Activity and stability of mesoporous CeO2 and ZrO2 catalysts for the self-condensation of cyclopentanone

Metal oxides containing acid-base pairs are widely used as catalysts for aldol condensation reactions, but they are often prone to secondary condensation and fast deactivation. In this contribution, high-surface area mesoporous CeO2 and ZrO2 have been synthesized and tested as catalysts for the self-condensation of cyclopentanone, an attractive molecular building block for several biomass upgrading strategies. The catalytic activity, stability, and overall kinetics behavior of these two catalysts have been compared in the liquid phase. A detailed kinetics analysis shows that on both oxides, the reaction is controlled by the initial unimolecular alpha-H abstraction step, in which cyclopentanone forms the crucial enolate intermediate. Subsequently, the enolate couples with another cyclopentanone molecule in a bimolecular C-C coupling step. CO2-TPD analysis and site titration with probe molecules indicate that initially the most active catalyst is the mesoporous ZrO2 sample with strong base character. However, this catalyst exhibits a very rapid deactivation. By contrast, the mesoporous CeO2 prepared in this study has lower basicity strength, which results in lower initial activity, but enhanced stability.