Understanding the role of halogen-containing ionic liquids in the hydrolysis of cellobiose catalyzed by acid resins

In the hydrolysis of cellobiose to glucose, the addition of 1-butyl-3-methylimidazolium chloride (BmimCl) ionic liquid (IL) enhanced the catalytic activity of Nafion^® NR50 substantially. Similar results were observed with 1-butyl-2,3-dimethylimidazolium chloride, 1-ethyl-3-methylimidazolium chloride, 1-allyl-3-methylimidazolium chloride, and 1-butyl-3-methylimidazolium bromide and iodide, but not acetate. The glucose yield was the highest when the amount of BmimCl added equalled the number of protons available in the resin catalyst, but further addition of BmimCl did not lead to substantial changes in the glucose yield. The extent of activity enhancement did not correlate well with the extent of swelling of the Nafion^® NR50 resin, but correlated well with the pH change of the reaction mixture due to the presence of ILs. It is believed that the principal contributor to the effect of ILs is displacement of the protons of the sulfonate groups in the resin by ion-exchange with imidazolium ions, and these released protons are the catalyst for the hydrolysis reaction. For the IL with acetate as the anion, the released protons associate with acetate to form acetic acid, which is a weak acid and has low hydrolysis activity.