Abstract
The mechanism of the condensation reaction of Al(OH)(4)(-) with D-gluconate is considered as a starting point in understanding the impact of organic substances on the Bayer process for obtaining alumina from bauxitic ores. Density functional theory (DFT) with a solvation model is used to study a model system of the different complexes formed between D-gluconate molecule and Al(OH)(4)(-) with monodentate or bidentate structures. The different mechanisms are studied to determine the most favorable pathway for the formation of the model complex. The DFT results show that the D-gluconate model forms an oxo bridge with aluminum through a first associative condensation step involving a pentacoordinated aluminum bonded to the carbon adjacent to the carbonyl of the ligand, and then a second bond is formed through a similar mechanism to obtain the most favorable bidentate structure. The rate-limiting activation barrier of the most favorable path is found to be 16.9 kcal mol(-1) with density functional and 6.5 kcal mol(-1) with CCSD(T) on geometries optimized with DFT and dispersion correction. The reaction energy for the preferred mechanism in solution is - 2.3 kcal mol(-1). These results are in agreement with experimental observations, proposing the formation of bidentate Al(OH)(4)(-) with D-gluconate through condensation reactions in a strongly alkaline medium.