A covalent organic framework-based route to the in situ encapsulation of metal nanoparticles in N-rich hollow carbon spheres

Covalent organic frameworks doped with metal cations can be used as novel precursors for the in situ encapsulation of metal NPs into N doped hollow carbon spheres. The integration of the hollow structure, N dopant and ultrafine Pd NPs gives the hybrid nanocomposites advanced catalytic performance.


Catalysts preparation
All chemicals were purchased from commercial sources and used without further treatments.

Synthesis of LZU1
1,3,5-triformylbenzene (48 mg) and 1,4-diaminobenzene (48 mg) were dissolved in 3 mL of 1,4-dioxane. The mixture was then transferred into a 25 mL tube. After adding 0.6 mL of 3.0 mol/L aqueous acetic acid, the mixture was flash frozen using a liquid-nitrogen bath. The tube was then sealed after degassing for 15 min. Upon warming to room temperature, the tube was heated at 120 °C for 3 days. The produced products were isolated by centrifugation, washed with DMF (30 mL × 3), THF (30 mL × 3), and dichloromethane (30 mL × 3) successively. The obtained solids were dried under vacuum at 100 °C for 10 h.

Synthesis of Pd II -LUZ1
LZU1 (80 mg) and palladium acetate (3.1 mg) were dispersed in 10 mL dichloromethane in a 25 mL flask. Then the mixture was kept stirring for 24 h at room temperature. The products were collected via centrifugation, washed with dichloromethane (3 x 20 mL). Finally, the sample was dried in air at room temperature for 1 h.

Synthesis of Pd@NHCS
The prepared Pd II -LUZ1 was carbonized at different temperatures under argon atmosphere (constant flow at 40 mL min −1 ) in a tubular furnace. The temperature was first heated to 150 °C at a heating rate of 1 °C min -1 and was held at this temperature for 2 h. The temperature was then raised to a particular value at a rate of 1 °C min −1 and was maintained at the final temperature for 3 h. The prepared catalyst was denoted as Pd@NHCS(X), where X indicated the pyrolysis temperature.

Synthesis of Pd/N-C(500)
The N-C support was synthesized by direct pyrolysis of ZIF-67, followed by removing the Co component in aqua regia for 24 h. Next, the N-C support was isolated by centrifugation and washed several times with deionized water and absolute ethanol before drying under vacuum at 150 °C overnight. Pd/N-C(500) was obtained by impregnating N-C (100 mg) with palladium acetate (8.4 mg) in dichloromethane (10 mL). The solid was treated at 500 °C under argon atmosphere following the same procedures for the preparation of Pd@NHCS(500).

Synthesis of Pd/C(500)
Pd/C(500) was obtained following the same procedures as for Pd/N-C(500) except using activated carbon as support.

Characterization
The BET surface area measurements were performed with N 2 adsorption isotherms at 77 K on a Micromeritics ASAP 2020M instrument. Before the analysis, the samples were evacuated at 100 C for 2 h. Powder X-ray diffraction patterns of the samples were recorded on a Rigaku diffractormeter (D/MAX-IIIA, 3 kW) employing Cu Kα radiation (λ = 0.1543 nm) at 40 kV, 40 mA at room temperature. TGA of Pd II -LZU1 was performed on a NETZSCH STA449C under argon atmosphere. X-ray photoelectron spectroscopy (XPS) measurements were performed on on a Kratos Axis Ultra DLD system with a base pressure of 10 −9 Torr. The size and morphology of samples were investigated by using a transmission electron microscope (JEM-2100F) with EDX analysis (XFlash 5030T) operated at 200 kV. The palladium contents of the samples were determined quantitatively by atomic absorption spectroscopy (AAS) on a HITACHI Z-2300 instrument.

Catalytic reactions
Typical procedures for hydrogenation of nitrobenzene: Nitrobenzene (0.1 mmol) and 1 mol% Pd catalyst (4.4 mg, 5.1 mg and 5.3 mg for Pd@NHCS (500) Typical procedures for aerobic oxidation of cinnamyl alcohol: cinnamyl alcohol (0.1 mmol) and 1 mol% Pd catalyst (4.4 mg, 5.1 mg and 5.3 mg for Pd@NHCS(500), Pd/C-N(500) and Pd/C(500), respectively) were added to 2 mL of toluene. The reaction mixture was stirred at 80 °C under air atmosphere. Upon reaction completion, the catalyst particles were removed from the solution by filtration and washed with toluene. The liquid phase was subsequently analyzed by GC/MS (Agilent Technologies 7890B-5977A equipped with a 0.25 mm × 30 m HP-5MS capillary column).

In Situ ATR-IR Spectroscopy
ATR-IR spectra were recorded on a Thermo Fisher iS10 equipped with a liquid nitrogen cooled MCT detector. The spectra were obtained by averaging 32 scans at a resolution of 1 cm -1 . The thin film of catalyst powder deposited on the ZnSe element for ATR-IR spectroscopic study was prepared as follow. A suspension of ca. 50 mg of catalyst powder in 2 mL of chloroform was placed in an ultrasonic bath for 1 h in order to obtain a uniform suspension. A thin layer of solution was spread onto a ZnSe internal reflection element (IRE) and dried out at room temperature. This procedure was repeated six times, subsequently the sample was dried in a vacuum oven for a complete evaporation of methanol.         [a] Measured by elemental analysis.
Appendix The MS spectra for the products listed in Figure 4 and Tables S3.