Biodiesel synthesis using calcined layered double hydroxide catalysts

The catalytic properties of calcined Li–Al, Mg–Al and Mg–Fe layered double hydroxides (LDHs) were examined in two transesterification reactions, namely, the reaction of glyceryl tributyrate with methanol and the reaction of soybean oil with methanol. While the Li–Al catalysts showed high activity in these reactions at the reflux temperature of methanol, the Mg–Fe and Mg–Al catalysts exhibited much lower methyl ester yields. CO2 TPD measurements revealed the presence of sites of weak, medium and strong basicity on both Mg–Al and Li–Al catalysts, the latter showing higher concentrations of medium and strong base sites; by implication, these are the main sites active in transesterification catalyzed by calcined Li–Al LDHs. Maximum activity was observed for the Li–Al catalysts when a calcination temperature of 450–500 °C was applied, corresponding to decomposition of the layered double hydroxide to the mixed oxide without formation of crystalline lithium aluminate phases.