The Influence of the Acidity of Ionic Liquids on Catalysis

Abstract

In the past 20 years, the concept of ionic liquids (ILs) have been extensively applied in the fields of chemistry, materials, and life sciences. [1] Undoubtedly, the ionic liquids composed of quaternary ammonium cations and anions, such as BF 4 À , PF 6 À , Cl À , and NTf 2 À , have been the backbone of this area since immidazolium cation ionic liquids were synthesized by Zaworotko et al. and were brought into catalysis and synthesis by Seddon, Rogers, Welton, Wasserscheid, and others. [2] Thousands of reactions have been performed in these ionic liquids and many of them exhibited better behavior than organic solvents. [3] Normally, the fine performance of these ionic liquids was attributed to the specific ionic environment of the ionic liquid. Nevertheless, the acidity of the air-and moisture-stable ionic liquids and its effect on catalysis is an interesting topic.As it is well known, a large amount of organic reactions can be catalyzed or promoted by an acid environment. [4] During our investigation of the function of ionic liquids in catalysis, especially air-and moisture-stable ones, we found that these ILs normally exhibit weak acidity in the presence of a small amount of water. That means the interpretations about the function of air-and moisture-stable ionic liquids in catalytic reactions are possibly wrong because the presence of trace amount of water is not avoidable in reality. Herein, we present our results on the study of acidity of air-and moisture-stable ionic liquids and their activity in some known acid-catalyzed reactions. We hope these results could be helpful for researchers in this area to reconsider the influence of the acidity of airand moisture-stable ionic liquids on catalysis and also in other fields. At the initial stage, the acidity of ionic liquids-water with different cations and anions were measured with a pH meter. The concentration of ionic liquid in water was 0.1 m. The operation was performed with methods given in the Annual Book of American Society for Testing and Materials Standards (ASTM) with slight modification. [5] As shown in Figure 1, ionic liquid-water mixtures with BF 4 À anions were all acidic. Interestingly, the acidity of the ionic liquids could be tuned via substituted alkyl variation. The pH value of EMImBF 4 ionic liquid reached 3.44(0.03) but the pH value of BMImBF 4 and HMImBF 4 were 4.27(0.14) and 6.61(0.03) respectively. BMImBF 4 ionic liquids purchased from Merck (lot code: S5204049909) and Sigma-Aldrich (lot code: 0001415814) were also measured for comparison. Under the same condition, their pH values were 4.70(0.05) and 4.30(0.09), respectively, which are exactly the same as the acidity of the ionic liquids that we synthesized. The incorporation of an hydroxyl group would further enhance the acidity of ionic liquids with BF 4 À anion. The pH value reached 3.12(0.01) and 3.11(0.02) with hydroxyethyl or hydroxypropyl groups. The substitution of the C2 position with a methyl group weakens the acidity of this kind of ionic liquid. For the ionic liquid BMMIm...

Ionic Liquids as Tools for the Acid‐Catalyzed Hydrolysis/Dehydration of Saccharides to Furanic Aldehydes

“…% fructose; 6 mol % acid). [41,75,88,[92][93][94] On the other hand, the stability of HCl may be influenced by the type of IL (possibly more stable in Cl-containing ILs). % fructose in [20 wt.…”

“…[110] Differences in acidity of the IL medium may affect the catalytic results. [92] Recycling of IL-based catalytic systems [92] Recycling of IL-based catalytic systems…”

pH Control of Ionic Liquids with Carbon Dioxide and Water: 1-Ethyl-3-methylimidazolium Acetate

“…30 In the absence of CO 2 , the addition of water can strongly effect the pH of an ionic liquid, with the most pronounced pH changes occurring for ionic liquid compositions greater than 80 wt % and less than 10 wt %. 31 In 32 Therefore, complete washing and removal of trace ionic liquid in the regenerated cellulose is necessary, which adds additional costs and limits commercial feasibility. Solubility data for systems of [emim][Ac] and CO 2 has previously been published; 21 however, the addition of water plays a valuable role in assisting acidification, and the measurement of pH is a novel element.…”

Rapid and Efficient Functionalized Ionic Liquid-Catalyzed Aldol Condensation Reactions Associated with Microwave Irradiation

“…The improved catalytic performance in the ILs was considered to be due to the stabilization of the iminium intermediate formed from the ketone and the secondary amine of the catalyst or because of the enhanced nucleophilicity of the enamine [34]. Therefore, a series of functionalized ILs incorporated with pyrrolidine and proline units have been synthesized and tested as organocatalysts for direct aldol reactions [46–49]. These new TSILs were found to effectively catalyze the direct aldol reactions of cyclic ketone or acetone and various aromatic aldehydes.…”