Kinetics of catalyzed acid/acid and acid/aldehyde condensation reactions to non-symmetric ketones

The kinetics and mechanism of acid and aldehyde condensations to produce non-symmetric ketones with CeO2-based catalysts were studied using a combination of conventional and pulse microreactor tests. The effects of oxygen and water on the reactions were also studied. Supported CeO2 catalysts effectively catalyze the ketonization of acids at essentially complete conversion for extended periods, at weight hourly space velocities of 4-5. The optimal temperature range is 400-430 degreesC, depending on feed. Time on stream and number of regeneration cycles improved catalyst performance. Selectivities are improved by promotion with small amounts of potassium. The acid/acid reaction to a typical methylketone proceeds roughly three times faster than the acid/aldehyde reaction, while the aldehyde/aldehyde initial reaction rate to desired methylketones is much slower; multiple aldol condensations predominate. When using acid/aldehyde feeds, water enhances ketone production, probably by supplying oxygen to the catalyst surface. While O-2 can fulfill a similar role, it also promotes combustion. Substitution of D2O and CD3COOH for water and acetic acid, respectively, led to kinetic isotope effects between 1.4 and 6.7, which is in the expected range for carboxylate decompositions. Experiments at low conversion using CD3COOH and either cyclopropanecarboxylic acid or its aldehyde showed that acetone and methylcyclopropylketone are formed preferentially as five D- and two D-atom isotopomers, respectively, for both acid/acid and acid/aldehyde feeds. This suggests the formation of a surface ketene intermediate, preferentially from acetic acid, which attacks a surface carboxylate to form the ketone, eliminating CO2. The same conclusions could be drawn from C-13 distributions in experiments using labeled acetic acid. (C) 2003 Elsevier B.V. All rights reserved.