Preparation of hollow spherical covalent organic frameworks via Oswald ripening under ambient conditions for immobilizing enzymes with improved catalytic performance

The hollow spherical covalent organic frameworks (COFs) have a wide application prospect thanks to their special structures. However, the controllable synthesis of uniform and stable hollow COFs is still a challenge. We herein propose a self-templated method for the preparation of hollow COFs through the Ostwald ripening mechanism under ambient conditions, which avoids most disadvantages of the commonly used hard-templating and soft-templating methods. A detailed time-dependent study reveals that the COFs are transformed from initial spheres to hollow spheres because of the inside-out Ostwald ripening process. The obtained hollow spherical COFs have high crystallinity, specific surface area (2,036 m(2).g(-1)), stability, and single-batch yield. Thanks to unique hollow structure, clear through holes, and hydrophobic pore environment of the hollow spherical COFs, the obtained immobilized lipase (BCL@H-COF-OMe) exhibits higher thermostability, polar organic solvent tolerance, and reusability. The BCL@H-COF-OMe also shows higher catalytic performance than the lipase immobilized on non-hollow COF and free lipase in the kinetic resolution of secondary alcohols. This study provides a simple approach for the preparation of hollow spherical COFs, and will promote the valuable research of COFs in the field of biocatalysis.

Automated summary

This study proposes a self-templated method for the preparation of hollow spherical COFs through the Ostwald ripening mechanism under ambient conditions. The obtained hollow COFs exhibit high crystallinity, stability, and yield, and can be used for immobilized enzyme applications with enhanced catalytic performance. This research will promote the valuable research of COFs in the field of biocatalysis.