Promising metal-free green heterogeneous catalyst for quinoline synthesis using Brønsted acid functionalized g-C₃N₄

Rationally designing distinct acidic and basic sites can greatly enhance performance and deepen our understanding of reaction mechanisms. In our current investigation, we studied the utilization of Brønsted acid sites within layered graphitic carbon nitride (g-C₃N₄) for the first time to enhance the rate of the Friedländer synthesis. The structural and surface analyses confirm the effective integration of -COOH and -SO₃H groups into the g-C₃N₄ lattice. The surface-functionalized g-C₃N₄-CO-(CH₂)₃-SO₃H exhibits a remarkable acceleration in quinoline formation, surpassing previously mentioned catalysts, and demonstrating notable recyclability under optimized mild reaction conditions. The heightened reaction rate observed over g-C₃N₄-CO-(CH₂)₃-SO₃H is attributed to its elevated surface acidity. By probing the Friedländer reaction mechanism through surface characterization, examination of reaction intermediates, and investigation of substrate scope, we elucidate the pivotal role of Brønsted acid sites. This study constitutes a comprehensive exploration of metal-free heterogeneous catalysts for the Friedländer reaction, offering a unique contribution to the field.