**4. Conclusions**

Zn-doped CaO nanospheroids were prepared by utilizing a simple method and were used as a heterogeneous catalyst for the amidation of a variety of natural triglycerides. Zn/CaO-400 nanospheroids were found to have a ~30 nm size, more than an 18.4 in basic strength, and 16.87 m<sup>2</sup>/g of surface area. All of these factors made it a highly efficient catalyst in amidation, as it took only a 4 wt % catalyst amount for the complete conversion of high FFAs containing JO triglyceride, with a 5:1 molar ratio of DEA/JO at 90 ◦C. The Zn/CaO nanocatalyst was also found to be efficient at room temperature (35 ◦C) for amidation reactions of JO. The Zn/CaO-400 nanospheroids were found to be most efficient when compared to other transition metal-doped CaO nanomaterials (Mn, Fe, Co, Ni, Cu, and Cd). In addition, CaO was found to be the most effective support material compared to MgO and ZnO. The presence of both CaO and Ca(OH)2 phases in Zn/CaO-400 made it the most efficient heterogeneous catalyst for the aminolysis of various triglycerides and FAMEs derived from them. Zn/CaO-400 was also found to have excellent recyclability, as it was used for six consecutive reaction cycles without losing much catalytic activity. The high reaction rate of 0.171 min–<sup>1</sup> with 47.8 kJ mol–<sup>1</sup> of activation energy and 4.75 X 10<sup>8</sup> pre-exponential factors made it a highly efficient heterogeneous catalyst.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/9/9/774/s1 (FTIR and NMR data), Scheme S1: Transesterification of triglycerides using 2-Zn/CaO-400 nanocatalyst, Figure S1: Comparative XRD spectrum of (a) CaO with Zn/CaO calcined at different temperatures and (b) Zn/CaO with different doping percentages of zinc ion; Figure S2: Recyclability studies of the catalyst in the aminolysis of JO; Figure S3: Progress of aminolysis using FTIR; Figure S4: Mass spectra of fatty acid amide derived from methyl laurate; Table S1: The chemical analysis of vegetable oils.

**Author Contributions:** D.K. was involved in the discovery and development of the photocatalyst platform. D.K., C.H.P., and C.S.K. analyzed the data and contributed to designing the experiments. D.K., C.H.P., and C.S.K. gave approval for the final version of the manuscript.

**Funding:** This work was supported by the National Research Foundation of Korea (NRF-2016R1D1A1B03934226 and 2019R1A2B5B02070092) Project. We are thankful to CURF, Chonbuk National University, South Korea, for the FESEM, XRD, FTIR, TEM, NMR, and mass spectroscopic analysis.

**Conflicts of Interest:** The authors declare no conflicts of interest.
