A Study of Heat-Treatment Induced Framework Contraction in Strontium-ETS-4 by Powder Neutron Diffraction and Vibrational Spectroscopy
journal contributionposted on 30.11.2001, 00:00 by Sankar Nair, Michael Tsapatsis, Brian H. Toby, Steven M. Kuznicki
The effects of heat-treatment on the structure of the strontium ion-exchanged titanosilicate ETS-4 have been studied by Rietveld analysis of powder neutron diffraction data and by FT-Raman spectroscopy. Hydrous Sr-ETS-4 (space group Cmmm), upon heat-treatment under inert atmosphere at temperatures between 423 and 573 K, exhibits framework contraction as evinced by the decrease in the unit cell dimensions. The effects of heat-treatment on the dimensions of the transport-controlling eight-membered ring (8MR) are elucidated by Rietveld analysis. It is also found that during heat-treatment: (a) the double three membered rings (D3MRs) in ETS-4 are sites of structural instability, (b) the titania chains running along  exhibit a large degree of disorder in the bridging oxygen atoms, and (c) significant relocations of the strontium cations take place, which may affect the separation properties of the heat-treated materials. Raman spectra of heat-treated ETS-4 crystals exhibit strong cation−framework interaction effects. Vibrational modes involving the atoms in the titania chains show progressive frequency shifts and loss of intensity with increasing heat-treatment temperature, in a manner consistent with the crystallographic results. The study indicates the potential for continuously varying the effective pore dimension of ETS-4 by combining heat-treatment with appropriate ion-exchange procedures.