Copper phthalocyanine and ionic liquid functionalized carbon nitride for catalytic cycloaddition of carbon dioxide

The advancement of efficient and environmentally sustainable heterogeneous catalysts that facilitate the transformation of carbon dioxide (CO₂) into chemicals has gained considerable attention. In this study, we synthesized a carbon nitride (C₃N₄) functionalized with copper phthalocyanine (CuPc) and ionic liquid (IL) (C₃N₄-CuPc-IL) and employed it as an efficient catalyst enabling the cycloaddition of CO₂ with epoxides. The presence of urea/urethane groups, Cu²⁺ ions, and I^- ions that can effectively activate and open the epoxide ring was confirmed by FTIR, XPS, UV-vis, TGA, XRD and SEM. Meanwhile, the multiple nitrogen-containing structures (copper phthalocyanine, C₃N₄, and quaternary ammonium cationic structures) facilitated the adsorption and activation of CO₂. Consequently, C₃N₄-CuPc-IL demonstrated high catalytic efficiency for the cycloaddition between CO₂ and epoxides. Specifically, with 5.0 wt% loading of C₃N₄-CuPc-IL catalyst under 2.0 MPa and 120 °C for 12 h, the yield of cyclic carbonate reached 98%. Additionally, the prepared catalyst demonstrated excellent structural stability and recyclability, alongside high catalytic activity toward various epoxides. Density functional theory (DFT) calculations indicated that the ring-opening reaction represents the rate-determining step in the C₃N₄-CuPc-IL catalyzed cycloaddition reaction, with an energy barrier of only 24.2 kcal•mol^-1. The impressive catalytic performance of C₃N₄-CuPc-IL confirmed the synergistic catalytic effect of hydrogen bond donor groups, Lewis acidic sites, and ionic active sites in the CO₂ cycloaddition reaction, providing theoretical guidance for the design of CO₂ conversion catalysts.