Molten-salt synthesis of highly condensed and nitrogen-tunable carbon nitride materials for metal-free and efficient synthesis of dimethyl carbonate

Dimethyl carbonate (DMC) is a building block in the modern chemical industry, and the transesterification of ethylene carbonate (EC) with methanol is a more sustainable strategy for the synthesis of DMC. Wherein, the design of metal-free and high-activity catalysts that can be easily prepared is a crucial research topic for clean synthesis of DMC. In this work, C₃N₄-T materials with non-metal components, high degrees of polycondensation, and adjustable distributions of nitrogen-containing species were successfully prepared by a facile molten-salt approach. Compared with g-C₃N₄ prepared by the traditional thermal polycondensation method, C₃N₄-T materials own larger specific surface areas and a higher degree of intralayer condensation. By simply adjusting the calcination temperatures, the fractions of bridging nitrogen species can be tuned, resulting in an improvement in alkaline strength. For the transesterification reaction of EC with methanol, the C₃N₄-500 materials demonstrated much higher activity than g-C₃N₄. Under a lower reaction temperature (100 °C) and a shorter reaction time (3 h), the maximum conversion of EC was 79.9 %. According to the characterization results of XPS and Hammett titration, the correlation between the catalytic activity of various C₃N₄-T materials and the alkaline properties was deliberated.