Bifunctional two-dimensional copper-organic framework for high catalytic performance on cycloaddition of CO2 with epoxides and deacetalization-Knoevenagel condensation

Abstract

Due to the larger external surface areas and more accessible active sites, two-dimensional (2D) metal-organic frameworks (MOFs) have aroused great interest in the catalytic field. Herein, the exquisite combination of Cu2+ and designed flexible tetracarboxylic acid of 1,1′-(1,3-phenylenebis(methylene))bis(1H-benzo[d]imidazole-5,6-dicarboxylic acid) (H4PBDA) under the solvothermal condition offers a highly robust 2D network of [Cu(H2PBDA)(H2O)]n (NUC-130), which are interlaced via hydrogen-bonding interactions to form a 3D skeleton. After removing associated solvent molecules by activation, NUC-130a is a scarcely reported Lewis acid/Brönsted acid mediated host framework, which can serve as a promising multifunctional catalyst due to the plentiful active sites including exposed penta-coordinated Cu2+ ions, Brønsted acidic sites of -COOH, and Lewis basic sites of Nbenzimidazole atoms and C=O groups. Performed experiments confirmed that NUC-130a had the high catalytic performance on the cycloaddition of CO2 with a series of epoxide derivatives under solvent-free mild conditions. Furthermore, for deacetalization-Knoevenagel condensation based on acid-base catalysis, NUC-130a also shows excellent catalytic activity, which should be ascribed to that dimethyl acetal can be more effectively hydrolyzed by free -COOH group into formyl group than simple Lewis acidic metal sites. Hence, this work not only provides a type of promising catalyst for efficient chemical fixation of CO2 and deacetalization-Knoevenagel condensation, but also opens up an avenue to creating multifunctional MOFs with integrating multiple active sites and larger external surface areas.