Phosphotungstic Acid Immobilized on Amine-Grafted Graphene

After 24 h, the solvent was removed by filtration to obtain the base-modified graphene oxide (GOA), which was extensively washed with ethanol and then...
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Phosphotungstic Acid Immobilized on Amine-Grafted Graphene Oxide as Acid/Base Bifunctional Catalyst for One-Pot Tandem Reaction Wenfeng Zhang, Qingshan Zhao, Tong Liu, Yuan Gao, Yang Li, Guoliang Zhang, Fengbao Zhang, and Xiaobin Fan* State Key Laboratory of Chemical Engineering, Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Collaborative Innovation Center of Chemical Science and Engineering, Tianjin University, Tianjin 300072, China S Supporting Information *

ABSTRACT: In this study, phosphotungstic acid immobilized on amine-grafted graphene oxide (GOAP) was prepared successfully by silylanization and electrostatic interaction. The obtained GOAP was characterized by Raman spectroscopy, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM), zeta potential measurements, and X-ray photoelectron spectroscopy (XPS). Systematic studies demonstrated that GOAP had excellent catalytic activities and robustness in the one-pot tandem deacetalization−nitroaldol reaction. The key role of the acid/base ratio in the catalytic performance of this bifunctional catalyst was also revealed by preliminary kinetic studies. This bifunctional catalyst might find promising applications in green chemistry, as it can not only reduce costs and waste by saving separation/purification steps and solvents/reagents, but also increase the yield by avoiding the separation of intermediate products.

1. INTRODUCTION 1

2. EXPERIMENTAL SECTION 2.1. Preparation of Acid/Base Bifunctional Graphene Oxide Catalyst. Graphene oxide (GO) was prepared and purified by the Hummers method.34 The resulting GO suspension (30 mL, 10 mg mL−1) was added to 150 mL of ethanol containing APTES (0.5280 g), and the mixture was stirred at room temperature for 30 min in a three-necked round flask (250 mL). Then, 0.4265 g of [3-(diethylamino)propyl]trimethoxysilane was added, and the reaction mixture was refluxed. After 24 h, the solvent was removed by filtration to obtain the base-modified graphene oxide (GOA), which was extensively washed with ethanol and then methanol. For the immobilization of phosphotungstic acid, the GOA was stirred with 150 mL of a methanolic solution of 0.2 g of phosphotungstic acid at 298 K. After 12 h, the solvent was removed by filtration, and the resulting solid was washed in warm water (333 K) and then dried under a vacuum, affording the acid/base bifunctional catalyst amine-grafted graphene oxide (GOAP). 2.2. Catalytic Ability. The reactions were carried out in a 50 mL three-necked round flask equipped with a condenser and a magnetic stirrer. Into the flask were placed 50 mg of the catalyst, 2 mmol of benzaldehyde dimethyl acetal, 10 mL of deionized water, and 10 mL of nitromethane. The resulting mixture was vigorously stirred at 100 °C under N2. After 18 h, the catalyst was separated by filtration, and the filtrate was analyzed by gas chromatography (GC).

2

Graphene, with its distinctive two-dimensional structure, huge surface area,3 and excellent mechanical4−6 and electrontransfer7−11 properties, has emerged as a promising support for heterogeneous catalysis. In particular, unlike other common catalyst supports, graphene has a two-dimensional structure that allows reactive species on it to be readily accessed with limited mass-transfer resistance. Therefore, many graphenebased metallic nanocatalysts12−26 such as CoFe2O4,13 Mn3O4,14 Au,17 Pd,18 FePt,19 and PtAu20 have shown superior catalytic performances in various heterogeneous reactions. Some organic molecules have also been immobilized on graphene as promising heterogeneous catalysts.27−33 For example, Ji et al. used benzenesulfonated graphene for the hydrolysis reaction of ethyl acetate.27 Yuan et al. investigated the catalytic behaviors of triethylamine-treated graphene oxide in the polymerization reaction.28 Liu et al. synthesized 3-aminopropyltriethoxysilane(APTES-) coated graphene oxide for the anchoring of phosphotungstic acid, which was an efficient catalyst for the oxidation of benzyl alcohol.29 Li et al. immobilized lipase on carboxyl-functionalized graphene oxide for the catalysis of olive oil hydrolysis.30 Xue et al. catalyzed the oxidation reaction of pyrogallol by hemin-grafted graphene.31 Recently, we found that a primary and tertiary amine bifunctional graphene oxide shows a good synergic effect in the classic trans-β-nitrostyrene-forming reaction.33 Herein, we report that amine-functionalized graphene oxide can be further functionalized with phosphotungstic acid through electrostatic interaction and shows interesting acid/base synergistic catalytic performance in the one-pot deprotection−nitroaldol reaction. © 2013 American Chemical Society

Received: Revised: Accepted: Published: 1437

October 9, 2013 December 20, 2013 December 27, 2013 December 27, 2013 dx.doi.org/10.1021/ie403393u | Ind. Eng. Chem. Res. 2014, 53, 1437−1441

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The recovered catalyst was washed with ethanol and deionized water and then dried under a vacuum. The catalyst was then reused for the above one-pot reaction. 2.3. Characterization. The samples were characterized by Raman spectroscopy (NT-MDT NTEGRA Spectra), scanning electron microscopy (SEM) (Hitachi S4800), energy-dispersive spectroscopy (EDS) (Hitachi S4800), high-resolution transmission electron microscopy (HRTEM) (Philips Tecnai G2 F20), X-ray photoelectron spectroscopy (XPS) (PerkinElmer, PHI 1600 spectrometer), and zeta potential measurements (Malvern, nano ZS). The reaction results were measured by GC (Agilent 6890N GC-FID system) and inductively coupled plasma optical emission spectrometry (ICP-OES) (Varian Company, Palo Alto, CA, VisTA-MPX).

3. RESULTS AND DISCUSSION Scheme 1 illustrates the preparation of the acid/base bifunctional catalyst from graphene oxide (GO). The base Scheme 1. Illustration of the Preparation of Acid/Base Bifunctional Catalyst

Figure 1. XPS spectra of (a) GO and (b) GOAP, of (c) GO and (d) GOAP in the C 1s region, and of GOAP in the (e) N 1s and (f) W 4p regions.

silane precursors. This conclusion was further supported by Raman analysis (Figure S2, Supporting Information). The corresponding high-resolution XPS N 1s spectrum of GOAP (Figure 1e) can be deconvoluted to three peaks. The two main peaks (integral area ratio of about 1:1) at 401.8 and 399.5 eV correspond to primary and tertiary amines, respectively. The last peak at 400.7 eV represents the H-bonding between amine groups. Note that the element atomic contents of N and W in GOAP were 4.27% and 3.21%, respectively. That is, the densities of the introduced amine and phosphotungstic acid groups were about 2.17 and 0.14 mmol g−1 (calculated using the W atomic ratio), respectively. Figure 2 shows the scanning electron microscopy (SEM) image and corresponding quantitative energy-dispersive X-ray spectroscopy (EDS) mapping of GOAP. Figure 2a reveals that the GOAP had crumpling structures, in accordance with the HRTEM observations (Figure S3, Supporting Information). The element distribution analysis demonstrated that the elements N, Si, O, P, and W (Figure 2b−f) were homogeneously distributed on the entire surface of GOAP, indicating the uniform attachment of the silanes with terminal amine and phosphotungstic acid groups on GO. Note that there is a discrepancy in the element content between EDS and XPS, probably due to the different resolutions of EDS and XPS, from element to element. In addition, given that XPS can penetrate only about nanometers below the surfaces whereas the sampling depth of EDS is in the range of 1 μm, some element signal from the backside of the GOAP sheet might be “shielded” in the XPS analysis, especially after functionalization that increases the thickness of GO. The crumpling characteristic of GOAP, along with the homogeneous distribution of the active molecules, should have potential advantages in catalytic reactions, because reactants and products can easily access or

catalyst (GOA) was obtained by grafting silanes with terminal primary and tertiary amine groups on GO through silylanization, including the hydrolysis and dehydration condensation of the silanes with the hydroxyl and epoxy groups on GO. Then, GOA was stirred in a methanolic solution of phosphotungstic acid (H3PW12O40), which was immobilized to give the acid/ base catalyst (GOAP) through the electrostatic interaction with the primary amine groups. By controlling the ratio of immobilized phosphotungstic acid, only part of the primary amine was occupied, creating acid/base catalytic sites containing phosphotungstic acid, primary amine, and tertiary amine on GO. The successful preparation of GOAP was first verified by Xray photoelectron spectroscopy (XPS) of the samples of GO and GOAP. Compared with GO (Figure 1a,c), GOAP showed a significant decrease in C−O−C and C−OH bonds (Figure 1d), accompanied by the emergence of N, Si, W, and P peaks (Figure 1b,f). A pronounced peak at 102.3 eV in the Si 2p XPS spectrum (Figure S1 of the Supporting Information) can also be easily observed and is due to the formation of abundant Si− O−C. The decrease in the C−O−C and C−OH bonds, together with the formation of Si−O−C, indicates the successful hydrolysis and dehydration condensation of the hydroxyl and epoxy groups on GO with the amine-terminal 1438

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Scheme 2. One-Pot Tandem Deprotection−Nitroaldol Reaction

deacetalization of A to benzaldehyde B, followed by the basecatalyzed reaction that converts B with nitromethane to the final product C. As summarized in Table 1, a high conversion of Table 1. Tandem Deacetalization−Nitroaldol Reaction Resultsa conversion (%)

yield (%)

rank

catalyst

substrate

A

B

C

1 2 3 4 5

GOAPb GOc GOAd GOA GOAP′e

A A B A A

100 100 − 100 100

0.8 100 − 20.3 20

99.2 0 99.5 79.7 80

a

Reaction conditions unless otherwise stated: A, 2 mmol; CH3NO2, 10 mL; H2O, 10 mL; catalyst, 50 mg; 100 °C; 18 h. bAcid/base ratio around 0.2:1. cReaction conditions: A, 2 mmol; CH3NO2, 10 mL; H2O, 10 mL; GO, 50 mg; 100 °C; 6.5 h. dReaction conditions: B, 2 mmol; CH3NO2, 10 mL; catalyst, 50 mg; 100 °C; 10 h. eAcid/base ratio around 0.5:1.

A (100%) and a satisfactory yield of C (99.2%) were obtained in reactions using GOAP (polyanion-to-amino ratio of 0.2:1) as the catalyst (entry 1 in Table 1). Compared with existing studies,35−37 the graphene-based bifunctional catalyst developed here exhibited a higher product yield with a lower catalyst loading. This superior performance should be attributed to the two-dimensional structure of GO, as it has a larger surface area for the immobilization of acid and base functional groups. In addition, the reactive species can readily reach or leave the catalytic active sites on GO with limited mass-transfer resistance. In control experiments with GO, although the successful conversion of A to B was observed, attributed to the acidic carbonyl groups on GO, no further reaction occurred in the second step (entry 2 in Table 1). Obviously, reactions starting from B verified that GOA (polyanion-to-amino ratio of 0:1) is a superior base catalyst for the second step, because it can convert benzaldehyde with nitromethane to C in a yield of 99.5% (entry 3 in Table 1). Interestingly, GOA and GOAP′ (polyanion-to-amino ratio of 0.5:1) with different compositions showed identical performances (both final yields of C were about 80% after 18 h). To probe the mechanisms involved, preliminary kinetic studies were carried out. As we expected, typical kinetics of two-step consecutive reactions were clearly observed during parallel reactions with GOA, GOAP′, and GOAP as catalysts. Figure 4 shows that the intermediate product B that resulted from the acid-catalyzed reaction increased quickly at the beginning and then decreased slowly because of its subsequent conversion during the basecatalyzed reaction. Meanwhile, the final product C gradually increased after a different onset time. According to the classic kinetic model of two-step consecutive reactions (eq S1, Supporting Information), the yield of product C can be expressed as

Figure 2. SEM image of (a) GOAP and corresponding quantitative EDS element mappings of (b) N, (c) Si, (d) O, (e) P, and (f) W.

leave the active sites on both sides of the two-dimensional GOAP. Figure 3 shows the zeta potentials of GO, GOA, and GOAP. Because of the negatively charged carboxylate groups, GO

Figure 3. Zeta potentials of GO, GOA, and GOAP.

displays a zeta potential of −32.6 mV under neutral conditions. After the silylation reaction, the obtained GOA showed positively charged surfaces with a zeta potential of 7.1 mV, which is assigned to the abundant grafted amine groups. The final result for the negatively charged surfaces in GOAP (−19.0 mV) provides direct evidence for the successful functionalization of phosphotungstic acid by electrostatic interaction with terminal primary amines. To test the catalytic performance of the bifunctional catalyst, the one-pot consecutive reaction of benzaldehyde dimethyl acetal A to trans-β-nitrostyrene C was used as a model reaction (Scheme 2). The first step of this reaction is the acid-catalyzed 1439

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Figure 5. Reuse experiments of the GOAP catalyst.

although the yield of C gradually decreased (by ∼10% after four runs). Inductively coupled plasma optical emission spectrometry (ICP-OES) characterization revealed that less than 1% of the tungsten content was lost from the GOAP catalyst even after the four-run experiment (Table S1, Supporting Information). In addition, the reused GOAP showed no obvious difference in morphology under SEM observations (Figure S4, Supporting Information). Therefore, we believe that the leakage of the immobilized phosphotungstic acid from GOAP can be neglected and the gradual decrease in the yield of C can be attributed to the weight loss of the GOAP catalyst during the interval separation processes.

4. CONCLUSIONS In summary, functional graphene oxide that combines catalytically active amines and phosphotungstic acid was successfully prepared by silylanization and electrostatic interaction. Systematic studies demonstrated that these active amines and polyanions could be readily introduced onto the graphene oxide sheets with homogeneous distributions. Notably, the obtained bifunctional graphene oxide (GOAP) shows excellent catalytic performances and synergistic catalytic effects in the one-pot tandem deacetalization−nitroaldol reaction. We also found by preliminary kinetic studies that the acid/base ratio of the bifunctional catalyst plays a key role in the consecutive acidand base-catalyzed reactions. Such bifunctional catalysts can not only reduce costs and wastewater by saving separation/ purification steps and solvents/reagents, but also increase the yield by avoiding the separation of intermediate products.

Figure 4. Yields of B and C as functions of time for (a) GOA, (b) GOAP′, and (c) GOAP.

⎧ k exp( −k 2t ) − k 2 exp( −k1t ) ⎫ ⎬[A 0] [C] = ⎨1 + 1 k 2 − k1 ⎩ ⎭

(1)

where k1 and k2 represent the rate constants of the first acidcatalyzed reaction and the later base-catalyzed reaction, respectively. They should be correlated with the densities of the catalytically active sites and sensitive to the acid/base ratio. For GOA, which has only a few carboxyl groups from GO (Figure 4a), k1 is smaller (compared with those for GOAP′ and GOAP), and only limited intermediate product B was available at the beginning, resulting in the longest onset time of product C (6 h) and an unsatisfactory final yield (about 79.7% after 18 h). In the case of GOAP′ (Figure 4b), abundant acid groups (including the immobilized phosphotungstic acid and the acidic carbonyl groups from GO) were available, and a larger k1 value should be expected. However, a relatively smaller k2 value limits the rapid conversion of B (yield of C was 80% after 18 h). Therefore, GOAP with an appropriate acid/base ratio shows the best catalytic performance, with a yield toward C of 99.2% after 18 h (Figure 4c). In a word, the deacetalization−nitroaldol reaction requires both acid and base to perform two distinct catalytic cycles for the consecutive reactions. That is, the substrate A first reacts to form an intermediate product B in a first acid-catalytic cycle, and this intermediate is converted to the final product C in an independent base-catalytic cycle. To achieve the best catalytic performance (“synergistic catalytic effect”), an appropriate acid/base ratio should be employed, because the performance is sensitive to the acid/base ratio. The stability and reusability of the GOAP catalyst were also tested by repeated experiments (Figure 5). Four-run experiments showed that the conversion of A was always 100%,

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ASSOCIATED CONTENT

S Supporting Information *

Characterization and kinetic model of the deacetalization− nitroaldol reaction. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: (+86) 22-27408778. Tel.: (+86) 22-27408778. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Funds for Excellent Young Scholars (No. 21222608), Program for New Century Excellent Talents in University (No. NCET12-0392), Research Fund of the National Natural Science Foundation of China (No. 21106099), Foundation for the Author of National Excellent Doctoral Dissertation of China (No. 201251), Tianjin Natural Science Foundation (No. 1440

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