Energetics of Al ₁₃ Keggin cluster compounds

The ε-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 ε-Keggin clusters, Al ₁₃ selenate, (Na(AlO ₄)Al ₁₂ (OH) ₂₄(SeO ₄) ₄·12H ₂O) and Al ₁₃ sulfate, (NaAlO ₄Al ₁₂(OH) ₂₄(SO ₄) ₄· 12H ₂O). The measured enthalpies of solution, ΔH _sol, at 28°C in 5 N HCl for the ε-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, ΔH _{f ,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, ΔH _{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 ε-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 ε-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 ε-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 ε-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.