At the moment, almost all commercial biodiesel productions used homogeneous base catalysts. However, the major disadvantage of homogeneous catalyst is the fact that it cannot be reused. Furthermore, the separation of catalyst from the reaction mixture requires further stages of washing, and the process is usually conducted in batch type rather than continuous type. In recent times, the research focusing on using heterogeneous base catalysts becomes more extensive in order to find the catalyst that can contribute to an economical process. The utilization of heteroge­neous catalyst means that they can be placed in a fixed bed reactor and allows con­tinuous biodiesel production.

Alkaline metal oxides are receiving considerable attention in biodiesel synthesis, as they contain basic sites that can catalyze the reaction. Examples of alkaline metal oxide catalysts are calcium oxide (CaO), strontium oxide (SrO), barium oxide (BaO), and magnesium oxide (MgO). CaO is one of the most established alkaline metal oxide catalyst used, owing to its relatively high basic strength and high avail­ability. It also holds the advantages of easy product recovery and easier handling as compared to NaOH and KOH.

A study on different kinds of metal oxides was conducted by Kawashima et al. (2008) for obtaining biodiesel using heterogeneous base catalysts. The catalysts were prepared from different types of metal oxides including calcium, barium, mag­nesium, and lanthanum. Among these, calcium-containing catalysts showed high yields of methyl ester. It is proposed that the weaker base strengths of Ba, Mg, and La series resulted in low activity for the transesterification reaction.

Liu et al. (2008) conducted the transesterification of soybean oil catalyzed by CaO. The biodiesel yield exceeded 95% for 3 h reaction time. Its lifetime was found to be better than K2CO3 and KF loaded on y-Al2O3 and maintained its catalytic activ­ity even after being used for 20 cycles, with only minimal reduction of biodiesel yield. The author suggested that the catalyst requires some amount of water for the reaction rate of transesterification to progress effectively, in this case was 2.03% of water content.

Calcium compounds were used as solid base catalysts in conversion of soybean oil into biodiesel (Kouzu et al. 2008). From the results, CaO resulted in the highest yield of FAME with the shortest time, followed by calcium hydroxide (Ca(OH)2), while calcium carbonate (CaCO)) seemed to be inactive for catalyzing the trans­esterification. It was found that CaO was deactivated when exposed to air. CO2 and moisture adsorbed on the basic sites of CaO rendered the catalyst and decreased its catalytic ability, based on the FAME yield.

Various attempts have been made to further increase the efficiency of CaO as heterogeneous base catalyst in biodiesel synthesis. One of the works was done by Yan et al. (2009). Lanthanum modified CaO catalysts were prepared for the trans­esterification process. The catalyst Ca3La1, defined by the molar ratio of La to Ca, successfully produced 94.3% FAME within 60 min at 58°C. The catalytic activity of the catalyst was comparable to NaOH in terms of FAME yield and even better than the other two catalysts (CajLa0 and Ca)La1). Furthermore, the mixed CaO — La2O3 catalyst is highly tolerable towards the fFa and water content in the feed­stock used.

Another attempt to improve the activity of CaO was done by Taufiq-Yap et al. (2011). CaO-MgO catalysts were synthesized using different Ca/Mg atomic ratios. The catalyst with the Ca/Mg atomic ratio of 0.5 (CM0.5) managed to obtain the highest biodiesel yield (90%), and the yield dropped as the Ca/Mg ratio increased. The author suggested that the decreased in the catalytic activity was caused by the low catalysts’ surface area. Higher loading above 0.5 at.% triggered the diffusion limitation between the reactant and the basic active sites. In addition, when CM0.5 was used, the FAME yield did not reduced significantly even after the fourth cycle.

The search for the efficient catalyst for biodiesel production is continued with the recent interest of catalyst made from cheaper source. The use of natural calcium sources for conversion of oils into biodiesel underlines the effort to cut down the cost of catalyst without affecting their catalytic performance.

The utilization of this type of catalyst adds value to the recycled waste, aside from being environmental friendly and green. The sources for the catalyst prepara­tion are presented in Table 9.3. Calcination step is essential for the decomposition of CaCO3-rich sources into CaO, which is the common heterogeneous base catalyst available. The process is conducted for several hours at high temperature to ensure that the catalyst prepared is viable for the biodiesel synthesis later.

Wei et al. (2009) investigated the use of egg shell for the transesterification of soybean oil. The author reported that the catalyst can be repeatedly used for 13 times without significant reduction in biodiesel yield and completely deactivates after being used more than 17 times. The study on the calcination temperature showed that biodiesel yield of 97-99% was achieved when the egg shell was calcined above 800°C, and the calcination below 600°C did not lead to the forma­tion of CaO.

Biodiesel was produced through transesterification of soybean catalyzed by combusted oyster shell (Nakatani et al. 2009). The process produced 73.8% of bio­diesel yield at optimum conditions, and it is comparable with the yield using CaO. Furthermore, the biodiesel purity was also quite high (98.4 wt%). Hu et al. (2011) synthesized catalyst from freshwater mussel shell, and biodiesel yield above 90% was achieved in 1.5 h reaction time. The author pointed out that the reduction of the catalytic activity was caused by the transformation of the catalyst into Ca(OH)2 and that the activity can be recovered after calcination in air at 600°C.