(281d) Aldol Condensation of Acetaldehyde over Titania, Hydroxyapatite and Magnesia

Aldol condensation is a well-known reaction that results in the formation of a C-C bond and is used in the production of important chemicals such as 2-ethylhexanal, isophorone, and crotonaldehyde among others. This work aims to elucidate the mechanism of aldol condensation of acetaldehyde over three materials: anatase titania (TiO₂), hydroxyapatite (HAP), and magnesia (MgO). Reactions were carried out in a fixed bed reactor with a total system pressure of 220 kPa at temperatures between 533 and 633 K and acetaldehyde partial pressures between 0.05 and 50 kPa. Crotonaldehyde was the only product observed over all three catalysts and severe catalyst deactivation occurred at acetaldehyde partial pressures of 5 kPa or greater. The aldol condensation reaction over all three catalysts was first order at low acetaldehyde partial pressure and approached zero order at high acetaldehyde partial pressure. No kinetic isotope effect (KIE) was observed with fully deuterated acetaldehyde reacting over TiO₂ orHAP implying C-H bond activation is not kinetically relevant. These measurements are consistent with a mechanism in which adsorption and desorption steps are kinetically significant during the reaction. Characterization of the catalysts by adsorption microcalorimetry of acetaldehyde and ethanol and diffuse reflectance Fourier transformed infrared spectroscopy of adsorbed acetaldehyde, crotonaldehyde and acetic acid revealed a very high reactivity of these catalysts, even at low temperatures.