Theoretical study of the adsorption of n-C4H10 on V2O5/SBA-15 by density functional theory

JOURNAL OF SCIENCE OF HNUE Chemical and Biological Sci., 2012, Vol. 57, No. 8, pp. 36-42 This paper is available online at THEORETICAL STUDY OF THE ADSORPTION OF n-C4H10 ON V2O5/SBA-15 BY DENSITY FUNCTIONAL THEORY Tran Thi Thoa and Nguyen Ngoc Ha Faculty of Chemistry, Hanoi National University of Education Abstract. The adsorption of n-C4H10 on V4O10, SV4, SV5, SV6, SV7 catalysts were investigated using the density functional theory (DFT). The obtained calculations indicated that SV4 was the best adsorbent and the interaction between the methylene group and V–O(1) was the most energetic and therefore favorable pathway among the studied reactions. Keywords: DFT, ODH, adsorption, activation energy. 1. Introduction The oxidative dehydrogenation (ODH) of light alkanes is one of the best routes to produce alkenes. Experimental studies have proposed that the ODH reaction of light alkanes over vanadium based catalysts occurs following a Mars-van Krevelen redox mechanism [3, 4] in which the first step is a weak adsorption of light alkanes by interaction with lattice oxygen. Theoretical and experimental studies of the adsorptions of n-butane over a catalyst are still rare. This paper presents the calculations in the adsorptions of n-butane over V4O10, SV4, SV5, SV6 and SV7 catalysts using the DFT method. 2. Content 2.1. Computational methodology All quantum chemical calculations of the geometry and energies of ground states and transition-state structures have been performed using the DFT method with nonlocal, gradient-corrected DFT. The Perdew, Burke and Ernzerhof (PBE) gradient-corrected functional [5] was employed in the calculation of the exchange-correlation energy. This method is implemented in the SIESTA code [6]. The Double Zeta basis plus Polarization orbitals (DZP) for valence electrons has been used for all calculations while core electrons are “frozen” in their atomic Received June 4, 2012. Accepted October 1, 2012. Contact Nguyen Ngoc Ha, e-mail address: hann@hnue.edu.vn 36 Theoretical study of the adsorption of n-C4H10 on V2O5/SBA-15 by density functional theory state and replaced by norm-conserving pseudopotentials [7] in its fully nonlocal (Kleinman-Bylander). Transition states have been found using the Nudged Elastic Band (NEB) method [8, 9] and the forces acting on dynamic atoms are all smaller than 0.1 eV/A˚. All calculations are applied using the spin polarization model. 2.2. Result and discussion 2.2.1. The catalytic systems Figure 1. Structures of some catalytic systems V4O10 is one of the most stable clusters of V2O5, confirmed by many experimental studies so it can be used to replace V2O5 crystal. SV4 stands for V4O10 placed on SBA-15 surface. SV5, SV6, SV7 catalysts obtained by substituting one, two, three silicon atoms in the SBA-15 structure by one, two, three vanadium atoms. All of the catalytic systems contain the vanadyl V–O(1) site and the V–O(2)–V site (see Figure 1). Some characters of the O(1) and O(2) sites, which influence the adsorption process of n-butane, are given as follows: - The possibility of losing/gaining electron of each site. In the ODH reaction, V+5 is the oxidizing agent while alkanes act as reducing agents and the O atom acts as a bridge. Electrons transfer from alkanes to V+5 [1, 13] through the O atoms. - Steric hindrance at each site. These factors decided a priority of each direction (O(1) and O(2)). 37 Tran Thi Thoa and Nguyen Ngoc Ha Many studies have shown that vanadyl V–O(1) is the most active site in the ODH of C4H10 [2], the most favorable step is an O(1) insertion into the C–H bond of the methylene group [2 and 10-12]. 2.2.2. The adsorption of n-C4H10 on the surface of SBA-15 The optimized result shows that the adsorption is only a physical adsorption with adsorption energy (Eads) Eads = Ef – Ei Eads = -7228.397 - (-7227.897) = -0.500 (eV) = -11.542 (kcal/mol) in which Ei and Ef denote the energies of initial and final states. 2.2.3. The adsorption of n-C4H10 on V4O10 Because two different lattice oxygen particles are available on the V4O10 surface (Figure 1), starting from the C4H10/V4O10 system, four possible interactions between lattice oxygen particles and butane were considered: CH2–O (1), CH2–O (2), CH3–O (1) and CH3–O (2). The activation energies (Ea ) and adsorption energies (Eads ) of the adsorption processes are shown in Table 1. Table 1. Ea and Eads corresponded to the interactions between lattice oxygen and n-butane in the adsorption process Energies CH2 CH3 O (1) O (2) O (1) O (2) Ea (kcal/ mol) 21.552 22.521 33.608 27.962 Eads (kcal/ mol) -17.182 -29.233 -15.727 -22.587 From the results presented in Table 1, it is clear that: - For n-butane, the interaction of the CH2 group with the surface was more likely than the CH3 group in all cases. - Regarding the catalyst, it was found that the O(1) was better than the O(2) site in comparison to the CH2 direction and in contrast to the CH3 direction. - The CH2–O (1) interaction was the most convenient path of the four reaction pathways studied because of the lowest energy barrier in relation to other ways. This conclusion is in good agreement with results from [1]. * Reaction at the O(1) site The CH2–O (1) interaction involved the insertion of the vanadyl oxygen into a methylen group to form the transition state denoted as TS.1.1 (see Figure 2). 38 Theoretical study of the adsorption of n-C4H10 on V2O5/SBA-15 by density functional theory At this TS.1.1, the distance between the C atom and the O(1) atom decreased from 3.036 to 2.580 A˚ and the O–H distance was shortened to 0.975 A˚ indicating that the OH group was nearly formed (0.960 A˚ in the experimental H–O bond length). In structure 3 (the related intermediate structure), the distances of the C–O (1) bond and H–O (1) bonds were 1.478 and 0.978 A˚, respectively. Using the NEB method, an activation barrier of 21.552 kcal/mol was calculated, indicating that this is the most energetically favorable path of the four reaction paths studied. Figure 2. Some structures in the CH2–O (1) direction The CH3–O (1) interaction started with the insertion of the O(1) into a methyl group to form the transition state. The calculation resulted in an activation barrier of 33.608 kcal/mol for this reaction, which is 12.056 kcal/mol higher than that of the CH2–O (1) interaction. Thus, this interaction is less favorable compared with that of CH2–O (1). This is understandable because the bonding energy of C–OH in the methylene group is 98.5 kcal/mol, which is weaker than it is in the methyl group (101.0 kcal/mol). 39 Tran Thi Thoa and Nguyen Ngoc Ha Furthermore, structures in the CH2–O (1) direction are more stable than those in CH3–O (1) due to the stronger hyperconjugations (called H) from CH3 and CH2 species. Structures in the CH2–O (1) reaction pathway were stabilized by 5 H, but those in CH3–O (1) were only stabilized by 2 H. * Reaction at the O(2) site The pathways of this site that were studied were similar to those of the O(1) site. The calculated results revealed that the process of forming transition states TS.2.1 and TS.2.2 have activation energies of 22.521 and 27.962 kcal/mol, respectively. So, the CH3 direction was less favorable compared to the CH2 direction at O(2) with regards to adsorption of n-butane. For the CH2–O (2) reaction pathway, CH2–O (2) interaction involved the breaking of a C–H bond in a methylene and the formation of an H–O (2) bond to form the transition state TS.2.1. This state was stabilized by the hyperconjugations from the CH3 and CH2 groups (5 H). Similarly, for the CH3–O (2) direction, CH3–O (2) interaction included the breaking of the C–H bond in a methyl and the formation of an H–O (2) bond to form the transition state TS.2.2. This TS was stabilized by the hyperconjugations from a CH2 group (2 H). In addition, when comparing the O(1) and O(2) pathways, it can be seen that the attaction of the CH2 group at the O(1) site was more convenient than at the O(2) site. The explanation for this is based on atoms’ ability to lose/ gain electrons and states in the electron transfer process of the adsorption step of n-butane on the catalyst [2]. Conversely, the O(2) site is more convenient than the O(1) site in the CH3 pathway. This is due to a steric effect. 2.2.4. The adsorption of n-butane on SV4, SV5, SV6 and SV7 catalysts The calculation results are revealed in Table 2. Table 2. Eads, Ea (kcal/ mol) corresponded to adsorption processes on SV4, SV5, SV6 and SV7 SV5 SV6 SV7 SV4 Systems Ea Eads Ea Eads Ea Eads Ea Eads CH2–O (1) 21.509 -14.495 21.168 -14.211 22.250 -12.856 19.331 -19.489 CH2–O (2) 26.523 -27.335 25.219 -31.586 28.289 -29.889 28.347 -25.568 CH3–O (1) 35.064 -9.424 32.561 -10.560 35.893 11.321 35.498 -9.186 CH3–O (2) 29.352 -21.831 31.163 -19.367 27.517 -20.605 30.241 -21.423 Table 2 indicated that in the adsorption process: - In each of the catalytic systems, the CH2–O (1) interaction was the most favorable pathway. - The difference between the adsorption and activation energies between pathways is very little. The CH2–O (2) and CH3–O (1) directions in adsorption on the SV6 catalyst, 40 Theoretical study of the adsorption of n-C4H10 on V2O5/SBA-15 by density functional theory the CH2–O (1) direction on SV4 and the CH3–O (2) direction on SV7 are most convenient in terms of thermodynamics because of the low value of activation energies (Ea). 2.2.5. Comparing the adsorption of n-butane on the surface of V4O10, SV4, SV5, SV6 and SV7 catalytic systems Table 3. Ea and Eads of adsorption steps of n-C4H10 on V4O10, SV4, SV5, SV6 and SV7 in the CH2 and CH3 direction Ea (kcal/ mol) Eads (kcal/ mol) Systems O(1) O(2) O(1) O(2) CH2 CH3 CH2 CH3 CH2 CH3 CH2 CH3 V4O10 21.552 33.608 22.521 27.962 -17.182 -15.727 -29.233 -22.587 SV4 19.331 35.498 28.347 30.241 -19.489 -9.186 -25.568 -21.423 SV5 21.509 35.064 26.523 29.352 -14.495 -9.424 -27.335 -21.831 SV6 21.168 32.561 25.219 31.163 -14.211 -10.560 -31.586 -19.367 SV7 22.250 35.893 28.289 27.517 -12.856 11.321 -29.889 -20.605 By comparing the activation energies of the adsorptions (see Tables 1 and 2), one can come to the following conclusions: - For each catalyst, the CH2–O (1) interaction is the most favorable path. - In pure V4O10, the interaction between CH2–O (2) is the most favorable. - In SV4, the CH2–O (1) and CH3–O (2) directions are the most favorable. - In SV6, the CH3–O (1) direction is pronounced. Thus, when V4O10 was placed on the SBA-15 forming SV4, SV5, SV6 and SV7 systems, the CH2–O (1) direction is still the most favorable in the adsorption step. For this direction, the SV4 system is the best catalyst in both the kinetic and thermodynamic aspects because of the low activation and adsorption energies. 3. Conclusion This study is a theoretical study on the mechanism of the adsorption step in the oxidative dehydrogenation of n-C4H10. The catalysts under investigation are based on V4O10 and V4O10/SBA-15. 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