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Enabling resource circularity through thermo-catalytic and solvent-based conversion of waste plastics
Abstract
The recent rise in plastic pollution has led to a growing environmental burden, motivating new and effective methods for circular repurposing of “end-of-use” plastics. In this review, we highlight recent advances in thermochemical and catalytic pathways toward circularity of plastics utilization; specifically, hydroconversion, solvent conversion, and catalytic conversion without solvent or gaseous reagent. We present advances in the design of supported metal catalysts (Pt, Ru, Zr) for the hydroconversion of plastics, especially polyolefins (POs) and polyesters. We deduce mechanistic insights from hydroconversion reactions toward realizing economic circularity. We also review two solvent treatments: solvolysis of condensation polymers and solvent extraction for composite polymers. Last, we discuss advances in hydrocarbon conversion, without solvent or gaseous reagent, to catalytically depolymerize POs. We highlight the challenges and envision the path forward in optimal catalyst and process design that will enable the development of chemical upcycling technologies for building a circular plastic economy.
... While several excellent reviews have been published in recent years on depolymerization of plastics by heterogeneous catalysis, 11,12 the importance of interfacial processes for plastics degradation, recycling, and upcycling is just starting to be recognized, leading to novel insights into the kinetics and design of such reactions. Thus, we spotlight the need to better characterize and study these phenomena in this Perspective. ...
Depolymerization of plastics is a leading strategy to combat the escalating global plastic waste crisis through chemical recycling, upcycling, and remediation of micro-/nanoplastics. However, critical processes necessary for polymer chain scission, occurring at the polymer–catalyst or polymer–fluid interfaces, remain largely overlooked. Herein, we spotlight the importance of understanding these interfacial chemical processes as a critical necessity for optimizing kinetics and reactivity in plastics recycling and upcycling, controlling reaction outcomes, product distributions, as well as improving the environmental sustainability of these processes. Several examples are highlighted in heterogeneous processes such as hydrogenation over solid catalysts, reaction of plastics in immiscible media, and biocatalysis. Ultimately, judicious exploitation of interfacial reactivity has practical implications in developing practical, robust, and cost-effective processes to reduce plastic waste and enable a viable post-use circular plastics economy.
... Solvolysis is the chemical breaking of bonds in the presence of a solvent [79], and as such is a synonym for depolymerization. The term is sometimes misused to describe solvent-based recycling [47,52,80]; at times, depolymerization processes have even been described as solvent-based [81]. ...
Polyolefin waste is the largest polymer waste stream that could potentially serve as an advantageous hydrocarbon feedstock. Upcycling polyolefins poses significant challenges due to their inherent kinetic and thermodynamic stability. Traditional methods, such as thermal and catalytic cracking, are straightforward but require temperatures exceeding 400 °C for complete conversion because of thermodynamic constraints. We summarize and critically compare recent advances in upgrading spent polyolefins and model reactants via kinetic (and thermodynamic) coupling of the endothermic C─C bond cleavage of polyolefins with exothermic reactions including hydrogenation, hydrogenolysis, metathesis, cyclization, oxidation, and alkylation. These approaches enable complete conversion to desired products at low temperatures (<300 °C). The goal is to identify challenges and possible pathways for catalytic conversions that minimize energy and carbon footprints.
The unmatched applications of plastic commodities are evident from the enormous plastic production, reaching over 400 million tons per year in recent times. Contrastingly, the lack of proper management leads to a large accumulation of plastic waste, majorly including polyolefins and polyesters. Conventional management methods possess significant drawbacks like cost‐ineffectiveness and greenhouse gas emissions. Over the last decade, chemical processes have shown promising potential for plastic management but only hold a 0.1% share in plastic recycling. The catalytic processes offer excellent protocols to obtain high‐value liquid fuels, waxes, and chemicals from plastic waste. This review presents an elaborate discussion on the state of the art in the reductive upcycling of polyolefins, polyesters, and mixed plastic waste. The review initially discusses the alarming statistics of plastics and conventional approaches followed by an introduction to chemical processes. Further, various recently reported catalytic upcycling strategies have been elaborated in detail followed by catalyst deactivation, technoeconomic analysis, and life cycle assessment to obtain a deeper understanding of the current state of this research field. Finally, a detailed summary of the current state of plastic management along with the existing challenges and countermeasures is discussed to open new avenues in plastic waste management research.
Waste plastic has imposed significant burdens on ecosystems. Photocatalytic waste plastic into high-value chemicals and
fuels has attracted widespread attention due to its mild reaction
conditions, high product selectivity, and eco-friendliness, which is a boon to alleviating severe environmental problems and the energy crisis. Nevertheless, there are still several technological obstacles including harsh pretreatment, low photocatalytic efficiency, excessive carbon emissions, and process scale limited to the laboratory, hindering practical applications. In this Perspective, we summarized the recent progress in photocatalytic waste plastics into (1) chemicals and H2 fuel via photoreforming technology, (2) oxide-containing chemicals derived from polystyrene photo-oxidation, as well as (3) syngas or C2 products converted by tandem photocatalytic CO2 reduction. Each panel focuses on the construction of efficient photocatalysts, the in-depth understanding of reaction mechanisms, and the design or optimization of reaction systems. The challenges and future research directions are also discussed in achieving a sustainable photocatalytic plastic upgrading approach. We believe this Perspective will help researchers to understand the development status of the photocatalytic plastic conversion and provide guidance for future exploration of efficient photocatalytic systems.
The upcycling of C-C bond linked polyolefins has drawn wide attention in recent years and lot of studies have been focused on the conversion of polyolefins to hydrocarbons, while the...
The rapidly increasing production and widespread application of plastics have brought convenience to our lives, but they have consumed a huge amount of nonrenewable fossil energy, leading to additional CO2 emissions and generation of an enormous amount of plastic waste (also called white pollution). Chemical recycling and upcycling of waste plastic products (also called waste plastic refineries) into recycled monomers and/or valuable chemicals can decrease the dependence on fossil energy and/or reduce the emission of CO2, enabling the full utilization of carbon resources for the development of a circular economy. Polyesters, a vital class of plastics, are ideal feedstocks for chemical recycling due to the easily depolymerizable ester bonds compared to polyolefins. Among them, polyethylene terephthalate (PET) is the most widely used product, making its chemical recycling to a circular carbon resource a hot topic with significant concerns. In this feature article, recent progress in depolymerization of waste polyesters (PET and/or PET-containing materials) and the subsequent upgrading of depolymerized monomers (or intermediates) to valuable chemicals was reviewed and prospected. Newly reported technologies, such as thermal catalysis, photocatalysis, electrocatalysis, and biocatalysis, were discussed. The achievements, challenges, and potential of industrial applications of chemical recycling of polyesters were addressed.
We demonstrate a novel approach of utilizing methanol (CH3OH) in a dual role for (1) the methanolysis of polyethylene terephthalate (PET) to form dimethyl terephthalate (DMT) at near‐quantitative yields (~97%) and (2) serving as an in‐situ H2 source for the catalytic transfer hydrogenolysis (CTH) of DMT to p‐xylene (PX, ~63% at 240 °C and 16 h) on a reducible ZnZrOx supported Cu catalyst (i.e., Cu/ZnZrOx). Pre‐ and post‐reaction surface and bulk characterization, along with density functional theory (DFT) computations, explicate the dual role of the metal‐support interface of Cu/ZnZrOx in activating both CH3OH and DMT and facilitating a lower free‐energy pathway for both CH3OH dehydrogenation and DMT hydrogenolysis, compared to Cu supported on a redox‐neutral SiO2 support. Loading studies and thermodynamic calculations showed that, under reaction conditions, CH3OH in the gas phase, rather than in the liquid phase, is critical for CTH of DMT. Interestingly, the Cu/ZnZrOx catalyst was also effective for the methanolysis and hydrogenolysis of C‐C bonds (compared to C‐O bonds for PET) of waste polycarbonate (PC), largely forming xylenol (~38%) and methyl isopropyl anisole (~42%) demonstrating the versatility of this approach toward valorizing a wide range of condensation polymers.
We demonstrate a novel approach of utilizing methanol (CH3OH) in a dual role for (1) the methanolysis of polyethylene terephthalate (PET) to form dimethyl terephthalate (DMT) at near‐quantitative yields (~97 %) and (2) serving as an in situ H2 source for the catalytic transfer hydrogenolysis (CTH) of DMT to p‐xylene (PX, ~63 % at 240 °C and 16 h) on a reducible ZnZrOx supported Cu catalyst (i.e., Cu/ZnZrOx). Pre‐ and post‐reaction surface and bulk characterization, along with density functional theory (DFT) computations, explicate the dual role of the metal‐support interface of Cu/ZnZrOx in activating both CH3OH and DMT and facilitating a lower free‐energy pathway for both CH3OH dehydrogenation and DMT hydrogenolysis, compared to Cu supported on a redox‐neutral SiO2 support. Loading studies and thermodynamic calculations showed that, under reaction conditions, CH3OH in the gas phase, rather than in the liquid phase, is critical for CTH of DMT. Interestingly, the Cu/ZnZrOx catalyst was also effective for the methanolysis and hydrogenolysis of C−C bonds (compared to C−O bonds for PET) of waste polycarbonate (PC), largely forming xylenol (~38 %) and methyl isopropyl anisole (~42 %) demonstrating the versatility of this approach toward valorizing a wide range of condensation polymers.
The large production volumes of commodity polyolefins (specifically, polyethylene, polypropylene, polystyrene, and poly(vinyl chloride)), in conjunction with their low unit values and multitude of short-term uses, have resulted in a significant and pressing waste management challenge. Only a small fraction of these polyolefins is currently mechanically recycled, with the rest being incinerated, accumulating in landfills, or leaking into the natural environment. Since polyolefins are energy-rich materials, there is considerable interest in recouping some of their chemical value while simultaneously motivating more responsible end-of-life management. An emerging strategy is catalytic depolymerization, in which a portion of the C–C bonds in the polyolefin backbone is broken with the assistance of a catalyst and, in some cases, additional small molecule reagents. When the products are small molecules or materials with higher value in their own right, or as chemical feedstocks, the process is called upcycling. This review summarizes recent progress for four major catalytic upcycling strategies: hydrogenolysis, (hydro)cracking, tandem processes involving metathesis, and selective oxidation. Key considerations include macromolecular reaction mechanisms relative to small molecule mechanisms, catalyst design for macromolecular transformations, and the effect of process conditions on product selectivity. Metrics for describing polyolefin upcycling are critically evaluated, and an outlook for future advances is described.
For polymer recycling research, consistent polymer substrates sourced from widely available vendors are useful to enable direct comparisons between studies. Additionally, when reporting new recycling approaches, it is essential to...
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.
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.
Chemical recycling of plastics, as a promising strategy to reduce waste pollution and yield value-added chemicals, is showing great potentials in circular economy. In this work, a photooxidation method was...
Polyolefins and especially polyethylenes (LLDPE, LDPE and HDPE) and polypropylene (PP) contribute a great deal to the growing amounts of plastic waste with a combined production share of almost 50 % by mass. While being almost universally applicable, they are mainly used for short‐lived packaging materials that constitute over 60 % of annual post‐consumer plastic waste. Thus, disproportionately high amounts of polyolefins end up as post‐consumer waste (PCW) and waste management strategies for these particularly inert plastics are needed. This necessity has promoted a great research effort dealing with valorization of these discarded but, nevertheless, valuable materials. This review aims to highlight the scientific advances made in chemical polyolefin recycling in recent years, focusing, though not exclusively, on catalytic processes to recycle polyolefin waste by various means at more moderate temperatures compared to pyrolysis such as deconstructing the polymer with the objective of upcycling in mind or by catalytic transformation to give access to functional chemicals.
Plastic waste represents an environmental threat and a huge economic loss. Catalytic hydroconversion of plastic wastes can enable valorization of plastic wastes to diverse value-added products at high selectivity under mild reaction conditions, attracting significant attention. In this review, we focus on the recent advances in the catalytic hydroconversion processes for upcycling plastics wastes to fuels and valuable chemicals. The hydrocracking and hydrogenolysis of polyolefins as well as the hydrogenolysis and hydrodeoxygenation of heteroatom-containing plastics are summarized and exemplified. Besides, hydroconversion of plastic wastes by using in situ generated hydrogen is also described. We emphasize the recent progress in catalyst design and its performance. Further, we systematically discuss the functions of typical catalysts and the reaction mechanisms to gain insight into the hydroconversion of plastic wastes. In addition, the effect of some key factors, including macro- and microstructure of plastics, additives, and reaction conditions, on the catalytic performance is demonstrated as well. Given the existing contributions, a perspective is provided to address the challenges and opportunities in the field and to evaluate some potential solutions and avenues for future research. We hope that this review will inspire future research on the rational exploration of optimal catalysts with hydroconversion processes for chemical upcycling of plastic wastes to build a circular plastic economy.
A catalytic architecture, comprising a mesoporous silica shell surrounding platinum nanoparticles (NPs) supported on a solid silica sphere (mSiO2/Pt-X/SiO2; X is the mean NP diameter), catalyzes hydrogenolysis of melt-phase polyethylene (PE) into a narrow C23-centered distribution of hydrocarbons in high yield using very low Pt loadings (∼10-5 g Pt/g PE). During catalysis, a polymer chain enters a pore and contacts a Pt NP where the C-C bond cleavage occurs and then the smaller fragment exits the pore. mSiO2/Pt/SiO2 resists sintering or leaching of Pt and provides high yields of liquids; however, many structural and chemical effects on catalysis are not yet resolved. Here, we report the effects of Pt NP size on activity and selectivity in PE hydrogenolysis. Time-dependent conversion and yields and a lumped kinetics model based on the competitive adsorption of long vs short chains reveal that the activity of catalytic material is highest with the smallest NPs, consistent with a structure-sensitive reaction. Remarkably, the three mSiO2/Pt-X/SiO2 catalysts give equivalent selectivity. We propose that mesoscale pores in the catalytic architecture template the C23-centered distribution, whereas the active Pt sites influence the carbon-carbon bond cleavage rate. This conclusion provides a framework for catalyst design by separating the C-C bond cleavage activity at catalytic sites from selectivity for chain lengths of the products influenced by the structure of the catalytic architecture. The increased activity, selectivity, efficiency, and lifetime obtained using this architecture highlight the benefits of localized and confined environments for isolated catalytic particles under condensed-phase reaction conditions.
Polyethylene terephthalate (PET) and CO2, two chemical wastes that urgently need to be transformed in the environment, are converted simultaneously in a one‐pot catalytic process through the synergistic coupling of three reactions: CO2 hydrogenation, PET methanolysis and dimethyl terephthalate (DMT) hydrogenation. More interestingly, the chemical equilibria of both reactions were shifted forward due to a revealed dual‐promotion effect, leading to significantly enhanced PET depolymerization. The overall methanol yield from CO2 hydrogenation exceeded the original thermodynamic equilibrium limit since the methanol was in situ consumed in the PET methanolysis. The degradation of PET by a stoichiometric ratio of methanol was significantly enhanced because the primary product, DMT was hydrogenated to dimethyl cyclohexanedicarboxylate (DMCD) or p‐xylene (PX). This synergistic catalytic process provides an effective way to simultaneously recycle two wastes, polyesters and CO2, for producing high‐value chemicals.
Collecting and removing ocean plastics can mitigate their environmental
impacts; however, ocean cleanup will be a complex and
energy-intensive operation that has not been fully evaluated. This
work examines the thermodynamic feasibility and subsequent
implications of hydrothermally converting this waste into a fuel
to enable self-powered cleanup. A comprehensive probabilistic
exergy analysis demonstrates that hydrothermal liquefaction has
potential to generate sufficient energy to power both the process
and the ship performing the cleanup. Self-powered cleanup
reduces the number of roundtrips to port of a waste-laden ship,
eliminating the need for fossil fuel use for most plastic concentrations.
Several cleanup scenarios are modeled for the Great Pacific
Garbage Patch (GPGP), corresponding to 230 t to 11,500 t of plastic
removed yearly; the range corresponds to uncertainty in the surface
concentration of plastics in the GPGP. Estimated cleanup times
depends mainly on the number of booms that can be deployed in
the GPGP without sacrificing collection efficiency. Self-powered
cleanup may be a viable approach for removal of plastics from the
ocean, and gaps in our understanding of GPGP characteristics
should be addressed to reduce uncertainty.
Plastics waste has become a major environmental threat, with polyethylene being one of the most produced and hardest to recycle plastics. Hydrogenolysis is potentially the most viable catalytic technology for recycling. Ruthenium (Ru) is one of the most active hydrogenolysis catalysts but yields too much methane. Here we introduce ruthenium supported on tungstated zirconia (Ru-WZr) for hydrogenolysis of low-density polyethylene (LDPE). We show that the Ru-WZr catalysts suppress methane formation and produce a product distribution in the diesel and wax/lubricant base-oil range unattainable by Ru-Zr and other Ru-supported catalysts. Importantly, the enhanced performance is showcased for real-world, single-use LDPE consumables. Reactivity studies combined with characterization and density functional theory calculations reveal that highly dispersed (WO x) n clusters store H as surface hydroxyls by spillover. We correlate this hydrogen storage mechanism with hydrogenation and desorption of long alkyl intermediates that would otherwise undergo further C−C scission to produce methane.
Upcycling of spent plastics has become a more emergent topic than ever before due to the rapid generation of plastic waste associated with the change of lifestyles of the human society. Polyethylene terephthalate (PET) is a major aromatic plastic and herein, the conversion of PET back into arenes was demonstrated in a one‐pot reaction combining depolymerization and hydrodeoxygenation (HDO) over a Co/TiO2 catalyst. The effectiveness of the Co/TiO2 catalyst in HDO and the underlining reaction pathway were established using the PET monomer terephthalic acid (TPA) as the substrate. Quantitative TPA conversion together with 75.2 mol% xylene and toluene selectivity under 30 bar initial H2 pressure at 340 °C was achieved after 4 h reaction. More encouragingly, the catalyst induced both depolymerization and HDO reaction via C−O bond cleavage when PET was used as a substrate. 78.9 mol% arenes (toluene and xylene) was obtained under optimized conditions.
Plastic waste, which is one of the major sources of pollution in the landfills and oceans, has raised global concern, primarily due to the huge production rate, high durability, and the lack of utilization of the available waste management techniques. Recycling methods are preferable to reduce the impact of plastic pollution to some extent. However, most of the recycling techniques are associated with different drawbacks, high cost and downgrading of product quality being among the notable ones. The sustainable option here is to upcycle the plastic waste to create high‐value materials to compensate for the cost of production. Several upcycling techniques are constantly being investigated and explored, which is currently the only economical option to resolve the plastic waste issue. This Review provides a comprehensive insight on the promising chemical routes available for upcycling of the most widely used plastic and mixed plastic wastes. The challenges inherent to these processes, the recent advances, and the significant role of the science and research community in resolving these issues are further emphasized.
We reviewed the current state of the art of poly(ethylene terephthalate) (PET) chemolysis used in the chemical recycling of PET.
The massive generation of plastic wastes without satisfactory treatment has induced severe environmental problems and gained increasing attentions. In this Minireview, recent progresses in the chemical upcycling of plastic wastes by using various methods (mainly in the past three to five years) is summarized. The chemical upcycling of plastic wastes points out a “plastic‐based refinery” concept, which is to use the plastic wastes as platform feedstocks to produce highly valuable monomeric or oligomeric compounds, putting the plastic wastes back into a circular economy. The different chemical methods to upcycle plastic wastes, including hydrogenolysis, photocatalysis, pyrolysis, solvolysis, and others, are introduced in each section to valorize diverse plastic feedstocks into value‐added chemicals, materials, or fuels. In addition, other emerging technologies as well as the new generation of plastic thermosets are covered.
The linear approach to resource utilization has led to the accumulation of waste plastic in the environment for decades. Unfortunately, both traditional mechanical recycling and incineration have faced their bottlenecks that have always resulted in quality deterioration and value recovery failures. Recently, chemical recycling and upcycling processes, including the conversion of plastics into their virgin monomers, liquid fuels, or chemical feedstocks to produce value‐added products, have been identified as the most promising strategy for recovering value from waste plastics. However, these methods are often cost prohibitive and relying on stringent conditions compared to current recycling methods. Accordingly, this Minireview summarizes recent trends and achievements in the chemical recycling and upcycling of waste plastics. We highlight three research topics: depolymerization of plastics into monomers; degradation of plastics into liquid fuels and waxes; and conversion of plastics into hydrogen, fine chemical feedstocks, and value‐added functional materials. Indeed, chemical recycling and upcycling is a bright path to a circular and environmentally friendly plastic economy.
The Sabatier principle, which states that the binding energy between the catalyst and the reactant should be neither too strong nor too weak, has been widely used as the key criterion in designing and screening electrocatalytic materials necessary to promote the sustainability of our society. The widespread success of density functional theory (DFT) has made binding energy calculations a routine practice, turning the Sabatier principle from an empirical principle into a quantitative predictive tool. Given its importance in electrocatalysis, we have attempted to introduce the reader to the fundamental concepts of the Sabatier principle with a highlight on the limitations and challenges in its current thermodynamic context. The Sabatier principle is situated at the heart of catalyst development, and moving beyond its current thermodynamic framework is expected to promote the identification of next-generation electrocatalysts.
Single-use plastics impose an enormous environmental threat, but their recycling, especially of polyolefins, has been proven challenging. We report a direct method to selectively convert polyolefins to branched, liquid fuels including diesel, jet, and gasoline-range hydrocarbons, with high yield up to 85% over Pt/WO 3 /ZrO 2 and HY zeolite in hydrogen at temperatures as low as 225°C. The process proceeds via tandem catalysis with initial activation of the polymer primarily over Pt, with subsequent cracking over the acid sites of WO 3 /ZrO 2 and HY zeolite, isomerization over WO 3 /ZrO 2 sites, and hydrogenation of olefin intermediates over Pt. The process can be tuned to convert different common plastic wastes, including low-and high-density polyethylene, polypropylene, polystyrene, everyday polyethylene bottles and bags, and composite plastics to desirable fuels and light lubricants.
The production, consumption, and waste of plastics have been rapidly growing worldwide in the last decades. A variety of data are needed to characterize plastics stocks and flows across space, time, and life cycle to derive insights for developing strategies to address various sustainability challenges from plastics and plastics waste. Here we review data sources on plastics stocks and flows to identify data gaps and research needs. We categorize the reviewed data sources by life cycle stages of plastics including material production, semi‐manufacturing, manufacturing, additives, consumption, in‐use stock, end‐of‐life, waste treatment, and trade. We identify four data gaps in these existing data for characterizing plastics stocks and flows, including inconsistent classification, missing data, conflicting data, and inexplicit data for plastics products and waste. These data gaps represent critical research needs including common platform for data sharing, standard methods for data reconciliation and estimation, consistent data collection and reporting, and new approaches for data collection and curation. This review establishes the state‐of‐the‐art of plastics stock and flow data and develops a roadmap for a high‐quality, comprehensive characterization of plastics stocks and flows to develop management strategies to address the sustainability challenges of plastics production, consumption, waste, and pollution.
Whilst plastics have played an instrumental role in human development, growing environmental concerns have led to increasing public scrutiny and demands for outright bans. This has stimulated considerable research into renewable alternatives, and more recently, the development of alternative waste management strategies. Herein, the aim was to highlight recent developments in the catalytic chemical recycling of two commercial polyesters, namely poly(lactic acid) (PLA) and poly(ethylene terephthalate) (PET). The concept of chemical recycling is first introduced, and associated opportunities/challenges are discussed within the context of the governing depolymerisation thermodynamics. Chemical recycling methods for PLA and PET are then discussed, with a particular focus on upcycling and the use of metal‐based catalysts. Finally, the attention shifts to the emergence of new materials with the potential to modernise the plastics economy. Emerging opportunities and challenges are discussed within the context of industrial feasibility.
The strong desire for a circular economy makes obtaining fuels and chemicals via plastic degradation an important research topic in the 21st century. Here, the first example of the H2‐free polyethylene terephthalate (PET) conversion to BTX (benzene, toluene and xylene) was achieved by unlocking hidden hydrogen in the ethylene glycol part over Ru/Nb2O5 catalyst. Among the whole process (hydrolysis, reforming and hydrogenolysis/decarboxylation), the parallel hydrogenolysis and decarboxylation were competing and the rate‐determining step. Ru/Nb2O5 exhibited superior hydrogenolysis and poorer decarboxylation performance in direct comparison with Ru/NiAl2O4, accordingly contributing to the distinct selectivity to alkyl aromatics among BTX. Ru species on Nb2O5, unlike those on NiAl2O4, showed more Ruδ+ species owing to the strong interaction between Ru and Nb2O5, restricting the undesired decarboxylation. Along with NbOx species for C−O bond activation, excellent reactivity towards the H2‐free conversion of PET back to BTX with alkyl aromatics as dominant species was achieved. This H2‐free system was also capable of converting common real PET plastics back to BTX, adding new options in the circular economy of PET.
Poly(butylene succinate) (PBS), a biodegradable plastic, is expected to have a huge consumer product markets in the near future. In this work, PBS was found to be an excellent source to produce valuable tetrahydrofuran (THF), through a solvent-less catalytic hydrogenolysis process over Pd/C catalyst. A high THF yield, 59.5 wt%, was achieved via a simple one-pot reaction under mild temperature conditions (240 oC). This technique would provide a distinctive but important way for green production of THF from spent PBS.
Closed-loop recycling offers the opportunity to mitigate plastic waste through reversible polymer construction and deconstruction. Although examples of chemical recycling of polymers are known, few have been applied to materials derived from abundant commodity olefinic monomers, which are the building blocks of ubiquitous plastic resins. Here we describe a [2+2] cycloaddition/oligomerization of 1,3-butadiene to yield a previously unrealized telechelic microstructure of (1,n′-divinyl)oligocyclobutane. This material is thermally stable, has stereoregular segments arising from chain-end control, and exhibits high crystallinity even at low molecular weight. Exposure of the oligocyclobutane to vacuum in the presence of the pyridine(diimine) iron precatalyst used to synthesize it resulted in deoligomerization to generate pristine butadiene, demonstrating a rare example of closed-loop chemical recycling of an oligomeric material derived from a commodity hydrocarbon feedstock.
Chemical upcycling of waste polyolefins via hydrogenolysis offers unique opportunities for selective depolymerization compared to high temperature thermal deconstruction. Here, we demonstrate the hydrogenolysis of polyethylene into liquid alkanes under mild conditions using ruthenium nanoparticles supported on carbon (Ru/C). Reactivity studies on a model n-octadecane substrate showed that Ru/C catalysts are highly active and selective for the hydrogenolysis of C(sp³)–C(sp³) bonds at temperatures ranging from 200 to 250 °C. Under optimal conditions of 200 °C in 20 bar H2, polyethylene (average Mw ∼ 4000 Da) was converted into liquid n-alkanes with yields of up to 45% by mass after 16 h using a 5 wt % Ru/C catalyst with the remaining products comprising light alkane gases (C1–C6). At 250 °C, nearly stoichiometric yields of CH4 were obtained from polyethylene over the catalyst. The hydrogenolysis of long chain, low-density polyethylene (LDPE) and a postconsumer LDPE plastic bottle to produce C7–C45 alkanes was also achieved over Ru/C, demonstrating the feasibility of this reaction for the valorization of realistic postconsumer plastic waste. By identifying Ru-based catalysts as a class of active materials for the hydrogenolysis of polyethylene, this study elucidates promising avenues for the valorization of plastic waste under mild conditions.
The upgrading of plastic waste is one of the grand challenges for the 21st century owing to its disruptive impact on the environment. Here, we show the first example of the upgrading of various aromatic plastic wastes with C−O and/or C−C linkages to arenes (75–85 % yield) via catalytic hydrogenolysis over a Ru/Nb2O5 catalyst. This catalyst not only allows the selective conversion of single‐component aromatic plastic, and more importantly, enables the simultaneous conversion of a mixture of aromatic plastic to arenes. The excellent performance is attributed to unique features including: (1) the small sized Ru clusters on Nb2O5, which prevent the adsorption of aromatic ring and its hydrogenation; (2) the strong oxygen affinity of NbOx species for C−O bond activation and Brønsted acid sites for C−C bond activation. This study offers a catalytic path to integrate aromatic plastic waste back into the supply chain of plastic production under the context of circular economy.
In this study, Pt nanoparticles on zeolite/γ-Al2O3 composites (50/50 wt) were located either in the zeolite or on the γ-Al2O3 binder, hereby varying the average distance (intimacy) between zeolite acid sites and metal sites from “closest” to “nanoscale”. The catalytic performance of these catalysts was compared to physical mixtures of zeolite and Pt/γ-Al2O3 powders, which provide a “microscale” distance between sites. Several beneficial effects on catalytic activity and selectivity for n-heptane hydroisomerization were observed when Pt nanoparticles are located on the γ-Al2O3 binder in nanoscale proximity with zeolite acid sites, as opposed to Pt nanoparticles located inside zeolite crystals. On ZSM-5-based catalysts, mostly monobranched isomers were produced, and the isomer selectivity of these catalysts was almost unaffected with an intimacy ranging from closest to microscale, which can be attributed to the high diffusional barriers of branched isomers within ZSM-5 micropores. For composite catalysts based on large-pore zeolites (zeolite Beta and zeolite Y), the activity and selectivity benefitted from the nanoscale intimacy with Pt, compared to both the closest and microscale intimacies. Intracrystalline gradients of heptenes as reaction intermediates are likely contributors to differences in activity and selectivity. This paper aims to provide insights into the influence of the metal–acid intimacy in bifunctional catalysts based on zeolites with different framework topologies.
Many plastic packaging materials manufactured today are composites made of distinct polymer layers (i.e., multilayer films). Billions of pounds of these multilayer films are produced annually, but manufacturing inefficiencies result in large, corresponding postindustrial waste streams. Although relatively clean (as opposed to municipal wastes) and of near-constant composition, no commercially practiced technologies exist to fully deconstruct postindustrial multilayer film wastes into pure, recyclable polymers. Here, we demonstrate a unique strategy we call solvent-targeted recovery and precipitation (STRAP) to deconstruct multilayer films into their constituent resins using a series of solvent washes that are guided by thermodynamic calculations of polymer solubility. We show that the STRAP process is able to separate three representative polymers (polyethylene, ethylene vinyl alcohol, and polyethylene terephthalate) from a commercially available multilayer film with nearly 100% material efficiency, affording recyclable resins that are cost-competitive with the corresponding virgin materials.
Background: Pollution – unwanted waste released to air, water, and land by human activity – is the largest environmental cause of disease in the world today. It is responsible for an estimated nine million premature deaths per year, enormous economic losses, erosion of human capital, and degradation of ecosystems. Ocean pollution is an important, but insufficiently recognized and inadequately controlled component of global pollution. It poses serious threats to human health and well-being. The nature and magnitude of these impacts are only beginning to be understood. Goals: (1) Broadly examine the known and potential impacts of ocean pollution on human health. (2) Inform policy makers, government leaders, international organizations, civil society, and the global public of these threats. (3) Propose priorities for interventions to control and prevent pollution of the seas and safeguard human health.
Methods: Topic-focused reviews that examine the effects of ocean pollution on human health, identify gaps in knowledge, project future trends, and offer evidence-based guidance for effective intervention.
Environmental Findings: Pollution of the oceans is widespread, worsening, and in most countries poorly controlled. It is a complex mixture of toxic metals, plastics, manufactured chemicals, petroleum, urban and industrial wastes, pesticides, fertilizers, pharmaceutical chemicals, agricultural runoff, and sewage. More than 80% arises from land-based sources. It reaches the oceans through rivers, runoff, atmospheric deposition and direct discharges. It is often heaviest near the coasts and most highly concentrated along the coasts of low- and middle-income countries. Plastic is a rapidly increasing and highly visible component of ocean pollution, and an estimated 10 million metric tons of plastic waste enter the seas each year. Mercury is the metal pollutant of greatest concern in the oceans; it is released from two main sources – coal combustion and small-scale gold mining. Global spread of industrialized agriculture with increasing use of chemical fertilizer leads to extension of Harmful Algal Blooms (HABs) to previously unaffected regions. Chemical pollutants are ubiquitous and contaminate seas and marine organisms from the high Arctic to the abyssal depths.
Ecosystem Findings: Ocean pollution has multiple negative impacts on marine ecosystems, and these impacts are exacerbated by global climate change. Petroleum-based pollutants reduce photosynthesis in marine microorganisms that generate oxygen. Increasing absorption of carbon dioxide into the seas causes ocean acidification, which destroys coral reefs, impairs shellfish development, dissolves calcium-containing microorganisms at the base of the marine food web, and increases the toxicity of some pollutants. Plastic pollution threatens marine mammals, fish, and seabirds and accumulates in large mid-ocean gyres. It breaks down into microplastic and nanoplastic particles containing multiple manufactured chemicals that can enter the tissues of marine organisms, including species consumed by humans. Industrial releases, runoff, and sewage increase frequency and severity of HABs, bacterial pollution, and anti-microbial resistance. Pollution and sea surface warming are triggering poleward migration of dangerous pathogens such as the Vibrio species. Industrial discharges, pharmaceutical wastes, pesticides, and sewage contribute to global declines in fish stocks.
Human Health Findings: Methylmercury and PCBs are the ocean pollutants whose human health effects are best understood. Exposures of infants in utero to these pollutants through maternal consumption of contaminated seafood can damage developing brains, reduce IQ and increase children’s risks for autism, ADHD and learning disorders. Adult exposures to methylmercury increase risks for cardiovascular disease and dementia. Manufactured chemicals – phthalates, bisphenol A, flame retardants, and perfluorinated chemicals, many of them released into the seas from plastic waste – can disrupt endocrine signaling, reduce male fertility, damage the nervous system, and increase risk of cancer. HABs produce potent toxins that accumulate in fish and shellfish. When ingested, these toxins can cause severe neurological impairment and rapid death. HAB toxins can also become airborne and cause respiratory disease. Pathogenic marine bacteria cause gastrointestinal diseases and deep wound infections. With climate change and increasing pollution, risk is high that Vibrio infections, including cholera, will increase in frequency and extend to new areas. All of the health impacts of ocean pollution fall disproportionately on vulnerable populations in the Global South – environmental injustice on a planetary scale.
Conclusions: Ocean pollution is a global problem. It arises from multiple sources and crosses national boundaries. It is the consequence of reckless, shortsighted, and unsustainable exploitation of the earth’s resources. It endangers marine ecosystems. It impedes the production of atmospheric oxygen. Its threats to human health are great and growing, but still incompletely understood. Its economic costs are only beginning to be counted. Ocean pollution can be prevented. Like all forms of pollution, ocean pollution can be controlled by deploying data-driven strategies based on law, policy, technology, and enforcement that target priority pollution sources. Many countries have used these tools to control air and water pollution and are now applying them to ocean pollution. Successes achieved to date demonstrate that broader control is feasible. Heavily polluted harbors have been cleaned, estuaries rejuvenated, and coral reefs restored. Prevention of ocean pollution creates many benefits. It boosts economies, increases tourism, helps restore fisheries, and improves human health and well-being. It advances the Sustainable Development Goals (SDG). These benefits will last for centuries.
Recommendations: World leaders who recognize the gravity of ocean pollution, acknowledge its growing dangers, engage civil society and the global public, and take bold, evidence-based action to stop pollution at source will be critical to preventing ocean pollution and safeguarding human health. Prevention of pollution from land-based sources is key. Eliminating coal combustion and banning all uses of mercury will reduce mercury pollution. Bans on single-use plastic and better management of plastic waste reduce plastic pollution. Bans on persistent organic pollutants (POPs) have reduced pollution by PCBs and DDT. Control of industrial discharges, treatment of sewage, and reduced applications of fertilizers have mitigated coastal pollution and are reducing frequency of HABs. National, regional and international marine pollution control programs that are adequately funded and backed by strong enforcement have been shown to be effective. Robust monitoring is essential to track progress. Further interventions that hold great promise include wide-scale transition to renewable fuels; transition to a circular economy that creates little waste and focuses on equity rather than on endless growth; embracing the principles of green chemistry; and building scientific capacity in all countries. Designation of Marine Protected Areas (MPAs) will safeguard critical ecosystems, protect vulnerable fish stocks, and enhance human health and well-being. Creation of MPAs is an important manifestation of national and international commitment to protecting the health of the seas.
The current global plastics economy is highly linear, with the exceptional performance and low carbon footprint of polymeric materials at odds with dramatic increases in plastic waste. Transitioning to a circular economy that retains plastic in its highest value condition is essential to reduce environmental impacts, promoting reduction, reuse, and recycling. Mechanical recycling is an essential tool in an environmentally and economically sustainable economy of plastics, but current mechanical recycling processes are limited by cost, degradation of mechanical properties, and inconsistent quality products. This review covers the current methods and challenges for the mechanical recycling of the five main packaging plastics: poly(ethylene terephthalate), polyethylene, polypropylene, polystyrene, and poly(vinyl chloride) through the lens of a circular economy. Their reprocessing induced degradation mechanisms are introduced and strategies to improve their recycling are discussed. Additionally, this review briefly examines approaches to improve polymer blending in mixed plastic waste streams and applications of lower quality recyclate.
Alkanol dehydration rates catalyzed by hydronium ions are enhanced by the dimensions of steric confinements of zeolite pores as well as by intraporous intermolecular interactions with other alkanols. The higher rates with zeolite MFI having pores smaller than those of zeolite BEA for dehydration of secondary alkanols, 3‐heptanol and 2‐methyl‐3‐hexanol, is caused by the lower activation enthalpy in the tighter confinements of MFI that offsets a less positive activation entropy. The higher activity in BEA than in MFI for dehydration of a tertiary alkanol, 2‐methyl‐2‐hexanol, is primarily attributed to the reduction of the activation enthalpy by stabilizing intraporous interactions of the Cβ‐H transition state with surrounding alcohol molecules. Overall, we show that the positive impact of zeolite confinements results from the stabilization of transition state provided by the confinement and intermolecular interaction of alkanols with the transition state, which is impacted by both the size of confinements and the structure of alkanols in the E1 pathway of dehydration.
Plastic solid waste (PSW) is an ever‐growing environmental challenge for our society, as it not only ends up in landfills but also in waterways and oceans and is consequently entering the food chain. A key strategy to overcome this problem while also preserving carbon resources is to use PSW as a feedstock, evolving towards a circular economy. To implement this, mechanical as well as chemical recycling technologies must be developed. Indeed, owing to the high volume of PSW generated each year, mechanical recycling alone is not adequate for addressing this global challenge. Because of this, chemical recycling via thermal and heterogeneous catalytic conversion has received growing attention. This process has the potential to take PSW and convert it into usable monomers, fuels, synthesis gas, and adsorbents under more sustainable conditions than thermal degradation. This Review highlights the recent research advances in catalytic technologies for PSW conversion and valorization.
The overconsumption of single-use plastics is creating a global waste catastrophe, with widespread environmental, economic and health-related consequences. Here we show that the benefits of processive enzyme-catalysed conversions of biomacromolecules can be leveraged to affect the selective hydrogenolysis of high-density polyethylene into a narrow distribution of diesel and lubricant-range alkanes using an ordered, mesoporous shell/active site/core catalyst architecture that incorporates catalytic platinum sites at the base of the mesopores. Solid-state nuclear magnetic resonance revealed that long hydrocarbon macromolecules readily move within the pores of this catalyst, with a subsequent escape being inhibited by polymer–surface interactions, a behaviour that resembles the binding and translocation of macromolecules in the catalytic cleft of processive enzymes. Accordingly, the hydrogenolysis of polyethylene with this catalyst proceeds processively to yield a reliable, narrow and tunable stream of alkane products.
A two-step aqueous alkaline hydrolysis was carried out on different types of real PET plastic waste under mild conditions.
Polyethylene terephthalate (PET) is selectively depolymerized by a carbon‐supported single‐site molybdenum‐dioxo catalyst to terephthalic acid (PTA) and ethylene. The solventless reactions are most efficient under 1 atmosphere of H2. The catalyst exhibits high stability and can be recycled multiple times without loss of activity. Waste beverage bottle PET or a PET + polypropylene (PP) mixture (simulating the bottle + cap) proceeds at 260 °C with complete PET deconstruction and quantitative PTA isolation. Mechanistic studies with a model diester, 1,2‐ethanediol dibenzoate, suggest the reaction proceeds by initial retro‐hydroalkoxylation/β‐C−O scission and subsequent hydrogenolysis of the vinyl benzoate intermediate.
To develop viable solutions for reducing plastic waste, spatially explicit data on the management of these materials are critical. Here we employ statistical and geospatial methods to present a comprehensive assessment of plastic waste in the United States by resin type at the state, county, and local levels. Of the estimated 44 Mt of plastic waste managed in 2019 domestically, approximately 86% was landfilled, 9% was combusted, and 5% was recycled. Landfilled plastics represented significant losses to the country's economy in 2019: an average of US$7.2 billion in market value, about 3.4 EJ as embodied energy (equivalent to 12% of energy consumption by the industrial sector), and 1.5 EJ as an energy source (equivalent to 5.5% and 5% of energy consumption by the industrial and transportation sectors, respectively). Lastly, we posit that substantial amount of landfilled plastic waste could be recovered through advanced sorting, existing, and emerging recycling processes.
The vast majority of commodity plastics do not degrade and therefore they permanently pollute the environment. At present, less than 20% of post-consumer plastic waste in developed countries is recycled, predominately for energy recovery or repurposing as lower-value materials by mechanical recycling. Chemical recycling offers an opportunity to revert plastics back to monomers for repolymerization to virgin materials without altering the properties of the material or the economic value of the polymer. For plastic waste that is either cost prohibitive or infeasible to mechanically or chemically recycle, the nascent field of chemical upcycling promises to use chemical or engineering approaches to place plastic waste at the beginning of a new value chain. Here state-of-the-art methods are highlighted for upcycling plastic waste into value-added performance materials, fine chemicals and specialty polymers. By identifying common conceptual approaches, we critically discuss how the advantages and challenges of each approach contribute to the goal of realizing a sustainable plastics economy. Methods for the transformation of plastics into materials with value, known as plastic waste upcycling, are outlined, and their advantages and challenges in terms of a sustainable plastics economy are discussed.
There is an urgent need for new technologies to enable circularity for synthetic polymers, spurred by the accumulation of waste plastics in landfills and the environment and the contributions of plastics manufacturing to climate change. Chemical recycling is a promising means to convert waste plastics into molecular intermediates that can be remanufactured into new products. Given the growing interest in the development of new chemical recycling approaches, it is critical to evaluate the economics, energy use, greenhouse gas emissions, and other life cycle inventory metrics for emerging processes, relative to the incumbent, linear manufacturing practices employed today. Here we offer specific definitions for classes of chemical recycling and upcycling and describe general process concepts for the chemical recycling of mixed plastics waste. We present a framework for techno-economic analysis and life cycle assessment for both closed- and open-loop chemical recycling. Rigorous application of these process analysis tools will be required to enable impactful solutions for the plastics waste problem.
Expected final online publication date for the Annual Review of Chemical and Biomolecular Engineering, Volume 13 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
Polyethylene terephthalate (PET) and CO2, two chemical wastes that urgently need to be transformed in the environment, are converted simultaneously in a one-pot catalytic process through the synergistic coupling of three reactions: CO2 hydrogenation, PET methanolysis and dimethyl terephthalate (DMT) hydrogenation. More interestingly, the chemicals equilibria of both reactions were shifted forward due to a revealed dual-promotion effect, leading to significantly enhanced PET depolymerization. The overall methanol yield from CO2 hydrogenation exceeded the original thermodynamic equilibrium limit since the methanol was in-situ consumed in PET methanolysis. The degradation of PET by a stoichiometric ratio of methanol was significantly enhanced because the primary product, DMT was hydrogenated to dimethyl cyclohexanedicarboxylate (DMCD) or p-xylene (PX). This synergistic catalytic process provides an effective way to simultaneously recycle two wastes, polyesters and CO2, for producing high-value chemicals.
Hydroconversion technologies have surged to the forefront of deconstructing plastic waste. Recent studies have been performed over several catalysts with varying conditions and plastics that make comparisons difficult. We compile and compare data from the literature by introducing various metrics and perform a simple energy analysis. We draw mechanistic similarities to and differences from the past literature on small alkane hydroconversion and leverage the former to propose standard approaches to tune product selectivity. We exemplify the plastics materials gap and the challenges it creates. Finally, we discuss the current limitations and suggest future work.
A Pt/W/Beta catalyst was developed for direct and efficient hydrogenolysis of low density polyethylene (LDPE) into C5-C12 gasoline alkanes with a high yield of 63.6 wt% (yield of C1-C13 light alkanes is 94.0 wt%) at 250 °C and 3.0 MPa H2 within 1.0 h. The obtained gasoline alkanes have a narrow carbon number distribution and contain abundant mono-branched isomers, which were attributed to well-dispersed Pt species and proximity of the cooperating active sites, especially the moderate acidity upon the introduction of 0.5 wt% tungsten species. The catalyst was also applicable to hydrogenolysis of light low density polyethylene (LLDPE), high density polyethylene (HDPE) and polypropylene (PP) to obtain the valuable gasoline alkanes (C5-C12) in high yield (50.8-73.5 wt%). The C-C bond cleavage of plastics over the Pt/W/Beta catalyst was found to occur through medium to long-chain hydrocarbons (n-Cx, x > 13) based on experiments performed using docosane (n-C22 alkane) as the model feedstock.
This perspective discusses a set of design principles for next-generation kinetically trapped, intrinsically circular polymers (iCPs) that are inherently, selectively, and expediently depolymerizable to their monomer state once their kinetic barriers of deconstruction are overcome, thereby enabling not only the ideal shortest chemical circularity but also tunable performance properties. After describing four elements of the design principles—thermodynamics and kinetics, strategies to overcome trade-offs and unify conflicting properties, predictive modeling, and supply-chain life-cycle assessment and techno-economic analysis, which are illustrated with state-of-the-art examples—it concludes with presenting key challenges and opportunities for sustainable development of iCPs.
Catalytic depolymerization of polyolefins is a promising chemical recycling strategy to create value-added products from waste plastics, which are accumulating in landfills and the natural environment at unsustainable rates. The cleavage of strong C-C bonds in polyolefins can be performed using a noble metal and hydrogen via a hydrogenolysis mechanism. Previously, we identified ruthenium nanoparticles supported on carbon (Ru/C) as a highly active heterogeneous catalyst for the conversion of polyethylene into liquid and gaseous n-alkanes under mild conditions. In the present study, we investigated the catalytic depolymerization of polypropylene (PP) under mild conditions (200-250 °C, 20-50 bar H2). We demonstrate that Ru/C produces C5-C32 iso-alkane yields above 68% in the absence of solvent and identify trade-offs between product yield and temperature, hydrogen pressure, and reaction time. We apply a rigorous analytical method to quantify all liquid and gaseous alkane products. The characterized catalyst was found to be recyclable after the complete depolymerization of high molecular weight PP (Mw ∼340,000 Da) to liquid and gaseous hydrocarbons and after depolymerization of a postconsumer PP centrifuge tube. Further, the catalyst was shown to be effective in depolymerizing a mixture of high-density polyethylene and PP to produce a mixture of linear and branched liquid alkanes, demonstrating feasibility for the depolymerization of streams of mixed polyolefin waste.
Plastics pollution is causing an environmental crisis, prompting the development of new approaches for recycling, and upcycling. Here, we review challenges and opportunities in chemical and biological catalysis for plastics deconstruction, recycling, and upcycling. We stress the need for rigorous characterization and use of widely available substrates, such that catalyst performance can be compared across studies. Where appropriate, we draw parallels between catalysis on biomass and plastics, as both substrates are low-value, solid, recalcitrant polymers. Innovations in catalyst design and reaction engineering are needed to overcome kinetic and thermodynamic limitations of plastics deconstruction. Either chemical and biological catalysts will need to act interfacially, where catalysts function at a solid surface, or polymers will need to be solubilized or processed to smaller intermediates to facilitate improved catalyst–substrate interaction. Overall, developing catalyst-driven technologies for plastics deconstruction and upcycling is critical to incentivize improved plastics reclamation and reduce the severe global burden of plastic waste. Plastics are invaluable materials for modern society, although they result in the generation of large amounts of litter at the end of their life cycle. This Review explores the challenges and opportunities associated with the catalytic transformation of waste plastics, looking at both chemical and biological approaches to transforming such spent materials into a resource.
Hydrocracking is an insufficiently explored route for chemical recycling of plastics waste. Platinum tungstated zirconia (Pt-WZr) was used in a batch reactor at low temperatures of 250 °C and 30 bar H2 pressure for 1-24 h reaction times using low-density polyethylene (LDPE, Mw∼76 kDa). We find Pt-WZr is a bifunctional catalyst for LDPE hydrocracking leading to higher value branched fuel- and lubricant-ranged alkanes. We demonstrate that the catalyst metal-to-acid site molar ratio (MAB) shifts the product distribution to larger cracked products and increases the isomerization degree in the residual polymer. We propose a new adhesive isomerization mechanism between the metal and Brønsted acid sites in parallel with slow polymer chain cracking, caused by competitive adsorption of the polymer over the liquid products and stereochemical hindrance of methines. This study provides a blueprint on how to engineer effective catalysts for hydrocracking polyolefin plastic wastes using the MAB as a catalyst descriptor.
Polyethylene (PE) is the most popular plastic globally, and the widespread use of plastics has created severe environmental issues. High energy consumption in the current process makes its recycling a challenging problem. In our report, the depolymerization of high-density PE was conducted in various liquid-phase solvents with the Ru/C catalyst under relatively mild conditions. The maximum yields of the jet-fuel- and lubricant-range hydrocarbons were 60.8 and 31.6 wt %, respectively. After optimization of the reaction conditions (220°C and 60 bar of H2), the total yield of liquid hydrocarbon products reached approximately 90 wt % within only 1 h. The product distribution could be tuned by the H2 partial pressure, the active-metal particle size, and the solvents. The solvation of PE in the different solvents determined the depolymerization reaction kinetics, which was confirmed by the molecular dynamics simulation results.
For the first time, propane-2,2-diyldicyclohexane, a jet fuel range C15dicycloalkane was directly produced by the aqueous phase hydrodeoxygenation (APHDO) of polycarbonate (PC). Among the investigated catalyst systems, the mixture of Rh/C and H-USY (denoted as Rh/C + H-USY) exhibited the best performance. Over it, a high yield of propane-2,2-diyldicyclohexane (94.9%) was achieved from the APHDO of pure PC pellets after a reaction was carried out at 473 K and 3.5 MPa H2for 12 h. The Rh/C + H-USY catalyst was stable under the investigated conditions. No evident deactivation was observed during three repeated cycles. When we used a chopped DVD disk (a representative of real PC wastes) as the substrate, a high yield (86.9%) of propane-2,2-diyldicyclohexane was obtained under the same reaction conditions. The propane-2,2-diyldicyclohexane as obtained had a high density (0.92 g mL⁻¹) and a high volumetric net heat of combustion (39.6 MJ L⁻¹). As a potential application, it can be blended into jet fuels to improve the range and payload of airplanes.
A series of terephthalamides were successfully produced by rapid catalyst-free microwave-assisted aminolysis of polyethylene terephthalate (PET). The produced terephthalamides from chemical recycling of PET were further utilized as reactants for fabrication of plastic films by a radical thiol-ene reaction or they were evaluated as plasticizers for polylactide (PLA). The chemical recycling by aminolysis was performed with four different amines: allylamine, ethanolamine, furfurylamine or hexylamine. The microwave-assisted process led selectively to good yields of well-defined terephthalamides with different terminal functional groups depending on the used amine. The reaction time varied from 10 to 60 min. The end-product obtained after aminolysis with allylamine was further reacted with a thiol through a radical thiol-ene reaction, producing good quality films with glass transition temperature (Tg) above room temperature. The three other terephthalamides were mixed with PLA at 10 wt% and evaluated as plasticizers. The terephthalamide produced by aminolysis with furfurylamine, increased the strain at break 20 times compared to the strain at break of neat PLA. The results illustrate that chemical recycling of PET by aminolysis is a viable and versatile option, producing a library of valuable chemical compounds ready for further use in material applications.
The rate of acid-base-catalyzed dehydration of alcohols strongly depends on the solvent and the environment of the acid sites. We find that Brønsted acidic sites in large-pore zeolites, but not in medium-pore zeolites, catalyze cyclohexanol dehydration in decalin at significantly higher rates than hydrated hydronium ions in aqueous phase. Specifically, the difference in turnover rates between the two solvents amounts to 2-3 orders of magnitude on H-BEA and H-FAU, while being very modest (within a factor of 2) for H-MFI. Combining kinetic, isotopic tracer, and 2H NMR measurements, it is established that cyclohexanol dehydration generally follows an E1-elimination pathway in decalin. A notable exception is the monomer dehydration route on H-MFI, which exhibits a much lower activation energy and a substantially negative activation entropy that appear to be associated with an E2-type mechanism. The C-O bond cleavage displays a dominant degree of rate control in decalin, which stands in contrast to deprotonation (C-H cleavage) being rate-limiting in aqueous-phase dehydration.
There is a growing interest in the depolymerization of polyethylene terephthalate (PET) waste for both environmental and economic reasons by chemical methods. In this work, PET waste was depolymerized through aminolysis using excess amount of monoethanolamine. Bis (2-hydroxy ethylene) terephthalamide (BHETA) was obtained as the main product. For the first time, the reaction conversion was calculated, by FTIR, DSC, TGA, and CHN methods, and compared with that of measured by conventional gravimetric method. As a result, TGA and DSC are the suitable methods for calculating the reaction conversion. The gravimetric method does not measure reaction conversion, but correctly measure the yield of reaction.
Reducing the carbon intensity of plastics production by sourcing sustainable feedstocks while simultaneously enabling effective polymer recycling represents a potential transformation of 21 st century manufacturing. To evaluate technologies that could enable such changes, it is imperative to compare the sustainability of bio-based and/or circular plastic flows to those of incumbent manufacturing paradigms. To that end, we estimate the supply chain energy requirements and greenhouse gas (GHG) emissions associated with US-based plastics consumption. Major commodity polymers, each of which has a global consumption of at least 1 MMT per year, account for an estimated annual 3.2 quadrillion Btutus (quads) of energy and 104 MMTCO 2e of GHG emissions in the US alone. This study serves as a foundation for comparing the supply chain energy requirements and GHG emissions of today’s plastics manufacturing to tomorrow’s disruptive technologies, to inform the development of bio-based plastics and the circular economy for synthetic polymers.
The accumulation of plastic waste in the environment has prompted the development of new chemical recycling technologies. A recently reported approach employed homogeneous organometallic catalysts for tandem dehydrogenation and olefin cross metathesis to depolymerize polyethylene (PE) feedstocks to a mixture of alkane products. Here, we build on that prior work by developing a fully heterogeneous catalyst system using a physical mixture of SnPt/γ-Al2O3 and Re2O7/γ-Al2O3. This heterogeneous catalyst system produces a distribution of linear alkane products from a model, linear C20 alkane, n-eicosane, and from a linear PE substrate (which is representative of high-density polyethylene), both in an n-pentane solvent. For the PE substrate, a molecular weight decrease of 73% was observed at 200 °C in 15 h. This type of tandem chemistry is an example of an olefin-intermediate process, in which poorly reactive aliphatic substrates are first activated through dehydrogenation and then functionalized or cleaved by a highly-active olefin catalyst. Olefin-intermediate processes like that examined here offer both a selective and versatile means to depolymerize polyolefins at lower severity than traditional pyrolysis or cracking conditions.
A new low-temperature catalytic upgrading of waste polyolefinic plastics to valuable chemicals such as liquid fuels and waxes by a heterogeneous catalyst is presented. CeO2-supported Ru (Ru/CeO2) acted as an effective and reusable heterogeneous catalyst, showing much higher activity than other metal-supported catalysts in hydrogenolysis of low-density polyethylene, and the catalyst worked even under mild reaction conditions such as low temperature of 473 K and low H2 pressure of 2 MPa, providing liquid fuel (C5-C21) and wax (C22-C45) in 77% and 15% yields (total 92 % yield), respectively. This catalyst was applicable to hydrogenolysis of various low-density polyethylenes, high-density polyethylene, polypropylene to provide the valuable chemicals (liquid fuel + wax) in high yields (83-90%). Furthermore, a commercial plastic bag and waste polyethylenes could be transformed to the valuable chemicals in high yields (91% and 88% yields).
A new future for polyethylene
Most current plastic recycling involves chopping up the waste and repurposing it in materials with less stringent engineering requirements than the original application. Chemical decomposition at the molecular level could, in principle, lead to higher-value products. However, the carbon-carbon bonds in polyethylene, the most common plastic, tend to resist such approaches without exposure to high-pressure hydrogen. F. Zhang et al. now report that a platinum/alumina catalyst can transform waste polyethylene directly into long-chain alkylbenzenes, a feedstock for detergent manufacture, with no need for external hydrogen (see the Perspective by Weckhuysen).
Science , this issue p. 437 ; see also p. 400
Exchange of deuterium (D) for hydrogen (H) on polyolefins enabled by heterogeneous catalysts is a versatile and relatively inexpensive technique to obtain matched pairs of isotopically labeled and unlabeled polymers. A bimetallic ultrawide pore silica-supported platinum-rhenium catalyst (PtRe/SiO2), originally designed for the hydrogenation of polystyrene (PS), can be used as an isotope exchange catalyst with various saturated hydrocarbon polymers, most notably polyethylene (PE). Recently, we discovered that under certain conditions a commercial linear low-density polyethylene (LLDPE) undergoes severe chain degradation during the H/D exchange reaction. In this study, we explored the effects of reacting various polymers on the PtRe/SiO2 catalyst. First, the extent of hydrogenolysis accompanying deuterium exchange was studied under the most severe reaction conditions (1:1 PtRe/SiO2-to-polymer by weight, 170 °C) with four different polymers: narrow-dispersity PS, perfectly linear PE, poly(ethylene-alt-propylene) (PEP), and a commercial LLDPE. PS was fully saturated to yield poly(cyclohexylethylene) (PCHE) without any detectable hydrogenolysis. Among the polyolefins, linear PE showed the least degradation, PEP incurred an intermediate extent of hydrogenolysis, and LLDPE experienced severe chain degradation; at these reaction conditions, the LLDPE was reduced in weight average molecular weight from 120 to under 11 kg/mol. A time-resolved experiment also revealed the exchange of hydrogen for deuterium on LLDPE coincident with hydrogenolysis following initial uptake of the heavy isotope. This loss of deuterium is due to the interaction of the hydrogenous solvent with the catalyst. Subsequently, the H/D exchange reaction conditions were varied to probe the process leading to LLDPE hydrogenolysis. For this purpose, Pt/SiO2 and PtRe/SiO2 catalysts were compared. When using Pt/SiO2, LLDPE maintained its molecular integrity at all catalyst loadings (1:1, 0.2:1, and 0.1:1 catalyst-to-polymer by weight) and reaction temperatures (130 and 170 °C). In the case of PtRe/SiO2, reducing the catalyst loading decreased but did not eliminate hydrogenolysis of LLDPE. Kinetic experiments and microstructural analysis of the hydrogenolysis products implicated a degradation mechanism involving C-C chain scission away from the tertiary carbon associated with the short (C4H9)-chain branches. These findings suggest a degradation mechanism mediated by the cooperative adsorption of the four-carbon side-chain and backbone units on the catalyst surface. The results of this study set important limitations on the conditions that can be employed to exchange deuterium for hydrogen on LLDPE and other polyolefins using the high-surface-area wide pore PtRe/SiO2 heterogeneous catalyst.
The constant increase of plastic waste released into the environment is a global problem which is of increasing concern to the general population. Although there are many different approaches to the recycling of plastics, chemical recycling is currently seen as one of the most promising technologies in that it allows plastic waste to fit into a sustainable, circular economy. Herein we investigate the chemical recycling of Bisphenol A polycarbonate (BPA-PC) using diols of different chain lengths to yield Bisphenol A and innovative carbonate-containing diols. Subsequently, the latter are polymerised into a series of unique value-added aliphatic polycarbonates (APC). The new polymers obtained by this method have shown promising values of ionic conductivity that make them attractive candidates to be implemented as sustainable polymer electrolytes for solid-state batteries. This procedure opens the way for recycling methods to produce unique, innovative materials using plastic waste as an alternative sustainable feedstock.