Activation Energies for the Reaction of Ethoxy Species to Ethene over

Nov 8, 2010 - Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan, and Department of ...
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J. Phys. Chem. C 2010, 114, 20107–20113

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Activation Energies for the Reaction of Ethoxy Species to Ethene over Zeolites Junko N. Kondo,*,† Daisuke Nishioka,‡ Hiroshi Yamazaki,† Jun Kubota,‡ Kazunari Domen,‡ and Takashi Tatsumi† Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan, and Department of Chemical System Engineering, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ReceiVed: July 29, 2010; ReVised Manuscript ReceiVed: October 15, 2010

Dehydration of ethanol to ethene on acidic OH groups of zeolites was observed by infrared (IR) spectroscopy and temperature programmed desorption (TPD). The rate of decomposition of surface ethoxy groups was measured by measuring the amount of recovered acidic OH groups by IR, while confirming the evolution of ethene using a mass spectrometer, for the estimation of activation energy. The obtained activation energies for the evolution of ethene from ethoxy groups on hydrogen form mordenite, ZSM-5, and ferrierite zeolites were 161 ( 6, 181 ( 2, and 188 ( 2 kJ · mol-1, respectively. A tendency was found that the activation energy became larger in smaller-pored zeolite. The cause of the difference of three zeolites in activation energy for ethene evolution from ethoxy species is discussed along with an energy diagram described from reported theoretical examinations. 1. Introduction The interaction and reactions of hydrocarbon and alcohol molecules on the active sites, acidic OH groups, on zeolites have been intensively studied by various methods.1-3 Among them, nuclear magnetic resonance (NMR)4 and infrared (IR) spectroscopy5 have been regarded as powerful tools to observe surface species directly under various conditions. In fact, a variety of mechanisms based on the observed species and their behaviors have been proposed,6-12 and theoretical calculation researches have profoundly evaluated the proposals.13-17 While reactions of methanol over zeolites are regarded as one of the most important reactions on zeolites due to methanol to olefins (MTO) or methanol to hydrocarbon (MTHC) processes, inevitable intermolecule reactions to generate products make the mechanism complicated.18,19 The simplest reaction of alcohols is deduced to be dehydration of ethanol to ethene and water, or hydrolysis of ethene to ethanol. From both reactions ethoxy groups are regarded as intermediate species. While the formation of ethoxy species from ethene adsorption on the acidic OH groups of zeolites was proposed by theoretical calculation as a result of proton transfer,20,21 such alkoxy groups were not observed experimentally but adsorbed ethene immediately oligomerized.22-25 On the other hand, the formation of stable ethoxy groups from dehydration of adsorbed ethanol is reported by NMR26 and IR27 spectroscopy. The next step of dehydration of ethanol, formation of ethene molecules from surface ethoxy groups, was quantitatively studied by IR spectroscopy with support from temperature programmed desorption (TPD) in the present research. Activation energies for the decomposition of ethoxy groups to produce ethene molecules and acidic OH groups were estimated on the hydrogen form of mordenite, ZSM-5, and ferrierrite zeolites. The obtained values of activation energy are discussed in line * To whom correspondence should be addressed. E-mail: jnomura@ res.titech.ac.jp. Fax: +81 (0)45 924 5239. Phone: +81 (0)45 924 5282. † Tokyo Institute of Technology. ‡ The University of Tokyo.

with the whole energy diagram of reactions of ethanol and ethene with the water system. 2. Experimental Section The hydrogen form of mordenite zeolite (JRC-Z-HM20, Catalysis Society of Japan, Si/Al ) 10), ZSM-5 (JRC-Z5-90H, Catalysis Society of Japan, Si/Al ) 45), and ferrierite (Tosoh, Si/Al ) 8.5) were used in the present study without further modifications. Each sample was pressed into a self-supporting disk (20 mm diameter, 20-30 mg) and placed in an IR cell attached to a conventional closed gas circulation system. The sample was pretreated by circulating O2 (15 kPa) at 773 K with a liquid nitrogen trap for 1 h followed by evacuation (10-3 Torr) at the same temperature for 15 min in order to remove residual contaminants. The IR spectra were obtained at a resolution of 4 cm-1 using a Jasco 6200 or 620 FT-IR spectrometer equipped with a mercury cadmium telluride (MCT) detector. A total of 64 scans were averaged for each spectrum. The IR spectra of the clean disk were recorded under evacuation at various temperatures as background spectra. Background-subtracted IR spectra showing adsorbed species are presented throughout this paper. For the quantification of the integrated intensity of the acidic OH groups the following frequency ranges were used: 3381-3696 cm-1 for mordenite, 3502-3648 cm-1 for ZSM-5, and 3401-3675 cm-1 for ferrierite. TPD spectra from ethanol-adsorbed zeolites were recorded with a quadrupole mass spectrometer (Anelva, AQA-100MPX), which is directly connected to the closed gas circulation system, while observing the surface species by FT-IR. All TPD spectra were measured at a heating rate of 3 deg · min-1. 3. Results and Discussion 3.1. Thermal Decomposition of Ethoxy Groups on Mordenite. TPD spectra of ethanol, water, and ethene from ethanol adsorption at 5 kPa on mordenite at room are shown in Figure 1A. The desorption profile of ethanol consists of two peaks at around 370 and 430 K, most probably due to desorption from

10.1021/jp107082t  2010 American Chemical Society Published on Web 11/08/2010

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Kondo et al. SCHEME 1: Schematic Illustration of Dehydration of Ethanol on the Acidic OH Groups of Zeolites: (A) Ethanol Adsorption, (B) Ethoxy Formation and Water Desorption, and (C) Ethene Evolution and Recovery of OH Groups

Figure 1. (A) TPD and (B) IR spectra of ethanol on mordenite: (a) 433, (b) 453, and (c) 473 K. Dotted line, m/e ) 45 (ethanol); solid line, m/e ) 28 (ethene); and dashed line, m/e ) 18 (water).

the external and internal surface of mordenite. On the other hand, desorption of water and ethene appeared in one peak at a similar temperature range (400-500 K), indicating the occurrence of dehydration of ethanol on the acidic OH groups of mordenite. The desorption of water from the reaction is supported from the fact that desorption of water from water-adsorbed mordenite started from room temperature with a maximun at around 390 K. No other molecules than ethanol, water, and ethene were detected. To observe the change of adsorbed species during the heating procedure, IR spectra were collected during TPD measurement. Typical spectra are represented by those observed at 433 (a), 453 (b), and 473 K (c) in Figure 1B. In spectrum a an intense negative band at 3610 cm-1 is attributed to decrease of isolated acidic OH groups due to consumption or/and conversion to the hydrogen-bonded state with ethanol. Ethanol molecules form strong hydrogen bonding to the acidic OH groups in the structure depicted in Scheme 1A. Bands attributable to the adsorbed ethanol are observed at 3700-3200 cm-1 (stretching of OH hydrogen-bonded to acidic OH groups), 2985 cm-1 and below (CH stretching), 1446 cm-1 (CH3 degenerate bending), and 1391 cm-1 (CH3 symmetric bending). The CH2 bending modes are too weak to be detected clearly. On the other hand, hydrogen-bonded acidic OH groups sprea s as a structured band from 3000 to 1300 cm-1 (known as ABC trio bands).28,29 This characteristic structure is interpreted as a result of resonance. Briefly, the broad OH stretching band is perturbed as negative bands of two fundamental bending modes (δ and γ) appearing below 1500 cm-1 and the corresponding overtones (2δ and 2γ).28,29 Thus, the complicated OH

absorption bands in the IR spectrum are evidence of the presence of molecularly adsorbed ethanol, which agrees with TPD spectra in Figure 1A.27 The structure of the baseline due to the adsorbed ethanol decreased gradually, simultaneously with the recovery of the isolated acidic OH groups when temperature was increased. While molecularly adsorbed ethanol is regarded as almost absent at 473 K (spectrum c), CH stretching and bending bands of ethyl groups were still observable The adsorbed species at 473 K, where ethanol desorption almost finished in Figure 1A, are identified as ethoxy groups. It should be noted that a similar hydrogen-bonded structure to that of ethanol is formed when water molecules adsorb on the acidic OH groups, resulting in a similar complicated IR absorption band.27-29 Thus, both ethanol and water formed by dehydration of ethanol are coexistent at 433 and 453 K. While desorption of ethene and water proceeded at 473 K in TPD spectra, they are supposed to be free from direct interaction with the acidic OH groups from the IR spectrum. Thus, ethoxy groups dominate the surface of mordenite at 473 K as illustrated in Scheme 1B. The extent of occupation of ethoxy groups can be roughly estimated from the integrated absorbance of the isolated acidic OH groups. In Figure 1B (line c) the integrated intensity of the negative band at 3610 cm-1 corresponds to about 10% of that in the background. When adsorption of ethanol was repeated at 453 K 10 times, the coverage of ethoxy groups gradually increased from 13% to 15%. Thus, 15% is regarded as the maximum coverage of ethoxy groups on the acidic OH groups of mordenite at 453 K. Since the formation and decomposition of ethoxy groups was found to proceed at 453 K, the time course of the decomposition of ethoxy species was observed (Figure 2). At the initial stage the absorption structure of the acidic OH groups hydrogenbonded to ethanol or water was observed. In fact, ethanol desorption was detected in the mass spectrometer by 10 min after starting the time course, while water and ethene constantly

Figure 2. Time course of IR spectra of adsorbed species formed from ethanol adsorption and immediate evacuation at 453 K on mordenite after (a) 5, (b) 10, (c) 15, and (d) 20 min.

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desorbed. Similarly to the previous TPD results, no other molecules than ethanol, water, and ethene desorbed. A small band at around 1480 cm-1 was observed as the time proceeded. This band could be attributed to diethyl ether-like species30 formed by a reaction among ethoxy groups in the time course. It should be mentioned that acid sites in the 8-menbered ring (MR) of mordenite are not accessible for ethanol, and thus, observed ethoxy groups on mordenite are located in 12 MR. This is supported by separate experiments: pyridine was adsorbed at 473 K followed by evacuation to inactivate acid sites in 12 MR for ethanol adsorption and ethoxy formation, where ethanol was successively introduced at the same temperature. Decrease in the intensity of OH band due to acid sites in 8MR, which was observed after pyridine adsorption, was not observed after exposure to ethanol. As mentioned above, the coverage of ethoxy species can be estimated from the integrated intensity of the band of OH groups, and the decrease of it is found in Figure 2. Thus, by measuring the time course of the recovery of the amount of the isolated acidic OH groups, the rate of decomposition of ethoxy groups can be obtained. This process corresponds to the reaction from (B) to (C) in Scheme 1. The coverage of ethoxy species can be given by the amount of free acidic OH groups in the background ([OH]0) and that of ethoxy-covered surface ([OH]) in the following equation

θ)

[OH]0 - [OH] [OH]0

(1)

Assuming the first-order dependence of the decomposition of ethoxy species on the coverage (θ), the rate constant k (s-1) can be expressed as

θ ) θ0 exp(-kt)

(2)

where t and θ0 are reaction time and initial coverage of ethoxy species, respectively, leading to the following equation

ln θ ) ln θ0-kt

(3)

Then, the activation energy (Ea) for the decomposition of ethoxy species is provided from the following equation:

ln k ) ln A -

Ea RT

(4)

where A, R, and T represent pre-exponential factor, gas constant, and temperature, respectively. The use of coverage instead of a definite number of ethoxy groups is to decrease errors in each experimental adjustment of the initial amount of ethoxy species. The time courses of a natural logarithm of coverage (ln θ) at temperatures between 453 and 473 K in Figure 3A are all in a straight line, which confirms the assumption of the first-order dependence of the reaction on the coverage. At the beginning of the reaction at low temperature, the effect of molecularly adsorbed ethanol on the quantification of the recovery of the isolated acid sites is not negligible, as observed in related IR spectra (Figures 1B (line b) and 2). So, the period where the effect of molecular adsorption can be negligible was used at each temperature. The Arrhenius plot using the rate constants

Figure 3. (A) Time course of a natural logarithm of coverage of ethoxy groups at (a) 453, (b) 463, (c) 468, and (d) 473 K, and (B) Arrhenius plot for the decomposition of ethoxy groups.

obtained in Figure 3A was fit to a linear correlation, and the activation energy was estimated as 161 ( 6 kJ · mol-1. 3.2. Thermal Decomposition of Ethoxy Groups on ZSM-5 and Ferrierite. The acid density may have an important effect for the reaction, so the consideration of it in the same framework structure is worth discussing. However, it was not possible to find stable high Al-containing ZSM-5 or high Si-consisting mordenite and ferrierite. Accorging to refs 31 and 32, the strength of the acid sites is independent of the acid density in the same topologies. Thus, we consider in the present study that the acid density does not affect the activation energy for the formation of ethene from ethoxy groups. TPD spectra of ethanol, water, and ethene from ethanol-adsorbed ZSM-5 are shown in Figure 4A. No other compounds were detected, probably due to the long distances among ethoxy groups. A large peak of ethanol at 380 K is attributed to desorption from the external surface of ZSM-5 similarly to the case of mordenite, and water and ethene were also observed as fragments of ethanol. The main peak of ethene desorption appeared at 495 K, which is much higher than that on mordenite (460 K). This may be due to the smaller pore size of ZSM-5 than that of mordenite, where more energy may be required to form the structure of transition state in smaller pore space, as discussed below. It is noted that desorption of water was not coincident with ethene desorption on ZSM-5; water desorption started from lower temperature than that of ethene. This indicates that ethoxy species are formed at lower temperature than their decomposition. IR spectra measured at the temperature indicated by lines a-e in Figure 4A are shown in Figure 4B. Similarly to spectra measured on mordenite (Figure 1B), OH stretching bands of ethanol (and/or water) and acid sites of ZSM-5 were observed at around 3500 cm-1 and as baseline structure at the low-

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Figure 5. Arrhenius plots for the decomposition of ethoxy groups on (a) ZSM-5 and (b) ferrierite. Figure 4. (A) TPD and (B) IR spectra of ethanol on ZSM-5: (a) 441, (b) 453, (c) 473, (d) 493, and (e) 513 K. Dotted line, m/e ) 45 (ethanol); solid line, m/e ) 28 (ethene); and dashed line, m/e ) 18 (water).

temperature range, followed by desorption of oxygenates and formation of ethoxy species. Monotonous decrease of bands of ethoxy species was observed in IR spectra during the time course of evacuation at 473 K together with detection of ethanol (minor species), water, and ethene in the mass spectrometer. When the adsorption-evacuation cycle was repeated 20 times at 473 K, coverage of ethoxy groups increased from 30% to 40%. The cause for the difference of mordenite and ZSM-5 in maximum coverage (15% at 453 K for mordenite) is not known at present but may be due to the difference in acid density, the distances among the acid sites, and the resulting ethoxy groups. It should be explained that ethoxy groups were formed at a sacrifice of silanol groups on the external surface. This is found from the presence of a negative band at 3750 cm-1 of silanol groups in Figure 4A. While a gradual recovery of the acidic OH groups was clear, the negative band of silanol groups did not changed at 513 K. Therefore, ethoxy groups formed on the external surface are very stable. Since the acidic OH groups were all recovered by decomposition of ethoxy groups in spectrum e, absorption bands of CH stretching and bending are attributed to ethoxy groups on the silanol position. This is not the only case on ZSM-5 but is also observed on mordenite (Figure 1B) and ferrierite (Figure 6B), while ZSM-5 appeared most evidently. Two bands, which were not observed on mordenite, appeared from the beginning at 1700 and 1450 cm-1 but did not increase nor decrease in intensity. Furthermore, these bands were only sometimes observed with less reproducibility. Therefore, the species were regarded as inoffensive to estimate the activation energy for decomposition of ethoxy species. The activation

Figure 6. (A) TPD and (B) IR spectra of ethanol on ferrierite: (a) 413, (b) 433, (c) 443, (d) 453, (e) 493, and (f) 513 K. Dotted line, m/e ) 45 (ethanol); solid line, m/e ) 28 (ethene); and dashed line, m/e ) 18 (water).

energy obtained from the Arrhenius plot in Figure 5 (line a) was 181 ( 2 kJ · mol-1. Since the desorption signal of ethanol from ethanol-adsorbed ferririte was extremely small in comparison with that of ethene, about one-third of the intensity of ethene is omitted in the TPD spectra in Figure 6A. The peak top of ethene desorption appeared at 480 K. Desorption of ethanol from the external surface was confirmed at 380 K and no other species than ethanol, water, and ethene were observed, similarly to the cases of other zeolites. IR spectra measured at temperatures indicated by lines from a to f are shown in Figure 6B. Spectrum a measured at 413 K is dominated by ethanol since ethanol still desorbed and because water attributed to ethoxy decomposition

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SCHEME 2: Energy Diagram of Ethanol and Ethane with Water on a Zeolite OH Group

was not obvious. The baseline structure due to hydrogen-bonded OH groups was observed up to 473 K, where desorption of ethanol was almost terminated. Thus, the counterpart of the hydrogen-bonded species of the acidic OH groups is supposed to be water. At higher temperature (spectra e and f), ethoxy groups were the only surface species, which decomposed gradually. Although the maximum coverage was attempted to estimate adsorption-desorption cycles (20 times) at 493 K, a totally different spectrum was observed after the second dosage of ethanol. Bands due to methylene groups gradually increased in intensity, and hydrogen-bonded acidic OH groups appeared at 3400 cm-1, which resulted from the interaction with alkyl groups.33 Therefore, ethoxy groups on ferrierite are considered to be reactive, and intermolecular reaction proceeded to form long-chained alkyl groups. In fact, TPD spectra of ferrierite, where 20 cycles of ethanol adsorption proceeded, clarified the desorption of butene with a peak at 540 K and other large molecules. For further confirmation, similar experiments were conducted for ethene adsorption. When ethene was dosed on ferrierite at room temperature, oligomeric species were observed in the IR spectrum. In the TPD measurement, desorption of ethene was observed at 420 K as a main product with larger molecules (m/e ) 60, 70, 84, and 85 mainly) desorbed at above 500 K. Thus, it was confirmed that ethoxy groups on ferrierite are reactive to form oligomers. 3.3. Ethoxy Species in the Energy Diagram of Ethanol and Ethene with Water on Zeolites. The proposed energy diagram for ethanol dehydration to ethene and water on the acidic OH groups of zeolites is summarized in Scheme 2.34-38 Described values are referred from experimental and calculated data. The estimation of these energy values and the structures of transition state and stable species was initiated by Kazansky’s group on the reaction of ethene with the acidic OH groups, which corresponds to the states from 4 to 3 via T2 in Scheme 2.21,32 While ethyl cation had been proposed to be formed as a

stable intermediate from the reaction, the theoretical calculation concluded that the cationic species was in the transition state, and that the stable intermediate was ethoxy groups. This is proposed to be due to the absence of stabilization of cationic species by solvation on the solid gas interface unlike that in homogeneous systems.21 On the other hand, ethoxy species were not observed by experiments in early time,22,23 followed by recent well-controlled rapid-scan24 and low-temperature25 observations. The resulting species of ethene and propene reaction on zeolites were oligomeric species,22-24,39,40 and dimeric species were observed by the reaction of isobutene41,42 and other olefins.25 The monomolecular speices formed from the reaction of an olefin molecule and the acidic OH group have not been observed experimentally. Thus, the reaction scheme was attempted to follow beginning with ethanol adsorption on acidic OH groups through ethoxy groups in the present study. The process of the formation of ethoxy groups from adsorption and dehydration of ethanol (states from 1 to 2 or 3 through T1) already has been reported by experimental results,26,27 although the value of the activation energy has not been estimated. The difference in energy of states 2 and 3 is referred to as the heat of adsorption of water on H-ZSM-5.36 In this study, the energy barrier for the generation of ethene from ethoxy groups (from state 3 to overcome the transition state, T2) was measured. The difference in energy of ethoxy groups and adsorbed ethene (states 3 and 4) was reported be ranging from 49 to 75 kJ · mol-1 depending on the cluster model and the environment included in theoretical calculations,38 and the activation barrier for adsorbed ethene to form ethoxy groups (from state 4 to overcome T2), between 123 and 135 kJ · mol-1.38,43 Accordingly, the activation energy of decomposition of ethoxy species can be considered to be in between 172 and 210 kJ · mol-1 from refs 38 and 43. Considering that the experimentally obtained values are 161 ( 6, 181 ( 2, and 188 ( 2 kJ · mol-1 for mordenite, ZSM-5, and ferrierite zeolites, the

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energy diagram described in Scheme 2 is regarded as reasonable. In other words, the proposed ionic structure of T2 (ethyl cation), the transition state between the conversion of ethoxy and ethene species, would be ideal. As described above, no adsorbed ethene but the acidic OH groups were observed after decomposition of ethoxy groups. This indicates that the activation energy for desorption of adsorbed ethene (heat of adsorption of ethene) is much smaller than that for protonation of adsorbed ethene to form ethoxy groups (from state 4 to T2), and is consistent with the suggested energy diagram described in Scheme 2. The difference among types of zeolites in the activation energy is most probably attributable to the stabilization of the T2 structure; the lager-spaced mordenite can better stabilize the ethyl cation species than the smaller-pored ferrierite, with ZSM-5 in between. Another possible cause of the difference of three zeolites in the activation energy measured is the difference in acid strength. If so, the order of the activation energy should be “ZSM-5 > mordenite > ferrierite”, where ethanol adsorption on OH groups in 8MR of mordenite is negligible as mentioned below, since the order of the acid strength of OH groups is reported to be those in “MOR 8MR > FER > MOR 12 MR > MFI” by IRMS-TPD of NH3 adsorption.42 On the other hand, when TPD spectra of water on three zeolites are compared in the present study, water desorption is observed at lower temperature on mordenite than ferrierite and ZSM-5, indicating the different activation energy in the formation of ethoxy groups. Thus, the energy level of state 3 can be suspected to be different on three types of zeolite. In such a case, discussion on the difference in activation energy on different zeolites cannot be concluded simply, but an attempt for the estimation of the energy level of the transition state in each zeolite by theoretical calculation would be required. Following the previous studies on the experimental observation of formation of ethoxy groups from adsorbed ethanol on the acidic OH groups on zeolites,26,27 decomposition of ethoxy groups and the activation energies were obtained in the present research. Scheme 2 is, thus, experimentally followed from the ethanol side to ethene, although some energetic values are still calculated ones. Therefore, whole states in Scheme 2 and their energetic relations are evidenced. The problem left in the diagram is the lack of experimental support for the formation of ethoxy groups from the adsorbed ethene side (from state 4 to 3 through T2). As mentioned above, it is not only the case for ethene but also for other olefins: the lack of observation of monomolecular intermediate in the activation of olefins. In the case of decomposition of alkoxy groups, there are no free molecules on neighboring sites of an alkoxy group in the transition state under the experimental conditions, which excludes the intermolecular reaction but results in formation and desorption of a corresponding olefin molecule. On the other hand, protonation of an olefin molecule occurs at much lower temperatures than that of decomposition of alkoxy species, and adsorbed olefin molecules are rapidly migrating on the surface of zeolite, leading to the intermolecular reaction between a molecule in the transition state and another adsorbed olefin molecule even at low coverage. Since there seems still to be some experimental consideration left, it should be worth pursuing research with isotopes as has been done by NMR on methanol to olefin (MTO) processes. 4. Conclusion In the energy diagram of dehydration of ethanol to ethene and water on the acidic OH groups of zeolites, the activation energies of the elemental step of decomposition of ethoxy

Kondo et al. groups, which were formed by adsorption of ethanol followed by water desorption to ethene and acidic OH groups, were estimated as 161 ( 6, 181 ( 2, and 188 ( 2 kJ · mol-1 on the hydrogen form mordenite, ZSM-5, and ferrierite zeolites, respectively, using IR spectroscopy. The energy barrier was in order to become larger for smaller-pored zeolite. Since the measured experimental values were in a range of those suggested by theoretical calculation results, the proposed structure of the transition state, ethyl cation species, was regarded as reliable. The variation of the activation energy on different types of zeolite could be attributed to differences in both pore size and acid strength. Detailed theoretical estimation of activation energies and energy differences on each zeolite would be necessary for further interpretation. References and Notes (1) Zecchina, A.; Spoto, G.; Bordiga, S. Phys. Chem. Chem. Phys. 2002, 7, 1627. (2) Hunger, M.; Weitkamp, J. Angew. Chem., Int. Ed. 2001, 40, 2954. (3) Kondo, J. N.; Yoda, E.; Ishikawa, H.; Wakabayashi, F.; Domen, K. J. Catal. 2000, 191, 275. (4) Haw, J. F.; Xu, T. AdV. Catal. 1998, 42, 115. (5) Lamberti, C.; Groppo, E.; Spoto, G.; Bordiga, S.; Zeccchina, A. AdV. Catal. 2007, 51, 1. (6) Wang, W.; Seiler, M.; Hunger, M. J. Phys. Chem. B 2001, 105, 12553. (7) Jiang, Y.; Hunger, M.; Wang, W. J. Am. Chem. Soc. 2006, 128, 11679. (8) Haw, J. F.; Nicholas, J. B.; Song, W.; Deng, F.; Wang, Z.; Xu, T.; Heneghan, C. S. J. Am. Chem. Soc. 2000, 122, 4763. (9) Haw, J. F.; Song, W.; Marcus, D. M.; Nicholas, J. B. Acc. Chem. Res. 2003, 36, 317. (10) Bjørgen, M.; Bonino, F.; Kolboe, S.; Lillerud, K.-P.; Zecchina, A.; Bordiga, S. J. Am. Chem. Soc. 2003, 125, 15863. (11) Bjørgen, M.; Lillerud, K.-P.; Olsbye, U.; Bordiga, S.; Zecchina, A. J. Phys. Chem. B 2004, 108, 7862. (12) Zhu, Q.; Kondo, J. N.; Setoyama, T.; Yamaguchi, M.; Domen, K.; Tatsumi, T. Chem. Commun. 2008, 5164. (13) Bhan, A.; Joshi, Y. V.; Delgass, W. N.; Thomson, K. T. J. Phys. Chem. B 2003, 107, 10476. (14) Maihom, T.; Boekfa, B.; Sirijaraensre, J.; Nanok, T.; Probst, M.; Limtrakul, J. J. Phys. Chem. C 2009, 113, 6654. (15) Svelle, S.; Kolboe, S.; Swang, O.; Olsbye, U. J. Phys. Chem. B 2005, 109, 12879. (16) Arstad, B.; Kolboe, S.; Swang, O. J. Phys. Chem. A 2005, 109, 8914. (17) Kolboe, S.; Svelle, S.; Arsted, B. J. Phys. Chem. A 2003, 113, 917. (18) McCann, D. M.; Lesthaeghe, D.; Kletrieks, P. W.; Guenther, D. R.; Hayman, M. J.; van Speybroeck, V.; Waroquier, M.; Haw, J. F. Angew. Chem., Int. Ed. 2008, 47, 5179. (19) Bjørgen, M.; Svelle, S.; Joensen, F.; Nerlov, J.; Kolboe, S.; Bonino, F.; Palumbo, L.; Bordiga, S.; Olsbye, U. J. Catal. 2007, 249, 195. (20) Senchenya, I. N.; Chuvylkin, N. D.; Kazansky, V. B. Kinet. Catal. 1985, 26, 926. (21) Kazansky, V. B. Acc. Chem. Res. 1991, 24, 379. (22) van den Berg, J. P.; Wolthuizen, J. P.; Clague, A. D. H.; Hays, G. R.; van Hooff, J. H. C. J. Catal. 1983, 80, 130. (23) Kofke, T. J. G.; Gorte, R. J. J. Catal. 1989, 115, 233. (24) Spoto, G.; Bordige, S.; Ricchiardi, G.; Acarano, D.; Zecchina, A.; Borello, E. J. Chem. Soc., Faraday Trans. 1 1994, 90, 2827. (25) Kondo, J. N.; Domen, K. J. Mol. Catal. A 2003, 119, 27. (26) Wang, W.; Jiao, J.; Ray, S. S.; Hunger, M. Chem. Phys. Chem. 2005, 6, 1467. (27) Kondo, J. N.; Ito, K.; Yoda, E.; Wakabayashi, F.; Domen, K. J. Phys. Chem. B 2005, 109, 10969. (28) Permenschikov, A. G.; van Santen, R. A. J. Phys. Chem. 1993, 97, 10678. (29) Krossner, M.; Sauer, J. J. Phys. Chem. 1996, 100, 6199. (30) NIST Chemistry WebBook, NIST Standard Reference Database No. 69, http://webbook.nist.gov/chemistry/. (31) Katada, N.; Suzuki, K.; Noda, T.; Sastre, G.; Niwa, M. J. Phys. Chem. C 2009, 113, 19208. (32) Sastre, G.; Katada, N.; Nuwa, M. J. Phys. Chem. C 2010, 114, 15424. (33) Kondo, J. N.; Domen, K.; Wakabayashi, F. J. Phys. Chem. B 1998, 102, 2259. (34) Senchenya, I. N.; Mikkeikin, I. D.; Zhidomirov, G. M.; Kazanskii, V. B. Kinet. Catal. 1980, 21, 1184.

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