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

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 condu cted at room temperature and under atmospheric pressure in aqueous media (o r, in the case of cyclohexane, in a water-neat substrate double phase). The oxidation of cyclohexanol involved step-by-step formation of cyclohexanone , epsilon -caprolactone and 6-hydroxyhexanoic acid, all of which have been identified in the reaction mixtures; in selected cases moderate over-oxidat ion 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 rutheniu m derivative exhibited the best performances in terms of rate and selectivi ty. Mono-persulfate was found to be a more convenient oxidizing reagent tha n hydrogen peroxide; related patterns were observed when H2O2 was used, how ever extended dismutation of the oxidant limited the overall yields. Cycloh exane underwent slow oxidation when reacted with persulfate (water-substrat e double phase) in the presence of the water-soluble metal catalysts; adipi c acid was selectively produced (95%) in the presence of RuPcS catalyst wit h yields as high as 21% (48 h). The catalytic performance of simpler ruthen ium derivatives, such as [RuCl2(DMSO)(4)] (RuDMS) and K-5[Ru(H2O)P11O39] (R uPW), was also examined for comparison purposes. A kinetic scheme for cyclo hexane oxidation is proposed.