(Invited) The Chemistry of the Sulfuric Acid Molten Salt Catalyst
Authors: Fehrmann Rasmus 
Published: November 2022

The most successful molten salt catalyst is undoubtedly the SO2 oxidation catalyst used for the production of sulfuric acid. Sulfuric acid is by far the most produced inorganic commodity chemical, with a world yearly production in 2021 of 250 Mtons, corresponding to a value of around 20 Billion USD.
The typical catalyst composition is reported as about 6.5% (w/w) V2O5 promoted by M2SO4 (with M being an alkali metal, mainly K, containing also Na and sometimes Cs) and supported on diatomaceous earth. BASF issued the first patent for this type of catalyst in 1914. For long time it was considered as a heterogeneous solid catalyst for the key proces of oxidation of SO2 to SO3 by air which by reaction with H2O forms sulfuric acid .
However, within the last 80 years evidence was found that the catalyst by uptake of the reaction product SO3 during operation at 400 - 600 °C became a molten salt mixture of alkali pyro- and hydrogen-sulfates wherein V2O5 dissolved. Thus, in the early 1980s it was recognized that the complex chemistry and compound formation of vanadium in the V2O5-M2S2O7-M2SO4-MHSO4 (M=alkali) molten salt system was key to the mechanistic understanding and systematic improvement of the working industrial catalyst.
The multiinstrumental research on the chemistry of the catalyst model system and operando investigations of the industrial catalyst, including thermal, spectroscopic and electrochemical methods, that have been applied for the last 40 years, will be reviewed here.
Of this the joint research with Marcelle Gaune-Escard and her group in Marseille, France has been concerned with the molten salts M2S2O7 and MHSO4, the binary molten salt systems M2S2O7–MHSO4, and the molten salt-gas systems M2S2O7–V2O5 and M2S2O7–M2SO4–V2O5 (M=Na, K, Rb, Cs) in O2, SO2, and Ar atmospheres. These catalyst model systems have been investigated by thermal methods such as calorimetry, differential enthalpic analysis, and differential scanning calorimetry. Fundamental thermodynamic data such as temperatures and molar heats of solid–solid transition and fusion, phase diagrams, heat capacities of solids and liquids, heat of mixing and heats of complex formation have been obtained.
Based on this accumulated research effort the mechanism of the catalytic reaction has been proposed around 20 years ago and it seems to be the state-of-the-art even today.
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