*3.3. Synthesis of Cu2O@Cu-BDC-NH<sup>2</sup>*

od-Cu2O@Cu-BDC-NH2: The od-Cu2O@Cu-BDC-NH<sup>2</sup> was prepared by an in situ assembly method. First, the as-prepared od-Cu2O (80 mg) was uniformly dissolved in 20 mL ethanol, and 181.1 mg of 2-aminoterephthalic acid was added into 20 mL of a mixed solution of ethanol and DMF (1:1 vol) under continuous stirring. After stirring well, the two separate solutions were mixed at room temperature with magnetic stirring for different periods (4 h–32 h) to control the growth of MOFs on od-Cu2O. The resulting products of od-Cu2O@Cu-BDC-NH2-xh (x = 4, 8, 12, 20 and 32) were washed by deionized water and ethanol several times, collected by centrifugation, and finally dried at room temperature under vacuum condition overnight.

cod-Cu2O@Cu-BDC-NH2: The preparation process of the cod-Cu2O@Cu-BDC-NH<sup>2</sup> was the same as that of the od-Cu2O@Cu-BDC-NH2, except that the precursor was changed from od-Cu2O to cod-Cu2O, and the resulting product of cod-Cu2O@Cu-BDC-NH2-8h was obtained after reaction for 8 h under magnetic stirring.

## *3.4. Synthesis of Cu-BDC-NH<sup>2</sup> Nanosheets*

The metal solution (named as M1) was prepared by dissolving 30 mg of Cu(NO3)<sup>2</sup> into a mixed solution of 3 mL of DMF and 1 mL of CH3CN. The ligand solution (named as L1) was synthesized by dissolving 30 mg of H2BDC-NH<sup>2</sup> into 4 mL of the mixed solution of DMF and CH3CN (1:3 vol). Then, the above solution M1 was added into L1, and the mixture was left to stand under an ambient environment for 24 h. Finally, the resulting product of Cu-BDC-NH<sup>2</sup> was collected by centrifugation, washed with DMF three times, and then stored in DMF.

#### *3.5. Characterization*

The morphology of the as-prepared nanomaterials was characterized using a scanning electron microscope (SEM, Regulus 8100, Hitachi, Japan) with an accelerating voltage of 5 kV. The crystallinity and structural details of materials were obtained through X-ray diffraction (XRD, Bruker D8 Advance, Bruker, Germany) using a Cu Kα radiation (λ = 1.541 Å). Fourier-transform infrared (FT-IR) spectra were conducted on a Nicolet 6700 spectrometer (Thermo Scientific, Waltham, MA, USA) using the potassium bromide (KBr) pellet technique over a range of 400–4000 cm−<sup>1</sup> . X-ray photoelectron spectroscopy (XPS) data were analyzed using a Thermo ESCALAB 250Xi spectrometer (Thermo Scientific, USA). The nitrogen adsorption–desorption isotherm was collected by using a Micromeritics ASAP2460 instrument (Micromeritics, Norcross, GA, USA) at 77 K. The thermal property was measured using thermogravimetric analysis (TGA, METTLER TOLEDO TGA/DSC3+, Mettler Toledo, Switzerland) in the range of 25–800 ◦C at a heating–cooling rate of <sup>±</sup><sup>10</sup> ◦C min−<sup>1</sup> under a nitrogen gas flow rate of 20 mL min−<sup>1</sup> .

#### *3.6. Catalytic Tests for Selective Oxidation of Styrene*

The catalytic tests for the selective oxidation of styrene were carried out as follows: the as-prepared catalyst (10 mg), styrene (2 mmol), and acetonitrile (5 mL) were added into three-neck round-bottom flask (25 mL) equipped with a reflux condenser. The device was immersed into an oil bath and heated to the desired temperature under magnetic stirring. Then, 6 mmol of 30 wt.% H2O<sup>2</sup> aqueous solution was added into the mixture to initiate the reaction. After a desired time, the reaction mixture was centrifuged, and the liquid layer was analyzed by a gas chromatography-mass spectrometer (GC-MS, Agilent 7890/5975C, Agilent, Santa Clara, CA, USA) using the external standard method for quantitative analysis.

The stability of the as-prepared catalyst was investigated by a mental leaching test. In a typical process, the catalytic reaction was stopped after 4 h and the solid catalyst of Cu2O@Cu-BDC-NH<sup>2</sup> was separated and removed by centrifugation. The reaction solution without the Cu2O@Cu-BDC-NH<sup>2</sup> catalyst was then stirred for a further 6 h, and the catalytic performance was analyzed by GC-MS.

#### **4. Conclusions**

In summary, a novel core-shell structured Cu2O@Cu-BDC-NH<sup>2</sup> heterogeneous catalyst with tunable Cu+/Cu2+ interface, variable composition, and structure was synthesized by a facile in situ self-assembly method. With H2O<sup>2</sup> as green oxidants, the resultant Cu2O@Cu-BDC-NH<sup>2</sup> catalysts exhibited significant catalytic performance for the selective oxidation of styrene at 40 ◦C under base-free conditions, and the optimized conversion of styrene and selectivity of benzaldehyde could reach 85% and 76%, respectively. The delicate combination of Cu2O and Cu-BDC-NH<sup>2</sup> not only provides a well-designed Cu+/Cu2+ active interface and porous MOF shells for mass transfer and protection of the active Cu2O component, but also provides appropriate acid–base regulation methods to improve the selectivity of the target product, thus suggesting a new perspective and simple strategies for the construction of a highly efficient and green catalytic system for the selective oxidation of styrene.

**Author Contributions:** Conceptualization, Methodology, X.Z.; Formal analysis, data curation, X.Z., M.H. and X.T.; Investigation, Data curation, P.W., Z.Z., X.B. and Y.J.; Writing-Original Draft Preparation, Review and Editing, X.Z., M.H. and X.T.; Writing-Review and Editing, X.T. and P.W.; Funding acquisition, X.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was financially supported by the National Natural Science Foundation of China (No. 52002029).

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

