  Adsorption in nanoporous materials is one strategy that can be used to store
hydrogen at conditions of temperature and pressure that are economically
viable. Adsorption capacity of nanoporous materials depends on surface area
which can be enhanced by incorporating a hierarchical pore structure. We report
grand canonical Monte Carlo (GCMC) simulation results on the adsorption of
hydrogen in hierarchical models of silicalite that incorporate 4 nm wide
mesopores in addition to the 0.5 nm wide micropores at 298 K, using different
force fields to model hydrogen. Our results suggest that incorporating
mesopores in silicalite can enhance adsorption by at least 20% if electrostatic
interactions are not included and up to 100% otherwise. Incorporating
electrostatic interactions results in higher adsorption by close to 100% at
lower pressures for hierarchical silicalite whereas for unmodified silicalite,
it is less significant at all pressures. Hydroxylating the mesopore surface in
hierarchical silicalite results in an enhancement in adsorption at pressures
below 1 atm and suppression by up to 20 % at higher pressures. Temperature
dependence at selected pressures exhibits expected decrease in adsorption
amounts at higher temperatures. These findings can be useful in the
engineering, selection, and optimization of nanoporous materials for hydrogen
storage.
