Co-MCM-41

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Energy & Fuels 2009, 23, 320–326

Skeletal Isomerization of 1-Hexene over Sulfided Co/Co-MCM-41 Catalysts Guojun Shi and Jianyi Shen* Laboratory of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China ReceiVed July 28, 2008. ReVised Manuscript ReceiVed October 14, 2008

Co-MCM-41 catalysts were synthesized and used to support the additional cobalt. Microcalorimetric adsorption of NH3 showed that the Co-MCM-41 and 5%Co/5%Co-MCM-41 exhibited stronger surface acidity than 5%Co/ MCM-41. The surface acidity was greatly enhanced upon the sulfidation. The 5%Co/5%Co-MCM-41 after sulfidation possessed strong surface acidity with an initial heat of about 180 kJ/mol and coverage of about 1060 µmol/g for the adsorption of ammonia. The catalytic tests showed that little skeletal isomerization occurred on the Co-Mo/γ-Al2O3, whereas it occurred substantially over the MCM-41 related catalysts. Over 60% skeletal isomerizations were observed for the 5%Co/5%Co-MCM-41 at 573 K. In addition, a combination of the Co/ Co-MCM-41 with a traditional Co-Mo/γ-Al2O3 catalyst showed not only the high hydrodesulfurization activity but also converted significant amounts of 1-hexene into branched hexanes. Thus, this technique might be used to compensate the loss of octane number of gasoline during its hydrodesulfurization by first converting linear olefins to branched ones followed by hydrogenating to branched alkanes.

Introduction Increasingly stringent legislative restrictions to require the production of low-sulfur gasoline impose high pressures on the developments of better hydrodesulfurization (HDS) catalysts.1 This includes not only high desulfurization capability but also skeletal isomerization function because the saturation of olefins of fluid catalytic cracking (FCC) naphtha during HDS brings about severe octane number losses. Conversion of linear olefins to branched ones followed by hydrogenating to isoalkanes would minimize the octane number losses.2 The olefins containing 5-7 carbon atoms contribute about 70% of olefins in FCC naphtha, and most of them are in linear or monosubstituted types.2 The use of 1-hexene as a probe molecule facilitates the analysis of products for the reactions of HDS and isomerizations. It is generally believed that the skeletal isomerization of a hydrocarbon over acidic sites was through the formation of protonated cyclopropane species.3,4 Moderate surface acidity, temperature, and residence time are favorable for the skeletal isomerizations of olefins.3 Numerous studies have been devoted to the HDS of thiophene and other organic sulfurs over sulfided catalysts, but only a limited number of studies have dealt with the isomerization and hydrogenation of olefins over sulfided catalysts, on which linear olefins mainly underwent the isomerization of a double bond shift with only minor skeletal rearrangements.1,5-8 Some groups studied the * To whom correspondence should be addressed. Telephone and 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) Li, D.; Li, M.; Chu, Y.; Nie, H.; Shi, Y. Catal. Today 2003, 81, 65–73. (3) Keller, V.; Barath, F.; Maire, G. J. Catal. 2000, 189, 269–280. (4) Sundaramurthy, V.; Eswaramoorthi, I.; Lingappan, N. Can. J. Chem. 2004, 82, 631–640. (5) Meerbott, W. K.; Hinds, G. P. Ind. Eng. Chem. 1955, 47, 749–752. (6) Toba, M.; Miki, Y.; Matsui, T.; Harada, M.; Yoshimura, Y. Appl. Catal., B 2007, 70, 542–547.

acidity of sulfided catalysts.9-12 Tanaka et al. suggested that the mono- (MoH) and dihydride sites (MoH2) were formed on the edges of MoS2 surfaces in H2, which catalyzed the isomerization and hydrogenation reactions, respectively.9,10 Alumina is usually used as a support of Co-Mo-S for HDS. It has been established that two types of species might exist in Co-Mo/γ-Al2O3 catalysts. Type I was the Co-Mo-S species that had Mo-O-Al linkages due to the strong interactions with the alumina support. Thus, this species was not completely sulfided and hence not active. In contrast, type II was the Co-Mo-S species that did not strongly interact with the support and was the active species for HDS.13 As compared to alumina, silica is a support with weaker interactions with molybdenum.9,13 In addition, mesoporous silica, like MCM-41 and SBA-15, with uniform mesopores and high surface areas, may facilitate the dispersion of active phases and the diffusion of reactants. Moreover, metal cations can be incorporated into the framework of such mesoporous silica. For example, Lim et al. synthesized Co-MCM-41 (cobalt in framework of MCM-41) for the production of single-walled carbon nanotubes.14 Corte´s et al. used CoMCM-41 for the oxidative dehydrogenation of isobutane.15 The Co-MCM-41 materials reported in the literature were usually prepared by the hydrothermal method with temperatures higher than 373 K. In addition, these Co-MCM-41 materials usually (7) Goddard, S. A.; Kukes, S. G. Energy Fuels 1994, 8, 147–150. (8) Yin, C.; Zhao, R.; Liu, C. Fuel 2005, 84, 701–706. (9) Tanaka, K. Appl. Catal., A 1999, 188, 37–52. (10) Tanaka, K.; Takehiro, N. J. Mol. Catal. A: Chem. 1999, 141, 39– 55. (11) He´doire, C.; Louis, C.; Davidson, A.; Breysse, M.; Mauge´, F.; Vrinat, M. J. Catal. 2003, 220, 433–441. (12) Gaborit, V.; Allali, N.; Danot, M.; Geantet, C.; Gattenot, M.; Breysse, M.; Diehl, F. Catal. Today 2003, 78, 499–505. (13) Topsøe, H. Appl. Catal., A 2007, 322, 3–8. (14) Lim, S.; Ciuparu, D.; Pak, C.; Dobek, F.; Chen, Y.; Harding, D.; Pfefferle, L.; Haller, G. J. Phys. Chem. B 2003, 107, 11048–11056. (15) Corbera´n, V. C.; Jia, M. J.; El-Haskouri, J.; Valenzuela, R. X.; Beltra´n-Porter, D.; Amoro´s, P. Catal. Today 2004, 91-92, 127–130.

10.1021/ef8005993 CCC: $40.75  2009 American Chemical Society Published on Web 12/03/2008

Skeletal Isomerization of 1-Hexene

had the low cobalt content (623 K), at which significant amounts of cracking product were produced.18-21 To the best of our knowledge, no studies have been reported so far for the isomerization of 1-hexene over CoMCM-41. Experimental Section Catalyst Preparation. Following is a typical way we used for the synthesis of Co-MCM-41 materials. Sodium metasilicate and cetyltrimethylammonium bromide (CTAB) were dissolved in hot water to form a solution. After the solution was cooled to 313 K, an appropriate amount of solution containing hexamminecobalt nitrate was added dropwise. The pH value of the mixture was adjusted to 9.0-9.5 by the addition of 1 M sulfuric acid. The mixture was then aged for 72 h at 313 K. The precipitate was isolated by centrifugation. The isolated precipitate was washed several times with deionized water and EtOH and then dried at room temperature. A series of samples with different contents of cobalt were synthesized this way. The samples were calcined at 823 K for 5 h in flowing air. The Co-MCM-41 samples prepared here are designated as XCo-MCM-41, where X is the Co/Si atomic ratio desired for the framework composition. A siliceous MCM41 was also prepared in the same way. The supported catalysts: 5%Co/MCM-41, 10%Co/MCM-41, 5%Co/5%Co-MCM-41, and Co-Mo/5%Co-MCM-41 were prepared by the incipient wetness impregnation method using the aqueous solutions of cobalt nitrate and ammonium heptamolybdate. The atomic ratios of Co/Mo ) 0.75 and (Co+Mo)/Si ) 0.10 were applied for the Co-Mo/5%CoMCM-41. The impregnated samples were dried at 333 K overnight and calcined in air at 773 K for 3 h. Catalyst Characterization. X-ray diffraction (XRD) patterns were collected at ambient atmosphere by a Philips X’Pert powder diffractometer with the Cu KR radiation (λ )1.5418 Å). For the low-angle XRD, the 2θ scans covered the range of 0.5 to 8° with a step of 0.02°, whereas for the wide-angle XRD, the 2θ scans covered the range of 10 to 80° with a step of 0.4°. The applied voltage and current were 40 kV and 40 mA, respectively. The BET surface areas and pore sizes were measured with an ASAP 2020 instrument. Experiments were performed at 77.3 K using N2 as the adsorbate. The average pore sizes were calculated according to the BET adsorption isotherms. (16) Vra˚lstad, T.; Øye, G.; Rønning, M.; Glomm, W. R.; Sto¨cker, M.; Sjo¨blom, J. Microporous Mesoporous Mater. 2005, 80, 291–300. (17) Karthik, M.; Tripathi, A. K.; Gupta, N. M.; Vinu, A.; Hartmann, M.; Palanichamy, M.; Murugesan, V. Appl. Catal., A 2004, 268, 139–149. (18) Logie, V.; Maire, G.; Michel, D.; Vignes, J. L. J. Catal. 1999, 188, 90–101. (19) Di-Gre´gorio, D.; Keller, V.; Di-Costanzo, T.; Vignes, J. L.; Michel, D.; Maire, G. Appl. Catal., A 2001, 218, 13–24. (20) Keane, M. A.; Alyea, E. C. J. Mol. Catal. A 1996, 106, 277–285. (21) Barath, F.; Turki, M.; Keller, V.; Maire, G. J. Catal. 1999, 185, 1–11. (22) Aad, E. A.; Rives, A.; Hubaut, R.; Aboukaı¨s, A. J. Mol. Catal. A 1997, 118, 255–260. (23) Talukdar, A. K.; Bhattacharyya, K. G.; Baba, T.; Ono, Y. Appl. Catal., A 2001, 213, 239–245. (24) Clark, M. C.; Subramaniam, B. Chem. Eng. Sci. 1996, 51, 2369– 2377.

Energy & Fuels, Vol. 23, 2009 321 The transmission electron microscopic (TEM) measurements were carried out using a JEOL electron microscope (JEM-2010), with an accelerating voltage of 200 keV. The UV-vis spectra were recorded by diffuse reflectance on a Cintra10e UV-vis spectrometer equipped with a diffuse reflectance attachment. BaSO4 was used as a reference. All spectra were recorded at room temperature under ambient atmosphere. A sample holder was used to support the wafer of a sample with about 100 mg. Elemental analysis was carried out on an X-ray fluorescence (XRF) ARL-9800 instrument. The samples were pressed into pellets before the analysis. Temperature programmed reduction (TPR) was performed by using a quartz U-tube reactor loaded with about 50 mg of a sample. A mixture of N2 and H2 (5.13% H2 by volume) was used as the reducing agent, and the flow rate was maintained at 40 mL/min. The temperature was raised at a programmed rate of 10 K/min from 303 to 1273 K. The microcalorimetric adsorption of ammonia was measured 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 (USA) for the pressure measurement and gas handling. The samples were evacuated at 573 K for 3 h before loaded into the calorimeter unit. After the thermal equilibrium was reached, heats of adsorption were measured when doses of NH3 were admitted sequentially. Samples were sulfided ex situ in a stainless steel reactor using 2% CS2 in heptane, first at 503 K for 2 h and then at 593 K for 4 h. The samples were cooled to room temperature in H2 before they were exposed to air. Then, the samples were evacuated at 573 K for 3 h to remove the possibly remained CS2 and heptane. The NH3 adsorption IR spectra were recorded on a Bruker Vector 22 FTIR spectrophotometer with a range of 4000-400 cm-1, a resolution of 2 cm-1, and 40 acquisition scans. The self-supporting wafer (10-15 mg, 13 mm diameter) was evacuated at 573 K for 3 h. A spectrum was then recorded as a background. The wafer was then exposed to NH3 (purity >99.9%) at room temperature for 30 min. After desorption at room temperature for 30 min, another spectrum was recorded for the adsorbed NH3. Catalyst Tests. A fixed-bed flow microreactor (φ10 i. d.) was used for the catalytic tests. About 2.7 g of a sample was loaded and sandwiched by quartz sand layers in the reactor. Catalysts were sulfided by 2% CS2 in heptane first at 503 K for 2 h and then at 593 K for 4 h and then were tested for the hydrodesulfurization (HDS) and isomerization reactions from 433 to 573 K at 1.5 MPa by using a heptane solution containing 500 ppmw sulfur (thiophene) and 20% 1-hexene. The liquid hourly space velocity (LHSV) was maintained at 2 h-1 with the H2/feed ratio of 300 (v/v). Reaction products were analyzed with a gas chromatograph equipped with a flame ionization detector (FID) for hydrocarbons and a flame photometric detector (FPD) for organic sulfurs. The hydrocarbon compositions of products were analyzed using a PONA-GC (0.25 mm × 50 m in Agilent 6890) and a GC-MS.

Results and Discussion Textural Properties of the Catalysts. The low-angle XRD patterns as shown in Figure 1 confirmed the MCM-41 structure of the samples. For the 5%Co-MCM-41, the (100), (110), and (200) peaks, characteristic of the hexagonally ordered MCM41 structure, can be clearly seen. The absence of any (210) peak is an indication of decreased ordering of the MCM-41 structure upon the incorporation of cobalt into the framework. The ordered structure of the 5%Co-MCM-41 was also confirmed by TEM (Figure 2). The incorporation of more cobalt in the framework as in the 10%Co-MCM-41 and 15%Co-MCM-41 further decreased the ordering of the MCM-41 structure. The longrange ordered structure was retained to some extent after loading cobalt on MCM-41 and the 5%Co-MCM-41 via the wet impregnation, since the (100) diffraction peak was still there,

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Figure 3. N2 adsorption and desorption isotherms of MCM-41, 5%CoMCM-41, and 5%Co/5%Co-MCM-41 at 77 K. Table 2. Results of Chemical Analysis by XRF for the Co-MCM-41 Samples Figure 1. Low-angle XRD patterns of the catalysts. sample

Co/Si molar ratio in solution

Co/Si molar ratio in sample

5%Co-MCM-41 10%Co-MCM-41 15%Co-MCM-41

0.05 0.10 0.15

0.031 0.062 0.081

Figure 2. TEM image of the 5%Co-MCM-41. Table 1. Textural Properties of Catalysts catalyst MCM-41 5%Co-MCM-41 10%Co-MCM-41 15%Co-MCM-41 5%Co/MCM-41 5%Co/5%Co-MCM-41 sulfided 5%Co/5%Co-MCM-41

pore volume pore diameter SBET (m2 g-1) (cm3 g-1)a (nm)b 1191 951 680 692 675 460 117

0.78 1.11 0.96 1.03 0.36 0.43 0.26

a BJH desorption cumulative volume of pores. pore diameter.

b

3.07 4.16 4.51 4.90 2.50 3.07 8.17 Adsorption average

but its intensity was decreased. No diffraction peak was found in the low-angle XRD pattern of the used 5%Co/5%Co-MCM41 catalyst, indicating the disappearance of the hexagonal mesoporous structure of the catalyst after the reaction. The poor stability of the ordered structure might be due to the relatively thin amorphous silica walls in MCM-41.25 In addition, no diffraction peaks were observed in the wide-angle XRD patterns (not shown) for any of the Co-MCM-41 samples, suggesting an amorphous framework and highly dispersed cobalt cations. Table 1 presents the textural properties of the samples. The average pore size of Co-MCM-41 was larger than that of MCM41 and increased with cobalt content. This might be an indication that cobalt was incorporated into the MCM-41 framework since the bond Co-O is longer than the Si-O bond.14 Otherwise, the existence of cobalt cations in pores might block the pores

(25) Khodakov, A. Y.; Zholobenko, V. L.; Bechara, R.; Durand, D. Microporous Mesoporous Mater. 2005, 79, 29–39.

Figure 4. DR UV-vis spectra of MCM-41 and Co-MCM-41 samples.

and reduce pore sizes.26,27 In fact, the samples with impregnated cobalt exhibited smaller pore diameters and lower surface areas and pore volumes. After sulfidation, the surface area of the 5%Co/5%Co-MCM-41 was greatly decreased, as seen in Table 1, showing the poor structural stability of the catalyst. The type IV nitrogen adsorption-desorption isotherms of MCM-41, 5%Co-MCM-41, and 5%Co/5%Co-MCM-41 (Figure 3) indicated the presence of mesopores. Table 2 gives the results of chemical analysis by XRF for the Co-MCM-41 samples. More than 50% of cobalt was successfully incorporated into the framework of MCM-41. DR UV-vis spectroscopy was used to probe the location of cobalt cations. As can be seen in Figure 4, no peaks were found for the siliceous MCM-41. The peaks around 530, 580, and 670 nm for the Co-MCM-41 samples could be attributed to tetrahedrally coordinated Co2+ in the MCM-41 framework.14 (26) Morey, M.; Davidson, A.; Eckert, H.; Stucky, G. Chem. Mater. 1996, 8, 486–492. (27) Ramı´rez, J.; Contreras, R.; Castillo, P.; Klimova, T.; Za´rate, R.; Luna, R. Appl. Catal., A 2000, 197, 69–78.

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Figure 5. Temperature programmed reduction profiles of the catalysts: (a) MCM-41, (b) 5%Co-MCM-41, (c) 10%Co-MCM-41, (d) 15%CoMCM-41, (e) 5%Co/MCM-41, and (f) 5%Co/5%Co-MCM-41.

No peaks around 300-400 nm for the octahedrally coordinated Co3+ were observed in the Co-MCM-41 samples. These results suggested that the cobalt cations were all in framework of the samples and no Co3+ cations were detected. Reducibility of the Catalysts. Figure 5 displays the TPR profiles of the samples. The sample 5%Co/MCM-41 prepared by the impregnation of cobalt on MCM-41 exhibited two reduction peaks around 600 and 622 K, which could be attributed to the sequential reduction of Co3+ f Co2+ f Co0.28 On the other hand, the sample 5%Co-MCM-41 prepared with the incorporation of cobalt cations in the framework exhibited a reduction peak around 974 K, 352 K higher than 622 K for the 5%Co/MCM-41. This can be taken as a strong indication that cobalt cations had been successfully incorporated into the framework of MCM-41 since it is well established that cobalt cations in the framework of MCM-41 were much more difficult to reduce than those dispersed on the surface.29 In addition, the main reduction peaks seemed to shift to higher temperatures with the increase of the content of cobalt cations in the framework. It is interesting to note the TPR profile for the 5%Co/5%Co-MCM-41, which showed the reduction of surface cobalt cations around 607 and 630 K and the reduction of those incorporated in the framework around 1030 K. Note that the intensities of the peaks between 600 and 630 K were significantly larger for 5%Co/5%Co-MCM-41 than those for 5%Co/ MCM-41, indicating a promoting effect of the incorporated cobalt on the reduction of supported ones. Surface Acidity of the Catalysts. Microcalorimetric adsorption results of NH3 were used to measure the surface acidity in terms of the strength and number of acidic sites on the catalysts. Part A of Figure 6 shows the population and strength of surface acidic sites of oxidic catalysts by NH3 microcalorimetric adsorption. MCM-41 and 5%Co/MCM-41 presented weak surface acidity since most surface sites exhibited the adsorption heats lower than 25 kJ mol-1. On the other hand, some strong acid sites were observed for the 5%Co-MCM-41, 10%CoMCM-41, and 5%Co/5%Co-MCM-41 with initial heats around 150 kJ mol-1. The most populated acid sites had adsorption heats around 40 kJ mol-1. The industrial Co-Mo/γ-Al2O3 (28) Liu, B.; Zhu, H.; Zhang, H.; Shen, J. Chinese J. Inorg. Chem. 2005, 21, 852–858. (29) Kong, Y.; Zhu, H.; Yang, G.; Guo, X.; Hou, W.; Yan, Q.; Gu, M.; Hu, C. AdV. Funct. Mater. 2004, 14, 816–820.

Figure 6. (A) Differential heat vs coverage for NH3 adsorption at 423 K on the catalysts before sulfidation. (B) Corresponding FTIR spectra for the adsorption of NH3 at room temperature.

catalyst possessed stronger surface acidity than 5%Co-MCM41, 10%Co-MCM-41, and 5%Co/5%Co-MCM-41 since most surface acid sites on the Co-Mo/γ-Al2O3 had heats of NH3 adsorption higher than 40 kJ mol-1. The IR spectra for adsorbed NH3 in part B of Figure 6 showed that Co-Mo/γ-Al2O3 exhibited mainly Lewis acidity (1300 and 1620 cm-1), whereas MCM-41 related catalysts displayed Lewis and Brønsted acidities. In particular, the 5%Co/5%Co-MCM-41 exhibited a significant amount of Brønsted acid sites (1460 and 1670 cm-1), which might favor the skeletal isomerization of olefins.3,4 After sulfidation, the surface acidity of the catalysts was greatly enhanced. Figure 7 shows the microcalorimetric and infrared spectroscopic results for the adsorption of ammonia. It is clearly seen that the heat of adsorption of NH3 was increased and NH3 coverage greatly enhanced. Figure 8 shows the effect of sulfidation. Before the sulfidation, the 5%Co/5%Co-MCM41 exhibited an initial heat of about 160 kJ mol-1 and a coverage of about 300 µmol/g for the adsorption of NH3. After sulfidation, the initial heat increased to 180 kJ mol-1 and the coverage increased to about 1060 µmol/g. Two main IR bands were observed for the catalysts after sulfidation, belonging to Lewis (1620 cm-1) and Brønsted (1470 cm-1). The Brønsted band was significantly more intensive than the Lewis band, especially for the 5%Co/5%Co-MCM-41. Isomerization Versus Hydrogenation. Figure 9 presents the results for the catalytic conversion of 1-hexene over the sulfided catalysts under the HDS conditions. Part A of Figure 9 shows that the conversion of 1-hexene was high over all of the catalysts

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Figure 7. (A) Differential heat vs coverage for NH3 adsorption at 423 K on the catalysts after sulfidation. (B) Corresponding FTIR spectra for the adsorption of NH3 at room temperature.

Figure 8. Comparison of surface acidity of the catalysts before and after sulfidation in terms of (A) differential heat vs coverage for NH3 adsorption at 423 K and (B) corresponding FTIR spectra for the adsorption of NH3 at room temperature.

Shi and Shen

studied here at temperatures higher than 493 K. Parts B and C of Figure 9 present the total isomerization selectivity (doublebond shift and skeletal) and the skeletal isomerization only, respectively, whereas part D of Figure 9 provides the hydrogenation activity. A comparison of the results in parts B and C of Figure 9 shows that the MCM-41 exhibited mainly the double-bond shift reaction at 433 K, probably due to its weak surface acidity. Part C of Figure 9 shows that little skeletal isomerization occurred on Co-Mo/γ-Al2O3 while it increased with the reaction temperature on the MCM-41-related catalysts. At 573 K, about 61% skeletal isomerizations were observed for the 5%Co/5%Co-MCM-41. The presence of cobalt cations in the framework of MCM-41 seemed to play an important role for the skeletal isomerizations of 1-hexene, probably due to the creation of Brønsted acidity, which was greatly enhanced upon the sulfidation. Figure 10 compares the results for the skeletal isomerization of 1-hexene over the 5%Co-MCM-41, 10%Co-MCM-41, 10%Co/ MCM-41, and 5%Co/5%Co-MCM-41. It was found that the 5%Co/5%Co-MCM-41 exhibited the highest activity for the skeletal isomerization of 1-hexene, probably due to that the sulfided 5%Co/5%Co-MCM-41 possessed more acidic sites than the other catalysts studied in this work. The total number of isomerizations of 1-hexene over Co-Mo/ γ-Al2O3 amounted to 62% at 473 K, and it decreased sharply with the increase of temperature from 473 to 573 K (part B of Figure 9), owing to the increased hydrogenation activity of 1-hexene over Co-Mo/γ-Al2O3 (part D of Figure 9). Thus, the hydrogenation and isomerization of 1-hexene seemed to be two competitive reactions, and Co-Mo/γ-Al2O3 exhibited relatively high activity for the hydrogenation of 1-hexene and therefore low selectivity to the isomerization of 1-hexene above 473 K. Table 3 gives the product distribution for the isomerization and hydrogenation of 1-hexene over the sulfided 5%Co/5%CoMCM-41 at 573 K. The products identified by PONA-GC and GC-MS suggested the following reactions for the conversion of 1-hexene: (1) direct hydrogenation of 1-hexene to n-hexane, (2) double-bond shift reaction of 1-hexene to 2- and 3-hexenes, (3) skeletal isomerization of 1-hexene to 3- and 4-methyl-2pentene, (4) hydroisomerization of 1-hexene to 2- and 3-methylpentanes, and (5) hydrocracking to lower hydrocarbons (C5-). No products of polymerization and aromatization were found. The main products from the conversion of 1-hexene were monosubstituted alkanes (44%). In contrast, 1-hexene was mainly converted into n-hexane over the traditional Co-Mo/ γ-Al2O3 catalyst under the HDS conditions. Two-Stage Bed of Catalysts for the Conversion of 1-Hexene. The above results suggest a pathway to convert n-alkenes first to isoalkenes followed by the subsequent hydrogenation to isoalkanes to reduce the octane number losses by the direct hydrogenation of n-alkenes. We checked this possibility by using Co-Mo/Co-MCM-41 and a two-stage bed of catalysts with Co/Co-MCM-41 (2.7 g) as the first and Co-Mo/γ-Al2O3 (2.7 g) as the second stage. Table 4 gives the results. The data in Table 4 shows that the Co-Mo/γ-Al2O3 exhibited high conversion of thiophene (97%) and also high activity for the hydrogenation of 1-hexene. Only 2% of 1-hexene underwent the skeletal isomerization over the Co-Mo/γ-Al2O3. On the other hand, the Co-Mo/5%Co-MCM-41 exhibited a lower conversion of thiophene (94%) but a significantly higher degree of skeletal isomerizations of 1-hexene (41%). The twostage catalysts showed an even higher conversion of thiophene (98%) and a high degree of skeletal isomerization of 1-hexene (37%). This result clearly indicated the sequential reactions of

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Energy & Fuels, Vol. 23, 2009 325

Figure 9. Conversion of 1-hexene over the sulfided catalysts MCM-41 (9), 5%Co-MCM-41 (O), 5%Co/MCM-41 (0), 5%Co/5%Co-MCM-41 (b), and Co-Mo/γ-Al2O3 (2). Reaction conditions: P ) 1.5 MPa, LHSV ) 2 h-1, and H2/liquid feed ) 300 v/v. Table 4. Hydrodesulfurization of Thiophene and Conversion of 1-Hexene over the Co-Mo/γ-Al2O3, Co-Mo/5%Co-MCM-41 and a Two-Stage Bed of Catalysts with 5%Co/5%Co-MCM-41 and Co-Mo/γ-Al2O3 as the First and Second Stagesa 1-hexene hydrogenation thiophene skeletal of 1-hexene conv. (%) isomerization (%) to alkanes (%)

catalysts

97 94 98

Co-Mo/γ-Al2O3 Co-Mo/5%Co-MCM-41 two-stage catalyst

2 41 37

99 75 91

a Reaction conditions: P ) 1.5 MPa, T ) 573 K, LHSV ) 2 h-1, and H2/liquid feed ) 300 v/v.

Figure 10. Skeletal isomerization of 1-hexene over the sulfided catalysts. Reaction conditions: P ) 1.5 MPa, T ) 573 K, LHSV ) 2 h-1, and H2/feed ratio ) 300 v/v. Table 3. Detailed Analysis of Products from the Hydrogenation and Isomerization of 1-Hexene over the Sulfided 5%Co/ 5%Co-MCM-41 with the Heptane Solution Containing 20% 1-Hexene and Thiophene with 500 ppmw of Sulfur Obtained by Using a PONA-GC and a GC-MS Instrumenta product

content (%)

C52,3-dimethyl-2-butene 2-methyl-2-pentene 3-methyl-2-pentene 4-methyl-2-pentene 2-methyl-pentane 3-methyl-pentane 1-hexene 2-hexene 3-hexene n-hexane

4.37 0.12 0.36 5.65 3.11 29.67 15.00 1.85 2.06 11.09 23.87

a Reaction conditions: P ) 1.5 MPa, T ) 573 K, LHSV ) 2 h-1, and H2/feed ratio ) 300 v/v.

some 1-hexene, that is the skeletal isomerization of 1-hexene followed by the hydrogenation to isoalkanes. Such results might be promising for the potential industrial applications since the linear olefins in a typical FCC naphtha may be as high as 23% (Table 5). Since the olefins lighter than

Table 5. Olefins in FCC Naphtha2 C4 C5 C6 C7 C8 C9 total

linear (%)

mono (%)

3.4 7.0 4.6 3.4 2.6 2.1 23.1

7.2 6.1 3.4 1.7 2.4 20.8

multi (%)

0.3 0.6

cyclo (%)

total (%)

0.7 1.8 2.1 1.0 0.1 5.7

3.4 14.9 12.7 9.6 5.2 4.5 50.4

hexene are easier to isomerize than hexene, whereasc those heavier are more difficult. We used 1-hexene as a representative to estimate the difference in octane number losses for two cases: (1) 100% 1-hexene were hydrogenated directly into n-hexane and (2) only 63% 1-hexene were hydrogenated to n-hexane and the remaining 37% were isomerized and then converted into 2-methylpentane. The reduction of octane number losses could be as high as 4 units for the second case as compared to the first one. Isomerization of parafins needs bifunctional catalysts, in which a strong solid acid is usually required and deactivation due to coking often occurs.30 Since the isomerization reactions proceed through the carbenium cations, whose formation is much easier via the interaction of a surface proton with an olefin (30) Ono, Y. Catal. Today 2003, 81, 3–16.

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than with a paraffin,31 the isomerization of olefins is much easier than that of parafins and requires only weak surface Brønsted acidity. Currently, about 60 million tons of gasoline are consumed annually in China, in which about 80% are from FCC process and contain about 50% olefins.2 The EU 2009 standard requires that the gasoline should contain less than 18% olefins. Apparently, the isomerization of olefins before they were hydrogenated during the HDS process is a way to compensate the losses of octane number due to the reduction of olefins. An increase of octane number by 1 unit increases the price of gasoline for about 100 RMB/ton, and the total annual gain may exceed 20 billion RMB ($2.7 billion U.S.) for the increase of octane number by 4 units for all of the gasoline consumed in China. In addition, an enhancement of octane number increases the efficiencies of internal combustion engines and reduces the consumption of fuels and therefore the emission of CO2. Conclusions Mesoporous Co-MCM-41 samples with 3-8% cobalt cations in the framework were synthesized, and more cobalt cations were supported. Microcalorimetric adsorption of NH3 showed (31) Dumesic, J. A.; Rudd, D. F.; Aparicio, L. M.; Rekoske, J. E.; Trevin˜o, A. A. The Microkinetics of Heterogeneous Catalysis; American Chemical Society: Washington D.C., 1993.

Shi and Shen

that the 5%Co-MCM-41 (with cobalt cations incorporated in the framework) exhibited stronger surface acidity than MCM41 and 5%Co/MCM-41 (with cobalt cations supported on the surface). Support of 5%Co on the surface of 5%Co-MCM-41 generated more Brønsted acidity. In addition, sulfidation greatly enhanced the surface acidity, as evidenced by the significantly increased NH3 coverage. The sulfided 5%Co/5%Co-MCM-41 was found to possess the most surface acid sites in the catalysts studied in this work. At 573 K, a significant degree of skeletal isomerization (61%) was achieved over the sulfided 5%Co/ 5%Co-MCM-41, higher than those over the 10%Co/MCM-41 and 10%Co-MCM-41. The traditional Co-Mo/γ-Al2O3 exhibited mainly the direct hydrogenation, with little isomerization of 1-hexene. A combination of the 5%Co/5%Co-MCM-41 with Co-Mo/γ-Al2O3 enhanced the hydrodesulfurization (HDS) of thiophene and converted a significant amount of 1-hexene into branched hexanes. Thus, Co/Co-MCM-41 might be used with the traditional Co-Mo/γ-Al2O3 to compensate the octane number losses caused by the direct hydrogenation of olefins in gasoline during HDS. Acknowledgment. This work was supported by the NSFC (20673055) and the High Tech. Program of Jiangsu Province of China (BG2006031). EF8005993