Methane to acetic acid over Cu-exchanged zeolites: Mechanistic insights from a site-specific carbonylation reaction

Article abstract of DOI:10.1021/ja5106927

The selective low temperature oxidation of methane is an attractive yet challenging pathway to convert abundant natural gas into value added chemicals. Copper-exchanged ZSM-5 and mordenite (MOR) zeolites have received attention due to their ability to oxidize methane into methanol using molecular oxygen. In this work, the conversion of methane into acetic acid is demonstrated using Cu-MOR by coupling oxidation with carbonylation reactions. The carbonylation reaction, known to occur predominantly in the 8-membered ring (8MR) pockets of MOR, is used as a site-specific probe to gain insight into important mechanistic differences existing between Cu-MOR and Cu-ZSM-5 during methane oxidation. For the tandem reaction sequence, Cu-MOR generated drastically higher amounts of acetic acid when compared to Cu-ZSM-5 (22 vs 4 μmol/g). Preferential titration with sodium showed a direct correlation between the number of acid sites in the 8MR pockets in MOR and acetic acid yield, indicating that methoxy species present in the MOR side pockets undergo carbonylation. Coupled spectroscopic and reactivity measurements were used to identify the genesis of the oxidation sites and to validate the migration of methoxy species from the oxidation site to the carbonylation site. Our results indicate that the Cu^II-O-Cu^II sites previously associated with methane oxidation in both Cu-MOR and Cu-ZSM-5 are oxidation active but carbonylation inactive. In turn, combined UV-vis and EPR spectroscopic studies showed that a novel Cu²⁺ site is formed at Cu/Al <0.2 in MOR. These sites oxidize methane and promote the migration of the product to a Bronsted acid site in the 8MR to undergo carbonylation.

Full text of DOI:10.1021/ja5106927

Article
pubs.acs.org/JACS
Methane to Acetic Acid over Cu-Exchanged Zeolites: Mechanistic
Insights from a Site-Specific Carbonylation Reaction
Karthik Narsimhan,† Vladimir K. Michaelis,‡ Guinevere Mathies,‡ William R. Gunther,†
Robert G. Griffin,‡ and Yuriy Roman-Leshkov*,†
́
†Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
‡Department of Chemistry and Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts
02139, United States
S
* Supporting Information
ABSTRACT: The selective low temperature oxidation of
methane is an attractive yet challenging pathway to convert
abundant natural gas into value added chemicals. Copper-
exchanged ZSM-5 and mordenite (MOR) zeolites have received
attention due to their ability to oxidize methane into methanol
using molecular oxygen. In this work, the conversion of methane
into acetic acid is demonstrated using Cu-MOR by coupling
oxidation with carbonylation reactions. The carbonylation
reaction, known to occur predominantly in the 8-membered
ring (8MR) pockets of MOR, is used as a site-specific probe to
gain insight into important mechanistic differences existing
between Cu-MOR and Cu-ZSM-5 during methane oxidation.
For the tandem reaction sequence, Cu-MOR generated drastically higher amounts of acetic acid when compared to Cu-ZSM-5
(22 vs 4 μmol/g). Preferential titration with sodium showed a direct correlation between the number of acid sites in the 8MR
pockets in MOR and acetic acid yield, indicating that methoxy species present in the MOR side pockets undergo carbonylation.
Coupled spectroscopic and reactivity measurements were used to identify the genesis of the oxidation sites and to validate the
migration of methoxy species from the oxidation site to the carbonylation site. Our results indicate that the CuII−O−CuII sites
previously associated with methane oxidation in both Cu-MOR and Cu-ZSM-5 are oxidation active but carbonylation inactive. In
turn, combined UV−vis and EPR spectroscopic studies showed that a novel Cu2+ site is formed at Cu/Al <0.2 in MOR. These
sites oxidize methane and promote the migration of the product to a Brønsted acid site in the 8MR to undergo carbonylation.
inorganic matrices. Most promising thus far are the iron-4 and
INTRODUCTION
■
copper-exchanged5 ZSM-5 zeolites, where FeIVO6 and CuII−
O−CuII species7 are believed to be key intermediates for the
selective oxidation of methane to methanol. Both reactive centers
oxidize methane into a surface bound methoxy group that is
extracted as methanol by reacting with water. The pioneering
work by Sels, Schoonheydt, and Solomon showed that methane
oxidation in copper-exchanged ZSM-5 (Cu-ZSM-5) occurs via
hydrogen abstraction by a bent mono-(μ-oxo) dicupric core to
form both a methyl radical and a CuI−OH−CuII intermediate,
which then combine through a rebound mechanism to form a
bound methoxy species on a copper center.7c Oxidation activity
is observed only for materials with Cu/Al molar ratios > 0.2,
which is the minimal metal content requirement to form the
mono-(μ-oxo) dicupric cores. To date, it is not clear if the methyl
radicals rebound onto the same reactive center to form a metal-
methoxy species,7c if they travel to another metal center,8 or if the
methoxy species readsorbs elsewhere on the zeolite framework.
The selective conversion of methane to liquid oxygenated
compounds, such as methanol or dimethyl ether, is an attractive
strategy for obtaining value-added chemicals from abundant
natural gas resources. Reforming natural gas followed by
Fischer−Tropsch synthesis is currently effective only at large
scales. In order to reach methane reserves at remote locations or
spread over large fields and to reduce unwanted emissions from
flaring, alternative conversion pathways must be implemented at
smaller scales.1 Low temperature oxidative routes for the
activation of C−H bonds are thermodynamically and kinetically
accessible. In nature, as in artificial systems, the key to selective
methane oxidation is the ability to generate reactive oxygen
species at metal active sites capable of attacking the strong C−H
bonds of methane while avoiding overoxidation into carbon
dioxide.
In enzymatic systems capable of converting methane into
methanol, such as methane monooxygenases, iron2 and copper3
can generate electrophilic metal−oxygen species adept at
attacking the strong C−H bonds of methane. Accordingly,
numerous methane oxidation schemes have focused on the
formation and reactivity of iron and copper−oxygen species in
Received: October 17, 2014
© XXXX American Chemical Society
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Scheme 1. Methane Oxidation and Carbonylation over Cu-MOR Exchanged from Acid or Sodium Precursors
In this work, the conversion of methane to acetic acid is
demonstrated in copper-exchanged mordenite (Cu-MOR) using
a tandem oxidation/carbonylation reaction sequence. Notably,
the carbonylation reaction serves as a site-specific probe capable
of identifying the location of methoxy groups within the zeolite,
thereby providing important mechanistic insight for methane
oxidation. Bhan et al. demonstrated that the carbonylation of
methoxy species generated from the interaction of methanol or
dimethyl ether with an acid site occurs preferentially in the eight-
membered rings (8MR) side pockets of MOR and ferrierite.9
MOR8,13 was not active for carbonylation. For samples prepared
from the acid form of the zeolite and with Cu/Al < 0.2, the
characterization data suggest the presence of an alternative
oxidation site in Cu-H-MOR that is responsible for methane
oxidation and subsequent methoxy migration.
RESULTS AND DISCUSSION
■
Reactivity Studies. Tandem methane oxidation−carbon-
ylation reaction sequences were investigated on Cu-MOR as a
function of copper and acid content. Methane oxidation over Cu-
MOR (Cu/Al = 0.36, Na/Al = 0.37) was first tested by activating
the zeolite at 823 K under O2 flow, cooling to 473 K under Ar,
and flowing methane for 0.5 h. Deuterium oxide extraction
yielded 12.3 μmol/gcat of methanol, which is in agreement with
previous extraction yields reported by Sels et al.5b (11 μmol
methanol/gcat from Cu-Na-MOR with Cu/Al = 0.43) and Alayon
et al.8 (13 μmol/gcat methanol from Cu-Na-MOR at Cu/Al =
0.38).
́
Roman−Leshkov et al. confirmed the same trend in carbon-
ylation reactivity by synthetically placing framework aluminum
sites within the 8MR pockets of ferrierite.10 Corma and co-
workers showed computationally that the carbonylation rates are
further enhanced by spectator copper(I) ions that facilitate CO
attack on surface methoxy species in the 8MR pockets.11 After
the oxidation/carbonylation sequence, we show that Cu-MOR
generates drastically higher amounts of acetic acid when
compared to Cu-ZSM-5 under identical conditions (vide
infra). We hypothesize that the oxidation-active copper species
are responsible of converting methane into methoxy species,
while the acid sites located in the 8MR pockets convert methoxy
species into acyl groups in the presence of CO (see Scheme 1).
Given that methoxy species bound onto copper sites are unlikely
to react with CO, we posit that the production of acetic acid
requires the migration of methoxy groups to a carbonylation
active Brønsted acid site. To support this hypothesis, we present
reactivity investigations using preferential titration and 13C- and
18O-labeled molecules coupled with spectroscopic and nuclear
magnetic resonance (NMR) characterization studies. Our results
show that, although some Cu sites in Cu-MOR are carbonylation
active, acetic acid is mainly obtained by migration of methoxy
groups into the 8MR pocket of H-MOR. Specifically, preferential
titration experiments indicated that the amount of acetic acid
produced was proportional to the number of Brønsted acid sites
in the 8MR pockets. NMR and mass spectroscopy (MS)
experiments using 13C- and 18O-labeled molecules confirmed
that the intermediate species for carbonylation was a methoxy
species localized on a Brønsted acid site. In addition, ultraviolet−
visible (UV−vis) spectroscopic studies on Cu-MOR showed that
the bent mono-(μ-oxo) dicupric species typically associated with
the oxidation-active sites of Cu-Na-ZSM-55b−d,7c,12 and Cu-Na-
Methane oxidation was coupled with carbonylation by flowing
CO at 1000 kPa and 473 K immediately after oxidizing methane
over the activated zeolite under strictly anhydrous conditions. In
Cu-MOR, carbonylation of surface methoxy species14 may occur
in three different locations: in acid sites located in the 8MR
pockets, in acid sites in the 12MR main channel, and in copper-
exchanged sites in both pore systems.14,15 Bhan and co-workers
demonstrated that methoxy species formed in the 8MR pockets
of H-MOR are the most carbonylation active,9a while those in the
main channel undergo carbonylation at much lower rates and
preferentially decompose into hydrocarbons above 473 K.9d
Importantly, carbonylation activity within Cu-MOR and H-
MOR can be modulated by partially exchanging acid sites with
sodium, which has been shown to preferentially titrate the
Brønsted acid sites in the 8MR pockets over those in the 12MR.9a
Following this approach, we investigated the carbonylation of
oxidized methane products as a function of Brønsted acid site
content at a constant Cu/Al ratio of 0.20−0.25 (Figure 1).
Deuterium oxide extraction showed that the parent Cu-MOR
(Cu/Al = 0.20 and H/Al = 0.5) generated 22.9 and 24.1 μmol/
g
cat of acetic acid and methanol, respectively (Figure 1). As the
acid sites in the 8MR pockets were progressively titrated from an
H/Al of 0.4 to 0.27, acetic acid production decreased from 18.5
to 12.3 μmol/gcat. These extraction levels persisted until almost
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products and from the HF dissolution method is nearly identical
for Cu-Na-MOR and slightly higher for Cu-H-MOR (1.40 vs
1.08). Since extraction in water appears to marginally favor acetic
acid, the relative amount of acetic acid is overestimated in the
deuterium oxide-extracted values and the differences in product
distribution observed for Cu-H-MOR and Cu-Na-MOR are
thereby caused by the lack of carbonylation activity in the sodium
sample and not because of selective extraction of methanol over
acetic acid. Taken together, these data suggest that the number of
active Cu sites is larger in Cu-H-MOR than in Cu-Na-MOR at
similar Cu/Al ratios, assuming a 1:1 stoichiometry between
active sites and products generated. The data are also in
agreement with previous reports by Bhan and co-workers
showing that for stop-flow carbonylation reactions of dimethyl
ether on partially sodium exchanged H-MOR, lower levels of
both acetic acid and total organic carbon are obtained when the
Na/Al ratio is increased (Figure S11).9a,14b
For comparison, control methane oxidation/carbonylation
reaction sequences were performed over Cu-ZSM-5. Although
carbonylation products were detected, the amount of acetic acid
in Cu-ZSM-5 was drastically lower when compared to that
obtained with Cu-H-MOR. Specifically, methane oxidation/
carbonylation on Cu-H-ZSM-5 generated only 4 μmol/gcat of
acetic acid out of 12 μmol/gcat of total oxygenates. Proportion-
ally, the methanol to acetic acid ratio produced by Cu-H-ZSM-5
was similar to that from a Cu-MOR zeolite without Brønsted acid
sites in the 8MR pockets (i.e., ca. 50% Na exchange or H/Al =
0.14), showing that acid sites in the 8MR pockets are necessary to
obtain high acetic acid yields. We note that Cu-Na-ZSM-5 failed
to produce acetic acid (Figure 2). Bhan and co-workers had
shown that the rate of dimethyl ether carbonylation in H-ZSM-5
was negligible compared to the rate in H-MOR due to the lack of
8MR side pockets to stabilize the acetyl intermediate.9a In batch
reactions of dimethyl ether carbonylation over H-ZSM-5, acetic
acid was produced but in very small amounts (0.02 mol acetic
acid/mol Al, Figure S12). On the basis of these results, we
Figure 1. Methane oxidation and subsequent carbonylation on Cu-H-
MOR at Cu/Al = 0.20−0.25. Methane oxidation conditions: Activation
at 823 K under O2, reaction T = 473 K, reaction time = 0.5 h.
Carbonylation conditions: T = 473 K, carbonylation time = 0.5 h, PCO
=
1000 kPa. [H+] was calculated from a propylamine desorption method
as described in previous studies (Supporting Information, Table S1).16
[Al3+] was calculated from elemental analysis using inductively coupled
plasma atomic emission spectroscopy (ICP-AES).
all of the Brønsted acid sites were titrated, at which point the
acetic acid production decreased to 6.0 μmol/gcat. These data
translate to an acetic acid production from copper sites, acid sites
in the 12MR, and acid sites in the 8MR of 6.0, 6.5, and 10.5
μmol/gcat, respectively. A total exchange of acid sites was not
achieved because, as observed by the groups of Bell,17 Hall,18 and
Gorte,16c Brønsted acid sites are always produced when copper is
exchanged into Na-exchanged zeolites. On the basis of the
observed dependence of carbonylation activity on Brønsted acid
site content within Cu-MOR with Cu/Al < 0.2, the acetic acid
yield would be expected to approach 6.0 in the absence of any
Brønsted acidity. Note that as the copper content in Cu-Na-
MOR was increased above Cu/Al of 0.33, the carbonylation
activity was completely quenched.
To directly measure the amount of organic carbon present in
the zeolite after reaction, Cu-MOR samples were dissolved in
hydrofluoric acid (HF) and the solution was analyzed with
1
quantitative H NMR. The total organic content from the
zeolites changed significantly as a function of Brønsted acid
content. The total amount of products on Cu-H-MOR and Cu-
Na-MOR was 60.9 and 22.0 μmolcarbon/gcat, respectively (Table
S7). The ratio of deuterium oxide−extracted methanol and acetic
acid to the total carbon content obtained via HF dissolution was
0.55 and 0.70 for Cu-H-MOR and Cu-Na-MOR, respectively.
Although the amount of deuterium oxide-extracted organics
from this Cu-H-MOR sample was slightly lower than that
calculated for a replicate experiment with the same zeolite (Table
S7, see Figure 5), the variability in extraction was within the
deviation previously found in Cu-ZSM-5.5b,c Overall, extraction
efficiencies and total carbon contents are in line with those
previously calculated for Fe−H-ZSM-519 and Cu-Na-ZSM-5
(Cu/Al = 0.58).5c The extraction efficiency could be increased by
utilizing other solvent combinations, such as 10% (v) water/
acetonitrile, but the overall change in extracted products would
be small.19 More importantly, we note that the ratio of acetic acid
to methanol calculated from the deuterium oxide-extracted
Figure 2. Methane oxidation and subsequent carbonylation on Cu-
ZSM-5 and Cu-MOR exchanged from sodium and acid precursors.
Zeolite precursors and Cu/Al contents are listed for each zeolite.
Methane oxidation conditions: Activation at 823 K under O2, reaction T
= 473 K, reaction time = 0.5 h. Carbonylation conditions: T = 473 K,
carbonylation time = 0.5 h, PCO = 1000 kPa.
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Figure 3. 13C[1H] cross-polarization (CP) MAS NMR spectra of Cu-MOR (H-MOR precursor, Cu/Al = 0.20) after 13CH4 oxidation (bottom
spectrum) and after 13CH4 oxidation and 13CO carbonylation (top spectrum). (Left) Full 13C[1H] CP MAS NMR spectra of Cu-H-MOR (Cu/Al =
0.20) after reaction with 13CH4 (bottom) and 13CH4 + 13CO (top). (Right) Enlarged spectral region containing methoxy resonances, with assignments.
Simulated Lorentzian peaks (solid) are shown below the experimental spectra (dashed).
surmise on the one hand, that the oxidation-active mono-(μ-oxo)
dicupric site typically formed in Cu-Na-ZSM5 under these
conditions is carbonylation inactive; and on the other, that the
methoxy species in Cu-H-ZSM-5 generated after the oxidation
step have either migrated to Brønsted acid sites or a different
copper site is formed in the presence of Brønsted acid sites that is
active for both oxidation and carbonylation.
methoxy species on Brønsted acid sites (55.9 ppm),11 and
methoxy species on a copper site (61.2 ppm, see Table S6, Figure
S7 for justification). After carbonylation with 13CO, the
resonance at 55.9 ppm decayed with the concomitant appearance
of resonances at 21.5 and 188.5 ppm, which are associated with
the methyl and carbonyl functional groups of acetic acid. A
similar trend was observed for Cu-H-MOR with a higher copper
content (Figure S4). In contrast, Cu-Na-MOR (Cu/Al = 0.36,
Na/Al = 0.33) only featured resonances at 53.4 and 61.2 ppm
after the methane oxidation step and did not feature resonances
associated with acetic acid after the carbonylation step (Figure
S5). These data offer strong evidence that the reaction
intermediate undergoing carbonylation in Cu-H-MOR is a
methoxy species on a Brønsted acid site.
Characterization Studies. The speciation of copper centers
in Cu-MOR with varying Cu/Al and H/Al ratios was investigated
with ultraviolet−visible (UV−vis) spectroscopy (Figure 4) for
samples that were treated first under Ar at 823 K, then under O2
at 823 K, and finally under methane at 473 K. Heat treating Cu-
H-MOR (Cu/Al = 0.2 and Na/Al = 0.03) under Ar generated a
broad peak centered at 13 300 cm−1 and a small shoulder at
16 700 cm−1 (Figure 4A). These bands have been previously
characterized as the d−d transitions of square pyramidal and
square planar Cu2+ species coordinated to the zeolite frame-
work.21 After the O2 treatment, the intensity of the 13 300 cm−1
and 16 700 cm−1 bands decreased to reveal a band at 12 500 cm−1
and a broad shoulder at 9600 cm−1. For both Cu-Na-MOR
samples, analogous visible-near-infrared (NIR) bands were
observed upon Ar and O2 heat treatments. The d−d transitions
at 13 600 cm−1 and 16 750 cm−1 featured reduced visible-NIR
intensities after calcination. In contrast to Cu-H-MOR, the Cu-
Na-MOR sample at Cu/Al = 0.36 featured a strong band at
22 200 cm−1 (Figure 4C), while both Cu-Na-MOR samples did
not possess the band at 12 500 cm−1 (Figure 4B−C).
Additionally, a very small shoulder at 9600 cm−1 was observed
in Cu-Na-MOR (Cu/Al = 0.22) compared to the large shoulder
in Cu-H-MOR.
To verify the origin of methoxy species undergoing carbon-
ylation, Cu-MOR and Cu-ZSM-5 were activated under 18O2
before reacting with methane (see Table S3). Previously, Sels et
al. had shown that 18O2 activation of Cu-Na-ZSM-5 followed by
methane oxidation generated methanol with 75% 18O enrich-
ment, thus demonstrating that the oxygen from the mono-(μ-
oxo) dicupric site became part of the methoxy group of
methanol.5b Analysis of water-extracted methanol from Cu-H-
ZSM-5 showed that enriched CH318OH constituted only 4.6%.
Similarly, the percentages of CH318OH extracted from Cu-H-
MOR and Cu-Na-MOR were 0.6 and 34%, respectively. In both
cases, these values show a large decrease in 18O content when
acid sites are present in the zeolite. To exclude the possibility of
isotopic scrambling by interaction of Me18OH with acid sites
during extraction, an aqueous CH318OH solution was mixed with
Cu-H-MOR (Cu/Al = 0.17) at room temperature for several
hours. The resulting solution contained 92.2% CH318OH (Table
S4), indicating that no significant 18O isotopic scrambling
occurred when mixing labeled methanol in the presence of acid
sites at conditions analogous to those of the water-extraction
process. These results are in agreement with previous
experimental and theoretical investigations showing the lack of
interaction of methanol with acid sites in zeolites at room
temperature.20 These data suggest that the oxygen in the
oxidation-active copper site is not incorporated into the extracted
methanol product when Brønsted acid sites are present.
The migration of the methoxy species from the copper sites to
acid sites after methane oxidation was investigated with 13C
magic-angle spinning nuclear magnetic resonance (MAS NMR).
As shown in Figure 3, three resonances are observed after
reacting 13CH4 on activated Cu-H-MOR (Cu/Al = 0.20, H/Al =
0.50), which are assigned to physisorbed methanol (53.4 ppm),
A different redox behavior was observed after reacting
methane over Cu-H-MOR or Cu-Na-MOR. For Cu-H-MOR,
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decayed after reaction with methane (Figure 4C). Additionally,
the 12 500 cm−1 band did not appear after calcination of either
Cu-Na-MOR zeolite, but it did appear in Cu-H-MOR. Reaction
with methane caused a very small decay in the shoulder at 9600
cm−1 in Cu-Na-MOR (Cu/Al = 0.22, Figure 4B), along with the
restoration of the 13 600 and 16 750 cm−1 bands in both Cu-Na-
MOR samples (Figure 4B−C). Thus, the appearance of the
methane oxidation active 9600 cm−1 band was unique to Cu-
MOR samples with at least trace amounts of Brønsted acidity and
carbonylation activity.
The exact nature of the methane oxidation active 9600 cm−1
band in Cu-H-MOR is still unknown. Sels and co-workers have
observed a similar shoulder at 9200 cm−1 in Cu-Na-MOR decay
upon reaction of calcined Cu-Na-MOR with methane,5b however
its appearance was inconsistent in Cu-MOR of different Si/Al
and Cu/Al ratios.5c For this band to be a d−d transition of the
square pyramidal or planar Cu2+ species absorbing at 13 300
cm−1 or 16 700 cm−1, two observations must be true. First, the
9600 cm−1 band should decay after thermal treatment in Ar for
any Cu-MOR sample regardless of Brønsted acidity since all Cu-
MOR samples contain both square pyramidal and planar species.
Second, the intensity of the d−d transitions should correlate to
the 9600 cm−1 band. However, as shown in Figure 4, the 9600
cm−1 band appeared prominently only in Cu-H-MOR and was
either absent in carbonylation inactive Cu-Na-MOR (Cu/Al =
0.36, Figure 4C, Figure 2) or was a small shoulder in Cu-Na-
MOR with trace Brønsted acidity (Figure 4A, Figure 1). Thus,
the 9600 cm−1 band appeared to form exclusively within Cu-
MOR samples that were active for carbonylation after methane
oxidation. Additionally, the 13 600 cm−1 and 16 700 cm−1 bands
gained intensity after thermal treatment in Ar, but the 9600 cm−1
band lost intensity (Figure 4A). Yet, the inverse trend was
observed upon calcination of Cu-H-MOR, showing that the 9600
cm−1 band could not represent the same Cu2+ species as the
other d−d transitions. Thus, we can exclude the possibility of the
9600 cm−1 band being an electronic transition of the 13 300 and
16 700 cm−1 d−d transitions and is likely associated with a
unique Cu center. While the 9600 cm−1 band was present in
carbonylation active Cu-Na-MOR (Cu/Al = 0.22 and Na/Al =
0.63), the surface methoxy groups formed were still highly
localized onto copper sites (Table S3). We note that the
percentage of CH318OH extracted from Cu-Na-MOR samples
with carbonylation inactive Cu/Al = 0.36 and carbonylation
active Cu/Al = 0.22 ratios was identical. Additionally, 13C[1H]
CP MAS NMR spectra showed no methoxy species on Brønsted
acid sites in Cu-Na-MOR (Cu/Al = 0.36). These data suggest
that the copper species represented by the 9600 cm−1 band in
Cu-H-MOR and Cu-Na-MOR (Cu/Al = 0.22) is both oxidation
and carbonylation active and it is much more readily formed in
the presence of Brønsted acidity.
Figure 4. Diffuse Reflectance UV−visible spectra of (A) Cu-H-MOR
(Cu/Al = 0.20, Na/Al = 0.03), (B) Cu-Na-MOR (Cu/Al = 0.22, Na/Al
= 0.63), and (C) Cu-Na-MOR (Cu/Al = 0.36, Na/Al = 0.37). Zeolites
were dried under argon at 823 K for 3 h (pink), calcined under oxygen at
823 K for 5 h (dashed gold), and reacted under CH4 at 473 K for 2 h
(navy). Difference spectra (calcined−argon, dash-dot orange; calcined−
CH4, dotted green) are shown.
The copper concentration range at which the 9600 cm−1 band
forms differs drastically from that required to form the mono-(μ-
oxo) dicupric species. As previously reported for Cu-Na-ZSM-5
and Cu-Na-MOR, the transition at 22 700 cm−1 and 30 000 cm−1
associated with the mono-(μ-oxo) dicupric site is only observed
for samples with Cu/Al > 0.20.5b−d,7c,8,12,21a,22 In Cu-Na-MOR,
the mono-(μ-oxo) dicupric species formed when the square
planar Cu2+ species was exchanged into the zeolite.5c,8,21a
Isolated square pyramidal Cu2+ species present at low copper
contents was inactive for methane oxidation, proved difficult to
reduce, and was shown not to form the mono-(μ-oxo) dicupric
site.21a In Cu-H-MOR samples with low Cu/Al ratios (ranging
from 0.17 to 0.10), the 9600 cm−1 band also decays after reaction
the d−d transitions at 9600 cm−1 and 12 500 cm−1 decayed while
the transitions at 13 300 cm−1 and 16 700 cm−1 were regenerated
(Figure 4A). Taken together, these spectra show how the sites
represented by the bands at 12 500 cm−1 and 9600 cm−1 in Cu-
H-MOR were created after high temperature activation under O2
and were consumed after reaction with methane. Unlike Cu-H-
MOR, the 22 200 cm−1 band in Cu-Na-MOR (Cu/Al = 0.36)
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with methane (Figure S9, A and B). At Cu/Al = 0.10, traces of the
d−d transition at 16 700 cm−1 were present, so it is possible that
square planar or pyramidal Cu2+ could be the precursors to the
methane oxidation active site. Interestingly, the yields of acetic
acid and methanol from Cu-H-MOR at Cu/Al = 0.17 were 22.6
and 26.0 μmol/gcat (Figure 5), respectively, which are values
a copper center and methane oxidation followed by migration of
methoxy species to a carbonylation-active Brønsted acid site. The
exact mechanism responsible for the migration of methoxy
species from the copper site is unknown and requires further
investigation. Previous methane oxidation studies over Cu-Na-
ZSM-5 and Cu-Na-MOR reported that methanol was not
evolved after flowing methane over the activated zeolite.5b Along
with DFT studies,7c the product after methane oxidation was a
methoxy species bound to the mono-(μ-oxo) dicupric site. This
was consistent with mechanisms of stoichiometric methane or
benzene oxidation over Fe-ZSM-5 resulting in (FeIII−
19,23
OCH3)α
or (FeIII−OPh)α groups.24 Recently, Panov and
co-workers25 proposed a quasicatalytic reaction mechanism over
Fe−H-ZSM-5 involving the surface diffusion of molecular
methanol from the FeIII−Oα active sites to the Brønsted acid
sites. Desorption of methanol into the gas phase was not
favorable under temperatures of 523 K; however, surface
diffusion had a lower activation energy that allowed methanol
to migrate within the zeolite at temperatures as low as 333 K.
Unfortunately, no explanation was provided as to how molecular
methanol was generated from surface methoxy species under
water-free reaction conditions. To gain insight into the nature of
the sites involved in the potential production of methanol from
methoxy species, we performed X-band continuous-wave
electron paramagnetic resonance (EPR) spectroscopy on Cu-
H-MOR and two Cu-Na-MOR samples that were dried under Ar
or calcined. The full-length spectra are shown in Figure S13. As
the values of gzz and Azz are the most informative for
identification of Cu2+ sites we show the low-field region of the
spectra in detail in Figure 6. The spectra of the two Cu-Na-MOR
samples (Cu/Al = 0.36, Na/Al = 0.37 vs Cu/Al = 0.22, Na/Al =
0.63), dried under Ar, are similar. Both spectra show features due
to at least two species: one species with gzz = 2.33 and Azz = 163 ×
10−4 cm−1 (referred to as site 2 hereafter, see Table 1) and a
second species to which we tentatively assign gzz = 2.27 and Azz =
183 × 10−4 cm−1 following Vandelderen et al.21a and Delabie et
al.21b (called site 3 hereafter). In the spectrum of Cu/Al = 0.22
(blue) the features of site 2 are more prominent than in the
spectrum of Cu/Al = 0.36 (black) and in addition this spectrum
shows an unidentified signal at 272.6 mT marked with an astrisk,
Figure 5. Methane oxidation and carbonylation on Cu-H-MOR with
varying copper content. Methane oxidation conditions: Activation at
823 K under O2, reaction T = 473 K, reaction time = 0.5 h.
Carbonylation conditions: T = 473 K, carbonylation time = 0.5 h, PCO
=
1000 kPa.
virtually identical to those observed for Cu-H-MOR with a Cu/
Al = 0.20. For samples with Cu/Al ratios of 0.25, the methanol
extracted from the zeolite increased to 35.5 μmol/gcat. At this
Cu/Al ratio, the mono-(μ-oxo) dicupric site should form within
Cu-MOR, implying that the mono-(μ-oxo) dicupric site is solely
responsible for the formation of the additional methanol and is
not associated with the formation of acetic acid. This observation
is consistent with the lack of carbonylation activity within Cu-Na-
ZSM-5 (Figure 2).
The reactivity and characterization data suggest the presence
of two coexisting pathways: methane oxidation/carbonylation at
Figure 6. EPR spectra (9.40 GHz) of (orange) Cu-H-MOR (Cu/Al = 0.20, Na/Al = 0.03), (black) Cu-Na-MOR (Cu/Al = 0.36, Na/Al = 0.37), and
(cyan) Cu-Na-MOR (Cu/Al = 0.22, Na/Al = 0.63). (Left) Cu-MOR thermally treated under Ar for 3 h at 823 K and (Right) Cu-MOR calcined under
O2 for 5 h at 823 K and purged under Ar before acquisition.
F
DOI: 10.1021/ja5106927
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Journal of the American Chemical Society
Article
Table 1. EPR Parameters for Cu2+ Species in Cu-MOR Treated Under Argon and Oxygen
Cu2+ site
gzz
Azz (× 10−4 cm−1
)
gxx = gyy
Axx (× 10−4 cm−1
)
d−d trans. (cm−1
)
geometry
1
2
3
2.37
2.33
2.27
149
163
183
−
2.06
−
18.2
12 500
13 600
16 750
sq. pyramidal
sq. planar
2.09
42.8
*. The spectrum of Cu-H-MOR (Cu/Al = 0.20, Na/Al = 0.03),
dried under Ar, clearly shows the features of site 2, and also a
small fraction of species 3 appears to be present. In addition the
spectrum shows a third species (called site 1) with gzz = 2.37 and
Azz = 149 × 10−4 cm−1. Upon calcination, the total signal of Cu2+
species reduces for all three zeolites. This was consistent with the
reduction of the 13 300 cm−1 and 16 700 cm−1 signals for square
pyramidal and planar Cu2+ species in Cu-H-MOR (Figure 4A)
and the 13 600 cm−1 and 16 750 cm−1 signals in Cu-Na-MOR
(Figure 4B−C). This signal loss in Figure 6 was attributed to the
formation of EPR silent copper species, which occurs in
significant quantities above Cu/Al of 0.20.5a,22 Multiple EPR
silent species could be responsible for methane oxidation activity
in Cu-H-MOR (Cu/Al = 0.20) and Cu-Na-MOR (Cu/Al =
0.22), so the lack of the 22 200 cm−1 band for the Cu−O−Cu site
in their UV−vis spectra (Figure 4A-B) may suggest alternate
copper active sites at low Cu/Al. After calcination, the relative
contribution of species 1 to the spectrum of Cu-H-MOR
increases. Species 2 is still clearly present, but the presence of
species 3 cannot be established with certainty. A similar effect is
seen in the spectra of the two calcined Cu-Na-MOR samples: the
presence of species 2 is obvious, whereas the fraction of species 3
has become very small. Other authors have observed species in
Cu-MOR with parameters similar to species 2 and 3. They are
generally associated with a square pyramidal and a square planar
Cu2+ site, respectively.7a,21,26 Sites with high g-values of 2.37−
2.40 have been observed in Cu-MOR27 and other zeolites that
were not thermally treated, in which case they were always
associated with hydrated Cu2+.28 For instance, electron spin echo
envelope modulation (ESEEM) spectroscopy attributed a gzz =
2.38 signal to a Cu2+ species coordinated to three water
molecules when the sample was evacuated at 323 K.29 In
addition, a hydrated Cu2+ site has been observed by X-ray
diffraction in Cu-MOR that was heated up to 783 K.30 Moreover,
the 12 500 cm−1 band we observe in calcined Cu-H-MOR
(Figure 4A) has been observed in untreated Cu-MOR and was
associated with hydrated Cu2+.21a It is tempting to speculate on
how a hydrated Cu species could be involved in the reaction
pathway. The presence of water molecules in close proximity to
the copper species would allow for surface methoxy species to
become hydrolyzed into molecular methanol, ultimately
enabling a reaction pathway involving surface methanol diffusion,
methoxy formation and subsequent carbonylation over the
Brønsted acid sites. While our data confirm the presence of site 1,
which has also been observed in Cu-H-MOR evacuated at 673
K,29 it is unclear if site 1 corresponds to a hydrated species. The
experiments were performed under strict anhydrous conditions
and it seems unlikely that that dehydration was incomplete for
our samples. Indeed, near-infrared (NIR) spectra on heat-treated
Cu-MOR do not show simultaneous bands around 5200 and ca.
7000 cm−1 corresponding to water molecules in the sample
(Figure S10). Evidently, more detailed investigations are needed
to understand the nature of the environment surrounding site 1.
CONCLUSIONS
■
Methane can be converted into acetic acid over copper
exchanged mordenite zeolites. Reactivity and spectroscopic
measurements on Cu-MOR with varying concentrations of
Brønsted acid sites revealed notable differences in the types of
Cu2+ species and product distributions from the tandem
oxidation/carbonylation sequences. In Cu-Na-MOR without
Brønsted acid sites, EPR and d−d transitions characteristic of
square pyramidal and planar species were present, along with the
characteristic 22 200 cm−1 band for the methane oxidation active
site previously identified for Cu-ZSM-5. Accordingly, these
materials showed methane oxidation activity but were carbon-
ylation inactive above Cu/Al = 0.30. In contrast, Cu-MOR
samples containing Brønsted acid sites were drastically more
carbonylation active, even at Cu/Al ratios <0.2. In these samples,
spectroscopic data shows the presence of a new Cu2+ site. After
reaction with methane, a unique band at 9600 cm−1 decayed,
suggesting the presence of a different methane oxidation active
site. The coupled methane oxidation and carbonylation reactions
showed that when trace amounts of Brønsted acid sites were
present, carbonylation activity was enhanced. Methane oxidation
over Cu-H-MOR has important consequences, since it generates
methoxy species that are located on the Brønsted acid sites of the
zeolite that can serve as intermediates in many other types of
reactions to create industrially relevant products.31 The detailed
characterization of the copper active sites and the migration of
the methoxy species is the focus of our current investigations.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details, 13C[1H] CP-MAS NMR spectra of surface
species on Cu-MOR, UV−visible−NIR spectroscopy of Cu-
MOR at low Cu/Al, control experiments with 18O2, dimethyl
ether carbonylation, full X-band EPR spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
yroman@mit.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Massachusetts Institute of
Technology Energy Initiative’s Seed Fund. The NMR used for
quantification at the Department of Chemistry’s Instrumentation
Facility was supported by the NSF (Award Nos. CHE-9808061
and DBI-9729592). We acknowledge the National Institutes of
Health for funding support of NMR and EPR at the Francis
Bitter Magnet Laboratory (EB-002026). V.K.M. is grateful to the
Natural Sciences and Engineering Research Council of Canada
and Government of Canada for a Banting Postdoctoral
Fellowship. G.M. gratefully acknowledges the Netherlands
Organization for Scientific Research (NWO) for a Rubicon
G
DOI: 10.1021/ja5106927
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Journal of the American Chemical Society
Article
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DOI: 10.1021/ja5106927
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