THE LIQUID-PHASE HYDROGENATION OF METHYLACETOACETATE USING NICKEL-EXCHANGED Y-ZEOLITE CATALYSTS .2. A KINETIC-STUDY

Kinetic data are reported for the liquid-phase hydrogenation of methylacetoacetate (MAA) over NiKY to a racemic methyl 3-hydroxybutyrate (MHB) product. The contribution of the reactant diffusion to the overall rate is considered from a measurement of the Thiele modulus and a study of the effects of altering a number of process variables, such as stirring speed, catalyst weight, particle size, and reaction temperature. A narrow range of experimental conditions is identified wherein the reaction operated under surface kinetic control with negligible diffusion constraints. Reaction orders with respect to MAA and hydrogen concentrations and an apparent activation energy were determined in the temperature interval 313-383 K. The apparent rate was observed to pass through a maximum as the concentration of MAA was increased. The rate data are fitted to a Langmuir-Hinshelwood model where both reactants adsorb competitively on the surface; the experimentally determined rates agree to +/-10% with the rates predicted by the kinetic model. The effect of the reaction solvent on the reaction rates is discussed and a linear relationship between apparent rate and dielectric constant, in the case of alcoholic solvents, is presented. Thiophene, acetic acid, sodium bromide, and water are identified as rate inhibitors, whereas the hydrogenated product (MHB) was observed to suppress the progress of the reaction at high conversion levels.