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.