Continuous Hydrogen Evolution from Tetrahydronaphthalene over

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Energy & Fuels 2005, 19, 2110-2113

Continuous Hydrogen Evolution from Tetrahydronaphthalene over Palladium Catalysts Supported on Activated Carbon Fibers Pham Dung Tien, Tetsuya Satoh,* Masahiro Miura,* and Masakatsu Nomura Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan Received April 1, 2005. Revised Manuscript Received June 13, 2005

The continuous production of hydrogen from tetrahydronaphthalene (1) is achieved effectively using Pd/ACF catalysts (where ACF is activated carbon fiber) in a common fixed-bed flow reactor. Thus, the reaction efficiency is almost five times higher than that in a batch reactor under similar reaction conditions. Furthermore, cyclohexane (3) and decahydronaphthalene (4) can also be used as a hydrogen source in the reaction system.

Introduction The reversible reaction pair, dehydrogenation-hydrogenation of carbocyclic compounds, is now recognized to be one of the potential candidate methods to safely supply and store hydrogen for proton exchange membrane (PEM) fuel cells,1,2 as well as for energy storage systems.3 Of the reaction pair, the dehydrogenation usually requires the use of effective transition-metal catalysts under rather severe conditions for efficient operation, because of its endothermic nature. Besides the thermodynamic problem, site-blocking by the corresponding dehydrogenated aromatic coproducts, which suppresses the approach of the starting saturated substrates to the catalysts, may intervene. In addition, for the practical utilization on PEM fuel cell vehicles, an effective process that allows the continuous operation of the reaction is required. Recently, several groups have reported that the dehydrogenation of cyclohexane, methylcyclohexane, and decahydronaphthalene can be conducted effectively in the presence of platinum catalysts under “liquidfilm”1 or “wet-dry multiphase” conditions,2 in which the liquid substrates/catalyst ratio must be kept low, not to suspend the catalyst but to keep it wet. Under these * Authors to whom correspondence should be addressed. E-mail addresses: [email protected]; miura@ chem.eng.osaka-u.ac.jp. (1) (a) Hodoshima, S.; Arai, H.; Saito, Y. Int. J. Hydrogen Energy 2003, 28, 197-204. (b) Hodoshima, S.; Arai, H.; Takaiwa S.; Saito, Y. Int. J. Hydrogen Energy 2003, 28, 1255-1262. (c) Shinohara, C.; Kawakami, S.; Moriga, T.; Hayashi, H.; Hodoshima, S.; Saito, Y.; Sugiyama S. Appl. Catal., A 2004, 266, 251-255. (d) Hodoshima, S.; Saito, Y. J. Chem. Eng. Jpn. 2004, 37, 391-398. (e) Hodoshima, S.; Takaiwa, S.; Shono, A.; Satoh, H.; Saito, Y. Appl. Catal., A 2005, 283, 235-242. (2) (a) Kariya, N.; Fukuoka, A.; Ichikawa, M. Appl. Catal., A 2002, 233, 91-102. (b) Kariya, N.; Fukuoka, A.; Utagawa, T.; Sakuramoto M.; Goto, Y.; Ichikawa, M. Appl. Catal., A 2003, 247, 247-259. (3) (a) Maria, G.; Marin, A.; Wyss, C.; Muller, S.; Newson, E. Chem. Eng. Sci. 1996, 51, 2891-2896. (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, 905-909. (c) Scherer, G. W. H.; Newson, E.; Wokaun, A. Int. J. Hydrogen Energy 1999, 24, 1157-1169.

conditions, the temperature of the catalyst layer becomes higher than the boiling point of the solution (superheated). It has been proposed that the hightemperature conditions accelerate the rates of both dehydrogenation and product desorption. However, to produce hydrogen continuously, a rather complicated flow reactor is used.1e,2b In the reactor, using a particular substrate feeder such as a spray-pulse system, the liquid phase of the substrates can reach a catalyst bed directly to realize liquid-film or wet-dry multiphase conditions in a continuous operation. Meanwhile, we have successfully performed the dehydrogenation of tetrahydronaphthalene (1) by an alternative approach, in which palladium catalysts supported on activated carbon fibers (ACFs) are used.4 ACFs have recently been regarded as promising catalyst supports,5 in distinction from traditional activated carbons, because of its morphological network, which is formed by short micropores with a narrow size distribution, ensuring fast adsorption/desorption. Actually, the reaction of 1 proceeded even under normal liquid-phase conditions with a high substrate/catalyst ratio. It is also known that the fibrous catalysts that are supported on ACFs have other advantages, such as low resistance to the passage of fluid and high fluid permeability, and are applicable to a fixed-bed flow reactor without any modification. Therefore, we have undertaken the continuous hydrogen production from 1 using our catalyst system in a simple, common flow reactor. The results are described herein. (4) (a) Tien, P. D.; Morisaka, H.; Satoh, T.; Miura, M.; Nomura, M.; Matsui, H.; Yamaguchi, C. Energy Fuels 2003, 17, 658-660. (b) Tien, P. D.; Satoh, T.; Miura, M.; Nomura. Energy Fuels 2005, 19, 731735. (5) For example: (a) Matatov-Meytal, Y.; Sheintuch, M. Appl. Catal., A 2002, 231, 1-16. (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, 237-246. (c) Joannet, E.; Horny, C.; Kiwi-Minsker, L.; Renken, A. Chem. Eng. Sci. 2002, 57, 3453-3460. (d) Landau, M. V.; Kogan, S. B.; Tavor, D.; Herskowitz, M.; Koresh, J. E. Catal. Today 1997, 36, 497-510. (e) Suzuki, M. Carbon 1994, 32, 577-586.

10.1021/ef050090z CCC: $30.25 © 2005 American Chemical Society Published on Web 07/14/2005

Hydrogen Evolution from Tetrahydronaphthalene

Figure 1. Experimental apparatus for the continuous hydrogen evolution from tetrahydronapthalene (1).

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Figure 2. Time course (b) the conversion of 1 and (O) turnover number (TON) in the dehydrogenation of 1 (4 mL/h) using Pd/ACF-2 (0.004 mmol Pd) under a nitrogen stream (5 mL/min) at 300 °C. Note: TON ) H2 (mmol)/Pd(mmol).

Table 1. Characterization Data for ACFs ACF sample

BET surface area (m2/g)

pore volume (cm3/g)

mean pore radius (nm)

ACF-1 ACF-2 ACF-3

957 1776 2020

0.40 0.87 1.13

0.84 0.98 1.10

Experimental Section Catalyst Preparation. The three types of ACFs (ACF-1, ACF-2, and ACF-3) employed were supplied by Osaka Gas Co., Ltd. The Brunauer-Emmett-Teller (BET) surface area, pore volume, and mean pore diameter for each are shown in Table 1. A conventional impregnation method was applied to a HCl aqueous solution that contained PdCl2 by stirring at 80 °C for 3 h, followed by a reduction treatment in water under hydrogen at room temperature for 16 h.6 The palladium contents of Pd/ACF-1, Pd/ACF-2, and Pd/ACF-3 prepared by this method were determined to be 0.84, 0.71, and 0.70 wt %, respectively, by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Dehydrogenation of Tetrahydronaphthalene (1). The reaction was performed in a fixed-bed flow reactor (10 mm inner diameter (ID) × 470 mm), as shown in Figure 1. In a typical experiment, 1 was injected (4 mL/h) with a microfeeder to the reactor containing a Pd/ACF catalyst (Pd ) 0.004 mmol) at 300 °C (external temperature) under a nitrogen stream (5 mL/min). For the case that used a purchased palladium catalyst supported on activated carbon powder,7 glass wool (ca 20 mg) was used to hold the catalyst in the flow reactor. The rate of hydrogen evolution was measured by a gas flow meter every 5 min for 120 min. After that, n-hexadecane (ca. 800 mg, as an internal standard) was added to the condensed mixture in a cold trap and then the amounts of produced naphthalene (2) and unconsumed 1 were determined by gas chromatography (GC) analysis. In all experiments, the hydrogen balance was kept among the reactant and products without serious deviations.

Results and Discussion In an initial attempt, the dehydrogenation of neat 1,2,3,4-tetrahydronaphthalene (1) was conducted in a (6) Mozingo, R. Organic Syntheses, Collective Vol. III; Wiley: New York, 1955; pp 685-690. (7) The Pd catalyst on activated carbon powder (5 wt %) was purchased from Wako.

Figure 3. Time course of (b) the conversion of 1 and (O) TON in the dehydrogenation of 1 (4 mL/h), using Pd/ACF-3 (0.004 mmol Pd) under a nitrogen stream (5 mL/min) at 300 °C. Note: TON ) H2 (mmol)/Pd (mmol).

fixed-bed reactor with the feed rate of 4 mL/h at 300 °C (external temperature) under a nitrogen stream (5 mL/ min). When Pd/ACF-2 (net palladium amount of 0.004 mmol) was used as a catalyst, the reaction proceeded efficiently to produce hydrogen continuously with a high turnover number (TON), which finally reached 2.2 × 104 after 120 min (Figure 2). [Note: TON ) amount of evolved hydrogen (mmol)/bulk Pd amount added (mmol).] The time course of the conversion of 1 calculated from the amount of evolved hydrogen during every 5 min has also been shown in Figure 2. Note that the TON and the average conversion (77%) during 120 min are five times higher than those for the reaction in a batch reactor under similar conditions (TON ) 4.5 × 103, 15% conversion).4b Although Pd/ACF-3 showed comparable activity (Figure 3), Pd/ACF-1 gave the somewhat-lower TON and conversion (Figure 4). The lower catalytic activity of the latter was also observed in our previous study in the batch-wise operation.4 In contrast, even under the continuous conditions, a conventional palladium catalyst supported on activated carbon powder7 showed very low activity. Thus, TON and the average conversion for 120 min were 1.3 × 103 and 4.5%,

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Figure 4. Time course of (b) the conversion of 1 and (O) TON in the dehydrogenation of 1 (4 mL/h) using Pd/ACF-1 (0.004 mmol Pd) under a nitrogen stream (5 mL/min) at 300 °C. Note: TON ) H2 (mmol)/Pd (mmol).


respectively, which are comparable to those obtained in the batch reactor using the same catalyst.4 As a common trend, the conversions at the early stage (∼30 min) are higher than the corresponding average conversions. This may be attributed to the relatively higher temperature of the catalyst surface in the early stage. It was usually observed that, when the endothermic dehydrogenation occurs on the catalyst, the temperature decreases gradually and then is kept at ca. 280 °C, as evidenced by the inserted thermometer (Figure 1). Expectedly, the external temperature significantly affected the reaction efficiency. Thus, the TON and average conversion increased up to 2.6 × 104 and 90% at 320 °C with Pd/ACF-2 (Figure 5), while they decreased to 1.6 × 104 and 55%, respectively, at 280 °C (Figure 6). Even at 300 °C, a considerably enhanced average conversion (97%) was obtained in the case with the reduced feed rate of 1 (2 mL/h), although the accumulated TON for 120 min decreased to 1.4 × 104 (Figure 7). In contrast to these factors, the reaction was not so much influenced by the flow rate and the type of carrier

Tien et al.




gas. Thus, the reaction proceeded with essentially similar efficiency under a relatively rapid nitrogen stream (10 or 15 mL/min) or a hydrogen stream (5 mL/ min).8 Under the latter conditions, a pure, amplified hydrogen stream could be obtained at the outlet.

Hydrogen Evolution from Tetrahydronaphthalene


We used 1 as the hydrogen source, because its aromatic ring may cause advantageous adsorption against site-blocking by 2 on the surface of the catalyst. Actually, in the previous batch-wise operation, cycloalkanes without any aromatic ring such as cyclohexane (3) and decahydronaphthalene (4) did not undergo dehydrogenation at all, even in the presence of Pd/ACF (8) Under the conditions with a hydrogen stream, no hydrogenation product of 1 (such as decahydronaphthalene) was detected.

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catalysts. However, to our surprise, their reaction occurred in the present continuous operation. Thus, the treatment of 3 and 4, using Pd/ACF-2 as a catalyst, under the conditions used for the reaction of 1, gave hydrogen and the corresponding aromatic coproducts (benzene (5) and 2, respectively) (Figures 8 and 9). In the continuous operation, the catalysis seems to be less sensitive to the site-blocking effect of the aromatic products. Although the reaction efficiency is not as high as that for the reaction of 1 at the present stage, the intrinsic hydrogen contents of these substrates are rather high. A further study to improve the catalytic efficiency is now underway in our laboratory. In summary, we have shown that the continuous hydrogen production from tetrahydronaphthalene (1) can be conducted effectively using Pd/ACF catalysts in a common fixed-bed flow reactor. The reaction temperature and the feed rate of 1 were determined to affect the reaction efficiency significantly, whereas the flow rate and the type of carrier gas were not important factors. Furthermore, cyclohexane (3) and decahydronaphthalene (4) could also be used as hydrogen sources in the reaction system. Acknowledgment. Partial financial support from Kansai Research Foundation (KRF) for technology promotion is acknowledged. We also thank Dr. Hisaji Matsui and Dr. Chiharu Yamaguchi of Osaka Gas for the supply of ACFs and helpful discussion. EF050090Z

Continuous Hydrogen Evolution from Tetrahydronaphthalene over

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