Water adsorption and hydrolysis on molecular Al oxides and hydroxides - solvation versus cluster formation

Journal article, 2001

Addition of water to the AlO(OH)(g) molecule and to the corresponding binuclear anhydride Al2O3(g) are studied by means of quantum chemistry. The AlO bond and the Al–O–Al bridge are found to be highly reactive towards water, and the final products are determined as Al(OH)3(g) and the dimer Al2(OH)6(g). The addition of further H2O molecules to Al(OH)3(g) was used to model micro-solvation. Two different types of cluster structures are found for Al(OH)3·nH2O. One type contains an Al(OH)4−/H3O+ ion-pair, for which the core anion displays a four-coordinated geometry on Al. The second type displays a six-coordinate Al, while tending to form a cationic core. Both structures require a Al(OH)3·6H2O(g) system as computational model to be observed. For neutral environments, polymerization of Al(OH)3 is preferred over the solvation of monomeric species. The results are understood to reflect the amphoteric dissolution chemistry of aluminium hydroxide under acidic or alkaline conditions.