Shape-Selective Diffusion of Olefins in 8‑Ring Solid Acid Microporous Zeolites

The diffusion of olefins through
8-ring solid acid microporous
zeolites is investigated using molecular dynamics simulations techniques
and using a newly developed flexible force field. Within the context
of the methanol-to-olefin (MTO) process and the observed product distribution,
knowledge of the diffusion paths is essential to obtain molecular
level control over the process conditions. Eight-ring zeotype materials
are favorably used for the MTO process as they give a selective product
distribution toward low-carbon olefins. To investigate how composition,
acidity, and flexibility influence the diffusion paths of ethene and
propene, a series of isostructural aluminosilicates (zeolites) and
silicoaluminophosphates (AlPOs and SAPOs) are investigated with and
without randomly distributed acidic sites. Distinct variations in
diffusion of ethene are observed in terms of temperature, composition,
acidity, and topology (AEI, CHA, AFX). In general, diffusion of ethene
is an activated process for which free energy barriers for individual
rings may be determined. We observe ring-dependent diffusion behavior
which cannot be described solely in terms of the composition and topology
of the rings. A new descriptor had to be introduced, namely, the accessible
window area (AWA), inspired by implicit solvation models of proteins
and small molecules. The AWA may be determined throughout the molecular
dynamics trajectories and correlates well with the number of ring
crossings at the molecular level and the free energy barriers for
ring crossings from one cage to the other. The overall observed diffusivity
is determined by molecular characteristics of individual rings for
which AWA is a proper descriptor. Temperature-induced changes in framework
dynamics and diffusivity may be captured by following the new descriptor
throughout the simulations.