Yolk–Shell-Structured Covalent Organic Frameworks with Encapsulated Metal–Organic Frameworks for Synergistic Catalysis

Yolk–shell composites offer a promising platform for integrating cores into hollow shells to create unique structures and properties. However, the concomitant functionality and tunability of yolk–shell nanocomposites is still a great challenge but highly desirable. Herein, we demonstrate a rational design for the fabrication of yolk–shell-structured covalent organic framework (COF)@metal–organic framework (MOF) (YS-COF@MOF) nanocomposites with COF as the external shell and MOF as the inner yolk. Series of the YS-COF@MOF composites with different MOF cores and COF shells were readily synthesized via a template-free solvothermal method. Control experiments showed that the formation of the hollow cavity between the core and the shell originated from the amorphous-to-crystalline transformation and the simultaneous shrinkage of the shell under the pyrrolidine-catalyzed conditions. The resultant YS-COF@MOF merges the inherent structure tunability and functionality of both COFs and MOFs. The functions of YS-COF@MOF can be regulated and optimized by judicious selections of metal ions and organic building blocks. Representative YS-TpPa@UiO-66-(COOH)2 with spatially distributed acidic and basic sites exhibited synergistically enhanced catalytic activity in one-pot deacetalization–Knoevenagel cascade reactions.