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Energy & Fuels 2008, 22, 2450–2454
Effect of Solvents on the Hydrogenation and Isomerization of 1-Hexene over Sulfided Co-Mo/γ-Al2O3 Catalysts for Hydrodesulfurization Guojun Shi, Hongying Zhao, Ligang Song, and Jianyi Shen* Lab of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China ReceiVed January 20, 2008. ReVised Manuscript ReceiVed March 26, 2008
The Co-Mo/γ-Al2O3 catalysts promoted with P and Mg were prepared and found to be highly active and stable for the hydrodesulfurization of commercial naphtha. The catalytic behavior of hydrogenation and isomerization of such catalysts were studied using model naphtha. The catalysts were found to be highly active for the hydrodesulfurization (HDS) of thiophene in hexane, but the activity was significantly inhibited by the presence of olefin. The model compound 1-hexene was found to be isomerized (mainly a double-bond shift with a little skeletal isomerization) and hydrogenated directly to n-hexane at the comparable rates over the sulfided Co-Mo/γ-Al2O3 catalysts. Microcalorimetric adsorption of ammonia showed the fairly strong surface acidity for the Co-Mo/γ-Al2O3 catalysts, which might be responsible for the isomerization activities. In addition, the hydrogenation and isomerization of 1-hexene seemed to proceed in a competitive way and were affected strongly by the solvents. The activity of hydrogenation was higher, while that of isomerization was lower for 1-hexene in heptane than in benzene. This result could be explained if it was supposed that the hydrogen transfer was the main pathway for the hydrogenation of olefins, considering that heptane is a better hydrogen donor than benzene. A pathway was also demonstrated in this work that 1-hexene could be first isomerized and then hydrogenated to isoalkanes of higher octane numbers. The other double-bond isomers generated from 1-hexene over the acidic sites were more difficult to be hydrogenated to n-alkanes, which might also reduce the octane number loss of naphtha during the hydrodesulfurization.
Introduction Organic sulfur compounds in gasoline are of serious environmental impact because of the emission of SO2 during the combustion of gasoline. This led to more and more severe regulations to reduce the content of sulfur in gasoline. For example, the European Union (EU) requires the reduction of sulfur in gasoline down to 10 ppmw or less by 2009.1 Commercial gasoline consists of different fractions from reforming, isomerization, alkylation, and fluid catalytic cracking (FCC) units. FCC gasoline, which represents 30-40% of the total gasoline pool, is by far the most important sulfur contributor (up to 85-95%) in commercial gasoline.2 Great efforts have been made to meet the requirements of lowering sulfur content while maintaining high octane numbers of gasoline. Two different strategies are often adopted: selective hydrodesulfurization (HDS), which preserves the octane number of gasoline, and deep desulfurization followed by octane number recovery through the isomerizations of alkanes. Selective HDS reduces the loss of octane number by avoiding the hydrogenation of olefins, but it might not meet the requirement of the EU 2009 that restricts olefins to 18% or even less. The deep HDS with subsequent octane recovery imposes disadvantages of high* To whom correspondence should be addressed: Lab of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China. Telephone/Fax: +86-25-83594305. E-mail:
[email protected]. (1) Brunet, S.; Mey, D.; Pe´rot, G.; Bouchy, C.; Diehl, F. Appl. Catal., A 2005, 278, 143–172. (2) Kaufmann, T. G.; Kaldor, A.; Stuntz, G. F.; Kerby, M. C.; Ansell, L. L. Catal. Today 2000, 62, 77–90.
energy consummation and low liquid yields. Conversion of linear olefins to branched ones followed by hydrogenation to isoalkanes would minimize the octane number loss.3 Additives were added to enhance the performances of the traditional Co-Mo/γ-Al2O3 catalysts for the HDS of gasoline.4–7 Hatanaka et al. found that the addition of K+ inhibited coke deposition and improved the stability of HDS catalysts.4 Predeposition of carbon was claimed to have the effect of selective HDS of FCC naphtha.5 Klimova et al. added magnesia into the alumina support for Mo catalysts and found that the hydrogenation capacity of the catalysts was substantially reduced, while the activity for the HDS of thiophene was much less affected.6 Prins et al. believed that the addition of phosphoric acid led to the formation of amorphous aluminum phosphate that weakened the interaction between Mo and the support and thus increased the active type II Co-Mo-S phase.7 Numerous studies have been devoted to the HDS of thiophene and other organic sulfurs over sulfided catalysts, and some reviews were published in the past 5 years.7–10 However, only (3) Li, D.; Li, M.; Chu, Y.; Nie, H.; Shi, Y. Catal. Today 2003, 81, 65–73. (4) Hatanaka, S.; Sadakane, O.; Hikita, S.; Miyama, T. Process for desulfurizing catalytically cracked gasoline. U.S. Patent 5,853,570, Dec 29, 1998. (5) Hatanaka, S.; Yamada, M.; Sadakane, O. Ind. Eng. Chem. Res. 1998, 37, 1748–1754. (6) Klimova, T.; Casados, D. S.; Ramı´rez, J. Catal. Today 1998, 43, 135–146. (7) Sun, M.; Nicosia, D.; Prins, R. Catal. Today 2003, 86, 173–189. (8) Topsøe, H. Appl. Catal., A 2007, 322, 3–8. (9) Kelty, S. P.; Berhault, G.; Chianelli, R. R. Appl. Catal., A 2007, 322, 9–15.
10.1021/ef800046f CCC: $40.75 2008 American Chemical Society Published on Web 05/07/2008
Co-Mo/γ-Al2O3 Catalysts for Hydrodesulfurization
limited studies dealt with the hydrogenation and isomerization of olefins over sulfided catalysts.11–17 Meerbott et al. found that n-olefins seemed to saturate faster than branched ones.11 Tanaka et al. suggested that mono- (MoH) and dihydride sites (MoH2) were formed on the edge of surface MoS2 in the presence of H2, which catalyzed the isomerization and hydrogenation reactions, respectively.13,14 In this work, we prepared Co-Mo/ γ-Al2O3 catalysts promoted by P and Mg. The catalysts were tested for the hydrodesulfurization of thiophene as well as the hydrogenation and isomerization of 1-hexene. The main purpose of this work was to study the effect of solvents on the hydrogenation and isomerization of olefins over the sulfided Co-Mo/γ-Al2O3 catalysts using 1-hexene as a probe molecule.
Energy & Fuels, Vol. 22, No. 4, 2008 2451 Table 1. Physical Properties of the Co-Mo/γ-Al2O3 Catalysts catalysta
surface area (m2/g)
pore volume (cm3/g)
average pore size (nm)
S1 used S1 (1200 h) regenerated S1 S2
204 176 211 181
0.47 0.34 0.46 0.47
6.57 5.91 6.61 7.20
a S and S denote the Co-Mo/γ-Al O catalysts in column and 1 2 2 3 trifolium shapes, respectively. Used S1 (1200 h) was the Co-Mo/ γ-Al2O3 catalyst in column shape after the hydrodesulrization of a commercial naphtha at 513 K for 1200 h. Regenerated S1 was the catalyst of used S1 regenerated at 773 K in air for 8 h.
Experimental Section Catalyst Preparation. The support was prepared by kneading γ-Al2O3, H3PO4, and MgO in the desired ratio, followed by extruding into column and trifolium shapes. The support was dried at 423 K for 6 h. The Co-Mo/γ-Al2O3 catalysts were prepared by impregnating the support with the aqueous solutions of cobalt nitrate and ammonium heptamolybdate followed by drying at 383 K for 12 h and calcination at 773 K in air for 3 h. The catalysts prepared contained 1.2% CoO, 7.9% MoO3, 1.8% P2O5, and 0.6% MgO by weight. Catalyst Characterization. X-ray diffraction (XRD) patterns were collected in an ambient atmosphere by a Philips X’Pert powder diffractometer with Cu KR radiation (λ ) 1.5418 Å). The 2θ scans covered the range of 10-80°, with a step of 0.02°. The applied voltage and current were 40 kV and 40 mA, respectively. The Brunauer-Emmett-Teller (BET) surface areas and pore sizes were measured by the ASAP 2020 instrument. Experiments were performed at 77.3 K using N2 as the adsorbate. The pore sizes were calculated from the desorption isotherm branch by means of the Barret-Joyner-Halenda (BJH) method.18 Microcalorimetric measurements of ammonia adsorption were performed to determine the surface acidity of the catalysts at 423 K. A C-80 calorimeter (Setaram, France) was connected to a volumetric system equipped with a Baratron capacitance manometer (Andover, MA) for the pressure measurement and gas handling. Prior to the microcalorimetric adsorption of ammonia, the catalysts were activated at 673 K in 500 Torr O2, followed by evacuation at the same temperature for 1 h. After the thermal equilibrium was reached, heats of adsorption were measured when doses of NH3 were admitted sequentially. Products from some catalytic runs were analyzed by a gas chromatograph (DB-5 column with 30 m × 0.32 mm and 0.25 µm film thickness) connected to a mass spectrograph (GC-MS). The column temperature was linearly programmed from 305 to 523 K, at a rate of 10 K/min. A mass selective detector (EI ) 70 eV) was used, and the NIST+ library was used for the identification of MS signals. The research octane number (RON) was determined using a standard test engine according to American Society for Testing and Materials (ASTM) D-2699. Catalytic Tests. Fixed-bed flowing reactors were used for the catalytic tests. About 5 mL (4 g) of a sample was loaded in the (10) Song, C. Catal. Today 2003, 86, 211–263. (11) Meerbott, W. K.; Hinds, G. P. Ind. Eng. Chem. 1955, 47, 749– 752. (12) Yin, C.; Zhao, R.; Liu, C. Fuel 2005, 84, 701–706. (13) Tanaka, K. Appl. Catal., A 1999, 188, 37–52. (14) Tanaka, K.; Takehiro, N. J. Mol. Catal. A: Chem. 1999, 141, 39– 55. (15) He´doire, C.; Louis, C.; Davidson, A.; Breysse, M.; Mauge´, F.; Vrinat, M. J. Catal. 2003, 220, 433–441. (16) Toba, M.; Miki, Y.; Matsui, T.; Harada, M.; Yoshimura, Y. Appl. Catal., B 2007, 70, 542–547. (17) Goddard, S. A.; Kukes, S. G. Energy Fuels 1994, 8, 147–150. (18) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373–379.
Figure 1. Typical hysteresis of adsorption-desorption isotherm for N2 adsorption at the liquid nitrogen temperature (a) and the corresponding pore size distribution (b) for the Co-Mo/γ-Al2O3 catalyst (S2).
microreactor (10 mm i.d.) for catalytic tests of model naphtha, while about 150 mL (120 g) of a catalyst was packed in a bench pilot reactor (40 mm i.d.) for the HDS of commercial naphtha. The Co-Mo/γ-Al2O3 catalysts were presulfided at 503 K for 2 h and then 593 K for 4 h in the naphtha stream containing 2% CS2. The feeds were then introduced into the catalyst beds with hydrogen. The reactions were performed at the temperatures from 433 to 573 K, a pressure of 1.5 MPa, a liquid hourly space velocity (LHSV) of 2 h-1, and an H2/feed ratio of 300 (v/v). Reaction products were analyzed by a gas chromatograph equipped with a flame ionization detector (FID) for hydrocarbons and a flame photometric detector (FPD) for organic sulfurs.
Results and Discussion Physical Properties of the Catalysts. The surface areas and pore structures of the catalysts are summarized in Table 1. Catalysts in different shapes were studied (S1 in column shape and S2 in trifolium shape). The two fresh catalysts possessed similar surface area (181 and 204 m2/g), pore volume (0.47 mL/ g), and pore size (6.52 and 7.20 nm). After a 1200 h reaction of hydrodesulfurization for commercial naphtha at 513 K, the surface area, pore volume, and pore size were all significantly decreased. These properties were resumed after the regeneration, indicating the pore blockage by the deposition of carbonaceous species during the reaction. Figure 1 shows that the Co-Mo/ γ-Al2O3 catalyst (S2) exhibited the type IV isotherm, with capillary condensation steps occurring at a partial pressure
2452 Energy & Fuels, Vol. 22, No. 4, 2008
Shi et al. Table 2. Typical Properties of FCC Naphtha Used for the Bench Pilot Reaction of Hydrodesulfurization in This Work
Figure 2. XRD patterns of the Co-Mo/γ-Al2O3 catalysts: (a) fresh S1, (b) fresh S2, (c) used S1, and (d) regenerated S1. Refer to Table 1 for definitions.
Figure 3. Differential heat versus coverage for NH3 adsorption at 423 K on the Co-Mo/γ-Al2O3 catalysts: fresh S1 (0), fresh S2 (9), used S1 (b), and regenerated S1 (O). Refer to Table 1 for definitions.
corresponding to pore sizes between 4 and 11 nm. The pore maximum was observed around 7.0 nm for this catalyst (S2). Figure 2 presents the XRD patterns for the fresh, used, and regenerated Co-Mo/γ-Al2O3 catalysts in column and trifolium shapes (S1 and S2). The base lines were noisy, and the peaks were not intensive for all of the XRD patterns, indicating that no crystalline phases were present in the catalysts. The strongest peaks could be assigned to γ-Al2O3 support. No diffraction peaks were observed for Co3O4 and MoO3 in the fresh catalysts (S1 and S2), indicating that the metal oxide precursors were welldispersed on the support. In addition, no diffraction peaks were observed for metal sulfides in the catalyst (S1) after the reaction of hydrodesulfurization. Finally, no diffraction peaks were observed for metal oxides even in the catalyst (S1) after the regeneration. These results indicated that the metal species were highly dispersed on the support, and they were stable upon the reaction of hydrodesulfurization and regeneration. Surface Acidity of the Catalysts. Microcalorimetric adsorption of NH3 was used to measure the surface acidity in terms of the strength and number of acidic sites on the Co-Mo/γAl2O3 catalysts. Figure 3 shows the results. The two fresh catalysts (S1 and S2) exhibited the similar surface acidity as revealed by the initial heats (∼150 kJ/mol) and coverage (∼620 µmol/g) for the adsorption of ammonia. After the reaction of hydrodesulfurization for 1200 h, the surface acidity of the used catalyst (S1) decreased significantly. The initial heat and coverage for the ammonia adsorption were decreased to 120 kJ/mol and 440 µmol/g, respectively, on the used catalyst. It is interesting to note that the surface acidity was completely restored after the regeneration. These results indicated that some surface acidic sites might be covered by carbonaceous species deposited during the reaction of hydrodesulfurization, agreeing with the characterization results of surface areas and pore structures. Catalytic Hydrodesulfurization in Bench Pilot Reactor. The typical properties of FCC naphtha are shown in Table 2. It
distillation temperature (K)
yield (vol %)
density (kg/m3)
total sulfur (µg/g)
olefin (vol %)
aromatics (vol %)
IBP-333 333-353 353-373 373-393 393-413 413-433 433-EP full fraction
32.76 8.81 13.34 7.33 11.95 10.99 12.82 98
654.5 690.2 724.5 746.7 775.9 798.8 830.5 735.6
439 452 831 955 1168 1179 2177 910
57.2 53.5 46.2 33.7 17.4 5.9 1.8 37.6
1.37 5.2 9.6 17.2 35.3 51 31 17.6
is important to have the detailed information about the distribution of sulfur contents with respect to distillation temperatures. Data in Table 2 show that the total sulfur content in the FCC naphtha increases with the distillation temperature. Yin et al. confirmed that thiophene sulfur represented a large fraction (60% and over) of the total sulfur in FCC naphtha.19 More aromatics and less olefins were found in the fractions with higher boiling points. Thus, the octane number loss may be reduced mainly by isomerization and aromatization of the low boiling fractions. Table 3 compares the properties of naphtha before and after the hydrodesulfurization by using the Co-Mo/γ-Al2O3 catalyst. The sulfur content in the naphtha decreased from 910 to 111 ppmw (87.7% conversion) after the HDS process over S2 (with trifolium shape). At the same time, as much as 29.3% of olefins were saturated during the process, with only 0.6 unit loss of the RON. Similar results were obtained for S1 (with column shape). Considering the significant differences of olefin and paraffin in RON, this result seemed to imply that the isomerization of olefins might occur during the HDS that offsets the octane number loss because of the saturation of olefins. The isomerization of olefin over the sulfided Co-Mo/γ-Al2O3 catalysts was addressed using 1-hexene as a model reactant in this work, and the results were presented below. Figure 4 shows the long-term run of the Co-Mo/γ-Al2O3 catalyst (S2) in a bench pilot reactor for the hydrodesulfurization of the commercial naphtha. It is clearly seen that the activity of the catalyst was stable and the sulfur level remained lower than 125 ppmw for the long period of 2500 h at 513 K. These results indicated that the Co-Mo/γ-Al2O3 catalysts studied in this work behaved well for the HDS of commercial naphtha. It is possible that the sulfur content might be further decreased to 50 ppmw or even lower at higher reaction temperatures. This has been confirmed in the HDS reactions of model naphtha. HDS of Thiophene in Model Naphtha. The Co-Mo/γAl2O3 catalyst (S2) was then tested for the hydrodesulfurization of thiophene in model naphtha using the microreactor. The model naphtha was composed of thiophene with 500 ppmw of sulfur and some 1-heptene in hexane. Figure 5 showed that the catalyst displayed high activity for the degradation of thiophene. For example, the sulfur level was decreased from 500 ppmw to 24 and 6 ppmw for the hydrodesulfurization of thiophene at 493 and 533 K, respectively, when hexane was used as the model naphtha. The addition of 1% 1-heptene to hexane reduced the effectiveness of HDS of thiophene; the sulfur level was increased from 6 to 17 ppmw after the reaction at 533 K. This effect was more evident when 20% 1-heptene was added. The sulfur level was found to be 142 ppmw after the HDS of thiophene at 493 K in hexane containing 20% 1-heptene. These results indicated that the presence of olefin inhibited the hydrogenation of thiophene, agreeing with the results obtained (19) Yin, C.; Zhu, G.; Xia, D. Prepr. Pap.-Am. Chem. Soc., DiV. Pet. Chem. 2002, 47 (4), 398–401.
Co-Mo/γ-Al2O3 Catalysts for Hydrodesulfurization
Energy & Fuels, Vol. 22, No. 4, 2008 2453
Table 3. Properties of Naphtha before and after the Hydrotreatment for 500 h over the Co-Mo/γ-Al2O3 Catalyst in the Bench Pilot Reactora FCC naphtha hydrotreatment S1 S2 a
before after before after
total sulfur (µg/g) 1047 126 910 111
sulfur conversion (%)
olefin saturation (%)
88.0
28.6
87.8
29.3
RON
alkanes (vol %)
olefins (vol %)
aromatics (vol %)
97.6 96.7 90.4 89.8
20.1 35.6 34.4 48.9
49.0 35.0 46.7 33.0
30.9 29.4 19.1 18.0
Reaction conditions: P, 1.6 MPa; T, 513 K; LHSV, 2.7 h-1; and H2/feed ratio, 500 (v/v).
Table 4. Products from the Hydrogenation and Isomerization of 1-Hexene over the Sulfided Co-Mo/γ-Al2O3 (S2) in the Microreactor with Different Solvents Containing Thiophene with 500 ppmw of Sulfura solvent
Figure 4. Sulfur level analyzed for the naphtha stream exiting out of the bench pilot reactor in a long-term run of the sulfided Co-Mo/γAl2O3 (S2). Reaction conditions: P, 1.6 MPa; T, 513 K; LHSV, 2.7 h-1; and H2/feed ratio, 500 (v/v).
Figure 5. Sulfur level analyzed for the stream of model naphtha exiting from the microreactor loaded with the sulfided Co-Mo/γ-Al2O3 catalyst (S2). The model naphtha containing thiophene with 500 ppmw of sulfur in hexane only (b), hexane with 1% 1-heptene (2), and hexane with 20% 1-heptene (9). Reaction conditions: P, 1.5 MPa; T, 493-573 K; LHSV, 2 h-1; and H2/feed ratio, 300 (v/v).
al.20
by Hatanaka et They ascribed the inhibition effect of olefin to the adsorption of olefin on the active sites. Effect of Solvents on the Hydrogenation of 1-Hexene. 1-Hexene may undergo reactions of hydrogenation, isomerization, aromatization, cracking, and polymerization over sulfided catalysts. Linear olefins might be isomerized first and then hydrogenated to isoalkanes. Model feeds containing 20% 1-hexene and 80% n-heptane or 80% benzene with thiophene (500 ppmw of sulfur) were used to investigate the effect of solvents on the hydrogenation and isomerization of 1-hexene. Table 4 gives the results. The main hydrogenation product of 1-hexene was n-hexane (∼82%), indicating that the direct hydrogenation of 1-hexene was the main pathway. The products from the double-bond shift of 1-hexene, skeletal isomerization of 1-hexene, hydrogenation of isomerized hexene, and cracking of hexene were also observed. These results are consistent with those reported by Meerbott et al.,11 who found that the leastbranched olefins saturated faster than the more branch ones over sulfided catalysts. Our experimental results showed no evidence of isomerization of n-hexane at the same conditions. Thus, the (20) Hatanaka, S.; Yamada, M.; Sadakane, O. Ind. Eng. Chem. Res. 1997, 36, 1519–1523.
product
heptane
benzene
2-methyl-2-butene 3-methyl-2-pentene 2-methyl-pentane 3-methyl-pentane 1-hexene 2-hexene 3-hexene n-hexane
0 0.12 0.11 0.32 6.22 3.94 7.48 81.81
0.24 0.16 0.16 0.41 1.42 4.63 10.86 82.12
a Reaction conditions: P, 1.5 MPa; T, 533 K; LHSV, 2 h-1; and H / 2 feed ratio, 300 (v/v).
Figure 6. Hydrogenation of 1-hexene over the sulfided Co-Mo/γ-Al2O3 (S2) in the microreactor with different solvent, benzene (9) or heptane (b), containing thiophene with 500 ppmw of sulfur. Reaction conditions: P, 1.5 MPa; LHSV, 2 h-1; and H2/feed ratio, 300 (v/v).
isoalkanes were produced from the hydrogenation of skeletally isomerized hexene. These results suggested that linear olefins could be first isomerized to branch ones followed by hydrogenation to isoalkanes over the sulfided catalysts, leading to the reduced loss of the octane number caused by the direct hydrogenation of olefins. Figure 6 compares the effect of solvents on the hydrogenation of 1-hexene over the sulfided Co-Mo/γ-Al2O3 (S2) at different temperatures. It was clearly seen that more 1-hexene were converted into alkanes in n-heptane than in benzene at the temperatures from 433 to 533 K. For example, about 53 and 72% of 1-hexene were converted into alkanes at 493 K when benzene and heptane were used as solvents, respectively. This could be explained by the different propensities of heptane and benzene in hydrogen-transfer reactions. Apparently, heptane is a better hydrogen donor than benzene, and it was found that the hydrogen-transfer reaction was the main pathway for the hydrogenation of olefins.21,22 A comparison of the data in Table 3 and Figure 6 indicated that the activity of hydrogenation of olefins at 513 K was much (21) Imamura, H.; Noda, Y.; Sakata, Y.; Tsuchiya, S. J. Alloy Compd. 2001, 323-324, 601–604. (22) Talukdar, A. K.; Bhattacharyya, K. G.; Baba, T.; Ono, Y. Appl. Catal., A 2001, 213, 239–245.
2454 Energy & Fuels, Vol. 22, No. 4, 2008
Figure 7. Isomerization of skeletal and double-bond shift of 1-hexene over the sulfided Co-Mo/γ-Al2O3 (S2) in the microreactor with different solvent, benzene (9) or heptane (b), containing thiophene with 500 ppmw of sulfur. Reaction conditions: P, 1.5 MPa; LHSV, 2 h-1; and H2/feed ratio, 300 (v/v).
higher for 1-hexene than for olefins in FCC naphtha. Such difference could be ascribed to the different hydrogenation activities for the terminal, nonterminal, linear, and branch olefins.11,23 The linear and terminal olefins, such as 1-hexene, are easier to be hydrogenated than the nonterminal and branch ones because the former exhibit less steric hindrances. Analysis showed that the ordinary FCC naphtha contained about 25% terminal olefins in the total olefin fraction.20 Effect of Solvents on the Isomerization of 1-Hexene. Deeper desulfurization requires more severe conditions that lead to more saturated olefins and thus more octane number losses. Isomerization of olefin and paraffin, especially skeletal isomerization, is an effective approach to decrease the octane number loss. Data in Table 4 show that the skeletal isomerization did occur, although it was in the low percentage. Figure 7 shows that the isomerization of 1-hexene could reach 60% over the Co-Mo/γ-Al2O3 (S2) at 453 K in benzene. The isomerization of 1-hexene might be catalyzed by the acid sites on the surface of the Co-Mo/γ-Al2O3 catalysts,22,24 as revealed by the microcalorimetric adsorption of ammonia (Figure 3). Figure 7 also shows that the activity of isomerization of 1-hexene was significantly higher in benzene than in heptane in the whole range of reaction temperatures used. This result might be explained by the competition reactions between hydrogenation and isomerization of 1-hexene over the sulfided Co-Mo/γAl2O3 catalysts. The more active hydrogenation reaction would definitely inhibit the isomerization. It has been mentioned above that the hydrogenation of olefins usually proceeds via the (23) Aad, E. A.; Rives, R.; Hubaut, R.; Aboukaı¨s, A. J. Mol. Catal. A: Chem. 1997, 118, 255–260. (24) Di-Gre´gorio, F.; Keller, V.; Di-Costanzo, T.; Vignes, J. L.; Michel, D.; Maire, G. Appl. Catal., A 2001, 218, 13–24.
Shi et al.
hydrogen transfer from hydrogen donors. Because heptane is a better hydrogen donor than benzene, hydrogenation of 1-hexene might be faster in heptane than in benzene, leading to the relatively lower isomerization activity of 1-hexene in heptane than in benzene. Because the percentage of skeletal isomerization was only about 1%, the double-bond shifts of 1-hexene were the main isomerization processes. The hydrogenation of the olefins produced from the double-bond shifts of 1-hexene was not as easy as 1-hexene, and they have higher octane numbers than 1-hexene.12 Thus, these olefins tended to remain in the products from the hydrodesulfurization of naphtha, so that the octane number loss might be reduced. Conclusions The Co-Mo/γ-Al2O3 catalysts with high activity and stability for the hydrodesulfurization of commercial naphtha were prepared and studied in terms of their catalytic behavior for the hydrogenation and isomerization reactions of model naphtha. The catalysts were found to be highly active for the HDS of thiophene in hexane. However, the activity for the hydrogenation of thiophene was significantly decreased by the presence of olefin. The model compound 1-hexene was found to be easier to isomerize at lower temperatures, while it was easier to hydrogenate directly to n-hexane at elevated temperatures over the sulfided Co-Mo/γ-Al2O3 catalysts. The isomerizations with both skeletal (∼1%) and double-bond shifts were observed. The surface acidity of the Co-Mo/γ-Al2O3 catalysts might be responsible for the isomerization activities. In addition, the hydrogenation and isomerization of 1-hexene seemed to proceed in a competitive way and were affected strongly by the solvent. The activity of hydrogenation was higher, while that of isomerization was lower for 1-hexene in heptane than in benzene. This result could be explained if it was supposed that the hydrogen transfer was the main pathway for the hydrogenation of olefins, considering that heptane is a better hydrogen donor than benzene. A pathway was also demonstrated in this work that 1-hexene could be first isomerized and then hydrogenated to isoalkanes of higher octane numbers. The other double-bond isomers generated from 1-hexene over the acidic sites were more difficult to be hydrogenated to n-alkanes, which might also reduce the octane number loss of naphtha during the hydrodesulfurization. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20673055) and the High Tech. Program of the Jiangsu Province of China (BG2006031). EF800046F
