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

Abstract

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.