Long- and Local-Range Structural Changes in Flexible Titanium Silicates with Variable Faulting upon Thermal Treatment and Corresponding Adsorption and Particle Size Polydispersity-Corrected Diffusion Parameters for CO₂/CH₄ Separation

Abstract

Sr²⁺-UPRM-5 is a titanosilicate containing adjustable structural faulting that prescribes changes in textural properties with temperature. In this work, we studied thermally induced structural changes in Sr²⁺-UPRM-5 variants prepared using tetrapropylammonium (TPA⁺) and tetrabutylammonium (TBA⁺) and their correlation to the diffusion of CO₂ and CH₄ at 25 °C. Both Sr²⁺-UPRM-5 materials contained different amounts of structural faulting that are correlated to the formation of 12-MR pores. In situ high-temperature X-ray diffraction revealed structural changes corresponding to orthorhombic phases up to 300 °C. Analysis of in situ high-temperature ²⁹Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy revealed new silicon environments surrounding the archetypical Si­(2Si, 2Ti_oct) and Si­(3Si, 1Ti_semioct) coordination centers. MAS NMR data analysis indicated that the Si environment in Sr²⁺-UPRM-5 (TPA) appears to be more susceptible to changes upon thermal treatment. A phenomenological volumetric transport model corrected for particle size polydispersity was used to estimate diffusion constants at 25 °C in adsorbents preactivated at different temperatures. At the optimal conditions, the CO₂/CH₄ kinetic selectivities were 41 and 30 for Sr²⁺-UPRM-5 (TBA) and (TPA), respectively