Green Catalytic Conversion of Some Benzylic Alcohols to Acids by NiO₂ Nanoparticles (NPNPs) in Water

The aqueous basic systems NiSO4 center dot 6H(2)O/K2S2O8 (pH = 14) and NiSO4 center dot 6H(2)O/KBrO3 (pH = 11.5) were investigated for the catalytic conversion of benzyl alcohol and some para-substituted benzyl alcohols to their corresponding acids in 75-97% yields at room temperature. The active species was isolated and characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction, EDX and FT-IR techniques, and identified as comprising NiO2 nanoparticles (NPNPs). The SEM and TEM images of the Ni peroxide samples showed a fine spherical-like aggregation of NiO2 molecules with a nearly homogeneous partial size and confirmed the aggregation's size to be in the range of 2-3 nm. The yields, turn over (TO) and turn over frequencies (TOF) were calculated. It was noticed that the aromatic alcohols containing para-substituted electron donation groups gave better yields than those having electron withdrawing groups. The optimum conditions for this catalytic reaction were studied using benzyl alcohol as a model. The mechanism of the catalytic conversion reaction was suggested, in which the produced NPNPs convert alcohols to acids in two steps through the formation of the corresponding aldehyde. The produced NiO because of this conversion is converted again to NPNPs by the excess of K2S2O8 or KBrO3. This catalytic cycle continues until all of the substrate is oxidized.