From trash to treasure: Chemical recycling and upcycling of commodity plastic waste to fuels, high-valued chemicals and advanced materials

Of all the existing materials, plastics are no doubt among the most versatile ones. However, the extreme increases in plastic production as well as the difficulty of the material for degradation have led to a huge number of plastic wastes. Their recycling rate after disposal is less than 10%, resulting in a series of serious environmental and ecological problems as well as a significant waste of resources. Current recycling methods generally suffer from large energy consumption, the low utilization rate of recycled products with low added value, and produce other waste during the process. Here, we summarized recentlydeveloped chemical recycling ways on commodity plastics, especially new catalytic paths in production of fuels, high-valued chemicals and advanced materials from a single virgin or a mixture of plastic waste,which have emerged as promising ways to valorize waste plastics more economically and environmentally friendly. The new catalyst design criteria as well as innovative catalytic paths and technologies for plastic upcycling are highlighted. Beyond energy recovery by incineration, these approaches demonstrate how waste plastics can be a viable feedstock for energy use with the generation of clean H_(2), high-quality liquid fuels and materials for energy storage, and help inspiring more catalytic process on plastic upcycling to overcome the economical hurdle and building a circular plastic economy.

Eco-friendly Technologies for Physical and Chemical Recycling of PVC-Related Wasteful Resources

The aim of this study is to enhance the recycled PVC （polyvinyl chloride） related material property by formulation technology and develop the recycling product processing technology furthermore develop the chemical recycling technology for last stage of PVC wastes. The formulation technology is composed of pre-treatment （crushing, separation etc.） and post-treatment （material ratio, additives, stabilizer etc.） to enhance the recyclate property. The formulation for recycled PVC by application basis and processing technology is applied to produce the structural product for civil and construction application such as pipe fittings and water drainage cap for environmental waterway. Also chemical recycling technology for end life PVC scrap which causes environmental pollution by incineration or landfill is studied for producing hydrocarbon and hydrogen chloride for VCM.

Studying the Utilization of Plastic Waste by Chemical Recycling Method

The rapid increase in the use of plastic materials in the recent years led to the accumulation of excessive amounts of plastic waste. The so-called thermoplastics such as PE, PP, PS, PVC and PET as well as materials that are derived from these are the type of plastic that is most used and consequently creates most of the waste. In this study, the original and waste forms of PE and PP plastic types have been chosen for thermal and catalytic degradation. As process parameter, 410oC - 450oC temperature interval and 600 mL/min constant flow rate nitrogen gas have been chosen as the carrier gas and the reaction time was considered to be 90 minutes for all experiments. Liquid products collected in experiments were separated by means of fractioned distillation process. For purposes of determining product distribution, the fractions, which were separated by distillation, were diluted in an appropriate solution for analysis of GC/MS. In the study conducted, it has been observed that the liquid product distribution obtained mainly consists of a mixture of saturated and unsaturated (heptane, heptane, octane, nonane, dodecane, etc.) hydrocarbons.

Rapid chemical recycling of waste polyester plastics catalyzed by recyclable catalyst

Waste plastics are serious environmental threats due to their low degradability and low recycling rate.Rapid and efficient waste plastics recycling technologies are urgently demanded for a sustainable future.Herein,we report a rapid,closed-loop,and streamlined process to convert polyesters such as poly(ethylene terephthalate)(PET)back to its purified monomers.Using trifluoromethanesulfonic acid or metal triflates as the recyclable catalyst,polyesters such as PET can be completely depolymerized by simple carboxylic acids within 1 h.By coupling this acidolysis with a subsequent hydrogenolysis process,the consumed carboxylic acid was recovered and the closed-loop of PET depolymerization could be established.All catalysts and depolymerization agents are fully recycled while only PET and hydrogen are consumed.

Chemical Recycling of Poly(ethylene furanoate)into Value-added Poly(ethylene-co-isosorbide furanoate)

We present here a novel strategy for the chemical recycling of bio-based poly(ethylene furanoate)s(PEF)to value-added high-performance bio-based poly(ethylene-co-isosorbide furanoate)(PEIF)copolyesters by the combination of cyclodepolymerization method with rapid cascade polycondensation-coupling ring-opening polymerization(PROP).The solution cyclodepolymerization of commercially available PEF affords cyclic oligo(ethylene 2,5-furandicarboxylate)s(COEFs),and the effects of reaction conditions on the yield of COEFs were studied.PEIF copolyesters with different isosorbide(IS)contents were synthesized via the cascade PROP of COEFs with IS,which show significant enhanced glass transition temperature.By melt spinning,PEIF fibers with different orientation factors were prepared,with excellent thermal stability and mechanical performance.The obtained PEIF fibers can lift a weight∼25000 times higher than its weight.The PEIF fibers are stable under ambient conditions but are biodegradable following the“surface erosion”mechanism.These sustainable value-added biodegradable PEIF fibers offer a solution to the environmentally friendly fibers.

Controlled synthesis and closed-loop chemical recycling of biodegradable copolymers with composition-dependent properties

The closed-loop recycling concept of the polymer wastes into building-block chemicals is attractive,but the closed-loop recycling of copolymers enabled by energy-efficient chemical recycling and cost-effective separations is still facing great challenges.Herein,for the first time,a one-pot sequential copolymerization of γ-butyrolactone(γ-BL) and p-dioxanone(PDO)using an economical ureas/alkoxides catalytic system is conducted to synthesize biodegradable and chemically recyclable poly-(γ-butyrolactone)-block-poly(p-dioxanone)(PγBL-b-PPDO) diblock copolymers with well-defined and controlled structures.The composition-dependent properties of PγBL-b-PPDO copolymers,including thermal properties and crystallization behavior,are investigated.The results show that the thermal stability and crystalline ability of PγBL are enhanced observably by introducing the PPDO block.Significantly,the PγBL-b-PPDO copolymers can be depolymerized efficiently into the corresponding co-monomers with a yield of over 95% by simply low-temperature pyrolysis under vacuum.Moreover,γ-BL and PDO monomers are selectively separated with an isolated purity of about 99% based on the difference in their physicochemical properties.Subsequently,their repolymerization is realized to obtain the copolymers with nearly identical structures and thermostability,demonstrating the closed-loop recycling of copolymers,i.e.,polymerization-depolymerization-repolymerization.This research provides important guidance for the design of novel sustainable polymers towards more efficient chemical recycling,separation and regeneration.

Life Cycle Assessment of Recycling High-Density Polyethylene Plastic Waste

Increasing production and use of various novel plastics products,a low recycling rate,and lack of effective recycling/disposal methods have resulted in an exponential growth in plastic waste accumulation in landfills and in the environment.To better understand the effects of plastic waste,Life Cycle Analysis(LCA)was done to compare the effects of various production and disposal methods.LCA shows the specific effects of the cradle-to-grave or cradle-to-cradle scenarios for landfill,incineration,and mechanical recycling.The analysis clearly indicates that increasing recycling of plastics can significantly save energy and eliminate harmful emissions of various carcinogens and GHGs into the environment.As recycling increases,the need for virgin-plastic production can be greatly reduced.Furthermore,the results of this study may help improve current mechanical recycling processes as well as potential future recycling methods,such as chemical recycling.Concerns about the current recycling/disposal methods for plastics have brought increasing attention to the waste accumulation problem.However,with the current COVID-19 pandemic,plastic accumulation is expected to increase significantly in the near future.A better understanding of the quantitative effects of the various disposal methods can help guide policies and future research toward effective solutions of the plastic waste problem.

MOF synthesis using waste PET for applications of adsorption, catalysis and energy storage

Polyethylene terephthalate (PET) as one of non-degradable wastes has become a huge threat to the environment and human health.Chemical Recycle of PET is a sustainable way to release 1,4-benzenedicarboxylic acid (BDC) the monomer of PET as common used organiclinker for synthesis of functional Metal–organic-frameworks (PET-derived MOFs) such as UiO-66, MIL-101, etc. This sustainable and costeffective“Waste-to-MOFs” model is of great significant to be intensively investigated in the past years. Attributes of substantial porosity, specificsurface area, exposed metal centers, uniform structure, and flexible morphology render PET-derived MOFs are well-suited for applications inadsorption, energy storage, catalysis, among others. Herein, in the present work, we have summarized recent advances in synthesis of PETderived MOFs using ex-situ and in-situ methods for typical applications of adsorption, catalysis and energy storage. Despite those improvementsin synthesis methods and potential applications, challenges still remain in development of green and economical routes to fully utilize waste PETfor massive manufacture of valuable MOF materials and chemicals. This review provides insights into the conversion of non-degradable PETwaste to value-added MOF materials, and further suggests promising perspectives to develop the sustainable “Waste-to-MOFs” model inaddressing environmental pollution and energy crises.

Catalytic recycling of plastics into value-added products

The overuse and ineffective management of plastics have led to significant environmental pollution. Catalytic upcycling into value-added chemicals has emerged as a promising solution. This review provides a comprehensive overview of recent advances in catalytic upcycling, focusing on the cleavage of chemical bonds such as carbon-carbon (C-C), carbon-oxygen (C-O), and carbon-hydrogen (C-H) in plastics. It systematically discusses plastics conversion via electrocatalysis, thermal catalysis, and photocatalysis. Additionally, it explores the conversion of plastics into value-added chemicals and functional polymers. The review also addresses the challenges in this field and aims to offer insights for developing sustainable and effective plastics upcycling technologies.

Polymer-to-Monomers Chemically Recyclable Poly(imide-imine)Plastics with Extreme-Condition Resistance and Flame Retardancy

Polyimides are a family of high-tech plastics that have irreplaceable applications in the fields of aerospace,defense,and opto-electronics,but polyimides are difficult to be reprocessed and recycled at the end of their service life,resulting in a significant waste of resources.Hence,it is of great significance to develop recyclable polyimides with comparable properties to the commercial products.Herein,we report a novel polymer-to-monomers chemically recyclable poly(imide-imine)(PtM-CR-PII)plastic,synthesized by cross-linking the amine-terminated aromatic bisimide monomer and the hexa-vanillin terminated cyclophosphazene monomer via dynamic imine bonds.The PtM-CR-PII plastic exhibits comparable mechanical and thermal properties as well as chemical stability to the commercial polyimides.The PtM-CR-PII plastic possesses a high Young’s modulus of≈3.2 GPa and a tensile strength as high as≈108 MPa,which also exhibits high thermal stability with a glass transition temperature of≈220℃.Moreover,the PtM-CR-PII plastic exhibits outstanding waterproofness,acid/alkali-resistance,and solvent-resistance,its appearance and mechanical properties can be well maintained after long-term soaking in water,highly concentrated acid and base,and various organic solvents.Furthermore,the cyclophosphazene moieties endow the PtM-CR-PII plastic with excellent flame retardancy.The PtM-CR-PII plastic exhibits the highest UL-94 flame-retarding rating of V-0 and a limiting oxygen index(LOI)value of 45.5%.Importantly,the PtM-CR-PII plastic can be depolymerized in an organic solvents-acid mixture medium at room temperature,allowing easy separation and recovery of both monomers in high purity.The recovered pure monomers can be used to regenerate new PtM-CR-PII plastics,enabling sustainable polymer-monomers-polymer circulation.

In situ combined chemical and biological assessment of estrogenic pollution in a water recycling system

Estrogenic pollution and its control in aquatic systems have drawn substantial attention around the world. The chemical and biological assessment approaches currently utilized in the laboratory or field cannot give an integrated assessment of the pollution when used separately. In this study, in situ chemical and biological methods were combined to detect pollution in a water recycling system. Data for the water quality index（WQI） demonstrated that the water treatment resulted in the decline of pollution from upstream to downstream.Wild male Nile tilapia, Oreochromis niloticus, was sampled in June and September. The concentrations of four common endocrine disrupting chemicals（EDCs） were determined in the tilapia liver by chromatographic analysis methods. The level of 17β-estradiol（E2） declined from upstream to downstream in both months. In contrast, the levels of bisphenol A（BPA）,di-（2-ethylhcxyl） phthalate（DEHP）, and perfluorooctane sulfonate（PFOS） did not display this declining tendency. The highest relative expression of vitellogenin 1（VTG1） was observed in tilapia from upstream, then the level significantly decreased along the water system. The relative expression levels of CYP1A1 in the water system were also significantly higher than that of the control. However, no declining trend could be observed along the water system. The change of VTG1 expression corresponded well with that of E2 levels in the tilapia liver. Overall,our study assessed the pollution by endocrine disruptors using chemical and biological data with good correspondence. This study also demonstrated the effectiveness of the water recycling system in eliminating estrogen pollution in municipal sewage.

Chemically recyclable copolyesters from bio-renewable monomers:controlled synthesis and composition-dependent applicable properties

The development of chemically recyclable polymers is a promising solution to address the dual challenges of the environment and resources caused by petroleum-based plastics.Despite recent advancements,it is highly desirable for developing recyclable polymers to meet the requirements of both practical uses and well-performed recyclability.Bio-renewable monomers have been paid great attention recently as promising potential candidates for establishing a sustainable circular polymer economy.Herein,a sequential copolymerization of various bio-renewable n-alkyl substituted δ-valerolactone((R)VLs)and p-dioxanone(PDO)is conducted to synthesize novel chemically recyclable diblock copolymers poly(p-dioxanone)-block-poly(n-alkyl-valerolactones)(PPDO-b-P(R)VLs)with well-defined and controlled structures.The properties of copolymers including thermal property,crystallization,mechanical property,hydrophilicity and transport property can be tuned effectively to meet the requirements of practical uses by alternating the alkyl substituents(R)and the P(R)VLs content.In addition,the high-efficiency and facile chemical recycling of copolymers to PDO and(R)VL comonomers is realized with a high yield of>96.5%and a high purity of 99%.

Green Synthesis of Chemically Recyclable Polyesters via Dehydrogenative Copolymerization of Diols

Preparation of chemically recyclable polyesters by ring-opening polymerization(ROP)has made a considerable progress over the past few years.However,this method involves cumbersome synthesis and minimal functional diversity of cyclic monomers.Therefore,it is of great significance to develop novel polymerization methods for direct polymerization of commercially available monomers to prepare recyclable polyesters with versatile functionalities.In present work,we report dehydrogenative copolymerization of commercialα,ω-diols to afford high molecular weight chemically recyclable aliphatic copolyesters(65.7 kg·mol^(-1))by using commercially available Milstein catalyst precursor.The thermal properties of the obtained copolymers could be finely tuned by simply adjusting the feeding ratio of two monomers.The incorporation of aliphatic or aromatic rings into polyester mainchain via copolymerization of 1,10-decanediol with 1,4-cyclohexanedimethanol and 1,4-benzenedimethanol could significantly improve the thermal properties of the resulting copolymers.More importantly,the obtained copolyesters were able to completely depolymerize back to original diols via hydrogenation by the same catalyst in solvent-free and mild conditions,thus offering a green and cost-effective route toward the preparation of widely used polyesters.

Beyond Conventional Degradation:Catalytic Solutions for Polyolefin Upcycling

Polyolefins(POs,i.e.,polyethylenes,ethylene/α-olefin copolymers,and polypropylenes)are the most ubiquitous synthetic macromolecular materials in modern life.Their widespread use and low recovery rate after extensive usage have caused significant resource waste and environmental concerns.Chemical recycling of POs provides an efficient approach to unravelling the polymer chain to various chemicals.However,conventional chemical recycling methods,including pyrolysis,hydrocracking,and oxidation,require high-energy input(typically>500℃)and/or the use of environmentally unfriendly chemicals,leading to complex product distribution.In this minireview,based on recent representative works,we summarize and highlight catalytic strategies addressing these issues in PO recycling from two perspectives:(1)employing advanced catalysts or technique designs to overcome the challenges in conventional chemical deconstruction approaches;and(2)developing novel tandem/cascade catalytic systems for highly selective PO upcycling under relatively mild conditions.We hope that this minireview will help researchers better understand the state of the art of PO chemical recycling and inspire more innovative and efficient ideas for this fast-developing field.

Continuous flow pyrolysis of virgin and waste polyolefins:a comparative study,process optimization and product characterization

Under optimal process conditions,pyrolysis of polyolefins can yield ca.90 wt%of liquid product,i.e.,combination of light oil fraction and heavier wax.In this work,the experimental findings reported in a selected group of publications concerning the non-catalytic pyrolysis of polyolefins were collected,reviewed,and compared with the ones obtained in a continuously operated bench-scale pyrolysis reactor.Optimized process parameters were used for the pyrolysis of waste and virgin counterparts of high-density polyethylene,low-density polyethylene,polypropylene and a defined mixture of those(i.e.,25:25:50 wt%,respectively).To mitigate temperature drops and enhance heat transfer,an increased feed intake is employed to create a hot melt plastic pool.With 1.5 g·min^(-1) feed intake,1.1 L·min^(-1) nitrogen flow rate,and a moderate pyrolysis temperature of 450℃,the formation of light hydrocarbons was favored,while wax formation was limited for polypropylene-rich mixtures.Pyrolysis of virgin plastics yielded more liquid(maximum 73.3 wt%)than that of waste plastics(maximum 66 wt%).Blending polyethylenes with polypropylene favored the production of liquids and increased the formation of gasoline-range hydrocarbons.Gas products were mainly composed of C3 hydrocarbons,and no hydrogen production was detected due to moderate pyrolysis temperature.

On Depolymerization

Polymers have become an essential part of modern life and the global economy on account of their costeffectiveness and versatile properties.However,most postconsumer polymer wastes are unrecycled,leading to environmental pollution and resource wastage.Depolymerization,as an efficient chemical recycling approach,holds great promise in establishing a circular polymer economy.In this review,we attempt to highlight recent and significant advancements in depolymerization methodologies.Two key research topics are discussed:(1)depolymerization of commodity polymers to produce reusable monomers and high-value chemicals;(2)depolymerization of intrinsically depolymerizable polymers.It is anticipated that this review will reflect the present status and future trends of this rapidly evolving realm of depolymerization.

Switchable Polyolefins from Polar Functionalization to Degradability

Polyolefins are synthetic plastics that exist on the largest scale and are ubiquitous in human life.They are also the most frequently discarded plastics.Consequently,the ability to either upgrade polyethylene(PE)plastic for value-added applications or to degrade PE plastic for value-added chemicals and monomers is highly desirable and sought after to mitigate the plastic waste problem.Herein,we report an advanced strategy for tackling the issue of PE plastics,first through a sequential upgrading and then through a degrading pathway.The optimal Diels–Alder-type polar comonomer diester-substituted norbornadiene is copolymerized with ethylene to produce the desired polar-functionalized PEs with both high comonomer incorporations of 42.4 mol%(-COOMe:as high to 59.6 mol%)and high molecular weights of up to 224 kg mol^(−1)in high catalytic activities of>100 kg mol^(−1)h^(−1).By means of a decisive retro-Diels–Alder reaction,this upgraded PE,namely polar-functionalized PE,can completely switch to a clean and soluble vinylene PE with a high content(28.7 mol%)of dispersed internal double bonds,which are degradable.Ethenolysis of the highmolecular–weight(∼30 kg mol^(−1))vinylene PE with ethylene yields industrially relevant telechelic oligomers(∼360 g mol^(−1))of long-chainα,ω-dienes and C9/C9+hydrocarbon products.This chemical upgrading and recycling method makes polyolefin plastic more sustainable.

Recycling strategies for vitrimers

Vitrimer is a new type of material that combine the advantages of thermoplastic and thermoset materials.The rapid dynamic exchange reactions at high temperature allow the topology of cross-linked networks to change and rearrange while keeping material structures and properties intact.The concept of vitrimer has emerged to provide a viable strategy for the recycling of highperformance polymer materials,and lots of research works have been carried out for the development of various types of vitrimers.In addition,the recycling strategies for vitrimers are particularly important to determine the performance and potential applications of the recovered materials.Therefore,it is an innovative and valu-able perspective to discuss vitrimer materials according to their different recycling strategies.In this review,we start with a brief overview of vitrimers,and then,focus on recycling strategies for vitrimers.Specifically,we highlight the advantages and disadvan-tages of the two different recycling strategies:physical and chemi-cal recycling methods,and then explore the feasibility of upcycling vitrimers using 3D printing technology.Finally,the impact of recy-cling strategies on vitrimer materials and the prospects for max-imizing the use of vitrimer materials are discussed.

Fully Chemical Recyclable Poly(γ-butyrolactone)-based Copolymers with Tunable Structures and Properties

The emerging chemical recyclable polymers,such as poly(γ-butyrolactone)(PGBL)and poly((R)-3,4-trans six-membered ring-fused GBL)(P((R)-M)),provide a good solution to the plastic pollution.However,these homopolymers suffer from limited structures and properties.Herein,we reported a fully chemical recyclable copolymer P(GBL-co-((R)-M))through ring-opening copolymerization(ROCOP)of GBL and(R)-M.By employing organomagnesium as the catalyst and regulating the reaction conditions,the chemical structures of copolymers were wellcontrolled(GBL content=13%-78%,Mn=6560-15600 g/mol,DM=1.08-1.59).The resultant P(GBL-co-((R)-M))exhibited fully chemical recyclability,which rapidly and quantitatively depolymerized into initial GBL and(R)-M monomer through chemolysis.By varying GBL content,tunable thermal properties were achieved for P(GBL-co-((R)-M)).The onset decomposition temperatures of copolymers varied from 193°C to 234°C.A linear evolution of glass transition temperature(T_(g))of P(GBL-co-((R)-M))versus GBL content was obtained as following equation of Tg=-1.06×GBL mol%×100+39.6.We hope that the reported fully chemical recyclable copolymers with tunable structures and properties would serve as the candidate material for sustainable applications.

Structure-Property Relationship Analysis of D-Penicillamine-Derivedβ-Polythioesters with Varied Alkyl Side Groups

The ring-opening polymerization of heterocyclic monomers and the reversed ring-closing depolymerization of corresponding polymers with neutral thermodynamics are broadly explored to establish a circular economy of next-generation plastics.Polythioesters(PTEs),analogues of polyesters,are emerging materials for this purpose due to their high refractive index,high crystallinity,dynamic property and responsiveness.In this work,we synthesize and polymerize a series of D-penicillamine-derivedβ-thiolactones(NRPenTL)with varied side chain alkyl groups,and study the structure-property relationship of the resulting polymers.The obtained PTEs exhibit tunable glass transition temperature in a wide range of 130–50℃,and melting temperature of 90–105℃.In addition,copolymerizations of monomers with different side chains are effective in modulating material properties.The obtained homo and copolymers can be fully depolymerized to recycle monomers.This work provides a robust molecular platform and detailed structure-property relationship of PTEs with potential of achieving sustainable plastics.