High conversion and yield of biodiesel can be achieved with the application of solid base catalysts. However, the sensitivity towards FFA in the feedstock limits their performance. To overcome this problem, heterogeneous acid catalysts are suitable alternatives than their counterpart as they are more tolerant towards feedstock pos­sessing high acid value. In addition, the catalysts can also be used to simultaneously catalyze esterification and transesterification, which eliminates the requirement for two-step processes of biodiesel production. They are good alternatives to the homo­geneous acid as they simplified the separation of catalyst from the reactants and can be recycled for further use. These catalysts significantly simplify biodiesel synthe­sis and, at the same time, reduce the production cost.

Among solid acid catalysts, zirconia has received considerable attention due to its high thermal stability and amphoteric nature, which can behave as both as an acid and as a base. Zirconia can be modified by incorporating suitable anions, such as sulfate ions to form a highly acidic or superacidic, depending on the required conditions.

Lopez et al. (2008) studied the catalytic performance of titania zirconia (TiZ), sulfated zirconia (SZ), and tungstated zirconia (WZ) for esterification of carboxylic acids and transesterification of triglycerides. Although SZ was the most active cata­lysts for the processes, the problem related to leaching of its sulfur loading was observed during the catalyst reusability test. The author suggested that WZ was more suitable for long-term use and can be easily regenerated by calcination in the air. The application of tungsten oxide zirconia (WO3/ZrO2), sulfated zirconia, and Amberlyst-15 in production of biodiesel was investigated by Park et al. (2010b). WO3/ZrO2 showed the highest activity among them, and there was also no apparent loss or leaching of WO3. Increasing the reaction temperature solved the low activity of the catalyst in feedstock with high FFA content.

The potential of ion exchange resin as catalyst for producing biodiesel has also been studied by researchers. Acidic ion exchange resin, such as Amberlyst-15, is inexpensive and commercially available as solid acid catalyst. This resin enables hassle-free separation step, aside from its excellent performance especially for esterification of FFA. Talukder et al. (2008) reported that Amberlyst-15 was a better choice than Novozym 435 (commercial enzyme for biodiesel catalysis) because of higher biodiesel yield and also low catalyst cost. The presence of organic solvent does not affect the performance of Amberlyst-15, instead increases the biodiesel yield. However, Amberlyst-15 performed worse than Novozym 435 when the water content in the feed was equivalent to 4 wt%, due to the hygroscopic nature of the catalyst and water may have been adsorbed on its surface.

In another study, Park et al. (2010a) confirmed that the esterification of FFA using Amberlyst-15 was hindered by water produced during the process. Another heterogeneous acid resin, Amberlyst BD20 maintained its catalytic activity even with water present. Characterization of the catalyst showed that it does not have pores on the surface, which prevents the adsorption of water on the surface.

Heteropolyacids (HPAs) possess high activity and stability, strong Brqrnsted acidity, and also excellent water tolerability. They can be employed as either hetero­geneous or homogeneous catalysts depending on their composition and the reaction medium. Tungstophosphoric acid (H3 PW12O40) was employed as the catalyst for promoting esterification of saturated and unsaturated fatty acids (Cardoso et al. 2008a). The conversion of oleic acid to ethyl oleate using HPA catalyst was compa­rable with those using catalysts such as H2 SO4 and PTSA. The catalyst was less productive with the inhibition of water, which resulted in decreased ester yield. On the brighter side, the catalytic activity of H3PW12O40 remained unchanged even after several recovery cycles.

A one-step solgel co-condensation was applied for incorporating tantalum per­oxide with tungstophosphoric acid to produce mesoporous composite catalyst, H3PW12O40/Ta2O5 of different H3PW12O40 loading (Xu et al. 2008). The catalyst hav­ing H3 PW12O40 loading of 10.8% demonstrated the highest catalytic activity. Any value higher than this would result in decreased porosity of the composite, which leads to blockage of the pores of Ta2O5 matrix. Complete esterification of myristic acid was achieved, and the yield from transesterification of soybean oil exceeds 75% using the synthesized catalyst. However, extended reaction time was required (24 h). No leaching of catalysts into biodiesel phase was reported for this study.

Xu et al. (2009) continued to further study the previously used catalyst. The hybrid catalyst was produced with the addition of either methyltrimethoxysilane (MeTMS) or phenyltrimethoxysilane (PhTMS) and silica (SiO2). The integration of the hybrid catalysts with both alkyl groups not only increased the yield for trans­esterification of soybean oil but also tuned the hydrophobic/hydrophilic balance of the catalyst. As a result, higher activity and much lower catalyst deactivation were obtained compared to alkyl-free catalysts. However, the preparation of the catalysts is complex when taking into account of the materials and processes required for this part.