Fluorine-oxygen dual sites engineered on carbon enable high efficiency in the cycloaddition of carbon dioxide:...

Fluorine (F)-doped carbon materials (FCMs) were one-pot synthesized and applied as the catalysts for the cycloaddition of carbon dioxide (CO₂) towards the cyclic carbonate for the first time. In this process, F dopants and oxygen (O)-containing groups on the carbon surface played a key role in enhancing the activity. The FCM synthesized at 500 °C (FCM-500) with 5.8 at.% F and 6.5 at.% O afforded 94.5 ± 2.5 % conversion, which was much higher than that of FCM synthesized at 800 °C (FCM-800, 78.2 ± 3.8 %) with 0.2 at.% F and 3.1 at.% O. The pseudo-first-order apparent rate constant and activation energy (E_a) of FCM-500 were found as 0.6 h^-1 and 36.7 kJ mol^-1, better than those over FCM-800 (0.3 h^-1 and 39.6 kJ mol^-1). The density functional theory (DFT) simulations revealed that the presence of F dopant and oxygen-containing groups would synergistically enhance the activity. The detailed structure-activity investigation was conducted and revealed that the semi-ionic CF bonds and oxygen-containing functional groups (hydroxyl, carboxyl, and quinonic carbonyl) on the surface of FCMs acted as the synergistic sites, which positively influenced the CO₂ cycloaddition reaction. Based on these findings, a reliable and quantitative kinetic model with the semi-ionic CF bonds, carboxyl, and quinonic carbonyl groups as the key sites was established for the FCM-catalyzed reaction system, in which the apparent reaction rate constant was 4.3 × 10^-2 mol^-0.9 L^0.9 min^-1, and the reaction orders of 1, 2-butylene oxide (BO), tetrabutylammonium bromide (TBAB), and FCM were 1, 0.6, 0.4, respectively. These kinetic parameters verified the feasibility of predicting the concentration of the substrates under different reaction conditions as well. This study offers a new strategy for the kinetic investigation of the CO₂ cycloaddition reaction catalyzed by the heteroatom-doped carbon catalysts.