THE ROLE OF CATALYSTS IN THE AIR OXIDATION OF ALIPHATIC-ALDEHYDES

Under preparative conditions, the liquid-phase air oxidation of 2-methylpropanal gives product profiles that differ with the catalyst. These distinctive profiles allow us to use this reaction to characterize the roles of some catalysts. The conversion of 2-methylpropanal to 2-methylpropanoic acid takes place in two stages. In stage 1 aldehyde is converted to peracid in a free radical chain reaction. In stage 2 peracid is converted to carboxylic acid. Paradoxically, noncatalyzed reactions give higher yields of 2-methylpropanoic acid than do reactions catalyzed by Mn or Co ions. The ions improve the efficiency of stage 2, but they also increase the rate of chain initiation in stage 1. Since these reactions are oxygen mass transfer limited, small increases in the reaction rate lead to oxygen starvation in the bulk liquid phase. This leads to increased formation of byproduct propane, propene, acetone, 2-propanol, CO, and CO2. Increases in CO formation are due to decomposition of acyl radicals and are diagnostic of severe oxygen starvation. Obviously, increases in the reaction rate must also increase the amount of chain termination which is the sole source of CO2. It is shown that Cu ion can inhibit the chain reaction and thus alleviate both conditions. © 1990, American Chemical Society. All rights reserved.