Preparation and Catalytic Properties of Highly Dispersed Molybdenum

Ind. Eng. Chem. Res. 1995,34, 3703-3712

3703

Preparation and Catalytic Properties of Highly Dispersed Molybdenum and Cobalt-Molybdenum Sulfide Catalysts Supported on Alumina Yasuaki Okamoto,*ftMitsuhiro Odawara, Hiroyuki Onimatsu, and Toshinobu Imanaka Department of Chemical Engineering, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, J a p a n

Highly dispersed molybdenum sulfides supported on alumina were found to be prepared by using MO(CO)Gas a starting material. The structure and catalytic activities for thiophene hydrodesulfurization (HDS) and butadiene hydrogenation (HYD)of the molybdenum sulfides and the effects of modification with Co by using C o ~ ( c 0or ) ~Co(NO)(C0)3as a precursor were examined a s a function of the dispersion of the molybdenum sulfide species. The structure and dispersion of molybdenum sulfides were studied by E M S techniques. The TOF (activity per Mo atom) of HDS remained invariant at a high dispersion of Mo, whereas the TOF of HYD increased with increasing dispersion of the molybdenum sulfide species. Dinuclear molybdenum sulfide species showed a significantly high TOF for the HYD. It was found with Mo/Alz03 catalysts promoted with Co that both the promotional ratio in the HDS activity and the amount of Co required for the maximum activity increased as the dispersion of the molybdenum sulfide increased.

Introduction Hydrotreatings of petroleum feedstocks have become more and more crucial not only for protecting environments but also for efficient utilization of natural resources, the reserves of which are necessarily limited. Extensive researches have been conducted to develop highly active catalysts for hydrotreatings, in particular for hydrodesulfurization (HDS) (Grange, 1980; Topsae et al., 1986; Delmon, 1993). One possible strategy in improving catalyst performances may be a significant improvement of the dispersion of catalytically active species over the supports. Industrial HDS catalysts are usually composed of Co(Nil-Mo sulfides supported on AlzO3. The origin of catalytic synergies between Co(Ni) and Mo are controversial (Prins et al., 1989; Delmon, 1993; Chianelli et al., 1994). At present, two main synergy models are under discussion. In one model the active sites are proposed to be a Co-Mo-S phase, where atomically dispersed cobalt sulfides are believed to be anchored on the edge surface of the MoSz host particles, forming Mo-S-Co bonds (Topsae and Clausen, 1984; Topsae et al., 1986). The local structures of Co and Mo atoms proposed on the basis of detailed XAFS studies for carbon supported Co-Mo catalysts are essentially consistent with the structure of the Co-Mo-S phase (Bouwens et al., 1990; Bouwens et al., 1991). On the basis of XAFS and Mossbauer techniques, it has been proposed that Ni-Mo-S (Nieman et al., 1990; Louwers and Prins, 1992) and Fe-Mo-S (Ramselaar et al., 1989) phases are also responsible for the catalytic synergies in Ni- and Fe-promoted molybdenum sulfide catalysts, respectively. In the other major model, promotional effects of Co or Ni are construed in terms of a “remote control” mechanism proposed by Delmon and co-workers (Karroua et al., 1989; Delmon, 1993). It is claimed in this model that the activity of molybdenum sulfide is enhanced by spillover hydrogen originally generated on highly dispersed Co or Ni promoter sulfide species in E-mail: [email protected].

the proximity. In both models, however, it is rational t o conjecture that an increase in the dispersion of the molybdenum sulfide species leads to an increase in the concentration of the Co(Ni)-Mo-S phase or in the number of the active sites on the molybdenum sulfide edge surface promoted by spillover hydrogen, thereby resulting in enhanced HDS activities. However, the preparation, structure, and catalytic behaviors of molybdenum sulfide species at an extremely high dispersion have rarely been studied in spite of indispensable information for the design of highly active HDS catalysts. With conventional sulfided Mo/Alz03 catalysts, the particle size of the molybdenum sulfide has been reported to be 1-10 nm in a diameter (Clausen et al., 1981; Candia et al., 1984; Nishijima et al., 1989; Breysse et al., 19911, where about 10 or more Mo atoms are involved, depending on the preparation parameters. The molybdenum oxide species and the extent of Mo aggregation in oxidic MoO3/A1203catalyst precursors are mainly controlled by the surface concentration of Mo in the catalyst, when they are prepared by conventional impregnation or equilibrium adsorption methods using ammonium heptamolybdate (AHM) (Hall, 1982; Okamot0 and Imanaka, 1988; Knozinger, 1988). Decreasing the molybdenum content t o aim at a higher Ma dispersion enhances interactions between the molybdenum oxide phase and the A1203 surface, thus producing molybdenum species less susceptible to reduction and sulfidation and thereby resulting in a decrease of the fraction of catalytically active molybdenum sulfides. Molybdenum complexes anchored on supports, for instance, may be possible starting materials instead of AHM for the preparation of highly dispersed molybdenum sulfides (Yermakov, 1976). Metal sulfide complexes including bimetallic sulfides have recently been examined as precursors for supported or unsupported hydrotreating catalysts (Stiefel et al., 1986; Elzner et al., 1986; Stiefel et al., 1989). With supported catalysts, the activity and selectivity of Co-MolAl203 catalysts prepared by using a complex Cp’zMozCo2S3(COk (Cp’: V-CEHdMe) are comparable t o those of conventional

Q888-5885/95/2634-3703$09.QQIQ 0 1995 American Chemical Society

3704 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995

catalysts (Curtis et al., 1988). Unfortunately, the dispersions of the active phases were not evaluated in these catalyst systems. Supported metal sulfide catalysts can also be prepared or synthesized by using metal complex precursors, such as metal carbonyl anchored on the support. Vrinat et al. (1980) examined the HDS activity for dibenzothiophene of the molybdenum sulfide catalysts prepared by a complete decomposition at 600 K of molybdenum hexacarbonyl Mo(COI6encaged in zeolites into metallic Mo, followed by sulfidation. The HDS activity of molybdenum sulfides on a NaY was fairly low. This is considered to be partly due to extensive agglomerations of Mo upon the complete decomposition of Mo(CO)6 as shown by Yon-Sing and Howe (1986). In our previous study (Okamoto et al., 1988, 1989a), Mo(CO)~ encaged in alkali metal cation exchanged Y- and X-type zeolites was directly sulfided at 673 K in a cautious way for the preparation of supported molybdenum sulfides. It was found that molybdenum sulfide species thus prepared are highly dispersed and highly active for the HDS of thiophene, compared with molybdenum sulfides prepared by a conventional impregnation method using AHM. The dispersion of molybdenum sulfide was estimated on the basis of NO adsorption capacity. More recently, Laniecki and Zmierczak (1991, 1993) showed that zeolite-supported molybdenum sulfides synthesized in a similar way exhibit enhanced activities for the HDS of thiophene and water gas shift reactions. The higher activities were ascribed t o a higher dispersion of molybdenum sulfides on the basis of the results of NO adsorption. It has been reported that a Mo-Ni/Y-zeolite catalyst prepared by incorporating Mo(CO)~ into a Niexchanged Y zeolite exhibits a higher thiophene HDS activity than conventional Mo-Ni/AlsOs catalysts (Sugioka et al., 1991). Thus, it appears that Mo(CO)~ is a possible precursor for the synthesis of highly dispersed molybdenum sulfide catalysts. In the present study, the dispersion, structure, and catalytic properties were examined for molybdenum sulfides prepared by using Mo(COI6adsorbed on A l 2 0 3 . The decomposition behavior and catalytic properties of Mo carbonyls anchored on A1203 have been intensively studied by several workers (Brenner and Bunvell, 1975; Goldwasser et al., 1989). In our preliminary study, Mo sulfide catalysts prepared from Mo(CO)d&O3 were found to exhibit higher catalytic activities for the thiophene HDS and butadiene hydrogenation than impregnation catalysts (Maezawa et al., 1988). Halbert et al. (1989, 1991) have recently shown that binary sulfide catalysts are synthesized by decorating unsupported or supported molybdenum sulfides by a low valence metal complex, such as Co2(CO)8 or Co(N0)(C0)3for Co-Mo catalysts. We also employed the cobalt carbonyls to prepare Co-Mo binary catalysts and to examine the effect of the dispersion of the molybdenum host sulfide upon the synergy generations.

Experimental Section 1. Preparation of Catalysts. The catalyst support, y-Al203, used here was supplied by the catalysis Society of Japan as a reference catalyst (JRC-ALO-4) (Murakami, 1983). The BET surface area was 163 m2 g-l. The nominal impurity levels are as follows: Na20,O.Ol wt %; 5 5 0 2 , 0.01 w t %; and Fe2O3,O.Ol wt %. The pore volume of the alumina is reported to be 0.66 mL g-l. After having been evacuated at 673 K for 1h (-=1 x Torr), the A1203 (0.1 g) powder was exposed to a vapor

of Mo(CO)~at room temperature for 30 min, followed by an evacuation at room temperature for 10 min to remove physisorbed Mo(CO)~.In this procedure, adsorbed Mo(CO)~ was partially decomposed t o Mo(CO), (x = 5-3), judging from deepening of yellow colors. The amount of Mo loading was 1.9 wt % by atomic absorption spectrometry after sulfidation at 673 K. In order t o increase the Mo loading, the Mo sulfide catalyst thus prepared and subsequently evacuated at 673 K was exposed again t o a Mo(CO)~vapor for 30 min at room temperature, followed by an evacuation at room temperature for 10 min. The Mo content was found to be 3.6 w t % after the second Mo loading and sulfidation. Supported Mo(COI6was sulfided in a stream of HzSI Hz (l/9 v/v, 0.8 dm3min-l). The sulfidation temperature was increased at 6 K min-l from room temperature to 673 K and kept at 673 K for 90 min, followed by cooling t o room temperature in the H2S/H2 stream. Molybdenum sulfide catalysts thus prepared by using Mo(CO)~ are designated MoSX/Alz03here, followed by a Mo loading in parentheses, e.g., MoSx/A1203(1.9)for 1.9 wt % Mo. Supported molybdenum catalysts were also prepared by a conventional pore volume impregnation of the alumina with an aqueous AHM solution for comparison. The impregnation catalysts, M o O d ~ z 0 3(Mo, 1.9 and 10 w t %), were calcined in air at 773 K for 5 h. The MoO3/A1203catalyst was sulfided for 90 min in a similar way as Mo(CO)dAl203. The sulfide catalysts prepared by the impregnation method are denoted MoSdAl203 in this study, although the formation of stoichiometric MoS2 was not confirmed. The addition of Co was conducted by exposing MoS,or MoSdAl203, which had been evacuated at 673 K for 1 h after the sulfidation, to a vapor of C02(C0)8 for 30 min at room temperature, followed by evacuation at room temperature for 10 min. Successive adsorption and evacuation were repeated to obtain the catalyst possessing a required amount of Co. Finally, C02(C0)~MoSx- or MoSdAl203 was sulfided again at 673 K for 90 min to prepare Ads-Co-MoS,- or MoSdAl203 catalysts. Co(NO)(C0)3was employed instead of Co2(CO)8 to prepare the catalysts having high Co contents (>2.5 wt % CO). A series of CoO-MoOdA1203 catalysts (Mo, 10 wt %) with a varying Co content was prepared by a conventional double impregnation method using aqueous cobalt nitrate solutions and precalcined MoOdA1203, followed by a second calcination at 773 K for 5 h. The CoOMoO3/A1203oxide precursors were similarly sulfided at 673 K for 90 min, providing Imp-Co-MoSdAlz03 catalysts. CoO/Al203catalysts were prepared by a pore volume impregnation method by using cobalt acetate and calcined at 773 K for 5 h (Okamoto et al., 1991a). Unsupported MoS2 was prepared by decomposing (NH&MoS4 at 673 K for 4 h in a stream of H2S/H2 (1/9 v/v, 0.27 dm3min-l). The BET surface area of the MoS2 catalyst thus prepared was 45 m2 g-l. 2. Reaction Procedures. The freshly sulfided catalyst was evacuated at 673 K for 1h before a catalytic reaction. The reaction was carried out under mild conditions by using an all-glass circulation system (0.2 dm3). The hydrodesulfurization (HDS)of thiophene was conducted at 673 K and an initial pressure of 20 kPa (HdC4H4S = 36). The thiophene pressure was kept constant (0.54 kPa) during the reaction by holding a small amount of a liquid thiophene reservoir kept at 273 K in the bottom of a U-tube in the circulation system.

Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 3706 Table 1. Catalytic Properties of Molybdenum Sulfide Catalysts for the Hydrodesulfurization (HDS)of Thiophene and the Hydrogenation (HYD)of Butadiene

MoS~

n

T O F Q / ~ Os-1 -~ Catalyst

Mo loadinp/wt %

HDS

HYD

1.9 1.9 3.6 1.9 1.9 10

2.2 2.5 2.9 0.0 1.6 2.5 0.57

13 6.2 2.6 0.0

M0Sx/&03 MoSX/Al203(2S) MOS,/&?03" MOSz/f&03 M0Sz/&03d MOSz/&03 MoSf

2.7 0.13

HYDIHDSb ~_____ 5.9 2.5 0.90 1.1 0.23

a TOF is defined as the activity per Mo atom after the correction of the activity of the alumina support. Reaction temperature: 673 K for HDS and 473 K for HYD. Ratio of TOF. Mo(CO)s was adsorbed on MoSx/&03(1.9), followed by a second sulfidation. Uncalcined. e Unsupported bulk MoS2.

The reaction products were analyzed by an on-line gas chromatography. The products were mainly C4 compounds, accompanying small amounts of C3 and C2 hydrocarbons. The HDS activity of the catalyst was calculated both from the total amount of hydrocarbons produced and from the H2S formation. They agreed with each other within an experimental accuracy (f5%). The hydrogenation (HYD)of butadiene was conducted over the freshly prepared sulfide catalyst at 473 K after evacuation at 673 K. The initial reaction pressure was 14 kPa (HdC4H6 = 2). The hydrogenation products were solely butenes and the catalyst activity was calculated on the basis of the total amount of butenes produced. 3. Catalyst Characterization. Characterization of the local structure of the Mo atoms in the sulfided molybdenum catalysts was carried out by using EXAFS (extended X-ray absorption fine structure) techniques. The Mo K-edge EXAFS was measured on the BL-1OB instrument of the Photon Factory at the National Laboratory for High Energy Physics (KEK) using a synchrotron radiation (2.5 GeV, 250-300 mA). The EXAFS spectra were obtained without exposing the sample to air by using an in situ EXAFS cell with Kapton windows (Okamotoet al., 1991b). Data analysis was carried out assuming a plane wave approximation. The X-ray photoelectron spectra ( X P S )of the sulfided catalysts were measured on a Hitachi 507 photoelectron spectrometer using an Al anode (1486.6 eV, 9 kV, 50 mA). The catalyst sample was mounted on a sample holder by using a double adhesive tape in a glovebox filled with a high purity nitrogen gas (nominal purity 99.999%), followed by evacuation in a pretreatment chamber at room temperature to ca. 1 x Pa. The base pressure of the spectrometer was ca. 1 x Pa during the XPS measurements. The binding energies were referenced to the Al(2p) band at 74.3 eV due to the support.

Results and Discussion 1. Molybdenum SulfiddAlumina Catalysts. Table HDS activity of the molybdenum sulfide catalyst supported on Al2O3. The activity is presented as a turnover frequency (TOF) with respect to the Mo atom after correction for the activity of the A1203 support itself (1.6 x mol 8-l h-l) by subtracting from the observed activity of the molybdenum sulfide/Al203 catalyst. The TOF of MoSx/&03(1.9) prepared from MO(CO)~ was close to that of the conventional MoSdA1203(10)catalyst. When MoSx/A1203(1.9) was sulfided twice at 673 K for 90 min [MoSx/&03(1.9,1 summarizes the

L

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230 B i n d i n g Energy/eV

1

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Figure 1. X-ray photoelectron spectra of the Mo(3d) level for AlnOa-supported molybdenum sulfide catalysts: (a) MoSX/Al2O3(1.91, (b) MOSz/&O3(1.9), and (c) MOSz/&03(10).

2S)l or when MoSx/A1203(1.9)was exposed to a Mo(CO)~ vapor to increase a Mo content [MoSx/A1203(3.6)1,the TOF was not affected for the HDS reaction. At 1.9 wt % Mo, the impregnation catalyst showed no activity for the HDS after the calcination, this being consistent with literature (de Beer et al., 1976; Okamoto et al., 1989b). The uncalcined catalyst in Table 1having 1.9 wt % Mo exhibited an activity for the HDS of thiophene. The TOF was, however, smaller than those of the MoSJ A1203 catalysts at similar Mo contents. With the HYD of butadiene, the A1203 support did not show any activity at 473 K. The TOF of MoSx/A1203(1.9) for HYD was much higher than that of MoSdAl203(10) as presented in Table 1. However, the TOF of MoSJAl203 decreased with prolonged sulfidation. On the addition of further Mo, the TOF of MoSx/A1203(3.6) approached that of Mo&/&03(10). MoSdAl203(1.9) showed no HYD activity at 473 K. The unsupported MoS2 catalyst exhibited only small HDS and HYD activities as summarized in Table 1. The sulfidation states of Mo in MoSx/A1203(1.9)and MoSdAl203 at 1.9 and 10 wt % were examined by using X P S . Figure 1compares the X P spectra of the Mo(3d) level for the catalysts. The spectra are composed of two sets of the Mo(3d) doublet peaks assigned to MoSz [M0(3dgi2),228.6 eV1 and MOW)(231.6 eV) species (Li and Hercules, 1984; Okamoto et al., 1989b)as indicated in Figure 1. Evidently, the sulfidation of MoS,/AlzO3(1.9) is almost complete. A major part of Mo is sulfided in MoSd&03(10), while only a small fraction of Mo is sulfided in MoSd&03(1.9). The sulfidation behaviors of the impregnation catalysts are in agreement with the previous results (Li and Hercules, 1984; Okamoto et al., 1989b). The XPS results in Figure 1 explain the very low activities of MoSd&Od1.9) catalyst for the HDS and HYD; strong interactions between molybdena and A1203 apparently inhibit the sulfidation or reduction of Mo(V1) species to Mo(IV) or lower valent species. The TOF of the HDS seems relatively invariant with the catalyst preparation and pretreatments. On the other hand, the TOF of MoSJAl203 for the HYD strongly

3706 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 5.40 I

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Figure 2. k3-weighted E M S functions of the Mo K-edge for AlzO3-supported molybdenum sulfide catalysts (solid line, observed; dotted line, calculated): (a) MoS,/&03(1.9), (b) MoS,/&03(1.9, 2S), (c) MoS,/Al203(3.6), and (d) MoS2/A1~03(10).

varies with the catalyst pretreatments, indicating that different active sites or configurations are responsible for HDS and HYD reactions. The active sites for the HYD of butadiene are understood better (Siegel, 1973; Tanaka and Okuhara, 1977; Wambeke et al., 1988; Okamoto et al., 1989b) than those for the HDS of thiophene. The HYD of diene has been demonstrated to proceed on triply coordinatively unsaturated Mo sites. Accordingly, it is estimated that the different selectivity, HYD/HDS, of the molybdenum sulfide catalyst in Table 1 results from the changes in the dispersion and structure of the molybdenum sulfide species with the catalyst preparation conditions. The local structure of Mo atoms in the catalyst was examined by using EXAFS techniques. Figure 2 shows the EXAFS function x(k) multiplied by K 3 for MoSJAl203 and MoSz/A1203(10) catalysts in Table 1. The K3weighted Fourier transforms (FT) of the catalysts are The FT for presented in Figure 3 (Ak= 4.0-15.0 kl). M o S ~ / A ~ ~ Ois~ very ( ~ O close ) to that for unsupported MoS2 (not shown) except for a relatively weak peak intensity due to the Mo-Mo bond at 0.275 nm (uncorrected for a phase shift) in agreement with other workers (Clausen et al., 1981; Bouwens et al., 1990).

This indicates the formation of highly dispersed MoS2like sulfides over the support. MoS,/Al203(3.6) and MoS,/Al203(1.9, 2s) sulfided for 3 h also exhibit MOST like FTs with significantly weakened Mo-Mo peak intensities, suggesting higher dispersions of the MOST like species than that in M o S ~ / A ~ ~ O ~On ( ~the O ) .other hand, MoSx/&03(1.9) shows a considerably different FT as shown in Figure 3. The distance of the Mo-Mo bond is evidently shorter than that of MoS2 and appears around 0.26 nm (uncorrected),indicating the formation of molybdenum sulfide species with a structure different from that of MoS2. More precise structural information was obtained from the EXAFS data by curve-fitting techniques. The EXAFS oscillations were found to be reasonably curvefitted by assuming only Mo-S and Mo-Mo bondings (hR = 0.15-0.33 nm). Polycrystalline MoS2 powders were used t o extract experimental EXAFS parameters. Crystallographic data for MoS2 are shown in Table 2 (Dickinson and Pauling, 1923). The results of the curve fittings are also illustrated in Figure 2 and the structural parameters derived from the EXAFS analysis are summarized in Table 2 together with those for some reference compounds relevant to the present catalyst

Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 3707

Distance/ A

Figure 3. Fourier transforms ( ~ =k 4-15 of k3-weighted EXAFS modulations of the Mo K-edge for AlzOs-supported molybdenum sulfide catalysts: (a) MoS,/&03(1.9), (b) MoSJAlz03(1.9, 2s);(c) MoS,/Al203(3.6), and (d) MoSz/&03(10).

systems (Ricard et al., 1973; Spivack et al., 1975; Muller et al., 1979; Cramer et al., 1984). The distances of the Mo-S and Mo-Mo bonds of MOs2/&03(10), MoSx/A1203(1.9,2S), and MoS,/Al203(3.6) are 0.240-0.241 and 0.315 nm, respectively, clearly indicating that the local structure of the molybdenum sulfides in these catalysts is very close to that of MoS2. The coordination numbers of the Mo-Mo bond, N(MoMo), for these catalysts are, however, well below the six of crystalline MoS2 and decrease in the order: MoSd &03(10) > MoSx/Al2O3(3.6)> MoSX/Al203(1.9,2s). It is, accordingly, concluded that the molybdenum sulfide species produced in these catalysts have a MoSz-like structure and that the particle size of the molybdenum sulfide decreases in the above order. It seems very rational that the dispersion degree of the molybdenum sulfide increases with decreasing Mo content. In the case of impregnation catalysts, the decrease in the Mo content induces the increase in the fraction of molybdenum species strongly interacting with A1203 and, thereby the increase in the fraction of unsulfided and catalytically inactive molybdenum species, as evidenced by the XPS results in Figure 1. It is revealed that the dispersion of molybdenum sulfide is easily controlled by using Mo(CO)s, since Mo is almost completely sulfided even at a very small content of Mo as illustrated in Figure 1. In a more rigorous sense, the Mo-Mo coordination numbers calculated from EXAFS results do not necessarily correspond to the particle size of the molybdenum sulfide species (Clausen et al., 1981; Chianelli et al., 1994). The EXAFS coordination numbers are also strongly affected by the disorder of the structure of the molybdenum sulfide species. The increase in the disorder of the structure lead t o a decrease in the calculated EXAFS coordination numbers. In this sense the term “dispersion” used above for simplicity may not be strictly correct and involves the effect of the structural disorder of the molybdenum sulfides. It is conceivable,

however, that the disorder of the structure of the molybdenum sulfide species increases with decreasing cluster size or increasing dispersion of the molybdenum sulfide species, so long as the catalysts prepared under similar conditions are compared. The Mo-Mo coordination numbers derived from the EXAFS results are thereby conjectured to overestimate the difference in the dispersion of the molybdenum sulfide species. The structural parameters of MoSx/&03(1.9) are apparently different from a MoS2-like structure; the Mo-Mo distance is considerably shorter compared with that of the MoS2 structure. Upon comparison with the Mo-Mo bond distances of the representative reference compounds in Table 2, we are inclined t o conclude the formation of dinuclear molybdenum sulfide species with bridging Sz2- and/or S2- ligands. The valence state of the molybdenum dinuclear sulfide species is considered to be Mo(IV) from the Mo(3d) XPS binding energies for MoSx/&03(1.9) in Figure 1. On the basis of X P S (Duchet et al., 1983) and Raman (Knozinger, 1988) spectroscopic studies, the formations of Sz2- ligands have been reported for M003/&03 and polycrystalline MoSz catalysts sulfided around 670 K. It was recently found (Okamoto and Katsuyama, manuscript in preparation) that the sulfidations at 473 and 373 K, respectively, of Mo(CO)e and molybdenum oxide dimer species (Okamoto et al., 1993) encapsulated in a NaY zeolite lead to the formations of the molybdenum sulfide dimer species possessing the EXAFS parameters identical with those for M0Sx/&03(1.9). The thermal stability of the dinuclear molybdenum sulfide species formed on A1203 is not very high. The structure is relaxed and transformed into a MoSz-like structure during further sulfidation at 673 K, accompanying an agglomeration of molybdenum species. The nuclearity of the molybdenum sulfide is considered to increase from two t o three or four on the treatment, taking into account the coordination number of the MoMo bonding for MoSx/&03(1.9, 2s) in Table 2. The TOF values of the thiophene HDS and the butadiene HYD are plotted in Figure 4 against the coordination number, N(Mo-Mo), of the Mo-Mo bonding of the molybdenum sulfide catalyst. The TOF of the HDS reaction is fairly constant over a wide range of N(Mo-Mo) of the molybdenum sulfide. The numbers of Mo atoms involved in the molybdenum sulfide particles are estimated to be 7-8 for MoSdA1203(10), 4-5 for MoS,/Al203(3.6), and 3-4 for MoSx/A1203(1.9, 2s) on the basis of the model proposed by Bouwens et al. (1990) for a Moss-like structure, neglecting the effects of the structural disorder mentioned above. The molybdenum sulfide in MoS,/Al203(1.9) is considered to involve two Mo atoms. With the molybdenum sulfide catalysts except for the unsupported MoSz crystal, therefore, almost all the Mo atoms in the molybdenum sulfide are expected to be exposed to the edge or corner of the sulfide, which are generally accepted as active sites for HDS reactions (Prim et al., 1989; Chianelli et al., 1994). Consequently, the fairly constant TOF of the HDS indicates that the HDS activity is not varied very much with the particle size or the local structure of molybdenum sulfide when the molybdenum sulfide involves more than two Mo atoms. This is consistent with the suggestions by Topsge et al. (1986) that with 4.0 and 8.6 w t % Mo/A.l203 sulfide catalysts, the specific activities for thiophene HDS per edge sites are very close to each other on the basis of the EXAFS results.

3708 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 Table 2. Structural Parameters= as Derived from the Mo K-Edge EXAFS for the Molybdenum Sulfide Catalysts and Reference Compounds catalyst Mo loading/wt % bondings CN RIA EdeV A&A2 MOS,/Al203

1.9

MoSZ/Al203(2S)

1.9

MoSJAl203

3.6

MOSdA203

10

Mo-S Mo-MO Mo-S Mo-MO Mo-S Mo-MO Mo-S Mo-MO reference compounds Mo-S Mo-MO Mo-S Mo-MO Mo-MO Mo-S Mo-MO Mo-S Mo-MO Mo-MO Mo-MO

MoS2' MOS3' [M03S(S2)6l2-

*

[Moz(S2)fiI2MOZ~-S)~[S~CN(C~H~)ZI~~ Mo2(~-S)2[(n-Pr)2dtcl~[SCN(n-Pr)zlzf

5.3 0.7 5.2 2.2 5.1 2.8 4.8 3.5

2.39 2.78 2.41 3.15 2.40 3.15 2.40 3.15

6 6 6 1 1 7 2 8 1 1 1

2.41 3.16 2.44 2.75 3.16 2.43 2.72 2.45 2.83 2.80

0.0056 0.0024 0.0038 0.0062 0.0031 0.0066 0.0001 0.0001

-1.1 -0.3 -1.8 -2.5 -3.5 -4.2 -0.7 -0.5

2.71

a CN; coordination number, R; bond distance, Eo; inner potential; u, Debye-Waller-like factor. Dickinson and Pauling, 1923. Cramer et al., 1984. d Muller et al., 1979. e Spivack et al., 1975. f Ricard et al., 1973. dtc: dithiocarbamate. n-Pr: C3H7. I

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Figure 4. Turnover frequencies (TOF) of the thiophene HDS ( 0 ) and the butadiene hydrogenation (0)as a function of the coordination number of Mo-Mo bondings for AlzO3-supported molybdenum sulfide catalysts.

107

-/,

I

I

d

Co-Loading I wtQ

The low TOF for the unsupported MoS2 is obviously due to a much lower concentration of catalytically active edge sites. The TOF of the butadiene HYD, on the other hand, varies with the dispersion of the molybdenum sulfide as illustrated in Figure 4. This is not ascribed simply to the change in the fraction of edge or corner sites with the dispersion or disorder. It is, therefore, cojectured that the proportion of triply coordinatively unsaturated Mo sites active for the hydrogenation depends on the Mo dispersion and structural disorder and that the increases in the dispersion andor in the disorder create highly unsaturated Mo sites. 2. Cobalt-Molybdenum Sulfide/Alumina Catalysts. Cobalt was added to the molybdenum sulfide catalyst, supported or unsupported, by exposing it to a vapor of C02(CO)8 or Co(NO)(C0)3. The cO2(co)8(cO(NO)(C0)3)-molybdenum sulfide catalyst was sulfided again at 673 K. Figure 5 illustrates the thiophene HDS activity of Ads-Co-MoSdAl203(10) catalyst as a function of the Co content as well as that of the conventional double impregnation catalyst, Imp-Co-MoSdAl203( 10). The latter catalyst showed a maximum activity at 2 w t % Co (Co/Mo = 0.33) as generally observed by many workers (de Beer et al., 1976; Grange, 1980). On the other hand, MoSdAl203(10) promoted by Co2(CO)s exhibits a steeper increase in the HDS activity up to about

Figure 5. HDS activities of Imp-Co-MoSz/Al~03(10) (01,AdsCo-MoSz/Al203(10) (O), and Ads-Co/Al203 ( 0 ) catalysts as a function of the Co content.

1wt % Co, followed by an activity plateau at a further addition of Co. Halbert et al. (1991) have recently reported similar observations that upon the addition of C02(CO)8 from a hexane solution, the HDS activity of molybdenum sulfide supported on A1203 is enhanced by a smaller amount of Co than that for the catalysts prepared by impregnation and that the activity levels off at an incremental addition of Co. As shown in Figure 5, the maximum HDS activity of Ads-Co-MoSdAl20~(10) is lower than that of the corresponding Imp-CoMOs2/A1203(10). The HDS activity of Ads-Co/AlzO3 prepared from Coz(CO)8 is also plotted in Figure 5 for comparison. The activities of the cobalt sulfide catalysts are considerably small compared with that of the Mo or Co-Mo catalysts. It is apparent that synergetic effects of Co are effectively generated by the introduction of Co as Coz(CO)8or Co(NO)(CO)3to the sulfided molybdenum species. Halbert et al. (1991) suggested direct reactions Of C02(CO)8with the edge sites of MoS2 on the basis of CO evolutions. In the present study, it was observed that the saturated amounts of Co2(CO)s adsorbed on the Mo&/AlzO3 catalyst always exceed that on the pure Al203.

Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 3709 o?

-

P

I

I

0

1

4 4

2

0

Co-Loading 1 wt%

Figure 6. HDS activities of Ads-Co-MoS,/AlzOs catalysts as a function of the Co content: ( 0 )MOS,/&03(1.9), and ( 0 )MoSJ Al203(3.6).

\ J

I

1

810

800

I

790 780 Binding Energy/eV

L

770

Figure 8. X-ray photoelectron spectra of the Co(2p)level for cobalt sulfide catalysts: (a) Ads-Co/Al~03(2.5 wt % Co), and (b) Impc0/&03(2.4). Table 3. Catalytic Properties’ of Co-Mo/AlzOa Catalysts for the Hydrodesulfurization of Thiophene at 673 K loadinglwt catalyst Ads-Co-MOSJAlzO3 Ads-Co-MoSJAlzOs Ads-Co-MoSz/AlzOa Imp-Co-MoS2/A1~03 Ads-Co-MoSz

1

0

I

1

I

1

2

3

Co-Loading / wt%

Figure 7. HDS activity of Ads-Co-MoSz Co loading.

catalyst against the

Figure 6 shows the HDS activities of Ads-Co-MoSJ &03(1.9) and 43.6) catalysts as a function of Co content. In both catalyst systems, the HDS activity increases as the Co concentration increases and reaches a plateau at a further Co addition as observed for AdsCo-MoSdAl203(10) catalyst. The HDS activity of the unsupported MoS2 crystalline catalyst was also increased by the modification with C02(CO)s and subsequent sulfidation as shown in Figure 7. Ads-Co-MoS2 catalyst shows a broad activity maximum with respect to a Co loading in contrast to the supported molybdenum sulfide catalysts in Figures 5 and 6. The chemical states of sulfided cobalt species supported on A1203 were examined by using XPS techniques. Figure 8 shows the Co(2p)spectra for Ads-Co/ A203(2.5 wt % Co) and Imp-Co/Al203(2.4) after sulfidation at 673 K, which were prepared by using C02(CO)s and Co acetate as starting materials, respectively. The Co(2p312)peak at 778.8 eV is ascribed to sulfided cobalt species and the peak a t 781.5 eV accompanied with satellite peaks is assigned to cobalt species incorporated into the A1203 surface phase (Patterson et al., 1976; Okamoto et al., 1980). The X P spectra in Figure 8 obviously indicate that the cobalt species prepared by using CO~(CO)S are completely transformed into sulfides, whereas a significant part of the cobalt species in ImpCo/Al203 remains unsulfided. It is well established

MO

1.9 3.6 10 10

co

co/ TOFb/ M ~ Q 10-4s-1

1.2 1.5 0.8 2 1.6

1.0 0.68 0.13 0.33 0.042

%a

16.7 14.8 7.0 8.1 0.84

RC 6.7d 5.1

2.8 3.2 1.5

a The optimum amount of Co required to attain the maximum activity was obtained by extrapolating the initial linear line of the Co loading vs activity curve to the plateau activity. bTOF is defined as the activity per Mo atom after correction for the activity of the support itself by subtraction. Promotional ratio is defined by the ratio of the TOF (per Mo atom) for the cobalt-molybdenum sulfide catalyst to the TOF for the host molybdenum sulfide catalyst (Table 1). The activity of MoSx/Al203(1.9,2S)is used for the calculation of R.

(Patterson et al., 1976; Okamoto et al., 1980; Chin and Hercules, 1982) that the cobalt species in conventional Co-Mo/AlsO3 impregnation catalysts are only partially sulfided. The promotional ratio of Co for the thiophene HDS is defined here as the ratio of the maximum TOF (per Mo) for the Co-Mo catalyst to the TOF for the host Mo catalyst. The promotional ratios for the present CoMo sulfide catalysts are summarized in Table 3 and plotted in Figure 9 against N(Mo-Mo) for the host molybdenum sulfide catalyst. The Mo-Mo coordination number of Imp-Co-MoSz/Al203(10) catalyst is assumed to be close to that of MoSdAl203(10). It is obvious that the promotional ratio increases with decreasing N(MoMo), that is, with increasing dispersion andor disorder of the host molybdenum sulfide. The “optimum” amount of Co, which is defined here t o be the minimum amount of Co required t o fully promote the preexisting molybdenum sulfide, was estimated by extrapolating the initial linear lines in Figures 5 and 6 t o the plateau activities. The optimum loading of Co apparently increases with increasing dispersion of the molybdenum phase as shown in Table 3. Figure 10 correlates the promotional ratio against the optimum C o M o atomic ratio. A linear correlation in Figure 10 indicates that the extent of the catalytic

3710 Ind. Eng. Chem. Res., Vol. 34, No. 11,1995 10 I

I

I

J

0

2

4

6

N(Mo-Mo)

Figure 9. Promotional ratio for the Co-Mo/AlzOs catalyst as a function of the coordination number of Mo-Mo bondings for the Ads-Co-MoS, (MoSz)/Alz03, molybdenum sulfide catalyst: (0) and ( 0 )Imp-Co-MoSz/AlzO3.

but rarely contributes to synergy generations under the present reaction conditions a t a low hydrogen pressure. With Ads-Co-MoS2 catalysts, excess amounts of Co are considered t o deposit on the cobalt species decorating the MoS2 edge sites as well as on the basal plane of the MoS2 crystal, resulting in the decrease of the HDS activity in Figure 7. The maximum HDS activity of Imp-Co-MoSdAl203(10) is higher than the plateau activity of Ads-CoMoSdA1203(10). This activity increase is considered to be a result of an increased dispersion of the molybdenum sulfide phase in the presence of Co (Okamoto et al., 1977). It is conjectured that the synergistic roles of Co in conventional Co-Mo/AlzOs catalysts is, at least, twofold; decorations of molybdenum sulfide phases with Co and, to a much lesser extent, increases in the Mo dispersion or stabilization of the molybdenum sulfide phase against agglomeration.

Conclusions

0

d 0,2

Ob

0,6

0,8

1,0

1,2

ColMo Atomic Ratio

Figure 10. Promotional ratio for the Co-Mo/AlzOs catalyst as a Ads-Co-MoS, function of the optimum Co/Mo atomic ratio: (0) (MoSz)/Al203,and ( 0 )Imp-Co-MoSz/Al~O3.

synergy increases linearly with the amount of Co. These findings in Figures 9 and 10 can be interpreted in terms of the edge decoration of the molybdenum sulfides with Co. The smaller the particle size or the more disordered the structure of the host molybdenum sulfide, the higher fraction of edge sites are decorated with Co, because of, probably higher stabilizing energies for the smaller and thereby more disordered molybdenum sulfide particles. When Coz(CO)8is introduced to MoS,- or MoSdAl203 catalysts, C02(CO)8molecules may be distributed among the molybdenum sulfide phases and bare A1203 surface. The adsorption of C O ~ ( C Oon ) ~A 2 0 3 has been reported by Schneider et al. (1984) and Iwasawa et al. (1986). In the present Co doping and sulfidation procedures, cobalt is completely sulfided in contrast to the cobalt species prepared by the impregnation method. The steeper activity increase of Ads-Co-Mo&/Al203( 10) catalyst at a smaller amount of Co is considered to result from a more preferential decoration of the preexisting MoS2like phase with Co than the modification in the conventional Co-Mo catalyst, in which a significant part of Co is incorporated into the A1203 phase. The levelings off of the HDS activity observed for Ads-Co-MoSd A1203 and Ads-Co-MoSX/Al2O3 catalysts suggest that the amount of Co required for the decoration of MoSz sites is limited and saturated at the optimum amount of Co. In this study, the maximum Co/Mo ratio is unity. The Co species exceeding the optimum amount is considered to deposit mainly on the bare A1203 surface,

The salient findings in the present study are as follows: (1)Highly dispersed molybdenum sulfides supported on A1203 are prepared by using Mo(CO)~.The molybdenum species thus prepared is almost completely sulfided even at very low Mo contents. (2) The formation of molybdenum sulfide dimer species is suggested, when Mo(C0)dAlzOa is carefully sulfided at 673 K for 90 min. The structure of the dinuclear species is thermally unstable and transformed into a stable MoS2-like structure, accompanying an agglomeration of Mo, on a prolonged sulfidation at 673 K. (3) The TOF of the HDS of thiophene remains invariant at a high dispersion of Mo, whereas the TOF of the butadiene hydrogenation increases with increasing dispersion and/or disorder. The molybdenum dimer species shows a significantly high TOF for the hydrogenation. (4)The thiophene HDS activity of the molybdenum sulfides supported on A1203 is effectively promoted by the addition of C02(CO)8 or Co(NO)(C0)3followed by sulfidation at 673 K. The promotional ratio and the amount of Co required for the maximum activity increase as the dispersion and/or disorder of the molybdenum sulfide increases. (5) The catalytic synergy for the HDS of thiophene can be interpreted in terms of the edge decoration of Moss-like species with Co under the present mild reaction conditions. (6) The synergistic effects of Co in the conventional double impregnation catalysts are, at least, twofold; edge decorations of molybdenum sulfide and, to a much lesser extent, increases in the Mo sulfide dispersion. The present study provides scientific insights into the structure and catalysis of highly dispersed molybdenum sulfides, the role of Co in Co-Mo catalysts, and the effects of the molybdenum sulfide dispersion on the catalyst performances and synergy generations. In addition to the above scientific findings, the industrial implications of the present study are that one of the strategies in the design of highly active hydrotreating catalysts resides in the preparation of highly dispersed molybdenum sulfide phases decorated with Co and/or Ni. A further investigation is required to synthesize and stabilize highly dispersed Co(Ni)-Mo sulfide species by employing preparation procedures more easily available in chemical industry. In line with this, we are studying

Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 3711 the preparation of highly dispersed molybdenum sulfide species in zeolite, where highly dispersed molybdenum sulfides are prepared even at a high Mo content (’10 w t %) (Okamoto and Katsuyama, manuscript in preparation). The results will be presented elsewhere.

Acknowledgment We are grateful to Professors H. Kuroda and N. Kosugi for providing us the EXAFS analysis program (EXAFS 1)and Professor M. Nomura and staff of the Photon Factory, National Laboratory for High Energy Physics, for assistance in measuring EXAFS spectra (Proposal: 89138). We also thank Mr. H. Katsuyama (Osaka University) for part of the experiments.

Abbreviation AHM ammonium heptamolybdate EXAFS extended X-ray absorption fine structure HDS hydrodesulfurization HYD

hydrogenation

TOF

turnover frequency (activity per Mo atoms here)

XAFS XPS

X-ray absorption fine structure X-ray photoelectron spectroscopy

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3712 Ind.Eng. Chem. Res., Vol. 34, No. 11, 1995 Encaged in a Zeolite and Preparation of Molybdenum Sulfide Catalysts. Proc. 9th Intern. Congr. Catal. 1988, 1, 11. Okamoto, Y.; Maezawa, A.; Kane, H.; Imanaka, T. Highly Dispersed Molybdenum Sulfide Catalysts Prepared from Mo(CO)6 Encaged in a Zeolite. J . Mol. Catal. 1989a, 52, 337. Okamoto, Y.; Maezawa, A.; Imanaka, T. Active Sites of Molybdenum Sulfide Catalysts Supported on A 1 2 0 3 and Ti02 for Hydrodesulfurization and Hydrogenation. J . Catal. 1989b, 120, 29. Okamoto, Y.; Adachi, T.; Nagata, K.; Odawara, M.; Imanaka, T. Effects of Starting Cobalt Salt upon the Cobalt-Alumina Interactions and Hydrodesulfurization Activity of CoO/AI203. Appl. Catal. 1991a, 73, 249. Okamoto, Y.; Imanaka, T.; Asakura, K.; Iwasawa, Y. Structure and Electronic State of Molybdenum Subcarbonyl Species Encaged in Zeolite. J.Phys. Chem. 1991b, 95, 3700. Okamoto, Y.; Kobayashi, Y.; Imanaka, T. Structure of Molybdenum Oxide Clusters Prepared in Zeolite Cages. Catal. Lett. 1993, 20, 49. Okamoto, Y.; Katsuyama, H. Manuscript to be submitted. Patterson, T. A.; Carver, J . C.; Leyden, D. E.; Hercules, D. M. A Surface Study of Cobalt-Molybdena-Alumina Using X-ray Photoelectron Spectroscopy. J . Phys. Chem. 1976, 80, 1700. Prins, R.; de Beer, V. H. J.; Somorjai, G. A. Structure and Function of the Catalyst and Promoter in Co-Mo Hydrodesulfurization Catalysts. Catal. Rev.-Sci. Eng. 1989,31, 1. Ramselaar, W. L. T. M.; Craje, M. W. J.; Gerkema, E.; de Beer, V. H. J.; van der Kraan, A. M. Sulphidation of Carbon-Supported Iron-Molybdenum Oxide Catalysts. Appl. Catal. 1989,54,217. Ricard, L.; Estienne, J.; Weiss, R. Formation of a Thiocarboxamidomolybdenum Complex by Oxidative Bond Cleavage. Inorg. Chem. 1973,12,2182. Schneider, R. L.; Howe, R. F.; Watters, K. L. Interactions of Cobalt Carbonyls with Oxide Surfaces. 2. Dicobalt Octacarbonyl and Tetracobalt Dodecacarbonyl on Silicas and Aluminas. Inorg. Chem. 1984,23, 4593. Siegel, S. Alkene Hydrogenation and Related Reactions: A Comparison of Heterogeneous with Homogeneous Catalysts. J . Catal. 1973, 30, 139. Spivack, B.; Dori, Z.; Stiefel, E. I. The Crystal structure of a Mo(V) Complex Having a Multiple Bonded Terminal Sulfur Atom. Inorg. Nucl. Chem. Lett. 1975, 11, 501.

Stiefel, E. I.; Pan, W.-H.; Chianelli, R. R.; Ho, T. C. Hydrotreating Using Self-promoted Molybdenum and Tungsten Sulfide Catalysts Formed from Bis(tetrathiometa1ate) Precursors. U S . Patent 4,581,125, 1986. Stiefel, E. I.; Halbert, T. R.; Coyle, C. L.; Wei, L.; Pan, W.-H.; Ho, T. C.; Chianelli, R. R.; Daage, M. Molecules, Clusters, Solids and Catalysts in Early Transition Metal Sulphide Systems. Polyhedron 1989, 8, 1625. Sugioka, M.; Takase, Y.; Takahashi, K. Hydrodesulfurization of Thiophene over Mo/Zeolite Catalysts. Proc. of JECAT91, 1991, 224. Tanaka, K.; Okuhara, T. Regulation of Intermediates on Sulfided Nickeland MOSSCatalysts. Catal. Rev.-Sci. Eng. 1977,15,249. Topscae, H.; Clausen, B. S. Importance of Co-Mo-SType Structures in Hydrodesulfurization. Catal. Rev.-Sci. Eng. 1984,26, 395. Topscae, H.; Clausen, B. S.; Topscae, N.-Y.; Pederson, E. Recent Basic Research in Hydrodesulfurization Catalysis. Ind. Eng. Chem. Fundam. 1986,25, 25. Vrinat, M. L.; Gachet, C . G.; de Mourgues, L. Catalytic Hydrodesulfurization of Dibenzothiophene over Y Type Zeolite. Catalysis by Zeolites; Imelik, B., Ed.; Elsevier: Amsterdam, 1980; p 219. Wambeke, A,; Jalowiecki, L.; Kastzelan, S.; Grimblot, J.; Bonnelle, J. P. The Active Site for Isoprene Hydrogenation on MoSz/Alz03 Catalysts. J . Catal. 1988, 109, 320. Yermakov, Yu. I. Supported Catalysts Obtained by Interaction of Organometallic Compounds of Transition Elements with Oxide Supports. Catal. Rev.-Sci. Eng. 1976, 13, 77. You-Sing, Y.; Howe, R. F. Adsorption and Decomposition of Mo(CO)6 in Zeolite Nay. J . Chem. SOC.,Faraday Trans. l 1986, 82, 2887. Received for review January 18, 1995 Accepted J u n e 6, 19955 IE950054V

Abstract published in Advance A C S Abstracts, September 15, 1995.