Hierarchically porous covalent organic frameworks assembled in ionic liquids for highly effective catalysis of C–C coupling reactions

Abstract

Although significant progress has been made in the synthesis of covalent organic frameworks (COFs) in recent years, the construction of hierarchical pores in such materials remains a great challenge. Herein, we report a facile synthesis of hierarchically porous COFs (HP-COFs) under mild conditions in ionic liquids 1-alkyl-3-methylimidazolium tetrafluoroborates ([C_nmim][BF₄], n = 4, 6, 10). It has been found that apart from the inherent micropores, a large mesoporous structure has been produced in the COFs in which the size of pores can be simply tuned by adjusting the alkyl chain length of the ionic liquids. These mesopores have been confirmed by N₂ sorption and electron microscopy techniques. Importantly, this approach is applicable for the preparation of various HP-COFs, such as imine and hydrazone based COFs, which are quite difficult to acquire through traditional methods. In addition, these HP-COFs show highly effective catalytic performance for C–C bond formation, especially for large size molecule based C–C coupling reactions in comparison with uni-pore COFs.