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Molybdovanadophosphoric anion ionic liquid as a reusable catalyst for solvent-free benzene oxidation to phenol by H2O2
Catalysis Communications 58 (2015) 215–218
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Molybdovanadophosphoric anion ionic liquid as a reusable catalyst for solvent-free benzene oxidation to phenol by H2O2 Chengyuan Yuan ⁎, Xionghou Gao, Zhishuang Pan, Xueli Li, Zhengguo Tan Petrochemical Research Institute, PetroChina, Beijing 100195, China
a r t i c l e
i n f o
Article history: Received 17 June 2014 Received in revised form 11 July 2014 Accepted 2 August 2014 Keywords: Molybdovanadophosphoric Oxidation Benzene Phenol Ionic liquid H2O2
a b s t r a c t Molybdovanadophosphoric anion (H2PMo11VO2− 40 ) room temperature ionic liquid was prepared and characterized by the methods of FT-IR and TG. Then, this ionic liquid was employed as catalyst for the oxidation of benzene to phenol by H2O2 without using solvent. The homogeneous emulsion formed by the catalyst and substrates during the reaction facilitated the catalytic process. After reaction, the catalyst could self-precipitate from the reaction media and proved to be recyclable. © 2014 Elsevier B.V. All rights reserved.
1. Introduction As an important chemical material, phenol is widely used for producing phenol resins, bisphenol-A, dyes, antioxidation agents and pharmaceuticals [1–5]. Traditionally, phenol is produced by cumene process which accounts for over 90% phenol production in the word. However, this method is restricted by its high energy consuming, less efficient, co-product acetone, and causes a lot of environmental problems [6]. Therefore, in recent years, many attempts have been made for direct oxidation of benzene to phenol using environmentally friendly oxidants through a single step process instead of cumene process [7,8]. Among the reported processes, direct oxidation of benzene to phenol with H2O2 as oxidant is considered as an attractive way for its facilitated handling, atom economy, as well as the fact that water is the only byproduct. V-based catalysts have been proved to be effective for benzene oxidation to phenol by H2O2 [9,10]. Up to now, various V-based catalysts such as (C5H8O2)2VO [11], [(CH3)4N]4PMo11VO40 [12], H5PMo10V2O40/SBA-15 [13], VOx/Clay [14] and VS-1 molecular sieve [15] have been made and used for direct benzene oxidation to phenol using H2O2 as oxidant. However, such processes mostly suffer from two major issues which severely restrict their applications in practice: first, the low phenol yield caused by over oxidation of phenol to various by-products; and second, the use of water-soluble solvents (such as methanol, acetic acid and acetonitrile) to blend organic benzene and aqueous H2O2, which ⁎ Corresponding author. Tel.: +86 931 7981621. E-mail address: [email protected] (C. Yuan).
http://dx.doi.org/10.1016/j.catcom.2014.08.004 1566-7367/© 2014 Elsevier B.V. All rights reserved.
inevitably increase the difficulty of the product separation. Therefore, how to realize direct benzene oxidation to phenol with high phenol yield by H2O2 in the condition of no solvent is of great significance. In recent years, ionic liquids (ILs) have attained increasing interest, especially in the area of catalytic reaction. As catalysts, ionic liquids have been successfully used for different reactions [16–19]. Among them, heteropolyanion ILs have attracted much attention for their unique acidity and redox property [20–22], and found their applications in different catalytic reactions such as esterification [23,24], epoxidation [25], Prins reaction [26] and Aldol reaction [27,28]. Qiao et al [29] prepared novel N-dodecylimidazolium peroxotungstate room temperature IL, and used it as catalyst for epoxidation of cyclooctene with H2O2. After reaction, the catalyst was found to be self-precipitating, which made the recovery and reuse of the catalyst very convenient. Wang et al [30] reported a Keggin-type phosphotungstate-fuctionalized IL. With ILs of [Bpy]BF4 and [Dopy]BF4 as solvents, it could effectively catalyze alkene epoxidation by aqueous H2O2. Furthermore, polyoxometalate-based IL was synthesized by Li et al. [31], and employed as catalyst for esterification of various alcohols with acetic acid. This IL catalyst showed high activity and excellent esters yields. During the reaction, the IL catalyst and substrates formed a homogenous emulsion system which could be broken by addition of weakly polar organic solvents to facilitate the catalyst separation after reaction. Though ILs have been used as solvents for benzene oxidation to phenol by H2O2 [32], to the best of our knowledge, there have been no reports on benzene oxidation to benzene using ILs as catalysts. Therefore, inspired by the above reports, in this paper, molybdovanadophosphoric anion ionic liquid was synthesized and used as highly active and reusable
216
C. Yuan et al. / Catalysis Communications 58 (2015) 215–218
catalyst for direct benzene oxidation to phenol with aqueous H2O2 in the condition of not using any organic solvents. As far as we know, there have been no similar reports for the moment. 2. Experimental Na2HPO4·12H2O, NaVO3·2H2O, Na2MoO4·2H2O, H2SO4, Pyridine, SOCl2, Polyethylene glycol monomethyl ether, 1-methylimidazole, benzene and 30 wt.% H2O2 were of analytical grade and were used as received. 2.1. Catalysts preparation H4PMo11O40 was prepared based on the report by Tsigdino et al. [33] with minor modifications. 7.16 g (20 mmol) of Na2HPO4·12H2O and 3.16 g (20 mmol) of NaVO3·2H2O were dissolved in 80 mL of water at room temperature. 2 mL of concentrated H2SO4 was added and the mixture was acidified to a red color, 53.2 g (220 mmol) of Na2MoO4·2H2O was dissolved in 80 mL of water and added to this mixture. Then, 34 mL of concentrated H2SO4 was added to the solution. After vigorous stirring of the mixture for 1 h, 160 mL of ethyl ether was added to extract the heteropoly acid. The preparation process of molybdovanadophosphoric anion ionic liquid catalyst is shown in Scheme 1. In step 1, 5 mmol polyethylene glycol monomethyl ether (mPEG750) and 5 mmol pyridine (acid-binding agent) were dissolved in 20 ml toluene. Then 10 mmol SOCl2 was added into the mixture dropwise. After that, the resulting mixture was refluxed at 110 °C for 8 h under Ar protection. After cooling to room temperature, the obtained mixture was filtrated to remove the insoluble salts. After evaporating toluene by vacuum, a pale yellow solid designated as mPEG-Cl was obtained. In step 2, mPEG-Cl obtained in step 1 and 10 mmol 1-methylimidazole were mixed in an autoclave, then sealed and stirred at 90 °C under 1.5 MPa Ar protection for 72 h. The obtained mixture was treated with ether adequately to get rid of excess 1-methylimidazole. A yellow solid was obtained and designated as [MIMmPEG]Cl. In step 3, 2 mmol [MIMmPEG]Cl was heated to 80 °C, then a solution of H4PMo11O40 (1 mmol) was added very slowly by drop. The resulting mixture was stirred at 80 °C for 24 h to make the ionic exchange completed. Finally, a dark brown viscous liquid was obtained and named as [MIMmPEG]PMoV. 2.2. Catalysts characterization The Fourier transform infrared spectroscopy (FT-IR) measurements were carried out with a Bruker IFS 120HR FT-IR spectrometer. Thermal gravimetric analysis (TGA) data was obtained using a Diamond TG/DTA/ DSC thermal analyzer.
2.3. Catalytic tests A typical procedure of the direct benzene oxidation to phenol with 30 wt.% H2O2 was as follows: A mixture of benzene (10 mmol, 0.9 ml), [MIMmPEG]PMoV (0.2 g) and 30 wt.% H2O2 (2.5 ml, 25 mmol H2O2) was stirred at 60 °C for 5 h. After reaction, the catalyst could self-precipitate from the system and be used for recycle. The unreacted benzene and products were extracted with ethyl acetate for further qualitative analysis and quantitative analysis. Qualitative analysis was carried out with a HP 6890/5973 GCMS. Quantitative analysis was carried out with an Agilent 6820 GC equipped with a FID using internal standard method. 3. Results and discussion 3.1. Characterization The FT-IR spectra of mPEG-Cl, [MIMmPEG]Cl, [MIMmPEG]PMoV and used [MIMmPEG]PMoV are illustrated in Fig. 1. It can be seen that there are four strong adsorption bands at around of 1070, 1215, 1270 and 1420 cm−1 for all the four samples, which can be attributed to the stretching and bending vibrations of C\O\C in mPEG molecules. Compared to the sample of mPEG-Cl, a new band near 1540 cm−1 is observed for [MIMmPEG]Cl, [MIMmPEG]PMoV and used [MIMmPEG] PMoV, which can be assigned as the C_N stretching vibration in 1-methylimidazole ring [34], confirming the successful linkage of 1-methylimidazole with mPEG. Three characteristic bands of H4PMo11O40 are found around 865, 950 and 1045 cm−1 for the sample of [MIMmPEG]PMoV [35], indicating the ion exchange of Cl− 1 with PMoV heteropolyanion. As compared with the fresh [MIMmPEG] PMoV, there is no notable difference in the IR spectrum of the used [MIMmPEG]PMoV after six recycles, indicating the highly chemical and structural stability of [MIMmPEG]PMoV catalyst. Fig. 2 shows the TG curves for [MIMmPEG]Cl and [MIMmPEG]PMoV. As it can be seen, the sample of [MIMmPEG]Cl gave a sharp weight loss process at around of 400 °C, which could be attributed to the rapid decomposition of the organic molecules of [MIMmPEG]Cl. In contrast with [MIMmPEG]Cl, the weight loss of the sample [MIMmPEG]PMoV at around of 400 °C was less than that of [MIMmPEG]Cl, which was because of the production of metal oxides from the decomposition rate of inorganic PMoV heteropolyanion, indicating the successful ion exchange of PMoV heteropolyanion. 3.2. Catalytic activity Table 1 shows the results of solvent-free benzene oxidation to phenol with H2O2 under various reaction conditions. As it is shown, in the
Scheme 1. Preparation IL of [MIMmPEG]PMoV.
C. Yuan et al. / Catalysis Communications 58 (2015) 215–218
1420
1540
Table 1 Oxidation of benzene to phenol with H2O2 under various conditionsa.
mPEG-Cl
1215 1270 1070
Transmittance (%)
[MIMmPEG]Cl [MIMmPEG]PMoV
Used [MIMmPEG]PMoV
950
Entry
Catalyst
Yield (%)b
1 2 3
Blank [MIMmPEG]Cl [MIMmPEG]PMoV
No reaction No reaction 1st 65 2nd 64 3rd 62 4th 62 5th 61
a Reaction conditions: 10 mmol benzene, 0.2 g catalyst, 2.5 ml of 30 wt.% H2O2, 60 °C, and 5 h. b Determined by GC using an internal standard technique. Side product was benzoquinone.
1045
865
800
217
1320
1000
1200
1400
1600
1800
2000
2200
A
2400
B
C
Wavenumber (cm-1) Fig. 1. FT-IR spectra of mPEG-Cl, [MIMmPEG]Cl, [MIMmPEG]PMoV and used [MIMmPEG] PMoV.
absence of catalyst (entry 1), no reaction happened. It gave the same result as that of no catalyst with the [MIMmPEG]Cl as catalyst (entry 2), indicating [MIMmPEG]Cl was not active for this reaction. When using IL of [MIMmPEG]PMoV as catalyst, it gave 65% phenol yield which was higher than that of previous reports (entry 3), confirming that PMoV heteropolyanion in [MIMmPEG]PMoV was the active component. Furthermore, a series of catalytic cycles were run to investigate the performance of [MIMmPEG]PMoV in recycling. As it is shown, our catalyst could be reused for five times at least with slight loss of phenol yield (entry 3). ICP analysis of the solution after reaction confirmed that the PMoV heteropolyanion contents were negligible, indicating that no leaching of active component occurred during the reaction. It should be noted that our reaction system could change from three phases to a homogeneous emulsion and then to all the catalysts selfprecipitating at the end of the reaction. As it shown in Fig. 3, before reaction, benzene, 30 wt.% H2O2 and catalyst were separated respectively. When rising to the reaction temperature, with the surfactant function of the amphipathic catalyst, the reaction system changed to a homogenous emulsion system, which extremely accelerated the catalytic process. At the end of reaction, after cooling to the room temperature, the catalyst could self-precipitate from the system completely, which greatly facilitated the recovery and reuse of the catalyst.
Fig. 3. Different stages of benzene oxidation to phenol with [MIMmPEG]PMoV as catalyst. (A) Before reaction; (B) during reaction; and (C) end of reaction.
4. Conclusions In this paper, a novel molybdovanadophosphoric anion ionic liquid catalyst has been successfully synthesized and used for the solventfree benzene oxidation to phenol by H2O2. This catalyst simultaneously possesses high activity of homogeneous catalyst and recyclability of heterogeneous catalyst, making the catalytic process more benign from the commercial point of view. Moreover, under the surfactant function of catalyst, there is no need of any organic solvents for the catalytic process, which makes the process environmentally friendly. The promising results encourage us to extend this catalyst to other catalytic oxidation reactions with H2O2 as oxidant. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2014.08.004. This data include MOL files and InChiKeys of the most important compounds described in this article.
Weight Loss
References
（2）
（1）
0
200
400
600
Temperature (oC) Fig. 2. TG curves of (1) [MIMmPEG]Cl and (2) [MIMmPEG]PMoV.
800
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Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Molybdovanadophosphoric anion ionic liquid as a reusable catalyst for solvent-free benzene oxidation to phenol by H2O2 Chengyuan Yuan ⁎, Xionghou Gao, Zhishuang Pan, Xueli Li, Zhengguo Tan Petrochemical Research Institute, PetroChina, Beijing 100195, China
a r t i c l e
i n f o
Article history: Received 17 June 2014 Received in revised form 11 July 2014 Accepted 2 August 2014 Keywords: Molybdovanadophosphoric Oxidation Benzene Phenol Ionic liquid H2O2
a b s t r a c t Molybdovanadophosphoric anion (H2PMo11VO2− 40 ) room temperature ionic liquid was prepared and characterized by the methods of FT-IR and TG. Then, this ionic liquid was employed as catalyst for the oxidation of benzene to phenol by H2O2 without using solvent. The homogeneous emulsion formed by the catalyst and substrates during the reaction facilitated the catalytic process. After reaction, the catalyst could self-precipitate from the reaction media and proved to be recyclable. © 2014 Elsevier B.V. All rights reserved.
1. Introduction As an important chemical material, phenol is widely used for producing phenol resins, bisphenol-A, dyes, antioxidation agents and pharmaceuticals [1–5]. Traditionally, phenol is produced by cumene process which accounts for over 90% phenol production in the word. However, this method is restricted by its high energy consuming, less efficient, co-product acetone, and causes a lot of environmental problems [6]. Therefore, in recent years, many attempts have been made for direct oxidation of benzene to phenol using environmentally friendly oxidants through a single step process instead of cumene process [7,8]. Among the reported processes, direct oxidation of benzene to phenol with H2O2 as oxidant is considered as an attractive way for its facilitated handling, atom economy, as well as the fact that water is the only byproduct. V-based catalysts have been proved to be effective for benzene oxidation to phenol by H2O2 [9,10]. Up to now, various V-based catalysts such as (C5H8O2)2VO [11], [(CH3)4N]4PMo11VO40 [12], H5PMo10V2O40/SBA-15 [13], VOx/Clay [14] and VS-1 molecular sieve [15] have been made and used for direct benzene oxidation to phenol using H2O2 as oxidant. However, such processes mostly suffer from two major issues which severely restrict their applications in practice: first, the low phenol yield caused by over oxidation of phenol to various by-products; and second, the use of water-soluble solvents (such as methanol, acetic acid and acetonitrile) to blend organic benzene and aqueous H2O2, which ⁎ Corresponding author. Tel.: +86 931 7981621. E-mail address: [email protected] (C. Yuan).
http://dx.doi.org/10.1016/j.catcom.2014.08.004 1566-7367/© 2014 Elsevier B.V. All rights reserved.
inevitably increase the difficulty of the product separation. Therefore, how to realize direct benzene oxidation to phenol with high phenol yield by H2O2 in the condition of no solvent is of great significance. In recent years, ionic liquids (ILs) have attained increasing interest, especially in the area of catalytic reaction. As catalysts, ionic liquids have been successfully used for different reactions [16–19]. Among them, heteropolyanion ILs have attracted much attention for their unique acidity and redox property [20–22], and found their applications in different catalytic reactions such as esterification [23,24], epoxidation [25], Prins reaction [26] and Aldol reaction [27,28]. Qiao et al [29] prepared novel N-dodecylimidazolium peroxotungstate room temperature IL, and used it as catalyst for epoxidation of cyclooctene with H2O2. After reaction, the catalyst was found to be self-precipitating, which made the recovery and reuse of the catalyst very convenient. Wang et al [30] reported a Keggin-type phosphotungstate-fuctionalized IL. With ILs of [Bpy]BF4 and [Dopy]BF4 as solvents, it could effectively catalyze alkene epoxidation by aqueous H2O2. Furthermore, polyoxometalate-based IL was synthesized by Li et al. [31], and employed as catalyst for esterification of various alcohols with acetic acid. This IL catalyst showed high activity and excellent esters yields. During the reaction, the IL catalyst and substrates formed a homogenous emulsion system which could be broken by addition of weakly polar organic solvents to facilitate the catalyst separation after reaction. Though ILs have been used as solvents for benzene oxidation to phenol by H2O2 [32], to the best of our knowledge, there have been no reports on benzene oxidation to benzene using ILs as catalysts. Therefore, inspired by the above reports, in this paper, molybdovanadophosphoric anion ionic liquid was synthesized and used as highly active and reusable
216
C. Yuan et al. / Catalysis Communications 58 (2015) 215–218
catalyst for direct benzene oxidation to phenol with aqueous H2O2 in the condition of not using any organic solvents. As far as we know, there have been no similar reports for the moment. 2. Experimental Na2HPO4·12H2O, NaVO3·2H2O, Na2MoO4·2H2O, H2SO4, Pyridine, SOCl2, Polyethylene glycol monomethyl ether, 1-methylimidazole, benzene and 30 wt.% H2O2 were of analytical grade and were used as received. 2.1. Catalysts preparation H4PMo11O40 was prepared based on the report by Tsigdino et al. [33] with minor modifications. 7.16 g (20 mmol) of Na2HPO4·12H2O and 3.16 g (20 mmol) of NaVO3·2H2O were dissolved in 80 mL of water at room temperature. 2 mL of concentrated H2SO4 was added and the mixture was acidified to a red color, 53.2 g (220 mmol) of Na2MoO4·2H2O was dissolved in 80 mL of water and added to this mixture. Then, 34 mL of concentrated H2SO4 was added to the solution. After vigorous stirring of the mixture for 1 h, 160 mL of ethyl ether was added to extract the heteropoly acid. The preparation process of molybdovanadophosphoric anion ionic liquid catalyst is shown in Scheme 1. In step 1, 5 mmol polyethylene glycol monomethyl ether (mPEG750) and 5 mmol pyridine (acid-binding agent) were dissolved in 20 ml toluene. Then 10 mmol SOCl2 was added into the mixture dropwise. After that, the resulting mixture was refluxed at 110 °C for 8 h under Ar protection. After cooling to room temperature, the obtained mixture was filtrated to remove the insoluble salts. After evaporating toluene by vacuum, a pale yellow solid designated as mPEG-Cl was obtained. In step 2, mPEG-Cl obtained in step 1 and 10 mmol 1-methylimidazole were mixed in an autoclave, then sealed and stirred at 90 °C under 1.5 MPa Ar protection for 72 h. The obtained mixture was treated with ether adequately to get rid of excess 1-methylimidazole. A yellow solid was obtained and designated as [MIMmPEG]Cl. In step 3, 2 mmol [MIMmPEG]Cl was heated to 80 °C, then a solution of H4PMo11O40 (1 mmol) was added very slowly by drop. The resulting mixture was stirred at 80 °C for 24 h to make the ionic exchange completed. Finally, a dark brown viscous liquid was obtained and named as [MIMmPEG]PMoV. 2.2. Catalysts characterization The Fourier transform infrared spectroscopy (FT-IR) measurements were carried out with a Bruker IFS 120HR FT-IR spectrometer. Thermal gravimetric analysis (TGA) data was obtained using a Diamond TG/DTA/ DSC thermal analyzer.
2.3. Catalytic tests A typical procedure of the direct benzene oxidation to phenol with 30 wt.% H2O2 was as follows: A mixture of benzene (10 mmol, 0.9 ml), [MIMmPEG]PMoV (0.2 g) and 30 wt.% H2O2 (2.5 ml, 25 mmol H2O2) was stirred at 60 °C for 5 h. After reaction, the catalyst could self-precipitate from the system and be used for recycle. The unreacted benzene and products were extracted with ethyl acetate for further qualitative analysis and quantitative analysis. Qualitative analysis was carried out with a HP 6890/5973 GCMS. Quantitative analysis was carried out with an Agilent 6820 GC equipped with a FID using internal standard method. 3. Results and discussion 3.1. Characterization The FT-IR spectra of mPEG-Cl, [MIMmPEG]Cl, [MIMmPEG]PMoV and used [MIMmPEG]PMoV are illustrated in Fig. 1. It can be seen that there are four strong adsorption bands at around of 1070, 1215, 1270 and 1420 cm−1 for all the four samples, which can be attributed to the stretching and bending vibrations of C\O\C in mPEG molecules. Compared to the sample of mPEG-Cl, a new band near 1540 cm−1 is observed for [MIMmPEG]Cl, [MIMmPEG]PMoV and used [MIMmPEG] PMoV, which can be assigned as the C_N stretching vibration in 1-methylimidazole ring [34], confirming the successful linkage of 1-methylimidazole with mPEG. Three characteristic bands of H4PMo11O40 are found around 865, 950 and 1045 cm−1 for the sample of [MIMmPEG]PMoV [35], indicating the ion exchange of Cl− 1 with PMoV heteropolyanion. As compared with the fresh [MIMmPEG] PMoV, there is no notable difference in the IR spectrum of the used [MIMmPEG]PMoV after six recycles, indicating the highly chemical and structural stability of [MIMmPEG]PMoV catalyst. Fig. 2 shows the TG curves for [MIMmPEG]Cl and [MIMmPEG]PMoV. As it can be seen, the sample of [MIMmPEG]Cl gave a sharp weight loss process at around of 400 °C, which could be attributed to the rapid decomposition of the organic molecules of [MIMmPEG]Cl. In contrast with [MIMmPEG]Cl, the weight loss of the sample [MIMmPEG]PMoV at around of 400 °C was less than that of [MIMmPEG]Cl, which was because of the production of metal oxides from the decomposition rate of inorganic PMoV heteropolyanion, indicating the successful ion exchange of PMoV heteropolyanion. 3.2. Catalytic activity Table 1 shows the results of solvent-free benzene oxidation to phenol with H2O2 under various reaction conditions. As it is shown, in the
Scheme 1. Preparation IL of [MIMmPEG]PMoV.
C. Yuan et al. / Catalysis Communications 58 (2015) 215–218
1420
1540
Table 1 Oxidation of benzene to phenol with H2O2 under various conditionsa.
mPEG-Cl
1215 1270 1070
Transmittance (%)
[MIMmPEG]Cl [MIMmPEG]PMoV
Used [MIMmPEG]PMoV
950
Entry
Catalyst
Yield (%)b
1 2 3
Blank [MIMmPEG]Cl [MIMmPEG]PMoV
No reaction No reaction 1st 65 2nd 64 3rd 62 4th 62 5th 61
a Reaction conditions: 10 mmol benzene, 0.2 g catalyst, 2.5 ml of 30 wt.% H2O2, 60 °C, and 5 h. b Determined by GC using an internal standard technique. Side product was benzoquinone.
1045
865
800
217
1320
1000
1200
1400
1600
1800
2000
2200
A
2400
B
C
Wavenumber (cm-1) Fig. 1. FT-IR spectra of mPEG-Cl, [MIMmPEG]Cl, [MIMmPEG]PMoV and used [MIMmPEG] PMoV.
absence of catalyst (entry 1), no reaction happened. It gave the same result as that of no catalyst with the [MIMmPEG]Cl as catalyst (entry 2), indicating [MIMmPEG]Cl was not active for this reaction. When using IL of [MIMmPEG]PMoV as catalyst, it gave 65% phenol yield which was higher than that of previous reports (entry 3), confirming that PMoV heteropolyanion in [MIMmPEG]PMoV was the active component. Furthermore, a series of catalytic cycles were run to investigate the performance of [MIMmPEG]PMoV in recycling. As it is shown, our catalyst could be reused for five times at least with slight loss of phenol yield (entry 3). ICP analysis of the solution after reaction confirmed that the PMoV heteropolyanion contents were negligible, indicating that no leaching of active component occurred during the reaction. It should be noted that our reaction system could change from three phases to a homogeneous emulsion and then to all the catalysts selfprecipitating at the end of the reaction. As it shown in Fig. 3, before reaction, benzene, 30 wt.% H2O2 and catalyst were separated respectively. When rising to the reaction temperature, with the surfactant function of the amphipathic catalyst, the reaction system changed to a homogenous emulsion system, which extremely accelerated the catalytic process. At the end of reaction, after cooling to the room temperature, the catalyst could self-precipitate from the system completely, which greatly facilitated the recovery and reuse of the catalyst.
Fig. 3. Different stages of benzene oxidation to phenol with [MIMmPEG]PMoV as catalyst. (A) Before reaction; (B) during reaction; and (C) end of reaction.
4. Conclusions In this paper, a novel molybdovanadophosphoric anion ionic liquid catalyst has been successfully synthesized and used for the solventfree benzene oxidation to phenol by H2O2. This catalyst simultaneously possesses high activity of homogeneous catalyst and recyclability of heterogeneous catalyst, making the catalytic process more benign from the commercial point of view. Moreover, under the surfactant function of catalyst, there is no need of any organic solvents for the catalytic process, which makes the process environmentally friendly. The promising results encourage us to extend this catalyst to other catalytic oxidation reactions with H2O2 as oxidant. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2014.08.004. This data include MOL files and InChiKeys of the most important compounds described in this article.
Weight Loss
References
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Temperature (oC) Fig. 2. TG curves of (1) [MIMmPEG]Cl and (2) [MIMmPEG]PMoV.
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