Oxidation of Lignin Model Compounds Containing an α-Carbonyl Group and a Ring-conjugated Double-bond by Hydrogen Peroxide-UV Photolysis

Studies were carried out on oxidation of lignin model compounds containing an α-carbonyl groups and a ring-conjugated double bond by hydrogen peroxide/UV photolysis. The results showed that the reaction is strongly pH dependent. Under conditions studied, optimum disappearance rates of substrates were observed with molar ratio of substrate:hydrogen peroxide (1 : 8.8) at pH 10.6 with irradiation of 254 nm photons. Under these conditions, acetovanillone was completely degraded into non-aromatic products after 2 hrs of irradiation, maleic acid and fumaric acid were found in trace amount in the reaction products. Under the same reaction conditions, iraii.t-ferulic acid was appeared more stable than acetovanillone, trans-cis isomerization was taken place, and maleic acid, fumaric acid and 4-hydroxy-benzoic acid were also found in the reaction products. These results indicate that oxidative cleavages of both aromatic ring, Cα-Cβ double bond, and demethoxylation have occurred during the hydrogen peroxide/UV photolysis. The possible pathways for the degradation of the model compounds have been discussed.