Pore Size Distribution in Hierarchical Materials: Insights from Molecular Simulations

Hierarchical materials, such as mesoporous zeolites, comprise of a distribution of pore sizes ranging from micropores (<2 nm) to mesopores (2–50 nm). Knowledge of pore size distribution is critical in designing and optimizing materials for various applications including, catalysis, separation, and gas storage and sequestration. Among the various available techniques for pore characterization, gas adsorption allows for determining a wide range of pore sizes in a non-destructive manner. Deducing the pore size distribution from the measured adsorption isotherms for argon, nitrogen, and/or carbon dioxide relies on theoretical procedures that utilize classical density functional theory (DFT) for the prediction of isotherms. These DFT predictions are limited to specific pore geometries (spherical, cylindrical, and slit pores) and represent the pore walls either as uniform surfaces or incorporating some degree of heterogeneity using interaction parameters tailored to entire classes of adsorbents, such as carbons, silicas, and zeolites.

In this work, large-scale Monte Carlo simulations are carried out to probe the adsorption in hierarchical materials. An attempt is made to understand the dependence of adsorption on confinement in mesoporous zeolites. By investigating large number of pore dimensions, a correlation between the capillary condensation pressure and the geometry of the pores is established. In addition to pore geometry, influence of adsorbate/adsorbent interactions on the adsorption behavior are also investigated.