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

Abstract

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.