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Jul 2, 2018 - Department of Chemistry, Sardar Bahadur Khan Women,s University, Quetta 87300, Pakistan. §. Department of Chemistry, University of ...
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Article Cite This: Inorg. Chem. 2018, 57, 13270−13278

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Hybrid Metal−Organic-Framework/Inorganic Nanocatalyst toward Highly Efficient Discoloration of Organic Dyes in Aqueous Medium Kanwal Iqbal,†,‡ Anam Iqbal,§ Alexander M. Kirillov,*,∥,⊥ Weisheng Liu,† and Yu Tang*,†

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†

State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, P.R. China ‡ Department of Chemistry, Sardar Bahadur Khan Women’s University, Quetta 87300, Pakistan § Department of Chemistry, University of Baluchistan, Quetta 87300, Pakistan ∥ Centro de Quimica Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, Lisbon 1049-001, Portugal ⊥ Peoples’ Friendship University of Russia (RUDN University), 6 Miklukho-Maklaya st., Moscow, 117198, Russian Federation S Supporting Information *

ABSTRACT: Nanoscale metal−organic frameworks (NMOFs) represent a unique class of solids with superior adsorption, mass transport, and catalytic properties. In this study, a facile and novel approach was developed for the generation of hybrid CuNMOF/Ce-doped-Mg-Al-LDH nanocatalyst through in situ self-assembly and solvothermal synthesis of a 2D Cu-NMOF, [Cu2(μ-OH)(μ4-btc)(phen)2]n·5nH2O {H3btc, trimesic acid; phen, 1,10-phenanthroline}, on a cerium-doped Mg-Al layered double hydroxide (Ce-doped-Mg-Al-LDH) matrix. Self-assembly between Cu-NMOF nanocrystals and exfoliated LDH led to their nanoscale mixing and prevented the formation of aggregated Cu-NMOF nanoparticles. In the resulting hybrid nanostructure, Cu-NMOF nanocrystals (∼10−20 nm particle size) are anchored uniformly on a Ce-doped-Mg-Al-LDH’s surface, possessing a dimension of several hundred nanometers. Catalytic activity of Cu-NMOF/Ce-doped-Mg-Al-LDH and Cu-NMOF was evaluated under ambient conditions in the reductive degradation (discoloration) of aqueous solutions of 4-nitrophenol (4-NP, model substrate) and a series of commercial organic dyes by applying sodium borohydride as a reducing agent. The Cu-NMOF/Ce-doped-Mg-Al-LDH nanocatalyst exhibited an outstanding catalytic activity toward degradation of 4-NP, with kapp (rate constant) of 0.03 and a catalyst TOF (turnover frequency) up to 7.1 × 103 h−1. Full and very quick discoloration of organic dyes {rhodamine B (RhB), methylene blue (MB), Congo red (CR), methyl orange (MO), and rhodamine 6G (R6G)} was also achieved with TOF values of up to 1.4 × 105/h. A superior activity of the hybrid nanocatalyst over Cu-NMOF can be regarded as a synergic effect among Cu-NMOF and Ce-doped-Mg-Al-LDH components, while the Ce-doped-Mg-Al-LDH carrier acts as a cocatalyst. The hybrid nanocatalyst can easily be recovered and reused successfully for the five consecutive reaction runs with the same catalytic performance. This study also shows that NMOFs can be easily incorporated onto conventional catalyst supports, resulting in hybrid nanocatalysts with a highly uniform structural architecture, controlled chemical composition, and excellent catalytic function.

1. INTRODUCTION

Conventional microbial wastewater treatment methods often suffer from low efficiency of dye decontamination, namely, due to a deterrent effect of an increased salt content in dye containing wastewater and a low biodegradability of dyes.8 Many different methods have been used for dye removal, namely, including oxidation, sorption,8 coagulation/flocculation,9 and photocatalytic processes.10,11 Despite a recognized efficiency of these methods, there are operational disadvantages and limitations in many cases. For instance, application of coagulants and polyelectrolytes generates a significant quantity of sludge during the coagulation and flocculation.12 Ozone-

Dyes represent an important class of organic compounds that are commonly used for the manufacturing of a high diversity of products and materials, such as nylon, wool, silk, acrylics, polyesters, leather food, cotton, lubricants, oils, waxes, and cosmetics. However, in the last decades, the discharge of wastewater effluents and consumption of organic dyes from different industrial sources have significantly increased, creating a serious public issue about the dye pollution in various fields.1−5 Given the high toxicity and difficult biodegradability of organic dyes,6 an overexposure to them can cause severe health problems in addition to well recognized environmental concerns.7 © 2018 American Chemical Society

Received: July 2, 2018 Published: October 8, 2018 13270

DOI: 10.1021/acs.inorgchem.8b01826 Inorg. Chem. 2018, 57, 13270−13278

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Inorganic Chemistry assisted oxidation methods or Fenton’s reagent (H2O2/Fe2+) allow a removal of dyes with no generation of sludge; nevertheless, these methods show a short oxidant lifetime and a significant cost of operation.13 Photoassisted methods for the discoloration of dyes are not usually applied at a large scale and require long operating times.14 Dye removal via adsorption can be simple and efficient but requires the disposal of used adsorbent materials.8 Given all of the above-mentioned reasons, the search for novel dye decontamination protocols, catalysts, and adsorbents represents an important research direction on an interface of green chemistry, sustainable catalysis, and environmental chemistry. Recently, the catalytic reductive discoloration of dyes assisted by NaBH4 has received great attention due to simplicity and very quick operating protocol.8 In this type of processes, nanoparticles of noble metals (e.g., Ag, Au, Ru, Pd, and Pt) supported on carbon or silica matrixes can be applied as efficient catalytic materials.15 However, high cost and scarcity of noble metals and derived catalysts impede their wide application in wastewater treatment procedures.16 As a result, the replacement of catalysts containing noble metals by cheaper analogues based on transition metals (non-noble metals) has emerged as a goal for further research. As an effort to bring cheaper and highly efficient catalytic materials to the reductive dye degradation, metal−organic frameworks (MOFs) have drawn a special attention17 due to their intrinsic porosity, high stability and crystallinity, rich diversity of architectures, recognized sorption, and catalytic applications.18,19 As potential catalyst supports, layered double hydroxides (LDHs) represent an abundant and low-cost type of clays, which can incorporate inorganic anions between the host brucite-like layers that bear a positive charge.20 Because of the structural flexibility of LDHs and unique composition, they have found notable applications in heterogeneous catalysis.21,22 Furthermore, doping of the LDH host layers by lanthanide ions can significantly improve the catalytic performance of the resulting materials due to an enhanced charge separation efficiency.23,24 Besides, the distortion of the lattice and vacant f-orbitals of lanthanides can have an effect on the surface charge injection process25 that can also be important for catalytic transformations. Despite representing a very promising type of materials, lanthanide-doped LDHs are still rarely applied in catalysis. By combining various features of LDHs and nanoscale MOFs (NMOFs) within one nanomaterial, it is possible to generate nanocatalysts and other functional materials with superior performance. However, to date, there are only limited reports regarding MOF/LDH hybrid nanomaterials.26 Taking into consideration the above discussion, the main objectives of the present study have been (1) to assemble a hybrid nanomaterial that would incorporate both the transitionmetal-based nanoscale MOF and Ln-doped LDH components and (2) to develop the application of the obtained nanomaterial as a recyclable, highly powerful, and low-cost nanocatalyst for reductive degradation (discoloration) of common organic dyes in H2O medium. For the first time, we report in this work the preparation of a hybrid Cu-NMOF/Ce-doped-Mg-Al-LDH nanostructure by in situ generation of 2D Cu-NMOF, [Cu2(μ-OH)(μ4-btc)(phen)2]n·5nH2O, on the Ce-doped-Mg-Al-LDH support using a solvothermal method. This hybrid nanostructure can be regarded as a very promising nanocatalyst toward

degradation of commercial organic dyes owing to a strong synergy among its Cu-NMOF and Ce-doped-Mg-Al-LDH components. Due to simplicity, this synthetic strategy is particularly attractive for the incorporation of MOFs on the layers of the LDH support and provides an efficient strategy toward the design of new hybrid nanocatalysts. Catalytic activity of both Cu-NMOF/Ce-doped-Mg-Al-LDH and CuNMOF for discoloration of different organic dyes and for 4nitrophenol reduction was studied. Hence, this work brings an original concept which is feasible toward the synthesis of hybrid and recoverable nanocatalysts, with an outstanding potential toward the reductive degradation of commercial dyes under ambient conditions and in H2O medium.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Cu(NO3)3·3H2O, Al(NO3)3·9H2O, Mg(NO3)2· 6H2O, trimesic acid, KOH, NaBH4, Ce(NO3)3·6H2O, 1,10-phenanthroline, and MeOH were supplied by Xiensi Reagent Company (Tianjin, China). 4-NP and all organic dyes were supplied by Guangfu Reagent Company (Tianjin, China). Analytical grade reagents were used in all experiments. A Milli-Q ultrapure (18.2 MΩ cm) system was used for the preparation of ultrapure water. 2.2. Methods and Instruments. The morphology of materials was investigated by SEM (scanning electron microscopy, JSM-6701F) and TEM (transmission electron microscopy, Tecnai G2 F30) equipped with an EDX (energy-dispersive X-ray) spectrometer. PXRD (powder X-ray diffraction) studies were carried out on a Rigaku D/max-2400 diffractometer (2θ = 20−80°) with Cu Kα radiation. XPS (X-ray photoelectron spectroscopy, PerkinElmer PHI5702) measurements were applied to investigate the electronic states of nanocatalyst. FT-IR (Fourier Transform Infrared Spectroscopy) spectra were recorded on a Nicolet 360 FTIR spectrometer (KBr pellet technique). ICP-AES (inductively coupled plasma-atomic emission spectroscopy) was applied for elemental analysis. Catalytic transformations were monitored using an Agilent Cary 5000 UV−vis spectrophotometer. 2.3. Synthesis of Mg-Al-LDH and Ce-Doped-Mg-Al-LD Carriers. Our previously reported hydrothermal procedure was applied for the preparation of Mg-Al-LDH and Ce-doped-Mg-AlLDH carriers; Mg(NO3)2·6H2O, Al(NO3)3·9H2O, and Ce(NO3)3· 6H2O were used as starting materials.27,28 2.4. Synthesis of Cu-Based MOF (Cu-NMOF). Cu-NMOF, [Cu2(μ-OH)(μ4-btc)(phen)2]n·5nH2O, was synthesized following a reported protocol after some modification.29 Cu(NO3)2·3H2O (2 mmol), trimesic acid (H3btc, 1,3,5-benzenetricarboxylic acid; 1 mmol), and 1,10-phenanthroline (phen, 2 mmol) were dissolved in a 50 mL anhydrous methanol and placed into a stainless steel Teflon lined reactor (100 mL), followed by heating for 24 h at 120 °C. Then, the reactor was coolded down to ambient temperature. Blue crystals of the obtained product were collected and then washed several times with H2O and MeOH, followed by drying for 12 h at 50 °C to give Cu-NMOF in 85% yield based on copper(II) nitrate. The purity of Cu-NMOF was investigated by FTIR spectroscopy, ICP, and PXRD analyses. 2.5. Synthesis of Cu-NMOF/Ce-Doped-Mg-Al-LDH Nanocatalyst. In a first step, a dispersion of Ce-doped-Mg-Al-LDH (1.6 mg/mL) in H2O was diluted with anhydrous methanol (50 mL) and further dispersed. A quantity of Ce-doped-Mg-Al-LDH (1.6 mg/mL) was selected to fit the amount of Cu-NMOF prepared with the same loadings of solvent and precursors. After 30 min of ultrasonication, trimesic acid (0.5 mmol) was introduced followed by stirring the obtained mixture at room temperature for 12 h. After that, Cu(NO3)2· 3H2O (2 mmol), 1,10-phenanthroline (2 mmol), and a further amount of trimesic acid (0.5 mmol) were introduced. The final reaction mixture was further stirred for 0.5 h at room temperature and then transferred to a 100 mL stainless steel Teflon lined reactor, which was heated for 24 h at 120 °C. After that, the reactor was cooled down to ambient temperature. An obtained solid was washed 13271

DOI: 10.1021/acs.inorgchem.8b01826 Inorg. Chem. 2018, 57, 13270−13278

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Inorganic Chemistry several times with H2O and MeOH and then dried for 12 h at 50 °C to furnish the Cu-NMOF/Ce-doped-Mg-Al-LDH material (0.89 g). Elemental analysis of Cu-NMOF and Cu-NMOF/Ce-doped-Mg-AlLDH indicated the Cu content of 16.53% in Cu-NMOF and 14.86% in Cu-NMOF/Ce-doped-Mg-Al-LDH. 2.6. Evaluation of Catalytic Performance. Typically, 30 μL of 0.01 M 4-NP (aqueous solution) was combined with deionized H2O (2.7 mL). Then, a freshly obtained solution of NaBH4 in H2O (0.1 M, 250 μL) was introduced, resulting in a solution with a bright yellow color. In the next step, 10 μL of nanocatalyst (5 mg/mL) was introduced, causing a gradual fading of yellow color. A progress of this fading process was investigated by UV−vis spectroscopy. Catalyst reusability was studied by repeating the reduction five times. In the end of each run, centrifugation allowed the isolation of the nanocatalyst; it was then reused in further runs. In the experiments with dyes as substrates instead of 4-NP, a similar procedure was employed. The only difference concerned the volume of the dye used, which was 15, 10, 20, 10, and 30 μL for RhB, MB, MO, R6G, and CR solutions (0.01 M), respectively.

over, it should be noted that both Cu-NMOF and Cu-NMOF/ Ce-doped-Mg-Al-LDH do not show any diffraction peaks corresponding to CuO (35.5 and 38.7°) or Cu2O (36.43°) phases.34 For Mg-Al-LDH, Ce-doped-Mg-Al-LDH, Cu-NMOF, and Cu-NMOF/Ce-doped-Mg-Al-LDH samples, the FT-IR spectra (Figure S1) reveal intense and rather broad bands (3000− 3500 cm−1) that are characteristic for ν(O−H) and ν(N−H) vibrations. In the samples containing Cu-NMOF, the peaks of the νas(COO) and νs(COO) vibrations appear at ∼1645 and ∼1513 cm−1, respectively (Figure S1c,d). The ν(NO3−) band is observed at 1380 cm−1. Due to the absence of peaks at 410, 500, 610, and 615 cm−1, we can deduce that there is no CuO or Cu2O in the obtained samples.34 SEM and TEM have been used to study the morphology and microstructure of the obtained samples (Figure 2). The TEM image of Ce-doped-Mg-Al-LDH (Figure 2a) reveals a small number of black dots due to the cerium impure phase, which agrees with the PXRD result. TEM images of Cu-NMOF show a well-defined cubic shape structure (Figure 2b,c). Further, TEM analysis of Cu-NMOF/Ce-doped-Mg-Al-LDH indicates the 10−20 nm Cu-NMOF cubic shape particles, forming a network via van der Waals forces on the Ce-doped-Mg-AlLDH layer (Figure 2d,e). The assembly of Cu-NMOF with Ce-doped-Mg-Al-LDH is further confirmed by analyzing a HRTEM (high-resolution transmission electron microscopy) image, wherein nanocatalyst clearly demonstrates the lattice fringes of two hybridized components with an interline distance of ∼0.31 and ∼0.21 nm for Cu-NMOF and the Cedoped-Mg-Al-LDH (inset, Figure 2d).35 The electron diffraction pattern (SAED) of Cu-NMOF/Ce-doped-Mg-AlLDH displays the bright circular rings that indicate a largely polycrystalline character of the as-synthesized Cu-NMOF (Figure 2f); this is clearly matched with the results of the PXRD study. All of these findings underscore a successful integration of Cu-NMOF into the Ce-doped-Mg-Al-LDH matrix, forming a hybrid nanocatalyst. The elemental distribution in Cu-NMOF/Ce-doped-Mg-AlLDH has been investigated by HAADF-STEM (high-angle annular dark field scanning transmission electron microscopy) (Figure 3). EDX (energy-dispersive X-ray) elemental mapping analysis indicates a uniform distribution of O, C, N, Al, Mg, Ce, and Cu in the hybrid nanostructure (full region), thus giving a clear proof of the homogeneous distribution of CuNMOF and Ce-doped-Mg-Al-LDH (Figure S2). The morphological structure (SEM image) of the CuNMOF/Ce-doped-Mg-Al-LDH nanocatalyst indicates its highly porous nature wherein Ce-doped-Mg-Al-LDH is uniformly decorated with cubic shaped particles of CuNMOF (Figure S4). XPS has been further used to determine the surface composition of Cu-NMOF/Ce-doped-Mg-Al-LDH (Figure 4). Full XPS spectrum survey reveals the signals of C 1s, Mg 1s, Al 2p, Ce 3d, and Cu 3d (Figure 4a). HXPS (highresolution XPS) signals with binding energies (BE) near 72.15 and 1301.26 eV most likely refer to Al 2p and Mg 1s, respectively (Figure S3). For the nanocatalyst, the Ce 3d spectrum (Figure 4b) shows eight peaks that were assigned as follows. Labels ν (ν0, ν1, and ν2) and μ (μ0, μ1, and μ2) correspond to the signals of Ce 3d5/2 and 3d3/2, respectively; these refer to the typical initial four-valent state peaks of the Ce 4f0.36 Highest BE peaks μ2 (914.25 eV) and ν2 (895.97 eV) represent the final Ce(3d94f0)O(2p6) state.37 The low BE

3. RESULTS AND DISCUSSION 3.1. Full Characterization of Cu-NMOF/Ce-DopedMg-Al-LDH Nanocatalyst. Powder X-ray diffraction (PXRD) was used to evaluate a potential structural medication and phase composition of Cu-NMOF/Ce-doped-Mg-Al-LDH against its parent precursors (Mg-Al-LDH, Ce-doped-Mg-AlLDH, and Cu-NMOF). On the 2θ scale, the most characteristic peaks correspond to (003) and (006) reflections at 8.25 and 19.47° (labeled as ● in Figure 1b and d), respectively;

Figure 1. Patterns of PXRD for (a) Mg-Al-LDH, (b) Ce-doped-MgAl-LDH, (c) Cu-NMOF, and (d) Cu-NMOF/Ce-doped-Mg-AlLDH. Labels ●, ⧫, and ∗ are peaks of Mg-Al-LDH, impure phase of cerium, and Cu-NMOF species, respectively.

these indicate the formation of nitrate-intercalated LDHs.30 A decreased diffraction intensity and crystallinity of Ce-dopedMg-Al-LDH is consistent with a bigger Ce3+ ionic radius (1.02 Å) in comparison with the one of Mg2+ (0.86 Å) and Al3+ (0.67 Å).31 In Ce-doped-Mg-Al-LDH, an appearance of a reflection at 26.4 (labeled as ⧫ in Figure 1b) is most likely due to cerium impure phase.32 The as-synthesized Cu-NMOF/Cedoped-Mg-Al-LDH material shows characteristic diffraction peaks of Cu-NMOF at 9.95, 10.46, 12.50, 14.37, 16.41, 17.26, 25.24, and 27.11° (labeled as ∗ in Figure 1d), which indicate the successful growth and formation of Cu-NMOF on the Cedoped-Mg-Al-LDH carrier.33 Importantly, we found that the diffraction peaks of the Cu-NMOF/Ce-doped-Mg-Al-LDH nanocatalyst are well shifted toward lower angles if compared with respective peaks of the parent Cu-NMOF sample. This further supports the presence of the relatively strong interactions between the Ce-doped-Mg-Al-LDH and CuNMOF components in the hybrid nanocatalyst.41−43 More13272

DOI: 10.1021/acs.inorgchem.8b01826 Inorg. Chem. 2018, 57, 13270−13278

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Figure 2. TEM images of (a) Ce-doped-Mg-Al-LDH, (b, c) Cu-NMOF, and (d, e) Cu-NMOF/Ce-doped-Mg-Al-LDH (the inset in part d is the HRTEM image). (f) SAED pattern of Cu-NMOF/Ce-doped-Mg-Al-LDH.

Figure 3. HAADF-STEM and EDX elemental mapping of the hybrid Cu-NMOF/Ce-doped-Mg-Al-LDH material.

states of μ1 (905.92 eV) and ν1 (886.36 eV) refer to Ce(3d94f1)O(2p5). The BE peaks μ0 (898.63 eV) and ν0 (879.99 eV) are derived from the Ce(3d94f2)O(2p4) final states. For Ce 3d, μ′ and ν′ labels are the typical Ce3+ peaks. The BE peaks ν′ and μ′ are observed at 882.53 and 904.41 eV, respectively; these refer to the Ce(3d94f1)O(2p5) state. The Cu 2p spectra of Cu-NMOF/Ce-doped-Mg-Al-LDH and CuNMOF (Figure 4c and d, respectively) are composed of the Cu 2p1/2 and Cu 2p3/2 signals. In the Cu-NMOF spectrum, the peak at 932.31 eV is the Cu 2p3/2 and a shakeup in the 939.05−952.04 eV range is characteristic for Cu2+ species; a signal at 960.22 eV corresponds to Cu 2p1/2. The Cu 2p3/2 peak emerging at an inferior binding energy indicates the Cu0/ Cu+ presence. In the Cu-NMOF/Ce-doped-Mg-Al-LDH nanocatalyst (Figure 4c), the Cu 2p exhibits main signals at 931.98 and 960.34 eV with their satellite signals shifted toward lower angles; this fact could be rationalized by the Ce3+ + Cu2+ ↔ Ce4+ + Cu0/Cu+ transformation.37 Such interactions have a great influence on the catalytic activity, since doping of Mg-AlLDH with Ce could affect chemical states of the Cu sites, namely, contributing to the reduction of Cu2+ to more catalytically active Cu0/Cu+ states. The spectrum of C 1s of Cu-NMOF exhibits two signals that belong to (COH) and (CO) groups of the organic linker

(285.14 and 288.61 eV, Figure 4f). In the Cu-NMOF/Cedoped-Mg-Al-LDH nanocatalyst, these C 1s peaks are at 285.14 and 288.26 eV, being 0.35 eV shifted to an inferior energy region (Figure 4e). This result indicates a couple of important points, namely, (i) an insertion of Cu-NMOF into Ce-doped-Mg-Al-LDH and (ii) an increase of the electron density at organic moieties as a result of the Cu-NMOF and Ce-doped-Mg-Al-LDH hybridization. This can be further confirmed by the O 1s XPS spectra of Cu-NMOF (Figure 4h) wherein the signal (529.96 eV) of the μ4-btc carboxylate groups is shifted to 529.21 eV in comparison with the nanocatalyst (Figure 4g). 3.2. Catalytic Activity of Cu-NMOF/Ce-Doped-Mg-AlLDH toward Reduction of 4-Nitrophenol and Reductive Degradation of Dyes. As a model reaction, the catalytic potential of the Cu-NMOF/Ce-doped-Mg-Al-LDH nanocatalyst was investigated in the reduction of 4-NP (4nitrophenol) to give 4-AP (4-aminophenol) under ambient conditions (room temperature and atmospheric pressure) in H2O medium; the aqueous solution of NaBH4 played the function of the reducing agent, and the reaction evolution was investigated by UV−vis. In the UV−vis spectrum of the aqueous solution of 4-NP, a maximum absorption band appears at 317 nm (Figure 5). After 13273

DOI: 10.1021/acs.inorgchem.8b01826 Inorg. Chem. 2018, 57, 13270−13278

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Figure 4. (a) Full survey XPS spectrum of Cu-NMOF/Ce-doped-Mg-Al-LDH and the corresponding high-resolution Ce 3d (b), Cu 2p (c), C 1s (e), and O 1s (g) spectra. High-resolution Cu 2p (d), C 1s (f), and O 1s (h) XPS spectra of Cu-NMOF.

the introduction of sodium borohydride, this band shifts to a higher wavelength (400 nm) as a result of the formation of 4nitrophenolate ions. The yellow color of an initial solution containing 4-nitrophenol quickly turns to a bright yellow. Although the standard electrode potential (ΔE⊖ = E⊖(4-NP/ 4-AP) − E⊖(H3BO3/BH4−) = −0.76 − (−1.33) = 0.67 V) for the reduction of 4-nitrophenol by sodium borohydride is favorable from the thermodynamics viewpoint, without an

appropriate catalyst, such a reduction process is restricted from the kinetics viewpoint.38 However, an introduction of the CuNMOF/Ce-doped-Mg-Al-LDH nanocatalyst into the system leads to a great effect on the reaction and the reduction of 4nitrophenol can be complete within 2 min. The color of the reaction medium weakens from bright yellow and completely disappears in the end of the reaction, followed by a gradual decrease of the absorption band (400 nm) of 4-nitrophenolate; 13274

DOI: 10.1021/acs.inorgchem.8b01826 Inorg. Chem. 2018, 57, 13270−13278

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if comparing with the Cu-NMOF/Ce-doped-Mg-Al-LDH nanocatalyst (Figure S5). Hence, Cu-NMOF/Ce-doped-MgAl-LDH (Figure S5, Table 1) shows an outstanding catalytic behavior toward all of the substrates tested as well as high reaction rates and remarkable TOF values that attain 7.1 × 103 h−1 in the 4-NP reduction. With this nanocatalyst, a prompt degradation of dyes has also been observed with reaction times varying from 60 s to max 120 s. Organic dyes that have less bulky molecules degrade more quickly, which can be associated with their lower steric hindrance. To further evaluate the catalytic efficiency of the tested catalysts, the kapp (apparent rate constant) was determined by the following equation: ln(Ct/C0) = ln(At/A0) = kappt. Because the reducing agent is used in excess relative to organic dye substrates, their concentration obeys a pseudo-first-order kinetics.38 Figures S5f, S6g, and S7g show the linear ln(Ct/C0) vs t (time of the reaction) dependences for the tested catalysts. From the linearly fitted slopes of ln(Ct/C0) vs t, the kapp values were obtained (Table 1), showing superior activity of CuNMOF/Ce-doped-Mg-Al-LDH over Cu-NMOF. This is also reflected by the high turnover frequency (TOF)39 values that are usually 1.86, 2.1, 3.73, 6.15, 36.8, and 48.2 (for RhB, R6G, 4-NP, MB, MO, and CR, respectively) times greater in the case of the nanocatalyst if compared to Cu-NMOF. Besides, the TOF values of the Cu-NMOF/Ce-doped-Mg-Al-LDH nanocatalyst are significantly superior if compared to other earlier reported catalysts for the reduction of 4-nitrophenol or organic dye discoloration (a summary is given in Tables S1 and S2).40 To probe the reusability of the Cu-NMOF/Ce-doped-MgAl-LDH nanocatalyst, several recycling experiments were performed under standard reaction conditions in the reduction of 4-nitrophenol. The obtained results show that the nanocatalyst maintains its original morphology, composition, and activity even after five consecutive reaction cycles, allowing a complete reductive degradation of 4-NP (Figures S8−S10).

Figure 5. UV−vis absorption spectra (after different periods of time) of the reaction mixture in the course of the reduction of 4-nitrophenol to 4-aminophenol by NaBH4 catalyzed by Cu-NMOF/Ce-doped-MgAl-LDH. The insets represent the fading of 4-NP aqueous solution (top left) and a plot of ln(Ct/C0) vs reaction time (top right). Conditions: time (0−120 s), H2O (2.7 mL), Cu-NMOF/Ce-dopedMg-Al-LDH (10 μL, 5.0 mg/mL), 4-NP (0.01 M, 30 μL), NaBH4 (0.1 M, 250 μL).

an intensity of the novel band (300 nm) of 4-aminophenol gradually increases (Figure 5). Apart from the reactions involving 4-nitrophenol as a model substrate, the catalytic potential of Cu-NMOF/Ce-doped-MgAl-LDH, Ce-doped-Mg-Al-LDH, and Cu-NMOF was also studied toward the discoloration of common organic dyes (RhB, MB, MO, R6G, and CR) with NaBH4; further details are given in Figures S5−S7. A summary of the catalytic performance of different catalysts is also given in Table 1. If Cu-NMOF is used, longer reaction times are required to achieve the 4-NP reduction and dye discoloration (Figure S6)

Table 1. Summary of the Catalytic Performance of Cu-NMOF/Ce-Doped-Mg-Al-LDH and Cu-NMOF toward Reduction of 4Nitrophenol and Discoloration of Dyesa catalyst Cu-NMOF/Ce-doped-Mg-Al-LDH Cu-NMOF Ce-doped-Mg-Al-LDH Cu-NMOF/Ce-doped-Mg-Al-LDH Cu-NMOF Ce-doped-Mg-Al-LDH Cu-NMOF/Ce-doped-Mg-Al-LDH Cu-NMOF Ce-doped-Mg-Al-LDH Cu-NMOF/Ce-doped-Mg-Al-LDH Cu-NMOF Ce-doped-Mg-Al-LDH Cu-NMOF/Ce-doped-Mg-Al-LDH Cu-NMOF Ce-doped-Mg-Al-LDH Cu-NMOF/Ce-doped-Mg-Al-LDH Cu-NMOF Ce-doped-Mg-Al-LDH

substrate (0.01 M)

reaction time (s)

4-nitrophenol 4-nitrophenol 4-nitrophenol methylene blue methylene blue methylene blue methyl orange methyl orange methyl orange Congo red Congo red Congo red rhodamine B rhodamine B rhodamine B rhodamine 6G rhodamine 6G rhodamine 6G

120 410 until 1320 210 510 until 660 60 210 until 660 60 270 until 660 120 210 1320 90 180 1620

kapp (s−1) −2

3.0 × 10 2.4 × 10−2 0.58 × 10−2 2.8 × 10−2 3.0 × 10−2 0.48× 10−2 3.1 × 10−2 2.2 × 10−3 0.78× 10−2 4.6 × 10−2 2.8 × 10−3 2.1 × 10−2 2.4 × 10−2 1.9 × 10−2 8.4 × 10−2 2.7 × 10−2 1.9 × 10−2 9.6 × 10−2

TOFb (h−1) 7.1 1.9 1.3 4.0 1.5 2.7 1.4 3.8 2.7 1.4 2.9 2.7 7.1 3.8 1.3 9.5 4.4 1.1

× × × × × × × × × × × × × × × × × ×

103 103 104 104 103 103 105 103 103 105 103 103 103 103 104 103 103 103

a

See Figure 5 and the experimental part for all reaction conditions. bTOF (turnover frequency) = [(molar concentration of reacted substrate)/ (molar concentration of metal (M) in catalyst)]/(reaction time) (h−1); M = Ce for Cu-NMOF/Ce-doped-Mg-Al-LDH and Ce-doped-Mg-AlLDH, or M = Cu for Cu-NMOF. 13275

DOI: 10.1021/acs.inorgchem.8b01826 Inorg. Chem. 2018, 57, 13270−13278

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Inorganic Chemistry A negligible decline in the activity (1−2%) observed after three cycles is due to a slight decrease in the amount of catalyst in recycling tests, catalyst isolation by centrifugation, and an experimental error. 3.3. Origin of Superior Catalytic Performance of CuNMOF/Ce-Doped-Mg-Al-LDH. The excellent catalytic behavior of Cu-NMOF/Ce-doped-Mg-Al-LDH has been rationalized in the following discussion. (1) Cu-NMOF has a strong electrostatic interaction with dyes due to their possible protonation and deprotonation behavior. Additionally, hydrogen bonding can also occur between dye and Cu-NMOF molecules; the latter have the COO− and OH− functionalities prone of H-bonding. (2) In Cu-NMOF/Ce-doped-Mg-AlLDH, there is an influence of the metal−organic framework and the Mg-Al-LDH structure on dye adsorption, thus facilitating the formation of adducts. Various transition metals can also reduce dyes in the presence of NaBH4. A significantly higher activity of Cu-NMOF/Ce-doped-MgAl-LDH over Cu-NMOF can be regarded to the presence of Ce-doped-Mg-Al-LDH, which operates as not only the carrier matrix but the cocatalyst as well. The following features of the Ce-doped-Mg-Al-LDH matrix can be identified. (A) It has a high surface area, good capacity toward exchange of anions, excellent stability, and capability to remove different pollutants from aqueous solutions. (B) Ce-doped-Mg-Al-LDH possesses base sites and large specific surface-active area, thus enabling it to function as both a support for Cu-NMOF and a cocatalyst. Hence, the hybridization of Ce-doped-Mg-Al-LDH with CuMOF can prevent the aggregation and afford additional surface-active sites for an enhanced catalytic efficiency. (C) It has a hydrophilic surface that speeds up the catalytic transformation by an adsorption of the donor BH4− at the Cu-NMOF surface. Moreover, Ce-doped-Mg-Al-LDH layers are very efficient adsorbents for dyes. (D) Doping Mg-Al-LDH by cerium enhances the mobility of electrons, increases the surface basicity, and improves the catalytic activity. (E) Also, a potential synergic effect can be observed owing to the combination of Ce and Cu within the hybrid nanocatalyst. Oxidation states of these metals can easily be modified, whereas there is also an interaction involving Cu and Ce (Cu2+/Cu+ ↔ Ce4+/Ce3+) which is important for the redox cycle. Hence, more reactive sites and enhanced electron mobility result in a superior catalytic performance of CuNMOF/Ce-doped-Mg-Al-LDH.41−43 Because of these features, hybrid nanocatalyst exhibits a synergic effect due to the presence of the Cu-NMOF and Cedoped-Mg-Al-LDH components, namely, via the formation of charge-transfer channels and an interfacial modification when Cu-NMOF and Ce doped ions are integrated into Mg-AlLDH. Catalytic reduction (discoloration of dyes, Figure 6) can proceed via the BH4− adsorption of on the surface of nanomaterial with the generation of very reactive nascent hydrogen.43 This then reduces molecules of dye adsorbed on a surface.44 Alternatively, the nascent hydrogen can reduce H2O molecules to give hydrogen that eventually serves as a reducing agent for an organic dye.43 Electrostatic interaction of CuNMOF by Ce-doped-Mg-Al-LDH can promote the transfer of electrons, thus leading to a synergic effect and enhanced catalytic performance of the hybrid nanomaterial. At last, molecules of dye (and the corresponding products of degradation) desorb from the support and the next catalytic cycle can take place.

Figure 6. Schematic representation of the simplified mechanism for the reductive discoloration of organic dyes catalyzed by Cu-NMOFs/ Ce-doped-Mg-Al-LDH.

4. CONCLUSIONS In short, by implementing an unusual strategy that combines the in situ self-assembly and hydrothermal techniques, a novel hybrid Cu-NMOF/Ce-doped-Mg-Al-LDH nanomaterial has been generated. In this material, the nanoscale Cu-NMOF [Cu2(μ-OH)(μ4-btc)(phen)2]n·5nH2O was immobilized on the Ce-doped-Mg-Al-LDH matrix. The obtained hybrid nanocatalyst has demonstrated an excellent catalytic performance toward the reductive discoloration of different organic dyes as well as the 4-nitrophenol degradation. Excellent TOFs have been achieved, namely, 7.1 × 103 and 1.4 × 105 h−1 in the reductive degradation of 4-NP and discoloration of Congo red, respectively. Because of the directing and dispersive behavior of the Ce-doped-Mg-Al-LDH support, Cu-NMOF can be generated in situ via a size-controlled protocol at the Cedoped-Mg-Al-LDH surface. By featuring a high surface area, a tunable morphology, a synergic effect, and a nanosize active phase, hybrid Cu-NMOF/Ce-doped-Mg-Al-LDH material displays excellent catalytic activity and recyclability. Moreover, by being assembled from low-cost Cu-NMOF and Ce-dopedMg-Al-LDH components, Cu-NMOF/Ce-doped-Mg-Al-LDH nanocatalyst outstands many other catalysts that were previously applied toward discoloration of commercial dyes; these usually comprise an expensive nobble metal component. The in situ generation strategy used in the assembly of the functional hybrid nanostructure in this study should also pave the new way toward design of nanomaterials with enhanced functional properties and promising applications in the fields of green and environmental chemistry, and sustainable catalysis. Further research on exploring such a type of nanocatalysts for the degradation of other types of pollutants in both aqueous and organic media will be explored.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01826. Additional catalyst characterization and catalytic activity data (Figures S1−S10, Tables S1 and S2) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Fax: +86 931-891-2582. Phone: +86 931-891-2552. 13276

DOI: 10.1021/acs.inorgchem.8b01826 Inorg. Chem. 2018, 57, 13270−13278

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Inorganic Chemistry ORCID

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Alexander M. Kirillov: 0000-0002-2052-5280 Weisheng Liu: 0000-0001-5448-6315 Yu Tang: 0000-0003-3933-043X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the National Natural Science Foundation of China (21471071, 21431002) and the Fundamental Research Funds for the Central Universities (lzujbky-2018-Ot01, lzujbky-2017-Ot05). A.M.K. acknowledges the FCT and Portugal 2020 (UID/QUI/00100/2013, LISBOA-01-0145-FEDER-029697) and the RUDN University (the publication was prepared with the support of the RUDN University Program 5-100).

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