Sequential Modification Strategy to Construct Crystalline Benzimidazole Covalent Organic Frameworks with Extraordinary Stability and Polymerization Degree

The intriguing proton-conducting and catalytic properties of benzimidazole-functionalized covalent organic frameworks (BIM-COFs) largely depend on their material quality. Here, we report a novel sequential modification (SM) strategy that enables the preparation of extraordinarily stable BIM-COFs with high polymerization degree. As a “proof of principle”, the designed TpBD-BIMSM was successfully synthesized using the SM strategy, which exhibited superior chemical stability (slight structural change after treatment in 6 M HCl or 6 M NaOH). Compared with the counterpart produced by the conventional direct condensation approach (TpBD-BIMDC), TpBD-BIMSM exhibited a higher Brunauer–Emmett–Teller (BET) surface area (213 m2 g–1 vs 52 m2 g–1) and significantly enhanced CO2, CH4, C2H6, and C2H4 uptake capacities. Furthermore, the proton conductivity of TpBD-BIMSM was measured to be 1.2 × 10–2 S cm–1, which is 2 orders of magnitude higher than that of TpBD-BIMDC (7.2 × 10–4 S cm–1) under identical conditions (80 °C and 98% RH) and also ranks it among the highest in all COF-based proton conductors. In addition, TpBD-BIMSM showed a lower activation energy (Ea) value than TpBD-BIMDC (0.16 eV vs 0.20 eV). Such high proton conductivity and low Ea value of TpBD-BIMSM can be attributed to its large surface area and high polymerization degree, which could provide extra proton transfer paths and accelerate proton movement. Furthermore, such a strategy can be readily extended to construct other BIM-COFs (TpOMe-BIMSM) that thus highlights the generality of sequential modification strategy. And this strategy thus paves a new way for constructing stable and highly polymerized BIM-COFs for related applications.