Kinetics of the complex of terbium o-methylbenzoate with 1,10-phenanthroline synthesis, decomposition mechanism

Article abstract of DOI:10.1023/A:1013186600293

The complex of [Tb₂(o-MBA)₆(PHEN)₂] (o-MBA: o-methylbenzoate and PHEN: 1,10-phenanthroline) were synthesized and chracterized by elemental analysis and IR spectroscopy. The thermal behavior of [Tb₂(o-MBA)_6<

Full text of DOI:10.1023/A:1013186600293

Journal of Thermal Analysis and Calorimetry, Vol. 66 (2001) 431–437
KINETICS OF THE COMPLEX OF TERBIUM
o-METHYLBENZOATE WITH 1,10-PHENANTHROLINE
Synthesis, decomposition mechanism
J.-J. Zhang1*, R.-F. Wang2 and H.-M. Liu3
1Experimental Center, Hebei Normal University, Shijiazhuang, 050016 P.R. China
2Department of Chemistry, Hebei Normal University, Shijiazhuang, 050016 P.R. China
3Department of Chemical and Pharmaceutical Engineering, Hebei University of Science and
Technology, Shijiazhuang, 050018 P.R. China
(Received January 4, 2001; in revised form May 12, 2001)
Abstract
The complex of [Tb2(o-MBA)6(PHEN)2] (o-MBA: o-methylbenzoate and PHEN: 1,10-phenanth-
roline) were synthesized and characterized by elemental analysis and IR spectroscopy. The thermal
behavior of [Tb2(o-MBA)6(PHEN)2] in dynamic nitrogen atmosphere was investigated by TG-DTG
techniques. The thermal decomposition process of the [Tb2(o-MBA)6(PHEN)2] occurred in three
consecutive stages at Tp 294, 427 and 512°C. The kinetic parameters and mechanisms of first de-
composition stage from analysis of the TG-DTG curves were obtained by the Malek method.
Keywords: non-isothermal kinetics, o-methylbenzoic acid, terbium complex, thermal decomposi-
tion, TG-DTG
Introduction
The rare earth complexes with aromatic acid and nitrogen-containing ligands have
many special structures and interesting luminous properties, thus arose people’s in-
terest to study them. They can be used in many areas such as extraction and separa-
tion, germicide, catalyst, luminous materials, functional materials and so on.
In the previous work [1–6], we reported the synthesis, structure, luminescence
spectrum and thermal decomposition behavior of some ternary lanthanide complexes
with aromatic acid and nitrogen-containing ligands. In this paper, we have prepared
the complex of terbium o-methylbenzoate with 1,10-phenanthroline. In order to
know more about the relationship between its structure and stability, we discussed its
thermal decomposition procedure by TG-DTG and IR techniques and the corre-
sponding non-isothermal kinetics by means of the Malek [7–8] method, which pro-
*
Author for correspondence: E-mail: jjzhang123@263.net
1418–2874/2001/ $ 5.00
Akadémiai Kiadó, Budapest
© 2001 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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ZHANG et al.: COMPLEX OF TERBIUM o-METHYLBENZOATE
vides a basis for evaluating the stability and compatibility of the composite luminous
materials of the rare earth complexes.
Experimental
Chemicals
All chemicals used were of analytical grade.
Synethesis of complex
3 mmol o-HMBA (o-methylbenzoic acid) was dissolved in 20 mL 95% C2H5OH and
its pH was controlled in a range of 6–7 with 1 mol L–1 NaOH solution, to which
1 mmol 1,10-phenanthroline dissolved in 10 mL 95% C2H5OH was added. The mixed
solution was dropped into the TbCl3 solution obtained by 1 mmol TbCl3 6H2O dis-
solved in 10 mL 95% C2H5OH. The mixture was heated under reflux with stirring for
8 h, when a white precipitate formed.
Apparatus and measurements
The carbon, hydrogen and nitrogen analyses were made using a Carlo-Erba model
1106 elemental analyzer. The metal content was assayed using EDTA titration
method.
Infrared spectroscopic analyses were recorded as KBr discs on BioRad FTS-135
spectrometer, between 4000 and 400 cm–1.
TG and DTG experiments for the title compounds were performed using a
Perkin Elmer TGA7 thermogravimetric analyzer under a nitrogen atmosphere, at a
flow rate of 40 mL min–1. The heating rate used were 3, 5, 7, 10°C min–1 from ambient
to 925°C and the sample size was 2.1±0.2 mg.
Methodology and kinetic analysis
Kinetics analysis method suggested by Malek et al. [7–8]
Calculation of the activation energy E
The calculation of the activation energy is based on multiple-scan methods where
several measurements at different heating rates are needed. In the present work, the
Kissinger [9] and Ozawa [10] methods were used to obtain E values.
Determination of kinetic model
Once the activation energy has been determined, two special functions Y(α) and Z(α)
which can easily be obtained by simple transformation of experimental data are de-
fined and a combination of the shape of function Y(α) with parameters αM and α ∞p , at
which the functions Y(α) and Z(α) have a maximum respectively, allows the determi-
J. Therm. Anal. Cal., 66, 2001

ZHANG et al.: COMPLEX OF TERBIUM o-METHYLBENZOATE
433
nation of the most suitable kinetic model. Y(α) and Z(α) functions can be expressed
as follows
dα
Y (α )=
ex
(1)
(2)
dt
dα T
Z(α )=Π(x)
dt
β
Calculation of the kinetic exponents n (or m)
To the calculation of the kinetic exponents, the appropriate equations depending on
the kinetic model determined in the above step can be employed.
Calculation of the pre-exponential factor
Knowing the value of the activation energy and kinetic model, the pre-exponential
factor can be calculated using equation:
βxp exp(xp )
A= −
(3)
′
Tf (α p )
All the computational works were done on an IBM computer using a QBASIC
program.
Results and discussion
Elemental analyses and infrared spectra
Analytical results for the complex compared with theoretical calculation results from
the proposed formulate are given in Table 1. It can be seen that the experimental data
agree with values of theoretical calculation.
Table 1 Elemental analyses of the complex
Elemental content/%
Complex
[Tb2(o-MBA)6(PHEN)2]
C
H
N
Tb
Theoretical values
Experimental data
58.07
58.27
3.93
4.20
3.76
4.18
21.35
21.44
Frequencies of characteristic absorption bands in IR spectra (cm–1 ) for ligands and
complex are listed in Table 2. The IR spectra of the complex show that the absorption va-
lency band of the C=O group, νC=O at 1689 cm–1 , disappear, whereas the bands of the
asymmetric vibrations ν
at 1606 cm–1 and of the symmetric vibrations ν
at
as(COO−
)
s(COO−
)
1418 cm–1 are apparent. The values of the splitting for the absorption bands of the valency
vibration ν
and ν
∆ν= νas –νs is very high (∆ν=188 cm–1). The IR spectra of
as(COO−
)
s(COO−
)
J. Therm. Anal. Cal., 66, 2001

434
ZHANG et al.: COMPLEX OF TERBIUM o-METHYLBENZOATE
the complex show that the absorption peak of νO–H (H2O) disappear at 3380 cm–1, the ab-
sorption band of the νC=N at 1644 cm–1 are observed to move lower wavenumber. The
spectroscopy data suggest that the terbium ion is coordinated with N atoms of
1,10-phenanthroline and O atoms of o-methylbenzoate [11, 12]. In the binuclear mole-
cule complex, each Tb3+ ions is bound not only to four bridging carboxylate oxygen at-
oms, but also two bidentate carboxylate oxygen atoms and two nitrogen atoms of phen
chelating ligand, giving coordination number of eight [5, 11, 13].
Table 2 Frequencies of characteristic absorption bands in IR spectra (cm–1 ) for ligands, complex
and some intermediate products of the thermal decompositions
ν
∆ν=
νas–νs
as(COO−
)
ν
s(COO−
)
νO–H
νC=N
νC=O
Compounds
PHEN
3380
1644
–
1689
–
–
–
–
–
–
–
o-HMBA
–
–
–
–
–
1630
–
[Tb2(o-MBA)6(PHEN)2]
Intermediate (Tb2(o-MBA)6)
End product (Tb4O7)
1606
1610
–
1418
1414
–
188
186
–
–
–
–
Thermogravimetric decomposition data
Thermal analytical data measured at β=5°C min–1 for title compound are presented in
Table 3. The percentages of mass loss and probable composition of the expelled
groups are also given. TG and DTG curves of [Tb2(o-MBA)6(PHEN)2] using a heat-
ing rate of 5°C min–1 are shown in Fig. 1.
The thermal decomposition process of [Tb2(o-MBA)6(PHEN)2] can be divided
into three stages. The first stage is at 229–309°C. The TG curve shows that the mass
loss corresponding to this temperature range is 24.05% which roughly coincides with
Fig. 1 TG-DTG curves of [Tb2(o-MBA)6(PHEN)2] using a heating rate of 5°C min–1
J. Therm. Anal. Cal., 66, 2001

ZHANG et al.: COMPLEX OF TERBIUM o-METHYLBENZOATE
435
the value of 24.20%, calculated for the loss of 2 mol phen from the complex. The IR
spectra of the residue at 309°C shows that the absorption band of C=N disappear at
1630 cm–1. The second-stage started from 309–486°C with the mass loss of 27.14%,
which corresponds to the loss of 3 mol o-MBA and the formation of Tb2(o-MBA)3.
The theoretical mass loss is 27.23%. The TG curve shows degradation in the third
step started at 486 and completed at 848°C which was considered to be removal of a
C24H21O2.5 with a mass loss of 24.58% (theoretical mass loss is 23.47%). As shown in
Table 2, the bands of the asymmetric vibrations ν
at 1610 cm–1 and of the sym-
as(COO−
)
metric vibrations ν
peared.
at 1414 cm–1 for the title complex heated 848°C disap-
)
s(COO−
By 848°C, the complex of terbium o-methylbenzoate with 1,10-phenanthroline
was completely degraded into Tb4O7, with a total loss of 75.77 mass% (theoretical
loss is 74.90 mass%). On the bases of experimental and calculated results, the thermal
decomposition of [Tb2(o-MBA)6(PHEN)2] is postulated to proceed as follows:
[Tb2(o-MBA)6(PHEN)2] → Tb2(o-MBA)6 → Tb2(o-MBA)3 → Tb4O7
Table 3 Thermal decomposition data for [Tb2(o-MBA)6(PHEN)2] in dynamic nitrogen atmo-
sphere from TG and DTG analysis
Mass loss/%
Probable comp.
of expelled
groups
Temperature DTG peak
range/°C
Stage
Intermediate
temp./°C
TG
Theory
I
229–309
309–486
294
427
24.05
27.14
24.20
27.23
–2C12H8N2
[Tb2(o-BA)6]
Tb2(o-BA)3
II
–3o-MBA
24.58
75.77a
23.47
74.90a
–C24H21O2.5
Tb4O7
III
486–848
512
aTotal loss of mass/%
Thermal decomposition kinetics
The activation energy values for the title compound determined by Kissinger and
Ozawa methods are listed in Table 4. It can be seen that the values of activation en-
ergy obtained by the two methods are in reasonable agreement, and E values calcu-
lated using the Ozawa method for the title compound are generally independent of the
value of α, which has provided a check for the constancy of E during the course of re-
action, therefore, the average value of E obtained using the Ozawa and Kissinger
methods was used in the rest of calculations.
The important feature and parameter values thus obtained, such as αM, α ∞p and
αp, corresponding to the maxima of both the Y(α) and Z(α) curves are summarized for
the title compound in Table 5. It is evident that the values of these parameters con-
form to the SB(m,n) model for title compound. The kinetic parameters calculation for
SB model are presented in Table 6.
J. Therm. Anal. Cal., 66, 2001

Table 4 The activation energy values for the first-stage decomposition of [Tb2(o-MBA)6(PHEN)2] obtained by Ozawa and Kissinger methods
E0a/
E0a/
Kissinger method
No.
α
r
No.
α
r
kJ mol–1
kJ mol–1
EK /kJ mol–1
r
1
2
3
4
5
6
7
0.1
128.28
130.81
134.06
131.69
127.35
123.84
133.13
0.9958
0.9992
0.9995
0.9994
0.9981
0.9989
0.9953
8
9
0.6
0.7
0.8
0.85
0.9
0.95
–
132.3
127.37
127.12
128.09
128.99
129.9
–
0.9977
0.9990
0.9982
0.9983
0.9982
0.9980
–
124.24
0.9980
0.15
0.2
–
–
–
–
–
–
–
–
–
–
–
–
10
11
12
13
–
0.3
0.4
0.5
0.55
aThe average value of E by the Ozawa method at various α values is 129.46 kJ mol–1. The average value of E by the Ozawa and Kissinger methods is
126.85 kJ mol–1

ZHANG et al.: COMPLEX OF TERBIUM o-METHYLBENZOATE
Table 5 The characteristic feature of the functions Y(α) and Z(α)
437
α p∞
αp
β/°C min–1
Shape of Y(α)
convex
αM
3
5
0.3782
0.4041
0.4238
0.3965
0.8388
0.7816
0.8187
0.8265
0.7759
0.7747
0.8214
0.7871
convex
7
convex
10
convex
Table 6 Kinetic parameters and mechanism
β/°C min–1
KMb
SB
m
n
lnA/s–1
E/kJ mol–1
3
5
0.2771
0.2696
0.3425
0.3014
0.4556
0.3975
0.4657
0.4587
26.16
25.68
25.12
24.95
126.85
SB
–
–
–
7
SB
10
SB
bKM: kinetic model
Conclusions
The thermal decomposition of the [Tb2(o-MBA)6(PHEN)2] occurs in three steps. In
the first step, the activation energy E is 126.85 kJ mol–1, the pre-exponential factor
lnA is 25.48 and the kinetic model was determined to be SB(m, n) model.
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