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Review

Sustainably Recycling and Upcycling of Single-Use Plastic Wastes through Heterogeneous Catalysis

by
Xiaoxia Zhang
1,
Shaodan Xu
1,*,
Junhong Tang
1,
Li Fu
1,* and
Hassan Karimi-Maleh
2,3,4
1
College of Materials & Environmental Engineering, Hangzhou Dianzi University, Xiasha Higher Education Zone, Hangzhou 310018, China
2
School of Resources and Environment, University of Electronic Science and Technology of China, Xiyuan Ave, Chengdu 611731, China
3
Department of Chemical Engineering, Quchan University of Technology, Quchan 9477177870, Iran
4
Department of Chemical Sciences, University of Johannesburg, Doornfontein Campus, 2028, Johannesburg 17011, South Africa
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(8), 818; https://doi.org/10.3390/catal12080818
Submission received: 4 July 2022 / Revised: 19 July 2022 / Accepted: 19 July 2022 / Published: 26 July 2022
(This article belongs to the Special Issue Environmental Catalytic Applications of Waste-Derived Materials)
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Abstract

:
The huge amount of plastic waste has caused a series of environmental and economic problems. Depolymerization of these wastes and their conversion into desired chemicals have been regarded as a promising route for dealing with these issues, which strongly relies on catalysis for C-C and C-O bond cleavage and selective transformation. Here, we reviewed recent developments in catalysis systems for dealing with single-use plastics, such as polyethylene, polypropylene, and polyethylene glycol terephthalate. The recycling processes of depolymerization into original monomers and conversion into other economic-incentive chemicals were systemically discussed. Rational designs of catalysts for efficient conversion were particularly highlighted. Overall, improving the tolerance of catalysts to impurities in practical plastics, reducing the economic cost during the catalytic depolymerization process, and trying to obtain gaseous hydrogen from plastic wastes are suggested as the developing trends in this field.

1. Introduction

Modern human life strongly relies on fossil-derived plastics for daily use, particularly the single-use plastics of polyethylene (PE), polypropylene (PP), and polyethylene glycol terephthalate (PET) for packaging food [1,2,3,4,5,6]. One of the most general routes for dealing with these waste plastics is to directly bury them underground, which takes hundreds of years for natural degradation because of the high stability of some plastics [7,8]. As reported, it was estimated that 4.8–12.7 million metric tonnes of plastics enter the oceans each year, and this has seriously damaged marine ecosystems [1,9]. For example, researchers have found microplastics in the deepest part of the ocean, the Mariana Trench [10]. These features make an urgency for dealing with waste plastics [11].
In addition to burying plastic wastes underground, another route is to burn them for obtaining heat, but this produces air pollutants (e.g., dioxin) [12,13]. A more recent trend is machining the plastics into particles for re-use. For most thermoplastics, the mechanical properties of the recycled plastics are usually reduced, meaning they cannot be used in the original fields and can only be considered as lower-value materials [14,15]. In the production of current polymer plastics, they are not designed for recycling, leading to downcycling processes because of reduced mechanical properties from the first-cycle use [16,17]. Compared with downcycling via a physical route, a chemical route is expected to convert plastic wastes into monomers that can be used to synthesize the same original materials, known as a recycling route [15,18,19]. Following this route, it is further expected to convert plastic wastes into other chemicals, which can be used to prepare value-added products rather than the original plastics, known as an upcycling process [20,21,22,23,24].
With regard to the recycling and upcycling processes, multiple strategies have been developed [25,26,27]. Due to the high stability of polymer plastics, catalysts with rationally designed functions have displayed a crucial role in C-C and C-O bond cleavage and have become an attractive topic in this field. In the past decade, multiple successful examples have been realized in plastic recycling and upcycling; hydrogenolysis on metal sites and catalytic cracking on acid sites have been extensively studied and are regarded as promising for dealing with polyolefins and polyesters [28,29,30]. With the rapid development in this field, we believe it is time to summarize the recent progress in the catalytic depolymerization of plastics. Different from previous reviews focusing on the whole lives of fossil-derived chemicals, we aim to summarize the development of various new catalysts and their performances in the conversion of polyolefins and polyesters. The catalyst structure for selective C-C and C-O bond dissociation is particularly highlighted. In addition, we also survey the developing trends in this field, which are the improvement of the tolerance of catalysts to impurities in practical plastics and the reduction of economic cost during the catalytic depolymerization process. In addition, attempting to obtain gaseous hydrogen rather than carbon-based chemicals is also discussed as a developing field for this topic.

2. Brief Introduction on Polymer Depolymerization

The depolymerization of waste plastics mainly includes biodegradation, thermal decomposition, and catalytic decomposition [31,32,33,34]. Conversion to platform chemicals can realize the utilization of resources [35]. Biodegradation is mostly catalyzed by microorganisms [36,37], which has a significant advantage in low cost but gives relatively low efficiency and takes a long time for the reaction. In addition to biodegradation, another classical process is the pyrolysis of polymers into smaller molecules [28], but this process leads to broad product distribution with limited selectivity for the desired products. Taking the thermal decomposition of polyolefin as an example, its process belongs to a typical one with random C-C cleavage, resulting in limited yields of the target products.
Compared with biodegradation and pyrolysis (noncatalytic), the catalytic depolymerization of plastics has significant advantages, including rapid reactions in short periods and controllable product distribution. For the catalytic depolymerization of polyolefin plastics, various solid-acid catalysts have been developed, such as acidic metal oxides and sulfides [38,39], zeolites [40,41], mesoporous aluminosilicates [42], and acid resins [43]. The reaction processes are strongly related to the acid strength, density, and mass transfer efficiency, which strongly determines the reaction channels to form gaseous alkanes, fuel-ranged hydrocarbons, and wax species [44,45,46]. Among these materials, zeolite is promising because of the unique shape selectivity for controlling product distribution, as well as its rigid framework, low cost for synthesis, and high thermal and hydrothermal stability. Inspired by the cracking process in the petrochemical industry, zeolite is also used for the catalytic cracking of polyethylene and polypropylene plastics. Owing to its controllable acid density and strength, zeolite has exhibited promising performances for obtaining various chemicals, but its one-pass durability is still an unsolved problem because of easy coke formation.
In addition to acid-catalyzed depolymerization, hydrogenolysis also provides an alternative route for C-C and C-O cleavage [47,48]. Compared with acid-catalyzed cracking, hydrogenolysis has significant advantages in coking resistance and minimizing the formation of light hydrocarbons (C1–C4) with obviously enhanced selectivity for fuel-ranged hydrocarbons (C5+) [49]. In addition, hydrogenation catalysts can be reasonably optimized for the cleavage of other groups, such as esters. Following this design, PET plastics, one of the most widely produced single-use ester plastics, has been decomposed into its original monomers and oligomers, which have been used to produce PET plastics again or transformed into other chemicals [49,50,51]. In addition to these routes, other techniques, such as the photothermal route and the hydride depolymerization route, have been developed, which are also discussed in the following sections.

3. Acid-Catalyzed Cracking of Polyolefins without Hydrogen

Polymers with C-C linkages represent the simplest and most widely used polymers, including polyethylene, polypropylene, and polystyrene [52,53,54]. These polymers also provide an ideal model for studying depolymerization by C-C cleavage [55,56,57,58,59]. In the thermal depolymerization of these polymers, the noncatalytic pyrolysis of the C-C bond forms reactive alky radicals, thus initiating the depolymerization reaction. In the catalytic cleavage of the C-C bond in these polymers, C-H activation has emerged as a central point that is crucial to the formation of active intermediates, such as carbocation, surface adsorbate, and some other radicals. These intermediate species can continuously promote C-C cleavage. Following this knowledge, various organometallic catalysts have been developed [57,60,61,62,63,64]. For example, Huang et al. reported a catalyst that could selectively convert polyethylene wastes into fuel-ranged hydrocarbons under mild conditions, where an alumina-supported Ir-based catalyst and a Re2O7 catalyst were combined for cascade dehydrogenation, olefin metathesis, and hydrogenation reactions [65]. In such a case, the recyclability of the precious Ir catalyst should be considered because it might leach during catalysis in a potential large-scale conversion. However, this feature makes its use uncertain for the future.
Among the multiple routes for C-C cleavage, the acidic catalysis route is a promising one, and it has been industrially used in petrochemistry. Most of the studied catalysts have been solid acids, which have significant advantages of low-cost preparation and easy recycling. Successful examples using proton-formed zeolites of ZSM-5 [66,67,68], FAU [69,70,71], and *BEA [72,73,74,75] have been reported. In these cases, the carbocation, initiated by the reaction of protons with hydrocarbon chains, acts as a crucial and active intermediate. Compared with the thermal depolymerization route, acid-catalyzed cracking occurs at lower temperatures [76,77,78,79]. However, due to strong zeolite acidity and the limitation of zeolite micropores, lower hydrocarbons (C1–C4) are usually obtained, showing limited selectivity for more valuable C5+ products. In addition, because of diffusion limitation in the micropores, coke easily forms that deactivates the catalyst in a short reaction period. This is an inherent disadvantage of zeolite catalysts in cracking.
To improve the coking resistance of zeolite catalysts, inducing mesopores into zeolite crystals and optimizing zeolite crystal morphology have been previously reported as efficient strategies, and various synthesis techniques have been developed accordingly [71,80,81,82,83,84]. For example, Reiprich et al. synthesized a layer-like FAU-type zeolite Y using an organosilane-assisted low-temperature hydrothermal method, catalyzing low-density polyethylene to obtain value-added C3–C4 gases and C5+ liquid with high selectivity [71]. Under the equivalent tests, abundant coke species easily formed on the conventional Y zeolite with obvious deactivation.
Lei et al. prepared hierarchical ZSM-5 catalysts using a successive treatment of desilication and lignin-mediated reassembly; open mesopores and moderately strengthened acidity in the hierarchical pores were obtained. They used this catalyst in the conversion of biomass and plastic feedstocks, giving more desired products than microporous ZSM-5. The surface and intracrystalline mesopores in the hierarchical ZSM-5 were beneficial for the conversion of macromolecules, such as aromatic oxygenates and long-chain hydrocarbons. In addition, the plastic-derived olefins synergistically interacted with the biomass derivatives, and the hierarchical structure extended the reactant accessibility from the catalyst surface into the microporous channels and internal acid sites to improve the catalytic performance and coking resistance (Figure 1) [85].
In addition, Inayat et al. explored ZSM-5 zeolites with different acid densities for the thermocatalytic pyrolysis of low-density polyethylene. They found that the catalyst with higher acid density benefitted the formation of more aromatics, while that with lower acid density benefitted the production of C2–C4 olefins. They further improved the reaction manner into two-stage catalysis and pyrolysis, and the concentration of aromatics in the pyrolysis oil reached over 77% with a mono-aromatics fraction of ca. 72% for the catalyst with higher acid density. Under equivalent tests, the catalyst with lower acid density formed C2–C4 olefins as the dominant products. These insights confirmed that reaction routes can be optimized by adjusting the zeolite acidity (Figure 2) [86].
Another attempt was performed by Choi and coworkers [87], who explored the catalytic decomposition of pine sawdust with polyethylene and polyethylene terephthalate in the presence of an HZSM-5 zeolite. The reactions were performed at different temperatures of 500 °C, 600 °C, and 700 °C. The oxygen-containing biomass, polyethylene terephthalate, and oxygen-free polyethylene preceded a redox reaction process, and the pine and PE ratio did not obviously influence the petrochemical concentration. More plastic in the feed led to higher wax production, and low wax production with high petrochemical production could be obtained by optimizing the feed composition. The best petrochemical yield was obtained with a sawdust-to-polyethylene ratio of 3.
Although the desired performances have been obtained for acidic-zeolite-catalyzed polyolefin plastic depolymerization, how to use the mixed products is still a challenge because of their complex compositions. The high reaction temperature in most cases is still energy-costing, and low reaction temperature and coking resistance are still unsolved problems in acid-catalyzed cracking processes.

4. Hydrogenolysis of Polyolefins Using Gaseous Hydrogen

Compared with the acid-catalyzed cracking process, the hydrogenolysis of polyolefins can occur at lower temperatures and can significantly suppress coke formation because hydrogen participation obviously reduces Gibbs free energy to be lower than zero at low temperature [88,89,90]. Following this route, Basset and coworkers showed that highly electrophilic Zr−H supported with a silica support could catalyze polyethylene’s hydrogenolysis into short-chain hydrocarbons [88]. In this process, C1–C4 hydrocarbon products were also formed from the deep hydrogenolysis. Tomishige and coworkers reported a Ru–CeO2 catalyst for the hydrogenolysis of heavier hydrocarbons into smaller molecules [91,92].
Supported Pt nanoparticles have been found to be more active in hydrogenolysis than other metals [93,94,95]. Delferro and coworkers loaded Pt nanoparticles on SrTiO3 nanocuboids via an atomic layered deposition method, obtaining Pt nanoparticles with sizes of 2.0 ± 0.5 nm crowding on the SrTiO3 support (Figure 3) [96]. In the hydrogenolysis of polyethylene macromolecules, this Pt–SrTiO3 catalyst could catalyze the selective conversion of polyethylene into lubricants and waxes with the presence of hydrogen at 300 °C, which are known as high-quality liquid products. Even for single-use plastic bags, similar products were still obtained, confirming the effectiveness of this strategy for dealing with practical-use plastics (Figure 4a,b).
In polyethylene hydrogenolysis, the accessibility of macroreactant molecules to the catalyst surface is regarded as rate control [97,98,99,100]. Generally, a solvent is employed for highly dispersing the long-chain molecules and promoting their reaction on the catalyst surface. For cases without a solvent, polyethylene–catalyst access is more challenging. Through 13C NMR characterization using 13C-enriched polyethylene adsorbing on a SrTiO3 support and a Pt–SrTiO3 catalyst, as well as theoretical simulations, it was found that the hydrocarbon chain adsorption on the SrTiO3 and TiO2 surfaces was unfavorable (Figure 4c). On the Pt–SrTiO3 catalyst, the hydrocarbon chain could be adsorbed on the Pt nanoparticles. On Pt(111) and Pt(100) surfaces, the hydrocarbon adsorption energies were −0.15 eV and −0.14 eV per CH2 group, respectively. In contrast, the hydrocarbon adsorption energy on the SrTiO3 surface was only −0.06 eV, confirming the negligible adsorption. This model was very similar to previous results reported by Carr et al., where hydrocarbons with heavier weights preferred to be adsorbed on alkali halide crystals, which could be used to preferentially extract the highest molecular weight fraction of a given mixture [101].
With regard to the challenge of product selectivity control in polyethylene hydrogenolysis, enzymes have provided an ideal blueprint [102,103,104]. In the deconstruction of flexible macromolecules, enzymes have ideal channels with active sites, where cleavage occurs to produce smaller molecules. After each cleavage, a smaller molecule is released, and the polymer threads further into the catalytic pore for another cleavage reaction. Inspired by this design, Perras and coworkers developed a mesoporous shell–active site–core catalyst by encapsulating silica-supported Pt nanoparticles within a mesoporous silica sheath [105]. This catalyst could catalyze the depolymerization of high-density polyethylene into a narrow distribution of diesel and lubricant-range alkanes. Importantly, the product selectivity could be adjusted by tuning the mesoporous sheath (Figure 4). Even for the hydrogenolysis of post-consumer HDPE, this catalyst was highly active and selective for obtaining fuel-ranged hydrocarbons. A more challenging investigation was performed in the hydrogenolysis of polypropylene, whose depolymerization is more difficult than the polyethylene. The yield of C9–C18 hydrocarbons reached as high as 79% in the hydrogenolysis of polypropylene, suggesting superior efficiency.
Catalysts 12 00818 g004 550
Figure 4. Processive deconstruction of macromolecules. (a) The processive mechanism through which many enzymes deconstruct large macromolecules. First, the polymer threads and binds into the catalytic pore or cleft. A catalytic cleavage reaction at the active site (represented by a pair of scissors) releases a low-molecular-mass fragment. The macromolecule then threads further into the pore to repeat the process. (b) An analogous mechanism proposed for a mSiO2–Pt–SiO2 catalyst, in which SiO2-supported Pt nanoparticles (orange) are located at the ends of nanopores in the mSiO2 shell. (c) Pore-diameter-dependent product distribution of polyethylene deconstruction [105].
Figure 4. Processive deconstruction of macromolecules. (a) The processive mechanism through which many enzymes deconstruct large macromolecules. First, the polymer threads and binds into the catalytic pore or cleft. A catalytic cleavage reaction at the active site (represented by a pair of scissors) releases a low-molecular-mass fragment. The macromolecule then threads further into the pore to repeat the process. (b) An analogous mechanism proposed for a mSiO2–Pt–SiO2 catalyst, in which SiO2-supported Pt nanoparticles (orange) are located at the ends of nanopores in the mSiO2 shell. (c) Pore-diameter-dependent product distribution of polyethylene deconstruction [105].
Catalysts 12 00818 g004
The aforementioned investigation provided an efficient strategy to change the product distribution, that is to optimize the mesopore diameter and length rather than changing the Pt nanoparticles. The balance between the polymer- and product-binding thermodynamics and the kinetic properties of C-C-bond-cleaving sites is crucial to affect the product selectivity. This insight is helpful for designing more efficient catalysts by controlling the synergism between active sites and nanopores.
In addition to mesopore engineering, the hydrocarbon selectivity in hydrogenolysis has been adjusted through tandem catalysis using zeolites [106,107,108]. Liu et al. developed a direct method to selectively convert polyolefins to diesel-, jet-, and gasoline-range hydrocarbons [109]. Key to the success is the employment of Pt, WO3, and ZrO2 catalysts with zeolite as tandem catalysts. The polyolefins were initially activated over Pt and then subsequently cracked and isomerized on the acidic WO3–ZrO2 and aluminosilicate zeolite, and olefin hydrogenated over Pt sites. Following this route, the combination of Pt, WO3, ZrO2, and HY catalysts exhibited up to an 85% yield of fuel-ranged hydrocarbons at temperatures as low as 225 °C. These data confirmed the high efficacy of the zeolite-promoted hydrogenolysis technique, and high yields of fuel-ranged hydrocarbons could be obtained in the conversion of multiple plastic wastes, including low- and high-density polyethylene, polypropylene, polystyrene, practical-use polyethylene bottles and bags, and composite plastics, to desirable fuels and light lubricants.
Scott and coworkers reported a tandem C-C hydrogenolysis and aromatization process using an alumina-supported Pt nanoparticle catalyst [110]. This route could be realized easily in a one-pot catalytic system at a low reaction temperature, converting various grades of plastic wastes into aromatics. Typically, in the conversion of a low-molecular-weight polyethylene (Mw = 3.5 × 103 g mol−1, Ð = 1.90) at 280 °C for 24 h, 80% yields of liquid and wax products were obtained. The overall ratio of Hα/Haromatic at 1.1 indicated that the major species were, on average, dialkylaromatics (Figure 5). In this process, some hydrogen was produced and detected in the reactor during catalysis. However, more than 90% of the hydrogen participated in the hydrogenolysis process to make the whole reaction process thermodynamically favorable. After the reaction, the detected hydrogen in the reactor was higher than that in a model reaction calculation, which might be due to polyethylene conversion to cycloalkanes and tetralins, which produces some amount of hydrogen. In this case, monoaromatic hydrocarbons related to naphthalenes could be optimized by shortening the reaction time and changing the partial pressure of H2. The amount of hydrogen in the reaction system should be sufficient for polyethylene hydrogenolysis but low enough to suppress aromatic hydrogenation, suggesting the possibility for further improvement in the yields of desired products.
As a typical acid support, aluminosilicate zeolite has been used to support Pt nanoparticles for polystyrene hydrocracking to liquid fuels. Through desilication or dealumination to modify the acidic properties of zeolite, the performance can be optimized. González-Marcos et al. found HBETA- and dealuminated-HBETA-zeolite-supported Pt catalysts were more active the those on Y-based zeolites [111]. Selectivity was also followed by analyzing the evolution of the yield to gas, gasoline, and diesel fractions, as well as the distribution of aromatics, naphthenics, olefins, isoparaffins, and paraffins in the majority gasoline fraction. The diesel fraction was favored for Pt/HZSM5, Pt/HBETA, and Pt/dealuminated HBETA and less favored for Pt/HY and Pt/desilicated HY, confirming the importance of zeolite support for the reactions.
Notably, the catalytic cracking of acid catalysts without metals requires high reaction temperatures, which suffer from the formation of undesired coke species and light hydrocarbons. Compared with thermal cracking, hydrogenolysis works at mild temperatures, which benefits the product selectivity control for obtaining the desired fuel-ranged hydrocarbons. However, many of these processes still rely on precious metal catalysts and gaseous hydrogen, which have high costs relative to acid catalyzed processes.

5. Hydrogenolysis of Polyesters Using Gaseous Hydrogen

The depolymerization of polymers containing heteroatoms, such as oxygen, is energy that is favorable compared with C-C cleavage. Solvolysis acts as one of the simplest methods for conversion, where the reactants of employed nucleophilic solvents react with carbonyl to generate products. This method has worked efficiently for the depolymerization of most oxygen-containing polymers and even nitro-containing ones. Following this trend, more solvents have been developed to facilitate the conversion, such as ionic liquids, which have a good solubilization capacity for polymers and can be functionalized with different catalytically active groups for catalysis [112,113,114]. Remarkably, enhancements in the activity of solvolytic depolymerization can be achieved through the use of catalysts such as zinc salts and some other metal salts [115]. In addition to these advantages, the separation and reuse of ionic liquids are still challenging.
Heterogeneously catalyzed hydrogenolysis has been developed for the depolymerization of polyesters to achieve monomer-related chemicals to realize recycling. For example, Wang and coworkers [116] reported Ru and Nb2O5 as efficient catalysts for the conversion of polyethylene terephthalate (PET) plastics into p-xylene in an aqueous solution, where the water molecules could strongly bind with the polar groups to benefit the conversion. Over the Ru and Nb2O5 catalysts, an exceptional yield of monomers (95.2%) and a high selectivity for arenes (87.1%) were realized (Figure 6A). Under equivalent reaction conditions, Nb2O5 supporting other metals, such as Pd and Pt, showed much lower yields of monomer molecules (~20%). The different performances of the Pd and Pt catalysts were explained by the favorable hydrogenation of the aromatic rings rather than the ester linkages, leading to chemically more robust C-O linkages. The unusual catalytic performances of the Ru and Nb2O5 catalysts were further confirmed by comparison with Ru nanoparticles loaded on other carriers, such as ZrO2, TiO2, and zeolite, all exhibiting insufficient performances that could not meet the level of Ru and Nb2O5. This phenomenon was due to the activated C-O bonds by the Nb2O5 support, which benefitted the cleavage of C-O linkage under hydrogenolysis conditions. Importantly, the good catalytic performances of Ru and Nb2O5 were not only limited to the hydrogenolysis of PET, but also could be extended to the hydrogenolysis of other various plastics with C-C or C-O linkages (Figure 6B).
Hydrogenolysis and solvolysis were efficient combined in the depolymerization of PE and its conversion into p-xylene products. Ma and coworkers [117] developed a cascade reaction system to realize this conversion by coupling three reactions: CO2 hydrogenation, PET methanolysis, and dimethyl terephthalate (DMT) hydrogenation. Considering that PET and CO2 are two chemical wastes that urgently need to be transformed, this cascade process simultaneously achieved goals of plastic waste conversion and CO2 utilization. Notably, PET methanolysis and CO2 hydrogenation are thermodynamically limited reactions individually, but they could accelerate each other in the cascade reaction system because of the continuous conversion of methane products and DMT products, realizing a dual-promotion process.
These investigations have confirmed that hydrogenolysis techniques can be optimized for the conversion of different plastics, where a catalyst with the appropriate structure for the selective hydrogenation of C-O bonds is highly crucial for this process. In a reasonably designed reaction system with appropriate catalysts, the reaction routes can be artificially adjusted for boosting the performance. In these processes, gaseous hydrogen is required for the cleavage of C-O and C-C bonds. Ma and coworkers [118] further explored C-O cleavage without hydrogen. Polylactic acid conversion was employed as a model. Although polylactic acid has been regarded as a biodegradable plastic, the natural degradation of polylactic acid requires a substantial period of time (Figure 6C). They developed a one-pot catalytic method to convert polylactic acid into alanine using an ammonia solution treatment with a Ru–TiO2 catalyst, realizing a 77% yield of alanine. In this process, hydrogen was not used, which is conceptually different from the general hydrogenolysis process, which might open a new route for plastic conversion to valuable chemicals using ammonia.

6. Conclusions and Future Perspective

The acid-catalyzed cracking process suffers from high energy cost and facile coke formation, but low-cost catalysts still make it promising for potential wide application. In contrast, although catalytic hydrogenolysis has achieved great success in multiple reaction systems, further optimizing of the catalysts and reaction conditions for improving the hydrocarbon and aromatic yields is still necessary. Notably, high weight ratios of the catalyst to the plastic substrate are still employed due to the close proximity between them during catalysis, which also leads to high catalyst cost during catalysis. In most hydrogenolysis processes, precious metals are still used, which further causes high cost when dealing with plastic wastes.
Another issue we have to emphasize is that current investigations have mostly focused on model polymer substrates and clean, commercial, single-use plastics. In the future, processes for dealing with practical-use waste plastics, impurities, and unknown solids that exist in wastes and affect catalysis should be investigated. For example, polyvinyl chloride provides -Cl species that can poison precious metal sites or corrode the zeolite framework on catalysts. Water and solid impurities in practical-use waste plastics can reduce catalyst recyclability. Further research work should focus on improving the performances of current catalysts with impurities and aim to obtain sufficient performances under mild reaction conditions for dealing with practical-use plastic wastes rather than model substrates.
With the current serious situation of waste plastics and promising strategies for dealing with them by chemical recycling and upcycling, we believe this field will be very active in the future. Developing new catalysts for the selective cleavage of target groups, reactor engineering to strengthen the access between polymers and catalysts, and product purification techniques for obtaining valuable molecules are desired techniques in the future for the recycling of carbon resources in waste plastics. With regard to catalyst development, the combination of artificial intelligence and machine-learning technology can help with the rational design of fine structures of heterogeneous catalysts for achieving high activity for C-C and C-O bonds, controllable selectivity for fuel and fine chemical products, and the desired tolerance of impurities. Such research would bridge the fields of material research and plastic waste recycling and upcycling.
Recently, hydrothermal liquefaction in sub- or supercritical water was developed for dealing with polycarbonate waste [119], an important type of modern plastic [120,121]. Phenol was the dominant liquefied product of polycarbonate, followed by other products of 4-isopropylphenol and 4-isopropenylphenol [119]. Following success, the combination of heterogeneous catalysts in sub- and supercritical water systems would benefit further the optimization of reaction rates and product selectivity. It requires efficient catalysts that are stable in hot water, where the synergism of heterogeneous catalysts and sub- or supercritical media might open a new route for dealing with polycarbonates and even some other heteroatom-containing plastics.
Another research trend for plastic conversion is to produce hydrogen through plastic waste reactions with water at high temperatures [122], followed by the operations of plastic thermal and hydrothermal depolymerization; solid, liquid, and gaseous product separation; reforming hydrocarbon species with water; and water–gas shifting to obtain hydrogen products. Compared with current hydrogen production using coal and methane, hydrogen production using plastic has significant advantages in the cost of the wastes. This might also guide the conversion of plastic wastes in the future.

Author Contributions

Conceptualization, S.X. and L.F.; methodology, X.Z. and S.X.; software, J.T. and H.K.-M.; validation, J.T. and H.K.-M.; formal analysis, X.Z. and H.K.-M.; writing—original draft preparation, X.Z.; writing—review and editing, S.X. and L.F.; supervision, S.X.; project administration, S.X.; funding acquisition, S.X. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Natural Science Foundation of Zhejiang Province (LY21B030002).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Scheme showing the catalytic conversion of low-density polyethylene (LDPE) with biomass [85].
Figure 1. Scheme showing the catalytic conversion of low-density polyethylene (LDPE) with biomass [85].
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Catalysts 12 00818 g002 550
Figure 2. Scheme showing low-density polyethylene (LDPE) pyrolysis–catalysis conversion of zeolite catalysts in one- and two-stage manners [86].
Figure 2. Scheme showing low-density polyethylene (LDPE) pyrolysis–catalysis conversion of zeolite catalysts in one- and two-stage manners [86].
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Catalysts 12 00818 g003 550
Figure 3. (a) Electron micrographs of Pt nanoparticles with an average size of 2.0 ± 0.5 nm on SrTiO3 support. (b) Hydrogenolysis of polyethylene into high-quality liquid products. (c) 13C MAS (red) and CPMAS (black) spectra of 13C-enriched polyethylene adsorbed on a SrTiO3 support (top) and a Pt–SrTiO3 catalyst (bottom). (d) Side view of optimized structures of n-hexane on Pt(111), Pt(100), and TiO2 double-layer-terminated SrTiO3(001) surface models [96].
Figure 3. (a) Electron micrographs of Pt nanoparticles with an average size of 2.0 ± 0.5 nm on SrTiO3 support. (b) Hydrogenolysis of polyethylene into high-quality liquid products. (c) 13C MAS (red) and CPMAS (black) spectra of 13C-enriched polyethylene adsorbed on a SrTiO3 support (top) and a Pt–SrTiO3 catalyst (bottom). (d) Side view of optimized structures of n-hexane on Pt(111), Pt(100), and TiO2 double-layer-terminated SrTiO3(001) surface models [96].
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Catalysts 12 00818 g005 550
Figure 5. (A) Schematic of reactor and product fractions with photographs of the powdered polymer and liquid products, as well as a transmission electron micrograph of the catalyst. (B) Hydrocarbon distribution after 24 h at 280 °C. (C) Overall PE conversion to alkylaromatics and alkylnaphthenes, and proposed mechanism of tandem polyethylene hydrogenolysis–aromatization via dehydrocyclization [110].
Figure 5. (A) Schematic of reactor and product fractions with photographs of the powdered polymer and liquid products, as well as a transmission electron micrograph of the catalyst. (B) Hydrocarbon distribution after 24 h at 280 °C. (C) Overall PE conversion to alkylaromatics and alkylnaphthenes, and proposed mechanism of tandem polyethylene hydrogenolysis–aromatization via dehydrocyclization [110].
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Figure 6. (A) Data characterizing the performances of various catalysts in the conversion of PET. Reaction conditions: 0.1 g PET, 0.1 g catalyst, 10 g H2O, 200 °C, 12 h, and 0.3 MPa H2 [116]. (B) Results of the conversion of various aromatic plastics over Ru–Nb2O5 catalyst [116]. (C) Scheme showing the processes of polylactic acid natural degradation and catalytic conversion [118].
Figure 6. (A) Data characterizing the performances of various catalysts in the conversion of PET. Reaction conditions: 0.1 g PET, 0.1 g catalyst, 10 g H2O, 200 °C, 12 h, and 0.3 MPa H2 [116]. (B) Results of the conversion of various aromatic plastics over Ru–Nb2O5 catalyst [116]. (C) Scheme showing the processes of polylactic acid natural degradation and catalytic conversion [118].
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Zhang, X.; Xu, S.; Tang, J.; Fu, L.; Karimi-Maleh, H. Sustainably Recycling and Upcycling of Single-Use Plastic Wastes through Heterogeneous Catalysis. Catalysts 2022, 12, 818. https://doi.org/10.3390/catal12080818

AMA Style

Zhang X, Xu S, Tang J, Fu L, Karimi-Maleh H. Sustainably Recycling and Upcycling of Single-Use Plastic Wastes through Heterogeneous Catalysis. Catalysts. 2022; 12(8):818. https://doi.org/10.3390/catal12080818

Chicago/Turabian Style

Zhang, Xiaoxia, Shaodan Xu, Junhong Tang, Li Fu, and Hassan Karimi-Maleh. 2022. "Sustainably Recycling and Upcycling of Single-Use Plastic Wastes through Heterogeneous Catalysis" Catalysts 12, no. 8: 818. https://doi.org/10.3390/catal12080818

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