Conversion of polyethylene waste to short chain hydrocarbons under mild temperature and hydrogen pressure with metal-free and metal-loaded MFI zeolites

Abstract

Catalytic hydrocracking on supported metal zeolites promisingly converts waste plastics into more valuable hydrocarbons. Although the presence of metal sites on solid acids is typically considered necessary for C-C bond cleavage of alkanes at modest temperatures, we show that polyethylene (PE) depolymerization on metal-free MFI zeolites proceeds at higher rates than analogous metal-loaded MFI zeolites (Pt, Ni) under mild reaction conditions. Higher rates on metal-free MFI are consistent with higher ratios of alkenes to alkanes, leading to subsequent alkene-mediated beta-scission events that form smaller molecules. Under varied reaction conditions (e.g., temperature, pressure, H2 versus N2), both metal-free and metal-loaded MFI catalysts demonstrate high selectivity to C3-C7 gaseous hydrocarbons, likely due to the 10-MR channel structure. Collectively, these findings demonstrate the importance of deconvoluting metal and acid sites contributions for PE hydrocracking, which has significant implications on rates and product selectivities of plastic upcycling reactions.