Loading Dependence of Self‐Diffusivities of Gases in Zeolites

Abstract

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 .

Diffusion of Methane and Carbon Dioxide in Carbon Molecular Sieve Membranes by Multinuclear Pulsed Field Gradient NMR

“…This would result in a decrease of methane diffusivity with decreasing loading in the limiting case of small methane concentrations inside the membranes. Such loading dependences of diffusivity and permeability have been observed at low loadings in other microporous materials, including select zeolites 34 (e.g., LTA, DDR), nanoporous glass, 31 and a CMS membrane derived from Kapton polyimide. 16 Note that in the low loading regime (i.e., θ ≈ 0) the thermodynamic factor approaches unity, and eqs 5 and 6 reduce to D T = D T0 ≅ D s .…”

Diffusion of Trimethylbenzenes, Toluene, and Xylenes in UWY Zeolite as a Catalyst for Transalkylation of Trimethylbenzenes with Toluene

“…The diffusivity is expected to be highly dependent on the loading. − The most simple expected behavior is that increasing loadings will lead to larger sorbate–sorbate repulsions and hence decreased diffusivity, but this will also be coupled with the extent to which each channel is occupied at different loadings. Hence, given the importance of this aspect, two extreme loadings have been considered, corresponding to a total of either 10 (low) or 21 (high) molecules in the 3 × 3 × 2 cell of UWY.…”

An atomistic simulation towards elucidation of operating temperature effect in CO2 swelling of polysulfone polymeric membranes

“…The fundamental equation underlying the COMPASS force field is provided in equations (S1) -(S4) of Supplementary Information. The electrostatic interactions have been characterized via the Ewald method (accuracy of 0.001 kcal/ mol), while the Lennard-Jones-6-12 function has been employed to describe the van der Waals interaction [59] with a cut-off-distance of ~18 Ǻ (spline width of 1 Ǻ and buffer width of 0.5 Ǻ). The cut-off distance has been employed since it is lower than half of the anticipated cell specification, which has been suggested in previous published works to be adequate to prevent atoms interrelating with their own image, while minimizing computational cost [60][61][62][63].…”