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The development of an efficient and environmentally sustainable chemical hydrolysis process for recycling waste plastics, based on green chemistry principles, is a key challenge. In this work, we investigated the role of subcritical CO2 on the hydrolysis of polyethylene terephthalate (PET) into terephthalic acid (TPA) at 180–200 °C for 10–100 min. The addition of CO2 into the reaction mixture led to the in situ formation of carbonic acid that helps to catalyze PET hydrolysis relative to hot compressed H2O (i.e. N2– H2O). The highest TPA yield of 85.0 ± 1.3% was obtained at 200 °C, PET loading of 2.5 g PET in 20 mL H2O for 100 min, and 208 psi of initial CO2 pressure. In addition, the subcritical CO2–H2O system demonstrated high selectivity toward hydrolyzing PET in a mixture with polyethylene (PE) at 200 °C for 100 min, thus providing “molecular sorting” capabilities to the recycling process. The robustness of the process was also demonstrated by the ability to hydrolyze both colored Canada Dry and transparent Pure Life® waste PET bottles into high yields of TPA (>86%) at 200 °C. In addition, subcritical CO2–H2O hydrolysis of colored PET bottles resulted in a white TPA product similar to that generated from transparent PET bottles. Overall, this work shows that, under optimized reaction conditions, subcritical CO2 can provide acid tunability to the reaction medium to favor waste PET hydrolysis for subsequent recycling. [ABSTRACT FROM AUTHOR]

Efficient recycling of polyethylene terephthalate (PET) plastics is a global concern due to the growing volume of plastic waste and its environmental impact. We studied PET hydrolysis and acidolysis processes to recover the PET monomer terephthalic acid (TPA) using various acid catalysts (zeolites, inorganic acids, ionic liquids, carboxylic acids, metal salts, and CO2) below the PET melting point and under identical conditions. TPA yield depended largely on the solution pH for some catalysts, especially aliphatic carboxylic acids, nitric acid, and CO2. However, TPA yields from hydrolysis with metal salts, ionic liquids, sulfuric acid, and aromatic carboxylic acids are also influenced by factors such as solubility limits, oxidation, and anion effects (for metal salts). Under mild hydrolysis conditions at 200 °C for 2 hours, carboxylic acids and metal salts achieved TPA yields > 80%, outperforming nitric acid, which required much more corrosive conditions at pH = 0.7. Zeolites have minimal impact on TPA yields in hydrolysis below the PET melting point. CO2 as a catalyst precursor to carbonic acid did not increase TPA yields significantly. We also explored using acetic acid as the sole reaction medium (acidolysis), which exhibited high TPA yields and a similar environmental energy impact to acid-catalyzed hydrolysis. Propanoic acid showed comparable efficiency, offering promising avenues for chemical recycling of PET. [ABSTRACT FROM AUTHOR]

Recycling spent polyesters, a widely used type of plastic, is highly important for a circular economy but remains a great challenge. Herein, biomass-derived dimethyl isosorbide (DMI) was developed as an emerging solvent for recycling waste polyethylene terephthalate (PET). DMI showed a good ability to rapidly dissolve PET in 10 min at 170 °C to obtain a solubility of 0.3 gPET gDMI -1, originating from the matched Hansen solubility parameters between PET and DMI. Moreover, the dissolved PET in DMI could be regenerated with a PET degradation ratio of only 2.9% using water as an anti-solvent. More importantly, DMIbased solvents were highly effective for alkaline (KOH) hydrolysis of PET to generate terephthalic acid (TPA). In a solvent composed of DMI and ethylene glycol (EG) with a volume ratio of 6: 4, complete conversion of PET could be achieved in 30 min at 100 °C, and the yield of TPA was nearly quantitative (99.6%). Systematic investigation revealed that the good performance of DMI/EG mixed solvents resulted from the formation of hydrogen-bonding interactions between DMI and EG. The renewability and good performance for PET recycling confirmed the great potential of DMI in practical applications of recycling waste plastics. [ABSTRACT FROM AUTHOR]

As a potential substitute for petroleum-based terephthalic acid (TPA) in the manufacture of polyesters, 2,5-furandicarboxylic acid has been obtained through several feasible routes. Herein, a new chemo-enzymatic strategy for generating furan-2,5-dicarboxylic acid (FDCA) precursors from sodium gluconate via bio-oxidation and dehydration is presented. By coupling the bio-oxidation of gluconate 5-dehydrogenase with carbonyl reductase PsCR for cofactor regeneration, 5-keto- D -gluconic acid (5KGA), a specific product of D -gluconic acid bio-oxidation and a stable intermediate, was obtained in 99% yield. The statin intermediate ethyl (S)-4-chloro-3-hydroxybutanoate was simultaneously generated as a co-product via cycling cascade catalysis. Subsequently, the yield of the FDCA precursor n-butyl-5-formyl-2-furancarboxylate (nBu-FFCA) generated via 5KGA dehydration and esterification reached 77.9%. Key intermediate characterization and theoretical calculations revealed that the decarboxylation and dehydration of cyclic 5KGA to furfural was the main side-reaction that prevented a better yield of dehydrated 5KGA. This route demonstrates good sustainability and market competitiveness compared to the existing FDCA synthesis methods. [ABSTRACT FROM AUTHOR]

With the rapid development of modern industry and the increasing scarcity of petroleum resources, bio-based polymeric materials using biomass resources as the primary raw material, with dual roles in resource conservation and low carbon, are promising alternatives to traditional petrochemicals. The efficient and low-cost preparation of terephthalic acid (TPA), 2,5-furandicarboxylic acid (FDCA) and ethylene glycol (EG), the monomers of the bio-based polyesters polyethylene terephthalate (PET) and polyethylene 2,5-furandicarboxylic acid (PEF), has attracted substantial attention. Both chemical and biocatalysis can oxidize biomass to produce bio-based monomers; however, chemical catalytic methods often require high temperatures, high pressure, and precious metal catalysts, resulting in high production costs. Compared with chemical conversion, bioconversion, with its mild reaction conditions and high selectivity, is an important development for the efficient use of biomass resources, but currently has a lower yield. In this review, we discuss and summarize strategies for the preparation of TPA, FDCA and EG and their applications, focusing mainly on biocatalytic approaches. Next, we illustrate the need to optimise the generation of efficient biocatalysts using strategies such as protein, enzyme, and metabolic engineering. Finally, the challenges and recommendations that must be addressed for each monomer's biosynthesis pathway are discussed, and an outlook for future directions and prospects is provided. This review provides theoretical guidance for developing efficient and cost-effective green manufacturing technologies for TPA, FDCA, and EG. [ABSTRACT FROM AUTHOR]

Enzymatic hydrolysis of poly(ethylene terephthalate) (PET) is a chemical recycling approach intended to enable a circular economy for polyesters. To quantitatively compare this technology to other recycling and synthesis approaches for PET, it is critical to conduct rigorous and transparent process analyses. We recently reported a detailed process model that was used to conduct economic, energy, and greenhouse gas emissions analyses for enzymatic PET recycling. Here, we expand upon this previous work by conducting a process-based life cycle assessment (LCA) of the same enzymatic hydrolysis system, to produce both terephthalic acid (TPA) and ethylene glycol (EG) for use in a closed-loop PET recycling scheme. This LCA shows that enzymatic hydrolysis currently performs 1.2 to 17 times worse than virgin TPA and PET production across most impact categories, excepting ecotoxicity and fossil fuel depletion. The top contributors to these impacts include post-consumer PET collection and processing, sodium hydroxide, and electricity. Sensitivity analysis shows that improving yields throughout the recycling process and eliminating certain process steps, such as amorphization pre-treatment and reaction pH control, can reduce the overall environmental impacts of enzymatic PET recycling to levels statistically equivalent to virgin TPA and PET production, thereby highlighting crucial areas for further research and innovation. [ABSTRACT FROM AUTHOR]

At temperatures supporting a glass transition in solid polyethylene terephthalate (PET), thermostable cutinases access the backbones of PET chains at the liquid–solid interface and hydrolyse ester bonds through endolytic and exolytic cuts. These lead to the production of three degradation intermediates [oligoethylene terephthalates (OETs), bis-hydroxyethyl terephthalate (BHET), and mono-hydroxyethyl terephthalate (MHET)] and two terminal degradation products [terephthalic acid (TPA) and ethylene glycol (EG)]. All five products are water-soluble to different degrees and diffuse away from PET to reach progressively higher concentrations in the surrounding solution, where they accumulate in proportion with the binding of cutinase to PET (through hydrophobic and cation–pi interactions). Importantly, however, the binding of cutinase to PET (which is essential for PET degradation) results in depletion of cutinase in solution, negatively impacting the conversion of degradation intermediates into terminal reaction products. The immediate and direct consequence of this is that the produced TPA and EG are contaminated by OET, BHET and MHET and, therefore, become unfit for recycling through re-condensation into PET. Here, we describe a 'two-enzyme' solution to the problem, consisting of a cocktail containing equimolar amounts of a PET-degrading enzyme [leaf-branch compost cutinase (LCC)] and a degradation intermediate-hydrolysing enzyme [Thermus thermophilus carboxylesterase (TTCE)]. The former produces OET, BHET, MHET, TPA and EG at PET's surface, while the latter works in solution to degrade the produced OET, BHET and MHET into TPA and EG. Through their synergy, the enzymes produce pure TPA and EG in yields that are 30–100% higher than those obtained with LCC alone. [ABSTRACT FROM AUTHOR]

The accumulation of polyethylene terephthalate (PET) wastes has caused severe environmental pollution and resource waste due to its widespread use. Hydrolysis of waste PET into terephthalic acid (TPA) is a promising recovery approach and can be a valuable supplement to the route starting from the petrochemical industry. However, the main roadblocks include low-efficiency degradation and tedious/ecounfriendly separation. Herein, we develop a mild non-aqueous hydrolysis for the selective ester cleavage of PET and facile product separation. This work applies ethylene glycol (EG, one of the monomers for PET polymerization) and tetrahydrofuran (THF) as the reaction solvent to obtain the hydrolysis product terephthalate in the presence of KOH. 100% of PET degradation and 97.5% of TPA yield can be achieved at 60 °C in 1 h. The addition of THF promotes mass transfer by etching of the PET surface while it makes the product auto-precipitate from the degradation solution sufficiently. Besides, THF showed an activating effect on the hydroxyl group of EG, and the ester bonds of PET were cleaved due to a synergy of potassium hydroxide and alkoxide. The degradation even took place at 30 °C. There was no decline in the degradation rate of PET after the degradation solution was recycled 5 times. The cosolvent system can also accomplish the selective degradation of PET into TPA while spandex remained unchanged for colored waste blended textiles. Besides, the system showed excellent elution properties for dyes on the textiles and made it possible to recycle complex wastes. This work provides a promising approach for the efficient and selective degradation of PET, as well as simple product separation by introducing a cosolvent and also provides a useful reference for the recycling of other waste polymers containing ester groups. [ABSTRACT FROM AUTHOR]

Common technologies for waste polyethylene terephthalate (PET) hydrolysis generally use strong acids or alkalis as catalysts; however, these processes are costly and generate a large amount of acid, alkali, and salt wastewater. In addition, these catalysts are difficult to recycle and reuse, which is not in line with the concept of sustainable development. This study describes the use of concentrated p-toluenesulfonic acid (PTSA) as an acid catalyst for PET hydrolysis under relatively mild conditions, enabling the degradation of approximately 100% of PET into 96.2% of terephthalic acid (TPA) within 90 minutes at 150 °C. A similar degradation efficiency can only be achieved at 150 °C using concentrated sulfuric acid for over 5 h. The generated TPA was easily separated from the hydrolysis system via filtration. The used PTSA was easily recovered from the filtrate after TPA collection using simple concentration and crystallization technologies. The recovered PTSA still maintained excellent catalytic efficiency for PET hydrolysis after five consecutive cycles. Furthermore, the kinetics of the reaction confirmed that the PET hydrolysis catalyzed by concentrated PTSA conforms to a first-order reaction with a relatively low apparent activation energy of 76.4–125.4 kJ mol−1. Finally, the technical feasibility and environmental impact of a scale-up were evaluated using Aspen Plus simulations. Overall, this work proposes a feasible and green strategy for the low-cost and environmentally friendly hydrolysis of waste PET. [ABSTRACT FROM AUTHOR]

2,5-Furandicarboxylic acid (FDCA) is a promising renewable building block, which can replace conventional petroleum-derived terephthalic acid (TPA). Here, we develop and evaluate a new catalytic process for the production of a renewable plastic monomer (FDCA) from lignocellulosic biomass-derived cellulose. In this process, cellulose is converted into FDCA (38.6% carbon yield) via a two-step catalytic conversion: dehydration of cellulose to 5-hydroxymethylfurfural (HMF, 42% molar yield) and oxidation of HMF to FDCA (93.6% molar yield). To effectively recover the products as well as recycle the solvent, we designed a detailed separation subsystem, which is integrated with the reaction subsystem. Importantly, to effectively minimize the required energy consumption, a heat pump is employed and heat integration is conducted. In addition, pioneer plant analysis is conducted to investigate the broad range of economic feasibility. In our techno-economic analysis, the integrated process leads to a minimum selling price of $1532 per ton of FDCA, meaning that the cellulose-derived FDCA has the potential to replace petroleum-derived TPA. In addition, sensitivity analysis reveals that the feedstock price and catalyst price are important parameters for the process. Furthermore, according to life-cycle assessment, the biomass-derived FDCA production is more environmentally favorable than the petroleum-derived TPA production. [ABSTRACT FROM AUTHOR]

The quest for green plastics calls for new routes to aromatic monomers using biomass as a feedstock. Suitable feedstock molecules and conversion pathways have already been identified for several commodity aromatics through retrosynthetic analysis. However, this approach suffers from some limitations as it targets a single molecule at a time. A more impactful approach would be to target bioprivileged molecules that are intermediates to an array of commodity and specialty chemicals along with novel compounds. Muconic acid (MA) has recently been identified as a bioprivileged intermediate as it gives access to valuable aliphatic and cyclic diacid monomers including terephthalic acid (TPA), 1,4-cyclohexanedicarboxylic acid (CHDA), and novel monounsaturated 1,4-cyclohexenedicarboxylic acids (CH1DA, CH2DA). However, accessing these cyclic monomers from MA requires to first isomerize biologically-produced cis,cis-MA to Diels–Alder active trans,trans-MA. A major impediment in this isomerization is the irreversible ring closing of MA to produce lactones. Herein, we demonstrate a green solvent-mediated isomerization using dimethyl sulfoxide and water. The mechanistic understanding achieved here elucidates the role of low concentrations of water in reducing the acidity of the system, thereby preventing the formation of lactones and improving the selectivity to trans,trans-MA from less than 5% to over 85%. Finally, a Diels–Alder reaction with trans,trans-MA is demonstrated with ethylene. The monounsaturated cyclic diacid obtained through this reaction (CH1DA) can be converted in a single step into TPA and CHDA, or can be directly copolymerized with adipic acid and hexamethylenediamine to tailor the thermal and mechanical properties of conventional Nylon 6,6. [ABSTRACT FROM AUTHOR]

The vast increase in the generation of post-consumer PET plastic waste, as well as fast increasing pledges of brand owners around the world to include recycled content have resulted in a pressing need for efficient recycling processes, such as chemical depolymerization. Although recycling rates of PET bottles are high, those of PET trays and films are still significantly lower due to the broad range of colours and multilayer structures, as well as due to a much poorer collection. In this study, a two-step aqueous alkaline hydrolysis was carried out on different types of real PET plastic waste under mild conditions (≤80 °C under atmospheric pressure). Reaction conditions such as temperature (50–80 °C), ethanol to water ratio (20–100 vol%), NaOH amount (5–15 wt%) and stirring rate (250–500 rpm) have been optimized by using pure PET pellets in order to maximize the product yield. At optimal conditions (60 : 40 vol% EtOH : H2O, 5 wt% NaOH and at 80 °C) product yields on a mass basis of approximately 95% have been achieved in less than 20 minutes. The purity of the obtained monomers, ethylene glycol (EG) and terephthalic acid (TPA), was characterized by NMR, UV-VIS and FTIR measurements. The experimental kinetic data are represented adequately using the diffusion model. Experiments performed at optimal conditions with different types of post-consumer plastic waste, revealed that the degradation rate increases inversely proportional to the particle size. Furthermore, the increased thickness of the samples and the presence of multilayers reduce the decomposition yield with a factor two as observed for monolayer (80%) versus multilayer PET trays (45%). In addition to transparent multilayer PET samples, by using the optimized alkaline hydrolysis with further cleaning processes different types of colours, including carbon black are removed from the hydrolysate successfully. A life cycle assessment (LCA) shows that the key to lower the carbon emissions is keeping the energy consumption low by increasing the solid/liquid (S/L) ratio and avoiding excess water addition during monomer purification. [ABSTRACT FROM AUTHOR]

In this study, 2-pyrone-4,6-dicarboxylic acid (PDC), a valuable monomer for bio-degradable plastics, was synthesised using terephthalic acid (TPA) derived from PET waste by a comprehensive chemo-microbial hybrid process. Depolymerisation of PET into TPA was performed via microwave-assisted hydrolysis using biomass-derived SiO2 catalysts with thiol functionalisation. The increased PET hydrolysis efficiency was investigated by reaction kinetics studies. The conversion of TPA obtained by depolymerisation of PET waste into PDC was achieved using two recombinant Escherichia coli strains expressing tphAabc and tphB genes for TPA degradation into protocatechuic acid (PCA) through 3,4-dihydroxy-cyclohexa-1,5-diene-1,4-dicarboxylic acid (DCD) (E. coli PCA strain) and using ligABC genes to convert PCA to PDC through 4-carboxy-2-hydroxymuconate semialdehyde (CHMS) (E. coli PDCPCA strain). Based on the high chemical depolymerisation of PET waste into TPA (over 97% efficiency), the double-catalyst system composed of the E. coli PCA strains with TPA degradation module and PDC synthesis module enables PET waste to be used in the PDC production platform with 96.08% of the overall efficiency. [ABSTRACT FROM AUTHOR]

Development of bio-based polyester packaging materials that adheres to the principles of green chemistry demands the simultaneous achievement of high performance and facile recycling performance but remains challenging. In this work, we fabricated a novel mica single-sheet (MSS)-modified furan-based biomimetic composite polyester (denoted as BCP), achieving strong, tough, and high-barrier films that exhibit a unique nature of aromatic ring and multi-phase architectures. BCP showed many beneficial structural characteristics: heterogeneous-induced nucleation and crystallization, multi-scale energy dissipation, strain-induced alignment and orientation, and multi-effect physical barrier effects. Benefitting from these features, the resultant BCP films showed a superior integration of high tensile strength (about 76 MPa), toughness (about 397%), and exceptional gas barrier properties (O2 0.0183 barrer, CO2 0.0244 barrer, and H2O 1.49 × 10–14 g cm cm−2 s−1 Pa−1), which are greater than those of most engineering plastics. More importantly, BCP also displayed impressive UV-shielding properties, solvent resistance, and easy physical and chemical recycling performance. Hence, the current work presents novel insights for the design and fabrication of strong, tough, high-barrier, and sustainable bio-based polyester materials that comply with the principles of green chemistry. [ABSTRACT FROM AUTHOR]

This review comprehensively explores various homogeneous and heterogeneous catalytic systems for the hydrogenolysis of oxygenated plastic waste (OXPs), presenting an adaptable solution to plastic pollution and generating valuable feedstock. Research demonstrates enhanced hydrogenolysis efficiency with reduced energy consumption, yielding alcohols, alkanes, alkenes, and aromatics. The effectiveness of depolymerization and the product distribution are influenced by factors such as solvents, ligands, metals, catalyst support, and reaction conditions. Scaling up these processes remains challenging, highlighting the need for non-toxic, highly active catalysts. Promising homogeneous catalysts, such as Ru(triphos-Xyl), and heterogeneous catalysts, such as Ru/Nb2O5, show potential in OXP depolymerization but face cost-related scalability issues. Homogeneous catalysts encounter commercialization obstacles due to harsh reaction conditions and difficulties in product separation, whereas heterogeneous catalysts like Ru/Nb2O5 provide effectiveness and stability with easier product separation. Nonetheless, challenges in scaling up, cost reduction, and catalyst reusability persist. Achieving economic viability is crucial for the commercialization of OXP hydrogenolysis and the reduction of plastic waste. The review emphasizes the shortage of depolymerization facilities for polyesters like poly(ethylene terephthalate) (PET), poly(bisphenol A carbonate) (BPA-PC), and epoxy resins (EP). It addresses recycling process challenges, focusing on sorting and supply chain issues, and identifies specific difficulties in recycling BPA-PC, PET, and EP materials, proposing chemical recycling as a viable solution to improve economic competitiveness and environmental sustainability. [ABSTRACT FROM AUTHOR]

Synthetic fibres cover most of the textile market, but their value chain is almost entirely linear. Common raw materials are non-renewable and oil-derived while requiring large amounts of (toxic) chemicals and energy for their processing into final products. In addition, synthetic textiles are usually non-biodegradable polymers; therefore, sustainable approaches for their depolymerisation into reusable monomers have not been implemented yet. As a result, most post-consumer synthetic textile waste ends up being landfilled, dispersed in the environment or incinerated, thus contributing significantly to global pollution. A possible solution to this issue is the design and use of advanced catalysts for their chemical recycling. This manuscript reviews the most significant approaches that appeared in the literature in the time span of 2015–2024, covering the selective depolymerisation process of synthetic waste textile to added-value reusable monomers using chemical catalysts. Unselective processes, for example, to produce fuel mixtures, biocatalytic methods and depolymerisation of polyolefins are not covered. The general aspects of the catalytic depolymerisation of synthetic polymers are briefly discussed, and the catalytic chemical recycling of synthetic textiles is detailed by the polymer type. While contributing to the overall achievement of the sustainable development goals, chemical recycling of synthetic textile waste may represent a useful strategy toward the circularity of the textile sector, which is almost unexplored. [ABSTRACT FROM AUTHOR]

The contemporary world faces issues related to energy, the environment, and food security. The use of carbon capture, storage, and utilization technologies can help reduce CO2 emissions from fossil fuels, which will result in major advancements toward dual carbon targets. In addition to promoting environmentally friendly manufacturing, chemical industries may replace fossil fuel-based raw materials with renewable biomass for the synthesis of organic acids and syngas. Although several studies are being conducted on co-valorization of CO2 and biomass feedstocks to produce organic acids and fine chemicals using biotechnology, thermocatalysis, electrocatalysis, and photocatalysis, there are still various obstacles in scaling up clean production, including (i) addressing environmental concerns, (ii) the intricate structure and chemical composition of biomass, (iii) conversion mechanisms and processes, (iv) designing catalyst materials with higher durability and recyclability, (v) greener solvent systems for catalysis and extraction, (vi) the deployment of modern technologies for characterization, (vii) training and guidelines for industrial operations, and (viii) governmental financing and policy. The sustainable manufacturing of biobased products from raw feedstocks produced from biomass has been made possible via technological breakthroughs in photo-/biorefineries, which are essential for the clean and environmentally friendly synthesis of organic acids. It is anticipated that clean production of organic acids from biomass will have a dominant market share, benefiting from both socioeconomic and environmental factors. With future technical developments, the valorization of feedstocks obtained from biomass together with CO2 for manufacturing fuels and fine chemicals will be more ecologically and economically feasible. [ABSTRACT FROM AUTHOR]

Plastic material performance is strongly correlated to the polymer's molecular weight. Obtaining a sufficiently high molecular weight is therefore a key goal of polymerization processes. The most important polyester polyethylene terephthalate (PET) and the new polyethylene furanoate (PEF) require metal catalysts and time-consuming production processes to reach sufficiently high molecular weights. Metal catalysts, which are typically antimony or tin for polyesters, end up in the plastic products which may result in sustainability and ecological challenges. When the less reactive comonomer isosorbide is introduced to produce (partly) biobased materials with enhanced thermal properties, such as polyethylene-co-isosorbide furanoate (PEIF), reaching high enough molecular weight becomes even more challenging. This study presents an easily implementable approach to produce high molecular weight PET and PEF polyesters and their isosorbide copolyesters PEIT and PEIF by coupling lower molecular weight polymer chains by the reactive diguaiacyl oxalate (DGO) chain extender. DGO is so reactive, that the use of metal catalysts can be completely avoided and it helps avoiding an extra solid-state polymerization step. In addition, DGO distinguishes itself from typical chain extenders by its ability to be completely removed from the resulting polymer, thereby avoiding the inherent drawbacks associated with typical chain extenders. [ABSTRACT FROM AUTHOR]

Due to the increasing demand for fossil fuel resources in modern society, attention is turning towards alternative sources. This paper firstly introduces the importance of the oxidation reaction of 5-hydroxymethylfurfural (HMF) and its widespread application in the field of biomass conversion. However, precise control over the selective oxidation of biomass-derived platform chemicals remains challenging, necessitating in-depth investigation into the mechanism of this oxidation process. Subsequently, the mechanism of the HMF oxidation reaction is discussed in detail, including the design and performance optimization of both traditional and novel catalysts, aiming to provide theoretical guidance and technical support for efficient and selective HMF oxidation. In the field of photocatalysis, strategies such as the introduction of photoresponsive catalysts, surface modification, and synergistic catalysis have been employed to enhance reaction rates and selectivity. In electrocatalysis, efficient conversion of HMF has been achieved through the modulation of catalyst structure and active sites. Meanwhile, photoelectrocatalysis hybrid systems, as emerging technologies integrating the advantages of both photocatalysis and electrocatalysis, demonstrate promising application prospects, with an overview of their research in HMF oxidation provided herein. Furthermore, the paper discusses the challenges faced by current selective HMF oxidation, including catalyst stability, selectivity, and product distribution, and proposes future research directions and prospects, including the design of multifunctional catalysts, optimization of reaction conditions, and in-depth exploration of catalytic mechanisms, to provide important references for achieving efficient biomass conversion. In summary, this paper systematically summarizes the latest research progress in selective photocatalysis, electrocatalysis, and photoelectrocatalysis for HMF oxidation, and provides prospects for future development, aiming to offer references and insights for relevant research fields. [ABSTRACT FROM AUTHOR]

The pervasive effects of plastic waste pollution affect both humanity and the environment, thus innovative and environmentally-friendly methods for recycling of plastics are crucially needed. The application of light to degrade or transform plastics into valuable products has gained significant attention. Numerous researchers have explored irradiation to achieve the photocatalytic breakdown of highly resilient plastic waste components into valuable monomers, which can be utilized for the synthesis of novel materials of synthetic or pharmacological interest. Many of these techniques have resulted in H2 evolution, while efforts were also made to reduce carbon emissions. In some cases, light was combined with additional energy sources, leading to development of photothermal or photoelectrochemical processes. With this tutorial review, our aim is to offer an overview of these novel photochemical upcycling protocols for the degradation of polymers, aspiring toward the introduction of novel processes in the near future. [ABSTRACT FROM AUTHOR]

Diels–Alder cycloaddition of biomass-derived 2-pyrone coumalic acid (CMA) with propylene provides an alternative pathway to produce toluic acid (TA), a precursor to terephthalic acid (TPA) which is a key component in the manufacture of polyethylene terephthalate (PET). To maximize yield and selectivity of the preferred isomer p-toluic acid (p-TA) we herein report a detailed reaction network that was established through investigating the influence of the dehydrogenation catalyst, solvent, temperature and reaction time on the formation of p-/m-TA. We further provide experimental results showing that the inverse electron demand Diels–Alder (IEDDA)/decarboxylation cascade of the reaction of CMA with propylene was not influenced by the catalyst, and, therefore, was solely dependent on temperature. Evident from changes in the p-/m-ratio of TA decreasing from 6.91 to 4.43 with increasing temperature and reaction time, p-TA was both the thermodynamic and kinetically favoured product. TA formation from CMA and propylene afforded an overall yield of ＞85 mol%, using γ-valerolactone (GVL) as the primary solvent. Throughout the reaction network analysis we observed that the TA intermediates 3-/4-methylcyclohexa-1,5-diene carboxylic acid were reactive towards by-product formation whereas TA was quite stable under reaction conditions. Henceforth, rapid dehydrogenation of the diene intermediates is crucial to maximize selectivity of the desired aromatic product. Lastly, CMA thermally decomposes under reaction conditions which was rapidly accelerated by having as low as 5 vol% water in the solvent, demonstrating the importance of water-free solvent to limit CMA decomposition and maximize product yield. [ABSTRACT FROM AUTHOR]

Amongst all synthetic polymers used in the clothing industry, polyethylene terephthalate (PET) is the most widely used polyester, its fibres representing half the total PET global market (in comparison bottle PET being less than a third). Compared to bottle PET, the recycling of fabric PET fibres represents a challenge, both due to intrinsic structural differences (chain length and crystallinity) and to the presence of various additives (dyes, protection or finishing agents). Effective waste management requires addressing these additives through elimination or recycling processes. This review article aims to give an overview about all the existing means to recycle PET fibres. Textile recycling encompasses primary (closed-loop), secondary (mechanical), tertiary (chemical), and quaternary (incineration with energy recovery) processes. Mechanical recycling faces challenges due to PET's characteristics, including lower molecular weight and additives. Chemical recycling, particularly solvolysis processes (hydrolysis in neutral, acidic, or alkaline media, alcoholysis, glycolysis, aminolysis or enzymatic hydrolysis), offers a more advanced approach and will be described in detail, focusing both on the specific recycling of fibres when available and enlightening the advantages and drawbacks of each method. To discuss the environmental impact of each process, a quantitative analysis was conducted by defining the experimental domain represented by the temperature range and reaction time, and then calculating the energy-saving coefficient, as a green metric adapted to the diversity of textile PET recycling processes and data provided in the literature. This coefficient allows for discussing the relevance of using complex or non-renewable catalysts in processes, the positioning of enzymatic pathways, and the choice of reaction mechanisms applicable to the industry. A prospective approach was employed to identify key criteria for future advancements in green recycling. Subsequently, a comparative analysis of depolymerisation methods will be presented within the context of sustainable development goals (SDGs), green chemistry, and green metrics. Finally, using ε factors, this analysis will facilitate the detection and highlighting of pathways that show the most promise in terms of greening PET recycling. [ABSTRACT FROM AUTHOR]

Plastic pollution has been a topic of grave concern worldwide due to major environmental, economic, health, and social impacts. Nevertheless, our society is far from casting plastics aside since they are essential for several applications (e.g. food packaging, automotive, electronics, building, and construction). Poly(terephtalate ethylene) (PET), an important synthetic polyester derived from petroleum, is extensively used in plastic packaging (mainly bottles and trays). Unfortunately, no greener polymer alternative has large-scale production to meet global demand. Consequently, plastic recycling is the most sustainable approach to manage the massive amounts of post-consumed PET materials, by recovering its monomeric units: terephthalic acid and ethylene glycol. However, current recycling methods are still costly, leading to more expensive reclaimed PET monomers than virgin ones obtained from fossil sources. Recently, PET upcycling emerged as a profitable strategy to value the reclaimed monomers by converting them into molecules with higher added value, enhancing the recycling process viability. In this review, we addressed all valuable commodities produced from the conversion of PET monomers catalyzed by microorganisms, as an eco-friendly process known as biological upcycling or bio-upcycling. We provided an outlook regarding potential markets and highlighted the benefits of the PET bio-upcycling route over the actual industrial production of each commodity. Moreover, we discussed different PET recycling methods employed to recover monomers prior to the bio-upcycling step, highlighting the pros and cons. Additionally, we addressed critical considerations about the major challenges regarding this novel recycling strategy, particularly in upscaling from lab-scale stage to real-world implementation. [ABSTRACT FROM AUTHOR]

The production of renewable chemicals and monomers is fundamental for transitioning to a future circular economy. Currently, cis,cis-muconic acid (ccMA) is a bio-sourced platform chemical with great potential for added-value chemicals, monomers, and specialty polymers. Among the three isomers, the trans,trans (tt-isomer) stands out due to its reactivity for polymerization and unique ability as a substrate for the Diels–Alder cycloaddition reaction. Whereas earlier research has focused on producing this isomer, the most promising solvent-driven isomerization in DMSO-containing water yields moderate ttMA due to a competitive ring-closing lactonization reaction, especially in highly concentrated systems. This work highlights the unique ability of DMSO, among several other solvents, to produce ttMA. In addition, we report the effect of the acidity of the initial MA concentration and the amount of water on the lactonization reaction. Control of reaction conditions and use of muconates (diethyl muconates = DEM) countered the competitive lactonization, reaching >90% tt-isomer selectivity. The involvement of water and DMSO in the isomerization mechanism was investigated in detail by probing the reaction mechanism with in situ NMR. Identifying the reaction products and several intermediates led us to propose a plausible mechanism. Based on this knowledge, condition optimization led to a significant thirty-fold ttDEM productivity improvement, viz. from 10 to 328 mM h−1. The DEM can be isolated almost quantitatively from the DMSO solvent system by extraction. [ABSTRACT FROM AUTHOR]

The global plasticizer market is projected to increase from $17 billion in 2022 to $22.5 billion in 2027. Various emerging/alternative plasticizers entered the market following the ban on several phthalate plasticizers because of their harmful effects. However, there are limited data (especially peer-reviewed) on emerging plasticizers' toxicity and environmental impact. This review compiles available data on toxicity, exposure, environmental effects, and safe production of emerging plasticizers. It identifies gaps in scientific research and provides evidence that emerging plasticizers are potential cases of regrettable substitution. Several alternative plasticizers, such as acetyl tributyl citrate (ATBC), diisononyl cyclohexane-1,2 dicarboxylate (DINCH), tris-2-ethylhexyl phosphate (TEHP), tricresyl phosphate (TCP), tris-2-ethylhexyl phosphate (TPHP), bis-2-ethylhexyl terephthalate (DEHT), and tris-2-ethylhexyl trimellitate (TOTM), show potential endocrine-disrupting properties and other toxic characteristics. Some chemicals like bis-2-ethylhexyl adipate (DEHA), diisobutyl adipate (DIBA), ATBC, DINCH, bis-2-ethylhexyl sebacate (DOS), diethylene glycol dibenzoate (DEGDB), DEHT, and phosphate esters showed the potential to cause toxicity in aquatic species. Plus, there is great lack of information on compounds like diisononyl adipate (DINA), dibutyl adipate (DBA), diisodecyl adipate (DIDA), dipropylene glycol dibenzoate (DPGDB), dibutyl sebacate (DBS), alkylsulfonic phenyl ester (ASE), trimethyl pentanyl diisobutyrate (TXIB), DEGDB and bis-2-ethylhexyl sebacate (DOS). Some compounds like epoxidized soybean oil (ESBO), castor-oil-mono-hydrogenated acetate (COMGHA), and glycerin triacetate (GTA) are potentially safer or less toxic. Alternative plasticizers such as adipates (log Kow 4.3–10.1), cyclohexane dicarboxylic acids (log Kow 10), phosphate esters (log Kow 2.7–9.5), sebacates (log Kow 6.3–10.1), terephthalates (log Kow 8.4), and vegetable oil derivatives (log Kow 6.4–14.8) have log Kow values that are comparable to phthalate plasticizers (log Kow 7.5–10.4), indicating potential bioaccumulation and health consequences. Field studies have demonstrated that phosphate esters can undergo bioaccumulation and biomagnification, but there is a lack of bioaccumulation studies for other compounds. We also discuss the metabolism of emerging plasticizers, though data are limited. Our article highlights that numerous alternative compounds display potential health and ecological risks, indicating they might not be suitable substitutes for legacy plasticizers. There is also a lack of scientific data on most emerging plasticizers. This way, we call for increased research and timely regulatory action to prevent global contamination and health risks. Finally, this study presents a scientifically robust protocol to avoid harmful substitutions and ensure the production of safer chemicals. [ABSTRACT FROM AUTHOR]

2,5-Furan dicarboxylic acid (FDCA) is one of the top-12 value-added chemicals derived from biomass that may serve as a ‘green’ substitute for terephthalic acid (TPA) in polyesters. FDCA can be synthesized chemically from 5-(hydroxymethyl)furfural (HMF), which is produced from fructose or glucose. To investigate the impact of the production chain of FDCA on the terrestrial ecosystem and unravel molecular pathways affected in animals, an ecotoxicogenomic study was performed to measure the transcriptome-wide response in soil invertebrates, namely Folsomia candida. First, we show that FDCA, HMF and TPA are biodegradable in natural soil. However, these chemicals can exert severe impact on the reproduction of F. candida in sterilized soil. Transcriptional changes were examined at EC50 concentrations of FDCA, HMF and TPA spiked in sterilized LUFA 2.2 soils. The results indicate that FDCA and TPA cause no significant changes in gene expression, which may be due to the low chemical water solubility and therefore slow uptake from the pore water. In contrast, a substantial number of genes were significantly regulated in F. candida after exposure to HMF. Gene ontology analysis showed many biological processes to be significantly affected, such as nucleic acid metabolism, the transcriptional metabolic process, cell developmental process and oxidation–reduction process. Moreover, the transcriptional profiles suggest that HMF might be biotransformed by F. candida into 5-sulfoxymethylfurfural (SMF) which is genotoxic and mutagenic. Current research shows that the environmental risk of the FDCA production chain from biomass is relatively low, but may only be affected by the release of the intermediate HMF compound. [ABSTRACT FROM AUTHOR]

Efficient recycling and upcycling strategies to retain the material in the economy and away from the ecosystems are important to achieve a sustainable plastic system. Poly(butylene adipate-co-terephthalate) (PBAT) is a biodegradable polyester that has gained considerable interest for various applications. Here, we report a study on enzymatic depolymerization of PBAT, recovery and purification of its monomers, and feasible routes for their recycling/upcycling. PBAT films (15 g L-1) were completely hydrolysed employing a leaf-branch compost cutinase variant (LCC-WCCG, 1.4 mg per gram polymer) to its monomers at a rate of 0.49 g L-1 h-1. LCC-WCCG kinetics were superior to that of other enzymes engineered for PBAT hydrolysis; the data were supported by in silico investigations. The released monomers were separated using membrane filtration and precipitation techniques and recovered with purity exceeding 95%. To close the loop, the monomers were re-polymerized and successfully cast into PBAT films. Moreover, adipic acid was reacted with hexamethylene diamine using Novozym®435 to form a polyamide, while 1,4-butanediol was oxidized to 4-hydroxybutyrate using Gluconobacter oxydans cells. The current study exemplifies a high-impact scientific approach toward a circular plastics economy. [ABSTRACT FROM AUTHOR]

To reduce global CO2 emissions in line with EU targets, it is essential that we replace fossil-derived plastics with renewable alternatives. This provides an opportunity to develop novel plastics with improved design features, such as better reusability, recyclability, and environmental biodegradability. Although recycling and reuse of plastics is favoured, this relies heavily on the infrastructure of waste management, which is not consistently advanced on a worldwide scale. Furthermore, today's bulk polyolefin plastics are inherently unsuitable for closed-loop recycling, but the introduction of plastics with enhanced biodegradability could help to combat issues with plastic accumulation, especially for packaging applications. It is also important to recognise that plastics enter the environment through littering, even where the best waste-collection infrastructure is in place. This causes endless environmental accumulation when the plastics are non-(bio)degradable. Biodegradability depends heavily on circumstances; some biodegradable polymers degrade rapidly under tropical conditions in soil, but they may not also degrade at the bottom of the sea. Biodegradable polyesters are theoretically recyclable, and even if mechanical recycling is difficult, they can be broken down to their monomers by hydrolysis for subsequent purification and re-polymerisation. Additionally, both the physical properties and the biodegradability of polyesters are tuneable by varying their building blocks. The relationship between the (chemical) structures/compositions (aromatic, branched, linear, polar/apolar monomers; monomer chain length) and biodegradation/hydrolysis of polyesters is discussed here in the context of the design of biodegradable polyesters. [ABSTRACT FROM AUTHOR]

Biodegradable plastics are considered as an alternative to commodity non-biodegradable plastics, but they still degrade slowly in the environment. Current strategies to promote their degradation rates often compromise their utility. Here, we enhance the degradation properties of poly(butylene terephthalate/adipate) (PBAT) by embedding an esterase mimic inside the polyester without compromising its processing and mechanical properties. This esterase mimic contains a binuclear zinc site and catalyzes the depolymerization of PBAT with a turnover frequency (TOF) an order of magnitude higher than that of conventional additives (ZnO). With 1 wt% of this enzyme mimic embedded, PBAT exhibits a 3.7-fold enhancement of the degradation rate under composting conditions. Mechanistic investigation indicates that the binuclear zinc binding center polarizes the ester carbonyl group and increases hydroxide uptake, thus leading to faster degradation. This work provides a possible approach to designing polymers that meet requirements of stability during use and fast degradation after use. [ABSTRACT FROM AUTHOR]

Ideonella sakaiensis PETase (IsPETase) is a unique polyethylene terephthalate (PET) hydrolase that displays great potential for mitigating PET waste at moderate temperatures. Although IsPETase exhibits greater specific activity towards PET, rapid activity loss limits its commercial application. Herein, semi-saturation mutation was carried out to stabilize the most flexible region in IsPETase and a thermostable S92P/D157A variant with a Tm value of 70.8 °C (ΔTm = 24.1 °C) was constructed, which enabled a 109.3-fold increase in products released from amorphous PET depolymerization at 40 °C. The further depolymerization of untreated post-consumer PET obtained 17.34 mM products (95.0% TPA). The crystal structure indicated that the "wobbling" W156 residue in IsPETaseWT was fixed in the substrate-binding conformation in the S92P/D157A variant, which contributed to an increase in thermostability and could provide steady interaction with the substrate. Previous studies emphasized that the W156 residue exhibiting a "wobbling effect" is critical to substrate binding, while the different structure–function relationship of the S92P/D157A variant indicated that the wobbling of W156 may not be a pre-requisite for efficient PET biodegradation. Instead, engineering PET hydrolases for fixing the conserved W156 residue in the substrate-binding conformation was proposed. The S92P/D157A variant exhibited a preference for PET in the gauche conformation, which is consistent with its efficiency towards the degradation of low crystallinity PET, further cementing the importance of conformation selection and providing complementary evidence for the function of the fixed W156 residue in PET binding and catalytic processes. Collectively, our results could contribute to the understanding and engineering of more effective PET hydrolases, promoting the industrial application of the enzymatic PET recycling process. [ABSTRACT FROM AUTHOR]

Incorporating hydrolytically sensitive functional groups into polymer backbones provides a feasible strategy to trigger their degradation to the starting monomers, thus enabling chemical recycling of the material. Here, we present two series of copolyesters in which a biobased spirocyclic ketal-functional diester monomer was incorporated into poly(butylene 2,5-furandicarboxylate) (PBLF) and poly(hexamethylene 2,5-furandicarboxylate) (PHLF), respectively. A two-step melt polycondensation resulted in copolyesters with moderate to high molecular weights, as confirmed by intrinsic viscosity and size exclusion chromatography data. Thermogravimetric analysis showed a thermal stability up to 275 °C, and increasing char yields upon incorporation of the spirocyclic monomer. The crystallinity and melting points of the copolyesters decreased with an increasing content of the spirocyclic ketal units in the backbone. Copolyesters containing up to 15% of the spiro-ketal units were semicrystalline, while those containing 20 and 50% spiro-ketal units were completely amorphous. The hydrolytic degradation of copolyesters from the PHLF series was investigated using 3–12 M aq. HCl, and were found to degrade faster than the corresponding homopolyesters. Acid-catalyzed cleavage of the randomly distributed spiro ketal units promoted the rapid fragmentation of the polymer chain into small oligomers, which were subsequently hydrolyzed to the original chemical building blocks. The ketone-terminated telechelic oligomers obtained after the degradation of spirocyclic ketal units were also investigated in a direct polymerization with pentaerythritol. The initial results implied that the oligomers can be re-polymerized into the original polymer. Hence, this work demonstrated a feasible pathway towards chemically recyclable 2,5-furandicarboxylate polyesters with a tuneable degree of crystallinity. [ABSTRACT FROM AUTHOR]

Diverse functionalized aromatic compounds are constructed from captodative dienophiles with exclusive regioselectivity. 100% biorenewable dimethyl terephthalate (DMT) from methyl coumalate and methyl pyruvate is achieved in a one-pot, Diels-Alder/ decarboxylation/elimination sequence in nearly quantitative yield. The DMT system is solvent-free and purification is accomplished through recrystallization. DMT hydrolysis reveals the co-monomer terephthalic acid (TPA) as a bio-based drop-in replacement for the polymer industry, avoiding harsh oxidation and petrochemicals. [ABSTRACT FROM AUTHOR]

The article focuses on liquid phase oxidation of p-xylene to terephthalic acid (TPA) at medium-high temperatures catalyzed by cobalt/Manganese/Bromine carried out in carbon dioxide-expanded solvents. It explores that the amount of yellow colored side-product is decreased using the above reaction, as confirmed by spectroscopic analysis. It adds that versatility of this reaction in homogeneous catalytic oxidations can be increased by maximizing the utilization of carbon feedstock.

Plastic waste is globally ubiquitous and ecologically harmful, but it can be recycled as an abundant carbon source to alleviate worldwide heavy dependence on fossil resources and reduce CO2 emissions. Therefore, research into the chemical recycling of plastic waste has become a critical and pressing area. Compared with polyolefins, polyesters, as represented by PET and PLA, can easily achieve selective depolymerization to their corresponding monomers due to the presence of weaker ester bonds, thus favoring their closed-loop recycling and upcycling. However, comprehensive reviews on this important topic remain scarce, especially from the standpoint of re/upcycling. In this review, we present significant progress in the catalytic depolymerization of different polyesters, including biodegradable polyesters and nonbiodegradable polyesters, and discuss the key factors that limit the efficacies of the different methods and formidable challenges towards closed-loop recycling and upcycling. Such insightful discussion may benefit the further development of advanced strategies to address the problems with the increasing polyester plastic wastes and stimulate their efficient recycling to value-added chemicals and materials. [ABSTRACT FROM AUTHOR]

The selective depolymerization of PET plastics to p-xylene (xylene) is very challenging owing to the full deoxygenation of the terephthalic acid monomer and the retainment of the benzene ring through saturation. In this study, the modification of Co–Al catalysts by introducing a second metal (Fe, Cu, Ni, or Zn) was conducted to tailor the activity of the cobalt species for the conversion of PET to xylene. The results indicated that the CoFe alloy in the Co–Fe–Al catalyst weakened the adsorption/activation of H2, achieving xylene yields of >99.0% from PET. Co–Fe–Al suppressed the ring hydrogenation of the benzene ring, while Co–Al and Co–Ni–Al catalyzed the hydrogenation of the benzene ring and cleavage of the C–C bond in the intermediates. Benzene ring-containing intermediates showed a high affinity for Co–Ni–Al and Co–Al, facilitating their hydrogenation. Co–Fe–Al could preferably adsorb and activate the oxygen-containing functionalities of the intermediates for the deoxygenation reactions. The hydrogenolysis reaction of C–OH in intermediates, like 1,4-benzenedimethanol (Ea: 137.8 kJ mol−1), was the rate-determining step, while the hydrogenation of the carboxylic intermediates was much easier. [ABSTRACT FROM AUTHOR]

The improper disposal of plastic waste raises significant concerns due to its severe threat to ecosystems and public health. Artificial photosynthesis, compared with the conventional strategies (e.g., pyrolysis) operated under harsh conditions, has emerged as a green and sustainable approach to convert plastic waste into commodity chemicals under ambient conditions. In this study, we have developed a partially oxidized CuIn5S8 nanosheet photocatalyst (O-CuIn5S8) to promote the conversion of polyethylene terephthalate (PET) waste into value-added chemicals while producing hydrogen fuel. After oxygen incorporation, the as-obtained O-CuIn5S8 catalyst demonstrates significantly improved activity, with a hydrogen evolution rate of 2.57 ± 0.02 mmol g−1 h−1, which is 7 times higher than that of the pristine CuIn5S8. Furthermore, O-CuIn5S8 allows an effective transformation of pretreated PET into a range of valuable commodity chemicals, including formate, acetate, and glycolate, via a hole-induced oxidation mechanism. This work paves the way for the rational development of photocatalysts to manage plastic pollution and meanwhile reclaim the carbon source within PET plastic waste. [ABSTRACT FROM AUTHOR]

While modern research into biodegradable and renewable plastics has expanded to cover a wide variety of applications, some characteristics, such as desirable elastic properties, are still difficult to achieve without the use of petroleum-based monomers. In addition, 100% biomass-based polymers are in high demand as environmental regulations increasingly require the use of biomass-based monomers. In this study, biomass-derived monomers, such as adipic acid (AA), 2,5-furandicarboxylic acid (FDCA), and 1,3-propanediol (PDO), were used to synthesize terpolyesters with elastic properties. The unsymmetrical structure of FDCA led to a spiral motif that biodegrades efficiently, and its ring structure also plays an important role as a hard elastomer domain. Applying the "odd–even effect" of PDO resulted in very high biodegradation rates, low melting points, and low degrees of crystallinity. An FDCA : AA ratio of 7 : 3 led to an elongation of around 600% and a recovery rate of 89% (to the original state) at a fixed PDO concentration. [ABSTRACT FROM AUTHOR]

To resolve white plastic pollution and the human carbon footprint, we report an efficient green approach to quantitatively convert polyethylene terephthalate (PET) to high-density C6–C8 cyclic hydrocarbons via single-step water-assisted hydrodeoxygenation (HDO) over a Ru/TiO2 catalyst in a biphasic system. To understand the structure–activity relationships of supported metal catalysts in PET HDO, acidic and neutral support-loaded Ru catalysts were prepared, characterized, and evaluated in mono- and biphasic systems. An experimental study revealed that Ru/TiO2 effectively catalyzed the production of C6–C8 cyclic hydrocarbons from PET. Under mild conditions (220 °C, 50 bar H2, 12 h), the catalyst completely converted PET into fuel-range C6–C8 cyclic hydrocarbons at a remarkably high selectivity of 96.8–99.2%. The catalyst successfully cleaved all C–O bonds in the aromatic plastic in a water–dodecane (O/W) biphasic system via the favorable effects of the metal-acid bifunction, hydrophilicity, and high dispersion of small Ru particles on the support. Due to its hydrophilic behavior, the nanosized catalyst formed stable Pickering emulsion droplets in the biphasic system. The presence of these droplets increased the selectivity towards C6–C8 hydrocarbons by forming a reaction interface with a large area between the oil and water layers. This improves accessibility to the active sites of the catalyst, stabilizes the intermediates, and enhances mass transfer and the separation of the target products. Furthermore, using a model compound study, we identified the pathway involved in the HDO reaction. Moreover, the addition of water to the biphasic system significantly increased the rate of proton transfer on the catalyst support. This study presents an excellent single-step catalytic pathway for application in synthesizing C6–C8 cyclic hydrocarbons using PET waste in a water-assisted biphasic system, providing a viable solution for plastic recycling. [ABSTRACT FROM AUTHOR]

Polyethylene terephthalate (PET) can be recovered as high-purity bis(2-hydroxyethyl terephthalate) (BHET) monomer by glycolysis in the presence of Fe3O4 nanoparticles (NPs). In this study, Fe3O4 NPs of various shapes, sizes, and surface areas were synthesized using different colloidal synthesis methods, and the conversion of PET glycolysis and BHET yield were compared. Spinel ferrite NPs, including Fe3O4, were synthesized using the coprecipitation (CP), thermal decomposition (TD), and the hydrothermal (H) methods. Among the NP catalysts, Fe3O4-CP exhibited the best glycolysis performance with a PET conversion of ∼100% and BHET yield of 93.5% at 195 °C for 2 h owing to its high surface area (146.6 m2 g−1). The larger the surface area and the better the dispersion, the higher the glycolysis activity. The glycolysis performance of the mixed spinel ferrite NPs was similar to that of the Fe3O4 NPs, indicating that replacing Fe2+ in the Fe3O4 NPs with other transition metals, M2+, did not significantly change the glycolysis performance. BHET monomers produced from commercial waste PET bottles in large quantities contained trace amounts of metal contaminants, because PET production uses various metal-based additives and catalysts. Amberlite IRC-120, a cation-exchange resin, effectively removed metal impurities from BHET. This study provides an effective strategy for producing recycled PET (r-PET) by waste PET glycolysis. [ABSTRACT FROM AUTHOR]

Plastic depolymerization usually requires rigorous conditions such as high temperature, high pressure, and caustic bases or acids, resulting in high energy costs and environmental issues. Recently, new technologies that operate under mild conditions are emerging. This review summarizes the progress made in the past decade in the conversion of waste plastics into high-purity monomers or value-added products under mild conditions by bio-, photo-, electro-, and low-temperature thermocatalysis. First, the enzymatic depolymerization of plastics is discussed, with emphasis on the progress in enzyme engineering to improve the thermal stability of enzymes. Next, electro- and photocatalytic approaches for the conversion of waste plastics into high value-added fuels and chemicals at room temperature are presented. Specifically, the status and problems of upgrading plastics via electrocatalysis and photoreforming are described, and the photocatalytic oxidation of plastics to chemical feedstocks using oxygen or air as the oxidant is also highlighted. Then, the recent progress in low-temperature thermocatalysis for plastic recycling and upgrading is surveyed. Finally, the challenges and opportunities of catalytic recycling and upgrading of waste plastics under mild conditions are discussed in terms of the catalyst rational design, catalytic system optimization and scalability, and economic feasibility. [ABSTRACT FROM AUTHOR]

Since its origin, green chemistry has headed the best-guiding philosophy for reducing pollution and safeguarding the environment. The twelve Principles of Green Chemistry provide us with enough tools to design sustainable transformations and implement industrial processes by means of renewable feedstocks. All these processes, to avoid or minimise waste production, require not only selective and efficient catalytic transformations with high atom and energy efficiency but also clean separation processes, and the use of non-toxic and safe products. Biocatalysts synthesized by the green chemistry and circular economy principles can constitute the most important and efficient strategy for achieving many of the 17 Sustainable Development Goals set by the United Nations. This will drive our society to a sustainable future by reducing the consumption of resources and drastically minimising the environmental impact of our recalcitrant wastes. This perspective illustrates by a series of selected examples how green and circular chemistry based on biocatalytic processes can pave synergies for sustainable development. [ABSTRACT FROM AUTHOR]

The use of enzymes in biochemical processes is of interest due to their ability to work under mild conditions while attaining high reaction rates. A limitation in the use of enzymes such as oxidoreductases on a large scale lies with their requirement for costly cofactors, e.g. NAD+, in stoichiometric quantities. Cofactor regeneration mechanisms using bienzymatic recycling systems is an attractive way to increase productivity and efficiency. The thermophilic enzyme aldehyde dehydrogenase (ALDHTt) was immobilized directly from E. coli cell lysate, containing the expressed enzyme, onto Ni2+ activated Sepharose®. The system displayed a rate of conversion of approx. 63% NAD+ with reuse achievable for up to 5 cycles and residual activity of the enzyme upon storage of 93% after 7 days. L -Lactate dehydrogenase was immobilized in a second reactor module downstream of ALDHTtvia two different methods, electrochemical entrapment in poly(3,4-ethylenedioxypyrrole) (PEDOP) and covalent attachment on glyoxyl agarose. Both reactors allowed for up to 100% conversion of NADH, however LDH@agarose proved superior in terms of reuse and storage. LDH@agarose displayed no reduction in activity after 6 cycles of use and retained 98% activity following 56 days storage. A coupled reactor containing immobilized ALDHTt–LDH was operated with the substrates hexanal, benzaldehyde, terephthalaldehyde and p-tolualdehyde. A particular advantage of the system is its ability to preferentially oxidise a single aldehyde group in substrates containing two aldehyde functional groups. The reactor demonstrated efficient cofactor regeneration under continual operation for up 24 h, with enhanced product yields. [ABSTRACT FROM AUTHOR]

Chemical recycling of polymers to monomers and chemicals is a promising pathway to valorize plastic waste that cannot be mechanically recycled, thus potentially minimizing resource consumption and the overall CO2 impact of the polymer industry. Among chemical recycling technologies, solvolytic depolymerization of poly(ethylene terephthalate) stands out as a selective process that maximizes monomer recovery. However, many challenges still remain regarding the optimization of these recycling technologies. Addressing these challenges could lead to these technologies becoming truly environmentally advantageous compared to alternative waste management solutions. Subcritical water has proven to be an outstanding media for a broad variety of reactions and its potential as a green solvent for PET depolymerization is reassessed based on a new nuclear magnetic resonance quantification method allowing for product characterization to a degree never reported before. In order to study the intrinsic product composition at every reaction condition (280 to 340 °C and 0 to 45 min reaction time), depolymerization experiments were performed in agitated micro-batch reactors, and NMR analysis was conducted prior to any alkaline-based purification. The highest recovery of terephthalic acid was achieved after 45 min at 340 °C, however, under these conditions ethylene glycol experiences a high degree of degradation. Collected data was then used to compare the environmental performance of different case scenarios leading to the preferable conditions to be 5 min at 310 °C, where the recovery of terephthalic acid, ethylene glycol, mono(2-hydroxyethyl) terephthalate and bis(2-hydroxyethyl) terephthalate is as high as 0.9 g g−1 PET. [ABSTRACT FROM AUTHOR]

Lignin represents the most abundant renewable aromatic resource, monophenolics, from which would serve as promising monomers for the synthesis of aromatic polymers in a sustainable manner. Hererin, we report that hydroxycinnamic derivatives, i.e., ethyl coumarate/ferulate (pCA-Et and FA-Et), with a conjugate and rigid structure can be selectively fragmented from herbaceous plant lignin over a ZnMoO4/MCM-41 catalyst, which upon a one-pot fluorosulfonation, carboxylation, and esterification, give aromatic diesters in high yields. Copolymerizing lignin-derived dicarboxylic species with holocellulose-derived butanediol and adipic acid results in a new family of aliphatic–aromatic polyesters, which display controllable mechanical, optical, and thermal performance, especially depending on the methoxyl group in lignin-derived units. The as-prepared copolyesters can be fully converted into monomers via ethanolysis at 80 °C, demonstrating excellent chemical recyclability. Overall, we provide a novel upstream and downstream route to transform sustainable biomass into functional polymers. [ABSTRACT FROM AUTHOR]

Given the current environmental crisis caused by polymers, we have seen that it is mandatory to make changes in the way we produce, consume, and, at the downstream, dispose, and manage polymeric materials after use. For applications where recycling might not be the adequate answer, or landfilling is highly likely to occur, (bio)degradable polymers can play an important role. This appraisal acknowledges the most important synthetic (bio)degradable polymers, noticeably polyesters, and their potential for substitution of traditional persistent ones, in terms of thermal and mechanical properties and more adequate fate in controlled compostable media or instead in natural environments. Special focus is given to bio-based poly(lactic acid), poly(butylene succinate) and furan-based polyesters. Alternatively, the most recent progress and contributions to rendering fossil-based polymers such as poly(caprolactone) and poly(glycolic acid) greener are discussed. [ABSTRACT FROM AUTHOR]

Less than 10% of the plastics generated globally are recycled, while the rest are incinerated, accumulated in landfills, or leak into the environment. New technologies are emerging to chemically recycle waste plastics that are receiving tremendous interest from academia and industry. Chemists and chemical engineers need to understand the fundamentals of these technologies to design improved systems for chemical recycling and upcycling of waste plastics. In this paper, we review the entire life cycle of plastics and options for the management of plastic waste to address barriers to industrial chemical recycling and further provide perceptions on possible opportunities with such materials. Knowledge and insights to enhance plastic recycling beyond its current scale are provided. Outstanding research problems and where researchers in the field should focus their efforts in the future are also discussed. [ABSTRACT FROM AUTHOR]

Isohexides (1,4:3,6-dianhydrohexitols) represent a family of bio-based heterocyclic diols attracting over 40 years' attention from both industry and academia. Their unique properties, including high structural rigidity, high hydrophilicity and distinct chirality, make them highly promising for developing novel sustainable polymers with higher performance, (bio)degradability and optical properties. Nevertheless, the preparation of high-quality polymers with sufficiently high molecular weight, an acceptable degree of discoloration and high/tunable contents of isohexides for commercial applications remains a great challenge due to their insufficient purity, lower reactivity and limited thermal stability. With respect to the possibility of commercialization, this review focuses on the efficiency of different short-process polymerization strategies for sustainable isohexide-based polyesters polymerized either directly or coupled with preferably in situ activation strategies. The influential factors for polymerization efficiency and the key catalytic systems of the polyesters will be also systematically discussed to gain clear insight into the challenges and prospects of isohexide-based polyesters for further practical applications. [ABSTRACT FROM AUTHOR]

The recycling of plastic waste is one of the most urgent issues in the 21st century owing to its detrimental impact on the environment and human health. However, it is greatly challenging to develop more efficient, low-cost and general methods capable of selectively converting plastic waste into added-value and separable chemicals while restricting their distribution. Here, we report a general electrocatalytic strategy for the selective upcycling of various polyester plastics into added-value chemicals by a CuCo2O4/Ni foam catalyst under mild conditions. In particular, polyethylene terephthalate (PET) was upcycled into terephthalate and formate with high efficiency and selectivity (>86%) even at a high current density of 100 mA cm−2. The electrocatalytic reaction mechanism was thoroughly elucidated by electrochemical in situ FTIR, NMR and DFT calculations. This work not only develops a superior electrocatalyst for upcycling various polyester plastics, but also provides guidance for the design of new and efficient electrocatalysts. [ABSTRACT FROM AUTHOR]

Bio-recycling of plastic waste is a promising solution to plastic pollution. As one of the most abundant plastic wastes, polyethylene terephthalate (PET) can be degraded by carboxylic ester hydrolases (EC 3.1.1). Nevertheless, the biological PET hydrolysis efficiency is always limited by the low activity and poor thermostability of the enzymes. Herein, to address the above barriers, we rationally mutated the relevant sites of Thermobifida cellulosilytica cutinase 1 (ThcCut1) involved in substrate binding. The wider substrate-binding pockets after mutation could facilitate the accessibility of the enzyme to the substrate. Divalent metal-binding sites were further predicted and substituted with disulfide bonds, with the aim of effectively improving the thermostability of the mutant ThcCut1. Coupled with sequence alignment and structural dynamics analysis, the ThcCut1-D205C/E254C/Q93G variant with a melting temperature exceeding the glass transition temperature of recycled PET was constructed. After comprehensively screening the active and thermally stable mutation sites, the resulting ThcCut1-G63A/F210I/D205C/E254C/Q93G (ThcCut1-AICCG) variant exhibited high enzymatic activity at a high temperature (70 °C). As result, 96.2% of the post-consumer PET bottle particles (without energy-intensive melt-quenching pretreatment) can be successfully degraded after 96 h of hydrolysis using ThcCut1-AICCG, which was 87.5 times higher than that using the wild-type ThcCut1. This novel strategy for amino acid site analysis will facilitate the modification of homologous cutinases to improve the catalytic performance, and provide a reliable technical method for constructing a PET hydrolase modification platform. [ABSTRACT FROM AUTHOR]

Finding ways to restore the environmental balance that has been disturbed by the rise in the use of plastic commodities is urgent for researchers and the wider community. It is imperative that the increasing amount of plastic waste and the high amount of petrochemical resources consumed during the constant replacement of single-use plastics be reduced. Poly(ethylene terephthalate) (PET) is one of the most commonly produced single-use polymers in the world, and its mechanical recycling is challenging due to the loss of properties during reprocessing. Chemical recycling is a feasible alternative to reclaim the monomers, however, its viability relies on establishing a straightforward, fast, and inexpensive procedure to turn the end-of-use polymer into new pure monomers. This work reports on the fastest known procedure for PET chemical recycling to produce terephthalic acid and ethylene glycol in an efficient and straightforward manner, thanks to microwave-assisted heating that permitted 100% PET conversion into TPA in just 1 minute at 120 °C. The depolymerization kinetics of this new procedure were studied and its improved efficiency over other reported hydrolysis procedures was attributed to a thicker shrinking layer. This new procedure may form a major breakthrough in chemical depolymerization. Evidence pointing to the higher reactivity of free OH species enabled us also to obtain dimethyl terephthalate (DMT) through the use of an anhydrous depolymerization system that is able to convert PET into DMT in 4 min at 80 °C, but requiring careful humidity control. The activation energy for the proposed depolymerization system was estimated as 120 kJ mol−1. An additional advantage of the proposed process is the production of potassium sulfate as a side-product, a valuable fertilizer with high market value. This product compensates for the lack of recoverability of the catalyst. Our KOH-in-methanol hydrolysis (KMH) process has the potential to become a widely used depolymerization solution for a wide range of (heterogeneous) condensation polymers, which is currently under study by our research group. [ABSTRACT FROM AUTHOR]