Synthesis and TG/DTA study on two new metallo(VI)-arsenato(V) heteropolyacids containing vanadium(V)

Article abstract of DOI:10.1023/B:JTAN.0000017337.47205.f9

An improved method for the synthesis of two heteropolyacids of the same type: H₅[AsMo₁₀V₂O₄₀]· 13H₂O and H₅[AsW₁₀V₂O₄₀] ·16H₂O was elaborated. Th

Full text of DOI:10.1023/B:JTAN.0000017337.47205.f9

Journal of Thermal Analysis and Calorimetry, Vol. 75 (2004) 153–158
SYNTHESIS AND TG/DTA STUDY ON TWO NEW
METALLO(VI)–ARSENATO(V) HETEROPOLYACIDS
CONTAINING VANADIUM(V)
A. Fodor1*, L. Ghizdavu2, A. Ÿuteu1 and A. Caraban1
1University of Oradea, Faculty of Science, Department of Chemistry, 3700 Oradea, Calea Armatei
Romane no 5, Romania
2Babes-Bolyai University, Department of Chemistry, 3400 Cluj-Napoca, Romania
(Received September 26, 2002; in revised form February 25, 2003)
Abstract
An improved method for the synthesis of two heteropolyacids of the same type:
H5[AsMo10V2O40]⋅13H2O and H5[AsW10V2O40]⋅16H2O was elaborated. The studied compounds
were characterized by elemental analysis, IR spectra and thermal behaviour over 20–800°C temper-
ature range.
Keywords: IR spectra, Keggin triheteropolyacids, thermal behaviour
Introduction
Keggin triheteropolyacids and their compounds have special redox properties that
have been the subject of electrochemical studies in order to investigate their electro-
chemical [1–7] or analytical behaviour [8–10].
In this work the following heteropolyacids with 12 metal centres:
H5[AsM10V2O40]⋅nH2O (M=Mo or W) were obtained and characterized (Table 1).
The thermal behaviour of these compounds was studied by means of TG/DTA
measurements. To characterise the compounds atomic emission spectrometry and IR
spectra were performed.
Experimental
The heteropolyacids were obtained by a modified version of Kokorin’s synthesis
Hx[EM' M'' O40]⋅nH2O (E=P, Si; M ' =Mo, W; M '' =V, W) methods [11–14]. The new
method used the following molar ratios between the reagents and the following rec-
ipe: an aqueous solution of As2O5 (0.005 mol) were treated with an aqueous solution
y
z
*
Author for correspondence: E-mail: alexandrinafodor@rdslink.ro
1388–6150/2004/ $ 20.00
Akadémiai Kiadó, Budapest
© 2004 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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FODOR et al.: KEGGIN TRIHETEROPOLYACIDS
of a mixture of: NaVO3⋅2H2O (0.02 mol) and Na2MoO4⋅2H2O (0.10 mol), respec-
tively, Na2WO4⋅2H2O (0.1 mol).
For either the compound, the obtained hot solution pH was adjusted to the value
of 1.5 using 1 M H2SO4 and then 2 h boiled. The free H5[AsM10V2O40]⋅nH2O (M=Mo
or W) with red-orange colours were obtained from the solution by double ether ex-
traction and recrystallized in the presence of water at room temperature in darkness,
with considerable yield, between 79 and 83% (Table 1).
Table 1 Some elemental analytical data of the studied heteropolyacids
Analysis/%
Compound
η
Component
calc.
found
W
V
62.35
3.45
2.54
1.34
9.77
62.21
3.36
2.45
1.25
9.68
H5[AsW10V2O40]⋅16H2O
1
2
83% As
H2O (constitution)
H2O (crystallization)
Mo
V
As
47.60
5.05
3.71
47.43
4.90
3.39
H5[AsMo10V2O40]⋅13H2O
79%
H2O (constitution)
H2O (crystallization)
2.00
11.62
1.64
11.48
The metal contents (Mo, W, V) of the sample were determined by atomic emis-
sion spectrometry with frame on Baird Spectrovac 2000 (The Netherlands). For the As
content determination the heteropolyacid was treated with Zn and conc. HCl therefore
AsH3 was obtained. Then AsH3 were captured in HNO3 to obtain H3AsO4 which was
spectrophotometric determinate as reduced arsenomolybdate at 840 nm [15].
The H2O content was calculated from the TG curves. The IR spectra were re-
corded in KBr pellets with an UR-20 spectrophotometer.
The thermal behaviour was studied with an OD-103 MOM derivatograph (Hun-
gary), sample mass: 200 1–2 mg, the sensitivity of the balance 50 mg, heating
rate: 10°C min–1, reference material: Al2O3, atmosphere: static air.
Results and discussions
In the modified synthesis method suggested in the present paper the output increases
from 50 to 83% comparatively to the classical method [11–14]. Two new factors opti-
mizing the synthesis reaction of the two compounds, namely the reagents ratios and
the value of the media pH-s stabilized at 1.5 (this class of heteropolyacids are stable
in aqueous medium at this pH value, as was suggested in the electrochemical cyclic
voltammetry studies [1–7]).
The obtained heteropolycompounds are highly soluble in water and decompose
in aqueous medium at pH>5.
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FODOR et al.: KEGGIN TRIHETEROPOLYACIDS
155
IR spectra
At room temperature the IR spectra of the two compounds present absorption bands
in the 468–1623 cm–1 region of the spectrum (Table 2).
Table 2 IR spectrum peaks and their assignment
Peaks/cm–1 at assign (M=W, Mo)
Compound
H2O
(crystallization)
V–O
M–O–M
M–O
As–O
1
2
H5[AsW10V2O40]⋅16H2O
H5[AsMo10V2O40]⋅13H2O
518
512
780
769
902
893
983
961
1623
1617
The bands from 1623 and 1617 were assigned to the crystallization water mole-
cules in agreement with X-ray diffraction effectuated on diheteropolyacids [15, 16].
The mentioned studies showed the involvement of these water molecules in hydrogen
bonds between the molecules placed inside the heteropolyacid molecule or between
the molecules themselves.
The other peaks were assigned to: V–O, M–O, M–O–M (M=Mo, W) and As–O
valence vibration in conformity to other Keggin triheteropolyacids IR spectra [17–23].
The two obtained triheteropolyacids are Keggin ungapping type because the
As–O valence vibration peak it is not fork [22–27], that means that all As–O bonds
are length equal.
All the bands from those two studied compounds spectra are lightly displace in
lower frequencies dominium 468–1623 cm–1 comparatively to 680–1700 cm–1 for the
following
triheteropolyacids:H5[PMo10V2O40]⋅nH2O,
H5[PW10V2O40]⋅nH2O,
H5[SiMo10V2O40]⋅nH2O and H4[PMo6W6O40]⋅nH2O, [22] fact that can be explained
by the nature of P or Si central atom. Therefore the As–O bounds strength for the
studied heteropolyacids are lower than the E–O (E=P, Si) bounds strength for the up-
per mentioned heteropolyacids.
Thermal analysis
The two studied compounds show similar thermal behaviour. Figure 1 shows one rep-
resentative TG, DTG and DTA curves recorded for H5[AsW10V2O40]⋅16H2O.
The main processes and their characterisation are given in Table 3. The dehydra-
tion is a two-step process. The first loss of the crystallisation water (also named lat-
tice water) occurs at ~120°C. The constitution water determines an increase of the lat-
tice cohesion by the formation of hydrogen bonds with the terminal oxygen atom
[16], confirmed by the tmax value of the reaction, which increases as t1<t2, in agree-
ment with the increase of number of the crystallisation water molecules.
The second step in the dehydration is also an endothermic process and it in-
volves the loss of the constitution water (also named structure water). This loss sig-
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FODOR et al.: KEGGIN TRIHETEROPOLYACIDS
Fig. 1 TG, DTG and DTA curves of H5[AsW10V2O40]⋅16H2O
nificantly influences the geometry of the molecule, leading to a new structural rear-
rangement of the polyanionic complex for the compounds at ~300°C.
Table 3 Thermal data of the triheteropolyacids
DTA peak/°C
TG data/%
Temp.
range/°C
Compound
Assign.
endo
exo
calcd.
found
115
–
350
–
303
–
9.77
1.53
3.91
10.12
1.94
3.56
16H2O
2.5H2O
0.5As2O5
20–400
400–1000
20–400
1 H5[AsW10V2O40]⋅16H2O
–
–
470
610
722
–
–
–
–
–
–
–
–
–
V2O5
and WO3
108
–
355
–
307
–
11.62
2.25
5.73
12.50
2.48
5.40
13H2O
2.5H2O
0.5As2O5
2 H5[AsMo10V2O40]⋅13H2O
–
–
485
600
738
–
–
–
–
–
–
–
–
–
400–1000
V2O5
and MoO3
In the DTG curve of the compounds the peak at 350°C, respectively, at 355°C
was assigned to a mass loss corresponding both to 0.5 mol As2O5 (in good agreement
with the transformation of As2O5 into As2O3 and the volatilization of As2O3) and to
the obtained residue witch corresponds to the stoichiometric ratio of the metal oxides.
The elemental analysis shows the absence of As in the solid residues.
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FODOR et al.: KEGGIN TRIHETEROPOLYACIDS
157
The endothermic reactions from 470–738°C in DTA curves correspond to the crys-
talline phase transformation of the oxides: at ~480°C WO3 changes its geometry from
tetragonal to rhombic, at ~600°C one can observe the melting of V2O5 and in
the 720–740°C temperature range, the tetragonal MoO3 and WO3 becomes rhombic [28].
The crystalline phase formed at 400°C is a mixture of metal oxides in stoichiometric
ratios 10WO3+V2O5 and 10MoO3+V2O5, results obtained by comparing the obtained and
the calculated calcinations residue.
Conclusions
The triheteropolyacids synthesis method suggested in the present paper increases the
efficiency comparatively to the classical method. The obtained compounds were char-
acterized by elemental analysis, IR spectra and thermal behaviour over 20–800°C tem-
perature range and their determinate formula is:
H5[AsW10V2O40]⋅16H2O, respectively, H5[AsMo10V2O40]⋅13H2O.
The crystalline phase formed at 400°C is a mixture of metal oxides in stoichio-
metric ratios, results that allow us to propose an alternative method for the synthesis
of a mixture of two oxides with a known composition, a method which consists of
calcinations of the compounds at ~400°C.
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