Alkali-free zn–al layered double hydroxide catalysts for triglyceride transesterification

Abstract

Zn–Al layered double hydroxides (LDHs) of general formula Al3+x(OH)2]x+(CO3Zn–Al2−)x/2·yH2O are promising solid base catalysts for the transesterification of lipids to biofuels. However, conventional synthetic routes employ alkali ydroxide/carbonate precipitants which may contaminate the final LDH catalyst and biofuel. The use of (NH3)2CO3 and NH3OH as precipitants affords alkali-free Zn–Al-LDHs spanning a wide composition range. The hydrothermal reconstruction of calcined Zn–Al-LDHs offers superior solid basicity and catalytic activity for the transesterification of C4–C18 triglycerides with methanol,compared with cold liquid phase or vapour phase reconstruction. Hydrothermally activated Zn3.3–Al-LDH was stable towards leaching during transesterification.