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Energy & Fuels 2007, 21, 1122-1126
Cyclohexane Ring Opening on Alumina-Supported Rh and Ir Nanoparticles Geonel Rodrı´guez-Gattorno,‡ Laura O. Alema´n-Va´zquez,*,‡ Xochitl Angeles-Franco,† Jose´ Luis Cano-Domı´nguez,‡ and Roberto Villago´mez-Ibarra† Centro de InVestigaciones Quı´micas, UniVersidad Auto´ noma del Estado de Hidalgo, Ciudad UniVersitaria, Carr. Pachuca-Tulancingo Km. 4.5, C.P. 42076 Pachuca, Hgo., Me´ xico, and Instituto Mexicano del Petro´ leo, Parque Ind. La Reforma, Carr. Pachuca-Cd. Sahagu´ n Km. 7.5, C.P. 42083, Pachuca, Hgo., Me´ xico ReceiVed February 24, 2006. ReVised Manuscript ReceiVed December 11, 2006
The opening of the cyclohexane ring over metallic nanoparticles of rhodium and iridium supported on alumina is reported in this paper. HRTEM micrographs of the catalysts, prepared by simple impregnation and in situ reduction, showed metallic nanoparticles 4.4 nm (S.D ( 1. 7) in size for Rh and 4.6 nm (S.D ( 1. 2) for Ir. The iridium catalyst showed a higher cyclohexane conversion and better stability than the rhodium catalyst. The main products with the iridium catalyst are n -hexane and n-pentane, while with the rhodium catalyst, there is also a significant production of benzene. Methylcyclopentane and light paraffins (C1-C4) are produced in small amounts with both catalysts. In general, the iridium catalyst has a higher selectivity toward n-hexane and a much lower selectivity toward benzene than the rhodium catalyst.
Introduction Gasoline and other motor fuels currently used are among the main causes of environment al pollution in major cities. Gasolines are classified according to their knocking characteristics, which are measured by their octane numbers. Branched paraffins have higher octane numbers than linear paraffins or cycloparaffins, making ring opening and isomerization an attractive route for improving the octane number of gasolines. Selective ring opening has recently been proposed as a potential route for improving distillate fuels.1-3 Yasuo et al.4 studied the cyclohexane and methylcyclopentane ring opening catalyzed by a nickel /alumina catalyst. The main products for both reactants were hexane isomers (n-hexane, 2-methylpentane, and 3-methylpentane). Kustov et al.5 described the ring opening of cyclohexane over Pt, Rh, and Ru to form n-hexane and alkanes with lower molecular weight. Their IR results show that Rh is the most active and selective metal when it is supported on alumina and that a higher acidity of the support tends to favor the ring opening with high selectivity to methylcyclopentane. The performance of a supported metal catalyst in this reaction depends on the particle morphology, the metal dispersion, and the electronic properties of the metal. Teschner et al.16 studied * To whom correspondence should be addressed. Phone: +52-7717170615. Fax: +52-771-7163059. E-mail:
[email protected]. † Centro de Investigaciones Quı´micas. ‡ Instituto Mexicano del Petro ´ leo. (1) McVicker, G. B.; Daage, M.; Touvelle, M. S.; Hudson, C. W.; Klein, D. P.; Baird, W. C.; Cook, B. R.; Chen, J. G.; Hantzer, S.; Vaughan, D. E. W.; Ellis, E. S.; Feeley, O. C. J. Catal. 2002, 210, 137. (2) McVicker, G. B.; Touvelle, M. S.; Hudson, C. W.; Vaughan, D. E. W.; Daage, M.; Hantzer; S.; Klein, D. P.; Ellis, E. S.; Cook, B. R.; Feeley, O. C.; Baumgartner; J. E. U.S. Patent 5 763 731, 1998. (3) McVicker, G. B.; Schorfheide, J. J.; Baird, W. C.; Touvelle, M. S.; Daage, M.; Klein, D. P.; Ellis, E. S.; Vaughan, D. E. W.; Chen, J.; Hantzer, S. S. U.S. Patent 6 103 106, 2000. (4) Yasuo, M.; Shoko, Y.; Masaaki, O. J. Catal. 1977, 49, 278-284. (5) Masloboishchikova, O. V.; Vasina, T. V.; Khelkovskaya-Sergeeva, E. G.; Kustov, L. M.; Zeuthen, P. Russ. Chem. Bull. Int. Ed. 2002, 51, 249-254.
Rh/Al2O3 catalysts prepared by the incipient wetness method in the ring opening of methylcyclopentane; the IR results indicate the presence of two forms of rhodium: one showing strong dependence of the ring-opening products distribution on reaction temperature and hydrogen partial pressure and the other exhibiting no variation in products distribution selectivity. The authors attribute this difference in the product distribution to different morphologies of Rh particles arising from changes in metal loading and reduction temperature. The effect of hydrogen partial pressure and temperature on the ring opening of methylcyclopentane over silica- and aluminasupported rhodium catalysts has been reported.6 Coq et al. showed the relationship between catalytic activity and particle size for different metals. The strong electron-confinement regime (at particle sizes near 1 nm) and strong metal-substrate interaction greatly affect the catalyst activity.7 Cunha and Cruz8 reported decreased activity for very small metal particles (1-2 nm) in the study of the hydrogenation of benzene and toluene over Ir /γ-Al2O3. In the present paper, the synthesis, characterization, and catalytic behavior in the ring opening of cyclohexane of aluminasupported metal nanoparticles (Rh and Ir) is explored. Although the performance of Rh toward ring-opening reactions is well documented,9 a systematic study focused on Ir /Al2O3 catalysts using cyclohexane as a model molecule and with the Rh/Al2O3 catalyst as a reference is still lacking. Experimental Section Alumina (Boehmite supplied by Almatis, 99.9%), RhCl3 (99%, Aldrich), and (NH4)2IrCl6 (43%, Strem Chemicals) were used as starting materials without further purification. Ultrapure water (8 (6) Teschner, D.; Paa´l, Z.; Duprez, D. Catal. Today 2001, 65, 185190. (7) Coq, B.; Dutartre, R.; Figueras, F.; Tazi, T. J. Catal. 1990, 122, 438447. (8) Cunha, D. S.; Cruz, G. M. App. Catal. A. 2002, 236, 55-66. (9) Dalla Betta, R. A.; Cusumano, J. A.; Sinfelt, J. H. J. Catal. 1970, 19, 343-349.
10.1021/ef060084i CCC: $37.00 © 2007 American Chemical Society Published on Web 01/23/2007
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Table 1. Products Distribution in the Ring Opening of Cyclohexane for 5% Rh/Al2O3 and 5% Ir/ Al2O3 Catalysts for Different H2/cyclohexane Ratiosa products distribution (wt %) conversion (wt %)
C1-C4
n-hexane
n-pentane
benzene
MCP
H2/cyclohexane ratio
Rh
Ir
Rh
Ir
Rh
Ir
Rh
Ir
Rh
Ir
Rh
Ir
20 40
21.20 21.49
36.54 60.27
4.14 3.97
3.16 5.10
60.90 34.57
71.92 71.51
20.90 44.11
16.63 21.50
10.24 7.03
0.46 0.21
0.71 2.52
0.19 0.10
a
Temperature ) 300 °C, pressure ) 30 bar, reaction time ) 8 h.
Figure 1. Cyclohexane conversion with rhodium catalysts prepared using different synthesis routes. In situ reduction refers to the activation process under hydrogen flow in the reactor.
Ω) was obtained from a Barnstead deionization system. The X-ray diffraction patterns were recorded on a Philips Analytical powder diffractometer with a graphite monochromator at 20 mA and 40 kV, using a copper anode and a nickel filter for λ ) 1.5406 Å. The HRTEM studies were performed in a JEM 2010 microscope. The surface area measurements of the samples were performed in a Micromeritics ASAP system. The Raman spectra were acquired in a Spectrum GX (NIR-FT-Raman) Perkin-Elmer equipped with a laser (λ ) 9394.69 cm-1). For local Raman studies a Jobin Yvon Horiba (T64000) spectrometer, equipped with a confocal microscope (Olympus BX41) with a 514.5 nm laser at a power level of 10 mW was used. The spectrometer is equipped with a CCD detector which is Peltier-cooled to 243 K to reduce thermal noise. Thermal analyses were carried out in Perkin Elmer equipment, using an ultradry air flow of 20 mL/min and a programmed heating rate of 10 °C/min. Catalyst Preparation. Initially, two different routes of synthesis were tested: the chemical reduction of the metal salt by means of a strong reducing agent, such as sodium borohydride, and the simple impregnation and subsequent in situ reduction under hydrogen flow in the reactor. Figure 1 shows an example (5% Rh/Al2O3) that clearly illustrates the higher activity of the catalyst obtained via impregnation. Therefore, the impregnation route was selected for all subsequent experiments. In the impregnation route, the alumina powder was dispersed in 10 mL of water with vigorous stirring, and the Rh or Ir salt, previously dissolved in 10 mL of water, was added to the alumina dispersion. Water was evaporated under vacuum at 115 °C, and the residual powder was heated at 120 °C overnight, deposited in the reactor, and activated at 450 °C and 30 bar under hydrogen flow for 2 h. Catalyst Evaluation. The catalyst (1 g) was placed in a continuous-flow stainless steel reactor (0.9 cm in diameter), and the temperature was raised to 300 °C under hydrogen flow at a pressure of 30 bar. Afterward, cyclohexane (99.9% purity) was fed to the reactor at a rate of 5 mL/h. The hydrogen flow was controlled with a Brooks 5850E mass-flow controller to maintain the desired H2/cyclohexane molar flow ratio. Relatively high H2/cyclohexane molar ratios (20 and 40) were used to protect the catalytic sites from carbon deposition and deactivation for a reasonable period of time.
Figure 2. Cyclohexane conversion over 5% Rh/Al2O3 and 5% Ir/Al2O3 catalysts. T ) 300 °C, P ) 30 bar, H2/cyclohexane molar ratio ) 40.
Figure 3. X-ray diffraction patterns of fresh and spent 5% Rh/Al2O3 catalyst. The line patterns corresponding to Boehmite (JCPDS 0211307) and γ-Al2O3 (JCPDS 010-0425) are shown as references.
Results and Discussion The specific area of the starting alumina, from BET surface area measurements, was 173 m2/g, which increased to 200 m2/g after activation. This effect is related to the topotactic transformation of Boehmite to γ-Al2O3, which produces a welldeveloped pore structure, resulting in a higher surface area.10 The conversion and product distributions obtained at a reaction time of 8 h over the 5% Rh/Al2O3 and 5% Ir/Al2O3 catalysts are shown in Table 1. The evolution of cyclohexane conversion over these catalysts with reaction time is shown in Figure 2. From these results it is readily apparent that the Ir/Al2O3 catalyst is more active than the Rh/Al2O3 catalyst at these conditions, with a cyclohexane conversion of 45% at 2 h and 62% at 8 h. The conversion of cyclohexane over the Rh/Al2O3 catalyst reaches a maximum of 37% at 4 h and decreases to 20% at 8 h. It is important to note that the main products with the Ir catalyst are n-hexane and n-pentane, while with the Rh catalyst there is also a significant production of benzene. (10) Paglia, G.; Buckley, C. E.; Udovic, T. J.; Rohl, A. L.; Jones, F.; Maitland, C. F.; Connolly, J. Chem. Mater. 2004, 16, 1914-1923.
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Figure 4. Raman and FT-IR spectra of samples of (A) the starting Al2O3 (Boehmite) and (B) 5% Rh/γ Al2O3 (after 8 h of reaction).
X-ray diffraction patterns of fresh and spent 5% Rh/Al2O3 catalysts are shown in Figure 3. The fresh catalyst (before activation) shows the presence of the starting aluminum oxide, with a Boehmite-type structure. During activation, the Boehmite was transformed into γ-Al2O3. It is worth noting that when the catalyst was prepared starting from γ-Al2O3 (previously prepared by heating the Boehmite at 450 °C for 2 h), the cyclohexane conversion was three times lower. Since the final alumina phase is the same in both cases, the most probable reason for this behavior is the metal-support interaction. Strong metal-support interactions change the chemical environment at the metalsupport interface and may affect the active sites of the catalyst.11 McVicker et al.12 and Soares et al.13 have reached s imilar conclusions about the strong interaction between metals and γ-Al2O3 supports. Figure 4 shows the IR (right) and Raman (left) spectra of the starting substrate (A) and of the 5% Rh/Al2O3 catalyst after 8 h of reaction. The broad Raman bands from 3000 to 3500 cm-1 (from 4000 to 2500 cm-1 in IR) correspond to physisorbed water and surface hydroxyl14 (physisorbed water was also detected by thermogravimetric analyses of both samples, see Figure S1 in Supporting Information). After 8 h of reaction, two small peaks appear at at 1600 and 1380 cm-1. These bands are the result of carbonaceous residues deposited during the reaction. The band at 1600 cm-1 is attributed to the stretching mode of the individual sheets in graphite (E2g mode, the socalled G band of graphite), and the band at 1380 cm-1 is assigned to an A1g-type mode, the disorder or D band. Local Raman spectra focused on selected areas of the sample and in the range from 1100 to 1800 cm-1 (Figure S2 in Supporting Information) show wider bands than those obtained for the bulk (Figure 4), indicating the heterogeneous nature of the carbonaceous deposits. The bands at 1600 and 1380 cm-1 were not observed in the iridium catalyst samples (see Figure S2 in (11) Del Angel, G.; Coq, B.; Dutartre, R.; Figueras, F. J. Catal. 1984, 87, 27-35. (12) McVicker, G. B.; Ziemiak, J. J. J. Catal. 1985, 95, 473-481. (13) Soares Netoa, T. G.; Cobob, A. J. G.; Cruz, G. M. Appl. Surf. Sci. 2005, 240, 355-365. (14) Uy, D.; Dubkov, A.; Graham, G. W.; Weber, W. H. Catal. Lett. 2000, 68, 25-32.
Supporting Information). The carbon content determined from the thermogravimetric analyses was 1.18% for the Ir catalyst and 1.9% for the Rh catalyst. Modifications of bands at frequencies under 1000 cm-1 in FT-IR are usually associated with the transition from Boehmite to γ-Al2O3, where aluminum changes its local coordination from octahedral to tetrahedral.15 The bands at 1072 and 1170 cm-1 correspond to OH bending modes in the Boehmite phase, which disappear after the phase transformation to γ-Al2O3. The HRTEM images were obtained after activation of the catalysts. Figure 5 shows selected micrographs of the 5% Rh/ Al2O3 and 5% Ir/Al2O3. The particle size distribution was obtained by counting a sample of ∼100 particles. The values of d111 obtained from the fast Fourier transforms (FFT) for selected nanoparticles in the micrographs at high resolutions (Figure 6) are very similar for both metals (d111(Rh) ) 2.19 Å and d111(Ir) ) 2.21 Å) and correspond to the reflections expected for the {111} family planes of the FCC crystal structures. Most metal particles have a cubo-octahedral nanocrystal shape, which is frequently found in metal nanoparticles with sizes greater than 2 nm.16,17,18 The carbonaceous deposits were not detected in the HRTEM micrographs of both catalysts. These deposits are usually detected as stacked hexagonal layers with a graphite-like arrangement.19 Carbon tends to deposit over the metal surface, and the metal in this case is well dispersed and embedded in the alumina matrix, which could explain the absence of carbonaceous deposits in the micrographs. According to the Raman and thermogravimetric analyses results, the better stability of the Ir catalyst in comparison with the Rh catalyst could be related to a lower formation of carbonaceous matter, perhaps, because of the higher hydrogen (15) Tarte, P. Spectrochim. Acta 1967, 23A, 2127-2143. (16) Teschner, D.; Matusek, K.; Paa´l, Z. J. Catal. 2000, 192, 335-343. (17) Gonza´lez, A. L.; Noguez, C.; Ortiz, G. P.; Rodrı´guez-Gattorno, G. J. Phys. Chem. B. 2005, 109, 17512-17517. (18) Gordon, M. B.; Cyrot-Lackmann, F.; Desjonqueres, M. C. Surf. Sci. 1979, 80, 359-367. (19) Torres-Garcı´a, E.; Rodrı´guez-Gattorno, G.; Ascencio-Gutie´rrez, J. A.; Alema´n-Va´zquez, L. O.; Cano-Domı´nguez, J. L.; Martı´nez-Herna´ndez, A.; Santiago-Jacinto, P. J. Phys. Chem. B 2005, 109, 17518-17525.
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Energy & Fuels, Vol. 21, No. 2, 2007 1125
Figure 5. HRTEM micrographs and plots of particle size distribution for 5% Ir/Al2O3 (left) and 5% Rh/Al2O3 (right).
Figure 6. HRTEM micrographs and FFT images of selected nanoparticles (inserts) for 5% Rh/Al2O3 (left) and 5% Ir/Al2O3 (right).
chemisorption bond energy in iridium,20,21 which leads to a higher local hydrogen concentration. The progressive deactivation of the Rh catalyst could also be related to the aromatic nature of the carbonaceous deposits, possibly resulting from the adsorption of benzene on the acid surface sites of this catalyst. These results are consistent with the observation that the selectivity in the ring-opening reaction is highly influenced by the nature of the metal and the acidic function of the support and the balance between acid and metal sites.22 Conclusions The 5% Rh/Al2O3 and 5% Ir/Al2O3 catalysts prepared in this work were very similar in metal dispersion, particle size, and
morphology. The iridium catalyst showed a higher cyclohexane conversion than the rhodium catalyst at all reaction times. Cyclohexane conversion for the iridium catalyst tended to increase with time, while for the rhodium catalyst it decreased considerably after 4 h. The main products with the iridium catalyst are n-hexane and n-pentane, while with the rhodium catalyst there is also a significant production of benzene. Methylcyclopentane and light paraffins (C1-C4) are produced in small amounts with both (20) Tomanek, D.; Mukhjerjee, S.; Kumar V.; Bennemann, K. H. Surf. Sci. 1982, 114, 11-22. (21) McVicker, G. B.; Baker, R. T. K.; Garten, R. L.; Kugler, E. L. J. Catal. 1980, 65, 207-220. (22) Lecarpentier, S.; van Gestel, J.; Thomas, K.; Houalla, M. J. Catal. 2007, 245, 45-54.
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catalysts. In general, the iridium catalyst has a higher selectivity toward n-hexane and a much lower selectivity toward benzene than the rhodium catalyst. The formation of benzene is somewhat reduced at higher H2/cyclohexane ratios for both catalysts. The progressive deactivation of the rhodium catalyst observed in these experiments could be related to the higher formation of carbonaceous matter for this catalyst and to the aromatic
Rodrı´guez-Gattorno et al.
nature of these deposits, according to the Raman and thermogravimetric analyses. Supporting Information Available: Figures showing thermogravimetric curves and Raman spectra of 5%Ir/Al2O3 and 5% Rh/ Al2O3. This material is available free of charge via the Internet at http://pubs.acs.org. EF060084I
