Synthesis of metal-free benzimidazole-based catalysts and its application in CO₂ cycloaddition

Ionic polymers functionalized with hydroxyl, carboxyl, and amino groups can enhance the catalytic activity of catalysts. However, the straightforward preparation of bifunctional ionic polymers containing abundant ionic active sites and hydrogen bond donors remains challenging. In this study, a series of porous ionic polymers (BZIs) containing different hydrogen bond donors (-NH₂, -OH, -COOH) were prepared through a simple one-pot Friedel-Crafts alkylation using benzimidazole derivatives and benzyl bromide. The structures and properties of BZIs were characterized by various techniques such as Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, solid-state nuclear magnetic resonance, and scanning electron microscopy. Among the prepared catalysts (BZI-NH₂, BZI-OH, and BZI-COOH), BZI-NH₂ exhibited the highest catalytic activity and recyclability, achieving a yield of 97% in the CO₂ cycloaddition. The synergistic effect of Br^-, hydrogen bond donors (-NH-, -NH₂), and N⁺ in BZI-NH₂ was found to contribute to its superior catalytic performance. DFT calculations were employed to study the effect of hydrogen bonds, Br^-, and N⁺ in BZI-NH₂ and BZI-OH on the CO₂ cycloaddition. Using BZI-NH₂ as an example, a mechanism was proposed for the synergistic effect between amino groups and bromide ions in catalyzing the CO₂ cycloaddition reaction.