Molecular Dynamics Kinetic Study on the Zeolite-Catalyzed
Benzene Methylation in ZSM‑5
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Abstract
The methylation of arenes is a key
step in the production of hydrocarbons
from methanol over acidic zeolites. We performed ab initio static
and molecular dynamics free energy simulations of benzene methylation
in H-ZSM-5 to determine the factors that influence the reaction kinetics.
Special emphasis is given to the effect of the surrounding methanol
molecules on the methylation kinetics. It is found that for higher
methanol loadings, methylation may also occur from a protonated methanol
cluster, indicating that the exact location of the Brønsted acid
site is not essential for the zeolite-catalyzed methylation reaction.
However, methylations from a protonated methanol cluster exhibit higher
free energy barriers than a methylation from a single methanol molecule.
Finally, comparison with a pure methanol solvent reaction environment
indicates that the main role of the zeolite during the methylation
of benzene is to provide the acidic proton and to create a polar environment
for the reaction. The metadynamics approach, which is specifically
designed to sample rare events, allows exploring new reaction pathways,
which take into account the flexibility of the framework and additional
guest molecules in the pores and channels of the zeolite framework.
This approach goes beyond the often applied static calculations to
determine reaction kinetics