13317
2008, 112, 13317–13319 Published on Web 08/07/2008
Highly Selective Catalytic Hydrogenation of Arenes using Rhodium Nanoparticles Supported on Multiwalled Carbon Nanotubes Bhalchandra A. Kakade,† Suman Sahoo,‡ Shivappa B. Halligudi,*,‡ and Vijayamohanan K. Pillai*,† Physical and Materials Chemistry DiVision, National Chemical Laboratory, Pune 411 008, India, and Catalysis DiVision, National Chemical Laboratory, Pune 411 008, India ReceiVed: June 02, 2008; ReVised Manuscript ReceiVed: July 19, 2008
Rhodium nanoparticles (RhNPs; 4.9 ( 0.4 nm) supported on multiwalled carbon nanotubes (Rh/MWNT), prepared by simple microwave treatment, show a remarkable catalytic activity for arene hydrogenation with enhanced turnover numbers of ∼10000; disubstituted arenes show selective conversion of thermodynamically less favorable cis products (>80%). A series of arenes have been tested using various Rh-based catalysts, and a comparison of the results with that of reported rhodium catalysts shows unique selectivity under mild conditions. Selective hydrogenation of arenes is a crucial step in the preparation of a wide variety of organic compounds of commercial interest. This selectivity is traditionally achieved by heterogeneous catalysts such as Rh/Al2O3, Raney nickel, and metal sulfide under drastic reaction conditions.1 Interestingly, heterogeneous catalysis using various nanoparticles/clusters has been an emerging theme, where many fascinating properties of nanomaterials like high surface area and different aspect ratio provide various specific sites that can bind and/or dissociate reactant molecules.2 Recently, molecular rhodium catalysts tethered to a palladium-silica support3 or an ionic copolymer4 have been reported as effective recyclable catalysts. Furthermore, the use of carbon nanotubes (CNT) as a support in heterogeneous catalysis has garnered much attention due to their favorable characteristics such as high surface area, mechanical strength and inert carbon network, and chemically tunable topography. They also show catalytic properties superior to those of catalysts prepared on activated carbon, soot, or graphite.5 The morphology and the size of the CNT supports can play a significant role in catalytic applications owing to their ability to disperse the active phase. A recent comparison between the interaction of transition-metal atoms with carbon nanotube walls and that with graphite indicates major differences in bonding sites, magnetic moments, and charge-transfer direction.6 Metallic nanoparticles stabilized by several capping agents like soluble polymers, quaternary ammonium salts, and polyoxoanions have shown a better catalytic performance for hydrogenation for various substrates.7 However, some of these nanoparticles are kinetically unstable and undergo agglomeration after several cycles, resulting in the deactivation of the catalyst in a few cycles. Further focus on their thermal stability, catalyst isolation, and precipitation during polarity changes has not been * To whom correspondence should be addressed. E-mail: vk.pillai@ ncl.res.in. Fax: +91-20-25902636 (V.K.P.); E-mail:
[email protected]. Fax: +91-20-25902633 (S.B.H.). † Physical and Materials Chemistry Division. ‡ Catalysis Division.
10.1021/jp804843h CCC: $40.75
understood adequately to date, despite the significance of soluble metal nanoparticles.8 In order to avoid such difficulties in using soluble metal catalysts, metal nanoparticles supported on high surface area carbon nanotubes would be a better option. Accordingly, several efforts have been attempted to explore the catalytic activity of metal nanoparticles anchored on CNTs.9 For example, CNT-supported Rh, Pd, and Pd/Rh nanoparticle catalysts synthesized by microemulsion-template show a higher yield in the Heck coupling reaction, although drastic conditions of pressure have been employed with the metal content more than 5 wt%.10 However, most of these reports deal with a tedious synthetic procedure involving supercritical conditions of solvents with more than 20% catalyst loading. Here, we report a rapid and simple ex situ microwave treatment for the synthesis of a Rh/MWNT hybrid material for the catalytic hydrogenation of various arenes with enhanced turnover frequencies (TOF) as compared to that of conventional catalysts reported to date. Rh nanoclusters capped by tridecylamine (RhTDA) used in these studies were synthesized by a method discussed elsewhere,11 and their catalytic activities for selective hydrogenation of geraniol to citronellol have been investigated systematically after supporting on various substrates.12 The details of the purification of MWNTs and the preparation adopted for Rh/MWNTs are discussed in S1 (Supporting Information). Rh/MWNT catalysts have been prepared by direct mixing of acidified (microwave treatment in acid mixture) MWNTs with tridecylamine-capped rhodium nanoparticles followed by microwave treatment for 1 min. In a typical experiment, a heterogeneous hydrogenation has been carried out using toluene as the test substrate and a substrate to catalyst (Rh content) molar ratio of 10000:1 in hexane at 40 °C and at 20 bar of H2. Methylcyclohexane is the only product after 2 h with a TON of 9900 (shown in Scheme 1). Interestingly, these materials could easily be recovered by simple filtration with a considerable degree of recyclability. 2008 American Chemical Society
13318 J. Phys. Chem. C, Vol. 112, No. 35, 2008 SCHEME 1: (a) Hydrogenation of Toluene Using Rh/MWNTs1 with a TON of 9900 with Almost 100% Conversion to the Methyl Cyclohexane; (b) Disubstituted Benzene upon Reduction Producing the Thermodynamically Less Favorable cis Product with 99% Conversion
Various catalysts including Rh/MWNTs1, Rh/MWCNTs2, Rh(0) (RhNPs by direct reduction using NaBH4), and RhTDA have been prepared along with a high surface area carbon-based catalyst, Rh/vulcanXC, and their catalytic activity has been compared with respect to their TON. Rh/MWNT catalysts, thus prepared, were characterized by HRTEM, XRD, XPS, and TG in order to explore the structural changes after decoration. Accordingly, Figure 1a shows the low-resolution TEM images of Rh/MWNTs, indicating a regular arrangement of RhNPs on the side walls of MWNTs. Figure 1b exhibits a HRTEM image of RhNPs, showing lattice fringes of the (111) plane with an interplaner distance of 0.220 nm, which matches with that determined from the XRD studies (S6, Supporting Information). A narrow diameter distribution of the Rh nanoparticles has been observed with an average size of 4.9 ( 0.4 nm (Figure 1c). This particle size variation could be attributed to the calcination effect, where few particles would have been fragmented into smaller sizes. The SAED pattern also reveals a few spots corresponding to the rhodium along with two bright rings
Letters matching with (002) and turbostratic (10) planes of MWNTs (Figure 1d). The lattice fringe profile of the particle clearly shows (Figure 1e) an interplaner distance of 0.220 nm corresponding to (111) planes of rhodium with fcc stacking. TG (S5, Supporting Information) and EDS (S7, Supporting Information) reveal approximately 1 wt% of Rh loading on the carbon nanotubes. A variation of pressure reveals that the conversion of toluene increases with an increase in pressure up to 20 bar of H2 followed by a decrease in activity at higher pressures. At initial pressure, the conversion is poor, and a gradual increase in conversion with time needs infinite time for a saturation limit. Similarly, an increase in the conversion is observed with the temperature (S4, Supporting Information), and an optimum of 40 °C has been selected for all studies. The recyclability of the Rh/MWNT catalyst has been tested for the hydrogenation of toluene by conducting five successive runs (S4, Supporting Information). After each run, the catalyst was repeatedly washed with hexane, dried at 150 °C for 5 h, and then used in the hydrogenation reaction with a fresh reaction mixture. The conversion of toluene is practically the same (99%) in the first five cycles, with a marginal decrease at the fifth cycle. Further experiments have been conducted using a range of arene substrates with Rh/MWNTs, showing excellent activity under selected conditions (Table S1, Supporting Information). In particular, benzene shows a similar trend as that of toluene, whereas naphthalene and anthracene show less reactivity for the selected reaction conditions, producing tetrahydro derivatives as the major products. Interestingly, hydrogenation of anisole gives a single product of methoxycyclohexane with a TOF of 880. Two different products like cyclohexyl phenol ether (major) and dicyclohexyl ether (minor) have been observed in the hydrogenation of diphenyl ether under identical conditions, whereas cis-4methylcyclohexanol has been found to be a major product in the hydrogenation of p-cresol. In addition, the hydrogenation results of disubstituted benzene derivatives such as xylenes are very valuable since these give selective conversion to cis-
Figure 1. (a) TEM image of Rh/MWNTs1 hybrid material by ex situ reduction, revealing a homogeneous distribution of RhNPs on CNTs. (b) HRTEM of Rh nanoparticles on a CNT with lattice fringes of the (111) plane. (c) The histogram indicates a size distribution (4.9 ( 0.4 nm) of NPs. (d) The SAED pattern reveals spots corresponding to RhNPs and bright rings corresponding to (002) and turbostratic (10) planes due to MWNTs. (e) Profile of lattice fringes of Rh(111) encircled in (b).
Letters
J. Phys. Chem. C, Vol. 112, No. 35, 2008 13319
TABLE 1: Catalytic Activity of Various Rh Catalysts under Identical Conditions of Temperature and Pressurea catalyst
product
conversion (%)
TON
TOF
Rh/MWNTs1 Rh/MWNTs1 Rh/MWCNTs2 Rh (0) RhTDA Rh/vulcanXC
methyl cyclohexane cyclohexaneb methyl cyclohexane methyl cyclohexane methyl cyclohexane methyl cyclohexane
99 99 53 72 52 63
9900 9900 5300 3600 2800 6300
4950 6600 442 1800 1300 6300
a Reaction conditions: 5 mmol of substrate, 5 mg of catalyst (0.0005 mmol), 50 mL of hexane, 40 °C, 20 bar of H2. b Benzene has been used as the substrate.
dimethylcyclohexane, a thermodynamically less favorable product. Selective formation of cis products has been reported in the literature on rhodium-catalyzed hydrogenation reactions performed with homogeneous,13 biphasic,14 gas/solid,15,16 and liquid/solid17,18 catalytic systems. Formation of the cis isomer is generally interpreted as the addition of six hydrogen atoms from the metal surface (in hydride form) to the aromatic ring from the same face, whereas the trans compounds (commonly observed as minor products) are related to the outcome of the partially hydrogenated intermediates. Indeed, these are formed when a dimethylcyclohexene derivative dissociates from the catalyst surface and then reassociates with the opposite “face” before further hydrogenation. In order to check the leaching of Rh nanoparticles into the reaction mixture, the reaction has been also carried out for 1 h under selected conditions using fresh Rh/MWCNTs1 at 40 °C and at 20 bar in hexane. The reaction has been stopped after 1 h and the filtrate has been stirred further for 2 h after removing the catalyst. In the absence of the catalyst, there is no further increase in the conversion of toluene, which indicates the absence of leaching of any Rh metal into the reaction mixture. This observation confirms that the reaction is catalyzed heterogeneously. A comparative study reveals that, despite a greater Rh loading in the case of Rh/MWCNTs2, the conversion of toluene is less than that in the case of Rh/MWNTs1 (Table 1), perhaps due to the difference in the hydrophobic/hydrophilic properties of catalysts. This has also been confirmed by carrying out the N2 adsorption studies (S9, Supporting Information), where a comparatively lower surface area of Rh/MWNTs has been observed due to the homogeneous coverage of nanoparticles through the pores/defect sites. However, Rh(0) and RhTDA show rather inferior conversion values as compared to that of Rh/MWNTs1. It could be due to the fact that the high surface area CNT provides a better support and could also be due to contributing factors such as the wall defects on CNTs (produced during pretreatment; S1, Supporting Information) and acid-base and wetting properties of the catalyst. Interestingly, Rh/ vulcanXC shows a comparatively higher efficiency for toluene (TOF: 6300) within 1 h, when compared with that of other catalysts. However, further conversion after 1 h has not occurred (reaction has stopped), perhaps due to catalyst poisoning pertaining to the less interaction of RhNPs with the vulcan carbon. Hence, upon comparison, Rh/MWCNTs1 shows remarkably higher efficiency in both the perspectives of conversion efficiency and better recyclability. In conclusion, our approach of using Rh/MWNTs for arene hydrogenation offers an unprecedented opportunity to obviate
many limitations of currently used heterogeneous catalysts, opening new possibilities of manipulating the properties and stability to give enhanced catalytic performance. Apart from a remarkable conversion of products, these hybrid materials are also capable of increasing the thermal stability that could arise from the incorporation of rhodium. However, several challenges have to be overcome before these advantages can be commercially exploited, including a rigorous evaluation of the chemical stability, durability, cost effectiveness, and finally possible mechanism of product conversion. Nevertheless, these results raise enough scope to design heterogeneous catalysts with better overall system efficiencies, recyclability, and simplified balance of plant using these new-generation hybrid materials. Acknowledgment. The authors thank CSIR for funding this work through a NMITLI programme. B.A.K. and S.S. thank UGC/CSIR, respectively, for financial support. Supporting Information Available: Experimental details and other characterizations like XRD and TG and detailed results. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Augustine, R. L. Heterogeneous Catalysis for the Synthetic Chemistry; M. Dekker: New York, 1996; Chapter 17. (b) Eisen, M. S.; Marks, T. J. J. Am. Chem. Soc. 1992, 114, 10358. (c) Keane, M. A. J. Catal. 1997, 166, 347. (2) (a) Ohde, M.; Ohde, H.; Wai, C. M. Chem. Commun. 2002, 2388. (b) Wilson, O. M.; Knecht, M. R.; Garcia-Martinez, J. C.; Crooks, R. M. J. Am. Chem. Soc. 2006, 128, 4510. (c) Bratlie, K. M.; Hyunjoo, J.; Komvopolous, K.; Yang, P.; Somorjai, G. A. Nano Lett. 2007, 7, 3097. (3) (a) Gao, H.; Angelici, R. J. J. Am. Chem. Soc. 1997, 119, 6937. (b) Yang, H.; Gao, H.; Angelici, R. J. Organometallics 2000, 19, 622. (4) Mu, X.; Meng, J.; Li, Z.; Kou, Y. J. Am. Chem. Soc. 2005, 127, 9694. (5) Auer, E.; Freund, A.; Pietsch, J.; Tacke, T. Appl. Catal., A 1998, 173, 259. (6) Menon, M.; Andriotis, A. N.; Froudakis, G. E. Chem. Phys. Lett. 2000, 320, 425. (7) Roucoux, A.; Schulz, J.; Patin, H. Chem. ReV. 2002, 102, 3757. (8) (a) Widegren, J. A.; Finke, R. G. J. Mol. Catal. A: Chem. 2003, 191, 187. (b) Dyson, P. J. Dalton Trans. 2003, 2964. (9) (a) Planeix, J. M.; Coustel, N.; Coq, B.; Brotons, V.; Kumbhar, P. S.; Dutartre, R.; Geneste, P.; Bernier, P.; Ajayan, P. M. J. Am. Chem. Soc. 1994, 116, 7935. (b) Ebbesen, T. W.; Hiura, H.; Bisher, M. E.; Treacy, M. M. J.; Shreeve-Keyer, J. L.; Haushalter, R. C. AdV. Mater. 1996, 8, 155. (c) Ang, L. M.; Hor, T. S. A.; Xu, G. Q.; Tung, C. H.; Zhao, S.; Wang, J. L. S. Chem. Mater. 1999, 11, 2115. (d) Ye, X. R.; Lin, Y. H.; Wai, C. M. Chem. Commun. 2003, 642. (e) Yen, C. H.; Cui, X.; Pan, H.B.; Wang, S.; Lin, Y.; Wai, C. M. J. Nanosci. Nanotechnol. 2005, 5, 1852. (f) Ye, X. R.; Lin, Y.; Wai, C. M.; Talbot, J. B.; Jin, S. J. Nanosci. Nanotechnol. 2005, 5, 964. (g) Georgakilas, V.; Gournis, D.; Tzitzios, V.; Pasquato, L.; Guldi, D. M.; Prato, M. J. Mater. Chem. 2007, 17, 2679. (10) Yoon, B.; Wai, C. J. Am. Chem. Soc. 2005, 127, 17174. (11) Kakade, B. A.; Shintri, S. S.; Sathe, B. R.; Halligudi, S. B.; Pillai, V. K. AdV. Mater. 2007, 19, 272. (12) Joseph, T.; Shintri, S. S.; Kakade, B. A.; Pillai, V. K.; Halligudi, S. B. J. Nanosci. Nanotechnol. 2007, 7, 1. (13) Fonseca, G. S.; Umpierre, A. P.; Fichtner, P. F. P.; Teixeira, S. R.; Dupont, J. Chem.s Eur. J. 2003, 9, 3263. (14) Bonilla, R. J.; James, B. R.; Jessop, P. G. Chem. Commun. 2000, 941. (15) Neyestanaki, A. K.; Ma¨ki-Arvela, P.; Backman, H.; Karhu, H.; Salmi, T.; Vayrynen, J.; Murzin, D. Y. J. Catal. 2003, 218, 267. (16) Park, I. S.; Kwon, M. S.; Kim, N.; Lee, J. S.; Kang, K. Y.; Park, J. Chem. Commun. 2005, 5667. (17) Park, K. H.; Jang, K.; Kim, H. J.; So, S. U. Angew. Chem., Int. Ed. 2007, 46, 1152. (18) Landre, P. D.; Richard, D.; Draye, M.; Gallezot, P.; Lemair, M. J. Catal. 1994, 147, 214.
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