Theory and simulation of cohesive diffusion in nanopores: Transport in subcritical and supercritical regimes

Publisher Website
Google Scholar
Abstract
We have studied a lattice model of self-diffusion in nanopores, to explore how loading, temperature, and adsorbate coupling influence benzene self-diffusion in Na–X and Na–Y zeolites. We propose a simple method for determining how adsorbate–adsorbate interactions modify activation energies of site-to-site jumps. We apply a mean-field approximation that describes transport semiquantitatively for a wide variety of system parameters, simplifying kinetic Monte Carlo simulations. We also derive an analytical diffusion theory that provides semiquantitative apparent activation energies, and qualitatively reasonable loading dependencies. We have found that supercritical systems exhibit three characteristic loading dependencies of diffusion, depending upon the degree of degeneracy of lattice sites. Subcritical diffusion systems are dominated by cluster formation, exhibiting intriguing loading dependencies with broad regions of constant diffusivity. Our model for benzene in Na–X is in excellent qualitative agreement with pulsed field gradient nuclear magnetic resonance (NMR) diffusivities, and in qualitative disagreement with tracer zero-length column (TZLC) data. We suggest that high-temperature TZLC experiments should be performed, to test whether the coverage of maximum diffusivity decreases with increasing temperature.