Synthesis of methylpseudoionones by activated hydrotalcites as solid base catalysts

Preparation of methylpseudoionones (precursors of methylionones) has been carried out by aldol condensation between citral and methyl ethyl ketone (Mek) using an amorphous aluminophosphate (ALPO), KF–Alumina and differently activated Al-Mg hydrotalcites as solid base catalysts. Activated hydrotalcites (calcined or calcined–rehydrated hydrotalcites) exhibited the best activity and selectivity to obtain methylpseudoionones. The effect of the chemical composition of the calcined hydrotalcites as well as the effect of the water content of the rehydrated samples have been studied. The main process variables such as reaction temperature and Mek∶citral molar ratio have been studied on these different activated catalysts, and in general it has been found that an increase of reaction temperature increases in rate of formation of methylpseudoionones, whereas the influence of the reaction temperature on the selectivity to this compound strongly depends on the type of activated hydrotalcite used. Owing to basicity/adsorption differences between calcined and rehydrated hydrotalcite, a different behaviour of reaction rate and selectivity to the desired product versus Mek∶citral ratio is observed, which should be attributed to the different nature and distribution of basic sites existing for both samples.