*2.3. Aminolysis Reaction*

The amidation of a variety of natural triglycerides, such as virgin soybean oil (VSO), waste soybean oil (WSO), jatropha oil (JO), animal fat (AF), Karanja oil (KO), and fatty acid methyl esters (FAMEs) derived from these oils, as well as methyl laurate (ML) with different molar concentrations of diethanolamine and different catalyst amounts, was performed at 90 ◦C. All amidation reactions for studying various reaction parameters were carried out with diethanolamine in the presence of 4 wt% of the 2-Zn/CaO-400 catalyst at 90 ◦C for 0.5 h (Scheme 1). The schematic for the FAMEs derivation from vegetable oils was performed with methanol (9:1 methanol/oil molar ratio) by using 5 wt% of the same catalyst at 65 ◦C (Scheme S1, Supplementary Materials).

The progress of the amidation reaction was monitored by taking out samples from the reaction mixture and analyzing them with FTIR and 1H-NMR (Nuclear Magnetic Resonance) techniques. The catalyst nanoparticles were removed by simple centrifugation of the final reaction mixture at 7000 rpm, and the organic layer was then washed with distilled water and dried over sodium sulfate. The amide derivatives thus obtained were further analyzed by FTIR (Figure 3A) and 1H-NMR (Figure 1B) analysis techniques. The final reaction product obtained from the methyl laurate amidation reaction was also characterized by mass spectrometry (Figure S4, Supporting Information) along with the 1H-NMR and FTIR studies. A shifting of the ester carbonyl peak to the ester amide peak from 1739 cm<sup>−</sup><sup>1</sup> to

1617 cm<sup>−</sup><sup>1</sup> indicated the formation of fatty acid diethanolamine (FAD) (Figure 3A). The formation of a diethanolamide derivative was also supported by the presence an –OH group peak at 3406 cm<sup>−</sup>1.

**Scheme 1.** (**a**) Waste soybean oil or jatropha oil; (**b**) vegetable oil-derived fatty acid methyl esters (FAMEs); and (**c**) methyl laurate-derived fatty acid diethanolamide preparation in the presence of a Zn/CaO solid catalyst.

On the other hand, in the 1H-NMR spectrum, the appearance of a multiplet at 3.48 and 3.78 ppm due to -NCH2- and -CH2OH protons (Figure 3Bb) and the disappearance of characteristic glyceridic proton signals at 4.13 and 4.30 ppm (Figure 3Bi) supported the conversion of triglyceride to corresponding FAD. Furthermore, in the case of a FAD derivative of JO-derived FAMEs and methyl laurate, the disappearance of a methyl ester proton signal at 3.65 ppm (Figure 3Biii,v) and the appearance of amide proton signals at 3.48 and 3.78 ppm (Figure 3Bd,f) (corresponding to -NCH2- and -CH2OH protons) confirmed the formation of respective FAD.

#### 2.3.1. Optimization of Different Parameters

The prepared nanocrystalline catalysts with different transition metals and different metal oxides were utilized for the amidation of natural triglycerides with diethanolamine. However, jatropha oil (JO) was selected for the optimization of parameters, as it has a high level of free fatty acid contents (8.2 wt % free fatty acids).

A series of transition metals was used for the doping in CaO to test their impact on the catalytic activity of CaO for amidation, and it was found that Mn/CaO, Fe/CaO, Co/CaO, Ni/CaO, Cu/CaO, Zn/CaO, and Cd/CaO showed 18%, 24%, 16%, 58%, 15%, 99%, and 25% FADs, which were yielded in 0.5 h at 90 ◦C. Bare CaO-400 was also tested and was found to have a 10% FAD yield (Figure 4a). Among all the prepared catalysts, Zn/CaO was found to be the most efficient, and it was selected for further optimization studies. Further, for the selection of a metal oxide as a base material, Zn was doped in CaO, MgO, and ZnO, as all of these metal oxides have been extensively reported to be efficient catalysts for different reactions. For the amidation of JO, Zn/CaO was found to be the most effective, with a 99% conversion yield for FADs, whereas Zn/MgO and Zn/ZnO also showed significant conversion rates, with 84% and 76% FAD yields, respectively (Figure 4b). The high surface area was the deciding factor for the catalytic activity.

**Figure 3.** Comparative ( **A**) FTIR spectrum and (**B**) 1H-NMR spectrum of (**a**) waste cotton seed oil, (**b**) fatty acid amide of waste cotton seed oil, (**c**) waste soybean oil (WSO)-derived FAMEs, (**d**) a fatty acid amide of WSO-derived FAMEs, (**e**) methyl laurate, and (**f**) an amide derivative of methyl laurate.

**Figure 4.** Effect of (**a**) different transition metal doping on CaO, (**b**) Zn ion doping on different metal oxides, (**c**) Zn ion concentration in CaO, and (**d**) calcination temperature in Zn/CaO on the amidation of jatropha oil (JO).

To optimize the concentration of Zn ions for a higher reaction rate, a series of aminolysis reactions was carried out by varying the Zn2<sup>+</sup> concentration from 0.5 to 5 wt%. There was a significant increase in the FAD yield from 20% to 99%, as the Zn2<sup>+</sup> concentration increased from 0.5 to 2 wt %, respectively. A further increase in Zn ion concentration had no effect on the reaction rate, and hence 2 wt% was chosen as an optimized amount of Zn for maximum efficiency (Figure 4c). The Zn/CaO nanoparticles prepared at different calcination temperatures were also used for the amidation of JO, and it was observed that the FAD yield was enhanced from 47% to 99% as the calcination temperature increased from 100 to 400 ◦C, respectively. Interestingly, calcination at a higher temperature such as 600 ◦C and 800 ◦C caused a reduction in the reaction rate as the FAD yield lowered to 85% and 68%, respectively. This could have been due to the fact that high temperatures caused the sintering of particles, which in turn decreased the surface area and basic strength (Table 1).

Different catalyst amounts from 1 to 10 wt % and a range of reaction temperatures from 30 to 130 ◦C were tried to figure out the optimized catalyst amount and reaction temperature. The FAD yield was increased from 25% to 99% when the catalyst amount was enhanced from 1 to 4 wt%. A further increase in the catalyst amount had no impact on the reaction rate (Figure 5a). Similarly, when the reaction temperature was increased from 30 ◦C to 90 ◦C, the FAD yield increased significantly from 24% to 99%, respectively. A higher reaction temperature did not show any effect on the reaction rate (Figure 5b). The optimization of the reaction temperature parameter was carried out by taking out samples at 5-, 15-, and 30-min time intervals and analyzing them through FTIR and 1H-NMR analysis.

After 90 ◦C, there was no change in the reaction rate, and hence 90 ◦C was used as the optimized reaction temperature.

**Figure 5.** Effect of (**a**) catalyst concentration, (**b**) reaction temperature, and (**c**) the diethanolamine (DEA)/JO molar ratio on the complete aminolysis of used cotton seed oil (reaction time = 0.5 h). (**d**) The effect of different feedstock on the time required and the fatty acid diethanolamine (FAD) yield for the amidation reaction. Reaction conditions: diethanolamine/feedstock = 5:1 (m/m), catalyst amount = 4 wt % of feedstock, temperature = 90 ◦C.

Six different molar ratios of diethanolamine and JO were used to optimize the diethanolamine (DEA) amount for maximum conversion in the minimum time. As the molar ratio increased from 3:1 to 5:1, the FAD yield also increased from 44% to 99%, respectively, in 0.5 h (Figure 5c). A diethanolamine/JO molar ratio of 5:1, a 4 wt% catalyst amount, and a 90 ◦C reaction temperature were the final optimum reaction conditions for the complete conversion of JO to fatty acid diethanolamides in the minimum possible time (0.5 h). However, the Zn/CaO nanospheroids were found to convert JO to FADs completely, with a 3:1 diethanolamine/JO molar ratio and a 1 wt% catalyst amount at room temperature (35 ◦C), but the reaction time increased to 4 h.

A variety of triglycerides, which included natural triglycerides as well as methyl laurate, were tested with an amidation reaction to check the efficiency of the prepared Zn/CaO nanospheroids. Zn/CaO was found to convert all of the triglycerides to FADs, where it took 30 min to complete the reaction in the case of JO, KO, WSO, and AF; and it took only 20 min for VSO and ML. The low free fatty acid (FFA) content was the reason for the lower reaction time for VSO (FFAs = 0.2%) and ML, whereas Zn/CaO was found to be highly efficient for high FFAs containing feedstock, viz. AF (1.4), WSO (2.1), KO (4.4), and JO (8.2). The high FFAs (free fatty acids) caused the partial deactivation of the catalytic sites of Zn/CaO.

To examine the reusability of Zn/CaO nanospheroids, after being recovered from the final reaction mixture through centrifugation, Zn/CaO was washed with hexane and dried at 400 ◦C. The recovered catalyst was tested for the six catalytic runs under the same reaction conditions and regeneration technique. The recycled and regenerated catalyst was also found to complete (>99 %, m/m) the amidation of JO, though it required 35 min for the second catalytic recycle and 70 min for the sixth catalytic run (Figure S2, Supplementary Materials). The partial loss of catalytic activity could have been due to the loss of Zn/CaO particles during successive centrifugation and the partial leaching of active species.
