The Effect of the Active-Site Structure on the Activity of Copper Mordenite in the Aerobic and Anaerobic Conversion of Methane into Methanol

Article abstract of DOI:10.1002/anie.201802922

Samples of the zeolite mordenite with different Si/Al ratios were used to synthesize materials with monomeric and oligomeric copper sites that are active in the direct conversion of methane into methanol. A comparison of two reactivation protocols with oxygen (aerobic oxidation) and water (anaerobic oxidation), respectively, revealed that such copper–oxo species possess different reactivity towards methane and water. We show for the first time that oligomeric copper species exhibit high activity under both aerobic and anaerobic activation conditions, whereas monomeric copper sites produce methanol only in aerobic processes.

Full text of DOI:10.1002/anie.201802922

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Effect of Active Sites Structure on Activity of Copper Mordenite in
Aerobic and Anaerobic Conversion of Methane to Methanol
Authors: Vitaly L. Sushkevich, Dennis Palagin, and Jeroen Anton van
Bokhoven
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201802922
Angew. Chem. 10.1002/ange.201802922
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Effect of Active Sites Structure on Activity of Copper Mordenite in
Aerobic and Anaerobic Conversion of Methane to Methanol
Vitaly L. Sushkevich,*[a] Dennis Palagin,[a] Jeroen A. van Bokhoven*[a,b]
Abstract: Zeolite mordenite with different Si/Al ratio was used to
synthesize the materials with monomeric and oligomeric copper
sites active in the process of direct conversion of methane into
methanol. Comparison of two reactivation protocols with oxygen
(aerobic oxidation) respectively water (anaerobic oxidation) revealed
that such copper-oxo species possess different reactivity towards
methane and water. We have shown for the first time that copper
oligomer species exhibit high activity under both aerobic and
anaerobic activation conditions, whereas copper monomer sites
produce methanol only in aerobic process.
inspired by natural enzymes[16] and proven experimentally,[2,5-8]
the possibility of formation of larger clusters has also been
suggested theoretically based on thermodynamic stability.[3,17-19]
Copper monomers, on the other hand, are believed to require
the formation of copper hydrides to facilitate the methane
oxidation, which is energetically unfavorable.[20] The intrinsic
activity of copper species of different size is still a subject of
debate. Furthermore, recent reports suggest the possibility of
formation of a mixture of active sites,[2-4] and even give evidence
to their dynamic structural behavior.[21,22]
At the same time, copper species of variable size are
expected to form during the synthesis via conventional ion
exchange of parent zeolites. As such, in the materials with high
Si/Al ratio (i.e. low Al content) the probability of the formation of
copper monomeric species is significantly higher than that for
the materials with low Si/Al (i.e. high Al content, high
concentration of Al pairs).[23] This method can thus be used for
modulation of the size of active copper clusters in zeolites and
their performance.
Herein we demonstrate the influence of Si/Al ratio of the
zeolite on the possible configurations of the active copper oxide
sites, and therefore on the activity of the material in the process
of aerobic and anaerobic conversion of methane into methanol.
We show that the samples with high Si/Al ratio (low Al content)
preferentially contain isolated copper cations leading to higher
methanol yield on a copper ion basis and selectivity. Such
stabilization comes at a price of a poor activity in the water
reactivation reaction, preventing these sites from stabilizing and
releasing molecular hydrogen while oxidizing Cu(I) into Cu(II).
The samples with lower Si/Al ratios, on the other hand, both
provide sufficient methanol yield, and facilitate an energetically
less costly release of hydrogen, thus offering optimal conditions
for the efficient selective direct methane conversion without
oxygen.
The samples were prepared by conventional ion exchange
of commercial zeolite mordenite with Si/Al ratio 6.5, 10 and 46
using copper nitrate solution followed by calcination in a flow of
dry air at 773 K (Table 1). They are denoted as CuMOR(x),
where x corresponds to Si/Al molar ratio in the parent zeolite.
Activation and reaction with methane was carried out using two
different procedures: i) standard pretreatment in oxygen
(“aerobic” activation), and ii) recently discovered activation in
inert gas (“anaerobic” activation).[13] The details on sample
synthesis, activation protocols and reaction conditions are given
in Supporting Information.
Conversion of methane to methanol is an industrially very
important process, as it provides a sustainable route from an
abundant and clean component of natural gas to one of the
main precursors for chemicals synthesis.[1]
A
promising
stepwise process over copper-exchanged zeolites has been
suggested; however, detailed understanding of the
a
mechanism of such a zeolite-catalyzed conversion is still
missing.[2-4] One of the ongoing debates in studying copper-
exchanged zeolites is the exact configuration of the available
active sites.[1-4] Another open question is the role of the nature of
the oxidant, and the mechanism of methane oxidation and
regeneration of the copper oxide active site.[5-8] Most crucially,
the existing methods often suffer from low selectivity or
costliness of oxidants.[9-12]
We have recently proposed an alternative solution to the
above problem by showing that selective anaerobic oxidation of
methane is possible,[13] where water can be used both to
provide oxygen to regenerate the zeolite active centers, and to
facilitate stabilization of the reaction intermediate, which
uniquely occurs in the copper zeolite, to drive the otherwise
endothermic Cu(I) oxidation reaction.[14,15] Instead of using
oxygen, only the presence of water is required, while the
reactivation of the zeolite material is done in an inert
atmosphere. Such a water-facilitated redox process requires at
least two copper atoms to stoichiometrically oxidize methane
into methanol, thus suggesting the presence of active sites
containing several copper atoms, such as dimers[5-7] and
trimers,[8] stabilized by the framework aluminum atoms.
While the existence of smaller active sites has been
[a]
Dr. Vitaly L. Sushkevich, Dr. Dennis Palagin, Prof. Dr. Jeroen A. van
Bokhoven
Laboratory for Catalysis and Sustainable Chemistry
Paul Scherrer Institut
First, the activity of the samples activated in oxygen has
been studied (Figure 1, blue lines). After interaction with
methane at 473 K and 7 bars, and subsequent introduction of
water-containing helium flow, desorption of methanol starts after
a certain induction period, which is associated with gradual
saturation of the samples with water. During the first 10 minutes,
the concentration of methanol in the stream reaches its
5232 Villigen PSI, Switzerland
E-mail: vitaly.sushkevich@psi.ch; jeroen.vanbokhoven@chem.ethz.ch
Prof. Dr. Jeroen A. van Bokhoven
Institute for Chemistry and Bioengineering
ETH Zurich
Vladimir-Prelog-Weg 1, 8093 Zürich, Switzerland
[b]
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201802922
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maximum and gradually decreases for the next 2–4 h. Only
traces of dimethyl ether were observed in the stream by mass
spectrometry, possibly due to the high partial pressure of water
with respect to methanol, which shifts the dehydration reaction
equilibrium. In the case of samples with Si/Al ratio of 6.5 and 10
the slowed desorption of methanol is related to the higher
amount of aluminum and residual Brønsted sites that can
interact with methanol molecules, hence possessing high affinity
to polar molecules. No hydrogen formation was observed after
the introduction of water on O2-activated samples (insets in Fig.
1), which is in line with our previous study.[11]
infrared spectroscopy of adsorbed NO was used[24] (Figure 2b).
The bands at 1804 and 1826, 1730 cm-1 are attributed to Cu(I)
mono- and dinitrozyls, respectively.[24-27] Similarly, the bands in
the region 2000-1900 cm-1 with three main peaks centered at
1995, 1950 and 1908 cm-1 are generally attributed to Cu(II)
interacting with NO. The bands with low vibration frequencies
(~1900 cm-1) are believed to be due to the copper monomeric
species, while the bands at high frequencies starting from 1940
cm-1 belong to oligomeric species interacting with NO.[28,29] The
small shoulder at ~1880 cm-1 corresponds to NO interacting with
zeolite surface (Figure S6). The formation of Cu(I) species in the
oxygen-activated samples is associated with the so-called
“autoreduction” process,[30] which implies the thermo-induced
conversion of Cu(II) into Cu(I) during the short evacuation at
673K after the oxidation in O2.
Table 1. Materials characteristics.
Sample
Si/Al molar ratio
Cu/Al molar ratio
Methanol yield,
CuMOR(6.5)
CuMOR(10)
CuMOR(46)
6.5
10
46
0.377
0.399
0.603
mol(MeOH)/
mol(Cu)
0.142
87
0.216
91
0.316
98
Aerobic
activation
protocol
Selectivity
towards
methanol, %
Methanol yield,
mol(MeOH)/
mol(Cu)
0.204
97
0.187
98
~0
Anaerobic
activation
protocol,
3rd cycle
Selectivity
towards
-
methanol, %
Quantitative assessment of the amount of methanol formed
over different samples (Table 1) points to the gradual increase
of the methanol yield per mole of copper with increase of Si/Al
ratio which is paralleled with different selectivity of methane
conversion over these samples. To address this point, in situ
infrared spectroscopy was used, which allowed observing the
surface species. The spectra obtained after the interaction of
methane at 473 K with oxygen-activated samples followed by
evacuation are given in Figures 2a, S4. The reaction with
methane led to appearance of three groups of bands attributed
to molecular methanol, carbon monoxide and surface methoxy
and formate species. IR intensities of the bands assigned to
δ(CH3) vibrations increase in the following order: CuMOR(46) <
CuMOR(10) < CuMOR(6.5), which is in line with the copper
content in these samples (Table 1). However, the intensities
ratio of the bands due to methanol overoxidation products, CO,
and formates to the bands of methanol increase in the same
order, indicating higher selectivity of the reaction in the case of
the CuMOR(46) sample. The quantitative analysis made by the
temperature-programmed desorption of carbon dioxide (Fig. S5)
after the reaction confirmed this, giving the highest selectivity of
98% to CuMOR(46), while for CuMOR(6.5) it amounted to only
87%.
Figure 1. Methanol desorption profiles from a) CuMOR(6.5), b) CuMOR(10)
and c) CuMOR(46) after the reaction with methane following aerobic (blue
lines) and anaerobic protocols measured by on-line mass-spectrometry (MS)
immediately after the introduction of water stream (2.6 vol% in He). The
reaction conditions: methane pressure 7 bars, 30 min contact time at 473 K.
The inserts represents corresponding MS response of molecular hydrogen.
Higher selectivity observed for CuMOR(46) may point to
different sites being present in the samples due to different
probabilities of formation of Al pairs in the structure of zeolite,
hence stabilizing the formation of copper CuxOy oligomeric
structures with x ≥ 2. To access the structure of copper sites,
While the spectra of adsorbed NO for CuMOR(6.5) and
CuMOR(10) are very similar, revealing the presence of identical
bands with close intensities, the CuMOR(46) sample shows
dramatic changes in the spectrum. In the region of Cu(II)
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species, only one intense band at 1908 cm-1, with a small
shoulder at 1950 cm-1 was observed, hence indicating the
dominant formation of copper monomeric species, in contrast to
the samples with Si/Al ratio higher than 10, where different
copper sites were detected (Figure 2b). Unfortunately, the
quantification of the amount of different copper species from the
IR data was not possible due to the unknown extinction
coefficients for each site.
In the next experiments, the reaction of different CuMOR
samples with different nature of copper species was carried out
using the anaerobic activation protocol in three consecutive
cycles (Figure 1). Being activated in a flow of He, CuMOR(6.5)
in the first cycle revealed methanol yield which is slightly lower
than that achieved for the oxygen-activated sample. However, in
the next two cycles, methanol yield increased by ~20%, which is
associated with the increase of selectivity due to avoiding
overoxidation (Fig. S11).[13] Shortly after water contact with the
methane reacted materials, hydrogen was observed in all three
cycles (insets in Figure 1).[13-15] Similarly, CuMOR(10) showed
As such, the difference in the nature of copper sites could lead
to different redox properties. In order to assess this, we used
temperature-programmed reaction of copper mordenite samples
with methane monitored by means of in situ XANES (Figures 2c, the presence of methanol and hydrogen during the cycling of
S7-S10).[31] It can be clearly seen that the reduction of all
samples starts at ~400 K and then progressively accelerates
with the rise of temperature. CuMOR(6.5) and CuMOR(10)
demonstrated similar redox properties, with almost full reduction
of Cu(II) into Cu(I) at 500 K. On the contrary, the reduction of
CuMOR(46) material showed a more gradual reduction until the
temperature of 600 K. This indicates lower reactivity of copper
monomeric species of CuMOR(46) towards methane.
the material in methane and water vapors. Treatment in oxygen-
free environment leads also to significant impact of
autoreduction process resulting in formation of additional
amount of Cu(I) as evidenced by FTIR of adsorbed NO (Figure
S12).
On the contrary, the CuMOR(46) sample demonstrated very
low methanol yield in any cycle when activated without oxygen.
No hydrogen was observed, indicating that this sample does not
undergo reoxidation with water. The rather small
methanol desorption peak could be attributed to
a
either
a small impact from residual copper
oligomeric species in CuMOR(46) sample or to
oxygen impurities in the flow of helium.
Therefore, it appears that monomeric copper
(II) species can interact with oxygen and
methane yielding methanol with high selectivity,
but the copper (I) species formed cannot interact
with water. On the contrary, copper-oxo
oligomeric sites exhibit high efficiency in the
synthesis of methanol from methane in both
aerobic and anaerobic protocols with satisfactory
yields and selectivity.
We have shown that over the CuMOR sample
with high Si/Al ratio neither the release of
hydrogen during interaction with water (Figure 1)
nor copper reoxidation (Figure S13) was
observed. Such a configuration of copper sites
does not allow water activation. Therefore, we
introduced the infrared spectroscopy of adsorbed
hydrogen to probe the affinity of copper sites in
different samples towards molecular H2. The
spectra given in Figure 2d showed the presence
of the bands at 4100, 4063 and 4031 cm-1. The
first band is assigned to hydrogen interacting
with Brønsted acid sites of the zeolite, while the
bands at lower frequencies are due to H2
adsorbed over the copper sites of the
materials.[32,33] The CuMOR(6.5) and CuMOR(10)
samples containing copper-oxo oligomeric species showed
intense bands for Cu sites interacting with hydrogen. This
observation is in line with the NO adsorption infrared
spectroscopy and the data on reactivity towards water. On the
contrary, CuMOR(46) shows the presence of a very low
intensity peak at 4031 cm-1 due to the copper oligomers, which
was also present in the spectra of adsorbed NO (Figure 2b),
indicating that molecular hydrogen cannot be stabilized over
copper monomeric species. This fact is in direct correlation with
Figure 2. a) in situ FTIR spectra of surface species formed after interaction of
methane with CuMOR samples; b) FTIR spectra of NO adsorbed over
CuMOR samples at 77K; c) results of linear combination fitting of XAS data
obtained in the temperature-programmed reaction of CuMOR materials with
methane at 1 bar; d) FTIR spectra of H2 adsorbed over CuMOR samples at
77K and 20 torr. All samples prior to the measurements were activated in
oxygen at 673 K for 1 h followed by evacuation at 673K for 20 min to remove
the traces of water.
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the data on methane oxidation in anaerobic conditions:
CuMOR(46) does not generate hydrogen and cannot undergo
reoxidation with water in any cycle (Figures 1, S13).
Figure 3 shows that the methane activation step is similar
for both the pair of monomers and the dimer, with the monomer
system exhibiting higher stabilization of the methoxy species
(214 kJ/mol vs 134 kJ/mol for the dimer). Such stabilization can
be explained by the larger number of hydrogen bonds formed in
the case of the monomeric system. Such an additional
stabilization makes it more energetically costly to remove the
products at the later steps of the reaction. Furthermore, the
larger distance between the two copper atoms in the case of
monomer system will necessarily lead to larger activation
barriers. Thus, the single water molecule cannot initiate the
reaction of copper reoxidation and simultaneous hydrogen
release. We model the formation of hydrogen with the
subsequent stabilization of the formed species with three
additional water molecules, according to the stabilization model
described in our previous work.[14] This is consistent with the
experimentally observed amount of water added to the system.
Thus, the formation of hydrogen involves the increase of ΔG in
the case of monomers, while the corresponding energy profile
for dimers rapidly goes down due to possible formation of very
stable bis(dihydroxy)dicopper sites. Although the energy of the
final water-stabilized species depends on the number of water
molecules, it is clearly much easier to stabilize the dimer
structure, following the release of hydrogen. Similar tendency
was observed for copper oxo trimers (Figure S19). This agrees
with the experimental evidence for the low yield of the reaction
products in the case of the samples with high Si/Al ratio.
The full reaction cycle corresponds to the net reaction of
CH4 + H2O → CH3OH + H2, and is completed only after the
removal of all water molecules, as strong adsorption of water
competes with the reaction of methane. This is achieved by
purging the CuMOR material in dry helium at 673 K. During
this process, fractions of water are removed from active
sites step by step, thus transforming a strong endothermal
effect of the net reaction (121 kJ/mol at 473 K) to a set of
thermodynamically feasible steps (Fig. S17 and Ref. 12),
helped by the strong entropy gain upon water desorption.
To conclude, our study shows that the Si/Al ratio can
To get a deeper insight into thermodynamic feasibility of the
CuMOR interaction with methane and reoxidation with water, we
carried out DFT calculations with copper sites modelling copper-
oxo species in Al poor and Al rich mordenite. We compared a
“conventional” mono(µ-oxo)dicopper species located in the 8-
member ring pore of mordenite with the system of two
monomers in the larger 12-member ring pore that can interact
with each other. It should be emphasized that the formation of
structured multicopper active sites is governed by the probability
of Al distribution, which might be especially restrictive for the
samples with higher Si/Al ratios. However, such structures are
statistically possible (see Supporting Information), with
probability greatly increased by the potential effects of dynamic
structural behavior.[21] In our previous work we have shown that
the presence of multiple copper atoms is essential for the two-
electron process of methane transformation into methanol and
subsequent copper reoxidation.[13] Whereas the presence of
isolated monomeric species leads to high energy barriers up to
175 kJ/mol and therefore renders the process energetically
unfeasible,[17] the presence of two copper atoms in a form of a
dimer facilitates the water-mediated release of methanol with
the barrier as low as 85 kJ/mol.[13] Such a substantial energetic
difference between the two systems is caused by the large
distance between the two copper atoms in the case of individual
monomeric species, making the formation of bridging species,
and, subsequently, the efficient proton transfer, impossible. We
therefore assumed the possibility of formation of the system of
be used to tune the configurations of the active sites in
copper zeolites, thus allowing direct control over the activity
of the materials in the conversion of methane to methanol.
The nature and redox properties of these sites can be
assessed by infrared spectroscopy of adsorbed nitric oxide
and hydrogen, as well as by X-ray absorption spectroscopy.
Monomeric and oligomeric copper species possess different
activity towards methane in both aerobic and anaerobic
pathways; and towards water under anaerobic conditions.
This difference is most probably associated with the
stabilization effect of the water molecules interacting with active
copper sites. Altogether, these data can serve as a basis for the
further improvement of existing and design of novel materials for
direct conversion of methane into methanol: for the anaerobic
methane conversion the materials must contain aggregated
copper oxo sites, while for aerobic conversion it is reasonable to
design the materials with, for instance, isolated copper
monomers.
two copper monomers in the same zeolite pore. While the 8-
member ring pore is too small to accommodate such a system,
the 12-member ring pore turned out to be of a suitable size
(Figures S14, 15).
Figure 3. Calculated reaction profiles and the corresponding intermediates of
the process of anaerobic oxidation of methane to methanol for the CuMOR
systems based on copper monomer (red lines) and dimer (blue lines). Zeolite
framework is omitted for the sake of clarity (Fig. S16). Full reaction cycle
leading to regeneration of original active centers is presented in Fig. S17.
Corresponding reaction profile for trimers is presented in Fig. S19.
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The authors acknowledge the use of the computing facilities of
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We thank the Energy System Integration platform of the Paul
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Keywords: methane to methanol conversion • copper zeolite •
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10.1002/anie.201802922
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Vitaly L. Sushkevich,* Dennis Palagin,
Jeroen A. van Bokhoven*
Page No. – Page No.
Effect of Active Sites Structure on
Activity of Copper Mordenite in
Aerobic and Anaerobic Conversion of
Methane to Methanol
Copper-exchanged zeolite mordenite with different Si/Al ratios was used to study a direct conversion of methane into methanol in
aerobic and anaerobic regimes. Al loading can be used to modulate the nature of the copper sites and the activity of the material.
This article is protected by copyright. All rights reserved.