Encapsulation of C–N-decorated metal sub-nanoclusters/single atoms into a metal–organic framework for highly efficient catalysis

A facile strategy was designed for the encapsulation of C–N-decorated Pd sub-nanoclusters/single atoms into MOF pores by the confined thermolysis of metal–organic polyhedra (MOPs). The obtained hybrids exhibited excellent catalytic performance in various important chemical processes.

obtained solid was washed with water, and activated by heating at 150 °C for 12 h under dynamic vacuum.

Synthesis of PCN⊂M
The M 6 L 4 ⊂MIL-101 hybrids were treated at 250°C under H 2 flow for 2 h. The heating rate and cooling rate were 0.5 °C/min. The obtained hybrids were washed with H 2 O and ethanol, and then dried at room temperature. PCN⊂M hybrids with different Pd contents were synthesized according to the above protocol by varying the amount of precursors. The obtained hybrids were named as XPCN⊂M (X = 0.33, 0.51, 0.64, 0.82), where X represents the actual Pd weight percentage, as measured by atomic absorption spectrometry (AAS). For 0.33PCN⊂M, 0.51PCN⊂M, 0.64PCN⊂M and 0.82PCN⊂M, the added L precursor was 3.0 mg, 4.7 mg, 6.3 mg and 7.4 mg, and the M mass was 3.5 mg, 5.2 mg, 7 mg and 8.6 mg, respectively. The theoretical Pd loading, the actual Pd loading and the doping efficiecy were shown in Table S2.
After stirring for 24 h, the suspension was filtered and washed with H 2 O for several times. The obtained solid was washed with water, activated by heating at 150 °C for 12 h under dynamic vacuum, and then heated at 250 °C in H 2 for 2 h. The heating rate and cooling rate were both 0.5 °C/min. The theoretical Pd loading, the actual Pd loading and the doping efficiecy were shown in Table S2.

Nitrobenzene hydrogenation reaction
Typically, nitrobenzene (0.1 mmol) and catalyst (0.0015 mmol Pd) were added to a Schlenk tube. Ethanol (2 mL) was then added under hydrogen at room temperature.
Then an hydrogen balloon was connected to the Schlenk tube. The reaction mixture was stirred at room temperature for the desired time. Afterwards, the reaction liquid was collected by centrifugation. the solid catalyst was washed with ethyl acetate (3 × 5 mL). The liquid was all collected and concentrated in vacuo. The crude product was quantified by GC-MS analysis.

Heterogeneity of the catalyst
To verify whether the catalysis of 0.64PCN⊂M was truly heterogeneous, the solid catalyst was filtered from the reaction solution after 2 min (Con. 52%) in nitrobenzene hydrogenation reaction. The reaction was continued with the filtrate in the absence of solid catalyst for an additional 5 min. The solution in the absence of solid did not exhibit any further reactivity. The results demonstrated that the reaction proceeded mostly on the heterogeneous surface.

Catalytic hydrogenation of furfural
Typically, FFA (0.52 mmol), catalyst (Pd/FFA = 1.15 * 10 -3 ) and water (4 mL) were added to a 10 mL stainless steel autoclave equipped with a magnetic stirrer, a pressure gauge, and an automatic temperature controller. Then the autoclave was sealed and purged with hydrogen at low pressure for several times to remove air.
Then the autoclave was purged with H 2 for the desired pressure and then set to the desired temperature. After reaction, the autoclave was cooled to room temperature and the reaction liquid was collected by centrifugation. The solid catalyst was washed with ethyl acetate (3 × 5 mL). The liquid was all collected and concentrated in vacuo.
The crude product was quantified by GC-MS analysis.
The conversions and selectivities were calculated based on the moles of furfural.
The conversion of furfural (mol%) and cyclopentanone selectivity (mol%) were calculated using the following equations: ( 1 -Moles of furfural

Recycling of the catalysts
The recyclability of the catalysts was tested for the nitrobenzene hydrogenation reaction and the catalytic hydrogenation of furfural under the investigated reaction conditions as described above except using the recovered catalysts. Each time, the catalyst was separated from the reaction mixture by centrifugation at the end of catalytic reaction, thereafter, washed with ethyl acetate and ethanol, dried at room temperature. The catalyst powder was reused as catalyst for a new run.

Materials characterization
BET surface area measurements were performed with N 2 adsorption/desorption isotherms at 77 K on a Micromeritics ASAP 2020M instrument. Prior to analysis, the samples were degassed at 150 C for 12 h. Powder X-ray diffraction patterns of the samples were recorded on a Bruker D8 ADVANCR using Cu Kα radiation (40 kV