Zhu et al. (2006) reported the high catalytic activity of heterogeneous solid super base CaO catalyst in the production of biodiesel from Jatropha oil. By using solid base catalyst, 93% of oil conversion was obtained within 2.5 h at 70°C, catalyst dos­age of 1.5% and methanol/oil molar ratio of 9:1. However, extra purification step was performed by using decalcifying agents (citric acid) to eliminate the remaining calcium species in biodiesel.

Besides, similar study using CaO was done by Taufiq-Yap et al. (2011a). The stability between single metal CaO and bimetal calcium-based oxides was inves­tigated via methanolysis of Jatropha oil. He and his co-researchers found that calcium-based mixed oxides catalysts (CaMgO and CaZnO) are able to produced high yield of biodiesel under mild condition with stronger stability. In optimum environment, more than 80% of biodiesel content was yielded at 65°C, catalyst amount of 4 wt%, methanol/oil ratio of 15 and 6 h reaction time. The result revealed that calcium-based mixed oxides rendered more feasible reaction than bulk CaO, this was due the drop of CaO catalytic activity throughout six cycles. This phenomenon was due to the leaching of active Ca2+ into the reaction medium and reacted with FFA content and resulted soap formation. Furthermore, the study was furthered by examining the optimum stoichiometric ratio between bimetal (Ca:Mg) catalysts in order to achieve maximum transesterification activity Taufiq — Yap et al. (2011b). A reaction model was design by using response surface meth­odology (RSM) to search for optimum condition in biodiesel mass production (Lee et al. 2011).

Endalew et al. (2011) tested the mixture of metal oxide catalysts in the simulta­neous esterification and transesterification of high FFA Jatropha oil in single-step reactions. According to Bender (1960), transition metal oxide catalysts are found to tolerate with high acid and water content, which are able to perform esterification and transesterification simultaneously. On the other hand, the presence of Lewis base and acid catalytic sites in amphoteric metal oxide catalyst is able to perform single-step esterification and transesterification reaction. Thus, the rare earth metal oxide catalysts loaded on transition metal oxide (La2O3-ZnO), amphoteric oxide (La2O3-Al2O3), and perovskite catalyst (La0 jCa0 9MnO3) were prepared for the cata­lytic test. However, the results showed a low transesterification activity due to the presence of weak base and acid strength. This indicated the catalysts required harsh reaction condition with longer reaction time, higher temperature, and pressure. Besides, alkaline earth metal oxide (CaO), alkali doped alkaline earth metal oxide (Li-CaO), and mixture of solid base (CaO or Li-CaO) with solid acid catalyst (Fe2(SO4)3) were studied in single-step simultaneous esterification and transesterifi­cation process. Soap formation was found in the CaO and Li-CaO catalyzed reac­tion in which FFA in the oil neutralized the Ca2+ on the surface of the catalyst. However, both CaO+Fe2(SO4)3 and Li-CaO+Fe2(SO4)3 showed complete conver­sion to biodiesel in single step process under mild condition. The presence of Lewis acid from Fe3 + promoted the esterification reaction for FFA reduction, and high basicity of CaO and Li-CaO transesterifies the oil to biodiesel.

The biodiesel production by transesterification of Jatropha oil with methanol using alumina-supported potassium nitrate (KNO2/Al2O3) was performed. Although the solid base catalyst gave high oil conversion (>84%) under mild condition of 70°C, 6% catalyst amount, 12:1 methanol/oil molar ratio for 6 h, the reusability for this catalyst was low. The results showed that conversion was less than 80% after third run (Vyas et al. 2009).

Kumar et al. (2010) investigated the catalytic activity of mesoporous Na supported on SiO2 catalyst (Na/SiO2) in the transesterification of Jatropha oil with methanol using ultrasonic irradiation. The optimum conditions to produce 98.53% of bio­diesel yield were 9:1 methanol/oil ratio, 3 wt% catalyst amount, and 50% ultrasonic wave amplitude for 15 min. The results showed that ultrasonic transesterification showed conversion rate that was similar to conventional method with shorter reac­tion time without mechanical stirring. The high stability of Na/SiO2 catalyst is capa­ble of maintaining biodiesel yield at above 90% for five consecutive cycles.