Enantioseparation of Mandelic Acid Enantiomers with Magnetic Nano-Sorbent Modified by a Chiral Selector

Article abstract of DOI:10.1002/chir.22524

In this study, R(+)-α-methylbenzylamine-modified magnetic chiral sorbent was synthesized and assessed as a new enantioselective solid phase sorbent for separation of mandelic acid enantiomers from aqueous solutions. The chemical structures and magnetic pr

Full text of DOI:10.1002/chir.22524

CHIRALITY 27:835–842 (2015)
Enantioseparation of Mandelic Acid Enantiomers With Magnetic
Nano-Sorbent Modified by a Chiral Selector
1
1
TUBA TARHAN,1,2 BILSEN TURAL, SERVET TURAL, AND GIRAY TOPAL1
*
1Department of Chemistry, Faculty of Education, Dicle University, Diyarbakir, Turkey
2Vocational High School of Health Services, Mardin Artuklu University, Mardin, Turkey
ABSTRACT
In this study, R(+)-α-methylbenzylamine-modified magnetic chiral sorbent was
synthesized and assessed as a new enantioselective solid phase sorbent for separation of
mandelic acid enantiomers from aqueous solutions. The chemical structures and magnetic prop-
erties of the new sorbent were characterized by vibrating sample magnetometry, transmission
electron microscopy, Fourier transform infrared spectroscopy, and dynamic light scattering.
The effects of different variables such as the initial concentration of racemic mandelic acid, dos-
age of sorbent, and contact time upon sorption characteristics of mandelic acid enantiomers on
magnetic chiral sorbent were investigated. The sorption of mandelic acid enantiomers followed
a pseudo-second-order reaction and equilibrium experiments were well fitted to a Langmuir iso-
therm model. The maximum adsorption capacity of racemic mandelic acid on to the magnetic
chiral sorbent was found to be 405 mg gꢀ1. The magnetic chiral sorbent has a greater affinity
for (S)-(+)-mandelic acid compared to (R)-(ꢀ)-mandelic acid. The optimum resolution was
achieved with 10 mL 30 mM of racemic mandelic acid and 110 mg of magnetic chiral sorbent.
The best percent enantiomeric excess values (up to 64%) were obtained by use of a chiralpak
AD-H column. Chirality 27:835–842, 2015. © 2015 Wiley Periodicals, Inc.
KEY WORDS: resolution; enantioselective sorption; mandelic acid enantiomers; magnetic chiral
nanoparticles; isotherm; kinetic
Mandelic acid is an alpha-hydroxy acid obtained from bitter
almonds. The (R)-form acts as a key intermediate in the
production of semisynthetic penicillins and cephalosporins.
At the same time, it is used as a chiral synthone and chiral
resolving agent in the synthesis of antitumor and antiobesity
agents.1 The (S)-form is an important starting material for
stereoselective transformations and a versatile intermediate
for pharmaceuticals.2 Also, the (R)-form has been used in
dermatology, treatment of skin disorders, skin care products,
skin inflammation, and resolving redness, for many years.
The enantiopure chiral compounds can be obtained by
asymmetric synthesis,3 fractional crystallization,4 or chiral
chromatography.5 Separations of enantiomers by chromatog-
raphy on chiral stationary phases is a widely established
methodology for both industry and academia.6–8
In recent years, nanoparticles have received more and
more attention for separation purposes. Nanoparticles have
large specific surface areas; thus, a large fraction of active
sites are available for appropriate chemical interaction.9–14
In the numerous nanoparticles, magnetic nanoparticles have
been widely researched because of their magnetic properties.
But the naked magnetic nanoparticles have low adsorption on
account of large solute molecules that cannot have specific
adsorption. Thus, many organic functional monomers or
polymers have been modified on magnetic nanoparticles
such as polyacrylic acid,15 tetrabenzyl,16 polyacrylamide,17
diphenyl,18 phosphatidylcholine,19 and so on.
Compared with other adsorption methods, the MSS method
possess many advantages, including adsorption efficiency
and time saving.11–20 A review of the literature on chiral
separation using the method of MSS reveals that few studies
have been conducted. Separation of the two enantiomers of
racemic N-(3,5-dinitrobenzoyl)-α-amino acid N-propylamides
was studied by Choi and Hyun.20 Enantioseparation of amino
acids enantiomers using β-cyclodextrins functionalized Fe3O4
21
nanospheres was reported by Chen et al.
In this study, R(+)α-methylbenzylamine (RMBA)-modified
magnetic nanoparticles were synthesized. This was achieved
by the interaction of RMBA with 3-glycidoxypropyldi-
methoxymethylsilane (GLYMO), which was then immobilized
on silica-coated magnetic nanoparticles (see Scheme 1).
Synthesized magnetic chiral sorbent (RMBAG-SCMPs) was
used as a chiral selector for the magnetic separation of
mandelic acid enantiomers.
To the best of our knowledge, this is the first report
concerning the use of RMBA-modified magnetic nanoparti-
cles as a magnetic chiral material to magnetically separate
mandelic acid enantiomers.
EXPERIMENTAL METHODS
Materials
3-Glycidoxypropyldimethoxymethylsilane (GLYMO, 98%), tetraethy-
lorthosilicate (TEOS), R(+)α-methylbenzylamine (RMBA, 98%), racemic
Magnetic-assisted sorption separation methods (MSS), a
novel form of sorption method, has been developed based
on the use of magnetic nanoparticles.11–20 In MSS methods,
functionalized magnetic nanomaterials are dispersed into
samples to adsorb target compounds, and then the sorbents
are separated rapidly by applying an external magnetic field.
© 2015 Wiley Periodicals, Inc.
*Correspondence to: B. Tural, Department of Chemistry, Faculty of Educa-
tion, Dicle University, 21280, Diyarbakir, Turkey.
E-mail: bilsentural@gmail.com
Received for publication 1 April 2015; Accepted 6 August 2015
DOI: 10.1002/chir.22524
Published online 14 September 2015 in Wiley Online Library
(wileyonlinelibrary.com).

836
TARHAN ET AL.
Scheme 1. Schematic illustration for the preparation steps of R(+)α-methylbenzylamine (RMBA) functionalized SCMPs.
mandelic acid (rac.-MA), (R)-(ꢀ)-mandelic acid (R-MA), and (S)-(+)-
mandelic acid (S-MA) were obtained from Sigma Chemical (St. Louis,
MO). All aqueous solutions were prepared with deionized water that
had been passed through a Millipore (Bedford, MA) milli-Q Plus water
purification system.
batchwise to examine the effect of contact time, sorbent dosage, and
initial concentration of rac.-MA. The experiments were conducted with
90 mg of RMBAG-SCMPs and 10 mL rac.-MA solutions within the
range of 2.5–30 mM at 25°C. The mixtures were shaken on a shaking
incubator at a constant agitation speed of 120 rpm for 45 min. After the
magnetic separation process, 0.5 mL of sample was taken from the
clear part to be extracted with 5 mL of diethyl ether five times (using
1 mL of diethyl ether each time). After mandelic acid had been
contained in the ether phase, ether was evaporated. The
hexane/isopropanol mixture, which had been prepared previously (at
a ratio of 1/1), was added to the rest solid and analyzed by HPLC
using a CHIRALPAK AD-H column and n-hexane/isopropanol/trifluoro
acetic acid (80/20/0.1) as mobile phase at a 0.8 mL/min flow rate at
25°C. The detection wavelength was 254 nm and each injection volume
was 20 μL.
Characterization Techniques
Magnetization measurements were conducted using
a vibrating
sample magnetometer (VSM) (Quantum Design PPMS-9T) at room
temperature. The size and morphology of the silica-coated magnetite
nanoparticles (SCMPs) were investigated using transmission electron
microscopy (TEM) (JEOL 2100 F, Japan). For TEM analysis, the
SCMPs were redispersed in pure water by sonication for 10 s and a
drop of suspension was placed onto SPI Double Copper Grids
100/200. Fourier-transformed infrared (FT-IR) spectra were measured
on a Thermo Scientific Nicolet IS10 FT-IR spectrometer (Pittsburgh,
PA). Sixteen scans were collected at a resolution of 4 cmꢀ1. The
particle sizes and distributions of the SCMPs and RMBAG-SCMPs
were determined by using the Dynamic Light Scattering Technique
(Zeta Sizer Nano-ZS, Malvern, PA). The measurement temperature
was kept at 25°C.
The selectivity for enantioseparation was calculated in terms of the per-
centage enantiomeric excess (%ee) of the samples:
SR ꢀ SS
e:e:ð%Þ ¼
ꢁ100
(1)
SR þ SS
Where S is the S-enantiomer peak area and R is the R-enantiomer peak
area obtained from HPLC.
Synthesis of RMBA Functionalized SCMPs
Synthesis of SCMPs was previously reported by our group.11–13 We
utilized a two-step postgraft method in order to synthesize the RMBA
functionalized magnetic nanoparticles (Scheme 1). GLYMO and RMBA
were reacted to prepare RMBA-bonded GLYMO (denoted RMBAG).
After dissolving 46.7 μL of RMBA in 20 mL of deionized water, the
pH value of the solution was measured and obtained at 9.5. To mini-
mize the hydrolysis of GLYMO, the solution was transferred into a
flask bottle placed in an ice-bath at 0°C, and 40 μL of GLYMO was
slowly added into the RMBA solution while continuously stirring the
mixture. The temperature of the mixed solution was raised to 40°C
and it was stirred for 6 h and subsequently was placed into an ice-bath
for 5 min to decrease the temperature to 0°C. After adding another
40μL of GLYMO, the temperature of the mixed solution was raised to
65°C and stirred for 6 h. Finally, the prepared RMBAG solution was
adjusted to pH 6 with concentrated HCl. To obtain surface functional-
ized magnetic particles, 30 mg of the SCMPs were mixed with 10 mL
of the prepared RMBAG solution in a flask at 75°C for 2 h with
stirring, and then the suspension was separated with the help of a
permanent magnet, the supernatant was removed, and RMBA function-
alized SCMPs, denoted as RMBAG-SCMPs (Scheme 1).
RESULTS AND DISCUSSION
Characterization
The FT-IR spectra of the SCMPs and the RMBAG-SCMPs
are shown in Figure 1 together with the spectrum for pure
RMBA molecules. Abroad absorption bands in the range of
900–1200 cmꢀ1 observed in the FT-IR spectrum (Fig. 1A,B)
is assignable to the Si-O, Si-O-C, Si-O-Fe, and Si-O-Si bonds.22
After the SCMPs surface modification with RMBA, a charac-
teristic peak appears at 3365 cmꢀ1. It is ascribed to N-H
stretching band from RMBA (see spectra RMBA (Fig. 1C)
and RMBAG-SCMPs (Fig. 1A)).23 Some alkyl C–H and aryl
C–H absorption bands at about 2800–3000 cmꢀ1 were
detected for the RMBAG-SCMPs and pure RMBA. A peak at
around ~1450 cmꢀ1 assigned to C-N deformation were clearly
visible in the spectra of the modified nanoparticle (Fig. 1A)
and pure RMBA (Fig. 1C). The results indicate that RMBA was
attached successfully to the surface of the SCMPs (Fig. 1).23
The magnetic properties of the Fe3O4 (MNPs) and the
RMBAG-SCMPs were investigated by VSM analysis at room
temperature. The saturation magnetization of pure magnetite
nanoparticles (Fig. 2A) and the chitosan-magnetite
Sorption Experiments
To verify the enantioseparation capacity of RMBAG-SCMPs for
mandelic acid enantiomers, sorption experiments were carried out
Chirality DOI 10.1002/chir

ENANTIOSEPARATION OF MANDELIC ACID ENANTIOMERS
837
Fig. 1. FT-IR spectra of silica coated magnetite particles (SCMPs) (B), the nanoparticles reacted with RMBA (RMBAG-SCMPs) (A), and with the spectrum of
pure RMBA molecules (C).
Fig. 2. (A) Magnetization versus magnetic field for the RMBAG-SCMPs, (B) TEM graphs of the RMBAG-SCMPs
nanocomposites (Fig. 2A) were about 68.0 emu gꢀ1 and to be
37.0 emu gꢀ1, respectively. The decrease in the saturation
magnetization of nanoparticles after surface modification
can be explained by the decrease in the amount of the mag-
netic moments per unit weight due to the diamagnetic contri-
bution of silica shell and RMBA. As can be seen from the
insert in Figure 2A, this saturation magnetization of
RMBAG-SCMPs makes them very susceptible to magnetic
fields, and therefore makes the solid and liquid phases easily
separated.14
The TEM brightfield micrograph for the RMBAG-SCMPs
is shown in Figure 2B. As can be seen from the TEM image,
the average size of the particles is ~10 nm.
The particle agglomeration size distributions of the SCMPs
and RMBAG-SCMPs are shown in Figure 3. Z-average
particle size and poly-dispersitivity index (PDI) of the SCMPs
were determined as 595 nm and 0.378, respectively, and
Z-average particle size and PDI of the RMBAG-SCMPs were
determined as 1060 nm and 0.397, respectively. These results
suggest that the particle size of the SCMPs was increased sig-
nificantly (from 595 nm to 1060 nm) when they were
functionalized with RMBA.
Sorption Studies
The separation of the enantiomers of rac.-MA onto
RMBAG-SCMPs as a function of contact time is shown in
Figure 4A,B. It can be seen that the amount of R-MA and
S-MA sorbed per unit mass of RMBAG-SCMPs increased
with the increase of initial concentration. The sorption equi-
librium was achieved at 45 min for each of the enantiomers
of rac.-mandelic acid. It can be seen that each of the enantio-
mers of chiral mandelic acid sorption onto RMBAG-SCMPs
is fast in the first 20 min and then it becomes slower near
equilibrium. Equilibrium was reached after 45 min. This
Chirality DOI 10.1002/chir

838
TARHAN ET AL.
can be explained by the large number of available vacant
sites on the RMBAG-SCMPs surfaces, which are gradually
occupied in time as a result of the sorption process.10 The
sorption order is S-MA > R-MA.
The effect of RMBAG-SCMPs dose on the separation of
the R-MA and S-MA is demonstrated in Figure 5A,B.
From this figure it is evident that the percentage sorption
of R-MA and S-MA onto RMBAG-SCMPs increases with
the increase in RMBAG-SCMPs dose, while the sorption
capacity, qe (mg/g), at equilibrium decreases. This can
be attributed to the increased sorbent surface area and
availability of more sorption sites of sorbent. It is apparent
that the uptake of solute markedly increased up to a chiral
sorbent dose of 90 mg and thereafter no significant
increase was observed. Hence, the excellent sorption of
R-MA and S-MA can be obtained by using 110 mg of
RMBAG-SCMPs. It was observed that RMBAG-SCMPs
had an affinity towards the R-MA and S-MA with different
percentages. Hence, RMBAG-SCMPs stood out with a
remarkable selective sorption. As can be seen in Figures 5,
the (R)-MA was sorbed with 41%, whereas the (S)-MA
was sorbed with 74% by 90 mg RMBAG-SCMPs. These
findings clarify that RMBAG-SCMPs has sorption affinity
towards enantiomers of rac.-MA. The sorption mechanism
Fig. 3. Z-average particle size distribution for the SCMPs and RMBAG-
SCMPs.
Fig. 5. Effect of sorbent dose for sorption of racemic mandelic acid enantio-
mers on RMBAG-SCMPs using 30 mM rac.-MA at 45 min agitation time (A)
percent removal and sorption capacity (mg gꢀ1) of S-MA. (B) Percent removal
and sorption capacity (mg gꢀ1) of R-MA.
Fig. 4. Effect of agitation time on sorption capacity (qe, mg gꢀ1) of racemic
mandelic acid (A) S-MA (B) R-MA on RMBAG-SCMPs at 10, 20, and 30 mM
initial concentrations of racemic mandelic acid.
Chirality DOI 10.1002/chir

ENANTIOSEPARATION OF MANDELIC ACID ENANTIOMERS
839
might proceed by forming significant hydrogen bonding
and host–guest interaction between the mandelic acid
and RMBAG-SCMPs.
Hence, the Freundlich equation can be written as the loga-
rithmic form27
1
Logqe ¼ LogKF
þ
LogCe
(3)
Table 1 shows the influence of the initial concentration of
rac.-MA and RMBAG-SCMPs dose on the optical resolution.
When the initial concentration of rac.-MA increased from 5
to 30 mM, the percent of ee decreased from 43% to 39%. This
was probably because more and more active sites became
covered by the isomer, thus leading to permeation of the
other isomer.1,5,24 Therefore, the amount of sorbent dose
was increased from 90 mg to 120 mg (Table 1). The %ee in-
creases with the increase in chiral sorbent dose from 90 mg
to 110 mg. A subsequent increment of dose formed a small
increase in ee. Therefore, the best enantioselectivity of
RMBAG-SCMPs was obtained by using 30 mM rac.-MA
and 110 mg RMBAG-SCMPs at 45 min and 25°C. HPLC
chromatograms showing the best ee value are shown in
Figure 6, including that of before and after separation by
magnetic assisted sorption. As can be seen from Figure 6,
the best ee value was obtained as 64%.
n
Sorption Isotherms
The equilibrium isotherms are used to describe the
sorbate–sorbent interactions, and their knowledge is im-
portant from both a theoretical and a practical point of
view. Various isotherm models have been published in
the literature to express experimental data sorption iso-
therms. In the present study, Langmuir, Freundlich, and
Dubinin-Radushkevich isotherms were employed.
The basic assumption of Langmuir isotherm is based on
monolayer coverage of the sorbate on the homogeneous
surface of the sorbent.25 The linear form of the Langmuir
isotherm equation is expressed as:26
1
1
1
1
¼
þ
ꢂ
(2)
qe Qm QmKL Ce
where Ce is the equilibrium concentration of sorbate (mg
Lꢀ1), qe (mg/g) is the amount of enantiomers of rac.-MA
sorbed per unit mass of sorbent, Qm the maximum sorption ca-
pacity (mg gꢀ1), and KL is the constant associated with the
free energy of sorption (L mgꢀ1). Where Qm and KL can be de-
termined from the linear plot of 1/qe versus 1/Ce (Table 2).
The Freundlich isotherm model is used to describe the
sorption characteristics for the heterogeneous sorbent
surface with sites that have diverse energies of sorption.
Fig. 6. HPLC chromatogram (% area) of racemic mandelic acid before (A)
and after (B) separation by magnetic assisted sorption carried out on
Chiralpak AD-H column and n-hexane/isopropanol/trifluoro acetic acid (80/
20/0.1) as mobile phase at 0.8 mL/min flow rate at 25°C. The detection wave-
length was 254 nm and each injection volume was 20 μL.
TABLE 1. Enantiomeric excess values of racemic mandelic acida
rac-MA Adsorbed Amount Residual Amount
Co (mg/10mL)
Dose (mg)
Co (mM)
S-MA(mg)
R-MA (mg)
S-MA (mg)
R-MA (mg)
aee %
90
90
90
90
90
90
100
110
120
5
10
15
20
25
30
30
30
30
7.6
15.2
22.8
30.4
38.0
45.6
45.6
45.6
45.6
3.0
5.8
8.6
11.3
14.1
16.9
18.5
20.1
20.5
1.7
3.3
4.8
6.3
7.8
9.4
9.6
10.5
10.5
0.8
1.8
2.9
4.0
5.0
5.9
4.3
2.7
2.3
2.1
4.3
6.6
43
41
40
39
39
39
51
64
67
9.0
11.2
13.5
13.2
12.3
12.3
aEnantiomeric excess was checked with HPLC chiral column (CHIRALPAK AD-H).
Mobile phase: n-hexane/isopropanol/trifluoro acetic acid (80/20/0.1).
Flow rate: 0.8 ml/min. Temperature: 25°C. Detector: 254 nm DAD. Co: initial concentration of rac-MA; volume is 10mL for each experiment.
Chirality DOI 10.1002/chir

840
TARHAN ET AL.
TABLE 2. Isotherms parameters for the sorption of racemic mandelic acid enantiomers on RMBAG-SCMPs
Langmuir constants
KL
Freundlich constants
Dubinin–Raduskvich Isotherm
Adsorbates
qm
qm, exp.
R2
KF
n
R2
K
qm
E
R2
)
( mg gꢀ1
)
(L/mg)
( mg gꢀ1
)
(L/g)
(mol2/kJ)
(mol/g)
(kJmolꢀ1
(R)-(ꢀ) MA
(S)- (+) MA
167
238
6.9E-03
0.268
114
188
0.94
0.98
3.08
52
1.54
1.98
0.91
0.83
2E-04
2.93
90.6
169
0.89
0.29
0.80
0.86
where qe is the amount of enantiomers of rac.-MA sorbed per
unit mass of sorbent in equilibrium (mg/g) and Ce is
1liquid phase concentration of rac.-MA enantiomers in equilib-
rium (mg Lꢀ1). The constant KF (mg1ꢀ(1/n) L1/n gꢀ1) is an
approximate indicator of sorption capacity, while 1/n is a
function of the strength of sorption in the sorption pro-
cess. The values of KF and n were obtained by plotting
Log(qe) vs. Log(Ce) and given in Table 2. The n value
shows the degree of nonlinearity between solution concen-
tration and sorption as follows: if n = 1, then sorption be-
comes linear; if n < 1, then sorption becomes a chemical
process; if n > 1, then sorption becomes a physical
process.28 As can be seen in Table 2, since n values of
R-MA and S-MA were calculated as 1.54 and 1.98,
respectively, the sorption of R-MA and S-MA on RMBAG-
SCMPs is a physical process.29
Langmuir and Freundlich sorption constants and correla-
tion coefficients (R2) are presented in Table 2. In both
cases, linear plots were gained, which reveal the applicabil-
ity of these isotherms on the ongoing sorption process.
The results revealed that Langmuir sorption isotherm was
a more linear model than Freundlich for the sorption of
mandelic acid enantiomers on RMBAG-SCMPs because
correlation Langmuir coefficients (R2) are greater than
Freundlich. The important features of the Langmuir
sorption isotherm parameter can be used to foresee the af-
finity between the sorbates and sorbent using a dimension-
less constant called a separation factor or equilibrium
parameter (RL), which is expressed by the following
relationship26,30
where qmax the theoretical saturation capacity, β is a constant re-
lated to the sorption energy, ε the Polanyi potential (mol2/J2),
calculated from Eq.:6
ꢀ
ꢁ
1
Ce
ε ¼ RTln 1 þ
(6)
where T is the solution temperature (K), R is the gas constant
(8.314 J molꢀ1 Kꢀ1), and Ce is the equilibrium concentration
of sorbate (mg Lꢀ1). By plotting ln qe versus ε2, it is possible
to determine the value of β from the slope and the value of
qmax (mg gꢀ1) from the intercept, which is ln qmax. The value
of mean free sorption energy, E (kJ/mol), can be estimated
by using B values as expressed in the following equation from
D–R parameter B as follows35
1
pffiffiffiffiffiffi
E ¼
(7)
2B
The extent of E is useful for guessing the type of sorption reac-
tion. If E is in the range of 8–16 kJ/mol, sorption is ruled by
chemical ion-exchange. In the case of E < 8 kJ/mol, physical
forces may affect the sorption.36 The parameters obtained using
Eqs. (6, 7) are evaluated in Table 2. The E values calculated using
Eq. 7 is 0.89 and 0.29 kJ molꢀ1 for the R-MA and S-MA, respec-
tively. This indicates that physico-sorption played a significant
role in the sorption of the R-MA and S-MA onto RMBAG-SCMPs.
Sorption Kinetics
The sorption kinetics of R-MA and S-MA onto RMBAG-
SCMPs were investigated by fitting the experimental data
with two kinetic models, namely, pseudo-first-order and
pseudo-second-order. The pseudo-first-order equation and
pseudo-second-order equation as expressed as37,38
1
RL ¼
(4)
1 þ bC0
where b (L/mg) is the Langmuir constant and C0 (mg/L)
is the initial concentration of mandelic acid enantiomers.
The value of RL indicated the type of Langmuir isotherm
to be irreversible (RL = 0), linear (RL = 1), unfavorable
(RL > 1), or favorable (0 < RL < 1).26,30 The RL values
between 0 and 1 indicate favorable sorption. The RL
value in the present investigation was found to be
between 0.80 and 0.95 for R-MA and S-MA, indicating that
the sorption of the R-MA and S-MA on RMBAG-SCMPs is
favorable.
A Dubinin–Radushkevich (D-R) isotherm was also ap-
plied to estimate the porosity apparent free energy and
the characteristics of sorption.31–33 It can be used to de-
scribe sorption on both homogenous and heterogeneous
surfaces. The D–R equation can be defined by the follow-
ing equation33,34
lnðqe ꢀ qtÞ ¼ lnqe ꢀ k1t
(8)
(9)
ꢂ
ꢃ
t
1
1
qe
¼
þ
t
qt k2q2e
Here, qt is the amount of mandelic acid enantiomers sorbed
at time t (mg/g), qe is the amount of mandelic acid enantio-
mers sorbed at equilibrium, k1 is the sorption rate constant
(minꢀ1) for the first order sorption, and k2 the pseudo-sec-
ond-order rate constant. The rate constants k1, k2 and qe were
calculated from the slopes and intercepts of the linear plot of
ln(qeꢀqt) or t/qt against t, respectively. Comparative linear
first-order and second-order sorption rate constants, calcu-
lated qe, experimental qe and R2 values are given for R-MA
and S-MA at 30°C in Table 3.
It can be seen that the linear correlation coefficients for the
first-order and second-order model are good and based on the
lnqe ¼ lnqmax ꢀ βε2
Chirality DOI 10.1002/chir
(5)

ENANTIOSEPARATION OF MANDELIC ACID ENANTIOMERS
841
R2
TABLE 3. Kinetic parameters of rac.-mandelic acid enantiomers on RMBAG-SCMPs
Pseudo-first-order
Pseudo-second-order
q(e, exp)
K1
q(e, cal)
R2
K2
q(e, cal)
(mg gꢀ1
)
(minꢀ1
)
(mg gꢀ1
)
(g mg-1 minꢀ1
)
(mg gꢀ1
)
(R)-(-) MA (mM)
30
20
10
104
69.5
36.5
0.087
0.092
0.089
97.3
60.3
36.4
0.98
0.95
0.95
1.14E-03
2.09E-03
2.58E-03
125
76.9
41.6
0.99
0.99
0.99
(S)-(+) MA(mM)
30
20
10
188
95
64
0.086
0.085
0.096
154
70.3
59.4
0.98
0.95
0.95
1.01E-03
2.18E-03
2.32E-03
200
101
69.9
0.99
0.99
0.99
complementary cellulosic stationary phases in supercritical fluid chroma-
tography. Chirality 2010;22:242–251.
4. Yokota M, Doki N, Shimizu K. Chiral separation of a racemic compound
induced by transformation of racemic crystal structures: dl-glutamic acid.
Cryst Growth Des 2006;6:1588–1590.
comparison between experimental and theoretically calculated
qe values, it was found that the pseudo-second-order model fitted
better than the pseudo-first-order model for sorption process.
CONCLUSION
5. Mayani VJ, Abdi SHR, Kureshy RI, Khan NH, Agrawal S, Jasra RV. Syn-
thesis and characterization of (S)-amino alcohol modified M41S as effec-
We have presented a simple and convenient chemical method
for the preparation of RMBAG-SCMPs using RMBA as a chiral
selector to conduct the enantiomeric separation of rac.-MA
enantiomers. The method using RMBA-modified magnetic
nano-sorbent effectively separates mandelic acid enantiomers.
The sorption capacity of mandelic acid enantiomers onto the
RMBAG-SCMPs depended on contact time, the initial concentra-
tion of mandelic acid enantiomers, and sorbent dose. The
sorption capacity increased with an increase in the initial concen-
tration of mandelic acid enantiomers. Kinetic studies have shown
that the reaction of sorption is pseudo-second-order. The values
of thermodynamic parameters obtained for the sorption
process indicated that the Langmuir isotherm model fitted quite
well with the experimental data (correlation coefficient R2 ≥
0.94), compared with the other two isotherm models. The maxi-
mum adsorption capacity of rac.-MA on the RMBAG-SCMPs was
found to be 405 mg gꢀ1. The enantioseparation of (R)-(ꢀ)-
mandelic acid (ee, 64%) was achieved by using 110 mg magnetic
chiral sorbent and 10 mL 30 mM rac.MA at 45 min and at 25°C.
Even though the complete separation of the two
enantiomers was not achieved in this study, the method of
magnetic field induced separation of enantiomers with the
use of SCMPs tagged to an appropriate chiral selector is
expected to be developed further and utilized as a successful
enantiomer separation technique in the future.
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This project is funded by the financial support from Dicle Uni-
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