Stepwise Methane-to-Methanol Conversion on CuO/SBA-15

Article abstract of DOI:10.1002/chem.201801135

The direct partial oxidation of methane to methanol is a challenging scientific and economical objective to expand the application of this abundant fuel gas as a major resource for one-step production of value-added chemicals. Despite substantial efforts to commercialize this synthetic route, to date no heterogeneous catalyst can selectively oxidize methane to methanol by O₂ with an economically acceptable conversion. Cu-exchanged zeolites have been recently highlighted as one of the most promising bioinspired catalysts toward the direct production of methanol from methane under mild conditions. In this work, Cu-based catalysts were prepared using mesoporous silica SBA-15 as an alternative support and their activity for this conversion was investigated. The results demonstrate that highly dispersed CuO species on SBA-15 are able to react with methane and subsequently produce methanol with high selectivity (>84 %) through water-assisted extraction. Furthermore, it was confirmed that the main intermediate formed after interaction of the catalyst with methane is a methoxyl species, which can be further converted to methanol or dimethyl ether on extraction with water or methanol, respectively.

Full text of DOI:10.1002/chem.201801135

A Journal of
Accepted Article
Title: Stepwise methane-to-methanol conversion on CuO/SBA-15
Authors: Ha Vu Le, Samira Parishan, Anton Sagaltchik, Hamideh Ahi,
Annette Trunschke, Reinhard Schomäcker, and Arne Thomas
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201801135
Link to VoR: http://dx.doi.org/10.1002/chem.201801135
Supported by

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Stepwise methane-to-methanol conversion on CuO/SBA-15
[
a]
[b]
[c]
[c],[d]
[d]
Ha V. Le, Samira Parishan, Anton Sagaltchik, Hamideh Ahi,
Annette Trunschke, Reinhard
[b]
[a]
Schomäcker, and Arne Thomas*
Abstract: The direct partial oxidation of methane to methanol is a
great scientific and economical objective to expand the application of
the abundant fuel gas as a major resource for one-step production of
value-added chemicals. Despite substantial efforts to commercialize
this synthetic route, to date, no heterogeneous catalyst can
oxidation strategies showed low yields and productivity due to
the much higher reactivity of the oxygenated products compared
[
1a, 3a, 4]
to methane.
Efficient activation of methane at milder
conditions and control of reaction selectivity toward the desired
products are therefore the main challenges in heterogeneous
catalysis.
selectively oxidize methane into methanol by
O
2
with an
economically acceptable conversion. Cu-exchanged zeolites have
been recently highlighted as one of the most promising bioinspired
catalysts toward the direct production of methanol from methane
under mild conditions. In this work, we prepared Cu-based catalysts
using SBA-15 as an alternative support and investigated their activity
for this conversion. Our results demonstrate that highly dispersed
CuO species on SBA-15 are able to react with methane and
subsequently produce methanol with high selectivity (> 84%) via
water-assisted extraction. Furthermore, it is confirmed that the main
intermediate formed after interaction of the catalyst with methane is
a methoxy species, which can be further converted to methanol or
dimethyl ether (DME) upon extraction with water or methanol,
respectively.
Biological catalysts have motivated many researchers to tackle
these challenges. Methane monooxygenase enzymes (MMOs)
in methanotropic bacteria are able to consume methane as their
sole source of both carbon and energy, selectively oxidizing
methane to methanol under ambient conditions by atmospheric
[
5]
O
2
.
Two forms of MMOs are located at different cellular
positions, including cytoplasmic MMOs (soluble form) and
membrane-bound MMOs (particulate form). A bis(µ-oxo)diiron
core is known as the active site in the soluble MMOs while the
particulate form contains di- and tricopper clusters that efficiently
[
6]
activate the C–H bond of methane. Inspired by these prototype
natural machines, in the last decade, Cu-, Co- and Fe-
exchanged zeolites have been used as heterogeneous catalysts
for the methane-to-methanol conversion due to their ability to
activate methane at low temperatures and reveal high
selectivities to methanol. This is typically achieved in a stepwise
Introduction
2 2
process, in which the catalyst is first oxidized in O or N O at
high temperatures (> 200 °C), and then reacts with methane at
lower temperatures (≤ 200 °C). The subsequent transformation
of the formed intermediate stabilized on the catalyst surface to
methanol is performed via a final treatment with water.
As the so far most active catalysts, Cu/zeolites have been
intensively studied to identify the possible structures of active Cu
As supplies of crude oil are declining, methane, which is the
major component of abundant resources such as natural gas,
methane hydrates, and biogas, has emerges as a potential
[7]
[1]
alternative feedstock for the chemical industry.
However,
methane is not widely utilized on a commercial scale because of
[2]
its high chemical inertness. In the current energy-intensive
route, methane is first converted to syngas typically at 1000 °C
and 30 bar, which can subsequently be transformed to methanol
and hydrocarbons via hydrogenation and Fischer–Tropsch
sites and the catalytic mechanisms to finally develop improved
[6a, 7d, 8]
procedures.
Interestingly, no steady state activity is
observed for large copper oxide nanoparticles supported on
pure silica MFI (average size 30 nm for copper oxides) or on
pure silica BEA (average size 40-60 nm for copper oxides) in a
[
3]
synthesis, respectively. Developing an alternative process to
produce value-added chemicals, such as methanol from
methane, preferably in one step, has been therefore a research
field of increasing interest in recent years. Unfortunately, direct
continuous isothermal process, in which
mixture of methane, O and water was introduced to the catalyst
while a small amount of methanol was
amorphous silica impregnated with
O in the original work of Groothaert et al., in which
a predetermined
2
[8a]
bed at 200 °C
produced over
Cu(OAc)
the above-described stepwise procedure was applied.
2
.H
2
[
7a]
[
a]
Ha V. Le, Prof. Dr. Arne Thomas
To
Institute of Chemistry-Functional Materials, BA2
Technische Universitat Berlin,
Hardenbergstraße 40, 10623 Berlin (Germany)
E-mail: arne.thomas@tu-berlin.de
expand this promising material class, we report here the first
demonstration of methane-to-methanol conversion over an
SBA-15-supported Cu catalyst. Unlike Cu-oxo complexes in
zeolite matrix previously suggested as the active sites for the
hydroxylation of methane, the catalytic activity of CuO/SBA-15
is attributed to small/ultrasmall CuO nanoclusters that are highly
dispersed throughout the SBA-15 framework. Our results are
expected to accelerate the development of novel Cu-based
catalysts using other supports than zeolites for this “dream
reaction”.
[b]
[c]
[d]
S. Parishan, Pro. Dr. Reinhard Schomäcker
Institute of Chemistry- Technical Chemistry, TC8
Technische Universitat Berlin,
Straße des 17. Juni 124, 10623 Berlin, (Germany)
A. Sagaltchik, Dr. H. Ahi
BasCat-UniCat BASF Joint Lab, Technische Universitat Berlin, EW
K 01
Hardenbergstraße 36, 10623 Berlin (Germany)
Dr. H. Ahi, Dr. Trunschke
Department of Inorganic Chemistry, Fritz Haber Institute of the Max
Planck Society
Faradayweg 4-6, 14195 Berlin (Germany)
Results and Discussion
Supporting information for this article is given via a link at the end of
the document.
Cu species were supported on SBA-15 by wet impregnation. In
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detail, the catalyst synthesized from Cu(II) acetylacetonate
impregnation and calcination (Figure 2). No nanoparticles are
detected in the TEM images of Cu-AA/SBA (Figure 2b) while
nanoparticles with various sizes (10–50 nm) are obviously seen
the Cu-OA/SBA sample (Figure 2c). Nitrogen sorption
measurements show a decrease of surface area and total pore
volume after the impregnation of the Cu species into SBA-15,
which is found to be more significant for Cu-AA/SBA (Table 1).
In addition, both mesopore and micropore volumes of Cu-
AA/SBA are lower than those of Cu-OA/SBA, respectively. In
combination with TEM and XRD results, it can be concluded that
Cu species are indeed mainly located within the pores of Cu-
AA/SBA while for Cu-OA/SBA larger CuO nanoparticles are also
located on the outer surface of SBA-15.
(
Cu(acac)
2
) was named Cu-AA/SBA while the one obtained
) was named Cu-OA/SBA. Similar
using Cu(II) acetate (Cu(OAc)
2
Cu loadings (~ 2.7 wt.%) were obtained for both Cu-AA/SBA and
Cu-OA/SBA.
The PXRD pattern at low angles of the unloaded SBA-15
showed three diffraction peaks indexed as (100), (110), and
(
200) plane corresponding to the well-ordered two-dimensional
[
9]
hexagonal structure of SBA-15 (Figure 1a).
The (100)
diffraction peak was as well observed in the for both CuO/SBA-
5 materials, indicating that the porous structure of the SBA-15
1
pore was sustained during impregnation. The decreased
intensity of the peaks for Cu-containing samples can be
attributed to the presence of Cu species in the SBA-15
[
10]
channels.
Wide-angle XRD measurements of the CuO/SBA-
15 samples showed different results, depending on the used
Cu(II) precursors (Figure 1b). No obvious peaks for any
crystalline phase are observed for Cu-AA/SBA, which indicates
that the Cu species are well-dispersed on the support. In
contrast, the diffractogram for Cu-OA/SBA shows to peaks at 2Ɵ
=
36° and 39° indicative of the formation of CuO particles on the
support.
(100)
(a)
Cu-OA/SBA
Cu-AA/SBA
(
110)
(
200)
SBA-15
0
1
2
3
4
5
2 (degrees)
(b)
Cu-OA/SBA
Cu-AA/SBA
SBA-15
Figure 2. TEM images of (a1, a2) pure SBA-15, (b1, b2) calcined Cu-AA/SBA,
and (c1, c2) calcined Cu-OA/SBA.
CuO
The catalytic activity of the CuO/SBA-15 materials was
investigated for the selective oxidation of methane to methanol
according to the well-known stepwise procedure first reported by
1
0
15
20
25
30
35
40
45
50
55
60
65
70
75
2
 (degrees)
[
7a]
Groothaert et al..
oxygen at 550 °C, then allowed to interact with methane at
00 °C. The last step for extraction of methanol could be
performed with either liquid water (off-line extraction) or steam
on-line extraction). For a catalytic comparison, Cu/mordenite
with a Cu loading of ~ 2.6 wt.% prepared by a recently reported
solid-state ion exchange between NH -mordenite and Cu(acac)
was applied as a reference zeolite-based catalyst.
Typically, the catalyst was activated in
Figure 1. PXRD patterns at (a) low angles and (b) wide angles of the
materials.
2
(
The structure of the CuO/SBA-15 samples was further
investigated by TEM measurements. Regular hexagonal
mesochannels are seen for all samples, before and after
4
2
[8b]
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result reported by Groothaert et al. using a Cu/mordenite
Table 1. Textural properties of SBA-15-based materials.
[7a]
catalyst.
the catalyst was activated in O
However, it should be noted that in the latter work,
at 450 °C and only 2 ml of water
2
[
c]
Cu
[a]
[b]
-1
[b]
-1
V
total
SA
V
meso
V
micro
3
-
to extract methanol was used while in the present study, the
activation step was performed at 550 °C and 10 ml of water was
added to the catalyst for the collection of methanol. It was
previously demonstrated the methanol yield can be significantly
Sample
loading
2
-1
3
3
(cm1 g
(
m g )
(cm g )
(cm g )
(
wt.%)
)
SBA-15
-
899.1
508.8
588.5
0.92
0.45
0.52
0.18
0.10
0.12
1.10
0.55
0.64
improved by increasing the activation temperature and using
Cu-AA/SBA
Cu-OA/SBA
2.71
2.78
[8b, c, 11]
more water.
Cu/mordenite,
Similar to our previous report on
2
was observed when the extraction was
[
8b]
CO
performed over 100 °C (Figure 3). Notably, the selectivity toward
methanol in the reaction using the CuO/SBA-15 catalysts (>
[
[
a]
SA = surface area calculated by the BET method.
b]
Vmeso = mesopore volume, and Vmicro = micropore volume calculated by the
84%) is comparable to the result obtained with Cu/mordenite.
NLDFT method.
[
c]
Besides Cu-oxo sites, small/ultra small copper oxide clusters
have been often proposed to be reactive to methane molecules
under appropriate reaction conditions in very recent studies
V
0
total = total pore volume calculated at p/p = 0.99.
Table 2. Catalytic performances of CuO/SBA-15 materials.
[8a, c]
using Cu-exchanged zeolite catalysts.
The well-ordered
Product yield
Oxidized Selectivity
[c]
Cu
loading
-1
porous structure of SBA-15 is advantageous to ensure a good
dispersion of active copper oxide species while large copper
oxide nanoparticles (30–60 nm) supported on pure silica were
(µmol gcat
)
CH4
µmol
to MeOH
Entry
Catalyst
[d]
(
& DME
[
a]
[b]
[b]
(
wt.%) MeOH DME CO2
-1
g
cat
)
(%)
[
8a]
previously proven to be inactive. The increased production of
methanol over Cu-AA/SBA is attributed to the high dispersion of
Cu species in the SBA-15 framework. It can be therefore
concluded that the Cu source and the method of material
preparation can significantly affect the size of formed Cu clusters
and their catalytic performance as well.
1
2
3
Cu-AA/SBA 2.71
Cu-
30.2
0
0
0
3.4
33.6
37.5
12.1
89.9
e
2.71
31.7
11.1
5.8
1.0
84.5
91.7
AA/SBA
Cu-OA/SBA 2.78
[
a]
b]
Methanol was analyzed by GC after off-line extraction with liquid water.
Gas-phase products were analyzed by MS during online extraction with
steam.
[
DME could be considered as a side product by the acidic
zeolite-catalyzed dehydration of in situ-produced methanol as
the extraction with steam is performed at an elevated
[
c]
Amount of oxidized methane was = moles(MeOH) + 2*moles(DME) +
moles(CO ).
2
[
6a, 7b]
[
d]
temperature (≥ 135°C).
However, our earlier study with the
Selectivity to MeOH and DME = [moles(MeOH) + 2*moles(DME)]/
moles(reacted CH
4
)
Cu/mordenite catalyst showed that methanol and DME were
detected at room temperature, suggesting that different
intermediates were generated on the catalyst surface, which
would be transformed into methanol and dimethyl ether,
[
e]
Reused catalyst for the second cycle.
[
8b]
respectively, by treatment of the catalyst with steam.
In this
study, no DME was produced over CuO/SBA-15 catalysts
-1
compared to 1.6 µmol gcat of DME obtained in the catalytic
cycle using Cu/mordenite (Table S1). To investigate the
mechanism of formation of DME on the Cu-based catalysts, an
online extraction of the samples with a methanol-saturated He
flow at room temperature was performed after their interaction
with methane.
Table 2. Production of DME by online extraction at room
remperature with different solvents..
[
a]
-1
DME amount produced (µmol gcat
)
Entry
Catalyst
Online
extraction with
methanol vapor
Online extraction
with steam
Figure 3. Mass-spectral signals of products after interaction of Cu-AA/SBA
with methane at 200 °C, followed by online extraction in an He flow saturated
with water.
1
Cu-AA/SBA
0
27.6
2
Cu/mordenite
1.6
48.6
[
a]
Analyzed by MS.
After the extraction with water, methanol was detected in
catalytic cycles over both CuO/SBA-15 catalysts indicating that
methane was indeed activated by Cu sites in SBA-15 at low
temperature (200 °C) and then converted to methanol upon the
treatment with water (Table 2). Cu-AA/SBA yielded 30.2 µmol
-
1
g
cat of methanol (Table 2, Entry 1) while a lower methanol
-1
amount of 11.1 µmol gcat was produced over Cu-OA/SBA
Table 2, Entry 3), which is even comparable to the original
(
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4.00
3.50
3.00
2.50
2.00
1.50
1.00
0.50
0.00
Cu/mordenite in the steam-assisted extraction step at room
temperature.
To gain more insight into the location of the Cu sites in SBA-15,
H
2
-TPR measurements were performed for both CuO/SBA-15
materials in comparison with the H -TPR results of Cu oxide
2
standards. The first reduction stage for CuO/SBA-15 samples is
observed from 200 to 300 °C, which is attributed to CuO
nanoparticles on the surface and at the facilely accessible
mesopores of SBA-15 (Figure 5). Most of Cu species in Cu-
OA/SBA-15 were reduced by H in this temperature range,
2
consistent with above characterization results that large CuO
nanoparticles are the phase of Cu in this sample. Besides,
further H reduction steps at 350 and 500 °C are found for the
2
Cu-AA/SBA sample, indicating that Cu species can be
distributed at less accessible sites of SBA-15, namely
Extraction with methanol
Extraction with water
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Time (h)
2
micropores in the main-channel wall, by using Cu(acac) for the
preparation of CuO/SBA-15. It is therefore suggested that the
formation of smaller CuO clusters in Cu-AA/SBA led to the better
catalytic performance of Cu-AA/SBA compared to the result
Figure 4. Comparison of mass-spectral signals of DME extracted from Cu-
AA/SBA in the online stage at room temperature using different solvents.
obtained with Cu-OA/SBA. Calculation of H
2
consumption in the
TPR measurements further indicates that after activated in O
2
,
the samples indeed contain Cu(II) species only as found in
Cu/zeolites, which are responsible for activation of methane in
[6a, 7d, 8b, 13]
the earlier studies.
CuO
Cu O
2
Cu-OA/SBA
Cu-AA/SBA
Scheme 1. Proposed formation of products on the Cu-based catalyst based
[
8a, b, 12]
on the former studies
and mass spectrometry.
2
00
400
600
800
-1
Notably, 27.6 µmol gcat of DME were achieved with Cu-AA/SBA
after the treatment with methanol vapor (Table 3). Also, the
production of DME on Cu/mordenite was remarkably increased
Temperature (°C)
2
Figure 6. H -TPR profiles of Cu oxides, calcined Cu-OA/SBA and calcined Cu-
-1
to 48.6 µmol gcat . These DME yields are close to the methanol
amounts produced upon the water-mediated extraction,
indicating that DME obtained in the experiment with a methanol-
saturated inert flow is a product of the reaction of methanol
molecules applied in the extraction protocol with the bound
intermediate species. It can be thus concluded that the
intermediate is indeed a methoxy species bound on the catalyst
surface after the reaction of active Cu species with methane
AA/SBA.
Since the first report on the stepwise manner for the gas-phase
direct production of methanol from methane over Cu-exchanged
zeolites,
supports for preparation of Cu-based catalysts. In spite of many
attempts over the last decade, the nature of active species in the
Cu/zeolite catalysts has been still unknown.
theoretical and spectroscopic analyses, several different
structures of Cu sites, which can be activated in O and
subsequently be able to react with methane molecules, have
been suggested using
exchangeable framework.
analysis is known as one of the essential characterization
techniques to yield more information on the activated Cu sites.
2
In earlier studies, the UV-vis spectra of O -activated Cu/ZSM-5
and Cu/mordenite showed a band at ~ 22 500 cm , whose
intensity of which is rapidly decreasing when methane was
[
7a]
only zeolites and zeotypes have been used as
(
Scheme 1). The rate of such a reaction could be significantly
[
8a]
Based on both
enhanced by acidic sites, which are abundantly available in
zeolite catalysts. The MS detected signals of DME with ~ 30
times higher intensities for Cu/mordenite after its contact with
the methanol-saturated He flow compared to the results
obtained from Cu-AA/SBA (Figure 4 and S6, red curves).
Furthermore, DME can be also produced in an online extraction
stage with water due to the attack of in situ-produced methanol
on unreacted intermediate species. Such as-formed methanol
species will be retained for a longer time within the zeolite
framework mainly possessing micropores in comparison with the
mesoporous SBA-15 material. This can explain the small
amount of DME generated along with methanol over
2
a
zeolitic model with
a
cation-
[
8e, 14]
In situ UV-vis spectroscopy
-1
[7a, 8d, 15]
purged to the samples.
active site of monooxo-dicopper (µ-Cu
This band is assigned to an
[
16]
2
O).
However, several
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Chemistry - A European Journal
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FULL PAPER
[
19]
studies have recently reported the absence of this band for
d-d transitions of Cu(II) ions.
Interestingly, similar to the
[
6a, 8a, c]
[6a, 17]
Cu/mordenite during the O
2
-activation step.
Instead of
results recently reported for Cu/mordenite,
a considerable
-1
that, Grundner et al. reported another broad band at ~ 31 000
development of the broad band centered at ~ 32 000 cm was
observed for both Cu-AA/SBA and Cu-OA/SBA during the
activation step (Figures 7a and S7a). It should be noted that the
~ 32 000 cm absorption commonly appears in UV-vis spectra
of Cu-based materials due to a charge transfer of O Cu in
-1
2
cm which was stable in O and disappeared after 30 min
[
6a]
2+
contact with methane. Also in this study, a [Cu
3
(µ-O)
3
]
core
-1
was suggested to be responsible for the activation of methane.
A similar result was further found in the very recent work by Kim
2
-
2+
[
17]
[19b, 20]
et al..
In addition, several other Cu sites embedded in the
CuO clusters.
This increasing feature observed in the UV-
zeolite channels have been proposed as potential cores for the
vis spectra of CuO/SBA-15 samples is therefore assigned to
activated CuO species deposited on SBA-15. As can be
expected, in the next step for interaction with methane at 200
[18]
activation of methane, namely a simple monocopper site,
2
+
2+ [8e]
larger Cu-oxo clusters such as [Cu
even small/ultrasmall CuO clusters.
4
O
4
]
and [Cu
5
O
5
] ,
and
[
8a, c]
-1
°C, the intensity of the 32 000 cm band started decreasing as
methane was sent to the sample (Figures 7b and S7b), proving
that the O -activated CuO species are reactive with methane.
2
Importantly, analyzing the outlet stream with the MS during the
contact of the samples with methane revealed that water which
is typically generated upon the simple reduction of CuO in the
1
0
0
.0
.5
.0
Heating, RT
o
1
1
2
2
3
00 C
[
21]
o
presence of methane at higher temperatures (> 500 °C), and
50 C
-
1
o
methanol were not detected. Such a decrease in the 32 000 cm
00 C
o
50 C
band intensity is therefore attributed to the activation of methane
by CuO species at 200 °C, which yields the corresponding
intermediate stabilized on the catalyst surface. However, the
incomplete disappearance of the band after 2 h interaction with
methane, which is more significantly observed for Cu-OA/SBA,
showed that the CuO species in the samples consist of both
active and inactive sites. Combining with the better production of
methanol observed for Cu-AA/SBA, we therefore conclude that
well-dispersed small CuO nanoparticles located within the SBA-
15 framework are more active for the methane-to-methanol
conversion. Also, according to the in situ UV-vis spectroscopy
o
00 C
o
350 C
o
400 C
o
450 C
o
Cooling, 200 C
(
a)
45000
10000
15000
20000
25000
30000
35000
40000
-1
Wavenumber (cm )
[
8b, c, 17]
results of Cu/mordenite catalysts previously reported,
it is
1
0
0
.0
o
After activation in O , 200 C
suggested that Cu/zeolites may possess both different active
sites including Cu-oxo complexes stabilized by aluminum in the
zeolite framework and small CuO nanoclusters.
In the second catalytic cycle, Cu-AA/SBA was able to produce a
similar amount of methanol under the identical conditions. The
mass-spectrometric analysis during the on-line extraction of
products with steam also showed similar results to those of the
first run (Figure S4). No considerable changes in structure and
morphology of the used catalyst were found based on TEM,
2
o
Contact with CH , 200 C, 5 min
4
15 min
30 min
60 min
90 min
120 min
.5
2
XRD, and N -sorption measurements. Therefore, it can be
concluded that CuO species are stable in SBA-15 and can be
reactivated after the first cycle.
(
40000
b)
45000
.0
0000
1
15000
20000
25000
30000
35000
-
1
Conclusions
Wavenumber (cm )
CuO/SBA-15 catalysts were prepared by wet impregnation and
tested for the partial oxidation of methane to methanol via a
three-step manner. This study shows that methanol is indeed
produced on CuO/SBA-15. CuO species are able to activate
methane into methoxy species and stabilize them on the surface.
Methanol or DME can be produced depending on the solvent
used in the subsequent extraction step. The reactivity of CuO
species on SBA-15 with methane was further demonstrated by
the in-situ UV-vis spectroscopy measurements, in which
Figure 7. In situ UV-vis spectra of Cu-AA/SBA (a) after activation in O
2
and (b)
subsequent reaction with methane at 200 °C.
It should be noted that SBA-15 is known as a porous silica
material without ion-exchangeable positions. Therefore, the
formation of the isolated Cu-oxo species, which are defined as
active sites for Cu/zeolites, in SBA-15 seems to be impossible.
-1
After the O
2
activation up to 550 °C, indeed no band in the
intensity of the band centered at 32 000 cm increased by an
-activation and considerably decreased after the contact with
methane. The catalyst prepared from Cu(acac) had a better
-1
region of 20 000–25 000 cm was found in the in situ UV-vis
O
2
spectra demonstrating the absence of the mono-µ-oxo dicopper
2
site. The spectra of the samples activated in O
absorption band centered at ~ 13 000 cm , that is assignable to
2
show an
catalytic performance due to the high dispersion of small/ultra-
small CuO species in the SBA-15 framework compared to the
-1
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Cu(OAc)
2
-based sample. However, further studies on supported
0.53 mm, and film thickness = 1.00 m). The oven was held at 60 °C for
-
1
CuO are needed to gain more insight in size of active clusters
and subsequently to improve the performance. While the active
sites for the hydroxylation of methane over Cu-exchanged
zeolites is not fully defined, CuO/SBA-15 can be seen as a much
simpler model catalyst. Based on this first demonstration, it
might be worthwhile to test a variety of further porous silica or
other materials as supports for Cu(II) for the challenging direct
conversion of methane to methanol.
8 min, and then heated at a rate of 30 °C min to 120 °C. Temperature of
inlet and detector was set constant at 180 °C.
In an experiment using the online extraction method, after the interaction
of the catalyst with methane for 60 min at 200 °C, the gas stream was
switched off and the catalytic bed was cooled down to room temperature.
The outlet stream was connected to a quadruple mass spectrometer
-
1
(MS, InProcess Instruments GAM 200). A 50 Nml min flow of water-
saturated He flow was introduced to the reactor at room temperature for
-
1
3
h before the reactor was heated at a rate of 1 °C min to 200 °C. This
temperature was kept constant for another 3 h. The temperature of the
gas-washing bottle was maintained at 25 °C during the whole
experiment. The products were identified based on the revolution of the
Experimental Section
Synthesis of materials
signal m/z = 31, 44, and 45 characteristic for methanol, DME, and CO
respectively. The He signal (m/z = 4) was used as an internal standard to
quantify DME and CO
2
,
2
.
Synthesis of SBA-15
[
9,
SBA-15 was prepared according to the procedure previously described.
2
2]
Characterization of materials
Typically, P123 (EO20PO70EO20, Mav = 5800, 2 g) were dissolved in a
mixture of water (30 g) and a 2 M hydrochloric acid solution (60 g) at 35
C. After vigorous stirring for 1 h, tetraethoxysilane (4.2 g) was added.
°
Transmission electron microscopy (TEM)
The resulting mixture was stirred at the same temperature for 24 h before
aged in a Teflon-lined autoclave at 100 °C for another 24 h. The white
solid was then filtered, washed with water, dried at 105 °C for 12 h, and
calcined under static air at 500 °C for 4 h.
TEM and energy dispersive X-ray spectroscopy (EDX) measurements
were recorded on a TECNAI G²20 S-TWIN electron microscope operated
at 200 kV, equipped with an EDAX EDX system (Si(Li)) SUTW detector,
energy resolution of 136 eV (for MnK(α)). For sample preparation, a drop
of the material dispersed in ethanol was deposited onto a carbon-coated
nickel grid via evaporation.
Synthesis of CuO/SBA-15
A Cu-based precursor (Cu(II) acetylacetonate or Cu(II) acetate, 0.45
mmol) was added to a mixture of SBA-15 (0.975 g) and absolute ethanol
N
2
sorption analysis
(10 ml). After sonication for 10 min, a slow evaporation of ethanol was
N
2
sorption analysis was performed at 77 K using a QUADRASORB SI,
conducted at 40 °C under vigorous stirring until a sludge-like phase was
obtained. The sample was dried at 80 °C for 12 h for complete removal of
ethanol. The resulting powder was then pressed into pellets at 100 bar
for 60 s, lightly ground, and sieved to a 200-400 m diameter fraction.
Calcination of the material was performed under static air at 550 °C for 4
h. The Cu content in the materials was determined by inductively coupled
plasma-optical emission spectrometry (ICP-OES).
equipped with automated surface area analyzer. Before analysis,
samples were degassed at 150 °C for 12 h. Specific surface area was
determined over a 0.05–0.30 P/P
o
range by Brunauer–Emmett–Teller
= 0.99. Average
(BET) method. Total pore volume was collected at P/P
o
pore width was calculated based on non-local density functional theory
(NLDFT) method.
Powder X-ray Diffraction (PXRD)
Synthesis of Cu/mordenite
PXRD was performed with a Bruker-AXS D8 Advanced diffractometer
with DAVINCI design using CuKα radiation (λ = 1.5418 Å) equipped with
a Lynx Eye detector.
Cu/mordenite (2.58 wt.% of Cu) was prepared by intensively grinding
Cu(acac)
2
(0.120 g) and NH
4
-form mordenite (Si/Al = 10, 0.975 g) in a
[
8b],[23]
mortar within 30 min.
The resulting mixture was pressed and sieved
In situ UV- visible (UV-vis) spectroscopy
to a 200-400 m diameter fraction before the Cu exchange was
UV-visible diffuse reflectance spectra were measured on a Cary 5000
spectrometer (Agilent) equipped with a Harrick Praying Mantis™ diffuse
reflectance attachment (DRP-P72) and a reaction chamber (HVC-VUV).
The in situ cell was connected to a gas delivery system for operation
performed in situ during the activation step of the catalytic testing.
Catalytic test
®
under flow conditions. Spectralon was used as a white standard.
Spectra were taken in the 200-800 nm range with a step size of 1 nm
every 3 min during the treatment of materials. Results are shown in the
Kubelka-Munk function (F(R)), which are calculated from the recorded
reflectance data.
A U-shaped quartz reactor (ID = 6 mm) was loaded with ~ 0.6 g of the
catalyst and placed in an oven. The catalyst bed was then treated at 550
-
1
-1
°
C (2 °C min ) in a 50 Nml min flow of O
2
for 8 h. After cooling to 60 °C
-
1
-1
(
10 °C min ), the excess gas-phase O
2
was removed by a 50 Nml min
-
1
-
1
-1
The in situ cell was heated to 550 °C with a rate of 10 °C min in a 50
flow of N for 5 min. A mixture of 5 Nml min CH
2
4
and 30 Nml min
N
2
-
1
Nml min flow of O
2
. The sample was held at 550 °C for 60 min, and then
was then introduced to the reactor. The temperature was kept
unchanged for 20 min before the catalyst was heated under the same
-
1
cooled to 200 °C with a rate of 10 °C min under the same flow. After O
2
-
1
-1
-
1
removal by a 50 Nml min flow of He for 5 min, a mixture of 15 Nml min
flow to 200 °C with a rate of 5 °C min . After interaction of the catalyst
with methane for 60 min, the methane-containing flow was switched off,
and the catalyst bed was cooled to room temperature. For quantification
of methanol, the resulting material was dispersed in 10 ml of water under
vigorous stirring for 2 h. After centrifuging and filtration, the liquid phase
was transferred to a volumetric flask, mixed with a predetermined volume
-
1
CH
Temperature-programmed reduction by hydrogen (H
-TPR experiments were conducted on BELCAT II instrument
Version 0.4.5.13). Prior to measurements, Cu/mordenites were
4
and 15 Nml min He was sent to the cell for 120 min at 200 °C.
2
-TPR)
H
2
a
(
-
1
pretreated in a 40 NmL min flow of O
2
at 550 °C for 3 h, and then cooled
-
to 40 °C. H
2
-TPR profiles of the samples were recorded in a 30 NmL min
of acetonitrile, as internal standard, and analyzed with
a gas
1
-1
flow of 5% H
/N
2 2
at a heating rate of 3 °C min up to 900 °C.
chromatograph (GC). Analysis of aqueous samples was performed using
a Shimadzu GC 2010-Plus equipped with a flame ionization detector
(
FID) and a SUPELCOWAX 10 column (length = 30 m, inner diameter =
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Chemistry - A European Journal
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Chemistry - A European Journal
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Table of Contents
FULL PAPER
Ha V. Le, Samira Parishan, Anton
Sagaltchik, Hamideh Ahi, Annette
Trunschke, Reinhard Schomäcker, and
Arne Thomas*
Page No. – Page No.
Stepwise methane-to-methanol
conversion on CuO/SBA-15
Cu-containing mesoporous SBA-15 was used for the first time as an efficient
catalyst for the stepwise synthesis of methanol from methane with high selectivity
(
> 84%) toward methanol and activity well comparable to the Cu-based zeolites so
far reported. Formation of the products dependent on the solvent used in the
subsequent extraction step was investigated (see picture).
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