Fabrication and Microstructure Tuning of a Pyrimidine-Bridged Organoalkoxysilane Membrane for CO₂ Separation

A novel pyrimidine-bridged organoalkoxysilane membrane was developed from 4,6-bis­(3-(triethoxysilyl)-1-propoxy)-1,3-pyrimidine (BTPP) via a sol–gel process. Self-catalyzed and HCl-catalyzed BTPP sols with different water molar ratios were prepared for membrane formation to tailor the microstructure of the BTPP membranes. A higher water molar ratio for the HCl-catalyzed sols led to the formation of a silica network with improved porosity and a well-connected structure. Gas adsorption measurements indicated that BTPP xerogels tended to show a dense silica network due to an organic-rich hybrid structure, and these also showed a higher level of CO₂/N₂ selectivity due to the presence of pyrimidine groups that could conduct special interactions with CO₂. Single-gas permeation testing was performed at different permeation temperatures using gases with different kinetic diameters: He (2.6 Å), H₂ (2.89 Å), CO₂ (3.3 Å), N₂ (3.64 Å), CH₄ (3.8 Å), and SF₆ (5.5 Å). The BTPP membranes showed a sharp kinetic diameter dependence of gas permeance with a higher level of H₂/SF₆ selectivity (>500). In addition, the relatively dense silica network and organic-rich properties of BTPP membranes resulted in activated diffusion for all gases considered, with the exception of SF₆ that could have permeated the BTPP membranes via larger pores or pinholes. CO₂ transport behaviors through BTPP membranes were compared according to activation energies for the permeation (E_p) of CO₂ and by the differences in E_p between CO₂ and N₂ (or CH₄). The BTPP-HCl-240 membrane that demonstrated the most-improved porosity and the best-connected silica network showed a lower E_p for CO₂ and a greater difference in E_p between CO₂ and N₂ (or CH₄). As a result, the BTPP-HCl-240 membrane exhibited great potential in CO₂ separation performance for both CO₂ permeance and CO₂/gas permselectivity. Compared with most of the reported amine-functionalized silica-based membranes, BTPP membranes showed great potential in CO₂ separation performance, which could lead to applications in CO₂ separation processes.