Sulfonic Acid Functionalized Ionic Liquids

The core of synthesizing sulfonic acid functionalized ionic liquid is to introduce -SO₃H group on the cationic group. The most prominent advantage is that it has Broňsted acidity. This feature makes it applicable to the synthesis of a variety of compounds or materials. The cations mainly include: 1-sulfonic acid propyl-3-methylimidazole, 1-sulfonic acid butyl-3-methylimidazole, 1-sulfonic acid propylpyridine and 1-sulfonic acid butylpyridine. Anions mainly include: chlorine, hydrogen sulfate, trifluoroacetic acid and trifluoromethanesulfonic acid.

Figure 1. Common structure of sulfonic acid functionalized ionic liquids

Applications:

• Catalyst: Sulfonic acid functionalized ionic liquid is the most acidic Broňsted acid ionic liquid. It can be used as a catalyst to catalyze Friedel-Crafts acylation, etherification, condensation, addition, esterification and other reactions that require acid catalysts. The use of this type of catalyst can reduce environmental pollution and meet the requirements of green and environmental protection.
• Hydrolyzed cellulose: The conversion of cellulosic biomass is the basic step for the preparation of biofuels. The sulfonic acid functionalized ionic liquid has the excellent characteristics of both inorganic acid and ionic liquid. In the hydrolysis of cellulose, it can act as a catalyst and a solvent. For sulfonic acid functionalized ionic liquids, Cl^- as an anion has a better hydrolysis effect.
• Oil desulfurization: Traditional desulfurization will use a large amount of organic solvents, causing serious pollution to the environment. Sulfonic acid ionic liquid not only has acid catalytic activity, but also can avoid environmental pollution and realize a green catalytic system for oil desulfurization.
• Preparation of biodiesel: The introduction of sulfonic acid functional groups on the cations improves the catalytic activity of the ionic liquid. The sulfonic acid functionalized ionic liquid can catalyze the transesterification reaction of soybean oil and methanol to generate biodiesel. In this application, the ionic liquid exhibits strong catalytic activity and good thermal stability.

Reference

• Yingnan Sun, Jianglei Hu, Sai An, Qingqing Zhang, Yihang Guo, Daiyu Song, Qingkun Shang. Selective esterification of glycerol with acetic acid or lauric acid over rod-like carbon-based sulfonic acid functionalized ionic liquids[J]. Fuel 2017, 207, 136-145.