## 4 Types of Catalysts Used in Transesterification
### Heterogeneous Catalysts
#### 4.2.1 Heterogeneous Base Catalyst  
Heterogeneous base catalysts overcome limitations associated with homogeneous base catalysts, for example hindrance in glycerol separation from biodiesel layer due to saponification. Metal oxides are known as the best heterogeneous base catalyst, these include Li/CaO, CaO, MgO, metal oxides (Mg/Zr and Mg/Al), CaO/ZnO as well as hydrotalcite; each of these have shown excellent catalysis performances in transesterification reactions. Heterogeneous alkaline catalysts also include MgZnAlO, CaO/Fe\({}_{3}\)O\({}_{4}\), CuO/Al\({}_{2}\)O\({}_{3}\), Li/MgO, K\({}_{2}\)CO\({}_{3}\) Sodium silicate, Cr/Al mixed oxide catalyst. Heterogeneous alkaline catalysts are designed in order to graft and lock in active molecules inside the pores and entrap on the surface of solid support such as alumina, ceria and silica [19; 25; 26]. Alkali-based heterogeneous catalysts have a high potential for catalysis for biodiesel production and overcome some of the drawbacks and limitations such as soap formation due to homogeneous catalysts. Soap formation makes the separation of by-product (glycerol) \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{6}{c}{**Heterogeneous Basic Catalysts**} \\ \hline \multirow{2}{*}{**Catalyst**} & \multirow{2}{*}{**Feedstock Oil**} & **Catalyst** & **Oil to** & \multirow{2}{*}{**Time** **Methanol**} & **Temp.** & **Yield of** \\  & & **(wt. \%)** & & & **(h: min)** & **(\%)** & **Biodiesel** \\ \hline CaO/Al\({}_{2}\)O\({}_{3}\) & Palm & 6 & 1:12 & 5 h & 65 & 98.6 \\ MgZnAlO & Jatropha & 8 & 1:11 & 6 h & 182 & 94.0 \\ CaO/Kf & Tallow seed oil & 4 & 1:15 & 2.5h & 65 & \textgreater{}96 \\ ZnO/CaO & Ethyl butyrate & 1.3 & 1:12 & 2h & 60 & \textless{}90 \\ CaO & Jatropha & 1.5 & 1:9 & 2.5 h & 70 & 93 \\ Mg/Al-hydrotalcite & Soybean & 5 & 1:13 & 1 h & 230 & 90.7 \\ Fe\({}_{3}\)O\({}_{4}\)/CaO & Jatropha & 2 & 1:15 & 1.33 h & 70 & 95 \\ MgO/Li & Soybean & 9 & 1:12 & 2 h & 60 & \textgreater{}94 \\ Mg/Al-hydrotalcite & Sunflower & 2 & 1:12 & 24 h & 60 & 75 \\ Al-Ca/K\({}_{2}\)CO\({}_{3}\) & Soybean & 2 & 1:13 & 2 h & 65 & 95.1 \\ Na-SiO2 & Jatropha & - & 1:9 & 0.25 h & - & 98.5 \\ Sodium silicate & Soybean & - & 1:7.1 & 1 h & 60 & \textasci{}100 \\ \hline \multicolumn{6}{c}{**Heterogeneous acidic catalysts**} \\ \hline \multicolumn{6}{c}{**Heterogeneous acidic catalysts**} \\ \hline \multicolumn{6}{c}{Zeolite X} & Sunflower & 4.2 & 1:6 & 7:00 & 60 & \textasci{}95 \\ SO\({}_{4}\)/ZrO\({}_{2}\) & Palm & 0.5 & 1:25 & 0:10 & 250 & 90 \\ WO\({}_{3}\)-ZrO\({}_{2}\) & Sunflower & 3 & 1:20 & 5:00 & 200 & 97 \\ KSF clay amberlyst & Jatropha & 5 & 1:12 & 6:00 & 160 & 70 \\ SO\({}_{4}\)\({}^{2-}\)/TiO\({}_{2}\)-SiO\({}_{2}\) & Cottonseed & 3 & 1:9 & 6:00 & 180 & 92 \\ SO\({}_{4}\)\({}^{2-}\)/ZrO\({}_{2}\)SiO\({}_{2}\) & Rocket seed & 5 & 1:8 & 24:00 & 65 & \textasci{}88 \\ \multicolumn{6}{c}{Waste Edible Oil} \\ \multicolumn{6}{c}{(WEO)} \\ \multicolumn{6}{c}{(Zr\({}_{0.7}\)H\({}_{0.2}\)PW\({}_{12}\)O\({}_{40}\)} \\ \multicolumn{6}{c}{Ferric manganese doped} \\ \multicolumn{6}{c}{Tungstated/molybdenum} \\ \multicolumn{6}{c}{Carbon based solid acid catalysts} \\ \multicolumn{6}{c}{ZS/Si} \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Heterogeneous base and acid catalyst used for biodiesel production [15; 20; 22; 23]. from fatty acid methyl ester layer. In contrast to homogeneous catalysts, these catalysts are easy to separate and easy to reuse because of their low solubility and their active sites during reaction are destroyed. However, they are also reconstructed in an easy way. The only disadvantage of heterogeneous catalysts that they absorb water when stored. Many metallic oxides such as alkalis, alkaline earth and transition metals have appeared as potential candidate for oils transesterification. As metal oxides have both bronsted acid (cations) and bronsted base characters (anions). Sometimes, composites are formed to be used as catalysts [26].