(473h) Microwave-Enhanced Catalytic Upcycling of Polymer

Chemical upcycling of polymer represents an attractive approach of using polymeric waste as feedstock in promoting the circular economy and decarbonizing chemical industry. Here we report a one-step microwave catalytic approach for the deconstruction of plastic polymers into monomeric ethylene and BTX aromatics (benzene, toluene, and xylene) that can be used to re-construct polymers.

Currently, the majority of polymer ends up in landfills or the environment, harming the ecosystem and affecting the natural environment. Thermochemical pathways (e.g., pyrolysis, gasification) and high pressure hydrogenolysis enable depolymerization films to produce small molecules that could be used as fuel or integrated into chemical refineries. However, these thermochemical processes suffer from severe challenges which include inevitably higher energy input leading to higher greenhouse gas (GHG) emission (i.e., ∼12 kg CO₂/1 kg H₂ production in gasification); poor product selectivity; low value of products, thus requiring significant upgrading for converting to valuable products.

Microwave catalytic process is a promising process for polymer upcycling with the advantages of lower reaction temperature, higher product selectivity, and simple one-step reaction process. Figure 1a presents the overall yields of LDPE under microwave and conventional thermal heating conditions at 300 â—¦C. Under microwave catalytic upcycling conditions, gas yield reached 92% and the residue yield was only 4%. While, under conventional thermal heating, the gas yield was only 74% and the residue yield rose to 12%. Microwave is an efficient method for the polymer depolymerization. The difference of using microwave and conventional thermal heating methods is also reflected by the selectivity to product, particularly for the desirable BTX (Benzene, Toluene, and Xylene) and olefins productions. Comparing the products obtained on microwave and thermal, evident difference was obtained. As shown in Figure 1b, the C₂⁼-C₄⁼ and C₂-C₄ are dominant of over 95% products in gas on conventional thermal heating route, and the BTX selectivity is only 2%. This is mainly caused by the self-decomposition of plastic polymers by random chain scission mechanism, as the PE is heated up together with catalyst. Microwave catalytic upcycling gave primarily BTX with 45% selectivity, ZSM-5 can effectively convert the alkene intermediates to BTX at the same reaction temperature under microwaves. The catalyst exhibits two functions under microwave, catalysis function and the function of converting electromagnetic energy into thermal heat energy (Figure 1c). The heat is initially generated on the catalyst particles, which have good dielectric loss under microwave irradiation. In contrast, polymer remains cold because it is transparent to microwave (Figure 1d). The hypothesis is that the “hot spots” presented at catalyst-polymer interface facilitate the β−scission to form C₂ olefinic intermediates in cracking LDPE. Further catalytic reaction will depend on the catalytic properties of catalyst.