseed, canola, soybean, and palm oil. In biodiesel production, waste oil and non-edible oils are preferable as feedstock compared to pure edible oils since the utilization of the latter might compete with food crops and increase the world food prices. The quality and characteristics of the biodiesel produced from vegetable oil are heavily influenced by the type of feedstock used since the type of catalyst used, process parameters, and final product purity and yield are substantially dependent on its chemical composition (Zulqarnain et al. 2021). One of the primary compounds in vegetable oil that is responsible for producing biodiesel is triglycerides (TG), the composition of which ranges between 90 and 98%. Other than that, crude vegetable oil usually contains water, free fatty acids (FFA), phospholipids, and other impurities, which can devalue the quality parameters of the produced biodiesel. Chemically, biodiesel is a fatty acid alkyl ester (FAAE) in which the alkyl group is dependent on the type of alcohol used in the reaction. The two important reactions in producing biodiesel are transesterification and esterification; the former involves the conversion of TG to FAAE while the latter engages in producing FAAE from FFA (Fig. 6.9). Transesterification, also termed alcoholysis, is the chemical reaction process of changing TG to FAAE using alcohol with glycerol as a byproduct. The catalyst used in transesterification reactions can be either acidic or basic. In addition, diverse types of alcohol can be utilized in biodiesel production, the most frequently used of which being ethanol and methanol, producing fatty acid ethyl ester (FAEE) and fatty acid methyl ester (FAME), respectively. Both ethanol and methanol are usually favorable due to their cost efficiency and industrial availability. For feedstocks with high FFA value, a complex problem can arise and affect transesterification reaction during and after the reaction. Therefore, a feedstock with a low FFA value is usually favored; otherwise, a compulsory pretreatment process is required through an esterification reaction. In the esterification reaction of FFA, 1 mol of alcohol is required for every mol of FFA to produce 1 mol of biodiesel and water, using an acidic catalyst. At present, the biodiesel industry is dominated by homogeneous catalysts due to their simple utilization, high efficiency, rapid reaction, inexpensive catalyst, and a

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Fig. 6.9 Biodiesel production via transesterification and esterification reactions

lower catalyst concentration requirement than heterogeneous catalyst reactions (Jamil et al. 2018). Some acidic catalysts commonly used in biodiesel production are hydrochloric acid, sulfuric acid, hydrofluoric acid, p-toluene sulfonic acid, and phosphoric acid. However, these catalysts are quite unsuitable for commercial and large-scale purposes since they can result in the corrosion of equipment. Moreover, acidic catalysts possess a slower rate of reaction than basic catalysts; the basic catalyzed reaction can be up to 4000 times better than that of the acidic catalyzed reaction. Basic catalysts include both alkaline and alkoxide-based catalysts, including sodium hydroxide, potassium hydroxide, sodium methoxide, potassium methoxide, and carbonates. In spite of the high reaction rate, basic catalyzed reactions require high-purity feedstock with FFA required to be less than 0.5% and in anhydrous condition. Basically, the presence of FFA and water will undesirably favor saponification reactions that later increase the production cost due to the complicated washing and purification steps. All these problems from homogenous catalysts become the driving forces and motivation for researchers to develop heterogeneous catalysts for biodiesel production. Heterogeneous catalyst is categorized under green technology due to its difference in phase from the feedstock, which allows for the separation and purification of

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the biodiesel, in turn facilitating the recovery of the catalyst to be recycled or reused again (Sahu et al. 2018). Notably, saponification reaction could be avoided using a heterogeneous catalyst as this type of catalyst is relatively insensitive to high FFA and water contents. Consequently, cheaper-priced low-grade vegetable oil is possible to be used. With all these advantages, the utilization of heterogeneous catalysts can decrease the total production cost of biodiesel. In recent years, TiO2 NPs have attracted a lot of interest as a material for heterogeneous catalysts due to their nontoxicity, high surface area, high resistance to corrosion, mechanical strength, high thermal stability, and acid–base property that favors both oxidation and reduction reactions even at low temperature and pressure (Oi et al. 2016). In addition, TiO2 NPs possess unique electronic and optical properties with a bandgap of 3.2 eV under ultraviolet light. Other than being a catalyst itself, TiO2 NPs are also a great candidate for catalyst support material since their high mesoporous surface area can contribute to high dispersion and stabilization of other catalyst material in its pores (Bagheri et al. 2014). Many works of research have shown that TiO2 NPs improve the performance of the catalytic activity of various catalysts, including hydrodesulfurization, epoxidation of propylene, thermal catalytic decomposition, dehydrogenation, water gas shift, and oxidation of alkanes. The current chapter will shed some light on several TiO2 NP–based catalysts used in transesterification, esterification reactions, and also simultaneous reaction from bifunctional catalysts to produce biodiesel

6.4.1

Transesterification Reaction Using TiO2 NP Catalyst

Currently, biodiesel production is operated mainly by transesterification reactions using basic catalysts (Endalew et al. 2011). As an alternative, researchers have developed a solid base catalyst via the incorporation of alkali and alkaline earth metal into the heterogeneous solid support (Table 6.3). Alsharifi et al. (2017) investigated the lithium-based TiO2 catalyst (Li/TiO2) for biodiesel production through the transesterification of canola oil. The synthesized catalyst can convert about 98.5% of triglycerides to FAME with optimum methanol to oil ratio of 24:1 and 5 wt% of catalyst loading at 55  C for 3 h. With 30 wt% of lithium loading on TiO2 support, the pore volume, pore size, and surface area of the catalyst increased and led to the high catalytic activity. However, this catalyst subsequently underwent leaching of lithium ions, which decreased the catalyst efficiency after every reaction cycle. In another study, canola oil was chosen to undergo a transesterification reaction using potassium supported TiO2 catalyst (K/TiNT) with 20% potassium loading (Salinas et al. 2010). Based on the carbon dioxide temperature-programmed desorption study, the catalyst behavior is proved to be correlated with weak basic sites, whereby a low potassium loading catalyst generated more active catalysts than a high potassium loading catalyst. As a result, the 20% potassium loading on TiO2 completely converted the canola oil to biodiesel with an optimum reaction

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Table 6.3 Different types of catalysts for biodiesel production via transesterification reaction Catalyst type TiO2– 0.5C4H5KO6

Feedstock Linseed oil

K/TiHT

Canola oil

Li/TiO2

Canola oil

Li2TiO3

Soybean oil

Cs-Ca/TiO2SiO2

Refined vegetable oil

NaFeTiO4/ Fe2O3-FeTiO3

Jatropha curcas L. oil

Sr-Ti mixed oxides

Soybean oil

TiO2-MgO

Waste cooking oil

Cu-TiO2

Palm oil

TiO2-Cu2O

Thumba oil

TiO2-ZnO

Palm oil

TiO2/RGO

Waste cooking oil

W/Ti/SiO2

Waste cottonseed oil

Reaction condition 60  C/3 h Methanol/oil 6:1 Catalyst 6 wt% 55  C/5 h Methanol/oil 54:1 Catalyst 6 wt% 55  C/3 h Methanol/oil 24:1 Catalyst 5 wt% 65  C/2 h Methanol/oil 24:1 Catalyst 6 wt% 60  C/2 h Methanol/oil 12:1 Catalyst 2 wt% 65  C/1 h Methanol/oil 12.47:1 Catalyst 13.8 wt% 60  C/15 min Methanol/oil 15:1 Catalyst 1 wt% 150  C/6 h Methanol/oil 30:1 Catalyst 5 wt% 45  C/45 min Methanol/oil 20:1 Catalyst 3% TiO2, 2% Cu 80  C/1 h Methanol/oil 6:1 Catalyst 1.6 wt% 60  C/5 h Methanol/oil 6:1 Catalyst 200 mg 65  C/3 h Methanol/oil 12:1 Catalyst 1.5 wt% 65  C/4 h Methanol/oil 30:1 Catalyst 5 wt%

FAME yield, % 98.5%

References Ambat et al. (2018)

Complete conversion

Salinas et al. (2012)

98%

Alsharifi et al. (2017)

98.5%

Dai et al. (2017)

98%

Feyzi and Shahbazi (2015)

93.24%

Gutierrez-Lopez et al. (2021)

98%

Rashtizadeh and Farzaneh (2013)

79.9%

Wen et al. (2010)

90.93%

De and Boxi (2020)

>60%

Patil et al. (2020)

92%

Madhuvilakku and Piraman (2013)

98%

Borah et al. (2018)

98%

Kaur et al. (2018)

(continued)

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Table 6.3 (continued) Catalyst type CeO-TiO2

Feedstock Palm oil

Ti-HTC

Waste frying oil

Lipase-PDATiO2

Jatropha curcas oil

TiO2/SO4

Soybean oil

Reaction condition 55  C/2.5 h Methanol/oil 11.05:1 Catalyst 1 wt% 71.16  C/2.12 h Methanol/oil 9.96:1 Catalyst 1.03 wt% 37  C/30 h Methanol/oil 6:1 Catalyst 10% Water content 0.5% 120  C/1 h Methanol/oil/catalyst 120:20:1

FAME yield, % 70.5%

References Oyenubi et al. (2016)

90.78%

Yusuff et al. (2020)

92%

Zulfiqar et al. (2021)

40%

De Almeida et al. (2008)

temperature of 55  C for 5 h with methanol to oil ratio of 54:1 and 6 wt% of 20% K/TiNT catalyst. The transesterification of linseed oil by nanocatalyst of TiO2-potassium L-tartrate monobasic (TiO2–0.5C4H5KO6) was performed by Ambat et al. (2018). Linseed oil has a higher content of linolenic acid compared to rapeseed, soybean, and sunflower oil. Without the addition of potassium ions, the TiO2 catalyst was unable to perform the transesterification reaction of linseed oil, most likely owing to its weaker base strength. After the loading of potassium ions onto the TiO2 particle, the basicity of the catalyst was enhanced, thus increasing its catalytic activity. The TiO2– 0.5C4H5KO6 catalyst can produce up to 98.5% of FAME yield under an optimum temperature of 60  C for 3 h of reaction with 6:1 methanol to oil ratio and 6 wt% of the catalyst loading. In contrast, de Almeida et al. (2008) attempted the use of superacid sulfated TiO2base catalyst for producing biodiesel via the transesterification of soybean oil. From nitrogen adsorption-desorption isotherm analysis, the superacid sulfated TiO2 exhibited a decrease in its specific surface area, average pores diameter, and pore volume when increasing TiO2 to sulfuric acid ratio, with the highest values being from the 5:1 ratio. Although with superacid properties, this catalyst can only produce 40% of biodiesel under a high reaction temperature of 120  C for 1 h reaction time with approximately 120:20:1 of methanol/oil/catalyst ratio. Utilizing the same type of feedstock, Dai et al. (2017) have synthesized Li2TiO3 for biodiesel production. In comparison to calcium oxide (CaO) and sodium hydroxide (NaOH), Li2TiO3 exhibits the highest conversion; the order of the conversion from high to low being Li2TiO3 > CaO > NaOH. The catalytic activity for the transesterification reaction was discovered as not necessarily directly related to the surface area but at some point depends on the catalyst base strength and basicity value, i.e., increasing

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the base strength and basicity value of the catalyst subsequently causes an increase in the catalytic conversion. The transesterification of soybean oil by Li2TiO3 catalyst produced 98.5% of biodiesel and effortlessly recovered and reused for up to 10 cycles without any indication of a significant decrease in its biodiesel conversion. Rashtizadeh and Farzaneh (2013) also reported on the use of soybean oil in transesterification reaction to produce biodiesel with a remarkable conversion of almost 98% within only 15 min. They synthesized strontium-titanium mixed oxide nanocomposite with an optimum of 0.81 molar ratio of strontium to titanium mixed oxides. Since alkaline earth oxides are naturally covered with peroxide, carbonates, and hydroxide, owing to their strong capabilities to adsorb water, carbon dioxide, and oxygen, the catalyst needs a high-temperature pretreatment of more than 700  C to remove impurities and eventually increase the catalytic activity. This strontiumtitanium mixed oxide nanocomposite can be recycled up to 4 times with a slight decrease in its catalytic conversion to 89%. Around the various feedstock types in producing biodiesel, Jatropha curcas L. oil has drawn a lot of interest owing to, among others, its low FFA content with only 1.38% oleic acid and high lipid content of around 50%. Due to this, Gutierrez-Lopez et al. (2021) employed Jatropha curcas L. oil and a synthesized low-cost catalyst designated as NaFeTiO4/Fe2O3-FeTiO3 to produce biodiesel via transesterification reaction. This magnetic catalyst has a unique morphology with a large agglomeration of needle-like particles approximately in the range of 5 μm. In fact, the incorporation of titanium or iron oxides into the catalyst structure is able to increase the Lewis acid sites on the surface of the catalyst. This type of acid acts as an electron acceptor species in the catalytic reaction, thus facilitating the proton extraction from the methanol ion to yield methoxide ion on the brønsted sites attacking the TG. Within 1 h of transesterification reaction, this catalyst can produce approximately 93.24% of biodiesel. However, this catalyst experienced a severe decrease in its conversion with less than 50% conversion after the fifth cycle. More recently, Zulfiqar et al. (2021) designed and synthesized a novel nanobiocatalyst made of Lipase-polydopamine-titanium dioxide nanoparticles, termed Lipase-PDA-TiO2 NPs. This hybrid material was tested for the enzymatic transesterification of Jatropha curcas oil. The functionalization of lipase enzyme on TiO2 surface by polydopamine linker resulted in higher enzyme stability, with the temperature variations found to be less influential toward the enzyme activity compared to that of free enzymes. Moreover, an optimum of 0.5% water content is compulsory in transesterification reactions using this catalyst to form a water–oil interface since direct interaction between alcohol and enzyme can deactivate the enzyme molecules. The utilization of Jatropha curcas oil, Lipase-PDA-TiO2 NP catalyst can produce 92% biodiesel at 37  C for 30 h with 6:1 methanol to oil ratio and 10% catalyst loading along with 0.5% water content. Nowadays, palm oil is one of the most widely used vegetable oils, with utilities ranging from frying oil, in the production of foods, cosmetics to pharmaceuticals, and many others. The sheer number of practicalities makes palm oil a global product and has become the world’s cheapest vegetable oil, thus preferable to be applied as feedstock for producing biodiesel. Using palm oil as feedstock, Oyenubi et al. (2016)

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synthesized ceria doped titanium nanomaterials (CeO2-TiO2) as the catalyst for the transesterification reaction. This catalyst utilized the crystalline anatase phase of TiO2 NPs with a mesoporous structure having a pore diameter in the range of 2–50 nm. These excellent properties of TiO2 NPs, along with cerium oxide, have increased the catalyst stability and catalytic activity with biodiesel yields of 70.5%. Although the catalytic conversion was a bit low compared to other catalysts, its reusability test showed an excellent result with only a 3% reduction in yield after five cycles. In another study, Madhuvilakku and Piraman (2013) synthesized binary metal oxides of titanium dioxide-zinc oxide (TiO2-ZnO) for the transesterification reaction of palm oil. Generally, divalent metal oxide (including ZnO) catalysts possess an extensive amount of covalent characters, and the combination of this transition metal oxide with TiO2 creates binary metal oxides with enhanced surface acidity that can facilitate the transesterification reaction. The XRD analysis indicated that the binary metal oxides did not form any specific structure and are present as separated oxides with the TiO2 and ZnO formed as tetragonal and hexagonal crystallites, respectively. A 92% yield of FAME was obtained using 200 mg of TiO2-ZnO under optimum reaction conditions at 60  C for 5 h with a 6:1 methanol to oil ratio. In a recent study, De and Boxi (2020) successfully synthesized copper impregnated TiO2 (Cu-TiO2) catalyst with an excellent catalytic activity that was able to produce 90.93% of biodiesel from palm oil within 45 min at a reaction temperature of 45  C and 20:1 methanol to oil ratio. By impregnation of only 2% of Cu, the bandgap energy of TiO2 was changed and impacted the catalytic activity of the catalyst. However, further increment in the Cu concentration within the catalyst subsequently decreased biodiesel production due to the loss of stoichiometry and the lattice mismatching that afterwards prompted the structural instability and quantum tunneling in the catalyst. Feyzi and Shahbazi in 2015 utilized refined vegetable oil as a feedstock for biodiesel using a nanocatalyst of cesium-calcium/titanium dioxidesilicon dioxide (Cs-Ca/TiO2-SiO2). The addition of calcium to the TiO2-SiO2 catalyst provided abundant basic sites on the catalyst surface with approximately 1.6  1020 carbon dioxide atoms that were desorbed per gram of the catalyst, and this high basic strength enabled a high rate of conversion of biodiesel. Moreover, the addition of another alkali metal, Cs, into the catalyst was also observed to increase the catalytic performance. Approximately 98% of biodiesel can be produced by this catalyst and maintained its conversion yield even after four cycles. Though some edible oils are widely available at a low price, their increasing consumption for biodiesel production might inadvertently compete with food crops and increase their price. Therefore, researchers are forced to find another source of feedstock; hence, non-edible oil and waste oil are the best choice. Thumba oil or scientifically known as Citrullus colocynthis is an underused non-edible oil. In 2020, Patil et al. utilized this oil for the production of biodiesel using synthesized titanium dioxide-copper(I) oxide (TiO2-Cu2O) nanoparticles composite under hydrodynamic cavitation method. The effect of the parameter showed that 60% of biodiesel could be produced under optimum conditions of 80  C for 1 h with a 6:1 methanol to oil ratio and 1.6 wt% of catalyst loading. Moreover, Kaur et al. (2018) successfully

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Fig. 6.10 Transesterification reaction on the surface of TiO2/RGO nanocomposite catalyst. (Reprinted from Borah et al. 2018 with permission from Elsevier)

synthesized tungsten-supported titanosilicate (W/Ti/SiO2) nanoflowers for transesterification of waste cottonseed oil. This heterogeneous catalyst can yield 98% biodiesel and be recycled up to four times without substantial loss in its activity. In addition, this synthesized catalyst can also withstand up to 4% of FFA without any loss or decrease in its biodiesel production. This astonishing property was contributed by the tungsten metal that increased the total acidity of the catalyst by more than double from 2.46 to 5.44 mmol/g. Waste cooking oil contains several FFA, including linolenic acid, stearic acid, oleic acid, palmitic acid, and linoleic acid, in considerably high percentages. A report by Wen et al. (2010) indicated that titanium dioxide-magnesium oxide (TiO2-MgO) mixed oxide catalyst successfully produced 92.3% of biodiesel using waste cooking oil. At a calcination temperature higher than 600  C, the Mg ions were substituted by Ti ions in the magnesia lattice. Conveniently, due to the higher valence electron of Ti ion compared to Mg ion, this substitution process created vacancies that further induced the structural defect within the catalyst and increased its catalytic performance. Without titanium substitution, the MgO catalyst suffered rigorous leaching and generated a high amount of methoxide magnesium that can be considered a homogeneous catalyst. With the substitution of Ti ion, the TiO2-MgO catalyst was able to perform the catalytic reaction up to four times, with the biodiesel yields maintained higher than 80%. Borah et al. (2018) delineated the transesterification process of waste cooking oil using synthesized titanium dioxide-graphene (TiO2/RGO) nanocomposite. In TiO2/ RGO catalyst, the strong chemical bond between TiO2 NPs and graphene distorted the TiO2 NP crystal structure and beneficially created structural defects. Owing to the strong chemical bond between them, the TiO2 NPs were observed to be uniformly dispersed and coupled on the graphene sheets. The mechanism for the transesterification reaction of TG over TiO2/RGO nanocomposites started with attaching the oxygens atoms from TG to the acidic Ti atoms (Fig. 6.10). This behavior enhanced the electrophilicity of the TG carbonyl sites hence facilitated the methanol to attack the carbonyl sites and created tetrahedral intermediates. Then, the TG is cleaved to produce 1 mol FAME and diglycerides. Afterwards, repeated

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steps with further addition of methanol produced another 2 mol of FAME and 1 mol of glycerol. With an optimum reaction condition at 65  C for 3 h with a 12:1 methanol to oil ratio and 1.5% TiO2/RGO nanocomposite loading, approximately 98% of biodiesel can be produced. Yusuff et al. (2020) exploited abandoned naturally occurring material of termite hill to synthesize titanium dioxide-termite hill composite for the transesterification of waste frying oil. Determined by XRD analysis, this aluminosilicate material possesses several crystalline structures, including quartz, cristobalite, tridymite, and kaolinite. Then, the addition of TiO2 on the surface of the termite hill showed a predominantly anatase phase pattern which indicated its successful dispersion. The basicity analysis of this titania-termite hill catalyst proved the presence of basic metal oxides with basic strength calculated to be 105.3  0.07 μmol/g. Using waste frying oil as feedstock, this catalyst can produce 90.78% of FAME. Nevertheless, this catalyst experienced a remarkable decrease in its catalytic conversion as only 51.9% biodiesel was produced after the third cycle. This decline in catalytic conversion was suggested to be caused by the pore plugging on the catalyst by oil.

6.4.2

Esterification Reaction Using TiO2 NP Catalyst

Inedible, cheaper, and common feedstocks of biodiesel from waste oils usually contain high FFA content of more than 5%. Therefore, the pretreatment process of waste oils by esterification reaction is sometimes mandatory to enhance the efficiency and overall quality of the biodiesel production from the transesterification reaction. Table 6.4 summarizes the TiO NP-based catalyst commonly used in esterification reactions in obtaining biodiesel. Berrones-Hernández et al., in 2019, synthesized sulfated titanium dioxide (STi) for the esterification of waste cooking oil with high oleic acid content. The TiO2 remained in the anatase phase even after impregnation with sulfuric acid. This observation proved the high dispersion of sulfate ions on the TiO2 surface as well as the stability and efficiency of the catalyst. The calcined STi produced a 71.1% yield of biodiesel then reduced greatly to 6.4% yield in the fourth cycle. Moreover, after each reaction cycle, the catalyst surface was coated with a gummy and oily layer; hence, extra purification steps are necessary. In 2020, Al-Qaysi et al. synthesized sulfated silica-titanium dioxide catalyst for esterification of oleic acid. The XRD patterns showed that this binary metal oxide catalyst had a smaller lattice constant and interplanar spacing compared to the pure anatase TiO2, suggesting the diffusion of Si ions into titanium dioxide lattice. Moreover, the absorption of sulfate active groups on the SiO2-TiO2 particles or those situated between the catalyst particles formed a highly stable sulfate-titania bond which further increased the catalyst acid strength to 1.386 mmol/g. This increment in catalyst acid strength enhanced the catalytic conversion of oleic acid to FAME from 69.3% to 94.1%. This simple catalyst has a great chance to be applied in industrial biodiesel production, given that its catalytic conversion was only reduced around 10% after the fifth cycle. The esterification of oleic acid using

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Table 6.4 List of catalysts for biodiesel production through the esterification reaction Catalyst type SO4/SiO2TiO2

Feedstock Oleic acid

SO4-TiO2

Stearic acid

STi-NC

Oleic acid

Ti-SBA-12

Oleic acid

Fe3O4@ ZIF-8/TiO2

Waste frying oil

Reaction condition 120  C/4 h Methanol/oil 9:1 Catalyst 3 wt% 80  C/1.5 h Methanol/oil 1:1 Catalyst 0.011 mol% 55  C/6 h Methanol/oil 6:1 Catalyst 5% 170  C/8 h Methanol/oil 10: 1 Catalyst 3 wt% 50  C/1 h 2.5 min Methanol/oil 30: 1 Catalyst 6 wt%

FAME yield, % 94.1%

References Al-Qaysi et al. (2020)

98%

Hosseini-Sarvari and Sodagar (2013)

86%

Berrones-Hernández et al. (2019)

90%

Kotwal et al. (2013)

93%

Sabzevar et al. (2021)

sulfated SiO2-TiO2 catalyst begins with the adsorption of lone pair electrons from the carbonyl group of oleic acid by the proton from the sulfate group. This step formed carbocation on the oleic acid, which was then transformed to a tetrahedral intermediate structure after the carbocation was attacked by the lone pair electrons of oxygen from methanol molecules. In the next step, the lone pair of electrons inside the tetrahedral intermediate rearranged and discarded one water molecule. Finally, once methyl ester was formed and the catalyst was revived to its original structure, the esterification reaction was repeated with other oleic molecules. In another study, Kotwal et al. (2013) synthesized mesoporous titanosilicates with a three-dimensional structure based on SBA-12 and SBA-16 for the production of biodiesel from oleic acid through an esterification reaction. This study revealed that Ti-SBA-12 have higher catalytic efficiency compared to Ti-SBA-16 as its high acidity values led to high oleic acid conversion even with a similar silica/titanium molar ratio. Other than that, 29Si magic angle spin nuclear magnetic resonance (29Si MAS NMR) signals revealed that Ti-SBA-16 was less hydrophilic than Ti-SBA-12. As a result, when methanol was changed to hydrophilic glycerol, the Ti-SBA-16 produced a higher biodiesel percentage than Ti-SBA-12. The hydrophilic glycerol strongly adsorbed on the hydrophilic surface of Ti-SBA-12 led to the lower conversion of oleic acid. With methanol, Ti-SBA-12 is able to convert 90% oleic acid to biodiesel at a reaction temperature of 170  C for 8 h with a 10:1 methanol to oil ratio and 3 wt% of catalyst loading. Other than that, another three-dimensional-based nanocatalyst was also synthesized for the conversion of oleic acid to biodiesel, namely TiO2-decorated magnetic

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zeolitic imidazolate framework-8 (Fe3O4@ZIF-8/TiO2) nanocomposite (Sabzevar et al. 2021). In Fe3O4@ZIF-8/TiO2 nanocomposite, the rutile phase TiO2 NPs were distributed on the surface of Fe3O4@ZIF-8, causing an increase in the catalyst acid strength to 0.74 mmol/g. With Fe3O4@ZIF-8/TiO2 nanocomposite, the carbonyl group in oleic acid was prone to nucleophilic attack due to its more positive character, thus yielding a higher percentage of biodiesel compared to the magnetic-ZIF-8 and pristine TiO2. Utilizing response surface methodology (RSM) and experimental method, 93% of biodiesel was obtained at a reaction temperature of 50  C for 1 h and 2.5 min with 6 wt% of Fe3O4@ZIF-8/TiO2 nanocomposite in 30:1 methanol to oil ratio. Surprisingly, this catalyst possesses high stability and continuous catalytic activity even after five cycles with only a 3% drop in its yield. Other than oleic acid, there are many other naturally occurring FFA in vegetable oils, including lauric acid, caprylic acid, stearic acid, and capric acid. Hosseinisarvari and Sodagar in 2013 reported the esterification of these FFA using synthesized sulfated-titanium dioxide (SO4-TiO2) nanocatalyst for biodiesel production. The esterification of lauric acid, caprylic acid, stearic acid, and capric acid with butanol using SO4-TiO2 nanocatalyst yielded a high percentage of biodiesel, about 98% for all of the FFA under an optimum reaction condition of 80  C, 1.5 h, 1:1 methanol to oil ratio and 0.011 mol% catalyst. Moreover, the reusability test of this catalyst using benzoic acid and butanol proved that this catalyst was able to work for up to five cycles while maintaining its catalytic activity. This high-catalytic activity and excellent reusability in conjunction with low toxicity and non-carcinogenic properties of the SO4-TiO2 nanocatalyst have attracted great interest in the potential of this catalyst as a green catalyst.

6.4.3

Simultaneous Transesterification and Esterification Reaction Using TiO2 NP Catalyst

As mentioned before, pretreatment step by step by esterification reaction for low-quality feedstocks with high FFA value is crucial to avoid any complicated problem when performing transesterification reaction for biodiesel production. However, these two-step reactions are somehow troublesome by way of time consuming and cost-ineffective. Therefore, many researchers have designed and developed bifunctional catalysts for biodiesel production that can perform both esterification and transesterification reactions simultaneously, as summarized in Table 6.5. Chen et al. (2014) synthesized and explored the application of Ti-SBA-15 for simultaneous transesterification and esterification of crude jatropha oil in the production of high-quality biodiesel fuel (BDF) under autogenous pressure at 200  C. Following the restricted European standard (EN 14214:2009), BDF should contain a minimum of 96.5 wt% of FAME with allowed impurities of total glycerol that needs to be less than 0.25 wt%. Using Ti-SBA-15 catalyst with methanol to oil ratio equal or higher than 108, BDF was produced with

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Table 6.5 Summary of reaction conditions for biodiesel production by bifunctional catalysts Catalyst type SO4/Fe-Al-TiO2

Feedstock Waste cooking oil

S-TSC

Waste soybean oil

Sulfated Ti-SBA-15

Unrefined canola oil

[Ti(SO4)O]

Used cooking oil

Ti-SBA-15

Crude jatropa oil

TiO2/PrSO3H

Used cooking oil

Reaction condition 90  C/2.5 h Methanol/oil 10:1 Catalyst 3 wt% 120  C/3 h Methanol/oil 20:1 Catalyst 10 wt % 200  C/4 h Methanol/oil 15:1 Catalyst 1 wt% 75  C/3 h Methanol/oil 9: 1 Catalyst 1.5 wt % 200  C/3 h Methanol/oil 27:1 Catalyst 15 wt % 60  C/9 h Methanol/oil 15:1 Catalyst 4.5 wt %

FAME yield, % 96%

References Gardy et al. (2018)

77%

Shao et al. (2013)

91 wt%

Sharma et al. (2014)

97.1%

Gardy et al. (2016)

90 wt%

Chen et al. (2014)

98.3%

Gardy et al. (2017)

97.8–98.6% of FAME and total glycerol of around 0.063–0.16 wt%. Outstandingly, with 2 and 30 wt% of water and FFA levels, respectively, the Ti-SBA-15 was still able to produce BDF according to the European standard. This high-tolerance property of Ti-SBA-15 catalyst was suggested to be attributed to its well-ordered mesostructured that possesses a hydrophilic surface that is suitable for water adsorption. Figure 6.11 illustrates the schematic mechanism in the production of BDF using Ti-SBA-15 catalyst. At the same time, Sharma et al. (2014) functionalized Ti-SBA-15 with a sulfated group and utilized it as a bifunctional catalyst for the conversion of unrefined canola oil to biodiesel. The catalyst characterization analysis showed that the addition of sulfate group to Ti-SBA-15 in no way changed the catalyst morphology, instead of transforming the titanium ion state from octahedral coordination to mixed tetrahedral–octahedral coordination. In addition, the free sulfate linkage on the surface of Ti-SBA-15 generated Brønsted acid sites, and unsaturation in the titanium

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Fig. 6.11 Reaction mechanism for the formation of BDF using Ti-SBA-15 catalyst. (Reprinted from Chen et al. 2014 with permission from Elsevier)

ion coordination provided the Lewis acid sites, which in turn yield catalyst with a stronger acidic strength in comparison to Ti-SBA-15 without a sulfate group. Under optimum reaction conditions, sulfated Ti-SBA-15 produced 91 wt% of biodiesel rather than only 18 wt% of the same product with non-sulfated Ti-SBA-15. Significantly, sulfated Ti-SBA-15 yielded almost the same amount of biodiesel even after three cycles confirmed its high stability and ability to retain its sulfate species without leaching. Although both the previously mentioned bifunctional catalysts are able to produce high amounts of biodiesel, the real challenge in this process is the reliance on the utilization of feedstock with high FFA contents, especially waste vegetable oils. Therefore, Shao et al. (2013) synthesized a bifunctional catalyst of sulfated mesoporous titania-silica (S-TSC) with waste soybean oil as a feedstock for biodiesel production. A comparative study using S-TSC catalyst showed that the feedstock type is crucial in biodiesel production as S-TSC catalyst successfully produced about 93.7% of biodiesel with oleic acid compared to only 77% of biodiesel from the utilization of waste soybean oil. Unlike sulfated Ti-SBA-15, S-TSC catalyst greatly suffered from leaching of sulfate species resulting in a massive drop in its waste

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soybean oil conversion to only 21% and 10% for the second and third run, respectively. In 2016, Gardy et al. successfully synthesized a novel sulfated titanium dioxide nanocatalyst [Ti(SO4)O] to produce biodiesel from used cooking oil with amazing reusability of up to eight times. Although used cooking oil has a high FFA content, the [Ti(SO4)O] nanocatalyst demonstrated a high conversion of around 97.1%, then steadily produced the biodiesel, up to eight cycles with only 3% decrease. This high stability and catalytic activity of [Ti(SO4)O] nanocatalyst was a result of the formation of polydentate sulfate species within the crystalline of TiO2 NPs that initiated the high tolerance toward high FFA content and increased the catalyst acid strength. Further study by Gardy et al. (2017) in the following year replaced the sulfate species on the surface of TiO2 NPs with a propyl sulfonic acid group to form novel TiO2/PrSO3H nanocatalyst. Although this catalyst possesses a surface morphology that is quite similar to the [Ti(SO4)O] nanocatalyst, it is only able to withstand four cycles with high conversion in the range of 98.3–94.16% before being drastically reduced to approximately 20.64% in the sixth run. Another excellent bifunctional catalyst by Gardy et al. (2018), namely SO4/FeAl-TiO2 solid acid catalyst with a superparamagnetic property, was reported for the production of biodiesel from waste cooking oil. This catalyst was observed to possess high FFA tolerance, as much as 20 wt%, and independent of FFA content as the biodiesel production remained at 95% throughout the whole process. The recycle study proved that the magnetic SO4/Fe-Al-TiO2 catalyst had excellent chemical and physical stability with an almost constant conversion of waste cooking oil over 10 recycles. This result confirmed that the SO4/Fe-Al-TiO2 catalyst was not affected by the sub-product of water from the esterification reaction that could deactivate or poison the active sites on the catalyst surface. This unusual behavior of SO4/Fe-Al-TiO2 catalyst was attributed to the aluminum and iron molecules that increased the strength of sulfate binding on the catalyst surface, thus inhibiting its leaching. The existence of paramagnetic properties in the catalyst shortens the separation time as its external magnetic field easily separates the catalyst from the reaction mixture.

6.5

Conclusion

This chapter highlights the biosynthesis of TiO2 NPs using various plants and microorganisms and their application in heterogeneous catalysis as a catalyst and catalyst support in biodiesel production. There has been proof that the design and optimization of the biosynthesis method are instrumental in achieving the desired properties such as morphology, size, shape, and crystalline structure. Compared to conventional methods, the biosynthesis method has significant advantages: less chemical usage, low cost, less toxic contaminants, and eco-friendly. In order to promote biodiesel production, there has been a substantial increase in the use of NPs, especially TiO2 NPs, as a catalyst and catalyst support. Optimization of the reaction

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parameters, including the percentage of TiO2 NP-based catalyst, temperature, time, and alcohol to oil ratio, successfully produces high biodiesel conversion with good recyclability. Nevertheless, biosynthesis of TiO2 NPs using microorganisms has a long way to go since, at present, only a few microorganisms are utilized for this purpose. On the other hand, some of the used plants are native plants that can only be found at specific places; hence, a lot of hard work is unavoidable in this area. In order to achieve the sustainable biosynthesis of TiO2 NPs, the focus should be on the discovery of suitable local and common plants alternative, which in turn can lead to a decrease in the development cost and further open an option to a large-scale biosynthesis of TiO2 NPs. In conclusion, the biosynthesis of TiO2 NPs using plants and microorganisms and its application for biodiesel production in this chapter provides some outstanding findings and insights for researchers that should offer valuable ideas to them in further developing and extending this field. Acknowledgments The authors would like to acknowledge funding from the Ministry of Education (MOE), Malaysia, through Fundamental Research Grant Scheme (FRGS/1/2021/STG04/UTM/ 02/2) and Universitas Negeri Malang for the matching grant (R.J130000.7354.4B686 PY/2021/ 00490).

References Ağçeli GK, Hammachi H, Kodal SP, Cihangir N, Aksu Z (2020) A novel approach to synthesize TiO2 nanoparticles: biosynthesis by using Streptomyces sp. HC1. J Inorg Organomet Polym Mater 30(8):3221–3229 Ahmad R, Mohsin M, Ahmad T, Sardar M (2015) Alpha amylase assisted synthesis of TiO2 nanoparticles: structural characterization and application as antibacterial agents. J Hazard Mater 283:171–177 Ahmad W, Jaiswal KK, Soni S (2020) Green synthesis of titanium dioxide (TiO2) nanoparticles by using Mentha arvensis leaves extract and its antimicrobial properties. Inorg Nanomet Chem 50(10):1032–1038 Ajmal N, Saraswat K, Bakht MA, Riadi Y, Ahsan MJ, Noushad M (2019) Cost-effective and eco-friendly synthesis of titanium dioxide (TiO2) nanoparticles using fruit’s peel agro-waste extracts: characterization, in vitro antibacterial, antioxidant activities. Green Chem Lett Rev 12(3):244–254 de Almeida RM, Noda LK, Goncalves NS, Meneghetti SMP, Meneghetti MR (2008) Transesterification reaction of vegetable oils, using superacid sulfated TiO2–base catalysts. Appl Catal A Gen 347(3):100–105 Al-Qaysi K, Nayebzadeh H, Saghatoleslami N (2020) Comprehensive study on the effect of preparation conditions on the activity of sulfated silica–titania for green biofuel production. J Inorg Organomet Polym Mater 30(10):3999–4013 Al-shabib NA, Husain FM, Qais FA, Ahmad N, Khan A, Alyousef AA, Arshad M, Noor S, Khan JM, Alam P, Albalawi TH, Shahzad SA (2020) Phyto-mediated synthesis of porous titanium dioxide nanoparticles from withania somnifera root extract: broad-spectrum attenuation of biofilm and cytotoxic properties against HepG2 cell lines. Front Microbiol 11:1–13 Alsharifi M, Znad H, Hena S, Ang M (2017) Biodiesel production from canola oil using novel Li/TiO2 as a heterogeneous catalyst prepared via impregnation method. Renew Energy 114: 1077–1089

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Chapter 7

Phyco-Nanotechnology: An Emerging Nanomaterial Synthesis Method and Its Applicability in Biofuel Production Gyanendra Tripathi, Aqsa Jamal, Tanya Jamal, Maryam Faiyaz, and Alvina Farooqui

Abstract The increased industrialization and globalization require high energy demand. In a way to fulfill this increased demand, many natural energy resources are being exploited resulted in its extension and global warming. This has shifted the attention of scientific communities toward sustainable and renewable sources of energy. Nowadays, nanotechnologies are the most promising area of research to solve this issue. It plays an active role by increasing the energy storage efficiency, conversion process, and improved design of device and performance. Its major application is seen in fuel cells, advanced batteries, biofuel, geothermal, solar cells, etc. Various physicochemical approaches are employed in manufacturing nano-sized particles, but these methods have certain limitations which include high energy input, high production cost, and synthesis of toxic by-products. To overcome these drawbacks, various green synthesis methods have been introduced. Among all the microorganisms used in this method, the alga-based nanoparticle biosynthesis is a blooming concept in recent years and has progressed as a separate branch known as phyco-nanotechnology. The characteristics of algae to grow rapidly, reduce ions, and accumulate metals make it suitable for nanoparticle synthesis. Various algal strains have been employed to perform green synthesis of nanoparticles, these may include micro- and macro-green algae, and cyanobacteria. These organisms comprise bioactive compounds and secondary metabolites that perform the function of capping, stabilizing, and reducing agents for manufacturing nanoparticles. These particles are a more efficient and reliable source of producing renewable energy which is much more affordable and cleaner. The chapter covered a few from a myriad of renewable energies which can be produced using nanoparticles. It unmasked the attributes of nanoparticles synthesized from algae and how they can revitalize the process of energy production in the field of environmental science.

G. Tripathi · A. Jamal · T. Jamal · M. Faiyaz · A. Farooqui (*) Department of Bioengineering, Integral University, Lucknow, Uttar Pradesh, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Srivastava et al. (eds.), Green Nano Solution for Bioenergy Production Enhancement, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-16-9356-4_7

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Keywords Nanotechnology · Renewable energies · Phyco-nanotechnology · Green synthesis · Algal biofuel

7.1

Introduction

Nanotechnology is a novel and developing technology having numerous applications. It includes the composition of nanoparticles which have one of the dimensions in the range of 1–100 nm. Nanoparticles are extensively utilized in various sectors including cosmetic industries, electrical appliances, hospital and biomedical fields, and biotechnology (LewisOscar et al. 2016). In the recent era, various physicochemical approaches are being used for manufacturing nanoparticles (Hussain et al. 2016). But these methods have certain limitations which include high-energy input, high production cost, and synthesis of toxic by-products (Benelli 2019). To overcome these drawbacks, various biological methods have been introduced. This biological process of production of nanoparticles which involves the use of animals, plants, and microorganisms is known as green synthesis for nanoparticle production. Biologically produced nanoparticles are found to be environment friendly, non-toxic, and less expensive (Hussain et al. 2016). Green synthesis is a developing branch of nanotechnology. Among all the microorganisms used in this method, the alga-based biosynthesis of nanoparticles is an originating trend in recent years and has progressed as a separate branch known as phyco-nanotechnology (Sathishkumar et al. 2019). Alga is an organism that is capable of producing oxygen on its own hence is referred to as photoautotrophic in nature. These are the most important group of ecologically and economically photosynthetic organisms (Thajuddin and Subramanian 1992; LewisOscar et al. 2014). Algae are eukaryotic, unicellular, or multicellular, and most aquatic organisms. The characteristics of algae to grow rapidly, reduce ions, and accumulate metals make it suitable for nanoparticle synthesis. Moreover, the average growth of its biomass is ten times faster than other plants (Farooqui et al. 2021; Arya and Chundawat 2020). The ability of algal biomass to grow rapidly at high rates, i.e., having a high doubling time (1–6 days usually), produces 10–20 times high oil than any other oil-producing plant (Borowitzka and Moheimani 2013). The photosynthetic rate of algae is also very high, i.e., 6.9  104 cells/mL/h. The solar energy conversion of alga biomass is around 4.5% (Bamba et al. 2014; Devi et al. 2012). Algal harvesting includes three basic steps: (1) recovery of biomass, (2) dewatering, and (3) drying. Techniques such as flocculation, centrifugation, gravity sedimentation, screening, filtration, and electrophoresis can also be used for harvesting. Utilization of these techniques mainly depends on the algae’s characteristics, like density, size, and the concentration of the targeted product (Jigar et al. 2014; Veillette et al. 2012). Triglycerides cover a major part of the oil that has been used for biofuel production. The conversion of algal to biofuel is mainly done by three techniques, including pyrolysis, transesterification, and micro-emulsification (Demirbas 2006; Pavan et al.

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2017; Echiegu 2016). Transesterification is the most commercially utilized method for the production of biofuel (Musa 2016). Various algal strains have been used for the green synthesis of nanoparticles; these may include micro- and macro-green algae, brown algae, red algae, or bluegreen algae. These organisms comprise bioactive compounds and secondary metabolites that act as capping, stabilizing, and reducing agents for manufacturing nanoparticles. The synthesis of nanoparticles can be either intracellular or extracellular depending on the site of synthesis of nanoparticles. The applications of nanoparticles can be broadly classified into biological and environmental. The twenty-first century marked an alarming rise in the development of renewable energy owing to the surge in demand which leads to depletion of natural energy resources and as an action of consequences degrades the quality of the environment (Tiquia-Arashiro and Rodrigues 2016). As far as environmental applications are concerned, nanoparticles are being exploited for the treatment of wastewater, bioremediation of various pollutants, and production of renewable energy. These particles are considered to be a more efficient and reliable source of producing renewable energy which is much more affordable and cleaner. Moreover, nanoparticles can be a source for developing countries to aid them in fulfilling their energy supply requirements and thus decreasing their dependence on non-renewable energy resources. The chapter covered a few from a myriad of renewable energies which can be produced using nanoparticles. It unmasked the attributes of nanoparticles synthesized from algae and how they are capable of revitalizing the process of energy production in the field of environmental science.

7.2

Nanoparticles

A nanometer (nm) is an SI unit that signifies 10 9 m in length (Jeevanandam et al. 2018) The US Food and Drug Administration (USFDA) discusses nanoparticles as materials having a minimum of one dimension in the limit of nearly 1–100 nm and shows dimension-dependent phenomena (Guidance 2011). As per the International Organization for Standardization (ISO), a nanoparticle materials are the one that have nanoscale dimensions, which is about 1–100 nm2. The mechanically derived or engineered nanoparticles have recently been exploited in the various sectors for research and development. However, these nanoparticles were present since ancient times in the environment, naturally around the earth in the form of minerals, clays, and bacterial products (Gupta and Xie 2018). Owing to their physicochemical properties like thermal and electrical conductivity, melting point, wettability, and catalytic activity, nanoparticles have gained importance in the research and technoeconomic sector (Jeevanandam et al. 2018). These unique properties of nanoparticles also lead to a biological response in various living systems. Nanoparticles have been broadly divided into two groups which include inorganic nanoparticles and organic nanoparticles. Inorganic nanoparticles comprise

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metals and metal oxides like iron oxide (Fe3O4), copper oxide (CuO), zinc oxide (ZnO), silver (Ag), gold, titanium oxide (TiO2) to name a few. On the other hand, some of the organic nanoparticles are N-halamine compounds, cationic quaternary polyelectrolytes, quaternary ammonium compounds, poly-ɛ-lysine, and chitosan (LewisOscar et al. 2016). Some of the nanoparticles are manufactured purposely using a particular method are known as engineered nanoparticles, e.g., fullerenes and carbon nanotubes (CNTs) (Taghavi et al. 2013). These are also inorganic in nature. NPs are also divided on the basis of their chemical and physical properties. Nanoparticles that are carbon-based majorly contains carbon and are used in many industrial sectors and research areas. They include fullerenes, carbon black, carbon nanofibers, carbon nanotubes (CNTs), and carbon anions. Chemical arc discharge, laser ablation, and vapor deposition are considered as the main approaches to synthesize carbon-based nanoparticles (Kumar and Kumbhat 2016). Next is metalbased nanoparticles which are common inorganic materials that comprise pure metals including silver, gold, zinc, titanium, platinum, cerium, thallium, and iron. They may also constitute different metallic oxides, sulfides, hydroxides, chlorides, or fluorides (Khan 2020). Metal-based nanoparticles have importance in various research areas due to their advanced optical properties (Sánchez-López et al. 2020). Ceramic nanoparticles are another type, mainly composed up of carbides, phosphates, oxides, and carbonates of metals and metalloids including silicon, calcium, titanium, etc. These nanoparticles have numerous applications due to their various relevant characteristics, such as resistance to chemical inertness and high heat (Thomas et al. 2015). Semiconductor nanoparticles, also referred to as quantum particles, are comprised of elements belonging to groups II–VI or III–V having exclusive electronic and optical properties due to their quantum confinement effects. Although the characteristic properties of these nanoparticles extremely relies on size and shape (Ishtiaq et al. 2020), they have unfastened a completely new field of research in the area of semiconductor nanocomposites to exploit their characteristics properly. These nano-sized materials have various applications including DNA detection, cell labeling, biosensors, cell tracking, in vivo imaging, telecommunication, LEDs, photovoltaic devices, and photodetectors (Jacob et al. 2016). A lipid-based nanoparticle possesses a solid core that comprises lipids and a matrix containing lipophilic molecules. Lipid nanoparticles are liposomal structures that can be developed to encapsulate various oligonucleotides (RNA and DNA) as well. The external surface of these nanoparticles is stabilized by surfactants or emulsifiers. Lipid nanotechnology is applied in several fields including cancer therapy, drug delivery, and others (Khan et al. 2019a, b). Based on catalytic nature, optical behavior, and large surface area, nanoparticles can be perfectly exploited for reaping energy. These particles are mainly utilized to produce energy from electrochemical and photoelectrochemical (PEC) water splitting (Ning et al. 2016). To preserve the energy in different forms at the nanoscale, nanoparticles can be used in energy storage applications.

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Physicochemical Properties of Nanoparticles

Nanoparticles possess unique properties in comparison to the bulky material because of their nano size that enables them to restrain their electrons and shows quantum effects (Rai et al. 2016). Various novel properties of nanoparticles include morphology, high reactivity, and nanoscale size and have contributed their application in various research fields and industrial sectors. Due to their nanoscale size, nanoparticles holds a large surface area to volume ratio which increases their number of active sites resulting in higher efficiency of producing various reactions and processes. Also, the efficacy of these nanomaterials to exert diversity in their shape and structure has expanded their role in several different fields such as bioimaging, drug delivery, environmental remediation, water treatment plant, and bioenergy production (Khoo et al. 2020). Various dynamic optical properties of nanoparticles, namely reflection, absorption, transmission, and light emission, are different from characteristics of similar bulk materials. The optical property of nanoparticles affects their color. For instance, a metallic, spherical, gold nanomaterial reflects the green color at 25 nm diameter and orange color in the range of 100 nm diameter. This largely depends on its varying morphology, and quantum effect showed by nanoparticles as a result of the electrical property is retained by these particles (Adewuyi and Lau 2021). Nanoparticles also have other unique features such as chemical stability, a high crystallinity, high adsorption capacity, catalytic activity, crystallinity, durability, and efficient storage, stability which makes them suitable for the production of bioenergy and biofuel. Nanodroplets, nanomagnets, and nanocrystals are some of the nanoparticles that have been added to improve the efficiency of fuel blends.

7.3

Synthesis of Nanoparticles

Nanotechnology is a critical discipline of cutting-edge studies handling design, synthesis, and manipulation of particles size ranging between 1 and 100 nm (Iravani et al. 2014). A number of techniques are stated for synthesizing NPs which include conventional or green route methods.

7.3.1

Conventional Methods

The conventional methods are mainly employed in producing monodispersed nanoparticles (Chokriwal et al. 2014). However, these methods are destructive in one or the other manner because they are toxic, flammable, and do not dispose-off into the environment easily (Kowshik et al. 2002). Nanoparticles are mostly used in biomedical applications, and toxic substances absorbed on the surface of

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Fig. 7.1 Conventional method of nanoparticle synthesis

nanoparticles synthesized by conventional means often show adverse effects (Jain et al. 2010). There are various chemical and physical methods to synthesize NPs, some of which are shown in Fig. 7.1.

Chemical Methods Ongoing advances in technology analysis have established a spread of strategies to synthesize nanoparticles (NPs) from various range of materials. Chemical decomposition, microemulsion, polyol, etc. are some of the chemical techniques used to prepare nanoparticles. Depending upon their synthesis methods, NPs possess unique structural, physicochemical, and morphological characteristics, which are important in an exceedingly wide variety of applications associated with the electronic, optoelectronic, optical, electrochemical, environment, and biomedical fields.

Microemulsions Microemulsions are clear and isotropic mixtures of stable nature. These are either a ternary or a quaternary mixture of water, oil, and surfactant along with a co-surfactant (Madhav and Gupta 2011; Malik et al. 2012). This formation of

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microdroplets depends upon the value of the surfactant based on hydrophilic– lipophilic balance (HLB) or the concentration of different components used in the mixture (Polychniatou and Tzia 2014). The microemulsion synthesis method is extensively used in producing inorganic nanoparticles (Malik et al. 2012). Oil and water upon mixing create an immiscible solution (Capek 1999) which requires the energy input to be mixed properly (Cele 2020). The microemulsion method is often exploited to produce metallic nanoparticles like gold, silver, etc. The preparation procedure involves the combining two microemulsions carrying a metallic salt and a reducing agent, respectively, which initiates the interchange between reactants as the water droplets collide. The exchange process followed by the formation of nanoparticles occurs in a series of events (Zhang et al. 2007), and the microemulsion droplets are subjected to Brownian motion which enhances the collision followed by surfactant layer opening and eventually leads to the formation of transient dimers. These dimers are kept in contact with the aid of material inter-micellar exchange. In the final step, metal ions are reduced to form nanoparticles. The nucleation reaction as well as particle growth happen within the micelles, and thus, the morphology of micelles impacts the morphology and size of synthesized nanoparticles (Tojo and Vila-Romeu 2014). The method is thermodynamically stable; however, it is extremely sensitive to changes like pH and temperature variations. Moreover, the concentration of nanoparticles synthesized is also limited (Teja et al. 2014).

The Polyol Method This method is extensively utilized for the synthesize metallic, oxide, and semiconductor nanoparticles. In this technique, the metal precursor is added into a glycol solvent and is subsequently heated at a constant temperature in a controlled manner (refluxing temperature) (Bensebaa 2013). The following series of events occur in the formation of nanoparticles by polyol method, in which the glycol is used as solvent for mixing with metal precursor and is heated followed by the metal precursor’s reduction and subsequent formation of an intermediate phase which acts as a reservoir for cations. This corresponds to the formation of the first clusters or nuclei which eventually leads to the growth step resulting in the formation of the metal nanoparticles (Fiévet et al. 2018). The synthesis of particles is under the control of reaction temperature and concentration of reactants (Kim et al. 2006). This conventional technique has several drawbacks as it is problematic to obtain nano-sized particles, and the solution has a high level of impurities which makes it difficult to separate the resultant particles (Der Wu et al. 2012).

Thermal Decomposition Thermal decomposition is used to synthesize stable monodispersed nanoparticles (Odularu 2018). In this method, the metal precursor undergoes thermal decomposition which results in the formation of metallic products. The solution is re-heated and

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then cooled to room temperature, the nanoparticles synthesized (Salavati-Niasari et al. 2008) are then precipitated using cold polar solvents such as deionized water, ethanol, and methanol. The process often takes a longer time to complete, and the optimization of particle size is also required (Unni et al. 2017).

Electrochemical Synthesis In this method, a metal precursor is mixed to an anode solution while the intermediate metal salt formed is reduced at the cathode (Rodriguez-Sanchez et al. 2000). The deposition of metal takes place at the interface of the electrolytic solution. The process leads to the formation of metallic nanoparticles stabilized by electrolytes like tetraalkylammonium salts. NPs showed dendritic growth, and a high concentration of precursor favors the growth (Singaravelan and Alwar 2015). This technique of nanoparticle synthesis is often associated with limitations related to the morphology and dimensions of nanoparticles (Tonelli et al. 2019).

Physical Methods Physical approach for the synthesis of nanoparticle comprises of methods like pulsed laser method, microwave irradiation, sonochemical method, gamma radiations, and others. It provides several advantages over chemical techniques as there is no solvent contamination and uniformity of distribution of NPs.

Pulsed Laser Method In this technique of nanoparticle synthesis, oxides of metal are dissolved in liquidlike acetone or deionized water. The solution is then mixed uniformly followed by ultrasonication. Next, the solution is irradiated by a pulsed laser such as Nd: YAG (neodymium-doped yttrium aluminum garnet; Nd: Y3Al5O12) for varying times (Park et al. 2008). The pulsed laser parameters are controlled such that they would not scatter the laser beam and interrupt the process. The resultant nanoparticles are obtained by drying them at temperatures and conditions depending upon the type of particle synthesized. For instance, TiO2 nanoparticles are obtained by drying the solution at room temperature under normal conditions while Ni nanoparticles are preferably stored under argon gas to avoid oxidation (Zuñiga-Ibarra et al. 2019). This method has the disadvantage of economical inconvenience as the high price of laser system and preparation of a large number of colloids incurs high investment cost. Also, a large amount of energy is needed to synthesize the nanoparticles (Sportelli et al. 2018).

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Microwave Irradiation Microwaves are electromagnetic waves comprising of two components, i.e., electric and magnetic fields (Sun et al. 2016). This method comprises a stock solution of metal oxide precursor, and ethanol is prepared to which the required amount of polyethylene glycol (PEG) is added. The solution is then placed in microwave reflux for an on and off cycle of variable times and power depending upon the type of nanoparticles being synthesized. For instance, CuO nanoparticles are synthesized under the working cycle of 6 s on and 24 s off (Wang et al. 2002) while Fe2O3 nanoparticles are synthesized under microwave operation which is on for 15 s and off for 15 s (Liao et al. 2001). The solution is then centrifuged, washed, and air-dried to obtain the synthesized nanoparticle. The disadvantages of microwave irradiation are its limitations in the yield of the product. Also, it has limited application for the production of nanoparticles of materials that can absorb microwaves like sulfur (Grewal et al. 2013).

Sonochemical Reduction The sonochemical reduction method is carried out in ultrasound irradiation setup, and metal precursor in an aqueous solution is poured into the reaction vessel and is sonicated under an atmosphere of argon–hydrogen (Salkar et al. 1999). The supply of constant ultrasonic power is maintained throughout the process and is measured by calorimetry. Next to irradiation, small amounts of the sonicated solutions were taken from the cell and added to a known volume of polyvinylpyrrolidone (PVP) solution to avoid the clustering of synthesized nanoparticles (Okitsu et al. 2005). The major drawback associated with sonochemical synthesis is the low efficiency of the technique.

Gamma Radiation In this method, gamma radiation interacts with an aqueous solution which results in the generation of reducing and oxidizing agents randomly distributed in the solution. Formed species reduce metal to the zero-valent state. The atmosphere of the solution can be altered by using additives like NO2 which can enhance the concentration of generated reducing and oxidizing agents (Flores-Rojas et al. 2020). The process is carried out inside a gamma chamber where radiolysis of water takes place when an aqueous solution containing metal precursor and acetone is purged with N2 and exposed to gamma irradiation. Then a series of chemical changes occur that will then involve other species present in the system, such as metal complexes, metal ions, monomers, or polymers. The change in color of the solution reflects the formation of nanoparticles say, development of pink color when an aqueous solution containing AuIII, PVP, 2-propyl alcohol, AgNO3, and acetone undergo irradiation process

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defines synthesis of gold nanoparticles. Polyvinylpyrrolidone (PVP) is used in the process as a capping agent (Misra et al. 2012).

7.3.2

Green Route Synthesis

Green route synthesis defines the production of nanoparticles that makes use of microorganisms and plants by green synthesis technology which is an alternative to traditional methods. It is a cost-efficient, environmentally friendly technique that produces non-hazardous by-products and requires low maintenance (Ciambelli et al. 2019). Green synthesis processes follow mild reaction conditions, and the use of nontoxic precursors has been highlighted in the development of nanotechnology which encourages environmental sustainability (Zhu et al. 2019). Green chemistry approach is another approach connecting nanotechnology with plants and is one involving the plant-mediated synthesis of nanoparticles (Parveen et al. 2016). Another approach is green syntheses using microorganisms. These are often referred to as “bionanofactories” due to their fast and affordable synthesis which does not harm the environment. Moreover, these organisms provide the advantage of their exclusive structures which provide a high capacity for metal uptake while maintaining safety levels (Ciambelli et al. 2019).

Plant Synthesis of nanoparticles from plant sources is the preferred method as it does not cause potential harm to the environment and is easy, single-step, and relatively reproducible (Ikram 2015). Different parts of a plant like leaves, flowers, fruit, root, seed, and bark are used to synthesize these particles. For instance, gold nanoparticle can be synthesized from the leaves of the mango (Mangifera indica) plant (Santhoshkumar et al. 2017). Also, the same type of metallic nanoparticles can be synthesized from the Rosaceae flower (Kahkha and Kahkha 2015). The initial step is the extraction of plant material. Further, the stock solution of plant extract is mixed with an aqueous solution containing the precursor for the synthesis of nanoparticles (Rajeshkumar and Bharath 2017), then the vessel is kept at varying temperature conditions to produce the nanoparticles. For instance, Ag nanoparticles from the fresh leaves of I. balsamina extract is kept at 60 in a water bath (Aritonang et al. 2019) while vessel for producing Ag nanoparticles from the fresh leaf of C. roseus is kept at room temperature. Color changes indicate the synthesis as a result of the reaction between plant extract or plant material and metal ions (Ponarulselvam et al. 2012). The particles synthesized can be used in different application from fields of environmental science, biomedical, etc., some of which are given in Table 7.1.

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Table 7.1 Examples of nanoparticles synthesized by plants Plant Trigonella foenum-graecum (fenugreek) Centella asiatica (Gotu kola) Punica granatum (pomegranate) Rosmarinus officinalis (rosemary)

Nanoparticle synthesized Gold (Au) Copper oxide (CuO) Iron oxide (γ-Fe2O3) Manganese dioxide (MnO2)

Application Catalytic activity Biodiesel Biofuel production Biofuel production

References Kuppusamy et al. (2016) Varghese et al. (2017) Duman et al. (2019) Stegarescu et al. (2019)

Table 7.2 Examples of nanoparticle synthesis carried out by bacteria Bacteria Pseudomonas aeruginosa Rhodopseudomonas capsulata Pseudomonas stutzeri Bacillus subtilis

Nanoparticle synthesized Gold (Au)

Application Biotechnological applications (bioremediation, biomineralization, bioleaching)

Gold (Au)

Biodiesel production

Silver (Ag)

Biomineralization

Silver (Ag)

Biofuel production

References Husseiny et al. (2007) Wang et al. (2011) Iravani et al. (2014) Razack et al. (2016)

Bacteria The synthesis of nanoparticles by employing bacteria has been extensively adopted as they are abundantly present in the environment and possess the nature of adapting extreme conditions, inexpensive culture, easy manipulations, and fast growth (Pantidos and Horsfall 2014). Bacteria produce nanoparticles either intra- or extracellularly. Extracellular synthesis can be done by using bacterial biomass, culture supernatant, or cell-free extract. The possible routes for synthesis are either the release of the nanoparticle to the external medium by bacteria that helps in the reduction of metal ions to nanoparticles or the particles are formed inside the cell and then secreted to the outside environment (Singh et al. 2015). Extracellularly synthesized nanoparticles may remain attached to the bacterial cell wall which can be recovered by sonication (Parikh et al. 2011). In order to carry the synthesis of NPs intracellularly, bacterial cells are added to the culture medium containing the precursor for the synthesis and grown under appropriate conditions. Synthesized particles are disposed of at periplasmic locations (Klaus et al. 1999), and products are recovered by ultrasonication (Kalishwaralal et al. 2010). Some of the nanoparticles synthesized by bacteria are enlisted in Table 7.2.

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Fungi The synthesis of nanoparticles by means of fungi can be done by either extracellular or intracellular means. In the latter technique, the metal precursor is mixed with the mycelial culture and gets absorbed into the biomass. As a result, synthesized nanoparticles are mined out by chemical treatment and centrifugation followed by filtration to disarrange the biomass and release the final product (Guilger-Casagrande and Lima 2019). For extracellular synthesis, an aqueous filtrate with the fungal biomolecules only is fed with the precursor which results in the formation of nanoparticles in the dispersion (Sabri et al. 2016). External synthesis does not require extra steps for the release of nanoparticles from cells. However, the dispersion containing nanoparticles should be purified to eradicate fungal residues. The methods that can be used for purification are membrane filtration, simple filtration, gel filtration, ultracentrifugation, and dialysis (Qidwai et al. 2018). Few of the nanoparticles synthesized by bacteria are listed in Table 7.3.

Algae The basic mechanism for the synthesis of nanoparticles via the algal route involves the preparation of an aqueous solution containing algal extract and metal precursor. The solution is then incubated for the required period under controlled conditions. The color change of the reaction mixture is indicative of the initiation of nucleation reaction which is followed by the growth of nanoparticles (Khanna et al. 2019). The synthesis route can either be intracellular wherein process occurs within the cell, and microalgal pretreatment is not required as synthesis relies on metabolic pathways occurring within algae (Sharma et al. 2016). Meanwhile, the extracellular route process occurs outside the cell and is aided by the presence of certain factors such as metabolites, pigments, ions, various proteins, RNA, DNA, lipids, and microbial by-products, involved in the cell metabolism (Mata et al. 2009). The nanoparticle synthesized are being currently utilized by many industries including pharmaceutical, nutraceutical, food industries, cosmetics, biofuel generating industries, etc. (Fig. 7.2). Few of the nanoparticles synthesized by bacteria are listed in Table 7.4. Table 7.3 Examples of nanoparticles synthesized by employing fungi Fungi Aspergillus niger

Nanoparticle synthesized Gold (Au)

Trichoderma viride

Silver (Ag)

Application Catalysis and photoelectronic material Biolabeling and sensors

Candida albicans

Gold (Au)

Biomedical

Lasiodiplodia theobromae

Silver (Ag)

Antibacterial, antifungal, anticancer

References Bhambure et al. (2009) Fayaz et al. (2010) Chauhan et al. (2011) Ranjani et al. (2020)

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Fig. 7.2 Nanoparticle synthesis from algae and its applications in various sectors

Table 7.4 Examples of nanoparticles synthesized by algae Nanoparticle synthesized Silver (Ag)

Algae Chlorella pyrenoidosa Lyngbya majuscula

Silver (Ag)

Application Photocatalytic activity Biofuel

Chlorella vulgaris Coelastrella sp.

Silver (Ag) Iron (Fe)

Biofuel production Biodiesel production

7.4

References Aziz et al. (2015) LewisOscar et al. (2016) Razack et al. (2016) Vignesh et al. (2020)

Alga-Mediated Nanoparticle Synthesis

Synthesis of nanoparticles exploiting algae as a source can be regulated by various parameters some of which include pH, temperature, contact time of the reaction mixture, the concentration of bio-extract as well as precursor and exposure to light. These factors depend upon the nucleation, agglomeration, stabilization, growth, and aging of nanoparticles which can modify the morphology and crystallinity of nanoparticles produced. After its synthesis, the characterization of nanoparticles occurs by several microscopic and spectroscopic techniques (Chaudhary et al. 2020). Biosynthesis of nanoparticles using algae can be performed from either route, that is, intracellular synthesis or extracellular synthesis as shown in Fig. 7.3. It is increasing tremendously because they contain an elevated number of peptides, pigments, and secondary metabolites which can act as nano-biofactories. Ease in harvesting, faster growth rate, and cost-effective scaleup are some of its characteristics that make algae a better source of nanoparticle synthesis (Patel et al. 2015).

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Fig. 7.3 Graphical representation of process involved in intracellular and extracellular nanoparticle synthesis from algae

(a) Intracellular Mode of Nanoparticle Synthesis from Algae The intracellular mode for synthesizing nanoparticles from algae deciphers a process in which there is no pretreatment required, and production of nanoparticles happens within the cell of algae. At the time of metabolism of this organism, various enzymes and cofactors have been released such as NADPH or NADPH-dependent reductase release during nitrogen fixation, photosynthesis, and respiration which acts as reducing agents (Chaudhary et al. 2020). The mechanism involved in the production of nanoparticles by intracellular means entails the collection of algal biomasses followed by the application of distilled water to wash the collected material. Then any metallic solution, for instance, AgNO3 is added to biomass. After this, it is incubated at particular parameters, and finally, the produced nanoparticles can be obtained by the centrifugation of the solution (Uzair et al. 2020). Numerous reports are analyzed which illustrate the nanoparticles are produced intracellularly. For example, the production of gold nanoparticles from cyanobacteria (Dahoumane et al. 2014; Parial et al. 2012) and diatoms (Senapati et al. 2012). Khanna et al. produced gold nanoparticles by providing the treatment of chloroauric acid at the temperature of 20  C for 72 h to the algal species, Ulva intestinalis and Rhizoclonium fontinale. The transition in thallus color from green to purple was detected as a confirmation of the production of nanoparticles. This change of thallus color inside the cells is due to the involvement of NADPH which shows the efficiency of entrapped cells to reduce salts of gold metal into its nanoparticles (Khanna et al. 2019). In another experiment, a solution of gold chloride was mixed with the livecell culture of the microalga Chlorella vulgaris, and the solution was incubated

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for 2 days. The metal ions reduced results in the formation of nanoparticles. These nanoparticles were harvested by centrifugation and were characterized by transmission electron microscopy (TEM), and further analysis was performed by using X-ray absorption spectroscopy or synchrotron-based X-ray powder diffraction which confirmed them as gold. Similarly, culturing of C. vulgaris to the salt solutions of rhodium, palladium, and ruthenium leads to the synthesis of corresponding metallic nanoparticles intracellularly (Luangpipat et al. 2011). (b) Extracellular Mode of Nanoparticle Synthesis from Algae When the synthesis of nanoparticles occurs on the outside of the cell, it is known as “extracellular” synthesis. There are several components involved in cell metabolism including metabolites, pigments, RNA, proteins, microbial by-products, DNA, and others which support this route of synthesis (Mata et al. 2009; Khanna et al. 2019). Extracellular means of synthesis do not require a long culturing process and laboratory equipment as the harvested algal biomass, before its use for the production of nanoparticles, is subjected to pretreatments like washing or blending. Also, by regulating the experimental parameters like temperature, pH, the concentration used, etc., the process can be standardized to impact the shape and the dimensions of nanoparticles (Dahoumane et al. 2016). For instance, pH in the basic range interacts with amine groups which aids in the process of capping and also helps to develop a stabilized nanoparticle (Namvar et al. 2015) while a higher range of pH enhances the reducing capacity of available functional groups (Parial et al. 2012). The main factors responsible for this method of nanoparticle synthesis from algae include the optimal concentration of precursor, active moieties present on the cell surface, and quantity of cells used in the process (Kalabegishvili et al. 2012a, b). The extracellular mode of nanoparticle synthesis begins with the collection of algal biomasses followed by their washing with distilled water. Next, the three methods are generally exploited to proceed further which include drying of algal biomass under shade for a defined period, then providing distilled water treatment to the dried powder followed by filtration of the solution. The other process is the sonication of algal biomass to get the cell-free extract. The third method is washing of algal biomass with distilled water followed by its incubation for approximately 8–16 h and finally filtration of this mixture to obtain the product (Uzair et al. 2020). To synthesize silver nanoparticles (AgNPs) from Desmodesmus sp., two different algal extracts, namely raw algal extract (RAE) and boiled algal extract (BAE), were taken for extracellular synthesis. To synthesize nanoparticles from RAE, the algal biomass is mixed with deionized water and then was centrifuged post-incubation for few days. The supernatant comprises algal biomass which was separated and had undergone several treatments including the addition of AgNO3 as a metal precursor for the extraction of nanoparticles. This is followed by several cycles of centrifugation at defined rotation per minute (rpm) for a definite period corresponding to each cycle. After synthesis, the extract undergoes separation to obtain the product (Öztürk 2019).

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Another study highlighted the extracellular mode of producing gold nanoparticles (AuNPs) by exploiting Tetraselmis suecica which is a marine microalga. To initiate the process, the microalgae were grown under specific conditions, and culture was taken during the logarithmic phase of growth. Then the supernatant was removed by centrifugation, and biomass was exposed to several treatments including washing, centrifugation, mixing, freezing, and re-centrifugation to collect the cell extract in the form of supernatant. The cell extract was then mixed with chloroauric acid and incubated at a certain temperature. A transition in the color of the solution to ruby-red which was previously yellow indicated the formation of AuNPs (Shakibaie et al. 2010).

7.4.1

Types of Algae Used for Nanoparticle Synthesis

Algae are aquatic, eukaryotic, photoautotrophic, unicellular/multicellular organisms that are found in various forms in nature. This class of microorganism is categorized depending upon the pigmentation released by them, which includes red algae (rhodophytes), brown algae (phaeophytes), and green algae (chlorophytes) (Prasad et al. 2013; Sahayaraj et al. 2012). Studies show that algae are extensively used in the synthesis of nanoparticles like metallic, metallic oxide, and other nanoparticles. The particles synthesized using algae are stable and can be produced in a relatively less time than other biological procedures (Thakkar et al. 2010).

Brown Algae Brown algae belong to the family Sargassaceae. Sterols are the most commonly found components in Fucales, the order of brown algae. These sterols include fucosterols, cholesterols, and others. There are number of functional groups present in brown algae which function as reducing or capping agents for nanoparticle synthesis. These include muramic acid, alginic acid, glucuronic acid, and vinyl derivatives (Kumar et al. 2011). As per literature studies, for the biosynthesis of a variety of NPs using brown algae, a diverse range of species have been already exploited, like Fucus vesiculosus (Mata et al. 2010), Sargassum polycystum (Khanna et al. 2019), Dictyota bartayresiana (Varun et al. 2014), Padina spp. (Singh et al. 2013), and others. To initiate the process of gold nanoparticle green synthesis using brown alga Cystoseira baccata, a metal precursor says, HAuCl4 in aqueous solution form was mixed with prepared extract of C. baccata and kept for 24 h at room temperature under continuous stirring. The synthesis progress was analyzed intermittently by UV-Vis spectrometry, the endpoint of AuNP synthesis was considered when surface plasmon resonance at 532 nm peak intensity showed no further variations (González-Ballesteros et al. 2017). One more species of brown algae, Padina pavonia, is exploited to synthesize silver nanoparticles. The collected algal sample

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was treated for few days and then mixed with an AgNO3 aqueous solution. The production began with the bioreduction of AgNO3 ions occurred at room temperature under light along with continuous stirring. The color change was evident in bioreduction. The characteristic analysis of newly synthesized NPs was performed using UV-Vis spectroscopy, TEM analysis, or any other characterization technique (Abdel-Raouf et al. 2019). Other than metal NPs, metallic oxide NPs are also produced by brown alga species. For instance, nanoparticles of zinc oxide were synthesized from brown macroalga, Sargassum muticum. To promote the synthesis process from this marine microalga, zinc acetate dehydrates aqueous solution was added to extract of algal biomass prepared in the aqueous form, and then the solution was kept in a water bath at 70  C with continuous stirring. The obtained solid product was collected via centrifugation technique, which was then washed and dried overnight. Pure ZnO NPs were obtained by heat treatment. The characteristic investigation of synthesized NPs was done using X-ray diffraction (XRD) analysis and FTIR spectrometer (Azizi et al. 2014).

Red Algae Prominent algae of the Rhodophyta family, red algae which are tremendously rich in various important vitamins and proteins (Yoon et al. 2006) which could be its favorable parameter for stabilization and reduction in the biosynthesis of NPs. The nanoparticles produced from seaweeds are however in the developmental stage because of their self-aggregation, gradual growth of crystals, and issues related to the stability of NPs (Singaravelu et al. 2007). The synthesis of silver nanoparticles can be achieved by harvesting red alga Acanthophora spicifera and washing it properly with distilled water. It was then dried and ground into fine powder. The algae were centrifuged at 4000 rpm for 20 min at RT, and then silver nitrate was added to its supernatant. A gradual transformation in the color to dark brown which was earlier in the shade of greenyellow was observed in the mixture which indicated the production of NPs. The characteristic analysis of the newly synthesized nanoparticles can be performed by X-ray diffraction (Ibraheem et al. 2016). Likewise, silver nanoparticles can also be produced by another red alga Portieria hornemannii which was added in deionized water and boiled for 10 min at 80  C. The extract was brought to normal temperature before Whatman filter paper was used for the filtration. This extract was then gradually added to the silver nitrate solution which progressively adapts a pale pink coloration and eventually showed a dark brown color after the incubation period of 24–48 h; this depicts the formation of nanoparticles. The properties of these Ag nanoparticles can be identified utilizing UV-Vis spectroscopy (Fatima et al. 2020). Hypnea valencia, another red alga, was utilized to produce zinc oxide nanoparticles. These algae were harvested from the Gulf of Mannar region, washed, dried, and boiled with distilled water for about half an hour. The extract so obtained was filtered through Whatman filter paper and cooled down. A small amount of this

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extract was mixed with an aqueous solution containing zinc nitrate which was kept on a magnetic stirrer to let the zinc salt completely dissolve in the solution. After that, the solution was kept in a water bath at 80  C for a pH between 5 and 10 and for 5–10 min. The characteristic study of synthesized nanoparticles was performed by numerous techniques which include scanning electron microscopy (SEM), dynamic light scattering (DLS), and UV-Vis spectroscopy analysis to name a few (Nagarajan and Kuppusamy 2013).

Blue-Green Algae Blue-green alga is classified under the order Chroococcales, which comprises two diverse families of Chroococcaceae and Entophysalidaceae. This type of alga shows inconsistent characteristics in the biological world as the two distinguished families of its order differs in their growth habitat-forming colonies (Chaudhary et al. 2020). These microorganisms show photosynthetic activity as they have two photopigments which are carotene and chlorophyll a. They also use water as an electron donor. Morphologically, blue-green algae are considered equivalent to unicellular bacteria (Khan et al. 2019a, b). There are many noteworthy examples where bluegreen algae were used to produce NPs including gold, silver, copper, and others. Some of the species are Nostoc ellipsosporum (Parial et al. 2016), Plectonema boryanum (Lengke et al. 2006), Synechocystis sp. (Focsan et al. 2011), Cylindrospermum stagnale (Husain et al. 2015), and others. One of the notable examples is exploitation of Spirulina platensis, blue-green algae for the production of gold nanoparticles. In this procedure, wet biomass of S. platensis mixed with an aqueous solution of chloroauric with different concentrations and placed to incubate at ambient temperature with continuous stirring for few days. The characteristic study of newly synthesized nanoparticles was done utilizing UV-Vis spectrometry and TEM (Kalabegishvili et al. 2012a, b). Another example to be considered is the green route production of silver NPs from blue-green algae, Nostoc muscorum, Calothrix marchica, and Anabaena oryzae. The method involved the mixing of silver nitrate solution with the aforementioned cyanobacterial cultures. The aqueous mixture was then placed at ambient temperature (25  C) for about a month in a dark room. The color transition in solution from red to brown was evident of silver nanoparticles produced. The characterization of the newly synthesized Ag nanoparticles was done using UV-Vis spectroscopy (Khalifa et al. 2016).

Green Algae Based on their habitat, green algae have been categorized into two forms: microgreen algae and macro-green algae (Kim et al. 2011). Microgreen algae are single-celled organisms and lives in freshwater bodies like lakes, ponds, rivers, etc. while macroalgae are mostly multicellular and can be found in marine water (Deglint

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et al. 2019). Nowadays, the synthesis of metallic and metal oxide NPs from green algae is widely practiced (Thangaswamy et al. 2021). The production of nanoparticles from green algae can be well explained by an example. Green alga Botryococcus braunii was collected from a lake situated in Rajasthan, India, and isolated by the method of serial dilution. Algal colonies were grown on Chu-13 medium which has been initially solidified and isolated after 3 weeks. The alga is then inoculated in a liquid medium at approximately 27  C by providing the light intensity of 1.2 Klux and light:dark hours cycle of 16:8 to the nutrient medium. The resultant biomass undergoes centrifugation and then is dried subsequently. Next, 100 mL of distilled water was added to 5 g of dried biomass. The solution is autoclaved and filtered, and the algal extract was obtained after centrifuging the filtered extract. Now, to synthesize copper nanoparticles, the extracted algal mass was gradually mixed into copper acetate solution with continuous agitation for 1 day at 100  C. The visible color changes of the solution, from a lighter shade of blue to dark brown directs the formation of Cu nanoparticles. These newly produced nanoparticles undergoes further separation steps and then characterized by UV-Vis NIR spectrophotometry. Similarly, silver nanoparticles can also be synthesized from the same species. The precursor used here is silver nitrate, and the stirring time was 3 h at room temperature. The biosynthesis of silver nanoparticles was marked by visible color changes of the solution from pale-yellow to reddish-brown. The produced NPs were characterized by UV-Vis spectrometry (Arya et al. 2018). Another example determines the biosynthesis of cadmium sulfide nanoparticles by exploiting a freshwater green Chlamydomonas reinhardtii. The algae were harvested and processed to obtain a pure algal extract, which was added to the water which is previously de-ionized and then kept in a water bath setup operating at 65  C temperature. After that, sodium sulfide and cadmium chloride were put into the extract solution, and the reaction was completed after 20 min. Cadmium sulfide NPs were isolated by centrifugation, and the characteristic analysis was done by UV-Vis absorption spectroscopy and high-resolution TEM (HR-TEM) (Rao and Pennathur 2017).

7.5

Potential of Nanoparticles in Biofuel Production

Algal biorefineries that are producing biofuel are facing many challenges, including (a) high energy consuming methods for disrupting cell walls, (b) high energy consuming method of biomass harvesting, and (c) less efficient methods for algal oil conversion to biofuel. As we discussed earlier, nanoparticles have both catalytic and magnetic properties that can benefit biofuel production. Silver nanoparticles have been employed for microalgal cultivation in photobioreactor, and it also helps in the uniform distribution of nutrients throughout (Hossain et al. 2019).

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Role of Nanotechnology in Algal Biofuel Production

The applicability of nanoparticles has been observed in every single step from production to harvesting, harvesting to oil extraction, and from oil extraction to oil conversion into fuel.

Nanotechnological Advancement in Light Exposure The basic need of algal culturing is illumination. Significant research has been done by many scientists, and an innovative nanomaterial has been designed for lightemitting diodes (LEDs) (Pompa et al. 2006; Pattarkine and Pattarkine 2012). The utilization of localized surface plasmon resonances (LSPR) along with the metallic nanoparticles is being considered most nowadays. The concentration of nanoparticle utilized needs to be considered most as the high concentration NP may hinder the light from reaching the algal cells. So, in closed photobioreactors, the nanoparticles are used outside of it to amplify light. At a metallic dielectric interface, the combined oscillations of the free electron are the basic principle behind the LSPR (Torkamani et al. 2010). The utilization of silver and gold nanoparticles suspension has been done for enhancing the efficiency of light uptake by Chlorella vulgaris (Eroglu et al. 2013).

Nanotechnological Advancement in Downstream Harvesting Another applicability of nanoparticles is observed in downstream harvesting technologies. Among all the harvesting technologies (like filtration, centrifugation, magnetophoretic separation, electrolysis, and flocculation), flocculation is the most viable and economical method (Vandamme et al. 2013; Farooq et al. 2013; Uduman et al. 2010). Currently, flocculation with the help of a magnetic field is seen as one of the most efficient technology of microalgal harvesting. Magnetic nanoparticle utilization for harvesting is an energy-efficient, inexpensive, and quick method. Iron oxide-based magnetic nanoparticles are being used for the removal of algae from freshwater and fishpond (Toh et al. 2012; Wang et al. 2013).

Lipid Extraction by Nanotechnology The algal cell wall is composed of complex glycoprotein and carbohydrates. It provides chemical and physical resistance to the algae. This makes lipid extraction a complex step and covers a major part of the entire cost of biofuel production (Lee et al. 2015). The most common method to extract lipid is the Soxhlet method that uses different solvents (hexane, methanol, chloroform, etc.) either individually or in

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different combinations. Development in this field has provided with safe, reliable, and cost-effective alternative methods. Apart from this high efficiency in oil extraction, NP is a nontoxic method for the extraction of lipid (Zhang et al. 2013). A study suggested the utilization of aminoclay nanoparticles with Chlorella sp. KR-1 resulted in high efficiency in algal biomass extraction along with oil extraction. The aminoclay nanoparticles act by reducing the water layer present between the cell wall and hydrophobic solvent. It results in more contact of the solvent with the cell wall for intracellular oil release. In another study, TiO2aminoclay has been utilized under UV irradiation at 365 nm for disruption of an algal cell (Lee et al. 2014).

Nanomaterials Application in Transesterification The process of converting algal oil into biodiesel is done through many means in which transesterification is mostly used. In this process, oils sourced from algae react with alcohol to produce biodiesel. Mainly four types of catalysts are used in transesterification for biodiesel production, namely, acid, enzyme, and heterogenous catalyst (Kumar et al. 2016) (Fig. 7.4). Catalyzation by acid is majorly done by using H2SO4 and HCL. It is a conventionally underutilized process as it is a costly process as it requires a large amount of methanol. Catalyzation by the enzyme is the most efficient method used for the transesterification process. Transesterification by lipase is one of the most utilized eco-friendly method. The only problem associated with the process of enzyme-catalyzed transesterification is its higher economical value and denaturation. Also, the by-products and substrates involved during the transesterification process inactivates the enzymes (Kumari et al. 2009).

Fig. 7.4 Nanomaterials used to enhance transesterification

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Conclusion

Nanotechnology involves designing, production, and application of matter with dimensions ranging between 1 and 100 nm, approximately. The nano-sized structures, devices, and systems are developed by manipulating their morphological characteristics under controlled environment. Nanoparticles are synthesized by various methods which involved both the chemical and physical techniques aforementioned in the chapter text. However, these methods have frequent limitations which make them not so well-grounded, so green synthesis lends a helping hand in nanoparticle production. The chemicals used are often toxic and cause harm to the environment as well. Green synthesis exploits a plethora of flora and microbial fauna to synthesize different types of nanoparticles including metal NPs, metal-oxide based NPs, and others. Cost-effectiveness, environment-friendly nature, reproducible techniques, and lower risk of contamination are some of the perks of green synthesis technology. Plants and microorganisms like bacteria, fungi, and algae are referred as “bionanofactories” as they are nanoparticle producer which is harmless to the environment. Algal biosynthesis of nanoparticles is the preferred route of synthesis due to its high peptides and secondary metabolite content along with the advantages of easy harvesting and rapid growth. This can be carried out either from the extracellular route or the intracellular route. In the former technique, nanoparticles or nanoclusters are formed in the cell-free extract while an additional purification step is needed in the latter method. Studies showed that a wide variety of algae are used to produce several nanoparticles. This variety is categorized based on pigment released by algae depending on which they are classified as red, brown, blue-green, and green algae. A few microorganisms from a myriad of them have been discussed in the chapter. Different species produce NPs depending upon the parameters like pH, concentration, light intensity, and others. These parameters are optimized and modified as per the requirement of the species involved in the process. Algal biorefineries face certain limitations related to high energy consumption during the production of biofuel. But nanoparticles can enhance biofuel production by exploiting their magnetic and catalytic properties. Nanoparticles can be used at multiple stages of biofuel production which include harvesting, extraction, and conversion. In recent years, natural sources are considered to have better potential in producing nanoparticles with great efficacy as these methods cause minimal adverse effects to the surrounding. Taking into consideration, the conventional methods can be concluded to impede the path of fulfilling the global fuel requirements. Therefore, a major consideration in the field of research and development has been focused on green synthesis. Fossil fuel combustion hass become increasingly harmful to the climate; moreover, the hike in prices of such fuels makes it necessary to find out more sustainable sources and methods for biofuel production. Nanotechnology provides precise and efficient properties which make it a promising technology for bioenergy production. Various industries like automotive, marine, and others are

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already using biofuel alternatives in place of conventional fuels as these fuels greatly reduce carbon emissions while maintaining similar energy density. Biofuels extracted from natural sources have shown several applications. In the future, it is expected that green transition along with technological advancement could possibly produce biofuel which will not harm the environment and can show further applications.

7.7

Future Aspects

Due to the rise in prices of oil and sudden change in climatic conditions around the globe which are occurring due to the combustion of fossil fuels, there is a surge in demand for sustainable and economically feasible biofuels. This fuel is being explored as a chief target to maintain and secure the market of energy in the future. Biofuel is often considered the fuel of the future as it is a low-carbon energy source. Moreover, it is cheaper than other conventional energy sources (Shanmugam et al. 2020a, b). The environmentalists and the researchers have been giving alarming signals regarding the overconsumption and hence depletion of fossil fuels and minerals (Kirsch 2020). Currently, the major concern for any nation is the efficient conversion of resources to produce enough energy and make it available as and when required; this greatly impacts its socio-economic and developmental status (Kumar and Chauhan 2013). Amidst the depletion scenario, biofuel can substitute the fossil fuel requirement with a reduced carbon footprint (Shanmugam et al. 2020a, b). Nanotechnology is one of the most promising modern bioenergy production approaches due to its precise structural organization and physiochemical properties (Shanmugam et al. 2020a, b). The photoautotrophic microorganisms like microalgae and cyanobacteria can produce oxygen and hydrogen by splitting water molecules in a process called biophotolysis (Srirangan et al. 2011). Hydrogen can be produced by hydrogenase enzyme under anaerobic conditions which are observed to get enhanced by incorporating certain nanoparticles such as iron (Fe) (Nath et al. 2015) and titanium oxide (TiO2) NPs (Pandey et al. 2015) in biophotolytic hydrogen production. This process provides the advantage of cost-efficiency, and a pure yield of product is obtained with a higher energy conversion efficiency (Das and Veziroglu 2008). Moreover, the majority of algae have the potential to accumulate high levels of oils in their dry biomass. For instance, Botryococcus spp. contain longchain hydrocarbons in their 50% dry biomass (Hannon et al. 2010). Algae as a source for biofuel production have several advantages over other terrestrial sources of biomass. They can even be grown on land which has a scarcity of nutrients and possesses the property of extracting nutrients from the water. Also, the algae possess a high content of lipids and fatty acids, making them an ideal source of biofuel production (Ganesan et al. 2020). These microorganisms can be easily engineered according to the requirement and can generate products that improve the potential of algal biofuel to compete with existing fuels (Hannon et al. 2010).

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The automotive industry is looking forward to minimizing the climatic variations to less than 2  C by 2040 which drives its attention toward the adoption of electric energy as a source. It is forecasted that more than 90% of vehicles including two-wheelers, three-wheelers, and passenger cars and trains will be battery dependent by 2060. However, certain means of transport like aviation and maritime required high-density fuels to operate. For instance, the fuel of a jet plane has an energy density approximately 50 times more than that of batteries available currently (Reid et al. 2020). Biofuels can be considered as a prominent substitute for diesel and petroleum fuels with a reduced level of carbon emission because of both exhibit similar energy densities (Shanmugam et al. 2020a, b). According to the estimates of the International Energy Agency (IEA), advanced biofuels are expected to account for 50% of the fuel for shipping and 70% of fuel for aviation demand by 2060 (International Energy Agency 2017). The production and applications of biofuels can be expanded in the future by harnessing the latest advancements like nanotechnology in the field of research and development. The inclusion of nanoparticles at optimum concentration can elevate the efficiency of production, and metallic nanoparticles like gold and silver can preserve the catalytic sites and prevent the feedback inhibition in various metalloenzymes during the process of biofuel production (Shanmugam et al. 2020a, b). It has been found that incorporating calcium oxide nanoparticles into the microalga biomass increases the biodiesel conversion efficiency up to 91% during catalytic transesterification (Hossain et al. 2019). Global warming is one of the most critical environmental concerns. In the future, BECCS (bioenergy with carbon capture and storage) can be opted as the low-cost alternative to reach the optimal temperature which is otherwise increasing 0.08  C per decade since 1880 and much more in recent years. In the process of executing BECCS, carbon can be captured and stored by extraction of bioenergy from cell biomass. The energy is extracted in various forms including biofuel, electricity, heat, etc. Thus, the carbon is removed from the atmosphere. Moreover, BECCS can also help in keeping the atmospheric CO2 concentrations at a level lower than 500 ppm by making ways for net negative emissions and carbon sequestration (Board 2019; Obersteiner et al. 2001; Rhodes and Keith 2008). The world is facing some major challenges to deal with the green transition to maintain sustainable environmental conditions. There is a constant demand for reducing carbon footprints, lowering harmful emissions, and hence replacing conventional fuels with biofuels. There are several evident applications of biofuels extracted from microbial biomass like algae already discussed in the aforementioned headings of the chapter. However, biofuel can be produced as a substitute for fossil fuels to be applied in various fields. In the future, by the exploitation of biomass along with technological advancements like nanotechnology, the biofuel requirement is estimated to be accomplished.

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Chapter 8

Fungi-Mediated Green Synthesis of Nanoparticles and Their Renewable Energy Applications Rani Padmini Velamakanni, Ragini Gothalwal, Rani Samyuktha Velamakanni, Sridhar Rao Ayinampudi, Priyanka Vuppugalla, and Ramchander Merugu

Abstract The biological synthesis of nanoparticles is gaining attention due to the wide area of applications it offers. Using microbes and plants synthesis of nanoparticles is the most eco-friendly approach and is also cost effective method. Plants, microorganisms will be able to consume the inorganic metal ions from other niche. Nanoparticles can be produced in two ways: extracellular method and intracellular method. The ability of a living entity to use the organic chemistry process with in itself to synthesize the nanoparticles has given scope in the area of biochemically analysis of biomolecules. Nanotechnology with Biology has led to opening up of a new branch called as Nanobiotechnology which is an advanced area and help us to study and understand better the living systems of both prokaryotic to eukaryotic origins. Every organism acts as a unique tool in its own way to synthesize the nanoparticles. The enzymatic processes and the metallic inorganic ions that are supplied by each living system is different and hence the nanoparticles formed is also different in size, shape, and composition with each system. The various extrinsic or intrinsic pathways cause reduction, or oxidation or biosorption, etc. or other process to occur leading to the formation of nanoparticles. These nanoparticles formed have applications in various fields like pharmaceuticals, cosmetics, drug delivery, gene therapy, bio-imaging, tissue engineering, etc. In this chapter, we tried to present an understanding on the biological synthesis of metallic nanoparticles using fungi and its renewable energy applications. The metallic nanoparticles are R. P. Velamakanni · P. Vuppugalla · R. Merugu (*) Department of Biochemistry, Mahatma Gandhi University, Nalgonda, India R. Gothalwal Department of Biotechnology, Barkatullah University, Bhopal, Madhya Pradesh, India R. S. Velamakanni Department of Pharmacy Practice, Anurag University, Hyderabad, India S. R. Ayinampudi Department of Chemistry and Pharmaceutical Chemistry, Mahatma Gandhi University, Nalgonda, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Srivastava et al. (eds.), Green Nano Solution for Bioenergy Production Enhancement, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-16-9356-4_8

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considered as one of the most important components in the above said applications. These nanoparticles are used in various forms like nanocrystals, nanopowders, nanoagglomerates, nanoclusters, etc. pertaining to the area and function. Keywords Nanoparticles · Biological synthesis · Metallic ions · Pharmaceutical · Extrinsic · Intrinsic

8.1

Introduction

With the advent of new technological developments, they have been employed in the biosynthesis of nanoparticles, as there is increasing need to develop a method for the synthesis of nanomaterials that is safe and economic and environment friendly. Nanoparticle chemistry is one the most recent and evolving branches. The presence of the nanoparticles in the form of dust or smoke in the environment around us would have astonished the scientists around 30 years ago. The use of nanoparticles in various fields like engineering, medicine, healthcare, agriculture have been in use even before their properties were understood or analyses (Heiligtag and Niederberger 2013) The use of transition metals resulted in synthesis of particles that acted as a heterogeneous catalysts and gave promising results in many areas especially in the petrochemical industry. By the end of the twentieth century nanotechnology evolved into a branch with tremendous capabilities and enhanced applications. The development of characterization techniques like the electron microscopy and other high resolution spectroscopic techniques left the researches across the world with potential and more suitable tools in studying and analyzing the nanomaterials and their applications. It has been realized by the scientific community that particles with uniform shape and size will have excellent interaction and hence better functions and thus there is a need for novel procedures for synthesis, purification, and characterization. Several methods can be employed for the synthesis of nanoparticles. These usually are found in liquid and solid solvents as dispersions and in the gaseous phase they are seen as an aerosol. It is also understood that solid phase method is the most feasible and flexible methods among the various methods and has been widely adopted by majority. In the solid phase methods the commonly used procedure is colloidal technique using nonpolar-polar solvents in the reaction medium. Important steps in the synthesis are nucleation and growth. A better understanding of these two steps will yield good particles (Talapin and Shevchenko 2016). Metal nanoparticles are arguably the most studied class of nanoparticles systems. Early works date back to the nineteenth century, including Michael Faraday’s synthesis of colloidal gold in the 1850s (Faraday 1857). Mie described the interaction of light with metal nanoparticles in 1908 (Mie 1908). There are examples of ancient Romans using gold nanoparticles to prepare stained glass (Heiligtag and Niederberger 2013). MRI imaging and magnetic data storage, requires magnetic nanoparticles. Some compounds that exhibit magnetic properties also have very good catalytic ability. Wu et al. (2016) gave a thorough information on various magnetic nanoparticles that are metallic, alloys of metals and oxides of metals and

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others that are multi components. The prepared nanoparticles can be used for various reactions in molecular chemistry as crystal particles like transformations, additions, substitutions. De Trizio and Manna offered a detailed discussion of one class of such transformations, namely cation exchange reactions and demonstrated the utility of these reactions for synthesizing novel nanomaterials (De Trizio and Manna 2016). In many cases, nanoparticle shape directly impacts its physical properties, such as the frequency of surface plasmon resonance in gold and silver nanoparticles, or luminescence polarization in semiconductor nanorods (Wang et al. 2016).

8.1.1

Techniques and Criteria Used in Characterization

The evolution of the nanotechnology can be correlated to the developments in the characterization techniques both in vivo and in vitro conditions assisting us to study and analyze the formed nanoparticles. Furthermore techniques like electron microscopy and scanning probe microscopy, X-ray scattering techniques, etc. are among the few techniques that play an important role in the characterization of nanomaterials. In deducing the size, shape, and distribution of size in a large cluster of nanoparticles, (SAXS) small-angle X-ray scattering technique is among the most reliable techniques. The technique has been an excellent tool for studies of nanoparticles synthesis in real time experimental conditions. Li, Senesi, and Lee offer the readers an authoritative review covering the fundamentals of SAXS and the applications of this technique for nanoparticle research (Li et al. 2016). To the best of our knowledge, it is the first review on this topic, and many scientists will find it useful. Another microscopy technique that reveal a detailed electronic arrangement inside the nanoparticle is Scanning tunneling microscopy (STM). Swart, Liljeroth, and Vanmaekelbergh in their review explained the importance of scanning tunnel microscopy in characterizing the nanoparticles. This technique provides reliable and promising results on par with a group of various spectroscopic techniques and at the single particle levels (Swart et al. 2016). There are also several other techniques that help in the determination of the structure, composition, size, and other basic features of the NPs. Fourier transform infrared spectroscopy (FTIR) is a technique based on the measurement of the absorption of electromagnetic radiation with wavelengths within the mid-infrared region (4000–400 cm 1) helps to study the functional groups of the particle under study (Blanco-Andujar 2014). Nuclear magnetic resonance spectroscopy is another important tools employed for the determination of structure. It is a both qualitative and quantitative. The principle of the technique is that the nuclei of any particle under study when placed in a strong magnetic field, the small energy change between the spin up and down states causes the transitions that can be recorded and probed by the electromagnetic radiation in the radio wave region. NMR is employed in the analyzing the interactions between the ligand and surface of diamagnetic substances or antiferromagentic substances. But the technique is less useful in studying the ferrimagnetic substance or ferromagnetic particles as they cause shifts in the signal frequency and reduced relaxation time due to the increased

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Table 8.1 Techniques used in NP characterization S. No 1

5

Name of the technique SEM (scanning electron microscopy) TEM (transmission electron microscopy) SAXS (small-angle X-ray scattering technique) STM (scanning tunnel microscopy) NMR

6

FTIR

2 3

4

Properties studied Size monodispersity, shape, aggregation state Size monodispersity, shape, aggregation state Interactions between the particles, physical properties like shape, size Detailed electronic arrangement Analyzing the interactions between the ligand and surface of diamagnetic substances or antiferromagentic, to identify the structure Identify the functional groups

References Reimer and Kohl (2009) Reimer and Kohl (2009) Chu and Liu (2000) Swart et al. (2016) Lu (2011)

BlancoAndujar (2014)

saturation magnetization of the particle. As a result, significant broadening of the signal peaks occurs, making the measurements practically inutile and unable to be interpreted (Lu 2011). Microscopy techniques like SEM, TEM can be used to determine NP size, size monodispersity, shape, aggregation state, detect, and localize/quantify NPs in matrices, study growth kinetics (Reimer and Kohl 2009). The following criteria is used as a background for most of the scattering techniques that are used in the characterization of nanoparticles (Table 8.1). Basic scattering functions: Scattering techniques involve the use of a probe radiation that may be (visible) laser light, X-rays, or neutrons, as we are dealing with sizes in the nanometer scale to those approaching the wavelength of visible light. By detecting the interactions between the particles and the probing radiation, physical properties, such as particle size, shape, and internal structure, can be determined. The theoretical backgrounds of SLS, SAXS, and SANS are quite similar, mainly dealing with the angular dependence of (short) time-average scattered intensity (I) versus the scattering angle, in terms of the scattering wave vector, q (Chu and Liu 2000). Size study: The size of the nanoparticles (Rg) can be characterized by studying the angular dependence of the scattered intensity from SLS, SAXS, or SANS measurements. Furthermore, additional information, related to specific systems was given by Zhou et al. (2009). Shape studies: The anisotropic particles shape can be analyzed by furnishing the form factor from small-angle X-ray scattering curve with the factors from other models. An important hypothesis of furnishing is that the polydispersity is tolerable or a good mathematical expression can be deduced from the effect due to polydispersity (Wu et al. 1993).

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Internal structure studies: Internal structures of nanoparticles, e.g., the core shell structures of the micelles and micro emulsions, as well as the multi-layer structures of microcapsules, can also be characterized by the scattering techniques. It is more obvious that the study of the internal organization and composition of nanoparticle is more tough that the external morphology and its surrounding interactions. Therefore most of the times instead of relying on a single scattering technique a combination of scattering techniques is employed for characterization (Wu et al. 1993). Determination of particle size distribution: Studies reveal that Z average distribution is proportional to the scattered intensity of particle with an apparent hydrodynamic radius. This Z average distribution is compared to the number average distribution. For polydispersed particle the number average molecular weight distribution is correlated to normalized weight-average molecular weight distribution [Fw (M)] (Munk 1989).

8.2

Biological Synthesis of Nanoparticles

The term “synthe” generally refers to the formation of something either naturally or by synthetic medium. It is the mechanism of mixing or combining two or more entities to give a new product. Nanoparticles which possess the unique characteristics make them useful in biological applications such as drug delivery system, regenerative medicine are called as Bio-nanomaterials. Nanoparticles of one or more dimensions of the order of 100 nm or less will have attracted great unusual and fascinating properties, and applications advantageous over their bulk counterparts (Balaji et al. 2009). These nanoparticles can be synthesized by physical, chemical, and biological techniques. For the synthesis of nanoparticles the methods used are physical, biological, and chemical methods. In recent times, the biological method of synthesis is more appreciated when compared to the chemical synthesis method as it suffers few limitations like the use of toxic chemical. The presence of chemical compounds might hinder its use in specific clinical fields. Microorganisms offer the excellent environment for the nanoparticles synthesis. The enzymes in the living system act as a catalyst for the formation of nanoparticles. These reactions can be performed at ambient pressure, temperature, pH, etc. With an enzymatic process, the use of expensive chemicals is reduced and the more acceptable “green” route is not as energy intensive as the chemical method and is also environment friendly. Microbes in their matrix inhabit several enzymes and other substrates that act as natural capping agents, catalysts for the formation of nanoparticles with greater surface area, improved interactions with the metals (Asharani et al. 2009). The biogenic approach is observed to host many advantages when compared to the other process as it is more flexible. Nanoparticles are biosynthesized when the microorganisms seize hold of target ions from their environment and then turn the metal ions into the element metal through enzymes generated by the cell activities. This procedure is further classified as extracellular and intracellular synthesis

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Fig. 8.1 Green synthesis of nanoparticles

depending on the site of synthesis in the living cell. In the intrinsic mechanism for the formation of nanoparticles there is a requirement of ions for the enzymes and other substrates to carry out the catalytic reactions that are being transported into cell. The extrinsic mechanism on the other way requires the trapping of metal ions, reducing ions for the reaction to be carried out for the formation of nanoparticles. These nanoparticles are used in diverse fields such as targeted drug delivery, cancer treatment, gene therapy, bio-imaging, DNA analysis, as antibacterial agents, biosensors, etc. (Balaji et al. 2009) (Fig. 8.1).

8.2.1

Factors That Play a Role in Nanoparticle Synthesis

The morphological characteristics of nanoparticles can be influenced by several variable parameters such as reaction time, pH, reactant concentration, and temperature. Such parameters are crucial to understand the effect of environmental factors for the synthesis of NP as these parameters may play a pivotal role in the course of the optimization of metallic NPs synthesis by biological means.

8 Fungi-Mediated Green Synthesis of Nanoparticles and Their Renewable Energy. . .

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pH For the formation of nanoparticles the reaction medium pH plays a crucial role in the formation of nanoparticles (Gardea-Torresdey et al. 1999). The pH of the medium or the solvents used in the preparation of nanoparticles will play a major role in the attainment of the shape, size, or the surface morphology. When the pH of the medium is acidic it results in the formation of large sized particles (Dubey et al. 2010; Sathishkumar et al. 2010). Armendariz et al. (2004) synthesized gold nanoparticles of the size 25–85 nm at pH 2 from the biomass of Oat plant (Avena sativa). On increasing the pH to 3 and 4, it was observed that the size of the particles is reduced to 5–20 nm that is smaller comparatively smaller to the size obtained at pH 2 (Armendariz et al. 2004). Additionally, the extract approachability of functional groups for particle nucleation the better extract was at pH 3 or 4. In comparison to the pH 2 scarcely functional groups were obtainable prompting particle aggregation to form larger Au nanoparticles. During the synthesis of Ag nanoparticles in Cinnamon zeylanicum bark at soaring pH (pH > 5) an increased number of spherical Ag nanoparticles were synthesized (Kumar and Yadav 2009). When Cinnamon zeylanicum bark extract was used for the synthesis of palladium (Pd) nanoparticles, a slight increase was monitored in particle size at higher pH and particle size was estimated from 15 to 20 nm at pH