Catalytic conversion of methane to methanol over Cu-mordenite

Article abstract of DOI:10.1039/c1cc15840f

Methane can be converted to methanol over copper-exchanged mordenite at 200 °C. Methanol could be recovered at the end of the reactor. This multi-step reaction opens the possibility for methane to methanol conversion in a closed catalytic cyclic reaction system.

Full text of DOI:10.1039/c1cc15840f

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COMMUNICATION
Catalytic conversion of methane to methanol over Cu–mordenitew
Evalyn Mae Alayon,ab Maarten Nachtegaal, Marco Ranocchiari and
Jeroen A. van Bokhoven*
a
a
ab
Received 20th September 2011, Accepted 27th October 2011
DOI: 10.1039/c1cc15840f
À1
Methane can be converted to methanol over copper-exchanged
mordenite at 200 8C. Methanol could be recovered at the end of the
reactor. This multi-step reaction opens the possibility for methane
to methanol conversion in a closed catalytic cyclic reaction system.
22 200 or at 22 700 cm was assigned as the spectroscopic
signature of the active copper site in Cu–MOR and Cu–ZSM-5,
1
0,11
À1
respectively.
The same UV-Vis signature at 22 700 cm
was related to the catalytic decomposition of nitric oxide and
was proposed to be similar in structure to a bis(m-oxo)
dicopper species by Cu K-edge X-ray absorption spectroscopy
Although much sought after, the use of methane as a chemical
1
feedstock is still greatly limited. The high C–H bond strength
1
2,13
(
XAS).
The identity of the active site leaves room for
discussion, since the percentage of total metal sites present that is
involved in the methane to methanol conversion is low. More recent
resonant Raman measurements coupled to normal coordinate and
density functional theory calculations suggested the structure of the
of methane and its perfect symmetry make it an extremely
2
challenging molecule for chemical attack. Oxidation reaction
routes are generally futile since the oxidation products of
methane are more reactive than methane itself. This pushes
the overall chemical system to total oxidation, unless methanol or
its precursor is stabilized. Partial oxidation has been achieved by a
homogeneous Pt–bipyrimidine-catalyzed process by the formation
of methyl bisulfate that can be hydrolyzed to methanol. The
reaction proceeds with excellent yields of methane partial oxidation
14,15
active site to be a bent mono(m-oxo) dicopper center.
Despite uncertainties about the structure of the active site,
metal–zeolite assisted methane conversion was the first step in
bridging a structure-specific and biochemically-controlled
enzymatic system to a heterogeneous catalytic system. The
main drawback, however, is that in all previous reported
systems, methanol was obtained only through extraction,
3
,4
products. However, such outstanding activity and selectivity
trades off with the use of harsh acidic conditions and the inability
to separate and recover the catalyst and product.
6
,10,11,16
and never directly.
During these previous studies, some
intermediate formed after reacting methane at temperatures
higher than 130 1C with a preoxidized catalyst did not desorb
to produce methanol. Extraction at room temperature, using a
A known mild and controlled oxidation is performed by an
enzymatic system, methane monooxygenase (MMO), which
converts methane to methanol under the ambient living
conditions of the host methanotroph. The active species consists
of diiron sites for the soluble MMO and dicopper sites for the
1
6
wet solvent, enabled the detection of methanol. Our goal was
to produce a complete route from methane to methanol, which
produces methanol without having to perform the solvent
extraction step, and such that methanol can be detected at
the end of the reactor.
5
particulate MMO. These binuclear centers create a bridged
oxidizing species sufficiently strong to attack the C–H bond of
methane. The microporous structure of pentasil zeolites, such as
ZSM-5, stabilized the binuclear centers analogous to those present
in MMOs. Fe–ZSM-5 converted methane to methanol at room
Cu–MOR was synthesized by ion exchange (S1 and S2,
2
ESIw). It was calcined in O at 450 1C and then reacted with
6,7
methane at 200 1C. Fig. 1 shows the UV-Vis spectra collected
of Cu–MOR after calcination and after interaction with
temperature. A Cu–FeZSM-5 system was also described to
8
2
convert methane to methanol with N O above 230 1C. High
À1
3+
temperature autoreduction of Fe
2+
to Fe
methane. The formation of an absorption band at 22 700 cm
under strong
after calcination and the disappearance of this band after reaction
with methane were observed. This suggests that the synthesized
Cu–MOR stabilized the dicopper active site previously reported
chemical stabilization in the zeolite matrix generates sites which
cannot be reoxidized by O but only by N O. These sites are
2
2
9
capable of selective hydrocarbon oxidation. Zeolites ZSM-5 and
mordenite (MOR) stabilize binuclear Cu centers, which convert
11
to activate methane.
To obtain complementary information about the structure
of Cu species, Cu K edge XAS measurements were performed
at the SuperXAS beamline of the Swiss Light Source, Villigen,
Switzerland. The X-ray absorption near edge structure
(XANES) region probes the empty p density of states, and
hence carries the chemical fingerprints of oxidation state,
coordination geometry and ligand electronegativity. Fig. 2
shows the XANES spectra of Cu–MOR after activation in
oxygen and reaction with methane. The presence of a weak
methane to methanol at 130–200 1C without a-oxygen from N O
2
10
but only using O
2
as an oxidant. The UV-vis absorption band at
a
b
Paul Scherrer Institute, Villigen, CH-5232, Switzerland.
E-mail: evalyn.alayon@psi.ch; Tel: +41 56 310 5310
ETH Zurich, Wolfgang Paulistrasse 10, Zurich, CH-8093,
Switzerland. E-mail: jeroen.vanbokhoven@chem.ethz.ch;
Tel: +41 44 632 5542
w Electronic supplementary information (ESI) available. See DOI:
0.1039/c1cc15840f
1
4
04 Chem. Commun., 2012, 48, 404–406
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Fig. 1 Room temperature UV-Vis spectra of Cu–MOR taken after
Fig. 3 Mass spectrometer-detected signal of m/z = 44 attributed to
calcination in O
2
at 450 1C (TT) and subsequent CH
4
interaction
À1
CO
CH
2
during heating in He at 5 1C min of Cu–MOR directly after
interaction at 200 1C.
at 200 1C (---).
4
the resulting CO
2 2
by mass spectrometry. The CO mass
spectrometer trace after methane interaction at 200 1C and
heating in helium (Fig. 3) indicated a broad desorption profile
of m/z = 44, which is attributed to CO , in agreement with
2
1
0
earlier findings. On the other hand, if after methane inter-
action and without changing temperature, a wet stream of
helium was allowed to flow through the catalyst material,
methanol was observed in the gas phase as shown by the mass
spectrometer data (Fig. 4). The m/z = 31 signal, which is
attributed to methanol, showed a sharp increase with the onset
of the m/z = 18 signal, which is attributed to water. By leading
the reactor effluent through a cold trap and using GC detection,
methanol was observed. Subsequent heating in helium after
methanol had been desorbed did not show CO desorption but
2
merely residual water (Fig. 4). This means that the intermediate
was removed and the only detected species in the gas phase was
methanol. S6 (ESIw) illustrates that methanol was obtained in a
second reaction cycle in equal amounts. This illustrates the
ability of Cu–MOR to be regenerated and enables collecting
methanol in a batch-wise manner from a single reactor.
Fig. 2 Cu K edge XANES spectra of Cu–MOR taken of the as-prepared
catalyst in He at room temperature (-Á-), during calcination in O at 350 1C
---), and during CH
interaction at 200 1C (TT).
2
(
4
pre-edge feature at 8977.5 eV and high intensity of the rising edge
2
+
feature at 8987 eV mark the Cu character of the Cu sites after
1
7
calcination. Interaction with methane formed a feature at
1
7
8
983 eV, which is attributed to cuprous ions. The accompanying
feature at 8986 eV has been assigned to the presence of a
+
-coordinate Cu or a very covalent Cu complex. Given the
2+
17
4
overall shape of the spectrum, reduction of part of the Cu sites took
place. These XAS data indicate that a large fraction of Cu changes
structure during the reaction steps, which includes the proposed
15
redox activity of the copper sites during methane conversion.
The amount of methanol that could be extracted from our
Cu-exchanged MOR, determined by taking out the material
after methane interaction and stirring in water at room
temperature with subsequent gas chromatographic (GC) analysis,
was 13 mmol MeOH per g Cu–MOR. This is comparable to the
À1
previously reported value of 11 mmol g using a 1 : 1 water/
acetonitrile extraction medium, and in agreement with the
10
2 2
Fig. 4 Mass spectrometer-detected signals of H O (m/z = 18), CO
changing carbon content in the sample (see S5, ESIw).
(
m/z = 44) and MeOH (m/z = 31) during the treatment of wet He at
An indirect method to quantify the amount of reacted
methane is to heat the catalyst bed in inert gas and detect
200 1C after methane interaction, and the subsequent heating in dry
À1
He at 5 1C min
.
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 404–406 405

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The Cu–MOR catalyst both activates methane and subsequently
strongly stabilizes the formed intermediate. We illustrate a
means to react this intermediate with water, which resulted in
methanol desorption. The inability to desorb the intermediate
Christian Koenig, David Stibal, Marco Servalli, Stephan
Rummelts, and Chen Lin is appreciated.
Notes and references
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2
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