Preparation and characterization of polyimide–silica composite membranes and their derived carbon–silica composite membranes for gas separation

Polyimide (PI)–silica composite membranes and their derived carbon–silica composite membranes were prepared for gas separation. The polyimide–silica composite membranes were prepared using the sol–gel technique, in which the polyimide matrix was synthesized by the condensation of pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA) while the inorganic phase was prepared by the in situ hydrolysis of tetraethyl orthosilica (TEOS) and a silane coupling agent, (3-aminopropyl)triehtoxysilane (APTES). The derived carbon–silica composite membranes were prepared by the pyrolysis of the polyimide–silica composite membranes at 900 °C under vacuum. The structures of the membranes were characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy, and X-ray diffraction (XRD) technique. The gas (He, CO2, N2 and O2) permeabilities of the polyimide–silica composite membranes and carbon–silica composite membranes were investigated. With the introduction of the silica, there was no significant enhancement of the gas separation in the resulting polyimide–silica composite membranes over the polyimide membrane, which still suffered a “trade-off” relationship between permeability and selectivity. However, the derived carbon–silica composite membranes exhibited better gas separation properties. The C–SiO2 28% composite membrane produced the highest permeances of 1042.18, 991.21, 296.03 and 155.26 GPU for He, CO2, O2 and N2, respectively, which were 21.61, 137.67, 103.15 and 150.74 times, respectively, of those of the pure carbon membrane. The C–SiO2 11% composite membrane produced the highest selectivities of 37.57, 36.61 and 7.09 for He/N2, CO2/N2 and O2/N2, respectively, which had surpassed the Robeson’s upper bound for these gas pairs. The agreement between the pure gas selectivity and the mixed gas selectivity for O2/N2 gas pair was reasonable, ranging from −8.30% to +16.61% using the pure gas selectivity as the base except for C–SiO2 23% composite membrane which showed +35.81% difference.

► Morphology of composite membranes was dependent on surface property of SiO2 particle.
► Permeability of gas (CO2, O2, N2, He) increased with increasing filler volume fraction.
► Composite membranes with modified SiO2 had comparable selectivity with pure P84.
► Composite membrane using modified SiO2 had higher selectivity than the pristine SiO2.
► Free volume theory provided acceptable predictions with the experimental data.