A molecular simulation study of adsorption of nitrogen and methane in titanium silicate (ETS-4)

Abstract

Adsorption isotherms of methane and nitrogen in porous titanium silicate ETS-4 (Engelhard titanium silicate) are calculated using grand canonical Monte Carlo (GCMC) simulations. Self-diffusion coefficients are determined using molecular dynamics (MD) simulations. Properties for pure gases were determined for two of the ideal ETS-4 polymorphs (ABAB-AA and ABAB-AC) dehydrated at different temperatures (423 and 573 K), taking into account only the framework atoms of the structure and ignoring the non-framework cations and water molecules. It was observed that equilibrium properties are slightly dependent on the structure selected for idealized polymorphs. However, it is not sufficient to explain the differences in adsorption capacity observed experimentally, which can only be explained with the combination of two polymorphs. In polymorphs with straight channels, self-diffusion in the direction of the main channel is two orders of magnitude larger than through the small rings that connect the main channels with some small cages. The trends observed in the self-diffusion coefficient with loading confirmed that crossing an eight-membered ring is an activated process.