Synthesis of mixed acid anhydrides from methane and carbon dioxide in acid solvents

Article abstract of DOI:10.1021/ol0348856

(Matrix presented) The reaction of CH₄ with CO₂ has been performed in anhydrous acids using VO(acac)₂ and K ₂S₂O₈ as promoters. NMR analysis establishes that the primary product is a mixed anhydride of acetic acid and the acid solvent. In sulfuric acid, the overall reaction is CH₄ + CO ₂ + SO₃ → CH₃C(O)-O-SO₃H. Hydrolysis of the mixed anhydride produces acetic acid and the solvent acid. When trifluoroacetic acid is the solvent, acetic acid is primarily formed via the reaction CH₄ + CF₃COOH → CH₃COOH + CHF₃.

Full text of DOI:10.1021/ol0348856

ORGANIC
LETTERS
2003
Vol. 5, No. 18
3193-3196
Synthesis of Mixed Acid Anhydrides
from Methane and Carbon Dioxide in
Acid Solvents
Mark Zerella, Sudip Mukhopadhyay, and Alexis T. Bell*
Department of Chemical Engineering, UniVersity of California-Berkeley,
Berkeley, California 94720
bell@cchem.berkeley.edu
Received May 20, 2003
ABSTRACT
The reaction of CH with CO2 has been performed in anhydrous acids using VO(acac)2 and K S2O8 as promoters. NMR analysis establishes
4
2
that the primary product is a mixed anhydride of acetic acid and the acid solvent. In sulfuric acid, the overall reaction is CH + CO2 + SO3
4
f CH C(O)−O−SO3H. Hydrolysis of the mixed anhydride produces acetic acid and the solvent acid. When trifluoroacetic acid is the solvent,
3
acetic acid is primarily formed via the reaction CH + CF3COOH f CH COOH + CHF3.
4
3
The discovery and development of catalytic processes for
the conversion of methane to commodity chemicals and
liquid fuels has been the subject of considerable research.1
Today, such products are produced in a multistep process,
which begins with the reforming of methane to synthesis
gas followed by the catalyzed conversion of this mixture of
H2 and COx (x ) 1, 2) to produce hydrocarbons and/or
oxygenated products (e.g., aldehydes, alcohols, carboxylic
acids). Since 65-75% of the capital cost of this indirect
approach is associated with methane reforming, there is an
ongoing interest in identifying strategies for the direct
conversion of methane to products.
conversion of methane to acetic acid is reported to be as
high as 97%. Spivey and co-workers have questioned
whether the acetic acid observed by Fujiwara and co-workers
was formed by the reaction of methane with carbon dioxide,
since this reaction is thermodynamically unfavorable (CH4
+ CO2 f CH3COOH, ∆G°rxn ) +55 kJ/mol), and, hence,
the fractional conversion of methane is predicted to be no
more than 1.6 × 10-6 at any operating conditions. As an
alternative, Spivey and co-workers proposed that acetic acid
is formed by the reaction of methane with the solvent,
CF3COOH.4 Since this reaction (CH4 + CF3COOH f
CH3COOH + CHF3, ∆G°rxn ) -52 kJ/mol) is thermo-
One of the targeted products for the direct conversion of
methane is acetic acid. Due to favorable thermodynamics
(CH4 + CO + 1/2O2 f CH3COOH, ∆G°rxn ) -202 kJ/
mol), a number of authors have focused on the liquid-phase
carbonylation of methane to acetic acid.2 As an alternative,
it is interesting to consider the reaction of CH4 with CO2.
Fujiwara and co-workers have reported that VO(acac)2 in
combination with K2S2O8 promotes the reaction of methane
with CO2 to acetic acid in CF3COOH as the solvent.3 The
(2) (a) Bagno, A.; Bukala, J.; Olah, G. A. J. Org. Chem. 1990, 55, 4284-
4289. (b) Lin, M.; Sen, A. Nature 1994, 368, 613-615. (c) Freund, H. J.;
Wambach, J.; Seiferth, O.; Dillman, B. German Patent 4428566, 1995. (d)
Chaepaikin, G. C.; Bezruchenko, A. P.; Leshcheva, A. A.; Boyko, G. N.;
Kuzmenkov, I. V.; Grigoryan, E. H.; Shilov, A. E. J. Mol. Catal. A: Chem.
2001, 169, 89-98. (e) Nishiguchi, T.; Nakata, K.; Takaki, K.; Fujiwara,
Y. Chem. Lett. 1992, 7, 1141-1142. (f) Nakata, K.; Yamaoka, Y.; Miyata,
T.; Taniguchi, Y.; Takaki, K.; Fujiwara, Y. J. Organomet. Chem. 1994,
473, 329-334. (g) Fujiwara, Y.; Kitamura, T.; Taniguchi, Y.; Hayashida,
T.; Jinoku, T. Stud. Surf. Sci. Catal. 1998, 119, 349-353. (h) Taniguchi,
Y.; Hayashida, T.; Shibasaki, H.; Piao, D.; Kitamura, T.; Yamaji, T.;
Fujiwara, Y. Org. Lett. 1999, 1 (4), 557-559. (i) Asadullah, M.; Taniguchi,
Y.; Kitamura, T.; Fujiwara, Y. Tetrahedron Lett. 1999, 40, 8867-8871. (j)
Asadullah, M.; Kitamura, T.; Fujiwara, Y. Angew. Chem., Int. Ed. 2000,
39 (14), 2475-2478.
* Corresponding author. Fax: 1 510 642 4778.
(1) (a) Gesser, H. D.; Hunter, N. R. Catal. Today 1998, 42, 183-189.
(b) Lunsford, J. H. Catal. Today 2000, 63, 165-174. (c) Otsuka, K.; Wang,
Y. Appl. Catal. A: Gen. 2001, 222, 145-161.
(3) Taniguchi, Y.; Hayashida, T.; Kitamura, T.; Fujiwara, Y. Stud. Surf.
Sci. Catal. 1998, 114, 439-442.
10.1021/ol0348856 CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/05/2003

dynamically favorable, it could readily explain the observed
conversions of methane to acetic acid. In the present study,
we have confirmed that the reaction proposed by Spivey and
co-workers occurs. More importantly, we have discovered
that under completely anhydrous conditions, CH4 and CO2
react with the acid solvent to form a mixed anhydride,
CH3C(O)-O-C(O)CF3. Upon hydration, the mixed anhy-
dride forms acetic acid and trifluoroacetic acid. We have
also found that formation of mixed acid anhydrides will occur
in sulfuric acid and trifluoromethanesulfonic acid.
In a typical reaction,5 CH4 and CO2 were reacted at 85 °C
in a high pressure, glass-lined autoclave. K2S2O8 and a small
amount of VO(acac)2 were dissolved in an anhydrous acid
(CF3COOH, H2SO4, or CF3SO3H). Reactions were carried
out for 16 h. Upon completion of the reaction, 2 g of water
were added to the liquid phase in order to hydrolyze any
anhydrides. The acetic acid thus formed was identified and
quantified by 1H NMR. The only major byproducts were the
methyl esters of the acid solvents (Figure 1).
Figure 2. 13C NMR of the reaction mixtures in CF3COOH.
Reaction conditions: CF3COOH (29.5 g), (CF3CO)2O (0.9 g),
K2S2O8 (10 mmol), VO(acac)2 (0.5 mmol), CH4 (80 psig), 13CO2
(115 psig), 17 h, 80 °C.
As seen in Figure 3, the presence of CHF3 is clearly iden-
tified by the peaks at -15.1 and -15.3 ppm; the peaks at
-12.4 and -13.5 ppm are due to the solvent. Although not
quantitatively, these data clearly demonstrate that CF3COOH
has reacted with CH4 to produce CH3COOH and CHF3.
To isolate the reaction of CH4 with CO2, CF3COOH was
replaced by other solvents, while holding all other reaction
conditions constant. Table 1 shows the effect of solvent
composition on the conversion of CH4 to acetic acid. Not
surprisingly, the highest conversion was obtained in tri-
fluoroacetic acid, since we now know the solvent directly
contributes to the yield. Approximately 7% conversion of
Figure 1. 1H NMR of the reaction mixture in CF3COOH. Reaction
conditions: CF3COOH (29.5 g), (CF3CO)2O (0.9 g), K2S2O8 (10
mmol), VO(acac)2 (0.5 mmol), CH4 (80 psig), CO2 (120 psig), 17
h, 80 °C. Yield (based on methane conversion) is 40% acetic acid
and ∼3.6% methyl trifluoroacetate.
Under the best conditions reported by Fujiwara et al.,3 we
observed a 40% methane conversion to acetic acid for CO2
pressure of 120 psig (Figure 1). This was lower than the
methane conversion reported by Fujiwara et al., 97%, because
of the larger headspace volume of our autoclave, which
results in a larger reservoir of methane. 13C NMR showed
that 13CH3COOH is formed when 13CH4 reacts with CO2. In
the absence of CO2, the conversion of CH4 to acetic acid
was 26%. To verify whether CO2 contributes to the formation
of acetic acid, we carried out the reaction of CH4 with 13CO2
in CF3COOH. As seen in Figure 2, CH313COOH was
detected by 13C NMR. Thus, 14% of the CH4 was converted
to CH3COOH via a reaction involving CO2. To confirm that
the balance of the acetic acid formed arises from the reaction
of CH4 with the solvent, CF3COOH, the liquid phase was
analyzed by 19F NMR to look for the anticipated coproduct,
CHF3.6
Figure 3. 19F NMR of the reaction mixture in CF3COOH. Reaction
conditions: CF3COOH (7 g), (CF3CO)2O (3 g), K2S2O8 (10 mmol),
VO(acac)2 (0.16 mmol), CH4 (100 psig), CO2 (120 psig), 85 °C,
10 h. The peaks at -12.4 and -13.3 ppm are associated with
CF3COOH. The peaks at -15.1 and -15.3 ppm are CHF3.
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Org. Lett., Vol. 5, No. 18, 2003

acid was run, but 1H NMR was performed prior to addition
of water to the product mixture (Figure 5). The product
Table 1. Direct Reaction of CH4 and CO2 in Different
Solvents
% conversion of
entry
solvent
CH4 to CH3COOH
byproducts
1
2
3
4
CF3COOHb
C9F20
H2SO4
CF3SO3Hd
16
0
7
CF3COOCH3
c
CH3OSO3H
CF3SO3CH3
13
a Reaction conditions: CH4, 80 psig; CO2, 120 psig; K2S2O8, 1 g (3.7
mmol); VO(acac)2, 0.043 g (0.16 mmol); solvent, 10.0 g; 85 °C; 16 h.
b Trifluoroacetic acid anhydride, 3.0 g, was used to make the system
anhydrous. c SO3, 3.0 g, was used. d Trifluoromethanesulfonic acid anhy-
dride, 3.0 g, was used.
CH4 to acetic acid was obtained when sulfuric acid was used
as the solvent, whereas 13% conversion of CH4 to acetic
acid was obtained using trifluoromethanesulfonic acid. Small
amounts of methyl esters of the acidic solvents were
produced as byproducts in each reaction. Interestingly, in
fuming H2SO4, the presence of CO2 seems to inhibit the
production of CH3SO3H. To ensure that any CO or CO2
produced by the oxidation of CH4 by K2S2O8 under the
reaction conditions was not responsible for acetic acid
formation, a blank reaction was performed in the absence
of CO2. The absence of an acetic acid peak in the 1H NMR
spectrum (Figure 4) demonstrates clearly that the only source
of CO2 is that which is originally supplied to the reactor.
To elucidate the pathway of acetic acid formation from
CH4 and CO2 in acid solvents, the same reaction in sulfuric
Figure 5. 1H NMR of the reaction mixtures in H2SO4. Reaction
conditions: H2SO4 (10.0 g), SO3 (3.0 g), K2S2O8 (3.7 mmol),
VO(acac)2 (0.16 mmol), CH4 (80 psig), 16 h, 85 °C. Spectrum was
taken prior to hydrolysis of the product solution.
obtained in this reaction was identified as the mixed
anhydride of acetic acid and sulfuric acid, CH3C(O)-O-
SO3H. Upon the addition of water, this mixed anhydride
hydrolyzes to produce acetic acid and H2SO4. The presence
of acetic acid was confirmed by distilling a water-acetic
acid azeotrope and then analyzing this mixture by 1H NMR
and Raman spectroscopy.7
Thus, it appears that the formation of acetic acid involves
the following stoichiometric reactions:
CH4 + CO2 + H2SO4 f CH3C(O)-O-SO3H + H2O (1)
∆G°rxn = +60 kJ/mol
H2O + SO3 f H2SO4
(2)
∆G°rxn ) -79 kJ/mol
CH3C(O)-O-SO3H + H2O f CH3COOH + H2SO4 (3)
∆G°rxn = -5 kJ/mol
While reaction 1 is thermodynamically unfavorable, when
it is coupled with reaction 2, the overall thermodynamics
(4) (a) Wilcox, E. M.; Gogate, M. R.; Spivey, J. J.; Roberts, G. W. Stud.
Surf. Sci. Catal. 2001, 136, 259-264. (b) Wilcox, E. M.; Roberts, G. W.;
Spivey, J. J. Appl. Catal. A: Gen. 2002, 226, 317-318.
(5) In a 100 mL glass-lined Parr autoclave, 3.7 mmol (1 g) of K2S2O8,
0.16 mmol (0.043 g) of VO(acac), 3 g of (CF3CO)2O, and 7 g of CF3COOH
were charged together with a small Teflon-coated magnetic stir bar. (The
solvent was chilled to 5-8 °C during these additions to minimize the thermal
decomposition of K2S2O8.) The reactor was then purged with N2 to expel
the air out of the system. It was then pressurized first with 120 psig of CO2
and then finally with 80 psig of methane from the adjacent connecting
cylinders. The reactor was heated to 85 °C under stirring and maintained
for 16 h. After the stipulated period of time, the reactor was quenched with
ice and opened to collect the reaction mixture. Then, 2.0 g of water was
slowly added to the mixture that was then filtered before taking it to 1H
NMR analysis. D2O was used in a capillary as the lock reference. The
corresponding chemical shift for acetic acid was 2.3 to 2.4 ppm, depending
on the concentration of acetic acid in the mixture.
Figure 4. 1H NMR of the reaction mixtures in H2SO4. Reaction
conditions: H2SO4 (10.0 g), SO3 (3.0 g), K2S2O8 (3.7 mmol),
VO(acac)2 (0.16 mmol), 80 psig of CH4 (24.6 mmol), 16 h, 85 °C.
(a) CO2 (120 psig, 34.9 mmol) was added to the autoclave, and
acetic acid was formed. (b) No CO2 was added to the reaction, and
only byproducts were formed. The excess water added after
completion of the reaction enables CH3OH to exist in equilibrium
with CH3OSO3H for case b.
(6) After the stipulated reaction period, the reactor was cooled to 5 °C,
and the gases were very slowly vented. Upon addition of 2 g of water, the
residual reaction mixture was immediately taken for 19F NMR analysis.
Org. Lett., Vol. 5, No. 18, 2003
3195

become favorable.8 Reaction 3 represents the postreaction
addition of water, which produces acetic acid and regenerates
sulfuric acid.
envisioned in the present study. By analogy to the reaction
of CH3 with SO3 to form CH3SO3 , it is possible to propose
•
•
•
•
that CH3 reacts with CO2 to form CH3CO2 . However, in
•
•
VO(acac)2 and K2S2O8 are both essential to the formation
of mixed acid anhydrides, since the exclusion of one or the
other results in no products. The amount of K2S2O8 required
is greater by an order of magnitude than the amount of
VO(acac)2. When the amount of K2S2O8 was reduced to the
level of that of VO(acac)2, no reaction was observed between
CH4 and CO2, using fuming sulfuric acid as the solvent. This
result stands in contrast to what we found in our studies of
methanesulfonic acid, where only small amounts of K2S2O8
contrast to CH3SO3 , CH3CO2 is thermodynamically un-
stable. We propose that the role of V(IV) in VO(acac)2 is to
•
-
stabilize CH3CO2 as CH3CO2 . In support of this proposal,
it is observed that peracetic acid decomposes to CH4 and
CO2 but that these products are not formed when VO(acac)2
is added to the reaction mixture.10 In fuming sulfuric acid,
the formation of CH3C(O)-O-SO3H can then be envisioned
to proceed via the reaction of CH3CO2 with SO3 and H+.
-
In conclusion, we have shown that in anhydrous acid, CH4
reacts with CO2 to form an anhydride of acetic acid and the
acid solvent. The water formed in this reaction must be
absorbed in an accompanying reaction for the overall
thermodynamics to be favorable. When the solvent is
CF3COOH, acetic acid is formed predominantly by the
reaction of CH4 with the solvent to produce CHF3 and
CH3COOH. In this case, the formation of a mixed anhydride
also produces acetic acid, but to a smaller extent. In all cases,
the addition of water to the mixed anhydride produces acetic
acid and the solvent acid.
were needed to promote the reaction CH4 + SO3
f
CH3SO3H in fuming sulfuric acid.9
While the specific functions of VO(acac)2 and K2S2O8 have
not been identified, it is possible to suggest a role for each
•
component. K2S2O8 cleaves thermolytically to KSO4 . In
studies of the K2S2O8-initiated sulfonation of CH4 to meth-
anesulfonic acid, it has been proposed that the radicals
produced by dissociation of the initiator react with CH4 and
•
produce CH3 radicals.9 A similar function for K2S2O8 is
(7) C-H stretch (methyl group) of acetic acid was 2951 cm-1
.
(8) Free energy of formation of each species was taken from standard
tables except the mixed anhydride for which the value was calculated using
group contributions.
Acknowledgment. This work was supported by a grant
from the Methane Conversion Cooperative, sponsored by BP.
(9) (a) Basickes, N.; Hogan, T. E.; Sen, A. J. Am. Chem. Soc. 1996,
118, 13111-13112. (b) Lobree, L.; Bell, A. T. Ind. Chem. Res. 2001, 40,
736-742. (c) Mukhopadhyay, S.; Bell, A. T. Ind. Chem. Res. 2002, 41,
5901-5905. (d) Mukhopadhyay, S.; Bell, A. T. Angew. Chem., Int. Ed.
2003, 42, 1019-1023. (e) Mukhopadhyay, S.; Bell, A. T. J. Am. Chem.
Soc. 2003, 125, 4406-4407. (f) Mukhopadhyay, S.; Bell, A. T. Org. Process
Res. DeV. 2003, 7, 161-163. (g) Mukhopadhyay, S.; Bell, A. T. Angew.
Chem., Int. Ed. 2003, 42, 2990-2993. (h) Mukhopadhyay, S.; Bell, A. T.
Chem. Commun. 2003, 1590-1591. (i) Mukhopadhyay, S.; Bell, A. T. Org.
Process Res. DeV. 2003, in press.
OL0348856
(10) Into a 50 mL Parr autoclave was charged 7 g of peracetic acid (32%
solution in dilute acetic acid) and heated to 110 °C under 110 psig N2 for
2 h. The reactor was then cooled, and the gases in the headspace were
analyzed by gas chromatography. The products contained a 1:1 mixture of
CH4 and CO2. A similar reaction was carried out in which 1 g of VO(acac)2
was added to the reaction mixture. Analysis of the autoclave headspace
gas showed no CH4 in this case.
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Org. Lett., Vol. 5, No. 18, 2003