*3.1. Structure and Morphological Characterization*

MOFs can be readily converted to metal-oxide composites, which take advantage of their original morphology and porosity. The as-prepared HKUST-1, HKUST-1/Fe3O4, and HKUST-1/Fe3O4/CMF composite samples were then calcinated to obtain the nanoporous metal-oxide particles, generating nanoporous CuO, CuO/Fe3O4, and carbon-doped CuO/Fe3O4 composite catalysts, respectively. Pyrolysis/thermolysis of HKUST-1/Fe3O4/CMF composites led to the formation of porous carbon-doped CuO/Fe3O4 composites.

The size and the morphology of the synthesized materials were investigated using a transmission electron microscope (TEM) and scanning electron microscope (SEM). Figure 1 shows the SEM and TEM images of HKUST-1/Fe3O4/CMF composites and carbon-doped CuO/Fe3O4 composites after calcination in air. HKUST-1 crystals and Fe3O4 nanoparticles were uniformly loaded onto the surface of CMF (Figure 1). The nanocellulose MOF-derived porous carbon-doped CuO/Fe3O4 composite was then obtained through a calcination process under air at 500 ◦C for 5 h. The Brunauer–Emmett–Teller surface area (SBET) of the carbon-doped CuO/Fe3O4 sample was 38.7 m2/g, which was much higher than that of the CuO/Fe3O4 sample (0.042 m2/g) obtained from the calcination of HKUST-1/Fe3O4. The resultant carbon-doped CuO/Fe3O4 composite inherited largely the original porous morphology of HKUST-1. Based on the mapping images, the conclusion was that both Cu and Fe elements dispersed well in the carbon-doped CuO/Fe3O4 composite catalyst.

XPS analysis was employed to investigate the elemental composition on the surface of different composites. For the XPS spectrum of carbon-doped CuO/Fe3O4 composite catalyst (Figure 2a), the main peaks were C*1s*, O*1s*, Fe*2p*, and Cu*2p*, centered at around 285 eV, 530 eV, 720 eV, and 930 eV, respectively. The C*1s* XPS pattern of the sample is shown in Figure 2b. The spectrum contains two peaks at 285 eV and 288.5 eV, which may be attributed to carbon (*sp<sup>2</sup>* hybridization) in the sample, and the Cu–O–C bonds or Fe–O–C bonds, respectively. Figure 2c shows the Cu*2p* core-level XPS spectrum of the composite catalyst. The Cu*2p1* and Cu*2p3* binding energies for the composite catalyst were 952.8 and 932.7 eV, respectively, indicating the presence of CuO in the composite catalyst. Similar results were reported in the literature [38,39]. The Fe*2p3* and Fe*2p1* binding energies (Figure 2d) for the composite catalyst were 710.7 and 725.4 eV, respectively, which agrees with published results [35,40], confirming the presence of Fe3O4 in the composite catalyst.

**Figure 1.** SEM images of (**a**) HKUST-1/Fe3O4/cellulose microfibril (CMF) composites and (**b**) carbon-doped CuO/Fe3O4 composite catalysts after calcination; TEM images of (**c**) carbon-doped CuO/Fe3O4 composite catalysts; (**d**–**f**) energy-dispersive X-ray spectroscopy (EDX) mapping of carbon-doped CuO/Fe3O4 composite catalysts.

**Figure 2.** (**a**) High-resolution X-ray photoelectron spectroscopy (XPS) survey spectra of the carbon-doped CuO/Fe3O4 composite catalysts. (**b**) High-resolution XPS scans over C*1s* peaks of the carbon-doped CuO/Fe3O4 composite catalysts. (**c**) High-resolution XPS scans over Cu*2p* peaks of carbon-doped CuO/Fe3O4 composite catalysts. (**d**) High-resolution XPS scans over Fe*2p* peaks of carbon-doped CuO/Fe3O4 composite catalysts.

The crystalline nature and the composition of the as-synthesized products were characterized using PXRD. The crystalline phases of CuO, such as (110), (11-1), (111), (20-2), (020), (202), (11-3), (31-1), and (220) are shown in Figure 3a, which are consistent with those reported in the literature [41,42]. These results support the conclusion that HKUST-1 was transformed into CuO via calcination under the present conditions. In the XRD pattern of carbon-doped CuO/Fe3O4 composite, the diffraction peaks and relative intensities were indexed to Fe3O4 (JCPD NO. 19-0629) and CuO (JCPD NO. 05-0661). This indicates that CuO and Fe3O4 were both obtained via calcining HKUST-1/Fe3O4/CMF composites under air. No other C-related impurity peaks were detected; a similar result was reported in the previous research [43].

**Figure 3.** (**a**) Powder X-ray diffraction (PXRD) patterns of carbon-doped Fe3O4/CuO composite catalyst. (**b**) Magnetization curve at 300 K of carbon-doped Fe3O4/CuO composite catalyst.

The magnetic behavior of the carbon-doped Fe3O4/CuO composite catalyst sample was evaluated at 300 K. Its magnetization saturation value (Ms) was 15.1 emu·g−1, suggesting a good magnetic property (Figure 3b). Thus, the magnetic carbon-doped Fe3O4/CuO composite catalyst could be readily separated from the reaction system using a magnet for the recycling process.
