Homogeneous Copper-Catalyzed Conversion of Methane to Methyl Trifluoroacetate in High Yield at Low Pressure

Article abstract of DOI:10.1002/cctc.201800412

The direct catalytic oxidation of methane to oxygenates, a reaction that garners significant scientific and industrial interest, is plagued by poor methane-based yields. Some of the best homogeneous catalytic systems reported to date convert methane to methyl esters using catalysts with complex organic ligands to reach high yields at relatively high temperature (>423 K) and pressure (20–70 bar). In our study, we used a simple copper compound, copper(II) oxide, to selectively convert methane to methyl trifluoroacetate at 363 K and low pressure (5 bar) resulting in yields as high as 63 % at a methane conversion of 71 %. The catalyst is easily recovered by treating the spent reaction mixture with a base, and the catalytic performance of the recovered material is highly comparable to that of the fresh catalyst. In terms of turnover, copper oxide (TON=33 for ester yield of 56 %) ranks higher than other simple metal compounds and is comparable to catalysts with NHC ligands. Thus, this work demonstrates the possibility of using a simple catalyst devoid of complex ligands to convert methane in high yields at low pressure.

Full text of DOI:10.1002/cctc.201800412

Accepted Article
Title: Homogeneous copper-catalyzed conversion of methane to
methyl trifluoroacetate in high yield at low pressure
Authors: Manoj Ravi and Jeroen A. van Bokhoven
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COMMUNICATION
Homogeneous copper-catalyzed conversion of methane to methyl
trifluoroacetate in high yield at low pressure.
Manoj Ravi , Jeroen A. van Bokhoven[a],[b]*
[
a]
[
[
a] Institute for Chemical and Bioengineering, ETH Zurich, CH-8093, Zurich, Switzerland
b] Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
Abstract: The direct catalytic oxidation of methane to oxygenates, a
reaction that garners significant scientific and industrial interest, is
plagued by poor methane-based yields. Some of the best
homogeneous catalytic systems reported to date convert methane to
methyl esters using catalysts with complex organic ligands to reach
high yields at relatively high temperature (>423 K) and pressure (20-
termination of the reaction. Much like the sulfuric acid-based
systems, there are a number of reports on trifluoroacetic acid-
based systems.[5,
9]
Palladium(II) complexes with chelating
bis(NHC) ligands catalyze the conversion of methane to methyl
trifluoroacetate in yields up to 980% relative to palladium in the
presence of potassium persulfate.[5] The NHC complexes were
very resistant under the acidic conditions required for methane
C-H activation. Methyl trifluoroacetate yields of 37% and 50%
7
0 bar). In our study, we used a simple copper compound, copper
II) oxide, to selectively convert methane to methyl trifluoroacetate at
63 K and low pressure (5 bar) resulting in yields as high as 63% at
(
3
were reported with manganese oxide[10] and cobalt acetate[11]
,
a methane conversion of 71%. The catalyst is easily recovered by
treating the spent reaction mixture with a base, and the catalytic
performance of the recovered material is highly comparable to that
of the fresh catalyst. In terms of turnover, copper oxide (TON=33 for
ester yield of 56%) ranks higher than other simple metal compounds
and is comparable to catalysts with NHC ligands. Thus, this work
demonstrates the possibility of using a simple catalyst devoid of
complex ligands to convert methane in high yields at low pressure.
respectively, as catalysts in a mixture of trifluoroacetic acid and
trifluoroacetic anhydride. The stoichiometry of methane oxidation
to methyl trifluoroacetate with potassium persulfate as the
oxidant is as follows: CH
CF COOCH + 2KHSO
4
+
CF
3
COOH
+
K
2
S
2
O
8

3
3
4
We report the homogeneous copper-catalyzed conversion of
methane to methyl trifluoroacetate in high yield at low
temperature and pressure. Unlike many other homogeneous
systems, which use complex organic ligands, this study relies on
copper (II) oxide, a simple copper compound. The activity of
numerous transition metal-based catalysts, such as
maganese,[10] vanadium[12] and cobalt[11], was tested in the
oxidation of methane to methyl trifluoroacetate. However,
Methane, the major component of natural gas, is, for the most
part, an underused resource.[1] The challenges in methane
transportation and conversion render its valorization
economically unfeasible leaving flaring as the solitary option.
studies of copper-based catalysts for this process are scarce
Industries, which use methane as
methanol indirectly, first converting methane to synthesis gas
CO and H ) and the subsequent synthesis of methanol. This
a
feedstock, produce
[13, 14]
and do not aim at high methane-based yields.
In this
contribution, we report a catalytic system that demonstrates the
possibility of reaching ester yields as high as 63%. Copper(II)
oxide is as an efficient pre-catalyst for the selective oxidation of
methane to methyl trifluoroacetate (Table 1, Fig. 1) and
dissolves easily in trifluoroacetic acid to form copper(II)
trifluoroacetate. We refrained from using copper salts with
organic counter-ions due to possible interferences under
reaction conditions designed for C-H activation. The conversion
of methane to methyl trifluoroacetate and methyl acetate esters
with copper acetate as the catalyst has been reported.[
However, the stability of the catalyst under oxidizing conditions
was mediocre. We tested the stability of copper acetate under
typical reaction conditions in the absence of methane and found
that more than 3 mol% of the catalyst disintegrated to form
methyl trifluoroacetate and a proportionate amount of formic acid
within two hours. This is undesirable in a catalytic process,
which typically lasts for long periods of time. Furthermore, the
ester product results not only from methane but also from side
reactions that consume the catalyst, thereby distorting the actual
yield. This has also been reported for palladium acetate
(
2
two-step process is energy-intensive, requiring high
temperatures and pressure.[ Hence, the quest for direct
methane-conversion routes has captivated the interest of the
scientific and industrial community for more than a century. One
such method of methane valorization is the selective oxidation of
methane to oxygenates such as methanol. However, the direct
oxidation of methane to methanol (DMTM) in high yield is
difficult due to the facile over-oxidation of primary oxygenates to
carbon dioxide. Systems that stabilize or derivatise methanol
during methane oxidation clearly outperform systems that do not
incorporate specific measures for methanol protection.[3-8] The
protection of methanol in the form of an ester, such as methyl
2]
15]
bisulfate
and
methyl
trifluoroacetate,
enables
liquid
homogeneous methane oxidation systems to reach high yields.
Methyl ester yields of 43% and 72% were reported with mercuric
bisulfate[
3]
[4]
2
and [(2,2′-bipyrimidine)PtCl ] , respectively, as
catalysts in sulfuric acid medium at 453 K and 35 bar. A more
recent study demonstrated the high catalytic activity and stability
of potassium tetrachloroplatinate (K
2
PtCl
4
) to selectively convert
[16]
catalysts and, hence, we abstained from using catalysts with
methane to methyl bisulfate in oleum. Under the suitable
conditions, primarily dictated by the concentration of sulfur
trioxide, the turnover frequency of the inorganic platinum salt
an acetate counter-ion in oxidation reactions. In this regard, pre-
catalysts without C-H bonds, such as cupric oxide, are attractive
alternatives.
Potassium persulfate as the oxidant enables the continuous
conversion of methane over long periods of time. Studies with
other oxidants, such as hydrogen peroxide, often report a
was significantly higher than that of the original homogeneous
version.[
7]
It is particularly noteworthy that potassium
tetrachloroplatinate in low concentrations with suitable
concentrations of sulfur trioxide did not deactivate until
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COMMUNICATION
Table 1. Catalytic results of copper-based homogenous catalysts for the oxidation of methane. [a]
Entry
Temp.
K]
K
[M]
2
S
O
2 8
Catalyst
[mMCu
Reaction
time [h]
Methyl ester
Selectivity [%]
Methane
conversion [%]
[
]
1
363
383
363
363
363
363
363
363
363
363
363
363
363
0.30
0.30
0.06
0.06
0.30
0.30
0.30
0
CuO (9.4)
CuO (9.4)
CuO (9.4)
CuO (9.4)
CuO (9.4)
CuO (9.4)
CuO (9.4)
CuO (9.4)
-
17
17
17
17
3
91.6
86.0
90.6
92.5
89.5
100
100
0
43.8
66.0
29.1
26.0
9.3
2
3
4[b]
5
6[c]
7[d]
8
3
<2
3
<1
17
17
17
17
17
17
tr.
9
0.30
0.30
0.06
0.15
0.15
52.1
88.9
91.6
91.5
73.6
6.0
1
1
1
1
0
CuO (18.8)
71.2
35.1
66.4
21.7
1
2
Cu O (10.4)
2[e]
3[f]
CuO (18.8)
CuO (18.8)
[
[
[
[
[
[
a] Standard reaction conditions: 5.2 bar CH
4
, 23 g TFA, 5 g TFAA, 875 rpm.
b] Using regenerated copper oxide catalyst.
c] In presence of 0.02 M benzophenone, selectivity for methyl ester in liquid phase recorded.
d] In presence of 0.05 M hydroquinone, selectivity for methyl ester in liquid phase recorded.
e] 19 g TFA, 9 g TFAA
f] Spent reaction mixture of entry 12 recycled for a second pass.
saturation in product yield within a few hours due to the much
faster rate of decomposition of hydrogen peroxide.[17] Our
approach with potassium persulfate is particularly rewarding due
to the stability of methyl trifluoroacetate, which is the primary
product under reaction conditions. A nearly constant selectivity
to the ester, independent of methane conversion (Fig. S1),
indicates that the over-oxidation of the ester to carbon dioxide is
strongly suppressed, facilitating the accumulation of the product
during methane oxidation. Under the tested conditions, there
was a linear relationship between catalyst mass and methane
conversion with ester selectivity remaining fairly independent of
the amount of catalyst used (Fig. S2).
(92% at 363 K and 86% at 383 K). This translates to an increase
in ester yield by more than 37%. We employed a sodium
hydroxide-based procedure to regenerate and recover the spent
catalyst. The X-ray diffraction pattern of the regenerated catalyst
closely resembles that of copper(II) oxide (Fig. S4).
At high oxidant loadings, the increase in methane conversion is
not proportional to the increase in the amount of the oxidant (Fig.
S3). This leveling-off suggests that the oxidant does not govern
the rate-limiting step of methane oxidation at higher loadings. At
high oxidant loadings (Entry 1, Table 1), the conversion in the
oxidant to potassium hydrogen sulfate is approximately 15%. It
is striking that, under these oxidant concentrations, the amount
of catalyst has a more tangible effect on methane conversion
(
Fig. S2), demonstrating its role in accelerating the rate-limiting
step. While most of the reactions in this study were carried out at
63 K, increasing the temperature to 383 K resulted in a
3
Figure 1. Comparison of product selectivity-methane conversion performance
of the copper oxide-based catalytic system with other homogeneous systems
significant increase in methane conversion (Entries 1-2, Table 1).
The methyl ester remained stable at higher temperature, as the
selectivity to methyl trifluoroacetate decreased to a minor extent
from[
8]
.
reported
to
date.
Figure
adapted
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COMMUNICATION
Table 2. Comparison of homogeneous catalysts for the oxidation of methane to methyl trifluoroacetate.
Entry
Catalyst
P [bar]
20
TON[a]
3.8
Methyl ester yield [%]
Ref.
[
[
5]
5]
1
2
Pd(acac)
2
n.a.
n.a.
30
30.0
[
[
[
18]
18]
19]
3
4
5
EuCl
FeCl
Pd(OAc)
3
10
10
25
5.3
n.a.
n.a.
n.a.
3
<0.5
0.7-3
2
/
benzoquinone/
H
5
PMo10
[Pd(hfacac)
[Cu(hfacac)
VO(acac)
2 40
V O
[14]
[14]
6
7
8
9
2
]
25-30
25-30
50
39.0[b]
13.0[b]
18.5
290
<13.7
<3.1
0.3
2 2
(H2O) ]
[
[
[
12]
12]
20]
2
H
H
4
PV
PV
1
Mo11
O
40
40
50
<4.4
60.2
56
1
0
1
5
2
Mo10O
10
128
1
CuO
5.2
33.0
This work
[
a] Moles of product / moles of catalyst, [b] Moles of methyl trifluoroacetate / moles of catalyst.
The catalytic performance of the regenerated material was
comparable to that of fresh copper oxide in terms of methane
conversion and ester selectivity (Entries 3-4, Table 1).
with cuprous oxide (Entry 11, Table 1) was comparable to that
with cupric oxide (Entry 3, Table 1).
A significant reason for the non-commercialization of a direct
methane-to-methanol process is poor methane-based yields.
One way to address this issue is to synthesize methanol
derivatives such as methyl esters to achieve yields that may
suggest the feasibility of commercialization.[8] Fig. 1 places the
results of this work in the context of the numerous other
homogeneous systems reported in the past and illustrates the
impressive performance of this catalytic system to retain high
product selectivity at high methane conversion (the sources for
the data points are in Ref.[8] ). This performance was achieved at
relatively low temperature and pressure compared to many of
the other methane oxidation processes reported to date.
Furthermore, the catalyst is a commercially available copper
compound with no complex organic ligands, which are usually
required to ensure the solubility of the active species by
preventing the precipitation of the metal.
Studies with potassium persulfate in alkane oxidation generally
propose a radical mechanism due to the high probability of
generating KSO °
radicals from K S O . Such a radical
4
2 2 8
mechanism seems to govern our system, because methane
conversion was strongly suppressed in the presence of
benzophenone and hydroquinone, known radical scavengers
(
Entries 5-7, Table 1). Control experiments performed in the
absence of the oxidant showed no conversion of methane (Entry
, Table 1). This implies that copper (II) cannot stoichiometrically
8
oxidize methane by reducing to copper (I) and underscores the
importance of potassium persulfate in methane activation. Thus,
it is likely that KSO °
abstracts a hydrogen from methane to form
4
the methyl radical. UV-visible spectroscopy is a useful technique
II
to investigate changes in the oxidation state of copper. Cu is
characterized by a d-d transition in the region of 600-800 nm
which is not present in Cu since it has a d10 configuration.[21] On
the other hand, absorbance at around 400 nm is associated with
I
The copper oxide pre-catalyst achieved turnovers (TON) of up to
33. Table 2 ranks the performance of this catalytic system with a
number of other catalysts for the oxidation of methane to methyl
I
[22]
a metal-to-ligand charge transfer transition in Cu species.
A
II
UV-visible spectrum of the solution after reaction indicates Cu -
to CuI conversion during the reaction, as indicated by the
appearance of an absorption band at 440 nm (Fig. S5). Thus,
we postulate that copper (II) oxidizes the methyl radical to
methyl trifluoroacetate and potassium persulfate facilitates the
re-oxidation of Cu to CuII to complete a catalytic cycle. This
suggests that CuI can also be used as a catalyst. The
conversion of methane and the selectivity to the ester achieved
trifluoroacetate.
Quantities such as conversion, yield and
selectivity are well defined, making it possible to compare
systems. However, TON and TOF cannot be compared directly,
often due to the pronounced effects of temperature and pressure
on these quantities.[23] Our experiments were performed at
considerably lower methane pressure (Table 2). Notwithstanding
this limitation, Table 2 shows that copper oxide has a higher
TON than other simple metal compounds. Furthermore, this is
comparable to values reported for catalysts with NHC
I
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of 0.12 M in two cycles of low-pressure reactions. Such or higher
concentrations of the ester were achieved only at higher
temperature and pressure (Table S1).
The catalytic system in this study leads to relatively high ester
yields due to the stability of the product and the catalyst under
the reaction conditions. The homogeneous catalyst can be
recovered by a simple base-involved procedure and re-used for
the oxidation of methane. The resistance of methyl
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Acknowledgements
The authors gratefully acknowledge the ESI platform, Paul
Scherrer Institute and ETH Zurich for financial support. Manoj
thanks Dr. René Verel for help with NMR measurements and
Sotiria Mostrou for discussions on micro GC measurements.
Keywords: C-H activation • Homogeneous catalysis • Methane
oxidation • Methyl trifluoroacetate • Copper catalysis
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Entry for the Table of Contents
COMMUNICATION
High-yield methane oxidation: This
work demonstrates the possibility of
using copper oxide, a simple catalyst
devoid of complex ligands, to convert
methane in high yield to methyl
trifluoroacetate at low pressure.
Manoj Ravi, Jeroen A. van Bokhoven*
Page No. – Page No.
Homogeneous copper-catalyzed
conversion of methane to methyl
trifluoroacetate in high yield at low
pressure.
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