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

Abstract

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.

1. 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]. MFe₂O₄ ferrite, which is a well-known ternary spinel structure with M²⁺ ions on B sites and Fe³⁺ 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]. CuFe₂O₄ 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].

More often, hybrid composites are fabricated through an in-situ method, where the templates have a high affinity towards metal ions that allows for the synthesis of metallic nanoparticles [8]. Cellulose nanocrystals (CNC) are derived from abundant cellulosic resources such as plants and microbial cellulose via sulfuric acid hydrolysis [9,10]. CNC have well-defined size and morphology, high specific surface area, high aspect ratio, high crystalline order, chirality, high mechanical strength, and controllable surface chemistry [11], which are appealing in a plethora of materials to catalytic applications. As reported in our previous research [12], the CuFe₂O₄/CNC nanocomposites show good dispersity and it has been suggested that the nanocomposites do catalyze 4-NP reduction.

To further improve the catalytic activity, various core/shell-structured magnetic nanocatalysts have been applied in nanocatalysis [13,14,15,16]. The as-obtained CuFe₂O₄ based core-shell nanocomposites showed excellent catalytic activity, magnetic separation, and magnetic carrying in nanocatalysis. Recently, the Ag modified magnetic composites have gained increasing attention because of the high catalytic activity of the Ag component, the good magnetic responsiveness of the magnetic core, and the relatively facile fabrication process [17,18,19]. Various noble metals, including Au, Pd, and Pt have been widely employed as catalysts for the reduction of 4-NP to 4-AP by NaBH₄ in aqueous media [20,21,22,23]. Moreover, Ag is more suitable for large-scale application than other noble metals (Au, Pd, and Pt) because of its low price [24]. Dopamine can self-polymerize under specific conditions to form a polydopamine (PDA) complex which has the ability of adhering onto the surface of various materials due to the strong stickiness [25,26,27] and the abundant catechol groups on the PDA complex can reduce metal into metal directly [28,29]. In this regard, Ag catalysts supported on ferrite, Fe₃O₄, Fe₃O₄@PDA, etc., have been used as magnetic catalysts for catalytic reduction. Recently, Wu et, al. fabricated Fe₃O₄@PDA-Ag nanocomposites and used them as an efficient catalyst for methylene blue reduction owing to Ag nanoparticles (NPs) [27]. CNC, crystalline cellulose nanofibers, and their derivatives, were good supports for the preparation of supported Ag NPs [11,30]. Moreover, CNC have eminent colloidal stability due to the interelectrostatic repulsions of the negatively charged sulfate ester groups on CNC [31,32]. These sulfate groups have been proven to favor stabilization and nucleation of Ag NPs in a recent research [11]. However, the introduction of CNC in magnetic catalyst is uncommon. The catalyst with CNC combines the advantages of magnetic and catalytic.

The ZIF-8 framework (Zn(MeIM)₂, MeIM = 2-methylimidazole) holds an intersecting 3D structure, high thermal and chemical stabilities, and large pore size and surface area, which are desirable for depositing metal NPs [33]; it can also rapid adsorb chemical pollutants from aqueous solutions. ZIF-8 has uniform but tunable cavities, tailorable chemistry, and is suited to stabilizing noble metal-NPs without blocking their surfaces, making them very attractive in catalysis [7]. Furthermore, proper design and construction of ZIF-8 nanocomposites by integrating functional materials (such as metal NPs, metal oxides, and polymers) are expected to present synergistically boosted catalytic activity, enhanced stability, and prolonged lifetimes [34,35] which would open new opportunity for fabricating highly efficient hierarchical core-shell catalysts [36].

Herein, we report the synthesis of cellulose nanocrystals (CNC) supported magnetic CuFe₂O₄@Ag@ZIF-8 nanocomposites. It consists of a paramagnetic CuFe₂O₄/CNC @Ag core and a porous ZIF-8 shell via a facile method. CNC was used as a template and dispersant for the incorporate with CuFe₂O₄ NPs and an absorbent via π–π stacking interactions of 4-NP. In addition, we used PDA as an intermediate, which can not only protect the CuFe₂O₄ NPs from corrosion in harsh environments and immobilize Ag NPs, but also induce the growth of ZIF-8 on the surface of the CuFe₂O₄/CNC@Ag. The as-synthesized CuFe₂O₄/CNC@Ag@ZIF-8 nanocomposites showed outstanding catalytic activity and reusability for the reduction of 4-NP. To our best knowledge, this is the first report on the integration of ZIF-8 with CuFe₂O₄/CNC@Ag into one nanostructure to significantly improve not only the intrinsic catalytic performances of Ag active species, but also the recyclability of catalysts.

2. Results and Discussion

2.1. Characterization of CuFe₂O₄/CNC@Ag and CuFe₂O₄/CNC@Ag@ZIF-8 Nanocomposites

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 CuFe₂O₄ 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]. CuFe₂O₄ 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 CuFe₂O₄/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 CuFe₂O₄/CNC@Ag, CuFe₂O₄/CNC@Ag@ZIF-8 nanocomposites had a distinct core-shell structure; the thickness of ZIF-8 shell surrounding the CuFe₂O₄/CNC@Ag was approximately 45 nm, and there was a controllable particle diameter ranging from 350 to 400 nm. Figure 1F displays a lattice resolved HRTEM image of CuFe₂O₄ nanocrystal on CNC. The distinct lattice fringes with interplanar spacings of 0.25 and 0.235 nm match well the (311) crystal plane of the CuFe₂O₄ cubic spinel structure [39] and the (111) plane of Ag, respectively [40].

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 concluded that the CuFe₂O₄/CNC@Ag@ZIF-8 nanocomposites with core-shell structure had been successfully achieved in Figure 2J.

Figure 3A presents the FTIR spectra of (a) CuFe₂O₄/CNC, (b) CuFe₂O₄/CNC@PDA, (c) CuFe₂O₄/CNC@Ag, and (d) CuFe₂O₄/CNC@Ag@ZIF-8. In line (a), the adsorption peaks at 430 cm⁻¹, 580 cm⁻¹, and 3425 cm⁻¹ correspond to the Fe–O, Cu–O, and O–H stretching vibrations, respectively [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 face-centered cubic (fcc) structure of Ag NPs [47,48], an indication of successful formation of Ag NPs via efficient in situ reduction by PDA layer. The XRD pattern of CuFe₂O₄/CNC@Ag@ZIF-8 nanocomposites indicated that the products were well crystallized and had high crystallinity even after coating ZIF-8 shell. Moreover, the diffraction peaks at 2θ = 10.4°, 12.8°, 14.7°, 16.5°, and 18.1° correspond to the (002), (112), (022), (013), and (222) lattice planes of ZIF-8 in the 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 m²/g, which is four times higher than that of CuFe₂O₄/CNC@Ag (38.68 m²/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 (M_s) 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 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 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.

2.2. 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 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(C_t/C₀) = −kt, can be applied, where C_t 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(C_t/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(C_t/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 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).

2.3. 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, ${\mathrm{BH}}_4^{-}$ was 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 ${\mathrm{BH}}_4^{-}$ 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⁺-Cu²⁺ and Fe²⁺-Fe³⁺ 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 Cu²⁺and Fe³⁺ ions present in the octahedral sites were exposed on the surfaces of particles. Due to that, there were transfers of electrons between Cu⁺-Cu²⁺ and Fe²⁺-Fe³⁺ ion pairs, which enhanced catalytic activity. Then the hydrogen atom transfers from ${\mathrm{BH}}_4^{-}$ 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.

It should be noted that the bleaching rate is considerably higher than the rates reported previously under the similar experimental conditions with Ag-based, CuFe₂O₄-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.

3. Materials and Methods

3.1. Chemicals

All the reagents used were of analytical purity and were used without further purification. Polyethylene glycol-6000 (PEG-6000), polyvinylpyrrolidone (PVP-1300000), dopamine hydrochloride (DA·HCl), and 2-methylimidazole (C₄H₆N₂) were purchased from Shanghai Aladdin Industrial Co., Ltd. (Shanghai, China). Cupric chloride anhydrous (CuCl₂) and other reagents, such as ferric chloride hexahydrate (FeCl₃·6H₂O), ammonium acetate (NH₄OAc), ethylene glycol (EG), silver nitrate (AgNO₃), 4-nitrophenol(4-NP), sodium borohydride (NaBH₄), and zinc nitrate (Zn(NO₃)₂·6H₂O) were purchased from Sigma-Aldrich (Shanghai, China). Cellulose nanocrystals (CNCs) were purchased from Tianjin Haojia Cellulose Co., Ltd. (Tianjin, China).

3.2. 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.

3.3. Preparation of CuFe₂O₄/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 CuCl₂·2H₂O and 3.2 mmol FeCl₃·6H₂O 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 NH₄OAc (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.

3.4. In Situ Reduction of Ag⁺ Ions

To coat CuFe₂O₄/CNC nanocomposites with the PDA shell, 50 mg CuFe₂O₄/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 CuFe₂O₄/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 AgNO₃ solution until brown precipitation was just dissolved. Portions 50 mg in size of the CuFe₂O₄/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, CuFe₂O₄/CNC@Ag nanocomposites were obtained.

3.5. Preparation of CuFe₂O₄/CNC@Ag@ZIF-8 Nanocomposites

Briefly, 50 mg CuFe₂O₄/CNC@Ag nanocomposites were added into Zn(NO₃)₂·6H₂O methanol solution (10 mL, 50 mM) and stirred for 10 min at 50 °C. Subsequently, the nanocomposites were dispersed in methanol solution (10 mL, 500 mM) under stirring for 30 min at 50 °C, collected by a magnet, cleaned with ultrapure water and ethanol, and dried under vacuum at 60 °C overnight.

3.6. General Procedure for the Reduction of 4-NP

The reduction of 4-NP by NaBH₄ was chosen as a model reaction for investigating the catalytic performance of the CuFe₂O₄/CNC, CuFe₂O₄/CNC@Ag, and CuFe₂O₄/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 NaBH₄ 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.

4. Conclusions

In summary, we demonstrated an effective strategy for the fabrication of novel cellulose nanocrystals (CNC)-supported magnetic CuFe₂O₄@Ag@ZIF-8 catalysts which consist of a paramagnetic CuFe₂O₄@Ag core and a porous ZIF-8 shell. The use of CNC include being a template and dispersant for the incorporation with CuFe₂O₄ 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 CuFe₂O₄/CNC@Ag catalysts, the core-shell structure CuFe₂O₄/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.

Author Contributions

Conceptualization, S.Z. and Y.X.; methodology, S.Z. and D.Z.; data curation, W.C. and H.L.; writing—review and editing, D.Z. and C.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Key Scientific Research Plan of Shaanxi Provincial Education Department (number 17JS016), the Guangxi Key Laboratory of Clean Pulping, the Papermaking and Pollution Control Opening Fund (KF 201711) and the National Demonstration Center for Experimental Light Chemistry Engineering Education (2018QGSJ02-09).

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

Sample Availability: Samples of the compounds are unavailable from the authors.

Figures, Schemes and Table

Scheme 1. Schematic representation of the synthesis of magnetic CuFe2O4/CNC@Ag@ZIF-8 nanocomposites.

Figure 1. TEM images of the (A) CuFe2O4/CNC nanocomposites; (B,C) CuFe2O4/CNC@Ag nanocomposites; (D,E) CuFe2O4/CNC@Ag@ZIF-8 nanocomposites; (F) HRTEM image of CuFe2O4 nanoparticles (NPs) and Ag NPs.

Figure 2. Representative high-angle annular dark field scanning TEM (HAADF-STEM) images and energy-dispersive X-ray (EDX) elemental mapping of Cu, Fe, Zn, Ag, C, O, and N (A–I) and the EDX spectrum (J) of the CuFe2O4/CNC@Ag@ZIF-8 nanocomposites.

Figure 3. (A) Fourier transform infrared (FTIR) spectra of (a) CuFe2O4/CNC, (b) CuFe2O4/CNC@PDA, (c) CuFe2O4/CNC@Ag, and (d) CuFe2O4/CNC@Ag@ZIF-8 nanocomposites; (B) XRD diffraction patterns of CuFe2O4/CNC, CuFe2O4/CNC@Ag, CuFe2O4/CNC@Ag@ZIF-8 nanocomposites, and simulated XRD pattern for ZIF-8.

Figure 4. (A) N2 adsorption-desorption isotherms; (B) the pore-distribution curves; (C) the magnetic hysteresis curves of the CuFe2O4/CNC@Ag@ZIF-8 and CuFe2O4/CNC@Ag nanocomposites (the inset shows the magnetic separation behavior of the CuFe2O4/CNC@Ag@ZIF-8 nanocomposites in the aqueous solution).

Figure 5. Typical time-dependent evolution of UV–Vis spectra showing the catalytic reduction of 4-NP to 4-AP by (A) CuFe2O4/CNC@Ag; (B) CuFe2O4/CNC@Ag@ZIF-8 nanocomposites; (C) plot of ln [4-NP] versus reaction time for the reduction of 4-NP using 3 mg CuFe2O4/CNC, CuFe2O4/CNC@Ag, and CuFe2O4/CNC@Ag@ZIF-8 nanocomposites; (D) conversion of 4-NP during six cycles of reaction by the CuFe2O4/CNC@Ag@ZIF-8 nanocomposites.

Scheme 2. The mechanism of the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by NaBH4 in the CuFe2O4/CNC@Ag@ZIF-8 nanocomposites.

Table 1

Comparison of k of different catalytic systems for the reduction of 4-NP (298K).

Entry	Nanocatalysts	k (min−1)	Reference
1	Ag/C	0.33	[55]
2	Fe3O4@SiO2@Ag	0.52	[56]
3	Au/TAPB-DMTP-COF	0.46	[57]
4	Au@TpPa-1	0.25	[53]
5	Ag NPs@CMG	0.204	[55]
6	CuFe2O4@Ag@ZIF-8	0.64	This work