GAO Xio-Zhen PANG Yu YANG Jing, YANG Xio-Chun SHEN Yu-Long② JIA Jing-Xin MENG Xing-Jun

a (Yangquan Municipal Key Laboratory of Quantum Manipulation, Shanxi Institute of Technology, Yangquan 045000, China)

b (Department of Chemistry, Tangshan Normal College, Tangshan 063000, China)

c (Department of Atmospheric Sciences, Texas A&M university, TX 77843, USA )

ABSTRACT We propose the complicated catalytic mechanisms for the acetic acid molecule catalyzed by transition metal oxide MoO2 based on density functional theory calculations. The geometries and energetic values of all stationaries and transition states involved in the three different reaction pathways (Channels I, II and III) are reported and analyzed. All reaction mechanisms are fully different from that of MoxOy catalyzing volatile organic compounds (VOCs) in previous studies. The completely new mechanisms catalyzed by MoO2 for acetic acid have been discovered for the first time. Channels I (IA and IB) and II are both addition reactions and channel III is hydrogen abstraction reaction by producing a leaving group. The barrier energies of reaction are also compared with other catalytic reactions, showing that MoO2 catalyst expresses a lower barrier energy (8.22 kcal/mol) by addition reaction, which represents MoO2 tends to absorb acetic acid pollution gas via addition reaction rather than release toxic substances. This also means that MoO2 is a more effective and representative catalyst and is suitable for further study of catalytic carboxylic acids, so the reaction mechanisms may provide a useful theoretical guidance and solution for the catalysis of carboxylic acids.

Keywords: reaction mechanism, acetic acid elimination, molybdenum dioxide, addition reaction, hydrogen abstraction reaction; DOI: 10.14102/j.cnki.0254-5861.2011-2485

1 INTRODUCTION

Transition metal oxides (TMOs), including molybdenum oxide, vanadium oxide, titanium dioxide, germanium dioxide, tungsten oxide, nickel oxide, and zinc oxide, have stimulated great interest because of their optical, electrical, and mechanical properties and catalytic performance. In particular, molybdenum oxide has drawn tremendous attention for its excellent properties[1,2]. It's reported that earth-abundant MoO3can selectively cleave the C-O bond in lignin under low H2pressure (≤1 bar)[3]to turn lignin into value-added chemicals or fuel[4], which makes full use of the biological energy of lignin. Besides, many research groups have investigated the reactivity of molybdenum oxide anions with several inorganic small molecules, including CO[5,6],and9]. They recently completed a series of experimental and computational studies on the reactions between molybdenum oxide cluster anions and water,+ H2O, showing that the molecular scale interactions between water and oxygen vacancies oncatalysts can lead to photocatalytic H2production[8-10]. Since H2from water has more energy-efficient than other fuels, the reaction of MoxOycluster with water is of outstanding value. Catalysts containing MoxOyas an active component have been extensively employed for a hydroprocessing reactions for industrial catalysis. For example, the development reactions for industrial catalysis. For example, the development in petroleum refining technology has raised hydroprocessing reactions to a high level of economic importance[11].

In recent years, MoxOycatalyed VOC pollution is a subject of continuing interest. Ethanol can be converted by MoxOycluster into other substances[12]. Alternatively, it has been shown that formic acid can be converted to carbon dioxide over MoxOy[13]. Carboxylic acids are ubiquitous and important chemical components of the troposphere, adding the acidity of atmosphere precipitation up to 64%[14]. The acid rain pollution has seriously affected human health and the natural environment. Besides, Kawamura. et al.[15]determined the presence and distribution of organic acids

(C(1)-C(10)) in the atmosphere. The results showed that acetic acid was the most abundant species followed by formic and propionic acids in the atmosphere[15]. Previously wet oxidation of various organic compounds including dyes, amides, and water-soluble polymers was carried out, and it was found that acetic acid was one of the most refractory compounds[16]. Organic pollutants degrade to lower molecular weight compounds and refractory acetic acid accumulates at the later stage of wet oxidation[16]. Therefore, complete catalysis of acetic acid has high research value. Furthermore, in our prior studies, we focused on the comparison of reaction mechanisms of MoOx+ HCHO (x= 1, 2, 3) and found that MoO2was the optimal catalyst[17]. Motivated by the rather poorly understand mechanistic details of MoxOy+ CH3COOH and with a desire to discover new class of reactions, we have embarked on a comprehensive program to study the reaction mechanism of MoO2+ CH3COOH.

In this study, optimization and design of MoO2are crucial due to the low abundance of active sites, which tends to be on defect sites such as oxygen vacancies[18]. So, in this paper, DFT method is used to systematically study the reaction mechanism. We hope our theoretical study can expand the direction of scientific study and lay the foundation for the next stage of experiment.

2 CALCULATION METHODS

The density functional theory (DFT) methods play a critical important role in the electronic structure, especially when organic compounds and transition metal oxides are included[19-28]. By trying different DFT methods, we found that B3LYP method is very precise in representing molecular structure[29-31]. Therefore, the structures of the reactants, intermediates, transition states and products have been fully optimized at the B3LYP/gen level and all calculations are carried out on the Gaussian09 software package[32].

The B3LYP method is an HF/DFT hybrid method using Becke's three-parameter functional (B3)[33]and the Lee- Yang-Parr generalized gradient correlation functional (LYP)[34]. ‘‘Gen'' is a general basis set obtained SDD[35,36](Stuttgart-Dresden ECP plus DZ) and DZP (Double-ξplus polarization)[37]. The SDD basis set with an effective core potential (ECP) was used for the molybdenum atoms. The core electrons of Mo atoms are replaced by an effective core potential (ECP) and the effective core approximation may become significant for the optimization of Mo atoms. For the C and O atoms, the allelectron DZP basis sets are used. They are the Huzinaga- Dunning contracted double-ζcontraction sets[38]plus a set of spherical harmonicdpolarization functions with the orbital exponentsαd(C) = 0.75 andαd(O) = 0.85. The DZP basis sets for C and O atoms may be designated as (9s5p1d/4s2p1d)[39]. For hydrogen, a set ofppolarization functionsαp(H) = 0.75 are added to the Huzinaga-Dunning DZ set[13,40]. The energy and harmonic frequency of the molecules are better described by using these basis sets.

To confirm the transition states and make zero point energy (ZPE) corrections[41], frequency analyses are done with the same basis set as that for the geometry optimizations. The zero-point energy corrections were taken from these reaction energy barriers. The Hartree-Fock (HF) levels theory[42,43]have contained geometric structures, energies, and harmonic frequencies, Gibbs free energies. Intrinsic reaction coordinate (IRC)[44,45]calculations are also performed to verify whether the reaction pathway of individual chemical reaction correctly connects the stationary points under consideration. Finally, all transition states are identified by having a single imaginary frequency. Mean- while, the calculations of reaction rate constant of rate- limiting steps utilize the transition state theory (TST) ex- pressed as Eq. 1[46].

wherecis the standard-state concentration (1 mol?L-1), △n is the change of the number of moles from reactions to transition states, kBis the Boltzmann constant, h is the Planck constant, ΔG(activation free energies) is the difference of the Gibbs free energy, andRis the gas constant.

3 RESULTS AND DISCUSSION

Acetic acid has three kinds of potential binding sites in the course of MoO2attacking: one is C atom in carboxy or methyl group, and the other is O atom of hydroxy group. Two different addition reaction mechanisms (Channels I and II) and a hydrogen abstraction reaction (Channel III) are found by studying the reaction of MoO2with acetic acid. All reaction pathways and molecular formulas are shown in Scheme 1. The optimized geometries of the reactants, intermediates, and products involved in the reaction at the B3LYP/gen level of theory are depicted in Figs. 1～4. For convenient discussion, the energy of CH3COOH + MoO2is set as zero for reference and relative energies of geometries are shown in Figs. 5～7. Harmonic vibrational frequencies, zero point energies, absolute energies and corrected energies computed for the stationary points and transition states at the B3LYP/gen level of theory are given in support information (Tables S1 and S2).

Scheme 1. An overview of all reaction mechanisms

Fig. 1. Optimized geometries of important intermediates and transition states computed at the B3LYP/gen level for the reaction of CH3COOH + MoO2. Bond lengths are in angstrom and angles in degree (Complex 1～C1-17)

3. 1 Mechanism of reaction I

Channel I describes the cleavage of a hydroxyl group to the central carbonyl carbon atom. When the MoO2approaches the C atom in the carboxy group, the first step in the reaction is for CH3COOH + MoO2to form a complex (Complex 1 in Fig. 1). As the initial step of reaction I, the energy difference from the reactant to stationary point 1 is 41.10 kcal/mol. The energies of all compounds are obtained on the basis of the reactants at 0 kcal/mol. There is an activation energy of 8.22 kcal/mol in going from complex 1 to compound C1-3. The Gibbs free energy difference between complex 1 and C1-TS2 is 30.71 kJ/mol, and the corresponding rate constant of the hydroxyl transfer step is 2.57 × 1030mol?L-1?s-1at 298.15 K. The stationary point C1-3 is considered to be a branching point, opening two complicated addition paths (Channels IA and Channel IB).

We first study the reaction mechanism of Channel IA. The compound C1-3 ejects a methyl group to produce stationary point C1-5. The formation of compound C1-5 is endothermic by 52.66 kcal/mol, causing large structural change and significant electronic reorganizations. Then, there is an activation energy of 95.35 kcal/mol in going from compound C1-3 to the transition state 4 (C1-TS4). By analyzing geometry configuration changes from C1-3 to C1-TS4, the C(1)-C(2) bond distance increases from 1.489 to 1.911 ?, and the ∠MoC(1)O(3) change most intensively increases from 80.91° to 148.96° (Fig. 1). The C1-TS4 structure has one imaginary frequency of 1005i·cm-1, which corresponds mainly to the expected movement of methyl group detaching from the C(1) atom and moving toward the O(3) atom. Finally, the total heat absorption from C1-3 to C1-5 is 52.66 kcal/mol. The following step comprises an isomerization process through C1-TS6 to compound C1-7, overcoming a barrier energy of 1.69 kcal/mol. It also indicats that the isomerization reaction for the rotation of H(1)-O(4) bond between stationary point C1-5 and C1-7 can be realized easily. The H atom transfer takes place continuously via C1-TS8 and C1-TS10 to the stationary point C1-11. Meanwhile, the C1-TS8 and C1-TS10 linking the compound C1-7 to C1-11 has been confirmed by IRC calcula- tion. The C1-TS8 presents imaginary frequency of 1642.06i·cm-1for the stretching of C(2)-H(3) bond, and the ∠C(1)O(3)C(2) reduces to 100.05° for the convenience of forming the H(3)-C(1) bond. The H(3) is attracted to the O(2) atom with strong electronegativity, resulting in the formation of O(2)-H(3) bond. After stationary point 11, another hydrogen atom transfer process occurs. The 4dand 5sorbitals of the outer layer of molybdenum atoms are half full, the C(1) and O(1) atoms in the acetic acid can generate a coordination bond with the Mo atom, and a three-membered ring composed of Mo, C and O atoms finally forms. Besides, in terms of the stability of the compound structure, compound C1-15 contains a single ring structure and therefore is more stable and easier to form. From C1-15 to C1-17 in Fig. 1, it is the ring opening of the epoxide. The C1-17 undergoes the C(1)-Mo bond breaking and the C(2)-H(4) bond rotating, making the Mo-O(1) bond slightly shrunk by 0.02 ? and ∠C(1)O(1)Mo turn larger to 159.29°. In this step, the IRC calculations at the B3LYP/gen level confirms that C1-TS16 is connected to the stationary point C1-17 in the forward direction.

According to our results, Channel IB has completely different reaction mechanism from Channel IA. Thus, in this section, we discuss the other addition reaction pathway (Channel IB) from compound C1-3. The barrier heights of C1-TS4 and C1-TS18 are 99.35 and 67.20 kcal/mol, respectively (Fig. 5). From the configurational stability view, the movement of methyl group requires more activation energy than the migration of H(4) atom. From the above discussion, we can conclude that Channel IB reaction mechanism by a compound C1-19 intermediate is more favorable. For Channel IB, the optimized geometry of compounds C1-21 and C1-23 is all stable three-ring structures. A can be seen from Fig. 2, the process from C1-21 to C1-23 is accompanied by the transformation of three-ring compound. In stationary point C1-23, there is a single bond character between C(2) and O(3) atoms. An analysis of the reaction process (Figs. 1 and 2) shows that C1-11 is the same compound in Channel IA and IB. Of course, for two competitive processes in Channel I, it is desirable to perform kinetic calculations, while such studies are beyond the scope of the present paper. However, we provide the vibrational frequencies and energies of the critical species of reaction channels to assist future kinetic studies. And the two addition reaction mechanisms are new research results, for the present study provides an important starting point.

Fig. 2. Geometries of C1-18～25 (see caption to Fig. 1)

Fig. 3. Geometries of complex 2～C2-9 (see caption to Fig. 1)

3. 2 Mechanism of reaction II

In the following discussions, we continue to describe and analyze another addition reaction mechanism Channel II. As an additional but qualitative probe into the kinetics of the reaction, we have carried out theoretical simulation calculation starting from the complex. The first step is the reaction of MoO2+ CH3COOH to form complex 2, in which the Mo atom is coordinated by O(4) and H(1) atoms. Meanwhile, the O(4)-H(1) bond of hydroxy group breaks. The mutual attraction Mo-H(1) bond having already been demonstrated theoretically is larger than the O(1)-H(1) bond at the beginning of reaction. Starting at the intermediate C2-3, the H(1) atom attached to the Mo atom migrates to the O(1) atom via C2-TS4 to form C2-5. After that, the interaction between H(2) and O(3) yields the stationary point C2-7. Also, it requires 38.74 kcal/mol to cross the C2-TS4 barrier to advance to the intermediate C2-5 and 56.38 kcal/mol to cross the C2-TS6 barrier to produce C2-7. For the transition state, the character of the stationary points is confirmed by normal- mode analysis, which yields only one imaginary frequency whose eigenvector corresponds to the reaction direction. The absolute values of imaginary frequencies at the B3LYP/gen level of theory for C2-TS4 and C2-TS6 are 1404.31 and 1955.89i·cm-1, respectively. It can also be seen that the transition state possesses a large absolute value of the imaginary frequency, indicaing that the quantum tunneling transmission coefficient should be larger. In fact, the H shifting reaction should be favored either thermodynamically or kinetically. What's more, the most obvious geometric change in this process is that the C(1)-O(3) single bond changes to the C(1)=O(3) double bond, and the C(1)-C(2) single bond goes to a C(1) = C(2) double bond (Fig. 2). Finally, the molecular configuration is adjusted to minimize energy in order to form the most stable compound C2-9 (Fig. 2). After isomerization, the ∠MoO(1)H(1) increases to 128.01°. Overall, for catalyzing CH3COOH by MoO2in Channel II, the first step of H· transfer is the rate-limiting step. The rate constant from complex 2 to C2-TS3 is 3.57 × 1019mol?L-1?s-1. By comparing the two addition reaction mechanisms of Channels I and II, they were found to be two rival reaction routes. The barrier energies for C1-TS2 are 8.22 kcal/mol in Channel I and 23.67 kcal/mol for C2-TS2 in Channel II, so it is obvious that the former requires less barrier energy.

3. 3 Mechanisms of reaction III

As can be seen in Fig. 4, the Channel III reaction mechanism is different from Channels I and II. Obviously, Channels I and II just involve addition process, whereas Channel III proceeds via more complicated hydrogen abstraction reaction process. We have obtained the reaction pathway with carboxy group as the binding site. Channel III involves various complicated geometries. Thus, we focus on discussing the reaction pathway mechanism. Furthermore, the mechanistic details are also investigated and discussed based on the extensive molecular structure analysis. Complex 3 is the starting point of Channel III, and then the electrons of C(1) atom in acetic acid first enter the orbit of the Mo atom, thereby shortening the distance between the C(1) and Mo atoms. In C3-TS2, the distance between C(1) and O(4) atoms is 2.332 ?. Additionally, the rate constant of complex 3 to the C3-3 of reaction III calculated at the B3LYP/gen level is 4.42 × 1027mol?L-1?s-1. With the formation of O(1)-H(4) bond, the O(3) atom gradually approaches to the Mo atom, and the C(1)-O(3) single double bond changes to the C(1)=O(3) double bond. The process of forming the Mo-O(3) single and C(1)=C(2) double bonds of the H(2) migration reaction takes place synergistically along MEP. For C3-TS6, the character of the stationary points is confirmed by normal-mode analysis, which yields only one imaginary frequency whose eigenvector corresponds to the direction of the reaction. Eventually, a metal four-membered ring was formed in C3-7. According to previous study[47], the four-membered ring system is generally unstable, which is most likely attributed to tension effects. Apparently, in next step, the C(1)=C(2) double bond is broken, activates the epoxide, and then induces the ring opening reaction. Even- tually, compound C3-7 converts to the C3-9. The Mo-C(2) bond length is 1.724 ?, a little shorter than that in C3-TS8. The ring-breaking in general encounters not only bond breaking but also the change of bond angle. For example, from transition state C3-TS8 to C3-9, ∠O(4)C(1)O(3) reduces from 108.96° to 105.94° (Fig. 4) and ∠C(2)MoO(3) increases to 103.09°. By observing the energy of the whole reaction path from Fig. 4, the barrier energy is 51.84 kcal/mol from compound C3-5 to C3-7, which is the largest activation energy among Channel III. It is considered to be the most difficult process in the reaction. C3-TS10 is the most critical transition state on Channel III, which connects intermediate C3-9 and products C3-11 and C3-12. The C(1)-O(3) bond is stretched to 1.433 ? with the shifting of H(1) atom, causing Route III to form the final products C3-11 and C3-12.

Fig. 4. Geometries of complex 3～C3-12 (see caption to Fig. 1)

Fig. 5. Reaction barrier energy diagram for CH3COOH + MoO2 calculated by the B3LYP/gen level of theory. The energies are given in kcal/mol and the energy values of compounds (complex 1～C1-25) are represented by zero-point energy corrections (ZPE corrections)

Fig. 6. Barrier energy diagram of MoO2 + CH3COOH reaction. Relative energies (including ZPE corrections) of the compounds (complex 2～C2-9) are located on the potential energy surfaces (see caption to Fig. 5)

Fig. 7. Barrier energy diagram of MoO2 + CH3COOH reaction. Relative energies (including ZPE corrections) of the compounds (complex 3～C3-12) are located on the potential energy surfaces (see caption to Fig. 5)

4 SUMMARY

The reaction mechanism of acetic acid with MoO2has been studied theoretically by using density functional theory. These molecular geometries have been fully characterized, and all of them were calculated at a good level[48]. Our research results show three rival reaction pathways from two reactants (Channels I, II and III), and the mechanisms are completely new. Channels IA and IB are included in Channel I. The mechanisms of Channels I and II are the addition reaction and for Channel III it is a hydrogen abstraction. With understanding the reaction mechanisms, we found a more efficient reaction pathway with the lowest activation energy to be only 8.22 kcal/mol (Channel IB). So, IB is preferred over the other routes, which means molybdenum dioxide tends to be directed towards the absorption of acetic acid rather than the release of gases. Meanwhile, the importance of the catalyzing step of CH3COOH depends not only on the barrier but also on the rate-limiting step of three kinds of reactions. The more favorable channel IB for the hydroxyl transfer of CH3COOH leads to the formation of C1-3, with the highest rate constant of 2.57 × 1030mol?L-1?s-1at 298.15 K compared with other channels. In all reaction channels, the imaginary frequency of the transition states in the hydrogen transfer process is large, which directly confirms that the transition states connect the corresponding compounds well. Previous theoretical studies of the potential energy surface of the CH3COOH + OH reaction by F. De Smedt. et al. predict the energy barriers of 9.5 and 11.6 kcal/mol[49]. This is an advantage of MoO2as a catalyst for degrading acetic acid with a lower activation energy in our study, that is, the barrier lowering with MoO2is significantly more effective. The acetic acid is proven to be the most damaging contaminant and the most difficult to degrade in carboxylic acids. So far, although there are a lot of researches on the decomposition of VOCs gas, only a few studies have been performed on catalyzing carboxylic acids reaction in both theoretical and experimental research[50-52]. Thus, the catalytic performances of MoO2are promising with a great impact at the catalyzed carboxylic acids domain.