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

Abstract

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 on 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 towards 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 can not solely be described 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.

Towards molecular control of elementary reactions in zeolite catalysis by advanced molecular simulations mimicking operating conditions

“…56 Next to reactivity, the catalyst topology and acidity were also found to impact diffusion rates of reaction products. 50,83,84 Next to the catalyst's topology, also its acid strength plays a determining role for the activity of HP species. Studies with isostructural catalysts with different framework compositions and thus different acid strength show distinct behavior in terms of product selectivity and deactivation rate during methanol conversion.…”

Methanol diffusion in zeolite HY: a combined quasielastic neutron scattering and molecular dynamics simulation study

“…There has been debate regarding the use of a flexible framework when the sorbate is significantly smaller than the pore aperture, and the influence of the force field used on the measured diffusivities. Examples of recent studies into the effect of pore breathing in diffusion of small molecules include a comprehensive study by Ghysels et al56 of MTG relevant small molecules in small pore zeolites and SAPO structures, introducing a new descriptor (the accessible window area) Fig 1. The 2x2x2 zeolite HY supercell used, viewed from (left) the 110 direction and (right) the 111 direction showing the structure of the faujasite cages.…”

An approach for predicting intracrystalline diffusivities and adsorption entropies in nanoporous crystalline materials

“…[77][78][79] The mainly caused by complex interaction between framework and molecules. 31,74,75 Different chemical composition and acidity of framework can alter the potential energy of diffusion. 31,75 The effects of chemical composition and acidity of framework need to be further studied with regard to the activation energy of diffusion.…”

“…31,74,75 Different chemical composition and acidity of framework can alter the potential energy of diffusion. 31,75 The effects of chemical composition and acidity of framework need to be further studied with regard to the activation energy of diffusion. Consequently, if may not be feasible to predict molecular diffusivity within zeolites and MOFs materials with strong acidity.…”