C O M M U N I C A T I O N S
Scheme 1. Catalytic Oxidation of Methane by Dioxygen through
an Electron-Transfer Chain
Figure 1. The yield of CF3COOCH3 versus the reaction time.
regeneration of Q but in CH2Cl2.11 We tested the feasibility of this
reaction (eq 4) in CF3COOH by adding NaNO2 to an air-stable
white suspension of H2Q in CF3COOH. NaNO2 is a convenient
nitrogen oxide source because it decomposes to NO in CF3COOH
and then rapidly reacts with O2 to form NO2 (eq 5).11 Once NaNO2
was introduced, the suspension cleared into a yellow solution under
air within 1 min and contained only Q as determined by NMR.
In summary, with the combination of the three redox couples
Pd2+/Pd0, Q/H2Q, and NO2/NO in CF3COOH, we have developed
a catalytic system for the one-pot aerobic oxidation of methane at
80 °C. The activation of methane with Pd2+ initiates an electron-
transfer chain, which carries the electrons from methane to O2,
similar to the biological oxidation processes. This is the first
example integrating an organic cocatalyst for the selective oxidation
of methane, which significantly increases the catalytic efficiency
of a transition metal. The TON of approximately 0.7 per hour is
limited by the initial activation step. Further improvement of
productivity could be possible if a more active catalyst than Pd2+
can be found for reaction 1.
When 20 µmol of each NaNO2 and Q were added to our system
in run 6, the yield doubled to 69 µmol CF3COOCH3 with respect
to runs 2 and 4. The molar ratios of CF3COOCH3 to Pd2+, Q, or
NaNO2 are clearly larger than 1. Moreover, all Pd2+ is practically
retained in the active state after the reaction. Thus, it can be
concluded that the selective oxidation of methane to methanol
has become catalytic by coupling Pd2+, Q, NaNO2, and O2 in
CF3COOH. Note that Pd0 can also be oxidized to Pd2+ directly by
NO2.12 However, without Q the yield of CF3COOCH3 is ca. 25%
lower even when 40 µmol NaNO2 is used (see SI-2, Supporting
Information). Run 7 shows that increasing the amounts of Q and
NaNO2 beyond twice the stoichiometric amount of Pd2+ does not
significantly promote the reaction further since the ester yield
remains constant. Figure 1 displays a nearly linear dependence of
the yield on the reaction time within 15 h under the conditions of
run 6. Also, the yield increases nearly linearly with the amount of
Pd2+, as seen in runs 6 and 8. This indicates that the turnover
number (TON) is approximately 0.7 per hour, which is consistent
with the yield of 60% methanol within 1 h on the basis of the Pd2+
reported previously.5 However, it is worthwhile to note that the
regeneration of Pd0 to Pd2+ by Q is not fast enough even in the
presence of NO/NO2 to prevent Pd0 from precipitation when the
ratio of Q/Pd2+ is too small as in run 9. Consequently the yield in
run 9 is not exactly twice as high as in run 6, as could be expected.
The origin of the methyl group in CF3COOCH3 was confirmed
by isotope experiments, replacing 14 out of 54 atm methane with
13CH4 in the reaction. Indeed, GC-MS analysis showed that the
ratio of the fragments COO12CH3 and COO13CH3 (m/e ) 59 and
60, respectively) was very close to 3:1. By 13C NMR, we also
detected the formation of CF3COO13CH3 (see SI-3). Therefore, the
methyl group in CF3COOCH3 is derived unambiguously from
methane. Note that we did not observe any other oxidation products
of methane such as formaldehyde, formic acid, or CO2.8
Acknowledgment. We acknowledge financial support from
the Ministry of Science and Technology of China (Grant 2005
CB221405).
Supporting Information Available: Gravimetric method for the
determination of Pd2+ left in the liquid after reaction; p-benzoqui-
none in the oxidation process; NMR analysis. This material is available
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(8) Care should be taken during the operations because of the danger of
explosion. The explosion limits of CF3COOH in air are between 3.8-21
v% and of CH4 in O2 between 5-61 v%. In a 50 ml autoclave, catalysts
and 3 ml CF3COOH were put in a glass liner with a Teflon-coated
magnetic stirrer. The reactor was three times purged with 30 atm CH4
and then pressurized with 54 atm CH4 and 1 atm O2. It was heated to 80
°C in an oil bath and kept for 10 h under stirring. After the reaction, the
reactor was cooled to 3 °C in ice water, and the pressure was slowly
reduced. The product was analyzed by GC-MS, NMR, and quantified
by GC. In all runs, approximately 20 µmol acetic acid was detected,
corresponding to the amount of the ligand of Pd(OAc)2. In addition we
found CO2 in all runs, but a control experiment with CF3COOH pressurized
with 1 atm O2 and 54 atm N2 yielded the same amount of CO2 within the
experimental error (5%) indicating that it is due to decomposition of
CF3COOH as reported in reference 3d.
This oxidation process is described in Scheme 1. Pd2+ is the
catalytically active center, oxidizing methane to CF3COOCH3 in
CF3COOH. The reduced Pd0 (eq 1) is subsequently regenerated by
Q, leading to Pd2+ and H2Q (eq 2). Then NO2 oxidizes H2Q to Q
and NO is produced (eq 4), which is finally reoxidized to NO2 by
O2 (eq 5). Thus the net oxidation reaction can be described as in
eq 6. Since CF3COOCH3 can be hydrolyzed to CH3OH and
CF3COOH (eq 7), the net reaction can be written as in eq 8.
(9) This is measured by gravimetry using dimethylglyoxime to precipitate
dissolved Pd2+ followed by selective oxidation of precipitated Pd0 and
centrifugation (see Supporting Information SI-1 for details).
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