Hydroxylation of Benzene via C–H Activation Using Bimetallic CuAg

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Letter pubs.acs.org/journal/ascecg

Hydroxylation of Benzene via C−H Activation Using Bimetallic CuAg@g‑C3N4 Sanny Verma,† R. B. Nasir Baig,† Mallikarjuna N. Nadagouda,‡ and Rajender S. Varma*,§ †

Oak Ridge Institute for Science and Education, 1299 Bethel Valley Road, Oak Ridge, Tennessee 37830, United States WQMB, WSWRD, National Risk Management Research Laboratory, U.S. Environmental Protection Agency, 26 West Martin Luther King Drive, Cincinnati, Ohio 45268, United States § Sustainable Technology Division, National Risk Management Research Laboratory, U.S. Environmental Protection Agency, 26 West Martin Luther King Drive, MS 443, Cincinnati, Ohio 45268, United States ‡

S Supporting Information *

ABSTRACT: A photoactive bimetallic CuAg@g-C3N4 catalyst system has been designed and synthesized by impregnating copper and silver nanoparticles over the graphitic carbon nitride surface. Its application has been demonstrated in the hydroxylation of benzene under visible light.

KEYWORDS: Graphitic carbon nitride, Bimetallic heterogeneous catalyst, Visible light, C−H activation, Hydroxylation of benzene

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support23−25 and developed a sustainable method for the room temperature conversion of benzene to phenol using visible light.

INTRODUCTION Phenol is a very important organic intermediate used for the synthesis of pharmaceutical products, polymer resins, fungicides and has widespread application as a preservative.1,2 Most of the phenol produced via cumene process requires high pressure and temperature in the presence of Lewis acid catalysts.3,4 The simplification of a process has been a great factor leading to the development of newer methods in phenol production5 as noble metals (Pd and Pt) and their heterogeneous analogues along with hydrogen peroxide are often deployed.6 Although these methods succeed in eliminating the use of compressed propylene gas at elevated temperatures, the main problem remains with the over oxidation of phenol under given conditions leading to the mixture of products.7−9 Even the use of less active metals such as iron, copper, etc. still requires high temperature for the generation of phenol.10−14 These metals, when immobilized over polymeric support or porous silica surface, brought down the reaction temperature significantly but required extended reaction time;15,16 success of this reaction lies in the sequential use of heat in combination with light.17,18 Bimetallic combinations have also been used for the synthesis of phenol at 100 °C over several hours of heating.19 Our aim was to convert benzene into phenol via C−H activation under ambient conditions and we sought milder visible light energy for the activation of C−H bond.20−22 Accordingly, we selected inherently photoactive graphitic carbon nitride (g-C3N4) as an economical, benign photoactive © 2017 American Chemical Society

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RESULT AND DISCUSSION Initially, iron, palladium, copper, nickel, silver nanoparticles were immobilized over the graphitic carbon nitride surface and screened for the room temperature conversion of benzene to phenol under visible light using hydrogen peroxide in acetonitrile (Table 1, entries 1−5). None of these catalysts gave encouraging results. These outcomes coerced us to explore bimetallic combinations for this transformation and accordingly: we synthesized FePd@g-C3N4, FeCu@g-C3N4, FeAg@g-C3N4 and FeNi@g-C3N4 and screened for the C−H activation and hydroxylation of benzene (Table 1, entries 6−9). The combination of iron with different metals gave slightly better conversion; however, none of the catalysts were active enough to oxidize benzene to phenol even after 12 h of stirring. Among the FePd@g-C3N4, FeCu@g-C3N4, FeAg@g-C3N4 and FeNi@g-C3N4, iron combination with copper and palladium Special Issue: Asia-Pacific Congress on Catalysis: Advances in Catalysis for Sustainable Development Received: March 12, 2017 Revised: March 17, 2017 Published: March 21, 2017 3637

DOI: 10.1021/acssuschemeng.7b00772 ACS Sustainable Chem. Eng. 2017, 5, 3637−3640

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ACS Sustainable Chemistry & Engineering

activated by CuAg@g-C3N4 catalyst resulting in the facilitation of C−H bond. The generated hydroxyl radical react with benzene leading to the formation of phenol (Figure 1).26

Table 1. Screening of Catalysts for Hydroxylation of Benzenea

entry

catalyst

time

conversionb

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15c 16d 17e 18f 19g 20h 21i

Fe2O3@g-C3N4 Pd@g-C3N4 Cu@g-C3N4 Ni@g-C3N4 Ag@g-C3N4 FePd@g-C3N4 FeCu@g-C3N4 FeAg@g-C3N4 FeNi@g-C3N4 PdCu@g-C3N4 PdNi@g-C3N4 PdAg@g-C3N4 CuNi@g-C3N4 CuAg@g-C3N4 CuAg@g-C3N4 CuAg@g-C3N4 CuAg@g-C3N4 CuAg@g-C3N4 CuAg@g-C3N4 CuAg@g-C3N4 CuAg@g-C3N4

12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 30 min 30 min 30 min 3h 30 min 30 min 30 min 12 h

15% 43% 39% 20% 32% 70% 67% 41% 29% 81% 72% 77% 57% 99% 99% 99% 99% 86% 83% 99% -

Figure 1. Plausible mechanism for the hydroxylation of benzene.

A set of experiments was performed to examine the recyclability of the catalyst for the hydroxylation of benzene. After completion of the first cycle, the catalyst was recovered using centrifuge, washed with acetone and dried in vacuum. A new reaction was then performed with fresh benzene, under similar conditions. The bimetallic copper−silver catalyst (CuAg@g-C3N4) could be recycled at least 5 times without losing its activity. The metal leaching of CuAg@g-C3N4 was studied by ICP-AES analysis before and after completion of the reaction. The concentrations of copper and silver were found to be 4.56% and 4.62% before the reaction, 4.55% and 4.60% after the fifth cycle. The ICP-AES of the mother liquor did not show the presence of metals, confirming the fact that the g-C3N4 holds the copper and silver nanoparticles via noncovalent interaction, which minimizes the metals leaching.

a

Benzene (1 mmol), CH3CN (5.0 mL), 30% H2O2 (1.1 mmol), 20 W domestic bulb, catalyst (100 mg). bPhenol conversion by GC−MS. c 50 mg of CuAg@g-C3N4. d25 mg of CuAg@g-C3N4. e15 mg of CuAg@g-C3N4. fMethanol as a solvent. gWater as a solvent. hEthanol as a solvent. iReaction was performed in the dark.

was relatively active (Table 1, entries 6−7). Therefore, it was imperative to find an active metal combination with palladium and copper to develop a suitable catalyst for this reaction. Accordingly, PdCu@g-C3N4, PdNi@g-C3N4, PdAg@g-C3N4, CuNi@g-C3N4 and CuAg@g-C3N4 were synthesized and assessed for the visible light mediated room temperature C− H activation hydroxylation of benzene (Table 1, entries 10− 14). Among them, the CuAg@g-C3N4 catalyst gave the best result leading to the selective hydroxylation of benzene in less than 30 min (Table 1, entry 14). After the active catalyst was obtained, it was important to optimize catalyst amount and reaction solvent. Reducing the catalyst amount from 100 to 50 mg and 25 mg gave the similar conversion within 30 min (Table 1, entries 15 and 16). However, further reduction in catalyst quantity led to the increase in reaction time and exposure to visible light (Table 1, entry 17). After optimizing the catalyst quantity, we screened an array of reaction solvents. The reaction in methanol and water diminished the reactivity of the CuAg@g-C3N4 and it turned out that acetonitrile is better solvent then methanol and water (Table 1, entries 18−20). Nevertheless, same reaction in ethanol resulted in the formation of corresponding phenol in less than 30 min (Table 1, entry 20). From this reaction, it became clear that ethanol and acetonitrile are the ideal solvents for phenol production at room temperature (Table 1, entry 16 and 20). The plausible mechanism of the reaction entails the degradation of hydrogen peroxide into hydroxyl radical over photo active bimetallic surface. Molecular benzene embraces the graphitic surface via noncovalent interactions and gets

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MATERIAL SYNTHESIS AND CHARACTERIZATION A pale yellow graphitic carbon nitride (g-C 3 N 4) was synthesized by calcination of urea at 500 °C27,28 and dispersed in water via sonicating over a period of 30 min. The copper and silver salts (Cu(NO3)2 and AgNO3) were sequentially added, and the reaction temperature was raised to 60 °C and stirred for 4 h. The copper and silver salts were reduced to Cu(0) and Ag(0) nanoparticles by adding excess of sodium borohydride (NaBH4). After reduction, the reaction temperature was brought down to room temperature and the contents centrifuged. The supernatant liquid was decanted out; solid residue was washed with acetone and dried under vacuum (Scheme 1). The CuAg@g-C3N4 catalyst was isolated as a brown solid and characterized using transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), scanning electron microscope, X-ray diffraction and inductive coupled plasma atomic emission spectroscopy (ICPAES). The TEM images of g-C3N423 and CuAg@g-C3N4 show the immobilization of copper and silver nanoparticles and 3638

DOI: 10.1021/acssuschemeng.7b00772 ACS Sustainable Chem. Eng. 2017, 5, 3637−3640

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ACS Sustainable Chemistry & Engineering Scheme 1. Synthesis of CuAg@g-C3N4

confirmed by EDX pattern. The exact percentages of silver and copper metal were determined using ICP-AES and found to be 4.62% and 4.56%, respectively (Figure 2). The SEM images of

Figure 3. SEM analysis of g-C3N4 (a, b) and CuAg@g-C3N4 (c, d).

Figure 2. (a) TEM analysis of g-C3N4; (b) TEM analysis of CuAg@gC3N4; (c) EDX spectra of CuAg@g-C3N4.

support g-C3N4 and CuAg@g-C3N4 catalyst show a clear difference in their morphology due to impregnation of silver and copper nanoparticles over nitrogen-rich graphitic surface (Figure 3). The XRD pattern of g-C3N421 shows a broad peak at 27° due to the amorphous nature of graphitic surface. After the immobilization the XRD pattern changes, its shows a broad peak at 27° and sharp peaks at 38.3°, 44.1°, 64.7° and 77.4° (Figure 4). The broad peak is the characteristic for amorphous graphitic support, whereas peaks at 38.3°, 44.1°, 64.7° and 77.4° represent the presence of crystalline silver nanoparticles (Figure 4).26 The presence of copper is confirmed using EDX and ICP-AES; however, it does not show any pattern in XRD spectra. The absence of XRD pattern for copper confirmed that copper nanoparticles in CuAg@g-C3N4 catalyst are amorphous in nature.

Figure 4. (a) XRD analysis of g-C3N4; (b) XRD analysis of CuAg@gC3N4.

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CONCLUSION A sustainable protocol has been developed for the hydroxylation of benzene via C−H activation using a photoactive CuAg@g-C3N4 catalyst. The synergic effect between copper and silver plays a crucial role for the activation of benzene and generation of active hydroxyl radicals.26 The nitrogenous 3639

DOI: 10.1021/acssuschemeng.7b00772 ACS Sustainable Chem. Eng. 2017, 5, 3637−3640

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surface of graphitic carbon nitride not only provides an election rich environment for holding copper and silver nanoparticles but also absorbs the visible and dissipates into the reaction media, which facilitates the C−H bond activation−hydroxylation of benzene.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00772. Synthesis of g-C3N4, synthesis of CuAg@g-C3N4 catalyst, general procedure for the hydroxylation of benzene, recycling of CuAg@g-C3N4 catalyst and NMR of product (PDF)

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AUTHOR INFORMATION

Corresponding Author

*R. S. Varma. Fax: 513- 569-7677; Tel: 513-487-2701; E-mail: [email protected]. ORCID

Rajender S. Varma: 0000-0001-9731-6228 Notes

The views expressed in this paper are those of the authors and do not necessarily represent the views or policies of the U.S. Environmental Protection Agency. Any mention of trade names or commercial products does not constitute endorsement or recommendation for use. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.V. and R.B.N.B. were supported by the Postgraduate Research Program at the National Risk Management Research Laboratory administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and the U.S. Environmental Protection Agency.

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DOI: 10.1021/acssuschemeng.7b00772 ACS Sustainable Chem. Eng. 2017, 5, 3637−3640