Improved photocatalytic performance for selective oxidation of amines to imines on graphitic carbon nitride/bismuth tungstate heterojunctions.

Imine compounds are vital intermediates in organic synthesis, and their cost-effective and green preparation has inspired numerous interests. Herein, we employ an environmentally friendly photocatalysis strategy to selectively oxidize amines to obtain imines. The heterojunction photocatalysts of graphitic carbon nitride/bismuth tungstate (CN/BW) were fabricated by hydrothermal growth of bismuth tungstate on the surface of graphitic carbon nitride. Characterization results demonstrate that the composites have been well composited and show improved separation ability of photo-generated charge carriers due to the formed type II band alignment. In comparison to the individual CN and BW, the heterojunctions exhibit apparent enhanced activity for the selective oxidation of amines to imines using O2 as the oxidant under irradiation of visible light. The optimized CN/BW heterojuntion with 75% BW content presents the high conversion (~98%) and selectivity (~98%) in 3 h oxidative coupling reaction of benzylamine. Furthermore, this heterojunction photocatalyst also has excellent cycling stability, as well as good catalytic performance for various benzylamine derivatives. On the basis of experimental results, the transfer pathway of photo-excited electrons and holes in the heterojunction photocatalyst was illustrated and a possible mechanism for the subsequent selective oxidation of amines to imines was proposed. This work provides an economical, sustainable and thus green process for the amine transformation.