Catalysis of Organic..

28Hydrogenation of a Schiff’s BaseThe formation rate of product C is dependent on the concentration of adsorbedSchiff’s base and the molecular hydrogen dissolved in the liquid phase. The rateconstant was determined in the temperature range of 10 to 45 °C. From the resultsan apparent activation energy ( Eap) of 40.2 kJ/mole and a pre-exponential factor Aof 2.64 x 10 5 mol min -1 g·cat -1 were calculated. The proposed kinetic modelsuggests that a lower hydrogen concentration in the solution may slow down thedesired transformation. The Schiff’s base is the most strongly bound component onthe palladium and has a high turnover frequency. In the case of gas-liquid masstransfer limitation (7), the reaction takes longer or may not even proceed tocompletion. The reduced Schiff’s base C has the potential to undergo a secondreductive alkylation with starting material A to form a dimer and subsequentlyproduce other hydrogenated dimer impurities.Maintaining the hydrogenation under kinetic control provides limited alcoholformation and avoids over reduction of product C. The performance of ahydrogenator depends on the gas-liquid mass transfer characteristics Kla (8).Possible operating scenarios with their observed impurity profiles are summarized inTable 5.Table 5. Impurity formation related to process conditionsMass TransferCharacteristics High catalyst loading Low catalyst loading .high Kla a. form over-reduction impurity E a. less impurities E & Db. decrease process yield b. less palladium residuelow Kla a. form side product D a. reaction will not go tob. over reduction impurity E completionincreased with time and b. C reacts with A to formcatalyst loadingdimers (not shown inthe reaction scheme)..All experiments were carried out in a 1 L Buchi hydrogenator.The mass transfer coefficient Kla range was 0.01 – 0.9 L/sec.ConclusionUsing the quasi-equilibrium and two-step reaction concepts in the catalytic cycle, thehydrogenation kinetics of Schiff’s base B were investigated. The analysis showedthat strong Schiff’s base adsorption provided rapid reduction and led to limitedbyproduct and impurity formation. The proposed mechanism suggested that lowercatalyst loading or hydrogen diffusion limitations would slow down the desiredtransformation and lead to enhanced impurity formation. This knowledge led to thedesign of a more robust process and a successful scale up.