Effect of immobilization site and membrane materials on multiphasic enantiocatalytic enzyme membrane reactors

In the experimental work reported here the optical resolution of racemic naproxen methyl ester with crude lipase immobilized in a membrane reactor was studied. The multiphasic enantiocatalytic enzyme membrane reactor consisted of an organic phase that dissolved the naproxen methyl ester, a lipase-loaded membrane, and an aqueous phase that extracted the reaction product. Lipase preferentially converted the (S)-naproxen methyl ester to (S)-naproxen acid that was simultaneously separated by a membrane. The effect of the immobilization site and membrane type on the performance of the enzyme membrane reactor was studied. Capillary polyamide membrane with 10 kDa nominal molecular weight cutoff (NMWCO) and polysulfone membrane with 30 kDa NMWCO were applied, with lipase loaded in the sponge layer or on a membrane thin layer. With various immobilization sites and membrane types, the enzyme membrane reactors showed different productivity and enantioselectivity resulting from the varying amounts of immobilized enzyme and varying microenvironment of the hydrolysis reaction. Higher amounts of immobilized enzyme led to increased productivity and generally higher enantioslectivity of the membrane reactor. It seems that the location of organic/aqueous interface on membrane, which plays an important role in a multiphasic enzyme membrane reactor, was influenced by the immobilization site and membrane type and this affected the productivity and enantioselectivity. As much as 90% enantioexcess was obtained with a polyamide membrane, and lower values with a polysulfone membrane. A sponge layer of polyamide membrane is the preferred immobilization site for its higher productivity than a thin membrane layer, and for its higher enantioselectivity than a polysulfone membrane. In comparison with the unstable hydrolysis activity of free lipase in stirred tank reactor, a stable lipase activity can be obtained with the lipase-immobilized membrane reactor, irrespective of whether a polyamide or polysulfone membrane is used.