Molybdenum- and tungsten-catalyzed reactions of acidic hydrogen peroxide with kraft lignin model compounds and enzymatically-liberated kraft lignin

For the purpose of evaluating the delignifying potential of acidic hydrogen peroxide, a representative selection of kraft lignin model compounds and enzymatically-liberated kraft lignin was reacted with acidic hydrogen peroxide in the presence of a metal catalyst (molybdenum or tungsten). Omission of the catalyst rendered the reagent unreactive towards the model compounds. Mo was a more effective catalyst than W. Phenols in which the phenolic hydroxyl group was etherified and ring-conjugated {dollar}alpha{dollar}-carbonyl structures were stable toward the metal-activated peroxide. Dimer structures showed higher reactivities than monomers and increased acidity of the peroxide solution decreased the reaction rate in all cases. The model compounds were demethoxylated and further oxidized to various quinonoid and hydroxylated structures. Only in the case of stilbenoid structures was there evidence of aromatic ring rupture leading to the formation of muconic acid derivatives. Side chain displacement occurred (20%) in benzyl alcohol structures and the olefinic bond in stilbene structures was cleaved. Evidence was obtained supporting the concept that diphenyl and dibenzyl ethers were formed in the reactions of monomer lignin model compounds and the molecular weight distribution of the material in the reaction mixtures indicated the formation of trimers and possibly low D.P. oligomers; The enzymatic kraft lignin was partially dissolved ({dollar}sim{dollar}6%) in the metal-activated acidic hydrogen peroxide treatment. Methanol constituted a major portion of the dissolved material. Mo proved a more effective catalyst than W and increased acidity had a beneficial effect on the reaction rate. Peroxide oxidation produced an increase in the carboxyl and carbonyl contents of the lignin and a decrease in the phenolic hydroxyl content. Following oxidation, the solubility of the lignin was drastically reduced suggesting the occurrence of inter- or intramolecular condensation reactions.