Boron doped graphitic carbon nitride with acid-base duality for cycloaddition of carbon dioxide to epoxide under solvent-free condition

The cycloaddition of CO2 and epoxides to yield cyclic carbonate under solvent-free conditions is an eco-friendly way to utilize CO2 in environmental science and green chemistry. In this paper, we report that boron doped carbon nitride (BCN) is highly active and selective for such reactions. BCN, especially if supported on mesoporous silica SBA-15 (i.e., B0.1CN/SBA-15), shows above 95% conversion and selectivity for cycloaddition of CO2 and styrene oxide (SO) to yield styrene carbonate (SC), even under solvent-free conditions. That is mainly due to the acid-base duality induced by B doping, which enables the co-activation of CO2 and epoxide. A mechanism based on acid-base duality is proposed, where CO2 is activated on the basic >NH sites and SO is on the acidic −B(OH)2 sites through a hydrogen bonding. The co-activated CO2 and SO react with each other to yield the SC. Density functional theory (DFT) calculations were conducted to support the mechanism, which show that the co-adsorption of CO2 and SO on BCN is energetically favorable and the reaction follows the Langmuir-Hinshelwood mechanism. The BCN with acid-base duality provides an option for cheap, green and efficient catalysts for CO2 utilization.