Applied Catalysis A: General 201 (2000) 191–202
Thermal stability and catalytic activity of Wells-Dawson
tungsten heteropoly salts
Laura E. Briand a,∗ , Horacio J. Thomas a , Graciela T. Baronetti b
a
Centro de investigación y Desarrollo en Procesos Cataliticos (CINDECA), CONICET, Universidad Nacional de La Plata,
Calle 47 N 257, (1900) La Plata, Buenos Aires, Argentina
b Instituto de Investigaciones en Catálisis y Petroquı́mica (INCAPE), Facultad de Ingenierı́a Quı́mica
(Universidad Nacional del Litoral)-CONICET, Santiago del Estero 2654, (3000) Santa Fe, Argentina
Received 10 June 1999; received in revised form 23 December 1999; accepted 23 December 1999
Abstract
Potassium-tungsten Dawson type salts, pure and monosubstituted with vanadium, are presented in this study. These compounds were thoroughly characterized by 31 P-NMR, 31 P MAS-NMR, scanning electron microscopy, EDX microprobe, X-ray
diffraction, infrared spectroscopy, X-ray photoelectron spectroscopy and thermal analysis. Both Dawson salts are thermally
stable up to 400◦ C. Above that temperature they recrystallize in a potassium–tungsten Keggin type salt and P2 O5 -WO3 (-K2 O)
compounds. Vanadium that was initially part of the tungsten framework is expelled out of the structure and covers the surface
of the new phases.
Dawson salts showed activity in methanol selective oxidation. Formaldehyde is the only product detected during methanol
reaction on intact Dawson salts, which indicates the presence of redox sites. After decomposition, dimethyl ether along with
formaldehyde is produced, due to the exposure of acidic phosphorous sites. The selectivity of the decomposed vanadium
containing Dawson salt towards dimethyl ether is lower than the pure potassium–tungsten salt, since vanadium is covering
acid sites. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Heteropoly compounds; Dawson salts; Methanol oxidation; Thermal stability
1. Introduction
The synthesis of heteropolyoxoanions dates back
to the beginning of this century, but these compounds
have only recently been applied to the field of catalysis [1–3]. The heteropolyoxoanions are composed of
a close-packed framework of metal-oxygen octahedrons, MOx (M=Mo+6 , W+6 ) surrounding a central
atom, X (Si+4 , P+5 , etc.). Such compounds are of in∗ Corresponding author. Fax: +54-221-4254277.
E-mail address: [email protected] (L.E. Briand)
terest because, apart from having well-defined structures [4], their M atom is easily replaced to modify the
acid–base and redox properties and, thus, the catalytic
behavior. Therefore, heteropolyoxoanions are potentially multipurpose catalysts. In this regard, there is
a necessity to produce materials that possess not only
catalytic activity, but also high thermal stability.
Most previous studies in the field of catalysis were
devoted to Keggin-type acids and salts, so research
focusing on other types of heteropolyoxoanions is in
the early stages of development [5,6]. Maksimov et al.
[7] conducted a comparative study of the activity of
0926-860X/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 6 - 8 6 0 X ( 0 0 ) 0 0 4 3 5 - X
192
L.E. Briand et al. / Applied Catalysis A: General 201 (2000) 191–202
Dawson, Keggin and Dexter-Silverton phases in the
synthesis of methyl tert-butyl ether in liquid phase
at 42◦ C. This early work concluded that Dawson
H6 P2 W18 O62 acid was the most active because of
its strong acidity, an essential requirement in this
process. More recently, Comuzzi et al. [8] studied
the thermal stability of a Wells-Dawson potassium
salt K6 P2 W18 O62 ·10H2 O. This compound showed a
structural rearrangement around 577◦ C, resulting in
a new material composed of a mixture of K3 PO4 , a
K3 PW12 O40 salt and a surface amorphous phase of
unknown composition generated by decomposition of
the Keggin phase. The new phase showed a higher
catalytic activity on the oxydehydrogenation of isobutane to isobutene than the starting Wells-Dawson salt
or a pure Keggin phase. The authors ascribed such
behavior to the amorphous phase, but they did not
identify the new active sites.
In the present work, a tungsten Wells-Dawson salt
(␣/␤ K6 P2 W18 O62 ·10H2 O) and the vanadium-monosubstituted salt (␣2 -1-K7 P2 VW17 O61 ·18H2 O) are
thoroughly characterized before and after thermal
treatment in order to obtain more insight on their
thermal stability. In addition, the nature (acid, basic
and/or redox) of the active sites was characterized by
methanol selective oxidation.
2. Experimental
2.1. Synthesis of the heteropolyoxoanions
The procedure used to synthesize the compounds
studied in this work was divided in three stages: (i)
synthesis of the mixture of ␣/␤-K6 P2 W18 O62 ·10H2 O
potassium salt isomers, (ii) preparation of the
␣2 -K10 P2 W17 O61 ·15H2 O lacunar compound by removing a WO unit from the capping atoms of the
P2 W18 O62 structure, and (iii), the hole left in the
lacunar compound was filled with a vanadium atom
to obtain ␣2 -1-K7 P2 VW17 O62 ·18H2 O monosubstituted Dawson-type structure. These stages are more
thoroughly described below.
(i) The ␣/␤-K6 P2 W18 O62 ·10H2 O isomer mixture
was prepared according to the method reported
by Lyon et al.: concentrated H3 PO4 was added
to a boiling solution of Na2 WO4 ·2H2 O in a 4:1
acid/salt molar ratio [9]. The mixture was kept
boiling for 8 h after which KCl was added to
precipitate the salt. The solid, thus obtained was
purified by re-crystallization and cooling to 5◦ C
overnight. The final product was filtered, washed
and vacuum-dried for 8 h at room temperature.
(ii) The ␣2 -K10 P2 W17 O61 ·15H2 O lacunar heteropolyoxoanion was obtained by treating the
␣/␤ isomer mixture with a KHCO3 solution at
40◦ C under stirring. The resultant precipitate was
filtered, re-dissolved in boiling water and cooled
overnight to 6◦ C. The crystals so obtained were
washed with pure ethanol and vacuum-dried for
8 h at room temperature.
(iii) The monosubstituted compound ␣2 -1-K7 P2 VW17
O61 ·18H2 O, having vanadium in its maximum
oxidation state (V+5 ), was synthesized from a
hydrochloric solution of the ␣2 -K10 P2 W17 O61 ·
15H2 O lacunar species and NaVO3 [9]. After
adding KCl to this solution, the precipitate was filtered and finally re-crystallized in a HCl solution.
The crystals were washed with distilled water
and vacuum-dried for 8 h at room temperature.
2.2. Catalysts characterization
2.2.1. 31 P-NMR spectroscopy
NMR spectra of the solutions were recorded on a
Bruker AM 500 spectrograph, operating at a frequency
of 202.459 MHz for 31 P at room temperature. According to the concentration, 5–10 ␮s pulses were used and
8–800 pulse responses were collected, the resolution
being 0.25 Hz per point.
31 P MAS-NMR spectra were recorded in a Bruker
MSL-300 equipment operating at a frequency of
121.496 MHz for 31 P at room temperature. A sample
holder of 5 mm diameter and 17 mm in height was
used. The spin rate was 2.1 kHz and several hundred
pulse responses were collected. Chemical shifts were
expressed in parts per million with respect to 85%
H3 PO4 as an external standard.
2.2.2. Scanning electron microscopy (SEM) and
EDX microprobe
The morphology of the compounds was analyzed
with a Philips SEM 505 microscope. In addition, the
percentages of tungsten, potassium and vanadium
were quantified by an EDX Philips 505 microprobe.
L.E. Briand et al. / Applied Catalysis A: General 201 (2000) 191–202
The equipment was calibrated by using standards
prepared with: H3 PW12 O40 , V2 O5 and KCO3 H. All
samples were homogenized and covered with a thin
gold layer for conductivity improvement.
2.2.3. X-ray diffraction
X-ray diffraction spectra were recorded for 2θ
values between 5 and 45◦ with a Philips PW 1390
equipment (CuK radiation and Ni filter). The following operating conditions were used: source voltage,
40 kV; source current, 20 mA; goniometer speed,
1(θ)=2◦ /min; chart speed, 2 cm/min; slit,1/0.1/1◦ .
2.2.4. Infrared analysis (FTIR)
Samples were analyzed by infrared spectroscopy
with a FTIR Bruker IFS 66 equipment.
2.2.5. X-ray photoelectron spectroscopy
X-ray photoelectron spectra were obtained with an
ESCA 750 Shimadzu instrument, using Mg K␣ radiation of 1251.6 eV. Samples were previously evacuated
in a chamber directly connected to the analysis system at room temperature. Surface charging was observed on all the samples. Binding energies of P2p ,
W4f , K2p3/2 , V2p3/2 levels were referenced to the C1s
line at 284.6 eV with an accuracy of ±0.2 eV.
2.2.6. TGA-DTA thermal analysis
TGA-DTA analyses were carried out in Shimadzu
TGA-50H and DTA-50 equipments, respectively.
Samples were heated in air from room temperature to
800◦ C at a rate of 10◦ C/min.
2.2.7. Catalytic test
Methanol selective oxidation was used as a probe
reaction to characterize acid, basic and redox sites.
The experiments were performed in a conventional,
continuous-flow reactor working under differential reactor conditions (conversion ≤10%). Full description
of the equipment and product analysis methodology
were given in a previous work [10]. The following
operating parameters were used in order to maintain
methanol conversion below 10%: sample weight,
∼200 mg, reaction temperatures, 350–450◦ C; flow
rate, 50 cm3 (NTP) min−1 and feed gas composition
methanol/oxygen/helium, 6/13/81 mol%.
193
3. Results and discussion
3.1. Spectroscopic characterization of fresh
heteropolyoxoanions
This section reviews the spectroscopic results
of the heteropolyoxoanions (vacuum-dried at room
temperature) characterized before any thermal
treatment was performed (fresh species). Table 1
shows 31 P-NMR chemical shift values for the
␣/␤-K6 P2 W18 O62 ·10H2 O isomer mixture (hereafter ␣/␤-P2 W18 ), lacunar ␣2 -K10 P2 W17 O61 ·15H2 O
(␣2 ) and the vanadium-monosubstituted compound ␣2 -1-K7 P2 VW17 O61 ·18H2 O (␣2 -V). The
18-metallo-2 phosphate anion, P2 W18 , which consists
of two PW9 half anions, presents two isomeric forms
that have already been described in the literature [11].
The ␤-P2 W18 isomer is different from the ␣ isomer,
because half its structure is rotated by π /3 around the
axis passing through such atoms. The ␣ isomer has
two equivalent phosphorus atoms and, consequently
it shows only one peak in the 31 P-NMR spectrum
at −12.3 ppm. In contrast, the ␤-isomer spectrum
shows slightly different chemical shifts (−10.8 and
−11.6 ppm) for its two phosphorus atoms [11]. The
sample used in the present study has 13% ␤-isomer,
according to the ␣/␤ ratio calculated from the NMR
signal intensities. Two signals at −6.4 and −13.6 ppm
are observed on the lacunar Dawson NMR spectrum. The first one is assigned to the phosphorus
atom whose surrounding is incomplete (PW8 ) and
the second, belongs to the unperturbed P atom. The
monosubstituted ␣2 -V compound possess a signal
at −10.7 ppm, in agreement with previous literature
reports for vanadium incorporation into a polar position of the PW8 half, and another at −12.8 ppm
corresponding to the unperturbed P atom [12]. The
observed 31 P-NMR chemical shifts coincide with
literature values, so the purity, homogeneity of the
Table 1
31 P-NMR chemical shifts of Dawson heteropolyoxoanions
Heteropolyaniona
Solvent
Chemical shiftb (ppm)
␣/␤-P2 W18
␣2
␣2 -V
Water
Water
Water
−12.3, −10.8, −11.6
−6.4, −13.6
−10.7, −12.8
a
b
K+ countercations.
Chemical shift from 85% H3 PO4 as external reference.
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L.E. Briand et al. / Applied Catalysis A: General 201 (2000) 191–202
Fig. 1. FTIR spectra of: (a) fresh ␣/␤ P2 W18 , (b) ␣/␤ P2 W18 calcined at 600◦ C, (c) fresh ␣2 -V and (d) ␣2 -V calcined at 600◦ C.
compounds and a clean heteroatom incorporation in
the lacunar structure can be ensured. More evidence
were obtained with infrared spectroscopy analysis
(Fig. 1). ␣/␤-P2 W18 and ␣2 -V spectra (Fig. 1a, c)
showed the characteristic bands of Dawson heteropolyoxoanions: PO4 tetrahedron (1089 cm−1 ), W=O
(950 cm−1 ) and W-O-W (912 and 779 cm−1 ) bonds
[9,13].
Finally, Table 2 presents bulk percentages of tungsten, potassium and vanadium obtained by EDX. The
agreement of experimental values and theoretical percentages is satisfactory.
substituted ␣2 -V compounds. A weight loss step
was detected between room temperature and 280◦ C
which, for both compounds, corresponds exactly
to the number of water molecules involved in the
K6 P2 W18 O62 ·10H2 O and ␣2 -1-K7 P2 VW17 O61 ·18H2 O
structures. From 280 to 800◦ C no additional weight
loss was observed. However, the DTA analysis revealed a small exothermic peak around 600◦ C. As
Table 2
Bulk concentrations of tungsten, potassium and vanadium
Sample
3.2. Thermal stability: characterization of the phases
generated by calcination
TGA and DTA analysis were carried out on
the ␣/␤-P2 W18 isomer mixture and the vanadium-
Wt.%
Tungsten Potassium Vanadium
Fresh ␣/␤-P2 W18 Theoretical
EDX
Fresh ␣2 -V
Theoretical
EDX
93.4
91.5
90.6
92.0
6.6
8.4
7.9
6.0
–
–
1.5
1.9
L.E. Briand et al. / Applied Catalysis A: General 201 (2000) 191–202
195
Fig. 2. (a) X-ray diffraction spectra of fresh ␣/␤ P2 W18 isomer mixture. Diagrams (b), (c) and (d) belong to samples calcined at 300, 400
and 600◦ C, respectively. Symbols: (䊉) Keggin K3 PW12 O40 salt; (䉱) P2 O5 -WO3 (-M2 O) compounds.
recently reported by Comuzzi et al. these compounds
undergo a structural rearrangement at such temperature without weight loss that results in the generation
of new phases [8]. In order to study this phenomenon
more deeply, fresh samples were calcined for 5 h at
300, 400 and 600◦ C, and the resultant phases were
characterized.
Figs. 2 and 3 show X-ray diffractograms of fresh
␣/␤-P2 W18 and ␣2 -V compounds and those of the corresponding samples calcined at different temperatures.
Fresh ␣/␤ isomer mixture (Fig. 2a) shows three intense
signals at 8.9, 9.0 and 9.4◦ , which coincide with those
reported for the ␣ isomer of the K6 P2 W18 O62 ·10H2 O
Dawson-type heteropolyoxoanion [14]. No modifications in the positions of the signals are observed at
300 and 400◦ C suggesting that, unlike in other heteropolyoxoanions, the secondary structure of these
Dawson heteropoly salts do not change after losing
the water of crystallization [15,16]. However, an increase in the background and intensity loss is observed in the XRD spectra of the samples calcined at
400◦ C (Figs. 2c and 3c). Previous studies reported a
similar effect in the XRD diffractograms of Keggin
and Dawson acids upon calcination at 250–400◦ C and
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L.E. Briand et al. / Applied Catalysis A: General 201 (2000) 191–202
Fig. 3. (a) X-ray diffraction spectra of the fresh, substituted ␣2 -V compound. Diagrams (b), (c) and (d) corresponds to samples calcined
at 300, 400 and 600◦ C, respectively. Symbols: (䊉) Keggin K3 PW12 O40 salt; (䉱) P2 O5 -WO3 (-M2 O) compounds.
300–600◦ C, respectively. The authors attributed the
phenomenon to the electrostatic repulsion of the dehydrated anions, causing a disorder of the secondary
structure [17,18]. After calcination at 600◦ C, a significant modification on the structure of both compounds is observed (Figs. 2d and 3d). 31 P MAS-NMR
spectra of the calcined salts show the presence of
three compounds with different phosphorous structures (Fig. 4a and b). The resonance around −14.8 ppm
is close to the resonance of the bulk Keggin struc-
ture but it must be noted the presence of other species
with well-defined signals in the −14 ppm zone [4].
These lines do not correspond either to lacunary or
to unsaturated species like PW11 O39 and P2 W17 O61 ,
or P2 W18 O62 and P2 W21 O71 [19]. On the other hand,
XRD spectra of ␣/␤-P2 W18 and the ␣2 -V salts calcined at 600◦ C (Figs. 2d and 3d) show exactly the
same features, which suggest that vanadium is not involved in the bulk structure of the decomposed vanadium substituted salt. In fact, some studies proposed
L.E. Briand et al. / Applied Catalysis A: General 201 (2000) 191–202
Fig. 4. 31 P MAS-NMR spectra for the heteropolyoxoanions calcined at 600◦ C. (a) ␣/␤ P2 W18 and (b) substituted ␣2 -V.
that vanadium could be expelled from the heteropolyoxoanion structure under reaction or thermal treatment. This topic will be discussed in detail in the following section.
XRD signals at 2θ: 10.9, 15.4, 18.9, 21.8, 26.7,
29.0, 30.9, 32.9, 34.8, 36.6 and 39.7◦ (denoted with a
filled circle in the diffractogram) coincide with those
calculated by the Rietvelt method for the Keggin
K3 PW12 O40 cubic salt with a0 =11.571 Å and spatial
group Pn3m [20]. The remaining signals at 2θ : 13.8,
23.3, 24.5, 27.2, 27.8, 28.3, 34.3 and 37.0◦ (denoted
with a triangle in the diffractogram) belong neither
to phosphorous oxides nor to pure WO3 , the decomposition products suggested for Keggin heteropoly
compounds [21–24]. Moreover, the signals do not
coincide with the ones published by Comuzzi et al.
for a Dawson salt decomposition at 800◦ C [8]. The
discrepancy between the results could be attributed
most probably to the temperature, since according to
Lunk et al. and Ziemer, the Dawson acid decomposes
197
in a WO3 -P2 O5 system whose chemical composition and crystallographic symmetry depends on the
temperature [25,26]. In fact, some of the observed
XRD signals (23.3, 24.5, 27.2, 28.3 and 34.3◦ ) could
be attributed to the orthorhombic W18 P2 O59 phase,
prepared by heating H6 P2 W18 O62 ·31H2 O for 8 h at
700◦ C [21]. This phase, described as P2 O5 .18 WO3 ,
is produced due to the introduction of phosphorus into the tetragonal WO3 matrix. An attempt to
identify the rest of the XRD signals by considering
a phosphorus-tungsten-potassium(vanadium) compound was not successful. It is not surprising that a
match between the observed XRD signals and published patterns was not found, since WO3 and P2 O5
oxides are able to generate compounds (crystalline and
amorphous) between a wide range of compositions,
and also admit an alkali metal [27–29]. More insights
on these phases were found with infrared analysis.
In accordance with the XRD and 31 P MAS-NMR
analysis, the infrared spectra (Fig. 1b and d) of
samples calcined at 600◦ C show the PW12 O40 3−
Keggin ion characteristic bands at: 1080 cm−1 (ν
P-O), 982 cm−1 (ν W-O), 888 cm−1 (ν W-O-W) and
801 cm−1 . Bands below 700 cm−1 could be assigned
to either lattice modes or bending modes of O-W-O
or O-P-O bond angles [30]. The additional infrared
bands have a resemblance with those belonging to
WO3 -P2 O5 or WO3 -P2 O5 -M2 O (where M is an alkali metal) systems [27]. In fact, clear differences
between the IR bands belonging to PW12 O40 3− ion
and tungsten-phosphorus compounds can be found
in the IR spectra. Bands in the 1000–1200 cm−1
region has been observed in WO3 -P2 O5 systems
with high phosphorus concentration and in general,
have been ascribed to P-O stretching vibrations of
many inorganic phosphate compounds [31]. These
are clearly different from the bands belonging to the
asymmetric P-O stretching mode for the PO4 tetrahedron of the PW12 O40 3− ion that only appear in
the 1070–1080 cm−1 wavenumber region. The signal
of the W-O stretching mode involving terminal oxygen atoms, appears at 980–985 cm−1 and 990 cm−1
for the Keggin ion and WO3 -P2 O5 (-M2 O) systems,
respectively [27,30]. A set of bands that are exclusively attributed to WO3 -P2 O5 systems can be found
in the 918–960 cm−1 region. These bands are assigned to symmetric P-O stretching modes for the
PO4 -tethraedra [27].
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L.E. Briand et al. / Applied Catalysis A: General 201 (2000) 191–202
Fig. 5. Scanning electron photomicrographs of ␣/␤ P2 W18 salt
(Magnification×1000).
Fig. 7. Scanning electron photomicrographs of ␣/␤ P2 W18 Calcined
at 400◦ C (Magnification×1000).
3.3. Dawson salts morphology: evolution during
calcination
Figs. 5–10 show the morphology of fresh ␣/␤-P2 W18
and ␣2 -V salts, calcined at 400◦ C and 600◦ C. Fresh
␣/␤-P2 W18 and ␣2 -V salts possess a smooth morphology (Figs. 5 and 6), in contrast with the rough
texture characteristic of an heteropoly compound of
the Keggin type (Fig. 11) [17]. Complete dehydration
of the salts is produced under calcination at 400◦ C,
but Dawson’s ion and crystal structures are preserved
as was discussed in the above section. SEM investigations confirmed that the morphology is not altered at
that temperature (Figs. 7 and 8). At 600◦ C the XRD
Fig. 6. Scanning electron photomicrographs of fresh ␣2 -V salt
(Magnification×1000).
Fig. 8. Scanning electron photomicrographs of ␣2 -V salt calcined
at 400◦ C (Magnification×1000).
Fig. 9. Scanning electron photomicrographs of ␣/␤ P2 W18 calcined
at 600◦ C (Magnification×1000).
L.E. Briand et al. / Applied Catalysis A: General 201 (2000) 191–202
199
morphology is similar to cubic WO3 modified with a
low content of phosphorus. This phase was identified
by Mioc et al. as a result of the calcination of Keggin
acid and an aluminum-tungsten Keggin salt at 650◦ C;
and by Lunk et al. after calcination of Dawson acid at
880◦ C [17,25]. The amount of particles belonging to
the Keggin phase is lesser than the amount of particles attributed to a WO3 -P2 O5 (-M2 O) compound(s),
which is in agreement with the low intensity of the
Keggin signal obtained in the 31 P MAS-NMR analysis. This observation could be considered as an
indication of a correct particle-structure assignment.
Fig. 10. Scanning electron photomicrographs of ␣2 -V salt calcined
at 600◦ C (Magnification×1000).
pattern, IR and 31 P MAS-NMR analysis show a recrystallization of the Dawson framework into at least
three new phases: Keggin type salt, W18 P2 O59 and
a WO3 -P2 O5 (-K2 O) compound. SEM micrographs
9 and 10 are a clear proof that calcination at 600◦ C
produces a complete modification of the heteropoly
compounds crystal structure and morphology. The
uniform smooth texture of the Dawson salts, evolved
to rectangular particles and most abundant, tabular
crystals. The first type of particles possess a rough
texture which resembles the morphology of a Keggin type heteropoly compound (compare Figs. 9 and
10, with Fig. 11). Platelet shaped particles may be
assigned to WO3 -P2 O5 (-M2 O) compound, since their
3.4. Surface analysis: vanadium mobility during
thermal treatment
X-ray photoelectron spectroscopy (XPS) surface
analysis of fresh and calcined heteropolyoxoanions are
presented in Table 3. The surface tungsten/potassium
atomic ratios of fresh ␣/␤-P2 W18 (P/K=2.82) and
␣2 -V (P/K=2.52) salts are in agreement with theoretical bulk atomic ratios (P/K␣/␤-P2W18 =3.00,
P/K␣2-V =2.42) indicating an homogeneous composition of the surface and bulk of the Dawson phase.
However, the concentrations of phosphorous and
vanadium do not coincides with the expected values.
These results may be attributed to the uncertainty of
the analysis due to the low concentrations of vanadium and phosphorous and/or the great size of the
heteropoly anion. The structure of an heteropoly
compound is normally described with a minimum arrangement of constituent atoms, that in the case of a
Table 3
Surface analysis of fresh and calcined at 300, 400 and 600◦ C
Dawson heteropoly saltsa
Sample
Fig. 11. Scanning electron photomicrographs of fresh Keggin acid
(Magnification×1000).
Surface atomic ratios
W/K
V/W
P/V
P/K
␣/␤-P2 W18
Fresh
Calcined at 300◦ C
Calcined at 400◦ C
Calcined at 600◦ C
2.82
3.04
2.14
2.89
–
–
–
–
–
–
–
–
0.59
0.52
0.56
0.51
␣2 -V
Fresh
Calcined at 300◦ C
Calcined at 400◦ C
Calcined at 600◦ C
2.52
2.72
2.55
1.55
0.04
0.04
0.03
0.42
3.96
3.60
0.54
0.59
0.35
0.39
0.36
0.39
a
W: tungsten, K: potassium, V: vanadium and P: phosphorus.
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L.E. Briand et al. / Applied Catalysis A: General 201 (2000) 191–202
Dawson compound is represented by a tungstophosphate anion P2 W18 O62 6− (two half unit composed
by a central PO4 tetrahedron surrounded by nine
WO6 octahedra). Information about this arrangement,
called primary structure, can be found in the literature for many heteropoly compounds. However, the
three-dimensional arrangement of cations, water of
crystallization and other molecules (secondary structure) of heteropoly salts or acids is barely known.
The available information indicates that the cell dimension of the cubic secondary structure of a fully
hydrated Keggin type compound is ∼23 Å [17]. Theoretical calculations performed on an arrangement
of two Dawson heteropolyanions, considering the
XRD studies of the structure, suggest that the unit
cell would have the following dimensions: a0 =11.6,
b0 =25 and c0 =15 Å [14,32]. The size of these structures is important when the results obtained with
X-ray photoelectron spectroscopy are discussed. This
technique is able to analyze the near surface region of
a sample. In fact, the photoelectrons that escape from
the solid, and are subsequently detected during the
analysis, belong to a narrow mean free path of only
10–20 Å for kinetic energies in the range 15–1000 eV
[33]. The fact that Phosphorous is in the center of
the Dawson structure and that only a fraction of
photoelectrons is suitable to be detected considering
the size of the heteropolyanion, lead to a nonreliable
quantitative determination.
Dawson salts are stable upon calcination at 300 and
400◦ C according to XRD and FTIR analysis and as
expected, the surface atomic ratios do not change due
to dehydration. At a higher temperature, the Dawson
heteropolyoxoanion decomposes in a mixture of Keggin and P-W-K compounds, which dramatically affects the surface composition. The surface is enriched
in vanadium as can be concluded from the increase in
the surface vanadium/tungsten atomic ratio. It has been
previously reported in the literature that during catalysis or after calcination, vanadium ions are ejected
from vanadomolybdic Keggin type heteropolyacids,
producing a Keggin salt free of vanadium [34]. Several studies suggested that the vanadium that goes out
of the Keggin structure produces a vanadyl salt, or
acts as a counter-cation of the Keggin anion. Catalytic
tests showed that the new phase was more active on
partial oxidation processes such as, isobutyric acid to
methylacrylic acid [15,34–36].
The similarity between the XRD, 31 P MAS-NMR
and FTIR analysis of the ␣/␤-P2 W18 and ␣2 -V salt
calcined at 600◦ C, suggests that the decomposed
vanadium-substituted Dawson salt does not have
vanadium into the Keggin structure. The fact that
vanadium is highly mobile and therefore, can be thermally spread on the surface of oxides even at 300◦ C,
allows one to propose that vanadium was expelled out
of the structure of the ␣2 -V salt during calcination
and is covering the surface [37,38].
3.5. Activity and selectivity on methanol selective
oxidation
Table 4 shows the activity and selectivity of Dawson salts towards methanol selective oxidation. The
compounds were tested at 350◦ C since this is a
characteristic temperature of selective oxidation processes. However, the salts calcined at 600◦ C were
tested at 450◦ C, because methanol conversion was
not detectable below that temperature. Catalytic activity values were extrapolated to 450◦ C for a better
comparison.
It is well known in the literature, that methanol reaction mechanism on oxides and heteropoly compounds
involves the formation of methoxy species. These intermediate species can either produce dimethyl ether
(acid–base pathway) or formaldehyde (redox pathway). Dawson salts calcined at 300◦ C only produce
formaldehyde under methanol reaction, which indicate clearly the presence of redox sites [39,40]. As
discussed previously, these salts maintain their Dawson structure intact at that temperature, however they
Table 4
Activity and selectivity of Dawson salts on methanol selective
oxidation
Sample
Calcination
Activity at
Selectivity
temperature (◦ C) 450◦ Ca (␮mol/g s) (%)b
HCHO DME
␣/␤-P2 W18 300
600
␣2 -V
300
600
2.77
0.31
2.14
0.59
100
79
100
89
–
21
–
11
a Activity based on overall methanol conversion under differential conditions.
b HCHO: formaldehyde, DME: dimethyl ether.
L.E. Briand et al. / Applied Catalysis A: General 201 (2000) 191–202
undergo a complete dehydration. Neither the preparation method nor the dehydration process generate
residual Brönsted acid sites on Dawson salts, unlike
what was observed in Keggin salts [41–43]. Although,
methanol activity of pure and vanadium substituted
Dawson salts are very similar, the activity per active
site (turnover frequency) should be known in order to
conclude about the effect of vanadium on the catalytic
behavior of these heteropolyanions.
The activity drops drastically after calcination at
600◦ C, which could be attributed to the modifications
of the structure. The exposure of phosphorus (initially
hidden into the Dawson cage), an acidic center, may
be the cause of dimethyl ether formation. Vanadium
spreading on the surface (covering acidic sites) of the
decomposed ␣2 -V salt agrees with the higher activity
and formaldehyde selectivity than the non-substituted
compound.
4. Conclusions
Pure Dawson type potassium-tungsten salt ␣/␤-K6
P2 W18 O62 ·10H2 O (␣/␤-P2 W18 ) and monosubstituted
with vanadium ␣2 -1-K7 P2 VW17 O61 ·18H2 O (␣2 -V)
possess similar thermal stability. These compounds
lose water up to 280◦ C, however, both structures are
stable up to 600◦ C. At this temperature, the Dawson
ion (P2 W18 O62 6− ) is destroyed, giving rise to three
new phases by solid–solid reaction. 31 P-NMR, XRD
and IR results lead to the conclusion that both Dawson
salts ␣/␤-P2 W18 and ␣2 -V, recrystallize in identical
phases at 600◦ C and that vanadium is not involved
in those new phases. The Keggin salt K3 PW12 O40 ,
W18 P2 O59 and a WO3 -P2 O5 -M2 O (M is an alkali
metal) compound are proposed as the decomposition
products. The assignment of these last structures is
difficult, since P2 O5 and WO3 are able to combine in
multiple proportions.
Surface studies showed that vanadium remains in
the ␣2 -V framework up to 300◦ C and is expelled out
of the structure at 600◦ C, being thermally spread on
the surface. This is an important topic that will be
further studied since the stability of vanadium (or
any element with redox properties) in the Dawson
ion, is a key to understand the behavior of these heteropoly compounds in selective oxidation processes.
Potassium-tungsten(-vanadium) Dawson salts studied
201
in this work possess low activity in methanol selective
oxidation but are highly selective towards formaldehyde. The fact that the poor activity of these materials
is related with their almost null specific surface area
(<1 m2 /g), makes interesting the synthesis of supported Dawson salts since their selectivity towards
partial oxidation products is promising.
Acknowledgements
The authors are grateful to Mrs. Graciela Valle for
performing the infrared analysis; Prof. Jose L. Garcia
Fierro for the XPS analysis and Mrs. Esther Ponzi for
TGA-DTA analysis.
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