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Scientists Find Out a Method to Convert Lignin into Valuable Chemicals

Published on 2014-11-10. Author : SpecialChem

Scientists disclosed a new method to convert lignin, a biomass waste product, into simple chemicals. The innovation is an important step toward replacing petroleum-based fuels and chemicals with biorenewable materials, said Shannon Stahl, an expert in "green chemistry" at the University of Wisconsin-Madison.

Lignin is the substance that makes trees and cornstalks sturdy, and it accounts for nearly 30 percent of the organic carbon in the biosphere. Stahl, senior author of a new report in the journal Nature, noted that lignin is 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 explained.

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 be broken down into the individual units.

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

Unfortunately, lignin is highly resistant to breakdown into the valuable subunits, especially in a cost-effective way. "People are constantly reminding me of the old adage, 'You can make anything from lignin, except money,'" he quipped.

 In work funded by the Great Lakes Bioenergy Research Center at UW-Madison, Stahl and his colleagues show that high yields of the aromatics may be obtained by 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," noted Alireza Rahimi, a UW postdoctoral researcher and first author of the Nature paper. The initial oxygen treatment step was reported by Rahimi and Stahl last year. "The acid then breaks the links."

Rahimi said he explored many 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," noted Rahimi.

Any process that competes in industry must be economical, and Stahl said avoiding metals in the process is one of several advantages. "The mild condition, 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 conceded that 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 said.

He is referring to the work of collaborator Josh Coon, a professor of chemistry and co-author of the Nature report. "Lignin is a complex polymer, and we didn't know how easy it would be to identify the products of this process," said Coon.

Coon is an expert in analytical chemistry and mass spectrometry, a technique that can pinpoint the identity of specific compounds. He and graduate student Arne Ulbrich showed that the 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. "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 described in the report, which is the basis of a patent application filed through the Wisconsin Alumni Research Foundation, represents an important step toward that goal.

About University of Wisconsin–Madison


The University of Wisconsin–Madison (also known as University of Wisconsin, Wisconsin, or regionally as UW, UW–Madison, or Madison) is a selective public research university located in Madison, Wisconsin, United States. Founded when Wisconsin achieved statehood in 1848, UW–Madison is the official state university of Wisconsin, and the flagship campus of the University of Wisconsin System. It was the first public university established in Wisconsin and remains the oldest and largest public university in the state. It became a land-grant institution in 1866. 

Source: University of Wisconsin–Madison


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