**1. Introduction**

The need for renewable energy sources to meet ever greater energy requirements is becoming increasingly urgen<sup>t</sup> [1]. Many researchers therefore advocate substituting traditional fuels with renewable alternatives that produce lower emissions of particulate matter and reduce greenhouse effects [2,3]. The current process used for producing fatty acid methyl esters (FAMEs) is the transesterification of vegetable oil catalyzed by alkaline catalysts. In addition, this reaction is associated with some difficulties. First, the catalysts cannot be recovered or reused, therefore they must be neutralized. Second, catalysts should be neutralized and separated from the methyl ester phase after the transesterification reaction, with much wastewater resulting [4,5]. Heterogeneous solid catalysts can be used for overcoming these problems. Heterogeneous solid alkaline catalysts are beneficial because of the good separation of the reaction mixtures and their recyclability. Their low price also decreases the overall production costs [6].

By the combination of silicon and lithium carbonate, our previous research discovered that the lithium silicate compound presented high basicity strength, high chemical stability, and thermo-stability. Such characteristics show extremely high applicability in the catalysis field, especially in transesterification [7]. The LiAlO2 structure has recently been published for transestrification. Under high temperatures, lithium aluminate acquired by calcinating Li2CO3 in the waste with aluminum

forms high basic solid catalyst [8]. Interestingly, both Li4SiO4 and LiAlO2 compounds demonstrate that these two metal oxides are the best solid basic catalyst for transestrification with high stability, lower cost, and solid basicity.

Bauxite contains a large percentage of Si and Al compounds. The expense of pure Si and Al makes the materials uneconomical. The method has been improved by replacing commercial materials with Si and Al from bauxite [9]. In recent years, newer and simpler methods based on the of bauxite containing a large percentage of Si and Al compounds have been introduced [8,9]. One simple method is based on a solid-state method at high temperatures, blending other base metals with bauxite, which contains Si and Al. The concept involves producing new solid basic catalysts, with the goal of reusing these new precursors and o ffering broad applications in various fields [10–12]. When bauxite is used as a low-cost solid base catalyst, it provides a twofold advantage in relation to environmental pollution. The use of low-cost materials in the manufacturing process is valuable for industrial applications, particularly for producing FAMEs through transesterification of various vegetable oils, where massive quantities of solid-alkali catalyst are processed [13–16]. The cheaper catalyst precursor from bauxite has some advantages, which are mainly economic and environmental. Another study we undertook applied solid base catalysts to transesterification. This indicated that solid-alkali catalysts provided high transesterification e fficiency [6,15,16].

In this paper, bauxite is used as a precursor to prepare alkali catalysts. The resulting catalyst was experimented in the transesterification reaction. The influence of various experimental methods—such as the catalyst amount, methanol–vegetable oil proportions, and catalyst reusability—on e fficiency was evaluated to detect the optimum conditions for the study of biodiesel production.
