Tailored Brønsted Acidic Sites Anchored Over Covalent Organic Framework and Its Potential in Acid-Catalyzed Reactions

For green chemical synthesis, designing a metal-free heterogeneous organocatalyst is very challenging as it offers an environment-friendly route over conventional metal-based catalysts. Covalent organic frameworks (COFs) have huge potential to be explored as heterogeneous organocatalysts because they contain several important features in one system, viz. high specific surface area, metal-free, desired organic functionalization, and outstanding stability. Herein, a ─SO₃H group anchored COF is synthesized, TFR-PDS-COF by employing a Schiff base extended condensation reaction. The material is highly crystalline in nature, exhibiting moderate BET surface area (115 m² g⁻¹) and very high NH₃ uptake capacity (1045 µmol g⁻¹). The high chemisorption property toward NH₃ suggests the highly acidic nature of the material, which motivate to explore it in the acid-catalyzed reaction. Initially, the amination of epoxide, which is a classic example of an acid-catalyzed epoxide activation reaction is chosen. TFR-PDS-COF exhibit good catalytic activity toward the amination of epoxides to β-amino alcohol synthesis for both simple and complex systems at room temperature under solvent-free conditions. The catalyst exhibits high recyclability for several cycles with the retention of its framework. The catalyst has been employed for other acid catalytic reactions such as cycloaddition and acetalization reactions, and displays excellent conversion with high selectivity. All of these results suggest that TFR-PDS-COF is a potential candidate for large-scale and highly sustainable acid-catalytic reactions.