- Email: [email protected]
γ-Al2O3 hydrodesulfurization catalysts
Applied Catalysis A: General 217 (2001) 287–293
On the synergy between tungsten and molybdenum in the W-incorporated CoMo/␥-Al2 O3 hydrodesulfurization catalysts John Vakros, Christos Kordulis∗ Department of Chemistry, Institute of Chemical Engineering and High Temperature Chemical Processes (ICE/HT), University of Patras, P.O. Box 1414, GR-26500 Patras, Hellas, Greece Received 12 October 2000; received in revised form 12 April 2001; accepted 12 April 2001
Abstract The aim of this study is to investigate the influence of incorporation of tungsten on an already prepared CoMo/␥-Al2 O3 hydrodesylfurization catalyst (base catalyst). The evaluation of W-incorporated CoMo/␥-Al2 O3 is described, in relation to the base catalyst, concerning their activities for hydrodesulfurization (HDS) of thiophene and hydrogenation (HYG) of its unsaturated products towards butane. The main conclusion of this study is that incorporation of a low amount of tungsten onto CoMo/␥-Al2 O3 catalyst increases its HDS and HYG activities. A synergy has not been found between Mo- and W-phase. The W-loading for which the maximum activity is observed highly depends on the details of the preparation method used. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Hydrodesulfurization; Cobalt–molybdenum/alumina catalysts; Tungsten; Catalyst modification; Hydrogenation; Promoter; Synergy; BET
1. Introduction Environmental legislation places increasingly severe restrictions on the gaseous and particulate emissions arising both from the refinery itself as well as from the use of heating and transportation fuels. For example, while the current EU specification of sulfur content in gasoline is 150 ppm, the new limit of sulfur for the year 2005 is anticipated to be 50 ppm [1,2]. The reason for this reduction is the already proven relationship between the sulfur amount in gasoline and the final exhaust emissions from gasoline-fueled automobiles [1,2]. The necessity for lowering of the sulfur content in the transportation fuels has resulted
∗ Corresponding author. Fax: +30-61-994-796. E-mail address: [email protected] (C. Kordulis).
in a world wide increase of the research activity focused to the improvement of the HDS catalysts. Typical HDS catalysts consist of molybdenum supported on an alumina carrier with either cobalt or nickel added as promoters for improving the catalytic activity. The scientific and patent literature on the development and the characterization of HDS catalysts is quite extensive [3–6]. Methods used for the preparation of these catalysts have been reviewed in detail [7,8]. There have been many efforts of improving the activity and extending the life of conventional HDS catalysts by adding to them metallic (W, Ti, Ni, Zn, Ru, Fe) [9–23] or non-metallic modifiers (P, F) [24–35]. An interesting attempt to improve CoMo and NiMo/␥-Al2 O3 catalysts by the addition of a small amount of tungsten has been described by Lee et al. [9–12]. They concluded that tungsten is a promising
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 1 ) 0 0 6 1 6 - 0
288
J. Vakros, C. Kordulis / Applied Catalysis A: General 217 (2001) 287–293
secondary promoter for the above catalysts. This is a very important conclusion, although it was not expectable as it is well established that Mo and W sulfided phases accelerate HDS reactions in a similar way and W-phase is less active than the Mo one [36,37]. Coordinatively, unsaturated surface sites of these phases are considered to be the active centers for HDS [38,39]. In contrast, it is well acceptable that the usual promoters of the HDS catalysts (Co, Ni) act in a different way. The later is the subject of a long discussion during the last decade [40,41] On the other hand, Lee et al. [9–12] did not took under consideration that by incorporating W onto CoMo/␥-Al2 O3 catalyst the total active phase increases. In this work, we try to investigate further the role of the tungsten incorporated onto CoMo/␥-Al2 O3 catalysts. More precisely, we attempt first to examine whether the increase observed in the HDS activity of the CoMo/␥-Al2 O3 catalysts upon the W incorporation is independent of the preparation conditions used. Thus, we prepared a series of W-incorporated CoMo/␥-Al2 O3 catalysts with increasing W-loading, following a different preparation procedure from those of Lee et al. [10–12]. We also attempt to investigate whether the increase in the activity, if any, is due to a synergy between Mo- and W-phase and cannot be attributed to a simple increase of the total active phase (Mo + W). Thus, we prepared two additional catalysts by substituting the W of the most active W-incorporated CoMo/␥-Al2 O3 catalyst with Mo using two different procedures. The activity of the prepared samples was tested using HDS of thiophene as probe reaction.
[(NH4 )6 Mo7 O24 ·4H2 O, minimum 99%, Merck]. Cobalt was then deposited by incipient wetness impregnation using a cobalt nitrate aqueous solution [Co(NO3 )2 ·6H2 O, minimum 99%, Merck]. The so prepared base catalyst contained 10 wt.% MoO3 and 2.8 wt.% CoO fulfilling, thus, the synergistic atomic ratio, [Co/(Co + Mo)] = 0.35, for these catalysts [3]. Using the above catalyst, a series of tungsten incorporated CoMo/␥-Al2 O3 catalysts, denoted by W(y)CM, was prepared by depositing various amounts of tungsten (0.4–3.0 wt.% WO3 ) on it. The incipient wetness impregnation method and ammonium tungstate [(NH4 )10 W12 O41 ·5H2 O, 99.9% Alfa] aqueous solutions were used for the above preparations. In the above series, by (y) we denote the loading (as wt.%) of tungsten oxide incorporated in each case. Two additional CoMo/␥-Al2 O3 catalysts were prepared having higher Mo content than that of the base one. The excess Mo content of these catalysts calculated in moles was equal to the W-incorporated in W(1.0)CM catalyst. In the one of them (CM-A), the excess molybdenum was added during the first impregnation step. For the preparation of the other (MCM), the base catalyst was used and the method of preparation of W(y)CM series was followed replacing the ammonium tungstate solution used in the last impregnation step by an ammonium heptamolybdate one. After the impregnation of each metal precursor, the catalysts were dried at 110◦ C for 2 h and then calcined in air at 500◦ C for 5 h. All the catalysts prepared are listed in Table 1. 2.2. Specific surface area measurements
2. Experimental 2.1. Preparation of catalysts Commercial ␥-Al2 O3 carrier (Houdry Ho 415, S BET = 123 m2 g−1 , water pore volume = 0.45 cm3 g−1 ) supplied as cylindrical extrudates was used for the preparation of the catalysts studied. The extrudates were crushed and sieved into particles of 90–150 m size. A CoMo/␥-Al2 O3 catalyst (denoted as CM) was prepared by depositing first molybdenum on the alumina carrier via wet impregnation and using an aqueous solution of ammonium heptamolybdate
The specific surface area (SSA) of the catalysts was determined by the dynamic BET method using a laboratory-constructed apparatus. Pure nitrogen and helium (Air Liquide 99.999%) were used as adsorption and carrier gas, respectively. 2.3. Catalytic activity measurements HDS and HYG activities of the prepared catalysts were measured in a continuous flow tubular fixed-bed microreactor operating in a differential mode at atmospheric pressure. The catalyst samples (50 mg) were presulfided in situ, with a stream of 15% (v/v)
J. Vakros, C. Kordulis / Applied Catalysis A: General 217 (2001) 287–293
289
Table 1 Compiles the notation, composition and atomic ratios of supported phases as well as the specific surface areas (SSA) of the catalysts prepared Catalysts
Notation
MoO3 (wt.%)
CoO (wt.%)
WO3 (wt.%)
Co/(Co + Mo + W)
W/(Mo + W)
SSA (m2 g−1 )
Mo/␥-Al2 O3 CoMo/␥-Al2 O3 W–CoMo/␥-Al2 O3 W–CoMo/␥-Al2 O3 W–CoMo/␥-Al2 O3 W–CoMo/␥-Al2 O3 CoMo/␥-Al2 O3 CoMo/␥-Al2 O3
M CM W(0.4)CM W(1.0)CM W(2.0)CM W(3.0)CM MCM CM-A
10.30 10.00 9.96 9.90 9.80 9.70 10.53 10.53
0.00 2.80 2.79 2.77 2.74 2.72 2.78 2.78
0.00 0.00 0.41 1.00 2.00 3.00 0.00 0.00
0.000 0.350 0.344 0.336 0.323 0.311 0.337 0.337
0.000 0.000 0.025 0.059 0.112 0.161 0.000 0.000
– 110 105 107 104 112 100 110
hydrogen sulfide in hydrogen, for 2 h at 400◦ C. The reaction mixture was prepared by passing hydrogen through a flow evaporator filled with liquid thiophene at 0◦ C. After an aging period of 15 h under the stream of the reaction mixture the rate of thiophene HDS and the relative yield to butane were determined over each catalyst at various temperatures in the range 250–320◦ C. A gas chromatograph (Pye Unicam) equipped with a flame ionization detector and a column (8 m × 1/8 in. o.d.) filled with carbowax 20M on chromosorb P A.W. was used for analyzing of the effluent of the reactor. HDS rate was defined as r = Fx/W, where F is the molar flow-rate (mol min−1 ) of thiophene at the reactor inlet, x the conversion measured and W the weight (g) of catalyst used. The relative yield to butane, Y = moles of butane produced/moles of thiophene consumed, was used to estimate the activity of the catalysts for the HYG.
3. Results and discussion The composition, the atomic ratios of the promoter (Co) over the rest of the supported phases, the atomic ratio of the W-incorporated over the total active phase loading and the SSA of the prepared catalysts are compiled in Table 1. Inspection of the table shows that: (i) the first atomic ratio remains always very close to the corresponding synergistic ratio for these catalysts (Co/(Co + Mo + W) ≈ 0.3), (ii) SSA does not differ significantly in all catalyst prepared, although the second atomic ratio (W/(Mo + W)) changes from 0 to 0.161. The above permit us to attribute any difference in the catalytic activity of the prepared samples to the incorporation of the W-phase.
Fig. 1 illustrates the thiophene hydrodesulfurization rates determined using the base and W-incorporated CoMo/␥-Al2 O3 catalysts for all temperatures studied. An inspection of this figure shows that the HDS activity initially increases with the W-content up to 1 wt.% WO3 and then decreases with a further increase of the W-loading. This behavior is in a general agreement with the results published by Lee et al. [10,12], who studied the influence of incorporation of W into CoMo and NiMo/␥-Al2 O3 catalysts. The only difference between our and their results is in the value of the atomic ratio, W/(Mo + W), where the maximum activity has been achieved. They found the maximum activity in an atomic ratio W/(Mo + W) = 0.025, while our results showed that the maximum activity is obtained over the W(1.0)CM sample where W/(Mo + W) = 0.059 (see Table 1). In order to investigate whether the observed increase in the HDS activity is due to a synergy between Mo- and W-phase, we compared the activity of the most active of the W containing samples, W(1.0)CM, with that for the two CoMo/␥-Al2 O3 catalysts containing the same amount of total active phase, CM-A and MCM. In Fig. 2, the HDS rates of the above catalysts are illustrated. An inspection of this figure reveals that all these catalysts exhibited the same catalytic activity for the HDS of thiophene. This clearly shows that it is not possible to accept any synergy between Mo and W-phase in the W(1.0)CM catalyst. The higher activity of the W(1.0)CM catalyst as it is compared with that of the CM catalyst should be attributed to the increased total active phase loading (W + Mo) in the W(1.0)CM catalyst. Let us try to explain the above-mentioned difference in the value of the atomic ratio, W/(Mo + W),
290
J. Vakros, C. Kordulis / Applied Catalysis A: General 217 (2001) 287–293
Fig. 1. Reaction rates of thiophene HDS measured over the base and W-incorporated CoMo/␥-Al2 O3 catalysts at various temperatures.
Fig. 2. Reaction rates of thiophene HDS measured over the W(1.0)CM, MCM and CM-A catalysts at various temperatures.
J. Vakros, C. Kordulis / Applied Catalysis A: General 217 (2001) 287–293
where the maximum activity has been achieved. In the present work, the molybdenum phase in the base catalyst was deposited on the alumina surface using wet impregnation. It is well known that this deposition method results to higher dispersion of the deposited phase as it compared to the incipient wetness impregnation used by Lee et al. [10–12]. Consequently, in the later case a significant amount of Mo-phase is expected to be badly dispersed forming MoO3 crystallites on the alumina surface. On other hand, the pH of the ammonium tungstate impregnating solution used in the present work was not adjusted. It was, pH 5.5, the same with that of the ammonium heptamolybdate impregnating solution. This was done in order to avoid any disturbance in the dispersion of the previously deposited Mo-phase. In contrast, the above authors had adjusted the impregnating pH to 9.5 in order to increase the relative concentration of monomeric W-species [42,43]. However, it is well known that MoO3 crystallites, weakly interacting with alumina surface, are easily dissolved in high pH aqueous solutions [44]. Thus, the high pH solution used in that case for the incorporation of W-species must probably provoked a re-dissolution of MoO3 crystallites of the base catalyst and resulted in the re-dispersion of
291
the Mo-phase after subsequent drying and calcination. Indeed, the above authors have concluded that the catalytic activity promotion observed with the incorporation of tungsten in the CoMo/␥-Al2 O3 catalysts “was due to the presence of monomeric sulfidation — resistant WO3 -species which played a role of increasing the dispersion of active Co–Mo–S phases over the support” [12]. Taking into account the above considerations, we can conclude that, in their case, it was the high pH of tungsten impregnating solution which played a key role in re-dispersing the Mo-phase and consequently, increasing the catalytic activity, rather than the small amount of tungsten added. As we have already mentioned, the HDS activity of W-incorporated CoMo/␥-Al2 O3 catalysts decreased with the W-content in all cases after a critical W-content. This decrease could be attributed to the fact that as the W-content increases the monolayer capacity of the ␥-Al2 O3 is exceeded and the low activity W-species cover some of the more active Mo-species. Fig. 3 shows the relative yield (Y) to butane obtained during the hydrodesulfurization of thiophene over the base and W-incorporated CoMo/␥-Al2 O3 catalysts. The above yield can be taken as a measure of HYG activity of the catalysts since butane is the only fully
Fig. 3. Relative yields to butane determined over the base and W-incorporated CoMo/␥-Al2 O3 catalysts at various temperatures.
292
J. Vakros, C. Kordulis / Applied Catalysis A: General 217 (2001) 287–293
saturated molecule produced by hydrogenation of intermediate products of the thiophene hydrogenolysis (butadiene and butanes). An inspection of the above figure reveals that the relative yield to butane for each sample is higher at low reaction temperatures. At the lowest temperature studied (250◦ C), the Y increases with W-content of catalysts. It reaches its maximum value at 1 wt.% WO3 and then decreases with a further increase of W-content. At the rest of the temperatures studied a similar behavior is observed, but the maximum Y is sifted to the sample containing 2 wt.% WO3 (W(2.0)CM). This is also another difference between our results and those published by Lee et al. [10,12]. In fact, using ethylene hydrogenation as probe reaction, they found that the maximum activity is also reached at an atomic ratio W/(Mo + W) = 0.025. However, we have to note that the increase of HYG activity provoked by the incorporation of W onto CoMo/␥-Al2 O3 catalysts was significantly more important than that of HDS. The above finding is in accordance with the fact that two kinds of active sites are appeared in the hydrotreating catalysts. To be specific, the presence of two distinct types of sulfur vacancies in Mo- and W-based hydrotreating catalysts has been reported [45,46]. One of them is responsible for hydrogenolysis of heteroatoms and the other one for hydrogenation. Specific geometric arrangement of sulfur vacancies with different degrees of coordination appear to be required for each one of the above-mentioned reactions. Corner sites and edge sites can display different degrees of sulfur vacancies and can accelerate different reactions [46,47]. It has been suggested that corner sites with two or three sulfur vacancies are primarily responsible for hydrogenation reactions. The hydrogenolysis sites could be edge sites at a lower oxidation state or combinations of a sulfur vacancy and of a SH group. The distribution of hydrogenation and hydrogenolysis sites in the hydrotreating catalysts would essentially depend on the kind of active phase (MoS2 or WS2 ) and their slab size. It is well known that W-phase posses higher HYG activity than the Mo-phase in hydrotreating catalysts [48,49]. Higher hydrogenation activity has been also correlated with better dispersion of the active phases, and thus, with higher number of active sites [50]. It is evident from the present study that tungsten incorporation in the CoMo base catalyst has consid-
erable influence only on its hydrogenation activity. Based on the above discussion, we can conclude that this incorporation probably provokes modification in the morphological characteristics of the MoS2 and WS2 slabs, thus, resulting to an increase in the number of active sites that favor hydrogenation reaction [51]. Further work will be useful in gaining a better insight on the role of W in these catalysts. Indeed, based on the present results we cannot decide whether formation of separate tungsten sulfide and CoMo sulfide phases or mixed tungsten–molybdenum sulfide phase doped by cobalt is formed in the W-incorporated CoMo/␥-Al2 O3 catalysts. However, Park et al. [52] studying W-incorporated NiMo/␥-Al2 O3 catalysts have found that the tungsten incorporation enhances the reducibility of Mo-phase and increases the concentration of octahedrally coordinated Mo6+ -species. These findings are strong evidences that incorporation of tungsten brings about structural and in turn morphological changes at least in MoS2 active phase precursor. These changes seem to correlate with the increase of the number of sulfur vacancies measured by NO adsorption on the final catalyst found by the above researchers. This is consistent with higher dispersion of the active components resulting, thus, to smaller slabs and, consequently, to a higher proportion of corner sites. This high proportion of corner sites might be a plausible explanation for the increased HYG activity observed in our case. 4. Conclusions The most important findings of the present study may be summarized as follows: 1. Low amounts of tungsten incorporated onto CoMo/␥-Al2 O3 catalysts increase their HDS and HYG activities. This effect is more pronounced in the case of HYG reactions. 2. Concerning HDS the above effect may be attributed to a simple increase of the total active phase (W + Mo) and not to a synergy between Mo- and W-phase. 3. Concerning HYG, the observed increase could be attributed to the higher activity of W-phase as it is compared with that of Mo-phase, but also in the structural and morphological changes, which the incorporation of W might provoke in the Mo-phase.
J. Vakros, C. Kordulis / Applied Catalysis A: General 217 (2001) 287–293
4. The exact values of W/(Mo + W) atomic ratio in which the maximum activities are observed depend on the experimental details of the preparation procedure followed.
Acknowledgements Financial support to J.V. from ICE/HT is gratefully acknowledged. References [1] The Auto-Oil II program: Air Quality Report, Final version 7.1, 3 October 2000. [2] K.L Rock, R.M. Foley, in: Proceedings of the Grace Davidson FCC Technology Conference, Lisbon, September, 1998. [3] P. Grange, Catal. Rev. Sci. Eng. 21 (1980) 1. [4] P. Ratnasamy, S. Sivashanker, Catal. Rev. Sci. Eng. 22 (1980) 401. [5] H. Topsoe, B.S. Clausen, F.E. Massoth, Hydrotreating catalysis, in: J.R. Anderson, M. Boudart (Eds.), Series Catalysis Science and Technology, Vol. 11, 1996. [6] E. Furimsky, Appl. Catal. A: Gen. 171 (1998) 177. [7] D.L. Trimm, Design of Industrial Catalysts, Elsevier, Amsterdam, 1980. [8] A.B. Stiles, Catalyst Manufacture, Chemical Industries Series, Vol. 14, Marcel Dekker, New York, 1983. [9] D.K. Lee, I.C. Lee, S.I. Woo, Appl. Catal. A: Gen. 109 (1994) 195. [10] D.K. Lee, I.C. Lee, S.K. Park, S.Y. Bae, S.I. Woo, J. Catal. 159 (1996) 212. [11] D.K. Lee, H.T. Lee, I.C. Lee, S.K. Park, S.Y. Bae, C.H. Kim, S.I. Woo, J. Catal. 159 (1996) 219. [12] D.K. Lee, W.L. Yoon, Catal. Lett. 53 (1998) 193. [13] B.C. Kang, S.T. Wu, H.H. Tsai, J.C. Wu, Appl. Catal. 45 (1988) 221. [14] A.R. Sainim, B.G. Johnson, F.E. Massoth, Appl. Catal. 40 (1988) 157. [15] G. Muralidhar, F.E. Massoth, J. Shabtai, J. Catal. 85 (1984) 44. [16] C.V. Caceres, J.L.G. Fierro, A. Lopez Agudo, F. Severino, J. Laine, J. Catal. 97 (1986) 219. [17] J. Laine, F. Severino, C.V. Caceres, J.L.G. Fierro, A. Lopez Agudo, J. Catal. 103 (1987) 228. [18] A. Lopez Agudo, J.L.G. Fierro, C.V. Caceres, J. Laine, F. Severino, Appl. Catal. 30 (1987) 185. [19] J.F. Cambra, P.L. Arias, M.B. Guemez, J.A. Legarreta, J.L.G. Fierro, Ind. Eng. Chem. Res. 30 (1991) 2365. [20] P.C.H. Mitchell, C.E. Scott, J.P. Bonnelle, J.G. Grimblot, J. Catal. 107 (1987) 482. [21] A.S. Hirschon, R.B. Wilson Jr., R.M. Laine, Appl. Catal. 34 (1987) 311. [22] M. Ternan, J. Catal. 104 (1987) 256.
293
[23] W.L.T.M. Ramselaar, M.W.J. Craje, R.H. Hadders, E. Gerkema, V.H.J. De Beer, A.M. Van Der Kraan, Appl. Catal. 65 (1990) 69. [24] Ch. Kordulis, F. Melo Faus, B. Delmon, Appl. Catal. 23 (1986) 367. [25] Ch. Papadopoulou, Ch. Kordulis, A. Lycourghiotis, React. Kinet. Catal. Lett. 33 (1987) 259. [26] Ch. Papadopoulou, A. Lycourghiotis, P. Grange, B. Delmon, Appl. Catal. 38 (1988) 255. [27] H.K. Matralis, A. Lycourghiotis, P. Grange, B. Delmon, Appl. Catal. 38 (1988) 273. [28] A. Spojakina, S. Damyanova, L. Petrov, Z. Vit, Appl. Catal. 56 (1989) 163. [29] Ch. Kordulis, A. Gouromihou, A. Lycourghiotis, Ch. Papadopoulou, H.K. Matralis, Appl. Catal. 67 (1990) 39. [30] Ch. Kordulis, E. Dalas, P.G. Koutsoukos, A. lycourghiotis, React. Kinet. Catal. Lett. 45 (1991) 277. [31] Z. Sarbak, S.L.T. Andersson, Appl. Catal. 69 (1991) 235. [32] P.M. Boorman, R.A. Kydd, T.S. Sorensen, K. Chong, J.M. Lewis, W.S. Bell, Fuel 71 (1992) 87. [33] J.M. Lewis, R.A. Kydd, P.M. Boorman, P.H. Van Phyn, Appl. Catal. 84 (1992) 103. [34] Ch. Papadopoulou, H. Matralis, A. Lycourghiotis, P. Grange, B. Delmon, J. Chem. Soc., Faraday Trans. I 89 (1993) 3157. [35] H. Matralis, Ch. Papadopoulou, A. Lycourghiotis, Appl. Catal. A 116 (1994) 221. [36] M.J. Ledoux, O. Michaux, G. Agostini, J. Catal. 102 (1986) 275. [37] J.K. Norskov, B.S. Clausen, H. Topsoe, Catal. Lett. 13 (1992) 1. [38] L. Portela, P. Grange, B. Delmon, Catal. Rev.-Sci. Eng. 37 (1995) 699. [39] P.T. Vasudevan, J.L.G. Fierro, Catal. Rev.-Sci. Eng. 38 (1996) 161. [40] A.N. Startsev, Catal. Rev.-Sci. Eng. 37 (1995) 353. [41] B. Delmon, G.F. Froment, Catal. Rev.-Sci. Eng. 38 (1996) 69. [42] K.H. Tytko, O. Glemser, Adv. Inorg. Chem. Radiochem. 19 (1976) 239. [43] C.F. Baes, R.E. Mesmer, The Hydrogenolysis of Cations, Wiley, New York, 1976, pp.257–260. [44] M.A. Goula, Ch. Kordulis, A. Lycourghiotis, J.L.G. Fierro, J. Catal. 137 (1992) 285. [45] F.E. Massoth, G. Muralidhar, in: H.F. Barry, P.C.H. Mitchell (Eds.), Climax Molybdenum, Proceedings of the International Conference on the Chemistry of Molybdenum, Ann Arbor, 1982, p. 343. [46] C. Moreau, P. Geneste, in: J.B. Moffat (Ed.), Theoretical Aspects of Heterogeneous Catalysis, Van Nostrand Reinhold, New York, 1990. [47] M. Daage, R.R. Chianelli, J. Catal. 149 (1994) 414. [48] D.S. Thakur, P. Grange, B. Delmon, J. Less Com. Met. 64 (1979) 201. [49] D.S. Thakur, P. Grange, B. Delmon, J. Catal. 91 (1985) 318. [50] S. Mignard, S. Kasztelan, M. Dorbon, A. Poillon, P. Sarrazin, Stud. Surf. Sci. Tech. 100 (1996) 209. [51] M. Absi-Halabi, A. Stanislaus, K. Al-Dolama, Fuel 77 (1998) 787. [52] Y.C. Park, E.S. Oh, H.K. Rhee, Ind. Eng. Chem. Res. 36 (1997) 5083.
On the synergy between tungsten and molybdenum in the W-incorporated CoMo/␥-Al2 O3 hydrodesulfurization catalysts John Vakros, Christos Kordulis∗ Department of Chemistry, Institute of Chemical Engineering and High Temperature Chemical Processes (ICE/HT), University of Patras, P.O. Box 1414, GR-26500 Patras, Hellas, Greece Received 12 October 2000; received in revised form 12 April 2001; accepted 12 April 2001
Abstract The aim of this study is to investigate the influence of incorporation of tungsten on an already prepared CoMo/␥-Al2 O3 hydrodesylfurization catalyst (base catalyst). The evaluation of W-incorporated CoMo/␥-Al2 O3 is described, in relation to the base catalyst, concerning their activities for hydrodesulfurization (HDS) of thiophene and hydrogenation (HYG) of its unsaturated products towards butane. The main conclusion of this study is that incorporation of a low amount of tungsten onto CoMo/␥-Al2 O3 catalyst increases its HDS and HYG activities. A synergy has not been found between Mo- and W-phase. The W-loading for which the maximum activity is observed highly depends on the details of the preparation method used. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Hydrodesulfurization; Cobalt–molybdenum/alumina catalysts; Tungsten; Catalyst modification; Hydrogenation; Promoter; Synergy; BET
1. Introduction Environmental legislation places increasingly severe restrictions on the gaseous and particulate emissions arising both from the refinery itself as well as from the use of heating and transportation fuels. For example, while the current EU specification of sulfur content in gasoline is 150 ppm, the new limit of sulfur for the year 2005 is anticipated to be 50 ppm [1,2]. The reason for this reduction is the already proven relationship between the sulfur amount in gasoline and the final exhaust emissions from gasoline-fueled automobiles [1,2]. The necessity for lowering of the sulfur content in the transportation fuels has resulted
∗ Corresponding author. Fax: +30-61-994-796. E-mail address: [email protected] (C. Kordulis).
in a world wide increase of the research activity focused to the improvement of the HDS catalysts. Typical HDS catalysts consist of molybdenum supported on an alumina carrier with either cobalt or nickel added as promoters for improving the catalytic activity. The scientific and patent literature on the development and the characterization of HDS catalysts is quite extensive [3–6]. Methods used for the preparation of these catalysts have been reviewed in detail [7,8]. There have been many efforts of improving the activity and extending the life of conventional HDS catalysts by adding to them metallic (W, Ti, Ni, Zn, Ru, Fe) [9–23] or non-metallic modifiers (P, F) [24–35]. An interesting attempt to improve CoMo and NiMo/␥-Al2 O3 catalysts by the addition of a small amount of tungsten has been described by Lee et al. [9–12]. They concluded that tungsten is a promising
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 1 ) 0 0 6 1 6 - 0
288
J. Vakros, C. Kordulis / Applied Catalysis A: General 217 (2001) 287–293
secondary promoter for the above catalysts. This is a very important conclusion, although it was not expectable as it is well established that Mo and W sulfided phases accelerate HDS reactions in a similar way and W-phase is less active than the Mo one [36,37]. Coordinatively, unsaturated surface sites of these phases are considered to be the active centers for HDS [38,39]. In contrast, it is well acceptable that the usual promoters of the HDS catalysts (Co, Ni) act in a different way. The later is the subject of a long discussion during the last decade [40,41] On the other hand, Lee et al. [9–12] did not took under consideration that by incorporating W onto CoMo/␥-Al2 O3 catalyst the total active phase increases. In this work, we try to investigate further the role of the tungsten incorporated onto CoMo/␥-Al2 O3 catalysts. More precisely, we attempt first to examine whether the increase observed in the HDS activity of the CoMo/␥-Al2 O3 catalysts upon the W incorporation is independent of the preparation conditions used. Thus, we prepared a series of W-incorporated CoMo/␥-Al2 O3 catalysts with increasing W-loading, following a different preparation procedure from those of Lee et al. [10–12]. We also attempt to investigate whether the increase in the activity, if any, is due to a synergy between Mo- and W-phase and cannot be attributed to a simple increase of the total active phase (Mo + W). Thus, we prepared two additional catalysts by substituting the W of the most active W-incorporated CoMo/␥-Al2 O3 catalyst with Mo using two different procedures. The activity of the prepared samples was tested using HDS of thiophene as probe reaction.
[(NH4 )6 Mo7 O24 ·4H2 O, minimum 99%, Merck]. Cobalt was then deposited by incipient wetness impregnation using a cobalt nitrate aqueous solution [Co(NO3 )2 ·6H2 O, minimum 99%, Merck]. The so prepared base catalyst contained 10 wt.% MoO3 and 2.8 wt.% CoO fulfilling, thus, the synergistic atomic ratio, [Co/(Co + Mo)] = 0.35, for these catalysts [3]. Using the above catalyst, a series of tungsten incorporated CoMo/␥-Al2 O3 catalysts, denoted by W(y)CM, was prepared by depositing various amounts of tungsten (0.4–3.0 wt.% WO3 ) on it. The incipient wetness impregnation method and ammonium tungstate [(NH4 )10 W12 O41 ·5H2 O, 99.9% Alfa] aqueous solutions were used for the above preparations. In the above series, by (y) we denote the loading (as wt.%) of tungsten oxide incorporated in each case. Two additional CoMo/␥-Al2 O3 catalysts were prepared having higher Mo content than that of the base one. The excess Mo content of these catalysts calculated in moles was equal to the W-incorporated in W(1.0)CM catalyst. In the one of them (CM-A), the excess molybdenum was added during the first impregnation step. For the preparation of the other (MCM), the base catalyst was used and the method of preparation of W(y)CM series was followed replacing the ammonium tungstate solution used in the last impregnation step by an ammonium heptamolybdate one. After the impregnation of each metal precursor, the catalysts were dried at 110◦ C for 2 h and then calcined in air at 500◦ C for 5 h. All the catalysts prepared are listed in Table 1. 2.2. Specific surface area measurements
2. Experimental 2.1. Preparation of catalysts Commercial ␥-Al2 O3 carrier (Houdry Ho 415, S BET = 123 m2 g−1 , water pore volume = 0.45 cm3 g−1 ) supplied as cylindrical extrudates was used for the preparation of the catalysts studied. The extrudates were crushed and sieved into particles of 90–150 m size. A CoMo/␥-Al2 O3 catalyst (denoted as CM) was prepared by depositing first molybdenum on the alumina carrier via wet impregnation and using an aqueous solution of ammonium heptamolybdate
The specific surface area (SSA) of the catalysts was determined by the dynamic BET method using a laboratory-constructed apparatus. Pure nitrogen and helium (Air Liquide 99.999%) were used as adsorption and carrier gas, respectively. 2.3. Catalytic activity measurements HDS and HYG activities of the prepared catalysts were measured in a continuous flow tubular fixed-bed microreactor operating in a differential mode at atmospheric pressure. The catalyst samples (50 mg) were presulfided in situ, with a stream of 15% (v/v)
J. Vakros, C. Kordulis / Applied Catalysis A: General 217 (2001) 287–293
289
Table 1 Compiles the notation, composition and atomic ratios of supported phases as well as the specific surface areas (SSA) of the catalysts prepared Catalysts
Notation
MoO3 (wt.%)
CoO (wt.%)
WO3 (wt.%)
Co/(Co + Mo + W)
W/(Mo + W)
SSA (m2 g−1 )
Mo/␥-Al2 O3 CoMo/␥-Al2 O3 W–CoMo/␥-Al2 O3 W–CoMo/␥-Al2 O3 W–CoMo/␥-Al2 O3 W–CoMo/␥-Al2 O3 CoMo/␥-Al2 O3 CoMo/␥-Al2 O3
M CM W(0.4)CM W(1.0)CM W(2.0)CM W(3.0)CM MCM CM-A
10.30 10.00 9.96 9.90 9.80 9.70 10.53 10.53
0.00 2.80 2.79 2.77 2.74 2.72 2.78 2.78
0.00 0.00 0.41 1.00 2.00 3.00 0.00 0.00
0.000 0.350 0.344 0.336 0.323 0.311 0.337 0.337
0.000 0.000 0.025 0.059 0.112 0.161 0.000 0.000
– 110 105 107 104 112 100 110
hydrogen sulfide in hydrogen, for 2 h at 400◦ C. The reaction mixture was prepared by passing hydrogen through a flow evaporator filled with liquid thiophene at 0◦ C. After an aging period of 15 h under the stream of the reaction mixture the rate of thiophene HDS and the relative yield to butane were determined over each catalyst at various temperatures in the range 250–320◦ C. A gas chromatograph (Pye Unicam) equipped with a flame ionization detector and a column (8 m × 1/8 in. o.d.) filled with carbowax 20M on chromosorb P A.W. was used for analyzing of the effluent of the reactor. HDS rate was defined as r = Fx/W, where F is the molar flow-rate (mol min−1 ) of thiophene at the reactor inlet, x the conversion measured and W the weight (g) of catalyst used. The relative yield to butane, Y = moles of butane produced/moles of thiophene consumed, was used to estimate the activity of the catalysts for the HYG.
3. Results and discussion The composition, the atomic ratios of the promoter (Co) over the rest of the supported phases, the atomic ratio of the W-incorporated over the total active phase loading and the SSA of the prepared catalysts are compiled in Table 1. Inspection of the table shows that: (i) the first atomic ratio remains always very close to the corresponding synergistic ratio for these catalysts (Co/(Co + Mo + W) ≈ 0.3), (ii) SSA does not differ significantly in all catalyst prepared, although the second atomic ratio (W/(Mo + W)) changes from 0 to 0.161. The above permit us to attribute any difference in the catalytic activity of the prepared samples to the incorporation of the W-phase.
Fig. 1 illustrates the thiophene hydrodesulfurization rates determined using the base and W-incorporated CoMo/␥-Al2 O3 catalysts for all temperatures studied. An inspection of this figure shows that the HDS activity initially increases with the W-content up to 1 wt.% WO3 and then decreases with a further increase of the W-loading. This behavior is in a general agreement with the results published by Lee et al. [10,12], who studied the influence of incorporation of W into CoMo and NiMo/␥-Al2 O3 catalysts. The only difference between our and their results is in the value of the atomic ratio, W/(Mo + W), where the maximum activity has been achieved. They found the maximum activity in an atomic ratio W/(Mo + W) = 0.025, while our results showed that the maximum activity is obtained over the W(1.0)CM sample where W/(Mo + W) = 0.059 (see Table 1). In order to investigate whether the observed increase in the HDS activity is due to a synergy between Mo- and W-phase, we compared the activity of the most active of the W containing samples, W(1.0)CM, with that for the two CoMo/␥-Al2 O3 catalysts containing the same amount of total active phase, CM-A and MCM. In Fig. 2, the HDS rates of the above catalysts are illustrated. An inspection of this figure reveals that all these catalysts exhibited the same catalytic activity for the HDS of thiophene. This clearly shows that it is not possible to accept any synergy between Mo and W-phase in the W(1.0)CM catalyst. The higher activity of the W(1.0)CM catalyst as it is compared with that of the CM catalyst should be attributed to the increased total active phase loading (W + Mo) in the W(1.0)CM catalyst. Let us try to explain the above-mentioned difference in the value of the atomic ratio, W/(Mo + W),
290
J. Vakros, C. Kordulis / Applied Catalysis A: General 217 (2001) 287–293
Fig. 1. Reaction rates of thiophene HDS measured over the base and W-incorporated CoMo/␥-Al2 O3 catalysts at various temperatures.
Fig. 2. Reaction rates of thiophene HDS measured over the W(1.0)CM, MCM and CM-A catalysts at various temperatures.
J. Vakros, C. Kordulis / Applied Catalysis A: General 217 (2001) 287–293
where the maximum activity has been achieved. In the present work, the molybdenum phase in the base catalyst was deposited on the alumina surface using wet impregnation. It is well known that this deposition method results to higher dispersion of the deposited phase as it compared to the incipient wetness impregnation used by Lee et al. [10–12]. Consequently, in the later case a significant amount of Mo-phase is expected to be badly dispersed forming MoO3 crystallites on the alumina surface. On other hand, the pH of the ammonium tungstate impregnating solution used in the present work was not adjusted. It was, pH 5.5, the same with that of the ammonium heptamolybdate impregnating solution. This was done in order to avoid any disturbance in the dispersion of the previously deposited Mo-phase. In contrast, the above authors had adjusted the impregnating pH to 9.5 in order to increase the relative concentration of monomeric W-species [42,43]. However, it is well known that MoO3 crystallites, weakly interacting with alumina surface, are easily dissolved in high pH aqueous solutions [44]. Thus, the high pH solution used in that case for the incorporation of W-species must probably provoked a re-dissolution of MoO3 crystallites of the base catalyst and resulted in the re-dispersion of
291
the Mo-phase after subsequent drying and calcination. Indeed, the above authors have concluded that the catalytic activity promotion observed with the incorporation of tungsten in the CoMo/␥-Al2 O3 catalysts “was due to the presence of monomeric sulfidation — resistant WO3 -species which played a role of increasing the dispersion of active Co–Mo–S phases over the support” [12]. Taking into account the above considerations, we can conclude that, in their case, it was the high pH of tungsten impregnating solution which played a key role in re-dispersing the Mo-phase and consequently, increasing the catalytic activity, rather than the small amount of tungsten added. As we have already mentioned, the HDS activity of W-incorporated CoMo/␥-Al2 O3 catalysts decreased with the W-content in all cases after a critical W-content. This decrease could be attributed to the fact that as the W-content increases the monolayer capacity of the ␥-Al2 O3 is exceeded and the low activity W-species cover some of the more active Mo-species. Fig. 3 shows the relative yield (Y) to butane obtained during the hydrodesulfurization of thiophene over the base and W-incorporated CoMo/␥-Al2 O3 catalysts. The above yield can be taken as a measure of HYG activity of the catalysts since butane is the only fully
Fig. 3. Relative yields to butane determined over the base and W-incorporated CoMo/␥-Al2 O3 catalysts at various temperatures.
292
J. Vakros, C. Kordulis / Applied Catalysis A: General 217 (2001) 287–293
saturated molecule produced by hydrogenation of intermediate products of the thiophene hydrogenolysis (butadiene and butanes). An inspection of the above figure reveals that the relative yield to butane for each sample is higher at low reaction temperatures. At the lowest temperature studied (250◦ C), the Y increases with W-content of catalysts. It reaches its maximum value at 1 wt.% WO3 and then decreases with a further increase of W-content. At the rest of the temperatures studied a similar behavior is observed, but the maximum Y is sifted to the sample containing 2 wt.% WO3 (W(2.0)CM). This is also another difference between our results and those published by Lee et al. [10,12]. In fact, using ethylene hydrogenation as probe reaction, they found that the maximum activity is also reached at an atomic ratio W/(Mo + W) = 0.025. However, we have to note that the increase of HYG activity provoked by the incorporation of W onto CoMo/␥-Al2 O3 catalysts was significantly more important than that of HDS. The above finding is in accordance with the fact that two kinds of active sites are appeared in the hydrotreating catalysts. To be specific, the presence of two distinct types of sulfur vacancies in Mo- and W-based hydrotreating catalysts has been reported [45,46]. One of them is responsible for hydrogenolysis of heteroatoms and the other one for hydrogenation. Specific geometric arrangement of sulfur vacancies with different degrees of coordination appear to be required for each one of the above-mentioned reactions. Corner sites and edge sites can display different degrees of sulfur vacancies and can accelerate different reactions [46,47]. It has been suggested that corner sites with two or three sulfur vacancies are primarily responsible for hydrogenation reactions. The hydrogenolysis sites could be edge sites at a lower oxidation state or combinations of a sulfur vacancy and of a SH group. The distribution of hydrogenation and hydrogenolysis sites in the hydrotreating catalysts would essentially depend on the kind of active phase (MoS2 or WS2 ) and their slab size. It is well known that W-phase posses higher HYG activity than the Mo-phase in hydrotreating catalysts [48,49]. Higher hydrogenation activity has been also correlated with better dispersion of the active phases, and thus, with higher number of active sites [50]. It is evident from the present study that tungsten incorporation in the CoMo base catalyst has consid-
erable influence only on its hydrogenation activity. Based on the above discussion, we can conclude that this incorporation probably provokes modification in the morphological characteristics of the MoS2 and WS2 slabs, thus, resulting to an increase in the number of active sites that favor hydrogenation reaction [51]. Further work will be useful in gaining a better insight on the role of W in these catalysts. Indeed, based on the present results we cannot decide whether formation of separate tungsten sulfide and CoMo sulfide phases or mixed tungsten–molybdenum sulfide phase doped by cobalt is formed in the W-incorporated CoMo/␥-Al2 O3 catalysts. However, Park et al. [52] studying W-incorporated NiMo/␥-Al2 O3 catalysts have found that the tungsten incorporation enhances the reducibility of Mo-phase and increases the concentration of octahedrally coordinated Mo6+ -species. These findings are strong evidences that incorporation of tungsten brings about structural and in turn morphological changes at least in MoS2 active phase precursor. These changes seem to correlate with the increase of the number of sulfur vacancies measured by NO adsorption on the final catalyst found by the above researchers. This is consistent with higher dispersion of the active components resulting, thus, to smaller slabs and, consequently, to a higher proportion of corner sites. This high proportion of corner sites might be a plausible explanation for the increased HYG activity observed in our case. 4. Conclusions The most important findings of the present study may be summarized as follows: 1. Low amounts of tungsten incorporated onto CoMo/␥-Al2 O3 catalysts increase their HDS and HYG activities. This effect is more pronounced in the case of HYG reactions. 2. Concerning HDS the above effect may be attributed to a simple increase of the total active phase (W + Mo) and not to a synergy between Mo- and W-phase. 3. Concerning HYG, the observed increase could be attributed to the higher activity of W-phase as it is compared with that of Mo-phase, but also in the structural and morphological changes, which the incorporation of W might provoke in the Mo-phase.
J. Vakros, C. Kordulis / Applied Catalysis A: General 217 (2001) 287–293
4. The exact values of W/(Mo + W) atomic ratio in which the maximum activities are observed depend on the experimental details of the preparation procedure followed.
Acknowledgements Financial support to J.V. from ICE/HT is gratefully acknowledged. References [1] The Auto-Oil II program: Air Quality Report, Final version 7.1, 3 October 2000. [2] K.L Rock, R.M. Foley, in: Proceedings of the Grace Davidson FCC Technology Conference, Lisbon, September, 1998. [3] P. Grange, Catal. Rev. Sci. Eng. 21 (1980) 1. [4] P. Ratnasamy, S. Sivashanker, Catal. Rev. Sci. Eng. 22 (1980) 401. [5] H. Topsoe, B.S. Clausen, F.E. Massoth, Hydrotreating catalysis, in: J.R. Anderson, M. Boudart (Eds.), Series Catalysis Science and Technology, Vol. 11, 1996. [6] E. Furimsky, Appl. Catal. A: Gen. 171 (1998) 177. [7] D.L. Trimm, Design of Industrial Catalysts, Elsevier, Amsterdam, 1980. [8] A.B. Stiles, Catalyst Manufacture, Chemical Industries Series, Vol. 14, Marcel Dekker, New York, 1983. [9] D.K. Lee, I.C. Lee, S.I. Woo, Appl. Catal. A: Gen. 109 (1994) 195. [10] D.K. Lee, I.C. Lee, S.K. Park, S.Y. Bae, S.I. Woo, J. Catal. 159 (1996) 212. [11] D.K. Lee, H.T. Lee, I.C. Lee, S.K. Park, S.Y. Bae, C.H. Kim, S.I. Woo, J. Catal. 159 (1996) 219. [12] D.K. Lee, W.L. Yoon, Catal. Lett. 53 (1998) 193. [13] B.C. Kang, S.T. Wu, H.H. Tsai, J.C. Wu, Appl. Catal. 45 (1988) 221. [14] A.R. Sainim, B.G. Johnson, F.E. Massoth, Appl. Catal. 40 (1988) 157. [15] G. Muralidhar, F.E. Massoth, J. Shabtai, J. Catal. 85 (1984) 44. [16] C.V. Caceres, J.L.G. Fierro, A. Lopez Agudo, F. Severino, J. Laine, J. Catal. 97 (1986) 219. [17] J. Laine, F. Severino, C.V. Caceres, J.L.G. Fierro, A. Lopez Agudo, J. Catal. 103 (1987) 228. [18] A. Lopez Agudo, J.L.G. Fierro, C.V. Caceres, J. Laine, F. Severino, Appl. Catal. 30 (1987) 185. [19] J.F. Cambra, P.L. Arias, M.B. Guemez, J.A. Legarreta, J.L.G. Fierro, Ind. Eng. Chem. Res. 30 (1991) 2365. [20] P.C.H. Mitchell, C.E. Scott, J.P. Bonnelle, J.G. Grimblot, J. Catal. 107 (1987) 482. [21] A.S. Hirschon, R.B. Wilson Jr., R.M. Laine, Appl. Catal. 34 (1987) 311. [22] M. Ternan, J. Catal. 104 (1987) 256.
293
[23] W.L.T.M. Ramselaar, M.W.J. Craje, R.H. Hadders, E. Gerkema, V.H.J. De Beer, A.M. Van Der Kraan, Appl. Catal. 65 (1990) 69. [24] Ch. Kordulis, F. Melo Faus, B. Delmon, Appl. Catal. 23 (1986) 367. [25] Ch. Papadopoulou, Ch. Kordulis, A. Lycourghiotis, React. Kinet. Catal. Lett. 33 (1987) 259. [26] Ch. Papadopoulou, A. Lycourghiotis, P. Grange, B. Delmon, Appl. Catal. 38 (1988) 255. [27] H.K. Matralis, A. Lycourghiotis, P. Grange, B. Delmon, Appl. Catal. 38 (1988) 273. [28] A. Spojakina, S. Damyanova, L. Petrov, Z. Vit, Appl. Catal. 56 (1989) 163. [29] Ch. Kordulis, A. Gouromihou, A. Lycourghiotis, Ch. Papadopoulou, H.K. Matralis, Appl. Catal. 67 (1990) 39. [30] Ch. Kordulis, E. Dalas, P.G. Koutsoukos, A. lycourghiotis, React. Kinet. Catal. Lett. 45 (1991) 277. [31] Z. Sarbak, S.L.T. Andersson, Appl. Catal. 69 (1991) 235. [32] P.M. Boorman, R.A. Kydd, T.S. Sorensen, K. Chong, J.M. Lewis, W.S. Bell, Fuel 71 (1992) 87. [33] J.M. Lewis, R.A. Kydd, P.M. Boorman, P.H. Van Phyn, Appl. Catal. 84 (1992) 103. [34] Ch. Papadopoulou, H. Matralis, A. Lycourghiotis, P. Grange, B. Delmon, J. Chem. Soc., Faraday Trans. I 89 (1993) 3157. [35] H. Matralis, Ch. Papadopoulou, A. Lycourghiotis, Appl. Catal. A 116 (1994) 221. [36] M.J. Ledoux, O. Michaux, G. Agostini, J. Catal. 102 (1986) 275. [37] J.K. Norskov, B.S. Clausen, H. Topsoe, Catal. Lett. 13 (1992) 1. [38] L. Portela, P. Grange, B. Delmon, Catal. Rev.-Sci. Eng. 37 (1995) 699. [39] P.T. Vasudevan, J.L.G. Fierro, Catal. Rev.-Sci. Eng. 38 (1996) 161. [40] A.N. Startsev, Catal. Rev.-Sci. Eng. 37 (1995) 353. [41] B. Delmon, G.F. Froment, Catal. Rev.-Sci. Eng. 38 (1996) 69. [42] K.H. Tytko, O. Glemser, Adv. Inorg. Chem. Radiochem. 19 (1976) 239. [43] C.F. Baes, R.E. Mesmer, The Hydrogenolysis of Cations, Wiley, New York, 1976, pp.257–260. [44] M.A. Goula, Ch. Kordulis, A. Lycourghiotis, J.L.G. Fierro, J. Catal. 137 (1992) 285. [45] F.E. Massoth, G. Muralidhar, in: H.F. Barry, P.C.H. Mitchell (Eds.), Climax Molybdenum, Proceedings of the International Conference on the Chemistry of Molybdenum, Ann Arbor, 1982, p. 343. [46] C. Moreau, P. Geneste, in: J.B. Moffat (Ed.), Theoretical Aspects of Heterogeneous Catalysis, Van Nostrand Reinhold, New York, 1990. [47] M. Daage, R.R. Chianelli, J. Catal. 149 (1994) 414. [48] D.S. Thakur, P. Grange, B. Delmon, J. Less Com. Met. 64 (1979) 201. [49] D.S. Thakur, P. Grange, B. Delmon, J. Catal. 91 (1985) 318. [50] S. Mignard, S. Kasztelan, M. Dorbon, A. Poillon, P. Sarrazin, Stud. Surf. Sci. Tech. 100 (1996) 209. [51] M. Absi-Halabi, A. Stanislaus, K. Al-Dolama, Fuel 77 (1998) 787. [52] Y.C. Park, E.S. Oh, H.K. Rhee, Ind. Eng. Chem. Res. 36 (1997) 5083.