Quantum Chemical Calculations On Fluoroaluminate Complexes With Alkali Metal Counterions

Structural study on molten electrolyte in aluminum electrolysis system helps to have a deeper understanding of electrode process and electrolysis mechanism.At present,domestic and foreign scientific researchers have not reached a consensus on that field.Furthermore,the electronic layer structure of the alkali fluoroaluminate complexes needs to be paid more attenton.In the present study,AlF4-,AlF52-,and AlF63-which are generally recognized existing in aluminum electrolytes,and nine types of ion-neutral ions formed by fluoroaluminate ions and the alkali metal cations Li+,Na+,and K+ respectively,LiAlF4,NaAlF4,KAlF4,Li2AlF5,Na2AlF5,K2AlF5,Li3AlF6,Na3AlF6 and K3AlF6,were selected as the research object.Quantum chemical calculation was adopted,and Gaussian 09 software was used.Firstly,the geometries of fluoroaluminate ions in ground state were optimized and the vibration frequencies were calculated.Secondly,the atomic net charge population of the ions was calculated.Thirdly,the energy-differences among the frontier molecular orbital were studied,and the type and intensity of the chemical bond were analysed.Finally,from the thermodynamic point of view,the mutual transformation process of the fluoroaluminate ions with or without alkali metal cations was investigated.It is found that with the increase of the coordination number of aluminum ion,the absolute value of the net charge of aluminum ion of fluoroaluminate ions gradually decreased,and that of fluorine ions gradually increased.The net charge of the same alkali metal cation in ion pairs decreased with the increase of the coordination number of the aluminum ions of fluoroaluminate ions.With the increase of nuclear charge number of alkali metal cations,in the same fluoroaluminate ion,the number of net charge of aluminum ion gradually decreased,and that of fluorine ions gradually increased.For the same fluoroaluminate ion,the charge transferred to the alkali metal cation decreased with the electronegativity of the alkali metal cation.The charge of the alkali fluoride cations contributed by fluoroaluminate ions increased with the coordination number of aluminum ion.With the increase of the coordination number of aluminum,the amount of charge involved in the repulsion polarization increased for the same alkali metal cation.The oxidation active sites of the three types of fluoroaluminate ions appeared around the fluorine ions.The strength of the Al-F bond was gradually weakened as the coordination number of the aluminum ion increased.The strength of the ion bond formed between the alkali metal cation and the fluoroaluminate ion was gradually weakened as the alkali metal cation nuclear charge number increased.The type of alkali metal cation had significant effects on the dissociation process of fluoroaluminate ions.