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Vanadium-catalyzed acetic acid synthesis from methane and carbon dioxide
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
439
V a n a d i u m - c a t a l y z e d acetic acid synthesis from m e t h a n e and carbon dioxide Yuki Taniguchi, Taizo Hayashida, Tsugio Kitamura, and Yuzo Fujiwara Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-81, Japan I. INTRODUCTION Carbon dioxide is one of the natural Cl-resources which is being watched with keenest interest as a substitute of toxic CO in the C~-chemistry. The chemical fixation of CO2, a process which is appealing for both scientific and environmental reasons since methane and CO2 are well known as the greenhouse gases, is an important task for the human being though it is difficult because of low reactivity of CO2. Alkane activation/functionalization by transition metals under mild conditions is also one of the most challenging problems in modern chemistry since small alkanes including methane, ethane, and propane are the most abundant natural resources of hydrocarbons. In continuing studies on C-H bond activations [1], we have found that methane, ethane, and propane give rise to the corresponding acetic, propionic, and butyric acids, respectively in good yields when allow to react with CO using the Pd(OAc)2/Cu(OAc)2/K2S2Os/CF3COOHcatalyst system [2]. We have also reported that oxygen can be used as an oxidant in lieu of K2S2Os in the Pd/Cu system, and that more interestingly CO2 can also react with methane to give acetic acid [3]. On the acetic acid synthesis from methane and CO2, there is only one example [4] in addition to ours [3]. We have found that carbon dioxide can also react with methane to give acetic acid in the presence of vanadium catalysts.
CH 4
+
CO 2
V cat., K2S20 8
CF3COOH
~
CH3COOH
2. EXPERIMENTAL In a 25-mL stainless steel autoclave fitted with a magnetic stirring bar, VO(acac)2 as catalyst, K2S2Os and CF3COOH were added. The autoclave was closed and then pressurized to 5 atm with CH4 and 5 atm with CO2. The mixture was heated with stirring at 80 ~ for 20 h. After cooling the autoclave was opened and the mixture was analyzed by GLC.
440
3. RESULTS AND DISCUSSION At first, we examined the reaction of methane (5 atm) with CO 2 (5 atm) in the presence of K2820 8 (5.0 mmol) in trifluoroacetic acid (TFA) (10 mL) using various transition metal compounds at 80 ~ for 20 h, and the results are summarized in Table 1. As is apparent from the table, VO(acac)2 (acac: acetylacetonate) gives the highest turnover number (TON) and the highest yield of acetic acid (entry 2). Vanadium(III) oxide is also effective in this reaction (entry 5). In the absence ofK2S208, the TONs of the catalyst are extremely low (entries 2-4), which suggests that K28208 acts as an oxidizing agent. Some vanadiumcontaining heteropolyacids also act as catalyst in this reaction (entries 6-9). This reaction requires strong acid as a solvent. The solvent effect for the synthesis of acetic acid is in the order: TFA (TON = 8.91) >> 2N TFA (1.60)- 2N HCI (1.36)-~ 2N HzSO 4 (1.07) < 2N NaOH (0.36) << H20 (0.06). Use of TFA (entry 1) gives the best result. Table 1 Effect of Catalysts a Entry
Catalyst
1 2 3 4 5 6 7 8 9
none VO(acac)2 NaVO 3 V205 V203 HsPV2Mo loO4oo30H20 HsPMo 12040~ H4PVW1104o~ H20 HsSiVW11040~
TONb 0 8.91 (0.09)d 3.28 (0.29)d 5.88 (0.15)d 8.25 2.67 2.76 5.86 5.30
Yield/%c 0.4 15.7 (0.2) d 6.0 (0.9) d 10.2 (0.4) d 14.7 4.3 4.6 9.6 8.5
a25-mL Stainless steel autoclave, CH4 (5 atm), CO 2 (5 atm), vanadium catalyst (0.05 mmol), K2S208 (5.0 mmol), CF3COOH (10 mL), 80 ~ 20 h. bTurnover number. CGLC yield based on CH 4. dNo K2S208 added. Table 2 shows the control experiments for the VO(acac)2]K2S208 catalyst system. In the absence of VO(acac)2 and/or K2S208, the carboxylation of CH4 did not occur (entries 1-3). Both of VO(acac)2 and K28208 are indispensable in this reaction. In order to improve the yield of acetic acid, we tested the effect of the amount of TFA (entries 1-3). As the amount of TFA increases, the amount of methane introduced into the autoclave decreases. As a result, the yield based on methane reaches up to 89% by use of 20 mL of TFA (entry 3). However the highest TON obtained in the case of the use of 15 mL of TFA (entry 2). To convert methane perfectly, an excess of CO2 gas was introduced in the autoclave (entries 4 and 5). When the CO2 pressure is increased to 20 atm, the yield of acetic acid based on CH4 reaches up to 97%.
441 Table 2 Control Experiments a Entry
K2S208 (mmol)
1
0
2 3 4 5
5 0 5 10
VO( acac)2 (mmol) 0
0 0.05 0.05 0.05
TONb
Yield/%c
0
0
0 0.1 9.8 13.4
0.4 0.2 15.7 21.6
a25-mL Stainless steel autoclave, CH4 (5 atm), CO 2 (5 atm), CF3COOH (10 mL), 80 ~ bTurnover number. CGLCyield based on CH4.
20 h.
Table 3 Quantitative Acetic Acid Synthesis from Methane and CO2 a Entry
1 2 3 4 5
CH4 (atm) (mmol) 5 5 5 5 5
3.05 2.00 0.95 0.95 0.95
CO2 (atm) (mmol) 5 5 5 10 20
3.05 2.00 0.95 1.89 3.78
TFA (mL)
TON b
10 15 20 20 20
13.4 24.0 16.9 17.5 18.4
a25-mL Stainless steel autoclave, VO(acac) 2 (0.05 mmol), K2S208 (10.0 mmol), 80 ~ bTurnover number. CGLCyield based on CH4.
Yield/%c
22 60 89 92 97 20 h.
Figure 1 shows the effect ofthe amount 0fK2S208. The yield of acetic acid increases in proportion to the amount of K2S208. The kinetic order of the reaction with respect to [K2S208] is clearly first order. The yield of acetic acid depends upon the concentration of VO(acac)2 catalyst (Figure 2). The reaction proceeds rapidly by the addition of very small amounts ofthe catalyst. And then the yield increases slowly. The effect of pressure of CH4 and CO2 is shown in Figure 3. The yield of acetic acid increases sharply in proportion to the pressure of CH4 until 5 atm, and then increases slowly until 30 atm of CH4 pressure and finally it became constant (Figure 3, a). The reaction proceeds with first order with respect to [CH4] at the initial period. This trend can be explained in the following way that the solubility of CH4 in TFA proportionally increases at low CH4 pressure, but at the higher pressure of CH4 it became saturated in TFA. After the saturation point, no CH4 dissolve further with increasing pressure. Moreover, the yield of acetic acid depends on the amount of CH4 dissolved in TFA. Interestingly, Figure 3 (b) indicates that the formation of acetic acid is independent to CO2 pressure, and that even in the absence of CO2 gas, the carboxylation of CH4 occurs. After the reaction, we detected small amounts of CO2 and CHF3 derived from the decomposition of TFA [2] in
442
1.0
1.4
0.8
1.2
i
y
"6 1.0 ~ 0.8
--
~0.6 9 0.4
0.6
<
0.4 0.2-
0.2 ~
...............
0
1 .................
2
~. . . . . . . . . . . . . . . .
7.................
T. . . . . . . . . . . . . . . . .
4 6 8 K2S208/mmol
0
10
..........
0
r ............
r ...........
r ............
r ..........
~ .............
r ...........
l
0.2 0.4 0.6 0.8 1.0 1.2 1.4 VO(acac) 2/mmol
Figure 1. Effect of the Amount of K2S208
Figure 2. Effect of the Amount of Catalyst
Reaction conditions: VO(acac)2 (0.05 mmol), CH4 and CO 2 (5.0 atm each), CF3COOH (20 mL), 80 ~ 20 h.
Reaction conditions: CH4 and CO2 (10 atm each), K2S208 (5.0 mmol), CF3COOH (20 mL), 80 ~ 20 h.
the residual gas by GLC analysis. Although the mechanism is not yet clear at this stage, VO(acac)2 would be converted to active VO(OCOCF3)3 by K2S208 and TFA. Then VO(OCOCF3)3 would abstract Ho from CH4 to form CH3V(OH)(OCOCF3)3 which reacts with CO2 to give (CH3COO)V (OH)(OCOCF3)3. Decomposition of this species gives rise to CH3COOH and VO(OCOCF3)3. The detailed results on acetic acid synthesis from methane and CO2 by vanadium catalysts will be presented and the mechanistic implication discussed.
1.0 a) CH 4 0.8 "6 ~ 0.6 9
b) CO 2
O 0.4 < 0.2 0 0
10
20
30
40 50
60
70 80
Pressure/atm Figure 3. Effect of Pressure of CH 4 and CO 2 Reaction conditions: VO(acac) 2 (0.05 mmol), CF3COOH (20 mL), 80 ~ 20 h. a) CO 2 (5 atm). b) CH 4 (5 atm).
REFERENCES 1. 2. 3. 4.
Y. Fujiwara, T. Jintoku, and K. Takaki, Chemtech, (1990) 636. Y. Fujiwara, K. Takaki, and Y. Taniguchi, Synlett, (1996) 591. M. Kurioka, K. Nakata, T. Jintoku, Y. Taniguchi, K. Takaki, and Y. Fujiwara, Chem. Lett., (1995) 244. H.-J. Freund, J. Wambach, O. Seiferth, B. Dillmann, WO 96/05163.
439
V a n a d i u m - c a t a l y z e d acetic acid synthesis from m e t h a n e and carbon dioxide Yuki Taniguchi, Taizo Hayashida, Tsugio Kitamura, and Yuzo Fujiwara Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-81, Japan I. INTRODUCTION Carbon dioxide is one of the natural Cl-resources which is being watched with keenest interest as a substitute of toxic CO in the C~-chemistry. The chemical fixation of CO2, a process which is appealing for both scientific and environmental reasons since methane and CO2 are well known as the greenhouse gases, is an important task for the human being though it is difficult because of low reactivity of CO2. Alkane activation/functionalization by transition metals under mild conditions is also one of the most challenging problems in modern chemistry since small alkanes including methane, ethane, and propane are the most abundant natural resources of hydrocarbons. In continuing studies on C-H bond activations [1], we have found that methane, ethane, and propane give rise to the corresponding acetic, propionic, and butyric acids, respectively in good yields when allow to react with CO using the Pd(OAc)2/Cu(OAc)2/K2S2Os/CF3COOHcatalyst system [2]. We have also reported that oxygen can be used as an oxidant in lieu of K2S2Os in the Pd/Cu system, and that more interestingly CO2 can also react with methane to give acetic acid [3]. On the acetic acid synthesis from methane and CO2, there is only one example [4] in addition to ours [3]. We have found that carbon dioxide can also react with methane to give acetic acid in the presence of vanadium catalysts.
CH 4
+
CO 2
V cat., K2S20 8
CF3COOH
~
CH3COOH
2. EXPERIMENTAL In a 25-mL stainless steel autoclave fitted with a magnetic stirring bar, VO(acac)2 as catalyst, K2S2Os and CF3COOH were added. The autoclave was closed and then pressurized to 5 atm with CH4 and 5 atm with CO2. The mixture was heated with stirring at 80 ~ for 20 h. After cooling the autoclave was opened and the mixture was analyzed by GLC.
440
3. RESULTS AND DISCUSSION At first, we examined the reaction of methane (5 atm) with CO 2 (5 atm) in the presence of K2820 8 (5.0 mmol) in trifluoroacetic acid (TFA) (10 mL) using various transition metal compounds at 80 ~ for 20 h, and the results are summarized in Table 1. As is apparent from the table, VO(acac)2 (acac: acetylacetonate) gives the highest turnover number (TON) and the highest yield of acetic acid (entry 2). Vanadium(III) oxide is also effective in this reaction (entry 5). In the absence ofK2S208, the TONs of the catalyst are extremely low (entries 2-4), which suggests that K28208 acts as an oxidizing agent. Some vanadiumcontaining heteropolyacids also act as catalyst in this reaction (entries 6-9). This reaction requires strong acid as a solvent. The solvent effect for the synthesis of acetic acid is in the order: TFA (TON = 8.91) >> 2N TFA (1.60)- 2N HCI (1.36)-~ 2N HzSO 4 (1.07) < 2N NaOH (0.36) << H20 (0.06). Use of TFA (entry 1) gives the best result. Table 1 Effect of Catalysts a Entry
Catalyst
1 2 3 4 5 6 7 8 9
none VO(acac)2 NaVO 3 V205 V203 HsPV2Mo loO4oo30H20 HsPMo 12040~ H4PVW1104o~ H20 HsSiVW11040~
TONb 0 8.91 (0.09)d 3.28 (0.29)d 5.88 (0.15)d 8.25 2.67 2.76 5.86 5.30
Yield/%c 0.4 15.7 (0.2) d 6.0 (0.9) d 10.2 (0.4) d 14.7 4.3 4.6 9.6 8.5
a25-mL Stainless steel autoclave, CH4 (5 atm), CO 2 (5 atm), vanadium catalyst (0.05 mmol), K2S208 (5.0 mmol), CF3COOH (10 mL), 80 ~ 20 h. bTurnover number. CGLC yield based on CH 4. dNo K2S208 added. Table 2 shows the control experiments for the VO(acac)2]K2S208 catalyst system. In the absence of VO(acac)2 and/or K2S208, the carboxylation of CH4 did not occur (entries 1-3). Both of VO(acac)2 and K28208 are indispensable in this reaction. In order to improve the yield of acetic acid, we tested the effect of the amount of TFA (entries 1-3). As the amount of TFA increases, the amount of methane introduced into the autoclave decreases. As a result, the yield based on methane reaches up to 89% by use of 20 mL of TFA (entry 3). However the highest TON obtained in the case of the use of 15 mL of TFA (entry 2). To convert methane perfectly, an excess of CO2 gas was introduced in the autoclave (entries 4 and 5). When the CO2 pressure is increased to 20 atm, the yield of acetic acid based on CH4 reaches up to 97%.
441 Table 2 Control Experiments a Entry
K2S208 (mmol)
1
0
2 3 4 5
5 0 5 10
VO( acac)2 (mmol) 0
0 0.05 0.05 0.05
TONb
Yield/%c
0
0
0 0.1 9.8 13.4
0.4 0.2 15.7 21.6
a25-mL Stainless steel autoclave, CH4 (5 atm), CO 2 (5 atm), CF3COOH (10 mL), 80 ~ bTurnover number. CGLCyield based on CH4.
20 h.
Table 3 Quantitative Acetic Acid Synthesis from Methane and CO2 a Entry
1 2 3 4 5
CH4 (atm) (mmol) 5 5 5 5 5
3.05 2.00 0.95 0.95 0.95
CO2 (atm) (mmol) 5 5 5 10 20
3.05 2.00 0.95 1.89 3.78
TFA (mL)
TON b
10 15 20 20 20
13.4 24.0 16.9 17.5 18.4
a25-mL Stainless steel autoclave, VO(acac) 2 (0.05 mmol), K2S208 (10.0 mmol), 80 ~ bTurnover number. CGLCyield based on CH4.
Yield/%c
22 60 89 92 97 20 h.
Figure 1 shows the effect ofthe amount 0fK2S208. The yield of acetic acid increases in proportion to the amount of K2S208. The kinetic order of the reaction with respect to [K2S208] is clearly first order. The yield of acetic acid depends upon the concentration of VO(acac)2 catalyst (Figure 2). The reaction proceeds rapidly by the addition of very small amounts ofthe catalyst. And then the yield increases slowly. The effect of pressure of CH4 and CO2 is shown in Figure 3. The yield of acetic acid increases sharply in proportion to the pressure of CH4 until 5 atm, and then increases slowly until 30 atm of CH4 pressure and finally it became constant (Figure 3, a). The reaction proceeds with first order with respect to [CH4] at the initial period. This trend can be explained in the following way that the solubility of CH4 in TFA proportionally increases at low CH4 pressure, but at the higher pressure of CH4 it became saturated in TFA. After the saturation point, no CH4 dissolve further with increasing pressure. Moreover, the yield of acetic acid depends on the amount of CH4 dissolved in TFA. Interestingly, Figure 3 (b) indicates that the formation of acetic acid is independent to CO2 pressure, and that even in the absence of CO2 gas, the carboxylation of CH4 occurs. After the reaction, we detected small amounts of CO2 and CHF3 derived from the decomposition of TFA [2] in
442
1.0
1.4
0.8
1.2
i
y
"6 1.0 ~ 0.8
--
~0.6 9 0.4
0.6
<
0.4 0.2-
0.2 ~
...............
0
1 .................
2
~. . . . . . . . . . . . . . . .
7.................
T. . . . . . . . . . . . . . . . .
4 6 8 K2S208/mmol
0
10
..........
0
r ............
r ...........
r ............
r ..........
~ .............
r ...........
l
0.2 0.4 0.6 0.8 1.0 1.2 1.4 VO(acac) 2/mmol
Figure 1. Effect of the Amount of K2S208
Figure 2. Effect of the Amount of Catalyst
Reaction conditions: VO(acac)2 (0.05 mmol), CH4 and CO 2 (5.0 atm each), CF3COOH (20 mL), 80 ~ 20 h.
Reaction conditions: CH4 and CO2 (10 atm each), K2S208 (5.0 mmol), CF3COOH (20 mL), 80 ~ 20 h.
the residual gas by GLC analysis. Although the mechanism is not yet clear at this stage, VO(acac)2 would be converted to active VO(OCOCF3)3 by K2S208 and TFA. Then VO(OCOCF3)3 would abstract Ho from CH4 to form CH3V(OH)(OCOCF3)3 which reacts with CO2 to give (CH3COO)V (OH)(OCOCF3)3. Decomposition of this species gives rise to CH3COOH and VO(OCOCF3)3. The detailed results on acetic acid synthesis from methane and CO2 by vanadium catalysts will be presented and the mechanistic implication discussed.
1.0 a) CH 4 0.8 "6 ~ 0.6 9
b) CO 2
O 0.4 < 0.2 0 0
10
20
30
40 50
60
70 80
Pressure/atm Figure 3. Effect of Pressure of CH 4 and CO 2 Reaction conditions: VO(acac) 2 (0.05 mmol), CF3COOH (20 mL), 80 ~ 20 h. a) CO 2 (5 atm). b) CH 4 (5 atm).
REFERENCES 1. 2. 3. 4.
Y. Fujiwara, T. Jintoku, and K. Takaki, Chemtech, (1990) 636. Y. Fujiwara, K. Takaki, and Y. Taniguchi, Synlett, (1996) 591. M. Kurioka, K. Nakata, T. Jintoku, Y. Taniguchi, K. Takaki, and Y. Fujiwara, Chem. Lett., (1995) 244. H.-J. Freund, J. Wambach, O. Seiferth, B. Dillmann, WO 96/05163.