Activated Carbon Catalysts in the Hydroxylation of

Article pubs.acs.org/IECR

Application of Fe/Activated Carbon Catalysts in the Hydroxylation of Phenol to Dihydroxybenzenes Mingming Jin, Ruiguang Yang, Meifang Zhao, Guiying Li,* and Changwei Hu* Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu, Sichuan, 610064, China S Supporting Information *

ABSTRACT: A series of Fe/activated carbon catalysts were prepared by impregnation of activated carbon with aqueous solution of ferric nitrate and employed in phenol hydroxylation to dihydroxybenzenes using hydrogen peroxide as oxidant. The samples were characterized by thermal analysis, inductively coupled plasma atomic emission spectrometry (ICP-AES), N2-adsorption, temperature-programmed oxidation mass spectrometry (TPO-MS), scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Part of the ferric (Fe(III)) species was reduced to ferrous (Fe(II)) species forming Fe3O4 when the Fe/activated carbon catalyst was heated at 400 °C for 3 h in air. Fe3O4 highly dispersed on activated carbon was found to be the active phase for the target reaction. The appearance of ferrous (Fe(II)) species greatly improved the catalytic activity. A phenol conversion of 41.3% and a yield of 36.0% to dihydroxybenzenes were obtained under the following optimal reaction conditions: catalyst amount, 0.1 g; reaction temperature, 30 °C; molar ratio of phenol/H2O2, 10.6/9.8; reaction time, 1 h.

1. INTRODUCTION Hydroquinone and catechol are important chemicals which can be used in many areas such as agrochemicals and the fine chemical industry.1 Traditionally, the catalysts used for the production of dihydroxybenzenes were inorganic acid or soluble metal salts,2,3 which had some distinct disadvantages such as waste producing, low yield to dihydroxybenzenes, and complexity of operation processes. In contrast, phenol hydroxylation over solid catalysts using H2O2 as oxidant attracted much attention, and various micro/mesoporous materials such as TS-1, MCM-41, MCM-48, HMS, APO, and carbon molecular sieve were applied in this reaction.4−9 The introduction of transition metals and their complexes, such as Fe, Cu, V, Pt, and Zr, into these solid materials can greatly improve the catalytic activities for phenol hydroxylation.4−11 Among these materials, Fe-containing catalysts exhibited higher phenol conversion and selectivity to dihydroxybenzenes. As one kind of typical solid material, carbon materials used in heterogeneous catalysis has a long history. They can act as catalysts by themselves or as supports for other active phases.12,13 Fe/activated carbon catalyst has been successfully used for the hydroxylation of benzene to phenol by hydrogen peroxide in acetonitrile.13 Furthermore, Fe/activated carbon catalyst was also usually used for the removal of phenol in wastewater.14−16 Very recently, Song et al.17 used multiwalled carbon nanotube (MWCNT) supported Fe3O4 as a catalyst to directly convert benzene to phenol. They suggested that the high catalytic activity of Fe/MWCNT catalyst was mainly attributed to the more efficient catalytic component of Fe3O4 and the beneficial topology of MWCNTs. The active phase Fe3O4 comes from the spontaneous reduction of Fe3+ on the defect sites of mutiwalled carbon nanotubes. Activated carbon and multiwalled carbon nanotubes are important porous carbon materials and have many similar characteristics,13 while © 2014 American Chemical Society

activated carbon is more inexpensive and is easily available. Therefore, it will be very interesting to investigate the catalytic performance of Fe/activated carbon catalyst in the hydroxylation of phenol to dihydroxybenzenes. In this work, activated carbon supported Fe3O4 catalyst was synthesized and used for the hydroxylation of phenol to dihydoxybenzenes.

2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. Commercially available wood-based activated carbon was purchased from Jiangsu Nantong Activated Carbon Corp. (China). First, the activated carbon (AC) was washed by distilled water and then dried at 110 °C for 12 h. After it was ground to 40−80 mesh and pretreated with 3 mol/L hydrochloric acid for 12 h, the material was washed with hot water until neutral pH was attained. Then the treated AC was dried at 110 °C for 10 h. Subsequently, the impregnation of activated carbon with 6% ferric nitrate solution (Fe3+ weight contents) was endured for 0.5 h, and the resulting sample was separated from the excess solvent by vacuum filtration. The sample obtained before calcination was named Fe/AC. The samples were then dried at 110 °C and calcined at 150, 300, 400, 500, or 600 °C in a muffle furnace for 3 h. The obtained samples were denoted as Fe/AC-T, namely, Fe/AC150, Fe/AC-300, Fe/AC-400, Fe/AC-500, and Fe/AC-600, where T represents the calcination temperature. The particle size of the samples changed slightly after the above treatment, and the Fe/AC-T samples of 80−120 mesh were selected and used as catalysts for the hydroxylation of phenol. Received: Revised: Accepted: Published: 2932

May 2, 2013 February 2, 2014 February 3, 2014 February 3, 2014 dx.doi.org/10.1021/ie404010u | Ind. Eng. Chem. Res. 2014, 53, 2932−2939

Industrial & Engineering Chemistry Research

Article

2.2. Catalyst Characterization. The actual iron content of the prepared samples was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). The samples were calcined in air for 3 h at 800 °C, and then dissolved with diluted hydrochloric acid (6 mol L−1) before ICP-AES analysis. Thermal gravimetric (TG) analysis was accomplished using a TGA/Q500 V20.10 in flowing air at a heating rate of 10 °C min−1, from room temperature (RT) to 750 °C. X-ray photoelectron spectroscopy (XPS) was performed in a Krotos-XSAM 800 multifunctional electron spectrometer using monochromatic Al Kα radiation. The anode was operated at 12 kV and 20 mA. The spectrometer was equipped with a DS300 unit for data acquisition. X-ray powder diffraction (XRD) patterns were obtained on a DX-1000 diffraction instrument with Cu Kα radiation at a wavelength of 0.154 nm. The data were collected over the 2θ range of 5−80° with a step size of 0.06 deg s−1. The samples were characterized by nitrogen adsorption at −196 °C using a Micromeritics Model TriStar 3020 vacuum volumetric sorption instrument. The surface area was determined with the Brunauer−Emmett−Teller (BET) method, and the pore volume and area distribution were determined with the Barrett−Joyner−Halenda (BJH) method. Prior to measurement of N2 adsorption, the samples were degassed at 200 °C for 12 h. Scanning electron microscopy (SEM) images were recorded on a FEI/PHILIPS Inspect F microscope, at an acceleration voltage of 20 kV and 10.1 mm working distance. Samples were coated with gold before measurements. The fresh catalyst sample was also characterized by temperature-programmed oxidation (TPO) in 30 mL min−1 air flow, with a ramp of 10 °C min−1. The evolved gas was analyzed by mass spectrometry (TPO-MS). CO2 arising from the oxidation of activated carbon was monitored by recording the m/z = 44 signal. NO2 arising from the decomposition of Fe(NO3)3 on activated carbon was monitored by recording the m/z = 46 signal. A multichannel mass spectrometer, HPR20QIC, was connected to the reactor outlet to analyze the outgas on-line. 2.3. Phenol Hydroxylation. The hydroxylation reaction of phenol was done in a 50 mL glass flask put into awater bath and fitted with a water-cooled reflux condenser. In a typical run, 1.0 g of phenol and 0.1 g of catalyst were added in the reactor, followed by addition of 20 mL of distilled water as solvent; that is, the initial concentration of phenol was about 0.53 mmol L−1 before the addition of H2O2. When the mixture was heated to the desired temperature, an aqueous solution of H2O2 (30% w/ v) was added. The pH value of the reaction system was measured to be 2.65 initially. After the reaction, the catalyst was separated by centrifugation and the products were analyzed by HPLC equipped with a UV detector using CH3CN/H2O [50/ 50 (v/v)] as mobile phase. To avoid the influence of adsorbed phenol on the activated carbon, the catalyst used after the reaction was washed successively with water and acetonitrile, and the solution was analyzed. The products were detected at 225 nm for phenol and dihydoxybenzenes and 254 nm for benzoquinone. The main products of dihydroxybenzenes and the byproducts were quantified using o-cresol as an internal standard. All products were also identified by coupled gas chromatography and mass spectroscopy (GC−MS, Agilent 5973 Network 6890N). The conversion of phenol (XPh), the effective utilization of H2O2 (UH2O2), and the yields of

dihydroxybenzenes and other products (Yothers) were defined as follows: XPh (%) =

0 − nPh nPh 0 nPh

UH2O2 (%) =

nHQ + nCAT + 2nBQ n H0 2O2

Ydihydroxybenzenes (%) =

Yothers (%) =

× 100%

× 100%

ndihydroxybenzenes 0 nPh

× 100%

0 − nPh − ndihydroxybenzenes nPh 0 nPh

× 100%

where n0Ph and nPh denote the initial and final amounts (moles) of phenol, respectively, and n0H2O2 denotes the initial amount (moles) of hydrogen peroxide, while nHQ, nCAT, and nBQ denote the produced amounts (moles) of hydroquinone, catechol, and p-benzoquinone, respectively. The amount of dihydroxybenzenes (ndihydroxybenzenes) is the sum of nHQ and nCAT. The subscript “others” denotes the sum of byproducts formed, including benzoquinone and tar.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Samples. 3.1.1. ICP-AES Analysis. The actual iron contents of the prepared samples determined by ICP-AES are listed in Table 1. It was indicated Table 1. Actual Iron Content Determined by ICP-AES Method sample

actual Fe content (wt %)

Fe/AC-150 Fe/AC-300 Fe/AC-400 Fe/AC-500 Fe/AC-600

5.1 15.7 25.2 43.5 39.6

that the actual iron content increased with calcination temperature up to 500 °C, while a slight decrease was also observed when the sample was calcined at 600 °C. The increase of iron content may be caused by the partial burn-up of the activated carbon support and the partial reduction of Fe oxides, while the slight decrease of iron content would be caused by the total oxidation of iron species (more oxygen entered the sample). 3.1.2. Thermal Analysis. Figure 1 shows the weight losses of AC and Fe/AC with increasing temperature. Without Fe impregnation, activated carbon showed only one weight loss peak at about 600 °C in air. Since above 90% of the sample was lost, this weight loss could be assigned to the oxidation of carbon. This result was in good agreement with that reported by Zazo et al.18 It was found that there were three weight loss peaks around 150, 385, and 600 °C for Fe/AC. The peak at about 150 °C could be attributed to the decomposition of ferric nitrate,19 and the peak at 600 °C was also assigned to the oxidation of activated carbon in air. Zazo et al.18 studied the weight loss of Fe/AC in air and N2 atmosphere, and found that the presence of Fe promoted the burnoff of activated carbon. The starting temperature of burnoff shifted from 550 °C in the absence of Fe to about 350 °C for Fe/AC in their work. Thus, 2933

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species, Fe species interacted with the activated carbon and this kind of interaction made the oxidation of activated carbon start at about 300 °C rather than 450 °C; that is, the ignition temperature of the oxidation decreased from 450 to 300 °C. However, the complete combustion of activated carbon still occurred at 600 °C. This confirmed the promoting effect of Fe species on the oxidation of activated carbon. The result was consistent with thermal analysis and the results of actual Fe content of the samples. 3.1.4. SEM Results. Scanning electron micrography (SEM) was used to perform the morphological examination of the original activated carbon and Fe/AC calcined at different temperatures (see Figure 3). The SEM micrographs of original activated carbon and the samples calcined at 150 and 300 °C were almost the same, though partial oxidation of activated carbon was confirmed by thermal analysis and actual Fe content analysis. Activated carbon granules could be seen clearly. With increasing calcination temperature, the particle size of the catalyst became smaller. The granules of activated carbon gradually vanished, leaving small seeds which were proven to be hematite by XRD characterization. That was due to the fact that more activated carbon could be oxidized at higher temperature. The result was consistent with that obtained by TGA and TPO. 3.1.5. Nitrogen Adsorption−Desorption Isotherm. Figure 4 shows the nitrogen adsorption−desorption isotherms and the pore size distribution for the activated carbon support and Fe/ AC-400. Both samples showed type I adsorption isotherms according to IUPAC classification. The BET surface area of the samples is given in Table S1 in the Supporting Information. For AC, the surface area was calculated to be 606.9 m2 g−1. The introduction of Fe increased the surface area when the calcination temperature was lower than 400 °C (that for Fe/ AC-400 was 638.0 m2 g−1). The surface area drastically decreased to 54.0 and 28.5 m2·g−1 when the samples were calcined at 500 and 600 °C, respectively, indicating the burnoff of activated carbon. BET analysis of Fe/AC-400 and AC samples indicated that the external surface area increased from 149.1 to 307.3 m2 g−1 after Fe loading, while the micropore area decreased from 457.8 to 330.7 m2·g−1. These data were different from those reported in the literature, where a decrease of BET surface area was observed.14,16 The increment of BET surface was most probably due to the catalytic oxidation of activated carbon by Fe species, whereas in the previous literature, Fe species coated the surface of activated carbon. 3.1.6. XRD Measurements. Figure 5 displays the XRD patterns of the Fe/AC catalysts calcined at different temperatures. XRD peaks of iron species could not be detected until the calcination temperature increased up to 500 °C, which indicated that Fe species dispersed highly on activated carbon when calcined at lower temperatures (≤400 °C).13,17,20 Considering the fact that Fe(NO3)3 decomposed at about 150 °C, the species on Fe/AC-150, Fe/AC-300, and Fe/AC400 might be iron oxides. As shown in Figure 4, the XRD patterns of Fe/AC-150, Fe/AC-300, and Fe/AC-400 were quite disperse; this was possibly caused by the low content of Fe and/or its high dispersion on the support. On Fe/AC-500 and Fe/AC-600 obvious diffraction peaks were detected at 32.9, 35.6, and 54.0°, corresponding to the (104), (110), and (116) diffraction peaks of Fe2O3 (JCPDS 33-0664). The crystalline of Fe species detected on Fe/AC-500 and Fe/AC-600 could be ascribed to hematite because of the burn-up of activated carbon. 3.1.7. XPS. Figures 6 and 7 show the XPS of the Fe 2p and O 1s spectra of the five samples. From the typical Fe 2p spectrum,

Figure 1. Differential thermal gravimetric curves of AC and Fe/AC in air atmosphere.

the peak around 385 °C in the present work was tentatively assigned to the catalytic oxidation of activated carbon by the supported Fe species. It also indicated that the activated carbon used in the present work could begin to be oxidized from 280 °C, but could not be totally burned off at around 385 °C, while the complete combustion occurred at about 600 °C. The above information would be helpful for studying the preparation procedure and the possible variation of the samples. Because the percentage of loss of activated carbon increased with calcination temperature up to 500 °C, the actual Fe content increased in the same tendency as shown in Table 1. 3.1.3. Temperature Programmed Oxidation Mass Spectra. To confirm the above assignment, TPO-MS characterization was carried out. Figure 2 shows the temperature programmed

Figure 2. Mass spectra from the temperature programmed oxidation off-gas of Fe/AC and AC: (A) NO2 from Fe/AC, (B) CO2 from Fe/ AC, and (C) CO2 from AC.

oxidation mass spectra profiles of AC and Fe/AC in air. As expected, there were two kinds of gaseous products emitted from sample Fe/AC, while only CO2 could be detected for AC. NO2 from sample Fe/AC was detected over the temperature range 140−350 °C. This confirmed the TGA results that the decomposition of nitrate iron started to occur at about 150 °C. CO2 started to emit at about 300 °C for Fe/AC, while for AC it started at 450 °C. It was indicated that, in the presence of Fe 2934



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Figure 3. SEM of original activated carbon and Fe/AC calcined at different temperatures: (A) original activated carbon, (B) Fe/AC-150, (C) Fe/ AC-300, (D) Fe/AC-400, (E) Fe/AC-500, and (F) Fe/AC-600.

Figure 4. (A) Nitrogen sorption isotherms of Fe/AC and AC. (B) The PSD curve is calculated from the desorption branch. 2935



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could not be oxidized at lower temperatures. Quintanilla et al. studied the existence of Fe species on the Fe/AC catalyst prepared in a similar way and found that the Fe species was Fe2O3.14 For Fe/AC-500 and Fe/AC-600, Fe 2p showed a broader Fe 2p3/2 peak with a satellite line situated at about 719 eV. The satellite line was the characteristic of Fe3+ in γ-Fe2O3. The result was consistent with XRD characterization. For Fe/ AC-400, Fe 2p did not show an obvious broader Fe 2p3/2 peak and the satellite line situated at about 719 eV was also very weak. This indicated that Fe3+ was mostly reduced to form Fe3O4.21,22 The O 1s peak at 530.3 eV observed on all five samples could be assigned to the Fe−O bond.21 The O 1s binding energy of 532.5 and 533.7 eV could be assigned to oxygen in SiO2 impurity (SiO) and phenol oxygen (C−O). Phenol oxygen disappeared for samples Fe/AC-500 and Fe/ AC-600. Fe/AC catalysts were prepared using a similar method by Song et al. Their results showed that Fe(III) species could not be reduced to Fe(II) species at 400 °C under N2 protection on activated carbon or graphite carbon.17 In the present work, the sample was calcinated at 400 °C in air and surface Fe3O4 species was obtained. Hence, it was reasonable to deem that the formation of Fe3O4 was attributed to the interaction of activated carbon and air. As seen from Table 2, the surface

Figure 5. XRD patterns of Fe/AC-150, Fe/AC-150, Fe/AC-300, Fe/ AC-400, Fe/AC-500, and Fe/AC-600.

Table 2. Surface Compositions of Samples Determined by XPS sample

atom

atom (%)

Fe/AC-150

Fe O Fe/O Fe O Fe/O Fe O Fe/O Fe O Fe/O Fe O Fe/O

0.94 16.81 0.06 1.62 21.13 0.08 2.32 24.02 0.10 5.41 39.12 0.14 6.84 46.05 0.15

Fe/AC-300

Fe/AC-400

Figure 6. Fe 2p XPS spectra for Fe/AC-150, Fe/AC-300, Fe/AC-400, Fe/AC-500, and Fe/AC-600.

Fe/AC-500

Fe/AC-600

iron content of the samples and the Fe/O ratio increased with calcination temperature, which implied the oxidation of the support. It was also indicated that the interaction of activated carbon and Fe did not promote the reduction of Fe(III) species for Fe/AC-150 and Fe/AC-300, and the valence of Fe species on these samples was still trivalence. For Fe/AC-500 and Fe/ AC-600, Fe3O4 could be oxidized to Fe2O3 by the excess oxygen in air when the activated carbon support was burned out. Nearly no activated carbon was left when the calcination temperature increased up to 500 °C, and only red Fe2O3 was left. Song et al. studied the Fe species on multiwalled carbon nanotubes (MWCNTs), activated carbon (AC), and graphite cabon (GC) supported Fe(NO3)3,17 and their results showed that the reduction reaction only happened on MWCNTs forming Fe3O4 when those materials were heated to 400 °C and kept for 3 h under N2 protection. Zazo et al. studied the weight loss of Fe/AC in air and N2 atmosphere. They attributed a small steep loss at around 700 °C appearing in the

Figure 7. O 1s XPS spectra for Fe/AC-150, Fe/AC-300, Fe/AC-400, Fe/AC-500, and Fe/AC-600.

the binding energies of Fe 2p3/2 and Fe 2p1/2 were located around 711.2 and 724.5 eV, indicating that Fe was directly bonded to O.14,21 The intensity of Fe 2p was lower for samples Fe/AC-150 and Fe/AC-300, consistent with the lower Fe content in these two samples. The lower Fe content was due to the presence of more initially added activated carbon which 2936



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TG curve in N2 atmosphere of Fe/AC to the reduction of Fe3+ by activated carbon.18 The present work showed that the reduction of Fe2O3 by activated carbon to form Fe3O4 could take place at about 400 °C in air. To further confirm the formation of Fe3O4 species, magnetic properties of the fresh Fe/WAC-400 were measured using a LakeShore 7410 vibrating sample magnetometer. As shown in Figure S1 in the Supporting Information, the average saturation magnetization of fresh Fe/WAC-400 was 0.53 emu g−1, indicating the formation of surface Fe3O4 species. 3.2. Phenol Hydroxylation Reaction. In the phenol hydroxylation reaction using Fe-containing catalysts, an induction period of 5−120 min was previously mentioned in the literature.6,23 It was reported that adding a small amount of CH3COOH could shorten and eliminate the induction period.6,24 The activity of phenol hydroxylation was measured at different temperatures in 5 min to test if an induction period also existed over Fe/AC-400. It was seen that the catalyst showed measurable phenol conversion at 30 °C, and phenol conversion increased with increasing temperature, as shown in Table S2 in the Supporting Information. However, the yield of dihydroxybenzenes was not high even at high phenol conversion. CH3COOH of 0.5 mL was added in the present system to promote the reaction at 30 °C. The HPLC and GC− MS analyses indicated that the products consisted of catechol, hydroquinone, and a small amount of benzoquinone after the reaction, while short chain compounds usually observed in the removal of phenol from wastewater were not observed. This might be caused by the lack of sufficient hydrogen peroxide and the difference of reaction conditions. 3.2.1. Effect of Calcination Temperature. The variation of catalytic activity over the catalyst calcinated at different temperatures is shown in Table 3. The Fe/AC-400 sample

different reaction conditions. For the removal of phenol from wastewater, the reaction was usually carried out with a high H2O2/phenol ratio, higher reaction temperature, and even higher pressure. To limit the partial oxidation to the stage of the formation of dihydroxybenzenes, the presence of Fe3O4 might be important under relatively mild conditions. In the preparation of Fe/Activated carbon catalyst for phenol removal and for the hydroxylation of benzene,13−16 the samples were heat treated at a calcination temperature below 300 °C; then Fe species existed as Fe(III). The higher selectivity to hydroquinone on Fe/AC-400 might be related to the presence of Fe3O4 and its unique microporosity.5,17,26,27 Since slight Fe leaching (about 5%) was observed in the experiment (as shown in the Supporting Information, Table S3), a control experiment to test the contribution of homogeneous Fe species was performed. It was observed that the conversion of phenol could afford 45% with only 26% yield to dihydroxybenzenes, when an equivalent amount of Fe to all the Fe contained in the catalyst was used. It was proven that Fe/AC-400 catalyst acted heterogeneously to promote the formation of dihydroxybenzenes. 3.2.2. Optimization of Reaction Conditions. Table 4 illustrates the influence of the reaction temperature on the Table 4. Effect of Reaction Temperature on the Title Reaction over Fe/AC-400a temp (°C)

XPh

YHQ

YCAT

YBQ

Yothers

UH2O2

30 40 50 60 70

41.3 42.0 42.1 42.2 42.9

17.3 18.2 18.6 19.0 18.9

18.7 19.0 20.2 20.4 19.9

0.8 0.3 0.0 0.0 0.0

4.6 4.5 3.3 2.8 4.1

40.7 40.9 42.0 42.6 42.0

a Reaction conditions: phenol, 1.00 g (10.6 mmol); H2O2, 1.00 mL (9.8 mmol); catalyst, 0.1 g; water, 20 mL; time, 1 h.

Table 3. Catalytic Activities of Catalysts Calcinated at Different Temperaturesa

a

hydroxylation of phenol. The reaction was carried out over the temperature range from 30 to 70 °C. The performance of the catalyst did not show a great difference with the increase of temperature from 30 to 70 °C. This was due to the high catalytic activity of Fe3O4. The effect of the reaction time was also tested at 30 °C, and the results are shown in Figure 8. It is shown that both the conversion and the yield to dihydrox-

exhibited excellent catalytic activity among the five samples. For Fe/AC-400, the conversion of phenol reached 41.3% with a selectivity of 88.9% to dihydroxybenzenes. For the other four samples, the conversions were all not beyond 20%. The catechol to hydroquinone ratio over Fe/AC-400 was about 1:1, while for the other four samples the ratios were all over 2:1. On the five samples, Fe(II) existed only on Fe/AC-400 as evidenced by XPS. It had been proposed that Fe3O4 was a more efficient catalytic component compared with Fe2O3 in the benzene hydroxylation reaction.17,25 It seemed that the high catalytic activity for Fe/AC-400 could also be interpreted by the fact that Fe3O4 was a more efficient catalytic component than Fe2O3 in the phenol hydroxylation reaction. Fe/activated carbon performed well in the hydroxylation of benzene and the removal of phenol in wastewater,13−16 while it could also give high selectivity to dihydroxybenzenes. This might be originated from the fine chemical environment of Fe and

Figure 8. Influence of reaction time on conversion of phenol and yield to dihydroxybenzenes at 30 °C. Reaction conditions: phenol 1.00 g; H2O2, 1.00 mL; Fe/AC-400 catalyst, 0.1 g; water, 20 mL.

calcination temp (°C)

XPh

YHQ

YCAT

YBQ

Yothers

UH2O2

150 300 400 500 600

12.7 12.8 41.3 13.2 18.8

2.6 0.9 17.3 1.3 4.0

5.4 5.9 18.7 5.6 8.3

2.1 3.2 0.8 3.5 2.6

2.6 2.8 4.6 2.7 4.0

13.2 14.5 40.7 15.3 19.0

Reaction conditions: phenol, 1.00 g (10.6 mmol); H2O2, 1.00 mL (9.8 mmol); catalyst, 0.1 g water, 20 mL; time, 1 h; 30 °C.

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Table 6. Recycle of Fe/AC-400 Catalysta

ybenzenes increased concomitantly with reaction time until 1 h, and then remained almost constant. The reaction reached a steady state in 1 h even at room temperature. At higher temperature, the reaction time was observed to be shorter,28,29 while both the increase of the hydroxylation rate and that of H2O2 decomposition might equilibrate, leading to equal yields of dihydroxybenzenes. For energy saving, 30 °C was chosen as the reaction temperature. The variation of catalytic activity with different amounts of catalyst is shown in Figure 9. It was observed that the phenol

0.1 0.3 0.5 1.0 1.5

8.7 19.7 26.4 41.3 56.0

1.2 4.8 9.0 17.3 22.4

YCAT 3.1 8.7 13.0 18.7 25.8

YBQ 1.7 2.2 0.8 0.8 0.0

Yothers 2.8 3.9 3.6 4.6 7.8

YCAT

YBQ

Yothers

UH2O2

first second third fourth

41.3 31.0 23.4 9.9

17.3 10.0 7.6 1.2

18.7 14.7 11.0 5.1

0.8 3.8 2.6 3.3

4.6 2.5 2.2 0.4

40.7 36.1 21.1 14.2

catalyst (Supporting Information, Table S4) by tar formed in the reaction. To sum up, Fe3O4 was formed on activated carbon when Fe/ AC was calcined at 400 °C in air for 3 h. The thus-formed activated carbon supported Fe3O4 showed excellent catalytic activities for phenol hydroxylation using H2O2 at 30 °C. A phenol conversion of 41.3% and a yield of 36.0% to dihydoxybenzenes were obtained using a phenol to H2O2 ratio of 1:1. The catalytic activity of Fe/AC was comparable to that of other reported catalysts, such as Fe/MCM-41 and CuAPO-11.6,8 Considering Fe/AC was easy to prepare with low cost, Fe/AC might be a promising catalyst for the hydroxylation of phenol, if the leaching of Fe species and the blocking of the pores are overcome.

4. CONCLUSION In the presence of Fe species, the starting temperature of catalytic oxidation of activated carbon decreased from 450 to 350 °C. The form of Fe species was Fe2O3 for Fe/AC-150 and Fe/AC-300. Highly dispersed Fe3O4 on activated carbon was formed when Fe/AC was calcinated at 400 °C for 3 h in air. At higher calcination temperatures (500 or 600 °C), Fe3O4 could be oxidized to Fe2O3 by the excess oxygen in air. The main active phase over Fe/AC was Fe 3 O 4 in the phenol hydroxylation reaction. Under optimal reaction conditions, a phenol conversion of 41.3% and a yield of 36.0% to dihydoxybenzenes were obtained using Fe/AC-400 as catalyst.

Table 5. Effect of Amount of H2O2 Used on the Title Reactiona YHQ

YHQ

Reaction conditions: Phenol, 1.00 g; H2O2, 1.00 mL; catalyst, 0.1 g; water, 20 mL; time, 1 h; 30 °C.

conversion increased from 32.3 to 41.6% with increasing amount of catalyst from 0.02 to 0.1 g. Further increment of catalyst amount to 0.2 g resulted in only a negligible increase of phenol conversion. This could be interpreted in terms of thermodynamic and mass transfer limitations at higher reaction rate.30−32 The results of hydroxylation of phenol with various amounts of H2O2 keeping the amount of phenol fixed (1.00 g, 10.6 mmol) are shown in Table 5. Although the conversion of

XPh

XPh

a

Figure 9. Effect of amount of catalyst on phenol hydroxylation. Reaction conditions: phenol, 1.00 g (10.6 mmol); H2O2, 1.00 mL (9.8 mmol); water, 20 mL; time, 1 h; 30 °C.

VH2O2 (mL)

run

■

UH2O2

ASSOCIATED CONTENT

* Supporting Information S

83.3 64.5 51.0 40.7 34.8

Experimental details and analytic data (BET, effect of reaction temperature, magnetic property, leaching of Fe species). This material is available free of charge via the Internet at http:// pubs.acs.org.

■

a

Reaction conditions: phenol, 1.00 g (10.6 mmol); water, 20 mL; catalyst, 0.1 g; time, 1 h; 30 °C.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

phenol and the yield to dihydroxybenzenes increased monotonically with increasing amount of H2O2, the selectivity to dihydroxybenzenes slightly decreased. This might be due to the deep oxidation of dihydroxybenzenes by an excess amount of H2O2. More importantly, H2O2 decomposition might become serious with higher concentration of H2O2.33 Therefore, it was favorable to keep a low H2O2 to phenol ratio for higher H2O2 efficiency. As shown in Table 6, the recycle of the catalyst showed that the catalyst deactivated after successive runs, which might be caused by gradual leaching of Fe species (shown in Supporting Information, Table S3) and the blocking of the pores of the

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The project is financially supported by the National Natural Science Foundation of China (Nos. 20502017, 20872102), PCRIRT (No. IRT0846), and characterization of the catalyst from the Analytic and Testing Center of Sichuan University is greatly appreciated. The suggestions by the reviewers are also gratefully acknowledged. 2938



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Activated Carbon Catalysts in the Hydroxylation of

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