MESA+ Institute for Nanotechnology

The research results described in the thesis “Mesoporous and microporous titania membranes” deal with the synthesis and properties of ceramic oxide membrane materials. Since most of the currently available inorganic membranes with required separation properties have limited performance in terms of membrane reliability and long-term stability, membranes made of new oxide materials were developed. Anatase and amorphous titania were chosen as mesoporous and microporous material for membrane development, respectively. By development of appropriate synthesis procedures, the relationship between synthesis conditions and properties of the obtained materials could be understood in terms of the theoretical background of sol-gel chemistry of transition-metal alkoxides.

Crack-free anatase-based titania layers with an average pore size of 7 nm can be obtained with a reproducibility of around 50%. On the basis of the hypothesis that cracking is caused by stresses that developed due to (partial) phase transformation at the calcination temperature and the low elasticity of the layers due to the low binder content, doping was employed to increase the reproducibility. Zirconia is a suitable dopant that not only retards the phase transformation of anatase into rutile to temperatures above 700 °C, but also leads to a material with higher porosity, smaller pore sizes, higher degrees of pore connectivities and larger internal surface areas. Highly reproducible defect-free amorphous microporous titania layers with a mean pore size of less than 0.9 nm can be obtained on both mesoporous g-alumina and titania/zirconia coated substrates. The material is chemically stable in the pH range from 2 to 13.

The development of mesoporous and microporous titania membranes introduces novel stacked membrane systems a-Al2O3 - TiO2 - SiO2 and a-Al2O3 - TiO2 - TiO2, along with the previously developed a-Al2O3 - g-Al2O3 - SiO2, for membrane technology applications. Since the chemical stabilities of mesoporous anatase and microporous amorphous titania membranes are higher than those of g-alumina and silica membranes, the application field of ceramic membranes can be expanded towards new processes that involve more corrosive chemicals. The mesoporous titania membrane can be applied in ultrafiltration, and possibly in nanofiltration. Since the g-alumina - silica membrane system already proved successful in gas separation and pervaporation, the system a-Al2O3 - TiO2 - SiO2 has potential in the same processes, with the additional advantage of higher chemical stability at low pH. The microporous titania membrane (system a-Al2O3 - TiO2 - TiO2) with pore sizes in the higher microporous range can be applied in pervaporation for the separation of some specific mixtures, and in nanofliltration. Applied in pervaporation, the system g-alumina/silica shows the highest separation factors in pervaporation, followed by the system titania/silica, and titania/titania. However, the chemical stability increases in the same order. Therefore, the choice of membrane and support material strongly depends of the requirements of the specific application.