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Density Functional Theory Study of the Dehydrogenation of Ethanol to Acetaldehyde over the Au-Exchanged ZSM‑5 Zeolite: Effect of Surface Oxygen Thana Maihom,†,‡,§ Michael Probst,∥ and Jumras Limtrakul*,‡,§,⊥ †

Department of Chemistry, Faculty of Liberal Arts and Science, Kasetsart University, Kamphaeng Saen Campus, Nakhon Pathom 73140, Thailand ‡ Department of Chemistry and NANOTEC Center for Nanoscale Materials Design for Green Nanotechnology and §Center for Advanced Studies in Nanotechnology and Its Applications in Chemical, Food and Agricultural Industries, Kasetsart University, Bangkok 10900, Thailand ∥ Institute of Ion Physics and Applied Physics, University of Innsbruck, 6020 Innsbruck, Austria ⊥ PTT Group Frontier Research Center, PTT Public Company Limited, 555 Vibhavadi Rangsit Road, Chatuchak, Bangkok 10900, Thailand S Supporting Information *

ABSTRACT: The transformation of ethanol, one of the low-cost biomass products, into fine chemicals has received considerable attention related to the development of clean industrial technologies. We calculated the reaction mechanism of ethanol dehydrogenation to acetaldehyde over Au-exchanged ZSM-5 zeolite with and without surface oxygen by means of the density functional theory. The reaction is proposed to proceed in two steps. In the first, dissociation of the ethanol O−H bond leads to an ethoxide intermediate which is then converted to acetaldehyde. The reaction barrier for the first step is 9.0 kcal/mol for the Au−O/ZSM-5 zeolite, much smaller than for the Au/ZSM-5 zeolite (43.0 kcal/mol). For step 2, the reaction barriers are 4.4 kcal/mol for Au−O/ZSM-5 and 30.8 kcal/mol for Au/ZSM-5, respectively. The results indicate that the presence of surface oxygen on the Au site assists both the ethanol O−H bond dissociation and the conversion of ethoxide to acetaldehyde.

1. INTRODUCTION Ethanol production from renewable biomass is receiving a great amount of attention due to the expanding use of ethanol as a fuel for motor vehicles and as a feedstock for producing hydrogen by catalytic reforming.1−3 Part of this stems from its convertibility to more valuable hydrocarbon compounds.4 Among them, acetaldehyde is one of the most important organic chemicals which can be obtained from ethanol via a catalytic dehydrogenation process. A large number of catalysts, in particular, metals and metalsupported materials, have been used for the dehydrogenation of alcohols to aldehydes.5−14 Among them, supported gold (Au) catalysts have been found to be promising. On various support materials, including silica, they exhibit high activity and good selectivity for the dehydrogenation of alcohols to aldehydes and also for various other transformation reactions of ethyl alcohol.15−22 Zeolites can serve as useful silica supports for metals such as Au. They have the advantages of being environmentally friendly and reusable and are able to be reactivated, and their nanopores give them size and shape selectivity. Au-exchanged NaY, NaZSM-5, ZSM-5, and Na-MOR zeolites have been synthesized previously.23−28 They showed high activity for various reactions © 2014 American Chemical Society

such as the direct decomposition of N2O and NO, the oxidation of CO, and the water gas shift (WGS) reaction at low temperature. Au+ was characterized to be the dominant active gold center in those reactions.28 Recently, we demonstrated the possible utilization of Au-exchanged zeolites for catalyzing various other reactions including methane C−H bond activation, the direct conversion of carbon dioxide and methane to acetic acid, nitrous oxide decomposition, and the conversion of carbon dioxide and ethane to propanoic acid.29−32 In those works, we found that the zeolite support increased the catalytic activity of Au. It further reduced the activation energies compared to the extreme case of a bare Au atom as a catalyst. In chemical reactions over zeolite catalysts, the zeolite framework is well known to play an essential role in reactive intermediates and the reactions of hydrocarbons.33−41 Hansen et al.38,39 have shown that highly reactive carbenium or carbonium ions as intermediates can be located inside the zeolite pore when using an appropriate cluster model of zeolite. They found protonated ethylbenzene to be a stable Received: May 21, 2014 Revised: July 14, 2014 Published: July 22, 2014 18564

dx.doi.org/10.1021/jp505002u | J. Phys. Chem. C 2014, 118, 18564−18572

The Journal of Physical Chemistry C

Article

Figure 1. Optimized geometries of the Au/ZSM-5 zeolite (a) and its orbital overlap between the s orbital of Au and the sp orbitals of framework oxygens (b) and the Au−O/ZSM-5 zeolite (c) and its orbital overlap between the s orbital of Au and the sp orbitals of surface oxygen (d).

calculations with dispersion including functionals became cost efficient enough to allow single-layer calculations on fairly large systems. For example, calculations with the M06 series of metahybrid density functionals by Zhao and Truhlar56,57 in which van der Waals interactions are taken into account in the parametrization have been successfully used to investigate the a d s o r p t io n an d r e ac t i o n m ec h a n is m s o v er z e o lites29−32,42−44,58−62 and also over metal−organic frameworks.63−66 In the present work, we investigate the mechanism of the dehydrogenation of ethanol to acetaldehyde over Au-exchanged ZSM-5 zeolite by using the M06-L functional. We study the reaction with and without oxygen on the Au active site and discuss the structures and energetics of reaction intermediates and transition states. To the best of our knowledge, this is the first time that ethanol oxidation on this catalyst is investigated theoretically.

intermediate of the benzene ethylation reaction by using a large cluster of zeolite (33T) in their calculations. Deng and coworkers40 suggested that the formation of reaction intermediates in the ethylbenzene disproportionation reaction can be controlled via the zeolite pore spaces. They also explained the shape selectivity with respect to the intermediates from the strain energies of them confined in the various zeolite pore structures. Previously we also demonstrated the effect of the zeolite framework in lowering the activation barriers and stabilization of intermediate species for various reactions such as n-hexane cracking,42 1-butene skeletal isomerization,43 and ethanol transformation on zeolites.44 These findings highlight that both van der Waals and electrostatic interactions with the zeolite walls significantly affect the relative stabilities of both intermediates and transition states of reactions and hence control the entire reaction mechanisms. Previously, the ONIOM method45 was found to be efficient for studying several reactions in zeolites.46−55 In its molecular mechanics layer, the van der Waals interactions are empirically present. The performance of this method depends, however, on the right partition of the system in the active region and environment and on the correct selection of high- and low-level methods. Alternatively, density functional theory (DFT)

2. MODELS AND METHOD Figure 1 illustrates the 12T (T refers to Si or Al atoms) cluster model that was used to model the ZSM-5 zeolite in this work. It was generated from its crystallographic structures.67 One aluminum atom was substituted for a silicon atom at the T12 18565



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Table 1. Optimized Geometrical Parameters of All Species Involved in the Dehydrogenation of Ethanol to Acetaldehyde on the Au/ZSM-5 Zeolite parameters

isolate

ethanol adsorption (Ads_1)

transition state 1 (TS1_1)

intermediate (Int_1)


product (Prod_1)

3.05 2.42 2.14 2.08 1.82

3.19 2.73 2.16 2.15 2.86

1.33 1.34 1.51 1.03

1.24 3.52 1.48 0.75

Distance (Å) Au−Al Au−O1 Au−O2 Au−Oe Au−H1 Oe−H1 Oe−C1 C2−H2 C1−C2 H1−H2

3.10 2.38 2.26

0.96 1.42 1.10 1.51

3.19 2.75 2.16 2.19 2.71 0.97 1.46 1.10 1.50 2.28

3.05 2.39 2.17 2.09 1.59 1.61 1.43 1.10 1.51 2.39

3.00 2.32 2.17 2.01 1.54 2.40 1.42 1.10 1.51 2.69 Angle (deg)

O1−Al−O2 Oe−Au−H1

92.9

93.7 19.2

91.4 49.7

91.1 83.8

91.6 66.7

93.9 72.1

Table 2. Optimized Geometrical Parameters of All Species Involved in the Dehydrogenation of Ethanol to Acetaldehyde on the Au−O/ZSM-5 Zeolite parameters

isolate

ethanol adsorption (Ads_2)


intermediate (Int_2)


product (Prod_2)

3.07 2.31 2.26 2.25 2.18 0.97

3.06 2.24 2.32 2.46 2.11 0.97

1.34 1.33 1.31 1.52

1.34 0.99 2.00 1.50

Distance (Å) Au−Al Au−O1 Au−O2 Au−O Au−Oe O−H1 Oe−H1 Oe−C1 O−H2 C1−H2 C1−C2

3.12 2.34 2.19 1.90

0.96 1.42 1.10 1.51

3.09 2.31 2.25 2.03 2.37 1.95 0.98 1.44 2.06 1.10 1.51

3.10 2.35 2.20 2.08 2.36 1.17 1.26 1.41 3.84 1.10 1.51

3.09 2.31 2.23 2.15 2.26 0.97 2.34 1.40 3.35 1.10 1.51 Angle (deg)

O1−Au−O2 O−Au−Oe O−H1−Oe

90.1

91.7 72.0 121.7

91.6 62.8 145.2

91.6 61.0 72.2

91.4 76.6

91.4 76.6

5 is displayed in Figure 1a. Au is coordinated to two bridging oxygen atoms of zeolite with Au···O1 and Au···O2 distances of 3.38 and 3.26 Å, respectively. The distance between Al and Au is 3.10 Å, which is similar to the experimental value of 3.20 reported for the Al···Au bond in the Au-exchanged FAU zeolite.72 The NBO analysis of this cluster shows the overlap between the s orbital of Au and the sp orbitals of oxygens O1 (s = 52%, p = 48%) and O2 (s = 17%, p = 83%) of the zeolite framework as shown in Figure 1b. Electron transfer to the Au cation decreases its atomic partial charge from +1e to +0.73e. Figure 1c shows the optimized structure of the Au−O/ZSM5 zeolite. ZSM-5 with Au−O active sites has not yet been synthesized in experiments, as far as we are aware. It could probably be created by the help of oxidizing agents such as hydrogen peroxide (H2O2) or nitrous oxide (N2O) from Au/ ZSM5. This decomposition of N2O on Au/ZSM-5 leading to Au−O/ZSM-5 has been reported in previous theoretical investigations.31,73,74 Similar to Au, the Au−O active site is placed on two bridging oxygen atoms of zeolite. Its Al···Au distance is slightly longer (∼0.02 Å), while the Au−O1 and Au−O2 distances are shorter than for Au alone. The Au−O bond distance in the surface oxygen species is 1.90 Å, similar to the bond length in (Au−O)+ from CCSD(T) calculations (1.88

site to generate the Brønsted acid site. The Au cation was exchanged on the Brønsted site to generate the Au-zeolite (Au/ ZSM-5). The M06-L density functional 56,57 was used in all calculations. The Al, Si, C, O, and H atoms were treated with the 6-31G(d,p) basis set, while the Au atom was described by the Stuttgart effective core potential (ECP)68 basis set. Only the 5T portion of the active region (Au/AlSi4O4 and Au−O/ AlSi4O4) and the probe molecules were allowed to relax during geometry optimizations. The lowest-energy spin state is a singlet for Au/ZSM-5 and a triplet for Au−O/ZSM-5, and these have been used in the calculations. Partial charges and population analysis were determined by the natural atomic orbital (NAO) and natural bond orbital (NBO) methods.69 Transition states were located with the Berny algorithm70 and then confirmed via frequency calculations at the same level of theory to guarantee that the transition-state structure has only one imaginary frequency. All calculations were performed with the Gaussian 09 code.71

3. RESULTS AND DISCUSSION 3.1. Model of Au/ZSM-5 and Au-O/ZSM-5 Zeolites. The optimized structure of the 12T cluster model of Au/ZSM18566



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Figure 2. Energy profile and structures of reactants, intermediates, and transition states for the dehydrogenation of ethanol to acetaldehyde on Au/ ZSM-5 zeolite (energies in kcal/mol).

Å).75 It is also nearly the same as the Au−O bond distance of 1.89 Å in AuO-ZSM-5 from B3LYP calculations with the 631G(d,p) basis set for light atoms and LANL2DZ for Au.74 The NBO analysis reveals a bond between the Au and O atoms resulting from the overlap of a mostly s-type orbital on Au (s = 86%, d = 14%) with an sp orbital of O (s = 14%, p = 86%). The corresponding orbitals are plotted in Figure 1d. The partial charges of Au and O are +0.88e and −0.29e, respectively. We furthermore examine the spin density on the Au−O/ZSM-5 zeolite. We found the majority of the spin density to be located on the O atom of the Au−O site (O = 1.38 and Au = 0.44). This free-radical character of the surface O on Au is responsible for its ability to bond to and abstract the hydrogen from the ethanol hydroxyl group in further steps. 3.2. Reaction Mechanism of Ethanol Dehydrogenation to Acetaldehyde. The reaction mechanism of ethanol dehydrogenation to acetaldehyde on both Au/ZSM-5 and Au− O/ZSM-5 zeolites is considered to proceed through two steps: the dissociation of the O−H bond of ethanol leads to the surface ethoxide intermediate which decomposes to the acetaldehyde product. These reaction steps are analogous to the mechanisms for the dehydrogenation of alcohols to aldehydes on the Au surface.22,76,77 Selected geometrical parameters for all reaction steps are shown in Tables 1 and 2

for the Au/ZSM-5 and Au−O/ZSM-5 cases, respectively. The energy profiles and selected geometrical parameters of the optimized structures along the reaction coordinates are shown in Figures 2 and 3. 3.2.1. Ethanol Dehydrogenation on Au/ZSM-5. The first step of the reaction is the adsorption of ethanol on the active site of Au through the lone pair interaction between the ethanol oxygen (Oe) and Au (Ads_1) with an adsorption energy with respect to the isolated molecules of −39.7 kcal/mol. The adsorption influences the structures of both Au/ZSM-5 and ethanol. In Au-ZSM-5, the Au···Al and Au···O1 distances are lengthened from 3.10 to 3.19 Å and from 2.38 to 2.75 Å, respectively, while the Au···O1 distance decreases to 2.16 Å. In ethanol, the Oe−H1 bond lengthens from 0.96 to 0.97 Å due to electron transfer from Oe to Au. The negative charge of Oe is reduced to −0.77e. Then, the reaction proceeds via the transition state (TS1_1) as the ethanol O−H bond is broken to form the ethoxide intermediate on the Au active site. The Oe−H1 bond distance is elongated from 0.97 to 1.61 Å, and the Au···H1 distance shortens to 1.59 Å. The normal-mode analysis discloses one imaginary frequency at 847i cm−1, which corresponds to movement along the reaction coordinate where the Oe−H1 bond finally breaks and the Au−Oe and Au−H1 bonds form 18567



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Figure 3. Energy profile and structures of reactants, intermediates, and transition states for the dehydrogenation of ethanol to acetaldehyde on the Au−O/ZSM-5 zeolite (energies in kcal/mol).

acetaldehyde product and hydrogen molecule (Prod_1) are adsorbed on the Au active site with an adsorption energy of −25.0 kcal/mol. 3.2.2. Ethanol Dehydrogenation on Au−O/ZSM-5. The same process on the Au−O/ZSM-5 zeolite starts again with the adsorption of an ethanol molecule (Ads_2) on the [Au−O] active site. The alcoholic OH interacts with the active 2-fold [Au−O] site. The lone-pair electron of the oxygen atom (Oe) binds to the Au atom with an O−Au distance of 2.37 Å, and a hydrogen bond between the ethanol hydrogen (H1) and the active surface oxygen (O) of the [Au−O] site with an O···H1 distance of 1.95 Å is formed. This adsorption structure is similar to the one for methanol and ethanol adsorbed on Fe− O/ZSM-5 and also for Ga−O/ZSM-5.44,83 In the adsorption complex, the Au−O bond is stretched from 1.90 to 2.03 Å upon ethanol adsorption, and the weakening of the O−H bond of ethanol changes its length from 0.96 to 0.98 Å. The negative partial charge of Oe decreases from −0.77e to −0.72e. The adsorption energy is calculated to be −14.4 kcal/mol. Following the ethanol adsorption, the ethoxide and hydroxide intermediate is generated via the transition state (TS1_2) where the alcoholic hydrogen atom starts to transfer to the active surface oxygen atom. The Oe−H1 bond distance is elongated from 0.98 to 1.26 Å, while the O···H1 distance of 1.17 Å shows that a real bond is being formed. The normalmode analysis shows one imaginary frequency at 1551.8i cm−1, corresponding to H movement along the reaction coordinate. The activation energy of this process is 9.0 kcal/mol. Then the transition-state ethoxide and hydroxide intermediate (Int_2) is

simultaneously. An activation energy of 43.2 kcal/mol is required to reach TS1_1. The result of this first step is the ethoxide intermediate (Int_1) and hydrogen adsorbed on the Au site of the Au/ZSM-5 zeolite (Figure 2). The Au−Oe distance becomes 2.01 Å, which can be compared to the Au−O distance of ∼2.2 Å for an ethoxy species on an Au surface.22 From DFT calculations, this surface ethoxy intermediate is also regarded as an important intermediate for ethanol dehydrogenation and decomposition over several other transition-metal surface catalysts such as Rh,78,79 Co,80 Pd,81 and Pt3Sn.82 The partial charges on the Oe and H1 atoms are −0.62e and 0.07e, respectively. The complexation of this ethoxide is endothermic by 3.6 kcal/mol with respect to the isolated molecules. It is less stable than that of the ethanol adsorption complex (Ads_1). Therefore, the O−H bond dissociation into ethoxide on Au/ ZSM-5 is kinetic, and thermodynamic energetics are unfavorable. This has also been observed for the dissociation of the alcoholic O−H bond on the surfaces of Au22,76,77 and Pd.81 The subsequent last step is the decomposition of the ethoxide intermediate into acetaldehyde and hydrogen products. In the transition state of this step (TS2_1), hydrogen H2 is transferred from the ethoxide methylene carbon (C2) to interact with hydrogen H1 on the Au site of the intermediate, resulting in H2 formation. The C2 hybridization changes from tetrahedral (sp3) to planar (sp2). The C2−H2 bond is lengthened from 1.10 to 1.34 Å, and the H1···H2 distance is contracted to 1.03 Å when H2 is formed. The TS2_1 transition state has one imaginary frequency at 2037i cm−1. The activation energy of this process is 30.8 kcal/mol. Finally, the 18568



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ZSM-5 and Au−O/ZSM-5 catalysts are placed over each other. The dissociation of the O−H bond is the rate-determining step for both of them. The activation of the O−H bond on the Au site in Au/ZSM-5 requires 43.0 kcal/mol. This barrier is reduced by almost a factor of 5 on the [Au−O] site in the Au− O/ZSM-5 zeolite (9.0 kcal/mol). The reason is the assistance of the active oxygen on Au for the O−H bond dissociation. As can be seen from the reaction profiles, the O of the alcohol binds to Au while the oxygen attached to Au takes care of the hydrogen atom. This is also seen from our charge and electron density analysis; the polarization of the Au−O with the density of the unpaired electrons mostly localized on the O explains the active role of the surface O in dissociating the O−H bond of the alcoholic group. The complexation of the ethoxide formed on the Au species is endothermic by 3.6 kcal/mol on Au/ZSM5 and becomes exothermic (−21.0 kcal/mol) on Au−O/ZSM5. The results show quantitatively how the surface oxygen facilitates ethanol O−H bond dissociation and ethoxide species formation and causes them to become both kinetically and thermodynamically favorable. These results agree with a former theoretical study of the methanol O−H bond dissociation on a Au(111) surface.77 They are also consistent with experimental observations stating that the surface O is essential to the activation of the O−H bond of methanol and other alcohols on Au.85,86 The ethoxide intermediate is stabilized by the zeolite framework. Its decomposition to acetaldehyde in the subsequent step requires lower activation energies of 30.8 and 4.4 kcal/mol for both reactions on the Au/ZSM-5 and Au− O/ZSM-5 zeolites, respectively, than the ones in step 1. The differences between Au−O/ZSM-5 and Au/ZSM-5 are mainly caused by the highly active hydroxyl OH species on the Au site in Au−O/ZSM-5 when abstracting the hydrogen from the methylene carbon of the ethoxide species. Furthermore, one can consider the energy barriers for both the hydrogenation of the ethoxide and thereby reversing the reaction back to ethanol and the forward reaction of decomposition of ethoxide to the acetaldehyde product. On the Au/ZSM-5 zeolite, the hydrogenation of ethoxide is kinetically and thermodynamically favored over ethoxide decomposition. In contrast, on Au−O/ ZSM-5 the forward reaction toward the acetaldehyde product is both kinetically and thermodynamically favored. We also calculated the relative enthalpies (ΔH) and free energies (ΔG) for the reaction on both Au/ZSM-5 and AuO/ZSM-5 at 298.15 K. They are given in Table S1 of the Supporting Information. The results show that the trends in energies, enthalpies, and free energies are the same. To investigate the effect of the zeolite framework beyond the studied cluster model, we performed single-point calculations on an extended 120T cluster (Figure S1 in Supporting Information). The relative energies of the systems involved in the reaction are summarized in Table 3. Only small changes occurred. It was found that all complexes in both Au/ZSM-5 and Au−O/ZSM-5 are stabilized by the zeolite framework in the range of 2−4 kcal/mol. On the other hand, the activation energies were almost unaffected by the confinement in the ZSM-5 framework. They were 44.9 and 31.7 kcal/mol for Au/ ZSM-5 and 9.6 and 5.0 kcal/mol for Au−O/ZSM-5. The fact that only small changes occur is also in agreement with previous works.31,32,60

formed with Au···Oe and Au···O distances of 2.26 and 2.15 Å, respectively. Again, the structure of this intermediate is similar to the ones found in the course of the alcoholic O−H bond activation in Fe−O/ZSM-5 and Ga−O/ZSM-5 systems.44,83,84 The complexation reaction is now exothermic by −21.0 kcal/ mol. The reaction to produce ethoxide via O−H bond dissociation is thermodynamically favorable. The activation energy for the dissociation of the ethanol O− H bond on Au−O/ZSM-5 is significantly lower than that found on the Fe−O/ZSM-5 zeolite (Ea = 17.7 kcal/mol).44 The reason might be the higher reactivity of the surface O on Au at the [Au−O]+ active site which originates from an unpaired electron there. In contrast, the electron density of this unpaired electron resides mostly on Fe in Fe−O.44,84 However, the activation energy for dissociating the alcoholic O−H bond at Au−O/ZSM-5 is higher than on Ga−O/ZSM-5.83 On the other hand, this dissociation produces a thermodynamically highly stable intermediate on the GaO site (−68 kcal/mol)83 which is difficult to convert to the desired product and might coke the zeolite pore and deactivate the Ga−O/ZSM-5 zeolite. In the case of Au−O/ZSM-5, the less-stable intermediate can much more easily be converted to the product. The second half of the process is the decomposition of the intermediate (Int_2) to the acetaldehyde product. There, the second hydrogen (H2) is abstracted from the methylene carbon (C1) of ethoxide to the hydroxyl group of the intermediate: in the transition state (TS2_2), the C1−H2 bond is broken, H2 transfers to O, and the hybridization of C2 changes from tetrahedral (sp3) to planar (sp2). The intramolecular C1−H2 distance increases to 1.31 Å while the O··· H2 distance decreases to 1.33 Å. One imaginary frequency at 770.1i cm−1 relates to the movement of H2 to O and to the C1−H2 bond breaking. The activation energy of this process with respect to the intermediate complex is 4.4 kcal/mol. After this migration, the acetaldehyde product and water molecule are formed (Prod_2) and acetaldehyde is adsorbed on the Au site with the neighboring water molecule. The Au···Oe and the Au···O distances are 2.11 and 2.46 Å, respectively. With respect to isolated compounds, the complexation energy is −25.0 kcal/ mol. 3.3. Comparison of Ethanol Dehydrogenation on Au/ ZSM-5 and Au-O/ZSM-5 Zeolites. In Figure 4, the energy profiles for the ethanol to acetaldehyde conversion on Au/

4. CONCLUSIONS The reaction mechanism of the ethanol dehydrogenation to acetaldehyde over Au/ZSM-5 and Au−O/ZSM-5 zeolites was

Figure 4. Comparison of the energy profiles from Figures 2 (Au/ ZSM-5, solid line) and 3 (Au−O/ZSM-5, dashed line). 18569



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Table 3. Relative Energies with Respect to the Reactants for the Dehydrogenation of Ethanol to Acetaldehyde on Au/ZSM-5 and Au−O/ZSM-5 Zeolites Calculated with M06-L/6-31G(d,p) relative energies (kcal/mol) Au/ZSM-5

Au−O/ZSM-5

reaction coordinates

12T

120T

12T

120T

ethanol adsorption transition state 1 intermediate transition state 2 product

−39.7 3.5 (Ea1 = 43.2) −3.6 27.2 (Ea2 = 30.8) −20.2

−43.6 1.3 (Ea1 = 44.9) −5.7 26.0 (Ea2 = 31.7) −24.5

−14.4 −5.4 (Ea1 = 9.0) −21.0 −16.6 (Ea2 = 4.4) −25.0

−16.8 −7.2 (Ea1 = 9.6) −23.5 −18.5 (Ea2 = 5.0) −26.6

National Center of Excellence for Petroleum, Petrochemical and Advanced Materials (NCE-PPAM)).

investigated by utilizing 12T cluster models and the M06-L density functional. The Au−O/ZSM-5 zeolite has been modeled to investigate the effect of surface oxygen on this reaction. The first step is the ethanol O−H bond dissociation leading to the formation of the ethoxide intermediate. The activation barriers for this step are 43.0 and 9.0 kcal/mol for the reaction on Au/ZSM-5 and Au−O/ZSM-5 zeolites, respectively. Consequently, the ethoxide intermediate was decomposed into an acetaldehyde product by way of transferring the hydrogen at the methylene carbon to interact with the hydrogen at the Au site for the Au/ZSM-5 zeolite and to the hydroxyl oxygen for the Au−O/ZSM-5 zeolite. In this step, somewhat smaller activation barriers of 30.8 and 4.4 kcal/mol for Au/ZSM-5 and Au−O/ZSM-5 zeolites, respectively, are encountered. The conversion of ethoxide to the acetaldehyde product on Au−O/ZSM-5 is again more favorable than on Au/ ZSM-5 because the onset of H2O formation stabilizes the transition state more than does the corresponding H 2 formation. Altogether, the surface oxygen, which might be easily generated from the decomposition of an oxidizing agent, assists the reaction by reducing the activation barrier for all steps and thus makes it both kinetically and thermodynamically feasible.

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ASSOCIATED CONTENT

S Supporting Information *

The 120T cluster model of Au/ZSM-5 and Au−O/ZSM-5 zeolites. Relative energies, ΔE, ΔH, and ΔG values at 298.15 K for the dehydrogenation of ethanol to acetaldehyde on Au/ ZSM-5 and Au−O/ZSM-5 zeolites calculated with M06-L/631G(d,p). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +66-2-562-5555 ext 2159. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported 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 the NANOTEC Center of Excellence funded by the National Nanotechnology Center), the Thailand Research Fund (TRF), and the Commission on Higher Education, Ministry of Education (the National Research University Project of Thailand (NRU) and the 18570



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