Formic-acid-induced depolymerization of oxidized lignin to aromatics

A method for the depolymerization of oxidized lignin under mild conditions in aqueous formic acid is described that results in more than 60 wt% yield of low-molecular-mass aromatics. Useful aromatics made simply from lignin The aromatic biopolymer lignin, a major component of plant cell walls and generally obtained from wood, is a valuable and renewable source of aromatic chemicals. Considerable progress has been made in the conversion of cellulose and hemicellulose to fuels and chemicals, but lignin has proved more recalcitrant. In this manuscript, the authors report a high-yield method for conversion of lignin to low molecular mass aromatics. The C–O cleavage reaction proceeds under mild conditions in aqueous formic acid. It produces small number of well-defined aromatic products, providing raw materials well suited for targeted conversion to a variety of valuable chemicals. Lignin is a heterogeneous aromatic biopolymer that accounts for nearly 30% of the organic carbon on Earth 1 and is one of the few renewable sources of aromatic chemicals 2 . As the most recalcitrant of the three components of lignocellulosic biomass (cellulose, hemicellulose and lignin) 3 , lignin has been treated as a waste product in the pulp and paper industry, where it is burned to supply energy and recover pulping chemicals in the operation of paper mills 4 . Extraction of higher value from lignin is increasingly recognized as being crucial to the economic viability of integrated biorefineries 5 , 6 . Depolymerization is an important starting point for many lignin valorization strategies, because it could generate valuable aromatic chemicals and/or provide a source of low-molecular-mass feedstocks suitable for downstream processing 7 . Commercial precedents show that certain types of lignin (lignosulphonates) may be converted into vanillin and other marketable products 8 , 9 , but new technologies are needed to enhance the lignin value chain. The complex, irregular structure of lignin complicates chemical conversion efforts, and known depolymerization methods typically afford ill-defined products in low yields (that is, less than 10–20wt%) 2 , 10 , 11 . Here we describe a method for the depolymerization of oxidized lignin under mild conditions in aqueous formic acid that results in more than 60wt% yield of low-molecular-mass aromatics. We present the discovery of this facile C–O cleavage method, its application to aspen lignin depolymerization, and mechanistic insights into the