Monomeric Copper(II) Sites Supported on Alumina Selectively Convert Methane to Methanol

Article abstract of DOI:10.1002/anie.201903802

Monomeric Cu^II sites supported on alumina, prepared using surface organometallic chemistry, convert CH₄ to CH₃OH selectively. This reaction takes place by formation of CH₃O surface species with the concomitant reduction of two monomeric Cu^II sites to Cu^I, according to mass balance analysis, infrared, solid-state nuclear magnetic resonance, X-ray absorption, and electron paramagnetic resonance spectroscopy studies. This material contains a significant fraction of Cu active sites (22 %) and displays a selectivity for CH₃OH exceeding 83 %, based on the number of electrons involved in the transformation. These alumina-supported Cu^II sites reveal that C?H bond activation, along with the formation of CH₃O- surface species, can occur on pairs of proximal monomeric Cu^II sites in a short reaction time.

Full text of DOI:10.1002/anie.201903802

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Accepted Article
Title: Monomeric Copper (II) Sites Supported on Alumina Selectively
Convert Methane to Methanol
Authors: Christophe Copéret, Jordan Meyet, Keith Searles, Mark A.
Newton, Michael Wörle, Alexander P. van Bavel, Andrew D.
Horton, and Jeroen A. van Bokhoven
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201903802
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http://dx.doi.org/10.1002/ange.201903802

10.1002/anie.201903802
Angewandte Chemie International Edition
COMMUNICATION
Monomeric Copper (II) Sites Supported on Alumina Selectively
Convert Methane to Methanol
Jordan Meyet, Keith Searles, Mark A. Newton, Michael Wörle, Alexander P. van Bavel, Andrew D. Horton, Jeroen A. van Bokhoven*,
Christophe Copéret*
Abstract: Monomeric CuII sites supported on alumina, prepared via
surface organometallic chemistry, selectively convert CH4 to CH3OH.
This reaction takes place via formation of CH3O surface species with
the concomitant reduction of two monomeric CuII sites to CuI,
according to mass balance analysis, IR, solid-state NMR, XAS and
EPR studies. This material contains a significant fraction of Cu active
sites (22 %) and displays a selectivity for CH3OH exceeding 83 %,
based on the number of electrons involved for this transformation.
These alumina-supported CuII sites show short reaction time and
demonstrate that C-H bond activation along with methoxy surface
species formation can occur on pairs of monomeric CuII sites.
A major challenge in these systems is associated with the
presence of a small fraction of active sites (often below 30%
even if the presence of 90% of active sites – 0.47 mol
CH3OH/mol Cu – have been recently reported),[18,19] rendering
unequivocal conclusions regarding active site structure
difficult.[13] Cu supported on non-zeolitic materials namely
amorphous silica, has also shown to be reactive for this
transformation, indicating that confinement is not required, but
the methanol yield does not exceed 3.6% CH3OH per Cu.[20,21]
Surface organometallic chemistry (SOMC)[22,23] combined
with thermolytic molecular precursors (TMP)[24] has emerged as
a powerful approach to generate supported isolated metal sites
with tailored nuclearity and oxidation state for a broad range of
metals.[25,26] This approach consists of grafting tailored molecular
precursors on supports with a controlled OH density, followed by
a post-treatment that removes organic ligands and generates
the desired isolated metal sites. Since nuclearity and local
environment are both key features for the selective CH4 to
CH3OH conversion on Cu sites, we reasoned that the
SOMC/TMP approach could be ideal to generate monomeric
CuII sites and to evaluate their reactivity for this reaction.
CH4, the main constituent of natural gas, is increasingly
produced by fracking and widely available at extraction sites for
fossil fuel. Its transport as liquefied natural gas is, however,
expensive and energy intensive, hence it is often flared despite
the highly negative environmental impacts and loss of resource.
Finding efficient routes to convert on-site CH4 to liquid fuels or
chemicals is, therefore, of significant economic and
environmental importance. As such, its direct conversion to
CH3OH is noteworthy since it provides a liquid that can be
directly used as chemical feedstock, fuel or fuel additive.
However, this process remains a grand challenge because over-
oxidation is favored due to the higher reactivity of CH3OH than
CH4.[1–5] In nature, bacterial enzymes, such as methane mono-
oxygenases, are able to oxidize CH4 to CH3OH in high selectivity
using O2 as primary oxidant.[6] These enzymes, containing Cu or
Fe metal centers, have been a source of inspiration to design
materials for the conversion of CH4 to CH3OH under mild
reaction conditions.[7] Attempts to perform this reaction
catalytically with tailored materials have been mostly
unsuccessful since at high conversion the selectivity towards
CH3OH is low. This hurdle can be in principle overcome by using
the concept of chemical looping, that consists in decoupling the
steps of oxidation of Cu sites and their reaction with CH4.[8] The
last step of the cycle consists of the desorption of the methoxy
species strongly bound to the copper sites using a protic solvent.
Of the various systems that have been investigated, Cu-
exchanged zeolites have shown potential for the selective
oxidation of CH4 to CH3OH using molecular oxygen as oxidant
Here, we report the synthesis of a monomeric CuII siloxide
molecular
precursor
[Cu(OSi(OtBu)3)2(TMEDA)]
(1;
tetramethylethylenediamine, TMEDA) and its use to generate
isolated CuII sites on alumina with the goal to preserve a silicon
and aluminum environment, as found in zeolites, without the
microporous environment (Figure 1b-c). We also prepared a
dimeric complex, [Cu(2,µ2-OSi(OtBu)3)(OSi(OtBu)3)]2 (2, Figure
1b), as a spectroscopic molecular probe for oxygen-bridged
dimeric Cu species and to evaluate its surface chemistry
towards the generation of CuII sites. Using a combination of
spectroscopic methods (UV-Vis, EPR, XAS), we show that
independently of the molecular precursors 1 or 2, mononuclear
CuII sites in an alumino-silicate environment are mostly formed
after thermal treatment at 700 ºC. These mononuclear sites
show unprecedented reactivity, activating CH4 within minutes at
200 °C and yielding CH3OH after hydrolysis in high selectivity
(>80%). CH4 activation takes place on a high fraction of proximal
monomeric CuII sites (22% of Cu) dispersed at the alumina
surface via a 2-electron oxidation process, indicating that di- or
tri-nuclear Cu-µ-oxo sites and/or micropores are not required for
the selective conversion of CH4 to CH3OH, thus contrasting the
expected situation in the corresponding zeolitic materials.
First, the dimeric complex (2) was prepared by reaction of
[Cu(OTf)2]x with 2 equiv. of NaOSi(OtBu)3 in THF and is further
converted to [Cu(OSi(OtBu)3)2(TMEDA)] (1) upon addition of
TMEDA. The complex (1) is isolated as light green single
crystals in 54% overall yield; it crystalizes in the P-1 space
group, where Cu occupies a distorted tetrahedral environment
(4’ = 0.65) as found in [Cu(OSi(OtBu)3)2(py)2].[27] This complex is
associated with (i) a typical EPR spectrum with g⊥ > g‖ ≈ ge and
a dz2 ground state (g⊥ = 2.27; g‖ = 2.00 A‖ ≈ 450 MHz, Fig. S1)
source.[9–13] The structure of the active sites is highly debated,
[9,14]
with evidence for both (µ-oxo) dicopper
and tris(µ-oxo)
trinuclear copper centers (Figure 1a);[15] recent theoretical
studies also suggest that monomeric Cu sites should not be
excluded.[16,17]
[*] J. Meyet, Dr. K. Searles, Dr. M. A. Newton, Dr. M. Wörle, Prof. Dr. C.
Copéret, Prof. Dr. J. A. van Bokhoven
Department of Chemistry and Applied Biosciences, ETH Zurich
Vladimir-Prelog-Weg 1-5, 8093 Zürich, Switzerland
E-mail: ccoperet@inorg.chem.ethz.ch
and (ii) an optical spectrum with characteristic max at 267 and
317 nm associated with the ligand to metal charge transfer
(LMCT) of the two terminal siloxides ligands and the two
nitrogen ligands bound to Cu, respectively (Fig. S2). For (2),
[Cu(2,µ2-OSi(OtBu)3) (OSi(OtBu)3)]2, isolated in 56% yield as
light green single crystals, the two Cu centers adopt a distorted
square planar geometry (4’ = 0.38 Cu1 and 0.27 Cu2),
Prof. Dr. J. A. van Bokhoven
Laboratory for Catalysis and Sustainable Chemistry, Paul Scherrer Institute
Villigen 5232, Switzerland
E-mail: jeroen.vanbokhoven@chem.ethz.ch
Dr. A. P. van Bavel, Dr. A. D. Horton
Shell Global Solutions International B.V.
Grasweg 31, 1031 HW Amsterdam, The Netherlands
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10.1002/anie.201903802
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coordinated by one terminal, one 2, and one bridging siloxide
ligand according to single crystal X-ray diffraction (Fig. S3).
Figure 1. a) Proposed active sites in Cu-exchanged zeolites b) synthesis and structure of CuII molecular precursors (1 and 2) c) strategy and results towards
generating monomeric CuII sites on alumina.
Its EPR spectrum shows g‖ > g⊥ > ge associated with a dx2
be fitted in the second coordination shell, which is fewer than
expected, but the Al/Si contribution to the scattering of the
second shell was found to be minor for 2 (Fig. S23).
2
-y
ground state, while the UV-Vis spectrum shows bands at 248
and 347 nm, associated with the LMCT bands arising from the
terminal and bridging oxygen atoms respectively (see ESI,
Section 1 for a complete characterization).
Grafting of 1 on alumina partially dehydroxylated at 700 °C
(Al2O3−700, = 0.7 OH.nm−2) in C6H6 yields trace amounts of
HOSi(OtBu)3 and 0.9 equiv. of isobutene per surface OH. The
blue solid recovered after washing and drying under high
vacuum (10-5 mbar) contains 0.46 wt% Cu, corresponding to
0.46 Cu.nm-2. IR spectroscopy of the grafted material shows the
disappearance of bands associated with surface OH groups and
concomitant appearance of (C-H) and (C-H) bands at
2975−2801 cm-1 and 1470−1365 cm-1, respectively (Fig. S8-9).
Cu K-edge X-ray absorption spectroscopy (XAS) reveals a
complex grafting process as evidenced by the appearance of a
pre-edge feature at 8982.0 eV in the spectrum (Fig. S14),
indicating partial reduction of a fraction of CuII to CuI; the
presence of CuII sites being confirmed by EPR (Fig. S.12).
However, treatment under a flow of dry synthetic air at 700 °C
yields a green solid 1-a700 free of organic ligands according to IR
spectroscopy (Fig. S8-9). This material contains exclusively CuII
sites as evidenced by XAS with an edge energy at 8986.0 eV
(Fig. S14). The UV-vis spectrum of 1-a700 shows a broad band
centered at 900 nm, corresponding to the d-d transition of CuII
species, and a sharp signal centered at 250 nm (Fig. S10),
corresponding to the LMCT of oxygen to a single Cu center
(Cu2+-O2-), consistent with the presence of monomeric CuII
ions.[28] The absence of absorption bands from 300–600 nm
shows that CuO clusters or peroxo- or µ-oxo species ,if present
at all, are only minor species, in contrast to the significant
amount of Cu associated to the reactivity with CH4 [29,30]. EPR
spectroscopy shows the presence of an anisotropic signal with
an axial configuration, implying the localization of the unpaired
electron in the d 2 orbital (g‖ = 2.31 A‖ ≈ 540 MHz; g⊥ = 2.11,
Figure 2. (left to right) Fourier transform of Cu K-edge EXAFS spectra for the
activated material (1-a700), the mononuclear (1) and dinuclear (2) complexes
Material
Element
O
CN[a]
3
R (Å)[b]
1.93
2.8
DW (2 2) [c]
0.016
R (%)[d]
22.8
1-a700
Si/Al
1.1
0.022
1
2
O
N
2
2
1.83
2.19
0.004
0.027
37
O
O
1
1
1.81
1.92
0.007
0.012
20.2
O
O
0.5
0.5
1
2.26
2.49
2.88
0.013
0.007
0.01
2
-y
x
Cu
Fig. S12). However, the signal cannot be completely described
by a single axial parameter due to the broadening induced from
Table 1. Curve fitting analysis of the activated material (1-a700) and the
mononuclear (1) and dinuclear (2) molecular precursors. [a] Coordination
number (± 10%) [b] Scatterer distance from central atom (± 1.5-2%) [c] Debye-
Waller factor where  is the root mean square inter-nuclear separation (Å) [d]
iN 1/i (ie (k) - it (k))2 x 100% (see ESI for complete definition).
magnetic
dipole-dipole
interactions
between
isolated
paramagnetic surface species.[31,32] Fitting the EXAFS (Fig. 2)
spectra is consistent with an average 3 O in the first coordination
sphere. The fitted bond distances are long for terminal Cu-O-
Si/Al but are expected for the constrained geometry 2,
determined from the crystal structure of the molecular
complexes (Fig S3). Interestingly, only a single Al/Si atom can
The absence of an extra Cu coordination in the fit of 1-a700 is
similar to monomeric 1 and in sharp contrast to what is observed
for dimeric 2 (Fig. 2), confirming the presence of monomeric CuII
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10.1002/anie.201903802
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species (See ESI Section 4 for detailed discussion on EXAFS).
Grafting of the dimeric complex 2 on Al2O3-700 followed by the
same thermal treatment provides a material containing mostly
monomeric sites according to all spectroscopic data. The
complete characterization of the grafted (1-a, 2-a) and activated
(1-a700 and 2-a700) materials are included in the ESI (Section 2).
The optimal conditions found for the partial oxidation of
CH4 to CH3OH using 1-a700 (Table S2) are as follows: activation
at 700 ºC under synthetic air, followed by reaction under 6 bars
of CH4 for 30 min at 200 ºC. While reaction of Al2O3-700 with CH4
did not provide any CH3OH after water extraction[33] (Table S2),
material 1-a700 yields 11% CH3OH per total Cu, indicating that
the reaction takes place on Cu sites and that 22% of them are
involved in this two-electron process (vide infra).[34] These
mononuclear sites react within minutes at 200 °C with CH4 to
give high CH3OH selectivity (>80%) on a significant fraction of
Cu surface sites (22% of Cu), clearly indicating that having di- or
the
EPR
signal
precludes
the
involvement
of
antiferromagnetically coupled centers (i.e., bridged µ-oxo Cu
dimer and Cu oxide clusters[36]) hence confirming the combined
CuII/CuI redox reaction of two proximal monomeric Cu sites,
which are not magnetically coupled with each other. The EPR
parameters and the UV-vis spectra (Fig. 3a and S17) before and
after reaction are similar, implying similitude in the local
environment of the monomeric Cu centers. We surmise that the
intrinsic limitation of this system must be related to
a
requirement for two sufficiently proximal reactive CuII monomers
on the oxide surface in order to perform this overall 2-electron
oxidation process rather than
a
structural configuration
requirement of the monomeric sites. The fraction of Cu
becoming EPR-silent (23%), from the reduction of CuII to CuI, is
consistent with the 22% expected from the CH3OH extraction.
In this work, we have demonstrated that oxygen-activated
monomeric CuII sites supported on alumina, generated via
SOMC/TMP, show fast reactivity and are selective for the
conversion of CH4 to CH3OH. It has been demonstrated that
these monomeric copper(II) sites on non-crystalline support with
specific spectroscopic signatures are selective for the formation
of methoxy intermediates and that di- or tri-nuclear centers are
not required for this transformation on oxide materials. This
discovery opens up opportunities to develop novel materials for
the selective partial oxidation of CH4 to CH3OH.
tri-nuclear sites on crystalline oxide materials is not
a
requirement for the selective conversion of CH4 to CH3OH.[35]
Moreover, in situ XAS (Fig. 3a) shows that about. 27% of the
total CuII is reduced to CuI, which is close to the expected 22%,
considering the CH3OH yield and the need of 2 electrons per
CH3OH formed. This performance corresponds to an electron
efficiency of the CuII sites for CH3OH above 83%, implying a
greater selectivity for CH3OH (> 83%), based on the electron
implied for overoxidized products formation. The low amount of
side products precludes their quantification in the gas phase but
the analysis of the reacted material and the product by IR and
NMR spectroscopy shows that CH3O-species along with trace
amounts of HCOO- and CO are formed upon reaction with CH4
as confirmed by using 13CH4 (Fig. S19-S21). The material 2-a700
has a similar performance yielding 9% CH3OH, 23% Cu
reduction and an electron efficiency of 75% (see Table S2 and
Fig. S22).
Acknowledgements
J.M. and M.A.N. thank Shell Global Solutions International B.V.
for financial support. We acknowledge the European
Synchrotron Radiation Facility, Grenoble, France, for
synchrotron resources (beamline BM31; CH 5579) and we thank
H. Emerich and D. Stoian for their assistance. We thank A.
Jenelle Knorpp and P. Šot for their help with XAS experiments,
T. Margossian and F. Allouche for discussions and A. Ashuiev
for his help with EPR.
a)
*
23% signal
disappearance
1-a700 activated
1-a700 CH4 reacted
Keywords: alumina • C-H activation • copper • methane •
methanol
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One plus one equals two: Methane
was selectively converted to methanol
by copper(II) sites generated on -
Jordan Meyet, Keith Searles, Mark A.
Newton, Michael Wörle, Alexander P.
van Bavel, Andrew D. Horton, Jeroen A.
van Bokhoven*, Christophe Copéret*
alumina
support
using
surface
organometallic chemistry and thermal
molecular precursor approach. The
activation of the material at high
temperature under oxygen generates
monomeric Cu sites. An important
fraction of these sites formed are
capable of partially oxidize methane to
methanol under mild conditions.
Page No. – Page No.
Monomeric Copper (II) Sites
Supported on Alumina Selectively
Convert Methane to Methanol
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