(178e) Understanding the Origin of Long-Chain Alkane Hydrogenolysis Selectivity for the Chemical Upcycling of Waste Polyolefins

Waste polyolefins such as polyethylene are among the most pervasive single-use plastics due to their strong C-C bonds and lack of functional handles for depolymerization. Thermal heterogeneous catalytic depolymerization is one method of breaking the strong C-C bonds in polyethylene to produce small molecules that could be used as fuel or as synthons for the production of inherently degradable plastics. But, these methods are often energy intensive and suffer from low control over product selectivity. In this investigation, carbon-supported Ru- and Co-based catalysts were identified as active catalysts for the hydrogenolysis of n-octadecane, a model compound for polyethylene. The effects of catalyst support structure, nanoparticle synthesis methods, and reaction conditions on hydrogenolysis activity and selectivity were investigated with the aim of controlling the location of C-C bond scission to produce desired narrow distributions of alkane products for use as fuels or transportable platform molecules. C-C bond scission at terminal locations along the carbon chain results in the formation of methane and a distribution of C₂-C₁₇ alkanes from octadecane, whereas C-C bond scission at central locations results in a narrower distribution of liquid alkanes and a decrease in methane formation. In order to understand the origin of C-C bond hydrogenolysis selectivity along the length of the chain, the mechanism and kinetics were evaluated using a combination of residence time studies, substrate effects, and microkinetic modeling. To support the proposed mechanism, a kinetic Monte Carlo model was developed in order to predict product distributions based on cleavage at different C-C bond locations, factoring in both concentration and chain length in predicting hydrogenolysis activity and selectivity. By understanding the factors that govern C-C bond hydrogenolysis selectivity, target reaction conditions and catalyst properties can be identified and applied to the selective hydrogenolysis of long chain polyolefins such as polyethylene, and real post-consumer plastic waste.