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New ionic liquid biomass pretreatment method eliminates need for enzymes, reduces water use

Simmons-biphasic
The new ionic liquid pretreatment of cellulosic biomass yields a biphasic solution, a lower phase rich in sugar and an upper phases rich in ionic liquid. (Courtesy of Simmons/JBEI) Click to enlarge.

Researchers at the US Department of Energy (DOE)’s Joint BioEnergy Institute (JBEI) have developed a new technique for pre-treating cellulosic biomass with ionic liquids that could reduce the cost of producing advanced biofuels from plant sugars.

This new technique requires none of the expensive enzymes used in previous ionic liquid pretreatments, and yields a biphasic solution—a lower phase rich in sugar and an upper phases rich in ionic liquid—making it easier to recover fuel sugars and recycle the ionic liquid. An open access paper describing this research has been published in the journal Biotechnology for Biofuels.

Most of our ionic liquid efforts at JBEI have focused on using enzymes to liberate fermentable sugars from lignocellulosic biomass after pretreatment, but with this new enzyme-free approach we use an acid as the catalyst for hydrolyzing biomass polysaccharides into a solution containing fermentable sugars.

We’re then able to separate the pretreatment solution into two phases, a sugar-rich water phase for recovery and a lignin-rich ionic liquid phase for recycling. As an added bonus, our new pretreatment technique uses a lot less water than previous pretreatments.

—Blake Simmons, corresponding author

Environmentally benign ionic liquids are used as green chemistry substitutes for volatile organic solvents. While showing great potential as a biomass pretreatment for dissolving lignocellulose and helping to hydrolyze the resulting aqueous solution into fuel sugars, the best of these ionic liquids so far have required the use of expensive enzymes.

Recent studies have shown that acid catalysts, such as hydrochloric or Brønsted, can effectively replace enzyme-based hydrolysis, but the subsequent separation of sugars and ionic liquids becomes a difficult and expensive problem can require the use of significant amounts of water.

Guided by molecular dynamics simulations carried out at DOE’s National Energy Research Scientific Computing Center (NERSC), Simmons and his colleagues at JBEI solved this problem by deploying the ionic liquid imidazolium chloride in tandem with an acid catalyst.

Imidazolium is the most effective known ionic liquid for breaking down lignocellulose and the chloride anion is amenable with the acid catalyst. The combination makes it easy to extract fermentable sugars that have been liberated from biomass and also easy to recover the ionic liquid for recycling. By eliminating the need for enzymes and decreasing the water consumption requirements of more traditional ionic liquid pretreatments we should be able to reduce the costs of sugar production from lignocellulose.

—Blake Simmons

Complete separation of the pretreatment solution into sugar-rich water and lignin-rich ionic liquid phases was attained with the addition to the solution of sodium hydroxide. The optimized sodium hydroxide concentration for both phase separation and sugar extraction was 15%, resulting in the recovery of maximum yields of 54% glucose and 88% xylose. The JBEI researchers believe these sugar yields can be increased by optimizing the process conditions and using more advanced methods of phase separation and sugar recovery.

After optimizing the process conditions, the next step will be to scale the process up to 100 liters, Simmons says.

Resources

  • Sun N, Liu H, Sathitsuksanoh N, Stavila V, Sawant M, Bonito A, Tran K, George A, Sale KL, Singh S, Simmons BA, Holmes BM (2013) Production and extraction of sugars from switchgrass hydrolyzed in ionic liquids. Biotechnol Biofuels. 6(1):39 doi: 10.1186/1754-6834-6-39

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