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

Abstract

Molecular dynamics (MD) simulations were performed to determine both the Maxwell-Stefan diffusivity, Ð_i, and the self-diffusivity, D_i,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 Ð_i - q_i dependence varies significantly with T. Furthermore, the activation energy changes with the loading q_i. 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 q_i and T on Ð_i. For other guest-host combinations that were investigated the Ð_i - q_i dependence was found to depend relatively weakly on T. The strength of correlations, quantified by (Ð_i/D_i,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 Ð_i - T and D_i,self - T variations are different; the difference is about 10% when the strength of correlations is large.