Immobilization of polyoxometalates on protonated graphitic carbon nitride: A highly efficient and reusable catalyst for the synthesis of pyridine-N-oxides

Abstract

N-oxides represent a significant class of compounds with increasing value due to their extensive applications in chemistry and biology. Herein, a series of heterogeneous catalysts were prepared based on the impregnation-precipitation method. In this strategy, protonated graphitic carbon nitride (pg-C3N4) was prepared first, and Cs3PW12O40 was immobilized over the surface of pg-C3N4 to obtain CsPW-CN composites. The prepared CsPW-CN composites achieved better catalytic activities than the pure Cs3PW12O40 in the N-oxidation of pyridine, and obtained 99% yield of pyridine-N-oxide in aqueous medium with H2O2 as a mild oxidant. Based on the characterizations of the catalyst morphology, structure, and chemical composition, the intimate interaction between Cs3PW12O40 and pg-C3N4 was verified. Meanwhile, the occurrence of the unique semi-embedded structure was an expected derivation of the pg-C3N4. Furthermore, the prepared CsPW-CN-1 composite was readily recovered and yielded 88.3% of the pyridine-N-oxide after 4 runs. This work could potentially provide a well-defined CsPW-CN composite for the N-oxidation of pyridine with a sustainable approach.