## 3 Results
### Influence of Reaction Conditions on the Transesterification  
CaNa/AC was applied to investigate the optimum conditions for the transesterification of Jatropha oil. The initial conditions were set as: 10:1 methanol-to-oil molar ratio, 338 K reaction temperature, 3 h reaction duration, and 600 rpm stirring speed. As shown in Figure 3a, the conversion increased from 27% to 80% when the catalyst amount increased from 1.0 to 3.0 wt.%, which was attributed to the availability of more basic sites by a larger amount of catalyst, as well as an increase in the frequency of interaction between the reactants and catalyst. However, no significant changes to the conversion were observed when the catalyst amount was increased from 3.0 to 5.0 wt.%. As a result, the optimal 3.0 wt.% catalyst amount was selected for the next experiments. Normally, the alkali-catalyzed transesterification of oil occurs at the boiling point of alcohol present in the reaction mixture [47; 48; 49]. In order to assess the influence of reaction temperature on the conversion, the transesterification was performed at 323, 328, 333, 338, and 343 K while other conditions were kept constant. As shown in Figure 3b, the conversion increased with increasing temperature from 323 to 338 K, and the maximum efficiency reached 80% at 338 K. As more energy was given for the reaction to proceed, a greater reaction temperature might reduce the viscosities of oils and raise the reaction rate [50; 51]. Thus, the conversion could be improved with increasing reaction temperature. However, the conversion slightly decreased when the temperature was beyond 338 K, which could be due to the fact that the boiling point of methanol is 337.7 K at atmospheric pressure. In general, a very high reaction temperature accelerates the loss of methanol by vaporization, which consequently led to decreased conversion. \begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**Sample**} & **Surface Area** & **Total Pore Volume** & **Average Pore** \\  & **(m\({}^{2}\)**-g\({}^{-1}\))** & **(cm\({}^{3}\)**-g\({}^{-1}\))** & **Diameter (nm)** \\ \hline AC & 138 & 0.27 & 6 \\ Ca/AC & 110 & 0.28 & 11 \\ Na/AC & 13 & 0.02 & 2 \\ CaNa/AC & 15 & 0.07 & 29 \\ \hline \end{tabular}
\end{table} 
Table 2: Surface area, total pore volume, and average pore diameter of AC, Ca/AC, Na/AC, and CaNa/AC. Moreover, the stability of the catalyst at different transesterification temperatures was investigated by analyzing the basicity of the used catalyst in the temperature ranges. In the reaction temperature range of 323-338 K, there was a slight loss in the basicity of the used catalyst compared to that of the fresh sample, as shown in Figure 4. However, the basicity loss was up to 30% at 343 K, indicating the formation of a homogenous catalyst [52]. Furthermore, the gas bubbles were observed to continually appear and burst on the surface of the catalyst in the reaction when the temperature was 343 K, Therefore, it could be deduced that this perturbing activity induced the dissociation of active sites from CaNa/AC. These results indicated that the optimal temperature for transesterification was 343 K (within the range of this study). Excess methanol is required to push the reaction toward the formation of FAME [53]. The methanol/oil molar ratio is commonly 6 for transesterification using a homogeneous base catalyst [54]. The reaction was performed with methanol/oil molar ratios ranging from 6:1 to 14:1 to investigate the effect of the methanol/oil ratio on the transesterification by CaNa/AC. The results in Figure 3c suggested that the conversion rate increased considerably with the increase in methanol/oil molar ratio, and the efficiency reached 88% at a 12:1 methanol/oil molar ratio. However, raising the methanol/oil molar ratio beyond 12:1 did not result in a significant promotion of the reaction rate. In conclusion, a methanol/oil molar ratio of 12:1 was deemed optimal.