ARTICLE pubs.acs.org/JPCA
Raman Spectroscopic Study of Tungsten(VI) Oxosulfato Complexes in WO3-K2S2O7-K2SO4 Molten Mixtures: Stoichiometry, Vibrational Properties, and Molecular Structure Andreas L. Paulsen,†,§ Angelos G. Kalampounias,† Rolf W. Berg,‡ and Soghomon Boghosian*,† †
Department of Chemical Engineering, University of Patras and Institute of Chemical Engineering and High Temperature Chemical Processes (FORTH/ICE-HT), GR-26500 Patras, Greece ‡ Chemistry Department, The Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark ABSTRACT: The dissolution reaction of WO3 in pure molten K2S2O7 and in molten K2S2O7-K2SO4 mixtures is studied under static equilibrium conditions in the XWO30 = 0-0.33 mol fraction range at temperatures up to 860 °C. High temperature Raman spectroscopy shows that the dissolution leads to formation of WVI oxosulfato complexes, and the spectral features are adequate for inferring the structural and vibrational properties of the complexes formed. The band characteristics observed in the WdO stretching region (band wavenumbers, intensities, and polarization characteristics) are consistent with a dioxo W(dO)2 configuration as a core unit within the oxosulfato complexes formed. A quantitative exploitation of the relative Raman intensities in the binary WO3-K2S2O7 system allows the determination of the stoichiometric coefficient, n, of the complex formation reaction WO3 þ nS2O72- f C2n-. It is found that n = 1; therefore, the reaction WO3 þ S2O72- f WO2(SO4)22- with six-fold W coordination is proposed as fully consistent with the observed Raman features. The effects of the incremental dissolution and presence of K2SO4 in WO3-K2S2O7 melts point to a WO3 3 K2S2O7 3 K2SO4 stoichiometry and a corresponding complex formation reaction in the ternary molten WO3-K2S2O7-K2SO4 system according to WO3 þ S2O72- þ SO42- f WO2(SO4)34-. The coordination sphere of W in WO2(SO4)22- (binary system) is completed with two oxide ligands and two chelating sulfate groups. A dimeric [{WO2(SO4)2}2(μ-SO4)2]8- configuration is proposed for the W oxosulfato complex in the ternary system, generated from inversion symmetry of a WO2(SO4)34- moiety resulting in two bridging sulfates. The most characteristic Raman bands for the WVI oxosulfato complexes pertain to W(dO)2 stretching modes (i) at 972 (polarized) and 937 (depolarized) cm-1 for the vs and vas W(dO)2 modes of WO2(SO4)22-, and (ii) at 933 (polarized) and 909 (depolarized) cm-1 for the respective modes of [{WO2(SO4)2}2(μ-SO4)2]8-.
’ INTRODUCTION The dissolution reactions of a number of metal oxides in molten alkali pyrosulfates, alkali sulfates, and mixtures thereof have been studied previously by us, revealing the formation of sulfato and oxosulfato complexes. Moreover, we have derived a formalism for inferring the stoichiometry of such and similar solute complexes in molten salt and ionic liquid solvents based on Raman intensity correlations.1 In particular, the stoichiometric, vibrational, and structural properties of the complexes formed by dissolution of V2O5 in molten M2S2O7,2 M2SO4,3 and M2S2O7-M2SO4 (M = K, Cs),4 of ZnO in K2S2O7 or Na2S2O75 and of Nb2O5 in K2S2O7 and K2S2O7-K2SO46 have been inferred by high temperature Raman spectroscopy. In addition, single crystal X-ray diffraction has been used for the structural characterization of VIII,7,8 VIV,9-12 VV,13,14 NbV,15 TaV,15 WVI,16-18 and MoVI19,20 crystalline sulfato or oxosulfato complexes, synthesized by precipitation from molten mixtures of the respective metal oxide and molten pyrosulfate or pyrosulfatesulfate salts. The dissolution of metal oxides in a molten salt at moderate or elevated temperatures (such as in molten pyrosulfates) has r 2011 American Chemical Society
drawn interest from the point of view of metal ore extraction and recovery of metal oxides (e.g., catalyst phases consisting of V2O5, WO3, Nb2O5, MoO3, ZnO, etc.). The molten mixtures V2O5-M2S2O7-M2SO4 (M = K, Cs) and V2O5-M2SO4 (M = K,Cs) have been studied extensively because they constitute the homogeneous liquid catalytic phase of the supported molten salt sulfuric acid catalysts.21 Notably, the VIII, VIV, and VV crystalline sulfato and oxosulfato complexes synthesized by precipitation from the vanadia-containing melts7-14 have been shown to be responsible for the deactivation of the sulfuric acid catalyst.21-24 Tungsten oxide (a highly inert yellow solid) alone, or in mixtures with other transition metal oxides, is a common constituent of supported metal oxide catalysts,25 and part of the “good mix” or “favorable balance” of its catalytic properties is due to its insolubility in acids. The type of complexes formed during the dissolution of tungstenVI oxide in pure molten alkali pyrosulfate is not known; WVI is expected to form anionic sulfato Received: September 29, 2010 Revised: February 16, 2011 Published: April 04, 2011 4214
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The Journal of Physical Chemistry A and/or oxosulfato complexes in molten pyrosulfates in analogy to VV1-3,26-30 and NbV.6 The present study aimed at studying the molecular structure and the vibrational properties of the complexes formed when dissolving WO3 in molten K2S2O7 and molten K2S2O7-K2SO4 mixtures. Raman spectroscopy is used at temperatures of 580-860 °C for studying the WO3-K2S2O7 molten mixtures with XWO30 = 00.33 (XWO30 denotes the initial WO3 mole fraction in the WO3K2S2O7 binary mixture) under static equilibrium conditions. Incremental amounts of K2SO4 are added to each binary mixture 2with n(SO24 )/n(W) = 0-2, where n(SO4 )/n(W) (hereinafter denoted Y) is the number of added moles of K2SO4 per W atom in each mixture. A quantitative exploitation of the relative Raman band intensities of the species present in equilibrium WO3K2S2O7 molten mixtures is shown to be adequate for determining the stoichiometry of the WO3 dissolution reaction. The composition effects on the Raman spectra of the binary WO3-K2S2O7 and ternary WO3-K2S2O7-K2SO4 molten systems are studied with a view to establish the structural configuration for the complexes in conjunction with consistent band assignments.
ARTICLE
Table 1. Relative Molar Compositions, XWO30,a and Indicators of Incremental Sulfate Content, n(SO24 )/n(W), of WO3-K2S2O7-K2SO4 Samples cell no.
’ EXPERIMENTAL SECTION Materials and Sample Preparation. The samples were
prepared by mixing WO3 (Alfa Aesar) with K2SO4 (Fluka) and K2S2O7 that was made by thermal decomposition of K2S2O8 (Fluka), as described previously.26 All handling of chemicals and filling of the Raman optical cells [made of cylindrical fused silica tubing (6 ( 0.1 mm o.d., 4 ( 0.1 mm i.d., and ∼3 cm long for the part containing the molten salts)] took place in a dry nitrogenfilled glovebox. The total amount of salt mixture added into each cell was 300-450 mg. Proper mixing of the components was necessary, as the melting point of WO3 (1472 °C) is very high compared to the fusion temperatures of the WO3-K2S2O7 and WO3-K2S2O7-K2SO4 mixtures. Thus, the optical cells were filled either by transferring WO3 and K2SO4 with approximately half the K2S2O7 into the cell and then adding the remaining K2S2O7 on top or by grinding all components intimately in an agate mortar before transferring into the optical cell. The samples were sealed under a low pressure (ca. 0.2 bar) of O2 (L’Air Liquide, 99.99%) in order to prevent self-reduction of WVI. Afterward they were equilibrated at 500-850 °C for several days (up to 4 weeks) before recording the Raman spectra. The long equilibration time was necessary due to the slow dissolution of WO3. Upon dissolution of tungsten oxide in potassium pyrosulfate, the resulting melts became transparent (pale yellow) and were extremely viscous. Often, it was necessary to remove bubbles and/or accelerate the dissolution of solids by torching the samples. Cooling of samples with high WO3 content often leads to formation of glasses. With further lowering of the temperature, several of the glassy samples exploded (tensions). The symbol X0i is used to denote the mole fractions of nonreacted components of the WO3-K2S2O7 binary mixture (weighed-in amounts) before any reaction had started. The composition of the ternary mixture is defined by combining XWO30 (neglecting K2SO4) with the ratio Y (Y = n(SO24 )/n(W)) of the number of sulfate groups added per tungsten atom, and this ratio was varied between 0 and 2. It was not possible to dissolve more than 33.4 mol % WO3 in K2S2O7 at 850 °C. A very steep increase of the melting point was observed on going from XWO30 = 0.20 (mp ∼ 640 °C) to XWO30 = 0.33 (mp ∼ 850 °C). The dissolution of WO3 is facilitated in ternary mixtures where
XWO30 a
Y = n(SO24 )/n(W)
1
0
-
2
0.050
-
3
0.100
-
4
0.150
-
5 6
0.201 0.251
-
7
0.334
-
8
0.198
0.492
9
0.199
1.00
10
0.196
1.98
11
0.250
0.492
12
0.252
0.990
13 14
0.253 0.330
1.99 0.499
15
0.332
1.01
16
0.332
2.01
17
0.493
0.996
a
XWO30 denotes the mole fractions of nonreacted components of the WO3-K2S2O7 binary mixture (weighed-in amounts) before any reaction had started and any K2SO4 added.
sulfate is also present. Table 1 summarizes the compositions of the cells made during the course of the present work. Raman Spectra. The Raman set up, the furnace for the optical cells, and the systematics for obtaining Raman spectra from molten salts and vapors at high temperatures have been described in detail elsewhere.1,3,4,31,32 Raman spectra were obtained with the 488.0 nm line of a Spectra Physics Model 164 argon ion laser operated at a power of 50 mW (at the sample). Spectra were recorded by using two polarizations of the incident scattered light to the scattering plane: the vertical-vertical (VV) and the horizontal-vertical (HV). The procedures for obtaining Raman spectra for the WO3K2S2O7 and WO3-K2S2O7-K2SO4 molten systems at high temperatures were similar to the ones described in ref 6 and are omitted, for brevity.
’ RESULTS AND DISCUSSION Raman Spectra of WO3-K2S2O7 Mixtures. Figure 1 shows representative Raman spectra obtained for WO3-K2S2O7/ O2(g) molten mixtures, together with the corresponding spectra obtained for pure molten K2S2O7 that are well-known4,33 and are included in Figure 1 for comparison. At 620 °C, the S2O72- ion in molten K2S2O7 exhibits its main bands at 1085 (terminal stretching), 730 (bridging S-O-S stretching), and 318 cm-1 (S-O-S deformation). Upon dissolution of WO3, complex formation becomes evident from the emergence of several new bands. Table 2 provides a summary of the observed band wavenumbers and characteristics in Raman spectra obtained for WO3-K2S2O7 molten mixtures. Bands due to complex formation are seen at 1195 (p), 1053 (p), 972 (p), 930 (dp), 672 (p), 627 (dp?), ∼450, ∼410, 375 (p), ∼280 (dp), and 243 (dp) cm-1. The band wavenumbers appear slightly red-shifted in spectra obtained at higher temperatures. The bands ascribed to 4215
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unit and is furthermore in agreement with the 972 and 928 cm-1 values for the νs and νas W(dO)2 modes measured for matrixisolated WVIO2 molecules at 4 K.34 Table 3 provides a summary of WdO stretching Raman wavenumbers for model compounds of interest to the present work. The 1053 cm-1 band as well as the majority of the rest of the bands due to the complex (Figure 1) are ascribed to sulfate modes in structural environments of moderate perturbation resulting from coordination and bridging. From the above, it turns out that the WVI complex species formed according to eq 1 consists of WO22þ and SO42units. The four sulfate fundamentals (ν1-ν4) for a tetrahedral Td configuration span the following irreducible representation: Γvib ¼ A1 ðν1 Þ þ Eðν2 Þ þ 2F2 ðν3 þ ν4 Þ
Figure 1. Raman spectra obtained for molten WO3-K2S2O7 mixtures (0 < XWO30 < 0.33) in oxygen atmosphere (pO2 = 0.2 bar) at temperatures as indicated by each spectrum. XWO30 denotes the mole fraction of WO3. Laser wavelength, λ0 = 488.0 nm; laser power, w = 50 mW; resolution, 6 cm-1. Asterisk: see text.
formation of complex increase in intensity relative to the S2O72solvent bands and become predominant in the Raman spectra of mixtures with high XWO30 (i.e., 0.25 and 0.33). Furthermore, there are no composition and/or temperature effects on the relative intensities of the bands ascribed to the complex(es) formed. The relative intensities increase monotonically, thereby indicating that they originate from one complex species. Thus, the dissolution reaction takes place at the expense of the S2O72- and leads to formation of one kind of WVI complex, following a general reaction scheme according to WO3 þ nS2 O7 2- f C2n-
ð1Þ
An inspection of Figure 1 points to the 972 and 1053 cm-1 polarized bands and to the 930 cm-1 depolarized band (marked by dashed lines in Figure 1) as the most prominent bands due to the WVI complex (C2n-). The 972 and 930 cm-1 bands lie in the WdO stretching region. Notably, the observation of a polarized and a depolarized band in the WdO stretching region is suggestive of a dioxo OdWdO unit occurrence. Indeed, a dioxo WO2 unit possesses two stretching modes (a symmetric νs and an antisymmetric νas mode). Moreover, for a transition metal dioxo unit, the Raman band due to symmetric stretching is much stronger in intensity and its wavenumber is expected to be 1040 cm-1 higher compared to the respective characteristics of the antisymmetric mode.38 In addition, contrary to the case of the polarized νs band, the Raman band due to the νas mode is expected to be depolarized. The observed “picture”, i.e., a strong polarized band at 972 cm-1 and a weaker depolarized band at 930 cm-1, is in full conformity with the existence of a WO22þ
Factor group analysis and symmetry considerations preview Raman activity for all vibrations, whereas only F2 is IR allowed. Vibrational modes labeled as ν1 and ν3 are stretchings and ν2 and ν4 are angle bendings. The four fundamentals are well-known from Raman work on aqueous solutions: ν1(A1) ≈ 980 cm-1, ν2(E) ≈ 450 cm-1, ν3(F2) ≈ 1100 cm-1, and ν4(F2) ≈ 615 cm-1.38 The weak band (marked by asterisk) seen in the Raman spectra of pure molten K2S2O7 at ∼965 cm-1 (e.g., see Figure 1a) is due to the ν1 symmetric stretching of free uncoordinated SO42- resulting from the partial dissociation reaction S2O72- T SO42- þ SO3.32 Coordination of the sulfate groups is expected to shift the bands, reduce the symmetry, and split the degeneracies of the ν2, ν3, and ν4 modes. Such vibrational properties for coordinated sulfate groups have been reported in Raman studies of molten V2O5-M2S2O7-M2SO4, V2O5-M2SO4 (M = K, Cs), and Nb2O5-K2S2O7-K2SO4 mixtures.3,4,6 In particular, the terminal S-O stretching is expected to be blue-shifted and the 1053 cm-1 band (see Figure 1) is accordingly assigned to terminal S-O stretching of a coordinated SO42- group. The observed Raman wavenumbers of the studied WO3-K2S2O7 and WO3-K2S2O7-K2SO4 molten mixtures are compiled in Table 2. The detailed assignments for the WO3-K2S2O7 system will be discussed in detail after the determination of the stoichiometry of eq 1. Stoichiometry of the WVI Complex in the Binary WO3K2S2O7 System. The assignment of the 972 cm-1 polarized band and of the 930 cm-1 depolarized band to the symmetric νs and antisymmetric νas W(dO)2 stretching modes of a dioxo WO22þ unit and the existence of one single band (1053 cm-1) in the SO terminal stretching region is supportive of the above postulation that points to formation of one complex species consisting of WO22þ and SO42- units. Below, a simple method is applied for establishing the stoichiometry of reaction 1, which correlates the relative Raman band intensities with the stoichiometric coefficient, n.1,2,6 Tungsten oxide, WO3, is assumed to be completely consumed during the dissolution reaction 1; hence, the equilibrium mixture consists of the complex species, C2n-, and S2O72-. The theoretical principles of the method, the various specific experimental procedures required, the limitations and the calculational systematics are described extensively in previous reports.1,2,6 For brevity, it is sufficient to state that the measured integrated Raman intensity due to a vibrational fundamental ν(i) of species j, Ij,ν(i), is related to the number of moles of species j contained in the scattering volume, Nj, according to: 1 Nj Ij, νðiÞ ¼ A f ðνðiÞ, TÞ 4216
ð2Þ
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Table 2. Raman Band Wavenumbers (cm-1) and Assignments for WVI Oxosulfato Complexes in Molten WO3-K2S2O7/O2(g) and WO3-K2S2O7-K2SO4/O2(g) Mixturesa WO3-K2S2O7(l) binary mixtures b
band wavenumber
tentative assignment for
WO3-K2S2O7-K2SO4(l) ternary mixtures WO2(SO4)22-(l)
band wavenumberc
tentative assignment for (WO2(SO4)3)28-(l)d
(1225)d w, dp
ν3(SO4)
1195 w, p
ν3(SO4)
1175 m, p
ν3(SO4)
1053 s, p 972 s, p
ν1(SO4) νs[W(dO)2]
1049 vs, p 933 s, p
ν1(SO4) νs[W(dO)2]
930 s, dp
νas[W(dO)2]
909 s, dp
νas[W(dO)2]
672 m, p
ν4(SO4)
664 s, p
ν4(SO4)
627 w, dp(?)
ν4(SO4)
602 m, dp
ν4(SO4)
∼450 w
ν2(SO4)
(460)e w, p(?)
ν2 (SO4)
∼410 w, p(?)
ν2 (SO4)
375 m, p (∼280)e m, dp
ν(W-O), νbending[W(dO)2](?) νbending[W(dO)2]
(378) m, dp 370 s, p 275 s, dp
ν(W-O), νbending[W(dO)2](?) νbending[W(dO)2]
243 m, dp
Abbreviations: s = strong; m = medium; w = weak; br = broad; v = very; p = polarized; dp = depolarized. b Band wavenumbers at 650 °C. c Band wavenumbers at 635 °C. d Occurring most probably in the [{WO2(SO4)2}2(μ-SO4)2]8- configuration (see text). e Obscured band. a
Table 3. WdO Stretching Wavenumbers for Different Mono-oxo and Di-oxo Tungsten Compounds compound
a
νs[W(dO)2],a cm-1
νas[W(dO)2],b cm-1
ν(W d O), cm-1
ref
WO2c
972
928
WO2Cl2
992
978
35
WO2Br2
1014d 991
974d 969
36 35
WO2Cl2(bpy)
955
916
37
WO2Cl2(phen)
943
916
37
WO2Br2(bpy)
954
907
37
WO2 (acac)2
960
912
37
34
[WF5O]-
987
38
WOCl4
1025
39
WOF4
1032 1055
35 38
1030
35
Symmetric stretching. b Antisymmetric stretching. c In neon matrix (4 K). d In N2 matrix.
where f(ν(i),T) equals 1 - exp(-hcv(i)/kT) and A embodies a number of factors such as the molecular scattering properties, excitation laser wavelength, spectrometric (instrumental) factors, scattering volume, etc. For the present analysis, it is convenient to recall the definition of the normalized intensity quotient I*:1 I ¼
ðIS2 O7 2- , νðS2 O7 2- Þ Þf ðνS2 O7 2- , TÞ=XS02 O7 20 ðIC2n- , νðCÞ Þf ðνC2n- , TÞ=XWO 3
ð3Þ
The inclusion of the Boltzmann thermal population factor, f(ν(i),T), in the nominator and denominator of eq 3 disentangles the experimentally measured Raman band intensities from temperature effects. On account of the stoichiometry of reaction 1 (which is assumed to be the only independent stoichiometric process taking place), the number of moles of S2O72- and C2nin equilibrium (Neq,S2O72- and Neq,C2n-, respectively), may be
expressed in terms of n as follows 0 Neq;S2 O7 2- ¼ NS02 O7 2- - nNWO 3
ð4Þ
0 Neq;C2n- ¼ NWO 3
ð5Þ
By summation of eq 4 and eq 5 the total number of moles is 0 Neq, total ¼ NS02 O7 2- - ðn - 1ÞNWO 3
ð6Þ
The number of moles C2n- in each equilibrium mixture equals the initial number of moles WO3 (see eq 5). Therefore, it turns out that the limit of the I* quotient for XWO30f0 is the ratio of the scattering power per ion of S2O72- divided by the scattering power per ion of C2n-. The relative Raman band intensities measured for a series of molten binary WO3-K2S2O7 mixtures (see Table 1 and Figure 1) are exploited for calculating the I* quotient, which is used for determining the stoichiometric coefficient, n. The 4217
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Table 4. Relative Integrated Raman Intensities (peak areas in arbitrary umits) of Representative Bands of the C2n- WVI Complex (at 972 cm-1) and of the S2O72- Solvent (at 1085 cm-1) Measured from the Raman Spectra of Molten WO3-K2S2O7 Mixtures XWO30
IC2n-,972
IS2O72-,1085
IC2n-,972/IS2O72-,1085
0.050
118
3526
0.0335
0.100 0.150
238 370
2925 2428
0.0814 0.1524
0.201
336
1288
0.2609
0.251
189
458
0.4127
0.334
320
367
0.8719
Figure 3. (A) A plausible structural model for the molten WO2(SO4)22- complex. (B) A dimeric formulation of the same species.
Figure 2. Open symbols and solid regression line: plot of the intensity quotient I* vs XWO30 (eq 3). An approximate value of n = 1 is found, corresponding to ideal crossing at XWO30 = 0.5. Filled symbols: intensity quotients calculated without the inclusion of the thermal population factors, f(ν(i),T).
integrated intensities (peak areas) of bands representative for each species were measured (see Table 4); the 972 cm-1 symmetric W(dO)2 stretching band was chosen to represent the WVI complex (C2n-), whereas S2O72- is represented by its strongest 1085 cm-1 band. I* is then plotted (with open symbols) versus XWO30 in Figure 2. It can be seen that I* extrapolates to zero for XWO30 = 0.488, indicating that the equilibrium mixture with such an initial composition would contain no S2O72- after complete dissolution/reaction of WO3. Figure 2 plots also (with filled symbols) the I* quotient without the inclusion of the temperature factors f(ν(i),T) in eq 3, for comparison. Thus, the results show that a 1:1 stoichiometry (n = 1) is evident, corresponding to ideal crossing at XWO30 = 0.5 in Figure 2. Thus, the product formula for reaction 1 is WS2O102-, and by taking into account the above preliminary structural interpretations pointing to the occurrence of a W(dO)2 unit and of coordinated sulfate groups, the most plausible form for reaction 1 is WO3 þ S2 O7 2- f WO2 ðSO4 Þ2 2-
ð7Þ
Reaction 7 is the only possible one, when considering the complete consumption of pyrosulfate ions for XWO30 = 0.5 (see Figure 2) and the appearance of bands which can be assigned to
symmetric and antisymmetric W(dO)2 stretching (972 cm-1 polarized and 930 cm-1 depolarized bands, respectively) and to coordinated sulfate groups (terminal S-O stretching at 1053 cm-1 and several split components of the ν2, ν3, and ν4 fundamental sulfate modes). Reaction 7 could also be regarded as a scheme predicting the formation of the WO2(SO4)22- unit which should occur as a monomer in dilute melts, whereas associated polymer [WO2(SO4)2]m2m- units may be progressively formed with increasing XWO30. Structural Model for WO2(SO4)22-. The proposed structural model for the WO2(SO4)22- complex is shown in Figure 3A. The WVI atom is in a six-fold coordination of a distorted octahedron, in conformity with the coordination chemistry of W. The W atom is shown to be surrounded by two oxide ligands (forming a ∼90° bent dioxo WO22þ unit) and two bidentate chelating sulfate groups. The oxide ligands occupy an apical and an equatorial position, and one bidentate chelating sulfate is positioned in the equatorial plane, while the second one occupies the remaining axial and equatorial positions of the octahedron. The resulting symmetries for the sulfate groups are mutatis mutandis similar, which justifies the moderate splitting of the degenerate ν2, ν3, and ν4 modes observed in the Raman spectra of Figure 1. Table 2 lists the band assignments as well as the intensity and polarization characteristics, in consistency with the WO2(SO4)22- structural model. Furthermore, it is known that transition metal oxosulfato complex units tend to associate to each other by bridging sulfate groups and forming polymeric anionic species.2-4,6,27-30 Such formation of polymeric and/or three-dimensional [WO2(SO4)2]m2m- networks is favored with increasing XWO30. For example, as depicted in Figure 3B, the formation of a dimer unit could take place by inversion symmetry of an independent WO2(SO4)22- moiety if a chelating bidentate sulfate opens up and transforms to bridging bidentate sulfate. In this way, a substitution of a chelating sulfate with two bridging sulfates (shared between the two moieties) takes place in the coordination sphere of each W atom, thereby maintaining the 6-fold coordination around W. Such a transformation would not affect significantly (with increasing XWO30) the remaining structural and vibrational features of the complex. This is in 4218
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Figure 5. Titration-like series of Raman spectra obtained in oxygen atmosphere (pO2 = 0.2 bar) for molten WO3-K2S2O7-K2SO4 mixtures with XWO30 = 0.25 at temperatures and incremental presence of K2SO4 (0 < Y < 2) as indicated by each spectrum. XWO30denotes the formal mole fraction of WO3 in the binary WO3-K2S2O7 mixture. Recording parameters: see caption of Figure 1. Asterisk: see text. Figure 4. Titration-like series of Raman spectra obtained in oxygen atmosphere (pO2 = 0.2 bar) at 635 °C for molten WO3-K2S2O7K2SO4 mixtures with XWO30 = 0.20 and incremental presence of K2SO4 (0 < Y < 2) as indicated by each spectrum. XWO30denotes the formal mole fraction of WO3 in the binary WO3-K2S2O7 mixture. Recording parameters: see caption of Figure 1. Asterisk: see text.
accordance with the Raman spectra shown in Figure 1 that do not exhibit composition and/or temperature dependent features on band positions and band relative intensities pertaining to the W oxosulfato complex (vide ante). Raman Spectra of WO3-K2S2O7-K2SO4 Molten Mixtures. By inspection through transparent tube furnaces during the equilibration of the samples, it was found that WO3 could be dissolved much easier in considerable amounts in molten K2S2O7 when K2SO4 was also present. For example, whereas the binary WO3-K2S2O7 mixture with XWO30 = 0.33 (sample no. 7, Table 1) had to be heated to 850 °C for several days before complete WO3 dissolution was achieved, the corresponding mixture with XWO30 = 0.33 that contained added sufate at a ratio Y = n(SO24 )/n(W) = 1 (sample no. 15, Table 1) melted readily at 620 °C. Figures 4- 6 show titration-like series of Raman spectra obtained for mixtures with XWO30 = 0.20 (Figure 4), XWO30 = 0.25 (Figure 5) and XWO30 = 0.33 (Figure 6 ) as a function of the ratio of added sulfate per tungsten atom, Y = n(SO24 )/n(W). All figures contain the Raman spectrum of pure molten K2S2O7 for comparison. The prominent presence of the strong sharp ν1(S2O72-) band in all spectra shown in Figures 46 indicates that all ternary WO3-K2S2O7-K2SO4 mixtures with XWO30 = 0.20-0.33 contain excess amounts of the solvent
S2O72-. Furthermore, an inspection of the titration-like features of the Raman spectra in Figures 4-6 shows that the gradual changes caused by the incremental addition of sulfate terminate for Y = n(SO24 )/n(W) = 1. The main spectral changes taking place include the following: (i) the 972(p)/930(dp) cm-1 νs/νas W(dO)2 doublet is gradually shifted to 933(p)/909(dp) cm-1. The occurrence of this pair of counterpart bands, of which the high frequency one is stronger and polarized whereas the low frequency one is weaker and depolarized, indicates the preservation of the WO22þ dioxo unit that was present in the binary system (vide ante), though in a slightly modified structural environment; (ii) the 1053 cm-1 sulfate (terminal S-O stretching) band progressively gains intensity (i.e., relative to the W(dO)2 νs/νas doublet) with increasing n(SO24 )/n(W) up to Y = 1. At the same time it gradually shifts to 1049 cm-1, indicating that more sulfate groups are now involved in the coordination sphere of W; (iii) a new depolarized band at 602 cm-1 can be seen and ascribed to a ν4(SO4) bending split component, indicating a slightly diversified sulfate coordination; (iv) the ν1(SO42-) band at 963 cm-1 due to dissolved free SO42- can be seen present in the Raman spectra obtained for all melts with Y = 0.5 and Y = 1. K2SO4 has a limited (though significant) solubility in molten K2S2O7 as seen by comparing, for example, spectra Figure 4a and Figure 4b; (v) the addition of excess amounts of SO42-, i.e., Y = 2 causes precipitation of K2SO4(s), evidenced from the appearance 4219
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Figure 6. Titration-like series of Raman spectra obtained in oxygen atmosphere (pO2 = 0.2 bar) for molten WO3-K2S2O7-K2SO4 mixtures with XWO30 = 0.33 at temperatures and incremental presence of K2SO4 (0 < Y < 2) as indicated by each spectrum. XWO30denotes the formal mole fraction of WO3 in the binary WO3-K2S2O7 mixture. Recording parameters: see caption of Figure 1. Asterisk: see text.
of a white solid cloud in the respective melts and the emergence of the strong and sharp ν1(SO42-) at ∼965 cm-1. It is seen that despite the structural modifications taking place in the ternary WO3-K2S2O7-K2SO4 molten system, the occurrence of a band pair at 933(p)/909(dp) cm-1 which is fully compatible with the characteristics of a νs/νas W(dO)2 stretching pair points to the existence of a dioxo WO22þ unit. Therefore, the complex species existing in the ternary system must still be consisting of WO22þ and SO42- units. However, it appears that with increasing amounts of sulfate added (vide ante, observation ii) more sulfate groups are coordinated to W, judging from the increase in the S-O stretching band intensity relative to the W(dO)2 stretching doublet band intensity. Furthermore, taking into account that the spectral changes terminate for Y = n(SO24 )/n(W) = 1, it is evident that there is one more sulfate group coordinated per W atom, compared to the case of the binary WO3-K2S2O7 system. Thus, we propose a reaction “following up” the one occurring in the binary system (eq 7) resulting in addition of sulfate at a ratio Y = n(SO24 )/n(W) = 1, i.e. WO3 þ S2 O7 2- þ SO4 2- f WO2 ðSO4 Þ3 4-
ð8Þ
which is the simplest and most plausible reaction scheme consistent with the spectral data in Figures 4-6 and the above observations i-v. Naturally, reaction 8 accounts for the proposed 1:1:1 stoichiometry and is the simplest one that shows the
Figure 7. Raman spectra of molten WO3-K2S2O7-K2SO4 mixtures with Y = 1 (i.e., n(SO24 )/n(W) = 1) obtained in oxygen atmosphere (pO2 = 0.2 bar) at 635 °C and XWO30 as indicated by each spectrum. XWO30 denotes the formal mole fraction of WO3 in the binary WO3K2S2O7 mixture. Recording parameters: see caption of Figure 1.
formation of a monomer WO2(SO4)34- unit. However, one cannot exclude the possibility that WO2(SO4)34- moieties existing in a molten mixture associate to each other to form larger (WO2(SO4)3)m4m- units. The Raman spectra of WO3K2S2O7-K2SO4 molten mixtures with formal WO3 contents XWO30 = 0.20, 0.25, and 0.33 and Y = n(SO24 )/n(W) = 1 are compiled in Figure 7. It is seen that with increasing XWO30 the bands due to the W oxosulfato complex increase in intensity relative to the respective “solvent” bands due to S2O72- and SO42-. The Raman band wavenumbers observed for the WO3K2S2O7-K2SO4 molten mixtures are compiled in Table 2. The vibrational properties and band assignments will be discussed below, in conjunction with the proposed structural configurations. Structural Models and Band Assignments for WO3K2S2O7-K2SO4 Molten Mixtures. A simple and “balanced” structural configuration for the WO2(SO4)34- unit that invokes a six-fold coordination around the W atom and results in a (distorted) WO6 octahedron is depicted in Figure 8A. One apical and one equatorial position of the octahedron are occupied by the oxide ligands of the bent W(dO)2 unit, whereas the remaining four positions are occupied by one bidentate chelating and two unidentate sulfates. As mentioned above, with increasing formal content of WO3 the chelating sulfate can “open up”, to become unidentate, thereby enabling association of neighboring WO2(SO4)34- units and formation of oligomeric or polymeric chains. To interpret the spectral observations and propose a consistent structural configuration for the WVI oxosulfato 4220
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Figure 8. (A) A plausible structural model for the molten WO2(SO4)34- complex. (B) A dimeric arrangement of the [{WO2(SO4)2}2(μ-SO4)2]8- molten species generated by inversion symmetry of the WO2(SO4)34- moiety.
complexes formed in the ternary WO3-K2S2O7-K2SO4 molten mixtures, we note the existence of dimeric (WO2(SO4)3)28units in the unit cells of the crystalline compounds Rb8(W2O4(SO4)6)16 and K8(W2O4(SO4)6),17 formed by slow cooling of the respective ternary WO3-M2S2O7-M2SO4 (M = Rb, K) molten mixtures. The unit cells of both aforementioned crystalline compounds contain the dimeric [{WO2(SO4)2}2(μSO4)2]8- unit that is depicted schematically in Figure 8B. It is reasonable to expect that a corresponding anionic precursor will exist in the molten mixtures, slow cooling of which results in formation of the crystals. Notably, the envisaged association (dimerization) of monomeric WO2(SO4)34- units leading to formation of the dimeric [{WO2(SO4)2}2(μ-SO4)2]8- unit shown in Figure 8B could take place by means of a mechanism of inversion symmetry resulting in two bridging sulfato ligands, thereby assuring a saturation in the coordination sphere of W. Indeed, there appears no possibility for further association to generate a larger polymer unit, and therefore the structural model of the dimeric unit shown in Figure 8B is proposed as a limiting model for the complex species existing in the WO3-K2S2O7K2SO4 molten mixtures. After establishing preliminarily the structural configuration of the complex, we proceed by discussing the pertinent band assignments. First we note the existence of a strong polarized band at 933 cm-1 and a depolarized shoulder band at 909 cm-1 (the wavenumber is deduced from the depolarized spectra (HV) in Figures 4-6). Both bands are located in the WdO stretching region and are fully consistent with the characteristics of a νs/νas pair of a WO22þ unit (see Table 3 and discussion above). The 1049 cm-1 band is assigned to S-O stretching of the coordinated sulfate groups, whereas the rest of the bands are assigned to ν2, ν3, and ν4 sulfate split components, W-O(-S) and (OdWdO)bending modes as compiled in Table 2. At this point, it is of special interest to compare the vibrational properties of the dimeric [{WO2(SO4)2}2(μ-SO4)2]8- complex formed in the ternary WO3-K2S2O7-K2SO4 system with the respective properties of the WO2(SO4)22- complex formed in the binary WO3-K2S2O7 molten system. The discussion pertains to the main W(dO)2 and S-O stretching band wavenumbers and characteristics. The νs/νas pair of the W(dO)2 stretching modes is found at 933/909 cm-1 for the
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dimeric [{WO2(SO4)2}2(μ-SO4)2]8- unit, i.e., at lower wavenumbers compared to the 972/930 cm-1 counterparts measured for the WO2(SO4)22- complex. Both sets are fully consistent with the expected band wavenumbers as well as intensity and polarization characteristics for symmetric and antisymmetric W(dO)2 stretching modes. The coordination around the W atom in the [{WO2(SO4)2}2(μ-SO4)2]8- species (Figure 8B) is more congested (in total, four sulfate groups are linked to the W atoms through W-O-S bridging bands, compared to two sulfate groups coordinated to the W atom of the WO2(SO4)22complex; see Figure 3A) thereby resulting probably in longer WdO distances within the dioxo unit and justifying the lower band wavenumbers of the νs/νas modes compared to the respective counterparts of WO2(SO4)22- (933/909 vs 972/ 930 cm-1). Correspondingly, the S-O terminal stretching of [{WO2(SO4)2}2(μ-SO4)2]8- is found at 1049 cm-1, compared to the 1053 cm-1 value seen for WO2(SO4)22-. The occurrence of unidentate sulfate groups within the dimeric unit depicted in Figure 8B justifies the lower wavenumber position for the S-O stretching.
’ CONCLUSIONS Solid WO3 can be dissolved in considerable amounts in molten K2S2O7 at high temperatures (580-860 °C) under O2 atmosphere. The dissolution is facilitated when K2SO4 is also present in the molten mixtures. By means of high temperature Raman spectroscopy, it is shown that the dissolution of WO3 in K2S2O7 and K2S2O7-K2SO4 melts leads to formation of anionic WVI oxosulfato complexes. The Raman band intensities measured for the binary WO3-K2S2O7 molten system in the mole fraction range 0 < XWO30 < 0.33 can be exploited for determining the stoichiometry of the dissolution reaction, and in conjunction with the vibrational properties it is shown that the reaction taking place is WO3 þ S2 O7 2- f WO2 ðSO4 Þ2 2The WVI atom is surrounded by 6 O atoms in a distorted octahedral configuration. The arrangement around W involves a bent dioxo OdWdO unit (the presence of which is evidenced by the 972 cm-1 symmetric W(dO)2 stretching (νs) polarized band and the 930 cm-1 antisymmetric (νas) W(dO)2 depolarized band) and two bidentate chelating sulfates, exhibiting their ν1(SO4) band at 1053 cm-1 and a moderate splitting of the degenerate ν2, ν3, and ν4 sulfate modes. A systematic study of the Raman spectra obtained for the WO3-K2S2O7-K2SO4 molten mixtures shows that a different WVI oxosulfato complex is formed in the ternary system. The composition effects (variation of XWO30 and n(SO24 )/n(W)) on the Raman spectra are adequate for inferring the stoichiometry of the reaction taking place as WO3 þ S2 O7 2- þ SO4 2- f WO2 ðSO4 Þ3 4A dimeric configuration [{WO2(SO4)2}2(μ-SO4)2]8- resulting from inversion symmetry between two independent WO2(SO4)34- moieties is the most plausible one. The tungsten atoms are octahedrally coordinated with two oxide ligands (bent dioxo WO22þ unit), two unidentate sulfate groups, and two bidentate bridging sulfate groups. The νs/νas W(dO)22þ stretching counterpart bands occur at 933 (strong, polarized) and 909 (medium, depolarized) cm-1. Structural models and consistent band assignments are proposed for the WVI complexes. 4221
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Notes §
Visiting scientist at FORTH/ICE-HT.
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