Molecular Dynamics Investigation of Surface Resistances in Zeolite Nanosheets

Most zeolite membranes contain an active layer of intergrown zeolite crystals that are hundreds of nanometers to a few microns thick. For such membranes, the effect of the surface resistances on transmembrane transport is expected to be small. Progress has been made in recent years in synthesizing zeolite nanosheets with unit cell-level thicknesses and converting them into ultrathin zeolitic membranes or catalysts with very small intraparticle diffusion length scales. In such situations, effects from surface resistance are likely to be amplified relative to large zeolite crystals. Little is currently known, however, about the characteristics of these resistances for zeolite nanosheets. In this paper, we use molecular dynamics to measure the surface resistance of different zeolite nanosheets for seven adsorbate species under a range of temperature and pressure conditions. We find that surface resistance dominates the resistance to mass transfer associated with the zeolite phase under almost all conditions we simulated, with low temperature and low-loading conditions exhibiting the highest surface resistance. We show that surface resistance is strongly dependent on the heat of adsorption of the diffusing molecule in the bulk zeolite and use this dependence to predict diffusion and ideal selectivity in nanosheets from six different frameworks using only data collected in the bulk material.