Amorphous-to-Crystalline Transformation: General Synthesis of Hollow Structured Covalent Organic Frameworks with High Crystallinity

Morphological control of covalent organic frameworks (COFs) isparticularly interesting to boost their applications; however, it remains a grand challengeto prepare hollow structured COFs (HCOFs) with high crystallinity and uniformmorphology. Herein, we report a versatile and efficient strategy of amorphous-to-crystalline transformation for the general and controllable fabrication of highlycrystalline HCOFs. These HCOFs exhibited ultrahigh surface areas, radially orientednanopore channels, quite uniform morphologies, and tunable particle sizes. Mechanisticstudies revealed that H2O, acetic acid, and solvent played a crucial role in manipulatingthe hollowing process and crystallization process by regulating the dynamic imineexchange reaction. Our approach was demonstrated to be applicable to various aminesand aldehydes, producing up to 10 kinds of HCOFs. Importantly, based on thismethodology, we even constructed a library of unprecedented HCOFs includingHCOFs with different pore structures, bowl-like HCOFs, cross-wrinkled COFnanocapsules, grain-assembled HCOFs, and hydrangea-like HCOFs. This strategy wasalso successfully applied to the fabrication of COF-based yolk-shell nanostructures with various functional interior cores.Furthermore, catalytically active metal nanoparticles were implanted into the hollow cavities of HCOFs with tunable pore diameters,forming attractive size-selective nanoreactors. The obtained metal@HCOFs catalysts showed enhanced catalytic activity andoutstanding size-selectivity in hydrogenation of nitroarenes. This work highlights the significance of nucleation-growth kinetics ofCOFs in tuning their morphologies, structures, and applications.

Automated summary

This study reports a versatile and efficient strategy of amorphous-to-crystalline transformation for the fabrication of highly crystalline HCOFs. The approach allows for the control of hollowing process and crystallization process by regulating the dynamic imine exchange reaction. Various HCOFs with ultrahigh surface areas and uniform morphologies were successfully synthesized, and COF-based yolk-shell nanostructures with functional interior cores were also fabricated. The obtained metal@HCOFs catalysts showed enhanced catalytic activity and size-selectivity, highlighting the significance of nucleation-growth kinetics of COFs in tuning their morphologies, structures, and applications.