Tailoring Ultramicroporosity to Maximize CO ₂ 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 CO ₂ . 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 (CO ₂ and N ₂ ). However, a surprising increase in CO ₂ permeance (>2000 GPU), with moderate CO ₂ /N ₂ selectivity (â20), 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 CO ₂ capture.