Engineering Surface Wettability of Reduced Graphene Oxide To

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Engineering Surface Wettability of Reduced Graphene Oxide (RGO) to Realize Efficient Interfacial Photocatalytic Benzene Hydroxylation in Water Jingyu Cai, Min Zhang, Dengke Wang, and Zhaohui Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04175 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018

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ACS Sustainable Chemistry & Engineering

Engineering Surface Wettability of Reduced Graphene Oxide (RGO) to Realize Efficient Interfacial Photocatalytic Benzene Hydroxylation in Water Jingyu Cai, Min Zhang, Dengke Wang, Zhaohui Li∗ Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350116, P. R. China

∗

Author to whom all correspondences should be addressed. E-mail: [email protected]. Tel (Fax): 86-591-22865855 1

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Abstract Selective one-step benzene hydroxylation to produce phenol is a green process, especially when the reaction can be carried out in water using economical carbon materials as catalysts under visible light. Herein, we reported that hydroxylation of benzene can be realized over reduced graphene oxide (RGO) in water under visible light using H2O2 as an oxidant. By engineering the surface wettability of RGO from hydrophilic (with a contact angle of 52°) to hydrophobic (with a contact angle of 127°), the performance for benzene hydroxylation can be significantly improved by more than 3 times. An optimum conversion of benzene reached 3.1 % with a H2O2/benzene ratio of 1:1 over hydrophobic RGO after irradiated for 20 h. The catalyst can be facilely recycled and no obvious decrease of the activity was observed after four successive runs. This study provides a green and sustainable method for phenol production based on economical carbon materials, and also highlights the important role of RGO as a promising photocatalyst for organic syntheses.

Keywords Reduced Graphene Oxide (RGO), Photocatalysis, Benzene hydroxylation, Phenol, Interfacial reaction

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Introduction Carbon is an abundant element on Earth and carbon-based materials are very charming for catalytic reactions since their have high stability, impressive surface areas and possess interesting properties.1-6 In addition to acting as support, carbon materials themselves have been used as catalytic active materials for a variety of organic reactions, such as alkanes dehydrogenation, alcohol oxidation, olefin hydrogenation and benzene hydroxylation.5,7-10 Among them, benzene hydroxylation to form phenol is a very important reaction since phenol has been widely used in industry for the syntheses of petrochemicals, agrochemicals and plastics etc. As compared to the common industrial three-step cumene process in production of phenol,11 which involves hazardous intermediates and produces equal amount of acetone as by-product, the direct benzene hydroxylation to form phenol using environmentally friendly H2O2 is greener and has attracted extensive recent research attention. The use of economical carbon materials to realize this reaction is especially appealing. Actually, different carbon polymorphs, including multi-walled carbon nanotubes (MWCNTs), activated carbon (AC) and reduced graphene oxide (RGO) have been used for selective benzene hydroxylation using H2O2 as oxidant.7,10,12-17 Over these carbon materials, the ⋅OH radicals generated by activating H2O2 can be stabilized on the carbon surface to form carbonyl and quinone groups, which react with the adsorbed benzene to generate phenol.15,16 However, the hydroxylation of benzene to produce phenol is challenging, not only because of the inertness of aromatic C-H bonds whose activation usually require harsh conditions, but also due to the increased reactivity of phonol compared to benzene.18,19 In order to develop processes based on renewable energy, the utilization of sunlight to drive chemical reactions has attracted widespread research interest.20-27 Compared to conventional thermal activation methods, photoinitiated reactions can be carried out under mild conditions, thus some undesirable thermal side reactions occur in high temperature can be suppressed.26,27 Benzene hydroxylation to produce phenol have also been realized over several different homogeneous and metal-doped heterogeneous catalysts,28 such as vanadium incorporated mesoporous titania,29 Fe-containing g-C3N4 as well as Fe-containing metal-organic frameworks (MOFs).30,31 However, for the metal doped heterogeneous catalysts, the leaching of the active species is usually unavoidable, 3



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while the homogeneous photocatalysts suffer from the problem of separation from the reaction systems. Developing a green and sustainable heterogeneous photocatalytic system based on cheap carbon materials for hydroxylation of benzene with high selectivity is significant yet demanding. As a member of the carbon family, RGO shows promising application in catalysis due to its peculiar properties and designability.32-42 Although RGO has not been applied for photocatalytic hydroxylation of benzene, the use of RGO for photocatalytic hydrogen production has already been reported.38,41 In addition, RGO has been found to activate H2O2 to generate ⋅OH under visible light via a Fenton-like pathway, which was used for the degradation of phenol.43 Since ⋅OH is highly oxidative and nonselective, to make use of ⋅OH in the selective benzene hydroxylation to produce phenol, most of previously reported benzene hydroxylation reactions were carried out in a mixture containing both H2O and organic solvent like CH3CN to extract produced phenol into the organic phase to inhibit the over-oxidation of phenol. A greener and promising strategy is to develop a H2O/benzene interfacial phase reaction by designing catalyst with suitable surface wettability, in which the desired product phenol can be desorbed quickly into the benzene phase. Stimulated by our previous studies that RGO-based aerogels with varied and controllable surface wettability can be prepared,44 herein we reported the preparations of RGO with controllable surface wettability, which were used for photocatalytic benzene hydroxylation in water under visible light with the oxidant of H2O2. Significantly improved performance for photocatalytic hydroxylation of benzene to produce phenol was obtained over RGO when its surface wettability was tuned from hydrophilic to hydrophobic to enable the reaction to occur in the water/benzene interface. This study provides a green and sustainable strategy for phenol production based on economical carbon materials. Experimental Synthesis of RGO, hydrophobic RGO-Cys and hydrophilic RGO-Lys Graphene oxide (GO) was firstly prepared from graphite flake by a modified Hummers method.45 RGO was then synthesized from GO following a modified method reported previously.10 An aqueous dispersion of GO (67 mL, 6 mg/mL) was diluted by 333 mL water and was sonicated for 1 h. Hydrazine hydrate (4 mL, 85 %) and 70 mL of 4


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ammonia solution were added into the above solution. The solution was heated at 95 °C for 3 h. The solid product was washed with deionized water and ethanol for several times, filtered over a membrane filter and dried in an air oven at 60 °C. RGO-Lys and RGO-Cys were synthesized similarly except that L-lysine or L-cysteine with a weight ratio of GO/amino acid to be 1:0.5 was added during the preparations. Characterizations Powder X-ray diffraction (XRD) data were collected over a Bruker D8 Advance X-ray diffractometer (Cu-Kα1 irradiation, λ=1.5406 Å). Fourier transform infrared (FT-IR) spectra were recorded in a transmittance mode with a resolution of 4 cm−1 over a Nicolet Nexus 670 FT-IR spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI Quantum 2000 XPS system (PHI, USA) with a monochromatic Al-Kα source and a charge neutralizer. Raman spectra were obtained on an invia-Reflex Micro-Raman Spectroscopy system (Renishaw Co.), with 532 nm line of an Ar ion laser at room temperature. The amount of residual metal was determined by an Inductively Coupled

Plasma-Optical

Emission

Spectrometer (ICP-OES) on

Optima 8000

(PerkinElmer). The transmission electron microscopy (TEM) image was obtained in a JEOL model JEM 2010 EX instrument at an accelerating voltage of 200 kV. The powder particles were supported on a carbon film coated on a 3 mm diameter fine-mesh copper grid. A suspension in ethanol was sonicated and a drop was dripped on the support film. Electron spin resonance (ESR) spectra were obtained over Bruker ESP 300E electron paramagnetic resonance spectrometer at room temperature. The contact angle was characterized by an OCA20 optical contact angle device. An automatic injection pump was used to reside droplets on a substrate for analysis. The contact angle of water on the sample was measured by the sessile drop method. 5 mg of the sample was pressed into a thin film with a diameter of 10 mm under a pressure of 10 MPa. All experiments were carried out at 298 K and 0.1 MPa. The sample was kept in air (ambient phase) and then water droplets of 2 µL were placed on the sample from a syringe at slow rate. BET surface area was carried out on an ASAP2020 apparatus (Micromeritics Instrument Corp., USA). The samples were degassed in vacuum at 160 °C for 12 h and then measured at 77 K. Benzene hydroxylation reactions 5



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The benzene hydroxylation reactions were carried out with a LED lamp (Beijing Perfect Light, PCX50B). In a typical reaction, deionized water (25 mL) and benzene (443 µL, 5 mmol) was transferred into a 50 mL reaction bottle containing 60 mg of catalyst. After stirred for 30 min, H2O2 (30 wt %, 515 µL, 5 mmol) was added into the reaction system. The system was irradiated by a LED lamp (65 mW/cm2) in 303 K. At the end of the reaction, 25 mL of acetonitrile was poured into the mixture followed by stirring for 1h. After removal of the catalyst, the solution was analyzed by High Performance Liquid Chromatography (Alliance e2695, Waters) using a C-18 column. The catalyst was washed with ethanol and then dried under vacuum at 60 °C for 10 h and then reused in the next run. Results and discussion RGO was synthesized via reduction of GO following a method reported previously. The XRD pattern of the as-obtained product shows a broad peak at 2θ value of 24.3°, corresponding to the (002) plane of graphene (Figure 1A (b)). The simultaneous disappearance of the peak at 2θ value of 9.8°, the characteristic peak of GO, clearly suggested that GO has been reduced to RGO (Figure 1A (a)).46,47 The successful reduction of GO to form RGO is also evidenced from the FT-IR and XPS. As shown in the FT-IR spectrum, the intensity of the peaks at 1259 and 1098 cm-1, which correspond to the surface C-O (epoxy) and C-O (alkoxy) groups, decreases dramatically, while those at 3412 and 1731 cm-1 assignable to O-H and C=O groups totally disappear (Figure 1B (a, b)).48 Decreased C-O content in RGO as compared to that in GO is also evidenced from the XPS results (Figure 1C (a, b)). The Raman spectrum of the resultant RGO shows typical D band at ca. 1341 cm-1 and G band at ca. 1590 cm-1. The former peak can be assigned to the defects and sp3-hydridized bonds while the latter peak is attributed to the graphitic sp2-bonded carbon. (Figure 1D (a, b)).49 The TEM image of the as obtained RGO clearly shows a morphology consisting of wrinkled thin paper-like structure (Supporting Information Figure S1A). The performance of the as-obtained RGO for the benzene hydroxylation under visible light was investigated. Considering that water is an environmental benign solvent, our reaction was initially carried out in water with a H2O2/benzene ratio of 1:1. It was found that only 0.1 % of benzene was converted after 4 h irradiation, with a selectivity of 6


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99 % to phenol (Figure 2A (a), 2B (a)). When prolonging the irradiation time to 20 h, the conversion of benzene reached 1.0 %, while the selectivity decreased slightly to 87 %, with benzoquinone produced as the main side product as shown in the HPLC result (Supporting Information Figure S2A). The ESR spectrum of an irradiated system containing RGO, benzene and H2O2 in the presence of 5, 5-dimethyl-1-pyrroline N-oxide (DMPO) as a trapping agent shows four typical signals and thus confirmed the formation of ⋅OH radicals (Supporting Information Figure S2B). Moreover, the addition of ethanol, a scavenger of ⋅OH radical, completely inhibited the benzene hydroxylation and no phenol was detected. Both the ESR result and the quenching experiment confirmed that benzene hydroxylation to produce phenol over RGO truly follows a ⋅OH radical mediated oxygenation pathway. The decrease of the selectivity to phenol upon increasing the reaction time is not unexpected since it is generally known that ⋅OH radicals involved in this reaction are non-selective and can further react with the phenol to form undesirable over-oxidized products.50 No products were detected in absence of RGO or without light irradiation. A control experiment using GO, which contains similar residual metal ions, for the same reaction show negligible activity, indicating that the conversion of benzene to form phenol is really catalyzed by RGO instead of the trace metal residues in RGO (Supporting Information Table S1). Based on the mechanism proposed previously on benzene hydroxylation to form phenol, the first step is the adsorption of benzene and H2O2 on the surface of the catalyst.10 Then the adsorbed H2O2 is activated by the active site on the surface of RGO to generate active ⋅OH, which react with the surface adsorbed benzene to form the desired product phenol. The rate for the generation of ⋅OH on the surface and its amount as well as the adsorption capability of the catalyst toward benzene and phenol, should be carefully manipulated in this reaction to achieve a high conversion of benzene while maintains a high selectivity to phenol, since ⋅OH not only can react with benzene to produce phenol, but also can oxidize the phenol to form undesirable over-oxidized products like catechol, benzoquinone etc. In this regard, catalyst with higher adsorption capability toward benzene, yet lower adsorption toward phenol should be beneficial for the selective benzene hydroxylation reaction, since the as-formed phenol can be easily desorbed from the surface to minimize the reaction between the phenol and the surface 7



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adsorbed ⋅OH to form over-oxidized products. The low photocatalytic activity for benzene hydroxylation in water over the as-obtained RGO may be due to its low adsorption toward benzene in the reaction medium. Even though most of the surface oxygenated groups have been removed during its formation, as evidenced from the FT-IR spectrum (Figure 1B (b)), the as-obtained RGO is hydrophilic with a contact angle of 78° and is dispersed in the water phase, which resulted in its low adsorption toward benzene (Figure 2C). To enhance its adsorption toward benzene, a frequently applied strategy is to introduce organic solvents like CH3CN to make a homogeneous reaction medium containing both water and benzene. However, in such reaction medium, the reaction between the desired phenol and H2O2 derived ⋅OH, leading to the formation of the over-oxidized products like catechol, benzoquinone or even decomposed to CO2, cannot be totally avoided. In addition, the use of organic solvents like CH3CN is environmentally harmful and is not green. An alternative yet promising strategy to enhance the adsorption capability of the catalyst toward benzene is to improve its surface hydrophobicity. It has already been reported that the performance for benzene hydroxylation to form phenol over vanadium-containing SBA-15 can be improved by silylation on SBA-15 to enhance its hydrophobicity, attributable to its improved adsorption toward non-polar benzene.51 Actually an ideal way to realize an efficient benzene hydroxylation is to develop a H2O/benzene interfacial phase reaction, in which the catalyst with suitable surface wettability can suspend in the interface between water and benzene to ensure an optimized interfacial reaction between benzene and H2O2, while in the meantime the desired product phenol can be desorbed quickly to the benzene phase to prevent it from being over-oxidized. To realize this, careful tuning the surface wettability of the catalyst is important. The presence of different functional groups on the surface of RGO makes it possible for a facile modification to tune its surface wettability. Our previous studies on RGO aerogels revealed that their surface wettability can be easily tuned using different amino acids.44 Following a similar strategy, we used L-cysteine as an additive during the reduction of GO to form RGO-Cys. Although the as-obtained RGO-Cys shows similar XRD pattern and FT-IR spectra (Figure 1A (c), 1B (c)), less wrinkled structure and fewer 8


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agglomerated graphene nanosheets are observed in its TEM image as compared with RGO (Supporting Information Figure S1B). It is believed that the presence of the a disulfide bond in RGO-Cys, as evidenced from the XPS spectra by showing peaks at 163.9 and 164.9 eV in the S 2p region, promoted the coalescence of adjacent edges between graphene nanosheets (Supporting Information Figure S3A). Unlike RGO obtained in absence of L-cysteine which is hydrophilic and suspends in the water phase, RGO-Cys is hydrophobic with a contact angle of 127°, which makes it capable to suspend in the interface between water and benzene phase during benzene hydroxylation reaction (Figure 2D). The hydrophobic surface of RGO-Cys can be attributed to the existence of the disulfide bond on the surface. To our anticipation, the conversion of benzene over RGO-Cys increased from the original 0.1 % for RGO to 0.5 % after 4 h irradiations (Figure 2A (b)). The conversion of benzene also increased with the irradiation time and reached 3.1 % after 20 h, while a relatively high selectivity of 90 % to phenol was still maintained (Figure 2B (b)). Even though phenol is more easily to be hydroxylated than benzene, a prolonged reaction of 32 h gave a benzene conversion of 3.4%, with only a slightly decreased phenol selectivity of 85%. Except benzoquinone, no other by-products have been detected in this system. Since RGO-Cys exhibits a smaller BET specific surface area (197 m2/g) than pristine RGO (407 m2/g), the higher activity observed over RGO-Cys cannot be attributed to the influence of specific surface area (Supporting Information Figure S4). Instead, it is the enhanced surface hydrophobicity of the RGO-Cys, which enables it to suspend in the interface of benzene/water to realize the two phase interfacial benzene hydroxylation reaction. To confirm this assumption, a more hydrophilic RGO, with a contact angle of 52° (Figure 2E), was also prepared by the addition of L-lysine as the additive during the reduction process and denoted as RGO-Lys (Supporting Information Figure S5). As expected, RGO-Lys showed an even poorer performance as compared with the pristine RGO, with a low benzene conversion of 0.8 % and a yield of 85 % to phenol in 20 h (Figure 2A (c), 2B (c)). Previous studies on the activation of H2O2 over carbon-based catalysts revealed that the oxygenated groups on the surface influence the catalytic performance.52 However, RGO-Cys, RGO and RGO-Lys show comparable FT-IR and XPS spectra in the C 1s 9



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region (Figure 1B (b, c), 1D (b, c), Supporting Information Figure S5B, C), excluding the effect of the surface functional groups on the catalytic performance. Although it’s reported that the defect on the surface of the carbon-based materials can promote the activation of H2O2 and the phenol yield exhibited a nearly linearly dependence on the degree of graphitization (ID/IG ratio), RGO-Cys shows a lower ID/IG (1.42) as compared with that of RGO (1.47) (Figure 1D (b, c), Supporting Information Figure S5D).13,53 These observations again confirm that the superior performance observed over RGO-Cys should be ascribed to its more hydrophobic surface. The RGO-Cys can be facilely recycled from the reaction and is stable during the benzene hydroxylation reaction. The cycling test show that there was no obvious loss of the photocatalytic activity over RGO-Cys after four successive runs (Figure 3). In addition, this reaction can be scaled up without obvious sacrificing its performance. A comparable benzene conversion (2.9 %) was still observed in 20 h when the reaction was scaled up by a factor of 5. The current system is appealing since the highly selective benzene hydroxylation to produce phenol can be realized over cheap carbon-based materials under ubiquitous solar light in an environmental benign solvent. Conclusions In summary, this study for the first time demonstrated that tuning of the surface wettability of RGO from hydrophilic to hydrophobic to enable the reaction to occur in water/benzene interface can significantly improve the photocatalytic performance of RGO for selective benzene hydroxylation. An efficient benzene hydroxylation to produce phenol can be realized over hydrophobic RGO-Cys in environmental friendly water as solvent and aqueous H2O2 as oxidant under visible light. This study not only provides a green, sustainable and economical method for phenol production, but also highlights the important role of RGO as a photocatalyst for organic syntheses. Associated content Supporting Information The Supporting Information is available free of charge on the ACS Publications website. ICP analyses, TEM images, XPS spectra, BET of RGO, RGO-Cys and RGO-Lys and other characterizations. 10


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Acknowledgements This work was supported by NSFC (21872031, U1705251) and 973 Program (2014CB239303). Z. L. thanks the Award Program for Minjiang Scholar Professorship for financial support.

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Captions for Figures Figure 1 (a) XRD patterns; (b) FTIR spectra; (c) XPS spectra in the C 1s region and (d) Raman spectra of GO, RGO and RGO-Cys. Figure 2 (a) Benzene conversion and (b) phenol selectivity over RGO (), RGO-Cys () and RGO-Lys (); photoes of reaction system in the presence of (c) RGO, (d) RGO-Cys and (e) RGO-Lys (inset: water contact angle). Reaction conditions: photocatalyst (60 mg), benzene (5 mmol), H2O (25 mL), H2O2/benzene (n/n = 1:1), LED (65 mW/cm2), 303 K. Figure 3 The recycling of the RGO-Cys for photocatalytic hydroxylation of benzene.

17



c

RGO-Cys

b

RGO

a 10

20

30

40

2θ (deg)

(C)

50

C-N/C-S C=OC-O

295

GO

C-C

60

C=C

RGO

b

GO

a 290

285

Binding Energy (eV)

C=C

c

RGO-Cys

b

RGO

a

C-O

(alkoxy)

GO C=O C-OH C-O (epoxy)

O-H

(D)

C 1s

c

RGO-Cys

(B)

70 4000 3500 3000 2500 2000 1500 1000 Wavenumber (cm-1)

Intensity (a.u.)

Intensity (a.u.)

(A)

Transmittance (a.u.)

Figure 1

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ID

IG

RGO-Cys

c

ID/IG=1.42

RGO

b

ID/IG=1.47

a

ID/IG=0.95

GO

280 1000

1200 1400 1600 1800 Raman shift (cm-1)

2000

18


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Figure 2

4

(A)

RGO-Lys RGO RGO-Cys

3

3.1

2.9 b

2 1.2

1

0.5 0.1

0

4

0.9

1.0

0.7

0.8

12 16 Time (h)

20

a 0.4 c

0.3

8

(C)

78° 78°

RGO

100

Phenol Selectivity (%)

Benzene Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


99

RGO-Lys RGO RGO-Cys

98

(B)

95 92

90

90

90

89

87

87

85

12 16 Time (h)

20

a 90

85

(D)

b

4

127° 127°

RGORGO-Cys

8

c

(E)

52° 52°

RGORGO-Lys

19



Figure 3

Benzene Conversion (%)

4

phenol selectivity

benzene conversion 3.1

3

90

2.9 89

2.9

88

2.8 88

100 90

2

80

1

70

0 0

1

2

3

Cycles

4

Phenol Selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 5

20


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Synopsis Hydrophobic reduced graphene oxide exhibits high activity for photocatalytic benzene hydroxylation to phenol in water and can be facilely recycled.

21


Engineering Surface Wettability of Reduced Graphene Oxide To

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