Ionic Liquid-Based Direct Acid Hydrolysis
In 2002 Rogers et al. reported [54] that high molecular weight pulp cellulose (DP ~ 1000) slowly dissolves (5-10g/100g ionic liquid) in 1-”butyl-3-methylimidazolium (BMIM+) salts, which are room temperature ionic liquids with Cl-, Br-, and SCN — anions when heated to 100°C, yielding viscous solutions [54]. The cellulose dissolution capability of BMIMCl can be explained in terms of non-hydrated chloride ions in the ionic liquid. It is believed that high chloride concentration and activity in [C4mim]+ Cl — is very effective in breaking the extensive hydrogen-bonding network present in cellulose, thus higher concentration of cellulose can be dissolved than in traditional solvent systems. Ionic liquids are effective in the dissolution of untreated lignocellulosic biomass as well [55, 56].
Since this first report on the dissolution of cellulose, considerable effort has been devoted to improve the solubility and to build on this initial discovery [57, 58]; a new branch in ionic liquid-cellulose research has emerged. Next, in 2007 Zhao et al. reported [59, 60] that cellulose could be hydrolyzed by adding catalytic amounts of sulfuric acid to the cellulose-ionic liquid solution. Since this development, ionic liquid-based cellulose depolymerization has branched out in a number of directions, and in many cases pure cellulose has been used as a model compound for biomass.
The new directions in ionic liquid-based cellulose depolymerization research can be categorized into three groups:
1. Use of catalytic amount of acids in cellulose/lignocel — lulosic biomass in ionic liquid solutions.
2. Use of catalytic amount of solid acids in cellulose/lig — nocellulosic biomass in ionic liquid solutions.
3. Use of acid group functionalized ionic liquids.