Fe-Supported SBA-16 Type Cagelike Mesoporous Silica with

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Fe-supported SBA-16 type cage-like 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 Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04976 • Publication Date (Web): 22 Mar 2016 Downloaded from http://pubs.acs.org on March 23, 2016

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Industrial & Engineering Chemistry Research

Fe-supported SBA-16 type cage-like mesoporous silica with enhanced catalytic activity for direct hydroxylation of benzene to phenol

Milad Jourshabani1,2, Alireza Badiei1,3*, Zahra Shariatinia2, Negar Lashgari1, Ghodsi Mohammadi Ziarani4

1

Department of Chemistry, College of Science, University of Tehran, Tehran, Iran 2

3

Department of Chemistry, Amirkabir University of Technology, Tehran, Iran

Nanobiomedicine Center of Excellence, Nanoscience and Nanotechnology Research Center, University of Tehran, Tehran, Iran 4

*

Department of Chemistry, Faculty of Science, Alzahra University, Tehran, Iran

Corresponding author, Fax: +98 2166405141; Tel.: +98 2161112614; E-mail: [email protected]

ACS Paragon Plus Environment


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Abstract In this work, a Fe-supported cage-like 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 (TEM), and elemental mapping analysis showed that the mesoporous structure of the support was retained during catalyst preparation and iron nanoparticles were dispersed on the SBA-16 surface. Moreover, ultraviolet-visible and X-ray photoelectron spectroscopic studies (XPS) revealed that the iron(III) oxidation state was dominant in the Fe-supported cage-like 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 of operating parameters such as the amount of H2O2, reaction temperature, reaction time, and catalyst dosage were investigated on the catalytic performance. Under the optimized conditions, 11.7% phenol yield and 96.4% selectivity to phenol were obtained; also the catalyst could be recycled for at least three times.

Keywords: Cage-like mesoporous silica; Fe/SBA-16 catalyst; Benzene hydroxylation; Phenol production.

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1. Introduction 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 petrochemical, agrochemical, polymer, and plastic industries.1,2 Amongst different oxidants, H2O2 is increasingly being used in the liquid phase reactions, because it has not only environmentally friendly features, but also water is its only by-product.3 In the several past 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/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 one-dimensional form of M41S family has been widely studied. Lee et al. investigated catalytic hydroxylation of benzene to phenol over VMCM-41, which showed only 1.39% conversion and 93% selectivity towards 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 hexagonally channel SBA-15 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 (cage like), a cubic structure, 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 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 the evaporation induced self-assembly, for direct hydroxylation of benzene to phenol; unfortunately, the yield of phenol was decreased 3



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obviously after second run in the reuse experiment of 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 on the catalytic performance. Although some catalysts with sufficient performances 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, remain a great challenge; because phenol is more active than benzene toward oxidation. In spite of 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 a 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 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 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.

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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 water. The solvent was evaporated at 40 °C while the reaction mixture was 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 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-ΙΙ). The chemical composition of the sample was analyzed by X-ray photoelectron spectrometer (XPS) equipped with an Al-K X-ray source operated at 1486.6 eV. A hemispherical energy analyzer (Specs EA 10 Plus) operating in 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 UVVis 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 chromatograph (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. 1,4Benzoquinone and hydroquinone were identified as by-products in some experiments.

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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 catalyst was immersed in 6 mL (114.8 mmol) acetonitrile and 1 mL 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 mole of phenol produced/initial mole of benzene. The phenol selectivity was calculated as mole of phenol/(mole of phenol + mole of hydroquinone + mole of 1,4-benzoquinone).

3. Results and discussion 3.1 Catalyst characterization Figure 1a displays the low-angle XRD patterns of SBA-16 and Fe/SBA-16. The prepared samples show a very sharp diffraction peak at 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) m space and (211) reflections and correspond to the SBA-16 cubic structure with the Im 3 group.18 This result obviously indicates that cubic mesostructure of support was not collapsed due to the incorporation of Fe species. Wide-angle XRD analysis of Fe/SBA-16 with a high Fe loading is presented in Figure 1b. There are no obvious diffraction peaks corresponding to iron oxides in the wide-angle region; this result exhibits 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 cage-like pore structures. After impregnation of iron nitrate, the isotherm of Fe/SBA-16 catalyst well maintains its cage-like structure; it can be deduced that the ordered mesoporous structure of the support is retained during catalyst preparation and does not destruct. 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 that the SBA-16 has a well-ordered mesostructure. As shown in Figures 3b and 3c, the mesostructure of the support is retained with the dispersion 6


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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 with the mean diameter of 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 in order to 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 cage-like mesoporous silica may present steric restriction to prevent the growth of nanoparticles and the iron species are highly dispersed into the pores of SBA-16. Totally, the XRD patterns, N2 adsorption–desorption isotherms, and TEM images confirm that the structure of the synthesized catalyst did not collapse during its preparation.

Figure 1

Figure 2

Figure 3

Figure 4 Table 1 Textural properties of SBA-16 and Fe/SBA-16. BET surface area

Pore diameter a

Total pore volume

(m2/g)

(nm)

(cm3/g)

SBA-16

964

3.7

0.819

10 wt.% Fe/SBA-16

604

3.6

0.488

Sample

a

Pore diameter was calculated from the desorption branch.

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 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 7



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

Figure 5

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

Figure 6

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 time, and catalyst dosage have significant effects on the catalytic performance. For this purpose, effect of reaction temperature, reaction time, volume of H2O2, and catalyst dosage were investigated on the performance of the Fe/SBA-16 catalyst.

3.2.1 The effect of reaction temperature The reaction temperature was evaluated on the catalyst activity in the range of 25 to 85 °C and the results are plotted in Figure 7. When the reaction temperature is raised to 65 8


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°C, the yield of phenol increases quickly and reaches to 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 is sharply dropped 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 equal to 65 °C.

Figure 7 3.2.2 The 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 equal to 8 h.

Figure 8 3.2.3 The 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 1,4benzoquinone. It is determined that the appropriate volume in this reaction is 2 mL of 30% aq. H2O2 (H2O2/benzene ratio = 1.77).

Figure 9 3.2.4 The effect of catalyst dosage The 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 raised up 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 9



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with a further increase in the molar ratio of catalyst to benzene from 0.015 up to 0.019, which can be caused by further oxidation of phenol to hydroquinone. Finally, 0.1 g 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, Supporting Information, respectively.

Figure 10

3.3 Reusability of the catalyst Beside 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 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.

Figure 11

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 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/NACH600N 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 three-dimensional connected channels of SBA16, resulting in high mass transfer of benzene on the Fe/SBA-16 surface. 10


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Table 2 Comparison of catalytic performances of various Fe-based catalysts for hydroxylation of benzene using H2O2 as the oxidant Benzene

Phenol

Phenol

Wcatalyst

T

t

conversion

yield

selectivity

(mg)

(K)

(h)

(%)

(%)

(%)

Fe-ACa

19.6

17.5

89.3

0.5

303

7

34

Fe/NACH-600Nb

50

20

40

0.1

336

5

5

Fe3O4/CMK-3c

18

-

92

0.06

333

4

11

Fe/MWCNTsd

10.8

-

95.5

0.05

333

2.5

6

Fe/GOe

15.9

15

94.1

0.04

338

3

35

Fe/SBA-16

12.1

11.7

96.4

0.1

338

8

this work

Catalyst

Other reaction conditions:

a

Reference

Benzene (11.2 mmol), H2O2 (48.5 mmol, 30 wt%), acetonitrile (20 mL).

b

The

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.

c

Benzene (1mL, 11.26 mmol), H2O2 (20 mmol, 30 wt%),

d

acetonitrile (6 mL). Benzene (11.3 mmol), H2O2 (13.5 mmol, 30 wt%), acetonitrile (8 mL). e Prepared 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.%).

3.5 Proposed mechanism for the catalytic reaction The published literature shows that both free-radical and non-radical mechanisms can be conceived for the oxidation of alcohols, olefins, and aromatic hydrocarbons over modified metal oxides using H2O2 as the oxidant.36-39 Based on 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 only covers 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 on Fe/SBA-16 by chemisorption on the surface of the supported Fe, together with the formation of an open biradical form of iron–peroxo complex. These radicals can coordinate to Fe in Fe/SBA-16, to form the Feperoxo 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.

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Scheme 1

4. Conclusions In summary, iron nitrate as a low cost precursor was dispersed on the cage-like SBA16 mesoporous silica, using a facile impregnation method (Fe/SBA-16). The XRD, N2 m) mesostructure adsorption–desorption, and TEM results indicated that the cage-like (Im 3 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 SBA16 pores. In addition, UV-Vis and XPS analyses confirmed that the majority of iron oxides on 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 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 Febased 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.

Supporting Information: 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), effect of the catalyst dosage on the conversion of H2O2 (Figure S4).

Acknowledgments The authors wish to thank University of Tehran for the financial support of this work.

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References (1) Stöckmann, M.; Konietzni, F.; Notheis, J. U.; Voss, J.; Keune, W.; Maier, W. F. Selective oxidation of benzene to phenol in the liquid phase with amorphous microporous mixed oxides. Appl. Catal., A 2001, 208, 343. (2) Zhang, J.; Tang, Y.; Li, G.; Hu, C. Room temperature direct oxidation of benzene to phenol

using

hydrogen

peroxide

in

the

presence

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Figure captions Figure 1. Low-angle XRD patterns of SBA-16 and Fe/SBA-16 (a) and wide-angle XRD pattern of Fe /SBA-16 (b). Figure 2. The N2 adsorption–desorption isotherms and pore size distributions (inset) of SBA16 and Fe/SBA-16. Figure 3. The TEM images of SBA-16 (a) and Fe/SBA-16 at different magnifications (b) and (c), some metal oxide particles are exhibited by the white arrows, and histogram showing the particle size distribution of iron nanoparticles (d). Figure 4. EDX analysis of Fe/SBA-16 and elemental mapping of Si, O and Fe. Figure 5. UV-Vis spectra of SBA-16 and Fe/SBA-16. Figure 6. The XPS spectrum of the synthesized Fe/SBA-16 catalyst: (a) survey spectrum and (b) Fe 2p spectrum. 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/SBA16 (0.1 g), acetonitrile (6 mL, 114.8 mmol), 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. 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. 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 11. Reusability of Fe/SBA-16 under the optimized reaction conditions. Scheme 1. The mechanism of the catalytic benzene hydroxylation by H2O2 in the presence of Fe/SBA-16.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 7

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Figure 9

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Figure 10

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Figure 11

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Scheme 1. The mechanism of the catalytic benzene hydroxylation by H2O2 in the presence of Fe/SBA-16.

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Fe-Supported SBA-16 Type Cagelike Mesoporous Silica with

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