Surface barriers to mass transfer in microporous catalysts and adsorbents

Experimental studies of transport in microporous materials such as zeolites and carbon molecular sieves have shown that a significant resistance to mass transfer can be present at the surfaces of the microporous regions. These surface barriers, believed to be caused by reconstruction effects at the free surfaces, can greatly modify the ability of a material to perform certain separations. In order to intelligently microengineer a surface barrier into a material, or to prevent undesired barriers, it is important to understand the relevant phenomena at the molecular level. The precision of surface-probing experimental techniques, such as atomic force microscopy, is not yet great enough to gain this understanding. In this work we apply computer simulation techniques and theory to the study of surface barriers in carbon molecular sieves and zeolites, especially barriers due to highly localized pore mouth resistances (on the order of a few Angstrom units). We develop an efficient method, related to the dual control volume methods recently appearing in the literature, for simulating the transport of molecules from the gas phase into a microporous solid under a concentration gradient. We also utilize transition state theory. Combining these techniques with solid surface models allows us to quantify the resistances of these surfaces in terms of mass transfer coefficients. We explore how the mass transfer coefficient is affected by pore size and geometry, temperature, gas pressure and composition, gas-solid interaction strength, gas-solid energy exchange, and solid lattice vibration. In the regime where the size of the pore mouth and the diffusing molecule are similar, we observe that changes of a few percent in the pore size cause order-of-magnitude changes in the mass transfer coefficient. Therefore it seems quite likely that for certain sorbate-sorbent systems, significant surface barriers could arise with essentially imperceptible changes in surface structure. We also observe that for a single-component gas, the gas-phase pressure has basically no effect on the transfer coefficient associated with crossing a pore mouth. Therefore, any observed concentration dependence of the surface barrier resistance in the uptake of a pure gas probably implies that the barrier is not highly localized, but rather distributed for some depth into the material. For mixtures, our simulations showed that the transfer coefficient of a smaller, less strongly adsorbed component can be reduced by the presence of a larger, more strongly adsorbed species. However, this reduction is no larger than that of molecules confined within the microporous channels; in fact, it is often less. In an unrelated study, we examine the thermodynamics of fluids confined to disordered microporous materials (e.g. silica gels). Building on the integral-equation formalism introduced by Madden and Glandt, we introduce two ways in which the pair-correlation functions can be used to calculate thermodynamic properties; one is analogous to the compressibility equation approach of liquid-state theory, while the other is based on scaled particle theory. We use the compressibility route, in combination with perturbation theory, to calculate the vapor-liquid phase coexistence envelopes of a confined model fluid