Loading Dependence of Self-Diffusivities of Gases in Zeolites

Experimental data on the self‐diffusivities, Di,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, qi. In LTA, Di,self appears to increase with qi for all molecules except N2. In FAU and in MFI the Di,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 Di,self with qi 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 Di,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 Di,self decreases with qi for all molecules. If the windows are narrow (CHA, DDR and LTA), the Di,self often exhibits a sharp increase with qi, reaches a maximum and reduces to near‐zero values at saturation. The sharpness with which Di,self increases with qi, is dictated by the degree of confinement at the window. Weakly confined molecules, such as Ne, do not exhibit an increase of Di,self with qi. Molecular self‐diffusion of gases within zeolites depends on the structure and dimensions of the cavities as well as the size and molar loading of the gas molecules. Molecular dynamics simulations indicate that self diffusion is generally a strong function of loading.