Heterogeneously Catalyzed Aerobic Oxidation of Methane to a Methyl Derivative

Article abstract of DOI:10.1002/anie.202104153

A promising strategy to break through the selectivity-conversion limit of direct methane conversion to achieve high yields is the protection of methanol via esterification to a more stable methyl ester. We present an aerobic methane-to-methyl-ester approach that utilizes a highly dispersed, cobalt-containing solid catalyst, along with significantly more favorable reaction conditions compared to existing homogeneously-catalyzed approaches (e.g. diluted acid, O₂ oxidant, moderate temperature and pressure). The trifluoroacetic acid medium is diluted (<25 wt %) with an inert fluorous co-solvent that can be recovered after the separation of the methyl trifluoroacetate via liquid–liquid extraction at ambient conditions. Silica-supported cobalt catalysts are highly active in this system, with competitive yields and turnovers in comparison to known aerobic transition metal-based catalytic systems.

Full text of DOI:10.1002/anie.202104153

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Accepted Article
Title: Heterogeneously-catalyzed aerobic oxidation of methane to a
methyl derivative
Authors: Jeroen A. van Bokhoven, Andrea N. Blankenship, Manoj Ravi,
and Mark A. Newton
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202104153
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10.1002/anie.202104153
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RESEARCH ARTICLE
Heterogeneously-catalyzed aerobic oxidation of methane to a
methyl derivative
Andrea N. Blankenship,[a]ǂ Manoj Ravi,[a]ǂ Mark A. Newton,[a] and Jeroen A. van Bokhoven*[a][b]
[a]
[b]
Ms. A. N. Blankenship, Dr. M. Ravi, Dr. M. A. Newton, Prof. J. A. van Bokhoven
Institute for Chemical and Bioengineering
ETH Zurich
Vladimir-Prelog-Weg 1, 8093 Zurich (Switzerland)
E-mail: jeroen.vanbokhoven@chem.ethz.ch
Prof. J. A. van Bokhoven
Laboratory for Catalysis and Sustainable Chemistry
Paul Scherrer Institute
5232 Villigen (Switzerland)
ǂ Authors contributed equally to the manuscript
Supporting information for this article is given via a link at the end of the document.
4c, 5d,
Abstract: A promising strategy to break through the selectivity-
conversion limit of direct methane conversion to achieve high yields
is the protection of methanol via esterification to a more stable methyl
ester. In this study, we present a novel aerobic methane-to-methyl-
ester approach that utilizes a highly dispersed, cobalt-containing solid
catalyst, along with significantly more favorable reaction conditions
compared to existing homogeneously-catalyzed approaches (e.g.
diluted acid, O2 oxidant, moderate temperature and pressure). The
trifluoroacetic acid medium is diluted (< 25 wt%) with an inert fluorous
co-solvent that can be recovered after the separation of the methyl
trifluoroacetate via liquid-liquid extraction at ambient conditions.
Silica-supported cobalt catalysts are highly active in this system, with
competitive yields and turnovers in comparison to known aerobic
transition metal-based catalytic systems. This work highlights the
need and potential to reinvigorate the development of methane
conversion technologies through the innovative application and
combination of concepts from across the community.
derivatives, over heterogeneous catalysts[2b,
6]. However,
existing approaches to partial oxidation are constrained by a
number of limitations, most notably, the selectivity-conversion
paradigm arising from the vulnerability of methanol to over-
oxidation[5d, 7]. Comparisons between diverse solid catalyst
systems reveal a clear trend of decreasing selectivity to methanol
with increasing conversion of methane, a limit that severely
restricts achievable yields and necessitates operating conditions
that are highly impractical for commercialization[5d, 7b, 7c, 8]. In order
to achieve industrially relevant methane conversion without
compromising high selectivity for methanol, it is imperative that
methanol is stabilized to prevent over-oxidation[7b,
interesting ‘product protection’ strategy, demonstrated primarily in
homogeneous catalytic systems, is the protection of methanol via
esterification to methyl bisulfate[10] and methyl trifluoroacetate[11]
These esters are less prone to further oxidation under typical
reaction conditions[7b] and, therefore, offer the possibility to
circumvent the selectivity-conversion limit during methane
conversion. In later steps, these esters can be hydrolyzed back to
methanol, thereby widening the array of potential commercial
applications for this chemistry. The translation of this protection
strategy to use with a heterogeneous catalyst has been largely
unexplored, with the notable exception of Schüth and co-workers
who synthesized a highly active solid catalyst based on the
homogeneous Periana platinum bipyrimidine complex for the
conversion of methane to methyl bisulfate in oleum using a SO3
7c,
9]. An
.
Introduction
In 2018, it is estimated that nearly 145 billion cubic meters
of unused natural gas, a methane-rich by-product of oil extraction,
were flared due to the lack of commercial technologies and
incentives to bring it to market[1]. Methane derived from natural
gas is a highly abundant resource that is widely used in the
production of commodity chemicals and liquid energy carriers,
notably methanol[2]. Although well-established industrial routes for
methane utilization exist, these processes generally proceed
through an indirect two-step pathway, with syngas as an
intermediate product. Consequently, these energy- and capital-
intensive processes are rendered economically unviable for
methane valorization at small- and mid-scale facilities[2a, 3], such
as remote and decentralized shale oil production sites[4]. The
challenge of developing a more scale-flexible direct methane
conversion route has therefore motivated the academic and
industrial communities over the previous years[5].
oxidant[6e]
.
The attractive yields of current methane-to-methyl ester
processes notwithstanding, these processes still fall short of
potential commercial application. Rather than focusing only on
obtaining high product yields, a more holistic consideration of
process conditions, cost and handling of reagents, and necessary
separations and recycle streams is essential. These factors,
which have been rigorously defined by industrial experts in this
field[12], constitute a further set of criteria for commercialization. In
particular, several key factors have precluded commercialization
of these processes, which include: (i) the homogeneous nature of
the catalysis, translating into challenges in product and catalyst
recovery; (ii) the use of harsh corrosive acids in an undiluted form,
which leads to greater operational hazards and costly equipment
materials[13]; (iii) the use of economically underwhelming oxidants,
Many diverse approaches have evolved to address this
challenge, including the selective functionalization of the methane
C-H bond to form primary oxygenates, such as methanol or other
1
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10.1002/anie.202104153
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RESEARCH ARTICLE
such as potassium persulfate and hydrogen peroxide[13]; and (iv)
the difficulty in hydrolyzing the ester to methanol due to the highly
exothermic interaction of water with the reaction mixture, which
potentially results in the undesired evaporation of methanol[14]. A
process that demonstrates the breakthrough performance of
solvent at room temperature. Acetonitrile-d3 is an aprotic polar
solvent used as the extractant to preserve the product as the
methyl ester and directly measure the product concentrations
using 1H NMR. Only residual amounts of the methyl ester remain
in the fluorous phase immediately after contact with the
deuterated acetonitrile phase as determined through 1H NMR
(Supporting Information, Figures S1 and S2), and therefore the
total product yield can be determined from the acetonitrile-d3
phase after extraction. Substituting water as the non-fluorous
extracting solvent could provide opportunities for product
separation with enhanced ester hydrolysis conditions compared
to processes that rely on undiluted reaction mediums. Importantly,
this simple and highly effective product separation method
provides a straightforward pathway for recycling the fluorous co-
solvent, thereby greatly reducing the overall usage of this
component. This unique advantage in product separation and
solvent recycle has not been previously described in published
methane-to-methyl-ester systems.
methane-to-methyl-ester
systems,
while
simultaneously
overcoming these conventional challenges has not, to our
knowledge, been previously demonstrated.
Herein, catalysts with cobalt loadings of 0.1 wt%, 0.5 wt%,
1.5 wt%, 5 wt% and 10 wt% were synthesized through an incipient
wetness impregnation method (Co/SiO2-IWI) and studied for the
catalytic partial oxidation of methane. Figure 2a illustrates the
dependence of the methyl ester productivity on the cobalt loading
of the Co/SiO2-IWI catalysts under the typical reaction conditions.
A methyl ester productivity of approximately 250 μmol/gcat•h is
obtained with the 0.5 wt% Co/SiO2-IWI catalysts. Increasing the
cobalt content of the catalysts results in only minor changes in the
productivity of the methyl ester up to 5 wt% (Figure 2a, blue
markers). Cobalt utilization is substantially higher for the low-
loaded catalysts, with a maximum of nearly 175 mmol/gCo•h
achieved using the 0.1 wt% Co/SiO2-IWI material, which
decreases rapidly with increased cobalt loading (Figure 2a, green
markers). The performance of the 10 wt% Co/SiO2-IWI catalyst is
substantially inferior to the catalysts with lower cobalt content in
terms of both productivity and cobalt utilization. In combination
with the average particle sizes from the corresponding
transmission electron microscopy (TEM) images (see Supporting
Information), these results suggest that the more active catalysts
are those with the higher cobalt dispersions. Consequently, the
most efficient utilization of cobalt is realized in the lowest cobalt-
loaded cases.
The heterogeneity of the reaction for the Co/SiO2-IWI materials
was confirmed via a hot filtration test and ICP-OES of the reaction
medium (Supporting Information, Figure S4). Additional tests
demonstrating the necessity of cobalt and molecular oxygen
oxidant are summarized in Supporting Information Table S3. To
further assess the stability of the Co/SiO2-IWI catalysts, the
catalyst material was recovered after an initial catalytic test and
recycled back into the reactor for a second run conducted under
the same conditions. Before recycling, some samples underwent
a thermal treatment consisting of a calcination to at least 250 ºC
in static air and are denoted as “reactivated” samples. Samples
that did not receive any treatment before the second run are
referred to as “spent” samples. The catalytic productivity obtained
with the spent 0.5 wt% Co/SiO2-IWI catalyst after reactivation is
comparable to that obtained with the freshly synthesized catalyst
(Figure 2b). Without the reactivation step, decreased catalytic
activity is observed for this catalyst.
Figure 1. Overview of designed methane-oxidation process and product
recovery. The conversion of methane to methyl trifluoroacetate occurs in an
autoclave charged with methane, air, solid catalyst, and a solution of TFA in
perfluorohexane. After reaction, methyl trifluoroacetate is extracted from the
fluorous reaction medium into a non-fluorous solvent.
To address the pressing limitations of direct methane
conversion, we propose an approach that combines the
heterogeneously-catalyzed partial oxidation of methane with
subsequent esterification of the product in a reaction medium of
trifluoroacetic acid (TFA) diluted in an inert perfluoroalkane co-
solvent. Perfluoroalkanes, such as perfluorohexane, are inert,
stable at elevated temperatures[15]
solubilities for gases[16]
By diluting TFA to manageable
, and can exhibit high
.
concentrations of below 25 wt% in a non-corrosive and oxidation-
resistant perfluoroalkane, a number of improvements are attained,
namely: a strongly reduced corrosivity of the reaction medium;
improved stability of a heterogeneous catalyst through operation
in a milder environment; enhanced recovery of the methyl ester
via a simple liquid-liquid extraction with a non-fluorous polar
solvent; and improved hydrolysis conditions.
Results and Discussion
Previous studies have shown that
a
number of
homogeneous transition metal catalysts display activity for
methane to methyl trifluoroacetate conversion, including
copper[11e], manganese[11c], and cobalt[11d]. As a preliminary step,
therefore, a variety of transition metals on solid supports were
synthesized and screened for activity in a batch reactor system
(Figure 1, screening results in Supporting Information). Based on
the initial screening, cobalt-containing silica catalysts synthesized
via an incipient wetness impregnation (Co/SiO2-IWI) with an
aqueous cobalt nitrate solution showed the most promising
activity.
On inspecting the fresh catalysts by TEM, we find large
cobalt-containing particles in those with high loadings, and X-ray
diffraction (XRD) of these catalysts confirms the presence of
The methyl ester is recovered from the fluorous reaction
medium through a facile liquid-liquid extraction with a polar
2
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Figure 2. Co/SiO2-IWI catalytic methane oxidation performance and characterization. (a) Effect of cobalt loading of Co/SiO2-IWI catalyst on methyl ester productivity
in methane-oxidation. Reaction conditions: 100 mg catalyst, 5 bar CH4, 2 bar air, 7 g of 14 wt% TFA/C6F14, 215°C, 3 h; (b) Methyl ester product (bar graph) and
ester yield (scatter point) obtained with fresh, spent and reactivated 0.5 wt% Co/SiO2-IWI catalyst. Reaction conditions: 5 bar CH4, 2 bar air, 7 g of 14 wt% TFA/C6F14
,
215 °C, 100 mg catalyst, 3 h; (c) Co K-edge XANES of 0.5 wt% Co/SiO2-IWI series. Inset includes two-component LCA results with the Co3O4 and Co(OH)2
standards; (d) Normalized absorbance of the Co K-edge XANES at 7725 eV for the spent Co/SiO2-IWI catalysts with respect to corresponding ester product of the
catalyst during reaction; (e) Methyl ester product obtained with fresh, spent and reactivated 0.1 wt% Co/SiO2-IWI catalyst. Reaction conditions: 5 bar CH4, 2 bar air,
7 g of 14 wt% TFA/C6F14, 215 °C, 100 mg catalyst, 3 h; (f) Co K-edge XANES of 0.1 wt% Co/SiO2-IWI series. Inset includes close-up of pre-edge feature.
Co3O4 (see Supporting Information). X-ray absorption
spectroscopy (XAS) further corroborates the dominance of a
spinel Co3O4 at each of the weight loadings in the fresh state and
suggests the presence of a minority CoII-like fraction, which
becomes less significant as the cobalt loading increases
(Supporting Information, Figure S9). Figure 2c shows the Co K-
edge X-ray absorption near edge structure (XANES) of the 0.5
wt% Co/SiO2-IWI catalyst in the fresh, spent, and reactivated
state. The lower binding energy feature around 7725 eV
associated with a CoII component is noticeably more prominent in
the spent and reactivated catalysts as compared to the fresh
catalysts. Linear combination analysis (LCA) fits to the XANES of
these materials (see Supporting Information) suggests that the
reactivated catalyst regains more of the CoIII/CoII character
indicative of the initial spinel structure, but still retains a significant,
loading-dependent amount of the character of the second
Co(OH)2-like species (Figure 2c, inset). For the higher-loaded
samples, such as the 5 wt% Co/SiO2-IWI catalyst, the cobalt
structure appears to be less perturbed after reaction in the spent
catalysts and the structure more closely resembles the initial
structure following reactivation (Supporting Information, Figure
S11).
the fresh catalyst (Figure 2e). Despite the decrease in activity
observed after the initial reaction, the spent and reactivated
samples still show better cobalt utilization than the higher-loaded
catalysts, with cobalt-based ester productivities around 80
mmol/gCo•h (corresponding to approx. 240 μmol/gcat in Fig 2e).
From an activity standpoint, the aerobic reactivation protocol does
not have a significant effect on the 0.1 wt% catalyst. The XAS of
both the spent and reactivated materials, which reveals a strong
resemblance between these materials (Figure 2f), supports this
observation. The effects of catalysis manifest principally through
the development of a shoulder in the XANES, associable with CoII
@ 7725 eV in the higher-loaded samples (Figure 2c). There is an
explicit linear relationship between the fraction of cobalt
associated with the 7725 eV CoII XANES feature and the obtained
methyl ester yield over the studied range of cobalt loadings
(Figure 2d). The 10 wt% Co/SiO2-IWI catalyst noticeably deviates
from this trend, which likely results from the greatly increased
domain size and reduced dispersion due to the formation of very
large cobalt particles and agglomerates, as evidenced by TEM
and XRD (see Supporting Information). In the case of the 0.1 wt%
material, the XANES feature @7725 eV CoII is transformed into a
strong peak rather than a shoulder (Figure 2f), and the overall
XANES envelopes of 0.1 wt% Co/SiO2-IWI in the spent and
reactivated state are markedly different from those of the higher
weight loaded samples.
In the case of the 0.1 wt% Co/SiO2-IWI catalysts, both the
spent and reactivated catalysts show equivalent ester
productivities that are approximately 50% of that obtained from
3
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10.1002/anie.202104153
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RESEARCH ARTICLE
octahedral (Oh) CoIII component of the Co3O4, has also been
consumed. Analysis of the EXAFS further indicates that a
complete structural transformation of the cobalt species present
on the 0.1 wt% catalyst from the Co3O4 starting phase into a highly
dispersed Oh CoII species has occurred (see Supporting
Information). The combination of catalytic data with the XAS
suggests that the new Oh CoII species can still be associated with
activity for methane oxidation. The most active cobalt species
across the series of catalysts, therefore, may not be the Co3O4
nanoparticles that are predominantly present in the fresh catalysts
across the range of loadings, but rather a newly formed species
that is best observed in the low-loaded, highly dispersed catalyst
materials.
By increasing the liquid hold-up in the reactor and reducing
the partial pressure of oxygen, the highest oxygen-based yield of
12% of the theoretical maximum was reached with 1.5 wt%
Co/SiO2 catalyst (Supporting Information, Table S5). Figure 3a
ranks the performance of the cobalt-catalyzed process reported
herein in comparison to other methane-oxidation approaches that
solely use dioxygen as the oxidant based on two important
parameters, namely productivity and oxygen conversion to the
desired product. This comparison reveals the considerable
performance-gap that exists between aerobic heterogeneous
Figure 3. Overview of the performance of different aerobic methane-oxidation
approaches. (a) Comparison of the productivity and oxygen-based yields of
previously reported methane-oxidation with molecular oxygen approaches; (b)
Comparison of the methane-based product yields of various methane-oxidation
with molecular oxygen approaches.
The pre-edge feature at ca. 7709 eV is indicative of a
fraction of the cobalt that initially exists with the tetrahedral (Td)
symmetry expected from the CoII component of the spinel
structure of Co3O4 (Figure 2f, inset)[17]. This feature is removed in
the spent catalyst, suggesting the removal Td symmetry from the
0.1 wt% sample post-reaction. Moreover, the overall shift of the
Co K-edge edge position to lower energy further suggests that the
Table 1. A comparison to homogeneous systems for the oxidation of methane to methyl trifluoroacetate.
Entry
Pre-catalyst
Pd(OAc)2
Oxidant
Pmethane
[bar]
Reported TO[a]
Productivity[b]
Ref
[molester molmetal-1 h-1]
[18]
[18]
1
2
K2S2O8
K2S2O8
20
30
3.8
30
0.2
2.1
[19]
3
4
5
6
H5PV2Mo10O40
CuO
K2S2O8
K2S2O8
O2
10
5
128 [c]
33
6.4 [c]
1.9
1
[11e]
[20]
Co(OCOCF3)3
EuCl3 / Zn
FeCl3
20
10
4
[21]
O2
5.3
5.3
[21]
[22]
7
8
O2
O2
10
25
< 0.5
< 0.5
Pd(OAc)2/BQ/
H5PMo10V2O40
3 – 118 [d]
0.4 – 14.8 [d]
[23]
[11c]
[24]
[24]
[25]
[25]
9
Co(OAc)2.4H2O
Mn2O3
O2
20
7
13.2
8.5
0.6
10
11
12
13
14
15
O2
2.8
[Pd(hfacac)2]
[Cu(hfacac)2(H2O)2]
VO(acac)2
H2O2
H2O2
H2O2
H2O2
O2
30
30
50
50
5
39
9.8
13
3.3
18.5 [c]
224 [c]
31
0.8 [c]
9.3 [c]
10.3
H4PV1Mo11O40
0.1% Co/SiO2-IWI
This study
[a] [Moles of methyl trifluoroacetate] / [Moles of metal in catalyst] measured over the reported reaction time; [b] Average rate of production based on reported
metal/catalyst loading and reaction time; [c] Including methyl acetate as a product; [d] Determined with respect to Pd(OAc)2.
4
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10.1002/anie.202104153
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RESEARCH ARTICLE
systems for methane-oxidation. The latter operate aerobically
with a NADH electron carrier and are characterized by a highly
designed by systematically addressing the limitations of
conventional systems. The use of a fluorous co-solvent as an acid
diluent brought several advantages, notably a milder reaction
environment that benefited working in a heterogeneous mode and
a facile product and solvent recovery method through a highly-
effective extraction with a non-fluorous solvent. The performance
of this cobalt-catalyzed process far exceeds other comparable
heterogeneous transition metal-based high-temperature catalytic
processes, which are generally restricted by much lower methane
conversion and product selectivity. Through future work to
elucidate the reaction mechanisms, target improvement of the
cobalt utilization at high metal loadings, and optimization of
process conditions, this novel catalytic approach holds the
promise of significant future improvement.
selective conversion of methane to methanol[26]
.
It is clear that high-temperature heterogeneous catalytic
approaches using different transition metals typically result in
much a poorer selectivity even at lower conversion, and
consequently, these systems appear on the very left in Figure 3a.
The performance of the Co/SiO2-IWI catalyst is distinct from these
other approaches and advances toward the more efficient bio-
enzymatic systems. Furthermore, methanol productivity achieved
per gram of the cobalt catalyst is of the same order as that
achieved per gram of dry / wet cells in the biological conversion
of methane. With the Co/SiO2-IWI catalyst, a productivity up to
0.03 kgmethanol/kgcat•h was attained at low cobalt loadings under
low-pressure conditions and a pronounced enhancement is
expected on increasing the feed pressure further. The volumetric
productivity, or space-time-yield (STY), is an additional metric
useful for comparing processes. Although optimization of this
parameter was not a focus of this work, volumetric productivity in
this case would likely be greatly enhanced through increasing the
amount of catalyst in the reactor, increasing methane partial
pressures, and targeting the retention of efficient cobalt utilization
at higher weight loadings.
In addition to excellent oxygen-based product yields,
working at lower partial pressure of methane showed improved
selective methane conversion to the methyl ester. Reactions
performed with 2.5% methane feed resulted in methane-based
ester yields of up to 17% (Supporting Information, Table S4).
While the transition metal-based catalysts depicted in Figure 3b
reach 2 - 3% methane-based yields at best, biological systems
consistently report yields close to 30%[26]. The high activity of
MMO enzymes coupled with the high selectivity in methane
conversion and oxygen utilization has proved difficult to replicate
in heterogeneous systems. The high activity of the cobalt-based
catalyst along with the use of esterification as a ‘product
protection’ strategy enables a performance that is comparable to
the bio-enzymatic systems.
Acknowledgements
This work was supported financially by ETH Zurich and the Swiss
National Science Foundation (Grant No. 200021_178943). The
authors would also like to acknowledge Dr. Frank Krumeich at
ETH Zurich for his work in acquiring the electron microscopy
images presented in this work, Prof. Christophe Copéret for
helpful discussions, and Dr. Adam Clark at the Swiss Light Source
for his expertise and support in acquiring the XAS data, as well as
the SuperXAS beamline for the provision of measurement time.
Keywords: Heterogeneous Catalysis • Methyl Ester • Cobalt •
Product Protection • Fluorous Solvents
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Table 1 lists the activity of the Co/SiO2-IWI catalyst with
known homogeneous catalytic systems that oxidize methane to
methyl trifluoroacetate. The performance of these systems is
compared on the basis of the reported turnovers (TO) over the
reaction period and the productivity. We note that the turnovers
reported typically represent a lower bound on the amount of
product formed per mole of catalyst, since in most cases it
appears that the catalyst may not be fully deactivated at the end
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homogeneous systems that employ stronger oxidants (K2S2O8,
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RESEARCH ARTICLE
Entry for the Table of Contents
Methane is aerobically converted to an oxidant-resistant methyl ester in high yields with a solid cobalt-containing catalyst at a moderate
temperature and pressure. Improved reaction conditions result from the dilution of the trifluoroacetic acid in an inert perfluoroalkane co-
solvent. The fluorous character of the solvent enables the facile recovery of the product ester through a simple liquid-liquid extraction
with a non-fluorous solvent.
7
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