Thermodynamic properties and thermal stability of the synthetic zinc formate dihydrate

Article abstract of DOI:10.1007/s10973-007-8587-5

Zinc formate dihydrate has been synthesized and characterized by powder X-ray diffraction, elemental analysis, FTIR spectra and thermal analysis. The molar heat capacity of the coordination compound was measured by a temperature modulated differential scanning calorimetry (TMDSC) over the temperature range from 200 to 330 K for the first time. The thermodynamic parameters such as entropy and enthalpy vs. 298.15 K based on the above molar heat capacity were calculated. The thermal decomposition characteristics of this compound were investigated by thermogravimetric analysis (TG) and differential scanning calorimetry (DSC). TG curve showed that the thermal decomposition occurred in two stages. The first step was the dehydration process of the coordination compound, and the second step corresponded to the decomposition of the anhydrous zinc formate. The apparent activation energy of the dehydration step of the compound was calculated by the Kissinger method using experimental data of TG analysis. There are three sharply endothermic peaks in the temperature range from 300 to 650 K in DSC curve.

Full text of DOI:10.1007/s10973-007-8587-5

Journal of Thermal Analysis and Calorimetry, Vol. 91 (2008) 3, 861–866
THERMODYNAMIC PROPERTIES AND THERMAL STABILITY OF THE
SYNTHETIC ZINC FORMATE DIHYDRATE
J. Zhang1,2, Y. Y. Liu1,2, J. L. Zeng1,2, F. Xu1*, L. X. Sun1**, W. S. You3 and Y. Sawada4
1Materials and Thermochemistry Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences
457 Zhongshan Road, Dalian 116023, P.R. China
2Graduate School of the Chinese Academy of Sciences, Beijing 100049, P. R. China
3Fac. Chem. and Chem. Engn., Liaoning Normal Univ., Dalian 116029, P. R. China
4Department of Nanochemistry, Faculty of Engineering, Tokyo Polytechnic University, 1583 Iiyama, Atsugi
Kanagawa 243-0297, Japan
Zinc formate dihydrate has been synthesized and characterized by powder X-ray diffraction, elemental analysis, FTIR spectra and
thermal analysis. The molar heat capacity of the coordination compound was measured by a temperature modulated differential
scanning calorimetry (TMDSC) over the temperature range from 200 to 330 K for the first time. The thermodynamic parameters
such as entropy and enthalpy vs. 298.15 K based on the above molar heat capacity were calculated. The thermal decomposition
characteristics of this compound were investigated by thermogravimetric analysis (TG) and differential scanning calorimetry
(DSC). TG curve showed that the thermal decomposition occurred in two stages. The first step was the dehydration process of the
coordination compound, and the second step corresponded to the decomposition of the anhydrous zinc formate. The apparent
activation energy of the dehydration step of the compound was calculated by the Kissinger method using experimental data of TG
analysis. There are three sharply endothermic peaks in the temperature range from 300 to 650 K in DSC curve.
Keywords: DSC, heat capacity, TG, TMDSC, zinc formate dihydrate
Introduction
other thermodynamic properties such as enthalpy and
entropy can be calculated. These parameters are
important for both theoretical and practical purposes.
Heat capacities determinations of various compounds
have attracted many researchers’ attention [10–13].
Temperature modulated differential scanning
calorimetry (TMDSC) was initially published by
Reading and co-workers in 1992, and has been greatly
developed for directly determining heat capacities of
various materials [14, 15]. TMDSC which applies a
small sinusoidal modulation of temperature
superimposed onto a linear underlying heating rate is
a recently developed extension to conventional DSC.
The structure and principle of the calorimeter have
been described in detail in the literatures [16–18].
In the present paper, molar heat capacity of zinc
formate dihydrate was measured by TMDSC and re-
ported for the first time. The accuracy of TMDSC is
established by comparing the measured heat capaci-
ties of standard sapphire (Al2O3) and benzoic acid
with previously reported values (NIST and
NBS) [19, 20]. The thermal decomposition character-
istics of this compound were investigated by TG and
DSC.
The formate anion, HCOO– is the simplest and
smallest carboxylate and is of benefit for the
formation of metal organic frameworks (MOFs) [1]
due to its small stereo effect. It has been reported that
the formates of large numbers of divalent metals
(Me=Mg, Mn, Fe, Cd, Co, Ni, Cu, Zn) form iso-
structural monoclinic crystals with general formula
Me(HCOO)2×2H2O (space group P21/c, No. 14) [2, 3].
In recent years, many metal organic frameworks
using the formate with a variety of divalent metal ions
have been synthesized, analyzed in crystal structure.
Gas sorption studies indicate they are porous and
promising candidates as sorbents for hydrogen
storage [4, 5].
It is well known that hydrated zinc formate is
one of the most important divalent metal formate.
And the crystal structure and the thermal dehydration
properties have been studies by some researchers
[3, 6–9]. However, no report about heat capacity and
thermodynamic properties of the coordination com-
pound is found in the literature. Molar heat capacities
of the materials at different temperatures are basic
data in chemistry and engineering, from which many
*
**
Author for correspondence: fenxu@dicp.ac.cn
Author for correspondence: lxsun@dicp.ac.cn
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2008 Akadémiai Kiadó, Budapest

ZHANG et al.
Experimental
capacity calibration was made by running a standard
sapphire (Al2O3) at each temperature. The calibration
method and the experiment were performed at the
same conditions as follows: 1) sampling interval:
1.00 s/pt, 2) zero heat flow at –253.15 K,
3) equilibrate at 133.15 K, 4) Modulate temperature
amplitude of ±0.5°C with period of 100 s,
5) isothermal for 5.00 min, 6) temperature ramp at
5 K min–1 to 373.15 K.
Sample preparation
All reagents not specifically listed below were
obtained from commercial sources of analytical grade
and were used as received. Zinc nitrate (10.0 g,
33.6 mmol) was dissolved in DMF/1, 4-Dioxane (2:1
by volume, 200 mL). The mixture was stirred at room
temperature until zinc nitrate entirely dissolved.
Then, excessive formic acid (3.9 g, 84.8 mmol) was
slowly added to the above transparent mixed solution.
The solution was distributed to twenty vials of 20 mL.
The vials were open, and the solvent was slowly
volatilized. After several weeks, white polyhedral
crystals were produced, collected and washed three
times with DMF (50 mL).
Elemental analyses for C2H6O6Zn found (%):
C, 12.10; H, 2.59; calc. (%): C, 12.55; H, 3.16;
FTIR (KBr pellet, 4000–400 cm–1): 3354, 1580
(OCO, str., asym.), 1398 and 1379 (OCO, deform.,
asym.), 1354 (OCO, str., sym.), 767 (OCO, deform.,
sym.), 567. A broad absorption band for v (hydroxyl)
appearing at 3354 cm–1 shows the presence of water
molecules in the complex.
Heat capacity ratios at T=200 to 330 K:
K1=0.9931; K2=1.008.
The masses of the reference and sample pans
with lids were measured to within 30±0.05 mg.
Samples were crimped in non-hermetic aluminum
pans with lids. Sample mass was weighed on a
Mettler Toledo electrobalance (AB135-S, Classic)
with an accuracy of (±0.01 mg).
Thermal analysis
The standard DSC of zinc formate dihydrate was
performed by heating at 5 K min–1 under dry nitrogen
flow (50 cm3 min–1) from 300 to 650 K, and samples
mass was about 10 mg.
A
thermogravimetric
analyzer
(Model:
The crystal structure of zinc formate dihydrate
was determined by the powder X-ray diffraction.
The resolved diffraction peaks match the standard
zinc formate hydrate sample perfectly, and no
characteristic peaks of impurities were observed [21].
Setsys 16/18, SETARAM Co., France) was used for
TG measurement of this sample under nitrogen
atmosphere (99.999%). Several experiments were
carried out at heating rates of b=5, 10, 15, 20,
25 K min–1 and the flow rate of nitrogen was
45 cm3 min–1. The mass of the sample was about
10 mg. Two Al2O3 crucibles were used (capacity:
100 mL). The reference crucible was filled with
a-Al2O3. The TG equipment was calibrated by the
CaC2O4×H2O (99.9%).
Methods
Instrument
Elemental analysis was carried on PE-2400 II Series
CHNS/O analyzer. A FTIR spectrum was recorded on
Bruker Equinox 55 infrared spectrometer using KBr
pellet in the range of 4000–400 cm–1. Powder X-ray
diffraction measurement was recorded on a X'Pert
PRO X-ray diffractometer using CuKa radiation
(40 kV, 40 mA).
Results and discussion
Heat capacity of standard sapphire (a-Al2O3) and
benzoic acid
Heat capacity measurements of each compound are
repeated three times unless stated elsewhere. The
emphasis of this work is to assess the reproducibility
and ensure accuracy of the measured data using
TMDSC (Q1000). For sapphire measurement, the
data of three reduplicate experiments and the
experimental standard deviation for standard sapphire
is given in Table 1. The experimental standard
deviation is below ±0.20, and the result shows that the
testing system of TMDSC is steady. Relative
deviations have been calculated by the following
equation:
Heat capacity measurement
The modulated temperature and standard DSC
measurements were performed on DSC Q1000
(T-zero DSC-technology, TA Instruments Inc., USA).
A liquid nitrogen cooling system was used for the
experiment at the low temperature range. And dry
nitrogen gas was used as a purge gas (50 cm3 min–1)
through the DSC cell. The temperature scale of the
instrument was initially calibrated in the standard
DSC mode, using the extrapolated onset temperatures
of the melting of indium (429.75 K) at a heating rate
of 10 K min–1. The energy scale was calibrated with
the heat of fusion of indium (28.45 J g–1). The heat
RD(%) = 102{Cp,m(exp) – Cp,m(ref)}/Cp,m(ref)
(1)
862
J. Therm. Anal. Cal., 91, 2008

SYNTHETIC ZINC FORMATE DIHYDRATE
where Cp,m(exp) is the experimental heat capacities
temperature range of 200 to 330 K, the RD (%) values
below ±3.1% are obtained as compared with the
recommended value [20].
and Cp,m(ref) is the referenced heat capacities.
In Table 1, the results show that the relative
deviation of our calibration data from the
recommended value [19] over the whole temperature
range was within ±1.3%.
Heat capacity of zinc formate dihydrate
To further attest to the reliability of the
measurements using TMDSC, the benzoic acid is
measured at the same experimental conditions and the
results are listed in Table 2. The experimental
standard deviations fluctuate below ±3.09 and also
show good reproducibility of our TMDSC data. In the
The molar heat capacities of zinc formate dihydrate
are listed in Table 3. The experimental standard
deviations below ±2.80 are obtained and show
reasonably good reproducibility in the temperature
range from 200 to 330 K.
Table 1 The experimental molar heat capacities of standard sapphire (molecular formula: Al2O3, molar mass: 101.96 g mol–1)
C
p,m (exp)/J mol–1 K–1
Standard
deviation
Cp,m (ref)
/
Relative
deviation
T/K
J mol–1 K–1
Test 1
Test 2
Test 3
Average
200
210
220
230
240
250
260
270
280
290
300
310
320
330
51.35
54.25
57.39
60.51
63.55
66.46
69.31
72.12
74.95
77.71
80.06
82.40
84.62
86.66
51.38
54.32
57.44
60.47
63.50
66.42
69.32
72.08
75.04
77.63
79.94
82.28
84.42
86.49
51.25
54.14
57.25
60.34
63.35
66.24
69.08
71.89
74.77
77.55
79.74
82.08
84.22
86.30
51.33
54.24
57.36
60.44
63.47
66.37
69.24
72.03
74.92
77.63
79.91
82.25
84.42
86.48
0.07
0.09
0.10
0.09
0.10
0.12
0.14
0.12
0.14
0.08
0.16
0.16
0.20
0.18
51.12
54.61
57.95
61.14
64.17
67.06
69.80
72.41
74.88
77.23
79.45
81.56
83.55
85.44
0.41
–0.68
–1.02
–1.14
–1.09
–1.03
–0.80
–0.52
0.05
0.52
0.58
0.85
1.04
1.22
Table 2 The experimental molar heat capacities of benzoic acid (molecular formula: C7H6O2, molar mass: 122.12 g mol–1)
C
p,m (exp)/J mol–1 K–1
Standard
deviation
Cp,m (ref)
/
Relative
deviation
T/K
J mol–1 K–1
Test 1
Test 2
Test 3
Average
200
210
220
230
240
250
260
270
280
290
300
310
320
330
103.44
107.01
111.34
115.98
120.89
125.42
130.30
135.80
141.78
147.52
153.26
157.53
162.18
167.43
100.16
103.53
107.72
112.18
116.86
121.66
126.64
132.01
137.39
143.00
147.89
153.14
158.02
163.15
101.86
105.41
109.87
114.60
119.52
124.56
129.94
135.43
141.17
147.03
152.28
157.78
163.52
169.14
101.82
105.32
109.64
114.25
119.09
123.88
128.96
134.41
140.11
145.85
151.14
156.15
161.24
166.57
1.64
1.74
1.82
1.92
2.05
1.97
2.02
2.09
2.38
2.48
2.86
2.61
2.86
3.09
102.89
107.11
111.40
115.78
120.22
124.71
129.24
133.81
138.40
143.03
147.66
152.30
156.96
161.60
–1.04
–1.67
–1.58
–1.32
–0.94
–0.67
–0.22
0.45
1.24
1.97
2.36
2.53
2.73
3.08
J. Therm. Anal. Cal., 91, 2008
863

ZHANG et al.
Table 3 The experimental molar heat capacities of zinc formate dihydrate (molecular formula: Zn(O2CH)2×2H2O, molar mass:
191.45 g mol–1)
C
p,m/J mol–1 K–1
Dataset 3
Standard
deviation
T/K
Dataset 1
Dataset 2
Average
200
205
210
215
220
225
230
235
240
245
250
255
260
265
270
275
280
285
290
295
300
305
310
315
320
325
330
170.51
172.23
174.64
177.05
179.48
181.99
184.56
187.26
189.96
192.79
195.47
198.15
200.83
203.89
206.96
209.83
212.70
215.76
218.83
222.27
224.95
228.21
231.85
235.29
238.74
242.38
246.59
170.43
172.04
174.30
176.78
179.22
181.72
184.33
186.93
189.48
192.02
194.70
197.58
200.26
203.13
206.00
208.68
211.55
214.62
217.68
220.93
223.61
226.87
230.70
234.14
237.59
241.23
245.44
166.50
168.15
170.51
172.84
175.16
177.59
180.17
182.78
185.40
188.14
190.97
193.75
196.43
199.49
202.17
205.23
208.11
211.17
214.23
217.30
220.36
223.23
226.68
230.12
233.57
237.40
242.18
169.15
170.81
173.15
175.56
177.95
180.43
183.02
185.66
188.28
190.98
193.71
196.49
199.17
202.17
205.04
207.91
210.79
213.85
216.91
220.17
222.97
226.10
229.74
233.18
236.63
240.33
244.74
2.29
2.30
2.29
2.36
2.42
2.47
2.47
2.51
2.50
2.49
2.41
2.39
2.39
2.35
2.53
2.39
2.39
2.39
2.39
2.57
2.36
2.58
2.71
2.71
2.71
2.61
2.29
The molar heat capacities of the sample are fitted
to the Shomate equation [22] of heat capacities (Cp,m
with reduced temperature (t) by means of the
nonlinear least squares fitter using the OriginPro 7.5
software:
Based on Eq. (2), the heat capacity of the sample
was calculated at 298.15 K to be 221.87 J mol–1 K–1.
From Table 3, it can be seen that the heat
capacity of the sample increases with increasing
temperature, and no phase transition or thermal
anomaly was observed. This indicates that the sample
is stable in this temperature region.
)
From 200 to 330 K
C
p,m (J mol–1 K–1) = –1157.93568 +
+ 10095.48024t – 28371.33053t2 +
+ 30241.08189t3 + 8.03318/t2
(2)
Thermodynamic functions of zinc formate dihydrate
Enthalpy and entropy of substances are basic
thermodynamic functions. The thermodynamic
functions relative to the reference temperature of
298.15 K were calculated in the temperature ranges
from 200 to 330 K with an interval of 5 K.
The thermodynamic relationships are as follows:
where t=T/1000, and T/K is the experimental
temperature. The correlation coefficient of the fitting,
R2=0.99997, and Chi2/DoF=0.0186 (DoF is the
abbreviation of the degree of freedom). The relative
deviations of all the experimental points from the
fitting heat-capacity values are within ±0.15%.
864
J. Therm. Anal. Cal., 91, 2008

SYNTHETIC ZINC FORMATE DIHYDRATE
T
ST - S298.15
=
(Cp,m /T )dT
(3)
ò
298.15
The values of thermodynamic function
HT–H298.15, ST–S298.15 are tabulated in Table 4.
Table 4 Calculated thermodynamic function data of zinc
formate dihydrate
Cp,m
/
HT–H298.15
kJ mol–1
/
ST–S298.15/
T/K
J mol–1 K–1
169.07
171.02
173.16
175.46
177.88
180.39
182.98
185.62
188.31
191.03
193.77
196.54
199.34
202.16
205.00
207.88
210.80
213.77
216.79
219.89
221.87
223.06
226.32
229.68
233.17
236.78
240.55
244.48
J mol–1 K–1
–76.70
–72.50
–68.35
–64.25
–60.19
–56.17
–52.17
–48.21
–44.27
–40.36
–36.48
–32.61
–28.77
–24.94
–21.14
–17.35
–13.58
–9.82
200
205
210
215
220
225
230
235
240
245
250
255
260
265
270
275
280
285
290
295
298.15
300
305
310
315
320
325
330
–19.03
–18.18
–17.32
–16.45
–15.56
–14.67
–13.76
–12.84
–11.90
–10.95
–9.99
–9.02
–8.03
–7.02
–6.00
–4.97
–3.93
–2.86
–1.79
–0.70
0.00
Fig. 1 TG and DSC curves of zinc formate dihydrate under
high purity nitrogen atmosphere at 5 K min–1
two peaks occur successively corresponding to the
decomposition of zinc formate, and the second peak
temperature is 552.82 K and the third peak
temperature is 565.68 K. The molar enthalpies (DHm)
of the second and third endothermic peaks are
determined to be 7.03 kJ mol–1 and 56.27 kJ mol–1,
respectively.
From the TG curve (Fig. 1), TG analysis of zinc
formate dihydrate shows that the two-step mass loss
occurs in the temperature range of 300 to 650 K at
5 K min–1. The first mass loss is about 18.80% in the
temperature range of 355–430 K, it is likely due to
dehydrate from zinc formate dihydrate to zinc
formate, which very coincides with the calculated
theory value of 18.82%. Further decomposition of
zinc formate occurs in the region of 430–650 K and
the mass loss is 37.91% (calculated 38.66%), which is
decomposed to zinc oxide in this step. The overall
mass loss of the sample was ca. 56.71% in accord
with the calculated percentage (57.48%).
–6.08
–2.35
0.00
0.41
1.38
1.53
5.09
Thermal kinetic study for TG analysis
2.67
8.80
3.83
12.50
Thermal kinetic parameter of zinc formate dihydrate
was analyzed using Kissinger method [23], which has
been widely applied to estimate the apparent
5.01
16.20
6.20
19.90
7.41
23.60
DSC and TG analysis
In the temperature range from 300 to 650 K, the DSC
curve of zinc formate dihydrate in Fig. 1 exhibits
three sharply endothermic peaks. The first significant
sharply endothermic peak corresponding to the
dehydration of zinc formate dihydrate is observed
with the peak temperature at 392.45 K. The onset
temperature and the molar enthalpy (DHm) of the
stage are determined to be 376.24
K
and
Fig. 2 TG curves of zinc formate dihydrate under high purity
nitrogen atmosphere at 5, 10, 15, 20, 25 K min–1
114.33 kJ mol–1, respectively. Next to the first peak,
J. Therm. Anal. Cal., 91, 2008
865

ZHANG et al.
References
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20, 25 K min–1, respectively. The basic data (b, Tp)
taken from the TG curves are used in the following
Kissinger equation:
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Conclusions
In this work, zinc formate dihydrate has been
synthesized and characterized by powder X-ray
diffraction, elemental analysis, FTIR spectra and
thermal analysis. The molar heat capacity, Cp,m, of
zinc formate dihydrate, was measured from 200 to
330 K using TMDSC for the first time. The thermo-
dynamic functions [HT–H298.15] and [ST–S298.15] were
derived from T=200 K to T=330 K with a temperature
interval of 5 K. The thermal stability of the compound
was investigated by the TG and DSC techniques.
The apparent activation energy of the dehydration
step of the complex was calculated to be
97.94 kJ mol–1, by the Kissinger method using
experimental data of TG analysis.
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23 H. E. Kissinger, Anal. Chem., 29 (1957) 1702.
Received: June 5, 2007
Accepted: October 16, 2007
DOI: 10.1007/s10973-007-8587-5
Acknowledgements
The authors gratefully acknowledge the National Nature
Science Foundation of China for financial support to this work
under Grant No. 20473091, 20573112 and 50671098.
866
J. Therm. Anal. Cal., 91, 2008