Structural regulation of ionic covalent organic frameworks for enhanced CO2 capture and catalytic conversion

Abstract

Ionic covalent organic frameworks (COFs), a subclass of ionic porous crystalline materials, have emerged as promising candidates for CO₂ capture and catalytic conversion. However, most reported ionic COFs rely on frameworks linked by reversible covalent bonds (e.g., imine –CN–), which compromises their structural stability and limits practical applications. To overcome these limitations, this work employs an industrially scalable and robust olefin-linked COF, NKCOF41, as a platform for constructing three ionic COFs (NKCOF41⁺–C₂H₅Br⁻, NKCOF41H⁺–Br⁻, and NKCOF41⁺–C₂H₄OHBr⁻) via quaternization and protonation strategies. These ionic derivatives preserve high crystallinity and specific surface area. Among them, NKCOF41⁺–C₂H₄OHBr⁻ exhibits exceptional CO₂ adsorption capacity (3.94 mmol g⁻¹, 72% higher than that of neutral NKCOF41), attributed to the synergistic effect of CO₂-philic –OH groups, Br⁻ sites, and a cationic framework. Catalytic studies demonstrate that NKCOF41⁺–C₂H₄OHBr⁻ effectively promotes the cycloaddition of CO₂. Under solvent-free and co-catalyst-free conditions, cyclic carbonates are obtained in yields of 89.4–99.1% at 60–100 °C, a CO₂ pressure of 0.1–0.5 MPa, and a reaction time of 24 h. Moreover, the catalyst exhibits excellent recyclability, retaining good catalytic activity over six consecutive cycles. This study presents a simple, scalable, and broadly applicable strategy for the design of high-performance, task-specific, and structurally stable ionic COFs.