EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Porous $\alpha$-Al$_2$O$_3$ monoliths and membranes were prepared using a novel synthesis approach. Compared to the conventional method of applying carcinogenic epoxides for the initiation of gelation, our synthesis technique is based on gelation by means of cross hydrolysis, thus, rendering the use of carcinogenic epoxides redundant. In this respect, a mutual hydrolysis between the aluminum salt and the aluminum alkoxide forms the sol. Form-stable lyogels can then be obtained from this sol if the synthesis is conducted at 100 °C under elevated pressure using an autoclave.
Furthermore, drying of the lyogels results in the formation of an intact monolithic xerogel after carrying out multiple solvent exchanges with solvents of increasing vapor pressures. In this way, the capillary stress which occurs during drying and which may lead to a fracturing of the material is reduced. Subsequent calcination at 1200 °C yields mechanically-stable and porous $\alpha$-Al$_2$O$_3$ monoliths with a BET surface of 11 m$^2$/g and a monomodal pore diameter of 170 nm. The monolith samples prepared in this manner have a hight of 6 mm and a diameter of 15 mm.
On the basis of this synthesis approach $\alpha$-Al$_2$O$_3$ membranes can be obtained in two different ways. Either, the $\alpha$-Al$_2$O$_3$ monolith is sawed into discs of < 1 mm thickness – the $\alpha$-Al$_2$O$_3$ membranes, or thinner lyogels are formed by reducing the volume of the sol used for synthesis. Further treatment of the lyogels in line with the above-described procedure gives rise to $\alpha$-Al$_2$O$_3$ membranes of 1 mm thickness.