14440 |Chem. Commun., 2014, 50, 14440--14442 This journal is ©The Royal Society of Chemistry 2014
Cite this: Chem. Commun., 2014,
50, 14440
Enhanced catalytic performance of Mn
x
O
y
–Na
2
WO
4
/
SiO
2
for the oxidative coupling of methane using an
ordered mesoporous silica support†
M. Yildiz,
ab
Y. Aksu,
c
U. Simon,
d
K. Kailasam,
e
O. Goerke,
d
F. Rosowski,
f
R. Schoma
¨cker,*
a
A. Thomas
e
and S. Arndt*
ag
The oxidative coupling of methane is a highly promising reaction for its
direct conversion. Silica supported Mn
x
O
y
–Na
2
WO
4
is a suitable catalyst
for this reaction. In this study, a variety of different SiO
2
materials have
been tested as supports. Surprisingly, the application of ordered meso-
porous silicas, here exemplarily shown for SBA-15 as support materials,
greatly enhances the catalytic performance. The CH
4
conversion
increasedtwofoldandalsotheC
2
selectivity is strongly increased.
The proven reserves of natural gas have enormous potential as
alternatives to the decreasing reserves of crude oil.
1
The main
component of natural gas is CH
4
, the most stable hydrocarbon.
Its conversion into value added products, particularly its direct
conversion, remains a difficult challenge.
2,3
One possible direct
conversion is the oxidative coupling of methane (OCM), as
shown in eqn (1).
CH
4
+O
2
-C
2
H
6
or C
2
H
4
+H
2
O (1)
Although, a large number of catalysts have been studied,
4
a
breakthrough has not been achieved yet, especially because many
catalysts deactivate due to the harsh reaction conditions.
5,6
Mn
x
O
y
–
Na
2
WO
4
/SiO
2
is a very active, selective and stable catalyst, a fact
which has been confirmed by several research groups.
7–9
The current
knowledge of this catalyst has recently been reviewed.
7
Moreover, a
fluidized bed processing procedure was developed for the large scale
preparation of this material
10
allowing its application in the OCM
mini-plant at the Technische Universita
¨tBerlin.
11
To optimize the catalytic performance and to understand the
structure–activity relationship, a variety of different support materials
for Mn
x
O
y
–Na
2
WO
4
are under investigation by our research groups.
12
However, it can be concluded that most support materials show
inferior or just comparable performance to SiO
2
as supports. On the
otherhandweobservedaremarkableinfluenceonthecatalytic
performance when different types of silica supports were used.
In this communication, we want to report on the observation that
the application of SBA-15 leads to a greatly enhanced catalytic
performanceincomparisonwithanyotherstudiedSiO
2
supports.
SBA-15 is an ordered mesoporous silica
13
whichiswidelyusedasa
catalyst support in fundamental research.
14
Indeed as very narrow
pore size distributions and highly ordered cylindrical mesopores can
be prepared in this material, SBA-15 has significant advantages to
study the dispersion of the supported active phase.
15
On the other
hand, so far SBA-15 has not found its way to industrial applications,
probably because much cheaper porous silicas can be prepared
by other approaches. Furthermore, the ordered cylindrical pore
structures have been described to be detrimental in terms of
transportofsubstratesinthematerial,thusmightcausesevere
diffusion limitations.
16,17
Here, we show that the application of
SBA-15 can largely enhance the catalytic performance of an OCM
catalyst, compared to commercially available porous silica supports.
This is most surprising as the silica support completely loses its
ordered mesoporous structure during the preparation of the catalyst.
The prepared catalysts are shown in Table 1. For the preparation
of the Mn
x
O
y
–Na
2
WO
4
/SiO
2
catalysts, a standard wet impregnation
procedure was used, as described in the literature.
7
The detailed
synthetic protocols can be found in the ESI.†Catalytic tests were
performed using a 6-fold parallel reactor set-up and 50 mg of the
catalyst. Details of the experimental setup can be found in the ESI.†
The measured surface areas of the prepared catalysts showed a
drastic reduction compared tothepuresupportmaterialcf. Table 1,
which is caused by the phase transformation from amorphous SiO
2
to a-cristobalite yielding complete collapse of the ordered meso-
porous structure of SBA-15.
7
This transformation is also observed by
a
Technische Universita
¨t Berlin, Institut fu
¨r Chemie, Straße des 17. Juni 124,
b
Gebze Institute of Technology, Department of Chemistry, 41400 Gebze, Kocaeli,
Turkey
c
Akdeniz University, Faculty of Engineering, Department of Material Science and
Engineering, Dumlupinar Bulvari, 07058 Antalya, Turkey
d
Technische Universita
¨t Berlin, Institut fu
¨r Werkstoffwissenschaften und
-technologien, Fachgebiet Keramische Werkstoffe, Secretariat BA 3,
Hardenbergstraße 40, 10623 Berlin, Germany
e
Technische Universita
¨t Berlin, Department of Chemistry, Functional Materials,
Hardenbergstraße 40, 10623 Berlin, Germany
f
UniCat BASF JointLab, Fakulta
¨t II, Secretariat BEL6, Marchstr. 6, 10587 Berlin,
Germany
g
PCK Raffinerie GmbH, Passower Chaussee 111, 16303 Schwedt/Oder, Germany
†Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cc06561a
Received 21st August 2014,
Accepted 26th September 2014
DOI: 10.1039/c4cc06561a
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XRD measurements, as shown in Fig. 1. The loss of surface area
during catalysis (before and after) is very small.
In Fig. 1, XRD patterns of the fresh and tested catalysts are
represented in darker and lighter colors, respectively. In the
fresh samples, a-cristobalite, tridymite, MnWO
4
,Na
4
WO
5
and
Na
2
WO
4
were detected. Moreover, Mn
2
O
3
or braunite is found,
but an unambiguous assignment was not possible due to
the small number of signals and their low intensity. For all
catalysts, a-cristobalite is formed as the main SiO
2
phase.
For the used samples of Cat-1 and Cat-3, quartz was additionally
found. It is important to note that substantial differences in the
phase compositions were not observed for all catalysts before and
after the OCM reaction.
Elemental analysis furthermore revealed that the Mn, W and
Na content on all silica supports is very similar (Table S2, ESI†). The
three catalysts thus show very similar results regarding the surface
area and composition and therefore no significant difference in
their catalytic activity would be expected at this point.
The results of the catalytic tests are shown in Fig. 2. All
tested catalysts exhibited a stable catalytic performance, with
slightly increased selectivities.
Cat-3 was almost inactive for the oxidative coupling of methane
under the applied reaction conditions, which is surprising, because
its catalytic performance is even worse than most catalysts reported
in the literature.
7
Cat-2 showed a comparable catalytic performance
when compared to the other commercial SiO
2
support materials
investigated in our laboratory (not shown). On the other hand, the
performance of Cat-1 (SBA-15 supported) was outstanding with
approximately 14% CH
4
conversion, i.e. it showed a two fold
increase in conversion compared to Cat-2 with an even higher C
2
selectivity. As stated above, this significant increase in catalytic
performance can be hardly explained by the surface area or
composition of the three catalysts. However, BET, XRD and
elemental analysis give no information on the distribution of
active components on the catalysts.
In Fig. 3 and 4, SEM images and EDX mapping measure-
ments are shown for the fresh catalyst Cat-1 and Cat-2. In Fig. 3,
the rod shaped morphology of the SBA-15 support can still be
seen in the SEM images of fresh Cat-1. The EDX mapping
shows the homogeneous distribution of elements, especially
tungsten. In contrast to this, for fresh Cat-2 irregular spherical
silica particles can be seen with a more inhomogeneous dis-
tribution of elements, cf. Fig. 4. In addition, EDX-mapping of
fresh catalyst Cat-1 showed even distribution of Mn with
smaller particle sizes, while in Cat-2, Mn rich phases are
observed as larger agglomerates on the silica support material.
Table 1 The codes of Mn
x
O
y
–Na
2
WO
4
/SiO
2
catalysts, the origin of support
materials and surface areas of the applied silica support materials and catalysts
Silica support material
Surface area of the catalyst
(m
2
g
1
)
Catalyst
code Origin Comment
Surface area
(m
2
g
1
)
Before
reaction
After
reaction
Cat-1 Synthesized SBA-15 616.9 6.8 4.2
Cat-2 BASF D 11-10 105.4 6.7 3.1
Cat-3 Sigma Silica gel,
grade 923
492.3 2.8 3.9
Fig. 1 XRD patterns of Cat-1 (green), Cat-2 (blue) and Cat-3 (red).
Fig. 2 CH
4
conversion as a function of time on stream (above) and C
2
selectivity as a function of time on stream (below) for Cat-1 (green), Cat-2
(blue) and Cat-3 (red) catalysts.
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14442 |Chem. Commun., 2014, 50, 14440--14442 This journal is ©The Royal Society of Chemistry 2014
For the first time in the research on the oxidative coupling of
methane, one catalyst exists, which can be reproducibly pre-
pared with good, medium, low and stable catalytic perfor-
mances. The ongoing detailed studies will enable a first real
view on the structure–activity relationship of this catalyst soon.
The catalytic performance of the Mn
x
O
y
–Na
2
WO
4
/SiO
2
catalyst
is greatly enhanced by the application of SBA-15 as the silica
precursor, approaching a level which might allow an industrial
application. The explanation for this enhancement might be that
the Mn
x
O
y
–Na
2
WO
4
precursors are better dispersed in the small
pores and the high surface area of SBA-15, which is reflected in
the enhanced dispersion of the final catalyst even when the
mesostructure of SBA-15 has collapsed after the thermal treat-
ment. Optimization of this silica precursor, e.g. via adjusting the
pore size and volume, could result in further improvement of the
catalytic performance. Moreover, detailed structural characterization
and comparison of these three catalysts, currently in progress, might
give a first insight into the structure–activity relationship of this
catalyst, a missing feature hindering the understanding of
many catalytic systems, particularly for metal oxides. Unraveling
such a kind of relationship could give room for further, perhaps
even concerted, improvements.
This work is part of the Cluster of Excellence ‘‘Unifying Concepts
in Catalysis’’ coordinated by the Technische Universita
¨tBerlin,
supported by the Deutsche Forschungsgemeinschaft. Mr Yildiz is
obliged to the Ministry of Education of the Republic of Turkey for
financial support. We thank Dr Caren Goebel for electron micro-
scopy measurements.
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Fig. 3 SEM images of fresh Cat-1 (Mn
x
O
y
–Na
2
WO
4
/SBA-15) and EDX-mapping of W L-edge (green) and Mn K-edge (red).
Fig. 4 SEM images of fresh Cat-2 (Mn
x
O
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–Na
2
WO
4
/SiO
2
) and EDX-mapping of W L-edge (green) and Mn K-edge (red).
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