Al2O3 catalysts for the deep hydrodesulfurization of dibenzothiophenes

Applied Catalysis B: Environmental 41 (2003) 171–180

Preparation of highly loaded, dispersed MoS2 /Al2 O3 catalysts for the deep hydrodesulfurization of dibenzothiophenes Jung Joon Lee, Heeyeon Kim, Sang Heup Moon∗ School of Chemical Engineering and Institute of Chemical Processes, San 56-1, Shillim-dong, Kwanak-ku, Seoul 151-744, South Korea Received 10 December 2001; received in revised form 5 April 2002; accepted 6 April 2002

Abstract Highly dispersed MoS2 /Al2 O3 catalysts containing various amounts of MoS2 were prepared by the sonochemical decomposition of molybdenum hexacarbonyl in the presence of sulfur and alumina, and tested for the hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT). The sonochemically synthesized catalysts exhibit an HDS activity about five-fold higher than that of catalysts prepared by a conventional impregnation method over a wide range of MoS2 loadings. The difference in HDS activity between the two types of catalysts is large, particularly at high loadings of MoS2 , because, in the case of the former catalysts, the activity increases with the amount of MoS2 to the point where the latter is as high as 25 wt.% while, on the latter catalysts, the activity reaches a saturation level when the amounts of MoS2 are larger than about 15 wt.%. The enhanced HDS activity of the sonochemically synthesized catalysts can be attributed to the improved dispersion of catalytically active species on the support, which is preserved up to high Mo loadings, and the absence of poisoning by sulfur chemisorbed on the catalyst. In both the cases of DBT HDS and 4,6-DMDBT HDS, hydrogenated products are obtained in larger amounts on the sonochemically synthesized catalysts than on the impregnated ones. With respect to the HDS of DBT and 4,6-DMDBT, the latter is enhanced more than the former on the sonochemically synthesized catalysts, which is again due to the high hydrogenation activity of the catalysts. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrodesulfurization; Sonochemistry; MoS2 ; Dibenzothiophene; 4,6-Dimethyldibenzothiophene

1. Introduction In the recent years, legislation in many countries concerning air pollution have forced refiners to considerably reduce the sulfur content of gas oil, from 0.05 wt.%, in 1996 to 0.035 wt.%, in 2000, and even lower levels are expected in the near future [1]. To satisfy these demands for the high purity products, it is important to remove alkyl-substituted aromatic sulfur ∗ Corresponding author. Tel.: +82-2-880-7409; fax: +82-2-875-6697. E-mail address: [email protected] (S.H. Moon).

compounds, such as 4,6-dimethyldibenzothiophene (4,6-DMDBT), from the gas oil. The latter are relatively resistant to desulfurization because of the steric hindrance to the sulfur atom offered by the methyl groups attached to the aromatic ring. Accordingly, catalysts of significantly high activity are required for the deep hydrodesulfurization (HDS) of gas oil. One of the strategies to improve the HDS activity is to increase the metal content, while maintaining the high dispersion of the catalytically active species, Mo and Co(Ni) sulfides, on the support. A commonly employed method for the preparation of molybdenum catalysts is the aqueous impregnation

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of the support with a solution of molybdenum precursors, e.g. ammonium heptamolybdate (AHM). However, this method is limited in the preparation of highly loaded, dispersed MoS2 on the support because the dispersed particles are easily agglomerated at high Mo loadings [2–4]. Recently, the sonochemical decomposition of volatile organometallic compounds has been widely applied to the synthesis of various nanostructured catalysts [5–7]. A sonochemical reaction is the result of acoustic cavitation. By irradiating a solution with ultrasound, small bubbles are formed and grown in the liquid until they eventually implode and collapse. Local hot spots, which have transient temperatures of ∼5000 K, pressures of ∼1800 atm, and cooling rates of ∼1010 K/s, are created in the collapsing bubbles. These extreme conditions provide energy that can initiate the decomposition of organometallic compounds, which are included in the solution or in the vapor phase of the bubbles and, finally, a variety of nanostructured materials can be produced [5–7]. Mdleleni et al. [8] synthesized unsupported MoS2 particles by the sonochemical decomposition of Mo(CO)6 in the presence of sulfur. The catalyst had an activity superior to that of a conventionally prepared catalyst in the HDS of thiophene. Dhas et al. [9] prepared highly dispersed, promoted MoS2 catalysts supported on alumina by the sonochemical method, which were more active than commercial catalysts in the HDS of thiophene and dibenzothiophene (DBT). However, they compared the catalytic activities of the two catalysts after normalizing their metal contents because the sonochemically prepared catalysts contained only 5.5 wt.% of Mo, much lower than the amount in the commercial catalysts, 10.6 wt.%. Moreover, the catalytic behavior of the sonochemically prepared catalysts was not investigated for the HDS of 4,6-DMDBT, a key reactant in deep HDS. In our recent study, we examined the activity of a sonochemically synthesized MoS2 /Al2 O3 catalyst containing 20 wt.% of Mo for the HDS of DBT and 4,6-DMDBT [10]. This catalyst exhibited HDS and hydrogenation activity higher than catalysts prepared by conventional impregnation, which was attributed to the high dispersion of catalytically active species and the reduced poisoning by sulfur of the former catalyst. It is well known that a conventional HDS catalyst shows a maximum activity at Mo contents of less

than about 15 wt.% because the dispersion of the Mo species decreases at higher loadings [2–4]. Accordingly, it would be desirable to investigate the effect of Mo loading on the activity of sonochemically prepared catalysts for deep HDS reactions, such as the HDS of 4,6-DMDBT. In this study, MoS2 /Al2 O3 catalysts containing various Mo loadings were prepared by sonochemical synthesis and tested for the HDS of DBT and 4,6-DMDBT. The catalyst surface was characterized by nitric oxide chemisorption, temperature-programmed reduction of sulfides (TPR-S) and X-ray photoelectron spectroscopy (XPS). 2. Experimental 2.1. Materials DBT and 4,6-DMDBT were purchased from Acros. A highly dispersed MoS2 /Al2 O3 catalyst, designated as Mo(I) was prepared following the method described in the literature [10]. Thus, a slurry of Mo(CO)6 , S8 and ␥-Al2 O3 (CONDEA, 214 m2 /g surface area and 0.77 cm3 /g pore volume) in a mixed solvent of hexadecane was irradiated with high intensity ultrasound (Sonics & Materials, model VCX-600, 1 cm diameter Ti horn, 20 kHz, 100 W cm−2 ) in a flowing argon (Ar) atmosphere at 333 K for 1.5 h. To adjust the Mo content of the catalysts, different amounts of ␥-Al2 O3 , e.g. from 0.5 to 3 g, were added to the slurry solution, which contained fixed amounts of other reactants. Thus, we were able to increase the Mo content of the catalyst by decreasing the amount of ␥-Al2 O3 . The resulting black powder, the sulfided catalyst, was filtered and washed several times with hexane in an inert atmosphere of a glove box in order to avoid oxygen contamination. The washed powder was then heated at 473 K in a vacuum to remove unreacted reactants. For comparison, a conventional MoS2 /Al2 O3 catalyst, designated as Mo(II), was prepared by impregnating ␥-alumina with an aqueous solution of (NH4 )6 Mo7 O24 ·4H2 O, followed by drying in air at 383 K for 12 h and calcination in air at 723 K for 4 h. The catalysts prepared in this study contained various amounts of Mo (3–25 wt.%), based on analyses using an inductively coupled plasma (ICP, SHIMADZU) as well as an elemental analyzer (EA, CE instrument).

J.J. Lee et al. / Applied Catalysis B: Environmental 41 (2003) 171–180

2.2. Reaction Mo(II), prepared by the impregnation method, was pre-sulfided in a flowing 12.9% hydrogen sulfide/ hydrogen mixture at 673 K for 2 h prior to the reaction. All HDS reactions were performed in a 100 cm3 autoclave reactor at 593 K under a 4.0 MPa hydrogen pressure for 2 h. The reactor, containing 0.3 g of catalyst, was charged with 0.06 g of DBT dissolved in 30 cm3 of n-pentadecane for DBT HDS. In the case of 4,6-DMDBT HDS, 0.3 g of catalyst and 0.03 g of 4,6-DMDBT dissolved in 30 cm3 of n-dodecane were added to the reactor. The liquid products were collected through a 1/16 in. diameter tube, and analyzed by gas chromatography (GC) using a silicone capillary column (OV-101; 0.53 mm in diameter and 30 m long) equipped with a flame ionization detector. 2.3. Characterization The amounts of nitric oxide chemisorbed on the catalysts were measured by a dynamic method. In the case of Mo(II), which was pre-sulfided in a hydrogen sulfide/hydrogen stream at 673 K, the sample was cooled to room temperature in helium for 1 h, and pulses of 5% nitric oxide/helium were then introduced into the reactor containing the sample. The amount of nitric oxide eluted from the reactor after each injection was measured by means of a mass spectrometer (VG Sensorlab), until three successive pulses gave a difference of <1%. The catalyst was then maintained in a flow of helium at 373 K for 1 h, and the procedure was

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repeated. The difference in nitric oxide uptake between the first and the second adsorption cycles was considered to be the amount of nitric oxide chemisorbed on the catalysts [11,12]. The temperature-programmed reduction of sulfided catalysts (TPR-S) was performed in a flow-control system with a mass spectrometer and a programmed heating unit. Before each run, the Mo(II) catalyst was pre-sulfided in a quartz reactor. The temperature of the reactor containing the sample was raised from 300 to 1300 K at a rate of 10 K/min, while 5% hydrogen/argon mixture was flowed through the reactor at 30 cm3 /min. H2 consumption as well as H2 S and CH4 production was measured by mass spectrometry. X-ray photoelectron spectra (XPS) of the catalysts were recorded by an ESCALAB 220i-XL equipped with an aluminum anode (Al K␣ = 1486.6 eV). The sulfided catalyst samples, the surface of which was covered with iso-octane for protection from air contamination, were mounted on double-sided adhesive tape for the XPS measurements. Binding energies (BE) were referenced to the Al 2p line at 74.6 eV.

3. Results 3.1. HDS of DBT Table 1 shows the results of DBT HDS for two types of MoS2 /Al2 O3 , Mo(I) and Mo(II), containing different amounts of the Mo species. The overall activity of Mo(I) is superior to that of Mo(II) over the entire

Table 1 Results of DBT HDS on MoS2 /Al2 O3 Mo contents (wt.%)

3 11 15 18 25 a

Mo(I)a

Mo(II)a

Conversion (%)b

Product distribution (10−5 mol)c BP

CHB

DCH

18 53 65 77 91

3.27 8.76 9.79 11.8 11.8

2.51 7.66 8.77 10.1 12.1

0 0.58 2.66 3.10 5.70

Conversion (%)b 10 19 20 23 24

Product distribution (10−5 mol)c BP

CHB

DCH

2.07 3.93 4.09 4.74 4.82

1.05 2.35 2.54 2.85 3.09

0 0 0 0 0

Mo(I), prepared by sonochemical synthesis; Mo(II), prepared by impregnation. Reaction condition: catalyst = 0.3 g, DBT = 0.06 g dissolved in 30 cm3 of n-pentadecane, temperature = 593 K, pressure = 4.0 MPa, reaction time = 2 h. c BP, biphenyl; CHB, cyclohexylbenzene; DCH, dicyclohexyl. b

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Fig. 1. HDS of DBT on MoS2 /Al2 O3 catalysts.

Fig. 2. Product selectivity in the HDS of DBT on MoS2 /Al2 O3 catalysts.

range of Mo loading. It is worthy noting, as shown in Fig. 1, that the activity of Mo(I) increases with Mo loading until the latter is as high as 25 wt.%, whereas, the activity of Mo(II) increases initially but approaches the saturation level at loadings beyond about 10 wt.%. This result for Mo(II) is in agreement with the previous reports that the optimum Mo loading of MoS2 /Al2 O3 for HDS is 8–14 wt% because higher Mo contents are accompanied by the agglomeration of the Mo species [2–4]. Table 1 also shows the amounts of major products of DBT HDS, which are biphenyl (BP), cyclohexylbenzene (CHB), and dicyclohexyl (DCH). DCH is not observed on the Mo(II). The products may be grouped into two, CHB + DCH and BP, depending on whether

their ring structure is saturated or not. Fig. 2, which shows the plot of the (CHB + BP)/BP ratios versus Mo loading, indicates that ring-saturated compounds are produced in relatively larger amounts than BP on Mo(I) than on Mo(II) over the entire range of Mo loading. In fact, this trend was observed in our previous report [10]. However, the above product distribution results may be caused by a simple kinetic effect because the product distribution is affected by the conversion [13,14], which is different for the two catalysts even at the same Mo loading, as shown in Fig. 1. Accordingly, we obtained an additional set of data using the catalysts having the same Mo loading, 15 wt.%, and changing the conversion by altering the

Table 2 Results of DBT HDS on MoS2 /Al2 O3 (Mo, 15 wt.%) Mo(I)a

Mo(II)a

Conversion (%)b

Product distribution (10−5 mol)c BP

CHB

DCH

16 54 67 76 93

2.78 7.96 9.57 10.2 11.5

2.21 8.09 8.67 10.3 11.9

0 0.75 2.63 3.22 5.61

a

Conversion (%)b 16 29 47 74 86

Product distribution (10−5 mol)c BP

CHB

DCH

3.12 5.26 8.17 10.9 12.5

1.85 3.79 5.98 9.31 10.7

0 0 0.55 2.79 3.62

Mo(I), prepared by sonochemical synthesis; Mo(II), prepared by impregnation. Reaction condition: catalyst = 0.3 g, DBT = 0.06 g dissolved in 30 cm3 of n-pentadecane, temperature = 593 K, pressure = 4.0 MPa, reaction time = 3.5 h (Mo(I)), 9 h (Mo(II)). c BP, biphenyl; CHB, cyclohexylbenzene; DCH, dicyclohexyl. b

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Fig. 3. Product selectivity vs. conversion of DBT on MoS2 /Al2 O3 (Mo, 15 wt.%).

reaction times. The results, listed in Table 2 and plotted in Fig. 3, consistently indicate that the (CHB + DCH)/BP ratio is higher on Mo(I) than on Mo(II) for all conversions examined. Therefore, we can conclude that Mo(I) produces larger amounts of the ringsaturated products than Mo(II), independent of the Mo loading and the extent of conversion. 3.2. HDS of 4,6-DMDBT Similar to the case of DBT HDS, Mo(I) exhibits an activity that is superior to that of Mo(II) in the HDS of 4,6-DMDBT for all Mo loadings. In Fig. 4, Mo(II) shows the maximum conversion at a Mo loading of

175

Fig. 4. HDS of 4,6-DMDBT on MoS2 /Al2 O3 catalysts.

about 15 wt.%, beyond which the conversion decreases with Mo content. Accordingly, the trend observed in DBT HDS that the activity of Mo(II) is limited at high Mo loadings is observed with a more stringent restriction in the HDS of 4,6-DMDBT. Table 3 shows that ring-saturated products are obtained in larger amounts in the HDS of 4,6-DMDBT than in DBT HDS, which has been attributed to steric hindrance by methyl groups bound to neighboring positions to the sulfur atom. In other words, the methyl substituents in the 4,6-positions sterically hinder the access of 4,6-DMDBT to the catalyst surface such that the rate of direct desulfurization, leading to DMBP, decreases [15,16]. Accordingly, most of the 4,6-DMDBT is desulfurized after the saturation of one aromatic

Table 3 Results of 4,6-DMDBT HDS on MoS2 /Al2 O3 Mo contents (wt.%)

3 11 15 18 25 a

Mo(I)a

Mo(II)a

Conversion (%)b

Product distribution (10−5 mol)c DMBP

MCHT

DMDCH

17 61 74 83 95

0.51 1.45 1.65 1.78 1.94

1.28 5.12 5.40 5.68 5.95

0.61 2.21 3.37 4.43 5.52

Conversion (%)b 10 17 21 18 15

Product distribution (10−5 mol)c DMBP

MCHT

DMDCH

0.37 0.54 0.63 0.57 0.47

0.76 1.24 1.43 1.29 1.24

0.21 0.73 0.92 0.72 0.43

Mo(I), prepared by sonochemical synthesis; Mo(II), prepared by impregnation. Reaction condition: catalyst = 0.3 g, 4,6-DMDBT = 0.03 g dissolved in 30 cm3 of n-dodecane, temperature = 593 K, pressure = 4.0 MPa, reaction time = 2 h. c DMBP, dimethylbiphenyl; MCHT, methylcyclohexyltoluene; DMDCH, dimethyldicyclohexyl. b

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Fig. 6. Nitric oxide chemisorption on MoS2 /Al2 O3 catalysts. Fig. 5. Product selectivity in the HDS of 4,6-DMDBT on MoS2 /Al2 O3 catalysts.

ring such that methylcyclohexyltoluene (MCHT) is obtained as a primary product. Table 3 shows that DMDCH is produced on Mo(II), which is unlike the case of DBT HDS, in which no DCH is produced on Mo(II). The (MCHT + DMDCH)/DMBP ratio is larger for the case of Mo(I) than Mo(II), similar to the results obtained for DBT HDS. Fig. 5 shows that the ratio increases with Mo loading on Mo(I) but levels off beyond 10 wt.% loading for Mo(II). This result is obtained because the conversion increases on Mo(I) but is limited, even lowered, on Mo(II) at high Mo loadings.

oxide uptake results, the intensity ratio is larger for Mo(I) than for Mo(II) at all Mo loadings, and levels off for Mo(II) at Mo loadings higher than 15 wt.%. The intrinsic activity of the catalysts was calculated by dividing the observed reaction rates with the amounts of nitric oxide, the results of which are as shown in Fig. 8. There are some scatterings of the activity data, which are believed to originate from errors in measuring the amounts of the nitric oxide uptake and from uncertainties in identifying the sites for the intrinsic HDS activity. Nevertheless, Fig. 8 suggests that the intrinsic activity for the HDS of either DBT or 4,6-DMDBT is almost the same for Mo(I)

3.3. Intrinsic activity of the catalysts Nitric oxide has been used as a probe molecule to measure the active sites of MoS2 /Al2 O3 catalysts, because it adsorbs largely to the edge and corner sites of MoS2 -like structures, which are known to be the active sites for HDS [17,18]. Fig. 6 shows the amounts of nitric oxide adsorbed on MoS2 /Al2 O3 catalysts for various Mo loadings. Nitric oxide uptake is consistently larger for Mo(I) than Mo(II). The adsorbed amount increases with Mo loading for Mo(II) but levels off beyond about 15 wt.% Mo loading, which is similar to the trend observed for the reactions. We also estimated the dispersion of MoS2 particles on the catalysts from the intensity ratio of the Mo 3d and Al 2p peaks in the XPS of the catalysts, as shown in Fig. 7 for different Mo loadings. Similar to the nitric

Fig. 7. Relative surface concentration of Mo in MoS2 /Al2 O3 catalysts.

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Fig. 8. Intrinsic activity of MoS2 /Al2 O3 catalysts in HDS reactions.

and Mo(II) and independent of a Mo loading up to 25 wt.%. Consequently, the enhanced conversions obtained for Mo(I) up to high Mo loadings were the results of high Mo dispersion in the sonochemically synthesized catalyst rather than the generation of new active sites that were not observed in the conventional impregnated catalyst, Mo(II). The highly dispersed Mo species in Mo(I) are not agglomerated even at high Mo loadings, which is not the case for Mo(II), and promotes ring hydrogenation to a greater extent than the direct desulfurization step. Although the above conclusion concerning the intrinsic activity is acceptable as a whole, we were also concerned about possible changes in the surface properties of the catalysts as the result of their different preparation methods. The following TPR-S results provide information about the catalyst surface properties. 3.4. Surface properties of the catalysts The TPR-S patterns of Mo(II) catalysts in Fig. 9 show two major peaks at different temperature regions: peak I with the maximum locations at 500–600 K, and peak II appearing above 1100 K with a low intensity. Peak I is assigned to the reduction of sulfur (Sx ), which has been deposited on the catalyst in the sulfidation step. It has been reported that sulfur poisons the catalysts because it is preferentially chemisorbed on the coordinatively-unsaturated edge and corner sites

Fig. 9. TPR-S patterns of Mo(II): (a) Mo, 3 wt.%; (b) Mo, 11 wt.%; (c) Mo, 18 wt.%; (d) Mo, 25 wt.%.

of MoS2 slabs, which are known to be the catalytically active sites [19–21]. Peak II originates from the reduction of the MoS2 species at high temperatures [19,20]. Peak I shows two changes when the Mo loading of the catalyst is increased. One is an enhancement in intensity, which is obviously due to the increased amounts of Sx chemisorbed on the active sites [19]. Peak I grows in intensity up to Mo loading of about 15 wt.%, which is consistent with the reaction and nitric oxide chemisorption results. The other change in peak I is that the peak shifts to lower temperatures with Mo loading. The position of peak I is affected by the interaction between the dispersed MoS2 and the support, which becomes stronger for small MoS2 particles [19]. Therefore, the peak shift to lower temperatures at high Mo loadings indicates a decrease in the interaction caused by the increase in MoS2 particle size. The TPR-S patterns of Mo(I) in Fig. 10 also show two major peaks similar to the case of Mo(II): peak I near 600 K and peak II above 1100 K. However, peak I for Mo(I) does not originate from the chemisorbed sulfur (Sx ), unlike the case of Mo(II), because H2 S was not detected in the product gases of the TPR-S experiments, as evidenced by mass analyses. On the

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In Fig. 10, the position of peak I is not shifted but its intensity is enhanced by an increase in Mo loading, which indicates that the reactivity of the surface carbon does not change but only its amounts change with Mo loading. 4. Discussion 4.1. Reaction mechanism of HDS

Fig. 10. TPR-S patterns of Mo(I): (a) Mo, 3 wt.%; (b) Mo, 11 wt.%; (c) Mo, 18 wt.%; (d) Mo, 25 wt.%.

other hand, CH4 was detected as the major gas of peak I, indicating that Mo(I) contained carbon instead of sulfur on the surface. Elemental analyses of the catalysts showed that carbon in amounts <2 wt.%, which was presumably produced by the decomposition of the alkane solvent or carbon monoxide during sonication [5], was present in Mo(I). Mo(II) contained no carbon, as evidenced by elemental analyses and by that CH4 was not produced in the TPR-S experiments. Thus, we conclude that sulfur is not chemisorbed on the surface of Mo(I) and, consequently, the coordinatively unsaturated sites of the catalyst are not poisoned by the chemisorbed sulfur. We believe that this result is obtained because Mo(I) was prepared by direct sonication without an additional sulfidation step [10]. To examine the possibility that the molybdenum species in the catalysts are present in forms other than MoS2 , which is known to be the catalytic material [3], we analyzed the binding energy of the Mo 3d electron of the catalysts based on the XPS results. The binding energy (BE) values for the Mo 3d5/2 and Mo 3d3/2 levels for all the catalysts prepared in this study were 229.1 and 232.2 eV, which are consistent with the literature values for MoS2 [22,23]. These results confirm that the MoS2 species is present in all the catalysts used in this study, including Mo(I).

The HDS of dibenzothiophenes (DBTs) occurs through two parallel reactions as illustrated in Fig. 11: (i) the direct desulfurization (DDS), which yields biphenyl-type compounds, and (ii) desulfurization after hydrogenation (HYD) of the aromatic ring, which leads to the production of cyclohexylbenzene-type compounds [24,25]. DBT is desulfurized mostly via the DDS route and, therefore, BP is produced in larger amounts than CHB. CHB may be additionally obtained by the hydrogenation of BP, particularly under conditions of high HDS conversions, but it has been reported that the rate of BP hydrogenation is significantly low in the presence of DBT, because DBT successfully competes with BP for the hydrogenation sites of the catalysts [26,27]. Accordingly, we may estimate the relative rates of DDS and HYD in DBT HDS simply by comparing the amounts of BP and CHB produced in the reaction so long as the HDS conversions are not very high. The same analysis of the relative rates of DDS and HYD based on product distribution may be made for 4,6-DMDBT HDS, although in this case MCHT is obtained as the major product because methyl groups attached to the aromatic rings of 4,6-DMDBT inhibit the DDS pathway more than the HYD route due to steric factors. The rate of DMBP hydrogenation is smaller than that of 4,6-DMDBT hydrogenation although the former reaction is suppressed by the presence of 4,6-DMDBT to smaller extents than in the case of DBT HDS. 4.2. Activity of the sonochemically synthesized catalysts As demonstrated in the above experimental results, sonochemically synthesized catalysts, Mo(I), exhibit HDS activity which is significantly higher than that of

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Fig. 11. HDS mechanisms of DBT and 4,6-DMDBT.

conventional impregnated catalysts, Mo(II), especially at high Mo loadings. The major reason for the activity enhancement of Mo(I) is because the sonochemical synthesis increases the number of active sites as the result of the enhanced dispersion of MoS2 in the catalyst, which is caused by the formation of small MoS2 crystallites under the highly dynamic preparation conditions. The high dispersion of the MoS2 crystallites is preserved on Mo(I) up to a Mo loading of 25 wt.%, which is in contrast to the case of Mo(II), which shows a limitation in activity at a loading of 10–15 wt.%. Between the two major routes of HDS, the HYD route is promoted to a larger extent than the DDS route on Mo(I), which contains many corner, rim, and edge sites on the small MoS2 crystallites. Since the corner and rim sites are largely responsible for the HYD route and the edge sites for the DDS route [28], the previously mentioned results suggest that the MoS2 crystallites of Mo(I) contain relatively larger amounts of the former sites than the latter. Mo(I) promotes the HYD route of DBT HDS to a larger extent than that of 4,6-DMDBT HDS, but enhances the overall HDS rate of 4,6-DMDBT more than that of DBT. The above somewhat contrasting trends in the product distribution and the overall HDS rates between the two

reactants originate from the facts that the major routes of HDS are different between DBT and 4,6-DMDBT and that Mo(I) preferentially promotes the HYD route. Irrespective of the methods used in the catalyst preparation, however, the intrinsic activity data of the catalysts, estimated based on the amounts of nitric oxide adsorbed on the catalysts, fall almost within the same value, which is independent of Mo loading and simply characteristic of the HDS of the specific reactant. Consequently, the intrinsic activity of the active sites is barely affected and only the number of the active sites is affected by the catalyst preparation methods. Besides the previously mentioned general conclusion concerning the intrinsic activity, we demonstrated previously that there is another reason for the enhanced activity of Mo(I). The TPR-S results showed that the active sites of the sonochemically synthesized catalysts were not poisoned by chemisorbed sulfur (Sx ), which is unlike the case of the impregnated catalysts. Consequently, we can conclude that the activity of the sonochemically synthesized catalyst, Mo(I), is enhanced for two reasons: primarily by the enhanced dispersion of MoS2 crystallites, and secondarily by the absence of chemisorbed sulfur.

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5. Conclusions In the present work, MoS2 /Al2 O3 catalysts of various Mo loadings were prepared by a sonochemical method, and their catalytic behavior in the HDS of DBT and 4,6-DMDBT was examined in comparison with that of conventional impregnated catalysts. From the previously mentioned experimental results and discussions, the following conclusions concerning the properties of sonochemically synthesized catalysts can be made. 1. Sonochemically synthesized catalysts show HDS activity higher than that of the conventionally prepared catalysts, especially at high Mo loadings. The catalytic activity increases up to a Mo loading of 25 wt.%, which is primarily due to the enhanced dispersion of MoS2 crystallites at the high metal contents. The absence of poisoning by chemisorbed sulfur is a secondary reason for the activity enhancement. 2. Sonochemically synthesized catalysts promote the hydrogenation route of HDS to larger extents than the direct desulfurization route and, consequently, hydrogenated products are obtained in larger amounts in the HDS of DBT compounds. 3. Concerning the sonochemically synthesized catalysts, the relative extent of hydrogenation promotion is larger for DBT HDS than for 4,6-DMDBT HDS, but the overall HDS rate is enhanced more for 4,6-DMDBT than for DBT. The characteristic trends in the product distribution and the overall HDS rate are observed because the HDS of DBT and 4,6-DMDBT proceeds via different major pathways. Acknowledgements This work was supported by the Brain Korea 21 project National Research Laboratory program. References [1] K.G. Knudsen, B.H. Cooper, H. Topsøe, Appl. Catal. A 189 (1999) 205.

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