However, alkalis and acids are not the only catalysts which can be used in the trans­esterification reaction and these include enzymes and solid catalysts. Some of the solid catalysts are listed in Table 7.11.

Transesterification using heterogeneous catalysts has been investigated using basic zeolites and alkaline metal compounds. Metal oxides, hydroxides and alkoxides have been used to transesterify rapeseed oil (Gryglewicz, 1999) where calcium oxide was the most effective. Metal oxides and those loaded with Al2O3, SiO2 and MgO were also used to treat rapeseed oil (Peterson and Scarrach, 1984).

Oil extracted from Pongamia pinnata has been transesterified using a solid Li/ CaO catalyst even in the presence of 0.48-5.75% free fatty acids (Meher et al.,

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2006b) and Jatropha curcas oil using CaO (Zhu et al., 2006). A number of modified zeolites have been used successfully to transesterify soybean oil (Suppes et al., 2004). Much of the research has been with solid base catalysts but solid acid catalysts have also been used. Tungstated zirconia, a solid super acid catalyst, has been used to transesterify soybean oil at 200-300°C, and has given a conversion of over 90% (Furuta et al., 2004). More recently, amorphous zirconia combined with titanium and aluminium has been shown to give over 95% conversion of soybean oil at 250°C (Furuta et al., 2006).

Microbial lipases have the ability to transesterify oils in the presence of metha­nol. These enzymes function in the presence of water and the catalyst and salts do not need removing at the end of the reaction (Table 7.12). However, the enzymes are more expensive than the simple inorganic catalysts. Some of the expense of using enzymes can be reduced by enzyme immobilization which allows a continu­ous process and increases the working life of the enzyme (Ban et al., 2001; Fukuda et al., 2001).

Transesterification has also been carried out using supercritical methanol, ethanol, propanol and butanol. The process does not require a catalyst but high tem­peratures (~300°C) and pressures (8 MPa) (Cao et al., 2005; Demirbas, 2006a, b).