Antioxidant Properties of Biodiesel-Diesel Blends and Non-Isothermal Kinetic Studies of Schiff-Based Isatin-Thiosemicarbazone Derivatives

1. Introduction

The prediction indicates that there will be an increase in the utilization of fossil fuels to meet the growing energy demands. In recent years, there has been a growing body of research dedicated to exploring alternative fuel sources as a response to the detrimental effects of fossil resource consumption on environmental contamination [1]. The utilization of biodiesel offers a wide range of advantages. The non-toxic nature of this specific characteristic provides both safety and cost-effectiveness, making it a significant advantage [2]. The production of biodiesel is a complex process that involves several chemical reactions. These reactions lead to the formation of free radicals, which are highly reactive molecules that can easily undergo oxidation when exposed to the surrounding atmosphere [3]. In a study conducted, it was shown that the oxidation process has a negative impact on both fuel efficiency and engine performance [4]. The storage stability of biodiesel is crucial in assessing its overall quality. The significance of standards and fuel quality cannot be overstated [5].

Antioxidants are a group of molecules that effectively halt the process of oxidation. In order to effectively manage the oxidation of biodiesel, one method employed is the inhibition of free radical formation or the scavenging of existing radicals [6]. Antioxidants are commonly composed of phenolic functional groups within their chemical structure [7]. In recent studies, it has been discovered that the addition of both natural and synthetic antioxidants to biodiesel is crucial for improving and maximizing its oxidative stability. The significance of reducing the amount of free radicals in biodiesel and delaying oxidation is highlighted by the use of antioxidants [8]. The auto-oxidation mechanism involves the interruption of the peroxide radical (ROO^.) progression by the antioxidant (AH), which inhibits the creation of an additional radical. The oxidation stability of biodiesel can be improved by incorporating antioxidants in specific proportions This addition serves to prevent the occurrence of oxidative reactions with antioxidant radicals [9]. The addition of antioxidants to biodiesel has been found to significantly improve its overall quality, extend its storage time, and enhance its durability [10].

The study of heterocyclic Schiff bases and their metallic complexes is of significant interest in the fields of chemistry, biology, and pharmacology [11,12]. These compounds are formed through the primary amino groups of ketones or the condensation reaction of aldehydes [11,13]. They exhibit various biological functions, including antioxidant [14], antifungal [14], antitumor [15], anticancer [16], antiviral [17], antimicrobial [18], and antimalarial properties [19]. Phenolic Schiff bases (SB-OH) exhibit antioxidant activity that is directly linked to their capacity to release hydrogen atoms [20]. Thiosemicarbazones are a notable category of synthetic chemicals that exhibit a diverse range of pharmacological and biological activities [21]. A diverse range of actions have been disclosed, including anti-HIV [22], anticancer [23], antimalarial [24], anticonvulsant [25], anti-inflammatory [26], antioxidant [27], anti-viral [28], enzymatic inhibition [29], antifungal [30], and antibacterial [31] properties. Isatin derivatives have demonstrated a wide range of biological activities, including antioxidant [32], antiviral [33], antifungal [34], antibacterial [35], antimicrobial [36], anti-HIV [22], antitubercular [37], and anticonvulsant [38] properties.

In this study, two previously synthesized isatin-thiosemicarbazones were used, whose chemical structures were elucidated by spectroscopic methods. Isatin-thiosemicarbazone derivatives were used to neutralize radical reactions in biodiesel-diesel fuel mixtures. Using a comparative analysis with Trolox, the antioxidant capacity of the compounds employed was confirmed. Trolox facilitates high-throughput screening for putative antioxidant capacity. These approaches are employed to evaluate the antioxidant capacity of biological samples such as plasma, individual chemicals, dietary components, or food extracts [40,41]. But it is believed that this synthetic antioxidant variant possesses hazardous and carcinogenic properties. Isatin-thiosemicarbazone derivatives were added, and a concentration of 3000 parts per million (ppm) was added to the mixtures. Several characterization techniques, including thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), fourier-transform infrared spectroscopy (FT-IR), and the 1,1-diphenyl-2-picryl hydrazyl (DPPH^.) assay, were used to evaluate the antioxidant activity of isatin-thiosemicarbazone derivatives. Also, the investigation of the breakdown kinetics of isatin-thiosemicarbazone derivatives was conducted using TGA. Non-isothermal experiments are more appropriate than isothermal kinetic studies for avoiding a rapid increase in temperature in the sample at the start. Conventional techniques that include fitting experimental data to the reaction model are not effective in providing accurate kinetic information for non-isothermal research. This is because these approaches fail to properly differentiate between the reaction model, f(α), and the temperature dependency, k(T). To address this drawback of model fitting, one might employ model-free isoconversional approaches. These approaches enable the determination of the activation energy based on conversion or temperature without making any assumptions about the reaction model [41].

2. Materials and Methods

2.1. Materials

Compounds from recognized suppliers such as Merck, Sigma, or Aldrich Chemical Company were used without further purification in their as-received form. Diesel fuel (D100) was given to OPET in Samsun, Turkey, while biodiesel (B100), made totally of waste sunflower and corn oil, was given to Aves Energy Oil and Food Industry in Mersin, Turkiye.

2.2. Methods

2.2.1. Schiff Based Isatin-Thiosemicarbazones Synthesis

Isatin-thiosemicarbazones were synthesized by reacting 5-methoxyisatin with thiosemicarbazides in aqueous ethanol and one drop of hydrochloric acid at reflux for three hours (Figure 1). The synthesis and characterization of the compounds have been performed previously by Muglu [42]. Physical data, melting points, and yields of the compounds are summarized in Table 1.

2.2.2. Preparations of Biodiesel-Diesel Blends

The ratio of biodiesel to diesel in diesel and biodiesel blends typically ranges from 20% to 80%. The mixture being blended is represented by the formula B20D80 [43]. In a recent study, researchers explored the potential benefits of combining extracts from various plants. The extracts were carefully blended together at a concentration of 3000 ppm. This concentration was chosen to ensure optimal effectiveness and maximize the potential benefits of the combined extracts. The researchers aimed to investigate the potential synergistic effects that could arise from this combination, as each plant extract may possess unique bioactive compounds. By combining these extracts, the researchers hoped to create a powerful formulation that could offer enhanced therapeutic properties [44]. Further research is needed to fully understand the implications and potential applications of Table 2. Various sample codes and content were presented.

2.2.3. Differential Scanning Calorimetry (DSC)

DSC approaches can be utilized for the purposes of characterizing, quantifying, and inferring. The primary objective of this study was to investigate the temperatures at which crystallization begins (measured in degrees Celsius) for the materials D100, B30D70, B30D70BHT, B30D70_2, and B30D70_1. The experiment was carried out with a TA Q-2000 type calorimeter equipped with an RCS90 and a cooling system. Aluminum pans were utilized for the purpose of conducting analysis. In the course of this experimental process, a sample with a weight of 5 ± 0.5 mg was carefully positioned within the pan. The cooling rate throughout the temperature range of 25°C to -90°C was determined to be 10°C per minute, while a nitrogen flow rate of 50 mL per minute was maintained [45].

2.2.4. Thermogravimetric Analysis (TGA)

TGA is commonly employed to investigate the correlation between changes in mass and temperature variations, either rising or constant, within controlled air circumstances. This analytical methodology is utilized to quantify vapors, analyze combustion reactions, examine degradation processes, and ascertain residual components in products. The TGA technique has been employed to observe the decomposition characteristics of various organic compounds and the potential improvement of their constituents. TGA was performed using a SDT Q-600 instrument manufactured by TA Instrument-Waters, located in the United States. The collection of samples was achieved by subjecting five powder samples, each weighing 5 ± 0.5 mg, to a heating process at a rate of 10 °C per minute. This heating process took place in an alumina pan, and an oxygen gas flow of 50 mL per minute was maintained during the process. The temperature was increased until it reached 400 °C, as documented in reference. Thermal studies of isatin-thiosemicarbazone compounds were conducted using a Hitachi STA 7300. TGA, DTG, and DTA curves were obtained by measuring the thermal behavior of 3 mg samples at a heating rate of 10 °K min⁻¹. The experiment was conducted within a temperature range of 25–900 °C in a nitrogen environment with a flow rate of 100 mL min^-1. A crucible made of Al₂O₃ is utilized for conducting measurements [46].

2.2.5. Fourier Transform Infrared Spectroscopy (FT- IR)

FT-IR (Perkin Elmer, Spectrum-Two, USA) was used to analyze the chemical functional groups in isatin-thiosemicarbazone derivatives. The spectral region ranging from 650 to 4000 cm^-1 was utilized for the purpose of scanning the surface of the sample. The ATR FT-IR spectra were obtained using an isothermal method at room temperature. At the beginning of our work, the background removal techniques, baseline correction, and data tune-up correction were implemented according to the procedure [47].

2.2.6. DPPH^. Method for Antioxidant Activity

The DPPH^. radical scavenging activity test was used to find out how effective the produced chemicals were as antioxidants. The free radical scavenging ability was assessed using the method of Brand-Williams et al. [48], with minor modifications. To assess antioxidant activity, the compounds were prepared in DMSO at a concentration of 250 µM. The compounds were dissolved in DMSO and then diluted to five different concentrations. The concentration of DPPH in ethanol was 50 µM. Different concentrations of compound solutions (5 µM, 10 µM, 20 µM, 50 µM, and 100 µM) were added to a previously prepared 5 mL DPPH solution, along with enough ethanol to make a total volume of 6 mL. The mixture was measured at 517 nm against a blank after being left for 30 minutes at room temperature in a dark room [49].

2.2.7. Kinetics of Isatin-Thiosemicarbazone Derivatives

The degradation of solids may be analyzed using a non-isothermal kinetic approach known as the model-free method. This method involves examining the temperature values associated with the reaction rate (α) in thermal analysis graphs obtained at various heating rates. Activation energies can also be calculated using this approach [50]. The thermal decomposition of isatin-thiosemicarbazone derivatives is a process that occurs in the solid state. The kinetics of this reaction may be broadly characterized by:

$$\frac{d\left(a\right)}{d\left(t\right)}=k\left(T\right).f\left(a\right)$$

(1)

The variables in the equation are as follows: t represents the reaction time in minutes, T represents the temperature in Kelvin, α represents the degree of conversion or reaction rate, k(T) represents the rate constant that depends on temperature, and f(α) represents the unique reaction model. The expression for k(T) can be represented using the Arrhenius equation:

$$k\left(T\right)=A.\exp \left(\frac{-E}{RT}\right)$$

(2)

Where A represents the exponential factor with units of min^-1, R represents the gas constant with units of 8.314 J mol^-1 °K^-1, and E represents the activation energy with units of J mol^-1. The kinetic triples, consisting of the E, A, and f(α) values, are used to forecast the conversion degree of a chemical given the temperature [51].

$$\mathit{ln}\left(\frac{\beta }{T^2}\right)=\mathit{ln} \left[\frac{E.A}{R.g\left(a\right)}\right]-\left(\frac{E}{R.T}\right)$$

(3)

$$g\left(a\right)={\int }_0^a{f\left(a\right)}^{-1} da$$

(4)

The activation energies for various α values may be determined by calculating the slope of the graph ln (β/T₂) – (1000/T) [52], using Equations (3) and (4).

$$\ln \left(\frac{\beta }{T^2}\right)=k1-\left(\frac{E}{R.T}\right)$$

(5)

$$\ln \left(\frac{\beta }{T}\right)=k2-\left(\frac{E}{R.T}\right)$$

(6)

$$\ln \beta =k3-\mathrm{1,051} \frac{E}{R.T}$$

(7)

Where k₁, k₂, and k₃ are fixed values. The activation energy was determined from the thermogravimetric curves using the Kissenger (Equation 5), Boswell (Equation 6), and Ozawa (Equation 7) techniques [41]. The thermogravimetric analysis tests were conducted using a Hitachi STA 7300 instrument at various heating rates (β = 5, 10, 15, 20 °K min^-1) in a N₂ environment with a flow rate of 100 mL min^-1. The temperature range for the studies was from ambient temperature to 1173 °K.

3. Results and Discussion

3.1. Differential Scanning Calorimetry (DSC)

DSC measurements were conducted on a blend of fuel containing isatin-thiosemicarbazone derivatives and unbleached fuel in a nitrogen gas atmosphere. The resulting data is graphically shown in Figure 2.

3.2. Thermogravimetric Analysis (TGA)

The compound of TGA-DTG graphs of several combinations were compared. It is observed that there is only one degree of degradation concerning temperature. The temperature at which degradation begins, known as the onset temperature, offers valuable information regarding thermal stability and the first boiling point. A positive association has been seen between the stability of the samples and the Tonset values, suggesting that an increase in sample stability is associated with a corresponding rise in Tonset values [53]. Figure 3 displays the graphical representations of the thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (DrTGA) curves. Upon closer inspection, the curves are pretty like one another. At these temperatures, sample mass is lost between 98.56 and 99.36%. The values taken from the thermometers and recorded on the thermograms are listed in Table 4.

3.3. Fourier Transform Infrared Spectroscopy (FT- IR)

The FT-IR spectra of the biodiesel-diesel blends are depicted in Figure 4 (a–f). The impact of incorporating antioxidants at a concentration of 3000 ppm into biodiesel samples is displayed in Table 5. The valence-stretching vibration frequency range of an unbounded hydroxyl group, v(O−H), is approximately 3661–3687 cm^-1. Spectral peaks in the range of 2925–3000 cm^-1 are often associated with the vibration of ν(C–H) bonds, indicating oxidation. Aldehyde and ketone compounds in biodiesel samples contain these bonds, which are also found in antioxidants. The spectral range of 2925–2990 cm^-1 is often linked to the ν(C−H) bending vibration, which is known to be involved in oxidation processes. Peaks in the 1079 cm^-1 wavenumber indicate stretching of the ν(C-O) bond. The spectrum shows strong absorption bands, indicating the presence of the ester carbonyl functional group (C=O). The absorption is observed in the spectral range of 1746 cm^-1. The lack of a nearby band could suggest that carboxylic acids are not present. The bending vibrations of v(N−H) are attributed to the observed vibrational modes at 1458 cm^-1. In isatin-thiocarbohydrazones, the bending vibrations of ν(C−N) occur within the range of 1199–1173 cm^-1. The long-term stability of biodiesel samples is linked to low oxidation levels [54]. Antioxidants, particularly phenolic chemicals, have been found to enhance peak intensity, according to studies.

3.4. DPPH^. Free Radical Scavenger Effect for Isatin-Thiosemicarbazones

The study assessed the samples' capacity to eliminate free radicals. Compound 1 had more activity with a value of 66.178 ± 0.11 μg/mL, whereas compound 2 showed lesser activity with a value of 79.927 ± 0.13 μg/mL. The arrangement of substituents in isatin-thiocarbohydrazones plays a vital role in determining their antioxidant activity, which is comparable to that of carbohydrazones [53]. The antioxidant activity of isatin-thiosemicarbazone derivatives can be attributed to the presence of substituted groups/atoms attached to the aromatic ring.

3.5. Thermal Characterization of Isatin-Thiosemicarbazones

Compound 1 is represented by the TGA, DTG, and DTA curves in Figure 5. The thermogravimetric (TGA) curve revealed that the thermal decomposition of compound 1 commenced at a temperature of 201 °C and exhibited two distinct stages of weight loss. The first and subsequent stages of weight loss account for 33.3% and 23.3%, respectively. The weight loss experienced from room temperature to 900 °C is precisely 69.8%. The differential thermogravimetric (DTG) curve of compound 1 exhibited two distinct peaks that corresponded to the thermogravimetric (TGA) curve. The initial stage was characterized by an exothermic peak at 213.3 °C observed in the DTA graph, indicating the melting point of the sample. The second stage was detected at a temperature of 265 °C on the DTA graph, and it manifested as an exothermic peak, indicating the thermal decomposition of compound 1.

Figure 6 displays the TGA, DTG, and DTA curves for compound 2. The thermogravimetric (TGA) curve indicates that the decomposition begins around 218 °C and follows a two-step process. The initial stage of weight reduction accounts for 37.3%, whereas the subsequent stage accounts for 18.5%. The weight loss experienced from a temperature of 25 °C to 900 °C amounts to 69.9%. The DTG curve of compound 2 exhibited two peaks that corresponded to the TGA curve. The initial stage is associated with the determination of the melting point, as evidenced by the presence of an endothermic peak at 243.3 °C recorded on the DTA curve. The second stage is linked to thermal deterioration, as evidenced by the exothermic peak at 267 °C observed on the DTA curve [54].

3.6. Investigating the Kinetics of Isatin-Thiosemicarbazones

The TGA curves of compound 1 are displayed in Figure 7, depicting four continuous heating rates: 5, 10, 15, and 20 °C min^-1. As the heating rate increases from 5 to 20 °C, the TGA curves exhibit a rightward shift. Additionally, the peak temperature also changes towards a higher value with an increasing heating rate. Substantial reductions in weight were seen across all measures within the temperature range of 200–400 °C. Thermal decomposition occurs within the temperature range of 200 to 400 °C. Given that this phase has also been detected in the sample and its decomposition happens within the temperature range under investigation, it unavoidably impacts the kinetic curves [51].

Figure 8a depicts the temperature-response rate relationship for compound 1, as represented by the curves. Figure 8b, Figure 8c, and Figure 8d display the diagrams used to calculate active tion energy using the Kissinger, Ozawa, and Boswell approaches, respectively. The activation energies obtained from the three approaches exhibit a high degree of similarity, as seen in Table 6. The activation energy values for the thermal decomposition process were determined using three different methods: the Kissinger approach yielded a value of 137.60 kJ mol^-1, the Ozawa method yielded a value of 146.58 kJ mol^-1, and the Boswell technique yielded a value of 142.09 kJ mol^-1. These calculations were performed under non-isothermal circumstances. Based on these results, it can be concluded that the activation energy needed for the decomposition of compound 1 falls within the range of 137–147 kJ mol^-1.

Figure 9 displays the thermogravimetric (TGA) curves of compound 2 obtained under several heating rates. Figure 9 demonstrates that the TGA curves changed towards higher temperatures as the heating rates increased. The temperature-α curves for compound 2 are shown in Figure 10a. The graphs illustrating the calculation of activation energy using the Kissinger, Ozawa, and Boswell approaches are displayed in Figure 10b, Figure 10c, and Figure 10d, respectively. The activation energy was calculated using the identical approaches for all three methods, and the corresponding findings are presented in Table 7. The activation energies for the thermal decomposition of compound 2, as determined by the Kissinger, Ozawa, and Boswell techniques, were 173.61, 182.48, and 178.04 kJ mol^-1, respectively. Based on these figures, the amount of energy needed to decomposition compound 2 is between 173 and 183 kJ mol^-1.

The non-isothermal kinetic investigations of isatin-thiosemicarbazone derivatives revealed that the activation energy needed for the thermal decomposition of compound 1 is lower than that of compound 2. The thermal stability of compounds is directly proportional to their activation energy; a more extensive activation energy value indicates more excellent thermal stability [57]. Based on the findings of non-isothermal decomposition kinetics, compound 2 exhibits more incredible activation energy than compound 1. Compound 2 has superior heat stability. Additionally, it is recognized that the molecule with a lower activation energy exhibits superior antioxidant capabilities [58]. Therefore, it has been verified that compound 1, which shows a greater yield, possesses superior antioxidant characteristics.

5. Conclusions

Biodiesel is an environmentally friendly and cost-effective alternative to traditional diesel fuel. Our research focused on addressing the instability of biodiesel caused by its high susceptibility to oxidation. The outcomes of our study are summarized below:

The DSC results are as follows: Using natural and synthetic antioxidants can significantly raise the crystallization temperature (Tc). The crystallization temperatures of multiple samples ranged from -11.76 °C to -13.97 °C. Lower temperatures were found to be a sign of increased purity in the samples being studied. The start of crystallization was determined by measuring the temperatures at which it began. A study was conducted to analyze different samples and record their temperatures. Two samples, D100 and B30D70, were tested for temperature. D100 had a temperature of -11.76 °C, while B30D70 had a slightly lower temperature of -11.87 °C. The B30D70TROLOX sample had a temperature of -12.03 °C, while B30D70_2 had a slightly lower temperature of -12.73 °C. The sample B30D70_1 had the lowest temperature of -13.97 °C among all the samples. The findings shed light on the temperature characteristics of the analyzed samples in this study. B30D70, B30D70TROLOX, B30D70_2, and B30D70_1 have lower crystallization tendencies than D100 in the realm of crystallization. The incorporation of antioxidants enhances oxidation stability. The substance mentioned has early crystallization, rapid oxidation, and is highly susceptible to oxidation. The stability of the characteristic increases sequentially. The addition of Schiff base_1 increased the fuel blends' crystallization point by 87.78%, while Schiff base_2 increased it by 72.16%.

The TGA results showed that the samples D100, B30D70, B30D70TROLOX, B30D70_2, and B30D70_1 underwent oxidative degradation between 25 and 275 °C. The degradation process occurred in one step. In thermal dynamics, mass loss occurs within certain temperature ranges. Recent studies have found that this occurrence happens between 98.09% and 99.44% in the different samples studied. The curves show similarities with slight differences in Tonset values among the biodiesel-diesel samples.

FT-IR was used to identify functional groups in the samples. The presence of functional groups in biodiesel-diesel blends, specifically B30D70_2 and B30D70_1, was found to be lower compared to other samples like B30D70TROLOX. This decrease can be attributed to the addition of 3000 ppm of Schiff bases. Incorporating Schiff bases into the composite has improved its oxidative stability, according to the mentioned discovery.

The compounds' antioxidant properties were tested using their DPPH^. free radical activities. The tests used the spectrophotometric method and scavenging techniques. The synthesized isatin-thiosemicarbazones showed IC₅₀ values of 66.178 ± 0.11 μg/mL and 79.927 ± 0.13 μg/mL for DPPH^. free radical scavenging activity. Compound 1 exhibited the highest antioxidant activity among the evaluated compounds, but had less activity than standard Trolox. A study was conducted to examine the connection between the compound's structure and its activity. The investigation studied substituents and their characteristics. Isatin-thiosemicarbazones were synthesized and used as antioxidants to minimize the disadvantages of biodiesel, which is a more efficient alternative to fossil diesel fuel.

The study also focused on examining the activation energies associated with the thermal decomposition of isatin-thiosemicarbazone derivatives. This was accomplished by examining non-isothermal thermogravimetric (TGA) curves at different heating rates (5, 10, 15, 20 °K min^-1). The activation energy values for compounds 1 and 2 were determined using the Kissinger, Ozawa, and Boswell techniques, yielding values of 137–147 kJ mol^-1 and 173–183 kJ mol^-1, respectively. As a result, it was discovered that the molecule with a lower activation energy exhibited superior antioxidant effects.

6. Patents

This section is not mandatory but may be added if there are patents resulting from the work reported in this manuscript.