Influence of synthesis conditions on surface heterogeneity of M41 type materials studied with lattice Monte Carlo

Abstract

The synthesis of mesoporous silica materials using surfactants as structure-directing agents is studied using lattice Monte Carlo simulations. The surfactant HmTn is modeled as a sequence of m hydrophilic segments or heads (H) followed by n hydrophobic segments or tails (T). Favorable interactions between the surfactant heads and the silica result in the formation of a surfactant-rich silica-rich phase in equilibrium with a dilute phase. Liquid crystal behavior is observed in the phase containing high-surfactant and high-silica concentration, with different structures depending on the overall system composition that are similar to the M41 family. The formation of a silica hexagonal phase is observed at low surfactant/silica ratios and lamellar or perforated lamellar phases are formed at high surfactant/silica ratios. The structure of the silica materials depends on the surfactant chemistry, the surfactant/silica ratios and the temperature. Heats of adsorption of simple gases on model MCM-41 type materials are calculated using Grand Canonical Monte Carlo simulations. Adsorption properties on model materials that were generated through a mimetic synthesis using lattice Monte Carlo simulations are compared with those on smooth cylindrical pores. Energetic heterogeneity in the materials studied is due to surface roughness and structural defects and not to the presence of areas with different chemical composition. This is in agreement with experimentally measured heats of adsorption of simple gases. At low coverage, heats of adsorption of argon and krypton on MCM-41 decrease with coverage, indicating that MCM-41 is not a homogeneous adsorbent even for non-polar spherical gas molecules, where the presence of polar groups in the adsorbent surface should have little effect.