Energetic Insight into the Formation of Solids from Aluminum Polyoxocations

The ε-Keggin [AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺ ion (AlAl₁₂⁷⁺) is a metastable precursor in the formation of aluminum oxyhydroxide solids. It also serves as a useful model for the chemistry of aluminous mineral surfaces. Herein we calculate the enthalpies of formation for this aqueous ion and its heterometal-substituted forms, GaAl₁₂⁷⁺ and GeAl₁₂⁸⁺, using solution calorimetry. Rather than measuring the enthalpies of the MAl₁₂^7/8+ ions directly from solution hydrolysis, we measured the metathesis reaction of the crystallized forms with barium chloride creating an aqueous aluminum solution monospecific in MAl₁₂^7/8+. Then, the contributions to the heat of formation from the crystallized forms were subtracted using referenced states. When comparing the aqueous AlAl₁₂⁷⁺ ion to solid aluminum (oxy)-hydroxide phases, we found that this ion lies closer in energy to solid phases than to aqueous aluminum monomers, thus explaining its role as a precursor to amorphous aluminum hydroxide phases. Downhill: Calorimetric measurements on the ε-AlAl₁₂⁷⁺ ion and its heterometal-substituted forms provide the first enthalpy of formation (ΔH_f,el) values on these species in aqueous form independent of formation pathway. Data reveal the AlAl₁₂⁷⁺ ion lies closer in energy to solid aluminum (oxy)hydroxide phases than aqueous monomers, thus explaining its role as a precursor to amorphous aluminum hydroxides (flocs).