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

We report on a novel sol-gel approach to synthesize highly porous alumina. Herein, an aluminum alkoxide is employed as a second homonuclear precursor alongside an aluminum salt. The use of carcinogenic epoxides can thus be avoided.
To date, the most well-established approach for the sol-gel synthesis of porous alumina is found in the epoxide-mediated route, starting from an aluminum salt. In our novel synthesis, the hitherto indispensable carcinogenic epoxide can be omitted as its function is superseded by the mutual hydrolysis of an aluminum alkoxide and an aluminum salt. We name this novel process cross hydrolysis.
The synthesis takes place in a solvent mixture of ethanol and 2-propanol. As an aluminum salt usually contains a stoichiometric amount of crystal water, it dissociates into solvated anions and aluminum-hexaaqua-complexes when dissolved in this non-aqueous but polar medium. These hexaaqua-complexes are in equilibrium with the deprotonated form, rendering the medium acidic, as equation (1) illustrates:

(1) [Al(H$_2$O)$_6$]$^{3+}$ + H$_2$O $\rightleftharpoons$ [Al(OH)H$_2$O)$_5$]$^{2+}$ + H$_3$O$^+$

However, the recombination of two pentaaqua-hydroxo-complexes into a dimer does not occur voluntarily. Instead of the commonly used carcinogenic epoxide, an aluminum alkoxide is now added to the reaction solution. Its OR-groups are readily protonated in the acidic medium, which simultaneously achieves two things: Firstly, hydrolysis of the alkoxide now occurs rapidly, and secondly, the pH of the reaction solution increases, shifting equilibrium (1) to the right hand side. Hydrolytic olation between the aluminum-aquohydroxo-complexes and the hydrolyzed alkoxide species now takes place. Thus, dimers and oligomers are formed, which grow into colloidal particles. Subsequently, these primary particles interconnect to yield an alumina network structure.
When the proper reaction conditions are applied, significantly increased porosities with a monomodal pore size distribution are attainable via this novel route.