7Fe2O4–graphene catalyst and its application in selective reduction of nitroarenes

Catalysis Communications 59 (2015) 161–165

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Short Communication

Preparation of magnetically separable Cu6/7Co1/7Fe2O4–graphene catalyst and its application in selective reduction of nitroarenes Haiyan Zhang, Ying Zhao, Weihua Liu, Shutao Gao, Ningzhao Shang, Chun Wang ⁎, Zhi Wang College of Sciences, Agricultural University of Hebei, Baoding 071001, China

a r t i c l e

i n f o

Article history: Received 6 August 2014 Received in revised form 4 October 2014 Accepted 17 October 2014 Available online 28 October 2014

a b s t r a c t A magnetically separable and highly active Co–Cu mixed spinel catalyst, Cu6/7Co1/7Fe2O4–graphene (Cu6/7Co1/7 Fe2O4–G), was fabricated by a hydrothermal method. The results demonstrated that the Cu6/7Co1/7Fe2O4–G possessed excellent catalytic activity for the reduction of a broad range of nitroarenes in the presence of NaBH4. The composite catalyst was efficient, stable, low-cost and could be easily recovered due to its magnetic separability. © 2014 Elsevier B.V. All rights reserved.

Keywords: Nitroarenes Reduction Cu6/7Co1/7Fe2O4–graphene Sodium borohydride

1. Introduction Aromatic amines are important starting materials and intermediates for the manufacture of a great variety of chemicals such as dyes, pesticides, herbicides, and agrochemicals [1–3]. At present, they are generally synthesized from the corresponding aromatic nitro compounds by a catalytic reduction or a non-catalytic process. The conventional noncatalytic method mainly uses metal/acid [2], such as Fe/HCl, that requires a stoichiometric excess of reagents, which presents difficulties in product isolation and eventually produces a large amount of secondary waste. Therefore, it is not considered an environmentally benign process. In contrast, catalytic reduction method has the merits of better product quality and less pollution, so an increasing number of researchers have shifted their attention to catalytic reduction in recent years. The catalytic reduction systems include catalytic hydrogenation [4], iron-based compound catalysts [5], nano-metal catalysts [6–8], noble metal catalysts [9,10] and so on. However, most of these methods have one or more shortcomings that need to be overcome. For example, catalytic hydrogenation needs high temperature and high H2 pressure, moreover the selectivity is a major issue. Iron-based catalysts are mainly explored for the reduction of aromatic nitro compounds in the presence of hydrazine hydrate. Hydrazine hydrate reduction delivers selectivity but the scale-up production of amines needs to be addressed due to hydrazine hydrate's explosive nature and toxicity. Most of the nano-metal catalysts contain Pd, Ag or other precious metals, which is expensive. The precious noble metal catalysts are expensive and usually sensitive to both air and moisture. All these problems led researchers to find ⁎ Corresponding author. Tel.: +86 312 7528291. E-mail address: [email protected] (C. Wang).

http://dx.doi.org/10.1016/j.catcom.2014.10.016 1566-7367/© 2014 Elsevier B.V. All rights reserved.

more economical, easily available, and environmentally benign alternatives for the catalytic reduction of nitroarenes. Carbon-based nanomaterials, such as carbon nanotubes, graphene and mesoporous carbon, are important supports for the metal and metal oxide nanoparticles in heterogeneous catalysis [11]. In particular, graphene, a novel one-atom-thick two-dimensional graphitic carbon system, has attracted particular research interest due to its high surface area, excellent electronic, thermal, optical, and mechanical properties [12,13]. To date, various kinds of catalysts, such as TiO2 [14], SnO2 [15], ZnO [16], Pt [17,18], Pt–Ru [19], Pt–Pd [20], Pd [21] and so on, have been supported on graphene-based templates for catalyzing different chemical transformations, energy conversion and photocatalytic reactions. Graphene-based nanomaterials have also been explored as an efficient catalyst for the catalytic reduction of nitroarenes. For instance, superparamagnetic graphene–Fe3O4 nanocomposite (G–Fe3O4) was fabricated and used as an efficient catalyst for the reduction of nitroarenes with hydrazine hydrate as a reductant [22]. Spinel ferrites MFe2O4 (M_Cu, Co, Mn, etc.), has a cubic closepacked arrangement of the oxygen ions with M2 + and Fe3 + ions at two different crystallographic sites [23]. In recent years, MFe2O4 has been widely applied in sensors, electronics and catalysis because it exhibits several advantages, such as relatively cheap, environmentally compatible, moisture insensitive, high dispersive, and easily separative [24]. Li et al. [25] explored mesoporous silica KIT-6 supported superparamagnetic CuFe2O4 nanoparticles for catalytic asymmetric hydrosilylation of ketones in air. Parella et al. [26] developed the catalytic application of CuFe2O4 nanoparticles for the Friedel–Crafts acylation. Binary spinel ferrites MFe2O4 have also been explored as efficient catalyst for the catalytic reduction of nitroarenes. Feng et al. [27] investigated the catalytic activity of CuFe2O4 nanoparticles for the reduction

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Intensity (a.u.)

Transmittance / (%T)

GO

G-CuFe 2O 4

CuFe 2O 4-G Cu 6/7 Co 1/7 Fe 2O 4-G

G-Cu 6/7 Co 1/7 Fe2O 4 3600

3200

2800

2400

2000

1600

1200

800

400

10

20

30

40

50

60

70

80

Wavenumbers / (cm-1)

2 Theta / (degree)

Fig. 1. FT-IR spectra of the GO, CuFe2O4–G(0.25) and Cu6/7Co1/7Fe2O4–G(0.25).

Fig. 3. XRD pattern of as-prepared CuFe2O4–G(0.25) and Cu6/7Co1/7Fe2O4–G(0.25).

of 4-nitrophenol to 4-aminophenol with excess amount of NaBH4. CuFe2O4 supported on graphene has been prepared and used as an efficient catalyst for the reduction of nitroarenes by our group [28]. The results demonstrate that the combination of CuFe2O4 with graphene can lead to a dramatic enhancement of the catalytic activity of CuFe2O4, which can be attributed to the remarkable synergistic effect between the CuFe2O4 and the graphene sheets. Recently, Cu1− xCoxFe2O4 system was found to be highly active for phenol alkylation with different alkylating agents [29]. Cu–Co combination in terms of synergism and 3d energy band overlap was suggested to be responsible for this efficient process [30]. The application of Cu1− xCoxFe2O4 system in the catalytic reduction of nitroarenes has not been reported yet. Considering that the introduction of Co into CuFe2O4 system might be an effective catalyst for the reduction of nitroarenes, in this paper, a series of magnetically separable Cu1− xCoxFe2O4–G composites were prepared through a hydrothermal method and the effect of Co on the catalytic activity of Cu1− xCoxFe2O4–G composites was investigated. It is believed that this method is an important addition to known procedures for reduction of aromatic nitro compounds either on a lab as well as on a larger scale.

and ethanol were all obtained from Chengxin Chemical Reagents Company (Baoding, China). Nitroaromatics were purchased from Aladdin Reagent Limited Company. The water used throughout the work was double-distilled on a SZ-93 automatic double-distiller from Shanghai Yarong Biochemistry Instrumental Factory (Shanghai, China). The infrared (IR) spectra (cm−1) were measured with a WQF-510 spectrometer. The size and morphology of the magnetic nanoparticles were observed by scanning electron microscopy (SEM) using a Hitachi S4800 field emission electron microscope operated at 30 kV and transmission electron microscopy (TEM) using a JEOL model JEM-2011(HR) at 200 kV. The X-ray diffraction (XRD) patterns of the samples were recorded with a Rigaku D/max 2500 X-ray diffractometer using Cu Kα radiation (40 kV, 150 mA). The stoichiometry of Cu and Co was determined by means of inductively coupled plasma atomic emission spectroscopy (ICP-AES) on Thermo Elemental IRIS Intrepid II. The energy dispersive X-ray spectroscopy (EDS) spectra were taken on TEAM energy spectrometer (EDAX USA). The Brunauer–Emmett–Teller (BET) surface areas were determined from the N2 adsorption at 300 K using V-Sorb 2800P. 2.2. Synthesis of Cu6/7Co1/7Fe2O4–graphene heteroarchitecture

2. Experimental Graphite oxide (GO) was prepared according to the procedure reported by us [31]. Cu1− xCoxFe2O4–G heteroarchitectures with graphene content (25 wt.%) were synthesized by a modified hydrothermal method reported in the literature [24]. A typical experiment procedure for the synthesis of Cu6/7Co1/7Fe2O4– graphene heteroarchitecture with 25 wt.% graphene content is as follows: 40 mg of GO was dispersed into 30 mL of ethylene glycol with sonication

2.1. Materials and apparatus Ferric trichloride hexahydrate (FeCl3·6H2O, 99%), copper sulfate pentahydrate (CuSO4·5H2O, 99%), Cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O, 98%), sodium borohydride (NaBH4, 97%), NH3·H2O (30%), polyvinyl pyrrolidone, glycol, sodium hydroxide (NaOH, 98%),

a

Fig. 2. SEM (a) and TEM (b) images of the Cu6/7Co1/7Fe2O4–G (0.25) sample.

b

H. Zhang et al. / Catalysis Communications 59 (2015) 161–165

2.3. Reduction reactions of nitroarenes

Table 1 Screening and control experiments for hydrogenation of p-nitrophenol.

Entry

Catalyst

Catalyst (mg)

Temperture (K)

Time (min)

Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

– G Fe3O4 Fe3O4–G(0.25) NiFe2O4–G(0.25) MnFe2O4–G(0.25) CoFe2O4–G(0.25) CuFe2O4–G(0.25) CuFe2O4–G(0.25) Cu6/7Co1/7Fe2O4–G(0.25) Cu6/7Co1/7Fe2O4–G(0.25) Cu6/7Co1/7Fe2O4–G(0.25) Cu6/7Co1/7Fe2O4–G(0.25) Cu6/7Co1/7Fe2O4–G(0.25) Cu3/4Co1/4Fe2O4–G(0.25) Cu1/2Co1/2Fe2O4–G(0.25) Cu6/7Co1/7Fe2O4/NP–G(0.25) Cu6/7Co1/7Fe2O4/N–G(0.25)

20 20 20 20 20 20 20 20 20 20 20 20 20 10 20 20 20 20

343 343 343 343 343 343 343 343 343 343 333 323 313 343 343 343 343 343

500 500 60 60 500 500 60 9 3 3 9 13 60 7 9 14 60 12

– Trace 26 39 Trace Trace 33 99 39 99 99 99 99 99 99 99 88 99

Reaction condition: p-nitrophenol (1 mmol), solvent (10 ml, EtOH: water = 1:1), NaBH4 (5 mmol).

for 12 h. 0.144 g (0.576 mmol) of CuSO4·5H2O, 0.028 g (0.096 mmol) of Co(NO3)2·6H2O and 0.3618 g (1.34 mmol) of FeCl3 were added to 10 mL of ethylene glycol and sonicated for 1 h. The above two solutions were then mixed together and stirred for 30 min. After that, the mixture was adjusted to pH of 8 with 6 mol L−1 NaOH aqueous solution and stirred for 30 min, yielding a stable homogeneous emulsion. The resulting mixture was transferred into a 70 mL Teflon-lined stainless steel autoclave and heated to 180 °C for 24 h under autogenous pressure. After the reaction mixture was cooled down to room temperature, the precipitate was filtered, washed with distilled water and ethanol, and dried in a vacuum oven at 30 °C for 12 h. The product was labeled as Cu6/7Co1/7Fe2O4–G (0.25). CuFe2O4–graphene (CuFe2O4–G) (0.25) was synthesized with the same method without adding Co(NO3)2·6H2O. For comparison, the same method was also used to synthesize Cu6/7Co1/7Fe2O4/NP–graphene (0.25) and Cu6/7Co1/7Fe2O4/N–graphene (0.25) by adjusting to pH of 8 with NH3·H2O (30%), while Cu6/7Co1/7Fe2O4/NP–graphene (0.25) with adding 0.32 g polyvinyl pyrrolidone.

0 min

Absorbance

3 min 5 min 7 min 9 min

300

350

400

163

450

Wavelength / (nm) Fig. 4. Successive UV–vis absorption spectra of the reduction of p-nitroaniline with NaBH4 in the presence of Cu6/7Co1/7Fe2O4–G (0.25) under 323 K.

In a 25 mL round bottom flask, nitroarenes (1.0 mmol, 1 eq.) were dissolved in a mixture of 10 mL H2O–EtOH (1:1, v/v). Then, sodium borohydride (5.0 mmol, 5 eq.) and Cu6/7Co1/7Fe2O4–G (0.25) (20 mg) were added. The mixture was stirred at 323–343 K for an appropriate time depending upon the nature of the substrate. Upon completion of the reaction (monitored by TLC), the mixture was cooled to room temperature and the catalyst was separated by a magnet for recycling tests. The reaction mixture was extracted with ether (3 × 10 mL). The organic phase were combined together and dried over anhydrous MgSO4. The solvent was evaporated under vacuum. The pure products were obtained by silica-gel column chromatography using petroleum ether: ethylacetate (4:1) as the eluent. 3. Results and discussion 3.1. Characterization of Cu6/7Co1/7Fe2O4–graphene The FTIR spectra were recorded to testify the hybrid material. As shown in Fig. 1, the spectrum of GO is in good agreement with previous work [32]. The broad, intense band at 3250 cm− 1 is assigned to the stretching of O–H. The peak at 1616 cm−1 (aromatic C_C) can be ascribed to the skeletal vibrations of unoxidized graphene domains. The C_O bond is associated with the band at 1047 cm− 1. From the FTIR spectrum of GO, it can be clearly seen that the graphene oxide exhibits an obvious characteristic absorption peak at about 1728 cm−1 corresponding to the stretching of the _C_O and _COOH groups. However, it cannot be seen from the FTIR spectrum of CuFe2O4–G (0.25) and Cu6/7Co1/7Fe2O4–G (0.25). It turned out that graphene oxide was reduced to graphene due to the strong reducing capability of ethylene glycol during the preparation process. From the FTIR spectrum of CuFe2O4–G (0.25) and Cu6/7Co1/7Fe2O4–G (0.25) samples, the spectra show a strong absorption corresponding to the stretching vibration of the tetrahedral and octahedral sites around 586 and 400 cm−1, respectively. The observed values illustrate that the frequency bands appearing at 586 and 400 cm−1 are responsible for the formation of metal oxide (CuFe2O4 and Cu6/7Co1/7Fe2O4). The absorption band at 1596 cm−1 on spectrum referred to the vibration of remainder H2O in the sample [4,27]. Fig. 2 shows the SEM and TEM images of Cu6/7Co1/7Fe2O4–G (0.25) sample. It can be seen that Cu6/7Co1/7Fe2O4–G were composed of quasi-sphere particle with particle sizes of about 20 nm. It clearly demonstrated that a crystal structure with spherical shape was formed. X-ray powder diffraction analysis was used to identify the crystal structure of the CuFe2O4–G (0.25) and Cu6/7Co1/7Fe2O4–G (0.25). As shown in Fig. 3, except some Cu impurity peaks at 43.5° and 50.6°, all peaks were indexed to be CuFe2O4 (JCPDS 77–0010), in detail, the peaks at 18.3° 30.0°, 35.5°, 53.5°, 57.1°, 62.6° and 74.1° are attributed to (111), (220), (311), (422), (511), (440) and (533) crystal planes of CuFe2O4 while no typical diffraction peak of reduced graphene oxide was observed. It is speculated that the graphene in the CuFe2O4–G (0.25) and Cu6/7Co1/7Fe2O4–G (0.25) heteroarchitecture was fully exfoliated due to the crystal growth of CuFe2O4 and Cu6/7Co1/7Fe2O4 nanoparticles between the interlayer of graphene sheets, which result in the low diffraction intensity of graphene. Furthermore, no visible peak corresponding to Co can be detected, which is ascribed to the fine dispersion of Co or the insertion into the CuFe2O4 matrix. The formation of copper was identified based on the reported data (JCPDS 85–1326) at 43.5° and 50.6°. The reason of existence of metallic copper may be that some of Cu2+ were reduced to copper metal in the formation process of CuFe2O4 and Cu6/7Co1/7Fe2O4 due to the strong reducing capability of ethylene glycol [27,28]. The result is consistent with that reported in the documents [33]. The energy dispersive X-ray spectroscopy was shown in Fig. S1 (see Supporting information). Obviously, Co, Fe, Cu, C and O in the

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Table 2 Chemoselective reduction of nitro compounds into amines by sodium borohydride catalyzed by Cu6/7Co1/7Fe2O4–G (0.25).

Entry

Carbonyl Compound

Product

Temperature (K)

Time (min)

Yield (%)

TOF (min−1)

1

323

13

99

1.22

2

323

8

99

1.98

3

323

9

99

1.76

4

333

10

96

1.54

5

343

35

95

0.43

6

323

40

99

0.40

7

333

15

91

0.97

8

343

30

96

0.51

9

343

25

95

0.61

10

323

12

99

1.32

11

343

25

91

0.58

Reaction condition: nitro compounds (1 mmol), NaBH4 (5 mmol), solvent (10 ml, EtOH: water = 1:1), catalyst (20 mg).

Cu6/7Co1/7Fe2O4–G (0.25) could be directly visualized from EDS. This also proved the presence of cobalt and carbon. EDS elemental mapping showed the atomic distribution of Co, Fe, Cu, C and O (Fig. S2). To further verify the existence of Co and the ratio of Cu and Co, the composition of the catalyst was determined by ICP-AES. The ratio of Cu to Co is 5.92:1, close to the stoichiometric ratio of Cu and Co is 6:1. The concentration of Co in the catalyst was 2.99 wt.%. The BET adsorption isotherm obtained for Cu6/7Co1/7Fe2O4–G (0.25) was shown in Fig. S3. The experimental multipoint BET surface area of Cu6/7Co1/7Fe2O4–G (0.25) was found to be 119 m2 g−1. 3.2. Catalytic activity Cu6/7Co1/7Fe2O4–graphene In the initial experiment, the p-nitrophenol was chosen as the model reactant in order to examine the efficiency of different catalysts, the effect of reaction temperature and the amounts of catalyst. As shown in Table 1, no product was obtained in the absence of the catalyst Table 3 The reduction of 4-nitrophenol catalyzed by different catalysts. Entry Catalyst 1 2 3 4 5 6

Cu6/7Co1/7Fe2O4–G (0.25) PdCu/G (2 mol% Pd) RHPrNH2@Ag Fe–phenanthroline/C (1 mol% Fe) Co–Mo2C/AC Resin-AuNPs

Temperature Time Yield Ref. (K) (min) (%) 323 323 373 373 353 313

13 90 65 600 180 20

99 98 98 97 100 82

This work [6] [34] [5] [10] [35]

(Table 1, entry 1), indicating that the catalyst was necessary for the reaction. Only low yields of the product were obtained with either graphene, Fe3O4, Fe3O4–graphene (0.25), NiFe2O4–graphene (0.25), MnFe2O4–graphene (0.25) or CoFe2O4–graphene (0.25) as catalyst (Table 1, entries 2–7). 99% yield was obtained when the reaction was performed at 343 K for 9 min with CuFe2O4–G (0.25) as the catalyst (Table 1, entry 8). However, the yield of the product is only 39% at 343 K for 3 min (Table 1, entry 9). The combination of CuFe2O4–G (0.25) with small amount of cobalt results in a dramatic enhancement of the catalytic activity of CuFe 2 O 4–G (0.25). The reaction time required to complete the reaction decreased from 9 min to 3 min with Cu6/7Co1/7Fe2O4–G (0.25) as the catalyst (Table 1, entry 10). The reaction still could be completed within a relatively short time (13 min) when the reaction temperature lowered down to 323 K (Table 1, entries 11, 12). But a longer reaction time (60 min) was required to complete the reaction with the reaction temperature at 313 K (Table 1, entry 13). When the dosage of Cu6/7Co1/7Fe2O4–G (0.25) decreased from 20 mg to 10 mg, the reaction can be completed within 7 min (Table 1, entry 14). Compared with CuFe2O4–G (0.25), the excellent catalytic activity of Cu6/7Co1/7Fe2O4–G (0.25) can be attributed to its unique heteroarchitechture, which provides the remarkable synergistic effect between the Cu6/7Co1/7Fe2O4 and the graphene sheets. The presence of graphene could enhance the adsorption of reactant molecules onto the catalytic sites of the Cu6/7Co1/7Fe2O4–G(0.25) through the π–π stacking and/or electrostatic interaction. For comparison, Pd@C, a conventional noble metal catalyst, was employed as the catalyst for the reduction of p-nitrophenol at 343 K for 4 min, the yield was 99%. However, the Pd@C catalyst needs to use noble metal

H. Zhang et al. / Catalysis Communications 59 (2015) 161–165

Pd and which is not so easy to separate by a magnet as the as-obtained catalyst. The effect of the loading amount of cobalt in CuFe2O4–G (0.25) on the catalytic activity of Cu1 − xCoxFe2O4–G was also investigated. The results indicated that the large amount of Co has a negative effect on the activity of Cu1− xCoxFe2O4–G (Table 1, entries 15, 16). Meanwhile, the catalytic activity of Cu 6/7Co1/7Fe2 O 4/NP–graphene (0.25) and Cu6/7 Co1/7Fe2O4/N–graphene (0.25) were lower than that of Cu6/7 Co1/7Fe2O4–G(0.25) (Table 1, entries 17, 18). Maybe the coordination effect of NH3·H2O and PVP with metal ions (Fe3 +, Cu2 + and Co2 +) was unfavorable for the formation of the crystal structure of CuCoFe2O4. Fig. 4 showed the UV–vis absorption spectra of the reduction of pnitroaniline by NaBH4 at various reaction times in the presence of Cu6/7Co1/7Fe2O4–G (0.25). The observed peak at 385 nm for the pnitroaniline shows a gradual decrease in intensity with time and a new peak appeared indicating the formation of p-phenylene diamine. The results indicated that Cu6/7Co1/7Fe2O4–G (0.25) exhibited a considerably high activity for the reduction of nitroarenes with sodium borohydride as the hydrogen donor. Using these newly developed conditions, we explored the scope and limitations of this method and the results are summarized in Table 2. As shown in Table 2, aromatic nitro compounds containing various electrons donating (Table 2, entries 1–6, 10) or electron-withdrawing groups (Table 2, entries 8, 11) were converted to the corresponding amino aromatics in good yields and all the reactions could proceed smoothly. Moreover, the reduction was also successfully carried out on bulkier molecule such as 1-nitronaphthalene with high yield (Table 2, entry 7). The reusability and recycling of Cu6/7Co1/7Fe2O4–G (0.25) was also investigated. The catalyst was separated from the reaction mixture using an external magnet, washed with ethanol for three times, dried at 60 °C in a vacuum oven for 2 h and reused in another reaction. The catalytic activity of Cu6/7Co1/7Fe2O4–G (0.25) did not show any significant decrease even after five runs (Table S1). In order to evaluate the efficiency of the as-prepared catalyst for the reduction of 4-nitrophenol, the present catalytic system was compared with other reported catalysts in terms of the yield of the product, reaction temperature and time (Table 3). The data in Table 3 indicated that the as-prepared Cu6/7Co1/7Fe2O4–G (0.25) can efficiently catalyze the reduction reaction with comparable or higher yield, and relative short reaction time. 4. Conclusions In conclusion, an inexpensive and magnetically recyclable catalyst, Cu6/7Co1/7Fe2O4–G (0.25), was synthesized by a hydrothermal method for the first time. Cu6/7Co1/7Fe2O4–G (0.25) was used as an efficient catalyst for the reduction of nitroarenes. The results demonstrate that the catalytic activity of CuFe2O4–G was enhanced by the combination of CuFe2O4–G with small amount of cobalt, which can be attributed to the remarkable synergistic effect between the Cu6/7Co1/7Fe2O4 and the graphene sheets. The Cu6/7Co1/7Fe2O4–G (0.25) catalyst can be readily recovered and reused at least five consecutive cycles without significant loss of its catalytic activity. We believe that this method might be an important addition to known procedures for reduction of aromatic nitro compounds either on a lab as well as on a larger scale.

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Acknowledgments This work was financially supported by the National Natural Science Foundation of China (nos. 31171698, 31471643), the Innovation Research Program of Department of Education of Hebei for Hebei Provincial Universities (LJRC009) and the Natural Science Foundation of Hebei Province (No. B2011204051).

Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2014.10.016. These data include MOL files and InChiKeys of the most important compounds described in this article.

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