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Changes in Desulfurization Reactivity of 4,6-Dimethyldibenzothiophene by Skeletal Isomerization Using a Ni-Supported Y-Type Zeolite Takaaki Isoda, Yoyoto Takase, Katsuki Kusakabe, and Shigeharu Morooka* Department of Materials Physics and Chemistry, Graduate School of Engineering, Kyushu University, Fukuoka 812-8581, Japan Received February 8, 1999
Skeletal isomerization of 4,6-dimethyldibenzothiophene (4,6-DMDBT) was performed at 270 °C for 1 h at a hydrogen pressure of 2.5 MPa using a Ni-supported Y-type zeolite catalyst. Alkyldibenzothiophenes (alkyl-DBTs), which were produced by methylmigration and transalkylation, were further desulfurized at 255-300 °C for 2-120 min at a hydrogen pressure of 2.5 MPa using a CoMo/Al2O3 catalyst. The produced alkyl-DBTs, which carried 1-3 methyl groups, could be classified into three groups on the basis of activation energy for desulfurization. Group I included 2,8-DMDBT and C1-DBT, which carried no methyl groups at either the 4- or 6-position. Group III included unreacted 4,6-DMDBT, and group II contained other alkyl-DBTs with 1-3 methyl groups. No alkyl-DBTs with 4 methyl groups were found in the products of 4,6-DMDBT. The steric hindrance of the methyl groups on 4,6-DMDBT was relieved by methylmigration and transalkylation, and, as a result, the desulfurization activation energy, 37.2 kcal/mol, for 4,6DMDBT was decreased to 24-33 kcal/mol for alkyl-DBTs in group II and to 23 kcal/mol for alkylDBTs in group I. Desulfurization reactivity of the alkyl-DBTs was also examined on the basis of molecular orbital calculation using the WinMOPAC program. The cross sectional area of the Sσn orbital on the sulfur atom, which was in contact with the catalyst surface, was 0.12 nm2 for 4,6-DMDBT, 0.70 nm2 for 4-MDBT, and 1.27 nm2 for DBT. Thus the desulfurization reactivity increases with increasing overlapping area of the Sσn orbital with the catalyst active site. A wider cross sectional area was achieved by migration of methyl groups using the zeolite catalyst.
Introduction Exhaustive desulfurization of gas oil constitutes is an environmental necessity in forms of its use in urban areas.1 To reduce the sulfur concentration in gas oil from 500 ppm to less than 100 ppm, desulfurization of 4-methyldibenzothiophene (4-MDBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) is required. LamureMeille et al.2 investigated the HDS reactivity of dibenzothiophene (DBT) and 4-MDBT over a conventional NiMo or CoMo/Al2O3 catalyst. The reaction for this process involved the removal of the sulfur atom, i.e., a direct desulfurization reaction and the removal of the sulfur atom after hydrogenation of benzene ring, i.e., a hydrodesulfurization reaction.3 However, the exhaustive desulfurization of these sulfur species using the conventional NiMo or CoMo/Al2O3 catalyst cannot be achieved under current refinery conditions, because the steric hindrance of methyl groups at the 4- and 6-positions on the dibenzothiophene skeleton prohibits access of the molecule to the catalyst active site.4 * Author to whom correspondence should be addressed. Telephone: +81-92-642-3551. Fax: +81-92-651-5606. E-mail: smorotcf@ mbox.nc.kyushu-u.ac.jp. (1) Octane Week 1997, 8 (15), Apr.14. (2) Lamure-Meille, V.; Schulz, E.; Lemaire, M.; Vrinat, M. Appl. Catal. 1995, 131, 143. (3) Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30, 2021. (4) Kabe, T.; Ishihara, A.; Zhang, Q.; Tsutsui, H.; Tajima, H. J. Jpn. Pet. Inst. 1993, 36, 467.
In an earlier study, we reported on the desulfurization reactivity of 4-MDBT and 4,6-DMDBT in decalin over a NiMo/Al2O3 catalyst, which showed a higher desulfurization activity for 4,6-DMDBT than a CoMo/Al2O3 catalyst.5 The major reaction pathway for 4,6-DMDBT involved both direct-desulfurization and desulfurization via hydrogenation of an aromatic ring, whereas 4-MDBT was desulfurized via the direct desulfurization route. The desulfurization rate for 4,6-DMDBT via hydrogenation was 100-fold higher than that via direct desulfurization. This suggests that the rate-determining step for desulfurization of 4,6-DMDBT, which is the most refractory sulfur species, over a conventional NiMo or CoMo/Al2O3 catalyst is the hydrogenation of the aromatic ring in the substrate, and that the desulfurization reaction is enhanced by eliminating the steric hindrance due to methyl groups. To eliminate the steric hindrance by methyl groups, Landau et al.6 proposed a desulfurization process via cracking of 4,6-DMDBT at elevated temperatures using a CoMo/Al2O3 catalyst including a Y-zeolite. Wada et al.7 reported that a Ni- and Pd-supported Y-type zeolite was highly active for the hydrocracking of 4,6-DMDBT. (5) Isoda, T.; Ma, X.; Mochida, I. J. Jpn. Pet. Inst. 1994, 37, 368. (6) Landau, M. V.; Berger, D.; Herskowitz, M. J. Catal. 1996, 158, 236. (7) Wada, T.; Murata, S.; Nomura, M. Prepr. Pap.- Am. Chem. Soc., Div. Fuel. Chem. 1995, 40, 978.
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When a mixture of Ni supported Y-type zeolite and CoMo/Al2O3 catalysts was used, as we reported,8,9 the desulfurization of 4,6-DMDBT proceeded via methylmigration and transalkylation over the zeolite catalyst at 270 °C, followed by desulfurization over the CoMo/ Al2O3 catalyst. The major products derived from 4,6DMDBT over the zeolite were 4-MDBT and 3,6DMDBT, suggesting that the methyl groups of 4,6DMDBT were removed or migrated from the 4-position to the 3-position. The second major products were DBTs, containing two or three methyl groups, although tetramethyl-DBTs (C4-DBTs) were not produced in this reaction. These alkyl-DBTs exhibited a wide variety of desulfurization reactivity over a conventional CoMo/ Al2O3 catalyst, and the rate constant was in the range of 4 × 10-3 to 2.5 × 10-2 min-1. Molecular orbital calculation showed that a large electron density existed on the HOMO (the highest occupied molecular orbital), as well as on the HOMO-5 (the orbital five levels lower than the HOMO), for the sulfur atom of alkyl-DBTs.10 A good correlation was found between electron densities on the Sσn orbital at the HOMO-5 of the sulfur atom and the rate constant of the alkyl-DBTs. This indicates that the Sσn orbital is directly and strongly related to the desulfurization reactivity of alkyl-DBTs. In the present study, the migration of methyl groups of 4,6-DMDBT using a Ni-supported Y-zeolite was carried out at 270 °C for 1 h at a hydrogen pressure of 2.5 MPa, and the products were further desulfurized over a CoMo/Al2O3 catalyst at 255-300 °C. The desulfurization reactivity of alkyl-DBTs was evaluated on the basis of the activation energy. To investigate the role of methylmigration for the desulfurization of 4,6-DMDBT, the molecular orbitals of alkyl-DBTs were calculated using the WinMOPAC program and were related to the cross sectional area of the Sσn orbital.
Isoda et al. Table 1. Properties of Catalysts
catalyst Ni-HY CoMo/Al2O3 a
metal surface amount of content area adsorbed NH3 [wt %] Si/2Al [m2/g] [mmol/g] 9.2a 19.3b
16.0
693 268
support
0.57 γ-alumina
NiO. CoO:MoO3 ) 4.4:14.9. b
Experimental Section Chemicals and Catalyst. 4,6-DMDBT was synthesized by procedures described in the literature.11 A CoMo/Al2O3 catalyst was prepared by a conventional impregnation procedure. A Ni-loaded HY zeolite (hereafter referred to as Ni-HY zeolite) was prepared as follows:12 A 10 g portion of USY zeolite was suspended in 100 mL of water at 75 °C, and 30 mL 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 0.5 h at 75 °C, 60 mL of an aqueous solution of 12 N HNO3 and Ni(NO3)2 (1.0 mol/L of Ni) was added with stirring. The pH of the slurry was maintained at pH 4.0 via the addition of 5% aqueous ammonia. After stirring for 0.5 h, the catalyst was filtered, washed with water, and dried. The properties of the catalysts are summarized in Table 1. The catalysts were presulfided at 360 °C for 2 h in a stream of H2S (5 vol %, H2 carrier) at atmospheric pressure prior to use. Two-Stage Reaction. Figure 1 shows the two-stage reaction procedure. The first-stage reaction was carried out in a 50-mL batch-autoclave equipped with a magnetic stirrer, rotating at 500 rpm, at 270 °C for 1 h under an initial H2 (8) Isoda, T.; Nagao, S.; Ma, X.; Korai, Y.; Mochida, I. Energy Fuels 1996, 10, 1078. (9) Isoda, T.; Takase, Y.; Takagi, H.; Kusakabe, K.; Morooka, S. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1998, 43, 547. (10) Isoda, T.; Takase, Y.; Kusakabe, K.; Morooka, S. Prepr. Pap.s Am. Chem. Soc., Div. Pet. Chem. 1998, 43, 575. (11) Grendil, R.; Lucken, E. A. C. J. Am. Chem. Soc. 1965, 87, 213. (12) Isoda, T.; Kusakabe, K.; Morooka, S. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1997, 42, 420.
Figure 1. Procedure for two-stage desulfurization reaction. pressure of 2.5 MPa, using 0.3 g of the powdered Ni-HY catalyst and 0.3 g of 4,6-DMDBT dissolved in 5 g of n-decane. After the catalyst was removed by filtration, the solution was diluted 10-fold with n-decane. In the second-stage reaction, alkyl-DBTs were desulfurized using 0.5 g of the CoMo/Al2O3 catalyst at 270 °C for 2-120 min at an initial H2 pressure of 2.5 MPa. The CoMo/Al2O3 catalyst was in the shape of columnar pellets, the size of which was 1 mm in diameter and 5 mm in length. To compare the activation energy, dibenzothiophene was also desulfurized under the above reaction conditions. The heating program was the same as reported previously.13 The final temperature was maintained for 2-120 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 255-300 °C and that it ended at the time of quenching. The H2 pressure increased from the initial value of 2.5 MPa to 4 MPa during the heating step and remained (13) Isoda, T.; Kusakabe, K.; Morooka, S.; Mochida, I. Energy Fuels 1998, 12, 493.
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Table 2. First-Order Reaction Rate Constants and Activation Energies for the Hydrodesulfurization of Alkyldibenzothiophenes substrate 4-MDBT C1-DBT 4,6-DMDBT 3,6-DMDBT 2,8-DMDBT C3-DBT #1 C3-DBT #2 C3-DBT #3 C3-DBT #4 2,3,4-TMDBT (DBT)a a
estimated structure 2-MDBT
2,6,7-TMDBT 2,4,7-TMDBT 3,4,7-TMDBT 2,3,6-TMDBT
rate constant × 105 [s-1] 255 °C
270 °C
285 °C
300 °C
activation energy [kcal/mol]
4.4 20.7 2.3 5.7 22.3 3.0 8.1 4.3 5.4 9.3 6.2
11.4 47.8 7.2 15.0 54.8 10.9 29.5 12.2 17.0 30.3 17.0
23.0 63.1 17.6 29.9 81.9 19.5 45.5 25.1 28.7 35.2 17.0
55.4 128.0 37.7 66.8 133.0 37.2 96.5 36.8 32.8 86.1 35.0
33.3 23.0 37.2 32.5 23.2 32.6 31.6 29.0 24.6 27.4 21.6
Other experiment results, reaction conditions; 2.5 MPa H2, 255-300 °C.
Figure 3. Classification based on the Arrhenius plot for desulfurization of alkyl-DBTs over the CoMo/Al2O3 catalyst.
Results and Discussion
Figure 2. First-order reaction plots of alkyl-DBTs over the CoMo/Al2O3 catalyst. then constant for the duration of the reaction. The catalytic activity of the zeolite was changed slightly during the reaction, while that of the metal-loaded zeolites remained unchanged under the reaction conditions utilized herein. The product was qualitatively and quantitatively analyzed using a GC-Ms and a GC-FPD, equipped with a silicone capillary column, as reported previously.8,9 Coke deposition on the spent catalyst was determined by elemental analysis. Molecular Orbital Calculation. Molecular orbital (MO) calculations for the alkyl-DBTs were carried out using WinMOPAC Vr.1 (Fujitsu Ltd.), based on a PM3 Hamiltonian method.14 This procedure has been reported previously.10
Skeletal Isomerization of 4,6-DMDBT over the Ni-HY Zeolite. Table 2 shows the distribution of alkylDBTs produced from the transalkylation of 4,6-DMDBT using the Ni-HY zeolite. The structures of the alkylDBTs were determined as reported previously.10 The major products were 4-MDBT and 3,6-DMDBT, indicating that the methyl groups of 4,6-DMDBT were removed or migrated from the 4-position to the 3-position. AlkylDBTs which contained three methyl groups (C3-DBTs) were also produced in this reaction. Alkyl fragments produced by cracking of the solvent could be attached to aromatic rings in the substrate.9 However, no tetramethyl-DBTs were found under the present reaction conditions. Thus the Ni-HY selectively enhanced the transalkylation of 4,6-DMDBT but induced no detectable desulfurization. (14) Stewart, J. J. P. Comput. Chem. 1989, 10, 221.
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Figure 4. Desulfurization pathways of 4,6-DMDBT with methylmigration over the Y-type zeolite catalyst (first stage) and the CoMo/Al2O3 catalyst (second stage).
The two-step loading procedure used in the present study enabled the use of Ni contents as high as 7.29.0 wt %, while only a few wt % of metal ions was incorporated onto the zeolite by conventional ionexchange procedures.13 Sulfide particles were located on the outer surface of the Ni-HY zeolite catalyst as evidenced by TEM observation.13 Coke precursors were extensively hydrogenated prior to being strongly adsorbed to the acid sites. This is consistent with the results herein. Metal-loaded HY-zeolites exhibit hydrocracking activity for paraffins and polyaromatics in a vacuum gas oil at reaction temperatures of 300-380 °C.13 The cracking is proton-catalyzed,13 while no hydrocracking reaction for 4,6-DMDBT is found in this temperature range. The low reaction temperature, 270 °C, permits selective skeletal isomerization of 4,6-DMDBT. The pore size of the Y-zeolite is 0.7 nm, while the size of 4-MDBT and 4,6-DMDBT, estimated by molecular orbital calculation, is 0.59 nm × 0.89 nm. Thus, the diffusion of these substrates through the zeolite pores may be difficult. This suggests that the transalkylation reaction may proceed on the outer surface of the zeolite particles. Desulfurization of Sulfur Species over CoMo/ Al2O3 Catalyst. Figure 2 shows the first-order reaction plots for 4,6-DMDBT, 4-MDBT, 2,8-DMDBT, and 2,3,4TMDBT at the second-stage reaction over the CoMo/ Al2O3 catalyst at 255 (1) and 270 °C (2). Desulfurization of alkyl-DBTs, which were produced from the transalkylation of 4,6-DMDBT, proceeded via a first-order rate reaction mechanism for all the sulfur species examined, and the desulfurization rates were strongly dependent on the molecular structures. As shown in Table 2, the rate constant increased with increasing reaction temperature, and the rate constant of 4,6DMDBT, which had been unreacted by the first stage reaction using the Ni-HY zeolite, was 2.3 × 10-5 s-1 at 255 °C and 37.7 × 10-5 s-1 at 300 °C. 2,8-DMDBT showed the largest rate constant, 22.3 × 10-5 s-1 at 255 °C and 133 × 10-5 s-1 at 300 °C, and 4-MDBT showed a rate constant, 4.4 × 10-5 s-1 at 255 °C and 55.4 × 10-5 s-1 at 300 °C. The reactivity of 2,3,4-trimethyldibenzothiophene (2,3,4-TMDBT) was higher than that of 4-MDBT and 4,6-DMDBT, and was 9.3 × 10-5 s-1 at 255 °C and 86.1 × 10-5 s-1 at 300 °C. C3-DBTs #1-#4, which carried three methyl groups, showed a slightly higher rate constant than that of 4,6-DMDBT. This indicates that the desulfurization reactivity was
Figure 5. Relationship between activation energy, Ea, and relative rate constant for desulfurization of alkyl-DBTs at 270 °C, k/k4,6-DMDBT.
accelerated by the combined effect of methylmigration and transalkylation. Figure 3 shows the Arrhenius plots for the desulfurization of alkyl-DBTs over the CoMo/Al2O3 catalyst. The alkyl-DBTs are classified into three groups on the basis of the activation energy, as well as the reaction rate shown in Table 2. Group I contains 2,8-DMDBT, which carries no methyl groups at the 4- or 6-position, and C1-DBT, which is estimated to be 2-MDBT. Group II contains 4-MDBT and other alkyl-DBTs, which carry three methyl groups. Sulfur species, such as 4-MDBT, 3,6-DMDBT, and 2,3,4-TMDBT, as well as unidentified C3-DBT #1-#4, are included in group II. Group III contains unreacted 4,6-DMDBT, which shows the highest desulfurization activation energy, 37.2 kcal/mol. This result indicates that the HDS reactivity of the alkylDBTs is related to the presence of methyl groups neighboring the sulfur atom, i.e., 4- and/or 6-positions. It should be noted that the activation energy is decreased from 37.2 kcal/mol for group III to 24-33 kcal/ mol for group II and to 23 kcal/mol for group I. This is consistent with the result of Kabe et al.,15 showing 40 kcal/mol for 4,6-DMDBT, 31 kcal/mol for 4-MDBT, and 24 kcal/mol for DBT. Figure 4 shows the pathways for the desulfurization of 4,6-DMDBT with accompanying methylmigration. (15) Kabe, T.; Ishihara, A.; Zang, Q. Appl. Catal., A 1993, 97, 1.
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Figure 6. Cross sectional area of the Sσn orbital on the sulfur atom of DBT, 4-MDBT, and 4,6-DMDBT. a, sulfur atom; b, cross section of the Sσn orbital on the sulfur atom; c, methyl group; d, catalyst plane.
Methyl groups on 4,6-DMDBT are removed or migrate during the first stage reaction at Bro¨nsted acid sites on the zeolite. The desulfurization of alkyl-DBTs, which are derived from 4,6-DMDBT, in the second-stage reaction proceeds via three pathways with different activation energies and gives rise to biphenyl and H2S as the desulfurization products. The differences in HDS reactivity and activation energy among the three groups were obvious in the range of 255-270 °C, as shown in Figure 3. In our earlier studies,9,10 the HDS reactivity of alkyl-DBTs over a CoMo/Al2O3 catalyst at 270 °C was clearly classified into four groups on the basis of firstorder reaction rates, which is consistent with the result in the present study. However, the difference in the activation energies was not clear at a reaction temperature of 300 °C. This suggests that the desulfurization schemes of alkyl-DBTs via methylmigration, shown in Figure 4, prevail in the range of 255-270 °C. Figure 5 shows the relationship between activation energy and relative rate constant for the desulfurization of alkyl-DBTs at 270 °C. The rate constant of 4,6DMDBT is taken as the reference. As a result of methylmigration, the activation energy decreases, while the relative rate constant increases. However, the relative rate constant of 2,8-DMDBT and 2,3,4-TMDBT is higher than those of DBT. Houalla et al.16 reported that desulfurization rates of DBTs were in the order of (16) Houalla, M.; Broderick, D. H.; Spare, A. V.; Nag, N. K.; De Beed, V. H.; Gates, B. C.; Kwart, H. J. Catal. 1980, 61, 523.
4,6-DMDBT < 4-MDBT < DBT < 3,7-DMDBT < 1,9DMDBT < 2,8-DMDBT. This order, which coincides with that obtained in the present study, suggests that the HDS reactivity of DBTs is affected by the electron donation from methyl groups to the sulfur atom. This effect strongly depends on the position of the methyl group on the benzene ring. The steric hindrance of methyl groups on 4,6-DMDBT is thus released by the skeletal isomerization using the zeolite catalyst at the first-stage reaction. Simulation. It has been reported that the desulfurization of thiophene proceeds when the lone pair electrons on the sulfur atom are coordinated to the unsaturated MoS2 edge plane.17 Thus, the energy of the lowest unoccupied molecular orbital (LUMO) does not contribute to the desulfurization reaction. We calculated energy levels of the molecular orbitals, as well as distributions of electron density, for the case of the 4,6DMDBT molecule.10 The energy level of the LUMO and HOMO is approximately -1 eV and -8.5 eV, respectively, irrespective of the structures of the alkyl-DBTs examined. A large electron density exists on both HOMO and HOMO-5 of the sulfur atom, forming Sp and Sσn bondings, respectively. Figure 6 shows the electron density distributions on the sulfur atoms of DBT, 4-MDBT, and 4,6-DMDBT. The HOMO-5 is described by the wire-frame model. The (17) Lipsh, J. M. J. G.; Schuit, G. C. A. J. Catal. 1969, 15, 179.
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produced from coexisting benzothiophene (BT).19 However, additive dibenzothiophene (DBT) did not affect the reactivity of 4,6-DMDBT. In this case, the HDS reactivity was in the order of 4,6-DMDBT < DBT < BT. Thus, coexisting species including alkyl-DBTs, the reactivity of which is low, could not be greatly involved in the HDS reaction of 4,6-DMDBT. Conclusions
Figure 7. Relationship between activation energy, Ea, and relative cross sectional area of the Sσn orbital on the sulfur atom of alkyl-DBTs, RS.
substrates approach the catalyst, keeping the DBT plane perpendicular to the surface plane of the catalyst. The cross sectional area of the Sσn orbital is then calculated from the intersection with the catalyst plane, to which the most projecting hydrogen atoms of each substrate contact. Figure 7 shows the relationships between the relative cross sectional area of the Sσn orbital on the sulfur atom and the activation energy for desulfurization of alkyl-DBTs. The cross sectional area of the Sσn orbital is approximately 0.12 nm2 for 4,6DMDBT, 0.70 nm2 for 4-MDBT, and 1.27 nm2 for DBT. The values of the cross sectional area for the other alkylDBTs are also in this range, because the accessibility of sulfur atom to catalyst plane is just related to the steric hindrance of methyl groups located on 4,6positions. The activation energy for desulfurization decreases with increasing relative cross sectional area. Thus, methylmigration of 4,6-DMDBT effectively decreases the steric hindrance in the neighborhood of the sulfur atom. In this simulation, the effects of adsorption and inhibition, which may play important roles in HDS for commercially available feedstocks, are not considered. Girgis et al.3 reported that HDS reactivity of sulfur species was hindered by sulfur-, oxygen-, and nitrogencontaining species. In an earlier study,18 we reported that the HDS reaction of 4,6-DMDBT was greatly decreased by hydrogen sulfide, which was added or
(1) The major products arising from the skeletal isomerization of 4,6-DMDBT at 270 °C were 4-MDBT and 3,6-DMDBT. Thus, the methyl group on the 4-position of 4,6-DMDBT was removed or migrated to the 3-position. No alkyl-DBTs with four methyl groups were formed under the present experimental conditions. (2) Alkyl-DBTs were classified into three groups on the basis of desulfurization activation energy. Groups I and II included C1-C 3-DBTs, the activation energy of which was 23 kcal/mol for group I and was 24-33 kcal/ mol for group II. Group I contained 2,8-DMDBT and C1DBT, which carried no methyl groups at the 4- and/or 6-positions. Other alkyl-DBTs with 1-3 methyl groups were assigned to group II. Unreacted 4,6-DMDBT was included in group III, which showed the highest desulfurization activation energy, 37.2 kcal/mol. (3) The activation energy of alkyl-DBTs was significantly decreased by the methylmigration of 4,6-DMDBT. The steric hindrance of methyl groups on 4,6-DMDBT was thus relieved by the skeletal isomerization using the zeolite catalyst at the first-stage reaction. (4) The cross sectional area of the Sσn orbital was approximately 0.12 nm2 for 4,6-DMDBT, 0.70 nm2 for 4-MDBT, and 1.27 nm2 for DBT. The other alkyl-DBTs exhibited cross sectional areas in this range. The activation energy for desulfurization decreased with increasing overlapping of the Sσn orbital with the catalyst surface. Acknowledgment. Dr. Jerzy M. Rudziski and Ms. Mayumi Matsushita of Fujitsu Kyushu System Engineering Ltd., Japan, supported the computer software used in this study. Useful discussion with Dr. Keiichiro Samejima of Fujitsu Ltd., Japan, is deeply acknowledged. EF990018Z (18) Isoda, T.; Nagao, S.; Ma, X.; Korai, Y.; Mochida, I. Energy Fuels 1996, 10, 487. (19) Isoda, T.; Ma, X.; Nagao, S.; Mochida, I. J. Jpn. Pet. Inst. 1995, 38, 25.
