Computational study of diffusion of propane in small pore acidic zeotypes AFX and AEI

• Molecular dynamics have been used to model diffusion of propane in small-pore zeotypes.
• The dimensions and flexibility of 8-ring windows influence the activation energy of propane.
• Zeolite and SAPO compositions, containing Brønsted sites, were modelled.
• The effects of loading and temperature were studied and related to pore-size and propane mobility.

Small pore zeolites, containing 8-rings as the largest, are widely employed in commercial processes such as methanol-to-olefins or methanol-to-hydrocarbons. Diffusion in these materials has mainly been studied in pure silica materials. In this study acidic versions of two zeolites (AFX and AEI) are considered under different chemical compositions, loading, and temperature. Propane molecule, one of the products in these processes, has a size very similar to those of zeolitic 8-rings and hence little variations in their dynamic size can affect diffusion coefficients by several orders of magnitude. The dynamics of intercavity motions, involving an 8-ring crossing, are thoroughly investigated with the help of atomistic molecular dynamics. Detailed analysis of trajectories, motions, orientations, conformations, 8-ring sizes and shapes, and energetic profiles including activation energies. The results show a preferential diffusion in SAPOs over zeolites, an influence of the diffusivity with the propane loading depending on the structure, and the need of high temperature to overcome the activation barriers defined by 8-ring crossing events. SAPO-18 (AEI) shows a certainly improved propane diffusivity due to the slightly larger size of its 8-rings compared to those of SAPO-56 (AFX). Framework deformations have also been found as a result of strong host–guest interactions in the AEI zeotype, especially in the case of the SAPO-18. The implications on diffusivity are analysed in detail depending on the loading and temperature.

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