Development of microporous hybrid membranes

Microporous hybrid materials and membranes

Membrane separation of molecules is often regarded as an energy efficient alternative for distillation. The development of amorphous silica membranes made it possible to separate even the smallest molecules by differences in size. The microporous network was simply formed via a sol-gel approach of silicon alkoxides

The limited hydrothermal and chemical stability of silica is a large drawback for application. Silica slowly decomposes in the presence of water, especially at higher processing temperatures (>100°C) as used in industry. In the last few years, hybrid silica membranes have been invented and developed in this laboratory. These materials have a much higher hydrothermal stability than silica. The incorporation of silicon-carbon-silicon linkages in the network improves the flexibility and therefore the crack and temperature resistance of membranes based on these materials. Furthermore, the hydrophobic carbon linkages are thought to shield polar silicon-oxygen bonds from water. However, long term degradation of these materials still occurs in the presence of water at 150°C. In one year the water permeance is reduced by approximately 20%.

An arts impression of a tubular asymmetric membrane for molecular separation

An alternative approach to improve the hydrothermal and chemical stability of hybrids is by doping transition metals such as zirconium into the network. However, the sol-gel polymerisation of metal alkoxides proceeds much faster than silicon alkoxides and metal oxides tend to form densely packed structures. Incorporation of such dopants in silica membranes typically leads to phase separation, yielding a final structure with crystalline transition metal oxide domains dispersed in a matrix of amorphous silica. The polymerisation rate of transition metal alkoxides can be reduced by the use of a variety of modifying agents, but intermixing of transition metals and silicon on atomic level is rarely observed.

This project will focus on the question how to incorporate transition metals into a hybrid silica matrix, without phase separation or densification. The challenge is to gain more insight into the sol-gel reaction mechanisms of mixed silicon oxide - transition metal oxide structures.

PhD student: Rogier Besselink

Daily Supervisor: Andre ten Elshof