Reactivity and Selectivity for the Hydrocracking of Vacuum Gas Oil

Energy & Fuels 1998, 12, 493-502

493

Reactivity and Selectivity for the Hydrocracking of Vacuum Gas Oil over Metal-Loaded and Dealuminated Y-Zeolites Takaaki Isoda, Katsuki Kusakabe, and Shigeharu Morooka* Department of Materials Physics and Chemistry, Graduate School of Engineering, Kyushu University, Fukuoka 812-8581, Japan

Isao Mochida Institute of Advanced Material Study, Kyushu University, Fukuoka 816, Japan Received August 5, 1997

Hydrocracking, hydrodenitrogenation (HDN), and hydrodesulfurization (HDS) of vacuum gas oil (VGO) were examined using Ni-, Co-, and Fe-loaded and dealuminated Y-zeolites at 300-380 °C under an initial hydrogen pressure of 5 MPa. The major fraction of the VGO was n-paraffins (45 wt %), and the second major fraction was alkyl-substituted 3- and 4-ring aromatics. Polar aromatics and 1- and 2-ring aromatics were minor components. Paraffins of 15-32 carbon chains and aromatic compounds of 3-4 rings were efficiently hydrocracked into a gasoline fraction over Ni-HY-A (Si/2Al ) 16.0) zeolite, which gave the least yield of gaseous byproducts among the catalysts examined. Extensively dealuminated Ni-HY-B zeolite (Si/2Al ) 50.0) produced a large amount of middle distillate fraction, while the yields of gas and coke that was deposited on the catalyst were markedly suppressed. Both zeolites exhibited excellent HDN and HDS activities. The Co-HY and Fe-HY zeolites showed a similar hydrocracking activity as did the Ni-HY-A zeolite, whereas the Fe-HY produced more gaseous hydrocarbons. The Co-HY and Fe-HY zeolites were inferior to the Ni-HY-A in HDS and HDN activities at 380 °C. The metal-free HY-A zeolite exhibited higher yields of gaseous hydrocarbons, 4-ring aromatics, coke, and hexaneinsoluble fractions than did the metal-loaded catalysts. The HY-A zeolite had essentially no HDS and HDN activities, and the concentration of alkyldibenzothiophenes in the product oil was increased by condensation and transalkylation during cracking reaction. The excellent activities of the metal-loaded HY-zeolites apparently originated from the optimized acidity, as a result of dealumination, in addition to the highly dispersed metal sulfides which enhanced hydrogenation activity. The acidic strength of the catalysts was a key factor in controlling the selectivity between gasoline and gas oil production.

Introduction Vacuum gas oil (VGO) is normally upgraded into gasoline and gas oil, which are important transportation fuels currently produced in refineries. Selective conversion to gasoline or gas oil must be controlled to meet changes in seasonal demand.1-5 In order to achieve these targets, the hydrocracking of n-paraffins and 3and 4-ring aromatic hydrocarbons in the feed represents the key technique. In the two-stage process proposed by the IFP, VGO is first hydrogenated over a NiMo/ Al2O3 catalyst, and the hydrotreated oil is then subjected to further cracking over an acidic zeolite.6 It has

been reported that the incorporation of Fe into a zeolite increases its activity with respect to mild cracking of atmospheric distillation residue directly to middle distillate,7 while most studies on upgrading of such residues have focused on hydrotreating and demetallization.8-10 A feasible selectivity between gasoline and gas oil can be attained by controlling the acidity of the zeolite. Coking, however, is unavoidable under the acid catalysis conditions which are utilized in conventional processes. At the same time, the sulfur and nitrogen contents in transportation fuels must be sufficiently low to satisfy regulations for minimizing environmental impact.11 Thus, hydrocracking with intensive hydrodenitrogena-

* Author to whom correspondence should be addressed. Fax: +8192-651-5606. E-mail: [email protected]. (1) Koyama, K. PETROTECH 1993, 16, 513. (2) Nakamura, I.; Yang, M.; Fujimoto, K. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1997, 42 (2), 401. (3) Ledoux, M. J.; Peter, A.; Blekkan, E. A.; Luck, F. Appl. Catal. 1995, A.133, 321. (4) Pai, P. A. Stud. Surf. Sci. Catal. 1995, 83, 489. (5) Kolesnikov, I. M.; Saidel, P.; Kolesnikov, S. I.; Yu Kilyanov, M. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1997, 42 (2), 461.

(6) Ishii, M. PETROTECH 1994, 17, 174. (7) Iino, A. J. J. Catal. Soc. 1993, 33, 21. (8) Kasztelan, S.; Harle, V.; Kressmann, S.; Morel, F. Prepr.sAm. Chem. Soc., Div. Pet. Chem. ACS. 1997, 42 (2), 340. (9) Mizutani, Y.; Nishizawa, A.; Yamamoto, Y.; Takehara, S.; Yamazaki, H. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1997, 42 (2), 343. (10) Den Ouden, C. J. J.; Bhan, O. K.; Boardman, S.; Street, R.D.; George, S. E. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1997, 42 (2), 373.

S0887-0624(97)00136-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/11/1998

494 Energy & Fuels, Vol. 12, No. 3, 1998

Isoda et al.

Table 1. Properties of Zeolites

catalyst HY-A Ni-HY-A Ni-HY-B Co-HY Fe-HY

Table 2. Properties and Composition of Vacuum Gas Oil

pore surface amount of metal content area adsorbed NH3 size (Å) (wt %) Si/2Al (m2/g) (mmol/g)a 0 9.2b 11.5b 9.9c 8.9d

16.1 16.0 50.0 16.0 18.0

794 693 568 708 710

0.83 0.57 0.50 0.61 0.68

18

a Adsorption of NH at 270 °C for 0.5 h, evacuation at 27 °C for 3 1 h, heating rate of 10 °C/min. b NiO. c CoO. d Fe2O3.

tion (HDN) and hydrodesulfurization (HDS) is required to practically upgrade VGO and to produce gas oil with low sulfur and nitrogen contents. In earlier studies,12,13 we examined a Ni-loaded dealuminated zeolite as an acid catalyst for desulfurization of gas oil. A combination of a conventional CoMo/Al2O3 catalyst and a Ni-HY zeolite catalyst effectively desulfurized refractory sulfur species such as 4,6-dimethyldibenzothiophene by enhancing methyl migration and hydrocracking prior to the desulfurization step. The hydrogenation activity of the zeolite originated from the highly dispersed Ni sulfide species, which also suppressed coke formation and color development in the HDS reaction. In the present study, a series of transition metals, including Ni, Co, and Fe, were loaded on a Y-zeolite, and its catalytic activities were evaluated in terms of the upgrading of VGO, which was distilled from Middle East crude. Reactions addressed in the study include hydrocracking of paraffins and polyaromatics into gasoline and gas oil fractions, as well as HDN and HDS of VGO. The effects of metal loading and dealumination of the zeolite on catalytic activity and selectivity were also evaluated, based on hydrogenation activity and acidity. Experimental Section Catalysts. Ni-, Co-, and Fe-loaded Y-zeolites were prepared according to the literature:13 A Na-Y zeolite (Na2O content ) 13.3 wt %, Si/2Al ) 5.0) was exchanged with ammonium ion and then calcined (hereafter referred to as HY catalyst). This catalyst was then further treated with steam at 670 °C to form Y zeolites having different degrees of dealumination (hereafter referred to as the HY-A and HY-B catalysts). The compositions of the HY-A and HY-B catalysts were Na2O ) 1.2 wt %, and Si/2Al ) 16.0 and 50.0, respectively. A 500 g portion of each Y zeolite was suspended in 5 L of water at 75 °C, and 3 L of an aqueous solution of 12 N HNO3 and Ni(NO3)2 (0.5 mol/ L, pH ) 1.1) was added into the slurry with stirring. After 0.5 h of stirring at 75 °C, 3 L of an aqueous solution of Ni(NO3)2 (1.0 mol/L) was added with stirring. The pH of the slurry was maintained at pH 4.0 by the addition of 5% aqueous ammonia. After stirring for 0.5 h, the catalyst was filtered, washed with water, and then dried at 120 °C for 4 h. The properties of the prepared zeolites are summarized in Table 1. The catalysts were presulfided at 360 °C for 2 h in a stream of H2S (5 vol %, H2 carrier) under atmospheric pressure just prior to use. (11) Takatsuka, T.; Wada, Y.; Suzuki, H.; Komatsu, S.; Morimura, Y. J. Jpn. Pet. Inst. 1992, 35, 197. (12) Isoda, T.; Nagao, S.; Ma, X.; Korai, Y.; Mochida, I. Energy Fuels 1996, 10, 1078. (13) Isoda, T.; Nagao, S.; Ma, X.; Korai, Y.; Mochida, I. J. Jpn. Pet. Inst. 1998, 41, 22.

density (15 °C) (g/mL) total sulfur (wt %) total nitrogen (wt %) V (wt ppm) Ni (wt ppm) boiling range (°C) IBP 5 (vol %) 10 (vol %) 20 (vol %) 30 (vol %) 40 (vol %) 50 (vol %) 60 (vol %) 70 (vol %) 80 (vol %)

0.9263 2.45 0.17 0.14 0.05 205 341 378 410 434 450 473 494 510 530

Feed and Reaction. A VGO from a Middle East crude was provided by a refinery in Japan. Table 2 summarizes the properties of the feed. It consisted of a 25 vol % middle fraction with a boiling range of 253-340 °C and a 75 vol % heavy fraction with a boiling range of 340-530 °C. Hydrocracking, HDN, and HDS reactions of the VGO were performed in a 50 mL batch autoclave equipped with a magnetic stirrer, rotating at 500 rpm, using 1 g of powdery catalyst and 5 g of feed at 300, 340, and 380 °C for 40 min under an initial H2 pressure of 5 MPa. The temperature was increased at 10.9 °C/min from 20 to 100 °C; at 13.5/min from 100 to 200 °C; at 14.2 °C/min from 200 to 300 °C; at 14.2 °C/min from 300 to 340 °C; and at 14.0 °C/min from 340 to 380 °C. The conversion at temperatures below 200 °C was negligible. Thus, the time periods which were required to reach 300, 340, and 380 °C were 6, 9, and 12 min, respectively, and the conversion obtained in these periods was 5 wt % at maximum. It was then assumed that the reaction started at the time when the temperature reached each prescribed reaction temperature, which was controlled within (2 °C. The H2 pressure increased from the initial value of 5 MPa to 6.0-6.6 MPa during the heating step and then gradually approached the final pressure, 5.5-6.0 MPa, which was dependent on hydrocracking reactivity. The reaction was quenched by placing the reactor into a water bath. The time required for quenching was less than 20 s, during which time the conversion was negligible. The catalytic activity of the NiHY zeolite was changed slightly during the reaction, while that of the metal-loaded zeolites remained unchanged under the reaction conditions utilized herein. The catalyst lifetime was not determined. Yields and compositions are expressed in weight fractions unless otherwise noted. The unit ppm is also based on a weight fraction. Fractionation and Analysis. Product oil was separated into hexane-soluble (HS) and -insoluble (HI) fractions by extraction. The HS fraction was qualitatively and quantitatively analyzed by GC-FID (Yanaco G-3800) equipped with a silicone capillary column (OV-101, 0.25 mm i.d. and 50 m long). Light, middle, and heavy fractions in the feed and product oils are referred to by boiling point in each fraction as follows: under 253 °C (n-C25). The symbols in the parentheses present the number of carbon atoms contained by n-paraffins in the fraction. C1-C4 hydrocarbon products were identified by comparison of their retention times with those of authentic gases. Weight loss of the feed after the hydrocracking reaction was defined as the gas yield, and carbon deposit on the catalysts was quantified by elemental analysis. The hydrocracking activity of the zeolites is described in terms of compositional changes in the products: gases, n-C25 faction, HI fraction, and coke. Figure 1 shows a diagram of the fractionation

Hydrocracking of Vacuum Gas Oil

Energy & Fuels, Vol. 12, No. 3, 1998 495

Results

Figure 1. Scheme for the separation of vacuum gas oil. procedure.14 VGO (2 g) was separated into fractions using a column packed with activated neutral alumina particles (100 g) as follows: The saturated fraction (PA) was eluted with 200 mL of hexane, the aromatic fraction with 500 mL of benzene, and the polar fraction (PO) with 150 mL of a 1:1 benzenemethanol mixture. These three fractions were recovered by evaporating the solvents and were then weighed. A portion of the aromatic fraction was further separated into five fractions based on their aromatic ring sizes14 using an HPLC (L4200, Hitachi) equipped with a UV detector (254 nm) and a Unisil Q 100-10 column (6.0 mm i.d. and 250 mm long). The mobile phase was n-hexane and was fed at a flow rate of 5 mL/min. Each fraction was weighed after removal of the solvent. Compounds in each fraction were quantified using GC-FID and GC-FPD (Yanaco G-6800) equipped with a silicone capillary column (OV-101, 0.25 mm i.d. and 50 m in length). Major compounds were identified by comparing their retention times with those of authentic standards.14 Sulfur species in the feed and product oils were also qualitatively and quantitatively analyzed by GC-FPD (Yanaco G-3800) using the same capillary column as was used for the GC-FID. 4,6-Dimethyldibenzothiophene (4,6-DMDBT) was synthesized according to published procedures,15 and 4,6DMDBT and benzonaphthothiophene (BNT) were used as standard samples to identify the sulfur species in VGO. Benzothiophenes substituted with three methyl and one ethyl group (C5-BT) were identified by comparison with literature data.16 Nitrogen contents in the product oils were determined by elemental analysis. HDN conversion was calculated from the difference in nitrogen contents in the starting VGO and the product oil. TPD, XPS, and HREM Analyses. The acidity of the zeolite was measured by temperature-programmed desorption (TPD) using NH3 in a He carrier. NH3 was adsorbed on the dried catalyst at 270 °C for 0.5 h and successively evacuated at 27 °C. The heating rate of the TPD was 10 °C/min. X-ray photoelectron spectra (XPS, Shimadzu, ESCA 1000) of the catalysts were obtained with Mg KR radiation of 1253.6 eV. Binding energies were identified based on the Al 2p (74.0 eV) in the catalyst.17-19 High-resolution electron micrographs (HREM, JEOL, JEM-2000 EX) were obtained at an acceleration voltage of 200 KV at magnifications of 100 000-400 000. Samples for HREM were prepared by grinding zeolite particles or by slicing epoxy resin specimens of embedded zeolite particles. Samples were supported on Cu grids and kept in an atmosphere of argon. (14) Sugimoto, Y.; Tunuukij, K.; Miki, Y.; Yamadaya, S. J. Jpn. Pet. Inst. 1992, 35, 339. (15) Gerdil, R.; Lucken, E. J. Am. Chem. Soc. 1965, 87, 213. (16) Ma, X.; Sakanishi, K.; Isoda, T.; Mochida, I. Ind. Eng. Chem. Res. 1995, 34, 748. (17) Houssenbay, S.; Kasztelan, S.; Toulhiat, H.; Bonnelle, J. P.; Grimblot, J. J. Phys. Chem. 1989, 93, 7176. (18) Wu, H.; Hercules, D. M. J. Phys. Chem. 1979, 83, 2003.

Hydrocracking Activity over Y-Zeolites. Figure 2 illustrates the distribution of products by hydrocracking of the VGO and product oils in terms of their boiling point. The hydrocracking reaction was carried out at 300, 340 and 380 °C using the Ni-HY-A, Ni-HY-B, FeHY, Co-HY, and HY-A catalysts, respectively. The NiHY-A showed an excellent activity for the hydrocracking of the heavy fraction (bp 340-530 °C), reducing its content to 21 wt % for a reaction at 340 °C and 11 wt % at 380 °C. A significant increase in the light fraction (bp < 253 °C) was found, with yields of 51 wt % at 340 °C and 66 wt % at 380 °C, while the gas yield was low. The fact that the middle fraction (bp 253-340 °C) was significantly reduced suggests that the heavy fraction was directly converted to the light fraction. Hexaneinsoluble (HI) components were not found in the product oils, while the coke deposition on the catalyst was approximately 3 wt % for all reaction temperatures used. The Ni-HY-B catalyst showed a hydrocracking activity at temperatures above 340 °C. The heavy fraction was markedly reduced to 55 wt % at 340 °C and 10 wt % at 380 °C. The total yield of light and middle fractions was increased to 80 wt % at 380 °C, and the gas and coke yields were less than 9 and 1 wt %, respectively. The Ni-free HY-A zeolite exhibited a higher activity for cracking of the heavy fraction than the other NiHY zeolites, reducing its content to 29 wt % at 340 °C and 14 wt % at 380 °C. The content of the light fraction remained at the level of 30 wt % for all reaction temperatures. However, the yields of gas and coke were as high as 30 and 4 wt %, respectively. The HY-A zeolite produced approximately a 3 wt % HI fraction at 300-380 °C. The Fe-HY and Co-HY zeolites exhibited product compositions similar to the Ni-HY-A zeolite, while the gas yield over the Fe-HY was 23 wt % at 380 °C, significantly higher than that over the other catalysts. Figure 3 illustrates component distributions of the VGO and the hydrocracked products over the Ni-HYA, Ni-HY-B, and Ni-free HY-A zeolites for a reaction at 380 °C. Products are classified into eight fractions: C1-C4 hydrocarbons (G), n-paraffins containing 9-32 carbons (PA), alkylbenzenes with 8-20 alkyl carbon substituents (1R), alkylnaphthalenes and alkylbenzothiophenes (2R), alkyldibenzothiophenes and alkylphenanthrenes (3R), alkylpyrenes, alkylchrysenes, and alkylbenzonaphthothiophenes (4R), polars (PO), and coke deposited on the catalyst (C). Their structures and symbols are shown in Table 3 and Figure 3. n-Paraffins were the major components in the feed VGO (45 wt %). The second largest components were 3- and 4-ring aromatics (16 and 13 wt %, respectively). Aromatics with 1- and 2-rings and polar compounds were minor components (10, 5, and 11 wt %, respectively). The fractionation using the HPLC equipped with the UV detector and the Unisil Q 100-10 column separated compounds by the number of aromatic rings. Benzene and tetralin are classified as 1-ring aromatics, and naphthalene and acenaphthene are classified as 2-ring aromatics. Large peaks in GC-FID profiles of the saturated fraction (PA) of the VGO were assigned to


Isoda et al.

Figure 2. Composition of product oils from vacuum gas oil as a function of reaction temperature over Ni-HY-A,-B, Co-, FeHY, and HY-A zeolites.

Figure 3. Product distributions from vacuum gas oil over NiHY zeolites at 380 °C.

n-paraffins with long alkyl chains. No peaks corresponding to naphthenes were observed. These results suggest that naphthenes were distributed in fractions 1R-5R and that the major components in each fraction were aromatics containing alkyl groups. After the reaction over the Ni-HY-A and Ni-HY-B at 380 °C, the contents of 4-ring aromatics and polar compounds were markedly reduced to 3 and 2 wt %, respectively, while the contents of 2- and 3-ring aromatics and paraffins were not greatly changed. The yields

of paraffins and 2-ring aromatics were 55 and 5 wt %, respectively, over both Ni-HY-A and Ni-HY-B. The yield of 3-ring aromatics was 9 wt % over the Ni-HY-A and 11 wt % over the Ni-HY-B, and the yield of 1-ring aromatics was 8 wt % over the Ni-HY-A and 14 wt % over the Ni-HY-B. The yield of gaseous hydrocarbons was 9 wt % over the Ni-HY-B and 15 wt % over the Ni-HY-A. Coke formation was reduced over the NiHY-B. Using the HY-A zeolite, on the other hand, the content of 4-ring aromatics increased to 22 wt % and that of 2and 3-ring aromatics greatly decreased. The content of paraffins was 34 wt % after the reaction. The HY-A zeolite produced the highest yields of hydrocarbon gases and coke, 26 and 4 wt %, respectively. This indicates that cracking and condensation reactions proceeded excessively over the zeolite without Ni loading under the acid catalysis conditions. Distribution of Paraffins (Fraction PA). Figure 4 illustrates the distribution of normal and branched paraffins in fraction PA of the feed and product oils over the Ni-HY-A, Ni-HY-B, and HY-A zeolites at 380 °C. The VGO contained C10-C32 n-paraffins, with C20-C30



Table 3. Classification of Products from Vacuum Gas Oil classification gaseous products n-paraffins aromatics 1 ring

compound C1-C4 hydrocarbons CH3(CH2)n-2(CH3) (9 e n e 32) alkyl derivatives (CH2)n–1CH3 (8 e n e 20) (+C2-C5)

2 rings

(+C3-C16)

Table 4. Yields of Naphthalene Derivatives in Fraction 2R from Vacuum Gas Oil and Product Oils over Zeolites at 380 °C yield (wt %) naphthalenes

feed Ni-HY-A Ni-HY-B HY-A

2,6- and 2,7-dimethyl1,3-dimethyl1,6- and 1,7-dimethyl2,3-, 1,5-, and 1,4-dimethyl1,2-dimethylethylpropyltrimethyl-

0 0.03 0.43 0.38 0.11 0.18 0.05 3.84

0.06 0.31 0.34 0.23 0.20 0.08 0 3.80

0.17 0.23 0.40 0.16 0.22 0 0.22 3.62

0 0.01 0 0 0 0 0 0.99

S

(+C1-C6)

3 rings S

(+C0-C4)

(+C0-C3)

4 rings

(+C0-C4)

(+C0-C4) S

polar compounds coke deposited on catalyst

paraffins as the major components. Neither straightchain paraffins of less than 10 carbons nor branched ones containing more than 10 carbons were detected in the feed. The Ni-HY-A effectively hydrocracked C10C32 paraffins in the feed and reduced their content to less than 0.1 wt %. Major products were branched decanes, nonanes, and dodecanes, the yields of which were 9.6, 9.0, and 8.9 wt %, respectively. The Ni-HY-B exhibited a hydrocracking activity slightly lower than the Ni-HY-A and reduced the content of C10-C32 paraffins to less than 0.5 wt %. Major products were branched C10-C13 paraffins, the yield of which was approximately 6 wt %. Using the HY-A catalyst, however, the yield of branched decanes was 7.0 wt %, while the content of C10-C32 paraffins was greatly reduced to 0.4 wt %. The smallest yield of C10C14 branched paraffins was attained over the HY-A. Distribution of Alkylbenzenes (Fraction 1R). Figure 5 illustrates the distribution of alkylbenzenes in fraction 1R in the feed as well as that in the product oils over the Ni-HY-A, Ni-HY-B, and HY-A zeolites at 380 °C. The VGO contained alkylbenzenes containing 8-20 carbons. Alkylbenzenes of 14-20 carbons were major components, and the yield of each component was in the range of 0.8-2.0 wt %. Alkylbenzenes containing 8-12 carbon chains were minor, and each content in the product was 0.1-0.2 wt %. The Ni-HY-A effectively cracked alkylbenzenes with long chains, and the yield of alkylbenzenes with alkyl chains of less than 10 carbons increased to 0.3-1.2 wt %. The yield of C11C20 alkylbenzenes was similar to that of C8-C10 alkyl-

Table 5. Yields of Phenanthrene Derivatives in Fraction 3R from Vacuum Gas Oil and Product Oils over Zeolites at 380 °C yield (wt %) phenanthrenes

feed

Ni-HY-A

Ni-HY-B

HY-A

phenanthrene unknown 1 C1unknown 2 C2C3C4-

0.10 0.98 0.94 0.35 2.98 2.58 2.48

0.14 0.27 1.34 0.22 1.60 1.46 0.39

0.29 1.25 1.39 0.99 2.10 1.41 0.86

0.03 0.14 0.37 0.10 0.69 0.44 0.24

benzenes. It is noteworthy that the Ni-HY-B zeolite gave a yield of C8-C20 alkylbenzenes, 0.8-1.4 wt %, which was larger than that observed for the Ni-HY-A zeolite. No specific selectivity with respect to chain length was observed. The HY-A zeolite exhibited the lowest activity for the hydrocracking of alkylbenzenes, giving a yield of less than 0.4 wt %. Distributions of Alkylnaphthalenes (Fraction 2R). Table 4 shows the yields of alkylnaphthalenes in fraction 2R of the feed and product oils. Trimethylnaphthalenes, in which the location of the methyl groups was not identified, were the major components in fraction 2R, with a total content of 3.8 wt %. The second major components were 1,4-, 1,5-, 1,6-, 1,7-, and 2,3-dimethylnaphthalenes, and the content of each was approximately 0.4 wt %. Other naphthalene derivatives were very minor in the feed. No significant changes in the content of alkylnaphthalenes were observed using the Ni-HY zeolites. The HY-A exhibited the highest activity and reduced the total content of methylnaphthalenes to 0.9 wt %. This indicates an excess cracking activity of the catalyst for polyaromatics, as shown in Figure 3. Distribution of Alkylphenanthrenes (Fraction 3R). Table 5 shows the yields of phenanthrenes in fraction 3R in the feed as well as the product oils. The VGO contained alkylphenanthrenes with 2-4 methyl groups as the major components, the total content of which was 8.0 wt %. Phenanthrene and methylphenanthrenes were minor components, and their contents were 0.1 and 0.9 wt %, respectively. The amount of phenanthrenes containing 2-4 methyl groups was reduced to 0.4-1.6 wt % by the reaction over the NiHY-A and 0.9-2.1 wt % over the Ni-HY-B. The HY-A was much more active for the cracking of phenanthrenes than the Ni-loaded zeolites. Distribution of Tetraaromatics (Fraction 4R). Table 6 shows the yields of 4-ring aromatic hydrocarbons in fraction 4R. The VGO contained pyrenes and chrysenes with 1-2 methyl groups, and the contents were 10.6 and 7.9 wt %, respectively. The Ni-HY-A


Isoda et al.

Figure 4. Yields of straight and branched paraffins of fraction PA in feed (A), product oils over Ni-HY-A (B), Ni-HY-B (C), and HY-A zeolites (D) at 380 °C.

Figure 5. Yields of alkylbenzene derivatives of fraction 1R in feed (A), product oils over Ni-HY-A (B), Ni-HY-B (C), and HY-A zeolites (D) at 380 °C. Table 6. Yields of 4-Ring Aromatic Hydrocarbons in Fraction 4R from Vacuum Gas Oil and Product Oils over Zeolites at 380 °C yield (wt %) 4-ring aromatics

feed

Ni-HY-A

Ni-HY-B

HY-A

C1-pyrenes C2-pyrenes chrysenes C1-chrysenes C2-chrysenes

5.17 5.38 0.16 4.24 3.65

2.56 2.95 0.08 0.99 0

3.16 1.70 0.02 0.85 0

5.66 5.71 0 3.32 8.82

and Ni-HY-B zeolites exhibited an excellent activity for the cracking of 4-ring aromatics. The yield of pyrenes was 5.5 wt % over the Ni-HY-A and 4.9 wt % over the Ni-HY-B. The content of chrysenes was reduced to less than 1.0 wt % using these catalysts. Content of pyrenes was no change over the Ni-free HY-A, whereas it

markedly increased the content of dimethylchrysenes. This suggests that aromatic rings were formed by condensation over the acidic catalysts used in this study. HDS Activity. Figure 6 illustrates changes in concentrations of representative sulfur species by HDS using the Ni-HY-A, Co-HY, Fe-HY, and HY-A zeolites as a function of reaction temperature. The Ni-HY-A exhibited the highest HDS activity for these sulfur species. Alkylbenzothiophenes (C5-BT), having three methyl groups and one ethyl group, are typical refractory benzothiophenes as reported previously.16 The content of C5-BT sharply decreased with increasing reaction temperature using the Ni-HY-A zeolite. At 300 °C, the content was decreased to 160 ppm, versus 250 ppm in the feed. No C5-BT was observed at reaction



Figure 6. Hydrodesulfurization of typical sulfur species in vacuum gas oil over metal-HY and HY zeolites: (O) Ni-HYA; (4) Co-HY; (0) Fe-HY; (b) HY-A. Feed: contents in original VGO; ppm in mass.

temperatures above 340 °C. The Ni-HY-B exhibited an activity similar to the Ni-HY-A for HDS of C5-BT. The HDS activity of the Co-HY was slightly lower than that of the Ni-HY-A. The Fe-HY zeolite showed the smallest activity among the metal-Y zeolites examined and reduced the sulfur content of C5-BT to 250 ppm at 300 °C and 40 ppm at 340 °C, although C5-BT was completely desulfurized at 380 °C. The metal-free HY-A had essentially no HDS activity for C5-BT. Benzonaphthothiophene (BNT) represents a typical sulfur species having four rings. Its concentration in the feed was 600 ppm, as shown in Figure 6. The HDS activity of BNT for the metal-loaded catalysts was in the order of Ni-HY-A ) Ni-HY-B > Co-HY-A ) FeHY-A. The Ni-HY-A reduced the BNT content to 240 ppm for a reaction at 300 °C, 120 ppm at 340 °C, and 110 ppm at 380 °C, while the Co-HY and Fe-HY reduced the content to 260 ppm at 300 °C, 330 ppm at 340 °C, and 150 ppm at 380 °C. The HY-A catalyst exhibited only low activity for HDS of BNT. 4,6-Dimethyldibenzothiophene (4,6-DMDBT) has been reported to be the most refractory sulfur species,20-22 and the initial concentration was 2700 ppm. The NiHY-A exhibited an excellent activity for HDS of 4,6DMDBT, reducing the content to 1000 and 1300 ppm by HDS at 300 and 380 °C, respectively. The Ni-HY-B exhibited an activity similar to the Ni-HY-A. However, the Fe-HY and Co-HY showed a lower HDS reactivity for 4,6-DMDBT. The content was decreased only to 1440 and 2330 ppm over the Fe-HY, and 1030 and 1860 ppm over the Co-HY, at 300 and 380 °C, respectively. The HDS reaction of 4,6-DMDBT using the HY-A zeolite at higher temperatures was negative. The 4,6-DMDBT content was increased because methyl fragments, which were produced by the cracking of n-paraffins, reacted with dibenzothiophenes as reported previously.12,13 HDN Activity. Figure 7 shows the effect of reaction temperature on HDN conversion of the VGO over the Ni-HY-A, Co-HY, Fe-HY, and HY-A catalysts. Among the catalysts examined, the Ni-HY-A exhibited the highest HDN activity at 380 °C, giving a conversion of 69 wt %. The HDN conversions using the Co-HY and Fe-HY zeolites were 51 and 40 wt %, respectively, and were lower than the HDN conversion using the Ni-HYA. The conversion over the Ni-HY-A at 300 °C was 5 wt %, which was equivalent to that over the Ni-HY-B (19) Nag, K.; Hercules, D. M. J. Phys. Chem. 1976, 80, 2094. (20) Isoda, T.; Ma, X.; Mochida, I. J. Jpn. Pet. Inst. 1994, 37, 368. (21) Isoda, T.; Ma, X.; Mochida, I. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1994, 39 (4), 584. (22) Kabe, T.; Ishihara, A.; Zhang, Q.; Tsutsui, H.; Tajima, H. J. Jpn. Pet. Inst. 1993, 36, 467.

Figure 7. Hydrodenitrogenation of vacuum gas oil over metal-HY and HY-A zeolites. Table 7. XPS Analysis of Ni 2p3/2 and S 2p on the Surface of Ni-HY Zeolites binding energy (eV) catalyst

Ni 2p3/2

Ni-HY-A (before sulfiding) Ni-HY-A (sulfided) Ni-HY-B (before sulfiding) Ni-HY-B (sulfided) Ni(9.2)NaYa (before sulfiding) Ni(9.2)NaYb (sulfided) Ni-mordenitec (reduced)

853.2 852.1 853.2 851.6 856.2 (Ni2+) 853.2 (Ni3S2) 857.3 (Ni2+) 854.9 (Ni+) 852.4 (Ni0)

a-c

S 2p 161.2 160.2

References 17-19, respectively.

and was lower than that over the Co-HY and Fe-HY. The metal-free HY-A was inactive for HDN of the VGO. Characterization of Ni-HY Zeolites. The amounts of desorbed NH3 determined by the TPD of the Yzeolites are shown in Table 1. Peaks in the TPD profiles appeared approximately at 130 and 300-400 °C. This suggests that there were two acidic sites with different strengths. By dealumination and Ni loading, the hightemperature peak of the HY-A was shifted from 332 to 397 °C, and the peaks became smaller. Table 7 shows the effect of sulfiding on the binding energies of Ni 2p. Before sulfiding, a large peak was found at 853.2 eV and was assigned to NiO species.17-19 After sulfiding, the peak was shifted to a lower binding energy of 852.1 eV and was assigned to Ni3S2 species.17-19 However, the degree of dealumination had no effect on the binding energy of Ni. Figure 8 shows HREM micrographs of the sulfided Ni-HY-A zeolite. The zeolite framework remained unchanged after the dealumination. Ni sulfide particles were observed on the Y-zeolite layer as shown in Figure 8A. The zeolite matrix was destroyed by the X-ray irradiation within a few seconds, whereas the particles of Ni sulfide did not change their size and location as indicated in Figure 8B. Ni sulfide particles were observed on the surface of the zeolite particle at a lower magnification as shown in Figure 8C. No Ni particles were observed on the inside of the zeolite particle which was sliced at a thickness of 50 nm, as indicated in Figure 8D. This result suggests that the majority of the Ni sulfide particles appear to be located on the outer surface of the zeolite particle. Figure 9 illustrates HREM micrographs of the sulfided Ni-HY-A (A), Ni-HY-B (B and C), and HY-A (D) zeolites. The zeolite matrix was also destroyed by the


Isoda et al.

(A)

(C)

(B)

(D)

Figure 8. HREM micrographs of sulfided Ni-HY-A zeolite. Sample thickness (A), (B), and (C) 90 nm; (D) 50 nm.

X-ray beam in this case. Highly dispersed Ni sulfide particles, 10 nm in size, were observed on the surface of Ni-HY-A. Very few Ni spots were observed on the surface of Ni-HY-B, as can be seen in Figure 8B. Fine Ni sulfide particles, which were aggregated on the edge of the zeolite particle, were observable under higher magnification. The HY-A zeolite contained a number of pores, the diameter of which was about 10-20 nm. No diffraction peaks, assigned to Ni oxide or sulfide before or after the sulfiding, were observed in the XRD

spectra of the Ni-HY-A and Ni-HY-B. This suggests that these Ni particles were of a low degree of crystallinity. Discussion Role of Metals on the Y-Zeolite. The present study has examined the role of metals, loaded on an HY zeolite in the hydrocracking of VGO. The heavy fraction, which contains refractory sulfur species, such as 4,6-DMDBT,


(A)

(B)


(C)

(D)

Figure 9. HREM micrographs of sulfided zeolites: (A) Ni-HY-A, (B) and (C) Ni-HY-B, and (D) HY-A.

is decomposed, giving minimum yields of hydrocarbon gases and coke over the dealuminated, metal-loaded Y-zeolites. The catalysts show hydrocracking activity due to protons in the starting zeolite as well as hydrogenation activity due to the metal sulfides which are highly dispersed on the zeolite surface. These combined activities are effective in the selective hydrocracking of paraffin and aromatic compounds, as described in the previous section. Longer chain paraffins are cracked into shorter and branched olefins, which are rapidly

hydrogenated without further cracking into C1-C3 hydrocarbons. It would be expected that hydrogenation moderates the deactivation of acidic catalysts due to coke deposition. Coke precursors are thought to be already extensively hydrogenated prior to being strongly adsorbed to the acid sites. The Ni sulfide-derived hydrogenation activity also promotes the HDN and HDS of heterocyclic compounds. Aromatic rings are first hydrogenated, and heteroatoms (X) are then released by the acidic fission


of C-X bonds. The Fe-HY and Co-HY zeolites exhibit lower HDS and HDN activities than the Ni-HY zeolite at a reaction temperature of 380 °C, because of their low hydrogenation activity. Mo¨ssbauer spectroscopy and transmission electron microscopy of an Fe-loaded Y-type zeolite suggest that two types of Fe species exist, namely, extremely fine particles of Fe(III) oxide formed in the super cage of the zeolite, and Fe(III) oxide which is bonded to the zeolite framework.23,24 The oxide particles maintain their size after sulfiding. In the present study, Ni2S3-like sulfides are clearly shown to be highly dispersed on the surface of the zeolite particle. These metal sulfides may be the origin of the high catalytic activity with respect to hydrogenation. The two-step Ni loading procedure developed in the present study permits the impregnation of a large amount of Ni. The Ni content reaches 7.2-9.0 wt %. In contrast, only a few wt % of metal ions are incorporated into the zeolite by conventional ion-exchange procedures. In contrast, the Y-zeolite extensively cracks the light fraction in the VGO to gaseous hydrocarbons. The strong acidity of the Y-zeolite, which has little hydrogenation activity, also enhances the polymerization of aromatic hydrocarbons in the feed, leading to 4-ring aromatic compounds, a hexane-insoluble fraction, and coke deposition on the catalyst. Some of the alkyl fragments produced by the cracking of paraffins became attached to the aromatic rings, leading to significant yield of alkylaromatics such as C5-BT and 4,6-DMDBT. Effect of Dealumination on Catalytic Activity and Product Selectivity. Dealumination gives rise to pores of 1.8 nm in diameter in the Y-zeolite structure, while the pore size of a conventional Y-zeolite is 0.7 nm. The size of aromatic molecules, as estimated by molecular orbital calculation, is 0.499 nm for benzene, 0.4990.681 nm for naphathalene, 0.499-0.931 nm for anthracene, and 0.685-0.929 nm for pyrene. Thus, aromatic molecules which contain methyl groups or long alkyl groups as substituents cannot enter the zeolite matrix. However, these molecules diffuse into macropores formed by the dealumination treatment, permit(23) Delgass, W. N.; Garten, R. L.; Boudart, M. J. Chem. Phys. 1969, 50, 4603. (24) Garten, R. L.; Delgass, W. N.; Boudart, M. J. Catal. 1970, 18, 90.

Isoda et al.

ting the hydrocracking reaction to proceed inside the dealuminated zeolites. Large aromatic molecules may be hydrogenated or cracked on the outer surface of the metal-loaded Yzeolite particles. In particular, hydrogenation progresses on the outer surface since the Ni sulfide particles are formed mainly on the peripheral of zeolite particles, as evidenced by TEM observation. Molecules in the light or middle fractions are smaller than those in the heavy fractions and can diffuse through micropores of zeolite crystals as well as through macropores in the dealuminated Y-zeolite. As a result, smaller molecules are further cracked on the Bro¨nsted acid sites located in the micropore walls. This is consistent with the findings that the yield of C1-C4 hydrocarbon gases increased with increasing yield of light fractions. The intensive dealumination also reduces the acidity of zeolite and suppresses excessive hydrocracking of light fractions inside the zeolite crystals. Thus, the selectivity for mild hydrocracking of VGO to middle distillate is improved with minimum yields of gases and coke deposition. Conclusions 1. The Ni-HY-A zeolite of Si/2Al ) 16.0 exhibited an excellent hydrocracking activity of the heavy fraction in VGO and gave the highest yield of the light fraction with the lowest gas and coke yields among the catalysts examined. The Ni-HY zeolites also exhibited the highest HDS activity for 4,6-dimethyldibenzothiophene at 380 °C. 2. The highly dealuminated Ni-HY-B catalyst of Si/ 2Al ) 50.0 gave a large yield of the middle fraction (gas oil), while the gas and coke yields were less than 9 and 1 wt %, respectively, under the conditions employed in this study. 3. The metal-free HY-A zeolite enhanced the formation of hydrocarbon gaseous, coke, and hexane-insoluble yield, while the light fraction yield remained unchanged at 30 wt %. Control of the optimal acidity originating from Si/Al ratio and the hydrogenation activity due to metal loading was a key factor in the selective hydrocracking of VGO over the metal supported Y-zeolites. EF970136R

Reactivity and Selectivity for the Hydrocracking of Vacuum Gas Oil

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