## 4 Types of Catalysts Used in Transesterification
### Mixed Metal-Based Catalysts  
Based on the catalytic mixture, mixed metal oxides have been used as a base catalyst and this nature is tuned by changing their structure, synthesis procedure, activation temperature, and chemical composition that strongly influences the final basicity.  
[PERSON] et al. (2018) reported on La\({}_{2}\)O\({}_{3}\) in ZrO\({}_{2}\) by sol-gel method followed by calcination at 600 \({}^{\circ}\)C and used it to produce biodiesel from canola oil via transesterification as basicity was enhanced [28]. A catalytic activity of heterogeneous catalyst, CaMgZn, mixed oxides (3:1:1) prepared via coprecipitation method in presence of precipitant, such as Na\({}_{2}\)CO\({}_{3}\) (0.75 mol/L) and CaMg(CO\({}_{3}\))\({}_{2}\) and CaZn(CO\({}_{3}\))\({}_{2}\) with 1.0 metal ion ratio to synthesize biodiesel from palm kernel oil and 97.5 wt. % and 6 wt. % catalyst loading, 20:1 methanol to oil ratio and 60 \({}^{\circ}\)C Na\({}_{2}\)ZrO3 as heterogeneous basic catalyst (3% loading) to prepare biodiesel (98.3%) at 65 \({}^{\circ}\)C for 3 h [29].  
Transition Metal Catalyst  
Among the most used materials for biodiesel synthesis are titanium oxide (TiO\({}_{2}\)) and zinc oxide (ZnO). Tungsten (W) supported TiO\({}_{2}\)/SiO\({}_{2}\) catalyst synthesized by [PERSON] et al. (2018) via sol-gel method and waste cotton seed oil was converted in to respective methyl esters with 5 wt. % catalyst loading, 30:1 M methanol/oil ratio at 65 \({}^{\circ}\)C for 4 h. Mixed oxides TiO\({}_{2}\)-ZnO (200 mg) on palm oil with 1:6 M oil to methanol ratio at 60 \({}^{\circ}\)C for 5 h and 83% yield was achieved [30; 31].  
Boron Group Catalyst  
Al\({}_{2}\)O\({}_{3}\) is widely used to support alloys, halides, metal oxides and nitrates. For methanolysis of coconut oil and palm oil kernel, Al\({}_{2}\)O\({}_{3}\) supported nitrates of alkali and alkaline earth metals were prepared via impregnation method and 450-850 \({}^{\circ}\)C calcination. These included NaNO\({}_{3}\)/Al\({}_{2}\)O\({}_{3}\), LiNO\({}_{3}\)/Al\({}_{2}\)O\({}_{3}\), KNO\({}_{3}\)/Al\({}_{2}\)O\({}_{3}\), Mg (NO\({}_{3}\))\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\)), Ca(NO\({}_{3}\))\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\), and effect of calcination temperature was thoroughly observed and recommended transesterification conditions with 15-20 wt. % Ca(NO\({}_{3}\))\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) for coconut oil and 10 wt. % for palm kernel oil with 1:65 oil/methanol molar ratio at 60 \({}^{\circ}\)C for 3 h [32; 33].  
Hydrotalcite-Based Catalyst  
These are anionic basic clays that are used as heterogeneous catalyst due to their tunable strength through a change in Mg/Al and intrinsic better morphology in comparison with alkaline earth metal oxides. The only demerit associated with their usability is their sensitiveness to FFA and non-homogeneity. [PERSON] (2020) studied Zr-doped Mg/ Al hydrotalcite and varied Zr/Mg molar ratios for biodiesel synthesis from refined rapeseed oil and tetravalent cation Zr\({}^{4+}\) improved the catalytic activity and 99.9% optimum conversion for (Zr/Mg/Al = 0.45:2.55:1 M) hydrotalcite, 4.8-5 atm pressure and 373-393 k temp for 6 h time [25]. [PERSON] et al. (2018) produced biodiesel from sunflower oil and Mg-Al hydrotalcite (1.5-5 M) ratios by co-precipitation and used 1:48 M oil: Methanol, 2 wt. % catalyst and 1 atm pressure in 24 h and achieved a 96% optimum conversion [34].  
The creation of cost-effective base catalyst is aided by waste produced from the environment surrounding the industries and more sustainable and eco-friendlier biodiesel is obtained. Calcium-rich wastes such as mussel, snail and oyster shells, animal bones and ash based catalysts are inexpensive and readily available [35]. Calcium derived from such wastes are transformed to CaO, the most versatile catalyst. [PERSON] (2019) used CaO from waste egg shell and compared with pure CaO. Biodiesel synthesis was optimized with at 1:9 M oil/methanol, 60 \({}^{\circ}\)C for 1h and the yield was 95.12% for waste eggshell and 96.81% for CaO.