Polyethylene Hydrogenolysis over Ru Supported on Mesoporous MFI Zeolite: Effects of Mesoporosity and External Acid Sites

Abstract

Efforts to produce liquid alkanes by hydrogenolysis of polyethylene (PE) have mostly focused on manipulating catalytic functions through the combination of active components and support materials (i.e., a rate-controlled approach). However, controlling reactant diffusion is also an important aspect, as bulky substances such as PE and their heavy fragments are prone to staying for a long time within support materials due to diffusion limitations, leading to overcracking into gaseous products. In this study, a series of Ru nanoparticles supported on MFI zeolites of varying mesoporosity were investigated for the reductive degradation of PE in order to identify support textural properties that enhance the yield of liquid products. Comprehensive characterization techniques, including N 2 isotherms, electron microscopy, chemisorption, and IR spectroscopy, were used to elucidate the catalyst properties. We found that increased mesoporosity improved the yield of liquid products (C 5 –C 20 ) by more than 1.3-fold and shifted the range of C 1 –C 20 hydrocarbon products from C 5 -centered to C 8 -centered, despite similar overall degrees of C–C bond scission across the catalysts. No enhancement in the C–C bond scission rate was observed upon introduction of mesoporosity under our reaction conditions. Furthermore, the selective poisoning of external acid sites using triphenylphosphine oxide significantly reduced the yields of liquid products. These findings indicate that both mesoporosity and external acid sites in MFI zeolites enhance liquid hydrocarbon production under mild reaction conditions by facilitating the diffusion of primary products and mitigating secondary overcracking. This study demonstrates that optimizing the mesoporosity of zeolite supports and exploiting external acid sites can effectively enhance the depolymerization and upcycling of polyethylene waste into valuable liquid hydrocarbons.