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Energy & Fuels 2006, 20, 2308-2313
Origin of Hydrocracking Functionality in β-Zeolite-Supported Tungsten Catalysts G. Muthu Kumaran, Shelu Garg, Manoj Kumar, N. Viswanatham, J. K. Gupta, L. D. Sharma, and G. Murali Dhar* Catalytic ConVersion Processes DiVision, Indian Institute of Petroleum, Dehradun 248 005, India ReceiVed June 2, 2006. ReVised Manuscript ReceiVed August 10, 2006
H-β-zeolite-supported tungsten sulfide catalysts with W loading varying between 10 and 25 wt % were prepared and characterized by BET surface area, pore size distribution in micro- and mesopore regions, in situ microcalorimetric ammonia adsorption, and oxygen chemisorption at low temperatures on sulfided catalysts. The thiophene HDS, cyclohexene hydrogenation (HYD), and cumene hydrocracking (HCK) reactions were carried out on sulfided catalysts. The differential heat curves and acid strength distributions indicated that sulfiding and WS2 content have a profound influence on the acidic properties of these catalysts. It was concluded that zeolite and WS2 both contribute to the acidic properties of the catalysts. There exists a correlation between strong acid sites and HCK activity. The relation between initial heat of adsorption and HCK activity suggested that acid sites g100 kJ/mol are involved in cumene hydrocracking on these catalysts.
Introduction Hydrocracking is an important unit operation in petroleum refining. Its utilization in petroleum processing is increasing at a rapid pace because of demand for clean fuels.1 Hydrodesulfurization, hydrogenation, and cracking of heavy organic molecules in the presence of hydrogen are important reactions in this operation. Hydrocracking catalysts are bifunctional with cracking and hydrogenation components.2 The cracking function used to be provided by mixed oxides in earlier days; nowadays, increasing use of Y-zeolite in catalytic formulations to augment cracking functionality is reported.3,4 The hydrogenation function is generally provided by Mo,W sulfides promoted with Co or Ni.5 It is well-documented that hydrogenation of aromatics and hydrogenolysis of C-N, C-S bonds are effected by promoted molybdenum or tungsten sulfide components.6 There are extensive studies on hydrogenation and hydrodesulfurization, hydrodenitrogenation etc., where as the cracking functionality of MoS2 and WS2 received limited attention, because it is believed that hydrocracking functionality is related to support only. Recently, it has been shown that Mo, CoMo catalysts contain significant cracking functionality.7,8 The cracking functionality of the support or active component is generated by the acid sites that are present on them. * Corresponding author: Fax: +91-135-2660202; E-mail:
[email protected]. (1) Scherzer, J.; Gruia, A. J. Hydrocracking Science and Technology; Marcel Dekker: New York, 1996. (2) Ward, J. W. In Applied Industrial Catalysis; Leach, B. E., Ed.; Academic Press: New York, 1984; Vol. 3, p 270. (3) Scott, J. W.; Bridge, A. G. AdV. Chem. Ser. 1971, 103, 113. (4) Marcilly, C.; Franck, J. P. In Catalysis by Zeolites; Imelik, B., et al., Eds.; Elsevier: Amsterdam, 1980; p 93. (5) Topsoe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating Catalysis Science and Technology; Anderson, J. R., Boudart, M., Eds.; SpringerVerlag: New York, 1996; Vol. 11. (6) Prins, R.; de Beer, V. H. J.; Somorjai, G. A. Catal. ReV.-Sci. Eng. 1989, 31, 1. (7) Rana, M. S.; Srinivas, B. N.; Maity, S. K.; Murali Dhar, G.; Prasada Rao, T. S. R. J. Catal. 2000, 195, 31. (8) Rana, M. S.; Maity, S. K.; Ancheyta, J.; Murali Dhar, G.; Prasada Rao, T. S. R. Appl. Catal., A 2004, 258, 215.
Acidic function can be evaluated by using well-known model compounds such as cumene, isooctene, etc., molecules. Many investigators studied the acidic function of the catalysts in the oxidic state.9,10 Massoth and co-workers11,12 investigated cracking of isooctene on Mo catalysts supported on a number of supports in the sulfided state and concluded that the catalysts exhibit significant activity for cracking of isooctene to isobutene. Small amounts of cracking products are observed in 1-hexene hydrogenation also.12 Appreciable dealkylation of m-diisopropyl benzene to cumene has been reported on CoMo catalysts.12 Dealkylation of 1-methyl naphthalene has been reported to take place over Co- and Ni-promoted molybdenum catalysts.13 Significant cracking of isooctene was observed on silicasupported CoMo catalysts, which indicates that sulfided CoMo is the seat of hydrocracking activity; it is well-known that SiO2 surfaces are not acidic. The cumene cracking reaction was also conducted on pure MoS2 and carbon-supported MoS2 catalysts, which indicated MoS2 and carbon-supported catalysts exhibit significant cracking activity.6 It has been shown by Mo and Co content variation on CoMo/SiO2-ZrO2 and CoMo/SiO2-TiO2 that supported MoS2 and CoMoS phases are indeed active for the cumene cracking reaction.7,8 From the above discussion, it is clear that MoS2 and its promoted analogues display significant cracking functionality of their own. Therefore, the question is whether the active component exhibits acid sites that are of strong enough acidity, and it is also pertinent to see whether these acidic sites can be detected by well-known basic probe molecules. Massoth has observed NH3 and pyridine adsorption on sulfided catalysts.14 Topsøe and Topsøe reported the presence of Bro¨nsted sites on sulfided Mo/Al2O3 when pyridine was used at high tempera(9) Ratnasamy, P.; Knozinger, H. J. Catal. 1978, 54, 155. (10) Segawa, K.; Hall, W. K. J. Catal. 1982, 76, 133. (11) Murali Dhar, G.; Massoth, F. E.; Shabtai, J. J. Catal. 1994, 85, 44. (12) Massoth, F. E.; Murali Dhar, G.; Shabtai, J. J. Catal. 1994, 85, 53. (13) Patzer, J. F.; Farruato, R. S.; Montagne, A. A. Ind. Eng. Chem. Process Res. DeV. 1979, 18, 625. (14) Cowley, S. W.; Massoth, F. E. J. Catal. 1978, 51 (1), 291.
10.1021/ef0602527 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/22/2006
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Table 1. Textural Properties of H-β-Zeolite and 17%W/H-β-Zeolite Samplesa sample
SBET (m2/g)
SMIC (m2/g)
SEXT (m2/g)
Vt (cm3/g)
VMIC (cm3/g)
% crystallinity of β-zeolite
β-zeolite 17%W
557 468
344 331
212 136
0.576 0.385
0.166 0.132
100 63.0
a
SMIC ) micropore area (m2/g), SEXT ) external surface area (m2/g), V ) total pore volume (cm3/g), VMIC ) micropore volume (cm3/g).
tures.15 Hou and Wise16 shown from NH3 chemisorption that MoS2 indeed contain acid sites in sufficient number and stated that NH3 titration provides a useful method for the determination of the acidic property of sulfhydryl groups. Petit et al.17 recently showed that Bro¨nsted acid sites are indeed present on Mo-, CoMo-, and NiMo-supported γ-Al2O3 and that the number of these sites increased in the presence of H2S. There is always a significant partial pressure of H2S in industrial reactors. It is likely that the sulfided MoS2 or WS2 phase always has significant acidity under actual reaction conditions. Although there are many studies about acidic properties of pure MoS2 and supported MoS2 promoted with Co or Ni,7,8,12,13 there are very few studies on the acidic function of WS2 and promoted catalysts. To bridge the gap, in this investigation, we have undertaken a systematic study of in situ acidity in sulfided state and its relation to hydrocracking functionality on sulfided WS2 and its promoted analogues supported over H-β zeolite, in order to understand the contribution of WS2 and its promoters in generating cracking functionality. Experimental Section Sample Preparation. The H-β-zeolite support used in this work is obtained commercially as the NH4 form of β-zeolite from Su¨d Chemie India Ltd, which was calcined at 550 °C for 6 h to get the H form. Supported W/H-β catalysts were prepared by incipientwetness impregnation on a H-β support by taking appropriate concentrations of ammonium m-tungstate hydrate (Fluka). The Coor Ni-promoted catalysts was prepared by impregnating the corresponding nitrate salts on an oven-dried 17%W/H-β-zeolitesupported catalyst. The impregnated catalysts were dried in air at 110 °C overnight, and all the catalysts were calcined at 550 °C for 6 h. Characterization. The calcined H-β-zeolite support and catalysts containing various amounts of tungsten were characterized by BET surface area, pore size distribution, microcalorimetric ammonia adsorption for acidity, and low-temperature oxygen chemisorption (LTOC). Pore size distribution was carried out using an ASAP-2010 pore size analyzer from Micromeritics. Microcalorimetric studies of adsorption of ammonia on various catalyst samples in the sulfided state have been performed to determined acidity and acid strength distribution using a Tian-Calvet type heat flux microcalorimeter (Setaram, model C-80, France) connected to a volumetric adsorption unit for sample pretreatment and probe molecule delivery.18 All the catalyst samples were sulfided in situ at 400 °C using a CS2/H2 mixture for 3 h and evacuated at the same temperature for 4 h prior to microcalorimetric measurements. The heats evolved from sequential doses of ammonia onto the sample were measures at 175 °C. The heat of adsorption generated for each dose was calculated from the resulting thermograms and the amount of ammonia adsorbed from the initial and final pressure. Sequential doses gave the differential heat of NH3 adsorption as a function of surface coverage (i.e., differential heat curves). It provides information about the number and strength of acid sites on samples. The low-temperature oxygen chemisorption (LTOC) was measure at -78 °C in a (15) Logadottir, A.; Moses, P. G.; Hinnemann, B.; Topsoe, N.-Y.; Knudsen, K. G.; Topsoe, H.; Norskov, J. K. J. Catal. 2006, 111, 44. (16) Hou, P.; Wise, H. J. Catal. 1982, 78, 469. (17) Petit, C.; Mauge, F.; Lavalley, J. C. Stud. Surf. Sci. Catal. 1997, 106, 157. (18) Chiranjeevi, T.; Muthu Kumaran, G.; Gupta, J. K.; Murali Dhar, G. Thermochim. Acta 2006, 443 (1), 87.
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 a double isotherm procedure reported by Parekh and Weller for reduced catalysts.19 The same system was used for the BET surface area measurements. Catalytic Activities. The hydrodesulfurization (HDS) of thiophene and hydrocracking of cumene (HCK) were carried out at 400 °C on a catalyst sulfided at the same temperature for 2 h in the 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. 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. The rates thus calculated for HDS, HYD, and HCK reactions are taken as a measure of their respective activities. The conversions were kept below 15% to avoid diffusion limitations.20
Results and Discussion Surface Area Analysis and Pore Size Distribution. A brief discussion about surface area and pore size distribution will be included in order to prepare a platform for the discussion on the acidity of sulfided catalysts and origin of catalytic functionalities. The surface areas of pure H-β-zeolite and W-loaded analogues are determined as a function of W loading. The pore size distribution of pure and tungsten-loaded β-zeolite in the micro- and mesopore regions are evaluated. The results of pore size analysis are shown in Table 1. It can be seen that the micropore area remains more or less the same and that there is a drop in β-zeolite crystallinity with tungsten addition to the zeolite. The surface area as a function of W loading in both the oxide and sulfided states based on per gram of support remained more or less constant up to 17 wt % W and then decreased. Massoth indicated that such a behavior indicates a highly dispersed state or monolayer formation of W on β-zeolite.21 Similar results were reported by Sai Prasada Rao et al. on WO3/ZrO220 and Chiranjeevi et al. on WO3/Al-HMS materials.22 The results suggest a highly dispersed nature of WS2 in these catalysts. Microcalorimetric Acidity Measurement. The acidic properties of WS2/H-β-zeolite catalysts in the sulfided state are obtained on in situ sulfided catalysts at 175 °C on a microcalorimeter coupled to a volumetric adsorption unit. Pure β-zeolite obtained as such, as well as after sulfiding in situ, was examined by NH3 adsorption. Various W catalysts with loadings of 1025 wt % W were in situ sulfided, and the ammonia adsorption was carried out. The number of adsorbed NH3 molecules at saturation provides the number of acid sites, whereas the corresponding heat of adsorption is a measure of strength of acid sites. In a calorimeter coupled with volumetric adsorption unit, procedures have been developed to relate the quantitative measurement of heat liberated to different processes that are taking place as a function of the increase in amount of NH3. (19) Parekh, B. S.; Weller, S. W. J. Catal. 1977, 47, 100. (20) Rao, K. S. P.; Murali Dhar, G. J. Catal. 1989, 115, 277. (21) Massoth, F. E. J. Catal. 1975, 36, 166. (22) 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.
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Figure 2. Acidity spectra of H-β-zeolite-supported tungsten sulfide catalysts.
Figure 1. Differential heat curves of H-β-zeolite-supported tungsten sulfide catalysts.
The calorimetric adsorption is generally shown as a plot of differential heats (kJ/mol) vs amount of ammonia adsorbed (mmol/g). The shape of calorimetric curve is strongly dependent on heterogeneity of the solid surface with respect to base molecule adsorption. A plot of the change in the number of adsorbed molecules for a corresponding change in heat of adsorption (dn/dq) vs heat of adsorption q (kJ/mol) gives the distribution of acid sites as a function of heat of adsorption. The differential heat curves of β-zeolite, sulfided β-zeolite, and various sulfided β-zeolite-supported W catalysts containing 10-25 wt % W are shown in Figure 1. It can be seen that NH3 interacts with acid sites present on the catalysts and that the interaction is heterogeneous, with heats of adsorption ranging from 60 to 160 kJ/mol. The interactions are manifested in the differential heat curves in the form of well-defined steps, plateaus, and abrupt changes in between. These changes are due to interaction of NH3 with a variety of acid sites that exhibit different strengths. The plateau abruptly changes to a lower level when interaction with the lower level becomes dominant. In some cases, a monotonic decrease is also observed.23-25 The heat of adsorption is arbitrarily classified into strong, medium, (23) Auroux, A. Top. Catal. 1997, 4, 71. (24) Auroux, A. Top. Catal. 2002, 19 (3-4), 205. (25) Martinez, N. C.; Dumesic, J. A. AdV. Catal. 1992, 38, 149.
and weak acid sites. The corresponding data on all the catalysts are shown in Table 3. In the differential heat curves in the case of β-zeolite, well-defined steps or plateaus can be clearly seen (Figure 1). The initial heat of adsorption is 142 kJ/mol. Sulfidation of β-zeolite decreases the acidity considerably across the board. Sulfidation resulted in a considerable decrease in acid sites higher than 120 kJ/mol. The total acidity of β-zeolite is 0.90 mmol/g, whereas in the case of sulfided β-zeolite, it is 0.62 mmol/g. The strong and medium acid sites are decreased. Addition of 10 wt % W to β-zeolite (sulfided) increases the total acidity compared to sulfided β-zeolite. Strong acid sites remained the same, whereas the medium acid sites increased to 0.24 mmol/g compared to 0.15 mmol/g on sulfided β-zeolite. The weak acid sites decreased further, whereas total acidity increased with an increase in W loading to 14 wt %. The total acidity increased to 0.70; the strong acidity increased to 0.31 mmol/g, the medium acidity remained more or less the same, and there is slight increase in weak acidity. With an increase in W loading to 17 wt %, the total acidity further increased to 0.88 mmol/g, the strong acidity to 0.32 mmol/g, and medium acidity to 0.28 mmol/g. The weak acidity also increased with an increase in W loading up to 17 wt %. At higher wt % W, the total acidity and strong and medium acidity showed a decreasing trend, whereas weak acid sites remained the same. It is interesting to see that a small number of very strong acid sites with high heats of adsorption are created with an increase in W loading up to 17 wt % W; however, the number and strength of these sites decreases at higher loading. The distribution of acid sites are better visualized from (dn/ dq) vs q plots; such plots for various W catalysts and pure and sulfided β-zeolites are shown in Figure 2. Only points at 100 kJ/mol and above are shown in Figure 2 for clarity as well as because of the awareness (which will be discussed later) that the sites with heat of adsorption g100 kJ/mol are important
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Energy & Fuels, Vol. 20, No. 6, 2006 2311
Table 2. Variation of Oxygen Uptake, NH3 Adsorption, and HCK Rates on H-β-Zeolite-Supported Tungsten Catalysts wt % W
O2 (µmol g-1)
initial heat of NH3 adsorption (kJ mol-1)
HCK rate (mol h-1g-1 × 10-3)
crystallite size of WS2 (Å)
0 10 14 17 19 21 23 25 pure WS2
10.0 20.4 39.5 45.0 32.0 26.5 21.7 17.8
140 154 157 162
252 176 180 224 132 125 127 119 60
25.8 18.7 19.9 31.3 41.8 55.9 74.0
136 132
Table 3. Microcalorimetric Results on Acidity and Acid Strength Distributions of β-Zeolite and Its Sulfided W Catalysts acid strength distributiona (mmol NH3/g of cat) sample no.
sample
total acidity (mmol NH3/g of cat)
strong
medium
weak
1 2 3 4 5 6 7
β-zeolite (pure) β-zeolite (sulfided) 10% W/β-zeolite (sulfided) 14% W/β-zeolite (sulfided) 17% W/β-zeolite (sulfided) 21% W/β-zeolite (sulfided) 25% W/ β-zeolite (sulfided)
0.90 0.62 0.67 0.70 0.88 0.65 0.63
0.38 0.29 0.29 0.31 0.32 0.22 0.20
0.37 0.15 0.24 0.23 0.28 0.16 0.16
0.15 0.18 0.14 0.16 0.28 0.27 0.27
a
Strong g 100 kJ mol-1; medium ) 100-75 kJ mol-1; weak < 75 kJ mol-1.
for the cumene cracking reaction. It can be seen from the figure that acid site distribution shows two peaks, one centered around 140 kJ/mol and another at 127 kJ/mol. Sulfidation eliminates acid sites represented by a high heat of adsorption and considerably reduces the area of the acid site peak centered around 127 kJ/mol. The peak maximum and also distribution is shifted to lower q values, indicating that not only are the number of acid sites reduced but also that the strength is weakened to considerable extent. It appears that sulfiding results in reduction of strong acid sites significantly. The addition of W to β-zeolite and sulfiding appears have a profound influence on acid site distribution. With the addition of 10 wt % W, the peak present at 127 kJ/mol is further reduced and a new peak at 115 kJ/mol is created. The peak at 140 kJ/mol that appeared in sulfided β-zeolite is almost eliminated. With a further increase in W up to 17 wt %, very strong acid sites in the range of 140160 kJ/mol are created. At higher loadings, the peak related to acid sites at 115 kJ/mol increases and the sites centered around 127 kJ/mol gradually reduce. It appears there is a perceptible difference in heats of adsorption of acid sites of zeolite and WS2, which allows us to distinguish acid sites that are related to zeolite or WS2 phase. It can also be noted that creation of strong acid sites is related to WS2 dispersion, indicating that highly dispersed tungsten has the strongest acid sites. Oxygen Chemisorption. Oxygen chemisorption on sulfides such as MoS2 and WS2 is well-known to represent the concentration of anion vacancies of the supported phase and the dispersion of the active component.11,26 The oxygen uptakes obtained at low temperatures on a series of catalysts with varying W content on sulfided catalysts as a function of W loading is shown in Figure 3, and the corresponding data on oxygen uptake, crystallite size, etc. are shown in Table 2. It can be seen from the figure that oxygen uptakes increase up to 17 wt % W loading and a tendency to decrease is shown at higher loadings. The crystallite size derived from oxygen uptakes indicates that the WS2 is in a highly dispersed state up to 17 wt % and the dispersion decreases at higher loadings. Sulfided β-zeolite chemisorbs O2 to a very small extent, and all the values reported are corrected for support contribution.
Catalytic Activities for Various Functionalities. The catalytic activities for hydrodesulfurization of thiophene, hydrogenation of cyclohexene, and hydrocracking of cumene were carried out at 400 °C on catalysts sulfided at the same temperature for 2 h. The support has a small contribution to hydrodesulfurization and hydrogenation, and the results are reported after correcting for support contribution. However, the sulfided pure support, as expected, exhibited high cracking activity (251.8 × 10-3 mol h-1 g-1). However, the addition of 10%W reduces the activity to 176 × 10-3 mol h-1 g-1; onward, the activity starts increasing up to 17 wt % W, followed by a decrease at higher loadings, as can be seen from the Figure 3. Therefore, the values reported in the case of hydrocracking are not support-corrected. It can be seen that hydrodesulfurization and hydrogenation activities increase with W loading up to 17 wt % W. However, at higher loading, the activities shows a decrease. Hydrocracking also shows a similar trend, except for the first point (10 wt % W) and a slight deviation at higher W loading. The oxygen uptakes shown in the same figure also follow the same trend as the three catalytic functionalities. Because anion vacancies are measured by oxygen chemisorption, it appears that anion vacancies are involved in all three of the reactions. It is wellestablished that oxygen chemisorbs on anion vacancies of MoS2 and WS2, and they are also known to be responsible for their hydrotreating activities.27 It is intriguing to see how anion vacancies are related to cracking functionality. The cracking functionality of WS2 is related to its sulfhydryl groups. These sulfhydryl groups are formed on WS2 in a H2S/H2 atmosphere and are controlled by the availability of anion vacancies. The formation of anion vacancies on WS2 is shown below. This may be the reason for a nice relationship between anion vacancies and cracking activity.
(26) Zmierczak, W.; Murali Dhar, G.; Massoth, F. E. J. Catal. 1982, 77, 432.
(27) Rao, K. S. P.; Ramakrishna, H.; Murali Dhar, G. J. Catal. 1992, 133, 146.
In the above discussion, we have seen the variation of cracking activity of WS2 as a function of W loading. It is also
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Figure 3. Variation of HDS, HYD, HCK, and O2 uptake of β-zeolitesupported catalysts with tungsten loading. Figure 5. Correlation of acidity and hydrocracking activity of cumene on tungsten sulfide catalysts supported over H-β-zeolite.
Figure 4. Effect of promoter contents (Co or Ni) of 17%W/H-β-zeolite catalyst on HCK and oxygen uptake.
interesting to see how the activity varies as a function of promoter content on 17 wt % W on β-zeolite. The promoter content against activity plots is shown in Figure 4. In the same figure, oxygen uptakes on sulfided catalysts are also shown. It can be seen that in the case of hydrocracking activity, the promotional effect is present and the activity is at a maximum at 3 wt % Co or Ni. A similar relationship is also obtained in the cases of hydrodesulfurization and hydrogenation. The activity of promoted catalysts follows the same trend as oxygen uptake as a function of promoter content. The similarity between trends of variation of HDS, HYD, HCK, and oxygen uptake suggests that hydrocracking activity predominantly originates from promoted WS2 phase in the (Co) Ni-WS2/β-zeolite system under the conditions employed in the study. It is interesting to discuss the cracking functionality further. It is well-known that zeolites are highly acidic and are known to display outstanding activities for cracking reactions. How is it that the activity correlates well with oxygen uptake and follows the same trend as HDS and HYD, which are known to be fundamental properties of WS2? It is also shown that sulfi-
dation decreases the acidity of β-zeolite. The cracking activity variation can be well understood, if we can assume that sulfided β-zeolite and WS2 have their own individual activities and that the high activity of β-zeolite is considerably suppressed by sulfiding. To learn about the cracking ability of pure WS2, we carried out the reaction on pure WS2, which showed considerable cracking activity (Table 2). The precursor of WS2 is wellknown to interact with hydroxyl groups on the support surface, and these hydroxyl groups are ones that are associated with Al. It is our general observation that W or Mo preferentially goes to Al in the case of zeolites and mesoporous materials.28-30 Therefore, it appears that WS2 interacting with Al is reducing the acidity of zeolites significantly. However, WS2 has its own cracking ability through acidic sulfhydryl groups. The acidity decrease due to elimination of hydroxyl groups during WS2 interaction with Al is partly compensated by WS2 acidity. The presence of sulfhydryl groups on sulfides is reported in the literature,17,31,32 and these are expected to be responsible for the cracking activity of WS2. In light of these observations, the acidity of the zeolite is considerably reduced by coverage of W on these sites and also by sulfidation. At 10 wt % W, some of the activity of zeolitic acidic sites is still present; WS2 sites start contributing predominantly from 14 wt % tungsten onward, and the contribution of WS2 sites dominates at higher loadings, reaching a maximum at 17 wt % W. It is interesting to see that the tungsten dispersion is also at a maximum at this loading. At higher loadings, the dispersion and the exposed WS2 surface both decrease, as do the acidity and hydrocracking activity. Earlier, we discussed acidity as a function of W loading. It is interesting to see how this measured acidity correlates with cumene hydrocracking activities. Such a relationship can be seen in Figure 5. It can be seen that a linear relationship passing through the origin is obtained in the case of strong acid sites. It can also be seen that, in the case of total acid sites, there is a lot of scatter and the relationship is weak. To further understand such a relationship, we plotted the strong acid sites and hydrocracking activity as a function of W loading (Figure 6). Both parameters follow the same trend exactly. The relationship between the parameters is true when strong and medium sites (28) Rawat, K. S.; Rana, M. S.; Murali Dhar, G. Stud. Surf. Sci. Catal. 1998, 135, 307. (29) Chiranjeevi, T.; Muthu Kumaran, G.; Gupta, J. K.; Murali Dhar, G. Catal. Commun. 2005, 6 (2), 101. (30) Muthu Kumaran, G.; Shelu Garg; Kapil Soni; Manoj Kumar; Sharma, L. D.; Murali Dhar, G.; Rama Rao, K. S. Appl. Catal., A 2006, 305 (2), 123. (31) Byskov, S. L.; Norskov, J. K.; Clausen, B. S.; Topsoe, H. J. Catal. 1999, 187, 109. (32) Hensen, E. J. M.; Lardinois, G. M. J. H.; de Beer, V. H. J.; van Veen, J. A. R.; van Santen, R. A. J. Catal. 1999, 187, 95.
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Energy & Fuels, Vol. 20, No. 6, 2006 2313
kJ/mol of NH3. A similar relationship is obtained in the case of Al-SBA-15 on NH3 adsorption and cumene cracking activity. In this case, it is also concluded that the cumene cracking reaction needs acid sites of g100 kJ acid strength.33 Summary and Conclusions
Figure 6. Variation of acidity and cumene cracking activity of β-zeolite-supported catalyst with tungsten loading.
Figure 7. Correlation of initial heats of adsorption of NH3 with cumene hydrocracking on tungsten sulfide catalysts supported over H-β-zeolite.
are taken together as well. Therefore, it appears only some of the acid sites of particular strength are responsible for HCK reaction on these catalysts. Microcalorimetry allows us to estimate the strength of acid sites; it is therefore interesting to see what strength of the acid sites is necessary to effect this reaction on these catalysts. Toward this end, we have plotted the initial heat of adsorption measured by microcalorimetry as a function of the hydrocracking rate constant in Figure 7. The initial heat of adsorption represents the strongest acid sites that are present on these catalysts. It can be seen that there is a linear relationship cutting the heat of adsorption axis at ∼100 kJ/mol. Therefore, from these results, it appears that cumene cracking reaction on these catalysts needs acid sites of strength g100
H-β-zeolite-supported tungsten catalysts with W loading from 10 to 25 wt % was prepared and characterized by BET surface area, pore size distribution, in situ microcalorimetric ammonia adsorption, and oxygen chemisorption in the sulfided state. The acidic properties of the catalysts were evaluated by cumene hydrocracking reaction. The HDS and hydrogenation reactions were also evaluated for a comparative study. The BET surface area analysis and oxygen chemisorption studies indicated that WS2 is highly dispersed on these catalysts. The acidity measurements indicated that on pure β-zeolite, sulfiding reduces the acidity and alters the acid strength distribution. The addition of WS2 causes profound changes in the number of acid sites, the strength of acid sites, and their distribution. The catalytic activities for HDS, HYD, and HCK and oxygen uptakes vary in a similar manner with W loading. It was concluded that both the zeolite and WS2 contribute to activity. The contribution of zeolite decreases with WS2 loading. The similarity between oxygen uptake variation and HCK activity suggested that the acidity is related to W dispersion. The promoter content variation also suggested that WS2 and promoted Ni-W-S or Co-W-S phases indeed have significant acidity. There exists a correlation between strong acid sites and HCK activity. The relationship between the initial heat of adsorption of NH3 and HCK activity suggested that acid sites with heats of adsorption g100 kJ/mol are responsible for the cumene hydrocracking reaction, suggesting that the strong acid sites are indeed involved in cumene hydrocracking on these catalysts. Acknowledgment. The authors are grateful to Dr. M. O. Garg, Director, Indian Institute of Petroleum, Dehradun, for his encouragement. G. M. Kumaran and Shelu Garg thank CSIR, India, for providing fellowships. EF0602527 (33) Muthu Kumaran, G.; Shelu Garg; Kapil Soni; Manoj Kumar; Gupta, J. K.; Sharma, L. D.; Rama Rao, K. S.; Murali Dhar, G. Microporous Mesoporous Mater. 2006, submitted.
