The aldol condensation of lower aldehydes over MFI zeolites with different acidic properties

Various MFI type zeolites were synthesized taking into account two factors affecting zeolitic acidity "by synthesis", namely the nature of the substituted heteroelement and the Si/Me-3- ratio. The surface properties with respect to chemical composition and acidity were investigated by temperature-programmed reduction, X-ray photoelectron spectroscopy, temperature-programmed desorption of ammonia, Fourier transform infrared spectroscopy and microcalorimetry. Vapor phase aldol condensation of acetaldehyde and formaldehyde over these catalysts has been carried out, and it has been observed that the structural features of the catalysts do play an important role in controlling conversion and selectivity. A reasonable acidity and activity relationship is established. A mechanism for cross-aldol condensation involving both Bronsted and Lewis acid sires is proposed. (C) 2001 Elsevier Science B,V. All rights reserved.