5.3f Ionic Liquids
One of the more usual methods of pretreatment of biomass uses ionic liquids. Ionic liquids (ILs) are organic salts that usually melt below 100°C and are strong solvation agents. A common salt that we are all familiar with is table salt, sodium chloride, NaCl. If dissolved in water, it separates into the ions of Na+ and Cl-, but it is not an organic salt like ILs. It has interesting properties, including the fact that, depending on the IL, it can solubilize whole cellulosic biomass or selectively dissolve components, e.g., lignin and cellulose. It is relatively easy to separate the dissolvent component from the organic salt by using an anti-solvent such as water, methanol, or ethanol. When cellulose has been dissolved by organic liquid and then re-precipitated by an anti-solvent, cellulose is less crystalline and easier to break down. Unfortunately, this is still a costly method of pretreatment, as there is difficulty in recycling ILs, and the ILs can be toxic to the enzymes and microbes used in processing cellulose to ethanol. One such IL is known as EmimAc (1-ethyl-3-methylimidazolium acetate), and is able to completely solubilize both cellulose and lignin in switchgrass. Figure 5.26 shows the chemical structure of EmimAc and the change in cellulose after reprecipitation of it using an antisolvent (T = 120°C). Figure 5.27 shows a schematic of the process diagram.