Dissolution of Lignin in Recalcitrant Biomass Using Ionic Liquid - Glycerol Mixtures

Lignocellulosic biomass has a recalcitrant structure including interwoven cellulose, hemicelluloses, and lignin biomacromolecules that is challenging to deconstruct. Many ionic liquids (ILs), salts molten below 100 °C, can be used to deconstruct lignocellulosic biomass and are less hazardous than the chemicals typically used. The environmentally-friendly ILs investigated in this study were 1-ethyl-3-methylimidazolium acetate and 1-ethyl-3-methylimidazolium formate. While effective and recyclable, these ILs are expensive. To reduce costs, dilution with other safe compounds was desired. Glycerol, a food additive, is inexpensive and becoming even more so since it is a by-product of the expanding biodiesel industry.  When glycerol was added to these ILs to pretreat loblolly pine or rice husks, lignin was dissolved into the IL- glycerol mixture and crystalline cellulose in the residue had been converted to more easily hydrolyzed amorphous cellulose.  Lignin could subsequently be precipitated using an anti-solvent.

 To determine the reasons for their efficacy, IL- glycerol mixtures were investigated to determine pH, viscosity, and density. The ILs investigated were 1-ethyl-3-methylimidazolium acetate and 1-ethyl-3-methylimidazolium formate, which were diluted with glycerol to give mass fractions of 0.25, 0.50, and 0.75. Values for pH for 1-ethyl-3-methylimidazolium acetate and its mixture with glycerol when diluted with water to be 10 mass% IL-glycerol mixture was 6.0, while 1-ethyl-3-methylimidazolium formate and its mixture with glycerol gave a pH of 8.3 when so diluted with water. Viscosity was measured for the IL-glycerol mixtures up to 140 °C.  While quite different at room temperature, glycerol, ILs, and mixtures had similar viscosities between 110 °C and 140 °C, the typical range for biomass IL pretreatment. Density measurements showed the IL-glycerol mixtures that gave more effective pretreatments to have positive excess molar volumes.