Molecular structure of supported molten salt catalysts for SO2 oxidation

In situ Raman spectroscopy has been used at 350–480 °C in various SO2/O2/H2O/N2 atmospheres to study the molecular structure of the vanadium complexes present in the active liquid phase of industrial supported molten salt sulfuric acid catalysts. The catalytic behavior and deactivation of the studied catalysts has been followed by separate activity measurements. Under oxidizing conditions (O2, 480 °C) the surface of the catalysts is covered by a molten salt in which the mononuclear VVO2(SO4)23− complex (in monomeric or oligomeric form) is predominant. Crystalline sulfate salts are also detected. After being subjected to activation (i.e., in an SO2/O2/N2 atmosphere at 480 °C), the catalysts take up SO3 and thereby crystalline sulfate is converted to molten pyrosulfate; the molten phase of the catalysts is shown to consist of (VVO)2O(SO4)44− (dimeric or binuclear fragments of oligomers) and VVO2(SO4)23−. Below a certain temperature, which strongly depends on catalyst composition, the Raman data are indicative of VV→VIV reduction and formation of the molten VIVO(SO4)22− complex, the accumulation of which results in precipitation of VIV crystalline compounds—mainly K4(VO)3(SO4)5—and depletion of the active phase in terms of vanadium. In reducing conditions (i.e., in SO2/N2 atmosphere) the VV→VIV reduction and VIV precipitation occur at higher temperature. The low-temperature (i.e., below 420 °C) catalytic activity is related to the stability of vanadium in the +5 state. Mixing of alkali promoters with the inclusion of Cs and/or the presence of H2O vapors in the feed gas contributes to the ability of the catalysts to stabilize the pentavalent vanadium complexes by preventing their reduction. The molecular structure of the components constituting the active liquid phase of industrial SO2 oxidation catalysts is studied by in situ Raman spectroscopy for the first time.