Acid Catalysts in Ionic Liquid Solutions
The first report on the use of a catalytic amount of acid in cellulose/ lignocellulosic biomass saccharification in ionic liquid solutions appeared in 2007 [59]. In this landmark study, Zhao et al. found that cellulose-BMIMCl solution with H2SO4/cellulose mass ratio of 0.92 produces total reducing sugars (TRS) and glucose in 59% and 36% yields, respectively, within 3 min. Further reducing the acid/ cellulose mass ratio to 0.46 produced higher yields after 42 min, and when the mass ratio was dropped to 0.11, the yields of TRS and glucose reached 77% and 43%, respectively, in 9 h. This is an exciting finding because the reaction system was operated under mild conditions using essentially a catalytic amount of H2SO4 and no pretreatment was required.
Later Dee and Bell also reported [61] the hydrolysis of cellulose dissolved in 1-ethyl-3-methylimidazolium chloride (EMIMCl) and 1-”butyl-3-methylimidazolium chloride (BMIMCl) catalyzed by mineral acids. Glucose, cellobiose, and 5-hydroxymethylfurfural (5-HMF) were observed as primary reaction products. The initial rate of glucose formation was determined to be of first order in the concentrations of dissolved glucan and protons and of zero order in the concentration of water. The absence of a dependence on water concentration suggests that cleavage of the P-1,4-glycosidic linkages near chain ends is irreversible. The apparent activation energy for glucose formation is 96 kJmol-1. The absence of oligosaccharides longer than cellobiose suggests that cleavage of interior glycosidic bonds is reversible due to the slow diffusional separation of cleaved chains in the highly viscous glucan/ionic liquid solution. Progressive addition of water during the course of glucan hydrolysis inhibited the rate of glucose dehydration to 5-HMF and the formation of humins. The inhibition of glucose dehydration is attributed to stronger interaction of protons with water than the 2-OH atom of the pyranose ring of glucose, a critical step in the proposed mechanism for formation of 5-HMF. The reduction in humin formation associated with water addition is ascribed to the lowered concentration of 5-HMF, since the formation of humins is suggested to proceed through the condensation polymerization of 5-HMF with glucose.