Introduction: Thermal decomposition kinetic analysis using Differential Scanning Calorimetry (DSC) provides a rapid and accurate method for describing reaction systems. One of the main advantages of this technique is the ability to predict material performance at temperatures and times that can be easily tested. The values of kinetic parameters significantly influence chemical and physical properties, including melting point, stability, and structural characteristics [1–3]. Compared to many other analytical methods, DSC offers the benefits of simple sample preparation and the flexibility to easily vary activation conditions. Both thermodynamic parameters (such as reaction enthalpy) and kinetic parameters (including activation energy and kinetic models) can be determined simultaneously [4–5].
Thermal studies also enable the exploration of pyrolytic synthesis of new metal complexes in the solid state that may not be obtainable via solution-based methods [6]. Metal complexes of transition metals, including Zn(II), Cd(II), Hg(II), Cu(II), and Mn(II), with organic ligands and azo-based Schiff bases have been extensively characterized through thermal analysis [7–8]. In the present study, the kinetic parameters of chromium complexes prepared using chromium trioxide (in 2-methyl-2-butanol) with benzoic acid, salicylic acid, and phthalic acid (in ethanol) were investigated. The aim was to gain insights into the thermal stability, enthalpy changes, and decomposition rates of these complexes.
Materials and Methods
Material Used: Details are presented in the table below:
Table 1: Details of the chemical used
| S.No. | Chemical | Manufacturer |
| 1. | Phthalic acid (C8H6O4) | S.d.fine-chem pvt. Ltd., Boisar, India |
| 2. | Benzoic acid (C7H6O2) | E.Merck (India) Ltd |
| 3. | Salicylic acid (C7H6O3) | S.d.fine-chem pvt. Ltd., Boisar, India |
| 4. | Chromic acid (CrO3) | Apex Chemical, Mumbai, India |
| 5. | Tertiary amyl alcohol (2-methyl-2-butanol) (C5H12O) | Merck-KGaA, Darmstadi, Germany |
| 6. | Ethyl alcohol (C2H5OH) | Merck-KGaA, Darmstadi, Germany |
Sample characterization by Differential Scanning Calorimetry (DSC)
Differential Scanning Calorimetry (DSC) analysis of the synthesized chromium complexes was performed using a Perkin Elmer DSC-7 instrument at the Department of Chemistry, IIT (ISM) Dhanbad, India. The thermal behavior of each complex was investigated under carefully controlled experimental conditions. For all measurements, perforated aluminum pans were employed to hold the samples, ensuring consistent heat transfer and preventing pressure build-up. The scans were conducted at a uniform heating rate of 50 °C per minute, starting from 50 °C and progressing up to a maximum temperature of 450 °C. This temperature range was chosen to capture all relevant thermal events, including melting, decomposition, or any phase transitions of the chromium complexes. The DSC analysis provided valuable information on the thermal stability, decomposition patterns, and energy changes associated with these complexes, which are critical for understanding their physicochemical properties and potential applications.
Results and Discussion
Cr/H₂pht/T Complexes:
Differential Scanning Calorimetry (DSC) thermograms of Cr/H₂pht/T complexes revealed various kinetic parameters associated with their thermal behavior. Both exothermic and endothermic transitions were observed, including temperature ranges and peak temperatures corresponding to decomposition processes. Key parameters such as enthalpy change (ΔH), activation energy (Eₐ), and reaction order were also determined from the DSC data.
The thermal decomposition of these complexes followed a two-step process. The first step was endothermic, with a peak temperature observed at 212.85 °C and a relatively high activation energy, indicating that significant energy input is required to initiate this decomposition stage. The second step was exothermic, with a peak temperature at 299.81 °C and a lower activation energy, suggesting that this stage releases energy and proceeds more readily than the first step. Both first-order and second-order reaction kinetics were observed during these decomposition stages.
It is important to note that the final decomposition step of the Cr/H₂pht complexes could not be recorded, as the DSC scanning was performed up to approximately 450 °C. Overall, the DSC studies indicate that Cr/H₂pht/T complexes are thermally stable at ambient temperature, while they may become labile or undergo significant structural changes at higher temperatures. These findings provide valuable insights into the thermal stability and decomposition behavior of the complexes, which could be useful for applications that require controlled thermal properties.
Table 3: Kinetic parameters of Cr/H2pht/T complex obtained from DSC thermogram
| Sample code | Temperature range(ºC) | Peak temp. (ºC) | Change in enthalpy (DH) (J/g) | Activation energy(Ea) (KJ/mol) | Order of reaction | |
| Cr/H2pht/T (Endothermic) | 181.58- 234.33 | 212.85 | 157.49 | 523.98 ±11.31 | 1.76±0.03 | |
| Cr/H2pht/T (Exothermic) | 257.13-334.13 | 299.81 | – 15.44 | 175.71 ±3.79 | 1.35±0.02 | |
Cr/H2sal complexes: In Cr/H2sal/T/1 and Cr/H2sal/T/2, second step decomposition was noticed to be an exothermic process, and heat flow took place at 295.75 and 291.05 ºC and showed a low value of activation energy, indicating lability of the complexes. The decomposition follows the first order of the reaction. The complexes showed an endothermic peak between 116.10 and 121.65 ºC for dehydration [10]. The last step change of Cr/H2sal complexes (monomeric, dimeric in nature) could not be recorded as scanning was done up to 450 ºC.
Table 4: Kinetic parameters of Cr/H2sal/T complex obtained from DSC thermogram
| Sample code | Temperature range(ºC) | Peak temp. (ºC) | Change in enthalpy (DH) (J/g) | Activation energy(Ea) (KJ/mol) | Order of reaction |
| Cr/H2sal/T/1 (Endothermic) | 73.3- 171.75 | 116.1 | 105.19 | 47.7±1.03 | 0.99±0.02 |
| Cr/H2sal/T/1 (Exothermic) | 265.23-330.67 | 295.75 | -17.9 | 126.51±2.73 | 0.88±0.01 |
| Cr/H2sal/T/2 (Endothermic) | 73.3- 171.75 | 121.65 | 141.66 | 76.31±1.64 | 1.35±0.02 |
| Cr/H2sal/T/2 (Exothermic) | 267.54-330.85 | 291.05 | -25.87 | 175.6±3.79 | 1.21±0.02 |
Cr/Hben/T complexes: In Cr/Hben/T complexes, second step decomposition is associated with the exothermic process, and heat flow took place at 223.28 ºC. The first step reaction associated with an endothermic process and having a low activation energy value shows the complexes’ lability nature. The decomposition follows the first and second order of reaction. Generally, decomposition’s first step is associated with the endothermic process, while the second is associated with the exothermic process. The last step change of Cr/Hben complexes couldn’t be recorded as scanning was done up to 450 ºC.
Table 5: Kinetic parameters of Cr/Hben/T complex obtained from DSC thermogram
| Sample code | Temperature range(ºC) | Peak temp. (ºC) | Change in enthalpy (DH) (J/g) | Activation energy(Ea) (KJ/mol) | Order of reaction |
| Cr/Hben/T (Endothermic) | 310.01-380.85 | 345.55 | 167.93 | 220.54±4.76 | 1.21±0.02 |
| Cr/Hben/T (Exothermic) | 132.96-286.42 | 223.28 | -118.3 | 31.31±0.67 | 0.66±0.01 |
Conclusion
Complexes were prepared by reacting chromium trioxide in 2-methyl-2-butanol with various carboxylic acids, including benzoic acid, salicylic acid, and phthalic acid (dissolved in ethanol), and subsequently characterized using Differential Scanning Calorimetry (DSC). The DSC thermograms of these complexes were analyzed to determine key kinetic parameters of thermal decomposition, including enthalpy change (ΔH), activation energy (Eₐ), and the order of the reaction (n).
The thermograms revealed that the complexes exhibited an endothermic peak in the temperature range of 73.3 to 394.61 °C, which can be attributed to the loss of water molecules (dehydration) from the complexes. Additionally, an exothermic peak was observed between 132.96 and 334.13 °C, corresponding to the decomposition of the organic moiety coordinated to the chromium center. Analysis of the kinetic parameters and enthalpy values indicated that these complexes possess relatively low activation energies, suggesting that they undergo thermal decomposition readily. Furthermore, the high heat of decomposition reflects the substantial energy released during the breakdown of the complexes, highlighting their significant thermal reactivity.
Overall, these findings provide valuable insights into the thermal behavior of chromium trioxide–organic acid complexes and demonstrate that the combination of DSC analysis with kinetic parameter evaluation can effectively elucidate the decomposition characteristics and stability of metal–organic complexes. Such information is essential for understanding their potential applications in catalysis, materials science, and other chemical processes where thermal stability is a critical factor.
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