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Selective oxidation of alcohols with H2O2 catalyzed by zinc polyoxometalate immobilized on multi-wall carbon nanotubes modified with ionic liquid
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CCLET 3909 1–6 Chinese Chemical Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters journal homepage: www.elsevier.com/locate/cclet
1
Original article
2
Selective oxidation of alcohols with H2O2 catalyzed by zinc polyoxometalate immobilized on multi-wall carbon nanotubes modified with ionic liquid
3 4
5 Q1
Robabeh Hajian* , Zahra Alghour
6
Department of Chemistry, Yazd University, Yazd 89195-741, Iran
A R T I C L E I N F O
Article history: Received 13 September 2016 Received in revised form 31 October 2016 Accepted 10 November 2016 Available online xxx Keywords: Immobilized ionic liquid [PZnMo2W9O39]5 Hydrogen peroxide Oxidation Alcohol Heterogeneous catalyst
A B S T R A C T
In this work, acid functionalized multi-wall carbon nanotubes (MWCNTs) were modified with imidazolium-based ionic liquids. The selective oxidation of various alcohols with hydrogen peroxide catalyzed by [PZnMo2W9O39]5, ZnPOM, supported on ionic liquids-modified with MWCNTs, MWCNTAPIB, is reported. This catalyst [ZnPOM@APIB-MWCNT], was characterized by X-ray diffraction, scanning electron microscopy (SEM) and FT-IR spectroscopic methods. This heterogeneous catalyst exhibited high stability and reusability in the oxidation reaction without loss of its catalytic performance. © 2016 Robabeh Hajian. Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
7
1. Introduction
8
Catalytic oxidation of hydroxyl groups to carbonyl compounds is one of the predominant reactions in organic synthesis [1]. In aldehyde synthesis by selecting an appropriate oxidant and/or catalyst creating carboxylic acids can be prevented. Traditional oxidation methods that use heavy metal complexes such as chromates and permanganates are often toxic, corrosive and expensive [2]. Consequently, application of heterogeneous catalysis and green oxidants, such as O2 or hydrogen peroxide, has received attention [3]. Ionic liquids (ILs) are salts with low melting points with a combination of a bulky organic cation and either an organic or inorganic anion [4]. Ionic liquids are regarded to be designer solvents whose chemical and physical properties (e.g. liquid range, conductivity, viscosity, density, thermal stability, vapor pressure, miscibility, wide liquid temperature range) may be fine-tuned by changing the combination of anions and cations, thereby allowing them to be utilized in various fields [5]. Polyoxometalates (POMs) are discrete molecular structures composed of cations and metal–oxygen-cluster anions with potential use in various applications in materials science, biochemistry, analytical, catalysis, medicine and solid-state devices [6]. Keggin-type polyoxometalates exhibit activity as both
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* Corresponding author. E-mail addresses: [email protected], [email protected] (R. Hajian).
acid and redox catalysts. When one or more addenda atom (Mo, W) is replaced with other heteroatoms, transition-metal-substituted polyoxometalates (TMSP) are obtained [7,8]. The zinc-containing heteropoly anion, [PZnMo2W9O39]5 (ZnPOM), has been known as a beneficial catalyst for oxidation reactions [9]. Because of very low surface area and high solubility of this homogeneous polyoxometalate in polar solvents, the development of supported POM catalysts is in demand. The combination of the advantages of ionic liquids with those of heterogeneous catalysts leads to the supported ionic liquid catalysis concept, which provides the suitable conditions for organic reactions [10,11]. In recent years, carbon nanotubes have attracted much attention in synthesis because of their superior properties and wide range of potential applications over other materials. As a result, important achievements have come up in different fields, including unique structural, mechanical, thermal and electronic properties [12,13]. Since CNTs are insoluble in most solvents, these materials can be used as a catalyst support. However, due to the strong intrinsic van der Waals forces, CNTs tend to aggregate spontaneously, which is the major limitation to their useful applications. The important work is attempting to have a homogeneous dispersion of CNTs. Physical and chemical functionalization is two main routes for dispersing CNTs in aqueous media [14–17]. The physical functionalization is based on van der Waals forces, being controlled by thermodynamics [18]. Ionic liquids are a kind of surfactant and were used for physical functionalization of CNTs to disperse them in aqueous solutions. Park et al. reported
http://dx.doi.org/10.1016/j.cclet.2016.12.003 1001-8417/© 2016 Robabeh Hajian. Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: R. Hajian, Z. Alghour, Selective oxidation of alcohols with H2O2 catalyzed by zinc polyoxometalate immobilized on multi-wall carbon nanotubes modified with ionic liquid, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.12.003
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R. Hajian, Z. Alghour / Chinese Chemical Letters xxx (2016) xxx–xxx
Scheme 1. Preparation of [ZnPOM@APIB-MWCNT]. 56
72
that the covalent modification of multi-wall carbon nanotubes (MWCNTs) with imidazolium salt-based ILs provided IL soluble functionalized MWCNTs [19]. Compounds of polyoxometalate and carbon nanotubes have attracted wide attention as they combine the unique chemical reactivity of POMs with high surface area and electrical conductivity of nanocarbons [20]. In this work, the carboxylic acid-functionalized multi-wall carbon nanotubes (MWCNT-COOH) surface was first functionalized with thionyl chloride (MWCN-Cl) and then with 1-(3aminopropyl) imidazole (MWCN-Im). In the next step, multiwall CNTs with imidazolium cation-based were functionalized with 1-bromobutan (MWCN-APIB) and then treated with ((n-C4H9)4N)5[PZnMo2W9O39] (ZnPOM) cluster. The imidazolium groups of the support interact electrostatically with the ZnPOM to produce the [ZnPOM@APIB-MWCN]. The catalytic activity of [ZnPOM@APIB-MWCN] in the oxidation of alcohols with hydrogen peroxide was also reported.
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2. Results and discussion
57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
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2.1. Preparation and characterization of catalyst, [PZnMoW@APIB-MWCNT] The preparation procedure for [PZnMoW@APIB-MWCNT] is shown in Scheme 1. In first step, imidazole modified MWCNT, MWCNT-API, was prepared by covalent attachment of
1-(3-aminopropyl)imidazole to MWCNT-COCl via an amide linkage. Then, MWCNT-API was reacted with 1-bromobutane at 90 C to prepare the supported ionic liquid MWCNT-APIB. In the next step, the ZnPOM was reacted with MWCNT-APIB to obtain the ionic liquid supported catalysts. The amounts of Mo and W were measured by ICP. Both values showed that the catalyst loading is about 0.004 mmol/g. The catalyst was characterized by FT-IR spectroscopy, XRD and SEM analyses. The four typical skeletal vibrations of the polyoxometalate, ((n-C4H9)4N)5[PZnMo2W9O39], appear at 1057 (nP–O), 953 (nM=O), 889 (nM–Ob–M) and 807 cm1 (nM–Oc–M) (Ob: cornersharing; Oc: edge-sharing), respectively. The observed bands at 2870 and 2960 cm1 can be assigned to the C–H vibration of tetrabutyl ammonium [12]. After an ion exchange with polyoxometalate anion, four typical skeletal vibrations of the ZnPOM and C¼O stretching vibration (1634) of the support are observed in the heterogeneous catalyst (Fig. 1). The FT-IR spectra denoted that ZnPOM had been successfully supported on ionic liquid-modified MWCNT-APIB. The X-ray diffraction patterns of MWCNT-APIB, ZnPOM and [ZnPOM@APIB-MWCNT] are shown in Fig. 2. From these patterns, it is clear that ZnPOM has been introduced in the alternative position of MWCNT-APIB. According to SEM images of MWCNT-APIB and [ZnPOM@APIBMWCNT] (Fig. 3), a clear change in the morphology of catalysts pointed out that the ZnPOM has been immobilized on the MWCNT-APIB.
Fig. 1. FT-IR spectrum of [ZnPOM@APIB-MWCNT].
Please cite this article in press as: R. Hajian, Z. Alghour, Selective oxidation of alcohols with H2O2 catalyzed by zinc polyoxometalate immobilized on multi-wall carbon nanotubes modified with ionic liquid, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.12.003
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CCLET 3909 1–6 R. Hajian, Z. Alghour / Chinese Chemical Letters xxx (2016) xxx–xxx
3
Fig. 2. XRD patterns of: (A) MWCNT-APIB; (B) ZnPOM and (C) [ZnPOM@APIB-MWCNT] composite.
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2.2. Catalytic activity
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The catalytic activity of the supported ZnPOM cluster was tested in the oxidation of benzyl alcohol to benzaldehyde with hydrogen peroxide. The reactions were continued until no further progress was observed. In order to choose the reaction media, various organic solvents were examined in the oxidation of benzyl alcohol. Among dichloromethane, acetonitrile, ethyl acetate, methanol and n-hexane, acetonitrile was selected as reaction media because the highest aldehyde yield was observed (Table 1, entries 1–5). The amounts of catalyst and H2O2 were also optimized and the higher yield was obtained using 0.004 mmol of catalyst and 5 mmol of H2O2 (Table 1, entries 6–9 and entries 10–13). The data of optimized of temperature was shown in Table 1 (entries 14–17). The time courses of ZnPOM, APIB-MWCN and MWCN under these reaction conditions are shown in Table 2. Under the optimized conditions, oxidation of different alcohols was investigated (Table 3). To study the catalytic activity of the
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Table 1 The effect of solvent, amounts of catalyst, H2O2, temperature on the oxidation of benzyl alcohol catalyzed by [ZnPOM@APIB-MWCNT] after 4 h. Entry
H2O2 (mL)
Catalyst (mmol)
Temp. ( C)
Solvent
Yield (%)a
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.125 0.25 0.50 0.75 0.50 0.50 0.50 0.50
0.004 0.004 0.004 0.004 0.004 0 0.002 0.004 0.008 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004
Reflux Reflux Reflux Reflux Reflux Reflux Reflux Reflux Reflux Reflux Reflux Reflux Reflux 25 45 65 Reflux
Dichloromethane Ethyl acetate n-Hexane Methanol CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN
50 75 80 40 95 5 75 95 90 75 90 95 95 7 16 40 95
a
GC yield based on the starting alcohol.
system, alcohol (0.5 mmol) and [ZnPOM@APIB-MWCNT] (50 mg, 0.004 mmol) in 3 mL CH3CN were introduced into a 25 mL roundbottom flask equipped with a magnetic stirrer (400 rpm). H2O2 (5 mmol, 30%) was added to this mixture and refluxed. The progress of the reactions was monitored by GC. At the end of the reaction, the catalyst was filtered and washed with Et2O (20 mL). The pure product was obtained after chromatography of organic layer on a short column of silica gel. In the presence of [ZnPOM@APIB-MWCNT] alcohols were converted to their corresponding aldehydes with 100% selectivity (Scheme 2). The electronic nature of the substituent exhibited little effect on the reaction process. It was assumed that reoxidation of the catalyst and not the dehydrogenation (a-H) step was ratelimiting [21]. The oxidation of the benzyl alcohol afforded benzaldehyde with an excellent yield of 95%. Substituted benzyl alcohol with donating and drawing groups such as NO2, Cl, OH, OMe, and t-Bu gave the desired products (Table 3, entries 2–6). Oxidation of 4-methoxy benzyl alcohol was oxidized to its corresponding aldehyde in excellent yield (95%). For 4-tert-butyl benzyl alcohol, the desired aldehyde was obtained in the yield of 70%. In the case of alcohols with electron-drawing substituents, 4nitro benzyl alcohol, 4-chloro benzyl alcohol, 4-hydroxo benzyl alcohol were oxidized to the corresponding aldehydes with yields of 60%–85%. Oxidation of 1,2-phenylethanol and 1,2-phenylpropanol were oxidized to 1,2-phenylethanal and 1,2-phenylpropanal with yield of 70% and 60% respectively.
Table 2 Oxidation of alcohols with H2O2 catalyzed by MWCNT, APIB-MWCNT and ZnPOM under reflux conditions.a Catalyst
MWCNT APIB-MWCNT ZnPOM
Yield (%)b 1h
2h
3h
4h
33 18 50
35 18 72
35 20 80
40 20 93
a Reaction conditions: alcohol (0.5 mmol), H2O2 (5 mmol), MWCNT and APIBMWCNT (50 mg) or ZnPOM (0.004 mmol), at reflux in CH3CN (3 mL). b GC yield based on the starting alcohol.
Please cite this article in press as: R. Hajian, Z. Alghour, Selective oxidation of alcohols with H2O2 catalyzed by zinc polyoxometalate immobilized on multi-wall carbon nanotubes modified with ionic liquid, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.12.003
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Fig. 3. Scanning electron micrographs of: (A) MWCNT-APIB and (B) [ZnPOM@APIB-MWCNT]. 148
2.3. Catalyst reuse and stability
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To assess lengthy cycle stability and reusability of [ZnPOM@APIB-MWCNT], oxidation of benzyl alcohol was selected as model reaction, and recycling experiments were carried out with a single
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Table 3 Oxidation of alcohols with H2O2 catalyzed by [ZnPOM@APIB-MWCNT] under reflux conditions.a Entry
Substrate
Product
b
1
Time (h)
Yield (%)
TOF (h
1
4
95
29.68
2
3.5
60
21.42
3
4
85
26.56
4
4
65
20.31
5
5
95
20
6
3.5
70
26.78
7
5
70
17.50
8
4
60
18.75
)
a Reaction conditions: alcohol (0.5 mmol), H2O2 (5 mmol), catalyst (50 mg, 0.004 mmol), CH3CN (3 mL). b GC yield based on the starting alcohol.
sample of the catalyst. After each experiment, the catalyst was removed by simple filtration, washed with acetonitrile and nhexane, dried at room temperature and reused. The catalyst was sequentially reused several times (Fig. 4). The filtrates were used for the determination of the catalyst leaching by atomic absorption spectroscopy (AAS). The results exhibited that in the first two runs some catalyst is leached from the support (1.27% for the first run and 0.12% for the second run), but in the next runs no leaching was monitored. The nature of the recovered catalyst was followed by FT-IR spectroscopy. The results indicated that the catalyst after reusing several times, demonstrated no change in its IR spectra (Fig. 5).
Scheme 2. Proposed reaction mechanism.
Fig. 4. The results obtained from catalyst reuse and stability in the oxidation of benzyl alcohol with H2O2 by [ZnPOM@APIB-MWCNT] under reflux conditions.
Please cite this article in press as: R. Hajian, Z. Alghour, Selective oxidation of alcohols with H2O2 catalyzed by zinc polyoxometalate immobilized on multi-wall carbon nanotubes modified with ionic liquid, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.12.003
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CCLET 3909 1–6 R. Hajian, Z. Alghour / Chinese Chemical Letters xxx (2016) xxx–xxx
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Fig. 5. FT-IR spectra of [ZnPOM@APIB-MWCNT] recovered in runs 1, 3, 5 and 7.
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3. Conclusion
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In this work, ((n-C4H9)4N)5[PZnMo2W9O39] was immobilized on ionic liquid-modified MWCNT-APIB. The zinc containing polyoxometalate was efficient in the oxidation of alcohols with H2O2 as an oxidant, producing the corresponding aldehydes in excellent selectivity. The major advantages of ionic liquidsupported catalyst were its high catalytic activity reusability.
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4. Experimental
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All materials were commercial reagent grade and obtained from Merck and Fluka. MWCNTs containing carboxylic acid groups (multi-wall carbon nanotube with purity, >95%; length, 10–50 mm; diameter, 10–30 nm, COOH content 2%, ash (catalyst residue) <1.5% and specific surface area >110 m2/g) was purchased from Neutrino Co. Ltd. Shenzen NTP Factory (China) and used without further purification. Hydrogen peroxide (30%) was titrated by a standard KMnO4 solution. FT-IR spectra were obtained as potassium bromide pellets in the range 400–4000 cm1 with a Bruker Equniox 55. Scanning electron micrographs (SEM) of the catalyst and support were taken on a SEM KYKY EM3200 instrument. Powder X-ray diffraction patterns were obtained on a D8 Advanced Bruker using Cu-Ka radiation (2u = 5–70 ). Gas chromatography experiments (GC) were performed with a Shimadzu GC-16A instrument using a 2 m column packed with silicon DC-200 or Carbowax 20 M. In GC experiments, n-decane was used as an internal standard. The ICP analyzes were performed on an ICP-Spectrociros CCD instrument. Atomic absorption analyses were carried out on a Shimadzu 120 spectrophotometer. ((n-C4H9)4N)5[PZnMo2W9O39] was prepared according to the literature [22]. Solid b-Na8HPW9O3424H2O was prepared by mixing Na2WO42H2O (60 g), H3PO4 (1.5 mL, 14.7 mol/L) and CH3COOH (11 mL, 17.4 mol/L) in 75 mL of water and the resulting solid was filtered. Then, white precipitate a-K7PMo2W9O3919H2O was obtained from aqueous solution of b-Na8HPW9O3424H2O (11 g), Na2MoO4 (20 mL, 1 mol/L), HCl (15 mL, 1 mol/L) and adjusting pH to 6–6.5 with HCl (1 mol/L) and KCl was added to this solution at the end. In the next step, a-K7PMo2W9O3919H2O (3.5 g) was dissolved in water (90 C) and ZnCl2 (0.5 g was dissolved in 5 mL water) was added and adjusting pH to 3 by HCl (1 mol/L). To
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this solution, [(n-C4H9)4N]Br was added and was stirred for 2 h, green solid, ((n-C4H9)4N)5[PZnMo2W9O39], was filtered off and then dried in vacuum. The ionic liquid support, APIB-MWCN, was prepared according to the method reported by Park et al. with some changes [19]. A suspension of MWCNTs (5 g) in SOCl2 (30.0 mL) was stirred at reflux for 3 h. After evaporation of the solvent, a solution of MWCNT-COCl (1 g) and 1-(3-aminopropyl) imidazole (3 mL) and triethylamine (3 mL) was stirred at reflux for 18 h under nitrogen atmosphere and thoroughly washed with CH2Cl2 and diethyl ether. Then, a mixture of MWCNT-API (80 mg) and 1-bromobutane (20 mL) was stirred reflux for 18 h at 90 C. The solid, APIB-MWCNT, was washed with anhydrous THF, HCl solution (1 mol/L), saturated NaHCO3 solution, and water successively. To a solution of ((n-C4H9)4N)5[PZnMo2W9O39] (1 g) in CH3CN, MWCNTAPIB (4 g) was added and refluxed for 24 h. Finally, the catalyst, [PZnMoW@APIB-MWCNT], was filtered and dried at room temperature.
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Acknowledgment
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We are thankful to the Yazd University Research Council for partial support of this work.
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References
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[1] M. Hudlicky, Oxidations in Organic Chemistry, American Chemical Society, 1990. [2] S.E. Davis, M.S. Ide, R.J. Davis, Selective oxidation of alcohols and aldehydes 224 over supported metal nanoparticles, Green Chem. 15 (2013) 17–45. [3] P. Tundo, G.P. Romanelli, P.G. Vázquez, F. Aricò, Multiphase oxidation of 225 alcohols and sulfides with hydrogen peroxide catalyzed by heteropolyacids, 226 Catal. Commun. 11 (2010) 1181–1184. [4] D. Betz, P. Altmann, M. Cokoja, W.A. Herrmann, F.E. Kühn, Recent advances in 227 oxidation catalysis using ionic liquids as solvents, Coord. Chem. Rev. 255 228 (2011) 1518–1540. [5] R.D. Rogers, K.R. Seddon (Eds.), A.C.S. Symposium Series, Ionic Liquids Q3 229 (Industrial Applications for Green Chemistry), American Chemical Society, 2002. [6] N. Mizuno, M. Misono, Heterogeneous catalysis, Chem. Rev. 98 (1998) 199–218. [7] S.S. Wang, G.Y. Yang, Recent advances in polyoxometalate-catalyzed reactions, 230 Chem. Rev. 115 (2015) 4893–4962. [8] H.N. Miras, J. Yan, D.L. Long, L. Cronin, Engineering polyoxometalates with Q4 231 emergent properties, Chem. Soc. Rev. 41 (2012) 7403–7430. [9] Y. Kikukawa, K. Yamaguchi, N. Mizuno, Sandwich-type zinc-containing 232 polyoxometalates with a hexaprismane core [{Zn2W(O)O3}2]4+ synthesized 233 by thermally induced isomerization of a metastable polyoxometalate, Inorg. 234 Chem. 49 (2010) 8194–8196.
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[10] C.P. Mehnert, Supported ionic liquid catalysis, Chem. Eur. J. 11 (2005) 50–56. [11] P. Zhao, M. Zhang, Y. Wu, J. Wang, Heterogeneous selective oxidation of sulfides with H2O2 catalyzed by ionic liquid-based polyoxometalate salts, Ind. Eng. Chem. Res. 51 (2012) 6641–6647. [12] Introduction to carbon materials research, in: M.S. Dresselhaus, P. Avouris (Eds.), Carbon Nanotubes, Springer, 2001. [13] A.V. Herrera-Herrera, M.Á. González-Curbelo, J. Hernández-Borges, M.Á. Rodríguez-Delgado, Carbon nanotubes applications in separation science: a review, Anal. Chim. Acta 734 (2012) 1–30. [14] V. Georgakilas, K. Kordatos, M. Prato, et al., Organic functionalization of carbon nanotubes, J. Am. Chem. Soc. 124 (2002) 760–761. [15] Y.-P. Sun, K. Fu, Y. Lin, W. Huang, Functionalized carbon nanotubes: properties and applications, Acc. Chem. Res. 35 (2002) 1096–1104. [16] K. Zhang, J. Lim, H. Choi, Amino functionalization and characteristics of multiwalled carbon nanotube/poly(methyl methacrylate) nanocomposite, Diamond Relat. Mater. 18 (2009) 316–318.
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CCLET 3909 1–6 Chinese Chemical Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters journal homepage: www.elsevier.com/locate/cclet
1
Original article
2
Selective oxidation of alcohols with H2O2 catalyzed by zinc polyoxometalate immobilized on multi-wall carbon nanotubes modified with ionic liquid
3 4
5 Q1
Robabeh Hajian* , Zahra Alghour
6
Department of Chemistry, Yazd University, Yazd 89195-741, Iran
A R T I C L E I N F O
Article history: Received 13 September 2016 Received in revised form 31 October 2016 Accepted 10 November 2016 Available online xxx Keywords: Immobilized ionic liquid [PZnMo2W9O39]5 Hydrogen peroxide Oxidation Alcohol Heterogeneous catalyst
A B S T R A C T
In this work, acid functionalized multi-wall carbon nanotubes (MWCNTs) were modified with imidazolium-based ionic liquids. The selective oxidation of various alcohols with hydrogen peroxide catalyzed by [PZnMo2W9O39]5, ZnPOM, supported on ionic liquids-modified with MWCNTs, MWCNTAPIB, is reported. This catalyst [ZnPOM@APIB-MWCNT], was characterized by X-ray diffraction, scanning electron microscopy (SEM) and FT-IR spectroscopic methods. This heterogeneous catalyst exhibited high stability and reusability in the oxidation reaction without loss of its catalytic performance. © 2016 Robabeh Hajian. Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
7
1. Introduction
8
Catalytic oxidation of hydroxyl groups to carbonyl compounds is one of the predominant reactions in organic synthesis [1]. In aldehyde synthesis by selecting an appropriate oxidant and/or catalyst creating carboxylic acids can be prevented. Traditional oxidation methods that use heavy metal complexes such as chromates and permanganates are often toxic, corrosive and expensive [2]. Consequently, application of heterogeneous catalysis and green oxidants, such as O2 or hydrogen peroxide, has received attention [3]. Ionic liquids (ILs) are salts with low melting points with a combination of a bulky organic cation and either an organic or inorganic anion [4]. Ionic liquids are regarded to be designer solvents whose chemical and physical properties (e.g. liquid range, conductivity, viscosity, density, thermal stability, vapor pressure, miscibility, wide liquid temperature range) may be fine-tuned by changing the combination of anions and cations, thereby allowing them to be utilized in various fields [5]. Polyoxometalates (POMs) are discrete molecular structures composed of cations and metal–oxygen-cluster anions with potential use in various applications in materials science, biochemistry, analytical, catalysis, medicine and solid-state devices [6]. Keggin-type polyoxometalates exhibit activity as both
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Q2
* Corresponding author. E-mail addresses: [email protected], [email protected] (R. Hajian).
acid and redox catalysts. When one or more addenda atom (Mo, W) is replaced with other heteroatoms, transition-metal-substituted polyoxometalates (TMSP) are obtained [7,8]. The zinc-containing heteropoly anion, [PZnMo2W9O39]5 (ZnPOM), has been known as a beneficial catalyst for oxidation reactions [9]. Because of very low surface area and high solubility of this homogeneous polyoxometalate in polar solvents, the development of supported POM catalysts is in demand. The combination of the advantages of ionic liquids with those of heterogeneous catalysts leads to the supported ionic liquid catalysis concept, which provides the suitable conditions for organic reactions [10,11]. In recent years, carbon nanotubes have attracted much attention in synthesis because of their superior properties and wide range of potential applications over other materials. As a result, important achievements have come up in different fields, including unique structural, mechanical, thermal and electronic properties [12,13]. Since CNTs are insoluble in most solvents, these materials can be used as a catalyst support. However, due to the strong intrinsic van der Waals forces, CNTs tend to aggregate spontaneously, which is the major limitation to their useful applications. The important work is attempting to have a homogeneous dispersion of CNTs. Physical and chemical functionalization is two main routes for dispersing CNTs in aqueous media [14–17]. The physical functionalization is based on van der Waals forces, being controlled by thermodynamics [18]. Ionic liquids are a kind of surfactant and were used for physical functionalization of CNTs to disperse them in aqueous solutions. Park et al. reported
http://dx.doi.org/10.1016/j.cclet.2016.12.003 1001-8417/© 2016 Robabeh Hajian. Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: R. Hajian, Z. Alghour, Selective oxidation of alcohols with H2O2 catalyzed by zinc polyoxometalate immobilized on multi-wall carbon nanotubes modified with ionic liquid, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.12.003
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Scheme 1. Preparation of [ZnPOM@APIB-MWCNT]. 56
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that the covalent modification of multi-wall carbon nanotubes (MWCNTs) with imidazolium salt-based ILs provided IL soluble functionalized MWCNTs [19]. Compounds of polyoxometalate and carbon nanotubes have attracted wide attention as they combine the unique chemical reactivity of POMs with high surface area and electrical conductivity of nanocarbons [20]. In this work, the carboxylic acid-functionalized multi-wall carbon nanotubes (MWCNT-COOH) surface was first functionalized with thionyl chloride (MWCN-Cl) and then with 1-(3aminopropyl) imidazole (MWCN-Im). In the next step, multiwall CNTs with imidazolium cation-based were functionalized with 1-bromobutan (MWCN-APIB) and then treated with ((n-C4H9)4N)5[PZnMo2W9O39] (ZnPOM) cluster. The imidazolium groups of the support interact electrostatically with the ZnPOM to produce the [ZnPOM@APIB-MWCN]. The catalytic activity of [ZnPOM@APIB-MWCN] in the oxidation of alcohols with hydrogen peroxide was also reported.
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2. Results and discussion
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2.1. Preparation and characterization of catalyst, [PZnMoW@APIB-MWCNT] The preparation procedure for [PZnMoW@APIB-MWCNT] is shown in Scheme 1. In first step, imidazole modified MWCNT, MWCNT-API, was prepared by covalent attachment of
1-(3-aminopropyl)imidazole to MWCNT-COCl via an amide linkage. Then, MWCNT-API was reacted with 1-bromobutane at 90 C to prepare the supported ionic liquid MWCNT-APIB. In the next step, the ZnPOM was reacted with MWCNT-APIB to obtain the ionic liquid supported catalysts. The amounts of Mo and W were measured by ICP. Both values showed that the catalyst loading is about 0.004 mmol/g. The catalyst was characterized by FT-IR spectroscopy, XRD and SEM analyses. The four typical skeletal vibrations of the polyoxometalate, ((n-C4H9)4N)5[PZnMo2W9O39], appear at 1057 (nP–O), 953 (nM=O), 889 (nM–Ob–M) and 807 cm1 (nM–Oc–M) (Ob: cornersharing; Oc: edge-sharing), respectively. The observed bands at 2870 and 2960 cm1 can be assigned to the C–H vibration of tetrabutyl ammonium [12]. After an ion exchange with polyoxometalate anion, four typical skeletal vibrations of the ZnPOM and C¼O stretching vibration (1634) of the support are observed in the heterogeneous catalyst (Fig. 1). The FT-IR spectra denoted that ZnPOM had been successfully supported on ionic liquid-modified MWCNT-APIB. The X-ray diffraction patterns of MWCNT-APIB, ZnPOM and [ZnPOM@APIB-MWCNT] are shown in Fig. 2. From these patterns, it is clear that ZnPOM has been introduced in the alternative position of MWCNT-APIB. According to SEM images of MWCNT-APIB and [ZnPOM@APIBMWCNT] (Fig. 3), a clear change in the morphology of catalysts pointed out that the ZnPOM has been immobilized on the MWCNT-APIB.
Fig. 1. FT-IR spectrum of [ZnPOM@APIB-MWCNT].
Please cite this article in press as: R. Hajian, Z. Alghour, Selective oxidation of alcohols with H2O2 catalyzed by zinc polyoxometalate immobilized on multi-wall carbon nanotubes modified with ionic liquid, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.12.003
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Fig. 2. XRD patterns of: (A) MWCNT-APIB; (B) ZnPOM and (C) [ZnPOM@APIB-MWCNT] composite.
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2.2. Catalytic activity
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The catalytic activity of the supported ZnPOM cluster was tested in the oxidation of benzyl alcohol to benzaldehyde with hydrogen peroxide. The reactions were continued until no further progress was observed. In order to choose the reaction media, various organic solvents were examined in the oxidation of benzyl alcohol. Among dichloromethane, acetonitrile, ethyl acetate, methanol and n-hexane, acetonitrile was selected as reaction media because the highest aldehyde yield was observed (Table 1, entries 1–5). The amounts of catalyst and H2O2 were also optimized and the higher yield was obtained using 0.004 mmol of catalyst and 5 mmol of H2O2 (Table 1, entries 6–9 and entries 10–13). The data of optimized of temperature was shown in Table 1 (entries 14–17). The time courses of ZnPOM, APIB-MWCN and MWCN under these reaction conditions are shown in Table 2. Under the optimized conditions, oxidation of different alcohols was investigated (Table 3). To study the catalytic activity of the
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Table 1 The effect of solvent, amounts of catalyst, H2O2, temperature on the oxidation of benzyl alcohol catalyzed by [ZnPOM@APIB-MWCNT] after 4 h. Entry
H2O2 (mL)
Catalyst (mmol)
Temp. ( C)
Solvent
Yield (%)a
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.125 0.25 0.50 0.75 0.50 0.50 0.50 0.50
0.004 0.004 0.004 0.004 0.004 0 0.002 0.004 0.008 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004
Reflux Reflux Reflux Reflux Reflux Reflux Reflux Reflux Reflux Reflux Reflux Reflux Reflux 25 45 65 Reflux
Dichloromethane Ethyl acetate n-Hexane Methanol CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN
50 75 80 40 95 5 75 95 90 75 90 95 95 7 16 40 95
a
GC yield based on the starting alcohol.
system, alcohol (0.5 mmol) and [ZnPOM@APIB-MWCNT] (50 mg, 0.004 mmol) in 3 mL CH3CN were introduced into a 25 mL roundbottom flask equipped with a magnetic stirrer (400 rpm). H2O2 (5 mmol, 30%) was added to this mixture and refluxed. The progress of the reactions was monitored by GC. At the end of the reaction, the catalyst was filtered and washed with Et2O (20 mL). The pure product was obtained after chromatography of organic layer on a short column of silica gel. In the presence of [ZnPOM@APIB-MWCNT] alcohols were converted to their corresponding aldehydes with 100% selectivity (Scheme 2). The electronic nature of the substituent exhibited little effect on the reaction process. It was assumed that reoxidation of the catalyst and not the dehydrogenation (a-H) step was ratelimiting [21]. The oxidation of the benzyl alcohol afforded benzaldehyde with an excellent yield of 95%. Substituted benzyl alcohol with donating and drawing groups such as NO2, Cl, OH, OMe, and t-Bu gave the desired products (Table 3, entries 2–6). Oxidation of 4-methoxy benzyl alcohol was oxidized to its corresponding aldehyde in excellent yield (95%). For 4-tert-butyl benzyl alcohol, the desired aldehyde was obtained in the yield of 70%. In the case of alcohols with electron-drawing substituents, 4nitro benzyl alcohol, 4-chloro benzyl alcohol, 4-hydroxo benzyl alcohol were oxidized to the corresponding aldehydes with yields of 60%–85%. Oxidation of 1,2-phenylethanol and 1,2-phenylpropanol were oxidized to 1,2-phenylethanal and 1,2-phenylpropanal with yield of 70% and 60% respectively.
Table 2 Oxidation of alcohols with H2O2 catalyzed by MWCNT, APIB-MWCNT and ZnPOM under reflux conditions.a Catalyst
MWCNT APIB-MWCNT ZnPOM
Yield (%)b 1h
2h
3h
4h
33 18 50
35 18 72
35 20 80
40 20 93
a Reaction conditions: alcohol (0.5 mmol), H2O2 (5 mmol), MWCNT and APIBMWCNT (50 mg) or ZnPOM (0.004 mmol), at reflux in CH3CN (3 mL). b GC yield based on the starting alcohol.
Please cite this article in press as: R. Hajian, Z. Alghour, Selective oxidation of alcohols with H2O2 catalyzed by zinc polyoxometalate immobilized on multi-wall carbon nanotubes modified with ionic liquid, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.12.003
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Fig. 3. Scanning electron micrographs of: (A) MWCNT-APIB and (B) [ZnPOM@APIB-MWCNT]. 148
2.3. Catalyst reuse and stability
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To assess lengthy cycle stability and reusability of [ZnPOM@APIB-MWCNT], oxidation of benzyl alcohol was selected as model reaction, and recycling experiments were carried out with a single
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Table 3 Oxidation of alcohols with H2O2 catalyzed by [ZnPOM@APIB-MWCNT] under reflux conditions.a Entry
Substrate
Product
b
1
Time (h)
Yield (%)
TOF (h
1
4
95
29.68
2
3.5
60
21.42
3
4
85
26.56
4
4
65
20.31
5
5
95
20
6
3.5
70
26.78
7
5
70
17.50
8
4
60
18.75
)
a Reaction conditions: alcohol (0.5 mmol), H2O2 (5 mmol), catalyst (50 mg, 0.004 mmol), CH3CN (3 mL). b GC yield based on the starting alcohol.
sample of the catalyst. After each experiment, the catalyst was removed by simple filtration, washed with acetonitrile and nhexane, dried at room temperature and reused. The catalyst was sequentially reused several times (Fig. 4). The filtrates were used for the determination of the catalyst leaching by atomic absorption spectroscopy (AAS). The results exhibited that in the first two runs some catalyst is leached from the support (1.27% for the first run and 0.12% for the second run), but in the next runs no leaching was monitored. The nature of the recovered catalyst was followed by FT-IR spectroscopy. The results indicated that the catalyst after reusing several times, demonstrated no change in its IR spectra (Fig. 5).
Scheme 2. Proposed reaction mechanism.
Fig. 4. The results obtained from catalyst reuse and stability in the oxidation of benzyl alcohol with H2O2 by [ZnPOM@APIB-MWCNT] under reflux conditions.
Please cite this article in press as: R. Hajian, Z. Alghour, Selective oxidation of alcohols with H2O2 catalyzed by zinc polyoxometalate immobilized on multi-wall carbon nanotubes modified with ionic liquid, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.12.003
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Fig. 5. FT-IR spectra of [ZnPOM@APIB-MWCNT] recovered in runs 1, 3, 5 and 7.
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3. Conclusion
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In this work, ((n-C4H9)4N)5[PZnMo2W9O39] was immobilized on ionic liquid-modified MWCNT-APIB. The zinc containing polyoxometalate was efficient in the oxidation of alcohols with H2O2 as an oxidant, producing the corresponding aldehydes in excellent selectivity. The major advantages of ionic liquidsupported catalyst were its high catalytic activity reusability.
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4. Experimental
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All materials were commercial reagent grade and obtained from Merck and Fluka. MWCNTs containing carboxylic acid groups (multi-wall carbon nanotube with purity, >95%; length, 10–50 mm; diameter, 10–30 nm, COOH content 2%, ash (catalyst residue) <1.5% and specific surface area >110 m2/g) was purchased from Neutrino Co. Ltd. Shenzen NTP Factory (China) and used without further purification. Hydrogen peroxide (30%) was titrated by a standard KMnO4 solution. FT-IR spectra were obtained as potassium bromide pellets in the range 400–4000 cm1 with a Bruker Equniox 55. Scanning electron micrographs (SEM) of the catalyst and support were taken on a SEM KYKY EM3200 instrument. Powder X-ray diffraction patterns were obtained on a D8 Advanced Bruker using Cu-Ka radiation (2u = 5–70 ). Gas chromatography experiments (GC) were performed with a Shimadzu GC-16A instrument using a 2 m column packed with silicon DC-200 or Carbowax 20 M. In GC experiments, n-decane was used as an internal standard. The ICP analyzes were performed on an ICP-Spectrociros CCD instrument. Atomic absorption analyses were carried out on a Shimadzu 120 spectrophotometer. ((n-C4H9)4N)5[PZnMo2W9O39] was prepared according to the literature [22]. Solid b-Na8HPW9O3424H2O was prepared by mixing Na2WO42H2O (60 g), H3PO4 (1.5 mL, 14.7 mol/L) and CH3COOH (11 mL, 17.4 mol/L) in 75 mL of water and the resulting solid was filtered. Then, white precipitate a-K7PMo2W9O3919H2O was obtained from aqueous solution of b-Na8HPW9O3424H2O (11 g), Na2MoO4 (20 mL, 1 mol/L), HCl (15 mL, 1 mol/L) and adjusting pH to 6–6.5 with HCl (1 mol/L) and KCl was added to this solution at the end. In the next step, a-K7PMo2W9O3919H2O (3.5 g) was dissolved in water (90 C) and ZnCl2 (0.5 g was dissolved in 5 mL water) was added and adjusting pH to 3 by HCl (1 mol/L). To
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this solution, [(n-C4H9)4N]Br was added and was stirred for 2 h, green solid, ((n-C4H9)4N)5[PZnMo2W9O39], was filtered off and then dried in vacuum. The ionic liquid support, APIB-MWCN, was prepared according to the method reported by Park et al. with some changes [19]. A suspension of MWCNTs (5 g) in SOCl2 (30.0 mL) was stirred at reflux for 3 h. After evaporation of the solvent, a solution of MWCNT-COCl (1 g) and 1-(3-aminopropyl) imidazole (3 mL) and triethylamine (3 mL) was stirred at reflux for 18 h under nitrogen atmosphere and thoroughly washed with CH2Cl2 and diethyl ether. Then, a mixture of MWCNT-API (80 mg) and 1-bromobutane (20 mL) was stirred reflux for 18 h at 90 C. The solid, APIB-MWCNT, was washed with anhydrous THF, HCl solution (1 mol/L), saturated NaHCO3 solution, and water successively. To a solution of ((n-C4H9)4N)5[PZnMo2W9O39] (1 g) in CH3CN, MWCNTAPIB (4 g) was added and refluxed for 24 h. Finally, the catalyst, [PZnMoW@APIB-MWCNT], was filtered and dried at room temperature.
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Acknowledgment
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We are thankful to the Yazd University Research Council for partial support of this work.
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References
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