Circular Plastics Technologies: Chemical Recycling 9781501523281

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Katrina Knauer Circular Plastics Technologies

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Katrina Knauer

Circular Plastics Technologies Chemical Recycling

Author Katrina Knauer

ISBN 978-1-5015-2328-1 e-ISBN (PDF) 978-1-5015-1561-3 e-ISBN (EPUB) 978-1-5015-1562-0 Library of Congress Control Number: 2023947821 Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the internet at http://dnb.dnb.de. © 2024 Walter de Gruyter Inc., Boston/Berlin Cover image: Igor Rupniewski, PhD Typesetting: TNQ Technologies Pvt. Ltd. Printing and binding: CPI books GmbH, Leck www.degruyter.com

Contents List of contributing authors

VII

Katrina Knauer 1 1 Circular plastics technologies: introduction 1 1.1 Plastics – the good, the bad, and the ugly 3 1.2 Recycling – why is it so challenging? 1.3 A circular economy – Is chemical recycling the answer? 6 References

5

Katrina M. Knauer, Cody Higginson and Minjung Lee 2 Circular plastics technologies: pyrolysis of plastics to fuels and 9 chemicals 9 2.1 Introduction 11 2.2 Technical scope 26 2.3 Summary and future perspectives 27 References Katrina Knauer, Cody Higginson, Yuanzhe Liang and Minjung Lee 3 Circular plastics technologies: depolymerization of polymers into parent 33 monomers 33 3.1 Introduction 35 3.2 Technical scope 35 3.2.1 Solvolysis 48 3.2.2 Thermal unzipping into monomers 52 3.3 Summary and future outlooks 53 References Katrina Knauer and Minjung Lee 4 Circular plastics technologies: open loop recycling of waste plastics into new 59 chemicals 59 4.1 Introduction 62 4.2 Technical scope 62 4.2.1 Solvolysis 65 4.2.2 Polyolefins 68 4.2.3 Oxidation 70 4.3 Summary and future trends 71 References Index

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List of contributing authors Cody Higginson Menlo Park, CA, USA

Minjung Lee Golden, CO, USA

Katrina Knauer Golden, CO, USA

Yuanzhe Liang Golden, CO, USA

https://doi.org/10.1515/9781501515613-201

Katrina Knauer*

1 Circular plastics technologies: introduction 1.1 Plastics – the good, the bad, and the ugly Since their inception in the 1900s, plastics have become a ubiquitous, and integral, aspect of modern life. When methods and processes to make synthetic polymers, the macromolecules that make up plastics, were first introduced to the world they were truly considered a “wonder material”. You could drop a piece of plastic on the floor, and it would not shatter like glass. You could make a flexible film with excellent barrier properties to preserve food significantly longer than paper wrapping. You could replace heavy metal parts in appliances, automobiles, and electronics. Plastics were incredibly cheap, lightweight, and versatile and were viewed as entirely disposable when they first entered the market in the late 1940s. This is evidenced by a now infamous photo in the 1955 Life Magazine issue “Throwaway Living” where a quintessential 1950s family is happily throwing single-use plastics into the air and boasting that throwing away plastic utensils and plates saves “forty hours of washing-up time” [1]. This was the beginning of the plastic linear economy. A linear economy is also referred to as the take-make-waste model. This is defined by the Ellen Macarthur Foundation (EMF) as a system where resources are extracted to make products that eventually end up as waste and thrown away, typically to a landfill. The linear economy model is one in which products are seen as disposable and replaceable rather than permanent and/or recyclable. The benefits of plastics are obvious in terms of the impact plastics have had on quality of life. Plastics are strong yet light-weight and have significantly contributed to reducing emissions in automobiles and airplanes when used to replace heavier components. Plastic packaging preserves food and increases shelf life in a society where food scarcity is an ever-growing concern as populations across the globe continue to rise. Plastics like polyethylene (PE) can be sterilized via gamma irradiation and when used in medical packaging have significantly reduced hospital acquired infectious diseases. Furthermore, in addition to societal benefits and quality of life improvements, plastics often have a lower environmental impact factor, when looking at a comprehensive life cycle assessment (LCA), when compared to non-plastic alternatives such as glass, paper, metals, and wood. This was demonstrated in a study by Ahamed et al. that summarized LCA metrics comparing single use high density PE (HDPE) plastic bags and reusable polypropylene (PP) nonwoven bags to paper and cotton grocery bags [2]. The LCA concluded that paper or cotton bags demonstrated the highest negative impacts for the

*Corresponding author: Katrina Knauer, Golden, USA, E-mail: [email protected]. https://orcid.org/ 0000-0002-0125-7532 As per De Gruyter’s policy this article has previously been published in the journal Physical Sciences Reviews. Please cite as: K. Knauer “Circular plastics technologies: introduction” Physical Sciences Reviews [Online] 2023. DOI: 10.1515/psr-2023-0020 | https://doi.org/10. 1515/9781501515613-001

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1 Circular plastics technologies

impact categories including abiotic fossil depletion, freshwater-, marine- and terrestrialecotoxicities, human toxicity, acidification and eutrophication potential when compared to the single use HDPE bag and reusable nonwoven PP bag [2]. The American Chemistry Council (ACC) published a study on the potential effects of shifting packaging from plastics to alternative materials such as aluminum, glass, steel, and paper in the US and Canadian markets [3]. They summarized that converting from plastic packaging to alternatives could generate 69 % higher CO2 emissions and 390 % more solid waste by weight and consume 90 % more energy and 481 % more water to deliver the equivalent volume of product [3]. The benefits of plastics notwithstanding, the astonishing growth of plastic production and the failure to address end-of-life (EOL) issues of today’s plastics has accelerated the depletion of finite natural resources (i.e., fossil fuels), caused severe worldwide plastics pollution problems, and led to enormous energy and materials value loss in the global economy [1, 4]. The vast majority of the chemical building blocks used to make plastics (i.e., monomers), are derived from fossil fuels and are a major product of the oil and gas industry. Virgin production of plastics (i.e., converting oil into plastics) can be reduced by the adoption of a circular economy model for plastics. The EMF defines a circular economy as a model where products and materials are kept in circulation through processes like maintenance, reuse, refurbishment, remanufacture, recycling, and composting. Recycling plastics seems like a no-brainer, yet plastic recycling rates across the globe remain abysmally low, with the most recent study estimating only 9 % of all plastic waste is recycled and 22 % is mismanaged (i.e., lost to the natural environment) [5]. None of the commonly used plastics, presented in Figure 1.1, are biodegradable. Thus, near-permanent contamination of the natural environment with plastic waste is a growing concern. Jambeck et al. reported that plastic waste has been found in all major oceans, with an estimated 4–12 million metric tons of plastic waste entering the marine environment in 2010 alone [6]. Contamination of freshwater systems and terrestrial habitats is also increasingly reported, as is the environmental contamination by microfibers that are shed from washing our clothes [7]. There is so much plastic in the

Figure 1.1: A summary of the most used commodity plastics and their associated chemical structures and recycling numbers.

1.2 Recycling – why is it so challenging?

3

environment that it is has formed a discernible layer on the earth’s crust that researchers in the future will be able to use to mark the current epoch, dubbed the Anthropocene, or maybe more accurately should be called the Plastic-cene [8]. Alan Weisman left a profound impression in his book The World Without Us when he suggested that out of all of the human activities that have affected the Earth’s natural cycle/environment, plastic waste is one of the most permanent [9]. The plastic waste problem is obvious but the solution, however, is not as evident.

1.2 Recycling – why is it so challenging? The alarming images of plastic waste in our oceans and natural ecosystems has created a massive spotlight on the recycling industry and sparked a revitalization of an outdated and inefficient infrastructure. Many consumers, governments, and non-government organizations (NGOs) are now asking why the recycling rates are so low and what can be done to improve them. The current state of the art for plastic recycling is a process known as mechanical recycling (MR). This process involves the sortation of plastics into their respective chemical families shown in Figure 1.1 followed by grinding/flaking, washing, extrusion, and pelletization of the sorted plastics. Extrusion is a melt blending technique that melts down the sorted plastic and blends the various plastic pieces together to yield a homogenized product that is then pelletized and sold back into the plastic supply chain for further processing into products. An example of this process in shown in Figure 1.2 and presents the MR of polyethylene terephthalate (PET) bottles into fibers used for

Figure 1.2: An example of the MR process where PET is sorted from other waste streams such as PE and PP, then flaked, washed, and melt extruded into a polyester fiber for applications in textiles. Adapted from Ref. [10] with permission from the Royal Society of Chemistry.

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1 Circular plastics technologies

textiles [10]. While MR is a useful tool for temporarily diverting plastics from the landfill, the number of re-cycles from an MR process is severely limited due to thermal degradation/oxidation that occurs during multiple heat cycles, contaminations that are not removed in the washing/sortation cycles, and little to no color control in the final product. Even before the plastic makes it to a recycling facility, some form of degradation has already occurred due to exposure to harsh conditions during the virgin plastic processing and use-life (Figure 1.3) [11]. As a result, very few plastics are recycled back into applications of their first use-life via MR and are instead usually funneled into a new application that is more tolerable to contamination. For example, we often see food/beverage packaging plastics recycled into something like textiles (Figure 1.2), car bumpers, park benches, or flowerpots. While this is one may to funnel single use waste into something more long term, these secondary applications do not have the same volumes as most single use plastic and thus the dent in waste mitigation is minimal. Furthermore, recycling plastics into new applications is not reducing the virgin production of these materials. In other words, we are not decreasing our reliance or usage of fossil fuels to produce plastics by just MR alone. Another significant challenge with MR processes is that MR requires highly precise separation/sortation of plastics into their respective chemical families. Even as low as 5 % cross plastic contamination can be detrimental to the recycled material’s performance/properties. Some countries, specifically in the EU, require sortation at the consumer/curbside level and have separate bins for separate kinds of plastics. However, the U.S. adopted a single stream plastic collection where all plastics are comingled into a curbside collection bin and sent to a Materials Recovery Facility (MRF) for sortation. The sortation process has seen tremendous improvements over the last decade with the implementation of optical and near-infrared (NIR) sortation methods to improve efficiency and purity of the sorted streams. Additionally, some MRFs have even implemented advanced robotics-based sortation to improve the purity of sorted waste streams and eliminate human error [12]. However, even with the technological advances made towards improved sortation, sorting remains a major cost driver for recycling and limits the feedstocks that can be recycled [13].

Exposure to T, UV, chemicals, pollutants, mechanical stress, etc.

Exposure to high T’s additives

Compounder

Polyolefin Producer

Converter

Exposure to high T’s additives

Product

Exposure to T, UV, chemicals, pollutants, mechanical stress, etc.

Use Life

Exposure to T, UV, chemicals, pollutants, mechanical stress, etc.

Exposure to high T’s, water, chemicals, and additives

Disposal

MRF

Exposure to T, UV, chemicals, pollutants, mechanical stress, etc.

Recycling

Mechanically Recycled Polyolefins

Figure 1.3: Typical lifecycle of a polyolefin pellet from the polymer producer to the recycler and associated exposure to degradation sources along the supply chain. Adapted from Ref. [11] with permission from Wiley.

1.3 A circular economy – Is chemical recycling the answer?

5

1.3 A circular economy – Is chemical recycling the answer? MR is a relatively low cost and low emissions solution to recycling plastics [14]. However, the challenges with MR discussed in the previous section have limited the feedstocks that are useable in MR processes and the MR rates are not enough to mitigate the enormous flux of plastic waste being produced every day [4]. To truly achieve a circular economy for plastics, new recycling processes are needed that are more tolerable to contamination, can yield virgin quality plastic, and can recycle materials infinitely without a loss in properties. Chemical recycling can be one way to achieve this. This process differs from MR and is a terminology used broadly to include many kinds of chemical processes. Depolymerization can break down polymers into their parent monomers for conversion back into new polymers. Pyrolysis can turn mixed plastic waste into fuels or naphtha, which can be cracked into petrochemicals to make plastics. There are even emerging technologies to convert plastics into entirely new chemical building blocks than what they were in their first use life that can be “upcycled” into higher value applications. In this text, we will refer to chemical recycling as any process that breaks the polymer bonds in plastics and converts the plastic into monomers or other chemical building blocks. Since the chemical processing of plastic waste allows for recyclers to build in unit operations to remove additives and contaminants from the deconstructed liquid products streams, chemical recycling offers a solution to the contamination problems that limit MR. Furthermore, since chemical recycling is breaking down the polymer to small molecular building blocks to rebuild into new polymers, the molecular degradation that plagues mechanically recycled plastics is not a problem. The plastics industry is placing a big bet on chemical recycling as it comes after significant pressure from consumers, governments, and NGOs to better mitigate plastic waste. This is evident by the dozens of startup companies that have emerged over the last decade that are attempting to scale chemical recycling and demonstrate profitable operation. Table 1.1 summarizes some of these companies and the associated funding/investment made thus far in their respective technologies and is an example of this expanding field. This book will provide a high-level overview of emerging circular plastics technologies through chemical recycling. We break down chemical recycling into three categories: 1. Pyrolysis of plastics to fuels and chemicals 2. Depolymerization and unzipping of polymers into parent monomers 3. Open loop recycling of waste plastics into new chemicals Each chapter will highlight notable technological advancements and identify companies that are attempting to commercialize and scale processes within each category. This is not meant to serve as a comprehensive, in-depth review, but rather act as a guide

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1 Circular plastics technologies

Table .: Summary of selected chemical recycling startup companies, associated target feedstocks, and level of investment. Company

Funding/Investment (USD)

Target plastic

Purecycle Brightmark Agilyx Nexus Ioniqa Loop Industries Worn Again Circ Novoloop Grn

$. M $ M $ M $ M $. M $. M $. M $. M $ M $. M

PP PE/PP/PS PE/PP/PS PE/PP/PS PET PET PET/cotton (textiles) Pet/cotton (textiles) PE PET

through the multiple types of chemical recycling being introduced both at pilot scales and in academic literature. This text will focus primarily on the chemical deconstruction step of the recycling process and will not discuss the challenges that remain in the collection of waste plastics. This is an important note to keep in mind as a reader of this text. Even with the massive growth of chemical recycling startups across the globe, the plastic waste feedstock volumes and consistency are a significant concern for recyclers. Most economic models show that chemical recycling is only profitable at massive, industrial scales [15]. This requires an enormous amount of plastic waste feedstock, and the current collection infrastructure is not enough to sustain this. As a result, many governments and NGOs are primarily focused on how to improve plastic waste collection, make collection for recycling more accessible, and incentivize consumers to participate [16]. However, this text will only discuss the innovations associated with the chemical deconstruction step of the cycle. Separate discussions on improving consumer participation are warranted.

References 1. Gonen R. The waste-free world: how the circular economy will take less, make more, and save the Planet. New York: Penguin; 2021. 2. Ahamed A, Vallam P, Iyer NS, Veksha A, Bobacka J, Lisak G. Life cycle assessment of plastic grocery bags and their alternatives in cities with confined waste management structure: a Singapore case study. J Clean Prod 2021;278:123956. 3. Associates F. Life cycle impacts of plastic packaging compared to substitutes in the United States and Canada: theoretical substitution analysis; 2018. Available from: https://www.americanchemistry.com/ better-policy-regulation/plastics/resources/life-cycle-impacts-of-plastic-packaging-compared-tosubstitutes-in-the-united-states-and-canada-theoretical-substitution-analysis. 4. Geyer R, Jambeck JR, Law KL. Production, use, and fate of all plastics ever made. Sci Adv 2017;3:e1700782.

References

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5. Diggle A, Walker TR. Environmental and economic impacts of mismanaged plastics and measures for mitigation. Environments 2022;9:15. 6. Jambeck JR, Geyer R, Wilcox C, Siegler TR, Perryman M, Andrady A, et al. Plastic waste inputs from land into the ocean. Science 2015;347:768–71. 7. Barnes DKA, Galgani F, Thompson RC, Barlaz M. Accumulation and fragmentation of plastic debris in global environments. Phil Trans Biol Sci 2009;364:1985–98. 8. Martin C. Plastic world. Curr Biol 2019;29:R950–3. 9. Weisman A. The world without us. New York: Macmillan; 2008. 10. Li H, Aguirre-Villegas HA, Allen RD, Bai X, Benson CH, Beckham GT, et al. Expanding plastics recycling technologies: chemical aspects, technology status and challenges. Green Chem 2022;24:8899–9002. 11. Westlie AH, Chen EYX, Holland CM, Stahl SS, Doyle M, Trenor SR, et al. Polyolefin innovations toward circularity and sustainable alternatives. Macromol Rapid Commun 2022;43:2200492. 12. AMP robotics launches automated secondary sortation facilities in Atlanta and Cleveland. Available from: https://www.amprobotics.com/news-articles/amp-robotics-launches-automated-secondary-sortationfacilities-in-atlanta-and-cleveland. 13. Fogt Jacobsen L, Pedersen S, Thøgersen J. Drivers of and barriers to consumers’ plastic packaging waste avoidance and recycling – a systematic literature review. Waste Manag 2022;141:63–78. 14. Uekert T, Singh A, DesVeaux JS, Ghosh T, Bhatt A, Yadav G, et al. Technical, economic, and environmental comparison of closed-loop recycling technologies for common plastics. ACS Sustainable Chem Eng 2023;11: 965–78. 15. Tullo AH. Plastic has a problem; is chemical recycling the solution. Chem Eng News 2019;97:39. 16. March A, Roberts KP, Fletcher S. A new treaty process offers hope to end plastic pollution. Nat Rev Earth Environ 2022;3:726–7.

Katrina M. Knauer*, Cody Higginson and Minjung Lee

2 Circular plastics technologies: pyrolysis of plastics to fuels and chemicals Abstract: Pyrolysis technologies are a staple in plastic chemical recycling because of the robustness to contamination and existing infrastructure. Pyrolysis is already considered to be a reasonably mature technology with numerous pilot plants operating to pyrolyze plastic waste into fuels and chemicals. This chapter will describe the pyrolysis process and important process parameters, the types of plastics that are suitable for pyrolysis recycling, the mechanism of pyrolytic degradation of various plastics, the products derived from different plastics, companies that have successfully scaled pyrolysis recycling, and recent innovations in the technology. keywords: circularity; degradation; fuels; plastic; pyrolysis; thermolysis.

2.1 Introduction One of the fastest scaling and expanding areas in plastic recycling is the conversion of waste plastics to petrochemicals and refined hydrocarbons via pyrolysis. Pyrolysis of plastics (also known as thermolysis or polymer cracking) has been a potential route for plastic waste management since the 1980s but has seen significant growth and expansion over the last five years [1]. Pyrolysis can be simply defined as the degradation of polymers at high temperatures in the absence of oxygen to yield oils consisting of gaseous and liquid hydrocarbon fractions. In other words, plastics can be transformed back into something like the crude oil that was originally pumped from the ground and converted into hydrocarbons in an oil refinery. In the three loops of plastic recycling outlined by the Ellen MacArthur Foundation (EMF), pyrolysis would fall in the molecular loop where the polymer backbone is broken down to a molecular level disparate from the parent monomers and further chemistry is required before repolymerization back into the original polymer is possible (Figure 2.1) [2]. Pyrolysis is an energy intensive, robust process that can accommodate common contaminations associated with post-consumer plastic waste such as food, dirt, aluminum laminates, printing inks, oil residues, and more [3]. Because of the high

*Corresponding author: Katrina M. Knauer, National Renewable Energy Laboratory, 15013 Denver W Pkwy, 80401-3393, Golden, CO, USA, E-mail: [email protected]. https://orcid.org/0000-0002-0125-7532 Cody Higginson, Novoloop, Inc. 3487 Edison Way Ste. Q, Menlo Park, CA 94025, USA Minjung Lee, National Renewable Energy Laboratory, 15013 Denver W Pkwy, 80401-3393, Golden, CO, USA. https://orcid.org/0000-0003-1687-897X As per De Gruyter’s policy this article has previously been published in the journal Physical Sciences Reviews. Please cite as: K. M. Knauer, C. Higginson and M. Lee “Circular plastics technologies: pyrolysis of plastics to fuels and chemicals” Physical Sciences Reviews [Online] 2023. DOI: 10.1515/psr-2022-0175 | https://doi.org/10.1515/9781501515613-002

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2 Pyrolysis of plastics to fuels and chemicals

Figure 2.1: The EMF has broken down plastic recycling into three loops: (1) the polymer loop where the polymer backbone remains unchanged (i.e., mechanical recycling); (2) the monomer loop where polymer is broken down (or “unzipped”) back into the parent monomer that can be directly repolymerized back into the original polymer; and (3) the molecular loop where the polymer backbone is broken down into useful chemicals that can be further processed into monomers or used as fuel (i.e., pyrolysis or gasification).

temperatures, even highly contaminated plastics can be processed without difficulty by pyrolysis [3]. Historically, pyrolysis has been employed at the large scale primarily as either a “waste-to-fuels” process or “waste-to-energy” process when combined with gasification (discussed later in this chapter). However, the current momentum towards a circular economy for plastics has shifted the focus to a “waste-to-plastics” pathway that involves converting the pyrolysis oil into monomers or other chemicals to be used for plastic remanufacturing [1]. Pyrolytic recycling of plastic waste has already been achieved on a commercial scale with new developments to improve the process such as microwaves and catalysts showing great commercial potential. Major polymer producers all around the world have announced strategies that plan to use the output of pyrolysis processes to utilize plastic waste as a raw material to remake plastics [4]. These commitments are likely reactions to the ambitious goals being set by consumer facing companies such as CocaCola, PepsiCo, and Unilever to incorporate up to 50% recycled material in their packaging in the next 5–10 years despite low volume outputs from mechanical recycling [4].

2.2 Technical scope

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Plastic films and flexibles that are used in many types of packaging are very difficult to recycle via conventional mechanical recycling pathways used today as they are not compatible with existing processing equipment (i.e., easily jams during sorting or in hoppers, feeders, and extruders). This limitation, coupled with the rising demand for recycled materials, has led to a rapid growth and expansion of the pyrolysis industry along with new innovations. This chapter will describe the pyrolysis process and important process parameters, the types of plastics that are suitable for pyrolysis recycling, the mechanism of pyrolytic degradation of various plastics, the products derived from different plastics, companies that have successfully scaled pyrolysis recycling, and recent innovations in the technology.

2.2 Technical scope Pyrolysis Thermal pyrolysis Process parameters Thermal degradation of macromolecules via pyrolysis can be accomplished in the presence of a catalyst (catalytic pyrolysis) or without (thermal pyrolysis) [5]. Thermal pyrolysis of plastics involves the decomposition of polymeric materials by means of high temperatures when it is applied in oxygen-free conditions. Since pyrolysis relies on the endothermic breaking of covalent bonds the supply of heat is essential to efficiently decompose the material [6]. Other key process parameters that influence the end products from pyrolysis are temperature, reactor type, pressure, residence time, and catalyst loading and type [7, 8]. Each of these parameters can be tailored to optimize the product yield and composition as well as reduce the energy requirements. However, temperature and reactor type have the most significant impact on product selectivity [9]. Temperature. Temperature is probably the most significant operating parameter in pyrolysis recycling since it controls the cracking reaction of the polymer backbone. The pyrolysis temperature range can be broad, between 300 and 900 °C for most plastics, and depends on a myriad of factors [10]. When the temperature is high enough, the energy will overcome the enthalpy of the C–C bond and result in a broken bond [11]. Temperature settings are based on the type of polymer being pyrolyzed and the preferred product composition/distribution. Typically, the pyrolysis temperature is dictated by the onset of degradation of the polymer which can be determined by thermal gravimetric analysis (TGA). Pyrolysis, for most plastics, begins at ∼300 °C or earlier depending on the polymer structure and types/concentration of heteroatoms [6]. Typically, polymers with robust carbon-carbon bonds, such as polyolefins, require

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2 Pyrolysis of plastics to fuels and chemicals

Table .: Summary of pyrolysis temperatures, products, and downstream applications for common plastics. Polymer

Polyethylene (PE)

Structure

Pyrolysis tem- Pyrolysis perature (°C) product

Downstream applications

–a Waxes, paraf[, ] fins, olefins –b [–] – [, ]

Oil refinery feedstock for conversion to diesel fuel or chemicals

– Waxes, paraf[, ] fins, olefins

Oil refinery feedstock for conversion to diesel fuel or chemicals

Polypropylene (PP)

Polystyrene (PS)

Nylon-

– Styrene, styrene [, , ] oligomers

– [] ε-caprolactam  [] – []

Monomers

Monomers

Polymethyl methacrylate (PMMA)

– Methyl [–] methacrylate

Polyethylene terephthalate (PET)

– [] Benzoic acid, vinyl terephthalate, aldehyde

Upgrading to chemicals or fuels

Polyurethane (PUR)

– [] Benzene, – [] methane, ethylene, NH, HNC

Upgrading to chemicals or fuels

Polyvinyl chloride (PVC)

– [] HCl, benzene

Upgrading to chemicals or fuels

a

Monomers

High density polyethylene (HDPE), blow density polyethylene (LDPE).

higher energy inputs and temperatures than other common plastics. Table 2.1 summarizes reported temperature ranges required for the pyrolysis of the most common plastics found in municipal solid waste (MSW). The pyrolysis temperature can also be influenced by the types of plastic additives in the feedstocks. All plastic formulations contain a complex concoction of additives, such as stabilizers, plasticizers and/or pigments [12]. Most additive types will evaporate during pyrolysis, but certain additives

2.2 Technical scope

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can affect the kinetics and mechanism of degradation and require additional tuning to the temperature [6]. Furthermore, additives that contain inorganic molecules can result in char formation and reactor fouling over time. The selected operating temperature also depends on the target products and distribution. In general, when the pyrolysis temperature is high, there is increased production of noncondensable gaseous fractions and lower liquid yields, such as diesel. Higher temperatures increase the yield of hydrogen, methane, acetylene, aromatics, and soot, whereas lower temperatures favor generation of aliphatic compounds and liquid products [13]. Therefore, if a gaseous or char product (e.g., carbon black) is preferred, higher temperatures >500 °C are suggested. If a liquid, fuel-like product is preferred instead, lower temperatures in the range of 300–500 °C are recommended, and this condition is applicable for all plastics [8]. When targeting diesel-like products, the optimal operating range for most common plastics is 390–425 °C according to the review by Sheirs et al. [3]. The high temperature requirements associated with pyrolytic recycling are one of the major limitations considered when scaling the technology as higher temperatures mean higher costs and greenhouse gas (GHG) emissions. Current innovations in pyrolysis focus on strategies to lower the required temperatures for plastic recycling and will be discussed in a later section. Reactor type. After temperature, the second most important parameter to consider when designing a pyrolysis process is the reactor type. A pyrolysis process design typically consists of a feeding section, reactor unit, and product collection vessels that contain downstream separation lines for product recovery and purification (Figure 2.2) [30–32]. Reactor type directly affects mixing of the plastics, quality of heat transfer, and final product yield in the pyrolysis process and requires configurations that have excellent mass and heat transfer characteristics. The type of catalyst that can be used in the process also depends on the reactor type applied. Several kinds of reactors can be used for pyrolysis including those involving high heat and mass transfer rates such as fluidized beds (bubbling and catalytic), autoclaves, melting vessels, plasma reactors, and unique designs to enable vacuum pyrolysis [33]. Similarly, it is also possible to find rotating cone [34], cyclonic reactors [35], and ablative process reactors [36] used in pyrolysis processes. At the commercial scale, the most common types of reactors used are fluidized bed reactors (FBRs), fluid catalytic cracking reactors (FCCRs), and screw/Auger kiln reactors (Figure 2.3) [5, 37, 38]. Emerging technologies are also exploring microwave assisted reactor designs and continuous extrusion designs for pyrolysis processes to lower energy inputs and increase throughput. The various reactors/configurations listed thus far have advantages and disadvantages in terms of technical and economic parameters. The literature contains many reports comparing the operation and performance of pyrolysis reactors [39–46]. Each configuration comes with benefits and trade-offs, as to be expected. Some known advantages and disadvantages of pyrolysis reactors identified in the literature are summarized in Table 2.2. The remainder of this section will focus on the most common reactor types used at the commercial scale.

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Figure 2.2: Schematic of a small-scale pyrolysis pilot plant. The configuration includes (1) transportation of the waste to the facility; (2) selective collection and sortation of target feedstocks for pyrolysis; (3) shredding of the plastic; (4) washing; (5) drying; (6) waste storage; (7) catalyst storage; (8) reactor unit; (9) heating gas storage; (10) separation unit; and (11) catalyst filter. Reprinted with permission from miskolczi et al. (Source: Miskolczi 2009) [32].

Figure 2.3: Common reactors used for pyrolytic recycling of plastics: (A) fluidized bed reactor; (B) fluid catalytic cracking reactor; and (C) screw/Auger kiln reactor.

Fluidized bed reactors. FBRs (Figure 2.3A) are the most widely used reactor type for plastic pyrolysis [8, 46]. In this type of reactor configuration the plastic waste is melted (i.e., converted to a “fluid”) and passed through a solid granular material (sand or solid

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15

Table .: Comparing advantages and disadvantages of common pyrolysis reactors (red = poor; yellow = satisfactory; green = good) [, –]

catalyst) at high enough speeds to suspend the solid and yield fluid-like properties [47]. This process, known as fluidization, imparts many important advantages to an FBR over other pyrolysis reactors. FBRs are characterized by excellent heat and mass transfer rates, resulting in consistent temperature control throughout the reactor and highly uniform products [31]. In a catalytic FBR, the catalyst sits on a distributor plate and is well mixed with the fluid in the bed providing a large surface area for reaction to occur [48]. From an economic point of view, FBRs are very attractive as they have low operating costs, can be operated in continuous mode, and the catalysts (when used) can be recovered and recycled in the process several times [8, 10]. Some disadvantages to using FBRs can be the complexity in the reactor design, solid fraction attrition, and bed defluidization which can lead to frequent shutdowns [49]. Solid fraction attrition is attributed to the solid/solid collisions in the hot solid media leading to attrition phenomena and producing fine particles in the resulting solid fraction [50]. These small particles hinder the separation process resulting in higher solids concentrations in the liquid fraction. This promotes aging, erosion, blockage, and combustion problems in the FBR design [41]. Regardless of the challenges associated with FBRs, they are considered robust and scalable reactors that are widely employed in demonstration and commercial pyrolysis plants [46]. An example of a successful commercial pyrolysis operation using an FBR is Recycling Technologies in the United Kingdom (UK). Recycling Technologies claim they can process up to 20 tons per day (TPD) of plastic waste into Plaxx™, a low sulfur heavy fuel oil, via their FBR pyrolysis configuration. Another example is the BP process that used an FBR to pyrolyze up to one TPD of plastic waste into light and heavy waxes, although this plant is no longer operational.

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2 Pyrolysis of plastics to fuels and chemicals

A variant of the FBR is the conical spouted bed reactor (CSBR) that is designed with more intense mixing in the bed to prevent defluidization events and reduce shutdowns. While CSBRs have not been demonstrated on a large scale, this type of reactor has been applied for the thermal pyrolysis of polyolefins on a laboratory scale, reporting very high yields of wax up to 80 wt% [51]. Other studies have reported successful conversion of polyolefin feedstocks to gasoline and diesel range products [52–54]. Fluid catalytic cracking reactors. FCCRs are common reactors found in conventional refineries and have historically been applied to convert vacuum gas oil (VGO) to gasoline. A primary challenge in achieving a continuous process for plastic pyrolysis is the continuous feeding of the feedstock in solid form into the reactor [55]. Fluid catalytic cracking reactors (FCCR) offers a solution to this problem by dissolving the plastic feed in a suitable solvent, followed by pyrolysis of the solution [55–57]. A common FCCR configuration is outlined in Figure 2.3B. First, a hot particulate catalyst is contacted with the dissolved plastic feedstock, creating cracking products and a coked catalyst. The plastic is cracked into gaseous components and separated to fuel gas [31]. The coked catalyst is separated, stripped of residual oil products, and regenerated by burning the coke. The hot catalyst is then recycled to the riser for additional cracking [58]. FCCRs are characterized by good solid polymer mixing, but some disadvantages associated with this type of reactor are high energy requirements (compared to other pyrolysis reactors), the costs associated with use of solvent, and very dilute feed streams [59]. One example of a large-scale operation using an FCCR is Reentech in South Korea. This process applies an FCCR pyrolysis configuration cracking of plastic wastes [3]. Screw/auger reactor. Screw/Auger Kiln reactors are a relatively simple design that overcome some of the problems of conveying heat for pyrolyzing plastics. Auger reactors are typically made up of a tubular reactor and a screw conveyer as shown in Figure 2.3C. The screw is used to convey a single feedstock or a blend with solid heat carriers down the length of a reactor tube. This is a continuous process where the residence time can by varied by the varying the speed of the screw and length of the reactor [60]. Typically, metal or ceramic spheres are added to the conveyer line that help avoid coke build-up and improve the heat transfer during the pyrolysis process. Some key benefits to using an Auger reactor for pyrolysis is the continuous operation, ability to purge through the length of the reactor (i.e., efficient chlorine removal), and selective zone heating [60]. Auger reactors have been historically used to pyrolyze tires and thermoset based waste electrical and electronic equipment (WEEE). Two wellknown examples of commercial scale use of screw/Auger reactors to pyrolyze plastic waste are Agilyx and Renewlogy. Agilyx is currently operating a pilot facility using a continuous, dual screw reactor design to process 10 TPD of plastic waste. Renewlogy is also operating a pilot plant facility at 10 TPD using an Auger/kiln reactor design. Brassard et al. recently reviewed auger reactor designs and the effect of various operational parameters on their performance [61].

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Pressure and residence time. Pressure and residence time are both temperature dependent factors that can influence product distribution and yield from plastic pyrolysis. Typically, in a pyrolysis process, applying higher pressures can shift the molecular weight of the product distribution and shifts the product yields to more gaseous products. However, this trend is only apparent at high temperatures and shows less impact at lower temperatures. The residence time can be defined as the average amount of time that the molten or dissolved plastic spends in the reactor [62]. Shorter residence times will yield longer carbon products and longer residence times will increase the overall yield of lighter hydrocarbons and noncondensable gases [63]. Studies applying auger reactors in pyrolysis processes have highlighted the importance of short residence time for obtaining a high yield of light olefins [3]. However, this affect is also temperature dependent as seen by Mastral et al. who studied the effect of residence time and temperature on product distribution of HDPE thermal cracking in an FBR. It was found that a higher liquid yield was obtained at longer residence times when the temperature was less than 685 °C. However, at higher temperatures, the residence time had less influence on the liquid and gaseous yield [62]. Degradation mechanisms and products The pyrolytic degradation mechanisms of plastics depend on the parent polymer. There are four types of degradation mechanisms that can occur: random-chain scission, endchain scission, chain-stripping, and crosslinking. These different mechanisms are related to the bond dissociation energies, the chain defects of the polymers, the aromaticity, and the presence of halogen and other hetero-atoms in the polymer chains. The products obtained from pyrolysis vary based on the parent polymer and are summarized for common plastics in Table 2.1. This section will discuss the different types of plastics that can be recycled by pyrolysis and their respective products. Additionally, companies that have demonstrated pilot scale pyrolysis recycling operations are summarized in Table 2.3 along with the respective primary feedstocks and products. Polyolefins. Of the most common waste plastics, polyolefins are probably the most ideal feedstocks for pyrolysis recycling due to their lack of hetero-atoms. Polyolefins include high density polyethylene (HDPE), low density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and polypropylene (PP). In high temperature environments, polyolefins degrade via random chain scission of carbon-carbon bonds where the polymer is broken randomly along the backbone into smaller molecules of varying chain lengths. As an example, the pyrolytic degradation mechanism of HDPE is outlined in Figure 2.4. Initiation consists of homolytic breaking of the carbon–carbon bonds by random chain scission, resulting in the formation of two radicals. This is followed by depropagation and the release of olefinic monomeric fragments from primary radicals. This mechanism perpetuates with hydrogen chain transfer occurs leading to the formation of olefinic species and polymeric fragments. Moreover, secondary radicals can also be formed from hydrogen abstraction through an intermolecular transfer reaction

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Table .: Select pyrolysis companies, primary feedstocks, and primary products as listed on company websites. Company

Primary feedstock

Primary product

Brightmark Energy (formerly RES Polyflow)

HDPE LDPE PP PS HDPE LDPE PP PS HDPE LDPE PP PS PS HDPE LDPE PP PS HDPE LDPE PP PS NAa HDPE LDPE PP PS HDPE LDPE PP PS HDPE LDPE PP NAa HDPE LDPE PP PS NAa HDPE LDPE PP PS Plastic Aluminum Laminates

Diesel, naphtha, wax

VadXX

Agilyx (formerly PlasFuel)

Pyrowave Nexus Fuels LLC

Renewlogy

Recycling Technologies UK New Hope Energy

Plastic Energy

Quantafuel

Fuenix Ecology Blest

Climax Global Energy EcoFuel Technology

Enval

Diesel, naphtha, syngas, carbon black

Light synthetic crude oil; styrene monomers

Styrene monomer and oligomers Light crude, diesel, gasoline, kerosene, wax

Crude oil and kerosene

Low sulphur hydrocarbon Plaxx™ Synthetic fuels

Diesel, naphtha

Purified diesel

Naphtha, paraffin and LPG

Crude oil, diesel Purified diesel and kerosene

Aluminum, crude oil

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Table .: (continued) Company

Primary feedstock

Primary product

Braven (formerly Golden Renewable Energy)

PET HDPE LDPE PP PS PE PP PS

Braven PyChem (Naphtha)

Reentech

Diesel, kerosene, gasoline

Figure 2.4: HDPE pyrolysis degradation steps and corresponding mechanisms.

between a primary radical and a polymeric fragment. β-cleavage of secondary radicals leads to an end-chain olefinic group and a primary radical which can ultimately lead to chain branching. Termination occurs in bimolecular mode with the coupling of two primary radicals or by disproportion of the primary macroradicals. The final pyrolysis product from polyolefins consists of a mixture of waxes, paraffins, and olefins that can be converted to diesel fuel or sold to an oil refinery as feedstock. The pyrolysis oil can be

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further refined to via hydrotreatment to produce olefins and other polymer precursors for remanufacturing of plastics similar to the process for purifying naphtha for plastic production [64]. The pyrolysis of polyolefins is not a trivial process and requires a considerable amount of energy. It has been reported that at temperatures above 250 °C, the activation energies required for depolymerization of polyolefins range from 150 to 300 kJ/mol [1, 65]. This clearly demonstrates the energy intensiveness of these processes. As a result, many efforts focus on lowering the temperatures required for pyrolyzing polyolefins (as discussed in later sections) and improving the value of the pyrolysis product. Unzipping polymers. Some common plastics undergo an “unzipping” mechanism in a pyrolysis process in which the polymer depolymerizes directly into parent monomers under pyrolysis conditions. These plastics include polystyrene (PS), polymethyl methacrylate (PMMA), and polyamide-6 (PA-6 or Nylon-6). The unzipping mechanism occurs by end-chain scission where the polymer is broken up from the end groups yielding the corresponding monomers (Scheme 2.1). Since the resulting pyrolysis oil is made up of mostly the original monomers, these types of plastic pyrolysis fall into the monomer loop (Figure 2.1) and will be discussed in more detail in Chapter 2 of this book. Polymers containing hetero-atoms. Ideal conditions for pyrolyzing plastics require the absence of oxygen. As a result, polymers containing hetero-atoms in the backbone are not ideal feedstocks for pyrolysis recycling. Common plastics that contain hetero-atoms are polyethylene terephthalate (PET), polyurethanes (PURs), polyamides, and polyvinyl chloride (PVC). Other hetero-atoms found in common plastics include bromine and fluorine. Under pyrolysis conditions hetero-atom containing plastics can yield hazardous and corrosive products such as ammonia, hydrogen cyanide, hydrogen chloride, hydrogen bromide, bromine, hydrogen fluoride, and others [3]. If these types of plastics are used as feedstocks for pyrolysis then careful selection of methods to neutralize or inhibit the effects of hazardous compounds formed are required. This imparts high costs and energy input for the overall recycling process and has significantly limited the widespread adoption of pyrolysis considering the large volumes of PET and PVC found in MSW. If pyrolyzed, the degradation of mechanism of PET, which contains oxygen and aromatic groups, degrades by random scission of the ester links in the main chain that form carboxylic acid and olefinic end groups [66]. The carboxylic acid end groups

Scheme 2.1: Schematic of end-chain scission of PA-6 producing ε-caprolactam

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undergo decarboxylation to yield phenyl end group compounds. Subsequently, benzoic acid and vinyl benzoate are formed by scission of the phenyl end group compounds. During pyrolysis, other volatile products are formed from vinyl benzoate and benzoic acids including benzene, toluene, styrene, and ethyl benzene. The final pyrolysis oil consists mostly of benzoic acid and vinyl terephthalate which can be upgraded to useful chemicals or fuel [10]. PURs are an incredibly complex group of plastics in that are either thermoplastics or thermosets (i.e., crosslinked) and can be synthesized from numerous types of monomers. PURs are classified based on a urethane linkage in the backbone, but in reality, this makes up a small percentage of the actual polymer structure. As a result of this complexity, several different types of degradation mechanisms can occur during pyrolysis such as random chain scission, chain stripping, and crosslinking. Overall, studies show that pyrolyzing PURs results in a complex mixture of compounds in the pyrolysis oil such as benzene, methane, ethylene, amines, and hydrogen cyanide [3]. PVC breaks down via chain stripping under pyrolysis conditions. In this mechanism, the reactive substituents or side groups on the polymer chain are eliminated, leaving an unsaturated chain. This polyene then undergoes further reaction, including β-scission, aromatization, and coke formation. This results in very little monomer recovery and the pyrolysis oil consists mostly of benzene and hydrochloric acid (HCl). The presence of HCl leads to corrosion of the reactor and formation of organochlorine compounds [67]. Additionally, the presence of chlorine limits the application of the pyrolysis oil as fuel. As a result, pyrolysis plants typically avoid PVC feedstock unless it has undergone a dichlorination pretreatment [7]. The presence of halogens is a particular problem in pyrolytic recycling as they can lead to the formation of highly toxic and dangerous compounds. Halogens can be present on the polymer backbone, such as the case with PVC, or in the additives such as brominated flame retardants used in many plastic formulations (present in some cases as high as 60% by mass of the polymer). As a result, dehalogenation has been a major focus in pyrolysis innovations. Dehalogenation technologies have been developed as a pretreatment step, using solvents or thermal treatment to remove halogens. Dehalogenation of liquid products using catalysts or antimony has also been studied. Scrubbing gas effluents from the pyrolysis process to remove HCl and HBr is also employed. Additionally, some efforts have even focused on capturing the bromine released during pyrolysis for reuse in flame retardants [68, 69]. Mixed plastic feedstocks. Pyrolysis is often presented as a solution for recycling of highly mixed, heterogeneous waste. Pyrolysis recycling of mixed waste plastics may be one avenue for recovering value from unwashed, commingled plastics and lowering the costs and logistics of plastic sortation typically required for recycling [3]. However, as demonstrated in the previous section, pyrolyzing different plastics can lead to varying compositions, yields, and quality of pyrolysis oil products. As a result, most commercial facilities have focused on single stream feedstocks. Several researchers have studied the

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resulting pyrolysis oil from mixed plastic waste streams [13, 20, 23, 48, 70]. These studies have focused primarily on mixtures of PE, PP, and PS, with low levels of PVC contaminations. Overall, the pyrolysis of mixed plastics produces lower liquid yields of target products liquid when compared to the pyrolysis of single plastic waste streams. Despite the lower liquid yield the quality of pyrolysis oil produced is typically comparable to the single plastic feedstocks. These studies have shown that the oil composition is still acceptable for further processing in petrochemical refineries. The challenge then becomes the techno-economics. Pyrolyzing mixed plastic streams will save on costs in the sorting steps. However, there is still the question on whether these cost savings justify the high energy and costs of pyrolysis when only 50% of the pyrolysis oil can be recovered. Assisted pyrolysis technologies Microwave assisted technology As noted, one of the major drawbacks to pyrolysis is the high energy inputs required to break the robust bonds in common plastics. Additionally, plastic feedstock has very low thermal conductivity which leads to a low rate of heat transfer through the materials and thus requires long residence times to achieve target yields [71]. In electric-heated pyrolysis, the plastic waste is heated by an external heating source which heats everything in the pyrolysis reactor, including the evolved volatiles. This results in significant energy losses and can also promote undesired side reactions that can lead to the formation of toxic compounds, increased production of char, and fouling of the reactor [72, 73]. Microwave heating can be applied as a more efficient means of heating the reactor. Microwave-assisted pyrolysis offers a lower energy pathway for breaking carbon bonds and recovering high quality pyrolysis oil. Microwave-assisted pyrolysis uses microwave heating to thermally crack the plastic waste. In this process, the waste material is mixed with a microwave-absorbent substance such as particulate-carbon. This substance absorbs microwave energy which then generates sufficient thermal energy to achieve the temperatures required for pyrolysis to occur [71]. Microwave radiation offers several advantages over conventional pyrolysis methods such as rapid heating, lower residence times, and lower production costs [8]. Unlike conventional methods, microwave energy is supplied directly to the material through the molecular interaction with the electromagnetic field, thus no energy is wasted to heat up the surrounding area of the pyrolysis chamber. One limitation to microwave-assisted pyrolysis is the lack of sufficient data to quantify the dielectric properties of various plastic waste streams [8]. The efficiency of microwave heating significantly depends on the dielectric properties of the material, which can vary depending on the plastic feedstock [71]. Therefore, most studies focus on single waste streams. Nevertheless, microwave-assisted pyrolysis continues to be explored, with some companies demonstrating the process at the pilot-scale such as Pyrowave, a startup company that uses microwave-assisted technologies to pyrolyze PS into styrene monomer. In-depth

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reviews of microwave-assisted pyrolysis have been presented by Lam and Chase [71] and Undri et al. [74]. Hydrothermal liquefaction Another method for improving the energy efficiency and yield in plastic pyrolysis is by incorporating subcritical or supercritical water and is known as hydrothermal liquefaction (HTL) [75]. When under subcritical or supercritical conditions water contains enough energy to break carbon-carbon bonds making it a useful solvent for advanced oxidation processes, including degradation and thermochemical conversion of plastic waste [76]. Super critical water (SCW) exists at a temperature above 374 °C and pressure higher than 22.1 MPa, and the physical and chemical properties are dramatically different compared to water at ambient/atmospheric conditions [77]. The high density and ionic product of SCW promotes the solvation of compounds, while the high diffusivity and low viscosity promotes faster mass transfer during pyrolysis [75, 78, 79]. The HTL pyrolysis process contains a series of reactions including hydrothermal cracking, hydrolysis, free radical, nucleophilic substitutions, and cyclization that converts plastics into monomers or chemical feedstock [80]. The SCW can serve as a solvent, catalyst, or reactant [81, 82]. Hydrocarbon yields and distributions are highly dependent on the residence time, but unlike conventional pyrolysis, the heating rate in HTL was reported to have an insignificant effect on product distribution [75]. HTL is not as heavily studied as conventional pyrolysis, but has started to attract more interest in recent years. Some challenges associated with using SCW-assisted pyrolysis are the high costs of equipment and operation and the corrosive effects of SCW on the reactor [5, 83]. One industrial scale example of an HTL process is the ReNew ELP technology that uses SCW to process plastic waste. ReNew ELP’s catalytic hydrocracking (Cat-HTR) technology uses SCW to break down plastic feedstocks and harvest hydrogen from the water for use in the creation of new, stable hydrocarbons. Catalytic cracking Over the years, various types of pyrolysis have been developed based on the kinetics of the degradation and are categorized as fast, catalytic fast, intermediate, slow, and vacuum pyrolysis [37]. Adding catalysts to the pyrolysis process can facilitate lower temperatures and shorter residence times [51, 67, 84, 85], increase the yield and selectivity of the pyrolysis oil [85, 86], and limit side reactions and the formations of undesired products [51]. Catalysts will impart better control over the product selectivity, but the reactor choice and operating conditions still have a significant effect on the product distribution and yields. Additionally, catalyst properties such as the acidity and pore structure also play a major role in the product distribution obtained [55–57]. Multiple studies have revealed that using catalysts can modify the selectivity towards the production of light olefins, gasoline, or diesel based on the type of catalyst used [31, 51–53, 56, 87]. Homogeneous and heterogeneous catalysts have been investigated with heterogenous catalysts being the most common in industrial practice due to the ease of

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2 Pyrolysis of plastics to fuels and chemicals

separation and recovery associated with these kinds of catalysts [88]. Homogenous catalysts include Lewis acids and fused metal tetracloroaluminates (M(AlCl4)n), where the metal can be lithium, sodium, potassium, magnesium, calcium, or barium [88, 89]. A wide variety of heterogeneous catalysts can be used including conventional solid acids (such as zeolites, silica–alumina, alumina, and other FCC catalysts), mesostructured catalysts, nanocrystalline zeolites (such as n-HZSM-5), and several others [88, 90]. FCC catalysts are the best choice for maximizing liquid oil production from plastics as they have the highest catalytic activity [8]. Additionally, FCCs can be recovered and reused and are economically attractive. FCC catalysts are typically made of zeolite crystals and a non-zeolite acid matrix known as silica–alumina. FCC catalysts are normally used in the petroleum refining industry to crack heavy oil fractions from crude petroleum into gasoline and liquid petroleum gas (LPG) fractions where aromatic and naphthenic compounds are selectively formed [6]. Catalysts are primarily used to reduce the overall costs of the process while also improving the value and selectivity of the products. Several reports show that the use of an acid catalyst may reduce the pyrolysis temperature, overall energy consumption, and total operating cost of the process [67, 91–95]. Catalysts can improve the hydrocarbon distribution to yield pyrolysis oil that resembles conventional fuel such as gasoline and diesel (e.g., C2–C4 olefins) without requiring further refining [8]. Schirmer et al. showed that the catalytic cracking of PE increases the gasoline yield when compared to conventional pyrolysis [96]. Catalytic cracking also involves several drawbacks. Catalysts can be easily deactivated by carbonaceous deposits, inorganic materials, and chloride and nitrogencontaining species making the process highly sensitive to contamination typically associated with plastic waste streams [55–57, 97]. Finally, many chemical recyclers have struggled to find an optimum catalyst that is ideal for cracking multiple types of plastic waste streams [57]. As a result, intensive pre-treatment steps are often required to limit deactivation of the catalyst. Furthermore, the required catalyst regeneration increases the process complexity and total operating cost [5].

Gasification Gasification is another important plastic recycling process that is typically classified in the same category as pyrolysis recycling and is the preferred route for heavily contaminated and otherwise unusable, waste streams [5]. Gasification applies high temperatures to convert organic solid material (coal, biomass, plastics, and organic waste) into a gaseous mixture (syngas) of carbon monoxide (CO), hydrogen (H2), carbon dioxide (CO2), and methane (CH4). Gasification consists of heating plastics with a substoichiometric amount of oxygen or air, and steam at temperatures of 700–1500 °C and atmospheric pressure [98, 99]. Process parameters such as temperature, O2/H2O ratio, and residence time can be tailored to target a specific H2/CO ratio in the resulting

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syngas. For example, more steam yields more H2 in the syngas when compared to pure oxygen or pure air gasification. The syngas produced from gasification can be used in Fischer–Tropsch processes (FTS) to produce fuels and chemicals or converted to methanol via catalytic hydrogenation [100]. Methanol can be further converted to olefins via downstream catalytic processes and used to produce new polymers from chemically recycled intermediates [5]. Gasification can be done in air which decreases the costs of gas separation compared to steam gasification, however, the presence of N2 in such a high-temperature environment triggers the production of harmful nitrogen oxides (NOx) which are 300X more damaging GHGs than CO2. Air gasification also requires higher volumes than steam processes, thus negatively impacting investment costs. Additional challenges associated with the gasification of plastic waste are the high amount of energy required and high production of char that occurs during the process. Typically, dual-stage gasification reactors are required to mitigate char production and reactor fouling, increasing the overall investment costs of the process [101]. Another key challenge is the high yields of toxins and aromatics that limit the direct use of the syngas in downstream processes. Overall, gasification is a well-established technology and the recent momentum towards a circular economy has led to the development of increasingly viable and sustainable solutions, such as the combination of gasification with pyrolysis or combustion and the cogeneration of different products (e.g., syngas, heat, and power) as summarized by Heidenreich and Foscolo [102]. Eastman has pioneered gasification as recycling technology for carpet waste feedstocks. Eastman’s gasification plant in Kingsport, TN uses plastic waste and coal feedstocks to generate syngas which they then feed into other processes at the Kingsport site in a completely integrated design. On a pilot scale, Texaco Inc. and Shell developed a proprietary gasification process, in which petroleum feedstocks were replaced by plastics (Figure 2.5) [6]. Ube Industries in Japan gasifies plastics in a pressurized FBR, developed jointly by Ube and Ebara Co. Plasma pyrolysis/gasification Plasma assisted pyrolysis combines pyrolysis with the thermochemical properties of plasma. The process is similar to gasification and uses extremely high temperatures, ranging between 1730 and 9730 °C, to convert plastic waste into syngas. Plasma assisted

Figure 2.5: Basic schematic of Texaco gasification process [6].

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2 Pyrolysis of plastics to fuels and chemicals

pyrolysis is extremely fast, lasting between 0.01 and 0.5 s, depending on the process temperature and type of waste feedstock [103]. The resulting syngas is composed mainly of CO, H2, and small amounts of higher hydrocarbons. Plasma pyrolysis has several advantages over conventional pyrolysis and gasification. Plasma pyrolysis is very fast leading to lower emissions and is suitable for mixed plastic feedstocks [97, 103]. The high temperatures decompose any toxic compounds in the syngas and limit the formation of HCl from PVC. The produced gas has a low tar content and high heating value which makes it suitable for electricity generation in turbines or hydrogen production in an integrated process. There are still several technical challenges to be addressed before the technology can meet plastic waste management requirements and become commercially available. The challenges differ depending on the specific plasma technology [104]. Thermal plasma technology is a well-established technology in metallurgy processing or material synthesis, but not in waste management [103]. For now, due to economic and legal aspects, the most important application of the technology is the destruction of hazardous waste rather than recycling, but some stakeholders predict that plasma assisted gasification of plastics may soon be commercialized [97].

2.3 Summary and future perspectives Pyrolysis technologies are leading the way in chemical recycling because of the robustness to contamination and existing infrastructure. Pyrolysis is already considered to be a reasonably mature technology with numerous commercial plants operating to pyrolyze plastic waste (Table 2.3). However, current energy requirements, variability of the waste streams, poor life cycle assessment metrics, and technoeconomics have presented limitations to implementation of pyrolysis as a major recycling route for plastic waste. At large scales (e.g., tons per day), using pyrolysis and/ or gasification technologies to produce fuels and/or chemicals from plastic can address the challenges of waste management and increasing global energy demand simultaneously. Overall, pyrolysis is considered a relatively simple, flexible, and suitable process for recycling plastic waste streams that are difficult to depolymerize. However, many still agree that an improved understanding of the chemistry and more innovative reactor designs are required to realize the full potential of pyrolysis and to achieve lower GHG emissions. Additionally, the absence of an optimum reactor technology to employ catalytic fast pyrolysis is another major bottleneck in industrial applications. Despite the drawbacks, consumer product companies are making big commitments to use recycled content in their packaging. Governments are also pushing companies to use more recycled plastic. California wants plastic packaging to contain 50% recycled content by 2030. The European Union is pushing for 30% by the same date. Shifting the endgame of pyrolysis to a waste-to-plastics model can help companies and governments achieve these goals. This will require collaborative efforts between the recyclers and oil

References

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refineries that are willing to purchase the pyrolysis oil for conversion into chemical feedstocks for plastics. The feed flexibility of thermal processes supports the potential of pyrolysis and gasification; however, the strong variability in plastic waste streams calls for further assessments through fundamental experiments and models. Additionally, innovations to lower the energy requirements and costs are needed to minimize the environmental footprint and provide an economically viable pathway to a circular plastics economy via pyrolysis recycling.

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Katrina Knauer*, Cody Higginson, Yuanzhe Liang and Minjung Lee

3 Circular plastics technologies: depolymerization of polymers into parent monomers Abstract: While most commodity plastics were not designed to easily depolymerize, some common plastics can be broken down into their parent monomers in the presence of heat, pressure, catalysts, and/or solvent. Here, we provide a high-level overview of the depolymerization technologies that have been studied and/or scaled as promising monomer-loop recycling processes for selective plastic waste streams. Namely, commodity plastics that are considered unzippable/depolymerizable include polyethylene terephthalate, polyamides, polymethyl methacrylate, and polystyrene. Monomer-loop recycling technologies are one of several pathways toward a circular economy for plastics. Keywords: Depolymerization; recycling; solvolysis; unzipping; circularity; plastics.

3.1 Introduction In contrast to the pyrolysis of plastic waste to produce fuel, naphtha, and other chemicals (Section 3.1) along the molecular recycling loop, some plastics can be depolymerized or “unzipped” to recover monomers directly in the so-called monomer recycling loop [1]. These monomers can be purified to remove additives, fillers, and pigments present in the plastic waste and used to resynthesize the parent polymer. In a sense, this is an idealized representation of a circular material – one that can be cycled through monomer and polymer repeatedly (Figure 3.1). Not all plastics are amenable to this kind of depolymerization. Most plastics manufactured today were designed for the “take-make-waste” model, based on a linear economy framework that does not adequately address materials’ end-of-life issues. In principle, any polymer could be rendered “depolymerizable” by manipulating the thermodynamic equilibrium of the polymerization and depolymerization by modifying temperature, pressure, concentration, state, etc. [3]. A thermodynamic measure of depolymerizability, or how easily a polymer can unzip to its parent monomers, can be

*Corresponding author: Katrina Knauer, National Renewable Energy Laboratory, 15013 Denver W Pkwy, Golden, CO, 80401-3393, USA, E-mail: [email protected]. https://orcid.org/0000-0002-0125-7532 Cody Higginson, Menlo Park, CA, USA Yuanzhe Liang, National Renewable Energy Laboratory, Golden, CO, USA Minjung Lee, Bioenergy Science and Technology, National Renewable Energy Laboratory, 15013 Denver W Pkwy, Golden, CO, 80401, USA. https://orcid.org/0000-0003-1687-897X As per De Gruyter’s policy this article has previously been published in the journal Physical Sciences Reviews. Please cite as: K. Knauer, C. Higginson, Y. Liang and M. Lee “Circular plastics technologies: depolymerization of polymers into parent monomers” Physical Sciences Reviews [Online] 2023. DOI: 10.1515/psr-2023-0014 | https://doi.org/10.1515/9781501515613-003

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Figure 3.1: Simplified schematic of the monomer loop for plastics recycling as defined by the Ellen McArthur Foundation (EMF), figure adapted from Ellis et al. [2].

measured by the ceiling temperature (Tc) of the polymer [4]. The Tc is defined as the temperature at which the polymerization and depolymerization reactions reach an equilibrium state. In other words, depolymerization will occur at temperatures above the Tc of the polymer. However, most commodity plastics have a Tc at or near the carbon degradation temperature and undergo random chain scission and crosslinking rather than an “unzipping” mechanism to monomer. Unzipping occurs when the bonds break at the polymer chain end (i.e., chain-end scission) to release the monomer in a systematic mechanism rather than random fragmentation along the backbone (Figure 3.2). Some commodity plastics can be broken down into one or more of their constituent monomers in sufficiently high yield and purity to be used to produce new polymers with properties comparable to the original material. For some condensation polymers like polyethylene terephthalate (PET), this depolymerization process requires the input of stoichiometric reactants and catalysts, while other materials, such as polystyrene (PS) or polymethyl methacrylate (PMMA), can be coaxed under appropriate conditions to unzip

Figure 3.2: Simplified representation of Tc and random chain scission verses chain-end unzipping mechanisms in polymer depolymerization.

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predominantly into their component monomers (styrene and methyl methacrylate, respectively). We categorize these depolymerization routes into two general branches: solvolysis and pyrolytic depolymerization (“unzipping”). Selected examples of plastics for which depolymerization is being explored as a means of chemical recycling, and the methods used to accomplish this are surveyed in this chapter. The conditions and processes described are not exhaustive, but are intended to be representative of chemical recycling strategies for the various classes of materials discussed herein [5].

3.2 Technical scope 3.2.1 Solvolysis Solvolysis is the depolymerization of a polymer by reaction with the chosen solvent, such as water, methanol, or ethylene glycol. Solvolysis is particularly suited for the breakdown of condensation polymers containing functional groups cleaved by hydrolysis and alcoholysis, and to some extent phosphorolysis and aminolysis, including polyesters, polyamides, polyurethanes, and polyethers. The primary goal behind the solvolysis chemistries discussed in this section is to collect monomers that can be purified and re-polymerized into their parent materials in an infinite cyclic process. 3.2.1.1 Polyethylene terephthalate PET, the condensation polymer of terephthalic acid (TA) and ethylene glycol (EG), is the third most produced thermoplastic (following polyethylene and polypropylene, respectively), and the most common thermoplastic polyester produced today. It is used in a wide range of applications such as textile fibers, thermoforming in manufacturing, and food, drink, and storage containers. Most of the demand for PET is for textile fibers, while beverage bottles constitute the second largest source of demand for PET. While mechanical recycling of PET is currently widespread, the mechanical, thermal, and rheological properties of the recycled plastic weaken over repeated processing cycles compared to virgin resin [6]. Furthermore, most mechanical recyclers will only take rigid, clear PET bottle flake as feedstock and other PET form factors (textiles, clam shells, film, etc.) are landfilled. It is perhaps for this reason that significant research efforts have focused on the chemical recycling of PET to recover virgin-like quality monomers, and headway has been made in the commercialization of several PET depolymerization strategies [7, 8]. Multiple reagents exists that can cleave ester bonds in PET backbones, but the primary strategies that enable recovery of chemicals useful for the synthesis of virgin-grade PET are hydrolysis, methanolysis, and glycolysis (Figure 3.3).

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Figure 3.3: Solvolytic routes for chemical depolymerization of PET into monomers TA, DMT, BHET, and EG.

3.2.1.1.1 Hydrolysis Hydrolysis of PET involves reacting the polymer with water at elevated temperatures to cleave the ester bonds, usually in the presence of a catalyst [8], the products of which are TPA and EG, and has been reviewed extensively in the academic literature [8–14]. The reaction can take place under acidic, alkaline, or neutral conditions, and research related to enzymatic hydrolysis of PET has also gained considerable momentum in recent years, as outlined below [8, 11, 15–18]. 3.2.1.1.1.1 Acidic hydrolysis. Acidic hydrolysis of PET can be carried out with concentrated mineral acids serving as catalyst, including nitric acid and phosphoric acid [11, 19]. However, the most common acid used in this process is sulfuric acid, as outlined in the following examples [8, 11, 20–22]. Brown and O’Brien described a process in 1976 employing at least 87 % sulfuric acid for the hydrolysis of PET [23]. In this case, the depolymerization was carried out at 100 °C for 5 min at atmospheric pressure, after which near-theoretical yields of TPA and EG were obtained in crude form. Thereafter, the crude hydrolysate was treated with sodium hydroxide to neutralize the sulfuric acid and TA (pH 7.5–9.0), forming soluble sodium terephthalate. The mixture was filtered to remove insoluble residues and passed through an ion exchange decolorizing column. The solution containing ethylene glycol, sodium sulfate, sodium hydroxide, and sodium terephthalate was acidified to pH 2.5–3.0 by addition of sulfuric acid to precipitate terephthalic acid, which was collected by centrifugation and found to be of 99+% purity after washing with water. The ethylene glycol in the aqueous centrifuge mother liquor was extracted with trichloroethylene and recovered at 99+% purity by distillation. In 1982, Pusztaszeri described a lower temperature process employing concentrated sulfuric acid to depolymerize PET scrap [21]. In the process, PET solid is stirred with water and concentrated sulfuric acid (volume ratio of 2:13 to 8.5:13) at room temperature for 5– 30 min at atmospheric pressure to produce a fully liquified crude product. Crude TPA, having solubility in water of 0.0017 g/100 g of solvent, is precipitated by the addition of an equivalent volume of cold water and collected by filtration [23]. In this case, the filtrate

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containing aqueous ethylene glycol, excess sulfuric acid, and other impurities is discarded as waste. The use of less concentrated sulfuric acid to depolymerize PET for recovery of TA and EG has been described by Yoshioka, et al. [22] However, the process requires higher temperatures (150–190 °C) and pressures than the preceding two examples to obtain comparable yields of TA, and reaction times of up to 12 h were reported. While the reaction efficiency and yield of TA and EG from acidic hydrolysis of PET are typically high, there are several shortcomings of this method that make it somewhat unattractive for industrialization. These include the use of large volumes of strong acids and the generation of copious aqueous and inorganic salt waste [11]. 3.2.1.1.1.2 Alkaline hydrolysis. The hydrolysis of PET in basic media is usually performed with aqueous sodium hydroxide solutions of 4–20 weight percent (wt%) to produce sodium terephthalate and ethylene glycol in good yield [11]. Furthermore, nearly complete conversion of PET is possible. On the other hand, alkaline hydrolysis is typically slower than acidic hydrolysis with conventional heating methods, and is often performed at high temperatures (in some cases greater than 200 °C) and elevated pressure (1.4–2 MPa) [11]. The process can be carried out in aqueous solution, or in non-aqueous co-solvents employing a phase transfer catalyst [11, 24, 25]. TA monomer can be recovered by neutralization of the hyrolysate with strong mineral acids (typically sulfuric acid or hydrochloric acid) and precipitation, while the EG product generated can be recovered by distillation or “salting out” from the filtrate. Such a process was patented as early as 1959; Pitat and colleagues reported the alkaline hydrolysis of PET in 18 wt% aqueous sodium hydroxide solution at 100 °C for 2 h, at which point the majority of the sodium terephthalate product formed precipitates from the reaction mixture [26]. The terephthalate salt is collected by filtration and redissolved to form a nearly saturated solution to which sulfuric acid is added to provide TPA as a filterable precipitate in 94 % yield. The filtrate containing EG hydrolysis product is recharged with NaOH and recycled into subsequent rounds of hydrolysis to accumulate EG, which is later recovered by distillation. Overall, the process requires approximately 0.8 kg of solid sodium hydroxide per kilogram of terephthalic acid recovered. In a more recent example of alkaline hydrolysis, Loop Industries patented their firstgeneration process in 2017 wherein an organic co-solvent, preferably dichloromethane (DCM), is used to swell the PET and accelerate depolymerization in a solution of C1–C4 alcohol and potassium or sodium hydroxide [27]. The organic solvent is present at 3– 5 volume percent relative to the alcohol (preferably methanol) containing dissolved potassium or sodium hydroxide. In contrast to the other examples of alkaline hydrolysis presented above, the exothermic depolymerization occurs at atmospheric pressure without additional external heating. The process enables recovery of TA and other TA derivatives such as 4-(methoxycarbonyl)benzoic acid) after dissolution of salts formed in the depolymerization with water, followed by acidification with sulfuric acid and collection of the precipitated TA. Ethylene glycol, in addition to dichloromethane and methanol used in the depolymerization, are recovered by a distillation process.

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Alkaline hydrolysis is not without drawbacks; as in acidic hydrolysis, separation processes required to obtain pure monomers can be energy intensive, and these methods still generate large amounts of inorganic salt wastes. One apparatus and method proposed in a 2013 patent application by Parravicini and colleagues from the Swiss recycling company Gr3n aims to increase the depolymerization efficiency and reduce the waste generated in the process [28]. The DEMETO method (depolymerization by microwave technology) utilizes a continuous reactor to carry out hydroglycolysis of PET, described as hydrolysis by alkaline hydroxide salts in an ethylene glycol solvolytic mixture, and is purportedly tolerant of contaminants present in the PET feedstock. The approach may also be applicable to polyamides (vide infra). The method involves co-feeding ground PET via an Archimedean screw system and solvolytic reagents (sodium or potassium hydroxide salts in ethylene glycol) via a separate feed line into an elongated horizontal microwave reaction chamber (Figure 3.4, upper left). The heterogeneous reaction mixture is carried through the reaction chamber and into a separation unit consisting of filtration and distillation modules. EG is distilled from terephthalate salts and can be reused in the solvolytic mixture and for synthesis of new PET resin. The terephthalate salts are acidified with hydrochloric acid to yield TA precipitate for new PET synthesis (Figure 3.4, right). The proposed handling of waste salts in the DEMETO process differs from other examples presented in this section. Namely the sodium or potassium chloride salts from the TA precipitation step are electrolyzed to regenerate NaOH (or KOH) reagents, and hydrogen and chlorine gas (Figure 3.4, bottom left). Hydrogen and chlorine are photochemically recombined to regenerate the hydrochloric acid used in precipitation of TA. While the Gr3n process has yet to be demonstrated at commercial scale, the consideration in the process design for addressing common waste streams in PET hydrolysis is noteworthy.

Figure 3.4: Hydroglycolysis unit proposed by Gr3n for scalable chemical recycling of PET [29].

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3.2.1.1.1.3 Neutral hydrolysis. PET hydrolysis under neutral conditions is performed with water or steam, typically at elevated pressures (1–4 MPa) and temperatures in the range of 200–300 °C, with a ratio of PET to water from 1:2 to 1:12 [11]. The neutral hydrolysis process occurs substantially faster when performed with a PET melt rather than the solid, leading to a preference for conditions above 245 °C [30]. While neutral hydrolysis can be conducted without additional catalysts, common transesterification catalysts can also be employed, including alkali-metal acetates, as well as acetates of zinc, calcium, and manganese [11]. Campanelli and colleagues report that addition of zinc acetate to neutral depolymerization reactions at 250–265 °C increases the rate constant by approximately 20 % compared to the uncatalyzed system [31]. Key benefits of performing PET hydrolysis under neutral conditions as described above compared to acidic and alkaline conditions are the avoidance of highly corrosive reaction conditions, simplifying apparatus maintenance, and the lack of large quantities of waste inorganic salts formed as process byproducts. Despite these advantages, a key drawback is the typically lower purity of the TA product due to contamination by other insoluble impurities and fillers that may be present in the parent polymer, thus requiring more complex purification approaches that negate some of the benefits of neutral hydrolysis. Furthermore, neutral hydrolysis of PET is typically slower than either acidic or basic hydrolysis processes. A strategy for the neutral hydrolysis of PET was described by Tustin and colleagues in a now-expired patent issued to Eastman Chemical Company [32]. In the process, PET is first heated with water at 200–280 °C and pressures greater than 1.7 MPa for 2 h to provide a crude TA precipitate upon cooling. EG could be isolated by a two-stage distillation of the filtrate. The crude TA was purified by heating in the presence of a flow of steam to produce a vapor composed of water and TA. The TA was collected as a solid deposit after cooling and was found to be 95.7 wt% pure by HPLC. The isolated TA could be used to resynthesize PET, but this was only demonstrated after diluting with virgin-grade TA such that the TA recovered from the neutral hydrolysis was only 36 wt% of the total TA feed. Few major industrial chemical recycling efforts for PET today seem to rely on the neutral hydrolysis strategy, likely because of the challenges associated with the lower purity of TA obtained from the method. 3.2.1.1.1.4 Enzymatic hydrolysis. An approach for PET hydrolysis that is gaining momentum is the use of esterase enzymes to facilitate depolymerization into TPA, EG, as well as bis(2-hydroxylethyl) terephthalate (BHET), and mono(2-hydroxyethyl) terephthalate (MHET) [5, 16–18]. Ester degrading enzymes (i.e., esterase) have been identified and studied in nature for decades. PET is an excellent candidate for esterase degradation given the high concentration of ester bonds in the polymer backbone. As a result, PET biodegradation via enzymatic hydrolysis has been studied for nearly two decades [33–40]. One of the most notable findings were those reported by Yoshida et al. in 2016 where they characterized the soil bacterium, Ideonella sakaiensis 201-F6, which employs a two-enzyme system to depolymerize PET to TPA and EG [41]. The first enzyme was named PETase which performs the initial attack on the PET backbone cleaving the ester bonds to form BHET, MHET, and TPA

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Figure 3.5: Scheme demonstrating PETase depolymerization of PET to BHET, MHET, and TPA. MHETase converts MHET to TPA and EG, adapted from Austin et al. [46].

(Figure 3.5). The PETase activity will also cleave the resulting BHET to yield MHET and EG. The second enzyme is called MHETase which further hydrolyzes the MHET to produce TPA and EG [41]. The structure, mechanism of hydrolysis, the evolution products, and engineering of the two-enzyme system for PET depolymerization have been reported in detail by Knott et al. [42]. The enzymatic recycling of actual PET waste was modeled and analyzed based on techno-economic analysis (TEA) and life cycle impacts by Singh et al. (Figure 3.6) [5]. The process involves first pretreating post-consumer polyester flakes to reduce the crystallinity. This is required since PETase does not demonstrate significant activity on crystalline substrates and will only attack amorphous domains. Most PET waste is ∼30–40 % crystalline, thus an amorphization step is required prior to enzymatic recycling. This is done by extruding the PET flake and cryo-grinding the extruded material into an amorphous powder. Following pre-treatment, the PET is enzymatically depolymerized in a bioreactor containing both PETase and MHETase. The gradual addition of sodium hydroxide (or a neutralizing base) is required to maintain the pH. Most enzymes are only active within a specific pH window and as PET is depolymerized, the pH gradually decreases due to the formation of TPA. To maintain the activity of the PETase and MHETase, the reaction requires pH control. Following depolymerization, the recycled TPA and EG are recovered via downstream processes. Costs and energy inputs of this process are dominated by the cost of the PET feedstock and the energy requirements for the PET flake pretreatment (i.e., cryogrinding). Uekert et al. conducted a life cycle assessment (LCA) of this process and reported that enzymatic hydrolysis currently performs 1.2 to 17 times worse than virgin TPA and PET production across most impact categories, excepting ecotoxicity and fossil fuel depletion

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Figure 3.6: Simplified process flow diagram of the PET enzymatic depolymerization process from Singh et al. [5].

[43]. The top contributors to these impacts include post-consumer PET collection and the required pretreatment, sodium hydroxide usage for pH control, and electricity inputs [43]. As a result, current studies are focused on discovering and engineering enzymes that are active on crystalline substrates and in wider pH windows [44, 45]. Carbios is a France based startup company founded in 2011 that is actively scaling a PETase based recycling technology for PET waste. The Carbios technology was published by Tournier et al. in 2020 [18]. The Carbios enzyme is only effective on amorphous PET and requires a amorphization pretreatment as well as the pH control reported previously in this text. Carbios claims 90 % amorphous PET depolymerization into target monomers in under 10 h [18]. In 2021, Carbios launched an industrial demonstration plant in ClermontFerrand, France to validate the efficiency of their enzymatic process. The demonstration plant includes a 20-cubic-meter depolymerization reactor capable of processing two metric tons of PET per cycle. Carbios intends to scale this process to a fully industrial scale in 2023 [47]. 3.2.1.1.2 Methanolysis into DMT and EG In the methanolysis of PET, methanol (MeOH) deconstructs the PET backbone via a transesterification reaction at the ester bond resulting in the methyl-ester analogue of TPA, DMT, and EG. Methanolysis has several advantages over other solvolysis processes such as higher robustness to contamination [48], existing infrastructure [49], and the insolubility of DMT in water allowing for easier separations and purification than TPA. Additionally, conventional PET manufacturing processes require the conversion of TPA to DMT for transesterification with EG to produce high molecular weight PET products. Thus, methanolysis products are ideal for immediate reuse in PET manufacturing. However, new trends for PET production processes are using TPA instead of DMT as the raw material [50]. This creates vulnerability in the economic model for methanolysis as the conversion of the DMT to TPA by hydrolysis adds considerable costs to the

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methanolysis process [51]. The methanolysis of PET waste has been studied for decades and several types of methanolysis processes have been explored with the primary pathways falling under liquid methanolysis, vapor methanolysis, and super critical methanolysis. 3.2.1.1.2.1 Liquid methanolysis. Liquid methanolysis is a catalytic process that involves high temperature (180–280 °C) and pressure (20–40 atm) to fully depolymerize the PET backbone. High temperatures are required to fully melt the PET and allow for full penetration of the methanol into the bulk of the polymer and high pressure is required to keep the MeOH in the liquid state at temperatures above the boiling point. Typical transesterification catalysts used for PET polymerization can also be used for liquid methanolysis such as zinc acetate (the most commonly used catalyst for methanolysis) [50], magnesium acetate, cobalt acetate, or titanium-based transesterification catalyst. Liquid methanolysis can be operated as a batch or continuous process. However, the high-pressure operation renders continuous mode challenging and expensive to operate and most processes at pilot or industrial scale are done in batch mode. Several efforts have been made to reduce the temperature and pressure required for complete methanolysis of PET to DMT and EG and improve the LCA metrics. These efforts typically include the addition of co-solvents (to better solubilize the PET and improve contact with the methanol), novel catalysts, or both. For example, Liu et al. reported that aromatic solvents assisted in the dissolution of PET due to intermolecular interactions and improved the depolymerization efficiency of PET in solvolysis processes [52]. Ionic liquids have also been explored as cosolvents to improve the yields and energy requirements for methanolysis [53], but have significant cost limitations when applied to PET recycling and the value of the DMT products are not sufficient to justify the use of ionic liquids. Pham et al. recently reported a low-energy catalytic PET methanolysis process using potassium carbonate. This study reported yields of DMT of 93.1 % at 25 °C in 24 h [54]. Yang et al. recently reported a study applying liquid methanolysis, with a toluene cosolvent, with zinc bis[bis(trimethylsilyl)amide] as the catalyst to depolymerize a mixture of polyesters including PET at temperatures below 120 °C and DMT yields >80 % (Figure 3.7) [55]. These results are particularly impactful as this demonstrates that other relatively common polyesters can be depolymerized into their parent monomers in the same pot as PET such as polylactic acid (PLA) and polybutylene adipate terephthalate (PBAT). Recently, Loop Industries pivoted from hydrolysis of PET to liquid methanolysis and filed a patent for a low-temperature, liquid methanolysis process to depolymerize PET (Figure 3.8) [56]. In the Loop process, stoichiometric (or sub-stoichiometric) amounts of alkali methoxides and co-solvents are used as active ingredients to depolymerize PET at temperatures less than 60 °C. 3.2.1.1.2.2 Vapor methanolysis. Vapor methanolysis is another method to depolymerize PET which uses super-heated vapor instead of liquid methanol. Vapor methanolysis is

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Figure 3.7: Graphical results from Yang et al. applying liquid methanolysis on mixed polyester substrates. Substrates in this study were (a) BPA-PC/PET mask, (b) PLA/PBS straw, and (c) PLA/PBAT bag [55].

Figure 3.8: Depiction of the Loop Industry methanolysis process to yield DMT and monoethylene glycol (MEG), taken from the Loop website [56].

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usually conducted at temperatures above 250 °C. Since it is not required to keep the methanol in a liquid state, vapor methanolysis does not need the high pressures used in liquid methanolysis. In the vapor methanolysis reactor, the PET exists in a melt phase and the methanol gas is passed through the PET melt. The influence of temperature on the gas–liquid interactions in vapor methanolysis is more complex than liquid methanolysis and high agitation is required to ensure good contact between the gas bubbles and the PET melt [48]. Some benefits of vapor methanolysis, when compared to liquid methanolysis, are the lower pressure which allow for easier removal of the DMT product as a vapor. Since products can be removed in-situ via vapor removal, the reaction equilibrium is also shifted promoting higher conversions. Thus, vapor methanolysis typically results in higher yields than liquid methanolysis [57], but the reaction rate is much slower. In the 1990s, Eastman-Kodak developed and patented an integrated methanolysis process with their plant in Kingsport, TN to produce high purity DMT from PET [58]. Since then, the company, now only Eastman, has revitalized the technology and is currently performing feasibility tests to commercialize a methanolysis facility to recycle PET waste [49]. 3.2.1.1.2.3 Supercritical methanolysis. Efforts have been made to improve product yields and economics for methanolysis of PET via the use of supercritical methanol. As discussed in Section 3.1, the high density and ionic product of supercritical fluids (SCFs) promotes the solvation of compounds. Depolymerization under a supercritical state of methanol allows for high conversion of PET and an improved DMT yield (up to 95 %) within 1 h under optimal reaction conditions (260–270 °C, 9–11 MPa) [59, 60]. Additionally, Sako et al. reported up to 100 % yields of DMT from PET in 30 min under super critical methanol conditions [61]. While the conversion rates in supercritical methanolysis are significantly faster than liquid or vapor methanolysis, the reaction must be conducted at a higher temperatures and pressure, and thus, high capital and operating costs are the primary disadvantages. Additionally, the high pressures render continuous processing of PET very difficult as noted for liquid methanolysis. Nevertheless, reports of successfully applying supercritical methanolysis, or other SCFs, to deconstruct PET are released regularly [57, 62]. 3.2.1.1.3 Glycolysis into BHET Glycolysis of PET involves transesterification with excessive glycol to generate BHET and EG as products. The reaction can take place with or without catalysts. Chen et al. reported in 1991 that PET was depolymerized at high temperature (200–240 °C) and pressure (2– 6 bar) in the absence of a catalyst, but the monomer yield remained low [63]. The glycolysis product was quenched to room temperature in the protection of nitrogen gas at the end of the reaction. Next, a large amount of water was used to wash the solid contents to remove the unreacted EG, followed by filtration to obtain the insoluble products. The final products were then transferred to boiling water, where BHETwas extracted from the suspension.

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Factors that influence the non-catalytic PET glycolysis include the reaction temperature, pressure and concentration ratio of EG to PET. Increasing pressure or temperature can accelerate the reaction rates. At a constant temperature, pressure and PET concentration, the glycolysis rate of PET is proportional to the square of EG concentration. It indicates that EG acts as both a reactant and catalyst in glycolysis. Furthermore, the system quickly reaches an equilibrium between BHET and PET oligomers (e.g., dimer and trimer) at a higher ratio of EG/PET, whereas the depolymerization ends incompletely with higher molecular weight oligomers at a lower ratio of EG/PET. Various types of catalysts were discovered to improve the PET glycolysis kinetics, conversion, and reaction conditions. Güçlü et al. reported the first employment of metal acetates as a PET glycolysis catalyst in 1989. In their experiment, xylene was used to form a multiphase reaction in zinc-acetate-catalyzed glycolysis of PET with EG at 170–245 °C. BHET was extracted constantly from the xylene layer to shift the equilibrium of the PET depolymerization, yielding ∼80 % monomer conversion [64]. Chen et al. found that manganese acetate could depolymerize ∼100 % PET into BHET and dimers at 190 °C after 1.5 h [65]. Light metal salts were reported to catalyze PET glycolysis by Troev et al. In their reaction, titanium phosphate successfully depolymerized PET into BHET up to 97 % selectivity at 190 °C after 2.5 h [66]. Fang et al. reported that polyoxometalates could achieve 85 % yield of BHET at 190 °C for 40 min [67]. Ionic liquids (ILs) were first used as PET glycolysis catalysts in 2009 by Wang et al., who found the ILs could achieve full conversion of PET to BHET at 180 °C at ambient pressure after 8 h [68]. Another attempt was made by Yue et al. to use basic ILs to catalyze glycolysis of PET into BHET with a yield of 71 % at 190 °C for 2 h [69]. Similar to ILs, deep eutectic solvents (DESs) were chosen for PET glycolysis-catalysts because of low cost, low toxicity and simple chemistry. In 2015, Wang et al. reported the first employment of DESs to catalyze PET glycolysis. Their optimal reaction condition took place at 170 °C for 30 min, achieving 83 % yield of BHET [70]. Sert et al. discovered that an effective DES made from potassium carbonate and EG was able to achieve BHET yield of ∼88 % at 180 °C after 2 h [71]. Organocatalytic PET glycolysis was discovered as the catalyst using the amine base 1,5,7-triazabicyclo[4.4.0] dec-5-ene (TBD) by Fukushima et al. in 2011. In this case, BHET yield reached 78 % at 190 °C and 1 atmospheric pressure after 3.5 h [72]. Jehanno et al. utilized a methanesulfonic acid and salt derived from TBD as a PET glycolysis catalyst, yielding 91 % BHET conversion at 180 °C after 2 h [73]. Emerging techniques, such as, microwave-assisted PET glycolysis can reduce energy consumption in conventional heating reaction conditions. Pingale et al. reported the first microwave irradiation study on PET glycolysis in 2008. In their reaction, microwave heating shortened the depolymerization timespan by 16 times, while providing the same yield of BHET [74]. Recently, Parrott patented a microwave-assisted PET glycolysis technique, which can achieve 94 % BHET yield with 0.1 wt% zinc acetate as a catalyst in 5 min of irradiation at 250 °C [75]. Last but not least, a variety of heterogeneous catalysts have been found to assist PET glycolysis. For example, 3 nm cobalt nanoparticles were reported as reusable catalysts for PET glycolysis by Veregue et al. The

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conversion of BHET reached 77 % at 180 °C after 3 h [76]. Wang et al. used a colloidal catalyst based on graphitic carbon nitride to catalyze PET glycolysis, achieving 80 % BHET yield in 30 min at 196 °C [77]. In addition to various lab practices, glycolysis is one of the most widely used industrial PET chemical recycling methods. Eastman Chemical Company patented their glycolysis process for recycling post-consumer or scrap polyester in 1995. In the first stage of the process, the polyesters are mixed with EG in a ratio of EG to dicarboxylic acid component in the polyester between 2 to 6 [78]. Then the reaction takes place between 180 °C to 240 °C for up to 4 h to produce an oligomeric material with salts of metals (e.g., Zn, Sb, Ti, Sn, Mn, or Ge) as catalysts. The second stage of the process involves dissolving the glycolysis products in a hot solvent (e.g., water, alcohols, ethers, nitriles, chlorinated hydrocarbons, aromatic hydrocarbons, or ketones). The hot stream is then treated with adsorbents (e.g., activated carbon, activated clay, silica, and alumina), followed by hot microfiltration to remove insoluble impurities, such as pigments and trace metals. The product will precipitate out when cooling the hot permeate to room temperature, and then the product is extracted through filtration or centrifugation. Ioniqa’s technology offers a closed-loop solution for PET recycling via a patented glycolysis process [79]. Their process involves using a combination of a catalyst complex, heat, and pressure to break down PET under the glycolysis condition using ethanediol. The catalyst comprises butylmethylimidazolim (bmim+) and FeCl4−, the bridging moiety is triethoxysilylpropyl, and the nanoparticle is magnetite/maghemite. The preferred combination of catalyst loading, temperature, pressure and reaction time is 2–5 wt%, 180 °C, 60 kPa and 1.5 h. The process also involves recovering the catalyst and retrieving trimers, dimers and/or monomers. In the first step of retrieving, water is added to dissolve monomers and solvent, whereas catalyst complex can be recovered in a separate phase under influence of an external electro-magnetic field gradient (e.g., 1T). The second step of retrieving involves crystallization of monomers. As Ioniqa stated in their patent, the degradation product is ready to be used without further need of purification. Jeplan’s glycolysis technology can break down PET plastic that has been contaminated with other materials, e.g., food residue or labels [80]. In the first step of the glycolysis process, PET is heated together with bis(β-hydroxylethyl) terephthalate (BHET) to pre-decompose the PET at 200–245 °C, under normal or increase pressure for up to 1.5 h. Then a reaction between pre-decomposition products with EG (3–5 wt%) takes place at 200–220 °C under normal or increase pressure for up to 2.5 h for BHET conversation. An ester exchange reaction catalyst (e.g., sodium and magnesium methylates, fatty acid salts and carbonates of Zn, Cd, Mn, Co, Ca, and Ba such as zinc borate and zinc acetate, metal Na and Mg, and oxides) can be added to the depolymerization reaction to facilitate smooth reaction. In the second step, the glycolysis product is brought into contact with activated carbon for decoloring treatment and then cation and/or anion exchange resins at 30–70 ° C for deionization treatment. In the third step, BHET is recovered by distillation or evaporation to distill off the compounds with a boiling point lower than that of BHET. The

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temperature and pressure in this step are selected to be 100–150 °C at 70–300 Pa. The BHET obtained from Jeplan’s glycolysis process can be used to produce high-quality PET fibers, films, and bottles. IBM-VolCat developed a glycolysis method of depolymerizing polyesters from postconsumer products, e.g., beverage bottles, to produce a high purity BHET [81]. The depolymerization reaction takes place with an alcohol of 25 carbons and an amine organocatalyst at 150–250 °C. The selection of an organocatalyst with a boiling point significantly lower than the boiling point of the alcohol allows for the easy recycling of the amine catalyst. Increasing the reaction temperature and pressure above the boiling point of alcohols can accelerate depolymerization rates and catalyst recovery. Upon completion of the depolymerization reaction, the product undergoes direct filtration to remove insoluble contaminations and unreacted polymers, then treatment with activated carbon to remove additional impurities including dyes, and treatment with ion exchange resins to remove catalyst residues. Lastly, a combination of crystallization and distillation is used to recover BHET and EG. In an exemplar application, VolCat demonstrated that the proposed glycolytic depolymerization of PET can close the loop in the PET bottle industry, i.e., depolymerization of post-consumer beverage bottles to product high purity BHET and then production of high-quality bottle grade PET from the recovered BHET. 3.2.1.2 Polyamides Polyamides are another class of polymers that contain cleavable heteroatoms in the backbone and are suitable for chemical depolymerization routes. The most common polyamide plastics found in municipal solid waste (MSW) are polyamide 6 (PA6) and polyamide 66 (PA66). The amide groups are stable enough to render PAs as highly useful materials in many high-performance applications such as fibers in carpets and textiles, automotive and aerospace applications, electronics, and building and construction materials. The amide bonds in PAs offer a point of attack for various degradation agents that can depolymerize the polymer to parent monomers via pyrolysis, hydrolysis, ammonolysis, or by applying supercritical fluids or ionic liquids. In Section 3.1, we discussed the thermal unzipping of PA6 (also known as Nylon 6) to its parent monomer, cyclic ε-caprolactam (CPL), under pyrolysis conditions. Studies have shown that PA6 will depolymerize to CPL in >80 % yields at temperatures 330–400 °C [82]. BASF patented a catalytic pyrolysis technology for depolymerizing PA6 (from carpet waste) into CPL in 1991 but is not practicing the technology today [83]. The amide bonds in PAs offer a point of attack for various degradation agents that can depolymerize the polymer to parent monomers using similar solvolytic approaches applied to PET such as hydrolysis, aminolysis, and methanolysis, albeit under different conditions [84]. PAs are associated with high glass transition temperatures (Tg), high melt temperatures (Tm), and high concentrations of hydrogen bonding. Thus, for complete depolymerization of these materials, more extreme conditions are often required then for PET. Most studies have focused on depolymerization of PA6. This is likely due to the

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lower crystallinity, lower Tm, and lower resistance to chemicals and acids than PA66 which makes it easier to break down chemo-catalytically. However, high operational temperatures (>250 °C), 13, 16 high pressures 16 and strong acidic or basic conditions often entail high energy input and operational difficulties that limit the applicability. Chemical recycling of PA66, on the other hand, is even more challenging, because PA66 polymer chains interact more strongly with one another since each secondary amide bond participates in two strong hydrogen bonds [85]. As a result, PA66 possesses a rigid semi-crystalline polymer structure, which translates into a higher tensile strength and higher melting point compared to PA6 on a macroscopic scale [86]. Both the intrinsic strength of the secondary amide bond and the resulting semi-crystalline polymer matrix pose severe challenges for the chemical recycling of PA66, which is typically tackled by applying high temperatures (>275 °C) [87–90], high pressures [91–93], supercritical fluids [94, 95], and/or strong acidic [84] or alkaline conditions [90, 96]. A process for depolymerizing PA6 scrap using high pressure steam was patented by AlliedSignal [97]. In the AlliedSignal process, PA6 was dissolved in high-pressure steam at 125–130 psi and 175–180 °C for 0.5 h in a batch process and then continuously hydrolyzed with super-heated steam at 35 °C and 100 psi (790 kPa) to yield 98 % CPT. The recovered monomer could be repolymerized without additional purification. Braun et al. 1999 reported the depolymerization of nylon 6 carpet in a small laboratory apparatus with steam at 3400 °C and 1500 kPa for 3 h to obtain a 95 % yield of CPT [98]. Aquafil Econyl and gr3n are two other companies that are commercially operating technologies for depolymerizing PA 6 and PA 66 via solvolysis. These technologies focus mostly on recycling of used carpets or fishing gear.

3.2.2 Thermal unzipping into monomers 3.2.2.1 Polymethyl methacrylate Polymethyl methacrylate (PMMA) is a widely used thermoplastic in various electronics, automotive, and building and construction applications. Notable products and tradenames for PMMA are Perspex®, Plexiglas® and Lucite® [99]. PMMA is one of the few commodity plastics that undergoes chain unzipping into the parent monomer, methyl methacrylate (MMA), using thermal of photochemical approaches [100, 101]. In a pyrolysis process, the liquid product from PMMA is comprised mostly of MMA that can be used to resynthesize PMMA products. Thermal pyrolysis of PMMA has been studied extensively [99, 100, 102, 103]. The effects of temperature, additives, fillers, and solids loading on the distribution of pyrolytic products were have been investigated [104, 105]. Studies have shown that PMMA thermal pyrolysis results in nearly 97 % recovery of MMA at relatively low temperatures at temperatures between 400 and 500 °C. In several papers it was reported that the liquid pyrolysis product was so pure that it could be polymerized again without any further

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treatment [105–107]. As outlined in Section 3.1, this process can be aided/assisted by the use of microwaves or plasma to improve monomer yields and reduce energy inputs and residence time [108–111]. Even though PMMA depolymerization via pyrolysis is more facile than other commodities such as polyolefins (Figure 3.9), we have not seen widespread adoption of this recycling process. One significant limitation is likely feedstock reliability. Unlike PET and polyolefins, PMMA is not collected in curbside recycling bins since it is not a common household plastic. As a result, there is no widespread collection system for PMMA and this requires special collections or drop-offs. Some companies, such as CompuPoint USA, are collecting used electronics and separating PMMA components. In 2022, Sumitomo Chemical announced that they were piloting a pyrolysis technology for PMMA recycling [112]. The Japan Steelworks Co. developed a continuous, twin-screw extruder technology to pyrolyze PMMA to MMA and improve the economics and energy inputs for the process. A schematic of The Japan Steelworks Co. is presented in Figure 3.10 [113].

Figure 3.9: Comparison of unzipping mechanism of PMMA and PS to polyolefins from Vollmer et al. [8].

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Figure 3.10: Schematic of twin-screw reactive extrusion design to depolymerize PMMA [113].

3.2.2.2 Polystyrene Polystyrene (PS) is another important commodity plastic used in packaging applications. General-purpose PS is a transparent, hard, and somewhat brittle plastic. PS can also be easily foamed (i.e., Styrofoam), to yield a lightweight, waterproof packaging material. Expanded polystyrene (EPS) is another similar foam material, which has been used as insulation, life vests and rafts, and food containers. PS foam is notoriously challenging from a recycling perspective due to the low density and porosity (i.e., uptake of contaminants) of the foam waste. Additionally, PS foam has been identified as a major contributor to ocean plastic debris and easily breaks apart in the natural world yielding a formidable amount of microplastics. As a result, researchers, companies, and NGOs have focused significant effort towards the effective recovery and recycling of PS. Thermal or thermo-catalytic pyrolysis of PS to styrene monomer has been studied extensively [114–119]. However, unlike PMMA, PS will undergo random chain scission at midtemperatures in pyrolysis (i.e., 300–400 °C) generating a liquid oil, consisting mainly of C6-C12 aromatic hydrocarbons, gasses, and solid residues. At higher temperatures (400– 500 °C) chain-end scission becomes the primary mechanism and the pyrolysis product consists primarily of styrene monomer. However, this process is not perfect. Styrene and α-methylstyrene have poor thermal-oxidative stability, resulting in the formation on undesired side products in the pyrolysis process (Figure 3.11). This increases the costs and energy for downstream separations to recover the styrene monomer in high yields. The oil product is not suitable for automotive fuels due to the high content of aromatic hydrocarbons that can cause carbon formation problems in the engine. Several factors inhibit the formation of styrene: heat transfer problems due to the difficulty of establishing contact between PS and the heat transfer material causing uneven heat supply, and intensified side reactions at high temperatures and long contact times. One approach to overcome the issues mentioned above is depolymerization with a temperature of less than 550 °C and a vapor resident time of less than 10 s in a uniform heat distributed fluidized bed reactor. Liu et al. have reported styrene yields of 72–79 % using this method [115]. Another approach is depolymerization in a hydrocarbon medium, which can avoid the heat transfer problem and side reactions mentioned above and achieve high styrene selectivity.

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51

Figure 3.11: Scheme of pyrolysis reaction for PS and undesired side reactions. (A) Hemolytic cleavage of backbone C–C bond in PS, (B) beta-cleavage of the backbone C–C bond and formation of styrene monomer, (C) hydrogen transfer and formation of trimer, and (D) thermal cracking of the trimer.

Several companies have emerged in the last decade that are piloting thermal pyrolysis of PS to recover styrene monomer. Most notably is Agilyx, a startup company that has pioneered a pyrolysis technology to depolymerize PS waste (Figure 3.12), with an emphasis on PS foam [120]. Agilyx has demonstrated a pyrolysis technology at their facility in Tigard, Oregon, which has a capacity of 10 tons per day of PS waste, according the to the Agilyx website. Agilyx claims their core differentiator from other pyrolysis technologies is their novel styrene purification process using Technip Energies’ process for purifying styrene monomer. The technology for the purification of Agilyx Styrene Oil has been pilot tested in Technip Energies’ Research Center in Weymouth, MA, USA. Pyrowave is another company attempting to optimize and scale the pyrolysis of PS waste

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Figure 3.12: Schematic of Agilyx pyrolysis process for PS waste as shown on the Agilyx website [120].

to recover styrene monomer. Pyrowave is pioneering a microwave assisted pyrolysis process and claim their process is less energy intensive and polluting than conventional PS pyrolysis.

3.3 Summary and future outlooks Monomer-loop recycling technologies are one of several pathways toward a circular economy for plastics. While most commodity plastics were not designed to easily depolymerize, some common plastics can be broken down into their parent monomers in the presence of heat, pressure, catalysts, and/or solvent. Here, we provided a high-level overview of the depolymerization technologies that have been studied and/or scaled as promising monomer-loop recycling processes for selective plastic waste streams. Namely, commodity plastics that are considered unzippable/depolymerizable include PET, PA6, PMMA, and PS. While several reports have demonstrated that these polymers

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can be depolymerized to recover high monomer yields, many gaps and challenges still exist before these technologies can achieve industrial reality. In a recent study by Uekert et al., the technical, economic, and environmental metrics of emerging closed-loop recycling processes were analyzed and compared against mechanical recycling [121]. Regardless of the high monomer yields and purity that can be achieved through these chemical depolymerization routes, mechanical recycling outperformed all other technologies, as well as virgin plastic production across economic and environmental considerations, but it exhibited lower material qualities and other technical metrics. Thus, chemists have their work cut out for them to continue to optimize and improve chemical depolymerization technologies and reduce the environmental impact of the proposed routes. However, the future is bright. In a recent Strategy for Plastics Innovation repot from the U.S. Department of Energy, chemical recycling was identified as a promising technology for mitigating plastic waste and highlighted as key area for innovation and optimization [122].

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Katrina Knauer* and Minjung Lee

4 Circular plastics technologies: open loop recycling of waste plastics into new chemicals Abstract: Open-loop recycling is any recycling process where the recycled materials are converted into new raw materials, often of higher value than the parent monomers. Typically, materials recycled through open-loop recycling go on to be used for purposes different from their former, pre-recycled purpose. This means that the input into the recycling process is converted to a new chemical building block, which can be used as an input into another manufacturing process. Open-loop recycling processes usually involve processing various types of products of similar material makeup and change the properties of the material itself (through heat, chemical reactions, or physical crushing). This chapter will highlight promising pathways for upcycling of various plastic waste streams into new applications via open loop chemical and biological recycling processes. Keywords: circular economy; plastics; recycling

4.1 Introduction One concept that many in the field of plastics agree on is that to sustainably manage the staggering amounts of plastic waste production that is projected in the next several decades, we need to shift from a linear “take-make-dispose” economy to a circular economy. Simply put, a circular economy is an economic system that aims to eliminate waste and establish continual use of resources and carbons (Figure 4.1). According to a 2020 report by Closed Loop Partners, a circular plastic economy is projected to be worth $4.5 trillion dollars by 2030 [1]. Some argue that a true circular economy for plastics in one in which plastics are cycled into the original application an infinite number of times. In chemical recycling, this means the polymer is decomposed to the original monomers and subsequently repolymerized into the parent polymer [2]. However, some researchers are challenging the definition of circularity by exploring chemical processes that can convert polymer chains into new chemical building blocks that can be used to create entirely different materials from the parent polymer. This concept has

*Corresponding author: Katrina Knauer, E-mail: [email protected]. https://orcid.org/0000-00020125-7532 Minjung Lee, https://orcid.org/0000-0003-1687-897X As per De Gruyter’s policy this article has previously been published in the journal Physical Sciences Reviews. Please cite as: K. Knauer and M. Lee “Circular plastics technologies: open loop recycling of waste plastics into new chemicals” Physical Sciences Reviews [Online] 2023. DOI: 10.1515/psr-2022-0178 | https://doi.org/10.1515/9781501515613-004

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Figure 4.1: Model depicting a circular economy for plastics where raw materials are only used once for initial design and then supplemented with recycled feedstocks.

recently been popularly referred to as “upcycling” [2]. This type of recycling aligns with the concepts of a circular economy which is restorative and regenerative by design. This means materials constantly flow around a ‘closed loop’ system, rather than being used once and then discarded, but do not necessarily have to stay in their own circle. In the case of plastic, circularity can also be defined as simultaneously keeping the value of plastics in the economy, without leakage into the natural environment. Upcycling of plastic via chemical processes offers another kind of route to valorize recycling and create value added intermediates from plastic waste (Figure 4.2) [2]. This type of

Figure 4.2: Open loop recycling of plastic waste where plastics are deconstructed into chemical building blocks disparate from the parent monomers that are used to create new, higher value materials. Adapted from Ref. [2].

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recycling falls into the “Molecular Loop” that is outlined and defined in Chapter 1 of this text, also known as closed-loop recycling. Unlike pyrolysis, the goal is not to use the new chemicals derived from the decomposed plastic as fuel, but to further process them into useful monomers and/or chemical building blocks that can re-enter the chemical supply chain. Many of the chemical recycling methods discussed in this chapter will overlap with those covered in Chapter 2, but instead of unzipping the polymer into the original monomers, these techniques are applied to yield new chemicals and impact new functionality. Most upcycling processes have focused on using specialty and engineering thermoplastics or thermosets such as polyurethanes (PURs), polyamides (PAs), polycarbonates (PCs), polyesters, and epoxies as the primary source of feedstock. These plastics fall into the number 7 category for recycling. The reason for this is that these types of plastics are complex in design and vary significantly in structure across applications. For example, PURs can be manufactured using several different types of monomers for varying applications but are still collectively referred to as PURs. As a result, it is challenging to identify an “unzipping” mechanism for these waste streams and pyrolysis of such polymers can yield highly variable oil. Large scale recycling efforts have mainly focused on the 1–6 plastics, as these materials are typically designed for single-use applications and make up most of the plastic waste stream. Number 7 plastics are usually used in applications where higher performance and longer lifetimes are required. However, these materials are starting to build up a substantial waste as consumerism and obsolescence continues to grow. For example, polyesters and PURs are a substantial component of most textiles and footwear. According to World Footwear, 24.2 billion pairs of show were manufactured in 2018, but 90 % of all shoes enters a landfill once disposed [3]. PAs make up a large portion of the fishnet industry, and in one clean-up alone 24 tons of PA fishnets were collected from the shores of Antarctica [4]. PCs are also starting to pile up with the rise of discarded and outdated electronics [5]. Polyolefins such as polyethylene (PE) and polypropylene (PP) are also often used in upcycling applications. In contrast with polymers linked by heteroatoms (as discussed below), C–C-linked polymers, exemplified by PE and PP, present opportunities for advances in catalysis science, considering their abundance and the technical challenges associated with cleaving C–C bonds. The resilience of C–C-linked polymers to the unzipping mechanism described in Chapter 2 has led to the emergence of new, catalytic technologies to break C–C bonds while also imparting new functionality to the polymer. In Chapter 1, the pyrolysis of polyolefins to fuels and chemicals was discussed in detail. This chapter will focus on emerging catalytic technologies such as metathesis, hydrogenolysis, oxidation, and other catalytic processes that can convert polyolefins into new chemical building blocks. The remainder of this chapter will cover other open loop recycling methods that fall in the molecular loop, outside of pyrolysis and gasification, that have been applied to break down plastic waste and which types of plastics are best suited for each method.

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4.2 Technical scope 4.2.1 Solvolysis In Chapter 2, the unzipping of polymers into parent monomers via solvolysis was discussed in detail. Solvolysis processes can also be applied and tuned to yield new chemical building blocks from plastic waste. Many of the processes described in this section will resemble those described in Chapter 2. This section will outline how solvolysis of plastics can be used to convert waste carbons into valuable chemical building blocks, disparate from parent monomers, to be funneled back into the chemical supply chain. 4.2.1.1 Glycolysis Glycolysis processes can effectively break ester, carbonate, and amide bonds in multiple types of plastics. In Chapter 2, the glycolysis of PET into bis(hydroxy ethylene) terephthalate (BHET) and a mixture of its oligomers was discussed in detail. This technology has been applied at the pilot scale as an effective means of “unzipping” PET into the starting monomers followed by purification and subsequent re-polymerization into PET. However, glycolysis has also been applied as a means of upcycling PET waste into aromatic polyester polyols which are key starting materials in the synthesis of rigid polyurethanes (Scheme 4.1) [6]. The best studied glycolysis reaction of PET is the one with an excess of 1,2-ethanediol (ethylene glycol, EG) or 2,2′-oxydietha-nol (diethylene glycol, DEG) resulting in polyols of high crystallinity [6]. Polyols are primary building blocks for PURs. The preparation of polyester polyols usually includes two stages. In the first stage, depolymerization of PET takes place, resulting in a glycolyzed product (GP) that consists of a mixture of BHET, oligomers, and unreacted glycols. In the second stage, GP is reacted with dicarboxylic acids, glycols and other additives resulting in polyester polyols (Scheme 4.1). The motivation to convert PET waste into polyols is that PURs are typically much higher value polymers than PET and thus provide an economic incentive to PET recycling. A basic schematic of this process is outlined in Figure 4.3. This technology has been scaled and commercialized by Resinate Materials Group (RMG) based in Plymouth, MI. RMG developed a one-step process to convert postconsumer PET into aromatic polyester polyols. RMG claims that the resulting aromatic polyols have desirable properties and attributes for formulating PUR products and are especially useful for PUR dispersions for coatings applications [7]. Huntsman has also announced that its polyols production facility in Kuan Yin, Taiwan, will begin to utilize

Scheme 4.1: Glycolysis of PET to aromatic polyols.

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Figure 4.3: Schematic of resinate process to upcycle PET into aromatic polyester polyols.

the company’s Terol polyols technology to recycle PET scrap into polyols to satisfy the growing demand from the regional PUR foam insulation market. Converting PET into polyol building blocks allows for the inclusion of post-consumer recycled content in PUR materials. PURs are not readily collected in curb side recycling programs. As a result, the PUR market has limited access to post-consumer recycled feedstocks. PET, on the other hand, is readily collected (globally) in curbside/home recycling and provides high volumes of potential recycled building blocks for the PUR market. Additionally, PET is sold at ∼$1.50/kg while PUR products can range from $4 to $15/kg depending on the grade and target application. Funneling PET waste into PUR products is a clear example of upcycling plastic waste into higher value products (Figure 4.3). Rorrer et al. also applied moderate glycolysis of PET to incentivize both higher extents of waste plastics reclamation and use of bio-based chemicals with waste-based intermediates [8]. Partially glycolyzed PET can be effectively upcycled into unsaturated polyesters that can be crosslinked and filled with glass or carbon fibers to yields a high-performance composite material for automotive and aerospace applications (Figure 4.4). Life cycle assessment (LCA) showed that this approach results in reductions in energy input and greenhouse gas emissions (GHGs) relative to standard composites manufacturing today [8]. Glycolysis has also been applied in deconstructing PCs [9], PAs [10], and PURs [11] into upcyclable chemical building blocks. Chemical deconstruction of the number 7 plastics via glycolysis becomes more complicated by the multiple types of polymer backbones that are associated each plastic. For example, in the PUR industry, the combinations of diols, diacids, ethers, polyols, diisocyanates, and chain extenders are endless. While glycolysis can effectively break bonds in PUR backbones, the significant diversity in these materials makes even open loop recycling very challenging.

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Figure 4.4: Schematic of conversion of waste PET to high performance composites. Adapted from Ref. [8].

4.2.1.2 Hydrolysis Any polymers that contain hydrolysable groups are susceptible to hydrolysis as in the reaction with water or steam. In terms of performance, hydrolysis is typically seen as an undesirable event leading to a loss in mechanical properties. However, in some cases, hydrolysis can be applied as a useful means of decomposing plastic waste once it is reached the end of its use-life. In Chapter 2, the hydrolysis of PET to yield terephthalic acid (TPA) and EG. However, hydrolysis as also been applied as a recycling method for other types of plastics as well such as PURs [12], PAs [13], and PCs [14]. The hydrolysis of PURs under elevated conditions, was the first type of recycling used explored for PURs by the Ford Motor Co. in the late sixties. Hydrolyzing a urethane bond results in the formation of an amine, alcohol, and carbon dioxide (CO2) as shown in Scheme 4.2. Hydrolytic decomposition of PURs results in the formation of diamines, diols,

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Scheme 4.2: Hydrolysis of urethane bond to yield an amine, alcohol, and CO2.

and polyols (typically either polyether diols or polyester diols depending on the type of PUR). This process requires a high amount of energy input and is typically carried out at temperatures between 300 and 500 °C. The energy consumption is further increased by the necessity to separate the amines from the polyols. The diols and polyols can be reused to manufacture PURs and the diamines can be used in the synthesis of other high performing materials such as PAs. The diamines can also be further converted into the original monomers, diisocyanates, via a phosgenation reaction. Phosgenation requires highly pure diamines and is considered a highly dangerous and toxic process [15]. Thus, applying the diamines in new polymers is the preferred upcycling route. Hydrolytic recycling of PURs is usually carried out on aliphatic PUR structures. The hydrolysis of aromatic PURs based on either methylene diisocyanate (MDI) or toluene diisocyanate (TDI) will yield aromatic diamines such as 4,4-diaminodiphenylmethane (MDA) or 2,4and 2,6-toluylene diamines. These are not only cancerogenic substances but also increase the viscosity of the recycled mixture making separation more difficult. Beyond glycolysis and hydrolysis, studies have been conducted applying methanolysis, acid hydrolysis, and other solvolysis methods as open loop recycling processing for polyesters, polyamides, polycarbonates, and polyurethanes.

4.2.2 Polyolefins 4.2.2.1 Olefin intermediate The C–C bond cleavage of PE is an energy intensive process that is industrially accomplished through pyrolysis or gasification at very high temperatures as outlined in Chapter 1. These technologies are often associated with an uncontrolled distribution of products. A more controllable and tailorable approach to upcycle polyolefins is through the installation of cleavable linkages along the polymer backbone that allow selective deconstruction as outlined in Figure 4.5. Incorporating cleavable linkages into polyolefins can be achieved by introducing tunable amounts of unsaturation. Subsequent C=C bond cleavage via oxidation, metathesis, and other approaches would produce telechelic polymers and/or upcyclable intermediates (Figure 4.5). This technology is still in early-stage development and has not been demonstrated at scale but shows promise as potential low energy pathways for upcycling polyolefin waste into higher value applications and reducing the overall environmental impact of polyolefin waste.

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Figure 4.5: Opportunities in polyolefin upcycling via open loop recycling via the olefin intermediate. Adapted from Ref. [16].

4.2.2.2 Hydrogenolysis Hydrogenolysis is an open loop recycling pathway that converts polyolefins into hydrocarbons that can be used as fuel or useful chemical building blocks (Scheme 4.3). The mechanism involves aliphatic molecules adsorbing to the surface of a noble metal catalyst followed by dehydrogenation (through C–H activation of the backbone, resulting in two carbon atoms adsorbed to a metal surface in a reactive state), C–C bond cleavage, and ultimately desorption [17]. A series of catalysts, including carbon-supported ruthenium (Ru), porous silica supported platinum (Pt), and a complex atomic layer deposition Pt on a perovskite support, have all been leveraged as hydrogenolysis catalysts capable of converting

Scheme 4.3: Basic schematic of hydrogenolysis of PE to alkanes. Adapted from Ref. [20].

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Figure 4.6: Schematic of hydrogenolysis of LDPE into liquid alkanes, adapted from Ref. [19].

polyolefins into alkane and iso-alkane mixtures in a solvent-free system [16]. Celik et al. obtained high yields of liquid hydrocarbons via hydrogenolysis of PE using well-dispersed Pt nanoparticles supported on SrTiO3 nanocuboids at 300 °C for 96 h [18]. Rorrer et al. was able to achieve depolymerization of both PE and PP substrates at even milder conditions (225 °C, 24 h) using a Ru nanoparticles supported on carbon (Figure 4.6) [19]. Deconstructing polyolefins via hydrogenolysis is in early phase development and has not been demonstrated at the pilot scale. Further studies are needed to better understand the effects of branching, feedstock variations, additives, and contaminants on the hydrogenolysis products from actual post-consumer polyolefin waste. Regardless, hydrogenolysis shows some promise as a viable, potentially lower-energy platform for deconstructing polyolefin waste into fuels or petrochemicals. While pyrolysis requires high temperatures (>600 °C) and offers very little control over the product selectivity, C–C bond cleavage via hydrogenolysis allows for selective depolymerization of polyolefins into liquid alkanes with targeted molecular weight ranges at lower temperatures (