Continuous enantioselective hydrogenation of ethyl pyruvate in "supercritical" ethane: Relation between phase behavior and catalytic performance

Continuous hydrogenation of an alpha -ketoester has been studied in a fixed -bed reactor over cinchonidine-modified Pt/Al2O3 using "supercritical" etha ne as a solvent. Application of ethane as a solvent offers good enantiosele ctivity and very fast conversion of ethyl pyruvate to ethyl lactate (averag e turnover frequency of 15 s(-1) at ambient temperature). The phase behavio r in the temperature range 15-50 degreesC and at pressures up to 140 bar ha s been investigated in a computer-controlled view cell, equipped with on-li ne video imaging and recording. The changes in reaction rate and enantiosel ectivity with pressure (density), temperature, and hydrogen concentration a re interpreted by considering the number and nature of phases present under the conditions applied. The results illustrate the obvious limitations of describing multicomponent phase behavior with that of the pure solvent. We demonstrate that consideration of the phase behavior of a binary fluid syst em is an ideal guide for rationalizing the phase behavior-related phenomena typical of multicomponent high-pressure reaction systems. (C) 2001 Academi c Press.