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, (sic)(i), and the self-diffusivity, D-i,D-self, of a variety of molecules (Ne, Ar, Kr, CH4, C2H4, CO2, O-2, and NA in five different zeolites (LTA, DDR, CHA, FAU, MR) for a range of temperatures, T, and molecular loadings, q(i). 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 (sic)(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 (sic)(i). For other guest-host combinations that were investigated the (sic)(i) - q(i) dependence was found to depend relatively weakly on T. The strength of correlations, quantified by ((sic)(i)/D-i,D-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 (sic)(i) - T and D-i,D-self - T variations are different: the difference is about 10% when the strength of correlations is large.