Kinetic Resolution on scCO2 - Design of Continuous Reactor Based ...

<strong>Kinetic</strong> <strong>Resolution</strong> on scCO2 - Design of Continuous Reactor
Based on Results of Batch Experiments
Edit Székely *1 , Margita Utczás 1 , Béla Simándi 1
1 Budapest University of Technology and Economics, Department of Chemical and Environmental Process
Engineering, Budapest, Hungary 1521
Corresponding author: sz-edit@mail.bme.hu; Phone: (+36)-1-463-3191; Fax: (+36)-1-463-3197
EXTENDED ABSTRACT
The production of single enantiomers has a still increasing importance in the pharmaceutical, food and
health industries. Asymmetric chemical synthesis and conventional resolution methods are widely applied for
preparing enantiopure compounds but biocatalysis also provides an alternative opportunity to prepare useful
chiral compounds [1]. Biocatalysis often offers advantages over chemical synthesis as enzyme-catalyzed
reactions are often highly enantioselective and regioselective. The well-known advantages of supercritical
carbon dioxide (scCO2) like gas-like diffusivity and low viscosity offer reduced mass transfer resistance between
the reaction mixture and the active sites of the enzyme, resulting in increased reaction rates. Additionally, scCO2
is non-toxic, non-flammable, low cost and the most important in technological respect is easy separation from
reaction mixture [2]. Enzymes, especially lipases are particularly suited to perform stereoselective reactions in
scCO2. The advantages of using enzymes in supercritical fluids have been investigated mostly in batch reactors
[3-5].
The enantiomers of trans-1,2-cyclohexandiol are used mainly as chiral auxiliaries, chiral building blocks or
chiral intermediates for the synthesis of pharmaceuticals, agrochemicals, or crown ethers. Numerous methods for
the chiral resolution of trans-1,2-cyclohexandiol were reported in literature. These include ester formation with
O-acetyl mandelic acid [6], selective dispiroketal formation [7] or selective molecular complex formation with L-
tartaric acid followed by supercritical fluid extraction [8]. <strong>Kinetic</strong> resolution of trans-1,2-cyclohexandiol was
carried out via asymmetric benzoylation [9], monophenyl ester formation [10], or enzymatically via selective
ester cleavage of diacetoxycyclohexane using the Bacillus subtilis recombinant lipase [11] or lipase AK [12].
We developed an efficient method for the lipase-catalyzed consecutive kinetic resolution of trans-1,2-
cyclohexandiol in scCO2 with vinyl-acetate acetyl donor catalysed by a commercial immobilized Candida
Antarctica lipase B (CALB). The reaction was optimized in scCO2 in batch reactor. The first acylation step is
moderately enantioselective, the formation of (1R,2R)-2-acetoxycyclohexan-1-ol is preferred, while the second
acylation step is fully enantioselective. Michaelis-Menten type reaction constants and turnover number values
were calculated from the yield over time functions. Based on the parameter requirements to achieve maximum
reaction rate with complete conversion and enantiopure products a continuous flow reactor was designed with a
few seconds of residence time (the time requirement of a batch reaction was several hours) assuming that turn
over number values in the batch reactor is equivalent to the turnover frequencies in a continuous flow reactor.
A combined extractor – enzymatic packed-bed reactor unit was developed. The cooled liquid CO2 was
pumped trough a preheater coil, where it becomes supercritical fluid. The scCO2 dissolved the solid substrate
from the extractor column and the solution was mixed with VA with a static mixer before entering into the
reactor column filled with the immobilised CALB. In a typical experiment, 230 mg CALB was filled into the

enzymatic reactor. The reactions were performed at 10 MPa and 45 °C. The amount of VA was in the range of
10 - 30 molar ratio to the substrate in each experiments. The residence time in the enzymatic reactor was varied
from 2 to 13 s by changing the flow rate of the CO2. The implemented continuous reactor was optimized to
achieve maximum productivity and enantiopure products. The optimum residence time in the fixed reactor
volume confirmed entirely the calculated operational parameters (calculated necessary residence time: 3.3 s,
measured optimal residence time: 4.3 s). No loss of enzyme activity was observed within 28 hours at continuous
operation.
ACKNOWLEDGMENTS
The research work was supported financially by Hungarian Scientific Research Fund (OTKA 72861) and by
TAMOP-4.2.1/B09/1/KMR-2010-0002 project.
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