Peter A. Rock Thermochemistry Laboratory and Nanomaterials in Environment, Agriculture and Technology Organized Research Unit, University of California, Davis, CA 95616, USA.
Proc Natl Acad Sci U S A. 2011 Sep 6;108(36):14775-9. doi: 10.1073/pnas.1111243108. Epub 2011 Aug 18.
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(2)O) and Al(13) sulfate, (NaAlO(4)Al(12)(OH)(24)(SO(4))(4)•12H(2)O). 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.
ε-Al(13) Keggin 铝氢氧化物簇是建立地球化学反应分子途径的重要模型。本文报道了两种以中心ε-Keggin 簇为基础的铝盐的生成焓，即 Al(13) 硒酸盐（(Na(AlO(4))Al(12)(OH)(24)(SeO(4))(4)•12H(2)O）和 Al(13) 硫酸盐（(NaAlO(4)Al(12)(OH)(24)(SO(4))(4)•12H(2)O）。在 28°C 下，5N HCl 中 ε-Al(13) 硒酸盐和硫酸盐的溶解焓（ΔH(sol)）分别为-924.57（±3.83）和-944.30（±5.66）kJ·mol(-1)。从元素得到的生成焓（ΔH(f,el)），对于 Al(13) 硒酸盐和硫酸盐，分别为-19656.35（±67.30）kJ·mol(-1)和-20892.39（±70.01）kJ·mol(-1)。此外，通过高温氧化物熔体溶液量热法测量数据计算得到了十水合硒酸钠的生成焓（ΔH(f,el)）：-4006.39（±11.91）kJ·mol(-1)。从基于氧化物的组成来看，两种 ε-Al(13) Keggin 簇化合物的形成是放热的，但从基于水铝石的组合来看，其能量是不利的。为了理解 ε-Keggin 簇对硒酸盐和硫酸盐的相对亲和力，计算了两个 S-Se 交换反应的焓。在固态下，硒在 Al(13) 化合物中比二元硫属元素更有利，而在 5N HCl 中，与水溶液相比，硫在簇化合物中更有利。这一贡献代表了对 ε-Al(13) 簇化合物的首次热力学研究，并为其他类似分子（包括为动力学研究创建的取代版本）建立了一种方法。这些数据强调了 ε-Al(13) 簇在自然和人为系统中的重要性，为从水溶液中的铝单体到氢氧化铝沉淀的形成途径中，Al(13) Keggin 簇是关键中间体的结论提供了确凿的热力学证据。
