Loading dependence of self-diffusivities of gases in zeolites

Abstract

Experimental data on the self-diffusivities, D_i,self, of a variety of gases (CH₄, N₂, Kr, C₂H₆, and C₃H₈) in three different zeolites, LTA, FAU, and MFI, show different dependences on the molar loading, q_i. In LTA, D_i,self appears to increase with q_i for all molecules except N₂. In FAU and in MFI the D_i,self shows a sharp decrease with increasing qi. In order to gain insights into the causes behind the loading dependences, molecular dynamics (MD) simulations were carried out to determine the self-diffusivities of seven gases (CH₄, N₂, Kr, C₂H₆, C₃H₈, Ar, and Ne) in six different all-silica zeolite structures (MFI, AFI, FAU, CHA, DDR, and LTA). The simulation results show that the variation of D_i,self with q_i is determined by a variety of factors that include molecular size and shape, and degree of confinement within the zeolite. For one-dimensional channels (AFI) and intersecting channel structures (MFI), the D_i,self invariably decreases with increasing qi. For zeolite structures that consist of cages separated by windows (FAU, CHA, DDR, LTA), the size of the windows is an important determinant. When the windows are wide (FAU), the D_i,self decreases with qi for all molecules. If the windows are narrow (CHA, DDR and LTA), the D_i,self often exhibits a sharp increase with q_i, reaches a maximum and reduces to near-zero values at saturation. The sharpness with which D_i,self increases with q_i, is dictated by the degree of confinement at the window. Weakly confined molecules, such as Ne, do not exhibit an increase of D_i,self with q_i.