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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]

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]

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]

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]

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]

The vast increase in the use of post-consumer polyethylene terephthalate (PET) plastic brings with it the challenge of recycling the increase in waste bottles and materials. The degradation of waste PET to its monomers in an environmentally benign solvent is a desirable way to process waste PET. Analysis of solubility parameters has inspired the successful application of biomass-derived γ-valerolactone (GVL) for PET dissolution, and further accelerated the alkaline hydrolysis of waste PET to its monomers. Hydrogen-bonding interactions between PET chains and GVL play a major role in the dissolution. Swelling differential scanning calorimetry (DSC) has indicated that GVL processing PET proceeds through two steps: swelling and dissolving. The optimal dissolving process by swelling DSC showed that waste PET bottle pieces could fully swell at 120 °C in 1 h, then dissolve at 170 °C in 2 min. The maximum solubility of PET in GVL is around 0.25 g·g−1. Here, water was used as an anti-solvent to induce PET to reprecipitate from the hot solution. Due to the virtue of fully pre-swelling and the very fast dissolution, the degradation ratio of PET during the dissolving process was only 5.7%. GVL processing greatly increased the effective surface area of PET, which promoted the following complete alkaline hydrolysis of PET in around 8 min. This study provides an environmentally benign, very fast, and energy efficient pathway for recycling waste PET. [ABSTRACT FROM AUTHOR]