# Bifunctional Heterogeneous Catalyst for Biodiesel Production from Waste Vegetable Oil
## 3 Results and Discussion
### Yield of Fatty Acid Methyl Esters  
According to the previous studies, a methanol-to-oil molar ratio of 9:1 is satisfactory for a high yield and complete conversion of triglycerides to methyl esters [28; 30]. The optimum catalyst loading of 4 wt % was selected for the transesterification of both waste oils. [PERSON] et al. [31] reported that an increase in catalyst loading above 5 wt % leads to an increase in the mixture viscosity and a decrease in the yield of FAME. The effect of the catalyst ratio on the yield of FAME was investigated by varying the CaO ratio at a range of 60-80%. The purification process was effective, as no traces of glycerol were observed in the FAMEs produced. The maximum FAME yield of 98% was achieved at 80%CaO/Al\({}_{2}\)O\({}_{3}\) when WSO was used, while a maximum yield of 89% was obtained at a 60%CaO/Al\({}_{2}\)O\({}_{3}\) catalyst ratio \begin{table}
\begin{tabular}{c c} \hline \hline
**Fatty Acids** & **wt \%** \\ \hline Heptane, 2,2,4,6,6-pentamethyl & 0.1808 \\
4-Methyldecane & 0.277 \\ Hexane, 3,3-dimethyl & 0.4396 \\
3,6-Dimethyldecane & 0.2217 \\ Octane, 5-ethyl-2-methyl & 0.1093 \\ Dodecane, 2-phenyl- & 0.3037 \\ Indanol-5 & 0.3677 \\
2,4-Dimethylheptane & 0.2818 \\ Pentadecane & 0.3578 \\ Diethyl Phtalate & 0.6016 \\ Nonadecane & 0.2484 \\ Phenol, 2,4-bis(1,1-dimethylethyl) & 1.4815 \\ n-Pentacosane & 0.3449 \\ Triacontane & 0.2417 \\ Hexadecanoic acid or Palmitic acid & 7.6173 \\ Heptadecanoic acid or Margaric acid & 0.8198 \\
9,12-Octadecadienoic acid or Linoleialic acid & 48.5629 \\
9-Octadecenoic acid or Oleic acid & 25.6451 \\ Octadecanoic acid or Stearic acid & 7.6528 \\ Linoleic acid & 1.0534 \\
5-Dodecyne & 0.2022 \\ Eicosanoic acid or Arachidic acid & 0.2815 \\ Docosanoic acid or Behenic acid & 1.4139 \\ Hexadecadienoic acid & 1.2934 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Characterization of WSO. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Fatty Acids** & **WSO (Mass \%)** & **WPO (Mass \%)** \\ \hline Palmitic acid (C16:0) & 5.99 & 16.56 \\ Stearic acid (C18:0) & 5.93 & 1.91 \\ Arachidic acid (C20:0) & 0.22 & 0.21 \\ Lignoeceric acid (C24:0) & 0 & 0 \\ Palmitoleic (C16:1) & 0 & 0 \\ Oleic acid (C18:1c) & 17.04 & 14.11 \\ Linoleic acid (C18:2c) & 0 & 3.41 \\ \hline Total & 29.18 & 36.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Analysis of total fatty acid (FA) content of the waste vegetable oil (WVOs). WPO: waste palm oil, WSO: waste sunflower oil. for WPO. Details are provided elsewhere [20]. This could be attributed to the fact that WPO consisted of a higher amount of FFAs (76.96 mg/g) compared to WSO (0.3825 mg/g), thus requiring a higher quantity of acidic sites (Al2O3) to esterify the FFAs and facilitate the conversion of triglycerides to FAME in conjunction with the basic sites (CaO) and thus achieve a high FAME yield. This decrease in FAME yield with increased CaO loading could also be an indication of the impact of external mass transfer constraints originated by the generation of products, for instance the FAME and by-product (e.g., glycerol). According to [PERSON] [25], such constraints are only avoidable at a higher stirring speed of approximately 1300 rpm. There was no stirrer with the capability of more than 1000 rpm readily available for use. This decrease in yield could also be attributed to the variation in the particle size of the CaO/Al2O3 catalysts, which might have caused a decrease in its catalytic activity during the reaction systems. [PERSON] et al. [4] reported that the activity of a catalyst is dependent on the particle size. Thus, smaller catalyst particles are supposed to demonstrate a higher reaction rate owing to an increase in the external surface area available. On the other hand, the bigger the particle size of a solid, the smaller the surface area available for the reaction to take place, and the reduction in the number of collisions between the molecules (solid-liquid phase) during the reaction.