A comparative thermal decomposition kinetic investigation on Fe(III) complexes of a antipyrine Schiff base ligand, 1,2-Bis(imino-4 ’ -antipyrinyl)ethane (GA)), with varying counter anions viz. CIO_{4}^{-}, NO_{3}^{-}, SCN^{-}, Cl^{-}, and Br^{-}, ha s been done by thermogravimetric analysis by using Coats - Redfern equation. The kinetic parameters like activation energy (E), pre-exponential factor (A) and entropy of activation (Δ S) were quantified. On comparing the various kinetic parameters, lower activation energy was observed in second stage as compared to first thermal decomposition stage. The same trend has been observed for pre-exponential factor (A) and entropy of activation ( ΔS ). The present results show that the starting materials having higher activation energy (E), are more stable than the intermediate products , however; the intermediate products possess well-ordered chemical structure due to their highly negative entropy of activation ( ΔS ) values . The present investigation proves that the counter anions play an important role on the thermal decomposition kinetics of the complexes.
Solid state thermal decomposition of metal complexes and its kinetic evaluation [
GA and its Fe(III) complexes were prepared and characterised as reported earlier [
Based on the various analytical, physicochemical and spectral studies [
The non-isothermal thermogravimetric analysis is generally used to investigate the thermal stability of solid state materials. In this method, the various kinetic parameters are calculated over an entire range of temperature in a continuous manner. The rate of a solid state reaction can be generally expressed by [
d α d t = K ⋅ f ( α ) (1)
(K = specific rate constant, α = the sample undergoing reaction and f(α) = conversion function).
In linear heating rate f, T = T_{0} + ft (T_{0}―temperature of initiation)
d T d t = ϕ (2)
Substituting (2) in (1)
d α d T = K ⋅ f ( α ) ϕ (3)
where, K is the specific rate constant, which is temperature dependent, can be expressed by the Arrhenius equation
K = A e^{−E/RT} (4)
(A―Pre-exponential factor, E―energy of activation, R―gas constant and T―temperature in Kelvin).
Substituting (4) in (3)
d α d T = A e − E / R T ϕ ⋅ f ( α ) (5)
On integration, taking initial temperature as zero, this assumption is correct as no reaction occurs between T = 0 and T = T_{i}.
∫ 0 α d α f ( α ) = A ϕ ∫ 0 T e − E / R T ⋅ d T (6)
g ( α ) = A ϕ ∫ 0 T e − E / R T ⋅ d T (7)
(g(α)―conversion integral).
The temperature integral can be evaluated by Coats and Redfern equation [
ln [ g ( α ) T 2 ] = ln A R ϕ E [ 1 − 2 R T E ] − E R T (8)
where, T―temperature, A―pre-exponential factor, R―gas constant, ϕ―heating rate and E―activation energy.
On plotting L.H.S. of the equation, which includes g(α) versus 1/T, will be a straight line, from the slope and intercept are used to calculate the activation energy (E) and the pre-exponential factor (A).
The entropy of activation (DS) can be calculated by the following equation.
A = k T s h e Δ S / R (9)
(k―Boltzmann constant, h―Planck’s constant and DS―Entropy of activation).
Complex | Stage of decomposition | TG plateau (˚C) | DTG peak (˚C) | Mass loss (%) |
---|---|---|---|---|
[Fe(GA)(ClO_{4})](ClO_{4})_{2} | I | 146 - 220 | 197 | 24.52 |
II | 220 - 482 | 369 | 54.72 | |
[Fe(GA)(NO_{3})_{2}]NO_{3} | I | 142 - 264 | 192 | 27.00 |
II | 264 - 497 | 325 | 31.00 | |
III | 497 - 910 | 583 | 31.50 | |
[Fe(GA)(SCN)_{2}]SCN | I | 190 - 313 | 303 | 31.00 |
II | 313 - 515 | 453 | 25.87 | |
III | 515 - 801 | 636 | 32.02 | |
[Fe(GA)Cl_{2}]Cl | I | 211 - 400 | 307 | 36.01 |
II | 400 - 874 | 637 | 36.17 | |
[Fe(GA)Br_{2}]Br | I | 240 - 408 | 350 | 29.10 |
II | 408 - 856 | 620 | 29.00 |
[Fe(GA)(NO_{3})_{2}]NO_{3}, the nitrate complex, decomposes in three stages. The thermal decomposition stage between 142˚C - 264˚C with a mass loss of 27% corresponds to the first stage. The mass loss happens at this stage corresponds to the decomposition of three nitrate ions. This observation was confirmed by the by the infrared spectral analysis of the intermediate residue at 264˚C. The second decomposition stage between 264˚C - 497˚C with a mass loss (31%) occurs, corresponding to the decomposition of half a molecule of ligand [
The thiocyanate complex, [Fe(GA)(SCN)_{2}]SCN, decomposes in three stages between the temperature range 190˚C - 801˚C. The thermal decomposition between 190˚C - 313˚C, the DTG peak at 303˚C having a mass loss of (31%) is owing to the removal of half a molecule of GA [
[Fe(GA)Cl_{2}]Cl, the chloride complex decomposes in two stages. The thermal decomposition occurs between 211˚C - 400˚C corresponds to the first stage. The mass loss (36.01%), occurring at this stage is owing to the decomposition of half a molecule GA [
[Fe(GA)Br_{2}]Br, the bromide complex, undergoes two-stage decomposition. The first stage starts at 240˚C and ends at 408˚C, with a mass loss (29.10%) is due to the decomposition of half a molecule of GA [
Activation energy (E), pre-exponential factor (A) and entropy of activation (DS) for the thermal decomposition of Fe(III) complexes are given in
The activation energies (E) of iron(III) complexes vary in the range 31.75 to 82.23 kJ∙mol^{−1}. The corresponding values of pre-exponential factor (A) of these complexes come in the range 1.56 × 10^{−2} to 66.20 S^{−1} while the respective values of entropy of activation (DS) of these complexes fall in the range −288.61 to −213.83 J∙mol^{−1}. As the decomposition proceeds, it has been observed that there is a decrease in the value of all the kinetic parameters as we move from first stage to the second stage may be due to the more ordered structure of intermediate compound attained after the first decomposition stage.
Based on the activation energies obtained from first and second stages of
Complex | Stage | E (kJ∙mol^{−1}) | A (S^{−1}) | DS (J∙mol^{−1}) |
---|---|---|---|---|
[Fe(GA)(ClO_{4})](ClO_{4})_{2} | I | 82.23 | 66.20 | −213.83 |
II | 35.36 | 5.11 ´ 10^{−2} | −275.99 | |
[Fe(GA)(NO_{3})_{2}]NO_{3} | I | 55.42 | 2.14 | −242.24 |
II | 42.77 | 7.94 ´ 10^{−2} | −271.75 | |
III | 81.09 | 0.28 | −264.20 | |
[Fe(GA)(SCN)_{2}]SCN | I | 76.18 | 33.11 | −220.22 |
II | 32.06 | 2.01 ´ 10^{−2} | −261.78 | |
III | 61.94 | 1.87 | −231.11 | |
[Fe(GA)Cl_{2}]Cl | I | 55.42 | 0.71 | −253.28 |
II | 31.75 | 1.87 ´ 10^{−2} | −287.26 | |
[Fe(GA)Br_{2}]Br | I | 57.04 | 0.74 | −253.43 |
II | 32.89 | 1.56 ´ 10^{−2} | −288.61 |
thermal decomposition of these complexes; perchlorate and nitrate complexes were found to be highly stable respectively, while chloride complex was found to be least stable in both stages. The negative value of the entropy of activation also indicates that the activated complexes are more ordered than reactants [
A comparative thermal decomposition kinetic investigation on Fe(III) complexes of 1,2-Bis(imino-4’-antipyrinyl)ethane (GA)), with varying counter anions ( ClO 4 − , NO 3 − , SCN^{−}, Cl^{−}, and Br^{−}), by using Coats-Redfern equation has been done by thermogravimetric traces. The kinetic parameters (activation energy (E), pre-exponential factor (A) and entropy of activation (∆S)) were quantified and compared. The present study proves that the counter anions play an important role on the thermal decomposition kinetic parameters of the complexes. The studies also give an insight into the energetic characteristics of solid propellants.
The authors declare no conflicts of interest regarding the publication of this paper.
Elemo, F., Gebretsadik, T., Gebrezgiabher, M., Bayeh, Y. and Thomas, M. (2019) Kinetics on Thermal Decomposition of Iron(III) Complexes of 1,2-Bis(Imino-4’-Antipyrinyl)Ethane with Varying Counter Anions. Advances in Chemical Engineering and Science, 9, 1-10. https://doi.org/10.4236/aces.2019.91001