Hydrogenolysis of Polyethylene by Metal-Organic Framework Confined Single-Site Ruthenium Catalysts

Upcycling polyolefins into value-added hydrocarbons via catalytic hydrogenolysis is challenging due to poor product selectivity, random C-C bond cleavage, and the formation of volatile alkanes. We have developed two isoreticular porous aluminum metal-organic framework (MOF) node-supported mononuclear ruthenium dihydride catalysts (DUT-5-RuH2 and MIL-53-RuH2), which are efficient in the hydrogenolysis of low-density polyethylene (LDPE) at 200 °C into a narrow distribution of liquid hydrocarbons (C8-C24). By systematic tuning of the pore sizes of the MOFs, high yields of desirable liquid alkanes were afforded with varying degrees of branching, achieving 80% selectivity. DUT-5-RuH2 produced a C22-centered bell-shaped alkane distribution with a polyethylene conversion of 98%, while MIL-53-RuH2, being selective for shorter alkanes, produced a C9-centered bell-shaped alkane distribution. Based on our spectroscopic and theoretical studies, the high catalytic activity and selectivity of these MOF catalysts are primarily attributed to the stabilization of single-site mono-RuH2 species at the MOF’s nodes via active-site isolation and the confinement of the active catalytic species within porous MOFs. Theoretical calculations suggest that RuH2-mediated polyolefin C-C bond cleavage primarily occurs via turnover-limiting σ-bond metathesis. This study underscores the significance of MOFs in the rational design of heterogeneous catalysts for the efficient upcycling of plastic waste. © 2024 American Chemical Society.