Investigation of metal oxide additives onto Na2WO4-Ti/SiO2 catalysts for oxidative coupling of methane to value-added chemicals

Article abstract of DOI:10.1016/j.cattod.2020.03.048

The oxidative coupling of methane (OCM) is a closely related reaction process involving the transformation of methane (CH₄) and O₂ mixtures into value-added chemicals such as ethylene and ethane (i.e. C₂₊). This work presents the effects of metal oxide additives into the Na₂WO₄-Ti/SiO₂ catalyst on the performance of the OCM reaction. Several metal oxide additives—including oxides of Co, Mn, Cu, Fe, Ce, Zn, La, Ni, Zr, Cr, and V—were investigated with the Na₂WO₄-Ti/SiO₂ catalyst. All of the catalysts were prepared using co-impregnation and the catalyst activity test was performed in a plug flow reactor at a reactor temperature range of 600–800 °C and atmospheric pressure. The physicochemical properties of the prepared catalysts relating to their catalytic activity were discussed by using the information of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) measurements. Na₂WO₄-Ti/SiO₂ added Mn was found to be the most active catalyst, involving shifts of binding energies of W 4f and Ti 2p toward lower binding energies. Moreover, a variety of operating conditions—including reactant- to-nitrogen gas ratio, catalyst mass, reactor temperature, and total feed flow rate—were intensively examined for the OCM reaction using the Na₂WO₄-Ti-Mn/SiO₂ catalyst. The maximum C₂₊ yield was subsequently discovered at 22.09% with 62.3% C₂₊ selectivity and 35.43% CH₄ conversion. Additionally, the stability of the Na₂WO₄-Ti-Mn/SiO₂ catalyst was also monitored with time on stream for 24 h.

Full text of DOI:10.1016/j.cattod.2020.03.048

Catalysis Today xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Investigation of metal oxide additives onto Na WO -Ti/SiO catalysts for
2
4
2
oxidative coupling of methane to value-added chemicals
Sarannuch Sringam , Phattaradit Kidamorn , Thanaphat Chukeawa,b, Metta Chareonpanicha,b,c
a
a
,
a,b,c,
Anusorn Seubsai
a
*
Department of Chemical Engineering, Faculty of Engineering, Kasetsart University, Bangkok, 10900, Thailand
Center of Excellence on Petrochemical and Materials Technology, Kasetsart University, Bangkok, 10900, Thailand
Research Network of NANOTEC–KU on NanoCatalysts and NanoMaterials for Sustainable Energy and Environment, Kasetsart University, Bangkok, 10900, Thailand
b
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
Catalyst
The oxidative coupling of methane (OCM) is a closely related reaction process involving the transformation of
methane (CH ) and O mixtures into value-added chemicals such as ethylene and ethane (i.e. C2+). This work
4
2
Manganese oxide
Oxidative coupling of methane
Sodium tungstate
Titanium dioxide
presents the effects of metal oxide additives into the Na
reaction. Several metal oxide additives—including oxides of Co, Mn, Cu, Fe, Ce, Zn, La, Ni, Zr, Cr, and V—were
investigated with the Na WO -Ti/SiO catalyst. All of the catalysts were prepared using co-impregnation and the
catalyst activity test was performed in a plug flow reactor at a reactor temperature range of 600–800 °C and
atmospheric pressure. The physicochemical properties of the prepared catalysts relating to their catalytic activity
were discussed by using the information of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and
2 4 2
WO -Ti/SiO catalyst on the performance of the OCM
2
4
2
2 4 2
scanning electron microscopy (SEM) measurements. Na WO -Ti/SiO added Mn was found to be the most active
catalyst, involving shifts of binding energies of W 4f and Ti 2p toward lower binding energies. Moreover, a
variety of operating conditions—including reactant- to-nitrogen gas ratio, catalyst mass, reactor temperature,
and total feed flow rate—were intensively examined for the OCM reaction using the Na
alyst. The maximum C2+ yield was subsequently discovered at 22.09% with 62.3% C2+ selectivity and 35.43%
CH conversion. Additionally, the stability of the Na WO -Ti-Mn/SiO catalyst was also monitored with time on
2 4 2
WO -Ti-Mn/SiO cat-
4
2
4
2
stream for 24 h.
1
. Introduction
OCM reaction to C2+, especially Mn modified with a variety of co-
catalysts, supports, and/or promoters, such as oxides of Mg, Na, and Ce.
The activity of those catalysts was approximately < 16% C2+ yield and
Methane (CH
4
) is the primary component of natural gas and biogas.
It is considered a greenhouse gas about 25 times stronger than CO
these two gases are released to the atmosphere in the same amount [1].
Since CH is abundant on Earth, it is highly attractive to worldwide
researchers to explore strategies to transform it into more useful pro-
ducts. One of the most challenging methods is the oxidative coupling of
methane (OCM), which is a type of chemical reaction using air or
2
if
25–78% C2+ selectivity. Lately, a solid mixture of Na
2 4
WO -Mn sup-
ported on SiO has been greatly attractive to researchers worldwide,
2
4
because this combination is highly active for the OCM reaction. The
catalyst has also been modified with many metals (e.g. Li, Na, K, Ba, Ca,
Fe, Co, Ni, Al, Ti, Ce, etc.) [7–9]. Some of these modified catalysts,
especially Na
improvement on both C2+ yield and selectivity (i.e. 16.8% C2+ yield
and 73% C2+ selectivity). The addition of a promoter into the Na WO
Mn/SiO catalyst (e.g. LiCl, NaCl, KCl, or CsCl) has also been found to
2 4 2 2
WO -Mn/SiO doped with TiO [10,11], exhibited an
4
oxygen directly reacting with CH to produce useful hydrocarbons such
as ethylene, ethane, propene, propane, etc. (C2+) [2–5]. In the absence
or presence of a catalyst, the OCM reaction is exothermic and normally
takes place in reaction temperatures of 600–1,000 °C [6]. Moreover, CO
and CO can be produced as byproducts. However, it is believed that if
2
a suitable catalyst is present, the products can be controlled and the
extreme reaction temperature can be reduced.
2
4
-
2
result in the incorporation of the promoter into the catalytic material,
thus increasing the number of strong basic sites, thereby enhancing the
catalyst activity [12–14].
Recently, we have found a combination of Na
2 4
WO and Ti-sup-
Previously, several potential catalysts have been reported for the
ported SiO exhibiting high activity for the OCM reaction, giving 4.9%
2
⁎
Corresponding author at: Department of Chemical Engineering, Faculty of Engineering, Kasetsart University, Bangkok, 10900, Thailand.
E-mail address: fengasn@ku.ac.th (A. Seubsai).
https://doi.org/10.1016/j.cattod.2020.03.048
Received 2 March 2019; Received in revised form 19 February 2020; Accepted 25 March 2020
0920-5861/ © 2020 Elsevier B.V. All rights reserved.
Please cite this article as: Sarannuch Sringam, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2020.03.048

S. Sringam, et al.
Catalysis Today xxx (xxxx) xxx–xxx
C
2+ yield with 71.7% C2+ selectivity and 6.8% CH
4
conversion at a
% CH
4
conversion× % C2+selectivity
%
C
2+ yield=
reactor temperature of 700 °C and atmospheric pressure [15]. The ac-
tivity of the catalyst had good stability over 24 h of on-stream testing.
Moreover, a crystalline structure of α-cristobalite of SiO
present along with TiO crystals was found to substantially enhance the
100
(4)
2
that was
2
.3. Catalyst characterization
2
activity of the catalyst for the OCM reaction to C2+. Therefore, improve
the activity of this catalyst is of great interest. Herein, we modify the
catalyst by adding a metal oxide additive into the Na WO -Ti/SiO
2 4 2
catalyst. The selected metal oxide additives include Co, Mn, Cu, Fe, Ce,
Zn, La, Ni, Zr, Cr, and V. The most active catalyst is subsequently
chosen to optimize the C2+ yield by varying operating conditions.
Additionally, advanced instrument techniques are also used to analyze
the prepared catalysts to acquire their physicochemical properties to
relate with their catalytic activity.
The X-ray diffraction (XRD) patterns of each catalyst were obtained
using a powder X- ray diffractometer (PXRD; JEOL JDX-3530 and
Philips X-Pert, using Cu-K radiation, 45 kV and 40 mA). The electronic
states of selected elements in each catalyst were examined using X-ray
photoelectron spectroscopy (XPS; Kratos Axis Ultra DLD) with Al Kα for
the X-ray source. The surface morphology of the samples was imaged
using a scanning electron microscope (SEM, FE-SEM: JEOL JSM7600 F).
α
3. Results and discussion
2. Experimental
3
.1. Activity of Na WO -Ti/SiO added metal oxide additives
2
4
2
2
.1. Catalyst preparation
In catalyst screening experiments for the OCM reaction in our la-
boratory, we have found that 5 wt% Na WO + 5 wt% Ti on SiO
denoted as Na WO -Ti/SiO ) showed a priming result for C2+ pro-
duction. We also found, from an XPS measurement of the catalyst, that
the form of Ti was TiO (data are not shown here). In this work, the
Na WO -Ti/SiO catalyst was further investigated by adding 11 ele-
ments, Co, Mn, Cu, Fe, Ce, Zn, La, Ni, Zr, Cr, and V. These 11 elements
were selected from the transition metals because of their availability,
non-toxicity, inexpensiveness, and, more importantly, inactiveness for
2
4
2
All of the catalysts were prepared using co-impregnation as follows.
(
2
4
2
Weights of Na
Ti, 97+%, Alfa Aesar), and X (X = Co, Mn, Cu, Fe, Ce, Zn,
La, Ni, Zr, Cr, or V) were determined and pipetted from each stock
precursor solution into amorphous fume SiO (surface area of 85-115
2 4 2 4 2
WO (Na WO •2H O, 98.0∼101.0%, Daejung), Ti
12 28 4
(C H O
2
2
4
2
2
2
m /g, Alfa Aesar) to obtain a desired weight percentage of the metal
components. Note that the precursor of the 11 elements was in the form
of metal nitrate hydrate, and the weight percentage of each component
on the support was calculated based on the formula appearing in each
catalyst’s name. For example, for the Na
weights of Na WO , Ti(0), and Mn(0) were determined and loaded onto
the SiO support. After that, the mixture was stirred at room tem-
perature for 2 h and heated to 120 °C until dried. Then, the dried
powder was set to calcine in an air furnace at 800 °C for 4 h. Finally, the
powder was ground until a fine powder was obtained. The weight
percentage of each component for each catalyst will be elaborated in
the results and discussion of that mentioned figure.
CH
metal oxide additive were prepared using a metal ratio of Na
5:5:2 (where X is an metal oxide additive and its weight is calculated
on the basis of the metallic form). The total metal loading was 12 wt%
for every catalyst, except Na WO -Ti/SiO (i.e. 5 wt% Na WO + 5 wt
Ti). The activity test results of the catalysts are presented in Fig. 1.
Under the same testing conditions, the catalyst without metal oxide
additive exhibited 4.43% C2+ yield with 33.8% C2+ selectivity and
3.34% CH conversion. The most promising catalyst was the addition
of Mn into the Na
2+ selectivity and 29.48% CH
which were added into the Na
yield higher than that of the catalyst without metal oxide additive, were
(in order) Co > Fe > Ce > Zn, giving 2+ yield in range of
4.50–7.65%. The addition of Mn clearly greatly improved the activity of
4
combustion [16]. All of the Na
2
WO
4
-Ti/SiO
2
catalysts with added
2
WO :Ti:X
4
2 4 2
WO -Ti-Mn/SiO catalyst, the
=
2
4
2
2
4
2
2
4
%
1
4
2
WO
4
-Ti/SiO
2
catalyst, yielding 9.97% with 35.0%
4
conversion. The other metal oxides,
C
2
.2. Catalytic activity test
2 4 2
WO -Ti/SiO catalyst and gave a C2+
The activity of each prepared catalyst for the OCM reaction was
C
a
evaluated in a plug flow reactor at atmospheric pressure. The reactor
temperature was set in the range of 600–800 °C. A catalyst (8–72 mg)
was packed in a quartz tube reactor (0.5 cm inner diameter) and
sandwiched between layers of quartz wool. The feed gas consisted of
methane (CH
nitrogen (N , 99.999%, Praxair) at a volume ratio of CH
:1:0, 3:1:2, or 3:1:4 with a total feed flow rate in the range of 35–95
4
, 99.999%, Praxair), oxygen (O
2
, 99.999%, Praxair), and
2
4
:O :N =
2
2
3
mL/min. The effluent gas was evaluated using an online gas chroma-
tograph (SHIMADZU, GC-14A) equipped with a flame ionization de-
tector (FID; for analyzing C
and a thermal conductivity detector (TCD; for analyzing CO, CO
CH ). The activity of each catalyst was analyzed after the system had
reached a set point of 2 h. Equations (1)–(4) show the formulas for
2 4 2 6 3 6 3 8 4 8 4 10
H , C H , C H , C H and C H , and C H )
2
, and
4
calculating the %CH
and %C2+ yield.
4
conversion, %C2+ selectivity, %CO
x
selectivity,
moles of CH4 input− moles of CH4output
%
CH4 conversion=
× 100
moles of CH4 input
(
1)
moles of C2+
× 100
%
%
C2+ selectivity=
COx selectivity=
Total moles of products
(2)
(3)
Fig. 1. Catalyst activity of Na
Reaction conditions: CH :O :N
feed flow rate of 35 mL/min, reactor temperature =700 °C.
2
WO
4 2
-Ti/SiO added metal oxide additives.
moles of COx
× 100
4
2
2
ratio = 3:1:0, catalyst weight =8 mg, total
Total moles of products
2

S. Sringam, et al.
Catalysis Today xxx (xxxx) xxx–xxx
Fig. 3. XPS spectra of Na
showing binding energy in the range of W4f.
2
WO
4
-Ti/SiO
2
added different metal oxide additives
Fig. 2. XRD patterns of Na
2 4 2
fixing wt% of Na WO :Ti:X = 5:5:2 and SiO balance.
2
WO
4
-Ti/SiO
2
catalyst added metal oxide additives,
2 4 2
Na WO -Ti/SiO . The characterization using XRD and XPS of these
catalysts will reveal how the activities of some catalysts improve, which
will be presented in the next section.
3.2. XRD and XPS analyses of Na
2 4 2
WO -Ti/SiO added metal oxide
additives
2 4 2
XRD patterns of the Na WO -Ti/SiO catalysts with added metal
oxide additives are presented in Fig. 2. The characteristic peaks of α-
cristobalite (2θ = 22.1, 28.6, 31.5, 36.1, 41.2, 42.9, and 44.9 (ICDD
No. 00-001-0438)) appeared for all catalysts. A small XRD peak in-
dicating the presence of α-tridymite (2θ = 21.8, 23.3, 27.3, and 30.1
(
ICDD No. 00-003-0227)) was also observed. The characteristic peaks of
Na WO (16.9, 27.8, 32.4, 48.8, 52.1, and 57.1 (ICDD No. 01-074-
369)) appeared in the Na WO catalysts with no added (-), Zn, Ni, Mn,
Fe, Cu, and Co. The characteristic peaks of TiO (2θ = 25.2, 37.0, 48.0,
4.1, and 55.0 (ICDD No. 01-073-1764)) were seen in the Na WO
catalysts with no added (-), Zr, Zn, V, La, and Ce. It should be noted that
the most active catalyst presented in Fig. 1 was Na WO -Ti-Mn/SiO . As
shown in Fig. 2, this catalyst consisted of α-cristobalite, crystalline
Na WO , and crystalline Mn (2θ = 33.1, 38.1, and 44.8 (ICDD No.
0-002-0896)). Hence, the presence of these crystalline phases is es-
sential for the OCM reaction. For the role of Ti, its combination with Mn
has been proposed in the form of the MnTiO phase; then, the MnTiO
phase plays an important role in remarkably improving the catalyst
activity [10,11]. However, no XRD peaks of MnTiO can be observed in
2
4
2
2
4
2
5
2
4
2
4
2
Fig. 4. XPS spectra of Na WO -Ti/SiO added different metal oxide additives
2
4
2
showing binding energy in the range of Ti2p.
2
4
2
O
3
0
35.12–35.24 eV for W 4f7/2 and 457.90–458.20 eV for Ti 2p3/2, while
3
3
the catalysts in group 2 gave the XPS signal in a range of 35.36–35.56
eV for W 4f7/2 and 458.34–458.68 eV for Ti 2p3/2. It is interesting to see
3
4
that the CH conversions of the catalysts in group 1 were significantly
Fig. 2, probably because a small amount of Mn was loaded and/or the
crystal species is too small to be detected.
The XPS spectra of all catalysts in the range of W (4f) and Ti (2p)
regions are presented in Figs. 3 and 4, respectively. The observed peaks
higher than those in group 2. Moreover, the catalysts in group 1 clearly
exhibited binding energies lower than the catalysts in group 2. Im-
portantly, these findings suggest that the metal additives—that are able
to shift the binding energies of W 4f and Ti 2p toward lower binding
corresponded to WO
2p1/2 ∼ 464 eV, 2p3/2 ∼ 459 eV). Of interest was that the peaks of
WO 2−and TiO of each catalyst were not identical. Perhaps, the ad-
dition of metal oxides influences the interaction of WO −2 or Ti
bonding with O2−, thus leading to shifts of the binding energies
17–19].
The most interesting results are presented in Fig. 5, showing the
plots of CH conversion of each catalyst versus its binding energy of W
7/2 (Fig. 5a) and Ti 2p3/2 (Fig. 5b). The plots in each figure can be
4 2
2−(4f5/2 ∼ 37.6 eV, 4f7/2 ∼ 35.4 eV) and TiO
2 4 2
energies when added into the Na WO -Ti/SiO catalyst—are highly
(
active for the OCM reaction.
4
2
It is possible that when the binding energy of W 4f or Ti 2p shifts
toward a lower binding energy, the energy to remove an electron from
the outer electron layer of W or Ti will become lower [18]. This reveals
that it becomes easier for an oxygen atom bonding with W or Ti to
dissociate or leave from the center. In other words, when an oxygen
4
[
2-
2+
4
molecule adsorbs onto an active site—WO
4
associated with Ti —it
4f
will dissociate and bond with the center of W or Ti. Due to the influence
of a metal oxide additive added into the catalyst, the oxygen atom can
easily detach from and leave the active metal center as H O [18]. This
2
divided into two groups: group 1 (the catalysts containing Mn, Fe, Co,
and Cr) and group 2 (the catalysts containing Cu, Zn, Ce, La, Ni, Zr, and
V). The catalysts in group 1 possessed binding energies in a range of
3

S. Sringam, et al.
Catalysis Today xxx (xxxx) xxx–xxx
Fig. 5. Plots of correlation between CH
4
conversions and binding energies of a) W 4f7/2 and b) Ti 2p1/2
.
proposed behavior can significantly enhance the reaction mechanism
for OCM. As a result, the catalysts in group 1 exhibit a higher CH
investigated by varying catalyst amounts and reactant gas feeding ra-
tios, while the total feed flow rate was fixed at 35 mL/min for every
4
conversion relative to those in group 2. Future studies, to deeply un-
derstand the reaction mechanism, are required.
condition. Three reactant gas feeding ratios of CH
4 2 2
:O :N were used,
including CH :O :N = 3:1:0, 3:1:2, and 3:1:4. The results are plotted
4
2
2
in Fig. 8. As expected, the activity of the catalyst increased on in-
creasing the catalyst amount because the number of active sites in-
crease as the catalyst amount increases. Consequently, more CH can
4
3.3. Optimization of Na
2
WO
4
-Ti-Mn/SiO
2
catalyst for production of C2+
react with the active sites, thereby increasing the C2+ yields. The cat-
alyst amount of each reactant gas feeding ratio that delivered the
highest C2+ yield was 24, 48, and 64 mg, giving a C2+ yield of 19.19%,
The Na WO -Ti-Mn/SiO
2+ yield by first varying the weight percentage of Mn into Na
2
4
2
catalyst was further optimized to increase
WO
from 0.0 to 2.5 wt%. The results are shown in Fig. 6. The C2+
yield was found to be optimized at 0.5 wt% Mn loading (12.16% C2+
yield with 39.7% C2+ selectivity and 30.62% CH conversion, then the
2+ yields slightly decreased to approximately 9.6%). The decrease in
the activity of the catalysts with Mn loading over 0.5 wt% could be due
to the formation of MnO multilayers, which affects the generation of
nucleophilic oxy species (e.g. O
C
2
4
-
Ti/SiO
2
19.61%, and 18.70% when the reactant gas feeding ratio of CH
4 2 2
:O :N
was 3:1:0, 3:1:2, and 3:1:4, respectively. It should be noted that the
4
optimal point of each condition is the point where the O
pletely consumed. Thus, after the optimal points, the CH
2
gas is com-
conversions
C
4
should have been steady. However, the C2+ conversions minimally
decreased with gradual decreases of C2+ yields, potentially because the
x
2–
2
2
–, O , and O–) [20]. SEM images of
C
2+ products can further react with some special active sites to produce
CO, CO , and even CH [21–27]. Thus, the CH conversion decreased as
the catalyst amount increased.
the catalysts shown in Fig. 6 are illustrated in Fig. 7. As shown, the
surface morphology and particle size of all catalysts are similar. All the
particles are irregularly shaped with various sizes, ranging from 50 nm
to about 1 μm in diameter of coral reef-shaped particles. Nevertheless,
the catalytic activity of each catalyst is different, probably due to dif-
2
4
4
The optimal catalyst amount of each condition found in Fig. 8 was
selected to study the effect of reactor temperature at three different
CH
ratio, the C2+ selectivities, CH
with increase in reactor temperatures, reaching an optimal value at 700
C. The highest C2+ yield was achieved at 19.61% with 60.40% C2+
selectivity and 32.48% CH conversion when the CH :O :N feeding gas
ratio was 3:1:2. At reactor temperatures above 700 °C, the catalyst
activity for C2+ formation rapidly dropped, while the CH conversions
slowly decreased. It is possible that the C2+ products quickly combust
in the presence of O or react further easily with active sites at high
reactor temperatures. Similar to the results in Fig. 8, the slow decreases
of CH conversion as reactor temperature rose were due to the addi-
tional CH from the decomposition of C2+ [17,28–31].
In general, a fuel-rich condition (i.e. CH :O ratio > 1.0) is more
favorable for C2+ production in the OCM reaction. The complete
combustion of CH is strongly favorable under fuel-lean conditions.
Therefore, the effect of CH :O ratio on the activity of the optimal
catalyst was studied, as shown in Fig. 10. The CH :O ratios were varied
4
:O
2
:N
2
feeding gas ratios, as shown in Fig. 9. For every feeding gas
ferences in the distribution of the active Mn species (Mn
in each catalyst.
2 3 3
O , MnTiO )
4
conversions, and C2+yields increased
2 4 2
The best Na WO -Ti-Mn/SiO catalyst found in Fig. 6 was further
°
4
4
2
2
4
2
4
4
4
2
4
4
2
4
2
from 1.0 to 3.0, for which the conditions ranged from fuel-lean to fuel-
rich. The results evidently showed that the C2+ yields slowly increased
as the CH
feed resulted in increases of C2+ yields and CH
theless, the C2+ selectivities significantly decreased. The results suggest
4
:O
2
ratios were lowered. In other words, O
2
increases into the
4
conversions. None-
that O
2
increases into the feed enhance the formations of CO and CO
2
Fig. 6. Effect of Mn loading into Na
CH :O :N ratio = 3:1:0, catalyst weight =8 mg, total feed flow rate of 35 mL/
min, reactor temperature =700 °C.
2 4 2
WO -Ti/SiO . Reaction conditions:
more than those of C2+ products [9,26,27].
4
2
2
The effect of total feed flow rate of the Na
2
WO
4
-Ti-Mn/SiO
2
catalyst
4

S. Sringam, et al.
Catalysis Today xxx (xxxx) xxx–xxx
Fig. 7. SEM images of Na
2
WO
4
-Ti-Mn/SiO
2
catalysts at different Mn loadings: a) 0 wt%, b) 0.5 wt%, c) 1.0 wt%, d)1.5 wt%, e) 2.0 wt%, and f) 2.5 wt%.
Fig. 8. Activity of Na
2
WO
4 2
-Ti-Mn/SiO catalyst at different catalyst amounts
Fig. 9. Activity of Na
tures and different CH
weight = 24, 48, and 64 mg for CH
tively, total feed flow rate =35 mL/min.
2
WO
4
-Ti-Mn/SiO
feeding gas ratios. Reaction conditions: catalyst
:O :N = 3:1:0, 3:1:2, and 3:1:4, respec-
2
catalyst at different reactor tempera-
and different CH :O :N
4
2
2
feeding gas ratios.
4
:O :N
2
2
4
2
2
for OCM reaction was also investigated as presented in Fig. 11. In
general, a short resident time inside a porous catalyst favors C2+ se-
lectivity because it reduces chances of combustion of C2+ products. The
results remarkably showed that the C2+ yields and selectivities were
achieved at approximately 20–22% and 60–62%, respectively, while
catalyst were much larger (about 50 times) than those of the fresh
catalyst. This indicated that one possible cause of the gradual deacti-
vation of the catalyst was the aggregation of the particles (i.e. catalyst
sintering). Comparison of the XRD spectra of the fresh and used cata-
lysts shows that the peaks of α-tridymite (2θ = 21.8, 23.3, 27.3, and
30.1 (ICDD No. 00-003-0227)) were clearer compared to those of the
4
the CH conversions were slightly changed at approximately 32–35%
when the total feed flow rate was in the range of 45–75 mL/min. At
above 75 mL/min, the activity of the catalyst slowly decreased, po-
tentially because the residence time or the contact time between the
reactants and the catalyst was greatly reduced.
fresh catalyst, while the NaWO peaks of the used catalyst were smaller
4
relative to those of the fresh catalyst. The peaks of α-cristobalite and
Lastly, a time-on-stream experiment over 24 h of the Na
Mn/SiO catalyst at the optimized operating conditions was conducted
to monitor its catalyst stability. The plots are presented in Fig. 12. It
was found that the CH conversions and the C2+ yields slightly de-
2
WO
4
-Ti-
TiO , however, exhibited no substantial change. This might suggest that
2
2
the crystallite size of α-tridymite could grow after many hours of
testing. However, it has been reported that the change of α-cristobalite
and α-tridymite does not substantially influence the activity of
Na WO -MnO -containing catalysts for OCM reactions [32]. Thus, it is
4
creased from the second to the last hour, while the C2+ selectivity re-
mained virtually unchanged. The C2+ yield was reduced from 21 to
2
4
x
not only catalyst sintering that causes the gradual deactivation, but also
the loss of Na WO .
16% within 24 h of experiment. The catalyst used for 24 h was then
2
4
observed to examine why the catalyst deactivated using SEM and XRD
analyses, as shown in Fig. 13. Apparently, the particle sizes of the used
5

S. Sringam, et al.
Catalysis Today xxx (xxxx) xxx–xxx
Fig. 12. Catalyst performance of Na
function of time on stream over 24 h. Reaction conditions: CH
:1:2, catalyst weight =48 mg, total feed flow rate =75 mL/min, reactor
temperature =700 °C.
2
WO
4
-Ti-Mn/SiO
2
of OCM reaction as a
Fig. 10. Effect of varying CH
4
:O
2 2 4 2
ratio of Na WO -Ti-Mn/SiO catalyst.
4
:O :N ratio =
2
2
Reaction conditions: (CH +O ):N ratio = 2:1, catalyst weight =48 mg, total
4 2 2
3
feed flow rate =35 mL/min, reactor temperature = 700 °C.
Fig. 11. Effect of varying total feed flow rate on catalytic performance of
Na
2 4 2 4 2 2
WO -Ti-Mn/SiO . Reaction conditions: CH :O :N ratio = 3:1:2, catalyst
weight =48 mg, reactor temperature =700 °C.
Fig. 13. SEM images (top) and XRD spectra (bottom) of fresh and used
Na WO -Ti-Mn/SiO catalyst.
2
2
2
4. Conclusion
gradually deactivated with time on stream because of catalyst sintering
and the loss of Na WO content. How to prevent catalyst deactivation
The purposes of this work were to study the effects of metal oxide
additives on the performance of the TiO -Na WO /SiO catalyst for the
OCM reaction to value-added chemicals and to optimize the C2+ yield
of the most promising catalyst. The plots between the CH conversions
2
4
2
2
4
2
and improve the C2+ selectivity of the Na
will be of great interest to further studies.
2 4 2
WO -Ti-Mn/SiO catalyst
4
and the binding energies of W 4f7/2 and Ti 2p3/2 of the prepared cat-
alysts have revealed that the catalysts that exhibited lower binding
energy values possessed high catalytic activity. This is because the bond
strength between W or Ti with O becomes weaker, thereby promoting
Acknowledgements
This work was financially supported by the Faculty of Engineering
at Kasetsart University, Bangkok, Thailand; the Kasetsart University
Research and Development Institute (KURDI); the Center of Excellence
on Petrochemical and Materials Technology, the National
Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and
Technology, Thai- land, through its program of Research Network
NANOTEC (RNN); the Thailand Research Fund and the Commission on
Higher Education (grant #MRG6180232).
the OCM reaction. Moreover, the addition of Mn into the Na
SiO catalyst was the most active catalyst. The crystalline phases of α-
cristobalite, Na WO , and Mn were present in the most promising
catalyst. In the attempts to optimize the C2+ yield by varying operating
conditions, the maximum C2+ yield of the Na WO -Ti/SiO catalyst
added 0.5 wt% Mn was achieved at 22.09% with 62.3% C2+ selectivity
and 35.43% CH conversion. The optimal conditions were a reactor
temperature of 700 °C, a CH :O :N feeding gas ratio of 3:1:2, a total
2 4
WO - Ti/
2
2
4
2 3
O
2
4
2
4
4
2
2
References
feed flow rate of 75 mL/min, and 48 mg of the catalyst. The stability of
the catalyst was also investigated, concluding that the catalyst
[
1] C.D. Ruppel, J.D. Kessler, Rev. Geophys. 55 (2017) 126–168.
6

S. Sringam, et al.
Catalysis Today xxx (xxxx) xxx–xxx
[17] W.C. Liu, W.T. Ralston, G. Melaet, G.A. Somorjai, Appl. Catal. A 545 (2017) 17–23.
[18] S. Gu, H.S. Oh, J.W. Choi, D.J. Suh, J. Jae, J. Choi, J.M. Ha, Appl. Catal. A. 562
(2018) 114–119.
[2] G.J. Hutchings, M.S. Scurrell, J.R. Woodhouse, Chem. Soc. Rev. 18 (1989) 251–283.
[3] M.Y. Sinev, Z.T. Fattakhova, V.I. Lomonosov, Y.A. Gordienko, J. Nat. Gas Chem. 18
(2009) 273–287.
[
4] M. Yildiz, Y. Aksu, U. Simon, T. Otremba, K. Kailasam, C. Göbel, F. Girgsdies,
[19] T.W. Elkins, H.E. Hagelin-Weaver, Appl. Catal. A 497 (2015) 96–106.
[20] A. Malekzadeh, M. Abedini, A.A. Khodadadi, M. Amini, H.K. Mishra, A.K. Dalai,
Catal. Lett. 84 (2002) 45–51.
O. Görke, F. Rosowski, A. Thomas, R. Schomäcker, S. Arndt, Appl. Catal. A 525
(2016) 168–179.
[
[
[
[
[
5] H.R. Godini, A. Gili, O. Görke, S. Arndt, U. Simon, A. Thomas, R. Schomäcker,
G. Wozny, Catal. Today 236 (2014) 12–22.
6] K. Khammona, S. Assabumrungrat, W. Wiyarath, J. Eng. Appl. Sci. 7 (2012)
[21] S.C. Oh, Y. Lei, H. Chen, D. Liu, Fuel 191 (2017) 472–485.
[22] V. Fleischer, R. Steuer, S. Parishan, R. Schomäcker, J. Catal. 341 (2016) 91–103.
[23] V. Fleischer, P. Littlewood, S. Parishan, R. Schomäcker, Chem. Eng. J. 306 (2016)
646–654.
4
47–455.
7] S. Ji, T. Xiao, S. Li, L. Chou, B. Zhang, C. Xu, R. Hou, A.P.E. York, M.L.H. Green, J.
Catal. 220 (2003) 47–56.
8] J.Y. Lee, W. Jeon, J.W. Choi, Y.W. Suh, J.M. Ha, D.J. Suh, Y.K. Park, Fuel 106
[24] Y. Gambo, A.A. Jalil, S. Triwahyono, A.A. Abdulrasheed, J. Ind. Eng. Chem. 59
(2018) 218–229.
[25] A. Aseem, G.G. Jeba, M.T. Conato, J.D. Rimer, M.P. Harold, Chem. Eng. J. 331
(2018) 132–143.
[26] M. Ghiasi, A. Malekzadeh, S. Hoseini, Y. Mortazavi, A. Khodadadi, A. Talebizadeh,
J. Nat, Gas Chem. 20 (2011) 428–434.
(2013) 851–857.
9] S.M.K. Shahri, A.N. Pour, J. Nat. Gas Chem. 19 (2010) 47–53.
[
[
[
10] P. Wang, X. Zhang, G. Zhao, Y. Liu, Y. Lu, Chinese J. Catal. 39 (2018) 1395–1402.
11] P. Wang, G. Zhao, Y. Liu, Y. Lu, Appl. Catal. A 544 (2017) 77–83.
12] F. Papa, D. Gingasu, L. Patron, A. Miyazaki, I. Balint, Appl. Catal. A. 375 (2010)
[27] C. Karakaya, H. Zhu, C. Loebick, J.G. Weissman, R.J. Kee, Catal. Today 312 (2018)
10–22.
1
72–178.
[28] M. Taghizadeh, F. Raouf, Inorg. Nano-Met. Chem. 47 (2017) 1449–1456.
[29] Z. Gao, J. Zhang, R. Wang, J. Nat. Gas Chem. 17 (2008) 238–241.
[30] A. Galadima, O. Muraza, J. Ind. Eng. Chem. 37 (2016) 1–13.
[31] G. Pantaleo, V.L. Parola, F. Deganello, R.K. Singha, R. Bal, A.M. Venezia, Appl.
Catal. B Environ. 189 (2016) 233–241.
[
[
[
13] V.H. Rane, S.T. Chaudhari, V.R. Choudhary, J. Nat. Gas Chem. 19 (2010) 25–30.
14] N. Hiyoshi, T. Ikeda, Fuel Process. Technol. 133 (2015) 29–34.
15] A. Seubsai, P. Tiencharoenwong, P. Kidamorn, C. Niamnuy, Eng. J. 23 (2019)
1
69–182.
[
16] W. Kumsung, M. Chareonpanich, P. Kongkachuichay, S. Senkan, A. Seubsai, Catal.
[32] J. Wang, L. Chou, B. Zhang, H. Song, J. Zhao, J. Yang, S. Li, J. Mol. Catal. A Chem.
245 (2006) 272–277.
Commun. 110 (2018) 83–87.
7