The microwave-assisted ionic liquid nanocomposite synthesis: platinum nanoparticles on graphene and the application on hydrogenation of styrene
© Lee et al.; licensee Springer. 2013
Received: 19 July 2013
Accepted: 18 September 2013
Published: 8 October 2013
The microwave-assisted nanocomposite synthesis of metal nanoparticles on graphene or graphite oxide was introduced in this research. With microwave assistance, the Pt nanoparticles on graphene/graphite oxide were successfully produced in the ionic liquid of 2-hydroxyethanaminium formate [HOCH2CH2NH3][HCO2]. On graphene/graphite oxide, the sizes of Pt nanoparticles were about 5 to 30 nm from transmitted electron microscopy (TEM) results. The crystalline Pt structures were examined by X-ray diffraction (XRD). Since hydrogenation of styrene is one of the important well-known chemical reactions, herein, we demonstrated then the catalytic hydrogenation capability of the Pt nanoparticles on graphene/graphite oxide for the nanocomposite to compare with that of the commercial catalysts (Pt/C and Pd/C, 10 wt.% metal catalysts on activated carbon from Strem chemicals, Inc.). The conversions with the Pt nanoparticles on graphene are >99% from styrene to ethyl benzene at 100°C and under 140 psi H2 atmosphere. However, ethyl cyclohexane could be found as a side product at 100°C and under 1,520 psi H2 atmosphere utilizing the same nanocomposite catalyst.
KeywordsGraphene Pt nanoparticle Ionic liquid Microwave Hydrogenation of styrene
Catalysts using metal nanoparticles have been one of the most interesting research areas in recent years since its relevance to chemical [1–4], pharmaceutical [5–8], and energy-related applications [9–11]. Recently, some researchers have shown that nanocatalysts with high dispersion and narrow size distributions stabilized by appropriate supports or capping materials can work under mild conditions with high activity and high selectivity when compared to conventional heterogeneous catalysts. It is known that the transition metal nanoparticles are effective catalysts, in which the shape, size, and surface structure of the solid supports all that contribute to the catalytic activity [1–4, 9–13]. The supports usually are alumina, zeolite, and carbon materials that further include the carbon black, carbon nanotubes, graphene, and nanoporous carbon [14–20].
The synthesis of graphite oxide and graphene followed the well-known Hammer's method . A 250-mL round bottomed flask filled with 25 mL concentrated sulfuric acid (98%, Adrich, St. Louis, MO, USA) was held in an iced bath. After 5 to 10 min, 10 mL fumed nitric acid was added slowly in 15 min. Then, graphite powder (1.0 g, with particle size <45 μm) was added into the mixture under vigorous stirring for 30 min with the flask held in the iced bath. Then 22 g potassium chlorate was added into the solution in 30 min, and the mixture was stirred at room temperature for 96 h. The solution was centrifuged with a suitable amount (about 200 to 300 mL) of deionized (DI) water added under an iced bath temperature. Removal of liquid phase, followed by addition of DI water and then centrifugation, was repeated for three times. The mud-like residue was dried at 80°C for 12 h to produce the graphite oxide.
The nanocomposite synthesis followed a procedure similar to that reported in our previous study . Graphite oxide (250 mg) was added in 250 mL DI water and stirred for 30 min before addition of 1.4 g NaBH4, and the mixture was kept at 80°C for 1 h. Prior to sulfonation, the solution was centrifuged for collection of residues that were rinsed with methanol for three times then dried at 80°C under the N2 atmosphere for 1 h. The graphite oxide was sulfonated and exfoliated to graphene with the following procedure: in a 500-mL round-bottomed flask, the residues (158 mg) in 300 mL DI water were dispersed using an ultrasonic bath for 30 min. Separately, sulfanilic acid (140 mg) and potassium nitrate (50 mg) were introduced into a 100-mL beaker containing DI water (40 mL) employing an iced bath. After being mixed well, the solution was added with 1 N HCl (1 mL) and then the solution was poured into the above mentioned round-bottomed flask and stirred for 2 h in the iced bath. Centrifugation followed by removal of aqueous solution resulted in the sulfonated graphene, which was rinsed with methanol for a few times then dried at 80°C under the N2 atmosphere.
Synthesis of the nanocomposites in ionic liquid 2-hydroxyethanaminium formate with microwave assistance
Substrate (100 mg)
Sphere/14 ± 6
Cube-like/18 ± 8
Cube-like/4 ± 7
The analytical instruments used were as the following: nuclear magnetic resonance (NMR) with Bruker AVA-400, Madison, WI, USA (400 MHz), element analysis (EA) by FLASH EA 1112 Series, Thermo Finnigan, Milano, Italy, X-ray diffraction (XRD) by Phillips PANalytical X'Pert PRO MPD, Amsterdam, The Netherlands (Cu, λ = 0.1541 nm, 2 theta: 5° to 80°), thermal gravity analysis (TGA) with Perkin Elmer 1 TGA, Waltham, MA, USA (2 to 5 mg samples in Pt plate with 5°/min heating rate), transmitted electron microscopy (TEM) with JEOL JEM-2010, Akishima-shi, Japan (LaB6, 200 kV), gas chromatography (GC) by Agilent Technologies 7890A GC system with Agilent Technologies 7683B Series injector, Santa Clara, CA, USA.
The hydrogenation of styrene was performed with a Parr 4762 (Q)* reactor, Moline, IL, USA, under two H2 pressure conditions: one at 100°C under 1,520 psi and the other at 100°C under 140 psi H2 atmosphere, both with a reaction time of 1 h. The hydrogenation of styrene with commercial Pd/C was loaded with catalyst 50 mg and styrene 1.22 g then 6 mL methanol was added in the Parr 4762 (Q)* reactor. Similar hydrogenation with commercial Pt/C was loaded with 50 mg of catalyst and 667 mg of styrene followed by 6 mL methanol in the reactor. For model catalyst (Pt/GE) experiments, it was added in the 4762 (Q)* reactor with 20 mg catalyst and 320 mg styrene with 6 mL methanol. After hydrogenation, the reactor was cooled down to room temperature; the mixed hydrogenation products were filtered with diatomite, and the liquid phases were analyzed with GC.
Results and discussion
The EA results of graphite oxide, sulfonated-graphite oxide and graphene
From the literature survey, CNT-supported palladium (Pd/CNT) and gold (Au/CNT) nanoparticles show negligible catalytic activity for the hydrogenation of benzene at room temperature. Using the Pd/CNT catalyst at 50°C with 10 atm H2, a conversion of benzene to cyclohexane (48.8% after 24 h) was observed. Rh/CNT and Pt/CNT show good catalytic activities for the hydrogenation of benzene at room temperature with ≥50% conversion to cyclohexane in 24 h. Cyclohexane is the only product detected in the hydrogenation of benzene , suggesting that the partially hydrogenated intermediates were only transient. The hydrogenation of styrene, employing the current nanocomposites Pt/GE, was on the side chain instead. The hydrogenation after 1 h could convert >99% of styrene to ethylbenzene. Benzene hydrogenation is an ideal reaction for such studies as it has been investigated extensively on single-crystalline Pt surfaces. Because this reaction has been shown to produce only cyclohexane on Pt(100) and both cyclohexene and cyclohexane on Pt(111), thus, suitable for probing nanoparticle shape-dependent reaction selectivity in catalysis . The Pd, Pt, and Ru species were investigated on the γ-Al2O3 supported catalysts for hydrogenation of styrene, and the group VIII metals were the best choices. The hydrogenation of styrene activity of metal catalysts on the supported alumina material followed the order Pd > Pt > Ru .
The results for hydrogenation of styrene from Pt/ G and commercial catalysts
100°C,140 psi,1 h
2 ~ 5
3 ~ 5
100°C,1520 psi,1 h
2 ~ 5
3 ~ 5
The low H2 pressure hydrogenation reaction condition exhibited a catalytic activity in the order Pd/C to Pt/C > Pt/GE. However, the high H2 pressure hydrogenation reaction condition gave an order of Pd/C > Pt/GE > Pt/C. The hydrogenation activity of Pt/GE was better than the commercial Pt/C but a little less than that of the commercial Pd/C.
The authors would like to thank Academia Sinica and National Central University for financially supporting this work.
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