In this work, the Co(II)-catalyzed oxidations of toluene and p-toluic acid have been carried out under mild conditions. A synergistic effect was found by using zirconium as a cocatalyst and ketones as promoters. Carbon dioxide, an alternative diluent gas to nitrogen, has a strong activation effect on the oxidation, especially on shortening the induction period. Under certain conditions, the reaction did not work at all with N-2/O-2 but proceeded effectively With CO2/O-2, affording high yields of benzoic acid.