Pathways of Methane Transformation over Copper-Exchanged Mordenite as Revealed by In Situ NMR and IR Spectroscopy

Article abstract of DOI:10.1002/anie.201912668

The reaction of methane with copper-exchanged mordenite with two different Si/Al ratios was studied by means of in situ NMR and infrared spectroscopies. The detection of NMR signals was shown to be possible with high sensitivity and resolution, despite the presence of a considerable number of paramagnetic Cu^II species. Several types of surface-bonded compounds were found after reaction, namely molecular methanol, methoxy species, dimethyl ether, mono- and bidentate formates, Cu^I monocarbonyl as well as carbon monoxide and dioxide, which were present in the gas phase. The relative fractions of these species are strongly influenced by the reaction temperature and the structure of the copper sites and is governed by the Si/Al ratio. While methoxy species bonded to Br?nsted acid sites, dimethyl ether and bidentate formate species are the main products over copper-exchange mordenite with a Si/Al ratio of 6; molecular methanol and monodentate formate species were observed mainly over the material with a Si/Al ratio of 46. These observations are important for understanding the methane partial oxidation mechanism and for the rational design of the active materials for this reaction.

Full text of DOI:10.1002/anie.201912668

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Accepted Article
Title: Pathways of Methane Transformation over Copper-Exchanged
Mordenite as Revealed by in situ NMR and IR Spectroscopy
Authors: Vitaly L. Sushkevich, René Verel, and Jeroen A. van
Bokhoven
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10.1002/anie.201912668
Angewandte Chemie International Edition
Pathways of Methane Transformation over Copper-Exchanged
Mordenite as Revealed by in situ NMR and IR Spectroscopy
Vitaly L. Sushkevicha,*, René Verelb, Jeroen A. van Bokhovena,b,*
a Laboratory for Catalysis and Sustainable Chemistry, Paul Scherrer Institut, 5232 Villigen PSI,
Switzerland
b Institute for Chemistry and Bioengineering, ETH Zurich, Vladimir-Prelog-Weg 1, 8093 Zurich,
Switzerland
Abstract
The reaction of methane with copper-exchanged mordenite with two different Si/Al ratios was
studied by means of in situ NMR and infrared spectroscopies. The detection of NMR signals was
shown to be possible with high sensitivity and resolution, despite the presence of a considerable
number of paramagnetic CuII species. Several types of surface-bonded compounds were found after
reaction, namely molecular methanol, methoxy species, dimethyl ether, mono- and bidentate formates,
CuI monocarbonyl as well as carbon monoxide and dioxide, which were present in the gas phase. The
relative fractions of these species are strongly influenced by the reaction temperature and the structure
of the copper sites and are governed by the Si/Al ratio. While methoxy species bonded to Brønsted acid
sites, dimethyl ether and bidentate formate species are the main products over copper-exchange
mordenite with a Si/Al ratio of 6; molecular methanol and monodentate formate species were observed
mainly over the material with a Si/Al ratio of 46. These observations are important for understanding
the methane partial oxidation mechanism and for the rational design of the active materials for this
reaction.
__________________________________________
*Corresponding authors: Tel.: +41563103518; E-mail addresses: vitaly.sushkevich@psi.ch ,
jeroen.vanbokhoven@chem.ethz.ch.
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1. Introduction
The search for novel pathways for the conversion of methane to valuable chemicals nowadays
attracts enormous interest from the industrial and scientific communities. [1-5] There is an abundance
of natural gas, mostly methane, and its price is relatively low, when calculated per carbon fraction.
However, methane is the least active of all the alkanes, and its selective transformation to the desired
product is challenging. The common areas of research include methane dry reforming, [6, 7] non-
oxidative aromatization [8, 9] and, more recently, direct oxidation to methanol [10-13]. The latter
direction has attracted considerable attention in the last decade after it was discovered that copper-
exchanged zeolites are able to activate methane. [14-17] To utilize this unique feature for practical
applications, a chemical looping approach was suggested, implying cyclic exposure of the material,
possessing redox properties, to an oxidant and to methane at different temperatures with the subsequent
extraction of the oxidation products, typically by using water. [14-19] The activation of copper-
containing materials is carried out at above 673K, while the reaction with methane and desorption of
methanol requires lower temperatures to avoid over-oxidation. [10-13]
Numerous copper-exchanged zeolites, including CuMFI [14-18], CuMOR [19-26], CuFAU [27],
CuMAZ [28-29] and CuCHA [30-34], were tested and revealed different methanol yields and
selectivity per cycle. The research targeting the achievement of the highest methanol productivity
allowed to obtain the methanol productivity close to the theoretical limit [35]. However, in most cases,
it does not exceed ~100-150 µmol/g, thus accounting for ~0.2-0.3 mol(MeOH)/mol(Cu) [21-25, 28, 29,
33]. Equally, significant attention was given to the elucidation of the nature of copper-oxo active sites,
hosted in different zeolites. Thus, PXRD, XAS, Raman and UV−vis data suggested that the formation
of copper monomers [30, 36], dimers in a form of bis-µ-oxo [14, 15] and mono-µ-oxo [16, 17, 20]
dicopper cores as well as trimers [21, 22] and copper oxide clusters [27, 32] is a possibility. The
formation of these sites is governed by the topology of the zeolite [33, 35, 37], Si/Al ratio [18, 26, 33,
+
35], copper loading [29] and the nature of the counter-cation (H+, Na+ or NH4 ) in the parent zeolite [22,
38]. During the reaction with methane, these sites undergo reduction, forming CuI species and methanol
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with established stoichiometry of 2×CuI per one methanol molecule as calculated for 100% selectivity
[29, 35, 39].
While copper transformation during activation in oxygen and reaction with methane has been
the topic of considerable research, very little attention has been paid to the fate of reaction products
originating from methane. Our first studies of this topic [23-25, 37] were based on in situ infrared
spectroscopy, which revealed the formation of several types of methoxy and formate species and
adsorbed carbon monoxide. Simultaneously, the formation of Brønsted acid sites was detected. [23]
Furthermore, Oord et al. confirmed the formation of methoxy species by means of near-IR
spectroscopy and detected ν(CH) overtones [40]. However, the nature, location and thermal stability of
the surface species, which formed during the interaction of oxygen-activated copper-exchanged zeolites
with methane, as well as the effect of the material composition and such characteristics as Si/Al ratio
and copper loading, have not yet been studied in detail.
Here we present a systematic spectroscopic study of copper-exchanged mordenite with two
different Si/Al ratios in the conversion of methane to methanol. Using in situ NMR and FTIR
spectroscopy, we identified for the first time that multiple types of methoxy species, corresponding to
molecular methanol, dimethyl ether (DME) and methoxy species bonded to Brønsted acid sites, form
during the oxidation of methane. Reaction at relatively low temperature leads to the formation of a
small amount of formaldehyde, while temperatures above 600 K lead to over-oxidation and formation
of carbon monoxide and carbon dioxide, which readily desorb to the gas phase. Importantly, the
samples with different Si/Al ratios, and, consequently, different structures of copper-oxo sites, react
differently and show different fractions of molecular methanol and strongly bonded methoxy species.
The study provides insight into methane oxidation over copper-exchanged zeolites at the molecular
level and provides important information for the further improvement of the process and active material.
2. Results and discussion
Table 1 and Figs S1 to S6 give the characteristics of the samples used in the present study.
Nitrogen adsorption-desorption data suggest a high micropore volume of the zeolites. CuMOR(6) and
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CuMOR(46) reveal reversible Type-I adsorption/desorption isotherms with a step at p/p0 < 0.01, typical
of microporous solids (Fig. S1). The powder XRD patterns of all the samples show typical features of
highly crystalline materials corresponding to a mordenite topology (Fig. S2). Reflections due to other
phases of crystalline impurity phases were not detected, thus excluding the presence of large copper
oxide particles. The nature of the CuII sites was also evaluated by infrared spectroscopy of adsorbed
nitrogen monoxide, the comprehensive analysis of which was discussed elsewhere [24, 41]. It shows
the preferential formation of copper-oxo monomeric species like (CuOH)+ for CuMOR(46), while in
the CuMOR(6) sample with a low Si/Al ratio, a mixture of copper sites with different nuclearity,
including copper-oxo dimers [20] was found (for the details, please see Supporting Information Figs
S3-S7). This is consistent with the low aluminum content of CuMOR(46), leading to the large
statistical distance between two neighboring Al atoms in the zeolite framework, hence disabling the
stabilization of copper oligomeric species. [42] Reactivity tests show the high activity of both
CuMOR(6) and CuMOR(46) samples in the methanol synthesis, achieving 0.14 and 0.3
µmol(MeOH)/mol(Cu), respectively (Table 1). The selectivity measured at 473 K is above 90%, in line
with previous studies [21, 23, 26]. Cycling the materials in five consecutive runs demonstrated stable
methanol yield and selectivity, hence indicating the absence of significant deactivation under the
selected conditions (Fig. S8).
First, the reaction of methane with CuMOR(6), activated in oxygen at 673 K to exclude the
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reduction with carbonaceous impurities [43], was studied at elevated temperature. H MAS NMR
spectra (Figs S9, S10) display the progressive development of the signal at 4.1 ppm due to the Brønsted
acid sites, in line with our previous study, confirming that the reaction of CuII species in the zeolite
pores with methane leads to the transfer of a hydrogen atom from CH4 to the framework as a proton
[23]. Simultaneously, the formation of CuI species is observed in Cu K edge XANES spectra acquired
during the temperature-programmed reaction with methane (Fig. S11). These observations indicates the
direct involvement of copper species to the reaction mechanism [26, 35, 39]. Fig. 1 shows 13C HPDEC
NMR spectra of reaction products as observed from 298 to 648 K. At ambient temperature, the only
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signal was the broad signal at ~-6 ppm, corresponding to methane. [44-47] The strong broadening is
associated with the presence of paramagnetic CuII species, which comprise most of the copper fraction
in the activated copper-exchanged mordenite. An increase in the reaction temperature leads to
narrowing of the methane signal due to the partial reduction of the paramagnetic CuII species to CuI
with methane. Simultaneously, the signal position drifts from -6 to -10 ppm, which might be associated
with different coordination and adsorption of methane over the surface of the material or different
strength of paramagnetic pseudo-contact shift. The latter might arise from the interaction of methane
with CuII sites leading to dipolar coupling between the magnetic moments of the 13C nuclei and of the
unpaired electron from CuII. [48] Starting at 448 K, a small signal appears at 58 ppm together with one
at 67 ppm at slightly higher temperature (498 K), suggesting the formation of new carbon-containing
species. The chemical shifts of these species are typical of compounds having C-O bonds and may
correspond to CH3O- fragments [49-55]. Further increase of the temperature leads to the development
of signal at 124 due to carbon dioxide. [51, 52] Moreover, the careful analysis of the spinning side
bands (Fig. S12) shows that another signal at 154 ppm having strong chemical shift anisotropy is
present in the spectrum. The position of this band corresponds to carbon monoxide, which also can
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form stable CuI carbonyls, possessing the signals around 175 ppm in C NMR spectrum. [53] Indeed,
the latter can be observed in the spectra giving very weak and broad signals at 174 and 178 ppm, and
can be only resolved by the analysis of spinning side bands appearing at 214, 218 and 253, 258 and 294,
298 ppm at 4 kHz MAS rate (for the details, please see Supporting Information, Fig. S12).
Simultaneously, the intensities of the peaks at 58 and 67 ppm gradually decrease, indicating the over-
oxidation of CH3O- species to carbon oxides. The increase of the intensity of the peak due to carbon
dioxide was not observed at high reaction temperatures. Most probably, this is associated with the
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adsorption of CO2 over zeolite having non-reacted CuII species, which suppress C signal (Fig. S13).
None of other products, including alkenes or aromatics, were found in the spectra.
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To access the structure and behavior of surface species, H-13C cross polarization spectra were
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collected (Fig. 2, Table S1). In contrast to C HPDEC spectra, there were no signals due to methane,
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carbon monoxide or carbon dioxide in the spectra. In the case of methane this suggests high mobility
and weak bonding to the surface of copper-exchanged mordenite, implying preferential presence in the
gas phase. The lack of carbon oxide signals in the CP/MAS spectra is associated with the absence of
hydrogen atoms in the vicinity of carbon atoms, which restricts polarization transfer. Notably, the peaks
within the range of 50 to 70 ppm persisted in the CP/MAS spectra, revealing the presence of at least
five resolved signals at 52, 58, 62, 64 and 67 ppm. The high intensity in CP/MAS suggests the presence
of multiple C-H bonds in these surface species, which, taken together with the chemical shift of the
peaks, enables the assignment to CH3O- fragments. Based on data reported previously and the
discussion of the transformation of methanol over zeolitic materials lead to the attribution of the signal
at 52 ppm to molecular methanol, which might bond weakly to the surface by H-bonding. [49-56] 2D
1H-13C heteronuclear correlation spectroscopy led to the assignment of the rest of the signals (Fig. 3).
The major cross peaks appear at (58.5; 4.15), (64.1; 4.11), (67.2; 3.83) and (67.2; 3.13) ppm,
confirming the attribution to the CH3O- fragments. The first peak is typically observed over most of
protonic forms of zeolites after adsorption and heating of methanol at elevated temperature and
corresponds to the methoxy species bonded to the Brønsted acid sites [49, 50, 54]. The presence of
multiple low-intensity cross-peaks at similar 13C chemical shift, but 1H chemical shift ranging from 4.8
to 5.8 ppm might be associated with the distortion of protons in methoxy species by the presence of
other adsorbed molecules, formed during the oxidation of methane, in the pores of zeolite, the
confinement effect of mordenite framework and presence of charged copper cations. Importantly, no
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resolved cross-peaks for 13C signal at 62 ppm were observed in 2D H-13C HETCOR, making it
difficult to attribute this signal with certainty. Literature data suggests that, during the transformation of
methanol over copper-free mordenite at elevated temperature, there are no observable signals near 62
ppm in the 13C NMR spectra [49, 50]. Equally, however, in copper-exchanged mordenite, the signal at
63 ppm was observed at 423 K, indicating the involvement of copper atoms in the coordination of these
methoxy species. [51]
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The signals at high frequencies are due to dimethyl ether formed from methanol via a
dehydration reaction [21], with the signal at (64.1; 4.11) characteristic of dimethyl ether adsorbed over
Brønsted acid sites, in side-on or end-on configuration. [51, 54] However, the presence of only one
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group of protons associated with this C signal suggests the equality of both CH3- groups in dimethyl
ether and, hence, enables the attribution of the peak at 64.1 ppm to the end-on adsorbed DME.
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Interestingly, the C peak at 67.2 ppm has two contributions from two different groups of protons at
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3.13 and 3.83 ppm in H NMR spectrum (Fig. 3). This clearly indicates the side-on coordination of
dimethyl ether, which implies the presence of two unequal CH3 groups. Furthermore, the shift to the
high frequency suggests the coordination of DME to the strong electron-accepting site, which might be
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a CuI site, formed after the reduction of CuII-oxo species. We therefore attribute the C signal to the
side-on adsorbed DME at the ion-exchange position occupied by a CuI cation. The same attribution was
proposed in the recent work by Zhou et al. [51]
With the exception of the formation of methoxy species and dimethyl ether, the heating of the
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sample to 498 to 548 K leads to the appearance of a new type of surface species with a C signal at
173 ppm (Fig. 2). This is the spectroscopic signature of formate species, which can form from methoxy
species or DME by further oxidation. Notably, this signal vanishes from the spectrum at 548 K, with
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simultaneous formation of carbon dioxide, as proven from the C HPDEC NMR spectrum (Fig. 1).
These observations suggest that the formate species are intermediates for carbon dioxide. The high
temperature leads to a decrease in the intensity of the signals due to methoxy species and DME.
Reaction at 598 K results in the disappearance of the signals at 64 and 67 ppm; further heating leads to
a decrease in the intensity of the peaks at 52 and 58 ppm, thus indicating gradual thermal
decomposition. Finally, at 648 K, there was only a small peak in the spectrum, due to the methoxy
species bonded to the Brønsted acid sites. As such, dimethyl ether possesses low stability over the
surface of copper-exchanged mordenite, and at temperatures as high as 598 K it undergoes oxidation to
the mixture of carbon oxides. In contrast, the methoxy species and molecularly adsorbed methanol
demonstrate high stability to over-oxidation at high temperature, probably due to the low mobility of
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these species and hence disabling their diffusion to and reaction over the CuII-oxo active sites. Other
products, such as aromatic or aliphatic compounds, were not observed at high temperature, which is
consistent with previous studies reporting the exclusive formation of methanol and carbon oxides
during methane oxidation over copper-exchanged mordenite. [14-35]
Due to the presence of a large amount of paramagnetic CuII in the activated sample, NMR
spectroscopy is not sufficiently sensitive to the low concentration of reaction products, formed at low
temperature when the conversion of CuII into CuI is low. In situ infrared spectroscopy was used to
analyze the initial reaction products that form at the lowest temperature. The reaction conditions and
experimental protocol for IR measurements were very similar to those used for the NMR measurements.
Fig. 4 shows the IR spectra of surface species formed during the reaction of CuMOR(6) with methane
at elevated temperature. Similar to the results of the NMR measurements, multiple bands,
corresponding to different reaction products, are visible. Analysis of the intensity of the bands at
increasing temperature enables assignment to five independent groups (Table 2, Fig. S14).
The bands belonging to the first and second groups appear in the spectra at the lowest
temperatures and are due to the stretching and deformation vibrations in the CH3O- fragments. These
bands are present in the spectra staring from low temperature and develop progressively up to 598 K.
Higher temperature leads to a decrease in the intensity; group (2) vanishes first. Comparing this
behavior with NMR spectra (Fig. 2) suggests that group (1) corresponds to methoxy species bonded to
the zeolite, while group (2) is associated with molecularly adsorbed methanol. This attribution is in line
with literature data, showing a blue shift of the C-H stretching vibrations in methoxy species with
respect to molecular methanol. [23, 57-59] The bands at 2159 cm-1 of group (3) are typical of carbon
monoxide adsorbed over CuI and formed by thermal decomposition of formate, as revealed by NMR
(Figs 1 and 2). The intensity of this band does not decrease, even at the highest temperature in these
experiments, indicating the formation of highly stable CuI(CO) monocarbonyl. [23, 60] Group (4),
together with the various C-H vibrations, shows the presence of the bands at 1696 and 1685 cm-1,
which are characteristic of carbonyl compounds having a C=O group. Taking the chemistry of the
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process into account, these bands may correspond to different forms of adsorbed formaldehyde, small
amounts of which form from methanol or methoxy species during oxidation at low temperature. [61, 62]
Formaldehyde is unstable at high temperature and decomposes rapidly, as visible in the IR spectra (Fig.
4). The last group (5) shows the superposition of several bands due to adsorbed water and asymmetric
vibrations of formate species. [23, 63, 64] Both are formed by partial over-oxidation of methanol or
methoxy species, formed from methane. The bands at 1619 and 1594 cm-1, present between 448 and
548 K, are due to bidentate formate species. The increase in the reaction temperature above 548 K leads
to the decomposition of formates and broadening of other signals, making the assignment challenging.
As well as the analysis of surface species, IR spectroscopy reveals the composition of the gas
phase as a function of the reaction temperature. Fig. 5 presents the spectra collected upon heating of the
CuMOR(6) sample in methane from 462 to 713 K. The bands at 2345 and 2144 cm-1 correspond to the
stretching vibrations in carbon dioxide and carbon monoxide, respectively, while the broad band with
fine structure, centered at 1300 cm-1, is due to gaseous methane. The reaction at low temperature up to
~500 K does not lead to the formation of over-oxidation products: the absence of gaseous products
suggests that the oxidation of formaldehyde observed in IR spectra (Fig. 4, Table S2) results in the
formation of species adsorbed on the surface, presumably formate or adsorbed carbon monoxide.
Progressive heating to 613 K leads to the appearance of the band due to carbon dioxide, the amount of
which stabilizes and does not change at higher temperature (Fig. 5, Table S2). Compared with the
behavior of surface formate species (Figs 2 and 4), the carbon dioxide that is released into the gas phase
originates from the oxidation of surface formate species, which are unstable at high temperature over
copper-exchanged mordenite. The formation of carbon dioxide from methoxy species or/and molecular
methanol is less likely due to the stable amount of carbon dioxide in the gas phase, while the methoxy
and methanol undergo progressive decomposition at temperatures above 600 K. Instead, at higher
temperatures, the band due to gaseous carbon monoxide appears and develops gradually in the spectra
(Fig. 5), which correlates with the decreasing intensity of the peaks due to methanol and methoxy
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species in the NMR and IR spectra (Figs 2 and 4). Taken together, this suggests that over-oxidation of
the latter at high temperature is complete and ceases at the stage of carbon monoxide formation.
Scheme 1 and Table 2 summarizes all these findings. The reaction of methane with activated
copper-exchanged mordenite leads to the formation of molecular methanol and dimethyl ether,
adsorbed on Brønsted acid sites, and methoxy species and carbon monoxide adsorbed over CuI sites. At
low reaction temperature (≤473 K) formaldehyde may form, together with formate species, the latter
being stable to ~550 K. Heating at higher temperatures results in the formation of carbon dioxide by
oxidation of formate species and of carbon monoxide by oxidation of methoxy species, methanol and
dimethyl ether.
Scheme 1. Methane transformation pathways to the reaction products over CuMOR(6), revealed
by NMR and IR spectroscopy.
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Next we elucidated the effect of the structure of copper-oxo sites on the methane oxidation
reaction. This required the study of the reaction over the CuMOR(46) sample, which predominantly
contains the CuII-hydroxo monomeric species. It was recently shown that monomeric species are active
in methane conversion and allows achieving higher selectivity with respect to that observed over the
CuMOR(6) sample, which contains active sites with multiple copper atoms (see the Supporting
Information). [24, 65] Similar results were reproduced for our CuMOR(46) material (Table 1). Fig. 6
13
shows the C HPDEC NMR spectra of the reaction products formed after the reaction of activated
CuMOR(46) with methane at elevated temperature. Compared to the spectra obtained for CuMOR(6),
the intensity of the signal at -7 ppm due to gaseous methane is considerably higher, which is accounted
for the smaller amount of paramagnetic copper loaded in the mordenite with a high Si/Al ratio.
Reacting the sample at a temperature as high as 473 K leads to the appearance of a small signal at 52
ppm due to molecular methanol. In contrast to what is observed over CuMOR(6) (Fig. 1), heating at
648 K does not lead to a significant decrease in this signal or to the appearance of new signals as a
result of over-oxidation. This indicates the high stability of the primary oxidation products over the
CuMOR(46) sample, probably due to the isolated nature of the active CuII-hydroxo species.
13C CPMAS NMR spectra of surface species acquired after reaction with methane (Fig. 7, S15)
shows three signals at 52, 58 and 62 ppm, corresponding to molecular methanol and two types of
methoxy species. Similar to CuMOR(6), the peak at 58 ppm is due to methoxy species bonded to
Brønsted acid sites, while the assignment of the signal at 62 ppm remains speculative; the signal is
probably associated with coordination of methoxy species to copper atoms. An increase in the reaction
temperature leads to a gradual increase in the intensity of all the signals, indicating an increase in
methane conversion and formation of molecular methanol rather than methoxy species in the studied
temperature range. There are no other observable intense signals, including those due to dimethyl ether
and formate species. The absence of DME as the reaction product might be associated with the low
concentration of aluminum, which leads to the low concentration of Brønsted acid sites required for
dehydration of methanol to dimethyl ether.
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Fig. 8 shows in situ infrared spectra collected during the reaction of the CuMOR(46) sample
with methane. Similar to that observed over CuMOR(6), the bands were associated with carbon
monoxide adsorbed over CuI (2159 cm-1) and adsorbed water (1628 cm-1). In the C-H stretching range,
there were no intense bands due to the methoxy species; the only intense bands at 2965(νas(CH3)), 2959
13
(νas(CH3)) and 2858 cm-1 (νs(CH3)) correspond to molecular methanol. This is in line with C NMR
spectroscopy, revealing the presence of mostly molecular methanol adsorbed after reaction over
CuMOR(46). Furthermore, a set of new bands, at 2921 (νas(HCO)), 2822 (νs(HCO)) and 1665
(νas(COO)) cm-1, starting from ~500 K, developed. The positions of these bands are typical of formate
species in monodentate coordination, which may form by over-oxidation of methanol and methoxy
species. Formation of formate species, in contrast to what was found for CuMOR(6), was associated
with the different nature of the copper-oxo sites in mordenite with a different Si/Al ratio. The intensity
of all the bands in the IR spectra increased from 448 to 648 K, indicating the high stability of adsorbed
methanol to over-oxidation as well as adsorbed carbon monoxide and formate species, which do not
decompose and desorb into the gas phase, as was observed for the CuMOR(6) sample (Figs 4 and 5).
No spectroscopic signatures of formaldehyde were detected in whole temperature range.
Scheme 2 summarizes the observations discussed above. Reaction of two isolated copper sites,
represented by CuOH+ species, leads to the formation of methoxy species, first coordinated to the
reduced CuI motif and Brønsted acid sites with coordinated water. The formation of the latter is also
confirmed by the FTIR spectroscopy, showing the appearance of the broad band at ~ 3400 cm-1 typical
for the H-bonded water (Fig. S16). Then, methoxy species can undergo hydrolysis with coordinated
water or other water molecules that form by over-oxidation of methane to form molecular methanol
bonded to bridged hydroxyl groups of zeolite.
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Scheme 2. Methane oxidation pathway over isolated copper-oxo species (a) and copper-oxo
dimers (b), stabilized in mordenite with a high and low Si/Al ratio, respectively.
In the copper-exchanged mordenite sample with a low Si/Al ratio, in addition to the copper-oxo
sites depicted in Scheme 2, the formation of copper-oxo oligomers, such as dicopper mono-μ-oxo
species, is possible [20, 26, 35]. The pathway, describing the reaction of such sites with methane is
given in Scheme 2b. Due to the absence of hydrogen atoms in mono-μ-oxo species, the reaction with
methane results in the formation of free Brønsted acid sites together with a reduced CuI motif (Scheme
2). Methoxy species can migrate to these Brønsted acid sites and form stable species, as observed by
NMR and IR spectroscopy.
The above schemes explain the dominant formation of molecular methanol over CuMOR(46)
with respect to CuMOR(6), where methoxy species are the main product. The reduction of two CuOH+
sites leads to the formation of one water molecule per one methoxy fragment. This water reacts with
methoxy species to give molecular methanol. In the case of the reaction of methane with mono-μ-oxo
species, no additional water is formed, hence stabilizing the methoxy species on Brønsted acid sites.
The only that is water formed by over-oxidation at high temperature and promotes hydrolysis, leading
to molecular methanol, a small amount of which is visible in the spectra starting from 548 K for
CuMOR(6) (Figs 2 and 4).
Further transformation of methoxy species and methanol implies the conversion to dimethyl
ether and over-oxidation to the formate species, CO and CO2. The formation of DME is pronounced
over the sample with a low Si/Al ratio, i.e. a high aluminum content, which forms Brønsted acid sites
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that catalyze the dehydration reaction of methanol. The sample with a high Si/Al ratio is more selective
to methanol, showing no significant signals due to carbon oxides in the NMR spectra (Figs 1 and 6).
The nature of formate species over copper-exchanged mordenites depends on the Si/Al ratio: For
CuMOR(6) mostly bidentate formate forms, while the reaction of CuMOR(46) with methane leads to
monodentate formate species. The reasons for this are presumably associated with the different
structure of the copper active sites in the studied materials.
3. Conclusions
Depending on the temperature, the reaction of methane with oxygen-activated copper-
exchanged mordenite leads to the formation of multiple surface species and gas phase products. The
Si/Al ratio of zeolite affects the structure of the copper species and is mainly responsible for the
formation of different products. At a temperature between 448 and 573 K the formation of molecular
methanol, methoxy species bonded to Brønsted acid sites and copper sites, as well as dimethyl ether
and formate species was detected. Dimethyl ether forms by dehydration of the methanol pathway,
while formate species are the unstable products of over-oxidation, which undergo thermal
decomposition to carbon dioxide. The reaction at higher temperatures from 573 to 648 K results in
severe over-oxidation of methoxy species, dimethyl ether and methanol to carbon monoxide.
The structure of the copper active sites, which can be modulated by changing the Si/Al ratio of
the parent zeolite, has a significant effect on the relative fraction and the nature of the reaction products.
Specifically, the reaction of CuMOR with s high Si/Al ratio, stabilizing the formation of isolated
copper-hydroxo sites in the zeolite pores, with methane, leads to the formation of mainly molecular
methanol. In contrast, over CuMOR with a low Si/Al ratio, methoxy species dominate. This is
associated with the contribution of the copper-oxo oligomers, such as dicopper mono-μ-oxo sites,
which react with methane, and does not lead to the formation of water, capable of methoxy hydrolysis.
Finally, NMR spectroscopy can be applied successfully to the study of oxygen-activated
copper-exchange zeolites, in spite of the presence of a large amount of paramagnetic CuII species. This
opens up new possibilities for studying the reaction mechanisms over this important class of materials.
14
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4. Supporting information
Nitrogen adsorption-desorption isotherms, XRD patterns, FTIR spectra of adsorbed nitrogen
monoxide and carbon dioxide, temperature-programmed reaction with methane followed by Cu K edge
1
1
XANES, H MAS NMR spectra, H-13C HETCOR of CuMOR(46), spinning side bands analysis, full-
range FTIR spectra after reaction with methane, table with NMR signals assignment.
5. Acknowledgements
V.L.S. and J.A.v.B thank the Energy System Integration platform of the Paul Scherrer Institute
for financial support.
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Table 1. Characteristics of studied copper-exchanged mordenites
Sample
Elemental composition
Vmicro, cm3/g
Methanol
yield,
Selectivity
towards
Si/Al ratio
Cu loading,
molMeOH/molCu methanol, %
wt%
4.36
1.22
CuMOR(6)
6.5
0.19
0.18
0.14
0.30
92
98
CuMOR(46)
46.1
Table 2. Assignment of IR bands observed during the reaction of copper-exchanged mordenites with
methane at elevated temperature. Mind that the deconvolution of the bands at 1458 and 1469 cm-1 due
to the deformation vibrations of CH3 group in methanol and methoxy species is complicated to and the
therefore the assignment was based on the literature data.
Group Assignment
IR band frequencies, cm-1
1
Methoxy species bonded to
2980 (νas(CH3)), 2871 (νs(CH3)), 1458 (δ(CH3))
Brønsted acid sites
Molecularly adsorbed
methanol
2
2967 (νas(CH3)), 2856 (νs(CH3)), 1469 (δ(CH3))
3
4
CuI monocarbonyl
Adsorbed formaldehyde
2159 (ν(CO))
2942 (νas(CH3)), 2836 (νs(CH3)), 1696 (ν(C=O)), 1685
(ν(C=O)), 1491 (δ(CH3))
5
Overlapping bands from
adsorbed water and formate
species
1632 (δ(H2O)), 1619 (νas(COO)), 1594 (νas(COO))
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Fig. 1. 13C HPDEC MAS NMR spectra of CuMOR(6) after the reaction with methane for 5 min at the
temperatures from 298 to 648K. Asterisks denote the broad contribution from NMR probe and spinning
side bands.
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Fig. 2. 13C CP/MAS NMR spectra of CuMOR(6) after the reaction with methane for 5 min at the
temperatures from 298 to 648K. Asterisks denote the spinning side bands.
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Fig. 3. 1H-13C HETCOR MAS NMR spectra of CuMOR(6) after the reaction with methane for 5 min at
523K.
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Fig. 4. IR spectra of surface species formed after the reaction of CuMOR(6) with 200 torr of methane
for 5 min at the temperatures from 298 to 648K.
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Fig. 5. IR spectra of gas phase acquired during the reaction of CuMOR(6) with 200 torr of methane at
elevated temperature.
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Fig. 6. 13C HPDEC MAS NMR spectra of CuMOR(46) after the reaction with methane for 5 min at the
temperatures from 298 to 648K. Asterisks denote the broad contribution from NMR probe and spinning
side bands, the wiggles around the signal at -7 ppm arise from the use of short acquisition times.
26
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Fig. 7. 13C CP/MAS NMR spectra of CuMOR(46) after the reaction with methane for 5 min at the
temperatures from 298 to 648K. Asterisks denote the spinning side bands.
27
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Fig. 8. IR spectra of surface species formed after the reaction of CuMOR(46) with 200 torr of methane
for 5 min at the temperatures from 298 to 648K.
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TOC Graphic
29
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