Covalent Organic Frameworks via In Situ Monomer Release for Humid CO₂ Uptake

Area of Science:

• Materials Science
• Chemistry
• Nanotechnology

Background:

• Covalent organic frameworks (COFs) are challenging to synthesize with high crystallinity and porosity, especially azine-linked systems due to limited bond reversibility.
• Existing strategies for improving crystallinity primarily focus on imine-linked COFs, with limited progress in azine-linked analogues.

Purpose of the Study:

• To develop a generalizable kinetic modulation strategy for enhancing crystallinity in azine-linked COFs.
• To synthesize novel crystalline azine-linked COFs with improved structural order and porosity.
• To evaluate the performance of these COFs in gas separation under humid conditions.

Main Methods:

• A kinetic modulation strategy involving the in situ slow release of hydrazine via trifluoroacetic acid-mediated hydrolysis of a tetrazine precursor during COF synthesis.
• Synthesis of two crystalline azine-linked COFs using either a nonplanar bicarbazole or a planar pyrene core.
• Characterization using Powder X-ray Diffraction (PXRD) and time-resolved UV-vis spectroscopy to analyze structural evolution and reaction kinetics.

Main Results:

• Successfully synthesized two crystalline azine-linked COFs with enhanced structural order (narrower PXRD FWHM) and higher surface areas compared to conventional methods.
• Demonstrated the role of reaction kinetics in framework evolution through time-resolved analyses.
• Observed S-shaped water vapor adsorption isotherms, indicating hydrophobic backbones and minimal water uptake below 40% relative humidity (RH).
• Confirmed that CO2 uptake is largely unaffected at 40% RH, with no interference from water in dynamic breakthrough experiments.

Conclusions:

• The kinetic modulation strategy provides a generalizable route for improving crystallinity in kinetically trapped COFs.
• The synthesized azine-linked COFs show promise as materials for efficient gas separation, particularly CO2 capture, under humid environments.
• This work overcomes limitations in azine-linked COF synthesis, opening new avenues for designing advanced porous materials.