Describing the diffusion of guest molecules inside porous structures

Abstract

The design and development of many emerging separation and catalytic process technologies require a proper quantitative description of diffusion of mixtures of guest molecules within meso- and microporous structures. In mesoporous materials with pore sizes 2 nm < d_p < 50 nm, there is a central core region where the influence of interactions of the molecules with the pore wall is either small or negligible; mesopore diffusion is governed by a combination of molecule-molecule and molecule-pore wall interactions. Within micropores with d_p < 2 nm, the guest molecules are always within the influence of the force field exerted with the wall; we have to reckon with the motion of adsorbed molecules, and there is no "bulk" fluid region. This article presents a unified, phenomenological, description of diffusion inside meso- and microporous structures using concepts and ideas that originate from James Clerk Maxwell and Josef Stefan. With the aid of extensive data sets of molecular dynamic simulations of unary and mixture diffusion in a wide variety of materials such as zeolites, metal-organic frameworks, covalent organic frameworks, carbon nanotubes, and cylindrical silica pores with a diverse range of pore topologies and pore sizes, we derive a molecular-level understanding of the various coefficients that arise in the phenomenological Maxwell-Stefan diffusion formulation. This understanding helps us to explain and describe a variety of experimental data and observations. We also demonstrate how a molecular level understanding aids separation and reaction process development.