SiO2

    Influence of temperature on hydroxylation of benzene to phenol using molecular oxygen catalyzed by V/SiO 2 Chao Guo, Weidong Du, Gang Chen, Lei Shi, Qi Sun PII: DOI: Reference:

S1566-7367(13)00101-5 doi: 10.1016/j.catcom.2013.03.018 CATCOM 3443

To appear in:

Catalysis Communications

Received date: Revised date: Accepted date:

18 January 2013 16 March 2013 20 March 2013

Please cite this article as: Chao Guo, Weidong Du, Gang Chen, Lei Shi, Qi Sun, Influence of temperature on hydroxylation of benzene to phenol using molecular oxygen catalyzed by V/SiO2 , Catalysis Communications (2013), doi: 10.1016/j.catcom.2013.03.018

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ACCEPTED MANUSCRIPT Influence of temperature on hydroxylation of benzene

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to phenol using molecular oxygen catalyzed by V/SiO2

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Chao Guo, Weidong Du, Gang Chen, Lei Shi, Qi Sun*

Institute of Chemistry for Functionalized Materials, Faculty of Chemistry and

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Chemical Engineering, Liaoning Normal University, Dalian 116029, Liaoning, China

Fax: +86-0411-82156989

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Tel: +86-0411-82159069

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Abstract

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E-mail : [email protected] (Qi Sun)

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Effect of temperature on hydroxylation of benzene to phenol with molecular oxygen as an oxidant was studied over V/SiO2 using different reductants. The V/SiO2 with highly dispersed vanadium species was prepared by a sol-gel process and characterized by diffuse reflectance UV-vis and ESR. This work shows that the onset temperature range of benzene hydroxylation and the temperature reaching the maximum phenol yield differ corresponding to each reductant. The reducing capacity of reductant can be showed by changing temperature and affects, in turn, benzene conversion, product selectivity, and the amount of leaching of V species.

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ACCEPTED MANUSCRIPT Keywords： Benzene; hydroxylation; molecular oxygen; reductant; phenol

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1． Introduction

Direct oxidation of benzene to phenol without byproducts is an attractive

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and challenging field from an economical and environmental point of view. A variety of oxygen sources such as O2 [1], H2O2 [2], and N2O [3] are used for this

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reaction. Among these oxygen sources, using the activation of O2 as an oxidant, many catalysts can efficiently catalyze the oxidation of benzene to phenol, but the reaction is truly catalytic only in the presence of a reductant able to recycle

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the oxidized catalyst. In 1954, Udenfriend et al. [4] first reported the oxidation

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of aromatics to produce phenol using the complex ferric sulfate-EDTA as a catalyst with O2 and ascorbic acid as the oxidant and reductant, respectively.

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From then on many reductants were tested to find a substitute for ascorbic acid.

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Masumoto et al. [5] developed seven different reductants over V/Al2O3 at 30 oC using O2 as oxidant, and concluded that all of reductants studied could not efficiently substitute ascorbic acid. Similarly, Battistel et al. [6] tested seven reductants at 50 oC over VCl3. Miyahara et al [7] studied eight reductants at 30 o

C over CuO-Al2O3. And they found that ascorbic acid was still the best

reductant. Thus，as reported in the current literature, the method for assessing different reductants is at the identical temperature over a catalyst in hydroxylation of benzene with O2. One critical parameter that influences the characteristic of a reductant should be reaction temperature. Therefore, we

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ACCEPTED MANUSCRIPT decided to carry out a study on the effect of temperature on hydroxylation of benzene with different reductants, with the aim of providing a method to select

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an effective reductant for hydroxylation of benzene using O2 as oxidant. Furthermore, vanadium compounds have been reported to be an active catalyst

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for phenol formation by liquid-phase catalyzed benzene oxidation using O2 as the oxidant [5, 6, 8]. Herein, a V/SiO2 xerogel was used to screen a suitable

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reductant.

2. Experimental

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2.1. Catalysts preparation

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The preparation of V/SiO2 was as follows: 18.61 cm3 of tetraethoxysilane (Tianjin Kermel Chemical Reagent Co. Ltd, AR grade) was mixed with 19.44 cm 3

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ethanol under stirring to form solution A. Solution B was prepared by dissolving

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NH4VO3 (0.5740g) (Tianjin Tianhe Chemical Reagent Factory, AR grade) and citric acid monohydrate (4.0114g) (Tianjin Beifang Tianyi Chemical Reagent Factory, AR grade) in 24 cm3 deionized water. Solution B was then added dropwise into solution A under stirring at room temperature followed by aging at 50 oC, drying at 110 oC for 20 h and calcining at 500 oC for 4 h. The vanadium loading per unit mass of the SiO2 matrix was 0.98mmol/g.

2.2. Catalyst Characterization Diffuse reflectance UV-vis spectra were recorded in the range of 200-900 nm on

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ACCEPTED MANUSCRIPT a JASCO Corp V-550 spectrophotometer. The ESR spectrometer was recorded at room temperature with a Bruker A 200 spectrometer. The UV-vis spectra of H2O2

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formed in the reaction solution were measured by a Shimazu Model UV-240 spectrophotometer. Vanadium content was determined by inductively coupled plasma

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(ICP) on a Perkin Elmer Plasma 2000. 2.3. Catalytic Tests

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The catalytic experiments were carried out in a 75 ml steel autoclave. The optimized reaction conditions were as follows: the catalyst (0.1 g), 10 cm3 of aqueous acetic acid solvent, 1 cm3 of benzene (11.3 m mol) and reductant (3

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mmol) were added to the reactor and stirred by a magnetic stirrer for 10 h under

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1 MPa of O2. After the reaction, the products were analyzed using Agilent-6890 gas chromatograph equipped with an FID using HP-5 capillary column (30

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m×0.32 mm×0.25 μ m) and N2 as the carrier gas.

3. Results and Discussion

3.1. Influence of temperature on hydroxylation of benzene We assessed the reductants at different reaction temperatures with V/SiO2 as a catalyst, and the results are summarized in Table 1. Each reductant corresponds to two temperatures. “a” is the temperature of reaching the maximum phenol yield, and “b” is the onset temperature range of hydroxylation of benzene. Beginning at 20 oC, ascorbic acid was the only one found to function

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ACCEPTED MANUSCRIPT as an effective reductant, and the phenol yield was 6.2% with selectivity of 100%. At the same temperature (20 oC), no activity was observed with other

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reductants investigated. When increasing the temperature from 20 to 40 oC, the onset of the catalytic reaction was observed using hydroquinone as the reductant.

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However, benzoquinone was found to be the only product and 2.3% benzene conversion was detected at 40 oC. Also, with benzoquinone at 40 oC, similar

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phenomenon was observed. Further increasing temperature from 40 to 60 oC, with pyrocatechol as the reductant the formation of phenol was observed. The phenol selectivity was 67.5% and remaining products were benzoquinone

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(5.5%), hydroquinone (7.8%) and other products (19.2%) at 60 oC. Using oxalic

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acid or glucose as the reductant, the direct oxygenation of benzene began in higher temperature range. The yields of phenol were 3.3% using oxalic acid at

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90 oC and 4.9% using glucose at 100 oC, respectively. Hence, ascorbic acid is the

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best reductant at lower temperatures. With each reductant the phenol yield gradually increased with increasing temperature. The maximum phenol yields were reached at different temperatures (Table1). The maximum phenol yield decreased in the following order: hydroquinone (120 oC) > ascorbic acid (90 oC) > oxalic acid (140 oC) > benzoquinone (120 oC) > pyrocatechol (120 oC) > glucose (120 oC). Thus, hydroquinone is more effective reductant at higher reaction temperatures. This suggests that elevated reaction temperature is presumably required to show the reducing capacity of reductant. Without the reductant the V/SiO2 showed no activity for phenol formation, indicating

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ACCEPTED MANUSCRIPT that the presence of the reductant is essential. Benzoquinone is also an effective reductant (Figure 1A). The controlled

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experiment indicated that benzoquinone can react with V5+ from the V/SiO2 form hydroquinone and V4+ at > 80 oC. Benzoquinone and hydroquinone may be

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transformed into each other in the catalyst system (Figure 1B). Similar phenomenon was also observed in other catalyst systems [9]. With either of reductants the catalyst

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system showed the similar activity and 100% phenol selectivity at 120oC. The percentage of leaching of V was obviously different with different reductants (Table 2). For example, with hydroquinone as the reductant the

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percentage of the leaching of V decreased from 88.7% at 40 oC to 32.5% at 120 oC.

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The difference in leaching of V may be attributed to the difference in the nature of reductant, such as coordination to the vanadium center and the reducing capacity.

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Effect of the leaching of vanadium species on the yield of phenol was

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investigated using the filtrate and the spend catalyst, respectively, at optimization temperature corresponding to each reductant. With the filtrates, yields of phenol were 1.8% for hydroquinone, 2.5% for ascorbic acid, 1.9% for oxalic acid, 1.3% for pyrocatechol and 2.4% for glucose, respectively. Furthermore, unsupported V2O5 (NH4VO3) was also used as the catalyst. Yields of phenol were 1.5%-5% (5% for ascorbic acid) using the five reductants mentioned above. Using the spend V/SiO2, yields of phenol using hydroquinone, ascorbic acid, oxalic acid, pyrocatechol and glucose as the reductant were 9.6%, 7.5%, 5.2%, 7.8% and 6.1%, respectively. The decrease of phenol yields may be due to the V leaching. Obviously, the

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ACCEPTED MANUSCRIPT heterogeneous catalysis plays a major role in the reaction.

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3.2. Characterization of catalyst

The colorimetric reaction was performed in a glass flask. A mixture of V/SiO2,

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reductant, benzene, and aqueous acetic acid was heated under a N2 atmosphere and the temperature was increased from 20 to 150 oC. With ascorbic acid as the reductant,

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the surface of the V/SiO2 in the solution at 20 oC gave a green color, corresponding to V4+ species [10]. With other reductants the surface of the V/SiO2 also showed green color at higher temperatures, i.e. 30, 60, 80 and 100 oC with hydroquinone,

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pyrocatechol, oxalic acid and glucose, respectively. The change in color caused by

each reductant.

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variation in temperature accords with the onset temperature range corresponding to

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The presence of V4+ species in the V/SiO2 can be confirmed by the diffuse

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reflectance UV-vis and ESR spectra. Taking hydroquinone as example, the results of the characterization of the fresh and hydroquinone-reduced V/SiO2 are shown in Figure 2 and 3. In Figure 2, for fresh V/SiO2, the bands in the range of 250-300 nm should be assigned to charge transfer transitions involving oxygen and vanadium (V) in tetrahedral coordination, present as isolated species [11]. The absorption band in the region of 333-500 nm is associated with the lower-energy charge transfer of O-V5+ electron transfer for octahedral coordination [12]. When fresh V/SiO2 was reduced in hydroquinone at 40 oC for 2h, two bands at around 250 and 282 nm are observed. The band at 250 nm was assigned to an isolated tetrahedral V 5+ [11]. The

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ACCEPTED MANUSCRIPT band at 282 nm is attributed to the lower-energy change transfer of O-V4+electron transfer [13]. For the fresh V/SiO2, the eight-line spectrum with very low intensity

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was observed (Figure 3), which is attributed to vanadyl VIVO (d1) species [6, 14]. After reduction, the signal becomes well-resolved and the intensity of the signal

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increases, which indicates the V4+ complexes appear to be isolated and highly dispersed [15]. Thus, the vanadium species on the surface of the V/SiO2 are highly

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dispersed.

To gain insight into the reaction mechanism, the formation of H2O2 was directly measured via UV-vis spectroscopy of the reaction solution with different reductants.

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The results in Figure 4 indicated that the appearance of an absorption peak at ca.220

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nm with each reductant (curves b-f), this absorption peak based on H2O2 [16]. Without reductant, the absorption peak at 220 nm was silent (curve g). Therefore, the present

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V/SiO2 catalytic system using both O2 and reductant is considered to proceed via a

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reaction scheme similar to a Fenton type reaction [16-18]: V5+ species in the V/SiO2 was reduced to V4+ species by reductant. H2O2 was formed by the reaction of the V4+species with O2 and a proton. Hydroxyl radicals produced by the reaction of H2O2 with the V4+ species and protons are considered to participate in the phenol formation.

4. Conclusions In spite of the differences in the reduction properties, all the tested reductants were found to exhibit excellent reactivity for hydroxylation of benzene at the selective temperature. It was suggested that screening reductant should be carried out

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ACCEPTED MANUSCRIPT at different temperatures. With each reductant the selectivity to phenol can be up to 100% corresponding to the maximum benzene conversion. Also, benzoquinone is also

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an effective reductant, and benzoquinone and hydroquinone may be transformed into each other. With each reductant, H2O2 was formed and hydroxylation scheme was

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similar to a Fenton type reaction.

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Acknowledgement

This work was supported by the National Natural Science Foundation of China

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(21173110)

References

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[1] H. Yang, J. Q. Chen, J. Li, Y. Lv, S. Gao, Applied Catalysis A: General 415 (2012)

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22-28.

[2] P. P. Zhao, L. Yan, J. Wang, Chemical Engineering Journal 204 (2012) 72-78.

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[3] A. J. J. Koekkoek, H. C. Xin, Q. H. Yang, C. Li, E. J. M. Hensen, Microporous and Mesoporous Materials 145 (2011) 172-181. [4] S. Udenfriend, C. T. Clark, J. Axelrod, B. B. Brodie Journal of Biological Chemistry, 208 (1954) 731-739. [5] Y. Masumoto, R. Hamada, K. Yokota, S. Nishiyama, S.Tsuruya Journal of Molecular Catalysis A: Chemical

184 (2002) 215-222.

[6] E. Battistel, R.Tassinari, M. Fornaroli, L. Bonoldi Journal of Molecular Catalysis A: Chemical 202(2003) 107-115. [7] T. Miyahara, H. Kanzaki, R. Hamada, S. Kuroiwa, S. Nishiyama, S. Tsuruya, Journal of Molecular Catalysis A: Chemical 176 (2001) 141-150. [8] H. Q. Ge, Y. Leng, C. J. Zhou, J. Wang, Catalysis Letters 124 (2008) 324-329. [9] A. Hanyu, E. Takezawa, S. Sakaguchi, Y. Ishii, Tetrahedron Letters 39 (1998) 9

ACCEPTED MANUSCRIPT 5557-5560. [10] K. Lemke, H. Ehrich, U. Lohse, H. Berndt, K. Jähnisch, Applied Catalysis A:

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General 243 (2003) 41-51.

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[11] S. Dzwigaj, M. Matsuoka, M.Anpo, M. Che, The Journal of Physical Chemistry.

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B 104 (2000) 6012-6020.

[12] Z. H. Luan, J. Xu, H.Y. He, J. Klinowski, L. Kevan, The Journal of Physical

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Chemistry 100 (1996) 19595-19602.

[13] G. Centi, S. Perathoner, F. Trifiro, A. Aboukais, C. F. Aissi, M. Guelton, The Journal of Physical Chemistry 96 (1992) 2617-2629. [14] C. W. Lee, W. J. Lee, Y. K. park, S. E. park, Catalysis Today 61 (2000) 137-141.

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[15] C. H. Lee, T. S. Lin, C. Y. Mou, The Journal of Physical Chemistry C 111 (2007) 3873-3882.

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[16] M. Ishida, Y. Masumoto, R. Hamada, S. Nishiyama, S. Tsuruya, M. Masai,

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Journal of the Chemical Society, Perkin Transactions 2 (1999) 847-853. [17] X. H. Gao, J. Xu, Catalysis Letters 111 (2006) 203-205.

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[18] A. Kunai, S. Hata, S. Ito, K. Sasaki, Journal of the American Chemical Society 108 (1986) 6012-6016.

List of Tables

Table 1 Effect of various reductants on hydroxylation of benzene over V/SiO2 a

the temperature of reaching the maximum phenol yield

b

the onset temperature range of hydroxylation of benzene

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benzoquinone selectivity is 100%

Reaction conditions: 0.1 g V/SiO2 , 1 ml benzene, 3 m mol reductant, 7 ml acetic

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acid, 3 ml H2O, 1 MPa O2 , and 10 h. All of oxidation reactions were carried out

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with fresh catalyst each time.

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Table 2 Influence of reaction temperature on the percentage of V leaching Reaction conditions are the same as table 1

The percentage of V leaching equals the difference of V contents between fresh and

List of Figures

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spent V/SiO2 divided by V content of fresh V/SiO2.

Figure 1 Effect of benzoquinone (A) or hydroquinone (B) as the reductant on the oxidation of benzene to phenol over V/SiO2 a. Selectivity to phenol (%) b. Conversion of benzene (%) c. Selectivity to benzoquinone (%) Reaction conditions are the same as table 1

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ACCEPTED MANUSCRIPT Figure 2 Diffuse reflectance UV-vis spectra of (a) fresh V/SiO2 ； (b)

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hydroquinone–reduced V/SiO2

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Figure 3 ESR spectra of (a) fresh V/SiO2；(b) hydroquinone–reduced V/SiO2

Figure 4 UV-vis spectra of H2O2 formed from the reaction solution with

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different reductants

a. aqueous solution of H2O2 (8×10-6 mol/l)

b. glucose ; c. pyrocatechol ; d. hydroquinone； e. ascorbic acid;

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f. oxalic acid ;

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g. without reductant

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Reaction conditions are the same as table 1

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80

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80

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a

60

60

40

40

20

20

b

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0 40

60

Conversion of benzene (%)

100

A

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Selectivity to phenol(%)

100

80

100

0

120

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Temperature ( C)

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60

60

40

40

a

20

b

20

0

Conversion of benzene

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80

c

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Selectivity (%)

80

100

B

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100

0 40

60

80

100

120

140

o

Temperature ( C)

Figure1 Effect of benzoquinone (A) or hydroquinone (B) as the reductant on the oxidation of benzene to phenol over V/SiO2 a. Selectivity to phenol (%);

b. Conversion of benzene (%)；

c. Selectivity to benzoquinone (%) Reaction conditions are the same as table 1

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1.0

(a)

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(b) 0.6

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Absorbance

0.8

0.4

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400

500

600

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800

900

Wavelength (nm)

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0.0 200

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0.2

Figure 2 Diffuse reflectance UV-vis spectra of (a) fresh V/SiO2；(b) hydroquinone-reduced V/SiO2.

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(a)

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(b)

2000

3000

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5000

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Gauss

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Figure 3 ESR spectra of (a) fresh V/SiO2；(b) hydroquinone-reduced

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V/SiO2.

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a g

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Absorbance

b c

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d e

250

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350

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wavelength(nm)

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200

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f

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Figure 4 UV-vis spectra of H2O2 formed from the reaction solution with different reductants a. aqueous solution of H2O2 (8×10-6 mol/L) b. glucose ; c. pyrocatechol ; d. hydroquinone e. ascorbic acid; f. oxalic acid ; g. without reductant Reaction conditions are the same as table 1

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Table 1

Ascorbic acid

40 (20-40)b

Hydroquinone

Phenol yield (%) 6.2 13.7 0

100 0.0c

14.3 0

100 67.5

10.0 9.0

100 100

9.7 3.3

100 100

10.1 4.9

100

9.5

the onset temperature range of hydroxylation of benzene

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c

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120 a 14.3 40 11.8 b Benzoquinone (20-40) 120 a 10.0 60 13.4 Pyrocatechol (40-60)b 120 a 9.7 90 3.3 b Oxalic acid (60-90) 140 a 10.1 100 4.9 b Glucose (90-100) 120 a 9.5 a the temperature of reaching the maximum phenol yield b

Phenol selectivity (%) 100 100 0.0c

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Reductant

Benzene conversion (%) 6.2 13.7 2.3

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Reaction temperature (oC) 20 90a

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Effect of various reductants on hydroxylation of benzene over V/SiO2

benzoquinone selectivity is 100%

Reaction conditions: 0.1 g V/SiO2 (5 wt%), 1 ml benzene, 3 m mol reducing agent, 7 ml acetic acid, 3 ml H2O, 1 MPa O2 , and 10 h. All of oxidation reactions were carried out with fresh catalyst each time.

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ACCEPTED MANUSCRIPT Table 2 Influence of reaction temperature on the percentage of V leaching Reaction temperature (oC)

Leaching of V (%)

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Reductant

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77.4

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Ascorbic acid 90

Hydroquinone 120

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60

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40

Pyrocatechol

32.5 51.0 19.5

90

89.3

140

30.6

100

41.6

120

87.1

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Glucose

88.7

120

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Oxalic acid

63.4

Reaction conditions are the same as table 1.

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The percentage of V leaching equals the difference of V contents between fresh and

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spent V/SiO2 divided by V content of fresh V/SiO2.

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights 1. V/SiO2 with highly dispersed vanadium species was prepared by sol-gel process

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2. Studying the effect of temperature on benzene oxidation with O2 over V/SiO2

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3. Reducing capacity of reductant can be showed by changing temperature 4. Both of reductants benzoquinone and hydroquinone can be interconvertible

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5. Presenting 100% selectivity to phenol with each reductant

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