Ind. Eng. Chem. Res. 2003, 42, 2269-2272
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Effect of Chlorine on the Properties of Ru/Al2O3 Vanina A. Mazzieri,† Pablo C. L’Argentie` re,† Fernando Coloma-Pascual,‡ and Nora S. Fı´goli*,† INCAPE (FIQ, UNL-CONICET), Santiago del Estero 2654, 3000 Santa Fe, Argentina, and Servicios de Instrumentacio´ n Cientı´fica y Apoyo a la Investigacio´ n, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain
Ru/Al2O3 catalysts with different chlorine contents were prepared using RuCl3 as the precursor. Different HCl concentrations in the impregnation solution or different treatments after impregnation were used. The catalysts were characterized by temperature-programmed reduction, X-ray photoelectron spectroscopy, and hydrogen chemisorption. They were tested in the selective hydrogenation of benzene to cyclohexene in a stirred tank reactor at 373 K and 3 MPa hydrogen pressure. It was found that different electronic states of Ru appear according to the chlorine content on Ru/Al2O3. The electronic state of Ru influences the selectivity to cyclohexene. The results seem to support that the most electron-deficient Ru species may adsorb cyclohexene more weakly. When cyclohexene is more weakly adsorbed, it could be more easily desorbed by avoiding its further hydrogenation to cyclohexane, thus increasing selectivity. Introduction The selective hydrogenation of benzene to cyclohexene is an important reaction because cyclohexene is the raw material for the production of important products. Because, unlike cyclohexane, cyclohexene does not form an azeotropic mixture with benzene, an expensive separation step is avoided that is customary in the manufacture of cyclohexene by dehydrogenation of cyclohexane. Cyclohexane, a side product of the reaction, is returned to the inlet stream and dehydrogenated directly to benzene.1 Various catalysts have been designed to improve the hydrogenation selectivity, among them Ru-based catalysts.2 Ruthenium catalysts employed in hydrogenation reactions in both the gas and liquid phases are most often supported on alumina, silica, titania, etc. It has been reported that the selectivity to cyclohexene depends on the precursor used for the preparation of the samples. Catalysts obtained from RuCl3 are more selective than the catalysts prepared by using Ru(acac)3 or [Ru(NO)](NO3)3.3 Conventional impregnation4 and the incipient wetness technique5 are the methods reported most frequently for the preparation of supported ruthenium catalysts. The activity and selectivity of the catalysts depend on the course and the experimental arrangement of the reduction and/or calcination of the catalyst precursor, bearing in mind that chlorides and other impurities may change the activity and selectivity of the final catalyst.6 The objective of this paper is to analyze the effect of the chlorine content on the catalytic selectivity of Ru/ Al2O3 during the benzene selective hydrogenation to cyclohexene. The chlorine content was varied by modi* To whom correspondence should be addressed. Fax: 54 342 4528062. E-mail:
[email protected]. † INCAPE (FIQ, UNL-CONICET). ‡ Universidad de Alicante, Spain.
fication of the HCl concentration of the impregnating solution and by different treatments after impregnation. Experimental Section Catalyst Preparation. The catalysts were prepared by wet impregnation of Al2O3 CK 300 from Ketjen (Sg ) 200 m2 g-1, pellets of 1.5 mm diameter and 4.0 mm length) using a solution of RuCl3 (Strem Chemicals) of adequate concentration such as to obtain catalysts containing 4 wt % ruthenium. To achieve several chlorine concentrations in the samples, different procedures were used. In one of them, the pH of the impregnating solution was changed by the addition of solutions having different HCl concentrations (2.4 and 3.8 M); in another case no HCl was added. The samples were then dried at 373 K for 24 h and reduced in a hydrogen stream at 673 K for 6 h. A part of the sample impregnated with a 3.8 M HCl solution was also calcined before reduction, and another part of the sample was treated with a 10 wt % NH4OH solution after calcination and before reduction. The procedures used for catalyst preparation and the nomenclature adopted for the different catalysts are summarized in Table 1. Catalyst Characterization. The ruthenium content was determined spectrophotometrically using a methanol solution saturated with N,N′-diphenylthiourea (DPTU), based on the Ru capacity to form a complex with DPTU, under certain conditions.7 Ru was previously extracted from the catalysts by heating the sample in the presence of a H2SO4 solution until total dissolution of the solid. Ruthenium dispersion was measured by H2 chemisorption in a volumetric equipment at 373 K. Each point was measured after 5 min of stabilization. For calculations, a H/Ru atomic ratio 1 was used.
10.1021/ie0209428 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/30/2003
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Table 1. Preparation Conditions for the Different Catalysts catalyst
HCl solution concn (M) used for impregnation
treatment after impregnation
A B C D E
3.8 2.4 0 3.8 3.8
reduction reduction reduction calcination + reduction calcination + washing with NH4OH - reduction
Ruthenium reducibility was determined by temperature-programmed reduction (TPR) using an Ohkura TP 2002S instrument equipped with a thermal conductivity detector. Nonreduced samples were heated to 975 K at 10 K min-1 in a gas stream of 5% hydrogen in argon. X-ray photoelectron spectroscopy (XPS) measurements were made using a VG-Microtech Multilab equipment, taken as reference the Al 2p signal at 74.5 eV. Determinations of the superficial atomic ratios were made by comparing the areas under the peaks after background subtraction and corrections due to differences in escape depth and in photoionization cross sections.8 The samples were reduced in situ in the pretreatment chamber of the XPS equipment following the same operational conditions as those used during the preparation of the catalysts. The benzene selective hydrogenation reaction was carried out in a stainless steel stirred tank reactor equipped with a magnetically driven stirrer operated at 125 rpm. The stirrer has a special design such as to obtain a good mixing; under these conditions, no diffusional external limitations were found. To investigate the possibility of internal diffusional limitation, experiments were carried out by decreasing the catalyst particle size up to 1/4 of the original length. No modifications in activity or selectivity were detected, thus neglecting the existence of internal diffusional limitations. The inner wall of the reactor was completely coated with Teflon in order to neglect the catalytic action of the steel of the reactor found by other authors.9 The reaction was carried out at 373 K and 3 MPa constant hydrogen pressure using a volume of liquid of 200 mL and 4 g of catalyst. Reactant and products were analyzed chromatographically, using a flame ionization detector and a capillary column CP-Wax 52 CB. Results and Discussion The Ru content of all catalysts was 4 wt %. The TPR profiles of the catalysts are presented in Figure 1. The profile of A shows a peak with a maximum at about 415 K, being similar to the reduction temperature of unsupported RuCl3.10,11 Another peak, smaller than the previous one, with a maximum at 468 K, is also observed and can be attributed to a ruthenium oxide because it was found that RuCl3 can be oxidized at the surface by air exposition at room temperature.11 Moreover, we have observed color changes (from dark red to black) in the RuCl3-impregnated catalyst upon air exposition, suggesting the transformation of the chloride into an oxide. Bossi et al.12 also observed a similar modification. It could also be considered the formation of the oxide during drying at 373 K for 24 h. The profile of B is similar to the one of A, but the first peak is smaller, probably because of the lower chlorine concentration used during impregnation; this is even more evident in the case of C, which was prepared
Figure 1. TPR profiles of catalysts prepared using different HCl concentrations and different treatments after impregnation. Table 2. Ru Oxidation State, Ru0/Ruδ+, and Cl/Al Atomic Ratios, Selectivity to Cyclohexene (SCHE, %) at 5 min of Operation and Dispersion (D) catalyst A D E B C
Ru 3d5/2 (eV)
Ru0/Ruδ+ (at./at.)
Cl/Al (at./at.)
SCHE at 5 min (%)
D (%)
280.0 281.5 279.6 280.9 279.7 281.1 279.6 281.1 n.m.
1.50
0.10
32.6
19
1.60
0.07
25.6
5
4.10
0.04
15.9
5
4.30
0.03
11.3
15
n.m.
n.m.
7.7
11
without HCl. The profile of D shows a peak with a maximum at about 415 K previously assigned to RuCl3 reduction, thus indicating that chlorine was not completely eliminated after calcination at 773 K. A shoulder at 425 K is also observed which may correspond, according to the literature,12 to ruthenium oxychloride. Another peak appears at about 470 K, which is related to a ruthenium oxide reduction; Koopman et al.11 and Betancourt et al.10 assigned a peak between 450 and 478 K to the reduction of RuO2. The profile of E presents a peak with a maximum at 470 K and a shoulder at about 427 K that can be attributed to the reduction of ruthenium oxide and of ruthenium oxychloride, respectively. The presence of RuCl3 and of the oxychloride indicates that chlorine is not completely eliminated after calcination and washing with a diluted NH4OH solution and that, according to the preparation conditions, different Ru species are present before reduction, which may lead to different species when the reduction step is performed. The presence of different Ru species was also found by XPS. Table 2 presents the binding energy (BE) corresponding to Ru species and the Ru0/Ruδ+ and Cl/ Al atomic ratios calculated from the XPS analysis,
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Figure 2. Ru 3d XPS spectrum of catalyst B. Deconvolutions of the Ru 3d3/2 and Ru 3d5/2 peaks are presented by solid lines.
where Ruδ+ means electron-deficient Ru species. The Ru dispersion (D, %) and selectivity to cyclohexene (SCHE, %) values are also shown. To compare more easily the values displayed in Table 2, the catalysts have been ordered following increasing Ru0/Ruδ+ atomic ratio values. In every case, the Ru 3d XPS peak was carefully deconvoluted. The referencing of the BE scale is difficult in the case of ruthenium because the Ru 3d3/2 peak appears superposed to the C 1s line. For this reason, the Al 2p signal at 74.5 eV was taken as the reference. There are also discrepancies in the BE reported in the literature for ruthenium compounds. Sample A shows a signal at 280.0 eV, attributed to Ru0, but not all Ru was reduced, because a signal at 281.5 eV (Ru in RuCl313) also appears. The position of the Ru 3d peak on sample D shows the presence of Ru0 species at 279.6 eV, although another peak at 280.9 eV was also detected, which may be ascribed to ruthenium oxychloride species.12 The position of the Ru 3d peak for catalyst E suggests mainly the presence of Ru0 (279.7 eV) and ruthenium oxychloride (281.1 eV). For catalyst B, mainly the presence of Ru0 (279.6 eV) and ruthenium oxychloride (281.1 eV) may be suggested. Obviously, decreasing the Cl/Al ratio increases Ru0/Ruδ+. Van der Steen and Scholten14 and Milone et al.3 affirmed that chlorine modifies the electronic state of Ru, favoring the formation of electron-deficient Ru species. Figure 2 shows, as an example, the Ru 3d spectra of catalyst B. Catalyst B presents the Cl 2p3/2 peak at 199.0 eV as shown in Figure 3. The Cl 2p XPS spectra of the other samples are very similar, although the Cl/Al atomic ratios change, as shown in Table 2. For A and B, the Cl/Al atomic ratio is related to the HCl concentration in the impregnating solution. For A, D, and E, prepared using the same HCl concentrations, the Cl/Al atomic ratio is related to the treatments after impregnation. It can be noticed that calcination and reduction produce a catalyst with a lower chlorine content than when only reduction was performed. Washing with a NH4OH solution before reduction reduces the chlorine content even more. It must be taken into account that, under these treatments, chlorine was not completely eliminated. Mieth and Schwarz15 also found that it is very
Figure 3. Cl 2p XPS spectrum of catalyst B.
difficult to eliminate chlorine from catalysts prepared from RuCl3. The only reaction products detected were cyclohexene and cyclohexane. Selectivity to cyclohexene is defined as the weight percentage of benzene transformed into cyclohexene. The selectivity to cyclohexene values after 5 min of operation (time at which the maximum selectivity values were observed for all catalysts), presented in Table 2, indicates a good correlation between the Ru0/Ruδ+ atomic ratio and selectivity to cyclohexene: the selectivity increases when the Ru0/ Ruδ+ ratio decreases. The total conversion values after 5 min of operation for all catalysts were within the 0.27-0.29 range. Catalysts A-C have been prepared in order to obtain different Cl contents following the same pretreatment procedure. To achieve this, several chlorine concentrations in the precursor solutions were used. These three catalysts have the same surface species but increasing Ru0/Ruδ+ ratios. For these catalysts, dispersion decreases with the chlorine content, as was also observed by other authors.11 A similar trend can be observed for selectivity, as was also found by Milone et al.3 Comparing catalysts B and E, having different pretreatments but similar Cl/Al ratios, we see that calcination decreases dispersion.
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Catalysts A and D were prepared using the same impregnating solution (which means the same chlorine concentration) but following different pretreatmrents. Catalyst D was calcined and catalyst A was not. It can be observed that calcination decreases the chlorine content. Catalysts D and E were prepared using the same impregnating solution, and they were both calcined. Catalyst E was also treated with a NH4OH solution to decrease its chlorine content; this last treatment did not affect the dispersion. Conclusions It can be concluded that different electronic states of Ru can be obtained by modifying the chlorine content of Ru/Al2O3 (prepared using RuCl3 as a Ru precursor) using different HCl concentrations in the impregnation solution or different treatments after impregnation. The electronic state of the ruthenium present at the surface influences the selectivity to cyclohexene. The increase in selectivity to cyclohexene when decreasing the Ru0/ Ruδ+ ratio suggests that the most electron-deficient Ru species may adsorb cyclohexene more weakly. When cyclohexene is more weakly adsorbed, it could be more easily desorbed by avoiding its hydrogenation to cyclohexane, thus increasing selectivity. Acknowledgment The experimental assistance of C. Ma´zzaro and M. Gonzalez and the financial assistance of CAI+D (UNL) and ANPCyT are greatly acknowledged. Literature Cited (1) Kluson, P.; Cerveny, L. Selective hydrogenation over ruthenium catalysts. Appl. Catal. A 1995, 128, 13. (2) Nagahara, H.; Ono, M.; Konishi, M.; Fukuoka, Y. Partial hydrogenation of benzene to cyclohexene. Appl. Surf. Sci. 1997, 121/122, 448.
(3) Milone, C.; Neri, G.; Donato, A.; Musolino, J. G.; Mercadente, L. Selective hydrogenation of benzene to cyclohexene on Ru/γ-Al2O3. J. Catal. 1996, 159, 253. (4) Narita, T.; Miura, H.; Ohira, M.; Hondou, H.; Sugiyama, K.; Matsuda, T.; Gonzalez, R. The effect of reduction temperature on the chemisorptive properties of Ru/Al2O3: Effect of chlorine. Appl. Catal. 1987, 32, 185. (5) Schoenmaker-Stolk, M. C.; Verwijs, J. W.; Don, J. A.; Scholten, J. J. F. The catalytic hydrogenation of benzene over supported metal catalysts. I. Gas-phase hydrogenation of benzene over ruthenium on silica. Appl. Catal. 1987, 29, 73. (6) Don, J. A.; Pijpers, A. P.; Sholten, J. J. F. Preparation and surface characterization of nonsupported ruthenium catalysts. J. Catal. 1983, 80, 296. (7) Knight, S. B.; Parks, R. L.; Leidt, S. C. Colorimetric determination of Ruthenium. Anal. Chem. 1957, 29, 571. (8) Briggs, D.; Seah, M. O. Practical Surface Analysis. Auger and X-ray Photoelectron Spectroscopy; Wiley: Chichester, U.K., 1990; Vol. 1. (9) Hu, S.; Chen, Y. Partial hydrogenation of benzene: a review. J. Chin. Chem. Eng. 1998, 29, 387. (10) Betancourt, P.; Rives, A.; Hubaut, R.; Scott, C. E.; Goldwasser, J. A study of the ruthenium-alumina system. Appl. Catal. A 1998, 170, 307. (11) Koopman, P. G. J.; Kieboom, A. P. G.; van Bekkum, H. Characterization of ruthenium catalysts as studied by temperature programmed reduction. J. Catal. 1981, 69, 172. (12) Bossi, A.; Garbassi, F.; Orlandi, A.; Petrini, G.; Zanderighi, L. Preparation aspects of Ru-supported catalysts and their influence on the final Products. Stud. Surf. Sci. Catal. 1979, 3, 405. (13) Briggs, D.; Seah, M. P. In Practical Surface Analysis, 2nd ed.; Wiley: New York, 1993; Vol. 1. (14) Van der Steen, P. J.; Scholten, J. J. F. Selectivity to cyclohexene in the gas-phase hydrogenation of benzene over ruthenium, as influenced by reaction modifiers. II. Catalytic hydrogenation of benzene to cyclohexene and cyclohexane. Appl. Catal. 1990, 58, 291. (15) Mieth, J.; Schwarz, J. The effect of catalyst preparation on the performance of alumina-supported ruthenium catalysts. J. Catal. 1989, 118, 218.
Received for review November 25, 2002 Revised manuscript received March 20, 2003 Accepted March 28, 2003 IE0209428
