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

Abstract

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).