A magnetic composite material derived from FeOOH decorated Cu-MOF and its catalytic properties

Inorganic Chemistry Communications 102 (2019) 162–170

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A magnetic composite material derived from FeOOH decorated Cu-MOF and its catalytic properties

T

Tursunjan Aydana, , Chao Yanga,d, , Yang Xub, Tongtong Yuana, Min Zhanga, Hui Lic, Xueming Liuc, Xintai Suc, Jide Wanga ⁎

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a

Ministry Key Laboratory of Oil and Gas Fine Chemicals, College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, China School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an 710062, China c Engineering and Technology Research Center for Environmental Nanomaterials, School of Environment and Energy, South China University of Technology, Guangzhou 510006, China d Xinjiang De'an Environmental Protection Technologies Inc, Urumqi 830046, China b

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Keywords: Metal organic framework Magnetic copper hybrid Catalytic reduction 4-NP

A Hybrid magnetic composite material Fe3O4@CuxOy/C (where, x = 1–2; y = 0–1) was prepared via pyrolysis of FeOOH decorated Cu3(BTC)2 (where, BTC = 1, 3, 5-benzenetricarboxylic acid) as a precursor. The obtained products was successfully applied to reduction reaction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) as a catalyst. Compared with other reported materials, Fe3O4@CuxOy/C exhibits high catalytic activity with excellent recyclability.

1. Introduction It is well known that the 4-nitrophenol (4-NP) is a common organic pollutant in industrial and agricultural waste waters. The United States environmental protection agency have listed 4-NP as one of the most hazardous and toxic pollutants because it is harmful to liver, kidney,

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eyes, and central nervous system [1–3]. However, 4-NP is very stable and difficult to decompose by natural microbial degradation [4–6]. It is not easy task to remove 4-NP from environmental and industrial waste waters. The catalytic degradation of 4-NP to 4-aminophenol (4-AP) is a more effective approach to remove 4-NP compared with electrochemical [7] and photo oxidation [8] methods. As a reduction product,

Corresponding author. Correspondence to: C. Yang, Xinjiang De'an Environmental Protection Technologies Inc, Urumqi 830046, China. E-mail addresses: [email protected] (T. Aydan), [email protected] (C. Yang).

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https://doi.org/10.1016/j.inoche.2019.02.014 Received 2 December 2018; Received in revised form 4 February 2019; Accepted 6 February 2019 Available online 07 February 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.

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Scheme 1. Presentation of synthetic procedure of Fe3O4@CuxOy/C.

Because of large surface area, permanent porosity and open metal sites, MOFs have been explored wide range applications in many fields such as catalysis, gas storage, drug delivery and separation [15–18]. In MOFs, the highly ordered metal ions are isolated by organic ligands regularly, which would play an important role in preventing metal aggregation during thermolysis. Metallic carbon composites have been easily obtained by direct pyrolysis of MOFs under an inert atmosphere while maintaining the original MOF skeleton [19,20]. Although noble metal nanomaterials exhibit excellent catalytic activities for the reduction of 4-NP [21–24], they are scarce and expensive, restricting their large scale applications. In contrast, copper nanoparticles and their composites are alternatives due to their less expensive, large abundance and high catalytic properties [25–27]. If a heterogeneous catalyst is functionalized with magnetic component, it would be facilitating the recovery and separation of catalyst [28–30]. However, Cu-MOF-derived magnetic CuxOy/C composite has been rarely employed as catalyst for the catalytic reduction of 4-NP so far. Here, we report the preparation of magnetic copper based composite Fe3O4@CuxOy/C by the direct pyrolysis of FeOOH-decorated CuMOF as the precursor. In contrast to conventional inner Fe3O4 core@ outer Cu shell, the as-prepared magnetic composite puts the Fe3O4 component outside. This construction would facilitate scalable production of magnetic catalyst using Cu-MOFs as template precursors, instead of Fe-based MOFs. During high-temperature pyrolysis under inert atmosphere, both Cu3(BTC)2 and FeOOH can be reduced into CuxOy/C(x = 1–2, y = 0–1) and Fe3O4, respectively. Subsequently, the as-prepared Fe3O4@CuxOy/C composite demonstrate good catalytic activity and recyclability during the reduction of 4-NP in the presence of NaBH4.

Fig. 1. XRD patterns of (a) Fe3O4, (b) CuxOy/C and (c) Fe3O4@CuxOy/C calcination products.

4-AP is important intermediate for manufacturing many analgesic and antipyretic drugs, hair dyeing agents, corrosion inhibitors in paints and fuels [9]. Therefore, the heterogeneous catalytic reduction of 4-NP to 4AP, not only important from the environmental protection point of view, but also beneficial to industrial practical applications. Transition metal nanoparticles (TMNs) for the catalytic reduction of 4-NP have been widely explored since this method was first used to treat 4-NP with Ag nanoparticles by Pradhan et al. [10]. However, TMNs are apt to coalesce and aggregate owing to their high surface energy. In order to prevent the aggregation, metal nanoparticles are usually immobilized on various solid supports such as zeolites, mesoporous aluminosilicates and other porous materials [11–14]. Metal organic frameworks (MOFs) are emerging class of hybrid functional materials built up with metal clusters and organic ligands.

2. Experimental 2.1. Chemicals and reagents All reagents were analytical reagent grade and used without further purification. Copper (II) nitrate tree hydrate (Cu(NO3)2·3H2O), ferric chloride hexahydrate (FeCl3·6H2O) and anhydrate ethanol (C2H5OH) were purchased from Sino pharm Chemical Reagent Co., Ltd. (Beijing, China). 4-Nitrophenol (4-NP) and Sodium borohydride (NaBH4) were purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). 1,3,5-

Fig. 2. SEM images of precursor FeOOH@Cu3 (BTC) 163

2

composite.

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Fig. 3. SEM images of (a,b) Fe3O4, (c,d) CuxOy/C and (e,f) Fe3O4@CuxOy/C calcination products.

Benzenetricarboxylic acid (BTC) was purchased from Alfa Aesar (USA). Ammonium bicarbonate (NH4HCO3) was purchased from Tianjin Zhiyuan chemical reagent Co., Ltd. (Tianjin, China). Purified water was purchased from Hangzhou wahaha group Co.Ltd. (Hangzhou, China).

temperature. Finally, the crude product HKUST-1 (Hong Kong University of Science and Technology) was filtered and washed with ethanol three times dried at 80 °C in an oven for 12 h to obtain octahedral Cu3(BTC)2. 2.2.2. Preparation of FeOOH materials In a 250 ml conical flask, 1.3515 g (5 mmol) of FeCl3·6H2O was dissolved in 200 ml of anhydrous ethanol under magnetic stirring. 1.1850 g (15 mmol) of NH4HCO3 was added to the above solution and continuously stirred for 12 h at ambient temperature. After the reaction, the product was filtered and washed with anhydride ethanol three times, and dried at 80 °C for 12 h.

2.2. Synthesis 2.2.1. Synthesis of Cu-MOF Cu-MOF was synthesized according to previously reported hydrothermal method [31] with some modification. Briefly, 2.188 g (9 mmol) of Cu(NO3)2·3H2O was dissolved in 20 ml of distilled water under magnetic stir forming homogenous solution A. The 1.05 g (3 mmol) of BTC was dissolved in 20 ml of anhydrous ethanol under ultra-sonication to form homogenous solution B. Then the solution B was added to the solution A under constant magnetic stirring for 10 min. The mixed solution was then transferred into an autoclave and heated to 110 °C. After reaction for 12 h, the autoclave was naturally cooled to room

2.2.3. Preparation of FeOOH@Cu3(BTC)2 composite materials 0.2703 g of FeCl3·6H2O (1 mmol) and 0.5000 g of Cu-MOF (prepared in Section 2.1.1) were added to 40 ml of anhydrous ethanol with ultrasonication for 10 min, then 0.2370 g of NH4HCO3 (3 mmol) was 164

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Fig. 4. (a) FT-IR spectra of each precursor FeOOH, FeOOH@Cu-MOF and Cu-MOF; (b) and their pyrolysis products Fe3O4, Fe3O4@CuxOy/C and CuxOy/C.

Fig. 5. Elemental mapping images of Fe3O4@CuxOy/C.

added and magnetically stirred for 12 h at ambient temperature. The asprepared composite material was filtered and washed several times with ethanol. The product was then dried at 80 °C for 12 h to obtain FeOOH@Cu3(BTC)2 precursor.

directly pyrolized under Ar atmosphere at 450 °C for 2 h with a heating rate of 5 °C·min−1. The resulting products were donated as CuxOy/C, Fe3O4, and Fe3O4@CuxOy/C, respectively. 2.3. Characterization

2.2.4. Preparation of Fe3O4@CuxOy/C As synthesized Cu-MOF, FeOOH and FeOOH@Cu3(BTC)2 were

The size and morphology of synthesized materials were explored by 165

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Fig. 6. (a) XPS survey spectra of Fe3O4@CuxOy/C; High-resolution XPS spectra of Fe3O4@CuxOy/C: (b) Cu 2p, (c) Fe 2p, (d) O 1 s, (e) C1s.

using Hitachi S-4800 scanning electron microscope with an energy dispersive X-ray spectrometer (FE-SEM, Tokyo, Japan). The X-ray powder diffraction patterns (PXRD) were recorded on a Bruker D/MAX3B X-ray diffractometer with Cu-Ka (λ = 0.154056) radiation scanning range from (2θ) 10° to 80° at the scan rate of 5°/min (Rigaku Corporation, Japan). Fourier transform infrared (FTIR) spectra of the products were obtained using a BRUKER EQUINOX55IR spectrometer. Elemental mappings of the sample were obtained by field emission scanning electron microscopy (FESEM; Carl Zeiss MERLIN Compact, Germany) equipped with an attached Oxford EDS detected. X-ray photo-electron spectroscopy (XPS) analyses were taken on Thermo

Fisher Scientific XPS ESCALAB 250Xi instrument with Al Ka (hν = 1486.8 eV) radiation. UV–Vis absorption spectrum was recorded by a Shimadzu UV-2450 spectrophotometer. 2.4. Catalytic reduction of 4-NP Briefly, 58 ml of aqueous solution of 4-NP (0.12 mM) was put into the 100 ml glass beaker and thermo stated at 25 °C. The solid NaBH4 (0.1053 g, 2.78 mmol) was added into the above solution and dissolved completely. Aliquot of 2.9 ml of mixture solution was take out using pipette and injected into a 4 ml quartz cuvette immediately, then 0.1 ml 166

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solvo-thermal method. Subsequently, the deposition of FeOOH on Cu3(BTC)2 was achieved by a non-aqueous precipitation approach. Finally, the precursor FeOOH@Cu3(BTC)2 was pyrolyzed into carbonized magnetic copper based composite (Fe3O4@CuxOy/C) under inert atmosphere. XRD patterns of Fe3O4, CuxOy/C and Fe3O4@CuxOy/C are shown in Fig. 1. The peaks at 2θ of 17°, 30°, 36°, 43°, 51°, 56°, and 63° corresponding to (111), (220), (311), (400), (422), (511) and (440) planes of Fe3O4 phase (Fig. 1a), can be matched with the characteristic peaks of Fe3O4 (JCPDS No. 75–0033). Three sharp diffraction peaks at 2θ = 43°, 50.5° and 74° can be assigned to (111), (200) and (220) crystal planes of solid copper phase (JCPDS No. 85–1326) along with a very weak one at ~37° can be estimated that small amount of CuO or Cu2O in CuxOy/C (Fig. 1b), respectively. Four characteristic peaks of Fe3O4 (17°, 36°, 43°, 63°), two characteristic peaks of Cu (43° and 74°), five peaks of Cu2O (30°, 37°, 43°, 63°, 74°) [9,11] and two main peaks 36°, 38° of CuO [26,33] appeared in the pattern of Fe3O4@CuxOy/C (Fig. 1c), confirming the co-existence of Fe3O4, Cu and Cu2O (or CuO) phases. The results suggesting that the Cu2+ ion in precursor FeOOH@Cu3(BTC)2 was not fully reduced to elemental copper Cu(0) during the calcinations. Fig. 2 shows typical SEM images of FeOOH@Cu3(BTC)2 at different magnifications. The morphology of the FeOOH@Cu3(BTC)2 precursor is octahedral shape with non-uniform particle size about 5–20 μm (Fig. 2a). The high magnification image showed that the octahedral particles became larger with relatively uniform size about 8 μm (Fig. 2b). Fig. 3a–f depict SEM images of Fe3O4, CuxOy/C and Fe3O4@CuxOy/ C. As shown in Fig. 3a and b, sample Fe3O4 are mainly composed of small sized particles and larger-sized quasi-octahedrons (average diameter of 300 nm and 0.87 μm). Fig. 3c and d reveal that sample CuxOy/ C approximately inherited the original morphology of HKUST-1. The rough surface embedded with many tiny particles. Structural collapses of MOF skeleton were also observed. Fig. 3e shows a SEM image of Fe3O4@CuxOy/C, suggesting that they did not fully retain the original morphology of the HKUST-1. Their surface are built from tiny nanorods with an average diameter of 44 nm (Fig. 3f). FTIR measurements of the precursors and calcination products are

Fig. 7. UV–Vis absorption spectra of 4-NP solution presence and non-presence of NaBH4.

of catalyst (Fe3O4@CuxOy/C, 0.2 mg·ml−1) dispersion was added. The absorbance of reaction mixture was monitored using UV–vis spectrophotometer at each 1 min time intervals over scanning range of 250–550 nm. Catalytic performance of CuxOy/C and Fe3O4 were also tested for comparison with same procedure. As for the recycle experiment, 2 mg of solid catalyst was added to 2.9 ml of 4-NP and NaBH4 mixed solution. After the reaction was finished, the catalyst Fe3O4@CuxOy/C was separated by external magnet, rinsed with water for a while and redispersed into a mixture of new reactants to produce next reaction cycle. 3. Results and discussion 3.1. Preparation and characterization of materials The synthetic process of the Fe3O4@CuxOy/C composite is illustrated in Scheme 1. First, Cu-MOF was prepared through a simple

Fig. 8. (a) Time dependent UV–Vis absorption spectra of 4-NP reduced by NaBH4 without and with catalysts: (b) Fe3O4, (c) CuxOy/C, (d) Fe3O4@CuxOy/C. 167

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Fig. 9. (a) Plots of Ct/C0 and (b) ln Ct/C0 versus reaction time t in different catalysts: Fe3O4, CuxOy/C and Fe3O4@CuxOy/C. Table 1 Comparison catalytic activity of Fe3O4@CuxOy/C and reported metal hybrid catalysts. Catalyst

4-NP (mM)

NaBH4 (mM)

Mass of catalyst (mg)

Rate constant (min−1)

Activity factora (min−1·mg−1)

Reference

Fe3O4@CuxOy/C Cu/MnO2 Porous Cu Zn0.3Co2.7@NC Au/Ag/ZIF-8 Au/Ag/MIL-101 Cu2O@CMK-8 Cu/AC CuFe2O4 Ni/C-800

0.12 0.1 0.1 0.1 0.103 N.Fb 0.074 0.092 0.05 0.092

47.9 20.6 40 86.6 163.1 N·F 9.1 3.84 25 92

0.02 0.02 0.05 0.06 3.46 N·F 0.2 1 2 1

1.03 0.685 0.558 0.683 0.298 0.2753 1.32 0.782 37.28 1.045

51.5 34.25 11.16 11.38 0.086

This work 3 13 19 21 23 26 27 29 30

a b

6.6 0.782 18.64 1.045

Activity Factor = rate constant/mass of catalyst. Not Found.

shown in Fig. 4. As shown in Fig. 4a, the peaks around 3306 cm−1 are attributed to stretching vibrations of –OH in FeOOH. The peaks around 1648 cm−1 can be assigned to characteristic peaks of Cu-MOF. The peaks at 3360 cm−1 are stretching vibrations of OeH in Cu-MOF@ FeOOH; the peak positions and intensities of 1043 cm−1 and 1621 cm−1 are almost the same with those of FeOOH (1042 cm−1 and 1614 cm−1); the peak positions and profiles of 3360 cm−1, 1650 cm−1, 1448 cm−1, 1371 cm−1, 729 cm−1 and 486 cm−1 are much the same with those of Cu-MOF. As shown in Fig. 4b, the peaks at 570 cm−1 are attributed to bending vibrations of FeeO in Fe3O4. As for the FTIR spectrum of Fe3O4@CuxOy/C, the peaks at 557 cm−1 are almost identical with bending vibrations of FeeO in Fe3O4. The peak positions and profiles of 1575 cm−1, 1022 cm−1 and 817 cm−1 for Fe3O4@CuxOy/C are much same as those of CuxOy/C. Elemental mapping were conducted to examine the distribution of elements within the material Fe3O4@CuxOy/C. The corresponding mapping images of Cu, Fe, O, C elements and some other impurities (Cl, S) of the sample reveal that the obtained catalytic material is not so pure as expected. The Cu, O and C elements distributed homogenously on the hybrid material. The Fe elements are might be deposited on the surface of the CuxOy/C as Fe3O4 (Fig. 5). This phenomena suggesting that the Fe3O4 might be constructed outside the magnetic hybrid material. Surface elemental composition and oxidation status of copper in Fe3O4@CuxOy/C was further investigated by X-ray photoelectron spectroscopy (XPS). A typical vide scan spectrum is shown in Fig. 6(a), the binding energy (BE) peaks at 284.24, 529.61, 712.16 and 933.14 eV originated from C, O, Fe and Cu elements, respectively, further indicating their coexistence in the material. The high resolution spectra of Cu, Fe, O and C are presented in Fig. 6(b–e), respectively. The peaks at BE of 933.14 and 953 eV could be attributed to the Cu 2p 3/2 and Cu 2p 1/2, respectively, which proved further the existence of Cu(0) or Cu2O in the as prepared composite [25,33]. The presence obvious satellite

peaks at 962 eV and a series overlapping peaks at 941, 943 eV, on the other hand, confirmed the existence of CuO species in the sample (Fig. 6b) [26]. BE peaks of Fe 2p3/2 and Fe 2p1/2 appear at 712.16 and 721 eV, respectively, indicating the presence of iron in composite (Fig. 6c). High resolution spectrum of O1s at BE = 529.61 eV, BE = 530.7 eV and BE = 533.3 eV could be arising from CeOeC, eC] O and eCeO, respectively (Fig. 6d). The main peak at BE of 284.24 and 285.6 eV could be fitted to CeC (sp2) and CeO groups of composite material, respectively (Fig. 6e). XPS results indicating that the existence of Cu in obtained hybrid material as Cu(0), Cu+ and Cu2+, that is to say, the magnetic hybrid material might be consisting of Fe3O4 and Cu/ CuO/Cu2O. 3.2. Evaluation of catalytic performance of materials The catalytic activity of hybrid Fe3O4@CuxOy/C was tested for the reduction reaction of 4-NP to 4-AP in the excess amounts of NaBH4. The catalytic performance of the as-synthesized Fe3O4 and CuxOy/C were also tested for the comparison. The process of all reduction reactions were monitored using UV–vis spectrophotometer. The absorption peaks of aqueous 4-NP solution in the absence and presence of NaBH4 are shown in Fig. 7. The distinct absorption peak of 4-NP is located at 314 nm, which was shifted to 400 nm in the presence of NaBH4 due to the formation of 4-nitrophenolate anions in alkaline condition. At the same time, the solution color changed from light yellow to grain yellow. The peak intensity at 400 nm nearly unchanged within 1 h (Fig. 8a), which indicated that the reduction reaction did not occurred without catalyst, even present excess amounts of NaBH4 (molar ratio of 4-NP: NaBH4 is 1: 400). The reduction of 4-NP to 4-AP by NaBH4 over different calcined products (Fe3O4, CuxOy/C, Fe3O4@CuxOy/C) were performed using time-dependent UV–vis absorption spectra. As shown in Fig. 8b, the peak height slightly decreased and no new peak was found at 300 nm while Fe3O4 using as catalyst. It shows that the catalytic 168

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The catalytic performance of hybrid Fe3O4@CuxOy/C was compared with reported materials in similar reaction conditions. As shown in Table 1, activity factor (defined as ratio of rate constant over the total mass of catalyst) of Fe3O4@CuxOy/C was calculated to 51.5 min−1·mg−1, which was higher than that of other copper based and noble metal catalysts. Porous CuxOy/C structure and synergetic effect of Fe3O4 and CuxOy/C should be responsible for the significant catalytic capability of Fe3O4@CuxOy/C. The stability and reusability of catalyst are also needed for each cycle in industrial practical applications. For this purpose, the recyclability of catalyst was tested in seven catalytic cycles (Fig. 10b). The conversion of 4-NP to 4-AP over Fe3O4@CuxOy/C still maintained 97.6% even after seven cycles. The results shows that the composite material Fe3O4@ CuxOy/C have not only superior catalytic activity but also an excellent recyclability and convenient reusability. 4. Conclusions In conclusion, magnetic Fe3O4@CuxOy/C powder was prepared through annealing the precursor FeOOH-decorated Cu-MOF synthesized by non-aqueous precipitation method. Such magnetic Fe3O4@CuxOy/C composite was used as a catalyst for the catalytic reduction of 4-NP to 4-AP. The catalytic performance of Fe3O4@CuxOy/C was improved by introducing Fe3O4 into the CuxOy/C composite. It was found that the apparent rate constant of 1.03 min−1 for Fe3O4@CuxOy/ C was higher than that of 0.74 min−1 for CuxOy/C. Another merit of this material is low cost, easy to preparation and use in many cycles without lost efficiency. Acknowledgement

Fig. 10. (a) Photograph of model reaction and (b) Reusability Fe3O4@CuxOy/C for seven cycles reduction of 4-NP with conversion rate.

This work was kindly supported by the Natural Science Foundation of China (No. 51564045), the Natural Science Foundation of Xinjiang University (No. BS150232, 209-61367), International Cooperation Project of Xinjiang Science and Technology Bureau (No. 20166020, 2017E0116, and 2017E01005).

of

reduction of 4-NP did not happen in the case of Fe3O4. In contrast, after respective introducing CuxOy/C and Fe3O4@CuxOy/C into the reaction solution, the absorption peak at 400 nm decreased rapidly. Meanwhile, a new peak at 300 nm appeared (Fig. 8c and d), which can be attributed to the absorption peak of colorless 4-AP [25,30,32–34]. The completion of reaction can be estimated by discoloration of mixture solution (Fig. 10a) and complete disappearance of the peak at 400 nm. Herein, 4-NP is completely reduced into 4-AP within 4 min for Fe3O4@CuxOy/ C, which is shorter than that of CuxOy/C (10 min). Therefore, the catalytic activity of three catalysts follows the order: Fe3O4@CuxOy/ C > CuxOy/C > Fe3O4. The catalytic model reaction could be regarded as pseudo first order kinetics. The pseudo first order rate constant can be determined according to the following equation:

ln (Ct / C0 ) = ln (At / A0 ) =

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kt

where Ct is the concentration of nitrophenolate ions at a reaction time t; C0 is the initial concentration of nitrophenolate ions; At is the absorbance at any time t; A0 is the absorbance at time t = 0 and k is the apparent rate constant, which is estimated from the slop of ln (Ct/C0) vs t. The relationship between ln (Ct/C0) and time for the reduction of 4NP in different catalysts are shown Fig. 9. k of Fe3O4, CuxOy/C and Fe3O4@CuxOy/C was 0.019, 0.74, and 1.03 min−1, respectively. Obviously, Fe3O4@CuxOy/C exhibited better catalytic activity than Fe3O4 and CuxOy/C. The greater catalytic activity of CuxOy/C can be ascribed to the simultaneous presence of Fe3O4, Cu, CuxO and C in its microstructures. Porous carbon derived from MOFs as supporting material may prevent agglomeration of metal catalyst and accelerating diffusion of reactants. Meanwhile, Fe3O4 derived from FeOOH may provide magnetic property for CuxOy/C improving the separation efficiency. 169

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