Chemistry
An In-Situ Neutron Diffraction Study of -Bi2Mo2O9 and -Bi2MoO6 as Partial
Oxidation Catalysts
Hamzah Fansui1 and Dong-ke Zhang2
1
Chemistry Department, Faculty of Mathematics and Natural Sciences
Institut Teknologi Sepuluh Nopember,
Kampus ITS Sukolilo, Surabaya, Indonesia, 60111.
2
Centre for Petroleum, Fuels and Energy
The University of Western Australia,
35 Stirling Highway, Crawley, Perth, Western Australia 6009
Abstract
The dynamics of lattice oxygen in β-Bi2Mo2O9 and γ-Bi2MoO6 as catalysts for partial oxidation of
propylene to acrolein was investigated using the neutron diffraction technique. The catalyst
characterization experiments were carried out under reaction conditions at 300, 350 and 400 oC
using the Medium Resolution Powder Diffraction (MRPD). The unit cell of the β-Bi2Mo2O9 was
seen to expand isotropically to the (111) face with increasing temperature, indicating that there was
no important atomic coordinate changes in the temperature range studied. On the other hand, the
unit cell of the γ-Bi2MoO6 expanded to the (110) face. The difference suggests that the two catalysts
have different activation mechanisms in catalysing the partial oxidation reaction. Structure
refinement of both catalysts under the in-situ conditions shows that certain lattice oxygen ions are
more mobile than others. The Oxygen ions No. 3 and 18 in β-Bi2Mo2O9 and No. 1 and 5 in γBi2MoO6 are the mobile oxygen ions in the lattice. The mobile lattice oxygen ions are the most
probable active oxygen for the selective oxidation of propylene to acrolein
Keywords: Acrolein, Bismuth molybdate, In-Situ Neutron Diffraction, Partial Oxidation, Propylene.
1. Introduction
Bismuth molybdates have long been known as active catalysts for selective oxidation of
olefins. There are several phases of bismuth molybdates but only three of them are known to be
active for partial oxidation of propylene to acrolein, namely α, β and γ bismuth molybdates.
Many researchers believe that lattice oxygen in bismuth molybdate catalysts plays an
important role in determining the activities of bismuth molybdates in catalysing the partial
oxidation of olefin. It has been proven that the oxidation reaction uses the lattice oxygen and
follows the Mars-van Krevelen mechanisms [1-2]. Several studies have also shown that the lattice
oxygen ions are involved in the oxidation process [1, 3-6]. Furthermore, Haber [7] mentioned that
the ease of oxygen movement in bismuth molybdates, by the formation of shear plane and
rearrangement of corner-linked metal oxides into edge-linked octahedrals of molybdenum as well as
tungstate oxide, favour their activities and selectivity towards acrolein formation.

Corresponding author: h.fansuri@chem.its.ac.id
2. Experimental
The catalysts used in the present study were prepared using the co-precipitation method
from Bi(NO3)3.5H2O and (NH4)6Mo7O24.4H2O solutions. Molar ratio of Bismuth to Molybdenum
was 1 and 2 for the preparation of β-Bi2Mo2O9 and γ-Bi2MoO6, respectively. The suspension was
kept in a water bath at 70oC and stirred well to evaporate the liquid slowly until it became a paste.
The paste was then dried at 120oC for 20 hrs in air. The dried cake was crushed and calcined in air
at 250o for 2 hrs followed by calcination at 650oC for 24 hrs for β-Bi2Mo2O9 and at 480oC for 20
hrs for γ-Bi2MoO6. Room temperature structure of the catalysts thus prepared was characterized
using both X-ray and Neutron diffraction techniques. XRD analysis employed Cu K radiation
operated at 40 kV and 30 mA. Neutron diffraction analysis was carried out using the High
Resolution Powder Diffraction (HRPD). A monochromatic neutron beam at a wavelength of
1.495Å was applied. In-situ neutron diffraction analyses were carried out using the Medium
Resolution Powder Diffractometer (MRPD) with a monochromatic neutron beam at a wavelength
of 1.665 Å. The catalyst characterisation was carried out at 300, 350 and 400oC in air and in a
simulated reaction atmosphere comprised of 1% C3H6, 2% O2 and 97% He in a special quartz cell
reactor [8] (Figure 1). All diffractograms were used to refine the lattice parameters, inter-atomic
distances and thermal parameters, using the LHPM Rietica refinement method [9].
Figure 1. A schematic of the special sample cell for the in-situ MRPD analyses
3. Results and discussion
Figure 2 shows HRPD diffractograms and refinement fit of room temperature -Bi2Mo2O9
and -Bi2MoO6. Search and Match results using Jade 6.0 and PDF (Powder Data File) database
version 2 reveals that the bismuth molybdate catalysts prepared in this experiment are -Bi2Mo2O9
and -Bi2MoO6.
The beta phase corresponds to ICSD (Inorganic Crystal Structure Database) collection No.
201742 based on structural model reported by Chen and Sleight [10]. The unit cell in the model is
monoclinic with unit cell parameters a=11.972(3), b=10.813(4), c=11.899(2), =90.0 =90.1(0)
=90.0 and unit cell volume V=1540.4. Meanwhile, the gamma phase corresponds to ICSD
collection No. 47139 where the unit cell is orthorhombic and a=5.482(0), b=16.199(1), c=5.509(0),
=90.0 =90.0 =90.0 and unit cell volume V=489.2 as reported by Teller et al. [11]. Structure
refinement results of the beta and gamma phases from their room temperature HRPD and XRD
using their corresponding structural model give slightly different unit cell parameters as shown in
Table 1.
(a)
(b)
Figure 2. Graphical representation of the refinement results of room temperature HRPD
diffractograms. (A) -Bi2Mo2O9 and (B) -Bi2MoO6.
Table 1. Refined unit cell parameters of bismuth molybdates of room temperature X-ray and
Neutron (HRPD) diffractograms.
Parameters
-Bi2Mo2O9
-Bi2MoO6
X-ray
Neutron
X-ray
Neutron
Volume (Å ) 1534.05(9) 1538.27(5) 489.41(3) 490.84(2)
3
a (Å)
11.954(0) 11.965(0)
b (Å)
10.799(0) 10.810 (0) 16.209(0) 16.225(0)
c (Å)
11.883(0) 11.893 (0)
 (o)
 (o)

5.484(0)
5.489(0)
5.506(0)
5.511(0)
90.000
90.000
90.000
90.143(3) 90.139(2)
90.000
90.000
90.000
90.000
90.000
90.000
90.000
The dynamic changes on the structure of beta and gamma phases under reaction condition
(increasing temperature and reaction atmosphere) were studied by HRPD. The unit cell of the βBi2Mo2O9 expanded isotropically as the temperature increased (Figure 2.a). The isotropic unit cell
expansion revealed that there is no important atomic coordinate change in the temperature range
studied. The thermal parameters did not increase as the temperature increased. However, some
oxygen ions (No. 3 and 18) in the lattice had larger thermal parameters than the others. These
oxygen atoms were laid in the cavities of the β-Bi2Mo2O9 crystal and bonded to molybdenum and
bismuth ions. On the other hand, the unit cell of the γ-Bi2MoO6 expanded more in the a and c
directions than in the b direction (Figure 2.b). Some oxygen ions in the lattice (No. 1 and 5)
experienced greater increases in their thermal parameters than the others. Oxygen No 1 was
situated between the Mo-O and Bi(2)-O layers while Oxygen No 5 was on the Mo-O layer and
closer to the Bi(1) layer than the other oxygen ions in the Mo-O layer. The specific thermal
parameters of Oxygen ions No 1 and 5 indicate that they are probably the key in controlling the
catalyst activity for the selective partial oxidation. In addition, the less lattice expansion in the b
direction suggests that oxygen atoms bridging the layers were strongly bonded to molybdenum
and/or bismuth, except Oxygen No 1. The lattice oxygen atoms with high thermal parameters at
high temperatures are the source of the oxidising oxygen responsible for the selective oxidation of
propylene to acrolein.
0.80%
2.50%
1.00%
0.70%
2.00%
0.80%
2.00%
1.50%
0.40%
1.00%
0.30%
a
b
c
beta
Cell Volume
0.20%
0.10%
0.00%
275
300
0.50%
a, b and c
0.50%
0.70%
0.60%
1.50%
0.50%
a
b
c
Cell Volume
0.40%
0.30%
1.00%
0.20%
Cell Volume
0.60%
Cell Volume
a, b, c, and beta
2.50%
0.90%
0.50%
0.10%
325
350
375
Temperature (oC)
(a)
400
0.00%
425
0.00%
0.00%
275
300
325
350
375
o
400
425
Temperature ( C)
(b)
Figure 2. Percentage of unit cell extensions of a) β-Bi2Mo2O9 and b) γ-Bi2MoO6
4. Conclusions
The in-situ neutron diffraction study revealed that the most probable active lattice oxygen
ions responsible for the partial oxidation of propylene to acrolein are oxygen No 3 and 18 in Bi2Mo2O9 and No. 1 and 5 in -Bi2MoO6. The active oxygen atoms in both bismuth molybdates are
slightly different in their nature. In the  phase, active oxygen ions are laying in the crystal’s cavity
and directly bonded to molybdenum and bismuth ions, while in the  phase, the active oxygen are
sandwiched between molybdenum and bismuth oxide layers.
References
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Appendices
1. Unit cell structure of -Bi2Mo2O9
Figure 3. A unit cell of -Bi2Mo2O9 (upper figure) and atom map of the unit cell (lower figure).
(Adapted from H.-Y. Chen and A.W. Sleight [10])
2. Unit cell structure of -Bi2MoO6
Figure 4.
From left to right, a unit cell of -Bi2MoO6 and its atom map. The figure is adapted
from Teller et al [11].