Energetics of Al13 Keggin cluster compounds

The epsilon-Al-13 Keggin aluminum hydroxide clusters are essential models in establishing molecular pathways for geochemical reactions. Enthalpies of formation are reported for two salts of aluminum centered epsilon-Keggin clusters, Al-13 selenate, (Na(AlO4)Al-12 (OH)(24)(SeO4)(4)center dot 12H(2)O) and Al-13 sulfate, (NaAlO4Al12(OH)(24)(SO4)(4)center dot 12H(2)O). The measured enthalpies of solution, Delta H-sol, at 28 degrees C in 5 N HCl for the epsilon-Al-13 selenate and sulfate are -924.57 (+/- 3.83) and -944.30 (+/- 5.66) kJ.mol(-1), respectively. The enthalpies of formation from the elements, Delta H-f,H-el, for Al-13 selenate and sulfate are -19,656.35 (+/- 67.30) kJ.mol(-1), and -20,892.39 (+/- 70.01) kJ,mol(-1), respectively. In addition, Delta H-f,(el) for sodium selenate decahydrate was calculated using data from high temperature oxide melt solution calorimetry measurements: -4,006.39 (+/- 11.91) kJ.mol(-1). The formation of both epsilon-Al-13 Keggin cluster compounds is exothermic from oxide-based components but energetically unfavorable with respect to a gibbsite-based assemblage. To understand the relative affinity of the epsilon-Keggin clusters for selenate and sulfate, the enthalpy associated with two S-Se exchange reactions was calculated. In the solid state, selenium is favored in the Al-13 compound relative to the binary chalcogenate, while in 5 N HCl, sulfur is energetically favored in the cluster compound compared to the aqueous solution. This contribution represents the first thermodynamic study of epsilon-Al-13 cluster compounds and establishes a method for other such molecules, including the substituted versions that have been created for kinetic studies. Underscoring the importance of epsilon-Al-13 clusters in natural and anthropogenic systems, these data provide conclusive thermodynamic evidence that the Al-13 Keggin cluster is a crucial intermediate species in the formation pathway from aqueous aluminum monomers to aluminum hydroxide precipitates.