RESEARCH NOTE pubs.acs.org/IECR
Direct Oxidation of Methane to a Methanol Derivative Using Molecular Oxygen Jiongliang Yuan,* Lanlan Wang, and Yan Wang Department of Environmental Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ABSTRACT: A direct selective oxidation of methane to a methanol derivative with the catalysis of palladium acetate/ benzoquinone/molybdovanado-phosphoric acid using molecular oxygen in trifluoroacetic acid has been suggested. Methyl trifluoroacetate is the only liquid product, and its highest yield could be obtained at 80100 °C. The addition of perfluorooctane could improve the yield of methyl trifluoroacetate significantly. The reaction product can be easily separated from the reaction system, using the difference of boiling points.
1. INTRODUCTION The activation and functionalization of methane, which is the most abundant hydrocarbon, has attracted much attention, because of its high abundance in natural gas and its low reactivity. Indirect reaction routes for obtaining methanol from methane via syngas have already been adopted; however, the production of syngas is an energy-intensive and cost-intensive process. Because of its energy- and cost-effectiveness, direct, lowtemperature oxidation of methane to methanol becomes promising. Many catalysts have already been developed to improve the yield and selectivity of methanol or its derivatives.14 For example, a stable platinum complex, (bpym)PtCl2, developed by Catalytica, could directly oxidize methane into a methanol derivative with high yield and selectivity.3 In a catalytic system of direct oxidation, both the oxidants and reaction solvents are also crucial to the activity of catalysts, and the yield and selectivity of the products. PtCl62 can oxidize methane to methanol and chloromethane with the catalysis of PtCl42 in aqueous solution; nevertheless, the yield and selectivity of methanol are very low.1 Because of its strong oxidative capacity and the high reactivity with methanol into stable products, fuming or concentrated H2SO4 are always adopted as the oxidant and reaction solvent in methane oxidation catalyzed by transition-metal compounds.24 In addition, concentrated H2SO4 facilitates the protonation of the bpym ligand, which is beneficial for catalytic activity.5 However, since water produced in methane oxidation decreases the concentration of H2SO4, the oxidative capacity of H2SO4 is weakened, and, thus, the catalysts deactivate rapidly.6,7 To eliminate the effect of water and promote the activity of catalysts, ionic liquids are introduced.8 In addition, ionic liquids can also act as dissolution media for otherwise insoluble Pt salts/oxide.8 Since CF3COOH and (CF3CO)2O are readily to react with methanol into stable CF3COOCH3,913 and have higher solubility of methane,11,12 they are also used as reaction solvents. In (CF3CO)2O and/or CF3COOH solvents, with the catalysis of vanadium-containing heteropolyacid, methane can be oxidized to CF3COOCH3 and other oxygenates.1113 Moreover, (CF3CO)2O could improve the yield of CF3COOCH3, because it removes the water generated in methane oxidation.913 r 2011 American Chemical Society
In addition to concentrated H2SO4, various peroxides and molecular oxygen in combination with carbon oxides and water have been adopted as the oxidants.9,1114 It has already been verified that hydrogen peroxide (or its equivalent) was formed in the system, starting with molecular oxygen and carbon monoxide in the presence of water.14 Unfortunately, the oxidation reactions using peroxides always involve a radical mechanism and, thus, have poor selectivity.1113 Molecular oxygen is an environmentally friendly, inexpensive oxidant; however, there are only a few reports on the partial oxidation of methane using molecular oxygen. Poor solubility of molecular oxygen is one of the important reasons for limiting its use. A one-pot aerobic oxidation of methane with the combination of the three redox couples Pd(II)/Pd(0), p-benzoquinone (BQ)/hydrobenzoquinone (H2Q), NO2/NO in CF3COOH was suggested, but the turnover number (TON, expressed in terms of moles of CF3COOCH3 per mole of Pd(II)) is very low.15 The Pd(II)/BQ/molybdovanadophosphoric acid/O2 system has been widely used in homogeneous catalysis of organic synthesis;16 however, there has been no successful report of the oxidation of methane. This is because of the absence of appropriate solvents and the weak oxidation capacity of molecular oxygen. Here, we demonstrate a Pd(OAc)2/BQ/H5PMo10V2O40 system for the direct oxidation of methane to methanol (in the form of CF3COOCH3 that can be hydrolyzed later) in CF3COOH at low temperature, in which the catalysts can be regenerated by molecular oxygen. Perfluorocarbons exhibit very high solubilities and transport properties of oxygen and, therefore, have become increasingly used in medicine and biotechnology (e.g., in artificial blood and in fermentation).1719 In this study, to improve oxygen solubility and transport property in CF3COOH, one of perfluorocarbons, C8F18, which is inert in catalytic oxidation, was introduced in methane oxidation as an oxygen carrier. Received: August 30, 2010 Accepted: April 15, 2011 Revised: April 14, 2011 Published: April 15, 2011 6513
dx.doi.org/10.1021/ie1018113 | Ind. Eng. Chem. Res. 2011, 50, 6513–6516
Industrial & Engineering Chemistry Research
RESEARCH NOTE
Table 1. Catalytic Oxidation of Methane to CF3COOCH3 in CF3COOH a BQ
O2
temperature,
CF3COOCH3
entry
(μmol)
(MPa)
T (°C)
(μmol)
TON
1 2
6 6
0.2 0.5
80 80
3.61 6.04
1.2 2.0
3
6
0.7
80
6.36
2.1
4
12
0.5
40
2.01
0.7
5
12
0.5
60
5.29
1.8
6
12
0.5
80
9.10
3.0
7
12
0.5
100
9.04
3.0
8
12
0.5
120
3.83
1.3
Conditions: CF3COOH, 10 mL; Pd(OAc)2, 3.0 μmol; H5PMo10V2O40, 0.5 μmol; CH4, 2.5 MPa; 80 °C; 8 h. a
Figure 1. Yield of CF3COOCH3 as a function of the molar ratio of BQ to Pd(OAc)2. (Conditions: CF3COOH, 10 mL; Pd(OAc)2, 3.0 μmol; H5PMo10V2O40, 0.5 μmol; CH4, 2.5 MPa; O2, 0.5 MPa; 80 °C; 8 h.)
2. EXPERIMENTAL SECTION 2.1. Materials. Pd(OAc)2, CF3COOH, and CF3COOCH3 were purchased from Shanghai Jingchun Reagent Co., Ltd. (PRC). BQ, (CF3CO)2O, and C8F18 were purchased from Sinopharm Chemical Reagent Co., Ltd. (PRC). Methane, oxygen, and nitrogen were obtained from Beijing Huayuan Gas Co., Ltd. (PRC). H5PMo10V2O40 was prepared from H3PO4, MoO3, and V2O5.20 H3PO4, MoO3, and V2O5 were obtained from Beijing Chemical Factory (PRC). 2.2. Experimental Method. Methane oxidation reactions were conducted in a 50-mL cylindrical stainless steel autoclave with a glass liner on the inside. Given amounts of Pd(OAc)2, BQ, and H5PMo10V2O40 were introduced in 10 mL of CF3COOH (or CF3COOH/(CF3CO)2O). Mild stirring was provided by a magnetic stirring bar coated with Teflon. The reactor was purged three times with 1.0 MPa of nitrogen and then pressurized with methane and oxygen (the pressures listed below are those charged to the reactor at room temperature). It was heated to a given temperature in an oil bath and kept under stirring. After the reaction, the reactor was cooled to 3 °C in an ice/water mixture; thus, the pressure was slowly reduced. 2.3. Characterization. The product was analyzed by gas chromatography-mass spectroscopy (GC-MS) and 1H NMR (Bruker AV600, Swiss). The gas phase was analyzed on a gas chromatograph (GC 4000A, East & West Analytical, PRC) with TDX-01 column, and the liquid was qualified on a gas chromatograph with a Porapak QS column.
3. RESULTS AND DISCUSSION 3.1. Methane Oxidation in CF3COOH. According to 1H NMR
and GC-MS analysis results, CF3COOCH3 is the only liquid product, and CO2 the only gas product in methane oxidation. The partial pressure of oxygen is an important factor in catalytic oxidation, and a higher partial pressure leads to a higher yield of CF3COOCH3 (see Table 1, entries 1, 2, and 3), because it can promote the regeneration reaction of H5PMo10V2O40.21 In contrast to the Catalytica system, a striking feature of the catalytic system is a lower reaction temperature, and the highest yield of CF3COOCH3 could be obtained at 80100 °C (see Table 1, entries 4, 5, 6, 7, and 8). In addition, the selectivity of CF3COOCH3 is higher (e.g., in comparison to 9.10 μmol of CF3COOCH3, 1.10 μmol of CO2 yield) (see Table 1, entry 6).
Figure 2. Yield of CF3COOCH3 as a function of the volume ratio of (CF3CO)2O to CF3COOH. (Conditions: (CF3CO)2O þ CF3COOH, 10 mL; Pd(OAc)2, 3.0 μmol; BQ, 12.0 μmol; H5PMo10V2O40, 0.5 μmol; CH4, 2.5 MPa; O2, 0.5 MPa; 80 °C; 8 h.)
In the catalytic system, the yield of CF3COOCH3 increases linearly with the molar ratio of BQ to Pd(OAc)2 when the molar ratio is
