Direct synthesis of adipic acid by mono-persulfate oxidation of cyclohexane, cyclohexanone or cyclohexanol catalyzed by water-soluble transition-metal complexes

Abstract

A catalytic system consisting of water-soluble metal sulfophthalocyanines (MPcS) or various ruthenium complexes and mono-persulfate as the oxidant was effective in the oxidation of cyclohexanone, cyclohexanol and cyclohexane to adipic acid with different yields and selectivity. Oxidations were conducted at room temperature and under atmospheric pressure in aqueous media (or, in the case of cyclohexane, in a water–neat substrate double phase). The oxidation of cyclohexanol involved step-by-step formation of cyclohexanone, ε-caprolactone and 6-hydroxyhexanoic acid, all of which have been identified in the reaction mixtures; in selected cases moderate over-oxidation of adipic acid to glutaric and succinic acid was also observed. Various MPcS catalysts were examined (M = Fe, Co, Ni, Cu and Ru), and the ruthenium derivative exhibited the best performances in terms of rate and selectivity. Mono-persulfate was found to be a more convenient oxidizing reagent than hydrogen peroxide; related patterns were observed when H₂O₂ was used, however extended dismutation of the oxidant limited the overall yields. Cyclohexane underwent slow oxidation when reacted with persulfate (water–substrate double phase) in the presence of the water-soluble metal catalysts; adipic acid was selectively produced (95%) in the presence of RuPcS catalyst with yields as high as 21% (48 h). The catalytic performance of simpler ruthenium derivatives, such as [RuCl₂(DMSO)₄] (RuDMS) and K₅[Ru(H₂O)P₁₁O₃₉] (RuPW), was also examined for comparison purposes. A kinetic scheme for cyclohexane oxidation is proposed.