Enantioselective hydrogenation of ethyl pyruvate -: Long-term performance of chirally modified Pt/zeolite catalysts

The long-term performance of 5 wt.% Pt/zeolite catalysts (HNaY with different degrees of ion exchange) modified with (-)cinchonidine was studied. The enantioselective hydrogenation of ethyl pyruvate to R(+)ethyl lactate was used as the model reaction. The Pt/zeolites are effective catalysts for long-term use, if the chiral modifier is added before each cycle of hydrogenation. The optimal ratio between the amount of catalyst and chiral modifier essentially depends on both the specific Pt surface area of the catalyst and the solvent used. It is found that the use of acetic acid as solvent results in a higher long-term performance than cyclohexane. This is due to the competitive adsorption between the chiral auxiliary and acetic acid on the Pt surface. Furthermore, the acidity of the solvent results in a weakening of the (-)cinchonidine/Pt interaction, which prevents an overloading of the Pt surface area with (-)cinchonidine or its decay products. This effect was determined by means of differential thermal analysis and elemental analysis. When cyclohexane is used as the solvent, the long-term performance is significantly improved by raising the carrier acidity. (C) 2000 Elsevier Science B.V. AU rights reserved.