Molecular simulation of the adsorption of methane in Engelhard titanosilicate frameworks

Molecular simulations were carried out to elucidate the influence of structural heterogeneity and of the presence of extra-framework cations and water molecules on the adsorption of methane in Engelhard titanosilicates, ETS-10 and ETS-4. The simulations employed three different modeling approaches, (i) with fixed cations and water at their single crystal positions, (ii) with fixed cations and water at their optimized positions, and (iii) with mobile extra-framework cations and water molecules. Simulations employing the final two approaches provided a more realistic description of adsorption in these materials, and showed that at least some cations and water molecules are displaced from the crystallographic positions obtained from single crystal data. Upon methane adsorption in the case of ETS-10, the cations move to the large rings, while in the case of ETS-4, the water molecules and cations migrate to more available space in the larger 12-membered ring channels for better accommodation of the methane molecules. For ETS-4, we also considered adsorption in all possible pure polymorph structures and then combined these to provide an estimate of adsorption in a real ETS-4 sample. By comparing simulated adsorption isotherms to experimental data, we were able to show that both the mobility of extra-framework species and the structural heterogeneity should be taken into account for realistic predictions of adsorption in titanosilicate materials.