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. 2024 Aug 5;14(16):12437–12453. doi: 10.1021/acscatal.4c03194

Industrial and Laboratory Technologies for the Chemical Recycling of Plastic Waste

Mason T Chin 1, Tianning Diao 1,*
PMCID: PMC11334192  PMID: 39169909

Abstract

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Synthetic polymers play an indispensable role in modern society, finding applications across various sectors ranging from packaging, textiles, and consumer products to construction, electronics, and industrial machinery. Commodity plastics are cheap to produce, widely available, and versatile to meet diverse application needs. As a result, millions of metric tons of plastics are manufactured annually. However, current approaches for the chemical recycling of postconsumer plastic waste, primarily based on pyrolysis, lag in efficiency compared to their production methods. In recent years, significant research has focused on developing milder, economically viable methods for the chemical recycling of commodity plastics, which involves converting plastic waste back into monomers or transforming it into other valuable chemicals. This Perspective examines both industrial and cutting-edge laboratory-scale methods contributing to recent advancements in the field of chemical recycling.

Keywords: plastic waste, recycling, SPI codes, polymer degradation, catalytic hydrogenolysis

1. Accumulation of Plastic Waste from Commodity Polymers and Recycling Strategies

Since their foundational commercial production in the 1950s, synthetic plastics have experienced enormous growth, attributed to low production costs, versatile applications, and widespread accessibility.1 According to a 2018 report by the International Energy Agency (IEA), the demand for commodity plastics has outpaced that of other bulk materials, such as cement and ammonia, with almost a doubling in demand since 2000 (Figure 1).2 The production of key commodity plastics will increase from 322 million metric tons (Mt) to 590 Mt by 2050, marking a 45% increase over three decades. As the global consumption of commodity plastics rises, the accumulation of plastic waste is expected to escalate correspondingly. A report by the Organization for Economic Co-operation and Development (OECD) highlights that, as of 2019, only 4% of plastic waste is recycled in the United States, with the global recycling rate estimated to be a mere 9%.3

Figure 1.

Figure 1

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Production of commodity plastics and management of plastic waste.2

There are four categories of recycling: primary, secondary, tertiary, and quaternary.4 Primary recycling, also known as closed-loop recycling, simply reuses postconsumer plastics for the same purpose as the original material without significantly altering its chemical and structural composition. For example, postconsumer plastics from water bottles is transformed into new plastic bottles. Secondary recycling, often referred to as mechanical recycling, processes a single stream of postconsumer plastic into resins, fibers, or sheets, with expanded applications. Due to its cost-effectiveness and simplicity, mechanical recycling is the most commonly practiced; however, this approach often results in materials of lower quality due to the partial degradation occurring during high-temperature reprocessing. The gradual degradation not only limits the potential applications of recycled plastics but also shortens their lifecycle. In addition, mechanical recycling necessitates rigorous sorting of plastic waste streams since only a single type of plastic can be reprocessed at a time.

On the other hand, quaternary recycling entails the combustion of plastic waste to produce energy, a method falling short of sustainability goals. Finally, tertiary recycling, known as chemical recycling, converts plastic waste into valuable chemicals or monomers, which can then be reformed into high-quality materials akin to original materials. Chemical recycling to monomers promotes a circular economy by allowing the continual reuse of materials, which has the added advantage of tolerating other additives and contaminants and potentially reducing the need for extensive sorting associated with mechanical recycling.5 Given these advantages, significant research is being directed toward improving the efficiency and cost-effectiveness of chemical recycling to provide a more sustainable strategy for plastic waste management.

2. Thermodynamic Considerations of Depolymerization

Polymerization is typically driven by a negative enthalpy change (ΔH°), while depolymerization is driven by a positive entropy change (eq 1). For example, in an exergonic olefin polymerization (ΔG° < 0), the cleavage of a C–C π-bond and formation of a C–C σ-bond lead to a negative ΔH°, whereas ΔS° is largely negative due to a decrease in the system’s degree of freedom.6 Increasing the temperature enhances the contribution of entropy and shifts ΔG°. The temperature at which ΔG° = 0, where the rates of polymerization and depolymerization are equal, is known as the ceiling temperature (Tc). The equilibrium favors polymerization when T < Tc and favors depolymerization when T > Tc. Several factors can influence Tc, including concentration, pressure, solvent, and monomer structure, all of which should be considered when comparing Tc values of different polymers.6−10 Additionally, the magnitude of the ratio between ΔH° and ΔS° provides insight into the difference in temperature from Tc required to theoretically achieve complete polymerization or depolymerization.5

2. 1

Despite the importance of ceiling temperature (Tc) in dictating polymerization/depolymerization equilibria, exploiting Tc alone to promote depolymerization is not effective for highly exergonic reactions such as ethylene polymerization or vinyl chloride polymerization.5 For polyethylene, thermolytic conditions result in byproducts such as long-chain waxes and other gases, achieving only moderate selectivity for ethylene.11 Similarly, the thermolysis of poly(vinyl chloride) can undergo side reactions before producing the monomer.12 This report discusses state-of-the-art approaches for achieving chemical recycling to monomer for these types of plastics, as well as other commodity plastics.

3. Commodity Plastics and Their SPI Codes

Each type of commodity plastic is assigned a unique identification code by the Society of the Plastic Industry, known as the SPI code. These SPI codes serve to distinguish the various types of common plastics, providing essential information to consumers about their recyclability. There are seven SPI codes designated for the most common commodity plastics, which are tabulated in Figure 2.13 This article discusses industrial and state-of-the-art lab methods for the chemical recycling of plastics corresponding to each SPI code number.

Figure 2.

Figure 2

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List of plastics and their SPI Codes.

3.1. Chemical Recycling of Poly(ethylene terephthalate) (PET, SPI Code 1)

Poly(ethylene terephthalate) (PET or PETE) is a polyester thermoplastic widely used in the beverage industry, serving as the primary material for packaging of bottled water, carbonated soft drinks, energy drinks, tea, and coffee. The United Nations University Institute for Water, Environment and Health reported that the estimated annual production of PET bottles in 2019 exceeded 600 billion units, equivalent to approximately 24 million tons of postconsumer PET waste.14 PET is marked with an SPI Code of 1, indicating its recyclability and encouraging consumers to participate in curbside recycling programs. In fact, PET is the most recycled plastic among commodity plastics, with approximately 29% of postconsumer PET waste being recycled in 2018.15 In commercial settings, PET is predominantly recycled mechanically into fibers which are further processed into products such as carpets, clothing, and other textiles. However, this mechanical recycling process, while economically viable, involves high temperatures for extruding PET into fibers. The forcing conditions lead to undesired degradation of PET through hydrolysis of the ester backbone, which decreases the molecular weight and reduces its quality. This degradation exacerbates with each repeated processing cycle.16

PET is typically synthesized through the condensation of terephthalic acid or dimethyl terephthalate with ethylene glycol 1, in the presence of a basic catalyst under heating conditions (Scheme 1A).17 Consequently, many industrial chemical recycling methods leverage the reverse process, hydrolysis, to recover the monomers.18,19 These recycling methods are categorized into three main types: hydrolysis (Scheme 1B), glycolysis (Scheme 1C), and methanolysis (Scheme 1D).

Scheme 1. Industrial PET Polymerization and Depolymerization.

Scheme 1

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Industrial hydrolysis of PET typically uses NaOH as a base catalyst due to its low cost, fast reaction rates, and high efficiency compared to other base-mediated depolymerization conditions (Scheme 1B).20 In these reactions, base-catalyzed hydrolysis of PET yields the disodium salt of terephthalic acid 2, which can be conveniently separated from insoluble reaction residues via filtration. Microwave irradiation has been employed to accelerate PET depolymerization, with a patent by GR3N reporting a 98% yield of terephthalic acid after just a 15 min residence time.21−23 Other patented processes by DePoly use UV irradiation with a TiO2 photocatalyst to promote hydrolysis. Additionally, enzymatic methods, such as those developed by Carbios, employ a modified leaf-branch compost cutinase (LCC) enzyme, proven to process up to 2 tons per batch, equivalent to approximately 100,000 bottles per cycle.24−26

Glycolysis conditions for converting PET into BHET 3 involve the use of ethylene glycol as the nucleophile to cleave the ester backbone (Scheme 1C). Patents from the industry have documented the use of Lewis acidic catalysts such as Zn(OAc)2 to promote the ester exchange of PET with an excess of ethylene glycol.27−31 Additionally, Ioniqa Technologies has developed a catalyst comprising an ionic liquid [Bmim]FeCl4, tethered to a magnetite nanoparticle via a bridging silane linker.32 By 2019, Ioniqa had implemented this innovative technology in a 10-kiloton recycling plant dedicated to the chemically recycling of postconsumer PET.33

The industrial methanolysis of PET employs methanol to hydrolyze the ester bonds within the polymer chain to afford 1 and 4, which can be purified through distillation (Scheme 1D). Patents from Loop Industries report high conversions to 4 at relatively mild temperatures between 60 and 100 °C, in the presence of cosolvents such as DCM and DMSO.34−37 Additionally, Recyc’Elit has developed patented conditions for low-temperature methanolysis, utilizing a mixture of alkoxide and amidine base catalysts alongside organic solvents.38 Another approach, vapor methanolysis, involves the feeding of methanol vapor into a reactor, allowing for the simultaneous collection of monomers through distillation during the reaction. Eastman Chemical has patented technologies based on vapor methanolysis and has announced commercialization plans to establish a facility with a 100,000 t capacity.39,40

Extensive research in academic settings has focused on improving these commercial technologies.41−43 More recently, significant efforts have been devoted to advancing enzymatic hydrolysis conditions, relevant to the technology reported by Carbios.44 Enzymes provide selectivity for PET within a mixed plastic waste stream, offering a clear advantage. However, challenges persist, such as the high costs associated with enzyme production and the separation of products postdepolymerization, which are more straightforward under traditional PET processing conditions.

3.2. Chemical Recycling of High-Density and Low-Density Polyethylene (HPDE, SPI Code 2; LDPE, SPI Code 4)

Polyethylene (PE) is a fully saturated thermoplastic and the most produced plastic worldwide. The two most prevalent types of PE are high-density polyethylene (HDPE), identified by an SPI code of 2, and low-density polyethylene (LDPE), identified by an SPI code of 4. HDPE is produced using Ziegler–Natta catalysts, including heterogeneous titanium complexes with triethylaluminum activators, as well as homogeneous group 4 metallocenes with methylaluminoxane activators.45,46 Applying low pressure ethylene results in linear structures with minimal branching.47 HDPE is highly crystalline and stiff, ideal for manufacturing bottles, buckets, pipes, and other containers. In contrast, the synthesis of LDPE with radical polymerization under high pressures of ethylene leads to irregular structures with significant branching.48 This gives LDPE low crystallinity and high flexibility, suitable for applications in disposable bags, food packaging, and wrapping films.

As of 2019, the annual production of PE in the United States reached approximately 23 million metric tons.49 Despite its widespread utilities, only about 930 thousand tons of PE were recycled in the United States.50 The mechanical recycling of PE, depending on extrusion conditions, can lead to either oxidation of the carbon backbone or extensive cross-linking and branching. Both alterations occur through hydrogen abstraction mechanisms in the presence of oxygen.51,52 These structural modifications can gradually degrade the quality of the recycled PE material over repeated extrusion processes.

Industry patents for the chemical recycling of PE, held by companies such as Chevron, SABIC, and Proctor & Gamble, employ pyrolytic conditions in catalytic reactors to produce complex mixtures of hydrocarbons (Scheme 2A).53−57 For example, GreenMantra Technologies has developed a [Fe–Cu–Mo-P] catalyst supported on aluminum oxide to convert postconsumer PE waste into commercially relevant C14–C41 waxes and greases.58 These pyrolysis conditions, however, are extremely energy intensive, often requiring high temperatures and pressures.

Scheme 2. Industrial and Forefront Methods for PE Chemical Recycling.

Scheme 2

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Several state-of-the-art methods for the chemical recycling of PE use hydrogenolysis conditions to fragment the carbon backbone into gas, liquid fuels, and wax products with chain lengths ranging from C1 to C45 (Scheme 2B).59 Heterogeneous transition metal catalysts based on Ru, Zr, Rh, Pt, and Co are typically employed at high temperatures and elevated pressures of hydrogen gas. These reactions may proceed either through σ-bond metathesis or β-alkyl transfer to achieve C–C bond cleavage of PE. For example, a recent study utilizes a heterogeneous Pt/γ-Al2O3 catalyst to convert PE waste into long-chain alkylbenzene products, which are crucial feedstocks for detergent manufacturing.60 Additionally, commercial Ru/C catalysts modified with ethylene glycol promote the formation of Ruδ+ species, which performs σ-bond metathesis with PE five times more efficiently than traditional Ru/C catalysts.61

Alternative PE depolymerization strategies include oxidation (Scheme 2B). Catalytic systems consisting of Co(II), Mn(II), and NHPI (NHPI = N-hydroxyphthalimide) can convert PE into lower molecular weight dicarboxylic acids.62,63 NHPI serves as an initiator by abstracting hydrogen atoms on PE, leading to the formation of carbon radicals. The radicals are trapped by O2 to form a peroxide intermediate, which is then homolyzed to an alkoxy radical through the Co(II)/Mn(II) catalytic system. β-scission of the alkoxy radical, followed by further oxidation, affords the lower molecular weight carboxylic acid products. These conditions have also been demonstrated to depolymerize PET and polystyrene (PS) into terephthalic acid and benzoic acid, respectively. Additionally, oxidative depolymerization of PE to dicarboxylic acids applies a heterogeneous Ru/TiO2 catalyst.64

More recently, an alternative approach employs a two-step dehydrogenation/ethenolysis reaction to convert PE waste into propylene (Scheme 2D).65 The first dehydrogenation step employs 6, an Ir hydride complex supported with a PCP pincer ligand, to incorporate up to 3.2% of internal alkene within the backbone of PE. This material is amenable to ethenolysis using the second generation Hoveyda-Grubbs catalyst (HG-II), in conjunction with 8, an alkene isomerization catalyst, under 25 bar of ethylene gas. The overall process can produce up to 80% yield of propylene from both HDPE and LDPE. Another tandem strategy that combines dehydrogenation, metathesis, and isomerization, uses dehydrogenation catalyst 9, olefin metathesis catalyst 10, and heterogeneous isomerization catalyst 11. This method is implemented in a semicontinuous reactor to convert PE into propylene with up to 94% selectivity.66

Although the leading methods are milder and more selective than those currently used in industry, the complexity of these reactions may render them impractical for commercial applications. Moreover, the use of some noble transition metal catalysts may increase the cost of the process. Nevertheless, these advances represent significant achievements in progressing toward a circular economy through postconsumer PE waste management.

3.3. Chemical Recycling of Poly(vinyl chloride) (PVC, SPI Code 3)

Poly(vinyl chloride) (PVC) is a high-strength thermoplastic and the third most produced plastic worldwide. Approximately 44 million metric tons of PVC were produced globally in 2018, with projections indicating a 35% increase by 2025.67 PVC is extensively used in the construction industry for window and door profiles, as well as drinking and wastewater piping, but it also finds applications in the automobile and packaging industries.68 Suspension polymerization is the most widely used commercial method to synthesize PVC.69 This process involves agitating a biphasic mixture of water and vinyl chloride, where vinyl chloride is polymerized using a monomer-soluble free radical initiator within aqueous droplets. As the polymerization progresses, PVC precipitates, facilitating isolation.

PVC, identified by an SPI code of 3, should not be mixed with other recyclable plastics due to its incompatibility during the recycling process. This incompatibility largely stems from additives used to enhance thermal and UV stability, including heavy metal stabilizers such as tin, cadmium, and lead.70 These additives also complicate the mechanical recycling of postconsumer PVC, necessitating their removal prior to recycling. Advanced sorting methods and separation techniques are available to separate PVC from other plastic waste and additives, but these processes are often costly and labor-intensive.71,72 Consequently, despite being the third most produced plastic globally, PVC has the lowest recycling rate among commodity plastics.41 Typically, 82% of postconsumer PVC waste is treated through landfilling, and 15% through incineration.73

Unlike other commodity plastics, PVC is generally not recommended for direct pyrolysis due to the emission of HCl gas and other hazardous chlorine-containing products, which have hindered its commercial-scale processing.74 Consequently, alternative chemical recycling strategies have been developed (Scheme 3A). Gasification involves treating PVC with controlled amounts of oxygen or steam under high temperatures to produce CO2, syngas (CO and H2), and HCl. The primary advantage of gasification is that it converts nearly all of the chlorine content in PVC waste into water-soluble HCl.67 Currently, two companies in Japan, Sumitomo Metals and Ebara, operate commercial-scale PVC recycling plants using gasification methods.75,76 Additionally, dehydrochlorination processes have been developed to remove chlorine from PVC waste before gasification or pyrolysis.77 For example, DSM Research in The Netherlands has developed the REDOP process to dechlorinate mixed plastic waste containing PVC.78 The generated HCl is quenched using a water-soluble base, and the dechlorinated plastic waste is recovered as insoluble granules. In Germany, Alzchem operates a plant that converts mixed plastic waste streams containing PVC into syngas, calcium carbide, and HCl, which is then recovered as an aqueous solution.79

Scheme 3. Industrial and Forefront Methods for PVC Depolymerization.

Scheme 3

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State-of-the-art methods have been developed to provide milder and more sustainable conditions for chemically recycling PVC. For example, the biodegradation of PVC has been reported through larvae consumption using a strain of Spodoptera frugiperda that can consume PVC films as its sole energy source (Scheme 3B).80,81 After an incubation period of 90 days, a biofilm formed on the surface of the PVC film, containing a mixture of oxidation products, including 12 and other long-chain esters, as detected by GC-MS analysis. A catalase-peroxidase enzyme is hypothesized to be responsible for this biodepolymerization of PVC.

A tandem dehydrochlorination-metathesis process has been reported to depolymerize PVC (Scheme 3C).82 Base-mediated elimination first introduces alkene functionalities into the PVC backbone. Applying allyl alcohol as a cosolvent grafts allyl ether groups onto the polymer, which serves as a handle to promote ring-closing metathesis. Applying Grubbs-II catalyst to 13 in the presence of 14 yields a complex mixture of alkenes, with 15 observed as the main low molecular product. A photocatalytic method can oxidize and degrade PVC into acetic acid using a Nb2O5 photocatalyst (Scheme 3D).83 The Nb2O5 photocatalyst is proposed to facilitate the formation of hydroxyl radicals, which mediate the oxidative C–C bond cleavage of PVC to generate CO2. The photoreduction of CO2 then generates CO2H radicals, which can dimerize and undergo further reduction to form acetic acid. On the other hand, chloride might be oxidized to form Cl2 or HOCl. This reaction is also effective in converting PE and polypropylene (PP) into acetic acid.

Despite the commercialization of various PVC recycling methods, significant opportunities remain to enhance the sustainability of these processes, especially considering the challenges associated with handling postconsumer PVC waste. Biodegradation represents an intriguing strategy, as PVC is traditionally considered a nonbiodegradable material. However, the complexity and efficiency of these methods pose substantial challenges in matching the simplicity and scalability of existing gasification and dehydrochlorination processes.

3.4. Chemical Recycling of Polypropylene (PP, SPI Code 5)

Polypropylene (PP) is a thermoplastic with the lowest density among commodity plastics.84 PP is more rigid and thermally stable compared to other polyolefins like HDPE and LDPE, making it suitable for applications requiring high temperature or chemical resistance, such as food packaging, automotives, construction and textiles.85 For example, food containers made from PP are typically microwave and dishwasher safe, unlike those made from PET. Due to its resistance to chemical degradation, PP is also widely used in medical and scientific fields for single-use laboratory consumables like pipettes, syringes, and tubes.

In 2018, approximately 56 million metric tons of PP were produced worldwide, representing the second most produced commodity plastic.86 PP is synthesized through chain growth polymerization of propylene using either heterogeneous supported transition metals or homogeneous group 4 metallocene Ziegler–Natta catalysts.87 PP can be produced with different stereochemical configurations: isotactic, syndiotactic, and atactic. Commercial Ziegler–Natta catalysts typically make isotactic PP, which means the methyl substituents are arranged on the same side of the polymer backbone. Atactic PP, which has the methyl substituents arranged randomly along the polymer, and syndiotactic PP, which have the methyl substituents arranged on alternating sides of the polymer, exhibit lower melting points compared to isotactic PP and are much less commercially relevant.

PP can be identified with an SPI code of 5 and is less commonly accepted by curbside recycling programs compared to other plastics such as PET and HDPE. Despite its widespread use across numerous industries, the recycling rate for PP in the United States was only 1% in 2017.88 Like many thermoplastics, mechanical recycling offers a straightforward method for managing postconsumer PP waste, but it has limitations, such as degradation of the PP backbone through repeated recycling processes. This degradation, which results in lower quality material,89 proceeds through hydrogen atom abstraction followed by chain scission and typically occurs at the high temperatures required during the extrusion processes of mechanical recycling. Therefore, incineration and landfilling remain the most common methods for disposing of PP waste.90

The chemical recycling of postconsumer PP waste is often treated as a mixture with other polyolefins.91 Therefore, many of the pyrolysis technologies discussed above also apply to PP waste, in addition to some commercial pyrolysis processes tailored for PP (Scheme 4A). Encina’s patented process uses a zeolite catalyst to convert PP into feedstock chemicals such as benzene, toluene, xylene, and propylene.92 Encina, operating in the United States, plans to build a $1.1 billion facility to implement this recycling technology.93 Additionally, Nexus Circular has patented pyrolytic treatments of PP and currently operates a 50 ton/day plant in the United States.94

Scheme 4. Industrial and Forefront Methods for PP Depolymerization.

Scheme 4

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State-of-the-art conditions implement hydrogenolysis to convert PP into mixtures of hydrocarbon products (Scheme 4B). A Ru/C catalyst produces C5–C32 iso-alkanes at 200–250 °C under 20–50 bar H2.95 This method has also been demonstrated to be effective with a mixture of PP and PE waste. Additionally, a Ru/TiO2 catalyst can convert PP into hydrocarbons that are valuable as lubricants.96 The reaction proceeds at 250 °C under 30 bar of H2.

Recently, methods that leverage temperature-gradient technologies have been developed to improve selectivity for specific products. A thermochemical depolymerization of PP employs a reactor that induces pyrolysis through electrified spatiotemporal heating (Scheme 4C).97 This depolymerization process features a spatial temperature gradient and a temporal heating profile. A porous carbon bilayer is electrically heated, conducting heat to an underlying reactor containing PP. The resulting temperature gradient promotes continuous melting and reaction as the plastic material moves through the carbon bilayer. Pulsing the electrical current allows for a temporal heating profile, enabling precise control over the duration the plastic material is exposed to peak temperatures. This process yields 36% propylene and has also proved effective with PET waste. Moreover, a temperature-gradient reactor has been designed to prevent the complete pyrolysis of PP into small molecules (Scheme 4D).98 The waxes produced after thermolysis are subjected to oxidation using a Mn stearate catalyst to produce surfactants. This reactor is also compatible with municipal HDPE and LDPE waste.

3.5. Chemical Recycling of Polystyrene (PS, SPI Code 6)

Polystyrene (PS) is a thermoplastic typically characterized by its rigidity and brittleness. It is available in two common forms: solid and foamed.99 Solid PS is utilized in various applications across numerous industries, including packaging, consumer goods, and the production of drinking cups and lids. Foamed PS, commercially known as Stryofoam, is a trademark of DuPont and extensively used in both packaging and construction sectors. In 2022, the global production reached approximately 15 million metric tons, making it the fourth most-produced commodity plastic.100

PS is predominantly synthesized through bulk free radical polymerization of styrene, employing a radical initiator. Styrene is also frequently copolymerized with butadiene to form high impact polystyrene (HIPS).101 Additionally, similar to PP, PS can exhibit various tacticity. In industrial settings, atactic PS is most common, where the stereochemistry α to the phenyl substituents is random.

PS is designated with an SPI code of 6, signaling to consumers that it is not suitable for regular curbside recycling programs and should be segregated from other recyclable plastics. Foamed PS, which can contain up to 95% air, presents particularly formidable recycling challenges due to the logistical and economic difficulties involved in transporting and processing this lightweight waste.102 Therefore, only about 1% of postconsumer PS is recycled annually. Like many thermoplastics, PS undergoes significant degradation through mechanical recycling, especially after multiple cycles. Chain scission represents the most prevalent form of degradation during reprocessing, although cross-linking may also occur at higher temperatures. Given these recycling obstacles, the end-of-life management for PS typically involves either landfilling or incineration.

Several industrial operations are dedicated to recycling PS through purification and reformation processes, including facilities operated by Polystyvert and PolystyreneLoop.103,104 Additionally, there are established industrial methods for the chemical recycling of PS, primarily through pyrolysis (Scheme 5A).105−109 Agilyx, in partnership with Ineos Styrolution, has announced plans to build a PS recycling plant in the United States with a capacity to process up to 100 tons of waste daily.110 Ineos also operates multiple plants in Europe, which collectively convert approximately 15,000 tons of PS into styrene each year.111 In Asia, Indian Oil Corp. has developed a catalytic pyrolysis process for the chemical recycling of PS. They are currently advancing plans to establish a commercial plant in India with the capacity to handle 387,000 tons of waste per year.112 Analysis of the oil fraction obtained after pyrolysis reveals the formation of styrene in 33% yield, along with monoaromatic products such as toluene, ethylbenzene, and α-methyl styrene. Additionally, poly aromatic products such as stilbene, 1,3-diphenyl propane, and 1,4-diphenyl-1-butene are also present.113

Scheme 5. Industrial and Forefront Methods for PS Depolymerization.

Scheme 5

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Recently, photoexcitation has been explored as a milder and more sustainable alternative to thermal methods for the depolymerization of PS. Irradiation of PS with white LED in the presence of FeCl3 as a catalyst at room temperature under ambient atmosphere produces a mixture of oxidized aromatic products with yields up to 23% (Scheme 5B).114 The mechanism involves the generation of chlorine radicals that abstract the benzylic hydrogen-atom from PS. The resultant radical is trapped by O2 and undergoes β-scission to furnish the cleaved product. Concurrently developed, another photoinduced method employs p-TsOH as an acid catalyst and 405 nm light under ambient temperature and atmosphere (Scheme 5C).115 Singlet oxygen, generated by blue LED light, abstracts a hydrogen atom from PS. The resulting radical is subsequently trapped by O2, leading to β-scission and the production of a mixture of oxidized aromatic products along with formic acid 16.

Recently, a photoinduced oxidative reaction uses a porphyrin derivative as the photocatalyst to generate singlet oxygen through UV light irradiation, yielding benzoic acid up to 71% (Scheme 5D).116 Additionally, a light-induced depolymerization of PS, utilizing catalytic AlCl3 under ambient temperature and pressure, produces benzene as the major product (Scheme 5E).117 With the addition of CH2Cl2, diphenylmethane 18 is formed in 87% yield via a Friedel–Crafts reaction. This process has been demonstrated to be scalable to a ton-scale batch reactor.

Although promising methods for PS chemical recycling have been developed in both industry and academia, significant challenges remain in the logistics and cost to collect and transport foamed PS. Currently, it is more cost-efficient to simply landfill or incinerate this type of postconsumer waste. Thus, there is still a need for more cost-effective chemical recycling processes.

3.6. Chemical Recycling of Other Commodity Polymers (SPI Code 7)

SPI code 7 includes a wide range of common plastics that lack standardized recycling protocols and are not produced on the same scale as the first six commodity plastics. Notable examples of plastics in this miscellaneous category include poly(methyl methacrylate) (PMMA), polycarbonate (PC), polyamide (PA, or Nylon), and polysiloxane.

3.6.1. Chemical Recycling of Poly(methyl methacrylate) (PMMA)

Poly(methyl methacrylate) (PMMA), also known as Plexiglas, is a transparent thermoplastic often used as a cheaper, lightweight, and shatter-proof alternative to glass. Annually, approximately 3.9 million metric tons of PMMA are produced globally through the free radical polymerization of methyl methacrylate (MMA).118,119 Despite its broad applications in consumer goods, electronics, construction, and automobiles, only about 10% of PMMA waste is recycled each year. Landfilling and incineration remain the predominant end-of-life treatment for postconsumer PMMA waste.

Current industrial technologies implement thermal gasification and pyrolysis to convert PMMA waste into MMA (Scheme 6A).120−124 The pyrolytic depolymerization mechanism is proposed to proceed through the homolytic scission of main-chain C–C bonds, followed by radical depropagation.125 In 2021, Agilyx, in collaboration with Mitsubishi Chemical Corp., successfully operated a pilot-scale facility for the pyrolytic depolymerization of PMMA, with plans to further commercialize the process.126 In Europe, a collaboration between Trinseo and Japan Steel Works leads efforts in the chemical recycling of PMMA, with a depolymerization plant set to be commissioned in Italy in 2024.127 Additionally, in Asia, Sumitomo Chemical has partnered with Japan Steel Works to build a pilot facility that will convert PMMA waste into MMA.128

Scheme 6. Industrial and Forefront Methods for Depolymerization of PMMA.

Scheme 6

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A recent advancement in chemical depolymerization directly converts PMMA to MMA under photothermal conditions (Scheme 6B).129 This method employs carbon quantum dots (CQDs) as a photothermal agent. Upon irradiation, a temperature gradient is established near the CQD surface, inducing the thermal cleavage of proximal C–C bonds. This process has also been successfully applied to other polymer classes, including poly(α-methylstyrene) (PAMS) and PS.

Additionally, modern depolymerization methods for PMMA leverage labile end-groups from reversible-deactivation radical polymerization (RDRP) or comonomers from bulk polymerization to reduce the initial decomposition temperature (Scheme 6C).130−138 These labile groups are typically activated by light, heat, or single-electron transfer (SET). While these approaches are efficient, mild, and yield high-quality MMA, they are not applicable to existing PMMA waste.

3.6.2. Chemical Recycling of Polycarbonate (PC)

Polycarbonate (PC) refers to a group of transparent thermoplastics often compared to PMMA but noted for their greater tensile strength and higher thermal stability.139 The most commonly used type is poly(bisphenol A carbonate) (BPA-PC), synthesized through a base-mediated addition of bisphenol A (BPA) to phosgene. Alternatively, nonphosgene methods using CO2 as a raw material can avoid using highly toxic phosgene.140 PCs are used across various industrial sectors, including consumer products, electrical, construction, and automobiles, and are also employed in specialized applications such as bulletproof glass.141 More recently, due to the potential adverse health effects of BPA, the FDA, EU, and Canada have prohibited its use in food packaging and infant bottles. As a result, other compounds, including benzoic acid, bisphenol A diglycidyl ether (BADGE), and butylated hydroxytoluene (BHT), have been explored as nontoxic alternatives.142 In 2016, global production of PCs totaled approximately 4.4 million metric tons.143

Similar to other plastics, the mechanical recycling of PCs results in materials with diminished properties, such as reduced impact resistance, after repeated recycling processes.144 Although pyrolysis of PCs yields complex mixtures of aromatic products, there are industrial methods that recover bisphenol A (BPA) through pyrolytic treatments (Scheme 7A).145−147 Covestro operates a pilot-scale pyrolysis plant in Germany that converts PC back into its monomers.148 Mitsubshi Chemical Corp. has also announced plans to build a pilot facility, with future expansions projected to reach a capacity of 10,000 tons per year.149 Hydrolysis of the PC backbone offers a milder and more selective method for recovering BPA compared to pyrolysis (Scheme 7A). This process typically involves the use of a metal hydroxide in aqueous solutions at elevated temperatures, yielding BPA and CO2. These conditions have been patented by companies such as SABIC and Teijin Chemicals.150−152 Modern methods for the depolymerization of PC employ catalysts to improve the efficiency and selectivity of hydrolysis or pyrolysis.153

Scheme 7. Industrial and Forefront Methods for Depolymerization of PC.

Scheme 7

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Hydrogenolysis presents a promising alternative to hydrolysis and pyrolysis for the depolymerization of PC, primarily producing BPA and methanol (Scheme 7C). Applying 0.5 mol % of Ru catalyst 19 at 140 °C under 100 bar of H2, this method can convert a PC compact disc (CD) into BPA with a 73% isolated yield.154 More recently, employing 5 mol % of Milstein’s pincer Ru catalyst 20 under the same conditions has been shown to achieve a quantitative yield of BPA.155 Additionally, a reaction using 0.5 mol % of Ru catalyst 21 at a lower temperature of 80 °C under 45 bar of H2 has also yield BPA quantitatively.156

3.6.3. Chemical Recycling of Nylon

Poly(amide), commonly known as Nylon, is a class of thermoplastics known for being soft with high elongation and abrasion resistance.157 Nylon is typically processed into fibers, which are widely used in textiles for clothing, carpets, and other consumer goods. Nylon is also molded into resins and used in the automobile industry. Nylon can be synthesized through polycondensation between diamines and dicarboxylic acids, or via the hydrolytic polymerization of lactams.158,159 The chain length of the monomers determines the nomenclature of the resulting polymer; for example, Nylon-6,6 is synthesized from adipic acid and hexamethylenediamine, both containing 6 carbon atoms, whereas Nylon-1,6 is formed from formaldehyde and adiponitrile. In 2019, 5.6 million tons of Nylon were produced, of which only 2% was manufactured using recycled materials.160

Mechanical recycling of Nylon can lead to hydrolysis of the polyamide backbone at elevated extrusion temperatures, resulting in a lower quality material.147 Due to the susceptibility of Nylon to hydrolyze, many industrial developments leverage this reactivity to recover monomers, a process that can occur both with and without acid or base catalysts (Scheme 8A).161−167 Notably, conditions reported by companies such as BASF, Du Pont, and Toray Industries focus on Nylon-6, with ε-caprolactam being recovered as the monomeric product. In 2023, Toray Industries announced a partnership with Honda Motor Co. to develop a 500 t/year pilot facility to recover ε-caprolactam from Nylon-6 waste.168 This technology utilizes supercritical water at elevated temperatures and operates catalyst-free. Asahi Kasei also has announced a partnership with Microwave Chemical to implement microwave technology for the depolymerization of Nylon-6,6 to recover adipic acid and hexamethylenediamine, which have plans to build a small-scale operation in 2024.169

Scheme 8. Industrial and Forefront Methods for Depolymerization of Nylon 6.

Scheme 8

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Advanced depolymerization techniques employ transition-metal catalysts to process Nylon under conditions that are milder and more selective than typical industrial methods. Hydrogenation with a pincer ruthenium catalyst 22 at 150 °C under 70 bar H2 allows for the conversion of Nylon-6 into amino alcohol 23 with a 24% yield (Scheme 8B).170 The proposed mechanism involves hydride transfer from a ruthenium dihydride intermediate to the polyamide, resulting in the formation of a hemiaminal. Under basic conditions, the hemiaminal undergoes C–N bond cleavage, yielding an amine and an aldehyde; the latter is then reduced to an alcohol. This reaction has proven effective across a wide spectrum of Nylon polymers.

The application of 5 mol % of lanthanide catalyst 24 can depolymerize Nylon-6 into ε-caprolactam with a 90% yield at 240 °C under a static vacuum of 10–3 Torr (Scheme 8C).171 This lanthanide catalyst acts as a Lewis-acid, facilitating the intramolecular cyclization of the polymer backbone to release ε-caprolactam. Reactivity has been further improved by using 1 mol % of lanthanide catalyst 25 at the same temperature, achieving 99% yield of ε-caprolactam. The bulky bis(cyclopentadienyl) (Cp*) ligands help minimize catalyst deactivation previously observed with 24.172 This method is effective across a broad range of postconsumer Nylon-6 waste materials.

3.6.4. Chemical Recycling of Polysiloxane

Polysiloxane is a class of thermoplastics characterized by a Si–O backbone with alkyl or aryl substituents. Depending on the polymer chain length, end groups, and the addition of fillers or additives, these materials can exist as either oils or rubber-like substances.173 Polydimethylsiloxane (PDMS), also known as dimethicone, is one of the most common silicone polymers. PDMS is used in a broad range of applications, from silicone oil and lubricants to silicone rubber and cosmetics.174 The industrial synthesis of PDMS involves the polymerization of dimethylchlorosilane with water, which releases HCl as a stoichiometric byproduct. In 2013, an estimated 2.1 million tons of polysiloxanes were produced worldwide.175

Depolymerization of PDMS typically occurs through Si–O bond cleavage, which can be mediated by catalytic or stoichiometric acids and bases.176−179 Initially, hydrolytic cleavage of a main chain Si–O bond forms a silanolate, which undergoes intramolecular σ-bond metathesis, known as “backbiting”, releasing cyclic siloxanes as products. This mechanism can also be initiated by nucleophiles and electrophiles or by heating above 350 °C in the presence of catalysts.163,180 The thermodynamically favored product is hexamethylcyclotrisiloxane (D3); however, octamethylcyclotrisiloxane (D4), decamethylcyclotrisiloxane (D5), and dodecamethylcyclotrisiloxane (D6) can also be formed at lower temperatures under specific conditions. At temperatures above 500 °C, homolytic scission of Si–C side chain bonds can occur, leading to the generation of methane radicals and cross-linking of PDMS chains.181

Industrial processes utilize both catalytic and noncatalytic pyrolysis to depolymerize PDMS into cyclic siloxanes (Scheme 9A).182 Dow Corning has utilized various heterogeneous Lewis acids to convert PDMS into a mixture of cyclic siloxanes at 350 °C.183 Additionally, Dow has announced a partnership with Circusil to build a silicone recycling plant in the United States, scheduled to commence operations in 2024.184 Similarly, Evonik has developed processes using Brønsted acids, which also generate cyclic siloxanes as depolymerization products at temperatures up to 600 °C.185,186 Wacker Chemie has developed noncatalytic methods where PDMS waste is transformed into metallurgic silicon at temperatures reaching 1900 °C.187

Scheme 9. Industrial and Forefront Methods for Depolymerization of PDMS.

Scheme 9

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State-of-the-art conditions aim to enhance base-mediated and nucleophilic mechanisms for PDMS recycling. A phosphazenium silanolate catalyst 26 facilitated the depolymerization of PDMS with a 0.1 mol % loading and at ambient temperature to afford a mixture of cyclic siloxanes in 82% yield (Scheme 9B).188 A crown ether-ligated silanolate catalyst 27 promoted the depolymerization of PDMS to cyclic siloxanes with up to 99% yield at 140 °C.189 The reaction proceeds without the need for a solvent, and products can be easily recovered through vacuum distillation. Leveraging the propensity for Si–F bond formation, 0.5 mol % of TBAF at ambient temperature depolymerizes PDMS to a mixture of cyclic siloxanes (Scheme 9C).190 The depolymerization process is initiated by the addition of fluorine to a silicon center within the polymer backbone. After the cleavage of the Si–O bond, a silanolate is formed, which can undergo backbiting to release cyclic siloxanes.

3.6.5. Chemical Recycling of Epoxy Resin

Epoxy resin is a class of thermoset plastics characterized by extensive cross-linking.191 Due to its favorable properties in electrical insulation, solvent resistance, and mechanical strength, epoxy resins have many applications, including packaging, adhesives, and reinforced composites.192 The synthesis of epoxy resins typically involves the condensation of an epoxide monomer with two or more oxirane groups with a curing agent, which can include both aliphatic and aromatic amines, amides, phenols, and acids. Bisphenol A (BPA) is the most used curing agent. In 2022, the global production of epoxy resins exceeded 6 million tons.193

Pyrolysis of epoxy resins presents a straightforward method to convert postconsumer waste into monomers, but necessitates high temperature conditions, which are not sustainable (Scheme 10A).194 State-of-the-art methods aim to offer milder conditions by applying transition-metal catalysis or base-mediated degradation. The use of ruthenium catalyst 19 has been shown to deconstruct epoxy resins into BPA (Scheme 10B).195 The reaction proceeds through dehydrogenation of the secondary alcohol, followed by oxidative addition of Ru into the adjacent PhO–C bond. The reaction is compatible with several types of commercial epoxy resins, recovering up to 81% yield of BPA. Nickel-catalyzed hydrogenolysis of epoxy resins produces BPA in 66% isolated yield from a diamine-cured epoxy resin (Scheme 10C).196 NaOH or KOtBu can mediate the degradations of epoxy resins to yield BPA in up to 81% and 71% yield, respectively (Scheme 10D and 10E).197,198 Lastly, a photocatalytic method with an acridine photocatalyst converts thiol-cured epoxy resins into BPA (Scheme 10F).199 The reaction operates through a proton-coupled electron transfer (PCET) mechanism, which promotes C–C cleavage through β-scission.

Scheme 10. Methods for Depolymerization of Epoxy Resins.

Scheme 10

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3.7. Forefront Methods Targeting Multiple SPI Codes

Currently, technologies for sorting postconsumer plastic waste are time-consuming, expensive, or have inherent limitations. A typical protocol applies a combination of techniques, including manual sorting, sink-float, and infrared (IR) spectroscopy.200 Manual sorting, which relies on recognizing SPI codes, is inefficient and cost-intensive. Sink-float methods, which exploit differences in the densities of each type of plastic, can be expensive due to the need for dedicated washing, drying, and wastewater treatments.201 Sorting processes based on IR spectroscopy use an IR detector to identify different types of plastics. Although these IR techniques are typically automated, they have limitations, such as difficulty analyzing colored or contaminated materials, which can interfere with IR detection and impede accurate identification.200

Conditions that convert various types of plastics into valuable chemicals or raw materials could eliminate the need for rigorous sorting of plastic waste. Several examples demonstrate compatibility with different materials, including those capable of recycling both HDPE and LDPE. For example, the oxidative degradation of PE could also be applied to PS and PET.55 Similarly, the photocatalytic conditions were shown to convert PE and PP, in addition to PVC, into acetic acid.76 Additionally, the photothermal conditions to depolymerize PMMA are also effective in depolymerizing PS and polylactic acid (PLA) into monomers.121

More recently, a photocatalytic conditions employing commercially available vanadium catalyst V(O)(acac)2 have been reported to convert SPI codes 2–7 into valuable chemicals such as formic acid, benzoic acid, and acetic acid (Scheme 11).202 The vanadium peroxide catalyst 28, generated from V(O)(acac)2 and O2, can be photoexcited through ligand-to-metal charge transfer (LMCT). The triplet excited state of the catalyst conducts H-atom abstraction to generate a radical within the polymer, which is trapped by V(IV) 29 to afford intermediate 30. Photoexcitation of 30 triggers LMCT to access its triplet excited state, which facilitates C–C bond cleavage through alkyl group transfer to generate 31. This process has been shown to be compatible with HDPE, PVC, LDPE, PP, PS, and SPI code 7 plastics such as polyvinyl acetate (PVAc) and poly(ethylene-vinyl acetate) (PEVA).

Scheme 11. Vanadium-photocatalyzed Depolymerization of SPI Codes 2-7.

Scheme 11

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4. Conclusion

Chemical recycling of plastics provides access to valuable chemicals and high-quality raw materials from postconsumer waste, which can be reprocessed into materials with properties comparable to those of virgin materials. This cyclical process of polymerization and depolymerization promotes a circular economy with favorable economic and environmental implications. Pyrolysis and methods involving inexpensive commodity chemicals are standard practice for chemical recycling of plastics in the industry due to their simplicity and cost-effectiveness. However, these methods often require high temperatures and/or pressures, making them energy-intensive and less sustainable for chemical recycling. Thus, there is a significant challenge and opportunity to develop more efficient, cheaper, and milder methods.

Two approaches can improve plastic waste management: (1) developing improved catalytic conditions for chemical recycling, and (2) creating new plastics more amenable to large-scale chemical recycling. The former approach relies on several considerations for catalyst and condition development, including low operating costs, stability against air, moisture, and contaminants, compatibility with heterogeneous systems, and efficient conversion to monomer.203 Ideally, these conditions would also be solvent-free, rapid, function at modest temperatures. To date, most advancements have involved adapting known reactions and catalysts from small molecules to polymers. However, the physical properties of plastics, including their poor solubility, have posed restrictions on many catalytic systems. Additionally, achieving selectivity in polymer remains a challenge; for example, reactions involving radical intermediates frequently lead to cross-linking. Looking ahead, developing new catalysts especially tailored for polymers to facilitate reaction occurring at the interface of the plastic and solution phases could have a significant impact.

Alternatively, novel plastic materials that retain favorable mechanical and thermal properties could be depolymerized under conditions or stimuli that are milder and more cost-effective than current industrial standards.204 While this approach offers a promising solution for the future, it does not address the issue of existing waste material and may encounter significant barriers to adoption within the commodity production industry.

Acknowledgments

This work receives support from the U.S. Department of Energy, Office of Basic Energy Sciences, through Catalysis Science under award number DE-SC0022300. M.C. is grateful for the Dean’s Dissertation Fellowship.

The authors declare no competing financial interest.

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