Cobalt-catalyzed oxidation of methane to methyl trifluoroacetate by dioxygen

Article abstract of DOI:10.1002/ejic.201300213

The cobalt-catalyzed oxidation of methane to methyl trifluoroacetate by molecular oxygen in trifluoroacetic acid has been studied in detail. Yields of up to 50 % based on methane were obtained. The catalytic activities were highly dependent on the anions of the cobalt salts (Co^II, Co^III) under investigation. Deactivation by precipitation of the cobalt catalyst could be prevented by the addition of trifluoroacetic anhydride. The selective cobalt-catalyzed oxidation of methane to methyl trifluoroacetate by dioxygen has been studied in detail. The catalytic activities of different cobalt precatalysts were investigated, with cobalt(II) nitrate proving to be the most efficient. The effects of solvent, reaction time, temperature, and catalyst loading have been studied. Copyright

Full text of DOI:10.1002/ejic.201300213

FULL PAPER
DOI:10.1002/ejic.201300213
Cobalt-Catalyzed Oxidation of Methane to Methyl
Trifluoroacetate by Dioxygen
Thomas Strassner,*[a] Sebastian Ahrens,[a] Michael Muehlhofer,[b]
Dominik Munz,[a] and Alexander Zeller[b]
Keywords: Alkanes / Oxidation / Cobalt / Oxygen / C–H activation
The cobalt-catalyzed oxidation of methane to methyl tri-
fluoroacetate by molecular oxygen in trifluoroacetic acid has
been studied in detail. Yields of up to 50% based on methane
were obtained. The catalytic activities were highly depend-
ent on the anions of the cobalt salts (CoII, CoIII) under investi-
gation. Deactivation by precipitation of the cobalt catalyst
could be prevented by the addition of trifluoroacetic anhy-
dride.
Introduction
The activation and functionalization of C–H bonds in
alkanes is a challenge for chemists worldwide.[1] In particu-
lar, the selective oxidation of methane to methanol, which
is difficult to achieve due to the highly inert nature of C–H
bonds, is of interest to the chemical industry. However,
some important progress has been made: After the early
work of Shilov and co-workers in 1972,[2] the “Catalytica”
system[3] in oleum set a new milestone in 1998. Our group
has contributed to this field through the functionalization
of methane to methyl trifluoroacetate (Scheme 1) by using
PdII and PtII complexes with chelating bis(NHC) ligands in
trifluoroacetic acid (TFA).[4,5] Methyl trifluoroacetate (b.p.
43 °C), which is stable under the reaction conditions, can
be removed from the reaction mixture by distillation (TFA
b.p. 72 °C) and then hydrolyzed to give methanol and tri-
fluoroacetic acid.[6] The latter could then be reprocessed.
Figure 1. Ligands based on the bis(NHC) and bis(pyrimidine) li-
gand motifs.
We combined the “Catalytica” bis-pyrimidine ligand
with the bis(NHC) ligand motif to synthesize the corre-
sponding palladium(II) and platinum(II) complexes (Fig-
ure 1).[5] All these palladium- and platinum-based systems
rely on well-defined molecular catalysts, which operate by
nonradical mechanisms.
Compared with oleum, which is used in the “Catalytica”
process, the main drawback of the PdII– and PtII–NHC-
based systems is the need for an external oxidant like
K2S2O8.[7] From an economic viewpoint, it would be advan-
tageous to use benign oxidants like dioxygen or air. Di-
oxygen has been shown to work together with reductive co-
reagents like CO[8] or dihydrogen.[9] However, with the pal-
ladium(II) and platinum(II) systems, we have not yet been
able to implement efficient redox cycles for the oxidation
step, as have been reported in the literature.[8b,9b,10] We
therefore decided to investigate the partial oxidation of
methane in the presence of molecular oxygen and cobalt
salts.
Scheme 1. Partial oxidation of methane to methanol in TFA.
The highly stable bis(NHC) catalysts (Figure 1) allow for
more than 40 turnovers under relatively mild conditions (T
= 90 °C).
[a] Physikalische Organische Chemie, Technische Universität
Dresden,
Bergstrasse 66, 01062 Dresden, Germany
Fax: +49-351-46339679
E-mail: thomas.strassner@chemie.tu-dresden.de
Homepage: www.chm.tu-dresden.de/oc3
[b] Department Chemie, Lehrstuhl für Anorganische Chemie,
Technische Universität München,
It has been reported that cobalt,[7g,11] copper,[7a,9a,12]
rare-earth-metal,[13] manganese,[14] and vanadium[7d,7i,9a,15]
salts as well as heteropolyanions[15d,16] are capable of oxid-
Lichtenbergstrasse 4, 85747 Garching bei München, Germany
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izing methane in trifluoroacetic acid by radical pathways. Table 1. Investigation of different cobalt and manganese salts in
the oxidation of methane.[a]
Although some[11a,11b,11e,11f,14] of these systems have been
shown to work with the oxidant dioxygen without the ad-
dition of a co-reductant, the reported chemoselectivities are
very modest as significant over-oxidation occurs. To the
best of our knowledge, the highest yield (only 37% based
on methane) has been reported for an Mn2O3-based sys-
tem.[14]
Cobalt catalysts in combination with dioxygen are well
known in oxidation catalysis and have, for example, been
used to oxidize aromatic compounds, aldehydes, and
alcohols.[17] Cobalt(III) trifluoroacetate is capable of
stoichiometrically oxidizing alkanes in trifluoroacetic acid
at ambient temperatures.[18] It is interesting to note that
high-spin CoIII oxides and complexes have also been shown
to be promising candidates for the selective activation of
methane.[19] Moiseev and co-workers showed that the reac-
tion of cobalt precatalysts with methane and longer alkane
Entry
Catalyst
mol-%
Yield [%]
1
2
3
Co(OAc)2·4H2O
CoCl2·6H2O
Co2O3
3.8
3.8
3.8
3.8
3.8
3.8
3.8
3.8
–
7
6
6
9
21
0
0
0
0
0
4
5
6
7
Co(acac)3
Co(NO3)2·6H2O
CoBr2·6H2O
CoF2
8
9
10
11
12
CoSO4·7H2O
–
Co(OAc)2·4H2O[b]
Co(OAc)2·4H2O[c]
Mn(OAc)3
3.8
3.8
3.8
0
4
[a] Reaction conditions: 20 bar CH4, 10 bar O2, T = 180 °C, 70 mL
TFA, 3 h; mol-% and yields are given relative to methane. [b] 0 bar
CH4, 10 bar O2. [c] 20 bar CH4, 0 bar O2.
homologues could be rendered catalytic by the addition of the underlying reaction mechanism. However, as the ad-
molecular oxygen.[11a,11b,11f] However, reports on the homo- dition of the radical scavenger benzophenone completely
geneous aerobic oxidation of alkanes by cobalt salts are stops the reaction, a radical mechanism appears likely. We
scarce, and the related chemistry is not very well under- carefully monitored the pressure differences and the volume
stood.[20] Herein, we report a detailed study of a catalytic changes of the reaction mixture and analyzed the composi-
system that relies on molecular oxygen as the terminal oxi- tion of the gas phase in detail for Co(OAc)2. We did not
dant and converts methane to methyl trifluoroacetate in re- see a considerable change in overall pressure of the gas
markably high yields of up to 50%.
phase during the course of the reaction, but observed that
about half of the dioxygen was consumed and about 30%
of the methane in the gas phase was replaced by CO2. Be-
cause only a small part of the solvent (ca. 1 mL) decom-
posed to CO2 during the reaction, we concluded that the
Results and Discussion
Several cobalt salts were tested as precatalysts for the formation of carbon dioxide is mainly due to over-oxidation
oxidation of methane (Table 1). A solution of trifluoro- of methane and not to the decomposition of TFA.
acetic acid with catalyst loadings of 3.8 mol-% (relative to
It has been suggested that the decomposition of tri-
·
methane) of the cobalt salts Co(OAc)2, CoCl2, Co2O3, and fluoroacetic acid leads to the formation of CF3 radicals,
Co(acac)3 yielded 6–9% of methyl trifluoroacetate (En- which readily react with hydrocarbons[1g,7d,22] and also
tries 1–4). The very similar catalytic activities of the CoII slowly with methane.[22c] The decomposition of trifluoro-
[Co(OAc)2 und CoCl2] and CoIII salts [Co2O3 und acetic acid does not seem to be coupled with the formation
Co(acac)3] indicate that under the reaction conditions CoII of the methyl ester in our case, as we found about the same
is oxidized to CoIII. As no conversion to methyl trifluoro- degree of solvent decomposition in experiments in which
acetate was obtained without the addition of oxygen (En- no product was formed. Consequently, and also considering
·
try 11), we have reason to believe that the “activation” of the relative speed of the reaction in comparison with CF3-
methane is mediated by a CoIII compound.
mediated processes,[22c] ·CF3 radicals do not play an impor-
Control experiments performed without the addition of tant role in the formation of the methyl trifluoroacetate.
catalyst (Entry 9) or methane (Entry 10) showed no conver- The yield of methyl trifluoroacetate was determined with
sion of methane into methyl trifluoroacetate. Likewise co- Co(OAc)2 as precatalyst under our standard reaction condi-
balt(II) bromide, cobalt(II) fluoride, and cobalt(II) sulfate, tions after 1, 3, 11, and 24 h (Table 2, Entries 1–4). Follow-
which are not soluble in trifluoroacetic acid, were com- ing an induction period of about 1 h, a significant amount
pletely inactive (Entries 6–8). Mn(OAc)3 was also tested of ester could be detected after 3 h. The increase in ester
and showed stoichiometric reactivity (Entry 12).
concentration slowed down significantly after 3–4 h, when
The highest yield (21%) was observed for CoII(NO3)2. It the catalyst precipitates in the form of a catalytically inac-
is very interesting to note in this context that nitrate salts tive pink solid. According to an energy-dispersive X-ray
can be used as a redox co-catalyst for the aerobic palla- analysis, this precipitate contained mainly cobalt fluorides
dium-catalyzed oxidation of hydrocarbons.[21] The draw- (ca. 20 atom-% cobalt) (cf. CoF2, Table 1). UV/Vis analysis
back of this catalyst is the very aggressive reaction mixture, indicated that cobalt(II) species remained in solution at low
which even attacked the gold-faced rupture disc of the auto- concentrations.
clave at 180 °C. We therefore decided to refrain from using
An increase in the concentration of the catalyst (Table 2,
Co(NO3)2 for further optimization studies at 180 °C. The Entries 5–10) and, in the case of CoCl2, longer reaction
harsh reaction conditions complicate the investigation of times had a beneficial effect on the yield (Table 2, Entries 8
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Table 2. Investigation of the oxidation of methane after different major tasks that might be connected: It removes water from
reaction times and catalyst loadings.[a]
the reaction mixture and prevents the precipitation of the
catalyst in the form of cobalt fluorides, which in combina-
tion leads to a higher conversion of methane to methyl tri-
fluoroacetate. The substitution of the solvent TFA by the
less acidic and less corrosive acetic acid was also investi-
gated (Entry 8). Indeed, we detected methyl acetate as the
reaction product, but the formation of the methyl ester was
not dependent on the presence of methane but rather due
to the degradation of the acetic acid. The oxidative cou-
pling of CO and methane to acetic acid, which presumably
forms part of the reverse reaction of this process, is well
known and has been investigated in detail by different
groups.[7b,7d,7g,8c–8e,8g,10b,24] The substitution of dioxygen by
N2O as the oxidant led to the formation of CO2 only.
Table 4 shows the results of an optimization study that
includes the use of the most active precatalyst cobalt(II)
nitrate from Table 1 under less demanding reaction condi-
tions (150 and 160 °C).
Entry
Catalyst
mol-% Reaction time [h] Yield [%]
1
2
3
4
5
6
7
8
9
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(acac)3
3.8
3.8
3.8
3.8
5.5
10.0
5.5
5.5
10.0
5.5
1
3
11
24
24
24
3
3
24
72
0
7
8
8
10
14
13
10
19
26
CoCl2·6H2O
CoCl2·6H2O
10
CoCl2·6H2O
[a] Reaction conditions: 20 bar CH4, 10 bar O2, T = 180 °C, 70 mL
TFA; mol-% and yields are given relative to methane.
and 9). Considering the induction period observed, we be-
lieve, in analogy to the proposal of others,[11a,11e,23] that
during the first hour of the reaction a cobalt catalyst in the
oxidation state +III or higher is formed, which then reacts
rapidly with methane. This interpretation is in agreement
with the observation that no product at all was formed in
the absence of molecular oxygen in the reaction mixture
(Table 1, Entry 11).
Table 4. Optimization of reaction temperature in the oxidation of
methane.[a]
Entry
Catalyst
T [°C] TFA/TFAA [mL] Yield [%]
For the functionalization of methane catalyzed by PdII–
bis(NHC) catalysts we could significantly reduce the wear
on the reaction vessel (without losing catalytic activity) by
adding trifluoroacetic anhydride (TFAA) to the reaction
mixture.[4d] We therefore also investigated the effect of add-
ing TFAA to the cobalt-based system. TFAA clearly pre-
vents catalyst deactivation, as even after 24 h no precipi-
tation of cobalt fluorides could be detected as long as an-
hydride remained in the reaction mixture (Table 3).
1
2
3
4
5
6
Co(NO3)2·6H2O
Co(NO3)2·6H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(NO3)2·6H2O
Co(NO3)2·6H2O
160
160
160
160
150
150
70:0
40:30
70:0
8
32
6
28
6
40:30
70:0
70:0[b]
5
[a] Reaction conditions: 20 bar CH4, 10 bar O2, 3.8 mol-% catalyst,
24 h; mol-% and yields are given relative to methane. [b] Addition
of 1 g of silica to the reaction mixture.
With the addition of TFAA we still obtained a yield of
32% with Co(NO3)2 [Co(OAc)2: 28%] compared with 8%
[Co(OAc)2, 6%] in pure TFA (Entries 1, 2 vs. 3, 4). Cobalt
nitrate still showed catalytic turnover at 150 °C (Entry 5).
We tried to prevent the precipitation of cobalt fluorides by
the addition of silica instead of trifluoroacetic anhydride, as
has been suggested in the literature.[14] However, we did not
observe any beneficial effect on the yield (Entry 6).
Table 3. Investigation of solvent effects on the oxidation of meth-
ane.[a]
Entry
Catalyst
mol-% TFA/TFAA [mL] Yield [%]
1
2
3
4
5
6
7
8
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
–
3.8
3.8
3.8
3.8
3.8
3.8
–
70:0
60:10
40:30
30:40
10:60
0:70
8
13
47
49
50
47
0
40:30
Conclusions
[b]
Co(OAc)2·4H2O
3.8
0
We have reported a catalytic system for the direct conver-
sion of methane into methyl trifluoroacetate in yields of up
to 50% based on methane. Various cobalt salts have been
tested [Co(OAc)2, CoCl2, Co(NO3)2, Co(acac)3, Co2O3,
CoBr2, CoSO4] as precatalysts, and cobalt nitrate was found
to be the most active. The use of dioxygen as oxidant offers
an opportunity for the development of an economically rea-
sonable process for the direct oxidation of methane to
methanol. The challenge of the precipitation and deactiva-
tion of the catalyst in the form of cobalt fluorides can be
addressed by the addition of TFAA to the reaction mixture.
[a] Reaction conditions: 20 bar CH4, 10 bar O2, T = 180 °C, 24 h;
mol-% and yields are given relative to methane. [b] 70 mL acetic
acid.
Higher concentrations of anhydride in the solvent led to
yields of up to 50% of methyl trifluoroacetate (Entries 1–
5). However, we observed thermal decomposition of TFAA
together with a higher degree of over-oxidation. Up to 50%
of the added anhydride was consumed, which we attribute
to the reactivity of trifluoroacetic anhydride towards radi-
cals.[22c] Analysis of the gas phase after the reaction re-
vealed a similar composition to that observed without the
addition of anhydride, albeit with a higher consumption of
oxygen (ca. 10% O2, ca. 35% CO2, ca. 55% CH4). The con-
trol reaction without catalyst did not lead to the formation
Experimental Section
General: All chemicals were reagent-grade and used as received
of products (Table 3, Entry 7). TFAA clearly fulfills two from commercial suppliers without further purification.
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Liquid-Phase Analysis: The autoclave was depressurized and the
reaction mixture analyzed by gas chromatography with an Agilent
6850 Series II Networked GC apparatus equipped with a flame
ionization detector (FID) and a Macherey–Nagel Optima-210
0.25 μm column (30 mϫ0.25 mm). The yield of the methyl ester
was quantified by the addition of p-xylene (25 μL) as standard to
the reaction mixture (1.00 mL) after the reaction and injection of
this mixture into the GC apparatus. All GC measurements were
repeated four times, given values are averaged over all measure-
ments. The yields based on methane were calculated according to
the ideal gas law.
[4]
Gas-Phase Analysis: Gas-phase analysis was performed with an
Agilent 6850 Series II Networked GC apparatus, equipped with a
thermal conductivity detector (TCD) and a J&W Scientific GS-
GasPro column (60 mϫ0.32 mm). Samples of gas were taken from
the autoclave with a custom-built 1.5 mL high-pressure gas mouse
during and after the reaction at pressures of up to 60 bar. The gas
sample was then expanded into an open-ended 1 mL sample loop
in the GC. After complete pressure equalization, the sample loop
was inserted into the stream of carrier gas with a computer-trig-
gered pneumatic valve. The detected gases were identified and
quantified by comparing the retention times on the column and
signal areas of the samples with previously measured gas mixtures
of known composition.
[5]
[6]
[7]
Energy-Dispersive X-ray Spectroscopy (EDX): The EDX measure-
ment was performed with a Zeiss DSM982 Gemini scanning elec-
tron microscope equipped with a Voyager 984A-1SUS Noran In-
struments energy-dispersive X-ray detector.
UV/Vis Spectrophotometry: The UV/Vis measurements were per-
formed with a Cary 3 Perkin–Elmer Lambda 25 UV/Vis spectro-
photometer at a resolution of 2 nm.
Acknowledgments
We are grateful for financial support from the Deutsche For-
schungsgemeinschaft (DFG) (STR 526/7-1 and STR 526/7-2) and
the Fonds der Chemischen Industrie (FCI). S. A. thanks the Kon-
rad-Adenauer-Stiftung (KAS) and D. M. the Studienstiftung des
Deutschen Volkes for their support. We are grateful for a generous
donation of TFA and TFAA by Solvay GmbH Hannover.
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Received: February 13, 2013
Published Online: July 2, 2013
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