**2. Results and Discussion**

### *2.1. Characterization of As-Prepared Catalyst*

Figures 1 and 2 present the X-ray powder di ffraction (XRD) results of catalysts. The structure of the bauxite underwent a phase change to Li4SiO4 (JCPDS-742145) and LiAlO2 (JCPDS-0440224) according to the solid-state preparation with Li2CO3. Although bauxite was a complex material and the main peaks of its XRD pattern usually overlaid one another, the major phase change in Li4SiO4 and LiAlO2 was observed after its calcination. Figure 1 shows di fferent calcination temperatures. At a calcination temperature of 600 ◦C, Li2CO3 (JCPDS-870728) structures had strong intensity. The catalysts can be observed to exhibit the di ffraction peak characteristics of Li4SiO4, LiAlO2, and Li2CO3. For calcination at 800 ◦C, a stronger intensity was present in the Li4SiO4 and LiAlO2 phase [17,18].

**Figure 1.** X-ray powder di ffraction (XRD) of bauxite mixed with Li2CO3 at di fferent calcination temperatures.

**Figure 2.** XRD patterns of prepared catalyst under different bauxite and Li2CO3 molar ratios.

As result of the above, the XRD patterns of the samples calcined at 600 ◦C corresponded to the Li2CO3 phase. When the calcination temperature reached the melting point of Li2CO3 (650 ◦C), Li2CO3 entered a molten state [19]. Subsequently, the catalyst (calcined at 800 ◦C) exhibits the different XRD diffraction peak due to phase transformation to crystalline Li4SiO4 and LiAlO2. It was found that the XRD patterns of catalyst (calcined at 800 ◦C to 1000 ◦C) were similar. These diffraction peaks indicated the presence of crystalline Li4SiO4 and LiAlO2. The XRD result is attributable to the phenomenon whereby Li4SiO4 and LiAlO2 started to form agglomerated blocks in the Li4SiO4 and LiAlO2 phase at high calcination temperatures. Therefore, regardless of the calcination temperature, the LiAlO2 and Li4SiO4phases were achieved.

Ordinary bauxite contains two major compounds: Al2O3 and SiO2. The XRD patterns of prepared catalysts for different bauxite and Li2CO3 ratios are shown in Figure 2. When the bauxite–Li2CO3 molar ratio was 0.5, the diffraction peaks of SiO2 were observed. With an increase in the bauxite–Li2CO3 ratio, the diffraction peaks of Li4SiO4 and LiALO2 became increasingly clear. When the bauxite–Li2CO3 ratio increased to 2, the heights of the diffraction peaks belonging to Li2CO3 increased further. Table 1 presents the efficiency of transesterification to determine the efficiency of all catalysts in the experiment. The reaction test did not provide optimal conditions for the transesterification procedure but provided a method to compare catalytic performance. Results from Table 1 demonstrate that the bauxite exhibited no catalytic performance. When Li2CO3 was modified bauxite, the catalysts exhibited catalytic activities. The earlier research of solid base catalysts for transesterification reactions is shown in Table 1. Comparing Li2CO3/Bauxite with other solid base catalysts, it could be clearly found that Li2CO3/Bauxite showed good catalytic performance for a transesterification reaction.

The structure morphology of catalysts was examined using field-emission scanning electron microscopy (FE-SEM). Figure 3 illustrates morphology noted under FE-SEM for a bauxite-Li2CO3 molar ratio of 4 calcined in air with various calcination temperatures. The catalyst has a uniform microscale block and agglomerated block composition with a 20–50 μm size range, as shown in Figure 3. Small mineral aggregates and agglomerated circular particles were present when the calcination was performed at high temperatures because of the oxide formation. Figure 3 demonstrates the catalyst results, and circular blocks of different sizes with diameters of approximately 50 μm accumulated on the surface. The particles and amorphous silica disappeared.


\* Reaction conditions: 12.5 g soybean oil; methanol/oil molar ratio, 12:1; catalyst amount, 6 wt.%; reaction time, 3 h; methanol reflux temperature and conventional heating method.

**Figure 3.** Field-emission scanning electron microscopy (FE-SEM) images of bauxite mixed with Li2CO3 at different calcination temperatures (**a**) 600 ◦C, (**b**) 700 ◦C, (c) 800 ◦C, (**d**) 900 ◦C, and (**e**) 1000 ◦C.
