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EnglishThe 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.
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| Titolo: | Rietveld refinement of the zorite structure of the ETS-4 molecular sieves |
| Autori: | |
| Data di pubblicazione: | 1998 |
| Rivista: | |
| Abstract: | 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. |
| Handle: | http://hdl.handle.net/11392/1199778 |
| Appare nelle tipologie: | 03.1 Articolo su rivista |
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