Ordered Mesoporous Carbon Nitrides with Tuneable Nitrogen Contents and Basicity for Knoevenagel Condensation

We report on the preparation of ordered mesoporous carbon nitrides (MCN) with a 3D porous structure, tuneable nitrogen contents and basicity and their basic catalytic activities on the Knoevenagel condensation of benzaldehyde with malononitrile. The chemical structure and the nitrogen contents of the materials are finely tuned with the simple adjustment of the calcination temperature from 350 to 550 °C. The samples prepared at 350 and 400 °C exhibit C₃N₅ structures with 1-amino/imino-1,2,4-triazole moieties, whereas the samples synthesised at 450, 500, and 550 °C possess C₃N₄ structures with 2-amino/imino-1,3,5-triazine moieties. The materials prepared at the temperature lower than 400 °C show much higher activity than those of the samples prepared at 450, 500 and 550 °C. The change in the catalytic activities of these materials is linked with the change of the structure, nitrogen contents and the functional groups on the surface of the materials prepared at different temperatures. CO₂ temperature programme desorption study reveals that 1-amino/imino-1,2,4-triazole moieties in C₃N₅ samples provide high basicity due to the strain in 5-membered 1,2,4-triazole rings; however, the samples with 1,3,5-triazine moieties have limited basicity which significantly affects the catalytic activity of the material. The optimised C₃N₅ catalyst shows an enhanced catalytic activity when compared to other mesoporous basic catalysts and C₃N₄. It is also found that the optimised catalysts are highly stable and can be recycled several times and no major change in the activity is observed.