Titanium Silicate Porous Materials for Carbon Dioxide Adsorption: Synthesis Using a Structure Directing Agent, Detemplation and Inclusion of Alkaline Earth Metal Cations

Abstract

A titanium silicate variant named UPRM-5 was prepared using tetraethylammonium hydroxide as a structure-directing agent (SDA). Characterization of the material by scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques showed a homogeneous and crystalline solid powder phase, with unit cell dimensions of a = 6.8 Å, b = 11.7 Å, c = 13.4 Å, α = 102.9°, β = 92.8°, and γ = 91.1°. Successful detemplation was achieved via ion exchange with NH4Cl as evidenced by thermal gravimetric analysis (TGA) and Fourier transform infrared (FT-IR) spectroscopy data. Effective functionalization was obtained after ion exchanging the detemplated material using SrCl2 and BaCl2. These ion exchanged variants were also characterized using XRD and porosimetry techniques. Thermal-vacuum activation of Sr-UPRM-5 at 90 °C resulted in a material with a surface area of ca. 240 m2/g, while activation at higher temperatures resulted in low surface areas and plausible structural distortion. On the other hand, the barium variant exhibited the best thermal stability, with an average surface area on the order of 250 m2/g after employing activation temperatures up to 180 °C. The differences in thermal stability may be a result of structurally coordinated water. Adsorption of CO2 at 25 °C in Sr- and Ba-UPRM-5 materials activated at different temperatures also co-corroborated the aforementioned thermal stability observations in addition to what appears to be cation relocation. Fitting of the CO2 adsorption data with the Dubinin−Astakhov model revealed a heterogeneous surface, which was corroborated by isosteric heats of adsorption estimated from the uptake data. For low partial pressures, the observed CO2 adsorption capacities increased as follows: NH4-UPRM-5 < Sr-UPRM-5 < Ba-UPRM-5. Both the Sr- and Ba-UPRM-5 materials exhibited outstanding selectivity for CO2 over CH4, N2, and O2.

Sr(ii)-UPRM-5 titanium silicate framework thermally induced contraction: in situ high temperature XRD and 29Si MAS NMR

“…Conventional and microwave-assisted hydrothermal syntheses and ion exchange with Sr(II) UPRM-5 was synthesized via conventional and microwaveassisted thermal methods, the former using the procedure reported elsewhere. 26 For microwave-assisted syntheses, two gel mixtures with the following composition: 3.4(TEA) 2 O : 7.3Na 2 O : 1.2 K 2 O : 1.3TiO 2 : 10SiO 2 : 200H 2 O were prepared. The pH of one of them was adjusted to 7 by employing a 6 M HCl aqueous solution.…”

“…Backfill was achieved using helium (High Purity Grade, Linde). Prior to analysis, the samples were degassed in vacuum of less than 10 mm Hg and a temperature of 90 • C. 26 Heating and vacuum rates used were 5 • C min -1 and 50 mm Hg s -1, respectively. The samples were loaded into vials fitted with isolation valves, which allowed transfer from degassing to analysis stages without exposing the adsorbent material to ambient conditions.…”

“…10,13,15,16,18-23 However, at the application end, this framework contraction evidently put limits on the ETS-4 ultimate adsorption capacity possibly requiring more operation/regeneration cycles. 10,19, [23][24][25][26] Recently, Hernandez-Maldonado and co-workers reported a new titanium silicate porous sorbent (UPRM-5), which exhibits similar flexible characteristics as ETS-4 but with larger surface area and CO 2 adsorption capacity. 26 This is possible due to the inclusion of a quaternary ammonium salt in the synthesis acting as a structure-directing agent (SDA).…”

“…10,19, [23][24][25][26] Recently, Hernandez-Maldonado and co-workers reported a new titanium silicate porous sorbent (UPRM-5), which exhibits similar flexible characteristics as ETS-4 but with larger surface area and CO 2 adsorption capacity. 26 This is possible due to the inclusion of a quaternary ammonium salt in the synthesis acting as a structure-directing agent (SDA). Upon detemplation and ion exchange with strontium(II) (Sr-UPRM-5), the framework exhibits contraction just like ETS-4.…”

“…However, under similar activation and adsorption conditions, the Sr-UPRM-5 material shows larger uptake capacity when compared to ETS-4, likely due to the presence of larger voids formed by the presence of the SDA. 26 Although the practical aspects of the thermal treatment of Sr-UPRM-5 are well understood, 26 further work is required to elucidate the changes being experienced by the framework at the atomic or framework level. Here we present and discuss in situ high temperature X-ray diffraction (XRD) and 29 Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy data to understand the framework contraction process.…”