Is Lignin the Future for Cheaper Chemicals and Biofuel Feedstock?

25 September 2016

In recent weeks, actually in recent decades, the biofuel industry has come under pressure to prove that it is economically viable. Headlines such as “Biofuels ‘Irrational and worse than fossil fuels’” (BBC April 2013) and “Greens reconsider biofuels mandate” (Bloomberg July 2016) have come in a rush of negative stories as the biofuel industry strives to prove its value.

But a new wave of thinking is to turn the bulk waste of plant matter, lignin, (from the stalks, stems and dry, woody pulp from many crops) into a useful chemical feedstock.

Up until now the only way to get any value from this waste product was to burn it for heat or electricity production, but now a research team from Sandia National Laboratories has found a possible route to breaking open lignin so that it can be utilised for its chemical properties.

The study began as a way to make biofuels a more realistic solution to replace fossil fuels, but has since turned into a potential route for cheaper chemicals production. The researchers started by examining the biofuels business model, and realised that in many cases it simply wasn’t viable. Growing corn to produce ethanol is a highly technical and expensive process that uses a lot of land and requires months of growing to produce a product that, for the most part, will simply be burned.

The team then looked at the fossil fuel industry, and saw many similarities in its structure to the biofuel industry. Crude oil production is a highly technical and expensive process given that is extracted from miles underground (or even under oceans), it is expensive to refine and then often needs to be shipped (across continents) to its market, simply to be burned. The business model only really becomes effective once the specialty chemicals industry has extracted the useful feedstock from crude.

As the study’s principal investigator Seema Singh outlined, when she said, “Gasoline is a low-value, high-volume product. This is balanced by the high-value chemicals derived from about 6-10 percent of every barrel of oil.” Her team realised, that what the biofuels industry needs is a way to add value to its business model, and turning crop waste products into a chemical feedstock would definitely achieve that.

The discovery began with chemists wondering how to access the chemical properties of lignin, and as often happens, they found inspiration from nature. As Singh says, “We know that over a long period of time fungus and bacteria do eventually break down lignin. If we can understand this process, we can use what nature already knows for biofuel and chemical production from lignin.”

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As the online journal Science Daily reports, “Since bacteria are easier to engineer for industrial production of desired chemicals, the researchers focused on bacteria. The best candidate was Sphingobium, or SYK-6, found in the lignin-rich waste stream from wood pulp production.

SYK-6 was extremely intriguing because it only feeds on lignin. Microbes generally live off sugar, which is much easier to break down and extract energy from. Imagine a choice between eating a corn kernel or a corn husk.”

Using a method called metabolic flux analysis, the team was able to follow the route of carbon from the feedstock, and find out how SYK-6 metabolises lignin. In doing so, “The Sandia team’s paper reports the method used to decipher the metabolic pathway of SYK-6.”

The researchers published their results in the scientific journal Proceedings of the National Academy of Science, where they explained the rationale behind their study as follows;

In this study we combined the unique approaches of both chemical engineering and biology to gain a deeper understanding of the metabolism of a soil bacterium, Sphingobium sp. SYK-6, that enables it to survive on lignin-derived monomers and oligomers. Understanding the central metabolism of SYK-6 will enable researchers to redesign the metabolic pathways of Sphingobium sp. SYK-6 more effectively to provide a renewable route for the production of products currently sourced from petrochemicals.”

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Sandia National Laboratories researchers Arul Varman, left, and Seema Singh.

However, this discovery is an unfinished story, as Patti Koning makes clear when she explains on the Sandia National Laboratory website how Singh and her team are already working on ways to genetically engineer SYK-6, in order to “…stop its metabolic process at a point when platform chemicals can be extracted from the lignin.” She also points out that, “Another path would be to splice the genes responsible for the important desired metabolic process in SYK-6 onto a strong industrial host like E. coli to create a chassis for desired fuels and chemicals. Platform chemicals, which can be used to derive valuable chemicals like muconic acid and adipic acid, are the goal.”

Ultimately the discovery opens a path that may turn plant waste into a chemical feedstock that could produce high-value products.

“Lignin is an untapped resource,” said Singh. “But as a basis for high-value chemicals, it is of immense value. Those high-value chemicals can be the basis for polyurethane, nylon, and other bioplastics. Decoding SYK-6 metabolic pathway is providing a roadmap for lignin valorization.”

While the science for industrial chemical production from lignin may still be a long way off, it is an amazing breakthrough to have found a way to turn waste plant matter into chemical feedstock. Given the impact that the chemical industry has on modern lives, how different would the world be if high-value chemicals were produced from the wheat stalks grown in Africa, or the corn husks of Peru?

 

Photo credit: Pacific Northwest National Laboratory
Photo credit: Dino Vournas
Main credit: Mike Belliveau