Aldol condensation of acetone over layered double hydroxides of the meixnerite type

Aldol condensation of acetone is performed at 273 K in liquid phase using as catalysts Mg–Al layered double hydroxides (LDH) with Cl− and/or CO32− as compensating anions. The products of the reaction are mainly diacetone alcohol and mesityl oxide. The catalytic activities increase with the Al content of the LDH and a conversion around 20% is reached when the carbonated compounds are calcined at 723 K where catalysts are mixed oxides of the Mg(Al)O type. The nature and the amount of the compensating anion in the LDH greatly influence the catalytic activity, particularly traces amount of Cl− are very detrimental. Mesityl oxide resulting from the dehydration of diacetone alcohol on acid sites strongly inhibits the reaction. By contrast, the addition in controlled amounts of H2O in acetone enhances the conversion with a higher selectivity into diacetone alcohol. At the same time the catalyst recovers the lamellar structure. A too high amount of H2O inhibits the reaction. Experiments have been performed by rehydration in vapor or in liquid phases of the mixed oxides, previously decarbonated by calcination at 723 K. With these samples, having the meixnerite-like structure, the conversion of acetone into diacetone alcohol reaches the thermodynamic equilibrium (23%) in less than 1 h. Therefore the rehydration of the mixed oxide into the meixnerite structure modifies the strength and the concentration of the active OH− sites.