Effect of Al-SBA-15 Support on Catalytic Functionalities of

Ind. Eng. Chem. Res. 2007, 46, 4747-4754

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Effect of Al-SBA-15 Support on Catalytic Functionalities of Hydrotreating Catalysts. II. Effect of Variation of Molybdenum and Promoter Contents on Catalytic Functionalities G. Muthu Kumaran,† Shelu Garg,† Kapil Soni,† Manoj Kumar,† L. D. Sharma,† K. S. Rama Rao,‡ and G. Murali Dhar*,† Catalytic ConVersion Processes DiVision, Indian Institute of Petroleum, Dehradun 248 005, India, and Indian Institute of Chemical Technology, Hyderabad 500 007, India

High surface area, ordered mesoporous SBA-15 and Al-SBA-15 were synthesized using published procedures and used as support for Mo and W catalysts promoted with Co and Ni. The molybdenum loading was varied on Al-SBA-15 from 2 to 12 wt %, and Co or Ni loading was varied on 8%Mo/Al-SBA-15 from 1 to 5 wt %. The support and catalysts were characterized by X-ray diffraction (XRD), Barett-Joyner-Halenda (BJH) pore-size distribution, Brunauer-Emmett-Teller (BET) surface area, Fourier transform infrared (FT-IR), temperature-programmed reduction (TPR), and oxygen chemisorption. The characterization results indicated that hexagonal mesoporous structure is retained on Mo, CoMo, and NiMo catalysts. The TPR studies indicated that there are significant differences in Mo reducibilities and reducible species that are present on SBA-15 and Al-SBA-15 supported catalysts. The catalytic activities for thiophene, hydrodesulfurization (HDS), and cyclohexene hydrogenation (HYD) were carried out as a function of Mo and CoMo or NiMo loadings at 400 °C on sulfided catalysts. The results indicated that the molybdenum is dispersed well up to 8% Mo loading and 8 wt % loading is optimum for catalytic activities. Oxygen chemisorption correlated well with catalytic activity. The reducibilities of the catalysts and their relation to catalytic activities are discussed. A comparison of the physicochemical properties and catalytic activities of Mo and W catalysts supported on SBA-15 and Al-SBA-15 is made. 1. Introduction Ever since the discovery of ordered mesoporous MCM-41, the research on mesoporous materials has grown by leaps and bounds.1,2 Many types of ordered mesoporous materials, like HMS, KIT, FSM-16, and SBA types of materials, have been discovered. These materials synthesized in acidic or basic media with a variety of templates offered the opportunity to synthesize materials with tunable pore size. It is also possible to increase the thermal stability of these materials by increasing the wall thickness. The large regular pore size, high surface area, and ability to substitute a variety of ions like Al3+, Ti4+, etc. emphasized their utility for applications as catalysts and catalytic supports.2 The materials like MCM-41, HMS, Al-HMS, AlMCM-41, SBA-15, and SBA-16 have been shown to have favorable catalytic activities for reactions of interest in hydrotreating and hydrocracking applications.3-11 Hydrotreating is a very important unit operation in petroleum refining and syncrude upgrading.12,13 Increasing environmental demands on quality of fuels necessitated removing sulfur to below the 50 ppm level.14 For fuel-cell applications, the sulfur needs to be reduced to the 0.1 ppm level; to achieve such a low sulfur level, the activity of catalysts needs to be increased seven times.15 In order to prepare such active catalysts, one of the approaches is to vary the support. A variety of supports have been tried with considerable success. Among these, carbon,16 oxides,17-21 mixed oxides,22-26 zeolites,27-30 and mesoporous materials3-11 are the prominent ones. We have earlier reported on Al-HMS material supported Mo and W * Corresponding author. Tel.: +91 135 2660146. Fax: +91 135 2660202. E-mail: [email protected]. † Indian Institute of Petroleum. ‡ Indian Institute of Chemical Technology.

catalysts and SBA-15 supported molybdenum catalysts.6-10 In this investigation, we are reporting the effect of Mo and Co content variation on physicochemical properties and catalytic activities of Al-SBA-15, of Si/Al ratio 10, supported molybdenum catalysts. A comparison of SBA-15 and Al-SBA-15 supported Mo and W catalysts are also made. 2. Experimental Section 2.1. Synthesis. The siliceous SBA-15 and Al incorporated SBA-15 materials were synthesized by following a published procedure.31,32 The incorporation of Al into the SBA-15 mesostructure was carried out by the direct-synthesis method using Al-isopropoxide as the Al source following the published procedure.32 In a typical synthesis, 9 mL of tetraethyl orthosilicate (TEOS) and a calculated amount of aluminum isopropoxide in order to obtain a Si/Al ratio equal to 10 is added to 10 mL of aqueous HCl solution at pH 1.5. This solution was stirred for over 3 h and then added to a second solution containing 4 g triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) copolymer (EO20/PO70/EO20) in 150 mL of HCl aqueous solution at pH 1.5 at 40 °C. The mixture is stirred for 1 h at the same temperature and allowed to react at 100 °C for 48 h. The solid product obtained was filtered off, dried at 110 °C, and finally calcined in air at 550 °C for 6 h with a heating rate of 1 °C/min. Supported molybdenum catalysts were prepared by the incipient-wetness impregnation method by taking appropriate concentrations of ammonium heptamolybdate tetrahydrate (Fluka) using Al-SBA-15 as the support. The Co or Ni promoted catalysts were prepared by impregnating the corresponding nitrate salts on oven-dried 8%Mo/Al-SBA-15 supported catalysts. The impregnated catalysts were dried in air at 110 °C

10.1021/ie060846x CCC: $37.00 © 2007 American Chemical Society Published on Web 06/06/2007

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overnight, and all the catalysts were calcined at 550 °C for 6 h. For comparison purposes, tungsten catalysts were also prepared using SBA-15 and Al-SBA-15 supports in a similar way by taking an appropriate concentration of ammonium metatungstate tetrahydrate (Fluka) and were tested under the same conditions as the molybdenum catalysts. 2.2. Characterization. The calcined mesoporous SBA-15 and Al-SBA-15 supports were characterized by low-angle X-ray diffraction (XRD), nitrogen adsorption-desorption analysis, and 27Al MAS NMR. All the catalysts were characterized by XRD, surface area, pore-size distribution analysis, temperatureprogrammed reduction (TPR), Fourier transform infrared (FTIR), and low-temperature oxygen chemisorption (LTOC). X-ray diffraction patterns were obtained using a model Rigaku D/Max111 B diffractometer using Cu KR radiation. Powder diffractograms were recorded over a range of 2θ values from 10 to 50° under the conditions of 40 kV and 40 mA on catalysts containing various amount of molybdenum. FT-IR spectra were carried out by M/S Perkin Elmer, model Spectrum GX. The samples were prepared by mixing the powder sample with KBr and making it into a thin wafer of 13 mm diameter. Each sample was scanned for 20 times at a resolution of 2 cm-1, and the average spectrum was taken. Temperature-programmed reduction profiles of catalysts were obtained using a TPD/TPR-2900 Micromeritics (U.S.A.) instrument for analyzing the nature of reducible metal species present on the support. TPR profiles were taken from ambient temperature to 1000 °C (10 °C/min), and then the temperature was kept isothermal for 30 min. A 5% H2/Ar mixture at a flow rate of 50 mL/min was used as the reducing gas. The hydrogen consumptions corresponding to the reduction of metal oxide at various stages of reduction were computed from the peak area calibrated with a standard Ag2O. The low-temperature oxygen chemisorption (LTOC) was measured at -78 °C in a conventional high-vacuum system, on a catalyst sulfided at 400 °C for 2 h using a CS2/H2 mixture at a flow rate of 40 mL/min, according to the double-isotherm procedure reported by Parekh and Weller33 for reduced catalysts. The same system was used for the Brunauer-Emmett-Teller (BET) surface area measurements. 2.3. Catalytic Activities. The hydrodesulfurization (HDS) of thiophene and hydrogenation (HYD) of cyclohexene were carried out at 400 °C on a catalyst sulfided at the same temperature for 2 h in a flow of CS2/H2 mixture, in a fixed-bed reactor operating at atmospheric pressure and interfaced with on-line gas chromatograph equipped with a six-way sampling valve for product analysis.20 The first-order rates were evaluated according to the equation x ) r(W/F), where r is rate in mol/ h/g, x is the fractional conversion, W is the weight of the catalyst in g, and F is the flow rate of the reactant in mol/h.20 The rates thus-calculated for HDS and HYD reactions are taken as a measure of their respective activities. The conversions were kept below 15% to avoid diffusion limitations. 3. Results and Discussion 3.1. Characterization of Support. 3.1.1. XRD Analysis. SBA-15 and Al-SBA-15 exhibit characteristic peaks in the lowangle region. Figure 1A shows the XRD pattern of SBA-15. Well-resolved low-angle diffraction peaks that can be indexed to (100), (110), and (200) can be noticed. The diffraction peaks are related to long-range 2D hexagonal ordering in the P6mm space group. The unit-cell parameter calculated from ao ) 2d100 x3 is 119.8 Å. These results are in agreement with the published literature.31 The XRD pattern of Al-SBA-15 is shown in Figure 1B. It can be noted that well-resolved low-angle diffraction

Figure 1. Low-angle XRD pattern of calcined (A) SBA-15 and (B) AlSBA-15 samples.

Figure 2. N2 adsorption-desorption isotherms of Mo and CoMo catalysts supported over Al-SBA-15.

peaks that can be indexed to (100), (110), and (200) reflections are present. The striking similarity in X-ray diffraction peaks suggests that the hexagonal structure is retained even after Al introduction. The unit-cell parameter calculated was found to be 126.0 Å. A comparison with SBA-15 indicates that the unitcell parameter ao increases from 119.8 Å for SBA-15 to 126.0 Å for Al-SBA-15. The shift in the unit-cell parameter can be attributed to the longer bond length of the Al-O bond compared to the Si-O bond. 3.1.2. N2 Adsorption-Desorption Analysis. The textural properties of the SBA-15 and Al-SBA-15 were examined by N2 adsorption-desorption isotherms at 77 K. The corresponding adsorption-desorption isotherms for SBA-15, Al-SBA-15, 8%Mo/Al-SBA-15, and 3%Co8%Mo/Al-SBA-15 are shown in Figure 2. It can be seen that a typical type IV isotherm with H1-type hysterisis loop is obtained in all cases. The initial

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Figure 3. BJH pore-size distribution of Mo and CoMo catalysts supported over Al-SBA-15.

Figure 5. FT-IR pattern of Al-SBA-15 supported molybdenum oxide catalysts.

Figure 4. XRD profile of Al-SBA-15 supported molybdenum hydrotreating catalysts.

increase in volume of adsorption at low pressure is due to monolayer adsorption in micropores and mesopores, and the upward deviation in the 0.6-0.8 P/Po range is associated with capillary condensation inside the mesopores. The pore-size distributions of SBA-15, Al-SBA-15, and 3%Co8%Mo/Al-SBA15 are shown in Figure 3. It can be seen that a narrow poresize distribution is obtained in all cases, indicating retention of the hexagonal mesoporous structure. The mean pore diameter for SBA-15 is 66.3 Å, while for Al-SBA-15 based materials, it is around 78 Å, as expected from the longer bond length of the Al-O bond. The examination by 27Al MAS NMR and IR results gave evidence for the incorporation of Al into the SBA-15 structure. The detailed characterization results on these materials are given elsewhere.34 3.2. Characterization of Supported Catalysts. 3.2.1. XRD Analysis. XRD analysis of molybdenum catalysts gives information about the presence or absence of various Mo phases. The XRD pattern of catalysts containing 2-12 wt % Mo in the range 10-50 2θ (theta) region are given in Figure 4. The

Figure 6. TPR profile of SBA-15 and Al-SBA-15 supported Mo, CoMo, and NiMo catalysts.

XRD of pure support as well as MoO3 is also given in the same figure for comparison purposes. It can be noted that, except for a broad hump in the 15-30 2θ (theta) region due to amorphous silica, there is no evidence for the presence of molybdenum phases up to 8 wt % Mo. At 10 wt %, however, peaks that can be attributed to MoO3 appear over the broad hump due to amorphous mesoporous silica. At 12 wt % Mo, the presence of Al2(MoO4)3 can also be noted in addition to MoO3. It is clear from the XRD pattern that molybdenum oxide is well-dispersed up to 8 wt % Mo on these catalysts.

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Table 1. TPR and Activity Data of SBA-15 and Al-SBA-15 Supported Mo, CoMo, and NiMo Catalysts reaction rates (mol h-1 g-1 × 10-3)

hydrogen consumption (mL) wt %Mo 0 4 6 8 10 12 8%Mo/ SBA-15 3%Co8%M o/SBA-15 3%Co8%M o/ Al-SBA-15 3%Ni8%Mo /SBA-15 3%Ni8%Mo /Al-SBA-15

per g of cat.

per g of Moa

(mol of H2)/ (mol of Mo)a

HDS

HYD

selectivity (kHYD/kHDS)

21.0 35.2 45.4 65.8 76.9 33.9 50.9 45.5 57.8 42.5

525 586 567 658 640 423 636 568 722 531

2.24 2.51 2.43 2.82 2.74 2.00 2.72 2.43 3.88 2.27

16.7 21.0 27.7 21.9 13.0 24.0 49.1 35.5 39.6 35.3

27.5 38.3 61.8 46.6 30.7 25.7 41.2 72.7 32.5 92.0

1.6 1.8 2.2 2.1 2.3 1.1 0.8 2.0 0.8 2.6

a H consumption per gram of Mo and (mol of H )/(mol of Mo) are calculated for promoted catalysts by assuming that only molybdenum is undergoing 2 2 reduction under this condition.

Table 2. BET Surface Area and Oxygen Chemisorption Data of Al-SBA-15 Supported Molybdenum Catalysts Mo (wt %) 2 4 6 8 10 12 8%Mo/SBA

BET surface area (m2/g) 396 (404)d 388 (403) 384 (407) 375 (405) 153 (168) 108 (121) 322 (348)

oxygen uptake (µmol/(g of cat.))

O/Mo × 100

EMSAa (m2/g)

% surface coverageb (θ)

crystallite size c (Å)

21.4 45.1 66.5 85.3 36.1 25.5 62.2

20.5 21.6 21.6 20.4 6.9 4.0 14.9

12.1 25.5 37.6 48.3 20.4 14.4 35.2

3.10 6.70 10.53 12.88 13.33 13.33 10.90

17.2 16.3 16.6 17.2 51.0 86.6 23.6

a EMSA ) O uptake × 0.566 616 (this constant value is obtained from pure MoS BET surface area divided by oxygen uptake). b Surface coverage ) 2 2 100 × (EMSA/surface area). c Crystallite size: 5 × 104/(F × M), where F is density of MoS2 (4.8 g mL-1) and M is the EMSA/(g of MoS2). d The value in the parenthesis indicates the BET surface area/(g of support).

3.2.2. FT-IR Spectroscopy. Infrared spectra are useful to ascertain the presence of various molybdates. The IR spectra of Mo/Al-SBA-15 with varying Mo loadings are shown in Figure 5. In the same figure, IR spectra of pure support are also shown. The framework region of the IR spectra of AlSBA-15 shown in the figure consists of band ∼1079, 799, and 1641 cm-1. The 1079 band is attributed to T-O asymmetric stretching vibrations, and the band at 800 cm-1 is due to T-O symmetric stretching vibrations due to intrinsic vibration of TO4 tetrahedra containing Al and Si. These bands are generally structure insensitive but dependent on composition. Introduction of Mo into the structure creates new features around 897.5, 958, and 655 cm-1, and these features increase in intensity with the increase of molybdenum loading. The features in this region are generally attributed to tetrahedral molybdenum, octahedral molybdenum, and other polymolybdates and crystalline MoO3. The vibrations due to Mo tetrahedral species appear at 930830 cm-1, and those for octahedral and other polyhedral species appear in 990-930 and 860-800 cm-1. The bands between 1070 and 799 cm-1 appear to be due to octahedral and tetrahedral molybdenum species. The characteristic band of bulk MoO3 appears at 820 cm-1.35 This band overlaps with the broad 799 cm-1 band; therefore, the presence of crystalline MoO3 is difficult to ascertain from IR studies alone. These spectral studies provide evidence for the presence and growth of tetrahedral and octahedral molybdenum species on Al-SBA-15 supported catalysts. It is interesting that the overtone peak at 1641 cm-1 decreases in intensity at very high loading. 3.2.3. Temperature-Programmed Reduction Studies. The TPR patterns give useful information about the reducibility of various phases that are present in molybdenum catalysts. The TPR patterns of various molybdenum oxide catalysts containing 2-12 wt % Mo are shown in Figure 6. In the same figure, the TPR pattern of Al-SBA-15 support is also shown. The TPR

pattern is flat and featureless, indicating that the support does not contribute to reduction throughout the temperature range studied. The TPR pattern of 4 wt % Mo displays a broad peak centered on 686 °C. There is also a shoulder in the low-temperature side at 578 °C. In 6 wt % Mo, a two-peak pattern emerges with peaks at 558 and 653 °C. The two-peak pattern continued up to 8 wt % Mo. The 8 wt % Mo TPR pattern contains a small shoulder in between these peaks. The two-peak pattern is also present at 10 and 12 wt % Mo. The low-temperature peak becomes sharp and prominent, while the high-temperature peak shifts to higher temperatures. Unsupported MoO3 is known to reduce in two steps MoO3 f MoO2 and MoO2 f Mo.36 As can be seen from the figure in the case of supported molybdenum catalysts also, a two-peak pattern can be noticed. However, the assignment in these cases is not straightforward because of the presence of different species of molybdenum interacting with the surface to different extents. It is well-known that Mo interacts with the support surface strongly at low loadings. At low loading, Mo is present predominantly as tetrahedral species, which are known to be difficult to reduce.37,38 As the Mo content increases, both tetrahedral and octahedral species coexist on the support surface, as observed in the IR spectra. It is also known that octahedral and other higher polyhedral species are reduced relatively easily. Further at very high loadings, in addition to the two abovementioned species, crystalline MoO3 is present. On the basis of the above discussion, the peak in the low-temperature region may be assigned to the reduction of octahedral species to MoO2 and the high-temperature peak may be assigned to the reduction of tetrahedral species as well as for reduction of MoO2 to Mo. The peak positions and quantitative hydrogen consumption data are shown in Table 1. It can be noted that the hydrogen consumption per gram of catalyst as well as per gram of Mo increases with Mo loading. H2/Mo molar ratio less than the


Figure 7. Effect of molybdenum loading on BET surface area of Al-SBA15 supported catalysts (A) before and (B) after sulfidation.

Figure 8. Effect of reaction rates and oxygen uptake on Al-SBA-15 supported catalysts with Mo loading.

theoretical value (3) indicates that the reduction is not complete under the conditions employed. However, in the case of high loadings, the value increases but is still lower than the theoretical value. A comparative study of reducibility of 8%Mo/SBA-15 and 8%Mo/Al-SBA-15 is shown in Figure 6. It can be seen that both exhibit two-peak patterns. The low-temperature peak of Al-SBA-15 is slightly higher, indicating a stronger interaction of Mo with Al-SBA-15 compared to SBA-15. The onset of reduction of Mo on Al-SBA-15 starts at a lower temperature, indicating there is also some portion of relatively easily reducible molybdenum on these catalysts. The presence of a shoulder on the peak envelope of Al-SBA-15 coupled with the broadness of the peak suggests that the surface of Al-SBA-15 is heterogeneous, accommodating a number of species with differing reducibilities. The quantitative data shown in Table 1 indicate that the hydrogen consumption is lower and the H2/Mo ratio is also lower, indicating that the reducibility of Mo is lower on Mo/SBA-15 compared to Mo/Al-SBA-15. A comparative study of Co and Ni promoted analogues of 8%Mo/SBA-15 and 8%Mo/Al-SBA-15 is also shown in Figure 6 and corresponding

Figure 9. Effect of promoters Co or Ni content on catalytic functionalities of Al-SBA-15 supported molybdenum hydrotreating catalysts.

quantitative data are shown in Table 1. It can be seen from the figure that the promoted catalysts also exhibit a two-peak pattern similar to that of the unpromoted catalysts. However, there are significant changes in intensities as well as peak temperatures. The low-temperature peak of CoMo catalysts supported on SBA15 and Al-SBA-15 occurs more or less at the same temperature. The NiMo catalysts display a significant shift of the lowtemperature peak to further lower temperatures. It can also be noticed that, in the case of NiMo catalysts, the hightemperature peak is broad and low in intensity compared to the cobalt-containing catalysts. It is interesting to note that Al-SBA15 supported CoMo and NiMo catalysts exhibit broader peaks compared to SBA-15 supported analogues, indicating the presence of a variety of species with varying reducibilities. H2 consumption per gram of catalyst, H2 consumption per gram of Mo, and H2/Mo molar ratios shown in Table 1 give a clear picture about the effect of promoting ions on the reducibility of molybdenum on these supports. It can be seen from the given data that, although molybdenum on Al-SBA-15 is marginally more reducible than on SBA-15, the promoted catalysts show different behavior. The reducibilities per gram of catalyst and per gram of Mo are higher in the case of CoMo and NiMo promoted catalysts supported on SBA-15, suggesting that the effect of promoter ions on Mo reducibility is more pronounced in the case of SBA-15 supported catalysts. This may be due to stronger interaction of molybdenum on Al-SBA-15 and consequent resistance to reduction. In the case of Al2O3 supported catalysts, it is known that molybdenum interacts strongly with the Al2O3 support compared to SiO2 supported catalysts.12 The H2/Mo molar ratios suggest that SBA-15 supported molybdenum catalysts promoted by Co and Ni are more reducible compared to corresponding catalysts based on Al-SBA-15. These differences in reducibilities have a bearing on catalytic activities for hydrodesulfurization, and this aspect will be dealt with after discussing catalytic activities.

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Figure 10. Comparison of hydrotreating catalytic functionalities of SBA-15 and Al-SBA-15 supported Mo, CoMo, NiMo,W, NiW, and CoW catalysts.

3.2.4. Surface Area Analysis. The Al-SBA-15 support and supported molybdenum catalysts obtained after calcination at 550 °C for 6 h were examined by BET surface area. The corresponding data are shown in Table 2. The surface area per gram of catalyst as well as per gram of support in both oxide and sulfided state are shown in Figure 7 as a function of Mo loading. It can be noted that the surface area per gram of catalyst decreases while the surface area per gram of support remains more or less constant in case of both oxide and sulfided catalysts up to 8 wt % Mo. Beyond this loading, however, there is a rapid decrease in all the cases. Massoth suggested that it is possible to analyze such behavior to extract information about monolayer formation of the supported species.39 In the case of Mo/γ-Al2O3 catalysts, Massoth has shown that the surface area per gram of support remains more or less constant as a function of Mo loading. The invariance of surface area per gram of support is interpreted as a formation of monolayer of MoO3 on γ-Al2O3 catalysts. The fact that similar behavior is shown in the case of Mo/Al-SBA-15 catalysts suggests that molybdenum is in the form of a monolayer up to 8 wt % Mo. Therefore, the surface area analysis suggests that MoO3 as well as MoS2 are in highly dispersed states up to 8 wt % Mo loading. Similar results were also presented in the case of Mo and W supported on other supports.40,41,20 3.2.5. Oxygen Chemisorption Studies. The results of oxygen chemisorption studies carried out at low temperature (-78 °C) on sulfided molybdenum catalysts are reported in Table 2. The values given here are after correcting for support contribution. The oxygen uptake variation as a function of molybdenum loading on sulfided catalysts is shown in Figure 8. It can be seen that the oxygen uptakes increase with molybdenum loading up to 8 wt % Mo and start to decrease at higher loadings. Oxygen is known to chemisorb on anion vacancies present in sulfided Mo and W systems.23 Therefore, such variation suggests that anion vacancies also increase up to 8 wt % Mo and then decrease at higher loading. It is also suggested by Zmierczak et al.42 that oxygen uptakes represent the general state of dispersion of MoS2 on supported catalysts. Therefore, the variation also indicates that MoS2 dispersion is passing through a maximum at 8 wt % Mo loading. It is possible to calculate parameters like % dispersion, equivalent molybdenum sulfide area (EMSA), % surface coverage, and crystallite size of MoS2 from oxygen uptakes. The results of such calculations are shown in Table 2. The dispersion is constant up to 8 wt % Mo and then decreases at higher loading. The EMSA increases up to 8 wt % Mo and then decreases beyond this loading. The crystallite size is more or less constant at ∼16 Å up to 8 wt % Mo and then onward

increases rapidly at higher loadings. It is important to mention at this juncture that presence of MoO3 peaks are noticed in the XRD pattern at 10 and 12 wt % Mo loading. The surface coverage by MoS2 is only 13%, indicating that molybdenum selectively attached to certain preferred portions of the support surface. It can also be noted from Table 2 that oxygen uptakes are higher and crystallite sizes are lower in the case of Mo/AlSBA-15 compared to Mo/SBA-15, suggesting that Mo is relatively well-dispersed on Al-SBA-15 compared to SBA-15 support. 3.3. Catalytic Activity Studies. The catalytic activities for hydrodesulfurization of thiophene and hydrogenation of cyclohexene carried out on sulfided catalysts at 400 °C as a function of Mo content are shown in Figure 8. It can be noted that catalytic activities represented by first-order rate constants for HDS and HYD increase with molybdenum loading up to 8 wt % Mo and then start decreasing. It can be noticed that similar behavior is also seen in the case of oxygen uptakes. It can be seen that there exists a correlation between oxygen uptakes and anion vacancies. Since it is well-known that anion vacancies are the seat for HDS as well as HYD and it is also welldocumented that oxygen chemisorption is associated with anion vacancies, such an observation, therefore, suggests that oxygen uptakes are related to the catalytic activities through anion vacancies.42 It can be noted that 8 wt % Mo is the optimum Mo loading to get maximum activity on this support. It is well-known that Co and Ni are excellent promoters for hydrotreating functionalities. Normally, the maximum in promotional effect occurs in the vicinity of 0.3 on γ-Al2O3 supported catalysts.12 It is interesting to see how the promotional effect of Co or Ni varied for HDS and HYD in the case of Mo/Al-SBA-15 catalysts as a function of Co or Ni content. Such a relationship is shown in parts A and B of Figure 9 for HDS and HYD, respectively. It can be seen that, in the case of HDS, a maximum is obtained at Co/[Co+Mo] ratio of 0.3 for cobalt catalysts, whereas in the case of Ni, the activities increase up to 5 wt % Ni. The HYD activity passes through a maximum at Co/[Co+Mo] or Ni/[Ni+Mo] ratio of 0.3 for both cobalt and Ni. This behavior is similar to that of γ-Al2O3 supported catalysts. Therefore, the ratio of 0.3 appears to be optimum for HDS and HYD for both the promoters. 3.4. Comparison of Mo or W Catalysts on Al-SBA-15 and SBA-15 Supports. A comparison of Mo and W catalysts supported on Al-SBA-15 is shown in Figure 10. In the same figure, results on Mo or W on SBA-15 are also shown. It can be seen that Mo/Al-SBA-15 is comparable in HDS activity to W/Al-SBA-15. However, in the case of hydrogenation activity,


W/Al-SBA-15 seems to be superior to molybdenum analogues of Al-SBA-15. A comparison of Mo/SBA-15 with W/SBA-15 indicates that Mo catalysts are superior to W supported catalysts for both HDS and hydrogenation. Although the HDS activities are comparable, it is interesting that Mo/Al-SBA-15 catalysts display higher hydrogenation activities compared to Mo/SBA-15, indicating that the acidic nature of Al-SBA-15 assists to augment hydrogenation functionality as reported by Breysse and coworkers.30 It is interesting to note that higher hydrogenation activities are also shown by W/Al-SBA-15. The oxygen uptakes plotted in the same graph follow the variation of catalytic activities for HDS and HYD, suggesting that dispersion differences in the catalysts may be responsible for the observed activity variations. This behavior is similar for HDS and HYD, suggesting that oxygen chemisorption is not specific to any one of the functionalities but measures the general state of dispersion of Mo or W. The observation that, for each of the Mo catalysts, the two functionalities varied independently suggests that these two functionalities may be originating from different sets of sites on the MoS2 surface.Topsoe and co-workers established on model systems that hydrogenation takes place on Brim sites.43 4. Summary and Conclusions In this investigation, Al-SBA-15 supported Mo, CoMo, and NiMo catalysts were characterized by BET surface area, IR, TPR, and oxygen chemisorption. The support characterization indicated that we are dealing with high surface area and hexagonal mesoporous Al-SBA-15 with Al predominantly in tetrahedral positions. The analysis of BET surface areas indicated that molybdenum is dispersed as a monolayer in both oxidic and sulfidic state. The pore-size distributions indicated that the hexagonal mesoporous structure is retained after Mo and promoter ion introduction. The X-ray diffraction analysis in the 10-50 2θ range indicated that MoO3 is likely to be present with crystallite sizes lower than 40 Å, suggesting that MoO3 is well-dispersed up to 8 wt % Mo. Beyond this loading, however, the presence of MoO3 as well as Al2(MoO4)3 are noted. The IR spectral studies 400 - 2000 cm-1 region gave evidence for the presence of octahedral and tetrahedral molybdenum and its growth with Mo loading. The TPR studies indicated that a twinpeak reduction pattern is obtained, which can be attributed to the reduction of octahedral and tetrahedral molybdenum species. A comparison between SBA-15 and Al-SBA-15 catalysts suggested that the reducibility of Al-SBA-15 supported Mo catalysts is marginally more than that of SBA-15 supported catalysts. However, in the case of promoted catalysts, the reducibilities are higher in the case of SBA-15 supported catalysts. The oxygen uptakes and crystallite size derived from them indicated that anion vacancy concentration increases up to 8% Mo loading and then decreases. The catalytic activities for HDS and HYD follow the same trend, suggesting that 8%Mo on Al-SBA-15 is optimum in terms of molybdenum dispersion and catalytic activities and there exists a relationship between oxygen uptakes and catalytic activities. The hydrogenation activities of Al-SBA-15 are higher for Mo, CoMo, and NiMo catalysts, indicating that the acidic nature of Al-SBA-15 is augmenting the hydrogenation activity. The results suggested that oxygen chemisorption is not specific to any one of the functionalities but measures the general state of dispersion of MoS2. The independent variations of HDS and HYD with promoters and support indicate that HDS and HYD may be originating from different sets of sites.

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ReceiVed for reView July 1, 2006 ReVised manuscript receiVed April 25, 2007 Accepted April 26, 2007 IE060846X

Effect of Al-SBA-15 Support on Catalytic Functionalities of

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