Molecular interplay of cations and nonpolar/polar sorbates in titanosilicate ETS-10

Abstract

Molecular simulations are performed to investigate the interplay of the nonframework Na+ cations and the nonpolar (CH4)/polar (H2O) sorbates separately in titanosilicate ETS-10. Five types of Na+ cations are identified in the ETS-10 framework based on the different geometric positions and site energies. Located in the 12-ring channels and with the lowest site energy, type V cations show substantially greater mobility than type I-IV cations that remain essentially static in the 7-ring channels and the 7- and 12-ring channel intersections. The predicted adsorption isotherms of both CH4 and H2O in Na-ETS-10 agree well with the literature data. Two and three adsorption sites are observed for CH 4 and H2O, respectively. Compared to CH4, H2O exhibits a greater extent of adsorption and a much smaller diffusivity. Diffusion of CH4 drops monotonically as loading increases due to the steric hindrance. The interplay between Na+ and CH4 is not pronounced because of the weak van der Waals interactions, and the number of each type of Na+ ion does not change appreciably upon the adsorption of CH4. However, the interplay is remarkable between Na+ and H2O as a consequence of strong electrostatic interactions. Redistribution of Na+ ions is observed upon the adsorption of H2O, in which the number of type V cations increases while other types slightly decrease. As the loading increases, diffusion of H2O initially rises and then drops as largely determined by H2O in the 12-ring channels. The mobility of Na+ follows H2O in the 7- and 12-ring channel intersections due to the formation of Na+-H2O complex. © 2008 American Chemical Society.