Fabrication of highly permeable polyamide membranes with large “leaf-like” surface nanostructures on inorganic supports for organic solvent nanofiltration

Organic solvent nanofiltration (OSN) has shown great potential in molecular separation and solvent purification. Polyamide (PA) thin-film composite (TFC) membranes on polymer supports have been widely used in OSN, but the development of solvent resist supports is still a big challenge. Inorganic supports have good solvent resistance, but their applications were hindered by their high price and low solvent permeability. How to enhance the membrane permeability while maintain a high solute rejection is another challenge. Here we report the use of an inexpensive inorganic support made of large alpha-alumina particles. The surface was smoothed by multilayer coatings first with smaller alpha-alumina particles followed by carbon nanotubes. The hierarchical inorganic support is 20 times more permeable than conventional polymer supports. A standard interfacial polymerization process was then used to make a PA TFC membrane on the inorganic supports. As the surface roughness and average pore size decreased with multilayer coatings, the PA layer showed increased size of the “leaf-like” surface structure and thus increased surface area for transport due to the superhydrophilicity of the inorganic support and the ultrafine porous structure of the carbon nanotube layer. The enlarged surface area combined with the highly permeable inorganic support resulted a significant improvement in membrane flux. A high ethanol permeance of 10.6 ± 0.3 L m-2 h-1 bar-1 and a high rhodamine B rejection of 99.8% ± 0.2% were achieved. The membrane permeance is approximately 4–13 times higher than that of conventional PA TFC membranes fabricated on polymer supports. Our results indicate that the potential benefits of inorganic supports are not limited to their good solvent resistance, strong mechanical strength, and high thermal and chemical stabilities and thus desires more attention in future studies.