Potassium modified alumina as a catalyst for the aldol condensation of benzaldehyde with linear C3–C8 aldehydes

Abstract

Different types of catalysts based on potassium modified alumina were prepared. The source of potassium was either potassium carbonate or potassium hydroxide. The prepared materials were characterized by UV–Vis spectroscopy, X-ray diffraction, thermogravimetry, nitrogen physisorption and tested as catalysts in the aldol condensation of benzaldehyde with C3–C8 linear aldehydes. Some of the products may serve as important fragrant compounds—especially jasmine aldehyde (prepared from heptanal) or 2-hexylcinnamylaldehyde (from octanal). It was found that the reaction rate increased with the increasing amount of potassium loaded on the catalyst. The reaction rate increased with decreasing the length of the aldehyde chain. The selectivity to the autocondensation product increased when the pore diameter of catalyst was decreased. The aldol condensation of benzaldehyde with propanal performed using alumina modified by 3.6 mmol potassium/g of material resulted in 97% propanal conversion with 89% selectivity to 2-methylcinnamylaldehyde and 6% selectivity to 2-methylpent-2-enal. The aldol condensation of benzaldehyde with octanal performed using alumina modified by 1.5 mmol potassium/g of material resulted in 29% propanal conversion with 17% selectivity to 2-hexylcinnamylaldehyde and 82% selectivity to 2-hexyldec-2-enal.