(229e) Catalyst Design Strategy for Polypropylene Upcycling to Lubricants Via Hydrogenolysis

Growth in plastic production over the years was accompanied by growth in plastic waste (PW), which is either landfilled (40%) or simply dumped into the environment (32%). To combat PW, significant effort is directed at PW recycling and upcycling to reduce waste generation and pollution. Very recently, several new strategies were developed to provide a relatively low-temperature chemocatalytic pathway from polypropylene (PP) to waxes, lubricants, fuel components, and aromatics. One of them includes catalytic hydrogenolysis of PO over Ru/TiO₂ catalyst, studied by our group. We showed that at 30 bar H₂ pressure, polypropylene (PP) is converted into liquid oligomers with the formation of methane as the main side-product. Major drawbacks of this process are long reaction times (12-16 h at 250 °C) and modest yields to target liquids (~65-70%). The aim of this study is to provide a strategy for catalyst design to improve catalyst activity and selectivity.

PP hydrogenolysis was studied in a batch reactor at 250 °C for 3-24 h at different H₂ pressures. Products were analyzed using GC,GPC and NMR. Kinetic studies, D₂ isotope labeling and kinetic modeling show that Ru active sites operate in a hydrogen deficient regime. This leads to low reaction rates and high methane selectivity. Boosting the surface hydrogen coverage accelerates the removal of hydrogenolysis intermediates from the surface. This reduces the impact of secondary terminal hydrogenolysis, which leads to methane formation. In order to increase the hydrogen coverage, we synthesized Ru/TiO₂ catalysts with different degrees of metal support interactions.

As a result, we reduced reaction time by a factor of 5 and simultaneously increased the liquid selectivity up to 85%. This constitutes a major advance compared to previously reported catalysts. It also shows how metal support interactions can be used to increase the activity of hydrogenolysis catalyst.