Investigation on polyethylene hydrogenolysis over a nickel catalyst via ReaxFF molecular dynamic simulation

Catalytic hydrogenolysis is a promising upcycling pathway to valorize low-value waste polyolefin plastics into high-value chemicals or fuels. To reveal the mechanism of Ni catalyzed hydrogenolysis of polyethylene, ReaxFF molecular dynamic simulation was carried out in this study. The results show that numerous polyethylene macromolecules and intermediates adsorb on the catalyst surface and undergo C-C and C-H bond breaking reactions in the catalytic hydrogenolysis process, which can be regulated by hydrogen pressure. High hydrogen pressure promotes the continuous adsorption and cleavage of polyethylene macromolecules and intermediates on the catalyst surface; however, excessively high hydrogen pressure leads to the generation of hydrogen radicals, which occupy the active sites and inhibit the adsorption process of polyethylene on the catalyst surface, thus decreasing the frequency of reaction occurrence. The reaction of hydrogen radicals with the intermediates followed by the subsequent desorption also assists in reducing the average unsaturation of the products to some extent. Additionally, a high selectivity of terminal C-C bond breakage that leads to the production of undesirable gas product was observed during catalytic hydrogenolysis, but this can be suppressed by increasing hydrogen pressure, at which the terminal carbon binds to the excess free hydrogen and desorbs timely from the catalyst surface. This work provides important insights into the detailed reaction mechanism and pathways of polyethylene catalytic hydrogenolysis over Ni catalyst and complements the existing experimental work to guide the catalyst design and product optimization prior to the large-scale application.