Nano-Cellulose/MOF Derived Carbon Doped CuO/Fe3O4 Nanocomposite as High Efficient Catalyst for Organic Pollutant Remedy

Metal–organic framework (MOF)-based derivatives are attracting increased interest in various research fields. In this study, nano-cellulose MOF-derived carbon-doped CuO/Fe3O4 nanocomposites were successfully synthesized via direct calcination of magnetic Cu-BTC MOF (HKUST-1)/Fe3O4/cellulose microfibril (CMF) composites in air. The morphology, structure, and porous properties of carbon-doped CuO/Fe3O4 nanocomposites were characterized using SEM, TEM, powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), and vibrating sample magnetometry (VSM). The results show that the as-prepared nanocomposite catalyst is composed of Fe3O4, CuO, and carbon. Compared to the CuO/Fe3O4 catalyst from HKUST-1/Fe3O4 composite and CuO from HKUST-1, this carbon-doped CuO/Fe3O4 nanocomposite catalyst shows better catalytic efficiency in reduction reactions of 4-nitrophenol (4-NP), methylene blue (MB), and methyl orange (MO) in the presence of NaBH4. The enhanced catalytic performance of carbon-doped CuO/Fe3O4 is attributed to effects of carbon preventing the aggregation of CuO/Fe3O4 and providing high surface-to-volume ratio and chemical stability. Moreover, this nanocomposite catalyst is readily recoverable using an external magnet due to its superparamagnetic behavior. The recyclability/reuse of carbon-doped CuO/Fe3O4 was also investigated.


Introduction
Recently, metal nanoparticles (NPs) were widely used in the fields of biomedicine and chemical reactions due to their high selectivity and catalytic efficiency [1][2][3]. Noble-metal nanoparticles (gold, silver, etc.) [4][5][6] and non-noble-metal nanoparticles (copper, zinc, and their oxides, sulfides, etc.) [7][8][9][10] are particularly noticeable. For example, Jiang et al. [11] reported that CuO and Au domains could greatly improve the photocatalytic activity and stability of Cu 2 O cubes in the photocatalytic degradation of methyl orange (MO). Rodríguez et al. [12] reported that potassium poly(heptazine imide) (PHIK)/Ti-based metal-organic framework (MIL-125-NH 2 ) composites had superior photocatalytic activity in rhodamine B (RhB) degradation under blue-light irradiation. Among the applications, metal nanoparticles can also be used for treating wastewater and drinking water due to their large surface areas and high activity [13,14]. With growing focus on the development 10 min under magnetic stirring (500 rpm) to remove the dissolved oxygen from the CMF suspension. Then, citric acid (1 mg) and diluted HCl solution (1 mL, 2 mol/L) were added to the above solution, followed by the successive additions of 1.3 mmol Fe(NO 3 ) 3 ·9H 2 O and 0.65 mmol FeSO 4 ·7H 2 O solid samples with nitrogen bubbling and magnetic stirring. The function of citric acid was to protect the as-prepared Fe 3 O 4 NPs from being oxidized by the dissolving oxygen in water. A dilute NaOH solution (10 mL, 2 mol/L) was added drop-wise into the above system, followed by bubbling with nitrogen and magnetic stirring (500 rpm) for 120 min at room temperature. The obtained Fe 3 O 4 /CMF composites (using a magnet) were washed three times with distilled water in a centrifugation step (4000 rpm, 10 min).
Copper (II) acetate monohydrate was dissolved in distilled water. Fe 3 O 4 /CMF nanocomposites and H 3 BTC were dispersed in ethanol and then treated in a sonicator (QSON-ICA) for 5 min. The ethanol solution containing Fe 3 O 4 /CMF composite and H 3 BTC was added to the aqueous system. The color of the solution changed from light blue to blue-black immediately, and the system was stirred continuously for 4 h. The final HKUST-1/Fe 3 O 4 /CMF composite was washed in a centrifugation step (4000 rpm, 10 min) with distilled water and absolute ethanol three times. Then, the drying was done at 50 • C under vacuum.
The synthesized HKUST-1/Fe 3 O 4 /CMF composites were placed in a ceramic boat, then transferred to a muffle roaster, which was operated under the conditions of 500 • C, air atmosphere, and a heating rate of 20 • C/min, before being held at 500 • C for 5 h; the procedures were similar to those described in the literature [36]. After cooling to room temperature, the sample was washed several times with distilled water and absolute ethanol in a centrifugation step (4000 rpm, 10 min), and finally dried in a vacuum at 60 • C for 5 h.

Characterization
Transmission electron microscope (TEM) analyses were conducted with a JEOL JEM 2011 transmission electron microscope at 200 kV. Scanning electron microscope (SEM) analyses were conducted with a JEOL JSM 6400 scanning electron microscope. The powder X-ray diffraction (PXRD) patterns of the prepared samples were collected using an X-ray diffractometer with Cu-Kα radiation (40 kV, 30 mA). The patterns were recorded in the region of 2θ from 10 • to 80 • with a scan step of 0.02 • . The chemical binding energies of the respective ions in the samples were measured using X-ray photoelectron spectroscopy (XPS, ESCALa-b220i-XL electron spectrometer, Thermo Fisher Scientific K-Alpha, UK) under 300-W Al-Kα radiation. The magnetic properties were measured with a vibrating sample magnetometer (VSM), a physical property measurement system, at 300 K, as a function of the applied magnetic field between −80 and 80 kOe. The ultraviolet-visible (UV-Vis) diffuse reflectance data were collected with a UV-Vis spectrophotometer (Evolution 201, Thermo Scientific) equipped with an integrated sphere.

Evaluation of Catalytic Performance
The catalytic reduction of 4-nitrophenol (4-NP) using NaBH 4 was chosen as the model reaction for investigating the catalytic performance of magnetic carbon-doped CuO/Fe 3 O 4 nanocomposite catalysts. Typically, catalytic reduction of a 30-mL 4-NP solution (1 mmol/L) was carried out in a beaker (100 mL) by continuously stirring at room temperature. Upon the addition of NaBH 4 (10 mM) into the 4-NP solution, its color changed immediately from light yellow to dark yellow due to the formation of 4-nitrophenolate ions (formed from the high alkalinity of NaBH 4 ) [37]. Then, the dark-yellow color faded with time (due to the conversion of 4-NP to 4-aminophenol (4-AP)) after the addition of carbon-doped CuO/Fe 3 O 4 nanocomposite catalyst (5 mg) (Scheme 1). UV-Vis adsorption spectra were recorded using a UV-Vis spectrophotometer in the range of 250-550 nm. When the reaction completed, the catalyst was easily separated from the solution using an external magnet due to its good magnetic performance. During the degradation process of organic dyes, 30 mL of dye solution (10 mg/L) and NaBH4 (10 mM) were mixed in a beaker (100 mL) by continuously stirring at room temperature. Then, 5 mg of carbon-doped CuO/Fe3O4 nanocomposite catalyst was added to the reaction system, and the catalytic process was monitored using the UV-Vis spectrophotometer. When the reaction ended, the catalyst was readily separated from the solution using an external magnet. 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. During the degradation process of organic dyes, 30 mL of dye solution (10 mg/L) and NaBH 4 (10 mM) were mixed in a beaker (100 mL) by continuously stirring at room temperature. Then, 5 mg of carbon-doped CuO/Fe 3 O 4 nanocomposite catalyst was added to the reaction system, and the catalytic process was monitored using the UV-Vis spectrophotometer. When the reaction ended, the catalyst was readily separated from the solution using an external magnet.

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/Fe 3  The size and the morphology of the synthesized materials were investigated using a transmission electron microscope (TEM) and scanning electron microscope (SEM). Figure Figure 2b. The spectrum contains two peaks at 285 eV and 288.5 eV, which may be attributed to carbon (sp 2 hybridization) in the sample, and the Cu-O-C bonds or Fe-O-C bonds, respectively. Figure 2c shows the Cu2p core-level XPS spectrum of the composite catalyst. The Cu2p1 and Cu2p3 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 Fe2p3 and Fe2p1 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 Fe 3 O 4 in the composite catalyst.
carbon-doped CuO/Fe3O4 nanocomposite catalyst was added to the reaction system, and the catalytic process was monitored using the UV-Vis spectrophotometer. When the reaction ended, the catalyst was readily separated from the solution using an external magnet. 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 m 2 /g, which was much higher than that of the CuO/Fe3O4 sample (0.042 m 2 /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. 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), , , 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/Fe 3 O 4 composite, the diffraction peaks and relative intensities were indexed to Fe 3 O 4 (JCPD NO. 19-0629) and CuO (JCPD NO. 05-0661). This indicates that CuO and Fe 3 O 4 were both obtained via calcining HKUST-1/Fe 3 O 4 /CMF composites under air. No other C-related impurity peaks were detected; a similar result was reported in the previous research [43]. 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 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].

Structure and Morphological Characterization
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.

Catalytic Reduction
In this study, we investigated the application of carbon-doped CuO/Fe3O4 composite catalysts for the catalytic reduction of 4-nitrophenol. In Figure 4a, the results are shown for the catalytic reduction of 4-NP to 4-AP in the presence of NaBH4 and carbon-doped CuO/Fe3O4 composite catalysts. The reduction process was followed based on the UV-Vis spectrophotometry. It shows that the absorbance at 400 nm (4-NP) decreased gradually as a function of time, while the absorbance at 290 nm (due to 4-AP) increased, confirming the catalytic reduction of 4-NP to 4-AP [44]. The catalytic reduction was almost complete within 10 min at room temperature, and the color of the solution changed from yellow to colorless. Similar results were also reported in the literature; Bordbar et al. [45] found that, using CuO/ZnO nanocomposites, catalytic reduction of 4-NP to 4-AP (using NaBH4 as the reducing agent) was completed in several minutes.
The kinetics of calcined CMF, CuO, CuO/Fe3O4, and carbon-doped CuO/Fe3O4 composites are shown in Figure 4b. In the absence of catalyst, the reduction of 4-NP by NaBH4 was negligible. In each case, the pseudo-first-order kinetic prevailed. The calcined CMF also had a negligible effect on the reduction of 4-NP. The rate constants (k) for CuO, CuO/Fe3O4, and carbon-doped CuO/Fe3O4 composite samples were 1.3 × 10 −3 s −1 , 3.6 × 10 −3 s −1 and 6.5 × 10 −3 s −1 , respectively. In the case of CuO, the catalytic activity was the lowest due to the aggregation of CuO nanoparticles, while the catalytic The magnetic behavior of the carbon-doped Fe 3 O 4 /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 Fe 3 O 4 /CuO composite catalyst could be readily separated from the reaction system using a magnet for the recycling process.

Catalytic Reduction
In this study, we investigated the application of carbon-doped CuO/Fe 3 O 4 composite catalysts for the catalytic reduction of 4-nitrophenol. In Figure 4a, the results are shown for the catalytic reduction of 4-NP to 4-AP in the presence of NaBH 4 and carbon-doped CuO/Fe 3 O 4 composite catalysts. The reduction process was followed based on the UV-Vis spectrophotometry. It shows that the absorbance at 400 nm (4-NP) decreased gradually as a function of time, while the absorbance at 290 nm (due to 4-AP) increased, confirming the catalytic reduction of 4-NP to 4-AP [44]. The catalytic reduction was almost complete within 10 min at room temperature, and the color of the solution changed from yellow to colorless. Similar results were also reported in the literature; Bordbar et al. [45] found that, using CuO/ZnO nanocomposites, catalytic reduction of 4-NP to 4-AP (using NaBH 4 as the reducing agent) was completed in several minutes.
The kinetics of calcined CMF, CuO, CuO/Fe 3 O 4 , and carbon-doped CuO/Fe 3 O 4 composites are shown in Figure 4b. In the absence of catalyst, the reduction of 4-NP by NaBH 4 was negligible. In each case, the pseudo-first-order kinetic prevailed. The calcined CMF also had a negligible effect on the reduction of 4-NP. The rate constants (k) for CuO, CuO/Fe 3 O 4 , and carbon-doped CuO/Fe 3 O 4 composite samples were 1.3 × 10 −3 s −1 , 3.6 × 10 −3 s −1 and 6.5 × 10 −3 s −1 , respectively. In the case of CuO, the catalytic activity was the lowest due to the aggregation of CuO nanoparticles, while the catalytic efficiency of carbon-doped CuO/Fe 3 O 4 composites was much better than that of CuO/Fe 3 O 4 , demonstrating that carbon doping is effective for enhancing the catalytic activity of the catalysts.
The catalytic reduction of 4-NP by NaBH 4 using metal-oxide nanoparticles (CuO) has two steps [46]: (1) borohydride ions are adsorbed onto the nanoparticle surface, forming active Nanomaterials 2019, 9, 277 7 of 12 surface-hydrogen, while 4-NP is also adsorbed onto the nanoparticle surface; (2) active hydrogen attacks the positively charged nitrogen in the nitro group of 4-NP, followed by the addition of two hydrogen atoms, producing 4-AP. hydrogen, while 4-NP is also adsorbed onto the nanoparticle surface; (2) active hydrogen attacks the positively charged nitrogen in the nitro group of 4-NP, followed by the addition of two hydrogen atoms, producing 4-AP. Cationic and anionic organic dyes were chosen to further investigate the catalytic properties of carbon-doped CuO/Fe3O4 composite catalyst. As shown in Figure 5a, for cationic dye (methylene blue), in the presence of NaBH4 and carbon-doped CuO/Fe3O4 composite catalyst, the absorbance at 660 nm (MB) gradually decreased as a function of time; furthermore, the catalytic reduction was completed within 6 min at room temperature (the color of the solution was colorless).
The results from methyl orange (an anionic dye) are shown in Figure 5c. Under otherwise the same conditions, the color change (from orange to colorless) was slower than that for MB (Figure 5a, from blue to colorless). The pseudo-first-order rate law was also valid here (Figure 5d). For the carbon-doped CuO/Fe3O4 composite catalyst, the rate constant (k) was 2.4 × 10 −3 s −1 for MO, while it was 12.9 × 10 −3 s −1 for MB.
We compared the catalytic performance of the carbon-doped CuO/Fe3O4 composite catalyst for the reduction of 4-NP, MB, and MO, with other related ones from the literature ( Table 1). As shown, the carbon-doped CuO/Fe3O4 composite catalyst showed much improved results, and the pseudofirst-order rate constant (k) for the nanocomposite catalyst from the present study was indeed consistently higher than that reported in the literature. The improved results may be attributed to the unique original morphologies associated with HKUST-1. Cationic and anionic organic dyes were chosen to further investigate the catalytic properties of carbon-doped CuO/Fe 3 O 4 composite catalyst. As shown in Figure 5a, for cationic dye (methylene blue), in the presence of NaBH 4 and carbon-doped CuO/Fe 3 O 4 composite catalyst, the absorbance at 660 nm (MB) gradually decreased as a function of time; furthermore, the catalytic reduction was completed within 6 min at room temperature (the color of the solution was colorless).
The results from methyl orange (an anionic dye) are shown in Figure 5c. Under otherwise the same conditions, the color change (from orange to colorless) was slower than that for MB (Figure 5a, from blue to colorless). The pseudo-first-order rate law was also valid here (Figure 5d). For the carbon-doped CuO/Fe 3 O 4 composite catalyst, the rate constant (k) was 2.4 × 10 −3 s −1 for MO, while it was 12.9 × 10 −3 s −1 for MB.
We compared the catalytic performance of the carbon-doped CuO/Fe 3 O 4 composite catalyst for the reduction of 4-NP, MB, and MO, with other related ones from the literature ( Table 1). As shown, the carbon-doped CuO/Fe 3 O 4 composite catalyst showed much improved results, and the pseudo-first-order rate constant (k) for the nanocomposite catalyst from the present study was indeed consistently higher than that reported in the literature. The improved results may be attributed to the unique original morphologies associated with HKUST-1.   From the viewpoint of practical application, the recycling/reuse of the catalyst is of critical importance. In the present study, after the catalytic degradation experiments, the magnetic carbon-doped CuO/Fe 3 O 4 composite catalyst was readily separated from the reaction system using an external magnet. The used catalysts were collected, and rinsed with distilled water several times. After a thorough washing process, the recovered magnetic catalyst was reused in the subsequent run of catalytic reduction of 4-NP under identical conditions, and the same process was repeated five times. The results are shown in Figure 6. The catalytic performance of the magnetic carbon-doped CuO/Fe 3 O 4 composite catalyst decreased only slightly (the 4-NP reduction ratio decreased from 100% to 96%) after five cycles. Similar results were obtained in the reuse/recycling experiments of the as-prepared magnetic carbon-doped CuO/Fe 3 O 4 composite catalyst during the catalytic reduction of MB and MO. Therefore, the as-prepared magnetic carbon-doped CuO/Fe 3 O 4 composite catalyst is a promising system for practical applications.

Conclusion
In this study, a nano-cellulose/MOF-derived carbon-doped CuO/Fe3O4 composite catalyst was successfully fabricated through pyrolysis/thermolysis of the HKUST-1/Fe3O4/CMF composite. The resultant carbon-doped CuO/Fe3O4 composite catalyst took advantage of the original porous morphology of HKUST-1; consequently, the carbon-doped CuO/Fe3O4 composite catalyst exhibited high catalytic activity for the reduction of 4-NP and organic dyes (MB and MO). In addition, the carbon-doped CuO/Fe3O4 composite catalyst showed good reusability/recyclability after five cycles. Notably, this strategy can be extended to the preparation of other functional MOF-based derivatives.

Conclusions
In this study, a nano-cellulose/MOF-derived carbon-doped CuO/Fe 3 O 4 composite catalyst was successfully fabricated through pyrolysis/thermolysis of the HKUST-1/Fe 3 O 4 /CMF composite. The resultant carbon-doped CuO/Fe 3 O 4 composite catalyst took advantage of the original porous morphology of HKUST-1; consequently, the carbon-doped CuO/Fe 3 O 4 composite catalyst exhibited high catalytic activity for the reduction of 4-NP and organic dyes (MB and MO). In addition, the carbon-doped CuO/Fe 3 O 4 composite catalyst showed good reusability/recyclability after five cycles. Notably, this strategy can be extended to the preparation of other functional MOF-based derivatives.