A Microkinetic Model for Isomerizing Ethenolysis

In isomerizing ethenolysis, tandem double bond isomerization and olefin metathesis catalysts convert polyethylene and ethylene coreactants into propylene. Isomerizing ethenolysis is particularly interesting among polymer upcycling strategies because of its potentially high selectivity to a specific value-added product. Following a theoretical analysis by Guironnet and Peters [ J. Phys. Chem. 124, 3935 (2020)], Conk et al. [ Science, 377, 1561 (2022)] demonstrated isomerizing ethenolysis in experiments using an iridium pincer dehydrogenation catalyst, a dimeric Pd(I) bromide isomerization catalyst, and a second-generation Hoveyda–Grubbs metathesis catalyst. This paper compares model predictions to the two-stage dehydrogenation and isomerizing ethenolysis experiments of Conk et al. In a model that accounts for the initial dehydrogenation and subsequent evolution of the chain length distribution, we show that the experimental propylene generation rates are consistent with an isomerizing ethenolysis rate that is zeroth order in the concentration of long chain ends. In contrast, Guironnet and Peters assumed a first order dependence on chain ends. To understand the discrepancy, we developed and solved a microkinetic model for the isomerizing ethenolysis reaction. Rate parameters in the microkinetic model are estimated from prior experiments and known equilibria. We find that the experiments of Conk et al. are performed near conditions of the theoretical maximum propylene production rate, where the kinetics are saturated with respect to both chain end and ethylene concentrations. For long chains, not preshortened by initial dehydrogenation and ethenolysis steps as in Conk et al., the model predicts lowered chain end concentrations and smaller propylene production rates that can become inhibited by high ethylene pressure.