Lignin is a highly abundant source of renewable carbon that could serve as a valuable source of biobased chemicals. However, its complex structural network of substituted aromatic rings connected by aliphatic side chains has hindered its broader use as a renewable raw material. If isolated, these aromatic ring structures could be used to make different value added chemicals and small molecules. The benzylic positions of lignin's polyaromatic network possess unique reactivity necessary for oxidative cleavage of the side chains and separation of aromatic rings. An iron-manganese catalyzed oxidation technique has been developed where molecular oxygen is used as the oxidant. Evaluation of a range of solvent systems and reaction conditions led to oxidation conditions able to convert lignin models to the corresponding carboxylic acids. Good to moderate yields were obtained in all cases, for example, the oxidation of 4-methoxyacetophenone gave 72% of 4-methoxycarboxylic acid. Further, this catalytic system is modified by incorporating a cobalt salt. The presence of cobalt in the catalytic system increases the overall yield in most cases. We have also examined the utility of this catalyst system for the conversion of organosolv lignin. HMQC and GC-MS analysis revealed that the oxidation of lignin using the iron, manganese, cobalt based catalytic system resulted in significant structural changes and production of low molecular weight materials. Once optimized, this catalytic oxidation technique may be useful for conversion of lignin to a family of useful aromatic compounds.

Metal catalyzed benzylic oxidation of lignin and lignin models with molecular oxygen

243rd National Meeting (2012)

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