Sol-gel method for synthesis of Mn-Na2WO4/SiO 2 catalyst for methane oxidative coupling

Article abstract of DOI:10.1016/j.cattod.2014.01.005

In this experimental study, a novel sol-gel method was developed to synthesize a 1.9%Mn-5%Na₂WO₄/SiO₂ catalyst for oxidative coupling of methane (OCM) reactions. The performance of the synthesized catalyst was investigated in fixed-bed and porous packed-bed membrane reactors. Particularly, the effects of operating temperature, methane-to-oxygen ratio and nitrogen dilution on the performance of this catalyst were investigated. It was observed that for high values of methane conversion, the sol-gel Mn-Na₂WO₄/SiO₂ catalyst provides 5-15% higher selectivity toward the desired products (C₂: C₂H₄ + C₂H₆) than the Mn-Na ₂WO₄/SiO₂ catalyst prepared by the incipient wetness impregnation method. It was also observed that for a similar set of experiments, the C₂-selectivity of the sol-gel catalyst is affected relatively less by the variation of methane-to-oxygen ratio. As a result, this catalyst can be exploited under the low methane-to-oxygen feed ratio which provides an efficient performance in both the OCM reactor and the OCM process scale. The best observed performance of the sol-gel catalyst in the packed-bed membrane reactor is 78% C₂-selectivity, 64% ethylene-selectivity and 24.2% C₂-yield under 20% nitrogen dilution.

Full text of DOI:10.1016/j.cattod.2014.01.005

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Contents lists available at ScienceDirect
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Sol–gel method for synthesis of Mn–Na WO /SiO catalyst
2
4
2
for methane oxidative coupling
a,∗,1
d
a,∗∗,1
b,1
c
b
H.R. Godini
, A. Gili
, O. Görke , S. Arndt , U. Simon ,
c
a
A. Thomas , R. Schomäcker , G. Wozny
a
Chair of Process Dynamics and Operation, Berlin Institute of Technology, Strasse des 17. Juni 135, Sekr. KWT-9, D-10623 Berlin, Germany
Department of Ceramic Materials, Institute for Material Science and Technologies, Berlin Institute of Technology, Hardenbergstrasse 40, D-10623 Berlin,
b
Germany
c
Institute for Technical Chemistry, Berlin Institute of Technology, Strasse des 17. Juni 124, D-10623 Berlin, Germany
Institute of Chemistry: Functional Materials, Berlin Institute of Technology, Hardenbergstrasse 40, D-10623 Berlin, Germany
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 August 2013
Received in revised form
In this experimental study,
a
novel sol–gel method was developed to synthesize
a
1
.9%Mn–5%Na2WO4/SiO2 catalyst for oxidative coupling of methane (OCM) reactions. The perfor-
mance of the synthesized catalyst was investigated in fixed-bed and porous packed-bed membrane
reactors. Particularly, the effects of operating temperature, methane-to-oxygen ratio and nitrogen
dilution on the performance of this catalyst were investigated. It was observed that for high values
of methane conversion, the sol–gel Mn–Na2WO4/SiO2 catalyst provides 5–15% higher selectivity
toward the desired products (C2: C2H4 + C2H6) than the Mn–Na2WO4/SiO2 catalyst prepared by the
incipient wetness impregnation method. It was also observed that for a similar set of experiments, the
C2-selectivity of the sol–gel catalyst is affected relatively less by the variation of methane-to-oxygen
ratio. As a result, this catalyst can be exploited under the low methane-to-oxygen feed ratio which
provides an efficient performance in both the OCM reactor and the OCM process scale. The best observed
2
2 November 2013
Accepted 5 January 2014
Available online xxx
Keywords:
Sol–gel method
Catalyst preparation
Oxidative coupling of methane
Mn–Na2WO4/SiO2 catalyst
Packed bed membrane reactor
performance of the sol–gel catalyst in the packed-bed membrane reactor is 78% C -selectivity, 64%
2
ethylene-selectivity and 24.2% C2-yield under 20% nitrogen dilution.
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
effective OCM catalysts so far, the Mn–Na WO /SiO catalyst is one
2 4 2
of the most stable and recommended catalysts [2,5]. The mechanis-
tic aspects of the OCM over this catalyst and the detailed analysis
of interaction of catalyst with the gaseous species have been inves-
tigated as the key information required for efficiently exploiting
this catalyst in a proper reactor feeding policy [6]. This catalyst can
be implemented in an appropriate reactor concept such as a mem-
brane reactor and therefore a selective stable OCM performance
can be accomplished using a packed-bed or a catalytic membrane
reactor. Catalytic membrane reactor not only offers a fine oxygen
feeding policy but it also addresses the exothermicity of this reac-
tion and the concern regarding hot-spot formation. However, an
efficient applicable method for implementing the catalyst over the
membrane structure should be developed based on the compre-
hensive understanding of the material and reaction engineering of
the system. Being able to provide a homogeneous activated struc-
ture with the desired diffusion and surface characteristics are the
main criteria for choosing the suitable method for this task.
Development of an efficient catalyst for oxidative coupling of
methane (OCM) remains to be the most challenging and crucial step
in the commercialization of this process. The performance of the
OCM catalyst not only affects the efficiency of the down-streaming
unit operations in the OCM process but also dictates the thermal-
reaction characteristics of the OCM reactor. In the last three decades
several catalysts and reactor concepts have been proposed for
oxidative coupling of methane (OCM) [1–3]. However, most of these
catalysts are either not stable enough for the expected long-term
operation on an industrial scale or do not provide a simultaneous
high ethylene yield and selectivity. A review over the research
activities on OCM catalyst and OCM reaction engineering has been
presented in some recent references [4]. Among the investigated
∗
Corresponding author. Tel.: +49 30 31421619; fax: +49 30 31426915.
Corresponding author. Tel.: +49 30 31424793; fax: +49 30 31428534.
E-mail addresses: hamid.r.godini@mailbox.tu-berlin.de, hgodini@gmail.com
H.R. Godini), a.gilidevillasante@tu-berlin.de (A. Gili).
In this work, a sol–gel synthesis procedure was developed for
preparing the Mn–Na WO /SiO catalyst and catalytic solution
∗
∗
2
4
2
which provides the desired characteristics for coating it over the
porous structure such as ceramic membrane. The distinguishing
(
1
These authors contributed equally to this paper.
0
920-5861/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2014.01.005
Please cite this article in press as: H.R. Godini, et al., Sol–gel method for synthesis of Mn–Na2WO4/SiO2 catalyst for methane oxidative coupling,
Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.01.005

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H.R. Godini et al. / Catalysis Today xxx (2014) xxx–xxx
[
9,19]. The specific role of the ␣-cristobalite phase on improving the
Nomenclature
C2-selectivity has been highlighted and comprehensively inves-
tigated in the context of the interaction between the support
structure and the cations [8,10,18–21].
It has been reported by several researchers [5,20,22] that using
the specific composition of 1.9–2% Mn and 5% Na WO /SiO for
this catalyst will lead to the best performance in terms of the C2-
selectivity and yield. The preparation method also has a crucial
impact on the performance of the resulting catalyst. It has been
observed that micro-pores are formed on the catalyst structure
when W and Na are implemented simultaneously [21]. This is an
important aspect since the preparation methods investigated in
this research – the wetness impregnation and sol–gel methods –
provide different characteristics in terms of the interaction of silica
and cations. Wang et al. have tested three preparation procedures
BOTZ
commercial glassy material used for coating the
ceramics
C2
ethane + ethylene
2
4
2
ICP
inductively coupled plasma
inside/outside diameter
incipient wetness impregnation
fixed bed reactor
ID/OD
IWI
FBR
Methane conversion portion of the inlet methane converted
to the desired and undesired products
OCM
oxidative coupling of methane
Selectivity portion of the whole consumed methane which
appears in the desired products
STP
standard temperature and pressure (298 K and
bar)
tetraethyl orthosilicate
which have resulted in three different levels of C -yield as reported
in Table 1 [22].
2
1
TEOS
In Table 1, the reported performance of the 1.9–2%Mn–
UniCat “Unifying Concepts in Catalysis”: research group in
Berlin
5
%Na WO /SiO catalyst produced via various preparation meth-
2 4 2
ods and tested in various reactors are reviewed. These data have
been obtained under different sets of operating conditions such
as gas dilution (%), GHSV (h ), which have also been reported in
Table 1. More information about the set of experimental conditions
employed for each case is available in the original references cited
in this table.
Yield
portion of the inlet methane which appears in the
desired products
−1
3
ꢀ
catalyst density (kg/m )
feature of synthesizing the Mn–Na WO /SiO with a sol–gel
method is that the precipitation of the catalytic precursors does
not occur before it is coated over the membrane surface.
As seen in Table 1, most of the researches so far have focused
on preparing the Mn–Na WO /SiO catalyst using the wetness
2
4
2
2
4
2
impregnation method. Only a few reports are available in which
the Mn–Na WO /SiO catalyst has been prepared using a kind of
In this research, a novel procedure for preparing the sol–gel
form of Mn–Na WO /SiO was developed. The resultant sol–gel
2
4
2
2
4
2
sol–gel method. In the approaches so far applied for sol–gel prepa-
ration of this catalyst, the same metal precursors as in the wetness
impregnation method have been used. These metal precursors are
mixed with tetraethyl orthosilicate (TEOS) as the source of silica.
During the catalyst synthesis via such an approach, an interaction
was then calcined to prepare the catalytic material in order to test
its activity and selectivity in the fixed-bed and packed-bed mem-
brane reactors. Moreover, the performance and characteristics of
the sol–gel catalyst were compared with those observed for the
Mn–Na WO /SiO catalyst prepared by a wetness impregnation
betweentheMn(NO ) andtheNa WO is likelytobethedominant
2
4
2
3 2 2 4
method. In this context, the effects of operating parameters such as
temperature, methane-to-oxygen ratio and nitrogen dilution were
comprehensively investigated.
phenomenon and precipitation is usually faster than the complete
establishment of the expected condensation reactions. Therefore,
establishing a homogeneous catalyst with uniformly distributed
species on the sol–gel network cannot be guarantied. Moreover,
homogeneous coating a porous ceramic support such as an alumina
membrane with the catalytic solution prepared by such a prepara-
tion procedure is usually not feasible. The microstructure of the
prepared catalyst by this method is strongly affected by different
parameters such as the type of precursors, the solvent, the dry-
ing process, and the calcination temperature. The drying process is
typically accompanied by densification. This is a critical step that
strongly affects the structure and the characteristics of the final
product (pore size distribution, porosity, surface area).
In order to achieve a homogeneous active surface, the source
and the procedure of implementing the metal precursors has to be
selected carefully. This is the main goal of the current research and
it has been accomplished via developing a novel route for synthe-
sizing this catalyst. In order to highlight the specifications of the
proposed sol–gel method, the preparation procedure and charac-
teristics of this method and the wetness impregnation method are
reviewed first. Then, the performance of the resulting catalysts, pre-
pared by each of these methods, is analyzed in the fixed-bed and
membrane reactors and finally compared to each other.
2
. Review of Mn–Na2WO4/SiO2 catalyst
The application of Mn–Na WO /SiO catalyst as an efficient
2
4
2
OCM catalyst has been investigated since 1992 [7] and its promising
potential for long-term operation has been highlighted since then
by several researchers [5,8,9]. This catalyst has been introduced as
one of the most stable catalysts for OCM [5,8–10]. Some funda-
mental and practical researchers have been conducted to further
improve the stability of this catalyst and evaluate its operating
2
aspects in different scales [11,12]. Our research group UniCat has
also been very active in analyzing different aspects of this catalyst,
from a fundamental understanding of the mechanisms on the cat-
alyst surface to large-scale preparation and testing of the catalyst
in a mini-plant scale fluidized bed reactor [9,10,13].
In the context of phenomenological analysis of this catalyst, Ji
et al. have contributed significantly to achieve the current under-
standing of the role of active sites on improving the C -selectivity
2
and yield [14–16]. It has been explained that Tungstate stabilizes
sodium on the catalyst surface and sodium facilitates the trans-
formation of amorphous silica to ␣-cristobalite [17,18]. Therefore,
it has been reported repeatedly that Na WO and WO4 (tetrahe-
2
4
3. Materials and methods
dron) have a crucial role in the selective conversion of methane
In this section, the wetness impregnation method and the
sol–gel method applied for preparation of the Mn–Na WO /SiO
2
4
2
catalysts are described in detail. The characterizations of the result-
ing catalysts as well as their typical performances are reported in
the next sections.
2
UniCat: “Unifying Concepts in Catalysis” is a research group coordinated by
Berlin Institute of Technology and funded by German Research Foundation –
Deutsche Forschungsgemeinschaft. (http://www.unicat.tu-berlin.de).
Please cite this article in press as: H.R. Godini, et al., Sol–gel method for synthesis of Mn–Na2WO4/SiO2 catalyst for methane oxidative coupling,
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3
Table 1
Comparative performance analysis of (1.9–2)%Mn–(4.7–5)%Na2WO4/SiO2 prepared by different methods and tested in various reactors. (If not stated otherwise, N2-dilution
has been used.).
◦
−1 −1
h )
Preparation method
Reactor
T ( C), CH4/O2 dilution(%)
GHSV/ꢀ (mL g
XCH4 (%)
SC2 (%)
YC2 (%)
C2H4/C2H6
Ref.
−1
h
IWI
IWI
IWI
Mix.Slur.
Ps.sol–geld
IWI
IWI
IWI
Micro
Micro
Micro
Micro
800, 3, 0
800, 3.2, 0
820, 4, 0
820, 4, 0
820, 4, 0
750, 4, 44
800, 4, 50a
800, 3, 0
36,000c
25,400
30,000
30,000
30,000
24,000
32.7
28.9
30.3
28
22.4
7
36.7
30.95
29.7
24
12.1
36.8
18
58.6
66.6
68.4
69.3
69.4
76
19.2
19.3
20.7
19.4
15.5
5.3
24.2
18.3
18.8
18
1.86
1.93
1.6
1.4
1.1
0.45
5
1.6
1.6
1.4
–
[8]
[19]
[22]
[22]
[22]
[9]
[23]
[20]
[20]
[24]
[25]
[26]
[5]
Micro
Fixed bed
Fixed bed
Micro
Micro
Fixed bed
Micro
Micro
Micro
Micro
2700c
h
−1
66
e
36,000
36,000
16,200
64,500
36,000
23,000c
2700
59.2
63.2
77
73.4
64.9
80
IWIf
IWI
IWI
IWI
IWI
IWI
800, 3, 0
775, 7.5, 0
800, 2, 60b
800, 3, 40
800, 7.5, 0
850, 4.5, 83
8.9
23.9
14
2.1
2.1
1.8
−1
h
33
80
26.4
[18]
a
Helium dilution.
b
Steam dilution.
c
d
e
f
−1
GHSV (h ).
Pseudo sol–gel (catalyst has been prepared via different procedure than the one reported in current paper).
1
2
.9%Mn–5%Na2WO4/SiO2.
%Mn–0.78%Na–4.22%W/SiO2.
3.1. Catalyst preparation
In order to obtain 1.9%Mn–5%Na WO /SiO catalyst, corre-
2 4 2
sponding amounts of sodium acetate anhydrous (Sigma–Aldrich:
CAS-No. 127-09-3), manganese (II) acetate tetrahydrate (Riedel-
de Haen: CAS-No. 6156-78-1) and ammonium metatungstate
hydrate (Sigma–Aldrich: CAS-No. 12333-11-8) were mixed in a
separate vessel containing ethanol and water with molar ratios
The incipient wetness impregnation method is considered as a
reference method to be reviewed here since it has been applied
and recommended by several researchers [5,9,10,13,17,25] for the
establishment of a stable and selective Mn–Na WO /SiO catalyst
2
4
2
(
see Table 1). The second method to be reviewed here is the novel
ethanol/H O = 4.4/6.4 (in reference to each mole of TEOS in the
2
proposed sol–gel preparation method which is described in this
paper for the first time.
pre-hydrolysis). Nitric acid was continually added until the pH of
the solution was controlled in the range of 2–3. After stirring the
solution, this solution was added drop-wise to the pre-hydrolysed
silica solution and the mixture was vigorously stirred for 2.5 h. No
interaction between the precursors was observed and a homoge-
neous gel was formed after few hours of stirring. This gel was then
3
.1.1. Mn–Na WO /SiO catalyst prepared by incipient wetness
2 4 2
impregnation method (IWI)
The detailed recommended recipe for preparing this catalyst has
been reported by Wang et al. [5] and was exactly replicated in this
research.
◦
allowed to dry at 90 C for 90 min and was annealed. Later the cal-
cined catalyst was crushed and sieved to reach the required range
of 250–500 ␮m to be tested in the packed bed reactor. The photo of
the catalyst crystal before and after calcination is shown in Fig. 1.
In this manner, the required amounts of Mn, Na, and W
were implemented in the catalyst’s structure to reach the
Amorphous SiO2 (Silica Gel Davisil Grade 636, Sigma–Aldrich:
CAS-No. 112926-00-8) with 35–60 mesh grain size (250–500 ␮m)
2
and specific surface area of 480 m /g was used in this preparation
method as the catalyst support material. A 5–20 g batch of silica
gel particles was impregnated by an aqueous solution containing
a proper amount of a Mn(NO ) × 4H O. After being air-dried at a
1
.9%Mn–5%Na WO /SiO catalyst composition.
2 4 2
A set of pre-experiments was devoted to determining the suit-
3
2
2
◦
able recipe for establishing the required amount of each component
and avoiding the interaction of precursors in the final resultant
catalyst. Moreover, the duration of gelation-time is an indicator
for the performance of the preparation procedure. It was found
that the minimum gelation-time of 2.5 h is adequate to ensure that
all precursors are hydrolyzed and have enough time to cast the
catalyst.
temperature of 130 C for at least 5 h, the granules were impreg-
nated with a second aqueous solution containing an appropriate
concentration of Na WO × 2H O [5]. The samples were then dried
2
4
2
◦
◦
for 8 h at 130 C and annealed at 800 C for 8 h under air. The heating
rate of 10 K/min was applied during the whole annealing proce-
dure. The resulting catalyst acquires 1.9% Mn and 5% Na WO (all
2
4
in weight percentage).
A quartz-washed powder with proven inert activity (already
washed with HCl and calcined) was purchased from Merck (CAS-
No. 14808-60-7) and used as inert packing to dilute the catalytic
bed.
4
. Catalyst characterization
An inductively coupled plasma (ICP) technique was applied
using a Horiba Ultima 2 ICP optical emission spectrometer to deter-
mine the bulk composition of the catalysts. The results confirmed
3
.1.2. Mn–Na WO /SiO catalyst prepared by sol–gel method
2 4 2
In the first step, TEOS (Sigma–Aldrich: CAS-No. 78-10-4) was
obtainment of the desired 1.9%Mn–5%Na WO4 composition of the
2
pre-hydrolysed due to the lowest hydrolysis rate of the silica pre-
cursor and in order to avoid the homo-condensation of the species.
A solution of water and nitric acid (Sigma–Aldrich: CAS-No. 7697-
catalysts. Using the ICP analysis, the concentration of Na, Mn, and
W in the sol–gel catalyst was measured and their values are respec-
tively 0.76%, 2.27% and 3.25%. Their corresponding concentrations
in the IWI catalysts are 0.7%, 1.9% and 3.16%.
3
7-2) was added drop-wise to a vessel containing the TEOS and
ethanol (Sigma–Aldrich: CAS-No. 64-17-5) while it was vigorously
stirring. The molar ratios used in this pre-hydrolysis step were
TEOS/ethanol/H O/HNO = 1/3.8/6.4/0.085. Then the mixture was
The observed X-ray diffraction patterns of the crushed catalyst
– characterized using a Philips PW 1830 – were also satisfactory
and matched expectations regarding the presence of ␣-cristobalite
and tungsten oxide on both the catalysts. Similar patterns with
2
3
stirred for 3 h at room temperature.
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Fig. 1. Sol–gel catalyst (a) before and (b) after annealing.
Table 2
(
a) Comparing the observed phases in both catalysts (sol–gel and IWI catalysts). (b) The measured specific surface area for four different sample of sol–gel catalyst and
comparing them with the reported values of specific surface area for the IWI catalyst. (c) Comparative EDX analysis of the sol–gel and IWI catalysts showing the percentage
of oxygen, silica and metal compounds in each sample catalyst.
Preparation method
a)
␣-Cristobalite (SiO2)
Tridymite (SiO2)
Quartz (SiO2)
Tungsten oxide (Na2WO4)
Bixbyite (Mn2O3)
Braunite (MnMn6SiO12)
(
IWI
Sol–gel
x
x
x
x
x
x
x
x
x
x
2
2
BET surface for sol–gel sample catalysts [m /g]
The reported BET specific surface area [m /g]
(
b)
1
1
1
1
.1
.3
.0
.2
2.5 [5]
3.8 [17]
1.9 [9]
2.2 [27]
O
Na
Si
Mn
W
(
c)
Sol–gel: magnification (10 k) – acceleration voltage (10 kV)
Sol–gel: magnification (10 k) – acceleration voltage (15 kV)
Sol–gel: magnification (5 k) – acceleration voltage (10 kV)
Sol–gel: magnification (5 k) – acceleration voltage (15 kV)
Theoretical bulk concentration
63.96
65.53
66
67.7
65.95
65.5
0.81
34.09
32.82
32.22
30.61
32.27
18.1
0.86
0.64
0.81
0.71
0.72
3.1
0.29
0.4
0.26
0.24
0.35
0.8
0.61
0.71
0.73
0.71
IWI catalyst [5]
12.5
Fig. 2. Comparative representation of the XRD patterns of the wetness impregnation catalyst (top-black) and the sol–gel catalyst (bottom-red): Na2WO4 (ꢀ), quartz (ꢁ),
tridymite (ꢂ), ␣-cristobalite (᭹) and MnMn6SiO12 (ꢁ). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this
article.)
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5
Fig. 3. SEM picture of the wetness impregnation (top) and the sol–gel catalyst (bot-
tom).
different peaks and intensity were detected for each catalyst (IWI
and sol–gel catalyst). This is in agreement with the observations of
other researchers who have reported that these observed phases
depend on the type of precursors used and on the sequence of
adding these precursors in the preparation recipe [22]. A compara-
tive representation of the XRD patterns for both catalysts is shown
in Fig. 2. Table 2(a) reveals the detected phases of both catalysts.
Some of these phases are known to have a crucial impact on the
activity and selectivity of the Mn–Na WO /SiO catalyst [18,19].
2
4
2
Scanning electron micrographs and energy dispersive X-ray
spectroscopy data were obtained using a ZEISS 1540 EsB machine
equipped with an EDX detector. The SEM pictures show that the
observed structures on the surface of the two catalysts are different.
This is partially due to the fact that the IWI catalyst has a stable pre-
defined structure, while the sol–gel catalyst has a bulky structure.
Fig. 3 shows the SEM pictures of the IWI and sol–gel catalysts.
Moreover, EDX analysis of the sol–gel catalyst was performed
to observe whether the Na, Mn and W cations had been homo-
geneously distributed over the catalyst structure. This is a crucial
aspect because a heterogeneous distribution of cations could cause
hot-spot formation near the surface with a dense population of
metal cations, which would consequently decrease the selectivity
of the OCM reactions.
Fig. 4. EDX analysis for the sol–gel catalyst showing the manganese distribution
(
bottom-green), the sodium distribution (pink-middle) and the SEM picture of the
catalyst (top-gray). (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
By analyzing several samples of the prepared sol–gel catalysts
in the pre-experimentation step, it was observed that the homo-
condensation should be avoided in order to achieve a structure
with highly disperse cations and minimize the local intensity of
the active sites. The results of this analysis for the prepared cata-
lyst via the proposed sol–gel method are shown in Fig. 4 where a
fine distribution of the cations is observed.
The spectrum of the components corresponding to this EDX
analysis is also reported here separately in Fig. 5 in order
to visualize the presence of the Na, Mn and W atoms over
the prepared catalyst. The EDX analysis confirmed that the
fine distribution of cations over the catalyst structure was
accomplished.
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As a result, the density of sol–gel catalyst is higher than the IWI
catalyst.
Moreover, the specific surface area of the sol–gel catalyst was
measured and reported in Table 2(b). As seen in this table, the spe-
cific surface area in the sol–gel catalyst is lower than in the IWI
catalyst. This is also partially due to the effect of sintering facilitated
by sodium. Sodium and other metal compounds are mostly avail-
able on the outer surface of the IWI catalyst. This is in contrast to
the homogeneously distribution of metal compounds all across the
sol–gel catalyst. This has been confirmed via the results of the EDX
analysis as shown in Table 2(c). Having considered the facilitating
effect of sodium in the sintering process of the silica structure, the
outer part of the IWI catalyst particle which has more concentration
of sodium significantly sinters while the total bulk shrinkage will
not be as significant as in the whole structure of the sol–gel catalyst.
Repeating the BET measurement tests for several samples of
the sol–gel catalyst and achieving similar results in terms of
specific surface area as is seen in Table 2(b) shows that the
Mn–Na WO /SiO catalyst can be successfully reproduced by the
2
4
2
sol–gel preparation method.
5. Experimental set-up and experimentation procedure
Both the sol–gel and wetness impregnated catalysts were tested
in a fixed-bed and also in a membrane reactor. The results of these
tests along with the general specifications of the reactor set-ups are
reported in this section.
5.1. Fixed bed reactor set-up and preliminary tests of the catalysts
The preliminary experiments for analyzing the performance of
the prepared catalysts were carried out in a packed-bed quartz
reactor with an inner diameter of 6 mm and a length of 55 mm.
Since the sol–gel catalyst is less active than the IWI catalyst, Usu-
ally 2 g sol–gel catalyst and below 1 g IWI catalyst are used in this
fixed bed reactor in order to (a) have more comparable level of
methane conversion for both catalysts, (b) address their difference
in density and ensure establishing the same volume of the catalytic
bed in both cases.
Before entering the reactor, the reactants are heated up to
◦
1
80 C. The temperature is controlled using an electric furnace.
The product gas stream is analyzed using an Agilent gas chromato-
graph 7890 A, equipped with a flame ionization detector, a thermal
conductivity detector, a HP-PLOT/Q and HP Molsieve columns.
The results of both catalysts under similar sets of operating con-
ditions are shown in Table 3.
Fig. 5. EDX analysis of the sol–gel catalyst, showing the counts as a function of the
excitation energy: the whole analysis spectrum (top) data has been separated into
two different graphs for better visualization.
The observed selectivity of the sol–gel catalyst in the fixed bed
reactor is significantly higher than for the IWI catalyst. Moreover,
usually the highest C -yield of the sol–gel catalyst in the fixed bed
2
reactor is achieved using lower nitrogen dilution or even in the
undiluted reaction environment. Nevertheless, the relatively lower
reactivity and higher selectivity of the sol–gel catalyst should be
properly exploited under proper set of operating parameters, such
as methane-to-oxygen ratio and nitrogen dilution, in order to syn-
chronize both the reaction and thermal performance of the system.
For instance, if the heat of reaction and feed flow rate are not enough
to maintain a proper temperature profile along the fixed bed reac-
tor, sometimes even slightly increase in the methane-to-oxygen
The bulk density of the IWI catalyst and sol–gel catalyst were
also measured which are respectively 0.56 g/cm3 and 1.22 g/cm .
The apparent difference between the density of these two cata-
lysts can be explained based on the difference in the mesoporous
structure of the IWI supported catalyst and the sol–gel bulk cat-
alyst. The sol–gel preparation route produces a highly branched
chained structure made of small clusters. This structure tends to
easily collapse and suffer shrinkage. In comparison to the IWI cat-
alyst, sintering is more intense in the entire of the sol–gel catalyst
since the sodium is present all across this bulk catalyst. Sodium
decreases the sintering temperature and leads to destruction of the
silica structure and shrinkage of the bulk volume. Moreover, the cal-
3
ratio reduces the C -yield and methane conversion significantly.
2
The produced ethylene/ethane ratio will be also affected signif-
icantly by the level of methane-to-oxygen ratio. Selected data
reported in Table 3 also represent these trends.
As is seen in this table, in order to secure the same level of
methane conversion as it has been achieved under methane-to-
oxygen ratio 2.5 for the sol–gel catalyst, methane-to-oxygen ratio
◦
cination temperature of the sol–gel catalyst (850 C), is higher than
◦
the calcination temperature employed for the IWI catalyst (800 C).
This will also intensify the sintering all through the sol–gel catalyst.
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Table 3
Selected performance of the sol–gel and IWI catalysts in a fixed bed reactor: effect of nitrogen dilution and methane-to-oxygen ratio.
Catalyst
CH4/O2
Methane conversion [%]
C2-selectivity [%]
C2-yield [%]
C2H4/C2H6
◦
a) For no inert gas dilution, T = 825 C, total flow rate = 210 (mL(STP)/min)
(
Sol–gel
IWI
IWI
2.5
2.5
3
26.5
35.4
26.8
72.2
58.1
59.6
19.1
20.6
16
2.6
2.75
1.7
Methane-to-oxygen
C2-selectivity [%]
C2-yield [%]
C2H4/C2H6
◦
b) For sol–gel catalyst, T = 825 C, total flow rate = 180 (mL(STP)/min), nitrogen dilution (0%)
(
2
3
.5
.5
72.1
77.5
18.8
13.2
2.62
1.51
for the IWI catalyst should be set in the range of 3–4. In gen-
eral, in order to achieve comparable results in terms of methane
conversion for the sol–gel and IWI catalysts, the catalyst weight,
methane-to-oxygen ratio or operating temperature should be dif-
ferent in testing these two catalysts. The observed lower activity
of the sol–gel catalyst can be attributed to its lower specific sur-
face area in comparison to the specific surface area in IWI catalyst
side, and (3) avoidance of building up a more than 4 bar pressure
drop across the membrane which potentially cause a membrane
breakage and safety issues, are secured. Therefore, the original
membranes should be modified in order to be exploited in an OCM
membrane reactor. In order to properly modify the membrane
and achieve the required oxygen distribution along the reactor, a
600 mm long tubular ␣-alumina membrane were purchased from
“Fraunhofer-Institute for Ceramic Technology” and modified in our
institute. The original fresh membrane has a support structure with
the specifications of 7/10 mm ID/OD, 1.5 mm thickness and 3 ␮m
average pore size. It has also a membrane layer with a thickness
of 10 ␮m and nominal pore size of 200 nm which cover the inside
layer of this porous ceramic tube.
(Table 2(b)). Moreover, as seen in Table 2(c), the sol–gel catalyst
has lower concentration of metal compounds distributed over its
surface. This was also qualitatively observed in Fig. 2, where the
peak for Na WO phase in the XRD analysis of the IWI catalyst
2
4
was shown to be higher than in the sol–gel catalyst. This might be
another reason for its lower activity and for the observed differ-
ence between the ethylene/ethane ratios in these two catalysts as
shown in Table 3.
For the IWI catalyst, some similar data with regard to the
effect of surface metal concentration on the methane conver-
The membrane was coated using the commercial BOTZ glaze
material. A homogeneous suspension of BOTZ glaze was prepared
to manually coat the ceramic membrane. The membrane was dip-
coated in this solution. This procedure was repeated six times, each
◦
sion, C -selectivity and ethylene/ethane products ratio can be
time followed by drying at 75 C for 15 min. As a result, a thin
2
found elsewhere. For instance in reference [8], by increasing
the concentration of the wolfram from 0.4% to 3.1%, the ratio
of ethane/ethylene products has changed from 0.3 to 1.6. The
and homogeneous layer was obtained over the membrane surface.
The membrane was then calcined at 950–1040 C for 30–45 min.
◦
Tailoring of the desired permeation profile along the membrane
and reduction of the contribution of the membrane to undesired
reactions were the main aims of the membrane modification, and
analyzing the results confirmed that these had been achieved. This
simple controllable modification procedure successfully provided
the required amount of oxygen permeation and ensured achieving
replicable results.
For more details about the procedure of membrane modification
and the design specifications of the membrane reactor set-up, read-
ers are referred to our recent publication [28]. The specifications of
the membrane reactor set-up do not affect the typical observed
trends of the performance analysis reported in this manuscript.
Nevertheless, the general specifications of the membrane reactor
set-up exploited for this analysis are also briefly reviewed here.
methane conversion and C -selectivity have respectively increased
2
and decreased. It seems that the generated reaction heat in the
intense reaction atmosphere facilitates the methane conversion,
ethane dehydrogenation and conversion of C2 products. There is
a similar situation when the observed conversion, selectivity and
ethylene/ethane ratio for the sol–gel and IWI catalysts are com-
pared. On the surface of the sol–gel catalyst, the concentration of
metal compounds is lower. According to this analogy, it is expected
and it was observed that the ratio of ethane/ethylene products in
the sol–gel catalyst is lower than in the IWI catalyst. This was shown
in Table 3.
5.2. Membrane reactor set-up for testing the catalysts
The prepared catalysts were also tested in a membrane reac-
tor set-up in which oxygen is distributed along the catalytic bed
through the surrounding tubular porous ceramic membrane. In this
manner, methane is axially fed into the catalytic bed positioned
inside the alumina tubular membrane and oxygen gradually per-
meates through the membrane into the catalytic bed, where it
reacts with methane. The distribution of the oxygen along the reac-
tor is a key parameter which crucially impacts the performance of
the OCM membrane reactor.
5.2.2. Reactor module and specifications
The structure of the membrane reactor module used in the
experimentation is shown in Fig. 6. The ceramic membrane is
implemented inside the metal reactor module and separates the
inner volume of the reactor module into the shell and tube sides.
Oxygen and nitrogen are injected into the shell side and on the tube
side, methane and nitrogen are fed into the catalytic bed.
According to the specifications of the reactor module, the pores
at each end of the membrane should be completely blocked. This
is needed to restrict the permeable area to the zone which is filled
with catalyst. Moreover, the permeation in the middle part should
be reduced as described earlier. Fig. 6 shows a picture of the result-
ing modified membrane along with the reactor set-up.
5.2.1. Membrane modification
The commercial inorganic membranes available on the mar-
ket do not simultaneously fulfill the key requirements of
being stable under the OCM reaction conditions and of pro-
viding the desired membrane permeation in the range of
The reactor module was made of alloy 800 (Fe/Cr alloy). The
annular-shell volume is filled with the Merck washed-inert quartz
packing in order to provide the conditions for safer operation and
efficient heat transfer. Moreover, quartz sand is used in the tube
3
−2 −1 −1
“
1–10 (cm cm min bar )”. In this manner, (1) establishment
of a fine oxygen distribution along the reactor, (2) prevention of
the back permeation of methane from the tube side into the shell
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Fig. 6. Picture of the membrane and membrane reactor module inside the experimental set-up.
side in the area outside the reaction zone to fix the position of the
catalyst bed.
heating zones individually. The recorded operating temperature
inside the packed bed is affected by two factors namely (1) the
reaction intensity and generated heat of reaction, and (2) the value
of the set temperature in the surrounding electrical tube furnace
(TW) [29]. In general, the amount of the provided external heat
and reaction heat should be controlled with regard to their impact
on the average operating temperature.
An infrared (IR) detector was used to monitor the concentration
of methane, oxygen, ethylene, ethane and carbon dioxide in the
reactor outlet gas stream. The precision of the IR measurements
were confirmed using gas chromatography (GC) sampling.
Two Teflon sealing rings were used to fix both ends of the
membrane inside the metallic reactor module before and after the
◦
reaction zone, where temperatures do not exceed 200 C. Right after
the reaction zone, there is a cooling water jacket which enables
cooling of the product gas stream immediately after leaving the
catalytic bed. Therefore, the reaction zone is practically restricted
to the area packed with the catalyst. Inside the ceramic membrane,
there is a multipoint thermocouple (13-points) with a 3 mm OD
stem extending along the reactor. This forms an annular space
between the thermocouple stem and inner diameter of the mem-
brane which is filled with the catalyst. In this manner, a very thin
catalytic bed (2 mm thick) is formed.
6. Performance analysis of the catalysts in the membrane
reactor
Up to 8 g of catalyst can be implemented in this reaction zone.
However, normally 2–3 g of the IWI catalyst and up to 4–7 g of the
sol–gel catalyst were applied in the experiments.
The sol–gel and wetness impregnated catalysts were tested in
the porous packed-bed membrane reactor in order to analyze the
effect of the operating parameters on the performance of these
catalysts under the fine oxygen dosing strategy and to highlight
the potential of each catalyst in terms of reactivity and selectivity.
The values of C -selectivity, methane conversion and C -yield were
recorded and analyzed as the main performance indicators of the
OCM membrane reactor using each catalyst.
It was generally observed that the sol–gel catalyst was less active
and more selective than the IWI catalyst. This conclusion was not
The set-points and the measured data were set and recorded
online through user-interface visualization of the control system.
Six mass flow controllers were employed to deliver the desired flow
rates and feed gas compositions of methane, oxygen and nitrogen
into the shell and tube sides. In this experimentation, the shell-side
outlet was completely closed and the whole shell-side gas stream
passed through the membrane as a dead-end configuration. In the
dead-end structure, the potential for back permeation of hydrocar-
bon from the tube side into the shell side and consequently the
probability of building an explosive atmosphere inside the shell is
reduced.
2
2
only based on the observed trends in terms of C -selectivity and
2
yield but also according to the thermal behavior of the OCM reac-
tor in the case of using each of these catalysts. The IWI catalyst
is a very active catalyst and observing local peak temperatures
The applied on/off heating/cooling procedure via the electrical
tube furnace enables one to maintain the reaction temperature at
the required level. The operating temperature was monitored at
◦
of 100 C higher than the average temperature along the packed
bed membrane reactor is not unusual. However, the observed hot
spot formation for the sol–gel catalyst was significantly lower for a
1
3 points inside the packed-bed membrane and controlled in two
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Fig. 7. Effect of nitrogen dilution and methane-to-oxygen ratio on the performance of the IWI catalyst in the OCM membrane reactor [methane feed flow
rate = 108 (mL(STP)/min); 3.5 g IWI catalyst, 3.5 g inert packing]. Left: analysis of C2-yield; Right: analysis of C2-selectivity.
comparable set of operating conditions. It should be also high-
lighted that the best performance of the sol–gel catalyst was
obtained in a higher average temperature in comparison to the
optimum operating temperature of the IWI catalyst.
been represented using the experimental data obtained from the
OCM membrane reactor set-up. The experimental data have been
highlighted on the graph for ease of tracking the trends.
The demonstrated trends in these graphs reveal that the IWI
catalyst is a very active catalyst. This can be seen by following the
effect of nitrogen dilution on C2-yield and selectivity. Increasing
the nitrogen dilution in this case decreases the significant impact
of hot-spot formation for this very active catalyst and therefore sig-
nificantly increases the C2-selectivity. However, despite reducing
the contact time because of the more diluted gas stream, this very
active catalyst converts a significant portion of the methane and as
a result, the C2-yield increases.
The observation of the performance of the sol–gel catalyst in the
OCM membrane reactor shows a different behavior. As is seen in
Fig. 8, the observed C2-yield for the sol–gel catalyst does not show a
monotonic trend by increasing the nitrogen dilution. Moreover, the
effect of methane-to-oxygen ratio on the observed C2-selectivity
is not as significant as it was for the IWI catalyst. As a result, the
C2-yield is reduced by exploitation of higher methane-to-oxygen
ratios. This is mainly because of the fact that a slight increase
in selectivity could not compensate for the drastic reduction in
methane conversion due to the use of higher methane-to-oxygen
ratios. The lower observed reaction-intensity and thermal perfor-
mance of the sol–gel catalyst is also important factors. All these
observations support the conclusion that the sol–gel catalyst is
more selective and less active than the IWI catalyst.
6.1. Effect of methane-to-oxygen ratio and nitrogen dilution
Methane-to-oxygen ratio is an important parameter which not
only affects the performance of the OCM reactor, but also has a sig-
nificant impact on the performance of the down-streaming units
in the whole OCM process. The same is true for the effect of nitro-
gen dilution. Higher nitrogen dilution mainly improves the thermal
performance of the OCM reactor reduces the hot spot formation
and increases the C -selectivity, but it will impose a significant cost
2
for having nitrogen to be separated in the down-streaming units,
especially in the cryogenic distillation.
Increasing the methane-to-oxygen ratio and nitrogen dilution
usually increases C -selectivity mainly because of reducing the par-
2
tial pressure of oxygen in the reaction environment. This is a typical
observation. However, a higher methane-to-oxygen ratio means
less available stoichiometric oxygen for converting methane which
consequently leads to lower methane conversion. Therefore, the
C2-yield, which by definition is the multiplication of methane con-
version and selectivity, might be increased or decreased due to the
variation of methane-to-oxygen ratio. Fig. 7 shows that for the IWI
catalyst, a higher C -yield and C -selectivity are achieved for a high
2
2
level of nitrogen dilution. However, depending on the range of vari-
Fig. 9 provides the possibility for comparing the performance
of these two catalysts in terms of the C2-yield and selectivity on
one graph. As is seen in this graph, sol–gel catalyst for all sets
of methane-to-oxygen ratios and under various levels of nitro-
gen dilution provide a more selective performance than the IWI
ation in the methane-to-oxygen ratio, the C -yield can be either
2
enhanced or degraded. This has been demonstrated in Fig. 7 where
the trends of the C -yield and selectivity for different methane-to-
2
oxygen ratio and various levels of nitrogen dilution observed have
Fig. 8. Effect of nitrogen dilution and methane-to-oxygen ratio on the performance of the sol–gel catalyst in the OCM membrane reactor [methane feed flow
rate = 132 (mL(STP)/min); 6.4 g sol–gel catalyst, 3.6 g inert packing]. Left: analysis of C2-yield; Right: analysis of C2-selectivity.
Please cite this article in press as: H.R. Godini, et al., Sol–gel method for synthesis of Mn–Na2WO4/SiO2 catalyst for methane oxidative coupling,
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Fig. 9. Effect of nitrogen dilution and methane-to-oxygen ratio on the performance of the OCM membrane reactor using the catalysts prepared by sol–gel and wet-
impregnation methods [methane feed flow rate = 120 (mL(STP)/min); 6.4 g sol–gel catalyst, 3.6 g inert packing; 3.5 g IWI catalyst, 3.5 g inert packing].
catalyst. However, depending on the range of these operating
operating parameter and in what range should be manipulated. Rel-
ative effects of those parameters on the performance of the sol–gel
and IWI catalysts were represented in Fig. 9. The performance of
the OCM downstream units will be also affected by the exploited
range of methane-to-oxygen ratio as well as the type of catalyst.
To highlight the promising performance aspects of the sol–gel
catalyst in a membrane reactor, the distinguished features of its
performance, which were repeatedly observed in several experi-
mentations, are reviewed here:
parameters it can result in a lower or higher C -yield.
2
As seen in Fig. 9, the best C2-yield of the sol–gel catalyst in the
OCM membrane reactor, within the investigated range of the oper-
ating conditions, is achieved at lower methane-to-oxygen ratios
where the methane conversion is higher. Moreover, this perfor-
mance is achieved in conjunction with a very high C -selectivity
2
under a low to moderate level of nitrogen dilution. All these are very
promising features and performance indicators especially when
these results are analyzed from the perspective of the whole OCM
process.
- The C -selectivity of the sol–gel catalyst is relatively less affected
2
By analyzing the performance of the whole OCM process it
by the methane-to-oxygen ratio. Therefore, the lowest methane-
to-oxygen ratios can be exploited for using the sol–gel catalyst
becomes clear that both C -yield and selectivity have a crucial
2
impact on the economy of the OCM process. However, there is
without reducing the C -selectivity significantly. As a result,
2
a minimum level of required C -yield above which achieving the
the amount of non-reacted methane and CO2 in the process
down-streaming units is reduced, which significantly reduces the
separation cost of the OCM process.
2
highest C -selectivity should be targeted as the main design crite-
2
ria for the whole OCM process. Therefore, in the higher ranges of
methane conversion, achieving a higher selectivity should be given
a priority. Ethylene is obviously the main desired product in the
OCM process and by analyzing the observed ratio of ethylene to
ethane production for the sol–gel catalyst, the potential of this
catalyst is further highlighted. As is seen in Fig. 10, the best ethy-
lene/ethane ratio for the sol–gel catalyst is obtained at the lower
methane-to-oxygen ratio and at low to moderate levels of nitrogen
dilution.
- Low to moderate level of inert dilution results in a higher C -yield
2
for the sol–gel catalyst which again improves the operation and
economy of the whole process.
In conclusion, this catalyst provides higher C -selectivity and
2
yield than the IWI catalyst in the desired range of operating con-
ditions which in turn improves the performance of the reactor, as
well as the performance of the whole OCM process.
However, the sol–gel catalyst shows a moderate level of activity
and maintaining a suitable reaction intensity and thermal perfor-
mance along the reactor as well as the optimum range of residence
times are the key factors in ensuring its efficient operation. Further-
more, a stability test should be performed to investigate the long
term operation of this catalyst.
The observed trends in Figs. 7–10 clearly demonstrate that
for improving the C -selectivity and C -yield in each case, which
2
2
Using the proposed sol–gel preparation method also has another
main advantage, which was the main motivation for developing
such a preparation method for this catalyst. The sol–gel preparation
method ensures the possibility of coating the sol-state of this cat-
alytic material over a porous inorganic membrane, thus efficiently
activating such porous structure.
7. Conclusions
A
novel sol–gel method was applied to prepare the
Mn–Na WO /SiO catalyst and efficiently coat it over an alu-
2
4
2
mina membrane support. According to the observed performance
of this catalyst in a fixed-bed reactor and a membrane reactor, this
catalyst provides a highly selective performance.
Fig. 10. Effect of nitrogen dilution and methane-to-oxygen ratio on the observed
production ratio of ethylene/ethane in the OCM membrane reactor using the sol–gel
catalyst [methane feed flow rate = 132 (mL(STP)/min); 6.4 g sol–gel catalyst, 3.6 g
inert packing].
The best C -yield for the sol–gel catalyst is achieved in low
2
methane-to-oxygen ratios and low nitrogen dilution which is
higher than the observed amount of C2-yield obtained for the
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11
IWI catalyst. Exploiting this sol–gel catalyst and its correspond-
ing optimum ranges of operating conditions ensures an efficient
performance for the whole OCM process since it reduces the cost of
down-streaming units significantly. The performance of the sol–gel
catalyst should be compared with the best reported performance
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Please cite this article in press as: H.R. Godini, et al., Sol–gel method for synthesis of Mn–Na2WO4/SiO2 catalyst for methane oxidative coupling,
Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.01.005