At present, commonly used homogeneous base catalysts are sodium hydroxide (NaOH), potassium hydroxide (KOH), and sodium methoxide (CH3ONa).

They are usually employed for catalyzing the conversion of triglycerides into biodiesel because of their high catalytic activity, which resulted in a relatively fast reaction (Hoda 2009). Besides, they are also preferred over acid catalysts in terms of their economical usage and available at cheaper price. Homogeneous base cata­lysts are proven to be appropriate choice for biodiesel synthesis, owing to the mild reaction conditions required for achieving high methyl ester yield (Phan and Phan 2008).

Although these catalysts are outstanding in terms of operating conditions, they also have some drawbacks. One of them is that they cannot tolerate feedstock

Fig. 9.2 Reactions between free fatty acid and basic catalysts to produce soap

containing high free fatty acid (FFA) content, especially when nonedible oils are used as the feedstock. FFA reacts with the basic catalyst and produces soap. The formation of soap is shown in Fig. 9.2. FFA reacts with the catalyst to form soap and water through saponification process when an alkali catalyst is added to oil or fat having significant amounts of FFA. This reaction is undesirable as it complicates the separation between products further downstream. Moreover, the saponification consumes alkali catalysts, decreasing transesterification rate and resulted in lower biodiesel yield.

Water is another element in oils or fats that is undesirable for producing bio­diesel. Hydrolysis of triglycerides and alkyl esters may occur if water is present. This leads to the formation of FFA, and thus produced undesired soap when an alkali catalyst is used. As water catalyzes the hydrolysis reaction, the substrates should have minimum water content, or nearly anhydrous.

Hydrolysis of esters not only reduced biodiesel yield, but it also contributes to increased soap formation as more fatty acids are produced (Leung and Guo 2006).