Proof of concept for continuous enantioselective liquid-liquid extraction in capillary microreactors using 1-octanol as a sustainable solvent

Article abstract of DOI:10.1039/c7gc01700f

The use of capillary microreactors for enantioselective liquid-liquid extraction (ELLE) was successfully demonstrated using a model system consisting of a buffered aqueous amino acid derivative (3,5-dinitrobenzoyl-(R,S)-leucine) solution (phosphate buffer, pH 6.58) and a chiral cinchona alkaloid (CA) host in an organic solvent. It was shown that 1-octanol is a suitable replacement for the commonly used chlorinated solvents like 1,2-dichloroethane. Experiments were conducted in a capillary microreactor set-up (0.8 mm internal diameter) operated in the slug flow regime at 294 K (residence times between 12 and 900 s, 1:1 flow ratio of the aqueous to organic phases, 1 mM of host and 1 mM of amino acid derivative). The enantiomeric excess (ee) was shown to be a function of the solvent and residence time and varied between 37% and 49% in 1,2-DCE and 28 and 46% in 1-octanol in the organic phase. The ee values in the organic phase at shorter residence times were higher than the independently determined equilibrium ee values (41% in 1,2-DCE and 31% in 1-octanol at a host concentration of 1 mM). This is an unprecedented observation with large implications for ELLE, as it implies that operation in the kinetic regime may lead to improved enantioseparation performance.

Full text of DOI:10.1039/c7gc01700f

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Page 1 of 26
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DOI: 10.1039/C7GC01700F
Proof of concept for continuous enantioselective liquid-liquid extraction
in capillary microreactors using 1-octanol as a sustainable solvent
a
a
b
c
b,d
Susanti, Tim G. Meinds, Erik B. Pinxterhuis, Boelo Schuur, Johannes G. de Vries,
b
a
a
a*
Ben L. Feringa, Jozef G. M. Winkelman, Jun Yue, Hero J. Heeres
a
Department of Chemical Engineering, ENTEG, University of Groningen, 9747 AG Groningen, The
Netherlands
b
Stratingh Institute for Chemistry, University of Groningen, 9747 AG Groningen, The
Netherlands
c
University of Twente, Sustainable Process Technology, 7500 AE Enschede, The
Netherlands
d
Leibniz Institut für Katalyse e.V. an der Universität Rostock, Catalytic Conversion of
Renewable Resources, 18059 Rostock, Germany
1

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Abstract
The use of capillary microreactors for enantioselective liquid-liquid extraction (ELLE) was
successfully demonstrated using a model system consisting of a buffered aqueous amino
acid derivative (3,5-dinitrobenzoyl-(R,S)-leucine) solution (phosphate buffer, pH 6.58) and a
chiral cinchona alkaloid (CA) host in an organic solvent. It was shown that 1-octanol is a
suitable replacement for the commonly used chlorinated solvents like 1,2-dichloroethane.
Experiments were conducted in a capillary microreactor set-up (0.8 mm internal diameter)
operated in the slug flow regime at 294 K (residence times between 12 and 900 s, 1:1 flow
ratio of the aqueous to organic phases, 1 mM of host and 1 mM of amino acid derivative). The
enantiomeric excess (ee) was shown to be a function of the solvent and residence time and
varied between 37%-49% in 1,2-DCE and 28-46% in 1-octanol in the organic phase. The ee
values in the organic phase at shorter residence times were higher than the independently
determined equilibrium ee values (41% in 1,2-DCE and 31% in 1-octanol at a host
concentration of 1 mM ). This is an unprecedented observation with large implications for
ELLE, as it implies that operation in the kinetic regime may lead to improved
enantioseparation performance.
2

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Introduction
The availability of enantiopure compounds is of great importance for the pharmaceutical,1,2
3
4
agrochemical, flavor and fragrances industries, as the individual enantiomers may show
significant differences in biological activity. Synthesis of racemic compounds followed by a
chiral separation is a well-established strategy to obtain enantiopure compounds.5,6
Classical resolution for instance by crystallization is the most used technique for racemate
separation.7-10 However, this approach has a number of limitations, such as excessive solids
handling.7,11 Alternative methods have been developed such as kinetic resolution,12-14 and
physical separations. Examples of the latter are membrane separations,15-
1
8
5
7
chromatographic separation, capillary electrophoresis, and enantioselective liquid-
liquid extraction (ELLE).7,19-40
ELLE involves contacting two immiscible liquid phases, one with the racemic mixture to be
separated (usually in the aqueous phase) and an organic phase containing a soluble chiral
host with a higher affinity for one of the enantiomers. After extraction, the enantio-enriched
organic phase is back extracted to recover and recycle the host. The principle of ELLE is
schematically shown in Figure 1.
Figure 1. Schematic representation of ELLE.
3

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Advantages of ELLE include ease of scale up due to the fact that liquid-liquid extraction is a
mature technology and the possibility to use one host family for the separation of multiple
racemates.7,41-42 Crystallization and filtration, which are the two major unit operations in
classical resolutions are also the two most problematic ones, causing a lot of problems in
the plant as a result of too small crystals, blocked filters and long filtration times. These
problems are largely avoided with ELLE. We have recently shown the proof of concept for
ELLE in a continuous mode using Centrifugal Contactor Separators (CCS), which are highly
process intensified devices that combine mixing and phase separation. By using a
countercurrent cascade of multiple CCS devices, good enantioselectivity and yield was
obtained for the separation of an aqueous solution of 3,5-dinitrobenzoyl-(R,S)-leucine
(DNB-(R,S)-Leu in Figure 2) with a cinchona alkaloid host (CA4 in Figure2) in 1,2-
dichloroethane (1,2-DCE).11,29,30 However, due to the relatively low selectivity, many stages
are required to obtain both enantiomers in high-enantiopurity,41,43 which has a negative
effect on capital costs. In addition, there is a clear incentive to minimize the host inventory
to reduce costs. Alternatives for CCS devices are intensified columns, as explored by
Kockmann and co-worker,22,23 and showing that a large number of stages is achievable per
meter column.
Another possible alternative to intensify enantioseparation by liquid-liquid extraction is the
use of microreactors.44 The use of microreactors for liquid-liquid extractions has been
reported,45-55 examples are chip-based microreactors, high capacity mini extractors, and
5
3
capillary or tubular microreactors (including coil-based flow reactors). The use of such
capillary microreactors offers good control over temperature and residence time, has
shown to lead to enhanced mass transfer rates, and is relatively easy to scale-up.51,54
Moreover, the use of microreactors may “green-up” ELLE e.g., by being more energy
efficient (as compared to CCS) and reduced solvent use due to low reactor volume. This
applies not only to the production phase, but also for the screening experiments in the lab.
Although capillary microreactors are of great interest for ELLE, its use has never been
reported in the literature and is an absolute novelty of this paper.
We here present an experimental study on enantioseparation by ELLE in capillary
microreactors in the slug flow regime. This flow regime is expected to be advantageous as it
is known to enhance mass transfer rates due to intensified circulation inside slugs and
4

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droplets.51,56 The experiments were performed in an integrated setup which combines
5
6
liquid-liquid extraction and separation. A model system consisting of the separation of
racemic 3,5-dinitrobenzoyl-(R,S)-leucine (DNB-(R,S)-Leu) using a chiral cinchona alkaloid
1
1
host was used. This reaction was selected as we have ample experience with it in CCS
devices11,57 and relevant thermodynamic data like complexation constants are available for
the reference solvent (1,2-DCE).
Several derivatives of cinchona alkaloid (CA) chiral hosts have been successfully applied for
the enantioseparation of DNB-(R,S)-Leu (Figure 2).44
Figure 2. DNB-(R,S)-Leu and examples of cinchona-based chiral hosts for ELLE reported in
the literature.44
In this study, we have used host CA3, the non-oxidized variant of the well-known extractant
CA4. It is the precursor for CA4 and as such one synthetic step in the host synthesis may be
eliminated. Two solvents were explored, viz. 1,2-DCE and 1-octanol. The former has been
used extensively for ELLE, though its use has strong drawbacks when considering
environmental performance. A such, the use of the environmentally more benign 1-octanol
has also been investigated and its performance will be compared with 1,2-DCE. Higher
alcohols are considered as green recommended solvents by a number of solvent selection
guides.58,59 In addition, 1-octanol is accessible from renewable resources ref.60 Besides
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solvent effects, the influence of process variables and particularly the residence time on the
separation performance was studied by varying the total flow rate and/or length of the
capillary microreactors.
Material and Methods
Materials
The host CA3 was synthesized according to literature two-step procedure (Figure S1,
6
1
Supplementary information). NMR and elemental analysis data are provided in Figure S2,
S3 and Table S1, Supplementary information. The amino acid derivative, 3,5-
dinitrobenzoyl-(R,S)-leucine (DNB-(R,S)-Leu), was obtained from DSM. Organic diluents,
viz. 1-octanol (99.8%) and 1,2-dichloroethane (1,2-DCE) (99.8%), were obtained from
Sigma-Aldrich. Disodium hydrogen phosphate (≥ 99.5%) and potassium dihydrogen
phosphate (≥ 99.5%), were obtained from Merck. All experiments were performed with
Milli-Q water.
Experimental setup
Two set ups were used for the experimental study reported i.e. a batch set up and a
continuous capillary microreactor. Experiments in the batch set up involved mixing an
aqueous phase with the racemate and an organic phase with the host in a vial using a Teflon
bar (1000 RPM) for a perdetermined amount of time. All continuous experiments were
carried out in a capillary microreactor set up as schematically shown in Figure 3. The
aqueous and organic phases were transferred to the capillary microreactor made of
poly(tetrafluoroethylene) (PTFE) tubings (Bola, Germany, 1.6 mm outer diameter and 0.8
mm inner diameter) via a Y-junction (120˚ angle between branches, 1 mm inner diameter,
made of polyether ether ketone (PEEK) for the use of 1,2-DCE and polymethylmethacrylate
(PMMA) for 1-octanol) using syringe pumps (model No. LA30, HLL Gmbh, Germany). The
end of the capillary microreactor was connected to a Y-splitter for the separation of both
liquid phases. The outlets of the Y-splitter consist of a PTFE tube and a glass tube of the
same dimension (i.e., 1.6 mm outer diameter and 0.8 mm inner diameter). The two tubes
were positioned in a prefabricated Y-shaped splitter made from PEEK or PMMA. The
6

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separation in the splitter is based on the preferential wettability (aqueous phase: glass,
organic phase: PTFE). The composition of the aqueous phase was analyzed (vide infra).
Figure 3. Schematic representation of the experimental setup
.
Experimental procedure
ELLE experiments in the capillary microreactor
The experiments in the slug flow capillary microreactor were carried out using 1 mM DNB-
R,S)-Leu in an aqueous phosphate buffer (pH=6.58) as the feed solution and 1 mM CA3 in
,2-DCE or 1-octanol as the extractant at room temperature. An overview of experimental
(
1
conditions is given in Table 1.
Table 1. Experimental conditions for the enantioselective extraction DNB-(R,S)-Leu in a
capillary microreactor.
Value
294
Ranges
Temperature (K)
Buffer concentration (M)
Buffer, pH
0.1
6.58
DNB-(R,S)-Leu concentration [mM]
Host, CA3 concentration [mM]
Capillary inner diameter (mm)
Capillary length (cm)
1.01 ± 0.01
1.03 ± 0.04
0.8
10-250
-
1
Q
Q
aq, Qorg [mL h ]
2.5-15.0
aq/
Q
org
1.0
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For all experiments, a 1 to 1 flow ratio of the aqueous to organic phases was applied. The
residence time (τ) is defined on the total flow rate and given in eq. 1.
π
4
2
d L
c
c
Vc
(1)
τ
=
=
Q + Q
Q + Q
aq
org
aq org
where V
c
is the geometrical volume of the microreactor, d
c
the inner diameter and L the
c
length of the capillary microreactor and Qaq and Qorg are the volumetric flow rates of the
aqueous and organic phases, respectively. In the set-up, residence times between 12 and
900 s are attainable by combining different microreactor lengths and the total flow rates.
All experiments were run for at least 2 residence times to ensure that the steady state was
obtained. During an experiment, the aqueous phase was collected and analyzed by HPLC. All
experiments were performed at least in duplicate. A good reproducibility of the
experiments was observed, with a relative standard deviation (RSD) of less than 6%.
ELLE experiments with DNB-(R,S)-Leu and CA3 in a batch set-up
To determine equilibrium concentrations and enantioselectivities, the extraction of DNB-
(R,S)-Leu with CA3 was performed in a batch set-up. Experiments were performed in 20 mL
vials, which were loaded with 5 mL of a 1 mM solution of CA3 in 1,2-DCE or 1-octanol and 5
mL of a buffered aqueous phase (pH 6.58) with a racemate concentration of 1 mM. The
solution was stirred using a Teflon bar (1000 RPM) for at least 14 h. After extraction, the
phases were allowed to settle and separated. The pH of the aqueous phase was measured
and its composition was analyzed by HPLC. The concentration of analyte in the organic
phase was calculated using an overall mass balance.
Analytical procedures
The concentration of DNB-(R,S)-Leu in the aqueous phase was analyzed by reversed phase
HPLC (a Shimadzu SIL-20A, with a CTO-20AC column oven and LC-20AD pumps) using a
chiral Astec Chirobiotic T column and a UV detector (270 nm). The eluent was a 3:1 (v/v)
mixture of acetonitrile and methanol with 0.25 vol% triethylamine and 0.25 vol% acetic
8

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acid. The flow rate was set at 1 mL/min and the injection volume was 15 μL. Before
injecting the aqueous phase samples to the column, the samples were filtered using a
syringe filter with a pore size of 0.45 μm (Sartorius). A calibration curve using pure DNB-
(R,S)-Leu was used to determine the concentrations in the samples.
The pH of the aqueous phase was measured using an InoLab pH 730 pH-meter equipped
with a SenTix 81 probe (WTW, Germany).
Theory and definitions
The principle of ELLE with the extractant/host in the organic phase (C) and the racemate to
be separated in the organic phase (R and S) is given in Figure 4.11,32 It involves transfer of
the enantiomers from the aqueous to organic phase followed by complexation to the chiral
host. The complexation constant is different for both enantiomers and this leads to
enentioselectivity.
Figure 4. Relevant reactions for ELLE 11
.
The concentrations of both enantiomers in the water phase were determined
experimentally (HPLC). Those in the organic phase were calculated by using overall mass
balances (eqs. 2 for batch operation and eqs. 3 for continuous operation). 11,23
−
(
2a)
Vaq[R]aq,o = Vaq
V [S] =Vaq
(
(
[R] +[R ]
)
+Vorg
(
[R]org +[RC]org
)
aq
aq
−
(2b)
[S] +[S ]
)
+Vorg
(
[S]org +[SC]org
)
aq
aq,o
aq
aq
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−
Q [R] = Qaq
(
[R] +[R ]
)
+ Qorg
(
[R] +[RC]
)
(3a)
(3b)
aq
aq,o
aq
aq
org
org
−
Q [S] = Qaq
(
[S] +[S ]
)
+ Qorg
(
[S] +[SC]
)
aq
aq,o
aq
aq
org
org
Here, [R]aq,0 and [S] aq,0 represent the inlet (continuous) or initial (batch) concentration of
the (R)- and (S)-enantiomer, Vaq and Vorg represent the volume of aqueous and organic
phase in batch operation, Qaq and Qorg are the volumetric flow rates of the aqueous and
organic phases for continuous operation, [RC] and [SC] represent the concentration of the
host-enantiomer complex .
The operational selectivity (αop ) is defined in eq. 4
(
4)
DS
DR
αop
=
,
if D > D
S R
Here, D is the distribution of the enantiomers between the two liquid phases, which is
defined as:
[
R]org,all [R]org +[RC]org
D =
=
R
(5a)
(5b)
[R− ] +[R]aq
[
R]aq,all
aq
[
S]org,all [S]org +[SC]org
D =
=
S
−
[
S]aq,all
[S ] +[S]
aq
aq
The enantiomeric excess, ee, is defined in eq 6.
[
[
R]i,all −[S]i,all
R]i,all + [S]i,all
ee =
×100%
(6)
i
Here, subscript i represents the organic phase or the aqueous phase.
A good enantioselective extraction process is not only determined by a high ee but also by a
high extraction yield, which is defined as the extracted amount into the organic phase
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1
1
compared with the inlet concentration in the aqueous phase. The yield of an enantiomer is
defined as:
[
R]org,all
[R]org +[RC]org
[R]aq,0
YR =
.SR =
.SR =
.SR
.SR
(7a)
(7b)
[
R]aq,0
[
S]org,all
[S]org +[SC]org
[S]aq,0
YS =
[
S]aq,0
Here SR is the solvent ratio, defined as Vorg/Vaq for batch operation and Qorg/Qaq for the
continuous microreactor.
Results and Discussion
Equilibrium experiments in batch with 1,2-DCE and 1-octanol
Batch experiments were performed in both 1,2-DCE and 1-octanol to determine the
equilibrium composition of the enantioselective extraction of DNB-(R,S)-Leu with CA3. 1,2-
DCE was used as the benchmark and 1-octanol as an example of a greener, environmentally
more benign solvent. Experiments were carried out with a fixed phase ratio of the aqueous
and organic phase (1 to 1) at room temperature and DNB-(R,S)-Leu and host concentrations
of 1 mM. To ensure equilibrium, the experiments were performed for at least 14 h. The
results for both 1,2-DCE and 1-octanol are given in Table 2.
Table 2. Batch equilibrium extraction results for DNB-(R,S)-Leu with CA3a
Organic solvent
number of
experiments, n)
eeaq
(%)
(± stdev)
eeorg
(%)
(± stdev)
α
op
Y
R
Y
S
(
(
± stdev)
(± stdev)
(± stdev)
1
1
,2-DCE (n=6)
-octanol (n=5)
3.2 ± 0.2
3.2 ± 0.2
0.15 ± 0.01 0.36 ± 0.02
0.31 ± 0.02 0.58 ± 0.01
14 ± 1
25 ± 2
41± 2
31 ± 3
aConditions: Vaq/Vorg = 1,phosphate buffer pH 6.58, 294 K, 1 mM DNB-(R,S)-Leu, 1 mM CA3
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The value for αop was 3.2
± 0.2 in both solvents. These values are close to those observed for
the well-known extractant CA4 for this extraction system (αop of 3.4) at similar conditions
1
1
in DCE. Thus, non-oxidized CA3 is a good alternative for CA4 with the advantage that one
synthetic step in the host synthesis can be eliminated. CA3 is selective for the S-enantiomer,
as is clear from the yield data in Table 2. When comparing performance of the two solvents,
the data show that the equilibrium ee values in the organic phase are higher for 1,2-DCE
(41%) than for 1-octanol (31%). However, the yield of the S enantiomer is higher in 1-
octanol than in 1,2-DCE, which is a promising feature and shows the potential of 1-octanol
to replace chlorinated solvents like 1,2-DCE.
Experiments in the continuous microreactor set-up
Experiments in 1,2-DCE
Enantioselective extraction of an aqueous solution of DNB-(R,S)-Leu with CA3 in 1,2-DCE
were carried out in a slug flow capillary microreactor (Figure 3) using various residence
times, by varying the flow rates and capillary lengths. All experiments were performed at
room temperature and a fixed flow ratio of the buffered aqueous to organic phases (1:1),
see the experimental section for details.
Initially, three experiments were performed to determine the effect of residence times on
performance. A microreactor with a capillary length of 10 cm was applied and operated at
three different flow rates (2.5-7.5 mL/h, residence times between 12 and 36 s). With these
flow settings, the reactor is operated in the slug flow regime (Figure S4, Supplementary
information). All experiments were run for at least 2 residence times to ensure that the
steady state was obtained. The outlet concentrations of DNB-(R,S)-Leu in both phases
versus residence times are shown in Figure 5.
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a)
b)
[
R]-enantiomer
S]-enantiomer
[R]-enantiomer
[S]-enantiomer
[
0.5
0.5
0.4
0.3
0.2
0.1
0
0
.4
.3
0
0.2
0.1
0
0
10
20
30
40
0
10
20
30
40
τ (s)
τ (s)
Figure 5. Outlet concentration of DNB-(R,S)-Leu in the aqueous phase (a) and organic
phase (b) versus residence time in the capillary microreactor (Figure 3) for 10 cm capillary
length. Conditions: see Table 1.
The results show that the (S)-enantiomer is preferentially extracted from the aqueous
phase to the organic phase, indeed proving that enantioselective extraction is possible in
the microreactor set-up. The ee values are calculated from the data presented in Figure 5
using eq. 6 and are provided in Figure 6a. The ee values are between 41-44 % for the
organic phase, though the error in the values is relatively large. However, the values are
within the range for the equilibrium value obtained in batch at similar conditions (41± 2 %,
Table 2). The operational selectivity is between 2.9 and 3.1 (Figure 6b) and also comparable
with the operational selectivity obtained in batch experiment (3.2
conditions.
± 0.2) at equilibrium
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a)
b)
4
60
50
40
30
20
10
0
Aqueous phase
Organic phase
3
2
1
0
0
10
20
30
40
0
10
20
30
40
τ (s)
τ (s)
Figure 6. Enantiomeric excess (a) and operational selectivity (b) of enantioselective
extraction of DNB-(R,S)-Leu and CA3 in DCE in a microreactor of 10 cm length. Conditions:
see Table 1.
Though the operational selectivity and the ee values seem to be close to the equilibrium
values obtained in batch, the concentration in the outlet are not constant and seem to be a
function of the residence time, see Figure 5 for details. This is an indication that the
experiments were not solely performed in the equilibrium regime. To further assess this,
additional experiments (in total 57, see Supplementary Information Table S2) were
performed at a wider range of residence times (12- 900 s). Residence times were set by
changing either the flow rates or the length of the microreactor. The results are given in
Figure 7 and indeed show that the concentrations in the outlet are not constant.
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a)
[R]-enantiomer
S]-enantiomer
equilibrium [R]
equilibrium [S]
b)
[R]-enantiomer
[S]-enantiomer
equilibrium [R]
equilibrium [S]
0.6
0.5
0.4
0.3
0.2
0.1
0
0.6
0.5
0.4
0.3
0.2
0.1
0
[
0
200
400
600
800
1000
0
200
400
600
τ (s)
800
1000
τ (s)
Figure 7. Concentration of DNB-(R,S)-Leu in the aqueous phase outlet (a) and organic
phase outlet (b) versus residence time after extraction with CA3 in 1,2-DCE in the slug flow
capillary microreactor. Conditions: see Table 1.
At low residence times, the concentrations are a function of the residence times, whereas
the values become constant at higher residence times and about equal to the equilibrium
values obtained in batch experiments. This implies that equilibrium is attained at higher
residence times, typically above 200 s. A similar trend was found for the ee values and the
operational selectivity (Figure 8). The ee’s varied between 37%-49% and of particular
interest is the observation that the ee values in the organic phase are higher in the kinetic
regime than at equilibrium. This implies that higher ee’s are possible when operating the
microreactor set-up in the kinetic regime thus at low residence times. This remarkable
aspect will be considered further when discussing the experiments with 1-octanol, which
showed an even more profound effect of residence times on ee.
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a)
b)
5
70
60
50
40
30
20
10
0
Aqueous phase
Organic phase
Equilibrium aqueous phase
Equilibrium organic phase
αop from microreactor
αop from batch equilibrium
4
3
2
1
0
0
200
400
600
800
1000
0
200
400
600
τ (s)
800
1000
τ (s)
Figure 8. Enantiomeric excess (a) and operational selectivity (b) versus the residence time
for the enantioselective extraction of DNB-(R,S)-Leu with CA3 in 1,2-DCE in the slug flow
capillary microreactor. The values are calculated from the data presented in Figure. 7 using
eq. 4 and eq. 6. Conditions: see Table 1.
Continuous experiments in 1-octanol
To find alternatives for 1,2-DCE and to probe solvent effect on separation performance, the
use of 1-octanol as the organic solvent was explored. A total of 29 experiments were
performed in the continuous set-up in the slug flow regime at a range residence times by
varying the total flow rate and tube length (Table S3, Supplementary information).
Conditions are given in Table 1.
The effect of residence times on the outlet concentrations are shown in Figure 9, and as
with 1,2-DCE, the outlet concentrations are a function of the residence time. For residence
times beyond about 300 s, the concentrations become constant and equilibrium is attained.
The outlet concentrations in this regime are indeed equal to those obtained in the batch
equilibrium studies (vide supra).
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a) 0.6
b)0.6
[
R]-enantiomer
[R]-enantiomer
[S]-enantiomer
equilibrium [R]
equilibrium [S]
[
S]-enantiomer
0.5
0.4
0.3
0.2
0.1
0
equilibrium [R]
equilibrium [S]
0.5
0.4
0.3
0.2
0.1
0
0
200
400
600
800
1000
0
200
400
τ (s)
600
800
1000
τ (s)
Figure 9. Aqueous phase outlet (a) and organic phase outlet (b) versus the residence time
for the enantioselective extraction of DNB-(R,S)-Leu with CA3 in 1-octanol in the slug flow
capillary microreactor. Conditions: see Table 1
Similar trends were observed for the ee values (Figure 10). The ee’s varied between 28-
46% and are a clear function of the residence time. Values are highest at the shortest
residence times and then level off to the equilibrium value (about 30%) at longer residence
times. The ee values in the equilibrium regime are similar to those obtained in batch
equilibrium studies (31%).
Aqueous phase
Organic phase
Equilibrium aqueous phase
Equilibrium organic phase
b)
αop from microreactor
αop from batch equilibrium
a) 50
4
3.5
3
40
30
20
10
0
2
1
0
.5
2
.5
1
.5
0
0
200
400
600
800
1000
0
200
400
600
800 1000
τ (s)
τ (s)
Figure 10. Enantiomeric excess (a) and operational selectivity (b) versus residence time for
the enantioselective extraction of DNB-(R,S)-Leu with CA3 in 1-octano. The values are
17

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calculated from the data presented in Figure 9 using eq. 4 and eq. 6. Conditions: see Table
1.
The observation of high ee values at short residence times is more profound in 1-octanol
than in 1,2-DCE. This kinetic effect may be explained by considering the intrinsic rates of
the complexation reactions in the organic phase (Figure 1) and the mass transfer rates of
both enantiomers from the aqueous to the organic phase. To gain insights in the intrinsic
rates of the complexation reactions, a number of batch experiments were performed with
DNB-(R,S)-Leu and CA3 in 1-octanol. The organic and aqueous phase were contacted
intensively for about 6 s and then phase separated within 30 s. Analysis of the aqueous
phase (HPLC) showed that equilibrium values were already attained within this timescale.
This implies that the intrinsic kinetics for complexation in the organic phase are faster than
the experimentally observed timescale of the kinetic regime in the microreactor (about 300
s in 1-octanol see Figure 9). As such mass transfer rates of the enantiomers from the
aqueous phase to the organic phase are also of importance and affect the overall rate of the
extraction processes. However, the extraction process in the kinetic regime in the
microreactor is not solely governed by mass transfer processes as in this case chiral
recognition is not possible and ee values at short residence times are expected to be
zero.43,62
This suggests that the experiments were carried out in the intermediate regime where both
mass transfer rates and intrinsic kinetics play a role and determine the overall rate of the
transfer processes. In this regime, the observed transfer rates of the enantiomers are
governed by physical properties of the system (diffusivities, mass transfer coefficients, etc.)
which are equal for both enantiomers and by the rates and equilibrium constants of the
complexation reactions. The latter are different for both enantiomers and will lead to
differences in transfer rates and thus ee values in the experiments performed in the
capillary microreactors. We have designated this as the kinetic regime, to make a
distinction with the equilibrium regime, but it should be noted that the behaviour of the
system is determined not only by the kinetics but also by the mass transfer characteristics.
Extensive reactor engineering modeling activities to obtain a better understanding of the
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complex interplay between reaction and mass transfer are in progress and will be reported
in due course.
Thus, we can conclude that higher ee values are attainable in the capillary microreactors in
the kinetic regime. This unprecedented finding has high potential and justifies further
studies as it suggests that equilibrium ELLE is not necessary the best when regarding ELLE
performance. However, for ELLE, not only the ee is of importance but the yields of the
individual enantiomers should also be considered and obviously a high yield is preferred.
The yields of the (R)- and (S)-enantiomer versus the residence times for the experiments in
1-octanol are given in Figure 11. Clearly, the yields are lower in the kinetic regime, for
which higher ee values are attainable. As such, a balance between ee and yield should be
considered. In this respect, the development of multistage microreactor concepts in which
each individual microreactor is operated in the kinetic mode to obtain high ee values holds
great promise.
1
.8
.6
.4
.2
0
Y
YR
R
Y
YS
S
q
q
E
E
E
q
u
u
u
u
i
i
l
l
i
i
i
i
b
b
b
b
r
r
r
r
i ui u
i ui u
m
m
Y
YR
YS
S
R
0
0
0
0
E
q
il
l
i
m
m
Y
0
200
400
τ (s)
600
800
1000
Figure 11. The extraction yields of both enantiomers for enantioselective extraction of
DNB-(R,S)-Leu with CA3 in 1-octanol. The values are calculated from the data presented in
Figure 9 using eq. 7a and 7b. Conditions: see Table 1.
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The results provided above only involve a single extraction step. Higher ee’s and yields are
possible by performing multistage extractions using various configurations (co-cross and
countercurrent operation). Prediction of ee’s and yields in the equilibrium regime for such
configurations by appropriate (reactor) engineering models are well established.32,63 For
instance, for an operational selectivity of 3 for a reactive extraction system, about 10
equilibrium stages are required in counter current operation to obtain ee values of 99%.
However, the introduction of non-equilibrium operation (in the kinetic regime) significantly
complicates the calculations. Here we want to discuss some preliminary results obtained
with operating the system in the kinetic regime, i.e. at short residence times, and in
combination with a multistage configuration for the system with 1-octanol as the organic
solvent. As an example, a multistage concept was applied by taking each time the aqueous
outlet to a next ELLE step where it is treated with a fresh organic stream containing only
1
mM of extractant, see Figure 12. In such a cross flow mode, it is possible to increase the
yield of (S)-enantiomer, while at the same time having ee values higher than the equilibrium
value for each step, by performing the extraction at a short residence time. Each extraction
step then operates in the kinetic regime. However, the ee values at such short residence
time will decrease from each step to the next due to the change in the aqueous inlet
concentration of S and R where the excess of R will increase with each step.
Figure 12. Multistage concept for ELLE using flow reactors applying fresh organic solution
to each step
Finally, we mention here that by suitably rearranging the inflow and outflow connections,
an overall countercurrent flow operation is possible, as illustrated in Figure 13. Reactor
engineering modeling activities to obtain a better understanding of the complex interplay
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between reaction and mass transfer and the consequences of various multistep operation
modes are in progress and will be reported separately.
Figure 13. Multistage concept for ELLE using flow reactors applying a countercurrent
scheme to the feed streams for each step.
Conclusions
We here report the proof of concept for ELLE in a capillary microreactor operated in the
slug flow regime. 1-Octanol was shown to be a good, environmentally more benign
alternative for conventionally used chlorinated solvents like 1,2-DCE. The use of
microreactors holds great potential for future exploration as it allows for continuous
operation, precise setting of residence times and small inventories of expensive host and
solvents. Furthermore, for new chemistry systems, it allows for fast screening and
optimization of process conditions. Of high interest is also the observation that the ee is
higher in the kinetic regime than in the equilibrium regime for particularly 1-octanol,
indicating that non-equilibrium ELLE may have high potential. Process studies are in
progress to demonstrate the use of microreactors for multistage operation, including a host
recycling step (back-extraction), to separate racemates and to give both enantiomers in
high yields and purity.
Author Information
Corresponding Author
*
Phone: +31 50 363 4174. E-mail: h.j.heeres@rug.nl
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Acknowledgement
The authors thank the technical staff of the Department of Chemical Engineering, University
of Groningen, The Netherlands, for the design of the experimental setup. Financial support
from STW through project no. 11404 (Chiral Separations by Kinetic Extractive Resolution in
Microfluidic Devices) and input from all user committee members of the project are
gratefully acknowledged.
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Table of contents entry
Separation of racemates in microreactors using liquid-liquid extraction with 1-octanol as the
solvent is successfully demonstrated.