Molybdenum(VI) Coordination in Tributyl Phosphate Chloride Based System

Fundamental coordination chemistry of Mo(VI) at its macroconcentrations in solvent extraction systems is of great importance for industrial processes that require the purification or recovery of large concentrations of Mo. The coordination of Mo(VI) in tri-n-butyl phosphate (TBP) from solutions of hydrochloric acid and up to 0.3 M Mo was investigated using UV, FTIR, and 31P NMR spectroscopies, as well as EXAFS. From these techniques we resolved near-neighbor atoms, speciation, structural information on the coordination environment, and thermodynamic parameters affiliated with the solvent extraction of Mo(VI) and HCl. The solvated extracted form of Mo(VI) as MoO2Cl2·2TBP was identified. High extraction yield of Mo at >5 M HCl concentration is driven by replacing HCl in the organic phase by Mo. The existence of additional organic Mo adducts is also discussed with the aid of density functional theory, however no evidence of dimeric or polymeric Mo species was found to be present in TBP.