Formic-acid-induced depolymerization of oxidized lignin to aromatics

Abstract

Lignin is a heterogeneous aromatic biopolymer that accounts for nearly 30% of the organic carbon on Earth1 and is one of the few renewable sources of aromatic chemicals2. 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 mills4. Extraction of higher value from lignin is increasingly recognized as being crucial to the economic viability of integrated biorefineries5,6. 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 processing7. Commercial precedents show that certain types of lignin (lignosulphonates) may be converted into vanillin and other marketable products8,9, 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 reaction. The broader implications of these results for lignin conversion and biomass refining are also considered.

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Figure 1: Lignin structure and strategies for depolymerization.
Figure 2: C–O cleavage of lignin model compounds with formic acid.
Figure 3: Depolymerization of aspen lignin with formic acid.
Figure 4: Mechanistic study of C–O cleavage in 2.

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Acknowledgements

We thank J. Ralph for numerous helpful discussions, H. Kim and A. Azarpira for assistance with the purification and NMR characterization of aspen lignin, and S. Chakraborty for assistance with gel-permeation chromatographic analysis of lignin samples. Financial support for this project was provided by the Great Lakes Bioenergy Research Center (Department of Energy Biological and Environmental Research Office of Science DE-FC02-07ER64494). The NMR facility was partly supported by the National Science Foundation (CHE-9208463) and the National Institutes of Health (S10 RR08389).

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A.R. and S.S.S. conceived the idea for the lignin depolymerisation. A.R. performed the chemical reactions of lignin and lignin model compounds, including NMR spectroscopic characterization of the products. A.U. and J.J.C. designed the analytical approach to characterize the lignin depolymerization products. A.U. performed LC/MS analysis. A.R. and S.S.S. wrote the manuscript in consultation with A.U. and J.J.C.

Corresponding author

Correspondence to Shannon S. Stahl.

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A patent application based on the results reported here has been submitted.

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Rahimi, A., Ulbrich, A., Coon, J. et al. Formic-acid-induced depolymerization of oxidized lignin to aromatics. Nature 515, 249–252 (2014). https://doi.org/10.1038/nature13867

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