New ‘green’ method to help upgrade biomass waste into valuable chemicals

UW-Madison chemists have identified an approach to use oxygen gas to convert lignin, a byproduct of biofuel production, into a form that could allow it to replace fossil fuels as a source of chemical feedstocks.

Lignin is a complex organic material found in trees and other plants and is associated with cellulose, the valuable plant matter used to make paper or biofuels. The complexity of lignin makes it difficult to convert into valuable products, says chemistry professor  Shannon Stahl. In the April 9 online edition of the Journal of the American Chemical Society, Stahl, post-doctoral associate Alireza Rahimi and coworkers describe a catalytic method for selective manipulation of lignin.

“Lignin is essentially a waste product of making biofuel, and it is usually just burned as a fuel or used as a low-cost additive in asphalt or concrete,” says Stahl. “If we could take this waste lignin and increase its value, we could improve the economics for biomass-derived fuels and provide a chemical feedstock that does not rely on fossil fuels.”

Chemical feedstocks are the raw materials in industrial processes that generate end products such as plastic. Catalysts — chemicals that accelerate chemical reactions, without being consumed themselves — are used across the chemical industry.

Use of lignin as a source of chemical feedstocks is part of “an emerging subfield of chemistry being built around biomass-derived chemicals,” Stahl says. “Rather than having tankers loaded with crude oil supplying the raw material for fuels, plastics and pharmaceuticals, we could start with renewable feedstocks.”

The research was performed in collaboration with John Ralph, a professor of biochemistry associated with the Great Lakes Bioenergy Research Center on the UW campus. The researchers have applied for a patent on the process, and patent rights have been assigned to the Wisconsin Alumni Research Foundation.

Chemists have taken lignin apart previously with oxidation, says Stahl. “But oxidation can be like hitting something with a sledgehammer, where you get shrapnel everywhere. You get some of what you want, but much of it ends up as unusable bits and pieces.” The focus of this work is to use selective catalysts, under very mild pressure and temperature. “It’s like using a scalpel and tweezers, rather than a sledgehammer, and getting exactly what we want from the molecule, and nothing else.

“Theoretically, the only byproduct of the process is water,” says Stahl, an expert in catalysis and green chemistry. “We are doing this very selectively, with a catalyst that is specific for a single structure on the lignin molecule.”

It’s not yet possible to know how the new intermediates would be used to make final products, Stahl admits. “The way the chemical industry is built, you take what nature gives you and find ways to convert it into interesting and important products that people want to buy. Ultimately, methods of this type can be used to limit our reliance on nonrenewable chemicals.”