A new catalytic system consisting of Mo for acetic acid synthesis from methane and CO

Article abstract of DOI:10.1246/bcsj.76.1677

In trifluoroacetic acid (TFA), methane and CO can be converted to acetic acid by a new catalytic system, Mo/CaCl₂/K₂S ₂O₈/TFA. The best result (89.4% of acetic acid) was obtained in a reaction of methane (20 atm) and CO (50 atm) using the catalytic system at 85 °C for 20 h.

Full text of DOI:10.1246/bcsj.76.1677

Short Articles
Bull. Chem. Soc. Jpn., 76, 1677–1678 (2003) 1677
Table 1. Effect of Molybdenum Compounds on Synthesis
aÞ
of Acetic Acid
A New Catalytic System Consisting
of Mo for Acetic Acid Synthesis from
Methane and CO
bÞ
Molybdenum
compound
Yield of acetic acid
Run
mmol
%
1
2
3
4
5
6
7
8
9
None
trace
0.03
1.29
0.04
trace
0.04
trace
0.11
0.11
trace
0.4
17.7
0.5
trace
0.5
trace
1.5
cÞ
Mo
Mo
c,dÞ
ꢀ
and Yuzo Fujiwara
y
y
Tsugio Kitamura, Yusuke Ishida, Teizo Yamaji,
y
MoO2
MoO2
MoCl5
[Mo(CO)6]
[MoO2(acac)2]
[MoO2(acac)2]
dÞ
Department of Chemistry and Applied Chemistry,
Faculty of Science and Engineering, Saga University,
Honjo-machi, Saga 840-8502
eÞ
1.5
a) Reaction conditions: methane (20 atm), CO (20 atm), a Mo
compound (0.2 mmol), K2S2O8 (5 mmol), and TFA (5 mL) in
yDepartment of Applied Chemistry,
Faculty of Engineering, Kyushu University,
Hakozaki, Fukuoka 812-8581
ꢁ
2
5-mL autoclave equipped with a glass tube at 85 C for 20 h.
b) GC yield based on methane. c) Molybdenum powder. d)
In the presence of CaCl2 (0.5 mmol). e) In the absence of meth-
ane.
(
Received April 2, 2003)
In trifluoroacetic acid (TFA), methane and CO can be
aÞ
Table 2. Effect of the Amount of Molybdenum Powder
converted to acetic acid by a new catalytic system, Mo/
CaCl2/K2S2O8/TFA. The best result (89.4% of acetic acid)
was obtained in a reaction of methane (20 atm) and CO (50
bÞ
Mo powder
mmol
Yield of acetic acid
Run
mmol
%
ꢁ
atm) using the catalytic system at 85 C for 20 h.
1
2
3
4
5
6
0
0.18
0.32
0.68
1.29
1.35
1.34
2.5
4.4
9.3
17.7
18.5
18.4
0.01
0.1
0.2
0.25
0.4
Methane, a major component of natural gas, is the most
abundant hydrocarbon and a promising compound as carbon
resources in the near future. To date, methane has been mostly
consumed as an energy source, like fuel. One reason for the
difficulties incurred in the use of methane is its low chemical
a) Reaction conditions: methane (20 atm), CO (20 atm), Mo
powder, CaCl2 (0.5 mmol), K2S2O8 (5 mmol), and TFA (5
mL) in 25-mL autoclave equipped with a glass tube at 85 C
for 20 h. b) GC yield based on methane.
ꢁ
1
reactivity. However, the development of new processes for
converting methane to useful substances, such as acetic acid
or methanol, is a challenging theme to contribute to the pros-
perity of society.
as the oxidant, as shown in Eq 1.
The combined catalyst of molybdenum powder and CaCl2
1
2
We have already reported some catalytic systems for acetic
2
–6
was the most effective (Table 1, run 3). Thus, acetic acid syn-
thesis was studied in detail with the Mo/CaCl2/K2S2O8/TFA
catalytic system. The effect of the amount of molybdenum
powder on acetic acid formation was examined under the reac-
tion conditions given in Table 2. When the reaction was car-
ried out in the absence of Mo, the yield of acetic acid was only
acid synthesis from methane and CO,
namely, Pd(OAc)2/
Cu(OAc)2/K2S2O8/trifluoroacetic acid (TFA), Cu(OAc)2/
7
K2S2O8/TFA, and [VO(acac)2]/K2S2O8/TFA. Hogeveen,
8
Olah, Sen,
synthesis from methane and CO.
9,10
11
and Shul’pin also reported on an acetic acid
In our search for an effective catalytic system for producing
acetic acid from methane and CO, we screened metals and
metal salts. Consequently, we found that molybdenum is an
active element as a new catalyst that is effective for acetic acid
synthesis. Here, we report on our new result concerning acetic
acid synthesis by a molybdenum-catalyzed reaction of meth-
ane with CO.
2
.5%. The yield of acetic acid increased constantly with in-
creasing amount of Mo up to 0.25 mmol, and reached 18.5%.
The effect of the amount of CaCl2 was then investigated us-
ing Mo powder (0.2 mmol) under the conditions indicated in
Table 2. In the absence of CaCl2, the yield of acetic acid
was only 0.4%. Similarly, the yield of acetic acid increased
with increasing amount of CaCl2 up to 0.4 mmol.
The above results indicate that the best molar ratio of Mo
and CaCl2 is 1:2.
To improve the yield of acetic acid, we further examined the
pressure of CO, the amount of oxidant (K2S2O8), and the
amount of TFA. The yield of acetic acid increased almost lin-
early with increasing pressure of CO in the range of 0–50 atm
CO pressure. When a reaction of methane (20 atm) with CO
(50 atm) in the presence of Mo (0.2 mmol), CaCl2 (0.4 mmol),
ð1Þ
Some molybdenum compounds were investigated as a cata-
lyst for acetic acid synthesis from methane and CO. The re-
sults are summarized in Table 1. In TFA, methane was con-
verted to acetic acid by molybdenum catalysts with K2S2O8
Published on the web August 15, 2003; DOI 10.1246/bcsj.76.1677

1
678 Bull. Chem. Soc. Jpn., 76, No. 8 (2003)
Ó 2003 The Chemical Society of Japan
aÞ
Table 3. Effect of the Amount of TFA
umn (H20M-PW30-300, liquid phase HR-20M) under the condi-
ꢁ
tions of injection/detection temperature (250 C) and column tem-
ꢁ
bÞ
TFA
mL
Yield of acetic acid
cÞ
perature (130 C) for acetic acid.
Run
TON
mmol
%
Carboxylation Reaction of Methane. A catalyst and K2S2O8
were placed in a 25-mL stainless-steel autoclave equipped with a
Teflon-coated magnetic stirring bar, and TFA was introduced.
The autoclave was closed and flushed with methane three times
for replacing the inside air of the autoclave, and then pressurized
with the desired pressures of methane and carbon monoxide. The
autoclave was heated with stirring at a fixed temperature for the
desired time. After the reaction, the autoclave was cooled on
an ice bath and then opened. A small amount of valeric acid
was added to the reaction mixture as an internal standard and ace-
tic acid was analyzed by GC.
1
2
3
4
3
5
8
10
1.45
2.42
3.33
2.95
16.1
33.2
68.0
89.4
7
12
17
15
a) Reaction conditions: methane (20 atm), CO (50 atm), Mo
powder (0.2 mmol), CaCl2 (0.4 mmol), K2S2O8 (5 mmol), and
TFA in 25-mL autoclave equipped with a glass tube at 85 C
for 20 h. b) GC yield based on methane. c) Turnover number
based on Mo.
ꢁ
ꢁ
and K2S2O8 (5 mmol) in TFA (5 mL) was carried out at 85 C
for 20 h, the highest yield of acetic acid (33.2%) was obtained.
Under the above conditions, the yield of acetic acid gradually
increased until the amount of K2S2O8 reached 5 mmol, and
then became constant at over 5 mmol of K2S2O8.
References
1
2
R. H. Crabtree, Chem. Rev., 95, 987 (1995).
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Finally, we examined the amount of TFA. With increasing
amount of TFA the yield of acetic acid increased and led to
3 K. Nakata, Y. Yamaoka, T. Miyata, Y. Taniguchi, K.
Takaki, and Y. Fujiwara, J. Organomet. Chem., 473, 329 (1994).
8
9.4% based on methane in a reaction using 10 mL of TFA,
4
M. Kurioka, K. Nakata, T. Jintoku, Y. Taniguchi, K.
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York (1998), p. 349.
as shown in Table 3. When a large amount of TFA was used,
the amount of methane introduced decreased and the yield
based on methane increased. However, the amount of acetic
acid was 2.95 mmol and the reaction in 8 mL of TFA gave
a larger amount of acetic acid (3.33 mmol).
Although the mechanistic aspects of the reaction still remain
unclear, the quenching reaction with radical scavengers strong-
ly supports the intervention of radicals, such as a methyl
radical. For example, the presence of 2,2,6,6-tetramethylpi-
peridin-1-oxyl completely prevented the formation of acetic
acid.
In summary, we have found a new catalytic system, Mo/
CaCl2/K2S2O8/TFA, for acetic acid synthesis. The new cat-
alytic system provided a high yield of acetic acid from meth-
ane and CO. Therefore, we believe that this process has wide
applicability in the synthesis of various carboxylic acids from
alkanes.
5
6
7
H. Hogeveen, J. Lukas, and C. F. Roobeek, J. Chem. Soc.,
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1
0
1
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12 Although we reported that CaCl was a good reagent for
2
1
3
acetic acid synthesis from methane and CO, we also found that
CaCl2 showed a wall effect on this reaction. The interesting effect
of CaCl2 encouraged us to use a combination of a metal and
CaCl2.
Experimental
General. All of reactions in this work were carried out in a
5-mL stainless autoclave. The product mixture was analyzed
2
by a Shimadzu GC-18A gas chromatograph equipped with a flame
13 M. Asadullah, T. Kitamura, and Y. Fujiwara, Angew.
Chem., Int. Ed., 39, 2474 (2000).
ionization detector by using a 30 m ꢂ 0.53 mm ꢀ capillary col-