B/g-C3N4 for Selectively Regulating Oxygen-Guided Photocatalytic Oxidation of Cinnamaldehyde to Benzaldehyde: Effects of Boron Sources

Natural benzaldehyde, an important spice-type flavor component and raw pharmaceutical material, is scarcely due to the escalating demand and consumer preference for natural foods. To mitigate this issue, the present study aimed to investigate the oxygen-driven selective oxidation of cinnamaldehyde to benzaldehyde under visible light. K 2 B 4 O 7 ·4H 2 O and KBH 4 were used as boron sources to modify the g-C 3 N 4 photocatalyst for regulating the cinnamaldehyde conversion rate and benzaldehyde selectivity. A series of characterization techniques revealed improvements in the photocatalytic performance of the modified catalysts─that is, g-C 3 N 4 doped with boron derived from K 2 B 4 O 7 ·4H 2 O or KBH 4 (BCN or KBCN, respectively; 10% or 25% increase in cinnamaldehyde conversion rate, 11% or 34% increase in benzaldehyde selectivity). These enhancements were due to the doped boron and the generated nitrogen vacancy. Moreover, the abilities of BCN and KBCN to adsorb oxygen and cinnamaldehyde improved. The conjugated aldehyde group and planar molecular structure of cinnamaldehyde are critical to the selective oxidation of the >C═C< unit to obtain the aldehyde. The mechanism underlying the photocatalytic action of KBCN was established, and the reasons for the superior photocatalytic performance of KBCN compared with that of BCN were determined. Essentially, the reducible boron source─KBH 4 ─produced a radical boron site on the surface of g-C 3 N 4, which collided with • O 2 – to generate another vital oxygen species 1 O 2 . Subsequently, the electrons transferred to the surface of KBCN could be captured by the adsorbed oxygen molecules to regenerate • O 2 – . Furthermore, the hole produced on the catalyst surface during visible-light illumination promoted its reaction with • O 2 – by obtaining an electron from the >C═C< unit of cinnamaldehyde. Overall, this study paves the way for future applications and research on the selective oxidation of the >C═C< group in α,β-unsaturated carbonyl compounds.