Structures and Energetics of the Methylation of 2-Methylnaphthalene

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J. Phys. Chem. C 2010, 114, 21611–21617

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Structures and Energetics of the Methylation of 2-Methylnaphthalene with Methanol over H-BEA Zeolite Karan Bobuatong,†,‡,§,⊥ Michael Probst,¶ and Jumras Limtrakul*,†,‡,§,⊥ Laboratory for Computational and Applied Chemistry, Department of Chemistry, Faculty of Science, Kasetsart UniVersity, Bangkok 10900, Thailand, Center of Nanotechnology, Kasetsart UniVersity Research and DeVelopment Institute, Kasetsart UniVersity, Bangkok 10900, Thailand, NANOTEC Center of Nanotechnology, National Nanotechnology Center, Kasetsart UniVersity, Bangkok 10900, Thailand, Center for AdVanced Studies in Nanotechnology and Its Applications in Chemical, Food and Agricultural Industries, Kasetsart UniVersity, Bangkok 10900, Thailand, and Institute of Ion Physics and Applied Physics, UniVersity of Innsbruck, A-6020 Innsbruck, Austria ReceiVed: September 8, 2010; ReVised Manuscript ReceiVed: October 24, 2010

The methylation of 2-methylnaphthalene (2-MN) with methanol to the 2,6 (2,6-DMN) and 2,7 (2,7-DMN) dimethylnaphthalenes catalyzed over nanoporous BEA zeolite has been investigated quantum chemically using the M06-2X density functional. The catalytic cycle consists of three elementary steps: (1) formation of a methoxy species from methanol that is bound to a zeolite oxygen atom, (2) methylation of 2-MN to DMN with methoxy leading to naphthalynic carbocations, and (3) formation of DMN by proton back-donation from naphthalynic carbocations. The reaction profiles are similar for both the 2,6 and the 2,7 isomer and are in agreement with the experimental observation that they are produced in equal amounts on acidic BEA zeolite. A possible side reaction, the formation of dimethyl ether via the self-activation of methanol, is also discussed. The stability of the intermediates inside the pores is, to a large extent, governed by the steric constraints and the van der Waals dispersion interactions induced by the pore structure of BEA zeolite. These are the key parameters for understanding the relationship between zeolite topology and catalytic activity. 1. Introduction The catalytic conversion of aromatic molecules via methylation, alkylation and isomerization is a very important process in the petrochemical industry.1 Nowadays, zeolites are used industrially for this purpose. These remarkable aluminosilicate minerals are capable of catalyzing chemical reactions that take place in their well-defined nanostructured pore cavity, and also often provide numerous advantages over the traditional Friedel-Crafts catalyst due to their high selectivity to the desired products and their thermal properties. In addition, the use of heterogeneous inorganic catalysts like zeolites facilitates the design of clean technologies and applications that have to take into consideration environmental concerns. In the past decade, the demand for 2,6-naphthalenedicarboxylic acid (2,6-NDA) as a raw material for the manufacture of a high-performance polyethylene naphthalene (PEN)2,3 has increased significantly. 2,6-Dimethylnaphthalene (2,6-DMN) is considered as the most suitable feedstock with its high reactivity toward oxidation into 2,6-NDA.4 Therefore, the synthesis of 2,6DMN from the ten DMN isomers has been studied.4-7 The * Corresponding author. E-mail: [email protected]. Telephone: +66-2562-5555, ext 2159. † Laboratory for Computational and Applied Chemistry, Department of Chemistry, Faculty of Science, Kasetsart University. ‡ Center of Nanotechnology, Kasetsart University Research and Development Institute, Kasetsart University. § NANOTEC Center of Nanotechnology, National Nanotechnology Center, Kasetsart University. ⊥ Center for Advanced Studies in Nanotechnology and Its Applications in Chemical, Food and Agricultural Industries, Kasetsart University. ¶ Institute of Ion Physics and Applied Physics, University of Innsbruck.

alternative process of 2,6-DMN production, the methylation of methylnaphthalene (MN), has also been investigated8-12 intensively. In all these studies, ZSM-5 showed a high β- and/or β,βselectivity, approximately 80%. T. Inui et al.13 optimized the performance of H-ZSM-5 by a mechanochemical method leading to a significant improvement in the life of the catalyst as well as the improvement of the shape-selectivity. However, the ratio of 2,6-DMN/2,7-DMN produced on ZSM-5 was almost 1, and their yields were still not high. S.-B. Pu et al.11 studied the same reaction over MFI metallosilicates, as well as over some large-pore zeolites, to improve especially the 2,6-DMN/ 2,7-DMN ratio. They reported that these large-pore zeolites significantly increase the yield of 2,6-DMN. In all the zeolites used, BEA showed the highest yield of 2,6-DMN and its deactivation was more gradual than the one of ZSM- 12 and Y. A three-dimensional large-pore structure without a supercage as is present in BEA, is necessary for binuclear aromatic hydrocarbon conversion. Furthermore, the catalytic life of BEA can be prolonged markedly by using tetralin as the solvent for β-MN. This is caused by the dehydrogenation of tetralin in the vicinity of the active sites to prevent coke formation and/or remove the coke formed. However, the reaction mechanisms of this reaction inside the zeolite pores are not well understood. Computational methods based on quantum chemistry have become reliable techniques to clarify the microstructural properties of various chemical systems.14 Especially, the computational investigation of the catalytic mechanisms in zeolites is of considerable practical importance. Nevertheless, zeolites that have a high impact in industrial processes usually possess hundreds of atoms per unit cell, which makes the use of sophisticated methods, such as periodic DFT calculations,

10.1021/jp108566c  2010 American Chemical Society Published on Web 11/17/2010

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computationally too expensive. Most DFT calculations do not take into account the van der Waals (vdW) contributions to the interaction energy.15 For example, the popular B3LYP functional has an unsatisfactory performance for such reactions. Besides issues such as an underestimation of reaction barrier heights, it is unable to describe van der Waals complexes bound by medium-range interactions, and it also exhibits incorrect trends in the bond energies of organometallic catalytic systems.16,17 Recently, Zhao and Truhlar18-21 have developed the M06 class of hybrid functionals, and they suggest that they are suitable for general-purpose applications. Both M06-L and M06 are intended for applications in transition metal chemistry and perform markedly better than B3LYP for main-group thermochemistry, barrier heights, and noncovalent interactions. M062X has an improved performance in these areas as compared with M06-L and M06, but it is not suitable for describing transition metal chemistry. In recent works22-24 it has been demonstrated that the zeolite confinement effect on reactions of unsaturated aliphatic, aromatic, and heterocyclic compounds can be studied by performing full quantum chemical calculations on sufficiently large clusters using the M06-2X method. This leads to a better overall accuracy than the somewhat arbitrary QM/MM schemes like the ONIOM method. Furthermore, it makes the selection of zeolite models more general and systematic. The purpose of the present study is to examine the reaction mechanisms of the methylation of 2-methylnaphthalene (2-MN) in a model of a large zeolite cluster. The reaction path for the side reaction, the formation of dimethyl ether, has also been studied. 2. Computational Details The crystal lattice structure of H-BEA was taken from the work of J. B. Higgins, et al.25 An extended 34T (34 tetrahedral sites) zeolite cluster, covering the active region of the H-β zeolite is used to represent the Brønsted acid site and the zeolite framework. It includes the 12-membered ring representing the main gateway to the intersection of two perpendicular 12 MR channel systems, where the reactions normally take place (See Figure 1). A silicon atom at a T5 position in β zeolite is substituted by an aluminum atom. A proton is added to the bridging oxygen atoms bonded directly to the aluminum atom, conventionally called the O5 position, which is the O1 position in this study. The M06-2X functional and the 6-31G(d,p) basis set were applied for the 34T zeolite cluster. In order to obtain more reliable results, single point calculations at the M06-2X/6311+G(2df,2p) level of theory were also carried out. All calculations have been performed using the Gaussian 03 code.26 During the structure optimization, the 5T portion of the activesite region [tSiO(tSiO)2AlO(H)OSit] and the adsorbates are allowed to relax while the rest of the active region is kept at the crystallographic coordinates. 3. Results and Discussion The results are organized in the following way. First, we examine the mechanism of the methylation of 2-MN to 2,6DMN and 2,7-DMN, the main issue of the present work. Finally, we explore the possibility of dimethyl ether formation as a competitive reaction to the methylation of 2-MN. 3.1. The Methylation of 2-Methyl Naphthalene. A graph of the reaction mechanism is depicted in Scheme 1. The energy profile containing all species involved is plotted in Figure 2. In the following, we employ the notation that a postfix ‘a’ (as in

Figure 1. 34T cluster models of H-BEA. (a) 34T quantum cluster viewed from the direction of the pore axis. (b) 34T quantum cluster viewed from the direction of the side pore.

Int_1a) denotes the 2,6-isomer, while a postfix ‘b’ is used for the 2,7-isomer. When energy values are followed by a second value in parentheses (as 2.6 (2.7) kcal/mol), this also refers to the 2,6- and 2.7-isomers. In addition, the optimized geometries of all species in this study are shown in the Supporting Information (Figure S1). This reaction is initiated by the adsorption of a methanol molecule on the acidic proton of H-BEA (denoted Ads in Scheme 1). The adsorption complex is stabilized by two hydrogen bonds between the OH group of methanol and the Brønsted acid site (O1-H1) of the zeolite: one between the Brønsted proton (H1) and the methanol oxygen atom (O4), and another between the methanol hydrogen atom

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SCHEME 1: Catalytic Cycles of the Methylation of 2-MN to 2,6-DMN and 2,7-DMN over H-BEA Zeolite; Initiation Step Is the Adsorption of Methanol (Ads)

(H2) and the basic oxygen atom of the zeolite active site (O3). No protonation of methanol by the Brønsted site is observed. Similar configurations have also been described in previous studies.27-30 The adsorption of methanol on the hydrogen form of ZSM-5 was studied in a calorimetric experiment by Gorte et al.31 At 80% coverage of the Brønsted sites, a value of 27.5 ( 1.2 kcal/mol (115 ( 5 kJ/mol) was reported. The average acidic strength of the ZSM-5 zeolite was found to be similar to or stronger than that of the BEA zeolite, as determined by various spectroscopic techniques,32-34 or NH3-TPD33 and hydrocarbon cracking.32,34,35 The calculated adsorption energy of -24.2 kcal/ mol therefore seems reliable. It is also compatible with typical van der Waals contributions inside the zeolite pore. At the first transition state, the methanol C-O bond is activated by the attack of the Brønsted proton H1 on the methanol oxygen atom O4. The protonated methanol OH group is leaving as the methyl group is being attacked by the active zeolite O atom (Figure S1b, Supporting Information). We observe the dissociation of the C1-O4 bond and the association of the C1-O2 bond, respectively. As summarized in Table 1, the C1-O4 bond is lengthened from 1.44 to 1.91 Å, whereas the C1-O2 distance contracts to 2.05 Å. The corresponding O4-C1-O2 angle is 171.4°. In this configuration, the geometry of the methyl group is altered from tetrahedral to trigonal planar. The active-site structure of the zeolite is only slightly affected in the course of the progression of the reaction. The activation barrier taken with respect to the adsorption of one methanol molecule at the acidic site (Ads) is predicted to be 39.8 kcal/mol (Figure 2a). This value is lower than the 51.4 kcal/mol obtained in previous calculations on smaller zeolite clusters.36,37 The stabilizing effect of the zeolite micropore on the ionic transition state is well-known and can reach 10-30% of the activation barrier as was found for small zeolite clusters.38 Upon methanol dehydration, methoxy species and a water molecule are created and coadsorb at the active site of BEA

zeolite, Int_1. The water molecule then desorbs (Int_2) and is replaced by the incoming 2-MN. The relative energies of the coadsorption complexes are exothermic by -20.2 (Int_3a; 2,6DMN) and -19.3 kcal/mol (Int_3b; 2,7-DMN). After the coadsorption, the methyl group from the zeolite framework is transferred to the β-position of 2-MN via transition states TS_2a and TS_2b. The barriers for this reaction are 29.5 and 28.1 kcal/mol for 2,6-DMN and 2,7-DMN, respectively. The transition states are also shown in Figure 2a. The normal mode corresponding to the single imaginary frequency was also calculated and indicates the proceeding of the methylation of 2-MN. In the transition structures, which have the characteristics of methyl carbocations with a trigonal planar geometry, the C-C bond between CH3 and MN starts to form. The C1-O2 bond of the methoxy species is elongated from 1.48 Å (1.49 Å) to 2.17 Å (2.12 Å) for 2,6-DMN and 2,7-DMN, respectively. Simultaneously, the lengths of the C1-C2 and C1-C3 bonds decrease from 3.77 Å to 2.20 Å and 3.35 Å to 2.20 Å for 2,6DMN and 2,7-DMN, respectively. The transition states lead to the formation of the key intermediates, 2,6- and 2,7-naphthalenium (Int_4a and Int_4b). These are adsorbed on the active site by -14.6 (-11.3) kcal/mol. The difference of the adsorption energies can be explained by the fact that the 2,6-naphthalenium intermediate is located closer to the active site than the 2,7isomer. The existence of aromatic cations in the zeolite cavity has been disclosed by the results of N. Hansen et al.38 and X. Rozanska et al.39 They found the aromatic carbocation as a reaction intermediate and concluded that large models of medium and micropore zeolites are able to stabilize the benzylic ionic intermediate. Our results from above show that the naphthalenium intermediates will stay adsorbed to the active site and can undergo a proton back-donation process, as shown in Figure 2b. The hydrogen atoms at the carbon center of the naphthalenium intermediates are not in close proximity to the three oxygen

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Figure 2. The calculated energy profile for the methylation of 2-MN to 2,6-DMN and 2,7-DMN (kcal/mol). Due to its size, this figure is split into parts (a) and (b).

TABLE 1: Geometrical Parameters from the 34T Model (M06-2X/6-311+G(2df,2p)//M06-2X/6-31G(d,p)) Calculations of the H-BEA Zeolite, Adsorption Complex (Ads), Transition States (TS_1 to TS_3b), Reaction Intermediates (Int_1 to Int_5b), and Products (Pro_a and Pro_b) for the Methylation of 2-MN to 2, 6-DMN and 2,7-DMN over H-BEA Zeolitea parameter O1-H1 C1-O2 C1-O4 C1-C2 C1-C3 C2-H3 O1-H3 Al-O1 Al-O2 Al-O3 ∠O1-Al-O2 a

H-BEA Ads TS_1 Int_1 Int_2 Int_3a TS_2a Int_4a Int_5a TS_3a Pro_a Int_3b TS_2b Int_4b Int_5b TS_3b Pro_b 0.98 1.85 1.70 1.68 93.1

1.10 3.76 4.61 3.17 2.05 1.48 1.48 1.44 1.91 4.01 1.81 1.72 1.69 1.69 1.70 1.76 1.86 1.86 1.72 1.70 1.69 1.69 97.1 97.2 94.3 94.3

1.48 3.92 3.77 1.09 6.08 1.69 1.86 1.69 94.4

2.17 2.20 2.81 1.09 4.88 1.68 1.71 1.67 96.7

3.12 1.54 2.52 1.16 4.57 1.72 1.71 1.72 100.0

3.38 1.54 2.55 1.13 2.26 1.72 1.72 1.72 99.1

3.24 1.54 2.56 1.24 1.60 1.72 1.72 1.72 98.8

3.18 1.51 2.53 2.46 0.99 1.84 1.69 1.69 94.9

1.49 3.58 3.35 1.09 5.69 1.69 1.85 1.69 94.6

2.12 2.75 2.20 1.09 5.01 1.71 1.75 1.71 97.8

3.20 2.55 1.53 1.12 4.36 1.72 1.71 1.72 99.9

3.36 2.55 1.54 1.13 2.24 1.72 1.72 1.72 99.1

3.32 2.51 1.55 1.26 1.68 1.71 1.72 1.72 99.16

3.45 2.53 1.51 2.56 0.99 1.84 1.69 1.69 94.9

Distances are in Å, and angles are in degrees.

atoms around the active site. To allow for the regeneration of the active site in the final step, the naphthalenium intermediates need to rotate along the zeolite pore axis which causes these hydrogen atoms to get closer to the active site. Due to the lack of data, we use an experimentally derived approximate value of 6.9 kcal/mol40,41 for the barriers of rotation of the protonated

species around C2. The reoriented naphthalenium intermediates stick to the active site (Int_5a and Int_5b) with adsorption energies of -17.6 and -17.9 kcal/mol, respectively. 2,6-DMN and 2,7-DMN can be generated through direct proton transfer from the tetrahedral carbon center of the naphthalenium intermediates to an oxygen atom attached to the

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SCHEME 2: Diagram Showing the Possible Competition between the Dimethyl Ether Formation and the Methylation of 2-MN over H-BEA Zeolite

aluminum atom at the active sites (TS_3a and TS_3b) with calculated barriers of 2.3 and 3.7 kcal/mol. The transition states are slightly perturbed by the zeolite framework which causes the optimized structures to be very close to the intermediates. At the transition state TS_3 where the C-H cleavage occurs, the C-H bond distances of the naphthalenium intermediates are elongated from 1.13 Å (1.13 Å) to 1.24 Å (1.26 Å). Finally, after this third transition state, the products, 2,6-DMN and 2,7DMN are adsorbed at the acidic active site via a π-bond interaction by -38.8 and -34.9 kcal/mol, respectively. These predicted reactions are exothermic, and finally 2,6-DMN and 2,7-DMN require desorption energies of 26.3 and 22.4 kcal/ mol, respectively. Overall, the reaction profiles for the formation of 2,6-DMN and 2,7-DMN are similar to each other, in agreement with the experimental observation11 that there is no preference for either of them when synthesized over acidic BEA zeolite. 3.2. Dimethyl Ether Formation. Finally, we consider another possible reaction of methanol over zeolite catalysts that could compete with the naphthalene methylation. It has been shown earlier37 that at low coverage the methoxy species described above can react with a second methanol molecule to dimethyl ether (DME). The proposed reaction mechanism of the dimethyl ether formation involves two elementary steps and is shown in Scheme 2. Its energetics are presented in Figure 3. We use the postfix ‘c’, when appropriate, for this reaction cycle. The first step starts with the initial adsorption of one methanol molecule at the zeolite acidic site, which is then dehydrated, leaving a methyl group attached to the basic oxygen of the zeolite. This step has been discussed already in section 3.1. The dehydration

process is now followed by the activation of another methanol molecule by the methoxy species. The adsorption of the second methanol to the methoxy intermediate (Int_3c) is exothermic by -10.3 kcal/mol. The adsorption energy of this structure is 9.9 kcal/mol higher than the coadsorbed methoxy species and 2-MN (Int_3a), indicating a weaker interaction with the BEA zeolite cavity. The transition state (TS_2c) has the characteristics of a methyl carbocation with a trigonal planar geometry that leads to the formation of the C-O bond with the methanol molecule. The distance between C1 and O2 of methoxy increases from 1.49 to 1.98 Å, and the C1 and O5 distance decreases from 3.22 to 1.92 Å as their bond is formed (Table 2). Here, the zeolite framework plays an important role in stabilizing both the adsorption complex and the transition structure, with the energy barrier lowered by 3.6 kcal/mol compared to the value obtained from the 3T quantum cluster model37 (Table 2). From the reaction profile in Figure 3 it can be seen that DME requires 26.2 kcal/mol to desorb from the active site. The rate-limiting step for DME formation according to this pathway is the dehydration of the first methanol molecule which requires 39.8 kcal/mol. The activation barrier in the latter step is 5.1 (6.4) kcal/mol higher than the methylation of 2-MN to 2,6-DMN (2,7-DMN). Therefore, also the second side reaction discussed here, the self-activation of methanol to dimethyl ether, is not supposed to be competitive with the methylation of 2-MN. 4. Conclusions In this theoretical investigation we have shown how steric constraints and van der Waals contributions of the zeolite

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Figure 3. The energy profile comparing the methylation of 2-MN to 2,6-DMN and the dimethyl ether (DME) formation.

TABLE 2: Geometrical Parameters from the 34T Model (M06-2X/6-311+G(2df,2p)//M06-2X/6-31G(d,p)) Calculations of the Intermediates (Int_2 and Int_3c), Transition State (TS_2c), and Product (Pro_c) for the Dimethyl Ether Formation over H-BEAa parameters

H-BEA

Int_2

Int_3c

TS_2c

Pro_c

O1-H1 C1-O2 C1-O5 C4-O5 O5-H5 O1-H5 Al-O1 Al-O2 Al-O3 ∠O1-Al-O2

0.98 1.85 1.70 1.68 93.1

1.48 1.69 1.86 1.69 94.3

1.49 3.22 1.41 0.97 4.05 1.66 1.80 1.65 93.0

1.98 1.92 1.44 0.97 3.97 1.71 1.77 1.71 98.1

2.99 1.43 1.43 1.51 1.10 1.80 1.70 1.70 95.8

a

Distances are in Å and angles are in degrees.

framework can affect the reaction pathways of the methylation of 2-methylnaphthalene and of competing reactions. Our results can be summarized as follows: (1) The methylation of 2-MN to 2,6-DMN and 2,7-DMN is initiated by the adsorption of a methanol molecule at the zeolite acidic site, which is then dehydrated, leaving a methyl group attached to the basic oxygen of the zeolite. Subsequently, the methyl group from the zeolite framework transfers to 2-MN at the β-positions yielding naphthalenium intermediates. Eventually, 2,6- and 2,7-DMN are generated by direct proton transfer from the naphthalenium intermediates to the active site. The formation of the methoxy species is considered to be the ratedetermining step. (2) The zeolitic framework plays an important role in the stabilization of the adsorption complexes, transition states, and intermediates. The calculated adsorption energies are in good qualitative agreement with experimental results. The stabilizing effect of the zeolite micropore on the ionic transition state causes the activation barrier to be about 20% lower than what has been calculated for small clusters lacking these contributions. The 34T cluster that is used to model BEA zeolite stabilizes the key reaction intermediates, the naphthalenium cations. (3) The energy profiles for the formation of 2,6- and 2,7DMN formation are similar and in agreement with the experimental data that showed no preference for either of them. (4) Considering a possibly competing process, the selfactivation barrier of methanol to DME is unfavorably high for

the C-O bond-formation step, compared to that for the formation of the C-C bond in 2-MN. Acknowledgment. This work was supported in part by grants from the National Science and Technology Development Agency (2009 NSTDA Chair Professor funded by the Crown Property Bureau under the management of the National Science and Technology Development Agency and NANOTEC Center of Excellence funded by the National Nanotechnology Center), The Thailand Research Fund (to J.L.), the Commission on Higher Education, Ministry of Education (the “National Research University Project of Thailand (NRU)” and the “National Center of Excellence for Petroleum, Petrochemical and Advanced Materials (NCE-PPAM)”). The support from the Kasetsart University Research and Development Institute (KURDI) and the Graduate School Kasetsart University (to K.B.) are also acknowledged. We are grateful to Donald G. Truhlar and Yan Zhao for their support concerning the M06-2X code. M.P. acknowledges an infrastructure grant from the Austrian Ministry of Science to the LFU scientific computing platform. Supporting Information Available: All Cartesian coordinates of the species in the paper. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Jacobs, P. A.; Martens, J. A. In Studies in Surface Science and Catalysis; van Bekkum, H., Flanigen, E. M., Jansen, J. C., Eds.; Elsevier: Amsterdam, 1991; Vol. 58, Chapter 12, pp 445-496. (2) Lillwitz, L. D. Appl. Catal., A 2001, 221, 337. (3) Song, C.; Schobert, H. H. Fuel Process. Technol. 1993, 34, 157. (4) Horita, H.; Takeuchi, G. Petrotech 1995, 18, 844. (5) Kraikul, N.; Rangsunvigit, P.; Kulprathipanja, S. Chem. Eng. J. 2005, 114, 73. (6) Kraikul, N.; Rangsunvigit, P.; Kulprathipanja, S. Appl. Catal., A 2006, 312, 102. (7) Sikkenga, D. Zaenger, I. C.; Williams, G. S. U.S. Patent 4,962,260, 1990. (8) Fraenkel, D.; Cherniavsky, M.; Ittah, B.; Levy, M. J. Catal. 1986, 101, 273. (9) Komatsu, T.; Araki, Y.; Namba, S.; Yashima, T. In Studies in Surface Science and Catalysis; Weitkamp, J., Karge, H. G., Pfeifer, H., Hölderich, W., Eds.; Elsevier: Amsterdam, 1994; Vol. 84, Part 3, p 1821. (10) Popova, Z.; Yankov, M.; Dimitrov, L. In Studies in Surface Science and Catalysis; Weitkamp, J., Karge, H. G., Pfeifer, H., Ho¨lderich, W., Eds.; Elsevier: Amsterdam, 1994; Vol. 84, Part 3, p 1829. (11) Pu, S.-B.; Inui, T. Appl. Catal., A 1996, 146, 305.

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