Rivera Ramos, Milton E.

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    Selective nanoporous sorbents for gas based separations: Ion exchanged silicoaluminophosphates and titanosilicates
    (2009-05) Rivera Ramos, Milton E.; Hernández Maldonado, Arturo J.; College of Engineering; Rinaldi, Carlos; Benítez, Jaime; Suleiman, David; Department of Chemical Engineering; López, Gustavo
    Two specific sorbents were targeted in this research work: silicoaluminosphosphates (SAPOs) and titanium silicates. For the former, Na+ -SAPO-34 sorbents were ionexchanged with several cations (Ag+, Mg2+, Ca2+, Sr2+, Ti3+, and Ce3+) to study their effect on the adsorption of similar size light gases. Adsorption isotherms for the functionalized Mn+-SAPO-34 materials displayed a non-linear behavior and, in many cases, did not follow the typical pore-filling mechanism. The surface interactions were analyzed according to electrostatic and non-specific contributions. Divalent cations were found to interact more with the sorbate when compared to the other charged species. Due to strong ion-quadrupole interactions, all the sorbent materials exhibited higher affinity for CO2 over the other gases tested (i.e., CH4, H2, N2 and O2). Mathematical modeling to estimate binary component adsorption performance during Vacuum Pressure Swing Adsorption (VPSA) corroborated that Sr2+-SAPO-34 sorbents are a suitable option for CO2 removal from CH4 mixtures, especially at low concentrations. In addition, these materials were subject of a diffusive transport study to elucidate the effect of particle size distribution on the estimates of diffusion time constants. A gamma particle size distribution was found to suitably describe the SAPO-34 characteristic dimensions. Many soft titanium silicates display sorption selectivity towards specific gases based on size exclusion principles. However, selectivity comes at the expense of sorption ultimate capacity, which decreases considerably due to the lack of pore volume. Here we present a novel titanium silicate polymorph (named UPRM-4d), which was synthesized by means of a structure-directing agent (SDA) to increase the working pore volume, and still exhibits the structural flexibility of Zorite based titanium silicates. The SDA was successfully removed from the as-synthesized sample by ion-exchange techniques to expose the pore volume. Afterwards, the UPRM-4d sorbent displayed selectivity towards CO2 and adsorbed almost twice the amount when compared to a commercial sorbent titanium silicate material known as ETS-4. Other UPRM-4 variants were studied as well, and their synthesis and characterization are presented here.