*3.3. Characterization*

The alkali strength analysis of the bauxite used as a precursor to prepare catalyst (H−) was defined as the acid–base indicator. All alkali catalysts were characterized using an X-ray di ffraction instrument with Cu K α radiation (MAC MXP18, Tokyo, Japan). The structure of the as-prepared alkali was observed employing a field emission scanning electron microscope (FE-SEM, JEOL JSM-6700F). FAME composition in the biodiesel was determined by using a Thermo trace gas chromatograph with a flame ionization detector. A Tr–biodiesel (30 m × 0.25 mm × 0.25 μm film thickness) capillary column was used. Samples were injected under the following conditions: the carrier gas was Nitrogen with a flow rate 2 mL/min., an injector temperature was 200 ◦C was 90 ◦C for 0.5 min and increased to 260 ◦C (programmed temperature) at a rate of 10 ◦C/min, and the detector temperature was 250 ◦C. This procedure was developed according to EN 14103.
