Sustainable Oxidation of Cyclohexane and Toluene in the Presence of Affordable Catalysts: Impact of the Tandem of Promoter/Oxidant on Process Efficiency

The oxygenation of cyclohexane and toluene by O-2 and H2O2 catalyzed by VO(acac)(2) and Co(acac)(2) was studied at 40-100 degrees C and 1-10 atm. Upon such conditions, the process can be remarkably (30x times) enhanced by the minute (6-15 mM) additives of oxalic acid (OxalH) or N-hydroxyphthalimide (NHPI). The revealed effect of OxalH on H2O2-piloted oxidation is closely associated with the nature of the catalyst cation and boosted by VO(acac)(2). Whereas the effectiveness of Co(acac)(2)-based systems was curbed by the addition of OxalH and remained much below the one displayed with the previous system. The observed conspicuous difference in activity was attributed to the substantially higher solubility of in situ formed VO(IV)oxalate compared to that of Co(II)oxalate. The exploration of H2O2 for the NHPI-promoted process leads to the decisively lower (5-7 times) yield in comparison to the O-2-driven reaction. Similarly, for the O-2-operated protocol, the yield cannot be improved by addition of OxalH either to VO(acac)(2) + NHPI or to Co(acac)(2) + NHPI mixture. By contrast, the combination of NHPI with VO(acac)(2) or Co(acac)(2) and particularly with the above two mixtures in O-2-piloted oxidation enhances the yield of the aimed products 3-6 times regardless of the substrate used. The revealed significant synergetic effect of the cobalt + vanadyl bicomponent catalyst was due to the participation of each of its moiety in the different stages of the process mechanism. Only benzyl alcohol and benzaldehyde were identified in VO(acac)(2)- or Co(acac)(2)-catalyzed toluene oxidation, while cyclohexane oxidation yields cyclohexylhydroperoxide in line with cyclohexanol and cyclohexanone. The putative mechanism of investigated processes is highlighted and discussed.