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

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 (C=N-C). The amounts and types of these nitrogen species in g-C3N4 are closely related to the ureaidicyandiamide 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) > C=N-C, 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. (C) 2016 Elsevier Inc. All rights reserved.