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.