Structure and reactivity of sodium aluminate complexes in alkaline solutions

Abstract

The underlying mechanism by which tetrahedrally-coordinated aluminate solution species react to precipitate octahedrally-coordinated Al3+ in gibbsite is unknown. Sodium aluminate solutions provide the opportunity to investigate the role of ion speciation and local structure in the reaction, as the solution metastability can be controlled by varying ratio of excess NaOD to Al3+. Here, capitalizing on this tunable precipitation behavior, aluminate speciation was investigated through a combination of Raman, infrared, and NMR spectroscopies, together with geometrical fitting of neutron and X-ray radial distribution functions. These techniques reveal a mixture of monomeric Al(OD)4− and dimeric Al2O(OD)62− in all compositions, but solutions avoiding precipitation for extended timescales (i.e. months to years) show an increased dimeric Al2O(OD)62− population formed at the expense of the Al(OD)4− monomer. Conversion between these species, as well the potential for further oligomerization and formation of larger pre-nucleation species is driven by the hydrogen/deuterium bonding (H/D-bonding) and proton/deuteron transfer reactions. Changes within the local environment around the aluminate anions, including solvation and the presence of solvent-shared ion pairs or contact ion pairs, will consequently govern the reactivity of these aluminate species to either promote or inhibit precipitation.