Sulfato Complex Formation of V(V) and V(IV) in Pyrosulfate Melts Investigated by Potentiometry and Spectroscopic Methods

Abstract

By combined potentiometric, electron paramagnetic resonance (EPR), and visible/near-infrared spectroscopic measurements, the coordination of SO 4 2-to V(IV) and V(V) in M 2 S 2 O 7 -M 2 SO 4 -V 2 O 5 (M ) K and Cs) melts, respectively, under SO 2 (g) and O 2 (g) atmospheres at 450-470°C has been investigated. The results for both systems are in accordance with the oxo sulfato vanadate equilibria VO(SO 4 ) 2 2-+ SO 4 2-a VO(SO 4 ) 3 4-for V(IV), (VO) 2 O(SO 4 ) 4 4-+ 2SO 4 2-a 2VO 2 (SO 4 ) 2 3-+ S 2 O 7 2-for V(V), and 2VO 2 (SO 4 ) 2 3-+ SO 2 + SO 4 2-a 2VO(SO 4 ) 3 4-for the V(V)-V(IV) redox reaction in melts saturated with sulfate. Constants for these equilibria have also been obtained, as well as characteristic EPR parameters and molar absorptivities for the complexes. In addition, the mole fractions of M 2 SO 4 in the M 2 S 2 O 7 -M 2 SO 4 /SO 2 (g) systems saturated with M 2 SO 4 were found to be 0.050 19 for M ) K at 450°C and 0.070 02 for M ) Cs at 470°C, respectively.

Oxidation of sulfur dioxide over supported solid V2O5/SiO2 and supported molten salt V2O5?Cs2SO4/SiO2 catalysts: molecular structure and reactivity

“…Thus, with increasing extent of vanadium reduction, the combination of the band due to the V IV complexes with the envelope of the 942 cm −1 band results in a gradual blue shift of the band observed in this region. The possible V V → V IV reactions taking place in the molten phase can be written as [14,18,19] …”

“…Thus, precipitation of V IV and V III crystalline compounds is responsible for the catalyst deactivation, experienced typically below ∼ 440 • C and high catalytic activity is related to the ability of the catalyst to stabilize vanadium in the +5 oxidation state and maintain the V V ↔V IV equilibrium shifted to the left. The complex chemistry of V IV and the equilibrium V V ↔V IV in the molten salt-gas system V 2 O 5 -M 2 S 2 O 7 -M 2 SO 4 /SO 2 -O 2 -SO 3 -N 2 (M = K, Cs) has been studied by high-temperature VIS/NIR, Raman and ESR spectroscopies, and potentiometry [14,18,19]. Of particular interest has been the use of high-temperature Raman spectroscopy, which enabled for the first time the establishment of the molecular structure of vanadium oxosulfato complexes present in the unsupported V 2 O 5 -Cs 2 S 2 O 7 -Cs 2 SO 4 /O 2 [12], V 2 O 5 -M 2 SO 4 /O 2 (M = K or Cs) [13], and V 2 O 5 -M 2 S 2 O 7 -M 2 SO 4 /SO 2 -O 2 (M = K or Cs) [14] molten salt/gas systems.…”

Thermal, Conductivity, NMR, and Raman Spectroscopic Measurements and Phase Diagram of the Cs2S2O7−CsHSO4 System

“…Given the change in gas components, the catalyst solvent is best described by the model melt system M 2 S 2 O 7 /MHSO 4 , because the presence of water in the system leads to an equilibrium (eq 1) between H 2 O, S 2 O 7 2- , and HSO 4 - Previously, we have reported phase diagrams and properties for the binary systems M 2 S 2 O 7 −MHSO 4 (M = Na and K) and published several investigations regarding spectroscopic and thermodynamic properties of the pure components (see ref and references therein). Cs 2 S 2 O 7 has been thoroughly investigated by numerous methods especially in relation to its complex formation and mixing properties with vanadium pentoxide. − …”

Supported Liquid Catalysts

“…The presence of the dimeric complex (VO) 2 O(SO 4 ) 4− 4 in a large composition range of the binary catalyst model system M 2 S 2 O 7 -V 2 O 5 and in supported industrial catalysts has been confirmed by potentiometric [73] and Raman spectroscopic measurements [74], respectively. It should be noted that the vanadium complexes of the catalytic cycle remain in the oxidation state +V.…”