Selective Methane Oxidation to Methanol on Cu-Oxo Dimers Stabilized by Zirconia Nodes of an NU-1000 Metal-Organic Framework

Article abstract of DOI:10.1021/jacs.9b02902

Mononuclear and dinuclear copper species were synthesized at the nodes of an NU-1000 metal-organic framework (MOF) via cation exchange and subsequent oxidation at 200 °C in oxygen. Copper-exchanged MOFs are active for selectively converting methane to methanol at 150-200 °C. At 150 °C and 1 bar methane, approximately a third of the copper centers are involved in converting methane to methanol. Methanol productivity increased by 3-4-fold and selectivity increased from 70% to 90% by increasing the methane pressure from 1 to 40 bar. Density functional theory showed that reaction pathways on various copper sites are able to convert methane to methanol, the copper oxyl sites with much lower free energies of activation. Combining studies of the stoichiometric activity with characterization by in situ X-ray absorption spectroscopy and density functional theory, we conclude that dehydrated dinuclear copper oxyl sites formed after activation at 200 °C are responsible for the activity.

Full text of DOI:10.1021/jacs.9b02902

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Article
Selective Methane Oxidation to Methanol on Cu-oxo Dimers
Stabilized by Zirconia Nodes of NU-1000 Metal–Organic Framework
Jian Zheng, Jingyun Ye, Manuel A Ortuño, John L. Fulton, Oliver Y. Gutiérrez, Donald M. Camaioni,
Radha Kishan Motkuri, Zhanyong Li, Thomas E. Webber, B. Layla Mehdi, Nigel D. Browning, R. Lee
Penn, Omar K. Farha, Joseph T. Hupp, Donald Truhlar, Christopher J. Cramer, and Johannes A. Lercher
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 22 May 2019
Downloaded from http://pubs.acs.org on May 23, 2019
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Selective Methane Oxidation to Methanol on Cu-oxo Dimers
Stabilized by Zirconia Nodes of NU-1000 Metal–Organic
Framework
Jian Zheng,†⊗ Jingyun Ye,‡⊗ Manuel A. Ortuño,‡ John L. Fulton,† Oliver Y. Gutiérrez,† Donald M.
Camaioni,† Radha Kishan Motkuri,# Zhanyong Li,⊥ Thomas E. Webber,§ B. Layla Mehdi,θ Nigel D.
Browning,θ R. Lee Penn,§ Omar K. Farha,⊥∥ Joseph T. Hupp,⊥ Donald Truhlar,‡ Christopher J. Cramer,‡
and Johannes A. Lercher*†Δ
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†Institute for Integrated Catalysis, and Fundamental and Computational Science Directorate, Pacific Northwest National Laboratory
Richland, Washington 99354, USA
‡ Department of Chemistry, Supercomputing Institute, and Chemical Theory Center, University of Minnesota, Minneapolis, Minnesota
55455, USA
# Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, USA
⊥ Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA
§ Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455-0431, USA
θ School of Engineering, University of Liverpool, Liverpool, L69 3GH, United Kingdom
∥ Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
Δ Department of Chemistry and Catalysis Research Institute, TU München, Lichtenbergstrasse 4, 85748 Garching, Germany
KEYWORDS: Methane oxidation, methanol production, metal-organic framework, ion-exchange, Cu XAFS.
ABSTRACT: Mononuclear and dinuclear copper species were synthesized at the nodes of NU-1000 metal–organic framework
(MOF) via cation exchange and subsequent oxidation at 200 °C in oxygen. Copper-exchanged MOFs are active for selectively
converting methane to methanol at 150–200 °C. At 150 °C and 1 bar methane, approximately a third of the copper centers are
involved in converting methane to methanol. Methanol productivity increased by 3-4 fold and selectivity increased from 70 % to
90 % by increasing the methane pressure from 1 to 40 bar. Density functional theory showed that reaction pathways on various
copper sites are able to convert methane to methanol, the copper oxyl sites with by far lower free energy barriers. Combining
studies of the stoichiometric activity with characterization by in situ X-ray absorption spectroscopy and density functional theory,
we conclude that dehydrated dinuclear copper oxyl sites formed after activation at 200° C are responsible for the activity.
INTRODUCTION
strict structure requirements. In the case of copper exchanged
zeolites, the dinuclear Cu species ([Cu–O–Cu]2+) for ZSM-5
and mordenite (MOR) was first proposed as the active
center.13 Recently, trinuclear copper-oxo clusters in MOR14
and ZSM-515 have been associated with high activity for
Direct conversion of methane to easily condensable
oxygenated energy carriers under mild condition is
conceptually a key reaction in the decentralized conversion of
shale gas.1-2 Enzymes such as particulate methane
monooxygenase (pMMO) are able to convert methane to
methanol at ambient temperature via active Cu sites.3
Bioinspired catalysts, for example, tricopper complexes
immobilized into mesoporous silica have shown activity for
selective methane oxidation.4-5 Similarly, copper- and iron-
exchanged zeolites have attracted much attention recently due
to their high methanol selectivity.6-11 With these materials,
methanol was produced by sequential dosing of oxygen and
methane, followed by methanol extraction with the assistance
of steam. Recently, Narsimhan et al. reported continuous
catalytic pathway to convert methane to methanol on copper-
exchange zeolites in a single pass.12 However, despite these
promising results, the low methanol yield caused by the low
concentration of the active centers is still the main challenge to
overcome.
methane partial oxidation. In contrast,
a mono-copper
[CuOH]+ core in eight-membered ring (8MR) zeolites such as
SSZ-13 has been suggested to an active site.16 Bokhoven and
co-workers have also shown that small dehydrated clusters
with several Cu atoms in MOR are active, but only active at
high methane pressure.17
Like zeolites, metal–organic frameworks (MOFs) have high
porosity, and they have customizable pore environments via
post-synthetic modification of linkers and nodes.18-25
Moreover, most MOFs are crystalline solids with permanent
ordered structures. This allows characterization by single-
crystal and/or powder-diffraction techniques, and it facilitates
the design of well-defined heterogeneous catalysts.26-28 Two
general types of active species stabilized by the MOFs have
been proposed for activating the C-H bond of alkane. Namely,
the own metal species of the MOF’s building units and extra-
framework metal sites formed via post-synthetic modifications
Elucidating the structure of the active site to the selectivity
of methanol is essential because methane activation poses
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of the MOF.29-33 For instance, the iron species in the structural
Thermal gravimetric analysis (TGA) shows that the exchanged
position of Fe-MOF-7427, 29 and Fe-MIL-5334 have been shown
active to convert alkanes to alcohols, although these sites may
suffer from instability or low catalytic performance. Other
examples are single-site cobalt and iron species formed via
post-synthetic metalation at the nodes of zirconium- and
hafnium-based MOFs that have been found active for C-H
bond cleavage.35-36 Recently, Baek et al. has shown that bis(µ-
oxo) dicopper species incorporated into the pores of MOF, e.g.
MOF-808, can selectively oxidize methane to methanol under
mild conditions.37
material is stable up to 300 ºC in air (Figure 1c). Transmission
electron microscopy (TEM, Figure S6) and high-angle annular
dark-field scanning transmission electron microscopy
(HAADF-STEM, Figure 1d) images show that there are no
observable Cu nanoparticles formed (> 1 nm). Elemental maps
obtained by scanning transmission electron microscope–
energy dispersive x-ray spectroscopy (STEM-EDS) show a
homogeneous distribution of Cu throughout the MOF crystal
(Figure 1e). Infrared spectroscopy shows that the spectral
features are largely unchanged after CuII exchange (Figure
S9a). However, the intensity decrease of the Zr-OH band at
3674 cm-1 indicates that CuII has exchanged protons of these -
OH groups on the Zr6 nodes (Figure S9b).39 Taken together,
these results show that the copper has been introduced into
NU-1000 without altering the MOF framework structure.
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NU-1000 is one of the most robust MOFs.38 The terminal –
OH groups on the zirconium oxide nodes of NU-1000 are
ideal for chemical functionalization such as ion exchange or
atomic layer deposition.39-40 There are a number of examples
highlighting the importance of dispersed metal atoms or
clusters in NU-1000 for heterogeneous catalysis.41-44 We have
reported that atomic layer deposition of (ALD) copper clusters
on the nodes of NU-1000 are selective for partial oxidation of
methane to methanol. We concluded that a trimeric Cu-
hydroxide-like structure bridging two nodes across the c-pore
of NU-1000 is the active site.45
d)
Cu-2.9-NU-1000
In order to probe a lower nuclearity of the Cu sites, we
report the synthesis, characterization, kinetic exploration, and
density functional theory (DFT) calculation of the catalytic
activity towards oxidative functionalization of methane on
monocopper and dicopper centers in NU-1000. The structural
determination is based on a combination of spectroscopic and
computational analysis. The structural changes of the copper
species accompanying the reaction were probed via in situ X-
ray adsorption spectroscopy (XAS). These findings are
important for understanding the structure-activity relationships
of metal exchanged MOFs for selective methane oxidation,
and they provide new insights into the design of catalytic
single atom and small clusters stabilized in the pores of MOFs.
e)
Cu-2.9-NU-1000
RESULTS AND DISCUSSION
Preparation and characterization of copper exchanged Cu-
NU-1000.
In this study, three Cu-NU-1000 samples were synthesized
via aqueous phase cation exchange.40 The loadings of Cu were
2.9 wt % (Cu-2.9-NU-1000, Cu/Zr6 molar ratio = 1), 1.9 wt %
(Cu-1.9-NU-1000, Cu/Zr6 molar ratio = 0.65), and 0.6 wt %
(Cu-0.6-NU-1000, Cu/Zr6 molar ratio = 0.2). The synthesis
and characterization of Cu-NU-1000 are detailed in the
Experimental Section. It is worth noting that following the Cu
exchange, we exchanged water with acetone as solvent before
drying. This step is key to preserving the integrity of the
framework, avoiding capillary-force-driven channel collapse
due to the high surface tension of water (Figures S3 and S4).46
Figure 1 shows characterizations of Cu-2.9-NU-1000. Similar
results were obtained for the 0.6 and 1.9 wt % Cu-loaded
samples. The XRD pattern shows that the crystal structure of
Cu-2.9-NU-1000 is the same as that of pristine NU-1000
(Figure 1a). There are small decreases in the BET surface area
(<10%), pore volume, and average size of the large pores,
consistent with Cu being deposited into the large hexagonal
channels of NU-1000 framework (Figure 1b and S30).
Figure 1. (a) XRD patterns and (b) N2 adsorption and
desorption isotherms measured at 77 K of Cu-2.9-NU-1000
compared with parent NU-1000; Insert of (b) shows pore size
distributions; (c) TG graphs of Cu-2.9-NU-1000 measured for
the sample in compressed air with temperature ramp of 3 K
per minute from room temperature to 700 °C; (d) High-angle
annular dark field (HAADF) image; and (e) STEM-EDS
elemental maps of Cu-2.9-NU-1000.
Cu K-edge X-ray absorption near-edge structure (XANES)
and extended X-ray absorption fine structure (EXAFS) were
used to assess the local structure of Cu in NU-1000. Figure 2a
shows the normalized XANES spectra for Cu-2.9-NU-1000,
Cu(OH)2, Cu2O, CuO, and aqueous CuII. The XANES of Cu in
NU-1000 is similar to that of the Cu(OH)2 reference. The pre-
edge feature at 8978 eV (inset) is assigned to the 1s  3d
transition of CuII species.47 The intensity of this spectral
feature is also close to that of Cu(OH)2. A pre-edge shoulder
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corresponding to CuI at 8983 eV was not observed.48 The 0.6
the pre-edge feature at 8973–8982 eV corresponding to the
1s→3d electronic transition for the CuII. SS = single
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and 1.9 wt. % Cu exchanged samples exhibited XANES
spectra nearly identical to that in Figure 2a for the 2.9 wt. %
sample (Figure S12 shows a comparison of the three Cu-
exchanged NU-1000 samples).
scattering, MS = multiple scattering.
Figure 2b shows the phase-uncorrected k2-weighted χ(R)
spectra for Cu-2.9-NU-1000 (black curve) and the reference
compounds. The first feature centered at ~1.5 Å is assigned to
the Cu-O single scattering. We observe that the first shell
structure of Cu in the MOF is nearly identical to that of
Cu(OH)2, as they show the same bond distance and amplitude.
The weak feature at ~2.0 Å is likely due to the elongated,
Jahn–Teller (JT) out-of-plane distortion of O atom(s) that
would be present in either the square pyramidal or the
distorted JT-octahedral symmetries.49-50 This feature is also
observable in Cu(OH)2, CuO, and aqueous CuII, but not in
tetrahedrally coordinated Cu2O.51-52 We also note that the Cu-
Cu single scattering feature (2.4 Å) in Cu-2.9-NU-1000 is
weak compared to that of Cu(OH)2, Cu2O, and CuO
references, and only slightly higher than that of isolated
aqueous CuII, which has no Cu neighbors. This Cu-Cu
coordination feature is especially weak in the samples (Cu-
1.9-NU-1000 and Cu-0.6-NU-1000) having lower Cu loadings
(Figure S13), and this suggest that Cu is present in very small
entities, i.e., dimeric or trimeric Cu species possibly in a
mixture with isolated Cu cations.
Activity of Cu-NU-1000 for methane-to-methanol
oxidation
Selective oxidation of methane was performed with Cu-NU-
1000. We used three sequential reaction steps in a typical
reaction cycle.45 We firstly varied the methane loading time
from 30 to 180 minutes to explore the impact of the methane
contact time on methanol productivity on Cu-2.9-NU-1000.
As shown in Figure 3a, the methanol yield increased from
~3.3 mmolMeOH/molCu (1.5 μmolMeOH/gcatalyst) (30 min) to 9.7
mmolMeOH/molCu (4.4 μmolMeOH/gcatalyst) (180 min). No obvious
increase in methanol production was observed by further
increasing the methane loading time. It suggests that the
surface methane coverage was nearly saturated after 3 hours.
Thus, activation with 1 bar oxygen at 200 °C and reaction with
1 bar methane at 150 °C yields the maximum amount of
methanol formed per Cu of about 0.01. This efficiency
observed here is comparable to that of ALD synthesized Cu-
NU-1000,45 but higher than that of Cu-MOR or Cu-ZSM after
activation with oxygen or NO at 150–200 °C.17, 53 It is also
worth noting that the methanol selectivity of 70 % is higher
than that of 40-60 % on previous ALD synthesized Cu-NU-
1000.45
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We also performed multiple catalytic cycles to show the
stability and applicability of Cu-2.9-NU-1000 (Figure 3b). The
methane loading time in the cycling test was fixed to 180
minutes. Methanol yield of about 9 mmolMeOH/molCu was
observed in each cycle, indicating only slight deactivation of
the catalyst. The selectivity to methanol was stable over the
five cycles at ~70%. Dimethyl ether (DME) was not observed.
In order to confirm the methanol selectivity, we have used 13C-
methane as the reactant in the first step and performed the
experiment under the same conditions. About
9
mmolMeOH/molCu of 13C-labelled methanol (m/z = 33) and ~4
mmolCO2/molCu 13C-labelled CO2 (m/z = 45) were observed.
Only minor amounts of 12CO2 (m/z = 44, < 1 mmolCO2/molCu)
were found. (Figure S17). Although some deactivation was
observed in the first cycle, it is less significant in subsequent
cycles. We attribute the initial deactivation to decarboxylation
of free carboxylate groups.45 This is consistent with the
observation of trace amounts of CO2 detected by the mass
spectroscopy during the activation step in oxygen at 200 °C
(Figure S16). These results demonstrate that Cu-NU-1000 is
stable for this reaction.
Compared to 4.4 μmolMeOH/gcatalyst obtained from Cu-2.9-
NU-1000, the methanol yields from Cu-1.9-NU-1000 and Cu-
0.6-NU-1000 were lower at 2.3 and 0.45 μmolMeOH/gcatalyst
respectively. A blank reaction under identical conditions using
the pristine NU-1000 MOF lead to ~0.3 μmolMeOH/gcatalyst
similar to our previous work reporting 0.4–0.5
,
,
45
μmolMeOH/gcatalyst
.
Assuming that the same amount of
Figure 2. (a) Normalized Cu-XANES spectra and (b) k2-
weighted Cu-EXAFS χ(R) spectra of the as-prepared Cu-2.9-
NU-1000 and reference compounds. The insert of a) shows
methanol is produced by the MOF itself from the Cu-
exchanged NU-1000, the methanol productivities are 4.1, 2.0,
and 0.15 μmolMeOH/gcatalyst for Cu-2.9-NU-1000, Cu-1.9-NU-
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1000 and Cu-0.6-NU-1000 (Figure 3c, red bars). These are mmolMeOH/molCu at 150 °C and 200 °C, respectively, at 1 bar.
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9.7, 6.2 and 1.4 mmolMeOH/molCu (Figure 3c, open bars) when
normalized by the molar ratio of Cu. Thus, the methanol
productivity per Cu atom of Cu-2.9-NU-1000 is 6.4-fold
higher than that of Cu-0.6-NU-1000, although all samples
exhibit the similar methanol selectivity of ~70% (inset in
Figure 3c).
The productivity increased to 24.5 and 34.8 mmolMeOH/molCu
(150 °C and 200 °C) as methane pressure was increased to 40
bar. We also observed 1.4 and 2.0 mmolDME/molCu of DME,
which equals additional 2.8 and 4.0 mmolMeOH/molCu of
methanol. In contrast, changing the methane pressure has only
negligible influence on the amounts of CO2, which is 2.7 and
4.3 mmolCO2/molCu at 150 °C and 200 °C at 40 bar. Taking
these into account, the selectivity to methanol is ~90% at 40
bar at both temperatures. Thus, the maximum of methanol
(including DME) productivity at 200 °C and 40 bar is 0.04
molecules per Cu atom. In terms of electron transfer, the
formation of one molecule of methanol, dimethyl ether, and
carbon dioxide requires 2, 4, and 8 electrons, respectively,
transferred to copper (CuII reduced to CuI). Thus, there are 70
(35 mmol  2), 8 (2 mmol  4), and 34.4 (4.3 mmol  8) mmol
electrons involved in the formation of methanol, dimethyl
ether, and carbon dioxide per each mol of copper. In a word, a
total of 112.4 mmol CuII were reduced to CuI from each 1000
mmol copper to activate methane (11.2% of total Cu). To the
best of our knowledge, this might be the highest value on the
MOF-based catalysts for methane oxidation so far.
Increasing methane pressure enhances the coverage of
activated species, which in turn enhances the extant of
methane conversion and the selectivity to methanol in
continuous operation.54-56 An increased yield of methanol with
increasing methane pressure was also recently observed for
Cu-MOR zeolites.7, 17 Thus, we further investigated the effect
of methane pressure on methanol productivity with Cu-2.9-
NU-1000. The catalyst was activated in 1 bar oxygen at 200
°C for 2 h. In the second step, methane at increasing pressure
was contacted with the material at 150 °C or 200 °C. We
observed an increase of the productivity of methanol
correlated with the increase of methane pressure (Figure S18).
Figure 3d shows a direct comparison of the catalytic
performance of Cu-2.9-NU-1000 exposed to methane at 1 and
40 bar. The methanol productivity was 9.7 and 14.5
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(CH3)2O
CH3OH
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Reaction Condition
Figure 3. Catalytic testing of copper-exchanged NU-1000 for selective oxidation of methane to methanol: (a) Effect of methane
loading time on methanol yield over Cu-2.9-NU-1000; (b) the activity over the first 5 catalytic cycles of Cu-2.9-NU-1000 and the
selectivity to methanol; (c) catalytic activity comparison of the Cu exchanged NU-1000 samples; and (d) effect of temperature and
methane pressure during the reaction. The catalyst was activated in 1 bar oxygen at 200 °C for 2 h, then 1 bar or 40 bar of methane
was applied at 150 °C or 200 °C (30 to 180 minutes), and finally steam-assisted product was extracted with 10% H2O and 90% He
for
3
h
at
135
°C.
The
products
were
analyzed
on-line
with
a
mass
spectrometer.
Structure probing along the reaction
In situ XAS of Cu-2.9-NU-1000 was performed to probe
structural variations in the active Cu sites along the reaction
cycle. The sample was firstly heated in oxygen for 120
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minutes at 200 °C. After cooling to 150 °C in O2, the XANES
spectrum was recorded in situ (Figure 4a, green curve). The
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XANES curve of this “O2 activated” Cu-2.9-NU-1000 sample
resembles the curve for the Cu(OH)2 reference (Figure S14).
There was a considerable decrease in the white line intensity
after activation, similar to that observed during activation of
Cu-zeolites.57-58 This change is due to the removal of
coordinating H2O ligands, which are also supported by the
EXAFS spectra that show (see as-synthesized and O2-activated
spectra in Figure 4b) a decrease in the amplitude of the first
Cu-O single scattering (1.5 Å) and the disappearance of the
second Cu-O scattering assigned to JT water (2.0 Å). Thus, we
conclude that the out-of-plane water molecules are removed
after heating in flowing O2. Furthermore, the weak Cu-Cu
single scattering feature at 2.4 Å demonstrates that the
nuclearity of the Cu sites in NU-1000 is not changed by the
heat treatment in oxygen.
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As discussed above, the pre-edge XANES signal at 8977–
8978 eV is assigned to the 1s→3d transition in CuII. Because
CuI ion does not have this transition owing to the filled d10
orbital, this feature can be used as a fingerprint for the
presence of CuII.59 We note this feature is enhanced (insets of
both XANES and first derivative XANES in Figure 4a) after
activation, suggesting an increased interaction of CuII ion with
the MOF framework due to the heat treatment, similar to that
of Cu-exchanged zeolites.60 We also note a loss of the
intensity of the multiple scattering feature at ~3.3 Å, which
suggests that the square planar structure of Cu is somewhat
distorted by the thermal activation. Interestingly, there is an
increase in the region at ~8983 – 8984 eV, which is commonly
considered to be the energy range for the 1s→4p transition in
CuI.51 However, considering that no intensity loss in the CuII
feature at 8977– 8978 eV was observed and that the decrease
of Cu center coordination symmetry upon dehydration, we
conclude that Cu remains 100 % as CuII in the O2 activated
Cu-2.9-NU-1000. Upon exposing Cu-2.9-NU-1000 to 1 bar
methane at 150 °C for 180 minutes, the white line as well as
the ~8978 eV features are largely unchanged. Only a very
small increase in the CuI 1s→4p transition intensity at ~8983
eV was observed. Linear combination fitting suggested that
CuII remains the dominant state, with a residual fraction of
approximate 6% CuI (Figures S19). The XANES of steam-
treated Cu-2.9-NU-1000 (Post) is nearly identical to that of as
synthesized samples, indicating that this sample is fully
hydrated at the end of the reaction cycle, which is in
accordance with the EXAFS curve in Figure 4b.
Figure 4. (a) Normalized Cu-XANES and (b) k2-weighted Cu-
EXAFS χ(R) spectra of Cu-2.9-NU-1000 measured during a
full cycle of selective methane oxidation to methanol. The top
(first derivative χµ(E)) and bottom (normalized χµ(E)) inset
show a magnification of pre-edge feature at 8973–8982 eV
corresponding to the 1s→3d electronic transition for the CuII .
Mag
0.5
fit
Img
fit
a)
b)
c)
0
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R (Å)
Figure 5. k2-weighted Cu K-edge Magχ(R) and Imgχ(R)
spectra of oxygen activated (a) Cu-2.9-NU-1000, (b) Cu-1.9-
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NU-1000, (c) Cu-0.6-NU-1000, and the corresponded FEFF
fits.
and 3.2 Å increase for Cu-2.9-NU-1000, which is indicative of
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a heavy backscatter (Cu). This effect is very small for Cu-0.6-
NU-1000. This hypothesis is also supported by the comparison
of FTIR and EPR spectra of these two samples (Figures S9,
S11, and Table S1). FTIR results show that the ratios of
deposited Cu ions to consumed terminal hydroxyl groups on
each Zr6 nodes are approximately 1:1 for Cu-0.6-NU-1000 and
2:1 for Cu-1.9-NU-1000. Quantification of the paramagnetic
signals in EPR spectra suggests that the isolated CuII species
account for 70–80% and 20-30 % of the total Cu species in the
Cu-0.6-NU-1000 and Cu-2.9-NU-1000, respectively.
Table 1. Parameters determined for Cu in Cu-NU-1000 by
fitting the experimental spectra with a Cu(OH)2 model derived
a
using FEFF9. Other parameters: amplitude reduction factor
(amp) = 0.83 ± 0. 08, R-factor = 0.009, and E0 = -7.6 ± 0.8.
Sample
Path
Cu-OSS
Cu-CuSS
CNa
Distance (Å)
1.954±0.007
2.871±0.017
3.908±0.014
1.960±0.0012
2.844±0.015
3.920±0.0024
1.962±0.013
/
DWFb
c
4.1±0.2
1.0±0.3
N/Ae
3.6±0.2
0.6±0.3
N/A
0.0046± 0.0005
0.0111± 0.0024
0.0184± 0.0020
0.0053± 0.0006
0.0169± 0.0069
0.0212± 0.0010
0.0069± 0.0010
/
Cu-2.9-NU-
1000
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d
Cu-OMS
Cu-OSS
Cu-CuSS
Cu-OMS
Cu-OSS
Cu-CuSS
Cu-Zr SS
Cu-OMS
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Cu-1.9-NU-
1000
Computed Structures for the Cu sites in Cu-NU-1000
3.5±0.6
f
/
Cu-0.6-NU-
1000
0.8±0.4
N/A
3.603 ±0.033
3.924±0.026
b
0.0178± 0.0051
0.0273± 0.0040
a CN = average coordination number; DWF = Debye–Waller
Factors; c SS = single scattering; d MS = multiple scattering;
e
f
NA= not available; Fitting Cu-0.6-NU-1000 with a Cu-Cu
path doesn't improve the overall fitting quality, which implies
Cu ions are isolated; Fits obtained with k-weighting of 2.
In order to determine the coordination environment of Cu-
NU-1000, we fitted the experimental EXAFS spectra of all the
three samples, using the Cu(OH)2 model as an initial model.
This structure was chosen, because the Cu(OH)2 model
contains the minimum set of parameters and the values of
these parameters for the Cu(OH)2 reference are closer than that
for other Cu(II) references. The best fit is shown in Figures 5
and S21, with all magnitude (Mag), imaginary (Img), and χ (k)
plots displayed. The fitted parameters are listed in Table 1. In
all samples Cu is coordinated with four O atoms with an
average bond distance of 1.95-1.96 Å. Taking into account the
∼3.91 Å multiple Cu-O scattering, one can conclude that these
four O atoms form a square planar configuration. For Cu-2.9-
NU-1000, the fit shows a Cu-Cu path with interatomic
distance of 2.87 Å and the coordination number (CN) of 1.0 ±
0.3 consistent with Cu present as a dinuclear cluster. For Cu-
1.9-NU-1000 sample, the Cu-Cu CN is 0.6 ± 0.3 suggesting
that a fraction of Cu is present as mononuclear. Fitted values
for the Cu-Cu parameters of Cu-0.6-NU-1000 were
insignificant suggesting that Cu is predominantly present in
isolated monomeric structures. Fitting with a Cu–O-Zr path
for Cu-2.9-NU-1000 and Cu-1.9-NU-1000 did not
significantly improve the fit quality nor changed it the Cu-O
and Cu-Cu fit parameters, suggesting Cu-Zr interactions are
significantly disordered.45 However, for Cu-0.6-NU-1000, a
Cu-Zr scattering with CN of about 0.8 and bond distance of
3.6 Å was obtained. Thus, it may be that for the samples with
higher Cu loading, the Cu-Zr interactions are not detected due
to disorder in their structures. Considering the variation in Cu-
Cu CNs (Table 1) discussed above, we conclude that the Cu in
the Cu-2.9-NU-1000, Cu-1.9-NU-1000 and Cu-0.6-NU-1000
is present as dinuclear Cu, a mixture of dinuclear and
mononuclear Cu, and mononuclear Cu, respectively. This
hypothesis is supported by the comparison of k1-, k2-, and k3-
weighted Img[χ(R)] spectra of Cu-2.9-NU-1000 and Cu-0.6-
NU-1000 (Figure S23). The Cu-O peak has been scaled so that
their amplitudes are approximately equivalent. As the k-
weight increases, the amplitudes of the features between 2.2
Figure 6. Possible DFT-optimized structure of dinuclear D-A
complex [Cull2(OH)4 (H2O)] on the MOF and and
mononuclear M-A complex [Cull (OH)2 (H2O)2] on the MOF.
Given the 4-coordinate square planar Cu(OH)2-like
structure determined by EXAFS, we used DFT with the M06-
L density functional to compute a series of dinuclear and
mononuclear Cu structures as shown in Figures S26 and
S28.28, 61 Optimized dinuclear (D-A) and mononuclear (M-A)
structures that best correspond with the experimental EXAFS
data are shown in Figure 6. In the D-A structure, the first Cu
atom is attached to the Zr6 node via two Cu-OH(μ3)-Zr
linkages, and the second Cu sits far away from the Zr6 node
bridged to the first Cu atom via another two O atoms. Four O
atoms coordinate each Cu ion in a slightly distorted square
planner phase. The calculated Cu-Cu distance is 2.80 Å, close
to the experimental value of 2.87 Å (Table 1). In the M-A
structure, a single Cu is attached to the Zr6 node via a Cu-
OH(μ3)-Zr linkage with Cu-O and Cu-Zr distances of 1.95 and
3.55 Å, respectively, which are in accordance with the EXAFS
determined values. The four O atoms show a nearly perfect
planar configuration with two O-Cu-O angles at 170.9 and two
at 177.3°.
We further compared the experimental EXAFS spectra of
Cu-2.9-NU-1000 and Cu-0.6-NU-1000 with spectra simulated
by calculating the Cu scattering paths using the D-A and M-A
model structures (Figures 7 and Figures S24, S26). The
simulated EXAFS spectra are in good agreement with the
respective experiments. Close match of all the Cu-OSS, Cu-
CuSS (only for D-A model), and Cu-OMS paths in Img[χ(R)]
and the oscillation in x(k) confirm that the computationally
defined structure are excellent agreement with the fact that the
majority of sites is dinuclear.
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left side, but in all cases, the actual calculations include the
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whole NU-1000 node with four benzoate and four
formate linkers as in the left sides of the panels.
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Figure 8. The cluster models for (a) CuII(OH)2(H2O)2, (b)
CuII (OH)4(H2O)2, (c) (CuII(O·)(OH) and (d) CuII (O·)(OH)3).
2
2
The NU-1000 node with the fragment shown in the form of
sticks in the left hand column of the figures are ignored to
clearly show the structures of copper species in the following
sections, which are shown in the right hand column.
First, we calculated the free energies for the following
reactions under the activation condition (pO =1) to identify the
2
Figure 7. k2-weighted Cu-EXAFS Img[χ(R)] spectra for (a)
Cu-2.9-NU-1000 and (b) Cu-0.6-NU-1000 and the DFT-
optimized D-A and M-A models. Insets show the respective
x(k) plots.
relative stability of the possible Cu species at temperatures
ranging from 298 K to 773 K. The results of this analysis are
summarized in Figure 9. At the activation temperature (473
K), three cupric ion species, CuII(OH)2, CuIIO, and CuII (OH)4,
2
are predicted to be the most stable species. The calculations
indicate that under the activation condition only a small
fraction of CuII was converted to the oxyl species
Thermodynamic analysis
CuII(O·)(OH) and CuII (O·)(OH)3.
The relatively low reactivity suggests that active and
inactive Cu sites are present after O2 activation at 200 °C. In
order to determine possible structures of the active sites after
O2 activation, a quantum mechanical thermodynamic study
has been performed with the M06-L exchange-correlation
density functional (the computational details and cluster
models of this study and the following study of the reaction
mechanism are provided in the computational section and the
SI). We started from the structures of the monocopper and
2
(
)
+
푧
Cu푥(OH)푦 H2O
2푙 + 2푚 ― 푦O2
4
→Cu푥O푙(OH)푚(H2O) + 푦 + 2푧 ― 푚 ― 2푛H2O
푛
2
dicopper species, CuII(OH)2(H2O)2 (M-A) CuII (OH)4(H2O)
2
(D-A), identified above by DFT and XAFS and shown in
Figure 6. These structures are also shown in Figure 8, where
we introduce shorthand formula notations and simplified
pictorial representations for cluster models to be used in a
thermodynamic analysis and a mechanistic exploration. The
thermodynamic and mechanistic calculations were carried out
with large cluster models illustrated as the left-side figures in
panels a, b, c, and d of Figure 8. Two of the cluster models for
monocopper and dicopper species before oxygen activation
are shown in Figures 8a and 8b, two for the copper oxyl
species are shown in Figures 8c and 8d, and the rest are in the
SI (Figure S31). Throughout the text and the SI, we use the
formulas under the structures in these figures as a shorthand to
represent the cluster models. We use the structures on the right
side of each panel as pictorial shorthand for those on the
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for the dominant and possibly active Cu species, with the
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3
results shown in Figure 10. The calculations do not take into
account the catalyst activation and methanol desorption steps,
which are separate processes that require additional steam.
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8
9
For the cupric hydroxide species, CuII(OH)2 and CuII (OH)4,
2
Figures 10a and 10d show that the reaction is initiated by
homolytic C–H bond dissociation, with H attaching to an OH
group to form an H2O ligand. Then the CH3 abstracts a OH
from the H2O ligand to produce CH3OH. The Gibbs free
energies of activation for these two steps are very large (39
and 59 kcal/mol for CuII(OH)2, 35 and 47 kcal/mol for
CuII(O·)(OH)
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CuII (O·)(OH)3
CuII (OH)4 ), suggesting their low activity for methane
2
2
oxidation.
Dehydration of CuII(OH)2 and CuII (OH)4 could potentially
2
generate CuIIO and CuII O(OH)2, respectively. Methane
2
oxidation on by CuIIO follows a similar pathway to that on
CuII(OH)2, and it shows better activity than CuII(OH)2 (Figure
10b) because of the smaller free energy barriers (33 and 59
kcal/mol). Figure 10e shows that for CuII O(OH)2, the reaction
2
CuII(O·)(OH)
CuII (O·)(OH)3
is again initiated by homolytic C–H bond dissociation, but
now the H atom attaches to the oxyl to form an OH ligand.
Then the CH3 abstracts OH from Cu to produce CH3OH. This
reaction of OH and CH3, adsorbed at the same Cu ion, is much
more facile than abstraction of OH from H2O, and the free
energy of activation is much lower. The Gibbs free energies
of activation for the two steps (Figure 10e) decrease to 23
kcal/mol (smaller than for the dinuclear Cu cluster studied
previously31) and 38 kcal/mol, indicating better activity than
2
Figure 9. Phase diagram of possible copper species formed
under O2 activation conditions. The bottom of the figure
shows the structures of the most dominant species [CuII(OH)2,
CuIIO and CuII (OH)4] in the temperature range of interest
2
(473 K - 523 K) and the two oxyl species [CuII(O·)(OH) and
CuII (O·)(OH)3]. See Figure S31 in the SI for the structures of
2
the full list of investigated species.
CuIIO. We noted that the triplet spin state for CuII O(OH)2 is
preserved for the first step, and the product of second step is a
closed-shell ground state, so the reaction involves a spin-
2
Understanding Reaction Mechanism
changing
transition.62-63
We also investigated the reaction mechanism for methane
oxidation by DFT calculations of free energies of activation
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Figure 10. The reaction mechanisms of methane oxidation over possible active copper sites for (a) CuII(OH)2, (b) CuIIO, (c)
CuII(O·)(OH), (d) CuII (OH)4, (e) CuII O(OH)2, and (f) CuII (O·)(OH)3. The quantities shown are free energies of equilibrium
2
2
2
species and free energies of activation of transition states, both relative to the leftmost species in the given panel. All results are for
a temperature of 298 K and a pressure of 1 bar. Illustrative atomic spin densities of the initial catalyst are shown in the figure,
where they are labeled ρ; S is the total electron spin. The doublet spin state of CuII(OH)2 and the triplet spin state of CuII (OH)4 are
2
preserved along the reaction path, and in these cases the spin density is mainly localized at Cu atoms for all the structures along the
pathway. For the triplet case, the spin coupling of the Cu centers is ferromagnetic. However, as shown in the figure, in some cases
the spin quantum number of the lowest-energy species is not the same for all structures on a given path.
Even though, copper oxyl species CuII(O·)(OH) and
2
CuII (O·)(OH)3, which are comparable to the previously
2
CuII (O·)(OH)3) are predicted by our thermodynamic analysis
reported values on Cu/zeolite or Cu/porphyrin systems.15, 64-66
to be present in relative small quantities, the Gibbs free energy
profiles shown in Figures 10c and 10f suggest their high
activity for methane oxidation to produce methanol. The
calculated free energy of activation for the first step is 17.2
kcal/mol and 18.9 kcal/mol, respectively for CuII(O·)(OH) and
The rebounding of the CH3 from Cu to OH is exergonic by 19
and 11 kcal/mol for CuII(O·)(OH) and CuII (O·)(OH)3
2
respectively. Subsequently, the CH3 abstracts the geminal OH
bound leading to the formation CH3OH with an free energy
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barrier 16 kcal/mol for CuII(O·)(OH) and 20 kcal/mol for
cupric hydroxide-like species, the latter being the most stable
CuII (O·)(OH)3.
component in the material (in agreement with its high
abundance found by EXAFS analysis). DFT calculation
demonstrate that copper oxyl species are active for selective
methane oxidation. However, the dominant Cu hydroxide-like
species are less active.
1
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2
The reaction paths showed in Figures 10c and 10f also
involve spin transitions. We found that the activity of the Cu
species increases with the increase of spin density of O, which
is consistent with previous studies.31The high activity of the
copper oxyl species [for example, CuII(O·)(OH)] may be
understand either in terms of the existence of an active oxygen
or the higher oxidizing power of a Cu that can be regarded as a
CuIII-oxo.67-69
At 150 °C and 1 bar methane, the formation of 9.7 mmol
methanol requires that 19.4 mmol Cu is reduced from CuII to
CuI, which is approximately 2 % of all Cu in Cu-2.9-NU-1000.
We observed, however, about 6 % of CuII was consistently
reduced to CuI during methane exposure (in situ XANES
spectra), i.e., three times more than needed for methanol
production. Thus, only a third of the reactive Cu centers is
involved in the selective methane oxidation to methanol under
these conditions. The other two thirds of reacting CuII are
involved in CO2 production, which requires eight electrons.
While, at 200 °C and 40 bar methane, the formation of ~38.8
mmol methanol (include 2 mmol DME) and 4.3 mmol CO2
requires that 78 and 34.4 mmol Cu is reduced from Cu(II) to
Cu(I), respectively. In another word, about 70% of the reactive
Cu centers is involved in the selective methane oxidation to
methanol.
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The calculations indicate that the cupric hydroxide species,
CuII(OH)2 and CuII (OH)4, which are predicted to be the
2
dominant species, have large free energies of activation and
therefore very low activity for the methane-to-methanol
reaction, but the minority copper oxyl species are the active
sites for methane to methanol. We conclude that the overall
low activity of Cu-NU-1000 is due to the small concentration
of the active oxyl species. These observations are consistent
with the experimental results.
In addition, the mononuclear CuII(O·)(OH) is found to be as
active as the dinuclear CuII (O·)(OH)3 for methane to
2
methanol reaction, which implies, if the catalyst contains only
the two copper oxyl species that we identified, that the
methanol yield should be found to be linearly related to the Cu
loading. However the methanol yields are 9.7 and 1.4
mmolMeOH/molCu for Cu-2.9-NU-1000 and Cu-0.6-NU-1000,
respectively, which are not linearly related to the Cu loading.
This observation suggests several possibilities. (1)
Overall, the activities of MOF-based materials for methane
oxidation are lower than those of Cu-zeolites. The latter,
however, are typically activated at temperatures of up to
500 °C, which is important for the formation of active (Cu–O–
Cu]2+ or [Cu3(µ-O)3]2+.13-14, 70 Due to the thermal stability limit
of MOFs, they have been activated at the milder temperatures
of 150–200 °C, which leads to a comparatively small fraction
of active sites. As an alternative strategy, here we found that
increasing the pressure of methane increases the methane
conversion and the methanol selectivity on Cu-exchanged NU-
1000. The reasons of the enhancing effect of the high methane
pressure is not clear yet. We hypothesize that either the
coverage of active molecular species increases, or to enabling
additional Cu sites to activate methane. A detailed kinetic and
mechanic study to understand how the methane pressure affect
the productivity of methanol and the distribution of products is
ongoing.
CuII O(OH)2, which is more active than CuIIO, may also be
2
involved in the reaction, leading to the increase the methanol
yield for Cu-2.9-NU-1000. (2) The identified M-A structure
for the monocopper species has a relatively low stability and
diffuses to and reacts with the linker,16 reducing the number of
active sites for Cu-0.6-NU-1000.
Structure-Activity Discussion
The in situ XAS, FTIR, and EPR characterizations and DFT
calculations confirmed the presence of dinuclear and
mononuclear copper sites on the Cu-exchanged NU-1000
MOFs. The methanol productivity per Cu atom of dicopper-
dominated Cu-2.9-NU-1000 is 6.4-fold higher than that of
monomer-dominated Cu-0.6-NU-1000 (1 bar methane and 150
ºC). Thus, we conclude that the dinuclear copper oxyl species
is the main contributor for methane oxidation over Cu-
exchanged NU-1000.
CONCLUSION
Using a liquid phase post-synthetic cation-exchange, 0.6–
2.9 wt % Cu was successfully anchored in nodes of NU-1000
MOF. X-ray adsorption spectroscopy suggests that Cu in the
as-synthesized MOFs has a local structure similar to Cu in a
fully hydrated Cu(OH)2. Heating the MOFs in oxygen at 200
°C does not change the overall structure and oxidation state of
Cu, but leads to the dehydration of Cu centers. The data also
show that, whereas the MOF with highest loading of Cu
contained mainly dinuclear Cu, the MOF with lowest Cu
loading contained predominantly mononuclear Cu. DFT
calculations of the Cu species are in good agreement with
experiments and thus lend confidence to our interpretation of
the data. The dinuclear species is predicted to have one Cu
attached to the Zr6 node via two μ3-OH groups and a second
Cu that sits far away from the Zr6 node, bridged to the first Cu
atom via two µ-OH groups. Evaluation of the resulting
material for methane oxidation to methanol showed that the
The active sites are generated during the oxygen activation
step at 200 ºC. After activation, there is a partial dehydration
of the hydroxyl species, but the overall structure of the
dinuclear Cu clusters does not change, which was confirmed
by a control experiment, where methanol was not produced on
the same material without an oxygen activation step. These
results allow us to conclude that dinuclear and mononuclear
Cu sites are formed during synthesis, which are precursors of
the active site, since the heat treatment only removes water,
but does not change the nuclearity of the Cu sites. The
computational thermodynamic analysis corroborates the
existence of different types of dinuclear Cu sites after O2
activation at 200 ºC, e.g., Cu “dry” oxyl-like and “hydrated”
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MOF with highest amount of Cu is 6.4 times more productive acquisition. Data analysis and background removal were
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per Cu than the MOF with lowest loading. Combined with the
stoichiometric activity, in situ structural characterization, and
density functional theory, we conclude that dinuclear copper
oxyl centers are responsible for the observed high selectivity
(~ 70%) to methanol. The sample gives a higher methanol
yield (3-4 fold) and higher methanol selectivity (~90%) by
increasing methane pressure from 1 bar to 40 bar. This implies
that about 11.2 mol % of Cu in the material is involved in
methane activation, which, to the best of our knowledge,
might be the highest value on MOF-based materials to date.
performed using ATHENA and ARTEMIS programs from the
iXAFS software package.72 The Fourier transform of the k-
space EXAFS data were fitted to theoretical models derived
using the FEFF9 code.73
EXAFS spectra simulation
EXAFS spectra of DFT-optimized Cu clusters were simulated
using ab initio scattering theory by applying approximate
global disorder parameters(σ2), corresponding to 300 K. The
computed coordinates were used to generate the primary input
for the ab initio EXAFS scattering code (FEFF9)73 that
includes all the single and multiple scattering paths out to 6 Å,
resulting in several hundred scattering paths for each Cu atom
in the structure. An approximate treatment of the bond
disorder at 300 K is applied by setting a universal value of the
DWF, σ2 = 0.005. The obtained spectra for each Cu atom in
the cluster are then averaged, and an overall E0 is applied to
match experimental values (oscillations in χ(k) converge at k =
0). While the global DWF is a good estimate of the first shell
disorder, it is an overestimation of the order in the higher
shells which manifests as an over-prediction of these
amplitudes, although the atom positions predicted by the
theory are correctly represented.
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EXPERIMENTAL SECTION
Chemicals
High-purity (99.995%) helium, oxygen, and methane were
purchased from Matheson. Methane-13C (99%) and Copper
acetate (99.99%) were obtained from Sigma-Aldrich. All
water was provided by a Mini-Q Plus water purification
system (Millipore). All the chemicals were used as received.
Materials Syntheses
The NU-1000 MOF was synthesized via a solvent thermal
reaction at 120°C, similar to the literature procedure reported
elsewhere.71 Cu-NU-1000 samples were synthesized via an
ion-exchange route in aqueous solution under ambient
condition. Copper acetate was chosen as the Cu precursor. In a
typical experiment, 0.5 g NU-1000 (0.23 mmol) was mixed
with 300 ml of 0.001 to 0.01 M Copper acetate solution. The
measured pH values of the solution were 5.0–6.0 during
exchange. A typical exchange time was 24ꢀh. After exchange,
the product was washed with deionized water at least 3 times
and separated by centrifugation. After the last water washing,
the sample was suspended in 50 ml acetone for 12 hours to
allow solvent exchange. This step was repeated for 3 times.
The solvent exchanged sample was then collected and dried in
vacuum oven overnight at 120 °C. The concentration of Cu in
Cu-NU-1000 samples was determined from inductively
coupled plasma–atomic emission spectroscopy (ICP-AES)
measurements.
Thermogravimetric Analysis (TGA)
TGA and differential scanning calorimetry (TG-DSC)
analyses were performed on a SENSYS EVO TG-DSC
(SETARAM Instrumentation) at the ramp rate of 3 K/min.
N2-Sorption
N2-physisorption for surface area and pore volumes was
obtained on a Micromeritics ASAP 2020 instrument. The
carbon slit pore model with a NL-DFT method was used.
Single-point adsorption close to P/P0 = 0.99 was used to
determine the total pore volume.
In situ FTIR
Infrared spectra of the MOFs were recorded on
a
ThermoScientific Nicolete FTIR spectrometer equipped with
CaF2 windows and a MCT detector with a resolution of 4 cm–
1. 128 scans were accumulated for each spectrum. The samples
for IR measurements were prepared as self-supporting thin
wafers with a density of approximately 2-3 mg/cm2. Upon
loading in the IR-cell, the samples were evacuated to 1.0×10-6
mbar at 150 ºC for 2 hours. The spectra were collected after
the cell cooled to room temperature.
XAS Measurements
The XANES and EXAFS experiments took place at the
Pacific Northwest Consortium/X-ray Science Division
(PNC/XSD) bending-magnet beamline at Sector 20 of the
Advanced Photon Source (APS) at Argonne National
Laboratory (ANL). Same to previous report,45 all experiments
were carried out in transmission mode with a focused beam
(0.7 × 0.6 mm) delivering 1010 photons through the sample. A
harmonic rejection mirror was used to reduce the effects of
harmonics. A Cu foil was placed downstream of the sample
cell, as a reference to calibrate the photon energy of each
spectrum. The XAFS reaction cell is based on a HiP medium-
pressure Hastelloy tee modified to house the four-sample
holder and allow gas flow-through capability.45 Glassy-carbon
discs (thickness = 0.75 mm, diameter = 5 mm) were used as
the X-ray windows. The Cu-NU-1000 samples were pressed
(0.25 ton) into pellets that were 0.5 to 2.0 mm thick. Prior to
data acquisition, all gas lines were purged with dry He, and
then with dry O2 or CH4, for activation and loading,
respectively. A flow rate of 2.5 mL/min was used during data
Powder X-ray Diffraction (PXRD)
PXRD patterns of Cu-NU-1000 were recorded on
EMPYREAN (PANalytical) with Cu Kα radiation (λ = 1.5406
Å, 45 kV, 40 mA).
Transmission Electron Microscopy (TEM)
The samples were infiltrated in LR White acrylic resin
(Electron Microscopy Sciences, Hatfield, PA) and
polymerized at 60 °C for 24 h. The embedded material was
sectioned to a 50 nm thickness on Leica ultramicrotome
(Ultracut E) using a Diatome diamond knife. The microtomed
samples were placed on 200-mesh lacey-carbon-coated Cu
grids (Ted Pella) and imaged with an aberration-corrected
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Journal of the American Chemical Society
JEOL 200F operating at 200 keV. The HAADF nominal probe
Page 12 of 16
ACKNOWLEDGMENT
1
size was ∼0.1 nm.
The authors gratefully acknowledge support for this work
from the Inorganometallic Catalyst Design Center, an EFRC
funded by the U.S. Department of Energy (DOE), Office of
Science, Office of Basic Energy Sciences (DE-SC0012702).
This research used resources of the APS, which is a DOE
Office of Science, Office of Basic Energy Sciences User
Facility. Sector 20 operations at the APS are supported by the
U.S. DOE (DE-AC02-06CH11357) and the Canadian Light
Source. We thank Dr. Mahalingam Balasubramanian (APS X-
ray Science Division) for assisting in the XAFS experiments.
Most of the experiments were performed at Pacific Northwest
2
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4
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6
7
8
9
Catalytic Testing
The activity of Cu-NU-1000 for methane oxidation to
methanol was tested either at atmospheric or elevated
pressure. The reaction was performed in a stainless-steel plug
flow reactor with a 4 mm inner diameter. The catalytic
reaction included three consecutive steps: (1) sample
activation in O2, (2) CH4 loading, and (3) water-steam purge
necessary for product desorption. Typically, 50 - 200 mg of
Cu-exchanged NU-1000 was loaded in the reactor and
activated in O2 flow (16 mL min–1) at 200 °C, followed by
flushing in He. Pure CH4 was flown subsequently in the flow
of 16 mL min–1 h at 150 or 200 °C for 3 h. After cooling to
135 °C in He flow, steam-assisted product desorption was
performed in 10/90 (molar) mixture of H2O/He purged at a
flow rate of 20 mL/min for up to 3 h. The oxidation products
were identified and quantified using online mass spectrometry
and by monitoring the time-dependent evolution of signals at
m/z of 31, 44 and 46, characteristic for methanol, CO2 and
dimethyl ether, respectively. The He signal (m/z = 4) was used
as an internal standard.
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National Laboratory (PNNL),
a multi-program national
laboratory operated by Battelle for the U.S. DOE. We also
acknowledge the Minnesota Supercomputing Institute (MSI)
at the University of Minnesota for providing resources that
contributed to the research results reported within this paper.
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Kulkarni, A. R.; Zhao, Z. J.; Siahrostami, S.; Norskov, J.
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