Turning down the heat

There are numerous ways to break down plant biomass and release its component sugars for conversion into fuels or chemicals, such as with steam, acids or bases. The problem with these different methods is that they tend to be quite expensive and energy-intensive, as the biomass needs be mixed with these substances at high temperatures for several hours. This has always made it difficult for fuels and chemicals derived from biomass to compete economically with those derived from petroleum.

Now, though, scientists are beginning to develop novel ways for breaking down biomass that can be performed at lower temperatures, making them much less energy-intensive and therefore cheaper. For example, two new methods developed by different research groups can break down wood and other biomass at temperatures of 80–85°C, compared with over 150°C for conventional methods.

One of the new methods was developed by a team of US engineers led by Basudeb Saha at the University of Delaware. As they report in a paper in ChemSusChem, their method involves exposing the biomass to a molten salt hydrate (MSH). In previous studies, MSHs have shown promise for converting cellulose into simple sugars. But Saha and his team have now found that an MSH comprising lithium bromide dissolved in aqueous sulfuric acid can both break down biomass, releasing the cellulose and hemicellulose from the tight embrace of lignin, and then convert the cellulose into simple sugars.

When Saha and his team heated poplar wood in this MSH at temperatures of just 85°C for one hour, they were able to release 90% of the sugars, in the form of the polysaccharides glucan and xylan, while also extracting the lignin as a solid. This just left the problem of how to separate the glucan and xylan from the MSH.

The solution Saha and his team came up with involved conducting a dehydration step, by adding aluminum chloride and the solvent ethyl acetate to the MSH. This converts the released glucan and xylan into furfural and hydroxymethylfurfural (HMF), which preferentially dissolve in the ethyl acetate. Because the ethyl acetate and MSH naturally form two immiscible phases, they can easily be separated from each other, allowing the furfural and HMF to be extracted and the MSH to be re-used. The furfural and HMF can then be chemically converted into various useful industrial chemicals.

What would be even better, though, is being able to extract the simple sugars themselves, as they would have a wider range of potential uses, including being fermented into bioethanol. In a recent paper in Science, a team of scientists from US and China, led by Junyong Zhu at the University of Wisconsin-Madison, reported a novel method for doing just that. This involves heating the biomass in a solution of p-toluenesulfonic acid (p-TsOH), a hydrotrope that can help water-repelling compounds such as lignin become soluble in water, at 80°C for just 20 minutes.

When Zhu and his team tried this with poplar wood, they found it broke down 90% of the wood into two fractions: a cellulose-rich water-insoluble fraction and an acid liquor stream containing mainly dissolved lignin. After extracting the cellulose-rich fraction by filtration, Zhu and his team showed that the cellulose in this fraction could be readily converted into simple sugars by enzymes. Alternatively, it could be used to produce paper or cellulose nanofibrils, say the scientists.

The dissolved lignin can then be extracted by simply diluting the p-TsOH solution, causing the lignin to precipitate out as nanoparticles that could be used as a feedstock for various chemicals (see Making sense of lignin). Finally, cooling the solution caused the p-TsOH to crystallize, allowing it to be extracted and then re-used for breaking down more biomass. And all this could be done without breaking too much of a sweat.

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.