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

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.


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(O 2 ) 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-C 5 H 5 N + (CH 2 ) 15 CH 3 ] 3 (PW 12 O 40 ) was modified on hydrophobic mesoporous silica gel, which was applied in the selective epoxidation of olefins with 15% aqueous H 2 O 2 , 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 tõ 2600 m 2 /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.

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 Na 2 WO 4 with H 2 O 2 , which provided a hydrophilic layer on GO; (2) H 3 PO 4 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 H 3 PO 4 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, H 2 O 2 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 H 3 PO 4 in the second step and H 2 O 2 in the final step are essential.

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.

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 (peroxooxygen 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.   [42]. The peaks at 935 cm −1 and 833 cm −1 were attributed to the stretching vibrations of the W-O and O-O (peroxooxygen 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, Ndimethylhexadecan-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].  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, Na 2 WO 4 reduced GO to form reduced GO and W (V), and the tungsten species were linked on the surface of the GO. H 2 O 2 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 H 2 O 2 ( 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 W 4f . The XPS N 1s 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].  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, Ndimethylhexadecan-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 31 P 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    TiO2-Photocatalyzed Epoxidation of 1-Decene by H2O2 under Visible Light. The solid-state 31 P 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   (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-Bu 4 N) 4 H 3 [PW 11 O 39 ]. 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.  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.   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.  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.    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 H 2 O 2 . An experiment was then conducted as follows: cyclooctene (1 mmol) was mixed with an oxidant (2 mmol) and catalyst Crystals 2020, 10, 1077 8 of 11 (26 mg) in water (10 mL) under air at 50 • C, then left to react for 20 h. The different oxidants, including H 2 O 2 , 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. H 2 O 2 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]BF 4 , 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). Crystals 2020, 10, x FOR PEER REVIEW 8 of 12 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).

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