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Energy & Fuels 1996, 10, 1078-1082
Hydrodesulfurization Pathway of 4,6-Dimethyldibenzothiophene through Isomerization over Y-Zeolite Containing CoMo/Al2O3 Catalyst Takaaki Isoda, Shinichi Nagao, Xiaoliang Ma, Yozo Korai, and Isao Mochida* Institute of Advanced Material Study, Kyushu University, Kasugakouen 6-1, Kasuga, Fukuoka 816, Japan Received March 25, 1996X
Hydrodesulfurization (HDS) reactivity of a refractory sulfur species, 4,6-dimethyldibenzothiophene (4,6-DMDBT), was examined over a Y-zeolite containing CoMo/Al2O3 (CoMo/Al2O3zeolite), conventional CoMo/Al2O3, and NiMo/Al2O3. Isomerization and considerable transalkylation of 4,6-DMDBT into 3,6-DMDBT and into tri- or tetramethyldibenzothiophenes, respectively, were observed characteristically over CoMo/Al2O3-zeolite catalyst. Migration of methyl groups enhances the HDS reactivity of the refractory sulfur species by diminishing the steric hindrance. Among the catalysts examined, CoMo/Al2O3-zeolite exhibited the best activity for HDS of the gas oil through the effective desulfurization of refractory alkyldibenzothiophenes.
Introduction An extensive desulfurization of refractory 4-methyland 4,6-dimethyldibenzothiophenes (4-MDBT and 4,6DMDBT, respectively) is essential to achieve the sulfur level of gas oil requested by current regulations.1 Their direct desulfurization through the interaction of their sulfur atom with the catalyst surface is sterically hindered by neighboring methyl groups.2 Kinetic data show that these molecules are hydrogenated at one of their phenyl rings prior to the desulfurization over conventional desulfurization catalysts. The steric hindrance is reduced by destruction of the planar configuration through the hydrogenation.2,3 However, this hydrogenation route suffers severe inhibition by other aromatic hydrocarbons in gas oil, such as naphthalene and tetralin, especially if they are in high concentration.3 There are two possible approaches for the efficient desulfurization of 4,6-DMDBT: (1) the selective hydrogenation of 4,6-DMDBT in the dominant aromatic partners and (2) hydrodesulfurization (HDS) reaction after the migration of substituted methyl groups in 4,6DMDBT. The present authors have reported that blends of Ru/Al2O3 and CoMo/Al2O3 catalysts4 or RuCoMo/Al2O3 catalyst5 showed higher hydrogenation selectivity for 4,6-DMDBT in a high concentration of naphthalene than conventional CoMo/Al2O3 and NiMo/ Al2O3. However, no isomerization reaction was found over the conventional catalysts.2,3 * Author to whom correspondence should be addressed [telephone (+81-092)5837555, ext. 7315; fax (+81-092)5753634]. X Abstract published in Advance ACS Abstracts, August 1, 1996. (1) Takatuka, T.; Wada, Y.; Suzuki, H.; Komatu, S.; Morimura, Y. J. Jpn. Pet. Inst. 1992, 35, 197. (2) Isoda, T.; Ma, X.; Mochida, I. J. Jpn. Pet. Inst., 1994, 37, 368. (3) Isoda, T.; Ma, X.; Mochida, I. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1994, 39, 584. (4) Isoda, T.; Nagao, S.; Ma, X.; Korai, Y.; Mochida, I. Energy Fuels 1996, 10, 482. (5) Isoda, T.; Nagao, S.; Ma, X.; Korai, Y.; Mochida, I. Energy Fuels 1996, 10, 487.
Table 1. Some Properties of Gas Oil total S (wt %) total N (ppm) density (g/mL) FIA (vol %) aromatic olefins saturates
0.7 92 0.84
color (Saybolt) pour point (°C) bp (°C)
+18 -10 232-340
17 5 78
In the present study, HDS reactivity of alkyldibenzothiophenes in a gas oil was examined over a CoMo/ Al2O3-containing Y-zeolite6 (CoMo/Al2O3-zeolite) as the test catalyst by inspecting the isomerization of the substrates prior to the desulfurization. The migration of methyl groups at the 4- or 6-position on the dibenzothiophene skeleton was expected on Brønsted acidity of this particular catalyst. Conventional CoMo/Al2O3 and NiMo/ Al2O3 were also studied for comparsion. The major objective of the present paper is to propose a new concept of HDS of refractory alkyldibenzothiophenes. Improvement of the performance life and optimization of the catalyst will be the next target for application in the current refinery. Experimental Section Chemicals and Catalysts. A gas oil from an Arabian crude was provided by a refinery in Japan. Some properties are summarized in Table 1. 4,6-DMDBT was synthesized according to ref 7. Commercially available CoMo and NiMo/ Al2O3 were used. CoMo/Al2O3-zeolite as a test catalyst was prepared by an impregnation procedure of Co and Mo salt solutions into the support which consisted of alumina and Y-zeolite of 5 wt %. Some of their properties are summarized in Table 2. Before the HDS reaction, the catalysts were sulfided at 360 °C for 2 h by flowing H2S (5 vol %) in H2 under atmospheric pressure just before its use. Reaction. HDS reaction of a gas oil was performed in a 50 mL batch autoclave at 340-380 °C under 3.0 MPa of H2 pressure for 20 min, using 1.0 g of catalyst and 10 g of feed. (6) Fujieda, S.; Morinaga, K.; Kondo, A. Petrotech (Tokyo) 1995, 18, 296. (7) Gerdil, R.; Lucken, E. J. Am. Chem. Soc. 1965, 87, 213.
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Energy & Fuels, Vol. 10, No. 5, 1996 1079
Table 2. Chemical Composition and Physical Properties of Catalysts zeolite containing CoMo/Al2O3 NiMo/Al2O3 CoMo/Al2O3 chem composn (wt %) 15.8 MoO3 NiO CoO 3.9 support Y zeolite containing Al2O3 phys properties surface area (m2/g) 220 pore vol (mL/g) 0.40 acid amt (mmol/g) 0.59
14.9 3.1
14.9
Al2O3
4.4 Al2O3
273 0.52 0.49
268 0.53 0.48
HDS reaction of 4,6-DMDBT was performed in a 50 mL batch autoclave at 270 °C under 3.0 MPa of H2 pressure for 0-3 h, using 1.0 g of catalyst and 10 g of substrate, including 0.1 wt % of 4,6-DMDBT in decane. Analysis. Desulfurized oil, desulfurized products in decane, and catalyst were separated by filtration. The separated catalyst was washed with 10 mL of acetone. Products dissolved in acetone were qualitatively and quantitatively analyzed by GC-MS, GC-FID, and GC-FPD (Yanaco G-3800) equipped with a silicone capillary column (OV-101, 0.25 mm × 50 m). Dibenzothiophene (DBT), 4-MDBT, and 4,6-DMDBT were identified with the standard samples according to ref 8. C1C4 hydrocarbons were also identified by their retention times. Other major peaks such as 3,6-DMDBT and C3- and C4-DBTs were identified by comparing molecular weight (GC-MS), confirming the sulfur species (GC-FPD) according to the relative retention times available in ref 9. Major products from 4,6-DMDBT, which was desulfurized through the hydrogenation of one or both phenyl rings, without apparent hydrogenation, or after the migration of a methyl group from the 4- to the 3- position, or simply hydrogenated were abbreviated B4,6, A4,6, C4,6, C3,6, and H, respectively. Their structures are illustrated in Table 3. Mass balance in the HDS reaction of 4,6-DMDBT was examined according to the following equation on carbon base:
missing product (%) ) [conversion of 4,6-DMDBT (%)] [total yields of products (%)] (1) No coke deposition was indicated by the elemental analysis of the used catalyst. Hence, it is suggested that gaseous products are missed through the hydrocracking. Analysis of Fluorescent Intensitiy. The fluorescent color of the desulfurized oil was examined using an RF-500S fluorescence spectrometer (Shimazu Co.) with an excited wavelength of 364 nm. The fluorescence intensity at 438 nm corrected relative to that of as-recieved gas oil was measured to quantify the fluorescent color of the product oils.8 Measurement of Acidity Strength on the Catalysts. The acidity of the catalysts was measured by temperature programmed desorption (TPD) of NH3. The catalyst was crushed to pass 16-24 mesh and dried at 400 °C. NH3 was adsorped to the dried catalyst at 100 °C under the flow and then vaccum. TPD of NH3 was measured from 100 to 600 °C at a heating rate of 10 °C/min. No desorption was assured at 600 °C for 20 min. Desorbed NH3 was quantified by a quadrapole mass spectrometer.
Results HDS of Gas Oil. Figure 1 illustrates the total sulfur content of the gas oil after the HDS reaction as a function of reaction temperatures over CoMo/Al2O3zeolite as well as reference catalysts under 3.0 MPa of (8) Ma, X.; Sakanishi, K.; Isoda, T.; Mochida, I. Ind. Eng. Chem. Res. 1995, 34, 748.
Figure 1. Hydrodesulfurization activities for gas oil over CoMo/Al2O3-zeolite, conventional NiMo, and CoMo/Al2O3 catalysts (3.0 MPa; 20 min; catalyst content 10 wt %): 4, NiMo/ Al2O3; 0, CoMo/Al2O3; b, CoMo/Al2O3-zeolite.
Figure 2. Hydrodesulfurization conversions of alkyldibenzothiophenes in a gas oil over CoMo/Al2O3-zeolite, conventional NiMo, and CoMo/Al2O3 catalysts (3.0 MPa; 20 min; catalyst content 10 wt %).
H2 pressure for 20 min. CoMo/Al2O3-zeolite exhibited an excellent activity for the HDS of gas oil among the catalysts examined, giving sulfur contents of 0.19 wt % at 340 °C, 0.12 wt % at 360 °C, and 0.08 wt % at 380 °C, respectively. NiMo and CoMo/Al2O3 were certainly inferior to the former catalyst, providing the contents of 0.19-0.2 wt % at 340 °C, 0.15 wt % at 360 °C, and 0.11-0.12 wt % at 380 °C, respectively. Figure 2 illustrates the conversions of alkyldibenzothiophenes in the gas oil as a function of reaction temperature. HDS reactivity markedly decreased in the order DBT, 4-MDBT, and 4,6-DMDBT over all catalysts. Three catalysts showed rather similar HDS activities for DBT. Higher HDS reactivities of alkyldibenzothiophenes over CoMo/Al2O3-zeolite were noted, giving conversions of 33 and 55% for 4-MDBT and of 25 and 35% of 4,6-DMDBT at 340 and 380 °C, respectively. HDS reactivities of 4-MDBT and 4,6-DMDBT were certainly smaller over the reference catalysts. Figure 3 illustrates the fluorescence intensities of the gas oil before and after the HDS reaction over the catalysts. NiMo and CoMo/Al2O3 developed similar relative intensities under the same conditions, giving 2.0 at 340 °C, 5.0 at 360 °C, and 11.0 at 380 °C, respectively. Fluorescent color developed markedly after the reaction over CoMo/Al2O3-zeolite above 360 °C, the relative intensities being 4.0 at 340 °C, 10.0 at 360 °C, and 24.0 at 380 °C, respectively. It should be noted that the color of the as-received oil was reduced effectively at 340 °C over all catalysts. HDS Reactivity of 4,6-DMDBT and Its Products. Figure 4 illustrates the conversions of 4,6-DMDBT vs
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Table 3. Product Distribution from 4,6-DMDBT over CoMo/Al2O3-Zeolite, NiMo, and CoMo/Al2O3 Catalysts product distribution (%) reaction time (h)
catalyst NiMo/Al2O3
CoMo/Al2O3
CoMo/Al2O3-zeolite
a
b
0 0.5 1 2 0 0.5 1 2 0 0.5 1 2 c
B4,6a
A4,6b
C4,6c
Hd
0 12 23 38 0 5 13 22
0 2 4 10 0 0 0 0
0 2 2 2 0 1 3 4 1 2 3 5
0 2 2 1 0 1 1 1 0 1 1 1
d
3,6-DMDBTe
C3,C4-DBTsf
missing producti
0 9 22 0 3 4 3
0 9 9 8
0 1 3 5
0 0 1 5
f
S
13 31 45
g
C3
h
HCh
9 0 17
e
S
C3,6g
S
+
C4 S
Boiling point 286-332 °C. i 100 - (yield of 4,6-DMDBT + products) (%).
Figure 3. Fluorescence intensity of desulfurized oil (3.0 MPa; 20 min; catalyst content 10 wt %): 4, NiMo/Al2O3; b, CoMo/ Al2O3-zeolite; 0, CoMo/Al2O3; - - -, feed oil.
Figure 4. Hydrodesulfurization conversions of 4,6-DMDBT over CoMo/Al2O3-zeolite, conventional NiMo, and CoMo/Al2O3 catalyst (270 °C; 3.0 MPa; 4,6-DMDBT 0.1 wt % in 10 g of decane; catalyst content 10 wt %): 4, NiMo/Al2O3; 0, CoMo/ Al2O3; b, CoMo/Al2O3-zeolite.
reaction time over CoMo/Al2O3-zeolite catalyst in decane at 270 °C under 3.0 MPa of H2 pressure. CoMo/Al2O3zeolite provided the highest conversion of 4,6-DMDBT among the catalysts examined, giving conversions of 29% by 0.5 h, 52% by 1 h, and 72% by 2 h, respectively. Conventional NiMo/Al2O3 was slightly inferior to CoMo/ Al2O3-zeolite, providing 27% conversion by 0.5 h, 52% by 1 h, and 68% by 2 h, respectively. CoMo/Al2O3 exhibited much lower HDS activity for 4,6-DMDBT than did NiMo/Al2O3,2,3 providing only 49% conversion by 2 h. The products over CoMo/Al2O3-zeolite were quite different from those over CoMo and NiMo/Al2O3. No B4,6 and A4,6, which were major products over the conventional catalysts, were found. Three types of products were detected: (1) methyl-migrated product of
3,6-DMDBT (m/z 212) and its desulfurized product, (2) tri- and tetramethyl-DBTs (C3- and C4-DBTs; m/z 226 and 240, respectively), and (3) hydrocracking products of their boiling range at 286-332 °C. Major desulfurization appeared to take place after inter- and intramethyl migration on the dibenzothiophene skeletons. Numbers of gaseous products were also observed in the gas chromatogram. The relative quantity of the product was 20 times greater over CoMo/Al2O3-zeolite than over CoMo and NiMo/Al2O3. The same amount was produced from the decane alone, indicating its major contribution. The amount increased proportionally with that of the catalyst. The alkylation of fragments from decane may also take place on the dibenzothiophene skeleton. Some of C10 hydrocarbons also originated from decane through its isomerization.10 Methylnanones were the major products. Product yields from 4,6-DMDBT are summarized in Table 3. CoMo/Al2O3-zeolite provided 3,6-DMDBT and C3- and C4-DBTs, of which yields were 3 and 8%, respectively, by 2 h, when the conversion of 4,6-DMDBT was 70%. Small yield of H (1%) was also observed. Yields of both C3,6 and hydrocracking products (HC) were 1-5% by 0.5-2 h. The former product came from 3,6-DMDBT through the desulfurization. It should be noted that there was a large amount of missing products over CoMo/Al2O3-zeolite catalyst. Secondary cracking products of the methyl-substituted biphenyls or its hydrogenated derivatives such as toluene and xylenes may be formed in the reaction route, but such products were not quantified. In contrast, the conventional catalysts provided B4,6 as the major HDS product from 4,6-DMDBT and A4,6 as the second major, the latter yield being 12% by 0.5 h, 23% by 1 h, and 38% by 2 h, respectively, over NiMo/ Al2O3. C4,6 was a minor HDS product from 4,6-DMDBT. Some products were also missing over the catalyst as shown in Table 3. Figure 5 plots the logarithmic conversions of 4,6DMDBT vs reaction time up to 3 h at 270 °C over the catalysts. First-order kinetics were observable to calculate the rate constants over all of the catalysts, (9) Report about Refining Technology of Gas Oil, Kagakuhin Kensa, Kyokai, Japan, 1990. (10) Martens, J. A.; Tielen, M.; Jacobs, P. A. Zeolites 1984, 4, 98.
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Figure 5. Logarithm conversions of concentration of 4,6DMDBT vs reaction time over CoMo/Al2O3-zeolite, NiMo, and CoMo/Al2O3 catalysts (270 °C; 3 MPa; 4,6-DMDBT 0.1 wt % in 10 g of decane; catalyst content 10 wt %). aC0/Ct ) initial amount of 4,6-DMDBT/remaining amount of 4,6-DMDBT.
Figure 6. Temperature programmed desorption profiles of NH3 on the surface of the catalysts.
indicating no deactivation within the present experimental conditions. No deactivation of CoMo/Al2O3zeolite was also reported within 700 h in the HDS of gas oil using a bench reactor.6 NH3-TPD of CoMo/Al2O3-Zeolite and Conventional Catalysts. Figure 6 illustrates NH3-TPD profiles of CoMo/Al2O3-zeolite, conventional CoMo, and NiMo/Al2O3 catalyst, respectively. The amounts of adsorbed NH3 over three catalysts are also summarized in Table 2. The amount of adsorbed NH3 over CoMo/ Al2O3-zeolite was more by 23% than that over the conventional HDS catalysts, while the desorption temperatures were located in a similar range regardless of the catalysts. CoMo/Al2O3-zeolite is estimated to carry a considerable Brønsted acidity which originates from the Y-zeolite, although the present procedure cannot distinguish Brønsted and Lewis acidity. Discussion Reaction Pathway of 4,6-DMDBT over Y-Zeolite Containing CoMo/Al2O3 Catalyst. Figure 7 illustrates the reaction pathway of 4,6-DMDBT over CoMo/ Al2O3-zeolite. Acid-catalyzed reactions take place certainly over CoMo/Al2O3-zeolite as suggested by a series of products. The first category of the reaction is to isomerize 4,6-DMDBT into 3,6-DMDBT. Considerable transalkylation of 4,6-DMDBT into C3- and C4-DBTs was also observed. Smaller yields of 3,6-dimethyl-, tri-, and tetramethyl-DBTs may suggest higher desulfurization reactivity of such isomerized as well as transalkylated products than that of 4,6-DMDBT, steric hindrance due to methyl groups of the 4- and 6-positions being moderated by such methyl migration. No C1DBTs were found in the product, their high reactivity
Energy & Fuels, Vol. 10, No. 5, 1996 1081
Figure 7. Reaction pathway of 4,6-DMDBT over zeolite containing CoMo/Al2O3 catalyst: a, HDS with isomerization route; b, hydrocracking route; c, direct desulfurization route.
being also indicated. The present study clarified a new route for the effective desulfurization of the refractory sulfur species in the gas oil. Zeolite containing CoMo/Al2O3 carried more acidic sites by 23% than did the conventional catalysts. Such acidity originating from the additive zeolite appears to be Brønsted nature to assist the methyl migration of alkyldibenzothiophenes to promote desulfurization. The extensive hydrocracking also takes place on acidic CoMo/Al2O3-zeolite, producing gaseous hydrocarbons hardly quantified in the present study. Deactivation by coking is believed to be a major disadvantage of the acidic catalyst. However, CoMo/ Al2O3-zeolite is reported not to suffer unusual deactivation in a 700 h run of the bench test under commercial conditions.6 Deactivation was not observed in the present model reaction of 4,6-DMDBT. Thus, no severe deactivation can be expected with such a test catalyst. Higher dispersion of MoS2 was on CoMo/Al2O3-zeolite than on a conventional CoMo/Al2O3 catalyst, probably suppressing the coking through the effective hydrogenation.6 Efficiency for the HDS of Gas Oil. CoMo/Al2O3zeolite exhibited excellent activity to remove sulfur from gas oil among the catalysts examined, although a conventional NiMo/Al2O3 exhibited a similar HDS activity for 4,6-DMDBT in decane. A significant amount of aromatic partners or produced H2S gas was reported to severely hinder the HDS reaction of alkyldibenzothiophenes over the NiMo/Al2O3 catalysts, because the inhibitors strongly hinder the hydrogenation step of phenyl rings in the substrate.3,11 The acidity of the CoMo/Al2O3-zeolite catalyst may not suffer such severe inhibitions for its isomerization activity. Gas oil from an Arabian crude includes nitrogen species of several dozens of parts per million; hence, their inhibition for the acidic site in the zeolite can be negligible in the gas oil. CoMo/Al2O3-zeolite catalyst was found to accelerate development of fluorescence color. Acidic condensation of aromatic substrates produces three to five ring aromatic hydrocarbons of particular shape which develop such color.12,13 The acidity on the CoMo/Al2O3zeolite catalyst causes such a condensation, although (11) Isoda, T.; Ma, X.; Nagao, S.; Mochida, I. J. Jpn. Pet. Inst. 1994, 38, 25. (12) Wakita, M.; Yanagisawa, K.; Matunaga, M. J. Jpn. Pet. Inst. 1995, 38, 25. (13) Ma, X.; Sakanishi, K.; Isoda, T.; Nagao, S.; Mochida, I. Energy Fuels 1996, 10, 91.
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the yield of colored products may be on the order of several parts per million. On the basis of the above discussion, combination of optimum acidity and higher hydrogenation activity will be the key issue to achieve the deep desulfurization of gas oil through moderation of steric hindrance by the methyl migration without extensive cracking and coke formation. The isomerization of methyl groups on the aromatic rings requires much less acid strength than the cracking and condensation. Hybrid catalyst which contains a commercially available CoMo/Al2O3 and less acidic zeolite can be worthwhile to be apply in the HDS of alkyldibenzothiophenes. Conclusions The present study is concluded as follows: (1) Among the catalysts examined, zeolite containing CoMo/Al2O3 (CoMo/Al2O3-zeolite) exhibited the best
Isoda et al.
activity for HDS conversion of alkyldibenzothiophenes in the gas oil. Isomerization of 4,6-DMDBT into 3,6DMDBT and considerable transalkylation into tri- or tetramethyl-DBTs were observed characteristically over CoMo/Al2O3-zeolite catalyst. Such methyl migrations moderate the steric hindrances of methyl groups at the 4- and 6-positions of the dibenzothiophene skeleton. (2) Identification of gaseous products is also suggested in hydrocracking route of hydrogenated 4,6-DMDBT and its desulfurization products. (3) CoMo/Al2O3-zeolite carried higher acidity by 23% than that of the conventional catalysts according to NH3-TPD measurement. The Brønsted acidity of the additive zeolite appears to enhance the mild hydrocracking as well as the isomerization of alkyldibenzothiophenes to promote the HDS reaction. EF960048R
