Search for an Efficient 4,6-DMDBT Hydrodesulfurization Catalyst: A

VOLUME 18, NUMBER 5

SEPTEMBER/OCTOBER 2004

© Copyright 2004 American Chemical Society

Articles Search for an Efficient 4,6-DMDBT Hydrodesulfurization Catalyst: A Review of Recent Studies Shyamal K. Bej,*,† Samir K. Maity,‡ and Uday T. Turaga§ Department of Chemical Engineering, University of Michigan, 3230 H.H. Dow Building, 2300 Hayward Avenue, Ann Arbor, Michigan 48109-2136, Instituto Mexicano del Petroleo, Eje Central Lazaro Cardenas 152, Mexico City, DF 07730, Mexico, and Fuel Science Program, Department of Energy and Geo-Environmental Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received November 19, 2003. Revised Manuscript Received May 11, 2004

Almost complete removal of 4,6-dimethyl dibenzothiophene (4,6-DMDBT) will perhaps be inevitable for reducing the sulfur content of diesel to a level of 50 wppm and lower. The hydrodesulfurization (HDS) of 4,6-DMDBT does not tend to occur through direct desulfurization, a pathway typically followed by reactive sulfur compounds over conventional CoMo/Al2O3 catalysts. Its reactivity can be enhanced either by increasing the rate of direct desulfurization or by transforming it to a more activated molecule through hydrogenation, isomerization, demethylation, and C-C bond scission. Attempts have been made to develop better catalysts using these concepts. Different additives such as phosphorus, fluorine, and lanthanum have been added to the alumina support for developing the required catalytic properties. Various other supports such as zeolite, zirconia, titania, etc., by themselves or in admixtures with alumina have also been used to improve the HDS activities of the catalysts. This article reviews the results of recent studies conducted in this area and summarizes the advances that have taken place in this direction.

Introduction Stringent environmental regulations are exerting pressure to reduce the maximum allowable sulfur content in diesel. In most advanced countries, the allowable limit on diesel sulfur will be restricted to 50 wppm within another 5 years.1,2 For example, Europe will tighten its diesel sulfur content to less than 50 wppm by 2005.3 The situation in the USA and Japan * Corresponding author: Tel: 734-763-5748. Fax: 734-763-0459. E-mail: [email protected]. † University of Michigan. ‡ Instituto Mexicano del Petroleo. § The Pennsylvania State University. (1) Shin, S.; Yang, H.; Sakanishi, K.; Mochida, I.; Grudoski, D. A.; Shinn, J. H. Appl. Catal. A: General 2001, 205, 101. (2) Yoshimura, Y.; Yasuda, H.; Sato T.; Kijima, N.; Kameoka, T. Appl. Catal. A: General 2001, 207, 303.

will also be quite similar.4 Sulfur present in diesel is usually removed through hydrodesulfurization (HDS) processes using alumina-supported CoMo- or NiMobased catalysts. Using these currently operating HDS technologies, the sulfur content in diesel is typically reduced to a level of 300-500 wppm. The molecules, which need to be removed for bringing down the sulfur content from existing levels of 300500 wppm to a future level of 3,7-DMDBT > 4,6-DMDBT Approaches for Developing Better Catalysts where 4,6-DMDBT has been found to exhibit the lowest rate. The electronic effects of the alkyl groups are known to be responsible for the relatively higher activity of 2,8DMDBT. The primary reason for the poor reactivity of 4,6-DMDBT has been attributed to the steric hindrance of the methyl groups, which makes the sulfur atom inaccessible to the active sites of the catalyst.10-14 As a result, the HDS of 4,6-DMDBT does not tend to follow the direct desulfurization route (shown below), typical

of other reactive sulfur compounds over CoMo/Al2O3 catalysts.15 This has, therefore, spurred intense interest among researchers to explore alternative catalytic pathways for activating this molecule. In addition, there has been considerable interest in developing improved process configurations and novel reactors to facilitate the efficient removal of these sulfur compounds. The possibilities as well as the challenges associated with these engineering options for producing ultralow sulfur diesel have been recently reviewed.16,17 Ongoing research is also focused on other non-HDS technologies for the efficient removal of these refractory sulfur molecules. These include desulfurization via adsorption, desulfurization via extraction, oxidative desulfurization, and desulfurization via precipitation, etc. Recent advances in these areas have been reviewed by Babich and Moulijn18 and Song.19 Among these, desulfurization by adsorption deserves special mention and is in a very advanced stage of development. Advances up to the late 1990’s in HDS catalysis and other related aspects have been reviewed in depth by Whitehurst et al.20 In this article, we will review recent (7) Schulz, H.; Bohringer, W.; Waller, P.; Ousmanov, F. Catal. Today 1999, 49, 87. (8) Kilanowski, D. R.; Teeuwen, H.; Gates, B. C.; Beer, V. H. J. D.; Schuit, G. C. A.; Kwart, H. J. Catal. 1978, 55, 129. (9) Houalla, M.; Broderick, D. H.; Sapre, A. V.; Nag, N. K.; Beer, V. H. J. D. J. Catal. 1980, 61, 523. (10) Houalla, M.; Nag, N. K.; Sapre, A. V.; Broderick, B. H.; Gates, B. C. AIChE J. 1978, 24 (6), 1015. (11) Landau, M. V. Catal. Today 1997, 36, 393. (12) Meille, V.; Schulz, E.; Lemaire, M.; Vrinat, M. J. Catal. 1997, 170, 29. (13) Kabe, T.; Ishihara, A.; Zhang, Q. Appl. Catal. A 1993, 97, L1. (14) Bataille, F.; Lemberton, J. L.; Michaud, P.; Perot, G.; Vrinat, M.; Lemaire, M.; Schulz, E.; Breysse, M.; Kasztelan, S. J. Catal. 2000, 191 (2), 409. (15) Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30 (9), 2021. (16) Bej, S. K. Fuel Process. Technol. 2004, 85, 1503. (17) Bej, S. K.; Dalai, A. K.; Maity, S. K. Rev. Process Chem. Eng. 2000, 3 (3), 203. (18) Babich, I. V.; Moulijn, J. A. Fuel 2003, 82, 607. (19) Song, C. Catal. Today 2003, 86 (1-4), 211. (20) Whitehurst, D. D.; Isoda, T.; Mochida, I. Adv. Catal. 1998, 42, 345.

The important routes through which the reactivity of 4,6-DMDBT can be enhanced are shown in Figure 1. Except for direct desulfurization, all other pathways mainly center on removing the steric hindrance of the methyl groups present at 4 and 6 positions. The first one, which has received considerable attention, is the hydrogenation of one of the phenyl rings. The hydrogenation of a phenyl ring imparts flexibility to the methyl group resulting in the reduction of steric hindrance.14,21,22 This is represented in Figure 2 which compares the flexibility of 4,6-DMDBT and its partially hydrogenated derivative for approach to the active site of the catalyst in head-on and side-on fashions. Another way of reducing the steric hindrance of the methyl groups is to shift these groups from 4,6 to 3,7 or to 2,8 positions through an isomerization reaction.23-25 The complete removal of one or both methyl groups through a dealkylation reaction offers another possibility.21,21 The scission of the single C-C bond in the thiophenic ring has also been attempted by various researchers.21 For this discussion, we will refer to these (isomerization, dealkyalation, and C-C bond scission) reactions as non-hydrogenative routes for desulfurization. The properties required in a catalyst for directing the HDS reaction through the above-mentioned pathways can be grouped into two classes. The saturation of one of the phenyl rings depends primarily on the hydrogenation capability of the catalyst which can be enhanced either by incorporating a suitable metal such as Ni, W, Pt, Pd, Ru, etc., and/or by providing a suitable support. All other routes such as isomerization, dealkylation, and C-C bond scission depend mainly on the acidic property of the catalyst.21 Support plays an important role in controlling the acidic property. Researchers have tried to use various mixed oxide supports from a combination of Al2O3, TiO2, ZrO2, etc., to achieve this goal.27 Besides the mixed oxide supports, other acidic materials such as different types of zeolites and amorphous silica alumina (ASA) have also received considerable attention.28,29 For the purpose of discussion, a general classification of the catalysts with potential for the removal of 4,6-DMDBT is shown in Figure 3. (21) Landau, M. V.; Berger, D.; Herskowitz, J. Catal. 1996, 159, 236. (22) Ma, X.; Sakanishi, K.; Iosda, T.; Mochida, I. Ind. Eng. Chem. Res. 1995, 34, 748. (23) Michaud, P.; Lemberton, J. L.; Perot, G. Appl. Catal. A: General 1998, 169, 343. (24) Lecrenay, E.; Sakanishi, K.; Mochida, I. Catal. Today 1997, 39, 13. (25) Ozaki, H. Catalysis Surveys from Japan 1997, 1 (1), 143. (26) Lecrenay, E.; Mochida, I. Stud. Surf. Sci. Catal. 1997, 106, 333. (27) Breysse, M.; Afanasiev, P.; Geantet, C.; Vrinat, M. Catal. Today, in press . (28) Isoda, T.; Takase, Y.; Kusakabe, K.; Morooka, S. Energy Fuels 2000, 14, 585. (29) Robinson, W. R. A. M.; van Veen, J. A. R.; de Beer, V. H. J.; van Santen, R. A. Fuel. Process. Technol. 1999, 61, 103.

Efficient 4,6-DMDBT Hydrodesulfurization Catalyst

Energy & Fuels, Vol. 18, No. 5, 2004 1229

Figure 1. Possible reaction pathways for enhancing the reactivity of 4,6-DMDBT.

methyl biphenyl) or simply BP (biphenyl) is generally the result of the demethylation propensity of the catalyst. Though desulfurized products formed through the direct desulfurization route can also undergo phenyl ring hydrogenation, the presence of relatively larger amounts of MCHT compared to DMBP generally indicates the preferential occurrence of HDS through hydrogenative desulfurization (i.e., prehydrogenation of 4,6-DMDBT). Similarly, the desulfurized products (formed through the prehydrogenation route) can also undergo isomerization or dealkylation. Specially designed experiments are conducted to understand the reactivities of various possible intermediates, which in turn help to elucidate the reaction pathways in a meaningful way. Oxide-Supported Catalysts Figure 2. Enhancement in flexibility of the partially hydrogenated 4,6-DMDBT molecule for approaching the active sites of the catalyst.

Studies aimed at developing better catalysts for 4,6DMDBT HDS have mostly been compared to either CoMo/Al2O3 or NiMo/Al2O3 catalysts. Conversion and/ or reaction rate constants have generally been used as a criterion for comparison. Analysis of intermediates and final products has often been employed to elucidate reaction pathways. For example, the preferential formation of dimethyl biphenyl (DMBP) compared to methyl cyclohexyl toluene (MCHT) over a catalyst indicates the dominance of a non-hydrogenative desulfurization route (refer Figure 1). The formation of DMBP could be the result of an increase either in the intrinsic hydrogenolysis capability or in the isomerization capability of the catalyst. On the other hand, formation of MMBP (mono

Alumina alone or in admixtures with other components has been used as supports for the active metals and promoters. In some studies other non-aluminabased oxide supports have also been used. Hydrodesulfurization catalysts are usually prepared by impregnation of active metals (Mo, W) and promoters (Co, Ni) onto the support surface following pore filling or incipient wetness techniques. Impregnations of metals are carried out either through a single step (coimpregnation) or a two-step (sequential impregnation) procedure. Finally, the impregnated sample is dried and calcined to disperse the active metals onto the support. The dispersion of the active metals, a key property of the finished catalyst, depends on a number of parameters such as impregnation procedure, solute concentration, pH, calcination temperature, etc. In the two-step impregnation procedure, the sequence of impregnating the active metal and the promoter sometime plays an

1230 Energy & Fuels, Vol. 18, No. 5, 2004

Bej et al.

Figure 3. A general classification of the catalysts holding potential for the removal of 4,6-DMDBT.

important role. The effects of these parameters on the properties of HDS catalysts have been discussed in several reviews.30-32 The oxides of the active metals are then converted to their sulfide forms, which are generally believed to be the active phases for the HDS reaction. Alumina-Supported Catalysts. Three combinations of metals, viz., CoMo, NiMo, and NiW have generally been used as the active components over the alumina support. Alumina-Supported CoMo Catalysts. As mentioned earlier, the rate of 4,6-DMDBT removal through a direct desulfurization route over CoMo/Al2O3 catalyst is slow because of the steric hindrance of the methyl groups. Early literature indicates that CoMo-based alumina catalysts have low activities for the prehydrogenation of the aromatic ring.13,33,34 Various efforts have been made to increase the activities of conventional CoMo-based alumina catalysts by incorporating more hydrogenation capabilities in these materials. These include loading of the active metals in greater amounts, improving dispersion of the active metals, and manipulating the acidity level of the alumina support. The first two objectives have been achieved by increasing the surface area of the support and also by using better metal loading techniques.35 A number of hydrotreating catalyst manufacturers such as Akzo Nobel, Criterion, Haldor-Topsoe, and United Catalyst have improved the performances of their CoMo-based alumina catalysts using various novel techniques, which have been recently summarized by Song and Ma.19,36

Addition of phosphorus and fluorine has been claimed to improve dispersion as well as to increase acidity of the alumina support.37,38 However, very little literature is available reporting the effects of these additives on the HDS of 4,6-DMDBT.39,40 A summary of recent findings has been provided by Moon.41 Lecrenay et al.42 compared the performances of CoMo/ Al2O3 and phosphorus-modified CoMo/Al2O3 catalysts. The Co and Mo contents of the catalysts were kept constant, respectively, at 3 and 10 wt % for both cases, whereas the phosphorus content in the second catalyst was 3 wt % (as P2O5). Upon introduction of phosphorus, the rate of 4,6-DMDBT HDS in decane increased 3-fold. They also compared cumene transalkylation and naphthalene hydrogenation rates over both these materials. The phosphorus-containing catalyst increased the cumene transalkylation and the naphthalene hydrogenation rates by approximately three and two times, respectively. The level in improvement depends on the amount of phosphorus added and the method of addition. Kwak et al.39 studied, in detail, the effect of phosphorus addition on the behaviors of alumina-supported CoMoSbased catalysts. The reaction was conducted at 350 °C under 4.0 MPa using 4,6-DMDBT in dodecane. The phosphorus-modified alumina support was prepared by impregnating γ-alumina with an aqueous solution of H3PO4. With the addition of phosphorus, the conversion of 4,6-DMDBT initially increased and then decreased, showing a maximum corresponding to a P2O5 content of 0.5 wt %. They proposed that the enhancement in conversion with the addition of phosphorus was due to

(30) Topsoe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating catalysis science and technology; Springer-Verlag: New York, 1996. (31) Zdrazil, M. Catal. Today 1988, 3, 269. (32) Startsev, A. N. Catal Rev.sSci. Eng. 1995, 37, 353. (33) Isoda, T.; Ma, X.; Mochida, I. Abstract of Papers of the ACS 208: 59-PETR Part 2 1994, AUG 21, 1994. (34) Ma, X.; Sakanishi, K.; Isoda, T.; Mochida, I. Abstract of Papers of the ACS 208: 83-PETR Part 2 1994, AUG 21, 1994. (35) Mignard, S.; Kasztelan, S.; Dorbon, M.; Billon, A.; Sarrazin, P. Stud. Surf. Sci. Catal. 1996, 100, 209. (36) Song, C.; Ma, X. Appl. Catal. B 2003, 41, 207.

(37) Atanasova, P.; Halachev, T.; Uchytil, J.; Kraus, M. Appl. Catal. 1988, 38, 235. (38) Matralis, H. K.; Lycourghiotis, A.; Grange, P.; Delmon, B. Appl. Catal. 1988, 38, 273. (39) Kwak, C.; Kim, M. Y.; Choi, K.; Moon, S. H. Appl. Catal. A 1999, 185, 19. (40) Kwak, C.; Lee, J. J.; Bae, J. S.; Choi, K.; Moon, S. H. Appl. Catal. A 2000, 200, 233. (41) Moon, S. H. Catal. Surveys from Asia 2003, 7 (1), 11. (42) Lecrenay, E.; Sakanishi, K.; Mochida, I.; Suzuka, T. Appl. Catal. A 1998, 175, 237.



the increase in molybdenum dispersion. This was also evident from nitric oxide chemisorption data. The decrease in HDS rate beyond this level of phosphorus content (0.5 wt % P2O5) was attributed to the formation of relatively stable Co-Mo-P compounds. The amounts of both DMBP and MCHT increased with increasing phosphorus content of the catalyst and attained a maximum value corresponding to a P2O5 content of 0.5 wt %. However, it was interesting to note that the increase in the rate of DMBP formation was higher than that of MCHT formation. This indicated that addition of phosphorus enhanced the rate of non-hydrogenative desulfurization to a greater extent compared to the rate of hydrogenative desulfurization. They proposed that addition of phosphorus increased the Bronsted acidity of the catalyst and this in turn facilitated the migration of the methyl groups. The migration of methyl groups was further supported by 2,2′-DMBP isomerization data. The effect of fluorine addition to CoMo-based catalysts for the HDS of 4,6-DMDBT was studied by Kwak et al.40 The method of successive impregnation was used to load the various components, where the order of impregnation was as follows: first fluorine, followed by molybdenum , then finally nickel. NH4F was used as the fluorine precursor. The HDS of 4,6-DMDBT (in dodecane) was performed at a temperature of 350 °C, under a hydrogen pressure of 4.0 MPa. It was observed that the conversion of 4,6-DMDBT increased with the increase in fluorine content of the catalysts. DMBP and MCHT were observed as the two major products of 4,6DMDBT HDS over fluorinated CoMoS catalysts. The concentrations of both these compounds increased with increasing fluorine content of the catalysts. This suggested that both hydrogenative and non-hydrogenative desulfurization routes were favored by the addition of fluorine. The enhancement in hydrogenative and nonhydrogenative desulfurization activities with the increasing amount of fluorine content was also confirmed by BP hydrogenation and 2,2′-DMBP isomerization data, respectively. The improvement in hydrogenation activity was attributed to the increased dispersion of the active phase CoMoS, whereas, the enhancement in isomerization capability was attributed to the increased acidity of the catalysts. However, for any particular level of fluorine incorporation, the concentration of MCHT was higher than DMBP, indicating that hydrogenation was the predominant pathway for the HDS of 4,6DMDBT over these catalysts. Unlike the zeolite-modified catalysts, cracking reactions producing light hydrocarbons were essentially absent over the fluorinated (as well as phosphorus-modified) catalysts, indicating that the acidity generated was not high enough to allow these undesirable side reactions. Alumina-Supported NiMo Catalysts. It has been reported that NiMo/Al2O3 catalysts normally have a greater aromatic hydrogenation capability as compared to that of CoMo/Al2O3 catalysts.43 Isoda et al.44 carried out a comparative study on the HDS of 4,6-DMDBT over alumina-supported commercial CoMo and NiMo catalysts. Their studies were conducted at a temperature of 270 °C, and a hydrogen pressure of 3.0 MPa. Under

identical experimental conditions, the conversion of 4,6DMDBT over the NiMo catalyst was 68%, whereas, that over the CoMo catalyst was only 49%. A comparison of the product distributions showed that hydrogenative desulfurization was the predominant pathway over both these materials. However, some minor differences were observed. For example, the extent of further hydrogenation of another aromatic ring in MCHT was appreciable over the NiMo catalyst, whereas, it was negligible over the CoMo catalyst. Although DMBP, the product of direct desulfurization, was formed in small quantities over both the catalysts, its content was lower for the NiMo-based one. Lecrenay et al.24 studied the HDS of 4,6-DMDBT (in decane solution) over both CoMo-based alumina and NiMo-based alumina catalysts. The CoO content of the CoMo catalyst was 4.2 wt % while the NiO content of the NiMo-based catalyst was 3.1 wt %. Though both catalysts had nearly similar cracking activities (as estimated from their isopropylbenzene transalkylation rates), the hydrogenation activity (as estimated from its naphthalene hydrogenation capability) of the NiMobased alumina catalyst was almost 2.5 times higher than the CoMo-based catalyst. As a result of the higher hydrogenation capability the NiMo-based material exhibited a higher HDS rate. The first-order rate constant for 4,6-DMDBT HDS over the CoMo-based catalyst was ∼0.006 min-1 g-1 while that over the NiMo-based one was ∼0.02 min-1 g-1. It was also interesting to note that the ratio of the amount of products obtained through the hydrogenative desulfurization route to those obtained through the direct desulfurization route was very high (∼12 at 270 °C) for the Ni-Mo catalyst as compared to ∼4 at 270 °C for the CoMo-based catalyst. This further suggested that the enhancement in HDS activity over the NiMo-based catalyst was possibly due to its increased hydrogenation capability. The relative performances of CoMo/Al2O3 and NiMo/ Al2O3 catalysts also depend on the aromatic content of the feedstock. Isoda et al.45 have compared the performances of these two catalysts using a mixture of 4,6DMDBT, decane, and naphthalene. Because of the dominance of naphthalene hydrogenation over the NiMo-based catalyst, the performance of this material for the conversion of 4,6-DMDBT was strongly retarded due to the presence of naphthalene. Conversely, the CoMo-based catalyst was found to be less retarded by naphthalene, and as a result was found to be superior to the NiMo-based one for the HDS of 4,6-DMDBT. In another study, Lecreany et al.46 reported that the CoMoS-based alumina catalyst showed a somewhat higher activity for the transalkylation of isopropylbenzene than that of a NiMoS-based alumina catalyst. Each catalyst contained 3 wt % of promoter metal (Ni or Co) and 10 wt % of molybdenum oxide. The NiMoSbased catalyst was also more active than the CoMoSbased one for the hydrogenation of naphthalene. The pseudo-first-order rate constant for the HDS of 4,6DMDBT (in decane) at 270 °C over the NiMo-based alumina catalyst was approximately double that of the CoMo-based alumina catalyst. This indicated that the

(43) Stanislaus, A.; Cooper, B. H. Catal. Rev. Sci. Eng. 1994, 36, 75. (44) Isoda, T.; Nagao, S.; Ma, X.; Korai, Y.; Mochida, I. Energy Fuels 1996, 10, 1078.

(45) Isoda, T.; Nagao, S.; Ma, X.; Korai, Y.; Mochida, I. Appl. Catal. A: General 1997, 150, 1. (46) Lecrenay, E.; Sakanishi, K.; Nagamatsu, T.; Mochida, I.; Suzuka, T. Appl. Catal. B 1998, 18, 325.


hydrogenation property of the NiMo-based material had more influence than the cracking property of the CoMobased catalyst for the HDS of 4,6-DMDBT. Knudsen et al.47 also reported that NiMo -based alumina catalysts were more active than the CoMobased ones for the removal of 4,6-DMDBT present in a blend of 50% SRGO and 50% LCO. In this case, both catalysts were tested at the same pressures, but LHSVs and reaction temperatures were adjusted to give the same level of HDS (∼97%). The effects of phosphorus addition have also been reported to improve the performance of NiMo/Al2O3 catalysts.42 Upon introduction of 3 wt % of P2O5 into a NiMo/Al2O3 catalyst containing 3 wt % Ni and 10 wt % Mo, the rate 4,6-DMDBT HDS could be increased by a factor of about 3. This enhancement in HDS rate was attributed to increase in both hydrogenation activity and transalkylation activity. Fluorine has also been used as an additive to improve the activity of NiMo/Al2O3 catalysts.48,49 Though addition of fluorine enhanced both non-hydrogenative desulfurization and hydrogenative desulfurization pathways, the effect was more pronounced in the former. Characterization of the catalysts revealed that both dispersion of the active metals (Ni and Mo) and acidity of the support were improved due to the addition of fluorine. XPS studies showed that addition of fluorine had a negligible effect on the electronic properties of either Mo or Ni. The increase in dispersion of the active metals caused an enhancement in the hydrogenative desulfurization route. The improvement in the non-hydrogenative desulfurization route was the result of methyl migration presumably due to the increased acidity of the catalysts. Pyridine IR and 2,2′-DMBP isomerization studies further supported the notion of an increase in Bronsted acid sites due to the addition of fluorine. Ogawa et al.50 attempted to improve the catalytic activity of NiMo/Al2O3 catalyst by introducing lanthanum as an additive. The rate constant for the HDS of 4,6-DMDBT increased by about 80% when La loading was low (0.7 wt %). Increasing La loading beyond this level decreased the surface Ni concentration which resulted in a decrease in the value of the rate constant. The structure of MoS2 crystallites was also affected by the addition of La. Up to a La loading of 0.7 wt %, the lateral size of the MoS2 crystallites increased. On the other hand, a higher La loading (more than 0.7 wt %) inhibited the lateral growth of MoS2 crystallites. Alumina-Supported NiW Catalysts. NiW-based materials are known to possess higher hydrogenating properties and hence hold potential for enhancing 4,6DMDBT HDS rates.51 Reinhoudt et al.52 conducted a comparative study on alumina-based CoMo, NiMo, and NiW catalysts. The Co and Mo contents of the CoMo/ (47) Knudsen, K. G.; Cooper, B. H.; Topsoe, H. Appl. Catal. A 1999, 189, 205. (48) Kim, H.; Lee, J. J.; Moon, S. H. Appl. Catal. B 2003, 44, 287. (49) Kim, H.; Lee, J. J.; Koh, J. H.; Moon, S. H. Stud. Surf. Sci. Catal. 2003, 145, 315. (50) Ogawa, Y.; Toba, M.; Yoshimura, Y. Appl. Catal. A 2003, 246, 213. (51) Gachet, G.; Breysse, M.; Cattenot, M.; Decamp, T.; Frety, R.; Lacroix, M.; de Mourges, L.; Portefaix, J. L.; Vrinat, M.; Duchet, J. C.; Housni, S.; Lakhdar, M.; Tilliette, M. J.; Bachelier, J.; Cornet, D.; Engelhard, P.; Gueguen, C.; Toulhoat, H. Catal. Today 1988, 4, 7. (52) Reinhoudt, H. R.; Troost, R.; van Langeveld, A. D.; Sie, S. T.; van Veen, J. A. R.; Moulijn, J. A. Fuel Process. Technol. 1999, 61, 133.

Bej et al.

γ-Al2O3 catalyst were 3.0 and 9.5 wt %, respectively. The NiMo/γ-Al2O3 catalyst contained 1.4 wt % of Ni and 7.9 wt % of Mo. The Ni and W contents of the NiW/γ-Al2O3 catalyst were 3.0 and 9.5 wt %, respectively. The HDS experiments were conducted using 4-E (4-ethyl), 6-M (6methyl) DBT in hexadecane at 390 °C and a total pressure of 6.0 MPa. The NiW-based material was found to be more active than the CoMo or NiMo catalyst presumably due to its greater hydrogenation capability. The higher activity of the NiW/Al2O3 catalyst as compared to that of the CoMo/Al2O3 or NiMo/Al2O3 catalyst has similarly been reported by Robinson et al.29 Catalysts Based on Zeolite Mixed Alumina Supports. Various types of zeolites have been added to alumina for increasing its acidity. The increase in acidity is expected to enhance the isomerization and dealkylation of the alkyl groups present in 4,6-DMDBT. Isoda et al.44 studied the performance of a Y-zeolite mixed alumina-supported CoMo catalyst. The Co and Mo were loaded over the mixed support containing 5 wt % Y-zeolite and balance alumina following an impregnation procedure. Commercially available CoMoand NiMo-based alumina catalysts were also tested for comparison. The loadings of Co in the commercial CoMo catalyst and the mixed supported CoMo catalyst were 4.4 and 3.9 wt % (as CoO), respectively. The Ni loading was 3.1 wt % (as NiO) in the commercial NiMo catalyst. The MoO3 contents in the commercial CoMo, NiMo, and the mixed supported CoMo catalysts were 14.9, 14.9, and 15.8 wt %, respectively. The HDS activity tests were conducted using 4,6-DMDBT in decane at 270 °C under a hydrogen pressure of 3.0 MPa. The Y-zeolite mixed alumina-supported CoMo catalyst showed the highest conversion. Under similar conditions, the conversion of 4,6-DMDBT obtained over the mixed-supported CoMo catalyst was 72%, whereas, the conversions over the commercial CoMo and NiMo catalysts were 49% and 68%, respectively. The product distribution over the zeolite mixed alumina-supported CoMo catalyst was significantly different from those obtained over the alumina-supported CoMo and NiMo catalysts. The product patterns obtained over the two alumina-based materials indicated that the HDS of 4,6-DMDBT took place mainly through the hydrogenation route. On the other hand, isomerization of 4,6-DMDBT to 3,6-DMDBT and dealkylation products were observed characteristically over the zeolite mixed alumina-supported catalyst. Significant amounts of various light hydrocarbons were produced over this catalyst presumably from the hydrocracking reactions. The dominance of methyl migration and hydrocracking reactions were the results of the higher acidity of the zeolite mixed alumina-supported CoMo catalyst (23% more acid sites) as compared to that of the conventional alumina-supported CoMo and NiMo catalysts. The absence of significant deactivation was also noticed over this zeolite mixed alumina-supported material despite the presence of a higher number of acid sites on it. Landau et al.21 also conducted a systematic study of 4,6-DMDBT HDS over alumina-supported CoMo and NiMo catalysts, a mixed HY-Al2O3-supported CoMo catalyst and a mixed HZSM-5-Al2O3-supported CoMo catalyst. The MoO3 contents of these catalysts were 25.1, 24.0, 20.9, and 24.8 wt %, respectively. The surface



Table 1. Performances of Various Oxide Mixed Alumina-Supported CoMo and NiMo Catalysts for the HDS of 4,6-DMDBT, Transalkylation of Cumene, and Hydrogenation of Naphthalene. The Co, Ni, and Mo Contents of All the Catalysts are 3, 3, and 10 wt %, Respectively42,46 catalyst 8 wt % TiO2-Al2O3 support

Al2O3 support

25 wt % TiO2-Al2O3 support

20 wt % ZrO2-Al2O3 support

15 wt % B2O3- Al2O3 support

properties

CoMo

NiMo

CoMo

NiMo

CoMo

NiMo

CoMo

NiMo

CoMo

NiMo

(m2/g)

240 5

440 10

252 14

252 14

247 18

247 14

230 11

230 15

288 19

288 32

7

4

10

4

35

12

26

7

43

35

3

9

5

8

10

15

6

17

6

22

surface area rate constant (× 10-3) for HDS of 4,6-DMDBT at 270 °C rate constant (× 10-6) for transalkylation of cumene at 250 °C rate constant (× 10-3) for hydrogenation of naphthalene at 250 °C

areas of the catalysts were 230, 140, 308, and 243 m2/ g, respectively. The zeolite-alumina mixed supports were prepared by coextrusion of aluminum hydroxide with zeolite powders. The HDS tests were carried out using 4,6-DMDBT dissolved in a mixed solvent containing 29% n-decane, 50% n-octadecane, and 30% tetralin. The first-order rate constants for the HDS of 4,6DMDBT at 360 °C for all the Co-Mo loaded materials are compared in Figure 4. Upon introduction of HZSM-5 into CoMo/Al2O3 catalyst, the HDS rate decreased by a factor of about 1.3. There was no significant change in the product distribution. A lower diffusion rate of the bulky 4,6-DMDBT molecule in the HZSM-5 channels was thought to be responsible for the decrease in the HDS activity. Because of this, the HZSM-5 component remained inactive and acted as an inert diluent. Conversely, the HDS rate increased by a factor of about 3 for the case of HY-zeolite mixed alumina-supported CoMo catalyst. Analysis of the products revealed that the reaction proceeded mainly through the scission of the C-C bond connecting the two aromatic rings in the 4,6-DMDBT molecule. In another study, Lecrenay et al.24 compared the performance of a zeolite mixed alumina-supported CoMo catalyst with those of alumina-supported commercial NiMo and CoMo catalysts. The CoO (or NiO) and MoO3 contents of the catalysts were in a similar range. The surface areas of the CoMo/Al2O3, NiMo/Al2O3, and zeolite mixed alumina-supported CoMo catalysts were 268, 273, and 220 m2/g, respectively. The first-order rate

constants for the HDS of 4,6-DMDBT (in decane) for these materials are shown in Figure 5. The value was highest for the alumina-zeolite mixed supported CoMo catalyst. It may be worth mentioning here that the zeolite mixed catalyst also showed the maximum cracking activity (as determined from the isopropylbenzene transalkylation test). The cracking activities of the alumina-supported NiMo and CoMO catalysts were very small. As expected, the alumina-supported NiMo catalyst exhibited the highest activity for the hydrogenation of naphthalene. They reported that as the acidic component was increased in the support, both cracking and hydrogenating capabilities of the catalysts were also enhanced. The desulfurization of 4,6-DMDBT over the zeolite mixed catalyst occurred mainly through isomerization and cracking routes. Catalysts Based on Other Oxide Mixed Alumina Supports. The use of mixed oxide alumina-supported CoMo- or NiMo-based catalysts have provided encouraging results for the HDS of thiophene and benzothiophene.53-57 This has prompted researchers to explore the potentials of these materials for the HDS of 4,6-DMDBT. The HDS of 4,6-DMDBT over alumina containing mixed oxide-supported CoMo- and NiMo-based catalysts were studied by Mochida and co-workers.42,46 The results are summarized in Table 1. Two different TiO2-

Figure 4. Comparison of first-order rate constants for the HDS of 4,6-DMDBT over alumina, HZSM-5 mixed alumina, and HY mixed alumina-supported CoMo catalysts (adapted form ref 21).

Figure 5. Comparison of first-order rate constants for the HDS of 4,6-DMDBT over NiMo/alumina, CoMo/alumina, and zeolite mixed alumina-supported CoMo catalysts (adapted from ref 24).


Bej et al.

Figure 6. Effects of various additives on the properties of alumina-supported HDS catalysts.

Al2O3 commercial supports containing 8 and 25 wt % of TiO2, respectively, were prepared by the co-hydrolysis of a mixture of alkoxides of Al and Ti. The active metals were loaded on these supports through the technique of successive impregnation in which Mo was first impregnated followed by either Co or Ni. Each catalyst contained 3 wt % of promoter metal (Co or Ni) and 10 wt % of molybdenum oxide. For the sake comparison, the metals were also loaded onto an alumina support. The HDS experiments were conducted using 4,6DMDBT dissolved in n-decane in the temperature range of 270-360 °C and under a hydrogen pressure of 2.45.0 MPa. The activities of the mixed Al2O3-TiO2supported catalysts were higher than that of the aluminasupported ones. The highest activity was observed for a CoMo-based catalyst supported on Al2O3-TiO2 mixture containing 25 wt % of TiO2. The major product was MCHT, indicating the dominance of the hydrogenative desulfurization route over the mixed supported materials. The acidity of the catalysts increased due to the addition of TiO2, as evidenced by their higher capabilities for the transalkylation of isopropylbenzene. The hydrogenation capability, as measured with the naphthalene hydrogenation reaction, also increased moderately with the increasing TiO2 content of the catalysts. They have concluded that the increased acidity may also have caused enhancement in the hydrogenation capability. The same group of researchers42 studied the activities of CoMo and NiMo catalysts supported on alumina mixed with other oxides such as ZrO2-Al2O3 and B2O3Al2O3 for the HDS of 4,6-DMDBT. The ZrO2 and B2O3 (53) Rana, M. S.; Srinivas, B. N.; Maity, S. K.; Murali Dhar, G.; Rao, T. S. R. P. Stud. Surf. Sci. Catal. 1999, 127, 397. (54) Gutierrez-Alejandre, A.; Gonzalez-Cruz, M.; Trombetta, M.; Busca, G.; Ramirez, J. Microporous Mesoporous Mater. 1998, 23 (56), 265. (55) Barrio, V. L.; Arias, P. L.; Cambra, J. F.; Guemez, M. B.; Campos-Martin, J. M.; Pawelec, B.; Fierro, J. L. G. Appl. Catal. A 2003, 248 (1-2), 211. (56) Grzechowiak, J. R.; Wereszczako-Zielinska, W.; Rynkowski, J.; Ziolek, M. Appl. Catal. A 2003, 250 (1), 95. (57) Murali Dhar, G.; Srinivas, B. N.; Rana, M. S.; Kumar, M.; Maity, S. K. Catal. Today 2003, 86 (1-4), 45.

contents in the mixed support were 20 and 15 wt %, respectively. The amounts of the active metals in these catalysts were the same as used in their earlier work (as described in the previous section).46 The results are shown in Table 1. The pseudo first-order rate constant at 270 °C over CoMo/Al2O3 was 5, and it increased to 11 and 19 over CoMo/ZrO2-Al2O3 and CoMo/B2O3Al2O3 catalysts, respectively. Similarly, the NiMo/Al2O3 catalyst had a pseudo first-order rate constant of 10, and it became 15 for NiMo/ZrO2-Al2O3 and 32 for NiMo/ B2O3-Al2O3 catalyst. The acidity of these supported materials, as measured from the rate of isopropylbenzene transalkylation, increased with the addition of the second oxide. In this case also, all the CoMo-based catalysts showed higher acidity as compared to that of the NiMo-based materials. The hydrogenation capabilities of the catalysts, as evident from the naphthalene hydrogenation activities, also increased. The enhancement in 4,6-DMDBT HDS rate was presumably due to the increase in their hydrogenation capabilities in which acidity might also have played a role. A mixture of alumina and amorphous silica-alumina was also used as a support for loading CoMo oxides.24 The activity of the alumina and amorphous silicaalumina mixed supported CoMo catalyst was higher than the conventional CoMo/Al2O3 catalyst but it could not outperform the NiMo/Al2O3 catalyst. Robinson et al.29 used amorphous silica alumina (ASA) as a support for NiMo catalysts for the HDS of 4-E,6-M-DBT in the hopes of taking advantage of the strongly acidic properties of ASA. The HDS rate for the ASA-supported NiMo catalyst was higher than that of alumina-supported commercial CoMo as well as NiMo catalysts. Based on the above information, Figure 6 summarizes the effects of various additives on the properties of alumina-supported HDS catalysts. Non-Alumina-Based Oxide-Supported Catalysts. Although γ-Al2O3, because of its excellent mechanical as well as dispersing properties, has been widely used as a support for commercial HDS catalysts, various other oxides have also been used for supporting the


active metals for the HDS of thiophene, benzothiophene, and dibenzothiophene.21,58-65 In fact, the non-aluminabased other oxide-supported catalyst formulations have opened up a new horizon in HDS catalysis research. Though there are a number of publications available regarding thoiophene HDS over these catalysts, only very few studies have been reported involving the HDS of 4,6-DMDBT. Landau et al.21 reported that the activity of a silica-supported NiMo catalyst for the HDS of 4,6DMDBT was higher than that of CoMo/Al2O3 and NiMo/ Al2O3 catalysts. Since only limited work has been done on the HDS of 4,6-DMDBT over non-alumina-based oxide-supported catalysts, there remain immense opportunities for advances in this area. Catalysts Supported on Non-Oxide-Based Materials Zeolite- and Mesoporous Materials-Supported Catalysts. Because of their higher surface areas, acidic properties, and well-defined pore structures, zeolites and mesoporous materials have attracted much attention as supports for CoMo/NiMo-based HDS catalysts.4,27,36,66 The higher surface areas allow loading of higher levels of the active metals without affecting dispersion. Isoda et al.28 studied the skeletal isomerization of 4,6DMDBT over a Y-type zeolite-supported Ni catalyst at 270 °C and a hydrogen pressure of 2.5 MPa. 4-MDBT and 3,6-DMDBT, produced through demethylation and methyl group migration, respectively, were found to be the major products of the reaction. Alkyl-DBTs containing three methyl groups were also detected. No desulfurized products were observed indicating that the Y-type zeolite-supported Ni catalyst induced no detectable desulfurization under the conditions of their investigation, however, enhanced the transalkylation of 4,6-DMDBT. Bataille et al.67 studied the HDS of 4,6-DMDBT over dealuminated HY-zeolite-supported Co and CoMo catalysts and compared the results with those of aluminasupported ones. The Mo/HY catalyst contained 8.3 wt % Mo. The Mo and Co contents of the CoMo/HY catalyst were 8.5 and 2.2 wt %, respectively. The reaction was carried out at a temperature of 330 °C and a hydrogen pressure of 3 MPa. Over all the materials, the HDS took place through two common routes, i.e., the direct desulfurization route and the prehydrogenation one. In addition to these, methyl group isomerization and (58) Cinibulk, J.; Kooyman, P. J.; Vit, Z.; Zdrazil, M. Catal. Lett. 2003, 89 (1-2), 147. (59) Venezia, A. M.; La Parola, V.; Deganello, G.; Cauzzi, D.; Leonardi, G.; Predieri, G. Appl. Catal. A 2002, 229 (1-2), 261. (60) Maity, S. K.; Rana, M. S.; Bej, S. K.; Ancheyta-Juarez, J.; Murali Dhar, G.; Rao, T. S. R. P. Appl. Catal. A: General 2001, 205, 215. (61) Maity, S. K.; Rana, M. S.; Srinivas, B. N.; Bej, S. K.; Murali Dhar, G.; Rao, T. S. R. P. J. Mol. Catal. A: Chemical 2000, 153, 121. (62) Maity, S. K.; Rana, M. S.; Bej, S. K.; Ancheyta-Juarez, J.; Murali Dhar, G.; Rao, T. S. R. P. Catal. Lett. 2001, 72 (1-2), 115. (63) Wang, D.; Qian, W.; Ishihara, A.; Kabe, T. J. Catal. 2002, 209, 266. (64) Dzwigaj, S.; Louis, C.; Breysee, M.; Cattenot, M.; Bellie´re, V.; Geantet, C.; Vrinat, M.; Blanchard, P.; Payen, E.; Inoue, S.; Kudo, H.; Yoshimura, Y. Appl. Catal. B 2003, 41, 181. (65) Damyanova, S.; Andonova, S.; Stereva, I.; Vladov, C.; Petrov, L.; Grange, P. React. Kinet. Catal. Lett. 2003, 79 (1), 35. (66) Turaga, U. T.; Song, C. Catal. Today 2003, 86 (1-4), 129. (67) Bataille, F.; Lemberton, J. L.; Perot, G.; Leyrit, P.; Cseri, T.; Marchal, N.; Kasztelan, S. Appl. Catal. A 2001, 220, 191.


transalkyaltion were also found to take place over the zeolite-based materials. Both the Mo/HY and CoMo/HY catalysts exhibited higher rates compared to the Mo/ Al2O3 and CoMo/Al2O3 catalysts. This enhancement in HDS rate was reported to be due to the migration of the methyl groups. Mesoporous materials such as MCM-41 possess potential for acting as good supports. These materials also offer less diffusional resistances to 4,6-DMDBT. Although some research68-71 has been conducted using Mo- or CoMo-loaded MCM-41 for the HDS of DBT, only a few definitive studies have been carried out for the HDS of 4,6-DMDBT over these materials.4,66 Further studies are required to understand and explore the full potential of these novel materials for the HDS of 4,6DMDBT. Carbon-Supported Catalysts. Recently, carbon has received considerable attention as a support for HDS catalysts. Carbon has a number of highly desirable properties such as high surface area, controllable pore volume and pore size, and perhaps favorable supportmetal interaction for HDS reactions.72 Farag et al.72,73 conducted a comparative study between carbon and alumina as a support for CoMo catalysts for the HDS of 4,6-DMDBT. Cobalt and Mo were loaded on two types of activated carbon using two different techniques. One support had a surface area of 907 m2/g with an average pore size of 12.5 Å, while the surface area and the average pore size of the other were 3213 m2/g and 9.9 Å, respectively. The Co and Mo contents of the catalysts were 2 and 10 wt %, respectively. A commercial CoMo/Al2O3 catalyst having 3.2 wt % of Co and 13.7 wt % of Mo was also tested. The pseudo first-order rate constant for the conversion of 4,6DMDBT at 340 °C for the 907 m2/g carbon-supported CoMo catalyst was about double that for the commercial CoMo catalyst. On the other hand, the catalyst prepared on the 3213 m2/g surface area carbon support exhibited a rate that was almost equal to that of the commercial CoMo catalyst; however, the rate was lower than that of the lower surface area-supported material. This was attributed to the pore diffusional resistances of 4,6DMDBT within the narrow pores of the high surface area carbon support. It was observed that the preferred pathway for HDS also depended on temperature. For example, at 340 °C, the two routes (direct desulfurization and hydrogenation) contributed equally to the overall rate. At lower temperature (∼300 °C), the hydrogenation route was the preferred one, while the direct desulfurization route became the dominant one at higher temperature (∼380 °C). Thermodynamic limitations for the hydrogenation reaction at higher temperature were probably responsible for such a shift in reaction pathways. (68) Song, C.; Reddy, K. M. Appl. Catal., A 1999, 176, 1. (69) Wang, A.; Wang, Y.; Kabe, T.; Chen, Y.; Yshihara, A.; Quian, W. J. Catal. 2001, 199, 19. (70) Wang, A.; Wang, Y.; Kabe, T.; Chen, Y.; Yshihara, A.; Qian, W.; Yao, P. J. Catal. 2002, 210, 319. (71) Klimova, T.; Calderon, M.; Ramirez, J. Appl. Catal. A 2003, 240, 29. (72) Farag, H.; Whitehurst, D. D.; Sakanishi, K.; Mochida, I. Catal. Today 1999, 50, 9. (73) Farag, H.; Mochida, I.; Sakanishi, K. Appl. Catal. A 2000, 194195, 147.


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proximately 1.6 times that of the commercial aluminabased catalyst. Other Catalysts

Figure 7. Proposed reaction scheme for the HDS of 4,6DMDBT over carbon-supported CoMo catalyst (adapted from ref 74).

Farag et al.74 carried out a kinetic analysis for the HDS of 4,6-DMDBT over a carbon-supported CoMo catalyst containing 2 and 10 wt % of cobalt and molybdenum, respectively. The study was conducted with a view to elucidate the mechanism of 4,6-DMDBT conversion pathways over the carbon-supported catalyst. They proposed a reaction network (refer to Figure 7) for the HDS of 4,6-DMDBT over the carbon-supported CoMo catalyst. The major products observed were 3,3′-dimethylphenylcyclohexane (3,3′-DMPC) formed through a hydrogenative desulfurization route and 3,3′dimethylbiphenyl (3,3′-DMBP) formed through the direct desulfurization route. Partially hydrogenated 4,6DMDBT was also detected in small quantities. Under the conditions of investigation, the formation of 3,3′DMPC through the hydrogenation of 3,3′-DMBP was found to be very slow. This confirmed the production of 3,3′-DMPC from the hydrodesulfurization of the partially hydrogenated 4,6-DMDBT. At a reaction temperature of 340 °C and a hydrogen pressure of 2.9 MPa, the ratio of the products generated from the direct desulfurization and hydrogenation routes were about 41:59. Sakanishi et al.75 studied the kinetics and mechanism of 4,6-DMDBT HDS over carbon-supported NiMo catalysts. Various types of carbon supports having a wide range of surface areas (480-3060 m2/g) were used. Regardless of the nature of carbon, NiMo/C catalysts exhibited higher activity than that of a commercial NiMo/alumina catalyst in the temperature range of 340-380 °C. In this range, the direct desulfurization route was the dominant one. On the other hand, the hydrogenative desulfurization route was the major one at lower temperatures (∼300 °C). In this range, the selectivity for various products was dependent on the level of 4,6-DMDBT conversion. Robinson et al.29 compared the performance of a carbon-supported CoMo catalyst with that of an aluminasupported commercial CoMo catalyst containing 3 wt % Co and 9.5 wt % Mo, respectively, for the HDS of 4-E,6-M-DBT. The carbon used for the support had a surface area of 1200 m2/g. The Co and Mo contents in this catalyst were 1.6 wt % and 8 wt %, respectively. The activity of the carbon-supported catalyst was ap(74) Farag, H.; Sakanishi, K.; Mochida, I.; Whitehurst, D. D. Energy Fuels 1999, 13, 449. (75) Sakanishi, K.; Nagamatsu, T.; Mochida, I.; Whitehurst, D. D. J. Mol. Catal. 2000, 155, 101.

Carbide-, Nitride-, and Phosphide-Based Catalysts. Recently, transition metal carbides and nitrides have received considerable attention for HDS reactions.76-87 Carbides and nitrides are a class of interstitial compounds of metal and, carbon or nitrogen, respectively. These materials can be prepared in high surface area form through temperature program reaction. Carbides and nitrides are also known to possess properties like those of the Pt-group materials.88 Furimsky87 has reviewed the potential of carbides and nitrides as catalysts for hydroprocessing reactions. Though carbide-based catalysts were tested for the HDS of various sulfur compounds, reports of their performances for the HDS of hindered DBTs are relatively few.78,79,86 Da Costa et al.78 investigated the 4,6DMDBT HDS over alumina-supported molybdenum carbide catalysts at a temperature of 340 °C and a pressure of 4 MPa. Over the carbide-based catalyst, the HDS took place following both direct desulfurization and hydrogenative desulfurization routes. A comparison of the selectivities for various products indicated that the direct desulfurization route was favored over the alumin-supported carbide catalyst in contrast to the conventional alumina-supported CoMo or NiMo catalysts, for which hydrogenative desulfurization was the preferred pathway. In another study,79 the same group of researchers compared the activity and stability of an alumina-supported molybdenum carbide to that of an alumina-supported sulfided molybdenum oxide and to that of a supported platinum catalyst for the HDS of 4,6-DMDBT. The ranking of HDS activity was as follows: MoS2/Al2O3 < Mo2C/Al2O3 < Pt/SiO2. The HDS reaction, as expected, mainly took place through the hydrogenative desulfurization route over both the supported platinum and sulfided molybdenum oxide catalysts. The Mo2C/Al2O3 catalyst favored the direct desulfurization route, despite its well-known hydrogenation capabilities. The influence of phosphorus and nickel on the properties of alumina-supported molybdenum carbide for the HDS of 4,6-DMDBT was studied by Manoli et al.86 The introduction of phosphorus increased the Lewis acid sites, which in turn enhanced the nonhydrogenative desulfurization routes. On the other (76) Dhandapani, B.; St. Clair, T.; Oyama, S. T. Appl. Catal. A 1998, 168, 219. (77) Sajkowski, D. J.; Oyama, S. T. Appl. Catal. A 1996, 134, 339. (78) Da Costa, P.; Potvin, C.; Manoli, J. M.; Lemberton, J. L.; Perot, G.; Mariadassou, G. D. J. Mol. Catal. A 2002, 184, 323. (79) Da Costa, P.; Potvin, C.; Manoli, J. M.; Breysse, M.; Mariadassou, G. D. Catal. Lett. 2003, 86 (1-3), 133. (80) Brian, D.; Stephanie, J.; Denise, H. B.; Rebekah, M.; Diana, C. P.; Scott, K.; Randy, S.; Mark, E. B. Catal. Today, in press. (81) Liu, Y. Q.; Liu, C. G.; Oue, G. H. Energy Fuels 2002, 16 (3), 531. (82) Trawczynnski, J. Catal. Today 2001, 65 (2-4), 343. (83) Trawczynnski, J. Appl. Catal. A 2000, 197 (2), 289. (84) Kim, D. W.; Lee, D. K.; Ihm, S. K. Catal. Lett. 1997, 43 (1-2), 91. (85) Oyama, S. T.; Yu, C. C.; Ramanathan, S. J. Catal. 1999, 184, 535. (86) Manoli, J. M.; Da Costa, P.; Brun, M.; Vrinat, M.; Mauge, F.; Potvin, C. J. Catal. 2004, 221, 365. (87) Furimsky, E. Appl. Catal. A: General 2003, 240, 1. (88) Levy, R. B.; Boudart, M. Science 1973, 181, 547.


hand, upon introduction of nickel an increase in the hydrogenative desulfurization rate was observed. Molybdenum nitride alone or in combination with other promoters has been reported to effectively catalyze the HDS of thiophene, benzothiophene, and vacuum gas oil.81-84 However, publications covering detailed studies for the HDS of 4,6-DMDBT over nitrides are not available in the literature. Similarly, only a few reports are available regarding the use of phosphides for the HDS of 4,6-DMDBT. Oyama et al.89 conducted the HDS of 4,6-DMDBT over nickel phosphide (Ni2P) catalysts supported on silica, alumina, and potassium ionexchanged USY. The catalytic activity was measured at 340 °C and 3.1 MPa in a trickle bed reactor. The activity was found to be Ni2P/K-USY > Ni2P/SiO2 . Ni2P/Al2O3, on the basis of equal sites loaded in the reactor. On the basis of the same concept (i.e., equal sites loaded in the reactor), the conversion of 4,6DMDBT over the Ni2P/K-USY catalysts was much higher (98% HDS) than that of a commercial sulfided CoMo/Al2O3 catalyst (55% HDS). Noble Metal-Based Catalysts. Because of their high hydrogenation capabilities, noble metal-based materials have also been considered and studied for HDS reactions.2,3,90-93 However, the use of these metals for HDS is limited because of their very low sulfur tolerances. Researchers2,3 have reported that a bimetallic Pd-Pt catalyst supported on ytterbium-modified ultrastable Y (USY)-zeolite is capable of effectively desulfurizing 4,6-DMDBT. Reinhoudt et al.90 studied the performances of several Pt catalysts supported on various materials such as amorphous silica alumina (ASA), γ-alumina, and stabilized Y-zeolite (XVUSY) for the HDS of 4-E,6-M-DBT. The study was conducted at a temperature of 360 °C and 6.0 MPa hydrogen pressure. They observed that with a similar level of Pt loading (about 0.8-1.0 wt %), the Pt/ASA catalyst exhibited much higher activity as compared to that of the Pt/γ-alumina-based catalyst. The activity of the commercial CoMo/γ-alumina catalyst was also higher than that of the Pt/γ-alumina-based material; however, it was lower than that of the Pt/ASA (89) Oyama, T. S.; Lee, Y. K.; Chaturvedula, H. Accepted for presentation at the 2003 annual AIChE meeting, San Francisco, November, 2003. (90) Reinhoudt, H. R.; Troost, R.; van Schalkwijk, S.; van Langeveld, A. D.; Sie, S. T.; van Veen, J. A. R.; Moulijn, J. A. Fuel Process. Technol. 1999, 61, 117. (91) Takashi, F.; Kazuo, I.; Takeshi, E.; Hirofumi, M.; Kazushi, U. Appl. Catal. A 2000, 192, 253. (92) Lourdes, I. Merin˜o; Centeno, Aristo´bulo; Sonia, A. Giraldo. Appl. Catal. A 2000, 197, 61. (93) Takashi, F.; Kazuo, I.; Katsuyoshi, O.; Hirofumi, M.; Kazushi, U. Appl. Catal. A 2001, 205, 71.


catalyst. Both Pt-based catalysts were quite stable under the conditions of reaction. On the other hand, the Pt catalyst supported on XVUSY, although showing a very high initial activity (even higher than the Pt/ASAbased catalyst), underwent a rapid deactivation within a time period of about 4 h. Catalyst characterization studies indicated that an appropriate tuning of the support acidity was very important for achieving high activity and stability. According to them, the effect of acidic supports on the high activity for the HDS of 4-E,6M-DBT might be bipartite. The combined effect of Pt and Pd was also studied by Reihoudt et al.52 4-E,6-M-DBT was used as a model compound for the HDS reaction. The Pd-Pt supported on ASA (amorphous silica alumina) catalyst showed higher HDS activity as compared to that of conventional CoMo/A2O3, NiMo/Al2O3, and NiW/Al2O3 catalysts. The beneficial effect of the noble metal was more prominent on the ASA support than on the Al2O3. When both Pd and Pt were present, the HDS activity was much higher than that of a single metal-containing catalyst. It was noted that a specific metal-support interaction was responsible for the higher activity of ASA-supported catalysts. A combination of various noble metals such as RePt, Ir-Pt, Sn-Pt, and Ge-Pt have also been used for the hydrogenation of LCO/SRLGO feedstock.91 However, these combinations of noble metals did not show any positive effects on the hydrogenation reaction; however, Pt or Pt-Pd-supported catalysts showed very high activities.91 Conclusions Relatively less research has been conducted in the area of 4,6-DMDBT HDS as compared to that for the HDS of thiophene and benzothiophenes. Among all the approaches studied for improving the activities of catalysts for the removal of 4,6-DMDBT, the inclusion of various additives such as phosphorus, fluorine, lanthanum, other oxides, and zeolites holds substantial promise. Most of these additives improve the catalytic activity either by improving the dispersion of the active metals (Mo and Co or Ni) and/or by generating acidic properties in the catalysts. Carbon is also a potentially useful support. Zeolites, when used alone as a support, do not provide very encouraging results. Much attention needs to be focused on mixed oxide-supported catalysts. Noble metal- and carbide-based formulations also bring lots of promise for the application. EF030179+

Search for an Efficient 4,6-DMDBT Hydrodesulfurization Catalyst: A

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