Mesoporous nitrogen-rich carbon nitride materials: synthesis and catalytic application as metal-free solid bases towards Knoevenagel condensation

Abstract

The base-catalyzed organic reactions represented by Knoevenagel condensation are important in the organic synthesis and chemical industry and most reported catalysts are homogeneous organic bases and metal-containing solids which suffer disadvantages in catalyst separation and metal contamination. Therefore, the development of metal-free and efficient heterogeneous base catalysts for the process is highly required. Herein, a series of mesoporous nitrogen-rich carbon nitride (mpg-CN) materials were synthesized using 5-amino-1H-tetrazole as a precursor via a hard-templating method. The synthesized mpg-CN, specially prepared under lower carbonization temperatures had higher nitrogen contents (C₃N₇, C₃N₆, and C₃N₅) than the ordinary carbon nitride (C₃N₄). The surface areas and distributions of nitrogen species were dependent on the carbonization temperatures. As metal-free solid bases, mpg-CN exhibited high activity in Knoevenagel condensation reactions. The mpg-C₃N₆ material showed the highest activity, achieving a benzaldehyde conversion of 80.6% and selectivity to the target product of 99.2%, attributed to its higher fraction of pyrrolic nitrogen species. In addition, the mpg-C₃N₆ catalyst can be reused five times without significant loss in activity and also demonstrated activity to several substrates.