Collect. Czech. Chem. Commun. 2010, 75, 145-164
Published online 2010-02-16 10:23:42

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

Flor R. Sipersteina, Martin Lísalb,c,* and John K. Brennand

a School of Chemical Engineering & Analytical Science, The University of Manchester, PO Box 88, Sackville Street, Manchester M60 1QD, UK
b Department of Physics, Faculty of Science, J. E. Purkinje University, 400 96 Ústí nad Labem, Czech Republic
c E. Hála Laboratory of Thermodynamics, Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, v.v.i., Rozvojová 135, 165 02 Prague 6-Suchdol, Czech Republic
d U.S. Army Research Laboratory, Weapons and Materials Research Directorate, Aberdeen Proving Ground, MD 21005-5066, USA


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.

Keywords: Adsorption isotherms; ETS-4; Grand canonical Monte Carlo; Molecular dynamics; Self diffusion.

References: 35 live references.