Activation of reconstructed Mg/Al hydrotalcites in the transesterification of microalgae oil

Mg/Al–CO3 hydrotalcites with a Mg/Al molar ratio of 4.0 were prepared using urea method. The microstructures and catalytic activities of the samples for the transesterification of microalgae oil with methanol were studied as compared with those prepared by co-precipitation. The as-synthesized and reconstructed hydrotalcites including their corresponding mixed oxides had been characterized by XRD, FT-IR, DSC and SEM analyses as well as the Hammett titration method. All the hydrotalcites possessed the hydrotalcite-like layered structure containing carbonate ions. Crystal sizes of reconstructed hydrotalcites were smaller than those of the as-synthesized ones, and the hydrotalcites prepared using urea method had larger crystal sizes than those by co-precipitation. The activity of the catalyst was closely correlated with its surface basicity, in which more basic sites displayed a higher catalytic activity. Again, the reconstruction of the hydrotalcites, the “memory effect”, had a significant influence on its surface basicity. The first calcination mixed oxide from the as-synthesized hydrotalcite using urea method had the highest catalytic activity with about 90.3% ester conversion. Furthermore, the Box–Behnken design, an experimental design for response surface methodology, was used to study the relationships of reaction temperature, methanol to oil molar ratio, and catalyst amount to the ester conversion. The results showed that it was a good agreement with the experimental data, demonstrating that the methodology may be useful for industrial process optimization.