Development of nanocarbon gold composite for heterogeneous catalytic oxidation

Materials Letters 87 (2012) 90–93

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Development of nanocarbon gold composite for heterogeneous catalytic oxidation Vishal J. Mayani, Suranjana V. Mayani, Sang Wook Kim n Department of Advanced Materials Chemistry, College of Science and Technology, Dongguk University, Gyeongju, Gyeongbuk 780-714, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 June 2012 Accepted 23 July 2012 Available online 31 July 2012

Gold nanoparticles unified on various materials have produced great advances in the field of catalysis and material science. Presently, we developed nanocarbon cages (CC) and carbon gold composite (CGC) using template methods with nanosilica ball, pyrolysis fuel oil and 1% HAucl4 solution. The morphology and porous structure of the silica spheres were closely shaped in the carbon cage. Due to their easy fabrication protocols and broad potential applications, the nanocomposite CC and CGC are preferred for specific interests. This nanocomposite combined the physical and chemical properties of the gold and carbon foundation. The activity of CGC for solvent-free heterogeneous catalytic oxidation of a secondary alcohol revealed with excellent conversion. & 2012 Elsevier B.V. All rights reserved.

Keywords: Carbon cage Carbon gold composite Pyrolysis fuel oil Heterogeneous catalysis

1. Introduction Low-cost pyrolysis fuel oil (PFO) with higher aromatic content from petroleum residue oil was used as a starting material for the production of pure naphthalene crystals and composite carbon materials [1,2]. Naphthalene and pitch residue were considered as a suitable source for synthesizing various carbon materials including carbon nanotubes and carbon nanocapsules due to their unique structural property [3–5]. Of several methods available for engineering porous carbon materials, the template method can be easily used to design a variety of porous networks with a broad range of pore sizes and well arranged morphology, along with combined chemical functionalities [6]. Supported gold composites have attracted great attention due to their characteristic properties and catalytic performances [7–9]. The nature of the catalyst supports plays an important role in synthesizing and maintaining particle dispersity, enhancing composite stability and raising the catalytic performance of the resulting composite material. Several metal oxides (TiO2, Al2O3, ZrO2, MgO and SiO2), mesoporous siliceous materials (SBA-15 and MCM-48) and carbon have been suggested as gold supports [10–12]. These composite materials have been used in several applications including biosensor, drug delivery, energy processing, methane combustion, SOx and NOx removal, and chemical synthesis with their commercial prospective visible [12–20]. Silica and carbon are the most widely used catalyst supports for heterogeneous catalysis [21,22]. Gold carbon materials exhibit not only the individual properties of gold and

n

Corresponding author. Tel.: þ82 54 770 2216; fax: þ82 54 770 2386. E-mail address: [email protected] (S. Wook Kim).

0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.07.071

carbon materials but also the combined properties of both materials and the emergence of new properties capable of supporting novel applications. These composites offer high mechanical and thermal strength, high surface area, carbon resistance to acid/base and the electric, optical and catalytic property of gold [23–27]. Many pharmaceuticals and fine chemicals have been manufactured by oxidation reaction, but usually in combination with noneco-friendly reaction systems. Removal of the resulting toxic reagents and unwanted products from the reaction mixture has proven difficult. Therefore, a need has arisen for an alternative process using a heterogeneous catalyst with mild reaction conditions. Herein, carbon gold composite (CGC) is designed by easy deposition of gold nanoparticles on a hierarchical carbon cage (CC). CGC offers great promise for heterogeneous oxidation reaction of a secondary alcohol with excellent conversion.

2. Experimental Synthesisof carbon cage (CC): The CC material was synthesized (Fig. 1) as described in our previously reported paper [2]. Synthesis of carbon gold composite (CGC): In order to prepare the CGC, 3.07 g of CC were added in 250 ml of sodium citrate dihydrate solution and the mixture was then exposed to ultrasonic process for 5 min. The suspension of CC was then transferred to 250 ml of distilled water. The diluted suspension was refluxed with stirring for 5 min, followed by the addition of 1% HAuCl4 (100 ml) to the suspension containing CC. The resulting suspension was stirred at boiling temperature for next 5 min. The resulting solid particles were filtered with a Buchner funnel and washed with distilled water. The powder was dried

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Pitch Residue O

NSB

Heterogeneous Catalytic Oxidation OH

CGC 120oC 4h, O2

(I)

HF (I) Thermal Treatment (II) HAuCl4

(I) CGC

Naphthalene Crystals

CSC

(II)

Au (111)

Filtrate SelfCrystalization

Relative Intensity (Arbitrary Units)

Vacuum Filtration

Heat Extraction

PFO

91

Au (200)

Au (220)

Au (311)

(002)

(C)

(002) (B)

(100)

CC

(002)

(A)

Fig. 1. Schematic illustration for the synthesis of CGC and its catalytic activities.

10

in air at 100 1C. Finally, the solid was calcined at 900 1C for 1 h in nitrogen to yield CGC (Fig. 1). Yield: 3.6 g, BET surface area of CGC: 283 m2/g, FTIR (KBr) 806, 1116, 1590, 3487 cm  1. Elemental analysis and ICP results showed that CGC contained 490 wt% carbon and 4 2.3 wt% Au after carbonization at 4900 1C (See Supplementary information). Catalytic oxidation using CGC: Heterogeneous catalytic oxidation of 1-phenylethanol was carried out at atmospheric pressure by solvent-free reaction condition using CGC as an eco-friendly catalyst (Fig. 1). Catalytic oxidation was carried out in a magnetically stirred 3-neck round-bottom flask under oxygen gas purging attached with a Dean–Stark trap and condenser to collect the water formed during the reaction. CGC (0.1 g) was added to 1-phenylethanol (41.4 m mol, 5.06 g) at room temperature. The reaction mixture was heated to 120 1C and continuously stirred for 4 h. The completion of the catalytic oxidation was monitored by thin layer chromatography. The product was filtered and the residue was washed with dry ethanol. The solvent was removed from the combined filtrate and the residue was purified by column chromatography by using n-hexane/EtOAc (9:1) to give acetophenone (yield, 4.78 g).

3. Results and discussion The composite materials CC and CGC were synthesized by simple template method using nanosilica ball (NSB) and easy deposition of gold nanoparticles on hierarchical CC, respectively. Both the composites were fully characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), N2 adsorption– desorption isotherm, scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), transmission electron microscope (TEM) and Fourier transform-infrared (FTIR). The detailed characterizations are given in Supplementary information. CGC was used as potential catalyst for heterogeneous oxidation reaction of a secondary alcohol. Characterization by powder XRD: The XRD profile of NSB showed (Fig. 2) one broad peak assigned at 2y ¼23.51, corresponding to the diffraction peak of amorphous silica [28]. Upon preparation of CC material from NSB, two additional peaks, with lower intensity corresponding to graphite-type reflection from the (002) and (100) planes, were observed at 2y values of 25.31 and 42.71, respectively [6,28]. Upon gold deposition in CC,

20

30

40

50

60

70

80

2 theta (Degree) Fig. 2. Powder XRD patterns of NSB (A), CC (B) and CGC (C).

the XRD of the CGC material showed four additional peaks at 2y ¼38.21, 44.4, 64.61 and 77.61, which corresponded to reflections of Au planes (111), (200), (220) and (311) respectively, denoting the formation of noble gold particles with a face centered cubic structure [9,29]. Characterization by SEM analysis: The SEM image of NSB showed that most of the particles were uniform and closely packed with 170 nm particle size diameter (Fig. 3a). It can be observed that the porous structure of the silica template is well imitated in CC (Fig. 3b). The consistent hollow cores of hierarchically porous CC were identical and strongly connected with each other. Additionally, CGC confirmed homogeneously dispersed and adhered Au particles on CC backbones (Fig. 3c). The EDS spectra recoded in the examined area of CGC have shown strong signals for both gold and carbon foundation (Fig. 3d). Fig. 3e and f shows the micrographs of CGC which again confirmed the presence of carbon and gold, respectively. During the template synthesis process and catalytic oxidation at higher temperature, the hierarchically type framework of CC and CGC is well-replicated and retained their well arranged hollow morphology. Heterogeneous catalytic oxidation using CGC: The large pores of carbonaceous composite material CGC render it a potential heterogeneous catalyst. We carried out the solvent-free oxidation of 1-phenylethanol with CGC in the presence of oxygen gas at 120 1C. CGC provided the oxidation product in  96% yield (Table 1, entry 1). An attempt was made to recycle the CGC catalyst in the catalytic oxidation reaction under the optimized reaction condition. After the first catalytic run the product was isolated by filtration and the solid mass was Soxhlet extracted with ethanol. The recovered CGC was then activated, dried in a vacuum desiccator overnight and subjected to the next catalytic cycle. Four catalytic cycles were completed successfully without loss in its catalytic performance (Table 1, entries 1–4). The CC support failed to catalyze the oxidation reaction under same reaction condition (Table 1, entry 5). As microwave assisted reaction are fast and convenient but CGC failed to catalyze the oxidation reaction under microwave irradiation (Table 1, entry 6).

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Fig. 3. SEM images of NSB (a), CC (b), CGC (c), SEM-EDS spectra of CGC (d), X-ray mapping of CGC detects of carbon (e) and gold (f).

Table 1 Heterogeneous catalytic oxidation of 1-phenylethanol using CGC and its recycling studya. Entry

Materialsb

Yieldd (%)

Catalytic run

1 2 3 4 5 6

CGC CGC CGC CGC CC CGCþ MWc

96 94 95 95 – –

1 2 3 4 – –

a The reaction was carried out by using 1-phenylethanol (5.06 g) and CGC (0.1 g) in presence of O2 at 120 1C for 4 h. b Two composite materials (CC/CGC) were used as catalyst. c The reaction was carried out by using 1-phenylethanol (5.06 g) and CGC (0.1 g) was irradiated for 1 min in a microwave oven. d Isolated yield after column chromatography.

The catalytic study clearly attributed the entire catalytic activity to the Au supported on the CGC material.

4. Conclusions Synthesized nanocomposites are a combination of carbon with a carbon gold foundation. The collective chemical and physical properties of these nanocomposites allow new kinds of catalytic application. We successfully used CGC in heterogeneous catalytic oxidation reaction with remarkable activity and selectivity for solvent-free catalytic oxidation of 1-phenylethanol to acetophenone, along with the added advantage of catalyst recycling. The composite material showed high conversion at mild oxidation reaction condition. These materials can be used as solid catalysts in different catalysis reactions. We are presently conducting

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adsorption experiments of various molecules using composite carbon materials for energy storage and reverse ‘greenhouse’ effect.

[6] [7] [8] [9]

Acknowledgments This work was supported by the Dongguk University research fund and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (no. 2012029239).

Appendix A. Supplementary information

[10] [11] [12] [13] [14] [15] [16] [17]

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2012.07.071.

[18] [19] [20] [21]

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