Nano/subnano-tuning of porous ceramic membranes for molecular separation

In sol-gel processing, porous ceramic membranes can be prepared by sol-coating porous substrates and drying for gelling, followed by a firing process. Ceramic membranes prepared by sol-gel processing can be categorized into amorphous materials such as silica, and crystalline materials such as alumina and titania. Amorphous silica networks, which can be prepared by the polymeric sol route, have ultra-microporous pores that allow small molecules such as helium and hydrogen to permeate. On the other hand, crystalline materials, which are mostly prepared by the colloidal sol route, have nano-sized pores in the range of one to several nanometers. In this article, sol-gel derived SiO2 and TiO2 membranes with controlled pore sizes in the range of sub-nano to nanometers will be reviewed with respect to membrane preparation and to their application in the separation of the gas and liquid phases. Ceramic membranes with high performance can be obtained by precise control of membrane structures (pore size, pore size distribution, thickness, pore shape, etc.) and membrane materials (SiO2, TiO2, composite oxide, hybrid materials, etc.). Nano/subnano-tuning of porous ceramic membranes is quite important for the improvement of membrane permeability and selectivity.