Reaction & Synthesis

Method Yields Aromatics from Lignin

Process transforms waste material into source of valuable feedstocks

By Chemical Processing Staff

A potentially low-cost method that converts lignin into aromatics could replace petroleum-based fuels and chemicals with biorenewable materials, say researchers at the University of Wisconsin-Madison (UW).

Lignin, a large-volume biomass feedstock that’s typically burned as a low-value fuel, is a complex material rich in aromatics. However, freeing the aromatics has been difficult and costly.

Now, researchers show that high yields of the aromatics (60%) may be obtained by exposing lignin to oxygen then treating it with a weak acid at mild conditions — relatively low temperatures (110°C/230°F) and low pressures. The process also is more economical because it avoids the use of metals.

“The oxidation step weakens the links in the lignin chains,” notes Alireza Rahimi, a UW postdoctoral researcher who worked on the project. “The acid then breaks the links.”
“What’s noteworthy from our study is the rigorous identification of the components of this mixture of aromatics. We have a very clear understanding of what the compounds are in that mixture. I think this is an ideal starting point for the development of new catalytic technology for various transformations of these types of materials,” says Shannon Stahl, professor at UW and lead researcher on the project.

“…if you can take specific components, the low molecular aromatics we generate, and incorporate those in a regular pattern into a polyester or other material, now you’re talking about an opportunity to discover new materials with useful applications,” adds Stahl.

More details appear in an article in a recent issue of the journal Nature.

The process still needs tweaking, notes Stahl. The oxidation step works on laboratory-derived lignin and its success depends on the lignin structure. In real-world applications, the step must work on a large variety of lignins. To address this, the team is aiming to develop a general catalyst that works with all classes of lignin.

“It’s hard to predict from our process to commercialization,” notes Stahl. “We made a major breakthrough, but … there’s still more to discover; for example, our project right now relies on an oxidation step and a depolyermization step. The oxidation step right now I would say is too costly. And the process[es] for the oxidation and depolymrization are not perfectly integrated. One works with organic solvents and one works with aqueous media. That will change going forward. Part of the development process is still at the level of new fundamental discovery.”

Stahl is hoping to start collaborating with partners and begin efforts towards accessing scalable lignin sources. “We have preliminary collaborations starting with other academic groups that can supply us with lignin…these are processes that hold promise for scalability but I would say they’re not yet a process lignin variety. It’s still at an academic level stage where essentially we’re going to have to reinvent the biomass industry to some extent, because if we we’re to take the average lignin that’s coming out of existing processes,  the lignin would be so badly damaged that it wouldn’t resemble native lignin,” he notes.

“… We would like to find engineers or other experts in complementary disciplines who would like to work with us in moving this process forward. Process development work is not something we’re particularly equipped to do. We have some collaborations in place, but in thinking of commercialization, we have not yet moved towards this pilot-scale type application.”
Stahl anticipates it’ll take at least five years to complete this next stage of development.