Catalytic condensation of aromatic aldehydes with acetone on activated Mg–Al mixed oxides

Abstract

The factors affecting the aldol condensation of some model aromatic aldehydes (benzaldehyde, the three isomers of F-benzaldehyde and 2-naphthaldeyde) with acetone have been investigated using heterogeneous catalysts obtained by the calcination of hydrotalcite-type (HT) precursors at 773 K for 8 h, followed by rehydration at room temperature for 7 h under a flow of N 2 saturated with water vapour. At 273 K the main product was the β-hydroxy-ketone, while at 318 K it mainly dehydrated to the corresponding trans-arylideneacetone, with small amounts (<10 wt%) of by-products as cis-arylideneacetone, 1,3-diarylideneacetone and 4-hydroxy-4-methyl-2-pentanone. High reaction times favoured the formation of trans-arylideneacetone. The best conversion values were observed for the three F-benzaldehydes and while using an HT catalyst with Mg/Al=2.5 as atomic ratio. Finally, a significant improvement of the productivity per kg of catalysts has been achieved by repeatedly adding new amount of aromatic aldehyde to the reaction mixture when the conversion value was about 80% ca., thus developing a semi-continuous process.