- Email: [email protected]
Al2O3 catalysts in the hydrogenation of o-xylene
Applied Surface Science 211 (2003) 341–351
Activity of Pd/Al2O3 and Ru/Al2O3 catalysts in the hydrogenation of o-xylene Effect of thiophene Adolfo Arcoya*, Xose´ L. Seoane, Luisa Ma Go´mez-Sainero Instituto de Cata´lisis y Petroleoquı´mica (CSIC), Cantoblanco, 28049 Madrid, Spain Received 10 January 2003; accepted 26 February 2003
Abstract Two catalysts containing 2 wt.% of ruthenium or palladium on alumina (CRu and CPd, respectively) were prepared by impregnation from chloride precursors and then calcined and reduced at 573 K. The catalysts were characterized by gas chemisorption, TPR and FT-IR using CO as probe molecule. The catalytic activity was evaluated in the o-xylene hydrogenation at 5 105 Pa, 77.6 h1 and 363–393 K, in a fixed-bed tubular reactor and the sulfur resistance analyzed at 373 K with different feeds contaminated with 20, 50 and 100 ppm of thiophene, respectively. CRu is more active and selective toward cis-1,2dimethylcyclohexane (c-DMCH) than CPd, but it is less sulfur resistant. The selectivity to 1,2-dimethylcyclohexane isomers (St and Sc) for each catalyst is scarcely influenced by the reaction temperature, but it is affected by both the presence of thiophene in the liquid feed and the degree of deactivation of the catalysts by sulfur. CO/FT-IR measurements of the fresh catalysts showed that, in addition to Pd0, an important part of palladium in CPd is present as Pddþ while Ru0 is by far the major species in CRu. The IR spectra of the deactivated samples suggest the presence of thiophene adsorbed on the surface, which is removed outgassing at 573 K and 103 Pa. After this treatment the deactivated samples do not completely recover either the metal surface or the capability to chemisorb CO. The catalytic performances are related to the nature of the metal, the ratio of the adsorption coefficients of thiophene and o-xylene (Kth/xil) and the electronic state of the active metal sites, which are modified by effect of thiophene. The decrease of the St/Sc ratio along the run suggests a parallel decrease of the surface electron deficiency produced by the chemisorbed thiophene. # 2003 Elsevier Science B.V. All rights reserved. Keywords: o-Xylene hydrogenation; Pd catalysts; Ru catalysts; Sulfur resistance; Thiophene poisoning; FT-IR
1. Introduction Catalytic hydrogenation is commercially used to reduce the aromatics content from petroleum derivatives such as medium distillates, kerosene and paraffinic *
Corresponding author. Tel.: þ34-91-585-48-04; fax: þ34-91-585-47-60. E-mail address: [email protected] (A. Arcoya).
solvents. Due to the stringent environmental regulations, supported group VIII metals based catalysts are required to reach high rates of reaction at mild operating conditions [1]. These catalysts are more active than the conventional hydrotreating one (sulfides of Co–Mo, Ni–Mo, or Ni–W), although they are highly sensitive to the sulfur compounds present in the industrial feedstocks [2]. For these reasons, the development of more active and sulfur resistant metal catalysts is, at present, a
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00354-4
342
A. Arcoya et al. / Applied Surface Science 211 (2003) 341–351
meaningful tool in the field of hydroprocessing. A very comprehensive review about the basic and industrial aspects of aromatic hydrogenation reactions was published a few years ago [3]. Most the works on aromatics hydrogenation are devoted to benzene [4,5], which is an useful probe molecule to evaluate hydrogenation catalysts. Few investigations, however, are focused to the hydrogenation of other aromatics, including xylene. So, Rahaman and Vannice [6] studied the hydrogenation of xylene isomers over Pd supported catalysts and observed that the more acid is the support the more active are the catalysts. Gomez et al. [7,8], working with group VIII metals based catalysts found a relationship between the selectivity towards cis-dimethylcyclohexane and the electronic heat capacity of the metal in the oxylene hydrogenation. More recently, Reyes et al. [9] reported the effect of the metal particle size for the same reaction. The effect of sulfur in this reaction has not been, however, analyzed. The main goal of this work is to compare the behavior of alumina supported Ru and Pd catalysts in the o-xylene hydrogenation and examine the effect of thiophene on the catalytic performance. Palladium is a well known hydrogenation catalyst while, up to now, ruthenium has been more scantly used in spite of the fact that it is cheaper than palladium. The catalysts were characterized by gas chemisorption, TPR and FT-IR. The catalytic performance in absence and in presence of thiophene is analyzed and interpreted in terms of the electronic state of the metal and the ratio of the adsorption coefficients of thiophene and o-xylene.
2. Experimental 2.1. Catalyst preparation Two catalysts (CPd and CRu) containing Pd or Ru on a g-alumina (Condea, high purity Puralox SCF grade, SBET ¼ 210 m2 g1 and particle size 0.5–0.8 mm) previously calcined at 823 K for 3 h, were prepared by incipient wetness impregnation with aqueous solutions of H2PdCl4 or RuCl3, respectively, in the adequate concentration to get 2 wt.% of metal on the support. H2PdCl4 was obtained treating PdCl2 with 0.1 M HCl solution. After drying at 393 K overnight, the precursors were heated under air stream at 1 K min1 from room
temperature up to 573 K for 4 h and then reduced under flow of hydrogen at 573 K for 3 h. 2.2. Characterization of the catalysts Temperature-programmed reduction (TPR) of the oxidized precursors was carried out in a flow system with a thermal conductivity detector. Samples of 0.5 g were outgassed at 573 K for 1 h in an Ar stream and then cooled down to 273 K. The TPR profiles were registered heating the samples from 273 to 773 K at 6 K min1 under a flow of 30 ml min1 of a 5% H2/Ar mixture. Metal dispersion of the catalysts (D) was calculated from the ratio D ¼ MS =MT , where MS is the number of metal atoms exposed per gram of catalyst and MT the total number of metal atoms per gram of catalyst obtained from ICP analysis. MS was determined by H2–O2 titration in a flow system with a thermal conductivity detector. The reduced samples were outgassed at 473 K under flow of argon for 2 h and then cooled to room temperature. Pulses of O2 (30 ml) were introduced into the carrier gas (30 ml min1) until total saturation of the sample surface. Titration of adsorbed O2 with H2 was performed at 353 K. The atomic stoichiometries used were Huptake =Pds ¼ 3 and Huptake =Rus ¼ 5 [8]. The electronic state of the metal was analyzed by FT-IR using CO as probe molecule. The spectra were recorded on a Nicolet 5ZDX FT-IR spectrometer with a resolution of 4 cm1. The self-supported wafer of the sample (25 mg cm2) was outgassed at RT for 1 h at 2 103 Pa and then treated at 573 K under a hydrogen pressure of 105 Pa, for 3 h. The sample was evacuated at 5 103 Pa and 573 K for 1 h and then cooled to room temperature. After recording the background spectrum, the sample was contacted with 2 103 Pa of CO for 10 min and then a new spectrum recorded. The spectrum of the adsorbed CO was obtained by subtraction of the preceding ones and the number and position of the component bands of the broad bands were estimated by Fourier self-deconvolution (FSD) after base line correction. FT-IR was additionally used to detect adsorbed species on the deactivated samples. 2.3. Catalytic activity measurements Hydrogenation of o-xylene was performed in a conventional catalytic system, with a fixed-bed flow
A. Arcoya et al. / Applied Surface Science 211 (2003) 341–351
reactor containing 1.5 g of catalyst. Liquid o-xylene (RPE grade) was diluted (5/95, w/w) with n-heptane (RPE grade) and fed to the reactor. A bed of SiC above the catalyst heated at 473 K was employed as a preheater of the liquid feed. The reaction conditions were 5 105 Pa, 333–393 K, hydrogen/o-xylene ¼ 30 mol/mol and liquid space velocity 77.6 h1. Under these conditions o-xylene is in the liquid phase during the reaction. High purity hydrogen (Air Liquid, purity: 99.995%) was successively passed through a Deoxo purifier and a 5A molecular sieve filter. The reactor was heated under the adequate hydrogen flow and then, when the reaction temperature was reached, the liquid was pumped in. The effluent of the reactor was condensed at 273 K and in order to estimate the stability of the catalyst several samples were periodically collected during the runs. To analyze the effect of sulfur on the catalysts at 373 K the liquid feed was doped with different concentrations of thiophene (20, 50 and 100 ppm). In these experiments the first product sample was collected after 10 min of the beginning of reaction and successive samples every 3 min until the activity was 20% of the initial one. The reaction product was analyzed by gas chromatography (GC) in a 4 m 3:18 mm OD column with 1,2,3-tris(2-cyanoethoxy) propane (10 wt.%) on Chromosorb P (60–80 mesh) at 353 K. To check the possible formation of methane during the reaction, the exit gas was also analyzed by GC at 313 K in a column of powdered activated charcoal. Helium (40 ml min1) was used as carrier gas. Prior to the kinetic measurements and following a conventional methodology [10] we had confirmed that in the reaction conditions used both external and internal diffusion limitations were absent.
3. Results and discussion 3.1. Characterization of the catalysts Metal loading, dispersion (D) and metal surface area (SM) of the catalysts are given in Table 1 and the TPR profiles of the calcined precursors are depicted in Fig. 1. The profile of CPd exhibits a single reduction peak at 318 K and the hydrogen uptake, calculated from the area under the curve, corresponds to the stoichiometric transformation of Pd2þ into Pd0
343
Table 1 Catalyst characterizationa Catalyst
Metal loading (wt.%)
D (%)
SM (m2/gcat)
CPd CRu
1.92 1.82
37.0 21.2
3.33 1.41
a D, dispersion; SM, metal surface area assuming SPd ¼ 8:3 1020 m2/Pd atom and SRu ¼ 6:13 1020 m2/Ru atom [23].
(H2 =Pd2þ ¼ 1). The TPR profile of CRu shows a double peak with maxima centered at 488 and 503 K, attributed to the reduction of different oxidized Ru species [11]. The CO–FT-IR spectrum of CPd (Fig. 2) shows three bands at 1936, 2100 and 2157 cm1. The broad band in the region 1800–2000 cm1 is formed by several overlapped single bands corresponding to different forms of bridged carbonyls on Pd. The band at ca. 2100 cm1 is assigned to linear carbonyl species adsorbed on different electronic palladium states [12,13]. The FSD of this band indicates that in addition to Pd0 (2099 cm1) electron deficient palladium species, Pddþ (2115 and 2135 cm1) [14] are also present. In a previous paper, we have reported that in catalysts prepared from PdCl2 acid solutions, even after carefully reduction below 723 K, Pddþ species would be formed by interaction of Pd0 with neighboring Hþ originated during the reduction step, through bridged structures stabilized by the Cl ions [15]: þ
Pd0 þ H þ Cl ! Pddþ Hð1dÞ Cl dþ
2þ
(1)
Accordingly, Pd is not unreduced Pd but Pd0 sharing the positive charge of the proton. Actually, CPd is completely reduced at 318 K, as the TPR results show. The weak band at 2160 cm1 corresponds to unreduced Pd2þ. These assignments was experimentally confirmed by the CO spectrum of an unreduced CPd sample (Fig. 2). The spectrum of CO adsorbed on CRu (Fig. 3) exhibits a broad band at low wavenumber (LFB) centered at 2025 cm1 which, in reduced catalysts, is unambiguously attributed to CO linearly bonded to metallic Ru [16]. The broad and weak band at ca. 2150 cm1 (HF1 band) and the small shoulder at 2080 cm1 (HF2 band) are typical of multicarbonyl species associated with partially oxidized ruthenium, Rudþ(CO)x. The source of such a species in reduced
344
A. Arcoya et al. / Applied Surface Science 211 (2003) 341–351
Fig. 1. TPR profiles of catalyst precursors.
Fig. 2. CO/FT-IR spectra of CPd and unreduced CPd.
A. Arcoya et al. / Applied Surface Science 211 (2003) 341–351
345
Fig. 3. CO/FT-IR spectra of CRu and unreduced CRu.
Ru-supported catalysts has been extensively discussed in the literature and the results are very controversial. So, for Chen et al. [17] they are due to CO adsorbed on Ru sites strongly interacted with electron acceptor sites on the surface. For other authors Rudþ(CO)x species are formed by either an oxidative disruption of Ru clusters by effect of CO [18] or a corrosive chemisorption of CO on Ru crystallites [16] in the IR cell. In our case, the HF bands are very small and, for this reason, whatever their origin might be, the spectrum showed in Fig. 3 clearly indicates that CRu is highly reduced, with Ru0 as the major species. 3.2. Hydrogenation activity Conversion of o-xylene and selectivity towards trans-1,2-dimethylcyclohexane (t-DMCH) and cis1,2-dimethylcyclohexane (c-DMCH) as a function of the reaction temperature in the range of 333– 393 K are plotted in Fig. 4. Conversion is defined as the number of molecules of o-xylene transformed per 100 molecules of o-xylene fed to the reactor and selectivity toward t-DMCH (St) and c-DMCH (Sc) as the number of o-xylene molecules transformed into
the respective isomer per 100 molecules of o-xylene converted. Hydrogenation activity of CRu, is higher than that of CPd, t-DMCH and c-DMCH being the only reaction products for both catalysts. The higher activity of CRu is in accordance with the results frequently reported in the literature for hydrogenation of light aromatic hydrocarbons over noble metal catalysts [2,19]. Since hydrogenation of aromatics is a structure-insensitive reaction [7], the difference of hydrogenation activity observed should be explained from the electron structure of the metals, which is characterized by the local density of unoccupied states at the Fermi level N(EF). This property measures the number of quantum states available for bonding reactants, i.e., the donor-acceptor ability of the metal [20]. However, the fact that Ru is more active than Pd, in spite of the fact that its N(EF) value is lower, suggests that o-xylene adsorbed on Ru is more distorted than on Pd, as it happens for toluene adsorbed on the same metal [19]. Thus, the subsequent hydrogen attack at the transition state in CRu is more favored than in CPd. Under the experimental conditions used St and Sc remain practically constant for each catalyst, with St
346
A. Arcoya et al. / Applied Surface Science 211 (2003) 341–351 Table 2 Deactivation parameters of the catalysts for different thiophene concentration in the feeda Catalyst
TR0
yn
SM(d)
CPd CRu
0.19 0.40
2.00 0.36
0.16 0.03
a TR0, initial sulfur resistance (thiophene molecule/metal atom); yn, thiophene molecules fed per surface metal atom necessary to deactivate the catalyst up to a=a0 ¼ 0:2; SM(d), metal surface (m2/gcat) of the deactivated catalysts (a=a0 ¼ 0:2).
Fig. 4. Conversion of o-xylene (solid line) and selectivity toward c-DMCH (dotted line) and t-DMCH (dashed line) as a function of temperature for CRu (*) and CPd (~).
being much higher for CPd (58%) than for CRu (1%). If t-DMCH is formed following the mechanism based on the ‘‘roll over’’ model [21] as suggested by Go´ mez and coworkers [22], results in Fig. 4 are the consequence that o-xylene is adsorbed on palladium more strongly than on ruthenium, as it was mentioned above. 3.3. Deactivation of the catalysts by thiophene The effect of the thiophene concentration in the feed (20, 50 and 100 ppm) on the catalysts performance was examined in the reaction at 373 K. Under these conditions the hydrogenation activity decreases as a function of the time on stream. Previously, we had verified that in absence of sulfur the reaction conversion does not change at least for 10 h on stream. Thus, deactivation by coke may be ruled out. Fig. 5 shows the normalized activity (a/a0) for CPd and CRu as a function of the number of thiophene
molecules fed to the reactor per surface metal atom (y). From these curves the initial sulfur resistance (TR0), i.e., the number of thiophene molecules required to poison one metal atom at t ¼ 0 was calculated (Table 2). For each catalyst, all the experimental values of a/a0 fit a single curve, whatever it is the thiophene concentration. However, while for CRu the normalized activity falls abruptly right from the beginning of reaction until a=a0 0:3, the initial slope of the CPd curve ( 5) clearly decreases for y values higher than 0.1. Thus, although the initial sulfur resistance of CRu (TR0 ¼ 0:40) is higher than that of CPd (TR0 ¼ 0:19), the number of thiophene molecules per metal atom required to deactivate the CRu bed up to a=a0 ¼ 0:2 (yn ¼ 0:36) is lower than that necessary to reach similar deactivation level for CPd (yn ¼ 2:00). Characterization measurements of the deactivated samples showed a dramatic loss of metal surface evidenced by both H2–O2 titration (SCPdðdÞ ¼ 0:16 m2/gcat and SCRuðdÞ ¼ 0:03 m2/gcat) and CO/FT-IR measurements, in spite of the fact that these were performed after outgassing at 373 K and 103 Pa (Figs. 6a and 7a). On the other hand, it is interesting to point out that the FT-IR spectra of the deactivated samples outgassed at RT and 103 Pa exhibit, in comparison to those of the fresh catalysts, two bands at 2931 and 2847 cm1 (Fig. 8). These bands are not observed in the spectra of the fresh catalysts impregnated with either thiophene or o-xylene and then evacuated, indicating that they correspond to species formed under the reaction conditions. By analogy to the spectrum reported by Blyholder and Bowen [24] for chemisorbed thiophene, these bands could be attributed to a carbon–hydrogen stretch of a saturated carbon atom, suggesting that the double bonds in thiophene become saturated by interaction with the surface. If it is so, deactivation of the
A. Arcoya et al. / Applied Surface Science 211 (2003) 341–351
347
Fig. 5. Deactivation of CPd and CRu in the hydrogenation of o-xylene at 373 K by effect of thiophene: 20 ppm (*, *), 50 ppm (~, ~) and 100 ppm (!, 5).
Fig. 6. CO/FT-IR spectra of the deactivated CPd samples outgassed at 103 Pa: (a) 373 K and (b) 573 K.
348
A. Arcoya et al. / Applied Surface Science 211 (2003) 341–351
Fig. 7. CO/FT-IR spectra of the deactivated CRu samples outgassed at 103 Pa: (a) 373 K and (b) 573 K.
Fig. 8. FT-IR spectra of the deactivated catalysts samples outgassed at 373 K and 103 Pa.
A. Arcoya et al. / Applied Surface Science 211 (2003) 341–351
catalysts is produced by the thiophene chemisorbed on the hydrogenation active sites. Adsorption of thiophene is stronger than that of o-xylene and it occurs competitively on the same sites. After evacuation at 573 K and 103 Pa the bands at 2931 and 2847 cm1 are no longer perceptible, while the CO/FT-IR spectra of these samples are partially restored. Thus, for CPd the CO spectrum (Fig. 6b) exhibits the linearly adsorbed CO band at 2098 cm1 completely recovered but this does not occur with that corresponding to bridged CO (1945 cm1). In a similar way, Fig. 7b shows that the intensity of the CO band on CRu (1997 cm1) is recovered in part. These results, in addition to the fact that the metal surface area of these treated samples is in part restored (SCPdðdÞ ¼ 1:06 m2/gcat and SCRuðdÞ ¼ 0:48 m2/gcat), suggest that some poisoning species strongly bonded to the metals are yet present. According to Boitiaux et al. [25] such a species could be metal sulfide originated be the dissociative adsorption of part of thiophene. The two very well defined zones in the CPd curve (Fig. 5) are consistent with the presence of two palladium species in the catalyst: Pd0, highly sensitive to the sulfur and Pddþ, more sulfur resistant [26]. In fact, the ratio between the IR bands intensity at 2138 and at 2106 cm1 for the deactivated catalyst (Fig. 6a) is higher than that for the fresh catalyst. The value TR0 ¼ 0:19, which means that one molecule of thiophene deactivates about five exposed Pd atoms, indicates that thiophene is bonded to the metal in a coplanar mode. For CRu, however, the value TR0 ¼ 0:40 suggests that thiophene is perpendicularly bonded to the surface by means of sulfur atom anchorage [27,28]. In this last case, moreover, the fact that the overall sulfur resistance up to an a/a0 value as low as 0.2 is similar to the initial sulfur resistance agrees
349
with the FT-IR result that Ru0 is the major species in CRu. 3.4. Effect of thiophene on the reaction selectivity Table 3 summarizes the St and Sc values obtained at the beginning of reaction in presence of 20, 50 and 100 ppm of thiophene. Comparing these results with those obtained for a poison free feed it is observed that the St/Sc ratio increases as the concentration of thiophene increases. Since at the beginning of the reaction the metal surface is practically clean, the changes of selectivity observed have to be attributed to a modification of the basicity of the liquid phase by effect of thiophene. Due to its aromatic-like character, thiophene is adsorbed on the catalyst in competition with o-xylene and so the ratio of the respective adsorption coefficients (Kth/xil) depends on the electronic properties of the molecules. Since the ionization energy of o-xylene (8.6 eV) is lower than that of thiophene (10 eV), o-xylene will be preferentially bonded to the more electrophilic metal species, i.e., Pddþ or Rudþ and thiophene on the less electron acceptor sites, i.e., Pd0 or Ru0, which will be deactivated. On the other hand, since the interaction xylene–Medþ is stronger than that of xylene–Me0 and the formation of t-DMCH requires a long stay of the intermediate precursor on the catalyst surface [22], it is reasonable to expect that the St/Sc ratio will increase in presence of thiophene, the more the higher is the thiophene concentration. This effect is scantly perceptible for CRu probably due to the fact that the fraction of electron deficient species in this catalyst is very small in comparison with that in CPd. Similar effect of change of basicity of the reacting medium on St/Sc has been observed in the competitive hydrogenation of
Table 3 Effect of the thiophene concentration (0, 20, 50 and 100 ppm) on the selectivity toward t- and c-DMCH for the fresh catalystsa Catalyst CPd
St (%) Sc (%) St/Sc a
CRu
0 ppm
20 ppm
50 ppm
100 ppm
0 ppm
20 ppm
50 ppm
100 ppm
55 45 1.32
60 40 1.50
73 27 2.70
75 25 3.00
1 99 0.01
2 98 0.02
4 96 0.04
4.6 95.4 0.05
St, selectivity toward t-DMCH; Sc, selectivity toward c-DMCH.
350
A. Arcoya et al. / Applied Surface Science 211 (2003) 341–351
Table 4 Effect of the deactivation degree (1 a=a0 ) of the catalysts on the St/Sc ratioa Catalyst CPd
St/Scb St/Scc St/Scd St/Sce
CRu
0
0.25
0.50
0.75
0
0.25
0.50
0.75
1.32 1.50 2.70 3.00
– 1.30 2.50 2.80
– 1.17 2.20 2.30
– – 2.00 2.00
0.01 0.02 0.04 0.05
– – – –
– – 0.3 –
– 0.1 0.2 0.2
a
St, selectivity toward t-DMCH; Sc, selectivity toward c-DMCH. Feed without thiophene. c Feed with 20 ppm of thiophene. d Feed with 50 ppm of thiophene. e Feed with 100 ppm of thiophene. b
o-xylene/benzene over CPd under similar experimental conditions. In this case, the value St =Sc ¼ 2, is consistent with the fact that benzene (ionization energy ¼ 9:8 eV) is less basic than o-xylene. The St/Sc values obtained for different deactivation degrees (0, 0.25, 0.5 and 0.75) reached with different concentration of thiophene are summarized in Table 4. For CPd, St/Sc clearly decreases as the poisoning degree increases, while for CRu the change of that ratio is scantly perceptible. Since the composition of the liquid feed is not modified along the deactivation process, these changes of St/Sc have to be now attributed to electronic modifications at the catalyst surface produced by thiophene during the course of the reaction, as it occurs for other reactions by effect of sulfur compounds [29]. The decreases of St/Sc observed in Table 4 indicate a parallel decrease of electron deficient character of the remainder active species, probably due to the fact that thiophene becomes more and more electron donor towards the metal along the run, the overall electron balance being a partial charge transfer from thiophene to the metal (Medþ).
4. Conclusions Catalytic performances of Ru and Pd supported on alumina have been analyzed at 333–393 K, 5 105 Pa and 77.6 h1 in absence and presence of thiophene. CRu is more active and selective toward c-DMCH than CPd but less sulfur resistant. This behavior is related to the nature of metal, the electronic state of the
active sites and the ratio of the adsorption coefficients of thiophene and o-xylene. The higher hydrogenation activity of CRu suggests that adsorbed o-xylene is more distorted on Ru than on Pd. The selectivity toward 1,2-dimethylcyclohexane isomers depends on the metal nature, irrespective of the reaction conditions, but it is strongly affected by the presence of thiophene in the feed and it changes during the catalyst deactivation process. Thiophene is preferentially chemisorbed on the lesser electron acceptor metal sites of the catalysts in competition with o-xylene, covers progressively the metal surface and deactivates the catalysts. The decrease of the St/Sc ratio as deactivation progresses evidences a parallel decrease of the surface electron deficiency, suggesting that thiophene becomes more and more electron donor towards the metal along the run, the overall electron balance being a partial charge transfer from thiophene to the metal.
Acknowledgements The authors gratefully acknowledge the financial support from CICYT, Spain (project MAT1999-0812).
References [1] M.F. Wilson, I.P. Fisher, J.F. Friz, Energy Fuel 1 (1987) 540. [2] A. Arcoya, A. Corte´ s, J.L.G. Fierro, X.L. Seoane, Stud. Surf. Sci. Catal. 68 (1991) 557.
A. Arcoya et al. / Applied Surface Science 211 (2003) 341–351 [3] L.A. Stanislaus, B.H. Cooper, Catal. Rev. Sci. Eng. 36 (1994) 75. [4] J.M. Basset, G. Dalmai-Imelik, M. Primet, R. Muti, J. Catal. 37 (1975) 2. [5] P. Chuo, M.A. Vannice, J. Catal. 107 (1987) 129. [6] M.V. Rahaman, A. Vannice, J. Catal. 127 (1991) 251. [7] R. Gomez, G. del Angel, V. Bertin, React. Kinet. Catal. Lett. 44 (1991) 517. [8] G. Del Angel, V. Bertin, A. Perez, R. Gomez, React. Kinet. Catal. Lett. 48 (1992) 259. [9] P. Reyes, M.E. Ko¨ nig, G. Pecchi, I. Concha, M. Lo´ pez Granados, J.L.G. Fierro, Catal. Lett. 46 (1997) 71. [10] O. Levenspiel, Chemical Reaction Engineering, Wiley, New York, 1966, Chapter 14. [11] K. Hadjiivanov, J.C. Lavalley, J. Lamotte, F. Mauge´ , J. SaintJust, M. Che, J. Catal. 176 (1998) 415. [12] F. Pinna, M. Selva, M. Signoretto, G. Strukul, F. Boccuzzi, A. Benedetti, P. Canton, G. Fagherazzi, J. Catal. 150 (1994) 356. [13] P. Marecot, A. Akhachane, J. Barbier, Catal. Lett. 36 (1996) 37. [14] W. Juszkyk, Z. Karpinski, I. Ratajczykowa, W.M.H. Sachtler, J. Catal. 120 (1989) 68. [15] N.S. Fı´goli, P.C. L’Argentiere, A. Arcoya, X.L. Seoane, J. Catal. 155 (1995) 95.
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
351
G.H. Yokomizo, C. Louis, A.T. Bell, J. Catal. 120 (1989) 1. H.W. Chen, Z. Zhong, J.M. White, J. Catal. 90 (1984) 119. F. Solimosy, J. Rasko´ , J. Catal. 115 (1989) 107. C. Minot, P. Gallezot, J. Catal. 123 (1990) 341. P.A. Cox, The Electronic Structure and Chemistry of Solid, Oxford University Press, Oxford, 1987, p. 63. Y. Inoue, J.M. Herrmann, H. Schmidt, R.L. Burwell, J.B. Butt, J.B. Cohen, J. Catal. 53 (1978) 401. M. Viniegra, G. Co´ rdoba, R. Go´ mez, J. Mol. Catal. 58 (1990) 107. J.J.F. Scholton, A.P. Pigpers, A.M.L. Hustings, Catal. Rev. Sci. Eng. 27 (1) (1985) 151. G. Blyholder, D.O. Bowen, J. Phys. Chem. 66 (1962) 1288. J.P. Boitiaux, J. Cosyns, F. Verna, Stud. Surf. Sci. Catal. 34 (1987) 105. X.L. Seoane, P.C. L’Argentiere, N.S. Fı´goli, A. Arcoya, Catal. Lett. 16 (1992) 137. K. Ahmed, D. Chadwick, L.S. Kershenbaum, Stud. Surf. Sci. Catal. 34 (1987) 513. A. Arcoya, X.L. Seoane, N.S. Fı´goli, P.C. L’Argentiere, Appl. Catal. 62 (1990) 35. J. Barbier, E. Lami-Pitara, P. Marecot, J.P. Boitiaux, J. Cosyns, F. Berna, Adv. Catal. 37 (1990) 279.
Activity of Pd/Al2O3 and Ru/Al2O3 catalysts in the hydrogenation of o-xylene Effect of thiophene Adolfo Arcoya*, Xose´ L. Seoane, Luisa Ma Go´mez-Sainero Instituto de Cata´lisis y Petroleoquı´mica (CSIC), Cantoblanco, 28049 Madrid, Spain Received 10 January 2003; accepted 26 February 2003
Abstract Two catalysts containing 2 wt.% of ruthenium or palladium on alumina (CRu and CPd, respectively) were prepared by impregnation from chloride precursors and then calcined and reduced at 573 K. The catalysts were characterized by gas chemisorption, TPR and FT-IR using CO as probe molecule. The catalytic activity was evaluated in the o-xylene hydrogenation at 5 105 Pa, 77.6 h1 and 363–393 K, in a fixed-bed tubular reactor and the sulfur resistance analyzed at 373 K with different feeds contaminated with 20, 50 and 100 ppm of thiophene, respectively. CRu is more active and selective toward cis-1,2dimethylcyclohexane (c-DMCH) than CPd, but it is less sulfur resistant. The selectivity to 1,2-dimethylcyclohexane isomers (St and Sc) for each catalyst is scarcely influenced by the reaction temperature, but it is affected by both the presence of thiophene in the liquid feed and the degree of deactivation of the catalysts by sulfur. CO/FT-IR measurements of the fresh catalysts showed that, in addition to Pd0, an important part of palladium in CPd is present as Pddþ while Ru0 is by far the major species in CRu. The IR spectra of the deactivated samples suggest the presence of thiophene adsorbed on the surface, which is removed outgassing at 573 K and 103 Pa. After this treatment the deactivated samples do not completely recover either the metal surface or the capability to chemisorb CO. The catalytic performances are related to the nature of the metal, the ratio of the adsorption coefficients of thiophene and o-xylene (Kth/xil) and the electronic state of the active metal sites, which are modified by effect of thiophene. The decrease of the St/Sc ratio along the run suggests a parallel decrease of the surface electron deficiency produced by the chemisorbed thiophene. # 2003 Elsevier Science B.V. All rights reserved. Keywords: o-Xylene hydrogenation; Pd catalysts; Ru catalysts; Sulfur resistance; Thiophene poisoning; FT-IR
1. Introduction Catalytic hydrogenation is commercially used to reduce the aromatics content from petroleum derivatives such as medium distillates, kerosene and paraffinic *
Corresponding author. Tel.: þ34-91-585-48-04; fax: þ34-91-585-47-60. E-mail address: [email protected] (A. Arcoya).
solvents. Due to the stringent environmental regulations, supported group VIII metals based catalysts are required to reach high rates of reaction at mild operating conditions [1]. These catalysts are more active than the conventional hydrotreating one (sulfides of Co–Mo, Ni–Mo, or Ni–W), although they are highly sensitive to the sulfur compounds present in the industrial feedstocks [2]. For these reasons, the development of more active and sulfur resistant metal catalysts is, at present, a
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00354-4
342
A. Arcoya et al. / Applied Surface Science 211 (2003) 341–351
meaningful tool in the field of hydroprocessing. A very comprehensive review about the basic and industrial aspects of aromatic hydrogenation reactions was published a few years ago [3]. Most the works on aromatics hydrogenation are devoted to benzene [4,5], which is an useful probe molecule to evaluate hydrogenation catalysts. Few investigations, however, are focused to the hydrogenation of other aromatics, including xylene. So, Rahaman and Vannice [6] studied the hydrogenation of xylene isomers over Pd supported catalysts and observed that the more acid is the support the more active are the catalysts. Gomez et al. [7,8], working with group VIII metals based catalysts found a relationship between the selectivity towards cis-dimethylcyclohexane and the electronic heat capacity of the metal in the oxylene hydrogenation. More recently, Reyes et al. [9] reported the effect of the metal particle size for the same reaction. The effect of sulfur in this reaction has not been, however, analyzed. The main goal of this work is to compare the behavior of alumina supported Ru and Pd catalysts in the o-xylene hydrogenation and examine the effect of thiophene on the catalytic performance. Palladium is a well known hydrogenation catalyst while, up to now, ruthenium has been more scantly used in spite of the fact that it is cheaper than palladium. The catalysts were characterized by gas chemisorption, TPR and FT-IR. The catalytic performance in absence and in presence of thiophene is analyzed and interpreted in terms of the electronic state of the metal and the ratio of the adsorption coefficients of thiophene and o-xylene.
2. Experimental 2.1. Catalyst preparation Two catalysts (CPd and CRu) containing Pd or Ru on a g-alumina (Condea, high purity Puralox SCF grade, SBET ¼ 210 m2 g1 and particle size 0.5–0.8 mm) previously calcined at 823 K for 3 h, were prepared by incipient wetness impregnation with aqueous solutions of H2PdCl4 or RuCl3, respectively, in the adequate concentration to get 2 wt.% of metal on the support. H2PdCl4 was obtained treating PdCl2 with 0.1 M HCl solution. After drying at 393 K overnight, the precursors were heated under air stream at 1 K min1 from room
temperature up to 573 K for 4 h and then reduced under flow of hydrogen at 573 K for 3 h. 2.2. Characterization of the catalysts Temperature-programmed reduction (TPR) of the oxidized precursors was carried out in a flow system with a thermal conductivity detector. Samples of 0.5 g were outgassed at 573 K for 1 h in an Ar stream and then cooled down to 273 K. The TPR profiles were registered heating the samples from 273 to 773 K at 6 K min1 under a flow of 30 ml min1 of a 5% H2/Ar mixture. Metal dispersion of the catalysts (D) was calculated from the ratio D ¼ MS =MT , where MS is the number of metal atoms exposed per gram of catalyst and MT the total number of metal atoms per gram of catalyst obtained from ICP analysis. MS was determined by H2–O2 titration in a flow system with a thermal conductivity detector. The reduced samples were outgassed at 473 K under flow of argon for 2 h and then cooled to room temperature. Pulses of O2 (30 ml) were introduced into the carrier gas (30 ml min1) until total saturation of the sample surface. Titration of adsorbed O2 with H2 was performed at 353 K. The atomic stoichiometries used were Huptake =Pds ¼ 3 and Huptake =Rus ¼ 5 [8]. The electronic state of the metal was analyzed by FT-IR using CO as probe molecule. The spectra were recorded on a Nicolet 5ZDX FT-IR spectrometer with a resolution of 4 cm1. The self-supported wafer of the sample (25 mg cm2) was outgassed at RT for 1 h at 2 103 Pa and then treated at 573 K under a hydrogen pressure of 105 Pa, for 3 h. The sample was evacuated at 5 103 Pa and 573 K for 1 h and then cooled to room temperature. After recording the background spectrum, the sample was contacted with 2 103 Pa of CO for 10 min and then a new spectrum recorded. The spectrum of the adsorbed CO was obtained by subtraction of the preceding ones and the number and position of the component bands of the broad bands were estimated by Fourier self-deconvolution (FSD) after base line correction. FT-IR was additionally used to detect adsorbed species on the deactivated samples. 2.3. Catalytic activity measurements Hydrogenation of o-xylene was performed in a conventional catalytic system, with a fixed-bed flow
A. Arcoya et al. / Applied Surface Science 211 (2003) 341–351
reactor containing 1.5 g of catalyst. Liquid o-xylene (RPE grade) was diluted (5/95, w/w) with n-heptane (RPE grade) and fed to the reactor. A bed of SiC above the catalyst heated at 473 K was employed as a preheater of the liquid feed. The reaction conditions were 5 105 Pa, 333–393 K, hydrogen/o-xylene ¼ 30 mol/mol and liquid space velocity 77.6 h1. Under these conditions o-xylene is in the liquid phase during the reaction. High purity hydrogen (Air Liquid, purity: 99.995%) was successively passed through a Deoxo purifier and a 5A molecular sieve filter. The reactor was heated under the adequate hydrogen flow and then, when the reaction temperature was reached, the liquid was pumped in. The effluent of the reactor was condensed at 273 K and in order to estimate the stability of the catalyst several samples were periodically collected during the runs. To analyze the effect of sulfur on the catalysts at 373 K the liquid feed was doped with different concentrations of thiophene (20, 50 and 100 ppm). In these experiments the first product sample was collected after 10 min of the beginning of reaction and successive samples every 3 min until the activity was 20% of the initial one. The reaction product was analyzed by gas chromatography (GC) in a 4 m 3:18 mm OD column with 1,2,3-tris(2-cyanoethoxy) propane (10 wt.%) on Chromosorb P (60–80 mesh) at 353 K. To check the possible formation of methane during the reaction, the exit gas was also analyzed by GC at 313 K in a column of powdered activated charcoal. Helium (40 ml min1) was used as carrier gas. Prior to the kinetic measurements and following a conventional methodology [10] we had confirmed that in the reaction conditions used both external and internal diffusion limitations were absent.
3. Results and discussion 3.1. Characterization of the catalysts Metal loading, dispersion (D) and metal surface area (SM) of the catalysts are given in Table 1 and the TPR profiles of the calcined precursors are depicted in Fig. 1. The profile of CPd exhibits a single reduction peak at 318 K and the hydrogen uptake, calculated from the area under the curve, corresponds to the stoichiometric transformation of Pd2þ into Pd0
343
Table 1 Catalyst characterizationa Catalyst
Metal loading (wt.%)
D (%)
SM (m2/gcat)
CPd CRu
1.92 1.82
37.0 21.2
3.33 1.41
a D, dispersion; SM, metal surface area assuming SPd ¼ 8:3 1020 m2/Pd atom and SRu ¼ 6:13 1020 m2/Ru atom [23].
(H2 =Pd2þ ¼ 1). The TPR profile of CRu shows a double peak with maxima centered at 488 and 503 K, attributed to the reduction of different oxidized Ru species [11]. The CO–FT-IR spectrum of CPd (Fig. 2) shows three bands at 1936, 2100 and 2157 cm1. The broad band in the region 1800–2000 cm1 is formed by several overlapped single bands corresponding to different forms of bridged carbonyls on Pd. The band at ca. 2100 cm1 is assigned to linear carbonyl species adsorbed on different electronic palladium states [12,13]. The FSD of this band indicates that in addition to Pd0 (2099 cm1) electron deficient palladium species, Pddþ (2115 and 2135 cm1) [14] are also present. In a previous paper, we have reported that in catalysts prepared from PdCl2 acid solutions, even after carefully reduction below 723 K, Pddþ species would be formed by interaction of Pd0 with neighboring Hþ originated during the reduction step, through bridged structures stabilized by the Cl ions [15]: þ
Pd0 þ H þ Cl ! Pddþ Hð1dÞ Cl dþ
2þ
(1)
Accordingly, Pd is not unreduced Pd but Pd0 sharing the positive charge of the proton. Actually, CPd is completely reduced at 318 K, as the TPR results show. The weak band at 2160 cm1 corresponds to unreduced Pd2þ. These assignments was experimentally confirmed by the CO spectrum of an unreduced CPd sample (Fig. 2). The spectrum of CO adsorbed on CRu (Fig. 3) exhibits a broad band at low wavenumber (LFB) centered at 2025 cm1 which, in reduced catalysts, is unambiguously attributed to CO linearly bonded to metallic Ru [16]. The broad and weak band at ca. 2150 cm1 (HF1 band) and the small shoulder at 2080 cm1 (HF2 band) are typical of multicarbonyl species associated with partially oxidized ruthenium, Rudþ(CO)x. The source of such a species in reduced
344
A. Arcoya et al. / Applied Surface Science 211 (2003) 341–351
Fig. 1. TPR profiles of catalyst precursors.
Fig. 2. CO/FT-IR spectra of CPd and unreduced CPd.
A. Arcoya et al. / Applied Surface Science 211 (2003) 341–351
345
Fig. 3. CO/FT-IR spectra of CRu and unreduced CRu.
Ru-supported catalysts has been extensively discussed in the literature and the results are very controversial. So, for Chen et al. [17] they are due to CO adsorbed on Ru sites strongly interacted with electron acceptor sites on the surface. For other authors Rudþ(CO)x species are formed by either an oxidative disruption of Ru clusters by effect of CO [18] or a corrosive chemisorption of CO on Ru crystallites [16] in the IR cell. In our case, the HF bands are very small and, for this reason, whatever their origin might be, the spectrum showed in Fig. 3 clearly indicates that CRu is highly reduced, with Ru0 as the major species. 3.2. Hydrogenation activity Conversion of o-xylene and selectivity towards trans-1,2-dimethylcyclohexane (t-DMCH) and cis1,2-dimethylcyclohexane (c-DMCH) as a function of the reaction temperature in the range of 333– 393 K are plotted in Fig. 4. Conversion is defined as the number of molecules of o-xylene transformed per 100 molecules of o-xylene fed to the reactor and selectivity toward t-DMCH (St) and c-DMCH (Sc) as the number of o-xylene molecules transformed into
the respective isomer per 100 molecules of o-xylene converted. Hydrogenation activity of CRu, is higher than that of CPd, t-DMCH and c-DMCH being the only reaction products for both catalysts. The higher activity of CRu is in accordance with the results frequently reported in the literature for hydrogenation of light aromatic hydrocarbons over noble metal catalysts [2,19]. Since hydrogenation of aromatics is a structure-insensitive reaction [7], the difference of hydrogenation activity observed should be explained from the electron structure of the metals, which is characterized by the local density of unoccupied states at the Fermi level N(EF). This property measures the number of quantum states available for bonding reactants, i.e., the donor-acceptor ability of the metal [20]. However, the fact that Ru is more active than Pd, in spite of the fact that its N(EF) value is lower, suggests that o-xylene adsorbed on Ru is more distorted than on Pd, as it happens for toluene adsorbed on the same metal [19]. Thus, the subsequent hydrogen attack at the transition state in CRu is more favored than in CPd. Under the experimental conditions used St and Sc remain practically constant for each catalyst, with St
346
A. Arcoya et al. / Applied Surface Science 211 (2003) 341–351 Table 2 Deactivation parameters of the catalysts for different thiophene concentration in the feeda Catalyst
TR0
yn
SM(d)
CPd CRu
0.19 0.40
2.00 0.36
0.16 0.03
a TR0, initial sulfur resistance (thiophene molecule/metal atom); yn, thiophene molecules fed per surface metal atom necessary to deactivate the catalyst up to a=a0 ¼ 0:2; SM(d), metal surface (m2/gcat) of the deactivated catalysts (a=a0 ¼ 0:2).
Fig. 4. Conversion of o-xylene (solid line) and selectivity toward c-DMCH (dotted line) and t-DMCH (dashed line) as a function of temperature for CRu (*) and CPd (~).
being much higher for CPd (58%) than for CRu (1%). If t-DMCH is formed following the mechanism based on the ‘‘roll over’’ model [21] as suggested by Go´ mez and coworkers [22], results in Fig. 4 are the consequence that o-xylene is adsorbed on palladium more strongly than on ruthenium, as it was mentioned above. 3.3. Deactivation of the catalysts by thiophene The effect of the thiophene concentration in the feed (20, 50 and 100 ppm) on the catalysts performance was examined in the reaction at 373 K. Under these conditions the hydrogenation activity decreases as a function of the time on stream. Previously, we had verified that in absence of sulfur the reaction conversion does not change at least for 10 h on stream. Thus, deactivation by coke may be ruled out. Fig. 5 shows the normalized activity (a/a0) for CPd and CRu as a function of the number of thiophene
molecules fed to the reactor per surface metal atom (y). From these curves the initial sulfur resistance (TR0), i.e., the number of thiophene molecules required to poison one metal atom at t ¼ 0 was calculated (Table 2). For each catalyst, all the experimental values of a/a0 fit a single curve, whatever it is the thiophene concentration. However, while for CRu the normalized activity falls abruptly right from the beginning of reaction until a=a0 0:3, the initial slope of the CPd curve ( 5) clearly decreases for y values higher than 0.1. Thus, although the initial sulfur resistance of CRu (TR0 ¼ 0:40) is higher than that of CPd (TR0 ¼ 0:19), the number of thiophene molecules per metal atom required to deactivate the CRu bed up to a=a0 ¼ 0:2 (yn ¼ 0:36) is lower than that necessary to reach similar deactivation level for CPd (yn ¼ 2:00). Characterization measurements of the deactivated samples showed a dramatic loss of metal surface evidenced by both H2–O2 titration (SCPdðdÞ ¼ 0:16 m2/gcat and SCRuðdÞ ¼ 0:03 m2/gcat) and CO/FT-IR measurements, in spite of the fact that these were performed after outgassing at 373 K and 103 Pa (Figs. 6a and 7a). On the other hand, it is interesting to point out that the FT-IR spectra of the deactivated samples outgassed at RT and 103 Pa exhibit, in comparison to those of the fresh catalysts, two bands at 2931 and 2847 cm1 (Fig. 8). These bands are not observed in the spectra of the fresh catalysts impregnated with either thiophene or o-xylene and then evacuated, indicating that they correspond to species formed under the reaction conditions. By analogy to the spectrum reported by Blyholder and Bowen [24] for chemisorbed thiophene, these bands could be attributed to a carbon–hydrogen stretch of a saturated carbon atom, suggesting that the double bonds in thiophene become saturated by interaction with the surface. If it is so, deactivation of the
A. Arcoya et al. / Applied Surface Science 211 (2003) 341–351
347
Fig. 5. Deactivation of CPd and CRu in the hydrogenation of o-xylene at 373 K by effect of thiophene: 20 ppm (*, *), 50 ppm (~, ~) and 100 ppm (!, 5).
Fig. 6. CO/FT-IR spectra of the deactivated CPd samples outgassed at 103 Pa: (a) 373 K and (b) 573 K.
348
A. Arcoya et al. / Applied Surface Science 211 (2003) 341–351
Fig. 7. CO/FT-IR spectra of the deactivated CRu samples outgassed at 103 Pa: (a) 373 K and (b) 573 K.
Fig. 8. FT-IR spectra of the deactivated catalysts samples outgassed at 373 K and 103 Pa.
A. Arcoya et al. / Applied Surface Science 211 (2003) 341–351
catalysts is produced by the thiophene chemisorbed on the hydrogenation active sites. Adsorption of thiophene is stronger than that of o-xylene and it occurs competitively on the same sites. After evacuation at 573 K and 103 Pa the bands at 2931 and 2847 cm1 are no longer perceptible, while the CO/FT-IR spectra of these samples are partially restored. Thus, for CPd the CO spectrum (Fig. 6b) exhibits the linearly adsorbed CO band at 2098 cm1 completely recovered but this does not occur with that corresponding to bridged CO (1945 cm1). In a similar way, Fig. 7b shows that the intensity of the CO band on CRu (1997 cm1) is recovered in part. These results, in addition to the fact that the metal surface area of these treated samples is in part restored (SCPdðdÞ ¼ 1:06 m2/gcat and SCRuðdÞ ¼ 0:48 m2/gcat), suggest that some poisoning species strongly bonded to the metals are yet present. According to Boitiaux et al. [25] such a species could be metal sulfide originated be the dissociative adsorption of part of thiophene. The two very well defined zones in the CPd curve (Fig. 5) are consistent with the presence of two palladium species in the catalyst: Pd0, highly sensitive to the sulfur and Pddþ, more sulfur resistant [26]. In fact, the ratio between the IR bands intensity at 2138 and at 2106 cm1 for the deactivated catalyst (Fig. 6a) is higher than that for the fresh catalyst. The value TR0 ¼ 0:19, which means that one molecule of thiophene deactivates about five exposed Pd atoms, indicates that thiophene is bonded to the metal in a coplanar mode. For CRu, however, the value TR0 ¼ 0:40 suggests that thiophene is perpendicularly bonded to the surface by means of sulfur atom anchorage [27,28]. In this last case, moreover, the fact that the overall sulfur resistance up to an a/a0 value as low as 0.2 is similar to the initial sulfur resistance agrees
349
with the FT-IR result that Ru0 is the major species in CRu. 3.4. Effect of thiophene on the reaction selectivity Table 3 summarizes the St and Sc values obtained at the beginning of reaction in presence of 20, 50 and 100 ppm of thiophene. Comparing these results with those obtained for a poison free feed it is observed that the St/Sc ratio increases as the concentration of thiophene increases. Since at the beginning of the reaction the metal surface is practically clean, the changes of selectivity observed have to be attributed to a modification of the basicity of the liquid phase by effect of thiophene. Due to its aromatic-like character, thiophene is adsorbed on the catalyst in competition with o-xylene and so the ratio of the respective adsorption coefficients (Kth/xil) depends on the electronic properties of the molecules. Since the ionization energy of o-xylene (8.6 eV) is lower than that of thiophene (10 eV), o-xylene will be preferentially bonded to the more electrophilic metal species, i.e., Pddþ or Rudþ and thiophene on the less electron acceptor sites, i.e., Pd0 or Ru0, which will be deactivated. On the other hand, since the interaction xylene–Medþ is stronger than that of xylene–Me0 and the formation of t-DMCH requires a long stay of the intermediate precursor on the catalyst surface [22], it is reasonable to expect that the St/Sc ratio will increase in presence of thiophene, the more the higher is the thiophene concentration. This effect is scantly perceptible for CRu probably due to the fact that the fraction of electron deficient species in this catalyst is very small in comparison with that in CPd. Similar effect of change of basicity of the reacting medium on St/Sc has been observed in the competitive hydrogenation of
Table 3 Effect of the thiophene concentration (0, 20, 50 and 100 ppm) on the selectivity toward t- and c-DMCH for the fresh catalystsa Catalyst CPd
St (%) Sc (%) St/Sc a
CRu
0 ppm
20 ppm
50 ppm
100 ppm
0 ppm
20 ppm
50 ppm
100 ppm
55 45 1.32
60 40 1.50
73 27 2.70
75 25 3.00
1 99 0.01
2 98 0.02
4 96 0.04
4.6 95.4 0.05
St, selectivity toward t-DMCH; Sc, selectivity toward c-DMCH.
350
A. Arcoya et al. / Applied Surface Science 211 (2003) 341–351
Table 4 Effect of the deactivation degree (1 a=a0 ) of the catalysts on the St/Sc ratioa Catalyst CPd
St/Scb St/Scc St/Scd St/Sce
CRu
0
0.25
0.50
0.75
0
0.25
0.50
0.75
1.32 1.50 2.70 3.00
– 1.30 2.50 2.80
– 1.17 2.20 2.30
– – 2.00 2.00
0.01 0.02 0.04 0.05
– – – –
– – 0.3 –
– 0.1 0.2 0.2
a
St, selectivity toward t-DMCH; Sc, selectivity toward c-DMCH. Feed without thiophene. c Feed with 20 ppm of thiophene. d Feed with 50 ppm of thiophene. e Feed with 100 ppm of thiophene. b
o-xylene/benzene over CPd under similar experimental conditions. In this case, the value St =Sc ¼ 2, is consistent with the fact that benzene (ionization energy ¼ 9:8 eV) is less basic than o-xylene. The St/Sc values obtained for different deactivation degrees (0, 0.25, 0.5 and 0.75) reached with different concentration of thiophene are summarized in Table 4. For CPd, St/Sc clearly decreases as the poisoning degree increases, while for CRu the change of that ratio is scantly perceptible. Since the composition of the liquid feed is not modified along the deactivation process, these changes of St/Sc have to be now attributed to electronic modifications at the catalyst surface produced by thiophene during the course of the reaction, as it occurs for other reactions by effect of sulfur compounds [29]. The decreases of St/Sc observed in Table 4 indicate a parallel decrease of electron deficient character of the remainder active species, probably due to the fact that thiophene becomes more and more electron donor towards the metal along the run, the overall electron balance being a partial charge transfer from thiophene to the metal (Medþ).
4. Conclusions Catalytic performances of Ru and Pd supported on alumina have been analyzed at 333–393 K, 5 105 Pa and 77.6 h1 in absence and presence of thiophene. CRu is more active and selective toward c-DMCH than CPd but less sulfur resistant. This behavior is related to the nature of metal, the electronic state of the
active sites and the ratio of the adsorption coefficients of thiophene and o-xylene. The higher hydrogenation activity of CRu suggests that adsorbed o-xylene is more distorted on Ru than on Pd. The selectivity toward 1,2-dimethylcyclohexane isomers depends on the metal nature, irrespective of the reaction conditions, but it is strongly affected by the presence of thiophene in the feed and it changes during the catalyst deactivation process. Thiophene is preferentially chemisorbed on the lesser electron acceptor metal sites of the catalysts in competition with o-xylene, covers progressively the metal surface and deactivates the catalysts. The decrease of the St/Sc ratio as deactivation progresses evidences a parallel decrease of the surface electron deficiency, suggesting that thiophene becomes more and more electron donor towards the metal along the run, the overall electron balance being a partial charge transfer from thiophene to the metal.
Acknowledgements The authors gratefully acknowledge the financial support from CICYT, Spain (project MAT1999-0812).
References [1] M.F. Wilson, I.P. Fisher, J.F. Friz, Energy Fuel 1 (1987) 540. [2] A. Arcoya, A. Corte´ s, J.L.G. Fierro, X.L. Seoane, Stud. Surf. Sci. Catal. 68 (1991) 557.
A. Arcoya et al. / Applied Surface Science 211 (2003) 341–351 [3] L.A. Stanislaus, B.H. Cooper, Catal. Rev. Sci. Eng. 36 (1994) 75. [4] J.M. Basset, G. Dalmai-Imelik, M. Primet, R. Muti, J. Catal. 37 (1975) 2. [5] P. Chuo, M.A. Vannice, J. Catal. 107 (1987) 129. [6] M.V. Rahaman, A. Vannice, J. Catal. 127 (1991) 251. [7] R. Gomez, G. del Angel, V. Bertin, React. Kinet. Catal. Lett. 44 (1991) 517. [8] G. Del Angel, V. Bertin, A. Perez, R. Gomez, React. Kinet. Catal. Lett. 48 (1992) 259. [9] P. Reyes, M.E. Ko¨ nig, G. Pecchi, I. Concha, M. Lo´ pez Granados, J.L.G. Fierro, Catal. Lett. 46 (1997) 71. [10] O. Levenspiel, Chemical Reaction Engineering, Wiley, New York, 1966, Chapter 14. [11] K. Hadjiivanov, J.C. Lavalley, J. Lamotte, F. Mauge´ , J. SaintJust, M. Che, J. Catal. 176 (1998) 415. [12] F. Pinna, M. Selva, M. Signoretto, G. Strukul, F. Boccuzzi, A. Benedetti, P. Canton, G. Fagherazzi, J. Catal. 150 (1994) 356. [13] P. Marecot, A. Akhachane, J. Barbier, Catal. Lett. 36 (1996) 37. [14] W. Juszkyk, Z. Karpinski, I. Ratajczykowa, W.M.H. Sachtler, J. Catal. 120 (1989) 68. [15] N.S. Fı´goli, P.C. L’Argentiere, A. Arcoya, X.L. Seoane, J. Catal. 155 (1995) 95.
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
351
G.H. Yokomizo, C. Louis, A.T. Bell, J. Catal. 120 (1989) 1. H.W. Chen, Z. Zhong, J.M. White, J. Catal. 90 (1984) 119. F. Solimosy, J. Rasko´ , J. Catal. 115 (1989) 107. C. Minot, P. Gallezot, J. Catal. 123 (1990) 341. P.A. Cox, The Electronic Structure and Chemistry of Solid, Oxford University Press, Oxford, 1987, p. 63. Y. Inoue, J.M. Herrmann, H. Schmidt, R.L. Burwell, J.B. Butt, J.B. Cohen, J. Catal. 53 (1978) 401. M. Viniegra, G. Co´ rdoba, R. Go´ mez, J. Mol. Catal. 58 (1990) 107. J.J.F. Scholton, A.P. Pigpers, A.M.L. Hustings, Catal. Rev. Sci. Eng. 27 (1) (1985) 151. G. Blyholder, D.O. Bowen, J. Phys. Chem. 66 (1962) 1288. J.P. Boitiaux, J. Cosyns, F. Verna, Stud. Surf. Sci. Catal. 34 (1987) 105. X.L. Seoane, P.C. L’Argentiere, N.S. Fı´goli, A. Arcoya, Catal. Lett. 16 (1992) 137. K. Ahmed, D. Chadwick, L.S. Kershenbaum, Stud. Surf. Sci. Catal. 34 (1987) 513. A. Arcoya, X.L. Seoane, N.S. Fı´goli, P.C. L’Argentiere, Appl. Catal. 62 (1990) 35. J. Barbier, E. Lami-Pitara, P. Marecot, J.P. Boitiaux, J. Cosyns, F. Berna, Adv. Catal. 37 (1990) 279.