Preparation of Magnetic CuFe2O4@Ag@ZIF-8 Nanocomposites with Highly Catalytic Activity Based on Cellulose Nanocrystals

A facile approach was successfully developed for synthesis of cellulose nanocrystals (CNC)-supported magnetic CuFe2O4@Ag@ZIF-8 nanospheres which consist of a paramagnetic CuFe2O4@Ag core and porous ZIF-8 shell. The CuFe2O4 nanoparticles (NPs) were first prepared in the presence of CNC and dispersant. Ag NPs were then deposited on the CuFe2O4/CNC composites via an in situ reduction directed by dopamine polymerization (PDA). The CuFe2O4/CNC@Ag@ZIF-8 nanocomposite was characterized by TEM, FTIR, XRD, N2 adsorption-desorption isotherms, VSM, and XPS. Catalytic studies showed that the CuFe2O4/CNC@Ag@ZIF-8 catalyst had much higher catalytic activity than CuFe2O4@Ag catalyst with the rate constant of 0.64 min−1. Because of the integration of ZIF-8 with CuFe2O4/CNC@Ag that combines the advantaged of each component, the nanocomposites were demonstrated to have an enhanced catalytic activity in heterogeneous catalysis. Therefore, these results demonstrate a new method for the fabrication of CNC-supported magnetic core-shell catalysts, which display great potential for application in biocatalysis and environmental chemistry.


Introduction
Functional nanocomposites represent an important class of nanomaterials and have attracted increased research interest due to their superior properties compared with individual components [1]. As an important member of nanocomposites family, the magnetic nanocatalyst is very attractive because it provides a convenient way to remove and recycle the nanocatalyst from the reaction system [2]. MFe 2 O 4 ferrite, which is a well-known ternary spinel structure with M 2+ ions on B sites and Fe 3+ ions located equally among A and B sites, has high thermal, mechanical, and chemical stability and versatile catalytic, electric, and magnetic properties, and it exhibits promise for applications in electronics, lithium ion batteries, sensors, catalysis, and diagnostic medicine [3,4]. CuFe 2 O 4 possesses high electronic conductivity, high thermal stability, and high activity for the hydrogenation of 4-nitrophenol (4-NP), which is a toxic and inhibitory in nature, to yield industrially important anilines like 4-aminophenol [5][6][7].
The general schemes for the synthesis of the CuFe2O4/CNC@Ag@ZIF-8 nanocomposites are illustrated in Scheme 1, which mainly involved three steps: (1) Firstly, one-pot solvothermal synthesis of cellulose nanocrystals supports CuFe2O4 NPs, and the network of cellulose nanocrystals can significantly enhance the dispersion stability. (2) Secondly, as a result of its unique self-adhesive and reductive nature, the PDA layer shows sufficient reductive capacity to reduce Ag + ions without the need for additional reducing agent [37]. By then, Ag NPs with sizes ranging from 20 to 30 nm in sphere shapes can be uniformly formed along the CuFe2O4 and CNC networks surface with the aid of adhesive and reductive PDA layer under alkaline conditions. (3) Thirdly, using Zn(NO3)2·2H2O and 2-methylimidazole as precursors, the ZIF-8 shell with controllable thickness was coated on the surface of CuFe2O4/CNC@Ag. The composite is prepared by layer-by-layer. Transmission electron microscopy (TEM) measurements revealed that the CuFe 2 O 4 NPs had good monodispersity in the CNC network with a mean size of about 250 nm ( Figure 1A). The pristine CNC) had a length of ca. 200-250 nm and a width of 15-20 nm, which is typical for CNC [38]. CuFe 2 O 4 NPs shown in Figure 1A were well dispersed in the presence of CNC substrate, which supported the conclusion that CNC can act as a good dispersant/support of nanoparticles, preventing the NPs' aggregation due to the excellent properties of CNC [38]. Figure 1B showed that uniform and dense Ag NPs, with an average size of 25 nm successfully distributed on the PDA surface, and no free Ag NPs were observed. The CNC networks became dark after coating PDA. The thin PDA shell layers formed around the CuFe 2 O 4 /CNC cores showed an average size of about 20 nm ( Figure 1C), displaying a distant core-shell structure. Figure 1E confirmed that the boundary between the ZIF-8 and PDA was obscure, which was attributed to the slight mass difference of the two components. Compared with CuFe 2 O 4 /CNC@Ag, CuFe 2 O 4 /CNC@Ag@ZIF-8 nanocomposites had a distinct core-shell structure; the thickness of ZIF-8 shell surrounding the CuFe 2 O 4 /CNC@Ag was approximately 45 nm, and there was a controllable particle diameter ranging from 350 to 400 nm. Figure 1F [39] and the (111) plane of Ag, respectively [40]. illustrated in Scheme 1, which mainly involved three steps: (1) Firstly, one-pot solvothermal synthesis of cellulose nanocrystals supports CuFe2O4 NPs, and the network of cellulose nanocrystals can significantly enhance the dispersion stability. (2) Secondly, as a result of its unique self-adhesive and reductive nature, the PDA layer shows sufficient reductive capacity to reduce Ag + ions without the need for additional reducing agent [37]. By then, Ag NPs with sizes ranging from 20 to 30 nm in sphere shapes can be uniformly formed along the CuFe2O4 and CNC networks surface with the aid of adhesive and reductive PDA layer under alkaline conditions. (3) Thirdly, using Zn(NO3)2·2H2O and 2-methylimidazole as precursors, the ZIF-8 shell with controllable thickness was coated on the surface of CuFe2O4/CNC@Ag. The composite is prepared by layer-by-layer. HAADF-STEM also confirmed the typical nanostructure of the nanocomposites (Figure 2A). The EDX mapping of the Cu and Fe elements revealed that CuFe 2 O 4 was mainly located within the nanocomposites ( Figure 2B). The diameters of the N element map were larger than that of the Cu and Fe, which further supported the fact that the PDA were successfully coated. The Ag element was distributed around the CuFe 2 O 4 because the density in the center was very low. In addition, the corresponding EDX spectra supported the conclusion that Ag NPs were embedded in the CNC substrate [38], forming the CuFe 2 O 4 /CNC@Ag@ZIF-8 nanocomposites. The diameters of the Zn element map were larger than that of the Ag, which further supported the fact that the Ag NPs were protected by ZIF-8. The Ag element was distributed on C element, which confirmed that Ag NPs were located on PDA shell ( Figure 2C-I). On the basis of the TEM and EDX mapping, it could be concluded that the CuFe 2 O 4 /CNC@Ag@ZIF-8 nanocomposites with core-shell structure had been successfully achieved in Figure 2J. HAADF-STEM also confirmed the typical nanostructure of the nanocomposites (Figure 2A). The EDX mapping of the Cu and Fe elements revealed that CuFe2O4 was mainly located within the nanocomposites ( Figure 2B). The diameters of the N element map were larger than that of the Cu and Fe, which further supported the fact that the PDA were successfully coated. The Ag element was distributed around the CuFe2O4 because the density in the center was very low. In addition, the corresponding EDX spectra supported the conclusion that Ag NPs were embedded in the CNC substrate [38], forming the CuFe2O4/CNC@Ag@ZIF-8 nanocomposites. The diameters of the Zn element map were larger than that of the Ag, which further supported the fact that the Ag NPs were protected by ZIF-8. The Ag element was distributed on C element, which confirmed that Ag NPs were located on PDA shell ( Figure 2C-I). On the basis of the TEM and EDX mapping, it could be   [41,42]. The band at 1645 cm −1 was due to the O-H bending vibration in CNC [43]. The absorbance bands at 2893 cm −1 , 1400 cm −1 and 1060 cm −1 were assigned to the C-H stretching vibration, the C-H deformation vibration and the C-O-C stretching of pyranose, respectively [15,44], which indicated that the CuFe 2 O 4 NPs were successfully immobilized on the CNC. In line (b), besides the characteristic adsorption peaks of line (a), the adsorption peaks at 1513 cm −1 are related to the C=C stretching vibrations of aromatic ring [29]; the broad peak at 3390 cm −1 is attributed to the O-H and N-H stretching vibrations [42]; the peak appearing at 1294 cm −1 can be assigned to the C-OH stretching vibration of phenol compounds [43]; all the peaks above demonstrate that the CuFe 2 O 4 /CNC@PDA nanocomposites were successfully prepared. After immobilizing Ag on the CuFe 2 O 4 /CNC@PDA (line c), the intensity of the peak at 1294 cm −1 become weaker owing to the interaction between Ag NPs and PDA. The band at 421 cm −1 (shown in line (d)) was attributed to the Zn-N stretch mode [44]. The bands in the spectral region of 500-1350 cm −1 and 1350-1500 cm −1 were assigned as the plane bending and stretching of imidazole ring, respectively [44]. The bands of 2500-3500 cm −1 could be ascribed to stretching vibrations of -CH 3 , -NH-and -OH (Zn-OH) within the internal structure of ZIF-8 [45]. concluded that the CuFe2O4/CNC@Ag@ZIF-8 nanocomposites with core-shell structure had been successfully achieved in Figure 2J.   [41,42]. The band at 1645 cm −1 was due to the O-H bending vibration in CNC [43]. The absorbance bands at 2893 cm −1 , 1400 cm −1 and 1060 cm −1 were assigned to the C-H stretching vibration, the C-H deformation vibration and the C-O-C stretching of pyranose, respectively [15,44], which indicated that the CuFe2O4 NPs were successfully immobilized on the CNC. In line (b), besides the characteristic adsorption peaks of line (a), the adsorption peaks at 1513 cm −1 are related to the C=C stretching vibrations of aromatic ring [29]; the broad peak at 3390 cm −1 is attributed to the O-H and N-H stretching vibrations [42]; the peak appearing at 1294 cm −1 can be assigned to the C-OH stretching vibration of phenol compounds [43]; all the peaks above demonstrate that the CuFe2O4/CNC@PDA nanocomposites were successfully prepared. After immobilizing Ag on the CuFe2O4/CNC@PDA (line c), the intensity of the peak at 1294 cm −1 become weaker owing to the interaction between Ag NPs and PDA. The band at 421 cm −1 (shown in line (d)) was attributed to the Zn-N stretch mode [44]. The bands in the spectral region of 500-1350 cm −1 and 1350-1500 cm −1 were assigned as the plane bending and stretching of imidazole ring, respectively [44]. The bands of 2500-3500 cm −1 could be ascribed to stretching vibrations of -CH3, -NH-and -OH (Zn-OH) within the internal structure of ZIF-8 [45].
To probe the presence of Ag NPs and ZIF-8 attached onto the CuFe2O4/CNC, XRD patterns were carried out during the experiments ( Figure 3B). It was observed that for CuFe2O4/CNC, two peaks at 2θ = 11.27 and 21.94° corresponded to the typical (101) and (020) lattice planes of cellulose [37], and the diffraction peaks located at 18.5°, 30.2°, 35.5°, 37.0°, 43.4°, 57.3°, and 62.6° corresponded to the (111), (220), (311), (222), (400), (422), (511), and (440) lattice planes, which matched well with those from the JCPDS card number 25-0283 for CuFe2O4 [46]. But for CuFe2O4/CNC@Ag and CuFe2O4/CNC@Ag@ZIF-8 nanocomposites, the XRD diffraction peaks derived from cellulose showed a slight decrease, and meanwhile, four diffraction peaks at 2θ = 39.86°, 44.23°, 64.47°, and 77.33° appeared, assigned respectively, to (111), (200), (220), and (311) lattice planes, and supported the facecentered cubic (fcc) structure of Ag NPs [47,48], an indication of successful formation of Ag NPs via The porosity of evacuated composites was investigated by nitrogen-sorption measurements. As presented in Figure 4A, the CuFe 2 O 4 /CNC@Ag and CuFe 2 O 4 /CNC@Ag@ZIF-8 nanocomposites exhibited a typical type V isotherm, validating a mesoporous characteristic [7]. The pore-size distribution ( Figure B) revealed that the CuFe 2 O 4 /CNC@Ag@ZIF-8 nanocomposites contained an average pore size of 4.0 nm which is lower than that of CuFe 2 O 4 /CNC@Ag (8.9 nm) in favor of the prevention of Ag active sites leaching. In addition, the specific surface area and the pore volume of the CuFe 2 O 4 /CNC@Ag@ZIF-8 nanocomposites were calculated to be 160.17 m 2 /g, which is four times higher than that of CuFe 2 O 4 /CNC@Ag (38.68 m 2 /g). The high external surface area and mesoporous structure endowed the CuFe 2 O 4 /CNC@Ag@ZIF-8 nanocomposites with high adsorption capacity and fast diffusion of reactants [17]. CuFe2O4/CNC@Ag@ZIF-8 nanocomposites, respectively, suggesting that the ZIF-8 materials synthesized using current protocol are highly crystalline. The porosity of evacuated composites was investigated by nitrogen-sorption measurements. As presented in Figure 4A, the CuFe2O4/CNC@Ag and CuFe2O4/CNC@Ag@ZIF-8 nanocomposites exhibited a typical type V isotherm, validating a mesoporous characteristic [7]. The pore-size distribution ( Figure B) revealed that the CuFe2O4/CNC@Ag@ZIF-8 nanocomposites contained an average pore size of 4.0 nm which is lower than that of CuFe2O4/CNC@Ag (8.9 nm) in favor of the prevention of Ag active sites leaching. In addition, the specific surface area and the pore volume of the CuFe2O4/CNC@Ag@ZIF-8 nanocomposites were calculated to be 160.17 m 2 /g, which is four times higher than that of CuFe2O4/CNC@Ag (38.68 m 2 /g). The high external surface area and mesoporous structure endowed the CuFe2O4/CNC@Ag@ZIF-8 nanocomposites with high adsorption capacity and fast diffusion of reactants [17].
The saturation magnetization (Ms) is a physical quantity that can reflect the magnetism of a substance. Both CuFe2O4/CNC@Ag and CuFe2O4/CNC@Ag@ZIF-8 nanocomposites were paramagnetic with little hysteresis and remanence, processing Ms values of 31.2 and 30.1 emu/g, respectively. Due to the coating of antimagnetic ZIF-8 shell, the saturation magnetization (Ms) decreased a little. As presented in Figure 4B, the CuFe2O4/CNC@Ag@ZIF-8 nanocomposites were well dispersed in water and presented a black suspension. However, rapid aggregation (≈1 min) of the CuFe2O4/CNC@Ag@ZIF-8 nanocomposites from the homogeneous suspension was obtained with the help of external magnet, and thus the dispersed solution became clear.

Catalytic Reduction of 4-Nitrophenol
Ag NPs have been generally used as excellent catalysts with high catalytic activity and selectivity for catalytic reduction or degradation of organic pollution in aqueous solution [38,39,46,49]. Many reports are available on the application of metal and metal oxides nanocatalysts for the reduction of The saturation magnetization (M s ) is a physical quantity that can reflect the magnetism of a substance. Both CuFe 2 O 4 /CNC@Ag and CuFe 2 O 4 /CNC@Ag@ZIF-8 nanocomposites were paramagnetic with little hysteresis and remanence, processing M s values of 31.2 and 30.1 emu/g, respectively. Due to the coating of antimagnetic ZIF-8 shell, the saturation magnetization (M s ) decreased a little. As presented in Figure 4B, the CuFe 2 O 4 /CNC@Ag@ZIF-8 nanocomposites were well dispersed in water and presented a black suspension. However, rapid aggregation (≈1 min) of the CuFe 2 O 4 /CNC@Ag@ZIF-8 nanocomposites from the homogeneous suspension was obtained with the help of external magnet, and thus the dispersed solution became clear.

Catalytic Reduction of 4-Nitrophenol
Ag NPs have been generally used as excellent catalysts with high catalytic activity and selectivity for catalytic reduction or degradation of organic pollution in aqueous solution [38,39,46,49]. Many reports are available on the application of metal and metal oxides nanocatalysts for the reduction of nitrophenols in the presence of NaBH 4 [50]. Herein, the catalytic reduction of 4-NP by NaBH 4 was used as a model reaction to investigate the catalytic performances of CuFe 2 O 4 /CNC, CuFe 2 O 4 /CNC@Ag, and CuFe 2 O 4 /CNC@Ag@ZIF-8 nanocomposites. Although the aqueous solution of 4-nitrophenol undergoes a rapid color change (with a UV-Vis absorption peak shift from 319 to 400 nm) after adding NaBH 4 due to the formation of 4-nitrophenolate ions, the reduction reaction does not proceed substantially in the absence of suitable catalysts (such as Au, Ag, Pd, and Pt NPs). Figure 5A suggested the catalytic reaction of CuFe 2 O 4 /CNC@Ag nanocomposites could be completed within 11 min. As shown in Figure 5B, the adsorption peak at 400 nm was observed to decrease in intensity rapidly and disappear eventually after 6 min, suggesting that the CuFe 2 O 4 /CNC@Ag@ZIF-8 nanocomposites do catalyze 4-NP reduction. Since the ZIF-8 itself, in the control experiment, exhibited no propensity to catalyze the reduction reaction, the above result indicates that the 4-nitrophenol molecules can diffuse quickly through the channels of ZIF-8 matrix and react on the surfaces of the active CuFe 2 O 4 @Ag NPs. In addition, the appearance of the new peak at ≈300 nm in the UV-Vis spectra suggested that the CuFe 2 O 4 /CNC@Ag@ZIF-8 nanocomposites catalyze the reduction of 4-NP to give 4-aminophenol as the sole product. nitrophenols in the presence of NaBH4 [50]. Herein, the catalytic reduction of 4-NP by NaBH4 was used as a model reaction to investigate the catalytic performances of CuFe2O4/CNC, CuFe2O4/CNC@Ag, and CuFe2O4/CNC@Ag@ZIF-8 nanocomposites. Although the aqueous solution of 4-nitrophenol undergoes a rapid color change (with a UV-Vis absorption peak shift from 319 to 400 nm) after adding NaBH4 due to the formation of 4-nitrophenolate ions, the reduction reaction does not proceed substantially in the absence of suitable catalysts (such as Au, Ag, Pd, and Pt NPs).  Figure 5A suggested the catalytic reaction of CuFe2O4/CNC@Ag nanocomposites could be completed within 11 min. As shown in Figure 5B, the adsorption peak at 400 nm was observed to decrease in intensity rapidly and disappear eventually after 6 min, suggesting that the CuFe2O4/CNC@Ag@ZIF-8 nanocomposites do catalyze 4-NP reduction. Since the ZIF-8 itself, in the control experiment, exhibited no propensity to catalyze the reduction reaction, the above result indicates that the 4-nitrophenol molecules can diffuse quickly through the channels of ZIF-8 matrix and react on the surfaces of the active CuFe2O4@Ag NPs. In addition, the appearance of the new peak at ≈300 nm in the UV-Vis spectra suggested that the CuFe2O4/CNC@Ag@ZIF-8 nanocomposites catalyze the reduction of 4-NP to give 4-aminophenol as the sole product.
To elucidate the reaction mechanism, the concentration of NaBH4 could be considered as constant throughout the reaction since it was in great excess (0.1 M). Therefore, pseudo-first-order kinetics with regard to the catalytic reduction of 4-NP, described as ln(Ct/C0) = -kt, can be applied, where Ct is the concentration of 4-NP at time t, C0 is the initial concentration of 4-NP, and k is the rate constant {51}. Figure 5C shows the linear relationship of ln(Ct/C0) as a function of reaction time t for the 4-NP reduction catalyzed by three as-synthesized catalysts. The values of kinetic rate constant k can be calculated from the rate equation ln(Ct/C0) = -kt. Impressively, the CuFe2O4/CNC@Ag@ZIF-8 nanocomposites exhibit the highest activity with a rate constant estimated to be 0.64 min −1 ,.5 and To elucidate the reaction mechanism, the concentration of NaBH 4 could be considered as constant throughout the reaction since it was in great excess (0.1 M). Therefore, pseudo-first-order kinetics with regard to the catalytic reduction of 4-NP, described as ln(C t /C 0 ) = −kt, can be applied, where C t is the concentration of 4-NP at time t, C 0 is the initial concentration of 4-NP, and k is the rate constant {51}. Figure 5C shows the linear relationship of ln(C t /C 0 ) as a function of reaction time t for the 4-NP reduction catalyzed by three as-synthesized catalysts. The values of kinetic rate constant k can be calculated from the rate equation ln(C t /C 0 ) = −kt. Impressively, the CuFe 2 O 4 /CNC@Ag@ZIF-8 nanocomposites exhibit the highest activity with a rate constant estimated to be 0.64 min −1 ,.5 and eight times higher than that of CuFe 2 O 4 /CNC@Ag and that of CuFe 2 O 4 /CNC, respectively, suggesting the higher catalytic efficiencies for Ag catalysts confined in ZIF-8 shell. This prominent catalytic activity can due to a porous ZIF-8 shell for stabilization of the encapsulated Ag NPs and rapid adsorption of chemical pollutants from aqueous solution. The catalytic active sites are both CuFe 2 O 4 and Ag NPs in the core-shell structures, which modified the electronic structure, and then enhanced the catalytic activity.
The reusability of CuFe 2 O 4 /CNC@Ag@ZIF-8 nanocomposites as the catalyst for the reduction of 4-NP was further confirmed by the observation of the similar conversion for the same reaction time (6 min) for six consecutive cycles (the slightly decreased conversions in the later catalysis cycles were presumably caused by the loss of catalyst during the washing process between cycles ( Figure 5D).

Reaction Mechanism of CuFe 2 O 4 /CNC@Ag@ZIF-8 Nanocomposites
As illustrated in Scheme 2, the mechanism of catalytic reduction of 4-NP by the CuFe 2 O 4 /CNC@Ag@ZIF-8 nanocomposites involved the traditional theory. In this work, the 4-NP can be adsorbed onto the mesoporous ZIF-8 shell via π-π stacking interactions because 4-NP is π-rich in nature [51]. Such chemical adsorption provides a high concentration of 4-NP near to the interface of the CuFe 2 O 4 @Ag and ZIF-8, leading to highly efficient contact between them. Simultaneously, BH − 4 was also adhered to the CuFe 2 O 4 /CNC@Ag surface and transferred electrons and hydride ions to the Ag NPs' surface. In addition, Zhou et al. claimed that when metal oxide closely contacted with metal, Fermi level alignment would lead to charge redistribution: electrons would escape from the metal and transfer into the semiconductor [52]. Liang et, al. used Ag/Fe 3 O 4 NPs as the catalyst for the reduction of 4-NP [53]. They reported that the electrons tended to leave Ag to Fe 3 O 4 and thus form a depleted region close to the Ag/Fe 3 O 4 interface. In our work, CuFe 2 O 4 was known as a p-type semiconductor with low band gap, so part of the electrons and hydride ion that injected from BH − 4 to Ag NPs could transfer to the neighboring CuFe 2 O 4 surface (Scheme 2). The existence of the surplus electrons on CuFe 2 O 4 provided large surface area and increased opportunities for reduction reaction, facilitating the capture of electrons by 4-NP molecules. Goyal et al. suggested that electron transfer between Cu + -Cu 2+ and Fe 2+ -Fe 3+ in the octahedral sites endowed CuFe 2 O 4 with enhanced catalytic activity [54] (Scheme 2). Herein, when the electrons and hydride ions were transfered to the CuFe 2 O 4 surface, both Cu 2+ and Fe 3+ ions present in the octahedral sites were exposed on the surfaces of particles. Due to that, there were transfers of electrons between Cu + -Cu 2+ and Fe 2+ -Fe 3+ ion pairs, which enhanced catalytic activity. Then the hydrogen atom transfers from BH − 4 to the 4-NP, resulting in the formation of 4-AP. Finally, the products of 4-AP are desorbed from the surface of the catalysts to the solution through the channels of the ZIF-8 shell.
Molecules 2019, 24, x FOR PEER REVIEW 9 of 14 eight times higher than that of CuFe2O4/CNC@Ag and that of CuFe2O4/CNC, respectively, suggesting the higher catalytic efficiencies for Ag catalysts confined in ZIF-8 shell. This prominent catalytic activity can due to a porous ZIF-8 shell for stabilization of the encapsulated Ag NPs and rapid adsorption of chemical pollutants from aqueous solution. The catalytic active sites are both CuFe2O4 and Ag NPs in the core-shell structures, which modified the electronic structure, and then enhanced the catalytic activity. The reusability of CuFe2O4/CNC@Ag@ZIF-8 nanocomposites as the catalyst for the reduction of 4-NP was further confirmed by the observation of the similar conversion for the same reaction time (6 min) for six consecutive cycles (the slightly decreased conversions in the later catalysis cycles were presumably caused by the loss of catalyst during the washing process between cycles ( Figure 5D).

Reaction Mechanism of CuFe2O4/CNC@Ag@ZIF-8 Nanocomposites
As illustrated in Scheme 2, the mechanism of catalytic reduction of 4-NP by the CuFe2O4/CNC@Ag@ZIF-8 nanocomposites involved the traditional theory. In this work, the 4-NP can be adsorbed onto the mesoporous ZIF-8 shell via π-π stacking interactions because 4-NP is π-rich in nature [51]. Such chemical adsorption provides a high concentration of 4-NP near to the interface of the CuFe2O4@Ag and ZIF-8, leading to highly efficient contact between them. Simultaneously, BH4was also adhered to the CuFe2O4/CNC@Ag surface and transferred electrons and hydride ions to the Ag NPs' surface. In addition, Zhou et al. claimed that when metal oxide closely contacted with metal, Fermi level alignment would lead to charge redistribution: electrons would escape from the metal and transfer into the semiconductor [52]. Liang et, al. used Ag/Fe3O4 NPs as the catalyst for the reduction of 4-NP [53]. They reported that the electrons tended to leave Ag to Fe3O4 and thus form a depleted region close to the Ag/Fe3O4 interface. In our work, CuFe2O4 was known as a p-type semiconductor with low band gap, so part of the electrons and hydride ion that injected from BH4 -to Ag NPs could transfer to the neighboring CuFe2O4 surface (Scheme 2). The existence of the surplus electrons on CuFe2O4 provided large surface area and increased opportunities for reduction reaction, facilitating the capture of electrons by 4-NP molecules. Goyal et al. suggested that electron transfer between Cu + -Cu 2+ and Fe 2+ -Fe 3+ in the octahedral sites endowed CuFe2O4 with enhanced catalytic activity [54] (Scheme 2). Herein, when the electrons and hydride ions were transfered to the CuFe2O4 surface, both Cu 2+ and Fe 3+ ions present in the octahedral sites were exposed on the surfaces of particles. Due to that, there were transfers of electrons between Cu + -Cu 2+ and Fe 2+ -Fe 3+ ion pairs, which enhanced catalytic activity. Then the hydrogen atom transfers from BH to the 4-NP, resulting in the formation of 4-AP. Finally, the products of 4-AP are desorbed from the surface of the catalysts to the solution through the channels of the ZIF-8 shell. It should be noted that the bleaching rate is considerably higher than the rates reported previously under the similar experimental conditions with Ag-based, CuFe 2 O 4 -based, and MOF-based catalysts.
The k values of different catalytic systems for the reduction of 4-NP were comparable to the values referenced in Table 1, and the results showed that the prepared catalyst possessed higher activity and lower activation energy.

Measurements
Sample morphologies with energy-dispersive X-ray spectroscopy (EDX) were characterized by transmission electron microscopy (TEM) on a TECNAI G2 TF20 (U.S.). FT-IR spectra of all samples in the wavenumber range 4000-400 cm −1 were obtained in KBr pressed pellets on a TENSOR model 27 FTIR spectrometer (Germany, Bruker). The powder X-ray diffraction spectra (XRD) were measured by X-ray diffraction (Germany, Bruker, D8Advance) with Cu Kα radiation, λ = 1.542 Å. The specific surface area was calculated by the Bruner-Emmett-Teller (BET) method. The pore size distributions were derived from the adsorption branches of the isotherms based on the Barrett-Joyner-Hollande (BJH) model. Magnetic hysteresis loops at room temperature were obtained using a vibrating sample magnetometer VSM 7304 (Lakeshore, Columbus, OH, USA). The chemical composition of nanocomposites was characterized by XPS (U.S. Thermos Scientific ESCALAB250). The UV-Vis spectra (China, Shanghai, Shimadzu UV-2501 PC spectrometer) were performed to study the catalytic reduction activity. The samples were placed in a 1 × 1 × 3 cm quartz cuvettes, and the spectra were recorded at room temperature.

Preparation of CuFe 2 O 4 /CNC Nanocomposites
In a typical preparation, the procedure was reported as per previous research [12]. CNC (0.2 g) was dispersed in 40 mL of glycol with vigorous stirring in an ultrasonic generator for 0.5 h. On the other hand, 1.6 mmol CuCl 2 ·2H 2 O and 3.2 mmol FeCl 3 ·6H 2 O were dissolved in 20 mL of glycol to form a clear solution. After complete dissolution, CNC solution was poured into the metal precursor solution and followed by the addition of 0.2 g PVP while stirring for 0.5 h. Addition of NH 4 OAc (90 mmol) in a stepwise manner was done to the mixture until homogeneous light green dispersion. Then, the mixture was transferred into a Teflon-lined stainless steel autoclave (80 mL capacity) and heated at 200 • C for 11 h. After the reaction, the autoclave was naturally cooled to room temperature, and the catalysts were collected and washed with redistilled water and ethanol three times, respectively. Finally, the catalysts were dried in a vacuum for 4 h at 60 • C.

In Situ Reduction of Ag + Ions
To coat CuFe 2 O 4 /CNC nanocomposites with the PDA shell, 50 mg CuFe 2 O 4 /CNC nanocomposites and 50 mg of dopamine hydrochloride were dissolved in 25 mL Tris buffer solution (10 mM, pH = 8.5). After shaking for 3 h at room temperature, the CuFe 2 O 4 /CNC@PDA were separated and washed with ultrapure water and ethanol several times. For the preparation of Ag NPs on PDA surfaces, Tollen's reagent (silver ammonia solution) was used as the Ag precursor solution. Silver ammonia solution was prepared by adding ammonia aqueous solution (2 wt %) into 10 mg·mL −1 AgNO 3 solution until brown precipitation was just dissolved. Portions 50 mg in size of the CuFe 2 O 4 /CNC@PDA nanocomposites were added to 25 mL of silver ammonia solution, and the mixture was shaken in a rotary shaker for 6 h at room temperature. The products were collected, washed with ultrapure water and ethanol several times, and dried under vacuum. Then, CuFe 2 O 4 /CNC@Ag nanocomposites were obtained.

General Procedure for the Reduction of 4-NP
The reduction of 4-NP by NaBH 4 was chosen as a model reaction for investigating the catalytic performance of the CuFe 2 O 4 /CNC, CuFe 2 O 4 /CNC@Ag, and CuFe 2 O 4 /CNC@Ag@ZIF-8 nanocomposites. Typically, 2.35 mL ultrapure water, 200 µL 5mM 4-NP solution, and 450 µL 200 mM of fresh prepared NaBH 4 aqueous solution were added into standard quartz cuvettes respectively, and the solutions turned bright yellow rapidly. Subsequently, 3 mg of each catalysts was added to start the reaction, and the intensity of the absorption peak at 400 nm was monitored by UV-Vis spectroscopy as a function of time.

Conclusions
In summary, we demonstrated an effective strategy for the fabrication of novel cellulose nanocrystals (CNC)-supported magnetic CuFe 2 O 4 @Ag@ZIF-8 catalysts which consist of a paramagnetic CuFe 2 O 4 @Ag core and a porous ZIF-8 shell. The use of CNC include being a template and dispersant for the incorporation with CuFe 2 O 4 NPs and a good absorbent via π-π stacking interactions of 4-NP. The framework matrix of the resulting composites retains its high surface areas, uniform mesoporous structure, porous crystalline structure, and good magnetic response. The core-shell magnetic catalysts were found to exhibit excellent catalytic performance for 4-nitrophenol reduction with good reusability. Compared to CuFe 2 O 4 /CNC@Ag catalysts, the core-shell structure CuFe 2 O 4 /CNC@Ag@ZIF-8 nanocomposites are ideal recyclable catalysts for liquid-phase reductions due to a porous ZIF-8 shell for stabilization of the encapsulated Ag NPs and rapid adsorption of chemical pollutants from aqueous solution. More importantly, with the merits of easy separation and porous shell structure, this simple and versatile method might provide a multitude of noble, ZIF-8, and magnetic catalysts for broad applications, such as environmental protection, chemical biosensors, and so on.