Facile alkali-assisted synthesis of g-C3N4 materials and their high-performance catalytic application in solvent-free cycloaddition of CO2 to epoxides

A series of graphitic carbon nitride materials were synthesized using guanidine hydrochloride (GndCl) as a precursor with the aid of alkali treatment. The introduction of alkali successfully enabled GndCl to be transformed into g-C3N4 at much lower calcination temperatures (450-475 degrees C). The g-C3N4 samples synthesized under various conditions have been characterized by several techniques including XRD, FTIR, UV-vis, C-13 NMR, and XPS spectroscopy. The results confirmed that the alkali could effectively accelerate further condensation of melem-like fragments to g-C3N4. Meanwhile, a possible mechanism of alkali-assisted synthesis of g-C3N4 from GndCl has been proposed. In solvent-free catalytic cycloaddition of CO2 to propylene oxide to propylene carbonate (PC), g-C3N4-NaOH and g-C3N4-KOH materials demonstrated high and stable catalytic performances, affording PC yields of ca. 90% under optimized reaction conditions. Moreover, the activities were superior to those obtained over g-C3N4 prepared without alkali treatment. In addition, the catalytic activity along with preparation method for the present g-C3N4 has also been compared with other reported g-C3N4-based catalysts.