Catalytic Functionalities of H-β-Zeolite-Supported Molybdenum

Jul 6, 2006 - Catalysts containing 2−12 wt % Mo and 1−5 wt % Co or Ni were prepared using commercial H-β-zeolite as a support. The support as wel...
0 downloads 12 Views 140KB Size
Download PDF
1784

Energy & Fuels 2006, 20, 1784-1790

Catalytic Functionalities of H-β-Zeolite-Supported Molybdenum Hydrotreating Catalysts G. Muthu Kumaran, Shelu Garg, Kapil Soni, V. V. D. N. Prasad, L. D. Sharma, and G. Murali Dhar* Catalytic ConVersion Processes DiVision, Indian Institute of Petroleum, Dehradun 248 005, India ReceiVed March 30, 2006. ReVised Manuscript ReceiVed June 4, 2006

Catalysts containing 2-12 wt % Mo and 1-5 wt % Co or Ni were prepared using commercial H-β-zeolite as a support. The support as well as various catalysts were characterized by X-ray diffraction (XRD), BET surface area, temperature-programmed reduction (TPR), and in situ oxygen chemisorption in the sulfided state. The XRD studies indicated that the crystallinity of β-zeolite decreases rapidly at higher loadings of molybdenum, and there is no evidence for the presence of crystalline Mo phases below 10 wt % Mo loading. At higher loadings, however, MoO3/MoS2 may be present in small quantities. The BET surface area analysis indicated that molybdenum is well-dispersed up to 6 wt % Mo loading in both oxide and sulfide states. The oxygen uptakes increased up to 6 wt % Mo and then decreased at higher loadings. The crystallite sizes evaluated by oxygen chemisorption are small, indicating that MoS2 is well-dispersed on the support up to 6 wt % Mo. The TPR results indicated that a Co or Ni promoter helps decrease the temperature of the Mo reduction. TPR results indicated that two molybdenum phases with differing reduction behavior exist before and after 6% Mo loading. Catalytic activities for HDS, HYD, and HCK followed the same trend as oxygen uptake. A comparison with γ-Al2O3 indicated that the β-zeolite support imparts superior HYD activities in Mo, CoMo, and NiMo catalysts. Y-zeolite catalysts exhibit higher HDS activities but lower HYD activities than β-zeolite-supported catalysts.

Introduction Hydrodesulfurization is an important unit operation in petroleum refining, and its importance is further emphasized because of the necessity of producing clean fuels that meet stringent environmental regulations.1 Starting from naphtha, kerosene, diesel, and vacuum gas oil, fuel oil and residue need to be hydrotreated at one stage or another during petroleum refining. In addition, it is also necessary to hydrotreat liquids derived from coal, oil shale, tar sands, etc. All these challenging tasks need highly active and balanced catalysts with respect to various catalytic functionalities such as hydrodesulfurization, hydrogenation, and hydrocracking.2,3 The main reactions involved in the above-mentioned processes are hydrogenolysis of C-S, C-N, and C-O bonds; hydrogenation of aromatic rings and olefins; and hydrocracking of C-C bonds.3 In addition to hydrogenolysis of substituted dibenzothiophenes such as 4,6-dimethyl dibenzothiophene, the hydrogenolysis of C-N bonds in nitrogen heterocyclic aromatic compounds includes ring hydrogenation prior to C-N bond cleavage. The processing of cracked feeds also involves hydrogenation of olefins. The acidic function of the support and sulfided phases contributes to hydrocracking. The acidic function is also useful in deep desulfurization of a 4,6-dimethyl diben* To whom correspondence should be addressed. E-mail: gmurli@ iip.res.in. Fax: 91-135-2660202. (1) Song, C. Catal. Today 2003, 86, 211. (2) Bertooacini, R. J.; Forgac, J. M.; Kim, D. K.; Pellet, R. J.; Robinson, K. K. In Proceedings of the Third International Conference on Chemistry and Uses of Molybdenum; Barry, H. F., Mitchell, P. C. H., Eds.; Climax Molybdenum Co.: Ann Arbor, MI, 1979; p 224. (3) Topsoe, H.; Clausen, B. S.; Massoth, F. E. In Catalysis: Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer Verlag: Berlin, 1996; Vol. 11.

zothiophene type of refractory sulfur compound to demethylate to reduce steric hindrance or isomerize to less-refractory isomers.1 Therefore, the catalysts used to process these liquids should have hydrogenolysis, hydrogenation, and hydrocracking functionalities. The above-mentioned functionalities are not only required but should also be appropriately balanced, depending on the application. To be able to prepare an effective, wellbalanced, sulfided catalyst, it is necessary to gain knowledge on the origin and interrelationship of functionalities of the catalyst and their dependence on the nature of support, composition, promoter, etc., variables. Support plays an important role in influencing the catalytic functionalities of the active component by varying dispersion and/or metal-support interactions.4-6 The influence of the support on catalytic properties of hydroprocessing catalysts has been studied with great interest.6-8 Many materials have been tested as supports to Mo and W active components with Co and Ni as promoters, some of which are SiO2,9 MgO,10 ZrO2,11,12 TiO2,13 carbon,14 and mixed oxides such as TiO2-ZrO2,15 (4) Okamoto, Y.; Breysse, M.; Murali Dhar, G.; Song, C. Catal. Today 2003, 86, 1. (5) Rana, M. S.; Maity, S. K.; Ancheyta, J.; Murali Dhar, G.; Prasada Rao, T. S. R. Appl. Catal., A 2004, 268, 165. (6) Breysse, M.; Portefaix, J.-L.; Vrinat, M. Catal. Today 1991, 10, 489. (7) Murali Dhar, G.; Srinivas, B. N.; Rana, M. S.; Manoj, K.; Maity, S. K. Catal. Today 2003, 86, 45. (8) Klimova; Lizama, L.; Amezcua, J. C.; Roquero, P.; Terres, E.; Navarrete, J.; Dominguez, J. M. Catal. Today 2004, 98, 141. (9) Delmon, B. In Proceedings of the Third International Conference on Chemistry and Uses of Molybdenum; Barry, H. F., Mitchell, P. C. H., Eds.; Climax Molybdenum Co.: Ann Arbor, MI, 1979; Vol. 73. (10) Chary, K. V. R.; Ramakrishna, H.; Rama Rao, K. S.; Murali Dhar, G.; Kanta Rao, P. Catal. Lett. 1991, 10, 27. (11) Afanasiev, P.; Cattenot, M.; Geantet, C.; Matsubayashi, N.; Sato, K.; Shimada, H. Appl. Catal., A 2002, 237 (1-2), 227.

10.1021/ef060134j CCC: $33.50 © 2006 American Chemical Society Published on Web 07/06/2006

H-β-Zeolite-Supported Molybdenum Hydrotreating Catalysts

ZrO2-Al2O3,16 TiO2-Al2O3,17 ZrO2-SiO2,18 SiO2-Al2O3,19-21 TiO2-SiO2,22 etc. Zeolites such as NaY,23,24 USY,25 ZSM-5,26 and β-zeolite27 have also been employed as supports. In recent years, mesoporous materials such as MCM-41,28 HMS,29,30 and SBA-1531 have been studied with great interest as supports for hydrotreating catalysts. In a continuation of our work on the effect of the support on catalytic functionalities, we are presenting our results on H-β-zeolite-supported molybdenum catalysts with a goal of understanding the origin and variation catalytic functionalities as a function of molybdenum and cobalt or nickel contents and relating the activities to results on physicochemical characterization of the catalysts. Experimental Section Sample Preparation. The H-β-zeolite support used in this work is obtained commercially as the NH4 form of β-zeolite from Sud Chemie India Ltd., which was then calcined at 550 °C for 6 h to get the H form. Supported Mo/H-β catalysts were prepared by the incipient-wetness impregnation method by taking appropriate concentrations of ammonium heptamolybdate ((NH4)6Mo7O24‚ 4H2O, Fluka) using H-β-zeolite as support. The Co- or Ni-promoted catalysts were prepared by impregnating the corresponding nitrate salts on an oven-dried 6%Mo/H-β-zeolite-supported catalyst. The impregnated catalysts were dried in air at 110 °C overnight, and all the catalysts were calcined at 550 °C for 6 h. For comparison purposes, γ-Al2O3-, NaY-, and ultrastable Y-zeolite-supported catalysts were also prepared in a similar way and tested under conditions similar to those for β-zeolite-supported catalysts. Characterization. The calcined H-β-zeolite support and catalysts were characterized by XRD, BET surface area, temperatureprogrammed reduction (TPR), and low-temperature oxygen chemisorption (LTOC). X-ray diffraction patterns were obtained using a (12) Maity, S. K.; Rana, M. S.; Srinivas, B. N.; Bej, S. K.; Murali Dhar, G.; Prasada Rao, T. S. R. J. Mol. Catal. A: Chem. 2000, 153 (1-2), 121. (13) Shimada, H. Catal. Today 2003, 86 (1-4), 17. (14) Abotsi, G. M. K.; Scaroni, A. W. Fuel Process. Technol. 1989, 22, 107. (15) Daly, F. P.; Ando, H.; Schmitt, J. L.; Sturm, E. A. J. Catal. 1987, 108, 401. (16) Damyanova, S.; Petrov, L.; Centeno, M. A.; Grange, P. Appl. Catal., A 2002, 224 (1-2), 271. (17) Borque, M. P.; Lo´pez-Agudo, A.; Olguı´n, E.; Vrinat, M.; Ceden˜o, L.; Ramı´rez, J. Appl. Catal., A 1999, 180 (1-2), 53. (18) Rana, M. S.; Srinivas, B. N.; Maity, S. K.; Murali Dhar, G.; Prasada Rao, T. S. R. J. Catal. 2000, 195, 31. (19) Massoth, F. E.; Murali Dhar, G.; Shabtai, J. J. Catal. 1994, 85, 52. (20) Pawelec, B.; Navarro, R. M.; Campos-Martin, J. M.; Lopez Agudo, A.; Vasudevan, P. T.; Fierro, J. L. G. Catal. Today 2003, 86, 73. (21) Barrio, V. L.; Arias, P. L.; Cambra, J. F.; Gu¨emez, M. B.; Pawelec, B.; Fierro, J. L. G. Fuel 2003, 82 (5), 501. (22) Rana, M. S.; Srinivas, B. N.; Maity, S. K.; Murali Dhar, G.; Prasada Rao, T. S. R. In Hydrotreatment and Hydrocracking of Oil Fractions (Studies in Surface Science and Catalysis); Delmon, B., Froment, G. F., Grange, P., Eds; Elsevier: New York, 1997; pp 127 and 397. (23) Li, D.; Nishijima, A.; Morris, D. E. J. Catal. 1999, 182 (2), 339. (24) Welters, W. J. J.; Vorbeck, G.; Zandbergen, H. W.; van de Ven, J. M.; van Oers, E. M.; de Haan, J. W.; de Beer, V. H. J.; van Santen, R. A. J. Catal. 1996, 161 (2), 819. (25) Bendezu´, S.; Cid, R.; Fierro, J. L. G.; Lo´pez Agudo, A. Appl. Catal., A 2000, 197 (1), 47. (26) Sugioka, M.; Sado, F.; Kurosaka, T.; Wang, X. Catal. Today 1998, 45 (1-4), 327. (27) Hedoire, C.-E.; Louis, C.; Davidson, A.; Breysse, M.; Mange, F.; Vrinat, M. J. Catal. 2003, 220 (2) 433. (28) Wang, A.; Wang, Y.; Kabe, T.; Chen, Y.; Ishihava, A.; Qian, W.; Yao, P. J. Catal. 2002, 210, 319. (29) Chiranjeevi, T.; Kumar, P.; Maity, S. K.; Rana, M. S.; Murali Dhar, G.; Prasada Rao, T. S. R. Microporous Mesoporous Mater. 2001, 44-45, 547. (30) Zepeda, T. A.; Halachev, T.; Pawelec, B.; Nava, R.; Klimova, T.; Fuentes, G. A.; Fierro, J. L. G. Catal. Commun. 2006, 7 (1), 33. (31) Murali Dhar, G.; Muthu Kumaran, G.; Manoj, K.; Rawat, K. S.; Sharma, L. D.; David Raju, B.; Rama Rao, K. S. Catal. Today 2005, 99, 309.

Energy & Fuels, Vol. 20, No. 5, 2006 1785 model Rigaku D/Max-111 B diffractometer using Cu KR radiation. Powder diffractograms were recorded over a range of 2θ values from 5 to 50° under the conditions of 40 kV and 40 mA on various molybdenum samples in both oxide and sulfide states. The samples were sulfided at 400 °C with a CS2/H2 mixture for 2h and then cooled to room temperature; the XRD patterns were obtained ex situ on these samples. 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 the temperature was then 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 consumption corresponding to the reduction of metal oxide at various stages of reduction was computed from the peak area calibrated with a Ag2O standard. The H2 consumption per Mo is calculated by dividing the total hydrogen consumption per gram of catalyst by the metal content. 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 Weller.32 The same system was used for the BET surface area measurements. Catalytic Activities. The hydrodesulfurization (HDS) of thiophene, hydrogenation (HYD) of cyclohexene, and hydrocracking of cumene (HCK) were carried out at 400 °C on a catalyst sulfided at the same temperature for 2 h in a flow of a 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.28 The first-order rates were evaluated according to the equation x ) r(W/F) where r is the rate in moles per hour per gram, x the fractional conversion, W the weight of the catalyst in grams, and F the flow rate of the reactant in moles per hour.10 The rates thus calculated for HDS, HYD, and HCK reactions were taken as a measure of their respective activities. The conversions were kept below 15% to avoid diffusion limitations.

Results and Discussion X-ray Diffraction. The β-zeolite-supported molybdenum catalysts obtained after calcination have been examined by X-ray diffraction in oxide as well as sulfide states. The X-ray diffraction patterns are shown in Figure 1A. The X-ray diffraction pattern of pure H-β-zeolite is also included in the same figure. It can be seen from the figure that the intensities of the zeolite lines decrease with the addition of Mo to the zeolite, indicating that the crystallinity of the zeolite is decreasing with increasing Mo loading up to 12% Mo. The crystallinity drops to 75% with the addition of 2 wt % Mo and then changes to a small extent up to 6 wt % Mo; beyond this loading, there is a rapid decrease. This observation has a bearing on the dispersion and crystallite size of the supported phase, which will be dealt with after discussing the oxygen chemisorption results. There is no evidence for the presence of any crystalline phases of molybdenum compounds except 12 wt % Mo on β-zeolite in the oxide state, where there are weak signals indicating the presence of MoO3 and Al2(MoO4)3 phases. The high-intensity lines of MoO3 fall in the same region (2θ ) 2025°) as the strong lines of β-zeolite; therefore, it is difficult to confirm the absence of these phases at lower loading in the oxide state. To get more insight, we have examined the catalysts in their sulfided state, as the MoS2 strong lines are in a different 2θ region (2θ ) 14°). These spectra are shown in Figure 1B. It can be noted that there is no indication for the presence of any crystalline MoS2 phases up to 8 wt % Mo, indicating that MoS2 in the sulfided state is of crystallite size smaller than ∼40 (32) Parekh, B. S.; Weller, S. W. J. Catal. 1977, 47, 100.

1786 Energy & Fuels, Vol. 20, No. 5, 2006

Kumaran et al.

Figure 2. BET surface area analysis of H-β-zeolite-supported molybdenum catalysts before and after sulfidation.

Figure 1. XRD profile of H-β-zeolite-supported molybdenum catalysts in the (A) oxide state (asterisk denotes the Al2(MoO4)3 crystallines) and (B) sulfide state.

Å. However, at higher loadings, the presence of MoS2 crystallites is difficult to ascertain because of weak signals. Therefore, it is clear that MoS2 is well-dispersed in both oxide and sulfide states throughout the range of molybdenum loadings studied in this investigation. Surface Area Analysis. The BET surface area of the catalysts was evaluated as a function of Mo loading in both oxidic as well as sulfided states. These results are shown in Figure 2 and Table 1. In the figure, the BET surface area per gram of support and per gram of catalyst is plotted as a function of Mo loading. It can seen that the BET surface area per gram of support remains more or less constant up to 6 wt % Mo, whereas the BET area per gram of catalyst continuously decreases with increasing Mo loading. Massoth suggested that for Mo/Al2O3 catalysts,33 the invariance of the BET surface area per gram of support indicates that MoS2 is dispersed as a monolayer or in a very highly dispersed state. Similar observations were noted by Chiranjeevi et al. on Mo/HMS,29 Saiprasad Rao et al.34 on WO3/ZrO2, and Murali Dhar et al. on Mo/SBA-15.31 Therefore, the surface area variation discussed above indicates that Mo is (33) Massoth, F. E. J. Catal. 1975, 36, 166. (34) Rao, K. S. P.; Ramakrishna, H.; Murali Dhar, G. J. Catal. 1992, 133, 146.

very well dispersed in the sulfided state, confirming the observations noted from X-ray diffractograms. Temperature-Programmed Reduction Studies. TPR is a very useful tool for gaining knowledge about the reducibility of various species present in the hydrotreating catalysts. MoO3/ H-β-zeolite catalysts containing different amounts of molybdenum in the range 2-12% Mo are subjected to TPR analysis. The β-zeolite has a flat reduction profile, indicating that the support does not contribute to reduction significantly (Figure 3). The 2 wt % Mo sample shows a single peak at 620 °C, with a shoulder on the low-temperature side. The 4 wt % Mo samples shows a peak at 520 °C and a shoulder on the high-temperature side. The 6 wt % Mo shows a pattern similar to that of 4% Mo, but the low-temperature peak at 537 °C is more intense. In 8% Mo, a two-peak pattern is displayed at 528 and 744 °C. There is also a small shoulder at 595 °C. The two-peak pattern continued up to 12% Mo. It can also be noted that the intensity of both the low- and high-temperature peaks increases with loading. The temperature peak maxima for the low-temperature peak did not a show clear trend, but the high-temperature peak is shifted further toward the higher-temperature side with increased Mo loading. Unsupported MoO3 is known to reduce in two steps, MoO3 f MoO2 and MoO2 f Mo.35 Two distinct peaks with the ratio 1:2 can be discerned from the spectrum. As can be seen from the figure in the case of the supported molybdenum catalysts, a two-peak pattern can be noted. However, the assignment in these cases is not straightforward because of the presence of different species of molybdenum interacting with surface to different extent. 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.12,36 As the Mo content increases, both tetrahedral and octahedral species coexist on the support surface. It is also known that octahedral and other higher polyhedral species are reduced relatively easily. At further very high loadings, in addition to the two above-mentioned species, crystalline MoO3 is present. On the basis of the above discussion, we may assign the peak in the low-temperature region to octahedral species and MoO2 and the high-temperature peak to the reduction of tetrahedral species as well as for reduction of MoO2 to Mo. The peak temperatures, low-temperature peak area to total area, and H2 consumption data are shown in Table 2 and Figure (35) Arnoldy, P.; de Jonge, J. C. M.; Moulijin, J. A. J. Phys. Chem. 1985, 89, 4517. (36) Chary, K. V. R.; Reddy, K. R.; Kishan, G.; Niemantsverdriet, J. W.; Mestl, G. J. Catal. 2004, 226 (2), 283.

H-β-Zeolite-Supported Molybdenum Hydrotreating Catalysts

Energy & Fuels, Vol. 20, No. 5, 2006 1787

Table 1. BET Surface Area and Oxygen Chemisorption Data of MoO3/H-β-Zeolite Catalysts SBET (m2/g) Mo (wt %)

per g of catalyst

per g of support

O2 uptake (µmol/g)a

O/Mo × 100

EMSA (m2/g)b

surface coverage (θ)c

crystallite size (Å)d

0 2 4 6 8 10 12

622 488 461 431 344 235 225

622 498 480 457 371 258 252

10.0 20.0 38.8 68.2 58.5 28.6 20.5

19.2 18.6 21.8 14.0 5.5 3.2

11.3 22.0 38.6 33.1 16.2 11.6

2.5 4.9 9.1 9.6 6.9 5.1

18.3 18.9 16.1 25.1 64.3 107.6

a The support contribution is corrected. b EMSA ) O uptake × 0.566616 (this constant value is obtained from pure MoS BET SA divided by oxygen 2 2 uptake). c Surface coverage ) 100 × (EMSA/SAcatalyst). d Crystallite size ) 5 × 104/(FM), where F is density of MoS2 (4.8 g mL-1) and M is the EMSA per gram of MoS2.

Figure 3. TPR profile of H-β-zeolite-supported molybdenum catalysts.

4. It can be noticed from the table that the H2 consumption per gram of catalyst increases with molybdenum loading, indicating an increase in the reducibility of molybdenum with increased Mo loading. The H2 consumption per gram of Mo, however, shows a decreasing trend up to 6% Mo and then starts increasing up to 12% Mo. The variation in reduction behavior before and after 6% Mo loading signifies the presence of two types of species with different reduction behaviors in the two regions. Figure 4 shows a graphical representation of H2 consumption per gram of Mo and the H2:Mo molar ratio against molybdenum loading. It can be seen that two distinct regions of reducibility can be noted in the 2-6 wt % Mo and 6-12 wt % Mo regions. These interesting results will be further discussed after discussion about oxygen chemisorption and activity data. The TPR patterns of promoted cobalt and nickel at the 3 wt % level on 6%Mo/β-zeolite are shown in Figure 5. It can be seen that reduction patterns are shifted to the low-temperature side, suggesting that the promoters help molybdenum species reduce at lower temperatures. Nickel appears to be more effective in this respect compared to cobalt. The H2 consumption per gram of Mo and H2:Mo molar ratios indicate that 6 wt % Mo sample MoO3 is not completely reduced, whereas the addition of a promoter helps increase the reducibility. For the addition of 5 wt % Co or Ni to 6 wt % Mo, however, the

H2:Mo molar ratio is higher than the theoretical value (3.0), indicating that free-promoter oxide reduction also contributes to total reduction at this stage. It is well-known that 3 wt % Co or 3 wt % Ni represents the optimum for promotion in supported Mo systems, as will be seen in this case and in the discussion that follows. Low-Temperature Oxygen Chemisorption. Oxygen uptake as a function of Mo loading is shown in Figure 6 and Table 1. It can be noted that oxygen chemisorption increases with molybdenum loading up to 6 wt % and then decreases with further loading. This point of inflection in the above figure is also seen at the same Mo loading in surface area per gram of support as well as in the H2:Mo molar ratio vs Mo loading plot. Oxygen is well-known to chemisorb on anion vacancies; therefore, the increase in oxygen uptake up to 6 wt % Mo and decrease at higher loadings suggest that anion vacancy variation on the molybdenum also follows a trend similar to that for the Mo loading. From the oxygen uptake data, it is possible to calculate parameters such as percent dispersion, equivalent MoS2 area (EMSA), percent surface coverage, and crystallite size of MoS2. These results are shown in Table 1. It can be noted from the table that only 10% of the total surface is covered by molybdenum, indicating that molybdenum selectively interacts with some portions of the support. The crystallite size is more or less constant up to 6 wt % Mo and then increases to reach ∼108 Å at 12% Mo. These crystallite size data are also in agreement with XRD data, indicating that Mo is well-dispersed up to 6 wt % Mo loading. To further understand the growth of MoS2 crystallites in this system, we have plotted crystallinity of the zeolite and crystallite size of MoS2 in the same figure (Figure 7) as a function of Mo loading. It can be noted that the crystallinity and crystallite size of MoS2 did not vary significantly up to 6 wt % Mo loading; the crystallinity of the zeolite then decreases rapidly, and the crystallite size of MoS2 also increases significantly. It appears that beyond 6 wt % Mo loading, the loss of crystallinity is responsible for the growth of crystallites of MoS2. Hydrotreating Catalytic Functionalities. Catalytic activities for the three functionalities viz. thiophene hydrodesulfurization, cyclohexene hydrogenation, and cumene hydrocracking on catalysts sulfided at 400 °C for 2 h obtained at 400 °C are shown in Figure 6. It can be seen that the three functionalities increase with Mo loading up to 6 wt % Mo and then start decreasing. The O2 uptake is also plotted in the same figure and varies in a manner similar to that of the activities. Because it is wellknown that hydrogenation and hydrodesulfurization takes place on anion vacancies and because oxygen is also known to chemisorb on anion vacancies, the similarity in behavior is not surprising. To further establish the relationship, we plotted the

1788 Energy & Fuels, Vol. 20, No. 5, 2006

Kumaran et al.

Table 2. TPR Data of H-β-Zeolite-Supported Molybdenum Catalysts hydrogen consumption (mL) wt % Mo on H-β 2 4 6 8 10 12 3%Co6%Moa 3%Ni6%Moa a

per g of catalyst

per g of Mo

mole H2:mole MoO3

relative peak area (A1/total)

13.0 23.9 34.6 50.0 65.2 80.0 42.6 42.1

654 598 576 625 652 666 710 701

2.80 2.56 2.46 2.67 2.79 2.87 3.00 3.00

0.46 0.54 0.66 0.54 0.55 0.50

Assuming that only MoO3 is undergoing reduction.

Figure 4. Variation in H2 consumption with Mo loading on H-βzeolite-supported catalysts.

Figure 5. TPR profile of H-β-zeolite-supported Mo, CoMo, and NiMo catalysts.

Figure 6. Effect of molybdenum loading on oxygen chemisorption and the reaction rate of H-β-zeolite-supported molybdenum catalysts.

oxygen uptake against the activities in Figure 8. It can be seen that a linear relationship passing through the origin is obtained in all three cases, indicating that oxygen uptake is indeed related to catalytic activities. It is interesting to note that such a

Figure 7. Effect of MoS2 crystallite size and percent crystallinity of H-β-zeolite with Mo loading on H-β-zeolite-supported catalysts.

Figure 8. Comparison between reaction rates and oxygen uptake of H-β-zeolite-supported molybdenum catalysts.

relationship is obtained in the case of hydrocracking, which is known to be a property of the zeolite support. Promotional Effects. Promotional effects of Co and Ni are examined by varying the promoter contents between 1 and 5 wt %. The variation in catalytic activities for HDS, HYD, and HCK as a function of promoter content are seen in Figure 9. It can be seen that activities for the three catalytic functionalities vary in the same manner viz attaining a maximum at 3 wt % promoter loading. It can also be noticed that similar behavior is obtained in the case of both Co and Ni for all three functionalities. As in the case of γ-Al2O3-supported Mo catalysts, the optimum promoter content appears to be 3 wt %. It is interesting that hydrocracking also shows the same trend as hydrodesulfurization and hydrogenation. At this juncture, it is necessary to dwell a little while on hydrocracking activity of these catalysts. Cumene hydrocracking reaction is a solid acid-catalyzed reaction, and β-zeolite is wellknown for its acidity.37 How is it that the trend in activity (37) Jacobs, P. A.; Leeman, H. E.; Uytterhoeven, Jan B. J. Catal. 1974, 33 (1), 17.

H-β-Zeolite-Supported Molybdenum Hydrotreating Catalysts

Energy & Fuels, Vol. 20, No. 5, 2006 1789

Table 3. Characterization and Catalytic Activity of Supported Mo, CoMo, and NiMo Catalysts supported catalysts

BET surface area (m2/g)

oxygen uptakea (µmol/g)

6%Mo/β-zeolite 1%Co6%Mo/β 3%Co6%Mo/β 5%Co6%Mo/β 1%Ni6%Mo/β 3%Ni6%Mo/β 5%Ni6%Mo/β

431 425 420 411 389 377 381

68.2 68.9 76.2 48.0 73.3 82.2 79.6

reaction ratea (mol h-1 g-1 of cat × 10-3) HDS HYD HCK 15.2 21.3 25.0 15.0 18.8 22.2 14.5

56.5 84.5 90.2 68.6 93.5 110.7 100.3

100.0 98.0 106.6 85.7 121.7 161.6 131.6

selectivity kHYD/kHDS 3.71 3.96 3.60 4.57 4.97 4.95 6.89

a Support corrected values are presented (β-zeolite support contribution for oxygen uptake ) 10.0 µmol g-1; for HDS ) 3.1 mol h-1 g-1 × 10-3, for HYD ) 10.8 mol h-1 g-1 × 10-3, and for HCK ) 251.8 mol h-1 g-1 × 10-3).

Figure 9. Effect of the Ni promoter content on hydrotreating functionalities of H-β-zeolite-supported molybdenum catalysts.

variation resembles that of HDS and HYD, which are wellknown to originate from Mo, CoMo, and NiMo components? It can be understood in the following way. β-zeolite is acidic and has high hydrocracking activity (251.8 × 10-3 mol h-1 g-1). The addition of 2% Mo increases the cracking activity further; the increase is continuous up to 6 wt % Mo, and after that, the activity start to decrease. Similar behavior is also noted in the case of promoters Co and Ni. This type of behavior clearly indicates that MoS2 and sulfided CoMo and NiMo phases contribute significantly to cracking activity. At 6 wt % Mo, the contribution of the MoS2 phase to total hydrocracking activity is 28.4%. In 3%Co6%Mo and 3%Ni6%Mo catalysts, the contributions of the Co-Mo-S and Ni-Mo-S phases increase to 29.7 and 39%, respectively. It can be clearly seen that Mo, CoMo, and NiMo indeed contribute significantly to the total cracking activity, even in the presence of highly acidic zeolite. The variation in hydrocracking activity in the same way as for HDS, HYD, and oxygen uptake with Mo and cobalt content also support this observation. SH groups are known to exist on this sulfided catalysts system.38 They may be responsible for the observed cracking activity. Similar behavior was noted by Nishizima and co-workers on MoO3/ Al2O339 and Murali Dhar and co-workers on Mo/SiO2-ZrO222 and Mo/SiO2-TiO240 systems. It appears that supported MoS2, Co-Mo-S, and Ni-Mo-S phases contribute to cracking activity of the catalysts. However, the contribution depends on the nature of support and MoS2 dispersion. Comparison with γ-Al2O3- and Y-Zeolite-Supported Catalysts. It is interesting to compare the catalytic activities of Mo, CoMo, and NiMo supported on H-β-zeolite for various func(38) Koranyi, T. I.; Moreau, F.; Rozanov, V. V.; Rozanova, E. A. J. Mol. Struct. 1997, 410-411, 103. (39) Nishijima, A.; Kameoka, T.; Sato, T.; Shimada, H.; Nishimura, Y.; Yoshimura, Y.; Matsubayashi, N.; Imamura, M. Catal. Today 1996, 29, 179. (40) Rana, M. S.; Maity, S. K.; Ancheyta, J.; Murali Dhar, G.; Prasada Rao, T. S. R. Appl. Catal., A 2004, 258, 215.

Figure 10. Comparison of hydrotreating functionalities of molybdenumbased catalysts on various supports.

tionalities (see Table 3) with γ-Al2O3- and Y-zeolite-supported analogues. Such a comparison is shown in Figure 10. In the same figure, oxygen uptake on the same catalyst is also shown. It can seen that HDS activities of β-zeolites are comparable to those of γ-Al2O3-supported catalysts. In the case of hydrogenation of cyclohexene, however, β-zeolite displays superior activities. A similar comparison with NaY and Mo/USY indicated that β-zeolite is less active for HDS of thiophene but in the case of cyclohexene, Mo, CoMo, and NiMo on β-zeolites showed higher activities. Breysse et al.27 observed high activity for hydrogenation of molybdenum catalysts supported on β-zeolite. They have also suggested that the proximity of acid sites to MoS2 enhances the hydrogenation functionality through electron transfer. The oxygen uptakes shown in the same graphs follow the same trend as catalytic activities. The oxygen is known to titrate anion vacancies in these catalysts. Therefore, such a relationship indicates that anion vacancies are indeed responsible for oxygen chemisorption as well as catalytic activities on these catalyst systems. It is also clear that the support can vary the catalytic functionality independently and that hydrogenation and HDS originate from different set of sites on these β-zeolite-supported Mo catalysts. It can also be noted that oxygen uptakes are not specific to any one of the functionalities but measure the number of anion vacancies that are connected to the active sites. Conclusions Mo, CoMo, and NiMo catalysts prepared by incipient-wetness impregnation using H-β-zeolite as the support have been examined by XRD, surface area measurement, temperatureprogrammed reduction, and oxygen chemisorption in the sulfided state. The XRD results indicated that the crystallinity starts decreasing after 6 wt % Mo as a function of Mo loading. There is no evidence for the presence of any molybdenum phases in oxide and sulfide states up to 6 wt % Mo. However at higher

1790 Energy & Fuels, Vol. 20, No. 5, 2006

loading in the vicinity of 12 wt % Mo, MoO3 and Al2(MoO4)3 phases may be present in small quantities. Surface-area analysis as a function of Mo content indicated that molybdenum is welldispersed on these catalysts up to 6 wt % Mo. At higher loading, the dispersion decreases rapidly. The temperature-programmed reduction studies indicated that the reduction behavior is different in the two regions before and after 6 wt % Mo, indicating that two types of phases exists in the two regions. The promoters are found to increase the reducibility of the molybdenum phase and decrease the temperature of reduction of Mo phase. The oxygen chemisorption studies indicated that anion vacancy concentration increases up to 6 wt % Mo and decreases at higher loading. The catalytic activities for thiophene hydrodesulfurization, cyclohexene hydrogenation, and cumene hydrocracking on sulfided catalysts followed the same trend as oxygen uptake as a function of Mo loading. There exists a linear correlation between oxygen uptake and the three functionalities.

Kumaran et al.

The similarity in variation of activities for HDS and HYD to HCK indicated that the supported molybdenum phase contributes to the cracking activity in addition to the activity due to zeolite support. The results also indicate that oxygen chemisorption is not specific to any one of the functionalities and may be measuring the general state of dispersion of molybdenum. It can also be concluded that the support varies the catalytic functionalities independently, which in turn leads to the conclusion that hydrogenation and HDS originate from different set of sites. Acknowledgment. The authors are grateful to Dr. M. O. Garg, Director of the Indian Institute of Petroleum, Dehradun, for his encouragement, and G.M.K. thanks CSIR, India, for a Senior Research Fellowship. EF060134J