Energetics of Al13 Keggin cluster compounds

摘要

The c-Al13 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 t-Keggin clusters, Al13 selenate, (Na(AIO4)AI12 (OH)24(SeO4)4»12H2O) and Al13 sulfate, (NaAIO4AI12(OH)24(SO4)4« ( 12H2O). The measured enthalpies of solution, AHSO|, at 28 °C in 5 N HCI for the e-AI13 selenate and sulfate are -924.57 (±3.83) and -944.30 (±5.66) kj-mol~1, respectively. The enthalpies of formation from the elements, Hfe[, for Al13 selenate and sulfate are -19,656.35 (±67.30) kJ-mol"1, and -20,892.39 (±70.01) kJ-mol"1. respectively. In addition, AH^ei 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 e-AI13 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 t-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 Al13 compound relative to the binary chalcogenate, while in 5 N HCI, sulfur is energetically favored in the cluster compound compared to the aqueous solution. This contribution represents the first thermody-namic study of e-AI13 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 f-AI13 clusters in natural and anthropogenic systems, these data provide conclusive thermodynamic evidence that the Al13 Keggin cluster is a crucial intermediate species in the formation pathway from aqueous aluminum monomers to aluminum hydroxide precipitates.