Super high selectivity of acrolein in oxidation of propene on molybdenum promoted hierarchical assembly of bismuth tungstate nanoflakes

Article abstract of DOI:10.1016/j.apcata.2014.05.021

The catalytic properties of well-defined hierarchically assembled Bi ₂WO₆ nanoflakes with surface dispersed molybdena have been studied with selective oxidation of propene as probe reaction. The adsorption of oxygen causes the decrea

Full text of DOI:10.1016/j.apcata.2014.05.021

Accepted Manuscript
Title: Super high selectivity of acrolein in oxidation of
propene on molybdenum promoted hierarchical assembly of
bismuth tungstate nanoflakes
Author: Yuming Cui Yifen Xia Jianbo Zhao Lei Li Teng Fu
Nianhua Xue Luming Peng Xuefeng Guo Weiping Ding
PII:
S0926-860X(14)00355-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2014.05.021
APCATA 14841
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
12-3-2014
1-5-2014
20-5-2014
Please cite this article as: Y. Cui, Y. Xia, J. Zhao, L. Li, T. Fu, N. Xue, L. Peng, X. Guo,
W. Ding, Super high selectivity of acrolein in oxidation of propene on molybdenum
promoted hierarchical assembly of bismuth tungstate nanoflakes, Applied Catalysis A,
General (2014), http://dx.doi.org/10.1016/j.apcata.2014.05.021
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Super high selectivity of acrolein in oxidation of propene on molybdenum promoted
hierarchical assembly of bismuth tungstate nanoflakes
Yuming Cui, Yifen Xia, Jianbo Zhao, Lei Li, Teng Fu, Nianhua Xue, Luming Peng, Xuefeng
Guo and Weiping Ding*
aKey Lab of Mesoscopic Chemistry, the School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210093, China
ABSTRACT
The catalytic properties of well-defined hierarchically assembled Bi2WO6 nanoflakes with
surface dispersed molybdena have been studied with selective oxidation of propene as probe
reaction. The adsorption of oxygen causes the decrease in the conductivity of Bi2WO6,
demonstrating its nation of n-type semiconductor. The adsorption of oxygen gains electrons
from the conduction band of Bi2WO6 to produce oxygen species active for propene reaction at
low temperatures, which, however, gives mainly COx as products. The modification by
surface molybdena significantly inhibited the total oxidation reaction of propene over the
Bi2WO6, while dramatically increased the conversion of propene and selectivity to acrolein
simultaneously, without the formation of acrylic acid. The characterization of conductivity
measurements, H2-TPR, O2-TPD, XPS and TPSR indicated that the thermally excited
electrons from the Bi2WO6 were combined with surface dispersed molybdena and resulted in
an extraordinary increase of selective oxidation performance. The unique effects of the
hierarchically assembled Bi2WO6 nanoflakes on the catalytic property of molybdena were
discussed and the mechanism of the catalyst for the selectively catalytic oxidation was
proposed. The results are useful to design novel catalysts for important reactions with nano
concepts.
Keywords: selective oxidation; bismuth tungstate; molybdenum oxide; propene; acrolein.
*Corresponding author. E-mail address: dingwp@nju.edu.cn (DWP). Tel: +86 25 83595077;
Fax: +86 25 83317761
Page 1 of 27

1. Introduction
The selective oxidation of propene into useful functionalized chemicals is an extremely
important reaction, since it produces a series of valued chemicals, including acrolein, acrylic
acid, acetone and propene epoxide [1-4]. In 1957, Standard Oil of Ohio (Sohio) discovered
the bismuth molybdate catalyst system, which yielded a fairly good selectivity but a low
propene conversion [5]. It is now clear that with bismuth-molybdenum-based catalysts, heavy
modifications, particularly with Bi2O3 and transition metal oxides, are generally required to
obtain moderate acrolein selectivity [6-7]. The selective oxidation reaction believed to occur
through a classical Mars van Krevelen mechanism. The selectivity depending on the relative
ease of desorption or further oxidation of the partially-oxidized chemisorbed species from
propene [8]. During the past decades, traditional processes to produce acrolein by selective
oxidation of propene are energy-intensive and inefficient since the processes involve multiple
steps and create large quantities of by-products. The main byproducts are acrylic acid and
carbon oxides in addition to minor products such as aldehyde, acetic acid and formaldehyde.
Thus, one challenge of this route is suppressing of by-products and increasing the selectivities
of target products in order to avoid trouble in the polymerization of the impurities.
The conventional catalysts for partial oxidation of propene, especially
bismuth-molybdena-based oxides [9-10], are reported to improve the catalytic activities by
finding the best combination of active components. Many studies concentrated on
multicomponent bismuth molybdate catalysts, however, have failed to take into account the
fact that specific structural interaction effects among the nano domains of a catalytic assembly
significantly promote their catalytic performances. With the progress of materials science, the
research on nanocatalysts has been widely spread, the catalysts with high activities and
selectivities have been reported using modern technology for material synthesizing featured
with the advantages of accurate control of catalytic structure [11-13]. Our previous studies
have reported an important finding that the Bi-Mo-O catalyst with bismuth molybdate
nanocrystals attached to molybdena nanobelts as oxidation catalyst showing high
performance for propene oxidation with only two metallic elements of Bi and Mo [14], due to
the interaction among them causes strong adjustment in structural as well as electronic
properties of bismuth molybdate. Similarly, Bi2WO6, as a typical Aurivillius oxide, is a
Page 2 of 27

unique photocatalytic material for solar-energy-conversion, which is ascribed to the excitation
of Bi2WO6 under visible-light irradiation to generate electron-hole pairs [15-16]. This
property makes it useful under thermal excitation to shape and to enhance the property of the
catalytic species it supported with the adjustment on the surface oxygen species, which are
extremely important for catalytic oxidation reactions [8,17-19].
For this purpose, we have attempted to further develop our recent exploration that MoO3
are immobilized on the surfaces of regular nanocrystals to obtain a model catalyst with clear
structure and highly catalytic performance [17,20]. Briefly, the bismuth tungstate nanoflakes
with loading of molybdena are highly active for propene conversion and especially selective
to the formation of acrolein. Those by-products mentioned above such as acrylic acid could
hardly be detected. The results are useful to design novel catalysts for important reactions
with nano concepts.
2. Experimental
2.1 Catalysts preparation
All chemicals were analytical-grade reagents and were used without further purification.
The hierarchical Bi2WO6 hollow assembled nanoflakes were synthesized by a hydrothermal
method. In a typical synthesis, 0.4548 g of Bi(NO3)3·5H2O and 0.1546 g of Na2WO4·2H2O
were added into 15 mL of Polyethylene Glycol (MW≈200) and 15 mL of deionized water,
respectively. These two solutions were mixed together and stirred for 30 min and then were
added into a 45 mL Teflon-lined autoclave up to 65 % of the total volume. The auto clave was
sealed in a stainless steel tank and heated at 453 K for 24 h. Subsequently, the autoclave was
cooled to room temperature naturally. The products were collected by filtration, washed with
distilled water for several times, and then dried at 353 K in air and calcined in air at 623 K for
5 h.
The molybdenum catalysts were prepared by impregnating the bismuth tungstate with a
appropriate amount of ammonium molybdate ((NH4)6Mo6O24·4H2O) solution for 5 h, then
drying at 353 K, and calcining in air at 623 K for 5 h. Hereafter, the MoO3/Bi2WO6 catalysts
are denoted as xMo/BWO, where x indicated the loading of MoO3 (mass fraction).
As reference catalysts, Mo-doped Bi2WO6 hierarchical microspheres in similar
compositions by a similar method using the mixture of aqueous solution of Bi-, Mo- and W-
salts were synthesized. The catalysts were denoted as BMoxW1-xO in the context.
Page 3 of 27

2.2 Catalytic activity
The reactor was vertical and made of 6 mm. o.d. quartz glass tube. The catalyst was held in
the middle of the tube between two layers of quartz wool. The catalyst was pretreated in the
quartz reactor with air flow (15 mL min-1) at 373 K for 2 h, then the system was cooled down
to the desired reaction temperature under the air flow to remove adsorbed moisture and any
adsorbed impurities. The reaction was started by introducing a reactant gas flow containing
propene and air. In kinetic measurement process, Helium was also used as a balance gas in the
reactant mixture to regulate the partial pressures of propene and air. For the measurement of
kinetics, the conversion of propene was kept between 3 and 15 % to enable calculation of
reaction rates from the results. The temperature around the reactor was maintained by a
furnace connected to a temperature controller. The catalytic performance was tested from 473
to 623 K. The products along with the unreacted propene were analyzed by on-line gas
chromatography. All of the lines and valves between the exit of the reactor and the gas
chromatographs were heated to 423 K, to prevent the condensation of organic products.
Propene conversion was calculated on a carbon basis from the concentrations of the products
detected, including aldehyde, propanal, acetone, acrolein, isopropanol, acrylic acid, CO, CO2
and the residual propene.
2.3 Characterization of the catalysts
XRD measurements were carried out on a Panalytical X’pert Pro Super X-ray
diffractometer with Cu Kα radiation (40 kV and 30 mA). The morphology and composition of
the samples were analyzed by Hitachi S-4800 field-emission scanning electron microscope
(FESEM) with an energy-dispersive X-ray (EDS) spectrometer. Transmission electron
microscopy (TEM), high-resolution TEM (HRTEM), and selected area electron diffraction
(SAED) of the hierarchical superstructures were performed on a JEOL JEM-2010UHR
instrument at an acceleration voltage of 200 kV. N2 physisorption experiments were
performed at 77 K on a Micromeritics Tristar. The samples were outgassed overnight at 423 K
under vacuum (2 Pa). The specific surface area was determined from the BET method in the
0.05~0.30 P/P0 range. The pore size distribution was derived from the desorption branch
using the BJH method. The average pore diameter is calculated as (4×pore volume/BET
Page 4 of 27

specific surface area). Raman spectra were recorded with a Renishaw invia system equipped
with a confocal microscope. A 514.5 nm exciting line was focused using a 50× objective lens.
The laser power at the sample was ~20 mW.
X-ray photoelectron spectroscopy (XPS) was recorded using a PHI 5000 Versa Probe
system, using monochromatic Al Kα radiation (1486.6 eV) operating at an accelerating power
of 15 kW. Before the measurement, the sample was outgassed at room temperature in a UHV
chamber (< 5×10-7 Pa). The sample charging effects were calibrated with binding energy (BE)
of the adventitious C 1s peak at 284.6 eV.
Temperature-programmed desorption of oxygen (O2-TPD) experiments were carried out in
a quartz micro-reactor (i.d. 2 mm) which was connected to an on line multifunctional gas
adsorption instrument PX200 (Tianjin golden eagle technology co., ltd.) equipped with a
thermal conductivity detector (TCD). Typically, 1 g of the catalyst was pretreated at 623 K for
1 h under continuous He flow (30 mL min-1) to remove any adsorbed moisture, then cooled to
323 K and saturated with O2 for 30 min. Subsequently, O2 in physical adsorption and gas
phase was purged in He flow (45 mL min-1) for a certain period at 323 K until a stable
baseline was obtained. O2-TPD experiment was then carried out in the range of 323~673 K at
a heating rate of 10 K min-1.
Temperature-programmed reduction of H2 (H2-TPR) experiments (Tianjin Xianquan
Industry and Trade Development Co., LTD) were carried out in a conventional setup similar
to O2-TPD. Typically, the sample (0.1 g) was pretreated at 623 K for 1 h under Ar flow to
remove water and other contaminants. Then the reduction step was carried out in a mixture
flow of H2/N2 (5 vol. % H2) from room temperature to 1073 K at a heating rate of 10 K min-1.
The H2 signal was monitored on line by a TCD.
2.4 Conductivity Measurements
The conductivities of the BWO and 5Mo/BWO catalysts were determined by measuring
the specific resistance of the sample discs with the two-electrode DC method. The sample to
be examined was compacted into a disc (2 mm thick and 13 mm diameter) containing two
electrodes, each made from conductive silver paste on to which silver leads (0.1 mm diameter)
were spot-welded. The final disc was further heated at 623 K for 5 h to remove impurities in
Page 5 of 27

order to improve the electric conductance. The aim was to detect and measure the relative
rates of reduction and oxidation of the samples, as preliminary experiments showed that
extensive conductivity changes occurred and varied with the reaction condition. Its chief
importance lies in the information it give about the net states of reduction of the catalyst
surface during the early stages of the selective and nonselective oxidation of propene [21].
2.5 C3H6-temperature programmed surface reaction (TPSR)
The C3H6-TPSR experiments were performed on a system equipped with an online
quadrupole mass spectro-meter. The method similar to that of Zhao and co-workers [22],
Typically, ~1.0 g of the 5Mo/BWO catalyst was loaded into a U-type quartz tube and initially
purged at 623 K with Air (30 mL min-1) for 1 h. The pretreated sample was cooled down in
the flowing air to 473 K, then the gas stream was subsequently switched to a high purity He
flow upon further cooling to 323 K maintained for 30 min. Propene adsorption was performed
by flowing a 4.76 % C3H6/He gas mixture (42 mL min-1) for 30 min at 323 K and the sample
was again purged with flowing helium for 30 min to remove residual physically adsorbed
propene. The TPSR experiments were then performed with a heating rate of 10 K min-1 in
either flowing He or 5 vol. % O2/He, and the desorbing products were monitored with the
online quadrupole MS. The m/e values used to detect the different desorption products were
m/e=41 for propene, m/e=56 for acrolein, m/e=43 for acetone, m/e=29 for aldehyde, m/e=44
for CO2. Unlike single crystal studies in vacuum, it is not possible to determine the kinetic
parameters from the FWHM of the TPSR curve since the desorbing products signals become
broadened due to dispersion of the gases as they pass through the thin catalyst bed.
Consequently, only the Tp temperature during C3H6-TPSR can be accurately determined by
use of the redhead equation [23].
3. Results and Discussion
The morphology and structure of the product was examined by a scanning electron
microscope and a transmission electron microscope (HRTEM). Fig. 1a shows a panoramic
FESEM image of the as-synthesized hollow structured Bi2WO6 microspheres.
Low-magnification FESEM images show that the sample consists of a large quantity of
Page 6 of 27

microspheres, indicating that Bi2WO6 microspheres can be prepared on a large scale by this
facile method. The sample mainly contains uniform spheres with diameters ranging in 3~5 μm.
Higher magnification microscopy (Fig. 1c) provides the detailed surface structures of Bi2WO6
microspheres. It can be clearly seen that the exterior surfaces of microspheres are rough and
each microspheres actually were constructed by many crisscrossing nanoflakes with thickness
of about 50 nm. Simultaneously, the small Bi2WO6 nanocrystallines were connected to form the
submicroscopical flake-like structure. (Fig. 1c).
b
a
5 μm
500 nm
c
d
B
50 nm
Bi2WO6
Orthorhombic
f
e
2.74 Å
(200)
5 nm
10
20
30
40
50
60
70
80
2-Theta(deg.)
Fig. 1. (a) and (b) FESEM and TEM images of as-prepared Bi2WO6 hollow microspheres using
polyethylene glycol (PEG 200) as the solvent; (c) Higher magnification microscopy FESEM images of
the Bi2WO6 hollow microspheres; (d) and (e) Higher magnification microscopy TEM images of the
Bi2WO6 nanoflakes. Inset of (d) shows the corresponding SAED image; (f) XRD pattern of the Bi2WO6
sample.
A typical TEM image of the sample shows that the microspheres have a dimension of about
Page 7 of 27

3 μm, and the relatively pale center observed on the TEM images of a single microsphere
further confirms that the as-prepared Bi2WO6 microspheres have a hollow center (Fig. 1b).
The shell thickness of the spheres is about 200 nm and the corresponding diffraction patterns
of the selected area electron diffraction (SAED) indicate that Bi2WO6 nanoflakes are single
crystalline (Fig. 1d). From the high resolution transmission electron microscopy (HRTEM)
image (Fig. 1e), the spacing between the adjacent fringes was measured to be 2.74 Ǻ, which
matches well with the (200) plane spacing of orthorhombic Bi2WO6. The phase structure is
also characterized by X-ray powder diffraction (XRD, Fig. 1f) and the diffraction peaks of the
Bi2WO6 can be clearly indexed to the well-crystallized orthorhombic phase of Bi2WO6 with
lattice constants a=5.456, b=5.436, and c=16.426 Å (JPCDS 73-1126). Moreover, the strong
and sharp diffraction peaks in the pattern show that the as-synthesized product is highly
crystalline, which is in accordance with HRTEM results. These results indicate that
as-prepared hollow Bi2WO6 microspheres were composed of Bi2WO6 nanoflakes with the
orthorhombic structure. It is speculated that the small Bi2WO6 nanocrystallines, connected
each other to form the nanoflakes, mostly provide the (100) and (010) facets as the exposed
surfaces in the view of crystalline model of the orthorhombic Bi2WO6 structure [24]. The
morphologies of the reference samples (not shown) are similar to the Bi2WO6.
In our previous study [25-27], the weak coordination among metallic species and PEG
chains had been proved to aggregate Mn+-PEG into globules. The metallic species wrapped in
the globules will preferentially hydrolyze to corresponding solid metallic compound nuclei, so
the globules act as the soft templates for the formation of Bi2WO6 microspheres. Studies have
clarified that Ostwald ripening will occur during the hydrothermal process. Typically, the
crystallites located in the central part of the spherical aggregates are smaller or less dense
compared to those in the outer parts, and thus they will be dissolved and re-crystallized on the
outer parts, resulting in hollow spheres [25,28-29]. Eventually, the core was dissolved and the
shell gets rougher and more crystallized, and the solid primary spherical particles on the
microsphere shell transfer into well-faceted Bi2WO6 particles with the increase of the reaction
time in synthesis.
Page 8 of 27

(B)
5Mo/BWO-used
7Mo/BWO
5Mo/BWO
3Mo/BWO
1Mo/BWO
Bi2WO6
10 20 30
60 70 80
2-Th4e0ta(5d0eg.)
7Mo/BWO
5Mo/BWO
7Mo/BWO
3Mo/BWO
1Mo/BWO
5Mo/BWO
3Mo/BWO
Bi2WO6
1Mo/BWO
Bi2WO6
200 400 600 800 1000 1200 1400
1350 1200 1050 900 750 600
-1
-1)
Wavenumber(cm
)
Raman shift ( cm
Fig. 2. (a) FESEM images of 5Mo/BWO sample (scale bar is 1 μm). (b) XRD patterns of the Bi2WO6
and xMo/BWO samples with different MoO3 contents. (c) and (d) Raman spectra and FTIR spectra of
the catalysts with different MoO3 loadings, respectively.
Fig. 2a shows the FESEM image of Bi2WO6 sample supported with 5 wt % molybdena.
Compared with pure Bi2WO6 sample (Fig. 1a), both samples exhibit flake-like morphology,
indicating that molybdena loading has no obvious influence on the morphology of Bi2WO6.
The XRD diffraction patterns of the samples are shown in Fig. 2b. All of the diffraction peaks
match the standard data for an orthorhombic Bi2WO6 structure and no crystalline MoO3 could
be detected in the XRD spectra, suggesting that only thin layers or very small crystals of
molybdena have been formed on the support. Moreover, neither possible impurities nor
changes of the lattice parameters of Bi2WO6 in all samples are found, which implies that the
loading of MoO3 does not result in the development of new phases. Raman and FTIR
techniques are also applied to detect the state changes of the catalysts upon MoO3 loading. In
Raman spectroscopy (Fig. 2c), the sample Bi2WO6 calcined at 623 K could be regarded as an
appropriate standard sample for high-purity single-phase crystalline Bi2WO6. The Raman
bands at the 996 and 291 cm-1 are attributed to the symmetric stretching and the asymmetric
bending modes of Mo=O bond, respectively. The bands at 821 cm-1 are assigned to the
Page 9 of 27

Mo-O-Mo symmetric stretch [30]. While, the intensity of the peak at 796 cm-1 in the Raman
spectra, assigned to one of the W-O stretches in the Bi2WO6 lattice, decreased with an relative
intensity of the increasing the MoO3 loading [31], as well as the peak at 309 cm-1. These
Raman band changes suggest the presence of tetrahedral coordinated Mo species on the
Bi2WO6 surface. Fig. 2d shows typical IR spectra of all samples. The high absorption in the
800~650 cm−1 spectral range can be related to the vibrations of W-O-W bridges between WO6
octahedra. The shoulder at 827 cm−1 is assigned to asymmetric stretching vibration modes of
apical oxygen atoms connecting the WO6 octahedra [32]. As it is seen, the characteristic
vibrations of octahedral WO6 and tetrahedral MoO4 units are in the same spectral region,
which makes difficulty to the precise assignment of the absorption bands [33]. According to
the Raman spectra data, it could be deduced that the molybdenum oxide dispersed on Bi2WO6
surface mainly as high dispersed or polymerized molybdenum oxide species. The dispersed
nature of molybdena species on supports is consistent with the absence of crystalline
structures in their X-ray diffraction patterns (Fig. 2b). X-ray photoelectron spectroscopy (XPS)
is applied to detect the chemical states of Mo species and the surface composition of the
catalysts and the results are listed in Table 1. Considering the Mo content in the 5Mo/BWO,
the results imply that the molybdenum is mainly located on the surface of the catalyst. Two
distinct peaks of binding energy corresponding to the Mo 3d5/2 and 3d3/2 (not shown),
respectively at 232.5 and 235.7 eV, which are lower than Mo in bulk MoO3 but similar to
those of the molybdenum in bismuth molybdate. It appears the molybdenum is highly
dispersed as surface bismuth molybdate-like species on the Bi2WO6, considering the results of
Raman and XRD results listed in Fig. 2.
Table 1. The surface composition of the Bi2WO6 and 5Mo/BWO catalysts.a
Samples
Bi2WO6
C
O
Mo
-
W
Bi
29.28
20.83
51.76
53.97
5.66
5.75
13.31
13.86
5Mo/BWO
5.58
a Determined by XPS analysis.
Page 10 of 27

Table 2. The specific surface area of the xMo/BWO and BMoxW1-xO catalysts.
Catalysts
MoO3 content BET surface Pore volume Surface density of MoO3
(wt %)a
area (m2 g−1) (cm3 g−1)
on support (atoms/nm2)
Bi2WO6
0
14.3
14.0
13.4
13.3
11.8
17.5
17.1
16.8
14.5
0.069
0.089
0.064
0.076
0.077
0.063
0.068
0.071
0.076
0
1Mo/BWO
3Mo/BWO
5Mo/BWO
7Mo/BWO
BMo0.01W0.99
BMo0.03W0.97
BMo0.05W0.95
BMo0.07W0.93
0.85
2.54
4.34
5.32
0.85
2.86
4.26
6.07
0.263
0.823
1.410
1.956
--
O
O
O
O
--
--
--
a Determined by XRF analysis.
The BET surface areas derived from N2-physisorption and the loading of MoO3 evaluated
from the XRF analysis are listed in Table 2. According to XRF results, the Mo contents in the
four Mo-based catalysts were slightly less than the amount of MoO3 added, just due to the
loss in the synthetic process. As shown in Fig.1, the exterior surfaces of hollow microspheres
are rough and each of microspheres actually is constructed by many crisscrossing nanoflakes
with thickness of about 50 nm. Recently, Ryoo et al. have also reported a simple method to
synthesize zeolites with crisscrossing nanoflakes structures [34]. Meanwhile, the large
number of acid sites on the external surface of the zeolites nanoflakes renders them highly
active for the catalytic conversion of large organic molecules, and the reduced crystal
thickness facilitates diffusion and thereby dramatically suppresses catalyst deactivation
through coke deposition during methanol-to-gasoline conversion. In current investigation, the
catalytic properties of Bi2WO6 nanoflakes supported with or without molybdena are tested
with selective oxidation propene by air.
The catalytic properties of the samples for the selective oxidation of propene are
carefully investigated and the results are shown in Fig. 3. As shown in Fig. 3a, with the
Page 11 of 27

increase in loading of MoO3 from 0 to 5 wt %, the catalytic properties of the catalysts are
significantly promoted. The selectivity to acrolein increases surprisingly from less than 5% to
~96% and the CO2 selectivity decreases to ~3%. Meanwhile, the production of aldehyde,
propanal, acetone and isopropanol are greatly diminished with the MoO3 amount increase. A
further increase in MoO3 loading has no obvious effect on catalytic property. The surface
molybdena plays pivotal roles in enhancing acrolein selectivity. Fig. 3b shows that both
propene conversion and acrolein selectivity over the 5Mo/BWO catalyst did not undergo
significant changes with time on stream. The propene conversion is ~62.9 % and is stable
during the time on stream. The selectivity to acrolein as high as 96 % is also stable during the
reaction. The catalytic performance of the model catalyst is better than many catalysts with
multicomponent documented [6]. Only very small amounts of by-products are detected.
Propene oxide and acrylic acid, however, are not formed in the oxidation of propene by
gaseous molecular O2. Introduction of MoO3 into the catalyst has a big promoting effect on
the selective oxidation propene to acrolein. The catalytic performance of the reference
catalysts was also investigated. In contrast, the reference catalysts are significantly less active
and less selective for oxidation of propene to acrolein (Table 3). The MoO3 promoted
hierarchical assembly of Bi2WO6 nanoflakes catalysts are specially active and selective for
propene converted to acrolein in a moderate temperature range. Some industry catalysts
patented in recent years with multi components are also listed for comparison.
(a)
(b)
100
80
60
40
20
0
100
80
60
40
20
0
Acrolein
CO2
Acrolein
CO2
Aldehyde
Others
Propene
Aldehyde
Propene
1
2
3
4
5
6
7
8
0
2
4
6
8
10 12 14
MoO3 content (wt%)
Reactin Time (h)
Fig. 3. (a) Catalytic performance of the catalysts with different MoO3 contents. (b) Variation of propene
conversion and acrolein selectivity with time on stream over the 5Mo/BWO catalyst. Reaction
Page 12 of 27

Conditions: T=623 K, W= 0.45 g, F= 25.5 mL min-1, WHSV=3400 mL g-1 h-1, P(C3H6)=1.99 kPa,
P(Air)=99.31 kPa.
Table 3. The catalytic performances of some reference catalysts.a
Temp. Conv. Selectivity of products
Catalysts
Acrolien CO2 Aldehyde Othersb
(%)
(K)
BMo0.01W0.99
BMo0.03W0.97
BMo0.05W0.95
BMo0.07W0.93
dMo12Fe2Bi1.5Co4.4K0.06Ox
eMo12Fe2.94Bi0.8Co7Si1.52K0.08Ox
O
O
O
O
623 29.4 21.5
623 37.9 47.4
623 23.3 63.6
623 21.2 64.8
52.4 7.4
35.0 10.1
24.9 6.7
20.7 5.6
-- --
-- --
--
18.7
7.8
4.9
8.9
1.62c
4.32c
-
593
591
615
619
99
95
97
97
90.50
91.68
98.56
87.42
fMo12Fe2.4Bi1.08Co9.6Al1.48V0.056Ag0.1764Pd0.0019K0.064Ox
gMo12Fe1.8Bi1.7Ni2.8Co5.2K0.1Ox
-- --
7.63c
a Reaction conditions: T=623 K, W= 0.45 g, F= 25.5 mL min-1, WHSV=3400 mL g-1 h-1, P(C3H6)=1.99
kPa, P(Air)=99.31 kPa. b Others products were acetone, propanal and isopropanol. c Acrylic acid. d LG
Chem. Ltd, WO 079980 (2005). e BASF, EP 1005908 (1998). f Sabic, WO 2000009260, US 6143928. g
Nippon Kayaku Co., Ltd., EP 807465 (1997).
The kinetic expression for the rate of propene oxidation to acrolein can be expressed as a
power law:
r = κ ⋅ Px ⋅ Py
(1)
C3H6
O
2
where r represents the rate of reaction to propene [mmol·g-1·h-1], k is the apparent rate
constant, x and y represent the reactant reaction orders. By respectively setting the pressures
of the propene and O2 as a constant, the reactant reaction orders can be measured. The results
are shown in Fig. 4a and Fig. 4b, with which the kinetic equation of the reaction for the
catalyst can be expressed as:
r = κ ⋅ P0. 89 ⋅ P0. 10
(2)
C3H6
O
2
Basically, the rate equation (2) reveals that the propene oxidation to acrolein reaction
follows around first-order kinetics with respect to propene and around zero-order kinetics
with respect to molecular O2 partial pressures. The partial reaction orders in both reactants
observed, however, may indicate that the kinetics is a composite of the reoxidation and
reduction kinetics. Similar results have been reported for the oxidation of propene over and
Page 13 of 27

for the oxidation of propene over iron-antimony catalyst [35-36] and bismuth molybdate
catalysts at low temperatures (<673 K) [37]. The apparent activation energy, Ea, for propene
selective oxidation to acrolein over the 5Mo/BWO is determined from the plot of ln(k) versus
1/T in the temperature range of 608~628 K, as shown in Fig. 4c. In order to obtain reasonable
results of the apparent activation energy, the catalytic conversion of propene is controlled less
than 13 %, by using small amounts of catalysts. The apparent activation energy for the
reaction is calculated as 32.8 kcal mol-1 from the Arrhenius equation, which is less than and
still in reasonable agreement with those recently reported for the selective oxidation of
propene to acrolein over bismuth molybdate catalysts [38-39]. The similarities in kinetics and
catalytic properties for these catalysts suggest that the catalytic reaction mechanism of these
catalysts is similar, but the catalyst Mo/BWO shows higher activity with much simple
structure and composition.
2.0
1.2
(a)
(c)
-13.0
-13.2
-13.4
-13.6
-13.8
-14.0
-14.2
(b)
1.5
1.0
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.5
0.0
y ~ 0.10
x ~ 0.89
Ea = 32.8 kcal/mol
-0.5
-1.0
0.2
0.4
0.6
0.8
1.0
1.59 1.60 1.61 1.62 1.63 1.64
0.2
0.4
0.6
0.8
1.0
1/T (×103 K-1)
Lg (P(O2))
Lg (P(C3H6))
Fig. 4. (a) The plots of the logarithm of r(C3H6) against the logarithm of P(O2). Reaction Conditions:
cat. 5Mo/BWO=0.05 g; Silica sand=0.40 g; T=623 K. P(C3H6)=4.51 kPa, F=60 mL min-1. (b) The plots
of the logarithm of r(C3H6) against the logarithm of P(O2). Reaction Conditions: cat. 5Mo/BWO=0.05
g; Silica sand=0.40 g; T=623 K. P(O2)=12.80 kPa, F=40 mL min-1. (c) The plots of the napierian
logarithm of against the 1/T. Reaction Conditions: cat. 5Mo/BWO=0.05 g; Silica sand=0.40 g;
P(C3H6)=1.99 kPa, F=25.5 mL min-1.
In order to understand the structure of the Mo/Bi2WO6 catalysts, Fig. 5 depicts the
structural model of the orthorhombic Bi2WO6 with the surface loading of molybdena. The
Page 14 of 27

crystal lattice parameters calculated are as follows: a = 5.4326, b=16.4302 and c=5.4584 with
a cell volume of 487.21 Ǻ3 for the orthorhombic Bi2WO6 [40], which is a layered structure
composed of WO6 octahedral layers and Bi-O-Bi layers (Fig. 5b). Each Bi atom is
coordinated with four O and four W atoms and, meanwhile, the WO6 octahedral are connected
to each other by corner-sharing O atom [40]. As mentioned above, the currently synthesized
submicroscopical flake-like Bi2WO6 are composed of small nanocrystals (Fig. 1c), which
provide mainly (100) and (010) facets as the exposed surfaces.
(b)
(a)
(c)
Fig. 5. (a) The FESEM images of the as-prepared 5Mo/BWO catalyst. (b) The two-dimensional
structure of the crystal planes: the (100) and (010) atomic planes of orthorhombic-phase Bi2WO6. (c)
The schematic diagram of the formation of MoO3 nanocrystal on the (100) and (010) atomic planes of
Bi2WO6 nanoflakes. Green bars mean the double bonds of Mo=O.
Molybdena loading has no obvious influence on the morphology and texture of Bi2WO6
nanoflakes, owning to the structural similarity between the Bi2MoO6 and Bi2WO6, which
causes the easy epitaxial growth or highly dispersion of molybdena on the surface of Bi2WO6
Page 15 of 27

by attaching to the interface oxygen atoms, i.e., the two-dimensional cell structures of the
(100) and (010) atomic planes of Bi2WO6, which are the two lateral facets of the Bi2WO6
nanoflakes (Fig. 5b and 5c). The tetrahedral MoO4 perfectly bonded to the surfaces can be
modeled with two manners, as shown in Fig. 5c, in which the terminal Mo=O species of
tetrahedral molybdate are exposed to the reaction atmosphere. For the selective oxidation of
propene, the nature of active sites and the present state of oxygen species selective are still
under debate, but it has been proposed that the Mo=O species is the selectively active oxygen
species for inserting to form acrolein [7,39]. According to the Raman results, the bands
around 996 and 291 cm-1 from xMo/BWO samples are attributed to the symmetric stretching
mode of Mo=O bonds of the tetrahedral molybdate (Fig. 2c). These results collectively
suggest that lattice oxygen with molybdenum connected fitly with the regular surfaces of
bismuth tungstate be important for the partial oxidation of propene (Fig. 5b and 5c).
(a)
(b)
Bi2WO6
20
5Mo/BWO
770 K
540 570
630
510Temperatu6r0e0(K)
103.3
52.2
7Mo/BWO
5Mo/BWO
3Mo/BWO
1Mo/BWO
Bi2WO6
300 400 500 600 700 800 900 1000
Temperature (K)
350 400 450 500 550 600 650
Temperature(K)
Fig. 6. (a) H2-TPR profiles of the catalysts with different MoO3 loadings. (b) O2-TPD profiles of the
catalysts with and without MoO3 loadings.
It is well accepted that the reducibility of the catalysts do have great influence on the
catalytic activity. For the xMo/BWO samples, the main reduction processes take place at the
temperatures higher than 700 K, as shown in Fig. 6a, which should be attributed to the
reduction of tungstate. With increasing MoO3 loadings, a shoulder peak at ~770 K appears
gradually. As reported in the earlier literatures [41], the peak at ~770 K is related to the first
step reduction of Mo(VI)→Mo(IV) of well dispersed octahedral Mo species. It is interesting
Page 16 of 27

that the introduction of molybdenum causes the significant decrease in consumption of
hydrogen during H2-TPR of Bi2WO4 at low temperatures of 500~650 K, implying that the
overactive oxygen species of the Bi2WO4 leading to unselective reaction are eliminated. It is
well in agreement with the catalytic properties of the xMo/BWO, which is drastically
modified by the molybdenum loading (Fig. 3).
That the selective oxidation of propene takes place with the chemisorbed allyl oxidized to
acrolein by lattice oxygen or chemisorbed oxygen is a general consideration [8]. The activity
or mobility of oxygen of the catalysts is very important. The results of
temperature-programmed desorption of oxygen (O2-TPD) are shown in Fig. 6b. The initial O2
evolution peak’s summit appears at ~369 K for Bi2WO6 catalyst. For 5Mo/BWO with
molybdenum loading, the O2 evolution peak’s summit shifts to a higher temperature (~389 K)
and the amount of oxygen evolution increases markedly as compared with Bi2WO6. It is
known that oxygen chemisorption depends strongly on the electrodonor properties [41].
According to the catalytic performance and the rate equation for the 5Mo/BWO catalyst, it
involves much more active oxygen species selective for oxidation of propene than the
Bi2WO6, also in consistent with the Raman results.
The measurement of conductivity is carried out to know the band information of the
catalysts, which can be correlated to the selectivity for the propene oxidation [42]. Fig. 7a
shows the variation in conductance of the 5Mo/BWO and Bi2WO6 in different atmospheres.
The conductance of the catalysts dropped immediately when the atmosphere is switched from
argon to oxygen and the removal of oxygen restores the conductance of the catalysts. For both
samples, the depleting adsorption of oxygen is deduced and indicates the n-type
semiconductor of the two samples. The adsorbed oxygen species should gain electrons from
the catalyst and turn to negative charged oxygen species. The higher conductance of
5Mo/BWO than Bi2WO6 means more carrier concentration and also stronger adsorption for
oxygen of the former. The Arrhenius plots of the dependence of conductance on temperature
for Bi2WO6 and 5Mo/BWO catalysts are shown in Fig. 7b. In comparison with Bi2WO6, the
5Mo/BWO catalyst possesses much less activation energy (0.22 eV versus 0.70 eV) for
electron’s moving. More electrons could be used for adsorption of oxygen on the Bi2WO6
with the assistance of surface molybdenum deposition. The Mo/BWO is more active than
Page 17 of 27

Bi2WO6 itself.
350
Sample disc
Silver leads
Bi2WO6(a)
5Mo/BWO(b)
-2.5
-3.0
-3.5
-4.0
-4.5
-5.0
-5.5
(b)
(a)
a)
b)
300
︶
Ea = 0.70 eV
a) y=9.13-8123.3x
Ea = 0.22 eV
b) y=-0.44-2525.3x
250
200
150
100
50
a
b
Ar or O2
5Mo/BWO
1.52 1.60 1.68 1.76 1.84 1.92
1/T( 10-3)
Bi2WO6
Bi2WO6
5Mo/BWO
Ar
Ar
Ar
O2
O2
O2
0
1080 1620 2160 2700 3240
540Contact Time(s)
520 540 560 580 600 620 640 660 680
Temperature(K)
Fig. 7. (a) Variation in conductance of the catalysts in different atmosphere. Inset shows the
conductivity cell. (b) Changes of the resistance with time on stream for the catalysts after different
temperature. Inset shows arrhenius plots for the change of conductance over Bi2WO6 and 5Mo/BWO
catalysts.
C3H6-TPSR measurements are carried out with the 5Mo/BWO catalyst to investigate the
catalytic surface reaction mechanism of propene selective oxidation to acrolein. The O2-free
C3H6-TPSR spectra are shown in Fig. 8a and the main reaction products from propene
oxidation by the lattice oxygen are acrolein, which appears in temperature ranges from
360~400 K. Chemisorbed propene desorbs at 425 K and the desorbing temperature makes it
cannot be considered as physically adsorbed propene [22]. Interestingly, the formation of CO2
at low temperature is suppressed under the O2-free condition and CO2 production primarily
occurs at the temperatures higher than 600 K. In the presence of gaseous oxygen, however,
the C3H6-TPSR experiment yields carbon dioxide as the main reaction product in two
different low temperature regions (Fig. 9b). Almost no selective oxidation products, acrolein
or aldehyde, are detected. These results reveal that the surface lattice oxygen from the
catalysts could be effectively activate propene and selectively oxidize propene to acrolein.
Introducing oxygen into propene stream, however, the C3H6-TPSR test yields CO2 as the main
reaction product. Thus, the oxygen atom of both acrolein and aldehyde should be originates
Page 18 of 27

from the lattice oxygen of the catalyst rather than from the gaseous molecular oxygen directly.
After the TPSR, the catalysts are checked by thermal gravity-mass spectrocopy measurement. It is
found that the catalyst surface is clean after TPSR with presence of gaseous oxygen but small
amount of carbon deposition is detected without the presence of gaseous oxygen. Similar results
are also reported by Wachs et al. [22,43].
(a)
(b)
CO2
CO2
Aldehyde
Aldehyde
Propene
Acrolein
Propene
Acrolein
350 400 450 500 550 600 650
Temperature (K)
350 400 450 500 550 600 650
Temperature(K)
Fig. 8. The C3H6-TPSR spectroscopy of the 5Mo/BWO catalyst in a flow of (a) He (O2-free) and (b)
O2/He.
Summing up the above results and discussions, the reaction routes of the selective
oxidation of propene to acrolein over the molybdena loaded on bismuth tungstate catalysts is
proposed and schematically illustrated in Fig. 9. With the temperature increasing, the
electrons in the valence band of Bi2WO6 are excited to its conduction band due to its intrinsic
excitations as a semiconductor [44], this is also a process similar to photo-generated electrons
and holes via visible-light irradiation. Then the conduction band electrons can interact with
-
adsorbed O2 to form chemisorbed oxygen species, O2 and/or atomic O- [45-46], which are
both strong oxidative species and play crucial roles in the oxidative of hydrocarbon. This kind
-
of electrophilic addition of O2 or O- results in the formation of peroxy- or epoxy-complexes,
which are only transient products of the degradation of the carbon chain and the total
oxidation under the conditions of catalytic oxidation. These reactions are so-called
electrophilic oxidation [45-46]. Accordingly, the oxidation of propene with the bismuth
tungstate catalyst also provided CO2 as the main product (73 %, Fig. 3a). On the other hand,
Page 19 of 27

according to Mars-van Krevelen mechanism [47], propene adsorbed on the surface of bismuth
oxide with hydrogen abstraction forms a π-allyl radical, which then reacts with the nearby
molybdenum oxo species to give rise to a C-O bond with the lattice oxygen. So, propene
reactions follow an allylic C-H bond activation and undergo a nucleophilic addition of the
lattice O2- ion, in which step causes the reduction of valence state of Mo(VI) and an oxygen
vacancy is left on the surface. The gaseous oxygen then fills in the vacancy and causes the
reoxidation of the Mo site for the next cycle of reaction. These reactions are classified as
nucleophilic oxidations [41,48].
The suitable cooperation between the sites of molybdnaum and bismuth on the catalyst
surface is extremely important, such the current catalyst with molybdena highly dispersed on
the surface of well crystallized Bi2WO4, over which the selectivity of acrolein increases with
temperature, while the selectivity of carbon dioxide decreases simultaneously. From the
apparent rate equation, the observed kinetics would be a composite of both the reduction and
reoxidation kinetics with partial dependencies on both propene and oxygen. In short, a
synchronous redox mechanism is involved in the selective oxidation of propene over the
current molybdena loaded bismuth tungstate catalysts.
Fig. 9. Schematic show of the reaction mechanism for the selective oxidation of propene by air over the
supported Mo/BWO catalyst.
4. Conclusions
Hierarchical Bi2WO6 nanoflakes, assembled as hollow microspheres, are easily realized by
solvenothermal synthesis using polyethylene glycol as the structure-directing agent. The
Page 20 of 27

hollow architectures with a diameter of ca. 4 μm are constructed by bismuth tungstate
nanoflakes with a thickness of about 50 nm. Molybdenum oxide species are immobilized onto
the nanoflakes surface in highly dispersed state via impregnation of ammonium molybdate.
The Bi2WO6 itself mainly catalyzes the formation of CO2 for propene oxidation. The
introduction of molybdena, however, dramatically increases the selectivity to acrolein and
decreases the formation of CO2 with significant promotion on propene conversion at the same
time. There is no acrylic acid detected during the propene oxidation, which is especially
interesting. The 5Mo/BWO catalyst exhibits the highest acrolein selectivity and propene
conversion among a series of molybdena loaded catalysts. In addition, the catalyst is very
stable and keeps an excellent catalytic performance in the selective oxidation of propene. In
this process, the thermally excited electrons are proposed to be incorporated in the rapid
regeneration of the active molybdenum sites, of which the lattice oxygen consumed in the
nucleophilic oxidation of propene to acrolein. A reasonable mechanism of catalyst
interactions and the routes of selective oxidation of propene are proposed to explain all the
experimental data.
Acknowledgment
The authors thank the financial supports from the Ministry of Science and
Technology of China (2009CB623504), the National Science Foundation of China
(20673054, 21273107), and Sinopec Shanghai Research Institute of Petrochemical
Technology.
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Page 23 of 27

Figure captions.
Fig. 1. (a) and (b) FESEM and TEM images of as-prepared Bi2WO6 hollow microspheres using
polyethylene glycol (PEG 200) as the solvent; (c) Higher magnification microscopy FESEM images of
the Bi2WO6 hollow microspheres; (d) and (e) Higher magnification microscopy TEM images of the
Bi2WO6 nanoflakes. Inset of (d) shows the corresponding SAED image; (f) XRD pattern of the Bi2WO6
sample.
Fig. 2. (a) FESEM images of 5Mo/BWO sample (scale bar is 1 μm). (b) XRD patterns of the Bi2WO6
and xMo/BWO samples with different MoO3 contents. (c) and (d) Raman spectra and FTIR spectra of
the catalysts with different MoO3 loadings, respectively.
Fig. 3. (a) Catalytic performance of the catalysts with different MoO3 contents. (b) Variation of propene
conversion and acrolein selectivity with time on stream over the 5Mo/BWO catalyst. Reaction
Conditions: T=623 K, W= 0.45 g, F= 25.5 mL min-1, WHSV=3400 mL g-1 h-1, P(C3H6)=1.99 kPa,
P(Air)=99.31 kPa.
Fig. 4. (a) The plots of the logarithm of r(C3H6) against the logarithm of P(O2). Reaction Conditions:
cat. 5Mo/BWO=0.05 g; Silica sand=0.40 g; T=623 K. P(C3H6)=4.51 kPa, F=60 mL min-1. (b) The plots
of the logarithm of r(C3H6) against the logarithm of P(O2). Reaction Conditions: cat. 5Mo/BWO=0.05
g; Silica sand=0.40 g; T=623 K. P(O2)=12.80 kPa, F=40 mL min-1. (c) The plots of the napierian
logarithm of against the 1/T. Reaction Conditions: cat. 5Mo/BWO=0.05 g; Silica sand=0.40 g;
P(C3H6)=1.99 kPa, F=25.5 mL min-1
Fig. 5. (a) The FESEM images of the as-prepared 5Mo/BWO catalyst. (b) The two-dimensional
structure of the crystal planes: the (100) and (010) atomic planes of orthorhombic-phase Bi2WO6. (c)
The schematic diagram of the formation of MoO3 nanocrystal on the (100) and (010) atomic planes of
Bi2WO6 nanoflakes. Green bars mean the double bonds of Mo=O.
Fig. 6. (a) H2-TPR profiles of the catalysts with different MoO3 loadings. (b) O2-TPD profiles of the
Page 24 of 27

catalysts with and without MoO3 loadings.
Fig. 7. (a) Variation in conductance of the catalysts in different atmosphere. Inset shows the
conductivity cell. (b) Changes of the resistance with time on stream for the catalysts after different
temperature. Inset shows arrhenius plots for the change of conductance over Bi2WO6 and 5Mo/BWO
catalysts.
Fig. 8. The C3H6-TPSR spectroscopy of the 5Mo/BWO catalyst in a flow of (a) He (O2-free) and (b)
O2/He.
Fig. 9. Schematic show of the reaction mechanism for the selective oxidation of propene by air over the
supported Mo/BWO catalyst.
Page 25 of 27

Graphical Abstract
Super high selectivity of acrolein in oxidation of propene on molybdenum promoted
hierarchical assembly of bismuth tungstate nanoflakes
Yuming Cui, Yifen Xia, Jianbo Zhao, Lei Li, Teng Fu, Nianhua Xue, Luming Peng, Xuefeng
Guo and Weiping Ding*
a Key Lab of Mesoscopic Chemistry, the School of Chemistry and Chemical Engineering, Nanjing
University, Nanjing 210093, China
*Corresponding author. E-mail address: dingwp@nju.edu.cn (DWP). Tel: +86 25 83595077;
Fax: +86 25 83317761
O2
CH3-CH=CH2
Propene
O=CH-CH=CH2
Acrolein
MoO3/Bi2WO6
Page 26 of 27

Highlights
ꢀ
ꢀ
ꢀ
ꢀ
Hierarchical Bi2WO6 microspheres composed by well shaped nanoflakes are obtained.
Surface modification of MoO3 dramatically enhances the catalyst as selective one.
Acrolein selectivity can surpass ~96% in oxidation of propene with high conversion.
Cooperation between surface molybdena and n-type semiconductor Bi2WO6 is important.
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