One‐Step Construction of Hydrophobic MOFs@COFs Core–Shell Composites for Heterogeneous Selective Catalysis

Abstract The exploration of novel porous core–shell materials is of great significance because of their prospectively improved performance and extensive applications in separation, energy conversion, and catalysis. Here, mesoporous metal–organic frameworks (MOFs) NH2‐MIL‐101(Fe) as a core generate a shell with mesoporous covalent organic frameworks (COFs) NUT‐COF‐1(NTU) by a covalent linking process, the composite NH2‐MIL‐101(Fe)@NTU keeping retentive crystallinity with hierarchical porosity well. Importantly, the NH2‐MIL‐101(Fe)@NTU composite shows significantly enhanced catalytic conversion and selectivity during styrene oxidation. It is mainly due to the hydrophilic MOF nanocrystals readily gathering the hydrophobic reactants styrene and boosting the radical mechanism path after combining the hydrophobic COFs shell. The synthetic strategy in this systematic study develops a new rational design for the synthesis of other core–shell MOF/COF‐based hybrid materials, which can expand the promising applications.


S 3 / S33
precipitate was isolated by centrifugation and washed with anhydrous 1,4-dioxane. The resultant material was purified by Soxhlet extraction using dichloromethane, dried in vacuum at 60 °C for 12 h to afford NTU-COF as a yellowish powder.
Synthesis of MIL@NTU-1 30 mg of NH 2 -MIL-101(Fe) was suspended in solution of 1, 4dioxane/mesitylene (1/1 v/v, 10 mL) in a 100mL thick walled pressure bottle. The suspension was sonicated for 30 min. Subsequently, various amounts of TAPB and 4-FPBA were added to the suspension such as 10 mg TAPB and 13 mg 4-FPBA. The suspension was continued to sonicate for another 30 min. After that, the reaction mixture was heated at 120 °C for 3 d. The precipitate was isolated by centrifugation and washed with anhydrous 1,4-dioxane. The resultant material was purified by Soxhlet extraction using dichloromethane, dried in vacuum at 60 °C for 12 h.

S 4 / S33
Adsorption experiment. The styrene (10 mM) was dissolved in acetonitrile. Well-dried adsorbent (10mg) was added to the 10 mL styrene solution (10 mM in acetonitrile). After adsorption for 10 minutes, the solution was removed. The remaining powder was dissolved in 10 mL acetonitrile and then the solution was collected by centrifugation, which was then diluted 100 times. The moles of adsorption were identified and quantified using GC-MS. Based on this, the amounts of styrene absorbed within materials were calculated.
Catalytic oxidation of styrene. The oxidation of styrene was performed in a 25 mL, three-necked flask equipped with a liquid condenser. In a catalytic run, the catalyst (10 mg), styrene (2 mmol) and tert-butyl hydroperoxide (TBHP, 6 mmol) were added to 10 mL of acetonitrile (CH 3 CN). Then the mixture was refluxed at 80 °C for 12 h. After the reaction, the solid catalyst was centrifuged, washed with acetonitrile and ethanol, dried in vacuum and reused without further purification. The products were identified and quantified using a gas chromatograph and argon gas as the carrier gas. Both the injector and detector temperatures were 250 °C. The reactant conversion and product for benzaldehyde were calculated as follows: Styrene conversion (mol%)=(moles of reactant converted)/(moles of reactant infeed)*100 Product selectivity (mol%)=(moles of product for benzaldehyde)/(moles of reactant converted)*100

Characterization
Field-emission scanning electron microscopy (FESEM) images were performed on a Hitachi SU8010 scanning electron microscope at 5.0 kV. Transmission electron microscopy (TEM) images were carried out using JEOL JEM-1400 at 120 kV. High angle annular dark field scanning transmission electron microscopy (HAADF-SEM) imaging and energy-dispersive X-ray spectroscopy (EDS) elemental mapping were carried out on JEOL ARM200 at 300 kV. Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku SmartLab diffractometer with Cu K ( = 1.540598 Å) radiation operating at 30 kV and 200 mA. X-ray photoelectron spectroscopy (XPS) were performed by a VG ESCALABMKII instrument. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Alpha spectrometer.
The content of Fe in different samples was determined by inductively coupled plasma spectrometer (ICP, Thermo Fisher Scientific). The contact angles were conducted using DSA30 (KRUSS GmbH,
[c] Blank means no catalyst was used.  Entry Catalysts Conv.