A novel method to introduce acidic and basic bi-functional sites in graphitic carbon nitride for sustainable catalysis: cycloaddition, esterification, and transesterification reactions

Abstract

Graphitic carbon nitride was prepared using urea, thiourea, and a mixture of urea–thiourea. Functional carbon nitride was prepared by reacting g-C₃N₄ with aqueous H₂SO₄. Characterizations revealed that bi-functional acidic (–SO₃H) and basic (–NH₂) sites were introduced after the aqueous H₂SO₄ treatment. The characterizations also revealed that the surface acidity and basicity were influenced by the concentration of aqueous H₂SO₄ solution. A detailed characterization of the catalyst was made using a complementary combination of powder X-ray diffraction, N₂ adsorption–desorption, scanning/transmission electron microscopy, thermogravimetric analysis, Fourier transform infrared spectroscopy, elemental analysis, and X-ray photoelectron spectroscopy. Furthermore, the bi-functional nature (acidity/basicity) of the catalyst was investigated with NH₃ and CO₂ temperature-programmed desorption techniques. The activation and utilization of CO₂ in the synthesis of cyclic carbonates and quinazoline-2,4(1H,3H)-dione over bi-functional graphitic carbon nitride was investigated. Kinetic and thermodynamic parameters (such as k, E_a, ΔH, ΔG, and ΔS) were calculated for the cycloaddition reaction of CO₂ to epichlorohydrin by varying the reaction parameters. Graphitic carbon nitride (prepared with urea–thiourea) treated with 60 wt% aqueous H₂SO₄ exhibited the highest catalytic activity and selectivity in the synthesis of cyclic carbonates and quinazoline-2,4(1H,3H)-dione. The catalyst was easily recovered and recycled with negligible loss of activity. In addition, the catalyst exhibited significantly high activity in the transesterification reaction of cyclic carbonate with methanol and the esterification reaction of oleic acid with methanol. The uniqueness of this study is the introduction of bi-functional (acid–base) sites by treating g-C₃N₄ with aqueous sulfuric acid, which was confirmed using NH₃ and CO₂ temperature-programmed desorption techniques and the various other physico-chemical characterization techniques mentioned above. Due to its simple and economical synthesis procedure, tuneable functional sites, efficient recyclability, and diverse catalytic activity, this catalyst is highly attractive for sustainable catalysis.