Ab Initio Simulations Reveal that Reaction Dynamics Strongly Affect Product Selectivity for the Cracking of Alkanes over H‑MFI

Product selectivity of alkane cracking catalysis in the
H-MFI zeolite
is investigated using both static and dynamic first-principles quantum
mechanics/molecular mechanics simulations. These simulations account
for the electrostatic- and shape-selective interactions in the zeolite
and provide enthalpic barriers that are closely comparable to experiment.
Cracking transition states for n-pentane lead to
a metastable intermediate (a local minimum with relatively small barriers
to escape to deeper minima) where the proton is shared between two
hydrocarbon fragments. The zeolite strongly stabilizes these carbocations
compared to the gas phase, and the conversion of this intermediate
to more stable species determines the product selectivity. Static
reaction pathways on the potential energy surface starting from the
metastable intermediate include a variety of possible conversions
into more stable products. One-picosecond quasiclassical trajectory
simulations performed at 773 K indicate that dynamic paths are substantially
more diverse than the potential energy paths. Vibrational motion that
is dynamically sampled after the cracking transition state causes
spilling of the metastable intermediate into a variety of different
products. A nearly 10-fold change in the branching ratio between C2/C3
cracking channels is found upon inclusion of post-transition-state
dynamics, relative to static electronic structure calculations. Agreement
with experiment is improved by the same factor. Because dynamical
effects occur soon after passing through the rate-limiting transition
state, it is the dynamics, and not only the potential energy barriers,
that determine the catalytic selectivity. This study suggests that
selectivity in zeolite catalysis is determined by high temperature
pathways that differ significantly from 0 K potential surfaces.