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Efficient Evolution of Hydrogen from Tetrahydronaphthalene upon Palladium Catalyst Supported on Activated Carbon Fiber Pham Dung Tien,† Hideaki Morisaka,† Tetsuya Satoh,† Masahiro Miura,† Masakatsu Nomura,*,†,‡ Hisaji Matsui,‡,§ and Chiharu Yamaguchi§ Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan, Collaborative Research Center for Advanced Science and Technology, Osaka University, Suita, Osaka 565-0871, Japan, and Research & Development Center, Osaka Gas Co. Ltd., 6-19-9 Torishima, Konohana-ku, Osaka 554-0051, Japan Received September 4, 2002
Catalytic hydrogen evolution from neat tetrahydronaphthalene under liquid-phase conditions was investigated. Among examined commercial Pd-, Pt-, and Rh-catalysts supported on activated carbon or alumina, Pd/C showed the highest activity. It was also found that Pd-catalysts supported on activated carbon fibers, which were prepared by an impregnation method, showed roughly three times higher activity than that of the commercial Pd/C.
Catalytic dehydrogenation of saturated or partially saturated carbocyclic compounds to produce the corresponding aromatic compounds is of broad utility in the field of organic synthesis.1 The reaction is also involved in the catalytic reforming of petroleum naphtha fractions.2 In our study on coal chemistry, we applied the reaction to coal-derived liquids which contain tetrahydronaphthalene or its methylated derivatives. By the catalytic treatment, these components were readily dehydrogenated to afford valuable naphthalene or methylnaphthalene-rich oils.3 Meanwhile, this reaction recently received a renaissance through applications as a new method to store and supply molecular hydrogen, which is due to stimulation by the advance in proton exchange membrane (PEM) fuel cell technology. Thus, it has been proposed that the reversible reaction pair, dehydrogenationhydrogenation of carbocyclic compounds, can be utilized for hydrogen storage.4 Of the reaction pair, the dehydrogenation usually requires rather severe conditions for carrying out effectively, due to its endothermic nature. Recently, several groups have reported that the dehydrogenation of cyclohexane, methylcyclohexane, and decahydronaphthalene can take place under relatively mild conditions.5,6 One of substantial problems * Corresponding author. † Department of Applied Chemistry, Faculty of Engineering, Osaka University. ‡ Collaborative Research Center for Advanced Science and Technology, Osaka University. § Research & Development Center, Osaka Gas Co. Ltd. (1) Fu, P. P.; Harvey, R. G. Chem. Rev. 1978, 78, 317. (2) Sinfelt, J. H. J. Mol. Catal. A 2000, 163, 123 and references therein. (3) Nomura, M.; Moritaka, S.; Miura, M. Energy Fuels 1995, 9, 936. (4) (a) Maria, G.; Marin, A.; Wyss, C.; Muller, S.; Newson, E. Chem. Eng. Sci. 1996, 51, 2891. (b) Newson, E.; Haueter, T. H.; Hottinger, P.; von Roth, F.; Scherer, G. W. H.; Schucan, T. H. H. Int. J. Hydrogen Energy 1998, 23, 905. (c) Scherer, G. W. H.; Newson, E.; Wokaun, A. Int. J. Hydrogen Energy 1999, 24, 1157.
comes from the aromatic products, which can retard the reaction by site-blocking on the catalyst.5b,6 To enhance the reaction efficiency, therefore, the reaction has to be conducted under “liquid-film” conditions, in which the amount of the liquid substrates is limited, not to suspend a catalyst but to keep it wet.5,6 Under these conditions, the temperature of catalyst layer in the “liquid-film” state becomes higher than the boiling point of the solution (superheated). It has been proposed that the high-temperature conditions accelerate rates of both dehydrogenation and product desorption.5b,5c,6 It is conceivable that tetrahydronaphthalene may be used as a hydrogen source, since it has an aromatic ring that may lead to advantageous adsorption on the catalyst. Consequently, we have examined the catalytic hydrogen evolution from the neat substrate and found that the reaction proceeds under normal liquid-phase conditions with significantly high efficiency, especially by using Pdcatalysts supported on activated carbon fibers. In view of hydrogen production, the dehydrogenation of 1,2,3,4-tetrahydronaphthalene (1) has to date been little explored. Thus, to look for an effective metal component for the catalysis, we first treated neat 1 (4 cm3) at 220 °C (oil bath temperature) in a batch reactor with a nitrogen balloon (1 atm)7 in the presence of commercial Pd-, Pt-, and Rh-catalysts supported on activated carbon or alumina powder (5 wt %, net metal amount: 0.01 mmol).7 The Rh-catalysts are less com(5) (a) Liu, C.; Sakaguchi, M.; Saito, Y. J. Hydrogen Energy Syst. Soc. Jpn. 1997, 22, 27: Chem. Abstr. 1997, 127, 164425. (b) Saito, Y. Syokubai 2001, 43, 259: Chem. Abstr. 2001, 135, 124741. (c) Hodoshima, S.; Arai, H.; Saito, Y. Int. J. Hydrogen Energy 2003, 28, 197. (6) Kariya, N.; Fukuoka, A.; Ichikawa, M. Appl. Catal. A 2002, 233, 91. (7) Nitrogen gas was purchased from Awao Sangyo (purity > 99.99%). Rh/C (5 wt %), Pd/alumina (5 wt %), Pd/C (5 wt %), Pt/alumina (5 wt %), and Pt/C (5 wt %) were purchased from Wako. Rh/alumina (5 wt %) was purchased from Aldrich.
10.1021/ef020187a CCC: $25.00 © 2003 American Chemical Society Published on Web 04/05/2003
Evolution of H2 from Tetrahydronaphthalene on Pd
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Table 1. Dehydrogenation of Tetrahydronaphthalene (1) Using Commercial Catalystsa
entry
catalyst
1
Pd/C
2
Pd/alumina
3
Pt/C
4
Pt/alumina
5
Rh/C
6
Rh/alumina
time/h
TONb
1 2.5 5 1 2.5 5 1 2.5 5 1 2.5 5 1 2.5 5 1 2.5 5
507 761 802 276 498 662 396 502 531 266 418 454 120 237 491 119 212 316
a Reaction conditions: 1 (4 cm3), metal catalyst (metal ) 0.01 mmol), under nitrogen (1 atm) at 220 °C (bath temp). b TON ) H2 (calcd, mmol)/metal (mmol).
mon, but we previously found that a soluble Rh complex can be used as a dehydrogenation catalyst for 1.8 During the reaction, a small portion of the liquid phase was periodically withdrawn by a syringe to monitor the amounts of naphthalene and unreacted 1 by GC. In each case, formation of the naphthalene and hydrogen gas was observed by GC and GC-MS. Since two molecules of hydrogen should be evolved with the production of one molecule of naphthalene, the turnover number (TON) for hydrogen production was defined as [amount of naphthalene formed (mmol)] × 2/bulk metal amount added (mmol), which was used to compare catalyst activities. As shown in Table 1, it was found that all of the Pd-, Pt-, and Rh-catalysts could induce the dehydrogenation of 1 and among them the Pd/C catalyst was the most effective. These results present striking contrast to those for the treatment of cyclohexane, in which only Pt among the three metals can trigger the reaction.6 Activated carbon fiber (ACF) has been regarded as a promising catalyst support because of various advantages such as the uniform distribution of microporosity.9 Therefore, we next prepared three Pd-catalysts supported on ACF-1, ACF-2, and ACF-3 (BET surface area: 957, 1776, and 2020 m2/g, respectively. See Supporting Information for more characterization data) according to the literature method.10 The Pd contents were determined to be 0.84, 0.84, and 0.70 wt %, respectively. Using the prepared fibrous catalysts (net metal amount: 0.005 mmol) the reaction of 1 (8 cm3) was conducted at 220 °C in a batch reactor with a (8) Nishinaka, Y.; Satoh, T.; Miura, M.; Morisaka, H.; Nomura, M.; Matsui, H.; Yamaguchi, C. Bull. Chem. Soc. Jpn. 2001, 74, 1727. (9) For example: (a) Matatov-Meytal, Y.; Sheintuch, M. Appl. Catal. A 2002, 231, 1. (b) deMiguel, S. R.; Vilella, J. I.; Jablonski, E. L.; Scelza, O. A.; Salinas-Martinez de Lecea, C.; Linares-Solano, A. Appl. Catal. A 2002, 232, 237. (c) Landau, M. V.; Kogan, S. B.; Tavor, D.; Herskowitz, M.; Koresh, J. E. Catal. Today 1997, 36, 497. (10) Mozingo, R. Organic Syntheses, Coll. Vol. III, 1955, 685.
Figure 1. Time course of the reaction of 1 using Pd/ACF-1 (O), Pd/ACF-2 (4), Pd/ACF-3 (+), and commercial Pd/C (0). Reaction conditions: 1 (8 cm3), palladium- catalyst (Pd ) 0.005 mmol for Pd/ACF, 0.01 mmol for the commercial one), under nitrogen (1 atm) at 220 °C. aTON ) H2 (calcd, mmol)/Pd (mmol).
nitrogen balloon (1 atm).7 The results, together with those using the commercial Pd/C catalyst under the same conditions, are shown in Figure 1. All experiments were carried out at least twice, and the average TONs are shown (the deviations were around (50). It is clear that all of Pd/ACF catalysts are more effective than the commercial one. Especially, when Pd/ACF-2 and Pd/ ACF-3 were used, TON reached ca. 2500 within 1 h. It should be cited that Mason et al. reported a new dehydrogenation system of 1 using a Pd/C catalyst, in which high-boiling ether solvents were used and sonication was applied to improve the reaction efficiency (TON ) ca. 1600 after 7 h).11 The present TONs using the simple system are significantly higher than theirs. When the reaction was performed at 300 °C using an electric heater, the three Pd/ACF catalysts produced hydrogen more smoothly after 1 h (Figure 2, the experimental deviations being around (300). Despite the higher external heating, the temperature of liquid phase was kept between 207 and 218 °C (bps of 1 and 2, respectively). The temperature of the catalyst surface seems to be similar to that of the liquid phase. This contrasts with the fact that the catalyst surface is superheated under liquid-film conditions.5,6 Therefore, it appears to be reasonable that the rate of dehydrogenation in the early stage was comparable to that at 220 °C (Figures 1 and 2). However, at 300 °C, the high dehydrogenation activities could be kept for longer reaction times. Using Pd/ACF-3, TON exceeded 7000 after 5 h. In this case, the amount of hydrogen evolved was confirmed by a gas buret (see Supporting Information for the detailed results). It was reported that a stream of CO2 to sweep hydrogen improved the efficiency of dehydrogenation of 1.12 However, in the (11) Mason, T. J.; Lorimer, J. P.; Paniwnyk, L.; Wright, P. W.; Harris, A. R. J. Catal. 1994, 147, 1. (12) It was reported that dehydrogenation of neat 1 proceeded with a high conversion under CO2 flow. However, since the reaction was also curried out with high Pd- and Pt-loadings, TONs were less than 50: Linstead, R. P.; Millidge, A. F.; Thomas, S. L. S.; Walpole, A. L. J. Chem. Soc. 1937, 1146.
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Figure 2. Time course of the reaction of 1 in the presence of Pd/ACF-1 (O), Pd/ACF-2 (4), Pd/ACF-3 (+), and commercial Pd/C (0) using an N2 balloon (1 atm), and in the presence of Pd/ACF-3 under N2 flow (×). Reaction conditions: 1 (8 cm3), palladium catalyst (Pd ) 0.005 mmol for Pd/ACF, 0.01 mmol for the commercial one), under nitrogen (1 atm) at 300 °C. a TON ) H2 (calcd, mmol)/Pd (mmol).
present case, the time course of TON under nitrogen flow conditions was found to be comparable to the standard conditions as shown in Figure 2. We selected 1 as the hydrogen source, since its aromatic ring may bring about advantageous adsorption against site-blocking by 2, as described above. However, it was found that the efficiency for the dehydrogenation of 1 was significantly affected by byproduct 2, rather than hydrogen. Thus, addition of 10% of 2 to substrate 1 before starting the reaction completely inhibited the dehydrogenation: treatment of the mixture of 1 and 2 (9:1, mol/mol) using Pd/ACF-2 at 220 °C for 3 h gave any observable change of the ratio. Therefore, it seems to be possible that the reduction of reaction efficiency observed within 1.5 h in the dehydrogenation reaction of 1 conducted at 220 °C even using Pd/ACF-2 and Pd/ ACF-3 (Figure 1) is attributable to the suppression by
Tien et al.
2 at least in part. During the reaction time, TONs were close to 3000, and thus, accumulated 2 reached ca. 10% in the reaction mixture. The mixture of 1 and 2 (9:1, mol/mol) was also treated over Pd/ACF-2 at 300 °C to result in no change of the ratio as at 220 °C, while the dehydrogenation of 1 could proceed with a higher conversion at this temperature (Figure 2). At the present stage, the reason the sensitivity toward 2 in the treatment of pure 1 at 300 °C is different from that using a mixture of 1 and 2 is not definitive. As described above, it was recently reported that the dehydrogenation of carbocyclic compounds such as cyclohexane and decahydronaphthalene using Pt-based catalysts can proceed smoothly under liquid-film conditions. In those cases, the reaction efficiency is very sensitive to the amount ratio of substrate to catalyst. Therefore, to carry out the reaction effectively, a significant amount of Pt loading was required. As a result, hydrogen productivity per one metal atom in a batch appears to be limited.12 Indeed, in the reaction of decahydronaphthalene, as well as even in that of tetrahydronaphthalene, TON was around 110 at 210 °C after 1 h.5c These facts suggest that, although the intrinsic hydrogen content of tetrahydronaphthalene is lower than those of cyclohexane and decahydronaphthalene, the greatly higher reactivity of the former toward dehydrogenation even under normal liquidphase conditions seems to make up the disadvantage. In summary, we have shown that catalytic hydrogen evolution from neat tetrahydronaphthalene can be performed under liquid-phase conditions effectively by using Pd-catalysts supported on activated carbon fibers. Further studies for obtaining insight into the relationship between the activity and characteristics of catalysts and attaining more enhanced reaction efficiency are now under way in our laboratory. Supporting Information Available: An experimental result to check the amount of actual hydrogen gas evolved and additional characterization data for ACFs. This material is available free of charge via the Internet at http://pubs.acs.org. EF020187A
