Conversion of methane to C2 and C3 hydrocarbons over TiO2/ZSM-5 core-shell particles in an electric field

Article abstract of DOI:10.1039/c9ra06927e

Catalytic conversion of methane (CH₄) to light olefins is motivated by increasing recoverable reserves of methane resources, abundantly available in natural gas, shale gas, and gas hydrates. The development of effective processes for conversion of CH₄ to light olefins is still a great challenge. The interface of ZSM-5 zeolite and TiO₂ nanoparticles is successfully constructed in their core-shell particles via mechanochemical treatment with high shear stress. The oxidative coupling of methane at a low temperature under application of an electric field may be induced by the O₂ activation via electrons running through the surface of TiO₂ located at the interface of TiO₂ and zeolite particles. Moreover, C₃H₆ was also produced by the ethylene to propylene (ETP) reaction catalyzed by Br?nsted acid sites in the ZSM-5 zeolite within core-shell particles.

Full text of DOI:10.1039/c9ra06927e

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Conversion of methane to C2 and C3 hydrocarbons
over TiO2/ZSM-5 core–shell particles in an electric
Cite this: RSC Adv., 2019, 9, 34793
field†
a
Qiao Han,
Atsuhiro Tanaka,a Masayuki Matsumoto,a Akira Endo,b
a
ac
*
Yoshihiro Kubota
and Satoshi Inagaki
Catalytic conversion of methane (CH4) to light olefins is motivated by increasing recoverable reserves of
methane resources, abundantly available in natural gas, shale gas, and gas hydrates. The development of
effective processes for conversion of CH4 to light olefins is still a great challenge. The interface of ZSM-5
zeolite and TiO2 nanoparticles is successfully constructed in their core–shell particles via
mechanochemical treatment with high shear stress. The oxidative coupling of methane at a low
temperature under application of an electric field may be induced by the O2 activation via electrons
running through the surface of TiO2 located at the interface of TiO2 and zeolite particles. Moreover,
C3H6 was also produced by the ethylene to propylene (ETP) reaction catalyzed by Brønsted acid sites in
the ZSM-5 zeolite within core–shell particles.
Received 31st August 2019
Accepted 17th October 2019
DOI: 10.1039/c9ra06927e
rsc.li/rsc-advances
workers.14 For most of the catalꢀysts listed in this review, a high
reaction temperature over 700 C is necessary to realize OCM;
1. Introduction
however, this can promote the over-oxidization of reaction
intermediates to CO and CO2, so that the yield of desirable
products, C2H6 and C2H4, is limited.14
Proven reserves of natural gas have signicantly increased in
the last decade mainly due to the exploitation of unconven-
tional natural gas sources such as shale gas, coal-bed methane
and gas hydrates.1 Methane (CH4), the principal component of
natural gas, has been proposed as an alternative feedstock to
petroleum for the production of high value-added chemicals as
well as an energy resource. Methane can be conventionally
converted through syngas (a mixture of CO and H2) as an
intermediate2–5 to high value-added chemicals such as meth-
anol, light olens, and aromatics; however, this indirect route
consumes a large quantity of external energy to produce
syngas.5 The development of energy-saving processes for the
direct conversion of methane to high value-added chemicals
thus remains a signicant challenge.
The formation of methyl radicals from CH4 has a high acti-
vation energy due to the high H–CH3 bond dissociation energy
(435 kJ molꢁ1).15 On the other hand, a low reaction temperature
is favorable for the selective production of desirable products
(C2H6 and C2H4), due to the suppression of deeper catalytic
oxidation and non-catalytic gas-phase oxidation of reaction
intermediates. To overcome these issues, non-equilibrium
plasma has been proposed as a promising technology to
realize high electron temperatures under low gas-phase
temperatures.16 Non-oxidative coupling of CH4 in non-
equilibrium plasma reaction systems such as corona
discharge16,17 and spark discharge18,19 have been widely investi-
gated. Liu et al.16,17 utilized corona discharge with zeolite cata-
lysts to produce higher hydrocarbons; C2 hydrocarbons (mostly
C2H2), and trace C3+ hydrocarbons were obtained with an input
power of 9 W (input voltage, 7 kV) in this reaction system. Kado
et al.18 reported the non-catalytic direct conversion of CH4 to
C2H2 using corona discharge. This process can also produce
a large amount of C2H2 (49% yield) with a supplied power of
approximately 50 W. However, a great deal of carbon deposition
occurred on the electrodes during reaction. Spark discharge was
also applied for producing C2H2 from CH4.19 High selectivity
toward C2H2 (>85%) and lesser amounts of deposited carbon
were achieved. Furthermore, the energy efficiency with spark
discharge was much higher (12.1 kW h (kg-C2H2)ꢁ1) than that in
dielectric barrier discharge (DBD) or corona discharge.19 Liu
Oxidative coupling of methane (OCM) is an efficient tech-
nology that directly converts CH4 to C2H6 and C2H4. Since Keller
and Bhasin proposed this process in the 1980s,6 Hinsen and
Baerns,7 Ito and Lunsford,8 and many other groups have re-
ported various OCM catalysts.9–13
A database of high-
performance OCM catalysts was reviewed by Zavyalova and co-
aDivision of Materials Science and Chemical Engineering, Yokohama National
University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan. E-mail:
inagaki-satoshi-zr@ynu.ac.jp; Tel: +81-45-339-3691
bNational Institute of Advanced Industrial Science and Technology (AIST), Tsukuba
Central 5, 1-1-1 Higashi, Tsukuba, 305-8565, Japan
cPRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi,
Saitama 332-0012, Japan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra06927e
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et al.20–22 also reported the direct conversion of CH4 and CO2 to zeolite cores and nanosized TiO2 shells form particles. ZSM-5
higher hydrocarbons via DBD with zeolite catalysts. The prod- zeolite (MFI topology) with intersectional 10–10-ring channel
ucts in this reaction system included syngas (CO + H2), light system was selected to form the core of the particles because it
hydrocarbons (C2 and C3), liquid hydrocarbons (C4+), plasma- has been most widely industrialized as a solid-acid catalyst for
polymerized lm, and carbon oxides (CO and CO2).21 The the selective production of light olens.47–51 Moreover, the ZSM-
+
selectivity toward CO and C4 was greater than 80%; however, 5 catalyst has exhibited high yield in the ethylene to propylene
C2H4 and C3H6 yields were extremely low (1.2% and 2.1%, (ETP) reaction.52,53 Therefore, the production of C2 hydrocar-
respectively) with an applied power of 500 W.22
bons as well as the selective production of C3H6 is expected
As an alternative, OCM can be achieved by application of using this type of catalyst with a core–shell structure because of
a direct current (DC) to the semiconductive Sr-doped La2O3 C2H4 produced via OCM in an electric eld can be catalytically
catalyst without excessive electric discharge.23 Sekine and co- converted to C3H6 by the ETP reaction on the Brønsted acid sites
workers recently reported a Ce–W–O catalyst derived from CeO2 of ZSM-5 within the core–shell particles.
modied with Ce2(WO4)3 polyoxometalate, which exhibited high
OCM activity due to a synergetic effect between the Ce2(WO4)3
structure and the electric eld to produce the reactive oxygen
species for the selective oxidation of CH4.24–26 The CePO4 nano-
2. Experimental
2.1. Catalyst preparation
rods with uniform surface Ce sites could work as a durable
TiO2 nanoparticles used in this study were commercially avail-
catalyst and showed the highest C2 yield of 18% in an electric
able from Aldrich. ZSM-5 zeolite (MFI topology, Si/Al ¼ 16, JGC
eld without the need for external heating.27 More recently,
Catalysts and Chemicals Ltd.) was used in this study. Silicalite-
McEwen and co-workers have reviewed some of the theoretical
methods that have been used to elucidate the inuence of
external electric elds on catalytic reactions, as well as the
application of such methods to selective methane activation.28
1, which is a pure-silica version of MFI-type zeolite, was
hydrothermally synthesized using tetrapropylammonium
bromide as a structure-directing agent (see ESI† for the detailed
synthetic procedure).
The zeolite–TiO2 core–shell particles were mechanochemically
prepared by the application of high shear stress using a high-
performance powder processing machine (Nobilta NOB-mini,
Hosokawa Micron Corporation). We have already conrmed
It is well-known that oxygen vacancies existing in the surface
and bulk of TiO2 (ref. 29–31) affect the catalytic activity in both
the photoreactions and non-photoreactions.32,33 The 18O isotopic
exchange reaction on metal oxide catalysts is a convincing
method for revealing the existence of the activated oxygen species
that the crystallinity and microporosity of the single ZSM-5
on the surface of catalyst to progress the OCM reaction. Mir-
zeolite were retained during this kind of mechanochemical
odatos and co-workers34 have investigated the elementary steps
treatment.54 The mechanochemical treatment was typically per-
dealing with the oxygen and methane activation in the OCM over
formed as follows: aer a portion of TiO2 nanoparticles (1.2 g)
lanthanum oxide catalyst using isotope transient experiments
was added to ZSM-5 (6.0 g), mechanochemical treatment was
using 18O2. They suggest that the fast interaction between
performed at 1000 rpm for 5 min, and then at 9000 rpm for
gaseous oxygen and lattice oxygen atoms, involving the formation
10 min. Another portion of TiO2 nanoparticles (1.2 g) was then
of active oxygen species possible to dissociate C–H bonds, could
be considered for most of the performing systems at higher
added to the treated mixture, which was mechanochemically
treated using the same procedure. These procedures were
reaction temperature. More recently, Ogo and co-workers have
repeated three more times until the weight of added TiO2 was
conducted the 18O2 isotope experiments and suggest that mobile
equal to that of ZSM-5. The time-courses of rotational speed and
the resulting output power are shown in Fig. S1.† The obtained
sample is designated as TiO2(mc)/ZSM-5. For comparison, the
core–shell particles of silicalite-1 (MFI topology) and TiO2 were
oxygen over La1ꢁxCaxAlO3ꢁd perovskite catalysts in an electric
eld promotes low-temperature OCM reaction.35
Zeolites are widely applied as a component of catalysts for
the direct conversion of CH4 to CH3OH and for the methane
also prepared according to the same procedure, and the prepared
dehydroaromatization (MDA) reaction due to their ion exchange
samples is designated as TiO2(mc)/silicalite-1. A physical mixture
ability, unique pore structure, and acid-catalytic properties. Cu-
of TiO2 and ZSM-5 with the same weights was prepared and is
designated as TiO2(pm)/ZSM-5. The prepared core–shell parti-
based zeolites36–38 and iron-containing zeolites38–40 have recently
been studied for their potential in the selective oxidation of CH4
cles, the physical mixture, and the simple TiO2 or ZSM-5 were
to CH3OH under oxidative conditions at low temperature.
calcined at 800 ꢀC for 12 h in a muffle furnace (denoted 800 in the
end of sample name; i.e., ZSM-5_800).
Under non-oxidative conditions, Zn,41 Mo,41–44 Mn,45 and Fe45
dispersed on zeolites have exhibited excellent catalytic perfor-
mance for the MDA reaction.
2.2. Characterization of catalysts
In the present study, we tried to promote that the O2 acti-
vation via electrons running through the surface of TiO2 located The prepared samples were measured using power X-ray diffrac-
at the interface of TiO2 and zeolite particles induces the OCM tion (XRD, Ultima-IV, Rigaku) with Cu Ka radiation at 40 kV and 20
reaction at a lower temperature under application of an electric mA to conrm the crystallinity and phase purity. The shape and
eld. Mechanochemical treatment using a powder composer particle size of the samples were observed using eld emission
with high shear stress46 was employed to prepare this charac- scanning electron microscope (FE-SEM, JSM-7001F, JEOL). Cross-
teristic catalyst with a core–shell structure, where microsized sectional observation of the core–shell particles and their EDS line-
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scanning and mapping were performed using another FE-SEM and the other contacted the lower side. The voltage in the
(SU9000, Hitachi High-Technologies) equipped with energy catalyst-bed was applied under constant-current conditions
dispersive X-ray spectroscopy (EDS; Genesis, EDAX), which was using a DC power supply (HAR-30N40, Matsusada Precision
measured at an accelerating voltage of 5.0 kV. Nitrogen adsorption Inc.). The supplied current was controlled from 2.0 to 9.0 mA to
and desorption isotherms were collected at ꢁ196 ꢀC with an make the electric eld stable. Time courses of the input current
Autosorb-iQ analyzer (Quantachrome Instruments). Prior to and the resulting voltage were recorded using an oscilloscope
measurement, each sample was heated in vacuo (less than 10 Pa) (TDS 3052B, Tektronix). Prior to the reaction, the catalyst bed
ꢀ
at 400 ꢀC for 12 h. The specic surface area (SBET) and micropore was heated at 300 C using a furnace for 30 min under Ar ow
volume (Vmicro) were estimated using the Brunauer–Emmett– (60 cm3 (SATP) minꢁ1). Aer cooling the furnace to 150 ꢀC, the
Teller (BET) and t-plot methods, respectively. The chemical reactants (CH4 and O2) and carrier gas (Ar) were fed into the
composition (Si/Al molar ratio) of the zeolites was determined reactor (CH4 : O2 : Ar ¼ 25 : 15 : 60 cm3 (SATP) minꢁ1, W/
using inductively coupled plasma-atomic emission spectroscopy Fmethane ¼ 1.6 g-cat. h molꢁ1). The temperature at the lower side
(ICP-AES; ICPE-9000, Shimadzu). The number of acid sites was of the catalyst-bed was monitored using a K-type sheathed
measured by ammonia temperature-programmed desorption thermocouple. The products aer passing a cold trap were
(NH3-TPD) measurement on a BELCAT-B (MicrotracBEL Corp.) analyzed using a gas chromatograph with a thermal conduc-
with a thermal conductivity detector (TCD). The catalyst was tivity detector (GC-TCD; GC-8A, Shimadzu) and two GC-FIDs
preheated at 600 ꢀC prior to the measurement under He ow. The (GC-2014, Shimadzu). The GC-TCD was equipped with
TPD data were collected at a ramping rate of 10 C minꢁ1. The a molecular sieve 5A packed column (2.0 m length; 3.0 mm i.d.).
ꢀ
number of acid sites was determined from the area of h-peak55–57 One GC-FID was equipped with a Porapak N packed column (4.0
in their proles. Coke deposition on the used catalyst was evalu- m length; 3.0 mm i.d.) and a methanizer (Ru/Al2O3 catalyst),
ated using thermogravimetric analysis (TGA, TG-8120, Rigaku) at while the other had a KCl/Al2O3 capillary column (50 m length;
a ramping rate of 10 ꢀC minꢁ1 from room temperature to 800 ꢀC 0.53 mm i.d.; 10 mm thick).
under air ow (30 cm3 (SATP) minꢁ1).
The conversion of CH4 and O2, product yield, and product
selectivity were determined by the following eqn (1)–(4):
2.3. Activity test on the ethylene-to-propylene (ETP) reaction
CH4 conversion (%) ¼ {(sum of C-atom moles of each product)/
(C-atom moles of input CH4)} ꢂ 100 (1)
The catalyst was pelletized to achieve a size of 355–500 mm. The
catalyst pellets were placed in a quartz tube (9.0 mm i.d.) as
a xed-bed reactor. Prior to the reaction, the catalyst was treated
at 550 ꢀC for 1 h under air ow (30 cm3 (SATP) minꢁ1). Aer the
pretreatment, the gas ow was changed from air to He (30 cm3
O2 conversion (%) ¼ {(moles of O2 consumed)}/
(moles of input O2)} ꢂ 100
(2)
(3)
(4)
(SATP) minꢁ1) followed by cooling to 400 ꢀC or the other specied Product yield (C%) ¼ {(C-atom moles of each product)/
reaction temperature. The ETP reaction was performed at three
(C-atom moles of input CH4)}
ꢀ
different temperatures: 400, 450, and 500 C. Ethylene (Wzeolite
/
Fethylene ¼ 16.4 g-cat. h molꢁ1) was introduced into the top of the
catalyst-bed with He as a carrier gas (30 cm3 (SATP) minꢁ1). The
C2H4 and products were analyzed using an online gas-
chromatograph with ame ionization detector (GC-FID; GC-
2014, Shimadzu) equipped with a DB-5 capillary column (60 m
length; 0.53 mm i.d.; 5.0 mm thick) and an offline GC-FID (GC-
2014, Shimadzu) with a KCl/Al2O3 capillary column (50 m
length; 0.53 mm i.d.; 10 mm thick). The conversion of C2H4, the
yield and distribution of products, and the material balance were
calculated on the carbon-basis of the input amount of C2H4.
Product selectivity (C%) ¼ {(product yield)/
(CH4 conversion)} ꢂ 100
Since reliable material balance is always achieved in the
range from low to high CH4 conversion which is based on the
appearance of carbon-containing products (hydrocarbons and
CO/CO2), the CH4 conversion is estimated by the eqn (1).
When the reaction was conducted without application of
a voltage in the catalyst bed, the catalꢀyst was heated at
prescribed temperatures (from 150 to 900 C) using a furnace
and the same reaction conditions described above.
The 16O2/18O2 isotopic oxygen exchange experiment at 150 ꢀC
in an electric eld was conducted. The details of experimental
procedures were described in ESI.†
2.4. Activity test on OCM reaction in an electric eld
The catalytic performance of each prepared catalyst for the
OCM reaction was tested with a xed-bed continuous-ow
reactor, of which a schematic image is presented in Fig. S2.†
The catalyst was pelletized without any binder, roughly crushed
and then sieved to obtain catalyst pellets with 355–500 mm in
size. The catalyst pellets (100 mg) were charged into a quartz
3. Result and discussion
3.1. Catalyst characterization
tube (4.0 mm i.d.) as a continuous-ow reactor. When the Three kinds of composed catalysts, TiO2(mc)/ZSM-5, TiO2(mc)/
reaction was conducted with application of a voltage in the silicalite-1, and TiO2(pm)/ZSM-5, were prepared in this study.
catalyst bed, two stainless-steel electrodes (2.0 mm i.d.) were Fig. 1 and S3† show XRD patterns of the samples before and
separately set in the reactor without contact with each other; aer thermal treatment at 800 ꢀC. The XRD patterns of as-
one electrode contacted with the upper side of the catalyst bed, received and calcined ZSM-5 showed only the MFI phase
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without any impurities. The as-received TiO2 consisted of two
crystalline polymorphs, anatase (P42/mmm) and rutile (I41/amd),
whereas calcined TiO2 was only the rutile phase. This is a good
consistency with a well-known phase transformation from
anatase to rutile between 600 and 700 ꢀC under atmospheric
pressure.58–60
In the physical mixture of TiO2 and ZSM-5, the peak intensi-
ties corresponding to an MFI structure were reasonably
decreased due to the weight ratio of TiO2 to ZSM-5. In contrast,
mechanochemical treatment led to a decrease in the peak
intensities of both TiO2 polymorphs, although those of MFI were
almost unchanged. This indicates that high shear stress during
mechanochemical treatment may cause partial amorphization of
TiO2 nanoparticles.54 With regard to the phase transformation of
TiO2 during thermal treatment at 800 ꢀC, TiO2 in TiO2(mc)/ZSM-
5_800 was still composed of both anatase and rutile, while that in
TiO2(pm)/ZSM-5_800 was only rutile. There are numerous reports
describing that SiO2 or several metal oxides act as an inhibitor to
the phase transformation into rutile;61–64 in particular, SiO2
species on the surface of TiO2 anatase induce a delay in the
nucleation of TiO2 rutile.60,61 It is thus supposed that strong
adhesion of TiO2 nanoparticles on the surface of ZSM-5 particles
occurs during mechanochemical treatment.
Fig. 2 FE-SEM images of (a) TiO2(pm)/ZSM-5 and (d) TiO2(mc)/ZSM-5.
(b) Low and (c) high magnification views of TiO2(pm)/ZSM-5_800. (e)
Low and (f) high magnification views of TiO2(mc)/ZSM-5_800.
surface of TiO2(mc)/ZSM-5_800 particles (Fig. 2e) was quite
different from those of the parent ZSM-5 (Fig. S4b†); the former
became rough (Fig. 2f) and the corners and edges of each
composed particle became rounded (Fig. 2e), which indicates
that the parent ZSM-5 particles were fully covered with a thin
layer of TiO2 nanoparticles to give a core–shell structure. The
same morphology was also observed in TiO2(mc)/silicalite-
1_800 (Fig. S5c and d†).
Fig. 2 shows FE-SEM images of TiO2(pm)/ZSM-5 and
ꢀ
TiO2(mc)/ZSM-5 before and aer thermal treatment at 800 C.
Typical FE-SEM images of TiO2 nanoparticles and the parent
ZSM-5 zeolite particles are shown in Fig. S4.† The particle size of
parent ZSM-5 was 1.5–2.0 mm (Fig. S4b†). There were no
signicant differences in the morphologies of parent ZSM-5 and
ZSM-5_800 (Fig. S4b and d†). On the other hand, the particle
size of TiO2 was signicantly increased from 25 to 100 nm by
thermal treatment at 800 ꢀC (Fig. S4a and c†). This is consistent
with the sharpening of the XRD peaks that correspond to the
TiO2 rutile polymorph.
To further verify the distribution of TiO2 within the composed
particles, EDS line-scanning and EDS-mapping of core–shell
particle (TiO2(mc)/ZSM-5) cross-section were performed. For line-
scanning (Fig. 3b), the outer layer of the particle, which is the
brightest region in the secondary electron image (Fig. 3a), is
mainly composed of Ti atoms within TiO2, while the core, the
central part of the slender particle, consists of Si and Al atoms,
which corresponds to ZSM-5 aluminosilicates. This indicates that
It was evident that TiO2(pm)/ZSM-5_800 consists of ZSM-5
zeolite particles and aggregates of TiO2 nanoparticles without
good contact with each other (Fig. 2b). The surface of the ZSM-5
particles in TiO2(pm)/ZSM-5_800 was still smooth (Fig. 2c), and
similar to those of ZSM-5_800 (Fig. S4d†). In contrast, the
Fig. 3 SEM, EDS line-scanning and EDS-mapping images of the cross-
section of TiO2(mc)/ZSM-5_800. (a) Secondary electron image, (b)
Fig. 1 Powder XRD patterns of (a) pristine and (b) thermally-treated EDS line-scanning of Si, Al and Ti, and mapping images of (c) Si and (d)
samples at 800 ꢀC.
Ti.
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the prepared particles have a core–shell structure of ZSM-5 distributions in the ETP reaction are shown in Fig. S8† and
covered with a thin TiO2 layer. The EDS-mapping (Fig. 3c and listed in Table S1.† As shown in Fig. S8a,† the parent ZSM-5
d) revealed the core–shell structure more clearly.
showed higher C2H4 conversion (85%) at 400 ꢀC, while C2H4
Table 1 lists the BET surface areas and micropore volumes of conversion decreased with an increase in the reaction temper-
samples estimated from N2 adsorption–desorption isotherms ature due to the formation of a large amount of coke during the
(see also Fig. S6†). Aer the thermal treatment, the BET surface reaction. TiO2(mc)/ZSM-5_800 showed half the conversion
area and micropore volume slightly decreased, probably due to (Fig. S8†) of the parent ZSM-5, although the contact time
dealumination from the MFI framework. This consideration is (Wzeolite/Fethylene, where W is the zeolite weight in the catalysts
supported by the lack of damage in the microporous structure and F is the ow rate of ethylene) was set to be the same.
of silicalite-1, because there were no changes in BET surface Possible reasons for the lower conversion are lowering of C2H4
area and micropore volume during thermal treatment. All the diffusion through the thin TiO2 layer to ZSM-5 and deactivation
composite samples of 50 wt% TiO2 and 50 wt% ZSM-5 exhibited of the external acid sites on ZSM-5 due to partial amorphization
reasonably smaller BET surface areas and micropore volumes; of the surface of ZSM-5 (ref. 54) or covering by TiO2 fragments.
i.e., the BET surface area (197 m2 (g-bulk)ꢁ1) of TiO2(mc)/ZSM-5
TiO2(mc)/silicalite-1_800 showed no C2H4 conversion
was almost half of that (399 m2 (g-bulk)ꢁ1) of the single ZSM-5. (Fig. S8c†), because there are no Brønsted acid sites, (^Al–
During both mechanochemical treatment and subsequent O(H)–Si^), in silicalite-1.
thermal treatment, is revealed from retaining the micropore
volumes corresponding to ZSM-5 zeolites within the core–shell
particles (see Table 1) were unchanged.
3.3. OCM reaction over TiO2(mc)/ZSM-5 catalyst in an electric
eld
It was concluded that the core–shell particles of TiO2(mc)/
ZSM-5 were successfully prepared by mechanochemical treat-
ment with high shear stress, without damage to the crystal-
linity.54 In these core–shell particles, small molecules (i.e., N2)
can diffuse through the voids within the thin TiO2 layer into the
micropores of ZSM-5.
The acidic nature of TiO2(mc)/ZSM-5_800 was evaluated by
NH3-TPD measurements (see Fig. S7†). Based on the h-peak
area in the NH3-TPD prole, the amount of acid sites of
TiO2(mc)/ZSM-5_800 (0.24 mmol (g-zeolite)ꢁ1) was almost the
same as that of ZSM-5_800 (0.25 mmol (g-zeolite)ꢁ1). Moreover,
the peak-top temperature of each h-peak was almost the same.
These results reveal that the acidic strength and amount of
ZSM-5 zeolite within the core–shell particles was unchanged
during mechanochemical treatment.
The TiO2/ZSM-5 composite and the related samples were
exposed to the CH4/O2 gas ow in an electric eld to evaluate
the catalytic properties for the OCM reaction. During the reac-
tion, a certain amount of H2O was detected. This strongly
supports that the OCM reaction successfully occurred in this
catalytic system. In addition, the production of H2 was also
observed. It may be caused by the partial oxidation of CH4 to
give H2 and CO; however, the selectivity to H2 from CH4 was still
low, which was less than 30%. The OCM reaction is thus
pointed out as the main reaction in this catalytic system.
Table 2 summarizes the OCM activities in an electric eld over
four catalysts: TiO2_800, TiO2(pm)/ZSM-5_800, TiO2(mc)/ZSM-
5_800, and TiO2(mc)/silicalite-1_800. The furnace temperature
ꢀ
was set at 150 C; however, during OCM and the over-oxidation
reaction to give CO or CO2, which are the exothermic reactions,
the temperature at the bottom of the catalyst bed was autoge-
nously increased. Although TiO2_800 catalyst made electric eld
3.2. Catalytic performance on ETP reaction
The parent ZSM-5, TiO2(mc)/ZSM-5_800, and TiO2(mc)/ stable during the input current changed from 8.0 to 4.0 mA
silicalite-1_800 were evaluated as catalysts for the ETP reac- (Fig. S9a†), the maximum of CH4 conversion was as low as 1.7%.
tion at various reaction temperatures. The product yields and The selectivity to CO and CO2 reached ca. 94.8% and the yields of
Table 1 Textural properties of samples prepared in this study
Sample
Specic surface areaa [m2 (g-bulk)ꢁ1
]
Micropore volumeb,c [cm3 (g-zeolite)ꢁ1
]
ZSM-5
ZSM-5_800
Silicalite-1
Silicalite-1_800
TiO2(pm)/ZSM-5
TiO2(pm)/ZSM-5_800
TiO2(mc)/ZSM-5
TiO2(mc)/ZSM-5_800
TiO2(mc)/silicalite-1
TiO2(mc)/silicalite-1_800
399
346
419
425
207
189
197
182
197
198
0.17
0.14
0.18
0.18
0.16
0.14
0.15
0.13
0.16
0.16
a
Specic surface area of catalysts were calculated using the Brunauer–Emmett–Teller (BET) equation based on N2 adsorption isotherms. Data in the
relative pressure range of 3–9 ꢂ 10ꢁ3 were employed for the surface area evaluation. b Micropore volumes of the catalysts were calculated by the t-
plot method based on N2 adsorption isotherms. Data in the relative pressure range of 0.75–0.95 were employed for the t-plot analysis. c The ideal
content of zeolite within the mixtures of TiO2 and zeolite was 50 wt%.
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Table 2 Catalytic activities over various catalysts under application of an electric field
Selectivity (C%)
Yield (C%)
Conversion
a
b
Run Catalyst
Input current (mA) Temp.c (ꢀC) of CH4 (%) CO
CO2 C2H6 C2H4 C2H2 C3H6 C2 C2H4 C3H6
A-1
A-2
A-3
TiO2_800
8.0
6.0
4.0
307
293
274
1.7
1.5
0.6
86.0 8.8
88.0 6.9
88.4 6.6
4.3
4.4
5.0
0.9
0.7
0.9
0.0
0.0
0.0
0.0
0.0
0.0
0.1 0.02 0.00
0.1 0.01 0.00
0.0 0.00 0.00
B-1
TiO2(pm)/ZSM-5_800
Operation impossiblec
—
—
—
—
—
—
—
—
—
—
C-1 TiO2(mc)/ZSM-5_800
C-2
C-3
8.0
6.0
4.0
432
416
350
18.4
11.9
2.9
64.2 3.4
61.5 2.8
64.5 3.2
8.5
15.7 6.5
1.7
2.6
0.0
5.6 2.89 0.32
3.9 2.06 0.31
0.9 0.24 0.00
12.3 17.4 3.5
14.5 8.3 9.5
D-1 TiO2(mc)/silicalite-1_800 8.0
435
418
387
8.3
7.9
1.1
56.2 5.7
55.2 5.8
65.6 4.4
10.9 16.4 10.1 0.7
3.1 1.36 0.06
3.0 1.35 0.05
0.3 0.08 0.03
D-2
D-3
6.0
4.0
15.1 17.1 6.0
15.9 7.3 3.8
0.7
3.1
a
Catalyst bed temperature measured using a thermocouple. b C2 yield means the sum of C2H6, C2H4, and C2H2 yields. c The intense discharge in
the catalyst bed occurred intermittently. The conversion of CH4, the selectivity toward products, and the yields of C2 and C3H6 over (A) TiO2_800, (B)
TiO2(pm)/ZSM-5_800, (C) TiO2(mc)/ZSM-5_800, and (D) TiO2(mc)/silicalite-1_800 in the electric eld. Reaction conditions: catalyst, 100 mg; preset
furnace temperature, 150 ꢀC; feed gas, CH4 : O2 : Ar ¼ 25 : 15 : 60 cm3 (SATP) minꢁ1. Pretreatment conditions: furnace temperature, 300 ꢀC; period,
30 min; Ar ow rate, 60 cm3 (SATP) minꢁ1
.
C2H6 and C2H4 were very low (<0.1%). TiO2(pm)/ZSM-5_800 TiO2 nanoparticles, facing to zeolite particles. Therefore, the CH4
caused excessive electric discharge in the catalyst bed, even at conversion is strongly affected by the external surface area of
an input current of 9.0 mA (Fig. S9b†), because ZSM-5 alumino- zeolite particles. Deactivation of the catalyst was not observed
silicates are typical insulators with extremely lower electric during the OCM reaction in electric eld. In the TGA of the used
conductivity; therefore, the catalytic results were not evaluated in catalyst, there was no signicant weight loss in the range from 300
this study. TiO2(mc)/ZSM-5_800 and TiO2(mc)/silicalite-1_800 to 700 ꢀC due to the deposited coke (Fig. S10†). This indicates that
maintained a stable voltage while a current of 9.0 mA was no coke formation occurs during the OCM reaction even in an
applied (Fig. S9c and d†), which indicates that the thin TiO2 layer electric eld. In addition, the crystallinities and morphology of the
within the core–shell particles enables electrons to conduct used catalyst were almost remained unchanged (data not shown).
between the two electrodes set in the catalyst bed. The TiO2(mc)/
We tried to conduct the 18O2 isotope exchange experiment on
ZSM-5_800 catalyst exhibited high CH4 conversion (18.4%) when TiO2_800 and TiO2(mc)/ZSM-5_800 catalyst in an electric eld
the input current was 8.0 mA, and a relatively high selectivity (see Fig. S11†). A certain amount of 18O16O molecules was
toward C2H6 and C2H4 (8.5 and 15.7%, respectively) with lower observed over TiO2(mc)/ZSM-5_800, while TiO2 yielded only
selectivity toward CO and CO2 (64.2 and 3.4%, respectively) was traces of 18O16O. We suppose that O2 close to an oxygen vacancy
achieved. The CH4 conversion over TiO2(mc)/ZSM-5 (1.5–2.0 mm) on the TiO2 surface can be activated by electrons running
was higher than that over TiO2(mc)/silicalite-1, which exhibits through the TiO2 surface, the active oxygen species subse-
a low external surface area corresponding to large particle size (ca. quently dissociates a H–CH3 bond to form a methyl radical,
15 mm). It is supposed that the active sites for the O2 activation yielding C2 hydrocarbons. This observation is just primitive;
and the subsequent CH4 activation are located on the surface of
Table 3 Influence of electric field on the catalytic activity of TiO2(mc)/ZSM-5_800
Selectivity (C%)
Temp.c (ꢀC) of CH4 (%) CO
Yield (C%)
Preset furnace
Input current
Conversion
a
b
Reaction conditions temperature (ꢀC) (mA)
CO2 C2H6 C2H4 C2H2 C3H6 C2
C3H6
Without EF
450
600
750
900
—
—
—
—
463
612
758
901
0.1
0.2
2.4
90.3
93.0
91.0
9.7
7.0
7.7
0.0
0.0
1.1
1.5
0.0
0.0
0.2
2.5
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0 0.00
0.0 0.00
0.0 0.00
0.7 0.01
16.9
84.9 11.0
With EF
150
6.0
417
11.3
58.4
2.7 13.6
15.1
7.8
2.5
4.1 0.28
a
Catalyst bed temperature measured using a thermocouple. b C2 yield means the sum of C2H6, C2H4, and C2H2 yields. Reaction conditions: catalyst,
100 mg; feed gas, CH4 : O2 : Ar ¼ 25 : 15 : 60 cm3 (SATP) minꢁ1. Pretreatment conditions: furnace temperature, 300ꢀC; period, 30min; Ar ow rate, 60
cm3 (SATP) minꢁ1
.
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therefore, the study focusing on the active oxygen species on the
3.5. Possible reaction pathway in electric eld
TiO2 surface in an electric eld will be reported elsewhere.
The inuence of the contact time (W/Fmethane) on the OCM
activity over the TiO2(mc)/ZSM-5_800 catalyst under application
of a constant current (7.0 mA) and lower reaction temperature
(150 ꢀC) was investigated to discuss the possible reaction
pathway in this catalytic system. The dependence of OCM
activity on the contact time is shown in Fig. 4 and S13,† and the
overall results are listed in Table S2.† The OCM reaction was
conrmed to occur as a catalytic reaction over TiO2(mc)/ZSM-
5_800 in this reaction system because there is a proportional
relation between W/Fmethane and each conversion of CH4 or O2,
as shown in Fig. S13.† A possible reaction pathway from C1
3.4. Electric eld effect on the catalytic activity in the OCM
Table 3 and Fig. S12† show the catalytic performance of
TiO2(mc)/ZSM-5_800 without application of a voltage but with
heating using a furnace at 450, 600, 750, and 900 ꢀC. At 450 ꢀC, no
catalytic reactions occurred, whereas at a high temperature of
750 ꢀC, 2.4% of CH4 conversion was achieved, and a higher
reaction temperature gave 16.9%, while high selectivity to CO and
CO2 was unchanged at more than 95%, regardless of the furnace
temperature. The maximum C2 yield (0.7%) with only heating
was much lower than that (4.1%) under application of a voltage at
a constant current of 6 mA in the catalyst bed. These results
indicate that the O2 activation via electrons running through the
surface of TiO2 located at the interface of TiO2 and zeolite
particles under a high voltage, even at lower temperature of
150 ꢀC, induces the OCM reaction without excessive formation of
CO and CO2, compared to the temperatures higher than 750 ꢀC.
We carefully studied the inuence of the input current at the
same reaction temperature (ca. 450 ꢀC) on the OCM activity over
TiO2(mc)/ZSM-5_800 catalyst in an electric eld (see Table 4).
ꢀ
When the preset temperature was xed at 150 C (runs 1–3 in
Table 4), the catalyst-bed temperature increased as the input
current increased. It is due to both exothermic reaction of OCM
and joule heating within solid catalyst (detail described in
Section 3.6). Hence, by tuning the preset temperature, the
catalyst-bed temperature was xed at ca. 450 ꢀC with the
different input current (runs 4–6 in Table 4). When the catalyst
bed temperature was xed, the CH4 conversion linearly
increases as the input current increases. This strongly supports
that the electrons through the surface of TiO2 within the
composite particles induce the O2 activation to promote the
OCM reaction, regardless of the reaction temperature, at least
ca. 450 ꢀC. In these reaction conditions, the selectivity to CO and
CO2 was stable at ca. 70%, as described above. When the OCM
reaction proceeded, C2H6 selectivity decreased while C2H4
selectivity increased; therefore, the possible reaction pathway in
this catalytic system will be discussed as described below.
Fig. 4 Relation between CH4 conversion and C2H6, C2H4, and C2H2
yield over TiO2(mc)/ZSM-5_800 under application of an electric field.
Reaction conditions: catalyst, 100 mg; input current, 7.0 mA; feed gas,
CH4 : O2 : Ar ¼ 5x : 3x : 12x (total flow rate: 50, 75, 100, 150, 200 cm3
(SATP) minꢁ1); preset furnace temperature, 150 ꢀC.
Table 4 Influence of different input current on the catalytic activity of TiO2(mc)/ZSM-5_800
Selectivity (C%)
Preset furnace
temperature
Conversion
of CH4 (%)
Run
Input current
Temp.c
CO
CO2
C2H6
C2H4
C2H2
CO + CO2
C2 yieldb (C%)
a
1
2
3
3.0
5.0
7.0
150
370
397
415
1.3
6.8
10.0
54.9
66.1
63.3
4.4
3.6
2.9
20.8
13.3
13.2
9.4
14.5
16.3
10.6
2.5
2.5
59
70
66
0.5
2.1
3.2
4
5
6
3.0
5.0
7.0
374
339
290
454
449
448
0.9
5.1
10.7
65.3
71.6
65.2
5.3
4.2
3.6
21.2
7.5
11.9
8.2
8.4
15.6
0.0
8.3
3.7
71
76
69
0.3
1.2
3.4
a
Catalyst bed temperature measured using a thermocouple. b C2 yield means the sum of C2H6, C2H4, and C2H2 yields. The selectivity of products,
conversion of methane and the yield of C2 in OCM reaction over TiO2(mc)/ZSM-5_800 with an electric eld. Reaction conditions: catalyst, 100 mg;
feed gas, CH4 : O2 : Ar ¼ 25 : 15 : 60 cm3 (SATP) minꢁ1. Pretreatment conditions: furnace temperature, 300 ꢀC; period, 30 min; Ar ow rate, 60 cm3
(SATP) minꢁ1
.
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radicals into C2 hydrocarbons (C2H6, C2H4 and C2H2) in this existence of CH4 and O2 in an electric eld realizes the OCM
reaction system is much more difficult to speculate. Fig. 4 reaction over this catalyst. In the low range of O2 partial pres-
shows each yield of C2 hydrocarbons in this catalytic system. sure (3 and 5 vol%), lower CH4 conversion (<1%) was obtained,
C2H2 was a minor C2 hydrocarbon product with a proportional whereas the higher O2 partial pressure (10 and 15 vol%) gave
increase dependent on the CH4 conversion. In the range of low higher CH4 conversion. The temperature at the bottom of the
CH4 conversion (from 4 to 8%), the yield of C2H6 was relatively catalyst bed further increased with the increase of O2 partial
higher than that of C2H4. In contrast, the yield of C2H4 jumped pressure, due to OCM and the over-oxidation reactions to give
up beyond that of C2H6 at higher CH4 conversion (from 10 to CO or CO2, which are the exothermic reactions. For instance,
20%). Although it seems that the dehydrogenation of C2H6 into when the O2 partial pressure reached 15%, the temperature at
C2H4 occurs along with the coupling of C1 radicals into C2H6 or the bottom of the catalyst bed increased to 432 ꢀC. The
C2H4, the variation of each C2 yield is even more complicated to temperature rise due to exothermic reaction was from 238 to
ꢀ
determine the reaction pathway from the results obtained here. 432 C, which is in good agreement with thermodynamic esti-
To evaluate the mechanism of C2 hydrocarbon formations mation from the conversion and selectivity.
during OCM in an electric eld, oxidative dehydrogenation of
ethane (ODH) over TiO2(mc)/ZSM-5 catalyst was conducted in
electric eld at 150 ꢀC. Reaction results are shown in Table S3.†
TiO2(mc)/ZSM-5 showed high C2H6 conversion and selectivity of
C2H4. The conversion of C2H6 increased with increasing the
input current, but the C2H2 selectivity was still low even at the
high C2H6 conversion. These results indicate the oxidative
dehydrogenation of C2H6 to C2H4 easily proceeded over
TiO2(mc)/ZSM-5 catalyst in an electric eld. The formation of
C2H2 may occur in other reaction pathways, such as micro-
discharge occurred at the interface between the ZSM-5 core
and the TiO2 shell of the core–shell particles, beside the dehy-
drogenation of C2H4. At least, we have conrmed that parent
ZSM-5 zeolite does not catalyze the reaction from H2 and CO
into hydrocarbons (see Table S4†) that is well known as Fischer–
Tropsch reaction,65,66 regardless of reaction temperature and H2
partial pressure.
3.7. Formation of C3H6 over TiO2/ZSM-5 catalyst
It should be noted that TiO2(mc)/ZSM-5_800 catalyst produced
a meaningful yield of C3H6 when the input current was in the
range of 4.0 to 8.0 mA, probably due to the ETP reaction catalyzed
by the Brønsted acid sites in ZSM-5 within the core–shell parti-
cles. In Table 2, the yield of C3H6 increased from 0 to 0.32% with
an increase in the yield of C2H4 from 0.24% to 2.89%. Such the
limited C3H6 yield may be due to the subsequent oxidation by O2
into CO or CO2. When the ETP reaction with O2 over TiO2(mc)/
ZSM-5 catalyst in an electric eld was also performed
(Fig. S14†), a relatively large amount of C3H6 as well as butenes
and pentenes were detected in the lower input O2 concentration.
The temperature at the bottom of the catalyst bed during the
ꢀ
reaction reached 432 C (when the input current was 8.0 mA),
because the OCM reaction is exothermic. At such temperature,
the ZSM-5 zeolite can catalyze the ETP reaction, as mentioned
above (see also Fig. S8†). The TiO2(mc)/silicalite-1_800 catalyst
only yielded a trace amount of C3H6, in spite of almost the same
CH4 conversion over TiO2(mc)/ZSM-5_800 at 387–435 ꢀC. These
3.6. Inuence of O2 partial pressure on the catalytic activity
in an electric eld
To conrm that active O2 species is necessary for dissociation of results also support that the production of C3H6 in this reaction
C–H bond in CH4, the inuence of the O2 partial pressure on the system is caused by the catalytic ETP reaction on ZSM-5 at the
OCM activity over the TiO2(mc)/ZSM-5_800 catalyst under desirable temperature.
application of a constant current (8.0 mA) and lower reaction
Fig. 5 shows a possible reaction pathway. O2 could be acti-
ꢀ
temperature (150 C) was also investigated. The results of the vated via electrons running through the surface of TiO2 located
catalytic activity of TiO2(mc)/ZSM-5_800 are listed in Table 5. No at the interface of TiO2 and zeolite particles. Such oxygen
catalytic reaction occurred in the absence of O2. In this case, the species could convert CH4 into CH3 radical to generate C2H6
temperature at the bottom of the catalyst bed increased from and C2H4. Moreover, C3H6 was also produced by the ETP reac-
150 to 238 ꢀC by Joule heating. On the other hand, the co- tion catalyzed by Brønsted acid sites in the ZSM-5 zeolite.
Table 5 Influence of O2 partial pressure on the catalytic activity in OCM over TiO2(mc)/ZSM-5_800 in an electric field
Reactant gas
(cm3 (SATP) minꢁ1
)
Selectivity (C%)
Conversion of
CH4 (%)
C2 yieldb (C%)
a
CH4
25
O2
Ar
Temp.c (ꢀC)
CO
CO2
C2H6
C2H4
C2H2
0
75
72
70
65
60
238
340
350
405
432
0.0
0.6
1.0
8.1
18.4
0.0
0.0
3.8
3.1
2.5
3.4
0.0
10.0
10.9
9.9
0.0
3.8
4.6
12.7
15.7
0.0
0.0
0.0
11.8
6.5
0.0
0.1
0.2
2.8
5.6
3
5
10
15
84.2
81.4
60.7
64.2
8.5
a
Catalyst bed temperature measured using a thermocouple. b C2 yield means the sum of C2H6, C2H4, and C2H2 yields. Reaction conditions: catalyst,
100 mg; total ow rate, 100 cm3 (SATP) minꢁ1; input current, 8.0 mA; preset furnace temperature, 150 ꢀC. Pretreatment conditions: furnace
temperature, 300 ꢀC; period, 30 min; Ar ow rate, 60 cm3 (SATP) minꢁ1
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400 ꢀC. In contrast, the CH4 conversion decreased signicantly
to 16.7% when the temperature was increased up to 500 ꢀC and
further decreased slightly from 500 to 700 ꢀC. The O2 activation
and the subsequent CH4 conversion may be related to the
temperature dependence of the TiO2 semiconductivity,67 in that
enhancement of electron conductivity in both TiO2 bulk and
interface between TiO2 nanoparticles may be suppressed by that
on the TiO2 surface facing to ZSM-5 particles at high tempera-
tures (to be discussed in detail elsewhere). The product selec-
tivity toward C2H6 and C2H4 with this catalytic system
continuously decreased with an increase of the furnace
temperature from 150 to 700 ꢀC due to over-oxidation to CO and
CO2. Therefore, at higher reaction temperatures, regardless of
whether a current is applied in the catalyst bed, only those TiO2
nanoparticles that contact each other within the core–shell
particles of TiO2(mc)/ZSM-5 may catalyze the complete oxida-
tion of CH4 into CO and CO2, which is supported by the high
Fig. 5 Illustration of the possible reaction scheme of OCM and ETP
reactions over TiO2(mc)/ZSM-5 composite catalyst in an electric field.
3.8. Temperature dependence of catalytic activity in an
electric eld
ꢀ
selectivity toward CO and CO2 (96.6%) at 900 C with 23.6% of
CH4 conversion over the single TiO2 catalyst.
The catalytic performance of TiO2(mc)/ZSM-5_800 with appli-
cation of a constant current of 7.0 mA with furnace heating from
150 to 700 ꢀC was evaluated, and the results are shown in Fig. 6
and Table S5.† External heating of the catalyst bed affects the
activation of CH4; the conversion of CH4 increased from 13.1 to
22.5% with an increase in the furnace temperature from 150 to
4. Conclusions
Core–shell ZSM-5 zeolite particles covered with a thin shell layer
of TiO2 nanoparticles were successfully prepared via mecha-
nochemical treatment under high shear stress using a high-
performance powder processing machine for application as
a unique catalytic system for OCM and subsequent C3H6
production. A constant current (4.0–8.0 mA) was applied to the
catalyst bed of TiO2(mc)/ZSM-5_800 at a low reaction tempera-
ture of 150 ꢀC, which resulted in high methane conversion
(18.4%) with selectivity toward C2H6 (8.5%) and C2H4 (15.7%),
whereas the selectivity toward CO and CO2 was relatively low at
64.2% and 3.4%, respectively. In this catalytic system, oxidative
coupling of methane at a low temperature under an electric eld
may be induced by O2 activation with electrons moving through
the surface of TiO2 at the interface between TiO2 and zeolite
particles under a high voltage. Furthermore, a good amount of
C3H6 was produced during this reaction due to the ETP reaction
catalyzed by Brønsted acid sites in ZSM-5 within the core–shell
particles. Further improvement of the procedure for preparing
a catalyst with a core–shell structure of semiconductive mate-
rials and zeolites, and a detailed investigation of the reaction
pathways, will be attempted in future work, to progress this
energy-sustainable catalytic system with assistance from elec-
tron semiconduction at lower reaction temperatures.
Conflicts of interest
There are no conicts to declare.
Fig. 6 Temperature dependence over TiO2(mc)/ZSM-5_800 under
application of an electric field. Relation between input furnace
temperature and (a) CH4 conversion and C2 yield, (b) selectivity of CO
and CO2, (c) selectivity of C2H4 and C2H6 over TiO2(mc)/ZSM-5_800.
Trace amounts of propylene selectivity are not shown in graph (c).
Reaction conditions: catalyst, 100 mg; input current, 7.0 mA; feed gas,
Acknowledgements
This work was supported by the JST PRESTO program
(JPMJPR16S2). We thank Prof. Yasushi Sekine, Associate Prof.
Shuhei Ogo, Ms Ayaka Sato, and Ms Yuna Takeno (Waseda
University) for fruitful discussion and technical assistance for
16O2/18O2 isotope oxygen exchange experiment.
CH4 : O2 : Ar
¼
25 : 15 : 60 cm3 (SATP) minꢁ1
; preset furnace
temperature, 150–700 ꢀC.
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¨
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I. Stensgaard and F. Besenbacher, Science, 2003, 299, 377–
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