Design of Silica-Cobalt Composite Microporous Structures with Dispersed Carbon Particles for Highly Permselective Gas Separation Membranes

Metal-doped silica membranes, fabricated via the sol–gel technique using metal nitrates, hold promise for high-temperature separation processes, such as H2 separation in steam reforming reactions. However, controlling the status of the doped metal is challenging and often leads to defect formation owing to the aggregation of metal oxides. In this study, we designed a uniform carbon–Co–SiO2 ceramic membrane using a one-pot sol–gel method with copolymerization, employing tetraethoxysilane and cobalt acetylacetone(III) (Co-(acac)3) as precursors. Organic chelate ligands within the amorphous silica network formed by the polymerization reaction were carbonized by calcination at 250–750 °C in an inert atmosphere. This approach suppressed defect formation and tailored the microporous structures to a wide range of separation systems. For example, the SiO2–Co-(acac)3 membrane calcined at 550 °C demonstrated a notable C3H6 permeance of 4.0 × 10–8 mol m–2 s–1 Pa–1 (GPU: 120), with a high C3H6/C3H8 selectivity of 46, attributed to the molecular sieving effect, whereas the membrane calcined at 650 °C exhibited a remarkable He permeance of 4.6 × 10–7 mol m–2 s–1 Pa–1 (GPU: 1400), with a high He/CH4 selectivity of 830. This study provides valuable insights into the development of defect-free carbon–cation–SiO2 ceramic membranes for a broad range of gas separation processes.