Sequential Modification Strategy to Construct Crystalline Benzimidazole Covalent Organic Frameworks with Extraordinary Stability and Polymerization Degree
Jinli Li, Qiong Lei, Xinglong Dong, Cailing Chen, Xiongli Liu, Zhiyuan Zhang, Feng Shui, Mao Yi, Baiyan Li, Xian‐He Bu
Abstract
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.