The use of a heterogeneous catalyst (base or acid) for biodiesel production has been recently and extensively explored. The main advantage of a heterogeneous catalyst over a homogeneous catalyst is that the former can be recycled and regenerated for repeated reac­tion cycles, resulting in minimum catalyst loss and improvement of the economic feasibility of biodiesel production. Furthermore, the catalyst can easily be separated at the end of the reaction using filtration; therefore, product contamination is reduced and the number of water washing cycles (purification) is minimized (Lam and Lee, 2012).

Heterogeneous catalysts can generally be divided into two categories:

1. Catalysts with basic sites, such as CaO, MgO, ZnO, and waste materials impregnated with KOH or NaOH. This type of catalyst can catalyze transesterification under mild reaction conditions with a high yield of biodiesel usually attained. Nevertheless, due to the basic properties of the catalyst, it is highly sensitive to the FFA content in the oil and results in soap formation instead of biodiesel. Leaching of active sites from the catalyst is another limitation that can cause product contamination and catalyst deactivation.

2. Catalysts with acidic sites, such as SO42~/ZrO2, SO42~/TiO2, SO42~/SnO2, zeolites, sulfonic ion-exchange resins, and sulfonated carbon-based catalysts. These catalysts are insensitive to the FFA content in oil and are able to perform esterification and transesterification simultaneously. However, the reaction rate is exceptionally slow; hence, extreme reaction conditions such as high temperature (more than 100 °C) with high alcohol-to-oil molar ratio (more than 12:1) are necessary to accelerate the overall reaction rate.

To date, the application of heterogeneous catalysts to algal biodiesel conversion is still scarcely reported in literature, primarily because algal lipids are a relatively new feedstock that is not commercially available in the market. In a recent study carried out by Umdu et al. (2009), CaO supported with Al2O3 was used as a heterogeneous base catalyst in transesterification of Nannochloropsis oculata lipids (Umdu et al., 2009). The highest algal bio­diesel yield attained was 97.5% under the following reaction conditions: reaction temperature of 50 °C, methanol-to-lipid molar ratio of 30:1, catalyst loading of 2 wt%, and reaction time of 4 h. The long reaction time was required mainly because of the initial three immiscible phases (lipid-alcohol-catalyst) that increase mass-transfer limitations in the system. When pure CaO was used as catalyst in the transesterification of algal lipids, insignificant biodiesel yield was recorded, but when CaO supported with Al2O3 (ratio 8:1 w/w) was used instead, signifi­cantly better results were achieved due to the increasing basic density and basic strength of the catalyst. Other heterogeneous catalysts such as Mg-Zr and hierarchical zeolites have also been investigated for transesterification of algal lipids; however, unsatisfactory biodiesel yield (less than 30%) was attained (Carrero et al., 2011; Krohn et al., 2011).