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Communication

Fabrication of Amphiphilic Graphene Oxide—Polyoxometalate with Temperature Response for Epoxidation of Olefin in Water

by
Jinghui Wu
1,2,3,
Hongwei Zheng
1,3,
Chi Wang
1,3,
Ya Gao
1,3,
Xianze Wang
1,3,*,
Xiaohong Wang
1,3,* and
Mingxin Huo
1,3
1
School of Environment, Northeast Normal University, Changchun 130117, China
2
Key Laboratory of Songliao Aquatic Environment, Ministry of Education, Jilin Jianzhu University, Changchun 130118, China
3
Key Lab of Polyoxometalate Science of Ministry of Education, Northeast Normal University, Changchun 130024, China
*
Authors to whom correspondence should be addressed.
Crystals 2020, 10(12), 1077; https://doi.org/10.3390/cryst10121077
Submission received: 30 October 2020 / Revised: 18 November 2020 / Accepted: 22 November 2020 / Published: 25 November 2020
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Abstract

:
Here, amphiphilic graphene oxide–polyoxometalate (GO-POM) was fabricated using a new strategy involving control of the stacking of GO lamellae through phosphoric acid and exfoliation by H2O2. The additions of H3PO4 and H2O2 were essential for the formation of the catalytic center of peroxo-POMs. The GO-POM hybrid had one side with hydrophilic properties and another side with hydrophobic properties, which conferred temperature-responding properties. GO-POM could catalyze the epoxidation of cyclooctene with complete conversion and 98% selectivity for epoxide at 50 °C for 12 h in water. Meanwhile, the catalyst could be easily recycled because of its thermosensitive property.

1. Introduction

Polyoxomolybdates (POMs) are built by transition metal–oxygen clusters with various structures and properties that have been used in various fields, such as materials [1,2], medicine [3], and catalysis [4,5]. POMs exhibit high Brønsted acidity and rapid redox transformation under mild conditions, which enables them to be used as efficient acid or oxidation catalysts [6]. Peroxo-polyoxometalates (PPOMs) [7] or peroxometalates have been found to be efficient catalysts for the epoxidation of olefin by hydrogen peroxide in biphasic catalysis [8,9,10,11,12,13,14,15,16,17], or in ionic liquid systems [18]. However, the recovery and separation of the catalysts were still problems [19]. One way to overcome these drawbacks is to immobilize the catalysts on mesoporous materials [20], dendrimers [21], and hyperbranched polymers [22]. However, the epoxidation of olefin in cheap, safe solvent such as water is more widely available, as well as being more in line with the concept of “green chemistry”. In earlier studies, the Cu(II) phthalocyanine–tetrasulfonic acid tetrasodium complex (CuPcS), loaded on silica coated with magnetic nanoparticles [23], was used for the epoxidation in water, and resulted in a 96% yield for cyclohexene using tetra-n-butylammonium peroxomonosulfate as an oxidant. Polymer-supported vanadium complexes exhibited a 100% conversion of cyclohexene and a 92% yield of oxidized by t-butyl hydroperoxide (TBHP) in water at 60 °C for 12 h [24]. Only a few studies have examined the epoxidation of olefin in water by POM catalysts, including their homogeneous form or supported on silica gel or polymer [25,26]. Peroxometalates [{W = O(O2)2}2(µ-O)] immobilized on magnetic support could act as magnetically separated catalyst in the epoxidation process in water with a 59.5% conversion of cyclooctene and a 58.9% yield of epoxide [26] at 60 °C for 3 h. [p-C5H5N+(CH2)15CH3]3(PW12O40) was modified on hydrophobic mesoporous silica gel, which was applied in the selective epoxidation of olefins with 15% aqueous H2O2, without the use of organic solvent, at 90 °C with 100% conversion and 98% yield [25]. There is thus much room for the improvement of the epoxidation of olefin with a green oxidant in water (Table 1).
Graphene has a two-dimensional (2D) structure, large specific surface area (theoretical value to ~2600 m2/g), and excellent charge mobility [28,29,30,31]. Graphene oxides (GO) have been used as catalysts or supports for chemical, electrochemical or optical reactions [32,33,34,35,36]. Graphene–POM hybrids have usually been obtained through the photo-reduction of GO using POMs as reducing agents [37,38,39,40], which are used as sensor agents or electrochemical catalysts. However, no studies have examined chemical catalysis by graphene–POM hybrids, and the activity of such POM catalysts is not sufficient compared with homogeneous ones because of limitations associated with the assessment organic substrates in POM sites. To develop an efficient heterogeneous catalyst for the epoxidation of olefin in water, we synthesized GO-supported POM materials (GO-POMs) as catalysts in the epoxidation reaction in water. The major advantages of this strategy include the (1) facile fabrication of GO-POM material using pH-controlled GO exfoliation/stacking, (2) one hydrophilic and one hydrophobic layer in a GO-POM, acting as a solid surfactant, (3) efficiency for the epoxidation in water, and (4) easy separation by decreasing the reaction temperature, and recoverability by filtration after completion of the reaction.

2. Materials and Methods

The fabrication of this material was completed as follows (Scheme 1) (Details in Supplementary Materials): (1) GO was dispersed in water and first coated with peroxotungstates obtained by reacting Na2WO4 with H2O2, which provided a hydrophilic layer on GO; (2) H3PO4 was added, which led to the stacking of GO lamellae as the pH decreased. pH is known to be very important in the exfoliation/stacking processes of GO lamellae [41], and a low pH leads to the stacking of GO lamellae. The other role of H3PO4 is to react with peroxotungstates to form peroxo-phosphotungstates (PPT), which provide the catalytic center for the epoxidation reaction; (3) Surfactant N,N-dimethylhexadecan-1-amine (DMA16) was added to the above mixture and reacted with peroxo-phosphotungstates through anion–cation interaction to fabricate a hydrophobic layer; (4) Lastly, H2O2 was added to exfoliate the GO lamellae to form the GO-POM hybrids containing one hydrophilic PPT side and one hydrophobic side. As result, amphiphilic GO-POM was fabricated using this strategy. To successfully fabricate this material, the additions of H3PO4 in the second step and H2O2 in the final step are essential.

3. Results

The graphene was characterized by TEM, BET, Raman, XPS, XRD, and HR-TEM (Figure 1). The graphene showed a large specific surface area and slice layer structure. The unique characteristics of this GO-POM included its temperature-responding property. That is, the dispersion of GO-POM in water can be modified by controlling the temperature. GO-POM hybrids aggregate in cold water (Figure 2a), and disperse homogeneously in hot water (50 °C) to form a highly uniform system. GO materials with temperature-induced transitions are known to result from both GO–water and GO–GO interactions, which are thought to be controlled by the intra/interlayer interactions of hydrogen bonds. At low temperatures, the effect of the hydrogen bond is strong, leading to the folding of GO-POMs with the hydrophilic layer inside and hydrophobic layer outside. Consequently, the GO-POM exhibited a hydrophobic property (Figure 2c left). With an increasing temperature, the hydrogen bond became weaker, leading to the folding of the GO-POM with the hydrophobic layer inside and the hydrophilic layer outside; as a result, the GO-POM had hydrophilic properties at higher temperatures (Figure 2c right).
In the water/toluene biphase, the behavior of the GO-POM further supported our hypothesis (Figure 2b). The structure of the GO-POM was similar to a surfactant, with a hydrophilic part and a hydrophobic part. At low temperatures, it was hydrophobic and could disperse well in toluene. When the temperature was increased to 50 °C, the toluene molecules were encapsulated by the GO-POM by its hydrophobic part (Figure 2d) to form the O/W system. This finding also confirmed the high efficiency of the reaction of olefin in water through the concentration of organic olefin. Therefore, the catalyst can be separated by simple filtration or centrifugation from the reaction mixture by controlling the temperature. The product can also be separated by extraction from the remaining aqueous solution.
The GO-POM material was firstly characterized by FT-IR (Figure 3). Nine peaks were observed at 521 cm−1, 564 cm−1, 607 cm−1, 723 cm−1, 771 cm−1, 833 cm−1, 935 cm−1, 1049 cm−1, and 1076 cm−1. The peaks at 522 cm−1 and 588 cm−1 were attributed to the vibration of the W-O-O-W bond [42]. The peaks at 935 cm−1 and 833 cm−1 were attributed to the stretching vibrations of the W-O and O-O (peroxo–oxygen bond) bonds, respectively [43]. The peaks at 1076 cm−1 and 1049 cm−1 were attributed to the stretching vibration of P-O [42]. The strong peaks at 2919 cm−1, 2851 cm−1, and 1470 cm−1 were attributed to alkyl chains and the methyl group of cetamine oxide [C16H33(CH3)2NOH]. ICP-AES showed that the tungsten content and the phosphorus content were 14.7 wt. % and 0.7 wt. %, respectively.
The XPS confirmed that two types of tungsten with oxidation states of 6+ and 5+ were present in the GO hybrid (Figure 4), while the peaks at 35.2 and 34.5 eV were corresponding to W (VI) and W (V) [44], respectively. The presence of W (V) was attributed to the formation procedure. Firstly, Na2WO4 reduced GO to form reduced GO and W (V), and the tungsten species were linked on the surface of the GO. H2O2 oxidized the reduced GO and W (V). Some surfactant N,N-dimethylhexadecan-1-amine was then added, which surrounded some tungsten atoms and prevented them from being oxidized by H2O2 (Table 2). Consequently, some W (V) was present. The molar ratio of W (VI) to W (V) was approximately 0.79 according to the relative peak intensity of W4f. The XPS N1s peak (Figure 5) at a binding energy of 402.7 eV was assigned to the nitrogen in cetamine oxide [45]. The O 1s XPS (Figure 6) features at 532.9 eV were attributed to the peroxide [46].
TiO2-Photocatalyzed Epoxidation of 1-Decene by H2O2 under Visible Light. The solid-state 31P NMR spectroscopy of GO-POMs was also measured to investigate the species of peroxophosphotungstate. The spectrum of the fresh catalyst (Figure 7) gave two main peaks at 4.6 and −11.5 ppm. The one at −11.5 ppm was assigned to heteropoly tungstophatosphates with a Keggin structure, which was close to the value (−11.7 ppm) described by Radkov and Beer [47] for (n-Bu4N)4H3[PW11O39]. The GO-POM gave a weak signal at −13.6 ppm related to the saturated Keggin structure because the highly nucleophilic monovacant lacunary Keggin units tended to be saturated by grafting with electrophilic C-OH groups, and this finding is consistent with a previous study [48]. According to the results of previous studies [14,49,50,51], it had been reported that the [31] P NMR peaks at 4.1, 2.6 and −1.5 ppm were attributed to [(PO4){WO(O2)2}2{WO(O2)2(H2O)}]3−, [(PO4){WO(O2)2}4]3−, and [(PO3(OH)){WO(O2)2}2]2−, respectively. Thus, the broad peak centered at 4.6 ppm could be decomposed into three peaks at 4.6, 2.6, and −2.0 ppm. These three peaks could be assigned to [(PO4){WO(O2)2}2{WO(O2)2(H2O)}]3−, [(PO4){WO(O2)2}4]3− and (PO3(OH)){WO(O2)2}2]2−. There were the three species of peroxotungstates in GO-POM.
Based on scanning electron microscope (SEM) images (Figure 8), the GO-POM sheets were found to be self-folded (Figure 2c). The EDS analysis of the GO-POM surface showed that the W loading amount was 39.0 wt. %. The EDAX results (Figure 9) suggest that the molar ratio of P:W was 2.4:1.
This GO-POM hybrid was used in the epoxidation of cyclooctene by H2O2. An experiment was then conducted as follows: cyclooctene (1 mmol) was mixed with an oxidant (2 mmol) and catalyst (26 mg) in water (10 mL) under air at 50 °C, then left to react for 20 h. The different oxidants, including H2O2, Oxone® and t-butylhydroperoxide (TBHP), were used to evaluate the catalytic activity of GO-POMs (Table 3, entries 1–3). Only trace amounts of the oxidation products were observed with Oxone® and TBHP catalyzed by GO-POM. H2O2 promoted the epoxidation reaction of cyclooctene, which resulted in a 92% conversion and 84% yield of the epoxide within 20 h. The influence of four solvents, including water, [Bmim]BF4, ethyl acetate, and acetonitrile, was also assessed, which revealed that ionic liquid could not promote the conversion of cyclooctene nor increase the yield of the epoxide.
Our results also showed that the yields were greatly affected by the amount of catalyst. The desired reaction rate and conversion were achieved by using 52 mg of the catalyst (Table 3, entry 7). To optimize the yields of the oxidation product, less water (5 mL) should be used. Under these optimal conditions, it was completely converted within 12 h.
After the epoxidation of cyclooctene, the mixture was lowered to room temperature. The GO-POM catalyst was decanted to the bottom of the reactor and separated easily because of its thermosensitive property (Figure 10). The epoxide was extracted with diethyl ether. After eight cycles, the yield of cyclooctene oxide obtained was as high as 92%. The stability of the GO-POMs was investigated by IR spectroscopy. No changes were observed before or after catalytic epoxidation (Figure 3).

4. Conclusions

In sum, we developed a highly efficient, thermosensitive GO-POM hybrid that could be used as an ecofriendly epoxidation catalyst in water. As an easily accessible catalyst, the ease of separation of the produce and the use of green water make this method particularly useful among contemporary methods of epoxide synthesis.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4352/10/12/1077/s1.

Author Contributions

J.W., X.W. (Xianze Wang), X.W. (Xiaohong Wang) and M.H. conceived and designed the experiments; J.W., H.Z., C.W., and Y.G. performed the experiments; J.W., X.W. (Xianze Wang) and X.W. (Xiaohong Wang) analyzed the data; J.W. and M.H. contributed analytical tools; J.W., X.W. (Xianze Wang) and X.W. (Xiaohong Wang) wrote the paper. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (51978134 and 51908241, 51708094), Jilin Provincial Science and Technology Department (20180414069GH), Fundamental Research Funds for the Central Universities (2412020FZ012) and Environmental Protection Scientific Research Project (JiHuanKeZi No. 2020-14).

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 10 01077 sch001 550
Scheme 1. Fabrication of GO-POMs.
Scheme 1. Fabrication of GO-POMs.
Crystals 10 01077 sch001
Crystals 10 01077 g001 550
Figure 1. Characterization of the GO (a) TEM-5 nm, (b) TEM-0.5 μm, (c) Raman, (d) XPS, (e) XRD, (f) HR-TEM.
Figure 1. Characterization of the GO (a) TEM-5 nm, (b) TEM-0.5 μm, (c) Raman, (d) XPS, (e) XRD, (f) HR-TEM.
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Figure 2. The thermo-responsive property of GO-POM: (a) photograph of the aqueous dispersions of GO-POM; (b) photograph of the emulsion stabilized by GO-POM with an oil-to-water volume ratio of 1:1 (left: room temperature; right: after heating to 50 °C); (c) hydrophobic and hydrophilic nature of GO-POM at different temperatures; and (d) emulsion stabilized by GO-POM.
Figure 2. The thermo-responsive property of GO-POM: (a) photograph of the aqueous dispersions of GO-POM; (b) photograph of the emulsion stabilized by GO-POM with an oil-to-water volume ratio of 1:1 (left: room temperature; right: after heating to 50 °C); (c) hydrophobic and hydrophilic nature of GO-POM at different temperatures; and (d) emulsion stabilized by GO-POM.
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Figure 3. FT-IR spectra of the catalyst: (a) fresh catalyst, (b) used catalyst.
Figure 3. FT-IR spectra of the catalyst: (a) fresh catalyst, (b) used catalyst.
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Figure 4. XPS spectrum of the W4f region for the catalyst.
Figure 4. XPS spectrum of the W4f region for the catalyst.
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Figure 5. XPS spectrum of the N1s region for the catalyst.
Figure 5. XPS spectrum of the N1s region for the catalyst.
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Figure 6. XPS spectrum of the O1s region for the catalyst.
Figure 6. XPS spectrum of the O1s region for the catalyst.
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Figure 7. 31P MAS NMR spectrum of the catalyst.
Figure 7. 31P MAS NMR spectrum of the catalyst.
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Figure 8. SEM image of GO-POM.
Figure 8. SEM image of GO-POM.
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Figure 9. EDAX spectrum of the catalyst.
Figure 9. EDAX spectrum of the catalyst.
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Figure 10. Different stages of epoxidation: (a) Before reaction; (b) during the reaction; (c) after the reaction.
Figure 10. Different stages of epoxidation: (a) Before reaction; (b) during the reaction; (c) after the reaction.
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Table
Table 1. The epoxidation of olefin by different catalysts.
Table 1. The epoxidation of olefin by different catalysts.
EntryCatalystSolutionConversionYield
1CuPcS @ASMNP [23]water92%92%
2c-PMAn-V [24]water100%92%
3MNP-[HDMIM]2[W2O11] [26]water59.5%58.9%
4[p-C5H5N+(CH2)15CH3]3(PW12O40) [25]water100%98%
5[(C4H9)4 N]4[SiW12O40] [27]water87.1%88%
Table
Table 2. Peak-fitting results of W4f XPS spectra of the catalyst.
Table 2. Peak-fitting results of W4f XPS spectra of the catalyst.
PeakPosition (eV)AreaFWHM (eV)% GL
037.374837.5421.02480
135.2241116.7221.03780
236.651058.7890.85580
334.51411.7190.82480
Table
Table 3. Epoxidation of cyclooctene with different catalysts, solvents and oxidants.
Table 3. Epoxidation of cyclooctene with different catalysts, solvents and oxidants.
EntryCatalystSolventOxidantTime (h)Conversion (%)Yield (%)Selectivity (%)
1CAT IH2OH2O220928492
2CAT IH2OOxone®209866
3CAT IH2OTBHP2030.310
4 aCAT I[Bmim]BF4H2O220764559
5CAT IEthyl acetateH2O22011980
6CAT IacetonitrileH2O22012325
7 bCAT IH2OH2O2121009494
Reaction conditions—catalyst: 0.026 g, H2O2: 2 mmol, cyclooctene: 1 mmol, solvent: 10 mL, temperature: 323 K, and reaction time: 20 h. Yield (%) = products (mol)/cyclooctene (mol) × 100. a: 1mL [Bmim]BF4. b: 0.052 g catalyst, 5 mL H2O. Conversions and selectivities were determined by gas chromatography using an internal standard technique.
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Wu, J.; Zheng, H.; Wang, C.; Gao, Y.; Wang, X.; Wang, X.; Huo, M. Fabrication of Amphiphilic Graphene Oxide—Polyoxometalate with Temperature Response for Epoxidation of Olefin in Water. Crystals 2020, 10, 1077. https://doi.org/10.3390/cryst10121077

AMA Style

Wu J, Zheng H, Wang C, Gao Y, Wang X, Wang X, Huo M. Fabrication of Amphiphilic Graphene Oxide—Polyoxometalate with Temperature Response for Epoxidation of Olefin in Water. Crystals. 2020; 10(12):1077. https://doi.org/10.3390/cryst10121077

Chicago/Turabian Style

Wu, Jinghui, Hongwei Zheng, Chi Wang, Ya Gao, Xianze Wang, Xiaohong Wang, and Mingxin Huo. 2020. "Fabrication of Amphiphilic Graphene Oxide—Polyoxometalate with Temperature Response for Epoxidation of Olefin in Water" Crystals 10, no. 12: 1077. https://doi.org/10.3390/cryst10121077

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