**4. Conclusions**

Bauxite was found to contain Si and Al essential compounds suitable as catalysts for biodiesel production. A highly e ffective catalyst for biodiesel production was synthesized using bauxite and Li2CO3 and by calcination at temperatures of 800 ◦C for 4h. The transesterification procedure of biodiesel reached over 96% and the optimal parameters were 65 ◦C reaction temperature, 3 h reaction time, 9: 1 methanol-to-oil molar ratio and 3 wt.% catalyst amount. Under the optimal parameters, the catalyst can be e ffectively reused for 6 runs with a minimal decrease of <10% in the conversion rate. In addition, the results demonstrate that the e fficiency with high free fatty acid concentration was significantly influenced. Consequently, the e fficiency of the same catalyst under the same reaction conditions di ffers with di fferent oil. In this work, the application of bauxite as a catalyst for biodiesel production not only provides a cost-e ffective and environmentally friendly way of using the bauxite, but also reduces the cost of biodiesel production.

**Author Contributions:** Data conceptualization, Investigation, Writing—original draft, Methodology, Y.-M.D. Methodology, Validation, Investigation, Software, C.-H.H. and J.-H.L. Writing—original draft preparation, F.-H.C. Supervision, Conceptualization, Visualization, Writing—review, Editing, Validation, C.-C.C.

**Funding:** This research was supported by the Ministry of Science and Technology of the Republic of China (MOST-108-2113-M-142-001).

**Acknowledgments:** We thanks for the financial support of this research by Ministry of Science and Technology of Taiwan. We also thank for the Center of Expensive Instruments at National Chung Hsing University to perform FE-SEM analysis.

**Conflicts of Interest:** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper
