(638e) Catalytic Hydroconversion of Polyethylene to Gaseous Hydrocarbons on MFI Zeolites Under Mild Conditions

In recent years, the rapid accumulation of plastic waste (e.g., polyolefins) in the environment, due in part to significant growth rate of plastic production, has resulted in global ecological concerns. Among different recycling technologies, chemical upcycling or conversion methods can serve as potential solutions to convert plastic waste into gaseous or liquid hydrocarbons that can be reutilized as fuels and chemical intermediates. Here, we performed solvent-free catalytic upcycling of PE to gaseous fuels (C₃-C₇) under mild conditions (473 K, 10 bar H₂, 12 h) on metal-free and metal-loaded (1 wt. % either Pt or Ni) MFI zeolites to elucidate the respective contribution of metal and Brønsted acid sites for PE conversion. Further, these catalysts were compared to acid-free Pt and Ni supported on SiO₂ (M/SiO₂), which would only exhibit hydrogenolysis activity; all samples were synthesized with similar nanoparticles sizes ((2.9-3.1) ± 1.6 nm) to minimize potential structure sensitivity and varied exposed metal surface area. Negligible solid conversions were measured on M/SiO₂, indicating that hydrogenolysis activities of Pt and Ni are extremely low at these conditions. Interestingly, higher PE conversions were observed on metal-free than metal-loaded MFI (473 K, 10 bar H₂, 12 h), which indicates that Brønsted acid sites alone are active for cleavage of C-C bonds in PE even at mild temperatures. The lower solid conversions observed on metal-loaded MFI zeolites are likely due to rapid hydrogenation of alkene species back to alkanes on metal sites that would otherwise be active for acid catalyzed beta-scission, hindering subsequent C-C bond cleavage events. These reactions mainly yield C₃-C₇ hydrocarbons with trace amount of low-value methane and >C₁₀ products, likely related to the selectivity of medium pores in MFI toward smaller species. Taken together, these results showcase the possibility of utilizing zeolite-based hydrocracking catalysts for conversion of plastic waste to fuels.