Effects of Acidic Properties on the Catalytic Performance of

Apr 12, 2008 - Prior to the catalytic reactions, all catalysts were in situ sulfided in 5 vol % H2S/H2 (balance) gas at 360 °C for 3 h. The HDS react...
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Energy & Fuels 2008, 22, 1456–1462

Effects of Acidic Properties on the Catalytic Performance of CoMo Sulfide Catalysts in Selective Hydrodesulfurization of Gasoline Fractions Takehisa Mochizuki,* Hideyuki Itou, Makoto Toba, Yasuo Miki, and Yuji Yoshimura National Institute of AdVanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ReceiVed October 30, 2007. ReVised Manuscript ReceiVed February 26, 2008

Improvement of the selectivity of hydrodesulfurization (HDS) to hydrogenation (HYD) of olefins is crucial to produce sulfur-free gasoline (S < 10 ppm) from fluid catalytic-cracked (FCC) gasoline. CoMo/SiO2 catalyst, which was less acidic than CoMo/Al2O3 catalyst, showed higher HDS/HYD selectivity than CoMo/Al2O3 catalyst under mild reaction conditions, although both HDS and HYD activities were lower for the former catalyst. However, under more severe reaction conditions, HDS/HYD selectivity over CoMo/SiO2 catalyst decreased to a significantly greater extent than that over CoMo/Al2O3 catalyst. NO adsorption measurements indicated that the formation of coordinatively unsaturated sites (CUS) over CoMo sulfides was more dependent on the reaction severity over the SiO2 supports. FT-IR data of adsorbed pyridine clearly showed that the strength of Lewis acidity over CUS, in particular, mainly formed under high-temperature conditions, was greater for CoMo/SiO2 catalyst than for CoMo/Al2O3 catalyst. These results suggest that strong Lewis acid sites over CUS promote a hydrocracking reaction in olefins: however, it is not more disadvantageous than HYD reactions in terms of octane loss. Controlling the Lewis acid properties over CoMo sulfides is effective in selective HDS of FCC gasoline, even when using nonacidic supports.

Sulfur-containing hydrocarbons are the origin of sulfur oxides (SOX), which cause corrosion of after-gas treatment devices.1 For this reason, the regulation of gas emissions from gasoline vehicles is becoming tighter, making production of sulfur-free gasoline (S < 10 ppm) a crucial option from an environmental point of view. Fluid catalytic-cracked (FCC) gasoline is the main component of gasoline. It contains about 30-40 vol % of olefins,2 which contribute to improvement of octane number. However, FCC gasoline also contains high levels of sulfur (0.005-1.5 wt %),2 depending on whether the FCC feed stocks have been prehydrodesulfurized or not. This makes the hydrodesulfurization (HDS) of FCC gasoline a prerequisite for producing sulfur-free gasoline (S < 10 ppm). In HDS of FCC gasoline, however, hydrogenation (HYD) reactions of olefins take place simultaneously and cause octane loss.3 Olefins are also known to convert to thiols by recombination reactions with H2S produced after HDS,4–7 resulting in reduced HDS conver-

sion. Therefore, suppressing the reactivity of olefins is very important for maximizing HDS while minimizing octane loss. It is accepted that HDS takes place on coordinatively unsaturated sites (CUS) of active metal sulfides over HDS catalysts such as CoMo/Al2O3.8–12 Many studies have been carried out to correlate HDS activities with the number of CUS by means of temperature-programmed reduction of sulfides13 or adsorption of probe molecules.8–10 Similarly, CUS will be involved in HYD of unsaturated compounds. Many studies have investigated the difference between HDS and HYD active sites and have suggested that HYD active sites have different properties to HDS active sites.4–7,14–16 Hatanaka et al. proposed that CUS can be classified as follows: (i) n-olefin HYD active sites, (ii) isoolefin HYD active sites, and (iii) HDS active sites.6 Although, they do not mention the difference in the structures among these active sites. Consequently, controlling the distribution of CUS over CoMo sulfides should be effective in improving the selectivity of HDS to HYD of olefins (HDS/ HYD selectivity).

* Corresponding author. Tel./Fax: +81-29-861-4532. E-mail address: [email protected]. (1) Gatta, R.; Harding, R.; Albro, T.; Chin, D.; Wormsbecher, R. F. Am. Chem. Soc. Prepr. DiV. Fuel Chem. 1992, 37 (1), 33. (2) Brunet, S.; Mey, C.; Pe´rot, G.; Bouchy, C.; Diehl, F. Appl. Catal. A: Gen. 2005, 278, 143. (3) Desai, P. H.; Lee, S. I.; Jonker, R. J.; De Boer, M.; Vrieling, J.; Sarli, M. S. Fuel Reformulation 1994, NoV./Dec., 43. (4) Hatanaka, S.; Yamada, M.; Sadakane, O. Ind. Eng. Chem. Res. 1997, 36, 1519. (5) Hatanaka, S.; Yamada, M.; Sadakane, O. Ind. Eng. Chem. Res. 1997, 36, 5110. (6) Hatanaka, S.; Yamada, M.; Sadakane, O. Ind. Eng. Chem. Res. 1998, 37, 1748. (7) Hatanaka, S.; Sadakane, O.; Okazaki, H. Sekiyugakkaishi (J. Jpn. Petrol. Inst.) 2001, 44 (1), 36.

(8) Wambeke, A.; Jalowiecki, L.; Kasztelan, S.; Grimblot, J. J. Catal. 1988, 109, 320. (9) Massoth, F. E., Muralidhar, G., Barry, H. F., Mitchell, P. C. H., Eds. Proceedings of the Climax Fourth International Conference on Chemistry and Uses of Molybdenum; 1982; pp 343. (10) Okamoto, Y.; Tomioka, H.; Imanaka, T.; Teranishi, S. J. Phys. Chem. 1980, 84, 1833. (11) Tanaka, K.; Okuhara, T. Catal. ReV.-Sci. Eng. 1977, 15, 249. (12) Tanaka, K. AdV. Catal. 1985, 33, 99. (13) Burch, R.; Collins, A. Appl. Catal. 1985, 18, 373. (14) Itou, H.; Koizumi, N.; Sakamoto, N.; Honma, T.; Ogawa, K.; Shingu, M.; Yamada, M. J. Jpn. Petrol. Inst. 2004, 47 (4), 258. (15) Mey, D.; Brunet, S.; Canaff, C.; Mauge´, F.; Bouchy, C.; Diehl, F. J. Catal. 2004, 227, 447. (16) Miller, J. T.; Reagan, W. J.; Kaduk, J. A.; Marshall, C. L.; Kropt, A. J. J. Catal. 2000, 193, 123.

1. Introduction

10.1021/ef700644e CCC: $40.75  2008 American Chemical Society Published on Web 04/12/2008

CoMo Sulfide Catalysts in SelectiVe HDS

In addition, the acidic properties of catalysts will influence the HYD of olefins, since basic olefins are subject to adsorption on Lewis acid sites and subsequent HYD. From the viewpoint of the improvement of HDS selectivity to olefin HYD, the following pieces of information are very interesting. Lewis acid sites or acidic OH groups of metal oxides are found to act as the adsorption sites for olefins.17–21 Moreover, it is reported that the interfacial OH groups of the supports, which interact with MoS2 via hydrogen bonds, are related to olefin HYD.22 It is therefore expected that using less acidic supports leads to suppression of olefin HYD and increasing HDS/HYD selectivity. Numerous studies have been carried out to control the acidic properties of supports, such as by the addition of alkali metals to CoMo/Al2O3,15 using basic supports such as hydrotalcite compounds,23 Al2O3-MgO24 and MgO.25 In addition to the acidities of the supports themselves, the overall acidity of supported catalysts will change after metal loading, since CUS of CoMo sulfides also exhibit Lewis acidity and SH groups exhibit Brønsted acidity. Moreover, the Lewis and Brønsted acidity are influenced by the reaction temperature and the reaction atmospheres. Therefore, information on CUS as well as acidic properties of sulfide catalysts will be necessary for improving the HDS/HYD selectivity. We have focused on amorphous SiO2 as a support, which is less acidic than Al2O3.26–28 Moreover, SiO2-supported CoMo sulfide catalysts are known to show higher turnover frequency (TOF) of HDS reactions due to the formation of Co-Mo-S in the type II phase,29,30 although the dispersion of MoS2 phases is usually lower than that over γ-Al2O3. Therefore, SiO2supported CoMo sulfide catalysts appear to have the potential to minimize the HYD of olefins and to maximize HDS/HYD selectivity. In this study, we investigated the effects of the acidic properties on HDS and HYD activities in gasoline fractions by using γ-Al2O3- and SiO2-supported CoMo sulfide catalysts. In addition, we characterized the CUS properties by using probe molecules such as nitric oxide (NO) and pyridine and correlated the catalytic activities of CoMo sulfide catalysts with the acidic properties of CUS and supports. 2. Experimental Details 2.1. Activity Tests. Commercial CoMo/Al2O3 of 202 m2/g-cat BET surface area with a pore volume 0.47 cm3/g-cat and in-house prepared CoMo/SiO2 catalysts with 216 m2/g-cat BET surface area (17) Busca, G.; Ramis, G.; Lorenzelli, V.; Janin, A.; Lavalley, J.-C. Spectrochim. Acta 1987, 43, 489. (18) Busca, G.; Ramis, G.; Lorenzelli, V. J. Chem. Soc., Faraday Trans. 1 1989, 85, 137. (19) Busca, G.; Lorenzelli, V.; Ramis, G.; Escribano, V. S. Mater. Chem. Phys. 1991, 29, 175. (20) Flego, C.; Parker, W. O., Jr. Appl. Catal. A: Gen. 1999, 185, 137. (21) Trombetta, M.; Buska, G.; Rossini, S. A.; Piccoli, V.; Cornaro, U. J. Catal. 1997, 168, 334. (22) Choi, J.-S.; Mauge´, F.; Pichon, C.; Olivier-Fourcage, J.; Jumas, J.-C.; Petit-Clair, C.; Uzio, D. Appl. Catal. A: Gen. 2004, 267, 203. (23) Zhao, R.; Yin, C.; Zhao, H.; Li, C. Fuel Process. Tecnol. 2003, 81, 201. (24) Klimova, T.; Solı´s, D.; Ramı´rez, J. Catal. Today 1998, 43, 135. (25) Hillarova´, E.; Vı´t, Z.; Zdrazeˇil, M. Appl. Catal. A: Gen. 1994, 118, 111. (26) Ca´ceres, C. V.; Fierro, J. L. G.; La´zaro, J.; Lo´pe Agudo, A.; Soria, J. J. Catal. 1990, 122, 113. (27) Okamoto, Y.; Ohhara, M.; Maezawa, A.; Imanaka, T.; Teranishi, S. J. Phys. Chem. 1986, 90, 2396. (28) Zaki, M. I.; Hasan, M. A.; Ahsagheer, F. A.; Pasupulety, L. Colloids Surf. A: Physicochem. Eng. Aspects 2001, 190, 261. (29) Okamoto, Y.; Maezawa, A.; Imakana, T. J. Catal. 1989, 120, 29. (30) Okamoto, Y.; Ochiai, K.; Kawano, M.; Kobayashi, K.; Kubota, T. Appl. Catal. A: Gen. 2002, 226, 115.

Energy & Fuels, Vol. 22, No. 3, 2008 1457 with a pore volume of 0.87 cm3/g-cat were used. Aqueous solutions containing ammonium heptamolybdate ((NH4)6Mo7O24 · 4H2O) and cobalt nitrate (Co(NO3)2 · 6H2O) were impregnated into SiO2 (Fuji Silisia, denoted as G10) using the incipient wetness method. After impregnation, the catalysts were evaporated at 60 °C for 2 h and calcined at 150 °C for 1 h, followed by calcination at 400 °C for 3 h. Metal loadings on each catalyst were 16.9 wt % of MoO3 and 4.5 wt % of CoO. Prior to the catalytic reactions, all catalysts were in situ sulfided in 5 vol % H2S/H2 (balance) gas at 360 °C for 3 h. The HDS reaction was carried out in a high-pressure fixed-bed continuous flow reactor. The reaction conditions were the following: hydrogen pressure of 1 MPa, reaction temperature of 200-260 °C, liquid hourly space velocity (LHSV) of 10 h-1, and volumetric ratio hydrogen to feed oil of 100 N L/L. Two kinds of feed oils were used: one was a feed oil containing model compounds such as 2,4,4trimethyl-2-pentene (244TM2PE, 5 wt %), 2-methylthiophene (2MT, 75 wt ppm-S), benzothiophene (BT, 75 wt ppm-S), n-heptane (55 wt %), ethylcyclohexane (10 wt %), 1,2,4-trimethylbenzene (30 wt %), and n-butylamine (5 wt ppm-N); the other one was FCC gasoline supplied by a refinery (n-paraffins 4.4 vol %, isoparaffins 25.8 vol %, olefins 25.4 vol %, naphthenes 12.6 vol %, aromatics 31.7 vol %, sulfur 62.6 wt ppm-S). The hydrocarbon compositions in feed oils and products were measured by GC-FID (Agilent 6890N with an HP-DHA1 column (102 m)) and GC-PIONA (Agilent 6890N (JIS K2536) Yokogawa Analytical Systems Co. GPI system). Sulfur compounds were analyzed by GC-SCD (Agilent 6890-Sievers 355), and elemental analysis of sulfur (Mitsubishi Chemicals Co., TS-100V) was also carried out. The HDS conversion and HYD conversion of 244TM2PE were calculated as follows: HDS[%] ) [(Sfeed - Sproduct)/Sfeed] × 100 where Sfeed and Sproduct indicate the concentration of sulfur in the feed and the products, respectively. HYD[%] ) [224TMPproduct/244TM2PEfeed] × 100 where 224TMPproduct and 244TM2PEfeed indicate the concentration of 2,2,4-trimethylpentane (224TMP) in the products and 244TM2PE in the feed, respectively. 2.2. Characterization of Catalysts. Uptakes of NO by the sulfided catalysts were determined by NO pulse adsorption measurements (Ohkura Riken, R-6015). The sample was in situ sulfided in H2S (5 vol %)/H2 (balance) stream at 360 °C for 1 h. After sulfidation, the sample was reduced in an H2 stream at various temperatures for 1 h, followed by purging with He at the same temperature for 3 min, then cooling to 50 °C. After these pretreatments, a sequential NO pulse was injected into the sample at 50 °C until no more NO was adsorbed onto the sample. FT-IR spectra of adsorbed NO or pyridine over the sulfided catalysts were measured using an FT-IR spectrometer (Thermo Electron, Nexus 670). To make FT-IR measurements of adsorbed NO, the pulverized catalyst sample was placed in the diffusereflectance FT-IR cell. The sample was in situ sulfided in an H2S (1000 ppm)/H2 (2%)/N2 (balance) stream at 360 °C for 1 h. After sulfidation, the sample was reduced in an H2 stream under the same conditions as the NO uptake measurements. After pretreatments, the background spectra were measured (256 scans, 4 cm-1 resolution) prior to NO introduction. NO was introduced at 50 °C until it was fully adsorbed onto the sample, followed by flushing with He at 50 °C for 0.5 h. The spectra were then measured and calculated using Kubelka-Munk functions. Prior to FT-IR measurement of adsorbed pyridine, pyridine was stored with molecular sieve 3A dried at 400 °C to minimize the moisture effect. The pulverized catalyst sample was prepared in self-supporting pellet form at about 10 mg/cm2. The sample was pretreated in the same as for the NO adsorption measurements. After pretreatment, the background spectra were measured (64 Scans, 4 cm-1 resolution) under vacuum (about 10-4 Torr). About 10 Torr of pyridine was introduced to the sample at room temperature for

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Table 1. Catalytic Activities of CoMo/SiO2 and Commercial CoMo/Al2O3 Catalystsi conversion of 244TM2PE (2,4,4-trimethyl-2-pentene) [%]a isomerization catalyst in-house prepared CoMo/SiO2 commercial CoMo/Al2O3

reaction temperature [°C]

totalb

HYDc

244TM1PEd

C8 skeletal isomerse

hydrocrackingf

HDS [%]g

HDS/HYD ratio [%/%]h

200 220 240 200 220 240

17.0 69.3 74.5 66.2 77.9 84.4

1.5 6.6 26.2 8.7 27.2 57.9

15.1 62.0 21.1 56.0 45.0 14.7

0.2 0.4 9.9 0.7 2.2 3.8

0.2 0.3 17.3 0.8 3.5 8.0

31.6 70.3 96.2 56.0 83.0 97.5

21.1 10.7 3.7 6.4 3.1 1.7

a Conversions were calculated by the results of GC-FID. b Total [%] ) [C c feed(244TM2PE) - Cproduct(244TM2PE)]/Cfeed(244TM2PE) × 100. HYD [%] ) Cproduct(224TMP)/Cfeed(244TM2PE) × 100. d 244TM1PE [%] ) Cproduct(244TM1PE)/Cfeed(244TM2PE) × 100. e C8 skeletal isomers [%] ) Cproduct(C8 skeletal isomers)/Cfeed(244TM2PE) × 100. f Hydrocracking [%] ) [(Cproduct(isobutane) + Cproduct(isobutene))/2]/Cfeed(244TM2PE) × 100. g HDS conversion was calculated by the results of GC-SCD and elemental analysis. h HDS/HYD ratio [%/%] ) HDS conversion [%]/HYD conversion [%] × 100. HDS [%] ) [Cfeed(S) - Cproduct(S)]/Cfeed(S) × 100. i Reaction conditions: 1 MPa (H2), 200-240 °C, H2/oil ratio 100 N L/L.

1.5 h. After pyridine adsorption, the sample was evacuated at 50 °C for 1 h, followed by heating stepwise to 100-200 °C. The spectra of the adsorbed pyridine were then measured. The acid strength and number of acid sites of the catalysts were determined by microcalorimetric measurements of the differential heat adsorption of ammonia (Tokyo Riko, CSA-450G) at 50 °C after the catalysts had been evacuated at 400 °C for 3 h.

3. Results and Discussion 3.1. Effects of Supports on Catalytic Activities of CoMo/SiO2 and Commercial CoMo/Al2O3 Catalysts. During the reaction of 244TM2PE, HYD product (224TMP), CdC double bond isomerization product (2,4,4-trimethyl-1-pentene, 244TM1PE), skeletal isomerization products (other C8 compounds), and hydrocracking products (isobutene and isobutane) were obtained for each catalyst. The catalytic activities at each reaction temperature were adopted after reaching a steady state (ca. 50 h onstream). Table 1 shows the activities and the HDS/ HYD selectivities over in-house-prepared CoMo/SiO2 and commercial CoMo/Al2O3 catalysts. For CoMo/Al2O3, HYD activity increased from 8.7% to 57.9% on increasing the reaction temperature. Whereas HDS activity was 56.0%, even at a reaction temperature of 200 °C, it increased to beyond 90% on increasing the reaction temperature from 200 to 240 °C. Moreover, the HDS/HYD selectivity over CoMo/SiO2 catalyst was much higher than that of CoMo/Al2O3 catalyst compared at the same temperature, though HYD and HDS conversion were lower for the former catalyst. Muralidhar et al. carried out HDS of thiophene, HYD of 1-hexene and hydrocracking of isooctene separately over different supported CoMo sulfided catalysts.31 They found that the HDS/HYD selectivity of CoMo/SiO2 -Al2O3 increased with increasing SiO2/Al2O3 ratio and, in particular, CoMo/SiO2 showed the highest HDS/HYD selectivity. Our results also indicate that CoMo/SiO2 shows higher HDS/ HYD selectivity than CoMo/Al2O3 in the case of simultaneous HDS and HYD reactions of 244TM2PE. CdC double bond isomerization (from 244TM2PE to 244TM1PE), skeletal isomerization (from 244TM2PE to other C8 compounds), and hydrocracking (formation of isobutene and isobutane) of 244TM2(1)PE also took place accompanied by HYD of 244TM2PE. At the lower reaction temperatures of 200 and 220 °C, activities for skeletal isomerization and hydrocracking of olefins over CoMo/SiO2 catalyst were lower than for CoMo/Al2O3 catalyst. In contrast, the catalytic activities exhibited the opposite pattern at 240 °C, i.e., higher skeletal isomerization and olefin hydrocracking conversions for CoMo/ SiO2 catalyst. These results suggest the appearance of stronger (31) Muralidhar, G.; Massoth, F. E.; Shabtai, J. J. Catal. 1984, 85, 44.

Figure 1. NO uptake of CoMo/SiO2 catalyst (b) and commercial CoMo/Al2O3 catalyst (0). Pretreatment: treatment with H2 at 200-340 °C for 1 h after sulfidation at 360 °C for 1 h. NO adsorption: pulse introduction of 10 vol % NO/He (1 mL/pulse) at 50 °C.

acid sites over CoMo/SiO2 catalyst, which promoted the skeletal isomerization and hydrocracking of olefins at 240 °C. 3.2. Effect of Support on the Distribution of CUS. Some previous studies have suggested that HYD active sites are different from HDS active sites and that these activities can be controlled independently.5,6,9 Moreover, the dependence of HDS/HYD selectivity on the same reaction temperature was quite different between CoMo/SiO2 and CoMo/Al2O3 catalysts. This might be due to the dependence of CUS formation on the reaction temperature as well as on the PH2S/PH2 partial pressure ratio around the sulfide phases. We investigated the CUS properties of CoMo sulfide catalysts in order to study the relationship between the activities and the surface structures. Uptake data for NO are shown in Figure 1 for CoMo/SiO2 and commercial CoMo/ Al2O3 catalysts which were treated with H2 at various temperatures after sulfidation at 360 °C. Uptake of NO on the sulfided CoMo/Al2O3 catalyst was higher than on CoMo/ SiO2 catalyst, regardless of H2 treatment temperature. This indicates that the dispersion of CoMo sulfides over SiO2 support is lower than that over γ-Al2O3 support due to the weaker interaction of CoMo sulfides with the SiO2 support. In addition, the stability of CoMo sulfides with respect to H2 treatment was quite different between over γ-Al2O3 supports and over SiO2 supports. Uptake of NO on CoMo/ Al2O3 catalyst increased almost linearly on increasing the H2 treatment temperature, which indicated a gradual formation of CUS over the sulfide surfaces. On the other hand,

CoMo Sulfide Catalysts in SelectiVe HDS

Energy & Fuels, Vol. 22, No. 3, 2008 1459

Figure 3. Equilibrium relationship during the reducing bulk MoS2 and Co9S8 phases: Mo-MoS2 equilibrium (2), Co-Co9S8 equilibrium ([).

Figure 2. FT-IR spectra of NO adsorbed on CoMo/SiO2 (a-e) and CoMo/Al2O3 catalysts (f-j). Pretreatment: sulfidation at 360 °C for 1 h (a, f), followed by treatment with H2 at 200 (b, g), 240 (c, h), 280 (d, i), and 340 °C (e, j) for 1 h. NO adsorption: flow introduction of 2 vol % NO/He at 50 °C for 0.5 h, followed by flushing with He for 0.5 h.

NO uptake on CoMo/SiO2 catalyst increased drastically beyond the H2 treatment temperature of 240 °C, whereas it changed little below 240 °C. This low-temperature behavior suggests that abstraction of sulfur atoms from CoMo sulfides over SiO2 support is more difficult than over γ-Al2O3 support, since low dispersed and probably highly crystalline MoS2 phases were formed on SiO2 supports. However, the abrupt CUS formation seen beyond 240 °C, in particular between 240 and 280 °C, was unexpected. This CUS formation might be accompanied by the formation of new Lewis acid sites. As shown in Table 1, an abrupt increase in skeletal isomerization and hydrocracking activities occurred between 220 and 240 °C. If this drastic increase in catalytic functions was due to the increased acidic properties of newly formed CUS, the increase in the number of CUS is clearly represented in Figure 1, although the temperature regions were shifted to the high-temperature range. Taking into account the fact that the hydrotreating conditions include high hydrogen pressure (1 MPa) and that an exothermic HYD reaction occurred over the CoMo sulfide phase, i.e., an increase in PH2/PH2S and in local temperature around the sulfide phases, a greater shift to higher temperatures under the NO uptake conditions than for hydrotreating conditions would be likely. To examine whether sulfur atoms are abstracted from Mo or Co sulfide phases, FT-IR spectra of NO adsorbed on the catalysts, which were treated with H2 at various temperatures after sulfidation at 360 °C, were measured. The FT-IR spectra are shown in Figure 2. Three bands appeared in the range of 2000-1500 cm-1. The bands at 1860 and 1800 cm-1 are assigned to NO adsorbed on CUS on the Co sulfide side and the bands at 1800 and 1700 cm-1 are assigned to NO adsorbed

on the CUS of Mo sulfide side.32 The whole band intensities over CoMo/SiO2 catalyst were lower than those over CoMo/ Al2O3 catalyst. These results are consistent with the results from NO uptake measurements. For CoMo/SiO2 catalyst after sulfidation at 360 °C, the band on the Co sulfide side at 1860 cm-1 appeared strongly, while that of Mo sulfide side appeared only faintly. This suggests that the dispersion of MoS2 phases was very low over SiO2 supports and the relative amount of the CUS of Co sulfide side to that of the Mo sulfide side became large. H2 treatment after sulfidation resulted in an increase in the number of CUS for both catalysts. For CoMo/SiO2 catalysts, above all, the number of CUS on the Co sulfide side increased more significantly than did CoMo/Al2O3 catalyst on increasing the H2 treatment temperature to 280 °C. Figure 3 shows the equilibrium relationship during the reduction of bulk MoS2 and Co9S8 phases. This relationship suggests that sulfur is desorbed more easily from Co9S8 than from MoS2 and is marked at high temperatures. These equilibriums for bulk phases will naturally be shifted from the dispersed Mo and Co sulfided phases on Al2O3 or SiO2. However, the reduction tendency for the bulk phases and dispersed phase will be similar to the CUS formation during reduction. For this reason, we believe that NO uptake on CoMo/SiO2 increases rapidly beyond the H2 treatment of 240 °C if some CoSX coexists with the CoMoS phase. These highly coordinating unsaturated sites will work as Lewis sites showing a hydrocracking function.33 By considering these results obtained from NO uptake measurements, sulfur atoms were mainly abstracted from the CoSX phases over CoMo/SiO2 catalyst during reactions at higher temperatures such as 240 °C. On the other hand, the intensity ratio of the band at 1860 cm-1 to the band at 1700 cm-1 was almost constant and independent of H2 treatment temperatures for CoMo/Al2O3 catalyst. It is reported that CUS formed over Ni sulfide catalysts cause the abstraction of hydride ions from hydrocarbons, leading to the formation of carbenium ions.34 Therefore, if such a phenomenon was dominant over Co sulfide catalysts in the same way as Ni sulfide catalysts, the catalytic performance of CoMo/SiO2 catalyst would be influenced more markedly than that of CoMo/ Al2O3 catalyst. (32) Topsøe, N.-Y.; Topsøe, H. J. Catal. 1983, 84, 386. (33) Infantes-Molina, A.; Me´rida-Robles, J.; Rodrı´guez-Castello´n, E.; Pawelec, B.; Fierro, J. L. G.; Jime´nez-Lo´pez, A. Appl. Catal. A: Gen. 2005, 286, 239. (34) Resasco, D. E.; Marcus, B. K.; Huang, C. S.; Durante, V. A. J. Catal. 1994, 146, 40.

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Figure 4. Heats of NH3 adsorption over oxidic CoMo/SiO2 catalyst (b) and commercial CoMo/Al2O3 catalyst (0). Pretreatment: evacuation at 400 °C for 3 h. Initial NH3 pressure: 200 Torr.

Figure 6. Evacuation temperature dependency of the band intensities of PyL-CoMoS at 1609 cm-1 for CoMo/SiO2 (b) and CoMo/Al2O3 (0) catalysts treated with H2 at 280 °C after sulfidation at 360 °C. Pyridine adsorption: exposure of 10 Torr of pyridine under vacuum at room temperature for 1.5 h. The spectra were measured after evacuation at various temperatures.

Figure 5. FT-IR spectra of pyridine adsorbed on CoMo/SiO2 (A) and CoMo/Al2O3 (B) catalysts treated with H2 at 280 °C after sulfidation at 360 °C. Pyridine adsorption: exposure of 10 Torr of pyridine under vacuum at room temperature for 1.5 h. The spectra were measured after evacuation for 0.5 h at various temperatures. Evacuation temperatures were 50 (a), 100 (b), 150 (c), and 200 °C (d).

Figure 7. HDS/HYD selectivity over CoMo/SiO2 catalyst (b) and commercial CoMo/Al2O3 catalyst (0).

3.3. Acidic Properties of CUS on the CoMo/SiO2 and commercial CoMo/Al2O3 catalysts. To clarify the effects of the acidic properties of supports on these reactions, we compared the adsorption behaviors of NH3 on CoMo/SiO2 catalyst and commercial CoMo/Al2O3 catalyst in oxidic states. The results are shown in Figure 4. The amount of adsorbed NH3 on oxidic CoMo/SiO2 was clearly smaller than that of oxidic CoMo/Al2O3 catalyst in all regions of acid strength. Moreover, the total amount of adsorbed NH3 (heat of adsorption >70 kJ/mol) on the former catalyst was about 25% smaller than that of the latter catalyst. These results indicate that higher hydrocracking activities of CoMo/SiO2 catalyst at higher reaction temperatures such as 240 °C cannot be explained in terms of the acidic properties of the supports. Therefore, the acid properties of CUS over CoMo sulfides are expected to contribute to enhanced hydrocracking activities. FT-IR measurements of adsorbed pyridine have frequently been used to investigate the distribution and strength of the acid sites. Figure 5 shows FT-IR spectra of pyridine adsorbed on CoMo/SiO2 and commercial CoMo/Al2O3 catalysts at room temperature followed by evacuation at several temperatures. The catalysts were treated at 280 °C in H2 after sulfidation at 360 °C. This reduction temperature is the temperature at which the quantity of NO adsorption signifi-

cantly increased. FT-IR spectra exhibited the bands assigned to pyridines adsorbed on Lewis acid sites and hydrogenbonded pyridines (PyL) at 1609 and 1596 cm-1, respectively, and those on Brønsted acid sites (PyB) at 1640 and 1545 cm-1.28 Notably, the band at 1609 cm-1 is ascribed to pyridine adsorbed on the strong Lewis acid sites formed on the CoMoS phases (denoted as PyL-CoMoS) since it appears only in CoMo sulfide catalysts, but not in Mo or Co sulfide catalysts.35 The band intensity of PyB for CoMo/SiO2 catalyst was smaller than that for CoMo/Al2O3, which was particularly marked at 1640 cm-1. On increasing the evacuation temperature, the band intensities of PyB and PyL gradually decreased and almost disappeared after evacuation at 150 °C for both catalysts, whereas the bands of PyL-CoMoS at 1609 cm-1 remained prominent, even at 200 °C. This indicates that CUS, which are formed on CoMoS phases over both CoMo/SiO2 and CoMo/Al2O3 catalysts, exhibited stronger Lewis acidities than other acid sites. The dependency of the band intensity of PyL-CoMoS on evacuation temperature for each catalyst is summarized in Figure 6. The dependency for the CoMo/SiO2 catalyst was distinct from that for the CoMo/Al2O3 catalyst. In a comparison of the band intensities of PyL-CoMoS after evacu(35) Berhault, G.; Lacroix, M.; Breysse, M.; Mauge´, F.; Lavalley J.-C.; Nie, H.; Qu, L. J. Catal. 1998, 178, 555.

CoMo Sulfide Catalysts in SelectiVe HDS

Energy & Fuels, Vol. 22, No. 3, 2008 1461

Table 2. Catalytic Activities of CoMo/SiO2 and Commercial CoMo/Al2O3 Catalysts toward Real FCC Gasoline hydrocarbon composition [vol%]a catalyst feed in-house prepared CoMo/SiO2 commercial CoMo/Al2O3

reaction temperature [°C]

n-paraffins

isoparaffins

olefins

naphtenes

aromatics

HDS [%]b

GC-RON loss [-]c

220 240 260 220 240 260

4.4 4.8 5.3 5.5 5.0 5.4 6.2

25.8 26.0 26.8 27.4 26.0 26.8 27.8

25.4 24.7 23.6 22.5 24.5 23.1 21.4

12.6 12.6 12.7 12.8 12.6 12.9 13.1

31.7 32.0 31.6 31.8 32.0 31.7 31.6

38.2 75.9 87.9 57.8 79.0 93.9

0.2 0.6 0.9 0.4 0.8 1.3

a Calculated by the results of GC-PIONA. b HDS [%] ) [C c feed(S) - Cproduct(S)]/Cfood(S) × 100. GC-RON (research octane number) was calculated by the results of GC-PIONA. GC-RONloss [-] ) GC-RONfeed - GC-RONproduct. d Reaction conditions: 1 MPa (H2), 220-260 °C, H2/oil ratio 100 N L/L.

ation at 50 °C, CoMo/Al2O3 catalyst showed double the amount for PyL-CoMoS than CoMo/SiO2 catalyst. This result is related to the higher degree of dispersion of CoMo sulfides over γ-Al2O3 supports. On increasing the evacuation temperature, the band intensities of PyL-CoMoS for CoMo/ SiO2 catalyst decreased only slightly, even after evacuation at 200 °C, whereas those for CoMo/Al2O3 catalyst decreased gradually. Above the evacuation temperature of 150 °C, the band intensities of PyL-CoMoS for CoMo/SiO2 catalyst exceeded those for CoMo/Al2O3 catalyst, in spite of the former catalyst being larger than the latter after evacuation at 50 °C. Hatanaka et al. found that reactions of olefins were involved at strong acid sites,6 since the HYD activity of CoMo/Al2O3 catalyst was depressed by modification with pyridine followed by heating at 300 °C, while HDS activity recovered by heating at 300 °C. As shown in Figure 6, our results indicate that strong acid sites will be more dominant on CoMo/SiO2 catalyst than CoMo/Al2O3 catalyst under severe reaction conditions where the adsorption affinity of reactants to Lewis acid sites will be weakening. It is generally thought that the formation of carbenium ions, which promote skeletal isomerization and hydrocracking reactions, takes place over Brønsted acid sites (SH groups over the sulfide phases).31 However, as shown in Figure 5, the relative band intensity of PyBs at 1640 and 1545 cm-1 with that of PyL-CoMoS at 1609 cm-1 was smaller for CoMo/SiO2 catalyst than for CoMo/Al2O3 catalyst, in particular at higher evacuation temperatures. Therefore, the strong Lewis acid sites appear to be attributable to abrupt enhancement in the skeletal isomerization and hydrocracking functions of CoMo/SiO2 catalysts under the higher temperature hydrotreating conditions shown in Table 1. Focusing on the HDS/HYD selectivity (Figure 7), selectivity over CoMo/SiO2 catalyst was double that over CoMo/ Al2O3 catalyst even under the severe conditions needed to produce sulfur-free gasoline (HDS conversion >95%). However, the HDS/HYD selectivity over CoMo/SiO2 catalysts decreased, on increasing the reaction temperature, more significantly than that over CoMo/Al2O3 catalysts. At higher reaction temperatures such as 240 °C, strong Lewis acid sites will be formed over CoMo/SiO2 catalyst that might be related to abrupt HYD and hydrocracking of 244TM2PE. However, it appears that the hydrocracking of 244TM2PE is not more disadvantageous than HYD of 244TM2PE in terms of octane loss, since the research octane number (RON) of isobutene formed by hydrocracking reactions is above 101.3 and equivalent to that of 244TM2PE (about 103.5) while 224TMP exhibits a RON of 99.3.36 3.4. HDS and Olefin HYD of Real FCC Gasoline. FCC gasoline, which is one of the major components of motor gasoline, contains high levels of sulfur derived from heavy gas oil and atmospheric residues used as FCC feedstock. More than 90% of the sulfur content in gasoline blendstocks derives from FCC gasoline. Reduction of sulfur content in

FCC gasoline is the most effective strategy for sulfur-free (S < 10 ppm) gasoline production. FCC gasoline also contains valuable olefins which contribute to the octane number of motor gasoline. Octane-boosting olefins in the FCC gasoline are often saturated during the hydrotreating reaction. Therefore, selective HDS which minimize octane loss are highly desirable in response to the ever-tightening legal controls on sulfur content37,38. It is worth revealing whether CoMo/SiO2 catalyst is effective in achieving selective HDS toward real FCC gasoline. We therefore investigated the activities of CoMo/SiO2 catalyst by using real FCC gasoline as well as 244TM2PE. Table 2 shows the activities of CoMo/SiO2 and commercial CoMo/Al2O3 catalysts toward real FCC gasoline. HDS activities increased on increasing the reaction temperature while GC-RON loss became more marked for both catalysts. A comparison of GC-RON losses clearly showed CoMo/SiO2 catalyst to cause a smaller octane loss at each reaction temperature than CoMo/Al2O3 catalyst. As shown in Table 1, CoMo/SiO2 catalyst showed enhanced hydrocracking activity at high temperatures, but its lower hydrogenation activity is of more importance for minimizing octane loss. These results show that CoMo/SiO2 catalyst has high potential to achieve selective HDS even toward real FCC gasoline. 4. Conclusions The following conclusions can be stated based on the results obtained from this study. (1) In the test reaction using simulated FCC gasoline containing 244TM2PE and thiophenes, HDS conversion increased with increasing reaction temperature, while HDS/HYD selectivity decreased for both CoMo/SiO2 and CoMo/Al2O3 catalysts. CoMo/SiO2 catalyst showed higher HDS/HYD selectivity than CoMo/Al2O3 catalyst. (2) Under higher temperature reduction conditions, abstraction of sulfur atoms resulted in an increase in the number of CUS exhibiting Lewis acidity. The formation of CUS under higher temperatures was significant for CoMo/SiO2 catalyst. (3) As confirmed by ammonia heat of adsorption measurements and FT-IR analysis of adsorbed pyridine, CoMo/SiO2 catalyst was less acidic than CoMo/Al2O3 catalyst. The lower acidity is linked with lowered reactivity of 244TM2PE for skeletal isomerization and hydrocracking. (4) Strength of Lewis acidity over CUS, in particular, formed under high-temperature conditions, was greater for CoMo/SiO2 (36) American Petroleum Institute, Eds. Technical data book-petroleum refining, 2nd ed.; 1970; Chapters 1-6, pp 1-22. (37) Toba, M.; Miki, Y.; Kanda, Y.; Matsui, T.; Harada, M.; Yoshimura, Y. Catal. Today 2005, 104, 64. (38) Toba, M.; Miki, Y.; Matsui, T.; Harada, M.; Yoshimura, Y. Appl. Catal. B: EnViron. 2007, 70, 542.

1462 Energy & Fuels, Vol. 22, No. 3, 2008

catalyst than for CoMo/Al2O3 catalyst. These sites promote skeletal isomerization and hydrocracking reactions of 244TM2PE, even at 240 °C, over CoMo/SiO2 catalyst. (5) When hydrotreating real FCC gasoline feedstock, the lower hydrogenation activity of CoMo/SiO2 catalyst compared with CoMo/Al2O3 catalyst more effectively minimized octane loss, even at nearly equivalent HDS activities.

Mochizuki et al.

These results indicate that controlling the Lewis acidic properties over supports and CoMo sulfides are likely to be effective in further minimizing octane loss and in maximizing HDS, allowing selective HDS of FCC gasoline. EF700644E