Catalysis field in orthorhombic Mo3VOx oxide catalyst for the selective oxidation of ethane, propane and acrolein

Article abstract of DOI:10.1016/j.cattod.2013.12.054

Orthorhombic Mo₃VO_x oxide catalysts in various crystal sizes with different external surface areas and the same micropore volumes were synthesized hydrothermally by adding sodium dodecyl sulphonate (SDS, C₁₂H₂₅SO₃Na) to preparation solution and by changing synthesis temperature. The synthesized catalysts were then tested for the selective oxidations of ethane, propane, and acrolein, in order to clarity catalysis field for the reactions. It was found that ethane converted to ethene in the heptagonal channel in the structure of the catalyst and propane was also oxidized to CO_x in the heptagonal channel. Acrolein, on the other hand, was converted to acrylic acid at the mouth of the heptagonal channel. It is concluded that the heptagonal channel is all responsible for the catalysis for ethane, propane, and acrolein. However, the catalysis field is different in a reflection of the molecular size of the reactants.

Full text of DOI:10.1016/j.cattod.2013.12.054

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Catalysis field in orthorhombic Mo3VOx oxide catalyst for the selective
oxidation of ethane, propane and acrolein
Satoshi Ishikawaa, Xiaodong Yib, Toru Murayamaa, Wataru Uedaa,∗
a Catalysis Research Center, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan
b State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen 361005, Fujian, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Orthorhombic Mo3VOx oxide catalysts in various crystal sizes with different external surface areas and
the same micropore volumes were synthesized hydrothermally by adding sodium dodecyl sulphonate
(SDS, C12H25SO3Na) to preparation solution and by changing synthesis temperature. The synthesized
catalysts were then tested for the selective oxidations of ethane, propane, and acrolein, in order to clarity
catalysis field for the reactions. It was found that ethane converted to ethene in the heptagonal channel in
the structure of the catalyst and propane was also oxidized to COx in the heptagonal channel. Acrolein, on
the other hand, was converted to acrylic acid at the mouth of the heptagonal channel. It is concluded that
the heptagonal channel is all responsible for the catalysis for ethane, propane, and acrolein. However,
the catalysis field is different in a reflection of the molecular size of the reactants.
Received 27 November 2013
Received in revised form
26 December 2013
Accepted 28 December 2013
Available online xxx
Keywords:
Orthorhombic Mo3VOx oxide catalysts
Micropore
© 2014 Elsevier B.V. All rights reserved.
Selective oxidation
Catalysis field
1. Introduction
treatment was conducted before the selective oxidation of propane
[13]. We concluded that the section surface of the rod-shaped crys-
Orthorhombic Mo3VOx is well known as an effective catalyst
for the selective oxidation of light alkanes [1–5], alcohols [6,7], and
acrolein [8]. Fig. 1 shows the structural model and SEM images of
orthorhombic Mo3VOx before and after grinding. The structure is a
layer-type with a slab comprising hexagonal and heptagonal chan-
nels of corner-sharing octahedra and pentagonal {(Mo)Mo5} units
[9]. In a reflection of the complex arrangement in the a–b plane and
the simple stacking in the c axis, crystals were rod-shaped. The rod-
shaped crystals are easily broken by grinding to shorter rod crystals
with keeping the diameter of the rod due to the crystal habit. In this
structure, the heptagonal channel works as a micropore adsorbing
small molecules like CO2, CH4, and C2H6 [10,11] because the hep-
tagonal channel is enough large in diameter to accept these small
molecules. Therefore, the heptagonal channel seems play a crucial
role for the selective oxidation of ethane, propane, and acrolein.
Recently, we have reported that ethane is converted in the
heptagonal channel micropore of orthorhombic Mo3VOx, which
leads the extremely high catalytic activity for the selective oxi-
dation of ethane [12]. In the case of the selective oxidation of
propane, we have reported that catalytic activities of Mo-V-Te-O
and Mo-V-Sb-O oxide catalysts, which assume the same structure
with orthorhombic Mo3VOx, were increased drastically when grind
tals of the catalysts attributed to the a–b plane in the orthorhombic
structure is active for the reaction. Shiju et al. have reported that
Mo-V-Te-Nb-O coated with alumina showed almost no catalytic
activity for the ammoxidation of propane, while the catalytic activ-
ity was increased drastically when the catalyst was ground [14].
The result shows that the a–b plane exposed by the grind treat-
ment is active for the ammoxidation of propane. We have reported
the relationships of the catalytic activity for the selective oxidation
of alkane to the crystal phase of crystalline Mo-V-O oxide catalysts.
We found that the catalysts which have heptagonal channel in the
structure showed the catalytic activity, on the other hand, no cat-
alytic activity was observed in the catalysts without heptagonal
channel in the structure [2,5].
In the case of the selective oxidation of acrolein, there are
many reports which deal with Mo-V-O oxide catalysts [15–17].
The active catalytic material for the selective oxidation of acrolein
was composed by Mo3VO11+y (y = 0–0.2). However, there are few
researches showing what kind of structural phase is active and
how catalysis can take place. Recently, we have reported that
crystalline Mo3VOx catalysts having heptagonal channel in the
structure showed extremely high catalytic activity for the selec-
tive oxidation of acrolein, while the catalysts with no heptagonal
channel showed no catalytic activity [8]. It is now no doubt that
the heptagonal channel of Mo3VOx is responsible for activating
acrolein. However, there are still no reports which investigate
actual catalysis field for the reactions.
∗
Corresponding author. Tel.: +81 11 762 9164; fax: +81 11 762 9163.
E-mail address: ueda@cat.hokudai.ac.jp (W. Ueda).
http://dx.doi.org/10.1016/j.cattod.2013.12.054
0920-5861/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: S. Ishikawa, et al., Catalysis field in orthorhombic Mo3VOx oxide catalyst for the selective oxidation
of ethane, propane and acrolein, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2013.12.054

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Fig. 1. Structural model (Mo, cyan; V, gray; O, red) (left) and SEM images (middle; ungound, right; gournd) of the orthorhombic Mo3VOx oxide catalyst. (For interpretation
of the references to color in this figure legend, the reader is referred to the web version of this article.)
More recently, we have reported that the crystal sizes of the
orthorhombic Mo3VOx oxide catalysts can be controlled by adding
sodium dodecyl sulphonate (SDS, C12H25SO3Na) and by changing
the synthesis temperature under hydrothermal preparative con-
ditions [12]. This method can afford no change of catalytically
affective properties such as crystal structure, micropore volume,
and chemical compositions except the external surface area with
these catalysts. Catalysis fields of orthorhombic Mo3VOx can be
clarified by evaluating the dependency of the catalytic activity upon
the external surface area. Here, we studied the selective oxidation
of ethane, propane, and acrolein over the catalysts in order to reveal
contributions of the external surface and micropore channel for the
reactions.
2.2. Characterization of synthesized materials
Catalysts were characterized by the following techniques. Pow-
der XRD patterns were measured with a diffractometer (RINT
Ultima+, Rigaku) using Cu-K␣ radiation (tube voltage: 40 kV, tube
current: 20 mA). Diffractions were recorded in the range of 4–60◦
with 5◦ min−1. FT-IR spectra were obtained using a spectrom-
eter (Paragon 1000, Perkin Elmer) at room temperature in the
range of 500–2000 cm−1. SEM images were taken using an elec-
tron microscope (JSM-7400F, JEOL). XPS (JPC-9010MC, JEOL) with
a non-monochromatic Mg-K␣ radiation was used for measuring
binding energy values of Mo and V. Binding energy was referred by
Au 4f7/2 (84.0 eV) which was deposited by an auto fine coater (JFC-
1600, JEOL). Elemental compositions in the balk were determined
by ICP-AES (ICPE-9000, Shimadzu). N2 adsorption isotherms at liq.
N2 temperature were measured by using an auto-adsorption sys-
tem (BELSORP MAX, Nippon BELL) for the samples both before and
after the catalytic tests. The samples before the reactions were heat
treated in air at 400 ◦C for 2 h. Prior to N2 adsorption, the catalysts
were evacuated under vacuum at 300 ◦C for 2 h. External surface
area was determined using a t-plot.
2. Experiments
2.1. Catalyst preparation
Catalysts were prepared according to our previous paper
[12]. An aqueous solution of Mo prepared by 8.83 g of
(NH4)6Mo7O24·4H2O (Mo: 50 mmol, Wako) dissolved in 120 mL
of distilled water was mixed with an aqueous solution of VOSO4
prepared by dissolving 3.29 g of hydrated VOSO4 (V: 12.5 mmol,
Mitsuwa Chemicals) in 120 mL of distilled water. The two solutions
were mixed and stirred for 10 min. Then, an appropriate amount of
sodium dodecyl sulphonate (SDS, C12H25SO3Na, Wako) was added
and stirred for another 10 min. The amount of SDS added was x = 0,
0.15, 0.30, and 0.60, where x corresponds to the molar ratio of
SDS/(Mo + V). Then, the obtained mixed solution was introduced
into an autoclave with a 300 mL-Teflon inner vessel and 4000 cm3
of Teflon thin sheet. After the introduction, N2 bubbling was con-
ducted in order to remove residual oxygen. The hydrothermal
reaction was started at 175 ◦C for 48 h. Gray solids formed on the
Teflon sheet was separated by filtration, washed with 1000 mL of
distilled water, and dried at 80 ◦C over night. Obtained solid con-
tained amorphous type of materials as an impurity phase, so that
the dried samples were treated with oxalic acid for purification. To
25 mL aqueous solution (0.4 mol L−1, 60 ◦C) of oxalic acid (Wako),
1 g of the dried material was added and stirred for 30 min, then
washed with 500 mL of distil water. When SDS was added, materi-
als were washed with 500 mL of distil water, followed by washing
with 300 mL of ethanol to remove SDS and drying at 80 ◦C over
night. FT-IR and CHN elemental analysis confirmed that SDS was
completely removed by ethanol washing. Hydrothermal synthesis
was also conducted at 230 ◦C for 20 h using 250 mL-Teflon inner
vessel. In this case, the amount of (NH4)6Mo7O24·4H2O, hydrated
VOSO4, and SDS were set to be half with the same concentration.
The other procedure was the same as described above. The general
abbreviation for the synthesized samples is MoVO-SDSx−y, where
x corresponds to the molar ratio of SDS/(Mo + V) and y corresponds
to the synthetic temperature.
2.3. Catalytic test
Selective oxidation of ethane was performed according to our
previous report [12]. Selective oxidation of propane in gas phase
was carried out at atmospheric pressure in a conventional vertical
flow system with a fixed bed Pyrex tubular reactor. As-synthesized
catalysts were ground with an agate mortar for 5 min, followed
by heat-treatment under N2 atmosphere at 400 ◦C for 2 h with a
fixed bed Pyrex tubular furnace. Then, 0.500 g of the treated cat-
alysts were diluted with 2.300 g of silica and put into the tubular
reactor for propane oxidation. The reactor was heated gradually
from room temperature at a rate of 10 ◦C min−1 to 360 ◦C under
mixed gas flow of nitrogen and helium flow (14.8 mL min−1) from
the top of the reactor. The temperature was measured with a ther-
mocouple inserted in the middle of the catalyst zone. When the
temperature reached 360 ◦C, a reactant gas with the composition of
C3H8/O2/H2O/(N2 + He) = 7.5/10.0/45.5/37.0 (mol%) was fed in with
total flow rate of 40 mL min−1 and started the reaction. Water was
supplied by helium bubbling with 8.1 mL min−1 of flow rate to hot
water at 90 ◦C. Reaction temperature was then decreased to 330,
300, 270, and 250 ◦C. Reactants and products were analyzed with
three online gas chromatographs (Molecular sieve 5A for O2, N2 and
CO with a TCD detector, Gaskuropack54 for CO2, C3H8 and C3H6
with a TCD detector, and Porapak Q for acetone, acetic acid and
acrylic acid with a FID detector). Blank runs showed that under
the experimental conditions used in this study, homogeneous gas-
phase reactions were negligible. Carbon balance was always ca.
95–100%.
Please cite this article in press as: S. Ishikawa, et al., Catalysis field in orthorhombic Mo3VOx oxide catalyst for the selective oxidation
of ethane, propane and acrolein, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2013.12.054

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Table 1
External surface area of MoVO-SDSx−y catalysts.
External surface area/m2 g−1
As synthesis
After catalysis
Ethane
Propane
Acrolein
MoVO-SDS0-175
7.3
–
7.2
6.8
7.2
7.3
14.0
5.3
1.2
3.9
5.5
6.4
8.3
–
6.7
4.0
–
–
19.2
4.5
1.6
–
MoVO-SDS0-175-non
MoVO-SDS0.15-175
MoVO-SDS0.30-175
MoVO-SDS0.60-175
MoVO-SDS0-230
MoVO-SDS0.15-230
MoVO-SDS0.30-230
MoVO-SDS0.60-230
5.9
5.7
15.4
2.9
0.8
2.4
2.8
–
15.1
4.8
0.6
–
–
–
Selective oxidation of acrolein was also carried out at atmo-
spheric pressure in the same conventional vertical flow system with
a stainless tubular reactor. As-synthesized catalysts were ground
with an agate mortar for 5 min, followed by heat-treatment under
N2 atmosphere at 400 ◦C for 2 h with a fixed bed Pyrex tubular
furnace. Then, 0.125 g of the treated catalysts were diluted with
1.200 g of silica and put into the tubular reactor for acrolein oxida-
tion. The reactor was heated at a rate of 10 ◦C min−1 to the desired
reaction temperature under mixed gas flow of nitrogen and helium
(70 mL min−1). When the temperature reached the desired reaction
temperature, 107.5 mL min−1 of reactant gas with the composition
of 2.3/7.4/25.2/65.1 (mol%) was fed in and started the reaction.
Acrolein was supplied by nitrogen bubbling with 20.6 mL min−1
of flow rate to acrolein solution at 0 ◦C. Water was supplied by
helium bubbling with 30.5 mL min−1 of flow rate to hot water at
80 ◦C. Reaction temperature was then decreased. Reactants and
products were analyzed with three on-line gas chromatographs
(Molecular Sieve 13X for O2, N2, and CO with a TCD detector,
Gaskuropack54 for CO2 with a TCD detector, and Porapak Q for
C2H3COH, CH3COOH, C2H3COOH with a FID detector). Blank runs
showed that no reaction took place without catalysts under the
experimental conditions used in this study. Carbon balance was
always ca. 96–100%.
Fig. 2. Side surface area (circle), section surface area (triangle), micropore volume
(square), and aspect ratio (lozenge) of the rod-shaped crystals of the catalysts after
the selective oxidation of ethane. Side surface area and section surface area were
calculated based on the measured external surface area by using the values of the
average diameter and the average length of the rod-shaped crystals obtained by
measuring 100 of the crystals in SEM images. Micropore volume was determined
by the t-plot method. Aspect ratio is the ratio of the average length to the average
diameter of the rod-shaped crystals.
of ethane were in the range of 0.22–1.04 ␮m in the diameter and
0.7–4.8 ␮m in the length, respectively. Aspect ratio is the ratio of
the average length to the average diameter of the crystals. The side
surface area and the section surface area of the rod-shaped crys-
tals were linearly increased with the external surface area, since
the aspect ratio of the rod-shaped crystals was almost the same
with the same rod-shaped crystals of the catalysts. On the other
hand, the micropore volume measured by N2 adsorption at liq-
uid N2 temperature was found almost constant. Micropore volume
measured by ethane adsorption was also almost the same and was
in the range of 16.1–18.8 × 10−3 cm3 g−1 after the ethane selective
oxidation. The molar ratio of V/Mo measured by ICP was in the
range of V/Mo = 0.36–0.39 in the catalysts. No change of the ele-
mental composition was observed after the selective oxidation of
ethane.
We now compare the catalytic performance of these catalysts for
the oxidation of ethane, propane, and acrolein in Fig. 3. As described
in the experimental section, MoVO-SDSx−y represents the cata-
lysts synthesized with x mol% of SDS adding (x = SDS/(Mo + V)) at
3. Results and discussion
The catalysts synthesized with SDS were characterized by XRD,
XPS, ICP, SEM, and N2 adsorption. Irrespective of the addition of
SDS and of the synthesis temperature, properties strongly relat-
ing to catalysis, such as crystal structure, elemental compositions
(bulk and surface), and micropore volume, were almost the same
in the synthesized catalysts. Size of the rod-shaped crystals was
only changed by the addition of SDS and by changing the syn-
thesis temperature, which thus caused change of external surface
area. The external surface area of the catalysts before and after
the selective oxidation of ethane, propane, and acrolein are listed
in Table 1. As can be seen in Table 1, the external surface area
was widely changed by altering the preparative conditions. After
the selective oxidations, the external surface areas of each cata-
lyst were slightly changed. Practically no XRD peaks and pattern
changes were observed after the catalytic tests of the selective
oxidation of ethane, propane, and acrolein, revealing that the cata-
lysts could be kept stable structurally under the catalytic reaction
conditions. Physicochemical properties of the catalysts after the
selective oxidation of ethane are summarized in Fig. 2. The side
surface area and the section surface area were calculated by using
the average diameter and the average length of the rod-shaped
crystals, which were obtained by measuring 100 of rod-shaped
crystals by using SEM photos. The average diameter and the aver-
age length of the rod-shaped crystals after the selective oxidation
y
◦C. The catalysts shown in Fig. 3 were MoVO-SDS0-175, MoVO-
SDS0.60-175, MoVO-SDS0-230, and MoVO-SDS0.15-230, of which
the external surface area were 7.2, 15.4, 2.8, and 0.8 m2 g−1 before
the reactions, respectively. These external surface areas were
slightly changed after the reaction as mentioned before (Table 1),
but a large difference in the external surface area among the cat-
alysts was still remained. Therefore we can evaluate the effects of
the external surface area on the reaction. The conversion of ethane
and the selectivity to ethene and COx are shown in Fig. 3(A) and (Aꢀ).
With increasing the reaction temperature from 250 ◦C to 380 ◦C, the
conversion of ethane increased. The selectivity to ethene decreased
with the reaction temperature. At the same time, the selectivity to
COx increased. Strikingly all the catalysts showed almost the same
conversion of ethane and the same selectivity to ethene and COx at
whole the reaction temperature range. Fig. 4(A) shows the relation-
ship between the external surface area and the conversion of ethane
at 300 ◦C. Irrespective of the increase of the external surface area of
the catalysts, the ethane conversion was ca. 30% for all the catalysts.
This figure clearly shows no dependency of the catalytic activity on
the external surface area of the catalyst. The result suggests that the
external surface area provides no practical impacts to the catalytic
activity. Therefore, catalysis field in the catalyst for the ethane oxi-
dation is considered to be the micropore channel. However, since
Please cite this article in press as: S. Ishikawa, et al., Catalysis field in orthorhombic Mo3VOx oxide catalyst for the selective oxidation
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Fig. 3. Conversion (top) and selectivity (bottom) changes as a function of reaction temperature in the selective oxidation of ethane (A), (Aꢀ), propane (B), (Bꢀ), and acrolein
(C), (Cꢀ). MoVO-SDS0-175 (closed triangle), MoVO-SDS0.60-175 (closed lozenge), MoVO-SDS0-230 (closed square), MoVO-SDS0.15-230 (closed circle), MoVO-SDS0-175-non
(open triangle). Selective oxidation of ethane, selectivity to ethene (solid line) and COx (dot line) are plotted. Selective oxidation of propane, selectivity to acrylic acid (solid
line) and COx (dot line) are plotted. Selective oxidation of acrolein, selectivity to acrylic acid (solid line) and COx (dot line) are plotted.
the section surface area of the catalysts was quite small compared
with the side surface area as shown in Fig. 2, the effects of section
surface for the selective oxidation of ethane are difficult to be eval-
uated. In order to investigate the contributions of section surface
area of the rod-shaped crystals for the reaction, a catalyst (MoVO-
SDS0-175-non) without grinding before catalysis was tested for
the reaction. The external surface area of MoVO-SDS0-175-non
was 6.8 m2 g−1, almost the same with that of MoVO-SDS0-175 as
Fig. 4. Conversion changes as a function of external surface area in the selective oxidation of ethane (A), propane (B), and acrolein (C). Substrates conversion of MoVO-SDS0-
175-non is represented by a closed star in either figure. External surface area changes by the reactions are represents by closed symbols (before reaction) and open symbols
(after reaction).
Please cite this article in press as: S. Ishikawa, et al., Catalysis field in orthorhombic Mo3VOx oxide catalyst for the selective oxidation
of ethane, propane and acrolein, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2013.12.054

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shown in Table 1. No change in the micropore volume measured
by ethane adsorption was observed between MoVO-SDS0-175-non
and MoVO-SDS0-175 (MoVO-SDS0-175-non, 22.3 cm3 g−1; MoVO-
SDS0-175, 17.7 cm3 g−1). Fig. 3(A), (Aꢀ) shows the conversion of
ethane and the selectivity to ethene and COx when MoVO-SDS0-
175-non was used. MoVO-SDS0-175-non (open triangle) showed
the same ethane conversion with the ground catalyst (closed tri-
angle) until 320 ◦C. The selectivity to ethene was slightly low and
the selectivity to COx was slightly high compared with the ground
catalyst at all the reaction temperature. This result implies that the
contribution of the section surface exposed by the grind treatment
for the ethane oxidation was negligible. The slight change of the
selectivity to ethene and COx was due to the deep oxidation of
ethane in the micropore channel, because the long diffusion dis-
tance of the heptagonal channel derived from the long length of the
rod-shaped crystals. MoVO-SDS0-175-non is considered to provide
an active sites in the heptagonal channel more chances for attack-
ing ethane, which leads the over oxidation. As a consequence, it
can be concluded that the micropore provides catalysis field for
the selective oxidation of ethane.
The orthorhombic Mo3VOx oxide catalyst has two micropores,
hexagonal channel and heptagonal channel in the a–b plane. The
size of the hexagonal channel in the a-b plane is calculated to be
0.25 nm and the size of the heptagonal channel is calculated to be
0.40 nm by using the structural model. The size of ethane is 0.40 nm,
so that the hexagonal channel is too small to accept ethane. On the
other hand, the heptagonal channel can adsorb ethane, which was
revealed by the ethane adsorption analysis and the Monte Carlo
simulation [12]. Therefore, the hexagonal channel can contribute
no catalytic activity for the selective oxidation of ethane, supporting
that the heptagonal channel responsible for the catalysis for the
reaction.
the apparent external surface area and rather suggests that the
propane oxidation takes place in the heptagonal channel like as
ethane.
The acrolein conversion and the selectivity to acrylic acid and
COx are shown in Fig. 3(C) and (Cꢀ). The acrolein conversion was
increased with the increase of the reaction temperature. The selec-
tivity to acrylic acid was in the range of 96–98% and was unchanged
by the increase of the reaction temperature. COx selectivity was also
unchanged irrespective of the reaction temperature. The catalysts
with high external surface area showed higher acrolein conversion
than that of the catalysts with low external surface area in all the
reaction temperature. Fig. 4(C) shows the relationship between the
conversion of acrolein at 220 ◦C and the external surface area of the
catalyst. With increasing the external surface area, the acrolein con-
version increased proportionally. The acrolein conversion over the
catalysts clearly depended on the external surface area, implying
that acrolein is converted on the external surface of the cata-
lyst. Then, in order to clarify the contribution of the side surface
and of the section surface of the rod-shaped crystals, a catalyst
without the grind treatment was tested for the selective oxida-
tion of acrolein. Fig. 3(C) shows the acrolein conversion and the
selectivity to acrylic acid and COx when MoVO-SDS0-175-non was
used. The acrolein conversion increased linearly with the increase
of the reaction temperature. The selectivity of the unground cat-
alyst was almost the same with the ground catalyst in all the
reaction temperature. However, the acrolein conversion was far
less compared with the ground catalyst, in spite of almost the
same external surface area. This fact clearly indicates that the sec-
tion surface exposed by the grind treatment is far more active
for the selective oxidation of acrolein than the side surface of
the rod-shaped crystals. As a consequence, acrolein is oxidized to
acrylic acid mainly on the section surface of the rod-shaped crys-
tals.
The propane conversion and the selectivity to acrylic acid and
COx are shown in Fig. 3(B) and (Bꢀ). Propane conversion was
increased with the reaction temperature. The selectivity to acrylic
acid was decreased with the reaction temperature and become
almost 0 at 370 ◦C. Instead, the COx selectivity increased with the
reaction temperature. The selectivity to acrylic acid and COx was
almost the same for all the catalysts. Different from the selec-
tive oxidation of ethane, the catalysts with low external surface
area showed lower propane conversion than that of the catalysts
with high external surface area at all the reaction temperature.
Fig. 4(B) shows the relationship between the external surface
area of the catalysts and the propane conversion at 315 ◦C. The
conversion of propane was increased from ca. 15% to ca. 22%
with increasing the external surface area, indicating an effect of
the external surface area on the conversion of propane. On the
other hand, the conversion of propane over MoVO-SDS0.60-175
and MoVO-SDS0-175 was almost the same, although the exter-
nal surface area of MoVO-SDS0.60-175 was almost twice as high
as that of MoVO-SDS0-175. In order to understand the details of
the catalysis field for the oxidation of propane, propane oxida-
tion by using MoVO-SDS0-175-non was conducted. Fig. 3(B), (Bꢀ)
shows the conversion of propane and the selectivity to acrylic acid
and COx when MoVO-SDS0-175-non was used. The conversion of
propane gradually increased with the reaction temperature. The
selectivity to acrylic acid was decreased with the increase of COx
selectivity, along with the increase of the reaction temperature.
The propane conversion of MoVO-SDS0-175-non was about the half
compared with that of MoVO-SDS0-175 in all the reaction temper-
ature. This result is also illustrated in Fig. 4(B). It is interesting to
note that the catalysts, MoVO-SDS0-175-non and MoVO-SDS0.15-
230, both of which have a very small section surface area but have
largely different external surface area, showed almost the same
propane conversion as can be seen in Fig. 4(B) (star mark). This
fact clearly indicates that the catalytic activity is independent of
Ethane with 0.40 nm in diameter evidently enters the heptago-
nal channel. This situation satisfactory explains the independence
of the conversion of ethane upon the external surface area shown in
Fig. 4(A). Therefore we can convincingly conclude that the conver-
sion of ethane to ethene can proceeds smoothly in the heptagonal
channel. The relationship for propane oxidation in Fig. 4(B) can be
similarly explained, since the molecular size of propane is sim-
ilar to that of the diameter of the heptagonal channel (0.40 nm
in diameter). However, due to more balky size of propane than
that of ethane for entering into the heptagonal channel, propane
should have much difficulty to go through the heptagonal chan-
nel. This kind of diffusion effect may become more prominent
when the diffusion length is long in the case of the long rod-
shaped catalysts with low external surface area. This might be
the reason for the observation that the propane conversion over
catalysts with low external surface area became clearly lowered
while the conversion became independent of the external surface
area when the catalyst is short rod-shaped (Fig. 4(B)). Obviously
we need further experiments for confirming the above discus-
sion.
In the case of the acrolein selective oxidation, it is pronounced
that the section surface of the rod-shaped crystals was exclusively
active. Since only the catalysts possessing the heptagonal channel
are active for the acrolein oxidation [8], the heptagonal structural
part is responsible for the reaction. Therefore we suggest that the
mouth of heptagonal channel on the surface can trap the alde-
hyde group of acrolein effectively and convert to the acid by active
oxygen species around the heptagonal channel, which leads to the
extremely high catalytic activity for the reaction. The heptagonal
channel existing in the section surface has strained bond derived
from the low symmetric shape. This situation may produce an
active oxygen species and is expected to contribute the catalytic
reaction.
Please cite this article in press as: S. Ishikawa, et al., Catalysis field in orthorhombic Mo3VOx oxide catalyst for the selective oxidation
of ethane, propane and acrolein, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2013.12.054

G Model
CATTOD-8853; No. of Pages6
ARTICLE IN PRESS
S. Ishikawa et al. / Catalysis Today xxx (2014) xxx–xxx
References
6
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Selective oxidations of ethane, propane, and acrolein were con-
ducted by using the orthorhombic Mo3VOx oxide catalysts with
different external surface area. The molecular size of ethane is small
enough to enter the heptagonal channel, so that ethane can enter
the heptagonal channel and convert to ethene. Propane is consid-
ered to convert in the heptagonal channel. However, because of
the balky molecular size of propane for entering into the heptago-
nal channel, the heptagonal channel with short length may prefer
to provide the catalysis field to propane. Acrolein is effectively con-
verted to acrylic acid on the section surface of the catalyst. Strained
bonds derived from the low symmetric shape of the heptagonal
channel in the section surface of the rod-shaped crystals are con-
sidered to produce an active oxygen species and to activate acrolein
effectively, resulting in the extremely high catalytic activity for the
reaction. Although the heptagonal channel is obviously responsible
for the catalysis of ethane, propane, and acrolein, the catalysis field
of the channel is found to be different in a reflection of the molec-
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Please cite this article in press as: S. Ishikawa, et al., Catalysis field in orthorhombic Mo3VOx oxide catalyst for the selective oxidation
of ethane, propane and acrolein, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2013.12.054