The use of ionic liquids (molten salts with melting points < 100 °C) as biomass solvents is an attractive alternative for the pretreatment of lignocellulosic biomass. Other work has shown that pretreatment with imidazolium-based ILs can greatly accelerate the subsequent enzymatic hydrolysis of biomass, or, alternatively, that ILs can be used with acid catlaysts to liberate the sugars without having to resort to enzymatic saccharification.

However, the separation of the sugars from the IL, as well as an effective means to recover and recycle the IL, also pose economic and sustainability challenges.

Rogers et al. reported that certain hydrophilic ILs could form an aqueous biphasic system (ABS) in the presence of concentrated kosmotropic salts. Subsequently, significant progress has been made that demonstrates the efficacy of this approach for separation of biomolecules, small organic molecules, biochemicals, and radiological isotopes.

...We describe a process that uses the phase separation behavior of imidazolium ILs/alkali/water solutions in tandem with acid catalyzed hydrolysis to extract the sugars liberated from biomass from the aqueous IL solutions. This approach offers the potential of reducing costs of sugar production from lignocellulose by eliminating the need for enzymes and decreasing the water consumption requirements of more traditional IL pretreatment approaches.

—Sun et al.

As reported in the paper, the process entails the mixing of milled switchgrass with the IL, followed by acid hydrolysis. This results in biomass sugars in a aqueous ionic liquid solution. The addition of NaOH (sodium hydroxide, an alkaline solution) induces the formation of the biphasic system: a bottom, sugar-rich phase, which is neutralized for fuel synthesis; and a top IL-rich phase which is recycled to the pretreatment system.

They found that the amount of sugar produced from this process was proportional to the extent of biomass solubilized. Pretreatment at high temperatures (e.g., 160 °C, 1.5 h) was more effective in producing glucose. Sugar extraction into the alkali phase was dependent on both the amount of sugar produced by acidolysis and the alkali concentration in the aqueous extractant phase.

Maximum yields of 53% glucose and 88% xylose were recovered in the alkali phase under the experimental conditions, based on the amounts present in the initial biomass.

It has been shown that certain concentrations of NaOH can phase separate with chloride based ILs...forming upper phase, IL rich and lower phase, alkaline rich. Both glucose and xylose prefer to partition to the alkaline rich phase. By combining this system with the acidolysis of biomass in IL, sugar monomers can be easily extracted from the aqueous ILs. The sugar yields depend on both the pretreatment conditions and alkali concentrations.

...Improved sugar yields could be achieved by further optimizing the amount of acid and water used in acidolysis step and the alkali salts used for sugar extraction. Molecular dynamics simulations can be used to predict sugar partitioning in the system, but the sugar partition coefficients are found to be affected by the presence of biomass.

—Sun et al.

The advantages of the process are, they summarized:

1. Sugars can be released in situ and extracted by alkaline solution with relatively high yields, and without the need for any enzymes;

2. The formation of an aqueous biphasic system enables facile recovery of the sugars and IL recovery at the same time; and

3. Significantly reduced volume of water (< 50 wt% of total mixture) is used as compared to more traditional IL-based pretreatment process.

The authors suggested that future research should be focused on recovery of the residual biomass in the IL rich phase; testing the IL recycling efficiency; and desalting of the alkaline rich phase to make it compatible with downstream fermentation microbes.

Resources

• Ning Sun, Hanbin Liu, Noppadon Sathitsuksanoh, Vitalie Stavila, Manali Sawant, Anaise Bonito, Kim Tran, Anthe George, Kenneth L Sale, Seema Singh, Blake A Simmons and Bradley M Holmes (2013) Production and extraction of sugars from switchgrass hydrolyzed in ionic liquids. Biotechnology for Biofuels 6:39 doi: 10.1186/1754-6834-6-39