Hydrocracking of Pyrenes over a Nickel-Supported Y-Zeolite Catalyst

Mar 2, 1999 - Figure 6 shows first-order reaction rate plots for the hydrocracking of Py, DPy, and HPy over the Ni−HY zeolite, as well as for hydrog...
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Energy & Fuels 1999, 13, 617-623

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Hydrocracking of Pyrenes over a Nickel-Supported Y-Zeolite Catalyst and an Assessment of the Reaction Mechanism Based on MD Calculations Takaaki Isoda, Seiichiro Maemoto, Katsuki Kusakabe, and Shigeharu Morooka* Department of Materials Physics and Chemistry, Graduate School of Engineering, Kyushu University, Fukuoka 812-8581, Japan Received September 22, 1998

The hydrocracking of pyrene (Py) was examined in a batch autoclave at 386 °C for 0-30 min under a hydrogen pressure of 5 MPa, using a Ni-supported Y-type zeolite (Ni-HY) catalyst. The hydrocracking of dihydropyrene (DPy) and hexahydropyrene (HPy) was also carried out under the same reaction conditions. In addition, a Ni-KY catalyst, whose Bro¨nsted acid sites were eliminated by ion exchange with potassium ions, was used to investigate the behavior of Py in the hydrogenation reaction network. Over the Ni-HY zeolite catalyst, Py was hydrogenated to DPy and HPy. Reaction involving the opening of aromatic rings proceeded after Py was hydrogenated to HPy via DPy. Mono- and diaromatics, which were produced from HPy, were further cracked into C1-C4 hydrocarbon gases. The dehydrogenation of DPy and HPy appeared to be the rate-determining step for the case of the Ni-HY zeolite. The Ni-HY zeolite catalyst thus possessed multifunctional activities of hydrogenation, dehydrogenation, and cracking. Cracking of polyaromatics is also discussed on the basis of molecular dynamics calculation using the Insight II and Discover 3 programs. Py and HPy molecules remained outside of the Y-zeolite pore, and acenaphthene and naphthalene were able to diffuse into the micropore. However, the overall potential energy of the Y-zeolite which adsorbed polycyclic aromatic products on the outer surface was lower than that of the Y-zeolite which contained the products in the pore. Thus, it is likely that the saturated rings of Pys were opened on the outer surface of the Y-zeolite catalyst.

Introduction Selective hydrocracking of long-chained paraffins and polyaromatics is an important area in meeting the increasing demands of transportation fuels.1-5 In our earlier studies,6-8 transition metals, such as Fe, Co, and Ni, were loaded on a Y-zeolite, and catalytic activities were evaluated based on the upgrading of vacuum gas oil (VGO) from Middle East crude. The major components in VGO were 9-32 carbon n-paraffins (45 wt %). The second largest components were 3-ring aromatics including dibenzothiophenes and phenanthrenes which were substituted with alkyl groups, as well as 4-ring aromatics including pyrenes, chrysenes, and benzonaphthothiophenes, also substituted with alkyl groups. The 3- and 4-ring aromatics constituted 16 and 13 wt %, * To whom correspondence should be addressed. Telephone: +8192-642-3551. Fax: +81-92-651-5606. E-mail: [email protected]. (1) Koyama, K. PETROTECH 1993, 16, 513. (2) Nakamura, I.; Yang, M.; Fujimoto, K. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1997, 42, 401. (3) Ledoux, M. J.; Peter, A.; Blekkan, E. A.; Luck, F. Appl. Catal. A 1995, 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. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1997, 42, 461. (6) Isoda, T.; Kusakabe, K.; Morooka, S.; Mochida, I. Energy Fuels 1998, 12, 493. (7) Isoda, T.; Kusakabe, K.; Morooka, S. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1997, 42, 420. (8) Isoda, T.; Kusakabe, K.; Morooka, S. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1997, 42, 424.

respectively. The Ni-HY zeolite catalyst was effective in hydrocracking pyrenes, but the Ni-free zeolite which had no hydrogenation activity yielded chrysenes and C1-C4 hydrocarbon gases. Song et al.9 hydrocracked phenanthrene using HY zeolites loaded with Ni, Fe, and La, as well as NiMo/ and CoMo/Al2O3 catalysts, at 400 °C for 1 h under a hydrogen pressure of 5-7 MPa. These workers also investigated the influence of catalysts on product distributions. The Ni-HY zeolite was the most effective, giving rise to high yields of 1-ring and 2-ring aromatics with coke deposition, whereas the NiMo/ and CoMo/ Al2O3 catalysts produced only hydrogenated phenanthrenes. Sato et al.10 reported that cracking of 1- and 2-ring aromatics was enhanced after hydrogenation of the aromatic ring. Girgis and Gates11 reviewed hydrogenation and cracking, as well as kinetics, of polyaromatics over NiMo/, CoMo/, and NiW/Al2O3, but did not address the issue to hydrocracking of polyaromatics over zeolite. The present study reports an examination of the hydrocracking reaction of pyrene, a typical refractory hydrocracking compound, as well as partially hydrogenated pyrenes, in a batch autoclave at 386 °C under a (9) Ueda, K.; Matsui, H.; Song, C.; Xu, W. J. Jpn. Pet. Inst. 1990, 33, 413. (10) Sato, Y.; Kamo, T.; Yamamoto, K.; Inaba, T.; Miki, K. J. Jpn. Pet. Inst. 1991, 34, 327. (11) Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30, 2021.

10.1021/ef9801928 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/02/1999

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Table 1. Properties of Catalysts

catalyst

metal content as NiO (wt %)

Ni-HY Ni-KY

9.2 9.2

Si/2Al

surface area (m2/g)

amount of adsorbed NH3 (mmol/g)

16.0 16.0

693 693

0.57 0

hydrogen pressure of 5 MPa using a Ni-loaded HYzeolite catalyst. The role of catalyst acidity was investigated using a Ni-KY zeolite catalyst without acid sites, and a scheme for the hydrocracking of pyrene is proposed. To discuss the mechanism of hydrocracking, the diffusion of Py, HPy, acenaphthene, and naphthalene through the Y-zeolite pore was evaluated on the basis of molecular dynamics calculation. Experimental Section Catalyst. (1) Ni-HY Zeolite. Ni-loaded HY zeolite (referred to as Ni-HY zeolite) was prepared as follows:6 A Na-Y zeolite (Na2O content ) 13.3 wt %, Si/2Al ) 5.0) was exchanged with ammonium ions and then calcined. The HY zeolite was then further treated with steam at 670 °C to remove aluminum atoms. The composition of the USY zeolite was Na2O ) 1.2 wt % and Si/2Al ) 16.0. A 500 g portion of USY 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 of Ni, pH ) 1.1) was added to the slurry with stirring. After stirring for 0.5h at 75 °C, 3 L of an aqueous solution of 12 N HNO3 and Ni(NO3) 2 (1.0 mol/L of Ni) was added with continued stirring. The pH of the slurry was maintained at pH 4.0 using 5% aqueous ammonia. After stirring for 0.5 h, the catalyst was filtered, washed with water, and dried at 120 °C for 4 h. (2) Ni-KY Zeolite. Protons which constituted Bro¨nsted acid sites on the HY zeolite were eliminated by ion-exchange using an aqueous 1 N KCl solution with stirring at 50 °C for 6 h. After the ion-exchange treatment, 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) an atmospheric pressure immediately prior to use. Chemicals. Pyrene (Py) and hexahydropyrene (HPy) (Tokyo Kasei) were used as substrate. Dihydropyrene (DPy) was synthesized by the following procedure: 5 g of Py was dissolved in 10 g of n-hexane and hydrogenated over a Pd/Al2O3 catalyst (Tokyo Kasei) in a 50 mL autoclave at 100 °C for 1 h under a hydrogen pressure of 10 MPa. After hydrogenation, the catalyst was removed by centrifugation, and the solvent was evaporated under vacuum. Products were recrystallized three times from methanol. Reaction. Hydrocracking and hydrogenation reactions were performed in a 50 mL batch autoclave equipped with a magnetic stirrer, rotating at 500 rpm, under an initial H2 pressure of 5 MPa. 0.3 g of each substrate and 0.3 g of powdered catalyst were mixed in 5 g of n-hexane. The latter solvent was chosen since the GC peaks of n-hexane and its cracked products came earlier than those of products of pyrene. The program of the temperature increase was the same as reported previously.6 The final temperature was maintained for 0-30 min, and the reaction was quenched by placing the reactor in a water bath. The time required for quenching was less than 20 s. It was assumed that the reaction started at the time when the temperature reached 386 °C and that it ended at the time of quench. 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 catalytic activity of the Ni-HY zeolite was changed slightly during the reaction,

Figure 1. Product distributions by hydrocracking of pyrene over Ni-HY zeolite. while that of the metal loaded zeolites remained unchanged under the reaction conditions utilized herein. The catalyst lifetime was not determined. The product was qualitatively and quantitatively analyzed using a GC-MS and a GC-FID, equipped with a silicone capillary column, as reported previously.6 Coke deposition on the used catalyst was determined by elemental analysis. The yield of each products is defined by

yield (wt %) ) 100 × (mass of product)/ (mass of substrate) (1) The yield of gases from hydrocracking reaction is as follows:

yield of C1-C4 gases (wt %) ) 100 (yield of unreacted pyrenes + yield of hydrocracking products + yield of coke on the catalyst) (2) Calculation. Molecular dynamics (MD) calculations were carried out using Discover 3 (Molecular Simulations Inc.) for MD calculation and Insight II Vr.97 (Molecular Simulations Inc.) for graphics, installed on a Silicon Graphics O2 workstation.

Results and Discussion Hydrocracking Products from Py over Ni-HY Zeolite. Figure 1 shows the distribution of products by hydrocracking of Py using the Ni-HY zeolite. Approximately 50% of the Py was reacted before the temperature reached 386 °C, and 15% of n-hexane was cracked over the entire reaction period. Major products were hydrogenated pyrenes and cyclohexane. The amount of unreacted pyrene and hydrogenated pyrenes decreased with increasing reaction time, while cracking products, such as toluene, xylene, tetralin with C0-C3 alkyl substituents, acenaphthene with C0-C1 substituents, indene with C0-C2 substituents, and C1-C4 hydrocarbon gases, increased. After 30 min of reaction, approximately 70 wt % of the pyrene had been hydrocracked to mono- and diaromatics and gases. No 3-ring aromatics such as phenanthrene were observed. The yield of coke on the catalyst was approximately 3 wt % at a maximum. The yields of alkylpyrenes, as well as unreacted pyrene, are summarized in Table 2. The yield of unreacted Py rapidly decreased from 53 wt % at 0 min to 15 wt % at 30 min. Alkylated pyrenes with C1 and C2 substituents were minor products, and the yield of each group increased gradually to approximately 2 and 3 wt

Hydrocracking of Pyrenes over a Ni-Supported Y-Zeolite Catalyst

Figure 2. Product distributions by hydrocracking of dihydropyrene over Ni-HY zeolite. Table 2. Yields of Unreacted Pyrene, Alkylpyrenes, and Hydrogenation Products by Hydrocracking Reaction of Pyrene over Ni-HY Zeolite

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Figure 3. Product distributions by hydrocracking of hexahydropyrene over Ni-HY zeolite.

yield (wt %) alkylpyrenes hydrogenated pyrenes reaction time (min) Py C1-Py C2-Py unknown DPy TPy HPy unknown 0 5 10 15 30

53 42 34 31 15

0 1 2 2 2

0 1 3 3 3

0 0 3 3 3

13 13 10 10 5

3 2 2 1 1

4 4 2 2 1

2 1 1 1 1

Table 3. Yields of Unreacted Dihydropyrene, Alkylpyrenes, and Hydrogenation Products by Hydrocracking Reaction of Dihydropyrene over Ni-HY Zeolite yield (wt %) alkylpyrenes hydrogenated pyrenes reaction time (min) Py C1-Py C2-Py unknown DPy TPy HPy unknown 0 5 10 15 30

59 52 48 47 41

0 0 3 1 2

0 1 5 3 5

0 0 6 3 3

23 17 20 13 9

4 1 1 1 2

7 2 2 1 1

5 1 0 0 0

%, respectively, at 30 min. Alkyl fragments produced by cracking of the solvent can be attached to aromatic rings of pyrenes.6 Table 2 also show the yields of hydrogenated pyrenes. Dihydropyrene was the major hydrogenated product, the yield of which was 13 wt % at reaction time t ) 0 min. The yields of TPy and HPy were smaller than the yield of DPy. The yields of the hydrogenated pyrenes decreased with increasing reaction time. No octa- and perhydropyrenes were observed. Hydrocracking Products from DPy over Ni-HY Zeolite. Figure 2 shows the distribution of products by hydrocracking of DPy. The major products were alkylpyrenes, the yield of which was 60 wt % at the start of the reaction at 386 °C. The yields of tetralin with C0C3 alkyl substituents, acenaphthene with C0-C1 substituents, and indene with C0-C2 substituents in the product increased with increasing reaction time. The yield of C1-C4 hydrocarbons in the product increased to 30 wt % at t ) 30 min. The yields of pyrenes produced by hydrocracking of DPy over the Ni-HY zeolite are summarized in Table 3. At the start of the reaction, a major portion of DPy was converted to Py with a yield of 59 wt %. With increasing reaction time, however, the yield of Py decreased to 41 wt % at t ) 30 min. Alkylated pyrenes with C1-C2 groups were minor products, and their

Figure 4. Yields of hydrogenated pyrenes by hydrogenation of pyrene over Ni-KY zeolite. Table 4. Yields of Unreacted Hexahydropyrene, Alkylpyrenes, and Hydrogenation Products by Hydrocracking Reaction of Hexahydropyrene over Ni-HY Zeolite yield (wt %) alkylpyrenes hydrogenated pyrenes reaction time (min) Py C1-Py C2-Py unknown DPy TPy HPy unknown 0 5 10 15 30

26 38 30 24 8

1 1 2 2 1

1 1 3 4 2

1 6 3 4 2

12 12 9 7 3

11 2 1 2 1

17 3 2 1 1

8 1 1 1 0

yields increased with reaction time. Table 3 also show the yields of DPy, TPy, and HPy. The yield of unreacted DPy was 23 wt % at t ) 0 min and decreased to 9 wt % at t ) 30 min. No octa- and perhydropyrenes were observed. The yields of TPy and HPy were small and decreased with reaction time. Hydrocracking Products from HPy over Ni-HY Zeolite. Figure 3 illustrates the distribution of products by hydrocracking of HPy over the Ni-HY zeolite. Hexahydropyrene was rapidly converted to smaller molecules, while the total yield of 1- and 2-ring aromatics and cyclohexane initially increased and then decreased with increasing reaction time. The total yield of gases increased and reached 57 wt % at t ) 30 min.

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Figure 5. Hydrocracking reaction pathway of pyrene over metal supported Y-zeolite.

Figure 6. First-order rate plots for the hydrocracking of pyrenes.

Figure 7. Intermediate formation for (i) hexahydropyrene and (ii) acenaphthene. Ball and stick model indicates a methane placed at carbon which is to be hydrocracked.

Ring-opening reaction of pyrene was thus enhanced by the partial hydrogenation of the aromatic rings in pyrene. Pyrene and alkylpyrenes were also produced by dehydrogenation of HPy, and the total yield of alkylpyrenes initially increased and then decreased to 11 wt % at t ) 30 min. The yields of pyrene and alkylpyrenes by hydrocracking of HPy over the Ni-HY zeolite are summarized in Table 4. The yield of pyrene was the highest, 38 wt %, at t ) 5 min and was 8 wt % at t ) 30 min. Alkylpyrenes with C1-C2 groups were minor products and initially increased and then decreased with increasing reaction time. The maximum yields of alkylpyrenes were at t ) 15-20 min, after the yield of pyrene had reached the maximum. Table 4 also show the yields of hydrogenated pyrenes by hydrocracking of HPy. The yield of DPy initially increased and then decreased to 3 wt % at t ) 30 min. The yields of TPy and HPy decreased rapidly

in the early stage of the reaction. No octa- and perhydropyrenes were observed. This suggests that HPy was cracked directly to light hydrocarbon gases by ringopening reactions over the Ni-HY zeolite. Hydrogenation Products from Py over Ni-KY Zeolite. Figure 4 shows the yields of products by hydrogenation of Py over the Ni-KY catalyst. Since the acidity of the HY zeolite was eliminated by ion-exchange treatment, the Ni-KY zeolite possessed only hydrogenation reactivity. Pyrene was converted to DPy, the yield of which increased with reaction time, reaching approximately 22 wt % at t ) 30 min. More extensively hydrogenated products, TPy and HPy, were minor but increased with reaction time. Yields of alkylpyrenes and gases were negligible. These results differ greatly from those obtained using the Ni-HY zeolite catalyst. Pathways and Kinetics for Hydrocracking of Pys. Figure 5 shows the reaction network for the hydrocracking of Py over the Ni-HY zeolite catalyst. Py is hydrogenated to DPy and to HPy. The dehydrogenation of DPy is preferential to the hydrogenation of Py, since a considerable amount of Py is produced from DPy before the temperature reaches 386 °C as shown in Figure 2. Dehydrogenation of HPy to DPy and Py also proceeds as is shown in Figure 3. When HPy is fed as the starting material, a maximum concentration of Py appears as shown in Table 4. This provides support for a reversible and consecutive reaction scheme. Hexahydropyrene is irreversibly hydrocracked to 1- and 2-ring aromatics, which are then further cracked to C1-C4 hydrocarbon gases irreversibly. These irreversible steps shift the reversible hydrogenation-dehydrogenation equilibria in the direction of hydrocracking and serves to decrease the yield of HPy. The aromatic rings must be opened after Py is hydrogenated to HPy via DPy, since hydrogenated compounds such as octa- and perhydropyrenes are not found under the reaction conditions employed. Figure 6 shows first-order reaction rate plots for the hydrocracking of Py, DPy, and HPy over the Ni-HY zeolite, as well as for hydrogenation of Py over the NiKY catalyst. The data are expressed by

ln [(yield of unreacted substrate)/100] ) - k(i)t (3) where k(i) is the overall reaction rate constant of substance i over the Ni-HY zeolite, and k′(Py) is the overall reaction rate constant of Py over the Ni-KY zeolite. The overall reaction rate constant of Py, k(Py), was 1.2 × 10-2 min-1, while that of DPy, k(DPy), was 6.2 × 10-3 min-1, over the Ni-HY zeolite. The overall reaction rate constant of HPy was 1.4 × 10-2 min-1. Thus, the Ni-HY zeolite catalyst possessed multifunctional activities of hydrogenation, dehydrogenation, and cracking. The overall cracking rate over the Ni-HY zeolite appears to be controlled by the dehydrogenation of DPy.

Hydrocracking of Pyrenes over a Ni-Supported Y-Zeolite Catalyst

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Figure 8. Images of adsorption and diffusion over Y-zeolite based on the MD calculation.

Metal-loaded HY-zeolites possess hydrocracking activity due to protons, as well as hydrogenation activity due to metal sulfides which are dispersed on the zeolite surface. The two-step Ni-loading procedure developed in the present study permits the impregnation of a large amount of Ni. The Ni content reached 7.2-9.0 wt %, while only a few wt % of metal ions was incorporated onto the zeolite by conventional ion-exchange procedures.6 These combined activities were effective in the ring-opening reaction. Coke precursors were extensively hydrogenated before they were strongly adsorbed to the acid sites. In our earlier study,6 it was also found that sulfide particles were located on the outer surface of the Ni-HY zeolite catalyst as evidenced by TEM observation.

Since the Ni-KY zeolite showed no cracking activity, DPy was the major product derived from Py. The overall reaction rate constant k′(Py) calculated from Figure 6 was 5.4 × 10-3 min-1. The longest time was required for the reaction to reach equilibrium, as shown in Figure 4. Simulation. In order to simulate the hydrocracking of HPy over the Y-zeolite, a formula, which represents the intermediate in the hydrocracking of HPy to acenaphthene, was composed by combining acenaphthene and four methane molecules. As shown in Figure 7 (i), three methanes are placed at the locations of three carbons in the saturated ring of HPy, and one methane is placed at the location of the remote carbon in the other saturated ring of HPy. These methane substituents are

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Figure 9. Time-dependent changes in overall potential energy for a Y-zeolite cell containing reactants and products.

not chemically bonded to acenaphthene. The intermediate is then placed outside, or inside, of the supercage of the Y-zeolite, which is constructed from the sodalite structure of eight units and is enclosed in a periodic boundary cell (50 × 25 × 25 Å). The temperature of the system is assumed to be 660 K. CVFF method was used as the force field parameter, and molecular dynamics of 10 fs step was calculated. The position of the intermediate on the zeolite is assumed to be the same as the optimized position of HPy. The intermediate is then hydrocracked at an infinite rate, producing acenaphthene and four methanes, and the product molecules begin to migrate in the periodic boundary cell. The same calculation is carried out using another formula, which simulates the intermediate for the hydrocracking of acenaphthene to naphthalene and two methanes. As shown in Figure 7 (ii), two methanes are located at positions of two carbons in the saturated ring of acenaphthene. The intermediate is placed outside, or inside, the Y-zeolite at the position which is optimized for acenaphthene. The products begin to migrate just after the intermediate is hydrocracked. Figure 8 shows the simulation for the diffusion of hydrocracking products from HPy. The zeolite skeleton is shown in red, and the products are indicated by the space-filling model. Carbon atoms are green, and hydrogen atoms are white. The pore size of the Y-zeolite is 0.7 nm, and the size of the aromatic molecules, as estimated by molecular orbital calculation, is 0.4990.681 nm for naphthalene, 0.65-0.681 nm for acenaphthene, and 0.685-0.929 nm for HPy. Thus, HPy cannot enter the pore and is adsorbed to the surface as shown in Figure 8a. The intermediate, which is assumed for the hydrocracking of HPy to acenaphthene, can enter the pore of the Y-zeolite. Thus, two cases are assumed: products which remains outside of the pore as shown in Figure 8, b and c (referred to as outside model), and inside of the pore, as shown in Figure 8, d and e (referred to as inside model). Figure 8b shows the image after the intermediate is hydrocracked to acenaphthene and four methanes. The produced acenaphthene remains outside

Table 5. Average Potential Energy of the Y-Zeolite Cell Containing Hydrocracking Products from Hexahydropyrene location of acenaphthene and naphthalene and polyaromatic compound 4 methanes (kcal/mol) 6 methanes (kcal/mol) outside inside

3210 3301

3204 3349

of the pore while methane diffuses freely in the micropore. Figure 8c shows the image when acenaphthene is hydrocracked to naphthalene and two methanes on the surface of the zeolite. Six methanes are produced altogether. When the intermediate, which is assumed for the hydrocracking of HPy, is placed in the pore, acenaphthene is formed as shown in Figure 8d. Acenaphthene remains inside the pore and is further hydrocracked to naphthalene and methane, as shown in Figure 8e. Figure 9 shows changes in overall potential energy for the zeolite, which contains hydrocracking products inside or outside the pore, as a function of calculation time. The data shown in Figure 9, b, c, d, and e correspond to the situations shown in Figure 8, b, c, d, and e, respectively. In this simulation, products change their locations in the periodic boundary cell. The overall potential energy of the cell, however, remains unchanged. As listed in Table 5, the average overall potential energy of the outside model is lower than that of the inside model. This suggests that the diffusion of 2- and 3-ring aromatics into the pore is not easy and that Py may be converted to acenaphthene via cracking of HPy on the outer surface of the metal-supported zeolite. Conclusions 1. Pyrene was cracked into 1- and 2-ring aromatics by partial hydrogenation of aromatic rings. The formation of acenaphthenes indicates that 3-ring aromatics such as phenanthrene were not major intermediates in the hydrocracking pathways of Py. Dihydropyrene was the major hydrogenated product and was further hydrogenated to HPy via TPy. No octa- and perhydropyrenes were observed.

Hydrocracking of Pyrenes over a Ni-Supported Y-Zeolite Catalyst

2. Dihydropyrene was hydrogenated to HPy, but in very low yield. Dehydrogenation reactions of hydrogenated pyrenes occurred even under a hydrogen atmosphere of 5 MPa. Hydrogenation of Py to DPy appeared to be the bottleneck in the pathways of hydrocracking of Py. 3. Neither octa- and perhydropyrenes were detected. This suggests that HPy was directly converted to 1- and 2-ring aromatics. Aromatic rings were opened by acid catalysis, and no gases were produced over the Ni-KY zeolite catalyst.

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4. Aromatics produced in hydrocracking of HPy were adsorbed outside of the pore, or transferred into the pore. The overall potential energy of the inside model was higher than that of the outside model. This suggests that diffusion of the polycyclic aromatic products into the zeolite pore is not easy and that these molecules are cracked on the surface of the zeolite rather than in the pore. EF9801928