Regulating the Acidity of SO₃ H-Functionalized Ionic Liquids: Hydrogen Bonding or Electrostatic Potential?

Area of Science:

• Catalysis
• Materials Science
• Physical Chemistry

Background:

• Sulfonic acid functionalized ionic liquids (ILs) are potent acid catalysts.
• Intramolecular hydrogen bonds (H-bonds) can significantly reduce the acidity of these ILs.
• Understanding factors affecting IL acidity is crucial for catalyst design.

Purpose of the Study:

• To investigate factors contributing to the reduced acidity of anionic SO3H-functionalized ILs.
• To identify strategies for enhancing the acidity of these ionic liquids.
• To correlate IL acidity with H-bond strength and electrostatic potential on the leaving proton (ESPLP).

Main Methods:

• Design and synthesis of anionic SO3H-functionalized ionic liquids.
• Catalytic experiments to assess IL performance in Friedel-Crafts alkylation.
• Atoms in Molecules (AIM) topology analysis and electrostatic potential calculations.

Main Results:

• IL acidity was found to correlate with the ESPLP, not H-bond strength.
• Strategies to enhance acidity include increasing cation electron-withdrawing ability, introducing H-bond acceptor sites on the cation, or altering substitution positions to minimize intramolecular H-bonds.
• Optimized ILs demonstrated a significant increase in tert-butylamine (TBA) conversion from 19% to 84% in Friedel-Crafts alkylation.

Conclusions:

• ESPLP is a more reliable indicator of acidity in SO3H-functionalized ILs than H-bond strength.
• Effective strategies were identified to tune and enhance IL acidity for catalytic applications.
• The developed strategies significantly improved catalytic efficiency in Friedel-Crafts alkylation.