Spectroscopic definition of the copper active sites in mordenite: Selective methane oxidation

Article abstract of DOI:10.1021/jacs.5b02817

Two distinct [Cu-O-Cu]²⁺ sites with methane monooxygenase activity are identified in the zeolite Cu-MOR, emphasizing that this Cu-O-Cu active site geometry, having a ∠Cu-O-Cu ~140°, is particularly formed and stabilized in zeolite topologies. Whereas in ZSM-5 a similar [Cu-O-Cu]²⁺ active site is located in the intersection of the two 10 membered rings, Cu-MOR provides two distinct local structures, situated in the 8 membered ring windows of the side pockets. Despite their structural similarity, as ascertained by electronic absorption and resonance Raman spectroscopy, the two Cu-O-Cu active sites in Cu-MOR clearly show different kinetic behaviors in selective methane oxidation. This difference in reactivity is too large to be ascribed to subtle differences in the ground states of the Cu-O-Cu sites, indicating the zeolite lattice tunes their reactivity through second-sphere effects. The MOR lattice is therefore functionally analogous to the active site pocket of a metalloenzyme, demonstrating that both the active site and its framework environment contribute to and direct reactivity in transition metal ion-zeolites.

Full text of DOI:10.1021/jacs.5b02817

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Article
Spectroscopic definition of the copper active
sites in Mordenite : selective methane oxidation
Pieter Vanelderen, Benjamin E. R. Snyder, Ming-Li Tsai, Ryan G. Hadt, Julie Vancauwenbergh,
Olivier Coussens, Robert A. Schoonheydt, Bert F. Sels, and Edward I. Solomon
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b02817 • Publication Date (Web): 26 Apr 2015
Downloaded from http://pubs.acs.org on April 28, 2015
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Spectroscopic definition of the copper active sites in
Mordenite : selective methane oxidation
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Pieter Vanelderen,†,‡,ǁ Benjamin E.R. Snyder, †,ǁ Ming-Li Tsai,† Ryan G. Hadt,† Julie Vancauwenbergh,‡
Olivier Coussens,‡ Robert A. Schoonheydt,*, ‡ Bert F. Sels*,‡ and Edward I. Solomon*,†
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†Department of Chemistry, Stanford University, Stanford, California 94305, United States
‡Center for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, 3000 Leuven,
Belgium
Abstract
Two distinct [CuꢀOꢀCu]2+ sites with methane monooxygenase activity are identified in the zeolite Cuꢀ
MOR, emphasizing that this CuꢀOꢀCu active site geometry, having a ∠CuꢀOꢀCu ~140°, is particularly
formed and stabilized in zeolite topologies. Whereas in ZSMꢀ5 a similar [CuꢀOꢀCu]2+ active site is located
in the intersection of the two 10 membered rings, CuꢀMOR provides two distinct local structures, situated
in the 8 membered ring windows of the side pockets. Despite their structural similarity, as ascertained by
electronic absorption and resonance Raman spectroscopy, the two CuꢀOꢀCu active sites in CuꢀMOR
clearly show different kinetic behaviors in selective methane oxidation. This difference in reactivity is too
large to be ascribed to subtle differences in the ground states of the CuꢀOꢀCu sites, indicating the zeolite
lattice tunes their reactivity through secondꢀsphere effects. The MOR lattice is therefore functionally
analogous to the active site pocket of a metalloenzyme, demonstrating that both the active site and its
framework environment contribute to and direct reactivity in transition metal ionꢀzeolites.
1. Introduction
The selective oxidation of hydrocarbons to oxygenated products is an extremely challenging, yet
economically important reaction. It requires very selective catalysts with high activity at sufficiently low
temperatures to avoid excessive formation of CO and CO2. Methane is a particularly challenging substrate
due to its high CꢀH bond strength (104 kcal/mol), and only a limited number of natural and synthetic
systems are capable of catalyzing this reaction1–5. Soluble methane monooxygenase and particulate
methane monooxygenase (pMMO) are, respectively, Feꢀ and Cuꢀcontaining enzymes that selectively
convert methane to methanol at room temperature, thus fulfilling the conditions of high activity and
excellent selectivity at low temperature6,7. To date, the reactive intermediate of the copperꢀcontaining
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pMMO has not been identified, as its reactive state is difficult to generate and trap in solution7–11
.
Zeolites, on the other hand, are microporous materials that can stabilize exceptionally reactive, wellꢀ
defined Fe/O and Cu/O sites12,13
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Zeoliteꢀbound Fe/O and Cu/O sites that selectively oxidize methane to methanol have been
identified14–18. CuꢀZSMꢀ5 is one such system19–21. CuꢀZSMꢀ5 can oxidize methane to methanol at low
temperature (<200°C) using O2 from the atmosphere. The methanol produced remains chemisorbed
(presumably as methoxide22), and can be removed with an appropriate solvent or by steam treatment.
Woertink et al.20 determined that the active Cu site in CuꢀZSMꢀ5 was a new species in Cu/O
chemistry using resonance Raman (rR) spectroscopy – a siteꢀselective method that enabled the distinction
of the 5 mol% active site from 95 mol% spectator Cu. The reactive copper site is characterized by an
absorption band at 22700 cmꢀ1. Tuning a laser into this absorption feature resonantly enhances vibrations
associated with the active site. rR vibrations at 237 cmꢀ1, 456 cmꢀ1 and 870 cmꢀ1 were assigned as the Cuꢀ
OꢀCu bending mode, the symmetric Cuꢀoxo stretch (νs) and antiꢀsymmetric Cuꢀoxo stretch (νas),
respectively, of a bent mono(µꢀoxo)dicopper site (140° ∠CuOCu). Essential for this assignment was a
feature at 1725 cmꢀ1, which is the second quantum of the antiꢀsymmetric stretch. It is more intense than
the fundamental because it is symmetric, and therefore rR allowed. In analogy to previously characterized
FeꢀOꢀFe sites23, the manifestation of strong symmetric and weak antisymmetric stretches led to the
assignment of a bent CuꢀOꢀCu core in CuꢀZSMꢀ5.
The capacity of the ZSMꢀ5 lattice to host a [Cu2O]2+ core was evaluated with density functional
theory (DFT) calculations. The best description is a [Cu2O]2+ core bound to the Al Tꢀsites of an Alꢀ(SiO)2ꢀ
Al sequence within the 10 membered ring (MR) of ZSMꢀ5, with each Cu coordinated in a bidentate
fashion to the zeolite lattice. The calculated Ea for the reaction of this copper core towards methane is 18.5
kcal/mol, which is in reasonable agreement with the experimental value of 15.7 kcal/mol. Moreover, the
DFT study showed that the reactivity has its origin in the strength of the OꢀH bond produced, which
provides a strong driving force, and the polarization of a frontier MO towards a CuIꢀoxyl radical at the
transition state20.
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To date, the [Cu2O]2+ core of CuꢀZSMꢀ5 is one of only two characterized intermediates in the
methane to methanol reaction20,24. Its discovery has had a broad impact on the fields of heterogeneous
catalysis and bioꢀinorganic chemistry25–28. The specific ZSMꢀ5 structure is expected to be a critical factor
in the stabilization of the active [Cu2O]2+ site, but aspects of the ZSMꢀ5 topology that stabilize Cu2O cores
are not fully understood. Studying other zeolite topologies should enable identification of structures that
stabilize and activate Cuꢀoxo species. This would provide insight into the influence of the zeolite lattice on
active site geometry and reactivity.
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Recently, there is increased interest in CuꢀMOR, which is also capable of selectively oxidizing
methane to methanol19,29–32. Literature suggests a high density of active sites for CuꢀMOR29,31, and the
larger pores of the MOR framework are favorable for product desorption and transportation. Analogous to
CuꢀZSMꢀ5, O2ꢀactivated CuꢀMOR has an absorption feature in the 22000 cmꢀ1 region16. A similar active
copper site is expected, but not yet evidenced29. In a recent study, we examined the copper distribution in
relation to the redox chemistry and spectroscopic features of CuꢀMOR29. It was found that 60% of all
coppers, including the 22200 cmꢀ1 related species, undergo autoreduction ꢀ in good agreement with the
amount of copper that changes structure during methane reaction obtained from in situ XAS experiments
by van Bokhoven et al31. The same group also showed that the active species can be generated and
consumed in a batchꢀwise operation mode, with product release by steam treatment32.
Here, we provide definitive evidence of the formation of [Cu2O]2+ cores in CuꢀMOR.
Experimental insight is obtained from in situ diffuse reflectance UVꢀvis and rR data, which are presented
in combination with a CH4 reactivity study. These data are then compared to previous studies on CuꢀZSMꢀ
5. While the geometric and electronic structures of the [Cu2O]2+ cores in the two zeolites are highly
similar, their reactivity differs, indicating the participation of the zeolite lattice in the reaction coordinate.
This combined catalytic and spectroscopic study represents an important step forward in understanding
general principles governing the geometric and electronic structure and reactivity of zeoliteꢀbound
[Cu2O]2+ sites.
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2. Materials and methods
Sample preparation The parent sodium mordenite (Südꢀchemie; Si/Al=5) was subjected to an extra ion
exchange in a sodium nitrate (NaNO3) solution to ensure the mordenite was completely in its sodium form
prior to copper cation exchange. This ensures a maximized copper exchange and prevents side reactions
with bronstëd sites or lewis acid extraꢀframework aluminum sites33 created by dehydration of bronsted
sites during high temperature treatments. Ionꢀexchange was performed with an aqueous solution of Cu(II)ꢀ
acetate, (Cu(CH3CO2)2.H2O), at ambient temperature for 24h. The Cu2+ exchange was repeated three
times, with a washing step in between. After the third exchange, the samples were washed and dried at
80°C. The Cu and Al contents were determined by an ICP analysis. The Cu content was 0.71 mmol/g and
the Cu/Al ratio was 0.43. The samples were pelletized to obtain grain sizes between 0.25 and 0.50 mm
diameter.
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Sample activation The standard pretreatment consists of a calcination in O2 at 450°C for 2 hours (1
°C/min), followed by He treatment at 500°C overnight. The standard pretreatment is either followed by a
pure O2 treatment at 250°C overnight or by dosing N2O (5% in Helium) at 250°C for 5 minutes.
18O labelled active copper sites were achieved by five cycles of autoreduction in He at 500°C and
18O2 activation at 250°C. This method was applied to saturate the zeolite lattice with 18O atoms, since 18O
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isotope exchange of TMI oxygen sites with the lattice 16O atoms has been observed for TMI zeolites34,35
.
UV-vis-NIR and rRaman spectroscopy All treatments prior to in situ UVꢀvisꢀNIR rRaman
spectroscopic measurements were performed in a quartz Uꢀtube/flow cell. The latter was equipped with a
window for inꢀsitu UVꢀvisꢀNIR diffuse reflectance spectroscopy (DRS) and a side arm for inꢀsitu rRaman
measurements. After UV‐vis measurement, the catalyst pellets were mounted in the quartz sideꢀarm of
the U‐tube, ensuring that the conditions for UVꢀvisꢀNIR and resonance Raman measurements were
identical. It should be emphasized that all successive steps were done inꢀsitu and that samples never came
in contact with air.
DRS spectra were recorded on a Varian Cary 5000 UV–VIS–NIR spectrophotometer at room
temperature against a halon white reflectance standard in the range 4000–50000 cmꢀ1. Resonance Raman
(rRaman) spectra were recorded using a Spex 1877 CP triple monochromator with 1200, 1800, and 2400
grooves/mm holographic spectrograph gratings and an Andor Newton CCD cooled to ‐80°C. Excitation
was provided by either a Coherent I90C‐K Kr+ ion laser (λex = 407 nm, 488 nm, 588 nm) or an
Innova Sabre 25/7 Ar+ CW ion laser (λex=351nm, 457.9 nm). The spectral resolution was ~2 cmꢀ1. Spectra
were recorded at room temperature. Baseline spectra were collected from a sample of CuꢀMOR containing
no CH4ꢀreactive sites.
Operando UV-vis study Kinetic studies were conducted by means of a set up that allowed spectral
changes to be monitored during the reaction. The reactions were carried out in a fixed bed reactor. The
setup consists of a plug flow reactor (i.d.=8 mm) fitted inside a furnace, a UV–Vis light source (Top
Sensor Systems DH‐2000 deuterium halogen light source) and a photodiode array detector
(Ocean Optics SD 2000) connected to the catalyst bed via optical fiber technology (Top Sensor Systems
FCB‐UV400 ME cable and FCB‐UV400G‐0.1‐XHT high temperature probe). Each spectrum is the result
of the superposition of 300 scans in the 38000ꢀ12000 cm‐1 region, each 50 ms in duration.
0.3 g amount of Cu−MOR catalyst (with an average crystal size of 1 ꢁm) was pelletized to obtain
grain sizes between 0.25 and 0.50 mm diameter and loaded in a quartz reactor (with a reactor volume, 1
cm3). The catalyst was then exposed to a flow of pure oxygen (2 h at 450°C), followed by a He treatment
overnight at 500°C. The reactor was cooled to 250°C and contacted with a flow of N2O in He (50
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cm3/min; 5 vol %). Subsequently, the reactor was cooled to the reaction temperature, ranging from 50 to
200 °C, and contacted with a flow of CH4 in He (50 cm3/min; 10 vol %).
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Temperature programmed desorption A 0.2 g amount of Cu−MOR catalyst (with an average crystal
size of 1 ꢁm) was pelletized to obtain grain sizes between 0.25 and 0.50 mm diameter and loaded in a
quartz reactor (with a reactor volume, 1 cm3). CuꢀMOR was activated with O2 at 250°C (vide supra).
Subsequently the activated CuꢀMOR is heated (5°C/min) in a He flow (20ml/min). The gas flow at the
reactor outlet is analyzed with a quadrupole mass spectrometer. Mass 32 was monitored to measure the O2
release from the CuꢀMOR surface as a function of the temperature.
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3. Results and analysis
3.1 In situ diffuse reflectance UV-vis study
Catalyst activation CuꢀMOR (Si/Al=5) with an atomic Cu/Al ratio of 0.43 was prepared according to the
procedure described in the materials and methods section. After synthesis, the sample was calcined in O2
at 450°C for 2h, followed by a He treatment at 500°C overnight. The sample was then oxidized by either
O2 or N2O at 250°C, and subsequently reacted with methane at 150°C. Figure 1 shows the UVꢀvis
absorption spectra after O2 treatment and subsequent methane reaction. O2 treatment at 250°C results in a
maximized intensity of the 22200 cmꢀ1 absorption band. This band is related to the methane oxidizing
copper site as it disappears during the methane reaction at 150°C. It is also notable that a small absorption
feature around 6500 cmꢀ1 disappears together with the 22200 cmꢀ1 band. Similar spectral changes have
been reported for CuꢀZSMꢀ5. The same spectrum was obtained for the N2O treatment, see Figure S1.
Figure 1. CuꢀMOR sample after O2 activation at 250°C (green) and after subsequent methane reaction at 150°C
(red).
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Stability in oxidizing atmosphere The stability of sites contributing to the 22200 cmꢀ1 absorption feature
was assessed by monitoring the Abs feature while heating (5°C/min) a sample in an O2 flow to 450°C
after O2 activation at 250°C. Figure 2A shows the behavior of the 22200 cmꢀ1 band between 300°C and
450°C, the interval between the spectra is 30°C. A strong decrease of the absorption band was observed
above 330°C, until it stabilized around 420°C. The difference between the spectra at 300 and 450 °C has a
maximum intensity at 21900 cmꢀ1 (Figure 2B, red). After 10h at 450°C the O2 flow was switched to
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Figure 2. (A) Intensity decrease upon heating a CuꢀMOR sample in O2 flow to 450°C after O2 activation at 250°C,
the spectra are measured at respectively 300°C (red), 330°C (orange), 360°C (yellow), 390°C (green), 420°C (blue)
and 450°C (purple). The spectrum after complete autoreduction in helium flow at 450°C is shown in black; (B)
difference of two consecutive spectra during heating in O2 to 450°C (red) and the difference between the spectrum
measured in O2 at 450°C and He at 450°C (black).
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helium, resulting in the complete disappearance of the remaining component of the Abs feature. The
difference between the spectra before and after helium is shown in Figure 2B in black and has its
maximum at 23100 cmꢀ1. These observations suggest that different sites contribute to the 22200 cmꢀ1
feature. One component has an absorption maximum at 21900 cmꢀ1 and appears to be unstable above
330°C in an O2 atmosphere, in contrast to a second component that persists in O2 above 330°C and has an
absorption maximum at 23100 cmꢀ1.
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Interestingly, attempts to restore the 21900 cmꢀ1 absorption band by cooling the sample,
containing the 23100 cmꢀ1 species, in O2 from 450°C to 250°C were unsuccessful. The autoreduction step
in helium at 450°C is necessary to return the catalyst to its initial state, enabling formation of the full
absorption band with O2 at 250 °C. This indicates that the sites contributing to the 21900 cmꢀ1 band are
inactivated and not merely desorbed in the higher temperature oxidizing conditions.
Stability in inert atmosphere The thermal stability of the two species in an inert atmosphere was studied
by O2 temperature programmed desorption (TPD) after O2 activation at 250°C. UVꢀvis absorption spectra
were measured at different temperatures. The TPD profile, shown in Figure 3A, revealed two broad
partially overlapping O2 desorption peaks with maxima at 360°C and 390°C. The difference between the
Figure 3. (A) MS signal of O2 during TPD of an activated CuꢀMOR (red) and its fitted peaks (dashed line is the
sum), (B) Difference spectrum between absorption spectra at 310°C and 350° and (C) difference spectrum between
absorption spectra at 350°C and 390°C, deconvoluted with absorption maxima at 21900 cmꢀ1 and 23100 cmꢀ1.
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absorption spectra obtained after desorption at 310 °C and 350 °C; and at 350°C and 390°C are shown in
Figure 3B and C. The 22200 cmꢀ1 Abs feature is resolved into two bands with maxima at 21900 cmꢀ1 and
23100 cmꢀ1. The intensity decrease of the UVꢀvis band in the low temperature part of the first desorption
peak is largely due to loss of the 23100 cmꢀ1 component, as can be seen from the difference spectrum of
the UVꢀvis spectra at 310°C and 350°C (Figure 3B yellow). The 21900 cmꢀ1 band more significantly
contributes to the intensity decrease associated with the higher temperature desorption peak (Figure 3C
red). Thus, O2ꢀTPD confirmed the presence of two absorption bands, one at 23100 cmꢀ1 and one at 21900
cmꢀ1, derived from distinct active sites with slightly different stabilities towards O2 desorption.
Note that the species stable at high temperature in O2, i.e. the 23100 cmꢀ1 related site, desorbs at
low temperature during TPD. The relative stability of the active sites in an inert atmosphere thus differs
from that under oxidizing conditions. This is explained by the observation that the disappearance of the
21900 cmꢀ1 band in O2 is not a desorption process, but reflects an irreversible conversion of one copper
site (vide supra) and thus is not related to the TPD experiment.
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Kinetics of CH4 Reaction Differences in the geometric and electronic structure of the two active sites of
CuꢀMOR may lead to differences in reactivity toward CH4. To assess this possibility, differences in
reactivity were quantified by monitoring the disappearance of the 22200 cmꢀ1 absorption envelope during
CH4 reaction at different temperatures ranging from 90°C to 200°C. Loss of individual components of the
Abs feature (at 21900 and 23100 cmꢀ1) was monitored by a Gaussian deconvolution of the 22200 cmꢀ1
band envelope. The decay of the absorption bands as a function of time was evaluated with the first order
rate equation:
ln(Abs0 – Absf)/AbstꢀAbsf) = kt
Abs0, Absf, and Abst are the intensities of the absorption bands at the start, finish and time t of the reaction
respectively. The plot of ln(Abs0 – Absf)/AbstꢀAbsf) as a function of time is a straight line for which the
slope is the rate constant (k) at that particular temperature (Figure S3). The change of ln (k/T) as a
function of 1000/T was used to construct Eyring plots for both copper oxygen sites, shown in Figure 4 .
From these Eyring plots the following activation enthalpy ꢂH‡ and activation entropy ꢂS‡ were obtained:
•
•
21900 cmꢀ1 absorption related copper site: ꢂH‡= 14.7 ± 0.5 Kcal molꢀ1 and ꢂS‡ = ꢀ36 ± 1.5 cal
molꢀ1 Kꢀ1
23100 cmꢀ1 absorption related copper site: ꢂH‡= 11.1 ± 0.5 Kcal molꢀ1 and ꢂS‡ = ꢀ44 ± 1.5 cal
molꢀ1 Kꢀ1
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Figure 4. Eyring Kinetics of CH4 Reaction with Activated CuꢀMOR. Eyring plots derived from the decay of Cuꢀ
MOR’s deconvolution bands at 23100 cmꢀ1 (blue) and 21900 cmꢀ1 (red) during reaction with CH4. Error bars
indicate the 95% confident interval of the linear regression.
Based on the slopes of the Eyring plots, it is clear that the two sites have different activation enthalpies for
methane oxidation. It is noteworthy that the Eyring plots obtained for CuꢀMOR have a common
intersection point, with an isokinetic temperature at 160°C. An isokinetic temperature in an Eyring plot is
often explained in the literature by a compensation effect, whereby different enthalpies of activation are
compensated by differences in the entropies of activation. However, a number of other factors may lead to
isokinetic points, so this observation should be interpreted cautiously.
3.2 Resonance Raman
The 22000 cmꢀ1 absorption feature of activated CuꢀMOR is uniquely identified with the CH4
reactive sites. Tuning a laser into the 22000 cmꢀ1 absorption feature resonantly enhances a number of
vibrations (Figure 5) that are not present before activation, disappear upon reaction with CH4, and are
demonstrated to profile the 22000 cmꢀ1 Abs feature (Figure 6). These features are therefore correlated
with the sites that contribute to the 22000 cmꢀ1 Abs feature. O2ꢀ and N2Oꢀactivated samples yield the same
resonance Raman (rR) features, demonstrating the same active sites are formed upon activation with either
O2 or N2O (Figure S5).
The rR spectrum of activated CuꢀMOR strongly resembles that of activated CuꢀZSMꢀ5 (Figure 5).
As with CuꢀZSMꢀ5, isotope sensitive features are seen at 455 (ꢀ16O/18O=5ꢀ8) cmꢀ1, ~850 (ꢀ16O/18O=40)
cmꢀ1, 1700 (ꢀ16O/18O=70ꢀ80) cmꢀ1, and 1850 (ꢀ16O/18O=30) cmꢀ1 (Figure 7). While similar frequencies
and isotope shifts are observed for CuꢀZSMꢀ5, the MOR features are broad and asymmetric, suggesting
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Figure 5. Comparison of the resonance Raman spectra (λex=458 nm) of O2 activated CuꢀMOR and CuꢀZSMꢀ5.
more than one species is present. This limits the quantitative analysis of isotope shifts for CuꢀMOR.
The rR profile spectra of the 22000 cmꢀ1 Abs feature of activated CuꢀMOR confirm the presence
of multiple active sites. The 22000 cmꢀ1 Abs feature contains two components at 21900 cmꢀ1 and 23100
cmꢀ1 (vide supra). Tuning a laser into the highꢀenergy shoulder of the Abs feature is therefore expected to
more strongly enhance vibrations associated with the 23100 cmꢀ1 component. Conversely, tuning a laser
into the low energy shoulder is expected to more strongly enhance vibrations associated with the 21900
cmꢀ1 component. This behavior is observed experimentally: the shape of each rR feature identified above
Figure 6. Resonance Raman spectra of N2Oꢀactivated CuꢀMOR collected at various excitation energies (left). Note
the shift in relative intensity of the features between 825ꢀ900 cmꢀ1 (left). The different laser excitation lines (in nm)
are indicated on the absorption spectrum of an O2 activated MOR (right).
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Figure 7. Resonance Raman spectra (λex=458 nm) of CuꢀMOR activated with 18O2 (red) and subsequent 16O2
activation (green).
is sensitive to excitation energy. Consider the feature at ~850 cmꢀ1 (Figure 6, inset). At low excitation
energies, the lower frequency component is most prominent. The relative intensity of this component
decreases with increasing excitation energy, however. Similar behavior is observed for the prominent
asymmetric features at 460 cmꢀ1 and 530 cm.ꢀ1, indicating each of these features is a also a composite.
The profiling behavior and asymmetry of the rR features of activated CuꢀMOR indicate two Cu/O
active sites are formed in CuꢀMOR. Each rR feature is therefore a composite of vibrations from separate
sites. Features of the individual sites could not be accurately distinguished based on the differential
enhancement observed during profiling studies. Alternatively, they could be distinguished by differential
reactivity of the sites with CH4. Kinetics data (Figure 4) indicate the site associated with the 21900 cmꢀ1
absorption feature reacts ~60% more rapidly with CH4 at 200°C than the site associated with the 23100
cmꢀ1 absorption feature. A sample of activated CuꢀMOR was therefore exposed to a pulse of CH4 (contact
time: 20 s) at 200°C, and rR data collected before and after partial CH4 reaction were compared (Figure
8). After baseline correcting these spectra against a fully CH4ꢀreacted sample, the rR features of the
individual MOR sites were distinguished through spectral subtraction. A constant multiple of the partially
reacted spectrum was subtracted from the unreacted spectrum. The constant was increased systematically
until only one set of features dominated. A small contribution of the subtracted species was kept to
prevent overꢀsubtraction. The process of spectra subtraction is expected to introduce uncertainties on the
order of ±5 cmꢀ1 to the frequencies reported in the difference spectra shown below in Figure 8. The
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Figure 8. (A) Resonance Raman spectra (λex=458 nm) of activated CuꢀMOR during reaction with CH4 at 200 °C.
Note the relative decrease of the 850 cmꢀ1 vibration after partial reaction. (B) rR spectra of individual MOR sites 1
(green) and 2 (yellow) generated by subtraction of spectra from activated and partially reacted samples (λex=458
nm).
difference spectra enable distinction of rR features of the individual [Cu2O]2+ sites in MOR. The
prominent symmetric stretch at 465 cmꢀ1 is correlated to the antisymmetric stretch at 850 cmꢀ1 (related to
the 21900 cmꢀ1 absorption band) and the symmetric stretch at 450 cmꢀ1 to the antisymmetric stretch at 870
cmꢀ1 (related to the 23100 cmꢀ1 absorption band).
The CH4 reactivity and characteristic spectroscopic features of the individual CuꢀMOR sites have
thus been identified, demonstrating that activated CuꢀMOR contains two distinct monoꢀoxoꢀbridged Cu
dimers. The 21900 and 23100 cmꢀ1 components of the Abs feature of CuꢀMOR are therefore assigned as
oxoꢀ>Cu(II) ligandꢀtoꢀmetal charge transfers (LMCTs), and the rR vibrations of the individual MOR sites
Table 1. Summary of rR features and NCA of ZSMꢀ5 and MOR sites.
ZSM-5
MOR 1
MOR 2
450
vsym (cm-1)
456
465
vasym (cm-1)
870
850
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<CuOCu (deg)
Cu-Ooxo FC (mdyn/Å)
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Cu-Ooxo/ Cu-Ooxo
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interaction FC: kMOM (mdyn/Å)
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are assigned in analogy to our past study of CuꢀZSMꢀ520. The frequencies and isotope sensitivities of νsym
and νasym were related to the bridge angle and CuꢀOoxo bond strengths of each [Cu2O]2+ core (table 1) using
a normal coordinate analysis (NCA) as given by the equations of Wing and Callahan for a triatomic MꢀOꢀ
M unit with C2v symmetry36. The NCA indicates that the [Cu2O]2+ cores of CuꢀMOR and CuꢀZSMꢀ5 are
comparable, yielding highly similar estimates of CuꢀO bond strengths and bridge angles for the three sites.
The subtle differences in νsym/νas for MOR 1 versus MOR 2 reflect limited differences in ∠CuOCu and
CuꢀO force constant. This analysis is robust. While spectral subtraction introduces uncertainties of up to
±5 cmꢀ1 for vsym and vasym, this has a minimal impact on the structural parameters estimated by this NCA
(e.g. ±1 deg for <CuOCu and ±0.1 mdyne/A for the CuꢀO force constant (FC)).
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4. Discussion
Previous studies of activated CuꢀZSMꢀ5 identified characteristic spectral features of a bent
[Cu2O]2+ site that exhibits selective, low temperature Hꢀatom abstraction (HAA) reactivity with CH4. The
mono(µꢀoxo) dicopper site in ZSMꢀ5 was, until now, the only characterized Cu/O site capable of selective
methane oxidation20,25. It has now been demonstrated that [Cu2O]2+ cores are also responsible for the lowꢀ
temperature, selective CH4 reactivity of activated CuꢀMOR. Unlike ZSMꢀ5, however, the MOR topology
stabilizes two distinct [Cu2O]2+ sites. These sites exhibit characteristic spectral features and reactivities
similar to ZSMꢀ5. The intense absorption feature of activated CuꢀMOR (around 22000 cmꢀ1) is assigned
as a composite of two oxoꢀ>Cu(II) LMCTs, each associated with a specific site. Tuning a laser into this
absorption feature resonantly enhances vibrations characteristic of bent [Cu2O]2+ sites: an intense
symmetric stretching mode of the bridging oxo ligand at 440ꢀ470 (delta = 5ꢀ10) cmꢀ1, and the
corresponding weak antisymmetric stretch between 850ꢀ870 (delta = 40) cmꢀ1. NCA of the frequencies of
these modes indicate the bond strengths and CuꢀOꢀCu bridge angles for the MOR and ZSMꢀ5 sites are
similar. Similarities between the spectral features and reactivity of the MORꢀ and ZSMꢀ5ꢀbound [Cu2O]2+
sites indicate that they have comparable geometric and electronic structures. These are summarized in
Table 2.
That these Cu2O cores have very similar geometric and electronic structures is interesting given
the different topologies of the MOR and ZSMꢀ5 lattices. Though a larger CuꢀOꢀCu angle might be
expected from the larger curvature of the 12 MR in MOR compared to the 10 MR in ZSMꢀ5, the apparent
tendency to solely form the observed geometry implies that a particular structural entity, present in both
zeolites, uniquely forms [Cu2O]2+ cores. This is consistent with our recent DFT study on CuꢀZSMꢀ537,
where it was found that, from all the possible 2CuI precursor sites in the 10 MR, there is one that is
particularly stable. Moreover, the CuꢀOꢀCu site derived from this 2CuI had the best fit with the
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experimental spectral features. The 2CuI precursor site is shown in Figure 9. In the same study the CuꢀOꢀ
Cu site formation from the 2CuI precursor with N2O was evaluated. N2O, instead of O2, was used since the
generation of the CuꢀOꢀCu core then involves only one O atom and only two electrons. Only a sufficiently
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Table 2. Summary of spectroscopic features and CH4 reactivities of ZSMꢀ5 and MORꢀbound [Cu2O]2+ sites.
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ZSM-5
390 °C
stable
22700
MOR 1
385 °C
unstable
21900
MOR 2
355 °C
stable
23100
O2 TPD peak
Stable in O2 at 450°C
Absorption feature (cm-1)
CH4 ΔH‡ (kcal/mol)
CH4 ΔS‡ (cal/mol K)
CH4 ΔG‡ @ 200 °C
15.7
-35
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vasym (cm-1)
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Cu-O force constant (mdyne/Å)
Figure 9. Structural model for the binuclear CuI precursor (in yellow) of the CuꢀOꢀCu site in ZSMꢀ537. Si atoms are
in gray and oxygen atoms in red.
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short CuꢀCu distance (<4.2 Å) will allow N2O bridging, enabling 2 electron activation to form the Cu2O
core with a low barrier (2 kcal/mol). The CuI centers prefer bidentate coordination with a close to linear
OlatꢀCuꢀOlat bite angle. The two coordinate CuI geometry with 150° bite angles is provided by the ZSMꢀ5
lattice by binding a CuI ion to a lattice oxygen ligand from an Al T site and the other from a Si T site.
These lattice oxygens therefore are separated by the appropriate distance by the curvature of the 10MR.
A comparison of the crystal structures of MOR and the MFI topology of ZSMꢀ5 reveals key
similarities between local structures where the 2CuI precursor site can be formed. The mordenite structure
is built from the same 5ꢀ1 secondary building38 unit as ZSMꢀ5; both framework structures are shown in
Figure 10. The MOR structure contains straight 12 MR channels (7 X 6.5 Å), with parallel compressed 8
MR channels (5.7 X 2.6 Å). The 12 MR and 8 MR channels are connected by another 8MR channel,
creating the soꢀcalled side pockets with 8 MR windows (3.4 X 4.8 Å). Two neighboring 12 MR channels
do not have a direct connection and the side pockets are staggered at the intersection with the 8 MR
channels. As a result of this structural organization, diffusion of molecules only takes place in one
dimension, i.e. along the 12MR channel. MOR is therefore considered as a 1D zeolite. The mordenite unit
cell has four distinct tetrahedral sites: T1 in the 12 membered ring (MR) channel, T2 and T4 at the
intersection of the 12MR channel with the sideꢀ pocket and T3 at the intersection of the 8MR channel with
the side pocket38. ZSMꢀ5 has a 3D pore system that consists of straight 10 MR channels (5.1 X 5.5 Å),
connected by sinusoidal 10 MR channels (5.3 X 5.6 Å). Calcined ZSMꢀ5 has 24 distinct tetrahedral sites
in its asymmetric unit38.
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There are two positions in the 12MR mordenite channel with a conformation of lattice O atoms
similar to the local environment of the CuI ions in ZSMꢀ5 shown in Figure 9. This is illustrated in Figure
S6 by placing CuI ions (in yellow) with bite angles of 150° at these two positions. Lattice O atoms from
other nearest neighbor Tꢀsites are about 0.3 Å too far from each other to achieve similar bite angles. One
Figure 10. Framework structures of mordenite (left) and ZSMꢀ5 (right). Si atoms are in grey and O atoms are in red.
The four nonꢀequivalent tetrahedral sites of mordenite are indicated on the structure. The blue arrow indicates the
sinusoidal 10 MR channel in ZSMꢀ5.
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CuI position is at a T4 site (indicated in green) and, as illustrated in Figure 11, placing a second
symmetrically equivalent CuI ion opposite to it in the side pocket results in a CuI pair coordinated to two
T4 sites (indicated in green) separated by two T2 sites. These two T4 atoms are separated by 7.54 A,
which is equivalent with the local structure around the active site in activated CuꢀZSMꢀ5 (7.55 A), as
shown in Figure 11 (bottom right). The striking resemblance with ZSMꢀ5 makes the side pocket at the
intersection with the 12MR a promising candidate to host one CuꢀOꢀCu site. Note that a highly similar
configuration of T sites is found at the intersection of the side pocket with the 8MR, viz two T3 sites
(Figure 11 in blue) separated by two T1 sites. Several binding modes of CuI in the confined space of the
compressed 8 MR channel can be envisioned. One possibility, analogous to that presented in Figure 11, is
shown in Figure S7.
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Figure 11. (top) Local structures of the MOR lattice at the intersection of the side-pocket with the 12 MR and 8 MR
channels. T4 and T3 atoms of a possible Al-Si-Si-Al configurations for Cu-O-Cu coordination are indicated
respectively in green and blue. Pair of CuI atoms (yellow) with bite angles of 150° are placed at two T4 sites.
Oxygen atoms at the outside of the 8MR and 12MR are omitted for clarity. (bottom) Comparison the local structure
stabilizing the Cu-O-Cu site in ZSM-5 with the T4-T2-T2-T4 and T3-T2-T2-T3 sequences in MOR. The distance
between two T atoms with hypothetical Al substitution is indicated.
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There is consensus that Al is substituted preferentially at the T3 and T4 sites (ca. 30ꢀ45%
occupancy by Al for Si/Al=5), while the percent occupation of Al at T1 and T2 is relatively low (ca. 5ꢀ
10%)39–43. The combination of Alꢀpairs and the proper local structure seems the ideal recipe for the
formation and stabilization of the active CuꢀOꢀCu moieties in zeolites. It appears that this configuration in
the zeolite lattice provides the appropriate positioning of CuI pairs required to form the CuꢀOꢀCu site, and
in addition provides a structure that stabilizes the CuꢀOꢀCu site once it is formed. Thus AlT3ꢀOꢀSiT1ꢀOꢀ
SiT1ꢀOꢀAlT3 and AlT4ꢀOꢀSiT2ꢀOꢀSiT2ꢀOꢀAlT2 sequences are particularly attractive candidates to stabilize Cuꢀ
OꢀCu cores. Based on the higher Al occupancy of T3 relative to T4, it is likely that MOR 2 (the major
species by O2 TPD) is stabilized by a pair of T3 Al sites, and is thus buried deep within the side pocket.
The minor species, MOR 1, is likely stabilized by a pair of T4 Al sites, and is therefore exposed directly to
the 12MR channel. Reactivity studies that will test this model and experimentally define the locations of
MOR 1 and MOR 2 are now in progress.
The substantial similarities between the ZSMꢀ5 and MORꢀbound [Cu2O]2+ sites defined
experimentally suggest ZSMꢀ5 and MORꢀbound [Cu2O]2+ sites exhibit exceptional reactivity toward CH4
for similar reasons. Previous DFT studies of CuꢀZSMꢀ5 defined two key contributions to the low barrier
for HAA from CH4 by a [Cu2O]2+ core20. First, the approaching substrate polarizes the reactive LUMOs
of the core toward the bridging oxo ligand, generating a Cu(I)ꢀoxylꢀlike electronic structure. This
enhances the electrophilicity of the bridging oxo, activating it toward HAA. The second contribution to
the low barrier comes from the high strength of the OꢀH bond in the resulting [Cu2OH]2+ first product.
Although [Cu2O]2+ sites bound to ZSMꢀ5 and MOR are likely reactive with CH4 for the same
reasons, one of these sites (MOR 2) exhibits a 3.6ꢀ4.6 kcal/mol lower ꢀH‡ for the CH4 oxidation. This
difference in reactivity is too large to be explained by the subtle differences in the MORꢀbound [Cu2O]2+
cores defined experimentally. However, the [Cu2O]2+ site with the lower ꢀH‡ (MOR 2 in table 2) has a
significantly higher ꢀS‡, consistent with a more confined reaction environment, again suggesting MOR 2
is located deep inside the side pocket coordinated to a pair of Al T3 sites (see Figure 11 in blue). The
lower ꢀH‡ may therefore be related to the influence of the zeolite lattice on the HAA reaction coordinate.
If an active site is located in a constricted space (e.g. in the side pockets or 8MR of MOR), the lattice may
influence the trajectory of CH4 approach. In this case, substrate confinement may influence ꢀH‡. For a
confined core, second sphere lattice O atoms could stabilize/destabilize the HAA transition state and/or
the [Cu2OH]2+ first product; or even enable a more activated [Cu2O]2+ geometry to form along the CH4
reaction coordinate.
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Based on the above considerations, the zeolite lattice appears functionally analogous to the activeꢀ
site pocket of a metalloenzyme. For example: tyrosinase, catechol oxidase, and hemocyanin all contain
planar ꢁꢀη2:η2 peroxoꢀbridged dicopper cores with very similar spectral features, yet each enzyme exhibits
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substantially different reactivity44. This difference in reactivity is attributed to the influence of the active
site pocket (e.g. second sphere residues) on substrate approach and interaction with the [Cu2(O2)]2+ core.
The MOR lattice likely exerts an analogous influence on the CH4 HAA reaction for at least one of the
MORꢀbound [Cu2O]2+ sites. Computational studies will enable assignment of these sites to specific 8MR
windows and define the origin of their differential reactivity with CH4.
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5. Conclusion
For the first time, the active species in CuꢀMOR have been unambiguously identified. It was
found that there are in fact two [CuO]2+ sites present. These two sites are highly similar to each other and
to the active [CuO]2+ site in ZSMꢀ5. This observation suggests a conserved structural motif is present in
both the ZSMꢀ5 and MOR topologies that stabilizes the [CuO]2+ geometries defined experimentally. The
two windows of the 8MR side pocket in MOR are appealing candidates given their resemblance to the
local structure around the CuꢀOꢀCu site in ZSMꢀ5. Testing a range of zeolites might further demonstrate
that only zeolites with a similar local structure stabilize [CuO]2+ sites capable of the CH4 to CH3OH
reaction.
It is unlikely that the difference in reactivity between the two [Cu2O]2+ sites in mordenite is due to
small differences in their ground states defined experimentally. Instead, it appears that the zeolite lattice
contributes to the reactivity of one of these sites (MOR 2), e.g. by directing the approach of methane to the
active sites, or by influencing the stability of the products and/or HAA transition state. The zeolite lattice
may therefore be functionally analogous to the active site pocket of a metalloenzyme. A detailed
computational analysis of the MOR active sites will elaborate this analogy, further developing the
correspondence between biological and heterogeneous Cu/O sites that exhibit selective, lowꢀtemperature
reactivity with methane.
Associated content
Supporting Information UVꢀvisꢀNIR and resonance Raman spectra of CuꢀMOR after N2O activation,
kinetics of the 22000 cmꢀ1 absorption band during reaction met methane and additional CuꢀMOR
structures are available free of charge via the internet at http://pubs.acs.org.
Author information
Corresponding Author
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edward.solomon@stanford.edu
robert.schoonheydt@biw.kuleuven.be
bert.sels@biw.kuleuven.be
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Author Contributions
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ǁ These authors contributed equally
Notes
The authors declare no competing financial interest.
Acknowledgment
P.V. acknowledges KU Leuven for a postdoctoral fellowship and the FWO for a travel grant during his
stay at Stanford University. B.E.R.S. acknowledges support from the National Science Foundation
Graduate Research Fellowship Program under Grant No. DGEꢀ11474, and from the Munger, Pollock,
Reynolds, Robinson, Smith & Yoedicke Stanford Graduate Fellowship. This work was supported by
National Science Foundation Grants CHEꢀ0948211 (to E.I.S.) and funded within the framework of FWO
(G.0596.11), M.ꢀL.T. received support from the Postdoctoral Research Abroad Program sponsored by the
National Science Council, Taiwan (R.O.C.) and R.G.H. acknowledges a Gerhard Casper Stanford
Graduate Fellowship and the Achievement Rewards for College Scientists (ARCS) Foundation.
References
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Figure 1. Cu-MOR sample after O2 activation at 250°C(green) and after subsequent methane reaction at
150°C (red).
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Figure 2. (A) Intensity decrease upon heating a Cu-MOR sample in O2 flow to 450°C after O2 activation at
250°C, the spectra are measured at respectively 300°C (red), 330°C (orange), 360°C (yellow), 390°C
(green), 420°C (blue) and 450°C (purple). The spectrum after complete autoreduction in helium flow at
450°C is shown in black; (B) difference of two consecutive spectra during heating in O2 to 450°C (red) and
the difference between the spectrum measured in O2 at 450°C and He at 450°C (black).
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Figure 3. (A) MS signal of O2 during TPD of an activated Cu-MOR (red) and its fitted peaks (dashed line is
the sum), (B) Difference spectrum between absorption spectra at 310°C and 350° and (C) difference
spectrum between absorption spectra at 350°C and 390°C, deconvoluted with absorption maxima at 21900
cm-1 and 23100 cm-1.
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Figure 4. Eyring Kinetics of CH4 Reaction with Activated Cu-MOR. Eyring plots derived from the decay of
Cu-MOR’s deconvolution bands at 23100 cm-1 (blue) and 21900 cm-1 (red) during reaction with CH4.
Error bars indicate the 95% confident interval of the linear regression.
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Figure 5. Comparison of the resonance Raman spectra (λex=458 nm) of O2 activated CuꢀMOR and CuꢀZSMꢀ
5.
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Figure 6. Resonance Raman spectra of N2O-activated Cu-MOR collected at various excitation energies
(left). Note the shift in relative intensity of the features between 825-900 cm-1 (left). The different laser
excitation lines (in nm) are indicated on the absorption spectrum of an O2 activated MOR (right).
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Figure 7. Resonance Raman spectra (λex=458 nm) of CuꢀMOR activated with 18O2 (red) and subsequent
16O2 activation (green).
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