
==== Front
ACS Appl Mater Interfaces
ACS Appl Mater Interfaces
am
aamick
ACS Applied Materials & Interfaces
1944-8244
1944-8252
American Chemical Society

39214617
10.1021/acsami.4c09315
Perspective
Interfacial Reactions in Chemical Recycling and Upcycling of Plastics
Ong Albert †
Teo Jerald Y. Q. †
https://orcid.org/0000-0002-8020-1720
Lim Jason Y. C. *†‡
† Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Innovis #08-03, Singapore 138634, Republic of Singapore
‡ Department of Materials Science and Engineering, National University of Singapore (NUS), 9 Engineering Drive 1, Singapore 117576, Singapore
* Email jason_lim@imre.a-star.edu.sg.
30 08 2024
11 09 2024
16 36 4697546987
07 06 2024
31 07 2024
29 07 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

Depolymerization of plastics is a leading strategy to combat the escalating global plastic waste crisis through chemical recycling, upcycling, and remediation of micro-/nanoplastics. However, critical processes necessary for polymer chain scission, occurring at the polymer–catalyst or polymer–fluid interfaces, remain largely overlooked. Herein, we spotlight the importance of understanding these interfacial chemical processes as a critical necessity for optimizing kinetics and reactivity in plastics recycling and upcycling, controlling reaction outcomes, product distributions, as well as improving the environmental sustainability of these processes. Several examples are highlighted in heterogeneous processes such as hydrogenation over solid catalysts, reaction of plastics in immiscible media, and biocatalysis. Ultimately, judicious exploitation of interfacial reactivity has practical implications in developing practical, robust, and cost-effective processes to reduce plastic waste and enable a viable post-use circular plastics economy.

catalysis
valorization
depolymerization
circular economy
sustainability
National Research Foundation Singapore 10.13039/501100001381 NRF-NRFF15-2023-0007 document-id-old-9am4c09315
document-id-new-14am4c09315
ccc-price
==== Body
pmc1 Introduction

Plastics are indispensable in modern society due to their low production cost, high durability, and versatility for a wide range of domestic and industrial applications. However, their prevalence and current unsustainable usage practices have resulted in tremendous environmental footprints from their end-of-life in landfills, incinerators, and irresponsible disposal in the environment. Particularly hazardous are microplastics (defined as plastic particles >100 nm and <5 mm)1 which are challenging to detect, yet are highly detrimental to the biosphere and human health due to their propensity of toxin sorption and translocation across the food chain. These problems are being tackled at multiple levels. While wastewater treatment plants are able to remove a large majority of microplastics,2 large quantities are still released in discharge affluents, and this has inspired development of alternative chemical methods such as Fenton oxidation for microplastic oxidation in wastewater.3 To reduce their environmental impact and waste accumulation, recent years have seen significant advances in strategies to address plastics’ unsustainable end-of-life. Chemical recycling of plastics such as PET to their monomers, which are then repolymerized to yield virgin plastics, keeps these polymers within circular material loops. Alternatively, chemical upcycling of plastic waste—their conversion to products of higher economic value instead of their monomers (e.g., PS to benzoic acid)4—offers opportunities to use these low-cost waste as chemical feedstock.5,6 It is thus unsurprising that depolymerization has been identified by the IUPAC as among the Top Ten Emerging Technologies in Chemistry in 2023.7

In many of the above-mentioned processes, interfacial reactions—the chemistry occurring at the boundary between different phases—are unavoidable (Figure 1). Because of the immiscibility of common plastic polymers (SPI codes 1–6) in many solvents, chemistry that occurs under such conditions is governed by the interface between the solid particles and the surrounding reagent/catalyst-containing fluid phase. These include the aforementioned Fenton chemistry for microplastic remediation under aqueous conditions,3 recycling of plastics (e.g., hydrolysis of insoluble PET particles in sodium hydroxide),8,9 and even upcycling of plastics such as oxidation of PS to benzoic acid in polymer-immiscible acetic acid.10 Additionally, interfacial reactions govern depolymerization of plastics over solid heterogeneous catalysts, such as during catalytic hydrogenolysis of polyolefin plastics to yield light hydrocarbon fractions. For biocatalytic plastics recycling and upcycling, which complement the aforementioned chemocatalytic approaches, interactions between enzymes and the surface of plastics are important in dictating substrate accessibility for reactions to occur. Other than these human interventions at addressing plastic pollution, interfacial reactions play crucial roles in the natural breakdown of plastic products in the environment through surface photooxidation under solar irradiation and biodegradation by microbial action. Clearly, these interfacial reactions have strong influences on the reaction kinetics of polymer scissions and the reaction outcomes in terms of product selectivity.

Figure 1 Importance of interfacial reactions in many aspects of plastics recycling and upcycling chemistry. The crystal structure of the PET hydrolase in “Biocatalysis” was generated from that reported by Guo and co-workers13 (PDB ID: 5XH2).

While several excellent reviews have been published in recent years on depolymerization of plastics by heterogeneous catalysis,11,12 the importance of interfacial processes for plastics degradation, recycling, and upcycling is just starting to be recognized, leading to novel insights into the kinetics and design of such reactions. Thus, we spotlight the need to better characterize and study these phenomena in this Perspective. We consider two scenarios that are generally relevant for all classes of commercial plastics, regardless of polymer backbone structures (e.g., presence of labile bonds such as esters), their reactivity, and sources (petroleum- or biomass-derived). First, we discuss bond cleavage over heterogeneous catalysts, which is most pertinent to industrial plastics recycling/upcycling, highlighting recent insights into how understanding of interfacial processes can be used to tune product selectivity. Thereafter, we discuss the much less studied reactions of solid plastic particles in a reactive fluid environment, which is pertinent for reactions in the condensed phase such as chemistry of plastics in nonmiscible solvents, microplastic remediation in wastewater treatment, and natural chemical breakdown of plastics in the environment by both abiotic and biological agents. Additionally, these considerations also apply to reactions of plastics under unconventional reactive gaseous or plasma environments. We hope that the insights and better appreciation of these phenomena at the molecular level will offer new methods to engineer reactive outcomes and new reaction modalities that further assist with alleviating global plastic pollution and enhance the value of waste plastics as societally relevant chemical feedstock.

2 Heterogeneous Reactions of Plastics over Solid Catalysts

Heterogeneous catalysis is likely to dominate commercial industrial technologies for chemical recycling and upcycling of plastics to produce fuel-range hydrocarbons. Of which, pyrolysis is one of the most mature technologies available, and is applied on industrial scales to upgrade mixed plastic waste to hydrocarbons, often with solid catalysts such as zeolites (e.g., HZSM-5), though interesting alternatives such as incineration ash have been explored.14 However, other than the high temperatures required (often >400 °C), pyrolysis often results in poor product selectivity and the substantial formation of undesirable carbonaceous coke and tar products.15 More recently, alternative technologies such as catalytic hydrogenolysis and hydrocracking are emerging, advantaged by milder reaction conditions in the presence of H2 for polymer backbone bond scission to afford high liquid yields.

Catalytic depolymerization by processes such as hydrogenolysis is strongly affected by the energetics and interfacial conformations of the polymers on the catalyst surface. Thus, catalyst design can be used as a tool to modulate these interactions by tuning the reactivity and product selectivity. In this regard, enthalpic and entropic considerations are important in the development of effective catalysts and processes for depolymerization of polymers. High energies are often required for bond cleavage during depolymerization, and an ideal catalyst design should encompass all aspects of the hydrogenolysis process as much as possible, ranging from C–H activation, H2 dissociation, and C–C bond dissociation to hydrogenation–desorption. Although conventional metallic catalysts such as Ru0 have demonstrated excellent reactivity in hydrogenolysis, they only accelerate certain steps (e.g., C–H activation and H2 dissociation) of an entire process. Recently, Hu et al.16 demonstrated the possibility of using a stable Ru0–Ruδ+ complex as the catalyst for the hydrogenolysis of polyolefins. It was proposed that the partial positive charge on Ruδ+ can shift the electron cloud density of the C–C bond to itself, thereby lowering the stability of the C–C bond and facilitating easier bond cleavage. To circumvent the instability of Ruδ+ under high H2 pressures used in the hydrogenolysis of polyolefins, ligands such as ethylene glycol, ethanolamine, and ethylenediamine were grafted on the surface of a commercial Ru/C catalyst. The ligated Ru atoms convert the surrounding active Ru0 sites into stable Ruδ+, which then aids in accelerating the C–C bond dissociation reaction. The modified catalyst boosted the reaction efficacy by 4.18 times relative to pristine Ru/C while maintaining good product selectivity for liquid alkanes.

Polyolefins such as PE have intrinsically high entropy, which results in energetically disfavored adsorption onto the solid catalyst surface during reaction. Indeed, mechanistic studies on the hydrogenolysis of linear alkanes have suggested the dominant role of entropy over enthalpy when predicting the rate and location of C–C bond cleavage.17 Therefore, certain catalyst design considerations should be implemented to tackle this problem. Considering space-confinement effects during catalyst design through the use of molecular framework, cages or channels could potentially lower the entropic effect during heterogeneous reactions (Figure 2A). For instance, Kang et al.18 successfully demonstrated this concept by designing a catalyst using Ru nanoparticles supported on stable mesoporous silica (SBA-15) for the hydrogenolysis of LDPE into high-value diesel. They capitalized on mesoporous channels within SBA-15 to exert a space-confinement effect to lower the entropy of PE, which allows for more favorable polymer adsorption onto the catalytic Ru surface. The best catalyst activity was obtained by precise impregnation of Ru nanoparticles on SBA-15 support with a 9.1 nm pore diameter. The precise impregnation of Ru resulted in a uniform distribution of catalytic sites, and the 9.1 nm pore size provided an optimal balance between the space-confinement effect and diffusion rate of the polymer. Although this catalyst has a narrow product distribution consisting of high-value diesels, the variation in the catalyst specifications only improves the reactivity but not product selectivity. To achieve higher product selectivity, further optimization of catalyst design to modify the localized chemical environments within the channels could be considered. Tennakoon et al.19 demonstrated this with a mesoporous silica shell architecture with catalytic Pt nanoparticles incorporated at the base of the mesopores. The catalyst made use of the space-confinement effects within the silica shell, thereby lowering the entropy of PE chains via the polymer–silica shell interaction. Because the active Pt sites are located at the base of the mesopores, hydrogenolysis occurs progressively, producing a range of hydrocarbon products with a specific variation of chain length. Additionally, an increase in the pore diameter of the catalyst was found to confer higher selectivity for longer-chain hydrocarbons.

Figure 2 (A) Schematic representation of space-confinement effects on polyolefin conformations, reducing entropy and achieving favorable polymer adsorption onto embedded catalyst surfaces. (B) Product selectivity for arenes from Ru/Nb2O5-catalyzed hydrogenation of mixed plastic waste (PET, PS, PPO, and PC).21 (C) Schematic representation of different polyolefin conformations on catalyst surfaces to determine how they affect catalytic performance. Figure adapted with permission from the Royal Society of Chemistry (copyright 2023), from Zare, M. et. al., Conformations of Polyolefins on Platinum Catalysts Control Product Distribution in Plastics Recycling, Chem. Sci., 2023, 14(8), 1966–1977 (ref (22)); permission conveyed through Copyright Clearance Center, Inc. (D) Importance of interfacial diffusion processes and physical proximities between acid and metal sites in heterogeneous bifunctional catalysis. Figure adapted with permission from the Royal Society of Chemistry (copyright 2022), from Kots, P.A. et. al., Polyolefin Plastic Waste Hydroconversion to Fuels, Lubricants and Waxes: A Comparative Study, React. Chem. Eng., 2022, 7(1), 41–54 (ref (24)); conveyed through Copyright Clearance Center, Inc.

Even though improving catalyst reactivity can enhance conversion efficiency, product selectivity is generally restricted to lighter hydrocarbons due to overcracking. Therefore, understanding the interfacial interactions between plastics and catalysts is also of paramount importance to optimize product selectivity. Strong polymer chemisorption onto the catalyst surface can result in depletion of adsorbed hydrogen species which impedes hydrogenolysis, necessitating higher H2 pressures to afford greater liquid product yields. On the other hand, catalyst surfaces with strong hydrogen affinity or acting as hydrogen reservoirs, such as through partially reduced WOx species in Ru/WO3/ZrO2 surfaces which store hydrogen as surface hydroxyls, can favor liquid yields during LDPE hydrogenolysis.20 These can alter polymer–catalyst surface interactions which favor desorption of hydrocarbons to prevent overcracking by reducing their residence time on the catalysts. This provides a method to favor production of economically valuable fuels and lubricant oils with higher molecular weight, while suppressing the formation of lower value products such as methane. Another example by Wang and Yan21 exemplified how interfacial interactions between plastics and catalysts may be exploited to tune product selectivity, whereby Ru/Nb2O5 catalysts enabled hydrogenation of mixed plastic waste (PET, PS, poly(propylene oxide), and polycarbonate) to afford high yields and selectivity for arenes, which contrasted with Ru/HZSM-5 catalysts that afforded mainly ring-saturated products (Figure 2B). Extensive surface mechanistic studies using techniques such as XANES to determine the Ru coordination environment and DRIFTS experiments to study the oxygen affinity and ability of different catalyst supports to activate C–O bonds shed light on the origins of these observations. The low coordination environment of the subnanometer Ru nanoclusters on Nb2O5 limited the surface area and restricted the coabsorption of aromatic rings and H2, which prevented aromatic ring hydrogenation. Synergistically, NbOx activates the benzene rings and C–O bonds, while C–C bonds are cleaved with the assistance of Bronsted acidic sites. Compared to the HZSM-5 support, Nb2O5 was able to activate benzene rings more strongly and showed stronger affinity and propensity for C–O bond activation. Additionally, the different coordination environment of Ru on HZSM-5 support, having higher coordination numbers than Nb2O5, favored aromatic ring hydrogenation reactions and substrate cracking reactions that contributed to the much lower overall product yields and drastically different substrate selectivity compared to Ru/Nb2O5. The high yield and selectivity of valuable arenes obtained using Ru/Nb2O5 offer a promising route to valorize mixed aromatic plastic waste.

The product distributions of heterogeneous depolymerization reactions will be strongly affected by the polymer’s conformation when it adheres to the catalyst surface, which dictates the number and position of bonds subjected to cleavage. To study these effects, Caratzoulas and Vlachos recently reported the use of replica-exchange molecular dynamic simulations to study various conformations of PE surrogates of different chain lengths (C20, C26, C44, C71, and C142) in the melt phase over Pt(111) catalyst surfaces at 473 K.22 Denoting these surface conformations by trains, tails, and loops (Figure 2C), the authors found that trains of ∼C20 in length have a high probability of being entirely on the surface, while more diverse conformational features were exhibited by longer chains. Although the average C–C surface train lengths was mostly unaffected by the polymer length, weakening the polymer–catalyst surface interaction strength can increase average train lengths by altering its relative interplay with alkane–alkane lateral interactions. The presence of branches along the polymer, which are prevalent among PE and PP, was found to exert a large effect on surface polymer conformations with trains being more structured. Since this allows different polymer segments to adsorb better on catalyst surfaces, it increases the susceptibility for C–C bond cleavage to afford more diverse distribution of products. Interestingly, when the authors studied the effects of mixing different proportions of C20 with C142 to model the real-life formation of shorter oligomeric hydrocarbons as the hydrogenolysis reaction progressed, they found that longer chains are still able to adsorb onto the Pt surface even when shorter chains were added at high proportions. This could explain prior experimental observations for Pt/SrTiO3-catalyzed PE upcycling, where longer reaction times reduced the polydispersity of the products formed,23 which was attributed to favorable adsorption and scission of longer chains rather than shorter fragments. However, the simulations also showed that C20 could change the C142 surface conformations by favoring formation of longer loops. The expected change in reactivity was experimentally verified when a PE and C20 mixture was reacted over Pt/Al2O3 to yield less C1–C4 products and greater amounts of liquid C10+ hydrocarbons. This intriguing modulation of surface conformation using lighter alkanes demonstrated through the computational study could offer a new method to favor production of longer and more economically valuable hydrocarbon products.

As described by Kots et al.,24 understanding the mass transport at the polymer–catalyst interface between different catalytic sites is also important to optimize and tune product distributions for bifunctional catalysts for processes such as polyolefin hydrocracking. For this process on metal–acid bifunctional catalysts, for example, the polymers can be first dehydrogenated at metal sites, followed by subsequent diffusion to acidic sites where protonation and carbenium isomerization/β-scission occurs, and finally to metal sites which can rehydrogenate any unsaturated products (Figure 2D). In a very recent study, Han et al. demonstrated that the proximity of metallic Pt sites on acidic USY greatly affected its catalytic performance for polyolefin hydrocracking.25 The best catalytic performance was observed for Pt nanoparticles localized exclusively on the USY surface which maximizes accessibility to Pt and provides optimal metal–acid proximity, affording high rates of reaction with excellent selectivity for C5–12 gasoline range fuels. In contrast, long diffusion paths between Pt and USY with long metal–acid site separation caused significant depreciation of catalytic performance. Another study by Liu et al.26 discovered that distance between the active Pt metal and acidic sites on HY zeolite significantly affects the catalytic performance and product distribution during hydrogenolysis. When Pt-loaded HY zeolite was mixed with WO3/ZrO2, the synergistic catalytic effects resulted in overcracking of PE, leading to predominantly lighter hydrocarbon products. Conversely, when Pt-loaded WO3/ZrO2 was mixed with HY zeolite, higher molecular weight hydrocarbons were obtained as the main products, likely because the active Pt metal is further from the acidic HY zeolite sites. While model short-chain alkanes are commonly used to study these effects, surface mass transfer phenomena in polymers will be further complicated by different adsorption strengths of polymers on catalyst surfaces, polymer structures, competition with shorter-chain products during reaction progression, and the physics of polymer mass transport.

Besides understanding the catalyst design and the interfacial reactions involved in the hydrogenolysis of polyolefins, the kinetics of such interfacial reactions can also impact the efficiency and product distribution of the process. Chen et al.27 discussed in detail the influence of various factors such as temperature and H2 pressure on the product distribution during hydrogenolysis of polyolefins. When conducting the reaction with a Ru/C catalyst, it was determined that hydrogenolysis of PE starts at 150 °C with liquid alkanes being produced up to a temperature of 200 °C. Mainly solids were obtained at temperatures below 150 °C, possibly due to lack of energy for polymer cleavage, whereas temperatures above 200 °C gave low-value gaseous products (e.g., methane) due to overcracking. H2 pressure was also found to have a profound impact on the product distribution. By increasing the H2 pressure, the gas competes with the polymer for binding sites on the catalyst surface, which results in the inability of the polymer to adopt a large surface ensemble commonly required for terminal C–C bond cleavage. Consequently, the reduction in terminal C–C bond cleavages suppresses methane formation. However, hydrogenolysis reactions are inherently disfavored at high H2 pressures as the dehydrogenated transition states require vacant neighboring metal sites to accommodate the dissociated hydrogen atoms. Therefore, the reaction will reach a maximum conversion efficiency, whereby a further increase in the H2 pressure will adversely affect the reaction efficacy.

3 Reactions of Solid Polymer Particles in a Reactive Fluid Medium

3.1 Models for Chemical Adsorption and Reaction on Plastic Surfaces

Interfacial phenomena for reactions involving solid plastic particulates or fragments in a reactive fluid environment are markedly different from those in heterogeneous catalysis. For these cases, reactions occurring on the plastic surfaces necessitate surface interactions of reagents and catalysts (including enzymes), such as chemi- or physisorption, before any chemistry can take place. Thereafter, the progress of the reactions, formation of products, and their influences on subsequent reaction propagation can be described by shrinking core models.

Physicochemical adsorption of reagents and catalysts onto solid plastic surfaces may be modeled using the Langmuir isotherm in the condensed (fluid) phase, taking into account the competition between solvent and solute(s) (i.e., dissolved reagents/catalysts) occupying binding sites on the particle surface (Figure 3A(i)). A number of important assumptions hold for this model to be valid: (1) adsorption occurs to give monolayer coverage, i.e., each adsorption site can only interact with one adsorbate (reagent) at any one time; (2) the interaction energies between the adsorbate and adsorbent (plastic) at all interaction sites are identical; (3) there are negligible interactions between adsorbates at different sites; and (4) the surface is homogeneous. Despite the nonideality of many practical situations, such as roughness (i.e., inhomogeneity) of plastic particle surfaces and inevitable variations in surface interaction strength between different sites, the Langmuir model may still be useful and valid under certain situations. For example, the model was applied to study the adsorption of PET hydrolases to PET plastic surfaces, although their interactions giving rise to an approximated monolayer coverage were largely found to be nonspecific.28 Nonetheless, heterogeneous surfaces with differing interaction energies between sites may be better modeled using the empirical Freundlich adsorption isotherm. Multilayered adsorption may be best modeled using the Brunauer–Emmett–Teller (BET) isotherm (Figure 3A(ii)), subjected to the assumptions that adsorption occurs on well-defined homogeneous sites, the only intermolecular interactions are that between a molecule already adsorbed onto a site and that of an upper layer, and the adsorption energies are identical on each molecular layer.

Figure 3 (A) Schematic illustration showing the differences between the (i) Langmuir and (ii) BET isotherm models. (B) Evolution of plastic particles in an immiscible bulk phase through formation of increasingly thick product layers inward from the polymer surface through the product layer model as well as the shrinking particle model involving gradual reduction in particle size from dissolution of surface products. (C) Examples of upcycling reactions involving plastics with saturated C–C backbones in an immiscible reactant-containing fluid phase, such as (i) organocatalytic PS oxidation to yield benzoic acid and 4-nitrobenzoic acid in acetic acid10 and (ii) uncatalyzed PE oxidation to dicarboxylic acids under aqueous microwave conditions.34 (D) Upcycling of PS using nonthermal H2 plasma at ambient pressure and temperature.35

Successful adsorption of reagents and catalysts onto the plastic surface prefaces subsequent reactions. For most scenarios involving plastics invoking the shrinking core model (SCM), it is assumed that the plastic particle is nonporous and reactions can only occur on the particle surface. In the product layer model, which is a subset of SCM, the overall particle size remains unchanged as the reaction progresses since the product formed is immiscible with the external bulk environment, and thus it does not get removed from the surface.29,30 As the reaction progresses, penetration and diffusion of the reagents across the product layer allow for continuous reactions at the surface of the unreacted core, which gradually shrinks as the product layer thickens inward (Figure 3B(i)). This model is applicable for postsynthetic chemical modification of plastics without excessive cleavage of the polymer chains, whereby reagents are able to diffuse across the product layer to progressively access the inner particle layers. Alternatively, it can apply to PET hydrolysis using nitric acid, which forms a layer of insoluble terephthalic acid on the surface of unreacted PET particles.31 On the other hand, the shrinking particle model (SPM) describes a scenario where the overall particle size decreases as the reaction progresses because of the high dissolution rate of the reaction product in the surrounding fluid medium, which then exposes the inner layers for further reaction (Figure 3B(ii)).30 This model is most applicable to processes such as PET hydrolysis under basic aqueous conditions, in which the formation of water-soluble anionic terephthalate-containing oligomers at the surfaces of the shrinking particles get dissolved in the bulk phase.

For both models, the overall reaction kinetics need to take into account diffusion and reaction rates, with the slowest process being rate-limiting. Diffusion rates are determined by factors such as size of the particle, diffusibility through the product layer (product layer model only), and the bulk fluid properties,32 giving rise to a diffusion-limited process if it is slower than reaction rates. Conversely, the process is reaction kinetics-limited if the rate of the reaction is rate determining.33 For SCM, the latter could be approximated by providing constant stirring to create strong turbulence around the solid particles and suppressing the influence of reactant diffusion. If both diffusion and reaction rates are comparable, then they need to be factored into the overall reaction kinetics modeling. It should be noted as well that both rates and their relative importance may vary as the reaction progresses due to factors such as a decrease in particle size and changes in particle temperature.

The reaction rates are greatly affected by changes in particle size and shape (i.e., affecting exposed surface area) and miscibility of products formed on the particle surface in the bulk medium. The shape of the particles can be accounted for in kinetic models by using a shape factor to account for the effects of irregular particle shapes in nonporous particles.29 Additionally, many kinetic models deviate from reality due to the possibility of particle breakup during reaction and the inevitable variations in particle size which can affect the reaction rates—factors which are usually not taken into consideration. For example, in Wan’s study of PET hydrolysis in aqueous KOH below the polymer’s melting point, their kinetic model assumed a constant surface area of the PET polymer during the reaction.8 However, deviations from experimental findings were found, likely due to nonconstant change in particle size during the reaction. Similarly, studies on hydrolysis of PET in sulfuric acid by Yoshioka et al.9 revealed that a realistic kinetic model needs to take into account the increase in effective surface area during interfacial reactions due to the formation of cracks and pores on the plastic surface during reaction. Finally, study of the kinetics of PET hydrolysis in nitric acid31 and sulfuric acid9 media by Yoshioka et al. showed the effects of product solubility in bulk environment after surface formation. In nitric acid, the hydrolysis was retarded by deposition of terephthalic acid on the surface of unreacted PET particles due to its poor solubility in the reaction medium. However, PET hydrolysis in sulfuric acid was thought to occur without surface deposition of terephthalic acid due to its better solubility.

3.2 Abiotic Plastic Recycling and Upcycling Reactions in Immiscible Media

Chemical recycling and upcycling of plastics in the solution phase allows crystalline polymer domains to be broken up for maximal accessibility to reagents and catalysts for reactive encounters. However, the excellent solvent tolerance of many of the most common commodity petroleum-based plastics in use today, including polyolefins (PEs and PPs) and PET, which makes them so useful for many applications, works against them. Polyolefins, for instance, can only be dissolved in environmentally- and health-hazardous chlorinated or aromatic solvents. Although a number of bioderived solvents have shown potential for dissolution of some of these recalcitrant plastics,5 some of them pose hazards of their own, and is often complicated by factors such as polymer branching (e.g., LDPE vs HDPE), molecular weights, and tacticity. In light of these factors, it is of value to develop processes for recycling and upcycling that are solvent-agnostic. This offers new opportunities for developing sustainable plastic degradation processes in solvents as benign as water, even for nonhydrolyzable plastics such as the polyolefins, PS, and PVC. Although not necessarily miscible, recycling of plastics with hydrolyzable and cleavable bonds (e.g., polyesters such as PET) in aqueous/alcoholic solvent media has been discussed in Section 3.1 and extensively reported in the literature.36 Hence, we will focus on emerging technologies for upcycling plastics containing unreactive saturated C–C backbones that occur in an immiscible fluid (liquid or gas) phase.

Oxidative upcycling of PEs and PS into organic acids offers unique opportunities to convert these low-cost plastics to oxygenated molecules of significant industrial demand, which complement the hydrocarbons produced from pyrolysis or hydrogenation processes. Unlike hydrolysis of PET, oxidative breakdown of polyolefins and PS necessitates the generation of radical species on the polymer backbone through hydrogen abstraction, which can either combine with oxygen-containing species present or undergo radical scission reactions that propagate chain cleavage. Thus, catalysts are designed to generate reactive oxygen species (e.g., 1O2) or form active species in situ that can directly perform hydrogen atom transfer (HAT) reactions on the polymer backbone under mild conditions. For instance, N-hydroxyl organocatalysts such as N,N′,N″-trihydroxyisocyanuric acid (THICA) are known to form reactive N-oxyl radicals under aerobic conditions, which are able to abstract hydrogen atoms from the PS polymer backbone to ultimately degrade the polymer into benzoic acid and 4-nitrobenzoic acid (Figure 3C(i)).10

In addition to catalyst selection, the choice of fluid medium is of high importance, as it affects both safety and reaction efficacy. As the reaction conditions involved are inherently highly oxidizing, the copious use of organic solvents (that may aid in polymer solubility) are often avoided for safety considerations. Hence, such reactions are preferably performed in aqueous media or with solvents that are suitable for oxidation, even though these reaction media can be immiscible with the plastic substrate. Oxidative breakdown of immiscible plastics into small molecules in these reactive fluid media occurs following the SPM model: the oxygenated oligomeric intermediates or products that are formed on the plastic particle surface during the reaction need to be removed by dissolution in the fluid media, thus progressively and continuously exposing the inner layers of the plastic particle for further reaction. The choice of solvent is therefore critical for ensuring reaction efficacy to avoid buildup of an immiscible product layer on the plastic surface that would inhibit further oxidations. For instance, the oxidation of PE in aqueous nitric acid under microwave conditions (180 °C, 3 h) to afford short-chain aliphatic dicarboxylic acids has been demonstrated (Figure 3C(ii)).34 Another study showed that Fe(III)-containing sulfonated PE solids, formed from pretreating PE with chlorosulfonic acid, followed by Fe(III) coordination, can be oxidized using aqueous H2O2 at 40 °C using Fenton chemistry to yield a range of dicarboxylic acids.37 Both of the aforementioned examples utilized water as a solvent that can effectively solubilize the targeted short-chain aliphatic dicarboxylic acid products. Other than water, a green solvent in which the polymer is not necessarily soluble in may be used. As aforementioned, the use of N-hydroxyl organocatalysts to convert PS to benzoic acid and 4-nitrobenzoic acid in acetic acid medium has been demonstrated (Figure 3C(i)). The untreated PS particles were initially insoluble in the solvent medium but were visually observed to gradually dissolve as the reaction progressed due to the effective dissolution of the smaller oxygenated products. It is unclear, however, whether the benzoic acid was formed directly on the plastic particle surface or from the oxidation of oligomeric styrene intermediates dissolved in the acetic acid medium through homogeneous catalysis, though both are likely to occur to some extent.

Plastic upcycling reactions in fluid media do not need to be limited to a condensed bulk phase. In an early study of oxidative plastic upcycling, Sen demonstrated that PS can be oxidized into benzoic acid and mixtures of nitrobenzoic acids in a pressured oxidizing atmosphere at high temperatures (275 kPa NO and 690 kPa O2 at 170 °C for 16 h).38 Modifying the partial pressures of NO and O2 also allowed HDPE, LDPE, PP, and nylon-6,6 to be converted to several dicarboxylic acids. Although not stated by the authors, these uncatalyzed reactions may have occurred between the gaseous oxidants and the polymer or its oxidized oligomeric intermediates in either the solid or liquid state, depending on their melting points. At the end of the reaction, no unreacted polymer starting materials were detected. Very recently, Peng and co-workers demonstrated a new method to degrade these plastics into hydrocarbons using nonthermal H2 plasma at ambient temperatures and pressures (Figure 3D).35 The H2 plasma contains a plethora of reactive hydrogen species such as ions and radicals, which were able to very rapidly break down the solid PS, yielding C1–C3 hydrocarbon gases and liquid paraffin and alkylbenzene products within 12 min. The solid polymer was observed to diminish throughout the reaction to afford the liquid product and increasing gaseous yields. Similarly, nonthermal plasma-assisted PE hydrogenolysis could also be achieved with catalysts such as Pt/C and SAPO-34 zeolites as they were found to improve the energy efficiency and product selectivity even further.39 Indeed, careful elucidation of the interfacial processes and reactions that occur for these emerging plastic upcycling technologies will be instrumental in engineering greater reaction efficiency and product selectivity.

3.3 Interfacial Reactions in Environmental and Biological Plastic Degradation

Interfacial processes play important roles in the degradation of plastics in the environment, triggered by both abiotic and biotic factors. Before being subjected to any chemical modifications, plastic particles can be broken down into microplastic fragments by physical processes such as abrasion, wave action, or even the mechanical action of chewing by larger animals. Additionally, microorganisms can also attach and grow in the micropores of plastics to propagate cracks and structural defects which mechanically weaken them and facilitate material disintegration.40 At the same time, surface chemical modifications via photooxidations can occur, triggering polymer bond scission, reducing polymer crystallinity, and increasing the surface hydrophilicity to enhance adhesion of microorganisms and enzymes for biological action. As plastic degradation occurs, oligomeric fragments can be dislodged from the plastic particle surface, dissolved in the surrounding aqueous media, or assimilated by microorganisms.

Photooxidative degradation is one of the most important low-temperature processes responsible for natural weathering of plastics in the environment41 that takes place primarily on the plastic surface and subsurface layers due to limited light penetration into the material. Plastics are susceptible to oxidative degradation initiated by UV light.42 The mechanism of photooxidative degradation consists of three steps (Figure 4): In the photoinitiation step, free radicals are formed from the breaking of chemical bonds in polymer chains induced by UV radiation, which is facilitated by chromophores that absorb light energy. Even though PE and PP are inert polyolefins exclusively made up of C–C and C–H bonds, they can undergo photoinitiated surface degradation to some extent in the natural environment since the direct photolysis of these bonds is energetically feasible.41 Furthermore, adventitious impurities (e.g., trace metals) or structural defects incorporated during manufacturing or subsequent weathering can behave as chromophores. For instance, chromophoric carbonyl groups formed from polymer oxidation during synthesis or processing can undergo Norrish Type I, Norrish Type II, and HAT reactions upon absorption of near-UV radiation to produce radicals that can initiate degradation processes.42−44 Alkene defects such as vinyl and vinylidenes present in HDPE and LDPE, respectively, are also readily oxidized by ozone or NOx to form UV-absorbing carbonyl groups.44

Figure 4 Types of interfacial chemical and biological reactions that occur on plastic surfaces in the environment, which lead to their degradation. From left to right: photo- and thermooxidative degradation, biodegradation, and hydrolytic degradation.

Thereafter during propagation, the polymer radical reacts with oxygen to form a peroxy radical, which subsequently generates photolabile hydroperoxide species. This intermediate readily decomposes via cleavage of the O–O bond with quantum yields close to unity to produce a macro-alkoxyl and hydroxyl radical that can effect further complex radical reactions.45 The resulting macro-alkoxyl radical can undergo reactions such as (1) β-scission of the polymer to form terminal aldehydes, (2) HAT reactions to produce hydroxyl functionalities, and (3) reaction between macro-alkoxyl and hydroxyl radical to form ketones.46 The formation of ketones further promotes photoinduced Norrish Type I and Type II reactions and ultimately results in polymer chain scission.42 As shown in Figure 4, ketone photolysis by Norrish Type I involves the formation of an acyl and alkyl radical, whereas Norrish Type II cleavage proceeds via γ-hydrogen abstraction to produce a vinyl and ketone chain-end. Finally, termination of the radical reactions is achieved through free radical combinations to form inert, nonradical products. As a consequence of polymer oxidation and random chain scission, macro-alkyl and -alkoxyl radicals may (re)combine to give branched or cross-linked chains or disproportionation end products.42,44,45 Expected products of radical termination reactions often include olefins, aldehydes, and ketones, all of which are susceptible to undergo photoinitiated degradation.

Other than photooxidation, thermooxidation and hydrolysis are amongst the most common abiotic pathways responsible for plastic breakdown (Figure 4). Thermooxidative reactions take place via radical processes when the thermal energy is greater than the energy required for homolytic bond scission. Hence, initiation reactions can occur by random chain scission to cause a reduction in polymer molecular weight as well as chain-end scission to generate volatile products.42 Although temperatures in the environment are usually maintained within fairly narrow intervals, solar-induced heating, especially by dark-colored plastic products, may result in localized elevated temperatures that facilitate this process. In general, polymers above their glass transition temperature (Tg) tend to get oxidized more rapidly than those in a glassy state because of the enhanced diffusion of oxygen into the polymer matrix.47 For polymers with hydrolyzable bonds (e.g., polyesters such as PET, PLA, and PCL), hydrolysis is one of the most important degradation mechanisms, though it can occur extremely slowly in nature. During this process, smaller oligomeric fragments containing carboxylic acid end-groups are typically formed; since the rate of hydrolysis is enhanced under acidic conditions, the hydrolytic degradation of these polymers becomes autocatalytic when carboxylic acid end-groups are increasingly formed.44 While this results in polymer chain scission, some degree of adventitious side chain or pendant hydrolysis can also take place,48 e.g., in poly(methyl methacrylate) (PMMA) and polyacrylonitrile (PAN). These can increase the surface hydrophilicity of the polymers, making them more susceptible to microbial or enzymatic adhesion.

Biological degradation of common petroleum-derived plastics is hindered by their immiscibility in water and the inability of the biocatalysts to penetrate the bulk polymer. Thus, catalysis is limited to the interface between the plastic and the surrounding reaction medium, making the process highly dependent on the surface properties of the plastic particles. As summarized by Wei and Zimmerman,49 a hydrophobic surface and smooth surface topology can hinder biofilm formation by polymer-degrading microorganisms and enzyme adsorption, which in turn affects catalytic activity. A low surface-area-to-volume ratio is detrimental to facile enzymatic hydrolysis by limiting the accessible surface area for enzymatic action. Additionally, polymer crystallinity also exerts a significant influence on the rate of biodegradation, with amorphous domains generally being more susceptible than their crystalline counterparts due to their less ordered molecular arrangements.50 In spite of this, crystalline domains can also be subjected to enzymatic attack. For instance, low-crystallinity PET films were completely hydrolyzed at 70 °C after 96 h by a polyester cutinase from Humicola insolens at an almost linear rate, suggesting that both amorphous and crystalline domains were attacked by the enzyme.51

Among plastics, the enzymatic biodegradation of PET is most well-studied, with great potential for industrial-scale biocatalytic recycling. An integral part of biocatalytic hydrolysis is the initial adhesion of the PETases to the surface of the PET particle, which is mediated by hydrophobic and electrostatic interactions between the surface polymers and the amino acids on the enzyme surface.52 Reducing the electrostatic repulsion between the enzyme and plastic substrate (e.g., from anionic carboxylate groups formed from hydrolyzed PET) and increasing the hydrophobicity of the enzyme surface can increase adhesion. For example, exposure of a hydrophobic domain by modification of Clostridium botulinum esterase led to enhanced PET sorption and hydrolysis.53 Surface adhesion allows superficial ester bonds to be accessible to the enzyme binding pocket for hydrolysis. An often-neglected, but important, factor affecting the surface polymer degradation is the influence of water beyond its roles as reactant and solvent. Water can act as a surfactant that lowers the Tg of the polymer surface compared to that of the bulk polymer, whose effects are being most pronounced up to a thickness of 13 nm on the surface,54 increasing surface polymer accessibility for biocatalysis. The specific mode of action of individual enzymes differs, with some resulting in predominantly endotype (random) chain scission, while others showing preferences for scission at the chain terminus (exotype). Hydrolysis of the insoluble PET polymers into smaller soluble degradation products, which can include oligomers, can also allow further enzymatic hydrolysis to occur in the homogeneous phase. At the same time, this gradually alters the surface topology of the polymer substrate, and it has been shown by SEM imaging that initial small shallow pits formed gradually increased in diameter with increasing exposure duration to the enzymes.55 The altered surface roughness, topology, and charges (from formation of carboxylates) in turn alter the successive adhesion of enzymes for further reaction. Additionally, enzymes can undergo an adsorption-induced denaturation process that leads to aggregation on the polymer surface, resulting in some changes in polymer surface topology.56 This process hinders further enzyme adsorption to the polymer surface, inhibiting further degradation. Finally, some degradation products, such as mono(2-hydroxyethyl) terephthalate (MHET), are also known to inhibit the PET hydrolases’ depolymerization efficiency.57

Like for PET, effective interactions and adhesion to the surface of recalcitrant hydrocarbon plastics (e.g., PE and PS) are essential prerequisites for polymer biodegradation to occur. Thereafter, enzymes secreted such as peroxidases and laccases lead to surface oxidations by installation of hydroxyls, carbonyls, and esters that facilitate chain scissions and degradation of the polymer material properties.58 These changes also increase the hydrophilicity of the plastic surface, thereby improving enzymatic adhesion and the availability for biodegradation. Once the polymer is reduced to shorter chains consisting of about 10–50 carbon atoms, they can be transported into the cell for subsequent metabolization.59 Despite the hydrophobicity of PE surfaces and the polymer’s unreactivity, these processes can happen remarkably quickly. Sanluis-Verdes et al. identified two enzymes (an arylphorin and a hexamerin named Demetra and Ceres, respectively) found in wax worm saliva that could oxidize and break down PE into small aliphatic ketones (10–22 carbons), 2,3-butanediol, and sebacic acid within a short period of time at room temperature.60 This embodies a promising alternative to the abiotic pretreatment of plastics in traditional microbial degradation pathways as well as the exclusive metabolic conversion of plastics to CO2.

4 Conclusions and Perspectives

Herein, we have broadly surveyed the critical roles played by interfacial processes in many aspects of plastic recycling, upcycling, and biodegradation. For heterogeneous catalysis, we have seen that controlling the surface interactions between the polymer and solid catalysts in hydrogenolysis can dramatically alter the selectivity of the products. Changing the proximity of the surface catalytic sites has also been demonstrated to affect the product distribution. Reactions of plastics in immiscible media are of direct importance to recycling of polyesters such as PET, with reaction kinetics dependent on interfacial processes and solubility of products formed on the surface. Other than these, such reaction conditions have also been shown to be emerging in the development of newer and potentially more sustainable methods of upcycling recalcitrant plastics with nonhydrolyzable C–C polymer backbones such as PE and PS. In the environment, interfacial processes govern abiotic breakdown of plastic particles through photooxidations and surface hydrolysis. The importance of interfacial processes cannot be overstated for biological degradation, regardless of the type of enzymes or microorganisms, being especially critical for initial biocatalyst adhesion to the plastic surface as a prerequisite for any reactions, polymer scission leading to polymer consumption, and even deactivation or inhibition of the biocatalysts.

While interfacial interactions and reactions have been investigated more extensively in heterogeneously catalyzed plastics breakdown, biocatalysis, and PET recycling, that for reactions of hydrocarbon plastics in immiscible media is still largely overlooked. Accordingly, the interfacial phenomenon governing upcycling reactions, such as oxidation of PE and PS into oxygenated small molecules, is still largely unknown. This includes important aspects of catalyst and reactant interaction with the polymer surface, potential plasticizing effect of the surrounding fluid phase on the polymer surface, and how formation of surface intermediates can affect the reaction kinetics or product distribution by altering the polymer–reactant/catalyst interactions. Several techniques have been utilized to study changes at the plastic interface. For example, SEM has shed light on how terephthalic acid formation on PET surfaces after hydrolysis with nitric acid results in deviations from SCM.31 AFM is especially useful for topographical imaging of surfaces and has been successfully applied to probe changes in polymer surfaces subjected to biocatalysis. FTIR and XPS are commonly used to track changes in surface functional groups, such as during PE surface oxidation by enzymes, though interference could come about from the presence of large quantities of additives used in real-life plastic waste, and even surface adhesion of catalysts and reactants. Although Tg is conventionally measured using DSC, changes at the surface of the polymer (such as when water acts as a plasticizer for PET) cannot be accurately reflected as this technique reflects the bulk properties of the polymer sample. Instead, interfacial Tg can be measured using an interfacial rheometer (possibly coupled to a Langmuir trough), as described by Machatschek.56 This technique also quantifies surface shear resistance, storage, and loss moduli, which gives information on changes in chain packing as reactions proceed. Computational modeling can also offer valuable insights on the difference in behavior of polymers at the interface compared to the bulk, effects of polymer conformations on reactivity (e.g., on solid catalysts), effects of solvent structure at the interface, and interactions of dissolved catalysts and reactants with the polymer surface, among many other effects.

More emphasis on elucidating the interfacial processes taking place during such reactions will greatly lead to better fundamental mechanistic understanding and allow rational design of more effective methods and catalysts for improved reaction outcomes. As a case in point, in recognition of the importance of strong enzyme adhesion to PET substrates for biocatalysis, plastic degrading enzymes can be fused with auxiliary binding domains such as fungal hydrophobins that boost substrate affinity. Using this strategy, bacterial cutinases covalently tethered to Trichoderma hydrophobins can result in >16-fold enhancement of PET hydrolysis in comparison to the native enzyme.61 Modulating the interfacial accessibility of catalytic sites to the plastic substrates could also alter reaction outcomes, facilitating catalyst customization for targeting specific classes of upcycled products. Such knowledge from heterogeneous catalysts could possibly be translated to design new families of high-performing catalysts such as metal–organic frameworks (MOFs)62 and Co/Al-layered double hydroxides,63 whose potential in recycling and upcycling neat plastics is just beginning to be realized. Effective exploitation of the thermodynamics and heterogeneous catalyst reactivity allowed tandem hydrogenolysis and aromatization of PE to occur to produce valuable, nonconventional long-chain alkyl aromatics in the complete absence of external H2 at a moderate reaction temperature of 280 °C.64 In addition, these interfacial studies can improve catalyst longevity and turnovers through understanding the mechanistic processes responsible for coke formation that can cause heterogeneous catalyst deactivation during hydrogenolysis, or how enzymes can undergo adsorption-induced denaturation. These can ultimately translate to practical economic outcomes by reducing the costs needed for catalyst replacement. The underappreciated roles of the solvent on the plastic interfaces are also only just beginning to be understood, such as the recent understanding of how water can act as a plasticizer on PET surfaces, which has important implications for both abiotic and biocatalytic reactivity. A significant knowledge gap still exists on the interfacial effects of solvent on other plastics such as polyolefins.

Finally, proper exploitation of interfacial reactivity will go a long way toward improving the sustainability of plastics valorization, recycling, and remediation. Upcycling reactions can be designed under less hazardous solvent conditions, or even under aqueous conditions, where interfacial catalysts can be designed to take advantage of preferential interaction on plastic surfaces for enhanced reactivity. Such innovations can find practical translation in developing highly efficient wastewater treatment processes, where micro- or nanoplastics can be removed before their irreversible release into the environment. The most successful and scalable processes suitable for large-scale real-world adoption need to be highly efficient, robust, and economical, and generate as little hazardous waste as possible. Ultimately, these will translate to economics through reduction in costs from waste disposal, energy consumption, and by maximizing productivity. A good understanding of interfacial processes and chemistries will go a long way toward achieving practical and economically viable plastics circularity, while simultaneously addressing the escalating plastic pollution crisis.

The authors are grateful to the NRF Fellowship (Award number: NRF-NRFF15-2023-0007) for financial support of this work.

The authors declare no competing financial interest.

Abbreviations

FTIR Fourier transform infrared spectroscopy

DSC differential scanning calorimetry

Tg glass transition temperature

PE polyethylene

PP polypropylene

PS polystyrene

PET poly(ethylene terephthalate)

PLA poly(lactic acid)

PCL poly(caprolactone)

AFM atomic force microscopy

XPS X-ray photoelectron spectroscopy

XANES X-ray absorption near edge structure

DRIFTS diffuse reflectance infrared Fourier transform spectroscopy.
==== Refs
References

Nguyen B. ; Claveau-Mallet D. ; Hernandez L. M. ; Xu E. G. ; Farner J. M. ; Tufenkji N. Separation and Analysis of Microplastics and Nanoplastics in Complex Environmental Samples. Acc. Chem. Res. 2019, 52 (4 ), 858–866. 10.1021/acs.accounts.8b00602.30925038
Lares M. ; Ncibi M. C. ; Sillanpää M. ; Sillanpää M. Occurrence, identification and removal of microplastic particles and fibers in conventional activated sludge process and advanced MBR technology. Water. Res. 2018, 133 , 236–246. 10.1016/j.watres.2018.01.049.29407704
Hu K. ; Zhou P. ; Yang Y. ; Hall T. ; Nie G. ; Yao Y. ; Duan X. ; Wang S. Degradation of Microplastics by a Thermal Fenton Reaction. ACS. EST. Engg. 2022, 2 (1 ), 110–120. 10.1021/acsestengg.1c00323.
Ong A. ; Wong Z. C. ; Chin K. L. O. ; Loh W. W. ; Chua M. H. ; Ang S. J. ; Lim J. Y. C. Enhancing the photocatalytic upcycling of polystyrene to benzoic acid: a combined computational-experimental approach for acridinium catalyst design. Chem. Sci. 2024, 15 (3 ), 1061–1067. 10.1039/D3SC06388G.38239702
Nguyen B.-N. T. ; Lim J. Y. C. Emerging green approaches for valorization of plastics with saturated carbon backbones. Trends in Chemistry 2024, 6 (3 ), 100–114. 10.1016/j.trechm.2024.01.001.
Jehanno C. ; Alty J. W. ; Roosen M. ; De Meester S. ; Dove A. P. ; Chen E. Y. X. ; Leibfarth F. A. ; Sardon H. Critical advances and future opportunities in upcycling commodity polymers. Nature 2022, 603 (7903 ), 803–814. 10.1038/s41586-021-04350-0.35354997
IUPAC Announces the 2023 Top Ten Emerging Technologies in Chemistry. https://iupac.org/iupac-2023-top-ten/ (accessed 2024-05-15).
Wan B. ; Kao C. ; Cheng W. Kinetics of Depolymerization of Poly(ethylene terephthalate) in a Potassium Hydroxide Solution. Ind. Eng. Chem. Res. 2001, 40 , 509–514. 10.1021/ie0005304.
Yoshioka T. ; Motoki T. ; Okuwaki A. Kinetics of Hydrolysis of Poly(ethylene terephthalate) Powder in Sulfuric Acid by a Modified Shrinking-Core Model. Ind. Eng. Chem. Res. 2001, 40 , 75–79. 10.1021/ie000592u.
Ong A. ; Teo J. Y. Q. ; Feng Z. ; Tan T. T. Y. ; Lim J. Y. C. Organocatalytic Aerobic Oxidative Degradation of Polystyrene to Aromatic Acids. ACS Sustain. Chem. Eng. 2023, 11 (34 ), 12514–12522. 10.1021/acssuschemeng.3c01387.
Borkar S. S. ; Helmer R. ; Mahnaz F. ; Majzoub W. ; Mahmoud W. ; Al-Rawashdeh M. m. ; Shetty M. Enabling resource circularity through thermo-catalytic and solvent-based conversion of waste plastics. Chem. Catalysis 2022, 2 (12 ), 3320–3356. 10.1016/j.checat.2022.09.003.
Martín A. J. ; Mondelli C. ; Jaydev S. D. ; Pérez-Ramírez J. Catalytic processing of plastic waste on the rise. Chem. 2021, 7 , 1487 10.1016/j.chempr.2020.12.006.
Han X. ; Liu W. ; Huang J.-W. ; Ma J. ; Zheng Y. ; Ko T.-P. ; Xu L. ; Cheng Y.-S. ; Chen C.-C. ; Guo R.-T. Structural insight into catalytic mechanism of PET hydrolase. Nat. Commun. 2017, 8 (1 ), 2106 10.1038/s41467-017-02255-z.29235460
Heng J. Z. X. ; Tan T. T. Y. ; Xing Z. ; Ong J. L. Y. ; Lin K. S. ; Koh X. Q. ; Jiang W. ; Zhang L. ; Zhu Q. ; Li Z. ; Loh X. J. ; Lim J. Y. C. ; Ye E. Unraveling the catalytic activity of CaClOH-rich incineration fly ash in the pyrolysis of single-use plastics. Mater. Today Chem. 2023, 31 , 101608 10.1016/j.mtchem.2023.101608.
Kunwar B. ; Cheng H. N. ; Chandrashekaran S. R. ; Sharma B. K. Plastics to fuel: a review. Renew. Sustain. Energy Rev. 2016, 54 , 421–428. 10.1016/j.rser.2015.10.015.
Hu P. ; Zhang C. ; Chu M. ; Wang X. ; Wang L. ; Li Y. ; Yan T. ; Zhang L. ; Ding Z. ; Cao M. ; Xu P. ; Li Y. ; Cui Y. ; Zhang Q. ; Chen J. ; Chi L. Stable Interfacial Ruthenium Species for Highly Efficient Polyolefin Upcycling. J. Am. Chem. Soc. 2024, 146 (10 ), 7076–7087. 10.1021/jacs.4c00757.38428949
Flaherty D. W. ; Iglesia E. Transition-state enthalpy and entropy effects on reactivity and selectivity in hydrogenolysis of n-alkanes. J. Am. Chem. Soc. 2013, 135 (49 ), 18586–99. 10.1021/ja4093743.24266427
Kang Q. ; Chu M. ; Xu P. ; Wang X. ; Wang S. ; Cao M. ; Ivasenko O. ; Sham T. K. ; Zhang Q. ; Sun Q. ; Chen J. Entropy Confinement Promotes Hydrogenolysis Activity for Polyethylene Upcycling. Angew. Chem., Int. Ed. 2023, 62 (47 ), e202313174 10.1002/anie.202313174.
Tennakoon A. ; Wu X. ; Paterson A. L. ; Patnaik S. ; Pei Y. ; LaPointe A. M. ; Ammal S. C. ; Hackler R. A. ; Heyden A. ; Slowing I. I. ; Coates G. W. ; Delferro M. ; Peters B. ; Huang W. ; Sadow A. D. ; Perras F. A. Catalytic upcycling of high-density polyethylene via a processive mechanism. Nat. Catal 2020, 3 (11 ), 893–901. 10.1038/s41929-020-00519-4.
Wang C. ; Xie T. ; Kots P. A. ; Vance B. C. ; Yu K. ; Kumar P. ; Fu J. ; Liu S. ; Tsilomelekis G. ; Stach E. A. ; Zheng W. ; Vlachos D. G. Polyethylene Hydrogenolysis at Mild Conditions over Ruthenium on Tungstated Zirconia. JACS Au 2021, 1 (9 ), 1422–1434. 10.1021/jacsau.1c00200.34604852
Jing Y. ; Wang Y. ; Furukawa S. ; Xia J. ; Sun C. ; Hülsey M. J. ; Wang H. ; Guo Y. ; Liu X. ; Yan N. Towards the Circular Economy: Converting Aromatic Plastic Waste Back to Arenes over a Ru/Nb2O5 Catalyst. Angew. Chem., Int. Ed. 2021, 60 (10 ), 5527–5535. 10.1002/anie.202011063.
Zare M. ; Kots P. A. ; Caratzoulas S. ; Vlachos D. G. Conformations of polyolefins on platinum catalysts control product distribution in plastics recycling. Chem. Sci. 2023, 14 (8 ), 1966–1977. 10.1039/D2SC04772A.36845916
Celik G. ; Kennedy R. M. ; Hackler R. A. ; Ferrandon M. ; Tennakoon A. ; Patnaik S. ; LaPointe A. M. ; Ammal S. C. ; Heyden A. ; Perras F. A. ; Pruski M. ; Scott S. L. ; Poeppelmeier K. R. ; Sadow A. D. ; Delferro M. Upcycling Single-Use Polyethylene into High-Quality Liquid Products. ACS Cent. Sci. 2019, 5 (11 ), 1795–1803. 10.1021/acscentsci.9b00722.31807681
Kots P. A. ; Vance B. C. ; Vlachos D. G. Polyolefin plastic waste hydroconversion to fuels, lubricants, and waxes: a comparative study. React. Chem. Eng. 2022, 7 (1 ), 41–54. 10.1039/D1RE00447F.
Han X. ; Zhou X. ; Ji T. ; Zeng F. ; Deng W. ; Tang Z. ; Chen R. Boosting the catalytic performance of metal-zeolite catalysts in the hydrocracking of polyolefin wastes by optimizing the nanoscale proximity. EES. Catal. 2024, 2 (1 ), 300–310. 10.1039/D3EY00180F.
Liu S. ; Kots P. A. ; Vance B. C. ; Danielson A. ; Vlachos D. G. Plastic waste to fuels by hydrocracking at mild conditions. Sci. Adv. 2021, 7 , eabf8283 10.1126/sciadv.abf8283.33883142
Chen L. ; Zhu Y. ; Meyer L. C. ; Hale L. V. ; Le T. T. ; Karkamkar A. ; Lercher J. A. ; Gutiérrez O. Y. ; Szanyi J. Effect of reaction conditions on the hydrogenolysis of polypropylene and polyethylene into gas and liquid alkanes. React. Chem. Eng. 2022, 7 (4 ), 844–854. 10.1039/D1RE00431J.
Badino S. F. ; Baath J. A. ; Borch K. ; Jensen K. ; Westh P. Adsorption of enzymes with hydrolytic activity on polyethylene terephthalate. Enzyme Microb. Technol. 2021, 152 , 109937 10.1016/j.enzmictec.2021.109937.34749019
Salmi T. ; Grénman H. ; Wärnå J. ; Murzin D. Y. Revisiting shrinking particle and product layer models for fluid-solid reactions - From ideal surfaces to real surfaces. Chem. Eng. Process. 2011, 50 (10 ), 1076–1084. 10.1016/j.cep.2011.05.015.
Zou Z. ; Yan D. ; Zhu J. ; Zheng Y. ; Li H. ; Zhu Q. Simulation of the Fluid-Solid Noncatalytic Reaction Based on the Structure-Based Mass-Transfer Model: Shrinking Core Reaction. Ind. Eng. Chem. Res. 2020, 59 (40 ), 17729–17739. 10.1021/acs.iecr.0c02805.
Yoshioka T. ; Okayama N. ; Okuwaki A. Kinetics of Hydrolysis of PET Powder in Nitric Acid by a Modified Shrinking-Core Model. Ind. Eng. Chem. Res. 1998, 37 , 336–340. 10.1021/ie970459a.
Bazyn T. ; Krier H. ; Glumac N. Evidence for the transition from the diffusion-limit in aluminum particle combustion. Proc. Combust. Inst. 2007, 31 (2 ), 2021–2028. 10.1016/j.proci.2006.07.161.
Ishida M. ; Wen C. Y. Comparison of zone-reaction model and unreacted-core shrinking model in solid-gas reactions - I Isothermal analysis. Chem. Eng. Sci. 1971, 26 , 1031–1041. 10.1016/0009-2509(71)80017-9.
Bäckström E. ; Odelius K. ; Hakkarainen M. Trash to Treasure: Microwave-Assisted Conversion of Polyethylene to Functional Chemicals. Ind. Eng. Chem. Res. 2017, 56 (50 ), 14814–14821. 10.1021/acs.iecr.7b04091.
Yao L. ; King J. ; Wu D. ; Ma J. ; Li J. ; Xie R. ; Chuang S. S. C. ; Miyoshi T. ; Peng Z. Non-thermal plasma-assisted rapid hydrogenolysis of polystyrene to high yield ethylene. Nat. Commun. 2022, 13 (1 ), 885 10.1038/s41467-022-28563-7.35173177
Jehanno C. ; Pérez-Madrigal M. M. ; Demarteau J. ; Sardon H. ; Dove A. P. Organocatalysis for depolymerisation. Polym. Chem. 2019, 10 (2 ), 172–186. 10.1039/C8PY01284A.
Chow C.-F. ; Wong W.-L. ; Ho K. Y.-F. ; Chan C.-S. ; Gong C.-B. Combined Chemical Activation and Fenton Degradation to Convert Waste Polyethylene into High-Value Fine Chemicals. Chem.—Eur. J. 2016, 22 (28 ), 9513–9518. 10.1002/chem.201600856.27168079
Pifer A. ; Sen A. Chemical Recycling of Plastics to Useful Organic Compounds by Oxidative Degradation. Angew. Chem., Int. Ed. 1998, 37 (23 ), 3306–3308. 10.1002/(SICI)1521-3773(19981217)37:23<3306::AID-ANIE3306>3.0.CO;2-B.
Yao L. ; King J. ; Wu D. ; Chuang S. S. C. ; Peng Z. Non-thermal plasma-assisted hydrogenolysis of polyethylene to light hydrocarbons. Catal. Commun. 2021, 150 , 106274 10.1016/j.catcom.2020.106274.
Skariyachan S. ; Taskeen N. ; Kishore A. P. ; Krishna B. V. Recent advances in plastic degradation - From microbial consortia-based methods to data sciences and computational biology driven approaches. J. Hazard. Mater. 2022, 426 , 128086 10.1016/j.jhazmat.2021.128086.34933258
Gijsman P. ; Meijers G. ; Vitarelli G. Comparison of the UV-degradation chemistry of polypropylene, polyethylene, polyamide 6 and polybutylene terephthalate. Polym. Degrad. Stab. 1999, 65 (3 ), 433–441. 10.1016/S0141-3910(99)00033-6.
Singh B. ; Sharma N. Mechanistic implications of plastic degradation. Polym. Degrad. Stab. 2008, 93 (3 ), 561–584. 10.1016/j.polymdegradstab.2007.11.008.
Hocking P. J. The Classification, Preparation, and Utility of Degradable Polymers. J. Macromol. Sci., Part C 1992, 32 (1 ), 35–54. 10.1080/15321799208018378.
Chamas A. ; Moon H. ; Zheng J. ; Qiu Y. ; Tabassum T. ; Jang J. H. ; Abu-Omar M. ; Scott S. L. ; Suh S. Degradation Rates of Plastics in the Environment. ACS Sustain. Chem. Eng. 2020, 8 (9 ), 3494–3511. 10.1021/acssuschemeng.9b06635.
Yousif E. ; Haddad R. Photodegradation and photostabilization of polymers, especially polystyrene: review. SpringerPlus 2013, 2 (1 ), 398 10.1186/2193-1801-2-398.25674392
Gardette M. ; Perthue A. ; Gardette J.-L. ; Janecska T. ; Földes E. ; Pukánszky B. ; Therias S. Photo- and thermal-oxidation of polyethylene: Comparison of mechanisms and influence of unsaturation content. Polym. Degrad. Stab. 2013, 98 (11 ), 2383–2390. 10.1016/j.polymdegradstab.2013.07.017.
Arya R. K. ; Thapliyal D. ; Sharma J. ; Verros G. D. Glassy Polymers—Diffusion, Sorption, Ageing and Applications. Coatings 2021, 11 (9 ), 1049 10.3390/coatings11091049.
Carter M. C. D. ; Wong M. S. ; Wang F. ; Lynn D. M. Influence of Side Chain Hydrolysis on the Evolution of Nanoscale Roughness and Porosity in Amine-Reactive Polymer Multilayers. Chem. Mater. 2020, 32 (16 ), 6935–6946. 10.1021/acs.chemmater.0c02095.
Wei R. ; Zimmermann W. Microbial enzymes for the recycling of recalcitrant petroleum-based plastics: how far are we?. Microb. Biotechnol. 2017, 10 (6 ), 1308–1322. 10.1111/1751-7915.12710.28371373
Thomsen T. B. ; Almdal K. ; Meyer A. S. Significance of poly(ethylene terephthalate) (PET) substrate crystallinity on enzymatic degradation. New Biotechnol. 2023, 78 , 162–172. 10.1016/j.nbt.2023.11.001.
Ronkvist Å. M. ; Xie W. ; Lu W. ; Gross R. A. Cutinase-Catalyzed Hydrolysis of Poly(ethylene terephthalate). Macromolecules 2009, 42 (14 ), 5128–5138. 10.1021/ma9005318.
Herrero Acero E. ; Ribitsch D. ; Steinkellner G. ; Gruber K. ; Greimel K. ; Eiteljoerg I. ; Trotscha E. ; Wei R. ; Zimmermann W. ; Zinn M. ; Cavaco-Paulo A. ; Freddi G. ; Schwab H. ; Guebitz G. Enzymatic Surface Hydrolysis of PET: Effect of Structural Diversity on Kinetic Properties of Cutinases from Thermobifida. Macromolecules 2011, 44 (12 ), 4632–4640. 10.1021/ma200949p.
Biundo A. ; Ribitsch D. ; Steinkellner G. ; Gruber K. ; Guebitz G. M. Polyester hydrolysis is enhanced by a truncated esterase: Less is more. Biotechnol. J. 2017, 10.1002/biot.201600450.
Shinotsuka K. ; Bliznyuk V. N. ; Assender H. E. Near-surface crystallization of PET. Polym. J. 2012, 53 (24 ), 5554–5559. 10.1016/j.polymer.2012.09.048.
Sonnendecker C. ; Oeser J. ; Richter P. K. ; Hille P. ; Zhao Z. ; Fischer C. ; Lippold H. ; Blázquez-Sánchez P. ; Engelberger F. ; Ramírez-Sarmiento C. A. ; Oeser T. ; Lihanova Y. ; Frank R. ; Jahnke H.-G. ; Billig S. ; Abel B. ; Sträter N. ; Matysik J. ; Zimmermann W. Low Carbon Footprint Recycling of Post-Consumer PET Plastic with a Metagenomic Polyester Hydrolase. ChemSusChem 2022, 15 (9 ), e202101062 10.1002/cssc.202101062.34129279
Tarazona N. A. ; Wei R. ; Brott S. ; Pfaff L. ; Bornscheuer U. T. ; Lendlein A. ; Machatschek R. Rapid depolymerization of poly(ethylene terephthalate) thin films by a dual-enzyme system and its impact on material properties. Chem. Catal. 2022, 2 (12 ), 3573–3589. 10.1016/j.checat.2022.11.004.37350932
Bååth J. A. ; Borch K. ; Jensen K. ; Brask J. ; Westh P. Comparative Biochemistry of Four Polyester (PET) Hydrolases**. ChemBioChem 2021, 22 (9 ), 1627–1637. 10.1002/cbic.202000793.33351214
Jin J. ; Arciszewski J. ; Auclair K. ; Jia Z. Enzymatic polyethylene biorecycling: Confronting challenges and shaping the future. J. Hazard. Mater. 2023, 460 , 132449 10.1016/j.jhazmat.2023.132449.37690195
Restrepo-Flórez J.-M. ; Bassi A. ; Thompson M. R. Microbial degradation and deterioration of polyethylene - A review. Int. Biodeterior Biodegradation 2014, 88 , 83–90. 10.1016/j.ibiod.2013.12.014.
Sanluis-Verdes A. ; Colomer-Vidal P. ; Rodriguez-Ventura F. ; Bello-Villarino M. ; Spinola-Amilibia M. ; Ruiz-Lopez E. ; Illanes-Vicioso R. ; Castroviejo P. ; Aiese Cigliano R. ; Montoya M. ; Falabella P. ; Pesquera C. ; Gonzalez-Legarreta L. ; Arias-Palomo E. ; Solà M. ; Torroba T. ; Arias C. F. ; Bertocchini F. Wax worm saliva and the enzymes therein are the key to polyethylene degradation by Galleria mellonella. Nat. Commun. 2022, 13 (1 ), 5568 10.1038/s41467-022-33127-w.36195604
Ribitsch D. ; Herrero Acero E. ; Przylucka A. ; Zitzenbacher S. ; Marold A. ; Gamerith C. ; Tscheließnig R. ; Jungbauer A. ; Rennhofer H. ; Lichtenegger H. ; Amenitsch H. ; Bonazza K. ; Kubicek Christian P. ; Druzhinina Irina S. ; Guebitz Georg M. ; et al. Enhanced Cutinase-Catalyzed Hydrolysis of Polyethylene Terephthalate by Covalent Fusion to Hydrophobins. Appl. Environ. Microbiol. 2015, 81 (11 ), 3586–3592. 10.1128/AEM.04111-14.25795674
Wu Y. ; Wang X. ; Kirlikovali K. O. ; Gong X. ; Atilgan A. ; Ma K. ; Schweitzer N. M. ; Gianneschi N. C. ; Li Z. ; Zhang X. ; Farha O. K. Catalytic Degradation of Polyethylene Terephthalate Using a Phase-Transitional Zirconium-Based Metal-Organic Framework. Angew. Chem., Int. Ed. 2022, 61 (24 ), e202117528 10.1002/anie.202117528.
Chu M. ; Wang X. ; Wang X. ; Xu P. ; Zhang L. ; Li S. ; Feng K. ; Zhong J. ; Wang L. ; Li Y. ; He L. ; Cao M. ; Zhang Q. ; Chi L. ; Chen J. Layered Double Hydroxide Derivatives for Polyolefin Upcycling. J. Am. Chem. Soc. 2024, 146 (15 ), 10655–10665. 10.1021/jacs.4c00327.38564662
Zhang F. ; Zeng M. ; Yappert R. D. ; Sun J. ; Lee Y. ; LaPointe A. M. ; Peters B. ; Abu-Omar M. M. ; Scott S. L. Polyethylene upcycling to long-chain alkylaromatics by tandem hydrogenolysis/aromatization. Science 2020, 370 , 437–441. 10.1126/science.abc5441.33093105
