Density Functional Study on the Transition State of Methane Activation

Chapter 25

Density Functional Study on the Transition State of Methane Activation over Ion-Exchanged ZSM-5 Yusuke Ueda, Hirotaka Tsuruya, Tomonori Kanougi, Yasunori Oumi, Momoji Kubo, Abhijit Chatterjee, Kazuo Teraishi, Ewa Broclawik, and Akira Miyamoto

Downloaded by UNIV OF ARIZONA on August 7, 2012 | http://pubs.acs.org Publication Date: April 8, 1999 | doi: 10.1021/bk-1999-0721.ch025

Department of Materials Chemistry, Graduate School of Engineering, Tohoku University, Aoba-ku, Sendai 980-8579, Japan In order to investigate the methane dissociation over Ga-ZSM-5, the transition state of the reaction pathway was determined by density functional calculations (DFT). We revealed that the activation energy is 25.8 kcal/mol and that the chemisorption state in which CH is attached to the Ga ion and H forms a hydroxyl group with the extraframework oxygen is the main product. We also investigated the influence of the cluster size on the transition state of methane dissociation. It was shown that [Al(OH) ] can express the trend and is adequate for use to model the zeolytic structure. The transition states of the methane dissociation reaction over Al-ZSM-5 and In-ZSM-5 were also investigated. 3

3+

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4

1. Introduction The main role of the catalyst is to lower the activation energy of the reaction by stabilizing the transition state. However, as the life time of the transition state is of pico seconds order, only a limited experimental research has so far been performed. Recently, with the development of the femtosecond pulsed laser (FPL), it has become possible to observe transition states experimentally (L2). However, complicated systems such as inhomogeneous catalytic reactions cannot be analyzed by FPL. On the other hand, computational technique to analyze the transition state is already established, and is routinely used to investigate the reaction pathway. Therefore, it would be the suitable method to study the catalytic reaction mechanism. Being responsible for the acid rain which destroy the global environment, nitrogen oxides must be removed from the automobile emission. The cationexchanged zeolitic catalysts such as Cu- (J), Co- (4,5), Mn- (6), Ni- (6), Pd- (7), Ga(8,9), In- (/OK Ce- (//), and H-ZSM-5 (12) as well as Cu- (13) and Co-mordenite (6) and Cu-SAPO-34 (14) have been actively investigated as catalysts for the selective reduction of nitrogen monoxide using hydrocarbons as reductants under the excess oxygen condition. In particular, as methane is present in the exhaust gas, catalysts which are active in NOx decomposition with methane as reductant are desirable. Gaand In- exchanged ZSM-5, introduced by Yogo and co-workers (8,10) are one of such catalysts and have been studied extensively. Methane is notoriously an inactive agent

© 1999 American Chemical Society

In Transition State Modeling for Catalysis; Truhlar, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

321

322 (15) and only the TPD (temperature programmed desorption) investigation has so far been carried out to study the dissociative mechanism over Ga-ZSM-5 (16). However, consensus has already been obtained that the dissociative adsorption of methane is the rate-determining step in many other heterogeneous catalytic processes such as steam reforming, alkylation of hydrocarbons and combustion (17). In this study, in order to investigate the reaction path of methane dissociation over Ga-ZSM-5, the transition state and activation energy were determined by density functional calculations (DFT). Two possible routes for the methane dissociation over Ga-ZSM-5 are considered here. The first route leads to a methyl attached to the Ga ion and a hydrogen forming a hydroxyl group with the extraframework oxygen (route 1). The second one corresponds to the reverse combination, i.e. H is attached to Ga * ion and CH^ is connected to the extraframework oxygen to make a methoxyl group (route 2). We already revealed by DFT calculations that the product of the former route is preferred in view of energy stabilization. Both reaction routes yield highly stable products in comparison to the methane physisorbed systems (18). Based on this result, we extended our investigations to fully describe the reaction pathway of methane dissociation in the present work. For this purpose, transition state search calculations were performed and the structures, molecular orbitals and the activation energies were analyzed. First of all, in order to clarify which reaction path is more feasible, either route 1 or route 2, the activation energies of both routes were compared to confirm the actual dissociation process (transition state of route 1 was already reported in ref. (19)). Secondly, in order to validate our choice of the cluster model, influence of cluster size on the transition state of the methane dissociation was investigated. Finally, because activity, selectivity and durability depend on the exchange cations present in ZSM-5, the catalytic performance of three exchange cations, namely, Ga, In and Al was investigated. The same calculations were thus performed on the physisorption state, chemisorption state and transition state of methane in Al-ZSM-5 and In-ZSM-5. 3+


3

2. Method and model Although the crystal structure was determined by X-ray diffraction study (20j, the physical and chemical state of the active gallium species in H-ZSM-5 zeolites has not been unequivocally established. There exists, however, strong evidence that Ga is present in a highly dispersed monomelic form, coordinated to basic oxygen within zeolite channels (21). In a reducing atmosphere it could be a reduced hydride moiety, while in the presence of excess oxygen an oxidized [GaO] unit may be proposed. Actually [GaO] species in Ga-ZSM-5 are suggested by some experiments (22.23). Furthermore, earlier quantum chemical studies have reported that T12 is the energetically favorable site for the incorporation of aluminum (24). Therefore in the present study [GaO] was assumed to be the active site in the framework of ZSM-5 whose T12 site was substituted by aluminum. MD calculation was performed under periodic boundary condition to determine the conformation of Ga-exchanged site within the zeolitic lattice (Figure la). A single A10 tetrahedron which represents the active site, was extracted as the cluster +

+

+

4



323

Figure 1 (a) Structure of Ga-ZSM-5 framework obtained by molecular dynamics, and (b) the [Al(OH) ]GaO model cluster of the Ga active center in GaZSM5. 4

model for quantum chemical calculations, where adjacent silicons were replaced by hydrogens and charge deficiency was compensated by [GaO] cationic species (Figure lb). This cluster model is the same as that used in our earlier works (18,19). Of this model, zeolitic framework including terminal hydrogen atoms were fixed and only [GaO] unit and reacting methane were optimized. Our preliminary calculation at LDA level using large cluster model (vide infra) revealed that relaxation of framework near Al site (Al(OSi) moiety) upon the dissociative adsorption of methane stabilizes the system by 7.0kcal/mol. Since only relative energies are discussed, relaxation effect should be even less than this value. Transition state search was also conducted within the same space of freedom by locating the point where the first derivative of the energy vanishes while the second derivative (Hessian) matrix has only one negative eigenvalue. MD calculations were carried out with the MXDORTO program developed by Kawamura (25) for the determination of the structure of the zeolite framework. The Verlet algorithm (26) was used for the calculation of atomic motions, while the Ewald method (27) was applied for the calculation of electrostatic interactions. DFT calculations were performed by solving Kohn-Sham equation self-consistently (28) as implemented in the DMol program (29-31). We employed local density approximation (LDA) with Vosko-Wilk-Nusair (VWN) functional (32) for geometry optimization. In order to calculate accurate energies, we applied the Beck-Lee-Yang-Parr (BLYP) (33) nonlocal functional as a correction. Double numerical with polarization functions (DNP) basis set was used. +

+

4


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3. Result and Discussion 3. 1 Reaction pathway and transition state for methane dissociation over GaZSM-5. As described in introduction, we considered two chemisorption models. The transition state of methane activation process (route 1) was calculated by DFT following the procedure described in ref. (19). Our cluster model is strictly based on the MD result, while in our earlier work Cs symmetry was applied in order to reduce the computational cost. In the first step the physisorbed state was calculated. This system may be described as the complete methane molecule interacting weakly with the Ga cation. From now on we will call the hydrogen which is abstracted from methane molecule dissociative hydrogen. We selected the distance between the dissociative hydrogen and the extraframework oxygen as the approximate reaction coordinate. In the second step the energy profile was calculated with respect to this coordinate keeping all other parameters fixed in order to approximately locate the transition state region. The energy maximum in this profile was found at the O-H and C-H distance being 1.6 and 1.8 A, respectively. This geometry was then taken as the initial coordinate in the next step of calculation in which the full transition state optimization was performed. Transition state structure was confirmed by the analysis of the eigenvector of Hessian which corresponds to the negative eigenvalue. The normal coordinate consists almost exclusively of the in-plane movement of the dissociative hydrogen between its neighboring carbon and oxygen atoms, supplemented by the Ga-0 stretch, which also supports our choice of the approximate reaction coordinate. Transition state of the route 2 was calculated by the similar procedure. The situation is more complicated for route 2 as only one bond length can not represent the reaction coordinate. Therefore in this route we varied two bond distances, namely, between the dissociative hydrogen and carbon and between carbon and extraframework oxygen, and drew an energy profile. The energy maximum in this profile was found at the O-C and C-H distance of 2.0 and 1.5 A, respectively. Also in this case, the structure at energy maximum was employed as the initial state for the transition state search. Figure 2 shows the reaction pathway and the transition state for methane dissociation. The adsorption energy was defined by eq. I: ^adsorption "~ ^host+guesi ~~ ^ h o s l "*"^gucst)

( ^)

Here £ , £ , and £ denote the total energy of the system including both the host zeolite cluster and the incorporated guest molecules, the bare zeolite cluster, and the adsorbate molecule in the gas phase, respectively. As seen from the figure, the adsorption energy of the methane at transition state and chemisorption state of route 1 are -1-28.6 kcal/mol and -39.8 kcal/mol. respectively. These values are slightly different from what are reported in ref. (19), as in our earlier work, the geometry of Ga-ZSM-5 cluster was different from the present model. The adsorption energy of the methane at transition state and chemisorption state of route 2 are +68.6 kcal/mol and -20.9 kcal/mol respectively. From the comparison of the adsorption energy, it was found that the activation energy of route 1 is about 40 kcal/mol lower than that of route 2. As a result, we can confirm that route 1 is the main reaction path, where hydrogen atom is abstracted by extraframework oxygen attached to Ga atom. host+auest

host

>iuest


325 H


I Transition state I (route 2) J 68.6 kcal/mol

\

-20.9 kcal/mol

Chemisorption state (route I) -39.8 kcal/mol Figure 2 Reaction pathway and transition state of methane dissociation in Ga ZSM-5


326 Table 1 Geometrical parameters of transition states.


C-Dissociative H Ga-Extraframework 0 Extraframework 0Dissociative H Ga-Dissociative H Ga-C Extraframework 0-C

(A) (A) (A) (A) (A) (A)

Transition state of route I 1.35 1.72 1.46

Transition state of route 2 1.63 1.73

-

1.64

-

2.21

-

-

2.10

The geometrical parameters at transition state of both routes are listed in Table 1. One can see that the C-H distance of route 1 is smaller than that of route 2. It means that in route 1 cleaving C-H bond and forming 0-H bond reaches the balancing point earlier than in the case of route 2 due to the strong O-H interaction. It may be considered that the interaction of H with O is more favorable than the interaction of H with Ga because O and Ga are electron-acceptor and electron-donor, respectively. As a result, activation energy of route I is lower than that of route 2. 3.2 Effect of cluster size. One of the problems that arises when target is large is the choice of the model. It is still computationally very difficult to include the entire zeolite into the quantum chemical calculation. In order to study efficiently, the model should be as small as possible but large enough to express the phenomena of interest. Thus we investigated the influence of cluster size on the activation energy and the structure of transition state so as to examine if the cluster size is adequate. The large cluster model shown in Figure 3 was constructed by replacing the terminating hydrogen of small cluster model with Si(OH)

Density Functional Study on the Transition State of Methane Activation

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