Study on the unsteady state oxidative coupling of methane: Effects of oxygen species from O2, surface lattice oxygen, and CO2on the C2+selectivity

Article abstract of DOI:10.1039/d0ra06065h

This study examined the effects of oxygen species on the unsteady-state oxidative coupling of methane (OCM) using a lengthy catalyst bed of Na2WO4/Mn/SiO2. The reaction conditions, including the methane-to-oxygen ratio, ratio of feed gas dilution by N2, quantity of catalyst, and feed flow rate were adjusted for the continuous flow fixed bed reaction system. While the O2 gas initiated methyl radical formation from methane, the surface lattice oxygen atoms improved the dehydrogenation of paraffins to olefins without significant activation of methane. The addition of CO2 as a mild oxidizing agent was also tested and slightly improved OCM selectivity with slightly lower methane conversion were observed. This journal is

Full text of DOI:10.1039/d0ra06065h

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Study on the unsteady state oxidative coupling of
methane: effects of oxygen species from O2,
surface lattice oxygen, and CO2 on the C2+
selectivity†
Cite this: RSC Adv., 2020, 10, 35889
bc
Suji Yoon,‡ab Seoyeon Lim,‡ab Jae-Wook Choi,a Dong Jin Suh,ac Kwang Ho Song
acd
*
and Jeong-Myeong Ha
This study examined the effects of oxygen species on the unsteady-state oxidative coupling of methane
(OCM) using a lengthy catalyst bed of Na2WO4/Mn/SiO2. The reaction conditions, including the
methane-to-oxygen ratio, ratio of feed gas dilution by N2, quantity of catalyst, and feed flow rate were
adjusted for the continuous flow fixed bed reaction system. While the O2 gas initiated methyl radical
formation from methane, the surface lattice oxygen atoms improved the dehydrogenation of paraffins to
olefins without significant activation of methane. The addition of CO2 as a mild oxidizing agent was also
tested and slightly improved OCM selectivity with slightly lower methane conversion were observed.
Received 11th July 2020
Accepted 17th September 2020
DOI: 10.1039/d0ra06065h
rsc.li/rsc-advances
the deep oxidation of methane to CO and CO2,6,8,10 many
selective oxidation catalysts have been prepared for the OCM
1. Introduction
reaction. For unpromoted oxides of alkali, alkaline-earth, and
rare earth metals, low CH4 conversion and high C2+ selectivities
were observed, along with lower C2+ yields (<12%),11 although
moderate OCM activity was achieved at lower reaction temper-
atures (<750 ꢀC).12 Perovskites, which are well-dened crystal-
line mixed oxides containing two different metals, were also
used for catalyzing OCM,13–15 exhibiting improved activity with
promoter addition.1,12 Among them, Na2WO4/Mn/SiO2 is one of
the most effective and stable OCM catalysts.9,16–21 An optimized
catalyst, composed of 2 wt% Mn and 5 wt% Na2WO4 on SiO2,
has exhibited ꢁ25% C2+ yield.6 The remarkable long-term
stability of Na2WO4/Mn/SiO2 (1000 h) was also reported for
continuous-ow reactions.21 Although the origin of its high
catalytic activity has not yet been elucidated,22 the unusual
formation of a-cristobalite SiO2 phase during calcination at
750–850 ꢀC has been reported as a distinguishing feature of
a good catalyst,23 and the active species has been suggested to
The vast reserves of natural and shale gas have led to global
interest in converting methane, their major component, to
valuable chemicals. The oxidative coupling of methane (OCM)
can directly produce ethylene using oxidizing agents including
oxygen gas at a high reaction temperature (>600 ꢀC), and hence
it is a promising way to use the highly stable methane as feed-
stock. The proposed mechanism for OCM is composed of
parallel and consecutive heterogeneous and gas-phase reac-
tions.1 The catalyst surface activated by the co-fed oxygen
produces methyl radicals by hydrogen abstraction from
methane. The methyl radicals can then be converted to the
desired ethane in the gas phase, or further oxidized to the
undesired CO and CO2. It was suggested that an industrially
feasible OCM process can be developed if the C2+ products
(hydrocarbons containing two or more carbon atoms) achieve
a yield above 25% and a selectivity of 75–80%.2–9
Because there is a critical technological barrier for the
selective formation of olens and paraffins while suppressing
be Na+ or Na2WO4 on silica surface with distorted WO4
2ꢂ
tetrahedra formed during phase transition.23–25 In addition to
the roles played by Na2WO4, manganese oxide was reported to
improve the oxygen mobility via reduction of Mn3+ to Mn2+.26
Because of the proposed oxygen activation by Mn in the Na–O–
Mn composites,27 the selective formation of ethylene, which can
occur by oxidative or non-oxidative dehydrogenation of ethane,
can be controlled by tuning the surface Mn concentration.24,28
Improved OCM activity by the addition of several additives to
Na2WO4/Mn/SiO2 has also been reported.9,29
aClean Energy Research Centre, Korea Institute of Science and Technology, Seoul
02792, Republic of Korea. E-mail: jmha@kist.re.kr
bDepartment of Chemical and Biological Engineering, Korea University, Seoul 02841,
Republic of Korea
cGraduate School of Energy and Environment (Green School), Korea University, Seoul
02841, Republic of Korea
dDivision of Energy and Environment Technology, KIST School, Korea University of
Science and Technology, Seoul 02792, Republic of Korea
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra06065h
Although a number of selective catalysts have been devel-
oped, a better understanding of the reaction conditions is also
‡ These authors equally contributed to this work.
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required to achieve a feasible OCM process. Because of the oxygen atoms were being consumed for the OCM, the unsteady
limited supply of O2 and the high CH4 conversion through the state reaction examined here is useful for understanding the
reactor, in which a reactant mixture ows downward through OCM reaction using lattice oxygen atoms. The spent catalysts
the catalyst bed, the conditions including reactant composi- were characterized using X-ray diffraction (XRD), X-ray photo-
tions in the upper catalyst bed may be signicantly different electron spectroscopy (XPS), and Raman spectroscopy to reveal
from those in the lower catalyst bed. When methane and changes in the catalysts during the OCM reaction.
oxidant are own through the oxygen-rich top bed, the oxidant
(mostly O2) can be adsorbed on the catalyst surface. The
adsorbed oxygen species activates methane by abstracting H to
form methyl radicals, which are coupled to form ethane. In the
2. Experimental
2.1. Materials
oxygen-decient bottom layer, methyl radicals, C2+ products,
and carbon oxides are present and can interact with the
unreacted methane and oxidant. The unreacted O2 can activate
methane by either forming methyl radical or deeply oxidize the
highly active C2+ products to CO and CO2, thereby decreasing
the C2+ selectivity. The amount of initially supplied oxidants
also inuences the level of deep oxidation products in the
bottom layer.
All chemicals were used without further purication. Sodium
tungstate dihydrate (Na2WO4$2H2O, 99+%) was purchased from
Yakuri Pure Chemicals (Kyoto, Japan). Manganese nitrate
hexahydrate (Mn(NO3)2$6H2O, 98%) was purchased from Kanto
Chemical (Tokyo, Japan). Silica gel (SiO2, 60–325 mesh, 99.5%)
was purchased from Johnson Matthey (Royston, United
Kingdom). Manganese(II) tungsten oxide (MnWO4, 99.9%) was
purchased from Alfa Aesar (Haverhill, Massachusetts, USA).
Manganese(III) oxide (Mn2O3, 99.9%) was purchased from
Sigma-Aldrich (St. Louis, Missouri, USA). Methane (CH4,
99.95%), oxygen (O2, 99.995%), nitrogen (N2, 99.9%), and
carbon dioxide (CO2, 99.99%) were purchased from Shinyang
Sanso (Seoul, Korea). DI water (18.2 MU m) was prepared using
an aquaMAX-Ultra 370 series water purication system (Young
Lin Instruments, Anyang, Korea).
By using less-active oxidizing agents, including H2O and
CO2, the selectivity of OCM products can be improved by sup-
pressing the burning of valuable hydrocarbons to form CO and
CO2.30 Water as an oxidant improved the coupling selectivity
over perovskite catalysts, exhibiting increased C2 production
with increasing partial pressure of steam.31 CO2 has also been
suggested as an alternative oxidant to O2.32–37 The formation of
surface lattice oxygen atoms from CO2 has been observed on
PbO–MgO and PbO–CaO catalysts, which improves the
abstraction of hydrogen atoms from methane.32,35 In Na2WO4/
Mn/SiO2, the presence of CO2 caused structural modication to
improve catalytic activity.38 The formation of hot spots, which
accelerate the deep oxidation to CO and CO2 because of
signicant local exothermic reaction, was observed in the
catalyst bed of a bench-scale OCM reactor,39 and these hot spots
can be reduced by the presence of CO2 with high heat capacity.40
In addition to the improved OCM activity, the presence of CO2
stabilized the lithium-doped MgO catalyst and suppressed its
deactivation.36 For oxidative dehydrogenation, it was suggested
that CO2 improves selective OCM and stabilizes the catalysts by
poisoning the nonselective active sites.41
2.2. Catalyst preparation
The catalyst containing 5 wt% Na2WO4 and 2 wt% Mn on SiO2
(Na2WO4/Mn/SiO2) was prepared using
a mixed slurry
method.39,42 Silica gel (28 g) was mixed with DI water (150 mL)
and stirred at 105 ꢀC for 1 h under reux. Mn(NO3)2$6H2O
(3.146 g) dissolved in DI water (10 mL) and Na2WO4$2H2O
(1.684 g) dissolved in DI water (10 mL) were added drop-wise to
the boiling mixture using a syringe pump over 15 min. The
prepared thick paste was dried in air at 105 ꢀC for 16 h and then
calcined in air at 800 ꢀC for 5 h. Prior to the reaction, the
prepared catalyst powder was passed through 70–140 mesh.
2.3. Catalytic activity measurement
The goal of this study is to understand the local OCM reac-
tion throughout the catalyst bed under continuously changing The catalytic reaction was performed using a xed bed cylindrical
reaction conditions, which can be attributed to the limited quartz reactor (ID ¼ 6 mm, length ¼ 370 mm) (Fig. S1†). The
supply of oxidizing reagents (particularly O2) in the xed bed unreacted O2 leaving the catalyst bed can contribute to further
system. The temperature (750–850 ꢀC), N2 dilution ratio, CH4/O2 homogeneous oxidation of the C2+ products to CO and CO2
ratio, catalyst bed thickness, total feed ow rate, and the co- before the product mixture was inactivated by cooling,38 which
feeding rate of CO2 were varied to study the behaviors of reac- hinders interpretation of the reaction results. To reduce homo-
tants and products under these continuously changing condi- geneous reaction of the product mixture, a narrow exit was
tions. The effects of O2 and CO2 (as strong and mild oxidizing introduced on the reactor to encourage its fast outow. The outlet
reagent, respectively) on the local OCM reaction were investi- of the reactor was narrower than the inlet (1 vs. 6 mm). The
gated in a long xed bed system, which is meant to simulate catalyst bed was located at the center of the reactor, with quartz
possible scaled-up reactors. CO2 as a mild oxidizing reagent can wool placed at its upper and lower ends. ZrSiO4 beads (Cenotec,
also function as a diluent in the OCM process, and it can be Co. Ltd.) were positioned on the top of the catalyst bed to mini-
recycled with the unreacted methane in the industrial scale mize the homogeneous reactions. A reaction mixture of CH4, O2,
process. With continuous consumption of O2 throughout the N2, and CO2 owed into the catalyst bed. Notably, the furnace
long catalyst bed and the formation of O2-decient region, the temperature, which was used as the reaction temperature in this
roles of lattice oxygen atoms on the OCM were also observed. study, was controlled (Fig. S2†). The measured temperature of
Although a steady state was not achieved while the lattice catalyst bed was 0–40 ꢀC higher, depending on the position in the
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Table 1 Reactions during the OCM process
catalyst bed and the reaction conditions. The temperature
measured at the bottom of catalyst bed was closest to the furnace
temperature. Because the change of temperature though the
catalyst bed was similar at each reaction temperature, we used
the furnace temperature as the reaction temperature for conve-
nience. Gas hourly space velocity (GHSV) was adjusted using
catalyst bed volume and reactant ow rate. Water formed during
the OCM reaction was removed at a cold trap (ꢂ2 ꢀC) connected
to a chiller (VTRC-620, Jeio Tech). Aer an hour of isothermal
operation, the product mixture was collected and analyzed using
a thermal conductivity detector (TCD) and a ame ignition
detector (FID) installed onto an online-gas chromatography
system (Agilent 7890A). CH4, O2, N2, CO, and CO2 were analyzed
using the TCD with a Carboxen 1000 column (Supelco, 60–80
mesh, 4.6 m ꢃ 1/8 in ꢃ 2.1 mm), and the hydrocarbons were
analyzed using the FID with an HP-PLOT Al2O3 S capillary
column (50 m ꢃ 320 mm ꢃ 8 mm). The catalytic performance was
characterized by the following parameters:
Reaction
3
CO production: CH4 + O2 / CO + 2H2O
CO2 production: CH4 +22O2 / CO2 + 2H2O
C2H6 production: CH4 + 14O2 / 21C2H6 + 21H2O
C2H4 production: CH4 + 12O2 / 21C2H4 + H2O
C3H8 production: CH4 + 13O2 / 31C3H8 + 32H2O
C3H6 production: CH4 + 12O2 / 31C3H6 + H2O
Olefins=paraffinsðmol=molÞ ¼
moles of ethylene and propylene produced
moles of ethane and propane produced
O2-consumption for product i, the fraction of O2 in the feed
required to convert CH4 into each product component i, was
obtained using the following relationship described in Table 1:
moles of CH4 consumed
CH4 conversion ðXðCH4Þ; %Þ ¼
moles of CH4 in the feed
ꢃ 100
number of carbon atoms per compound i molecule ꢃ moles of compound i
Selectivity to compound i ðSðiÞ; %Þ ¼
ꢃ 100
moles of CH4 consumed
2 ꢃ moles of C2 hydrocarbons þ 3 ꢃ moles of C3 hydrocarbons
C2þ selectivity ðSðC2þÞ; %Þ ¼
ꢃ 100
moles of CH4 consumed
2.4. Catalyst characterization
moles of O2 consumed
O2 conversion ðXðO2Þ; %Þ ¼
ꢃ 100
XRD results were collected to observe the catalysts' crystal
structures using a Shimadzu XRD-6000 device equipped with
a CuKa1 (l ¼ 0.15406 nm) source. Raman spectroscopy analysis
was performed using a Renishaw InVia Raman microscope. The
surface electronic structures were analyzed using a PHI 5000
Versa Probe X-ray photoelectron spectroscope at Korea Basic
Science Institute (Busan, Korea). The XPS spectra were cali-
brated using C 1s peak at 284.6 eV.
moles of O2 in the feed
CH4 conversion ꢃ C2þ selectivity
C2þ yield ðYðC2þÞ; %Þ ¼
100
O2-consumption for compound i ðO2-cons:ðiÞ; %Þ ¼
SðiÞ ꢃ moles of O2 molecules to convert CH4 to compound i
ꢃ 100
3
1
1
1
1
2
SðCOÞ ꢃ þ SðCO2Þ ꢃ 2 þ SðC2H6Þ ꢃ þ SðC2H4Þ ꢃ þ SðC3H8Þ ꢃ þ SðC3H6Þ ꢃ
2
4
2
3
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3. Results and discussion
3.1. Effects of CH4/O2 ratio and N2 dilution on catalysis
Before investigating the effects of reaction conditions on the
OCM results, the effects of O2 concentration or the CH4/O2 ratio
on the OCM activity were studied. CH4/O2 ¼ 3 mol/mol was
selected as an optimal reaction condition in this study because
of the high C2+ selectivity with moderate CH4 conversion,
although the highest C2+ yield was not achieved using this ratio
(Fig. 1 and Table S1†). Notably, in this study, C2+ compounds
include ethane, ethylene, propane, and propylene. When CH4/
O2 was adjusted from 1 to 6 mol/mol at 800 ꢀC, the highest C2+
yield of 21.5% (44.0% CH4 conversion and 49.0% C2+ selectivity)
was achieved at 800 ꢀC when CH4/O2 ¼ 2 mol/mol. The C2+
selectivity sharply increased with increasing CH4/O2 ratio up to
3 mol/mol, and then steadily increased. Based on these obser-
vations, we selected CH4/O2 ¼ 3 mol/mol for the good CH4
conversion and high C2+ selectivity. The observed increase in
C
2+ selectivity and decrease in CH4 conversion at a higher CH4/
O2 ratio (i.e., reducing the limited supply of oxygen reactant)
were consistent with the results of previous studies.43 At higher
CH4/O2 ratios, the amount of O2 was insufficient for activating
the large number of methane molecules, thus the CH4 conver-
sion decreased. At lower CH4/O2 ratios, the formation of methyl
radicals was improved by the high partial pressure of O2 to
achieve high CH4 conversion, but the methyl radicals were also
easily oxidized to CO and CO2 to decrease the C2+ yield (Table
S1†). These observations suggest that a moderate CH4/O2 ratio
is required to achieve a high C2+ yield by combining the CH4
conversion and C2+ selectivity.
Fig. 2 CH4 conversion (circles), C2+ selectivity (triangles), and O2
conversion (squares) using different contents of N2 diluent at (a)
750 ꢀC, (b) 800 ꢀC, and (c) 850 ꢀC. CH4/O2 ratio ¼ 3 mol/mol, GHSV ¼
10 000 hꢂ1
.
With the CH4/O2 ratio xed at 3 mol/mol, an optimized N2
dilution ratio of 20% (v/v) or CH4/O2/N2 ¼ 3/1/1 mol/mol/mol
produced the best CH4 conversion and C2+ selectivity (Fig. 2
and Table S2†). High concentrations of methane and O2 can
accelerate the reaction rates, and the deep oxidation of methane
to CO and CO2 may occur along with the formation of hot spots
because of the large heat of reaction.39 Diluting the reactant
mixture decreases the partial pressures of methane and O2 and
thus improves the selectivity for coupled C2+ hydrocarbons
while suppressing the formation of CO and CO2.39,44,45 This
change in the OCM results was more signicant at the higher
temperatures of 800–850 ꢀC (with CH4 conversions > 20%) and
ꢀ
less signicant at 750 C (with CH4 conversions < 20%). When
Fig. 1 CH4 conversion (circles), C2+ selectivity (triangles), and C2+
yield (rectangles) over Na2WO4/Mn/SiO2 catalyst at 800 ꢀC and
different CH4/O2 (mol/mol) ratios. The O2 conversions were 100% for
the feed was highly diluted at the low reaction temperature, the
formed methyl radicals may not be able to undergo gas-phase
coupling with other methyl radicals in the highly diluted envi-
ronment, which may reduce the C2+ selectivity. The general
all cases. GHSV ¼ 10 000 hꢂ1. CH4 flow rate was fixed at 18 mL minꢂ1
,
and the flow rates of O2 and N2 were adjusted to achieve the total flow
rate of 50 mL minꢂ1
.
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3.2. Effects of catalyst bed thickness and feed ow rate on
catalysis
Using the xed CH4/O2 ratio and N2 dilution, the effects of
oxygen species on the OCM activity were investigated. The
oxygen species activating methane and other hydrocarbons
change throughout the long catalyst bed at a xed ow rate.
Both O2 gas and surface oxygen species contribute to OCM,
while only the latter are present at the O2-decient bottom part
of the bed. With increasing catalyst bed thickness and the same
ow rates of reactants, along with increasing residence time
and decreasing GHSV, the reaction results did not signicantly
change when the O2 was completely converted (Fig. 4a, c, S4,
Tables S3 and S4†). While incomplete conversion of O2 was
observed at lower GHSV and lower reaction temperature, the
100% O2 conversion produced almost the same CH4 conversion
(32.1–35.9%) along with 50.9–59.8% C2+ selectivity and 16.8–
20.5% C2+ yield. The negligible C2+ production and CH4
conversion in the O2-decient region (when 100% O2 conver-
sion was reached) indicated that the O2 gas was required for
signicant conversion of methane to the C2+ compounds.
Compared to the smaller changes in CH4 conversion and C2+
selectivity, the olen selectivity increased with increasing cata-
lyst bed thickness, indicating that the olens were formed by
the oxidative dehydrogenation of paraffins at the O2-decient
region. Based on these observations, the oxidative dehydroge-
nation of paraffins to olens can be initiated by the surface
lattice oxygen atoms. In contrast, the production of methyl
radicals that contributed to CH4 conversion was less initiated by
these surface lattice oxygen atoms because there was no
signicant increase of CH4 conversion at the O2-decient
region. Therefore, the surface lattice oxygen atoms, which were
formed by diffusion of bulk lattice oxygen atoms to the catalyst
surface, mainly dehydrogenated the paraffins to olens.
Meanwhile, the surface-adsorbed oxygen atoms from the bulk
O2 gas preferentially converted methane to methyl radicals,
which were further coupled to produce paraffins. In addition to
the formation of olens, the production of CO2 also slightly
increased with increasing catalyst bed thickness, indicating that
dehydrogenation using surface lattice oxygen species was
accompanied by deep oxidation to form CO2 as by-product.
The OCM was also studied at a xed catalyst bed thickness
(0.18 mL or 6.4 mm) and adjustable feed ow rates (15–90
mL minꢂ1) to conrm the role of each oxygen species at CH4/O2/
N2 ¼ 3/1/1 mol/mol/mol (Fig. 4b, d, S5, Tables S5 and S6†).
When 100% O2 conversion was achieved, the CH4 conversion
(28.5–35.5%) and C2+ selectivity (56.5–64.2%) were not signi-
cantly adjusted at 800–850 ꢀC, indicating no signicant
dependence of OCM activity on the feed ow rate.
Fig. 3 Ratios of olefins/paraffins and CO/CO2 for the dehydrogena-
tion of paraffins and the oxidation of CO, depending on the content of
N2 diluent at 800 ꢀC.
trend of increasing C2+ selectivity with increasing CH4 conver-
sion, under different reaction conditions for the same catalysts,
has also been observed in our previous studies.46–48
The ratios of CO/CO2 and olens/paraffins increased at lower
dilution by N2 or higher partial pressures of the reactants (Fig. 3,
S3, and Table S2†). Because higher partial pressures of the
reactants are expected to improve the formation of CO2 and
increase the olens/paraffins ratio,44 the improved formation of
CO rather than CO2 we observed must come from another
reaction pathway, which will be discussed later in the Section
3.2. Note that the amount of O2 required for forming CO2 is 1.33
times larger than that for forming CO, and the production of
olens from methane requires more than twice the amount of
O2 required to produce paraffins. Because of the limited supply
of O2 in this study, the deep oxidation to CO and CO2 consumed
more O2 and suppressed the methane activation and the
dehydrogenation, and this lowered the olens/paraffins ratio.
While the dehydrogenation of paraffins to olens occurred at
the O2-decient region using surface lattice oxygen species, as
depicted in Fig. 4 and Table S3,† decreasing the feed ow rate,
which has the same effect as increasing the catalyst bed thick-
ness at a the xed ow rate, did not signicantly affect the
dehydrogenation activity (Table S5†). A turbulent ow in the
catalyst bed may suppress formation of the O2-decient region,
obscuring the effects of surface lattice oxygen species.
Fig. 4 (a and b) Catalytic activities and (c and d) ratios of olefins/
paraffins and CO/CO2 for the dehydrogenation of paraffins and the
oxidation of CO, depending on the catalyst bed thickness (a and c) and
feed flow rate (b and d) at 800 ꢀC.
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Fig. 5 Reactions by oxygen species.
In addition to dehydrogenation, the preferred formation of
CO2 over CO was observed, which indicates deeper oxidation to
CO2 at a lower GHSV, lower feed ow rate, or longer residence
time. Particularly at 850 ꢀC, the CO/CO2 ratio signicantly
decreased at lower feed ow rate or longer residence time (Table
S5†). The improved formation of CO2, which consumed a large
quantity of oxygen species, also reduced the CH4 conversion
because of the lack of oxidizing reagents. The preferred
formation of CO at a shorter residence time can be attributed to
the gas-phase formation of CO from methyl radicals.5,49 CO can
be preferentially produced when a methyl radical reacts with O2
instead of another methyl radical, increasing the CO/CO2 ratio.
The reactions preferred by the adsorbed O2 and the surface
lattice oxygen species are depicted in Fig. 5. Methane molecules
were activated by the surface-adsorbed O2 to form methyl
radicals, which were then coupled to ethane. Ethane molecules
were oxidatively dehydrogenated by the surface lattice oxygen
species to form ethylene. The activation of methane molecules
to methyl radicals by the surface lattice oxygen species was
negligible; however, the dehydrogenation of paraffins and deep
oxidation of hydrocarbons can be preferred by the surface
lattice oxygen species.
Fig. 6 (a) Catalytic activities, (b) ratios of olefins/paraffins and CO/
CO2, and (c) oxygen consumption for the dehydrogenation of ethane
(or propane) and the oxidation of CO, depending on the catalyst bed
thickness at CH3/O2/N2/CO2 ¼ 3/1/0.2/0.8 mol/mol/mol/mol and
800 ꢀC.
3.3. Effects of the catalyst bed thickness on the catalysis with
co-fed CO2
Based on the improved formation of CO2 at lower feed ow
rates, we studied the effects of a mild oxidant such as CO2 on
the OCM selectivity (or the product distributions of paraffins,
olens, CO, and CO2). As one of the products of OCM, CO2 can
act as a diluent or a mild oxidant replacing the diluent N2 and
the oxidant O2.36,38,50 As a mild oxidant, a moderate concentra-
tion of CO2 has been reported to improve the C2+ selectivity and
reduce the CH4 conversion.38,51,52 For example, MgO or Sm2O3-
based catalysts exhibited increased C2+ selectivity with a low
partial pressure of CO2.32,53 Suppressed OCM activity with CO2
has also been reported: Li/MgO was poisoned by CO2,54 and
Na2WO4/Mn/SiO2 was deactivated by concentrated CO2.38
Nevertheless, it may be possible to modify the OCM activity by
using an appropriate amount of co-fed CO2 as mild oxidant to
replace the mixture of N2 and O2. Thus, the OCM reaction was
tested at a xed feed ow rate, variable catalyst bed thickness,
and the 20% N2 in the feed was replaced by 16% CO2 + 4% N2 in
order to investigate this possibility and understand the effects
of the adsorbed oxygen atoms (Fig. 6, S6, Tables S7 and S8†).
When increasing the catalyst bed thickness (or decreasing
the GHSV), CH4 conversion and C2+ selectivity reached 30.8–
34.5% and 51.1–61.9%, respectively at 100% O2 conversion
(Table S5†), which is similar to the results using N2 as diluent
(32.1–35.9% and 50.9–59.8%, respectively, Table S3†).
Compared with the use of N2 diluent, the co-feeding of CO2
slightly lowered CH4 conversion and slightly improved C2+
selectivity by 1.5–4.7%, achieving almost identical C2+ yields.
The improved C2+ selectivity was more distinct when the O2 was
not completely consumed (O2 conversion < 100%), particularly
when GHSV >20 000 hꢂ1 at 750 C.
ꢀ
In terms of the selectivity between CO and CO2, a larger CO/
CO2 ratio was observed when CO2 was co-fed compared to using
the N2 diluent alone (Tables S3 and S7†). The improved
formation of CO with CO2 co-feeding can be attributed to the
catalytic conversion of CO2 to CO. While this conversion can
occur by either the reduction or catalytic dissociation of CO2,
a highly oxidizing environment can suppress the reduction
reaction by hydrogen generated in the dehydrogenation of
paraffin or reduction of water. The catalytic dissociation of CO2
to CO and O may occur to increase the concentration of CO,50,51
and the produced oxygen atoms may improve the C2+ selectivity.
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Fig. 8 Raman spectra of Na2WO4/Mn/SiO2 catalyst: (a) in the fresh
state, after the reaction with N2 diluent at the (b) top and (c) bottom of
the catalyst bed, after the reaction with co-fed CO2 at the (d) top and
(e) bottom of the catalyst bed.
MnWO4 powder (Fig. S9†). While the co-existing Na2WO4 and a-
cristobalite SiO2 are known as key components for good OCM
catalysts,22,23,57 it is not clear whether the formation of MnWO4
reduced the OCM activity or not in this study.
When heated to 800 ꢀC in air, the Na2WO4 and Mn2O3 phases
were found to disappear for the fresh catalyst at room temper-
ature, indicating the melting of these solid materials
(Fig. S8(a)†). Compared to these, MnWO4 survived, while
maintaining its solid structure at the OCM reaction tempera-
ture. a-Cristobalite SiO2 exhibited a peak shi toward smaller 2q
or larger d-spacings, indicating thermal expansion of the crystal
structure.
Fig. 7 XRD results of Na2WO4/Mn/SiO2 spent in the reaction with N2
diluent from the (a) top and (b) bottom of the catalyst bed, and in the
reaction with co-fed CO2 from the (c) top and (d) bottom of the
catalyst bed. The peaks were assigned to a-cristobalite SiO2 (B),
Na2WO4 (A), MnWO4 (P), and Mn2O3 (+).
The actual temperatures of the top, middle, and bottom
layers of the 35.3 mm catalyst bed were measured (Fig. S2†). The
temperature was observed to decrease from the top to the
bottom layer, indicating that the involved reactions occurred at
the entrance of the catalyst bed.
We also summarized all the reaction results obtained in this
study in terms of C2+ yield, depending on CH4 conversion and
O2 conversion (Fig. S7†). Regardless of reaction conditions, CH4
conversion, O2 conversion (for <100%), and C2+ yield increased
proportionally with each other, indicating that the formation of
C2+ compounds was not signicantly adjusted by varying the
reaction conditions, as long as O2 was present. Thus, the effects
of surface lattice oxygen species may not be signicant while O2
is present.
3.5. Study of catalyst surface by Raman spectroscopy and
XPS
Raman spectra of the spent catalysts also conrmed the
formation of MnWO4 in the O2-decient region (bottom layer of
catalyst bed) by the distinct MnWO4 peak at 885 cmꢂ1 for both
CO2-free and CO2-added feeds (Fig. 8). Raman bands indicating
Na2WO4 were observed in the fresh catalyst and spent ones at all
regions of the bed. Notably, the bands at or below 500 cmꢂ1 can
be attributed to a-cristobalite SiO2, and those at 500–700 cmꢂ1
can be attributed to manganese oxides.24 When heated to 800 ꢀC
in air, Mn2O3 and a-cristobalite SiO2 peaks became weaker than
those of the fresh catalyst, indicating that these phases became
weaker or less ordered at high temperatures (Fig. S8(b)†).
In terms of surface electronic structures, the spent catalysts
3.4. Identication of crystal structures
To understand the effects of catalyst structures on the OCM
activity, the spent catalysts were characterized (Fig. 7). There exhibited more intense Na 1s and Mn 2p peaks when only N2
was no signicant difference between catalysts spent using N2 was used as diluent in the reaction (Table S9†). The O 1s peaks
as diluent alone and with CO2 co-feeding. However, when using at 530.9 eV indicating the presence of Na2WO4 were stronger
a longer catalyst bed or smaller GHSV, catalysts on the top and than those with co-fed CO2 (Fig. S10†). The peaks at 532.6 eV are
bottom of the bed exhibited different structures. The formation assigned to SiO2. Compared to the fresh catalyst, the spent
of MnWO4 was distinct at the bottom (O2-decient region) but catalysts aer reaction with N2 as the only diluent or with co-fed
not clearly observed at the top (O2-rich region). While MnWO4 CO2 had much higher surface concentrations of Mn as
had been reported in the literature as either an active or inactive measured by XPS (Table S9†). This growing surface concentra-
component,22,28,47,55,56 we observed moderate OCM activity on tion of Mn can be attributed to the formation of MnWO4, even
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Table 2 OCM results using spent catalyst prepared in the O2-deficient region (bottom of catalyst bed)a,b
X(CH4)
(%)
X(O2)
(%)
S(C2+
(%)
)
Y(C2+
(%)
)
Olens/paraffins
Reaction temperature (ꢀC)
(mol molꢂ1
)
Fresh Na2WO4/Mn/SiO2
750
800
850
35.2
35.3
33.4
100
100
100
54.5
51.5
45.9
19.2
18.2
15.4
2.12
2.49
3.77
Spent Na2WO4/Mn/SiO2 prepared in the O2-decient region
750
800
850
35.0
35.8
34.1
98.2
100
100
59.6
53.4
49.9
20.8
19.1
17.0
2.11
2.68
3.98
a
b
GHSV ¼ 10 000 hꢂ1, CH4/O2/N2 ¼ 3/1/1 mol/mol/mol, total feed ow rate ¼ 30 mL minꢂ1
C2+ selectivity, Y(C2+): C2+ yield.
. X(CH4): CH4 conversion, X(O2): O2 conversion, S(C2+):
though the distinct peaks could not be identied because the
Mn may have protruded to the surface by complexation with W
species.47
Conflicts of interest
There are no conicts to declare.
The spent catalyst from the O2-decient region (which
exhibited the formation of MnWO4) was re-used for the OCM,
and the activity was compared with the fresh one (Table 2). The
spent catalyst exhibited higher C2+ selectivity, higher C2+ yield,
and higher olens/paraffins ratio by 3.7–9.4%, 4.9–10.4%, and
0–7.6%, respectively. Therefore, the spent catalysts containing
MnWO4 produced more C2+ compounds along with a higher
dehydrogenation activity. These observations conrmed that
dehydrogenation occurred preferably in the O2-decient region,
which exhibited the formation of MnWO4.
Acknowledgements
This research was supported by C1 Gas Renery Program
through the National Research Foundation of Korea (NRF)
funded
by
the
Ministry
of
Science
and
ICT
(2015M3D3A1A01064900).
Notes and references
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