Lignin, a component of plant cell walls, remains a significant barrier to efficiently transforming bioenergy crops into biofuels and bioproducts. Its complex structure and composition makes it notoriously difficult to process, so it often ends up being burned or going unused. 

Researchers at the Joint BioEnergy Institute (JBEI) are investigating how to add value to lignin, which would make biorefineries more profitable. One strategy is to break down lignin into molecules that microbes can eat and, through fermentation, convert into bio-based products. 

But when it comes to breaking down lignin, things get tricky. Lignin is made of a variety of individual units that don’t repeat uniformly throughout its structure. These individual units also bond together in many different ways. Often, approaches focus on deconstructing lignin by breaking these inter-unit bonds, but this leads to the creation of multiple different products.  

“You never know exactly what you have in the deconstructed stream,” said Hemant Choudhary, Director of Catalytic Lignin Depolymerization at JBEI. “And when you have this huge pool of products in small concentrations, it’s very expensive to separate them into something useful.” 

These molecules also usually aren’t bioavailable, meaning microbes don’t like to eat them, and they can sometimes even be toxic to microbes. 

In a new study, Choudhary’s team shifted their focus from breaking lignin’s inter-unit bonds to instead dismantling its carbon-based backbone. 

“When we do that, we are able to create simple, carbon-based small molecules that microbes are happy to eat,” Choudhary said. “Instead of making all these different kinds of molecules in an unpredictable way, we have an idea of what we are going to make each time. Essentially, we are streamlining the molecules we are making.” 

To do this, the researchers used a catalyst made with polyoxometalate (POM) salts, which caused a chemical reaction that added oxygen to lignin’s backbone, breaking down its core structure, instead of its inter-unit bonds. This resulted in the creation of a range of organic acids that can be used as a standardized carbon source material for subsequent biorefinery processes. 

This “lignin-first” strategy is a reversal from traditional approaches that prioritize extracting sugars from plants to feed to microbes, often treating lignin as a waste stream generated by this process. 

“Instead of considering lignin at the very end of the process, let’s think about lignin at the very beginning,” Choudhary said.  

Though this is a relatively new technology that JBEI is developing, Choudhary sees this research as revisiting the idea of how to make lignin deconstruction more predictable. This could unlock new opportunities in biomanufacturing and make biorefineries more economical. 

“I like to say that my work focuses on developing chemistries that enable biology, and this research is a perfect example of that,” he said. 

“Lignin is often viewed as the Gordian knot of the biofuels enterprise,” said Blake Simmons, Director of the Biological Systems and Engineering Division and JBEI’s Chief Science & Technology Officer and Vice President for Deconstruction. “This technology can potentially untie that knot by converting the lignin into bioavailable streams suitable for use within a biorefinery context.”