Controlled synthesis of graphitic carbon nitride and its catalytic properties in Knoevenagel condensations

Abstract

Graphitic carbon nitride (g-C3N4) with high nitrogen content and surface area was synthesized by the thermal condensation of dicyandiamide and urea. This method gave a much higher product yield than the direct thermal decomposition of urea. The prepared g-C3N4 is active for the Knoevenagel condensation of an aldehyde or ketone with methylenic compounds, but the activity depends on the structure of basic nitrogen species. Fourier transform infrared and X-ray photoelectron spectroscopy show that at least four types of nitrogen species are present in g-C3N4, including primary amines (NH2), secondary amines (NH), tertiary nitrogen (N(C)3), and pyridinic nitrogen (CNC). The amounts and types of these nitrogen species in g-C3N4 are closely related to the urea/dicyandiamide ratio in its precursor mixture. The catalytic activity of these nitrogen species in the inactivated Knoevenagel condensation system decreases in the order of NH2>NH>N(C)3>CNC, while it is similar for the NH2 and NH species in the activated reaction system. This trend provides strong evidence for the abstraction of a proton from the methylenic compounds as the rate-determining step. The prepared g-C3N4 is highly stable and reusable in the Knoevenagel condensation reactions.