Abstract:During the precipitation of gibbsite from supersaturated sodium aluminate solution, the main aluminum containing species in solution will transform from tetrahedral ［Al(OH)₄］^- to sixfold octahedral ［(H₂O)₂Al-(OH)₄］^-. In order to elucidate the mechanisms responsible for above transformation, the formation Gibbs free energy as well as frontier orbits of a wide range of aluminum species are studied by ab initio method at B3LYP/6-31G** level. Based on theoretical calculation results, thermodynamic possibility and coordination possibility for aluminate ion transforming from ［Al(OH)₄］^- to ［(H₂O)₂Al(OH)₄］^- are analyzed and thermodynamic permitted reaction pathways are extracted. It is found that ［Al(OH)₄］^- can not react directly with H₂O to carry out the variation of coordination number. Transformation of tetrahedral ［Al(OH)₄］^- to octahedral ［(H₂O)₂Al(OH)₄］^- is involved in two reaction pathways, one is realized by neutral ［Na(H₂O)⁺₄·Al(OH)^-₄］ acting mediator, the other is carried by neutral ［(H₂O)Al(OH)₃］. Though there is a strong thermodynamic trend for the transformation of ［Al(OH)₄］^- to［(H₂O)₂Al(OH)₄］^-,the practical transformation is very slow. Thus, it can be concluded that there is a great kinetic resistance during the transformation from ［Al(OH)₄］^- to ［(H2O)₂Al(OH)₄］^-.

Key words:  ab initio calculation; aluminate ion; Gibbs free energy;frontier orbital theory