Tailoring Ultramicroporosity To Maximize CO2 Transport within Pyrimidine-Bridged Organosilica Membranes

Amine-functionalized organosilica membranes have attracted an increasing amount of attention because of significant potential for the capture of postcombustion CO2. The appealing separation performance of these membranes, however, is generally obtained via compromises to gas permeance. In the present study, a novel, ultramicroporosity-tailored composite (organo)­silica membrane with high flux was synthesized via sol–gel cocondensation of a pyrimidine-bridged organoalkoxysilane precursor 4,6-bis­(3-(triethoxysilyl)-1-propoxy)-1,3-pyrimidine (BTPP) with a second intrinsically rigid network precursor (1,2-bis­(triethoxysilyl)­ethane or tetraethylorthosilicate). The surface chemistry, ultramicroporosity, and chain-packing state of the initial BTPP-derived membranes can be carefully tuned, which has been verified via Fourier transform infrared spectroscopy, water-contact angle measurement, X-ray diffraction, and positron annihilation lifetime spectroscopy. The composite (organo)­silica xerogel specimens presented a slightly improved ultramicroporosity with noticeable increases in gas adsorption (CO2 and N2). However, a surprising increase in CO2 permeance (>2000 GPU), with moderate CO2/N2 selectivity (∼20), was observed in the resultant composite (organo)­silica membranes. Furthermore, gas permeance of the composite membranes far surpassed the values based on Maxwell predictions, indicating a possible molecular-scale dispersion of the composite networks. This novel, porosity-tailored, high-flux membrane holds great potential for use in industrial postcombustion CO2 capture.