Ultrasound and photo-assisted oxidation of toluene and benzyl alcohol with oxidovanadium(V) complexes

The aroylhydrazone Schiff bases 2-hydroxy-N'-(1-(pyrazin-2-yl)ethylidene)benzohydrazide and N,N'-bis(2-hydroxybenzylidene)oxalohydrazide, have been used to synthesize the dinuclear complexes [VO(HL1)(µ-O)]2 (1) and [{VO(OEt)(EtOH)}2(L2)] (2)·2 H2O. Complex 1 is newly synthesized and 2 was reported earlier. Structural characterization of 1 was accomplished by single crystal X-ray diffraction, elemental analysis, and IR spectroscopy. The catalytic performance of both vanadium complexes was investigated for the peroxidative oxidation of toluene and benzyl alcohol through advanced oxidation processes (AOPs). Besides the conventional heating (CONV), other energy inputs, i.e., ultrasound-assisted (US_PROBE and US_BATH) and photo-assisted (PHOTO) methods were explored. The effects of a series of parameters were studied to improve the catalytic reaction. For the peroxidative oxidation (H2O2, 30% aq.) of toluene for 1 h at room temperature, the oxygenated product yields followed the trend: US_PROBE (8.1%) > US_BATH (5.4%) > PHOTO (3.9%) > CONV (no oxygenated products). The effect of other energy inputs (besides conventional heating) in the formation of oxygenated products is even more pronounced in the peroxidative oxidation of benzyl alcohol carried out for 1 h at room temperature, with the following trend: US_PROBE (59.5%) > PHOTO (30.9%) US_BATH (21.5% of oxygenated products) > CONV (no oxygenated products detected). Complexes 1 and 2 are promising catalysts for the conversion of toluene and benzyl alcohol under very light reaction conditions when assisted by ultrasound irradiation, and this sonochemical approach is disclosed as a promising advanced oxidation process (AOP).