## 4 Types of Catalysts Used in Transesterification
### Heterogeneous Catalysts
#### 4.2.1 Heterogeneous Base Catalyst
Alkali and Alkaline Earth Metallic Catalysts  
Among frequently exploited heterogeneous catalysts, metal-based oxides are common catalyst for transesterification. [PERSON] et al. (2020) demonstrated that the amount of basic site/unit wt. corresponds to the surface area approx. and sequence CaO > MgO > SrO > BaO: oxides of strong basic sites enhance the reaction rate more efficiently [25]. Among these oxides of metals, CaO is most widely used catalyst, owing to its comparatively high basic strength, long life, low solubility in methanol, high activity and working under moderate conditions. Previous studies reported basic strength by FTIR analysis as: MgO-KOH > MgO-NaOH > MgO-CeO\({}_{2}\). Alkaline fluoride is homogeneous strong base catalyst, and it is mixed with other compounds and composites which are used for transesterification of oil or fats, and biodiesel is synthesized. These catalysts are KF/ZnO, KF/CaO and KF/CaO and other catalysts are supported. Catalysts are synthesized from calcium carbonate (CaCO\({}_{3}\)) present in organic based materials such as agricultural shells, bones, and marine shells. Through calcination at 800-1000 \({}^{\circ}\)C CaCO\({}_{3}\) is converted into CaO and CO\({}_{2}\). [PERSON] and [PERSON] (2018) treated the CaO catalyst with methanol and glycerol and its activity of was enhanced after this process. The resultant catalyst derived from CaO was denoted by CADG (Calcium diglyceroxide) that presents high catalytic activity compared to CaO when used for transesterification of waste cooking oil. The optimization parameters for CaO were 12:1 methanol to oil molar ratio, 65 \({}^{\circ}\)C temperature, 3% catalyst amount, 400 rpm agitation and 90 min reaction duration and a 93% yield was obtained. However, CADG gave a 96% biodiesel yield under optimized conditions such as 10:1 methanol to oil molar ratio, 60 \({}^{\circ}\)C temperature, 1.5% catalyst amount, 300 rpm agitation and 50 min reaction time. Hence, it was concluded that CADG was more efficient catalyst than CaO [26]. A heterogeneous base catalyst appears to be superior to homogeneous catalyst as it is easy to separate and purify reaction products, has less sensitivity to moisture and free fatty acids, is less corrosive, and is ecofriendly, recyclable active at less reaction temperature. Additionally, these catalysts are able to catalyze transesterification of low-quality oils with high content of free fatty acids in it. [PERSON] et al. (2020) reported limitations of heterogeneous base catalysts such as synthesis cost and reaction rate. Hence, such facts are needed to be focused for the application of heterogeneous alkaline catalysts for utilization at a large scale. If the reusability potential is regarded as a point of consideration, then the shortcoming of synthesis budget can be eliminated. However, further investigations are prerequisite for raw material and reaction rate for catalysts [25]. [PERSON] et al. (2018) used strontium oxide (SrO) to produce biodiesel from palm oil via ethanolysis under optimized conditions of 1:12 oil/ethanol molar ratio, 5 wt. % catalyst loading at 80 \({}^{\circ}\)C for 3h and 98.2% yield was achieved and reused for 5 times with >90% yield. [PERSON] et al. (2018) prepared LiFe\({}_{5}\)O\({}_{8}\)-LiFeO\({}_{2}\) and this catalyst was prepared by mixing Li\({}_{2}\)CO\({}_{3}\) (aq. soln.) and Fe\({}_{2}\)O\({}_{3}\) followed by calcination (800 \({}^{\circ}\)C) and then cooling. This catalyst was used with 8 wt. % loading, 1:36 oil to methanol molar ratio at 65 \({}^{\circ}\)C for 2 h and 96.5% yield was obtained. Furthermore, it was reused 5 times with a 94% yield [27].