Characteristics of CoMo Catalysts Supported on Modified MCM-41

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Characteristics of CoMo Catalysts Supported on Modified MCM-41 and MCM-48 Materials for Thiophene Hydrodesulfurization Murid Hussain, Seon-Ki Song, Jun-Hee Lee, and Son-Ki Ihm* National Research Laboratory for EnVironmental Catalysis, Department of Chemical and Biomolecular Engineering, Korea AdVanced Institute of Science and Technology (KAIST), 373-1 Gusung-dong, Yusung-gu, Daejeon 305-701, South Korea

The potential of two different mesoporous materials, MCM-41- and MCM-48-supported sulfided CoMo bimetallic catalysts, for hydrodesulfurization (HDS) of thiophene at specific conditions was explored to investigate the effect of channel connectivity on HDS activity. The hydrothermal stability of MCM-41 and MCM-48 was improved remarkably by a post-salt treatment technique. Aluminum incorporation to mesoporous material frameworks improved acidity and hydrothermal stability. Mesoporous-material-supported CoMo catalysts with different Co/Mo atomic ratios were prepared by coimpregnation of Co(NO3)2‚6H2O and (NH4)6Mo7O24‚4H2O. The characterization of the catalysts was carried out by X-ray diffraction (XRD), N2 adsorption/ desorption, NH3-temperature-programmed desorption (NH3-TPD), transmission electron microscopy (TEM), CO chemisorption, elemental analysis (EA), and 13C NMR. MCM-41-supported catalysts showed a small decrease in activity compared to MCM-48-supported catalysts. The optimal Co/Mo atomic ratio for MCM41- and MCM-48-supported series of catalysts was 3:7. The aluminosilicate-supported CoMo catalysts showed higher activities than their siliceous counterparts. 1. Introduction Development of highly active hydrodesulfurization (HDS) catalysts is one of the most urgent subjects in the petroleum industry, not only to protect the environment but also to efficiently utilize limited natural resources. Sulfided Co-Mo or Ni-Mo (W)-based catalysts have been extensively used in industry for HDS reactions.1 Over the past few decades, several different models have been proposed for explaining the promotional effect of Co. The addition of a small amount of Co to Mo/Al2O3 dramatically improves the HDS activity of Mo.2 More than 17 theories or models have been made in order to explain this synergy, including Co-Mo-S model, contact synergy or remote control model, intercalation model, and monolayer model.1,3,4 The catalytic activities of Mo and Co-Mo sulfide catalysts for HDS are strongly affected by the support employed.5 The strong support interactions allow for highly dispersed MoS2 structures to be easily prepared, which remain stable during operation. Strong support interactions in alumina-supported catalysts have negative aspects due to the formation of relatively less active Type-I Co-Mo-S. Weaker support interactions of the Mo (W) species were reported on silica as compared with those of alumina. However, silica OH-groups play a role in dispersing Mo on the carrier surface.1 Mesoporous molecular sieves, such as M41S, are a new family of amorphous siliceous materials with large surface area and strictly controlled mesoporosity. Mesoporous-materialsbased catalysts prepared by incorporating metals, metal oxides, organometallic complexes, and heteropoly acids into mesopores have been reported, showing good catalytic activities for many types of reactions.6,7 Corma et al.8 used Al-MCM-41-supported Ni-Mo catalysts for hydrocracking of vacuum gas oil. They found that AlMCM-41-supported catalysts were more active in hydrodes* To whom correspondence should be addressed. Tel.: +82-42-8693910/3955. Fax: +82-42-869-5955. E-mail: [email protected].

ulfurization (HDS), hydrodenitrogenation (HDN), and hydrocracking than those supported over ultrastable Y (USY) zeolites or γ-Al2O3. Reddy et al.9 used supported Co-Mo and Ni-Mo over Al-MCM-41 to investigate the hydrodesulfurization of petroleum residues and atmospheric and vacuum residues. The Al-MCM-41-supported catalyst was superior in HDS activity to its γ-Al2O3-supported counterpart but not as active as the commercial catalyst. Song and Reddy10 investigated the hydrodesulfurization of dibenzothiophene (DBT) over Co-Mo/Al-MCM-41 at both high and normal metal loading. Co-Mo/Al-MCM-41 catalyst at a normal metal-loading level was less active in converting DBT than the γ-Al2O3-supported one at 350 or 375 °C. However, the Al-MCM-41-supported catalyst at high metal loading was more active than its γ-Al2O3-supported counterpart in DBT conversion. They concluded that sulfided Co-Mo/AlMCM-41 catalysts were promising for deep hydrodesulfurization of distillate fuels under relatively mild conditions. Ramı´rez et al.11 found that strong acidity of the support (Al2O3 + MCM-41) was detrimental to the HDS activity of the catalysts. Only weak and medium acid sites on the surface favored the formation of HDS active sites. Klimova et al.12 reported that the dispersions of Mo and Ni oxidic species were increased with the incorporation of aluminum in MCM-41 support because of the strong interaction of Mo and Ni oxidic species with aluminum atoms of the support. Cui et al.13 found that MCM-41-supported Ni and Mo catalysts were more active than similar NaY-supported catalysts but were more strongly dependent on the dispersion of metal oxides, owing to the onedimensional channel system of MCM-41. They also showed that KIT-1-supported MoO3 and/or NiO catalysts exhibited higher catalytic activities for thiophene hydrodesulfurization than similar MCM-41- and NaY-zeolite-supported catalysts, because the three-dimensional disordered network of short channels in KIT-1 reduced the risk of blockage in the catalysts and facilitated the transport of reactant and product molecules.14 Park et al.15 found that naphthalene and tetralin conversion was increased with the acid amount of the supports in Pt/Al-

10.1021/ie058064b CCC: $33.50 © 2006 American Chemical Society Published on Web 11/09/2005

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MCM-41 catalysts. Wang et al.16 reported that a deep HDS catalyst, active to DBT, could be prepared by depositing CoMo species over siliceous MCM-41. The optimal Co/Mo atomic ratio for these catalysts was 0.75, higher than the conventional γ-Al2O3-supported catalysts. They also prepared a deep HDS catalyst by depositing Ni-W species over siliceous MCM-41, which exhibited high activity in desulfurization of DBT and a high-sulfur gas oil.17 In addition, they described HDS of DBT over siliceous MCM-41-supported catalysts by using sulfided NiMo catalysts. The optimum HDS activity was observed at a Ni/Mo atomic ratio of 0.75, similar to that of CoMo/MCM41.18 The objective of this study was to investigate the interactions and synergistic effects between Co and Mo at different pore structures (MCM-41 and MCM-48) for siliceous and aluminosilicate mesoporous supports of different channel connectivity in thiophene HDS. 2. Experimental Section 2.1. Catalyst Preparation. MCM-41 and MCM-48 supports were hydrothermally synthesized19-22 with cetyltrimethylammonium chloride (CTACl, 25 wt %, Aldrich), cetyltrimethylammonium bromide (CTABr, Aldrich), and tetraoxyethylene dodecyl ether (Brij30, Aldrich) surfactants and were added to an aqueous solution of sodium chloride (NaCl, extra pure, Junsei).21-23 The pH was adjusted to 10 by addition of concentrated ethylenediamine tetraacetic acid tetrasodium salt (EDTANa4, pure, Junsei) solution, followed by heating in an oven for 12 days at 100 °C. The precipitate was filtered and washed with 100 °C hot distilled water before cooling. Samples were dried at 100 °C, washed with an ethanol-HCl mixture, and calcined in air under static conditions at 550 °C for 5 h. Al-MCM-41 and Al-MCM-48 (Si/Al ) 5, 20) were prepared by the post-aluminum incorporation technique.21,24-26 Siliceous MCM-41 and MCM-48 were heated to remove water in a drying oven at 100 °C for 10 h. AlCl3 anhydrate (extra pure, Junsei) in absolute ethanol (99.9%, Merck) was added to siliceous MCM-41 and MCM-48 in polypropylene bottles and stirred vigorously for 30 min. The mixtures were filtered and washed with absolute ethanol, dried at 110 °C in air, and calcined in air at 550 °C for 5 h. CoMo bimetallic catalysts (Co/Mo atomic ratios of 10:0, 9:1, 7:3, 5:5, 3:7, 1:9, and 0:10) supported on siliceous MCM-41 or MCM-48 were prepared by the incipient wetness technique. A 7 wt % MoO3 loading level was chosen for preparing this series of catalysts. The content of cobalt was determined by varying the atomic ratio of Co/Mo. The total number of atoms loaded on 1 g of the support at the basis of 7 wt % of Mo was 4.72 × 1020 atoms/(g of cat). The Mo precursor was ammonium heptamolybdate tetrahydrate [(NH4)6Mo7O24‚4H2O, 99.9%, Aldrich], and the Co precursor was cobalt nitrate hexahydrate [Co(NO3)2‚6H2O, 98%, Aldrich]. After impregnation, each catalyst was dried in a vacuum-drying oven at 70 °C for 12 h. The catalysts were calcined at 550 °C for 6 h in the presence of air. CoMo catalysts supported on aluminosilicate MCM-41 and MCM-48 mesoporous materials and siliceous supports were prepared in the same manner. 2.2. Characterization. X-ray diffraction (XRD) analyses were conducted with a Rigaku powder diffractometer at 40 kV and 45 mA using Cu KR radiation. The scanning range and speed were 1.2-50° and 6 deg/min, respectively. The acidic property of the catalysts was analyzed by temperatureprogrammed desorption (TPD) of chemisorbed ammonia using a Pulse Chemisorb 2705 (Micromeritics). A 0.02 g aliquot of

sample was placed in a quartz tubular reactor, pretreated in a helium flow heated to 500 °C at a rate of 10 °C/min, and retained at 500 °C for 2 h. Samples were cooled to 100 °C, and an ammonia pulse was injected. After physisorbtion of ammonia, it was purged with helium. TPD was carried out up to 600 °C at 5 °C/min. Transmission electron microscopic (TEM) images were collected from thin edges of the sample particles with a Phillips CM20 electron microscope operating at 200 keV. TEM specimens were prepared by dipping a carbon-coated 300 mesh copper grid into a suspension of mesoporous materials in ethanol that was presonicated for 10 min. CO chemisorption was made on the bimetallic catalysts to determine metal dispersion by ASAP 2010 (Micromeritics). The catalysts were evacuated at 150 °C for 5 h followed by chemisorption of carbon monoxide at 25 °C. Total weight of coke deposit on catalysts was determined by the combustion method using an elemental analyzer (Elemental Analyses system, GmbH Vario EL). CP/ MAS-13C NMR spectra were analyzed to determine the aromatic content of coke deposit on the catalysts using an FT NMR spectrometer (Bruker AM-300). 2.3. Catalyst Activity Measurement. HDS activity was measured in a stainless-steel microflow reactor. Presulfidation was made with H2S/H2 (10 vol % H2S) mixed gas (30 cm3/ min) at 400 °C for 2 h, and the reactor was purged with He at 400 °C for 30 min. The thiophene (99%, Aldrich) HDS reaction was carried out at 400 °C in the microflow reactor operated at 20 atm H2 with a H2/thiophene mole ratio of 15. Reaction products were analyzed using a gas chromatograph (Hewlett Packard 5710A) equipped with a thermal conductivity detector (TCD). 3. Results and Discussion Most HDS catalysts are used in the sulfided state. The active site of HDS catalysts is the sulfur anion vacancy. Thus, presulfidation becomes an important factor in HDS activity.27 The presence of Co is essential for the activation of Co-Mo/ Al2O3 by sulfiding, indicating a close relationship between the activating effect of presulfiding and the promoting effect of Co or Ni.2 Thiophene was the sulfur compound used in this study to investigate HDS activities for sulfided CoMo catalysts supported over siliceous as well as Al-incorporated MCM-41 and MCM48 supports. Thiophene conversions for different CoMo bimetallic catalysts (Co/Mo atomic ratios were 1:9, 3:7, 5:5, 7:3, and 9:1) supported over siliceous MCM-41 and MCM-48 were shown in parts a and b of Figure 1, respectively. The lowest conversions (%) were observed with the catalysts with the higher Mo loading. For low conversion, a little difference was observed between Co/Mo 10:0, 9:1, and 7:3. For high conversion, a little difference was shown between Co/Mo 3:7 and 1:9. The addition of Co improves the HDS activity in all unsupported catalysts.28 However, Co showed different behavior in supported catalysts than in unsupported catalysts. It is believed that only the Co-Mo-S structure is active in HDS. However, not all of the cobalt added to the formulation resulted in the formation of unique Co-Mo-S species. Cobalt preferentially interacts with the edge of the MoS2 crystallite surface as it is added, forming the desired active species. When all of the edges are covered, cobalt forms a separate phase of stable sulfide Co9S8, which does not contribute to desulfurization.1,16 The atomic size of the Mo particle is larger than the atomic size of the Co particle. Therefore, in supported catalysts, in the ratio of CoMo (9:1), Co particles capture the Mo particles from all

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Figure 1. Thiophene HDS activity over supported CoMo catalysts with time on stream: reaction temperature ) 400 °C, reaction pressure ) 20 atm, H2/thiophene ) 15, for siliceous supports W/F ) 2.85 ((g of cat)‚ min)/cm3 thiophene, and reaction time ) 10 h for (a) CoMo/MCM-41 catalysts and (b) CoMo/MCM-48 catalysts.

the sides and Mo becomes nonactive during the HDS reaction. An atomic ratio of CoMo (3:7) showed maximum conversion due to a synergistic effect between Co and Mo. In contrast, Mo-supported catalysts showed higher conversion than the Cosupported catalysts due to the more acidic nature of Mo. HDS activity was affected by the nature of the support used.5 The relative ratio of catalytic sites for hydrodesulfurization, hydrogenation, and hydrocracking changes with the type of support materials and the metal-loading level.10 Support channel connectivity effect in thiophene HDS activity using MCM-41supported catalysts showed a small decrease compared to that using MCM-48-supported catalysts (Figure 1). MCM-41 structure consists of a parallel one-dimensional channel forming a hexagonal array, while MCM-48 structure consists of two independent and intricately interwoven networks of mesoporous channels.29,30 The three-dimensional channel network of MCM-48 showed high conversion and stability and was much more desirable than the one-dimensional channel of MCM-41 from a diffusion and catalytic point of view. The catalytic performance of catalysts is strongly correlated with the amount and the nature of acid sites.11,12,31 The activity and selectivity depends on the characteristics of the metal sites and acid sites. The balance between metal and acid sites influences the performances in hydrocracking and hydroisomerization.15 CoMo catalysts supported on Al-MCM-41 and AlMCM-48 showed higher activity than their siliceous counterparts. The acidity of the aluminum incorporated supports might help to crack the thiophene so as to improve the HDS activity. CoMo (3:7)/Al-MCM (Si/Al ) 5) showed the maximum conversion, whereas the CoMo (10:0)/Al-MCM (Si/Al ) 20) showed the minimum conversion (Figure 2 parts a and b). The aluminum-incorporated series showed a similar synergistic effect

Figure 2. Thiophene HDS activity over supported CoMo catalysts with time on stream: reaction temperature ) 400 °C, reaction pressure ) 20 atm, H2/thiophene ) 15, for aluminum-incorporated supports (Si/Al ) 5, 20) W/F ) 2.159 ((g of cat)‚min)/cm3 thiophene, and reaction time ) 10 h for (a) CoMo/Al-MCM-41 catalysts and (b) CoMo/Al-MCM-48 catalysts.

at CoMo (3:7) to that of the siliceous series described earlier. Thiophene conversions were greater in aluminosilicate supports even at a lower space time (W/F). A ratio of Si/Al ) 5 gave higher conversions than Si/Al ) 20 in all the cases. MCM-41 and MCM-48 supports have low hydrothermal stability in water and aqueous solutions because of structure loss involving silicate hydrolysis. Low hydrothermal stability is also a difficult problem when transition metals and metal oxides such as Pt clusters and molybdenum oxides are incorporated in hot aqueous solutions.22-24,32,33 Therefore, most researchers used Al2O3 as a support material in HDS catalysis.5,34-36 Improved hydrothermal stability by post-salt addition was attributed to increase pore wall thickness by secondary synthesis. Exceeding the pore wall thickness by 1 nm, which is typical for MCM-41 and MCM-48, makes the product more stable against water and water vapor.37 A disappearance of small XRD peaks was observed due to impregnation of transition metals on the supports. Even after impregnation of metals, the supports retained their mesoporosity and crystallinity, which was observed from the first XRD peak intensities (Figure 3 parts a and b). Mesoporous materials show their characteristic XRD peaks below a 2-theta value of 10°. It is clear that CoO and MoO3 show several peaks at a 2-theta value of 20-50°.28 However, no peak was observed for 2-theta values between 20° and 50°. This means that the metals were highly dispersed on mesoporous supports. The disappearance of XRD peaks suggests that the metal oxides are, to some extent, highly dispersed on the surface of the mesoporous channels.

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Figure 3. XRD patterns of supported CoMo catalysts calcined at 500 °C with metal loading showing metal dispersion, scan range ) 1.2-50°, for (a) CoMo/MCM-41 (post-salt treated) catalysts, (b) CoMo/MCM-48 (post-salt treated) catalysts, (c) CoMo/Al-MCM-41 (post-salt treated, Si/Al ) 5) catalysts, and (d) CoMo/Al-MCM-48 (post-salt treated, Si/Al ) 5) catalysts. Table 1. Surface Area, Pore Volume, and Average Pore Diameter of Supports and Catalysts supports/catalysts

SBET (m2/g)

PV (cm3/g)

Si-MCM-41-supported catalysts MCM-41 1117 1.12 CoMo(10:0)/MCM-41 1025 1.03 CoMo(9:1)/MCM-41 994 1.00 CoMo(7:3)/MCM-41 980 0.99 CoMo(5:5)/MCM-41 941 0.95 CoMo(3:7)/MCM-41 785 0.73 CoMo(1:9)/MCM-41 727 0.62 CoMo(0:10)/MCM-41 901 0.90 CoMo(10:0)/MCM-41a 731 0.75 CoMo(3:7)/MCM-41a 599 0.57 CoMo(0:10)/MCM-41a 706 0.71 Al-MCM-41-supported catalysts Al-MCM-41(20) 1056 1.03 Al-MCM-41(5) 1008 0.97 CoMo(10:0)/Al-MCM-41(20) 967 0.93 CoMo(10:0)/Al-MCM-41(5) 923 0.88 CoMo(3:7)/Al-MCM-41(20) 913 0.90 CoMo(3:7)/Al-MCM-41(5) 825 0.83 CoMo(0:10)/Al-MCM-41(20) 918 0.92 CoMo(0:10)/Al-MCM-41(5) 879 0.85 a CoMo(10:0)/Al-MCM-41(5) 701 0.73 CoMo(3:7)/Al-MCM-41(5)a 655 0.60 CoMo(0:10)/Al-MCM-41(5)a 699 0.70 a

APD (Å) 30 30 30 30 31 32 32 31 32 32 32 30 29 29 29 30 30 31 31 32 32 32

supports/catalysts

SBET (m2/g)

PV (cm3/g)

Si-MCM-48-supported catalysts MCM-48 1163 1.26 CoMo(10:0)/MCM-48 1004 1.09 CoMo(9:1)/MCM-48 1035 1.13 CoMo(7:3)/MCM-48 997 1.11 CoMo(5:5)/MCM-48 953 1.03 CoMo(3:7)/MCM-48 820 0.78 CoMo(1:9)/MCM-48 812 0.71 CoMo(0:10)/MCM-48 933 0.98 CoMo(10:0)/MCM-48a 852 0.86 CoMo(3:7)/MCM-48a 771 0.75 CoMo(0:10)/MCM-48a 715 0.76 Al-MCM-48-supported catalysts Al-MCM-48(20) 1115 1.19 Al-MCM-48(5) 1049 1.09 CoMo(10:0)/Al-MCM-48(20) 999 1.05 CoMo(10:0)/Al-MCM-48(5) 937 0.98 CoMo(3:7)/Al-MCM-48(20) 921 0.93 CoMo(3:7)/Al-MCM-48(5) 884 0.85 CoMo(0:10)/Al-MCM-48(20) 931 0.97 CoMo(0:10)/Al-MCM-48(5) 893 0.87 a CoMo(10:0)/Al-MCM-48(5) 850 0.84 CoMo(3:7)/Al-MCM-48(5)a 795 0.76 CoMo(0:10)/Al-MCM-48(5)a 712 0.77

APD (Å) 30 31 31 31 31 31 32 31 32 31 31 29 30 30 30 31 31 31 31 31 31 32

Catalyst ) used catalyst, SBET ) BET surface area, PV ) pore volume, and APD ) average pore diameter.

Al-incorporated mesoporous materials showed similar XRD results to those of the siliceous supports. Mesoporous materials retained their mesostructures even after the impregnations of metals because of improved hydrothermal stability by pH adjustment, post-salt treatment, and aluminum incorporation. This was confirmed by the XRD patterns of mesoporoussupported catalysts in which small peaks were observed even after metal impregnation (Figure 3 parts c and d].

MCM-41 and MCM-48 mesoporous supports showed a large surface area and pore volume (Table 1). A decrease in these characteristics on supports was shown after metal impregnation because of increased density by depositing metals and from pore blocking by these metal species. MCM-48-supported catalysts showed a smaller decrease in surface areas and pore volume than MCM-41-supported catalysts because of the threedimensional pore structure of the MCM-48 support.

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Figure 4. NH3-TPD of (a) CoMo/MCM-41 catalysts and (b) CoMo/MCM48 catalysts.

Figure 6. CO chemisorption of (a) CoMo/mesoporous material, (b) CoMo/ Al-MCM-41 (Si/Al ) 20, 5), and (c) CoMo/Al-MCM-48 (Si/Al ) 20, 5) catalysts.

Figure 5. NH3-TPD of CoMo catalysts supported on (a) Al-MCM-41 (Si/Al ) 20, 5), (b) Al-MCM-48 (Si/Al ) 20, 5), Si/Al (5) vs Si/Al (20).

A small decrease in the surface area and pore volume of the supports was due to aluminum incorporation (Si/Al ) 5, 20). Further small decreases were seen after metal impregnation. There was also a further decrease in the surface area and pore volume of the catalysts after HDS reaction. NH3-TPD spectra of siliceous supports and supported catalysts showed that impregnation of Co, Mo, and CoMo in the supports increased the amount of acid sites of catalysts compared with supports only (Figure 4 parts a and b). After calcination, Mo was present as MoO3 while Co was in the form of CoO. More oxygen atoms attached with MoO3 compared with CoO made it more electronegative and, hence, increased the acidity. There was a small difference in the amount of acid sites between MCM-41-based and MCM-48-based catalysts. There was a significant increase in the response of the Alincorporated series compared to the NH3-TPD of the siliceous series. A significant increase in the acidity of the catalysts was observed because of improved acidity by the post Al-incorporation (Figure 5 parts a and b). These results confirmed the

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Figure 7. TEM images of (a) MCM-41 and CoMo/MCM-41catalysts and (b) MCM-48 and CoMo/MCM-48 catalysts. Table 2. Amount and Aromaticity of Coke Deposit on Supported CoMo Catalysts after Thiophene HDS catalysts

coke amounta (wt %)

aromaticityb (wt %)

catalysts

coke amounta (wt %)

aromaticityb (wt %)

CoMo(0:10)/MCM-41 CoMo(3:7)/MCM-41 CoMo(10:0)/MCM-41 CoMo(0:10)/MCM-48 CoMo(3:7)/MCM-48 CoMo(10:0)/MCM-48

8.0 7.1 5.9 12.2 11.2 10.2

11.3 11.4 11.3 12.1 12.3 11.9

CoMo(0:10)/Al-MCM-41 CoMo(3:7)/Al-MCM-41 CoMo(10:0)/Al-MCM-41 CoMo(0:10)/Al-MCM-48 CoMo(3:7)/Al-MCM CoMo(10:0)/Al-MCM-48

16.2 14.2 12.1 17.0 16.0 13.5

12.7 12.8 12.5 12.8 12.9 12.4

a

Coke wt./cat. wt., analyzed by EA. b Aromatic wt./coke wt., analyzed by

reaction activity results (Figure 2), where more acidic sites were helpful in the cleavage of the C-S bond and gave a higher conversion in thiophene HDS. Higher hydrogenation activity has been correlated with better dispersion of the active phases and, thus, with higher number of active sites.36 CO chemisorption of different CoMo ratios supported on MCM-41 and MCM-48 (Figure 6a) were comparable with thiophene conversion results (Figure 1). The

13C

NMR.

relationship between thiophene conversion and CO chemisorption indicated that high conversion was obtained with highest dispersion. The effect of Al-incorporation into MCM-41 and MCM-48 on the adsorbed CO volume showed that acidity was increased because of more Al-incorporation and that more metals were dispersed (Figure 6 parts b and c). These results are similar to NH3-TPD and thiophene HDS reaction results. Al-MCM-48

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support, because of its three-dimensional pore structure, was superior in CO chemisorption to the Al-MCM-41 support, which had a one-dimensional channel network. TEM is a powerful tool to visualize different pore orderings.38 The TEM image of post-salt treated MCM-41 showed the particles in a uniform order, whereas for post-salt treated MCM48, particles were not in uniform order. Both samples showed mesoporosity. Metal dispersion was very clear in the dark areas for MCM-41- and MCM-48-supported CoMo catalysts. TEM images, CO chemisorption, and XRD results confirmed particle size and metal dispersion (Figure 7 parts a and b). Coke amount and aromaticity are very important for predicting catalyst life, from which the catalyst design can be further improved.39 The variation in coke wt % of the catalysts was due to a higher acidity for Mo than for Co (Table 2). This was confirmed in the Al-incorporated series, where the coke amount was greater because of increased acidity of the catalysts. In both series, MCM-48-supported catalysts showed more coke amount than MCM-41-supported catalysts, or similar results to those seen with NH3-TPD. There was a constant aromaticity for all the catalysts measured by 13C NMR, since the reaction conditions for all the catalysts were the same. 4. Conclusions The hydrothermal stability of mesoporous materials was improved significantly with post-salt treatment. There was a small decrease in surface area and pore volume of the catalysts. Co and Mo showed a synergistic effect at a ratio of 3:7 to give the highest thiophene HDS conversion. Both acidity and hydrothermal stability were improved by post Al-incorporation. There was an increase in catalyst activity due to post Alincorporation, compared to siliceous mesoporous-materialssupported catalysts. This study demonstrated that MCM-48 with a three-dimensional pore channel is a better catalyst support compared with MCM-41, which has a one-dimensional pore channel in thiophene HDS. The pore structure of MCM-48 could reduce the risk of deactivation in catalysts and facilitate the transport of reactant and product molecules. On the basis of our study, CoMo catalysts with a Co/Mo atomic ratio of 3:7 incorporated into three-dimensional pore structures would be better catalysts for HDS than the same catalyst incorporated into one-dimensional pore structures for the HDS process. Acknowledgment This work was partially supported by the grant of Brain Korea 21 (BK21) Project and also by the National Research Laboratory (NRL) Project of Korea. M.H. is also thankful to Korea Science and Engineering Foundation (KOSEF) for the scholarship. Literature Cited (1) Topsøe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating Catalysis; Springer: Berlin, 1996. (2) Yamada, M. Surface Fine Structure of Hydrodesulfurization Catalysts. New Aspects of Promoting Effects of Co and Ni. Catal. SurV. Jpn. 1999, 3, 3. (3) Grange, P.; Vanhaeren, X. Hydrotreating Catalysts, an Old Story with New Challenges. Catal. Today. 1997, 36, 375. (4) Fujikawa, T.; Ribeiro, F. H.; Somorjai, G. A. Hydrodesulfurization of Tetrahydrothiophene over Evaporated Mo, Co and Mo-Co Model Catalysts. Catal. Lett. 1999, 63, 21. (5) Okamoto, Y.; Ochiai, K.; Kawano, M.; Kobayashi, K.; Kubota, T. Effects of Support on the Activity of Co-Mo Sulfide Model Catalysts. Appl. Catal., A 2002, 226, 115.

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ReceiVed for reView July 14, 2005 ReVised manuscript receiVed October 5, 2005 Accepted October 6, 2005 IE058064B