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, 2Tiocta) and Si(3Si, 1Tisemi-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.
Thermal Gravimetric Analysis Isosteric Heat External Surface Area Diffuse Reflectance Infrared Fourier Transform Micropore Surface Area
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This publication is based upon work supported by the National Aeronautics and Space Administration under Grant No. NNX09AV05A. Partial support was also provided by the National Science Foundation (NSF) under Grant No. HRD 0833112 (CREST Program). We also wish to acknowledge support from the PR-LSAMP Bridge to the Doctorate Program and the Puerto Rico Institute for Functional Nanomaterials. The NMR measurements were performed at the National High Magnetic Field Laboratory (NHMFL) supported by NSF Cooperative Agreement No. DMR-0654118, the State of Florida, and the U.S. Department of Energy.