Effect of macrovoids in nano-silica/polyimide mixed matrix membranes for high flux CO2/CH4 gas separation

Macrovoid structured mixed matrix membranes (MMMs) composed of nano-size (200 nm) silica particles and co-polyimide were prepared from 6FDA–ODA–DAM (6FDA = 4,4′-(hexafluoroiso-propylidene)diphthalic anhydride; ODA = 4,4′-oxidianiline; DAM = 1,3,5-trimethyl-2,6-phenylenediamine) with different proportions (1 : 1 and 1 : 4) and tetraethoxysilane (TEOS) via the sol–gel method. The separation performance of MMMs with 6FDA–ODA–DAM treated at high temperature (450 °C) was excellent for CO2/CH4 separation (for 6FOD–ODA–DAM (1 : 1): CO2 permeability ∼265 Barrer and CO2/CH4 selectivity ∼32; for 6FOD–ODA–DAM (1 : 4): CO2 permeability ∼302 Barrer and CO2/CH4 selectivity ∼25). Remarkably, the best membrane could resist pressure up to 600 psi without any loss of permselectivity. The CO2/CH4 separation performance of a series of silica–6FDA–ODA–DAM(11) MMMs with different SiO2 loadings is theoretically predicted using a modified Maxwell model where both gas permeability and macrovoid shape factor are simultaneously considered as adjustable parameters. Applying the optimized values, the modified Maxwell model predictions were in excellent agreement with experimental permeability data (less than 2% deviation).