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-BIM SM 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-BIM DC ), TpBD-BIM SM exhibited a higher Brunauer–Emmett–Teller (BET) surface area (213 m 2 g –1 vs 52 m 2 g –1 ) and significantly enhanced CO 2, CH 4, C 2 H 6, and C 2 H 4 uptake capacities. Furthermore, the proton conductivity of TpBD-BIM SM was measured to be 1.2 × 10 –2 S cm –1, which is 2 orders of magnitude higher than that of TpBD-BIM DC (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-BIM SM showed a lower activation energy ( E a ) value than TpBD-BIM DC (0.16 eV vs 0.20 eV). Such high proton conductivity and low E a value of TpBD-BIM SM 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-BIM SM ) 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.