Phosphorous-modified bulk graphitic carbon nitride: Facile preparation and application as an acid-base bifunctional and efficient catalyst for CO₂ cycloaddition with epoxides

A series of phosphorus (P) modified graphitic carbon nitride (P-C₃N₄) were facilely prepared by direct thermolysis of melamine and hexachlorotriphosphazene. Based on the results of XPS and ³¹P NMR analyses, it is deduced that there is replacement of C atoms located at the bay and edge terminal positions by P atoms. The P-C₃N₄ materials are easy-to-handle, stable, and metal-free. They function well as a catalyst for the synthesis of cyclic carbonates through carbon dioxide (CO₂) cycloaddition to epoxides under mild conditions without the need of a solvent. This is the first time that an acid-base bifunctional P-C₃N₄ was prepared and used as catalyst for cycloaddition reactions. It was observed that the catalytic activity of P-C₃N₄ increases with the rise of P content as a result of the enrichment of acid sites. The excellent performance of Bu₄NBr/P-C₃N₄-2 is attributed to the synergetic effect of acid sites and halide anions for ring opening of epoxide as well as to basic sites for adsorption and activation of CO₂. A possible multi-synergetic mechanism is proposed for the cycloaddition reaction over P-C₃N₄. Moreover, P-C₃N₄ can be easily separated and reused for at least five times without showing significant loss of activity.