UPRM-5 titanium silicates prepared using tetrapropylammonium and tetrabutylammonium cations: framework stability, textural properties, and carbon dioxide adsorption

UPRM-5 is a flexible titanium silicate first prepared using tetraethylammonium (TEA⁺) and that exhibited improved structural and adsorption properties when compared to other titanium silicates. In order to further tailor these properties, we have employed tetrapropylammonium (TPA⁺) and tetrabutylammonium (TBA⁺), as structure directing agents (SDAs), respectively. Analysis of the local-range structure using ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy suggested silicon environments corresponding to Si(2Si, 2Ti_octa) and Si(3Si, 1Ti_semi-octa), as expected for a flexible titanium silicate. A quantitative analysis, however, revealed that the amount of semi-octahedral titanium centers was greater in the variant prepared with TPA⁺ suggesting that the nature of the NR₄ ⁺ cation plays an important role in the formation of framework faulting. Both UPRM-5 variants were detemplated and modified to include extraframework Sr²⁺ and produce materials for carbon dioxide adsorption. Their thermal stability and pore contraction were first investigated by means of in situ high-temperature X-ray powder diffraction and nitrogen porosimetry. Materials prepared with TBA⁺ showcased better thermal stability when compared to variants prepared with TPA⁺ and even TEA⁺, probably due to the relative low level of structural faulting. All variants, however, displayed a pore contraction process associated with the release of tenacious water. Carbon dioxide uptakes varied considerably depending on the choice of SDA employed and the isosteric heat of adsorption profiles correlated with a heterogeneous surface. The results suggest that Sr²⁺–UPRM-5 (TPA) materials could be tailored for purification applications, whereas Sr²⁺–UPRM-5 (TBA) materials could be tailored for bulk-level separation applications.