Energetics of Al 13 Keggin cluster compounds

Christopher R. Armstrong, William H. Casey, Alexandra Navrotsky

Research output: Contribution to journalArticlepeer-review

27 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)14775-14779
Number of pages5
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number36
StatePublished - Sep 6 2011
Externally publishedYes


  • Aluminum hydrolysis
  • Polyoxocations

ASJC Scopus subject areas

  • General


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