Chiral fixed bed reactor for stereoselective heterogeneous catalysis: modification, regeneration, and multiple product syntheses

A chiral fixed bed reactor (CFBR) was used for continuous enantioselective hydrogenations of ethyl pyruvate (EP) and ethyl benzoylformate (EB) on cinchonidine (Cind) and cinchonine (Cin) modified Pt/Al2O3. The reactions were carried out with ethanol (EtOH) as solvent at 0 and 20 °C, under a typical feed hydrogen mole fraction of xH2≈0.007 and total pressures of 60–200 bar. Due to the configuration used, the feeds of reactants as well as total pressure could be varied independently. In addition, the analytical method used was different from the commonly used method. HPLC with tandem UV–vis/circular dichroism were used to assess conversion and selectivity. The primary findings concerning the chemistry and reaction engineering include the following: (i) the method of contacting ethyl pyruvate is important (EP was cold stored and contacted with ethanol immediately prior to reaction to minimize hemiketal formation); (ii) 0 °C provides a more stable hydrogenation than 20 °C; (iii) e.e. is independent of system pressure; (iv) effective demodification and subsequent remodification in the CFBR is possible, thereby allowing regeneration of the catalytic system where activity and enantioselectivity are maintained; and (v) multiple product syntheses can be achieved using the same Pt/Al2O3 catalyst with multiple substrates and multiple modifiers. These results demonstrate the technological feasibility of using one-and-the-same supported-metal packed bed for many diverse syntheses of chiral products.