Interfacial resistance of gas transport through rigid and flexible zeolites

Zeolite is widely used in the fields of adsorption, catalysis and membrane separation due to its well-defined structure. Equilibrium molecular dynamics (EMD) simulations were employed to investigate the effects of lattice flexibility on the transport diffusion of gases including CH4, Ar and H2 in LTA zeolite, associated with the effects of molecular size and gas loading being discussed. Our results reveal that the lattice vibration of zeolite leads to a pore breathing effect, which results in significantly enhanced interfacial resistance and much slower transport diffusion for CH4 and Ar, compared with the rigid lattices. Nevertheless, the diffusion of H2 with a small diameter, light weight and weak interactions, receives negligible impact from the lattice vibration. We find that the diffusivities of CH4, Ar and H2 increase with the thickness of zeolites and converge to constant values after the critical thicknesses, over which the interfacial resistance becomes negligible. For CH4 and Ar, the critical thicknesses are in the sub-micrometer scale and are around one order of magnitude higher than that of H2, emphasizing the interfacial resistance plays a dominant role for the molecules with diameters comparable to the aperture of zeolite and carrying relatively greater inertia. Further, as the loading increases from - 1 to - 3 mol per u. c., the interfacial resistances of CH4, Ar and H2 are reduced both in the rigid and flexible lattices, as a consequence of the prevalent fluid-fluid interactions. However, the ratios of the interfacial resistance to the total resistance, (Rf/Rt), and the critical thicknesses demonstrate different loading dependencies for CH4, Ar and H2. Our simulations essentially prompt the understanding of gases transport in nano-porous media, and benefit the optimal design of nano-porous membrane.

Automated summary

The lattice flexibility of zeolite significantly affects the transport diffusion of gases such as CH4, Ar and H2, with a pore breathing effect leading to enhanced interfacial resistance for CH4 and Ar but negligible impact on H2. The diffusivities of gases increase with zeolite thickness and reach constant values after critical thicknesses, emphasizing the dominant role of interfacial resistance for larger molecules. Loading dependency on interfacial resistances and critical thicknesses varies for CH4, Ar and H2, highlighting the importance of fluid-fluid interactions.