Computational study of structural and thermal properties of the microporous titanosilicate ETS-10

Abstract

A theoretical approach to evaluate structural and physical chemical properties of ETS-10 has been chosen, employing calculation strategies that have proven to be accurate in the case of zeolites. The non-framework cation sites for K + and Na + ions have been modelled, combining a Monte Carlo Packing (MCP) procedure with the Lattice Energy Minimization (LEM) technique. A set of four distinct extra-framework cation sites distributed along the Ti-O-Ti rods has been obtained. They are surrounded on both sides by silicon five rings, and located between two Ti atoms of the rods. The lowest lattice energy has been obtained by placing K + cations on the non-framework sites. This is consistent with cation exchange properties of ETS-10. Na + ions are easier to exchange by H + than K + , despite the fact that mobility of hydrate K + is 50% higher than that of hydrated Na+ ions. In the case of the K-ETS-10 minimum-energy structure, the Ti-O bond length along the Ti-O-Ti rod (1.97) increases in about 3.5%, with respect to the proposed structure. Average Ti-O distances of 2.13along the Ti-O-Si linkages have been obtained. The Ti-O-Ti bond angle (163°) is about 5% larger than in the proposed starting structure. Lattice energy minimizations were performed for Al substitution into two different Si chemical environments in the ETS-10 structure; Si(4Si, 0Ti) and Si(3Si, 1Ti). The calculated stability difference for the two studied Al insertion positions is striking, indicating a clear preference for the sites connected to tetrahedral silicon, as opposed to those connected to octahedral Ti sites. This result is in good agreement with the Al, Ti avoidance in the ETAS-10 structure, indicated by 29 Si and 27 Al MAS NMR spectra. Molecular dynamic simulations suggest a significantly higher thermal stability for the polymorph B of ETS-10, compared to the experimentally observed for the disordered material.