Continuous hydrogenation of 1-phenyl-1,2-propanedione under transient and steady-state conditions: Regioselectivity, enantioselectivity and catalyst deactivation

Continuous enantioselective hydrogenation of 1-phenyl-1,2-propanedione was investigated at 25 °C and 5 bar H₂ over a knitted Pt silica fiber catalyst modified with cinchonidine. The main reaction parameters were the reactant and modifier concentrations, the liquid space time and the pre-treatment of the catalyst. The initial hydrogen uptake rate decreased about 25-45% within 20 min time-on-stream. The catalyst deactivation was quantitatively described with a mechanistic model, which assumed that the deactivation is of first-order in reactant concentration. The amount of modifier did not affect the reaction rate. The steady-state regioselectivity of 1-hydroxy-1-phenylpropanone and enantioselectivity of the main product, (R)-1-hydroxy-1-phenylpropanone, were independent of reactant inlet concentration and space time. The enantioselectivity of the main product, (R)-1-hydroxy-1-phenylpropanone, increased from zero to the highest enantiomeric excess (ee = 60%) with increasing time-on-stream, after which it attained a steady-state value, dependent on the modifier concentration. Continuous feeding of the modifier was necessary to maintain a constant ee.