Continuous application of chemzymes in a membrane reactor: asymmetric transfer hydrogenation of acetophenone

The application of homogeneously soluble catalysts is limited by the recovery in cases where the price of the catalyst is high. Biological catalysts, enzymes, can be efficiently recycled by means of an ultrafiltration membrane due to their high molecular weight, for example, in the continuously operated membrane reactor. In order to transfer this principle to chemical catalysis, we have attached a transfer hydrogenation catalyst, first invented by Gao and Noyori, to a polymer. The resulting homogeneously soluble, polymer-bound catalyst (chemzyme) can now be retained by ultrafiltration membranes like enzymes. On applying this catalyst in continuously operated membrane reactors, a continuous isopropoxide dosage is necessary in order to compensate deactivation caused by water residues in the reed stream. Thus, high space-time yields up to 578 g L-1 d(-1) and enantioselectivities up to 94% can be achieved. These results were compared to an enzyme catalyzed system consisting or a carbonyl reductase that also utilizes 2-propanol as a hydrogen source for the cofactor regeneration of NADH

Membrane aerated hydrogenation: enzymatic and chemical homoeneous catalysis

Among the most successful systems for homogeneous catalysis, hydrogenation catalysts capable of activating molecular hydrogen, take outstanding roles in research laboratories and in industry. To open up the field of continuous catalytic hydrogenations a novel membrane reactor concept was developed and successfully applied for hydrogenations with dihydrogen both for chemical and for enzymatic catalysis. The hydrogenase I of the archaeon Pyrococcus furiosus was utilized for the continuous hydrogenation of NADP+ to NADPH with recycling of the enzyme by means of ultrafiltration. The well known PyrPhos-Rh system was used for the enantioselective synthesis of an amino acid derivative by hydrogenation