Highly Porous Metalloporphyrin Covalent Ionic Frameworks with Well-Defined Cooperative Functional Groups as Excellent Catalysts for CO2 Cycloaddition

Graphical Biomimetic heterogeneous catalysts : Multifunctional metalloporphyrin covalent ionic frameworks with a highly porous structure for cooperative catalysis by Lewis acidic metal sites and nucleophilic Br − ions showed efficient catalytic activity in CO 2 cycloaddition to epoxides (see figure), and the catalytic performance could be further enhanced by altering the functional groups and the BET surface area. The development of multifunctional heterogeneous catalysts with high porosity and remarkable catalytic activity still remains a challenge. Herein, four highly porous metalloporphyrin covalent ionic frameworks (CIFs) were synthesized by coupling 5,10,15,20-tetrakis(4-nitrophenyl)porphyrin (TNPP) with 3,8-diamino-6-phenylphenanithridine (NPPN) or 5,5′-diamino-2,2′-bipyridine (NBPy) followed by ionization with bromoethane (C 2 H 5 Br) or dibromoethane (C 2 H 4 Br 2 ) and then metalization with Zn or Co. The resulting CIFs showed high efficiency in catalyzing the cycloaddition of propylene oxide (PO) with CO 2 to form propylene carbonate (PC). All of the Zn-containing CIF catalysts were able to catalyze the cycloaddition reaction with a PC yield greater than 97 %. The TNPP/NBPy (CIF2) catalyst ionized with C 2 H 4 Br 2 and metalized with Zn (Zn-CIF2-C 2 H 4 ) exhibited the highest catalytic activity among the synthesized catalysts. The high catalytic performance of Zn-CIF2-C 2 H 4 is related to its high porosity (577 m 2 g −1 ), high Br:metal ratio (1:3.89), and excellent synergistic action between the Lewis acidic Zn sites and the nucleophilic Br − ions. Zn-CIF2-C 2 H 4 is sufficiently stable that greater than 94 % PC yield could be obtained even after six cycles. In addition, Zn-CIF2-C 2 H 4 could catalyze the cycloaddition of several other epoxides with CO 2 . These highly porous materials are promising multifunctional and efficient catalysts for industrially relevant reactions.