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Review
Recent Advances in the Chemobiological Upcycling of Polyethylene Terephthalate (PET) into Value-Added Chemicals
1Department of Food Science and Technology, Chungnam National University, Daejeon 34134, Republic of Korea
2Department of Biochemical Engineering Gangneung-Wonju National University, Gangneung 25457, Republic of Korea
3Synthetic Biology Research Center, Korea Research Institute of Bioscience and Biotechnology, Daejeon 34141, Republic of Korea
4Department of Biological and Chemical Engineering, Hongik University, Sejong 30016, Republic of Korea
5Center for Bio-based Chemistry, Korea Research Institute of Chemical Technology (KRICT), Ulsan 44429, Republic of Korea
J. Microbiol. Biotechnol. 2023; 33(1): 1-14
Published January 28, 2023 https://doi.org/10.4014/jmb.2208.08048
Copyright © The Korean Society for Microbiology and Biotechnology.
Abstract
Keywords
Graphical Abstract

Introduction
Plastics are ubiquitous in daily life as they are used in packaging, construction, and clothing [1]. These materials are significant in modern society owing to their adaptability, low manufacturing costs, and desirable physical features, which include flexibility, lightness, impermeability, and durability [2, 3]. Consequently, the demand for plastics has continuously increased; however, improper recycling of plastic waste causes plastic pollution [4]. Plastic accumulation threatens human survival and the environment [5]. According to Geyer
Reuse, incineration, and landfilling are conventional methods for managing plastic waste [9]. Plastic incineration provides energy but also emits particulate matter and toxic gases such as unburned hydrocarbons, nitrous and sulfurous oxides, furans, and dioxins, which cause severe environmental damage [11] For these reasons, landfilling and incineration are not recommended for plastic garbage disposal, and instead, more sustainable and environmentally friendly methods are required [12].
Due to its exceptional mechanical properties, thermal stability, and impermeability to gases and liquids, polyethylene terephthalate (PET) is the most popular synthetic plastic material applied in beverage packaging, food containers, bottles, and textiles [13]. PET is produced by polycondensation of terephthalic acid (TPA) and ethylene glycol (EG) or trans-esterification of dimethyl terephthalate (DMT) and EG. Its stability and resistance to hydrolytic or enzymatic degradation have led to PET being the most commonly found plastic waste in the environment [1]. Currently, PET is primarily recycled via physical and chemical methods; various studies have also reported the use of biodegradation using microorganisms to break down PET into its monomers: TPA and EG. Physical recycling involves heat treatment at high temperatures, and PET is degraded into downcycled products because of the loss of mechanical properties, which are different from those of virgin PET [1]. Chemical recycling uses chemicals and expensive catalysts to decompose waste into monomers; recycled plastics are typically more expensive than virgin plastics and are not economically viable [13]. To solve the plastic pollution problem, various approaches, including biological depolymerization and upcycling of plastic wastes, and even plastic-eating microbes, have been considered [14]. Circular repurposing of plastic materials is critical for building a socioeconomic ecosystem without contributing to plastic waste [15]. Upcycling converts waste materials into products of greater worth and quality in their second life [16]. Upcycling also helps achieve the circular economic feasibility of plastic waste with complete recyclability and no loss of value or usability, in contrast to other recycling methods.
In this review, we discuss the development of the upcycling processes suitable for PET, comprising two steps: 1) substrate production from PET waste, and 2) upcycling of the substrate into value-added chemicals (Fig. 1). For producing substrates by degrading PET, we suggest processes suitable for bioconversion to obtain value-added chemicals after reviewing various existing processes, including pyrolysis, gasification, and depolymerization, along with their advantages and disadvantages. Following the review of chemical and biological upcycling models, we propose an efficient upcycling model for producing value-added chemicals. Finally, we suggest a method for PET upcycling to achieve economic feasibility and a circular economy regarding the plastic life cycle, as a means to address plastic environmental pollution.
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Fig. 1. Overall scheme representing the primary aspects of the review.
Production of Substrates from PET Waste for Bioconversion Processes
Various processes, including pyrolysis, gasification, and depolymerization, can be applied to produce substrates from PET waste (Fig. 2, Table 1). Although individual processes for substrate production from PET waste are well established, selecting a suitable upcycling process, particularly regarding biological upcycling, is crucial because of the economic feasibility and establishment of refineries. Therefore, in this section, we discuss the status of research, along with the advantages and disadvantages, to suggest a process suitable for upcycling.
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Table 1 . Degradation product types and yields obtained from PET using various PET degradation processes.
Process Substrate Product Yield Reference Pyrolysis PET bottle sheets1 Solid
Gas35.67 wt%
40 wt%[22] PET water bottles with rice husk Gas
Biocrude
char39.9-68.9 wt%
12-29%
15.7-31.7%[23] PET bottles2 Gas
Oil
Solid residue13.3±0.6 wt%
46.7±1.9 wt%
39.7±1.6 wt%[24] Pure PET pellets Gas: CO, CO2
Solid: TPA-
25%[25] Gasification Virgin PET pellets Gas: H2, CO2, CO
Char
Tar90.93 wt%
6.15 wt%
2.9 wt%[29] Virgin PET pellets Gas: H2
CO2
Biphenyl- [28] Depolymerization Aminolysis Post-consumer PET
DHTA
BHTA
BFTA
DAA64%
91%
82%
61%[34] PET flakes Monomers, Dimers, Trimers, Oligomers - [35] Methanolysis PET DMT 89.1% [39] PET DMT
EG78%
76%[37] PET DMT
EG95% [38] PET DMT
EG91% [40] PET DMT
EG93.5% [41] Glycolysis PET BHET 80.30% [46] PET BHET 70% [49] Post-consumer PET bottles TPA
MHET
BHET62.79-80.66%
17.22-34.79%
0.54-0.59%[50] PET bottles BHET, TPA, EG Quantitative [51] Biological depolymerization PET BHET
EG68.6% [45] Post-consumer PET waste TPA 49±2% [76] Amorphous PET TPA 16.7 g/L/h [79] Chemoenzymatic depolymerization PET BHET
MHET
PET oligomers84.8%
7.7%
8.7%[8] PET BHET 35% [47] PET TPA
EG99.9% [3] Note: DHTA, dihexylterephthalamide; BHTA, bis(2-hydroxyethyl) terepthalamide; BFTA, bis(furan-2-ylmethyl) terepthalamide; DAA, diallyterepthalamide; DMT, dimethyl terephthalate; TPA, terephthalic acid; BHET, bis(2-hydroxyethyl) terephthalate; MHET, mono-(2-hydroxyethyl) terephthalate; EG, ethylene glycol.
1-solid products contained benzoic acid, 4-vinyl benzoic acid, monovinyl terephthalate, and divinyl terephthalate.
2-Oil components include paraldehyde (54.7 wt%), ethylene glycol (23.65 wt%), and benzoic acid and benzoates (11.5 wt%). The solid carbonaceous residue contained carbon, ash, nitrogen, and sulfur). Gas contained CO and CO2 (more than 90 vol%), a few C1 -C 4 hydrocarbons (~7 vol%), and hydrogen (~3 vol%).
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Fig. 2. Substrate production processes from PET waste into respective products, including different processes used to produce substrates that are employed in biological upcycling.
Substrate Production by Pyrolysis of PET
Pyrolysis is the thermal transformation of polymers into liquid fuels and value-added products at considerably high temperatures (300-900°C) in the absence of oxygen [17-19]. PET pyrolysis releases products such as TPA, vinyl terephthalate, aromatic compounds (namely toluene and benzene); esters such as vinyl benzoate; carboxylic acids such as ethyl and methyl benzoic acid; and aliphatic hydrocarbons such as ethane and methanol. TPA formed during pyrolysis can be used for upcycling after purification; however, TPA clogs the equipment [20]. Pyrolysis is affected by factors like substrate type and time, and temperature, which influence the polymer degradation rate; and catalyst loading, which improves the process efficiency through time reduction. Slow, fast, and flash pyrolysis methods are based on reaction times, and microwave pyrolysis is based on the apparatus type [21].
PET waste was pyrolyzed for energy recovery at 500°C, yielding solids, gas, and residue products of 24.33, 35.67, and 74.67 wt%, respectively [22]. The effect of plastic waste loading was studied using microwave co-pyrolysis of PET and rice husk mixtures at 600°C for 4-13 min, which yielded syngas (29%) affected by increasing plastic waste loading, biocrude, and char [23]. Slow pyrolysis of PET waste was conducted at 400°C producing gas, solid residue, and oil at 13.3 ± 0.6, 39.7 ± 1.6, and 46.7 ± 1.9 wt%, respectively [24]. Catalytic pyrolysis of PET was performed to study the effect of temperature and catalyst on PET degradation (500°C in a fixed-bed, semi-batch reactor using a Lewis-Brønsted acid side catalyst). The increase in temperature increased the PET conversion rate, forming liquid-, gas-, and solid-phase products with benzoic acid and TPA. This process is primarily affected by the formation of char, which influences the conversion rate and product yield [25]. Pyrolysis does not require intense waste sorting and feedstock pre-treatment, and is consequently convenient and flexible; hence, it is economical and less labor-intensive. Moreover, pyrolysis reduces the dependence on conventional energy sources (such as fossil fuels), the volume of PET waste, and the carbon footprint of plastic products by reducing carbon monoxide and carbon dioxide emissions [26]. However, pyrolysis produces detrimental materials such as biphenyls, which create environmental pollution and health problems [27]. The substrates required for biological upcycling in this process are expensive as they require purification.
Substrate Production by PET Gasification
Gasification is the degradation of plastics at high temperatures (700-1300°C) using oxidizing agents such as oxygen, steam, and air to produce syngas or producer gas consisting of methane, carbon monoxide, and hydrogen gas [28]. Depending on the gasifying agent and heterogeneous char gasification reactions, gasification involves drying, devolatilization, tar cracking, combustion, and shifting [28]. This process is mainly affected by factors including the type of plastic (aliphatic or aromatic), bed material, temperature, and gasifying agent [29]. Depending on the oxidizing agent, steam gasification, air gasification, and co-gasification, wherein different compounds are mixed before gasification, have been explored. Dolomite is used in the fluidized bed to reduce agglomeration and coke formation on active carbon. Active carbon is predominantly used as a tar remover, improving syngas quality [30]. Steam gasification uses steam as the primary hydrogen source. Considering the effect of the operating conditions on the tar and gas composition during the steam gasification of PET, increasing the temperature improves the hydrogen and carbon monoxide yields as the residence time and steam-to-fuel ratio are increased; and carbon monoxide and carbon dioxide contents are decreased [28]. Air gasification of PET was performed in a two-stage gasifier with active carbon. Air was used instead of steam to provide hydrogen and carbon monoxide, yielding producer gas (90.93 wt%), char (6.15%), tar (2.90%), and condensate liquid (0.03 wt%)[29]. Gasification is considerably flexible in treating various feedstock composites and can be integrated into current energy and fuel production systems. However, PET exhibits low effective carbon conversion, which reduces syngas heat. During steam gasification, less than 30% of carbon in PET is converted into gaseous products at 700°C. This conversion is required to form CO2, which is an important constituent of syngas; however, PET produces 3-4 times less carbon dioxide [28].
Substrate Production via Plastic Depolymerization
depolymerization is the application of polymer chemistry to undo polymerization reactions to yield PET monomers,
Chemical Depolymerization
Chemical depolymerization uses chemicals and catalysts to influence the breakdown of PET into its monomers, BHET, MHET, TPA, and EG obtained from glycolysis, DMT from methanolysis, and TPA derivatives from aminolysis. Chemical depolymerization methods include alcoholysis (methanolysis), aminolysis, hydrolysis (steam, mineral acids, water, and alkalis), and glycolysis [31]. Inorganic catalysts, organocatalysts, and ionic liquids have been used to accelerate depolymerization.
Aminolysis involves the reaction of PET in primary amine-rich solutions, namely, hydrazine, ethanolamine, methylamine, allylamine, and ethylamine, to produce TPA and EG diamides. PET is used in the form of powder or fibers (temperature range = 20-100°C) [32]. Waste PET bottles were converted into hydrogel adsorbents through aminolysis. PET was reacted with tri- and tetraamines, diethylene amine, and diethylene-tetraamine, producing monomers, dimers, and oligomers, respectively, which were interlinked with ethylene glycol diglycidyl ether to form hydrogels [33]. Additionally, using an ultrafast microwave, aminolysis of PET could be conducted without catalysts in the primary amine-rich solutions at 180-200°C. Consequently, terepthalamides with different functional groups, namely, dihexylterephthalamide (DHTA), bis-(2-hydroexylethylterephtalamide (BHETA), bis-(furan-2-ylmethyl terepthalamide (BFTA), and diallyterepthalamide (DAA) were produced. These terepthalamides were used as plasticizers for polylactide and polylactic acid to reduce brittleness or as resin components for photopolymerizable film production [34]. PET was reacted with 1,2-diamino propane at temperatures in the range of 100-130°C for 20-24 h, yielding a combination of products, namely, monomers, dimers, trimers, and oligomers; the reaction of the monomers with salicylaldehyde produced a Schiff base, which can be used as a precursor for biologically active ligands, complexes, and catalysts [35]. Aminolysis requires minimal energy and time, and a simple purification step for the synthesized products. Because of these advantages, we expect that BHETA, DHTA, BFTA, and DAA could potentially be applied in biological upcycling for synthesizing high value-added chemicals after further depolymerization into TPA and EG, with simultaneous catabolism of certain microorganisms.
In methanolysis, PET is degraded by methanol at high temperatures (160-300°C) and pressures of up to 7 MPa in the presence of transesterification catalysts, consequently forming DMT and EG as the primary products. Additionally, PET oligomers (dimers, trimers, and tetramers) are possibly formed. Methanolysis involves two steps: depolymerization and purification of DMT through crystallization and distillation [36]. Methanolysis is influenced by factors including type and amount of catalyst, temperature, and time. PET waste methanolysis was done at 200°C for 2 h with bamboo leaf as a green heterogeneous catalyst; DMT and EG were the primary products with yields of 78% and 76%, respectively [37]. PET was also depolymerized with methanol in the presence of calcined sodium silicate at 180-200°C for 30 min, resulting in DMT (95% yield) and EG (depolymerization rate = 100% [38]. Using poly ionic liquids, namely, PIL-Zn2+ and PIL-Co2+ as catalysts, PET was depolymerized with methanol into DMT and EG under optimized conditions, resulting in DMT (89.1% yield) with 100% PET conversion [39]. PET was depolymerized at 200°C for 30 min using MgO/NaY (4 wt%), producing DMT (91%yield) and EG with 99% conversion of PET [40]. A low-energy catalytic methanolysis process was developed to depolymerize PET using methanol, with potassium carbonate as a catalyst, at 20-35°C. A high yield of 93.1% DMT and EG was obtained at 25°C, indicating that methanolysis could be performed at low temperatures [41]. Relatively low-quality PET can be used for methanolysis because of a simplified purification process involving the DMT product. Moreover, increased levels of contamination are tolerated, thereby offsetting chemical processing costs. However, methanolysis is expensive and sensitive to water presence, which is linked with catalyst poisoning [36]. DMT cannot be directly used in biological upcycling but can be converted into TPA, which is the required substrate. DMT was hydrolyzed to TPA in the presence of Nb/HZSM-5, a solid acid catalyst in the range of 160-220°C. The conversion was influenced by temperature changes; the DMT conversion and TPA yield increased gradually with an increase in temperature. At 160°C, DMT conversion and TPA yield were 33.4 and 21.8%, respectively; at 200°C, the DMT conversion was complete and the rate was 100% (DMT yield = 93.5%) [42]. Similar to the aminolysis products (BFTA, BHETA, DAA, and DHTA), the methanolysis product, DMT, can be applied to biological upcycling as a substrate after applying a suitable hydrolysis process to produce TPA and EG from DMT.
Glycolysis is a significant chemical depolymerization method and is used to obtain BHET and EG, which are used in bio-upcycling. Glycolysis is typically performed on a commercial scale and is consistently mediated by trans-esterification catalysts using EG as a solvent at a high temperature range (180-250°C), producing BHET, oligomers, and EG [43, 44]. Glycolysis uses different types of catalysts, namely metal-based ceramics, biomass-derived materials [45], organocatalysts [46], metallic chlorides, acetates, enzymes, and eutectic solvents based on ionic liquids and metal salts [47, 48]. Catalysts influence the speed of glycolysis; however, different catalysts have different costs and affect the process following glycolysis by influencing the enzymes/cells used, thereby requiring purification and increasing the economic cost of the entire process. Catalysts typically harm the environment; hence, biomass-derived materials are currently being investigated. Glycolysis is a simple, low-cost, and flexible method that produces a high yield of BHET monomer [47]. PET was depolymerized in the presence of EG and various catalysts, namely, graphite carbon nitride, melamine, potassium chloride, and sodium chloride at 160-96°C for 5-120 min; using graphite carbon nitride nanocatalysts resulted in BHET with 80.30% yield. Owing to better yields, lower total costs, and pollution reduction, graphite carbon nitride nanocatalysts are superior to metal-based and metal-free ionic catalysts [46]. Virgin PET pellets were depolymerized via glycolysis to estimate the efficiency of BHET production with modified conditions: temperature = 180-220°C and time = 10-180 min. PET was glycolyzed in the presence of a pure zinc acetate catalyst at a pressure of 3 bar, producing oligomers, dimers, and BHET at 180 and 220°C in short periods (e.g., 10 min) [49]. Because glycolysis depends on temperature and time, determining the optimum conditions is important. Post-consumer PET samples were broken down through glycolysis in the presence of deep eutectic solvents: choline chloride-thiourea and choline chloride-urea as catalysts, producing residual PET which was hydrolyzed in the presence of sodium carbonate and EG for a microwave irradiation time of 3 min; TPA, MHET, and BHET were obtained with yields of 62.79-80.66, 17.22-34.79, and 0.54-0.59%, respectively, with 99% PET conversion. Microwave irradiation was used because it reduces the reaction time, proving that PET can be rapidly depolymerized [50]. PET from a soft drink company was recycled using conventional heating and microwave radiation as energy sources in the presence of Lewis acids: t-BuNH2/LiBr in glycolysis and t-BuNH2/NaCl in hydrolysis. Conventional heating was performed at 197°C for 25 h, whereas microwave radiation was performed at 210°C for 30 min, producing EG and TPA. Microwave radiation is recommended as an energy-conserving process because PET is completely depolymerized within a short duration [51]. An oyster-derived (biomass-derived) catalyst was used in glycolysis optimization, which could break down PET at 195°C for 1 h into BHET and EG with 68.6% yield. By replacing conventional metal-based catalysts with biomass-based catalysts, an environmentally friendly and economically competitive glycolysis process can be established [45]. Glycolysis has several advantages, including simplicity, flexibility, low cost, and the synthesis of TPA, EG, and BHET, which can be used in the upcycling process with or without purification [52]. However, this process involves high pressure and temperature, catalysts, and difficulty in separating and purifying oligomers from the desired products [53].
Biological Depolymerization
Biological depolymerization produces intermediates by biologically degrading plastic, which is subsequently subjected to downstream bioconversion to obtain value-added products. This process generally uses a PETase-MHETase dual enzyme or a single enzyme system. Enzymatic depolymerization involves the use of enzymes with promiscuous activities, such as lipases, esterases, cutinases, carboxyl esterases, and MHETase, to release BHET and MHET monomers, which are further degraded to TPA and EG as the final products [47, 54]. These enzymes have been discovered in various microbes, such as
Several microorganisms with PET-degrading abilities, such as
Therefore, biological depolymerization is an environmentally friendly method because it uses mild conditions (temperature and pH) in the absence of hazardous chemicals [47]. Enzymes are specific and selective to specific substrates, rendering the process effective, as they can be used to act on targeted substrates. Enzymatic depolymerization (e.g., hydrolysis) is crucial for producing substrates suitable for bioconversion (e.g., TPA and EG). However, biological depolymerization incurs high operating costs [82]. To establish economically feasible biological depolymerization processes, possible further developments include reducing the enzyme production costs and improving the catalytic properties and stability.
Chemoenzymatic Depolymerization
Several combined chemoenzymatic depolymerization processes have been developed, wherein chemical depolymerization, including glycolysis, yields BHET, oligomers, and EG, and enzymatic hydrolysis breaks down BHET and MHET into TPA. TPA and EG are the substrates used for bioconversion. PET was glycolyzed in the presence of urea/NaOAc·3H2O, a eutectic solvent-based catalyst, and EG to obtain the main product: BHET (35%yield and 73.6% PET conversion). The synthesized BHET was further hydrolyzed by
Production of High-Value Products from PET Monomers by Upcycling Processes
Upcycling involves the conversion of degraded products from chemical transformations or biological degradation into various value-added biochemicals via chemical or biological routes [84]. Substrate production processes for upcycling using various degradation methods, such as pyrolysis and gasification, have been efficiently developed. However, because recently published research is predominantly related to upcycling BHET, TPA, and EG, we focus on the upcycling status of these chemicals. Therefore, in this section, after reviewing the biological upcycling of TPA and EG by TPA/EG-metabolizing microorganisms and whole-cell conversion into high-value-added chemicals, as shown in Fig. 3 and Table 2, we suggest a promising convergence process for upcycling PET.
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Table 2 . Chemicals produced by whole-cell conversion.
Substrate Recombinant organism Products Production yields References TPA E. coli PCA 81.4% [13] Gallic acid 15.9% Catechol 97.8% Pyrogallol 39.0% Vanillic acid 41.6% Muconic acid 85.4% EG G. oxydans KCCM 40109Glyoxylic acid 98.6% TPA E. coli PCA -1PCA 90.4% [3] EG G. oxydans KCCM 40109Glyoxylic acid 91.6% TA E. coli RARE-pVanXVanillin 79% [67] TPA P. stutzeri TPA-3PPHB 11.56 wt% [75] TPA P. umsongensis GO16 KS3HAA 35 mg/L [73] TPA P. putida GO16, G019, PHA, 3-hydroxydecanoic acid 4.4 mg/L/h [74] P. frederiksbergensis (GO23)8.4 mg/L/h TPA I. sakaiensis PHA 0.75 ± 0.09 g/L [98] Note: PCA, protocatechuic acid; PHB, polyhydroxy butyrate; HAA, hydroxy alkanoyl oxy-alkanoate; PHA, polyhydroxyalkanoate
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Fig. 3. Catabolic pathways of TPA to aromatic chemicals: catechol, PCA, and other high value-added chemicals. The α-ketoadipate pathway (blue line) consists of genes, namely, PcaHG (protocatechuate 3,4-dioxygenase), PcaB (3-carboxy-cis, cis-muconate cycloisomerase), PcaC (γ-carboxy-muconolactone decarboxylase), and PcaD (β-ketoadipate enol-lactone). The targeted products include: CMA, β-carboxy-cis,cis-muconic acid, CML-γ-carboxymuconolactone, MCL, Muconolactone,KA,β-ketoadipic acid. The PCA 2,3-cleavage pathway (light green lines) consists of genes, namely, PraA, (PCA 2,3-dioxygenase), PraH, (5 CHMS decarboxylase), 5C-2HMS dehydrogenase, XyHI,4-oxalocrotonate isomerase, XyIJ,2- oxopent-4-enoate hydratase,XyIK, 4-hydroxy-2-oxovalerate aldolase, XyIQ, acetaaldehyde dehydrogenase. The targeted molecules are: CHMS, 5-Carboxy-2-hydroxymuconate-semialdehyde, ICA, isocinchomeronic acid, HMS, 2-Hydroxymuconate semialdehyde, PCL-picolinic acid, HPD/OEA, 2-Hydroxypenta-2,4-dienoic acid/2-oxopent-4-enoic acid, HOA, 4-Hydroxy- 2-oxovaleric acid. The other pathway (grey line) is encoded by LigAB,4,5-PDC,Lig C,CHMS dehydrogenase, Lig I, PDC hydrolase, LigJ, OMA hydratase, LigK, CHA(4-carboxy-4-hydroxy-2-oxoadipate) aldolase yields energy compounds. The targeted compounds include: HCMS, 4-Carboxy-2-hydroxymuconate-semialdehyde, DPA-dinicotinic acid, PDC, 2-pyrone- 4,6-dicarboxylic acid, CHA, 4-Carboxy-4-hydroxy-2-oxoadipic acid. Catechol is catabolized by CatA, Catechol 1,2- dioxygenase,CatB, Muconate cycloisomerase and Cat C, Muconolactone isomerase into MA, cis,cis-Muconic acid.
Biological Upcycling of TPA or EG by TPA- or PET-Metabolizing Microorganisms
TPA, a PET hydrolysis product, is not widely regarded as a bacterial growth substrate, due to its toxicity to cells [74]. However, PET biodegradation using
EG is metabolized via various pathways, namely oxidization to ethanol and acetaldehyde, which are converted to acetate via acetyl-CoA, followed by substrate-level phosphorylation to form ATP (Fig. 4). Ethanol can be oxidized and the reducing equivalents can be reused by CO2 reduction to acetate in the Wood-Ljungdahl pathway. EG can be catabolized to acetaldehyde by propanediol dehydratase (PduCDE) and CoA-dependent propionaldehyde dehydrogenase (PduP) proteins encoded by the
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Fig. 4. Metabolic pathway of ethylene glycol to glyoxylate and other energy compounds. This consists of PQQ (pyrroloquinoline quinone), which is reduced to PQQH2 upon conversion of ethylene glycol to glycolaldehyde and other compounds synthesized from glycolate. The enzymes shown are PedE/PedH, PQQ-dependent alcolhol dehydrogenase, Pedl, peroxisomal 3-ketoacyl-COA thiolase, GlcDEF, glycolate dehydrogenase, GlxR, tatronate semialdehyde reductase, Hyi,hydroxypyruvate isomerase and PykA/Pyk, pyruvate kinase.
Although TPA or EG can be easily converted into target chemicals by metabolic engineering, several drawbacks include a low growth rate [85] and limited engineering tools [92] for direct application to microbial chassis for producing value-added chemicals. Future studies can focus on chassis engineering to improve growth and productivity, remove unnecessary metabolic pathways, and reinforce substrate utilization capability to generate industrial strains.
Whole-Cell Conversion of TPA and EG
The whole-cell conversion uses engineered cells containing TPA-metabolizing genes or wild-type cells with the ability to degrade TPA, thus enabling the conversion of TPA and EG into other chemicals for further use. Whole-cell microbial catalysts containing
Whole-cell conversion is cost-effective and less time-consuming, with no enzyme purification; rapid validation of new synthetic pathways is possible. However, this process exhibits a shortcoming of low substrate loading; hence, chassis engineering, adaptive laboratory evolution, and the introduction of heterologous TPA transport systems are required [94]. Further studies considering substrate loading improvement by transporter introduction and chassis engineering enabling the endurance of high TPA loading are recommended.
Development of Convergence Technology for PET Waste Upcycling
Plastic upcycling technologies are instrumental in solving current plastic waste problems, in addition to biodegradable plastic production technology, because all conventional plastics cannot be replaced by biodegradable plastics (Fig. 5). Although disposable plastics for packaging should be replaced with biodegradable plastics, the long-term use of plastics with less biodegradability, such as plastics for processed food and electronics, would continuously generate potential plastic waste. The development of plastic upcycling technologies consists of two important steps: i) substrate production for bioconversion via PET waste depolymerization, and ii) upcycling of the produced substrates.
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Fig. 5. Lifecycle of plastics, including the linear economy, wherein plastics from fossil fuels end up as plastic waste in the environment, and the circular economy, wherein plastic waste from fossil fuels undergoes various processes to yield high value-added chemicals, thus conserving the environment.
The depolymerization of PET is essential because it provides substrates that can be used in biological upcycling to produce high value-added chemicals through a circular loop of plastic upcycling, thereby reducing the amount of plastic in the environment. Chemical depolymerization breaks down PET waste into BHET, which can be used as a substrate in enzymatic depolymerization. This method has an increased tolerance for high levels of contamination, and lower-quality PET can be used. However, high reaction conditions (high temperatures and pressures) and expensive catalysts increase the overall cost of the process and produce low-purity target chemicals, for example BHET, including lots of unnecessary chemicals due to additives and side reactions. Besides, the produced substrates require transformation to be applied for bioconversion. The produced substrates require transformation to be applied for bioconversion. Biological depolymerization can be developed as an alternative to chemical depolymerization with various advantages: operation at low temperatures, no pressure, and selective depolymerization of PET waste from mixed plastic waste. However, significant disadvantages are observed in terms of operating costs, including enzyme production. Recently, chemoenzymatic depolymerization has been considered an efficient method to produce substrates for the bioconversion of PET waste, because it adopts the advantages of chemical and biological depolymerization processes. Through chemical depolymerization, a substantial amount of PET waste can be easily transformed into smaller chemicals such as BHET. The products can then be transformed into suitable substrates for bioconversion by enzymatic hydrolysis processes using enzymes such as esterases. In the future, the development of more efficient chemical depolymerization methods using biocompatible catalysts with advanced processes, along with a singular enzymatic system operating at a biocompatible temperature (e.g., 37°C) and improved hydrolysis capability, can provide the economic feasibility to contribute to plastic waste reduction.
Several chemical and biological upcycling processes have been reported thus far. Considering BHET, chemical upcycling is advantageous because it can efficiently produce valuable items without further hydrolysis of BHET. For example, fiberglass-reinforced plastics were successfully developed by polymerizing BHET produced from PET waste with renewably sourced monomers, including acrylic acid, methacrylic acid, and MA [95]. However, the product portfolio to produce value-added chemicals is limited compared with that of the biological upcycling process; biological upcycling provides better benefits because it can directly produce various value-added chemicals by combining multiple enzymes. Biological upcycling involves the application of PET monomers, TPA, and EG, using whole-cell conversion and TPA-metabolizing microorganisms, to convert them into expensive chemicals. Depolymerization products are converted into high value-added products via biological upcycling. Moreover, it enables the circular utilization of PET, reduces the amount of plastic waste produced, and encourages the proposed circular economy to ultimately solve the prevalent greenhouse gas emission and crude oil consumption problems. However, biological upcycling is affected by limitations such as low substrate loading, and fewer known PET-degrading microorganisms. Therefore, additional studies on TPA/EG-metabolizing microorganisms, consolidated bioprocessing (CBP) for reducing enzyme usage, and advanced heterologous TPA transport systems toward efficient TPA uptake will be required.
Although the use of petroleum-based TPA and EG is more economically feasible, the continuous use of petroleum fossil fuels causes serious damage to the environment. Meanwhile, due to the reduction in fossil fuel reserves [96], TPA/EG synthesis has shifted from fossil fuels to biomass [97], hence creating further possibilities, including the suggested use of TPA/EG from recycled PET. Therefore, the application of TPA and EG from recycled PET enables the use of PET in a circular economy producing high value-added chemicals and reducing the plastic burden. The TPA/EG amounts currently obtained are so low that further research is needed to find more reliable microorganisms to break down PET into TPA/EG while also increasing the produced amounts to encourage a shift from laboratory scale to industrial scale.
Conclusions
In this review, the current status of PET upcycling approaches was comprehensively addressed. Different forms of substrate fabrication using chemical, biological, and chemobiological methods were described. Subsequently, the value-added chemical production process by biological upcycling was discussed. However, considering the economic feasibility, each process presents disadvantages in terms of establishing a circular plastic economy. In the future, advancing individual technologies, including the production of substrates for bioconversion and biological upcycling, should be considered along with integrating each technology to design an economically feasible PET upcycling process. Based on this development, further studies should be conducted to develop and take practical technologies from the laboratory scale to an industrial scale to reduce PET waste and provide economic profit.
Acknowledgments
This work was supported by a research fund from Chungnam National University (2022-0559-01).
Conflict of Interest
The authors have no financial conflicts of interest to declare.
References
- Hiraga K, Taniguchi I, Yoshida S, Kimura Y, Oda K. 2019. Biodegradation of waste PET.
EMBO Rep. 20 : e49365. - Dissanayake L, Jayakody LN. 2021. Engineering microbes to bio-upcycle polyethylene terephthalate.
Front. Bioeng. Biotechnol. 9 : 656465. - Kim DH, Han DO, In Shim K, Kim JK, Pelton JG, Ryu MH,
et al . 2021. One-Pot Chemo-bioprocess of PET depolymerization and recycling enabled by a biocompatible catalyst, betaine.ACS Catal. 11 : 3996-4008. - Shi L, Liu H, Gao S, Weng Y, Zhu L. 2021. Enhanced extracellular production of
Is PETase inEscherichia coli via engineering of the pelB signal peptide.J. Agric. Food Chem. 69 : 2245-2252. - Knott BC, Erickson E, Allen MD, Gado JE, Graham R, Kearns FL,
et al . 2021. Characterization and engineering of a two-enzyme system for plastics depolymerization.Proc. Natl. Acad. USA 117 : 25476-25485. - Geyer R, Jambeck JR, Law KL. 2017. Production, use, and the fate of all plastics ever made.
Sci. Adv. 3 : e1700782. - Hou Q, Zhen M, Qian H, Nie Y, Bai X, Xia T,
et al . 2021. Upcycling and catalytic degradation of plastic wastes.Cell Rep. Phys. Sci. 2 : 100514. - Kim HT, Hee Ryu M, Jung YJ, Lim S, Song HM, Park J,
et al . 2021. Chemo-biological upcycling of poly(ethylene terephthalate) to multifunctional coating materials.ChemSusChem 14 : 4251-4259. - Son HF, Cho IJ, Joo S, Seo H, Sagong HY, Choi SY,
et al . 2019. Rational protein engineering of thermo-stable PETase fromIdeonella sakaiensis for highly efficient PET degradation.ACS Catal. 9 : 3519-3526. - Law KL, Starr N, Siegler TR, Jambeck JR, Mallos NJ, Leonard GH. 2020. The United States' contribution of plastic waste to land and ocean.
Sci. Adv. 6 : eabd0288. - Heidari M, Garnaik PP, Dutta A. 2019. The valorization of plastic via thermal means: industrial scale combustion methods.
Plastics Energy 2019 : 295-312. - Kumar S, Singh E, Mishra R, Kumar A, Caucci S. 2021. Utilization of plastic wastes for sustainable environmental management: a review.
ChemSusChem. 14 : 3985-4006. - Kim HT, Kim JK, Cha HG, Kang MJ, Lee HS, Khang TU,
et al . 2019. Biological valorization of poly(ethylene terephthalate) monomers for upcycling waste PET.ACS Sustain. Chem. Eng. 7 : 19396-19406. - Wei R, Zimmermann W. 2017. Biocatalysis is a green route for recycling recalcitrant plastic polyethylene terephthalate.
Microb. Biotechnol. 10 : 1302-1307. - Pantelic B, Ponjavic M, Jankovic V, Aleksic I, Stevanovic S, Murray J,
et al . 2021. Upcycling biodegradable PVA/starch film to a bacterial biopigment and biopolymer.Polymers 13 : 3692. - Sung K, Cooper T, Kettley S. 2015. A review on upcycling: the current body of literature, knowledge gaps and a way forward - IRep -Nottingham Trent University.
17th International Conference on Environmental, Cultural, Economic and Social Sustainability 17 : 28-40. - Dyer AC, Nahil MA, Williams PT. 2021. Catalytic co-pyrolysis of biomass and waste plastics as a route to upgraded bio-oil.
J. Energy Inst. 97 : 27-36. - Klaimy S, Lamonier JF, Casetta M, Heymans S, Duquesne S. 2021. Recycling of plastic waste using flash pyrolysis - effect of mixture composition.
Polym. Degrad. Stab. 187 : 109540. - Orozco S, Alvarez J, Lopez G, Artetxe M, Bilbao J, Olazar M. 2021. Pyrolysis of plastic wastes in a fountain confined conical spouted bed reactor: Determination of stable operating conditions.
Energy Convers. Manag. 229 : 113768. - Osman AI, Farrell C, Al-Muhtaseb AH, Al-Fatesh AS, Harrison J, Rooney DW. 2020. Pyrolysis kinetic modeling of abundant plastic waste (PET) and
in-situ emission monitoring.Environ. Sci. Eur. 32 : 112. - Suresh A, Alagusundaram A, Kumar PS, Vo DVN, Christopher FC, Balaji B,
et al . 2021. Microwave pyrolysis of coal, biomass and plastic waste: a review.Environ. Chem. Lett. 19 : 3609-3629. - Liu Y, Fu W, Liu T, Zhang Y, Li B. 2022b. Microwave pyrolysis of polyethylene terephthalate (PET) plastic bottle sheets for energy recovery.
J. Anal. Appl. Pyrolysis 161 : 105414. - Suriapparao DV, Kumar DA, Vinu R. 2022. Microwave co-pyrolysis of PET bottle waste and rice husk: effect of plastic waste loading on product formation.
Sustain. Energy Technol. Assess. 49 : 101781. - Straka P, Bičáková O, Šupová M. 2022. Slow pyrolysis of waste polyethylene terephthalate yielding paraldehyde, ethylene glycol, benzoic acid, and clean fuel.
Polym. Degrad. Stab. 198 : 109900. - Shahi A, Roozbehani B, Mirdrikvand M. 2022. Catalytic pyrolysis of waste polyethylene terephthalate granules using a Lewis-Brønsted acid sites catalyst.
Clean Technol. Environ. Policy 24 : 779-787. - Vijayakumar A, Sebastian J. 2018. Pyrolysis process to produce fuel from different types of plastic - A review.
IOP Conf. Ser.: Mater. Sci. Eng. 396 : 012062. - Lee J, Lee T, Tsang YF, Oh JI, Kwon EE. 2017. Enhanced energy recovery from polyethylene terephthalate via pyrolysis in CO2 atmosphere while suppressing acidic chemical species.
Energy Convers. Manag. 148 : 456-460. - Li S, Cañete Vela I, Järvinen M, Seemann M. 2021. Polyethylene terephthalate (PET) recycling via steam gasification - The effect of operating conditions on gas and tar composition.
Waste Manage. 130 : 117-126. - Choi MJ, Jeong YS, Kim JS. 2021. Air gasification of polyethylene terephthalate using a two-stage gasifier with active carbon for the production of H2 and CO.
Energy 223 : 120122. - Jeong Y-S, Kim J-W, Ra HW, Seo MW, Mun T-Y, Kim J-S. Two-stage air gasification of ten different types of plastic using active carbon as a tar removal additive.
SSRN Electronic J. : 1-38. - Biermann L, Brepohl E, Eichert C, Paschetag M, Watts M, Scholl S. 2021. Development of a continuous PET depolymerization process as a basis for a back-to-monomer recycling method.
Green Process Synth. 10 : 361-373. - Gupta P, Bhandari S. Chemical depolymerization of PET bottles via ammonolysis and aminolysis, pp. 109-134.
In: Recycling Polyethylene Terephthalate Bottles . - Chan K, Zinchenko A. 2021. Conversion of waste bottles' PET to a hydrogel adsorbent via PET aminolysis.
J. Environ. Chem. Eng. 9 : 106129. - Bäckström E, Odelius K, Hakkarainen M. 2021. Ultrafast microwave-assisted recycling of PET to a family of functional precursors and materials.
Eur. Polym. J. 151 : 110441. - Otaibi A, Alsukaibi A, Rahman M, Mushtaque M, Haque A. 2022. From waste to schiff Base: Upcycling of aminolysed poly(ethylene terephthalate) product.
Polymers 14 : 1861. - Spychaj T. 2002. Chemical recycling of PET: methods and products. pp. 1252-1290. Handbook of thermoplastic polyesters: homopolymers, copolymers, blends, and composites.
- Laldinpuii ZT, Khiangte V, Lalhmangaihzuala S, Lalmuanpuia C, Pachuau Z, Lalhriatpuia C,
et al . 2022. Methanolysis of PET waste using a heterogeneous catalyst of bio-waste origin.J. Polym. Environ. 30 : 1600-1614. - Tang S, Li F, Liu J, Guo B, Tian Z, Lv J. 2022a. Calcined sodium silicate as a solid base catalyst for the alcoholysis of poly(ethylene terephthalate).
J. Chem. Technol. Biotechnol. 97 : 1305-1314. - Jiang Z, Yan D, Xin J, Li F, Guo M, Zhou Q,
et al . 2022. Poly(ionic liquid)s an efficient and recyclable catalysts for methanolysis of PET.Polym. Degrad. Stab. 199 : 109905. - Tang S, Li F, Liu J, Guo B, Tian Z, Lv J. 2022b. MgO/NaY as modified mesoporous catalyst for methanolysis of polyethylene terephthalate wastes.
J. Environ. Chem. Eng. 10 : 107927. - Pham DD, Cho J. 2021. Low-energy catalytic methanolysis of poly(ethyleneterephthalate).
Green Chem. 23 : 511. - Guo B, Liu J, Tang S, Liu Y, Tian Z, Lv J. 2022. Hydrolysis of dimethyl terephthalate to terephthalic acid on Nb-modified HZSM-5 zeolite catalysts.
J. Chem. Technol. Biotechnol. 97 : 1695-1704. - Goje AS, Thakur SA, Chauhan YP, Patil TM, Patil SA, Diware VR,
et al . 2005. Glycolytic aminolysis of poly(ethylene terephthalate) waste at atmospheric pressure for recovery of a value-added insecticide.Polym. Plast Technol. Eng. 44 : 163-181. - Thachnatharen N, Shahabuddin S, Sridewi N. 2021. The Waste management of polyethylene terephthalate (PET) plastic waste: A Review.
IOP Conference Series: Mater. Sci. Eng. 1127 : 012002. - Kim Y, Kim M, Hwang J, Im E, Moon GD. 2022. Optimizing PET glycolysis with an oyster shell-derived catalyst using response surface methodology.
Polymers 14 : 656. - Wang Z, Jin Y, Wang Y, Tang Z, Wang S, Xiao G,
et al . 2022. Cyanamide as a highly efficient organocatalyst for the glycolysis recycling of PET.ACS Sustainable Chem. Eng. 10 : 7965-7973. - Neves Ricarte G, Lopes Dias M, Sirelli L, Antunes Pereira Langone M, Machado de Castro A, Zarur Coelho MA,
et al . 2021. Chemoenzymatic depolymerization of industrial and assorted post-consumer poly(ethylene terephthalate) (PET) wastes using a eutecticbased catalyst.J. Chem. Technol. Biotechnol. 96 : 3237-3244. - Wang R, Wang T, Yu G, Chen X. 2021. A new class of catalysts for the glycolysis of PET: Deep eutectic solvent@ZIF-8 composite.
Polym. Degrad. Stab. 183 : 109463. - Mendiburu-Valor E, Mondragon G, González N, Kortaberria G, Eceiza A, Peña-Rodriguez C. 2021. Improving the efficiency for the production of bis-(2-hydroxyethyl) terephthalate (BHET) from the glycolysis reaction of poly(ethylene terephthalate) (PET) in a pressure reactor.
Polymers 13 : 1461. - Azeem M, Fournet MB, Attallah OA. 2022. Ultrafast 99% polyethylene terephthalate depolymerization into value-added monomers using sequential glycolysis-hydrolysis under microwave irradiation.
Arab. J. Chem. 15 : 103903. - Trejo-carbajal N, Ambriz-luna KI, Herrera-gonz AM. 2022. Efficient method and mechanism of depolymerization of PET under conventional heating and microwave radiation using t-BuNH2/Lewis's acids.
Eur. Polym. J. 175 : 111388. - Park SH, Kim SH. 2014. Poly(ethylene terephthalate) recycling for high-value added textiles.
Fash. Text 1 : 1-17. - Yang Y, Lu Y, Xiang H, Xu Y, Li Y. 2002. Study on methanolytic depolymerization of PET with supercritical methanol for chemical recycling.
Polym. Degrad. Stab. 75 : 185-191. - Kawai F, Kawabata T, Oda M. 2019. Current knowledge on enzymatic PET degradation and its possible application to waste stream management and other fields.
Appl. Microbiol. Biotechnol. 103 : 4253-4268. - Huang X, Cao L, Qin Z, Li S, Kong W, Liu Y. 2018. Tat-independent secretion of polyethylene terephthalate hydrolase PETase in
Bacillus subtilis 168 mediated by Its native signal peptide.J. Agric. Food Chem. 66 : 13217-13227. - Roberts C, Edwards S, Vague M, León-Zayas R, Scheffer H, Chan G,
et al . 2020. Environmental consortium containingPseudomonas andBacillus species synergistically degrade polyethylene terephthalate plastic.MSphere 5 : e01151-20. - Alisch-Mark M, Herrmann A, Zimmermann W. 2006. Increase of the hydrophilicity of polyethylene terephthalate fibers by hydrolases from
Thermomonospora fusca andFusarium solani f . sp. pisi.Biotechnol. Lett. 28 : 681-685. - Vertommen MAME, Nierstrasz VA, van der Veer M, Warmoeskerken MMCG. 2005. Enzymatic surface modification of poly(ethylene terephthalate).
J. Biotechnol. 120 : 376-386. - Kawai F, Thumarat U, Kitadokoro K, Waku T, Tada T, Tanaka N,
et al . 2013. Comparison of polyester-degrading cutinases from genusthermobifida .ACS Symp. Series 1144 : 111-120. - Ribitsch D, Hromic A, Zitzenbacher S, Zartl B, Gamerith C, Pellis A,
et al . 2017. Small cause, large effect: Structural characterization of cutinases fromThermobifida cellulosilytica .Biotechnol. Bioeng. 114 : 2481-2488. - Kawai F, Kawase T, Shiono T, Urakawa H, Sukigara S, Tu C,
et al . 2017. Enzymatic hydrophilization of polyester fabrics using a recombinant cutinase Cut 190 and their surface characterization.J. Fiber Sci. Technol. 73 : 8-18. - Ronkvist ÅM, Xie W, Lu W, Gross RA. 2009. Cutinase-catalyzed hydrolysis of poly(ethylene terephthalate).
Macromolecules 42 : 5128-5138. - Müller RJ, Schrader H, Profe J, Dresler K, Deckwer WD. 2005. Enzymatic degradation of poly(ethylene terephthalate): Rapid hydrolyse using a hydrolase from T. fusca.
Wiley Online Library 26 : 1400-1405. - Chen S, Su L, Chen J, Wu J. 2013. Cutinase: Characteristics, preparation, and application.
Biotechn. Adv. 31 : 1754-1767. - Jerves C, Neves RPP, Ramos MJ, Da Silva S, Fernandes PA. 2021. Reaction mechanism of the PET degrading enzyme PETase studied with DFT/MM molecular dynamics simulations.
ACS Catal. 11 : 11626-11638. - Han X, Liu W, Huang JW, Ma J, Zheng Y, Ko TP,
et al . 2017. Structural insight into the catalytic mechanism of PET hydrolase.Nat. Commun. 8 : 2106. - Sadler JC, Wallace S. 2021. Microbial synthesis of vanillin from waste poly(ethylene terephthalate).
Green Chem. 23 : 4665-4672. - Then J, Wei R, Oeser T, Gerdts A, Schmidt J, Barth M,
et al . 2016. A disulfide bridge in the calcium-binding site of a polyester hydrolase increases its thermal stability and activity against polyethylene terephthalate.FEBS Open Bio 6 : 425-432. - Franden MA, Jayakody LN, Li WJ, Wagner NJ, Cleveland NS, Michener WE,
et al . 2018. EngineeringPseudomonas putida KT2440 for efficient ethylene glycol utilization.Metab. Eng. 48 : 197-207. - Austin HP, Allen MD, Donohoe BS, Rorrer NA, Kearns FL, Silveira RL,
et al . 2018. Characterization and engineering of a plasticdegrading aromatic polyesterase.Proc. Natl. Acad. Sci. USA 115 : E4350-E4357. - Furukawa M, Kawakami N, Tomizawa A, Miyamoto K. 2019. Efficient degradation of poly(ethylene terephthalate) with
Thermobifida fusca cutinase exhibiting improved catalytic activity generated using mutagenesis and additive-based approaches.Sci. Rep. 9 : 16038. - Li Q, Zheng Y, Su T, Wang Q, Liang Q, Zhang Z,
et al . 2022. Computational design of a cutinase for plastic biodegradation by mining molecular dynamics simulations trajectories.Comput. Struct. Biotechnol. J. 20 : 459-470. - Tiso T, Narancic T, Wei R, Pollet E, Beagan N, Schröder K,
et al . 2021. Towards bio-upcycling of polyethylene terephthalate.Metab. Eng. 66 : 167-178. - Kenny ST, Runic JN, Kaminsky W, Woods T, Babu RP, Keely CM,
et al . 2008. Up-cycling of PET (polyethylene terephthalate) to the biodegradable plastic PHA (Polyhydroxyalkanoate).Environ. Sci. Technol. 42 : 7696-7701. - Liu P, Zhang T, Zheng Y, Li Q, Su T, Qi Q. 2021. Potential one-step strategy for PET degradation and PHB biosynthesis through cocultivation of two engineered microorganisms.
Eng. Microbiol. 1 : 100003. - Kaabel S, Daniel Therien JP, Deschênes CE, Duncan D, Friščic T, Auclair K. 2021. Enzymatic depolymerization of highly crystalline polyethylene terephthalate enabled in moist-solid reaction mixtures.
Proc. Natl. Acad. Sci. USA 118 : e2026452118. - Puspitasari N, Tsai SL, Lee CK. 2021. Fungal hydrophobin RolA enhanced PETase hydrolysis of polyethylene terephthalate.
Appl. Biochem. Biotechnol. 193 : 1284-1295. - Son HF, Cho IJ, Joo S, Seo H, Sagong HY, Choi SY,
et al . 2019. Rational protein engineering of thermo-stable PETase fromIdeonella sakaiensis for highly efficient PET degradation.ACS Catal. 9 : 3519-3526. - Tournier V, Topham CM, Gilles A, David B, Folgoas C, Moya-Leclair E,
et al . 2020. An engineered PET depolymerase to break down and recycle plastic bottles.Nature 580 : 216-219. - Qi X, Ma Y, Chang H, Li B, Ding M, Yuan Y. 2021. Evaluation of PET degradation using artificial microbial consortia.
Front. Microbiol. 12 : 778828. - Kumar V, Maitra SS, Singh R, Burnwal DK. 2020. Acclimatization of a newly isolated bacteria in monomer terephthalic acid (TPA) may enable it to attack the polymer polyethylene terephthalate (PET).
J. Environ. Chem. Eng. 8 : 103977. - Mahmood N, Yuan Z, Schmidt J, Xu CC. 2016. Depolymerization of lignins and their applications for the preparation of polyols and rigid polyurethane foams: A review.
Renew. Sustain. Energy 60 : 317-329. - Werner AZ, Clare R, Mand TD, Pardo I, Ramirez KJ, Haugen SJ,
et al . 2021. Tandem chemical deconstruction and biological upcycling of poly(ethylene terephthalate) to β-ketoadipic acid byPseudomonas putida KT2440.Metab. Eng. 67 : 250-261. - Gao R, Pan H, Kai L, Han K, Lian J. 2022. Microbial degradation and valorization of poly(ethylene terephthalate) (PET) monomers.
World J. Microbiol. Biotechnol. 38 : 89. - Yoshida S, Hiraga K, Takehana T, Taniguchi I, Yamaji H, Maeda Y,
et al . 2016. A bacterium that degrades and assimilates poly(ethylene terephthalate).Science 351 : 1196-1199. - Salvador M, Abdulmutalib U, Gonzalez J, Kim J, Smith AA, Faulon JL,
et al . 2019. Microbial genes for a circular and sustainable bio-PET economy.Genes 10 : 373. - Kincannon WM, Zahn M, Clare R, Beech JL, Romberg A, Larson J,
et al . 2022. Biochemical and structural characterization of an aromatic ring-hydroxylating dioxygenase for terephthalic acid catabolism.Proc. Natl. Acad. USA 119 : e2121426119. - Johnson CW, Salvachúa D, Rorrer NA, Black BA, Vardon DR, St. John PC,
et al . 2019. Innovative chemicals and materials from bacterial aromatic catabolic pathways.Joule 3 : 1523-1537. - Trifunović D, Schuchmann K, Müller V. 2016. Ethylene glycol metabolism in the acetogen
Acetobacterium woodii .J. Bacteriol. 198 : 1058-1065. - Li WJ, Jayakody LN, Franden MA, Wehrmann M, Daun T, Hauer B,
et al . 2019. Laboratory evolution reveals the metabolic and regulatory basis of ethylene glycol metabolism byPseudomonas putida KT2440.Environ. Microbiol. 21 : 3669-3682. - Welsing G, Wolter B, Hintzen HMT, Tiso T, Blank LM. 2021. Upcycling of hydrolyzed PET by microbial conversion to a fatty acid derivative.
Methods Enzymol. 648 : 391-421. - Tang HZ, Jiang JD, Wu XL, Qi X, Yan W, Cao Z,
et al . 2021. Current advances in the biodegradation and bioconversion of polyethylene terephthalate.Microorganisms 10 : 39. - Kang MJ, Kim HT, Lee MW, Kim KA, Khang TU, Song HM,
et al . 2020. A chemo-microbial hybrid process for the production of 2-pyrone-4,6-dicarboxylic acid as a promising bioplastic monomer from PET waste.Green Chem. 22 : 3461-3469. - Devi Salam M, Varma A, Prashar R, Choudhary D. 2021. Review on efficacy of microbial degradation of polyethylene terephthalate and bio-upcycling as a part of plastic waste management.
Appl. Ecol. Environ. Sci. 9 : 695-703. - Rorrer NA, Nicholson S, Carpenter A, Biddy MJ, Grundl NJ, Beckham GT. 2019. Combining reclaimed PET with Bio-based monomers enables plastic upcycling.
Joule 3 : 1006-1027. - Huber GW, Iborra S, Corma A. 2006. Synthesis of transportation fuels from biomass: Chemistry, catalysts, and engineering.
Chem. Rev. 106 : 4044-4098. - Pang J, Zheng M, Sun R, Wang A, Wang X, Zhang T. 2016. Synthesis of ethylene glycol and terephthalic acid from biomass for producing PET.
Green Chem. 18 : 342-359. - Fujiwara R, Sanuki R, Ajiro H, Fukui T, Yoshida S. 2021. Direct fermentative conversion of poly(ethylene terephthalate) into poly(hydroxy alkanoates) by
Ideonella sakaiensis .Sci. Rep. 11 : 1-7.
Related articles in JMB

Article
Review
J. Microbiol. Biotechnol. 2023; 33(1): 1-14
Published online January 28, 2023 https://doi.org/10.4014/jmb.2208.08048
Copyright © The Korean Society for Microbiology and Biotechnology.
Recent Advances in the Chemobiological Upcycling of Polyethylene Terephthalate (PET) into Value-Added Chemicals
Joyce Mudondo1†, Hoe-Suk Lee2†, Yunhee Jeong1, Tae Hee Kim1, Seungmi Kim1, Bong Hyun Sung3, See-Hyoung Park4, Kyungmoon Park4, Hyun Gil Cha5*, Young Joo Yeon2*, and Hee Taek Kim1*
1Department of Food Science and Technology, Chungnam National University, Daejeon 34134, Republic of Korea
2Department of Biochemical Engineering Gangneung-Wonju National University, Gangneung 25457, Republic of Korea
3Synthetic Biology Research Center, Korea Research Institute of Bioscience and Biotechnology, Daejeon 34141, Republic of Korea
4Department of Biological and Chemical Engineering, Hongik University, Sejong 30016, Republic of Korea
5Center for Bio-based Chemistry, Korea Research Institute of Chemical Technology (KRICT), Ulsan 44429, Republic of Korea
Correspondence to:H.G. Cha, hgcha@krict.re.kr
Y.J. Yeon, yjyeon@gwnu.ac.kr
H.T. Kim, heetaek@cnu.ac.k
†These authors contributed equally to this work.
Abstract
Polyethylene terephthalate (PET) is a plastic material commonly applied to beverage packaging used in everyday life. Owing to PET’s versatility and ease of use, its consumption has continuously increased, resulting in considerable waste generation. Several physical and chemical recycling processes have been developed to address this problem. Recently, biological upcycling is being actively studied and has come to be regarded as a powerful technology for overcoming the economic issues associated with conventional recycling methods. For upcycling, PET should be degraded into small molecules, such as terephthalic acid and ethylene glycol, which are utilized as substrates for bioconversion, through various degradation processes, including gasification, pyrolysis, and chemical/biological depolymerization. Furthermore, biological upcycling methods have been applied to biosynthesize value-added chemicals, such as adipic acid, muconic acid, catechol, vanillin, and glycolic acid. In this review, we introduce and discuss various degradation methods that yield substrates for bioconversion and biological upcycling processes to produce value-added biochemicals. These technologies encourage a circular economy, which reduces the amount of waste released into the environment.
Keywords: Polyethylene terephthalate (PET), substrate production for bioconversion, biological upcycling, value-added chemicals
Introduction
Plastics are ubiquitous in daily life as they are used in packaging, construction, and clothing [1]. These materials are significant in modern society owing to their adaptability, low manufacturing costs, and desirable physical features, which include flexibility, lightness, impermeability, and durability [2, 3]. Consequently, the demand for plastics has continuously increased; however, improper recycling of plastic waste causes plastic pollution [4]. Plastic accumulation threatens human survival and the environment [5]. According to Geyer
Reuse, incineration, and landfilling are conventional methods for managing plastic waste [9]. Plastic incineration provides energy but also emits particulate matter and toxic gases such as unburned hydrocarbons, nitrous and sulfurous oxides, furans, and dioxins, which cause severe environmental damage [11] For these reasons, landfilling and incineration are not recommended for plastic garbage disposal, and instead, more sustainable and environmentally friendly methods are required [12].
Due to its exceptional mechanical properties, thermal stability, and impermeability to gases and liquids, polyethylene terephthalate (PET) is the most popular synthetic plastic material applied in beverage packaging, food containers, bottles, and textiles [13]. PET is produced by polycondensation of terephthalic acid (TPA) and ethylene glycol (EG) or trans-esterification of dimethyl terephthalate (DMT) and EG. Its stability and resistance to hydrolytic or enzymatic degradation have led to PET being the most commonly found plastic waste in the environment [1]. Currently, PET is primarily recycled via physical and chemical methods; various studies have also reported the use of biodegradation using microorganisms to break down PET into its monomers: TPA and EG. Physical recycling involves heat treatment at high temperatures, and PET is degraded into downcycled products because of the loss of mechanical properties, which are different from those of virgin PET [1]. Chemical recycling uses chemicals and expensive catalysts to decompose waste into monomers; recycled plastics are typically more expensive than virgin plastics and are not economically viable [13]. To solve the plastic pollution problem, various approaches, including biological depolymerization and upcycling of plastic wastes, and even plastic-eating microbes, have been considered [14]. Circular repurposing of plastic materials is critical for building a socioeconomic ecosystem without contributing to plastic waste [15]. Upcycling converts waste materials into products of greater worth and quality in their second life [16]. Upcycling also helps achieve the circular economic feasibility of plastic waste with complete recyclability and no loss of value or usability, in contrast to other recycling methods.
In this review, we discuss the development of the upcycling processes suitable for PET, comprising two steps: 1) substrate production from PET waste, and 2) upcycling of the substrate into value-added chemicals (Fig. 1). For producing substrates by degrading PET, we suggest processes suitable for bioconversion to obtain value-added chemicals after reviewing various existing processes, including pyrolysis, gasification, and depolymerization, along with their advantages and disadvantages. Following the review of chemical and biological upcycling models, we propose an efficient upcycling model for producing value-added chemicals. Finally, we suggest a method for PET upcycling to achieve economic feasibility and a circular economy regarding the plastic life cycle, as a means to address plastic environmental pollution.
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Figure 1. Overall scheme representing the primary aspects of the review.
Production of Substrates from PET Waste for Bioconversion Processes
Various processes, including pyrolysis, gasification, and depolymerization, can be applied to produce substrates from PET waste (Fig. 2, Table 1). Although individual processes for substrate production from PET waste are well established, selecting a suitable upcycling process, particularly regarding biological upcycling, is crucial because of the economic feasibility and establishment of refineries. Therefore, in this section, we discuss the status of research, along with the advantages and disadvantages, to suggest a process suitable for upcycling.
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Table 1 . Degradation product types and yields obtained from PET using various PET degradation processes..
Process Substrate Product Yield Reference Pyrolysis PET bottle sheets1 Solid
Gas35.67 wt%
40 wt%[22] PET water bottles with rice husk Gas
Biocrude
char39.9-68.9 wt%
12-29%
15.7-31.7%[23] PET bottles2 Gas
Oil
Solid residue13.3±0.6 wt%
46.7±1.9 wt%
39.7±1.6 wt%[24] Pure PET pellets Gas: CO, CO2
Solid: TPA-
25%[25] Gasification Virgin PET pellets Gas: H2, CO2, CO
Char
Tar90.93 wt%
6.15 wt%
2.9 wt%[29] Virgin PET pellets Gas: H2
CO2
Biphenyl- [28] Depolymerization Aminolysis Post-consumer PET
DHTA
BHTA
BFTA
DAA64%
91%
82%
61%[34] PET flakes Monomers, Dimers, Trimers, Oligomers - [35] Methanolysis PET DMT 89.1% [39] PET DMT
EG78%
76%[37] PET DMT
EG95% [38] PET DMT
EG91% [40] PET DMT
EG93.5% [41] Glycolysis PET BHET 80.30% [46] PET BHET 70% [49] Post-consumer PET bottles TPA
MHET
BHET62.79-80.66%
17.22-34.79%
0.54-0.59%[50] PET bottles BHET, TPA, EG Quantitative [51] Biological depolymerization PET BHET
EG68.6% [45] Post-consumer PET waste TPA 49±2% [76] Amorphous PET TPA 16.7 g/L/h [79] Chemoenzymatic depolymerization PET BHET
MHET
PET oligomers84.8%
7.7%
8.7%[8] PET BHET 35% [47] PET TPA
EG99.9% [3] Note: DHTA, dihexylterephthalamide; BHTA, bis(2-hydroxyethyl) terepthalamide; BFTA, bis(furan-2-ylmethyl) terepthalamide; DAA, diallyterepthalamide; DMT, dimethyl terephthalate; TPA, terephthalic acid; BHET, bis(2-hydroxyethyl) terephthalate; MHET, mono-(2-hydroxyethyl) terephthalate; EG, ethylene glycol..
1-solid products contained benzoic acid, 4-vinyl benzoic acid, monovinyl terephthalate, and divinyl terephthalate..
2-Oil components include paraldehyde (54.7 wt%), ethylene glycol (23.65 wt%), and benzoic acid and benzoates (11.5 wt%). The solid carbonaceous residue contained carbon, ash, nitrogen, and sulfur). Gas contained CO and CO2 (more than 90 vol%), a few C1 -C 4 hydrocarbons (~7 vol%), and hydrogen (~3 vol%)..
-
Figure 2. Substrate production processes from PET waste into respective products, including different processes used to produce substrates that are employed in biological upcycling.
Substrate Production by Pyrolysis of PET
Pyrolysis is the thermal transformation of polymers into liquid fuels and value-added products at considerably high temperatures (300-900°C) in the absence of oxygen [17-19]. PET pyrolysis releases products such as TPA, vinyl terephthalate, aromatic compounds (namely toluene and benzene); esters such as vinyl benzoate; carboxylic acids such as ethyl and methyl benzoic acid; and aliphatic hydrocarbons such as ethane and methanol. TPA formed during pyrolysis can be used for upcycling after purification; however, TPA clogs the equipment [20]. Pyrolysis is affected by factors like substrate type and time, and temperature, which influence the polymer degradation rate; and catalyst loading, which improves the process efficiency through time reduction. Slow, fast, and flash pyrolysis methods are based on reaction times, and microwave pyrolysis is based on the apparatus type [21].
PET waste was pyrolyzed for energy recovery at 500°C, yielding solids, gas, and residue products of 24.33, 35.67, and 74.67 wt%, respectively [22]. The effect of plastic waste loading was studied using microwave co-pyrolysis of PET and rice husk mixtures at 600°C for 4-13 min, which yielded syngas (29%) affected by increasing plastic waste loading, biocrude, and char [23]. Slow pyrolysis of PET waste was conducted at 400°C producing gas, solid residue, and oil at 13.3 ± 0.6, 39.7 ± 1.6, and 46.7 ± 1.9 wt%, respectively [24]. Catalytic pyrolysis of PET was performed to study the effect of temperature and catalyst on PET degradation (500°C in a fixed-bed, semi-batch reactor using a Lewis-Brønsted acid side catalyst). The increase in temperature increased the PET conversion rate, forming liquid-, gas-, and solid-phase products with benzoic acid and TPA. This process is primarily affected by the formation of char, which influences the conversion rate and product yield [25]. Pyrolysis does not require intense waste sorting and feedstock pre-treatment, and is consequently convenient and flexible; hence, it is economical and less labor-intensive. Moreover, pyrolysis reduces the dependence on conventional energy sources (such as fossil fuels), the volume of PET waste, and the carbon footprint of plastic products by reducing carbon monoxide and carbon dioxide emissions [26]. However, pyrolysis produces detrimental materials such as biphenyls, which create environmental pollution and health problems [27]. The substrates required for biological upcycling in this process are expensive as they require purification.
Substrate Production by PET Gasification
Gasification is the degradation of plastics at high temperatures (700-1300°C) using oxidizing agents such as oxygen, steam, and air to produce syngas or producer gas consisting of methane, carbon monoxide, and hydrogen gas [28]. Depending on the gasifying agent and heterogeneous char gasification reactions, gasification involves drying, devolatilization, tar cracking, combustion, and shifting [28]. This process is mainly affected by factors including the type of plastic (aliphatic or aromatic), bed material, temperature, and gasifying agent [29]. Depending on the oxidizing agent, steam gasification, air gasification, and co-gasification, wherein different compounds are mixed before gasification, have been explored. Dolomite is used in the fluidized bed to reduce agglomeration and coke formation on active carbon. Active carbon is predominantly used as a tar remover, improving syngas quality [30]. Steam gasification uses steam as the primary hydrogen source. Considering the effect of the operating conditions on the tar and gas composition during the steam gasification of PET, increasing the temperature improves the hydrogen and carbon monoxide yields as the residence time and steam-to-fuel ratio are increased; and carbon monoxide and carbon dioxide contents are decreased [28]. Air gasification of PET was performed in a two-stage gasifier with active carbon. Air was used instead of steam to provide hydrogen and carbon monoxide, yielding producer gas (90.93 wt%), char (6.15%), tar (2.90%), and condensate liquid (0.03 wt%)[29]. Gasification is considerably flexible in treating various feedstock composites and can be integrated into current energy and fuel production systems. However, PET exhibits low effective carbon conversion, which reduces syngas heat. During steam gasification, less than 30% of carbon in PET is converted into gaseous products at 700°C. This conversion is required to form CO2, which is an important constituent of syngas; however, PET produces 3-4 times less carbon dioxide [28].
Substrate Production via Plastic Depolymerization
depolymerization is the application of polymer chemistry to undo polymerization reactions to yield PET monomers,
Chemical Depolymerization
Chemical depolymerization uses chemicals and catalysts to influence the breakdown of PET into its monomers, BHET, MHET, TPA, and EG obtained from glycolysis, DMT from methanolysis, and TPA derivatives from aminolysis. Chemical depolymerization methods include alcoholysis (methanolysis), aminolysis, hydrolysis (steam, mineral acids, water, and alkalis), and glycolysis [31]. Inorganic catalysts, organocatalysts, and ionic liquids have been used to accelerate depolymerization.
Aminolysis involves the reaction of PET in primary amine-rich solutions, namely, hydrazine, ethanolamine, methylamine, allylamine, and ethylamine, to produce TPA and EG diamides. PET is used in the form of powder or fibers (temperature range = 20-100°C) [32]. Waste PET bottles were converted into hydrogel adsorbents through aminolysis. PET was reacted with tri- and tetraamines, diethylene amine, and diethylene-tetraamine, producing monomers, dimers, and oligomers, respectively, which were interlinked with ethylene glycol diglycidyl ether to form hydrogels [33]. Additionally, using an ultrafast microwave, aminolysis of PET could be conducted without catalysts in the primary amine-rich solutions at 180-200°C. Consequently, terepthalamides with different functional groups, namely, dihexylterephthalamide (DHTA), bis-(2-hydroexylethylterephtalamide (BHETA), bis-(furan-2-ylmethyl terepthalamide (BFTA), and diallyterepthalamide (DAA) were produced. These terepthalamides were used as plasticizers for polylactide and polylactic acid to reduce brittleness or as resin components for photopolymerizable film production [34]. PET was reacted with 1,2-diamino propane at temperatures in the range of 100-130°C for 20-24 h, yielding a combination of products, namely, monomers, dimers, trimers, and oligomers; the reaction of the monomers with salicylaldehyde produced a Schiff base, which can be used as a precursor for biologically active ligands, complexes, and catalysts [35]. Aminolysis requires minimal energy and time, and a simple purification step for the synthesized products. Because of these advantages, we expect that BHETA, DHTA, BFTA, and DAA could potentially be applied in biological upcycling for synthesizing high value-added chemicals after further depolymerization into TPA and EG, with simultaneous catabolism of certain microorganisms.
In methanolysis, PET is degraded by methanol at high temperatures (160-300°C) and pressures of up to 7 MPa in the presence of transesterification catalysts, consequently forming DMT and EG as the primary products. Additionally, PET oligomers (dimers, trimers, and tetramers) are possibly formed. Methanolysis involves two steps: depolymerization and purification of DMT through crystallization and distillation [36]. Methanolysis is influenced by factors including type and amount of catalyst, temperature, and time. PET waste methanolysis was done at 200°C for 2 h with bamboo leaf as a green heterogeneous catalyst; DMT and EG were the primary products with yields of 78% and 76%, respectively [37]. PET was also depolymerized with methanol in the presence of calcined sodium silicate at 180-200°C for 30 min, resulting in DMT (95% yield) and EG (depolymerization rate = 100% [38]. Using poly ionic liquids, namely, PIL-Zn2+ and PIL-Co2+ as catalysts, PET was depolymerized with methanol into DMT and EG under optimized conditions, resulting in DMT (89.1% yield) with 100% PET conversion [39]. PET was depolymerized at 200°C for 30 min using MgO/NaY (4 wt%), producing DMT (91%yield) and EG with 99% conversion of PET [40]. A low-energy catalytic methanolysis process was developed to depolymerize PET using methanol, with potassium carbonate as a catalyst, at 20-35°C. A high yield of 93.1% DMT and EG was obtained at 25°C, indicating that methanolysis could be performed at low temperatures [41]. Relatively low-quality PET can be used for methanolysis because of a simplified purification process involving the DMT product. Moreover, increased levels of contamination are tolerated, thereby offsetting chemical processing costs. However, methanolysis is expensive and sensitive to water presence, which is linked with catalyst poisoning [36]. DMT cannot be directly used in biological upcycling but can be converted into TPA, which is the required substrate. DMT was hydrolyzed to TPA in the presence of Nb/HZSM-5, a solid acid catalyst in the range of 160-220°C. The conversion was influenced by temperature changes; the DMT conversion and TPA yield increased gradually with an increase in temperature. At 160°C, DMT conversion and TPA yield were 33.4 and 21.8%, respectively; at 200°C, the DMT conversion was complete and the rate was 100% (DMT yield = 93.5%) [42]. Similar to the aminolysis products (BFTA, BHETA, DAA, and DHTA), the methanolysis product, DMT, can be applied to biological upcycling as a substrate after applying a suitable hydrolysis process to produce TPA and EG from DMT.
Glycolysis is a significant chemical depolymerization method and is used to obtain BHET and EG, which are used in bio-upcycling. Glycolysis is typically performed on a commercial scale and is consistently mediated by trans-esterification catalysts using EG as a solvent at a high temperature range (180-250°C), producing BHET, oligomers, and EG [43, 44]. Glycolysis uses different types of catalysts, namely metal-based ceramics, biomass-derived materials [45], organocatalysts [46], metallic chlorides, acetates, enzymes, and eutectic solvents based on ionic liquids and metal salts [47, 48]. Catalysts influence the speed of glycolysis; however, different catalysts have different costs and affect the process following glycolysis by influencing the enzymes/cells used, thereby requiring purification and increasing the economic cost of the entire process. Catalysts typically harm the environment; hence, biomass-derived materials are currently being investigated. Glycolysis is a simple, low-cost, and flexible method that produces a high yield of BHET monomer [47]. PET was depolymerized in the presence of EG and various catalysts, namely, graphite carbon nitride, melamine, potassium chloride, and sodium chloride at 160-96°C for 5-120 min; using graphite carbon nitride nanocatalysts resulted in BHET with 80.30% yield. Owing to better yields, lower total costs, and pollution reduction, graphite carbon nitride nanocatalysts are superior to metal-based and metal-free ionic catalysts [46]. Virgin PET pellets were depolymerized via glycolysis to estimate the efficiency of BHET production with modified conditions: temperature = 180-220°C and time = 10-180 min. PET was glycolyzed in the presence of a pure zinc acetate catalyst at a pressure of 3 bar, producing oligomers, dimers, and BHET at 180 and 220°C in short periods (e.g., 10 min) [49]. Because glycolysis depends on temperature and time, determining the optimum conditions is important. Post-consumer PET samples were broken down through glycolysis in the presence of deep eutectic solvents: choline chloride-thiourea and choline chloride-urea as catalysts, producing residual PET which was hydrolyzed in the presence of sodium carbonate and EG for a microwave irradiation time of 3 min; TPA, MHET, and BHET were obtained with yields of 62.79-80.66, 17.22-34.79, and 0.54-0.59%, respectively, with 99% PET conversion. Microwave irradiation was used because it reduces the reaction time, proving that PET can be rapidly depolymerized [50]. PET from a soft drink company was recycled using conventional heating and microwave radiation as energy sources in the presence of Lewis acids: t-BuNH2/LiBr in glycolysis and t-BuNH2/NaCl in hydrolysis. Conventional heating was performed at 197°C for 25 h, whereas microwave radiation was performed at 210°C for 30 min, producing EG and TPA. Microwave radiation is recommended as an energy-conserving process because PET is completely depolymerized within a short duration [51]. An oyster-derived (biomass-derived) catalyst was used in glycolysis optimization, which could break down PET at 195°C for 1 h into BHET and EG with 68.6% yield. By replacing conventional metal-based catalysts with biomass-based catalysts, an environmentally friendly and economically competitive glycolysis process can be established [45]. Glycolysis has several advantages, including simplicity, flexibility, low cost, and the synthesis of TPA, EG, and BHET, which can be used in the upcycling process with or without purification [52]. However, this process involves high pressure and temperature, catalysts, and difficulty in separating and purifying oligomers from the desired products [53].
Biological Depolymerization
Biological depolymerization produces intermediates by biologically degrading plastic, which is subsequently subjected to downstream bioconversion to obtain value-added products. This process generally uses a PETase-MHETase dual enzyme or a single enzyme system. Enzymatic depolymerization involves the use of enzymes with promiscuous activities, such as lipases, esterases, cutinases, carboxyl esterases, and MHETase, to release BHET and MHET monomers, which are further degraded to TPA and EG as the final products [47, 54]. These enzymes have been discovered in various microbes, such as
Several microorganisms with PET-degrading abilities, such as
Therefore, biological depolymerization is an environmentally friendly method because it uses mild conditions (temperature and pH) in the absence of hazardous chemicals [47]. Enzymes are specific and selective to specific substrates, rendering the process effective, as they can be used to act on targeted substrates. Enzymatic depolymerization (e.g., hydrolysis) is crucial for producing substrates suitable for bioconversion (e.g., TPA and EG). However, biological depolymerization incurs high operating costs [82]. To establish economically feasible biological depolymerization processes, possible further developments include reducing the enzyme production costs and improving the catalytic properties and stability.
Chemoenzymatic Depolymerization
Several combined chemoenzymatic depolymerization processes have been developed, wherein chemical depolymerization, including glycolysis, yields BHET, oligomers, and EG, and enzymatic hydrolysis breaks down BHET and MHET into TPA. TPA and EG are the substrates used for bioconversion. PET was glycolyzed in the presence of urea/NaOAc·3H2O, a eutectic solvent-based catalyst, and EG to obtain the main product: BHET (35%yield and 73.6% PET conversion). The synthesized BHET was further hydrolyzed by
Production of High-Value Products from PET Monomers by Upcycling Processes
Upcycling involves the conversion of degraded products from chemical transformations or biological degradation into various value-added biochemicals via chemical or biological routes [84]. Substrate production processes for upcycling using various degradation methods, such as pyrolysis and gasification, have been efficiently developed. However, because recently published research is predominantly related to upcycling BHET, TPA, and EG, we focus on the upcycling status of these chemicals. Therefore, in this section, after reviewing the biological upcycling of TPA and EG by TPA/EG-metabolizing microorganisms and whole-cell conversion into high-value-added chemicals, as shown in Fig. 3 and Table 2, we suggest a promising convergence process for upcycling PET.
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Table 2 . Chemicals produced by whole-cell conversion..
Substrate Recombinant organism Products Production yields References TPA E. coli PCA 81.4% [13] Gallic acid 15.9% Catechol 97.8% Pyrogallol 39.0% Vanillic acid 41.6% Muconic acid 85.4% EG G. oxydans KCCM 40109Glyoxylic acid 98.6% TPA E. coli PCA -1PCA 90.4% [3] EG G. oxydans KCCM 40109Glyoxylic acid 91.6% TA E. coli RARE-pVanXVanillin 79% [67] TPA P. stutzeri TPA-3PPHB 11.56 wt% [75] TPA P. umsongensis GO16 KS3HAA 35 mg/L [73] TPA P. putida GO16, G019, PHA, 3-hydroxydecanoic acid 4.4 mg/L/h [74] P. frederiksbergensis (GO23)8.4 mg/L/h TPA I. sakaiensis PHA 0.75 ± 0.09 g/L [98] Note: PCA, protocatechuic acid; PHB, polyhydroxy butyrate; HAA, hydroxy alkanoyl oxy-alkanoate; PHA, polyhydroxyalkanoate.
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Figure 3. Catabolic pathways of TPA to aromatic chemicals: catechol, PCA, and other high value-added chemicals. The α-ketoadipate pathway (blue line) consists of genes, namely, PcaHG (protocatechuate 3,4-dioxygenase), PcaB (3-carboxy-cis, cis-muconate cycloisomerase), PcaC (γ-carboxy-muconolactone decarboxylase), and PcaD (β-ketoadipate enol-lactone). The targeted products include: CMA, β-carboxy-cis,cis-muconic acid, CML-γ-carboxymuconolactone, MCL, Muconolactone,KA,β-ketoadipic acid. The PCA 2,3-cleavage pathway (light green lines) consists of genes, namely, PraA, (PCA 2,3-dioxygenase), PraH, (5 CHMS decarboxylase), 5C-2HMS dehydrogenase, XyHI,4-oxalocrotonate isomerase, XyIJ,2- oxopent-4-enoate hydratase,XyIK, 4-hydroxy-2-oxovalerate aldolase, XyIQ, acetaaldehyde dehydrogenase. The targeted molecules are: CHMS, 5-Carboxy-2-hydroxymuconate-semialdehyde, ICA, isocinchomeronic acid, HMS, 2-Hydroxymuconate semialdehyde, PCL-picolinic acid, HPD/OEA, 2-Hydroxypenta-2,4-dienoic acid/2-oxopent-4-enoic acid, HOA, 4-Hydroxy- 2-oxovaleric acid. The other pathway (grey line) is encoded by LigAB,4,5-PDC,Lig C,CHMS dehydrogenase, Lig I, PDC hydrolase, LigJ, OMA hydratase, LigK, CHA(4-carboxy-4-hydroxy-2-oxoadipate) aldolase yields energy compounds. The targeted compounds include: HCMS, 4-Carboxy-2-hydroxymuconate-semialdehyde, DPA-dinicotinic acid, PDC, 2-pyrone- 4,6-dicarboxylic acid, CHA, 4-Carboxy-4-hydroxy-2-oxoadipic acid. Catechol is catabolized by CatA, Catechol 1,2- dioxygenase,CatB, Muconate cycloisomerase and Cat C, Muconolactone isomerase into MA, cis,cis-Muconic acid.
Biological Upcycling of TPA or EG by TPA- or PET-Metabolizing Microorganisms
TPA, a PET hydrolysis product, is not widely regarded as a bacterial growth substrate, due to its toxicity to cells [74]. However, PET biodegradation using
EG is metabolized via various pathways, namely oxidization to ethanol and acetaldehyde, which are converted to acetate via acetyl-CoA, followed by substrate-level phosphorylation to form ATP (Fig. 4). Ethanol can be oxidized and the reducing equivalents can be reused by CO2 reduction to acetate in the Wood-Ljungdahl pathway. EG can be catabolized to acetaldehyde by propanediol dehydratase (PduCDE) and CoA-dependent propionaldehyde dehydrogenase (PduP) proteins encoded by the
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Figure 4. Metabolic pathway of ethylene glycol to glyoxylate and other energy compounds. This consists of PQQ (pyrroloquinoline quinone), which is reduced to PQQH2 upon conversion of ethylene glycol to glycolaldehyde and other compounds synthesized from glycolate. The enzymes shown are PedE/PedH, PQQ-dependent alcolhol dehydrogenase, Pedl, peroxisomal 3-ketoacyl-COA thiolase, GlcDEF, glycolate dehydrogenase, GlxR, tatronate semialdehyde reductase, Hyi,hydroxypyruvate isomerase and PykA/Pyk, pyruvate kinase.
Although TPA or EG can be easily converted into target chemicals by metabolic engineering, several drawbacks include a low growth rate [85] and limited engineering tools [92] for direct application to microbial chassis for producing value-added chemicals. Future studies can focus on chassis engineering to improve growth and productivity, remove unnecessary metabolic pathways, and reinforce substrate utilization capability to generate industrial strains.
Whole-Cell Conversion of TPA and EG
The whole-cell conversion uses engineered cells containing TPA-metabolizing genes or wild-type cells with the ability to degrade TPA, thus enabling the conversion of TPA and EG into other chemicals for further use. Whole-cell microbial catalysts containing
Whole-cell conversion is cost-effective and less time-consuming, with no enzyme purification; rapid validation of new synthetic pathways is possible. However, this process exhibits a shortcoming of low substrate loading; hence, chassis engineering, adaptive laboratory evolution, and the introduction of heterologous TPA transport systems are required [94]. Further studies considering substrate loading improvement by transporter introduction and chassis engineering enabling the endurance of high TPA loading are recommended.
Development of Convergence Technology for PET Waste Upcycling
Plastic upcycling technologies are instrumental in solving current plastic waste problems, in addition to biodegradable plastic production technology, because all conventional plastics cannot be replaced by biodegradable plastics (Fig. 5). Although disposable plastics for packaging should be replaced with biodegradable plastics, the long-term use of plastics with less biodegradability, such as plastics for processed food and electronics, would continuously generate potential plastic waste. The development of plastic upcycling technologies consists of two important steps: i) substrate production for bioconversion via PET waste depolymerization, and ii) upcycling of the produced substrates.
-
Figure 5. Lifecycle of plastics, including the linear economy, wherein plastics from fossil fuels end up as plastic waste in the environment, and the circular economy, wherein plastic waste from fossil fuels undergoes various processes to yield high value-added chemicals, thus conserving the environment.
The depolymerization of PET is essential because it provides substrates that can be used in biological upcycling to produce high value-added chemicals through a circular loop of plastic upcycling, thereby reducing the amount of plastic in the environment. Chemical depolymerization breaks down PET waste into BHET, which can be used as a substrate in enzymatic depolymerization. This method has an increased tolerance for high levels of contamination, and lower-quality PET can be used. However, high reaction conditions (high temperatures and pressures) and expensive catalysts increase the overall cost of the process and produce low-purity target chemicals, for example BHET, including lots of unnecessary chemicals due to additives and side reactions. Besides, the produced substrates require transformation to be applied for bioconversion. The produced substrates require transformation to be applied for bioconversion. Biological depolymerization can be developed as an alternative to chemical depolymerization with various advantages: operation at low temperatures, no pressure, and selective depolymerization of PET waste from mixed plastic waste. However, significant disadvantages are observed in terms of operating costs, including enzyme production. Recently, chemoenzymatic depolymerization has been considered an efficient method to produce substrates for the bioconversion of PET waste, because it adopts the advantages of chemical and biological depolymerization processes. Through chemical depolymerization, a substantial amount of PET waste can be easily transformed into smaller chemicals such as BHET. The products can then be transformed into suitable substrates for bioconversion by enzymatic hydrolysis processes using enzymes such as esterases. In the future, the development of more efficient chemical depolymerization methods using biocompatible catalysts with advanced processes, along with a singular enzymatic system operating at a biocompatible temperature (e.g., 37°C) and improved hydrolysis capability, can provide the economic feasibility to contribute to plastic waste reduction.
Several chemical and biological upcycling processes have been reported thus far. Considering BHET, chemical upcycling is advantageous because it can efficiently produce valuable items without further hydrolysis of BHET. For example, fiberglass-reinforced plastics were successfully developed by polymerizing BHET produced from PET waste with renewably sourced monomers, including acrylic acid, methacrylic acid, and MA [95]. However, the product portfolio to produce value-added chemicals is limited compared with that of the biological upcycling process; biological upcycling provides better benefits because it can directly produce various value-added chemicals by combining multiple enzymes. Biological upcycling involves the application of PET monomers, TPA, and EG, using whole-cell conversion and TPA-metabolizing microorganisms, to convert them into expensive chemicals. Depolymerization products are converted into high value-added products via biological upcycling. Moreover, it enables the circular utilization of PET, reduces the amount of plastic waste produced, and encourages the proposed circular economy to ultimately solve the prevalent greenhouse gas emission and crude oil consumption problems. However, biological upcycling is affected by limitations such as low substrate loading, and fewer known PET-degrading microorganisms. Therefore, additional studies on TPA/EG-metabolizing microorganisms, consolidated bioprocessing (CBP) for reducing enzyme usage, and advanced heterologous TPA transport systems toward efficient TPA uptake will be required.
Although the use of petroleum-based TPA and EG is more economically feasible, the continuous use of petroleum fossil fuels causes serious damage to the environment. Meanwhile, due to the reduction in fossil fuel reserves [96], TPA/EG synthesis has shifted from fossil fuels to biomass [97], hence creating further possibilities, including the suggested use of TPA/EG from recycled PET. Therefore, the application of TPA and EG from recycled PET enables the use of PET in a circular economy producing high value-added chemicals and reducing the plastic burden. The TPA/EG amounts currently obtained are so low that further research is needed to find more reliable microorganisms to break down PET into TPA/EG while also increasing the produced amounts to encourage a shift from laboratory scale to industrial scale.
Conclusions
In this review, the current status of PET upcycling approaches was comprehensively addressed. Different forms of substrate fabrication using chemical, biological, and chemobiological methods were described. Subsequently, the value-added chemical production process by biological upcycling was discussed. However, considering the economic feasibility, each process presents disadvantages in terms of establishing a circular plastic economy. In the future, advancing individual technologies, including the production of substrates for bioconversion and biological upcycling, should be considered along with integrating each technology to design an economically feasible PET upcycling process. Based on this development, further studies should be conducted to develop and take practical technologies from the laboratory scale to an industrial scale to reduce PET waste and provide economic profit.
Acknowledgments
This work was supported by a research fund from Chungnam National University (2022-0559-01).
Conflict of Interest
The authors have no financial conflicts of interest to declare.
- Abstract
- Introduction
- Production of Substrates from PET Waste for Bioconversion Processes
- Substrate Production by Pyrolysis of PET
- Substrate Production by PET Gasification
- Substrate Production via Plastic Depolymerization
- Chemical Depolymerization
- Biological Depolymerization
- Chemoenzymatic Depolymerization
- Production of High-Value Products from PET Monomers by Upcycling Processes
- Biological Upcycling of TPA or EG by TPA- or PET-Metabolizing Microorganisms
- Whole-Cell Conversion of TPA and EG
- Development of Convergence Technology for PET Waste Upcycling
- Conclusions
- Acknowledgments
- Conflict of Interest
Fig 1.

Fig 2.

Fig 3.

Fig 4.

Fig 5.

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Table 1 . Degradation product types and yields obtained from PET using various PET degradation processes..
Process Substrate Product Yield Reference Pyrolysis PET bottle sheets1 Solid
Gas35.67 wt%
40 wt%[22] PET water bottles with rice husk Gas
Biocrude
char39.9-68.9 wt%
12-29%
15.7-31.7%[23] PET bottles2 Gas
Oil
Solid residue13.3±0.6 wt%
46.7±1.9 wt%
39.7±1.6 wt%[24] Pure PET pellets Gas: CO, CO2
Solid: TPA-
25%[25] Gasification Virgin PET pellets Gas: H2, CO2, CO
Char
Tar90.93 wt%
6.15 wt%
2.9 wt%[29] Virgin PET pellets Gas: H2
CO2
Biphenyl- [28] Depolymerization Aminolysis Post-consumer PET
DHTA
BHTA
BFTA
DAA64%
91%
82%
61%[34] PET flakes Monomers, Dimers, Trimers, Oligomers - [35] Methanolysis PET DMT 89.1% [39] PET DMT
EG78%
76%[37] PET DMT
EG95% [38] PET DMT
EG91% [40] PET DMT
EG93.5% [41] Glycolysis PET BHET 80.30% [46] PET BHET 70% [49] Post-consumer PET bottles TPA
MHET
BHET62.79-80.66%
17.22-34.79%
0.54-0.59%[50] PET bottles BHET, TPA, EG Quantitative [51] Biological depolymerization PET BHET
EG68.6% [45] Post-consumer PET waste TPA 49±2% [76] Amorphous PET TPA 16.7 g/L/h [79] Chemoenzymatic depolymerization PET BHET
MHET
PET oligomers84.8%
7.7%
8.7%[8] PET BHET 35% [47] PET TPA
EG99.9% [3] Note: DHTA, dihexylterephthalamide; BHTA, bis(2-hydroxyethyl) terepthalamide; BFTA, bis(furan-2-ylmethyl) terepthalamide; DAA, diallyterepthalamide; DMT, dimethyl terephthalate; TPA, terephthalic acid; BHET, bis(2-hydroxyethyl) terephthalate; MHET, mono-(2-hydroxyethyl) terephthalate; EG, ethylene glycol..
1-solid products contained benzoic acid, 4-vinyl benzoic acid, monovinyl terephthalate, and divinyl terephthalate..
2-Oil components include paraldehyde (54.7 wt%), ethylene glycol (23.65 wt%), and benzoic acid and benzoates (11.5 wt%). The solid carbonaceous residue contained carbon, ash, nitrogen, and sulfur). Gas contained CO and CO2 (more than 90 vol%), a few C1 -C 4 hydrocarbons (~7 vol%), and hydrogen (~3 vol%)..
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Table 2 . Chemicals produced by whole-cell conversion..
Substrate Recombinant organism Products Production yields References TPA E. coli PCA 81.4% [13] Gallic acid 15.9% Catechol 97.8% Pyrogallol 39.0% Vanillic acid 41.6% Muconic acid 85.4% EG G. oxydans KCCM 40109Glyoxylic acid 98.6% TPA E. coli PCA -1PCA 90.4% [3] EG G. oxydans KCCM 40109Glyoxylic acid 91.6% TA E. coli RARE-pVanXVanillin 79% [67] TPA P. stutzeri TPA-3PPHB 11.56 wt% [75] TPA P. umsongensis GO16 KS3HAA 35 mg/L [73] TPA P. putida GO16, G019, PHA, 3-hydroxydecanoic acid 4.4 mg/L/h [74] P. frederiksbergensis (GO23)8.4 mg/L/h TPA I. sakaiensis PHA 0.75 ± 0.09 g/L [98] Note: PCA, protocatechuic acid; PHB, polyhydroxy butyrate; HAA, hydroxy alkanoyl oxy-alkanoate; PHA, polyhydroxyalkanoate.
References
- Hiraga K, Taniguchi I, Yoshida S, Kimura Y, Oda K. 2019. Biodegradation of waste PET.
EMBO Rep. 20 : e49365. - Dissanayake L, Jayakody LN. 2021. Engineering microbes to bio-upcycle polyethylene terephthalate.
Front. Bioeng. Biotechnol. 9 : 656465. - Kim DH, Han DO, In Shim K, Kim JK, Pelton JG, Ryu MH,
et al . 2021. One-Pot Chemo-bioprocess of PET depolymerization and recycling enabled by a biocompatible catalyst, betaine.ACS Catal. 11 : 3996-4008. - Shi L, Liu H, Gao S, Weng Y, Zhu L. 2021. Enhanced extracellular production of
Is PETase inEscherichia coli via engineering of the pelB signal peptide.J. Agric. Food Chem. 69 : 2245-2252. - Knott BC, Erickson E, Allen MD, Gado JE, Graham R, Kearns FL,
et al . 2021. Characterization and engineering of a two-enzyme system for plastics depolymerization.Proc. Natl. Acad. USA 117 : 25476-25485. - Geyer R, Jambeck JR, Law KL. 2017. Production, use, and the fate of all plastics ever made.
Sci. Adv. 3 : e1700782. - Hou Q, Zhen M, Qian H, Nie Y, Bai X, Xia T,
et al . 2021. Upcycling and catalytic degradation of plastic wastes.Cell Rep. Phys. Sci. 2 : 100514. - Kim HT, Hee Ryu M, Jung YJ, Lim S, Song HM, Park J,
et al . 2021. Chemo-biological upcycling of poly(ethylene terephthalate) to multifunctional coating materials.ChemSusChem 14 : 4251-4259. - Son HF, Cho IJ, Joo S, Seo H, Sagong HY, Choi SY,
et al . 2019. Rational protein engineering of thermo-stable PETase fromIdeonella sakaiensis for highly efficient PET degradation.ACS Catal. 9 : 3519-3526. - Law KL, Starr N, Siegler TR, Jambeck JR, Mallos NJ, Leonard GH. 2020. The United States' contribution of plastic waste to land and ocean.
Sci. Adv. 6 : eabd0288. - Heidari M, Garnaik PP, Dutta A. 2019. The valorization of plastic via thermal means: industrial scale combustion methods.
Plastics Energy 2019 : 295-312. - Kumar S, Singh E, Mishra R, Kumar A, Caucci S. 2021. Utilization of plastic wastes for sustainable environmental management: a review.
ChemSusChem. 14 : 3985-4006. - Kim HT, Kim JK, Cha HG, Kang MJ, Lee HS, Khang TU,
et al . 2019. Biological valorization of poly(ethylene terephthalate) monomers for upcycling waste PET.ACS Sustain. Chem. Eng. 7 : 19396-19406. - Wei R, Zimmermann W. 2017. Biocatalysis is a green route for recycling recalcitrant plastic polyethylene terephthalate.
Microb. Biotechnol. 10 : 1302-1307. - Pantelic B, Ponjavic M, Jankovic V, Aleksic I, Stevanovic S, Murray J,
et al . 2021. Upcycling biodegradable PVA/starch film to a bacterial biopigment and biopolymer.Polymers 13 : 3692. - Sung K, Cooper T, Kettley S. 2015. A review on upcycling: the current body of literature, knowledge gaps and a way forward - IRep -Nottingham Trent University.
17th International Conference on Environmental, Cultural, Economic and Social Sustainability 17 : 28-40. - Dyer AC, Nahil MA, Williams PT. 2021. Catalytic co-pyrolysis of biomass and waste plastics as a route to upgraded bio-oil.
J. Energy Inst. 97 : 27-36. - Klaimy S, Lamonier JF, Casetta M, Heymans S, Duquesne S. 2021. Recycling of plastic waste using flash pyrolysis - effect of mixture composition.
Polym. Degrad. Stab. 187 : 109540. - Orozco S, Alvarez J, Lopez G, Artetxe M, Bilbao J, Olazar M. 2021. Pyrolysis of plastic wastes in a fountain confined conical spouted bed reactor: Determination of stable operating conditions.
Energy Convers. Manag. 229 : 113768. - Osman AI, Farrell C, Al-Muhtaseb AH, Al-Fatesh AS, Harrison J, Rooney DW. 2020. Pyrolysis kinetic modeling of abundant plastic waste (PET) and
in-situ emission monitoring.Environ. Sci. Eur. 32 : 112. - Suresh A, Alagusundaram A, Kumar PS, Vo DVN, Christopher FC, Balaji B,
et al . 2021. Microwave pyrolysis of coal, biomass and plastic waste: a review.Environ. Chem. Lett. 19 : 3609-3629. - Liu Y, Fu W, Liu T, Zhang Y, Li B. 2022b. Microwave pyrolysis of polyethylene terephthalate (PET) plastic bottle sheets for energy recovery.
J. Anal. Appl. Pyrolysis 161 : 105414. - Suriapparao DV, Kumar DA, Vinu R. 2022. Microwave co-pyrolysis of PET bottle waste and rice husk: effect of plastic waste loading on product formation.
Sustain. Energy Technol. Assess. 49 : 101781. - Straka P, Bičáková O, Šupová M. 2022. Slow pyrolysis of waste polyethylene terephthalate yielding paraldehyde, ethylene glycol, benzoic acid, and clean fuel.
Polym. Degrad. Stab. 198 : 109900. - Shahi A, Roozbehani B, Mirdrikvand M. 2022. Catalytic pyrolysis of waste polyethylene terephthalate granules using a Lewis-Brønsted acid sites catalyst.
Clean Technol. Environ. Policy 24 : 779-787. - Vijayakumar A, Sebastian J. 2018. Pyrolysis process to produce fuel from different types of plastic - A review.
IOP Conf. Ser.: Mater. Sci. Eng. 396 : 012062. - Lee J, Lee T, Tsang YF, Oh JI, Kwon EE. 2017. Enhanced energy recovery from polyethylene terephthalate via pyrolysis in CO2 atmosphere while suppressing acidic chemical species.
Energy Convers. Manag. 148 : 456-460. - Li S, Cañete Vela I, Järvinen M, Seemann M. 2021. Polyethylene terephthalate (PET) recycling via steam gasification - The effect of operating conditions on gas and tar composition.
Waste Manage. 130 : 117-126. - Choi MJ, Jeong YS, Kim JS. 2021. Air gasification of polyethylene terephthalate using a two-stage gasifier with active carbon for the production of H2 and CO.
Energy 223 : 120122. - Jeong Y-S, Kim J-W, Ra HW, Seo MW, Mun T-Y, Kim J-S. Two-stage air gasification of ten different types of plastic using active carbon as a tar removal additive.
SSRN Electronic J. : 1-38. - Biermann L, Brepohl E, Eichert C, Paschetag M, Watts M, Scholl S. 2021. Development of a continuous PET depolymerization process as a basis for a back-to-monomer recycling method.
Green Process Synth. 10 : 361-373. - Gupta P, Bhandari S. Chemical depolymerization of PET bottles via ammonolysis and aminolysis, pp. 109-134.
In: Recycling Polyethylene Terephthalate Bottles . - Chan K, Zinchenko A. 2021. Conversion of waste bottles' PET to a hydrogel adsorbent via PET aminolysis.
J. Environ. Chem. Eng. 9 : 106129. - Bäckström E, Odelius K, Hakkarainen M. 2021. Ultrafast microwave-assisted recycling of PET to a family of functional precursors and materials.
Eur. Polym. J. 151 : 110441. - Otaibi A, Alsukaibi A, Rahman M, Mushtaque M, Haque A. 2022. From waste to schiff Base: Upcycling of aminolysed poly(ethylene terephthalate) product.
Polymers 14 : 1861. - Spychaj T. 2002. Chemical recycling of PET: methods and products. pp. 1252-1290. Handbook of thermoplastic polyesters: homopolymers, copolymers, blends, and composites.
- Laldinpuii ZT, Khiangte V, Lalhmangaihzuala S, Lalmuanpuia C, Pachuau Z, Lalhriatpuia C,
et al . 2022. Methanolysis of PET waste using a heterogeneous catalyst of bio-waste origin.J. Polym. Environ. 30 : 1600-1614. - Tang S, Li F, Liu J, Guo B, Tian Z, Lv J. 2022a. Calcined sodium silicate as a solid base catalyst for the alcoholysis of poly(ethylene terephthalate).
J. Chem. Technol. Biotechnol. 97 : 1305-1314. - Jiang Z, Yan D, Xin J, Li F, Guo M, Zhou Q,
et al . 2022. Poly(ionic liquid)s an efficient and recyclable catalysts for methanolysis of PET.Polym. Degrad. Stab. 199 : 109905. - Tang S, Li F, Liu J, Guo B, Tian Z, Lv J. 2022b. MgO/NaY as modified mesoporous catalyst for methanolysis of polyethylene terephthalate wastes.
J. Environ. Chem. Eng. 10 : 107927. - Pham DD, Cho J. 2021. Low-energy catalytic methanolysis of poly(ethyleneterephthalate).
Green Chem. 23 : 511. - Guo B, Liu J, Tang S, Liu Y, Tian Z, Lv J. 2022. Hydrolysis of dimethyl terephthalate to terephthalic acid on Nb-modified HZSM-5 zeolite catalysts.
J. Chem. Technol. Biotechnol. 97 : 1695-1704. - Goje AS, Thakur SA, Chauhan YP, Patil TM, Patil SA, Diware VR,
et al . 2005. Glycolytic aminolysis of poly(ethylene terephthalate) waste at atmospheric pressure for recovery of a value-added insecticide.Polym. Plast Technol. Eng. 44 : 163-181. - Thachnatharen N, Shahabuddin S, Sridewi N. 2021. The Waste management of polyethylene terephthalate (PET) plastic waste: A Review.
IOP Conference Series: Mater. Sci. Eng. 1127 : 012002. - Kim Y, Kim M, Hwang J, Im E, Moon GD. 2022. Optimizing PET glycolysis with an oyster shell-derived catalyst using response surface methodology.
Polymers 14 : 656. - Wang Z, Jin Y, Wang Y, Tang Z, Wang S, Xiao G,
et al . 2022. Cyanamide as a highly efficient organocatalyst for the glycolysis recycling of PET.ACS Sustainable Chem. Eng. 10 : 7965-7973. - Neves Ricarte G, Lopes Dias M, Sirelli L, Antunes Pereira Langone M, Machado de Castro A, Zarur Coelho MA,
et al . 2021. Chemoenzymatic depolymerization of industrial and assorted post-consumer poly(ethylene terephthalate) (PET) wastes using a eutecticbased catalyst.J. Chem. Technol. Biotechnol. 96 : 3237-3244. - Wang R, Wang T, Yu G, Chen X. 2021. A new class of catalysts for the glycolysis of PET: Deep eutectic solvent@ZIF-8 composite.
Polym. Degrad. Stab. 183 : 109463. - Mendiburu-Valor E, Mondragon G, González N, Kortaberria G, Eceiza A, Peña-Rodriguez C. 2021. Improving the efficiency for the production of bis-(2-hydroxyethyl) terephthalate (BHET) from the glycolysis reaction of poly(ethylene terephthalate) (PET) in a pressure reactor.
Polymers 13 : 1461. - Azeem M, Fournet MB, Attallah OA. 2022. Ultrafast 99% polyethylene terephthalate depolymerization into value-added monomers using sequential glycolysis-hydrolysis under microwave irradiation.
Arab. J. Chem. 15 : 103903. - Trejo-carbajal N, Ambriz-luna KI, Herrera-gonz AM. 2022. Efficient method and mechanism of depolymerization of PET under conventional heating and microwave radiation using t-BuNH2/Lewis's acids.
Eur. Polym. J. 175 : 111388. - Park SH, Kim SH. 2014. Poly(ethylene terephthalate) recycling for high-value added textiles.
Fash. Text 1 : 1-17. - Yang Y, Lu Y, Xiang H, Xu Y, Li Y. 2002. Study on methanolytic depolymerization of PET with supercritical methanol for chemical recycling.
Polym. Degrad. Stab. 75 : 185-191. - Kawai F, Kawabata T, Oda M. 2019. Current knowledge on enzymatic PET degradation and its possible application to waste stream management and other fields.
Appl. Microbiol. Biotechnol. 103 : 4253-4268. - Huang X, Cao L, Qin Z, Li S, Kong W, Liu Y. 2018. Tat-independent secretion of polyethylene terephthalate hydrolase PETase in
Bacillus subtilis 168 mediated by Its native signal peptide.J. Agric. Food Chem. 66 : 13217-13227. - Roberts C, Edwards S, Vague M, León-Zayas R, Scheffer H, Chan G,
et al . 2020. Environmental consortium containingPseudomonas andBacillus species synergistically degrade polyethylene terephthalate plastic.MSphere 5 : e01151-20. - Alisch-Mark M, Herrmann A, Zimmermann W. 2006. Increase of the hydrophilicity of polyethylene terephthalate fibers by hydrolases from
Thermomonospora fusca andFusarium solani f . sp. pisi.Biotechnol. Lett. 28 : 681-685. - Vertommen MAME, Nierstrasz VA, van der Veer M, Warmoeskerken MMCG. 2005. Enzymatic surface modification of poly(ethylene terephthalate).
J. Biotechnol. 120 : 376-386. - Kawai F, Thumarat U, Kitadokoro K, Waku T, Tada T, Tanaka N,
et al . 2013. Comparison of polyester-degrading cutinases from genusthermobifida .ACS Symp. Series 1144 : 111-120. - Ribitsch D, Hromic A, Zitzenbacher S, Zartl B, Gamerith C, Pellis A,
et al . 2017. Small cause, large effect: Structural characterization of cutinases fromThermobifida cellulosilytica .Biotechnol. Bioeng. 114 : 2481-2488. - Kawai F, Kawase T, Shiono T, Urakawa H, Sukigara S, Tu C,
et al . 2017. Enzymatic hydrophilization of polyester fabrics using a recombinant cutinase Cut 190 and their surface characterization.J. Fiber Sci. Technol. 73 : 8-18. - Ronkvist ÅM, Xie W, Lu W, Gross RA. 2009. Cutinase-catalyzed hydrolysis of poly(ethylene terephthalate).
Macromolecules 42 : 5128-5138. - Müller RJ, Schrader H, Profe J, Dresler K, Deckwer WD. 2005. Enzymatic degradation of poly(ethylene terephthalate): Rapid hydrolyse using a hydrolase from T. fusca.
Wiley Online Library 26 : 1400-1405. - Chen S, Su L, Chen J, Wu J. 2013. Cutinase: Characteristics, preparation, and application.
Biotechn. Adv. 31 : 1754-1767. - Jerves C, Neves RPP, Ramos MJ, Da Silva S, Fernandes PA. 2021. Reaction mechanism of the PET degrading enzyme PETase studied with DFT/MM molecular dynamics simulations.
ACS Catal. 11 : 11626-11638. - Han X, Liu W, Huang JW, Ma J, Zheng Y, Ko TP,
et al . 2017. Structural insight into the catalytic mechanism of PET hydrolase.Nat. Commun. 8 : 2106. - Sadler JC, Wallace S. 2021. Microbial synthesis of vanillin from waste poly(ethylene terephthalate).
Green Chem. 23 : 4665-4672. - Then J, Wei R, Oeser T, Gerdts A, Schmidt J, Barth M,
et al . 2016. A disulfide bridge in the calcium-binding site of a polyester hydrolase increases its thermal stability and activity against polyethylene terephthalate.FEBS Open Bio 6 : 425-432. - Franden MA, Jayakody LN, Li WJ, Wagner NJ, Cleveland NS, Michener WE,
et al . 2018. EngineeringPseudomonas putida KT2440 for efficient ethylene glycol utilization.Metab. Eng. 48 : 197-207. - Austin HP, Allen MD, Donohoe BS, Rorrer NA, Kearns FL, Silveira RL,
et al . 2018. Characterization and engineering of a plasticdegrading aromatic polyesterase.Proc. Natl. Acad. Sci. USA 115 : E4350-E4357. - Furukawa M, Kawakami N, Tomizawa A, Miyamoto K. 2019. Efficient degradation of poly(ethylene terephthalate) with
Thermobifida fusca cutinase exhibiting improved catalytic activity generated using mutagenesis and additive-based approaches.Sci. Rep. 9 : 16038. - Li Q, Zheng Y, Su T, Wang Q, Liang Q, Zhang Z,
et al . 2022. Computational design of a cutinase for plastic biodegradation by mining molecular dynamics simulations trajectories.Comput. Struct. Biotechnol. J. 20 : 459-470. - Tiso T, Narancic T, Wei R, Pollet E, Beagan N, Schröder K,
et al . 2021. Towards bio-upcycling of polyethylene terephthalate.Metab. Eng. 66 : 167-178. - Kenny ST, Runic JN, Kaminsky W, Woods T, Babu RP, Keely CM,
et al . 2008. Up-cycling of PET (polyethylene terephthalate) to the biodegradable plastic PHA (Polyhydroxyalkanoate).Environ. Sci. Technol. 42 : 7696-7701. - Liu P, Zhang T, Zheng Y, Li Q, Su T, Qi Q. 2021. Potential one-step strategy for PET degradation and PHB biosynthesis through cocultivation of two engineered microorganisms.
Eng. Microbiol. 1 : 100003. - Kaabel S, Daniel Therien JP, Deschênes CE, Duncan D, Friščic T, Auclair K. 2021. Enzymatic depolymerization of highly crystalline polyethylene terephthalate enabled in moist-solid reaction mixtures.
Proc. Natl. Acad. Sci. USA 118 : e2026452118. - Puspitasari N, Tsai SL, Lee CK. 2021. Fungal hydrophobin RolA enhanced PETase hydrolysis of polyethylene terephthalate.
Appl. Biochem. Biotechnol. 193 : 1284-1295. - Son HF, Cho IJ, Joo S, Seo H, Sagong HY, Choi SY,
et al . 2019. Rational protein engineering of thermo-stable PETase fromIdeonella sakaiensis for highly efficient PET degradation.ACS Catal. 9 : 3519-3526. - Tournier V, Topham CM, Gilles A, David B, Folgoas C, Moya-Leclair E,
et al . 2020. An engineered PET depolymerase to break down and recycle plastic bottles.Nature 580 : 216-219. - Qi X, Ma Y, Chang H, Li B, Ding M, Yuan Y. 2021. Evaluation of PET degradation using artificial microbial consortia.
Front. Microbiol. 12 : 778828. - Kumar V, Maitra SS, Singh R, Burnwal DK. 2020. Acclimatization of a newly isolated bacteria in monomer terephthalic acid (TPA) may enable it to attack the polymer polyethylene terephthalate (PET).
J. Environ. Chem. Eng. 8 : 103977. - Mahmood N, Yuan Z, Schmidt J, Xu CC. 2016. Depolymerization of lignins and their applications for the preparation of polyols and rigid polyurethane foams: A review.
Renew. Sustain. Energy 60 : 317-329. - Werner AZ, Clare R, Mand TD, Pardo I, Ramirez KJ, Haugen SJ,
et al . 2021. Tandem chemical deconstruction and biological upcycling of poly(ethylene terephthalate) to β-ketoadipic acid byPseudomonas putida KT2440.Metab. Eng. 67 : 250-261. - Gao R, Pan H, Kai L, Han K, Lian J. 2022. Microbial degradation and valorization of poly(ethylene terephthalate) (PET) monomers.
World J. Microbiol. Biotechnol. 38 : 89. - Yoshida S, Hiraga K, Takehana T, Taniguchi I, Yamaji H, Maeda Y,
et al . 2016. A bacterium that degrades and assimilates poly(ethylene terephthalate).Science 351 : 1196-1199. - Salvador M, Abdulmutalib U, Gonzalez J, Kim J, Smith AA, Faulon JL,
et al . 2019. Microbial genes for a circular and sustainable bio-PET economy.Genes 10 : 373. - Kincannon WM, Zahn M, Clare R, Beech JL, Romberg A, Larson J,
et al . 2022. Biochemical and structural characterization of an aromatic ring-hydroxylating dioxygenase for terephthalic acid catabolism.Proc. Natl. Acad. USA 119 : e2121426119. - Johnson CW, Salvachúa D, Rorrer NA, Black BA, Vardon DR, St. John PC,
et al . 2019. Innovative chemicals and materials from bacterial aromatic catabolic pathways.Joule 3 : 1523-1537. - Trifunović D, Schuchmann K, Müller V. 2016. Ethylene glycol metabolism in the acetogen
Acetobacterium woodii .J. Bacteriol. 198 : 1058-1065. - Li WJ, Jayakody LN, Franden MA, Wehrmann M, Daun T, Hauer B,
et al . 2019. Laboratory evolution reveals the metabolic and regulatory basis of ethylene glycol metabolism byPseudomonas putida KT2440.Environ. Microbiol. 21 : 3669-3682. - Welsing G, Wolter B, Hintzen HMT, Tiso T, Blank LM. 2021. Upcycling of hydrolyzed PET by microbial conversion to a fatty acid derivative.
Methods Enzymol. 648 : 391-421. - Tang HZ, Jiang JD, Wu XL, Qi X, Yan W, Cao Z,
et al . 2021. Current advances in the biodegradation and bioconversion of polyethylene terephthalate.Microorganisms 10 : 39. - Kang MJ, Kim HT, Lee MW, Kim KA, Khang TU, Song HM,
et al . 2020. A chemo-microbial hybrid process for the production of 2-pyrone-4,6-dicarboxylic acid as a promising bioplastic monomer from PET waste.Green Chem. 22 : 3461-3469. - Devi Salam M, Varma A, Prashar R, Choudhary D. 2021. Review on efficacy of microbial degradation of polyethylene terephthalate and bio-upcycling as a part of plastic waste management.
Appl. Ecol. Environ. Sci. 9 : 695-703. - Rorrer NA, Nicholson S, Carpenter A, Biddy MJ, Grundl NJ, Beckham GT. 2019. Combining reclaimed PET with Bio-based monomers enables plastic upcycling.
Joule 3 : 1006-1027. - Huber GW, Iborra S, Corma A. 2006. Synthesis of transportation fuels from biomass: Chemistry, catalysts, and engineering.
Chem. Rev. 106 : 4044-4098. - Pang J, Zheng M, Sun R, Wang A, Wang X, Zhang T. 2016. Synthesis of ethylene glycol and terephthalic acid from biomass for producing PET.
Green Chem. 18 : 342-359. - Fujiwara R, Sanuki R, Ajiro H, Fukui T, Yoshida S. 2021. Direct fermentative conversion of poly(ethylene terephthalate) into poly(hydroxy alkanoates) by
Ideonella sakaiensis .Sci. Rep. 11 : 1-7.