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.