No abstract

Aluminum hydroxide ions in the e-Keggin structure provide geochemical models for how structure affects reactivity, and consequently, how aqueous ions evolve to bulk precipitates. Here we report a systematic comparison of heterometal substitution into the MAl 12 e-Keggin structure, where M = Ga III , Al III , or Ge IV . We use direct solution calorimetric techniques to compare the energetics of these substituted structures and complement these measurements with density functional theory (DFT) calculations to further examine this structure as a host to alternative heterometals. The measured enthalpy of solution (DH soln ) at 28 °C in 5 N HCl for the selenate salts of GaAl 12 7+ and AlAl 12 7+ , was measured as -869.71 ± 5.18 and -958.04 ± 2.79 kJ/mol, respectively. The enthalpies of formation from the elements, DH°f ,el , for the selenate salts of GaAl 12 7+ and AlAl 12 7+ , are -23 075.02 ± 61.68 and -23 334.18 ± 60.38 kJ/mol, respectively, supplanting previous values. We compare structural relationships to both experimental and calculated energies to identify the driving forces that control these substitutions and stability, and establish that tetrahedral M-O bond lengths are closely related to the strain and stability of the structure. We show that substitution depends on the size and valence of the heterometal through energetics, and we extend our thermodynamic and structural relationships to other not yet synthesized MAl 12 clusters (M = Si IV , Fe III , Be II , Mg II , or Zn II ).

a b s t r a c tWe report enthalpies of formation from the elements and oxides, D f H = À (22,673.44 ± 30.19) kJ Á mol À1 and D f;ox H = À(869.68 ± 28.75) kJ Á mol À1 , for [GeO 4 Al 12 (OH) 24 (H 2 O) 12 ](SeO 4 ) 4 Á12H 2 O (cr) (GeAl 12 ) measured using high temperature oxide-melt solution calorimetry. This material is the selenate salt of the germanium-substituted polynuclear Al 7þ 13 ion in the e-Keggin structure. We also report heat capacities from temperatures of (2 to 300) K on this GeAl 12 selenate and its other heterometal substituted forms, Na[AlO 4 Al 12 (OH) 24 (H 2 O) 12 ](SeO 4 ) 4 Á12H 2 O (cr) (AlAl 12 ) and Na[GaO 4 Al 12 (OH) 24 (H 2 O) 12 ](SeO 4 ) 4 Á12H 2 O (cr) (GaAl 12 ), measured using a Quantum Design Physical Property Measurement System (PPMS). These measurements were used to calculate entropies and subsequently free energies of formation for these three materials. All three MAl 12 selenates, where M = Al(III), Ga(III), or Ge(IV), have similar heat capacities from temperatures (2 to 300) K and similar characteristic Debye temperatures, H D , suggesting similar lattice vibrational densities of states.

The ε-Keggin [AlO4Al12(OH)24(H2O)12](7+) ion (AlAl12(7+)) 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, GaAl12(7+) and GeAl12(8+), 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 AlAl12(7+) 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.

a b s t r a c tWe report enthalpies of formation from the elements and oxides, D f H = À(26195.31 ± 34.46) kJ Á mol À1 and D f;ox H = À(1402.09 ± 33.36) kJ Á mol À1 , for crystalline [Al 13 (l 3 -OH) 6 (l 2 -OH) 18 (H 2 O) 24 ](NO 3 ) 15 Á 11H 2 O (cr) (flat-Al 13 ) measured using aqueous solution room temperature calorimetry. This material is a nitrate salt of a polynuclear aluminum hydroxide ion with thirteen aluminum metal centers, Al 13 15+ .Unlike the e-Al 13 7+ Keggin ion, it contains solely octahedrally coordinated aluminum in a more flat topology. We also report heat capacities from temperatures of (2 to 300) K on this flat-Al 13 nitrate using a Quantum Design Physical Property Measurement System (PPMS). These measurements were used to calculate the standard molar entropy and subsequently the free energy of formation for this material. We observed that during the heat capacity measurements, the flat-Al 13 sample underwent a second order phase transition near T = 214 K.