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.