Diffusion behavior of gas molecules in the one-dimensional channel of AlPO4-5 molecular sieves

Abstract

Understanding of molecular diffusion mechanisms in zeolite plays a crucial role in the processes of adsorption and separations and can greatly facilitate the design of zeolite at molecular level. However, compared with the case of free gas phase, the diffusion behaviors in channel of zeolite are much more complicated due to the confined pore environments. To dig more detailed information about the diffusivity mechanism of gas molecules in the zeolite pore, diffusion behaviors of H 2 , He, N 2 , CH 4 and CO 2 molecules in the one-dimensional (1-D) channel of AlPO 4 -5 (AFI) zeolite-like materials were investigated using a combined classic molecular dynamics (MD) simulations and density functional theory (DFT) calculations. It is found that the diffusion behavior for light molecules (H 2 and He) at low loading in AlPO 4 -5 channel is more freedom because of their small molecular diameter and low energy variation (ΔE) in the longitudinal direction of the pore. Then, transport of light gas molecules can be described as Knudsen diffusion and the molecular mass plays a key role in this case. Transports of N 2 , CH 4 or CO 2 , with larger molecular diameter and higher ΔE value, were proposed to compete with surface diffusion and their diffusivity should be controlled by the adsorption energy between gas molecule and pore wall. Therefore, the diffusion degree of gas molecules at low loading is decreased as follows: H 2 > He > N 2 > CH 4 > CO 2 . When the gas molecule loading increases, the gas diffusion freedom will be limited due to the increased gas-gas and gas-porous media collisions. Thus, the diffusion of these gas molecules with higher concentration is dominated by the gas-gas and the gas-AFI interactions. DFT calculation results demonstrate that both gas-gas and the gas-AFI interaction energies depend on their van der Waals diameter. As a result, the diffusion of these gas molecules at higher loading in AlPO 4 -5 channel is decreased as: He > H 2 > N 2 > CH 4 > CO 2 . Our results confirmed that the diffusivity of gas molecules within the 1-D channel of AlPO 4 -5 is controlled by the molecular mass or gas-wall interactions at low loading, while is dominated by the gas-gas/gas-wall interactions at high loading. • DFT calculation results confirmed that AlPO 4 -5 unit cell with P 1 symmetry is much more stable than that with P 6 cc symmetry. • Transport of light gas molecules (H 2 and He) with small d m and low ΔE at low loading in AlPO 4 -5 channel can be described as Knudsen diffusion and the molecular mass plays a key role in this case. • Transports of N 2 , CH 4 and CO 2 , with larger molecular diameter and higher ΔE value, at low loading in AlPO 4 -5 channel were considered as surface diffusion and their diffusivity should be controlled by the gas-AFI interactions. • The diffusion of these gas molecules in AlPO 4 -5 channel with higher concentration is dominated by the gas-gas and the gas-AFI interactions.