Support material variation for the MnxOy-Na 2WO4/SiO2 catalyst

Article abstract of DOI:10.1016/j.cattod.2013.12.024

Mn_xO_y-Na₂WO₄/SiO₂ is an active catalyst for the oxidative coupling of methane, with a remarkable stability and a suitable performance for an industrial application. Mn _xO_y-Na₂WO₄ was supported on different support materials and the catalytic activity was investigated with a parallel reactor system, allowing a direct comparison of all results. Considering the C₂ yield and the potential practical application, SiO₂ is indeed the best support material for this catalyst. However, the comparison in an X-S plot with a reference Mn_xO _y-Na₂WO₄/SiO₂ catalyst indicates that SiC, Fe₂O₃-based oxides and TiO₂-rutile are also promising support materials.

Full text of DOI:10.1016/j.cattod.2013.12.024

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Contents lists available at ScienceDirect
Catalysis Today
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Support material variation for the MnxOy-Na2WO4/SiO2 catalyst
M. Yildiza, U. Simonb, T. Otrembaa, Y. Aksuc, K. Kailasamd, A. Thomasd,
R. Schomäckera, S. Arndta,∗
a Technische Universität Berlin, Institut für Chemie, Technische Chemie, Straße des 17. Juni 124, 10623 Berlin, Germany
b Technische Universität Berlin, Institut für Werkstoffwissenschaften und -technologien, Fachgebiet Keramische Werkstoffe, Hardenbergstraße 40, 10623
Berlin, Germany
c Akdeniz University, Faculty of Engineering, Department of Material Science and Engineering, Dumlupinar Bulvari, 07058 Antalya, Turkey
d Technische Universität Berlin, Institut für Chemie, Funktionsmaterialen, Hardenbergstraße 40, 10623 Berlin, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 June 2013
Received in revised form 5 December 2013
Accepted 7 December 2013
Available online xxx
MnxOy-Na2WO4/SiO2 is an active catalyst for the oxidative coupling of methane, with a remarkable stabil-
ity and a suitable performance for an industrial application. MnxOy-Na2WO4 was supported on different
support materials and the catalytic activity was investigated with a parallel reactor system, allowing a
direct comparison of all results. Considering the C2 yield and the potential practical application, SiO2 is
indeed the best support material for this catalyst. However, the comparison in an X-S plot with a reference
MnxOy-Na2WO4/SiO2 catalyst indicates that SiC, Fe2O3-based oxides and TiO2-rutile are also promising
support materials.
Keywords:
Oxidative coupling of methane
OCM
© 2014 Elsevier B.V. All rights reserved.
MnxOy-Na2WO4/SiO2
Support material variation
Silica
TiO2-rutile
1. Introduction
The formation of a surface cluster species of tetrahedral WO4
species with one W O bond and three W
O Si bonds were sug-
The oxidativecouplingof methane(OCM, see Eq. (1)) is an attrac-
tive direct route for the conversion of methane into value added
compounds.
gested and presumed to be the active site by Jiang et al. [16]. Wu
et al. showed that the tetrahedral WO4 is distorted, most probably
due to the presence of Na ions [17].
It was reported that at the surface of the reduced catalyst in the
presence of gas phase O2, F-centers (an oxygen ion vacancy with
two trapped electrons) exist, which are able to activate molecular
O2 above 80 ◦C, forming lattice oxygen. A redox mechanism with
aCH4 + bO2 → cC2H6 + dC2H4 + eH2O
(1)
In the search for a suitable catalyst, hundreds of materials have
been tested in the past [1–5]. The MnxOy-Na2WO4/SiO2 catalyst has
attracted a lot of attention as an OCM catalyst for a practical appli-
cation [6–9,14]. The literature about this material has recently been
reviewed by our research group [3]. The stability of this material
is remarkable and confirmed by different research groups [10–15].
Moreover, the catalytic performance (CH4 conversions of 20–30%
at C2 selectivities of approximately 70%) makes this catalyst system
a potential candidate for practical applications.
W
O Si bonds and electron transfer between a tungsten species
and F-centers was suggested [18], which was later modified to a two
metal site model, including an oxygen spillover from the Mn2O3 to
the Na2WO4 [19]. The two site model seemed to be in accord with
XAFS (X-ray Absorption Fine Structure) and XPS (X-Ray Photoelec-
tron Spectroscopy) studies before and after the reaction [20].
The exact composition and functionality of the active centers are
still not clear. Palermo et al. [21] studied the oxides of Mn, Na and W
supported on SiO2 alone and/or in combination. They reported that
amorphous SiO2 undergoes a phase transition to highly crystalline
␣-cristobalite during calcination at 750 ◦C, far below the usual tran-
sition temperature of 1500 ◦C. This is caused by the presence of Na
[21]. A suitable catalyst requires all three metal atoms/oxides, so
it was concluded that strong synergies are present between these
three metal oxides and a Na-induced crystallization of the sup-
port material occurred, converting it from an active and unselective
∗
Corresponding author. Tel.: +49 30 31424973.
E-mail addresses: mahmut.yildiz@mailbox.tu-berlin.de (M. Yildiz),
ulla.simon@tu-berlin.de (U. Simon), torsten.otremba@tu-berlin.de (T. Otremba),
yilmazaksu@akdeniz.edu.tr (Y. Aksu), kamalakannan@mailbox.tu-berlin.de
(K. Kailasam), arne.thomas@tu-berlin.de (A. Thomas), schomaecker@tu-berlin.de
(R. Schomäcker), arndt@chem.tu-berlin.de, arndt@bluebottle.com (S. Arndt).
0920-5861/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2013.12.024
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Table 1
The determined factors [fi = ki(CD4)/ki(CH4)] for the reaction constants obtained
from CD4 and CH4 with 100 mg catalyst at 750 ◦C. Reprinted from Yilidz, M., Arndt,
S., Simon, U., Otremba, Y. A. T., Berthold, A., Görke, O., Thomas, A., Schubert, H.,
Schomäcker, R., 2012. Mn-Na2WO4/SiO2 – an industrial catalyst for methane cou-
pling. In: Ernst, S., Balfanz, U., Buchholz, S., Lichtscheidl, J., Marchionna, M., Nees
F., Santacesaria E., (Eds.), Reducing the Carbon Footprint of Fuels and Petrochemi-
cals, Deutsche Wissenschaftliche Gesellschaft für Erdöl, Erdgas und Kohle e.V., pp.
125–132., with permission from DGMK.
fi
Feed gas composition (CH4:O2:N2)
2:1:4
4:1:4
8:1:4
f1
f2
f3
f4
f5
0.6
0.7
0.3
0.5
0.2
0.5
0.7
0.4
0.5
0.2
0.6
0.7
0.5
0.4
0.2
Fig. 1. Simplified reaction network [2]. Adapted from Ref. [2].
can be prevented only by a development of a new highly selective
material or an improvement of a catalyst.
One issue which has not been thoroughly studied for the MnxOy-
NaWO4 catalyst is the effect of support materials other than SiO2.
Therefore, in the present manuscript we investigated a detailed
variation of different metal oxides as support material for the
MnxOy-Na2WO4 in order to study their influence on the catalytic
performance. Moreover, we studied in detail how the different
components of the active materials (MnxOy and Na2WO4) act
on these support materials alone. Furthermore, a test with pure
MnxOy-Na2WO4 was performed to study the catalytic activity with-
out any support material.
All catalytic materials have been studied in a parallel reactor
under identical conditions, allowing a direct comparison of the
obtained results; this is important because results reported in dif-
ferent publications in literature are often not comparable, due to
very different reaction conditions [48].
material into an inert support material. However, the exact active
site could not be revealed.
Wang et al. compared MnxOy-Na2WO4/SiO2, MnxOy-
Na2WO4/MgO and NaMnO4/MgO [22]. Due to the identical
catalytic performance of all three materials, it was suggested that
a common active site, consisting of Na–O–Mn species exists. This
is inconsistent with the suggestion of Wu et al. [17], who proposed
that the W O bonds of a distorted WO4 tetrahedron could be the
active site.
Besides determination of the active center, the production of
large amounts of this catalyst is important. Therefore, Simon and
co-workers developed an up-scaled preparation of this catalyst
[14], which is crucial for any practical application.
A variation of the composition has been investigated, trying to
find substitutes for Mn [23–26], for Na [27–30] and for W [31–33],
respectively. It was found that any other elements are also active,
but the combination of the oxides of Mn–Na–W was superior to all
tested catalysts.
2. Experimental part
2.1. Catalyst preparation
Despite the research performed till now, the role of the SiO2
support material remains unclear. Moreover, there are only very
few attempts using other support materials than SiO2. Liu et al.
reported the successful application of SiC as support material [34],
giving a similar performance compared to SiO2. Yu et al. reported a
study of different support materials. The applied support materials
were rare earth oxides (La2O3, CeO2, Pr6O11, Nd2O3, Sm2O3, Dy2O3
and Yb2O3) and SiO2 for Na2WO4 as active component [31]. CeO2
and Pr6O11 were mainly total oxidation catalysts. Covered with
Na2WO4 they exhibited good C2 selectivities. However, it is unclear
why typical support materials such as Al2O3, TiO2, etc., were not
used in these experiments. Moreover, the applied rare earth oxides
are active OCM catalysts themselves, Sm2O3 and Nd2O3 in par-
ticular. A blank experiment with only support material was not
reported in this study. Generally, it is questionable if supporting
an active OCM catalyst on a material which is an active OCM cat-
alyst itself, leads to results, which could significantly increase the
knowledge on the supported material due to expectable complex
interactions.
In the previous work, we performed kinetic isotope effect exper-
iments with CD4 for MnxOy-Na2WO4/SiO2 catalyst to contribute to
the understanding of the reaction pathways and the origin of COx
products [35]. The results of these experiments in Table 1 indicate
that the most influenced reactions were the consecutive reactions
which convert the C2 products towards the total oxidation products
(Fig. 1). The kinetic isotope effect is as discussed in the literature
[36–45] agrees with the observed results. The consecutive reac-
tions might be suppressed by reaction engineering (e.g. applying
new reactor concepts, such as membrane reactor) [46,47]. How-
ever, oxidation of CH4 molecules towards COx products, because of
unselective catalysts or unselective sites of the selective catalysts,
The prepared catalysts, the loading of the active components
and the abbreviations used in the figures and tables are presented
in Table 2. Manganese is present in the form of manganese oxides
or Mn-containing mixed oxides, however, the loading is calculated
for pure Mn.
The applied support materials (some of them e.g. La2O3, CaO,
MgO, are also known active components themselves in the OCM
[49]), their origin and specific surface area are shown in Table 3.
2.1.1. WO3/support
Supported tungsten oxide catalysts were prepared via
a
wet impregnation method of the different support materi-
als (Table 3) with the appropriate aqueous solution of the
(NH4)10H2(W2O7)6·xH2O (Aldrich, 99.99%, the number of hydrate
was assumed four for calculations) at room temperature. Samples
were dried in air overnight at 65 ◦C. The calcination process was
performed in air, heating the samples in 4 h from room tempera-
ture to 750 ◦C (with a heating rate of approximately 3 K/min) and
holding the temperature for 1 h at 750 ◦C, consecutively. The pre-
pared catalysts were then ground into fine powder and sieved. The
particle size of the catalysts used for the reaction was ≤200 ␮m.
Table 2
Prepared catalysts, applied loadings and used abbreviations.
Catalyst
Abbreviation
Blank
Support
5 wt% WO3/Support
5 wt% Na2WO4/Support
2 wt% MnxOy/Support
2 wt% MnxOy–5 wt% Na2WO4/Support
W/Support
Na/W/Support
Mn/Support
Mn/Na/W/Support
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Table 3
themselves. The catalytic performance of the bare untreated sup-
port materials as purchased is omitted, because this information
was considered irrelevant for the purpose of this study.
The origins and specific surface areas of the used support materials.
Support material
Origin of support material
BET (m2/g)
La2O3
CaO
Al2O3
ZrO2
SiO2
SiC
Sigma Aldrich, min. 99.9%
Riedel-de Haën
BASF
3% Yttria stabilized, Sigma Aldrich
BASF
3
6
338
105
120
1
124
40
39
2.2. Catalyst characterization
2.2.1. ICP
The determination of the metal loading was performed via
inductively coupled plasma atomic emission spectroscopy (ICP-
AES), using a Horiba Scientific ICP Model Ultima 2.
Sigma Aldrich
MgO
Sigma Aldrich, ≤ 99%
Fe2O3
Fe3O4
SrO
TiO2-rutile
TiO2-anatase
Sigma Aldrich
Alfa Aesar, 98% metal basis
Alfa Aesar, 99.5% metal basis
Aldrich
2
20
127
2.2.2. BET
The specific surface area was determined by a Micromeritics
Gemini III 2375 Surface Area Analyzer, using N2 adsorption at
−196 ◦C. Before measuring, the samples were degassed at 300 ◦C
and 0.15 mbar for at least 30 min. The surface areas were calculated
by the method of Brunauer, Emmett and Teller (BET).
BASF
The loading of WO3 was calculated from the recipe according to Eq.
(2).
WO3(g)
WO3(g) + Support(g)
WO3(wt%) =
× 100%
(2)
2.2.3. XRD
Powder X-Ray diffractograms were obtained (CuK␣1 radia-
tion, wavelength 0.154 nm) using Bruker AXS D8 ADVANCE X-ray
diffractometer. The angle variation was performed from 2 to 90◦,
with a step size of 0.008◦. The diffractograms were analyzed with
the program Diffrac.suite EVA.
2.1.2. Na2WO4/support
Supported sodium tungstate catalysts were prepared with the
same method as described in Section 2.1.1. Na2WO4·2H2O (Sigma-
Aldrich, min. 99%) was used for impregnating the support and the
loading was calculated according to Eq. (3).
2.3. Catalytic tests
Na2WO4(g)
Na2WO4(g) + Support(g)
Na2WO4(wt%) =
× 100%
(3)
A 6-fold parallel reactor (Integrated Lab Solutions Berlin and
Premex Reactor AG) was used for the determination of the cat-
alytic performance, with packed-bed, linear, tubular reactors made
of quartz glass. For each catalytic run, 50 mg catalyst were diluted
with approximately 1.5 ml quartz sand (quartz sand: purchased
from Merck, already washed with HCl and calcined, ca. 60% of the
particles have the size 0.2–0.8 mm). Below and above the cata-
lyst bed, a small amount of pure quartz sand was put to ensure
proper heat transfer. The particle size of the catalyst was below
200 ␮m in each experiment to exclude internal mass transfer
effects.
2.1.3. MnxOy/support
Supported manganese oxide catalysts were prepared with the
same method as described in Section 2.1.1. The applied MnxOy
precursor was (CH3COO)2Mn·4H2O (Fluka, >99%). The loading of
MnxOy was calculated according to Eq. (4).
Mn(g)
Mn(g) + Support(g)
MnxOy(wt%) =
× 100%
(4)
The applied reaction conditions were: 750 ◦C reactor tem-
perature, 60 ml/min gas flow and a feed gas composition of
CH4:O2:N2 = 4:1:4 (methane and synthetic air as oxygen source).
The analysis was performed with a gas chromatograph Agi-
lent 7890 A, equipped with a flame ionization detector (FID), a
thermal conductivity detector (TCD), an HP-PLOT/Q and an HP
Molsieve column. He (Air Liquide, purity 99.999%) was used as
carrier gas. The analyzed compounds were O2, N2, CO2 via TCD,
CH4, C2H4 and C2H6 via FID. N2 was used as internal standard.
The reproducibility of conversion (X) and selectivity (S) is suffi-
cient.
The conversion and selectivity were calculated from a mass bal-
ance based on the inlet and outlet concentration of reactants and
products, see Eqs. (7) and (8). The carbon balance was always higher
than 95%. The desired reaction products of the oxidative coupling of
methane are C2H4 and C2H6. The selectivity of these two products
was discussed as a sum, the C2 selectivity.
2.1.4. MnxOy-Na2WO4/support
The MnxOy-Na2WO4 active compounds on different support
materials were prepared via an adapted two-step wet impreg-
nation method [15]. First, the different support materials were
impregnated with an aqueous solution containing appropriate
concentration of (CH3COO)2Mn·4H2O (Fluka, >99%) at room tem-
perature. These materials were then dried in air at 65 ◦C overnight.
After that, the obtained materials were impregnated with an
aqueous solution containing the appropriate concentration of
Na2WO4·2H2O (Sigma-Aldrich, min. 99%) at room temperature. The
preparation was then followed by the same drying, calcination and
sieving procedures as described in Section 2.1.1. The loading of
Na2WO4 was calculated according to the Eq. (5) and the MnxOy
loading was calculated according to Eq. (6).
Na2WO4(g)
Mn(g) + Na2WO4(g) + Support(g)
Na2WO4(wt%) =
MnxOy(wt%) =
× 100%
(5)
ꢀ
Products
ꢀ
X =
(7)
Products + Unconverted Reactant
Product A
Mn(g)
× 100%
(6)
Mn(g) + Na2WO4(g) + Support(g)
ꢀ
SA
=
(8)
Reaction Products
2.1.5. Blank
The described procedure in Section 2.1.1 was applied to the sup-
port materials exactly the same way, without adding any of the
active compounds but only distilled water. These samples were
prepared for blank tests for the activity of the support materials
Under the applied reaction conditions (reactor with quartz sand,
no catalyst), as described above, the results of thermal OCM reac-
tion were: O2 conversion: 1.2%, CH4 conversion: 0.2% at 93.8% C2
selectivity.
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Table 4
The intended and determined loadings of the selected catalysts.
Active components
Intended loading
Determined loading
Mn/Na/W/SiO2
Mn/Na/W/TiO2-rutile
Mn/Na/W/SiC
Mn (wt%)
Na (wt%)
W (wt%)
2.00
0.78
3.13
2.16
0.85
2.95
2.02
0.73
3.12
1.99
0.65
3.04
Table 5
The specific surface areas in m2/g of the prepared fresh catalysts and spent MnxOy-Na2WO4/Support catalysts in the OCM (Pure Support: Support material without any
treatment. Blank Material: Support material after applying the preparation method of the catalysts but without any active components, see Section 2.1.5., S: Support
material).
Support material
Pure support
Blank material
W/S
Na/W/S
Mn/S
Mn/Na/W/S
Mn/Na/W/S (spent)
La2O3
CaO
Al2O3
ZrO2
SiO2
SiC
3
6
338
105
120
1
124
40
39
6
12
217
51
113
≤1
37
3
3
7
5
3
10
10
222
49
104
2
22
6
10
4
9
7
7
8
94
1
5
3
11
3
3
186
50
52
≤1
73
10
9
260
46
16
≤1
21
5
242
36
11
≤1
15
5
MgO
Fe2O3
Fe3O4
SrO
12
3
4
4
5
3
2
2
2
TiO2-rutile
TiO2-anatase
20
127
11
29
7
22
8
30
10
6
6
13
4
7
3. Results & discussion
3.1. Characterization
For the pure support material, as purchased and blank, the
expected phases are found. Additionally, for certain compounds,
such as La2O3, CaO or SrO, hydroxides and carbonates are also
present, for MgO, only Mg(OH)2 and no carbonates are found. This
could be due to storage in air. TiO2 in the rutile form, also contains
TiO2 in the anatase form.
Treating the purchased support material with the preparation
procedure obtaining the blank samples, does not lead to any observ-
able changes in the material.
Loading only WO3 on the support materials, led again to the
formation of hydroxides and carbonates for a number of sup-
port materials. The formation of WO3 is detected for Al2O3, SiO2,
SiC, Fe2O3, Fe3O4 and TiO2-rutile. However, for a number of sup-
port materials, the formation of tungstate compounds is found,
e.g. La10W22O81, CaWO4, Ca3WO6, MgWO4, Sr3WO6 and SrWO4.
Supporting WO3 on TiO2-anatase and ZrO2 results in amorphous
materials, indicating a strong structural change of the whole sup-
port material.
Similar to that of WO3, loading of MnxOy on the support mate-
rials leads to the formation of hydroxides and carbonates. Mn2O3
is observed as a separate phase for a number of support materials.
The formation of mixed metal oxides is observed with Mn on only
3.1.1. Elemental analysis
Due to the large number of catalysts, it was not possible to deter-
mine all loadings for all prepared materials via chemical analysis.
Therefore, the loading for a few chosen catalysts was determined
(Table 4) and found to agree with the intended loading, within the
range of experimental errors.
3.1.2. Specific surface area
The BET surface areas of all prepared materials are shown in
Table 5. Surface areas of the selected metal oxides vary between
very low (e.g. La2O3, CaO, SiC, SrO) and high values (Al2O3, ZrO2,
SiO2, MgO, TiO2-anatase). The surface areas of the blank samples
are lower than those of pure support materials because of the cal-
cination process, except for La2O3, CaO and SrO. There is a strong
reduction of the surface area upon the addition of the active com-
ponent, except for La2O3, Al2O3, CaO, SiC and SrO. Especially for
SiO2, the most substantial decrease occurs in the samples which
contain Na, because of the phase transformation of the amorphous
SiO2 into ␣-cristobalite [3].
As expected the surface areas of most MnxOy-Na2WO4/Support
catalysts decrease after 16 h time on stream, while the surface areas
of CaO and SiC supported catalysts increase. The strongest reduc-
tions happen for Al2O3 and ZrO2 supported samples.
Table 6
The detected phases for the different support and blank materials.
Support materials
Support material
Pure support material
Blank material
3.1.3. Phase analysis
La2O3
CaO
Al2O3
ZrO2
SiO2
SiC
MgO
Fe2O3
Fe3O4
SrO
TiO2-rutile
TiO2-anatase
No support
La2O3, La(OH)3
CaO, Ca(OH)2, CaCO3
Amorphous
Y0.15Zr0.85O1.93
Amorphous
SiC
MgO, Mg(OH)2
Fe2O3
Fe2O3
La2O3, La(OH)3
CaO, Ca(OH)2, CaCO3
Amorphous
Y0.15Zr0.85O1.93
Amorphous
SiC
MgO
Fe2O3
Fe2O3
A summary of the phase analysis of the detected XRD patterns
for the pure support materials, the blank samples, the supported
single metal oxides and the mixed metal oxides are presented in
Tables 6–8. XRD patterns and phase analysis of all blank samples
and catalysts can be found in the supporting information (Appendix
A). Generally, the prepared materials are multi-phase materials,
leading to numerous different phases being present. Therefore, only
unambiguously identified patterns are presented. This does not
exclude the presence of other phases, either in minor amounts or
amorphous states. Moreover, many prepared catalysts are amor-
phous, prohibiting a structural analysis via XRD.
SrO, SrCO3, Sr(OH)2·H2O
Rutile, anatase
Anatase
SrO, SrCO3, Sr(OH)2·H2O
Rutile, anatase
Anatase
–
–
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Table 7
The detected phases for the different supported single metal oxides.
Single metal oxides
Support material
W/Support
Mn/Support
La2O3
CaO
Al2O3
La2O3, La(OH)3, La10W22O81
CaO, Ca(OH)2, CaWO4, Ca3WO6
WO3, amorphous
La2O3, La(OH)3
CaO, Ca(OH)2, Mn2O3, Ca2MnO4
Amorphous
ZrO2
Amorphous
Y0.15Zr0.85O1.93
SiO2
SiC
WO3, amorphous
SiC, WO3
Mn2O3 or MnMn6SiO12, amorphous
SiC, Mn2O3
MgO
MgO, Mg(OH)2, MgCO3, MgWO4
WO3, Fe2O3
WO3, Fe2O3
SrO, SrCO3, Sr3WO6, SrWO4
Ti4H2O9.1.9H2O, rutile, anatase, WO3
Amorphous
MgO, Mg(OH)2
Fe2O3
Fe2O3
SrO, SrCO3, Sr(OH)2.H2O
Rutile, anatase
Anatase, rutile, Mn2O3
Mn2O3
Fe2O3
Fe3O4
SrO
TiO2-rutile
TiO2-anatase
No support
WO3
MnxOy/CaO. MnxOy does not change TiO2-anatase and ZrO2 into
amorphous materials, like WO3.
support material reacts with the metal oxides forming a new com-
pound in low concentration or in an amorphous state, prohibiting
any detection via XRD.
When supporting Na2WO4, only for SiO2, SiC, Fe2O3 and TiO2-
rutile detectable amounts of Na2WO4 were found. The known
transition of amorphous SiO2 into ␣-cristobalite and tridymite
phases is found while TiO2-anatase is turned into an amorphous
material.
3.2. Catalysis
A good method to compare different catalysts is to determine
via kinetic modeling the best performance which can be achieved
over a range of operating conditions. However, this is in any case
time consuming and many catalysts are instable, making a compar-
ison difficult. In the present manuscript, we measured the catalytic
activity only for one set of reaction conditions (feed gas com-
position, gas flow, mass of catalyst, reactor temperature) but for
16 h time on stream. Because especially for the very temperature-
sensitive OCM reaction it is really a challenging issue to compare
the results with reported results in literature due to very different
conditions [48]. We are aware that this comparison is not the opti-
mal way for a catalyst selection. However, the obtained results can
be used to exclude catalysts with an inappropriate performance,
for example due to instability or very low C2 selectivities. More-
over, an estimation if a support material is a candidate for further
investigations, or not, should also be possible.
Supporting MnxOy-Na2WO4, hydroxides or carbonates are only
found for MgO, La2O3 and SrO. Mixed oxides of support-Mn
or support-W phases are found for CaO and SrO. Al2O3, ZrO2
and TiO2-anatase support materials lead the MnxOy-Na2WO4 cat-
alysts to totally amorphous materials. However on TiO2-rutile
supported catalyst, Na2WO4, TiO2-anatase and TiO2-rutile phases
were detected to the contrary of the MnxOy-Na2WO4/TiO2-anatase.
Fe2O3 phase is dominating for MnxOy-Na2WO4/Fe3O4, but for
Fe2O3 supported catalyst Na2WO4 phase is also present addi-
tionally. On SiO2 and SiC supported catalysts mainly same active
component phases (Na2WO4 and Mn2O3) were detected. Amor-
phous and unselective silica support material transformed into
␣-cristobalite phase after preparation process for the MnxOy-
Na2WO4/SiO2.
For the MnxOy/SiO2 and MnxOy-Na2WO4/SiO2 catalysts, pat-
terns identified as Mn2O3 might also be explained as braunite
(MnMn6SiO12) phase. Since patterns of Mn2O3 and braunite are
very similar and overlap, it is too difficult to distinguish whether
the patterns belong to Mn2O3 or braunite.
It can be seen in the phase analysis of the catalysts (Appendix
A) that in some catalysts both amorphous contributions and crys-
talline phases are present together. However, certain oxides, such
as La2O3 or Fe3O4, Al2O3 and TiO2-anatase do not show any distinct
patterns when the material is supported on them. A reason could be
either that the supported material remains amorphous or that the
3.2.1. Stability
The stability of a catalyst is a crucial point for its application
potential. However, due to the large amounts of measured data, it
is not feasible to present the time on stream trajectories for all pre-
pared catalysts here. The complete data are shown in Appendix B.
However, in order to provide information about the stability of the
catalysts here, the first data point for the CH4 conversion, obtained
approximately 60 min after reaching the desired reactor tempera-
ture, is compared with the last data point for the CH4 conversion,
Table 8
The detected phases for the different supported mixed metal oxides, Na2WO4 and MnxOy-Na2WO4.
Mixed metal oxides
Support material
Na/W/Support
Mn/Na/W/Support
La2O3
La2O3, La(OH)3
La2O3, La(OH)3
CaO
CaO, Ca3WO6
CaO, Ca3WO6, Ca2MnO4
Al2O3
Amorphous
Amorphous
ZrO2
Y0.15Zr0.85O1.93
Amorphous
SiO2
SiO2 (cristobalite, tridymite), Na2WO4, Na4WO5
SiO2 (cristobalite, tridymite), Mn2O3 or MnMn6SiO12, Na2WO4, Na4WO5
SiC
MgO
SiC, Na2WO4
MgO
SiC, Mn2O3, Na2WO4
MgO, Mg(OH)2, Mn2O3
Fe2O3, Na2WO4
Fe2O3
Fe3O4
SrO
TiO2-rutile
TiO2-anatase
No support
Fe2O3, Na2WO4
Fe2O3
SrO, Sr(OH)2, SrCO3, Sr(OH)2.H2O
Rutile, anatase, Na2WO4
Amorphous
Fe2O3
SrO, SrCO3, Sr(OH)2.H2O, Sr(OH)2, Sr2MnO4, MnO
Rutile, anatase, Na2WO4
Amorphous
Na2WO4
Mn2O3, Na2WO4
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Table 9
The residual catalytic activity for CH4 conversion in % after 16 h time on stream.
Support material
Blank material
W/Support
Mn/Support
Na/W/Support
Mn/Na/W/Support
La2O3
CaO
Al2O3
ZrO2
SiO2
SiC
MgO
Fe2O3
Fe3O4
SrO
TiO2-rutile
TiO2-anatase
No support
98
80
82
100
22
60
42
16
75
90
74
54
–
51
55
125
80
104
94
103
105
36
64
101
40
77
102
57
33
74
68
79
101
49
112
88
160
63
83
72
80
180
100
88
98
108
67
67
89
200
200
32
16
85
83
100
100
121
108
82
89
115
82
38
71
obtained after approximately 16 h time on stream. The result is
the residual catalytic activity. The calculated data is presented in
Table 9.
The catalytic activity of some catalysts, like WO3/SiC,
Na2WO4/TiO2-anatase, Na2WO4/MgO increases with time on
stream. This is probably caused by structural changes during the
reaction. In Figs. 2–4, XRD patterns of some fresh and spent catalysts
are shown for catalysts with a strongly increased catalytic activity
after 16 h time on stream. Some of them demonstrate structural
changes different from fresh catalysts.
It was detected for the XRD patterns of WO3/SiC that the
ratio of SiC to WO3 has changed. For the Na2WO4/TiO2-anatase,
a phase transformation from amorphous state into anatase, rutile
and Na2WO4 phases occurred during reaction, showing very strong
crystallization. The patterns of Na2WO4/MgO catalysts before and
after reaction are exactly the same, no changes are observed.
The stability of the blank samples was high, except for SiO2,
MgO, Fe2O3 and TiO2-anatase. WO3/Fe2O3 and WO3/Fe3O4 cata-
lysts strongly deactivated, while the activity of the Al2O3, SiC and
MgO supported WO3 samples increased. SrO supported MnxOy
catalyst was stable during the reaction whereas MnxOy/SiO2,
MnxOy/TiO2-anatase and MnxOy/Fe2O3 lost their activity substan-
tially. Although supported Na2WO4 catalysts were stable, except
for the ZrO2 supported sample, with the combination of three
active components, nearly all supported catalysts were highly
stable except MnxOy-Na2WO4/ZrO2 and MnxOy-Na2WO4/TiO2-
anatase which strongly deactivated. Among all catalysts Al2O3
and MgO supported samples did not exhibit any sign of deacti-
vation. However, the unselective performance of Al2O3 supported
catalysts is an important disadvantage with respect to the
desired C2 products. Furthermore no interpretation can be derived
for the La2O3, MnxOy/La2O3, MnxOy-Na2WO4/La2O3, MnxOy/CaO,
MnxOy-Na2WO4/CaO, MnxOy/Al2O3, MnxOy-Na2WO4/Al2O3, ZrO2,
MnxOy/ZrO2 and MnxOy/MgO catalysts because of full O2 conver-
sion (Appendix B).
3.2.2. Comparison of catalytic performances
For the blank support material, WO3/Support, Na2WO4/Support,
MnxOy/Support and MnxOy-Na2WO4/Support the catalytic per-
formances after approximately 16 h are presented in Fig. 8. In Fig. 5
the CH4 conversions are presented, in Fig. 6 the C2 selectivities and
in Fig. 7 the C2 yields. The CH4 conversions of Al2O3, SiO2, SiC, Fe2O3,
SrO, TiO2-rutile and TiO2-anatase support materials are rather poor,
on the other hand La2O3, CaO, ZrO2, MgO and Fe3O4 are active them-
selves, but C2 selectivities of these active support materials are very
low. It becomes evident, that certain support materials (i.e. La2O3,
CaO or MgO) are by no means inert carriers. In particular, La2O3
exhibits a high activity, which is hardly changed upon the loading
of the metal oxides, except for WO3. However, most of the other
support materials show a rather low performance.
Supporting WO3, does not lead to any significant CH4 conver-
sion. In fact, it does decrease the conversion of the active support
materials. This is especially prominent for La2O3, ZrO2, MgO and
Fe3O4, while it is not distinct for SiO2 [50], SiC and CaO support
materials. WO3 improved slightly the activity only for SrO. Since
most of the supported WO3 catalysts have low CH4 conversion,
their C2 selectivities are very high.
However, supporting Na2WO4, the CH4 conversion increases
compared to supporting WO3. La2O3, CaO and Al2O3 supported
Na2WO4 catalysts are highly active in comparison to the other
Fig. 2. XRD patterns of W/SiC before and after 16 h time on stream (᭹: WO3,
SiC).
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Fig. 3. XRD patterns of Na/W/TiO2-anatase before and after 16 h time on stream (ꢀ: Na2WO4, ꢁ: TiO2-anatase, ꢂ: TiO2-rutile).
Fig. 4. XRD patterns of Na/W/MgO before and after 16 h time on stream (ꢂ: MgO).
Na2WO4/Support samples, but their C2 selectivity is very poor.
When Al2O3, SiO2 and SiC supported Na2WO4 catalysts are
compared to the blank samples of the corresponding support mate-
rials, their CH4 conversions increase explicitly in contrast to the
other catalysts.
Supporting only MnxOy, leads to relatively high CH4 con-
version. For MnxOy on SiO2 and MgO, this has already been
reported [51–53]. The CH4 conversions over La2O3, CaO, Al2O3,
ZrO2, MgO and Fe3O4 supported manganese oxide catalysts are
outstanding, some of them are even higher than conversion of the
25
20
15
10
5
0
Lanthana
Calcium
Alumina
oxide
Zirconia
Silica
Mn/Na/W/Support
Mn/Support
Na/W/Support
W/Support
Support
Silicon
carbide
Magnesia
Iron (II,III)
oxide
Iron (III)
oxide
Stronꢀa
Titania-
Ruꢀle
Titania-
Anatase
Without
Support
Fig. 5. The CH4 conversion determined after approximately 16 h time on stream for the different materials. Reaction conditions: 750 ◦C, 50 mg catalyst, flow rate of 60 ml/min
and feed gas composition of CH4:O2:N2 = 4:1:4.
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90
80
70
60
50
40
30
20
10
0
Lanthana
Calcium
oxide
Alumina
Zirconia
Mn/Na/W/Support
Mn/Support
Silica
Silicon
carbide
Magnesia
Na/W/Support
W/Support
Iron (II,III)
oxide
Iron (III)
oxide
Stronꢀa
Support
Titania-
Ruꢀle
Titania-
Anatase
Without
Support
Fig. 6. The C2 selectivity determined after approximately 16 h time on stream for the different materials. Reaction conditions: 750 ◦C, 50 mg catalyst, flow rate of 60 ml/min
and feed gas composition of CH4:O2:N2 = 4:1:4.
MnxOy-Na2WO4/Support catalysts. However the C2 selectivities are
mostly drastically reduced, total oxidation is dominating.
Supporting MnxOy-Na2WO4, it becomes evident that the com-
bination of these three metal oxides results in the best catalytic
performance, c.f. C2 yield. La2O3, CaO, Al2O3, ZrO2 and SiO2 sup-
ported trimetallic catalysts show the highest CH4 conversions
amongst all MnxOy-Na2WO4/Support samples. While the well-
known catalyst MnxOy-Na2WO4/SiO2 is highly selective towards
C2 products, for the La2O3, CaO, Al2O3, ZrO2 supported trimetallic
catalysts total oxidation products are prevailing.
Considering the C2 yield, presented in Fig. 7, it can be seen that
SiO2 is not the only suitable support material. Support materials,
such as La2O3, Al2O3, CaO or ZrO2 exhibit a similar C2 yield than
SiO2, but their CH4 conversion is much higher at much lower C2
selectivity. This catalytic behavior is not beneficial for a practical
application. However, TiO2-rutile and SiC exhibit a similar per-
formance than SiO2, with relatively low conversions at high C2
selectivities.
Regarding catalyst components without support material,
MnxOy shows substantial activity itself with poor C2 selectivity,
whereas the CH4 conversion over WO3 and Na2WO4 is extremely
low. Furthermore, it is interesting to note, that MnxOy-Na2WO4
without support material also displays a considerable activity
and selectivity, questioning the actual contribution of the support
material.
The results of the catalytic tests of all tested materials are sum-
marized in a conversion selectivity diagram (Fig. 8) by comparing
the obtained data points with the trajectory of a reference MnxOy-
Na2WO4/SiO2 catalyst, which is described in more detail in [14]. If
only the number of active sites changes, the resulting data points
would fall on the reference trajectory. However, if the nature of
active sites is changed, the data points might be below (aggrava-
tion) or above (improvement) the reference trajectory, indicating
possible changes of the catalytic properties.
WO3/Support and Na2WO4/Support catalysts are concentrated
mostly in the low CH4 conversion region with 0–3%. MnxOy/Support
10
9
8
7
6
5
4
3
2
1
0
Lanthana
Calcium
Alumina
Mn/
Na/
W/S
upp
ort
oxide
Zirconia
Silica
Mn/
Suppo
rt
Silicon
carbide
Na/
W/S
ppo
uppo
rt
Magnesia
rt
Iron (II,III)
oxide
W/Su
Iron (III)
oxide
Stronꢀa
Supp
ort
Titania-
Ruꢀle
Titania-
Anatase
Without
Support
Fig. 7. The C2 yield determined after approximately 16 h time on stream for the different materials. Reaction conditions: 750 ◦C, 50 mg catalyst, flow rate of 60 ml/min and
feed gas composition of CH4:O2:N2 = 4:1:4.
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Fig. 8. Active components on the different support materials were compared in an X-S diagram for the oxidative coupling of methane. Results were recorded after
approximately 16 h time on stream for each data point. Reaction conditions: 750 ◦C, 50 mg catalyst, flow rate of 60 ml/min and feed gas composition of CH4:O2:N2 = 4:1:4.
catalysts and bulk materials (pure support materials) exhibit high
conversions but they are mostly unselective towards C2 products.
Even if they showed high or low CH4 conversion, nearly all data
points are below the reference trajectory. For MnxOy-Na2WO4 sup-
ported on SrO, MgO, CaO, La2O3, Al2O3 and ZrO2, the performance is
also worse in comparison to the reference trajectory. Despite their
often high CH4 conversion, they usually lack C2 selectivity. More-
over, some of these materials strongly deactivate, e.g. ZrO2. This
means that the prepared catalysts contain active sites with higher
tendency towards COx than the reference catalysts.
The materials supported on TiO2 (rutile or anatase), Fe2O3,
Fe3O4, SiC and SiO2 are either on the reference trajectory or well
above, indicating that they could also be suitable support mate-
rials. If stability of the catalytic performance is additionally taken
into account, then TiO2-anatase has actually to be excluded.
However, without knowing the exact nature of the active sites
it is possible to say, that they are at least as good if not better than
that of the reference MnxOy-Na2WO4/SiO2. That could mean, that
their active sites are either less unselective or less active for the
consecutive combustion of the C2 products.
have active sites which are as suitable (SiC, Fe-based) or even more
suitable (TiO2-rutile) for the formation of C2, but seem to be present
in lower numbers.
Acknowledgments
This work is part of the Cluster of Excellence “Unifying Concepts
in Catalysis” coordinated by the Technische Universität Berlin.
Financial support by the Deutsche Forschungsgemeinschaft (DFG)
within the framework of the German Initiative for Excellence is
gratefully acknowledged. We would like to thank Prof. Dr. Helmut
Schubert† for his precious contributions. Mr. Yildiz is obliged to
the Ministry of Education of the Republic of Turkey for fundings.
We also thank Harald Link for ICP measurements and Maria Unter-
weger for XRD measurements. We are also obliged to Mr. Schiele
and the workshop for their support with the equipment. More-
over, we would like to thank Evonik and BASF for supplying support
material.
Appendices A and B. Supplementary data
4. Summary & conclusion
Supplementary data associated with this article can
be found, in the online version, at http://dx.doi.org/
10.1016/j.cattod.2013.12.024.
For the MnxOy-Na2WO4, a variety of different support mate-
rials has been studied. Most of the materials show a catalytic
performance inferior to the reference MnxOy-Na2WO4/SiO2 and/or
deactivate, although MnxOy-Na2WO4 without a support material
exhibits also significant catalytic activity, questioning suggestions
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Please cite this article in press as: M. Yildiz, et al., Support material variation for the MnxOy-Na2WO4/SiO2 catalyst, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2013.12.024