Sr(ii)-UPRM-5 titanium silicate framework thermally induced contraction: in situhigh temperature XRD and 29Si MAS NMR

Abstract

The thermally induced contraction process of a titanosilicate prepared with tetraethylammonium hydroxide and ion exchanged with Sr(II) (Sr-UPRM-5) after detemplation has been characterized via in situhigh temperature X-ray diffraction (XRD) and ²⁹Si magic angle spinning nuclear magnetic resonance (MAS NMR). The as-synthesized material was prepared via conventional and microwave-assisted hydrothermal methods, the latter resulting in a reaction time an order of magnitude shorter than the former case. In situhigh temperature XRD tests performed on Sr-UPRM-5 indicate that at 120 °C, water coordinated to the structure is released initiating the collapse of the framework. At much higher temperatures, the material eventually becomes an amorphous phase. Indexing of the XRD patterns indicates that lattice constant a was more affected by the heat treatment, probably related to the material’s pore system, while the unit cell volume experienced a 44% reduction. ²⁹Si MAS NMR analyses for as-synthesized UPRM-5 confirmed two different silicon environments: Si(2Si, 2Ti_octa) and Si(3Si, 1Ti_semi-octa), which are similar to those exhibited by titanosilicate ETS-4. On the other hand, in situ high temperature ²⁹Si MAS NMR analyses for Sr-UPRM-5 demonstrated that changes in the silicon environment due to the presence of titanium centers possessing additional multiple coordination states, which arise from elimination of framework coordinated water molecules, are responsible for the structure collapsing. In general, these results underline the importance of avoiding complete removal of tenacious water molecules in order to preserve the Sr-UPRM-5 properties suitable for adsorption and catalysis applications.