Methane Oxidation to Methanol Catalyzed by Cu-Oxo Clusters Stabilized in NU-1000 Metal-Organic Framework

Article abstract of DOI:10.1021/jacs.7b02936

Copper oxide clusters synthesized via atomic layer deposition on the nodes of the metal-organic framework (MOF) NU-1000 are active for oxidation of methane to methanol under mild reaction conditions. Analysis of chemical reactivity, in situ X-ray absorption spectroscopy, and density functional theory calculations are used to determine structure/activity relations in the Cu-NU-1000 catalytic system. The Cu-loaded MOF contained Cu-oxo clusters of a few Cu atoms. The Cu was present under ambient conditions as a mixture of ～15% Cu⁺ and ～85% Cu²⁺. The oxidation of methane on Cu-NU-1000 was accompanied by the reduction of 9% of the Cu in the catalyst from Cu²⁺ to Cu⁺. The products, methanol, dimethyl ether, and CO₂, were desorbed with the passage of 10% water/He at 135 °C, giving a carbon selectivity for methane to methanol of 45-60%. Cu oxo clusters stabilized in NU-1000 provide an active, first generation MOF-based, selective methane oxidation catalyst.

Full text of DOI:10.1021/jacs.7b02936

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Article
Methane oxidation to methanol catalyzed by Cu-oxo
clusters stabilized in NU-1000 metal-organic framework
Takaaki Ikuno, Jian Zheng, Aleksei Vjunov, Maricruz Sanchez-Sanchez, Manuel A. Ortuño, dale Pahls,
John L. Fulton, Donald M. Camaioni, Zhanyong Li, Debmalya Ray, B. Layla Mehdi, Nigel D. Browning,
Omar K. Farha, Joseph T. Hupp, Christopher J. Cramer, Laura Gagliardi, and Johannes A. Lercher
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 14 Jun 2017
Downloaded from http://pubs.acs.org on June 14, 2017
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Methane oxidation to methanol catalyzed by Cu-oxo clusters stabilized in
NU-1000 metal-organic framework
Takaaki Ikuno,† Jian Zheng,‡ Aleksei Vjunov,‡,∞ Maricruz SanchezꢀSanchez,† Manuel A. Ortuño,# Dale R. Pahls,# John L. Fulton,‡
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Donald M. Camaioni,‡ Zhanyong Li,§ Debmalya Ray,# B. Layla Mehdi,‡,ꢁ Nigel D. Browning,‡ Omar K. Farha,§,¶ Joseph T. Hupp,§
Christopher J. Cramer,# Laura Gagliardi,# and Johannes A. Lercher †,‡,
*
†Department of Chemistry and Catalysis Research Institute, TU München, Lichtenbergstrasse 4, 85748 Garching, Germany
‡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
ꢁMaterials Science and Engineering, University of Washington, Seattle, WA 98195, USA
§Department of Chemistry, and Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road,
Evanston, Illinois 60208, USA
¶Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
KEYWORDS Methane oxidation, methanol production, metalꢀorganic framework, Cu XAFS spectroscopy.
ABSTRACT: Copper oxide clusters synthesized via atomic layer deposition on the nodes of the metalꢀorganic framework NUꢀ
1000 are active for oxidation of methane to methanol under mild reaction conditions. Analysis of chemical reactivity, in situ Xꢀray
absorption spectroscopy, and density functional theory calculations are used to determine structure/activity relations in the CuꢀNUꢀ
1000 catalytic system. The Cuꢀloaded MOF contained Cu as clusters of a few atoms each. The Cu was present at ambient
conditions as a mixture of ~15 % Cu+ and ~ 85 % Cu2+. The oxidation of methane on CuꢀNUꢀ1000 was accompanied by the
reduction of 9 % Cu in the catalyst from Cu2+ to Cu+. The products, methanol, dimethyl ether and CO2, were desorbed with the
passage of 10% water/He at 135 °C, giving a carbon selectivity for methane to methanol of 45–60 %. CuꢀNUꢀ1000 is an interesting
firstꢀgeneration of MOFꢀbased selective methane oxidation catalysts stabilizing Cu oxoꢀclusters.
based nodes that provide reactive sites for postꢀsynthetic
INTRODUCTION
The catalyzed conversion of shale gasꢀderived light
deposition of metalꢀions, e.g., through atomic layer deposition
(ALD).4ꢀ5
hydrocarbons, e.g., methane to methanol, for further
application as automotive fuels and/or bulk chemicals is
especially attractive in light of improved methods of
hydrocarbon extraction.1 However, the lack of a transportation
infrastructure or an efficient method to upgrade gas to liquid
hydrocarbons at or near the well site often leads to flaring of
large quantities of natural gas causing both environmental
damage and economical loss. While methane can be converted
to methanol at very high temperature either in the presence of
water vapor or via a methaneꢀsyngasꢀmethanol process,2
a
direct methane oxidation pathway to methanol at low
temperatures would have a substantial advantage over the
existing routes. In the last decade, several groups have
reported that the formation of Cuꢀnanoclusters in the
micropores of zeolites leads to catalysts that are active and
selective for methane oxidation at moderate temperatures
(150ꢀ200 °C).3 The main limitation of such catalysts is the
strong adsorption of methanol on the hydrophilic zeolite.
Here, we describe the use of the NUꢀ1000 metalꢀorganic
framework (MOF),4 Zr6(ꢀ3ꢀOH)8(OH)8(1,3,6,8ꢀtetrakis(pꢀ
benzoate)pyrene)2, as a support (Figure 1) for copper oxo
clusters. This MOF is chosen because it has wellꢀdefined Zrꢀ
Figure 1. Views of the NUꢀ1000 crystal structure looking down
the c axis (left) and a/b axes (right) with a blowup of the Zr6 node
(Zr6(ꢀ3OH)8(OH)8) on which the Cu is deposited.
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grey and light grey spheres represent Zr, O, C, and H atoms,
respectively.
While MOFs have previously been demonstrated to be active
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for a range of catalytic reactions, e.g., selective hydrogenation
of 1ꢀoctene6 and ethylene,7 oxidative dehydrogenation of
propane,8 oxidation of CO to CO2,9 oxidation of organic
substrates,10 hydrodesulfurization reactions,11 and activation of
ethane,12 their use as catalysts for partial oxidation of methane
has not been attempted.
CuꢀXANES analysis. CuꢀXANES measurements were
performed to determine the electronic structure of Cu in Cuꢀ
NUꢀ1000 during the course of the reaction, i.e., ambient
conditions, activation in O2, and methane loading. Figure 3
shows the normalized CuꢀXANES spectra for CuꢀNUꢀ1000 as
well as reference compounds, Cu2O, CuO, aqueous Cu2+,
Cu(OH)2 and Cuꢀfoil.
We describe in this communication the synthesis,
characterization, and application of CuꢀNUꢀ1000 as a direct
methaneꢀtoꢀmethanol oxidation catalyst. In addition to
exploring the chemical reactivity, in situ Xꢀray absorption
spectroscopy (XAS) under catalytic conditions has been used
to follow the modification of the Cuꢀspecies and directly probe
the structure/activity properties of the CuꢀNUꢀ1000 system.
The insights from this first investigation of a metalꢀorganic
framework material as catalysts for methane oxidation lay the
foundation for the development of subsequent generations of
materials.
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RESULTS AND DISCUSSION
Properties of CuꢀNUꢀ1000. The CuꢀNUꢀ1000 investigated
here for methane oxidation to methanol was synthesized via
atomic layer deposition in metalꢀorganic frameworks (AIM).
The synthesis and characterization of CuꢀNUꢀ1000 are
detailed in the Methods section and Supporting Information
(SI). The concentration of Cu was 10 wt %, corresponding to
an average of approximately 4 Cu atoms per node. The crystal
structure of CuꢀNUꢀ1000 was analyzed by powder Xꢀray
diffraction (PXRD). The pattern (Figure S2) is like that of the
pristine NUꢀ1000, except for a loss of intensity of diffraction
peaks at 2θ = 7–10, corresponding to a decrease in the
coherency of planes with spacing of 0.8–1.2 nm. Recently, a
difference envelope density analysis of synchrotronꢀbased Xꢀ
ray scattering data has shown that ALD of Cu in NUꢀ1000
leads to the formation of Cuꢀoxo clusters within the small
pores that connect the triangular and hexagonal channels.13
Textural properties obtained by N2 sorption at 77 K (Figure
S17 and Table S5), show that the surface area and pore volume
decreased by ~30%, consistent with the installation of Cuꢀoxo
clusters and in line with observations for Znꢀ and NiꢀNUꢀ
1000,4, 14 Figure 2 shows HAADF STEM images for pristine
NUꢀ1000 and CuꢀNUꢀ1000. The lattice sites of NUꢀ1000 are
not changed by Cu deposition. The interꢀnode distances of
approximately 1.7 nm of both samples agree well with the
theoretical values of the parent NUꢀ1000 with a view of
looking down the aꢀaxis (Figure 2c). These results show that
the crystallinity and porosity of NUꢀ1000 are largely retained
after Cu deposition.
Figure 3. The normalized CuꢀXANES spectra of the CuꢀNUꢀ1000
and references are shown. The inserts demonstrate the preꢀedge
feature at 8977ꢀ8978 eV corresponding to the 1s to 3d electronic
transition for the distorted symmetry of Cu2+ as well as the 1s to
4p transitions for Cu+ and Cu2+ at 8982ꢀ8984 and 8985.5 eV,
respectively. The colorꢀcoding is reported in the legend.
CuꢀNUꢀ1000 most resembles the structure of Cu in
Cu(OH)2. There are several important observations in the Cuꢀ
XANES spectrum of CuꢀNUꢀ1000. First, the intensity of the
preꢀedge peak at 8977ꢀ8978 eV, attributed to the Cu2+ 1s→3d
electronic transition,15 indicates that most of Cu is present as
Cu2+ in an octahedral symmetry. As its normalized intensity is
lower than that for the Cu2+ standards, we conclude that a
fraction of Cu is present as Cu+. The presence of Cu+ would
reduce the overall intensity of the preꢀedge peak because Cu+
is a d10 ion and thus a 1s→3d transition is not allowed.16 The
second important spectral feature is observed at 8982ꢀ8984 eV
and is assigned to the 1s→4p electronic transition in Cu+.17
Previously Kau et al. demonstrated that the peak position for
this transition also reflects the coordination number of Cu+.
The 8982ꢀ8984 eV preꢀedge feature corresponds to twoꢀ and
threeꢀcoordinated geometries. The feature will shift to higher
energy in the case of 4ꢀcoordinate tetrahedral structure.18 Thus,
we assign the peak to twoꢀ or threeꢀcoordinated Cu+. The
pronounced shoulder at 8985.5 eV is attributed to the 1s→4p
transition of Cu2+ present in tetragonal symmetry.19 Finally,
there is no evidence of the presence of Cu0 in CuꢀNUꢀ1000 at
ambient conditions. We have also performed XANES linear
combination fitting to determine the concentrations of Cu+ and
Cu2+ using Cu2O and Cu(OH)2 as respective references.20 The
fit suggests that ~15 % of Cu is present as Cu+ and ~85 % as
Cu2+. These values are important as they allow us to determine
the extent of oxidation state modification during the catalytic
reaction steps discussed later.
Figure 2. HAADF STEM images of the polycrystalline NUꢀ1000
(a) and CuꢀNUꢀ1000 with the view of the aꢀaxis. The bright areas
in (a) represents (Zr6(µ3ꢀOH)8(OH)8) nodes and (b) represents the
Cu deposited (Zr6(µ3ꢀOH)8(OH)8) nodes with the pores between
them. The scale bar in (a) and (b) is 5 nm. (c) View of the parent
NUꢀ1000 crystal structure looking down the aꢀaxis. Blue, red,
Figure 4 shows the CuꢀXANES spectra acquired during Cuꢀ
NUꢀ1000 activation in flowing O2 (top) and subsequent
exposure to 1 bar CH4 (bottom) at 150 ºC. We observe an
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evolution of the XANES preꢀedge features during CuꢀNUꢀ spectra of CuꢀNUꢀ1000 and selected references. Cuꢀfoil and
1
1000 heat up to 150 ºC in 2.75 mL/min O2 flow. The minor
differences between the 25 ºC and 150 ºC spectra are
attributed to structural distortions of the Cuꢀspecies with
increasing. Overall, little change was observed in the Cuꢀ
XANES (as well as the EXAFS, see Figure S8) during 1 h
activation in flowing O2 at 150 ºC. Giordanino et al. reported
similar observations of 400 ºC oxygenꢀactivated CuꢀSSZꢀ13
zeolite and proposed that the minor changes are induced by the
partial dehydration (removal of physisorbed water) in the
vicinity of the Cuꢀspecies.17 It should be noted in passing that
Cu+ and Cu2+ species were concluded to be in equilibrium,
because both species coꢀexist even after treatment in O2 or He
at 400 ºC.17, 21 When the catalyst was exposed to CH4 (Figure
3), we observe an increase in the Cu+ 1s → 4p transition
intensity. The XANES linear combination fitting suggests that
~9 % of Cu was reduced from Cu2+ to Cu+ upon contact with
methane (see SI). However, because the spectral intensity at
8977ꢀ8978 eV remained largely unchanged, we conclude that
the majority of Cuꢀions in the MOF retains the “2+” oxidation
state.
CuO are not shown because these standards are least relevant
to the Cuꢀstructure in CuꢀNUꢀ1000. We begin the comparison
to references by examining the first shell CuꢀO single
scattering (SS) signal observed at R~1.4 Å in Figure 5. The
CuꢀO in CuꢀNUꢀ1000 has a somewhat shorter bond (peak is
shifted left in the plot) and has lower amplitude compared to
Cu(OH)2. This suggests multiple Cuꢀspecies and agrees well
with the XANESꢀdetermined distribution of ~15 % Cu+ and ~
85 % Cu2+. Also, we note that CuꢀNUꢀ1000 has a CuꢀCu SS
feature like in Cu(OH)2. Because this spectral feature is absent
in aqueous Cu2+, which represents an isolated Cuꢀion species,
we conclude that Cu in the MOF is most likely present as Cuꢀ
clusters rather than isolated cations (one Cu per Zr6ꢀnode
facet). We hypothesize that either small Cuꢀclusters of few Cu
atoms (e.g. 2ꢀ4 atoms) or widelyꢀspaced sheets similar to
Cu(OH)2 exist.22 Finally, similar to Cu(OH)2 and aqueous
Cu2+, there is a significant CuꢀO multiple scattering (MS) peak
at R=3.3 Å (2 × CuꢀO SS, distance uncorrected for phase shift)
that is characteristic of square planar Cu.23
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Figure 5. The k2ꢀweighted CuꢀEXAFS Img[χ(R)] spectra of Cuꢀ
NUꢀ1000 (black), Cu(OH)2 (red), aqueous Cu2+ (green) and Cu2O
(blue) are shown. The spectral regions most affected by the CuꢀO
single scattering (SS) and multiple scattering (MS) as well as Cuꢀ
Cu SS are also shown. Spectra recorded at room temperature.
Figure 4. The normalized CuꢀXANES spectra of CuꢀNUꢀ1000
during activation in oxygen (a) and methane loading (b) at 150 ºC
are shown. The inserts demonstrate the preꢀedge feature at 8977ꢀ
8978 eV corresponding to the 1s 3d electronic transition for the
distorted symmetry of Cu2+ as well as the 1s 4p transitions for Cu+
and Cu2+ at 8982ꢀ8984 and 8985.5 eV, respectively. The colorꢀ
coding is reported in the legend.
Figure 6. The k2ꢀweighted CuꢀEXAFS Img[
χ(R)] spectra of
CuꢀNUꢀ1000 (black) and the CuꢀNUꢀ1000 fit (magenta)
obtained using the Cu(OH)2 model are shown. The spectral
regions most affected by the CuꢀO single scattering (SS) and
multiple scattering (MS) as well as CuꢀCu SS are also shown.
CuꢀEXAFS analysis. CuꢀEXAFS analysis was performed in
order to determine the structural environment around the Cuꢀ
atoms. Figure 4 shows the k2ꢀweighted CuꢀEXAFS Img[
χ(R)]
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Because the spectral features of the present sample resemble but slightly shorter than the experimental value of 2.931 Å
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those of Cu(OH)2, we used Cuꢀhydroxide as a model to fit the
experimental Cu EXAFS spectra and to determine the CuꢀO
and CuꢀCu coordination numbers (CNs). The best fit is shown
in Figure 6 and the respective fitted parameters are reported in
Table 1. The fit result suggests that Cu is coordinated with
four (CN = 3.5±0.2) O atoms in the first shell with an average
CuꢀO distance of ~1.94 Å. The presence of the multiple
scattering distances at ~3.9 Å indicates that these four O atoms
are arranged in a square planar conformation. There is further
an O atom with a CuꢀO distance of ~2.33 Å. There is likely a
second axial O at even longer distance as in Cu(OH)2. We
suggest these are JahnꢀTeller24 distorted outꢀofꢀplane atoms,
which is typical for square pyramidal or strongly distorted
octahedral Cuꢀcoordination.22,25 The fit also indicates CuꢀCu
scattering at ~2.93 Å with an average fit coordination number
(Table 1). A simulated EXAFS spectrum of A (Figure S22 in
SI) exhibits a Cu–O SS peak in excellent agreement with that
found for CuꢀNUꢀ1000, and the Cu–Cu SS region is also in
overall reasonable agreement with the experimental spectrum.
The length of A is about 10 Å, a value close to the diameter
of the cꢀpore of NUꢀ1000 (Figure 1). Thus, we envision a
structure like A fitting into the cꢀpore (Figure 1 right) and be
anchored at either end by two different nodes.
A
computational model was designed to mimic that environment.
From periodic DFT calculations,26 we extracted a cluster
model containing two [Zr6(µ3ꢀO)4(µ3ꢀOH)4(OH)4(OH2)4]8+
nodes. The four [(1,3,6,8)ꢀtetrakis(pꢀbenzoate)pyrene]4–
(TBAPy4–) linkers connecting the two nodes were simplified
to [(1,6)ꢀbis(pꢀbenzoate)pyrene]2–; the eight TBAPy4– linkers
that coordinate to only one node were truncated to formate
groups, and finally, we removed one proton from the
coordinating aquo group on the bridged face of each node so
that a [Cu3(OH)4]2+ fragment could be inserted and anchored
by two hydroxyl groups of each node at either end. During the
optimization, the organic linkers were kept frozen to maintain
the rigidity of the MOF framework. The optimized structure,
A-2node, is shown in Figure 8. As surmised, the trimer fits
perfectly in the pore bridging the two nodes. The average Cu–
Cu distance is 2.979 Å, which is longer than for the bare
cluster A and in excellent agreement with the reported value of
2.931 Å (Table 1). The inclusion of dispersion interactions
during geometry optimization using M06ꢀLꢀD3 and PBEꢀD3
generated similar structures with average Cu–Cu distances of
2.979 Å and 2.980 Å, respectively. A structure was also
obtained from periodic PBEꢀD3 calculations. All the
computational models are structurally indistinguishable to
within the uncertainty in the experimental EXAFS distances
(cf. Table S6).
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of 1.3
±
0.3. The coordination number suggests that Cu is
present as Cuꢀhydroxo dimers, trimers or tetramers in the Cuꢀ
NUꢀ1000. Although not shown here, the addition of CuꢀZr
scattering to the EXAFS fitting did not improve the overall fit
quality. It indicates that the CuꢀOꢀZr sequences are either few
or significantly distorted.
Table 1. The average interatomic distances and
Debye-Waller factors (DWF) determined for Cu ions
in Cu-NU-1000 by fitting the experimental k2-
weighted spectrum to the Cu(OH)2 model derived
using FEFF9. a
Shell
CN
3.5±0.2 1.94±0.01
1 (set)b
2.33±0.01
Distance (Å)
DWF
CuꢀO1 SS
CuꢀO2 SS
0.0044±0.0007
0.0024±0.0020
0.0083 (set)b
0.011±0.008
0.011±0.008
CuꢀCu SS 1.3±0.3 2.93±0.02
CuꢀO1 MS
CuꢀO1 MS
ꢀ
ꢀ
3.88±0.02
3.97±0.05
a CN = average coordination number, SS = single
scattering, MS = multiple scattering; other parameters:
amplitude reduction factor (amp) = 0.83, Rꢀfactor = 0.013,
and E0 = ꢀ8.6. bThe value for Cu(OH)2 (Table S2).
Computational modeling of CuꢀNUꢀ1000. Having analyzed
the bulk features of CuꢀNUꢀ1000 based on EXAFS data, we
turn to theory to gain insight at the atomic level. All
calculations described below were performed at the density
functional theory (DFT) level using the M06ꢀL functional (cf.
Computational methods in Experimental Section).
Figure 8. DFTꢀoptimized structure of trimer A-2node. The two
CuꢀCu distances to the central metal atom are 2.968 and 2.990 Å.
Figure 7. DFTꢀoptimized structure of trimer A.
To further assess the relevance of this structure, we
simulated the EXAFS spectrum of A-2node and compared it
to experiment for CuꢀNUꢀ1000 (Figure 9). The Cu–O SS
peaks at ~1.4 Å and ~1.9 Å are in good agreement, and the
positions of the Cu–Cu SS peaks of the simulated spectrum
also match well with experiment (except for the signal at ~2.4
Å which has a slightly longer bond in the computational model
Given the strong similarities identified in the EXAFS of
Cu(OH)2 and CuꢀNUꢀ1000 (Figure 5), we initially optimized a
bare trinuclear cluster, [Cu(OH)2]3·2H2O A (Figure 7). All
three CuII atoms have fourꢀcoordinate, squareꢀplanar
environments. The average Cu–Cu distance is 2.845 Å, similar
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than seen in experiment). Additionally, from the kꢀ dimethyl ether and CO2. We note that water was also produced
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representation we observe that the Cu–Cu and Cu–Zr
scatterings appear to be in phase (see Figure S23 in SI).
during CH4 oxidation, however, the amount is much lower
compared to the steam used in the process, and as it does not
contribute to the carbon balance, we neglected its
concentration. We performed three cycles of the three process
steps to determine the activity and the stability of CuꢀNUꢀ
1000. As shown in Table 2 (condition A), in the first cycle we
observed products equivalent to 43.9 µmolcarbon/gCuꢀNUꢀ1000, the
selectivity to methanol and dimethyl ether (17.7 and 2.0 µmol,
respectively) being ~45 %. A decrease in activity in the
subsequent two cycles was observed, with overall yields of
30.7 and 21.8 µmolcarbon/gCuꢀNUꢀ1000, respectively. However, the
formation of CO2 decreased significantly (40% per cycle),
while the methanol and dimethyl ether decreased only 14% per
cycle. Thus, the selectivity towards methanol and dimethyl
ether increased to 53% and 61 %, respectively. When the
methaneꢀloading step was omitted (condition B), then no
methanol or dimethyl ether was produced.
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We also tested the methane to methanol reaction with using
50 % H2O and 50 % He to desorb the products in the third
step, but this produced more CO2 and less methanol and
dimethyl ether (Table 2, conditions C). While total products
equivalent to 60 µmol carbon/g CuꢀNUꢀ1000 were obtained in
the first catalytic cycle, the combined yield of methanol
dimethyl ether was only approximately 8 µmolcarbon/g (14 %
selectivity). We speculate that the increased amount of CO2
formed stems mainly from decarboxylation of linkers in Cuꢀ
NUꢀ1000, since 60 % of the pore volume was lost after the
first cycle (Table S5) and even NUꢀ1000 produces CO2 when
subjected to the same conditions (Table 2 condition D).28 In
contrast, when 10% steam in He was used, the change in the
pore volume and surface area of the CuꢀNUꢀ1000 was only 1 ꢀ
2 %.
Figure 9. The k2ꢀweighted CuꢀEXAFS Img[
χ(R)] spectra are
shown for CuꢀNUꢀ1000 (black) and the A-2node (simulated, red)
structure in Figure 7
Catalytic testing of CuꢀNUꢀ1000. The activity of CuꢀNUꢀ
1000 for methane oxidation to methanol was tested in a
process consisting of three sequential reaction steps.27 First,
CuꢀNUꢀ1000 was pretreated in O2 flow at 200 ºC to remove
physisorbed water. Then, the reactor was cooled to 150 ºC and
purged with pure He. In the second step, the pretreated Cuꢀ
NUꢀ1000 was exposed to a flow of CH4 at 150 ºC, which
causes partial reduction of Cu2+ to Cu+, as monitored by
XANES. In the final step, water was introduced in the form of
10% steam in He at 135 °C to desorb the products methanol,
Table 2. The activity and selectivity for the oxidation of methane by Cu-NU-1000.
Conditionsa Cycle
Carbon Yield (µmol/g)
CO2
Carbon selectivity (%)
Methanol
DME
Totalb
Methanol + DME
CO2
A
1
2
3
1
1
2
3
17.7
15.8
13.2
0
2.0
0.6
0.1
0
24.2
14.3
8.5
43.9
30.7
21.8
11.8
59.9
35.4
33.0
45
53
61
0
55
47
39
100
86
90
91
B
C
11.8
51.7
31.7
30.1
6.9
2.9
2.2
1.3
0.7
0.7
14
10
9
D
1
2
0.5
0.4
0.5
0.6
34.5
21.7
35.5
22.7
3
4
97
96
aConditions: (A) CuꢀNUꢀ1000 with activation in O2 at 200 °C for 3 h, CH4 loading at 150 °C for 3 h, and steamꢀassisted product
desorption in the flow of 10% H2O and 90% He for 2 h at 135 °C; (B) same conditions as A except methane loading step was skipped;
(C) CuꢀNU1000 with activation in O2 at 150 °C for 1 h, CH4 loading at 150 °C for 3 h, and steamꢀassisted product desorption in the
flow of 50% H2O and 50% He for 30 min at 135 °C; (D) same conditions as C except using NUꢀ1000 in place of CuꢀNUꢀ1000. bThe
values are calculated based on all the collected products.
The origin of the Cu selectivity in CuꢀNUꢀ1000. Let us now
explore the origin of the Cuꢀactivity in CuꢀNUꢀ1000 in
relation to the fraction of Cu(II) that is observed to be reduced
by XANES analysis. We suggest that the 15 % Cu+ already
present under ambient conditions does not contribute to
methane conversion. Because Cu+ has only a very limited
oxidation potential29 and we do not observe any formation of
Cu0 via EXAFS. Only minor changes were observed in the
Cu2+/Cu+ ratio after treatment at 200 °C. Upon methane
loading in the second step of the process, we observed an
increase in the fraction of Cu+ from 15 to 24 %, which is
suggested to be the consequence of a Cu(II)/methane redox
couple.30 We surmise that the extent of Cu2+ reduction in Cuꢀ
clusters of a few atoms via methane loading at 150 ºC is
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thermodynamically limited.31 We, hence, conclude that the however, leads to a gradual increase in the fraction of Cu+,
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methane conversion and selectivity levels attributable to the
metal deposited on the MOF nodes are achieved by just a
fraction (9 %) of the Cu in CuꢀNUꢀ1000. This fraction
amounts to 149 µmol/g of the total 1656 µmol/g Cu species on
the node. The stoichiometries for forming methanol and CO2
can be calculated by equations 1 and 2, respectively, where the
active Cu(II) species is represented as Cu(OH)2. The oxidation
of methane to methanol and methane to CO2 require 2 and 8
equivalents of Cu(OH)2, respectively.
from 15 to 24 %, indicative of methane activation. While the
activity of CuꢀNUꢀ1000 for methane oxidation to methanol
indicates that a significant fraction of the Cu atoms appears to
be spectators, active Cu species deposited in NUꢀ1000 convert
methane at 150 °C with a 45–60 % selectivity to methanol and
dimethyl ether, the remainder being CO2. Excess CO2
produced during the first cycle derives from partial
decarboxylation of the NUꢀ1000 linkers under reaction
conditions.
9
CH4 + 2Cu(OH)2 ⇌ CH3OH + Cu2O + 2H2O
CH4 + 8Cu(OH)2 ⇌ CO2 + 4Cu2O + 10H2O
(1)
(2)
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EXPERIMENTAL SECTION
Chemicals. Highꢀpurity (99.995%) helium, oxygen and methane
gases were purchased from Matheson and used without further
purification. Deuterated dimethylsulfoxide (DMSOꢀd6) was obtained
from Cambridge Isotope Laboratory and deuterated sulfuric acid
D2SO4 (d2) (96−98% solution in D2O) was obtained from Sigma
Aldrich.
Thus, in the first cycle, it requires 43.9 µmol CH4 to reduce
233 µmol Cu(OH)2 to form 19.7 µmolcarbon methanol plus
dimethyl ether and 24.2 µmol CO2. As this is 84 µmol
Cu(OH)2 more than the 149 µmol predicted to be converted by
the XANES analysis (Figure 3), it is concluded that excess
CO2 is produced during the steaming step via decarboxylation
of CuꢀNUꢀ1000. We note that there are precedents for
decarboxylation of carboxylic acids by Cu(II) ions32 and the
proto decarboxylation of aromatic carboxylic acids is
catalyzed by Cu(I) ions.33 Thus, assuming that approximately
150 µmol of Cu(OH)2 are converted during the methane
loading step, which transform 33.5 µmol CH4 to 19.7 µmol
methanol plus dimethyl ether and 13.8 µmol CO2, the 10.4
µmol CO2 is attributed to the decarboxylation of linkers.
Indeed, in line with this estimate, 11.8 µmol/g of CO2 was
observed as a sole product during steam treatment in 10% H2O
and 90% He in a blank test (Table 2, condition B) in which the
catalyst was not contacted with methane. Taking this into
account, the selectivity for methane oxidation to methanol and
dimethyl ether in the first cycle is as high as 56 %, which is
comparable to the selectivity measured in the subsequent
cycles. As the yields of CO2 in the second and third cycles are
much lower than in the first cycle, we conclude that
decarboxylation of linkers becomes less significant in
subsequent cycles. It should be noted that nevertheless a
partial deactivation of the catalyst with each cycle is evident
from the decrease in methanol and DME yield from 19.7
µmol/g to 13.3 µmol/g. We tentatively attribute this to the
deactivation of a fraction of the active Cu species. The
evidence raises interesting questions about why only a small
fraction (~0.5%) of the available carboxylates seems
susceptible to degradation. It might be, for example, that a
minor fraction of carboxylates in CuꢀNUꢀ1000 feature metal
coordination that differs from that indicated by the singleꢀ
crystal Xꢀray structure of the parent material.
Material synthesis. The NUꢀ1000 metalꢀorganic framework as well
as the Cuꢀcontaining CuꢀNUꢀ1000 were synthesized via atomic layer
deposition similar to the literature procedure reported elsewhere.4
Bis(dimethylaminoꢀ2ꢀpropoxy)copper(II), Cu(dmap)2 (reactant A,
Figure S1), was chosen as the Cu precursor. Room temperature
deionized H2O was used as the coreactant (reactant B). In a typical
experiment, a customꢀmade stainless steel powder sample holder
containing microcrystalline NUꢀ1000 (60.0 mg, 0.028 mmol) was
placed in the ALD chamber, which was held at 110 °C for 30 min to
remove any physisorbed water before dosing with the Cu precursor. A
cylinder containing Cu(dmap)2 was held at 100 °C, and each of its
pulses followed the time sequence of t1−t2−t3, where t1 is the precursor
pulse time, t2 is the substrate exposure time, and t3 is the N2 purge
time (t1 = 1 s, t2 = t3 = 240 s). To ensure full metalation of the Zr6 sites
throughout the microcrystals, the Cu(dmap)2 pulsing cycle was
repeated 80× before exposing the MOF to H2O pulses, using the same
time sequence as for the Cu pulse (t1 = 0.015 s, t2 = t3 = 120 s). The
concentration of Cu in CuꢀNUꢀ1000 samples was determined from
inductively coupled plasma–atomic emission spectroscopy (ICP–
AES) measurements.
Nearꢀedge structure (XANES) and extended Xꢀray absorption fineꢀ
structure (EXAFS) measurements. The XAFS measurement
experiments took place at the Pacific Northwest Consortium/Xꢀray
Science Division (PNC/XSD) bendingꢀmagnet beamline at Sector 20
of the Advanced Photon Source (APS) at Argonne National
Laboratory (ANL). 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 Cuꢀfoil was placed downstream of the
sample cell, as a reference to calibrate the photon energy of each
spectrum. The XAFS reaction cell used in this work is shown in the
SI. The cell is based on a HiP© medium pressure Hastelloy® tee
modified to house the 4ꢀsample holder and allow gas flowꢀthrough
capability. Glassyꢀcarbon discs (thickness = 0.75 mm, diameter = 5
mm) were used as the Xꢀray windows. The CuꢀNUꢀ1000 samples
were pressed (0.25 ton) into pellets (see SI) that were ~0.5 mm thick.
The pellets are held in the sample holder (see SI). Prior to data
acquisition, all gas lines were purged with dry He, then with dry O2 or
CH4, for activation and loading, respectively. A flowꢀrate of 2.75
mL/min was used during data acquisition. Data analysis and
background removal were performed using ATHENA and ARTEMIS
programs from the iXAFS software package.34 The Fourier transform
CONCLUSION
Cu oxide clusters deposited on the ZrO2 nodes of NUꢀ1000
by AIM catalyze partial oxidation of methane. The state of the
Cu was determined by XAS analysis to contain ~15 % Cu+ and
~85% Cu2+ at ambient conditions. The average structure of the
Cu is square planar coordinated by four O atoms (CuꢀO =
~1.94 Å) with an additional one or two outꢀofꢀplane Jahnꢀ
Tellerꢀdistorted O atoms, one of them has CuꢀO = ~2.33 Å). A
CuꢀCu distance of ~2.93 Å was observed and the coordination
number of Cu was determined at 1.3±0.3. A combined EXAFS
and DFT study indicates that the cluster that predominates in
CuꢀNUꢀ1000 is likely to be a trimeric Cuꢀhydroxideꢀlike
structure that bridges two nodes across the cꢀpore of the MOF.
of the kꢀspace EXAFS data (both the real and imaginary parts of
χ(R)
were fitted to theoretical models derived using the FEFF9 code.35
EXAFS spectra of calculated Cuꢀclusters were simulated using ab
initio scattering theory by applying approximate global disorder
parameters, σ2, corresponding to 300 K (e.g., Figure 9). The computed
coordinates were used to generate the primary input for the ab initio
EXAFS scattering code (FEFF9)35 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
Catalyst preꢀtreatment in O2 flow does not change the
Cu2+/Cu+ ratio. Subsequent loading with CH4 at 150 ºC,
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universal value of the DWF, σ2=0.005. The obtained spectra for each
1*1*1 kꢀpoint grid was used for the Brillouin Zone sampling. An
energy convergence criteria of 10−6 eV and a force convergence
criteria of 0.05 eV/Å were used. All calculations were performed
considering spin polarization.
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 overꢀprediction of these amplitudes, although the atom positions
predicted by the theory are correctly represented.
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ASSOCIATED CONTENT
Supporting Information
Thermogravimetric analysis (TGA). Thermogravimetric analysis
and differential scanning calorimetry (TGꢀDSC) analyses were
PXRD and STEM characterizations, the design of the XAFS cell,
details of XAFS fitting analysis, the NUꢀ1000 and CuꢀNUꢀ1000
performed on
a
SENSYS EVO TGꢀDSC (SETARAM
1
Instrumentation) at the ramp rate of 3 K/min.
stability analysis under reaction conditions, HꢀNMR spectrum of
9
N2ꢀsorption. N2ꢀphysisorption for surface area and pore volumes
was obtained on a Micromeritics ASAP 2020 setup. The carbon slit
pore model with a NLꢀDFT method was used. Single point adsorption
close to P/P0 = 0.99 was used to determine the total pore volume.
Powder XRD. Powder XRD patters of CuꢀNU1000 were recorded
on EMPYREAN (PANalytical) with CuꢀKα radiation (λ = 1.5406 Å,
45 kV, 40 mA).
STEM. 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 200ꢀmesh
laceyꢀcarbonꢀcoated Cuꢀgrids (Ted Pella) and imaged with an
aberrationꢀcorrected JEOL 200F operating at 200 KeV. The high
angle annular dark field (HAADF) nominal probe size was ~0.1nm.
1HꢀNMR. Spectra were recorded on a Bruker 500 FTꢀNMR
spectrometer. 1H chemical shifts are reported in ppm with the residual
solvent resonances as the reference. MOF samples (~2–5 mg) were
dissolved in 4–6 drops of concentrated D2SO4 and then diluted with
DMSOꢀd6 before analysis.
CuꢀNUꢀ1000 dissolved in D2SO4/DMSOꢀd6, DFTꢀoptimized
clusters CunꢀNUꢀ1000 (n = 1–3) and Periodic DFTꢀoptimized
Cu3ꢀNUꢀ1000 unit cell Cartesian coordinates. The Supporting
Information is available free of charge on the ACS Publications
website.
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AUTHOR INFORMATION
Corresponding Author
*Johannes.Lercher@pnnl.gov, Johannes.Lercher@ch.tum.de
Author Contributions
T.I. and J.Z. contributed equally.
Present Addresses
∞BASF Corporation, 100 Park Ave, Florham Park, NJ 07932
ACKNOWLEDGMENT
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 (DEꢀSC0012702). 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 (DEꢀAC02ꢀ06CH11357) and
the Canadian Light Source. We thank Dr. Mahalingam
Balasubramanian (APS Xꢀray Science Division) for assisting in
the XAFS experiments. The STEM work was performed using the
Catalytic testing. The activity of CuꢀNUꢀ1000 for methane
oxidation to methanol was tested at atmospheric pressure. The
reaction was performed in a stainlessꢀsteel plug flow reactor with a 4ꢀ
mm inner diameter. The catalytic reaction included three consecutive
steps: 1) CuꢀNUꢀ1000 activation in O2, 2) CuꢀNUꢀ1000 loading with
CH4, and 3) waterꢀsteam purge necessary for product desorption. In a
typical experiment: 20 mg of CuꢀNUꢀ1000 was loaded in the reactor
and activated in O2 flow (16 mL minꢀ1) at 200 °C, followed by
flushing in He. CH4 was loaded subsequently in the flow (16 mL minꢀ
1) as a mixture of 90 % CH4 and 10 % He for 4 h at 150 °C. After
cooling to 135 °C in He flow, steamꢀassisted product desorption was
performed in either 50/50 or 10/90 mixture of H2O/He purged at a
flow rate of 20 mL/min for 2 h. The oxidation products were
identified and quantified using online mass spectrometry and by
monitoring the timeꢀdependent evolution of signals at m/z 31, 44 and
46, characteristic for methanol, CO2 and dimethyl ether, respectively.
The He signal (m/z = 4) was used as an internal standard. Selectivity
and yield were determined from the integral of the product
concentrations as a function of time. Dimethyl ether was assumed to
have been formed by condensation of two molecules of methanol via
loss of water.
Computational methods. All calculations were performed at the
density functional theory (DFT) level using the M06ꢀL functional36 as
implemented in Gaussian 09.37 The M06ꢀL functional performs well
for mediumꢀrange electron correlation effects, transition metal
chemistry,38 and the modeling of zeolites.38a For selected structures
longerꢀrange dispersion interactions were included by adding the D3
correction39 to both the M06ꢀL and PBE40 levels of theory. Numerical
integrations were performed with an ultrafine grid. To reduce
computational cost, an automatic densityꢀfitting set generated by the
Gaussian program was used. The 6ꢀ31G(d) basis set was used for H,
C, and O;41 the SDD pseudopotential and its associated double zeta
basis set was employed for Cu and Zr.42 During optimization, the Zr6ꢀ
node and Cu clusters were relaxed while the linkers were kept frozen.
All Cu(II)3ꢀclusters were calculated in the highꢀspin state (quartet).
Minima were confirmed by frequency calculations for all species
(except for A-2node) at the same level of theory (298.15 K).
Environmental Molecular Sciences Laboratory (EMSL),
a
national scientific user facility sponsored by the DOE Office of
Biological and Environmental Research and located at Pacific
Northwest National Laboratory (PNNL), a multiꢀprogram national
laboratory operated by Battelle for the U.S. DOE. B.L.M.
acknowledges support by PNNL’s Laboratory Directed Research
and Development (LDRD) program. N.D.B. acknowledges
support by PNNL’s Chemical Imaging Initiative LDRD Program.
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.
ABBREVIATIONS
XAS, Xꢀray Absorption Spectroscopy; XANES, Xꢀray
Absorption Near Edge Structure; EXAFS, Extended Xꢀray
Absorption Fine Structure; MOF, MetalꢀOrganic Framework;
AIM, Atomic layer deposition In Metalꢀorganic frameworks;
CN, Coordination Number; DWF, DebyeꢀWallerꢀFactor;
XRD, Xꢀray diffraction; TGA, Thermogravimetric analysis;
DSC, differential scanning calorimetry; DFT, density
functional theory; HAADFꢀSTEM, Highꢀangle annular darkꢀ
field Scanning transmission electron microscopy.
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Periodic calculations were performed using the Vienna Ab Initio
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approximation exchange correlation functional PBE40 with the D3
correction.39 A plane wave kinetic energy cut off of 520 eV and a
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