Synthesis, thermal investigations and kinetic data of Zn(BF 4)2?6H2O

Article abstract of DOI:10.1007/s10973-008-8996-0

The thermal dehydration and decomposition of Zn(BF₄) ₂?6H₂O have been studied by TG, DTA and DSC analyses. It is found that the dehydration occurs in two steps. Following the experimental results a thermal decomposition sc

Full text of DOI:10.1007/s10973-008-8996-0

Journal of Thermal Analysis and Calorimetry, Vol. 95 (2009) 1, 319–321
Short Communication
SYNTHESIS, THERMAL INVESTIGATIONS AND KINETIC DATA OF
Zn(BF4)2×6H2O
Deyanka Nikolova* and M. Georgiev
Department of Inorganic Chemistry, University of Chemical Technology and Metallurgy, 8 Kl. Ohridski Str. 1756 Sofia, Bulgaria
The thermal dehydration and decomposition of Zn(BF4)2×6H2O have been studied by TG, DTA and DSC analyses. It is found that
the dehydration occurs in two steps. Following the experimental results a thermal decomposition scheme of the compound under in-
vestigation is proposed. The enthalpies of dehydration have been determined as well as the formal kinetic parameters are presented.
Keywords: DSC, DTA, kinetic parameters, phase transitions, zinc(II) tetrafluoroborate hexahydrate
Introduction
ambient temperature. The resulting solution was fil-
tered and concentrated by vacuum at 328 K for 6 h.
The colorless crystals were obtained after cooling and
dried in a desiccator over CaCl2.
There are scarce data about the synthesis and proper-
ties of tetrafluoroborates in the literature. Phase tran-
sition, thermal decomposition and kinetic parameters
of Cd(BF4)2×6H2O have been studied in [1]. Crystal-
line compounds of the type [M(H2O)6](BF4)2, where
M=Mn, Fe, Co, Ni, Zn have been prepared and TG,
DTA curves presented in [2]. Synthesis and dehydra-
tion process of [Cd(H2O)6](BF4)2 is described in [3].
Only TG, DTA curves of hydrates and ammoniacates
of zinc-, cadmium- and copper tetrafluoroborates are
presented in [4]. There are several recent publications
dealing with the properties of 1-butyl-3-methyl-
imidazolium tetrafluoroborate (BMIBF4) [5] and
1-butylpiridinium tetrafluoroborate (BPBF4) [6]. Fur-
thermore, new complexes [Co(MH)2(Thio)2]BF4·H2O
and [Co(DH)2(NH3)2]BF4 have been synthesized and
their crystal structure has been determined [7].
The compound was characterized by quantita-
tive analysis: Zn2+ complexometrically and BF– ac-
4
cording to [8].
Thermogravimetric (TG) and differential ther-
mal analysis (DTA) measurements were performed
using a Paulik–Paulik–Erdey MOM OD-101 appara-
tus at a heating rate of 3 K min–1 and a sample mass
of 184 mg up to 673 K. Differential scanning calori-
metric (DSC) measurements were performed on a
Perkin-Elmer DSC-4 apparatus in the temperature
range 303–483 K at a heating rate of 3 K min–1 and
sample mass of 6.4 mg. The X-ray analysis of the
solid products of the decomposition was performed
on a Zeiss TUR-M 62 apparatus with CuKa radiation.
The aim of the present study is to prepare a com-
pound with defined composition and to investigate
the thermal decomposition of Zn(BF4)2×6H2O, the ki-
netic parameters of the dehydration process as well as
the enthalpy changes accompanying the most impor-
tant phase transitions determined by DSC.
Results and discussion
The TG and DTA curves of Zn(BF4)2×6H2O are
shown in Fig. 1. A small endothermic peak of the
DTA curve in the temperature range 333–353 K,
which is not related to any mass loss in TG curve was
observed. It corresponds to a phase transition in the
solid-state. The next endothermic process of complex
character in the range 353–443 K was observed. It is
related to the dehydration of Zn(BF4)2×6H2O to
Zn(BF4)2×4H2O at Tmax=413 K.
Experimental
Zinc(II) tetrafluoroborate hexahydrate was synthe-
sized by treating zinc(II) oxide (Merck p.a.) with
tetrafluoroboric acid (35%, Merck p.a.) and stirring at
*
Author for correspondence: ddnikolova@yahoo.com
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2009 Akadémiai Kiadó, Budapest

NIKOLOVA, GEORGIEV
Fig. 2 Schematic diagram of the X-ray diffraction lines for:
a – intermediate phase at 593 K, b – phase at 750 K
Fig. 1 TG and DTA curves of Zn(BF4)2×6H2O
Zn(BF4)2×6H2O® Zn(BF4)2×4H2O+2H2O
The decrease in sample mass for two water mole-
cules is 11.1%, which is an, agreement with the value
calculated – 10.4%. The small endothermic effect in
the DTA curve at Tmax=433 K which is not connected
with any mass loss by TG, corresponding to the sam-
ple melting:
Fig. 3 DSC curves of Zn(BF4)2×6H2O
Tmax=402 K. It is connected with the ‘solid–solid’
transformation, releasing and evaporation two water
molecules, thus forming Zn(BF4)2×4H2O. The value of
D H for this temperature range is 141.9 kJ mol–1. An
endothermic process of complex character is ob-
served in the range 408–483 K. The peak at 441 K is
connected with the process of melting Zn(BF4)2×4H2O
which is in agreement with the data from DTA curve.
Subsequent heating of the melted sample leads to the
dehydration and evaporation of four water molecules,
thus producing an anhydrous salt, which decomposes
immediately. The total enthalpy for this complex pro-
cess is D HPh.tr.=487.6 kJ mol–1. The basic part of this
value is due to D Hvap of water of crystallization
38.63×4=154.52 kJ mol–1 [11].
Zn(BF4)2×4H2O(s)® Zn(BF4)2×4H2O(l)
The endothermic peak observed in DTA curve at
Tmax=493 K is connected with dehydration of
Zn(BF4)2×4H2Oat a liquid phase and simultaneous de-
composition of Zn(BF4)2 to BF3 and ZnF2.
Zn(BF4)2×4H2O(l)® ZnF2+2BF3+4H2O
The X-ray diffraction patterns for the intermedi-
ate phase confirm the formation of ZnF2 (Fig. 2a) [9].
The decrease in sample mass by TG during the
whole process of thermal dehydration and decompo-
sition is 70.0%, which was compared with that calcu-
lated – 70.2%.
Based on TG data, the kinetics of the dehydra-
tion and decomposition in the temperature range
333–433 and 443–533 K was characterized in the iso-
thermal mode. A computer program was used, includ-
ing 25 kinetic equations described in the literature, for
which – upon comparison with experimental data
from the TG curve – the value of E* and the correla-
tion coefficient are obtained [12]. The kinetic data
show that the both processes take place in the
diffusion area (Table 1).
The last small endothermic peak of the DTA curve
at Tmax=750 K is connected with partial oxidation of
ZnF2 to ZnO. The phase isolated at 750 K is proved by
X-ray diffraction data in Fig. 2b. In this figure reflec-
tions for both ZnF2 and ZnO [10] can be seen.
DSC curve for Zn(BF4)2×6H2O as registered on
heating rate of 3 K min–1 in the temperature range
of 300–500 K is shown in Fig. 3. A large endothermic
process is observed in the range 353–408 K with
Table 1 Kinetic parameters of the thermal decomposition of Zn(BF4)2×6H2O
Temperature range/K
Kinetic equation
E*/kJ mol–1
R
333–433
443–533
F=[(1–a )–1/3–1]–1
F=[(1–a )–1/3–1]–1(1–a )4/3
73.9
0.9504
0.9876
290.2
320
J. Therm. Anal. Cal., 95, 2009

Zn(BF4)2×6H2O
3 E. Mikuli, A. Migdal-Mikuli and R. Gajerski,
Conclusions
J. Therm. Anal. Cal., 68 (2002) 861.
4 F. Kütek, Coll. Czech. Chem. Comm., 32 (1967) 2353.
5 Z. H. Zhang, Z. C. Tan, Y. S. Li and L. X. Sun,
J. Therm. Anal. Cal., 85 (2006) 551.
The scheme of thermal dehydration and decomposi-
tion of Zn(BF4)×6H2O is proposed.
• The endothermic peak of the DTA curve in the tem-
perature range 333–353 K corresponds to a phase
transition in the solid-state.
6 Z. H. Zhang, L. X. Sun, Z. C. Tan, F. Xu, X. C. Lv,
J. L. Zeng and Y. Sawada, J. Therm. Anal. Cal.,
89 (2007) 289.
7 S. T. Malinovski, O. A. Bologa, E. B. Coropceanu,
R. Luboradzki and N. V. Gerbeleu, Russ. J. Coord. Chem.,
30 (2004) 339.
• The dehydration process starts at Tmax=413 K and
Zn(BF4)×4H2O is obtained.
• Zn(BF4)×4H2O melts at Tmax=433 K.
• Dehydration at Tmax=493 K of Zn(BF4)×4H2O in a
liquid phase and simultaneous decomposition to
BF3 and ZnF2 are observed.
8 Analytischeskaia chimia elementov – Bor, Isd., M.,
Nauka 1964, p. 199 in Russian.
9 JCPDS 07 – 0214.
10 JCPDS 36 – 1451.
• Partial oxidation of ZnF2 to ZnO.
11 U. Rabinovich, Vlianie istopii phys. svoistva zidkostei M.,
Nauka 1961, p. 71.
The kinetic data of dehydration and decomposi-
tion in the temperature range 333–433 and 443–533 K
are defined.
12 J. žesták, Thermophysical Properties of Solids, Akademia,
Prague 1984, pp. 240, 430.
Received: January 10, 2008
Accepted: October 2, 2008
References
1 M. Georgiev and M. Maneva, J. Thermal Anal.,
47 (1996) 1729.
DOI: 10.1007/s10973-008-8996-0
2 E. Mikuli, A. Migdal-Mikuli and S. Wrobel,
Z. Naturforsh., 54a (1999) 225.
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