Characterization of Ca - Bi - Mo oxide catalyst for selective propane ammoxidation, using XRD, XPS, TPRX/TPRO, and IR/Raman

Article abstract of DOI:10.1021/jp049840g

The physicochemical properties of highly active and selective Ca-Bi-Mo oxide for ammoxidation of propane to acrylonitrile were studied. The modified γ-bismuth molybdate (γ-Bi₂-MoO₆) and defective Ca-Bi-Mo phases produced by addition

Full text of DOI:10.1021/jp049840g

J. Phys. Chem. B 2004, 108, 8941-8946
8941
Characterization of Ca-Bi-Mo Oxide Catalyst for Selective Propane Ammoxidation, Using
XRD, XPS, TPRX/TPRO, and IR/Raman
Seong Ihl Woo,* Jong Seob Kim, and Hee Kwon Jun
Department of Chemical & Biomolecular Engineering and Center for Ultramicrochemical Process Systems,
Korea AdVanced Institute of Science and Technology, 373-1 Guseong-dong,
Yuseong-gu Daejeon, 305-701, Republic of Korea
ReceiVed: January 13, 2004; In Final Form: April 23, 2004
The physicochemical properties of highly active and selective Ca-Bi-Mo oxide for ammoxidation of propane
to acrylonitrile were investigated by XRD (powder), IR/Raman, XPS, and TPRX/TPRO (temperature
programmed reaction/temperature programmed reoxidation) techniques. It was found that the phases in the
Ca-Bi-Mo oxide varied with the composition of the oxide. The modified γ-bismuth molybdate (γ-Bi
2
-
MoO ) and defective Ca-Bi-Mo phases produced by addition of Bi oxide played an important role in propane
6
ammoxidation to acrylonitrile. XPS results revealed that the mole ratio of elements on the surface was different
from that in the bulk. The concentration of bismuth decreased from the surface to the inner layer of
Ca Bi12-xMo12 oxide, while the concentration of calcium increased from the surface to the inner layer. The
x
Ca-Bi-Mo oxide showed good catalytic performance when an appropriate amount of calcium oxide was
present on the surface along with Bi and Mo oxides. The addition of Ca oxide into Bi-Mo oxide decreased
the number of active sites for complete oxidation and increased the ability of reoxidation (O
of reduced catalysts.
2
consumption)
Introduction
oxide composition.10 However, the phases present in this oxide
system were not known. In light of the importance of this
catalyst, the physicochemical properties of the Ca-Bi-Mo
oxide were investigated with XRD (powder), XPS, IR/Raman
Most industrial organic chemicals are made from petroleum
or natural gas. About 25% of them are manufactured by the
selective partial (amm)oxidation over heterogeneous catalysts
which are mostly multicomponent mixed-metal oxide catalysts.
The raw materials of the present partial oxidation and ammoxi-
dation are mainly olefinic hydrocarbons. However, extensive
studies have been performed to manufacture the required
chemicals from paraffinic hydrocarbons. The most successful
partial oxidation of alkanes is the production of maleic anhydride
from n-butane over a VPO (vanadium phosphorus oxide)-based
,and TPRX/TPRO to explain the catalytic properties. In addition,
this study was intended to correlate the physicochemical
properties of catalyst (ability of redox, atomic composition of
surface and bulk, kind of phases) with their catalytic perfor-
mances for propane ammoxidation.
Experimental Section
1
catalyst. This is the only process used in the U.S.A. due to its
(a) Catalyst Preparation. The Ca-Bi-Mo oxides with
low feed cost and reduced environmental pollution.
In recent years many researchers have been trying to develop
catalysts for the selective conversion of propane to acrylonitrile
because of the considerable price difference between propane
and propene.2 However, the selective conversion of propane
to acrylonitrile is much more difficult than that of propene due
to the low reactivity of propane. All existing commercial
processes for the production of acrylonitrile use propene as a
feedstock. BP/Sohio Chemicals recently announced an additional
production line for direct conversion of propane to acrylonitrile.
various compositions were reproducibly prepared by a precipita-
tion method including the exact control of experimental
parameters such as pH in mixture solution and increasing rate
-4
10
of temperature during calcination. Ammonium molybdate
((NH4)6Mo7O24‚4H2O, Aldrich, 99.98%) was dissolved in a
basic ammonia solution (pH 10). This solution was added
dropwise to a nitrous solution of bismuth nitrate (Bi(NO ) ‚
3
3
5H O, Aldrich, 99.99%) and calcium nitrate (Ca(NO ) ‚4H O,
2
3 2
2
Aldrich, 99.99%), and then dilute ammonia was added until the
pH of the solution reached 5. After heating at 80 °C with
vigorous stirring to evaporate water, a viscous slurry was
obtained and subsequently dried at 120 °C for 24 h, precalcined
at 320 °C for 3 h in an air stream, and ground. A 100/200 mesh
sieve was used to collect powders with sizes between 100 and
5-7
Thus far, two families of catalysts, Sb-
and Mo-based
catalysts,8 have been discovered for this purpose. In the case
of propene ammoxidation, the Mo-based system is superior to
the Sb-based catalysts. Similar to the acrylonitrile case, extensive
efforts are being expended to replace the propene-based acrylic
acid process with a propane-based process.
,9
2
00 mesh. Finally, this product was calcined at 520 °C for 6 h
in an air stream. The catalysts were pale yellow.
b) Reaction Procedure. The propane ammoxidation was
We found that a Ca-Bi-Mo oxide catalyst was highly active
and selective for ammoxidation of propane to acrylonitrile, and
that the performance of the catalyst depended sharply on the
(
carried out at atmospheric pressure with a continuous flow
reactor system. The reaction products were analyzed by using
an on-line gas chromatograph (HP 5890) with two different
*
To whom correspondence should be addressed. E-mail: siwoo@
kaist.ac.kr. Phone: +82-42-869-3918. Fax: +82-42-869-8890
1
0.1021/jp049840g CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/26/2004

8942 J. Phys. Chem. B, Vol. 108, No. 26, 2004
Woo et al.
Figure 1. SEM micrograph for Ca
x
Bi12-xMo12 oxide: (a) x ) 0, (b) x ) 3, (c) x ) 5, (d) x ) 7, (e) x ) 9, and (f) x ) 12.
sequential GC columns (Gaskuropack-54, 5 m long, and M.S.
of the surface oxygen species. Reduced oxides were flushed
5
A, 2 m long). The detailed procedures were described
with N2 at 500 °C for 1 h, and then cooled to room temperature.
N2 was changed with 10% O2 gas at a flow rate of 30 cm /min.
1
1
3
elsewhere.
(
c) Methods of Characterization. X-ray powder diffraction
The reactor was heated from 298 to 793 K at a rate of 10 deg
K/min and then cooled to room temperature. The consumption
of O2 arising from the reoxidation of catalystys was monitored
by the computer-interfaced mass spectrometer.
patterns were obtained with a Rigaku GF 2035 X-ray diffrac-
tometer with Cu K radiation. The IR spectra were recorded with
a Bomem infrared spectrophotometer (Michelson-102), using
a conventional KBr disk technique. The Raman spectra were
collected with a Spectra-Physics argon ion laser (Model 171)
for excitation. About 100-200 mg of each bismuth molybdate
sample was pressed onto a thin layer of KBr to provide
mechanical support. The pressed sample was then mounted onto
a sample holder capable of spinning at 2000 rpm to avoid a
local heating effect caused by the focused laser beam. X-ray
photoelectron spectra were measured with the Leybold, LHS-
0. The magnesium anode (hν ) 1253.6 eV) was used for
excitation and the X-ray power supply was run at 10 kV and
00 mA. The samples were deposited as a thin layer on double-
sided adhesive tape and introduced into the preparation chamber.
The samples were outgassed in the preparation chamber of the
spectrometer. The pressure in the analysis chamber was
maintained at 3 × 10 mbar. The spectra of the C 1s, O 1s,
Ca 2p, Mo 3d, and Bi 4f levels were recorded. For calibration
of the binding energy a small gold film was vapor-deposited
on part of the sample. The Au 4f7/2 peak (84.0 eV) was used as
a reference. To investigate the reactivity of surface oxygen
species and redox ability, TPRX/TPRO was carried out. The
ammoxidation of propene to acrylonitrile in the absence of
Results and Discussion
(a) Effect of the Oxide Composition on the Structure. In
1
0
our previous research, the selectivity of propane toward
acrylonitrile according to the values of x (moles of Ca) in
CaxBi12-xMo12 oxide catalysts was reported. The maximum
selectivity of propane to acrylonitrile occurred when x was
between 6 and 9. For x > 9, carbon oxides increased with
increasing x. For x < 6, propene was the principal product.
Because the basicity of the catalyst increased with the increase
of x (content of calcium), the oxidation reaction of propene and/
or acrylonitrile to carbon oxides was enhanced. The optimum
content of Ca for reaction of propane to acrylonitrile was
obtained at values of x from 6 to 9. It can be suggested that the
difference in catalytic performance depends on the presence of
adequate redox and acid-base properties of the catalyst system,
because these properties strongly affect the transfer of lattice
oxygen and electron between catalyst and reactants. In this
paper, the physicochemical properties of Ca-Bi-Mo oxide
were investigated with XRD (powder), XPS, IR/Raman, and
TPRX/TPRO to explain the catalytic properties of propane
ammoxidation. Scanning electron micrographs (SEM) of cata-
lysts with different values of x are shown in Figure 1. Surface
morphologies of the catalysts are of variable shape and size
depending on the composition of the catalyst. Figure 2 shows
the XRD patterns of the Ca-Bi-Mo oxide catalysts according
1
1
-
9
3
gaseous oxygen was performed at a flow rate of 30 cm /min.
The reactant mixture of C3H6/NH3/N2 (5/5/90%) was passed
through the catalyst. The reactor was heated from 298 to 793
K at a rate of 10 deg K/min, held for 30 min, and then cooled
to room temperature. During each run, effluent gases were
monitored for CO2 and acrylonitrile to investigate the reactivity

Characterization of Ca-Bi-Mo Oxide Catalyst
J. Phys. Chem. B, Vol. 108, No. 26, 2004 8943
Figure 2. XRD patterns of (a) Ca
oxide, and (c) Ca Bi Mo12 oxide.
3 9 6 6
Bi Mo12 oxide, (b) Ca Bi Mo12
9
3
to composition. The phases of the Ca-Bi-Mo oxides varied
with the Ca/Bi atomic ratio. Ca3Bi9Mo12 oxide (Ca/Bi ) 1/3)
consisted of two phases, â-bismuth molybdate (â-Bi2Mo2O9)
and CaMoO4. Ca6Bi6Mo12 oxide (Ca/Bi ) 1/1) consisted of R-
and γ-bismuth molybdate (R-Bi2Mo3O12 and γ-Bi2MoO6) and
CaMoO4. Ca9Bi3Mo12 oxide (Ca/Bi ) 3/1) consisted of
γ-bismuth molybdate (γ-Bi2MoO6) and CaMoO4. Conclusively,
the addition of Ca oxide to Bi-Mo oxide caused the formation
of different Bi-Mo oxide phases. As mentioned in our previous
results the selectivity of propane toward acrylonitrile on Ca6-
Bi6Mo12 and Ca9Bi3Mo12 oxides was much higher than that of
Ca3Bi9Mo12 oxide. γ-Bismuth molybdate was not contained in
Ca3Bi9Mo12, while it was contained in both Ca6Bi6Mo12 and
Ca9Bi3Mo12 oxides. Thus γ-bismuth molybdate oxide phase was
considered to be a selective phase for propane ammoxidation.
To investigate the synergy effect of the phases present in the
Ca6Bi6Mo12 oxide which are composed of R-, γ-bismuth
molybdate and CaMoO4, the reaction were performed with the
physical mixtures of pure corresponding molybdates. The
acrylonitrile selectivity (18%) obtained with the mechanical
mixture (R- + γ-bismuth molybdate + CaMoO4) was lower
than that of Ca6Bi6Mo12 oxide catalyst in the propane ammoxi-
dation. This indicated that Ca6Bi6Mo12 oxide catalyst did not
consist of the mixture of pure molybdates (R- + γ-bismuth
molybdate + CaMoO4), but consisted of modified multicom-
ponent oxide phases. Therefore, it can be suggested that the
higher selectivity of Ca6Bi6Mo12 oxide could be attributed to
the modified molybdate phases produced by the solid solution
reaction, even though the XRD results could not identify this
phase.
Figure 3. Infrared spectra of Ca-Bi-Mo oxide.
and Mo cations. The IR spectrum of CaMoO4 oxide (Figure
-
1
3g) showed that the characteristic band at 820 cm was
associated with tetrahedral MoO4 species. With the addition of
bismuth (x ) 1), the IR spectrum of Ca11BiMo12 oxide (Figure
3f)) is slightly different from that of pure CaMoO4. With
increasing bismuth concentration of x ) 3, additional adsorption
-
1
-1
bands appear at ca. 900 cm and ca. 700 cm as shown in
1
0
-1
Figure 3e. The broad band at 720 cm can be assigned to the
modified γ-bismuth molybdate. The bands at 900 and 890 cm
-1
are arising from the formation of the higher order Mo-O bond.
It was known that the Mo-O bond located next to the cation
1
3
vacancies has bond orders greater than one. In the case of
Pb-Bi-Na-Mo oxide, the Mo-O stretching band at higher
wavenumbers clearly revealed that this band was arising from
the cation vacancy and that the intensity of this band was
proportional to the concentration of cation vacancy.14
In the case of Ca6Bi6Mo12, the IR bands were observed at
-
1
930, 895, 840, 800, and 730 cm . These bands correspond to
the characteristic IR bands of CaMoO4 and R- and γ-bismuth
molybdate. The IR spectrum of Ca3Bi9Mo12 showed broad bands
-
1
between 650 and 850 cm . This oxide is composed of
â-bismuth molybdate and CaMoO4. These IR results are
correlated with the XRD results. It was noteworthy that highly
active and selective catalysts were Ca6Bi6Mo12 and Ca9Bi3Mo12
oxide having high order Mo-O. In Figure 4, Raman spectra of
Ca1-3xBi2xφxMoO4 oxide are shown as a function of bismuth
content (or cation vacancies). The spectrum of CaMoO4 (at x
1
2
) 0) is in good agreement with previous results. Four bands
-1
(
b) Formation of Defect Evidenced by IR, XRD, and
at 875, 817, 815, and 790 cm are assigned to Mo-O stretching
frequencies. The bismuth (at x ) 0.05) was added to the
Raman. The infrared spectra of Ca12-xBixMo12 oxides are
shown in Figure 3. The CaMoO4 oxide has the ABO4 scheelite
structure (space group C4h) in which all Mo atoms were
tetrahedrally coordinated by oxygen at a distance of ca. 1.775
-
1
CaMoO4 phase and its new Raman spectra at 920 cm was
shown in Figure 4b, which is arising from Mo-O bonds located
next to cation vacancies. Figure 5 shows the Raman spectra of
Ca-Bi-Mo oxides. The Raman spectrum of Ca3Bi9Mo12 oxide
1
2
Å. In addition, all oxygen atoms were bridged between Ca

8944 J. Phys. Chem. B, Vol. 108, No. 26, 2004
Woo et al.
TABLE 1: Surface Concentration of Ca-Bi Molybdate
Oxide Catalysts
BiMo Ca
57.0
3
9 6 6 9 3 4
Bi Mo12 Ca Bi Mo12 Ca Bi Mo12 CaMoO
O (1s)
Mo (3d) 18.7
58.4
20.0
18.5
2.8
61.2
21.8
10.3
6.7
59.8
21.4
7.6
57.7
20.6
Bi (4f)
24.3
Ca (2p)
11.2
20.3
TABLE 2: Atomic Concentration of Ca-Bi Molybdate
Oxide after 30 min of Sputtering
Ca
3
Bi
9
Mo12
Ca
6
Bi
6
Mo12
9 3
Ca Bi Mo12
O
Mo
Bi
57.7
26.3
13.0
3.2
56.8
26.9
6.5
57.0
27.1
4.8
Ca
9.8
13.1
because of the tendency of the defect to migrate into the surface
and the higher coordinative unsaturated degree of the surface.
The catalytic properties are greatly influenced by the small
number of defects. The role of the defects must be to stimulate
allyl formation and to facilitate the diffusion of the lattice
oxygen, resulting in the higher selectivity toward acrylonitrile.
In the oxidation of propane to acrolein, the catalytic performance
of bismuth vanadomolybdate oxide varied with the concentration
of the defect and the highest yield was obtained with the catalyst
Figure 4. Raman spectra of the Ca1-3xBi2x
and (b) x ) 0.05.
x 4
φ MoO oxide: (a) x ) 0
1
7
Bi0.85V0.55Mo0.45. In the ammoxidation of propane to acrylo-
nitrile, Bi0.85V0.55Mo0.45 oxide catalyst also showed a good
catalytic performance. It was thought that there are appropriate
cation vacancies in Bi0.85V0.55Mo0.45. These cation vacancies
generate higher order Mo-O centers, which make lattice oxygen
available for the partial oxidation. Also, vacancies improve the
mobility of the lattice oxide ions, which in turn enhance the
replacement of oxygen ion in the redox cycle of the catalyst.
(
c) XPS Study of Ca-Bi-Mo Oxides. The X-ray photo-
electron spectra of the Ca-Bi-Mo oxide are shown in Figure
. The elements Bi, Mo, Ca, and O were detected on the catalyst
6
6
+
surface. The elements Mo, Bi, and Ca were assigned to Mo ,
Bi , and Ca , respectively.
3
+
2+
18,19
The variations of binding
energies for these elements were very low in Ca-Bi-Mo oxide.
From the Mo 3d spectra of the catalysts, we could ensure that
6
+
most of the Mo presented as Mo in oxidic surroundings. In
addition, the broadening of Mo 3d and O 1s spectra was
observed, indicating that there were several molybdenum and
oxygen species having slightly different binding energy levels
on the surface and that the catalyst was modified by adding the
bismuth oxide to CaMoO4. Tables 1 and 2 showed composition
differences between the surface and the bulk, respectively. The
mole ratio of elements on the surface depended on components
constituting the oxides due to the difference of surface tension
of each element. It was clear that the bismuth was mostly
observed on the surface of Ca-Bi-Mo oxide. The mole ratio
Figure 5. Raman spectra of the (a) Ca
3 9 6 6
Bi Mo12 oxide, (b) Ca Bi -
Mo12 oxide, and (c) Ca Bi Mo12 oxide.
9
3
+
of elements in the bulk was measured after sputtering of Ar
(
Figure 5a) indicates the presence of â-bismuth molybdate and
ion with the ion energy of 2.5 keV. Bismuth decreased and
calcium increased, compared with the results of the surface. It
indicated that the concentration of bismuth in the Ca-Bi-Mo
oxide decreased from the surface to an inner layer of the
catalysts, while the concentration of calcium increased from the
surface to an inner layer. Considering that Ca6Bi6Mo12 and Ca9-
Bi3Mo12 were selective for the reaction of propane to acrylo-
nitrile and that an appropriate amount of Ca oxide was also
present on the surface of these catalysts along with Bi and Mo
oxides in Table 1, it was clear that these catalysts could generate
the active site such as defective Ca-Bi-Mo oxide, which had
strongly influenced the reaction of propane to acrylonitrile.
(d) Redox Properties. To investigate the reactivity of the
surface oxygen species on Ca-Bi-Mo oxide, the ammoxidation
CaMoO4, whereas the Ca6Bi6Mo12 oxide is composed of R-,
γ-bismuth molybdate and CaMoO4. The Raman spectrum of
Ca9Bi3Mo12 oxide (Figure 5c) indicates the presence of γ-bis-
muth molybdate and CaMoO4.15 A new band at 920 cm was
also observed in all of the Ca-Bi-Mo oxides. From these IR
and Raman results, it can be concluded that defective Bi-
Mo-O oxide having the scheelite structure was produced by
adding bismuth oxide to CaMoO4 oxide.
In the case of CaMoO4 with the scheelite structure, it is
possible to form cation vacancies and the ternary Ca-Bi-Mo
phase by the replacement of Ca with Bi in the CaMoO4
phase. Although a small number of defects are formed in the
bulk, a large number of defects must be present on the surface
-1
2+
3+
1
6

Characterization of Ca-Bi-Mo Oxide Catalyst
J. Phys. Chem. B, Vol. 108, No. 26, 2004 8945
Figure 6. XPS spectra of Bi 4f, Mo 3d, Ca 2p, and O 1s in Ca-Bi-Mo oxide: (a) CaMoO
Ca Bi Mo12 oxide, and (e) BiMo oxide.
4 9 3 6 6
, (b) Ca Bi Mo12 oxide, (c) Ca Bi Mo12 oxide, (d)
3
9
CO2 with acrylonitrile, the amount of CO2 was higher than that
of acrylonitrile, indicating that the sites for complete oxidation
on the surface were more than those for selective oxidation. In
addition, the relative ratio of CO2 to acrylonitrile depended on
the composition of catalysts. In the case of BiMo oxide, the
amount of CO2 was higher than that of the other catalysts. It
was supposed that the BiMo oxide had the larger amount of
-
-
-
-
O2 or O species than other catalysts, in that the O or O2
species were strongly electrophillic reactants which are respon-
2
0
sible for the complete oxidation. In the case of olefins, this
-
-
electrophilic addition of O2 or O results in the formation of
peroxy or epoxy complexes, respectively which were the
intermediates of the degradation of the carbon skeleton and total
oxidation in the conditions of heterogeneous catalytic oxida-
2
1
tion. When the content of calcium oxide gradually increased,
the amount of CO2 rapidly decreased, while acrylonitrile was
nearly constant. It indicated that the active sites of complete
oxidation on the surface were diminished by addition of calcium
oxide. The reduced state of the catalysts was investigated by
using the TPRO technique. The TPRO profiles of the Ca-Bi-
Mo oxides were obtained and shown in Figure 8. Two peaks at
about 230 and 450 °C were observed for all catalysts except
CaMoO4, indicating that the reduced Ca-Bi-Mo oxides might
be reoxidized in these two temperatures. In the case of CaMoO4
oxide, only one peak was observed at 420 °C and the
consumption of gaseous oxygen was the smallest in the Ca-
Bi-Mo oxides, indicating that the CaMoO4 oxide had a small
ability to redox cycle. The first peak in TPRO profiles came
from the reoxidation of the reduced bismuth. Miura et al.
reported that the reoxidation of reduced bismuth oxide (25% at
Figure 7. TPRX profile of the Ca-Bi-Mo oxide system.
of propene in the absence of gaseous oxygen was performed.
The CO2 and acrylonitrile in the products were monitored by a
quadrupole mass spectrometric analyzer as shown in Figure 7.
At the reaction temperature of about 400 °C, the surface oxygen
species was initiated with reactants. Comparing the amount of
22
400 °C) took place at 230 °C. The reduction of bismuth oxide
should produce metallic bismuth only. The second reoxidation
step suggested by the peak at 450 °C may be the reoxidation of

8946 J. Phys. Chem. B, Vol. 108, No. 26, 2004
Woo et al.
reoxidation (O2 consumption) of reduced catalysts. The catalysts
were highly active and selective when an appropriate amount
of Ca oxide was present on the surface along with Bi and Mo
oxides.
Acknowledgment. This research was funded by Tong-Suh
Petrochemical company and CUPS (Center for Ultra-micro-
chemical Process Systems) sponsored by KOSEF (2003-2004).
The authors thank Professor I. Wachs and Dr. D. S. Kim for
their help in obtaining the Raman spectra.
References and Notes
(
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1
2
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Mo oxide varied with the composition of the oxide. The
γ-bismuth molybdate (γ-Bi2MoO6) in both Ca6Bi6Mo12 oxide
and Ca9Bi3Mo12 oxide is considered to be the active phase for
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