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Highly Efficient and Stable Vanadia-Titania-Sulfate Catalysts for Methanol Oxidation to Methyl Formate: Synthesis and Mechanistic Study Ziyu Liu, Ruifang Zhang, Shibin Wang, Na Li, Rui Sima, Guojuan Liu, Ping Wu, Gaofeng Zeng, Shenggang Li, and Yuhan Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12621 • Publication Date (Web): 03 Mar 2016 Downloaded from http://pubs.acs.org on March 8, 2016
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The Journal of Physical Chemistry
Highly Efficient and Stable Vanadia-Titania-Sulfate Catalysts for Methanol Oxidation to Methyl Formate: Synthesis and Mechanistic Study
Ziyu Liu,a Ruifang Zhang,a Shibin Wang,a,b Na Li,a,c Rui Sima,a Guojuan Liu,a Ping Wu,a Gaofeng Zeng,a Shenggang Li,a,b,* Yuhan Suna,b,*
a
CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced
Research Institute, Chinese Academy of Sciences, 100 Haike Road, Shanghai 201210, China b
School of Physical Science and Technology, ShanghaiTech University, 319 Yueyang Road,
Shanghai 200031, China c
Department of Chemistry, Shanghai University, 99 Shangda Road, Shanghai 200444, China
*E-mail:
[email protected],
[email protected] Keywords: Methanol Selective Oxidation; Density Functional Theory; Periodic Slab and Cluster Models; Hemiacetal and Formyl Mechanisms; Redox and Acid Sites.
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ABSTRACT Vanadia-titania-sulfate nanocatalysts for methanol oxidation to methyl formate (MF) were prepared by co-precipitation. When calcinated at 400°C, both methanol conversion and MF selectivity reached ~98.5% at reaction temperatures of 140−145°C. Characterizations with several experimental techniques revealed the catalysts as highly dispersed vanadia supported by anatase titania with acid sites of significant strength and density. The catalysts also showed very high stability with lifetime exceeding 4500 hours. Extensive density functional theory calculations using both cluster and surface models revealed MF to form via the hemiacetal mechanism, involving the condensation of methanol and formaldehyde at acidic sites and methanol and hemiacetal oxidations at redox sites. The alternative formyl mechanism was predicted kinetically much less favorable, showing these catalysts to work in a distinct mechanism from the rutile titania photocatalyst. Interestingly, methanol chemisorption at redox sites leads to the formation of acidic sites capable of catalyzing the condensation reaction.
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The Journal of Physical Chemistry
1. INTRODUCTION Methanol has been proposed as a fundamental chemical and energy feedstock of the future replacing fossil fuels.1 In addition to being used directly as a fuel2 and a fuel additive, methanol has also been used by the industry to manufacture formaldehyde (CH2O, FA), 3 dimethyl ether (CH3OCH3, DME),4 as well as hydrocarbons such as ethylene and propylene.5 Methyl formate (HCOOCH3, MF) and dimethoxymethane (CH3OCH2OCH3, DMM)6 are also products obtainable from methanol oxidation, and the former was proposed as a building block molecule in C1 chemistry. 7 At present, MF is commercially produced by carbonylation of methanol using sodium methoxide as the catalyst and dry CO as the carbonylation reagent, and there is significant interest in developing more efficient and greener processes for MF synthesis from methanol. Friend, Bäumer, and co-workers8 showed that nanoporous Au catalysts could catalyze methanol oxidation with very high MF selectivity (> 97%) at relatively low temperature (< 80°C). Extensive studies on Au- and Pd-based catalysts were carried out,9,10,11,12,13,14,15 and the highest MF yield of ~90% was reported for graphene-supported Au-Pd nanocatalysts.10 The rutile TiO2(110) surface was found to be a photocatalyst for this reaction,16,17,18,19 and loading Au-Ag nanoparticles onto the TiO2 surface can raise the MF yield to ~77% under UV irradiation.13 Supported transition metal oxide catalysts such as ReOx/CeO2,20 RuOx/ZrO221 and V2O5/TiO26 can also catalyze this reaction. With the V2O5/TiO2 catalysts, DMM is the main product at low temperature ( VTS-350 > VTS-450 >> VTS-500, so the highest MF yield was obtained with the VTS-400 catalyst. With a given catalyst,
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increasing the reaction temperature from 135 to 145°C usually improve both methanol conversion and MF selectivity, although much stronger effects of the reaction temperature were observed for the VTS-450 and VTS-500 catalysts than the VTS-350 and VTS-400 catalysts. The highest MF yield was achieved with the VTS-400 catalyst at reaction temperatures of 140−145°C, with both methanol conversion and MF selectivity reaching ~98.5%. The VTS-400 catalyst outperformed all the known catalysts for MF synthesis by one-step methanol oxidation. In addition, the VTS-400 catalyst also has very high stability with lifetime exceeding 4500 hours. With its very high activity, selectivity and stability, it is a very promising catalyst for commercialization.
Table 1. Methanol Conversion (X), Product Selectivities (SX; X = MF, FA, DMM, DME), and MF Yield (YMF) for the VTS Catalyst at Different Reaction Temperatures (T). Catalyst
VTS-350
VTS-400
VTS-450
VTS-500
T (oC)
X (%)
135
95.8
95.4
4.4
0.0
0.2
91.4
140
97.8
96.5
3.3
0.0
0.2
92.3
145
97.5
96.3
3.5
0.0
0.2
93.9
135
98.0
96.8
3.1
0.0
0.1
94.9
140
98.5
98.3
1.7
0.0
0.0
96.8
145
98.7
98.6
1.1
0.0
0.0
97.3
135
80.9
84.9
14.8
0.0
0.3
68.7
140
90.9
92.3
7.6
0.0
0.1
84.0
145
95.6
96.0
3.9
0.0
0.1
91.7
135
50.6
49.9
22.3
27.7
0.1
26.0
140
55.3
59.0
24.0
16.8
0.2
32.7
145
60.1
60.6
25.0
14.2
0.2
36.7
SMF (%) SFA (%) SDMM (%) SDME (%)
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YMF (%)
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For the VTS-400 catalyst, we further carried out reaction kinetic studies. We first attempted to perform least square fit of the product distribution using simple expressions for the formation rates of the major products, r = A•exp(−Ea/RT)•PCH3OHα•PO2β. However, we were unable to obtain good convergence for the fit. In order to get the fit converged, we used a different expression for the formation rate of MF, r = A•exp(−Ea/RT)•PDMMα•PO2β, where MF was considered to form from the oxidation of DMM, and DMM was formed from the oxidation of CH3OH. With the modified rate expression for MF formation, the pre-exponential factor A and the activation energy Ea were determined to be 8.041010 mol•g−1•atm−0.9•h and 0.80 eV, whereas the reaction orders α and β were calculated to be 0.5 and 0.2. Our difficulty in determining the reaction order of CH3OH for the rate of MF formation may be partially due to the fact that the selectivity of DMM was much higher than that of MF in our current kinetic studies, and further kinetic experiment and data fitting need be performed to elucidate the kinetics for this complex reaction. The fact that the VTS-400 catalyst has the best performance for methanol oxidation to MF suggests that the selective formation of MF requires highly dispersed vanadia species on the anatase titania support. Furthermore, in addition to redox sites, acidic sites with sufficient acidic strength and number density must also be present. It can be expected that highly dispersed vanadia can function as redox sites responsible for methanol oxidation to formaldehyde, whereas acidic sites are required to catalyze the condensation reaction of methanol and formaldehyde leading to the formation of hemiacetal, which can be further oxidized at the redox sites.
3.3 Mechanistic Study
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In order to verify the above hypothesis, we carried out DFT calculations to predict the mechanism of MF formation on the anatase TiO2-supported vanadia catalysts. As the amount of sulfur in our VTS catalysts is very limited ( Formyl Mechanism 4250
4300
4350
4400
4450
4500
4550
Time (hour)
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