Preparation and characterization of mixed ammonium salts of Keggin phosphomolybdate

Article abstract of DOI:10.1016/j.ica.2009.04.049

Keggin-type modified ammonium salts of molybdophosporic acid were prepared by partial substituting of the ammonium ions by X (X = Sb^III, Bi^III or Sn^II) ions. They were characterised by BET method, XRD, ³¹P NMR,

Full text of DOI:10.1016/j.ica.2009.04.049

Inorganica Chimica Acta 362 (2009) 3896–3900
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Preparation and characterization of mixed ammonium salts of Keggin
phosphomolybdate
a
b
a
a,*
L. Dermeche , R. Thouvenot , S. Hocine , C. Rabia
a
Laboratoire de Chimie du Gaz Naturel, Faculté de Chimie, BP32, El-Alia, 16111 Bab-Ezzouar, Alger, Algeria
Laboratoire de Chimie Inorganique et Matériaux Moléculaires, Université Pierre et Marie Curie, 4, Place Jussieu, 75252 Paris Cedex 05, France
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Keggin-type modified ammonium salts of molybdophosporic acid were prepared by partial substituting
Received 2 September 2008
Received in revised form 22 April 2009
Accepted 28 April 2009
III
III
II
31
of the ammonium ions by X (X = Sb , Bi or Sn ) ions. They were characterised by BET method, XRD,
P
NMR, UV–Vis, Raman, IR spectroscopies and thermal analysis (TG and DTA). It appeared that introducing
an element X = Sb or Sn led to partially reduced compound corresponding to the electron exchange occur-
ring between Sb(III) or Sn(II) and Mo(VI) without modification of Keggin structure.
Published by Elsevier B.V.
Available online 14 May 2009
Keywords:
Polyoxometalate
Keggin
Spectroscopies
Characterization
1
. Introduction
electron transfer during redox processes. We report here the syn-
thesis and characterization by BET method, ICP measurements,
3
1
The catalytic activity of molybdophosphoric heteropolycoum-
XRD, P NMR, UV–Vis, Raman, IR techniques and thermal analysis
(TGA and DTA) of ammonium–antimony, ammonium–bismuth
and ammonium–tin mixed phosphomolybdates with Keggin struc-
ture in the aim to find correlation between redox properties and
the nature of counter-cation.
pounds (HPA) having the Keggin structure, in oxidation reactions,
depends on their reducibility. Several attempts pointed out the
importance to have a reduced Keggin anion for obtaining of better
selectivities towards products of partial oxidation [1]. Many stud-
ies have focused on the effect of the counter-cation nature in the
polyoxometalate catalytic performances. Thus, it has been shown
that the nature of cation can play a significant role in the determi-
nation of the redox processes. It has been reported that ammo-
nium, pyridinium or quinolinium salts of phosphomolybdic acid
efficiently catalyze the oxidation of propane and isobutane [2–6].
The oxidative sensitivity of these organic counter-cations allows
to obtain a strong auto-reduction under thermal treatment and
thus to change the redox state of the HPA without addition of re-
agents. Moreover, it has been shown that using Fe(III), vanadyl
2. Experimental
2.1. Synthesis
(NH
salts (NH
4
)
3
PMo12
4
O40 (noted NH PMo12) and the mixed ammonium
n+
3+
3+
2+
4
)
x
X
y
PMo12O40 where X = Sb , Bi , Sn (noted XPMo12)
were precipitated at pH < 1 as described by Cavani et al. [11]. For
each compound a solution (A) was prepared as follows: 25 g of
ammonium heptamolybdate (0.02 mol) was dissolved in 100 mL
of water by heating and 1.15 mL of phosphoric acid (85%,
0.01mol) were added. The solution becomes pale yellow. The pH
2
+
(
VO ), antimony (Sb(III)) or cobalt (Co(II) ion as counter-cation
or incorporated in the HPA structure modifies the redox properties
of the HPA, with development of a more favourable distribution of
both reduced Mo(V) and oxidized Mo(VI) sites [1,7–10].
4 3
is then equal to 5.5. (NH ) PMo12O40 was precipitated by slow
In this study, we have used the ammonium salt of 12-molybdo-
phosphoric acid ((NH ) PMo12O40) that is micro and mesoporous
4 3
with a large surface area and which exhibits better thermal stabil-
ity than the parent acid and we have partially replaced the ammo-
nium ions by antimony, bismuth and tin in order to facilitate
addition of concentrated hydrochloric acid (ca. 24 mL, 37%) to a
solution A (final pH 0.6). The yellow compound was filtered off
and dried overnight at 50 °C. For the preparation of the mixed
ammonium salts (XPMo12), the solution A was added with
3 2
0.01 mol of XCl (Sb or Bi) or XCl (Sn) previously dissolved in
the minimum amount of concentrated hydrochloric acid. XPMo12
salt was precipitated by slow addition of concentrated hydrochlo-
ric acid until pH 0.6 (ca. 25 mL). The precipitate was filtered off and
*
Corresponding author. Tel./fax: +213 21247311.
E-mail address: c_rabia@yahoo.fr (C. Rabia).
0
020-1693/$ - see front matter Published by Elsevier B.V.
doi:10.1016/j.ica.2009.04.049

L. Dermeche et al. / Inorganica Chimica Acta 362 (2009) 3896–3900
3897
2
dried overnight at 50 °C. BiPMo12 and (NH
4
)
3
PMo12 and amount of the substituting cation (ca. 100 m /g). The mea-
O
40 are yellow
but SnPMo12 and SbPMo12 are dark green.
4
sured surface area of NH PMo12 is in agreement with literature
data [12].
2.2. Characterization
4 3
2 – The FTIR spectra of (NH ) PMo12O40 and of XPMo12, X = Bi,
Sb or Sn (Fig. 1), in the low wavenumber region (1100–
ꢀ
1
Inductively coupled plasma spectrometry (ICP) measurements
500 cm ), are very similar and the characteristic bands of the Keg-
gin structure were observed. According to Rocchiccioli-Deltcheff
were carried out using
Spectrometer.
a Perkin–Elmer, Optima 2000 D.VS
ꢀ1
et al. [13], the bands at 1063, 961, 863, 786 and 561 cm corre-
spond to as(P–O ), as (Mo–O ), as (Mo–O –Mo), as (Mo–O –Mo)
and d (P–O) vibrations respectively. In Keggin-type unit, O refers
to O atom common to PO tetrahedron and a trimetallic group;
to O atom connecting two trimetallic groups, O to O atom con-
necting two MoO octahedral inside a trimetallic group and O to
the terminal O atom. This structure has Td symmetry and corre-
BET surface area measurements were performed at liquid nitro-
gen temperature using a Micrometrics Accusorb 2100E apparatus.
Prior to each adsorption–desorption measurement, the samples
were degassed at T = 150 °C for 24 h. The specific surface areas
were determined using the linear part of the BET equation.
Infrared spectra were recorded on a Bruker IFS 66 FTIR spec-
trometer with samples prepared as KBr disks on the 400–
m
a
d
m
b
m
c
a
4
O
b
c
6
d
sponds to
a
isomer. In addition, the vibration band attributed to
ꢀ
1
ꢀ1
4
000 cm range. Laser Raman spectroscopy data were obtained
ammonium ions was observed at 1403–1407 cm . The compari-
with a Kaiser Optical Systems Holollab 5000R model, equipped
with near-IR laser diode (kexc = 785 nm) and CCD detector. The la-
ser power was adjusted to 10 mW at the sample position to pre-
vent local heating effects.
son of the IR frequency of vibration bands of XPMo12 salts
4
(Fig. 1) with those of NH PMo12 did not reveal significant differ-
ꢀ1
ence within the range 1500–500 cm . This suggests that primary
structure remains intact and that the ammonium ions are partially
replaced in XPMo12 salts by the X ions.
31P MAS NMR spectra were measured at room temperature on a
Bruker Avance 400 spectrometer. 85% H
nal reference.
3
PO
4
was used as an exter-
3 – The Raman spectra of different salts are shown in Fig. 2. The
4
spectrum of NH PMo12 in the low wavenumber region (1000–
ꢀ1
UV–Visible diffuse reflectance spectra were recorded in the
00–800 nm regions on a Varian Cary 5E spectrometer equipped
240cm ) exhibits the characteristic bands of the Keggin unit.
According to literature data [14], the bands at 988, 876, 608 and
2
ꢀ
1
with a polytetrafluoroethylene (PTFE) integration sphere. PTFE
was used as a reference.
X-ray diffraction powder patterns were obtained on a D8 Ad-
vance Bruker AXS diffractometer using Cu K
240 cm correspond to
M) and (Mo–O ) metal-oxygen vibrations respectively. The
shoulder at 971 cm is attributed to
tion of PO tetrahedron is Raman-inactive. The observed vibration
bands at 988 and 240 cm are the more intense. The Raman spec-
trum of BiPMo12 is similar to that of NH PMo12. While the partial
s d b s c
t (Mo = O ), mas (M–O –Mo), m (Mo–O –
m
s
a
ꢀ
1
d
tas(Mo = O ). The P–Oa vibra-
a
radiation and VAN-
4
ꢀ1
TEK fast detector in HTK16 ANTON PAAR room.
Thermogravimetric and differential thermal data were collected
on a SDT-2960 thermal analyzer. The thermogravimetry and differ-
ential thermal analysis experiments were performed under air flux,
using 25–70 mg samples and a heating rate of 10 °C/min.
4
substitution of ammonium ions by antimony or tin compared to
bismuth, has an influence on the metal-oxygen vibration bands.
So, in Raman spectra, excepted the
not modified, that corresponding to
intensity and its frequency is passed from 988 to 979 cm . On
m
s
(Mo–O
a
) band which was
t
s
(Mo = O
d
) decreased in
ꢀ1
3
. Results and discussion
– The chemical composition determined by ICP and surface
the other hand, the shoulder assigned to
t
as(Mo = O
d
) vibration
1
area of the heteropolycompounds studied in the present study
are shown in Table 1. The results of chemical analysis of salts were
adjusted considering 12 atoms of molybdenum per Keggin unit
according to the nature of HPA and were found in good agreement
with desired stoichiometries for phosphorous and molybdenum for
NH4+
M-Od
P-Oa
a
b
M-Oc M-Ob
BiPMo12 and (NH
4
)
3
PMo12
O40 and only for molybdenum for both
SbPMo12 and SnPMo12
O
40. In these latter’s, the amount of phos-
1
403
1
063
864
962
786
c
phorous was found in excess compared to the desired value. This
has been attributed to the existence of species containing phos-
phorous other than HPA, as it was shown in the 31P MAS NMR anal-
ysis. The ICP analysis of Bi, Sb or Sn element led to stoichiometries
coefficient given in Table 1. It is noted that the amount of bismuth
contained in the salt is very low, compared with those of tin and
antimony. This could be due to hydrolysis phenomena where the
bismuth ions could be sensible.
d
2000
1800
1600
1400
1200
1000
800
600
400
Transmittance / Wave number
The surface areas of mixed salts are significantly lower than that
2
of NH
4
PMo12 (222 m /g) and nearly independent of both nature
4
Fig. 1. FTIR spectra of NH PMo12, BiPMo12, SbPMo12 and SnPMo12.
Table 1
Chemical composition from ICP and surface area of heteropolycompounds.
2
Formula Chemical composition (wt%)
Stoichiometries coefficient
Surface area (m /g)
P
Mo
X
P
Mo
X
(NH
(NH
(NH
(NH
4
)
4
)
4
)
4
)
3
PMo12
2.988Bi0.004PMo12
0.5Sn1.25PMo12
40
0.75Sb0.75PMo12
40
O
40
1.71
1.68
1.75
2.07
58.42
57.86
38.94
33.32
1.05
1.04
1.30
2.20
12
12
12
12
222
101
96
O
40
60 ppm
4.41
4.32
0.004
0.75
1.25
O
O
100

3
898
L. Dermeche et al. / Inorganica Chimica Acta 362 (2009) 3896–3900
2
d
c
d
c
1
.5
1
b
a
9
88
71
876
.
5
0
2
40
169
9
608
500
b
a
2
000
1500
1000
0
2
00
300
400
500
600
700
800
Raman Shift (cm-1
)
λ (nm)
4
Fig. 2. Raman spectra of NH PMo12 (a), BiPMo12 (b), SbPMo12 (c) and SnPMo12 (d).
4
Fig. 4. UV–Visible spectra of NH PMo12 (a), BiPMo12 (b), SbPMo12 (c) and SnPMo12
(
d).
and the bands attributed to
b s c
mas(M–O –Mo) and m (Mo–O –M) are
not clearly visible on spectra in particular in presence of SnPMo12
.
band in the spectrum of Keggin-type compound is attributed to
different oxygen ions and to inter-anion charge transfer transitions
These perturbations were probably caused by the partially reduced
state of heteropolyanion, confirmed by the characteristic blue col-
our of reduced HPA of Keggin-type [15]. Compared to IR spectros-
copy, the Raman spectroscopy seems to be sensible to the
reduction of molybdenum (VI) induced by antimony and tin.
[
8,16–18]. On the other hand, these energies have been also asso-
ciated to cationic composition of the Keggin secondary framework
as the crystallinity of the compound and oxidation potential of the
oxometal. In addition to this broad absorption band, another large
charge transfers band was observed above 700 nm for both
SnPMo12 and SbPMo12 samples, that may be attributed to the d–
3
1
4
–
P MAS NMR technique is very sensitive to local chemical
3
1
environment and surrounding symmetry of phosphor. In
NMR analysis, the whole NH PMo12 and BiPMo12 samples gave
only one peak, at ꢀ4.4 ppm. (Fig. 3) The chemical shift of P has
been found at ꢀ4.4 ppm for H PMo12 40, a value close to those ob-
served for our samples. However, for SnPMo12 and SbPMo12 sam-
P
4
1
d transition band of d Mo(V) species in octahedral coordination
3
1
[
10,19–21]. This result suggested a partial reduction of Mo(VI) to
3
O
Mo(V) in both SnPMo12 and SbPMo12 salts corresponding to the
electron exchange occurring between Sb(III) or Sn(II) and Mo(VI)
as follows:
3
1
ples, their P NMR spectrum could be decomposed in three
peaks, one at ꢀ4.4 ppm, as major peak, and the two others, as min-
or peaks, at ꢀ0.8 and ꢀ7.0 ppm for SnPMo12 and at ꢀ2.0 and
II
VI
IV
V
III
VI
V
V
Sn þ 2Mo $ Sn þ 2Mo ; Sb þ 2Mo $ Sb þ 2Mo
ꢀ
6.0 ppm for SbPMo12. These latter’s disappear after pre-treatment
The increase of the intensity of this band corresponds to the in-
crease of the concentration of Mo(V) ions in the polyoxometalate
at 400 °C under nitrogen. This can indicate that the amount of
impurities, if it is present, it must be low. Theses impurities explain
the excess of phosphorous observed par ICP analysis for these two
salts.
[
8]. Thus, these results suggest that the reduction of this last one
is more important in the presence of tin than of antimony.
The brown colour of both SnPMo12 and SbPMo12 salts disap-
pears after a pre-treatment at 400 °C under molecular oxygen, dur-
5
– UV–Visible diffuse reflectance seems to be a very appropri-
ate technique to distinguish the electronic properties of the Mo
ions. In the UV–Vis spectra of salts (Fig. 4), a large band in the do-
main of wavelengths 200–400 nm was observed with multiplicity
for all samples. It is constituted of several components (200–250,
ing 4 h and becomes yellow, colour of the (NH
the other hand, the UV–Vis spectrum of pre-treated SnPMo12 salt
Fig. 5) shows that the intensity of adsorption band at 700 nm
4 3
) PMo12O40 salt. On
(
has decrease, indicating that the Mo(V) ions were oxidized to
Mo(VI) ions. Similar results were obtained with SbPMo12 (figure
not represented here). These results, in agreement with those of
the Raman spectroscopy, confirm that the introduction of tin and
antimony led to partially reduced phosphomolybdate.
2
70–350, and 400–450 nm ranges) having different energies asso-
ciated to ligand–metal charge transfers (LMCT) from oxygen to
Mo(VI) in the Keggin anion. This result has been also observed by
other authors that suggested that the presence of more than one
6
– The thermal stability of the salts was investigated by means
of thermogravimetric (TG) and differential thermal analysis (DTA)
-
4.4
2
-
0.8
-
7.0
-6
d
a
1
0
.5
1
-
2
c
b
b
.5
0
a
49
25
0
-25
-49
200
300
400
500
600
700
800
(
ppm)
λ (nm)
Fig. 3. 31P MAS NMR spectra of NH
SnPMo12 (d).
Fig. 5. UV–Visible spectrum of SnPMo12, (a): before thermal treatment, (b): after
thermal treatment (400 °C/4h/O ).
4
PMo12 (a), BiPMo12 (b), SbPMo12 (c) and
2

73
L. Dermeche et al. / Inorganica Chimica Acta 362 (2009) 3896–3900
3899
b
a
c
b
1
1
05
00
0 ,. 02 exo
105
100
95
489
0 ,. 02
0 ,. 01
0
4
88
exo
0
0
,. 01
a
b
9
9
8
8
5
0
5
0
420
-
-
-
0 ,. 01
0 ,. 02
0 ,. 03
-0 ,. 01
-0 ,. 02
-0 ,. 03
2
15
90
71
85
-0 ,. 04
80
-0 ,. 04 endo
2
5
125 225 325 425 525 625
ee nn dd oo
e ex ox o
27
127
227
327
427
527
627
T(°C)
T(°C)
d
d
c
1
1
05
00
00 , ,0. 0 22
11 00 55
11 00 00
99 55
0 ,. 05
exo
441
411
00 , ,0. 0 11
d
0 ,. 025
0
c
00
9
5
0
5
0
505
6
79 - -00 , ,0. 0 11
-00 , ,0. 0 22
-00 , ,0. 0 33
- -00 , ,0. 0 44
9
8
8
99 00
-0 ,. 025
-0 ,. 05
-
88 55
-
7
7
2
63
40
60
125
endo
88 00
-0 ,. 075
2
5
125 225 325 425 525 625
25
225
325
425
525
625
e en nd do o
T(°C)
T(°C)
4
Fig. 6. TG–DTA diagrams of NH PMo12 (a), BiPMo12 (b), SbPMo12 (c) and SnPMo12 (d).
*
*
a
*
b
b
Δ
*
a
*
Δ
Δ
b
*
*
*
*
*
*
*
500°C
450°C
400°C
*
*
500°C
5
00°C
4
50°C
4
50°C
4
00°C
400°C
2
5°C
2
5°C
25°C
1
2
16 21 25 29 33 37 42 46 50 54 58 63
1
2 16 21 25 29 33 37 42 46 50 54 58 63
8
8 1
12 16 21 25 29 33 37 42 46 50 54 58 63
2 16 21 25 29 33 37 42 46 50 54 58 63
8
12 16 21 25 29 33 37 42 46 50 54 58 63
2
θ/deg
22 θθ // dd ee gg
2 θ/deg
2θ eg
/d
2
θ/deg
c
d
c
d
* Δ
c
*
Δ *
Δ
d
*
*
Δ
*
*
*
*
*
50 00 0° C° C
45 50 0° C° C
40 00 0° C°
*
5
4
4
C
*
*
50
45
40
25
0
°
*
5
4
4
2
0
0
°C
°C
°C
5
0
0
°
00
0°
2
25 5° C° C
5
°
°
C
C
8
1
1
2
2
1
1
6
6
2
2
1
1
2
2
5
5
2
2
9
9
3
3
3
3
3
3
7
7
4
4
2
2
4
4
6
6
5
5
0
0
5
5
4
4
5
5
8
8
6
63
3
8
12 16 21 25 29 33 37 42 46 50 54 58 63
2
2
θ/deg
θ/deg
22 θθ //dd eeg g
4
Fig. 7. XRD patterns of NH PMo12 (a), BiPMo12 (b), SbPMo12 (c) and SnPMo12 (d).
(
Fig. 6). The TG curve of NH
are three steps of weight loss. Before 200 °C, weight loss
5.7 wt%) attributed to crystallisation water desorption and be-
4
PMo12 salt (Fig. 6a) shows that there
note that the amount of weight released from both SbPMo12 and
SnPMo12 is higher than that from both NH PMo12 and BiPMo12.
This is probably due to high hydration degree of Sb and Sn cations
whose the stoichiometric coefficient is the highest (0.74 and 0.94,
respectively).
4
(
tween 200–500 °C weight loss (5.5 wt%) which was conducted in
two steps, assigned to ammonium ions departure. In the DTA curve
(
Fig. 6a), two endothermic peaks were observed at 73 and 228 °C,
associated to the different mass losses observed in TG. A wide exo-
thermal signal assigned to NH PMo12 decomposition with maxi-
mum at 488 °C and another small exothermic peak resulting
The area of endothermic peaks assigned to the losses of mass
corresponding to the water and to the ammonium ions, in the case
of substituted salts, seems to be related with both number and nat-
ure of cations incorporated as observed in TG curves. Thus, it is
more important in the case of SnPMo12 and SbPMo12. The complete
decomposition of substituted salts corresponds to the broad exo-
4
2 5 3
from the exothermic crystallization of both P O and MoO oxides
at 559 °C were observed.
The TG curves of mixed salts (Fig. 6b–d) are similar to that of
NH PMo12 salt with weight-losses divided into three major events.
The total mass loss is of 11.73, 10.68, 12.52 and 18.47 wt% for
NH PMo12, BiPMo12, SbPMo12 and SnPMo12 respectively. It is to
thermic effect with maxima at 481, 510 and 441 °C for BiPMo12,
SbPMo12 and SnPMo12 respectively.
According to the TG–DTA studies, the decomposition of BiPMo12
and that of SbPMo12 take place at a temperature similar to that of
4
4

3
900
L. Dermeche et al. / Inorganica Chimica Acta 362 (2009) 3896–3900
NH
4
PMo12 (488–505 °C). However, for the SnPMo12 sample, the
4
ammonium salt NH PMo12. The presence of the charge transfer be-
thermal stability is lower, probably due to strong reducing charac-
ter of tin ions as shown in UV–Visible.
tween the tin (or antimony) counter-cation and molybdenum (VI)
in the Keggin unit has been shown by UV–Visible analysis. Thus,
the introduction of tin and antimony can lead to partially reduced
phosphomolybdates with a lower thermal stability for tin based
salt.
7
– Fig. 7 shows that the XR diffractograms of XPMo12 are sim-
ilar to that of NH PMo12 whose the structure is cubic (JCPDS 09-
412). This is suggested that the X ions have occupied the coun-
4
0
ter-ion position in the polyoxometalate.
Besides, the examined thermal stability by TG–DTA, the struc-
tural stability of salts has been studied as a function of the temper-
ature of thermal treatment. Fig. 7 shows X-ray diffractograms of
salts at 25 °C and after treatment under air at following tempera-
tures 400, 450 and 500 °C. It was found a similar evolution of
XRD profiles for all compounds independent of their starting
composition.
The XRD spectra’s of all salts after treatment at 400 °C, are sim-
ilar to those observed at 25 °C. This indicates that all samples
maintain the cubic phase when most of its ammonium cations
were eliminated as observed in TG analysis. This phenomenon
agrees with that observed by Sultan et al. [5]. After treatment at
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[
[
11] F. Cavani, R. Mezzogori, A. Pigamo, F. Trifiro, Surf. Chem. Catal. 3 (2000) 523.
12] S. Albonetti, F. Cavani, F. Trifiro, M. Gazzano, M. Koutyrev, F.C. Aissi, A.
Aboukais, M. Guelton, J. Catal. 146 (1994) 491.
4
50 °C, for all salts, the XRD analysis shows the formation of
molybdenum trioxide in the monoclinic form (b MoO ) and in
the orthorhombic form ( MoO ). These two species were already
[13] C. Rocchiccioli-Deltcheff, M. Fournier, R. Frank, R. Thouvenot, Inorg. Chem. 22
1983) 207.
14] C. Rocchiccioli-Deltcheff, R. Thouvenot, R. Frank, Spectrochim. Acta 32A (1976)
87.
3
(
a
3
[
observed by Rocchiccioli-Deltcheff et al. [22] in the case of the
5
decomposition of 12-molybdophosphoric acid. After treatment at
[15] C. Marchal-Roch, C. Julien, J.F. Moisan, N. Leclerc-Laronze, F.X. Liu, G. Hervé,
App. Catal. A 278 (2004).
5
00 °C only orthorhombic
a
3
MoO was detected showing that b
[
16] C. Rocchiccioli-Deltcheff, A. Aoussi, M.M. Bettahar, S. Launay, M. Fournier, J.
Catal. 164 (1996) 16.
MoO was transformed into
3
a
MoO .
3
The decomposition temperature of salts to oxides obtained by
the XRD analysis is weaker than that observed in DTA (450 °C
against 489, 481 and 510 °C).
[17] Ch. Srilakshmi, N. Lingaiah, I. Suryanarayana, P.S. Sai Prasad, K. Ramesh, B.G.
Anderson, J.W. Niemantsverdriet, Appl. Catal. A: Gen. 296 (2005) 54.
[
[
[
[
18] M. Fournier, C. Louis, M. Che, P. Chaquin, D. Masure, J. Catal. 119 (1989) 400.
19] Carmen I. Cabelloa, Irma L. Botto, Horacio J. Thomas, App. Catal. A 197 (2000).
20] C. Thomazeau, V.Martin.P. Afanasiev, Appl. Catal. A: Gen. 199 (2000) 61.
21] F. Cavani, R. Mezzogori, A. Pigamo, F. Trifiro, E. Etienne, Catal. Today 71 (2001)
4
. Conclusion
97.
[
22] C. Rocchiccioli-Deltcheff, A. Aouissi, S. Launay, M. Fournier, J. Mol. Catal. A
Chem. 114 (1996) 33l.
The obtained results in this study showed that the substituted
salts have the Keggin structure and the cubic structure of pure