Energetics of heterometal substitution in  -Keggin [MO4Al12(OH)24(OH2)12]6/7/8+ ions

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 ).

Heat capacities and thermodynamics of formation of ε-Keggin MAl12 Selenates (M = Al(III), Ga(III), or Ge(IV))

“…These materials were stored in a temperature and humidity controlled room for the duration of these studies. XRD showed no changes to the sample integrity, no extraneous peaks were visible, and the TGA results were also consistent with our previous work on these materials, in particular their water content was unchanged [23]. A summary of the sample descriptions and purities can be found in table 1.…”

“…This is a continuation of previous work [23] and all MAl 12 selenates were checked with powder X-ray diffraction (XRD) and thermogravimetric analysis (TGA) to ensure the sample integrity was maintained and the hydration state was unchanged. These materials were stored in a temperature and humidity controlled room for the duration of these studies.…”

“…The enthalpies of solution, D sol H, of the Na[(MO 4 ) Al 12 (OH) 24 (H 2 O) 12 ](SeO 4 ) 4 Á12 H 2 O (cr) , where M = Al(III) or Ga(III), were measured previously [23], and the thermodynamic cycles to calculate the enthalpy of formation, D f H , for these compounds are found in the prior work. High temperature oxide melt solution calorimetry was used to obtain the D f H of [(GeO 4 ) Al 12 (OH) 24 (H 2 O) 12 ](SeO 4 ) 4 Á12H 2 O (cr), as a suitable germanium reference material necessary to the thermodynamic cycle could not be found for room temperature aqueous solution calorimetry.…”

“…The aqueous [AlO 4 Al 12 (OH) 24 (H 2 O) 24 ] 7+ ion (AlAl 7þ 12 ) and its heterometal-substituted forms, [GaO 4 Al 12 (OH) 24 (H 2 O) 24 ] 7+ (GaAl 7þ 12 ) [15][16][17][18][19], and [GeO 4 Al 12 (OH) 24 (H 2 O) 24 ] 8+ (GeAl 8þ 12 ) [20,2 1], can be crystallized with sulfate or selenate ions. These crystals are used for structural determination [1][2][3]15,21,22] and recently, for calorimetric studies of heat of formation [23,24].…”

“…As part of an ongoing study of the thermodynamic properties of these compounds, we complement recent measurements of enthalpy of formation [23] with low temperature heat capacity measurements. Together these measurements enable complete thermodynamic characterization of standard enthalpies, entropies, and free energies of formation of these solid heterometal -substituted MAl 12 selenates (M = Al(III), Ga(III) or Ge(IV)).…”