Catalytic oxidative conversion of methane and ethane over polyoxometalate-derived catalysts in electric field at low temperature

Article abstract of DOI:10.1016/j.cattod.2017.05.013

We examined oxidative coupling of methane (OCM) and oxidative dehydrogenation of ethane (ODE) over various polyoxometalate supported CeO₂ catalysts at 423 K in an electric field. Tetrabutylammonium (TBA) salt of Keggin-type phosphotungstate (PW₁₂O₄₀) supported CeO₂ catalysts ((TBA)₃PW₁₂O₄₀/CeO₂) showed high activities for OCM and ODE in the electric field (3 mA) at 423 K, although the catalyst without the electric field showed extremely low activities at such low temperature. FT-IR spectra and XRD patterns confirmed that the structure of (TBA)₃PW₁₂O₄₀/CeO₂ was changed to Ce₂(WO₄)₃/CeO₂ after reactions with the electric field, and it acted as an active site structure for OCM and ODE with the electric field. Results of activity tests revealed that the C₂H₆ production was a main reaction in OCM with the electric field, and C₂H₄ was formed through a successive oxidative dehydrogenation of the formed C₂H₆. Periodic operation tests demonstrated that reactive oxygen species suitable for ODE were formed on the catalyst surface of Ce₂(WO₄)₃ in the electric field.

Full text of DOI:10.1016/j.cattod.2017.05.013

CatalysisTodayxxx(xxxx)xxx–xxx
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Catalysis Today
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Catalytic oxidative conversion of methane and ethane over
polyoxometalate-derived catalysts in electric field at low temperature
⁎
Shuhei Ogoa,b, , Kousei Iwasakia, Kei Sugiuraa, Ayaka Satoa, Tomohiro Yabea, Yasushi Sekinea
a
Department of Applied Chemistry, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan
PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
We examined oxidative coupling of methane (OCM) and oxidative dehydrogenation of ethane (ODE) over
various polyoxometalate supported CeO2 catalysts at 423 K in an electric field. Tetrabutylammonium (TBA) salt
of Keggin-type phosphotungstate (PW12O40) supported CeO2 catalysts ((TBA)3PW12O40/CeO2) showed high
activities for OCM and ODE in the electric field (3 mA) at 423 K, although the catalyst without the electric field
showed extremely low activities at such low temperature. FT-IR spectra and XRD patterns confirmed that the
structure of (TBA)3PW12O40/CeO2 was changed to Ce2(WO4)3/CeO2 after reactions with the electric field, and it
acted as an active site structure for OCM and ODE with the electric field. Results of activity tests revealed that the
C2H6 production was a main reaction in OCM with the electric field, and C2H4 was formed through a successive
oxidative dehydrogenation of the formed C2H6. Periodic operation tests demonstrated that reactive oxygen
species suitable for ODE were formed on the catalyst surface of Ce2(WO4)3 in the electric field.
Methane oxidative coupling
Ethane oxidative dehydrogenation
Electric field
Polyoxometalate
Cerium tungstate
C–eH bond dissociation
1. Introduction
anticipating methane activation at low temperatures. Results show that
various low-temperature catalytic reactions such as methane steam
Natural gas is being discovered in many countries around the world.
The United States has extracted large amounts of shale gas.
Nevertheless, the state of natural gas, especially methane, is gaseous
at room temperature and atmospheric pressures. Therefore, it is
transported using gas pipelines or LNG systems. Small to medium-sized
natural gas fields have difficulty using such methods. Therefore,
efficient conversion of methane to valuable chemicals and fuels is
necessary for such cases. We specifically focused on direct catalytic
methane conversion to C2 hydrocarbons by oxidative coupling of
methane (OCM) [1–10]. The formula can be described as presented
below (Eq. (1)).
reforming [11–17] can proceed in the electric field. We also reported
that OCM proceeded at a low temperature (423 K) in the electric field
over Sr-La2O3 (Sr/La = 1/20) catalyst [14,17–19] and Ce-W-O system
catalysts [17,20]. Especially, Ce-W-O system catalysts with
a
Ce2(WO4)3 structure, which derived from Keggin-type polyoxometa-
lates (POMs) supported CeO2, showed the high CH4 conversion and C2
selectivity in the electric field at 423 K.
In this study, we specifically examined various polyoxometalate
supported catalysts for catalytic oxidative conversions of methane and
ethane in the electric field at low temperature. The effects of catalyst
composition, imposed current, contact time, and other parameters on
the catalytic activity and selectivity were investigated. An active
structure was characterized using powder X-ray diffraction and FT-IR
spectroscopy.
CH4 + 1/2O2 → 1/2C2H4 + H2O (ΔH°298 = −140.1 kJ mol−1
)
(1)
Because of its stable tetrahedral structure, methane activation
requires temperatures higher than 973 K. Furthermore, the reactivity
of ethylene is higher than that of methane. Consequently, C2 selectivity
decreases because gas-phase non-selective and sequential oxidation
with oxygen to form CO and CO2 is unavoidable at such high
temperatures. Therefore, it is extremely difficult to obtain high C2
yield with OCM.
2. Experimental
2.1. Catalyst preparation
Tetrabutylammonium (TBA) and Cesium salts of Keggin-type poly-
oxometalates (POMs), such as (TBA)3PM12O40 and Cs3PM12O40
(M = Mo, W) (denoted as TBA-POMs and Cs-POMs), were prepared
To resolve the difficulties described above, we adopted a non-
conventional catalytic system, a catalytic reaction in an electric field,
⁎
Corresponding author at: Department of Applied Chemistry, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan.
E-mail address: ogo@aoni.waseda.jp (S. Ogo).
http://dx.doi.org/10.1016/j.cattod.2017.05.013
Received 31 October 2016; Received in revised form 20 March 2017; Accepted 4 May 2017
0920-5861/©2017ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:Ogo,S.,CatalysisToday(2017),http://dx.doi.org/10.1016/j.cattod.2017.05.013

S. Ogo et al.
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C2Yield
according to the published procedure with some modifications [20–24].
They were analyzed using FT-IR spectroscopy (see Supporting Informa-
tion). H3PMo12O40 and H3PW12O40 (denoted as H-POMs) were used as
supplied (Wako Pure Chemical Industry Ltd.). All other chemicals were
reagent-grade; they were used as supplied.
C2Selectivity (%, C-based) =
× 100
CH4Conversion
(5)
2.2.2. Catalytic dehydrogenation of ethane
Catalytic conversions of ethane with or without the electric field in
the presence of oxygen were conducted using the reactor as described
above. The catalyst was sieved into 355–500 μm and 100–200 mg of it
was charged in the reactor. The reactant feed gases were ethane,
oxygen, and argon (C2H6:O2:Ar = 25:15:60, total flow rate 100 SCCM).
The W/FC2H6 was 1.5–3.0 gcat h mol−1. The reactor temperature was set
to 423 K to avoid the condensation of water produced by the reactions,
except for reactions that used no electric field. Product gases after
passing a cold trap were analyzed using the GC-FID and the GC-TCD. In
gaseous products, CO, CO2, CH4, C2H4, and C2H2 were detected
however C3+ hydrocarbons were not detected. The calculation formula
for C2H6 conversion in this study is shown below (Eq. (6)).
Keggin-type TBA-POMs supported on CeO2 (JRC-CEO-1) catalysts
were prepared by impregnation method with acetone as the impregna-
tion solvent [20]. The loading amount of TBA-POMs was 40 wt%. First,
acetone (30 mL) and CeO2 (0.6 g) were added to a 300 mL eggplant
flask and were stirred for 2 h using a rotary evaporator. Subsequently,
TBA-POMs (0.4 g) dissolved into acetone (10 mL) were added to the
flask and were stirred for 2 h again. The resulting suspension was dried
up on a hot plate while stirring. Then the resulting solid was dried
overnight at 393 K.
Keggin-type Cs-POMs or H-POMs supported on CeO2 catalysts were
prepared using a similar method to that for TBA-POMs/CeO2, except
that the impregnation solvent was water.
As
a reference catalyst, Ce2(WO4)3/CeO2 catalyst containing
Carbonmolesof(CO,CO2,CH4,C2H4,andC2H2)
C2H6Conversion(%)=
× 100
(6)
11.9 wt% W was prepared using an impregnation method with water
as the impregnation solvent, as described in previous reports [20,25].
An ammonium metatungstate hydrate ((NH4)6H2W12O40·H2O) was used
as a precursor. After impregnation, the resulting suspension was dried
up on a hot plate while stirring. Then the resulting solid was dried at
393 K overnight, followed by calcination for 3 h in air at 1173 K under
Carbonmolesofinputethane
A periodic operation test was conducted to elucidate surface active
species on the catalyst in the following steps. In the first step, oxygen
and Ar were supplied to the reactor with an electric field for 10 min for
oxidation of the catalyst surface. For the second step, residual oxygen in
the gas phase of the reactor was removed with Ar purge for 5 min. For
the third step, ethane and Ar were supplied to the reactor with an
electric field for 12 min to evaluate the oxidation catalysis of the
surface oxygen species on the catalyst. As the final step, Ar purge was
conducted for 20 min to remove all residual gases. The steps described
above were repeated for four cycles. Product gases were analyzed at
5 min after oxygen+Ar supply, and at 2 min after ethane+Ar supply
(COx and desorbed C2H6 were detected at 5 min after from oxygen+Ar
supply). Gas flow was O2:Ar = 5:50, total flow rate 55 SCCM (for
oxidation of the catalyst surface) and C2H6:Ar = 5:50, total flow rate 55
SCCM (for oxidation of supplied ethane by surface oxygen species). The
reactor temperature was fixed at 473 K. The imposed current was set at
3.0 mA.
a ramping rate of 0.5 K min−1
.
2.2. Activity test
2.2.1. Catalytic oxidative coupling of methane
Catalytic oxidative coupling of methane was conducted with a fixed
bed flow-type reactor equipped with a quartz tube (4.0 mm i.d.). A
schematic image of the reaction system is presented in Supporting
Information Fig. S1. The catalyst was sieved into 355–500 μm. Then
100 mg of it was charged in the reactor. The reactant feed gases were
methane, oxygen, and Ar (CH4:O2:Ar = 25:15:60, total flow rate 100
SCCM). The effect of contact time (W/FCH4) was investigated by
changing the total flow rate. The standard W/FCH4 was
1.5 gcat h mol−1. For the reaction in the electric field, two stainless
steel electrodes (2.0 mm o.d.) were inserted contiguously into the
catalyst-bed in the reactor. And a thermocouple was inserted into the
catalyst-bed to measure the reaction temperature. The electric field was
controlled using a constant current (3, 5, or 7 mA) with a DC power
supply. The imposed voltage depended on the electric properties of the
catalyst. Current and voltage profiles were measured using an oscillo-
scope (TDS 2001C; Tektronix Inc.). The reactor temperature was set to
423 K to avoid the condensation of water produced by the reactions,
except for reactions that used no electric field. Product gases after
passing a cold trap were analyzed using a GC-FID (GC-14B; Shimadzu
Corp.) with a Porapak N packed column and methanizer (Ru/Al2O3
catalyst), and using a GC-TCD (GC-2014; Shimadzu Corp.) with a
molecular sieve 5A packed column. In gaseous products, CO, CO2,
C2H6, C2H4, and C2H2 were detected however C3+ hydrocarbons were
not detected. The respective calculation formulae for conversion, C2
yield, and C2 selectivity in this study are shown below (Eqs. (2)–(5)).
2.3. Characterization
FT–IR spectra were recorded on a spectrometer (FT-IR/6200; Jasco
Corp.) using a KBr pelletizing method. The crystalline structure was
characterized using powder X-ray diffraction (XRD, RINT-Ultima III;
Rigaku Corp.) operating at 40 kV and 40 mA with Cu-Kα radiation.
3. Results and discussion
3.1. Oxidative coupling of methane over POMs/CeO2 in an electric field
First, catalytic oxidative coupling of methane (OCM) over various
polyoxometalates (POMs) supported CeO2 catalysts in the electric field
were conducted at furnace temperature of 423 K. Table 1 shows the
results of the activity tests. Table S1 shows the temperature increase
and the reaction enthalpy with the imposed electric power. CH4
conversion and C2 yield were in following order: TBA-PW12O40
CeO2 > Cs-PW12O40/CeO2 > TBA-PMo12O40/CeO2 > Cs-
PMo12O40/CeO2 > H-PW12O40/CeO2 > H-PMo12O40/CeO2.
/
Carbonmolesof(CO,CO2,C2H6,C2H4,andC2H2)
CH4Conversion(%)=
× 100
(2)
Carbonmolesofinputmethane
PW12O40/CeO2 series showed higher activity and C2 selectivity than
those of PMo12O40/CeO2 series. Counter cation of POMs also affected
on the activity and the selectivity. Among the tested POMs/CeO2
catalysts, TBA-PW12O40/CeO2 catalyst showed the highest CH4 conver-
sion (14.9%) and C2 yield (6.5%) in the electric field (3.0 mA) at
furnace temperature of 423 K. However, as shown in Fig. S2, the
counter cations of PW12O40/CeO2 had the small impact on C2 hydro-
carbon selectivity. Similar to our previous report [20], the OCM activity
ConsumptionmolesofO2
Inputoxygenmoles
O2Conversion(%)=
× 100
(3)
(4)
Carbon moles of (C2H6, C2H4, and C2H2)
Carbon moles of input methane
C2Yield (%, C-based) =
× 100
2

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Table 1
Oxidative coupling of methane over various polyoxometalate supported catalysts in the electric field.a
b
Catalyst
Voltage
/V
Ttc
Conv./%
CH4
Selectivity/C-%
C2 yield
/K
O2
20.6
11.4
14.2
16.2
2.6
C2H6
C2H4
C2H2
CO
49.2
52.3
39.4
61.7
100
58.9
1.1
CO2
/%
(TBA)3PW12O40/CeO2
H3PW12O40/CeO2
Cs3PW12O40/CeO2
(TBA)3PMo12O40/CeO2
H3PMo12O40/CeO2
Cs3PMo12O40/CeO2
CeO2
1300
800
900
1100
400
900
689
624
642
651
540
623
652
14.9
5.8
12.7
12.5
0.1
13.8
13.3
16.1
4.9
18.4
15.3
19.7
9.7
11.2
7.9
10.3
16.2
0.0
7.5
11.1
14.5
7.5
0.0
8.2
6.5
2.1
5.9
3.9
0.0
2.2
0.0
0.0
0.0
6.8
28.2
8.1
100
9.9
0.1
12.6
0.0
10.5
0.0
200
98.7
a
CH4:O2:Ar = 25:15:60, total flow rate 100 SCCM; current, 3.0 mA; catalyst weight, 100 mg; furnace temperature, 423 K.
Catalyst-bed temperature measured using a thermocouple.
b
of POMs/CeO2 was derived from the supported POMs, because bare
CeO2 contributed to complete oxidation of methane to produce CO2
with 98.7% selectivity (O2 conversion was 100%) in the electric field.
Moreover, the TBA-PW12O40/CeO2 catalyst showed stable OCM activity
for at least half an hour (Fig. S3).
Next, to investigate the effect of the imposed current on the catalytic
activity and selectivity, OCM reaction was conducted over TBA-
PW12O40/CeO2 catalyst at 423 K in the electric field with current of
1.5-7.0 mA as summarized in Table 2. The conversions of CH4 and O2
and the C2 yield increased with higher current. Increased in the current
resulted in decreased applied voltage, but the input power (curren-
t × voltage) increased. Therefore, the catalyst-bed temperature in-
creased because of Joule heating with the higher input power (Table
S1). However, TBA-PW12O40/CeO2 catalyst showed the low OCM
activity (CH4 conv., 5.0%; C2 yield, 0.2%) without the electric field,
even at the high temperature of 1073 K, so the effect of Joule heat by
the induced electric field was not important in the system.
Moreover, as shown in Fig. 1, C2 hydrocarbon selectivities were
affected by the imposed current. The C2H6 selectivity was high (23.2%)
at the low imposed current of 1.5 mA, and then it decreased with
increased the imposed current. The C2H4 selectivity increased with
increasing imposed current from 1.5 mA to 3.0 mA, and then the
selectivity decreased with further increasing imposed current. On the
other hand, the C2H2 selectivity increased concomitantly with increas-
ing imposed current.
Furthermore, to clarify the reaction mechanism, the influence of
contact time (W/FCH4) on OCM activity over TBA-PW12O40/CeO2
catalyst at 423 K in the electric field was investigated. As presented
in Supporting Information Table S2, CH4 conversion, O2 conversion,
and C2 yield increased concomitantly with increasing contact time.
Fig. 2 shows the relation between CH4 conversion and C2H6, C2H4, and
C2H2 selectivity over TBA-PW12O40/CeO2 catalyst in the electric field
Fig. 1. Effect of imposed current on conversion and selectivity for the oxidative coupling
of methane over (TBA)3PW12O40/CeO2 catalyst: (+) CH4 conversion; (▲) C2H6
selectivity; (●) C2H4 selectivity; (□) C2H2 selectivity.
(3.0 mA). As shown in Fig. 2, as CH4 conversion approaches 0%, C2H6
selectivity increase and C2H4 and C2H2 selectivity decreases. In the
range of very low CH4 conversion, C2H6 production was the main
reaction in the OCM with the electric field. One can infer that methyl
radical was produced on the catalyst surface in the electric field. On the
other hand, C2H4 selectivity increased concomitantly with increased
CH4 conversion and decreased C2H6 selectivity. These results suggested
that C2H4 was formed through an oxidative dehydrogenation of C2H6.
C2H2 selectivity also increased with increased CH4 conversion and/or
imposed current (Fig. 1). From these results, it was considered that
Table 2
Oxidative coupling of methane over (TBA)3PW12O40/CeO2 catalyst in the electric field with various imposed currents.a
b
Current
/mA
Voltage
Ttc
Conv./%
CH4
Selectivity/C-%
C2H6
C2 yield
/%
/V
/K
O2
C2H4
C2H2
CO
CO2
0.0
–
423
0.0
0.1
0.1
0.4
1.3
1.3
5.0
0.0
0.7
2.1
4.2
3.8
5.9
14.8
0.0
0.0
0.0
0.0
0.0
0.0
3.2
0.0
0.0
0.0
0.0
0.0
0.0
0.3
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.2
573c
673c
773c
873c
973c
1073c
37.6
36.3
40.7
50.8
86.3
84.5
62.4
63.7
59.3
49.2
13.7
12.0
1.5
3.0
5.0
7.0
1700
1300
800
574
689
772
863
5.8
14.5
20.6
40.1
63.3
23.2
13.8
5.8
13.5
18.4
12.4
5.9
7.0
48.8
49.2
53.7
60.6
7.5
7.5
6.4
7.4
2.5
6.5
13.0
16.9
14.9
32.5
52.8
11.2
21.7
23.9
700
2.2
a
CH4:O2:Ar = 25:15:60, total flow rate 100 SCCM; current, 1.5–7.0 mA; catalyst weight, 100 mg; furnace temperature, 423 K (c573–1073 K).
Catalyst-bed temperature measured using a thermocouple.
b
3

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[C2]
[COx] + [C2]
1
C2selelecticity =
=
γt2 + 2
(11)
1
γt2 = k4 [Oact]t
(12)
2
From the Eq. (11), in the kinetic region (γt2 ≪ 2: surface reaction
control), C2 selectivity should be constant value (50%), not depending
on the contact time (t). Moreover, in the kinetic region (γt2 ≪ 2), C2
formation rate (rC2) was represented by the following equation (Eq.
(13), detailed calculation process is shown in Supporting Information).
d[C2]
dt
rC
=
= 2γt = rCO
x
2
(13)
where γt2 ≪ 2
These equations indicated that the C2 formation rate increased in
proportion to the contact time (t), but C2 selectivity become constant
(50%), not depending on the contact time in the kinetic region
(γt2 ≪ 2). The assumption and the proposed mechanism could explain
the experimental results (Table S2). Thus, it was considered that the
reaction mechanism in the OCM reaction in the electric field at low
temperature significantly differed from that in the conventional OCM
reaction [27] without the electric field at high temperature. In this
catalytic system with the electric field, because the OCM proceeded
using the active oxygen species (Oact) on the catalyst surface in the
electric field even at low gas-phase temperature, a non-selective
oxidation and OCM reaction in the gas-phase did not proceed at such
low temperature, and thereby C2 selectivity was high in the OCM
reaction in the electric field at low temperature.
Fig. 2. Relation between CH4 conversion and C2H6, C2H4, C2H2, CO, CO2 selectivity over
(TBA)3PW12O40/CeO2 catalyst in the electric field (423 K, 3.0 mA): CH4/O2 = 1.67; (▲)
C2H6 selectivity; (●) C2H4 selectivity; (□) C2H2 selectivity; (◊) CO selectivity; (♦) CO2
selectivity.
C2H2 was generated through the oxidative dehydrogenation of C2H4 or
through a coupling of CH species, which was formed from CH4 with
electric energy [26]. Although it was previously reported that COx was
formed via sequential oxidation of the C2 hydrocarbons over Ce-W-O
system catalysts in the electric field [20], the CO selectivity (50%)
independent on the contact time (Fig. 2). Therefore, the following
reaction equations were proposed (Eqs. (7)–(10)).
The structure of POMs/CeO2 catalysts before and after OCM
reaction with the electric field were analyzed using FT-IR spectroscopy
(Supporting Information Fig. S4) and XRD (Fig. S5). As shown in Fig.
S4, as-made samples were attributed to the POMs (PW12O40 or
PMo12O40), however the peaks corresponding to POMs disappeared
after OCM reaction. As shown in the XRD patterns of PW12O40/CeO2
series (Fig. S5(a)), new peaks, which were attributable to Ce2(WO4)3,
appeared after OCM reaction with the electric field. In the case of
PMo12O40/CeO2 series (Fig. S5(b)), TBA-PMo12O40 was changed to
Ce2(MoO4)3; H-PMo12O40 was changed to amorphous phase and others;
Cs-PMo12O40 was changed to Ce2(MoO4)3, CsCeMoO8, and Cs2MoO4.
We previously reported that the active site structure of Ce-W-O system
catalysts for OCM in the electric field was Ce2(WO4)3, and OCM
occurred using lattice oxygen of Ce2(WO4)3, which was consumed
and reproduced by redox mechanism [20]. Therefore, in the present
catalytic system, it is likely that the formed Ce2(WO4)3 or Ce2(MoO4)3
contribute to OCM activity.
k
1
Olat + EP → Oact
(7)
(8)
k
2
Oact + CH4 → ·CH3
k
3
2·CH3 → C2
(9)
k
4
Oact + C2 → COx
(10)
Abbreviations in these equations were as follows: Olat, lattice
oxygen of the catalyst; EP, electric power; Oact, active oxygen species
on the catalyst; C2, C2 hydrocarbons. In this proposed mechanism,
assuming that both the ·CH3 radical formation and the sequential
reaction of C2 hydrocarbon to COx proceeded using the active oxygen
species (Oact) over the catalyst surface. Based on the reaction equations
and the assumption, C2 selectivity was represented by the following
equations (Eqs. (11) and (12)). Detailed calculation process is shown in
the Supporting Information.
Table 3
Oxidative dehydrogenation of ethane over various oxide catalysts in the electric field.a
b
Catalyst
Current
/mA
Voltage
/V
Ttc
Conv./%
C2H6
Selectivity/C-%
/K
O2
C2H4
C2H2
CH4
CO
CO2
(TBA)3PW12O40/CeO2
Ce2(WO4)3/CeO2
CeO2
3.0
5.0
7.0
700
600
400
553
603
637
2.2
3.7
8.5
9.4
9.4
13.4
63.9
74.3
73.3
0.0
0.0
0.9
3.7
6.0
7.2
21.0
13.0
12.2
11.4
6.7
6.4
3.0
5.0
7.0
400
300
300
529
552
579
0.9
2.7
3.9
5.2
10.0
14.3
55.5
47.1
49.1
0.0
8.1
4.1
2.1
3.7
2.7
30.7
34.2
35.3
11.7
6.9
8.8
3.0
5.0
7.0
200
100
100
572
579
587
20.1
20.3
20.4
99.0
100
100
22.3
23.2
23.6
0.0
0.0
0.0
0.2
0.3
0.3
5.3
5.3
5.4
72.2
71.2
70.7
a
C2H6:O2:Ar = 25:15:60, total flow rate 100 SCCM; current, 3.0–7.0 mA; catalyst weight, 100 mg; furnace temperature, 423 K.
Catalyst-bed temperature measured using a thermocouple.
b
4

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3.2. Oxidative dehydrogenation of ethane in an electric field
Table 4
Results of periodic operation test (after 2 min from C2H6 supply) over (TBA)3PW12O40
/
CeO2 catalyst in the electric field.a
To evaluate the mechanism of C2 hydrocarbon formations during
OCM in the electric field, catalytic conversions of ethane over TBA-
PW12O40/CeO2, Ce2(WO4)3/CeO2, and CeO2 catalysts were conducted
in the electric field at 423 K. Results of activity tests are shown in
Table 3. In gaseous products, C2H4, C2H2, CH4, CO, and CO2 were
b
Cycle number Voltage Ttc
C2H6 conv.
/%
Selectivity/C-%
C2H4 C2H2 CH4 CO
/-
/V
/K
CO2
1
2
3
4
600
600
600
500
562 0.9
545 0.7
535 2.3
552 0.4
36.0
46.3
57.9
67.5
11.4
8.9
4.0
23.1 12.1 17.4
13.7 12.1 19.0
detected however C3+ hydrocarbons were not detected. TBA-PW12O40
/
CeO2 and Ce2(WO4)3/CeO2 catalysts showed high C2H4 selectivity and
low C1 (CO, CO2, and CH4) selectivities, which indicated that dissocia-
tion of C–H bond of C2H6 was a dominant reaction compared to
dissociation of C–C bond over the catalysts with the electric field. We
suppose that it derives from the bond dissociation energy of C–H and
C–C. Moreover, the C2H6 conversion increased with increasing the
imposed current or the catalyst weight, but the C2H2 selectivity was still
low even at the high C2H6 conversion (Table 3, Table S3, and Fig. S6).
These results indicated that the oxidative dehydrogenation of C2H6 to
C2H4 easily proceeded however a successive dehydrogenation of C2H4
to C2H2 was unfavorable reaction over these catalysts in the electric
field. In addition, the dehydrogenation activity of TBA-PW12O40/CeO2
and Ce2(WO4)3/CeO2 catalysts derived from the supported POMs or
Ce2(WO4)3, because bare CeO2 catalyst showed high C2H6 conversion
(∼20%) and high CO2 selectivity (∼70%).
Next, we evaluated the effects of temperature on the dehydrogena-
tion activity over Ce2(WO4)3/CeO2 catalyst. Fig. 3 and Table S4 show
the results of the catalytic ethane conversions over Ce2(WO4)3/CeO2
catalyst at 473–973 K without the electric field. Although C2H4 was
selectively produced without the electric field at temperature higher
than 773 K, C2H6 conversion was extremely low and no C2H4 was
formed over Ce2(WO4)3/CeO2 catalyst without the electric field in the
low temperature range (< 773 K). Similar results were observed over
TBA-PW12O40/CeO2 catalyst (Fig. S7 and Table S5).
17.8
17.7
8.2 12.1
9.0 5.8
0.0
a
C2H6:Ar = 5:50, total flow rate 55 SCCM; current, 3.0 mA; catalyst weight, 100 mg;
furnace temperature, 473 K.
b
Catalyst-bed temperature measured using a thermocouple.
formed over the catalyst in the electric field without gas-phase oxygen
and that it was maintained during four cycles. These results demon-
strate that reactive oxygen species suitable for ODE were formed on the
catalyst surface in the electric field. The synergetic effect of Ce2(WO4)3
and electric field created the reactive oxygen species suitable for the
selective C–H bond dissociation and activated ethane because the active
structure for ODE with the electric field was Ce2(WO4)3.
From these results, imposing the electric field to the catalyst-bed
enabled selective oxidative dehydrogenation of ethane to ethylene at
low temperature, such as 423 K.
4. Conclusion
Oxidative coupling of methane (OCM) was conducted over various
polyoxometalate supported catalysts in the electric field at low
temperature. The 40wt%TBA-PW12O40/CeO2 catalyst showed high
OCM activity in the electric field at 423 K, although the catalyst
showed low OCM activity, even at the high temperature of 1073 K
without the electric field. After OCM reaction, XRD patterns and FT-IR
spectra confirmed that the structure of TBA-PW12O40/CeO2 catalyst was
changed to Ce2(WO4)3/CeO2. The formed Ce2(WO4)3 species might act
as an active site structure for OCM in the electric field. Results of
activity tests indicated that C2H6 production was the main reaction in
the OCM with the electric field, and C2H4 was formed through a
successive dehydrogenation of the formed C2H6.
The structure of TBA-PW12O40/CeO2 and Ce2(WO4)3/CeO2 catalysts
before and after the ethane conversion with the electric field were
analyzed using the XRD (Fig. S8). As shown in Fig. S8, the structure of
Ce2(WO4)3/CeO2 after the reaction with the electric field was not
markedly different from as-made. However, the structure of TBA-
PW12O40/CeO2 changed to Ce2(WO4)3/CeO2 after the reaction with
the electric field. Similar to the OCM reaction, an active structure for
the oxidative dehydrogenation of ethane (ODE) with the electric field
was Ce2(WO4)3.
To evaluate the mechanism of C2 hydrocarbon formations during
OCM in the electric field, the oxidative dehydrogenation of ethane
(ODE) was also conducted in the electric field. Results revealed that
imposing the electric field to the catalyst-bed enabled selective
oxidative dehydrogenation of ethane to ethylene at low temperature,
such as 423 K. TBA-PW12O40/CeO2 and Ce2(WO4)3/CeO2 catalyst
showed the high C2H4 selectivity and was maintained even at the high
C2H6 conversion. XRD measurements confirmed that an active site
structure for ODE in the electric field was Ce2(WO4)3. Periodic
operation tests demonstrated that reactive oxygen species suitable for
ODE were formed on the catalyst surface in the electric field. Therefore,
in the OCM reaction with the electric field, C2H4 was formed through
the oxidative dehydrogenation of C2H6 over the catalyst surface of
Ce2(WO4)3 in the electric field.
To confirm that reactive oxygen species are formed on the catalyst
surface in the electric field, the periodic operation test at 473 K over
TBA-PW12O40/CeO2 catalyst was conducted with the electric field.
Results are presented Table 4. The results show the activity at 2 min
after from starting ethane+Ar supply. Table 4 shows that C2H4 was
Acknowledgements
This work was supported by JST PRESTO and JST CREST.
Appendix A. Supplementary data
Fig. 3. Oxidative dehydrogenation of ethane over Ce2(WO4)3/CeO2 catalyst with or
without the electric field (3–7 mA): (●) C2H6 conversion with EF; (○) C2H6 conversion
without EF; (■) C2H4 selectivity with EF; (□) C2H4 selectivity without EF.
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.cattod.2017.05.013.
5

S. Ogo et al.
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