Fabrication of mesoporous carbon tube and foam: Application as supports in enantio-selective separation of optically pure amino acid from racemates

Article abstract of DOI:10.1166/jnn.2015.8369

The mesoporous monolithic carbon (MMC) foams and carbon tubes were newly fabricated in cmscale using the mixture of triblock copolymers and phenol/HCHO resin precursors. The regular mesoporosity were formed in the body of MMC foam and carbon fibers. In this work, the organic phases containing chiral ARCA adsorbent and a phase transfer catalyst were coated on the surfaces of mesoporous carbon support, and this ARCA/carbon mixture was adopted for the enantioselective separation of amino acid in the circulation system. (S)-ARCA coated MMC support showed high selcetivity up to 90% for the separation of D-type phenylalanine, serine and tryptophan from racemic mixtures.

Full text of DOI:10.1166/jnn.2015.8369

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Vol. 15, 334–338, 2015
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Fabrication of Mesoporous Carbon Tube and
Foam: Application as Supports in
Enantio-Selective Separation of Optically
Pure Amino Acid from Racemates
Hoon-Gi Jeon, Ju-Hyun Lee, Gye-Ryung Kim,
Da-Min Park, and Geon-Joong Kim∗
Department of Chemical Engineering, Inha University, Incheon, 402-751, Korea
The mesoporous monolithic carbon (MMC) foams and carbon tubes were newly fabricated in cm-
scale using the mixture of triblock copolymers and phenol/HCHO resin precursors. The regular
mesoporosity were formed in the body of MMC foam and carbon fibers. In this work, the organic
phases containing chiral ARCA adsorbent and a phase transfer catalyst were coated on the surfaces
of mesoporous carbon support, and this ARCA/carbon mixture was adopted for the enantioselective
separation of amino acid in the circulation system. (S)-ARCA coated MMC support showed high
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selcetivity up to 90% for the separation of D-type phenylalanine, serine and tryptophan from racemic
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mixtures.
Copyright: American Scientific Publishers
Keywords: Mesoporous Carbon, Foam, Amino Acid, Chiral Separation, Adsorption.
1. INTRODUCTION
After carbonization of carbon/silica composite at 900 ꢀC
and dissolution of silica by HF, large fractions of macro-
porosity in three dimensions were formed in the monolith
body, and the remaining carbon walls exhibited uniform
mesoporosity.
Mesoporous materials with a pore diameter in the range
of 2∼10 nm may be suitable for a wide range of
applications.1ꢀ2 Formation of porosity on two or three dif-
ferent length scales with hierarchical structure may pro-
vide the easy diffusion of reactants to the active sites for
catalysis and adsorption process. The synthesis of meso-
porous composites by various procedures has been stud-
ied widely to develop the new catalysts.3–6 Mesoporous
silica, alumina and carbons are fundamental candidates
for the use as a support. Among these materials, porous
carbon has the big potential for practical uses.7ꢀ8 In this
study, we have fabricated the mesoporous monolithic car-
bon (MMC) foams and tubes. MMC foams in large size
could be fabricated from the mixture of triblock copoly-
mers and phenol/HCHO resin precursors. At the drying
Optically pure D-amino acids (AA) are industrially
important chiral building blocks for the synthesis of phar-
maceuticals, food ingredients, and drug intermediates.9–12
The syntheses of optically pure chemicals have gained
significant potential over recent years. Chemoenzymatic
dynamic kinetic-resolution processes have recently been
developed for deracemization of amino acids.13 (S)-ARCA
(Alanine Racemase Chiral Analogue) is a good candidate
for the selective adsorption of D-AA through imine forma-
tion reaction.14 In this work, the organic phases containing
(S)-ARCA adsorbent and Ionic Liquid (as a phase trans-
fer catalyst; PTC) were coated on the surfaces of meso-
porous carbon supports for selective separation of D-AA
in the circulation system. The heterogeneous adsorbents
were loaded in the continuous flow type packing bed.
This heterogenized system has overcome the disadvan-
tage of homogeneous liquid–liquid extraction process for
ꢀ
temperature higher than 300 C, the sol precursors were
expanded and solidified to make the MMC foams. As well
the mesoporous carbon tubes were synthesized by coating
of above sol on the surface of silica fibers.
∗Author to whom correspondence should be addressed.
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doi:10.1166/jnn.2015.8369

Jeon et al.
Application as Supports in Enantio-Selective Separation of Optically Pure Amino Acid from Racemates
the separation of D-AA using ARCA. (S)-ARCA coated
on MMC support has a unique property for the selective
adsorption of D-AA (up to 90% selcetivity) from racemic
mixtures.
fully dissolved. This solution was impregnated on MMC
support and then it was packed in the tubular type reac-
tor. The aqueous solution of 2.0 eq. D/L-phenylalanine,
2.0 eq. NaOH and 1.8 eq. DIPA dissolved in 200 ml of dis-
tilled water was pumped and circulated through the reac-
tor packed with (S)-ARCA/carbon support for adrsorption
of D-AA. The adsorption analysis was performed for one
hour, and after changing the circulating solution as 3 M-
HCl, the hydrolysis and back-extraction of adsopted AA
into the acidified water was performed for additional 1 h
with circulation by pump.
2. EXPERIMENTAL DETAILS
2.1. Materials Synthesis
The starting carbon sol and the low-molecular-weight phe-
nolic resin/silica sol precursors were prepared according
to the method reported previously in the papers.15ꢀ16 This
carbon sol or carbon/silica composite sol was concentrated
by evaporating the solvent, until the volume of sol mix-
ture was reduced to 1/3 as compared to the starting vol-
ume. Then it was kept at 300 ꢀC for self-expansion during
the solidification of sol into gel foam. The typical proce-
dure to fabricate the mesoporous carbon foam is shown
in Scheme 1(a). The carbon precursor sol was coated on
the surfaces of silica fibers modified by PVP and NH4F.
This carbon/silica comꢀposite was dried without washing,
and carbonized at 900 C in N2 stream.
3. RESULTS AND DISCUSSION
The centimeter-scale MMC foam could be fabricated
by using the mixture of triblock copolymers and phe-
nol/HCHO resin precursors basically. The optical and
SEM, TEM images of MMCs are shown in Figure 1. For
the fabrication of MMC foams, by incorporating TEOS sol
in the carbon sol mixture, it was expanded more efficiently
to make the foam structure. The size, density and bulk
shape of MMC foam could be controlled, according to the
container and treated heating temperatures (Figs. 1(a), (b)).
The body volume of MMC foam was reduced after car-
bonization at 900 ꢀC. Figure 1 also shows the shape of
the carbonized MMC foam made by replicating the shape
of container. It is clearly found that large fractions of
The procedure to prepare the mesoporous carbon tube
is shown also in Scheme 1(b).
2.2. Separation of D-AA from Racemic AA
Organic solution of 10 g of (S)-ARCA, 1.4 eq. of Ionic
Liquid, 20 ml of EDC solvent was mixed until ARCA was
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macroporosity in three dimensions are observed on cross-
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sectional SEM images of MMC foam (Fig. 1(d)). The
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formation of mesopores in MMC foam walls could be con-
firmed by TEM (Fig. 1(e)) and BET analysis (Fig. 1(g)).
This carbon showed high surface areas such as 1100 m2/g
and pore volume of 0.71 cm3/g with mean pore size
of 55 Å.
The mesopores were successfully incorporated inside
the wall of carbon tubes by carbonization of phe-
nol/HCHO/F127 carbon sols coated on the silica fibers
after dissolution of silica by HF (shown in Scheme 1(b)).
The analytical results by SEM, XRD and TEM revealed
that the materials had well developed pore with a large
specific area. Optical microscopic images (Fig. 2(A)) and
SEM photographs (Fig. 2(b)) show the vacant inside of
carbon tubes. The bundle type carbon tubes were easily
obtained by coating of carbon sols (Fig. 2(c)).
TEM image of monolith sample (Fig. 2(d)) displays a
hexagonal array of pore structure for the carbon tubes.
However the mesoporosity was observed throughout the
whole carbon tubes. N2 adsorption–desorption isotherm of
carbon tubes display steep changes with pressure, which is
an indication of the presence of mesoporosity (Fig. 2(e)),
showing surface area of 800 m2/g and pore volume of
0.54 cm3/g with mean pore size 50 Å. The wall thickness
of carbon tubes was around 1 ꢁm.
The racemic D/L-AA is consisting of 50% D-AA and
50% L-AA. (S)-ARCA forms an imine group with D-AA
in major under the basic condition around pH 10. As a
Scheme 1. Procedure for the fabrication of MMC foam (a) and meso-
porous carbon tubes (b).
J. Nanosci. Nanotechnol. 15, 334–338, 2015
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Application as Supports in Enantio-Selective Separation of Optically Pure Amino Acid from Racemates
Jeon et al.
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Figure 1. OM image of dried MMC in beaker (a), removal of container after polymerization (b), after carbonization (c). SEM (d) and TEM (e)
image, XRD patterns (f) and N2 adsorption–desorption isotherm (g) of MMC.
maximum, only 1.0 eq. (S)-ARCA in organic solvent
reacts theoretically with 1.0 eq. D-AA from 2.0 eq.
racemic D/L-AA, leaving 1.0 eq. of L-AA in the aqueous
layer. High molecular weight chiral aldehyde compound
such as ARCA is soluble only in organic solvent such as
methylene chloride. The conventional adsorption is going
on through liquid–liquid extraction process. Since AA is
soluble in an aqueous phase as well ARCA is dissolved
in the organic phase, PTC is absolutely needed for fast
diffusion of AA into organic layer to contact with ARCA.
This Adsorption/Desorption process is briefly summarized
in Scheme 2. For the successful application in our work,
ARCA, PTC and ionic liquid were dissolved in the organic
solvent, and then the mixture was impregnated on the
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J. Nanosci. Nanotechnol. 15, 334–338, 2015

Jeon et al.
Application as Supports in Enantio-Selective Separation of Optically Pure Amino Acid from Racemates
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Figure 2. OM (a), SEM (b), (c) and TEM (d) images and N2 adsorption–desorption isotherm (e) of mesoporous carbon tubes.
MMC foams or carbon tubes. The AA dissolved in basic
water was efficiently contacted to ARCA on carbon sup-
port during the circulating process. At the high loading of
new ionic liquid, a full dissolution and homogeneous coat-
ing of ARCA were achieved. In the heterogenized ARCA
system, phase separation into organic layer and aqueous
layer was unnecessary. Adsorpted D-AA was desorpted
and released into the aqueous HCl solution from ARCA
adsorbents coated on the carbon support through hydroly-
sis. To increase the adsorption rates, (S)-ARCA and PTC
were coated on the carbon support as thin layers. The
mesoporous support provided excellent property for homo-
geneous loading of chiral ARCA on the surfaces as thin
layer. In the repeated adsorption, maximum adsorption of
95% D-amino acid was achieved in the batch and con-
tineous circulation system. The high purity of D-AA up
to 95% could be obtained by adsorptive separation in the
continuous system.
Scheme 2. Selective adsorptive separation of chiral AA by ARCA.
J. Nanosci. Nanotechnol. 15, 334–338, 2015
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Application as Supports in Enantio-Selective Separation of Optically Pure Amino Acid from Racemates
Jeon et al.
were used for the selective separation of D-AA from
racemic AA in the continuous flow type packed bed reac-
tor system. The unique separation properties in heteroge-
nized adsorption system may expand the applications of
porous materials for commercial processes in the future.
Such carbon materials with bimodal meso/macroscopic
pores would be useful for many potential applications,
such as heterogeneous catalysts when bulky reactant
molecules are involved.
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Copyright: American Scientific Publishers
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4. CONCLUSION
The porous carbon foam and tube materials having
bimodal pore structures were fabricated for anchoring the
enantioselective chiral adsorbents. These carbon supports
Received: 19 March 2013. Accepted: 19 April 2013.
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J. Nanosci. Nanotechnol. 15, 334–338, 2015