Catalytic processing of plastic waste on the rise

Abstract

The exponential growth of plastic waste is a clear threat to the United Nations’ sustainable development goals: Sustainable cities and communities, Responsible consumption and production, and Life below water. Social awareness, policies, and plastic waste processing are three pillars toward a paradigm shift in the plastic economy. While conventional treatments like incineration or mechanical routes cannot cope with the waste generation rate and increasingly stricter regulations, emerging chemical paths targeting fuels, refinery feedstocks, monomers, chemicals, and materials are regarded as a prospective solution. This review intends to support the development of technologies driven by catalysis, given the potential of the latter to maximize economic and ecologic efficiency. To this end, it outlines the state of the art from lab scale to commercial implementation and proposes ideas to foster catalyst design in the light of parallelisms with other established catalysis applications. Approximately 80% of the >350 Mt of plastics produced annually is turned into waste, which equals 30% of the European electricity consumption. Chemocatalytic strategies giving life to fuels, chemicals, and monomers promise to transform plastic pollution, barely addressed by incineration and mechanical recycling, into an opportunity. Under the motto “catalysis generates plastics and should handle their fate,” this review integrates fundamentals and applications of catalytic transformations of seven major plastics. Following algorithmic analysis of >400 articles and patents, it outlines relevant systems for 13 frontrunner routes based on heterogeneous, homogeneous, and biocatalysis. Given the predominance of off-the-shelf catalysts, it stresses transposable state-of-the-art systems of developed fields to foster design of materials. Based on recent system engineering analyses, it highlights more sustainable paths for implementation, unattended by current trends in industry and academia. It closes with a unified view and critical thoughts on research priorities to accelerate progress toward a sustainable plastic economy. Approximately 80% of the >350 Mt of plastics produced annually is turned into waste, which equals 30% of the European electricity consumption. Chemocatalytic strategies giving life to fuels, chemicals, and monomers promise to transform plastic pollution, barely addressed by incineration and mechanical recycling, into an opportunity. Under the motto “catalysis generates plastics and should handle their fate,” this review integrates fundamentals and applications of catalytic transformations of seven major plastics. Following algorithmic analysis of >400 articles and patents, it outlines relevant systems for 13 frontrunner routes based on heterogeneous, homogeneous, and biocatalysis. Given the predominance of off-the-shelf catalysts, it stresses transposable state-of-the-art systems of developed fields to foster design of materials. Based on recent system engineering analyses, it highlights more sustainable paths for implementation, unattended by current trends in industry and academia. It closes with a unified view and critical thoughts on research priorities to accelerate progress toward a sustainable plastic economy. In 1907, Leo Baekeland invented bakelite, the first fully synthetic polymer and coined the term plastic. A century later, plastics are ubiquitous in our lives. Longer preservation and lower cost for food packaging, new textiles, or effective thermal isolations for housing are just a few examples of the undisputable revolution brought by this family of 26 moldable polymers. Approximately 360 Mton of plastics1Geyer R. Production, use, and fate of synthetic polymers.in: Letcher T.M. Plastic Waste and Recycling: Environmental Impact, Societal Issues. Academic Press, 2020: 13-32Crossref Google Scholar are currently manufactured per year, almost exclusively from light (C1-C4) fractions of oil or from natural gas. Notably, seven types of plastics meet 80% of the global plastic demand, with low- and high-density polyethylene (LDPE and HDPE) and polypropylene (PP) being the top contributors (Figure 1A, solid bars).1Geyer R. Production, use, and fate of synthetic polymers.in: Letcher T.M. Plastic Waste and Recycling: Environmental Impact, Societal Issues. Academic Press, 2020: 13-32Crossref Google Scholar Among the fraction of less-impactful plastics some carry special features, such as the toxic bisphenol A-containing polycarbonates, the biodegradable polylactic acid, or the rapid-growing polyamides, and should not be neglected. Plastic waste is quickly accumulating and currently represents a serious economic and environmental challenge. Aproximately $100 billion are lost every year solely due to disposal of plastic packaging and, by 2050, there will be more plastic waste in the ocean than fish by weight, if the current trend continues.1Geyer R. Production, use, and fate of synthetic polymers.in: Letcher T.M. Plastic Waste and Recycling: Environmental Impact, Societal Issues. Academic Press, 2020: 13-32Crossref Google Scholar,2Ryberg M.W. Laurent A. Hauschild M. Mapping of global plastics value chain and plastics losses to the environment.http://wedocs.unep.org/handle/20.500.11822/26745Date: 2018Google Scholar The waste problem is far from simple and, thus, requires prioritized efforts for our sustainable development. Focusing on the main plastics, a quick inspection of their annual production identifies those that are targeted the most by anti-waste measures, but another factor to consider is that plastic production and waste generation differ to a large extent. Indeed, plastics are manufactured in mega plants, whereas waste is created in a distributed manner, which shifts the focus to their end use. It is also relevant to note that each polymer type becomes a waste at a different pace, as easily concluded comparing dashed (waste) and solid (production) bars in Figure 1A. Ca. 95% of polyethylene terephthalate (PET) produced is transformed into waste within a year, whereas only 46% of polyvinyl chloride (PVC) is. The reason behind this discrepancy is the very different lifetime of the products where these plastics are mainly applied, as studied by Geyer et al.1Geyer R. Production, use, and fate of synthetic polymers.in: Letcher T.M. Plastic Waste and Recycling: Environmental Impact, Societal Issues. Academic Press, 2020: 13-32Crossref Google Scholar Figure 1B shows the contribution of the main plastic types to the principal economy sectors (vertical axis) and their estimated lifetime in years as well as the contribution of each sector to plastic waste generation (horizontal axis).1Geyer R. Production, use, and fate of synthetic polymers.in: Letcher T.M. Plastic Waste and Recycling: Environmental Impact, Societal Issues. Academic Press, 2020: 13-32Crossref Google Scholar LDPE, HDPE, and PP are very versatile plastics, in contrast to PET, for example, which is only suitable for packaging. Packaging, textiles, and consumer goods are directly liable for 75% of all plastic waste, while plastic products employed in transport, building, and electrical devices sum up to only 15%. Unfortunately, the former items are also the most finely scattered ones, which calls for ramified and well-planned collection infrastructures, similar to CO2 and biomass, which are abundant but highly dispersed resources worldwide. Furthermore, they involve a variety of plastics, demanding effective sorting methods. In summary, the intersection of the four main types of plastics (LDPE, HDPE, PP, and PET) with the chief applications in packaging, textiles, and consumer goods accounts for 54% of all plastic waste and, hence, must nucleate recovery initiatives comprising collection, sorting, and processing. A large fraction (29%, Figure 2) of plastic waste follows an unknown fate, such as plastics escaping the collection system or being lost in the processing. Of special relevance is the apparently small fraction (4%) of plastics reaching the oceans, having a deep impact on the marine ecosystem. Plastic waste tends to concentrate in certain areas following gyres. The largest floating patch of trash primarily composed of plastics is called the Pacific Trash Vortex and has a size equivalent to Mongolia (ca. 1.6·106 km2). Macroplastics (mainly bags, fishing lines and nets, and ropes) cause problems for aquatic creatures due to ingestion and entanglement, whereas microplastics “reduce activity, rate, or capacity, induce particle toxicity, adsorb toxic pollutants, and transport invasive species.”2Ryberg M.W. Laurent A. Hauschild M. Mapping of global plastics value chain and plastics losses to the environment.http://wedocs.unep.org/handle/20.500.11822/26745Date: 2018Google Scholar Landfilling and incineration have comprised the main strategies with strong environmental consequences to tackle plastic waste (Figure 2). Approximately 60% of all plastic ever produced has been landfilled.1Geyer R. Production, use, and fate of synthetic polymers.in: Letcher T.M. Plastic Waste and Recycling: Environmental Impact, Societal Issues. Academic Press, 2020: 13-32Crossref Google Scholar In spite of the devastating ecological impact,9Moberg Å. Finnveden G. Johansson J. Lind P. Life cycle assessment of energy from solid waste - part 2: landfilling compared to other treatment methods.J. Clean. Prod. 2005; 13: 231-240Crossref Scopus (112) Google Scholar this is still the fate for 40% of plastic waste (Figure 2), as it is the least costly option.10Hundertmark T. Mayer M. McNally C. Simons T.J. Witte C. Modeling a virtuous circle of plastics recycling worldwide.https://www.mckinsey.com/industries/chemicals/ourinsights/%0Ahow-plastics-waste-recycling-could-transform-the-chemical-industry#Date: 2017Google Scholar Only a handful of countries have totally or partially banned this practice (e.g., Switzerland or Austria), whereas it remains widely unrestricted even in most developed countries (ca. 80% in Malta or Greece). Energy recovery by incineration gained increasing relevance over the years. Approximately 10% of all plastics produced has been incinerated,1Geyer R. Production, use, and fate of synthetic polymers.in: Letcher T.M. Plastic Waste and Recycling: Environmental Impact, Societal Issues. Academic Press, 2020: 13-32Crossref Google Scholar and today, this method manages 27% of waste worldwide (Figure 2).10Hundertmark T. Mayer M. McNally C. Simons T.J. Witte C. Modeling a virtuous circle of plastics recycling worldwide.https://www.mckinsey.com/industries/chemicals/ourinsights/%0Ahow-plastics-waste-recycling-could-transform-the-chemical-industry#Date: 2017Google Scholar Plastics possess a high heating value (HHV), between 20 and 40 MJ kg−1 (for PET and PP, respectively), which is comparable to that of crude oil (45 MJ kg−1).11Tsiamis D.A. Castaldi M.J. Determining accurate heating values of non-recycled plastics (NRP).https://plastics.americanchemistry.com/Energy-Values-Non-Recycled-Plastics.pdfDate: 2016Google Scholar However, the CO2 fingerprint of plastic incineration is approximately 50% larger than that of burning crude oil due to the energy intensiveness of plastic manufacturing.12Vollmer I. Jenks M.J.F. Roelands M.C.P. White R.J. van Harmelen T. de Wild P. van der Laan G.P. Meirer F. Keurentjes J.T.F. Weckhuysen B.M. Beyond mechanical recycling: giving new life to plastic waste.Angew. Chem. Int. Ed. Engl. 2020; 59: 15402-15423Crossref PubMed Scopus (45) Google Scholar Moreover, though it is often more environmentally benign than landfilling, plastics incineration needs to be associated with advanced pollution control systems to avoid the uncontrolled release of harmful compounds such as dioxins.13Ragaert K. Delva L. Van Geem K. Mechanical and chemical recycling of solid plastic waste.Waste Manag. 2017; 69: 24-58Crossref PubMed Scopus (438) Google Scholar Approximately 7% of all plastic ever produced has been processed into new products.1Geyer R. Production, use, and fate of synthetic polymers.in: Letcher T.M. Plastic Waste and Recycling: Environmental Impact, Societal Issues. Academic Press, 2020: 13-32Crossref Google Scholar Nowadays, 13% of waste is handled this way (Figure 2),10Hundertmark T. Mayer M. McNally C. Simons T.J. Witte C. Modeling a virtuous circle of plastics recycling worldwide.https://www.mckinsey.com/industries/chemicals/ourinsights/%0Ahow-plastics-waste-recycling-could-transform-the-chemical-industry#Date: 2017Google Scholar with very distinct contributions worldwide, i.e., 0% in most undeveloped countries, 8.4% in the USA, and 30% in the European Union (EU).14United States Environmental Protection Agency Facts and figures about materials, waste and recycling.https://www.epa.gov/facts-and-figures-about-materials-waste-and-recyclingDate: 2018Google Scholar The public opinion is increasingly pressing toward a more sustainable plastic economy and the EU has taken the lead by issuing stringent regulations on single-use plastic items and setting a target of 65% plastic recycling (excluding incineration) by 2025.15European Union Directive of the european parliament and of the council on the reduction of the impact of certain plastic products on the environment.https://data.consilium.europa.eu/doc/document/PE-11-2019-REV-1/en/pdfDate: 2019Google Scholar This ambitious target surpasses current capabilities and, therefore, calls for new and/or optimized strategies in a very constrained time frame. A first symptom of this stress and an example of the impact of legislation is the increasing demand for recycled PET in spite of its higher price than the virgin analoge.16Brooks B. Recycled plastics market will feel the heat from consumer demand in 2020.2020https://www.spglobal.com/platts/en/market-insights/blogs/petrochemicals/012220-recycled-plastics-market-will-feel-the-heat-from-consumer-demand-in-2020Google Scholar In this context, it is reasonable to expect that regulations that share the same objectives will spread to other territories. The vast majority of the plastic waste processing industry is based on mechanical recycling (ca. 99%, Figure 2), which is favored by the simplicity of the operations, e.g., agglomeration or extrusion. In principle, all types of thermoplastics can be mechanically recycled with little or no quality impairments, but in practice, LDPE and HDPE, PP, and polystyrene (PS) are hardly manageable by this option. The strict preprocessing steps, including cleaning and thorough sorting to avoid phase separation, largely hinder its wide application.13Ragaert K. Delva L. Van Geem K. Mechanical and chemical recycling of solid plastic waste.Waste Manag. 2017; 69: 24-58Crossref PubMed Scopus (438) Google Scholar,17Al-Salem S.M. Lettieri P. Baeyens J. Recycling and recovery routes of plastic solid waste (PSW): a review.Waste Manag. 2009; 29: 2625-2643Crossref PubMed Scopus (1035) Google Scholar Additionally, the handling of complex mixtures involving coatings and paints adds complexity and contaminants that cannot be completely eliminated may have strong negative impacts on mechanical properties, impairing the original product quality.17Al-Salem S.M. Lettieri P. Baeyens J. Recycling and recovery routes of plastic solid waste (PSW): a review.Waste Manag. 2009; 29: 2625-2643Crossref PubMed Scopus (1035) Google Scholar These drawbacks are mitigated through the use of stabilizers and compatibilizers,13Ragaert K. Delva L. Van Geem K. Mechanical and chemical recycling of solid plastic waste.Waste Manag. 2017; 69: 24-58Crossref PubMed Scopus (438) Google Scholar but the resulting goods are non-food-grade and thus limited to the manufacturing of detergent bottles or paint buckets, for instance. Chemical routes are applicable to all kinds of plastic waste and are to date mostly directed to the production of heavy and light oil fractions (Figure 2). Despite gaining attention over the last decade,12Vollmer I. Jenks M.J.F. Roelands M.C.P. White R.J. van Harmelen T. de Wild P. van der Laan G.P. Meirer F. Keurentjes J.T.F. Weckhuysen B.M. Beyond mechanical recycling: giving new life to plastic waste.Angew. Chem. Int. Ed. Engl. 2020; 59: 15402-15423Crossref PubMed Scopus (45) Google Scholar,18Plastic upcycling.Nat. Catal. 2019; 2: 945-946Crossref Scopus (1) Google Scholar they are currently applied to only 1% of plastic waste (Figure 2).1Geyer R. Production, use, and fate of synthetic polymers.in: Letcher T.M. Plastic Waste and Recycling: Environmental Impact, Societal Issues. Academic Press, 2020: 13-32Crossref Google Scholar It was already explored by BASF in the 1990’s but later abandoned due to problems associated with contaminants in plastic waste and especially owing to corrosion linked to chlorine when handling PVC.18Plastic upcycling.Nat. Catal. 2019; 2: 945-946Crossref Scopus (1) Google Scholar Chemical routes have been, since then, silently developed in research institutions and other companies17Al-Salem S.M. Lettieri P. Baeyens J. Recycling and recovery routes of plastic solid waste (PSW): a review.Waste Manag. 2009; 29: 2625-2643Crossref PubMed Scopus (1035) Google Scholar,18Plastic upcycling.Nat. Catal. 2019; 2: 945-946Crossref Scopus (1) Google Scholar and are now on the verge of a quick escalation in both academic and industrial arenas.12Vollmer I. Jenks M.J.F. Roelands M.C.P. White R.J. van Harmelen T. de Wild P. van der Laan G.P. Meirer F. Keurentjes J.T.F. Weckhuysen B.M. Beyond mechanical recycling: giving new life to plastic waste.Angew. Chem. Int. Ed. Engl. 2020; 59: 15402-15423Crossref PubMed Scopus (45) Google Scholar A vivid debate has emerged in the literature to categorize the developing plastic chemical processing technologies based on the products obtained, giving rise to the terms down-, re-, and upcycling. There is still a lack of consensus on which product attribute should be used as the main criterion among its nature, functionality, or economic value, as reflected in Figure 3. “Recycling” has been used to encompass any route, including4Philip F.B. Report of the basic energy sciences roundtable on chemical upcycling of polymers. Bethesda, US.https://www.osti.gov/servlets/purl/1616517/Date: 2019Google Scholar and excluding5Ellen Macarthur FoundationTowards the circular economy.https://www.mckinsey.com/∼/media/mckinsey/dotcom/client_service/sustainability/pdfs/towards_the_circular_economy.ashxDate: 2013Google Scholar incineration. It can also refer to processes targeting products with the same quality and purpose or with similar or lower economic value compared to the original plastic. Specifically, the terms “mechanical recycling” and “downcycling” have been widely employed to name methods that transform plastics without altering their basic chemical structure, leading to comparable or inferior quality/value materials. In contrast, “chemical recycling” has been linked to depolymerization routes6Coates G.W. Getzler Y.D.Y.L. Chemical recycling to monomer for an ideal, circular polymer economy.Nat. Rev. Mater. 2020; 5: 501-516Crossref Scopus (59) Google Scholar or to strategies yielding fuels, chemicals, and high-value products, including7Thiounn T. Smith R.C. Advances and approaches for chemical recycling of plastic waste.J. Polym. Sci. 2020; 58: 1347-1364Crossref Scopus (7) Google Scholar and excluding4Philip F.B. Report of the basic energy sciences roundtable on chemical upcycling of polymers. Bethesda, US.https://www.osti.gov/servlets/purl/1616517/Date: 2019Google Scholar monomers. Another controversial term is “upcycling,” with “chemical upcycling” being linked to the production of chemicals, fuels, or higher-value products as chemical recycling4Philip F.B. Report of the basic energy sciences roundtable on chemical upcycling of polymers. Bethesda, US.https://www.osti.gov/servlets/purl/1616517/Date: 2019Google Scholar or, alternatively, to products with high added value.8Sohn Y.J. Kim H.T. Baritugo K.A. Jo S.Y. Song H.M. Park S.Y. Park S.K. Pyo J. Cha H.G. Kim H. et al.Recent advances in sustainable plastic upcycling and biopolymers.Biotechnol. J. 2020; 15: e1900489Crossref PubMed Scopus (5) Google Scholar A generic definition of upcycling reads: “the process of converting materials into new materials of higher quality, economic value, and/or increased functionality”.5Ellen Macarthur FoundationTowards the circular economy.https://www.mckinsey.com/∼/media/mckinsey/dotcom/client_service/sustainability/pdfs/towards_the_circular_economy.ashxDate: 2013Google Scholar Toward a more unified terminology, we stress the urgent need for considering environmental implications, via comprehensive lifecycle analysis (LCA), to support categorization and prioritization, as highlighted in Status and future of implementation. Particular potential is seen in chemical paths driven by catalysis, owing to the possibility of reaching superior control over product distribution and purity under milder conditions than otherwise required due to the highly stable nature of plastics. This review surveys catalytic routes transforming plastic waste into products that, upon further processing, give rise to plastics, refinery feedstocks, fuels, chemicals, and materials (Figure 2). This set of routes is labeled as catalytic plastic waste processing and arguably shows the most brilliant future for expansion, as it currently treats only ca. 0.1% of waste based on the capacities of the industrialized processes reported. From a unifying perspective, catalytic plastic waste processing shall lay the grounds for a plastic waste refinery, the final products of which may vary according to market and regulation changes (Figure 2). To date, catalytic processing of plastic waste has been partially reviewed in articles focusing on (1) specific subfields, such as the paper by Dove et al.19Jehanno C. Pérez-Madrigal M.M. Demarteau J. Sardon H. Dove A.P. Organocatalysis for depolymerisation.Polym. Chem. 2019; 10: 172-186Crossref Google Scholar on organocatalysis; (2) processes like pyrolysis20Anuar Sharuddin S.D. Abnisa F. Wan Daud W.M.A. Aroua M.K. A review on pyrolysis of plastic wastes.Energy Convers. Manag. 2016; 115: 308-326Crossref Scopus (0) Google Scholar or glycolysis;21Ügdüler S. Van Geem K.M. Roosen M. Delbeke E.I.P. De Meester S. Challenges and opportunities of solvent-based additive extraction methods for plastic recycling.Waste Manag. 2020; 104: 148-182Crossref PubMed Scopus (5) Google Scholar and (3) different plastics, where catalysis-driven reactions are described as particular cases.22Sinha V. Patel M.R. Patel J.V. Pet waste management by chemical recycling: a review.J. Polym. Environ. 2010; 18: 8-25Crossref Scopus (282) Google Scholar Recently, wider reviews on chemical processing did not provide a separated analysis of catalytic efforts,7Thiounn T. Smith R.C. Advances and approaches for chemical recycling of plastic waste.J. Polym. Sci. 2020; 58: 1347-1364Crossref Scopus (7) Google Scholar,12Vollmer I. Jenks M.J.F. Roelands M.C.P. White R.J. van Harmelen T. de Wild P. van der Laan G.P. Meirer F. Keurentjes J.T.F. Weckhuysen B.M. Beyond mechanical recycling: giving new life to plastic waste.Angew. Chem. Int. Ed. Engl. 2020; 59: 15402-15423Crossref PubMed Scopus (45) Google Scholar and another account on heterogeneously catalyzed reactions focused on operating conditions and reactor design.23Mark L.O. Cendejas M.C. Hermans I. The use of heterogeneous catalysis in the chemical valorization of plastic waste.ChemSusChem. 2020; 13: 5808-5836Crossref PubMed Scopus (0) Google Scholar Based on these premises, this review aims to profile the catalytic processing of plastics from the molecular level to commercial technologies for heterogeneous, homogeneous, and biobased routes. A first analysis of available literature (Figure S1) highlights the quick evolution of plastic waste1Geyer R. Production, use, and fate of synthetic polymers.in: Letcher T.M. Plastic Waste and Recycling: Environmental Impact, Societal Issues. Academic Press, 2020: 13-32Crossref Google Scholar compared to that of scientific articles and patents, suggesting that this approach has not mobilized large resources yet. Scientific papers and patents are considered as activity metrics in academia and industry, respectively. As for the main areas of interest, process development greatly surpassed fundamental understanding in both arenas. PET is the more frequently reported plastic, but the focus has fluctuated over the years, with PET, PP, and LDPE showing a consistently increasing trend (Figure S2). A more profound examination of the literature suggests the structure depicted in Figure 4 as most appropriate to analyze the field. The first level is defined by the catalysis type (heterogeneous, homogeneous, and biocatalysis), which largely determines process design. Homogeneous processes, followed by biocatalytic processes, triggered interest in this field, but heterogeneously catalyzed reactions rapidly evolved with some delay, later dominating the research landscape (Figure S1B). The second level comprises the main reported chemical routes. For the sake of simplicity, some chemically and technologically similar (glycolysis and hydrolysis) and less researched (methanolysis, cyclic depolymerization, aminolysis, and hydrogenolysis) paths have been grouped. Finally, the third level encompasses the plastic(s) processed following each strategy, informing on their flexibility. The diameters of the category circles are an expression of the number of publications (papers and patents) since 1980 (see Table S1). At the process level, the flexible heterogeneous pyrolysis stands out as the preferred route for fuel production, whereas homogeneous glycolysis and enzymatic hydrolysis are key to recover monomers for the attractive market of recycled PET. An interesting follow-up analysis comparing papers and patents for each process (pie charts) reveals that industrial efforts represent a considerable portion only for pyrolysis and homogeneous glycolysis, which indeed are the technologies of choice of companies (see Status and future of implementation). Research highlights at the lab scale mirrors the structure depicted in Figure 4 to develop a systematic examination aided by mining of literature data with emphasis on catalyst development. Lessons from other fields provides guidelines for future research based on the knowledge gathered in petro-, biomass, and halogen chemistry. Status and future of implementation covers the current implementation efforts and discusses the future of commercialization based on available environmental and economic analyses. Finally, Conclusions and outlook critically assesses where the center of gravity of this field currently lies, highlights gaps affecting its development, and suggests future opportunities. Research on heterogeneous catalytic processing of plastics started in the mid-1990’s focusing on pyrolysis and hydrocracking, with industry taking quicker steps forward in the former transformation (Figures S1 and S2). From the mid-2000s, investigations had expanded to glycolysis and hydrolysis and gasification-carbonization and had embraced dehydrochlorination by the end of the same decade. A strong intensification of activities on all these routes has been witnessed in the last 10 years (Figure S2). This field has demonstrated to be very interdisciplinary, with a sustained influence of polymer scientists and an additional intermixing of efforts from the catalysis community, analytical chemists, and chemical and process engineers in this century. The however limited participation of catalysis groups shall explain that little catalyst design has been pursued so far. Off-the-shelf catalysts are largely applied, and catalyst development typically encompasses screening of materials known for other processes occasionally that have been modified to a minor extent for tailoring to the targeted reaction. The plethora of contributions and richness of the chemistry resulted into a variegated landscape reflected in the open literature, which, after an analysis based on algorithmic categorization, is graphically depicted in Figure 5. Therein, the size of nodes indicates the relative relevance of reactions, types of plastics used as feedstock, main classes of catalysts employed, and type of products attained, while edges connecting the nodes offer a visual account of their interrelations. The more frequently found relations are highlighted in shades of red. Pyrolysis has been the most widely studied reaction with PS, LDPE and HDPE, and PP as relevant plastic streams, often fed as a binary or tertiary mixture. Since these plastics are oxygen-free, products attained comprise a wide range of liquid and, to a lesser extent, gaseous hydrocarbons (HCs) and waxes for application as fuels and chemicals. While initial research has dealt with non-catalytic pyrolysis, the use of catalysts had soon shown potential to increase the quality of the mix generated by narrowing the broad product distribution. The chief catalysts are zeolites, with metal oxides and supported metals playing a substantial role as well. Notably, two auxiliary practices have gained traction, i.e., the addition of plastics to petroleum residues and, more recently, to biomass feedstocks. The former aims at increasing the yield of liquid HCs providing a co-substrate featuring long HC chains, whereas the latter oppositely targets an improved product distribution in the upgrading of the highly oxygenated biomass feedstocks toward fuels and fuel additives. Hydrocracking, the second most investigated reaction, is equivalent to pyrolysis in practical terms with the addition of hydrogen to the feed to even better enclose the span of the HCs chain length. Catalysts employed are also similar, additionally featuring a redox function to activate hydrogen, if not al