A biomimetic approach to lignin degradation: Metalloporphyrins catalyzed oxidation of lignin and lignin model compounds

An overview of the state of the art for the use of synthetic metalloporphyrins in the catalytic oxidation of lignin and lignin model compounds is presented. The biomimetic oxidation of 5-5′ condensed and diphenylmethane lignin model compounds with several water soluble anionic and cationic iron and manganese porphyrins in the presence of hydrogen peroxide is described. Manganese porphyrins were found more effective in degrading lignin substructures than iron porphyrins. Among them the cationic manganese mesotetrakis(N-methyl-pyridinio) porphyrin pentaacetate [TPyMePMn(CH3COO)5], never used before in lignin oxidation, proved to be the best catalyst. The catalytic activity of porphyrins in hydrogen peroxide oxidation of residual kraft lignin was also investigated. TPyMePMn(CH3COO)5 was able to perform the most extensive degradation of the lignin structure, as demonstrated by the decrease of aliphatic hydroxyl groups and increase of carboxylic acids, as measured by quantitative 31P-NMR. No significant condensation reactions occurred during manganese porphyrin catalyzed oxidations of residual kraft lignin, while in the presence of iron porphyrins an increase of condensed moieties was detected.