Strategic valorization

As the only renewable source of aromatic compounds on Earth, lignin offers a potential route to a whole load of useful chemicals, especially being an abundant waste product of both the pulp and paper and biofuel industries. The problem has always been finding efficient, cost-effective ways to derive these useful chemicals from lignin, a process known as valorization, as lignin has proved highly resistant to both chemical and microbial attack. Not to be dissuaded, however, two research groups have recently come up with new strategies of attack: one chemical and one microbial.

The chemical strategy developed by Sandip Singh and Paresh Dhepe at CSIR - National Chemical Laboratory in Pune, India, is based on an ionic liquid, a salt that is liquid at room temperature, as ionic liquids have already shown great potential for breaking down plant biomass (see Together at last). Up to now, this has involved immersing the biomass in the ionic liquid. But that requires a lot of ionic liquid, which can be expensive, and also creates the problem of how to separate the breakdown products, such as cellulose, from the ionic liquid.

So when looking to employ ionic liquids specifically for lignin valorization, Singh and Dhepe decided to try creating a solid catalyst version, as this would use less ionic liquid and also be easier to separate from the soluble compounds produced by the breakdown of lignin. To produce such a solid catalyst, they took a Brønsted acidic ionic liquid, which previous studies had showed could break down lignin, and developed a way to immobilize it on a silica gel.

As they report in a paper in ChemistrySelect, the resultant solid catalyst could break down lignin into various soluble furan-based products, including syringyl, guaiacyl and p-hydroxyphenyl, with yields of up to 90%. Singh and Dhepe didn’t have any control over what products were produced by the catalyst, but their strategy clearly works and they can now try adapting it to produce specific chemicals from lignin.

The waste lignin generated by the biofuel industry has already been partly broken down, as a result of the need to treat plant biomass with heat and acids to release fermentable sugars. This produces a liquid hydrolysate containing various compounds derived from lignin, such as guaiacol, phenol, syringol and vanillin. Unfortunately, these compounds aren’t much use on their own and are also toxic to the yeast that produce ethanol from the released sugars in the hydrolysate. They can, however, potentially be converted into more useful and less toxic compounds.

For example, vanillin can theoretically be converted into catechol, a widely used intermediate chemical, although an effective process for doing this has been lacking. Now, Seema Singh and her colleagues at Sandia National Laboratories in the US have produced a genetically-engineered (GE) strain of Escherichia coli that can do this with great efficiency.

To produce this strain, they obviously needed to design and introduce a metabolic pathway for converting vanillin to catechol. Often, however, a GE strain will only start expressing the enzymes that make up a novel metabolic pathway when a chemical known as an inducer is added to the growth media, but these inducers can be expensive.

So Singh and her colleagues instead decided to introduce a gene for a promoter that only becomes active in the presence of vanillin and link it to the metabolic pathway. This meant that vanillin could now act as its own inducer, ensuring that the catechol-synthesis pathway only turned on in its presence. To speed up the conversion process, they also introduced a gene for a cellular transporter that works with aromatic compounds, which could actively transport vanillin inside the E. coli cell.

As they report in a paper in the Proceedings of the National Academy of Sciences, the resultant GE strain could readily convert vanillin to catechol. Although the conversion rate was quite low, Singh and her colleagues are confident that it could be increased by optimizing the metabolic pathway. Their work also raises the possibility of combining this GE strain of E. coli with yeast in a single bioreactor, allowing both ethanol and catechol to be produced from treated biomass.

What is more, the catechol could have more uses than just as an intermediate chemical. In a recent paper in Advanced Sustainable Systems, Swedish scientists reported developing a fuel cell that can generate electricity from lignin-derived catechol.

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.