Molecular Models for Templated Mesoporous materials: Mimetic Simulation and Gas Adsorption

Abstract

The complex structures of the Templated Mesoporous Materials (TMMs) are difficult to capture using experiments. On the other hand, detailed structural information is required in order to study the confinement effects and predict material properties. We therefore present a methodology to prepare realistic molecular models of the TMMs using molecular simulations. Mimetic simulations are used to simulate the synthesis of the TMMs resulting in mesoscale models of the materials. Using this technique, we have developed models for SBA-15 and the Mesostructured Cellular Foams (MCF). The mimetic simulations also allow us to study the phase diagrams of the surfactants involved in the synthesis. We have investigated the ternary phase diagrams (surfactant-oil-water and surfactant-silica-water) of model triblock surfactants and have highlighted the effects of oil on the ordered structures. The simulation results for the effect of oil are in partial agreement with the experiments. Next, we devise a technique to convert the mesoscale TMM models into atomistic ones. The method has been demonstrated by preparing atomistic models for SBA-15. The physical properties of the models (pore size distribution, surface area, TEM and AFM images) are compared to the experimental ones. The porosities and the surface areas of the models are in quantitative agreement with those of the experimental SBA-15, whereas the pore size distribution and TEM results agree qualitatively with the experiments. We also present new methods for characterizing model structures including a fast technique for computing pore size distributions. The results from our new technique show speed increases of several orders of magnitude compared to the existing method. Finally we simulate the adsorption of Argon inside the model SBA-15 using Grand canonical Monte Carlo simulations. The adsorption isotherm from the model is in semi-quantitative agreement with that of an experimental SBA-15. The adsorption behavior of several different pore models are investigated, which provides new light on the roles of surface roughness and micropores in determining adsorption properties. We conclude by saying that the pore models developed in this work may be used in studying phase transitions, adsorption, diffusion and reactions inside nanopores, and in preparing new mesoporous material models such as the CMK carbons.