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Jointly published by Akadémiai Kiadó, Budapest and Springer, Dordrecht

React.Kinet.Catal.Lett. Vol. 87, No. 2, 343-348 (2006)

R4913 THE ACTIVITY OF Au SUPPORTED ON VARIOUS TYPES OF CARBON IN THE RING TRANSFORMATION REACTIONS OF METHYLOXIRANE András Fásia, Klára Hernádia, István Pálinkób*, Gábor Galbácsc and Imre Kiricsia

Department of Applied and Environmental Chemistry, University of Szeged, Rerrich B. tér 8, H-6720 Szeged, Hungary b Department of Organic Chemistry, University of Szeged, Dóm tér 8, H-6720 Szeged, Hungary c Department of Inorganic Chemistry, University of Szeged, Dóm tér 7, H-6720 Szeged, Hungary Received October 21, 2005 Accepted November 2, 2005

a

Abstract In this work the catalytic activity of gold catalysts supported on various forms of carbon is described and compared in the ring transformations of propylene oxide (methyloxirane). The catalysts were Au/multiwall carbon nanotube (Au/MWNT), Au/activated carbon and Au/graphite. The reactions were studied in a pulse microreactor in the 363 ­ 473 K temperature range. Methyloxirane underwent deoxygenation and isomerisation over the Au/MWNT and Au/activated carbon catalysts but deoxygenation only occurred over Au/graphite. At 363 K the Au/MWNT was only active, and it proved to be the most active catalyst at each temperature. This finding was attributed to an optimum mix of defect sites at the metal-support interface and electric conductivity of the support ­ both believed to be crucial in the high catalytic activity observed. Keywords: Au/multiwall carbon nanotube, Au/activated carbon, Au/graphite, methyloxirane, transformation pathways

INTRODUCTION Not long ago gold was not considered to be a useful metal in catalysis. The only application in this area was its use in multimetallic catalysts as a diluent. It

____________________________ Dedicated to Professor Zoltán Paál on the occasion of his 70th birthday. *Corresponding author. Tel.: +36 62 544 288 Fax: +36 62 544 200; E-mail: [email protected]

0133-1736/2006/US$ 20.00. © Akadémiai Kiadó, Budapest. All rights reserved.

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is not the case any more. Since the discovery of Haruta et al. [1] tremendous research activity went and still goes into studying the low-temperature oxidation of CO over supported Au catalysts [2]. It is intriguing that other molecules were scarcely investigated. An exception is, e.g., the epoxidation of propylene with H2O2 over Au/TiO2 giving propylene oxide [3]. Recently, we have studied the ring transformation reactions of this molecule (propylene oxide that is) over Au/MgO and unsupported gold as well [4]. Appreciable catalytic activity was found over Au/MgO when the activation of the catalyst was finished with H2. After similar treatment even unsupported gold showed some activity. On changing the support to multiwall carbon nanotube (the preparation of Au/carbon nanotube catalysts was not a trivial exercise[5]) the catalyst was still working although the activity was considerably lower than over Au/MgO. In further work the support was changed to other carbon types and the performance of these catalysts is compared to that of Au/multiwall carbon nanotube (Au/MWNT) in this contribution.

EXPERIMENTAL Multiwall carbon nanotube (MWNT) (BET surface area: 368 m2/g) was prepared by the catalytic chemical vapor deposition (CCVD) method applying Fe,Co/Al(OH)3 as the starting catalyst. MWNTs were grown at 973 K by the decomposition of acetylene and were purified as described in [5]. Both the activated carbon (BET surface area: 640 m2/g) and the graphite powder (BET surface area: 70 m2/g) were Aldrich products. Gold (3 wt.%) was deposited onto the supports from a colloid prepared from a HAuCl4 solution (the HAuCl4 solution was decomposed with NaOH and stabilized with tetrakis(hydroxymethyl)-phosphonium chloride) by sonicating the suspension for one hour (this was found to be optimal). Metal loading was determined by the ICP (induction-coupled plasma) method. T(ransmission)E(lectron)M(icroscopy) images of the supported catalysts were taken on a Philips CM-300 FEG instrument and displayed in Fig. 1. The reactions of propylene oxide (methyloxirane, 1) were studied in a pulse microreactor (1 hydrogen as carrier gas with a 363 K (I), 393 K (II), 423 K (III) and 473 K (IV) pulse sequence with 1 hour waiting time between the pulses. Blank experiments (experiments over the metal-free supports) have been performed at the highest temperature applied in catalytic experiments and no transformations of methyloxirane have been found.

gniylppa egnar erutarepmet K 374 ­ K 363 eht ni )ezis eslup lµ

FÁSI et al.: Au/MULTIWALL CARBON NANOTUBE

345

Fig. 1. TEM images of the catalysts: (a) Au/activated carbon, (b) Au/graphite and (c) Au/MWNT

Product analysis was performed by the GC-MS method (Hewlett-Packard [HP] 5890 gas chromatograph equipped with a HP 5970 quadrupole mass selective detector). Good separation was achieved on a 50-m long CPWAX 52CB coated CHROMPACK WCOT fused silica capillary column by applying a temperature program (303 K for 15 min, 323 K for 10 min and 473 K for 10 min). Product identification was based on clean samples. Before reaction the catalysts (10 mg) were pretreated in hydrogen flow (45 cm3/min) at 623 K for an hour.

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RESULTS AND DISCUSSION Methyloxirane underwent various reactions over the Au/MWNT and Au/activated carbon catalysts but only deoxygenation occurred over Au/graphite and only at the highest temperature applied (473 K). The reaction pathways are depicted in Scheme 1.

2

CHO O

3 4

CH3 O

1

OH

5

CH2OH

6

H2O

7 Scheme 1. Transformation pathways of methyloxirane

At the lowest temperature (363 K) the Au/MWNT was only active giving overwhelmingly propylene (2) ­ a deoxygenation product, i.e. the reverse reaction of epoxidation mentioned in the Introduction took place. Although deoxygenation remained the main reaction over the entire temperature range studied, isomerization products were also formed and at higher temperatures hydrogenative ring opening occurred as well. Interestingly, at all temperatures Au/activated carbon had lower activity than Au/MWNT. Its activity became appreciable only at 473 K and the scope of transformations widened relative to that experienced at 423 K, when propylene was formed exclusively. Beside deoxygenation (which was still the main reaction), acetone (3) and propionaldehyde (4) were formed in comparable amounts. The Au/graphite catalyst was inactive in the 363 ­ 423 K temperature range. It displayed some activity at 473 K resulting in deoxygenation exclusively. The composition of the effluent after reaction is summarized in Table 1.

FÁSI et al.: Au/MULTIWALL CARBON NANOTUBE

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Table 1 Composition of the effluent after the ring transformation reactions of propylene oxide (methyloxirane) over 10 mg of various types of carbon-supported Au catalysts Composition/mol% Au/MWNT I 1 2 3 4 5 6 7 98.6 1.1 0.3 0 0 0 0 II 96.1 2.2 0.5 0.1 0.7 0.1 0.3 III 92.2 3.6 0.9 0.2 1.8 0.6 0.7 IV 82.4 8.7 1.1 0.8 3.4 0.7 1.2 I 100 0 0 0 0 0 0 Au/activated carbon II 100 0 0 0 0 0 0 III 99.0 1.0 0 0 0 0 0 IV 86.9 10.2 1.6 1.3 0 0 0 I 100 0 0 0 0 0 0 Au/graphite II 100 0 0 0 0 0 0 III 100 0 0 0 0 0 0 IV 99.7 0.3 0 0 0 0 0

It was observed earlier that the gold-support interface plays a crucial role in catalysis and the defect sites of the support are involved in the transport of certain reactive species [4, 6]. The defect sites of the Au/graphite are probably fully consumed upon preparation, thus it is hardly active even at 473 K. The activated carbon is full of defects. Even though a portion of them was surely used up during the synthesis of the Au/activated carbon, enough should have remained to result in high activity in ring transformations. The catalyst was active indeed, especially at 473 K, but the Au/MWNT was even more active at all temperatures applied. This finding was surprising since the walls of carbon nanotubes are believed to be perfectly graphitic and the Au/graphite catalyst displayed almost no activity. It has been proven, however, that the CCVD method makes many defects allowing the deposition of gold up to 50 wt.% [4]. This means that depositing only 3 wt.% leaves many defects available for cooperation with the metallic sites. However, this is not enough for explaining the higher catalytic activity than that was observed over Au/activated carbon. It seems that the conductive property of nanotube also contributes. Thus, the optimum mix of electronic property and the available defect sites makes the Au/MWNT catalyst superior to the others.

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CONCLUSIONS The defect sites and the conductive property of the support play important role in the gold-catalysed reactions of methyloxirane. In these respects Au/MWNT catalyst was the optimal compared to Au/graphite (lack or very small concentration of defect sites) or Au/activated carbon (no or very small electric conductivity). Acknowledgement. This work was supported by the National Science Fund of Hungary through grant T046491. The financial help is highly appreciated. REFERENCES

1. 2. M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.J. Genet, B. Delmon: J. Catal., 144, 175 (1993). Some most recent examples: P. Landon, J. Ferguson, B.E. Solsana, T. Garcia, A.F. Albert, A.A. Herzing, C.J. Kiely, S.E. Golunski, G.J. Hutchings: Chem. Commun., 3385 (2005); I. Dobrosz, K. Jiratova, V. Pitchon, J.M. Rynkowski: J. Mol. Catal. A, 234, 187 (2005); W. Yan, B. Chen, S.M. Mahurin, V. Schwartz, D.R., Mullins, A.R. Lupini, S.J. Pennycook, S. Dai, S.H. Overbury: J. Phys. Chem. B, 109, 10676 (2005); B. Solsona, M. Conte, Y. Cong, A. Carley, G. Hutchings: Chem. Commun., 2351 (2005); X. Zhang, H. Wang, B.-Q. Xu: J. Phys. Chem. B, 109, 9678 (2005); I.N. Remediakis, N. Lopez, J.K. Norskov: Angew. Chem., Int. Ed., 44, 1824 (2005); S.T. Daniells, M. Makkee, J.A. Moulijn: Catal. Lett., 100, 39 (2005). G. Mul, A. Zwijnenburg, B. van der Linden, M. Makkee, J.A. Moulijn: J. Catal., 201, 128 (2001). A. Fási, I. Pálinkó, K. Hernádi, I. Kiricsi: Catal. Lett., 81, 237 (2002). A. Fási, I. Pálinkó, J.W. Seo, Z. Kónya, K. Hernádi, I. Kiricsi: Chem. Phys. Lett., 372, 848 (2003). A. Abad, P. Concepción, A. Corma, H. García: Angew. Chem. Int. Ed., 44, 4066 (2005).

3. 4. 5. 6.

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