Thermal decomposition of several layered zirconium salts

Article abstract of DOI:10.1023/A:1010136403670

The thermal decomposition of zirconium molybdate, tungstate and arsenate were investigated. The total mass losses of the investigated materials were 12.5, 11 and 8.5%, respectively. Despite having different crystal dimensions and structure the thermal decomposition of the samples takes place in a similar way. During heating two main endothermic processes with mass loss were observed. At the end of the thermal decomposition, oxides of the original materials were observed. The mentioned mass losses originate partly from the crystal water loss of the materials. The calculated crystal water content in the original molecule was 1.3 and 1 mole/molecule unit, respectively. Furthermore, for zirconium arsenate, a sublimation process was recorded above 960 K.

Full text of DOI:10.1023/A:1010136403670

Journal of Thermal Analysis and Calorimetry, Vol. 63 (2001) 117–123
THERMAL DECOMPOSITION OF SEVERAL
LAYERED ZIRCONIUM SALTS
L. Szirtes1, J. Megyeri1, L. Riess1 and E. Kuzmann2
1Institute of Isotope and Surface Chemistry, Chemical Research Centre of the Hungarian Academy
of Sciences, H-1525 Budapest, P.O. Box 77, Hungary
2Department of Radiochemistry, Eötvös University H-1518 Budapest, P.O. Box 32, Hungary
(Received January 30, 2000; in revised form August 29, 2000)
Abstract
The thermal decomposition of zirconium molybdate, tungstate and arsenate were investigated. The
total mass losses of the investigated materials were 12.5, 11 and 8.5%, respectively. Despite having
different crystal dimensions and structure the thermal decomposition of the samples takes place in a
similar way. During heating two main endothermic processes with mass loss were observed. At the
end of the thermal decomposition, oxides of the original materials were observed. The mentioned
mass losses originate partly from the crystal water loss of the materials. The calculated crystal water
content in the original molecule was 1.3 and 1 mole/molecule unit, respectively. Furthermore, for
zirconium arsenate, a sublimation process was recorded above 960 K.
Keywords: thermal decomposition, zirconium arsenate, zirconium molybdate, zirconium tungstate
Introduction
Zirconium phosphate in various forms (amorphous and crystalline) and their deriva-
tives or intercalates were investigated in detail [1, 2]. The thermal behaviour of this
kind of materials was discussed earlier [3, 4]. Less interest has been devoted to other
zirconium salts like molybdate, tungstate and arsenate. The possibility of obtaining
them in well-defined crystalline form is more difficult than those of phosphates and,
in addition, their ion exchange capacity is lower than that of the latter materials [5–9].
Perhaps the ‘awkward’ behaviour of these materials explains why less interest has
been devoted to these zirconium salts. Bearing in mind the above mentioned factors
zirconium molybdate, tungstate and arsenate in crystalline form were prepared, and
subsequently identified by the XRD method with the aim of carrying out thermal
analysis of the prepared materials in order to obtain more information about the ther-
mal behaviour of these salts. Our results are presented in this paper.
1418–2874/2001/ $ 5.00
Akadémiai Kiadó, Budapest
© 2001 Akadémiai Kiadó, Budapest
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Experimental
All the chemicals used were of MERCK analytical grade production.
Preparation
Zirconium molybdate (ZrMo) was synthesized as follows: an aqueous solution of am-
monium-heptamolybdate, the pH of which had previously been adjusted to 1.9 with
6 M solution of hydrochloric acid was added dropwise (at vigorous stirring) to the
stoichiometric amount of zirconium oxychloride in 1 M solution of hydrochloric
acid. The precipitate (amorphous gel) remained in contact with its mother liquid for
96 h at 360 K. The product was filtered, washed with redistilled water until pH=4 and
it was then dried at room temperature in a vacuum dessicator over P2O5. The zirco-
nium tungstate (ZrW) was prepared in the same way as described above except that
ammonium-paratungstate solution was used as the starting material. Zirconium arse-
nate (ZrAs) was produced in the following way: 100 g of zirconium oxychloride was
dissolved in 50 cm3 1 M hydrochloric acid solution. 4 M solution of arsenic acid
(120 g of As2O5) was added dropwise to this solution at room temperature (during
continuous vigorous stirring). Furthermore, 50 cm3 of conc. hydrochloric acid solu-
tion was slowly added. The mixture was then refluxed at 360 K for 100 h. Next, the
precipitate was filtered, washed until pH=4 and dried at room temperature in a vac-
uum dessicator over P2O5.
Analytical
In the case of ZrMo, 0.2 g was dissolved in 10 cm3 of hot conc. sulphuric acid and the
solution was diluted to 200 cm3. The molybdenum was precipitated with 5% AgNO3
solution. After ignition MoO3 was gravimetrically determined. The zirconium was
also gravimetrically determined in the form of ZrO2 from the filtrate, fuming it with
conc. aqua regia and following precipitation with aqueous ammonia and ignition at
1000°C. With ZrW, 0.2 g of it was dissolved in 10 cm3 of conc. sulphuric acid solu-
tion and then diluted to 200 cm3. Tungsten was determined using the method de-
scribed by Heyne [10], Zr as described above. In the case of ZrAs, also 0.2 g was dis-
solved in 10 cm3 of 1 M solution of hydrochloric acid then the solution was diluted to
25 cm3. As was determined iodometrically from 15 cm3 of the previous solution; Zr
was determined as described by Alberti et al. [11].
Identification
Sample identification was carried out by XRD method using DRON-2 type computer
controlled diffractometer with CoKα–Fe filtered radiation. The diffractograms were
done in the range of 2Θ=3–135° at a goniometer speed of 1° min–1, then they were
evaluated by a computer program [12].
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119
Thermal analysis
A computer controlled Mettler TA-1-HT type thermobalance was used for analysis under
the following conditions: temperature range 298–1023 K, heating rate= 278 K min–1,
gravimetric sensity=7993 units mg–1, sample mass=100 mg, ambience: air (dynamic),
crucible Pt, sample period 1 s.
Result and discussion
Based on the analytical data the following stoichiometric ratio was found: M/Zr=2,
where M=Mo, W and As, respectively.
The prepared materials were identified by XRD analysis. Their possible struc-
ture and some main crystallographic data were determined and calculated, respec-
tively based in each case on 42 peaks of the given diffractogram. The calculated data
were accepted only if the results simulated with CaRine W 3.1 program were in
agreement with them. The most informative fragment (in reduced form) of the given
diffractograms is shown in Fig. 1.
Fig. 1 A fragment of each of the X-ray diffractograms; 1 – ZrMo, 2 – ZrW, 3 – ZrAs
As a result of the evaluation, we found that all three prepared materials are crys-
talline. Zirconium molybdate and tungstate had a tetragonal structure (with different
crystal size), and the zirconium arsenate monoclinic one. The crystal dimensions of
zirconium molybdate were a=1.1944±0.0001 nm, and c=1.7870±0.0001 nm, space
groups I4 c,d. For zirconium tungstate the crystal dimensions were:
a=1.1870±0.0001 nm, c=1.2741±0.0001 nm, space groups I4,c,d. The dimensions of
a unit cell of zirconium arsenate in space groups P21/n are: a=0.5926±0.0001 nm,
b=0.9093±0.0001 nm, c=1.1870±0.0001 nm, and β=101.7°. If we compare these data
with that of for α-zirconium phosphate (a=0.9078 nm, b=0.5297 nm, c=1.5414 nm
and β=101.7°, space groups P21/n [13]), we can suppose that the zirconium arsenate
also has a layered structure similarly to that of found for α-zirconium phosphate.
On evaluating the thermal analytical data we found the following:
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Crystalline zirconium molybdate exhibited three endothermic processes with a
mass loss of 1.1, 6.0, 3.0% and an exothermic process with a mass loss of 2.4% result-
ing in a total mass loss of 12.5%. The above mentioned processes take place in the
temperature ranges 298–360, 470–570, 570–610 and 610–630 K, respectively
(Fig. 2). As a consequence of these data, it is apparent that the first and the second en-
dothermic processes cover parts of the crystal water loss. Based on these mass loss
data one mole/molecule unit of crystal water was calculated. The third one covers the
structural water loss originating from the decomposition of the molybdate part. The
validity of the latter value was checked by titration of the sample with potassium ion.
On the thermoanalytical curve of potassium salt for the third endothermic process a
consequent lower mass loss was observed, in addition, the quantity of the exchanged
potassium ion (calculated from the titration data) was found to be equal to the quan-
tity of hydrogen ions originating from one mole of water. The exothermic process
characterizes the oxygen loss taking place during the reorganization of molybdenum
oxide.
Fig. 2 TG, DTA curves of zirconium molybdate
Judging from the analytical and thermal decomposition data the initial material
has the composition ZrH4Mo6O21·H2O and the final product can be characterized by
the formula ZrO2·3Mo2O5. Taking these data into consideration, the following mode
of thermal decomposition can be proposed:
–H2 O
–2H2 O
ZrH4Mo6O21·H2O
→ ZrH4Mo6O21
→ ZrMo6O19 –O→2 ZrO2·3Mo2O5
up to 570 K
570–610 K
up to 630 K
For zirconium tungstate two characteristic endothermic processes with a mass
loss were observed in the temperature ranges of 370–520 and 520–690 K, respec-
tively (Fig. 3). A total mass loss of 11% was found. Based on the analytical and ther-
mal decomposition data the initial material can be characterized by the composition
ZrH2W7O24·2.9 H2O and the final product identified as a mixture of oxides of
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SZIRTES et al.: SEVERAL LAYERED ZIRCONIUM SALTS
121
Fig. 3 TG, DTA curves of zirconium tungstate
ZrO2·7WO3. If the sample was titrated with potassium ion the same result was found
as described for zirconium molybdate.
–3H2 O
–H2 O
ZrH2W7O24·2.9H2O
→ ZrH2W7O24
→ ZrO2·7WO3
370–520 K
520–610 K
In the case of zirconium arsenate, a total mass loss of 8.5% was found, 4.0% of
which belong to the first endothermic process, the second one exists in the tempera-
ture range of 450–540 K, without a mass loss, and the third one to 4.5% (Fig. 4). The
analytical data of the initial product requires 31.67% of ZrO2, 59.07% of As2O5 and
9.25% of H2O. Based on these data, for the original molecule we suggest the compo-
sition Zr(HAsO4)2·H2O, while the final product after thermal decomposition showed a
mixture of ZrO2·As2O5 (up to ~960 K), above which a sublimation process occurred.
For the thermal decomposition of this material our proposition is:
–H2 O
phase trans
–H2 O
Zr(HAsO4)2·H2O
→ Zr(HAsO4)2
→ Zr(HAsO4)2
→ ZrO2·As2O5
up to 390 K
450–540 K
540–650 K
Comparison of the thermal decomposition of zirconium arsenate and α-zirco-
nium phosphate showed considerable similarities, namely, in both cases the crystal
water, one mole/molecule unit was lost (up to 420 and 430 K, respectively). This loss
of crystal water is followed by a solid phase transition without a mass loss between
420–540 K and between 470–520 K, respectively. Then the materials lost their struc-
tural water in one step between 540–650 and 810–1000 K, respectively. Knowing the
similarities of the crystalline structure of both materials no wonder the thermal de-
compositions found are almost the same for them.
Contrary to the zirconium arsenate the other two materials (molybdate and
tungstate) have, as we know it from our earlier experiences, three-dimensional net-
work structure in which the crystal water takes place likewise as it is in the layered
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Fig. 4 TG, DTA curves of zirconium arsenate
structure. This behaviour of the crystal water can motivate the similarities found in
the case of the thermal decompositions of zirconium molybdate, tungstate and arse-
nate regardless of the difference that exists between their crystalline structure. The
similar situation of the crystal water in the structure determined in each case the first
mass loss which is then followed by different decomposition, appropriate to the
chemical composition of the given molecule.
Conclusions
Zirconium molybdate and tungstate have a tetragonal structure, that of zirconium ar-
senate is monoclinic. It means that the latter material is a crystalline monohydrogen
arsenate with layered structure, similar to that of α-zirconium phosphate.
Despite having a different structure, the investigated materials showed similar
thermal behaviour. They decomposed in two main steps practically within the same
temperature range. During the first step they lost their crystal water without changing
the crystal structure. In the second step the anionic part of the molecule is decom-
posed, as a result of which a water loss is observed and, finally, at the end of thermal
heating a mixture of metal oxides of the original molecules remained. This phenome-
non exists perhaps because of the similar position of crystal water inside the various
crystalline structures. In the case of molybdate an additional oxygen loss was ob-
served as a consequence of the reorganization of molybdenum oxide. For arsenate a
sublimation process starting at about 960 K was recorded.
* * *
The support from OTKA (M-027367) is greatly appreciated.
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