Biomass Valorization: Sustainable Methods for the Production of Chemicals [1 ed.] 3527347178, 9783527347179

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Biomass Valorization

Biomass Valorization Sustainable Methods for the Production of Chemicals

Edited by Davide Ravelli Chiara Samorì

Editors

Prof. Davide Ravelli University of Pavia Department of Chemistry viale Taramelli 12 27100 Pavia Italy Prof. Chiara Samorì University of Bologna Department of Chemistry “Giacomo Ciamician” via S. Alberto 163 48123 Ravenna Italy

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Contents Foreword xi Preface xiii 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7

Role of Biomass in the Production of Chemicals 1 Layla Filiciotto, Evan Pfab and Rafael Luque Introduction 1 Biomass Valorization 3 Lignocellulosic Biomass 5 Key Biomolecules 6 Solvents 10 Pretreatment of Lignocelluloses 12 Conclusions and Perspectives 15 References 15

Section I 2

2.1 2.1.1 2.2 2.3 2.4

3 3.1 3.2 3.2.1 3.2.2

Catalytic Strategies 23

Biomass Processing via Acid Catalysis 25 Iurii Bodachivskyi, Unnikrishnan Kuzhiumparambil and D. Bradley G. Williams Introduction 25 Is an Acid the Best Catalyst? 26 Acid-Catalyzed Processing of Cellulosic Polysaccharides 29 Acid-Catalyzed Processing of Lignin 44 Conclusions and Perspectives 47 References 47 Biomass Processing via Base Catalysis 57 Lichen Liu, Maria J. Climent and Sara Iborra Introduction 57 Aldol Condensation 60 Aldol Condensation of Furanic Aldehydes 60 Self-Aldol Condensation of Acetone 63

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3.2.3 3.3 3.4 3.4.1 3.4.2 3.5

Aldol Condensation Between Alcohols: Guerbet Coupling Reaction 64 Ketonization Reaction of Carboxylic Acids 65 Transesterification Reaction 68 Biodiesel Production 68 High Value-Added Chemicals from Transesterification Reactions 70 Conclusions and Perspectives 73 References 74

4

Biomass Processing via Metal Catalysis 81 Sofia Capelli and Alberto Villa Introduction 81 Synthetic Strategies for Supported Metal Nanoparticles 83 Impregnation 83 Precipitation 84 Sol Immobilization 85 Furfural 86 Furfural Hydrogenation 87 Furfural to Furfuryl Alcohol 87 Furfural to Tetrahydrofurfuryl Alcohol 88 Furfural to Pentanediols 89 Furfural to 2-Methylfuran 90 Furfural Oxidation 92 Furfural to Furoates 92 5-Hydroxymethylfurfural (HMF) 92 HMF Hydrogenation 93 HMF to 2,5-Dimethylfuran (DMF) 94 HMF to 2,5-Dihydroxymethyltetrahydrofuran (DHMTHF) 95 HMF Oxidation 96 HMF to 2,5-Furandicarboxylic Acid (FDCA) Using Monometallic Systems 96 HMF Oxidation over Bimetallic Catalysts 100 Conclusions and Perspectives 103 References 103

4.1 4.2 4.2.1 4.2.2 4.2.3 4.3 4.3.1 4.3.1.1 4.3.1.2 4.3.1.3 4.3.1.4 4.3.2 4.3.2.1 4.4 4.4.1 4.4.1.1 4.4.1.2 4.4.2 4.4.2.1 4.4.2.2 4.5

5 5.1 5.2 5.3 5.4 5.5 5.5.1 5.5.2 5.5.3

Biomass Processing with Biocatalysis 113 Roger A. Sheldon Introduction 113 Generations of Renewable Biomass: Advantages and Limitations 113 Advantages and Limitations of Biocatalysis 116 Enzyme Discovery and Optimization of Enzyme Performance 117 Enzyme Immobilization 118 Enzyme Immobilization by Cross-linking Enzyme Molecules 119 Advantages and Limitations of Cross-Linked Enzyme Aggregates (CLEAs) 120 Magnetically Separable Immobilized Enzymes 120

Contents

5.6 5.7 5.8 5.8.1 5.9 5.10 5.10.1 5.10.2 5.10.3 5.10.4 5.11 5.12

Enzymatic Hydrolysis of Starch to Glucose 121 Enzymatic Depolymerization of Lignocellulose 122 Enzymatic Hydrolysis of Cellulose and Hemicellulose 123 Magnetizable Immobilized Enzymes in Lignocellulose Conversion 124 Enzymatic Hydrolysis of 3rd Generation (3G) Polysaccharides 124 Commodity Chemicals from Carbohydrates (Monosaccharides) 126 Fermentative Production of Commodity Chemicals 126 Deoxygenation via Dehydration of Carbohydrates to Furan Derivatives 129 Polyethylene Furandicarboxylate (PEF) as a Renewable Alternative to PET 129 Enzymatic Synthesis of Bio-based Polyesters 131 Enzymatic Conversions of Triglycerides: Production of Biodiesel and Bulk Chemicals 132 Conclusions and Perspectives 133 References 133

Section II Thermal Strategies 147 6 6.1 6.2 6.2.1 6.2.2 6.2.3 6.2.4 6.3 6.4 6.4.1 6.4.1.1 6.4.1.2 6.4.1.3 6.4.1.4 6.4.2 6.4.2.1 6.4.2.2 6.4.2.3 6.4.3 6.4.4 6.4.5 6.5

Biomass Processing via Pyrolysis 149 Daniele Fabbri, Yunchao Li and Shurong Wang Brief Introduction 149 Chemicals from Cellulose Pyrolysis 151 General Aspects 151 Levoglucosan 154 Levoglucosenone 156 LAC, (1R,5S)-1-Hydroxy-3,6-Dioxabicydioxabicyclo-[3.2.1]octan-2-one 157 Chemicals from Lignin Pyrolysis 160 Pyrolysis of Biomass 161 Levoglucosan 161 Effects of Metal Oxides 162 Effects of Alkali and Alkaline Earth Metals 162 Effects of Acid Impregnation 162 Effects of Other Components 163 Levoglucosenone 163 Effects of Metal Chlorides 163 Effects of Acid Catalysts 163 Others 164 Furfural 164 Aromatic Hydrocarbons 167 Phenolic Compounds 169 Conclusions and Perspectives 170 References 171

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7.1 7.1.1 7.1.2 7.2 7.2.1 7.2.2 7.2.3 7.2.4 7.3 7.3.1 7.3.2 7.3.3 7.3.4 7.4 7.4.1 7.4.2 7.4.3 7.4.4 7.5

Biomass Processing via Thermochemical–Biological Hybrid Processes 181 Cristian Torri, Alessandro Girolamo Rombolà, Alisar Kiwan and Daniele Fabbri Introduction 181 Hybrid Thermochemical/Biological Processing with Single-Strain Microorganisms 183 Hybrid Thermochemical/Biological Processing with Microbial Mixed Consortia (MMC) 183 Pyrolysis Products (PyP) from the Microorganism’s Standpoint 185 What Pyrolysis Can Do for Microorganisms: Yields and Bioavailability of PyP 186 Viable Pathways According to Thermodynamics Laws 188 Rate of MMC Biological Conversions in Relationship with PyP Treatment 191 Toxicity of PyP Toward MMC 193 Conversion of PyP with MMC: Survey of Experimental Evidence 198 Syngas Conversion to Methane 203 Syngas Conversion to H2 , Volatile Fatty Acids (VFA), and Alcohols 203 Conversion of Condensable PyP to Methane 205 Conversion of Condensable PyP to VFA and Other Intermediates 206 Feasible Pathways for Producing Chemicals from PyP with MMC 207 Hybrid Pyrolysis Fermentation and Extraction of Mixed VFA/Alcohols 207 Alkaline Fermentation of Pyrolysis Products to VFA Salts, Ketonization, and Hydrogenation to C3–C6 Mixed Alcohols 209 Alkaline Fermentation of Pyrolysis Products to VFA Salts and Polyhydroxyalkanoates (PHA) Production via Aerobic MMC 211 Direct Alcohol Production by Means of Fermentation of PyP under High Hydrogen Pressure 213 Conclusions and Perspectives 215 References 216

Section III Advanced/Unconventional Strategies 225 8

8.1 8.2 8.3 8.4 8.4.1 8.4.1.1

Biomass Processing via Electrochemical Means 227 Roman Latsuzbaia, Roel Johannes Martinus Bisselink, Marc Crockatt, Jan Cornelis van der Waal and Earl Lawrence Vincent Goetheer Introduction 227 Electrochemical Conversion of Bio-Based Molecules 228 Conversion of Sugars 230 Conversion of Furanics 234 5-(Hydroxymethyl)furfural (5-HMF) 234 5-HMF Oxidation 235

Contents

8.4.1.2 8.4.2 8.5 8.6 8.7 8.8 8.9

5-HMF Reduction 238 Furfural 240 Conversion of Levulinic Acid 244 Conversion of Glycerol 246 Lignin Depolymerization 248 Scale-up of Electrosynthesis of Biomass-Derived Chemicals 248 Conclusions and Perspectives 254 References 254

9

Biomass Processing via Photochemical Means 265 Andrey Shatskiy and Markus D. Kärkäs Introduction 265 Fundamental Aspects of Photoredox Catalysis 266 Photochemical Valorization of Lignin 267 Strategies for Cα —Cβ Bond Cleavage 268 Strategies for Lignin Oxidation and Cβ —O Bond Cleavage 272 Strategies for Ar—O Bond Cleavage 278 Conclusions and Perspectives 281 References 282

9.1 9.2 9.3 9.3.1 9.3.2 9.3.3 9.4

10 10.1 10.2 10.3 10.4 10.5 10.6

11 11.1 11.2 11.3 11.4 11.4.1 11.4.2 11.4.3 11.5 11.5.1 11.5.2 11.6

Biomass Processing via Microwave Treatment 289 Roberto Rosa, Giancarlo Cravotto and Cristina Leonelli Introduction 289 Microwave–Matter Interaction: Advantages and Limitations in the Processing of Biomass 291 Microwave Pyrolysis 296 Microwave-assisted Hydrolysis 299 Microwave-assisted Extraction of Phytochemical Compounds 303 Conclusions and Perspectives 306 References 307 Biomass Processing Assisted by Ultrasound 315 Cezar A. Bizzi, Daniel Santos, Gabrielle D. Iop and Erico M. M. Flores Introduction 315 Ultrasound Background 316 Ultrasound-Assisted Biomass Pretreatments 319 Ultrasound-Assisted Biomass Conversion 322 Thermochemical Conversion Assisted by Ultrasound 323 Biochemical Conversion Assisted by Ultrasound 324 Chemical Conversion (Synthesis) Assisted by Ultrasound 325 Ultrasound-Assisted Extraction of Value-Added Compounds 326 Ultrasound Contribution to Biomass Extraction Processes 326 Uses of Alternative Approaches for Biomass Extractions Assisted by Ultrasound 328 Alternative Solvents 331

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11.7

Conclusions and Perspectives 332 References 333

12

Biomass Processing via Mechanochemical Means 343 George Margoutidis and Francesca M. Kerton Overview and Introduction 343 Background to the Method 343 Properties of a Typical Laboratory Mixer/Mill 346 Crystallinity Reduction in Biopolymers via Mechanochemistry 348 Mechanochemical Transformations of Polysaccharides 352 Cellulose Depolymerization 352 Cellulose Modification Toward Composite Materials 355 Transformations of Chitin 355 Mechanochemical Transformations of Amino Acids, Nucleotides, and Related Materials 357 Mechanochemical Treatment of Lignin 359 Biominerals from Mechanochemical Processing of Biomass 360 Conclusions and Perspectives 361 References 361

12.1 12.1.1 12.1.2 12.2 12.3 12.3.1 12.3.2 12.3.3 12.4 12.5 12.6 12.7

Section IV Closing Remarks 367 13 13.1 13.2 13.2.1 13.2.2 13.2.3 13.3 13.4 13.5

Industrial Perspectives of Biomass Processing 369 Tommaso Tabanelli and Fabrizio Cavani Replacing Existing Petrochemicals with Alternatives from Biomass: An Introduction 369 Oleochemical Biorefinery: A Consolidated and Multifaceted Example of Biomass Processing 371 Biofuels and Coproduced Chemicals from Oils and Fats 371 Skeletal Isomerization of Unsaturated Fatty Acids for Isostearic Acid Production 379 Bio-based Synthesis of Azelaic and Pelargonic Acids: A Renewable Route Toward Bio-based Polyesters and Cosmetics 382 From Sugar to Bio-monomers: The Case of 2,5-Furandicarboxylic Acid (FDCA) 385 From Bioethanol to Rubber: The Synthesis of Bio-butadiene 388 Conclusions and Perspectives 391 References 391 Index 411

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Foreword Why bother with biomass conversion? It seems so difficult compared to converting hydrocarbons into products. People used to think that we were about to run out of fossil fuels, but that was a red herring – there is enough left in the ground to serve us for hundreds of years. So, if running out of hydrocarbons is not the problem, then why are we trying to convert biomass? The problem is in the consequences of hydrocarbon usage, not in the depletion of hydrocarbons. Those consequences are severe. Fossil fuel resources are heavily contaminated with sulfur, mercury, and other pollutants. Many products from such feedstocks are persistent, leading to problems such as plastics in the oceans. However, most importantly, burning fossil-based fuels or incinerating fossil-based products generates carbon dioxide while the production of the feedstocks consumes no CO2 . Now, imagine a sustainable future – a future in which all of our needs are met with products made from renewable resources, and those same products are themselves recycled into new products. The feedstocks are nontoxic, the products are biodegradable, and greenhouse gas emissions are completely offset by CO2 consumption. Everyone would agree with that as a desirable goal, but we are a long way from that future. There is so much work to do before we get there. We as a society are married to our petrochemical past in so many ways, from the products we choose to make, the ways in which we make those products, and even what we teach our future chemists and chemical engineers at the universities. We need to divorce ourselves from our petrochemical past in order to bring about that sustainable future. Make no mistake about it. That divorce is going to be messy, but it is still worth doing. At first, we thought that it would be easy. Just make existing products from biomass instead of fossil fuels. It sounds so simple, but study after study has shown that the environmental impact of transforming biomass into traditional chemical products is usually more harmful than making the same products from fossil fuels. So, what are we going to do? We have to be smarter about it. Those traditional chemical products were chosen in the past because they were easy to make from hydrocarbons. Why cannot we choose new chemical products? Let us choose chemical products that are easy to make from biomass!

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Foreword

Choosing new products is not the only task on our To Do list. There is a long list of tasks ahead of us. We have to ● increase our knowledge of biomass conversions. ● evaluate new conversion processes and products in terms of potential environmental impact, not just as an afterthought but during the design stage, ● use only renewable energy in the biomass conversion processes, ● develop less energetically costly ways of removing organic products from water, given that water is often the solvent for fermentations and other standard techniques for biomass transformations, ● build up supply chains from producers of raw biomass, through transportation and conversion to platform chemicals, all the way to manufacture finished products, ● seek new sustainable feedstocks and new platform chemicals, keeping in mind their potential availability at industrial scale and the environmental impact of using such feedstocks at scale, ● design our new products so that they can be recycled efficiently, can biodegrade if discarded, and do not themselves lead to environmental crises, ● change how organic chemistry is taught at the universities so that biomass feedstocks become the norm. That sounds like a lot of work, but do not despair. There are so many talented people working on biomass conversion, such as the authors and editors of this volume, that each of these items on our “To Do” list can be achieved. The sheer variety of approaches described in the following chapters assures me that there is great hope for the future of biomass conversion. Many of you, the readers, are also developing new technologies for sustainable chemical manufacturing. We will attain that sustainable future, and this book demonstrates that we are making progress toward that goal. My commendations to Chiara Samorì and Davide Ravelli for putting this work together, to the authors for their many contributions to the book and the field, and indeed to everyone in the green and sustainable chemistry research community for their efforts in developing the chemistry that will make sustainable living a reality. August 2020

Philip Jessop Queen’s University

xiii

Preface It is nowadays apparent that the chemistry of the future will involve the exploitation of biomass-based renewable materials, the currently available stock of fossil resources being doomed to exhaustion. This transition may indeed bring about several benefits because having recourse to renewable resources should limit the impact human activities are having on climate change. Currently, the use of biomass by mankind is limited to addressing a few specific needs, notably fulfilling the feed demand and supplementing energy production in addition to the fossil fuel portfolio. The impact of these activities on the net primary production (NPP) of terrestrial and marine biomass can be accounted for by considering the human appropriation of net primary production (HANPP) parameter, which corresponds to all the human alterations of photosynthetic production in the ecosystems. This constant HANPP represents a significant fraction of the NPP and has a huge impact on ecosystems because it reduces the amount of energy available to other species, influences biodiversity, and alters water, energy, and carbon flows within food webs, also modifying the distribution of resources. In the prospected future scenario of a massive use in industry, biomass will at some point become a scarce resource, and its utilization should be considered wisely, accordingly. In particular, the entire substitution of fossil fuels with biomass for energy production purposes is unrealistic because of the huge amount of biomass that should be devoted at this end. Furthermore, one may argue if this kind of application is the best use possible of biomass, fully exploiting its characteristics in terms of chemical composition. Through history, a variety of biomass constituents have been employed in the preparation of valuable drugs, flavors, and fragrances, or to provide, especially in the second half of the nineteenth century, commodity materials such as cellulose esters (nitrate and acetate) and oxidized linseed oil (linoleum). Indeed, there exist different options of using biomass to produce chemicals. Nowadays, let apart the use of wood in the paper industry, bio-based surfactants, lubricants, coatings/dyes, additives for plastics and solvents (mostly based on vegetable oils/animal fats, sugar, or starch) are the most important applications of biomass in chemistry. As for future applications, the question is still open; however, it can be anticipated that the use of biomass for chemicals production is a much more sustainable option than having recourse to it for energetic purposes. Furthermore, in addition

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to merely duplicating existing products deriving from fossil resources, the chemistry of biomass opens the opportunity to develop a new portfolio of products, having no equivalence among those presently manufactured by classical synthetic routes from hydrocarbons. A subsidiary advantage is that the development of bioproducts requires fewer legislative constraints. Accordingly, there is an increasing interest in developing suitable techniques to tackle the valorization of biomass to produce chemicals and this area is expected to further expand in the future. Along the same line, bio-based waste materials, to be included in a circular economy perspective, can likewise contribute significantly. Independently from the actual biomass employed, this is a challenging area because inhomogeneous materials with variable composition must be processed with tailored technologies. The book “Biomass Valorization: Sustainable Methods for the Production of Chemicals” is intended to present the state of the art of the different strategies available nowadays to convert biomass into useful building blocks/commodity chemicals. Each chapter features an introductory section, detailing the core details of the described technology and showcasing the typical chemical pathways that can be activated by having recourse to it. Next, peculiar advantages and limitations of the described strategy in the processing of biomass are described. Finally, relevant examples from the recent literature are reported, with attention to the organic chemistry perspective, also indicating how the different approaches can modify and valorize the native functionalities present in the starting biomass. After an introductory section (Chapter 1), intended to set the stage and describe how biomass can contribute to the production of chemicals, the rest of the book has been organized according to the diverse approaches that can be exploited, also highlighting the potential, challenges, and innovative solutions associated with them. Biomass valorization processes have been explored using catalytic routes, including acid catalysis (Chapter 2), base catalysis (Chapter 3), metal catalysis (Chapter 4), and biocatalysis (Chapter 5). Various thermal strategies that can be applied for the valorization of biomass involve pyrolysis (Chapter 6) and thermochemical–biological hybrid processes (Chapter 7). Different advanced/unconventional strategies have also shown great promise, such as those involving electrochemical (Chapter 8) and photochemical (Chapter 9) means, microwave treatment (Chapter 10), ultrasound-assisted approaches (Chapter 11), and mechanochemical approaches (Chapter 12). As a final contribution, biomass processing from an industrial perspective is assessed (Chapter 13). There is no doubt that in the future, the production of chemicals will be based on the exploitation of biomass and the time has come to find the best methods to address this challenge and put it into practice. November 2020

Chiara Samorì, University of Bologna, Italy Davide Ravelli, University of Pavia, Italy

1

1 Role of Biomass in the Production of Chemicals Layla Filiciotto, Evan Pfab, and Rafael Luque Universidad de Córdoba, Campus de Rabanales, Departamento de Quimica Organica, Edificio Marie Curie (C-3), Ctra Nnal IV-A, Km 396, Córdoba, Spain

1.1 Introduction Chemistry is a fundamental part of everything around us. Nature is largely responsible for all of the chemistry that occurs and has been so from the dawn of time. However, societal advances and technological developments in recent years have allowed us to contribute far more chemistry than in the past. Quality-of-life improvements for major parts of the world with better food distribution, clothing, technological devices, and medical treatments have required the chemistry to progress further with detrimental unknown effects on the environment. Policies and scientists worldwide are now striving toward the development of a truly sustainable society, culminating into the implementation of the UN’s 17 Sustainable Development Goals that tackle various issues including infrastructures, education, equality, peace, and environmental protection [1]. In the active search for solutions, biomass valorization has emerged as the most viable option for a more sustainable chemical industry. The impact that sustainability could have on the chemical industry is best reflected in the magnitude of the chemical industry itself. Today, the chemical industry generates approximately $4 trillion in global sales with the production of more than 95% of all commodities [2]. One of the biggest turning points in the chemical industry, and what arguably led it to such heights, was the advent of catalytic cracking in the nineteenth century for the refining of fossil resources. Catalytic cracking allowed for most of the products we use daily to be easily sourced from petroleum [3]. Biomass valorization processes were also being explored around the same time. However, the complex nature of biomass and the wide availability of fossil resources gained all of society’s attention on the use of the latter [4]. As such, petroleum processes have been the major focus of scientists and engineers for the past two centuries. Although significant developments have been achieved considering this with higher resource efficiency and cleaner technologies, the resulting environmental concerns driven by the emissions and spills have led much attention back to renewable processes such as biomass valorization. Biomass Valorization: Sustainable Methods for the Production of Chemicals, First Edition. Edited by Davide Ravelli and Chiara Samorì. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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1 Role of Biomass in the Production of Chemicals

Biomass valorization and more sustainable practices are important steps for overturning the “disposable society” mindset where resources are viewed as infinite, cheap, and harmless. One of the most straightforward examples of this can be seen with plastic. Advances in chemistry not only created plastics but also helped notice the alarming consequences. Plastics were developed in conjunction with the advent of petroleum processes. Plastic products possess desirable characteristics (lightweight, durable, etc.) that allow for endless applications at a low manufacturing cost. Plastics found their way into daily use with things such as clothing and packaging. However, we were unequipped to properly handle this new technology. The characteristics that make plastic so appealing for a wide variety of applications (i.e. durable and heat resistant) are the same that make plastic so difficult to deal with. Its inherent non-degradability, along with extremely careless handling and littering, created a plastic waste crisis with the now widespread problem of microplastics in our oceans [5, 6]. Biomass is a more attractive feedstock that can create bio-based and/or biodegradable plastics to help overturn the drastic impact from petroleum-based products. Much initial research has focused on using biomass for drop-in solutions, i.e. plastics with the same composition and properties as the traditional ones (e.g. polyethylene [PE] and polyethylene terephthalate [PET]). However, the process chemistry limits the efficiency to sugars. On the other hand, other bio-based plastics with new properties have been developed, e.g. polyethylene furanoate (PEF) or poly-lactate (PLA). The former is a durable plastic based on furan and the latter a compostable plastic. Developing bio-based plastics that are also biodegradable – a fundamental challenge in biomass valorization – can ensure a higher sustainability at the waste management stage, as their waste is less dangerous to animals and humans (microplastics, trapped in fishing nets). However, differentiation in the lifetimes of plastics will also require the development of durable bioplastics. Accumulating plastic waste is just one of the many concerns that is helping to drive sustainable practices forward. Other concerns from the fossil-driven industrial revolution include the following: ●

●

●

Irreversible depletion of fossil fuels (i.e. oil and gas) and their detrimental environmental issues [7, 8]. Higher average temperatures and aggravation of weather conditions worldwide (e.g. heavier rains) from an increase of greenhouse gases and record levels of CO2 in the atmosphere [9]. Global population growth (>9 billion projected by 2050) leading to higher energy, food, and chemical demands [10].

These concerns require a sustainable chemical industry that embraces the concepts of green chemistry [11], circular [12] and low-carbon economies [13], and high resource efficiency [14]. As such, biomass valorization and conversion of renewable feedstocks through green processes are advancing to fully shift toward a safer and sustainable chemical industry.

1.2 Biomass Valorization

1.2 Biomass Valorization The sustainable production of chemicals and products can be achieved from conversion of biomass, an inherently renewable source. Biomass covers a wide range of bio-based resources from plants or animals. These resources include plant-based materials, biowastes, and aquatic organisms. Valorizing renewable biomass feedstocks can offer environmental benefits that include reduced emissions, safer feedstocks, better geographic distribution of resources, and achievement of a circular economy [12, 15–18]. In a circular economy, resources – such as carbon, nitrogen, and phosphorous compounds – are used with a circular “take–make–reuse/recycle” approach, as opposed to a linear “take–make–dispose” approach [12]. A closed cycle can be achieved with biomass valorization processes by recycling the generated CO2 through natural photosynthetic processes [19, 20]. This process happens particularly with biodegradable plastics. Further, the existence of nonedible and rapidly growing plants parallel to the development of high-throughput agricultural technologies can lead to a carbon-neutral cycle in short periods of time, readjusting the increased levels of CO2 emission given by the fossil industries [21]. In the context of biofuels, biomass has been subdivided in three categories given as follows along with the major evidenced drawbacks: (1) First-generation biomass: This includes all edible biomasses (e.g. sugarcane, corn, whey, barley, and sugar beet) that are composed of sucrose or starchy carbohydrates, hence relatively simple macromolecules with low recalcitrance. Biological fermentation of said sugar polymers yields bioethanol, one of the most studied drop-in biofuels with current industrial production [22]. Food-derived vegetable oils are also considered as first-generation biomass and they yield biodiesel through transesterification [23]. The main issue of this type of biomass is the clear competition with food resources (which will be continuously more precious, given the increase of world population) as well as the intensive use of water and land for the growth of said crops [24]. (2) Second-generation biomass: Nonfood raw materials, including by-products and waste materials. Generally, second-generation biofuels are produced from lignocelluloses (e.g. grasses, soft or hard wood, and forestry residues) or various wastes/by-products (e.g. agricultural: stover, wheat straw, corn cob, rice husk, and sugarcane bagasse; industrial: glycerol, grains from distilleries, and paper sludge; or urban: household and municipal solid wastes). Given the structural composition of these feedstocks (mixtures of cellulose, hemicellulose, and lignin), pretreatment is usually required for fermentation to biofuels and biochemicals, and the process economics are hindered by the use of multiple steps, leading to lower overall conversions [25–30]. The main technological challenge of these feedstocks is, in fact, the structural complexity that hinders the efficient use of the lignocelluloses as a whole, calling for pretreatments that in turn possess drawbacks depending on the method (vide infra).

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1 Role of Biomass in the Production of Chemicals

(3) Third-generation biomass: This includes nonedible feedstocks that do not require agricultural lands for their cultivation, namely, aquatic biomass, such as algae and other microorganisms (e.g. cyanobacteria). Depending on the strain, these feedstocks may contain mono/polyunsaturated hydrocarbons to produce gasoline-like fuels via cracking or higher lipid content for biodiesel applications via transesterification. When considering algae, the main issue is correlated with the high water content that hinders transportation or requires significant energy inputs or long times to dry them, whereas microorganisms require specific operating conditions. Furthermore, the economic challenges of these feedstocks limit their industrial application, given the low cultivation volumes and resource efficiency in processing [31–33]. A fourth generation of biomass is also contemplated and exemplified as modified microorganisms considered in the third generation, finally used to harvest solar energy through photosynthetic processes [34, 35]. However, these microbial species require improvements of genomics-based breeding and carry the usual concerns of modified organisms, such as unexpected microbial resistance. The available volumes of these types of biomass will play a major role in identifying the biggest driver for chemical sustainability. According to a 2018 report from the European Union (EU), the annual production of agricultural biomass (i.e. first generation) was estimated at 956 million tonnes (Mt) of dry matter of which 54% directly used for food consumption and 46% of residues (e.g. leaves and stems) partially used for animal bedding or bioenergy production. In fact, 80% of the agricultural biomass is used as food and feed, showing the limited potential of using first-generation biomass for chemicals and energy production. As it concerns third-generation biomass, in particular algae (including macro and micro), only 0.23 Mt of wet matter was estimated, corresponding to a mere 0.027 Mt of dry mass. On the other hand, the total woody biomass (above ground, second generation) was estimated at 18 600 Mt of dry weight [36]. Looking at the quantities of the different biomasses, the high availability of lignocelluloses in Europe makes them the most attractive. The >18 000 Mt of woody resources can make Europe competitive worldwide and support sustainable processes. Particularly, the efficient use of lignocelluloses and residues would improve the long-term sustainability of the chemical industry, given the volumes and little impact on the food resources, although these feedstocks still rely on forest management constraints. Other waste materials (e.g. food and municipal) are increasing in volumes, given the concomitant increase of world population and improvement of their living conditions. For example, 61 Mt of food waste are produced yearly in the EU alone [37]. However, the major challenges of these products are the variable seasonal composition as well as the implementation of a proper supply chain of these anthropological side streams to biorefineries [38]. Conversion strategies of biomass, however, generally come with low resource efficiency, causing higher production costs and limited competitiveness with the well-established petroleum market. Thus, for economic advantage, high volumes, ease of production, and limited competition with other markets (e.g. food) are required. In this sense, the use of lignocellulosic biomass may again offer a

1.3 Lignocellulosic Biomass

promising alternative to the fossil-based industry. From an energetic perspective, lignocelluloses and other waste materials possess lower energy densities compared to nonrenewable resources such as coal, oil, and natural gas. However, biopower possesses negative emissions thanks to the photosynthetic process, whereas fossil fuels cause significant life cycle greenhouse gas emissions [39]. Also, conversion of biomass to key molecules (e.g. ethanol, 2-methylfuran, and hydrogenated ethers and fatty acids) can offer biofuel diversification with various energy contents for different transport applications, including aviation; these processes rely on the separation of the different biomass components [21, 40]. From a chemical point of view, the use of lignocelluloses can offer a wide variety of platform chemicals for the synthesis of not only traditional but also new products to satisfy different areas in the chemical industry (pharmaceuticals, textiles, and materials), which are discussed in the following paragraph. A separation of bio-components will be required and explained therein.

1.3 Lignocellulosic Biomass Of all types of biomass, lignocelluloses are the most available on the planet, ranging from wood and forestry waste to straw and agricultural waste. Lignocellulosic biomass is composed of cellulose (40–50%), hemicellulose (15–20%), lignin (25–35%), and other elements (Figure 1.1). Both cellulose and hemicellulose are carbohydrate-based polymers, while lignin is an aromatic polymer. Cellulose is a linear, glucose-based polymer, making it a good source of this C6-sugar. Cellulose cross-links with hemicellulose, a branched polymer composed of different C5-carbohydrates, uronic acids, and C6-sugars. Lignin, perhaps the most irregular component of lignocellulose, is a polyaromatic macromolecule composed of phenylpropane derivatives. Lignin is mostly responsible for structural rigidity within the OH R O HO

OH

HO O

O

O

OH

R

O

O HO

n

OH

HO O

O

OH OCH3 O

HOCH2

OCH3 O

HOCH2

Lignin

O O O O

O H3CO

O

OCH3

H3CO

Softwood

Hemicellulose

n

OCH3 O

OH HOH2C

R

O

OH

O

Grasses

Cellulose

R

O

OH

OH

OH

OCH3 OH

Hardwood CH2OH

Other elements (e.g. chlorophyll, inorganic metals)

Figure 1.1

Schematic representation of the components of lignocelluloses.

5

6

1 Role of Biomass in the Production of Chemicals

lignocellulose. Further, lignocellulosic bio-feedstocks include variable quantities of pigments; terpenes; inorganic elements such as Mn, K, P, Cl, Ca, Mg, and Na, as well as Al, C, Fe, N, S, Si, and Ti to a smaller extent; and various extractives, e.g. carbohydrates, proteins, lipids, waxes, chlorophyll, terpenes, tannins, and uronic acids. An extensive and systematic review on the composition of various types of biomass shows the significant changes in the composition of these elements depending on the type of biomass [41]. Overall, lignocelluloses are made of highly oxygenated C5- and C6-derivatives. The oxygen functionalities make lignocelluloses a much different feedstock to petroleum sources that are mainly hydrocarbons. The oxygen functionalities in lignocelluloses are in some cases advantageous because they can minimize oxidation reactions, which usually have a negative environmental impact, and favor reduction reactions, which are typically milder processes and have less environmental impact. Further, the propensity to produce coke/humins and ash obliges the use of mild temperatures for these by-products’ minimization, as opposed to the traditional catalytic cracking/reforming of fossils. In fact, the presence of plenty of oxygen functionalities and low volatility tend to lead to the molecules’ decomposition at high temperatures, generating carbonaceous residues. Lignocelluloses have variable composition in their singular components depending on the plant origin. Water and inorganic residue contents also vary significantly from grass to wood. Although composition does vary significantly, biomass can source several useful compounds, including carbohydrates, aromatics, terpene, and fatty esters. These different components can be isolated and converted for use in many applications including pharmaceutical, cosmetics/perfumes, plastics, textiles, and specialty chemicals. For this, several different biomass valorization routes can be envisioned with a wide range of obtainable products.

1.4 Key Biomolecules During the first attempts of biomass valorization, drop-in energy solutions have been investigated as they could directly substitute the use of fossil resources for transportation vehicles. The most common examples are the use of bioethanol and biodiesel as additives to common automotive fuels. Bioethanol is mostly produced in industry using yeast fermentation of C6-sugars. With an increase of 25 billion gallons (roughly 75 Mt) worldwide, bioethanol is one of the most mass-produced bio-based molecules. However, starchy feedstocks (i.e. first generation) are mostly used in the production of bioethanol, causing direct competition with the food market, widespread deforestation, and concerns on the presence of enough food sources for both humans and animals [42]. Also, bioethanol has limited competitiveness with petroleum options because of low product value and relatively high price, especially when considering food sustainability. To add perspective, the price of oil would have to be above $70–80 per barrel for bioethanol to be competitive from a cost standpoint, while today, oil is at 90%) and high selectivity to monoester (∼98%), being a promising strategy for the sustainable production of fatty acid monoesters of PEG. Because of the growing production of biodiesel through transesterification, glycerol is becoming a low-cost raw chemical that can be further transformed into other value-added products [90]. In this context, the production of glycerol carbonate (GC) from glycerol by transesterification or carbonylation with urea using base catalysts is of industrial interest because GC is widely used as a solvent, an additive, and a versatile building block used in the manufacture of polymers [91]. GC can be produced by transesterification of a cyclic carbonate (ethylene carbonate) with glycerol (Scheme 3.13). For instance, Climent et al. showed that basic solids such as MgO- and MgAl-mixed oxides derived from hydrotalcites were efficient catalysts for this reaction, and among them, MgAl-mixed oxide with a Mg/Al molar ratio of 0.25 showed the highest yield of GC [92]. Furthermore, the catalytic performance could be further improved by tuning the chemical composition of the mixed metal oxide. Solid basic catalysts with stronger basicity such as LiAl- and CaAl-mixed oxides derived from hydrotalcites presented even higher activity as well as good stability for the transesterification of ethylene carbonate and glycerol. Another route for the production of GC is through the base-catalyzed carbonylation reaction between glycerol and urea. The carbonylation mechanism involves O O

Base catalyst

OH O + HO

OH

O

O HO

O

OH

O

O

OH Ethylene carbonate

GC

O

+

HO

OH

OH

Scheme 3.13 Transesterification of cyclic carbonate with glycerol for the production of glycerol carbonate.

71

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3 Biomass Processing via Base Catalysis O

OH HO

OH

Scheme 3.14

+

H2N

Base catalyst

OH HO

NH2

O

NH2

O

HO

O + 2NH3 O

O Glycerol carbamate

GC

Synthesis of glycerol carbonate by carbonylation with urea.

two steps: firstly, the nucleophilic attack of one hydroxyl group of glycerol to a carbonyl group of urea will lead to glycerol carbamate and liberate 1 mol of ammonia. The second step is the carbonylation of the glycerol carbamate intermediate into GC with the elimination of a second mol of ammonia (Scheme 3.14). In this transformation, both acid and basic sites are required, in such a way that the acid site activates the carbonyl group of the urea increasing its electrophilic character, and the conjugated basic site activates the hydroxyl group of the glycerol by abstracting a proton and increasing its nucleophilic character. Various solid base catalysts have been tested in the carbonylation reaction between glycerol and urea; among them, the ZnAl-mixed oxide derived from hydrotalcite showed high activity with highest selectivity to GC, which can be associated with its good balance in Lewis acidity and basicity [92]. In the literature, various Zn-containing solid basic catalysts have been prepared and tested for the reaction between glycerol and urea. As shown in Table 3.1, the selectivity is dependent on the catalyst composition, which is not fully addressed in the literature. One possible reason to account for such differences could be related to the structural transformation occurring with the solid basic catalyst. For instance, it has been revealed in a recent work that when using the ZnAl-mixed oxide catalyst for the reaction of glycerol and urea, homogeneous Zn complex (zinc isocyanate) is formed because of the leaching of Zn species into the liquid phase and such Zn Table 3.1 Summary of catalytic reaction performance of glycerol carbonylation with urea over several solid catalysts.

Catalysts

Zirconium phosphate

Glycerol/ urea (mol ratio)

Catalyst (%)a)

Catalyst recycling

Glycerol Conv (%)

GC Select. (%)

References

1:1

1

Yes

76

∼100

[93]

ZnAlOx

1:1

5

Yes

82

88

[92]

Au/MgO

1 : 1.5

∼1.8

Yes

81

68

[94]

Zinc glycerolates

1:1

5

n.d.

71

92

[95]

Co3 O4 /ZnO

1:1

6

n.d.

69

97

[96]

ZnO

1:1

∼5.4

n.d.

61

69

[97]

ZnAlOx

1:1

∼5.4

n.d.

82

80

[97]

p-ZnAlOx

1:1

5

n.d.

72

63

[98]

c-ZnAlOx

1:1

5

n.d.

83

70

[98]

a) wt% of the catalyst with respect to glycerol. n.d., not determined.

3.5 Conclusions and Perspectives

complex can be adsorbed by the ZnAl-mixed oxide. As a result, the reaction between glycerol and urea could proceed in a combination of homogeneous and heterogeneous pathway [99]. Later, these authors have compared the activity and selectivity of ZnAl-mixed oxide materials prepared by different methods, and they have proposed that the zinc isocyanate complex adsorbed on ZnAl-mixed oxide can selectively generate GC, while the zinc isocyanate complex in the liquid phase can produce both GC and zinc glycerolate [98]. Because of the above-mentioned leaching problem of Zn in the solid basic catalysts, development of more stable solid catalysts is still a challenge. Thus, recently, Lari et al. have reported the application of Zn-free, MgAl-mixed oxide as the catalyst for glycerolysis of urea to produce GC under continuous conditions [100]. By optimizing the catalyst composition and the reaction conditions, it is possible to obtain high selectivity (nearly 100%) and 40–60% yield of GC for more than 100 hours of operation.

3.5 Conclusions and Perspectives Biomass processing via base catalysis commonly involves reactions such as aldol condensation of carbonyl compounds, Guerbet reaction (aldol condensation between alcohols), ketonization of carboxylic acids, and transesterification of vegetable oils. Base-catalyzed C—C bond forming reactions such as aldol condensation and ketonization are important tools that allow to upgrade biomolecules to larger functionalized oxygenates. These compounds may have direct applications in the fine chemical industry or could be hydrodeoxygenated to produce hydrocarbon fuels and lubricants. Transesterification of vegetable oils with short alcohols to produce biodiesel is one of the most studied processes, which, besides to produce environmentally friendly fuel, generates high amount of glycerol. Through base-catalyzed transesterifications, glycerol and FAME produced from the biodiesel industry can be used as starting materials for the production of biobased fine chemicals such as biodegradable surfactants and glycerol carbonate with applications as a solvent, an additive, and in the manufacture of polymers. Homogeneous basic catalysts have been extensively used for these transformations. However, heterogeneous catalysts present important advantages. Besides the possibility of recovery and reuse of solid catalysts, they can be designed with the adequate basicity or combining the acid–base pairs required for a given reaction. Thus, alkali metals supported on alumina, rare earth oxides, alkaline oxides, alkaline earth oxides, transition metal oxides, and mixed oxides derived from hydrotalcites, alkali-loaded, or alkali-exchanged zeolites have been used for these transformations. However, the design of active, selective, and stable catalysts avoiding leaching and deactivation processes is still needed. Particularly promising for the production of fine chemicals will be the development of cascade processes, where several reaction steps occur in a one-pot process, avoiding separation and purification operations, with the consequent economic and environmental benefits. This might be achieved by developing catalytic systems with multiple active sites (e.g. combining basic sites with redox, metal, and acid sites). Consequently, process development along with catalyst improvement will continue to be an important research area.

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´ Ž., Lukic, ´ I., Zdujic, ´ M. et al. (2016). Assessment of CaTiO3 , CaMnO3 , 79 Kesic, CaZrO3 and Ca2 Fe2 O5 perovskites as heterogeneous base catalysts for biodiesel synthesis. Fuel Processing Technology 143: 162–168. 80 Molaei Dehkordi, A. and Ghasemi, M. (2012). Transesterification of waste cooking oil to biodiesel using Ca and Zr mixed oxides as heterogeneous base catalysts. Fuel Processing Technology 97: 45–51. 81 Woodford, J.J., Dacquin, J.-P., Wilson, K. et al. (2012). Better by design: nanoengineered macroporous hydrotalcites for enhanced catalytic biodiesel production. Energy & Environmental Science 5: 6145–6150. 82 Bennett, J.A., Wilson, K., and Lee, A.F. (2016). Catalytic applications of waste derived materials. Journal of Materials Chemistry A 4: 3617–3637. 83 Shan, R., Lu, L., Shi, Y. et al. (2018). Catalysts from renewable resources for biodiesel production. Energy Conversion and Management 178: 277–289. 84 Chen, G., Shan, R., Li, S. et al. (2015). A biomimetic silicification approach to synthesize CaO–SiO2 catalyst for the transesterification of palm oil into biodiesel. Fuel 153: 48–55. 85 Xie, J., Zheng, X., Dong, A. et al. (2009). Biont shell catalyst for biodiesel production. Green Chemistry 11: 355–364. 86 Bancquart, S., Vanhove, C., Pouilloux, Y. et al. (2001). Glycerol transesterification with methyl stearate over solid basic catalysts: I. Relationship between activity and basicity. Applied Catalysis A: General 218: 1–11. 87 Barrault, J., Pouilloux, Y., Clacens, J.M. et al. (2002). Catalysis and fine chemistry. Catalysis Today 75: 177–181. 88 Corma, A., Hamid, S.B.A., Iborra, S. et al. (2005). Lewis and Brönsted basic active sites on solid catalysts and their role in the synthesis of monoglycerides. Journal of Catalysis 234: 340–347. 89 Climent, M.J., Corma, A., Hamid, S.B.A. et al. (2006). Chemicals from biomass derived products: synthesis of polyoxyethyleneglycol esters from fatty acid methyl esters with solid basic catalysts. Green Chemistry 8: 524–532. 90 Lari, G.M., Pastore, G., Haus, M. et al. (2018). Environmental and economical perspectives of a glycerol biorefinery. Energy & Environmental Science 11: 1012–1029. 91 Sonnati, M.O., Amigoni, S., de Givenchy, E.P.T. et al. (2013). Glycerol carbonate as a versatile building block for tomorrow: synthesis, reactivity, properties and applications. Green Chemistry 15: 283–306. 92 Climent, M.J., Corma, A., De Frutos, P. et al. (2010). Chemicals from biomass: synthesis of glycerol carbonate by transesterification and carbonylation with urea with hydrotalcite catalysts. The role of acid–base pairs. Journal of Catalysis 269: 140–149. 93 Aresta, M., Dibenedetto, A., Nocito, F. et al. (2009). Valorization of bio-glycerol: new catalytic materials for the synthesis of glycerol carbonate via glycerolysis of urea. Journal of Catalysis 268: 106–114. 94 Hammond, C., Lopez-Sanchez, J.A., Ab Rahim, M.H. et al. (2011). Synthesis of glycerol carbonate from glycerol and urea with gold-based catalysts. Dalton Transactions 40: 3927–3937.

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95 Turney, T.W., Patti, A., Gates, W. et al. (2013). Formation of glycerol carbonate from glycerol and urea catalysed by metal monoglycerolates. Green Chemistry 15: 1925–1931. 96 Rubio-Marcos, F., Calvino-Casilda, V., Bañares, M.A. et al. (2010). Novel hierarchical Co3 O4 /ZnO mixtures by dry nanodispersion and their catalytic application in the carbonylation of glycerol. Journal of Catalysis 275: 288–293. 97 Fujita, S.I., Yamanishi, Y., and Arai, M. (2013). Synthesis of glycerol carbonate from glycerol and urea using zinc-containing solid catalysts: a homogeneous reaction. Journal of Catalysis 297: 137–141. 98 Nguyen-Phu, H. and Shin, E.W. (2019). Disordered structure of ZnAl2 O4 phase and the formation of a Zn NCO complex in ZnAl mixed oxide catalysts for glycerol carbonylation with urea. Journal of Catalysis 373: 147–160. 99 Nguyen-Phu, H., Park, C.-Y., and Shin, E.W. (2018). Dual catalysis over ZnAl mixed oxides in the glycerolysis of urea: homogeneous and heterogeneous reaction routes. Applied Catalysis A: General 552: 1–10. 100 Lari, G.M., de Moura, A.B.L., Weimann, L. et al. (2017). Design of a technical Mg–Al mixed oxide catalyst for the continuous manufacture of glycerol carbonate. Journal of Materials Chemistry A 5: 16200–16211.

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4 Biomass Processing via Metal Catalysis Sofia Capelli and Alberto Villa Università degli Studi di Milano, Department of Chemistry, 20133 Milano Italy

4.1 Introduction Lignocellulose materials are the most abundant renewable sources that can be used to meet our needs in terms of chemicals, food, and energy for a long-term solution. It is well known that lignocellulose is mainly composed of cellulose (30–55 wt%), hemicellulose (25–30 wt%), and lignin (25–30 wt%). The high potential of cellulosic biomass as a source of oil-sparing substances and fuels is widely reported in the scientific literature and its interest is continuously growing (Figure 4.1). Cellulose is a water-insoluble polysaccharide composed of glucose units linked by β-1,4 glycosidic bonds. Besides enzymatic hydrolysis, acid catalysis is a promising alternative way to convert cellulose, but the use of mineral acids (H2 SO4 , HCl, and HBr) is subject to environmental concerns (Chapter 2). The use of these homogeneous catalysts leads to safety and serious operational issues because of the difficulty to separate them for recycling. This problem often leads to contamination of the products and equipment damages because of their corrosive behavior. Therefore, the attention is turning to heterogeneous catalysts, which are easily separated and recovered from the reaction mixture and can be reused many times before deactivating. Among the different heterogeneous catalysts, the use of supported metal nanoparticles (NPs) is a promising route to enhance biomass conversion to chemicals [1–3]. The possibility to design and tune metal catalysts with improved activity, selectivity, and stability is opening different strategies for the valorization of biomass in the liquid phase. The catalyst design process must consider that metal NPs should not leach in the liquid phase because of the chelating behavior of the reactants (and/or the products) and the pH. Moreover, the choice of the metal and the supporting materials should be carefully made considering the corrosion under reaction conditions. Transition-metal-supported NPs are known for their capability to catalyze a wide range of chemical reactions. Their ability to promote oxidation, hydrogenolysis, hydrogenation, and decarbonylation/decarboxylation reactions makes them perfect candidates for biomass conversion. In the past decades, supported metal NPs were Biomass Valorization: Sustainable Methods for the Production of Chemicals, First Edition. Edited by Davide Ravelli and Chiara Samorì. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

82

4 Biomass Processing via Metal Catalysis OH

OH

OH

HO OH

OH

OH

OH

O

HO

OH

OH OH

O

Gluconic acid

O2

H2

OH

O2 OH

OH

Sorbitol

O

HO

OH

OH

Saccharic acid

OH H+

cellulose

HO

O

OH

O

O

H+

O H+ HO

OH

OH OH Glucose

O 5-Hydroxymethylfurfural

Levulinic acid H+

Hemicellulose

O

H+

O

H+

OH OH OH Xylose

O

OH

O

H2

OH

H2

Furfural

Furfuryl alcohol

O2

Lignin OH O

OH HO

OH

HO

OH OH Xylonic acid

OH OH Xylitol

O

O2 OH

OH

O

HO

OH OH OH Xylaric acid

Figure 4.1 A representation of various pathways for the transformation of lignocellulosic biomass to important value-added chemicals.

widely used for the conversion of biomass to furans, trying to limit the use of mineral acids [4]. Generally, particles at nano- and sub-nanometers show high surface activity because they are more disordered than in the bulk. The activity of metal-based nanocatalysts highly depends on particle morphology, composition, size and shape, and surface modification. Even though metal NPs are highly versatile, they must be used under the correct operating conditions. In fact, metal NPs are subject to different deactivation mechanisms such as sintering, leaching, and poisoning (Figure 4.2). The sintering of metal particles results in loss of active surface area, which causes an irreversible catalyst deactivation. As far as the supported metal NPs are concerned, sintering due to temperature-driven migration and coalescence of metal on support is improbable because most liquid-phase reactions are carried out up to 200 ∘ C. However, sometimes sintering of supported metal NPs occurs close to room temperature Support Sintering

Metal

Leaching

Poisoning Selective

Non-selective

Figure 4.2 Representation of phenomena resulting in the modification of supported metal nanoparticles during the reaction.

4.2 Synthetic Strategies for Supported Metal Nanoparticles

because of Ostwald ripening (atomic migration process) involving the extraction and transport of surface metal atoms by chelating molecules such as polyhydroxylic molecules [5]. In contrast, unsupported metal particles can easily sinter even at temperatures lower than 100 ∘ C. Leaching of catalyst in the reaction medium is the main cause of deactivation in liquid-phase reactions (Figure 4.2). Leaching of metal atoms depends on bulk and surface metal properties and upon reaction medium (pH, chelating properties of the molecules, and oxidation potential). Platinum metals are much more stable to leaching and can be even used both in acidic and oxidizing media. In several cases, the loss of metal from the support is mainly due to support leaching. Thus, leaching of Pd during oxidation of 2-methyl-phenoxyethanol on Pd/CaCO3 has been reported because of partial support dissolution [6]. Both Pt and Al of Pt/Al2 O3 catalyst leached in the oxidation of aldopentose solutions [7]. Instead, carbon supports are known for their very high resistance to acidic and chelating media. The total coverage of metal surface by strongly adsorbed species, such as sulfur compounds, poisons irreversibly the activity of metals under usual conditions of liquid-phase reactions. Whenever poisons are present in very low amount in the feedstock, deactivation increases steadily with time. Recent investigations and publications focused on the design and mechanism of action of multifunctional catalysts [8]. In fact, the addition of a second metal acting as a promoter of selectivity, stability, and activity is currently widely investigated. Moreover, theoretical modeling has been applied to understand the reaction mechanism of biomass-derived molecules on the surface of metal NPs [2, 9]. Because the influence of organic impurities contained in bio-sourced raw materials has a great influence on catalyst deactivation, the choice of metal and support must be wisely taken [10, 11].

4.2 Synthetic Strategies for Supported Metal Nanoparticles Supported metal NPs can be synthesized using different strategies: impregnation, precipitation and sol immobilization. Using impregnation technique, metallic NPs can be easily prepared, while a calcination step is mandatory to decompose the metal salt precursor to metallic or metal oxide NPs. Finally, precipitation method allows obtaining metal hydroxide NPs that can be reduced or oxidized to metallic or metal oxide NPs, respectively, varying the calcination conditions.

4.2.1

Impregnation

The impregnation method is a procedure where a certain amount of the metal precursor solution is added with the support (or another active phase), which in a subsequent step, is dried, and removed by calcination [12]. There are two principal impregnation methods that can be distinguished on the basis of the volume of the

83

84

4 Biomass Processing via Metal Catalysis

Evaporation

Figure 4.3

Calcination or activation

Representation of wet impregnation (WI) procedure. Metal solution addition

Metal Mixing

Drying

Volume = support pore volume

Figure 4.4

Representation of incipient wet impregnation (IWI) procedure.

precursor solution: wet impregnation (WI; Figure 4.3) and incipient wet impregnation (IWI; Figure 4.4). In WI, an excess of liquid volume is used. After a certain time, the separation of the solid from the solution is performed and the excess of the liquid is removed by drying. Conversely, IWI is a commonly used technique for the synthesis of heterogeneous catalysts. Typically, the active metal precursor (metal chloride, nitrate, sulfate, acetonate, etc.) is dissolved in water or an organic solvent. Then, the metal precursor solution is added to a support containing the same pore volume as the volume of the solution that was prepared. The solution of the precursor enters into the pores because of capillary action. After the impregnation, the catalyst can be dried and calcined to drive off the volatile components, thus depositing the metal on the surface of the catalyst. Two mechanisms are involved in impregnation methods according to the type of the method. In the IWI method, capillary action draws the solution into the pores, while in the WI method, the transport changes from capillary process to diffusion ones [13]. Impregnation methods are cheap, fast, and allow to tune the final properties of the catalytic materials. However, it is difficult to prepare high-loading catalysts and to obtain a homogeneous metal dispersion on the support.

4.2.2

Precipitation

Deposition–precipitation (DP) is one of the most widely employed catalyst preparation methods and it can be used to prepare either single-component or supported mixed catalysts. In this process, an aqueous phase of the metal salt is mixed with an alkaline solution to produce an insoluble metal carbonate and/or hydroxide. The precipitation process can be induced by a change in operating conditions such as metal salt concentration, evaporation rate, temperature, and pH. These parameters

4.2 Synthetic Strategies for Supported Metal Nanoparticles

influence the crystal growth and their aggregation, thus particles size and their distribution. The precipitation procedure involves four main steps. The first is the dissolution of the precursor of the active component in water or a suitable medium to form a homogeneous solution. The second step implies the precipitation of the metal precursor as a result of a change of pH or after the starting of an evaporation treatment. Then, the solid is dried and ground to powder form. A binder can then be added to help the anchoring of the precursor to the support and it must be carefully chosen so that it is burned and decomposed during calcination or activation. The last step (calcination or activation) converts the salt or hydroxide form into the corresponding oxide by reacting with air at a suitable temperature. The precipitation mechanism can be divided into three steps: supersaturation, nucleation, and growth. In the supersaturation region, the system is highly unstable, and precipitation occurs due to small perturbation (i.e. change in temperature or pH and solvent evaporation). During the nucleation, there is the formation of small elemental particles, which are stable at the chosen operating conditions. Finally, the particles grow (or agglomerate) because under supersaturation condition, the rate of nucleation is much higher than the crystal growth rate. Therefore, small particles are formed and amorphous precipitates can be obtained [14]. The precipitation method often leads to a wide particle size distribution and uncontrolled particle morphology, but a highly homogeneous metal distribution can be obtained.

4.2.3

Sol Immobilization

Sol immobilization (or colloidal) methods are well established for preparing small metal NPs with narrow particle size distribution and achieving high degree of dispersion of metal NPs. The main parameters that affect the NP synthesis are the NP colloidal concentration, the amount of the stabilizer, the amount of the reducing agent, and the reduction temperature. Considering catalytic reactions, capping agents (i.e. stabilizers) are considered to have a great impact on the performance of metal NPs because of the hindered access of the substrate on the catalyst surface [15]. In particular, the amount of capping agent and its chemical structure might affect the catalyst activity (i.e. acting as promoter) and/or change and eventually control the selectivity in liquid-phase reactions [16]. The sol immobilization synthesis starts with the addition of a metal salt precursor to water (or a solvent able to dissolve the precursor) (Figure 4.5). Then, the stabilizing agent in a proper amount is added. The stabilizers can be polymers or nonaromatic or aromatic heterocyclic compounds that are able to bind or coordinate the metal precursor. Then, the reducing agent is added to completely reduce the metal precursor to metal NPs. Different reducing agents can be used, such as NaBH4 , gaseous hydrogen, or organic compounds (tetrakis(hydroxymethyl)phosphonium chloride, glucose, etc.). After the reduction step, the support is introduced and the immobilization step occurs. When all metal NPs are immobilized, the catalyst is filtered and washed.

85

86

4 Biomass Processing via Metal Catalysis

Stabilizer Mn+

Reducing agent Filtration and drying Support

Figure 4.5

M

Representation of sol immobilization procedure.

Sol immobilization method is a simple method to synthesize controlled NP shape and size, and it allows to also prepare core–shell and alloy NPs. The disadvantages are the difficulty to prepare concentrated metal NP colloids and the sensibility to impurities and pH.

4.3 Furfural Furfural has been identified as one of the most promising chemicals by the American Department of Energy, and it offers great prospects for the development of sustainable processes for the production of furan derivatives [4, 17]. This compound is the precursor for a wide range of products with potential applications such as solvents, resins, biofuels, or building blocks for polymers, pharmaceuticals, and fine chemicals. Furfural can be obtained via acid treatment of agricultural wastes such as rice hulls, oat hulls, and corncobs. Their hemicellulosic component contains high levels of pentose polysaccharides, such as xylan and arabinan [18], which constitute the main precursors of furfural (Figure 4.6). Until now, the acid treatment of isolated hemicellulose is far from giving a quantitative yield because of the high activation barrier for furfural production. The average weight yield of furfural considering the dry weight of the feedstock is around 10%. Furfural is commercially produced in batch continuous digesters [19], where C5 polysaccharides are hydrolyzed to pentoses, which are further cyclo-dehydrated to furfural. Different catalysts can be used to improve the yield of the reaction, like strong inorganic acids; the high yield of the process, however, is not sufficient to overcome the drawbacks arising from the use of these compounds such as the corrosive nature or the difficult disposal, making them less appealing from an industrial scale

4.3 Furfural

CHO H3O Pentosan

+

O

H

C

OH

HO

C

H

H

C

–H3O

H

O

OH

CH2OH

Figure 4.6

Representation of the transformation of a generic pentosan to furfural.

prospective [20]. The main alternative is the production of acetic acid in situ from the biomass waste; in this case, steam is usually employed to remove the furfural from the reaction zone as a vapor. Another possibility, which attracted a lot of academic attention, concerns the use of solid catalysts [17]. For example, the treatment of xylose in a water–toluene mixture at 160 ∘ C with the use of a modified zirconia catalyst gave rise to a 45% selectivity of furfural at 95% conversion [17]. Titanate, niobite [21], and silica heteropolyacid catalysts [22] have also been investigated. Unfortunately, in each case, the selectivity to the dehydration was always limited. The best results have been obtained through the use of a microporous silica functionalized with sulfonic acidic groups with an 82% selectivity toward furfural at a 91% conversion of xylose [23]. Furfural conversion with metal NPs leads to many derivatives, which include furfuryl alcohol (FFA), furan, 2-methyl tetrahydrofuran (2-MeTHF), and methyl furoate (Figure 4.7). These derivatives can be obtained from both hydrogenation and oxidation reactions under different conditions. The metal and the support must be carefully chosen to drive the reaction toward the desired products. Metal-supported NPs are widely used for this purpose because with different synthetic methods, it is possible to tune the shape, oxidation state, coordination, and size of the catalyst.

4.3.1

Furfural Hydrogenation

Furfural hydrogenation reactions lead to the production of furan-based chemicals such as FFA, tetrahydrofurfuryl alcohol (THFA), 2-methylfuran (2-MF), furan, and tetrahydrofuran. FFA can be used in the manufacturing of foundry resins, adhesives, and wetting agents, while 2-MF is considered as a promising biofuel [24]. Hydrogenation reactions of furfural involve hydrogenation of carbonyl group to alcohol and hydrodeoxygenation (HDO) to furan. Hydrogenation reactions of furfural also involve ring-opening reactions where furfural or THFA are converted to 1,5-pentanediol (1,5-PDO) (and sometimes to 1,2-pentanediol). 1,5-PDO is an interesting chemical that can be used for the synthesis of polyurethanes and polyesters [25]. 4.3.1.1 Furfural to Furfuryl Alcohol

Hydrogenation products of furfural are by far the most important compounds because they find application in many different fields.

87

88

4 Biomass Processing via Metal Catalysis

O

O

O

OH Furan

2-Furoic acid O2

–CO2 O

O

O

O2

OR

O

OH

O

H2

ROH Furfuryl alcohol (FFA)

Furfural

Furoate H2 O

H2 O

OH 2-Methylfuran

Tetrahydrofurfuryl alcohol (THFA) H2

OH

O OH

HO 2-Methyl tetrahydrofuran

Figure 4.7

1,5-Pentanediol

HO 1,2-Pentanediol

Possible products obtained from furfural conversion.

One of the most useful furfural derivatives is FFA, which is mainly used in resins, lubricants, and fuel manufacturing and as a solvent [26]. The conversion of furfural to FFA represents a key synthetic transformation, which is attracting both academia and industry. Nowadays, 62% of the worldwide furfural production is devoted to the production of FFA [27]. The catalytic hydrogenation of furfural occurs by the selective hydrogenation of the carbonyl group (C=O) of furfural via an alkoxide intermediate producing FFA [28]. As reported in Table 4.1, different metal NPs can be used. Often, furfural hydrogenation to FFA is performed in 2-propanol (2-PrOH) as a hydrogen source, using it for hydrogen transfer mechanism. One of the most studied catalysts is copper-chromite (Cu2 Cr2 O5 ). With a copper-based catalyst at mild operating conditions, it is possible to fully convert furfural to FFA. In particular, Cu/SiO2 prepared by precipitation method is the most promising catalyst that is able to give 99.8% of conversion and an FFA yield of 98.1% at 130 ∘ C and 0.1 MPa [35]. Even Pd-based catalysts were studied, but higher pressures are required and a maximum of 70% of yield of FFA was reached. 4.3.1.2 Furfural to Tetrahydrofurfuryl Alcohol

THFA is an important downstream product of furfural, which can be used as an organic solvent, and it is widely used in the industrial and agricultural fields [39]. Because of its benign nature and low toxicity, it is also widely used as a resin in the industry. Supported Ni NPs are the most widely used catalysts for furfural and

4.3 Furfural

Table 4.1

Catalysts for the hydrogenation of furfural to furfuryl alcohol.

Catalyst

Pressure Temperature X Furfural FFA (H2 ) (MPa) (∘ C) (%) yield (%) Solvent

Preparation

10% Cu/SiO2

0.1

290

71

63

—

IWI 3.2 nm

[29]

5% Ni/SiO2

0.1

230

84

32

—

IWI

[29]

1% Pd/SiO2

0.1

230

69

14

—

IWI

[29]

Cu-Cr2

6

200

95

85

Octane

—

[27]

PtSn0.3

1

100

100

96

2-PrOH

Ion exchange [30]

PtSn0.8

1

100

71

98

2-PrOH

Ion exchange [30]

Ni34.1 Fe36 B29.9 1

100

100

100

EtOH

Sol

[31]

3% Ru/Zeolite 0.5

30

3

36

Dioxane

IWI

[32]

ReOx(1.4) /SiO2 5

250

85

42

Dodecane IWI

[33]

3% Pd/HPS

120

60

80

2-PrOH

[34]

6

WI

References

Cu/SiO2 -HDP 0.1

130

99

98

—

DP

[35]

Cu/SiO2 -EA

130

99

98

—

DP

[35]

0.1

89

Cu0.4 Mg5.6 Al2 2

110

100

100

Water

DP

[36]

Rh/KIT-6

—

100

98

99

2-PrOH

WI

[37]

Pd/TiO2

0.5

50

80

70

2-PrOH

Sol-imm

[38]

FFA hydrogenation to THFA (Table 4.2). Reactions can be performed in light alcoholic solvents such as ethanol (EtOH) and 2-PrOH, and in all the reported cases, a full conversion (>99%) with high THFA yield was obtained. Pd-supported NPs gave excellent results also at low temperature using 2-PrOH as the solvent [40]. 4.3.1.3 Furfural to Pentanediols

Terminal-diols, which have a linear carbon–carbon chain and carbons at both edges with the OH group, have been used as monomers for the production of polymers. In particular, these α,ω-diols are widely used for industrial polyester and polyurethane production. 1,5-PDO can be an alternative to these conventional terminal-diols because of the analogous molecular structure and physical properties, such as a relatively low viscosity and glass transition temperature, and good flexibility [47]. The current world capacity of 1,5-PDO production is about 3000 tons per year [47], because of limited readily accessible C5 petroleum feedstock. Another interesting diol is 1,2-PDO, which is currently produced from nonrenewable petroleum resources via a costly multistep process involving selective oxidation of pentene to pentene oxide and subsequent hydrolysis [48]. 1,2-PDO is widely used as a monomer for polyester synthesis and the key intermediate for low-toxicity antibiotics. Because of the continuous increment of demand of diols, the development of catalysts and catalytic reactions for their production from renewable resources is needed [25]. Supported noble metal NPs gave high conversion of furfural, THFA, and FFA, but Rh-based catalysts showed the best performance in terms of PDO

90

4 Biomass Processing via Metal Catalysis

Table 4.2 alcohol.

Catalysts for the hydrogenation of furfural or furfuryl alcohol to tetrahydrofurfuryl

Catalyst

Pressure Temperature THFA (H2 ) (MPa) (∘ C) Substrate X (%) yield (%) Solvent Preparation References

10% Ni/CNT

3

130

Furfural

98

83

EtOH

WI

[40]

Cu–Ni/CNT

4

130

FFA

100

90

EtOH

WI

[40]

3.2% Ru/ aectorite

2

40

FFA

100

99

MeOH

WI

[41]

10% Ni/ Ba–Al2 O3

4

140

Furfural

>99

100

Water

WI

[42]

Pd/HAP

1

40

Furfural

100

100

2-PrOH Ion exchange

[43]

Pd/TiO2

1

40

Furfural

100

89

2-PrOH Ion exchange

[43]

5% Ru/TiO2

2.7

90

FFA

95

92

MeOH

[44]

5% Ru/TiO2

2.7

90

FFA

92

80

2-PrOH DP

[44]

15% Ni/Al2 O3

4

80

FFA

99

93

EtOH

WI

[45]

Ni/SiO2

0.1

140

Furfural

99

93

—

WI

[46]

Table 4.3

DP

Catalysts for the hydrogenation of furfural to pentanediols.

Catalyst

Pressure (H2 ) (MPa)

Temperature (∘ C)

Substrate

X (%)

PDO yield (%)

Solvent

References

Rh/MoOx /SiO2

8

120

THFA

94

90 (1,5)

Water

[50]

Rh–ReOx /SiO2

8

100

THFA

96

90 (1,5)

Water

[50]

Rh–ReOx /C

8

100

THFA

99

95 (1,5)

Water

[49]

Ni/SiO2

—

140

Furfural

100

94 (1,5)

—

[46]

1.9% Pt/HT

3

150

Furfural

>99

73 (1,2)

2-PrOH

[51]

1.9/Pt/MgO

3

150

Furfural

>99

68 (1,2)

2-PrOH

[51]

PtO2 /Al2 O3

0.1

5

FFA

100

80 (1,2)

EtOH

[52]

3% Pd/MMT

3.5

220

Furfural

100

65 (1,2)

2-PrOH

[53]

4% Ru/MnOx

0.5

120

FFA

>99

24 (1,2)

Water

[48]

yield. In particular, Rh–ReOx catalyst afforded 99% conversion and 95% yield when carbon was used as a support [49]. Using Pt-based catalysts, almost full conversion of furfural was obtained, leading to the formation of 1,2-PDO in good yield using an alcoholic solvent (Table 4.3). 4.3.1.4 Furfural to 2-Methylfuran

2-MF is a liquid with high solvent power. Recently, it has been proposed as a promising biofuel additive because some of its physical properties [54, 55]. Besides, 2-MF

4.3 Furfural

is used for the synthesis of antimalarial drugs, functionally substituted aliphatic compounds, and heterocycles [56, 57]. HDO reactions of furfural to 2-MF occur mainly via hydrogenation and hydrogenolysis of C=O in parallel to some reactions that produce γ-valerolactone and tetrahydrofurfuryl alcohol. Therefore, to obtain high selectivity to the desired product, a catalytic site with excellent hydrogenation/hydrogenolysis abilities is the ideal candidate for this conversion. Generally, Cu-based catalysts are the most studied for HDO of aldehyde to methyl group, whereas the Pd-based ones show good performance in the hydrogenation of unsaturated C=O bonds or furan rings [58] (Table 4.4). Therefore, a combination of these two metals could provide high furfural conversion to 2-MF. Furfural conversion to 2-MF is mainly performed with Cu NPs supported on metal oxides. When gaseous hydrogen is used as a reducing agent, full conversion of furfural was obtained with Cu/SiO2 with 2-MF yield of 89.5%. If hydrogen is provided by the solvent (2-PrOH) via hydrogen transfer mechanism and Pd is added as a second metal, a lower yield of 2-MF was reached (≈60%). Ni–Fe- and Ni–Cu-based catalysts were also tested and the best result was obtained with 10% Ni–10% Cu/Al2 O3 , which gave 75.6% of 2-MF yield. Finally, Ir-based catalysts were tested on different supports and, when carbon was used in this role, 100% conversion with 2-MF yield of 80% was obtained (Table 4.4). Table 4.4

Catalysts for the hydrogenation of furfural to 2-methylfuran. Pressure Temperature XFurfural (MPa) (∘ C) (%)

2-MF yield (%) Solvent

10Cu-3Pd/ ZrO2

0.1 (N2 )

220

100

62

2-PrOH IWI

[59]

10Cu-3Pd/ TiO2

0.1 (N2 )

220

100

61

2-PrOH IWI

[59]

10Cu-1Pd/ ZrO2

0.1 (N2 )

220

98

64

2-PrOH IWI

[59]

5% Ni 5% Fe/SiO2

0.1 (H2 )

250

96

39

—

IWI

[60]

22.9% Cu/SiO2

0.1 (H2 )

220

100

89

—

DP

[61]

26.0% Cu/Al2 O3

0.1 (H2 )

220

99

71

—

DP

[61]

23.5% Cu/ZnO

0.1 (H2 )

220

94

61

—

DP

[61]

Catalyst

5% Pd/TiO2 0.3 (H2 ) 4% Ir/C

0.7 (H2 )

4% Ir/ZrO2

0.7 (H2 )

10% Ni 10% 1 (N2 ) Cu/Al2 O3

Preparation References

25

65

24

Octane

WI

[62]

220

100

80

2-PrOH WI

[63]

220

92

46

2-PrOH WI

[63]

210

97

76

2-PrOH IWI

[64]

91

92

4 Biomass Processing via Metal Catalysis

Table 4.5

Catalysts for the oxidative esterification of furfural. Pressure O2 (MPa)

Catalyst

Temperature (∘ C)

XFurfural (%)

Fuorate yield (%)

Solvent

Preparation

References

5% Au/CMK-3 1.5

120

99

>99

MeOH

WI

[66]

1.5% Au/CeO2

0.6

120

74

100

MeOH

DP

[67]

1.5% Au/ZrO2

0.6

120

100

100

MeOH

DP

[68]

0.4% Au/ZrO2

0.6

120

100

90

MeOH

DP

[69]

Au/TiO2

0.1

25

100

100

MeOH

Au8 Pd2 /HAP

-

120

94

99

MeOH

DP

[71]

Au8 Ag2 /HAP

-

120

90

96

MeOH

DP

[71]

1.5%Au/CeO2

0.6

120

>99

>99

MeOH

Sol-imm

[72]

4.3.2

[70]

Furfural Oxidation

Furfural oxidation reactions are not widely reported in the literature, because only recently the potentialities of the derived products were discovered. Here, we reported as an example furfural esterification to methyl furoate. 4.3.2.1 Furfural to Furoates

Oxidative esterification of furfural to furoate is attracting companies’ interest. Methyl-2-furoate is a specialty fragrance with a market value of around $50–100 kg, i.e. about 2 orders of magnitude higher than the furfural value. The main metal used for oxidation of furfural to furoates is gold. In fact, gold has been acknowledged as a very good catalyst for selective oxidation with molecular O2 when in the form of NPs [65]. Gold-supported NPs on oxides are very active catalysts under mild conditions, even below room temperature, and this feature makes them unique (Table 4.5). To have a very active catalyst, gold NP size is the key parameter ruling the catalytic performance. Corma and Serna performed a systematic study about the influence of Au particle size and content and a linear relationship between activity and total number of external gold atom was found [73]. Among all the oxide supports, zirconia showed the best catalytic performance. This oxide possesses many interesting properties such as redox properties, tuneable porosity, and surface area. Indeed, surface acidity/basicity of zirconia can be easily tuned by adding different dopants, such as sulfates. These dopants ensure higher gold dispersion because of the high surface area of ZrO2 and the presence of SO2− 4 groups plays a key role on depositing dispersed Au clusters in close contact with the support [74].

4.4 5-Hydroxymethylfurfural (HMF) 5-Hydroxymethylfurfural (HMF) is an important platform molecule that can be obtained by acid-catalyzed depolymerization of cellulose to glucose, isomerization

4.4 5-Hydroxymethylfurfural (HMF)

of glucose to fructose, and consecutive dehydration [75–79]. HMF is a versatile and multifunctional compound and it is a precursor to high-value pharmaceuticals, thermoresistant polymers, and biofuels [76, 80, 81]. HMF is a heteroaromatic molecule (furanic ring) with an aldehyde functionality and a hydroxymethyl group at the C5 position. The typical reactions that HMF can undergo are: hydrogenation, oxidation, decarbonylation, acetalization, Knoevenagel condensation, etherification, and esterification. HMF can also produce diols through the furanic ring opening by hydrogenolysis of the C—O—C bonds. In this section of the chapter, we will focus the attention to the most studied HMF transformation involving metal NPs, including oxidation and hydrogenation/hydrogenolysis.

4.4.1

HMF Hydrogenation

HMF hydrogenation can produce a variety of products, including fuels and chemicals, such as 1,2,6-hexanetriol (1,2,6-HT), 2,5-dihydroxymethylfuran (DHMF), 1,6-hexanediol (1,6-HD), 2,5-dimethyltetrahydrofuran (DMTHF), 2,5-dimethylfuran (DMF), and 2,5-dihydroxymethyltetrahydrofuran (DHMTHF; Figure 4.8). Among all products, DMF and DHMTHF are the main targets. DMF is a potential transportation fuel [82, 83]. DHMTHF finds application as a solvent because it is degradable, less toxic, and more chemically stable than other unsaturated furan compounds and acts as a building block for polymer synthesis [84–86]. For example, DHMTHF can be converted to 1,6-hexanediol, a valuable chemical for producing high-performance polyesters, polyurethane resins, and adhesives through hydrogenolysis and dehydration reactions [87]. Different metal NPs including Pt, Pd, Ru, Cu, and Ni, and solvents (e.g. methanol, 2-butanol, and water) were used [88, 89] to hydrogenate HMF. Hydrogenation reactions are performed Figure 4.8 Representation of possible products obtained via hydrogenation of HMF.

OH HO

OH

HO

+

OH

1,6 HD H2

1,2,6 HT

H+

OH

OH

OH O

H2

O O

–H2O

O

–CO

–CO O

FA

H2 –H2O

DMTHF

O

MFA H2 –H2O

O

O

MF

FMF

HMF = (Hydroxymethyl)furfural 5MF = Methyl furfural MFA = 5-Methyl furfuryl alcohol FA = Furfuryl alcohol MF = 2-Methyl furan DMF = 2,5-Dimethylfuran

O

OH H2

5MF

OH

H2

DMF

O H2

HMF

O H2

DHMF

DHMTHF

OH

OH O

DHMF = 2,5 Dihydroxymethylfuran DMTHF = 2,5 Dimethyltetrahydrofuran DHMTHF = 2,5 Dihydroxymethyltetrahydrofuran 1,6 HD = 1,6 Hexanediol 1,2,6 HT = 1,2,6 Hexantriol

93

94

4 Biomass Processing via Metal Catalysis

Table 4.6

Catalytic hydrogenation and hydrogenolysis of HMF over (bi)metallic catalysts.

Catalyst

H2 -source

Temperature (∘ C) Solvent

Time Conversion Product (hours) (%) (yield %)

References

Pd/C

Formic acid

120

THF

15

100

DMF (95)

[90]

Ru/C

Isopropanol

190

Isopropanol

6

100

DMF (80)

[91]

180

THF

3

100

DMF (96)

[92]

3

100

DMF (91)

[93]

75

DMF (75)

[94] [94]

7Ni-30W2C/ H2 (40 bar) AC Ni–Fe/CNT

H2 (30 bar)

200

Butanol

Ru/AC

H2 (20 bar)

150

2-Butanol

Ru/CNFs

H2 (20 bar)

150

2-Butanol

68

DMF (65)

Co/rGO

H2 (20 bar)

200

Ethanol

1

100

DMF (94.1) [95]

Pd–Ir/SiO2

H2 (80 bar)

2

water

4

100

DHMTHF [97] (95)

Ru/CeOx

H2 (28 bar)

130

Butanol/H2 O

12

100

DHMTHF [98] (91)

2Pd/rGO

H2 (10 bar)

20

Water

6

77.4

DHMF (92.9)

2Ru/rGO

H2 (10 bar)

20

Water

6

99.5

DHMF (93) [96]

Ru1 Pd1 /rGO H2 (10 bar)

20

Water

6

99.9

DHMTHF [96] (87)

[96]

in the presence of molecular hydrogen but also using hydrogen donor molecules such as 2-propanol, 2-butanol, or formic acid [89]. In general, Ni- and Pd-supported catalysts were selective toward products resulting from the decarbonylation of the HMF carbonyl group, whereas hydrogenolysis products were observed on Cu-supported catalysts [89]. Table 4.6 reports examples of catalytic systems used for HMF hydrogenation. 4.4.1.1 HMF to 2,5-Dimethylfuran (DMF)

DMF is formed by hydrogenolysis of C=O and C—O bonds of HMF (Figure 4.8). Various noble metals, such as Pd, Ru, Pt, Ni, Co, and Rh, have been reported in the hydrogenation of HMF toward DMF [87, 99–101]. Ru- and Pd-based catalysts are the most studied for this reaction. Ru/C using 2-propanol as a solvent produced DMF with a yield of 80% [91]. It was observed that partially oxidized Ru is effective for the selective production of DMF, suggesting a synergistic effect of the Ru metal and the Lewis acid RuOx sites on the surface of the catalyst. The presence of RuO2 oxygen vacancies favors the scission of the C—O bond [102]. Activated carbon (AC) and carbon nanofibers (CNFs) were used to investigate the effect of the support on Ru NPs for the hydrogenation of HMF in 2-butanol as a solvent [94]. It was demonstrated that, when using Ru on AC, DMF was obtained as the main product (selectivity of 75%), whereas Ru on CNFs produced alkoxymethyl furfurals (selectivity of 65%). The different selectivity was related to the different locations of the metal NPs and not to

4.4 5-Hydroxymethylfurfural (HMF)

the surface properties of the carbon as expected. To reduce the cost of the catalyst, non-noble metal NPs were also used. Co NPs supported on graphene oxide showed high activity and selectivity to DMF (94.1%) [95]. The high activity in the hydrogenolysis reaction was addressed to the strong alignment of single Co atoms/Co clusters with the graphene plane. Ohyama et al. studied the influence of the particle morphology on the activity. They reported that small Au particles (99

20

80

[112]

Au/CNF

3

60

HMF:NaOH 1:2

6

>99

94

6

[112]

Au/CNT

3

60

HMF:NaOH 1:2

6

>99

61

39

[112]

Au/graph

3

60

HMF:NaOH 1:2

6

>99

74

26

[112]

Pd–MnO2

1

100

HMF:K2 CO3 1:2

4

100

n.d.

88.1

[113]

Pt–NCs

1

100

HMF:NaHCO3 10 1:4

100

—

90

[114]

Pt–NOs

1

100

HMF:NaHCO3 10 1:4

79

48

22

[114]

Pt–NSs

1

100

HMF:NaHCO3 10 1:4

100

10

48

[114]

Au/TiO2

10

95

HMF:NaOH 1:4

4

100

88

12

[115]

Au–Cu/TiO2 10

95

HMF:NaOH 1:4

4

100

69

31

[115]

Au6 –Pt4 /AC

3

60

HMF:NaOH 1:2

3

>99

38

62

[112]

Au6 –Pd4 /AC 3

60

HMF:NaOH 1:2

2

>99

5

95

[112]

Au8 –Pd2 /AC 3

60

HMF:NaOH 1:2

3

>99

1

99

[112]

Au/PS–CNF

3

60

HMF:NaOH 1:2

2

54

53

43

[116]

Pt/PS–CNF

3

60

HMF:NaOH 1:2

2

43

73

22

[116]

Au–Pt/ HHT–CNF

3

60

HMF:NaOH 1:2

2

88

30

68

[116]

Au–Pt/ 3 P–HHT-CNF

60

HMF:NaOH 1:2

1

98

2

96

[116]

Au/HT

1

95

No base

7

100

—

>99

[117]

Au/MgO

1

95

No base

7

73

64

1

[117]

Au-Pd/CNT

5

100

No base

12

100

—

94

[118]

Au-Pd/nNiO 10

90

No base

6

95

2

70

[119]

Au-Pd/HT

60

No base

6

86

65

9

[120]

1

98

4 Biomass Processing via Metal Catalysis

O

O O

2,5-Diformyl furan (DFF) OH

O

O

O

O O

O

OH 5-Formyl-furan carboxylic acid (FFCA)

5-Hydroxymethylfurfural (HMF) OH

O

O O

HO

OH 2,5-Furan dicarboxylic acid (FDCA)

O OH 5-Hydroxymethyl-2-furan carboxylic acid (HMFCA)

Figure 4.9 5-Hydroxymethyl furfural (HMF) oxidation to 5-hydroxymethyl-2furancarboxylic acid (HMFCA) and FDCA.

CeO2 , and Mg(OH)2 as support [128]. The superior activity was attributed to the small Au particles size but also to the interaction between the hydroxyl groups in the supercage of the zeolite and the Au nanoclusters, which leads to electronic modification of the Au NPs. Au/Al2 O3 catalyst showed high activity and selectivity with a >99% FDCA yield after 4 hours by using only 4 equiv of NaOH at 70 ∘ C. Au/Al2 O3 superiority compared to similar gold-based systems might be related to the different catalyst preparation methods and, more specifically, to the stronger Au–support interaction originated. The stability/reusability study of the catalyst in five subsequent cycles shows both a slight deactivation in terms of FDCA yield (12% from the first to the fifth cycle) and an unaltered HMF conversion (always 100%). Alumina modification with ceria (20 wt%) leads to an increased FDCA yield, which is attributed to the particle size stabilization and to the ability of ceria to undergo fast Ce4+ /Ce3+ redox cycles, resulting in greater oxygen mobility [111]. The presence of a capping agent (polyvinylpyrrolidone, PVP) was demonstrated to influence the activity, modulating the metal–support interaction [129]. PVP-stabilized Ag–PVP/ZrO2 catalysts with a large particle size (>10 nm) exhibited superior catalytic activity and HMFCA selectivity compared to Ag/ZrO2 with a smaller particle size (≈8.5 nm), indicating that the addition of PVP has a promoting effect on the aerobic oxidation of HMF into HMFCA, with 97.8% conversion of HMF and 78.7% yield of HMFCA. Different studies demonstrated the beneficial use of basic oxides such as hydrotalcite and MgO in the selective oxidation of HMF to FDCA, even in base-free conditions [117, 130]. The superior performance of these supports was attributed in part to the contamination of hydrotalcites by the homogeneous base employed to synthesize the inorganic basic supports, such as Na2 CO3 /NaOH [131, 132]. In the absence

4.4 5-Hydroxymethylfurfural (HMF)

of a soluble base, competitive adsorption between strongly bound HMF and formed oxidation intermediates block the Au-active sites. The addition of NaOH restored the activity of the catalysts, liberating free gold sites able to activate the alcohol function within the metastable HMFCA reactive intermediates. The particle size effect has been demonstrated to influence the activity in the HMF conversion and, in particular, in terms of FDCA yield. Au NPs with different sizes were obtained by using different carbons, namely, activated carbon (AC), carbon nanofibers (CNFs), carbon nanotubes (CNTs), and graphite (graph) as a support [112]. TEM analyses of the Au catalysts revealed that the average particle sizes increased with decreasing oxygen content and amorphous character of the support: Au/AC (2.9 nm) > Au/CNFs (3.8 nm) > Au/CNTs (4.6 nm) > Au/graph (5.4 nm). Only Au/AC, which shows the smallest particles (20 000) polymers, which are produced using organometallic catalysts at elevated temperatures (>190 ∘ C). Such processes suffer from problems of discoloration and difficult removal of residual amounts of metals from the polymer. Hence, the sustainability of these polymers can probably be further enhanced by employing greener enzymatic methodologies for their manufacture. Polyesters [187–189] and polyamides [190] can be produced enzymatically. For example, PBS with a molecular weight of 38 000 was produced by Novozyme 435 catalyzed reaction of diethyl succinate with 1,4-butanediol at 95 ∘ C in diphenyl ether as a solvent to create a single phase [191]. Bio-based PEF and homologs can be produced by Nov435-catalyzed reaction of FDCA diesters with diols (Figure 5.10) [192]. The diol produced by hydrogenation of HMF (2,5-bis(hydroxymethyl)furan [BHMF]) can similarly be converted to polyesters by Nov435-catalyzed reaction with esters of dicarboxylic acids, including FDCA. Similarly, bio-based polyamides can be produced by the corresponding Nov435-catalyzed reactions of FDCA with bio-based diamines [193].

131

132

5 Biomass Processing with Biocatalysis O

O OH

HO

+

Nov 435

EtO

bio-1, 4-BDO

OEt O

O

PhOPh / 95 °C

O n

O PBS MW = 38 000

HO

CH2

OH n

n = 4,6,8,10,12

Figure 5.10

+ MeO

OMe O O

O

Nov 435 PhOPh / 95 °C

m

O

O O

O

CH2

O n

Enzymatic synthesis of bio-based polyesters.

5.11 Enzymatic Conversions of Triglycerides: Production of Biodiesel and Bulk Chemicals Although in this chapter we have primarily been concerned with the biocatalytic conversion of 1G, 2G, and 3G polysaccharides to biofuels and commodity chemicals, it is worth noting that triglycerides contained in biomass can be converted to a mixture of fatty acid esters and glycerol by chemocatalytic or biocatalytic methods. The former is exploited as bio-based diesel and the latter is another potential source of commodity chemicals. The production of biodiesel comprises the transesterification of a triglyceride with a primary alcohol, generally methanol or ethanol, to afford glycerol together with fatty acid methyl or ethyl esters, respectively. Biodiesel has a comparable fuel economy to petroleum-based diesel but with reduced emission of hydrocarbons, carbon monoxide, and particulates. As with polysaccharides, we can also discriminate between 1G, 2G, and 3G triglycerides. 1G triglycerides are derived from edible oil seeds and are not perceived as a sustainable option in the longer term because of direct or indirect competition with food production. 2G triglycerides comprise deliberate production of nonedible oils such as Jatropha or, preferably, valorization of waste oils and fats, e.g. waste cooking oils. 3G triglycerides are derived from algal oils and are not in competition with food production. Conventional chemocatalytic production of biodiesel involves catalysis by bases such as sodium methoxide and is an energy-intensive operation with high waste treatment costs. Moreover, the feedstock must have a low free fatty acid content because any free fatty acids will neutralize the base catalyst, resulting in catalyst losses and the formation of soap emulsions. In contrast, enzymatic trans-esterification employing lipases [194–196] is characterized by mild reaction conditions, low energy demand, low waste treatment costs, and more flexibility with regard to free fatty acids in the feedstock, as they are also esterified to biodiesel. Limitations of current lipase-based methods include the high cost of the enzymes, inactivation of the lipase by the methanol and/or impurities in the feedstock, and poor miscibility of oils and fats with methanol, resulting in heterogeneous systems. Obviously, it is very difficult for an enzyme to compete directly with sodium

References

methoxide on a cost-of-goods basis, but for a meaningful comparison, the fully integrated process costs of the two routes, including the energy consumption and costs of waste treatment and disposal, need to be considered. Yet another possibility is to use a combination of a lipase with a phospholipase, which enables the use of less-expensive unrefined, non-degummed oils by performing a one-pot degumming and transesterification, resulting in a substantial increase in efficiency [197].

5.12 Conclusions and Perspectives The costs of the hydrolytic conversion of first- and second-generation biomass to fermentable sugars are crucial factors in determining the commercial viability of the bio-based economy. They can be optimized by reducing the costs of the enzymes involved, which are currently used on a single use, throw-away basis. This can be achieved by enabling the recycling of the enzymes via immobilization. However, this can be challenging owing to the heterogeneous nature of the reaction mixture, which contains various suspended solids. A potentially attractive methodology is to produce magnetic immobilized enzymes, which can be separated from other suspended solids, on an industrial scale, using commercially available equipment. 2G biofuels are still at the beginning of the learning curve and significant cost reductions are still needed to achieve commercial viability. In this context, cost-effective immobilization and recycling of the complex enzyme cocktail involved can make an important contribution. 3G feedstocks are in an even more premature stage of development but certainly have many advantages in the context of mitigation of climate change and environmental pollution, which warrant their consideration. They can also be utilized for the production of a variety of commodity chemicals and industrial monomers.

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6 Biomass Processing via Pyrolysis Daniele Fabbri 1 , Yunchao Li 2 , and Shurong Wang 2 1

University of Bologna, Department of Chemistry “Giacomo Ciamician”, Via F. Selmi 2, Bologna, 40126, Italy Zhejiang University, College of Energy Engineering, State Key Laboratory of Clean Energy Utilization, 38 Zheda Road, Hangzhou, 310027, P. R. China 2

6.1 Brief Introduction Biomass, as the only carbon-containing renewable energy source, has the potential to replace petroleum in producing liquid fuels and chemicals [1]. In the 1920–1930s, the chemurgy movement in the United States advocated the use of biomass as a source of chemicals. It is considered that anything that can be obtained from hydrocarbons could be obtained from carbohydrates (biomass) [2]. In recent years, and have published a number of articles on the production of high-value chemicals from biomass, indicating that this approach has become an important research direction for future biomass utilization [3–7]. The trend of biomass development is to change from “electricity substitution” to “resource substitution.” The use of first-generation biomass feedstock, such as corn and edible oil seeds, is not considered a long-term sustainable option, at least in the eyes of many policymakers and the general public, as it competes directly or indirectly with food production. In contrast, second-generation bio-based high-value chemicals will utilize lignocellulosic biomass, particularly residues/wastes from forestry and agricultural activities, as a feedstock for integrated biorefineries. A significant challenge in the conversion of second-generation biomass feedstock lignocellulose to valuable additional chemicals compared to first-generation biomass feedstock (sugar, starch, and vegetable oil) is related to its structural complexity. This is due in part to the properties of the three main components of the biomass, namely [8]: (1) cellulose (35–50%), a crystalline, insoluble, linear glucose polymer linked by glycosidic bonds. Its crystallinity gives plant cell walls tensile strength; (2) hemicellulose (20–35%), a cross-linked fibrous amorphous polymer consisting mainly of different pentoses, hexoses, uronic acids, and sugar monomers usually substituted with acetoxy and methoxy groups; Biomass Valorization: Sustainable Methods for the Production of Chemicals, First Edition. Edited by Davide Ravelli and Chiara Samorì. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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6 Biomass Processing via Pyrolysis OH

OH

OH

O OH OH p-Coumaryl alcohol Coniferyl alcohol H

O

O OH Sinapyl alcohol

G

S S G H

H

Macrofibril Plant

S

G

G

H

H S

Plant cell

G

Macrofibrill

G

S

S

H G

H G

Lignin

Cell wall Lignin

10–20 nm

Hemicellulose

Pentose Hexose

n-3

Crystalline cellulose

n-3 Glucose Cellodextrin

Figure 6.1 Elsevier.

n-3

n-3 n-3 Hydrogen bond

Schematic of biomass composition. Source: From Nitsos et al. [9]. © 2013,

(3) lignin (15–30%), which gives plants structural rigidity, including polyaromatic/ phenolic polymers that are difficult to depolymerize (Figure. 6.1). The proportions of the main biomass components vary according to the types of biomass. In general, compared to wood (about 0.3–0.4 wt% inorganics, about 2–3 wt% extractives, such as waxes), herbaceous biomass usually contains a large amount of inorganics (90%). The stability of the cathode materials followed the order Cd > Pb > Zn > In [5]. The electrochemical reduction of levulinic acid to valeric acid proceeds at low pH and high overpotential, whereas at low pH and low overpotential, γ-VL is the main product [95]. Xin et al. proposed levulinic acid conversion into valeric acid or γ-VL as a means of energy storage and reported electricity storage efficiency of 70% with an energy consumption for the production of valeric acid of 1.5 kWh/L VA [95]. The authors performed the electrolysis in a polymer electrolyte membrane (PEM) electrocatalytic flow cell, obtaining a selectivity of 95% to VA adopting an acidic electrolyte (pH = 0), and a 100% selectivity to γ-VL in a neutral electrolyte (pH = 7.5) [95]. Qiu et al. reported an integrated approach for processing a mixture of levulinic and formic acids, resulting from the acid hydrolysis of cellulose. The authors selectively reduced levulinic acid in an electrochemical flow reactor on Pb electrode in an acidic electrolyte and then fed the mixture containing valeric and formic acids into a direct formic acid fuel cell to produce electricity and an aqueous solution of valeric acid [93]. The exact mechanism of LA reduction to VA is still unclear, possibly going through ECH or through another reduction pathway [5].

8.6 Conversion of Glycerol Glycerol is an abundant bio-based chemical intermediate used in the synthesis of a large number of compounds used in the industry [102]. Glycerol is produced in surplus, as it is a side product of the growing biodiesel production. Biodiesel is produced via acid- or base-catalyzed transesterification of fats and oils (triglycerides) with methanol to result in a mixture of fatty acid methyl esters (FAMEs, biodiesel) and glycerol [103]. Therefore, in recent decades, valorization of glycerol into useful products attracted significant attention [103]. Various glycerol conversion routes to

8.6 Conversion of Glycerol O OH

O

OH

OH O Dihydroxyacetone Hydroxypyruvic acid O OH Glycerol

OH

OH Glycolic acid

OH O

OH

OH

OH O

O

O

O OH

O Glyoxylic acid

OH

Mesoxalic acid O

O

OH O Glyceraldehyde OH

OH

OH

OH

OH

OH Glyceric acid

Tartronic acid

OH OH

OH O

Oxalic acid

Figure 8.17 Reaction pathways of the electrochemical glycerol oxidation. Source: Adapted from Simões et al. [119].

value-added chemicals have been proposed, such as biocatalytic fermentation [104], thermocatalytic conversion [105], photocatalytic conversion [106], hydrogenation [107], hydrogenolysis [108], dehydration [109], carbonylation [110], carboxylation [44], photo-electrochemical [111] and electrochemical conversion [10, 112–115], and many more. The electrochemical conversion of glycerol is primarily investigated for energy storage purposes, with foreseen applications in fuel cells [112, 116], and for the production of hydrogen [117] or formic acid (as hydrogen carrier) [118]. Furthermore, its electrochemical conversion can lead to the production of value-added chemicals, mainly tartronic acid (TA), dihydroxyacetone (DHA), glyceraldehyde (GLYD), glyceric acid (GLYC), and hydroxypyruvic acid (HYDP), see Figure 8.17 [114, 115]. Direct one-pot production of lactic acid from glycerol [120] or its indirect formation, via oxidation of glycerol-derived 1,2-propandiol, are promising routes toward polylactic acid [26, 121]. The main reaction pathways involved in the electrochemical oxidation of glycerol toward various products are mainly determined by the electrocatalyst used, applied potential, and pH of the electrolyte [114]. Typically, electrochemical oxidation of glycerol is performed on Pt-, Pd-, Au-, and Ni-based alloys in aqueous electrolytes [114]. As an example, Koper and coworkers reported the electrochemical oxidation of glycerol to DHA in an acidic electrolyte at 0.5–0.6 V vs. RHE. The process occurred with a 100% selectivity on a carbon-supported Pt catalyst containing Bi as an additive, which blocked the catalytic sites of Pt, active for the oxidation of primary alcohols [113]. Lee et al. reported 90% conversion of glycerol with a DHA yield of 61% on a PtSb/C catalyst in acidic media. Noteworthy, bond cleavage to form C1 and C2 products occurred at higher electrode potentials under otherwise identical conditions [122]. James et al. reported glycerol conversion in two types of electrolysis reactors, undivided, and divided cell (with an ion-selective membrane) to prevent product/reactant reactions on “opposite” electrodes [33]. During electrolysis in both types of cells, glyceraldehyde and 1,3-propanediol were reported as the main products on Pt anode and Pb or Zn cathodes in chloride containing electrolytes (NaCl, KCl, and HCl) [33]. Dai et al. reported a one-pot glycerol electrolysis to lactic acid occurring in a two-compartment cell on AuPt alloy electrocatalysts in alkaline electrolyte. The process occurred through initial formation of dihydroxyacetone, and then base-catalyzed dehydration and Cannizzaro rearrangement to lactic acid led to the final product [123].

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8.7 Lignin Depolymerization Lignin is one of the most abundant, natural organic polymers, showing a highly branched structure. It currently finds limited applications, mainly as a low-grade fuel for pulp industry [124]. The annual lignin production is estimated to be between 40 and 60 million tons, mostly as a non-commercialized waste product [125]. However, lignin’s complex structure composed of linked aromatics represents a promising opportunity for the production of liquid biofuels, chemicals, and polymers [124]. One of the most common strategies for lignin valorization is depolymerization in order to obtain the oligomer and monomer components. Depolymerization is usually performed via thermal oxidation with heterogeneous or homogeneous catalysts via photochemical oxidation or with selective enzymes [124–130]. Lignin valorization via electrochemical conversion is a promising approach because of the mild electrolysis conditions and potentially low costs. However, development of sustainable processes with high selectivities and yields in terms of valuable products is nowadays still a great challenge [124–126]. The major issues remain the low selectivity of the process and the overoxidation of target products to organic acids and CO2 [124, 128]. In general, the electrochemical lignin depolymerization is performed in alkaline aqueous electrolytes (1 M KOH or NaOH) on nickel, cobalt, Ni–Co alloys, lead oxide (IV), or platinum catalysts [128, 131]. Porous electrodes, such as wires, foams, and fleeces, are often used in batch or flow electrochemical reactors [124, 129]. The main product of interest of lignin oxidation is vanillin, which is usually obtained with yields