An efficient partial oxidation of methane in trifluoroacetic acid using vanadium-containing heteropolyacid catalysts

Article abstract of DOI:10.1016/S0022-328X(98)00931-0

The new catalytic system has been examined for the partial oxidation of methane in liquid phase. It was found that the vanadium containing heteropolyacids/K₂S₂O₈/(CF₃CO) ₂O/CF₃COOH catalyst system converts methane to methyl trifluoroacetate along with a trace amount of methyl acetate in a 95% yield based on methane. The activation energy of the reaction was estimated to be 27.9 kcal mol^-1.

Full text of DOI:10.1016/S0022-328X(98)00931-0

Journal of Organometallic Chemistry 574 (1999) 116–120
An efficient partial oxidation of methane in trifluoroacetic acid using
vanadium-containing heteropolyacid catalystsꢀ
Dong-guo Piao, Kensuke Inoue, Hiroyasu Shibasaki, Yuki Taniguchi, Tsugio Kitamura,
Yuzo Fujiwara *
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu Uni6ersity, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan
Received 26 June 1998; received in revised form 3 August 1998
Abstract
The new catalytic system has been examined for the partial oxidation of methane in liquid phase. It was found that the
vanadium containing heteropolyacids/K2S2O8/(CF3CO)2O/CF3COOH catalyst system converts methane to methyl trifluoroacetate
along with a trace amount of methyl acetate in a 95% yield based on methane. The activation energy of the reaction was estimated
to be 27.9 kcal mol−1. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Vanadium; Heteropolyacid; Methane; Partial oxidation; Methyl trifluoroacetate; Activation energy
1. Introduction
Herein we report an efficient partial oxidation of
methane to its esters in TFA by V-containing hetero-
polyacid catalysts.
The lower alkanes such as methane are the most
abundant of the hydrocarbons but the least reactive.
Thus, the partial oxidation of methane is of great
practical interest which is appealing for the liquefaction
of natural gas and the chemical conversion to more
useful chemical products. Recently, various transition
metal catalyst systems have been investigated for liquid-
phase oxidation of methane to methanol derivatives
[1–12]. In continuing studies on C–H bond activations
[13], we have found that methane in trifluoroacetic acid
(TFA) can be also converted to methyl trifluoroacetate
(1) along with a small amount of methyl acetate (2) in
the presence of a catalytic amount of H5PV2Mo10O40
and K2S2O8 as an oxidizing agent, and (CF3CO)2O
(TFAA) under mild conditions Eq. (1):
2. Results and discussion
First, we examined the activity of various catalyst
systems for the partial oxidation of methane in TFA
solution. Consequently, some vanadium-substituted
Keggin type heteropolyacids such as H5PV2Mo10O40
gave good results as shown in Table 1. As is apparent
from the table, the oxidation of methane with K2S2O8
proceeds to afford small amounts of esters 1 and 2 even
in the absence of the catalyst (entry 1). By the addition
of the catalyst, especially vanadium containing hetero-
polyacids, the yields of the products (1 and 2) in-
creased. Among them, H5PV2W10O40 gave the highest
turnover number (TON) and the highest yields of esters
(entry 21). On the selectivity of ester 1, the
H5PV2Mo10O40 catalyst gave higher selectivity than
H5PV2W10O40 in similar TON and yield (entry 4). It is
noteworthy that with the increasing numbers of vana-
(1)
ꢀ This article is dedicated to the memory of the late Professor R.
Okawara.
* Corresponding author. Tel.: +81-92-642-3548; fax: 81-92-642-
3548; e-mail: yfujitcf@mbox.nc.kyushu-u.ac.jp.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)00931-0

D.-g. Piao et al. / Journal of Organometallic Chemistry 574 (1999) 116–120
117
dium atom in the heteropoly anion the yield and the
selectivity of the products decrease (entries 3–7 and
20–22) because of the over-oxidation of the products
[14]. Vanadium(V) oxide and vanadyl acetylacetonate
are also more effective than MoO3 and Na2WO4 in this
reaction (entries 23, 24, 30, and 31). One can see that
the active site of the heteropoly anion as a catalyst is
the oxo–vanadium moiety. Consequently, we selected
H5PV2Mo10O40 as a catalyst on the basis of the yield
and the selectivity of the product (entry 4).
This reaction requires strongly acidic conditions;
TFA works best as a solvent. Furthermore, addition of
TFAA accelerates the reaction rate. The effect of the
amount of TFAA is summarized in Fig. 1. The yield of
products 1 and 2 are dramatically increased by the
addition of 10 mmol of TFAA. Excess use of TFAA of
more than 10 mmol decreased the yield of 1. The role
of TFAA is not clear, but it seems that TFAA works as
Fig. 1. Effect of the amount of TFAA. Conditions: CH4 (20 atm),
H5PV2Mo10O40 (0.022 mmol), K2S2O8 (5.00 mmol), TFA (5 ml),
80°C, 20 h.
desiccant of the hydrate on the catalyst and activates
the catalyst since heteropolyacid catalysts usually exist
as 30 hydrates.
Fig. 2 shows the effect of the amount of an
H5PV2Mo10O40 catalyst. The yield of products in-
creased with an increasing amount of the catalyst. The
highest yield of products (1 and 2) was obtained by
using 0.013 mmol (TON=490) of the catalyst. Excess
use of the catalyst resulted in lower yields because the
complex reaction took place.
The effect of oxidants was studied using an
H5PV2Mo10O40 catalyst in TFA. The peroxide K2S2O8
works best as the oxidant for the partial oxidation of
methane. The use of other oxidants such as Na2S2O8,
(NH4)2S2O8, MnO2, KMnO4, and H2O2 in lieu of
K2S2O8 gave inferior results. Fig. 3 shows the effect of
the amount of K2S2O8. The yield of products increased
with an increasing amount of K2S2O8. The best result
was obtained by using 5 or 6 mmol of K2S2O8. Excess
use of the oxidant resulted in lower yields because of
lower efficiency of stirring and the complex reaction.
The time course of the reaction at several tempera-
tures under the same conditions is shown in Fig. 4. The
initial reaction rate (within 10 h) increased as the
Table 1
Catalyst effect for the partial oxidation of methanea
Entry
Catalyst
TON Yield (%)b
Ratio/1:2
1
2
3
4
5
6
7
8
9
None
–
5
0.4
0.1
2.4
4.7
2.8
2.1
1.8
0.1
0.2
0.4
0.3
0.3
0.5
0.4
0.1
0.8
0.4
3.5
0.5
1.0
6.4
3.3
4 0
3.5
3.1
1.5
0.7
0.8
2.1
0.3
1.0
0.9
0.7
2.0
79:21
76:24
83:17
88:12
86:14
73:27
67:33
83:17
90:10
73:27
54:46
72:28
76:24
72:28
80:20
56:44
59:41
73:27
74:26
76:24
73:27
55:45
83:17
71:29
76:24
54:46
88:12
87:13
63:37
78:22
96: 4
91: 9
93: 7
91: 9
H3PMo12O40
H4PVMo11O40
H5PV2Mo10O40
H6PV3Mo9O40
H7PV4Mo8O40
H8PV5Mo7O40
H3PW2Mo10O40
H3PW4Mo8O40
H3PW6Mo6O40
H3PW8Mo4O40
H3PW10Mo2O40
H3PW12O40
H4SiW12O40
H4SiW4Mo8O40
H4SiW6Mo6O40
H4SiW9Mo3O40
H5SiVW11O40
H4SiMo12O40
H4PVW11O40
H5PV2W10O40
H6PV3W9O40
V2O5
80
161
92
76
62
4
8
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
17
14
12
22
15
6
32
18
161
19
50
241
139
57
56
37
17
9
VO(acac)2
V2O3
NaVO3
Ta2O5
Nb2O5
TiO2
MoO3
Na2WO4
12
31
5
16
14
11
Pd(OAc)2
Cu(OAc)2
Pd(OAc)2/CU(OAc)2 31
a Reaction conditions: CH4 (20 atm), catalyst (50 mg for hetero-
polyacid, 0.05 mmol each for entries 23–34), K2S2O8 (5.00 mmol),
TFAA (10.0 mmol), TFA (5.0 ml), 80°C, 20 h.
Fig. 2. Effect of the amount of catalyst. Conditions: CH4 (20 atm),
K2S2O8 (5.00 mmol), TFA (5.0 ml), TFAA (10.0 mmol), 80°C, 20 h.
b GLC yield of 1 and 2 based on CH4.

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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.

D.-g. Piao et al. / Journal of Organometallic Chemistry 574 (1999) 116–120
119
Table 2
Quantitative conversion of methane to methyl trifluoroacetatea
3.3. Typical procedure for the partial oxidation of
methane by heteropolyacid catalyst
Entry TFA (ml)
TON (1+2) Product (1+ Yield (%)b
2)/mmol
In a 100 or 25-ml stainless steel autoclave fitted with
a glass tube and a magnetic stirring bar, were added the
catalyst, K2S2O8, TFA and TFAA, successively. The
autoclave was closed and then pressurized to 10 atm
with CH4. The mixture was heated with stirring at 80°C
for 20 h. After cooling the autoclave was opened and
the mixture in TFA was directly analyzed by GLC
using ethanol as an internal standard. Methyl trifl-
uoroacetate (1) was obtained along with a small
amount of methyl acetate (2) as a by-product of this
reaction. The formation of the esters 1 and 2 was
1
2
3
4
5
5
36
118
128
98
0.79
2.57
2.72
2.11
1.80
13.9 (93:7)
54.3 (93:7)
67.7 (89:11)
87.6 (89:11)
95.0 (82:18)
7.5
9
9c
9d
81
a Reaction conditions: 25-ml autoclave, H5PV2Mo10O40 (0.022
mmol), CH4 (10 atm), K2S2O8 (5.00 mmol), TFAA (10.0 mmol),
80°C, 20 h.
b GC yield based on CH4. Numbers in parentheses are the ratio of
1 and 2.
c CH4 (6 atm).
1
confirmed by H- and 13C-NMR spectra of the reaction
d N2 (5 atm) added to the reaction mixture, CH4 (5 atm).
mixture. Methyl trifluoroacetate (1): 1H-NMR
(CF3COOH)
l
4.04 (s, CF3COOCH3); 13C-NMR
H3PO4 (5 and 0.5%) on 80/100 mesh Uniport HP] and
Porapak QS) attached with a switch valve. 1H- and
13C-NMR spectra were obtained on a JEOL JNM-
AL300 FT-NMR spectrometer in TFA solution, and
chemical shifts (l) were expressed in ppm downfield
from 4,4-dimethyl-4-sila-pentanesulfonic acid sodium
salts (DSS).
(CF3COOH) l 54.8 (s, CH3), 115.1 (q, CF3, JC–F=282
Hz), 162.6 (q, CꢀO, JC–F=43.5 Hz). Methyl acetate
1
(2): H-NMR (CF3COOH) l 2.17 (s, CH3CO, 3 H),
3.81 (s, CH3O, 3 H); 13C-NMR (CF3COOH) l 19.5 (s,
CH3), 53.2 (s, CH3O), 178.8 (s, CꢀO).
Acknowledgements
3.2. Materials
This work was supported in part by Grant-in-Aid for
Scientific Research on Priority Area no. 283 Innovative
Synthetic Reactions and Scientific Research (A) no.
09355031 from Monbusho. We are grateful to the
Japan New Metals Co. Ltd., and Nippon Inorganic
Colour & Chemical Co. Ltd. for generous gifts of
heteropolyacids.
Methane (Sumitomo Pure Chemical Co. Ltd.) and
nitrogen (BOC gases Co. Ltd.) gases were commercial
grade. Trifluoroacetic acid (TFA), trifluoroacetic anhy-
dride (TFAA), and potassium peroxodisulfate were
purchased from Wako Pure Chemicals Co. Ltd. All
heteropolyacids were gifted and were used without fur-
ther purification.
Fig. 7. Possible mechanism for the . formation of methyl trifluoroacetate (1) from methane.

120
D.-g. Piao et al. / Journal of Organometallic Chemistry 574 (1999) 116–120
[8] R.A. Periana, D.J. Taube, E.R. Evitt, D.G. Lo¨ffler, P.R. Wen-
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.