U Wisc. scientists develop new method to convert lignin to simple chemicals under mild conditions

3 November 2014

Researchers at the University of Wisconsin have disclosed a new method to convert lignin, an important component of biomass waste, into simple chemicals. Lignin, which accounts for nearly 30% of the organic carbon in the biosphere, is a complex material containing chains of six-carbon rings. These aromatics could be the basis for a sustainable supply of useful chemicals, but only if the chains of lignin can be broken down into the individual units. Lignin, however, is highly resistant to breakdown, especially in a cost-effective way.

Prof. Shannon Stahl and his colleagues developed, in work funded by the Great Lakes Bioenergy Research Center at UW-Madison, a method for the depolymerization of oxidized lignin under mild conditions in aqueous formic acid that results in more than 60 wt% yield of low-molecular-mass aromatics. A paper on the method is published in the journal Nature.

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The initial oxygen treatment step (producing the oxidized lignin) was reported by Rahimi and Stahl last year. Rahimi explored many different approaches to break down the lignin. Thee team was trying various metals under acidic conditions, when it discovered that acid without metals gave the best result.

The oxidation step weakens the links in the lignin chains. The acid then breaks the links. Under these conditions, the aromatics formed in significantly higher yields than anyone has observed previously.

Any process that competes in industry must be economical, and Stahl says avoiding metals in the process is one of several advantages.

The mild conditions, with relatively low temperatures (110 degrees Celsius/230 degrees Fahrenheit) and low pressures, as well as the lack of need for expensive metal catalysts, makes it different from many other approaches.

Josh Coon, a professor of chemistry and co-author, and graduate student Arne Ulbrich showed that the resulting product mixture closely matches the distribution of subunits in the natural lignin.

Stahl sees lignin as a key to future biorefineries that would use renewable biomass rather than petroleum as the feedstock to produce fuels or chemicals while reducing environmental impact.

Lignin is the only large volume renewable feedstock that contains aromatics. Aromatics are used to make many things, from plastic soda bottles to Kevlar to pesticides and pharmaceuticals. Today, the aromatics are almost exclusively derived from petroleum. We need to find an economical way to convert lignin to value-added materials. Most of the focus in this field has been on cellulose, but I don’t think there will be sufficient value to compete with petroleum unless we can generate value from lignin, too.

The Wisconsin Alumni Research Foundation has filed a patent application on the process.

Resources

• Alireza Rahimi, Arne Ulbrich, Joshua J. Coon Shannon S. Stahl (2014) “Formic-acid-induced depolymerization of oxidized lignin to aromatics,” Nature doi: 10.1038/nature13867