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

Abstract

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.

Landscaping Covalent Organic Framework Nanomorphologies

“…Using macrostructural (Al 2 O 3 , SiO 2 , Fe 3 O 4 , etc.) or molecular templates (e.g., micelles) to generate core–shell morphologies or hollow spheres is a straightforward method to synthesize 0-D COF morphologies. − Direct mixing of macrostructural templates into the concentrated solution (10–18 mmol L –1 ) of COF precursors leads to the assembly of the amorphous polymer on the template surface. The thermodynamic conditions drive the transition from the amorphous polymeric phase to the crystalline networks of COF .…”

Donor–Acceptor Pairs in Covalent Organic Frameworks Promoting Electron Transfer for Metal-Free Photocatalytic Organic Synthesis

“…Covalent organic frameworks (COFs) consist of ordered organic molecular building units via covalent bonds and possess unprecedented structural designability, crystallinity, and porosity . These materials have been researched in many fields, including small-molecule storage and separation, sensing, drug delivery, energy storage, and heterocatalysis. − COFs with 2-dimensional structures (2D COFs) possess π-conjugated structures in-plane, while the tailorable structures endow them with systematic modulation of light absorption, band structure, and electron mobility, making them promising photocatalysts. − Moreover, the clear atom location in COFs contributes to insights into the catalytic mechanism . Therefore, in recent years, 2D COFs as photocatalysts for solar-energy transfer have been explored in organic synthesis, H 2 evolution, and CO 2 reduction …”

Combining metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs): Emerging opportunities for new materials and applications

“…Not merely growing COFs onto the surface of MOFs, alternatively, the study of COF@MOF hybrids has been proposed in recent years [30,69,139,140]. For example, Lan' s group [28] utilized a one-step strategy to achieve MOF growth onto the surface of COF by covalent bonds, namely NH 2 -UiO-66@TpPa-1-COF (Tp = 1,3,5-triformylphloroglucinol, Pa = pphenylenediamine).…”

“…In brief, the stable and environmentally friendly immunosensor exhibited the advantages of convenience, rapid response and low-cost for antigen GL-3 detection, compared with traditional analytical techniques. Obviously, MOF/COF hybrid materials have shown great potential in detection [140,188,189].…”