Catalytic Mechanisms of Methanol Oxidation to Methyl Formate on

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Catalytic Mechanisms of Methanol Oxidation to Methyl Formate on Vanadia-Titania and Vanadia-Titania-Sulfate Catalysts Na Li, Shibin Wang, Qinghua Ren, Shenggang Li, and Yuhan Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10289 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 8, 2016

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The Journal of Physical Chemistry

Catalytic Mechanisms of Methanol Oxidation to Methyl Formate on Vanadia-Titania and Vanadia-Titania-Sulfate Catalysts

Na Li,a,b Shibin Wang,b,c Qinghua Ren,a Shenggang Li,b,c,* Yuhan Sunb,c

a

Department of Chemistry, Shanghai University, 99 Shangda Road, Shanghai 200444, China

b

CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced

Research Institute, Chinese Academy of Sciences, 100 Haike Road, Shanghai 201210, China c

School of Physical Science and Technology, ShanghaiTech University, 100 Haike Road,

Shanghai 201210, China

*E-mail: [email protected]

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ABSTRACT Density functional theory calculations were carried out using both cluster and slab models to investigate the catalytic reaction network and the effect of sulfate p romoter on methanol selective oxidation using the V2 O 5 /TiO 2 catalyst. The hemiacetal mechanism was reaffirmed to be the most favorable reaction pathway for the formation of methyl formate (MF) by the prediction of another reaction pathway involving formic acid. Molecular oxygen was found to assist the desorption of the reaction products from the reduced catalyst active site, both formaldehyde and methyl formate (MF). The mechanism of catalyst regeneration was elucidated based solely on first principles calculations, which involves the conversion of another methanol molecule to formaldehyde over a peroxo species. This leads to our formulation of a complete catalytic reaction network for methanol selective oxidation to formaldehyde and MF on the V2 O 5 /TiO 2 catalyst. In addition, the detailed mechanism for the formation of formaldehyde and MF was also predicted on the sulfate-promoted V2 O5 /TiO 2 catalyst. Based on the calculated reaction networks, the preferred formation of MF on the V2 O5 /TiO 2 -based catalysts was attributed to the lower energy barrier of the oxidative dehydrogenation (ODH) of the CH3 OCH2O* intermediate than that of CH3 O*. Furthermore, our calculated energy barriers also suggest that the sulfate-promoted V2 O5 /TiO 2 catalyst not only leads to higher catalytic activity for methanol conversion but also higher selectivity of MF over CH2 O, consistent with previous experimental observations. The sulfate promoter was found to increase the positive charge at the V site, leading to a lower energy barrier for the ODH of CH3 O* than the unpromoted catalyst.

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1. INTRODUCTION Methyl formate (HCOOCH3 , MF) is a versatile intermediate widely used by the industry to produce commodity chemicals such as formic acid (HCOOH) and formamide (HCONH2 ),1 and it was proposed as a building block molecule in C1 chemistry.

2

MF is currently

manufactured by reacting methanol (CH3 OH) with CO catalyzed by a strong base such as sodium or potassium methoxide (CH3 OM, M = Na, K).3 Due to the poor efficiency of the above process,1,3,4 , 5 , 6 there is significant interest in developing greener catalytic processes for MF synthesis. During the past decade, a number of different routes have been explored, and MF can be synthesized at varying efficiency by CH3 OH dehydrogentation,

7 , 8 , 9 , 10 , 11

CH3OH

oxidation,1,3,4,6,12 , 13 , 14 ,15 , 16 , 17 ,18 ,19 ,20 ,21 ,22 ,23 ,24 ,25 ,26 ,27 ,28 ,29 ,30 ,31 ,32 ,33 ,34 ,35 ,36 ,37 ,38 dimethyl ether (CH3 OCH3 , DME) oxidation, 39 CO2 hydrogenation or reduction in CH3 OH,5,40 ,41 , 42 ,43 , 44 , 45 CH3 OH carbonylation,46,47 as well as directly from synthesis gas.48,49,50 MF synthesis from CH3 OH via selective oxidation is more attractive than dehydrogenation, as the former is not thermodynamically constrained. Heterogeneous catalysts for CH3 OH oxidation to form MF can be categorized into three classes: noble metal-based catalysts, TiO 2 -based photocatalysts, and transition metal oxide-based catalysts. Nanoporous Au catalysts were first shown by Friend, Bäumer, and co-workers to catalyze this reaction with very high MF selectivity of 97% at low reaction temperature of 80°C.17 Following this work, a number of noble metal (Au,

Pd, Pt) based catalysts were investigated

for this

reaction,3,4,21,22,27,28,29,32,34 and the highest MF yield of ~90% was reported for graphenesupported Au-Pd nanocatalysts at 70°C.22 However, the use of noble metals in these catalysts may prevent them from large scale industrial application. TiO 2 was first found by Kominami and co-workers to be an effective photocatalyst for this reaction with MF selectivity of 91% at room

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temperature,18 and later works focused on elucidating the photocatalytic mechanism23,24,25,33 and developing TiO 2 -supported noble metal (Au, Ag, Pd) photocatalysts,1,26,30,35 although supported Cu photocatalysts were also found to catalyze this reaction. 36,37 MF yield up to 77% was obtained with Au-Ag/TiO 2 photocatalysts at room temperature, much higher than that of ~50% with Cu/CuZnAl- ZnO photocatalysts. Many transition metal oxide-based catalysts can also catalyze

this reaction,

such as

RuO x /ZrO 2 ,15,20

ReOx /CeO2 ,16

and

V2 O5 /TiO 2 -based

catalysts.12,13,31,38 With the V2 O5 /TiO 2 catalyst, dimethoxymethane (CH3 OCH2 OCH3 , DMM) is formed at relatively low temperature of 70−100 °C with selectivity of 88−95%, whereas MF is produced at relatively high temperature of ~150 °C with selectivity of ~85%.31 Most of the studies on the V2 O5 /TiO 2 -based catalysts were focused on the selective synthesis of DMM, 51,52,53,54 ,55,56,57 ,58,59,60,61 ,62,63,64 ,65,66,67 although we recently synthesized a vanadia-titaniasulfate (VTS) catalyst, which can catalyze CH3 OH oxidation with very high MF selectivity of 98.6% at very high CH3 OH conversion of ~98.7% at 145°C.38 The very high MF yield of ~97.3% is crucial for the efficient production of MF from CH3 OH. In our previous work, we also carried out density functional theory (DFT) calculations to elucidate the formation mechanism of MF from CH3 OH on the V2 O 5 /TiO 2 catalyst,38 and compared to that predicted for the TiO 2 photocatalyst by Liu and co-workers. 68 Although they also proposed the hemiacetal mechanism involving the combination of CH3 O* and CH2 O* to form CH3 OCH2 O* favorable to the formyl mechanism involving the dehydrogenation of CH2 O* to form HCO*, they failed to predict the kinetic barrier of the above combination reaction. On the other hand, we predicted this combination reaction to have very low energy barriers of 0.2 to 0.4 eV, but it must be catalyzed by an acidic site such as the V−OH site formed upon CH3 OH chemisorption, which leads to the formation of the hemiacetal molecule (CH3 OCH2 OH). The

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oxidative dehydrogenation (ODH) of CH3 OCH2 OH yields MF, which has similar energy barriers as that of CH3 OH to yield CH2 O. The mechanism that we derived from first principles calculations is thus a confirmation of that proposed by Liu and co-workers for the RuO x /ZrO 2 catalyst a decade ago.15 Recent experimental studies by Kaichev and co-workers on CH3 OH selective oxidation using a monolayer V2 O5 /TiO 2 catalyst have revealed much details about this reaction,31 and in this study we performed further DFT calculations with aim of providing an even better understanding on their as well as our previous experimental observations. We are particularly interested in elucidating the role of molecular oxygen (O 2 ) in the catalyst regeneration for constructing a complete catalytic mechanism, as well as the effect of sulfate (SO 4 2−) on the V2 O 5 /TiO 2 catalyst for the selective formation of MF.

2. COMPUTATIONAL METHODS In our previous study, we combined DFT calculations using a molecular cluster (VTi3 O7 (OH)3 ) and a periodic slab (V(=O)(−O−)3 /anatase-TiO 2 (001)) as the catalyst models for the highly dispersed V2 O5 /TiO 2 catalyst.38 We took advantage of the synchronous transit- guided quasi-Newton algorithm 69 for the efficient search of possible transition states in molecular systems, which can be difficult to imagine for complex catalytic reactions, as well as the intrinsic reaction coordinate approach 70 , 71 for their unequivocal identification, which are both well implemented in the Gaussian 09 program package. 72 This was complemented by more realistic modeling of the most important elementary steps using a periodic slab, which allows for the study of effects of the extended surfaces as well as the surface coverages. Results from these two types of calculations are in general in good agreement, for example the energy barrier for

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hydrogen transfer from the OH group in CH3 OH to the terminal =O at the V(V) site was predicted to be 1.11 and 1.19 eV with the cluster and slab models of the V2 O 5 /TiO 2 catalyst.38 The agreement can be attributed to good electron localization of metal oxide systems and the use of DFT functionals of similar accuracy in both cases. This approach proves invaluable for establishing reliable catalytic mechanisms for complex heterogeneous catalytic reactions. In the present study, the molecular cluster calculations were carried out with the VTi3 O7 (OH)3 and VTi3 O6 (SO 4 )(OH)3 clusters as models of the V2 O5 /TiO 2 and VTS catalysts, respectively. These cluster models are chosen as the V site has the similar local structure as that in the slab models in that it is surrounded by three bridge −O(Ti) and one terminal =O or SO4 group. The B3LYP 73 hybrid exchange-correlation functional was employed along with the augcc-pVDZ 74 and aug-cc-pVDZ-PP 75 , 76 , 77 basis sets for the non- metal and metal atoms, respectively. Geometry optimizations were carried out in redundant internal coordinates with the Berny algorithm 78 for local minima, and with the above-mentioned approaches for transition states. Analytic harmonic vibrational frequencies were calculated to verify the nature of the stationary states and to obtain the zero-point energy corrections. Molecular visualization was accomplished using the AGUI graphical interface from the AMPAC program package.79 The periodic slab calculations were performed with the V(=O)(−O−)3 and V(SO 4 )(−O−)3 clusters covalently bound to the anatase-TiO2 (001) surface with a p(2x2) supercell as models of the V2 O 5 /TiO 2 and VTS catalysts, respectively. The choice of the support was justified in our previous work38 considering that TiO 2 in the most active VTS catalyst calcinated at 400 °C was determined by powder X-ray diffraction to be predominately in the anatase phase, and its (001) surface has been shown to be more reactive than the more stable (101) surface. The PBE80 exchange-correlation functional in the generalized gradient approximation was used along with

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the projector-augmented wave (PAW) potentials. 81,82 The default potentials were used for the non- metal atoms, and for the transition metals the semi-core 3s2 3p6 states were treated as valence states. The TiO 2 slab consists of four Ti atomic layers with each layer having four Ti atoms. The bottom two Ti atomic layers were kept at their bulk positions, and a vacuum layer of 15 Å was added between the adjacent slabs. Adsorptions and reactions were allowed only on the relaxed side of the slab. An energy cutoff of 520 eV, a Monkhorst-Pack83 k-point mesh of (3x3x1), and a force convergence of 0.02 eV/Å were used for the ionic relaxation. Transition states were located with the climbing image nudged elastic band (CI-NEB) approach. 84,85 These calculations were carried with VASP, 86 , 87 and visualized by Materials Visualizer from Materials Studio. 88 All calculations were performed on the computing clusters at Shanghai Advanced Research Institute.

3. RESULTS AND DISCUSSION 3.1 Methanol Oxidation on the V2 O5 /TiO2 Catalyst 3.1.1 Formation mechanism of methyl formate Based on the calculated energy barriers of the elementary steps for CH3 OH oxidation to MF using the cluster model, we have previously shown that the hemiacetal mechanism is more favorable than the formyl mechanism. 38 This is primarily due to the very high energy barrier predicted for the dehydrogenation of CH2 O* to form HCO*, 2.04 or 1.85 eV depending on whether the terminal =O or –OH group is the target for the hydrogen transfer. On the other hand, the condensation reaction between CH3 O* and CH2 O* catalyzed by the acidic –OH group to form CH3 OCH2 OH* was predicted to have an energy barrier of merely 0.37 eV, and the ODH of CH3 OCH2OH* to form MF*, which consists of hydrogen transfer from the OH group to the terminal =O (Ea = 1.13 eV) following by that from the CH2 group to the bridge O (Ea = 1.19 eV),

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was calculated to have similar energy barriers as that of CH3 OH* to form CH2 O* (Ea = 1.11 and 1.29 eV, respectively).

a 3 (TS1) 1.99 5 (TS2) 1.29 4 0.84

1 (Ti3VO7(OH)3)

HCOOH

6 0.74

CH2O

1 0 eV

7 0.89

2 −0.05

b 12 (TS4) 1.28

9 (TS3) 0.99

HCOOH CH3OH 10 0.40 1 0 eV

8 −0.12

11 0.24

H2O HCOOCH3

13 −0.37

14 −0.11

1 −0.01

Figure 1. Potential energy surfaces (ΔE0K, eV) for the formation of HCOOCH3 via HCOOH calculated with the cluster model (B3LYP). The C, O, Ti, V atoms are shown as large gray, red, white, and turquoise balls, and the H atoms are shown as small white balls.

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Besides the above two types of mechanisms, earlier studies suggested the formation of formic acid (HCOOH) as an intermediate for MF formation, 13,89 and here we further examine this pathway using the cluster model. As shown in Figure 1a, two elementary steps are involved in the formation of HCOOH at the V=O redox site, the dehydrogenation of CH2 O* to form HCO* with an energy barrier of 2.04 eV as predicted in our previous report 38 and the combination of HCO* and the resulting terminal –OH group leading to HCOOH with an energy barrier of only 0.45 eV. The second step has a very similar energy barrier as that of 0.45 eV for the combination of HCO* and CH3 O* to form MF. Upon the formation of HCOOH, the V site is reduced to the +3 formal oxidation state, and the total reaction (−O−)3 V=O + CH2 O → HCOOH + (−O−)3 V is quite endothermic at 0.89 eV. The formation of MF from the reaction of HCOOH and CH3 OH also occurs at the V=O site, as shown in Figure 1b. It also involves two elementary steps, CH3 OH* chemisorption at the V=O site to form CH3 O* and the acidic V−OH site with an energy barrier of 1.11 eV as predicted in our previous work38 and the reaction of HCOOH and CH3 O* catalyzed by the acidic V−OH site with an energy barrier of 1.04 eV. The last step leads to the formation of MF and H2 O physisorbed at the V=O site, and the catalyst is regenerated upon their desorptions. Compared to the very low energy barrier of 0.37 eV for the condensation of CH2 O and CH3 O* also catalyzed by the V−OH site, that for the reaction of HCOOH and CH3 O* has a much higher energy barrier, showing the later may require a stronger acid catalyst. For the reaction shown in Figure 1b, the V site remains the +5 formal oxidation state, and the total reaction, HCOOH + CH3 OH → HCOOCH3 + H2 O, is slightly exothermic by −0.01 eV, which is to be compared with that of −0.20 eV (∆H298K) derived from the experimental heats of formation. 90 As pointed out by Tatibouët,89 the formation of HCOOH was rarely observed in CH3 OH

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selective oxidation. This is consistent with our calculations on its formation mechanism in Figure 1. The formation of HCOOH by the ODH of CH2 O* is unfavorable at the V=O redox site as it must overcome a significant energy barrier of ~2.0 eV to form HCO*, which is much higher than that of ≤1.3 eV predicted for the ODH of both CH3 O* and CH3 OCH2 O*. In addition, the formation of HCOOH by the dissociative adsorption of HCOOCH3 at the V=O site involving a co-adsorbed H2 O* is also unfavorable with an energy barrier of 1.65 eV from Figure 1b, which may be due to the simultaneous breaking of the C−OCH3 and forming of the C−OH bond. Thus, for the formation of MF, the formic acid mechanism shown in Figure 1 can be considered as closely related to the formyl mechanism, both involving the formation of HCO*, which is unfavorable due to its very high energy barrier. The hemiacetal mechanism is more favored as much lower energy barriers were predicted based on our previous study. This reaffirms our previous conclusion that the hemiacetal mechanism is the mo st favorable pathway for MF formation from CH3 OH oxidation.

3.1.2 O2 -assisted product desorption The ODH of CH3 O* to form CH2 O* and that of CH3 OCH2 O* to form MF* both occur at the V(V) site leading to its reduction to the V(III) site regardless whether the target for the hydrogen transfer is the bridge −O− or the terminal –OH group. In both cases, desorption of the physisorbed CH2 O* or MF* and H2 O* leads to the formation of the bare V(III) site. The reoxidation of the V(III) site by O 2 to regenerate the V(V) site is usually considered as a fast reaction, although the detailed mechanism has yet been elucidated by first principles calculations. In the following, we provide a catalyst regeneration mechanism based solely on quantum chemical calculations, which is divided into two parts, O 2 -assisted product desorption and

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regeneration of the V(V) site.

a HCHO 2b 0.51

3b (TS1b) 0.61

4b 0.34

3a −0.14 1 0 eV

2a (TS1a) 0.10

H 2O

4a 0.21

5b 0.73

O2

O2

H 2O 5a −1.26

6 −1.12

HCHO

7 −1.59

b HCOOCH3 9b 10b (TS2b) H2O 0.61 0.51 11b 0.34

8 0 eV

11a 9a (TS2a) 10a 0.29 0.50 0.14 H2O

12b 0.73

O2

O2

12a −1.38

6 −1.11

HCOOCH3 7 −1.58

Figure 2. Potential energy surfaces (ΔE0K, eV) for desorption of (a) CH2 O and (b) HCOOCH3 without (red) or with (black) O 2 -assistance calculated with the cluster model (B3LYP). Species shown in blue are on the triplet energy surface.

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As discussed in our previous work, with the cluster model the ODH of CH3 O* to form CH2 O* was predicted to be a highly endothermic reaction (∆Er = 0.89 eV) with a significant energy barrier (∆Ea = 1.29 eV),38 and thus the reverse reaction is very exothermic with a very low energy barrier (∆Ea = 0.40 eV). As shown in red in Figure 2a, the desorption energy of CH2 O* of 0.51 eV is higher than the above reverse energy barrier, so there is significant chance for the conversion of CH2 O* back into CH3 O*. Upon the desorption of CH2 O*, hydrogen transfer occurs with a very low energy barrier of 0.10 eV leading to the formation of H2 O* at the V(III) site, and the further desorption of H2 O* is endothermic by 0.38 eV leading to the formation of the bare V(III) site. The above is the desorption pathway presented in our previous report. Although no high energy barrier was predicted, the whole desorption process is endothermic by 0.73 eV with the first step endothermic by 0.51 eV. There is, however, an energetically more favorable desorption pathway involving O 2 as shown in black in Figure 2a. Hydrogen transfer first occurs leading to the formation of H2 O* coadsorbed with CH2 O* at the V(III) site, and this step is slightly exothermic by −0.14 eV with a very low energy barrier of 0.10 eV. The desorption of H2 O* then occurs with an endothermicity of only 0.35 eV leaving CH2 O* still physisorbed at the V(III) site. Both the energy barrier of the first step and the endothermicity of the second step are lower than the energy barrier of 0.40 eV for the reverse reaction of the ODH of CH3 O*. Although the further desorption of CH2 O* is endothermic 0.52 eV, with the addition of O 2 the energy of the system drops significantly by 1.47 eV, and CH2 O* is no longer strongly attached to the V site as its desorption energy is merely 0.14 eV. O2 -assisted desorption of CH2 O can be considered as a competitive adsorption process. We failed to find any transition state between structures 4a and 5a, so we think that this process is barrierless. Thus, the pathway shown in black in Figure 2a is energetically much more

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favorable than that shown in red. There is some complication when introducing O 2 into the system, as it has a triple ground state and this is the spin state used in our calculations in Figure 2, as the triple ground state of O 2 is much lower in energy than the lowest energy singlet state, and direct conversion between these two states is also spin forbidden. In fact, the stationary points shown in blue in Figure 2, 5a, 12a and 6, are on the triplet energy surface due to the addition of O 2 , and a spin crossover must occur when (−O−)3 V(−O−O−) converts from the triplet to the singlet state, which is exothermic by 0.47 eV and is assumed to occur without much difficulty. For the ODH of CH3 OCH2 O* to form MF*, the reverse reaction was predicted to have a much higher energy barrier of 0.97 eV.38 As shown in red in Figure 2b, the direct desorption of MF* is endothermic by 0.51 eV, and the further desorption of H2 O* involves a very low energy barrier of 0.10 eV and a desorption energy of 0.38 eV. All these steps is energetically favorable. The alternative pathway shown in black in Figure 2b is energetically even more favorable, which involves hydrogen transfer to form H2 O* with an energy barrier of 0.50 eV, the desorption of H2 O* with an endothermicity of 0.32 eV, and O 2 -assisted desorption of MF* with a desorption energy of 0.27 eV. After a spin crossover, the (−O−)3 V(−O−O−) structure in the singlet state is also formed, where the V site can be considered to be in the +5 formal oxidation state, although the (−O−)3 V(=O) structure has yet been regenerated.

3.1.3 Regerenation of the V(V) site

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5b (TS2b) 2.14 CH2O 6a 1.09 3 (TS1) 0.64

10 (TS4) 1.93

8 (TS3) 2.67 9 1.74

7 1.53

5a (TS2a) 1.51

CH3OH

1 0 eV

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4 0.24 2 −0.10

12 (TS5) −1.49

CH2O

6b −2.77

11 −2.13

H 2O

13 −2.70

1 −2.60

Figure 3. Potential energy surfaces (ΔE0K, eV) for the regeneration of the catalyst calculated with the cluster model (B3LYP).

In our previous work, we assumed the re-oxidation of the V(III) site by O 2 as a fast process,38 and here we provide a possible mechanism for the regeneration of the V(V) site. As shown in Figure 3, we start with the (−O−)3 V(−O−O−) structure arising from Figure 2 upon O 2 assisted product desorption. Although the V site in this structure can be considered as in the +5 formal oxidation state, it differs from that the starting structure of (−O−)3 V(=O) in our catalyst model. Two possible pathways for converting the (−O−)3 V(−O−O−) structure into the (−O−)3 V(=O) structure were found, and the one with the lower energy barrier for each elementary step is shown in black, which involves five transition states. The first step is CH 3 OH chemisorption leading to the formation of CH3 O* with a modest energy barrier of 0.74 eV. The 14


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second step is the dehydrogenation of CH3 O* to form CH2 O* with a higher energy barrier of 1.27 eV due to the change of the formal oxidation state from +5 to +3 for the V site. Upon the desorption of CH2 O*, hydrogen transfer occurs from the bridge OH group to the V metal site with a slightly lower energy barrier of 1.14 eV, and the formal oxidation state of the V site changes from +3 back into +5. The first three steps are all endothermic, with an endothermicity of 0.34 eV for the first step, 0.85 eV for the second one, and 0.21 eV for the third one. The fourth step involves hydrogen transfer from the V metal site to the O atom in the OOH group with a very low energy barrier of 0.19 eV, which leads to the breaking of the O−O bond and the formation of two OH groups. The final step involves hydrogen transfer from one of the OH group to the other one with a relatively low energy barrier of 0.64 eV, and upon the desorption of H2 O*, the (−O−)3 V(=O) structure is regerenated. The last two steps are both exothermic, −3.87 eV for the fourth step and −0.57 eV for the fifth step. In the above pathway, a maximum energy barrier of 1.27 eV was predicted for the step involving the dehydrogenation of CH3 O*, which is rather close to that of 1.29 eV predicted for a similar step in the ODH of CH3 OH at the (−O−)3 V(=O) site. An alternative pathway shown in red in Figure 3 was also found, and here H2 O* is directly formed upon hydrogen transfer from CH3 O* to the OOH group, which has a much higher energy barrier of 1.90 eV. Thus, the pathway shown in black in Figure 3 is kinetically more favorable to that shown in red.

3.1.4 Catalytic reaction network

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CH2O

[9] 0.37 (0.20)

[12] 0.50

H 2O + O2 HCOOCH3

Figure 4. Reaction network for CH3 OH oxidation by O2 to form CH2 O ([1] to [8]) and HCOOCH3 ([9] to [12]) predicted for the V2 O 5 /TiO 2 catalyst. Energy barriers (ΔE0K, eV) for the elementary steps along the most favorable pathway were calculated at the B3LYP and PBE levels with the cluster and slab models shown in black and red, respectively.

We have previously presented a reaction network for the formation of C H2 O and MF by CH3 OH oxidation on the V2 O5 /TiO 2 catalyst.38 However, we were unable to provide a closed catalytic cycle in that study, and with our present investigation on the catalyst regeneration mechanism, we are now able to provide a complete catalytic cycle as shown in Figure 4. We note that the steps for the formation of CH2 O are parts of those for the formation of MF, as the former is an intermediate in the formation of the later.

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The catalytic cycle for CH3 OH oxidation by O 2 to form CH2 O and H2 O consists of eight transition states, labeled with [1] to [8] in black in Figure 4. These steps are: [1] CH3 OH chemisorption at the V=O t pair site (Ot labels the terminal O) to form CH3 O−(V*) and HOt −(V*), where X−(V*) denotes X species adsorbed at the V* site, [2] hydrogen transfer from CH3 O−(V*) to O b−(V*) (O b labels the bridge O) to form CH2 O−(V*) and HOb−(V*), [3] hydrogen transfer from HOb−(V*) to HOt −(V*) to form O b−(V*) and H2 Ot −(V*) followed by the desorption of H2 Ot and O2 -assisted desorption of CH2 O to form the V=O 2 pair site (O 2 labels the peroxo) upon spin- flip, [4] CH3 OH chemisorption at the V=O 2 pair site to form CH3 O−(V*) and HO2 −(V*), [5] hydrogen transfer from CH3 O−(V*) to O b−(V*) to form CH2 O−(V*) and HOb−(V*) followed by the desorption of CH2 O, [6] hydrogen transfer from HO b−(V*) to V to form Ob−(V*) and H−(V*), [7] hydrogen transfer from H−(V*) to the HO2 −(V*) to form two HOt −(V*) by breaking the O−O bond, [8] hydrogen transfer from one HO t −(V*) to the other one to form H2 O t −(V*) and O t =(V*) followed by the desorption of H2 Ot to regenerate the V=O t pair site. The total reaction for the above catalytic cycle is 2CH3 OH (g) + O 2 (g) → 2CH2 O (g) + 2H2 O (g) with (−Ob−)3 V(=O t ) as the active site for the V2 O 5 /TiO 2 catalyst. Our proposed catalytic mechanism for CH3 OH oxidation to CH2 O can be considered as a Mars and Van Krevelen mechanism, 91 as the terminal O at the V(V) site, which can be viewed as a lattice O, first reacts with CH3 OH to form the product H2 O (the O in the product CH2 O comes from CH3 OH), and the reduced V(III) site is then re-oxidized by O 2 to regenerate the V(V) site. However, based on our mechanism, the O in the second H2 O formed actually comes from O 2 , and upon the completion of the first catalytic cycle, the regenerated terminal O at the V(V) site also comes from O 2 , so without interchange between the terminal and bridge O atoms, the bridge O will not be incorporated into the product.

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The additional steps involved in the formation of MF from CH3 OH oxidation are also shown in Figure 4 and labeled as [9] to [12] in blue. These steps are: [9] the reaction of CH2 O with CH3 O−(V*) catalyzed by the acidic HO t −(V*) to form physisorbed hemiacetal CH3 OCH2OH and O t −(V*), [10] CH3 OCH2 OH chemisorption at the V=O t pair site to form CH3 OCH2O−(V*) and HO t −(V*), [11] hydrogen transfer from CH3 OCH2 O−(V*) to O b−(V*) to form MF−(V*) and HO b−(V*), [12] hydrogen transfer from HO b−(V*) to HO t −(V*) to form Ob−(V*) and H2 Ot −(V*) followed by the desorption of H2 Ot and O 2-assisted desorption of MF to form the V=O 2 pair site, which can further react with CH3 OH leading to the regeneration of the catalyst active site.

3.1.5 Alternative active sites With (−Ob−)3 V(=Ot ) as the active site for the V2 O5 /TiO 2 catalyst, we have previously predicted CH3 OH chemisorption to have substantial energy barriers of 1.19 and 1.11 eV with the slab and cluster models, respectively.38 This appears to be inconsistent with the experimental observations of Kaichev and co-workers,31 as their in situ Fourier transform infrared (FTIR) studies suggested the formation of CH3 O* at temperature as low as 50 °C, while MF was produced at temperature as low as 70 °C, although CH3 OH conversion is also low at 10%. Different explanations are possible for these experimental observations. One of them is that these are due to active sites of much higher reactivities, 92 such as surface defect sites, and due to their limited quantities, the catalyst does not show very high catalytic activity at relatively low temperature.

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5b (TS2b) 6b 1.60 1.55 1b V(=O)(−O−)3 CH3OH

3b (TS1b) 1.03

2a −0.15

1 0 eV

2b −0.16

6a (TS2a) 1.41

4b 0.41 3a (TS1a) 0.58

5a −0.24 4a −0.54

7 0.93

H 2O

1a V(−OH)(−O−)4

Figure 5. Potential energy surfaces (ΔE0K, eV) for the formation of CH2 O on two different slab models of the V2 O5 /TiO 2 catalyst calculated at the PBE level. The H, O, Ti, and V atoms in the catalyst models (1a and 1b) are shown as white, red, gray, and turquoise balls. The C atoms in the other structures are displayed as gray balls.

Here we present a relatively stable catalyst model with higher reactivity for CH 3 OH chemisorption. As shown in Figure 5, the (−Ob−)4 V(−Ot H) structure (1a), which was calculated to be higher in energy than the (−O b−)3 V(=O t ) structure (1b) by 0.53 eV, can also react with CH3 OH. CH3 OH physisorption energies predicted for these two structures are very close, but CH3 OH chemisorption at the V−Ot H pair site was predicted to be exothermic by −0.38 eV, whereas that at the V=O t pair site was calculated to be endothermic by 0.57 eV. The energy barrier for the former of 0.74 eV is also much lower than that of the later of 1.19 eV, so CH3 OH chemisorption is much more favored both thermodynamically and kinetically at the (−O b−)4 V(−O t H) structure than the (−O b−)3 V(=O t ) structure. Upon the desorption of H2 O, the

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(−Ob−)4 V(−O t CH3 ) structure is formed for the slab model 1a, and hydrogen transfer from CH3 O* was predicted to be endothermic by 1.17 eV with a high energy barrier of 1.65 eV. This energy barrier is much higher than that calculated for the corresponding step for the slab model 1b of 1.19 eV, so the slab model 1a can be expected to be much less reactive for the ODH of CH 3 O*, despite its being more reactive for CH3 OH chemisorption to form CH3 O*. However, if the effective energy barrier for CH3 OH reaction with the slab model to form CH2 O* is considered, that for 1a of 1.41 eV is considerable lower than that for 1b of 1.60 eV, so the former may still be considered to be more reactive with CH3 OH than the later. In the next section, we shall provide a model which is more reactive for both CH3 OH chemisorption and the ODH of CH3 O*.

5 (TS2) 1.56 1 Ti2V2O8(OH)2 CH3OH 1 0.00

6 1.06

3 (TS1) 0.92 4 0.29

2 −0.11

Figure 6. Potential energy surface (ΔE0K, eV) for the formation of CH2 O on the Ti2 V2 O8 (OH)2 cluster model calculated at the B3LYP level.

Although the vanadia species in the VTS catalysts has been shown to be highly dispersed

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on the titania support,38 in addition to the VO x monomer, its polymers may be present on the catalyst surface, so we further examined the effect of its polymerization on the reactivity using the Ti2 V2 O8 (OH)2 cluster as a dimer model. As shown in Figure 6, the two hydrogen transfer steps in the ODH of CH3 OH to form CH2 O on this catalyst model were predicted to have energy barriers of 1.03 and 1.27 eV, slightly lower than those of 1.11 and 1.29 eV on the Ti3 VO 7 (OH)3 cluster as a monomer model, so oligermerization of the VO x species can slightly promote the ODH of CH3 OH, consistent with vanadia being a better oxidant than titania. However, the support effect of titania has been shown by our previous experiment to be essential for the high catalytic activity of the VTS catalyst, as crystallization of the vanadia species reduce s the catalytic activity, partially due to the decrease in the number of surface vanadia species.

3.2 Methanol Oxidation on the VTS Catalyst 3.2.1 Formation of formaldehyde The catalysts that we synthesized in our previous work actually contain a small amount of sulfur presumably in the form of sulfate, and there we carried out some preliminary calculations on a VTS catalyst model based on our cluster model for the V2 O5 /TiO 2 catalyst.38 Here, we further investigate the effect of sulfate on the V2 O5 /TiO 2 catalyst for CH3 OH oxidation using both cluster and slab models. As shown in Figure 7, both our cluster and slab models are based on the corresponding V2 O5 /TiO 2 catalyst models, where the terminal =O is replaced by the sulfate (−O−SO 2 −O−) group as a bidentate ligand for the V(V) site.

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a

3b (TS1b) 0.99

1b V(=O)(−O−)3

4b 0.40

CH3OH 2b −0.12

1 0 eV 1a V(SO4)(−O−)3

5b (TS2b) 1.69 6b 1.29 6a 0.37 5a (TS2a) 0.96

2a 3a (TS1a) 4a −0.32 −0.15 −0.22

b

5b (TS2b) 1.60 1b V(=O)(−O−)3

3b (TS1b) CH3OH 1.03

4b 0.41

2b −0.16 1 0 eV

6b 1.55 6a 0.52

5a (TS2a) 0.80

2a 3a (TS1a) 4a −0.51 −0.07 −0.61

1a V(SO4)(−O−)3

Figure 7. Potential energy surfaces (ΔE0K, eV) for the formation of CH2 O on the cluster models (a) and the slab models (b) of the V2 O5 /TiO2 and VTS catalysts. The S atoms are shown as gold balls.

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Figure 7a slightly differs from the figure presented in our previous work38 in that the target for hydrogen transfer from CH3 O* arising from CH3 OH chemisorption is the bridge O instead of the HSO 4 group. When hydrogen is transferred to the OH group on the HSO 4 group, physisorbed CH2 O*, H2 O*, and SO 3 * are simultaneously formed on the reduced V(III) site, which is highly endothermic by 1.51 eV with a very high energy barrier of 1.57 eV. When hydrogen is transferred to the bridge O, physisorbed CH2 O* is also formed along with chemisorbed HSO 4 * and HO b*, which is endothermic by 0.59 eV with a much lower energy barrier of 1.18 eV. Thus hydrogen transfer to the bridge O is more favored, whose energy barrier is actually ~0.1 eV lower than that for the corresponding step on the cluster model of the V2 O 5 /TiO 2 catalyst. From Figure 7a, the SO 4 species at the V(V) site mainly promotes CH3 OH chemisorption with its energy barrier reduced from 1.11 eV to 0.17 eV. In additio n, our present calculations further suggest that the SO 4 species also promotes the ODH of CH3 O* to form CH2 O* albeit to a much less extent. Similar conclusions can be drawn from our slab calculations as shown in Figure 7b. The energy barrier for CH3 OH chemisorption is significantly reduced from 1.19 eV to 0.44 eV, although that for the ODH of CH3 O* is somewhat increased from 1.19 eV to 1.41 eV. We note that with a p(2x2) supercell for our VTS catalyst model, adsorbed species in neighboring cells on the slab surface tend to weakly interact with each other, which may affect our results and a larger supercell must be used in order to avoid such interaction.

3.2.2 Formation of methyl formate Using the above cluster and slab models for the VTS catalyst, we further investigated the effect of sulfate on the formation of MF. Results from the cluster model is shown in Figure 8,

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whereas those from the slab model is given as Supporting Information. Due to the substantial interaction between surface species in neighboring cells in MF formation, only thermodynamics was predicted for the slab model, but similar trend is observed for the cluster and slab models.

10b (TS4b) 1.13

3b (TS1b) 0.99 CH3OH

4b 0.40

CH2O

2a −0.32

8b (TS3b) 0.61

5b 0.21

2b −0.12 1 0 eV

6b (TS2b) 0.58

7b −0.52 3a (TS1a) 4a −0.15 −0.22

5a −0.63

11b 0.16

9b −0.06

6a (TS2a) −0.02

11a 10a (TS4a) −0.73 0.46

8a (TS3a) 9a 7a −0.55 −0.54 −0.61

Figure 8. Potential energy surfaces (ΔE0K, eV) for the formation of HCOOCH3 on the cluster models of the V2 O5 /TiO 2 and VTS catalysts.

As shown in Figure 8, the mechanism for the formation of MF on the VTS catalyst model is very similar to that on the V2 O5 /TiO 2 catalyst model, both involve four hydrogen transfer steps. The first step is CH3 OH chemisorption, which was already discussed above and whose energy barrier was predicted to be greatly reduced from 1.11 eV on the V2 O5 /TiO 2 catalyst model to 0.17 eV on the VTS catalyst model. The second step is acid-catalyzed condensation of 24


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CH3 O* and CH2 O* to form physisorbed CH3 OCH2OH*, whose energy barrier was slightly increased from 0.37 eV to 0.61 eV mostly due to the stronger physisorption of CH2 O* on the VTS catalyst. The third step is CH3 OCH2 OH chemisorption, whose energy barrier was calculated to greatly decrease from 1.13 eV to 0.07 eV. The last step is the ODH of CH3 OCH2 O* to form physisorbed MF, whose energy barrier also reduces from 1.19 eV to 1.01 eV. From the above comparison on the calculated energy barriers of the individual elementary step, it is clear that the formation of MF on the VTS catalyst model is kinetically much favorable to that on the V2 O 5 /TiO 2 catalyst model. In addition, thermodynamic consideration also leads to the same conclusion from our cluster as well as slab model calculations in that these elementary steps on the VTS catalyst model are usually more exothermic or less endothermic than those on the V2 O 5 /TiO 2 catalyst model.

3.2.3 Catalytic reaction network Assuming a similar mechanism for the regeneration of the VTS catalyst, we can build a reaction network for CH3 OH oxidation to CH2 O and MF on this catalyst as well. As shown in Figure 9, two elementary steps are involved in the formation of CH2 O and three additional ones are involved in the formation of MF, besides those for catalyst regeneration. For the formation of CH2 O, CH3OH first chemisorbs at the V=SO 4 pair site leading to the formation of CH3 O−(V*) and HSO 4 −(V*) in step [1], and hydrogen transfer from CH3 O−(V*) to O b−(V*) then occurs to form CH2 O−(V*) and HO b−(V*) in step [2]. For the formation of MF, CH2 O first reacts with CH3 O−(V*) catalyzed by the acidic HSO 4 −(V*) leading to the formation of physisorbed CH3 OCH2OH at the regenerated V=SO 4 pair site in step [4], and this is followed by CH3 OCH2OH chemisorption at the V=SO 4 pair site to form CH3 OCH2 O−(V*) and HSO 4 −(V*)

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in step [5], and hydrogen transfer from CH3 OCH2 O−(V*) to Ob−(V*) leading to the formation physisorbed MF and HO b−(V*) in step [6].

CH3OH

[1] 0.17 (0.44)

[6] 1.01

[5] 0.07

Figure 9. Reaction network for CH3 OH oxidation to form CH2 O ([1] to [3]) and HCOOCH3 ([4] to [7]) predicted for the VTS catalyst model. Energy barriers (ΔE0K, eV) for the elementary steps along the most favorable pathway were calculated at the B3LYP and PBE levels with the cluster and slab models (shown in black and red, respectively).

3.3 Effects of Sulfate on the Catalytic Activity and Product Selectivity 3.3.1 Effect on the catalytic activity

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Acid- modified V2 O5 /TiO 2 catalysts such as the VTS catalyst have been shown by a number of experimental works to promote not only the conversion rate of CH3 OH but also the selective formation of DMM and MF.38,51,58,61 With this study, we can give a detailed explanation for the effect of sulfate on the activity and selectivity in CH3 OH selective oxidation to MF. As shown in Figure 4 for the V2 O5 /TiO 2 catalyst and Figure 9 for the VTS catalyst, with the cluster model the highest energy barrier for the elementary steps during CH3 OH oxidation to CH2 O and MF was predict to be that of the ODH of CH3 O* for both catalyst models (assuming there is no higher energy barrier for catalyst regeneration with the VTS catalyst). For the V2 O 5 /TiO 2 catalyst, this energy barrier was predicted to be 1.29 eV, whereas for the VTS catalyst, this was calculated to be 1.18 eV. Thus, the highest energy barrier predicted for the VTS catalyst is lower than that for the V2 O5 /TiO 2 catalyst by 0.11 eV, and at ~140 °C, which is the optimal reaction temperature for the selective formation of MF, this shall result in a n increase in the reaction rate constant of this elementary step by about exp(∆Ea /RT) ≈ 22, where ∆Ea is the difference in their energy barriers and R is the ideal gas constant. Here, we assume a similar prefactor for the rate constants for the ODH of CH3 O* on the V2 O5 /TiO2 and VTS catalysts due to their similarity in nature. Although this is consistent with the fact that acid-promoted especially sulfate- modified V2 O 5 /TiO 2 catalysts are catalytically more active for CH3 OH selective oxidation, it is difficult for us further make a quantitative comparison against the experimental data as this difference is not available experimentally. We here provide an explanation for the beneficial effect of the sulfate group on the catalytic activity based on the calculated natural charges. As discussed above, the rate determining step in the formation of both CH2 O and MF is the hydrogen transfer step from the CH3 O* group to the bridge O, i.e. from structure 4a to 6a for the VTS catalyst model and from

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structure 4b to 6b for the V2 O5 /TiO 2 catalyst model in Figure 7a. The natural charges were calculated to be 1.11 and −0.33 e for the V and HSO 4 groups in structure 4a, and 1.08 and −0.21 e for the V and OH groups in structure 4b. They deviate significantly from the formal changes of +5 and −1 e. Nevertheless, it shows that the HSO 4 group carries more negative charge than the OH group, leading to more positive charge at the V site in structure 4a than structure 4b, so the V site in the former can be expected to be a better oxidant than that in the latter. Our calculations further show that from structure 4a to 5a, the positive charge on the V atom decreases by 0.14 e, corresponding to the reduction of the V site, which further reduces by 0.04 e from structure 5a to 5b. On the other hand, from structure 4b to 5b, the positive charge on the V atom was calculated to slightly increase by 0.02 e, in conflict with the intuition that the V site is reduced, although its positive charge does decrease from structure 5b to 6b by 0.02 e leading to negligible change in the charge on the V atom during this elementary step. The above analysis is consistent with the lower energy barrier for the above step predicted for the VTS catalyst model of 1.18 eV than that for the V2 O5 /TiO2 catalyst model of 1.29 eV. However, the notion that the V(V) site is reduced to the V(III) site in the ODH of CH3 O* to form CH2 O* departs substantially from the insight gained from the charge analysis.

3.3.2 Effect on the MF selectivity From the reaction network shown in Figure 4 for CH3 OH oxidation on the V2 O5 /TiO 2 catalyst, we can attribute the selectivities of CH2 O and MF to the differences in the maximum energy barriers (∆Ea) for steps [2] to [3] and steps [9] to [12], although steps [4] to [5] also lead to the formation of CH2 O, and other products such as DMM and DME can also be formed under certain conditions, which will be investigated in our future work. With the cluster model, the

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energy barrier of step [2] of 1.29 eV is the highest for the formation of CH2 O, whereas that of step [11] of 1.19 eV is the highest for the formation of MF. The fact that the highest energy barrier for MF formation is about 0.10 eV lower than that for CH2 O formation can explain the selective formation of MF in CH3 OH oxidation on the V2 O5 /TiO 2 catalyst. We can further estimate the ratio of the selectivity of MF to CH2 O as the ratio between the rate constants of these two elementary steps. As both steps involve hydrogen transfer from the C to O b, they can be considered to be similar in nature and to have similar prefactors, and the ratio of their rate constants (kMF/kCH2O) can be estimated to be exp(∆Ea/RT) ≈ 16 at T ≈ 140 °C, where ∆Ea is the difference in their energy barriers. The ratio of the selectivity of MF to CH2 O reported by Kaichev and co-workers range from 16 to 28 with CH3 OH conversion rates ranging from 45% to 85%,31 so our estimate based on the cluster model calculations is in reasonable agreement with their results, although our simple model does not allow us to estimate the effect of the CH3 OH conversion rate on the product selectivity. With the slab model, the energy barrier was calculated to be 1.19 eV for step [2] and 1.01 eV for step [11], which has a larger difference of 0.18 eV. However, step [10] was predicted to have a higher energy barrier of 1.16 eV than step [11], so our simple model may not work properly. Nevertheless, even with the slab model, the energy barrier for the ODH of CH3 OCH2 O* was predicted to be lower than that of CH3 O*, which should be the primary reason for the preferred formation of MF in CH3 OH oxidation. From the reaction network for CH3 OH oxidation shown in Figure 9 for the VTS catalyst, we can also use a similar approach to estimate the ratio of the selectivity of MF to CH2 O. Here, we attribute the difference in the selectivities to the differences between the energy barriers of step [2] and the maximum for steps [4] to [6]. With the cluster model, the energy barrier of step [2] for the formation of CH2 O was predicted to be 1.18 eV, whereas that of step [6] of 1.01 eV

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was calculated to be the highest for the formation of MF. Thus, the highest energy barrier for the formation of MF was predicted to be lower than that for the formation of CH2 O by 0.17 eV, which can explain the selective formation of MF in CH3 OH oxidation on the VTS catalyst. We can further estimate the ratio of the rate constants (kMF/kCH2O) at 140 °C as ~119 for the VTS catalyst model, which is much higher than that of ~16 for the V2 O5 /TiO 2 catalyst model. The ratio of the selectivity of MF to CH2 O reported in our previous work reaches a maximum of 58 with the VTS catalyst calcinated at 400 °C, which is about half of our estimate. Apparently, the ratio of the selectivity of MF to CH2 O is affected by other factors including the CH3 OH conversion rate, the amount of the sulfate content, as well as the formation of other products, which cannot be accommodated with our simple model. Nevertheless, it is clear that the difference in the energy barriers for the dehydrogenation of CH3 O* and CH3 OCH2 O* is an crucial factor in determining the ratio of the selectivities of CH2 O and MF.

4. CONCLUSIONS We carried out density functional theory calculations to investigate the catalytic mechanism of methanol oxidation on the V2 O5 /TiO 2 -based catalysts to form methyl formate (MF). The VTi3 O7 (OH)3 and VTi3 O6 (SO 4 )(OH)3 molecular clusters as well as the V(=O)(−O−)3 and V(SO4 )(−O−)3 monomers supported by the anatase-TiO 2 (001) periodic slab surface were used as the catalyst models. Consistent results were obtained from these cluster and slab calculations, enabling us to establish a reliable and complete catalytic reaction net work for methanol oxidation to formaldehyde and MF on the V2 O 5 /TiO 2 catalyst. Our calculations show that molecular oxygen can assist the desorption of these two reaction products, and the catalyst regeneration involves a peroxo species and the conversion o f another molecule of methanol into

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formaldehyde. From the calculated energy barriers in the reaction network, the favorable formation of MF over formaldehyde at temperature around 140 °C is attributed to the lower energy barrier of the oxidative dehydrogenation (ODH) of CH3 OCH2 O* than that of CH3 O*. The ratio of the selectivity of MF to formaldehyde was estimated from the difference in the energy barriers of these two elementary steps to be ~16 and ~119 for the unpromoted and sulfatepromoted V2 O5 /TiO 2 catalysts, which compare reasonably well with previous experimental observations. The sulfate promoter was also predicted to enhance the catalytic activity of the V2 O 5 /TiO 2 catalyst by lowering the energy barrier for the ODH of CH3 O*, also consistent with previous experimental findings.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Complete lists for refs. 4, 22, 37, and 72, coordinates of the catalyst models used, potential energy surfaces for the formation of methyl formate on the slab models of the VTS catalyst, structures and coordinates of the stationary states on the potential energy surfaces for Figures 1-3 and 5-8.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interests.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Nos. 21473233 and 21506243), the Science and Technology Commission of Shanghai Municipality (Nos. 14DZ1203700 and 14DZ1207602), the Lu’an Mining Group (Changzhi, Shanxi, China), and the Shanghai Huayi (Group) Company (Shanghai, China).

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Page 44 of 44

TOC GRAPHIC 1.29 1.18 CH2O

1.19 1.01 CH2O

HCOOCH3

Unpromoted V2O5/TiO2

HCOOCH3

The Maximum Energy Barrier in eV @ B3LYP

44


SulfatePromoted V2O5/TiO2

Catalytic Mechanisms of Methanol Oxidation to Methyl Formate on

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