Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Energetic Trends in Monomer Building Blocks for Uranyl Peroxide Clusters Lei Zhang,*,† Mateusz Dembowski,‡ Ana Arteaga,§ Sarah Hickam,† Nicolas P. Martin,§ Lev N. Zakharov,§ May Nyman,§ and Peter C. Burns†,‡ †
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Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, Indiana 46556, United States ‡ Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States § Department of Chemistry, Oregon State University, Corvallis, Oregon 97331, United States S Supporting Information *
ABSTRACT: The uranyl triperoxide anionic monomer is a fundamental building block for uranyl peroxide polyoxometalate capsules. The reaction pathway from the monomer to the capsule can be greatly altered by the counterion: both the reaction rate and the resulting capsule structure. We synthesized and characterized uranyl triperoxides Mg2UO2(O2)3·13H2O (MgUT), Ca2UO2(O2)3·9H2O (CaUT), Sr2UO2(O2)3·9H2O (SrUT), and K4UO2(O2)3·3H2O (KUT) and compared their thermodynamic stabilities. The enthalpies of formation from oxides and elements of these compounds were calculated by thermochemical cycles from measurements by high temperature oxide melt drop solution calorimetry. Their formation enthalpies from oxides become more negative linearly as a function of the increasing basicity of the respective oxides on the Smith scale. This relationship holds for previously Li and Na analogues. Further affirming the trend, ΔHf,ox of MgUT departs from linearity, due to the distinct bonding environment of Mg2+, as compared to the other alkalis and alkaline earths in the series.
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INTRODUCTION More than 60 nanoscale uranyl peroxide clusters have been isolated and characterized over the past dozen years.1−7 Most of these clusters are cages with diameters ranging from 1.5 to 4 nm and consist of 20−124 uranyl ions bridged by bidentate peroxide and other linkers, including oxalate and pyrophosphate. Uranyl peroxide clusters spontaneously self-assemble in water at room temperature and can be crystallized for detailed structural analysis. The thermodynamics of uranyl peroxide cage clusters, as well as their fundamental building blocks including the uranyl triperoxide monomer [UO2(O2)34−], are important to understand their self-assembly. Uranyl triperoxide monomers are important components of the uranyl−peroxide−water system, in particular as intermediates at high pH.8 In recent years, high temperature oxide melt drop solution calorimetry has been used extensively to study the thermochemistry of uranyl minerals,9−17 uranium oxide solid solutions,18−20 and salts of uranyl peroxide monomers and nanoclusters.21−23 This methodology is straightforward for materials containing U(IV) and/or U(VI).24,25 In particular, bubbling oxygen through the oxide melt enhances the dissolution of the sample pellets and provides sufficiently high oxygen fugacity to ensure all U will be present as U(VI) after dissolution. Recently, we also expanded our efforts in hightemperature calorimetry to neptunium oxides.26 © XXXX American Chemical Society
Thermochemical data for uranyl peroxide compounds in general are scarce relative to the rapidly growing number of described compounds. Studtite [(UO2(O2)(H2O)2)·2H2O] and metastudtite [(UO2(O2)(H2O)2], the only uranyl peroxide minerals found in natural settings, are built of uranyl hexagonal bipyramids bridged by peroxide to form chains.27,28 Calorimetric studies confirmed that formation of studtite and metastudtite from hydrogen peroxide in water is energetically favorable, facilitated by alpha radiolysis of water in uranium deposits.12,29 Thermal stability studies of studtite showed that it is stable below 30 °C under dry conditions and up to 90 °C in the presence of water.30 Previously, we synthesized and isolated alkali and alkaline earth salts of uranyl peroxide monomers, [MxUO2(O2)3·nH2O] (x = 2 or 4),2,31−33 and determined the formation enthalpies of Na and Li salts of these monomer building blocks.21 Here, we describe the synthesis and characterization of the K salt of uranyl triperoxide, and extend calorimetric measurements to the K, Mg, Ca, and Sr salts of the uranyl monomer. A distinct trend is noted between the enthalpy of formation of these monomer salts from oxides and the Smith scale34 of acidity of the alkali and alkaline earth oxides that serve as counterions for the uranyl peroxide units. Received: September 18, 2018
A
DOI: 10.1021/acs.inorgchem.8b02661 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Table 1. Reactions and Thermodynamic Cycles Used to Calculate Enthalpies of Formation of Mg2UO2(O2)3·13.31H2O, Ca2UO2(O2)3·8.53H2O, and Sr2UO2(O2)3·8.69H2O at 25 °C from Oxides (ΔHf,ox) and from the Elements (ΔHf,el) in kJ/mol ΔH (kJ/mol)
reaction
(1) M 2UO2 (O2 )3 ·nH 2O(c,25°C) → 2MO(sln,700°C) + UO3(sln,700°C) +
ΔH1 = ΔHDS(M 2UO2 (O2 )3 ·nH 2O)
3 O2(g,700°C) + nH 2O(g,700°C) 2
(2) MO(c,25°C) → MO(sln,700°C)
ΔH2 = ΔHDS(MO)39
(3) UO3(c,25°C) → UO3(sln,700°C)
ΔH3 = ΔHDS(UO3)
(4) O2(g,25°C) → O2(g,700°C)
ΔH4 = ΔHheatcontent(O2 ) = 21.840
(5) H 2O(l,25°C) → H 2O(g,700°C)
ΔH5 = ΔHheatcontent(H 2O) = 6940
(6) 2MO(c,25°C) + UO3(c,25°C) +
3 O2(g,25°C) + nH 2O(l,25°C) 2
ΔH6 = ΔHf,ox = − ΔH1 + 2ΔH2 + ΔH3 +
→ M 2UO2 (O2 )3 ·nH 2O(c,25°C)
1 O2(g,25°C) → MO(c,25°C) 2 3 (8) U(c,25°C) + O2(g,25°C) → UO3(c,25°C) 2 1 (9) H 2(g,25°C) + O2(g,25°C) → H 2O(l,25°C) 2 (10) 2M(c,25°C) + U(c,25°C) + nH2(g,25°C) + (8 + n)/2O2(g,25°C) → M2UO2(O2)3·nH2O(c,25°C) (7) M(c,25°C) +
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3 ΔH4 + nΔH5 2
ΔH7 = ΔHf,el(MO)40 ΔH8 = ΔHf,el(UO3)40 ΔH9 = ΔHf,el(H 2O)40 ΔH10 = ΔHf,el(M2UO2(O2)3·nH2O) = ΔH6 + 2ΔH7 + ΔH8 + nΔH9
Discovery diffractometer equipped with Cu Kα radiation, using a scan range of 5−60° 2θ. Comparison of the measured patterns to those calculated from single-crystal X-ray data confirmed both the identity and purity of the synthesized phases (Figure S1).2,33 Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Crystalline samples were dissolved in and diluted with 5% nitric acid to a total volume of 10 mL to achieve elemental concentrations ranging from 1 to 20 ppm. Five ppm of yttrium was added to each sample to serve as an internal standard. Samples were analyzed by a PerkinElmer Optima 8000 inductively coupled plasma optical emission spectrometer. Elemental concentrations were calculated from a calibration curve of known standards, and ratios of cations to uranium were determined. Thermogravimetric Analysis (TGA). TGA of the MUT compounds were performed to determine water content using a Mettler Toledo TGA-DSC-1. About 10 mg of each sample was heated from 25 to 900 °C at a heating rate of 5 °C/min under N2. The water content of each compound was calculated from the percent weight loss during the heating process (Figure S2). The calculated water content (listed in Table 3) of each sample was slightly different from the ideal formula and was used in the thermochemical cycle calculations. Calorimetry. High temperature oxide melt drop solution calorimetry was performed using a Setaram AlexSYS 1000 high temperature Tian-Calvet calorimeter. The calorimeter was calibrated using the heat content of 5 mg α-Al2O3 pellets.37−39 For each compound, approximately 5 mg of pure sample was pressed into a 1.5 mm diameter pellet and dropped into 10 g of molten sodium molybdate solvent contained in silica glass crucibles in the AlexSYS calorimeter operated at 700 °C. The calorimetric assembly was flushed continuously with oxygen at 43 mL/min and oxygen was bubbled in the molten solvent at 5 mL/min to facilitate dissolution of uranyl peroxide compounds in an oxidizing atmosphere and to remove the evolved water. The thermochemical cycles given in Tables 1 and 2 were used to calculate the enthalpies of formation of each compound from the oxides and the elements (Table 3).
EXPERIMENTAL PROCEDURES
Caution! All experiments were done using isotopically depleted uranium (238U, α = 4.267 MeV) and conducted by trained personnel in purpose-designed laboratories for handling such radioactive isotopes. Syntheses of Uranyl Triperoxide Monomer Compounds (MUT). Alkaline uranyl triperoxide monomer compounds M2UO2(O2)3·nH2O (MUT, where M = Mg, Ca, Sr) were synthesized by combining aqueous solutions of 0.5 M UO2(NO3)2, 30% H2O2 (EMD), 25% N(CH3)4OH (TMAOH, Alfa Aesar), and 0.25 M M(NO3)2 (Alfa Aesar), forming yellow crystals of MUT. Details of the synthesis are in the literature.33 Potassium uranyl triperoxide monomer compound K4UO2(O2)3· 3H2O (KUT) was synthesized by dissolving 0.211 M uranyl nitrate (0.1 g) in 1.2 mL of deionized water, followed by addition of 0.6 mL of 30% hydrogen peroxide while stirring in an ice bath (molar ratio of H2O2/U = 23.18), immediately yielding an insoluble yellow precipitate (studtite, determined by X-ray diffraction). Next, 4.0 M KOH (0.80 mL) was added to the solution, dissolving the studtite and forming a clear orange solution. Within 2.5 to 3 min, a yellow precipitate formed, and 40 mL of acetone was then added to the slurry mixture. This mixture was quickly centrifuged; the solid was isolated by decantation and was then moved to a vacuum oven to dry at room temperature for 3 h. Orange block-shaped crystals formed (KUT), along with an orange slurry byproduct. For calorimetric experiments, the slurry was separated from the crystals with a disposable plastic pipet, and ethanol was quickly added to the vial. The crystals were removed from the vial immediately prior to the measurements, washed with ethanol, and left to dry on filter paper. The yield of KUT was 64%. Note: retaining the crystals in the mother liquor just prior to measurements is important to avoid its conversion to oligomers. Single-Crystal X-ray Diffraction of KUT. Single crystal diffraction intensities for KUT were collected at 173 K on a Bruker Apex2 CCD diffractometer using Cu Kα radiation, λ = 1.54178 Å. An absorption correction was applied using SADABS.35 The space group was determined based on intensity statistics. The structure was solved by direct methods and Fourier techniques and refined on F2 using full matrix least-squares procedures. All non-H atoms were refined with anisotropic displacement parameters. H atoms of water molecules were not located. All calculations were performed using the Bruker SHELXL-2014/7 package.36 Powder X-ray Diffraction (PXRD). The bulk materials of MUT were characterized by powder X-ray diffraction using a Bruker
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RESULTS The crystalline structures of Mg2UO2(O2)3·13H2O (MgUT), Ca2UO2(O2)3·9H2O (CaUT), Sr2UO2(O2)3·9H2O (SrUT), Li4UO2(O2)3·10H2O (LiUT), Na4UO2(O2)3·9H2O (NaUT), and K4UO2(O2)3·3H2O (KUT) are illustrated in Figure 1, B
DOI: 10.1021/acs.inorgchem.8b02661 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Table 2. Reactions and Thermodynamic Cycles Used to Calculate Enthalpies of Formation of the K4UO2(O2)3·3.00H2O at 25 °C from Oxides (ΔHf,ox) and from the Elements (ΔHf,el) in kJ/mol ΔH (kJ/mol)
reaction (1) K4UO2(O2)3·3H2O(c,25°C) → 2K2O(sln,700°C) + UO3(sln,700°C) + 3/2O2(g,700°C) + 3H2O(g,700°C)
ΔH1 = ΔHDS(K4UO2 (O2 )3 ·3H 2O)
(2) K 2O(c,25°C) → K 2O(sln,700°C)
ΔH2 = ΔHDS(K 2O)39
(3) UO3(c,25°C) → UO3(sln,700°C)
ΔH3 = ΔHDS(UO3)
(4) O2(g,25°C) → O2(g,700°C)
ΔH4 = ΔHheatcontent(O2 ) = 21.840
(5) H 2O(l,25°C) → H 2O(g,700°C)
ΔH5 = ΔHheatcontent(H 2O) = 6940
(6) 2K 2O(c,25°C) + UO3(c,25°C) + 3/2O2(g,25°C)+ 3H 2O(l,25°C)
ΔH6 = ΔHf,ox = − ΔH1 + 2ΔH2 + ΔH3 +
→ K4UO2 (O2 )3 ·3H 2O(c,25°C)
3 ΔH4 + 3ΔH5 2
(7) 2K(c,25°C) + 1/2O2(g,25°C)→ K2O(c,25°C)
ΔH7 = ΔHf,el(K 2O)40
(8) U(c,25°C) + 3/2O2(g,25°C) → UO3(c,25°C)
ΔH8 = ΔHf,el(UO3)40
(9) H2(g,25°C) + 1/2O2(g,25°C) → H2O(l,25°C)
ΔH9 = ΔHf,el(H 2O)40
(10) 4K(c,25°C) + U(c,25°C) + 3H2(g,25°C) + 11/2O2(g,25°C) → K4UO2(O2)3·3H2O(c,25°C)
ΔH10 = ΔHf,el(K4UO2 (O2 )3 ·3H 2O) = ΔH6 + 2ΔH7 + ΔH8 + 3ΔH9
Table 3. Summary of Thermodynamic Data for Uranyl Triperoxide Compounds in kJ/mola compound
composition
MgUT CaUT SrUT KUT
Mg2UO2(O2)3·13.31H2O Ca2UO2(O2)3·8.53H2O Sr2UO2(O2)3·8.69H2O K4UO2(O2)3·3.00H2O
ΔHDS 1042.72 597.36 616.56 213.27
± ± ± ±
ΔHf,ox 36.69 5.73 9.86 4.74
−93.72 −147.20 −249.16 −600.08
± ± ± ±
ΔHf,el 36.72 6.94 12.54 7.95
−6324.90 −5079.31 −5137.80 −3407.71
± ± ± ±
36.74 7.21 12.71 9.02
ΔHDS: enthalpy of drop solution; ΔHf,ox: enthalpy of formation from the oxides at 25 °C; ΔHf,el: enthalpy of formation from the elements at 25 °C.
a
volume of 587.73(5) Å3, reflecting the surprisingly low lattice water content (3−4 times less than the other chemically analogous salts). The nearly linear uranyl ion (UO22+) is coordinated equatorially by three bidentate peroxide (O22−) ligands. The K cations are 6−8 coordinated (K−O = 2.7−3.0 Å) and are bonded both to end and side-on peroxide ligands, yl-oxos of the uranyls, and to at most one water molecule. Details concerning data collection, structure determination, and refinement are summarized in the Supporting Information, in addition to tables of pertinent bond distances and bond valence sums (Tables S1−S3). All MUT compounds contain uranyl ions that are coordinated by three bidentate peroxide located in equatorial positions of hexagonal bipyramidal [UO2(O2)3]4− monomers. In the structures of CaUT, SrUT, and KUT, the alkali and alkaline earth cations are bonded to peroxide oxygen atoms and also the yl-oxos, forming linkages between adjacent monomers. Also, LiUT and NaUT both exhibit alternating layers of the uranyl triperoxide anion and alkali cations bonded to water molecules. In LiUT, the Li cations are also bound to the yl-oxos, whereas in NaUT, the Na cations are bonded to both the yl-oxos and peroxo ligands. In the case of MgUT, the Mg cations are coordinated by six H2O groups in an octahedral arrangement about the central cation, and thus, the uranyl triperoxide monomers are linked into the overall structure only through H bonds in this structure. The identity and purity of each MUT compound studied here was confirmed by X-ray powder diffraction (Figure S1), and chemical analysis confirmed the cation to uranium ratios. The water content of each compound from TGA (Figure S2) is listed in Table 3. The hydration states of bulk samples determined by TGA were used in the thermochemical cycle calculations.
emphasizing the bonding between the uranyl triperoxide units and the alkali/alkaline earth cations. The synthesis and structure of KUT are reported here for the first time. KUT crystallizes in P1̅ with a small unit cell
Figure 1. Ball-and-stick views of the crystal structures of (a) MgUT,33 (b) CaUT or SrUT,33 (c) LiUT,32 (d) NaUT,32 (e) KUT. Yellow, red, cyan, green, magenta, orange, and purple polyhedral/spheres represent uranium, oxygen, magnesium, calcium/strontium, lithium, sodium, and potassium atoms, respectively. C
DOI: 10.1021/acs.inorgchem.8b02661 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 2. Comparison of enthalpies of formation from oxides for MUT (M = Li, Na, K, Mg, Ca, Sr) (a) as a function of acidity of alkali and alkalineearth oxides on the Smith scale34 and (b) as a function of six-coordination ionic radius. Note*: Previously published data from Armstrong et al.21
analysis greatly improved accuracy and reproducibility of thermochemical measurements, as described in the Experimental Procedures. As shown in Figure 2a, we find that the enthalpies of formation from oxides (ΔHf,ox) for uranyl peroxide monomer compounds correlate linearly (R2 = 0.978; R2 = 0.985 excluding ΔHf,ox of MgUT) with the acidity of the alkali and alkaline earth metal oxides on the Smith scale.34 The ΔHf,ox decreases as the acidity of the metal oxides decrease. This suggests the increasingly exothermic lattice associations between the uranyl peroxide monomer and the alkali/alkaline earth cation reflect strong acid−base interactions. Similar trends have been observed in sheet structured uranyl minerals including uranyl silicates,13,14 uranyl phosphates,9 uranyl vanadates,41 and uranyl arsenates.17 In the case of various uranyl minerals, the structural units consist of sheets of [(UO2)(HSiO4)]1−, [(UO2)(PO4)]1−, [(UO2)2(V2O8)]2−, or [(UO2)(AsO4)]1− with mono-, di-, or trivalent interstitial cations between successive sheets. In the structures of salts of uranyl triperoxide monomers, [UO2(O2)3]4− hexagonal bipyramids are linked by counter-cations, except in the case of MgUT, whose structure is distinctly different (Figure 1a). Instead of bonding directly to the uranyl peroxide anions, the Mg2+ cations are octahedrally coordinated by six water molecules and linked with the uranyl peroxide only by H-bonding of the Mg-coordinated water molecules. Increasing exothermic enthalpies of formation from MgUT to KUT from their respective oxides reflect the importance of acid−base chemistry of the oxides involved. As the entropies of formation from oxides will be similar, the generally very exothermic ΔHf,ox of the MUTs will be the dominating term in the Gibbs free energy terms; this indicates that the most thermodynamically stable phases with the same structural building units will be those with the strongest acid−base chemical reactions, where both the yl-oxo and peroxide ligands are the acid (oxygen donor) and the alkali or alkaline earth metals are the base (oxygen acceptor). The deviation of MgUT from the linear trend in Figure 2a is likely the result of the unique bonding environment of the divalent cation. A correlation between the enthalpy of formation and the sixcoordinated ionic radius of the alkali cation was reported earlier for uranyl peroxide monomer compounds.21 ΔHf,ox generally decreases as the cation radius increases. With the newly obtained thermochemical data we find two similar but separate trends for the alkali and alkaline earth series for uranyl
The enthalpies of drop solution measured by high temperature calorimetry for the synthesized single-phase MUT salts were used to calculate their enthalpies of formation from oxides and elements via thermochemical cycles listed in Tables 1 and 2, which are provided in Table 3. Uranyl triperoxide monomer compounds have exothermic enthalpies of formation from oxides ΔHf,ox (Table 3), 2MO(c,25° C) + UO3(c,25
° C)
+
3 O2(g,25 C) + nH 2O(1,25° C) ° 2
→ M 2UO2 (O2 )3 ·nH 2O(c,25° C) (M = Mg, Ca, Sr) 2K 2O(c,25° C) + UO3(c,25
° C)
+
(1)
3 O2(g,25 C) + 3H 2O(1,25° C) ° 2
→ K4UO2 (O2 )3 ·3H 2O(c,25° C)
(2)
Regardless of countercation identity, the same amount of oxygen is involved in this reaction for all MUTs (eqs 1 and 2); thus, the entropies of formation from oxides for these MUTs will be similar, and the enthalpies of formation for different MUTs indicate their relative thermodynamic stability among these compounds. Enthalpies of formation from oxides vs acidity of the oxides on the Smith scale34 are shown in Figure 2a. For uranyl triperoxide monomers, ΔHf,ox decreases linearly with the basicity of the metal oxides.34 The Smith scale quantifies acidity/basicity of oxides against H2O as the acidic oxygen donor, as the standard state. More negative values correspond to basicity and more positive values correlate with acidity.
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DISCUSSION In a prior study,21 we reported formation enthalpies of a compound designated as K4UO2(O2)3·9H2O, where its ΔHf,ox of −453.4 ± 9.3 kJ/mol was determined. During our attempts to repeat these experiments,21 we synthesized LiUT and NaUT, and obtain consistent thermochemical data with those reported in the earlier paper; however, attempts to obtain a K-monomer with analogous calorimetric results were unsuccessful. This is due to the kinetic instability of this compound in its powder dry form. We observed it polymerize and start to form capsules via peroxide decomposition in the solid-state, and this will be detailed in a future publication. Additional synthetic efforts led to crystallization of a different uranyl triperoxide monomer, KUT (K4UO2(O2)3·3H2O), which was characterized and used for thermochemical studies here. Moreover, retaining the crystals of KUT in the mother liquor until just prior to characterization and D
DOI: 10.1021/acs.inorgchem.8b02661 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Table 4. Enthalpies of Reactions Involving Uranyl Triperoxide Compounds at Room Temperature (25 °C), Calculated Using Data Refered to Standard State of All the Species MgUT formation with peroxide: + 2+ 2Mg(aq) + UO3(c) + 3H 2O2(l) + 12.31H 2O(l) → Mg 2UO2 (O2 )3 ·13.31H 2O(c) + 4H(aq)
(A)
ΔH = −86.88 ± 36.79 kJ/mol MgUT formation without peroxide: + 2+ 2Mg(aq) + UO3(c) + 15.31H 2O(l) + 1.5O2(g) → Mg 2UO2 (O2 )3 ·13.31H 2O(c) + 4H(aq)
(B)
ΔH = 208.50 ± 36.80 kJ/mol CaUT formation with peroxide: + 2+ + UO3(c) + 3H 2O2(l) + 7.53H 2O(l) → Ca 2UO2 (O2 )3 ·8.53H 2O(c) + 4H(aq) 2Ca(aq)
(C)
ΔH = −55.41 ± 7.57kJ/mol CaUT formation without peroxide: 2+ + 2Ca(aq) + UO3(c) + 10.53H 2O(l) + 1.5O2(g) → Ca 2UO2 (O2 )3 ·8.53H 2O(c) + 4H(aq)
(D)
ΔH = 239.96 ± 7.60 kJ/mol SrUT formation with peroxide: + 2+ + UO3(c) + 3H 2O2(l) + 7.69H 2O(l) → Sr2UO2 (O2 )3 ·8.69H 2O(c) + 4H(aq) 2Sr(aq)
(E)
ΔH = −52.38 ± 12.80 kJ/mol SrUT formation without peroxide: 2+ + 2Sr(aq) + UO3(c) + 10.69H 2O(l) + 1.5O2(g) → Sr2UO2 (O2 )3 ·8.69H 2O(c) + 4H(aq)
(F)
ΔH = 243.00 ± 12.82 kJ/mol KUT formation with peroxide: + + + UO3(c) + 3H 2O2(l) + 2H 2O(l) → K4UO2 (O2 )3 ·3.00H 2O(c) + 4H(aq) 4K (aq)
(G)
ΔH = −43.96 ± 9.08 kJ/mol KUT formation without peroxide: + + 4K (aq) + UO3(c) + 5H 2O(l) + 1.5O2(g) → K4UO2 (O2 )3 ·3.00H 2O(c) + 4H(aq)
(H)
ΔH = 250.04 ± 9.10 kJ/mol LiUTa formation with peroxide: + + + UO3(c) + 3H 2O2(l) + 9H 2O(l) → Li4UO2 (O2 )3 ·10H 2O(c) + 4H(aq) 4Li(aq)
(I)
ΔH = −68.18 ± 14.06 kJ/mol LiUTa formation without peroxide: + + 4Li(aq) + UO3(c) + 12H 2O(l) + 1.5O2(g) → Li4UO2 (O2 )3 ·10H 2O(c) + 4H(aq)
(J)
ΔH = 227.20 ± 14.08 kJ/mol NaUTa formation with peroxide: + + 4Na(aq) + UO3(c) + 3H 2O2(l) + 8H 2O(l) → Na4UO2 (O2 )3 ·9H 2O(c) + 4H(aq)
(K)
ΔH = −113.98 ± 9.98 kJ/mol NaUTa formation without peroxide: + + 4Na(aq) + UO3(c) + 11H 2O(l) + 1.5O2(g) → Na4UO2 (O2 )3 ·9H 2O(c) + 4H(aq)
(L)
ΔH = 181.40 ± 10.00 kJ/mol a
Recalculated from previously published data from Armstrong et al.21
enthalpies for reactions of interest, especially those involving aqueous species. As shown in Table 4, the formation of the alkaline earth uranyl triperoxide monomers (MgUT, CaUT, and SrUT) in solution from metal ions, uranium trioxide, water,
peroxide monomer compounds (Figure 2b). This is likewise true for alkali salts of hexaniobate polyoxometalates.42 By employing the calculated ΔHf,el values of each compound reported here and previously, it is possible to calculate E
DOI: 10.1021/acs.inorgchem.8b02661 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry and peroxide (reactions A, C, and E, in Table 4) are exothermic,
Crystal data and structure refinement for KUT, bond lengths and bond valence analysis for KUT, comparison of experimental and calculated powder X-ray diffraction spectra for MUTs, and thermogravimetric analysis for MUTs (PDF)
2+ 2M(aq) + UO3(c) + 3H 2O2(l) + (n − 1) ·H 2O(l) + → M 2UO2 (O2 )3 ·nH 2O(c) + 4H(aq)
(3)
Accession Codes
in contrast to the reactions without peroxide (reactions B, D, and F), which are endothermic,
CCDC 1856094 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing data_
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
2+ 2M(aq) + UO3(c) + (n + 2) ·H 2O(l) + 1.5O2(g) + → M 2UO2 (O2 )3 ·nH 2O(c) + 4H(aq)
(4)
■
where M = Mg, Ca, Sr. This is also the case for alkali uranyl peroxide monomers KUT, LiUT, and NaUT. Their formation enthalpies in solution from peroxide (reactions G, I, and K, in Table 4) are exothermic,
*E-mail:
[email protected]. ORCID
+ 4M(aq) + UO3(c) + 3H 2O2(l) + (n − 1) ·H 2O(l)
→ M4UO2 (O2 )3 ·nH 2O(c) +
+ 4H(aq)
Lei Zhang: 0000-0002-0134-4142 Mateusz Dembowski: 0000-0002-6665-8417 May Nyman: 0000-0002-1787-0518 Peter C. Burns: 0000-0002-2319-9628
(5)
while the reactions without peroxide (reactions H, J, and L) are endothermic,
Notes
The authors declare no competing financial interest.
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+ 4M(aq) + UO3(c) + (n + 2) ·H 2O(l) + 1.5O2(g) + → M4UO2 (O2 )3 ·nH 2O(c) + 4H(aq)
ACKNOWLEDGMENTS The postdoctoral salary of L.Z. and graduate stipend of M.D. were funded by the Office of Basic Energy Sciences of the U.S. Department of Energy as part of the Materials Science of Actinides Energy Frontier Research Center (Grant DE-SC0001089). The graduate stipend of S.H. and A.A. and participation of P.C.B. and M.N. were supported by the Department of Energy, National Nuclear Security Administration under Award DE-NA0003763. L.N.Z. and N.P.M. provided crystallography support for KUT. Calorimetry; TGA for MgUT, CaUT, and SrUT; and powder diffraction data were collected at the Materials Characterization Facility in the Center for Sustainable Energy at the University of Notre Dame. ICP-OES data were collected at the Center for Environmental Science and Technology (CEST) at the University of Notre Dame.
(6)
where M = K, Li, Na. Formation enthalpies for LiUT and NaUT (reactions I, J, K, and L in Table 4) were recalculated from the earlier report21 to reflect formation from UO3 instead of UO2. This is consistent with our experimental observations that the formation of the uranyl triperoxide monomers is only favorable in the presence of hydrogen peroxide. The thermochemical data of the alkaline earth uranyl triperoxide compounds MgUT, CaUT, and SrUT and the KUT (K4UO2(O2)3·3H2O) studied here show a clear correlation between their formation enthalpies from oxides and the basicity of the countercation oxides. Interestingly, the KUT with the most negative ΔHf,ox is also the most kinetically unstable, and we have yet to synthesize RbUT and CsUT due to even greater kinetic instability.
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CONCLUSIONS Four uranyl triperoxide compounds MgUT, CaUT, and SrUT and the new KUT were synthesized and characterized. Their formation enthalpies were measured by high temperature oxide melt drop solution calorimetry and are compared to the priorinvestigated LiUT and NaUT. A linear correlation exists between the formation enthalpies from oxides and the acidity of the alkali and alkaline earth oxides based on Smith’s scale. This suggests the acid−base associations of M−O22− and M−Oyl−U (M = alkali or alkaline earth metal cations) are the driving force of crystallization of these salts. Consistent with this is the departure of MgUT from this trend, in which Mg2+ does not bond to the uranyl peroxide units, except through hydrogen-bonding of H2O molecules that are bonded to Mg2+.
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AUTHOR INFORMATION
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REFERENCES
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DOI: 10.1021/acs.inorgchem.8b02661 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
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DOI: 10.1021/acs.inorgchem.8b02661 Inorg. Chem. XXXX, XXX, XXX−XXX
