Continuous flow kinetic resolution of a non-equimolar mixture of diastereoisomeric alcohol using a structured monolithic enzymatic microreactor

Abstract

Kinetic resolution of a non-equimolar mixture of optical isomers may be a problem, and no study on the very fast, high-yield continuous-flow resolution process has been reported. In this paper, we demonstrate the exceptional performance of the microstructured monolithic reactor in the continuous kinetic resolution of the non-equimolar mixture (S : R = 85 : 15) of the secondary allylic alcohol (+)-1-[(1S,5R)-6,6-dimethylbicyclo[3.1.0]hex-2-en-2-yl]ethanol using Pseudomonas cepacia lipase (PCL) as the biocatalyst. The siliceous rods (6 × 40 mm), which were synthesized by the sol–gel method combined with phase separation and were applied as microreactors, featured a very open 3D tortuous pore structure with flow-through channels 20–30 μm dia. and mesopores of 20 nm (4 cm³ g⁻¹ total pore volume, surface area 285 m² g⁻¹). After hydrophobization with hexadecyl groups, a 7.25 mg enzyme was attached by adsorption to a single monolith (0.25 g). The continuous-flow experiments showed that high substrate conversions can be obtained even for residence times (τ) that are less than 2 min, to give remarkable space-time yield values of approximately 100 g h⁻¹ L⁻¹. For the flow rate of 0.45 mL min⁻¹ (τ = 2.14 min), the targeted substrate was fully transformed (de > 99%) to the corresponding ester. Stability of the system was confirmed during the 10 day continuous operation. A comparative study with a batch slurry system, using the same lipase immobilized on the silica gel powder, proved the vast superiority of the developed system.