The supercritical fluid (SCF) extraction technique was extended by its application to SCF-solid-catalyzed reactions that otherwise take place in a two-fluid-phase reactor. By use of CO2as a solvent, toluene was contacted with air in the presence of redox or acid catalysts and underwent partial oxidation to primarily benzaldehyde. A screening study of several oxide and mixed-metal oxide catalysts revealed that supported CoO, partly oxidized to Co(III), was the most active (10-5/s turnover number) and selective (no multiring condensation products) catalyst. The relative activity of the supported CoO catalyst was not unexpected, because the Co2+/Co3+redox couple is the most effective catalyst for this reaction in solution. The oxide was capable of duplicating the liquid-phase behavior, although at lower activity levels than promoted (for example, with Br- ions) homogeneous cobalt catalysts.
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Dooley et al. (1987) studied this question.