Breaking bonds

In theory, breaking down lignin into useful aromatic compounds shouldn’t be too difficult. Lignin is a polymer made up of three different aromatic monomers that are mainly held together by what are known as β-O-4 ether bonds, which can be cleaved by exposing the lignin to hydrogen over a metal catalyst, releasing the monomers. But while this process works well on model lignin compounds, it is much less effective on raw plant biomass, where it tends to produce a wide range of different compounds, only some of which are useful. In addition, the metal catalyst is often difficult to separate from the remaining cellulose and hemicellulose.

The process doesn’t even work well when biomass has first been broken down by acids to separate the lignin from the cellulose and hemicellulose. This acid treatment tends to alter the structure of the separated lignin, converting the β-O-4 bonds to carbon-carbon bonds, which are much more difficult to cleave.

Now, a team of chemists at Xiamen University in China, led by Ye Wang, has come up with a novel method for efficiently cleaving the β-O-4 bonds in lignin that works well on raw biomass. Not only does it produce useful aromatic compounds in high yields, but it also allows the hemicellulose and cellulose to be extracted from the biomass at high yields. Furthermore, when combined with the findings of another recent study, the method might offer a way to derive an even greater variety of useful compounds from plant biomass.

The centerpiece of this novel method are semiconducting nanocrystals known as quantum dots. Wang and his team were testing the catalytic ability of various nanoparticles for cleaving the β-O-4 bonds in model lignin compounds when irradiated with light, finding that cadmium sulfide (CdS) nanoparticles performed best. Knowing that quantum dots can also be made from CdS, Wang and his team decided to test their catalytic ability as well. They discovered that CdS quantum dots just 4nm in size were four times more effective at cleaving the bonds in the model compounds than the CdS nanoparticles.

So they next tested the quantum dots on raw biomass, specifically on birch wood. This involved first coating the quantum dots with 3-mercaptopropionic acid, to ensure they would disperse when added to a methanol solution rather than just clump together. Then the chemists exposed the birch wood to the quantum dot-containing methanol solution while irradiating it with light for around eight hours.

As they report in a paper in Nature Catalysis, the quantum dots were equally as effective at cleaving β-O-4 bonds in the birch wood as in the model lignin compounds. This resulted in 84% of the aromatic monomers being released from the wood, mainly as ketones, which are widely used in the production of pharmaceuticals and plastics.

After releasing the monomers, the quantum dots could easily be extracted from the solution by adding acetone, which caused them to clump together. This meant they could be reused multiple times. What is more, because the process is so gentle, the remaining hemicellulose and cellulose were not damaged, allowing them to be extracted. Using a mild acid treatment, Wang and his team found they could extract 84% of the hemicellulose in the birch wood as xylose, and then use enzymes to extract 91% of the cellulose as glucose.

Another challenge with lignin is that its composition can differ substantially between different types of plant biomass. So although β-O-4 bonds are the most prevalent in all types of lignin, their actual proportion can vary between 45% and 85%, meaning this new method may work better with some plants than others. In another recent study, however, US scientists found that the lignin in vanilla seeds, as well as in various related plants, contains almost 100% β-O-4 bonds (see Researchers find value in unusual type of plant material). This is because the lignin in vanilla seeds is a simplified form that consists almost entirely of catechyl monomers held together by β-O-4 bonds.

If the genes responsible for this simplified lignin could be transferred into energy grasses, then it should offer a highly efficient way to produce catechols, which are widely used in the production of pesticides, perfumes and pharmaceuticals. Especially when combined with a few CdS quantum dots.

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.