A molecular dynamic investigation of the diffusion of methane-ethane and methane-propane mixtures in zeolites

Abstract

Molecular Dynamic (MD) simulations were carried out to determine the Maxwell-Stefan (M-S) diffusivities, Ð _i, and self-diffusivities, D_i,self, of methane (C1), ethane (C2), and propane (C3) for a variety of molecular loadings, q_i, in three classes of zeolite topologies: (1) intersecting channels (MFI, ISV, BEA), (2) one-dimensional (1D) channels (AFI, TON, FER, MOR, LTL), and (3) cages separated by windows (FAU, LTA, ERI, CHA, DDR). The Ð _i are strongly dependent on loading, decreasing to zero at saturation loading in all cases. For 1D channels, the decrease of Ð_i with q_i is severe. For cages separated by narrow windows (LTA, ERI, DDR, CHA), the Ð_i increase sharply with qi before eventually reducing to zero at saturation loading. Correlation effects are reflected in the ratio of the self- to M-S diffusivity, D_i,self/ Ð_i; this ratio is seen to be strongly dependent on the topology. Correlation effects are negligibly small in zeolite structures with cages separated by narrow windows. For binary C1-C2 and C1-C3 mixtures in both intersecting channel structures and 1D channels, the D_i,self of the more mobile species, C1, is reduced significantly due to the presence of the more tardy C2 or C3. The mobility of the tardy species is enhanced due to the presence of the mobile C1. For cages separated by narrow windows, the inter-cage hops are practically independent and there is no accelerating or decelerating effects during mixture diffusion.