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.

Recent Progress in the Chemical Upcycling of Plastic Wastes.

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.

Waste to Wealth: Chemical Recycling and Chemical Upcycling of Waste Plastics for a Great Future.

Polypropylene Plastic Waste Conversion to Lubricants over Ru/TiO2 Catalysts

Hydrogenolysis of Polypropylene and Mixed Polyolefin Plastic Waste over Ru/C to Produce Liquid Alkanes

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

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).

Low-temperature catalytic upgrading of waste polyolefinic plastics into liquid fuels and waxes

We scrutinized the effect of supports (types and calcination temperatures) in Ru/support catalysts in the hydrogenolysis of low-density polyethylene and found that zirconia-supported Ru (Ru/ZrO2) with high-temperature (1073 K) calcined ZrO2 was an effective and reusable catalyst, showing about 3-fold higher activity on catalyst amount basis than the previously reported Ru/CeO2 catalyst with similar selectivites. The catalyst was applicable to the reactions of various polyolefins including high-density polyethylene and polypropylene. Experimental studies and catalyst characterizations such as TPR, XRD, TEM and XAS with various Ru/ZrO2 and reference catalysts such as Ru/CeO2, Ru/SiO2 and Ru/γ-Al2O3 showed good correlations between the Ru particle sizes and catalytic performance in Ru catalysts except for Ru/γ-Al2O3: The catalyst with a moderate size of ~2.5 nm Ru particles provided the highest conversion. The activity per surface Ru metal was higher at larger Ru particle sizes. The selectivity to cheap gas products was low at middle to small Ru particle sizes (1–7 nm), but high at large Ru particles (>7 nm). 

Mifumi Tanji

Papers

Low-temperature catalytic upgrading of waste polyolefinic plastics into liquid fuels and waxes

Structure-activity relationship in hydrogenolysis of polyolefins over Ru/support catalysts