Study of the citral/acetone reaction on MgyAlOx oxides:: Effect of the chemical composition on catalyst activity, selectivity and stability

The liquid-phase citral/acetone reaction was studied on MgO, Al2O3 and MgyAlOx mixed oxides with Mg/Al molar ratios of 0.11-3. The density, strength and nature of surface acid sites were determined by NH3 thermodesorption and FTIR of pyridine. The surface basic properties were probed by temperature-programmed desorption of CO2. The rate of the citral/acetone cross-aldolization to pseudoionones increased linearly with the density of strong O2- base sites. Thus, the pseudoionone synthesis was efficiently promoted on MgO and Mg-rich MgyAlOx samples. In contrast, the initial rate of the parallel acetone self-condensation reaction increased with the density of Lewis acid sites, and diacetone alcohol was more rapidly formed on Al2O3 and Al-rich MgyAlOx samples. Differences in the reactant adsorption strength on surface Al3+ sites may explain the observed differences between the acid-catalyzed mechanisms of cross- and self-aldolization reactions. In situ catalyst deactivation was determined by performing two consecutive catalytic runs. The activity decay of MgyAlOx samples during the citral/acetone reaction was between 15 and 20%, irrespective of the catalyst composition. (C) 2008 Elsevier B.V. All rights reserved.