posted on 2022-12-13, 13:06authored byShubham Malviya, Peng Bai
Proton-exchanged zeolites are effective Brønsted-acid
catalysts
that exhibit molecular-level shape selectivity. Yet, the nonuniform,
heterogeneous porous environment makes it challenging to understand
and predict site-dependent reactivity that originates from the intricate,
noncovalent interactions exerted by the catalyst pore walls, especially
with large, flexible reactants. In this work, we developed a computational
procedure consisting of generating quasi-transition-state (TS) complexes
using force-field-based configurational-biased sampling and subsequent
reaction path optimization using a density-functional theory-based
nudged-elastic-band method. This approach allows us to capture how
the TS configurations adapt to the local environment and to obtain
a rough estimate of TS entropy via the number of accessible TS configurations
at each active site. The resulting site-dependent TS energetics further
enable the calculation of ensemble-averaged activation barriers. Using
this approach, we studied the protolytic cracking of n-butane in TON, MFI, LTA, and FAU zeolites. It is found that in the
MFI zeolite, while the tighter zig-zag and straight channels have
lower TS energies, the more spacious intersection region potentially
supports a much larger number of TS configurations. In addition, it
is also found that if the reactant state is restricted to the vicinity
of an active site, the computed site-specific barriers show a large
variation across different zeolites and among different sites within
the same zeolite, with values ranging from 182 to 218 kJ/mol. We argued
that for kinetics-limited reactions, the reactant state should be
taken to be the globally most stable configuration and showed that
this treatment leads to ensemble-averaged barriers that are substantially
more similar across zeolites, with the FAU and LTA zeolites having
intrinsic barriers around 220 kJ/mol and the MFI and TON zeolites
around 200 kJ/mol.