New Approach To Utilizing Lignin

The irregular and complex structure of the biopolymer lignin, which surrounds cellulose and hemicellulose in plant cell walls, presents a challenge in trying to use the material as a biobased chemical feedstock. In an effort to improve lignin processing, Alireza Rahimi, Shannon S. Stahl, and coworkers at the University of Wisconsin, Madison, have come up with a stepwise approach to selectively oxidize lignin’s alcohol groups and then selectively cleave C–C and C–O bonds in the oxidized product to obtain valuable small aromatic compounds (J. Am. Chem. Soc., DOI: 10.1021/ja401793n). Rahimi described using an acetamido derivative of the nitroxyl radical compound TEMPO as an organocatalyst, along with molecular oxygen and mineral acid cocatalysts, to convert select alcohol groups to carbonyl groups in a lignin model compound. The oxidation set up the model compound for a subsequent peroxide-based oxidation that chops it into aromatic bits. The initial oxidation step also works well on lignin itself, Rahimi said. The next step for the Wisconsin team will be to try the cleavage step on oxidized lignin. They envision that the new process could serve as a platform for obtaining high yields of various aromatics from lignin by varying the reaction conditions of the stepwise oxidations.