Rietveld refinement of the zorite structure of ETS-4 molecular sieves

The crystal structure of ETS-4 titanosilicate [Na9Si12Ti5O38(OH). 12H(2)O] was refined using synchrotron X-ray powder diffraction and the Rietveld method (R-P=0.050; R-wP=0.063; R-B=0.169). Our diffraction data provide direct evidence that ETS-4 has the same structure as the zorite mineral. The framework of zorite can be described as nenadkevichite-like chains, laterally connected by 4[SiO4]/[TiO6] units, corresponding to two different chemical environments for Si. Similar to previous observations, a discrepancy was found between the population ratio of Si positions as calculated by the occupancy refinement of atomic sites related to these two environments, and that inferred from Si-29 MAS NMR. This is tentatively explained here on the basis of the large amounts of protons in the proximity of the Si2 site as indicated by Si-29 CP/MAS NMR. The low thermal stability of the zorite/ETS-4 structure is explained in terms of relevance of the hydrogen bonding between extra framework water molecules and framework oxygens. (C) 1998 Elsevier Science B.V. All rights reserved.