4.3f 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 in 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. The first figure below shows the chemical structure of EmimAc and the change in cellulose after reprecipitation using an antisolvent (T = 120°C). The second figure below shows a schematic of the process diagram.
Schematic of Iconic Liquid (EmimAc) pretreatment process. Biomass enters IL pretreatment (120-140 C) with EmimAC. This pretreatment takes hours. After pretreatment the products head to cellulose regeneration which washes the products with anti-solvents (water, acetone+ethanol). The cellulose is extracted, and the remaining products go into lignin precipitation which precipitates the lignin and recycles the anti-solvents and IL.