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Graphite-supported rhodium catalysts for naphthalene hydrogenation in supercritical carbon dioxide solvent
Catalysis Communications 10 (2009) 1681–1684
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Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Graphite-supported rhodium catalysts for naphthalene hydrogenation in supercritical carbon dioxide solvent Norihito Hiyoshi, Aritomo Yamaguchi, Chandrashekar V. Rode, Osamu Sato, Masayuki Shirai * Research Center for Compact Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST), 4-2-1 Nigatake, Miyagino, Sendai 983-8551, Japan
a r t i c l e
i n f o
Article history: Received 17 March 2009 Received in revised form 30 April 2009 Accepted 13 May 2009 Available online 20 May 2009 Keywords: Supercritical carbon dioxide Naphthalene Tetralin Decalin Graphite-supported rhodium catalysts
a b s t r a c t Activities of graphite-supported rhodium catalysts were examined for the hydrogenation of naphthalene in supercritical carbon dioxide (scCO2) solvent at 313 K, and results were compared with those of activated carbon-supported rhodium catalysts. The graphite-supported rhodium catalysts exhibited higher activities on a catalyst-weight basis and higher turnover frequency values for the naphthalene hydrogenation in scCO2 than activated carbon-supported rhodium ones. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Hydrogenation of naphthalene ring is a very important reaction for the production of diesel and jet fuels [1], fine chemicals [2], and hydrogen storage system using organic chemical hydride for the proton exchange membrane fuel cells [3–5]. Vapor and liquid phase hydrogenation of naphthalene has been investigated at high temperature (>473 K) [6,7]. However, the high reaction temperature causes deactivation of supported metal catalysts because of the formation of coke on the active sites. Low temperature hydrogenation of naphthalene using organic solvents is also possible [8]; however, low reaction rates and the difficulty in the separation of pure products from solvents became a critical issue. Supercritical carbon dioxide (scCO2) solvent is known to be effective for various hydrogenation reactions over supported metal catalysts [9–12]. We have already reported that the hydrogenation over activated carbon-supported rhodium catalysts in scCO2 could overcome the above problems to produce decahydronaphthalene (decalin) [13,14]. This naphthalene hydrogenation method has following advantages: (i) low reaction temperature, (ii) easy separation of products from solvents and catalysts, (iii) non-toxicity and nonflammability of the scCO2 solvent, and (iv) high selectivity to cisdecalin which is more preferable than trans-decalin for the hydrogen storage system [13,14]. In addition, higher reaction rates can be expected in scCO2 than in organic solvent because of higher miscibility of hydrogen with scCO2. Nevertheless, it was found that * Corresponding author. Fax: +81 22 237 5224. E-mail address: [email protected] (M. Shirai). 1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2009.05.007
the hydrogenation rate of naphthalene was not always higher in scCO2 than in organic solvents, depending on the type of catalysts [14,15]. Therefore, the development of a suitable supported metal catalyst for the naphthalene hydrogenation in scCO2 still remains an important topic of research. Herein, we report the role of graphite as a support in enhancing the high activity of rhodium catalysts for the hydrogenation of naphthalene in scCO2. 2. Experimental 2.1. Catalyst preparation Graphite-supported rhodium catalysts containing 5 wt% of rhodium (Rh/HSAG300 and Rh/HSAG100) were prepared by an incipient wetness technique using rhodium(III) chloride trihydrate (RhCl3 3H2O, 99.5%, Wako Pure Chemical Industries, Ltd.) as rhodium source and high surface area graphite TIMREX HSAG300 and HSAG100 (TIMCAL Ltd.) as support. Aqueous solution of RhCl3 3H2O (0.25 mol g 1) was added to HSAG300 and HSAG100 and dried at 373 K in air. This procedure was repeated three times for HSAG300 and four times for HSAG100 to obtain catalysts containing 5 wt% rhodium. Activated carbon-supported rhodium catalysts containing 5 wt% rhodium with different rhodium particle sizes (Rh/AC, Rh/AC-0.3u and Rh/AC-1.0u) were prepared by impregnation methods using activated carbon (Wako Pure Chemical Industries, Ltd., 031-02135) and aqueous solution of RhCl3 3H2O [15]. Rh/AC was prepared by impregnation of activated carbon with aqueous solution of RhCl3 3H2O (0.025 mol dm 3) at room temperature for 1 h followed by the evaporation of water in
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vacuo at 343 K for 1 h and then at 363 K. Rh/AC-0.3u and Rh/AC-1.0u were prepared as follows. Activated carbon was stirred in an aqueous solution of RhCl3 3H2O (0.025 mol dm 3) at room temperature for 1 h, and then aqueous solution of urea (0.076 mol dm 3 for Rh/ AC-0.3u and 0.250 mol dm 3 for Rh/AC-1.0u) was added. The molar ratio of urea to rhodium was 0.3:1 for Rh/AC-0.3u and 1:1 for Rh/AC1.0u. After stirring at room temperature for 1 h and 363 K for 1 h, the solution was evaporated in vacuo at 363 K. The graphite- and activated carbon-supported catalysts were reduced in a flow of hydrogen at 573 K for 2 h. The graphite supports were significantly different from the activated carbon support in surface area and porosity. The BET surface areas of HSAG100, HSAG300 and the activated carbon used in this study were 98, 325 and 1450 m2 g 1, respectively. The micropore surface area determined by the t-plot analysis were 25, 107 and 638 m2 g 1 for HSAG100, HSAG300 and the activated carbon, respectively. The granule size values of HSAG100, HSAG300 and activated carbon were 13, 6 and 500 lm, respectively. 2.2. Characterization Transmission electron microscopy (TEM) was performed using an FEI TECNAI G-20 instrument operating at accelerating voltages up to 200 kV with a single crystal LaB6 filament. The X-ray powder diffraction (XRD) patterns of the catalysts were obtained using a Rigaku RINT 2200 instrument with Cu Ka1 radiation. Hydrogen adsorption isotherms of the catalysts were measured using an automatic adsorption apparatus (AUROSORB-1C, Quantachrome). After an activated catalyst was treated with 80 kPa of hydrogen at 573 K followed by the evacuation at 573 K for 1 h, the hydrogen adsorption isotherm was measured at 323 K in the pressure range 0.4–7 kPa. 2.3. Catalytic reaction The hydrogenation of naphthalene and tetralin were carried out using a batch system. The amounts of benzothiophene in naphthalene and tetralin (>97%, Wako Pure Chemical Ind. Ltd.) used in this study were less than detection limit of FID-GC (<0.0002 mol%). The weighed amounts of catalyst and substrate (2.34 mmol) were placed in a stainless steel high-pressure reactor (50 cm3 capacity), and the reactor was flushed three times with carbon dioxide. After the reactor was heated to 313 K in an oil bath, hydrogen (3 MPa, 57 mmol) was introduced to the reactor from a hydrogen cylinder, and then carbon dioxide was introduced through a pump (SCF-Get, Jasco) to total pressure of 18 MPa and the contents were stirred magnetically. After the reaction period, the reactor was cooled down rapidly with an ice bath, the pressure was released slowly and the contents were discharged to separate the catalyst by simple filtration. The unreacted substrate and the products formed were recovered with acetone and analyzed with GC-FID (HP6890), which showed a material balance of more than 95%. n-Heptane (Wako Pure Chemical Ind. Ltd., Japan) was used as a solvent (20 cm3) for studying liquid phase hydrogenation of naphthalene and tetralin under 3 MPa of hydrogen and 0.1 MPa of carbon dioxide.
3. Results and discussion Fig. 1 shows TEM images and particle size distribution of the graphite-supported rhodium catalysts. For the Rh/HSAG300 catalyst (Fig. 1a), rhodium particles (ca. 2 nm) were observed around the edge of large graphite crystallites (ca. 1 lm in diameter) and few rhodium particles were observed on the basal plane of the graphite crystallites. The Rh/HSAG300 catalyst had narrow rho-
dium particle size distribution with an average particle size of 1.9 nm, and no particles more than 4 nm were observed (Fig. 1c). For the Rh/HSAG100 catalyst (Fig. 1b), rhodium particle (<4 nm) also existed on the edge of the large graphite crystallites (1 lm). However, the Rh/HSAG100 catalyst had larger rhodium particle (10 nm) on the basal plane of the graphite crystallites, by which the particle size distribution of the Rh/HSAG100 catalyst was broadened compared to the Rh/HSAG300 catalyst (Fig. 1d). In contrast to the graphite-supported catalysts, TEM observation showed that the rhodium particles of the activated carbon-supported catalysts were distributed homogeneously on the whole range of the activated carbon support [15]. Dispersion values, which were the number of surface rhodium atoms normalized by the number of total rhodium atoms, were determined by a hydrogen adsorption method (dispersion (%) = hydrogen atoms adsorbed (mol)/rhodium atoms loaded (mol) 100), and the average particle diameters (dH2) were estimated from the dispersion values assuming that particles are spherical in shape (Table 1). In addition, the average particle diameters were also estimated by TEM (dTEM) and XRD (dXRD) (Table 1). The dispersion values of the Rh/HSAG100, Rh/HSAG300 and Rh/AC catalysts determined by hydrogen adsorption measurements were similar (2728%); however, dTEM values were different between the three catalysts. For the Rh/HSAG100 catalyst, the dH2 value was quite consistent with the dTEM and dXRD values. In contrast, the dH2 value of the Rh/AC catalyst was estimated to be four times larger than the dTEM value, indicating that that the dispersion value of the Rh/AC catalyst was smaller than that expected by the TEM observation. The dispersion values, which were determined by hydrogen adsorption, were also compared for the catalysts having the comparable dTEM values (Rh/HSAG100 vs. Rh/AC-1.0u, and Rh/ HSAG300 vs. Rh/AC-0.3u) (Table 1). The graphite-supported catalysts had the larger dispersion values than the activated carbonsupported catalysts having the similar dTEM values. In particular, the dispersion value of the Rh/HSAG100 catalyst was three times larger than that of the Rh/AC-1.0u catalyst. These results suggest that the hydrogen adsorption is substantially inhibited for the activated carbon-supported rhodium catalysts. These differences in hydrogen adsorption ability between the graphite- and activated carbon-supported catalysts would result from the location of rhodium particles on the supports. The rhodium particles on the edge of the graphite crystallites are well exposed on the surface facilitating an easy adsorption of hydrogen molecules. In contrast, the rhodium particles of the activated carbon-supported catalysts would be inside the pores of activated carbon causing less number of surface rhodium atoms available for hydrogen adsorption. The activities of the catalysts for naphthalene hydrogenation were examined by measuring the initial consumption rate of naphthalene in both scCO2 and n-heptane at 313 K. We confirmed that the initial naphthalene consumption rate for all catalysts did not changed by grinding catalysts, indicating that the granule size of the catalysts were not influence the activities. The amount of naphthalene decreased linearly from the beginning of the reaction in both the solvents, and mainly tetralin (90–95% of selectivity which depended on catalysts and solvents), small amounts of cis-, transdecalin (3–8% and 0.2–1.0%, respectively), and octahydronaphthalene (0.8–1.7%) were formed (not shown). Fig. 2a shows the rate of hydrogenation on a catalyst-weight basis (rw). The graphite-supported catalysts showed high rw values in scCO2 than the activated carbon-supported ones. It is noteworthy that the graphite-supported catalysts were highly active particularly in scCO2, whereas these catalysts showed comparable rw values to some of the activated carbon-supported catalysts in n-heptane. Turnover frequency (TOF) values were also calculated on the assumption that number of adsorbed hydrogen atoms is the number of active sites (Fig. 2b). The graphite-supported catalysts showed higher TOF
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c
d
0.8
0.6
Fraction
Fraction
0.6
0.8
0.4
0.2
0.4
0.2
0.0
0.0 0
1
2
3
4
5
6
7
8
9
10
0
1
2
3
4
5
6
7
8
9 10
Particle diameter/nm
Particle diameter/nm
Table 1 Dispersion and particle size of supported rhodium catalysts. Catalysta
Db (%) (dH2c (nm))
dTEMd (nm)
dXRDe (nm)
Rh/HSAG100 Rh/HSAG300 Rh/AC Rh/AC-0.3u Rh/AC-1.0u
26.8 27.1 27.9 19.7 10.1
5.4 1.9 <1f 2.3 5.1
3.2 – – 1.5 3.6
(4.1) (4.1) (3.9) (5.6) (10.9)
a
Rhodium loading was 5 wt%. Dispersion determined by a hydrogen adsorption method. c Average particle size calculated using the dispersion value on the assumption that particles were sphere. 3 2 d Average particle sizes determined by TEM. dTEM = R(nidi )/R(nidi ). Where, ni and di denotes number and diameter of particles, respectively. e Average particle sizes estimated by XRD. f Under detection limit.
a
10
rw/mmol g-1 min-1
Fig. 1. TEM images and particle size distributions of Rh/HSAG300 (a), (c) and Rh/HSAG100 (b), (d).
8 scCO2 6
n-Heptane
4 2
b
0
b
70 60
40 30 20 10
u
u
.0 R h/ AC -1
-0 .3 AC R h/
R h/ AC
HS AG 30 0 R h/ HS AG 10 0
0
R h/
values in scCO2 than the activated carbon-supported ones, whereas all the catalysts showed comparable TOF values in n-heptane. We have reported that the TOF values of activated carbon-supported rhodium catalysts increased with increasing metal particle size in scCO2; however, they were almost constant in n-heptane. The high TOF values of the graphite-supported catalysts in scCO2 could be partially explained by the particle size effect of rhodium. The higher TOF values of the Rh/HSAG100 and Rh/HSAG300 catalysts than the Rh/AC catalyst could be explained by the larger dTEM values of these graphite-supported catalysts. Besides the effect of particle size, other reasons for the high TOF values of the graphite-supported catalysts in scCO2 should be also considered because the graphite-supported catalysts exhibited higher TOF values in scCO2 than activated carbon-supported catalysts having the comparable dTEM values (Rh/HSAG100 vs. Rh/AC-1.0u and Rh/HSAG300 vs. Rh/AC-0.3u) (Fig. 2b). One possible explanation is that the location of rhodium particles influences the TOF values. In scCO2, the diffusion rate of naphthalene to rhodium particles on the edge of graphite crystallites would be higher than that to ones in the pore
TOF/min-1
50
Fig. 2. Caption reaction rate on catalyst-weight basis (rw) (a) and turnover frequency (TOF) (b) of supported rhodium catalysts for naphthalene hydrogenation in scCO2 (total pressure: 18 MPa) and in n-heptane (20 cm3). Initial hydrogen pressure: 3 MPa (57 mmol); initial naphthalene: 2.34 mmol; reaction temperature: 313 K; reactor capacity: 50 cm3; catalyst weight: 0.003 g.
of activated carbon support. However, other factors such as electronic state of rhodium particles and/or particle morphology, which affects the coordination number of surface rhodium atoms,
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a
0.90
10
scCO2 n-Heptane
Final cis ratio
rw/mmol g-1 min-1
8 scCO2 6
n-Heptane
4
0.85
2 0
TOF/min-1
50
0u -1 .
3u -0 . AC
R h/ AC
R h/
60
R h/
70
R h/ AC
HS AG 30 0 R h/ HS AG 10 0
b
0.80
Fig. 4. Final cis ratio of decalin for tetralin hydrogenation over supported rhodium catalysts in scCO2 (total pressure: 18 MPa) and in n-heptane (20 cm3). Initial hydrogen pressure: 3 MPa (57 mmol); initial tetralin: 2.34 mmol; reaction temperature: 313 K; reactor capacity: 50 cm3.
40 30 20 10
0u -1 .
.3
u R h/ AC
AC R h/
R h/ AC -0
R h/
HS AG 30 0 R h/ HS AG 10 0
0
Fig. 3. Reaction rate on catalyst weight basis (rw) (a) and turnover frequency (TOF) (b) of supported rhodium catalysts for tetralin hydrogenation in scCO2 (total pressure: 18 MPa) and in n-heptane (20 cm3). Initial hydrogen pressure: 3 MPa (57 mmol); initial tetralin: 2.34 mmol; reaction temperature: 313 K; reactor capacity: 50 cm3; catalyst weight: 0.003 g.
could be also responsible for the high TOF values of the graphitesupported catalysts in scCO2. Hydrogenation rate on a catalyst-weight basis (rw) is influenced by the number of active sites as well as the TOF values of active sites. The result of hydrogen adsorption showed that the graphite-supported catalysts had the larger number of rhodium atoms exposed to the surface that could act as active sites than the activated carbon-supported catalysts, even though the particle sizes of rhodium were similar in both the cases. Therefore, the higher rw values of the graphite-supported catalysts in scCO2 can be attributed to both the larger number of active sites on the surface and the higher TOF value. Hydrogenation of naphthalene over the graphite- and activated carbon-supported rhodium catalysts proceeded consecutively, and naphthalene was completely transformed to decalin via intermediate tetralin. Therefore, we examined the catalytic activity for the consecutive hydrogenation step using tetralin as a reactant. The amount of tetralin decreased linearly from the beginning of the reaction, and mainly cis-decalin (73–84% of selectivity), small amounts of trans-decalin (9–19%) and octahydronaphthalene (8– 12%) were formed in both the solvents. Fig. 3 shows the rw and the TOF values of the tetralin hydrogenation. The higher rw values were also obtained for tetralin hydrogenation particularly in scCO2 over the graphite-supported catalysts than those for the activated carbon-supported ones. The larger number of surface active sites and the higher TOF value in scCO2 are responsible for the higher rw values of the graphite-supported catalysts for the tetralin hydrogenation. Another finding in the tetralin hydrogenation was that
the graphite-supported catalysts exhibited high selectivity to cisdecalin (Fig. 4), which is a preferable product than trans-decalin for the hydrogen storage system because of higher dehydrogenation rate of cis-decalin [4]. After all the tetralin was hydrogenated to decalin, final cis ratio reached 89 and 87% for Rh/HSAG100 and Rh/HSAG300 in scCO2, respectively, which were higher than those for the activated carbon-supported catalysts in scCO2 (84–86%). 4. Conclusions The graphite-supported rhodium catalysts were more efficient than the rhodium catalysts supported on activated carbon for the hydrogenation of naphthalene in scCO2 because rhodium particles were on the edge of graphite crystallites leading to higher hydrogen adsorption; however, this was not the case in n-heptane solvent. Acknowledgement This study was supported by Industrial Technology Research Grant Program from New Energy and Industrial Technology Development (NEDO) of Japan. References [1] B.H. Cooper, B.B.L. Donnis, Appl. Catal. A 137 (1996) 203. [2] C. Song, CATTECH 6 (2002) 64. [3] S. Hodoshima, S. Takaiwa, A. Shono, K. Satoh, Y. Saito, Appl. Catal. A 283 (2005) 235. [4] N. Kariya, A. Fukuoka, T. Utagawa, M. Sakuramoto, Y. Goto, M. Ichikawa, Appl. Catal. A 247 (2003) 247. [5] B. Wang, D.W. Goodman, G.F. Froment, J. Catal. 253 (2008) 229. [6] S.R. Kirumakki, B.G. Shpeizer, G.V. Sagar, K.V.R. Chary, A. Clearfield, J. Catal. 242 (2006) 319. [7] P.A. Rautanen, M.S. Lylykangas, J.R. Aittamaa, A.O.I. Krause, Ind. Eng. Chem. Res. 41 (2002) 5966. [8] W. Weitkamp, Adv. Catal. 18 (1968) 1. [9] P.G. Jessop, W. Leitner (Eds.), Chemical Synthesis Using Supercritical Fluids, Wiley, New York, 1999. [10] A. Baiker, Chem. Rev. 99 (1999) 453. [11] M.G. Hizler, M. Poliakoff, Chem. Commun. (1997) 1667. [12] T. Seki, J.-D. Grunwaldt, A. Baiker, Ind. Eng. Chem. Res. 47 (2008) 4561. [13] N. Hiyoshi, T. Inoue, C.V. Rode, O. Sato, M. Shirai, Catal. Lett. 106 (2006) 133. [14] N. Hiyoshi, M. Osada, C.V. Rode, O. Sato, M. Shirai, Appl. Catal. A 331 (2007) 1. [15] N. Hiyoshi, C.V. Rode, O. Sato, Y. Masuda, A. Yamaguchi, M. Shirai, Chem. Lett. 37 (2008) 734.
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Graphite-supported rhodium catalysts for naphthalene hydrogenation in supercritical carbon dioxide solvent Norihito Hiyoshi, Aritomo Yamaguchi, Chandrashekar V. Rode, Osamu Sato, Masayuki Shirai * Research Center for Compact Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST), 4-2-1 Nigatake, Miyagino, Sendai 983-8551, Japan
a r t i c l e
i n f o
Article history: Received 17 March 2009 Received in revised form 30 April 2009 Accepted 13 May 2009 Available online 20 May 2009 Keywords: Supercritical carbon dioxide Naphthalene Tetralin Decalin Graphite-supported rhodium catalysts
a b s t r a c t Activities of graphite-supported rhodium catalysts were examined for the hydrogenation of naphthalene in supercritical carbon dioxide (scCO2) solvent at 313 K, and results were compared with those of activated carbon-supported rhodium catalysts. The graphite-supported rhodium catalysts exhibited higher activities on a catalyst-weight basis and higher turnover frequency values for the naphthalene hydrogenation in scCO2 than activated carbon-supported rhodium ones. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Hydrogenation of naphthalene ring is a very important reaction for the production of diesel and jet fuels [1], fine chemicals [2], and hydrogen storage system using organic chemical hydride for the proton exchange membrane fuel cells [3–5]. Vapor and liquid phase hydrogenation of naphthalene has been investigated at high temperature (>473 K) [6,7]. However, the high reaction temperature causes deactivation of supported metal catalysts because of the formation of coke on the active sites. Low temperature hydrogenation of naphthalene using organic solvents is also possible [8]; however, low reaction rates and the difficulty in the separation of pure products from solvents became a critical issue. Supercritical carbon dioxide (scCO2) solvent is known to be effective for various hydrogenation reactions over supported metal catalysts [9–12]. We have already reported that the hydrogenation over activated carbon-supported rhodium catalysts in scCO2 could overcome the above problems to produce decahydronaphthalene (decalin) [13,14]. This naphthalene hydrogenation method has following advantages: (i) low reaction temperature, (ii) easy separation of products from solvents and catalysts, (iii) non-toxicity and nonflammability of the scCO2 solvent, and (iv) high selectivity to cisdecalin which is more preferable than trans-decalin for the hydrogen storage system [13,14]. In addition, higher reaction rates can be expected in scCO2 than in organic solvent because of higher miscibility of hydrogen with scCO2. Nevertheless, it was found that * Corresponding author. Fax: +81 22 237 5224. E-mail address: [email protected] (M. Shirai). 1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2009.05.007
the hydrogenation rate of naphthalene was not always higher in scCO2 than in organic solvents, depending on the type of catalysts [14,15]. Therefore, the development of a suitable supported metal catalyst for the naphthalene hydrogenation in scCO2 still remains an important topic of research. Herein, we report the role of graphite as a support in enhancing the high activity of rhodium catalysts for the hydrogenation of naphthalene in scCO2. 2. Experimental 2.1. Catalyst preparation Graphite-supported rhodium catalysts containing 5 wt% of rhodium (Rh/HSAG300 and Rh/HSAG100) were prepared by an incipient wetness technique using rhodium(III) chloride trihydrate (RhCl3 3H2O, 99.5%, Wako Pure Chemical Industries, Ltd.) as rhodium source and high surface area graphite TIMREX HSAG300 and HSAG100 (TIMCAL Ltd.) as support. Aqueous solution of RhCl3 3H2O (0.25 mol g 1) was added to HSAG300 and HSAG100 and dried at 373 K in air. This procedure was repeated three times for HSAG300 and four times for HSAG100 to obtain catalysts containing 5 wt% rhodium. Activated carbon-supported rhodium catalysts containing 5 wt% rhodium with different rhodium particle sizes (Rh/AC, Rh/AC-0.3u and Rh/AC-1.0u) were prepared by impregnation methods using activated carbon (Wako Pure Chemical Industries, Ltd., 031-02135) and aqueous solution of RhCl3 3H2O [15]. Rh/AC was prepared by impregnation of activated carbon with aqueous solution of RhCl3 3H2O (0.025 mol dm 3) at room temperature for 1 h followed by the evaporation of water in
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vacuo at 343 K for 1 h and then at 363 K. Rh/AC-0.3u and Rh/AC-1.0u were prepared as follows. Activated carbon was stirred in an aqueous solution of RhCl3 3H2O (0.025 mol dm 3) at room temperature for 1 h, and then aqueous solution of urea (0.076 mol dm 3 for Rh/ AC-0.3u and 0.250 mol dm 3 for Rh/AC-1.0u) was added. The molar ratio of urea to rhodium was 0.3:1 for Rh/AC-0.3u and 1:1 for Rh/AC1.0u. After stirring at room temperature for 1 h and 363 K for 1 h, the solution was evaporated in vacuo at 363 K. The graphite- and activated carbon-supported catalysts were reduced in a flow of hydrogen at 573 K for 2 h. The graphite supports were significantly different from the activated carbon support in surface area and porosity. The BET surface areas of HSAG100, HSAG300 and the activated carbon used in this study were 98, 325 and 1450 m2 g 1, respectively. The micropore surface area determined by the t-plot analysis were 25, 107 and 638 m2 g 1 for HSAG100, HSAG300 and the activated carbon, respectively. The granule size values of HSAG100, HSAG300 and activated carbon were 13, 6 and 500 lm, respectively. 2.2. Characterization Transmission electron microscopy (TEM) was performed using an FEI TECNAI G-20 instrument operating at accelerating voltages up to 200 kV with a single crystal LaB6 filament. The X-ray powder diffraction (XRD) patterns of the catalysts were obtained using a Rigaku RINT 2200 instrument with Cu Ka1 radiation. Hydrogen adsorption isotherms of the catalysts were measured using an automatic adsorption apparatus (AUROSORB-1C, Quantachrome). After an activated catalyst was treated with 80 kPa of hydrogen at 573 K followed by the evacuation at 573 K for 1 h, the hydrogen adsorption isotherm was measured at 323 K in the pressure range 0.4–7 kPa. 2.3. Catalytic reaction The hydrogenation of naphthalene and tetralin were carried out using a batch system. The amounts of benzothiophene in naphthalene and tetralin (>97%, Wako Pure Chemical Ind. Ltd.) used in this study were less than detection limit of FID-GC (<0.0002 mol%). The weighed amounts of catalyst and substrate (2.34 mmol) were placed in a stainless steel high-pressure reactor (50 cm3 capacity), and the reactor was flushed three times with carbon dioxide. After the reactor was heated to 313 K in an oil bath, hydrogen (3 MPa, 57 mmol) was introduced to the reactor from a hydrogen cylinder, and then carbon dioxide was introduced through a pump (SCF-Get, Jasco) to total pressure of 18 MPa and the contents were stirred magnetically. After the reaction period, the reactor was cooled down rapidly with an ice bath, the pressure was released slowly and the contents were discharged to separate the catalyst by simple filtration. The unreacted substrate and the products formed were recovered with acetone and analyzed with GC-FID (HP6890), which showed a material balance of more than 95%. n-Heptane (Wako Pure Chemical Ind. Ltd., Japan) was used as a solvent (20 cm3) for studying liquid phase hydrogenation of naphthalene and tetralin under 3 MPa of hydrogen and 0.1 MPa of carbon dioxide.
3. Results and discussion Fig. 1 shows TEM images and particle size distribution of the graphite-supported rhodium catalysts. For the Rh/HSAG300 catalyst (Fig. 1a), rhodium particles (ca. 2 nm) were observed around the edge of large graphite crystallites (ca. 1 lm in diameter) and few rhodium particles were observed on the basal plane of the graphite crystallites. The Rh/HSAG300 catalyst had narrow rho-
dium particle size distribution with an average particle size of 1.9 nm, and no particles more than 4 nm were observed (Fig. 1c). For the Rh/HSAG100 catalyst (Fig. 1b), rhodium particle (<4 nm) also existed on the edge of the large graphite crystallites (1 lm). However, the Rh/HSAG100 catalyst had larger rhodium particle (10 nm) on the basal plane of the graphite crystallites, by which the particle size distribution of the Rh/HSAG100 catalyst was broadened compared to the Rh/HSAG300 catalyst (Fig. 1d). In contrast to the graphite-supported catalysts, TEM observation showed that the rhodium particles of the activated carbon-supported catalysts were distributed homogeneously on the whole range of the activated carbon support [15]. Dispersion values, which were the number of surface rhodium atoms normalized by the number of total rhodium atoms, were determined by a hydrogen adsorption method (dispersion (%) = hydrogen atoms adsorbed (mol)/rhodium atoms loaded (mol) 100), and the average particle diameters (dH2) were estimated from the dispersion values assuming that particles are spherical in shape (Table 1). In addition, the average particle diameters were also estimated by TEM (dTEM) and XRD (dXRD) (Table 1). The dispersion values of the Rh/HSAG100, Rh/HSAG300 and Rh/AC catalysts determined by hydrogen adsorption measurements were similar (2728%); however, dTEM values were different between the three catalysts. For the Rh/HSAG100 catalyst, the dH2 value was quite consistent with the dTEM and dXRD values. In contrast, the dH2 value of the Rh/AC catalyst was estimated to be four times larger than the dTEM value, indicating that that the dispersion value of the Rh/AC catalyst was smaller than that expected by the TEM observation. The dispersion values, which were determined by hydrogen adsorption, were also compared for the catalysts having the comparable dTEM values (Rh/HSAG100 vs. Rh/AC-1.0u, and Rh/ HSAG300 vs. Rh/AC-0.3u) (Table 1). The graphite-supported catalysts had the larger dispersion values than the activated carbonsupported catalysts having the similar dTEM values. In particular, the dispersion value of the Rh/HSAG100 catalyst was three times larger than that of the Rh/AC-1.0u catalyst. These results suggest that the hydrogen adsorption is substantially inhibited for the activated carbon-supported rhodium catalysts. These differences in hydrogen adsorption ability between the graphite- and activated carbon-supported catalysts would result from the location of rhodium particles on the supports. The rhodium particles on the edge of the graphite crystallites are well exposed on the surface facilitating an easy adsorption of hydrogen molecules. In contrast, the rhodium particles of the activated carbon-supported catalysts would be inside the pores of activated carbon causing less number of surface rhodium atoms available for hydrogen adsorption. The activities of the catalysts for naphthalene hydrogenation were examined by measuring the initial consumption rate of naphthalene in both scCO2 and n-heptane at 313 K. We confirmed that the initial naphthalene consumption rate for all catalysts did not changed by grinding catalysts, indicating that the granule size of the catalysts were not influence the activities. The amount of naphthalene decreased linearly from the beginning of the reaction in both the solvents, and mainly tetralin (90–95% of selectivity which depended on catalysts and solvents), small amounts of cis-, transdecalin (3–8% and 0.2–1.0%, respectively), and octahydronaphthalene (0.8–1.7%) were formed (not shown). Fig. 2a shows the rate of hydrogenation on a catalyst-weight basis (rw). The graphite-supported catalysts showed high rw values in scCO2 than the activated carbon-supported ones. It is noteworthy that the graphite-supported catalysts were highly active particularly in scCO2, whereas these catalysts showed comparable rw values to some of the activated carbon-supported catalysts in n-heptane. Turnover frequency (TOF) values were also calculated on the assumption that number of adsorbed hydrogen atoms is the number of active sites (Fig. 2b). The graphite-supported catalysts showed higher TOF
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c
d
0.8
0.6
Fraction
Fraction
0.6
0.8
0.4
0.2
0.4
0.2
0.0
0.0 0
1
2
3
4
5
6
7
8
9
10
0
1
2
3
4
5
6
7
8
9 10
Particle diameter/nm
Particle diameter/nm
Table 1 Dispersion and particle size of supported rhodium catalysts. Catalysta
Db (%) (dH2c (nm))
dTEMd (nm)
dXRDe (nm)
Rh/HSAG100 Rh/HSAG300 Rh/AC Rh/AC-0.3u Rh/AC-1.0u
26.8 27.1 27.9 19.7 10.1
5.4 1.9 <1f 2.3 5.1
3.2 – – 1.5 3.6
(4.1) (4.1) (3.9) (5.6) (10.9)
a
Rhodium loading was 5 wt%. Dispersion determined by a hydrogen adsorption method. c Average particle size calculated using the dispersion value on the assumption that particles were sphere. 3 2 d Average particle sizes determined by TEM. dTEM = R(nidi )/R(nidi ). Where, ni and di denotes number and diameter of particles, respectively. e Average particle sizes estimated by XRD. f Under detection limit.
a
10
rw/mmol g-1 min-1
Fig. 1. TEM images and particle size distributions of Rh/HSAG300 (a), (c) and Rh/HSAG100 (b), (d).
8 scCO2 6
n-Heptane
4 2
b
0
b
70 60
40 30 20 10
u
u
.0 R h/ AC -1
-0 .3 AC R h/
R h/ AC
HS AG 30 0 R h/ HS AG 10 0
0
R h/
values in scCO2 than the activated carbon-supported ones, whereas all the catalysts showed comparable TOF values in n-heptane. We have reported that the TOF values of activated carbon-supported rhodium catalysts increased with increasing metal particle size in scCO2; however, they were almost constant in n-heptane. The high TOF values of the graphite-supported catalysts in scCO2 could be partially explained by the particle size effect of rhodium. The higher TOF values of the Rh/HSAG100 and Rh/HSAG300 catalysts than the Rh/AC catalyst could be explained by the larger dTEM values of these graphite-supported catalysts. Besides the effect of particle size, other reasons for the high TOF values of the graphite-supported catalysts in scCO2 should be also considered because the graphite-supported catalysts exhibited higher TOF values in scCO2 than activated carbon-supported catalysts having the comparable dTEM values (Rh/HSAG100 vs. Rh/AC-1.0u and Rh/HSAG300 vs. Rh/AC-0.3u) (Fig. 2b). One possible explanation is that the location of rhodium particles influences the TOF values. In scCO2, the diffusion rate of naphthalene to rhodium particles on the edge of graphite crystallites would be higher than that to ones in the pore
TOF/min-1
50
Fig. 2. Caption reaction rate on catalyst-weight basis (rw) (a) and turnover frequency (TOF) (b) of supported rhodium catalysts for naphthalene hydrogenation in scCO2 (total pressure: 18 MPa) and in n-heptane (20 cm3). Initial hydrogen pressure: 3 MPa (57 mmol); initial naphthalene: 2.34 mmol; reaction temperature: 313 K; reactor capacity: 50 cm3; catalyst weight: 0.003 g.
of activated carbon support. However, other factors such as electronic state of rhodium particles and/or particle morphology, which affects the coordination number of surface rhodium atoms,
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N. Hiyoshi et al. / Catalysis Communications 10 (2009) 1681–1684
a
0.90
10
scCO2 n-Heptane
Final cis ratio
rw/mmol g-1 min-1
8 scCO2 6
n-Heptane
4
0.85
2 0
TOF/min-1
50
0u -1 .
3u -0 . AC
R h/ AC
R h/
60
R h/
70
R h/ AC
HS AG 30 0 R h/ HS AG 10 0
b
0.80
Fig. 4. Final cis ratio of decalin for tetralin hydrogenation over supported rhodium catalysts in scCO2 (total pressure: 18 MPa) and in n-heptane (20 cm3). Initial hydrogen pressure: 3 MPa (57 mmol); initial tetralin: 2.34 mmol; reaction temperature: 313 K; reactor capacity: 50 cm3.
40 30 20 10
0u -1 .
.3
u R h/ AC
AC R h/
R h/ AC -0
R h/
HS AG 30 0 R h/ HS AG 10 0
0
Fig. 3. Reaction rate on catalyst weight basis (rw) (a) and turnover frequency (TOF) (b) of supported rhodium catalysts for tetralin hydrogenation in scCO2 (total pressure: 18 MPa) and in n-heptane (20 cm3). Initial hydrogen pressure: 3 MPa (57 mmol); initial tetralin: 2.34 mmol; reaction temperature: 313 K; reactor capacity: 50 cm3; catalyst weight: 0.003 g.
could be also responsible for the high TOF values of the graphitesupported catalysts in scCO2. Hydrogenation rate on a catalyst-weight basis (rw) is influenced by the number of active sites as well as the TOF values of active sites. The result of hydrogen adsorption showed that the graphite-supported catalysts had the larger number of rhodium atoms exposed to the surface that could act as active sites than the activated carbon-supported catalysts, even though the particle sizes of rhodium were similar in both the cases. Therefore, the higher rw values of the graphite-supported catalysts in scCO2 can be attributed to both the larger number of active sites on the surface and the higher TOF value. Hydrogenation of naphthalene over the graphite- and activated carbon-supported rhodium catalysts proceeded consecutively, and naphthalene was completely transformed to decalin via intermediate tetralin. Therefore, we examined the catalytic activity for the consecutive hydrogenation step using tetralin as a reactant. The amount of tetralin decreased linearly from the beginning of the reaction, and mainly cis-decalin (73–84% of selectivity), small amounts of trans-decalin (9–19%) and octahydronaphthalene (8– 12%) were formed in both the solvents. Fig. 3 shows the rw and the TOF values of the tetralin hydrogenation. The higher rw values were also obtained for tetralin hydrogenation particularly in scCO2 over the graphite-supported catalysts than those for the activated carbon-supported ones. The larger number of surface active sites and the higher TOF value in scCO2 are responsible for the higher rw values of the graphite-supported catalysts for the tetralin hydrogenation. Another finding in the tetralin hydrogenation was that
the graphite-supported catalysts exhibited high selectivity to cisdecalin (Fig. 4), which is a preferable product than trans-decalin for the hydrogen storage system because of higher dehydrogenation rate of cis-decalin [4]. After all the tetralin was hydrogenated to decalin, final cis ratio reached 89 and 87% for Rh/HSAG100 and Rh/HSAG300 in scCO2, respectively, which were higher than those for the activated carbon-supported catalysts in scCO2 (84–86%). 4. Conclusions The graphite-supported rhodium catalysts were more efficient than the rhodium catalysts supported on activated carbon for the hydrogenation of naphthalene in scCO2 because rhodium particles were on the edge of graphite crystallites leading to higher hydrogen adsorption; however, this was not the case in n-heptane solvent. Acknowledgement This study was supported by Industrial Technology Research Grant Program from New Energy and Industrial Technology Development (NEDO) of Japan. References [1] B.H. Cooper, B.B.L. Donnis, Appl. Catal. A 137 (1996) 203. [2] C. Song, CATTECH 6 (2002) 64. [3] S. Hodoshima, S. Takaiwa, A. Shono, K. Satoh, Y. Saito, Appl. Catal. A 283 (2005) 235. [4] N. Kariya, A. Fukuoka, T. Utagawa, M. Sakuramoto, Y. Goto, M. Ichikawa, Appl. Catal. A 247 (2003) 247. [5] B. Wang, D.W. Goodman, G.F. Froment, J. Catal. 253 (2008) 229. [6] S.R. Kirumakki, B.G. Shpeizer, G.V. Sagar, K.V.R. Chary, A. Clearfield, J. Catal. 242 (2006) 319. [7] P.A. Rautanen, M.S. Lylykangas, J.R. Aittamaa, A.O.I. Krause, Ind. Eng. Chem. Res. 41 (2002) 5966. [8] W. Weitkamp, Adv. Catal. 18 (1968) 1. [9] P.G. Jessop, W. Leitner (Eds.), Chemical Synthesis Using Supercritical Fluids, Wiley, New York, 1999. [10] A. Baiker, Chem. Rev. 99 (1999) 453. [11] M.G. Hizler, M. Poliakoff, Chem. Commun. (1997) 1667. [12] T. Seki, J.-D. Grunwaldt, A. Baiker, Ind. Eng. Chem. Res. 47 (2008) 4561. [13] N. Hiyoshi, T. Inoue, C.V. Rode, O. Sato, M. Shirai, Catal. Lett. 106 (2006) 133. [14] N. Hiyoshi, M. Osada, C.V. Rode, O. Sato, M. Shirai, Appl. Catal. A 331 (2007) 1. [15] N. Hiyoshi, C.V. Rode, O. Sato, Y. Masuda, A. Yamaguchi, M. Shirai, Chem. Lett. 37 (2008) 734.