Fe-Supported SBA-16 Type Cagelike Mesoporous ... - ACS Publications

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Fe-Supported SBA-16 Type Cagelike Mesoporous Silica with Enhanced Catalytic Activity for Direct Hydroxylation of Benzene to Phenol Milad Jourshabani,†,‡ Alireza Badiei,*,†,§ Zahra Shariatinia,‡ Negar Lashgari,† and Ghodsi Mohammadi Ziarani∥ †

School of Chemistry, College of Science, University of Tehran, Tehran, Iran Department of Chemistry, Amirkabir University of Technology, Tehran, Iran § Nanobiomedicine Center of Excellence, Nanoscience and Nanotechnology Research Center, University of Tehran, Tehran, Iran ∥ Department of Chemistry, Alzahra University, Tehran, Iran ‡

S Supporting Information *

ABSTRACT: In this work, an Fe-supported cagelike mesoporous silica type SBA-16 catalyst (Fe/SBA-16) was successfully synthesized using iron nitrate as the precursor through a simple impregnation method. Results of X-ray diffraction, N2 adsorption−desorption, transmission electron microscopy, and elemental mapping analysis showed that the mesoporous structure of the support was retained during catalyst preparation and that iron nanoparticles were dispersed on the SBA-16 surface. Moreover, ultraviolet−visible and X-ray photoelectron spectroscopic studies revealed that the iron(III) oxidation state was dominant in the Fe-supported cagelike mesoporous silica. It was found that the Fe/SBA-16 was an appropriate catalyst for the benzene hydroxylation to phenol using H2O2 as the oxidant. The effects on the catalytic performance of operating parameters such as the amount of H2O2, reaction temperature, reaction time, and catalyst dosage were investigated. Under the optimized conditions, 11.7% phenol yield and 96.4% selectivity to phenol were obtained; in addition, the catalyst could be recycled at least three times.

1. INTRODUCTION

has been widely studied. Lee et al. investigated catalytic hydroxylation of benzene to phenol over V-MCM-41, which showed only 1.39% conversion and 93% selectivity toward phenol in acetic acid at 343 K.12 In another study, V-MCM-48 gave 10% benzene conversion, but the selectivity to phenol was only 38% in acetonitrile at 343 K.13 Unlike M41S materials, metal supported on a one-dimensional hexagonal channel SBA15 system has shown better catalytic performance for direct hydroxylation of benzene. Kong et al. reported that the catalyst consisting of CuO/SBA-15 gave 20.6% benzene conversion and a phenol selectivity of 92.4% in acetic acid at 338 K.14 Among mesoporous materials, SBA-16 is one of the newly ordered mesoporous materials that possesses three-dimensional connected channels (cagelike), a cubic structure, and thicker pore walls and higher hydrothermal stability than those of other mesoporous silica materials.15,16 Also, each isolated nanocage is interconnected by eight neighboring pore entrances which can

The one-step hydroxylation of benzene to phenol by H2O2 is one of the most important reactions for producing phenol which has recently attracted much attention from economic and eco-friendly standpoints. The corresponding phenol is a valuable chemical intermediate for the petrochemical, agrochemical, polymer, and plastic industries.1,2 Among different oxidants, H2O2 is increasingly being used in the liquid-phase reactions because it not only has environmentally friendly features but also has water as its only byproduct.3 In the past several decades, a variety of catalysts have been developed for the direct hydroxylation of benzene with hydrogen peroxide as the oxidant. Various studies have been developed on this reaction using metal-based catalysts on various micro- and mesoporous supports, such as graphitic carbon nitride,4 activated carbon,5 and carbon nanotube.6 Recently, metal-supported mesoporous silica materials were found to be active in the direct oxidation of benzene to phenol because of their high surface area, uniform pore sizes, and large pore volumes.7−11 The introduction of transition-metal oxides and their complexes, such as Fe, Cu, and V, into mesoporous supports such as the one-dimensional form of the M41S family © 2016 American Chemical Society

Received: Revised: Accepted: Published: 3900

December 29, 2015 March 3, 2016 March 22, 2016 March 22, 2016 DOI: 10.1021/acs.iecr.5b04976 Ind. Eng. Chem. Res. 2016, 55, 3900−3908

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Industrial & Engineering Chemistry Research

stirred vigorously. The collected powder was dried at 80 °C for 8 h and calcined at 550 °C for 5 h. The final product containing 10 wt % of iron (2.2 mmol/g) was obtained and labeled as 10 wt % Fe/SBA-16. 2.4. Catalyst Characterization. N2 adsorption−desorption isotherms were obtained using a BELSORP-mini II instrument at 77 K. The Brunauer−Emmett−Teller (BET) equation was used to calculate specific surface areas, and the Barret−Joyner−Halenda equation was employed to determine the pore size distributions and the pore volumes. X-ray diffraction (XRD) measurements were performed using Cu Kα radiation (X’Pert-PRO X-ray diffractometer). The transmission electron microscopy (TEM) measurements were conducted on a Philips CM30 microscope with an acceleration voltage of 150 kV. The elemental mapping analysis of the catalyst particles was performed using an energy-dispersive X-ray (EDX) analyzer (XMU, VEGA-II). The chemical composition of the sample was analyzed by an X-ray photoelectron spectrometry (XPS) instrument equipped with an Al−K X-ray source operated at 1486.6 eV. A hemispherical energy analyzer (Specs EA 10 Plus) operating in a vacuum better than 10−7 Pa was used to determine the core-level binding energies of photoelectrons emitted from the surface. Binding energy (BE) values were calibrated by fixing the C (1s) core level with a BE of 284.5 eV. UV−vis spectra were obtained using a Rayleigh UV-1600 spectrophotometer. The UV−vis absorption spectrum of a solid sample was obtained by the addition of a known sample to spectral grade n-decane; a quartz window with a path length of 0.5 mm was used. The solid sample had very low light scattering in n-decane. A reasonable-quality spectrum was obtained because the reflective index of n-decane is very close to that of silica SBA-16. The liquid products were analyzed using a gas chromatography (GC) instrument (PerkinElmer 8500) equipped with a flame ionization detector. Quantitative analysis of the liquid products was performed based on calibration curves, with toluene as an internal standard. 1,4-Benzoquinone and hydroquinone were identified as byproducts in some experiments. 2.5. Catalytic Performance Evaluation. Catalytic hydroxylation of benzene was tested in a 50 mL round-bottom flask equipped with a reflux condenser and a magnetic stirrer. In a typical run, 0.1 g of catalyst was immersed in 6 mL of acetonitrile (114.8 mmol) and 1 mL of benzene (11.26 mmol). The resulting mixture was heated to 65 °C, and then 2 mL (20 mmol) 30% aq H2O2 was added dropwise for a period of 20 min. The reaction mixture was stirred for 8 h. After the reaction mixture was cooled, the catalyst was separated by centrifugation, and a small amount of ethanol was added to the liquid product, resulting in the formation of a single-phase liquid for the GC analysis. The phenol yield was calculated as (moles of phenol produced)/(initial moles of benzene). The phenol selectivity was calculated as (moles of phenol)/(moles of phenol + moles of hydroquinone + moles of 1,4-benzoquinone).

be more resistant to metal particle aggregation.17 Its unique three-dimensional channel system is believed to present an excellent porous host for guest species, thereby facilitating mass transfer of reactants throughout the pore channels without pore blockage.18 Zhu et al. investigated the catalytic hydroxylation of benzene to phenol over VOx/SBA-16 prepared by an impregnation method; they achieved 13.8% benzene conversion and 97.5% selectivity for phenol.19 More recently, Dong et al. used Co-doped SBA-16 which was synthesized through evaporation-induced self-assembly for direct hydroxylation of benzene to phenol; unfortunately, the yield of phenol was decreased obviously after the second run in the reuse experiment of the catalyst.20 Moreover, Fe-SBA-16 catalyst prepared via direct hydrothermal method exhibited good catalytic performance in the oxidation of cyclohexene.21 In this modification, the isolated metal species which were highly dispersed on the surface of support played an important role in the catalytic performance. Although some catalysts with satisfactory performance have been explored for hydroxylation of benzene to phenol, from an economic viewpoint, the preparation of low-cost catalysts with high activity and selectivity toward phenol remains a great challenge because phenol is more active than benzene toward oxidation. Despite the numerous advantages of SBA-16, it has not been studied extensively as a support in direct hydroxylation of benzene to phenol. In our previous work, a mesoporous silica-supported chromium catalyst with high selectivity toward phenol was successfully prepared.22 In this contribution, we prepared an Fe/SBA-16 catalyst using iron nitrate as a low-cost precursor and SBA-16 as a support via a simple impregnation method. The effects of four operating factors, namely the amount of H2O2, reaction temperature, reaction time, and catalyst dosage, on the catalytic performance were investigated and are discussed in detail. The phenol yield and selectivity under the optimized conditions were determined, and the reusability of Fe/SBA-16 catalyst was also investigated. It was found that Fe-supported mesoporous silica can be an effective catalyst for the direct hydroxylation of benzene to phenol using H2O2 as the clean oxidant.

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. Pluronic F127 (EO106PO70EO106, Mw = 13 600), sodium silicate solution (SiO2 26%, Na2O 8%) as a silica source, HNO3 (65%), Fe(NO3)3·9H2O, acetonitrile, benzene, H2O2 (30%), and toluene were purchased from the Merck Company. All other reagents used in this work were analytically pure and used without further purification. 2.2. Synthesis of SBA-16 Support. Mesoporous silica SBA-16 was prepared using the method described in the literature.23 Pluronic F127 (14.1 g) was dissolved in HNO3 (65%, 144 mL) and deionized water (900 mL). The solution was stirred at 30 °C. Then, sodium silicate solution (62.4 g) was added and the reaction mixture was stirred at 300 rpm for 3 h at 70 °C. The product was kept at 100 °C for 24 h. The surfactant was extracted using ethanol and HCl (2 M), and the obtained solid was calcined at 550 °C for 5 h. 2.3. Preparation of Fe/SBA-16 Catalyst. The catalyst was prepared via an impregnation method as follows: 1 g of SBA-16 was first dried at 120 °C for 2 h under vacuum to remove the adsorbed water and then suspended in a solution obtained by dissolving Fe(NO3)3·9H2O (0.87 g) in 50 mL of water. The solvent was evaporated at 40 °C while the reaction mixture was

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. Figure 1a displays the lowangle XRD patterns of SBA-16 and Fe/SBA-16. The prepared samples show a very sharp diffraction peak at a 2θ value of 0.74° and two minor diffractions at 2θ values of 1.06° and 1.30°, respectively; these are indexed to (110), (200), and 3901

DOI: 10.1021/acs.iecr.5b04976 Ind. Eng. Chem. Res. 2016, 55, 3900−3908

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Figure 2. N2 adsorption−desorption isotherms and pore size distributions (inset) of SBA-16 and Fe/SBA-16.

Table 1. Textural Properties of SBA-16 and Fe/SBA-16 sample SBA-16 10 wt % Fe/SBA-16 a

BET surface area (m2/g)

pore diametera (nm)

total pore volume (cm3/g)

964 604

3.7 3.6

0.819 0.488

Pore diameter was calculated from the desorption branch.

that the SBA-16 has a well-ordered mesostructure. As shown in Figure 3b,c, the mesostructure of the support is retained with the dispersion of iron nitrate and iron nanoparticles (the white arrow) on the SBA-16 surface, although few aggregates of iron oxide particles could be seen. In addition, the number of the particles formed was analyzed to determine the size distribution of particles on the surface of the catalyst, and the resultant data are plotted in a histogram (Figure 3d). Most of the particles have sizes in the range of 1.1−3.9 nm; the mean diameter is 2.5 nm. The catalyst composition was explored by EDX analysis as demonstrated in Figure 4. The peaks are clearly related to O, Si, and Fe elements. The elemental mapping was collected as further proof of the distribution of particles, indicating the Fe element is uniformly distributed on the support (Figure 4, inset). It can be deduced that the cagelike mesoporous silica may present steric restriction to prevent the growth of nanoparticles and that the iron species are highly dispersed into the pores of SBA-16. In total, the XRD patterns, N2 adsorption−desorption isotherms, and TEM images confirm that the structure of the synthesized catalyst did not collapse during its preparation. The coordination geometry of metal oxides depends strongly on the support type and composition, metal loading, heat treatment, and support surface chemistry.26 Coordination of the iron species over SBA-16 was investigated by ultraviolet− visible (UV−vis) spectroscopy (Figure 5). The absorption band at 237 nm can be attributed to ligand-to-metal charge transfer (LMCT) of isolated Fe3+ species on the SBA-16, which are in the form of the tetrahedral geometry. In fact, a central Fe3+ ion could react with two silanol groups on the support surface. The absorption band at 395 nm and a shoulder band at 526 nm can

Figure 1. Low-angle XRD patterns of SBA-16 and Fe/SBA-16 (a) and wide-angle XRD pattern of Fe/SBA-16 (b).

(211) reflections and correspond to the SBA-16 cubic structure with the Im3̅m space group.18 This result obviously indicates that the cubic mesostructure of support was not collapsed because of the incorporation of Fe species. Wide-angle XRD analysis of Fe/SBA-16 with a high iron loading is presented in Figure 1b. There are no obvious diffraction peaks corresponding to iron oxides in the wide-angle region; this result indicates that iron species can uniformly be dispersed on SBA-16, which is consistent with previous reports.24,25 The N2 adsorption− desorption isotherms and pore size distributions of SBA-16 and Fe/SBA-16 are shown in Figure 2. It is clear that both samples demonstrate the type IV isotherm with typical H2 hysteresis loop, indicating their cagelike pore structures. After impregnation of iron nitrate, the isotherm of Fe/SBA-16 catalyst well maintains its cagelike structure; it can be deduced that the ordered mesoporous structure of the support is retained during catalyst preparation and does not destroyed. The data on textural characteristics of the samples are also given in Table 1. A decrease in the pore volume and surface area could be caused by the presence of iron species inside the pores. TEM was used for further investigation of the structural properties of the prepared samples. Figure 3a clearly reveals 3902


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Figure 3. TEM images of SBA-16 (a) and Fe/SBA-16 at different magnifications (b, c). Some metal oxide particles are indicated by the white arrows, and the histogram shows the particle size distribution of iron nanoparticles (d).

Figure 5. UV−vis spectra of SBA-16 and Fe/SBA-16.

Figure 4. EDX analysis of Fe/SBA-16 and elemental mapping of Si, O, and Fe.

X-ray photoelectron spectroscopy was performed to further determine the chemical composition of Fe-supported cagelike mesoporous silica and the valence states of iron species present therein. Figure 6a illustrates the XPS survey spectrum of the

be assigned to oligomerized and aggregated iron species, respectively.27−29 3903


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time, and catalyst dosage have significant effects on the catalytic performance. For this purpose, the effects of reaction temperature, reaction time, volume of H2O2, and catalyst dosage on the performance of the Fe/SBA-16 catalyst were investigated. 3.2.1. Effect of Reaction Temperature. The effect of reaction temperature on the catalyst activity was evaluated in the range of 25−85 °C, and the results are plotted in Figure 7.

Figure 7. Effect of the reaction temperature on the hydroxylation of benzene. Reaction conditions: benzene (1.0 mL, 11.26 mmol), 30% aq H2O2 (2 mL, 20 mmol), Fe/SBA-16 (0.1 g), acetonitrile (6 mL, 114.8 mmol), t = 8 h.

When the reaction temperature is raised to 65 °C, the yield of phenol increases quickly and reaches 11.7%. This may be a result of H2O2 decomposition to active species with increasing temperature.33 As the reaction temperature is further increased to 85 °C, the yield of phenol drops sharply to 4.23%. This might be explained as the self-decomposition of H2O2 to water at high temperature. Decrease in the phenol selectivity was caused by further oxidation of phenol to hydroquinone, which was identified by the GC analysis. Finally, the optimum temperature is considered to be 65 °C. 3.2.2. Effect of Reaction Time. The effect of reaction time variations on the catalyst activity at a constant reaction temperature of 65 °C are depicted in Figure 8. It can be observed that the phenol yield is increased from 2 to 8 h because the reaction time is enough to produce phenol from benzene. The selectivity and yield toward phenol are decreased up to 24 h, which can be due to phenol oxidation to hydroquinone. Therefore, the optimum time is considered to be 8 h. 3.2.3. Effect of H2O2 Amount. The effect of the molar ratio of H2O2 to benzene on the direct hydroxylation of benzene to phenol was investigated, and the results are shown in Figure 9. It can be observed that the yield of phenol is increased while the H2O2/benzene ratio is increased from 0.88 to 1.77, and then it is sharply decreased by adding more H2O2. This decreasing trend from 1.77 to 4.44 could be due to conversion of phenol to 1,4-benzoquinone and hydroquinone, which were confirmed by the GC analysis. In addition, Figure 9 illustrates that when the H2O2/benzene ratio is increased from 2.66 to 4.44, hydroquinone can be converted to 1,4-benzoquinone. It is

Figure 6. XPS spectrum of the synthesized Fe/SBA-16 catalyst: (a) survey spectrum and (b) Fe 2p spectrum.

Fe/SBA-16 catalyst that consists of Si, O, C, and Fe elements, with sharp photoelectron peaks appearing at binding energies of 103.6 (Si 2p), 533 (O 1s), and 284.5 eV (C 1s) and a too weak photoelectron peak around 711 eV (Fe 2p). The origin of the carbon peak is attributed to the environmental contamination from the XPS instrument itself. The XPS spectrum (Figure 6b) of the Fe 2p region shows two peaks at 711.1 (for Fe 2p3/2) and 723.5 eV (for Fe 2p1/2) with a shakeup satellite at 718.9 eV. This is characteristic of Fe3+ in Fe2O3, which is in good agreement with earlier similar reports.30 Furthermore, the binding energy value of Fe 2p3/2 obtained here is higher than that of Fe3+ in pure Fe2O3 (at 710.9 eV), which may be due to the strong interaction between Fe3+ incorporated in the cagelike mesoporous silica with silanol groups to form Si− O−Fe bonds.31,32 Ultraviolet−visible and X-ray photoelectron spectroscopic analyses reveal that the iron(III) oxidation state is dominant in the Fe/SBA-16 catalyst. 3.2. Catalytic Activity. In addition to developing suitable catalyst preparation processes, optimization of the operating variables plays a key role in achieving a good catalytic performance. The literature survey shows that parameters such as the amount of H2O2, reaction temperature, reaction 3904


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Figure 10. Effect of the catalyst dosage on the hydroxylation of benzene. Reaction conditions: benzene (1.0 mL, 11.26 mmol), 30% aq H2O2 (2 mL, 20 mmol), acetonitrile (6 mL, 114.8 mmol), T = 65 °C, t = 8 h.

Figure 8. Effect of the reaction time on the hydroxylation of benzene. Reaction conditions: benzene (1.0 mL, 11.26 mmol), 30% aq H2O2 (2 mL, 20 mmol), Fe/SBA-16 (0.1 g), acetonitrile (6 mL, 114.8 mmol), T = 65 °C.

3.3. Reusability of the Catalyst. In addition to the reaction conditions, reusability is also a crucial parameter for a heterogeneous catalyst. In this regard, the reusability of the Fe/ SBA-16 catalyst was evaluated in the direct synthesis of phenol from benzene for three runs, and the results are shown in Figure 11. After each reaction, the catalyst was separated by

Figure 9. Effect of the amount of H2O2 on the hydroxylation of benzene. Reaction conditions: benzene (1.0 mL, 11.26 mmol), Fe/ SBA-16 (0.1 g), acetonitrile (6 mL, 114.8 mmol), T = 65 °C, t = 8 h.

determined that the appropriate volume in this reaction is 2 mL of 30% aq H2O2 (H2O2/benzene ratio = 1.77). 3.2.4. Effect of Catalyst Dosage. The effect of molar ratio of catalyst to benzene was evaluated on the direct synthesis of phenol from benzene in the range of 0.011 to 0.027 (Figure 10). When the catalyst/benzene ratio is increased to 0.019, the phenol yield is increased from 2.1% to 11.7%, resulting probably from the presence of more catalytic active sites. The yield and selectivity of phenol are decreased with a further increase in the molar ratio of catalyst to benzene from 0.015 to 0.019, which can be caused by further oxidation of phenol to hydroquinone. Finally, 0.1 g of catalyst (catalyst/benzene ratio 0.019) is chosen as the optimum catalyst amount. The achieved phenol yield and selectivity are 11.7% and 96.4%, respectively (reaction temperature, 65 °C; reaction time, 8 h; H2O2 content, 2 mL; and catalyst dosage, 0.1 g). Effects of the reaction temperature, reaction time, amount of H2O2, and catalyst dosage on the conversion of H2O2 are also shown in Figures S1, S2, S3, and S4, respectively.

Figure 11. Reusability of Fe/SBA-16 under the optimized reaction conditions.

centrifugation, washed several times with acetonitrile, and dried at 100 °C. The recovered catalyst was used under the optimum conditions for the next run so that the selectivity in the second and third runs was 100%. As shown in Figure 11, the phenol yield decreases obviously after the first run over the reused catalyst, but the catalyst activity is maintained after the second run. This could be attributed to the low leaching of iron nanoparticles from the extra-framework of the support in the first reaction. 3.4. Comparison of the Catalytic Efficiency with Literature Results. The catalytic performance of Fe/SBA-16 in the direct hydroxylation of benzene was compared with some 3905


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Table 2. Comparison of Catalytic Performances of Various Fe-Based Catalysts for Hydroxylation of Benzene Using H2O2 as the Oxidant catalyst

benzene conversion (%)

phenol yield (%)

phenol selectivity (%)

Wcatalyst (mg)

T (K)

t (h)

ref

Fe-ACa Fe/NACH-600Nb Fe3O4/CMK-3c Fe/MWCNTsd Fe/GOe Fe/SBA-16

19.6 50 18 10.8 15.9 12.1

17.5 20 − − 15 11.7

89.3 40 92 95.5 94.1 96.4

0.5 0.1 0.06 0.05 0.04 0.1

303 336 333 333 338 338

7 5 4 2.5 3 8

34 5 11 6 35 this work

a Benzene (11.2 mmol), H2O2 (48.5 mmol, 30 wt %), acetonitrile (20 mL). bThe catalyst was prepared using impregnation of iron nitrate monohydrate with Norit activated carbon; the molar ratio of benzene:H2O2:acetonitrile was 1:3:4.65. cBenzene (1 mL, 11.26 mmol), H2O2 (20 mmol, 30 wt %), acetonitrile (6 mL). dBenzene (11.3 mmol), H2O2 (13.5 mmol, 30 wt %), acetonitrile (8 mL). ePrepared through Fe(NO3)3·9H2O and graphene oxide as starting materials; benzene (1 mL), H2O2 (3.5 mL, 30 wt %), acetic acid (10 mL, 80 wt %).

Scheme 1. Mechanism of the Catalytic Benzene Hydroxylation by H2O2 in the Presence of Fe/SBA-16

on Fe/SBA-16 by chemisorption on the surface of the supported Fe, together with the formation of an open biradical form of the iron−peroxo complex. These radicals can coordinate to Fe in Fe/SBA-16, to form the iron−peroxo complex. Similarly, it has been reported that vanadium oxide species in contact with H2O2 turned to peroxovanadate species.40 This formed radical may capture a hydrogen atom from a benzene molecule to form a carbon free radical and the hydroxyl radical, as shown in Scheme 1. The latter is able to attack a carbon free radical to produce a phenol molecule.

of the recently reported results using Fe-based catalysts on other supports (Table 2). The phenol yields gained over the catalysts such as Fe-AC and Fe/NACH-600N are higher than those of their counterparts; however, they show lower selectivities toward phenol. The Fe/SBA-16 catalyst illustrates the highest selectivity for phenol compared with other Fe-based catalysts. The good activity for such a catalyst may be attributed to its high surface area, large pore volume, and the threedimensional connected channels of SBA-16, resulting in high mass transfer of benzene on the Fe/SBA-16 surface. 3.5. Proposed Mechanism for the Catalytic Reaction. The published literature shows that both free-radical and nonradical mechanisms can be conceived for the oxidation of alcohols, olefins, and aromatic hydrocarbons over modified metal oxides using H2O2 as the oxidant.36−39 On the basis of previous studies, we propose here a plausible reaction pathway for the hydroxylation of benzene, using H2O2 in the presence of Fe/SBA-16 (Scheme 1). Of particular note is that the mechanism covers only isolated species of tetrahedral Fe3+ as the active sites on the catalyst surface, and a similar mechanism may also be envisaged for other active sites. H2O2 is activated

4. CONCLUSIONS In summary, iron nitrate as a low-cost precursor was dispersed on the cagelike SBA-16 mesoporous silica, using a facile impregnation method (Fe/SBA-16). The XRD, N2 adsorption−desorption, and TEM results indicated that the cagelike (Im3̅m) mesostructure of the support was maintained after impregnation with iron nitrate. Furthermore, iron oxide nanoparticles with different particle sizes were well dispersed on the internal surface of SBA-16 pores. In addition, UV−vis and XPS analyses confirmed that the majority of iron oxides on 3906


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(7) Jiang, T.; Wang, W.; Han, B. Catalytic hydroxylation of benzene to phenol with hydrogen peroxide using catalysts based on molecular sieves. New J. Chem. 2013, 37, 1654. (8) Taguchi, A.; Schüth, F. Ordered mesoporous materials in catalysis. Microporous Mesoporous Mater. 2005, 77, 1. (9) Weitkamp, J.; Hunger, M.; Rymsa, U. Base catalysis on microporous and mesoporous materials: recent progress and perspectives. Microporous Mesoporous Mater. 2001, 48, 255. (10) Corma, A. From microporous to mesoporous molecular sieve materials and their use in catalysis. Chem. Rev. 1997, 97, 2373. (11) Arab, P.; Badiei, A.; Koolivand, A.; Mohammadi Ziarani, G. Direct hydroxylation of benzene to phenol over Fe3O4 supported on nanoporous carbon. Chin. J. Catal. 2011, 32, 258. (12) Lee, C. W.; Lee, W. J.; Park, Y. K.; Park, S.-E. Catalytic hydroxylation of benzene over vanadium-containing molecular sieves. Catal. Today 2000, 61, 137. (13) Lemke, K.; Ehrich, H.; Lohse, U.; Berndt, H.; Jähnisch, K. Selective hydroxylation of benzene to phenol over supported vanadium oxide catalysts. Appl. Catal., A 2003, 243, 41. (14) Kong, A.; Wang, H.; Yang, X.; Hou, Y.; Shan, Y. A facile direct route to synthesize large-pore mesoporous silica incorporating high CuO loading with special catalytic property. Microporous Mesoporous Mater. 2009, 118, 348. (15) Rivera-Muñoz, E. M.; Huirache-Acuña, R. Sol gel-derived SBA16 mesoporous material. Int. J. Mol. Sci. 2010, 11, 3069. (16) Maheswari, R.; Pachamuthu, M. P.; Ramanathan, A.; Subramaniam, B. Synthesis, characterization, and epoxidation activity of tungsten-incorporated SBA-16 (W-SBA-16). Ind. Eng. Chem. Res. 2014, 53, 18833. (17) Ma, Z.; Yang, H.; Qin, Y.; Hao, Y.; Li, G. Palladium nanoparticles confined in the nanocages of SBA-16: Enhanced recyclability for the aerobic oxidation of alcohols in water. J. Mol. Catal. A: Chem. 2010, 331, 78. (18) Cheng, C.-F.; Lin, Y.-C.; Cheng, H.-H.; Chen, Y.-C. The effect and model of silica concentrations on physical properties and particle sizes of three-dimensional SBA-16 nanoporous materials. Chem. Phys. Lett. 2003, 382, 496. (19) Zhu, Y.; Dong, Y.; Zhao, L.; Yuan, F. Preparation and characterization of mesopoous VOx/SBA-16 and their application for the direct catalytic hydroxylation of benzene to phenol. J. Mol. Catal. A: Chem. 2010, 315, 205. (20) Dong, Y.; Zhan, X.; Niu, X.; Li, J.; Yuan, F.; Zhu, Y.; Fu, H. Facile synthesis of Co-SBA-16 mesoporous molecular sieves with EISA method and their applications for hydroxylation of benzene. Microporous Mesoporous Mater. 2014, 185, 97. (21) Jermy, B. R.; Kim, S.-Y.; Bineesh, K. V.; Selvaraj, M.; Park, D.-W. Easy route for the synthesis of Fe-SBA-16 at weak acidity and its catalytic activity in the oxidation of cyclohexene. Microporous Mesoporous Mater. 2009, 121, 103. (22) Jourshabani, M.; Badiei, A.; Lashgari, N.; Mohammadi Ziarani, G. Highly selective production of phenol from benzene over mesoporous silica-supported chromium catalyst: Role of response surface methodology in optimization of operating variables. Chin. J. Catal. 2015, 36, 2020. (23) Kosuge, K.; Kikukawa, N.; Takemori, M. One-step preparation of porous silica spheres from sodium silicate using triblock copolymer templating. Chem. Mater. 2004, 16, 4181. (24) Huang, R.; Lan, B.; Chen, Z.; Yan, H.; Zhang, Q.; Bing, J.; Li, L. Catalytic ozonation of p-chlorobenzoic acid over MCM-41 and Fe loaded MCM-41. Chem. Eng. J. 2012, 180, 19. (25) Li, Y.; Chen, Y.; Li, L.; Gu, J.; Zhao, W.; Li, L.; Shi, J. A simple co-impregnation route to load highly dispersed Fe (III) centers into the pore structure of SBA-15 and the extraordinarily high catalytic performance. Appl. Catal., A 2009, 366, 57. (26) Weckhuysen, B. M.; Wachs, I. E.; Schoonheydt, R. A. Surface chemistry and spectroscopy of chromium in inorganic oxides. Chem. Rev. 1996, 96, 3327.

the support existed mainly as the Fe(III) oxidation state. The catalytic activity was investigated in the direct hydroxylation of benzene to phenol using H2O2 as the clean oxidant. Various operating variables were optimized in the catalytic reaction, namely, reaction temperature, reaction time, amount of H2O2, and catalyst dosage. The optimum conditions were as follows: reaction temperature, 65 °C; reaction time, 8 h; H2O2 content, 2 mL; and catalyst dosage, 0.1 g. Under these conditions, Fe/ SBA-16 catalyst showed an appropriate phenol yield (11.7%) and a higher selectivity (96.4%) in comparison with those of previously reported Fe-based catalyst systems. The good activity of this catalyst is suggested to be a result of its high surface area, large pore volume, and the three-dimensional connected channels of SBA-16, which result in high mass transfer of benzene on the Fe/SBA-16 surface.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b04976. Effect of the reaction temperature on the conversion of H2O2 (Figure S1), effect of the reaction time on the conversion of H2O2 (Figure S2), effect of the amount of H2O2 on the conversion of H2O2 (Figure S3), and effect of the catalyst dosage on the conversion of H2O2 (Figure S4) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Fax: +98 2166405141. Tel.: +98 2161112614. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank University of Tehran for financial support of this work.

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REFERENCES

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