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Energy & Fuels 1998, 12, 298-303
Hydrocracking of Dibenzothiophenes Catalyzed by Palladium- and Nickel-Coloaded Y-type Zeolite Kazuyoshi Kaneda, Takema Wada, Satoru Murata, and Masakatsu Nomura* Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565, Japan Received July 21, 1997
Three kinds of metal-loaded Y-type zeolites (nickel-loaded, palladium-loaded, and palladiumand nickel-coloaded Y-type zeolites) were examined as catalysts for hydrocracking of dibenzothiophene and 4,6-dimethyldibenzothiophene. The results indicated that palladium- and nickelcoloaded Y-type zeolite (Pd-Ni-Y) had very high activity for the above reaction under following conditions (at 320 °C for 1 h under an H2 pressure in the presence of the above catalyst): dibenzothiophene was completely converted to hydrocarbon gases (ethane, propane, and butane), mono-ring compounds (benzene and cyclohexane derivatives), and hydrogen sulfide. At slightly higher temperature or for longer reaction time compared with above reaction conditions, 4,6dimethyldibenzothiophene was also hydrocracked to a higher extent. We also conducted characterization of these metal-loaded zeolites using X-ray photoelectron spectroscopy, NH3 temperature-programmed desorption, and FT/IR spectroscopy. These analyses indicated that Pd-Ni-Y has a large number of Bro¨nsted acid sites. This might be one of the reasons for this high catalytic activity of Pd-Ni-Y.
Introduction Hydrodesulfurization is one of the most important processes for upgrading of petroleum-derived oils. Therefore, numerous studies have been extensively conducted so far. Recently, these studies have been focused on deep desulfurization of gas oil required due to current regulations. Generally, it is well-known based on the detailed analysis of sulfur compounds in petroleum heavy oils that dibenzothiophene (DBT) is one of the sulfur-containing model compounds which tend to show high resistivity toward deep hydrodesulfurization (HDS). Therefore, DBT becomes a typical target molecule.1-4 Generally, binary metal-loaded alumina, for example, Ni-Mo/Al2O3, Co-Mo/Al2O3, and so on, were used as the catalyst for desulfurization in petroleum industry.5-9 Although these catalysts have high activities, these activities are thought not to be enough for deep desulfurization. Therefore, development of the catalyst with high performance is expected. We have been conducting hydrocracking (HC) reaction of polycyclic aromatic compounds like phenanthrene and pyrene in the presence of transition metal-loaded zeo(1) Curtis, C. W.; Chen, J. H.; Tang, Y. Energy Fuels 1994, 8, 249. (2) Kabe, T.; Qian, W.; Ishihara, A. J. Catal. 1994, 149, 171. (3) Qian, W.; Ishihara, A.; Ogawa, S.; Kabe, T. J. Phys. Chem. 1994, 98, 907. (4) Ozkan, U. S.; Shuangyan, N.; Zhang, L.; Moctezuma, E. Energy Fuels 1994, 8, 249. (5) Qian, W.; Ishihara, A.; Ogawa, S.; Kabe, T. J. Phys. Chem. 1994, 98, 912. (6) Ma, X.; Sakanishi, K.; Isoda, T.; Mochida, I. Energy Fuels 1995, 9, 33. (7) Nagai, M.; Sato, T.; Aiba, A. J. Catal. 1996, 97, 52. (8) Kabe, T.; Ishihara, A.; Zhang, Q.; Tsutsui, H.; Tajima, H. Sekiyu Gakkaishi 1993, 36, 467. (9) Ma, X.; Sakanishi, K.; Isoda, T.; Mochida, I. Ind. Eng. Chem. Res. 1995, 34, 748.
lite.10 We examined three kinds of metal-loaded Y-type zeolites; i.e., nickel-loaded, palladium-loaded, and palladium- and nickel-coloaded Y-type zeolites (abbreviated as Ni-Y, Pd-Y, and Pd-Ni-Y) and found that the binary metal-loaded one has the highest activity for hydrocracking of these substrates among the three. In a recent study, we examined the availability of PdNi-Y catalyst for hydrocracking reaction of dibenzothiophene and its 4,6-dimethyl derivatives and found that these sulfur-containing compounds could be also hydrocracked to a great extent. In this report, we would like to publish the details of this reaction along with the results of its characterization. Experimental Section Samples. 4,6-Dimethyldibenzothiophene was prepared by the method reported previously.11 NH4-substituted Y-type zeolite (No. 23650) was purchased from Shokubai Kasei Co. Ltd., Japan. The other reagents employed in this study were commercially available and purified by the conventional distillation or recrystallization before use. Preparation of the Metal-Loaded Zeolites. The NH4substituted Y-type zeolite (50 g) was stirred in 1000 mL of aqueous solution of Ni(NO3)2 (0.25 M) at 90 °C for 24 h, and then filtered and dried at 110 °C to obtain the nickel cation substituted zeolite (Ni-Y). For the Pd-supported zeolite (PdY), the NH4-substituted Y-type zeolite (15 g) was treated in aqueous solution (200 mL) of [Pd(NH3)4](NO3)2 (0.025 M) at 40 °C for 24 h. For palladium- and nickel-coloaded one (PdNi-Y), NH4-substituted Y-type zeolite was treated with aqueous [Pd(NH3)4](NO3)2 solution (0.025 M), followed by aqueous Ni(NO3)2 solution (0.25 M). The resulting cation-exchanged (10) Akagi, K.; Murata, S.; Nomura, M. Energy Fuels 1995, 9, 435. (11) Gerdil, R.; Lucken, A. C. J. Am. Chem. Soc. 1965, 20, 213.
S0887-0624(97)00123-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/14/1998
Hydrocracking of Dibenzothiophenes zeolites were calcined in a stream of air (52 mL/min) at 450 °C for 4 h and then subjected to reduction with H2 flow (52 mL/min) at 450 °C for 1 h. The contents of nickel and palladium in each zeolite were determined by using a Rigaku Denki System 3270 type fluorescence X-ray analyzer. The amounts of metal loading were as follows: Ni 3.7 wt % (NiY), Pd 5.5 wt % (Pd-Y), Ni 3.3 wt %, and Pd 3.6 wt % (PdNi-Y). Hydrocracking Reaction of Dibenzothiophenes. The substrate (1 g) and the catalyst (0.5 g) were placed into a 50 mL SUS 316 autoclave, which was pressurized to 70 kg/cm2 with hydrogen and then heated to 320 °C at the rate of 8 °C/ min. Reaction time here means the duration at a desired temperature. The gaseous products were passed through an aqueous iodine solution (1 mol/L, 20 mL) to recover the hydrogen sulfide produced. The collected gaseous products from which hydrogen sulfide was eliminated were analyzed quantitatively by a Shimadzu GC-3BT (active carbon column, 2 m) and a Shimadzu GC-8AIT (silica gel column, 60/80 mesh, 3 m). The resulting iodine solution was diluted to 200 mL by deionized water, 20 mL of which was titrated with sodium thiosulfate solution (0.1 mol/L) using aqueous starch solution as an indicator. The liquid and solid products were recovered by washing the inside of the autoclave with CH2Cl2. The components of liquid products (CH2Cl2-soluble fraction) were identified using a JEOL JMS-DX-303HF type GC-MS, and quantified using a Shimadzu GC-14APFSC (CBP-1 capillary column, i.d. 0.5 mm × 25 m) after adding an appropriate internal standard (usually dicyclohexyl phthalate). Coke was defined as the carbonaceous material that remained on the surface or inside of the pore of the catalyst after the washing with dichloromethane. The yield was calculated by taking the carbon weight from the microanalysis of the catalyst recovered into consideration. X-ray Fluorescence (XRF). Quantitative analysis of metals loaded was carried out by XRF, a Rigaku system 3270. Before measurements, zeolite samples were calcined at 450 °C for 4 h under an air flow (52 mL/min) and reduced at 450 °C under an H2 flow (52 mL/min). Ammonia Temperature-Programmed Desorption (NH3TPD). NH3-TPD was measured, using a Bell Japan MultiTask TPD instrument. The experimental conditions employed were as follows: At first, the zeolite was heated to 450 °C, kept at this temperature for 1 h, and cooled to 100 °C. NH3 was adsorbed at 100 °C under 20 mmHg of NH3 pressure for 15 min and kept at this temperature in vacuo for 1 h to remove an excess amount of NH3. Then, the sample was heated to 600 °C at a heating rate of 10 °C/min under 50 mL/min of helium stream, the resulting ammonia being quantified by mass spectrometer. Measurement of FT/IR Spectra of Pyridine-Sorbed Zeolites. After calcination and reduction of each zeolite catalyst, it was placed in a glass vessel. Then, pyridinesaturated nitrogen stream was passed through the glass tube for 3 min and kept for 1 h. Then, the apparatus was heated at 150 °C (bath temperature) under a nitrogen stream to remove excess pyridine. The resulting zeolite (5 mg) was mixed with potassium bromide (500 mg), ground with agate mortar, extruded to a disk (10 mm diameter), and subject to FT/IR measurement. The spectra were recorded on a Shimadzu FTIR-8100M spectrometer and processed with a computer equipped with it. X-ray Photoelectron Spectroscopy (XPS). XPS spectra were recorded on a Perkin-Elmer ULVAC-THI Model 5500 by using Mg KR as X-ray source. Before zeolite samples were subject to measurement, they were calcined at 450 °C for 4 h under air flow (52 mL/min) and reduced at 450 °C under an H2 flow (52 mL/min).
Energy & Fuels, Vol. 12, No. 2, 1998 299
Figure 1. Hydrocracking reaction of DBT in the presence of metal-loaded zeolite catalysts.
Results and Discussion Hydrocracking Reaction of Dibenzothiophenes. Three kinds of metal-loaded zeolites (Ni-Y, Pd-Y, and Pd-Ni-Y) were examined as a hydrocracking catalyst of dibenzothiophene. The reaction of DBT (1 g) was conducted in a 50 mL stainless autoclave at 300 °C under 70 kg/cm2 of hydrogen pressure in the presence of these zeolite catalysts (0.5 g), and the results are shown in Figure 1. In this case, product distribution represents the product recovery (more than 100 wt % of total products distribution means the incorporation of hydrogen and less than 100 wt % the loss of products recovered). With Ni-Y as a catalyst, about 90% of DBT was consumed while alkylbenzenes, alkylcyclohexanes, and gases (ethane, propane, iso- and n-butanes, and hydrogen sulfide) were obtained. However, sulfurcontaining compounds (mainly consisting of hydrogenated DBT derivatives and small amounts of mercaptans) still remained in 11% yield and formation of coke (12%, dichloromethane-insoluble materials remained on the catalyst surface) was observed. Activity of Pd-Y catalyst was similar to that of Ni-Y catalyst. On the other hand, Pd-Ni-Y catalyst showed the highest activity among the three. This agrees well with the fact that this catalyst showed the highest activity for hydrocracking of polycyclic aromatic hydrocarbons like phenanthrene and pyrene. Yield of biphenyl and its hydrogenated derivatives, which are the major products in the hydrodesulfurization reaction of DBT in the presence of Ni-Mo/Al2O3 or Co-Mo/Al2O3, was very low (less than 2% yield in the case of Ni-Y). These compounds are thought to have been hydrocracked to mono-ring compounds and hydrocarbon gases because the catalysts employed in this study have very high activity for hydrocracking of aromatic compounds. One of the reviewers pointed out the role of metal function of the catalyst on the increased activity for coloaded catalyst. We thought that the extent of dispersion of metal on zeolite is closely related to the increased activity for hydrocracking other than the acid function. Therefore, we measured dispersion of metal by XRD; however, due to the low concentration of each metal we could not obtain exact information about its crystallinity. We plan to study this point further. By using Pd-Ni-Y catalyst, rather high conversion of DBT could be attained, but 6% of DBT was still remained. Therefore, we examined the effects of both duration and temperature of the reaction to optimize
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Figure 3. Hydrocracking reaction of DBT in the presence of Pd-Ni-Y catalyst. DBT 1.0 g, catalyst 0.5 g, H2 70 kg/cm2, 320 °C, 1 h.
Figure 2. Effects of both duration and temperature of the reaction on the conversion of DBT (b), and yields of H2S (9) and coke (2). Reaction conditions: DBT 1.0 g, Pd-Ni-Y catalyst 0.5 g, H2 70 kg/cm2; (a) at 300 °C for 0-2 h, (b) at 280-320 °C for 1 h.
reaction conditions. The results are shown in Figure 2,a and b. Both increase of the reaction duration (from 1 to 2 h) and increase of the reaction temperature (from 300 to 320 °C) showed similar effects on product distribution of each reaction: rather high conversion of DBT (more than 99%) and minor amounts of coke (around 4%). In these cases, yields of hydrogen sulfide reached 13.8% (at 320 °C for 1 h) and 15.1% (at 300 °C for 2 h), these values corresponding to 80% and 89% of the amounts of sulfur contained in starting DBT, respectively. It is interesting to note that the amount of coke formed during reaction decreased under these conditions (from 10% to 4% from the reaction at 300 °C for 0 h to the one at 300 °C for 2 h, and from 9% to 3% from the reaction at 280 °C for 1 h to the one at 320 °C for 1 h). In the present study, we defined the carbonaceous materials (dichloromethane-insoluble materials) remaining on the catalyst surface and in the catalyst pore after the reaction as “coke fraction”. These results suggest that the coke fraction consist of oligomers of DBT and this seems to be easily cracked under the present reaction conditions. As described above, we could successfully conduct hydrocracking of DBT by using Pd-Ni-Y catalyst at 300-320 °C for 1-2 h under a hydrogen pressure of 70 kg/cm2; however, the main products were mono-ring compounds and lower hydrocarbon gases. In general, these products may be somewhat undesirable products because hydrocracking to these compounds requires a large amount of hydrogen. At first, we reduced the amount of the catalyst employed. In the reaction at 320
°C for 1 h with the use of 0.1 or 0.2 g of the catalyst brought about the following results: DBT conversion decreased to 42% (catalyst 0.1 g) or 64% (catalyst 0.2 g), and the yield of H2S also decreased considerably. These results indicate that effective conversion of 1 g of DBT requires 0.5 g of the catalyst. We also investigated the effect of amount of catalytic species on Y-type zeolite on the reaction. Figure 3 shows the results of hydrocracking of DBT by using Y-type zeolite with various amounts of nickel and palladium. Decrease of concentration of nickel keeping palladium almost at a similar level resulted in decreasing DBT conversion and vice versa. These results also indicate that about 3 wt % of catalytic species, nickel and palladium, should be effective for hydrodesulfurization reaction of DBT. Hydrotreatment of DBT in n-Dodecane. Using Pd-Ni-Y as a catalyst, hydrotreatment of a mixture of n-dodecane and DBT (95:5, weight ratio) as a model of gas oil was examined. The reaction of model mixture (5 g) was conducted at 320 °C for 1 h under an H2 pressure (70 kg/cm2) in the presence of the catalyst (0.5 g); the results are shown in Table 1. DBT was converted completely to gases, H2S, and mono-ring compounds, while a part of n-dodecane (approximately 20 wt %) was converted to C12 isomers and lighter alkanes. To suppress the conversion of n-dodecane, we decreased the amounts of the catalyst. When the amount of the catalyst was decreased from 0.5 to 0.15 g via 0.25 g, n-dodecane conversion decreased from 20% to 4% via 18%. Consequently, we found that only DBT could be cracked to lighter compounds, with very minor extent of dodecane decomposition (about 4%), at 300 °C under an H2 pressure of 70 kg/cm2 for 1 h (in the presence of 0.15 g of the catalyst). In the reaction at 320 °C with 0.25 or 0.15 g of the catalyst, the amount of coke was 3.5 or 3.2 wt %, respectively. On the other hand, in the reaction at 300 °C with 0.15 g of the catalyst, the coke yield reached 12.8 wt %. Hydrocracking Reaction of 4,6-Dimethyldibenzothiophene over Pd-Ni-Y Catalyst. HDS of 4,6dimethyldibenzothiophene (DMDBT) is known to be considerably more difficult than that of DBT, since the two methyl groups in DMDBT prevent approach of the sulfur atom to active site of catalysts. However, for deep HDS of gas oil, HDS of DMDBT has to also be effectively performed. Therefore, the hydrocracking reaction of DMDBT was examined using Pd-Ni-Y at 320 °C or
Hydrocracking of Dibenzothiophenes
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Table 1. Hydrocracking of Model Gasoil by Pd-Ni-Y Catalysta amount of the catalyst (g)
reaction temp (°C)
recovery of n-dodecane (mol %)b
yield of dodecane derivatives (mol %)b
conversion of DBT (mol %)c
yield of H2S (mol %)d
yield of coke (wt %)e
0.5 0.25 0.15 0.15
320 320 320 300
80 82 96 96
5 5 4 4
100 100 99 96
84 78 78 74
6.0 3.5 3.2 12.8
a The reaction of 5 g of the model mixture (n-dodecane:DBT ) 95:5, wt/wt) was conducted under 70 kg/cm2 of hydrogen pressure in a 70 mL stainless autoclave. b Recovery or yield based on n-dodecane employed was determined by GC analysis. c Determined by GC analysis. d Yield based on DBT employed was determined by titration method. e Yield based on DBT employed was determined by elemental analysis.
Figure 4. Hydrocracking reaction of DMDBT in the presence of Pd-Ni-Y catalyst. DMDBT 1.0 g, Pd-Ni-Y catalyst 0.5 g, H2 70 kg/cm2; (a) at 320 °C for 0-2 h, (b) at 320-340 °C for 1 h.
340 °C under H2 pressure (70 kg/cm2) for 1 or 2 h. The results are shown in Figure 4. At 320 °C, more than 97% of DMDBT was converted to gaseous products and derivatives of benzene and cyclohexane, but hydrogenated DMDBT derivatives and the other sulfur-containing compounds still remained (ca. 6%). Either elongation of the reaction time to 2 h or increase of the reaction temperature up to 340 °C resulted in an almost complete conversion of DMDBT, only a trace amount of sulfurcontaining compounds being observed in the products. In these reaction conditions, yields of hydrogen sulfide reached 15.1 wt % (at 320 °C for 2 h) and 15.4 wt % (at 340 °C for 1 h), these values corresponding to 94% and 96% of the amount of sulfur contained in starting DMDBT, respectively. Moreover, coke formation was suppressed from 5 wt % to 2 wt % (both at 320 °C for 2 h and at 340 °C for 1 h). These results indicate that DMDBT with Pd-Ni-Y catalyst could be almost completely hydrocracked under hydrogen pressure of 70 kg/ cm2 at 320 °C for 2 h or at 340 °C for 1 h. Mechanism for Hydrocracking of 4,6-Dimethyldibenzothiophene. As mentioned above, hydrodesulfurization of DMDBT is known to be difficult because of its methyl groups at 4- and 6-positions. Many studies on this subject were extensively conducted by using conventional Ni-Mo/Al2O3 or Co-Mo/Al2O3.12-14 These studies revealed that in the initial stage of the reaction, hydrogenation of the benzene ring takes place and the resulting cyclohexyl group can reduce steric hindrance around the sulfur atom. In our case, the situation is very different from the above studies because Al2O3 has large pores while Y-type zeolite has small pores: in the case of Y-type zeolite, DMDBT cannot enter into the channel of zeolite. Therefore, we investigated the initial stage of the reaction to obtain insight into the reaction mechanism. (12) Ma, X.; Sakanishi, K.; Isoda, T.; Mochida, I. Ind. Eng. Chem. Res. 1995, 34, 748. (13) Landau, M. V.; Begger, D.; Herskowitz, M. J. Catal. 1996, 158, 236. (14) Isoda, T.; Nagao, S.; Ma, X.; Korai, Y.; Mochida, I.; Landau, M. V. Energy Fuels 1996, 10, 1078.
Figure 5. Total ion chromatogram and selected ion chromatograms of the products from HDS reaction of DMDBT at 320 °C for 0 h under 70 kg/cm2. Two arrows indicate 4-methyldibenzothiophenes (ca. 3300 scans) and DMDBT (ca. 3600 scans).
There are four possible reactions to reduce steric hindrance, i.e., (a) demethylation reaction to monomethylated DBT or unsubstituted DBT, (b) hydrogenation of DMDBT to its hexahydrogenated derivative (reaction similar to that catalyzed by Ni-Mo/Al2O3 or Co-Mo/Al2O3), (c) intramolecular migration of methyl groups (isomerization) to other dimethylated DBT derivatives, and (d) intermolecular migration of methyl groups (disproportionation). Among these four possibilities, the first one could be eliminated, because no detectable amount of methane was observed in the gaseous products from HC reaction of DMDBT. Zeolites are known to have strong acidities, which could catalyze isomerization and disproportionation of alkylated aromatic compounds. To obtain information concerning the reaction mechanism of HC reaction of DMDBT, we analyzed the reaction products at an early stage of the reaction. At first, a mixture of DMDBT (1 g) and the catalyst (0.5 g) was heated in the autoclave under an H2 pressure (70 kg/cm2) from room temperature to 320 °C at a heating rate of 8 °C/min. When the temperature of the apparatus reached 320 °C, the reaction was quenched by rapid cooling of the apparatus to room temperature. Then, the resulting products were subjected to GC and GC-MS analysis. Their total ion chromatogram is shown in Figure 5 along with selected ion chromatograms (m/z ) 198, 212, 226, and 240). Major reaction products identified were three kinds of monomethylated (m/z ) 198), seven kinds of dimethylated (m/z ) 212), seven kinds of trimethylated (m/z ) 224), and minor amount of tetramethylated DBT derivatives (m/z ) 240), their yields being 11, 28, 22, and 5% (by GC analysis), respectively. As for hydrogenated compounds, no detectable amount of these products was
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Figure 7. FT-IR spectra for the pyridine-sorbed zeolites. Table 3. FT-IR Spectra for the Pyridine-Sorbed Zeolites absorbance of pyridine sorbed with Bro¨nsted Lewis acid site (cm-1) acid site (cm-1) H-Y Ni-Y Pd-Y Pd-Ni-Y a
Figure 6. NH3-TPD spectra of Y-type zeolite and metalloaded Y-type zeolites.
1543.2 1543.2 1543.2 1541.3
H-Y Ni-Y Pd-Y Pd-Ni-Y
amount of acid site (mequiv/g)
1.36 2.15 1.41 1.82
0.72 1.21 0.82 1.07
observed. These results seem to indicate that inter- and intramolecular migrations of methyl groups to sterically less hindered compounds play an important role in the early stage. There is another interpretation of the above data: Both migration of methyl groups and hydrogenation of aromatic ring took place and only hydrogenated products cracked to mono-ring compounds and gases, and therefore only methyl-migrated compounds were observed. Now that we do not have enough evidence to conclude which one is more reasonable, further investigation will be needed. Characterization of the Pd- and Ni-Coloaded Y-type Zeolite. We conducted NH3-TPD experiment to evaluate total acid sites of these zeolites. In these TPD spectra, we observed two broad peaks as shown in Figure 6, overlapping each other. Each spectrum could be divided into two Gaussian peaks by curve fitting. On the basis of these data, we evaluated the amount of acid sites of these zeolites; the results are summarized in Table 2. The amount of acid sites increased according to the following sequence: H-Y < Pd-Y < Pd-Ni-Y < Ni-Y. To obtain more detail information about characteristics of acid sites, we measured infrared
not measd 1448.7 1456.4 1448.7
not measd (0.477) (0.044) (0.110)
1635.8 1637.8 1637.8 1637.8
Relative intensity for absorbance of CdN bonds to CdC bonds. Table 4. Results of XPS Analysis of the Zeolites chemical shift (eV)
Table 2. Results for NH3-TPD Experiments total adsorption of NH3 (mmol/g)
(0.195)a (0.215) (0.289) (0.341)
absorbance of CdC bonds (cm-1)
Ni(2p3/2) Ni-Y Pd-Y Pd-Ni-Y
Pd(3d5/2)
857.0 856.6
334.6 335.1
spectra of the pyridine-adsorbed zeolites (Figure 7). There observed two specific peaks at 1543 and 1448 cm-1 in these spectra. According to the literature, these could be assigned as CdC stretching of pyridine adsorbed at Bro¨nsted and Lewis acid sites, respectively. The relative absorbance of the peak for Bro¨nsted acid sites to CdC stretching in aromatic rings followed the sequence H-Y < Ni-Y < Pd-Y < Pd-Ni-Y; on the other hand, peak strength corresponding to the Lewis acid sites followed the sequence Pd-Y < Pd-Ni-Y < Ni-Y (Table 3). These results may indicate that nickel loading increases Lewis acid sites while palladium loading increases Bro¨nsted acid sites. Bro¨nsted acid sites are thought to yield proton as active species for cracking of aromatic ring, this agreeing well with the results that Pd-Ni-Y catalyst showed the highest activity for DBT conversion. It is noteworthy that the peak assigned as Bro¨nsted acid sites of Pd-Ni-Y catalyst shifted to lower wavenumber than that of the other pyridine-adsorbed zeolite, indicating that PdNi-Y catalyst has more strong acidity than the others. X-ray photoelectron spectroscopic analysis of these zeolites was conducted. Chemical shifts of Ni(2p3/2) and Pd(3d5/2) are summarized in Table 4. Chemical shifts for Ni(2p3/2) were 857.0 eV for Ni-Y and 856.6 eV for Pd-Ni-Y, corresponding to nickel(II) hydroxide (855.5-
Hydrocracking of Dibenzothiophenes
857 eV), while chemical shifts for Pd(3d5/2) were 334.6 eV for Ni-Y and 335.1 eV for Pd-Ni-Y, similar to that of palladium metal (334.9 eV). In Pd-Ni-Y zeolite, the chemical shift of Pd shifted to higher energy region slightly and that of Ni shifted to the lower energy region by comparison with chemical shifts of single metalloaded zeolites. These results may indicate that there is weak interaction between palladium and nickel on the surface of Pd-Ni-Y zeolite. As to concentration of metal species on Pd-Ni-Y zeolite, the atomic ratio of Pd to Ni was 0.14 on the base of peak areas of Ni(2p3/2) and Pd(3d5/2). On the other hand, the ratio of 0.59 was obtained by bulk analysis of the zeolite (XRF analysis, see Experimental Section). Conclusions Palladium- and nickel-coloaded Y-type zeolite showed high activity for hydrocracking of DBT. In the reaction at 300 °C for 2 h or at 320 °C for 1 h, the substrate almost completely cracked to gases (ethane, propane, butane, and hydrogen sulfide) and mono-ring compounds (derivatives of cyclohexane and benzene). This catalyst could convert not only DBT but also DMDBT.
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Using a mixture of n-dodecane and DBT as model of gas oil, this catalyst could convert only DBT with suppression of the decomposition of n-dodecane. Analysis of the product in the early stage of hydrocracking reaction of DMDBT indicated that intra- and intermolecular migration of methyl groups occurred to a great extent, leading to the conversion of bulky DMDBT to sterically less hindered compounds. The latter should be cracked more easily than DMDBT. Characterization of these metal-loaded Y-type zeolites with XPS, XRF, NH3-TPD, and FT-IR showed the following insights. Pd-Ni-Y catalyst has the most amount of Bro¨nsted acid sites among the catalysts employed in this study, this being considered one of reasons why this catalyst showed the highest activity for the hydrocracking of DBT. Acknowledgment. This work was supported by Grant-in-Aid for Scientific Research No. 08232255 from the Ministry of Education, Science and Culture, Japan. The authors gratefully thank BELL Japan, Inc., for the measurement of NH3-TPD. EF970123D
