The influence of alkaline metals on the strong basicity of Mg–Al mixed oxides: The case of transesterification reactions

Several Mg/Al mixed oxides, prepared by three different co-precipitation procedures, have been tested in different transesterification reactions. The catalytic activity for sunflower oil transesterification correlates with the leachable basicity, and not with the total basicity, measured by titration methods. The presence of residual alkaline ions after crystallization is the main source for the strong basicity in this kind of solids, and the difficulty in the complete elimination of the alkaline traces are probably the origin of the strong discrepancies in basicity detected along the literature data. The effect of these alkaline traces is more evident with a more demanding reaction, such as that of methyl palmitate with isobutanol. In such case the alkaline-free solids did not show any catalytic activity. The nature of these strong basic sites makes the recovery problematic, given that part of the alkaline cation is lost to the reaction medium, a process favored by an increase in the polarity of the medium. This fact is also a general limitation for the application of this kind of solids to liquid phase reactions requiring high basicity.

The strong basic sites of Mg/Al mixed oxides are mainly due to traces of alkaline metals, as shown by the linear correlation between yield and leachable basicity. This effect is more clearly shown when highly demanding reactions, such as transesterification with branched higher alcohols, are carried out.Figure optionsDownload as PowerPoint slide