The role of rehydration in activating Mg6Al2 hydrotalcites for catalytic biodiesel production

Abstract

In this work, the use of Mg6Al2 as heterogeneous base catalyst was investigated in the transesterification of sunflower oil reaction. The performance of calcined or rehydrated Mg6Al2 solids was tested for efficient biodiesel production. The solids were calcined at different temperatures ranging from 400°C to 600°C and then rehydrated in hot water for 15 minutes. A preliminary catalytic activity screening showed that calcination at 450°C prior to rehydration is the most effective. The corresponding highest obtained FAME yield was around 56%. The most efficient catalyst, Mg6Al2-450RH, was then characterized with different physico-chemical techniques. XRD, FTIR and DSC/TG analyses showed that the hydrotalcite crystalline structure was destroyed after calcination at 450°C. However, a 15 minutes rehydration treatment was sufficient to enable the reconstruction of the hydrotalcite structure. The basicity of the calcined and rehydrated solids was probed using CO2-TPD technique. It is clear that the rehydrated solid has a larger total number of basic sites, which appear to be stronger than those encountered in the freshly calcined solid. Several reaction parameters were varied in order to enhance the reaction using the Mg6Al2-450RH catalyst. The best conditions are: a rehydration duration of 15 minutes, a reaction temperature of 60°C, a methanol to oil molar ratio of 12:1 and a catalyst to oil ratio of 10wt%.