Study of Te and V as counter-cations in Keggin type phosphomolybdic polyoxometalate catalysts for isobutane oxidation

Article abstract of DOI:10.1016/j.jcat.2008.11.013

Keggin-type phosphomolybdic acid and cesium salts with protons partially substituted by tellurium and by both tellurium and vanadium have been prepared, characterized using several techniques and tested as catalysts in the partial oxidation of isobutane i

Full text of DOI:10.1016/j.jcat.2008.11.013

Journal of Catalysis 261 (2009) 166–176
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Journal of Catalysis
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Study of Te and V as counter-cations in Keggin type phosphomolybdic
polyoxometalate catalysts for isobutane oxidation
∗
Q. Huynh, Y. Schuurman, P. Delichere, S. Loridant, J.M.M. Millet
Institut de Recherches sur la Catalyse et l’Environnement de Lyon, IRCELYON, UMR5256 CNRS-Université Claude Bernard Lyon 1, 2 avenue A. Einstein, F-69626 Villeurbanne cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 July 2008
Revised 12 November 2008
Accepted 12 November 2008
Available online 11 December 2008
Keggin-type phosphomolybdic acid and cesium salts with protons partially substituted by tellurium and
by both tellurium and vanadium have been prepared, characterized using several techniques and tested
as catalysts in the partial oxidation of isobutane into methacrylic acid (MAA). The results showed that
tellurium when introduced as counter-cation was present as Te4+ capping the Keggin anion and was
randomly distributed in the acid or in the cesium salt. This cation induced a positive effect on the
selectivity to MAA and methacrolein (MA) without significant effect on the activity except in the acid
at low loading where it also increased the activity. The co-substitution of protons by vanadyl cations had
a slight effect on the selectivity but increased the activity especially at low level of substitution, which
led to a very efficient catalyst. Selectivity to MAA and MA and isobutane conversion rate of 65 and 17%
Keywords:
Isobutane oxidation
Polyoxometalates catalyst
Heteropolyacid
Methacrylic acid
Vanadyl and tellurium countercation
◦
respectively were reached at 350 C and were both very constant with time on stream. The catalytic
results obtained in both stationary and transient conditions allowed to propose a reaction mechanism
very close to one already proposed with four intermediates amongst which one is common to both MAA
and MA. These results were used to understand the catalytic effect of tellurium and vanadium.
© 2008 Elsevier Inc. All rights reserved.
1. Introduction
tune the acidic and redox properties of these sites to optimize
their catalytic properties.
This has been used to design very efficient catalysts for isobu-
tane partial oxidation most of them being molybdenum-based het-
eropolyacids with Keggin structure and a complex formula that en-
compasses partial substitutions of the central (As to P) or addenda
atoms (V to Mo) in the anion and several metallic counter-cations
[6–10]. The individual effect of these substituting elements is not
clearly understood and the catalysts obtained while being efficient
enough for an industrial application, lack stability.
In this work, we have undertaken the rational design of a
new catalyst formula based on the effect of the partial substi-
tution of protons in either pure phosphomolybdic heteropolyacid
or mixed protons and cesium salt, by tellurium and vanadium
cations. Tellurium was selected for its hydrogen abstracting effi-
ciency reported in the literature on oxidation catalysts [11]. Vana-
dium cations when substituting Mo cations in the Keggin structure
were shown to lead to more efficient catalysts [12–14]. However,
vanadium substituted phosphomolybdic anions are not stable over
long-term use at the catalytic reaction temperature and a restruc-
turing of the compound with the diffusion of vanadium in counter-
cationic position and a partial destruction of the anions occurs
and is detrimental to the catalytic properties [12,15]. Consequently
we chose to prepare compounds with pure phosphomolybdic an-
Selective oxidations of light alkanes or olefins can be considered
as processes consisting of several successive steps corresponding
either to dehydrogenation of the organic molecule or oxygen in-
sertion. Such complex reactions are generally achieved using multi-
component catalysts, with different catalytic sites able to carry out
all these steps. The different reaction steps can also be achieved
on sites of different phases that cooperate, enhancing the catalytic
performances with respect to each component phase and leading
to the well-known synergetic effect [1,2]. However, having each re-
action step occurring on catalytic sites at the surface of the same
phase is preferred. This is particular true for isobutane oxidation
reaction to methacrylic acid for which it has been proposed that no
or few intermediate compounds that are susceptible to desorb and
be selectively further transformed on another phase are formed
[3].
Polyoxometalates, which are the best catalysts for this reaction,
have the unique ability to undertake partial substitution in the an-
ionic structure as well as in cationic position [4,5]. This allows
generating different catalytic sites on the same compound and to
Corresponding author.
E-mail address: jean-marc.millet@ircelyon.univ-lyon1.fr (J.M.M. Millet).
*
0021-9517/$ – see front matter © 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2008.11.013

Q. Huynh et al. / Journal of Catalysis 261 (2009) 166–176
167
ions and introduce directly the vanadium in counter-cationic posi-
tions.
Table 1
Parameters used for the Mo 3s and V 2p3/2 decomposition in the XPS spectra.
Peak
Binding energy
(eV)
Line width
(eV)
Line shape
(GL%)a
2. Experimental
2.1. Preparation of the catalysts
Mo 3s
V 2p3/2
510.65 0.1
517.50 0.15
5.9 0.1
2.5 0.1
30
30
a
GL corresponds to the % of Gaussian in Gaussian–Lorentzian fit.
The prepared compounds corresponded to pure Keggin-type
phosphomolybdic acids or salts. They are all constituted of
(PMo12O40) anions with protons, Cs, VO and Te as counter-cations
and designated according to their metal counter-cation content.
X-ray diffraction patterns were recorded using a Siemens D5005
◦
diffractometer and CuKα radiation with 0.02 (2θ) steps over the
◦
3–80 angular range with 1 s counting time per step.
For example, Cs2Te0.2 denotes
a solid with the stoichiometry
Surface analyses by X-ray photoelectron spectroscopy (XPS)
were performed in a Kratos Axis Ultra DLD equipped with a mag-
netic immersion lens, a hemispherical analyzer and a delay line de-
tector. Experiments was performed using a monochromated AlKα
X-ray source, with a pass energy of 20 eV and a spot size aperture
Cs2Te0.2HxPMo12O40, x depending of the reduction level of molyb-
denum cations. HxTey, (NH4)1Te1, Cs2Tex and Cs2TexVy compounds
have been prepared. The tellurium containing phosphomolybdic
acid samples have been synthesized using a method described for
the pure acid [16]. 5.64 g of MoO3 were dissolved in 150 mL of
hot water. 0.34 g of phosphoric acid and various amounts of tel-
of 300 × 700μm with a pressure in the analysis chamber below
−8
5 × 10
Pa. XPS spectra of Mo 3d, Mo 3s and V 2p levels were
◦
lurium were added to the solution and maintained at 80 C under
◦
measured at 90 (normal angle with respect to the plane of the
surface). A charge neutralizer was used to control charges effects
on samples. V 2p3/2 and Mo 3s peaks have been used to determine
the surface V/Mo atomic ratio. Because these peaks were very close
to each other, the spectra have been decomposed using parameters
for the Mo 3s peak determined with respect to the Mo 3d peak
(Table 1).
◦
reflux for 12 h at 80 C. The final yellow solution was evaporated
under reduced pressure and the solid obtained, dried at 125 C and
◦
◦
calcined under a flow of air at 360 C for 6 h.
To synthesize tellurium containing cesium salts, 140 mL aque-
ous solutions of 8.16 g of phosphomolybdic acid (FLUKA 79560)
and various amounts of telluric acid were prepared. To this solu-
tion, 0.4 mL of a second aqueous solution containing 1.3 g of ce-
sium carbonate was added dropwise and a precipitate was formed.
EXAFS spectra at the Te K-edge of the (NH4)1Te1 compound
before and after heat treatment have been recorded on the exper-
imental station D44 (LURE-DCI storage ring, Orsay, France) using
Si/Li mono-element Eurysis solid-state detector and a Si(111) dou-
ble crystal monochromator. The acquisitions were performed in the
range of 31650–32900 eV. The spectra were analyzed by a stan-
dard procedure of data reduction using the IFEFFIT code [19]. For
the EXAFS analysis, structural parameters were obtained from least
squares fitting in R-space, using amplitude and phase of backscat-
tering as well as mean-free-path taken from FEFF codes [20].
XANES V and Te K-edge spectra of the Cs2Te0.2V0.2 compounds
have been recorded before and after catalytic testing. The analy-
ses of V and Te K-edge spectra were based on the observations of
the pre-edge and edge positions respectively. For vanadium spec-
tra, a linear relation between the energy position of the pre-peak
and the formal oxidation state of vanadium has been observed [21,
22]. It allowed determining the average oxidation state of vana-
dium in the Cs2Te0.2V0.2 compound by a simple measure of the
position of the center of mass of the pre-peak. For the Te spectra,
the edge position has been compared to that of reference com-
pounds TeO2 and (NH4)6TeMo6O24·7H2O collected under the same
conditions.
◦
After 1 h under stirring at 80 C, the solution was dried at re-
duced pressure and the solid obtained, treated similarly to the
phosphomolybdic acid samples [17]. An ammonium salt with tel-
lurium has also been synthesized using a similar protocol except
that the precipitate was obtained by adding a 37% chloridric acid
solution (5 mL) to a 150 mL aqueous solution of ammonium hepta-
molybdate (8.27 g), phosphoric acid (0.53 g) and telluric acid (0.89
g). This compound has not been studied as catalyst but used for
the characterization of the environment of the tellurium cations in
counter-cation position by X-ray absorption spectroscopy (XAS).
The vanadium containing compounds were prepared by main-
taining the hydrated acid or cesium salt synthesized as described
above for 6 h, at room temperature and under stirring in a solu-
tion of toluene containing vanadium acetylacetonate. The phospho-
molybdic compounds were not soluble in toluene and an exchange
of the protons of the compounds by the vanadium cations took
place [18]:
+
V[C5O2H7]3 + 3H + 6H2O → V3+[H2O]6 + 3CH3COCH2COCH3.
◦
The solids dried under reduced pressure at 60 C were not calcined
but directly heated under the catalytic reaction flow when tested.
The compounds were designated according to their metal counter-
cation content.
2.3. Catalysts testing
The oxidation of isobutane to methacrylic acid was carried out
at atmospheric pressure in a dynamic differential micro-reactor
containing 1 to 2 g of catalyst. A gas feed O2/iBu/H2O/N2 = 27/
2.2. Characterization techniques
Chemical analysis was determined by atomic emission for Mo,
V, Te, and P, using an induced plasma technique, and for Cs, us-
ing an air–acetylene flame. The specific surface areas were deter-
13.5/10/49.5 kPa was used with
a total flow rate equal to
6 cm3 min−1; the gases at the outlet of the reactor were analyzed
on line by gas chromatography using a TCD gas chromatograph
with CP-Molsieve 5 Å, Silicaplot and Porapak Q columns. The or-
ganic substrates were condensed during the reaction and analyzed
off-line using a chromatograph equipped with a FID detector and a
CPWAX58/FF column. During the first hours, all the catalysts were
undergoing a deactivation and the catalytic data were thus system-
atically collected after at least 8 h. Methacrolein (MA), methacrylic
acid (MAA), acetic acid (AcA), CO, and CO2 were the five major
by-products formed under the reaction conditions used. Traces
of acetone and acrylic acid have been detected, but most of the
◦
mined by the BET method using nitrogen adsorption at −196 C.
The infrared spectra were measured as KBr pellets on a Brücker
Vector 22 FTIR spectrometer. Raman spectra were recorded using a
LabRam HR spectrometer (Jobin Yvon) equipped with a CCD detec-
+
+
tor. The exciting line at 514.53 nm of an Ar –Kr ion laser (Spectra
Physics, Model 2018RM) was focused on the samples using a ×100
objective. The influence of the laser on the heating of samples has
been previously investigated and a very limited power of 0.1 mW
has been used. The homogeneity of the samples has been checked
by performing several analyses for each samples.

168
Q. Huynh et al. / Journal of Catalysis 261 (2009) 166–176
Table 2
Chemical analyses and BET surface areas of the synthesized solids.
Compound
Atomic ratios
12Cs/Mo
BET surface
−1
)
area (m2
g
12V/Mo
12Te/Mo
12P/Mo
H3
-
–
–
–
–
–
–
–
–
–
–
–
–
0.6
1.0
1.5
0.9
–
0.07
0.20
0.50
0.24
0.25
0.24
0.31
–
1.2
1.3
1.7
1.9
–
H1,8 Te0,6
1.2
1.2
1.3
1.2
1.3
1.1
1.3
1.1
1.3
1.1
1.3
1.2
H1Te1
Te1.5
(NH4)1Te1
Cs2H1
Cs2Te0.05
Cs2Te0.2
–
2.1
2.2
2.1
2.2
2.0
2.2
2.1
2.0
11.1
14.4 (27.4)a
9.2 (20.4)a
6.9 (11.7)a
8.3
Cs2Te0.5
–
Cs2Te0.2V0.05
Cs2Te0.2V0.10
Cs2Te0.2V0.20
Cs2Te0.3V0.10
0.06
0.12
0.21
0.10
10.7
12.0
11.4
a
Values in parentheses correspond to surface areas before catalytic test.
time, were not considered in the calculations. The carbon bal-
ances were always between 96 and 100%. The analysis of acids was
checked by chemical analysis using phenolphthalein as indicator.
Liquid NMR analysis of the condensed products has periodically
been used to insure that no other products were formed even as
traces.
Fig. 1. X-ray diffraction pattern of the compound Cs2Te0.2
.
In order to gain information on the reaction mechanism, exper-
iments have been conducted using the method of TAP (Temporal
Analysis of Products). Transient pulse experiments were carried out
in a commercial TAP-2 reactor system (Mithra Technologies Inc.)
[23]. The system was equipped with four high-speed pulse valves,
a liquid trapped vacuum system, and a quadrupole mass spec-
trometer located directly underneath the micro-reactor exit. The
reactor is typically loaded with 40 mg of 0.2–0.5-mm-size catalyst
in the centre of the reactor between two layers of 0.2–0.3-mm-size
quartz. Lean mixtures were simulated by introducing alternatively
pulses of isobutane and oxygen or 1:1 water–oxygen mixture, over
◦
the bare surface of the catalysts at 360 C, until steady-state cover-
age was reached.
3. Results
3.1. Chemical analyses and BET surface area measurements of the
catalysts
Fig. 2. Variation of the cell parameter of the Cs2Tex compounds as a function of the
tellurium content.
The chemical analyses data of the prepared samples (salts and
acids) are presented in Table 2 with the specific surface area
measured after catalytic test. The results of the chemical analy-
ses were in good agreement with the theoretical stoichiometries.
The specific surface areas of the acids samples were rather low
and increased slightly with the tellurium content. Those of the
cesium salts were higher but decreased with the tellurium con-
tent. In the case of the cesium salts, the surface areas of sev-
eral compounds have also been determined before catalytic test.
The results obtained showed that a strong decrease of the sur-
face area was observed during catalytic testing. Such sintering
effect, which occurred when the catalysts were brought into the
reaction conditions has already been observed by several authors
[24,25]. It explained the strong decrease of the activity of this type
of catalysts, taking place systematically during the first hours on
stream. The chemical analyses of the Cs2Te0.2 samples with vari-
ous amounts of vanadyl cations as counter-cations were in good
agreement with the desired stoichiometries. Their specific surface
areas measured after catalytic test were comparable, although they
slightly increased with the vanadium content. The chemical anal-
ysis of several samples has been performed after catalytic testing.
They all showed that the chemical composition did not vary under
catalytic reaction and more specifically no loss of tellurium was
detected.
3.2. X-ray diffraction and XAS spectroscopy characterization of the
catalysts
The X-ray diffraction patterns of the tellurium containing
◦
acids and cesium salts after heat treatment at 360 C have been
recorded. The diffractograms of the Cs2Te0.2 compound is given in
Fig. 1 to illustrate the data obtained. The patterns of the acid sam-
ples were similar to those previously reported for the acid phases
hydrated at about 8 and 1.5 H2O [26,27]. The cesium salts patterns
exhibited only the cubic phase (Pn3m) commonly associated with
the pure alkaline heteropolysalts [28]. The cell parameter of the
cubic phase has been calculated for the different cesium salts of
the series (Fig. 2). It decreased linearly when the tellurium content
increased which tends to show that tellurium was randomly dis-
tributed in the cesium salt and that the phase corresponded to a
solid solution. The X-ray diffraction patterns of the vanadium con-
taining compounds were similar, exhibiting only the cubic phase
with no significant variation of the cell parameter with the vana-
dium content.
In order to characterize the local environment of Te cations in
the heteropolycompounds, a tellurium containing ammonium salt
(NH4)1Te1 has been prepared and characterized by EXAFS at Te
K-edge just after precipitation and after heat treatment. Its X-ray

Q. Huynh et al. / Journal of Catalysis 261 (2009) 166–176
169
Fig. 3. Magnitudes of Fourier-transformed spectra of the Te K-edge spectra of the
compound before (top part) and after (bottom part) heating under air at 360 C;
Fig. 4. Te K-edge XANES spectra of the Cs2Te0.2V0.1 compound before (b) and after
catalytic test (d) and of reference phases (NH4)6TeMo6O24 (a) and TeO2 (c).
◦
experimental data full lines and calculated ones dashed lines.
Table 3
Te K-edge EXAFS fit results for the (NH4)Te compound before and after heat treat-
ment and edge energy calculated from the XANES spectra compared to reference
phases.
Compounds
Site
Edge energy
(eV)
R
(Å)
σ2
(Å2)
N
(NH4)6TeMo6O24
Te6+
Te6+
Te4+
Te4+
31,838.3
31,838.3
31,829.6
31,829.2
◦
(NH4)Te-25
TeO2
(NH4)6Te-360
C
1.92
1.90
0.0019
0.0040
6
4
◦
C
diffraction after heat treatment corresponded to that of a cubic
phase and its chemical composition was in agreement with the
desired one (Table 2). Fig. 3 shows the modulii of the Fourier
transformed spectra of k3-weighted EXAFS of the solids. A good
fit of the first peak was found when considering 6 Te–O dis-
tances at 1.92 Å for the solid before heat treatment and 4 Te–O
distances at 1.90 Å after heat treatment (Table 3). Before heat
treatment, the Te cation should be surrounded by 6 OH groups
like in H6TeO6 and the observation in the infra-red spectrum be-
Fig. 5. V K-edge XANES spectra of the Cs2Te0.2V0.1 compound before (a) and after
catalytic testing (b).
sides the bands corresponding to the Keggin heteropolyanion [29]
−1
of a band at 668 cm
attributed to a Te–O–H vibration confirmed
this interpretation (data not shown) [30]. It is likely that telluric
acid has been precipitated besides the heteropolycompound. Fur-
thermore, the XANES spectrum of the compound showed that tel-
lurium was +6 before heat treatment (Table 3, Fig. 4). After heat
treatment, both the oxidation state and the environment of tel-
lurium have changed. Tellurium was present as Te4+ cation with
4 close oxygen neighbors forming with the electron lone pair E
a distorted trigonal bipyramid as in several compounds like crys-
talline α or βTeO2 [31]. This was confirmed by the profile of the
XANES spectrum obtained with the absence of a shoulder on the
high-energy side of the first peak similarly to that simulated for
such [TeO4] clusters with FEFF8 [32]. To exhibit such environ-
ment with the metal–oxygen bond determined, Te cation should
be capping Keggin anions and be bound to four oxygens of the
same Keggin unit. This coordination has already been observed for
vanadyl or molybdyl cations when capping the same type of anion
[33,34].
5). This result was compatible with the binding energy attributed
to V4+ measured by XPS. It may be postulated that vanadium was
present in counter-cation position as vanadyl cations capping the
Keggin anions as shown previously [34].
3.3. XPS and Raman spectroscopy characterization of the catalysts
Raman spectroscopy has been used to characterize the Cs2Tex
compounds and the Cs2Te0.2V0.1 compound, before and after cat-
alytic testing; the spectra of several compounds are presented in
Fig. 6. The spectrum of the Cs2H1 compound, which corresponded
to that of the cesium Keggin-type phosphomolybdic salt showed
few changes before and after catalytic testing [36]. On the contrary,
all the tellurium containing compounds exhibited after catalytic
testing besides peaks characteristic of the Keggin-type salt, new
peaks at 1024, 1008, 844, 790, 732, 676, 628, 545, 495, 306, 283
−1
The XANES spectra at V K-edge recorded for the Cs2Te0.2V0.1
compounds before and after catalytic testing showed an average
oxidation state for vanadium respectively equal to 4 and 4.1 (Fig.
and 183 cm
(Table 4). These peaks, which were only weakly
visible before catalytic test and more intense after, did not corre-
spond to MoO3 neither to heteropolyanions fragments as described

170
Q. Huynh et al. / Journal of Catalysis 261 (2009) 166–176
Table 4
Raman bands positions in the spectra of the compound Cs2Te0.2 before and after catalytic testing. Assignments of the main bands 1: νs Mo=Od; 2: νas Mo=Od; 3: νs P–O;
4: νas Mo–Ob–Mo.
−1
Compound Frequencies (cm
Cs2Te0.2av. 1607 1560
)
1
2
3
4
990 973 961 880
607
386 348
258 238 229
168 137
Cs2Te0.2ap. 1600 1572 1024 1008 991 974 962 880 844 790 732 676 628 605 545 495 386 348 306 283 258 238 227 183 170 143
Table 5
◦
◦
Results of the XPS analysis of the tellurium and cesium salts analyses before catalytic testing (after calcination at 360 C) and after catalytic testing at 340 C in standard
conditions.
Catalyst
Cs2H1
Element
Binding energy (eV)
Before testing
Atomic ratios
Before testing
M/P
After testing
After testing
M/P
Mo5+/(Mo5+ + Mo6+
)
Mo5+/(Mo5+ + Mo6+
)
Cs 3d5/2
Mo 3d5/2
724.1
233.2
231.4
134.2
724.1
232.9
230.9
134.2
0.97
1.08
0.10
0.17
P 2p
Cs2Te0.05
Cs2Te0.2
Cs2Te0.5
Cs 3d5/2
Te 3d5/2
Mo 3d5/2
724.2
577.7
233.3
231.5
134.5
723.9
577.4
233.8
232.5
134.3
1.33
0.04
1.53
0.08
0.03
0.10
0.09
0.02
0.05
0.07
P 2p
Cs 3d5/2
Te 3d5/2
Mo 3d5/2
724.2
577.5
233.3
231.2
134.2
724.1
477.4
233.8
232.5
134.2
2.00
0.22
2.00
0.22
P 2p
Cs 3d5/2
Te 3d5/2
Mo 3d5/2
724.2
577.2
233.3
231.4
134.2
724.0
577.5
233.9
232.6
134.2
1.82
0.46
2.07
0.51
P 2p
The surface chemical composition of several Cs2Tex compounds
has been determined by XPS. The results showed that the surface
Te and Cs compositions were comparable to the bulk ones (chemi-
cal analyses) confirming the formation of solid solutions (Table 5).
The molybdenum (Mo 3d5/2) peaks could be decomposed into two
peaks corresponding to Mo5+ and Mo6+ species. All the solids
appeared reduced at the surface but no relation between the pres-
ence of tellurium and the relative ratio of Mo5+ could be drawn.
After catalytic testing the surface composition as well as the ratio
of Mo5+ did not really vary and no loss of tellurium was observed.
It may be noted that for the catalysts after catalytic testing with
a high Te content, the best fits of the Mo signal were obtained
with a second Mo6+ with a larger binding energy (234.00 eV).
These new species, which represented only few % (<6%) of the to-
tal Mo species have not been interpreted and are presently under
study.
The variation of the surface vanadium content as a function of
the bulk vanadium content of the Cs2Te0.2Vx compounds has been
the focus of a specific study. A surface enrichment compared to
the bulk was evidenced particularly at low V content (Fig. 7). It
could be related to the method of addition of vanadium as counter-
cation, which proceeded through a cationic exchange without dis-
solution of the cesium and tellurium salt.
3.4. Isobutane oxidation
Fig. 6. Raman spectra of the compounds Cs2H, Cs2Te0.2 and Cs2Te0.2V0.1 before (a,
b, c) and after catalytic testing (d, e, f).
The catalytic results obtained for the synthesized compounds
in conventional testing apparatus are presented in Table 6. They
showed that the substitution of protons by tellurium in the acid
and the cesium salt has a strong positive effect on the selectiv-
ity to methacrylic acid (MAA) and methacrolein (MA). In parallel,
a small decrease of the activity was observed for the acid and a
by Mestl et al. [15]; these new peaks have been observed for
the reference sample Mo(H2O)2(MoO)2PMo12O40 and attributed to
capped heteropolyanions bonds vibrations [37].

Q. Huynh et al. / Journal of Catalysis 261 (2009) 166–176
171
Fig. 8. Variation of isobutane conversion (2) and selectivity to AcA (P), CO (e), CO2
(!) and MAA + MA (1) as a function of the vanadium content of the Cs2Te0.2 salt
◦
Fig. 7. Bulk and surface composition in vanadium determined from chemical analy-
ses and XPS data.
at 335 C; contact time 4.8 s, catalyst mass 2 g, iBu/O2/H2O/N2 = 27/13.5/10/49.5.
Table 6
Catalytic properties of Te substituted acid and the phosphomolybdic cesium salt.
Testing conditions: contact time 4.8 s, mass of catalyst 2 g, composition of the reac-
tion mixture iBu/O2/H2O/N2 = 27/13.5/10/49.5. AcA: acetic acid, MA: methacrolein,
MAA: methacrylic acid.
Catalyst
Tempera-
ture
Conver-
sion
(%)
1.3
3.5
4.4
Selectivity (%)
Yield (%)
AMA +MA
CO
CO2
AcA
MAA
MA
◦
( C)
H3
310
345
355
16
23
32
45
44
34
11
15
18
11
9
6
16
9
10
0.4
0.6
0.7
H1.8 Te0.6
310
330
355
2.3
3.9
6.3
11
14
19
15
16
18
9
10
13
27
25
27
39
36
22
1.5
2.4
3.1
H1Te1
325
340
360
2.6
3.4
4.8
4
6
10
7
10
13
4
4
5
41
40
33
43
40
39
2.2
2.7
3.5
Te1.5
310
345
355
2.2
3.4
4
7
8
11
15
14
15
4
6
7
46
41
37
28
31
30
1.6
2.4
2.7
Fig. 9. Effect of addition of water on the isobutane conversion (2) and on the selec-
tivity to methacrolein and methacrylic acid (1) at 330 C.
◦
Cs2Te0.05
Cs2Te0.2
Cs2Te0.5
324
347
369
7
9.9
14.4
22
28
27
23
21
23
7
11
11
33
30
25
15
10
13
3.4
3.9
5.4
Table 7
Catalytic properties of the compound Cs2Te0.3V0.1. Testing conditions: contact time
4.8 s, mass of catalyst 2 g, composition of the reaction mixture: iBu/O2/H2O/N2
27/13.5/10/49.5. AcA: acetic acid, MA: methacrolein, MAA: methacrylic acid.
=
330
352
367
7
10
12
9
13
15
9
12
13
5
7
10
60
55
50
16
13
12
5.3
6.8
7.4
Catalyst
Tempera-
ture
Conver-
sion
(%)
10.4
14.9
16.1
Selectivity (%)
Yield (%)
AMA +MA
CO
CO2
AcA MAA MA
◦
( C)
325
345
370
5.4
7.9
10.8
4
5
9
6
7
10
4
5
10
71
67
52
13
16
19
4.2
6.6
7.8
Cs2Te0.3V0.1
330
345
350
5
11
12
9
13
14
6
8
9
66
56
54
14
12
11
7.6
10.1
10.5
optimal yield for the same water content. These results suggest
that water influences not only the number or accessibility of the
active sites but also these sites intrinsic properties. The decreased
conversion observed at high water content could arise from a pref-
erential adsorption of water on these sites.
The catalyst composition has been optimized varying concomi-
tantly the vanadium and tellurium contents. The best properties
have been obtained for the compound Cs2Te0.3V0.1 (Table 7) [38].
The influence of the gas phase composition in terms of isobutane
to oxygen ratio and dilution into nitrogen, has been studied on this
catalyst. The results obtained are shown in Fig. 10.
small increase for the cesium salt. In the later case the conver-
sion decreased, but since the specific surface area decreased even
more significantly, the intrinsic rate of isobutane transformation
increased. The substitution of protons by vanadyl cations in the
Cs2Te0.2 catalysts increased the conversion with only a slight de-
crease of the selectivity in MAA and MA (Fig. 8).
The effect of the addition of water to the gas feeds has been
studied on the Cs2Te0.2 sample (Fig. 9). Isobutane conversion in-
creased with increasing water pressure up to 10% and then de-
creased. A positive effect on the selectivity toward MAA and MA
was observed. This effect was more pronounced at low water con-
tent, which led to an optimal yield in MAA and MA with a water
content of 10%. The effect of water has been studied on the vana-
dium containing catalysts. It appeared to be the same with an
To better understand the reaction mechanism, we have con-
ducted a study using a TAP reactor. We have pulsed alternatively
isobutane and O2 or O2 + H2O (in Ar 50%) over a Cs2Te0.2 cata-

172
Q. Huynh et al. / Journal of Catalysis 261 (2009) 166–176
(a)
(b)
Fig. 10. Schematic representation of the charge composition and evolution of the isobutane conversion (2), selectivity to MAA (Q), MA (F), AcA ("), CO (!), CO2 (1) as a
◦
function of the i-C4H10 /O2 ratio (a) and dilution in N2 (b) on Cs2Te0.3V0.1, at 340 C in the standard testing conditions.
(a)
(b)
(c)
(d)
Fig. 11. Mass intensity vs. time curves of desorbing reaction intermediates produced by (a) pulsing successively isobutane and a 1:1 O2/H2O mixture (after 0.5 s) over Cs2Te0.2
◦
◦
at 360 C, (b) pulsing successively isobutane and a 1:1 O2/H2O mixture (after 0.5 s) over Cs2Te0.2 at 360 C; (c) and (d) correspond to the normalized responses, respectively,
obtained on isobutane pulses.
◦
lyst at 360 C. In all cases the observed conversion was rather low,
probably due to both the low contact time in the TAP reactor and
the low concentration of species adsorbed at the surface of the cat-
alyst. When O2 was pulsed alternatively with isobutane, only traces
of CO2 were obtained on the O2 pulse, but CO, CO2 (major prod-
uct), water and methacrolein were formed on the isobutane pulse
(Fig. 11a). Fig. 11b, where the peak maxima were normalized to fa-
cilitate time comparisons, shows the pulse response sequence of

Q. Huynh et al. / Journal of Catalysis 261 (2009) 166–176
173
the reactants and products: first isobutane, followed by isobutene,
then methacrolein and carbon oxides. No methacrylic acid was de-
tected. The peak of carbon dioxide was large, probably because it
arised mainly from the subsequent transformation of the adsorbed
intermediates into CO2 on the catalyst surface.
When O2 +H2O (1:1 ratio) was pulsed instead of O2, methacry-
lic and acetic acids were observed on the isobutane pulse (Figs. 11c
and 11d). These results showed that the acids were strongly ad-
sorbed at the surface and that water was a key parameter in their
desorption. The products always came out in the order “isobutane,
isobutene, methacrolein and CO2.” When the acids were formed,
their signals came after that of methacrolein although some acid
appeared at the beginning of the pulse. This observation led us
to postulate that a certain amount of acid was already present on
the surface before the adsorption and was simply displaced by the
later.
catalyst report that isobutene is oxidized about 500 times faster
than it is formed via isobutane [10]. Secondly the catalytic oxi-
dation of isobutene did not take place just at the surface of the
polyoxometalates catalysts but a significant number of surface lay-
ers participated to the reaction. This was clearly shown when the
oxidation of isobutene was studied by DRIFT on this type of com-
pounds (data not shown). This feature increases the probability for
isobutene molecules to be oxidized before reaching and leaving the
surface.
The pathway proposed with two main adsorbed intermediates
corresponding respectively to an alkoxide and a dioxyalkyldiene
appears to be a good working model and this proposal was taken
as the basis [4]. However we propose a slightly different and more
complete pathway with the existence of other adsorbed interme-
diates, which can lead to isobutene and to MAA and MA as shown
in the following scheme,
4. Discussion
The characterization of the phosphomolybdic acid and cesium
salt with tellurium and vanadium as counter-cations showed that
the later elements were present as Te4+ and (VO)2+ cations cap-
ping the phosphomolybdic Keggin unit. Tellurium was randomly
distributed in the acid and the cesium salt whereas en enrichment
in vanadium was observed at the surface. The tellurite entities
corresponded thus to TeO4 E with one lone electron pair (E) stere-
ochemically active and credibly oriented in opposition to the four
oxygen of the Keggin unit similarly to the oxygen of the vanadyl
species. To balance charges in the capped anions, molybdenum
cations have to be reduced. It was not possible to determine the
extent of reduction of Mo in the capped anions neither whether
the anions are single or bi-capped as it has been shown to be pos-
sible in several compounds [33,34]. In the Cs2Tex compounds, XPS
analyses showed a reduction of molybdenum corresponding ap-
proximately to 5 Mo5+ per Te cation except at high Te content
where it is lower.
and in Fig. 12. The pulse responses of isobutane, isobutene and
methacrolein (MA) obtained from TAP experiments have been
modeled according to the scheme (Fig. 13). This scheme contains
two parallel routes, one through surface intermediates, the second
through the formation of gas phase isobutene. The oxidation of
MA has not been taken into account in the modeling and therefore
the predicted MA response is much broader than the experimen-
tal one. The values of the apparent rate constants for the differ-
ent steps are reported in the same scheme. The low conversion
leads to rather large errors on the individual rate constants, but
the relative rates still give a good indication on the different re-
action routes. The ratio of the values for the surface reaction of
I0 to I1 to the desorption rate of I0 into isobutene amounts to
100, thus favoring a surface route rather than the gas phase route.
This explained that isobutene was not observed in steady-state ex-
periments but completely converted due to the significant longer
contact times than in the TAP reactor. The ratio of 10 between the
adsorption of isobutane to the adsorption of isobutene explains the
much higher reactivity for isobutene. This ratio was however lower
than what was observed by Schindler et al., probably due to the
very different conditions in the TAP reactor.
The intermediate I2, which is common to both methacrylic acid
and methacrolein would undergo either a dissociation of a C–O
bond leading to methacrolein or an hydration followed by a subse-
quent dehydrogenation to form methacrylic acid as shown in Fig.
12. Water in such case would not only help to the desorption of
the reactions products but also be involved in the reaction mech-
anism. This could add an explanation to the positive role of water
both on the activity and the selectivity of the catalysts. The results
of the TAP experiments however tend to show that its main role is
the first one since the observations of methacrylic and acetic acid
in the products both completely depend upon the addition of wa-
ter in the pulse.
This could be consistent if single or bi-capped anions are con-
sidered, with respective formula of the type [PMo54+Mo86+O40
-
Te4+ 3−
]
or [PMo58+Mo46+O40Te4+
]
. At high Te content all Te
3−
2
cations may not be capping the heteropolyanions, which would
explain the lower reduction rate of Mo observed by XPS. With ad-
dition of vanadium, [PMo56+Mo66+O40(VO2+)2]5− polyanions could
be formed as reported previously [35]. In compounds with or with-
out cesium, protons would balance anions charges. Complementary
characterization have presently been undertaken in order to de-
termine the oxidation state of molybdenum cations and the exact
structure of the anions.
The results of the isobutane oxidation over the studied com-
pounds revealed that either in the acid form or as the cesium salt,
tellurium has a strong positive effect on the selectivity to MAA and
MA whereas vanadium has an effect on the activity.
The nature of the real pathway in the oxidation of isobutane
into methacrylic acid has been extensively debated but controver-
sies still remain. Several authors proposed the intermediate forma-
tion of isobutene [10,39,40] while others assumed a direct forma-
tion of methacrolein [2,4,41,42]. Among the latter, some authors
although they do not include isobutene in the reaction network,
do not exclude its formation but with a rapid further oxidation
[2,4]. We did not detect any isobutene in our conventional testing
experiments, like in most of the experiments of the same type re-
ported in the literature, but our TAP experiments clearly showed
that isobutene was first formed and should be considered as an
intermediate. Two reasons may explain why isobutene was not
observed in conventional testing experiments. First isobutene is
oxidized much faster than isobutane. Schindler et al. who have
studied the oxidation of isobutane and isobutene on the same
In view of the results of the catalyst testing study, the role of
tellurium may be clarified. The variation of selectivity to MA has
been plotted as a function of the selectivity to MAA for all the
acids and the cesium salts tested (Fig. 14). Over the cesium salts,
upon increasing Te content the selectivity to MAA alone increased.
Over the acids, the same feature was observed at high Te content
but at low content the selectivity to MA also increased. This led
to propose that the main role of Te was to favor the transforma-
tion of the I2 intermediate into MAA. This would explain the large
increase in MAA selectivity. The effect is all the more visible as
methacrylic acid is quite stable and acetic acid and carbon oxides

174
Q. Huynh et al. / Journal of Catalysis 261 (2009) 166–176
Fig. 12. Proposed mechanism for isobutane oxidation; the degradation reactions of MAA, MA, isobutene or intermediates have not been schematized.
are mainly formed from the oxidative degradation of methacrolein.
Te4+ should intervene in the dehydrogenation step, because of its
hydrogen abstracting efficiency already reported in many oxidation
reaction of alkenes and alkanes and common to elements possess-
ing a lone pair of electrons like Bi3+ or Sb3+ [1,2]. However it
cannot be excluded that it also played a role in the hydration step.
In the case of the acid, at low content, Te should also help the
transformation of the I1intermediate into I2. This transformation
implies dehydrogenation steps that Te could promote.
The effect of vanadium was less easy to be interpreted from the
results. The isobutane conversion increased almost linearly with
the vanadium content (Fig. 8) but when the intrinsic rate of isobu-
tane transformation is plotted as a function of the vanadium con-
tent, an increase was observed at low V content but not at high;

Q. Huynh et al. / Journal of Catalysis 261 (2009) 166–176
175
Fig. 13. Experimental (symbols) and calculated (lines) pulse responses of isobutane,
isobutene and methacrolein over the Cs2Te0.2 catalyst at 360 C.
Fig. 15. Variation of the rate of transformation of isobutane as a function of the
surface vanadium content of the Cs2Te0.2Vx compounds determined by XPS.
◦
termined by XPS is plotted as a function of the bulk composition
(Fig. 15). The V surface composition was shown to follow the same
evolution that the intrinsic rate. The surface enrichment detected
at low V content, could be related to the method of addition of
V as counter-cation, which proceeded through a cationic exchange
without dissolution of the cesium and tellurium salt. It might be
postulated that new more efficient sites were formed when vana-
dium was present. Such hypothesis is supported by the fact that a
change of the apparent activation energy for the transformation of
isobutane was systematically observed for the V containing cesium
salts (76 2 instead of 82 2 kJ/mol).
Finally, it is rather difficult to compare the catalytic properties
of our catalysts to those patented or published since turnover fre-
quencies or intrinsic rates of isobutane conversion are never given.
The only comparison that could be made is a comparison of pro-
ductivity in MAA + MA produced per gram of catalyst and per
hour. Based upon this number, the catalysts described in this pa-
per, reached productivity between 0.85 and 0.9, which is superior
to many of the patented catalysts and close to the best one with
around 1.0 obtained at the same total pressure and temperature
[43]. These results are quite encouraging, as the preparation of
the catalysts can still be improved and defined and because of the
fact that the catalysts were very stable with time on stream. This
stability, which is a very positive attribute for a possible industri-
alization, may be due to a higher stability of the heteropolyanions
in relation with their purely molybdic composition or to their cap-
ping.
(a)
5. Conclusion
The results presented in this paper showed that the substi-
tution of protons by Te in phosphomolybdic heteropolyacid and
cesium salt was possible. The characterization of obtained com-
pounds showed that after dehydration, Te cations capped the Keg-
gin anions similar as molybdyl and vanadyl cations. They have a
strong effect on the selectivity to MAA of both the Keggin phos-
phomolybdic acid and its cesium salt. In the case of the pure acid
they have also a positive effect on the selectivity to MA. Such
effect may be attributed to their hydrogen abstracting efficiency
already reported in the literature in other oxidation reactions. The
concomitant substitution of protons by vanadyl cations allowed
increasing the activity of the catalysts without loosing too much
selectivity to MAA and MA. The catalyst composition has been op-
timized and corresponds according to the characterization results
(b)
Fig. 14. Variation of the selectivity in methacrylic acid (MAA) as a function of that in
methacrolein (MA) on the different Te containing acids HxTey (a) and cesium salts
Cs2TexVy (b) studied.
this shows that the increase in activity could be partially related
to the increase of the specific surface area of the compounds but
that there was also a modification of catalytic sites. This can be
evidenced when the evolution of the V surface composition de-

176
Q. Huynh et al. / Journal of Catalysis 261 (2009) 166–176
to the formula Cs2Te0.3(VO)0.1. The reduction level of the phospho-
molybdic anions and the residual protons content has still to be
determined. The catalyst has an efficiency comparable to numer-
ous patented catalysts of the same type with a simpler formulation
and most of all a very good stability. A reaction pathway rela-
tively similar to one already published has been proposed. It takes
into account the formation of isobutene, which was clearly demon-
strated from TAP experiments.
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