High Catalytic Activity in the Phenol Hydroxylation ... - ACS Publications

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High Catalytic Activity in the Phenol Hydroxylation of Magnetically Separable CuFe2O4−Reduced Graphene Oxide Yitao Zhao, Guangyu He,* Wen Dai, and Haiqun Chen* Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Changzhou University, Jiangsu Province, Changzhou 213164, China S Supporting Information *

ABSTRACT: We reported a highly active CuFe2O4 catalyst modified with reduced graphene oxide (CuFe2O4−RGO) by a solvothermal method. The composite catalyst was fully characterized by FTIR, XRD, Raman, TEM, and XPS, which demonstrated that the CuFe2O4 nanoparticles (NPs) with a diameter of approximately 17.8 nm were densely and compactly deposited on the reduced graphene oxide (RGO) sheets. The as-prepared CuFe2O4−RGO composites were used to catalyze phenol hydroxylation for the first time, which exhibited great catalytic activity. The conversion rate of phenol to dihydroxybenzenes reached 35.5% with a selectivity of 95.2% obtained, which is much higher than for reported systems (25.0%). The catalytic activity remained high after six cycles. More importantly, the catalyst can be easily recovered due to its magnetic separability and the organic solvent-free nature of the phenol hydroxylation process. A possible mechanism in phenol hydroxylation by H2O2 over CuFe2O4−RGO20 catalyst was also proposed. disadvantages.13−17 Lately, ferrite catalyzed synthesis of phenol hydroxylation has become a research focus due to its simple preparation, good catalytic performance, and magnetic separability.18−22 Research20−22 showed that the spinel ferrite MFe2O4 (M = Mg2+,20 Cd2+,21 Co2+, and Cu2+ 22) have stable catalytic performance. Zhang et al.16 used magnesium ferrite synthesized by coprecipitation to catalyze phenol hydroxylation. The conversion rate of phenol reached 20.2% with hydroquinone selectivity of 39.0% and catechol selectivity of 61.0% obtained. However, additional initiator for the reaction process is necessary when MgFe2O4 is used as catalyst and a large dosage of catalyst is needed. For catalyst CdFe2O4,21 toxic metal Cd is used as the metal source. As for CoFe2O4 and CuFe2O4,22 a small amount of rare metal, such as La and Sr, is commonly needed to be doped. In order to make the phenol hydroxylation more valuable in industrial applications, a catalyst with lower cost, higher stability, and better catalytic performance is desired. Our previous studies showed that when metal oxide nanoparticles (NPs), such as Fe3 O 4, 23 CoFe 2 O 4, 24 or ZnFe2O4,25 were loaded on reduced graphene oxide (RGO) sheets, highly active and selective catalysts were obtained. That is because graphene possesses not only large specific surface area, high chemical stability, good adsorption capacity, but also a planar structure as well as many excellent electrical properties. The combination of the NPs and the graphene sheets affords the composite better performance due to the synergies generated between the NPs and the graphene sheets. In this regard, we prepared CuFe 2O4−RGO composites using graphene oxide (GO), copper nitrate, and ferric nitrate as

1. INTRODUCTION Phenol hydroxylation to produce dihydroxybenzenes has attracted much attention in the current chemical industry since the hydroxylation products, such as hydroquinone (HQ) and catechol (CAT), are widely used to synthesize many valuable chemical products.1,2 For example, HQ can be used to prepare photosensitive and thermosensitive materials as well as film developer. CAT is an important intermediate of pesticides and composed spicery. There are some traditional industrial processes for the synthesis of dihydroxybenzenes, such as aniline oxidation3 and diisopropyl benzene oxidation.4 Aniline hydroxylation is losing its market due to its low yield and damage to the environment. On the other hand, diisopropyl benzene oxidation is a mature technology. However, the long synthetic route, the numerous byproducts which are difficult to separate, and the variable market requirements of the byproduct, acetone, limit its further development. Recently, with the concept of green chemistry being widely spread, the direct hydroxylation of phenol has become a research hot spot due to its simple operating protocol and environmentally friendly features, with bright prospects for future industrial applications.5−7 The main methods were well reviewed by Karakhanov et al.8 in 2010, including those of Rhone-Poulenc,9 Ube,10 Brichima,11 and Enichem.12 Among these, Enichem gave relatively better results using TS-1 zeolite heterogeneous as catalyst.12 TS-1 molecular sieve lowered the concentration of the oxidant, hydrogen peroxide, and the conversion rate of phenol reached 25.0%. However, the synthesis of TS-1 molecular sieve was complex with an unstable performance obtained. Besides, acetone was needed for the regeneration of the catalyst. Karakhanov et al. reported an improved method for hydroxylation of phenol to catechol using copper and iron complexes with N,N and N,O ligands as catalysts.8 However, the stability of the catalyst still needs to be enhanced. Some other catalysts also have insurmountable © 2014 American Chemical Society

Received: Revised: Accepted: Published: 12566

April 22, 2014 July 9, 2014 July 15, 2014 July 15, 2014 dx.doi.org/10.1021/ie501624u | Ind. Eng. Chem. Res. 2014, 53, 12566−12574

Industrial & Engineering Chemistry Research

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cooled to room temperature, and the precipitate was filtered, washed with deionized water and ethanol five times, and then vacuum dried at 60 °C for 10 h. The product was labeled as CuFe2O4−RGO20. For comparison, the same method was used to synthesize pure CuFe2O4 and pure RGO. 2.3. Characterization of the Catalyst. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 370 FTIR spectrometer (Thermo Nicolet, USA) using pressed KBr pellets. Raman spectra of the samples were acquired using a Renishaw inVia Reflex Raman microprobe. X-ray diffraction (XRD) measurements were carried out using a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.5405 Å). Transmission electron microscopic (TEM) images were taken with a JEOL JEM-2100 microscope (JEOL, Japan). X-ray photoelectron spectroscopy (XPS) was carried out on an RBD upgraded PHI-5000C ESCA system (PerkinElmer) with Mg Kα radiation (hν = 1253.6 eV). The contents of Cu and Fe in the catalyst were analyzed using inductively coupled plasma (ICP; novAA 300, Analytik Jena AG). The reaction process was monitored using a high performance liquid chromatograph (HPLC; Shimadza, LC-20A, Japan) equipped with a UV/vis detector (Shimadza, Japan). The product was analyzed by HPLC−mass spectrometry (MS) (Shimadza, Japan). 2.4. Phenol Hydroxylation and Analytical Procedure. The catalytic hydroxylation of phenol using 30 wt % H2O2 solution as oxidant was carried out in a round-bottom tube immersed in a water bath maintained at different temperatures. The following is a typical oxidation process: 0.94 g (10 mmol) of phenol, 10 mL of water, and 10 mg of catalyst were mixed in the flask at a reaction temperature of 55 °C. Then 10 mmol (1 mL) of H2O2 was added into the reaction system. The reaction products were analyzed by HPLC with a UV/vis detector at 277 nm. Anisole was used as an internal standard. Aliquots of 10 μL were collected at given time intervals during the reaction and filtered in order to remove any catalyst and injected into an XDB C-18 column (Agilent, 4.6 × 250 mm, 5 μm, USA) in reverse phase mode with methanol−water (40:60 v/v) as eluent at a rate of 1.0 mL/min. After the reaction, catalysts were filtered off, washed with deionized water and ethanol, vacuum dried at 60 °C for 3 h, and then recycled directly in the reaction. The conversion of phenol (XPhenol), the selectivity of dihydroxybenzenes (SDHB), the effective utilization of H2O2 (UH2O2), the turnover number (TON), the molar ratio of CAT to HQ (RCAT/HQ), and the yield of other products (YOthers) except CAT and HQ were defined as follows: nb,Phenol − na,Phenol XPhenol = ·100% nb,Phenol

raw materials and ethanol as solvent by a one-pot method. The deposition of CuFe2O4 NPs and the reduction of GO were accomplished simultaneously. The as-prepared CuFe2O4−RGO composite has a good catalytic effect on the preparation of dihydroxybenzenes by hydroxylation of phenol. Moreover, it could be easily recovered under an external magnetic field.

2. EXPERIMENTAL SECTION 2.1. Materials. Natural graphite powder (99.9%, 500 mesh), Cu(NO3)2·3H2O, Fe(NO3)3·9H2O, hydrogen peroxide (30 wt %), phenol, and other materials were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). All chemicals were of analytical grade or above and used as received. 2.2. Preparation of the Magnetic Catalyst. Colloidal graphite oxide (CGO) was synthesized from natural graphite powder according to the modified Hummers method.26,27 CuFe2O4−RGO composites with different RGO contents (10, 20, 30, and 40 wt %) were synthesized by a hydrothermal method. A typical experimental procedure for the synthesis of CuFe2O4−RGO composite with 20 wt % RGO content is as follows: 2.223 g of CGO (with GO content of 2.6 wt %) was dispersed into 50 mL of ethanol with sonication for 30 min. A 0.242 g sample of Cu(NO3)2·3H2O and 0.808 g of Fe(NO3)3· 9H2O were then added to 20 mL of ethanol with stirring for 30 min at room temperature. The above two solutions were then mixed together and stirred for 30 min. After that, the mixture was adjusted to pH 12 with 6 M NaOH solution and stirred for 3 h, yielding a stable green homogeneous emulsion. The resulting mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave and maitained at 180 °C for 24 h under autogenous pressure (Figure 1). The reaction mixture was

SDHB =

UH2O2 = TON =

nCAT + nHQ nb,Phenol − na,Phenol

·100%

nb,Phenol − na,Phenol n H2O2

·100%

nb,Phenol − na,Phenol ncatalyst

R CAT/HQ = nCAT /nHQ YOthers =

Figure 1. Illustration of the synthesis procedure of CuFe2O4−RGO20 composite. 12567

(nb,Phenol − na,Phenol) − (nCAT + nHQ ) nb,Phenol

·100%

dx.doi.org/10.1021/ie501624u | Ind. Eng. Chem. Res. 2014, 53, 12566−12574


Article

where nb,Phenol and na,Phenol are the molar amounts of phenol before and after the reaction, ncatalyst is the molar amount of the CuFe2O4 in the catalyst, and nCAT and nHQ are the molar amounts of CAT and HQ, respectively.

indicate a small size of the spinel CuFe2O4 NPs, which is about 17.8 nm according to the Scherrer equation. Raman spectra of GO and CuFe2O4−RGO20 composite are presented in Figure 3b. The Raman spectrum of CuFe2O4− RGO20 composite also contains the characteristic D and G bands of GO (at 1354 and 1606 cm−1, respectively). However, the D/G intensity ratio (ID/IG = 0.97) of CuFe2O4−RGO20 is larger than that of graphite oxide (ID/IG = 0.86), which indicates that GO was partially reduced.31,32 The 2D, D + G, and 2G bands observed at 2694, 2961, and 3220 cm−1, respectively, provide further evidence for the reduction of GO. The D band shifts from 1360 to 1354 cm−1, which may be attributed to the reduction of GO to RGO.33,34,35 The TEM image of CuFe2O4−RGO20 composite in Figure 4a shows that the RGO sheets were decorated with a large quantity of CuFe2O4 NPs and the edges of RGO are clearly seen. The size distribution histogram of CuFe2O4 NPs shows that the average size of CuFe2O4 NPs is approximately 17.8 nm, which is in accordance with the XRD result. The homogeneous distribution of the CuFe2O4 NPs was expected to offer an enhanced catalytic activity. Figure 4b shows a high-resolution TEM image of CuFe2O4 NPs; the crystal lattice fringes with a d-spacing of 0.25 nm can be assigned to the (311) plane of CuFe2O4 NPs. The presence of CuFe2O4 NPs on the surface of RGO sheets was further confirmed by the XPS spectra of CuFe2O4−RGO20 shown in Figure 5. The XPS wide-scan spectrum presented in Figure 5a shows the compositional elements of the CuFe2O4− RGO20 NPs. Parts c and d of Figure 5 show the high-resolution XPS spectra for Fe 2p and Cu 2p, respectively. From the Cu 2p spectrum, it was found that the peak at 932.7 eV is from Cu 2p3/2, with a satellite peak at 942.0 eV. The result provides clear evidence for the presence of Cu2+.32 All the Fe 2p spectra show two main peaks with binding energies of 711.1 and 724.6 eV, which were respectively assigned to Fe 2p3/2 and Fe 2p1/2. Two accompanying satellite peaks visible at binding energies of around 718.4 and 732.6 eV are indicative of the presence of Fe3+ cations.36,37 Figure 5b indicates a decrease of oxygen content in CuFe2O4−RGO20 NPs compared with that of GO,25,32 indicating the reduction of GO. Among the oxygencontaining groups on the carbon sheets, the epoxy groups were largely reduced in the composite compared with the starting graphite oxide. This showed that GO was reduced to RGO via hydrothermal reaction, which is in accordance with the FTIR, XRD, and Raman results. ICP test results showed that the contents of Cu and Fe out of 10 mg of CuFe2O4−RGO20 were 2.118 and 3.713 mg (i.e., 21.18 and 37.13%), respectively. This means 7.944 mg of

3. RESULTS AND DISCUSSION 3.1. Characterization of CuFe2O4−RGO20. The FTIR spectra of GO and the CuFe2O4−RGO20 composite are shown in Figure 2. As expected, the FTIR spectrum of GO is in good

Figure 2. FTIR spectra of GO and CuFe2O4−RGO20 composite.

agreement with those from previous works.28,29 The broad and intense band observed at 3437 cm−1 is ascribed to the stretching vibration of O−H. The bands at 1716, 1380, and 1090 cm−1 correspond to the C−O stretching vibration, the C−O−H deformation vibration, and the CO stretching vibration, respectively. The peak at 1622 cm−1 can be assigned to the aromatic skeletal CC stretching vibration of the unoxidized graphitic domains. Compared with that of GO, new prominent absorption bands at about 529 and 436 cm−1 appeared in the FTIR spectrum of the CuFe2O4−RGO20 composite, which can be assigned to CuFe2O4 NPs.30 It can also be clearly seen that the bands at 2918, 2850, 1716, 1380, and 1090 cm−1 disappeared, suggesting that GO in the composite had been reduced to RGO.28 The XRD diffraction patterns of the as-prepared GO, RGO, and CuFe2O4−RGO20 composite are shown in Figure 3a. For the CuFe2O4−RGO20 composite, all the diffraction peaks can be indexed as spinel CuFe2O4 while no typical diffraction peak of GO (001) or RGO (002) is observable. It is speculated that the GO in the CuFe2O4−RGO20 composite was exfoliated due to the crystal growth of CuFe2O4 NPs between the interlayer of GO sheets.24,25 The diffraction peaks at 2θ = 18.5, 30.1, 35.6, 38.7, 42.9, 53.1, 57.6, and 62.8° can be indexed to the (111), (220), (311), (320), (400), (422), (511), and (440) planes of cubic CuFe2O4 (JCPDS 25-0283). The broad diffraction peaks

Figure 3. (a) XRD patterns of GO, RGO, and CuFe2O4−RGO20. (b) Raman spectra of GO and CuFe2O4−RGO20. 12568



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Figure 4. (a) TEM image of CuFe2O4−RGO20; (inset) size distribution histogram of CuFe2O4 NPs. (b) High-resolution TEM image of CuFe2O4− RGO20.

Figure 5. (a) Wide-scan and (b−d) deconvoluted XPS spectra of as-prepared CuFe2O4−RGO20.

CuFe2O4 was loaded on RGO, and the RGO content of CuFe2O4−RGO20 is about 20.56%, which is consistent with thermogravimetric analysis (TGA) results (Supporting Information, Figure S1). 3.2. Catalytic Hydroxylation of Phenol. In this study, the catalytic activities of all catalysts were tested under designed reaction conditions. The effects of various reaction parameters on hydroxylation of phenol were also investigated. 3.2.1. Effect of RGO Content on the Catalytic Hydroxylation of Phenol. The effect of RGO content on phenol hydroxylation over different catalysts is presented in Table 1. No conversion of phenol was detected when no catalyst was added, which indicated that CuFe2O4 NPs were responsible for the activity in the hydroxylation. However, all the CuFe2O4− RGO samples showed obvious catalytic activity for phenol hydroxylation. With the RGO content in the catalysts increased from 10 to 40%, the conversion rate of phenol first increased from 27.2 to 35.5% and then decreased to 28.6%. The same changing trend was found with SDHB. This showed that the introduction of RGO enhanced the catalytic activity of the

Table 1. Effect of RGO Content on Catalytic Hydroxylation over Different Catalystsa catalyst none CuFe2O4 CuFe2O4− RGO10 CuFe2O4− RGO20 CuFe2O4− RGO30 CuFe2O4− RGO40

GO (wt %)

XPhenol (%)

SDHB (%)

RCAT/HQ

YOthers (%)

TON

0 0 10

0.9 27.2 29.2

0 95.2 91.8

− 2.2 2.0

0.9 1.3 1.4

− 6.2 71.5

20

35.5

95.2

2.3

1.7

101.4

30

30.8

94.1

2.1

1.8

99.4

40

28.6

92.3

2.0

2.1

105.6

a

A 0.94 g sample of phenol reacted with H2O2 in a molar ratio of 1:1 over 10 mg of the catalysts in 10 mL of water at 55 °C for 30 min.

composites, which made it possible for the CuFe2O4 NPs to disperse uniformly on RGO sheets and prevented them from agglomerating, thus increasing the number of active centers on CuFe2O4 NPs. Meanwhile, the π−π stacking and electrostatic 12569



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interactions between phenol and RGO38−40 induced the adsorption of reactant on the catalyst and increased the collision probability between reactants, leading to a high conversion rate. However, when the RGO content exceeded 20 wt %, the conversion rate started to decrease. This might be because the active content of CuFe2O4 decreased too much to provide sufficient active copper/iron centers, which promoted less decomposition of H2O2. Accordingly, CuFe2O4−RGO20 was used as catalyst in the subsequent experiments due to its best catalytic activity, unless specifically stated. The selectivity also varied with the increase of RGO content. This might be because the strong adsorption of RGO toward phenol can promote hydroxylation. Meanwhile, the product can also be adsorbed on RGO sheets, which would result in deeper oxidation to generate byproducts. 3.2.2. Effect of Catalyst Dosage on the Catalytic Hydroxylation of Phenol. The effect of different dosages of CuFe2O4−RGO20 on phenol hydroxylation is shown in Table 2. As catalyst, 5, 10, 15, and 20 mg of CuFe2O4−RGO20 were

Table 3. Effect of Solvent on the Phenol Hydroxylation over CuFe2O4−RGO20 Catalysta

XPhenol (%)

SDHB (%)

RCAT/HQ

YOthers (%)

5 10 15 20

33.0 35.5 35.8 35.7

95.7 95.2 95.1 95.0

2.3 2.3 2.1 2.0

1.4 1.7 1.8 1.8

XPhenol (%)

SDHB (%)

RCAT/HQ

YOthers (%)

35.5 0.4 0.6 0.5

95.2 0.1 0.0 0.1

2.3 − − 0.2

1.7 0.3 0.5 0.4

a

A 0.94 g sample of phenol reacted with H2O2 in a molar ratio of 1:1 over 10 mg of CuFe2O4−RGO20 in 10 mL of different solvents at 55 °C for 30 min.

3.2.4. Effect of Molar Ratio of Phenol to H2O2 on the Reaction. As shown in Table 4, the reaction was carried out Table 4. Effect of Molar Ratio of Phenol to H2O2 on the Reaction over CuFe2O4−RGO20a

Table 2. Effect of Dosage on the Reaction over CuFe2O4− RGO20 Catalysta dosage (mg)

solvent H2O MeOH EtOH CH3CN

molar ratio (PhOH:H2O2)

XPhenol (%)

SDHB (%)

RCAT/HQ

YOthers (%)

UH2O2 (%)

2:1 1:1 1:2 1:3 1:4 1:5

15.2 35.5 56.8 68.7 78.2 79.1

79.6 95.2 85.8 77.1 70.7 70.5

3.5 2.3 2.0 1.8 1.8 1.8

3.1 1.7 7.4 14.9 22.9 23.3

30.4 35.5 28.4 22.9 19.6 15.8

a

a A 0.94 g sample of phenol reacted with H2O2 in different molar ratios over 10 mg of CuFe2O4−RGO20 in 10 mL of water at 55 °C for 30 min.

used, respectively. The conversion rate increased from 33.0 to 35.7% with the rise of CuFe2O4−RGO20 from 5 to 20 mg. However, no significant difference was observed when the amount of catalyst was increased from 10 to 20 mg. Theoretically, the amount of active sites on CuFe2O4−RGO20 rose with increasing dosage of catalyst, which increased the decomposition of H2O2 to •OH, and would promote the reaction to reach a higher conversion rate. However, when the dosage of catalyst exceeded 10 mg, a large amount of gas was released from the reaction solution and no obvious increase of the conversion rate was observed. This means that H2O2 decomposed more rapidly, but the reaction was not rapid enough to consume the •OH generated. Therefore, 10 mg of CuFe2O4−RGO20 was used in the subsequent experiments unless specifically stated. 3.2.3. Effects of Different Solvents on the Catalytic Hydroxylation of Phenol. It had been reported that solvents had significant effects on phenol conversion and the yield of products.42,43 In order to study the influence of the reaction medium, the reaction was performed in different solvents. Table 3 presents a summary of the solvent effect on the catalytic hydroxylation of phenol over CuFe 2O4−RGO20 composites. It shows that the conversion rate and selectivity were both the highest when the reaction was carried out in water. On the other hand, when MeOH, EtOH, or CH3CN was used as solvent, the conversion rate and selectivity were less than 1.0%. The reason is probably that it is easier for H2O2 to decompose to highly reactive •OH in water than in other solvents. Therefore, water was used as solvent in the subsequent experiments, which is environmentally friendly compared to the organic solvents reported.15

with a fixed amount of phenol to varying amount of H2O2 in molar ratios of 2:1, 1:1, 1:2, 1:3, 1:4, and 1:5, respectively. The maximum phenol conversion (79.1%) was obtained in the phenol to H2O2 molar ratio of 1:5. However, the efficiency of the H2O2 utilization was highest when the reaction was carried out at the phenol to H2O2 molar ratio of 1:1, as is shown in Table 4. Though the conversion rate of phenol was only 35.5%, the highest selectivity (95.2%) was achieved. Actually, excess H2O2 is undesirable, which decreases the utilization of H2O2.42 When the molar ratio of phenol to H2O2 was raised to 1:4, the selectivity of products decreased from 95.2 to 70.7%, while the yield of byproducts increased rapidly from 1.7 to 22.9%. It might be caused by the direct oxidation of phenol into byproducts, but also by the reoxidation of CAT and HQ when H2O2 was excessive. Thus, we chose the molar ratio of 1:1 as the optimum ratio to improve the efficiency of hydrogen peroxide utilization and achieved high conversion rate with high selectivity. 3.2.5. Effect of Reaction Temperature on the Catalytic Hydroxylation of Phenol. The effect of reaction temperature on phenol hydroxylation using CuFe2O4−RGO20 as catalyst is presented in Table 5. The reaction was respectively carried out at 45, 50, 55, 60, 65, and 70 °C. Though only slight changes of the selectivity were observed with the increase of reaction temperature, the conversion rate increased first to 35.5% at the reaction temperature of 55 °C and decreased thereafter. The rise of temperature might promote the conversion of H2O2 to • OH. The concentration of •OH reached its maximum level when the temperature was increased to 55 °C. However, when the temperature was increased further, the concentration of • OH decreased due to the decomposition of H2O2 to H2O and O2, leading to the decrease of the conversion. Therefore, to achieve high conversion and selectivity, the subsequent

A 0.94 g sample of phenol reacted with H2O2 in a molar ratio of 1:1 over different amounts of CuFe2O4−RGO20 in 10 mL of water at 55 °C for 30 min.

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shown in Figure 6, CuFe2O4−RGO20 was still stable even after being recycled six times. Slight decrease of phenol conversion

Table 5. Effect of Temperature on the Reaction over CuFe2O4−RGO20 Catalysta temp (°C)

XPhenol (%)

SDHB (%)

RCAT/HQ

YOthers (%)

45 50 55 60 65 70

20.2 21.1 35.5 32.3 27.9 27.4

95.0 94.7 95.2 87.3 92.4 90.0

3.5 2.4 2.3 2.3 2.0 2.0

1.0 1.1 1.7 4.1 2.1 2.0

a A 0.94 g sample of phenol reacted with H2O2 in the molar ratio of 1:1 over 10 mg of CuFe2O4−RGO20 in 10 mL of water at different temperatures for 30 min.

Figure 6. Catalytic stability of CuFe2O4−RGO20 in the hydroxylation of phenol. A 0.94 g sample of phenol reacted with H2O2 in the molar ratio of 1:1 over 10 mg of CuFe2O4−RGO20 in 10 mL of water at 55 °C for 30 min.

experiments were carried out at 55 °C. Unlike the conversion rate and selectivity, the R decreased from 3.5 to 2.0 all the way with the rise of temperature, which is because the ohydroxylation of phenol is kinetically controlled whereas the p-hydroxylation is thermodynamically controlled. 3.2.6. Effect of Reaction Time on the Catalytic Hydroxylation of Phenol. The effect of reaction time on phenol hydroxylation using CuFe2O4−RGO20 as catalyst is presented in Table 6. It was reported that there was an

and variation of product selectivity were observed. Characterization of the catalyst showed no obvious difference before and after the reaction (Supporting Information, Figures S2 and S3). 3.2.8. Proposed Mechanism. CuO−RGO (Supporting Information, Figure S3) and Fe2O3−RGO (Supporting Information, Figure S4) composites were prepared for the comparison. The CuO system was found to have some catalytic activity, while the Fe2O3 system did not have any. Also, MgFe2O4−RGO20 composite was prepared and no catalytic activity was found either, which means the real active center was Cu rather than Fe (Supporting Information, Figure S5). The catalytic mechanism of phenol hydroxylation over CuFe2O4−RGO20 was proposed and is shown in Scheme 1, which is similar to Fenton’s reaction. The Cu(II)−O−Fe(III) species are catalytically active centers for phenol hydroxylation with H2O2 being used as the hydroxylation reagent.44 The •OH radicals were generated via the decomposition of H2O2 over Cu(II)−O−Fe(III) due to the existence of Fe(III) in the catalyst through the formation of the Fenton-like reaction. Cu(II)−O−Fe(III) was converted to Cu(III)−O−Fe(III) simultaneously.45,46 The strong oxidative •OH radicals oxidized phenol to intermediates (IM1 and IM2), which immediately reacted with Cu(III)−O−Fe(III) to produce HQ and CAT. Meanwhile, Cu(III)−O−Fe(III) was regenerated into the catalytically active Cu(II)−O−Fe(III). A side reaction (Scheme 1, reaction 4) might occur along with the hydroxylation of phenol, where Cu(III)−O−Fe(III) reacted with H2O2 to produce O2, leading to less utilization efficiency of H2O2.45,46 3.2.9. Effect of Different Catalytic Systems on the Catalytic Hydroxylation of Phenol. The phenol conversion rate over CuFe2O4−RGO20 was compared with those over some other Cu-based catalysts reported in the literature (Table 7). The catalytic activity of CuFe2O4−RGO20 is far higher than those of homogeneous catalysts1,47,48 as well as heterogeneous copper(II) complexes.45 A higher conversion rate was reached in a shorter time at lower temperature, which is comparable with those of industrial hydroxylation processes.12

Table 6. Effect of Time on the Reaction over CuFe2O4− RGO20 Catalysta reaction time (min)

XPhenol (%)

SDHB (%)

RCAT/HQ

YOthers (%)

5 10 15 30 45 60 75

27.5 32.3 34.8 35.5 35.6 35.7 35.8

93.5 96.7 96.6 95.2 94.9 94.7 94.6

2.6 2.4 2.4 2.3 1.9 1.8 1.6

1.1 1.1 1.2 1.7 1.8 1.8 1.9

a A 0.94 g sample of phenol reacted with H2O2 in the molar ratio of 1:1 over 10 mg of CuFe2O4−RGO20 in 10 mL of water at 55 °C for different times.

induction period at the beginning of the hydroxylation reaction before phenol conversion increased promptly.41,43 Compared with the reaction carried out without catalyst (Supporting Information, Table S1), the induction period was greatly shortened. The conversion rate increased from 0.5 to 27.5% after 5 min. The selectivity of CAT and HQ reached 96.7% within 10 min, which should be closely related to the introduction of RGO to the composite catalyst. The conversion rate reached 27.5% in 5 min, which indicated a large amount of • OH was produced in the presence of CuFe2O4−RGO20. The • OH then oxidized phenol immediately. Another explanation is that CuFe2O4−RGO20 could lower the activation energy of the reaction, which makes o- and p-hydrogens of phenol more active. No obvious increase of the conversion rate of phenol was detected when the reaction time was extended from 30 to 75 min, while the selectivity decreased slightly, which might be because prolonging the reaction time led to deeper oxidation of CAT and HQ to byproducts. 3.2.7. Catalytic Stability of CuFe2O4−RGO20. The catalytic stability of the magnetically separable CuFe2O4−RGO20 was also studied. After the reaction, the catalyst was separated by applying an external magnetic field, washed with deionized water and EtOH, and then vacuum dried at 60 °C for 3 h before being used in the subsequent recycling experiment. As is

4. CONCLUSION A novel magnetically separable CuFe2O4−RGO nanocomposite catalyst was synthesized using a solvothermal method under autogenous pressure. The dense and homogeneous deposition of CuFe2O4 on RGO sheets afforded the composite catalytic activity toward hydroxylation of phenol with H2O2, which should be attributed to the synergetic action between CuFe2O4 12571



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Scheme 1. Proposed Mechanism of Phenol Hydroxylation with H2O2 over CuFe2O4−RGO20

Table 7. Phenol Hydroxylation Catalyzed by Different Catalytic Systems

a

catalyst

solvent

[PhOH]/[H2O2]

time (min)

temp (°C)

conversion (%)

ref

CuFe2O4−RGO20 CuCl2−H4SiW12O40 CuUSY CuH β [Cu-Imace-H][NO3] LaCuO4 LaSrCuO4 CuO

water water water water water water water water

1 1 1 1 1 1 1 1

30 270 120 120 180 120 120 120

55 70 60 60 70 70 70 70

35.5 39 9a 35a 27 50.9 2.2 11.7

this work 47 48 48 1 45 45 45

Tars.

■

ACKNOWLEDGMENTS The financial support from the National Natural Science Foundation of China (No. 51202020), the Science and Technology Department of Jiangsu Province (BY2012099, BY2013024-04), Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology (BM2012110), and the PAPD of Jiangsu Higher Education Institutions are gratefully acknowledged.

NPs and RGO sheets. The optimal reaction condition was explored, and better conversion rate of phenol and higher selectivity of CAT and HQ were obtained compared with the reported results. Water was proved to be the optimum solvent of the reaction, which is environmentally friendly. In addition, the magnetically separable catalyst has good catalytic stability with high conversion and selectivity on phenol hydroxylation remaining after six cycles.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details and analytic data (TG curve of the CuFe2O4−RGO20 composite; XPS spectra and TEM image of CuFe2O4−RGO20 composite after reaction; XRD patterns of CuO−RGO, Fe2O3−RGO, and MgFe2O4−RGO20 composites; ICP test results of contents in different CuFe2O4−RGO composites). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86 519 86330088. Fax: +86 519 86330086. *E-mail: [email protected]. Tel.: +86 519 86330257. Notes

The authors declare no competing financial interest. 12572



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