Peking University discusses the catalytic strategy of plastic waste upgrading and conversion

On September 7, 2022, Professor Martin of Peking University published a forward-looking article titled “Catalytic strategies for upvaluing plastic wastes” in the journal Chem.

This paper explores three strategies for constructing a catalytic route for depolymerizing and converting plastic polymers into high value-added chemicals, and provides new possibilities for the development of catalytic upgrading routes with plastic waste as raw materials in the future. The corresponding authors of the paper are Martin and Wang Meng; The first author is Zhang Meiqi.

Nowadays, plastic waste has gradually become another important type of carbon resources in addition to fossil fuels, CO2, biomass, exploring the chemical transformation process of plastic polymers at the molecular level, helping to find a reasonable solution to plastic waste, is an important basis for alleviating the energy and environmental problems caused by the mass production and mass disposal of plastic products. With the help of catalytic means, the upgrading of plastic polymers can be efficiently and highly selectively converted into fuels, small molecule chemicals, other polymer materials, carbon materials and other processes. This paper focuses on the process of depolymerizing and upgrading plastic polymers into small molecule high-value chemicals, and discusses the following three catalytic strategies based on whether there is a clear and specific intermediate molecule on the reaction route and the type of such intermediate: (1) First, the polymer is depolymerized into a corresponding monomer or oligomer (or derivative thereof), and then the monomer or oligomer (or derivative) is converted into a high-value chemical; (2) First, the polymer is degraded into small molecules on the platform such as CO2, CH4, formic acid, methanol, etc., and the small molecules on the platform are further converted into high-value chemicals; (3) Based on the activation and fracture of specific chemical bonds, the plastic polymer is directly converted.

Route 1: Use monomers or oligomers (or derivatives thereof) as intermediates for dispolymerization and transformation

Polymer polymers may be depolymerized under certain conditions into corresponding monomers or oligomers (or derivatives thereof), for example, by solvolysis can be polyester, polyolic acid, etc. depolymerized into monomers or derivatives thereof, through catalytic pyrolysis, alkane recompression and other processes can be polyolefins and the like depolymerized or oligomers (or derivatives); Further catalytic hydrogenation, ammonization, oxidation, and hydroformylation can further increase the value of these depolymerization products (Figures 1A, 1B). For example, by coupling CO2 hydrogenation to methanol, PET methanol lysis and hydrolysis of alcohol solutions, the group realized the co-conversion of PET and CO2 of two wastes (Angew. Chem. Int. Ed. Engl. 2022, 61, e202117205); The process of efficient preparation of alanine from PLA was realized without external hydrogenation by PLA ammonialysis, lactamide hydrolysis, and ammonium lactate dehydrogenation-amination-rehydrogenation steps (J. 2007). Am. Chem. Soc. 2021, 143, 16358-16363)。 These processes add new options to the realization of a circular economy.

Route 2: Platform small molecules as intermediates for depolymerization and transformation

Under specific conditions, some plastic polymers can be slowly degraded into small molecules such as CO2, CH4, H2O; The activation and transformation of small molecules of C1 such as CO2, CH4, formic acid, and methanol have been extensively and deeply explored. Looking for a catalytic system that can efficiently and highly selectively degrade plastic polymers as platform small molecules, combined with the successful experience of platform small molecules synthesizing high-value chemicals, it is expected to achieve the upgrading and transformation of plastic polymers with platform small molecules as intermediates (Figure 1C). Recently, the route of photocatalytic conversion of plastic polymers into acetic acid or syngas with CO2 as an intermediate has been reported, and the transformation process with other platform small molecules such as CH4 as intermediates is also worth studying.

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Figure 1: Indirect catalytic conversion path

Route 3: Direct catalytic conversion

Microscopically, plastic polymers are long chains with specific chemical bonds (C–C, C–O, C–N, etc.) between structural units (monomers) with high repetition, providing the possibility of precise activation and precise breaking of certain chemical bonds in monomers or between monomers, directly converting polymers and obtaining valuable products with high selectivity. For example, the depolysis of polyolefins, polyesters, polyamides and the like can be effectively achieved by hydrolysis of C–C, C–O, C–N and other chemical bonds in monomers or between monomers (Figure 2A). Polymers with C–C bonds as the main chain such as polyolefins, polystyrene, and polyvinyl chloride are widely used in actual plastic products, and activating inert chemical bonds to achieve the conversion of polymers into high-value chemicals is both a challenge and an opportunity. In addition to the catalytic conversion of PE into long-chain alkyl aromatic hydrocarbons by hydrolysis and aromatization, heteroatoms such as O, N, S, P, B, halogen and other heteroatoms can be introduced during the depolymerization process to directly convert the polymer into a chemical with higher value (Figure 2C). For example, recently the research group reported for the first time the process of efficient preparation of aromatic oxidants such as benzoic acid by direct oxidation of PS by means of heterogeneous photocatalysis (Nat. Commun. 2022, 13, 4809)。 In addition, the conversion of polymers into high-value chemicals by causing the weakening and breaking of adjacent C–C bonds by C–H bond functional grouping is also a very feasible but yet unexplored strategy (Figure 2C).

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Figure 2: Direct catalytic conversion path

From the perspective of catalytic route, we discuss three strategies for understanding and converting plastic polymers into high value-added chemicals (monomers/oligomers/derivatives as intermediates, platform small molecules as intermediates, and direct catalytic conversion). In addition, the problems of mass/heat transfer and catalyst poisoning that are not discussed in this article still need a lot of research to be deeply understood and overcome. It should also be mentioned that plastic waste in the actual system is usually a mixture state, and it is urgent to combine multiple catalytic techniques to deal with these complex multi-component systems. (Source: Science Network)

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