Reactivity of Pentamethylcyclopentenyl Cations toward Olefin Formation in the Methanol-to-Olefin (MTO) Process

Polymethylcyclopentenyl cations are frequently observed experimentally in zeolite catalysis but are investigated to a lesser extent than aromatics or acyclic olefins. Here, the reactivity of the pentamethylcyclopentenyl cation (pentaMCP+) for the formation of olefins in H-SSZ-13 is investigated by using density functional theory (DFT). We find that pentaMCP+ can react similarly to aromatics in the side-chain mechanism with a selectivity for ethylene. The selectivity is due to a specific transition state that favors the elimination of ethylene over the further methylation steps. Generally, our calculations indicate that methylation of the unsaturated side chain is rate-limiting, as also observed for the aromatic cycle. This is because the formation of the neutral species with an unsaturated side chain from the more stable cationic resting state of the hydrocarbon is already unfavorable and adds to the intrinsic barrier for methylation. To estimate the catalytic activity, microkinetic modeling is performed based on the computed Gibbs free energies. Analogous to the aromatic cycle, we find that a second accessible acid site can lower the overall barriers by making stepwise methylation more favorable. Based on our findings, we suggest that there is a third catalytic cycle operating during MTO conversion that is located between the olefin and aromatic cycle and produces primarily ethylene with overall free energy barriers comparable to those of the olefin and aromatic cycle.