An investigation of porous and non-porous fillers in mixed matrix membranes for gas separation applications

Mixed matrix membranes (MMMs) containing inorganic and organic materials have attracted interest for gas separation applications, with the aim of using the advantages of both types of materials for enhancing gas separation performance, compared to either pure polymer or purely inorganic membranes. When these two materials are combined, the difference in properties of the inorganic and organic phases may cause the creation of voids at the polymer-filler interface, especially for glassy polymers, which decreases the gas separation performance. Both porous and non-porous materials can be used as the inorganic phase. Porous material with high selectivity and molecular sieve properties can increase the gas separation performance. The free volume between the polymer chains increases due to the polymer chain packing disruption by addition of the non-porous material. Among the various porous inorganic materials used for membrane fabrication, zeolites are one of the best fillers for gas separation, due to excellent chemical stability and high permselectivity. Fabricating commercial hollow fibre membranes with an optimal selective layer thickness of about 100 nm with micron-sized zeolite particles is not possible, and the thicker membrane results in lower permeation. Moving to a nano-sized zeolite allows for the fabrication of a thinner selective layer in membranes with consequently higher permeation. However, nano-sized zeolite particles tend to agglomerate due to their high interaction forces, and this may also cause void formation when introduced into the polymer phase, which affects the gas separation performance. In general, the MMMs containing zeolite nanoparticles had lower permeabilities but higher selectivities than the MMMs containing TiO2 nanoparticles, demonstrating the difference in properties of these materials due to different interactions with the same polymer.