Correlation of thermal and spectral properties of chromium(III) picolinate complex and kinetic study of its thermal degradation Zeinab M. Abou-Gamra & Michel F. Abdel-Messih
Journal of Thermal Analysis and Calorimetry An International Forum for Thermal Studies ISSN 1388-6150 Volume 117 Number 2 J Therm Anal Calorim (2014) 117:993-1000 DOI 10.1007/s10973-014-3768-5
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Author's personal copy J Therm Anal Calorim (2014) 117:993–1000 DOI 10.1007/s10973-014-3768-5
Correlation of thermal and spectral properties of chromium(III) picolinate complex and kinetic study of its thermal degradation Zeinab M. Abou-Gamra • Michel F. Abdel-Messih
Received: 20 September 2013 / Accepted: 15 March 2014 / Published online: 12 April 2014 Ó Akade´miai Kiado´, Budapest, Hungary 2014
Abstract Chromium(III) picolinate complex, namely [Cr(pic)3]H2O, was prepared and characterized by the methods of the elemental analysis, infrared spectroscopy, X-ray diffraction, and thermal analysis (TG/DTG, DTA). The correlation of the thermal and spectral properties of the complex with its structure is discussed in the study. The correlation of the spectral data with the structure leads to the accord, and coherence was found between thermal properties and structure of the complex for both steps of the thermal decomposition (dehydration and pyrolysis of organic ligand). Activation parameters were evaluated using the theory of absolute reaction rate. Keywords Kinetic Chromium(III) Picolinic acid Thermal Decomposition
Introduction Picolinic acid is one of the most important chelating agents present in the human body. It is generated in liver and kidney during the catabolism of the essential amino acid, tryptophan, and subsequently transported to the pancreas. During digestion, it is secreted to the intestine and is used as a complexing agent in the absorption of essential metals [1, 2]. Pyridine carboxylic acids (nicotinic and picolinic) are involved in several essential biochemical processes. Effect of insulin in the body is optimized by glucose tolerance factor (GTF), formulated by two nicotinic acid ligands coordinated to chromium(III) ion [3]. Zinc picolinate has healing effect against Herpes Simplex virus [4]. Nutritional,
biochemical, and medical studies with [Cr(pic)3] [1, 5] pointed out that 200 lg per day decreased total cholesterol and LDL cholesterol and resulted in significant losses in body fat and increases in lean muscle mass [6]. Pyridine carboxylic acid complexes are also important from the industrial point of view, for example, in nuclear reactor decontamination, where low oxidation state metal ion decontamination processes use V(II)/V(III) complexes in decontamination solutions [7, 8]. Thermal decomposition of solid complexes was extensively studied since it is a measure of their structures and stabilities. Thermal decomposition of chromium(III) complexes was the subject of earlier work by Bear and Wendlandt [9], Vaughn et al. [10], Correbella et al. [11], Guindy et al. [12], Haines [13], Zhou et al. [14], Arii et al. [15], and Abou-Gamra [16]. The present study is extension of these studies which deal with thermal decomposition and effect of heating rates on the kinetic parameters of non-isothermal decomposition of chromium picolinate.
Experimental Chemicals All chemicals were of pure grade (B.D.H. and Merck) and used without further purification. Chromium(IIII) chloride hexahydrate was purchased from Merck. Picolinic acid was purchased from B.D.H. Preparation of [Cr(pic)3]H2O complex
Z. M. Abou-Gamra M. F. Abdel-Messih (&) Chemistry Department, Faculty of Science, Ain Shams University, Abbassia, Cairo, Egypt e-mail:
[email protected]
Chromium(III) picolinate complex was prepared by dissolving 15 mmol of chromium(III) chloride hexahydrate and 45 mmol of picolinic acid in 100 mL water (at pH
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&4.5 adjusted by NaOH). The solution was heated to 40 °C for 2 h and then the temperature was raised to 60 °C to concentrate the solution. The solution was left for 20 days until violet crystal was formed. The solid obtained was washed with ethanol and then dried in a vacuum desiccator. The mechanism of formation of the complex was studied by Abdel-Messih and Abou-Gamra [17].
Z. M. Abou-Gamra, M. F. Abdel-Messih Table 1 Chemical composition and IR data for chromium(III) picolinate Composition of complex Element
Experimental %
Theoretical %
Cr
12.021
11.9266
C
49.491
49.541
N
9.617
9.633
Apparatus
H
3.421
3.211
O
25.495
25.688
IR spectra were measured by Perkin Elmer spectrophotometer 1000 between 400 and 4,000 cm-1. Samples were prepared in the form of KBr disk. Carbon, hydrogen, and nitrogen were determined by Perkin Elmer 2400 CHN. Chromium was determined by atomic absorption Perkin Elmer model 3100. TG and DTA were carried out by using a Shimadzu model 50 using dynamic N2 purging gas atmosphere at a constant heating rate. X-ray diffraction data were recorded using a Philips Analytical X’PERT MPD diffractometer using CuKa radiation. The XRD patterns were recorded in a diffraction angle range from 5° to 45° with a step of 0.03° and integration time of 4 s per step. The diffraction pattern has been treated with the Rietveld refinement method.
IR data/cm-1
Results and discussion The elemental analysis of the chromium picolinate complex, [Cr(pic)3]H2O, is given in Table 1 which shows the similarity of experimental results with the theoretical content of the complex as confirmed by IR data, Table 1. IR data show that after complex formation, all bands that involve OH (2400–2800 cm-1) disappeared. Difference of symmetric and asymmetric vibrations of the carbonyl group i.e., Dm (C=Oasym - C=Osym) provided valuable information about the coordination mode of COOH group in the complexes. In free ligand, bands were observed at 1657.9 cm-1 and 1453.2 cm-1 (Dm: 204.5 cm-1) for C=Oasym and C=Osym, respectively, while in the complex were observed at 1679.4 and 1450.6 cm-1 (Dm: 228.8 cm-1) for C=Oasym and C=Osym, respectively. These shifts in frequencies and the higher magnitude of Dm indicate the monodentate coordination mode of carboxylic group of the ligand with the chromium(III) ion in center [18–20]. New bands appeared in the spectra of the complex at 543.2 cm-1, corresponding to N ? Cr and 474.4 cm-1 due to the O ? Cr vibrations which support the involvement of N and O atoms in complexation with chromium(III) [21]. IR spectrum of picolinic acid displayed a band at 1602.4 cm-1 which could be assigned to mC=N of picolinic acid. The shift in this band to a lower frequency 1565.9 cm-1 indicates that the C=N group of picolinic acid is coordinated to the chromium(III) [22]. The water molecule which is not involved in metal bonding gives rise to
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Picolinic acid
[Cr(pic)3]H2O
Assignments
3430 w, br
3525.7, 3455.9
m (O–H) water m (O–H) acid
2600 3110.6 m/3054.7 w
3110.2, 3080.3, 3055.8, 3034.1
m (C–H)
1657.9
1679.4 vs
mas (C=O) acid
1610.3 s
1606.2 s
m (ring)
1526.9 m
1565.9 w
1453.2 m
1472.8 m
ms (C=O) acid
1311.3 s, 1299.9 s
1290.5 s, 1258 m
d (C–H)
1250.5 m, 1199.7 m
1139.1 w, 1156.9 m
1159.8 m, 1084.5 m
1050.7 m d (O–H) acid
991.2 m, 966.8 w 1602.9
1565.9
m (C=N)
754 s
767 m
c (C–H)
630.8 m
692.1 m
U (C–C)
543.2 w
m (Cr–N)
474.4 s
m (Cr–O)
vs very strong, s strong, m medium, w weak, br brood
strong split IR bands at 3525.7 and 3455.9 which can be assigned to OH stretching vibration of this molecule. XRD pattern of the studied chromium picolinate and theoretical XRD pattern of [Cr(pic)3]H2O (the calculated X-ray pattern obtained using crystallographic data published by Stearn Armstrong [23] ) are shown in Fig. 1. As shown in Fig. 1, there are coincidences between peaks of our experimental data and the calculated pattern. From the observed data, the structure of the complex under investigation is
O N O
O
O Cr N
N O O
[Cr(pic)3].H2O
Author's personal copy Correlation of thermal and spectral properties of [Cr(pic)3]
995
TG for the chromium(III) picolinate complex studied at the heating rates of 5, 10, and 15 °C min-1 and DTA at the heating rate of 10 °C min-1 are shown in Fig. 2. Figure 2 shows that the TG curve of chromium(III) picolinate complex exhibits inflection point at *95 °C due to the loss of lattice water [24, 25], the second inflection point at *380 °C corresponding to the loss of first mole of picolinic acid [19], the third inflection point at *420 °C corresponding to the loss of second mole of picolinic acid, and the fourth inflection point at *490 °C attributed to the loss of third mole of picolinic acid and formation of Cr2O3 [19]. TG data were confirmed by DTA measurements, as showin in Fig. 2, which show three endothermic peaks located at 112 °C due to the loss of lattice water and a split peak at 375 and 420 °C attributed to the loss of two picolinic acid, and an intensive broad exothermic peak at 507 °C due to the elimination of the remaining picolinic acid [26], and simultaneously Cr2O3 is formed [9, 12, 16]. It is observed that the step (1) is a pure step, while other steps were mixed (step 3 started before completing step 2 and step 4 started before completing step
Fig. 1 a XRD pattern for studied complex, b theoretical XRD pattern for [Cr (pic)3]H2O
3), but each step is pure for &75 % depending on the heating rate. Consequently, heating rate (5 °C min-1) is one in agreement with calculated mass loss, Table 2. The scheme of thermal decomposition can be represented as 95 C
CrðpicÞ3 H2 O ! CrðpicÞ3 þ H2 O
CrðpicÞ3
380600 C
!
3 picolinic þ Cr2 O3
IR spectra for each step of decomposition were measured, Fig. 3, which show the disappearance of the band at 3525.7, 3455.9 cm-1 attributed to the loss of lattice water and at 3116.8, 3193 and 493.7 cm-1 due to loss of picolinic acid [19, 26]. Figure 3 also shows the appearance of a band at 555.5 cm-1 attributed to tCr–O of Cr2O3 [18]. Arrhenius plots obtained from TG data are shown in Fig. 4 for the thermal decomposition chromium picolinate complex. The non-isothermal plots were evaluated using Eq. (1) [26–30].
a
10
5
15
20
25
30
35
40
2θ
b Theoretical XRD pattern [Cr(pic)3].H2O
5
10
15
20
25
30
35
40
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Z. M. Abou-Gamra, M. F. Abdel-Messih 0.004
a
10
b
0.010
0.002
0.005
0.000 4 –0.002 3 –0.004
8
Mass/mg
1st Derivative
Mass/mg
5
0.000 6
–0.005 –0.010
4
2
–0.015 –0.006
2
1 100
200
300
400
500
600
700
–0.008 800
–0.020 100
200
300
Temperature/°C DTA UV 40.00
c 4
0.000
1st Derivative
–0.005 2 –0.010
1
600
700
800
600
d
20.00
10.00 112.5 °C
0.00
375.1 °C
–10.00 0.00
0 400
500
30.00
3
200
400
Temperature/°C
5
Mass/mg
1st Derivative
6
800
200.00
741.2 °C
400.00
600.00
800.00
Temperature/°C
Temperature/°C
Fig. 2 TG–DTG curves at heating rates, a 5 °C min-1, b 10 °C min-1, c 15 °C min-1, and d DTA at the heating rate of 10 °C min-1
Table 2 Observed and calculated mass loss during the thermal decomposition of chromium(III)–picolinate complex Step
Observed mass loss % -1
-1
Calculated mass loss %
Process for 1 mol of complex
-1
5 °C min
10 °C min
15 °C min
Step (1)
-4.326
-4.129
-4.393
4.1096
Loss of 1 mol of H2O
Step (2)
-35.743
-39.534
-39.411
28.211
Loss of 1 mol picolinic acid
Step (3)
-27.382
-26.484
-23.254
28.211
Loss of 1 mol picolinic acid
Step (4)
-15.302
-14.690
-15.170
21.808
Loss of 1 mol picolinic acid and formation of 0.5 mol of Cr2O3*
* Calculated remaining mass is 17.31 % which equivalent to formation of Cr2O3 is coincident with practical remaining mass, 17.121 %
log
ðda =dTÞb E ¼ log k ¼ log A f ðaÞ 2:303 RT
ð1Þ
where da/dT is the fraction decomposed per degree, b is the heating rate, f(a) is the kinetic differential functions, Table 3, T is the temperature in degree Kelvin, E is the energy of activation, A is the Arrhenius frequency factor, and R is the general gas constant. Values of the energy of activation were evaluated from Arrhenius plots, Fig. 4 and are given in Table 4 show that
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the dehydration process for chromium picolinate follows evaporation mechanism. Loss of the first mole of picolinic acid involves the break of Cr–N bond and Cr–O. Zero-order and the first-order kinetics were confirmed by applying Eq. 2 [9] log
ðdw=dtÞ E=2:303 R ð1=TÞ ¼x log wr log wr
ð2Þ
where wr = w1 - wt, w1 is the mass loss at the completion of the reaction, wt that at time t, and x at the order of
Author's personal copy Correlation of thermal and spectral properties of [Cr(pic)3] Fig. 3 IR spectra of the product of different steps
997
heated at 620 °C
Transmittance
heated at 440 °C
heated at 395 °C
heated at 120 °C
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers/cm–1
Table 3 The selective mechanism functions for evaluation of the kinetics equations Name of functions
Kinetics mechanism
f(a)
Zero order
Formal chemical reaction
1
First order
Formal chemical reaction
(1 - a)
Second order
Formal chemical reaction
(1 - a)2
Nth order
One-dimensional diffusion
0.5a-1
Parabola law
Two-dimensional diffusion
-1/ln(1 - a)
Valensi equation
Three-dimensional diffusion
1.5(1 - a)2/3[1 - (1 - a)1/3]-1
Jander equation
Three-dimensional diffusion
1.5[(1 - a)-1/3 - 1 -]-1
Cylindrical symmetry
Phase boundary reaction
2(1 - a)1/2
Spherical symmetry Prout–Tompkins equation
Phase boundary reaction
3(1 - a)2/3 (1 - a) a
Expanded Prout-Tompkins equation
Auto_catalysis Ramiform nucleation
(1 - a)n aa
Avrami–Erofeev equation
Two-dimensional nucleation and growth
(1 - a)[-ln(1 - a)]1/2
Three-dimensional nucleation and growth
3 (1 - a)[-ln(1 - a)]2/3
N-Dimensional nucleation and growth
n (1 - a)[-ln(1 - a)](n-1)/n
Ginsting–Brounstein contracting sphere
reaction. A plot of log (dw/dt)/log wr vs. (1/T)/log wr yields straight line, Fig. 5. From the values of intercepts, steps 1, 2, and 4 (where x & 1) follow first-order kinetics, while step 3 (where x & 0) follows zero-order kinetics which mean that the loss of second picolinic acid was very fast in comparison to steps 2 and 4. Values of the energy of activation obtained from Eq. (2) are the same order of magnitude as that given in Table 4. The data given in Table 4 indicate that the dehydration process follows evaporation mechanism, whereas the loss of picolinic acid,
which involves the break of Cr–N and Cr–O bonds, is the rate-determining step. Values of DS*, the entropy of activation, were calculated from the theory of absolute reaction rate Eq. (3) A ¼ K
T e DS =R h
ð3Þ
are also given in Table 4 which shows that the activated complex is less restricted in rotation (DS* B 0) for all steps (loss of lattice water and loss of picolinic acid processes)
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Author's personal copy Z. M. Abou-Gamra, M. F. Abdel-Messih 0.0 –0.2 –0.4 –0.6 –0.8 –1.0 –1.2 –1.4 –1.6 –1.8 –2.0 –2.2 –2.4 –2.6 –2.8 –3.0 2.35
–1
5 °C/min
10 °C/min
15 °C/min
15 °C/min
2.40
2.45
2.50
2.55
2.60
2.65
–4 1.40
2.70
1.42
1.44
1.46
–1
1.48
3
1/T x 10 /K
–2.7
1.50
1.52
–1
5 °C/min
–2.0
10 °C/min
–2.1
5 °C/min
15 °C/min
–2.2
10 °C/min
–2.3
15 °C/min
(logdα /dT.β )/F(α )
–2.8 –2.9 –3.0 –3.1 –3.2
1.24
–3
Step (2)
3
–3.3
–2
Step (1)
1/T x 10 /K
(logdα /dT.β )/F(α )
5 °C/min
10 °C/min
(logdα /dT.β )/F(α )
(logdα /dT.β )/F(α )
998
–2.4 –2.5 –2.6 –2.7 –2.8 –2.9 –3.0
Step (3)
–3.1 1.26
1.28
1.30
1.32 3
1/T x 10 /K
1.34
1.36
1.38
1.05
1.40
Step (4) 1.10
1.15
–1
1.20
1.25 3
1/T x 10 /K
1.30
1.35
–1
Fig. 4 Arrhenius plots for non-isothermal of different steps
Table 4 Values of the Arrhenius parameters for thermal decomposition of chromium(III)–picolinate complex Temp/K
Kinetic parameters
5 °C min-1
Step (1) loss of lattice water 385
Step (2) loss of 1st picolinic acid
648.1
E/kJ mol
-1
Step (4) loss of 3rd picolinic acid and formation of 0.5 Cr2O3
693
780
15 °C min-1 63.362
70.677
63.362
4.97E ? 08
765561.4
765561.4
DS*/J K-1 mol-1
-127.07
-180.82
-180.82
E/kJ mol-1
372.17
440.14
330.8.80
A/s
2.16551E ? 2
9.09829E ? 30
5.26381E ? 22
DS*/J K-1 mol-1
?205.70
?294.06
?136.6
E/kJ mol-1
161.09
111.80
135.64
A/s-1
245307033.2
39774.0661
1536809.371
DS*/J K-1 mol-1
-136.89
-137.89
-210.325
E/kJ mol-1 A/s-1
102.29 224517.3978
65.10 6927.55528
113.8342 902111.044
DS*/J K-1 mol-1
-196.751
-225.622
-185.207
except the step 2 for chromium picolinate complex compared to the reactant [30, 31]. As for the second step, loss of first picolinic acid at all heating rates, values of DS*,
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10 °C min-1
A/s-1
-1
Step (3) loss of 2nd picolinic acid
Thermogravimetry
Table 4, indicate high activity of escaping molecule by the rotation of reactant on the surface and/or by formation of a mobile surface layer.
Author's personal copy Correlation of thermal and spectral properties of [Cr(pic)3]
999 1.9
2.4 1.8
(logdw/dt)/logwr
(logdw/dt)/logwr
2.2
2.0
1.8
1.6
1.7
1.6
1.5
1.4 1.4 –0.75
step(1) –0.70
step(2) –0.65
–0.60
–0.55
–0.50
1.3 –0.54 –0.52 –0.50 –0.48 –0.46 –0.44 –0.42 –0.40 –0.38 –0.36 –0.34
–0.45
(1/T )/logwr
(1/T )/logwr 2.0
2.0
1.9
(logdw/dt)/logwr
(logdw/dt)/logwr
1.8
1.6
1.4
1.8 1.7 1.6 1.5
1.2 1.4
step(3)
step(4)
1.0 –0.45
–0.40
–0.35
–0.30
–0.25
1.3 –0.44 –0.42 –0.40 –0.38 –0.36 –0.34 –0.32 –0.30 –0.28 –0.26 –0.24
(1/T )/logwr
(1/T )/logwr
Fig. 5 Kinetics of the thermal decomposition of different steps at heating rate = 5 °C min-1
Conclusions The chromium(III) complex with picolinic acid was obtained as compound with the formula, [Cr(pic)3]H2O. The proposed structure is confirmed by FTIR spectrum and X-ray analysis of the complex. This is in good agreement with the data obtained from thermal analysis. Thermal decomposition was accomplished by four steps, (1) the loss of lattice water, (2) the loss of first picolinic acid, (3) the loss of second picolinic acid, (4) the loss of third picolinic acid and formation of Cr2O3. Also, kinetics and thermodynamic parameters were calculated for each step. Acknowledgements The authors are grateful to M.S. Al-Kotb, Physics Department, Faculty of Science, Ain Shams University, Egypt, for his helpful discussion of theoretical X-ray.
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