NUCLEATION MECHANISMS OF GIBBSITE AS SUGGESTED BY COMPUTATIONAL MOLECULAR MODELLING

Soar, T.J. and Gerson, A.R.

An extensive study of the possible mechanisms of gibbsite nucleation under caustic conditions has been carried out using semi-empirical quantum mechanical molecular modelling. The entropy, enthalpy, Gibbs free energy and species transformation activation energies have been calculated for a wide range of OH- and H2O coordinated Al-containing monomers, dimers, trimers and tetramers while in a solution environment. From these calculations we have found that the Al(OH)4 - (Moolenaar et al., 1970) monomer is the most likely monomer to form while [(OH)3Al-(O)-Al(OH)3]2- (Moolenaar et al., 1970) is the most likely dimer to form. In addition it was found that due to the stability of the Al(OH)4- monomer that no dimerisation reaction involving this species was possible to give an energetically favourable dimer (i.e. ΔG°r was always greater than zero). We have therefore proposed a mechanism by which the dimer is formed through reaction of two unstable monomers (possibly Al(OH)52- or Al(OH)63-) which may be present in very small concentrations. The reaction of minor monomeric species to form dimers and subsequently oligomers may be the rate determining step in the nucleation process. Polymerisation occurs through further addition of these species. Formation of the double -OH- bridging found between neighbouring Al atoms in gibbsite occurs by dissociative addition of H2O to the already existing -O- bridge.