BCN materials with adjustable active sites as heterogeneous acid–base catalysts for Knoevenagel condensation reaction

Abstract

The Knoevenagel condensation reaction is one of the most effective methods for the formation of C–C bonds. However, developing efficient heterogeneous catalysts for the Knoevenagel condensation reaction is imperative. In this study, a series of boron carbon nitride (BCN) catalysts with adjustable active sites were designed via molecular self-assembly and calcination-temperature adjustment. The BCN materials synthesized at a calcination temperature of 750 °C (BCN-750) had the largest number of active sites and exhibited excellent activity, with a yield of 96% in the condensation reaction of benzaldehyde and malononitrile in only 4 min at room temperature. The excellent catalytic activity was attributed to the B and N species at the B–N–C sites, which functioned as acids and bases, respectively, to catalyze the Knoevenagel condensation reaction. This result was confirmed by temperature-programmed desorption of CO2 (CO2 TPD), temperature-programmed desorption of NH3 (NH3 TPD), X-ray photoelectron spectroscopy (XPS), and 11B solid-state magic-angle spinning nuclear magnetic resonance spectroscopy. In addition, aromatic and aliphatic aldehydes were successfully converted into their corresponding products. Analysis of the catalytic mechanism revealed that the deprotonation of malononitrile to form a carbanion was the rate-determining step.