Ionic liquids (ILs) have unique properties applicable to a variety of industrial processes. Nearly universal solvating capabilities, low vapor pressures, and high thermal stabilities make these compounds ideal substitutes for a wide range of organic solvents. To date, the best performing ILs are derived from nonrenewable sources such as petroleum or natural gas. Due to their potential for large-scale deployment, ILs derived from inexpensive, renewable reagents are highly desirable. Herein, we describe a process for synthesizing ILs from materials derived from lignin and hemicellulose, major components of terrestrial plant biomass. With respect to overall sugar yield, experimental evaluation of these compounds showed that they perform comparably to traditional ILs in biomass pretreatment.

—Socha et al.

What if we could turn what is now a bane to the bioenergy industry into a boon? Lignin is viewed as a waste stream that is typically burned to generate heat and electricity for the biorefinery, but if other uses for lignin could be found with higher economic value it would significantly improve the refinery’s overall economics.

—Blake Simmons

Simmons and Seema Singh, who directs JBEI’s biomass pretreatment program, are the corresponding authors of an open access paper describing this research in the Proceedings of the National Academy of Sciences (PNAS). The lead author is Aaron Socha. Other co-authors are Ramakrishnan Parthasarathi, Jian Shi, Sivakumar Pattathil, Dorian Whyte, Maxime Bergeron, Anthe George, Kim Tran, Vitalie Stavila, Sivasankari Venkatachalam and Michael Hahn.

The cellulosic sugars stored in the biomass of grasses and other non-food crops, and in agricultural waste, can be used to make advanced biofuels that could substantially reduce the use of the fossil fuels responsible for the release of nearly 9 billion metric tons of excess carbon into the atmosphere each year.

More than a billion tons of biomass are produced annually in the United States alone and fuels from this biomass could be renewable substitutes for gasoline, diesel and jet fuel on a gallon-for-gallon basis. Unlike ethanol, “drop-in” transportation fuels derived from biomass have the potential to be directly dropped into today’s engines and infrastructures at high levels—greater than 50%%without negatively impacting performance.

However, if biofuels, including cellulosic ethanol, are to be a commercial success, they must be cost-competitive with fossil fuels. This means economic technologies must be developed for extracting fermentable sugars from cellulosic biomass and synthesizing them into fuels and other valuable chemical products. A major challenge has been that unlike the simple sugars in corn grain, the complex polysaccharides in biomass are deeply embedded within a tough woody material called lignin.

Researchers at JBEI have been cost-effectively deconstructing biomass into fuel sugars by pre-treating the biomass with ionic liquids—salts that are composed entirely of paired ions and are liquid at room temperature. The ionic liquids that have emerged from this JBEI effort as a benchmark for biomass processing are imidazolium-based molten salts, which are made from nonrenewable sources such as petroleum or natural gas.

Imidazolium-based ionic liquids effectively and efficiently dissolve biomass, and represent a remarkable platform for biomass pretreatment, but imidazolium cations are expensive and thus limited in their large-scale industrial deployment. To replace them with a renewable product, we synthesized a series of tertiary amine-based ionic liquids from aromatic aldehydes in lignin and hemicellulose.

—Seema Singh

The JBEI researchers tested the effectiveness of their bionic liquids as a pre-treatment for biomass deconstruction on switchgrass, one of the leading potential crops for making liquid transportation fuels. After 73 hours of incubation with these new bionic liquids, sugar yields were between 90- and 95-percent for glucose, and between 70- and 75-percent for xylose. These yields are comparable to the yields obtained after pre-treatment with the best-performing imidazolium-based ionic liquids.

Lignin and hemicellulose are byproducts from the agricultural industry, biofuel plants and pulp mills, which not only makes these abundant polymers inexpensive, but also allows for a closed-loop bio-refinery, in which the lignin in the waste stream can be up-cycled and reused to make more bionic liquid.

—Lead author Aaron Socha, Director of the Center for Sustainable Energy at the Bronx Community College

The current batch of bionic liquids was made using reductive amination and phosphoric acid, but Socha says the research team is now investigating the use of alternative reducing agents and acids that would be less expensive and even more environmentally benign.

This research was supported by the DOE Office of Science.

Resources

• Aaron M. Socha, Ramakrishnan Parthasarathi, Jian Shi, Sivakumar Pattathil, Dorian Whyte, Maxime Bergeron, Anthe George, Kim Tran, Vitalie Stavila, Sivasankari Venkatachalam, Michael G. Hahn, Blake A. Simmons, and Seema Singh (2014) “Efficient biomass pretreatment using ionic liquids derived from lignin and hemicellulose,” PNAS doi: 10.1073/pnas.1405685111