Recovery of Arenes from Polyethylene Terephthalate (PET) over a Co/TiO2 Catalyst

Catalyst- and Solvent-Free Upcycling of Poly(Ethylene Terephthalate) Waste to Biodegradable Plastics

Poly(ethylene terephthalate) (PET) is an important polymer with annual output second only to polyethylene. Due to its low biodegradability, a large amount of PET is recycled for sustainable development. However, current strategies for PET recycling are limited by low added value or small product scale. It is urgent to make a breakthrough on the principle of PET macromolecular reaction and efficiently prepare products with high added value and wide applications. Here, the catalyst- and solvent-free synthesis of biodegradable plastics are reported through novel carboxyl-ester transesterification between PET waste and bio-based hydrogenated dimer acid (HDA), which can directly substitute some terephthalic acid (TPA) units in PET chain by HDA unit. This macromolecular reaction can be facilely carried out on current equipment in the polyester industry without any additional catalyst and solvent, thus enabling low-cost and large-scale production. Furthermore, the product semi-bio-based copolyester shows excellent mechanical properties, regulable flexibility and good biodegradability, which is expected to substitute poly(butylene adipate-co-terephthalate) (PBAT) plastic as high value-added biodegradable materials. This work provides an environmental-friendly and economic strategy for the large-scale upcycling of PET waste.

Hydrodeoxygenation of Oxygen-Containing Aromatic Plastic Wastes into Cycloalkanes and Aromatics

Chemical recycling and upcycling offer promising approaches for the management of plastic wastes. Hydrodeoxygenation (HDO) is one of the appealing ways for conversion of oxygen-containing plastic wastes, including polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polycarbonate (PC), polyphenyl ether (PPO), and polyether ether ketone (PEEK), into cyclic alkanes and aromatics in high yields under mild reaction conditions. The challenge lies in achieving C-O activation while preserving C-C bonds. In this review, we highlight the recent advancements in catalytic strategies and catalysts for the conversion of these oxygen-containing plastic wastes into cycloalkanes and aromatics. The reaction systems, including multi-step routes, direct HDO and transfer HDO methods, are exemplified. The design and performance of HDO catalysts are systematically summarized and compared. We comprehensively discuss the functions of the catalysts' components, reaction pathway and mechanism to gain insights into the HDO process for efficient valorization of oxygen-containing plastic wastes. Finally, we provide perspectives for this field, with specific emphasis on the non-noble metal catalyst design, selectivity control, reaction network and mechanism studies, mixed plastic wastes management and product functionalization. We anticipate that this review will inspire innovations on the catalytic process development and rational catalyst design for the HDO of oxygen-containing aromatic plastics to establish a low-emission circular economy.

Selective Upcycling of Polyethylene Terephthalate towards High-valued Oxygenated Chemical Methyl p-Methyl Benzoate using a Cu/ZrO2 Catalyst

Upgrading of polyethylene terephthalate (PET) waste into valuable oxygenated molecules is a fascinating process, yet it remains challenging. Herein, we developed a two-step strategy involving methanolysis of PET to dimethyl terephthalate (DMT), followed by hydrogenation of DMT to produce the high-valued chemical methyl p-methyl benzoate (MMB) using a fixed-bed reactor and a Cu/ZrO2 catalyst. Interestingly, we discovered the phase structure of ZrO2 significantly regulates the selectivity of products. Cu supported on monoclinic ZrO2 (5 %Cu/m-ZrO2 ) exhibits an exceptional selectivity of 86 % for conversion of DMT to MMB, while Cu supported on tetragonal ZrO2 (5 %Cu/t-ZrO2 ) predominantly produces p-xylene (PX) with selectivity of 75 %. The superior selectivity of MMB over Cu/m-ZrO2 can be attributed to the weaker acid sites present on m-ZrO2 compared to t-ZrO2 . This weak acidity of m-ZrO2 leads to a moderate adsorption capability of MMB, and facilitating its desorption. Furthermore, DFT calculations reveal Cu/m-ZrO2 catalyst shows a higher effective energy barrier for cleavage of second C-O bond compared to Cu/t-ZrO2 catalyst; this distinction ensures the high selectivity of MMB. This catalyst not only presents an approach for upgrading of PET waste into fine chemicals but also offers a strategy for controlling the primary product in a multistep hydrogenation reaction.

Advances in solar-driven, electro/photoelectrochemical, and microwave-assisted upcycling of waste polyesters

Plastic waste in the environment causes significant environmental pollution. The potential of using chemical methods for upcycling plastic waste offers a dual solution to ensure resource sustainability and environmental restoration. This article provides a comprehensive overview of the latest technologies driven by solar-driven, electro/photoelectrochemical-catalytic, and microwave-assisted methods for the conversion of plastics into various valuable chemicals. It emphasizes selective conversion during the plastic transformation process, elucidates reaction pathways, and optimizes product selectivity. Finally, the article offers insights into the future developments of chemical upcycling of polyesters.

Hydrodeoxygenation of Oxygen-Containing Aromatic Plastic Wastes to Liquid Organic Hydrogen Carriers

To address the global plastic pollution issues and the challenges of hydrogen storage and transportation, we report a system, based on the hydrodeoxygenation (HDO) of oxygen-containing aromatic plastic wastes, from which organic hydrogen carriers (LOHCs) can be derived. We developed a catalytic system comprised of Ru-ReOx /SiO2 +HZSM-5 for direct HDO of polycarbonate (PC), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyphenylene oxide (PPO), and their mixtures, to cycloalkanes as LOHCs, with high yields up to 99 %, under mild reaction conditions. The theoretical hydrogen storage capacity reaches ca. 5.74 wt%. The reaction pathway involves depolymerization of PC into C15 aromatics and C15 monophenols by direct hydrogenolysis of the C-O bond between the benzene ring and ester group, and subsequent parallel hydrogenation of C15 aromatics and HDO of C15 monophenols. HDO of cyclic alcohol is the rate-determining step. The active site is Ru metallic nanoparticles with partially covered ReOx species. The excellent performance is attributed to the synergetic effect of oxophilic ReOx species and Ru metallic sites for C-O hydrogenolysis and hydrogenation, and the promotion effect of HZSM-5 for dehydration of cyclic alcohol. The highly efficient and stable dehydrogenation of cycloalkanes over Pt/γ-Al2 O3 confirms that HDO products can act as LOHCs.

PET Waste Recycling into BTX Fraction Using In Situ Obtained Nickel Phosphide

The annual production of plastic waste is a serious ecological problem as it causes substantial pollution of the environment. Polyethylene terephthalate, a material usually found in disposable plastic bottles, is one of the most popular material used for packaging in the world. In this paper, it is proposed to recycle polyethylene terephthalate waste bottles into benzene-toluene-xylene fraction using a heterogeneous nickel phosphide catalyst formed in situ during the polyethylene terephthalate recycling process. The catalyst obtained was characterized using powder X-ray diffraction, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy techniques. The catalyst was shown to contain a Ni₂P phase. Its activity was studied in a temperature range of 250-400 °C and a H₂ pressure range of 5-9 MPa. The highest selectivity for benzene-toluene-xylene fraction was 93% at quantitative conversion.

Chemical Recycling Processes of Waste Polyethylene Terephthalate Using Solid Catalysts

Polyethylene terephthalate (PET) is a non-degradable single-use plastic and a major component of plastic waste in landfills. Chemical recycling is one of the most widely adopted methods to transform post-consumer PET into PET's building block chemicals. Non-catalytic depolymerization of PET is very slow and requires high temperatures and/or pressures. Recent advancements in the field of material science and catalysis have delivered several innovative strategies to promote PET depolymerization under mild reaction conditions. Particularly, heterogeneous catalysts assisted depolymerization of post-consumer PET to monomers and other value-added chemicals is the most industrially compatible method. This review includes current progresses on the heterogeneously catalyzed chemical recycling of PET. It describes four key pathways for PET depolymerization including, glycolysis, pyrolysis, alcoholysis, and reductive depolymerization. The catalyst function, active sites and structure-activity correlations are briefly outlined in each section. An outlook for future development is also presented.

Chemical Upcycling of Waste Plastics to High Value-Added Products via Pyrolysis: Current Trends, Future Perspectives, and Techno-Feasibility Analysis

Chemical upcycling of waste plastics into high-value-added products is one of the most effective, cost-efficient, and environmentally beneficial solutions. Many studies have been published over the past few years on the topic of recycling plastics into usable materials through a process called catalytic pyrolysis. There is a significant research gap that must be bridged in order to use catalytic pyrolysis of waste plastics to produce high-value products. This review focuses on the enhanced catalytic pyrolysis of waste plastics to produce jet fuel, diesel oil, lubricants, aromatic compounds, syngas, and other gases. Moreover, the reaction mechanism, a brief and critical comparison of different catalytic pyrolysis studies, as well as the techno-feasibility analysis of waste plastic pyrolysis and the proposed catalytic plastic pyrolysis setup for commercialization is also covered.

Mechanochemistry Milling of Waste Poly(Ethylene Terephthalate) into Metal-Organic Frameworks

Converting poly(ethylene terephthalate) (PET) into metal-organic frameworks (MOFs) has emerged as a promising innovation for upcycling of waste plastics. However, previous solvothermal methods suffer from toxic solvent consumption, long reaction time, high pressure, and high temperature. Herein, a mechanochemical milling strategy was reported to transform waste PET into a series of MOFs with high yields. This strategy had the merits of solvent-free conditions, ambient reaction temperature, short running time, and easy scale-up for large-scale production of MOFs. The as-prepared MOFs exhibited definite crystal structure and porous morphology composed of agglomerated nanoparticles. It was proven that, under mechanochemical milling, PET was firstly decomposed into 1,4-benzenedicarboxylate, which acted as linkers to coordinate with metal ions for forming fragments, followed by the gradual arrangement of fragments into MOFs. This work not only promotes high value-added conversion of waste polyesters but also offers a new opportunity to produce MOFs in a green and scalable manner.

A systematic review on plastic waste conversion for a circular economy: recent trends and emerging technologies

Our biosphere has been adversely affected by plastic waste pollution, especially non-biodegradables in landfills, which induces hazardous chemical leaching and toxic gas emissions on burning into the atmosphere.

Plastic Waste Upcycling: A Sustainable Solution for Waste Management, Product Development, and Circular Economy

Plastic waste pollution, including non-biodegradable landfills, leaching of toxic chemicals into soil and waterways, and emission of toxic gases into the atmosphere, is significantly affecting our environment. Conventional plastic waste recycling approaches generally produce lower value materials compared to the original plastic or recover inefficient heat energy. Lately, upcycling or the valorization approach has emerged as a sustainable solution to transform plastic waste into value-added products. In this review, we present an overview of recent advancements in plastic waste upcycling, such as vitrimerization, nanocomposite fabrication, additive manufacturing, catalytic transformation, and industrial biotechnology, envisaged with technical challenges, future developments, and new circular economy opportunities.

Converting waste PET plastics into automobile fuels and antifreeze components

With the aim to solve the serious problem of white plastic pollution, we report herein a low-cost process to quantitatively convert polyethylene terephthalate (PET) into p-xylene (PX) and ethylene glycol (EG) over modified Cu/SiO₂ catalyst using methanol as both solvent and hydrogen donor. Kinetic and in-situ Fourier-transform infrared spectroscopy (FTIR) studies demonstrate that the degradation of PET into PX involves tandem PET methanolysis and dimethyl terephthalate (DMT) selective hydro-deoxygenation (HDO) steps with the in-situ produced H₂ from methanol decomposition at 210 °C. The overall high activities are attributed to the high Cu⁺/Cu⁰ ratio derived from the dense and granular copper silicate precursor, as formed by the induction of proper NaCl addition during the hydrothermal synthesis. This hydrogen-free one-pot approach allows to directly produce gasoline fuels and antifreeze components from waste poly-ester plastic, providing a feasible solution to the plastic problem in islands.

To solve the serious problem of white plastic pollution many degradation routes are being investigated. Here the authors show a H₂-free low-cost Cu/SiO₂ catalyzed process to quantitatively convert polyethylene terephthalate into p-xylene and ethylene glycol in one pot with methanol as both the solvent and hydrogen source at 210 °C.

Closing the Carbon Loop in the Circular Plastics Economy

Today, plastics are ubiquitous in everyday life, problem solvers of modern technologies, and crucial for sustainable development. Yet the surge in global demand for plastics of the growing world population has triggered a tidal wave of plastic debris in the environment. Moving from a linear to a zero-waste and carbon-neutral circular plastic economy is vital for the future of the planet. Taming the plastic waste flood requires closing the carbon loop through plastic reuse, mechanical and molecular recycling, carbon capture, and use of the greenhouse gas carbon dioxide. In the quest for eco-friendly products, plastics do not need to be reinvented but tuned for reuse and recycling. Their full potential must be exploited regarding energy, resource, and eco efficiency, waste prevention, circular economy, climate change mitigation, and lowering environmental pollution. Biodegradation holds promise for composting and bio-feedstock recovery, but it is neither the Holy Grail of circular plastics economy nor a panacea for plastic littering. As an alternative to mechanical downcycling, molecular recycling enables both closed-loop recovery of virgin plastics and open-loop valorization, producing hydrogen, fuels, refinery feeds, lubricants, chemicals, and carbonaceous materials. Closing the carbon loop does not create a Perpetuum Mobile and requires renewable energy to achieve sustainability. This article is protected by copyright. All rights reserved.