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

• Published in: Microporous and mesoporous materials 1998-04, Vol.21 (1-3), p.143-153
• Main Authors: Cruciani, G, De Luca, P, Nastro, A, Pattison, P
• Format: Article
• Language: English
• Subjects: Chemistry; ETS-4; Exact sciences and technology; General and physical chemistry; Ion-exchange; Rietveld refinement; Surface physical chemistry; Titanosilicates; Zeolites: preparations and properties; Zorite
• ISSN: 1387-1811; 1873-3093
• DOI: 10.1016/S1387-1811(97)00051-6

The crystal structure of ETS-4 titanosilicate [Na9Si12Ti5O38(OH)·12H2O] was refined using synchrotron X-ray powder diffraction and the Rietveld method (Rp=0.050; Rwp=0.063; RB=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 29Si 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 29Si 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.