**1. Introduction**

The rapid depletion of fossil fuel resources and global warming has motivated researchers to expand different technologies that utilize renewable energy sources [1–3]. Natural triglycerides [4,5] and lignocellulosic biomass [6–8] have been converted in various platforms to value-added products. Triglycerides in vegetable oils and animal fats have been used in industries as a feedstock for the preparation of fatty amides, nitriles, amines, and alcohols, which in turn are useful for the preparation of various commodity chemicals such as surfactants and different types of polymers [9].

Fatty acid amides have a wide range of applications, viz. in surfactants, cosmetics, fungicides, lubricants, foam-control agents, water repellents, shampoos, detergents, corrosion inhibitors, and antiblocking agents, in plastics processing technologies [5,10,11]. Fatty acid amides possess better ignition properties than simple esters and hence are more useful in biodiesel development technology [12]. Moreover, fatty acid amide derivatives of natural triglycerides (vegetable oils or animal fats) or fatty acids have been found to be free from sulfur or any other aromatic compounds and thus help to lessen the greenhouse effect, showing an improvement in cetane number and cold flow properties and a beneficial effect on particulate matter emissions [12]. The industrial synthesis of fatty acid amides involves a two-step process: first, the conversion of triglycerides into fatty acid methyl/ethyl esters, followed by a high-temperature treatment to prepare fatty acid amides [5].

Due to their grea<sup>t</sup> importance, some methodologies to prepare fatty acid amides from fatty acids or fatty acid alkyl esters or triglycerides through treatments with different amines have been proposed

previously [13,14]. At the industrial level, homogeneous catalysts such as sodium ethoxide [15], sodium methoxide [16], and calcium chloride [17] are used for the preparation of fatty acid amides. Enzymes have been frequently reported [18,19] as a heterogeneous catalyst for amidation reactions, though they require a longer reaction duration. In addition, solvent-free conditions [20,21], SmIII complexes [22], SnIV complexes [23], the Deoxo-Fluor reagen<sup>t</sup> [bis(2-methoxyethyl)amino-sulfur trifluoride] [24], and other chemicals [25] have been utilized to obtain the desired amide. However, a few drawbacks are associated with these methods, viz. low product yields, a longer reaction duration, difficult product separation, a lack of catalyst reusability, a large molar excess of reactants, contamination of the product, and the creation of stoichiometric amounts of undesired products. Therefore, it is necessary to extend new, efficient, environmentally friendly, ecologically correct, and reusable catalytic methods for the amidation of fatty acids and triglycerides [26–28]. In the recent past, heterogeneous catalysts have attracted considerable attention, as they are nonhazardous, have good selectivity and recyclability, and are easy to separate from reaction medium [29–31].

In the present report, zinc-doped CaO, MgO, and ZnO were prepared in nanoparticle form using an incipient-wetness impregnation method and were used as heterogeneous catalysts for solvent-free direct amidation of natural triglycerides. The effect of transition metal ion impregnation on CaO activity and calcination temperature was also studied by preparing a series of catalysts with Fe, Co, Cu, Zn, and Cd doped on CaO.

#### **2. Results and Discussion**

#### *2.1. Brunauer–Emmett–Teller (BET) Surface Area and Hammett Indicator Test*

The basic strength (p*<sup>K</sup>*BH+) and surface area of prepared catalysts were analyzed using the Hammett indicator test and Brunauer–Emmett–Teller (BET) surface area measurement, respectively. The basic strength of CaO was found to be increased from 9.8–10.1 to 11.1–15.0 after 2 wt% doping of Zn, which further increased to a maximum of 18.4 after calcination at 400 ◦C. However, a further increase in calcination temperature decreased the basic strength. The improvement of the basic strength of CaO after zinc ion doping with an increase in calcination temperature up to 400 ◦C could have been due to the partial dehydration and strong increase in the surface area [32]. An increase in the Zn ion concentration did not improve the basic strength. However, after Zn doping on MgO and ZnO, the basic strength was increased to the range of 15.0–18.4 (Table 1). The BET surface area was another critical factor that had a direct impact on catalytic efficiency. Bare CaO had a surface area of 3.56 m<sup>2</sup>/g, which improved to 16.87 m<sup>2</sup>/g after 2 wt% doping of Zn along with calcination at 400 ◦C. However, calcination at high temperatures, viz. 600 ◦C and 800 ◦C, caused a reduction in surface area to 10.12 m<sup>2</sup>/g and 5.25 m<sup>2</sup>/g, respectively, which could have been due to the sintering of material at high temperatures. The doping of the Zn ion on MgO and ZnO also caused an increase in surface area from 10.4 m<sup>2</sup>/g to 14.89 m<sup>2</sup>/g and 4.72 m<sup>2</sup>/g to 12.13 m<sup>2</sup>/g, respectively (Table 1). Hence, the doping of the Zn ion and calcination were critical factors responsible for the increase in basic strength and surface area.

#### *2.2. Structural Analysis of Catalyst*

The effect of calcination temperature, Zn ion concentration, and different Zn doping on different metal oxides was studied by powder XRD analysis (Figure 1 and Figure S1 in Supporting Information). The presence of the cubic phase of CaO was confirmed by peaks at 2θ values of 32.27◦, 37.47◦, 53.89◦, and 67.39◦ (JCPDS (Joint Committee on Powder Diffraction Standards) card no. 821691), as shown in Figure 1a. After zinc doping (2 wt%) and calcination at 200 ◦C (2-Zn/CaO-200), the cubic phase of CaO was converted into a hexagonal form of Ca(OH)2, as supported by the peaks at 18.05◦, 28.6◦, 34.17◦, 47.17◦, 50.78◦, and 62.57◦ (JCPDS 84-1276), as shown in Figure S1a. A further increase in the calcination temperature to 400 ◦C showed the coexistence of both the cubic and hexagonal phases, which might have been the reason for the abrupt enhancement in the basic strength of 2-Zn/CaO-400. However, a further increase in calcination temperature to 600 ◦C showed the presence of only the cubic phase. The increase of the Zn ion concentration had no impact on the structure of Zn/CaO-400 (Figure S1b).


**Table 1.** Effect of calcination temperature, Zn ion concentration, and different oxides on the basic strength, surface area, and crystallite size. BET: Brunauer–Emmett–Teller.

<sup>\*</sup> on (200) plane by Debye–Scherrer method [33].

**Figure 1.** Comparative powder XRD patterns of (**a**) CaO with 2-Zn/CaO-400, (**b**) MgO with 2-Zn/MgO-400, and (**c**) ZnO with 2-Zn/ZnO-400.

XRD patterns for the 2-Zn/MgO-400 catalysts showed sharp diffraction peaks at 2θ = 36.89◦, 42.92◦, 62.28◦, 74.58◦, and 78.53◦, which were attributed to the crystalline phase of the MgO (JCPDS 4-829) (Figure S1b). The XRD analysis of 2-Zn/ZnO-400 showed the presence of a hexagonal phase of ZnO (JCPDS: 80-0075), as indicated by the peaks at 31.79◦, 34.51◦, 36.2◦, 47.55◦, 56.65◦, 62.84◦, 66.31◦, 67.9◦, and 69.03◦ (Figure 1c). The diffraction pattern of Zn was not observed in any XRD spectrum, which might have been due to its high degree of dispersion on CaO/MgO/ZnO or it being below the detection limit of XRD. The particle size of prepared nanoparticles was also calculated from powder XRD analysis data using the Debye–Scherrer method [33]. Bare CaO particles were found to have a 108-nm size, and after Zn doping, it was found to decrease to 33–39 nm in range (Table 1). However, a change in Zn ion concentration and calcination temperature was not found to alter the particle size significantly.

Dynamic light scattering analysis (DLS) was performed for the measurement of the particle size distribution of CaO and 2-Zn/CaO-400 and showed that the average particle size of CaO and 2-Zn/CaO-400 was 115 nm and 35 nm, respectively (Figure 2a,b). The particle size and surface morphology of the prepared catalyst were analyzed through field emission scanning electron microscopy (FESEM), and the average particle size was observed in the range of 100–200 nm, with irregular surface

morphology (Figure 2c). The same particles were analyzed using transmission electron microscopy (TEM) for clear observation of the particle size, and it was found that the 2-Zn/CaO-400 nanoparticles had an average particle size of ~30 nm with an oblate spherical shape (Figure 2d).

**Figure 2.** Particle size distributions of (**a**) CaO and (**b**) 2-Zn/CaO-400: (**c**) field emission scanning electron microscopy (FESEM) and (**d**) transmission electron microscopy (TEM) images of 2-Zn/CaO-400.
