Carboxyl Functionalized Ionic Liquids

Functionalized ionic liquids can meet diversified application requirements by introducing specific functional groups into anions and cations. The introduction of carboxyl groups into the ionic liquid can make the functionalized ionic liquid have both the properties of ionic liquid and carboxylic acid. It can be used as an acid catalyst in organic synthesis, as a benign solvent to dissolve biological macromolecules, as an absorbent to absorb harmful substances, and as a reaction medium to prepare and modify nanoparticles. Carboxyl functionalized ionic liquids can be divided into cationic functionalized and anionic functionalized. Cationic functionalization is mainly the introduction of carboxylic acid carbon chain on the nitrogen atom of imidazole. Anionic functionalization is the direct use of carboxylate anions. Carboxyl functionalized ionic liquids can be applied in the fields of biology, organic synthesis, nanomaterials and environmental fields.

Figure 1. Common structure of carboxyl functionalized ionic liquids

Applications:

• Dissolved cellulose: Cellulose is cheap and easy to obtain, non-toxic, non-polluting, renewable and biodegradable. It is widely used in textiles, papermaking, plastics, food and energy. A large number of intramolecular and intermolecular hydrogen bonds contained in cellulose lead to high crystallinity, making it insoluble in water and general organic solvents, and seriously hindering the application of cellulose. For functionalized ionic liquids, carboxyl functionalized ionic liquids can increase the solubility of cellulose by forming a large number of hydrogen bonds and anion and cation interactions.
• Catalyst: Carboxyl functionalized ionic liquid is a type of proton acid functionalized ionic liquid. The introduction of carboxyl groups into the structure of ionic liquids can achieve the catalysis of specific chemical reactions, showing high catalytic activity, selectivity and good reusability. For example, carboxyl functionalized ionic liquids can catalyze esterification, alkylation, and Mannich reactions.
• Modified nanoparticles: Nanoparticles are easy to agglomerate due to their huge surface energy. On the one hand, the use of carboxyl ionic liquid as a reaction medium to prepare nanoparticles can improve the performance of the resulting nanoparticles. On the other hand, carboxyl functionalized ionic liquids can modify the surface of nanoparticles, prevent agglomeration, and stabilize the crystal structure of nanoparticles.
• Absorption of SO₂: The carboxylate in the carboxylate anion functionalized ionic liquid can coordinate with SO₂ to achieve high SO₂ absorption at a lower temperature. In addition, the absorbent can be recycled many times through analysis and has excellent stability.

Reference

• Nikolett Bagány, Aleksandar Tot, Milan Vraneš, Slobodan Gadžurić. Influence of the carboxyl group on the physicochemical and hydration properties of the imidazolium-based ionic liquid[J]. Journal of Molecular Liquids 2021, 328, 115474.