Oxidative coupling of methane over Mn-Na2WO4 catalyst supported by monolithic SiO2

Article abstract of DOI:10.1016/j.apcata.2017.08.020

In this work, oxidative coupling of methane was studied over monolithic Mn/Na₂WO₄/SiO₂ catalysts. The monolithic catalysts were prepared by three different ways as (1) preparation of monolithic silica (monosil) followed by impregnation of Mn and Na₂WO₄ (2) preparation of monolithic silica with Mn addition during gelation (Mn-monosil) followed by impregnation of Na₂WO₄ and (3) preparation of monolithic silica as MCM-41 structure (MCM-monosil) followed by impregnation of Mn and Na₂WO₄. The catalysts were tested in a microflow reactor and the results were compared with those obtained over particulate catalysts. It was found that Mn-monosil performed best and produced C₂ yield of 16.2%, which is close to the values obtained over particulate catalyst (19.3%). Mn-monosil was also passed the 10 h stability and hysteresis tests successfully. SEM characterization revealed that monolithic catalysts prepared by three different methods had different pore structure; it was also seen from SEM images that Mn-monosil preserved its original form better, and this was also verified by XRD analysis.

Full text of DOI:10.1016/j.apcata.2017.08.020

Accepted Manuscript
Title: Oxidative Coupling of Methane over Mn-Na2WO4
Catalyst Supported by Monolithic SiO2
Authors: Cagla Uzunoglu, Aybuke Leba, Ramazan Yildirim
PII:
S0926-860X(17)30400-3
DOI:
Reference:
http://dx.doi.org/10.1016/j.apcata.2017.08.020
APCATA 16375
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
17-6-2017
9-8-2017
13-8-2017
Please cite this article as: Cagla Uzunoglu, Aybuke Leba, Ramazan Yildirim, Oxidative
Coupling of Methane over Mn-Na2WO4 Catalyst Supported by Monolithic SiO2,
Applied Catalysis A, Generalhttp://dx.doi.org/10.1016/j.apcata.2017.08.020
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Oxidative Coupling of Methane over Mn-Na2WO4 Catalyst Supported by Monolithic SiO2
Cagla Uzunoglu, Aybuke Leba, Ramazan Yildirim*
Department of Chemical Engineering, Boğaziçi University, 34342, Bebek-Istanbul/ Turkey
*Corresponding Author:
e-mail: yildirra@boun.edu.tr Phone: +90 212 359 7248 Fax: +90 212 287 2460
Graphical abstract
90
83
82.8
2%Mn/5%Na2WO4/SiO2
T=725 oC, CH4/O2=5
79.8
80
70
60
50
40
30
20
10
0
61.1
22.3
20.2
18.9
18.5
16.4
16.0
16
8.7
CH4 Conversion
C2 Selectivity
C2 Yield
Particulate Catalyst Mn-monosil MCM-monosil Monosil

2
Highlights
 Oxidative coupling of methane was studied on Mn-Na2WO4 /monolithic SiO2 (Monosil) prepared
by various methods
 Catalyst prepared by Mn insertion during SiO2 gelation (Mn-monosil) followed by Na2WO4
impregnation gave highest C2 yields among monolithic catalysts tested.
 C2 yield obtained over Mn-monosil was comparable to that obtained over particulate catalyst
 Mn-monosil preserved its structure better during reaction
ABSTRACT
In this work, oxidative coupling of methane was studied over monolithic Mn/Na2WO4/SiO2 catalysts.
The monolithic catalysts were prepared by three different ways as (1) preparation of monolithic silica
(monosil) followed by impregnation of Mn and Na2WO4, (2) preparation of monolithic silica with Mn
addition during gelation (Mn-monosil) followed by impregnation of Na2WO4 and (3) preparation of
monolithic silica as MCM-41 structure (MCM-monosil) followed by impregnation of Mn and
Na2WO4. The catalysts were tested in a microflow reactor and the results were compared with those
obtained over particulate catalysts. It was found that Mn-monosil performed best and produced C2
yield of 16.2%, which is close to the values obtained over particulate catalyst (19.3%). Mn-monosil
was also passed the 10 hr stability and hysteresis tests successfully. SEM characterization revealed
that monolithic catalysts prepared by three different methods had different pore structure; it was also
seen from SEM images that Mn-monosil preserved its original form better, and this was also verified
by XRD analysis.
Keywords: Oxidative coupling of methane, Mn/Na2WO4/SiO2, monolithic silica, monosil, monolith.

3
1. Introduction
Oxidative coupling of methane (OCM) provides a direct route for ethylene and ethane
production from natural gas; CO and CO2 are also produced as undesired by-products [1]. Although
OCM has been studied as one of the potentially suitable process to utilize natural gas, there is a major
obstacle preventing its commercialization: the desired C2 products (ethylene and ethane) are more
reactive than methane at temperatures high enough for significant CH4 conversion [1-2]. Hence either
conversion or selectivity toward C2 products is low resulting always low C2 yields. However, the
researchers have been still working to overcome this problem for last three decades considering the
attractiveness of the process; those works involves testing various catalyst formulations as well as
finding optimum reactor configuration and operational conditions to improve C2 yield [3-5].
Various catalysts have been investigated for OCM process to enhance the C2 selectivity and
yield; among them, Mn/Na2WO4/SiO2 was found as one of the most effective catalyst [6-11]. It is
believed that each element (Mn, Na and W) in the catalyst has a special role, and interaction among
them provide the formation of the active centers on the surface of the support. Manganese acts as an
oxygen supplier for tungsten due to its high electronic conductivity and increase the stability of the
catalyst; sodium, which acts as a structural promoter, has a dual role to produce desired products as
well as to inhibit by-products. Tungstate, on the other hand, provides the formation of active sites
through both W=O and W-O-Si groups [10-12]. Several researchers have studied various
compositions of catalyst in order to obtain the optimum performance [6-7, 9, 13-15]; it generally
suggested that the composition should be around 2% Mn and 5% Na2WO4 for maximum yield. The
replacement of the elements in Mn/Na2WO4/SiO2 with other alternatives has not been produced any
improvement verifying that the combination of manganese, sodium and tungsten in the ratios
mentioned above is required for high performance [7,11,16].

4
It is observed, at various studies that, the temperature in the range of 600-800 °C is needed to
break the C-H bonds in methane and to reach significant conversion. However, higher temperature
also causes the catalyst deactivation and the formation of by-products. This problem is worsening by
the difficulties in temperature control of the catalyst bed; the studies have shown that there were about
150 °C temperature difference between the catalytic bed and the reactor furnace [17].
Desirable yields strongly depend on the feed composition as well; finding the optimum feed
composition is crucial to minimize the gas phase reactions leading undesired by-products [18-19].
The oxygen concentration in the feed is vital; a higher concentration lead to an increment of the side
reactions while too low O2 concentration causes low CH4 conversion [6, 11].
The physical structure of Mn/Na2WO4/SiO2 catalyst also seems to play an important role in
OCM reactions. It is believed that the reaction proceeds through the formation of CH3 radicals on the
catalyst surface followed by their combination in gas phase [20-21]; this may be indicating that some
empty space around the catalytic material is required. However, undesired COx species are also
formed in the gas phase as it is evident from the fact that filing the empty space before and after the
catalyst bed with quartz chips improves the C2 yield [9, 17, 23]. As the results, there should be an
optimum void structure to have desired C2 yield, and even a successful particulate catalyst worked
well in the laboratory may not do so in industrial applications because the void structure will be
completely changed in scale up process. Hence, using a catalyst (like monoliths), of which the void
structure can be preserved during scale up process, may be beneficial for the commercialization of
OCM process. Indeed, various works have been reported in the literature involving OCM over
monoliths (with well-defined void structure) like metallic monolits [23], ceramic monolits [24],
porous SiC [25] and granulated silica based support [26] with limited success.

5
The monolithic silica (monosil) is a porous single piece of material that can be produced in the
dimension of centimeters with various geometries using molds (for example as cylinders). They can
be prepared in larger dimensions and used directly in larger reactors, or large number of small size
monoliths can be placed into multiple channels; this way risk of changing the void structure of the
material during scale up is avoided. Hence, monolithic silica can be a good alternative to the forms of
silica used for OCM process by various investigators cited above; however, as far as we know, it has
not been used for OCM process although there are some published works involving its preparation
and use in other reactions [27-29].
In this work, OCM process over monosilic Mn/Na2WO4/SiO2 catalyst was studied; the monosil
is prepared using three different methods (1) preparation of monosil followed by Mn and Na2WO4
impregnation, (2) Na2WO4 impregnation on monosil, in which Mn was inserted during gelation of
monosil, and (3) Mn and Na2WO4 impregnation over monosil prepared as MCM-41 silica. The
reactions test were conducted at various temperatures and feed compositions; the catalysts were also
characterized by SEM/EDX and XRD analyses.
2. Experimental
2.1. Catalyst Preparation
Particulate Catalyst: 2 wt. %Mn- 5 wt.% Na2WO4/SiO2 was prepared by incipient to wetness
sequential impregnation over 60-100 mesh (0.250-0.149 mm) silica gel; Mn was impregnated first.
Mn(NO3)2. 4H2O and Na2WO4. 2H2O were used as precursor. After impregnation of each metal, the
slurry was dried at 130 °C in the oven, and then calcined at 800 °C for 8 hours.
Monosil Catalyst: Monolithic silica (monosil) catalysts were prepared using three different
procedures. First, the distilled water, 69% nitric acid, TEOS and PEG 20 000 was added and

6
continuously stirred at 0 °C in a programmable water bath until the solution was turned into a gel. The
mixture was then poured into the plastic molds, which had the appropriate diameters for catalysis to
be fixed in the reactor after all steps were finished. Then, the molds were kept in the oven at 40 °C for
5 days in order to complete the gelation. Later, monolithic columns were removed from the molds,
washed with distilled water and treated with ammonium solution in order to obtain well-ordered
mesopores (Fig. 1.a). Then they were dried and calcined at 550 °C for 5 h with a ramp rate of 1
°C/min (Fig. 1.b). Based on the weight of the monosil, appropriate amount of the Mn(NO3)2
tetrahydrate solution and then Na2WO4 dihydrate solution were poured over the monosil using a
disposable plastic transfer pipette. The impregnated monosils were dried (Fig. 1.c), and finally
calcined at 800 °C for 8 h (Fig. 1.d); the catalyst prepared this way was labeled as monosil in the
remaining part of the manuscript. Although there are some local non-homogeneities in the colors of
monoliths, the procedure was reproducible; the catalyst prepared in different batches gave
approximately the same results.
In the second procedure, the manganese was added to the solution before the gelation took place
to increase the homogeneous dispersion of the metal precursors. The remaining steps were similar to
those in the first method. The monosil prepared this way was labeled as Mn-monosil. Finally, as the
third method, MCM-41 silica monoliths were prepared by adding CTAB (Cetyltrimethylammonium
Bromide) into the solution as a final component to increase the mesoporosity in the monosil followed
by calcination and impregnations steps as explained in the first method. This monosil was labeled as
MCM-monosil.
o
The last calcination step (800 C) was initially performed in both muffle furnace (at stationary
air) and in the reactor (in flowing air); the catalysts calcined in muffle furnace gave better selectivity
and yield; hence the experiments were continued with the samples calcined this way.

7
2.2. SEM/EDX and XRD Characterization
SEM/EDX characterization tests were carried out using Philips XL 30 ESEM-FEG system with
the resolution of 2.0 nm (at 20 kV). XRD analysis was performed using Rigaku D/MAX-Ultima+/PC
X-Ray diffraction (XRD) equipment having a X-ray generator with Cu Kα radiation (λ =0.154 nm),
scanning angle range of 3-90° at a rate of 2°/min, accelerating voltage of 40 kV and a current of 40
mA in Advanced Technologies Research and Development Center of Boğaziçi University.
2.3. Reaction Tests
The catalytic reaction tests were carried out in 10 mm ID, 800 mm long flow quartz reactor,
which was narrowed to 2 mm ID after the catalyst bed. In each experiment, 0.3 g particulate catalyst
or 2 pieces of monosil (equivalent to 0.3 gr particulate catalyst in weight) were loaded into the
reactor; the feed flowrate was 120 ml/min. The upper and lower parts of the reactor were filled with
quartz chips to minimize the pre and post-catalytic gas phase reactions. Quartz wool was placed
above and below the catalyst bed to prevent the mixing of the catalyst and quartz chips. Temperature
was controlled with a K-type thermocouple which was attached to the outer wall of the reactor. The
catalyst bed was heated to the reaction temperature under the nitrogen flow. The inlet gas flowrates
were measured using mass flow controllers while the total gas flowrate in the reactor outlet was
measured by soap bubble meter. The exit stream was analyzed by using a Shimidzu 14A type of GC
equipped with TCD detector and Carboxen-1000 60/80 column.
3. Results and Discussion
Some researchers suggested that the reactor diameter should be reduced after the catalyst bed
and/or the empty space after (and before) the catalyst bed should be filled with quartz chips in order
to improve C2 selectivity [9, 17, 23]. Indeed, we decreased reactor diameter from 10 mm to 2 mm

8
after the catalyst bed, and improved C2 selectivity significantly even though the conversion was not
affected. Similarly, the performance was also improved by filling the empty space before and after the
catalyst bed by quartz chips; hence we continued all reactions under these conditions. We did not
change the chemical composition of the catalyst considering that 2 wt. %Mn- 5 wt.% Na2WO4/SiO2
was found to be optimum by various investigators.
Following equations were used to calculate CH4 conversion, C2 selectivity and yield.
(1)
푋
×퐹 −푋
×퐹
표푢푡
표푢푡
퐶퐻4
퐶퐻4
푖푛
푖푛
퐶퐻4 퐶표푛푣푒푟푠푖표푛 =
푋
×퐹
퐶퐻4
푖푛
푖푛
2 × 퐹표푢푡(푋퐶2퐻4 + 푋퐶2퐻6
)
퐶2 푆푒푙푒푐푡푖푣푖푡푦 =
(2)
(3)
푋퐶퐻4 × 퐹 − 푋퐶퐻4 × 퐹표푢푡
푖푛
표푢푡
푖푛
퐶2 푌푖푒푙푑 = 푆푒푙푒푐푡푖푣푖푡푦 × 퐶표푛푣푒푟푠푖표푛
where Xs stand for mole fractions of gases while Fin and Fout are total flowrates in the reactor inlet
and outlet respectively. The error in conversion and selectivity was less than 10%
3.1. SEM/EDX chracterization of catalysts
Fig.2. shows the SEM images of 2 wt.%Mn-5 wt.% Na2WO4/SiO2 particulate catalyst; the
images in left hand side are for the fresh catalysts at different magnification ratios while the images in
right hand side are taken after the reaction. The grey roads appearing in images (b) and (d) are pieces
of quarts wool that mixed with the catalyst during emptying the reactor. White shiny pieces in (e) and
(f) represent Na2WO4; Mn could not be clearly identified.

9
The shape and pore structure of the particulate catalyst were generally preserved during reaction
with some erosion, which is apparent from the comparison of images (e) and (f); the same images also
indicates that catalyst had about 1 µm pores; and the pore sizes seem to increase with reaction (due to
erosion). Some of the Na2WO4 was also lost during reaction as it is evident from the decrease of
white pieces in (f) compared to (e).
The SEM images for monolithic catalyst prepared using three different procedures were also
presented in Fig. 3; we just presented the magnified images for three monoliths before and after the
reaction because their differences were not clear at lower magnification ratios. As it is apparent from
the images of fresh catalysts with the magnification ratio of 1000, the void structure of regular and
Mn-monosils (images a and d) are similar while the MCM-monosil (image g) is quite different. At the
higher magnification ratio (10000x), on the other hand, all three monosils have completely different
pore (void) structure (images b, e and h); however all seem to have similar interparticle pore sizes
(about 5-10 µm) while only the intraparticle pores of Mn-monosil (less than 1 µm) are apparent at this
magnification ratio.
The SEM images obtained for used catalyst were more interesting. The regular monosil lost its
original structure (image c) significantly after reaction (there seems to be some agglomeration) while
the effects of the reaction on the void structure of Mn-monosil was minimum (image f). MCM-
monosil turned into a more porous material indicating some erosion on the surface or sintering of
some material (image i). Apparently all three monosils also lost some of their Na2WO4 content as the
amount of white color structures were decreased after reaction.
It should be noted that Mn-monosil, which preserved the original structure most during
reaction, also performed best among the monosils in the reaction test as we will discuss latter; MCM-

10
monosil gave very close C2 yield to Mn-monosil while the regular monosil, which lost its original
structure most, has much lower C2 yield than other two catalysts. These results are in agreement with
the arguments presented in Introduction that the size and structure of the empty space around the
catalytic material may be indeed important.
3.2. XRD chracterization of catalysts
The XRD pattern of Mn/WO4/SiO2 catalysts in all forms tested can be seen in Figure 4. Various
SiO2 structures such as cristobalite low, quartz and tridymite as well as braunite ((Mn2O3).3MnSiO3),
Na2WO4, and Mn peaks were observed. Unfortunately, the peaks of most species were overlapping
with each other, hence it was not easy to draw any definitive conclusion; however two general
observations may help to explain the similarity and differences in the performance of four catalytic
forms as will discussed in the following sections.
First of all, the XRD patterns of Mn-monosil (both fresh and used) are more similar to those of
particulate catalyst; as presented in the following sections, these two forms also showed the highest
C2 yield. Second, when the XRD patterns of fresh and used samples were compared, it is clear that the
particulate and Mn-monosil also preserved their structure most as apparent from the peaks in 2ϴ
range of 15-40o; this was also evident from the SEM images in Figure 3. Na2WO4 (especially peak at
2ϴ=17°), also more apparent in particulate and Mn-monosil while Mn seems to be in metallic form
(overlapped with cristobalite low and tridymite at 2ϴ=22°)

11
3.3. Reaction Tests with Particulate Catalyst
First, 45-60 mesh size Mn/Na2WO4/SiO2 particulate catalyst (0.3 g) was tested under the
temperature range of 600-860 °C with the CH4/O2 ratio of 5; the feed flow rate was 120 ml/min. CH4
conversion, C2 selectivity and C2 yield are shown in Fig. 5. No significant CH4 conversion was
observed until 650 °C. Then both CH4 conversion and C2 selectivity increased with further increase of
o
temperature until 725 C, at which they reached the values of 22 % and 83 % respectively; these
values corresponds to the C2 yield of 19 % (12.6 % ethylene and 6.6% ethane), which is within the
range of values reported in the literature (15-20%) for this catalyst formulation [7]. On the other hand,
the oxygen conversion increased dramatically with increasing temperature after 650 oC and it reached
o
o
to 97% at 725 C; meanwhile CO2 selectivity decreased with increasing temperature up to 725 C
while CO remained almost constant (with increasing yield due to increasing CH4 conversion).
After 725 oC, the CH4 conversion and C2 selectivity values did not change significantly (as also
o
COx selectivity) until 800 C; the oxygen conversion continued to increase and reached to 100% at
o
750 C. Then, with the further increase of temperature, the C2 selectivity (and the yield) decreased
drastically while the CO selectivity increased (accompanied with a slight increase in CH4 conversion).
Apparently reactions like steam reforming of ethylene (C2H4+2H2O2CO+4H2) is taken place after
this temperature as it was proposed in literature [30-31]; the detection of H2 (not quantified) in the
effluent gas also supported this proposition.
Fig. 6 shows the effect of the feed compositions on the performance of particulate catalyst. The
experiments were performed at CH4/O2 ratio between two and 10 at 725 °C, which was determined as
the optimum temperature. The results indicated that OCM reaction was favored at low CH4/O2 ratios;
when the ratio was 2, the highest CH4 conversion and therefore C2 yield (19.3 %) were obtained.
However, the gas phase side reactions (COx formation) became also important at lower methane to

12
oxygen ratio ratios as expected [32]; this is the reason that C2 selectivity decreased while CH4
conversion was increased when the CH4/O2 ratio changed from five to two. The C2 selectivity
decreased at the CH4/O2 ratio of seven and increases again (also observed in monolithic catalyst in
Fig. 9), and we are not clear about the cause of this behavior at this stage.
3.4. Reaction Tests with Monolithic Structures
We started with comparing the reaction test of three different monolithic structure labeled as
monosil, Mn-monosil and MCM-monosil as it was explained in Section 2; Fig. 7 presents the CH4
conversion, C2 selectivity and C2 yield obtained in those structures. The results of particulate catalyst
o
were also added for comparison; the reaction tests were performed at the temperature of 725 C and
CH4/O2 ratio of five; the results obtained over Mn-monosil and MCM-monosil are comparable to
those obtained over the particulate catalyst while regular monosil performed considerably lower than
the others; these results are in accordance with the work of Yildiz et. Al (2016) reporting that
variation in the structure of silica support may have significant impact on the performance [33]. It
should be remembered from the SEM analysis (Fig. 2 and 3) that Mn-monosil (and the particulate
catalyst) also preserved their structure best during the reaction while regular monosil lost its structure
most; this may be another indicator of the significance of support structure for the performance of the
catalyst. Hence the works presented in the remaining sections were carried out using only the Mn-
monosil as the best performing monolithic structure.
Next, the effects of reaction temperature over Mn-monosil was investigated at a temperature
range 600-860 ºC using the feed CH4/O2 ratio of 5 (Fig. 8); the trends were similar to those obtained
over the particulate catalyst. The maximum yield was 16.0 %, which was close to the results of
particulate catalyst (18.5%). The C2 selectivity increased with temperature and reach to its maximum
at 700 °C; the CH4 conversion, hence C2 yield continued to increase resulting maximum C2 yield at

13
o
725 C (selectivity of CO+CO2 was the lowest). With further increase of temperature, C2 yield
o
decreased slowly until 800 C, and sharply dropped after this temperature. Apparently the gas phase
oxidation reactions of C2 products become dominate (as it was discussed above for particulate
catalyst); this was also apparent from the sharp increase of CO selectivity after this temperature.
The effects of feed composition (CH4/O2) ratio were also tested over Mn-monosil, and the
results are presented in Fig. 9. Conversion increased significantly if the CH4/O2 ratio decreases
(especially from five to two) while the C2 yield remains the same (16.2% total, 10.9% ethylene and
5.3% ethane) due to decreasing C2 selectivity. Although the CH4 conversion and C2 selectivity over
Mn-monosil and particulate catalysts are quite close to each other, they are not so under CH4/O2 ratio
of 2; the C2 selectivity decreases more over Mn-monosil (from 79.3 % to 40.8 %) than that over the
particulate catalyst (from 82.8 % to 72.3%). Similarly CH4 conversion increases more over Mn-
monolith (20.2% to 39.8%) than particulate catalyst (22.3 % to 27 %) resulting to close values of C2
yields. Apparently Mn-monosil is more sensitive to excess O2, and the reasons for this are not clear at
this stage.
The C2 selectivity improved with increasing methane to oxygen ratios; however the conversion
was lower at these ratios probably due to the complete depletion of oxygen in the feed; this also
resulted in lower C2 yields. Then, it can be deduced from these results that the CH4/O2 ratio of five is
optimum for both particulate and Mn-monosilic Na2WO4.
3.5. Stability Tests for Mn-monosil
To test the stability of M-monosil, the reaction tests were performed for 600 minutes at 725 °C
and the methane to oxygen ratio of five; although 600 min is not sufficient to show the stability of a

14
catalyst, the results in Fig. 10 clearly shows that Mn-monosil performed well during this period
showing a potential for a stable catalyst.
The stability of Mn-monosil was also measured by applying the hysteresis test as shown in
Table 1. The reaction test was started at 725 ºC by measuring CH4 conversion, C2 yield and
selectivity; then the reactor furnace was heated up until reaching 860 ºC. As expected, C2 yield
decreased significantly at this temperature. Later, the catalyst bed was cooled down gradually until
the initial temperature of 725 ºC; and another sample was taken at this temperature. As Table 1
clearly shows, the yield and selectivity were improved back and almost reached to their initial value at
725 ºC (conversion was slightly higher while selectivity was slightly lower). This test, together with
the SEM analyses and stability test in Fig. 11 indicate that Mn-monosil is indeed quite stable.
3.6. Blank test with quartz chips
Finally, in order to make sure that the results show the true performance of the OCM catalysts, a
blank test was also performed by filling the reactor with quartz chips in the absence of catalyst, and
running the reaction test under the same temperature range and feed conditions. In the conditions that
particulate and monosilic catalyst tested (725oC and CH4/O2 of 5), the conversion reached to 5.4 with
C2 yield of 2.13; both values are considerable lower than those obtained in the presence of catalyst
(over both particulate and monosil form) indicating that the catalytic effects to produce C2+ products
are indeed real.
4. Conclusion
Monolithic Mn/Na2WO4/SiO2 catalyst were prepared three different ways as (1) preparation of
monolithic silica (monosil) followed by impregnation of Mn and Na2WO4 (2) preparation of

15
monolithic silica with Mn addition during gelation (Mn-monosil) followed by impregnation of
Na2WO4 (3) preparation of monolithic silica as MCM-41 structure (MCM-monosil) followed by
impregnation of Mn and Na2WO4. Then they were characterized and tested for OCM performances.
The following conclusions can be drawn from the results presented and discussed above:
 SEM analysis showed that monolithic catalysts prepared three different ways had different
pore structures as was seen in SEM analysis; it was also seen that Mn-monosil preserved
its original form most while the form of regular monosil changed significantly, this results
was also verified by XRD analysis
 Mn-monosil produced C2 yield of 16.0%, which is quite close to the values obtained over
particulate catalyst (18.3%) under the same conditions (at 725 ºC and CH4/O2=5); these
values are also within the limits reported in literature.
 Over both particulate and Mn-monosil catalyst, the C2 selectivity is lower at low CH4/O2
rations while CH4 conversion is higher resulting higher C2 yields; however more CH4 is
also converted to COx products at low CH4/O2 ratios, hence increase in C2 yield is not as
high as CH4 conversion.
 Mn-Monosil catalyst is quite stable; no significant change is performance was observed at
600 min long experiment, and hysteresis test performed.

16
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18
A
B
C
D
Fig. 1. Preparation stages of monosil catalyst. (A) after removal from mold (B) after first calcination,
(C) after impregnation (D) after second calcination.

19
(a)
(b)
(d)
(c)
(e)
(f)
Fig. 2. SEM images of 2 wt.%Mn-5 wt.%Na2WO4/SiO2 particulate catalyst. (a) fresh-250x, (b) used -
250x, (c) fresh-1000x, (d) used -1000x, (e) fresh-10000x, (f) used -10000x

20
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
Fig. 3. SEM images for 2 wt.%Mn-5 wt.%Na2WO4/SiO2, (a) fresh monosil (1000x), (b) fresh monosil
(10000x), (c) used monosil (10000x), (d) fresh Mn-monosil (1000x), (e) fresh Mn-monosil (10000x),
(f) used Mn-monosil (10000x), (g) fresh MCM-monosil (1000x), (h) fresh MCM-monosil (10000x),
(i) used MCM-monosil (10000x),

21
Fig. 4. XRD patterns of all Mn/Na2WO4/SiO2 catalysts tested

22
100
90
80
70
60
50
40
30
20
10
0
19
18
18
11
6
2
600 625 650 675 700 725 750 775 800 825 850 875
Temperature (°C)
CH4 Conversion
CO Selectivity
C2 Selectivity
C2 Yield
CO2 Selectivity
O2 Conversion
Fig. 5. Influence of the temperature over particulate catalyst at CH4/O2=5.

23
100
90
80
70
60
50
40
30
20
10
0
85.1
82.8
77.0
73.0
27.0
22.3
19.7
19.3
18.5
15.2
15.0
12.8
2
5
7
10
CH4/O2
CH4 Conversion
C2 Selectivity
C2 Yield
Fig. 6. The influence of the feed composition over particulate catalyst 725 °C.

24
100
90
80
70
60
50
40
30
20
10
0
83.4
82.8
79.8
61.1
22.3
20.2
18.9
18.5
16.4
16.0
15.7
8.7
CH4 Conversion
C2 Selectivity
C2 Yield
Particulate Catalyst Mn-monosil MCM-monosil Monosil
Fig. 7. Comparison of the results of different monolithic structures at 725 ºC and CH4/O2=5.

25
100
90
80
70
60
50
40
30
20
10
0
15.6
13.3
16.0
12.0
9.4
5.8
600 625 650 675 700 725 750 775 800 825 850 875
Temperature (°C)
CH4 Conversion
CO selectivity
C2 Selectivity
C2 Yield
CO2 Selectivity
O2 Conversion
Fig. 8. Influence of the temperature over Mn-monosil t at CH4/O2=5.

26
100
90
80
70
60
50
40
30
20
10
0
86.9
79.3
78.8
40.8
39.8
20.2
18.1
16.2
16.0
14.2
12.7
10.4
2
5
7
10
C2 Yield
CH4/O2
CH4 Conversion
C2 Selectivity
Fig. 9. The influence of the feed composition over supported Mn-monosil at 725 °C.

27
90
70
50
30
10
0
100
200
300
Time (min)
Conversion
400
500
Yield
600
Selectivity
Fig. 10. Stability results of Mn-monosil at 725 °C and CH4/O2 ratio of 5.

28
Table 1. Hysteresis test for Mn-monosil with CH4/O2 ratio of 5.
CH4
C2
C2 Yield
Step
Conversion (%) Selectivity (%) (%)
Initial test at 725 oC
20.2
15.1
79.3
79.3
72.7
16.0
5.8
Temperature increased to 860 oC
Temperature declined to 725 oC back 23.7
16.3