A molecular dynamics investigation of a variety of influences of temperature on diffusion in zeolites

Molecular dynamics (MD) simulations were performed to determine both the Maxwell–Stefan diffusivity, ĐiĐi, and the self-diffusivity, Di,self, of a variety of molecules (Ne, Ar, Kr, CH4, C2H4, CO2, O2, and N2) in five different zeolites (LTA, DDR, CHA, FAU, MFI) for a range of temperatures, T, and molecular loadings, qi. The simulation results show that for cage-type zeolites with narrow windows (LTA, CHA, and DDR) with guest molecules such as CH4 and Kr, that are tightly constrained at the window regions, the ĐiĐi − qi dependence varies significantly with T. Furthermore, the activation energy changes with the loading qi. The model of Reed and Ehrlich (D.A. Reed, G. Ehrlich, Surf. Sci. 102 (1981) 588) provides a convenient description of the combined influence of qi and T   on ĐiĐi. For other guest–host combinations that were investigated the ĐiĐi − qi dependence was found to depend relatively weakly on T  . The strength of correlations, quantified by (ĐĐi/Di,self − 1) is found to be practically T-independent for all guest–host combinations examined. Generally speaking, for a specified loading qi  , the activation energy for ĐĐi − T and Di,self − T variations are different; the difference is about 10% when the strength of correlations is large.