Boron-doped carbon nitride as a metal-free catalyst for the cycloaddition of CO2 with epoxides under ambient conditions

Abstract

The synthesis of cyclic carbonates via the coupling of CO2 and epoxides represents an efficient, synthetic pathway that supports net-zero emissions and promotes atom economy. However, achieving the eco-friendly fixation of CO₂ with epoxides under ambient, solvent-free conditions requires an efficient catalyst, offering a sustainable alternative to conventional methods that rely on toxic carbon monoxide or phosgene. Herein, the effectiveness of a metal-free catalyst with synergistic dual active sites, boron atoms acting as Lewis acids and nitrogen groups in PCN acting as Lewis bases, has been investigated to contribute to the current doping modification strategy for boron-doped polymeric carbon nitride (PCN-B). After the detailed structural characterization of PCN and PCN-B by utilizing many advanced analytical techniques, they were tested as metal-free catalysts for the chemical coupling reaction of epoxides with CO2 to yield cyclic carbonates under ambient conditions. Among the tested catalysts, the boron-doped PCN-B1 with 2.5 wt% boron showed the highest catalytic activity with the epichlorohydrin conversion up to 97% and 99% selectivity under mild conditions due to the abundant oxygen vacancies and more acidic site formation, which provides sufficient active sites for CO2 transformation into cyclic carbonates as fine chemicals. Moreover, the synergistic dual-active sites engineering in the PCN-B catalytic system demonstrated exceptional recyclability, exhibiting no drop in catalytic activity over five consecutive runs while maintaining a yield of over 90% with the selectivity exceeding 93%. Kinetic experiments indicate that the PCN-B catalytic system shows a lower energy barrier for the ring-opening step in the chemical conversion of CO2, further facilitating the CO2 transformation. The kinetics of this cycloaddition reaction were estimated to be pseudo-first order, and the rate constant was 0.914 h−1 at 100 °C under similar reaction conditions.