Novel catalysts in catalytic upcycling of common polymer wastes

Catalytic upcycling is a promising waste management strategy that enhances the circularity of polymer wastes by transforming them into high-value-added products. This review presents the latest developments in novel catalysts, their applications and reaction mechanisms in upcycling approaches at various temperatures. High-temperature upcycling approaches include catalytic pyrolysis of polymers with biomass-derived biochar and carbonization with metal-based catalysts, which mainly produce hydrogen gas (H2), mono-aromatic hydrocarbons and carbon nanomaterials. Electro-reforming, photo-reforming, glycolysis and enzyme-assisted depolymerisation take place in the low- and intermediate-temperature with metal-based catalysts, organo-catalysts and biocatalysts. Diverse value-added products are obtained from these approaches, such as terephthalic acid, formic acid, H2, bis(2-hydroxyethyl) terephthalate, mono(2-hydroxyethyl) terephthalate, etc. The biomass-derived biochar has abundant functional groups, porous structure and large surface area that favour the depolymerisation of polymer wastes. However, its catalytic activity declines after long-term reuse due to coke deposition and reduced essential components. Metal-based catalysts, on the other hand, have large amounts of active sites, ensure high electron transport capability, and encourage the generation of electron-hole pairs. However, they can suffer from formation of by-products, accumulation of organic intermediates, and declining activities during the separation process. Biocatalysts have surface regions (e.g., serine residue, tryptophan residue, amino acids) for effective hydrolysis of polymer wastes, but display limited thermostability and low activities at a wide pH range. To advance the field, future research should focus on developing novel catalysts with excellent thermostability and catalytic activities.