Converting Crop Waste to Chemicals

Tuesday, November 4, 2014 @ 12:11 PM gHale

There is a new method to convert lignin, a biomass waste product, into simple chemicals, which could be the next step toward replacing petroleum-based fuels and chemicals with biorenewable materials, researchers said.

Lignin is the substance that makes trees and cornstalks sturdy, and it accounts for nearly 30 percent of the organic carbon in the biosphere, said Shannon Stahl, an expert in “green chemistry” at the University of Wisconsin-Madison.

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Lignin ends up being a waste product of the paper industry, where cellulose is the valuable product. “Lignin is burned as a low-value fuel, but if biofuels are to become a reality, we need to get more value from lignin,” he said.

Lignin is a complex material containing chains of six-carbon rings. These rings, called “aromatics,” could be the basis for a sustainable supply of useful chemicals — but only if the chains of lignin can end up being broken down into the individual units.

“Lignin is the only large volume renewable feedstock that contains aromatics,” Stahl said. “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.”

The catch is, though, lignin is resistant to breakdown into the valuable subunits, especially in a cost-effective way.

In work funded by the Great Lakes Bioenergy Research Center at UW-Madison, Stahl and his colleagues show high yields of the aromatics are possible from exposure of lignin to oxygen followed by treatment with a weak acid under mild conditions.

“The oxidation step weakens the links in the lignin chains,” said Alireza Rahimi, a UW postdoctoral researcher and first author of a paper on the subject. “The acid then breaks the links.”

Rahimi said he looked at different approaches to break down the lignin. “For example, hydrogen peroxide works, but it decomposes some of the aromatic products.”

They were trying various metals under acidic conditions, when they discovered that acid without metals gave the best result. “Under these conditions, the aromatics formed in significantly higher yields than anyone has observed previously,” Rahimi said.

Any process that competes in industry must be economical, and Stahl said 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.”

Stahl said the chemicals they obtain from their process still require further manipulation before they have real market value. “But we have a head start on this, because we know the main products that we’re making.”

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.

“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 process is going through a patent application filed through the Wisconsin Alumni Research Foundation.

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