NARA is optimizing a conversion process that converts the carbohydrates in woody biomass into bio-jet fuel and other chemical products. Depending on the tree species, up to 60% of the dry weight of wood is carbohydrates. That leaves the remaining 40% of the woody biomass as a byproduct. In order to maximize the economic sustainability of a wood-to-biofuel industry, converting the byproduct into valuable commercial products is an important focus for the NARA project.
Much of that byproduct is lignin, a complex organic polymer. Lignin has been used for a variety of purposes. Many wood-products industries burn the lignin to reduce fuel and electricity expenses. Lignin is also used as a raw material for chemical production, as a dispersant, and as a road-dust suppressant. Research is underway to use lignin to make other products such as plastics, activated carbon and biofuels.
To fully realize lignin’s potential as a feedstock for a wide array of high-value products, abundant and chemically consistent lignin forms need to be made available.
NARA researchers recently published a peer-reviewed article that evaluates a novel method used to extract lignin from wood. This method uses deep eutectic solvents (DES), which effectively extract lignin at high purity.
This technique could deliver a low molecular weight lignin product that has high industrial potential. This work is the first detailed evaluation using DES to extract lignin from wood.
DES is a mixture of two or more chemicals. One chemical is a hydrogen acceptor and the other functions as a hydrogen donor. For this experiment, four chemical combinations were tested. Choline Chloride (ChCl) acted as the hydrogen acceptor and was combined with one of four hydrogen donor chemicals: acetic acid (Aa), lactic acid (lac), levulinic (lev) and glycerol (gly). The DES mixtures were applied to Douglas-fir (softwood) and poplar (hardwood) samples at set temperatures ranging from 90°C to 180°C.
The lignin was recovered after treatment, quantified, and the structural parameters were determined. The potential mechanisms involved during the extraction process are described.
All of the DES mixtures, except ChCl-Gly, could extract lignin (and hemicellulose) from both wood samples. The amount of lignin extracted reached a maximum at 140°C with ChCl-Lac providing the highest lignin extraction yield of 78.5% for poplar and 58% for Douglas-fir.
The purity of the lignin produced by the described DES procedure is 95%, which is very pure among available lignin products. The DES lignin represents a new form of lignin. It has a lower and more narrowly distributed molecular weight compared to other available lignin forms and no ether linkages.
The authors conclude their work by stating, “ The findings from this study may provide a breakthrough toward truly realizing the high value potential of lignin”.