RETRACTED ARTICLE: Conversion of Methane into Methanol and Ethanol over Nickel Oxide on Ceria–Zirconia Catalysts in a Single Reactor

Article abstract of DOI:10.1002/anie.201704704

The conversion of methane into alcohols under moderate reaction conditions is a promising technology for converting stranded methane reserves into liquids that can be transported in pipelines and upgraded to value-added chemicals. We demonstrate that a catalyst consisting of small nickel oxide clusters supported on ceria–zirconia (NiO/CZ) can convert methane to methanol and ethanol in a single, steady-state process at 723 K using O₂ as an abundantly available oxidant. The presence of steam is required to obtain alcohols rather than CO₂ as the product of catalytic combustion. The unusual activity of this catalyst is attributed to the synergy between the small Lewis acidic NiO clusters and the redox-active CZ support, which also stabilizes the small NiO clusters.

Full text of DOI:10.1002/anie.201704704

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Accepted Article
Title: Conversion of Methane to Methanol and Ethanol over Nickel
Oxide on Ceria-Zirconia Catalysts in a Single Reactor
Authors: Chukwuemeka Okolie, Yasmeen F. Belhseine, Yimeng Lyu,
Matthew M. Yung, Mark H. Engelhard, Libor Kovarik, Eli
Stavitski, and Carsten Sievers
This manuscript has been accepted after peer review and appears as an
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201704704
Angew. Chem. 10.1002/ange.201704704
Link to VoR: http://dx.doi.org/10.1002/anie.201704704
http://dx.doi.org/10.1002/ange.201704704

Angewandte Chemie International Edition
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COMMUNICATION
Conversion of Methane to Methanol and Ethanol over Nickel
Oxide on Ceria-Zirconia Catalysts in a Single Reactor
[
a]
[a]
[a]
[b]
Chukwuemeka Okolie, Yasmeen F. Belhseine, Yimeng Lyu, Matthew M. Yung, Mark H.
[c]
[c]
[d]
[a]
Engelhard, Libor Kovarik, Eli Stavitski, and Carsten Sievers*
Abstract: Conversion of methane into alcohols under moderate
reaction conditions is a promising technology for converting stranded
methane reserves into liquids that can be transported in pipelines
and upgraded to value-added chemicals. We demonstrate that a
catalyst consisting of small nickel oxide clusters supported on ceria-
zirconia (NiO/CZ) can convert methane to methanol and ethanol in a
over Cu/SiO
2
catalysts at atmospheric pressure (15).
An interesting route for methane activation that might avoid
the economy of scale of established routes is direct partial
oxidation into oxygenates like methanol, formic acid and
formaldehyde or methanol precursors such as methyl bisulphate
(
6). In nature, bacterial methanotrophs convert methane to
methanol with metalloenzymes, such as methane
single, steady-state process at 723 K using O
available oxidant. The presence of steam is required to obtain
alcohols rather than CO as the product of catalytic combustion. The
2
as an abundantly
monooxygenases (16). Soluble monooxygenase contains a
carboxylate bridged diiron center, whereas a dicopper cluster
has been proposed as the active site of particulate
monooxygenase (17). While monooxygenases enable an
interesting path for methane conversion, they only have limited
potential for large scale industrial processes. Inspired by
2
unusual activity of this catalyst is attributed to the synergy between
the small Lewis acidic NiO clusters and the redox-active CZ support,
which also stabilizes the small NiO clusters.
methane
monooxygenases,
supported
phthalocyanine
complexes of Fe, Cu, and Co have been investigated (18-20). In
other studies, methane was converted to methanol over small
metal oxide clusters in zeolites, such as bis(µ-oxo) dicopper
complexes in Cu/ZSM-5 (10, 21-26). However, prohibitively
The availability of methane from conventional and
unconventional reserves has revolutionized the world’s energy
mix in recent years (1). In conventional processes, methane is
reformed into syngas (2), which can be used for methanol
synthesis (3) or be converted to higher hydrocarbons by Fischer-
Tropsch synthesis (4). Unfortunately, these processes are only
economical on large scales, and reforming requires high
temperatures (typically > 1100 K). Since transporting gas over
long distances is economically challenging, many natural gas
reserves are “stranded”, and methane is often flared on site
rather than used in a productive way. In addition to the loss of a
valuable resource, this practice results in greenhouse gas
emissions (5). Catalytic processes that could enable conversion
of methane to liquid products in a single reactor have enormous
potential to enable more effective use of stranded gas reserves.
This situation has led to intense interest in converting methane
to value-added chemicals and fuels (6-12).
expensive oxidants like tert-butyl hydroperoxide (18), H
2 2
O (10),
or N O (27) had to be used in many of these studies to generate
2
reactive oxygen species. In other studies, the “catalyst” was
reactivated by calcination before it was able to undergo the next
turnover (24, 28). Thus, the reaction did not occur in a
conventional catalytic cycle, in which all elementary reactions
occur under the same conditions.
Methane can also be activated at rather mild temperatures
(
373 – 423 K) by strong Lewis acid sites that are formed when
alumina is calcined at high temperature (29, 30). However, in
many cases, the methyl groups remained on the surface rather
than engaging in catalytic turnovers. Nevertheless, this
illustrates the potential of Lewis acid sites as parts of bifunctional
catalysts.
The conventional route for methanol production uses
In this study, we developed a catalyst that is capable of
converting methane to alcohols at steady state and moderate
temperatures in a single reactor. Steam was necessary to
Cu/ZnO/Al
2
O
3
, ZnO/Cr
2 3
O or similar catalysts at 30-350 bar (3,
1
3). The reaction occurs by sequential hydrogenation of CO or
CO . Synthesis of ethanol and higher alcohols was reported over
2
2
release the alcoholic products, and O acted as an oxidant. This
Rh, Mo, and modified Fischer–Tropsch or methanol synthesis
catalysts and typically also requires elevated pressure (14). The
proposed routes for C-C bond formation include methanol and
CO homologation and CO insertion into metal carbon bonds.
Ethanol formation from methanol and syngas was also reported
catalyst combines Lewis acid sites with a redox-active ceria-
zirconia (CZ) support to enable catalytic turnovers. Additionally,
coupling and/or methanol homologation reactions occur, to form
ethanol, which is more valuable than methanol.
The CZ support with a molar CeO
obtained by coprecipitation following a procedure published
before (31, 32). Al , CoO , FeO , PdO, and NiO clusters on
2 2
:ZrO ratio of 83:17 was
2
O
3
x
x
[
a]
C. Okolie, Y. F. Belhseine, Y. Lyu, Dr. C. Sievers
School of Chemical & Biomolecular Engineering
Georgia Institute of Technology
CZ were prepared by dry impregnation of nitrate precursors. The
loading was 2 wt.% (on a metal basis) in all cases. The desired
Lewis acidic metal oxide clusters were obtained by calcination at
311 Ferst Drive NW, Atlanta, GA, 30332, United States
2
723 K. NiO/SiO with the same Ni loading and NiO/CZ with 5
Email: carsten.sievers@chbe.gatech.edu
Dr. M. M. Yung
National Renewable Energy Laboratory
wt.% Ni were prepared as reference samples.
[
[
b]
c]
The concentration of acid sites on each catalyst was
determined by pyridine adsorption followed by FTIR
spectroscopy. Only the characteristic band of pyridine on Lewis
15013 Denver West Parkway Golden, CO, 80401, United States
Dr. L. Kovarik, Dr. M.H. Engelhard
Environmental Molecular Sciences Laboratory
Pacific Northwest National Lab
-1
acid sites (LAS) (1445 cm ) was observed, while none of the
spectra contained the characteristic peak of pyridinium ions on
3335 Innovation Blvd., Richland, WA, 99354, United States
-1
Brønsted acid sites (1540 cm ) (Figure S1). Pure CZ had a low
[
d]
Dr. E. Stavitski
-1
concentration of LAS (2.9 μmol.g ), whereas significantly higher
National Synchrotron Light Source II
Brookhaven National Laboratory
Upton, NY, 11973, United States
values of 80±4 μmol.g-1 were measured for the samples with
2 3
additional metal oxide clusters except for Al O /CZ (Figure S1b).
This analysis shows that 26% of the Ni atoms in 2 wt.% NiO/CZ
are involved in forming a Lewis acid site (Table S1).
Supporting information for this article is given via a link at the end of
the document
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COMMUNICATION
The ability of the Lewis acidic catalysts to activate
methane was probed by in-situ IR spectroscopy. All samples
were activated in vacuum at 723 K. After 1 h of exposure to 530
mbar of methane at 323 K and evacuation for 1 min, CZ only
retained a small amount of physisorbed methane as indicated by
a small peak at 3014 cm-1 (Figure S4a). At temperatures
between 373 and 523 K, no stable surface species were
observed. This shows that CZ does not have sufficient Lewis
acidity for methane activation. On catalysts with additional metal
oxide clusters, peaks corresponding to asymmetric and
dispersed NiO, several larger particles (up to 10 nm) were also
occasionally observed. In contrast, the EDX map of NiO/SiO
2
only revealed Ni in the form of larger particles of 10 – 25 nm
(Figure 2b). An increase of the Ni loading on CZ to 5 wt.% also
resulted in the formation of larger NiO clusters (Figure 2c). In
agreement with these results, the X-ray diffractogram of NiO/CZ
only contained peaks corresponding to CZ, whereas diffractions
2
of NiO crystallites were observed for NiO/SiO (Figure S3). This
shows that the concentration of crystalline particles in 2 wt.%
NiO/CZ is below the detection limit for XRD. It is suggested that
the high dispersion of NiO on CZ is one prerequisite for the
unique activity of these species. In addition, CZ could alter the
net charge of the supported NiO clusters or provide interfacial
active sites.
3
symmetric CH stretching modes were observed around 2957
-1
and 2875 cm , respectively, starting at 373 – 423 K (Figure 1
and S4b-g). The intensity of the peaks varied between catalysts
with different supported metal oxide clusters, but they were
observed for all the samples used in this study. In agreement
Further insight into the synergy between the NiO clusters
and the CZ support was obtained by probing the oxidation state
of Ni and Ce with in-situ XANES experiments (Figure 3). When
NiO/CZ was exposed to a flow of 4% methane in helium at 723
K, the Ce L3-edge shifted to lower energy over the course of 4 h
with previous work on methane activation on γ-Al
conclude that methane is activated and that surface methyl or
methoxy groups are formed. In case of NiO/CZ and FeO /CZ,
additional bands were observed around 2921 and 2851 cm-
Figure 1 and Figure S4e). These peaks are attributed to
asymmetric and symmetric CH stretching modes, respectively
33). The presence of these bands indicates that surface methyl
groups are coupled to longer alkyl or alkoxy chains. Methane
activation was also studied on NiO/SiO to probe the role of the
CZ support in the activation and coupling of methane. Only the
peaks of methyl groups were observed, but no CH groups were
2 3
O (30), we
x
1
(
(
Figure 3a), indicating reduction of the CZ support. After 4 h, no
2
further shift of the Ce L3-edge could be observed. The final edge
position and the line shape with two maxima, which is typical for
Ce 4+, indicated that the majority of Ce atom remained in an
oxidation state of 4+ with a small contribution from Ce 3+ (35,
(
2
36). The relatively low white line intensity is attributed to strong
2
Ce fluorescence absorption by ceria matrix. Complementary
XPS experiments showed that 49% of the Ce atoms within 3 nm
formed. This shows that CZ plays a direct or indirect role in
enabling the formation of alkyl chains with more than one carbon
atom.
from the surface were reduced to Ce 3+ upon exposure to CH
4
for 4 h at 723 K (Figure S13). Thus, it is suggested that the
reduction of CZ at 723 K is mostly limited to surface sites,
whereas the bulk essentially remains oxidized (31, 36).
(
a)
(b)
(c)
2
0 nm
2
0 nm
20 nm
Ni
0
17
Ni
0
101 Ni (K)
Ni
0
92
NiO
particle
Well-dispersed
NiO
NiO
particle
NiO
particle
Figure 2. HAADF images and corresponding EDX maps of Ni for (a) 2 wt.%
NiO/CZ (b) 2 wt.% NiO/SiO (c) 5 wt.% NiO/CZ. The scales in the Ni EDX
maps correspond to the integrated signal from the ionization peak of Ni(K).
Figure 1. Difference IR spectra of products from CH
73 K.
4
on selected catalysts at
2
4
Based on these results, we hypothesized that CZ stabilizes
very small, well-dispersed NiO or FeO clusters that are capable
The spectrum at the Ni K-edge did not change in the first 4 h,
while the reduction of CZ was proceeding (Figure 3b). However,
a reduction of the white line intensity and a shift of the edge to
higher energy occurred within 30 minutes once the reduction of
Ce stopped. This shows that NiO clusters are readily converted
to metallic Ni when they are present on a reduced CZ surface.
Together, the changes of the Ce L3-edge and Ni K-edge
indicate that CZ can act as an oxygen reservoir that can supply
a certain amount of oxygen to NiO clusters to keep them in the
oxidized state. Importantly, CZ has been reported to efficiently
activate oxygen even at room temperature via a so-called
multiple exchange mechanism (37). The combined abilities of
activating oxygen and transferring activated oxygen species to
x
of activating and coupling methane at low temperatures.
Previous reports showed that similar non-metallic Pt and Au
species can be stabilized by pure ceria and that these species
are highly active as part of water-gas-shift catalysts (34). To
verify our hypothesis, EDX maps and X-ray diffractograms of
2
NiO/CZ, NiO/SiO , and were obtained. The EDX maps of
NiO/CZ with 2 wt.% Ni showed the presence of well-dispersed
Ni on the entire surface of the CZ support (Figure 2a and S8).
The exact size distribution of the Ni clusters cannot be quantified
by EDX mapping due to the detectability-stability limits.
Nevertheless, it can be concluded that they should be present
as NiO clusters below ~2-3 nm in size. In addition to the well
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Angewandte Chemie International Edition
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COMMUNICATION
the supported particles, such as the NiO clusters in this study,
make CZ an effective component of bifunctional oxidation
catalysts (38, 39).
3
3
.5
.0
1
.5
.0
(a)
1
In addition to studying the redox behavior of NiO/CZ, we
used in-situ XANES to probe the interaction of steam with the
supported NiO clusters, since in previous studies the formation
of metal hydroxides prevented selective oxidation of methane
0
.5
CeO2
Standard
2
2
.5
.0
0.0
5716 5720 5724 5728
x x
over CuO and FeO clusters in steady state (24, 28). No
changes were observed at the Ni K-edge when NiO/CZ was
exposed to steam at 723 K for 4 h (Figure 3c) indicating that the
NiO clusters are not converted into hydroxides under these
conditions. The derivative of the spectrum provides even clearer
evidence for the absence of hydroxyl species (Figure S15).
1
.5
1.0 Mild reduction
of ceria within
0.5 4h
2
While the white line of the Ni(OH) standard showed two distinct
maxima, this feature was not observed for the NiO standard or
the NiO/CZ catalyst after exposure to steam at 723 K. Thus, it is
safe to conclude that NiO clusters are stable in this environment.
5
h 4 h 3 h 2 h 1 h 10 mins 0 h
0
.0
Based on the observation of higher alkyl or alkoxy chains on
NiO/CZ (Figure 1) and the characterization of this material, we
hypothesized that methanol and higher alcohols can be formed
over these catalysts. It was shown that steam can hydrolyze
surface methoxy species to form methanol (26). However, this
elementary reaction needs to be coupled with a selective
oxidation step to make the process thermodynamically feasible
5700 5710 5720 5730 5740 5750 5760 5770
Photon Energy /eV
(b)
3
2
.0
.5
1
.25
Ni(OH)2
Standard
0 h
0 mins
1 h
1.00
(
see SI). Thus, the conversion of methane over 2 wt.% NiO/CZ
was studied in a packed bed reactor at 723 K with a feed steam-
to-carbon ratio of 1.0, an O to carbon ratio of 0.2, and a balance
1
0
.75
2.0
.5
8352 8356
2
8348
NiO
Standard
of nitrogen (Figure 4). The initial methane conversion was 18%.
It declined to 15% over the first 3 h on stream, but no further
changes were observed after that. If one assumed that every Ni
atom of the catalyst constitutes an active site, the conversion of
2
h
1
Sudden
reduction of
NiO after 4h
4 h
4 h 10 mins
-1
1.0
0.5
1
5% would correspond to a turnover frequency of 50 h .
Methanol, ethanol, and CO were observed as the main carbon
2
5
h
containing products along with hydrogen. The ethanol and
methanol yields reached 3.7% and 2.9% on a C atom basis,
respectively. This corresponds to a cumulative alcohol selectivity
of up to 43%, and an ethanol selectivity of 24% on a C atom
basis. The formation of methanol and ethanol was confirmed by
NMR spectroscopy (Figures S24a and S25) and MS (Figure
S24b). The production of methanol and ethanol throughout the
reaction remained almost constant. No products with three or
more carbon atoms were observed under the conditions applied
0
.0
320
8
8330
8340
8350
8360
8370
Photon Energy /eV
(c)
3
.0
2.5
.0
1.5
2 2
here. The formation of H and CO can be attributed to oxy-
reforming or a sequence of steam reforming and the water-gas-
shift reaction, but oxidation of other surface species could also
0
1
2 h
3
4
h
h
Ni(OH)2
Standard
contribute to the formation of CO
2
.
8348
8352
h
h
2
In the initial stage of the reaction, 2.6% of the carbon
atoms entering the reactor remained unaccounted for in the
product stream and 1.5% were converted to aromatics (Figure 4).
Within 5 h, the fractions of unaccounted carbon and aromatics
decreased to 0.23% and 0.14%, respectively, and remained at
approximately this level for the remaining 3 h of the reaction.
Previous reports indicated that coking is typical for Ni particles
that have a certain minimum size (40, 41). Thus, it is suggested
that carbonaceous deposits are formed on the few large NiO
clusters (Figure 2a). Some aromatic precursors of these
deposits are desorbed instead of being permanently deposited.
Once the larger particles are covered, the formation of deposits
and aromatics almost ceases. In contrast, the well-dispersed
NiO clusters remain accessible and active. Combustion analysis
of the spent catalyst indicated the presence of 2.9 wt.% of
combustible deposits.
NiO
Standard
1
0
.0
.5
0.0
320 8330 8340 8350
8
8360 8370
Photon Energy /eV
Figure 3. In-situ XANES spectra: (a) Ce L
NiO/CZ at 723 K (b) Ni K-edge during reaction of CH
c) Ni K-edge during exposure to H O at 723 K.
3
-edge during reaction of CH
4
over
4
over NiO/CZ at 723 K
(
2
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Angewandte Chemie International Edition
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COMMUNICATION
Further evidence for this scenario was obtained when
methane was converted under identical conditions over 2 wt.%
Acknowledgements
The work was performed with funds from the Petroleum
Research Fund of the American Chemical Society (grant
number: 53873) and the U.S. Department of Energy (DOE)
Office of Basic Energy Science under Contract No. DE-
SC0016486. The XAS data was obtained at beamline 9-BM of
the Advanced Photon Source, a U.S. Department of Energy
NiO/SiO
particles. Over 2 wt.% NiO/SiO
but the catalysts deactivated completely over the course of 6 h
Figure S23a). The amount of deposited carbon on the spent
2
and 5 wt.% NiO/CZ, which contained larger NiO
2
, the initial conversion was 15%,
(
catalyst as determined by CHN analysis was 5.8 wt.%, which is
significantly higher compared to 2 wt.% NiO/CZ. Only CO and
CO were observed as carbon-based products over this catalyst,
2
(
DOE) Office of Science User Facility operated for the DOE
Office of Science by Argonne National Laboratory under
Contract No. DE-AC02-06CH11357. EDX mapping and XPS
was performed using EMSL a DOE Office Science User Facility
sponsored by the Office of Biological and Environmental
Research. The Renewable Bioproducts Institute at the Georgia
Institute of Technology is thanked for the use of its facilities. We
thank Michael Buchanan, Leah Filardi, Sankar Nair, Evgeny
Pidko, Andrew J. Medford, and Johannes A. Lercher for
experimental support and fruitful discussions.
which demonstrates further that the synergy between NiO and
CZ plays a critical role in the production of alcohols. The initial
conversion over 5 wt.% NiO/CZ was 23%, but it declined to 13%
within 5 h (Figure S22a), and the cumulative alcohol selectivity
was only around 30%. These observation show that 5 wt.%
NiO/CZ contains fewer small NiO clusters that are active for
sustained production of alcohol than 2 wt.% NiO/CZ.
To illustrate the function of steam, 2 wt.% NiO/CZ was
used in a reaction with a feed that only contained methane and
O
1
2
in N
4% over the first 3 h on stream and remained constant for the
rest of the experiment. CO was the only carbon containing
2
(Figure S17b). The conversion dropped from 17% to
2
18
product detected. This shows that steam is necessary to obtain
alcohols, whereas the combustion of methane dominates in the
absence of steam in the feed. With a feed containing only
methane, steam and nitrogen, the initial methane conversion
was 2% and some alcohols were formed (Figure S17a).
However, the catalytic activity ceased within about 4 h, which
correspond to the time at which the reduction of Ce stopped
during the in-situ XANES experiment under similar conditions.
1
1
6
4
H O Conv.
2
CH Conv.
4
H2
CO2
O2 Conv.
EtOH
MeOH
C Deposit
12
Monoaromatics
Multi-ring aromatics
1
0
8
6
4
2
0
The observations described above show that NiO clusters
can activate methane, similar to the one in previous reports of
methane activation on other types of Lewis acidic sites (29, 30).
In the case of sufficiently large clusters, carbonaceous deposits
are formed, and the clusters are deactivated. The CZ support
appears to stabilize NiO clusters that are sufficiently small to
avoid this deactivation path. In the presence of steam, the
surface species can be hydrolyzed and desorbed as alcohols.
Based on the in-situ XANES results (Figure 3a and 3b) and well-
0
100
200
300
400
500
Time /min
2
documented activity of CZ for activating O (37), it is suggested
that the active oxygen species required for the selective
oxidation reaction originate on CZ. However, the exact origin of
O atoms in the products will be the subject of future studies. A
reference experiment showed that methanol formation over
NiO/CZ is also possible from syngas and steam (Figure S18b),
but CO was not completely converted.
2
Figure 4. Conversion of methane and O and yields of products formed during
reactions of methane, steam and oxygen in a packed bed reactor setup at 723
K and 1 atm over 2 wt.% NiO/CZ. Yields of carbon-containing products are
-
1
calculated on a carbon atom basis. A space velocity of 3000 h of reactant
gas diluted in nitrogen was used. Other experimental conditions are detailed in
the SI.
The strong peaks of alkyl or alkoxy groups with two or
more carbon atoms in in-situ IR spectra (Figures 1 and S5)
seems to point at a reaction path involving coupling of surface
alkyl or alkoxy species. In a reference experiment, ethanol was
formed from a mixture of methanol and syngas at the same
temperature and pressure (Figure S19). This indicates that
homologation might be an alternative reaction path (14), but the
observation of residual CO in this experiments (as opposed to
the absence of CO in the product stream from methane
conversion) indicates that homologation is less likely to be the
dominant reaction path.
Keywords: C-H activation • Heterogeneous catalysis • Lewis
acids • Methane • Nickel
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Angewandte Chemie International Edition
10.1002/anie.201704704
COMMUNICATION
COMMUNICATION
Methanol and ethanol are
produced continuously from
C. Okolie, Y. F. Belhseine, Y. Lyu,
M.M. Yung, M.H. Engelhard, L.
Kovarik, E. Stavitski, C. Sievers*
methane and oxygen in a single
reaction over a catalyst consisting
of NiO clusters on ceria-zirconia.
Oxygen is used as an abundantly
available oxidant for this reaction
and the presence of steam
Page No. – Page No.
Conversion of Methane to
Methanol and Ethanol over Nickel
Oxide on Ceria-Zirconia in a
Single Reactor
ensures the production of alcohols
as opposed to products of
complete combustion of methane.
This article is protected by copyright. All rights reserved.