Continuous application of polyglycerol-supported salen in a membrane reactor: Asymmetric epoxidation of 6-cyano-2,2-dimethylchromene

In the current work, suitability of hyper-branched polyglycerol as a high loading catalyst support is demonstrated. A polyglycerol-supported manganese-salen complex (chemzyme) is applied as a homogeneous catalyst in the epoxidation of 6-cyano2,2-dimethylchromene. The recyclability of the corresponding catalyst was investigated in repetitive batch experiments as well as a continuous operation of the reaction in an ultrafiltration membrane reactor. An enhanced stability of the catalyst in repetitive batches was observed as a result of immobilization whereby the total turnover number increased from 23 in a single batch to 80 in four repetitive batches. To enable continuous operation, a continuously operated, stirred tanked reactor (CSTR) was equipped with an ultrafiltration membrane (MPF-50) and a retention of 98% was determined. The continuous chemzyme membrane reactor was operated over the course of 20 residence times. After approximately 12 residence times, the steady state was reached yielding 70% conversion as well as an enantiomeric excess up to 92%. A space-time yield (sty) of 458 gL(-1)d(-1) and a turnover frequency (TOFreaction) of up to 18 h(-1) was reached in the steady state. It was determined that the total turnover number (TTN) was enhanced by a factor of 10 from 24 (batch) up to 240 for 20 residence times in CSTR operation.