Lignin as the second most abundant natural biopolymers is well-known for the difficulty to convert it into value-added chemicals. Most of the researches on lignin were focused on converting it into liquid hydrocarbons and aromatics via the thermal depolymerization and subsequent selective hydrodeoxygenation. However many of the valuable bulk chemicals are small oxygenate molecules such as aldehydes, diols, and carboxylic acids. Thus recently the catalytic conversion of lignin into valuable oxygenates under relatively mild conditions has attracted great attention. Catalytic oxidation of lignin is one of the most promising way to produce monophenols1, dicarboxylic acids2, and aromatic aldehydes3. However most of these catalytic oxidation processes used the expensive hydrogen peroxide as the oxidant. It is known that the oxidation occurred in a radical reaction pathway.2 The activation of oxygen and generation of peroxide radical could be critical for lignin oxidation. N-hydroxylphthalimide(NHPI) is known to generate phthalimide-N-Oxyl (PINO) and catalyzes the oxidation of liquid hydrocarbons with oxygen.
In this work, the lignin oxidation using oxygen as oxidant catalyzed by various substituted NHPI and certain transition metal cations were studied. A series of bifunctional metal ion-NHPI oxidation catalysts were designed and prepared. The synergy effect of the substituted NHPI and transition metal cations were examined as well as the effect of solvent, reaction temperature, the ratio of NHPI to the transition metal cation. The structure activity relationship between the properties of metallic ion, spatial proximity of both active centers and the catalytic activity and selectivity of the catalyst were carefully examined.
1. Ouyang, X.-p.; Tan, Y.-d.; Qiu, X.-q., Oxidative degradation of lignin for producing monophenolic compounds. Journal of Fuel Chemistry and Technology 2014, 42 (6), 677-682.
2. Ma, R.; Guo, M.; Zhang, X., Selective Conversion of Biorefinery Lignin into Dicarboxylic Acids. ChemSusChem 2014, 7 (2), 412-415.
3. Zhang, J.; Deng, H.; Lin, L., Wet Aerobic Oxidation of Lignin into Aromatic Aldehydes Catalysed by a Perovskite-type Oxide: LaFe1-xCuxO3 (x=0, 0.1, 0.2). Molecules 2009, 14 (8), 2747-2757.
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