Abstract
This chapter describes supercritical SO2 for the preparation of sulfur dioxide complexes and for the recovery of precious metals from used catalysts. Reaction of the bis(hydrogensulfito) complex [Ru(SO3H)2(NH3)4] with anhydrous sulfuric acid (molar ratio 1:2) in supercritical SO2 yielded the bis(sulfur-dioxide) complex [Ru(SO2)2(NH3)4](HSO4)2, as shown by analytical data and IR spectra. A similar reaction with molar ratio 1:1 seemingly yielded the compounds [Ru(SO3H)(SO2)(NH3)4](HSO4) and [Ru(SO3H)(SO2)(NH3)4]2SO4 as a mixture, and these results have to be confirmed further. As a result, a way is opened to prepare oligosulfur-dioxide complexes from oligosulfito complexes of platinum-group metals through elimination of H2O from hydrogensulfito complexes obtained by protonation with strong acids such as HBF4, HPF6, H2SiF6, or the like. It is found that treatment of used catalytic converters with supercritical sulfur dioxide, also in presence of oxidants such as SO3 or SO2Cl2 and with prolonged reaction time, does not improve the extractability of the catalytically active metal.
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