Valorisation of plastic waste via metal-catalysed depolymerisation

Open- and Closed-Loop Recycling: Highly Active Zinc Bisguanidine Polymerization Catalyst for the Depolymerization of Polyesters

In this study, the aliphatic N,N-bisguanidine zinc complex [Zn(DMEG2ch)2](OTf)2 ⋅ THF is introduced as a promising candidate for the chemical recycling of (bio) polyesters. This catalyst is highly active in the ring-opening polymerization (ROP) of lactide (LA) and ϵ-caprolactone (CL). The combination of polymerization and depolymerization activity creates new pathways towards a sustainable circular economy. The catalytic activity of [Zn(DMEG2ch)2](OTf)2 ⋅ THF for the chemical recycling of polylactide (PLA) via alcoholysis was investigated by detailed kinetic and thermodynamic studies. It is shown that various high value-added alkyl lactates can be obtained efficiently under mild reaction conditions. Catalyst recycling was successfully tested using ethanol for the degradation of PLA. In addition, LA can be recovered directly from PLA, enabling either open- or closed-loop recycling. Selective PLA degradation from mixtures with polyethylene terephthalate (PET) and polymer blends are presented. For the first time, a cascade recycling reaction of a PLA/polycaprolactone (PCL) blend is tested with a zinc-based bisguanidine catalyst, whereby PLA is degraded selectively at first and subsequent modification of the reaction conditions leads to efficient degradation of the remaining PCL. The highly active, universally applicable benign zinc catalyst allows the implementation of a circular plastics economy and thus the reduction of plastic pollution in the environment.

Chemoselective Hydrogenolysis of Urethanes to Formamides and Alcohols in the Presence of More Electrophilic Carbonyl Compounds

The development of methods for the chemical recycling of polyurethanes is recognized as an urgent issue. Herein, we report the Ir-catalyzed hydrogenolysis of the urethane C-O bond to produce formamides and alcohols, where both formamides and ester and amide functionalities are tolerated. The chemoselectivity observed is counterintuitive to the generally accepted electrophilicity order of carbonyl compounds. Hydrogenolysis of urea and isocyanurate, potential byproducts in the polycondensation process of polyurethanes, is also achieved alongside the selective degradation of polyurethanes themselves, which affords diformamides and diols. The time-course of the hydrogenative polyurethane degradation reveals that the bond cleavage occurs not from the terminal, but from any part of the polymer chain. The present catalysis offers a novel method for the recycling of polyurethane-containing polymer waste.

Factorial Analysis and Thermal Kinetics of Chemical Recycling of Poly(ethylene terephthalate) Aided by Neoteric Imidazolium-Based Ionic Liquids

Poly(ethylene terephthalate) (PET) waste accumulation poses significant environmental challenges due to its persistent nature and current management limitations. This study explores the effectiveness of imidazolium-based neoteric solvents [Emim][OAc] and [Bmim][OAc] as catalytic co-solvents in the glycolysis of PET with ethylene glycol (EG). Reaction thermal kinetics showed that both ionic liquids (ILs) significantly enhanced the depolymerization rate of PET compared to traditional methods. The use of [Emim][OAc] offered a lower activation energy of 88.69 kJ·mol-1, thus making the process more energy-efficient. The contribution of key process parameters, including temperature (T), plastic-to-ionic liquid (P/IL) mass ratio, and plastic-to-solvent (P/S) mass ratio, were evaluated by means of a factorial analysis and optimized to achieve the maximum PET conversion for both neoteric solvents. The relevance sequence for both ionic liquids involved the linear factors T and P/S, followed by the interaction factors T×P/S and T×P/IL, with P/IL being the less significant parameter. The optimal conditions, with a predicted conversion of 100%, involved a temperature of 190 °C, with a P/IL of 1:1 and a P/S of 1:2.5, regardless of the IL used as the catalytic co-solvent.

Counterintuitive chemoselectivity in the reduction of carbonyl compounds

The reactivity of carbonyl functional groups largely depends on the substituents on the carbon atom. Reversal of the commonly accepted order of reactivity of different carbonyl compounds requires novel synthetic approaches. Achieving selective reduction will enable the transformation of carbon resources such as plastic waste, carbon dioxide and biomass into valuable chemicals. In this Review, we explore the reduction of less reactive carbonyl groups in the presence of those typically considered more reactive. We discuss reductions, including the controlled reduction of ureas, amides and esters to aldehydes, as well as chemoselective reductions of carbonyl groups, including the reduction of ureas over carbamates, amides and esters; the reduction of amides over esters, ketones and aldehydes; and the reduction of ketones over aldehydes.

Polymer Waste Recycling of Injection Molding Purges with Softening for Cutting with Fresnel Solar Collector-A Real Problem Linked to Sustainability and the Circular Economy

A plastic injection waste known as "purge" cannot be reintegrated into the recycling chain due to its shape, size, and composition. Grinding these cannot be carried out with traditional mills due to significant variations in size and shape. This work proposes a process and the design of a device that operates with solar energy to cut the purges without exceeding the degradation temperature. The size reduction allows reprocessing, revalorization, and handling. The purges are mixtures of processed polymers, so their characterization information is unavailable. Some characterizations were conducted before the design of the process and after the cut of the purges. Some of the most representative purges in a recycling company were evaluated. The flame test determines that all material mixtures retain thermoplasticity. The hardness (Shore D) presented changes in four of the purges being assessed, with results in a range of 59-71 before softening and 60-68 after softening. Young's modulus was analyzed by the impulse excitation technique (IET), which was 2.38-3.95 GPa before softening and 1.7-4.28 after softening. The feasibility of cutting purges at their softening temperature was evaluated. This was achieved in all the purges evaluated at 250-280 °C. FTIR allowed for corroboration of no significant change in the purges after softening. The five types of purges evaluated were polypropylene-ABS, polycarbonate-ABS-polypropylene, yellow nylon 66, acetal, and black nylon 66 with fillers, and all were easily cut at their softening temperature, allowing their manipulation in subsequent process steps.

Catalytic thiolation-depolymerization-like decomposition of oxyphenylene-type super engineering plastics via selective carbon-oxygen main chain cleavages

As the effective use of carbon resources has become a pressing societal issue, the importance of chemical recycling of plastics has increased. The catalytic chemical decomposition for plastics is a promising approach for creating valuable products under efficient and mild conditions. Although several commodity and engineering plastics have been applied, the decompositions of stable resins composed of strong main chains such as polyamides, thermoset resins, and super engineering plastics are underdeveloped. Especially, super engineering plastics that have high heat resistance, chemical resistance, and low solubility are nearly unexplored. In addition, many super engineering plastics are composed of robust aromatic ethers, which are difficult to cleave. Herein, we report the catalytic depolymerization-like chemical decomposition of oxyphenylene-based super engineering plastics such as polyetheretherketone and polysulfone using thiols via selective carbon-oxygen main chain cleavage to form electron-deficient arenes with sulfur functional groups and bisphenols. The catalyst combination of a bulky phosphazene base P4-tBu with inorganic bases such as tripotassium phosphate enabled smooth decomposition. This method could be utilized with carbon- or glass fiber-enforced polyetheretherketone materials and a consumer resin. The sulfur functional groups in one product could be transformed to amino and sulfonium groups and fluorine by using suitable catalysts.

Highly selective pressure-dependent (transfer) hydrogenative depolymerization of polybutylene succinate

Ru-MACHO®-BH is an effective catalyst for controlled depolymerization of polybutylene succinate. Under low pressure hydrogen the catalyst produces gamma-butyrolactone via a novel transfer hydrogenation wherein dehydrogenation and hydrogenation deconstruct the polymer chain. Simply increasing the hydrogen pressure selectively generates 1,4-butanediol.

Hydroxylation-Depolymerization of Oxyphenylene-Based Super Engineering Plastics To Regenerate Arenols

Super engineering plastics, high-performance thermoplastic resins, show high thermal stability and mechanical strength as well as chemical resistance. On the other hand, chemical recycling for these plastics has not been developed due to their stability. This study describes depolymerization of oxyphenylene super engineering plastics via carbon-oxygen main chain cleaving hydroxylation reaction with an alkali hydroxide nucleophile. This method is conducted with cesium hydroxide as a hydroxy source and calcium hydride as a dehydration agent in 1,3-dimethyl-2-imidazolidinone, which provides hydroxylated monomers effectively. In the case of polysulfone, both 4,4'-sulfonyldiphenol (bisphenol S) and 4,4'-(propane-2,2-diyl)diphenol (bisphenol A) were obtained in high yields. Other super engineering plastics such as polyethersulfone, polyphenylsulfone, and polyetheretherketone were also applicable to this depolymerization.

Depolymerization of robust polyetheretherketone to regenerate monomer units using sulfur reagents

Super engineering plastics, high-performance thermoplastic resins such as polyetheretherketone, and polyphenylene sulfide have been utilized in industries, owing to their high thermal stability and mechanical strength. However, their robustness hinders their depolymerization to produce monomers and low-weight molecules. Presently, chemical recycling for most super engineering plastics remains relatively unexplored. Herein, we report the depolymerization of insoluble polyetheretherketone using sulfur nucleophiles via carbon-oxygen bond cleavages to form benzophenone dithiolate and hydroquinone. Treatment with organic halides converted only the former products to afford various dithiofunctionalized benzophenones. The depolymerization proceeded as a solid-liquid reaction in the initial phase. Therefore, this method was not affected by the shape of polyetheretherketone, e.g., pellets or films. Moreover, this depolymerization method was applicable to carbon- or glass fiber-enforced polyetheretherketone material. The depolymerized product, dithiofunctionalized benzophenones, could be converted into diiodobenzophenone, which was applicable to the polymerization.

Changes in physicochemical and leachate characteristics of microplastics during hydrothermal treatment of sewage sludge

Sewage sludge is an important source for microplastics (MPs) entering into environment. Hydrothermal treatment has been considered a promising method for reducing MPs in sewage sludge. However, MPs degradation characteristics and mechanism during sludge hydrothermal treatment are not fully understood. In the study, three common MPs, i.e. polyethylene (PE), polystyrene (PS) and polyethylene terephthalate (PET) were used to explore the effect of hydrothermal treatment on the properties of MPs in sewage sludge. The hydrothermally-treated (HT) MPs in sludge feature more broken and rougher surfaces with higher O-containing functional groups in the sludge than those in water. The dissolved leachates from the HT MPs in the sludge show higher concentrations than the counterparts, implying that certain components in sludge serve to promote the MP degradation and leaching during hydrothermal treatment. Three model components in the sludge, including protein, carbohydrate, and SiO₂, were further investigated for their individual effects on the hydrothermal degradation of MPs. Compared with those in water, the HT MPs in the protein and carbohydrate solutions show greater changes in the surface micro-morphologies and carbonyl index, and generate more leachates. However, the SiO₂ solution results in similar difference in the MPs changes with the water solution, indicating that organic components of sludge play a more critical role in the enhanced MPs hydrothermal degradation than inorganic components. The HT PET leads to more pronounced changes in the physicochemical and leaching characteristics than the HT PE and PS, possibly due to more susceptible hydrolysis of the PET. Hydrothermal degradation of the MPs is found to be mainly driven by depolymerization of the polymer and leaching of the plastics additives. The findings imply that the sludge organic components significantly promote the MPs aging and degradation during hydrothermal treatment, and potential changes in the environmental risk of the treated MPs upon their subsequent land applications.

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.

Cell-Free Biosynthesis of ω-Hydroxy Acids Boosted by a Synergistic Combination of Alcohol Dehydrogenases

The activity orchestration of an unprecedented cell-free enzyme system with self-sufficient cofactor recycling enables the stepwise transformation of aliphatic diols into ω-hydroxy acids at the expense of molecular oxygen as electron acceptor. The efficiency of the biosynthetic route was maximized when two compatible alcohol dehydrogenases were selected as specialist biocatalysts for each one of the oxidative steps required for the oxidative lactonization of diols. The cell-free system reached up to 100 % conversion using 100 mM of linear C₅ diols and performed the desymmetrization of prochiral branched diols into the corresponding ω-hydroxy acids with an exquisite enantioselectivity (ee>99 %). Green metrics demonstrate superior sustainability of this system compared to traditional metal catalysts and even to whole cells for the synthesis of 5-hydroxypetanoic acid. Finally, the cell-free system was assembled into a consortium of heterogeneous biocatalysts that allowed the enzyme reutilization. This cascade illustrates the potential of systems biocatalysis to access new heterofunctional molecules such as ω-hydroxy acids.

Recent Advancements in Plastic Packaging Recycling: A Mini-Review

Today, the scientific community is facing crucial challenges in delivering a healthier world for future generations. Among these, the quest for circular and sustainable approaches for plastic recycling is one of the most demanding for several reasons. Indeed, the massive use of plastic materials over the last century has generated large amounts of long-lasting waste, which, for much time, has not been object of adequate recovery and disposal politics. Most of this waste is generated by packaging materials. Nevertheless, in the last decade, a new trend imposed by environmental concerns brought this topic under the magnifying glass, as testified by the increasing number of related publications. Several methods have been proposed for the recycling of polymeric plastic materials based on chemical or mechanical methods. A panorama of the most promising studies related to the recycling of polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), and polystyrene (PS) is given within this review.