Density Functional Theory Studies on the Mechanism of Activation Formic Acid Catalyzed by Transition Metal Oxide MoO①

GUAN Jun-Xia LIANG Yan YANG Jing YANG Xiao-Chun JIA Jing-Xian

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Density Functional Theory Studies on the Mechanism of Activation Formic Acid Catalyzed by Transition Metal Oxide MoO①

GUAN Jun-Xia LIANG Yan YANG Jing②YANG Xiao-Chun JIA Jing-Xian

(063000)

This paper systematically studies the reaction mechanisms of formic acid catalyzed by transition metal oxide MoO. Three different reaction pathways of Routes I, II and III were found through studying the reaction mechanism of transition metal oxide MoOcatalyzing the formic acid. The transition metal oxide MoO interacts with the C=O double bond to form chiral chain compounds (Routes I and II) and metallic compound MoOH2(Route III). In this paper, we have studied the mechanisms of two addition reaction pathways and hydrogen abstraction reaction pathway. Routes I andII are both addition reactions, and their products are two different chiral compounds MoO3CH2, which are enantiomeric to each other. In Route III, metal compounds MoOH2and CO2are obtained from the hydrogen abstraction reaction. Among them, the hydrogen abstraction reaction occurring in Route III is more likely to occur than the others. By comparing the results of previous studies on the reaction of MO?+ ROH (M = Mo, W; R = Me, Et), we found that the hydrogen abstraction mechanism is completely different from the mechanism of oxygen-containingorganic compound catalyzed by MO.

reaction mechanism, formic acid activation, transition metal oxide MoO, hydrogenabstraction reaction, addition reaction;

1 INTRODUCTION

Transition metal oxides (TMOs) play an important role in industrial catalysis, such as oxidation, ammoxidation and dehydrogenation[1]. The research on TMOs catalyzed organic compounds still has great prospects because TMOs have different reaction mechanisms for different reactants, and there are multiple pathways for the same reactants. The active sites such as oxygen vacancies and interstitials are present in the defect sites, which not only explains the transport properties of ionic solids but also plays an important role in determining the surface properties[2]. In recent decades, many research groups have begun to study small molecule TMO because it has great scientific research and practical value[3-5]. One of their research interests is to study the reactivity of transition metal oxide clus- ters with inorganic or organic small molecules[6, 7]. For example, molybdenum oxide has been used to catalyze the oxidation of methane to produce methanol[8]. In particular, the reaction with H2O is significant because it could produce hydrogen from water[9-12]. Since hydrogen has a wide range of uses, such as substitutes for fuels, the reaction of TMO cluster with water is of great value.

A large number of catalysts containing molyb- denum oxides and tungsten oxides have been used for industrial production[13]. Many studies on the reaction of molybdenum oxides with ROH have been more thorough and comprehensive[14, 15], in which the optimization of molybdenum oxide struc- tures is critical. Molybdenum and tungsten are in the same group (VI) so that a structurally similar oxide can be formed, or it can be combined with each other to form an oxide[16, 17], both of which exhibit a variety of unique chemical and electronic properties in their oxides[18].

Researchers have used the density functional theory (DFT) methods to study the MO?+ ROH (M = Mo, W; R = Me, Et) reaction, and the results show that there are four mechanisms: oxidation, addition, abstraction of OR and OH. The products are MO?+1+ RH, MO+1RH?, MO+1R?+ H?, and MO+1H?+ R?, respectively. According to the study, MO?+ ROH rate coefficients are higher than analogous WO?clusters, so we decided to choose MoO as the object of this experiment[19]. In order to develop a more comprehensive transition metal catalyst, it is necessary to expand its reaction object and continue to study the reaction with hydroxyl- containing organic compounds, which are important for improving the overall mechanisms. We also reviewed some related literatures, with its research content as follows: the thermal decomposition of formic acid was investigated in the temperature range of 1000～2000 K and in the density range of (0.5～2.5)×10?5mol/cm3. In the absence of a catalyst for the same gas conditions, two reactions occur with formic acid, and the products of which are CO + H2O and CO2+ H2. Besides, the result shows that the main reaction is HCOOH → CO + H2O[20]. However, so far, TMOs + RCOOH reaction does not have clear mechanisms, so we begin to study the mechanism of MoO + HCOOH reaction to determine whether this reaction is consistent with the above mechanisms. In this paper, we use the DFT methods to study the three reaction mecha- nisms of MoO activates formic acid, and explored the new mechanism. This experiment fills the gaps in the mechanisms of TMOs + RCOOH reaction and lays the foundation for the future tests.

2 METHOD OF CALCULATION

The density functional theory (DFT) methods have been applied to our calculations to account for the electron correlation effect[21], which gives good performance in calculating, particularly for organic molecules and metal-nonmetallic compounds[22-31]. DFT methods can reliably simulate the actual solid-phase catalytic reaction and predict the reactivities of TMOs in the gas-phase[32,33]. All calculations of stationary points and transition states in this study use the "Becke-3-LYP" method and are performed with the Gaussian 09 suit of programs[34-36].

By comparing a variety of optimization methods, we decided to use the B3LYP/gen method to optimize, because it can provide a more precise structure[37, 38]. “Gen” is a general basis set obtained using SDD (Stuttgart-Dresden ECP plus DZ) for Mo and Double-plus polarization (DZP) basis sets for C, H and O[39]. The DZP basis sets used for carbon and oxygen add one set of pure spherical harmonicfunctions with orbital exponentsα(C) = 0.75 andα(O) = 0.85 to the standard Huzinaga-Dunning contracted DZ sets and are designated (951/421)[40, 41]. For hydrogen, a set ofpolarization functionsα(H) = 0.75 are added to the Huzinaga-Dunning DZ set. The B3LYP method with general basis sets (SDD for Mo and DZP for C, H and O) not only reduces the time required for the calculation but also does not have a lot of deviation. Eventually, we use single-point calculations at the B3LYP/gen level of theory to determine the energy parameters of the molecule[42].

Geometric structures, energies, and harmonic frequencies have been obtained at the Hartree-Fock (HF) levels theory[43]. Moreover, intrinsic reaction coordinate (IRC) analysis identified that each tran- sition state connects the reactant and product minima[9, 44]. The most stable species were analyzed by vibrational frequency calculations[45]. Vibra- tional frequency calculations verified all frequen- cies of stationary points are positive, while transi- tion states had a single imaginary frequency, and zero-point energy corrections were taken from these frequency calculations[46-49].

3 RESULTS AND DISCUSSION

Scheme 1 shows the overall reaction pathway and indicates the molecular formula and its number for all compounds. By studying the reaction of MoO with formic acid, we found three different mecha- nisms: addition reactions and hydrogen abstraction reaction. By observing Scheme 1 we can see in the stationary point 1, there is a three-membered ring structure consisting of carbon, oxygen and molybdenum atoms. The reason for the formation is that the free orbital in the molybdenum atom can accept electrons from the carbon and oxygen atoms. Then, from the stationary point 1, Route I is separa- ted from the other two routes. In addition, RoutesII andIII are separated at stationary point 11. Of all the compounds in this study, stationary points 1 and 11 were the two most critical species, because the different rotation angle of hydroxyl will lead todifferent reaction mechanisms. The final products 9 and 22 are the products of addition reactions (Routes I and II) and 29 and 30 are the products of the extraction reaction (Route III). In addition, the products obtained in Routes I (9) and II (22) are enantiomeric. The geometries of all compounds are optimized by the B3LYP method with gen basis sets.

Scheme 1. An overview of various mechanisms and illustration of the nomenclature used to designate molecules

3. 1 Reaction mechanism of Route I

We first study the mechanism of the addition reactions. We can use the B3LYP method in con- junction with the gen basis set to determine the position of the three-membered ring compound 1. The C and O(2) atoms of the carbonyl group in the formic acid can be coordinated with the Mo atom because the half-full 4and 5orbitals of the molybdenum atom can accept electrons from two atoms. As the first step of all paths, the formation of stationary point 1 is accompanied by an exotherm of 49.91 kcal/mol. Then, from the stationary point 1 to transition state 2 (TS2), the reaction is endothermic by 14.92 kcal/mol. All the energies of the com- pounds on the basis of the energy of the reactants are 0 kcal/mol. By observing the structural changes from 1 to TS2, we can see that the C–H(1) bond is increased from 1.097 to 1.272 ?, and the distance between the H and Mo atoms is reduced to 2.029 ? (see Fig. 1). The imaginary frequency of 390i cm?1also proves that the C–H(1) bond is stretched (Table S1). The change from TS2 to stationary point 3lies in the cleavage of C–H(1) bond and the formation ofH(1)–Mo bond. Besides, TS2 connects the stationary points 1 and 3 confirmed by the IRC calculation. The total heat released from 1 to 3 is 22.29 kcal/mol, and finally the Mo–H(1) bond was reduced to 1.96 ?.

Fig. 1. Equilibriums geometries of 1～9 (see Scheme 1) calculated using B3LYP method along with the gen basis set (Bond lengths are given in angstrom and angles in degree)

Fig. 2. Geometries of 10～22 (see Scheme 1 and caption to Fig. 1)

Fig. 3. Geometries of 23～30 (see Scheme 1 and caption to Fig. 1)

The configuration of the molecule continues to change through TS4 to another three-membered ring compound 5, with the barrier energy of this step to be 37.81 kcal/mol. There is an imaginary frequency of 1588cm?1for the stretching of O(3)–H(2) bond in TS4. It can be seen from Fig. 1 that the O(3)–H(2) bond increases from 0.973 to 1.297? in this step, and the bond angle centered on C and Mo atoms is also reduced to short the distance between the H(2) andO(1) atoms. The increase of the O(3)–H(2) bond length indicates that the H(2) atom is attracted by the O(1) atom. At the same time, Mo atom also attracts the O(2) atom,resulting in C=O(2) double bond breaking into a single bond. The bondlengths ofO(3)–C(2) andC=O(2) increase to 1.287 and 1.277?, respectively. The change from 5 to 9 is that the C–O(2) bond is cleaved and the H(1) atom separated from Mo connects to the O(2) atom to form a new hydroxyl group. In Fig. 4, the energy relationship of these steps has been described. IRC calculation has confirmed that TS6 and TS8 connected to the corresponding compounds. Take chiral chain pro-duct MoO3CH29 and reactants to compare, and the result is that the Mo atom inserted into the C–O(2) bond. And for two hydrogen atoms H(1) and H(2), they are separated from the C and O(3) atoms, respectively, and eventually bonded with the O(2) and Mo atoms. Figs. 1 and4 show the geometries and energy relationships of the compounds from 5 to 9.

Fig. 4. Relative energies (including ZPE corrections) of the stationary points located on the potential energy surfaces. All energies are relative to the same zero value, MoO + HCOOH at infinite separation. The energy values are given in kilocalories per mole and are calculated using the B3LYP method with the gen basis set

3. 2 Reaction mechanism of Route II

As shown in Scheme 1, the stationary point 11 can be divided into two reaction routes: Routes II and III. Stationary point 11 is derived from sta- tionary point 1 by rotating hydroxyl, so the two structures are similar and connected by TS10. The barrier energies for TS2 are 14.92 kcal/mol in Route I and 0.11 kcal/mol for TS10, so it is obvious that the later requires less activation energy. From the molecular perspective, the migration of H1 atom requires more energy than the rotation of O(3)–H(2) bond. According to the above two reasons, we can conclude that the reaction from 1 to TS10is more favored. The hydroxyl group of compound 11 continues to rotate around the C–O(3) bond due to the attraction of O(2) atom. Finally, H(2) atom does not bond with O(2) and continues to rotate to the geometry shown in stationary point 15. The rotation of the hydroxyl group results in the change in the relative position of the atoms in the molecule.

In Route II, each of the stationary point and transition states after 15 have an enantiomer in Route I, because the reaction mechanisms of these two routes (Routes I and II) are both addition. In addition, the high degree symmetry of the structure of 7 determines its very low energy. As a result, 7 is the same intermediate in Routes I and II. The final products 22 (Route II) and 9 (Route I) are enan- tiomers centered on Mo atoms. The two routes emit equal amounts of energy, and it is clear that Route II requires less energy and therefore is more likely to occur. According to the study of the above two paths, the mechanism of its addition reaction is the same as the previous research results, and it is consistent with the general formula: MO?+ ROH (M = Mo, W; R = Me, Et) = MO+1RH?.

3. 3 Reaction mechanism of Route III

In this section, we continue to describe the hydro- genabstraction mechanism of MoO catalyzing the formic acid. This mechanism is different from the previous studies, so we focus on the process of its occurrence and analyze the causes of its product formation. The geometries of the compounds and the energy relationships between them are shown in Figs. 3 and 4.

As can be seen from Scheme 1, the three-mem- bered ring compound 11 is the dividing point of the two paths. Unlike TS12 in Route II, the hydroxyl is rotated in the opposite direction shown in TS23 in Fig. 3. At this point, the H(2) atom is attracted by Mo atoms and the O(3)–H(2) bond is increased to 1.206 ?. Then the H(2) atom is completely separated from the O(3) atom, and the O(3) and H(2) atoms are bonded to the Mo atom to form the double-ring compound 24. Comparing Routes II and III from the energy level, the barrier energy for 11 to TS12 is 31.64 kcal/mol in Route II and 21.81 kcal/mol in Route III. Thus, we can conclude that Route III is more likely to happen than Route II. Besides,regarding the stability of the molecular structure, compound 24 contains a double ring structure and therefore is more stable than the single ring compound 13, so it is easier to form. From 24 to 26, it is the process of opening the bicyclic ring. In this step, the C–Mo and O(2)–Mo bonds were broken. The C–O(3) bond is free to rotate so that the H(1) atom can be closer to the Mo atom. From interme- diate 26 to the transition state TS27, the distance between H(1) and Mo reduced to 2.224 ?. Eventually, the C atom bonded to Mo and the second hydrogen atom H(1) have migrated from the C atom to the Mo atom, leading to complex 5. TS28 is the last transition state on Route III, which connects products 29 and 30 and 5. From TS28 to the products, the O(3)–Mo and C–Mo bonds of the carbon dioxide as the leaving group have been stretched to 2.373 and 3.286 ?, respectively. The resulting products of Route IIIare MoOH229 and CO230. We can summarize the reaction of Route III into the following formula: MoO + RCOOH = MoOHR + CO2.

4 SUMMARY

In our paper, the reaction of MoO with formic acid has been studied at the DFT level, and the results are compared with the conclusion of MO?+ ROH (M = Mo, W; R = Me, Et) reaction. There are total three reaction paths. The mechanism of Routes I and II is the addition reaction, which is slightly different with the previous study. But we found a complete new reaction mechanism in Route III, and it's a top priority path compared to the others. The mechanism of Route III is the hydrogenabstraction reaction distinguished from the above mechanisms. We also discussed the reasons for the different reaction pathways and products, and also predicted the results of further reactions. The reaction begins with the coordination of C and O(2) atoms with the Mo atom, and then the stationary point 3 and three-membered ring compound 11 were obtained by overcoming the activation energies of 14.92 and 0.11 kcal/mol, respectively. The formation of compound 11 is easier than compound 3, which means RoutesII and III are more likely to occur than Route I. After compound 11, two kinds of distinguishable reaction paths have been found as follows: addition reaction (Route II) and hydrogen abstraction reaction (Route III). The barrier energy for 11 to TS12 is 31.64 kcal/mol in Route II, which is 21.81 kcal/mol higher than the barrier for the corresponding step 11 to TS23. So, Route III is preferred over the other two routes, which means the hydrogenabstraction reaction takes place first.For the autocatalytic reaction of formic acid in the gas phase, the barrier energies of its two routes (63.4 and 66.3 kcal/mol) are much higher than that of hydrogen abstraction reaction (0.11 kcal/mol) because MoO is a highly hydrogen-absorbing material whose participation reduces the activation energy of the reaction.

There are three paths in the reaction because of the free rotation of the hydroxyl group. The rotation of the hydroxyl group in a different direction causes the hydrogen to be attracted by different atoms. In Route I, the hydroxyl group rotates to a position parallel to the O(1) atom and then is attracted, eventually H atom transfers to the Mo atom. In Route II, the rotation of the hydroxyl group leads to the generation of enantiomers, so Route II is also an addition reaction. Especially in Route III, the hydroxyl group rotates to the position near the Mo atom and then H atom migrates to it, because the H atom is directly attracted by Mo atom, thus forming different structures of molecules.

Analysis of the hydrogen abstraction mechanism can be seen: MoO catalyst can absorb not only the hydrogen atoms on the hydroxyl, but also those bonded to carbon atom. By comparing the barrier energy of the two dehydrogenation steps, we found that the second step (12.08 kcal/mol) is lower than the first one (21.81 kcal/mol). In the reaction of formic acid catalyzed by transition metal oxide MoO, the hydrogen abstraction reaction is more likely to occur. The barrier energy of the first step (6.64 kacl/mol) is less than the barrier energy (0.11 kacl/mol) of methanol catalyzed by MoO. Therefore, the reaction of formic acid catalyzed by MoO prefers to the reaction of methanol catalyzed by MoO.

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18 December 2017;

9 April 2018

① This work was supported by the National Natural Science Foundation of China (No. 21373025), and the major projectof Tangshan Normal College(No. 2017B01)

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10.14102/j.cnki.0254-5861.2011-1859