Stoichiometry, Vibrational Modes, and Structure of Niobium(V) Oxosulfato Complexes in the Molten Nb₂O₅−K₂S₂O₇−K₂SO₄ System Studied by Raman Spectroscopy

Abstract

The structural and vibrational properties of NbV oxosulfato complexes formed in Nb2O5-K2S2O7 and Nb2O5-K2S2O7-K2SO4 molten mixtures with 0 <or= XNb2O50 <or= 0.25 have been studied by high-temperature Raman spectroscopy under static equilibtrium at temperatures up to 700 degrees C. The spectral features for the binary Nb2O5-K2S2O7 molten system indicate that the dissolution of Nb2O5 proceeds with consumption of S2O72- leading to the formation of a NbV oxosulfato complex according to Nb2O5 + nS2O72- --> C2n-; a simple formalism exploiting the relative Raman band intensities is used for determining the stoichiometric coefficient, n, pointing to n = 3 and to the following reaction: Nb2O5 + 3S2O72- --> 2NbO(SO4)33-, which is consistent with the Raman spectra of the molten mixtures. Nb2O5 could be dissolved much easier when K2SO4 was present in an equimolar (1:1) SO42-/Nb ratio; the incremental presence of K2SO4 in Nb2O5-K2S2O7 melts induces composition effects in the Raman spectra that terminate when n(SO42-)/n(Nb) = 1. The composition effects and the temperature-dependent features of the Raman spectra obtained for Nb2O5-K2S2O7-K2SO4 molten mixtures together with the spectral changes occurring upon freezing are accounted for by a Nb2O5.3K2S2O7.2K2SO4 stoichiometry for the complete reaction taking place: Nb2O5 + 3S2O72- + 2SO42- --> NbO(SO4)4S2O77- + NbO2(SO4)23-. The spectral data are discussed in terms of the most plausible structural models, for which consistent band assignments are made. The most characteristic Raman bands for the NbV oxosulfato complexes pertain to Nb=O modes: (i) at 937 cm-1 for the mono-oxo Nb=O mode of NbO(SO4)33-; (ii) at 958 cm-1 for the mono-oxo Nb=O mode of NbO(SO4)4S2O77-; and (iii) at 926 cm-1 for the symmetric dioxo Nb(=O)2 mode of NbO2(SO4)23-.

Raman Spectroscopic Study of Tungsten(VI) Oxosulfato Complexes in WO3−K2S2O7−K2SO4 Molten Mixtures: Stoichiometry, Vibrational Properties, and Molecular Structure

“…The assignment of the 972 cm −1 polarized band and of the 930 cm −1 depolarized band to the symmetric ν s and antisymmetric ν as W(O) 2 stretching modes of a dioxo WO 2 2+ unit and the existence of one single band (1053 cm −1 ) in the S−O terminal stretching region is supportive of the above postulation that points to formation of one complex species consisting of WO 2 2+ and SO 4 2− 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 . ,, Tungsten oxide, WO 3 , is assumed to be completely consumed during the dissolution reaction 1; hence, the equilibrium mixture consists of the complex species, C 2 n − , and S 2 O 7 2− . The theoretical principles of the method, the various specific experimental procedures required, the limitations and the calculational systematics are described extensively in previous reports. ,, For brevity, it is sufficient to state that the measured integrated Raman intensity due to a vibrational fundamental ν( i ) of species j , I j ,ν( i ) , is related to the number of moles of species j contained in the scattering volume, N j , according to: I normalj , normalν false( i false) = A 1 f ( normalν false( i false) , T ) N normalj 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.…”

“…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, I j,ν(i) , is related to the number of moles of species j contained in the scattering volume, N j , according to: …”

Molybdenum(VI) Oxosulfato Complexes in MoO3–K2S2O7–K2SO4 Molten Mixtures: Stoichiometry, Vibrational Properties, and Molecular Structures

“…One bidentate chelate sulfate group is coordinated to the equatorial plane, and the other is coordinated to the remaining axial and equatorial vertexes of the octahedron. However, it is known that transition-metal oxosulfato complexes tend to associate to each other through bridging bidentate sulfate groups, forming chainlike or networklike polymeric anionic complexes. − ,,,− It is evident that the extent of association of MoO 2 (SO 4 ) 2 2– units is favored with increasing X MoO 3 0 . Figure B shows possible alternative structural models for the [MoO 2 (SO 4 ) 2 ] m 2 m – chains and/or networks.…”

“…Notably, the precipitation of V III , V IV , and V V crystalline salts has been shown to be responsible for the deactivation of the sulfuric acid catalyst, causing depletion of the active phase from its V content. 4,24−26 Although the type of complexes formed during the dissolution of molybdenum(VI) oxide in pure molten alkali pyrosulfate is not known, it is expected that Mo VI forms anionic sulfato and/or oxosulfato complexes in molten pyrosulfates in analogy to V V , 1−3 Nb V , 7 and W VI . 8 The present study is concerned with the structural and vibrational properties of the complexes formed in the molten binary MoO 3 −K 2 S 2 O 7 and ternary MoO 3 −K 2 S 2 O 7 −K 2 SO 4 systems.…”

“…The dissolution of metal oxides at moderate or elevated temperatures (such as in molten pyrosulfate) has drawn interest from the point of view of metal ore extraction and recovery of metal oxides (e.g., catalyst active phases consisting of V 2 O 5 , WO 3 , Nb 2 O 5 , MoO 3 , ZnO; catalyst carriers such as TiO 2 , ZrO 2 ). Thus, high-temperature Raman spectroscopy has been used for establishing the stoichiometry as well as the structural and vibrational properties of the metal oxosulfato complexes formed by dissolution of ZnO in molten K 2 S 2 O 7 and Na 2 S 2 O 7 and of Nb 2 O 5 and WO 3 in molten K 2 S 2 O 7 and K 2 S 2 O 7 –K 2 SO 4 . , In addition, by cooling the respective precursor melts under appropriate gas atmospheres, a large family of crystalline compounds has been synthesized and subjected to single-crystal X-ray analysis, thereby enabling the structural characterization of V III , − V IV , − V V , , Nb V , Ta V , W VI , − and Mo VI , crystalline sulfato or oxosulfato complexes. Notably, the precipitation of V III , V IV , and V V crystalline salts has been shown to be responsible for the deactivation of the sulfuric acid catalyst, causing depletion of the active phase from its V content. ,− …”