Catalytic activities of alkali ion-exchanged zeolites, alumina, alkali treated alumina and hydrotalcite having varied Mg/Al molar ratio were evaluated for aldol condensation of propanal in liquid phase under solvent free conditions. The maximum conversion (97%) of propanal with 99% selectivity of 2-methylpentenal was obtained at 100 degrees C and 10 h using activated hydrotalcite of Mg/Al molar ratio of 3.5 as a catalyst. The basicity of the activated hydrotalcite of Mg/Al ratio 1.5, 2.5 and 3.5 was evaluated by isomerization of beta-isophorone to a-isophorone as a model test reaction reported for basicity measurement. The effect of amount of catalyst and temperature on the conversion of propanal, selectivity of 2-methylpentenal and initial rate of reaction were studied in detail. The activation energy for propanal condensation under solvent free condition was found to be 58 kJ using activated hydrotalcite of Mg/Al molar ratio 3.5 as a catalyst. The catalyst was recycled six times without any significant loss in conversion and selectivity. (c) 2007 Elsevier B.V. All rights reserved.