CO-Assisted Direct Methane Conversion into C1 and C2 Oxygenates over ZSM-5 Supported Transition and Platinum Group Metal Catalysts Using Oxygen as an Oxidant

Article abstract of DOI:10.1002/cctc.202000168

Conversion of methane towards chemical feedstocks such as oxygenates and hydrocarbons has been studied along with the development of natural gas technology. In this study, CO-assisted direct conversion of methane into C₁ and C₂ oxygenates was demonstrated over ZSM-5 supported transition and platinum group metal catalysts. Besides previously investigated Rh, other platinum group metals (e. g., Ru and Ir) were found to be a potential element as the catalyst. Presence of CO was critical for the reaction, which would work as a ligand, a reductant, and a reactant. On the other hand, the competing undesired CO oxidation toward CO₂ was found to be the major issue. Partial oxidation toward methanol or formic acid (C₁ oxygenates formation) was parallel to oxidative carbonylation to acetic acid (C₂ oxygenate formation).

Full text of DOI:10.1002/cctc.202000168

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: CO-assisted direct methane conversion into C1 and C2
oxygenates over ZSM-5 supported transition and platinum group
metal catalysts using oxygen as an oxidant
Authors: Takahiko Moteki, Naoto Tominaga, and Masaru Ogura
This manuscript has been accepted after peer review and appears as an
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To be cited as: ChemCatChem 10.1002/cctc.202000168
Link to VoR: https://doi.org/10.1002/cctc.202000168
01/2020

10.1002/cctc.202000168
ChemCatChem
COMMUNICATION
CO-assisted direct methane conversion into C1 and C2
oxygenates over ZSM-5 supported transition and platinum group
metal catalysts using oxygen as an oxidant
Takahiko Moteki,‡,*[a] Naoto Tominaga,‡,[a] and Masaru Ogura[a]
[a]
Dr. T. Moteki, N. Tominaga, Prof. M. Ogura
Institute of Industrial Science
The University of Tokyo
4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan
E-mail: moteki@iis.u-tokyo.ac.jp
‡
T. Moteki and N. Tominaga contributed equally to the paper
Supporting information for this article is given via a link at the end of the document.
Abstract: Conversion of methane towards chemical feedstocks such
as oxygenates and hydrocarbons has been studied along with the
development of natural gas technology. In this study, CO-assisted
direct conversion of methane into C1 and C2 oxygenates was
demonstrated over ZSM-5 supported transition and platinum group
metal catalysts. Besides previously investigated Rh, other platinum
group metals (e.g., Ru and Ir) were found to be a potential element as
a catalyst. Presence of CO was critical for the reaction, which would
work as a ligand, a reductant, and a reactant. On the other hand, the
competing undesired CO oxidation toward CO2 was found to be the
major issue. Partial oxidation toward methanol or formic acid (C1
oxygenates formation) was parallel to oxidative carbonylation to acetic
acid (C2 oxygenate formation).
developments.[11–14] As far as we know, the highest one-pass yield
(70%) of the C1 oxygenates production was reported over
homogeneous platinum-based catalyst by Periana et al.[15]
Compared to this benchmark result, far low oxygenates yield has
been achieved on heterogeneous catalysts.[13] In 2005,
Groothaert et al. developed a heterogenous ZSM-5 supported
copper catalyst and succeeded in the methanol synthesis with
high selectivity by “chemical loop” system.[16] Because of the
structural similarity in copper sites formed on zeolite to the
enzyme, particulate methane monooxygenase (pMMO),[17–19] a lot
of studies about zeolite supported copper catalysts have been
reported and summarized in review papers.[14,20,21] However,
together with the low oxygenates (i.e., methanol) yield (<< 1%),
non-catalytic “chemical loop” process has also been a major
drawback.
Chemical industries have required a technology innovation to the
demand of the times to follow the changes in the feedstocks.
Since around 2010, the market price of natural gas has been
decreasing due to the shale gas revolution, and the major
feedstock for energy and base chemicals is shifting from crude oil
to natural gas.[1,2] Because the main component of natural gas is
methane (CH4, 70–90%), a lot of interest have been focused on
methane conversion into valuable products such as C1
oxygenates (i.e., methanol and formic acid), syngas (carbon
monoxide with hydrogen), and resulting synthetic gasoline.[3–5]
Methane conversion into liquid product would also contribute to
the transportation problem of the combustible gas from isolated
mines to consuming region.[6,7]
Another catalyst developed recently was single-atom Rh catalyst
supported on ZSM-5 reported in 2017[22] and 2018[23] by two
groups. It catalysed the methane conversion into methanol
(CH3OH), formic acid (HCOOH), and acetic acid (CH3COOH) at
423 K under the assist of CO with production rate of 8.5, 1.6×10,
and 3.1×10 mol·molRh−1·h−1, respectively. The values were higher
than the previous copper-based zeolite catalysts.[24–27] In the
report of 2017, CO was denoted as a “cocatalyst”.[22] This
interesting reaction system was, however, not well-understood
especially in the role of CO and the origin of C2 oxygenates (i.e.,
CH3COOH). A new strategy for direct methane conversion would
be established by a further investigation of this CO-assisted
reaction system.
The two-step chemicals production via syngas has been
industrialized for a long time although its high energy consumption
due to the high temperature process (>1073 K) has been
considered as a huge drawback and it limited the application only
in a large-scale industry.[8] Formation of oxygenates via partial
oxidation with several types of oxidants (e.g., O2, H2O2, and N2O)
is an alternative approach. Among them, low-temperature direct
partial oxidation of methane using gaseous oxygen as an oxidant
(i.e., CH4 + 1/2O2 → CH3OH) has been expected as the most
desirable methane converting method to replace the current
energy-intensive and large-scale approach.[9,10] A huge number of
studies have been reported on the process and catalyst
Herein, the CO-assisted methane conversion into C1 and C2
oxygenates was demonstrated over ZSM-5 supported transition
and platinum group metals (i.e., Fe, Co, Ni, Cu, Ru, Rh, Pd, Ir,
and Pt). In the system, CO worked as a ligand to the transition
metal and/or a reductant to form active oxo-species on the metal.
Furthermore, it also worked as a co-reactant to form acetic acid
via an oxidative carbonylation reaction. Competing simple
oxidation of CO into CO2, however, suppressed the desired
reactions. The detailed reaction route was investigated, and
sequential partial oxidation from CH4 to CH3OH to HCOOH was
found parallel to oxidative carbonylation of CH4 to CH3COOH.
1
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Metal catalysts supported on ZSM-5 were prepared by
impregnation method. Practically, 600 μL of metal nitrate or
chloride salts solution were step-wisely added to 1 g of NH4-ZSM-
5 (Si/Al = 11.9) powder and mixed homogeneously, and then
pretreated by H2 at 823 K for 3 h. Catalytic tests were conducted
with a high-pressure batch reactor at 423 K under the presence
of O2, CO, and CH4 gases (0.2, 0.5, and 2.0 MPa, respectively)
with 40 mg of catalyst and 8 mL of solvent water. Gaseous and
liquid products were analysed by GC-FID equipped with
methanizer and 1H NMR with DANTE presaturation pulse,[28]
respectively. For the detailed experimental procedures and
product quantification, please see Supporting Information.
Besides previously tested Rh,[22,23] four other platinum group
metals (i.e., Ru, Pd, Ir, Pt) supported on ZSM-5 were screened as
a catalyst for CO-assisted direct methane conversion. In addition,
transition metals including Cu were also tested as reference
catalysts. Especially, Cu has previously been well-studied in the
CO-free chemical loop system.[14,20,21] In the current study, all the
tested metals demonstrated the formation of C1 and C2
oxygenates under the presence of CO (Fig. 1), and precious
metals exhibited higher catalytic performance than Cu. In any
case, only CH3OH and HCOOH were formed as C1 oxygenates,
and CH3COOH was formed as a C2 oxygenate. Other potential
chemicals, such as acetaldehyde (CH3CHO), were not observed
either by 1H NMR or GC-FID in our reaction conditions.
Furthermore, it should be noted that CO was a critical additive and
no oxygenates were formed without CO over precious metal
catalysts (Fig. S7, in SI). In the case of Cu, small amount of
CH3OH was formed (4.6 μmol·g−1) without CO (Fig. S7 in SI),
maybe similar to a minor catalytic route found in the chemical loop
system.[25,29]
oxygenates formation, the rates were in the same magnitude of
order over Ru, Rh, Pd, and Ir catalysts (Fig. 1). These results
would suggest that similar type of CO-assisted oxidation reaction
proceeded over precious metals, implying the key factor is in the
general feature of those atoms, which would explain their
reactivity and selectivity.
Generally, under the presence of CO as a ligand, platinum group
metals are thought to be reduced by the electron donation from
coordinated CO, for instance RhIII is reduced by CO to RhI–(CO)x
over ZSM-5.[22] CO coordination is considered to make the
electronic state around the metal more nucleophilic, and it
enables electrophilic activation as a catalyst.[30,31] Because the
presence of CO should be essential for the reaction (Figs. 1 and
S7), the C–H activation might proceed over CO-coordinated metal
site (i.e., CO works as a metal ligand). Another possibility might
be the coordinated CO forms active oxo-species (dissociated
single O atom, i.e., Rh=O) over metal site (i.e., CO works as a
reductant, Rh–CO + O2  CO2 + Rh=O). In either case, CO
should preliminarily coordinate on the metal site. Detailed reaction
mechanism was assumed in the end of present literature. The
coordination of CO was confirmed by FTIR spectroscopy over Rh-
ZSM-5 (Fig. S8 in SI). Two bands at 2112 and 2034 cm–1
appeared after CO introduction, which was assigned as a
symmetric and an asymmetric stretching of C=O in carbonyl
species over Rh, respectively.[22] The investigation of other ligand
instead of CO would be an approach for higher catalytic activity.
However, as far as our attempts testing halogens and pyridines,
a potential candidate has not been found yet (Table S1 in SI). It
would be because the well-known unique interaction between
metal and CO accompanied with both electron donation and back-
donation.[32] We believe that the investigation of an alternative
ligand is critical for the further catalyst development because the
added CO was simply oxidized and consumed as discussed
below, which hindered the desired reaction and formed a lot of
byproduct (i.e., CO2).
The consumption of CO into CO2 was found to be the major issue
in this catalysis system, as it was added as a “cocatalyst”,[22]
a
catalyst ligand, a reductant, or a reactant. Fig. 2 showed CO
consumption and CO2 production along with the reaction time
over representative metal catalysts, Ru and Rh supported on
ZSM-5. Over both catalysts, the increase of CO consumption was
corresponding to the CO2 production along with the reaction time
(Fig. 2). The right axis in Fig. 2 represented the theoretical
conversion of CO, and it was approaching to 100% with the
increase of reaction time (i.e., the theoretical maximum (4.6
mmol) limited by the initial O2 amount in gas phase (2.3 mmol)).
The results suggested that the CO2 was formed in a large amount
mostly by CO oxidation. Therefore, qualitative investigation on the
total oxidation of methane into CO2 would be difficult due to the
far lower reactivity of methane conversion. Besides CO oxidation,
water gas shift reaction (CO + H2O  CO2 + H2) might also be
possible, which can be critical in SCR reaction at higher
temperature.[33] The route was, however, denied because CO2
formation was not confirmed by a reference reaction of water and
CO over Rh–ZSM-5 under the absence of O2 and CH4 (data not
shown). Then, the issue was that the most of CO added as a
“cocatalyst”,[22] a catalyst ligand, a reductant, or a reactant was
simply oxidized into CO2 via a side reaction. Again, finding an
alternative instead of CO should be crucial for future catalyst
development.
Figure 1. Product formation rates [molproduct·molmetal−1·h−1] of CH3OH (light blue
dot), HCOOH (blue diagonal line), and CH3COOH (red filled) over ZSM-5
supported transition and platinum group metal (Fe, Co, Ni, Cu, Ru, Rh, Pd, Ir,
Pt) catalysts (0.2 MPa O2, 0.5 MPa CO, 2.0 MPa CH4, 8 mL H2O, 40 mg catalyst,
423 K, 3 h).
Whereas the selectivity toward each product changes along with
the metal atoms (Fig. 1), the reaction was not specific for Rh
catalyst tested before.[22,23] And other precious metals could also
be a potential candidate (especially Ru and Ir, because of their
high production rates in CH3OH and HCOOH, respectively). If
CH3OH and HCOOH formations are integrated as
a C1
2
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One indicator for partial oxidation of methane is the yield of
methanol, which has been often compared among catalyst.[13] The
highest yield of methanol was 1.6% and 0.2% over Ru (96 h) and
Rh (72 h) catalysts, respectively (Fig. S9 in SI). The value by Ru
catalyst prepared in this study (yield of 1.6%) represented one of
the highest yields compared with other heterogeneous catalysts
reported before.[13]
In order to investigate the reaction pathway in this system,
potential intermediates (i.e., CH3OH, HCOOH, CH3COOH and
CH3CHO) were reacted with the presence of O2 and CO (Fig. 4).
Starting from CH3OH as a reactant, the formation of HCOOH was
observed along with its consumption (Fig. 4a), and from HCOOH,
no liquid phase products were formed (Fig. 4b). In both cases,
moles of the reactant consumed was larger than that of product
formed in liquid phase, suggesting the over oxidation toward CO2.
These results are in good agreement with the discussion above
(Fig.3), in which CH3OH and HCOOH were assumed as primary
and secondary products, respectively, in the sequential partial
oxidation. Interestingly, formation of CH3COOH was not
confirmed in both cases. This suggested that carbonylation (or
CO insertion) of CH3OH to CH3COOH, similar to Monsanto
process,[34] did not proceed in this system. Together with the
results of CH3COOH formation as a primary product (Fig 3), we
propose the direct oxidative carbonylation of methane as a
Figure 2. CO consumption (grey open diamond) and CO2 formation (black filled
diamond) over ZSM-5 supported (a) Ru and (b) Rh catalysts (0.2 MPa O2, 0.5
MPa CO, 2.0 MPa CH4, 8 mL H2O, 40 mg catalyst, 423 K). Right axis shows the
theoretical CO conversion limited by O2 amount.
Because of the consumption of CO (Fig. 2), the catalytic reaction
(i.e., oxygenates formation) might be suppressed especially over
50 h in our reaction condition. Fig. 3 showed the formation of
oxygenates (i.e., CH3OH, HCOOH, and CH3COOH) along with
reaction time over Ru and Rh catalysts. The formed amount of
each product initially increased along with the time, and then
those of CH3OH and HCOOH decreased whereas that of
CH3COOH was maintained (Fig. 3). The behaviors of C1
oxygenate products would be explained by the over oxidation
toward CO2 and the suppression of their formation due to the lack
of CO at longer reaction time. On the other hand, CH3COOH was
formed as like an apparent terminated product unaccompanied
with further oxidation. Changes in the product selectivity along
with the reaction time (Fig. 3 top) indicated that the formation of
CH3OH and CH3COOH behaved as like primary products, on the
other hand, that of HCOOH behaved like a secondary product
over both catalysts. The results suggested the direct formation of
CH3OH and CH3COOH and the subsequent oxidation of CH3OH
to HCOOH. Furthermore, over oxidation toward CO2 would
proceed from C1 oxygenates.
plausible CH3COOH formation route (CH4 + CO + 1/2O2

CH3COOH). Formation of acetaldehyde, CH3CHO, should also be
considered, although its formation had not been experimentally
confirmed (Fig. 3). By using CH3CHO as a starting reactant (Fig.
4d), it was found that the consumption of CH3CHO was much
faster than any other reactant and formation of CH3COOH (in 80%
carbon selectivity). Thus, even if it forms, CH3CHO assumed to
be an unstable intermediate and quickly oxidized into CH3COOH.
When the reaction started from CH3COOH, its consumption was
much slower than any other reactants (Fig. 4c). Due to the slower
oxidation (or decomposition) of CH3COOH, it observed as like an
apparent terminated product in high yield (Fig. 3). As
demonstrated in the Periana-Catalytica system,[35] in which
activated methane was preserved in the form of CH3–X (X = Cl,
OSO3H), protecting a reactive product is an effective approach in
partial oxidation of methane. Based on these results, the overall
reaction pathways in this system were summarized in Scheme 1.
For the further consideration about reaction mechanism, we
speculated following mechanism as a more plausible case: 1)
formation of oxo-species with using CO as a reductant, 2) C–H
activation via σ metathesis over surface oxo-species (M=O + CH4
 CH3–M–OH), and 3) subsequent oxygenates formation[30,36]
(Scheme S1(a) in SI) rather than the C–H activation by metal and
subsequent O (and CO) insertion (Scheme S1(b) in SI) discussed
previously.[22] Although our speculation would agree with the no
CH3CHO observation (Fig. 1) and the large CO consumption (Fig.
2), further studies should be necessary to elucidate the
speculated reaction mechanism in the future.
Figure 3. Amount of formed CH3OH (light blue open circle), HCOOH (blue open
square), and CH3COOH (red filled square) and their selectivities over ZSM-5
supported (a) Ru and (b) Rh catalysts (0.2 MPa O2, 0.5 MPa CO, 2.0 MPa CH4,
8 mL H2O, 40 mg catalyst, 423 K). Selectivities were calculated by the amount
of product divided by the sum amount of CH3OH, HCOOH, and CH3COOH.
Dashed lines are meant to guide the eye.
3
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In summary, it was demonstrated that ZSM-5 supported platinum
group metals catalyzed methane conversion into C1 and C2
oxygenates under the assist of CO. In the reaction system studied
here, the CO was unique and critical maybe as a ligand, a
reductant, and a reactant. Partial oxidation of methane toward
methanol or formic acid was parallel to oxidative carbonylation of
methane to acetic acid. The major drawback of the system was
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Keywords: zeolite • Rh-ZSM-5 • Methane conversion • Partial
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5
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Entry for the Table of Contents
ZSM-5 supported platinum group metal catalysts achieved direct methane conversion into methanol, formic acid, and acetic acid
under the assist of CO. Ru and Ir besides Rh were found to be the potential candidates. Presence of CO was critical for the reaction,
whereas undesired CO oxidation into CO2 also proceeded. C1 formation route (i.e., partial oxidation) was parallel to C2 formation
route (i.e., oxidative carbonylation).
6
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