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D.-g. Piao et al. / Journal of Organometallic Chemistry 574 (1999) 116–120
Fig. 3. Effect of the amount of K2S2O8. Reaction conditions: CH4 (20
atm), H5PV2Mo10O40 (0.022 mmol), TFA (5.0 ml), TFAA (10 mmol),
80°C, 20 h.
Fig. 5. Arrhenius plot of the V-catalyzed oxidation of methane.
Conditions: CH4 (10 atm), H5PV2Mo10O40 (5.0 mmol), K2S2O8 (5.0
mmol), TFA (5.0 ml), TFAA (10 mmol), 5 h.
temperature increased. The yield of the reaction at
100°C increased up to 10 h and then decreased rapidly.
The best yield of products was obtained in the reaction
at 80°C for 20 h. The activation energy of this reaction
was calculated by the Arrhenius plot of initial reaction
rate within 5 h. From Fig. 5, the activation energy (DE)
is estimated to be 27.9 kcal mol−1. A lower value (15.3
kcal mol−1) for the similar partial oxidation of
methane with the CuCl2/K2PdCl4/H2O/TFA/CO/O2
catalyst system was reported by Sen and co-workers [6].
Then, we investigated the effect of the CH4 pressure.
The representative results are summarized in Fig. 6.
The TON of the catalyst increased with an increasing
pressure of methane. The best result was obtained at 20
atm of CH4. Under a higher pressure than 20 atm, the
yield of the ester 1 decreased whereas the yield of ester
2 increased with the increasing pressure of CH4.
In order to improve the yield based on methane, the
reaction using a 25-ml autoclave was examined. The
representative results are listed in Table 2. The yield
based on methane increased with increasing amounts of
the solvent (entries 1–3).
surizing inert nitrogen (5 atm) (entry 5).
Although the details of the mechanism of the partial
oxidation of methane by vanadium catalysts are not yet
clear, a possible mechanism is shown in Fig. 7. A high
oxidation state oxo-vanadium species V(V)=O would
abstract H from CH4 to form a methyl radical (CH3 )
which would be oxidized by V(V)=O to a methyl
cation (CH3+). The CH3+ cation would then react with
CF3COO− to give CF3COOCH3 (1). The by-product
CH3COOCH3 (2) would be formed via ester exchange
between 1 and acetic acid which would be formed from
the reaction of CH4 and CO, derived from the decom-
position of TFA and/or TFAA [13,15].
3. Experimental
3.1. General
Analytical GLC evaluations of the products were
performed on a Shimadzu GC-8A gas chromatography
equipped with a flame ionization detector by using two
2.0 m×3.0 mm i.d. stainless columns ([Unisole 10T+
The reaction under the lower pressure of methane (6
atm) afforded the product in an 87.6% yield (entry 4).
Furthermore, we succeeded the almost quantitative
conversion (95%) of methane to methyl esters by pres-
Fig. 4. Time course of the reaction. Conditions: CH4 (20 atm),
H5PV2Mo10O40 (0.022 mmol), K2S2O8 (5.00mmol), TFA (5.0 ml),
TFAA (10 mmol).
Fig. 6. Effect of CH4 pressure. Conditions: H5PV2Mo10O40 (0.022
mmol), K2S2O8 (5.0 mmol), TFA (5.0 ml), TFAA (10 mmol), 80°C,
20 h.