Effective Dawson type polyoxometallate catalysts for methanol oxidation

Article abstract of DOI:10.1016/j.molcata.2011.12.024

Dawson type polyoxometallates K₆P₂Mo _xW_18-xO₆₂ (x = 0, 5, 6) and α1 and α2-K₇P₂Mo₅VW₁₂O₆₂ were prepared and characterized by BET, IR, UV-vis and ³¹P NMR spectroscopies and thermal analysis (TG and DTA) and tested in methanol oxidation at 260°C in the presence of molecular oxygen. The Dawson compounds were found to be active in this reaction and the product distribution (formaldehyde, methyl formate, dimethylether, dimethoxymethane) depends on the polyanion composition and on the framework symmetry. α-K₆P ₂W₁₈O₆₂ exhibits an excellent catalytic performance with ca. 27% of methanol conversion and 98% of dimethylether selectivity. α1-K₇P₂Mo₅VW ₁₂O₆₂ and α2-K₇P₂Mo ₅VW₁₂O₆₂ show a similar activity (17-19% of conversion) with 49% of methyl formate selectivity and 41% of formaldehyde selectivity respectively. α-K₆P₂Mo₆W ₁₂O₆₂ is the most oxidizing catalyst and the most selective toward the methyl formate (ca. 53%).

Full text of DOI:10.1016/j.molcata.2011.12.024

Journal of Molecular Catalysis A: Chemical 356 (2012) 29–35
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Effective Dawson type polyoxometallate catalysts for methanol oxidation
Leila Dermechea,b, Nassima Salhi , Smain Hocine , René Thouvenot , Chérifa Rabia
a
a
b
a,∗
a
Laboratoire de Chimie du Gaz Naturel, Faculté de Chimie, Université des Sciences et de la Technologie Houari Boumediène, USTHB, BP 32, El-Alia, 16111 Bab-Ezzouar,
Alger, Algérie
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:
Received 8 November 2011
Received in revised form
Dawson type polyoxometallates K P Mo W
prepared and characterized by BET, IR, UV–vis and P NMR spectroscopies and thermal analysis (TG
and DTA) and tested in methanol oxidation at 260 C in the presence of molecular oxygen. The Dawson
compounds were found to be active in this reaction and the product distribution (formaldehyde, methyl
formate, dimethylether, dimethoxymethane) depends on the polyanion composition and on the frame-
work symmetry. ␣-K6P2W18O62 exhibits an excellent catalytic performance with ca. 27% of methanol
conversion and 98% of dimethylether selectivity. ␣1-K7P2Mo5VW12O62 and ␣2-K7P2Mo5VW12O62 show
a similar activity (17–19% of conversion) with 49% of methyl formate selectivity and 41% of formaldehyde
selectivity respectively. ␣-K6P2Mo6W12O62 is the most oxidizing catalyst and the most selective toward
the methyl formate (ca. 53%).
18−x 7 2 5
O62 (x = 0, 5, 6) and ␣1 and ␣2-K P Mo VW12O62 were
6
2
x
31
◦
1
9 December 2011
Accepted 21 December 2011
Available online 2 January 2012
Keywords:
Dawson anions
Methanol oxidation
Dimethyl ether
Formaldehyde
Methyl formate
© 2011 Elsevier B.V. All rights reserved.
1
. Introduction
The methanol oxidation is an attractive on-site source of H2 for
experimental conditions. Thus, Tatibouët [11] showed that the
decomposition of methanol could be used as a model reaction
to characterize the acid–base and redox properties of a catalyst.
The DME formation can describe the pure dehydration ability of a
catalyst, which is generally related to its acidic character. The for-
mation of DMM can be assigned to a dual site including a redox
dehydrogenating site and a Lewis acid site, while the FA formation
requires only a redox dehydrogenating site. Concerning, the MF and
fuel cells [1–3] and can also lead to valuable oxygenated prod-
ucts such as formaldehyde (FA), methyl formate (MF), dimethyl
ether (DME) and dimethoxymethane (DMM) which have various
applications as solvent and/or precursor to many other chemi-
cal compounds. DME and DMM can also be used as oxygenated
additives to fuel, owing to their high cetane number. In addition
to these properties, DME, compound non-toxic, non-corrosive and
biodegradable in air, can act as a clean fuel. The simplicity of this
short carbon chain compound leads during combustion to very
low emissions of particulate matter. Selective methanol oxidation
to these different products was the subject of several studies. It
has been reported that Sb V mixed oxide [4] and supported rhe-
CO , their formation can be related to strong oxidative character of
2
the catalyst.However, few works have tested the polyoxometal-
lates (POMs) having the Dawson structure as catalysts compared
to those of the Keggin structure. Dawson type compounds may
be promising catalysts in homogeneous and heterogeneous sys-
tems because their redox and acidic properties can be controlled
at atomic/molecular levels. They showed excellent catalytic activi-
ties in several reactions such as ethyl-tert-butyl ether [12], methyl
tert-butyl ether [13] synthesis, both oxidative dehydrogenation of
isobutyric acid [14] and isobutane [15] and hydroxylation of phenol
[16].
nium oxide systems [5] are selective to formaldehyde, V O5/TiO2
2
[
6], H3+nPMo
VnO40 Keggin [7] and Mn/Re/Cu catalysts [8] to
12−n
dimethoxymethane and V Ti O materials [9] to methyl formate.
On silica-supported 12-molybdophosphoric acid catalysts, it was
shown that the distribution of reaction products depends on the
loading and experimental conditions [10].
These works evidenced that methanol decomposition reaction
is very sensitive to acidic and redox properties of solid and to
The aim of this work was to prepare a series of Dawson-type
POMs, as potassium salts, namely K P MoxW O62 (x = 0, 5, 6)
6
2
18−x
and ␣1 and ␣2-K7P Mo5VW12O62, in order to examine the effect
2
of the chemical composition on their catalytic performance in
methanol oxidation. POMs were characterized by BET, IR, UV–vis
31
and P NMR spectroscopies and thermal analysis (TG and DTA). The
◦
methanol conversion was carried out at 260 C under atmospheric
∗ Corresponding author. Tel.: +213 21 24 73 11; fax: +213 21 24 73 11.
E-mail addresses: c rabia@yahoo.fr, crabia@usthb.dz (C. Rabia).
pressure in the presence of molecular oxygen.
1
381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.12.024

3
0
L. Dermeche et al. / Journal of Molecular Catalysis A: Chemical 356 (2012) 29–35
2
. Experimental
a
2.1. Materials
K P W18O62 was prepared according to the literature data
6
2
2
n−
O62]
18−(x+y)
b
c
[
(
[
17]. Mixed W/Mo/V polyoxometallates [P MoxVyW
2
x = 5, 6 and y = 0, 1) were obtained from the hexavacant anion
1
2−
H P W12O56
]
according to the method described by Contant
2
2
et al. [18] including the successive condensation and hydrolysis.
K7P Mo5VW12O62 was obtained in the form of two ␣1 and ␣2 iso-
2
mers. The different POMs were noted: ␣-P W18, ␣2-P Mo5W13,
2
2
␣
-P Mo W , ␣1-P Mo5VW and ␣2-P Mo5VW12.
0
2
2
6
12
2
12
2
000
1750
1500
1250
1000
750
500
250
-
1
wavenumber (cm )
2.2. Characterization
Fig. 1. IR Spectra of ␣-P2W18 (a), ␣-P2Mo6W12 (b) and ␣1-P2Mo5VW12 (c).
BET surface area measurements were performed at liquid
nitrogen temperature using a Micrometrics ASAP 2020 V1.05 G
apparatus. Prior to each adsorption–desorption measurement, the
sample was degassed at T = 150 C for 3 h. The specific surface areas
anion, has been identified by four characteristic IR bands in the
1100–770 cm range [20]. In addition to these vibration bands,
◦
−1
were determined using the linear part of the BET equation.
several shoulders or small bands are observed for substituted
−
1
−1
Infrared spectra were recorded on the 4000–400 cm range on
a Bruker IFS 66 FT-IR spectrometer using samples prepared as KBr
disks.
heteropolyanions. The P O band appears in the 1084–1100 cm
−
1
range while the M Od band appears in the 970–945 cm range.
The inter group M Ob M and the intra-group M Oc M bands
31P MAS NMR spectra were measured at room temperature on a
Bruker Avance 400 spectrometer. 85% H PO was used as an exter-
−1
−1
appear around 910–908 cm and 782–779 cm , respectively. In
addition of these four IR bands, another band is observed around
3
4
−
1
nal reference.
530 cm corresponding to ı(P O) vibration. The replacement of W
atoms by Mo atoms only does not seem to have an effect on the IR
vibration bands of Dawson polyanion, while an important pertur-
bation is observed when vanadium is introduced into the Dawson
unit. As shown on the IR spectrum of ␣1-P Mo VW (Fig. 1c), two
UV–vis diffuse reflectance spectra were recorded in the
00–200 nm region on a Varian Cary 5E spectrometer equipped
8
with a polytetrafluoroethylene (PTFE) integration sphere. PTFE was
used as a reference.
2
5
12
−
1
Thermogravimetric (TG) and differential thermal (DTA) data
were collected on a SDT-2960 thermal analyzer. The TG and DTA
experiments were performed under air flux, using 30 mg of sample
shoulders appear at 1140 and 1100 cm on the P O vibration band
and one at 1012 cm on the M Od vibration band.
−
1
31
In P NMR spectroscopy, the chemical shift values depend
on the composition and on the relative positions of the W, Mo
and V atoms in the polyanion framework (Table 1). As already
reported by Contant et al. [18], when the two half-anions are
identical, the symmetric species show one signal, and when the
species are unsymmetrical, two signals are observed. For ␣-P W
◦
and a heating rate of 1 C/min.
2.3. Catalytic measurements
◦
The methanol oxidation tests were carried out at 260 C under
2
18
and ␣-P Mo W , only one 31P line is observed at −12.51 and
atmospheric pressure using a Pyrex tubular flow micro reactor. A
thermocouple was installed within the reactor, in contact with cat-
alyst bed. The catalyst (20 mg) was first pretreated in situ for 1 h
2
6
12
−9.51 ppm, respectively in agreement with the equivalence of the
3−
3−
]
31
two half-anions [PW O
]
and [PW Mo O
, respectively.
9
31
6
3
◦
at 300 C under an oxygen stream with a rate flow of 20 ml/min
While P Mo W , ␣1-P Mo VW , and ␣2-P Mo VW species
2
5
12
2
5
12
2
5
12
and then, the reactor was cooled down to the reaction temperature
gave two peaks at −9.39 and −10.08 ppm, at −9.73 and −10.10 ppm
and at −8.95 and −9.76 ppm, respectively, in agreement with the
lower symmetry of these polyanions due to two unequivalent half
units.
◦
(
260 C). The flow of the initial reaction gas mixture (7.0 ml/min of
N and 1.5 ml of O ) passed through a saturator filled with methanol
2
2
at a partial pressure of 58.2 Torr. The flows of the initial reaction
vapor–gas mixture and the final reaction mixture were connected
in turn to the analyzer using the flow switcher. The analyzer is a
gas-chromatographic (Cp-3800), instrument embedded in the sys-
tem; it included a flame-ionization detector (FID) with a Carboxen
Fig. 2 shows the polyhedral representation of the Dawson
6−
polyanion structure [P W18O62
]
and numbering of the metallic
2
atoms according to IUPAC recommendations. In Dawson het-
eropolyanions, two different types of clusters are present: two
terminal trimetallic groups M O (numbers 1–3 and 16–18) and
1
000 column, a thermal-conductivity detector (TCD) with a chro-
3
13
mosorb 107 column. The FID line allowed to analyze methanol,
methyl formate (MF), formaldehyde (FA), dimethyl ether (DME)
and dimethoxymethane (DMM) and the TCD line carbon monoxide
six dimetallic groups M O (numbers 4–15) arranged in a double
crown. The overall framework having a D3h symmetry corresponds
2 10
to ␣ isomer. Mo O and Mo O clusters are connected to each
3
13
2
10
(
CO) and carbon dioxide (CO ) noted COx.
other only by single ␮-oxo-bridges and the local symmetry of
all W(VI), Mo(VI) and V(V) is octahedral.Based on the results of
tungsten, vanadium and phosphorus NMR spectroscopy, Contant
et al. [18] assigned and marked the positions of molybdenum and
vanadium atoms in the mixed W/Mo/V Dawson heteropolyanions
as follows: 1,4,9,10,15,16-P Mo W12; 1,9,10,15,16-P Mo5W13;
2
3
. Results
3
.1. Characterization
2
6
2
The specific surface areas of Dawson POMs are very low
1–2 m /g) and similar to that of the Dawson heteropolyacid,
1,9,10,15,16-4-P Mo5VW (␣1-P Mo5VW12) and 4,9,10,15,16-1-
P2Mo5VW12 (␣2-P Mo5VW12).
2
2 12 2
2
(
H P W18O62 [19].
UV–vis/diffuse reflectance spectroscopy (Fig. 3) shows that
Dawson heteropolyanions exhibit strong absorption in the
650–200 nm wavelengths domain with multiplicity associated
to ligand–metal charge transfers (LMCT) from oxygen to W(VI),
6
2
Fig. 1 shows the IR spectra of ␣-P W , P Mo W and ␣1-
2
18
2
6
12
6−
P2Mo5VW12. [P W18O62
]
Dawson heteropolyanion, constituted
units corresponding to the lacunary Keggin
2
3
−
of two [PW O31]
9

L. Dermeche et al. / Journal of Molecular Catalysis A: Chemical 356 (2012) 29–35
31
Table 1
3
1P NMR chemical shifts and numbering of the metallic atoms according to the IUPAC for Dawson POMs.
POMs
Notation
ı (ppm)
P1
P2
ꢀıexp
ꢀı [19]
K6[P2W18O62]
␣-P2W18
−12.51
−9.51
−9.39
−9.73
−8.95
−2.51
−9.51
−0.08
−0.10
−9.76
0
0
0.69
0.37
0.81
0
0
0.72
0.28
0.98
1
1
1
4
,4,9,10,15,16-K6[P2Mo6W12O62]
,4,9,10,15-K6[P2Mo5W13O62]
,9,10,15,16-4-K7 [P2Mo5VW12O62]
,9,10,15,16-1-K7 [P2Mo5VW12O62]
␣-P2Mo6W12
␣2-P2Mo5W13
␣1-P2Mo5VW12
␣2-P2Mo5VW12
ꢀıexp = ı P1 − ı P2.
The width of the absorption band varies as follows: ␣2-
P2Mo5VW12 > ␣1-P Mo5VW > P Mo W > P Mo5W > P W18.
2
12
2
6
12
2
13
2
The introduction of vanadium into the POM framework increases
the negative charge of the polyanion, therefore the size of the
cluster increases with the number of potassium atoms. This result
is in agreement with literature data [24].
The thermal stability of the POMs was investigated by means
of thermogravimetric (TG) and differential thermal analysis (DTA).
The TG curve of P W (Fig. 4) shows a very low weight loss (∼1%)
2
18
◦
below 200 C, attributed to the release of physisorbed water and
between 200 and 500 C, no other weight loss was observed. On the
◦
DTA diagram, two endothermic peaks are observed with maximum
◦
at 61 and 105 C, associated to the mass loss observed in TG. No
exothermal signal assigned to the POM decomposition is observed,
◦
suggesting that until 500 C, the P W salt is stable.
2
18
The TG curves of P Mo5W and P2 Mo W salts (Fig. 5) are
2
13
6
12
similar to that of P W with also a low weight-loss (∼1%) below
2
18
◦
2
00 C. The DTA curves show three endothermic peaks at ca. 64, 100
and 140 C in the case of P Mo5W and two endothermic peaks at
ca. 110 and 140 C in the case of P Mo W , associated also to the
water mass loss. Small exothermal signal is observed at ca. 240 C in
presence of P Mo W that could result from a change of isomeric
◦
2
13
Fig. 2. Polyhedral representation of the Dawson structure of [P2W18O62]6− polyan-
ion.
◦
2
6
12
◦
2
6
12
Mo(VI) and V(V) in the Dawson anion. The position of the bands
form. The decomposition of both salts takes place at the same tem-
perature (490 C) indicating that the replacement of W atoms of
P2W18 Dawson unit by Mo atoms decreases the thermal stability
of solid.
around 255 and 275 nm is attributed to M
O M bridges (M: W, Mo
◦
and V) and the lowest energy absorption band shifts toward higher
wavelength (from 350 to 420 nm) and broadens is related to size
of the counter-ion. Similar results were obtained by other authors
The DTA diagrams of ␣1 and ␣2-P Mo5VW (Fig. 5) show one
2
12
[
21–25] which suggested that the presence of more than one band
◦
broad endothermic peak with maximum at 86 and 78 C respec-
tively, associated to the release of physisorbed water, a wide
in the UV–vis spectra of Keggin-type or Dawson-type compounds
results of the effect of several parameters such as the presence of
exothermal signal assigned to ␣1 and ␣2-P Mo5VW decompo-
2
12
different oxygen atoms (Oa, O , O and O ), local symmetry, overall
b
C
d
◦
sition with maximum at 430 and 450 C, respectively. In the case of
1-P Mo5VW12, another small exothermic peak resulting probably
symmetry, polarizing and size of the counter-cation. An increase of
cluster size or an increase of the polarizing effect and/or a decrease
of the size of the counter-cation lead to a broadening and a red shift
of the absorption band. It has also been reported that the broaden-
ing of the band can be assigned to the existence of interactions
between polyanions.
␣
2
from the exothermic crystallization of P O5, WO , MoO and V O5
2
3
3
2
◦
oxides is observed at ca. 460 C. In the TG diagrams of these salts,
only a continuous weight loss (∼2%), taking place mainly below
◦
2
00 C is observed. ␣2-P Mo5VW seems slightly more stable than
2
12
␣
1-P Mo5VW12.
2
According to the DTA data, the thermal stability of Dawson POMs
decreases as follows: P2W18 ꢀ P Mo5W ≈ P Mo W > ␣2-
2
13
2
6
12
d
P2Mo5VW12 > ␣1-P Mo5VW . More W atoms are substituted
2
12
c
0
0
,4
,2
0
5
0
-5
b
b
exo
-10
a
e
-
-
-
-
0,2
0,4
0,6
0,8
-15
-20
endo
-
25
30
35
-40
a
-
6
1°C
-
200
300
400
500
600
700
800
900
105°C
-
1
Absorbance / Nanometers
21
49
100
152
204
256
308
359
410
461
T (°C)
Fig. 3. UV–vis DR spectra of ␣-P2W18 (a), ␣2-P2Mo5W13 (b), ␣-P2Mo6W12(c), ␣2-
P2Mo5VW12 (d) and ␣1-P2Mo5VW12(e).
Fig. 4. TGA (a) and DTA (b) diagrams of ␣-P2W18.

3
2
L. Dermeche et al. / Journal of Molecular Catalysis A: Chemical 356 (2012) 29–35
(
2)
490°C
90
4
0,4
0,2
0
30
20
0
,2
0
20
(
1)
236°C
exo
4
90
b
b
1
0
0
10
-
0,2
0,4
0,6
0,8
exo
0
-
-
0,2
0,4
0,6
0,8
-10
-10
-
a
-
6
4°C
-20
-30
-40
endo
-20
-30
-40
-
endo
-
110°C
100
-
1
1
02°C
a
1
41°C
1
39°C
-
-1,2
4
0
50
152
204
256
308
359
b
410
460
2
8
49
100
152
204
256
307
359
b
410
460
T (°C)
T (°C)
4
50
0,2
0
0
,2
0
10
0
(4)
4
30
0
(
3)
-10
-
-
-
-
0,2
0,4
0,6
0,8
-0,2
0,4
0,6
-0,8
exo
-10
exo
-20
-30
-40
-50
-60
-
-20
-
-30
-40
-50
-1
endo
endo
a
-
-
-
-
1,2
1,4
1,6
1,8
-1
a
-1,2
78°C
99
-60
86°C
-1,4
-70
2
1
48
152
204
256
307
359
410
460
2
7
48
99
152
204
256
307
359
409
460
T(°C)
T (°C)
Fig. 5. TGA (a) and DTA (b) diagrams of ␣2-P2Mo5W13 (1), ␣-P2Mo6W12 (2), ␣1-P2Mo5VW12 (3) and ␣2-P2Mo5VW12 (4).
more the thermal stability is reduced, results in agreement with
the lower symmetry of the polyanion.
3.2. Catalytic tests
The catalytic properties of K P MoxW O62 (x = 0, 5, 6) and
18−x
Table 2 shows the IR bands frequencies of POMs after treatment
6
2
◦
at 300 C for 1 h under oxidizing atmosphere (conditions of catalytic
␣1 and ␣2-K7P Mo5VW12O62 Dawson type salts were examined
2
◦
tests). No major modifications of frequencies of IR bands typical
in methanol oxidation by molecular oxygen at 260 C after pre-
treatment in situ for 1 h at 300 C under an oxygen stream. The
−
1
◦
of the Dawson polyanion: ꢁs(P Oa) (1093–1081 cm ), ꢁas(P Oa)
−1
−1
(
(
1021–1015 cm ), ꢂas(M O ) (976–948 cm )
ꢂas(M Ob M)
Dawson compounds are active and the major products detected
are formaldehyde (FA), methyl formate (MF), dimethylether (DME),
and dimethoxymethane (DMM) and carbon oxides (noted COx)
rather minor products.
d
−1
−1
916–881 cm ) and ꢂs(M Oc M) (791–780 cm ), are observed
for W/Mo and ␣2-P Mo5VW , exceptedfor ␣1-P Mo5VW where
2
12
2
12
−
1
1
the shoulder at 1102 cm disappeared and the ꢁs(P Oa) shifted
from 1082 to 1076 cm
−
indicating that ␣1-P Mo5VW solid is
Figs. 7–9 show the variations of the activity and the selectivities
2
12
more sensitive to thermal pretreatment in agreement with DTA
data. This study shows that the Dawson structure remains intact
and that the substitution of W atoms by Mo and V atoms does not
modify the stability of the polyanion framework during pretreat-
ment of the solid.
as a function of time for P W , P Mo5W and ␣1-P Mo5VW12.
No significant variation of both catalytic activity and DME selec-
tivity was observed during more than 3 h of reaction in presence
2 18 2 13 2
of P W (Fig. 7) suggesting a very good catalytic stability of this
2
18
catalyst. This result is expected to have working at low temperature
◦
These results are confirmed by those of the UV–vis study
(260 C) and also since Shikata et al. [13,19] showed that the reaction
(
Fig. 6), in which no major modification of the absorption band was
takes place in the solid bulk without the structure of the polyanion
is modified. On the other hand, the study of the stability of ␣1-
◦
observed after thermal treatment under (300 C/O /1 h).The whole
2
results on the thermal behavior of Dawson compounds showed that
the structure of the polyanion was preserved under the conditions
of catalytic test.
KxP Mo17MO40 examined by IR, XRD, and by solution 31P NMR,
after the isobutane oxydehydrogenation reaction at 450 during
100 h, showed that the catalyst was stable [15].
2
◦
b
a
b
(2)
(
1)
a
2
00
300
400
500
600
700
800
900
1000 200
300
400
500
600
700
800
900
Absorbance / Nanometers
Absorbance / Nanometers
◦
Fig. 6. UV–vis spectra of ␣-P2Mo6W12 (1), ␣1-P2Mo5VW12 (2), before (a) and after (b) thermal treatment (300 C/1 h/O2).

L. Dermeche et al. / Journal of Molecular Catalysis A: Chemical 356 (2012) 29–35
33
Table 2
◦
FT-IR results of Dawson POMs before (a) and after (b) thermal treatment at 300 C/1 h/O2.
IR frequencies (cm 1
−
)
ꢁ
s(P Oa)
ꢁ
as(P Oa)
ꢂas(M Od)
ꢂas(M Ob M)
ꢂs(M Oc M)
␣
␣
␣
␣
␣
␣
␣
␣
-P2W18 (a)
-P2W18 (b)
1093
1090
1089
1081
1102–1082
1076
1085
1021
1021
1017
1016
1015
1015
1016
1015
960
961
954
956
948
976
948
945
910
912
910
906
911
881
916
915
782
783
780
791
785
788
786
787
-P2Mo6W12 (a)
-P2Mo6W12 (b)
1-P2Mo5VW12 (a)
1-P2Mo5VW12 (b)
2-P2Mo5VW12 (a)
2-P2Mo5VW12 (b)
1083
1
1
20
00
(18.7%) and ␣2-P2Mo5VW12 (17.3%). P2Mo6W12 seems to be the
less active with only 11.8% of conversion. Contrary to the cat-
alytic activity, product distribution is much more sensitive to both
Conv
HCHO
CH3OCH3
position and nature of transition metal in the framework. P W
8
0
0
0
0
2
18
catalyst exhibits excellent catalytic performance in methanol oxi-
dation with 98% of DME selectivity and 27% of conversion. The DME
production by a bimolecular dehydration of methanol reflects the
dehydration ability of a catalyst, generally related to its Brönsted
or Lewis acidic character [8,11,13,16,19]. This result indicates that
P2W18 heteroplysalt have a strong acidic character of Lewis type
assigned to high charge of W (VI). The dehydration of methanol has
already observed on metal oxides as Mo/Al O where only dimethyl
6
4
2
0
5
43
81
119
157
195
233
271
2
3
t (min)
ether was obtained as reaction product. The authors have attributed
the Lewis acidic character of the catalyst to Mo(VI) and Al (III) [26].
The substitution of 5 W atoms by 5 Mo atoms led to a stronger
decrease of the DME selectivity from ca. 98 to ca. 19% indicating a
strong decrease of the acidic strength of the catalyst at the detri-
ment of oxidative power. It is reported that the dimethyl ether
formation can be considered as a parallel pathway of the methanol
transformation which is controlled only by surface acidity [11]. It
Fig. 7. Methanol conversion and product selectivities as a function of reaction time
at 260 C on ␣-P2W18, pretreated at 300 C/1 h/O2.
◦
◦
For ␣1-P Mo5VW (Fig. 8), the catalytic activity and the for-
2
12
mation of products vary slowly with time-on-stream. Among the
methanol oxidation products, MF seems to be the most stable.
While in presence of P Mo5W catalyst, the variations of the cat-
2
13
has been observed that the disappearance of the acidity of both SiO2
alytic activity and selectivities of different reaction products are
more pronounced with time (Fig. 9). The methanol conversion
increases gradually from 20 to 32% after more than 3 h of test. The
selectivity toward DMM strongly increases with time from ca. 10%
to ca. 35%, while that of FA decreases from ca. 35% to ca. 25%. This
decrease can be attributed to the condensation reaction between
methanol absorbed and FA to give DMM. The formation of the other
reaction products is relatively stable during the catalytic test with
MF and DME selectivities of ca. 20% and ca. 25–32%, respectively.
Table 3 shows the catalytic test results of K P W Mox (x = 0,
, 6) and ␣1 and ␣2-K7P Mo5VW systems in methanol selec-
2 12
tive oxidation, obtained after around 3 h, when nearly steady-states
were reached.
supported H PMo12O
40
[10] and SiO2 supported HMgPMo12O40
3
[
27] catalysts, pretreated at different temperatures, leads to a dra-
matic decrease in the rate of dimethyl ether formation to the benefit
of that of redox products formed on oxidative centers. Thus, the
presence of molybdenum favored the formation of oxidation prod-
ucts such as FA and MF with 27 and 28% of selectivity, respectively.
DMM is obtained with ca. 25% of selectivity. These results indi-
cate that P Mo5W catalyst presents both Lewis acid sites and
2
13
oxidative sites with similar strengths.
6
2
18−x
The substitution of another W atom by one Mo atom in the
5
P2Mo5W system returns the methanol oxidation more selective
13
to MF (ca. 53% of selectivity) and more selective to deep oxidation
product (ca. 5% of carbon oxides selectivity). The other reaction
products are FA and DME with ca. 18 and ca. 23% of selectivity,
The catalytic activities of P Mo5W and P W are similar with
7.0 and 27.7% of conversion, higher than those of ␣1-P Mo5VW
2
13
2
18
2
2 12
Conv
CO2
Conv
HCHO
HCOOCH3
CO2
CH3OCH3
CH3OCH2OCH3
6
5
4
3
2
1
0
0
0
0
0
0
HCHO
CH3OCH3
CH3OCH2OCH3
HCOOCH3
4
3
3
2
0
5
0
5
20
1
1
5
0
5
0
0
5
43
119
157
195
5
43
119
t (min)
157
195
t(min)
Fig. 8. Methanol conversion and product selectivities as a function of reaction time
at 260 C on ␣1-P2Mo5VW12, pretreated at 300 C/1 h/O2.
Fig. 9. Methanol conversion and product selectivities as a function of reaction time
◦
◦
◦ ◦
at 260 C on ␣2-P2Mo5W13, pretreated at 300 C/1 h/O2.

3
4
L. Dermeche et al. / Journal of Molecular Catalysis A: Chemical 356 (2012) 29–35
Table 3
Catalytic performance of Dawson POMs, pretreated at 300 C/2 h/O2, for the methanol oxidation at 260 C.
◦
◦
%
POMs
␣-P2W18
␣2-P2Mo5W13
␣-P2Mo6W12
␣1-P2Mo5VW12
␣2-P2Mo5VW12
Conv
CO2
27.3
0.0
27.7
0.4
11.8
4.7
18.7
1.4
17.3
2.9
HCHO
CH3OCH3
HCO2CH3
1.3
98.4
0.0
26.6
19.2
28.4
25.3
18.4
23.4
53.4
0.0
18.2
18.2
48.8
13.3
40.8
26.4
18.9
11.0
(
CH3O)2CH2
0.0
+
1/2 O2
+1/2 O2
+
1/2 O2
Oxidation reactions
HCOOH
CO2
CH OH
H2CO
3
(
CH O) CH
HCOOCH3
+ H2O
CO + H2O
Dehydration reactions
CH OCH3
3
2
2
3
+
H O
2
+
H O
2
Scheme 1. Reaction pathways in methanol conversion [11].
respectively. DMM product has not been observed. With respect to
the position of Mo atoms in the polyanion structure (1,4,9,10,15,16-
P Mo W12), the methanol oxidation to MF, FA and COx (sum of
selectivities = 76%) over this catalyst, could occur on the polyanion
face that could correspond to a concentration of oxidative active
pseudoliquid phase. It is generally accepted in acid catalysis, that
the reaction takes place in the bulk of the polyoxometallate, assimi-
lated to a pseudoliquid phase, where the solid catalyst behaving in a
sense like a concentrated solution. Thus, the bulk phase is the reac-
tion field. The high-activity state of the pseudoliquid phase and the
rapid absorption–desorption of reactive and products are probably
2
6
sites related to Mo O and Mo
O Mo bonds. Thus, P Mo W
2 6 12
presents a very strong oxidative character compared to P Mo5W13
,
the reasons for the high activity of P W18. Moreover, this perfor-
2
2
but also an acidic character with a moderate strength.
mance can be related to the shape of Dawson-type heteropolyanion
which is not spherical as that of Keggin-type and that the secondary
structure of Dawson polyoxometallate may be less ordered and less
rigid.
In P/W/Mo/V systems, the vanadium position in framework
polyanion have an important effect on the product distribution
and therefore on acid and redox properties of the catalyst. Thus,
␣
1-P Mo5VW (1,9,10,15,16-4-P Mo5VW12), with the vanadium
FA formation that requires only a redox dehydrogenating site
is observed after introduction of Mo and V atoms. Its selectivity is
particularly high when the V atom is in position 1 in the trimetallic
group cluster. The oxidative power introduced by Mo and V atoms
has already been observed in the case of both Keggin and Dawson
type heteropolyanions [7,14]. The redox sites were attributed to
the metal-terminal oxygen bond (M Od).
2
12
2
atom, in position 4 in a dimetallic group M O cluster, is selective
to MF with ca. 50% of selectivity. FA and DME are obtained with sim-
ilar selectivity of ca. 18% higher than that of DMM (ca. 13%). While
2
10
␣
2-P Mo5VW (4,9,10,15,16-1-P Mo5VW12) with the vanadium
2 12 2
atom, in position 1 in a trimetallic group M O13 cluster, is selec-
3
tive toward FA with ca. 41% of selectivity. MF, DME and DMM are
obtained with selectivities of ca. 19, 26 and 11%, respectively. These
results show that both catalysts exhibit a good synergistic effect
of the reducibility and acidic property in the methanol oxidation
with a moderate relative acidic density and a high oxidative power.
However, when the vanadium atom is in position 4, the catalyst
seems to be more oxidative (sum of FA, MF, COx selectivities: ca. 67
against 63%). In position 4, the active sites are probably V O and/or
DMM formation can be assigned to the condensation reaction
between methanol, rapidly absorbed into the pseudoliquid phase
and the formed product (FA) and MF formation can be assigned to
an esterification reaction between methanol and formic acid which
would come from the FA oxidation. The presence of the formic
acid considered as an intermediate in the formation of MF, has
not been reported in the literature works. The products, DMM and
MF, require both redox sites and acid sites. The redox sites are in
charge of transferring the lattice oxygen to adsorbed methanol to
form formaldehyde and formic acid, while the acid sites catalyze
the condensation of the intermediates with methanol to form DMM
and MF [28]. These different reactions may occur at the surface and
solid bulk of the polyanions as shown the scheme de reaction path-
ways (Scheme 1) suggested by Tatibouët [11], where the oxidation
of methanol involves several sequential and parallel reactions.
This study evidenced the high sensitivity of methanol oxidation
to both composition and relative positions of the W, Mo, and V
atoms in the framework of Dawson polyanion. These two param-
eters lead to different acidic and redox properties that control
the selectivity orientation between the formations of dehydration
and/or redox products.
Mo
Mo
O
O
V, more oxidizing sites and in position 1, W
V.
O V and/or
From the catalytic properties of Dawson-type POMs in the
methanol oxidation reaction, the oxidative power of the substituted
POMs decreases in the following order: ␣-P Mo W > ␣1-
2
6
12
P2Mo5VW12 > ␣2-P Mo5VW ꢀ ␣2-P Mo5W . P W
presents
2
12
2
13
2
18
only an acidic character.
The DME formation reflecting the acidic character of catalyst is
related to the presence of W atoms. The acidic character increases
with W atom number. The efficiency of P W18, in its acid form has
already been reported by Misono et al. [13,19] in the gas phase
synthesis of methyl tert-butyl ether from methanol. The authors
showed that the catalytic activity of P W heteropolyacid for this
2
2
18
reaction was at least 13 times greater than that of Keggin type HPAs
HnXW12O40, X = P, Si, Ge, B, and Co). From this result, the authors
suggested that the activity of catalyst was not governed only by
both acid strength and surface area of H P W18O60 which are lower
(
4. Conclusion
6
2
than those of parent Keggin type heteropolyacids, H PW12O40 and
H SiW12040, but by another property lied to the presence of the
4
The V-substituted
diphosphates, K P Mo W O62 (x = 0, 5, 6) and ␣1 and
18−x
Dawson
type
molybdo-tungsto-
3
6
2
x

L. Dermeche et al. / Journal of Molecular Catalysis A: Chemical 356 (2012) 29–35
35
␣
2-K7P Mo5VW12O62
,
synthesized, characterized by different
[8] H. Li, J.P. Li, F.K. Xiao, W. Wei, Y.H. Sun, J. Fuel Chem. Technol. 37 (2009) 613–617.
[9] E. Tronconi, A.S. Elmi, N. Ferlazzo, P. Forzatti, Ind. Eng. Chem. Res. 26 (1987)
2
31
techniques (BET, IR, UV–vis and
P NMR spectroscopies) and
1269–1275.
thermal analysis (TG and DTA) and tested in a probe reaction such
as methanol oxidation display a series of interesting properties
in relation with the combination of various atomic composi-
tions and the framework symmetry: (i) ␣-K P W18O62 very
[
10] C. Rocchiccioli-Deltcheff, A. Aouissi, S. Launay, M. Fournier, J. Mol. Catal. A:
Chem. 114 (1996) 33l–342l.
11] J.M. Tatibouët, Appl. Catal. A 148 (1997) 213–252.
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Catal. A: Gen. 298 (2006) 217–224.
[
[
6
2
selective to dimethylether reflecting its high acidic character;
ii) ␣1-K7P Mo5VW12O62 selective methyl formate and ␣2-
[13] S. Shikata, S. Nakata, T. Okuhara, M. Misono, J. Catal. 166 (1997) 263–271.
[14] D.R. Park, H. Kim, J.C. Jung, S.H. Lee, I.K. Song, Catal. Commun. 9 (2008) 293–298.
(
2
[
[
15] F. Cavani, R. Mezzogori, A. Trovarelli, J. Mol. Catal. A 204–205 (2003) 599–607.
16] J. Yu, P. Yang, Y. Yang, T. Wu, Catal. Commun. 7 (2006) 153–156.
K7P Mo5VW12O62 to formaldehyde reflecting their bi-functional
2
acid-oxidative character; (iii) ␣-K P Mo W12O62 very selective to
[17] J. Randall, D.K. Lyon, R.J. Domaille, R.G. Finke, A.P. Ginsberg, A Wiley-
6
2
6
methyl formate reflecting its high oxidative character.
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the W, Mo, and V atoms in the framework of Dawson polyanion lead
to different acidic and redox properties that control the selectivity
orientation between the formations of dehydration and/or oxida-
tion products. Thus, the Dawson POMs appear as a good design to
direct the selective oxidation of methanol toward the desired prod-
ucts by the choice of elements constituting the heteropolyanion.
[
3
597–3604.
[19] S. Shikata, T. Okuhara, M. Misono, J. Mol. Catal. A: Chem. 100 (1996) 49–59.
[
[
[
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