Finding the value within lignin

If cellulosic biofuels ever really take off, the world is quickly going to be awash in lignin: for example, meeting the US target for producing 79 billion liters of cellulosic ethanol in 2022 will result in the generation of around 62 million tons of lignin. At the moment, lignin is mainly burnt for heat, but this is a waste of a potentially valuable resource that can theoretically be converted into whole range of useful chemicals. These could provide an additional revenue stream to improve the economics of cellulosic biofuels, helping them to take off in the first place.

The problem is that turning theory into practice has turned out to be quite difficult. There are two basic approaches to breaking down lignin, a complex phenolic polymer, into useful chemicals: chemical and biological. The chemical approach involves treating the lignin with harsh chemicals such as strong acids, often over expensive catalysts. It’s fast, taking just hours, but it tends to break the lignin down into many different compounds, only some of which are useful.

"You get a little bit of whole lot of various chemicals when you break down lignin this way," says Seema Singh, a bioengineer at Sandia National Laboratory, US. "The quantities yielded are not terribly useful."

The biological approach involves breaking down the lignin with microbes, which are often genetically engineered. This approach can be more specific, as the microbes are usually engineered to favour the production of specific compounds, but it is much slower, with the whole process taking weeks or months.

Now, though, scientists are beginning to try out variations on these approaches. One is to combine the chemical and biological approaches, in order to obtain the benefits of both. This is what Singh, together with colleagues at Sandia and the Joint BioEnergy Institute, US, is doing. They start by breaking down lignin with a weak solution of hydrogen peroxide, which quickly converts the lignin into vanillin and syringate as the major products, without producing too much unwanted by-products.

Next, they feed this vanillin and syringate to a genetically-engineered strain of Escherichia coli containing genes from various different bacteria, including a species known to be able to break down vanillin and syringate. The end result is a strain able to convert vanillin into muconic acid, an intermediate chemical in the production of nylon, plastics, resins and lubricants, and syringate into pyrogallol, an intermediate in the production of certain antibiotics and insecticides. According to Singh, these chemicals have a combined market value of $256 billion.

As Singh and her colleagues report in a paper in Science Advances, that was just the start, because they also tried another novel approach, which did away with the chemical step entirely. This involves genetically engineering the plant feedstock, which in this case was tobacco, to modify the metabolic pathway that normally synthesises phenylalanine, a precursor of lignin, so that it instead produces a compound known as protocatechuate (PCA).

After extracting PCA from tobacco leaves with methanol, they feed it to their genetically-engineered E. coli, which converts it to muconic acid. Because the strain is now only producing muconic acid, it does so at a higher yield than possible with the combination of the chemical and biological approaches.

"We basically skipped three quarters of the steps we were doing previously by engineering the plant to grow intermediate chemicals," says Singh. "PCA can be easily extracted from the modified tobacco and converted into muconic acid with little effort."

In contrast, Igor Slowing and his colleagues at Ames Laboratory, US, are focusing on improving the chemical approach. They have developed a catalyst that can efficiently break down lignin at low temperatures and pressures in the presence of propanol, without the need for any strong acids. This catalyst is made from ceria and palladium doped with sodium, and in early tests has proved able to convert phenol, a simple version of the monomers making up lignin, into nylon precursors. Slowing and his colleagues are now investigating whether the catalyst, which is described in papers in the Journal of Materials Chemistry A and the Journal of Catalysis, can do the same with lignin. 

The views represented here are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. or of the SCI.