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" ethane as a solvent. Application of ethane as a solvent offers good enantioselectivity and very fast conversion of ethyl pyruvate to ethyl lactate (average turnover frequency of 15 s(-1) at ambient temperature). The phase behavior in the temperature range 15-50 degreesC and at pressures up to 140 bar has been investigated in a computer-controlled view cell, equipped with on-line video imaging and recording. The changes in reaction rate and enantioselectivity with pressure (density), temperature, and hydrogen concentration are 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 system is an ideal guide for rationalizing the phase behavior-related phenomena typical of multicomponent high-pressure reaction systems. (C) 2001 Academic Press.