Transformation of monomer aluminate ions from tetrahedron to octaheron
(College of Chemistry and Chemical Engineering,
Central South University, Changsha 410083, China)
Central South University, Changsha 410083, China)
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