602
Energy & Fuels 2007, 21, 602-609
Hydroisomerization of n-Octane over Nickel-Tungsten Supported on Silica-Alumina Catalysts Yacine Rezgui* and Miloud Guemini Laboratoire de Recherche de Chimie Applique´ e et Science des Mate´ riaux, UniVersite´ d’Oum El Bouaghi, B.P. 358, Route de Constantine, Oum El Bouaghi 04000, Alge´ rie ReceiVed August 24, 2006. ReVised Manuscript ReceiVed December 3, 2006
The hydroisomerization of n-octane over Ni-WOx/silica-alumina based catalysts has been investigated in a down flow fixed-bed quartz reactor by changing operating conditions such as time on stream, reaction temperature, SiO2/Al2O3 and H2/n-octane ratio, andspace velocity. This study aims to understand the activity and selectivity properties of the prepared catalysts and to analyze the relationship between the metal content, the acidity (nature and strength) of the samples, and the main reaction paths. It was found that all catalysts deactivate with time on stream, with the conversion remaining steady after 100 min. On increasing the nickel loading, conversion increases as does selectivity to isomerization, which reaches a maximum and then decreases. Moreover, the obtained results showed that the H2 partial pressure and SiO2/Al2O3 ratio positively affect isomerization selectivity while the opposite effect was observed for activity (conversion). In addition, the main reaction pathways in n-octane hydroconversion over the prepared solids were found to be isomerization, cracking, and hydrogenolysis. At steady-state conditions (after 100 min TOS), the catalyst with 15% of nickel and 10% of tungsten (WT(15,10)) showed the best results (69% of selectivity at 32.5% conversion).
Introduction As environmental regulations on motor gasoline became stricter throughout the world, petroleum refiners were compelled to search for new routes to enhance the octane rating. For this objective, refiners have relied on a variety of options, including the increase of the aromatics content and the use of oxygenated compounds.1-3 However, new restrictions to aromatics content in gasoline (reformulated gasoline) have further tightened the octane number issue.4 On the other hand, boosting gasoline with oxygenates turned out to be limited due to recent concerns about the contamination of water natural sources by water-soluble MTBE coming from spills of stored tanks.5 A potential solution resides in increasing the light isoparaffins content in gasoline,6,7 which could be obtained by the hydroisomerization/hydrocracking of heavy long chain n-alkanes, but they were surplus refinery cuts of little economic value. Nowadays, considerable efforts have been invested on the understanding of the reaction network that leads to the formation of isomers as well as cracked products. Zhang and Smirniotis studied the effect of zeolites structure on the reaction mechanism for n-octane hydroisomerization, and hydrocracking.8 Weitkamp and co-workers reacted C8-C10 long-chain alkanes over Pt/Y * Corresponding author. E-mail:
[email protected]. (1) Rangel, M. C.; Carvalho, L. S.; Reyes, P.; Parera, J. M.; Figoli, N. S. Catal. Lett. 2000, 64, 171-178. (2) Gonzalez, M. R.; Fogash, K. B.; Kobe, J. M.; Dumesic, J. A. Catal. Today 1997, 33, 303-312. (3) Yori, J. C.; Pieck, C. L.; Parera, J. M. Appl. Catal. 1999, 181, 5-14. (4) Ledoux, M. J.; Gallo, P. D.; Huu, C. P.; York, A. P. E. Catal. Today 1996, 27, 145-150. (5) Silva, R. D.; Catalun˜a, R.; Menezes, E. W.; Samios, D.; Piatnicki, C. M. S. Fuel 2005, 84, 951-959. (6) Watanabe, K.; Kawakami, T.; Baba, K.; Oshio, N.; Kimura, T. Appl. Catal. 2004, 276, 145-153. (7) Kuchar, P. J.; Bricker, J. C.; Reno, M. E.; Haizmann, R. S. Fuel Process. Technol. 1993, 35, 183-200. (8) Zhang, W.; Smirniotis, P. G. J. Catal. 1999, 182, 400-416.
zeolites9,10 and over Pt/H-ZSM-5 zeolites,11 where it was shown that residual alkali cations could influence the catalytic conversion of n-octane on x,y zeolites.12 On the other hand, Lugstein et al.13 investigated the hydroconversion of n-octane, 2,5dimethylhexane, and 2,2,4-trimethylpentane on Ni-containing ZSM-5, mordenite, and β-catalysts, while Brillis and Manos studied the deactivation of Y-zeolite during catalytic cracking of C8 aliphatic hydrocarbons.14 Furthermore, Grau and coworkers have published several papers on the hydroisomerization-cracking of n-C8 over Pt promoted oxoanion promoted superacids15,16 and over Pt supported on WO4-ZrO2, SO4ZrO2, Al2O3, and their mixtures.17-19 In the literature, several other studies deal with n-octane hydroconversion20-23 and n-decane,24-29 n-dodecane,30,31 and n-hexadecane32-35 hydroconversion. The aim of this study was to use a new type of catalysts “Ni-WOx/SiO2-Al2O3” for studying the n-octane hydroconversion and then investigate the role of metallic function content (9) Weitkamp, J. Ind. Eng. Chem. Prod. Res. DeV. 1982, 21, 550-558. (10) Steijns, M.; Froment, G.; Jacobs, P. A.; Uytterhoeven, J. B.; Weitkamp, J. Ind. Eng. Chem. Prod. Res. DeV. 1981, 20, 654-660. (11) Weitkamp, J.; Jacobs, P. A.; Martens, J. A. Appl. Catal. 1983, 8, 123-141. (12) Weihe, M.; Hunger, M.; Breuninger, M.; Karge, H. G.; Weitkamp, J. J. Catal. 2001, 198, 256-265. (13) Lugstein, A.; Jentys, A.; Vinek, H. Appl. Catal. 1999, 176, 119128. (14) Brillis, A. A.; Manos, G. Catal. Lett. 2003, 91 (3-4), 185-191. (15) Grau, J. M.; Parera, J. M. Appl. Catal. 1993, 106, 27-49. (16) Grau, J. M.; Parera, J. M. Appl. Catal. 1997, 162, 17-27. (17) Grau, J. M.; Vera, C. R.; Parera, J. M. Appl. Catal. 2002, 227, 217230. (18) Grau, J. M.; Yori, J. C.; Parera, J. M. Appl. Catal. 2001, 213, 247257. (19) Grau, J. M.; Vera, C. R.; Parera, J. M. Appl. Catal. 1998, 172, 311326. (20) Kuznetsov, P. N. J. Catal. 2003, 218, 12-23. (21) Lee, S.; Lee, D.; Shin, C.; Paik, W. C.; Lee, W. M.; Hong, S. B. J. Catal. 2000, 196, 158-166.
10.1021/ef060430w CCC: $37.00 © 2007 American Chemical Society Published on Web 01/30/2007
Hydroisomerization of n-Octane
Energy & Fuels, Vol. 21, No. 2, 2007 603
Table 1. Acidity Measured by NH3-TPD (mmol NH3/g) and Chemical Composition of the WT(x,y) Catalysts37 composition (wt %)
Table 2. Bro1 nsted and Lewis Acid Sites Measured by IR after Saturation with Pyridine at 100 °C
acidity
catalyst
Ni
W
weak
medium
total
WT(12,8) WT(12,10) WT(15,8) WT(15,10) WT(15,30) WT(17,8) WT(17,10)
12 12 15 15 15 17 17
8 10 8 10 30 8 10
0.12 0.13 0.15 0.19 0.20 0.06 0.10
0.15 0.11 0.17 0.11 0.05 0.24 0.26
0.29 0.25 0.32 0.28 0.23 0.34 0.38
and the effect of process variables, like reaction time on stream, temperature, H2/n-hydrocarbon molar ratio, and space velocity in order to get further insight into the catalytic behavior of the prepared samples. Experimental Section Catalyst Preparation. The catalysts used throughout the experiments consisted of nickel-tungsten oxide supported on silicaalumina, hereafter abbreviated WT(x,y) (where x and y indicate the weight percentage of nickel and tungsten in the catalyst, respectively), with ponderal composition listed in Table 1. They were prepared by a sol-gel method as described in refs 36 and 37: a series of seven WT(x,y) catalysts with x values equal to 12, 15, and 17 and y values equal to 8, 10, and 30. A constant SiO2/Al2O3 ratio equal to 1.83 and three other samples containing 15% Ni and 10% W but having different SiO2/Al2O3 ratios (3, 5, and 7) were prepared using the sol-gel method by mixing the required amounts of aluminum sulfate, sodium tungstanate, and nickel nitrate. To the sol obtained, under vigorous stirring, an aqueous solution of sodium silicate was added. To exchange undesirable ions, such as Na+, the prepared gel was activated under reflux conditions in a thermostat with ammonium sulfate (liquid to solid ratio of 30) at 60 °C over a period of 48 h (this unit operation was repeated several times), washed with hot water (60 °C), dried at 120 °C for 4 h, and finally calcined at 500 °C for 5 h. A heating rate of 10 °C/min was used. Ammonia Temperature Programmed Desorption (NH3-TPD). NH3-TPD measurements were carried out to determine the total acidity and the acid strength distribution of catalysts. All the catalysts were treated in situ in an oxygen flow at 500 °C for 3 h, then outgassed in helium flow at 300 °C, and finally reduced in hydrogen flow at 430 °C for 2 h. After reduction, the sample was (22) Narasimhan, C. S. L.; Thybaut, J. W.; Marin, G. B.; Jacobs, P. A.; Martens, J. A.; Denayer, J. F.; Baron, G. V. J. Catal. 2003, 220, 399-413. (23) Meriaudeau, P.; Tuan, V. A.; Nghiem, V. T.; Sapaly, G.; Naccache, C. J. Catal. 1999, 185, 435-444. (24) Martens, J. A.; Parton, R.; Uytterhoeven, L.; Jacobs, P. A.; Froment, G. F. Appl. Catal. 1991, 76, 95-116. (25) Alvarez, F.; Ribeiro, F. R.; Perot, G.; Thomazeau, C.; Guisnet, M. J. Catal. 1996, 162, 179-189. (26) Galperin, L. B.; Bradley, S. A.; Mezza, T. M. Appl. Catal. 2001, 219, 79-88. (27) Elangovan, S. P.; Bischof, C.; Harmann, M. Catal. Lett. 2002, 80 (1-2), 35-40. (28) Elangovan, S. P.; Harmann, M. J. Catal. 2003, 217, 388-395. (29) Roussel, M.; Lemberton, J.; Guisnet, M.; Cseri, T.; Benazzi, E. J. Catal. 2003, 218, 427-437. (30) Campelo, J. M.; Lafont, F.; Marinas, J. M. Appl. Catal. 1998, 170, 139-144. (31) Fang, K.; Wei, W.; Ren, J.; Sun, Y. Catal. Lett. 2004, 93 (3-4), 235-242. (32) Zhang, S.; Zhang, Y.; Tierney, J. W.; Wender, I. Fuel Process. Technol. 2001, 69, 59-71. (33) Park, K.; Ihm, S. Appl. Catal. 2000, 203, 201-209. (34) Calemma, V.; Peratello, S.; Perego, C. Appl. Catal. 2000, 190, 207218. (35) Keogh, R. A.; Davis, B. H. Catal. Lett. 1999, 57, 33-35. (36) Rezgui, Y.; Guemini, M.; Tighezza, A.; Bouchemma, A. Catal. Lett. 2003, 87, 11-24. (37) Rezgui, Y.; Guemini, M. Appl. Catal. 2005, 282, 45-53.
catalyst
Br (µmol of Pyr/g)
Le (µmol of Pyr/g)
Br/Le
WT(12,8) WT(12,10) WT(15,8) WT(15,10) WT(15,30) WT(17,8) WT(17,10)
35 52 70 102 50 90 130
140 121 148 100 10 154 89
0.25 0.43 0.47 1.02 5.00 0.58 1.46
∆Br (Br2-Br1)
∆Le (Le1-Le2)
catalysts with 12% Ni 17 19 catalysts with 15% Ni 32 48 catalysts with 17% Ni 40 65
further dried in flowing He at 300 °C for 1 h and then cooled to room temperature. When the system became steady, ammonia was passed at 100 °C for 30 min (using a 10% NH3/He carrier gas). The samples were then purged, at the same temperature, with helium flow for 1 h, and finally the temperature was raised at 10 °C/min up to 700 °C in a helium flow. Ammonia desorption was continuously measured by means of thermal conductivity detector, and the water evolved was trapped in a KOH trap located immediately before the TCD. FT-IR Spectroscopy. FT-IR spectra of adsorbed pyridine were recorded on a 170-SX Nicolet FT-IR spectrometer. The sample was finely grounded and pressed into self-supporting wafer (14 mg/ cm2) and then placed in a heatable glass cell equipped with KBr windows, which permitted us to follow the changes of the spectrum with thermal treatments. Prior to IR measurements, the sample was treated in situ (in the cell) with an hydrogen flow at 430 °C for 2 h, evacuated to 0.1 Pa at the same temperature for 1 h, and then exposed to pyridine (1 Torr) at 423K for 20 min, outgassed 1 h at room temperature (pressure 0.1 Pa), and then heated up to the desired temperature using a linear program. Catalytic Test. The n-octane hydroisomerization.hydrocracking tests were carried out at temperatures from 150 to 300 °C at atmospheric pressure, a weight hourly space velocity (WHSV) varying form 4 to 12 h-1 and a hydrogen/hydrocarbon (H2/n-octane) molar ratio ranging from 5 to 15 in a down flow fixed-bed quartz reactor equipped with a sampling valve for gas chromatographic analysis, containing 1 g of catalyst and operating under isothermal conditions. The reactor was heated by mean of a controlled temperature electrical oven. Before starting the run, the catalyst was kept in a hydrogen stream under 430 °C for 3 h in order to prereduce the metallic function. Conversion was calculated as the disappearance of n-C8. Isomerization selectivity was defined as the yield of C8 isocomponents divided by n-C8 conversion.
Results and Discussion NH3-TPD. From Table 1, it appeared clearly that catalysts total and medium acidities were promoted by an increase in Ni content, while weak acidity increased to reach a maximum for 15% Ni and then decreased. On the other hand, with a rise in tungsten amount, weak acidity increased. The effect became more significant at higher nickel loading, while no correlation between medium, total acidity, and tungsten content could be obtained. FT-IR Spectroscopy. In order to find a possible correlation between tungsten, nickel content, and the nature of the acidic sites of the prepared samples, the peak areas corresponding to Bro¨nsted and Lewis sites after saturation (100 °C) were determined. These areas as well as the Bro¨nsted to Lewis ratio (Br/Le) for each catalyst are summarized in Table 2. The collected data indicate that, except for WT(15,30), with increasing tungsten content, the density of protonic acid sites increased while the concentration of Lewis acid sites decreased. These effects were more pronounced at high nickel content ((∆Br (12% Ni) ) 17) < (∆Br (15% Ni) ) 32) < (∆Br (17% Ni) ) 40) and (∆Le (12% Ni) ) 19) < (∆Le (15% Ni) ) 48) < (∆Le
604 Energy & Fuels, Vol. 21, No. 2, 2007
Rezgui and Guemini
Figure 1. Evolution of the catalysts catalytic activity with time on stream. (A) Reaction temperature ) 150 °C. (B) Reaction temperature ) 300 °C.
(17% Ni) ) 65)). These results rule out that the presence of tungsten generates Bro¨nsted sites and that nickel promotes the beneficial effect of tungsten. On the other hand, the presence of nickel tends to increase the concentration of both Bro¨nsted and Lewis acid sites. Effect of Time on Stream. Conversion and isomerization selectivity evolution with time on stream were studied using the above-mentioned operating conditions. As shown in Figure 1, the catalytic activity declined with time on stream (TOS), the deactivation being more rapid during the first few minutes, and stationary conversions were obtained after 100 min without any further observable trend to deactivation. WT(17,8) and WT(17,10) catalysts gave the highest conversions of 38.5% and 51.5 (for WT(17,8)) and 39.2% and 52.2% (for WT(17,10)) at 150 and 300 °C, respectively, while the WT(15,30) catalysts gave the lowest one of 20.10% and 27.30%, respectively. In addition, the conversion over the WT(15,10) catalyst was between those of these three catalysts, but the latter stabilized value was higher than that of WT(17,10), which leads to the fact that the higher the activity showed by the catalyst, more severe is its deactivation. Catalysts deactivation was also more promoted by an increase in reaction temperature (Figure 1B), although the conversion level in the initial stage of run was raised. Besides, the different samples lose their activity with a rate that depends on their nickel content: the greater the nickel loading, the lower the observed activity drop. This decrease in activity may be attributed to a metallic function poisoning, or to a loss of the support acidity, or finally to the loss of the hydrogen dissociation capacity of WOx species, which is rapidly deactivated by coke.38 On the other hand, with increasing time on stream cracking reactions decreased, the effect was more marked during the first few minutes. It seems that this decrease is dependent on two parameters: the density of medium acid sites and the nickel content. At higher medium acid sites density, the cracking deactivation rate was more pronounced [catalysts WT(17,8) and WT(17,10) that possessed the highest cracking activity in accordance with their greatest acidity of 0.26 mmol of NH3/g, (38) Falco, M. G.; Canavese, S. A.; Comelli, R. A.; Figoli, N. S. Appl. Catal. 2000, 201, 37-43.
exhibited the highest activity deactivation from 60.21 to 47.56%, which means a decrease of 12.65%, in the case of the former catalyst and from 48.62 to 36.22%, which means a decrease of 12.40% for the latter, whereas WT(15,30), which showed the lowest cracking activity in agreement with its lower acidity of 0.05 mmol of NH3/g, showed the lowest decay in the rate of cracking reactions with TOS]. These results rule out that the high acidity of the catalysts with 17% Ni is not only the cause of their catalytic activity but also the principal cause of their rapid deactivation by producing carbonaceous deposits catalyzed by the strong and medium acid sites. On the other hand, for the same medium acid sites density, with increasing the nickel loading the decay in cracking activity became less significant [the WT(15,10) sample with 15% Ni and a medium acid site density of 0.11 mmol of NH3/g, exhibited a less cracking deactivation rate than the WT(12,10) sample with 12% Ni and a medium acid site density of 0.12 mmol of NH3/g]. On the basis of the data collected in Table 3, the main cracking products are C1, C2, C3, i-C4, n-C4, i-C5, and n-C5, with i-C4 always being the most important. During the run, the cracked products yields decreased except for C1, which is a result of hydrogenolysis occurring over metal sites. This result infers that the cracking reactions decay is due to the loss of catalysts acidity and not to the metallic function poisoning. Besides, the iso-C8 production increased, which means that i-C8 is an intermediate. The n-octane isomerizes to isooctane, which is further cracked. Moreover, on the samples with 17% Ni, the production of C6 and C7 hydrocarbons was null because the higher acidity catalyzes these hydrocarbons cracking. Furthermore, an increase in TOS induces an increase in isomerized products to cracked products ratio (I/C). On the other hand, isomerization selectivity depends on the same parameters: the number of medium acidity sites present on the catalyst and the nickel amount; the higher the number of medium acid sites, the higher the increase in isomerization selectivity, and for comparable medium acidities, an increase in the nickel loading induces the weakening of the increase in the isomerization reactions. As mentioned by Grau et al.,18 the i/n (ratio of branched isomers to their normal counterparts) butane and the i/n-pentane ratios give an idea on the isomer-
Hydroisomerization of n-Octane
Energy & Fuels, Vol. 21, No. 2, 2007 605
Table 3. Effect of Time on Stream on the Main Possible Reactions and on the Distribution of Their Productsa cat
WT(12,8)
WT(12,10)
WT(15,8)
WT(15,10)
WT(15,30)
WT(17,8)
WT(17,10)
t(min)
20
60
100
20
60
100
20
60
100
20
60
100
20
60
100
20
60
100
20
60
100
conv CH isom crack hyd C1 C2 C3 i-C4 n-C4 i-C5 n-C5 i-C6 n-C6 i-C7 n-C7 i-C8 i/n-C4 i/n-C5 C1 + C2
35.2 50.3 49.7 44.9 5.4 3.7 1.7 20.1 38.4 13.4 10.0 11.4 0.2
23.4 37.3 62.6 31.7 5.6 4.7 0.9 10.2 35.9 11.9 9.4 9.1 0.7 0.5 0.2
20.2 35.3 64.7 29.7 5.6 4.8 0.8 10.0 34.0 11.0 9.1 8.3 0.8 0.4 0.2
35.5 42.5 57.5 36.9 5.6 4.6 1.0 10.5 37.2 13.5 11.1 11.9 0.9 1.0 0.1
23.5 33.6 66.4 28.2 5.4 4.8 0.6 5.5 34.9 12.2 10.6 10.1 0.7 0.4 0.2
19.8 32.6 67.4 27.1 5.5 5.0 0.5 7.8 32.0 11.0 8.7 8.0 0.6 0.3 0.2
39.1 48.7 51.3 38.7 10 8.4 1.6 20.0 38.1 13.6 8.9 5.1 0.2
28.9 35.9 64.1 25.8 10.1 9.1 1.0 9.0 35.7 12.1 8.5 4.5 0.3 0.2 0.1
26.7 33.9 66.1 23.9 10 9.1 0.9 8.3 33.7 11.2 8.1 4.2 0.3 0.2 0.1
44.3 37.5 62.5 29.1 8.4 7.9 0.5 10.2 32.3 13.4 8.4 5.2 0.3 0.3
34.3 32.0 68.0 23.5 8.5 8.1 0.4 7.1 29.6 11.7 7.9 4.6 0.3 0.1
32.5 31.0 69.0 22.6 8.4 8.0 0.4 7.0 28.8 11.2 7.6 4.3 0.2 0.1
23.8 28.8 71.2 26.7 2.1 2.1
15.0 26.1 73.9 23.9 2.3 2.3
12.3 25.9 74.1 23.6 2.3 2.3
13.5 2.8 3.3 3.5 7.1 1.4 0.7
13.1 2.3 3.2 2.7 7.0 1.3 0.6
46.7 80 20 36 44 31.8 12.2 9.3 14.4 20.3 5.0 6.4
38.2 65.2 34.8 22.1 43.1 33.3 9.8 4.7 12.3 13.7 3.6 3.7
35.9 56.3 43.7 14.2 42.1 33.8 8.3 1.5 9.9 9.8 3.2 2.9
47.4 81.4 18.6 38.1 43.3 32.2 11.1 13.7 14.2 20.3 4.0 4.1
39.4 67.0 33.0 27.8 38.6 33.8 5.4 11.3 12.1 13.1 4.0 3.4
37.7 64.3 35.7 22.9 41.4 34.3 7.1 10.5 11.7 9.3 4.0 3.1
1.1 16.1 20.6 2.87 3.02 3.09 0.88 1.03 1.10 5.4 5.6 5.6
13.0 2.0 3.0 2.5 6.9 0.9 0.6 0.1 0.1 8.3 20.0 25.9 4.1 19.5 23.9 12.1 27.0 32.4 22.0 54.9 68.6 0.6 18.9 20.3 0.4 16.9 20.0 2.75 2.87 2.91 2.80 2.94 3.00 2.41 2.57 2.57 0.84 0.71 0.67 0.71 0.90 1.01 0.72 0.92 1.04 0.93 1.05 1.09 1.73 1.87 1.93 1.61 1.73 1.77 0.49 0.39 0.36 0.78 0.97 1.10 0.98 1.18 1.30 5.6 5.4 5.5 10 10.1 10 8.4 8.5 8.4 2.1 2.3 2.3 44 43.1 42.1 43.3 38.6 41.4
a Reaction conditions: reaction temperature ) 250 °C; H /n-octane ratio ) 5; and WHSV ) 4 h-1. Key: cat, catalyst; conv, conversion; CH, cracking 2 + hydrogenolysis; isom, isomerization; crack, cracking; hyd, hydrogenolysis.
Figure 2. Effect of the Ni content on the activity and selectivity of WT(x,y) catalysts. (A) Catalysts with 8% W. (B) Catalysts with 10% W. Reaction conditions: reaction temperature ) 250 °C, time on stream ) 100 min, H2/n-octane ratio ) 5, and WHSV ) 4 h-1.
ization/cracking ratio. These ratios increase with increasing TOS, particularly in the case of WT(17,8) and WT(17,10) because of their significant deactivation that generates a decrease in the cracking reactions and thus allows more isomerization of the adsorbed “C8” carbenium ions. The ratios i/n-C4 and i/n-C5 are above the thermodynamic equilibrium value for the isomerization reaction,39,40 indicating the cracking of isoalkanes C6, C7, and C8. This can be explained by the fact that in hydrocracking reactions branched products cannot be formed by secondary isomerization of the linear fragments13,41 since competitive adsorption at the acid sites becomes less favorable with decreasing chain length of a fragment. In addition, if the i-C8 intermediate had been monobranched, similar amounts of n-C4 and i-C4 have been produced,17 or in our experiments the i-C4 was produced in higher proportion and therefore the i-C8 intermediate was multibranched. This can be explained by the (39) Parera, J. M. Catal. Today 1992, 15, 481-490. (40) Derouane, E. G. J. Catal. 1986, 100, 541-544. (41) Martens, J. A.; Jacobs, P. A.; Weitkamp, J. Appl. Catal. 1986, 20, 239-281.
fact that the more C8 is ramified before the C-C bond cleavage, the higher is the probability of its β-scission, which generates a higher amounts in i-C4 and i-C5. Effect of Nickel Loading. The influence of the nickel content on catalytic activity and selectivity toward isomerization was investigated at 250 °C, for a time on stream of 100 min and a reduction temperature of 430 °C. As shown in Figure 2, increasing the Ni content increased conversion, which suggests that Ni induces a beneficial effect on the WT(x,y) catalysts catalytic performance. These results are in agreement with the data of total acidity of the prepared solids. The higher the Ni content, the higher is the catalyst total acidity. Consequently, the higher is the conversion. Similar observations were reported in the open literature,42,43 where it was mentioned that nickel was effective in enhancing catalyst activity. Besides, selectivity to isomerization increased with nickel content until a maximum was reached at about 15% Ni and then it decreased for higher (42) Yori, J. C.; Parera, J. M. Appl. Catal. 1995, 129, 83-91. (43) Welters, W. J. J.; Waerden, O. H. V.; Beer, V. H. J.; Santen, R. A. Ind. Eng. Chem. Res. 1995, 34, 1166-1171.
606 Energy & Fuels, Vol. 21, No. 2, 2007
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Table 4. n-Octane Isomerization Products Distribution over WT(x,y) Catalystsa 2MC7
3MC7
4MC7
DMC6
TMC5
iC8
catalyst
sel
yield
sel
yield
sel
yield
sel
yield
sel
yield
sel
yield
WT(12,8) WT(12,10) WT(15,8) WT(15,10) WT(15,30) WT(17,8) WT(17,10)
1.78 6.11 9.63 9.06 20.63 7.92 8.14
0.3 1.21 2.57 2.94 2.54 2.84 3.07
8.8 8.9 9.06 11.47 25.55 7.38 7.43
1.78 1.76 2.42 3.73 3.14 2.65 2.80
5.1 3.6 1.39 4.6 7.79 1.7 0.89
1.03 0.71 0.37 1.50 0.96 0.61 0.34
2.61 4.11 2.02 4.63 9.75 1.80 1.93
0.53 0.81 0.54 1.50 1.20 0.65 0.73
2.31 3.18 1.8 2.64 4.88 1.50 1.61
0.47 0.63 0.48 0.86 0.60 0.54 0.61
20.6 25.9 23.9 32.4 68.6 20.3 20.0
4.17 5.13 6.38 10.53 8.44 7.29 7.54
a
Reaction temperature, 250 °C; reduction temperature, 430 °C; and time on stream, 100 min.
contents. In addition, amounts of branched isomers increased with increasing nickel content, in the case of catalysts with tungsten 8%, whereas in the case of catalysts with tungsten 10%. They increased to reach a maximum at 15% Ni and then decreased (Tables 4). Such a catalytic behavior is typical of bifunctional catalysis.44-47 As reported by Sinha and Sivasanker,48 the reason for the initial increase in isomerization selectivity (I/C ratio) with increasing Ni content was the greater availability of Ni sites in the vicinity of acid sites enabling rapid dehydrogenation of the carbenium ions (and olefins) and desorbing them as alkanes before they underwent cracking reactions. In contrast, in the range 15-17% nickel there are enough metallic sites to form olefins for feeding all the acid sites, and further increased metal function prevents the isomerization reaction and increases the rate of cracking reactions. Finaly, it is worthy noting that metallic nickel assures the formation of dissociated hydrogen necessary to generate active acid sites. The availability of the dissociated hydrogen is not only important for the generation of the acid sites but also for the elimination of ionic intermediates from the surface before β-scission events occur, avoiding cracking and polymerization reactions and thus increasing the isomerization selectivity. The same results were observed by Falco et al.38 during their studies on the influence of platinum concentration on tungsten oxide-promoted zirconia during n-hexane isomerization. Effect of Tungsten Loading. The effect of tungsten concentration on the catalytic activity for n-octane isomerization over WT(x,y) catalysts at 250 °C is shown in Figure 3a. The activity was strongly dependent on tungsten concentration. Increasing tungsten concentration led first to an increase in catalytic activity, and then a noticeable activity drop is observed (30% W, catalyst WT(15,30)). This conversion loss can be due to the appearance of an inactive tungsten oxide species. Similar results were reported by Ouafi et al.49 during their studies on the nature and the structure of tungsten surface species present on NiO-WO3/Al2O3 hydrotreating catalysts, where they determined that the limit in the tungsten concentration was at about 40% WO3 and that bulk WO3 was detected after this limit. The same effect was also mentioned by Benitez et al.,50 who reported that the WO3 limit for the WO3/γ-Al2O3 system is between 24 and 30%, and by Hua and Sommer,51 who mentioned that the tungsten limit in the Pt/WOx/ZrO2 catalysts was at about 10%. (44) Perez, M.; Armendariz, H.; Toledo, J. A.; Vasquez, A.; Navarrete, J.; Montoya, A.; Garcia, A. J. Mol. Catal. 1999, 149, 169-178. (45) Leu, L. J.; Hou, L.-Y.; Kang, B.-D.; Li, C.; Wu, S.-T.; Wu, J.-C. Appl. Catal. 1991, 69, 49-63. (46) Demirci, U. B.; Garin, F. Catal. Lett. 2001, 76, 45-51. (47) Chen, J.-K.; Martin, A. M.; Kim, Y. G.; John, V. T. Ind. Eng. Chem. Res. 1988, 27, 401-409. (48) Sinha, A. K.; Sivasanker, S. Catal. Today 1999, 49, 293-302. (49) Ouafi, D.; Mauge, F.; Lavalley, J. C.; Payen, E.; Kasztalan, S.; Houari, M.; Grimblot, J.; Bonnelle, J. P. Catal. Today 1988, 4, 23-37. (50) Benitez, V. M.; Querini, C. A.; Figoli, N. S.; Comelli, R. A. Appl. Catal. 1999, 178, 205-218. (51) Hua, W.; Sommer, J. Appl. Catal. 2002, 232, 129-135.
It was reported that reduced tungsten species possessed activity toward isomerization, which has been attributed to their metallic character.52-55 It was also reported that the acidity and number of Bro¨nsted acid sites increased with the loading of tungsten oxide up to monolayer coverage.56,57 As the surface WOx density increases above monolayer coverage, formation of polytungstate species is responsible for generating Bro¨nsted acid sites via the partial reduction of W6+ Lewis acid centers and delocalization of the negative charge among several oxygen atoms, thus stabilizing carbenium ion intermediates.58-60 These reported results could explain the increase of the catalytic activity with the amount of tungsten oxide. On the other hand, increasing the tungsten loading increased the isomerization selectivity and I/C ratio; this increase was more pronounced at 15% Ni (Figure 3b, R is the I/C ratio and I is the i-C8). In addition, an enhancement in the selectivity toward i-C8 was observed with increasing the tungsten content, the effect being more pronounced at 15% Ni. It was reported that oxygen atoms of WOx species could bond dissociated hydrogen atoms formed in H2 dissociation (by metallic nickel function) or C-H bond activation steps, consequently providing the Bro¨nsted W-OxH acidic sites and thus enhancing the rate of isomerization reaction.51,61,62 According to Meijers et al.,63 catalyst activity and stability are related to both Lewis and Bro¨nsted acid sites. Addition of tungsten oxide increases the Bro¨nsted site density at the expense of Lewis sites while maintaining the density of total sites constant.64 It is known that Lewis sites catalyze favorably cracking reactions associated with the formation of coke.65,66 Thus addition of tungsten increases the catalyst selectivity to isomerization. Furthermore, (52) Katrib, A.; Logie, V.; Saurel, N.; Wehrer, P.; Hilaire, L.; Maire, G. Surf. Sci. 1997, 377-379, 754-758. (53) Katrib, A.; Logie, V.; Peter, M.; Wehrer, P.; Hilaire, L.; Maire, G. J. Chim. Phys. 1997, 94, 1923-1937. (54) Bigey, C.; Hilaire, L.; Maire, G. J. Catal. 1999, 184, 406-420. (55) Katrib, A.; Hemming, F.; Wehrer, P.; Hilaire, L.; Maire, G. J. Electron Spectrosc. Relat. Phenom. 1995, 76, 195-200. (56) Scheithauer, M.; Cheung, T. K.; Jentoft, R. E.; Grasselli, R. K.; Gates, B. C.; Kno¨zinger, H. J. Catal. 1998, 180, 1-13. (57) Baertsh, C. D.; Soled, S. L.; Iglesia, E. J. Phys. Chem. B 2001, 105, 1320-1330. (58) Kuba, S.; Heydorn, P. C.; Grasselli, R. K.; Gates, B. C.; Che, M.; Kno¨zinger, H. Phys. Chem. Chem. Phys. 2001, 3, 146-154. (59) Barton, D. G.; Soled, S. L.; Iglesia, E. Top. Catal. 1998, 6, 87-99. (60) Shimizu, K.; Venkatraman, T. N.; Song, W. Appl. Catal. 2002, 224, 77-87. (61) Finelli, Z. R.; Figoli, N. S.; Comelli, R. A. Catal. Lett. 1998, 51, 223-228. (62) Iglesia, E.; Barton, D. G.; Biscardi, J. A.; Gines, M. J. L.; Soled, S. L. Catal. Today 1997, 38, 339-360. (63) Meijers, S.; Gielgens, L. H.; Ponec, V. J. Catal. 1995, 156, 147153. (64) Soled, S. L.; McVicker, G. B.; Murrell, L. L.; Sherman, L. G.; Dispenziere, N. C.; Hsu, S. L.; Waldman, D. J. Catal. 1988, 111, 286295. (65) Arenamnarta, S.; Trakarnpruk, W. Int. J. Appl. Sci. Eng. 2006, 4, 21-32. (66) Chang, C. D.; Chu, C. T.-W.; Socha, R. F. J. Catal. 1984, 86, 289296.
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Figure 3. (a) Effect of tungsten loading on the activity and selectivity of WT(x,y). A, Catalysts with 12% Ni; B, catalysts with 15% Ni; C, catalysts with 17% Ni. Reaction conditions: reaction temperature ) 250 °C, time on stream ) 100 min, H2/n-octane ratio ) 5, and WHSV ) 4 h-1. (b) Variation of the I/C ratio and iC8 differences with tungsten loading for different Ni amounts. Reaction conditions: reaction temperature ) 250 °C, time on stream ) 100 min, H2/n-octane ratio ) 5, and WHSV ) 4 h-1.
it is believed that the surface residence time of carbenium ions is much shorter in the case of PtWZ catalysts (by comparison with PtSZ), thereby preventing β-fission reactions that lead to acid-catalyzed cracking.67,68 This effect is related to the WOx (x e 2) species that are also present in our catalysts. Besides, the presence of nickel is favorable for improving the tungsten dispersion in catalysts.49 This phenomenon could explain the effect that, at the same tungsten loading, the isomerization level increases with increasing nickel content. Effect of SiO2/Al2O3 Ratio. n-Octane transformation over WT(x,y) catalysts containing the same nickel (15%) and tungsten (10%) contents but having different SiO2/Al2O3 ratios (1.83, 3, 5, and 7) at 250 °C and 100 min TOS is illustrated in Figure 4. These results evidence that an increase in the SiO2/Al2O3 ratio induces a decrease in both conversion and cracking reactions, while the selectivity toward isomerization as well as the I/C ratio increase. Moreover, the amounts of the formed isomers (67) Larsen, G.; Petkovic, L. M. Appl. Catal. 1996, 148, 155-166. (68) Vaudagna, S. A.; Comelli, R. A.; Figoli, N. S. Appl. Catal. 1997, 164, 265-280.
Figure 4. Effect of SiO2/Al2O3 ratio on the conversion and selectivity over WT(15,10) catalysts. Reaction conditions: reaction temperature ) 250 °C, time on stream ) 100 min, H2/n-octane ratio ) 5, and WHSV ) 4 h-1.
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Figure 5. Effect of H2/n-C8 molar ratio on the conversion, selectivity, cracking, and I/C ratio of n-octane treated with WT(15,10) catalyst. Reaction conditions: reaction temperature ) 250 °C, time on stream ) 100 min, and WHSV ) 4 h-1.
decrease with increasing the SiO2/Al2O3 ratio. These effects can be interpreted by a decrease in the number of acid sites available for the cracking reactions. As reported by Ravishankar and Sivasanker,69 there is a general consensus on the existence of a significant electron transfer between the support and the transition metal atoms, and electron depletion occurs in the case of acidic supports. In a given supported metal catalyst, any variation in the acid site content of the support is expected to influence the net electronic property of the metal and hence
changes in activity or selectivity for specific products are likely when SiO2/Al2O3 ratio of the support is changed. Besides, for reactions where migration of intermediates occurs between the metal and acid sites, as in isomerization and cracking, the change in the relative densities of the two species could affect selectivities. Thus in catalysts with high SiO2/Al2O3 ratios, where the acid density is lower, each intermediate olefin formed at a nickel dehydrogenating site comes into contact with very few acid sites between two nickel sites, which could favorably lead to the formation of isomerized products, whereas in catalysts with lower SiO2/Al2O3 ratios, each intermediate olefin comes into contact with more acid sites between two nickel sites and thus the cracking tendency clearly predominates. Effect of H2/n-Octane Molar Ratio. The influence of H2/ n-C8 molar ratio on conversion and I/C ratio, was investigated at 250 °C by varying the hydrogen flow rate and maintaining the hydrocarbon partial pressure constant. As shown in the Figure 5, the conversion of n-octane decreased with increasing the H2/n-C8 molar ratio from 5 to 15 while selectivity to isomers increased (I/C ratio increases). The conversion decrease may be due probably to an increase in total space velocity (n-octane + H2), while the I/C ratio increase may be ascribed to the fact that the isomerization reactions are faster than the cracking ones. In addition, since the cracking of isoalcanes is easier than the cracking of normal alkanes, cracking reactions are more at high conversions where the isoalcanes are present in important quantities. Effect of Space Velocity. The space velocity was varied, at a constant reaction temperature, by proportionally changing the flow of the feed components (H2 and n-C8), so as to keep the molar ratio of H2 to n-C8 at a constant value of 5. As shown in
Scheme 1. n-Octane Isomerization Mechanism
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over Ni-WOx/Al2O3-SiO2 catalysts can be written as in Scheme 1. Conclusions
Figure 6. Effect of space velocity on n-octane hydroconversion. Reaction conditions: reaction temperature ) 250 °C, time on stream ) 100 min, and H2/n-octane ratio ) 5.
Figure 6, the conversion of n-octane decreased with space velocity, while selectivity to isomers and isomerized to cracked n-octane (I/C) ratio increased. From the open literature, it is well-known that cracking reactions are slower than the isomerization ones, thus they will be less favored at higher space velocities (lower residence times). Consequently, the I/C ratio increases with increasing space velocity. This result verifies that the formation of cracking products apparently succeeds the formation of branched isomers. In addition, the ratio of monoto multi-branched isomers (mono/multi) also increased with space velocity, namely, with the conversion of n-octane, which infers that the isomerization first leads to mono-branched isomers mainly and then to multi-branched ones. With regard to the above results, the mechanism of n-octane transformation (69) Ravishankar, R.; Sivasanker, S. Appl. Catal. 1996, 142, 47-59.
In this work the hydroconversion of n-octane over Ni-WOx/ SiO2-Al2O3 catalysts, prepared by the sol-gel method, was investigated. The data collected reveal the following: (i) Conversion decreased with TOS, the higher the activity showed by the catalyst, the more severe is its deactivation. Furthermore, medium acidity has been found to be of paramount importance in activity decay because the strong acidity of the WT(x,y) solids with 17% Ni appeared to be the main reason for the high coking forming propersity as compared to the samples with 15% Ni. On the other hand, the stronger deactivation of cracking reactions as compared to isomerization resulted in a net increase with TOS of octane isomers. (ii) The lower the activity of the prepared samples, the lower the selectivity to cracking and the higher the selectivity to branched octanes. Besides, isomerization occurs in consecutive steps: monobranched isomers are formed in large amounts before dibranched ones appeared. (iii) Nickel induced a benificial effect on the stability of th WT(x,y) catalysts, while their isomerization selectivity increase with Ni content to reach a plateau at about 15% Ni. On the other hand, the catalytic activity was strongly dependent on surface WOx loading. (iv) An increase in SiO2/Al2O3 ratio induced a decrease in both conversion and cracking reactions, while selectivity to isomers as well as I/C ratio increase. In addition, it is suggested that the hydroisomerization selectivity of n-octane is significantly affected by the hydrogen pressure and space velocity. (v) WT(15,10) catalyst appeared to provide optimal balance between acid availability and strength. It is selectivity to isomers amouted to 69% at a conversion of 32.5%. EF060430W
