The effects of firing temperature and Ti/Si ratio on the H2 permeation characteristics of microporous TiO2-SiO2-organic chelating-ligand composite membranes

Abstract

Titania-silica composite materials were synthesized and successfully employed in the preparation of microporous gas separation membranes via sol–gel processing. Acetylacetone (ACA), an organic-chelating ligand (OCL), was introduced as a reaction modulator to control the hydrolysis/condensation reactions of titan alkoxide, which resulted in formation of well combined titania and silica structures. The membrane material had a molar ratio of Ti/ACA = 1/1 and was coated onto an intermediate layer with an average pore diameter of about 1 nm. The effect of firing temperature on gas permeability was examined at 300 and 500 °C under a nitrogen atmosphere. As a result, for the case of Ti/Si/ACA = 1/1/1, the membrane fired at 300 °C under nitrogen (300 N) showed a molecular sieving permeation mechanism and a permeance that depended on molecular diameter. This membrane achieved H2 permeance of 0.63 × 10-6 mol/(m2 s Pa), H2/CH4 selectivity of 72.8, and H2/SF6 selectivity of 534.7. The membrane fired at 500 °C under nitrogen (500 N) showed a similar molecular sieving performance, albeit with smaller values for H2 permeance and H2/SF6 selectivity. For this study, the Ti/Si ratios were also altered and studied. Membranes with ratios of Ti/Si/ACA = 1/2/1 and 2/1/2 were prepared by firing under nitrogen at the optimal temperature of 300 °C, and the Ti/Si molar ratio effect on the gas permeance was examined. H2/SF6 selectivity was increased with an increase in the silica content (Ti/Si/ACA = 1/2/1 membrane: H2 permeance 1.26 × 10-6 mol/(m2 s Pa), H2/SF6 = 1350.7). On the other hand, H2/SF6 selectivity became smaller as the titania composite content was increased (Ti/Si/ACA = 2/1/2 membrane: H2 permeance 1.16 × 10-6 mol/(m2 s Pa, H2/SF6 = 53.0). Therefore, by optimizing the titania content, it is expected to achieve the H2 gas separation performance of the TiO2-SiO2-ACA membrane according to the application of the membrane.