Molecular Dynamics with Chemical AccuracyAlkane Adsorption in Acidic Zeolites

For the adsorption
of methane to hexane on acidic zeolites with
varying pore sizes (Socony Mobil-5 (MFI), chabazite (CHA), faujasite
(FAU)) and Brønsted acid site concentrations, heats of adsorption
are predicted. The widely applied “standard model” of
computational catalysis, density functional theory with some account
of dispersion (DFT-D) and the harmonic approximation for local sampling
of the potential energy surface (PES), leads to a large mean absolute
deviation (MAD) from experiment of 17.2 kJ mol–1, far outside chemical accuracy limits (±4 kJ mol–1). Passing either to molecular dynamics (MD) at the DFT-D level or
to wave function-based electron correlation methods (second-order
Møller–Plesset perturbation theory  MP2) for energies
at local minimum structures reduces the MAD to 8.7 and 5.9 kJ mol–1, respectively, still outside the chemical accuracy
range. We present MD simulations on an MP2 quality PES, which strongly
reduces the MAD to 1.9 kJ mol–1. This has been achieved
by finding two descriptors for the MP2–DFT-D energy differences,
which reduces the total number of required MP2 calculations to only
36 and, hence, the computational demand by several orders of magnitude.
The predicted heats of adsorption at reaction temperatures (650 K)
support experimental results derived from spectroscopic measurements.
They show that the observed decrease of experimental apparent barriers
from propane to pentane for alkane cracking in MFI (28 kJ mol–1) is largely due to increasing adsorption strengths
(21 kJ mol–1) and to a much smaller extent to decreasing
intrinsic barriers (7 kJ mol–1).