Loading Dependence of Self‐Diffusivities of Gases in Zeolites

Experimental data on the self‐diffusivities, D i,self , of a variety of gases (CH 4 , N 2 , Kr, C 2 H 6 , and C 3 H 8 ) in three different zeolites, LTA, FAU, and MFI, show different dependences on the molar loading, q i . In LTA, D i,self appears to increase with q i for all molecules except N 2 . In FAU and in MFI the D i,self shows a sharp decrease with increasing q i . 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 (CH 4 , N 2 , Kr, C 2 H 6 , C 3 H 8 , 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 D i,self with q i 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 D i,self invariably decreases with increasing q i . 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 D i,self decreases with q i for all molecules. If the windows are narrow (CHA, DDR and LTA), the D i,self often exhibits a sharp increase with q i , reaches a maximum and reduces to near‐zero values at saturation. The sharpness with which D i,self increases with q i , is dictated by the degree of confinement at the window. Weakly confined molecules, such as Ne, do not exhibit an increase of D i,self with q i .