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Clicked graphene oxide as new support for the immobilization of peroxophosphotungstate: Efficient catalysts for the epoxidation of olefins
Accepted Manuscript Title: Clicked graphene oxide as new support for the immobilization of peroxophosphotungstate: Efficient catalysts for the epoxidation of olefins Authors: M. Masteri-Farahani, M. Modarres PII: DOI: Reference:
S0927-7757(17)30635-0 http://dx.doi.org/doi:10.1016/j.colsurfa.2017.06.073 COLSUA 21760
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
6-3-2017 23-6-2017 26-6-2017
Please cite this article as: M.Masteri-Farahani, M.Modarres, Clicked graphene oxide as new support for the immobilization of peroxophosphotungstate: Efficient catalysts for the epoxidation of olefins, Colloids and Surfaces A: Physicochemical and Engineering Aspectshttp://dx.doi.org/10.1016/j.colsurfa.2017.06.073 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Clicked graphene oxide as new support for the immobilization of peroxophosphotungstate: Efficient catalysts for the epoxidation of olefins M. Masteri-Farahani*, M. Modarres Faculty of Chemistry, Kharazmi University, Tehran, Islamic Republic of Iran
*Corresponding author. Tel.: +98 263 4551023; fax: +98 263 4551023.
E-mail address: [email protected] Graphical Abstract
Research highlights:
The surface of graphene oxide was modified with grafting and tethering methods via click reaction. Peroxopolyoxotungstate anions were immobilized on the surface of clicked graphene oxides. The prepared materials acted as active, selective, and reusable catalysts in the epoxidation of olefins with H2O2.
Abstract
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Peroxophosphotungstate (PW) anions were successfully immobilized on the surface of clicked graphene oxides and used as heterogeneous catalysts for the epoxidation of various olefins using H 2O2 as oxidant. Graphene oxide (GO) was functionalized with azide groups through both grafting and tethering methods. Then, supported ionic liquids were formed on the surface of GO through the Cu (I)-catalyzed azide-alkyne cycloaddition (CuAAC) click reaction. The clicked GO supported ionic liquids were used as appropriate supports for the immobilization of PW anions. Characterization of the prepared materials by various physicochemical methods showed that PW anions have been successfully immobilized on the surface of clicked GOs. Catalytic activities of these heterogeneous catalysts were evaluated in the epoxidation of olefins which showed that not only the catalysts are active, but also they could be reused several times without significant loss of their activities. Keywords: Click Chemistry; Heterogeneous Catalysis; Epoxidation; Peroxopolyoxotungstate; Graphene Oxide.
1. Introduction
Catalytic epoxidation of olefins is an important process in the synthetic chemistry as the produced epoxides are essential precursors for the production of fine chemicals, pharmaceuticals, and chemical intermediates [1-3]. Polyoxometalates (POMs) as effective catalysts have been extensively utilized for the epoxidation of olefins due to their high performance [4-6]. Among the POMs, tungsten-based ones have attracted great interest as green and environmental friendly epoxidation catalysts owing to high catalytic activities and excellent selectivities in the presence of H 2O2 as oxidant [7-11]. From green chemistry point of view, H2O2 is a beneficial oxidant because of its availability and cheapness compared to organic peroxides and peracids. Moreover, it has high active oxygen content and generates water as only by-product [12-14]. However, the use of POMs homogeneous catalysts is restricted owing to disadvantages such as high cost of recovery and separation from the reaction mixtures. Thus, various approaches have been used for heterogenizing these homogeneous catalysts on solid supports such as mesoporous silica [[15-19], metal-organic frameworks (MOFs) [20-22],
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functionalized polymers [23,24], and magnetic nanoparticles [25-30]. Recently, graphene oxide (GO) has been utilized as one of the most promising supports due to its remarkable properties such as two dimensional (2D) structure, large surface area, outstanding mechanical strength and the existence of many oxygen-containing functional groups on its basal planes and edges [31-38]. Mungse et al. and Verma et al. reported the immobilization of oxo-vanadium Schiff base complex on the surface of GO nanosheets for the oxidation of various alcohols, fatty acids and esters in the presence of tert-butyl hydroperoxide (TBHP) as oxidant which exhibited high catalytic activity comparable to their homogeneous counterpart [39,40]. Scheuermann et al. reported that Pd nanoparticles immobilized on GO showed high activity in the Suzuki-Miyaura coupling reaction along with little palladium leaching [41]. Su et al. reported the immobilization of transition metal (Fe2+, Co2+, VO2+, Cu2+) Schiff base complexes on the surface of GO as efficient and reusable catalysts for the epoxidation of styrene [42]. Click reactions, especially Cu (I)-catalyzed 1,3-dipolar azide-alkyne cycloaddition (CuAAC), have attracted great attention for surface functionalization owing to high selectivity, easy reaction conditions and the absence of side products [43,44]. While click chemistry approach has been used to functionalize solid supports such as graphene and carbon nanotube (CNT) [45-50], to the best of our knowledge, the immobilization of POMs on the surface of graphene oxide by using click chemistry has not been reported yet. Here, we report the design and characterization of two new heterogeneous catalysts. First, the surface of GO nanosheets was modified with azide groups through grafting and tethering approaches. Then, click reaction of azide-functionalized GOs with 3-ethynyl-1-methylimidazolium cations resulted in the covalent attachment of ionic liquids on the surface of GO nanosheets. Finally, the ionic liquid modified GO nanosheets were used to immobilize peroxophosphotungstate (PW) anions. Catalytic activities of both heterogeneous catalysts were studied in the epoxidation of various olefins with using H2O2 as oxidant. 2. Experimental
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2.1. Materials and methods The graphite powder was purchased from Sinchem Company. Other chemicals were purchased from Merck chemical company. All of the solvents were used after distillation. Fourier transform infrared (FT-IR) spectra were taken with a Perkin-Elmer Spectrum RXI FT-IR spectrometer. X-ray diffraction (XRD) analyses were carried out by PANalytical X’pert Pro diffractometer with Cu Kα (λ=0.154nm) radiation. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were recorded using ZEISS-DSM 960A microscope and Zeiss-EM10C-100 kV transmission electron microscope, respectively. The elemental compositions of the samples were measured with VARIAN VISTA MPX ICP-OES. Nitrogen content of the samples was determined by Perkin Elmer 2400 SERIES II CHN Analyzer. The Brunauer–Emmet–Teller (BET) surface areas of the materials (outgassed under high vacuum at 120°C) were determined using Microtrac Bel Corp instrument at liquid nitrogen temperature (-196°C). The progress of the catalytic experiments was controlled by Agilent 6890N gas chromatograph (GC) equipped with a HP-5 capillary column. 2.2. Preparation of the catalysts 2.2.1. Preparation and modification of GO with azide groups via grafting and tethering methods GO was prepared by improved Hummers approach [51]. GO-N3 was prepared through the grafting of azide groups on the surface of GO according to the literature [52]. On the other hand, in tethering method, GO (1 g) was dispersed in 40 ml toluene using ultrasonication. Then, (3chloropropyl)trimethoxysilane (3 mmol) was added and the reaction mixture was refluxed for 24 h under N2 atmosphere. The obtained solid (called GO-Cl) was washed with methanol to remove the excess of silylating agent and dried under vacuum at 80°C. Then, the prepared GO-Cl (1 g) was stirred in a solution of 2 mmol NaN3 in 100 ml dimethylformamide (DMF) for 8 h at 80°C. After cooling to room temperature, the prepared GO-pr-N3 was filtered and washed with DMF to remove the excess of NaN3.
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2.2.2. Preparation of 3-ethynyl-1-methylimidazolium bromide To a mixture of 1-methylimidazole (1 g) in 50 ml of acetonitrile, 1.9 ml of propargyl bromide was added dropwise and the mixture was refluxed for 24 h. Then, the solvent was removed by rotary evaporator to obtain a dark-red viscous liquid. 2.2.3. Immobilization of ionic liquids on GO with click reaction GO-N3 or GO-pr-N3 (1 g) was dispersed in a mixture of H 2O/DMF (1:1, 100 ml) by using ultrasonication for 30 min. Then, 3-ethynyl-1-methylimidazolium bromide (1.6 g), copper (II) sulfate (0.1 g) as catalyst, and sodium ascorbate (1.6 g) as reductant were added to the mixture and stirred at room temperature for 24 h. The products (GO-pr-IL and GO-IL) were separated with centrifugation, washed with excess of water, and dried at 80°C in a vacuum oven overnight. 2.2.4. Immobilization of peroxophosphotungstate (PW) anions on the surface of GO supported ionic liquids PW sodium salt was prepared according to the procedure of Ishii et al. [55]. For the immobilization of PW anions, the as-prepared PW anions (2 mmol) was added to a mixture of GO supported ionic liquid (1 g) in 50 ml deionized water and the mixture was stirred for 8 h at room temperature. The resulting solid was filtered and washed several times with distilled water and then dried in vacuum oven overnight. The two prepared heterogeneous catalysts by grafting and tethering methods are referred as GO-IL-PW and GO-pr-IL-PW, respectively. 2.3. Catalytic epoxidation of olefins In a typical catalytic run, a mixture of olefin (4 mmol) and catalyst (50 mg) in acetonitrile (5 ml) were placed in a 10 ml round-bottom flask equipped with a reflux condenser. After addition of H 2O2 30% (6 mmol), the mixture was refluxed under nitrogen atmosphere. The reaction progress was followed at various times (2, 4, 6, 8 and 24 h) by using gas chromatography. Finally, the catalyst was separated,
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washed with methanol, and vacuum dried at 80°C. The recovered catalyst was reused in the next epoxidation reactions in similar conditions. 3. Results and discussions 3.1. Preparation and characterization of the catalysts The synthetic pathway of GO-pr-IL-PW and GO-IL-PW catalysts is illustrated in Scheme 1. In tethering method, the GO was covalently modified through the reaction of (3-chloropropyl) trimethoxysilane with surface hydroxyl groups of GO. In the next step, the substitution reaction of chloro groups with azide ions leads to the formation of GO-pr-N3. Then, an imidazolium-based ionic liquid (IL) was immobilized on the surface of GO nanosheets through click reaction of the supported azide groups with pre-synthesized 3-ethynyl-1-methylimidazolium bromide. Eventually, PW species were immobilized through the electrostatic interaction with positively charged imidazolium-based groups on the surface of GO support to achieve GO-pr-IL-PW heterogeneous catalyst. On the other hand, in grafting approach, GO-N3 was prepared by ring opening of GO surface epoxy groups through the reaction with NaN3 [52]. In comparison with earlier reports [53,54], this method is simple and more efficient. CuAAC click reaction between 3-ethynyl-1-methylimidazolium bromide and supported azide groups resulted in the preparation of imidazolium-based ionic liquid on the surface of GO nanosheets and formation of GO-IL. Finally, PW species were attached electrostatically to the imidazolium-based groups to obtain GO-IL-PW heterogeneous catalyst. FT-IR spectra of pristine GO, GO supported ionic liquids with both methods, as well as corresponding catalysts are illustrated in Figure 1. In pristine GO, the bands at 3403, 1723, 1225, and 1055 cm-1 are assigned to the stretching vibrations of O-H, COOH, C-OH, and C-O (epoxy) groups, respectively, and reveal the presence of several oxygen-containing functional groups. The peak at 1612 cm-1 can be assigned to the aromatic C=C stretching vibrations in GO [52,56]. Modification of GO with azide groups in both methods can be confirmed by the appearance of new bands at ~2122 cm-1 due to the
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azide stretching vibrations. In addition, the bands in the range of 2850-2940 cm-1 can be attributed to the stretching vibrations of CH2 groups of alkyl chains in modified GO with tethering method. The loading amounts of azide groups on GO-pr-IL-PW and GO-IL-PW, as determined by elemental analysis, were found to be 1.2 and 0.71 mmol.g -1, respectively. The vibrational bands corresponding to the azide groups were disappeared after click reaction with 3-ethynyl-1-methylimidazolium bromide which confirmed their conversion to triazole rings and immobilization of imidazolium based ionic liquid on the surface of GO. In the FT-IR spectra of the catalysts, the bands appeared at around 1080, 979, and 812 cm-1 can be assigned to νP-O, νW=O, νW-O-O, respectively [8]. These observations clearly indicate that PW species have been successfully immobilized on the surface of GO nanosheets. Further evidence for the immobilization of PW species on the surface of GO was provided by inductively coupled plasma optical emission spectroscopy (ICP-OES) chemical analysis which reveals that the fresh GO-pr-IL-PW and GO-IL-PW catalysts include 0.032 and 0.019 mmol.g-1 PW, respectively (Table 1).
XRD analysis was used to study the structural changes of GO after the modification process (Figure 2). Pristine GO represented a sharp and strong diffraction peak at 2θ=11°, corresponding to (001) reflection plane, which indicates the oxidative conversion of graphite to graphene oxide nanosheets [39]. This diffraction peak was disappeared after reacting with PW species and very broad peaks appeared in higher angles which indicate the successful well-dispersion of PW species on the surface of GO nanosheets. The surface properties of the materials were investigated by nitrogen adsorption-desorption analysis. The N2 adsorption–desorption isotherms of pristine GO, GO-pr-IL-PW and GO-IL-PW are shown in Figure 3. All of the isotherms are of type IV with appearance of hysteresis loop which is characteristic of the mesoporous structure [42,57]. The BET surface area of pristine GO is 280 m2.g-1, while those of GO-pr-IL-PW and GO-IL-PW are 18 and 8 m2.g-1, respectively. The results illustrate that the surface
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area decreased due to the immobilization of PW species on the surface of GO nanosheets although the supported catalysts maintain the mesoporous characteristics. The surface morphology of GO was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) which are shown in Figure 4. According to SEM and TEM images, pure GO has a typical layered structure. It can be seen in TEM images that GO-pr-IL-PW and GO-IL-PW catalysts have some crumpled layered structures due to the presence of PW species between the layers. 3.2. Investigation of catalytic activities of the prepared catalysts The catalytic activities of the prepared catalysts were explored in the epoxidation of some olefins and allylic alcohols using H2O2 as oxidant under optimized conditions and the results are summarized in Figures 5 and 6. The results showed that both catalysts are highly active with high turnover frequency (TOF) values and selectively produce the desired epoxides. The high TOF values of the catalysts can be attributed to the role of produced ionic liquids as co-catalyst according to the earlier reports [58,59]. It can be seen that the conversions in the presence of GO-pr-IL-PW catalyst are more than GO-IL-PW which may be due to its higher PW loading. Moreover, the TOF values are higher in the case of GO-pr-ILPW catalyst. This observation can be explained by considering the more accessibility of the catalytic active centers in the GO-pr-IL-PW catalyst. The longer spacer group in this catalyst allows the substrates to reach the PW species more readily. In accordance with our previous results [15,16], the olefins with higher electron density on their C=C bonds show higher reactivities than terminal ones which confirm the electrophilic attack of the active centre of the catalyst to the olefin. As can be seen in the proposed mechanism of epoxidation reaction (Figure 7), the catalytic active site in the catalyst has electrophilic character and the mechanism of oxygen transfer to the olefin is electrophilic attack of electron deficient oxygen to the olefin double
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bond as nucleophilic species. So, more electron density of the double bond accelerates the epoxidation reaction. On the other hand, the pristine GO showed negligible cyclooctene conversion (2%) under the same reaction conditions which indicated the critical role of the catalysts. 3.3. Investigation of the recyclabilities of the catalysts After each run, the catalyst was recovered for reusing in the next reaction. The recyclabilities and stabilities of GO-pr-IL-PW and GO-IL-PW catalysts were examined in the epoxidation of cyclooctene under similar reaction conditions. Both catalysts were recycled five times and the results showed that they could be reused without significant loss of activity and selectivity (Figure 8). Moreover, in order to clarify the plausible contribution of the leached PW species, in the catalytic epoxidation of cyclooctene, the catalyst was filtered at the reaction temperature after 8h and the filtrate was allowed to react further 16 h. The results showed that in this time interval the conversion of cyclooctene increased only 2% which shows that there is no significant PW species in the filtrate and the catalytic process is truly heterogeneous. These observations show the high stabilities and recyclabilities of the prepared catalysts.
Conclusion In summary, we demonstrated that PW species can be successfully immobilized on GO supported ionic liquid through tethering and grafting methods. In order to introduce ionic liquid groups, modification of GO nanosheets was carried out by CuAAC click reaction. Structural characterizations showed that PW species were dispersed on the surface of GO nanosheets. Both of the prepared heterogeneous catalysts were used in the selective epoxidation of olefins and allylic alcohols with H2O2. The heterogeneous catalysts were shown to be highly active and stable in the reaction conditions. The catalysts easily recovered without significant loss in activity and selectivity for five successive runs.
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Acknowledgement The authors are grateful to Kharazmi University for financial support.
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[55] Y. Ishii, K. Yamawaki, T. Ura, H. Yamada, T. Yoshida, M. Ogawa, Hydrogen Peroxide Oxidation Catalyzed by Heteropoly Acids Combined with Cetylpyridinium Chloride: Epoxidation of Olefins and Allylic Alcohols, Ketonization of Alcohols and Diols, and Oxidative Cleavage of 1,2-Diols and Olefins, J. Org. Chem. 53 (1988) 3587-3593. [56] K. Krishnamoorthy, M. Veerapandian, K. Yun, S.-J. Kim, The chemical and structural analysis of graphene oxide with different degrees of oxidation, Carbon, 53 (2013) 38-49. [57] Z. Li, S. Wu, H. Ding, H. Lu, J. Liu, Q. Huo, J. Guan, Q. Kan, Oxovanadium (IV) and iron (III) salen complexes immobilized on amino-functionalized graphene oxide for the aerobic epoxidation of styrene, New J. Chem. 37 (2013) 4220-4229.
16
[58] L. Liu, C. Chen, X. Hu, T. Mohamood, W. Ma, J. Lin, J. Zhao, A role of ionic liquid as an activator for efficient olefin epoxidation catalyzed by polyoxometalate, New J. Chem. 32 (2008) 283–289. [59] P. Zhao, M. Zhang, Y. Wu, and J. Wang, Heterogeneous selective oxidation of sulfides with H2O2 catalyzed by ionic liquid-based polyoxometalate salts, Ind. Eng. Chem. Res. 51 (2012) 6641-6647.
17
Scheme 1. Schematic synthetic routes of the GO-pr-IL-PW and GO-IL-PW catalysts.
18
Figure 1. FT-IR spectra of (a) pristine GO, (b) GO-pr-N3, (c) GO-N3, (d) GO-pr-IL-PW, and (e) GOIL-PW.
19
Figure 2. XRD patterns of (a) pristine GO, (b) GO-pr-IL-PW, and (c) GO-IL-PW.
20
Figure 3. N2 adsorption–desorption isotherms of (a) pristine GO, (b) GO-pr-IL-PW, and (c) GO-ILPW. The values represent averages of two experiments and in order to clarity the error bars are omitted.
21
Figure 4. SEM (a) and TEM images of pristine GO (b), GO-pr-IL-PW (c), and GO-IL-PW (d).
22
Figure 5. Results of the catalytic epoxidation of olefins with H2O2 in the presence of (top) GO-ILPW, and (bottom) GO-pr-IL-PW. Reaction conditions: Olefin (4 mmol), 30% H2O2 (6 mmol), Catalyst (50 mg), refluxing CH3CN (5 ml). TOF values were calculated as mmol of product per mmol of PW in catalyst per time (h) (The points indicate the average values and the error bars show deviation from the average value. In order to clarity the other error bars are omitted).
23
Figure 6. Results of the catalytic epoxidation of allylic alcohols with H2O2 in the presence of (top) GO-IL-PW, and (bottom) GO-pr-IL-PW. Reaction conditions: Allylic alcohol (4 mmol), 30% H2O2 (6 mmol), Catalyst (50 mg), refluxing CH3CN (5 ml). TOF values were calculated as mmol of product per mmol of PW in catalyst per time (h).
24
Figure 7. Proposed mechanism of epoxidation of olefins with H 2O2 in the presence of supported PW species.
Figure 8. Investigation of catalyst reusability in the epoxidation of cyclooctene with H 2O2. Reaction conditions: Cyclooctene (4 mmol), 30% H2O2 (6 mmol), Catalyst (50 mg), refluxing CH3CN (5 ml).
25
Table 1. Results of chemical analyses of GO-pr IL-PW and GO-IL-PW P
W
PW
(mmol.g−1)
(mmol.g−1)
(mmol.g−1)
Fresh GO-pr-IL-PW
0.032
0.101
0.032
1st reuse
0.030
0.098
0.030
2nd reuse
0.029
0.099
0.029
3rd reuse
0.029
0.097
0.029
4th reuse
0.028
0.098
0.028
Fresh GO-IL-PW
0.019
0.069
0.019
1st reuse
0.016
0.059
0.016
2nd reuse
0.016
0.061
0.016
3rd reuse
0.014
0.051
0.014
4th reuse
0.014
0.050
0.014
Sample
26
S0927-7757(17)30635-0 http://dx.doi.org/doi:10.1016/j.colsurfa.2017.06.073 COLSUA 21760
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
6-3-2017 23-6-2017 26-6-2017
Please cite this article as: M.Masteri-Farahani, M.Modarres, Clicked graphene oxide as new support for the immobilization of peroxophosphotungstate: Efficient catalysts for the epoxidation of olefins, Colloids and Surfaces A: Physicochemical and Engineering Aspectshttp://dx.doi.org/10.1016/j.colsurfa.2017.06.073 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Clicked graphene oxide as new support for the immobilization of peroxophosphotungstate: Efficient catalysts for the epoxidation of olefins M. Masteri-Farahani*, M. Modarres Faculty of Chemistry, Kharazmi University, Tehran, Islamic Republic of Iran
*Corresponding author. Tel.: +98 263 4551023; fax: +98 263 4551023.
E-mail address: [email protected] Graphical Abstract
Research highlights:
The surface of graphene oxide was modified with grafting and tethering methods via click reaction. Peroxopolyoxotungstate anions were immobilized on the surface of clicked graphene oxides. The prepared materials acted as active, selective, and reusable catalysts in the epoxidation of olefins with H2O2.
Abstract
1
Peroxophosphotungstate (PW) anions were successfully immobilized on the surface of clicked graphene oxides and used as heterogeneous catalysts for the epoxidation of various olefins using H 2O2 as oxidant. Graphene oxide (GO) was functionalized with azide groups through both grafting and tethering methods. Then, supported ionic liquids were formed on the surface of GO through the Cu (I)-catalyzed azide-alkyne cycloaddition (CuAAC) click reaction. The clicked GO supported ionic liquids were used as appropriate supports for the immobilization of PW anions. Characterization of the prepared materials by various physicochemical methods showed that PW anions have been successfully immobilized on the surface of clicked GOs. Catalytic activities of these heterogeneous catalysts were evaluated in the epoxidation of olefins which showed that not only the catalysts are active, but also they could be reused several times without significant loss of their activities. Keywords: Click Chemistry; Heterogeneous Catalysis; Epoxidation; Peroxopolyoxotungstate; Graphene Oxide.
1. Introduction
Catalytic epoxidation of olefins is an important process in the synthetic chemistry as the produced epoxides are essential precursors for the production of fine chemicals, pharmaceuticals, and chemical intermediates [1-3]. Polyoxometalates (POMs) as effective catalysts have been extensively utilized for the epoxidation of olefins due to their high performance [4-6]. Among the POMs, tungsten-based ones have attracted great interest as green and environmental friendly epoxidation catalysts owing to high catalytic activities and excellent selectivities in the presence of H 2O2 as oxidant [7-11]. From green chemistry point of view, H2O2 is a beneficial oxidant because of its availability and cheapness compared to organic peroxides and peracids. Moreover, it has high active oxygen content and generates water as only by-product [12-14]. However, the use of POMs homogeneous catalysts is restricted owing to disadvantages such as high cost of recovery and separation from the reaction mixtures. Thus, various approaches have been used for heterogenizing these homogeneous catalysts on solid supports such as mesoporous silica [[15-19], metal-organic frameworks (MOFs) [20-22],
2
functionalized polymers [23,24], and magnetic nanoparticles [25-30]. Recently, graphene oxide (GO) has been utilized as one of the most promising supports due to its remarkable properties such as two dimensional (2D) structure, large surface area, outstanding mechanical strength and the existence of many oxygen-containing functional groups on its basal planes and edges [31-38]. Mungse et al. and Verma et al. reported the immobilization of oxo-vanadium Schiff base complex on the surface of GO nanosheets for the oxidation of various alcohols, fatty acids and esters in the presence of tert-butyl hydroperoxide (TBHP) as oxidant which exhibited high catalytic activity comparable to their homogeneous counterpart [39,40]. Scheuermann et al. reported that Pd nanoparticles immobilized on GO showed high activity in the Suzuki-Miyaura coupling reaction along with little palladium leaching [41]. Su et al. reported the immobilization of transition metal (Fe2+, Co2+, VO2+, Cu2+) Schiff base complexes on the surface of GO as efficient and reusable catalysts for the epoxidation of styrene [42]. Click reactions, especially Cu (I)-catalyzed 1,3-dipolar azide-alkyne cycloaddition (CuAAC), have attracted great attention for surface functionalization owing to high selectivity, easy reaction conditions and the absence of side products [43,44]. While click chemistry approach has been used to functionalize solid supports such as graphene and carbon nanotube (CNT) [45-50], to the best of our knowledge, the immobilization of POMs on the surface of graphene oxide by using click chemistry has not been reported yet. Here, we report the design and characterization of two new heterogeneous catalysts. First, the surface of GO nanosheets was modified with azide groups through grafting and tethering approaches. Then, click reaction of azide-functionalized GOs with 3-ethynyl-1-methylimidazolium cations resulted in the covalent attachment of ionic liquids on the surface of GO nanosheets. Finally, the ionic liquid modified GO nanosheets were used to immobilize peroxophosphotungstate (PW) anions. Catalytic activities of both heterogeneous catalysts were studied in the epoxidation of various olefins with using H2O2 as oxidant. 2. Experimental
3
2.1. Materials and methods The graphite powder was purchased from Sinchem Company. Other chemicals were purchased from Merck chemical company. All of the solvents were used after distillation. Fourier transform infrared (FT-IR) spectra were taken with a Perkin-Elmer Spectrum RXI FT-IR spectrometer. X-ray diffraction (XRD) analyses were carried out by PANalytical X’pert Pro diffractometer with Cu Kα (λ=0.154nm) radiation. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were recorded using ZEISS-DSM 960A microscope and Zeiss-EM10C-100 kV transmission electron microscope, respectively. The elemental compositions of the samples were measured with VARIAN VISTA MPX ICP-OES. Nitrogen content of the samples was determined by Perkin Elmer 2400 SERIES II CHN Analyzer. The Brunauer–Emmet–Teller (BET) surface areas of the materials (outgassed under high vacuum at 120°C) were determined using Microtrac Bel Corp instrument at liquid nitrogen temperature (-196°C). The progress of the catalytic experiments was controlled by Agilent 6890N gas chromatograph (GC) equipped with a HP-5 capillary column. 2.2. Preparation of the catalysts 2.2.1. Preparation and modification of GO with azide groups via grafting and tethering methods GO was prepared by improved Hummers approach [51]. GO-N3 was prepared through the grafting of azide groups on the surface of GO according to the literature [52]. On the other hand, in tethering method, GO (1 g) was dispersed in 40 ml toluene using ultrasonication. Then, (3chloropropyl)trimethoxysilane (3 mmol) was added and the reaction mixture was refluxed for 24 h under N2 atmosphere. The obtained solid (called GO-Cl) was washed with methanol to remove the excess of silylating agent and dried under vacuum at 80°C. Then, the prepared GO-Cl (1 g) was stirred in a solution of 2 mmol NaN3 in 100 ml dimethylformamide (DMF) for 8 h at 80°C. After cooling to room temperature, the prepared GO-pr-N3 was filtered and washed with DMF to remove the excess of NaN3.
4
2.2.2. Preparation of 3-ethynyl-1-methylimidazolium bromide To a mixture of 1-methylimidazole (1 g) in 50 ml of acetonitrile, 1.9 ml of propargyl bromide was added dropwise and the mixture was refluxed for 24 h. Then, the solvent was removed by rotary evaporator to obtain a dark-red viscous liquid. 2.2.3. Immobilization of ionic liquids on GO with click reaction GO-N3 or GO-pr-N3 (1 g) was dispersed in a mixture of H 2O/DMF (1:1, 100 ml) by using ultrasonication for 30 min. Then, 3-ethynyl-1-methylimidazolium bromide (1.6 g), copper (II) sulfate (0.1 g) as catalyst, and sodium ascorbate (1.6 g) as reductant were added to the mixture and stirred at room temperature for 24 h. The products (GO-pr-IL and GO-IL) were separated with centrifugation, washed with excess of water, and dried at 80°C in a vacuum oven overnight. 2.2.4. Immobilization of peroxophosphotungstate (PW) anions on the surface of GO supported ionic liquids PW sodium salt was prepared according to the procedure of Ishii et al. [55]. For the immobilization of PW anions, the as-prepared PW anions (2 mmol) was added to a mixture of GO supported ionic liquid (1 g) in 50 ml deionized water and the mixture was stirred for 8 h at room temperature. The resulting solid was filtered and washed several times with distilled water and then dried in vacuum oven overnight. The two prepared heterogeneous catalysts by grafting and tethering methods are referred as GO-IL-PW and GO-pr-IL-PW, respectively. 2.3. Catalytic epoxidation of olefins In a typical catalytic run, a mixture of olefin (4 mmol) and catalyst (50 mg) in acetonitrile (5 ml) were placed in a 10 ml round-bottom flask equipped with a reflux condenser. After addition of H 2O2 30% (6 mmol), the mixture was refluxed under nitrogen atmosphere. The reaction progress was followed at various times (2, 4, 6, 8 and 24 h) by using gas chromatography. Finally, the catalyst was separated,
5
washed with methanol, and vacuum dried at 80°C. The recovered catalyst was reused in the next epoxidation reactions in similar conditions. 3. Results and discussions 3.1. Preparation and characterization of the catalysts The synthetic pathway of GO-pr-IL-PW and GO-IL-PW catalysts is illustrated in Scheme 1. In tethering method, the GO was covalently modified through the reaction of (3-chloropropyl) trimethoxysilane with surface hydroxyl groups of GO. In the next step, the substitution reaction of chloro groups with azide ions leads to the formation of GO-pr-N3. Then, an imidazolium-based ionic liquid (IL) was immobilized on the surface of GO nanosheets through click reaction of the supported azide groups with pre-synthesized 3-ethynyl-1-methylimidazolium bromide. Eventually, PW species were immobilized through the electrostatic interaction with positively charged imidazolium-based groups on the surface of GO support to achieve GO-pr-IL-PW heterogeneous catalyst. On the other hand, in grafting approach, GO-N3 was prepared by ring opening of GO surface epoxy groups through the reaction with NaN3 [52]. In comparison with earlier reports [53,54], this method is simple and more efficient. CuAAC click reaction between 3-ethynyl-1-methylimidazolium bromide and supported azide groups resulted in the preparation of imidazolium-based ionic liquid on the surface of GO nanosheets and formation of GO-IL. Finally, PW species were attached electrostatically to the imidazolium-based groups to obtain GO-IL-PW heterogeneous catalyst. FT-IR spectra of pristine GO, GO supported ionic liquids with both methods, as well as corresponding catalysts are illustrated in Figure 1. In pristine GO, the bands at 3403, 1723, 1225, and 1055 cm-1 are assigned to the stretching vibrations of O-H, COOH, C-OH, and C-O (epoxy) groups, respectively, and reveal the presence of several oxygen-containing functional groups. The peak at 1612 cm-1 can be assigned to the aromatic C=C stretching vibrations in GO [52,56]. Modification of GO with azide groups in both methods can be confirmed by the appearance of new bands at ~2122 cm-1 due to the
6
azide stretching vibrations. In addition, the bands in the range of 2850-2940 cm-1 can be attributed to the stretching vibrations of CH2 groups of alkyl chains in modified GO with tethering method. The loading amounts of azide groups on GO-pr-IL-PW and GO-IL-PW, as determined by elemental analysis, were found to be 1.2 and 0.71 mmol.g -1, respectively. The vibrational bands corresponding to the azide groups were disappeared after click reaction with 3-ethynyl-1-methylimidazolium bromide which confirmed their conversion to triazole rings and immobilization of imidazolium based ionic liquid on the surface of GO. In the FT-IR spectra of the catalysts, the bands appeared at around 1080, 979, and 812 cm-1 can be assigned to νP-O, νW=O, νW-O-O, respectively [8]. These observations clearly indicate that PW species have been successfully immobilized on the surface of GO nanosheets. Further evidence for the immobilization of PW species on the surface of GO was provided by inductively coupled plasma optical emission spectroscopy (ICP-OES) chemical analysis which reveals that the fresh GO-pr-IL-PW and GO-IL-PW catalysts include 0.032 and 0.019 mmol.g-1 PW, respectively (Table 1).
XRD analysis was used to study the structural changes of GO after the modification process (Figure 2). Pristine GO represented a sharp and strong diffraction peak at 2θ=11°, corresponding to (001) reflection plane, which indicates the oxidative conversion of graphite to graphene oxide nanosheets [39]. This diffraction peak was disappeared after reacting with PW species and very broad peaks appeared in higher angles which indicate the successful well-dispersion of PW species on the surface of GO nanosheets. The surface properties of the materials were investigated by nitrogen adsorption-desorption analysis. The N2 adsorption–desorption isotherms of pristine GO, GO-pr-IL-PW and GO-IL-PW are shown in Figure 3. All of the isotherms are of type IV with appearance of hysteresis loop which is characteristic of the mesoporous structure [42,57]. The BET surface area of pristine GO is 280 m2.g-1, while those of GO-pr-IL-PW and GO-IL-PW are 18 and 8 m2.g-1, respectively. The results illustrate that the surface
7
area decreased due to the immobilization of PW species on the surface of GO nanosheets although the supported catalysts maintain the mesoporous characteristics. The surface morphology of GO was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) which are shown in Figure 4. According to SEM and TEM images, pure GO has a typical layered structure. It can be seen in TEM images that GO-pr-IL-PW and GO-IL-PW catalysts have some crumpled layered structures due to the presence of PW species between the layers. 3.2. Investigation of catalytic activities of the prepared catalysts The catalytic activities of the prepared catalysts were explored in the epoxidation of some olefins and allylic alcohols using H2O2 as oxidant under optimized conditions and the results are summarized in Figures 5 and 6. The results showed that both catalysts are highly active with high turnover frequency (TOF) values and selectively produce the desired epoxides. The high TOF values of the catalysts can be attributed to the role of produced ionic liquids as co-catalyst according to the earlier reports [58,59]. It can be seen that the conversions in the presence of GO-pr-IL-PW catalyst are more than GO-IL-PW which may be due to its higher PW loading. Moreover, the TOF values are higher in the case of GO-pr-ILPW catalyst. This observation can be explained by considering the more accessibility of the catalytic active centers in the GO-pr-IL-PW catalyst. The longer spacer group in this catalyst allows the substrates to reach the PW species more readily. In accordance with our previous results [15,16], the olefins with higher electron density on their C=C bonds show higher reactivities than terminal ones which confirm the electrophilic attack of the active centre of the catalyst to the olefin. As can be seen in the proposed mechanism of epoxidation reaction (Figure 7), the catalytic active site in the catalyst has electrophilic character and the mechanism of oxygen transfer to the olefin is electrophilic attack of electron deficient oxygen to the olefin double
8
bond as nucleophilic species. So, more electron density of the double bond accelerates the epoxidation reaction. On the other hand, the pristine GO showed negligible cyclooctene conversion (2%) under the same reaction conditions which indicated the critical role of the catalysts. 3.3. Investigation of the recyclabilities of the catalysts After each run, the catalyst was recovered for reusing in the next reaction. The recyclabilities and stabilities of GO-pr-IL-PW and GO-IL-PW catalysts were examined in the epoxidation of cyclooctene under similar reaction conditions. Both catalysts were recycled five times and the results showed that they could be reused without significant loss of activity and selectivity (Figure 8). Moreover, in order to clarify the plausible contribution of the leached PW species, in the catalytic epoxidation of cyclooctene, the catalyst was filtered at the reaction temperature after 8h and the filtrate was allowed to react further 16 h. The results showed that in this time interval the conversion of cyclooctene increased only 2% which shows that there is no significant PW species in the filtrate and the catalytic process is truly heterogeneous. These observations show the high stabilities and recyclabilities of the prepared catalysts.
Conclusion In summary, we demonstrated that PW species can be successfully immobilized on GO supported ionic liquid through tethering and grafting methods. In order to introduce ionic liquid groups, modification of GO nanosheets was carried out by CuAAC click reaction. Structural characterizations showed that PW species were dispersed on the surface of GO nanosheets. Both of the prepared heterogeneous catalysts were used in the selective epoxidation of olefins and allylic alcohols with H2O2. The heterogeneous catalysts were shown to be highly active and stable in the reaction conditions. The catalysts easily recovered without significant loss in activity and selectivity for five successive runs.
9
Acknowledgement The authors are grateful to Kharazmi University for financial support.
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Scheme 1. Schematic synthetic routes of the GO-pr-IL-PW and GO-IL-PW catalysts.
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Figure 1. FT-IR spectra of (a) pristine GO, (b) GO-pr-N3, (c) GO-N3, (d) GO-pr-IL-PW, and (e) GOIL-PW.
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Figure 2. XRD patterns of (a) pristine GO, (b) GO-pr-IL-PW, and (c) GO-IL-PW.
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Figure 3. N2 adsorption–desorption isotherms of (a) pristine GO, (b) GO-pr-IL-PW, and (c) GO-ILPW. The values represent averages of two experiments and in order to clarity the error bars are omitted.
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Figure 4. SEM (a) and TEM images of pristine GO (b), GO-pr-IL-PW (c), and GO-IL-PW (d).
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Figure 5. Results of the catalytic epoxidation of olefins with H2O2 in the presence of (top) GO-ILPW, and (bottom) GO-pr-IL-PW. Reaction conditions: Olefin (4 mmol), 30% H2O2 (6 mmol), Catalyst (50 mg), refluxing CH3CN (5 ml). TOF values were calculated as mmol of product per mmol of PW in catalyst per time (h) (The points indicate the average values and the error bars show deviation from the average value. In order to clarity the other error bars are omitted).
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Figure 6. Results of the catalytic epoxidation of allylic alcohols with H2O2 in the presence of (top) GO-IL-PW, and (bottom) GO-pr-IL-PW. Reaction conditions: Allylic alcohol (4 mmol), 30% H2O2 (6 mmol), Catalyst (50 mg), refluxing CH3CN (5 ml). TOF values were calculated as mmol of product per mmol of PW in catalyst per time (h).
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Figure 7. Proposed mechanism of epoxidation of olefins with H 2O2 in the presence of supported PW species.
Figure 8. Investigation of catalyst reusability in the epoxidation of cyclooctene with H 2O2. Reaction conditions: Cyclooctene (4 mmol), 30% H2O2 (6 mmol), Catalyst (50 mg), refluxing CH3CN (5 ml).
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Table 1. Results of chemical analyses of GO-pr IL-PW and GO-IL-PW P
W
PW
(mmol.g−1)
(mmol.g−1)
(mmol.g−1)
Fresh GO-pr-IL-PW
0.032
0.101
0.032
1st reuse
0.030
0.098
0.030
2nd reuse
0.029
0.099
0.029
3rd reuse
0.029
0.097
0.029
4th reuse
0.028
0.098
0.028
Fresh GO-IL-PW
0.019
0.069
0.019
1st reuse
0.016
0.059
0.016
2nd reuse
0.016
0.061
0.016
3rd reuse
0.014
0.051
0.014
4th reuse
0.014
0.050
0.014
Sample
26