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.
Experiment
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.
Results
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”.