Alkali-Free Zn–Al Layered Double Hydroxide Catalysts for Triglyceride Transesterification

Abstract

Zn⁻Al layered double hydroxides (LDHs) of general formula [Zn2+(1−x)Al3+x(OH)2]x+(CO32−)x/2·yH2O are promising solid base catalysts for the transesterification of lipids to biofuels. However, conventional synthetic routes employ alkali hydroxide/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.

Keywords

• biodiesel
• layered double hydroxides
• transesterification
• solid base
• triglycerides