Primera Pedrozo, José N.

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    Flexible titanium silicate porous materials for selective carbon dioxide adsorption
    (2013) Primera Pedrozo, José N.; Hernández Maldonado, Arturo J.; College of Engineering; Suleiman Rosado, David; Curet Arana, María C.; Martínez Iñesta, María M.; Department of Chemical Engineering; Hernández Rivera, Samuel P.
    Flexible titanium silicates are Zorite-like synthetic materials in which their framework atoms’ configuration exhibits mixed oxides containing tetrahedrally coordinated silicon and squaredpyramidal/octahedral titanium atoms. One of these titanium silicates is the novel variant named UPRM-5 that was prepared using conventional and microwave-assisted hydrothermal methods, and it was prepared by employing tetraethylammonium hydroxide as a molecular structuredirecting agent (SDA). Liquid-phase ion exchange with an NH4Cl salt solution was employed to remove the SDA, and several characterization techniques could provide evidence of this. Functionalization of the detemplated material was achieved after ion exchange with SrCl2 and BaCl2 salt solutions. The resulted functionalized materials exhibited higher surface areas and CO2 uptake compared with a counterpart commercial ETS-4 material, as well as remarkable selectivity for CO2 over CH4, N2, and O2. The barium variant showed the best thermal stability, and the strontium variant was the more flexible with thermal vacuum activation. The latter was characterized with in situ high temperature XRD, XRD-DSC, DTG, and 29Si MAS NMR techniques to study the framework contraction process. With these techniques, it was found that the removal of the framework-coordinated water molecules after 120 °C caused internal migration of the cations, producing rearrangement of the framework atoms. Although the UPRM-5 XRD pattern differed from that of ETS-4, both materials possess similar silicon environments, represented by Si(2Si, 2Tiocta) and Si(3Si, 1Tisemi-octa), respectively, evidenced by standard 29Si MAS NMR on the as-synthesized material. Therefore, simulations of faulted XRD patterns were employed to determine the level of polymorphism or faulting in the UPRM-5 crystal structure, resulting in a combination of two orthorhombic polymorphs with faulting probabilities of 90 % and 10 % in the a- and c-direction of the unit cell, respectively. The Na-UPRM-5 crystal structure was approached using a dual-phase Rietveld refinement method employing an orthorhombic with a faulted triclinic phase, having a major contribution in the orthorhombic phase. In addition, the results revealed a structural faulting and distortion product from the presence of the TEA+ cations such that, after their removal, the material exhibits higher adsorption capacity than ETS-4 in similar conditions.