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.
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