Multi-stage chiral separation with electrospun chitin nanofiber membranes

Separation and Purification Technology 118 (2013) 300–304

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Multi-stage chiral separation with electrospun chitin nanofiber membranes Kenta Shiomi, Masakazu Yoshikawa ⇑ Department of Biomolecular Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 606-8585, Japan

a r t i c l e

i n f o

Article history: Received 14 May 2013 Received in revised form 29 June 2013 Accepted 2 July 2013 Available online 12 July 2013 Keywords: Cascade Chiral separation Chitin Multi-stage separation Nanofiber membranes

a b s t r a c t Chitin nanofiber membranes were fabricated via electrospray deposition. The nanofiber membrane adsorbed L-isomer from racemic mixture of Glu and Phe in preference to the corresponding antipodes, while D-Lys was preferentially incorporated into the membrane. The adsorption isotherms revealed that all enantiomers studied in the present study were incorporated into the nanofiber membrane without any specific interaction, in other words, those were simply adsorbed. In membrane transport experiments, D-isomers were preferentially transported through the nanofiber membrane. In order to attain higher permselectivity, a multi-stage cascade separation was applied in the present study. As expected, the permselectivity toward D-enantiomer was enhanced exponentially with the increase in number of stages from one to five stages via three by applying a multi-stage cascade membrane separation. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Membrane separation is ecologically and economically competitive to other conventional separation methods, since membrane separations, excepting pervaporation, are operated without phase separation [1–3]. Furthermore, membrane separation is continuously operated under mild conditions. In these days, membrane separation has been applied in various areas, such as production of drinking water from sea water by reverse osmosis (RO), separation and concentration of macromolecules and colloidal particles by ultrafiltration (UF), production of ultrapure water by nanofiltration (NF), removal of microorganisms by microfiltration (MF), concentration or removal of ionic materials by electrodialysis (ED), gas separation for recovery of H2, concentration of O2, and removal of CO2, dehydration or purification of biofuel by pervaporation (PV), hemofiltration, hemodialysis, and so forth. In membrane separation, flux values and permselectivities, which often show a trade-off relationship, are important membrane performances. In a sense, the enhancement of flux is more important than that of permselectivity. This frustrating trade-off relationship between throughput (flux) and permselectivity for one component of a mixture has caused sluggish spread of membrane-based separation. The authors’ research group reported that molecularly imprinted nanofiber membranes, which are fabricated by simultaneously applying an alternative molecular imprinting ⇑ Corresponding author. E-mail address: [email protected] (M. Yoshikawa). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.07.004

and an electrospray deposition, have potential that both flux and permselectivity can be simultaneously enhanced [4–6]. Nanofiber fabric is one of plausible membrane forms, which can sidestep that trade-off problem. Even though the permselectivity expressed by one stage membrane separation with a nanofiber membrane is low, higher permselectivity with an applicable flux would be obtained by applying a multi-stage cascade in membrane separation [7–9]. Multi-stage cascade membrane separation will give relatively high flux value, since nanofiber membrane intrinsically gives high flux value. To this end, chitin was adopted as a membrane material for nanofiber fabric in the present study. Since chitin nanofiber membrane revealed to show chiral separation ability [10,11]. And chitin is one of highly abundant green polymer occurring mainly in the exoskeletons of shellfish and insects and is synthesized at rate of 1010 to 1011 tons per year [12]. Eelectrospun chitin nanofiber membrane was fabricated as a model membrane. Using those electrospun chitin nanofiber membranes, multi-stage cascade separation was demonstrated adopting racemic mixture of amino acid. Since the demand of chiral separation by membranes has been required in industries, involving pharmaceuticals, agrochemicals, food additives, perfumes, and so forth [13–16]. In the present study, three types of amino acid, such as glutamic acid (Glu), which have very polar anionic side chain, phenylalanine (Phe), having aromatic side chain, and lysine (Lys) with very polar cationic side chain, were adopted as as model permeants (racemates). Among those three types of amino acid, Phe and Lys are essential amino acids.

K. Shiomi, M. Yoshikawa / Separation and Purification Technology 118 (2013) 300–304

2. Experimental 2.1. Materials Chitin powder from crab shells was purchased from Nacalai Tesque, Inc., Kyoto, Japan. D-Isomer and L-isomer of three types of amino acid, such as gluatamic acid (Glu), phenylalanine (Phe), and lysine (Lys) were obtained from commercial source and used as received.

1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP) was adopted as a solvent and the polymer concentration was fixed to be 0.40 wt.% in the present study. Esprayer ES-2000 (Fuence Co., Ltd., Wako, Japan) was adopted as the electrospray deposition device. Polymer solution was electrosprayed at ambient temperature using an applied voltage of 30.0 kV. The syringe used in the present study had a capillary tip of 0.52 mm diameter. The feeding rate was 2.50 mm3 min1. A grounded aluminum foil used as a counter electrode was placed 10 cm from the tip of the capillary. The morphologies, such as diameter of fiber and the thickness of the electrosprayed nanofiber membrane, were determined with KEYENCE VE-7800 scanning electron microscope (SEM). A small section of the membrane was placed on the SEM sample holder and sputter-coated with platinum prior to the analysis. 2.3. Adsorption phenomena The membrane samples (ca. 1 cm  2 cm) were immersed in a 10.0 cm3 of racemic Glu, Phe, or Lys solution with concentrations of 1.0  104 and 2.0  104 mol dm3, and the mixture was allowed to be equilibrated at 40 °C. A 0.02 wt.% sodium azide was added as a fungicide. The amount of amino acid adsorbed in the nanofiber membrane was low so that adsorption selectivity was studied at lower concentration than that for the membrane transport. Quantitative measurements of aliquots of the solution at the initial stage and after equilibrium had been reached were made using liquid chromatography (LC) [JASCO PU-2080, equipped with a UV detector (JASCO UV-2075)] employing a CHIRALPAK MA(+) column (50 mm  4.6 mm (i.d.)) (Daicel Co., Osaka, Japan) for the measurement of racemic Glu’s and Phe’s, and a CROWNPAK CR(+) column (150 mm  4.0 mm (i.d.)) (Daicel Co., Osaka, Japan) for the measurement of racemic Lys’s. Aliquots of solution after 5 and 10 days were adopted as those of solution. An aqueous copper sulfate/acetonitrile mixed solution was used as a mobile phase for Glu and Phe analyses, and a perchloric acid solution as eluent for Lys analysis. The determined concentrations after 5 days and 10 days gave identical values, which meant the equilibrium has been reached within 5 days. The adsorption selectivity SA(i/j) is defined as

SAði=jÞ ¼ ðði  AAÞ=ðj  AAÞÞ=ð½i  AA=½j  AAÞ

40.0 cm3. An aqueous solution of racemic mixture of amino acid was placed in the left-hand side chamber (feed side) and an aqueous solution in the right-hand side chamber (permeate side). Each concentration of racemic amino acid was fixed to be 1.0  103 mol dm3. A 0.02 wt.% sodium azide was added as a fungicide. All experiments were carried out at 40 °C. The amounts of the D- and L-isomers that transported through the membrane were determined by liquid chromatography (LC) as above. The flux, J (mol cm cm2 h1), is defined as:

 J ¼ ðd½i  AAR =dtÞðV R =1000Þd =A

2.2. Preparation of nanofiber membranes

ð2Þ

where [i-AA]R (mol dm3) is the concentration of i-enantiomer in the right-hand side chamber (permeate side), t is time (h), VR (cm3) denotes the volume of the right-hand side chamber, d (cm) is membrane thickness, and A (cm2) represents membrane area, respectively. The permselectivity toward D-isomer, aD/L, is defined as the flux ratio, JD/JL, divided by the concentration ratio [D-AA]/[L-AA]:

aD=L ¼ ðJD =JL Þ=ð½D  AA=½L  AAÞ

ð3Þ

2.5. Membrane conductance (Membrane resistance) Membrane conductance was estimated from membrane resistance [17,18]. The resistances of each enantiomer solution with membrane and without membrane were measured by Portable Kohlrausch Bridge TYPE BF-62A (Shimadzu Rika Instruments Co.) and CO-1305 oscilloscope (KENWOOD). The area of each platinum electrode was 1.0 cm2 and the distance between the electrodes was fixed to be 7.0 cm. An aqueous solution of optically pure D- or L-enantiomer of amino acid, of which concentration being 1.0  103 mol dm3, was poured into both chambers (D-side and L-side). The resistance was measured as a constant temperature of 40 °C with stirring. The measurement of resistance was completed within a few minutes. 3. Results and discussion 3.1. Chitin nanofiber membranes The scanning electron microscope (SEM) photograph of the electrospayed chitin nanofiber membrane is shown in Fig. 1. The diameter of nanofiber in membrane for multi-stage cascade study was determined to be 181 ± 63 nm using Image J software program by measuring at least 30 fibers from each SEM image. The thickness of the membrane was also determined to be ca. 250 lm from SEM images.

ð1Þ

where (i-AA) and (j-AA) denote the amount of enantiomer of amino acid adsorbed in the membrane, and [i-AA] and [j-AA] are the concentrations in the solution after equilibrium had been reached, respectively. 2.4. Membrane transport A membrane (area, 3.0 cm2) was secured tightly between two chambers of a permeation cell. In the study of multi-stage cascade membrane transport, each membrane was separated by poly(ethylene terephtharate) spacer with 1 mm thickness. The volume of each chamber, such as feed side and permeate side, was

301

Fig. 1. SEM image of chitin nanofiber membrane.

302

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3.2. Adsorption phenomena First, adsorption selectivity of the present nanofiber membrane was studied so that chiral recognition ability of the present membrane could be investigated. Table 1 summarizes the amounts of amino acids adsorbed in the membrane for three types of amino acid, such as Glu, Phe, and Lys. Those results are plotted as functions of the concentration of aqueous solution, which had been equilibrated with the membrane and shown in Fig. 2. The relationships for those enantiomers gave straight lines, implying that those isomers were adsorbed in the membrane without any specific interaction. In other words, those enantiomers were simply adsorbed in the membrane. Non-specific adsorption of each enantiomer was occurred without the interference of non-specific adsorption of antipode. From this, the relationships given in Fig. 2 can be regarded as adsorption isotherms, though data were determined from adsorption experiments of racemic mixtures of each amino acid. The adsorption selectivity from racemic mixture can be also determined by using the slopes of adsorption isotherms. In this case, the adsorption selectivity can be determined as follows:

SAði=jÞ ¼ mi =mj

ð4Þ

where mi and mj are the slope of the adsorption isotherm for i- and respectively. The L-isomers were preferentially incorporated into the membrane from racemic mixture of Glu or Phe. Contrary to adsorption selectivity toward L-Glu and L-Phe, D-Lys was preferentially incorporated into the chitin nanofiber membrane. The adsorption selectivity toward L-Glu was determined to be 1.14 and that toward L-Phe to be 1.30, respectively. The adsorption selectivity toward D-Lys was determined to be 1.14. Those adsorption selectivities determined by adopting slopes for adsorption isotherms coincided with those determined by adsorption studies, which are natural results. The inclination of adsorption selectivity observed in the present chitin nanofiber membrane artificially fabricated from powder sample by electrospray deposition was same as that prepared by a different way [10], though difference in value of adsorption selectivity was observed. j-enantiomer,

3.3. Chiral separation Permselectivity of membrane is often dependent on its membrane thickness. The thinner the membrane thickness, the higher the probability of the formation of membrane defect. In the study on multi-stage cascade membrane transport, each membrane used for the present study should give its intrinsic or identical permselectivity. To this end, relationship between membrane thickness and its permselectivity was studied. The results are summarized in Table 2. Permselectivity for each membrane was determined by membrane resistance measurement, which was proposed as a

facile method to predict permselectivity of racemic mixture of permeant with charge [17,18]. Permselectivities were also determined by membrane transport of racemic mixture to make sure of those determined by membrane resistance. Both permselectivities tended to give similar values and increased with the increase in membrane thickness asymptotically to an intrinsic value. The results led the conclusion that over the membrane thickness of 250 lm gives its intrinsic or identical permselectivity. From this, membranes with thickness of around 250 lm were hereafter used as membrane samples. Fig. 3 shows time-transport curves of racemic mixture of amino acids with five-stage cascade chitin nanofiber membranes. As observed in the optical resolution with natural chitin nanofiber membrane [10], D-isomers were transported in preference to the corresponding L-isomers through the present membranes. The permselectivity for the enantioselective separation of racemic mixture of Lys reflected its adsorption selectivity. Contrary to this, those for Glu and Phe were opposite to their adsorption selectivities. In other words, the D-isomers of Glu and Phe were selectively transported through the membranes though both L-Glu and L-Phe were selectively incorporated into the membrane. Such transport phenomena were often observed in chiral separation [19–26]. The discrepancy in adsorption selectivity and permselectivity for membrane transport of Glu and Phe can be explained as follows: a relatively strong interaction between the L-isomer of Glu or Phe and the membrane retarded the diffusion of the L-isomer in the membrane. As a result, the enantiomer less incorporated into the membrane was transported faster than that preferentially incorporated into the membrane. The permselectivity toward D-isomer for a given amino acid through the one-stage membrane separation is to be aD/L,1, that for n-stage cascade separation would be given by

aD=L;n ¼ ðaD=L;1 Þn

ð5Þ

or

logðaD=L;n Þ ¼ logðaD=L;1 Þ  n

ð6Þ

Fig. 4 shows relationship between permselectivities for the separation of racemic mixture of amino acids with three types of multi-stage cascade separation, such as one, three, and five stages, respectively, and the number of cascade (n). The plots of logarithm of aD/L,n vs. number of stage n gave a straight line, passing through origin. Following Eq. (6), the permselectivity for one stage chiral separation can be determined from the slope. The determined permselectivities were 1.10 for Glu, 1.16 for Phe, and 1.13 for Lys, respectively. Next, dependence of flux on number of cascade will be described. The relationship between flux and number of stage for membrane transport for three types of amino acid are shown in Fig. 5. The total flux, in other words, sum of that for D-enantiomer and L-enantiomer, is plotted against reciprocal of stage number n.

Table 1 Adsorption selectivity of chitin nanofiber membrane toward racemic mixture of amino acid. D-AA

AA D,L-Glu

D,L-Phe

D,L-Lys

a b

104 [AA] (mol dm3)

L-AA

(D-AA)/mem. (mol/g-mem.) 5

(D-AA)/CRUa (mol/mol) 3

(L-AA)/mem. (mol/g-mem.) 5

(L-AA)/CRUa (mol/mol)

SA(D/L)b

SA(L/D)b

2

1.0

4.50  10

5.15  10

1.04  10

0.90

1.12

2.0

8.94  105

1.81  102

1.12  104

2.05  102

0.88

1.14

1.0

1.23  105

2.50  103

1.60  105

3.26  103

0.77

1.31

2.0

2.59  105

5.25  103

3.38  105

6.85  103

0.77

1.31

1.0

2.39  105

4.86  103

2.12  105

4.31  103

1.13

0.89

2.0

4.87  105

9.88  103

4.29  105

8.70  103

1.14

0.88

CRU, constitutional repeating unit. SA(i/j) = ((i-AA)/(j-AA))/([i-AA]/[j-AA]); (i = D, j = L or i = L, j = D).

9.13  10

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Fig. 2. Adsorption isotherms of D-isomer and L-isomer of Glu (a), Phe (b), and Lys (c) in the chitin nanofiber membrane at 40 °C.

Table 2 Dependence of membrane performance on membrane thickness. Amino acid

Glu

Membrane

72

250

450

JD/mol cm cm2 h1 JL/mol cm cm2 h1

2.23  109 2.13  109 1.05 250 270 1.08

2.67  109 2.40  109 1.12 322 377 1.17

2.28  109 1.98  109 1.15 469 550 1.17

1.61  109 1.48  109 1.08 3.62  104 3.83  104–4.17  104 1.06–1.15

2.27  109 1.85  109 1.23 1.27  105 1.54  105 1.22

1.76  109 1.45  109 1.21 2.85  105 3.49  105 1.23

2.12  109 2.02  109 1.05 657 703 1.07

2.06  109 1.81  109 1.14 687 815 1.19

1.31  109 1.11  109 1.17 713 866 1.21

aD/L RD/O RL/O GD/GL* Phe

JD/mol cm cm2 h1 JL/mol cm cm2 h1

aD/L RD/O RL/O GD/GL* Lys

JD/mol cm cm2 h1 JL/mol cm cm2 h1

aD/L RD/O RL/O GD/GL* *

Membrane thickness (lm)

Performance

GD/GL = RL/RD.

Fig. 3. Time-transport curves of racemic mixture of Glu (a), Phe (b), and Lys (c) through five-stage cascade chitin nanofiber membranes at 40 °C. ([D-Lys]L,0 = [L-Lys]L,0 = 1.0  103 mol dm3.).

Fig. 4. Relationship between permselectivities for the separation of racemic mixture of Glu (a), Phe (b), and Lys (c), and three types of multi-stage cascade separation (number of cascade, n).

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Fig. 5. Relationship between flux and number of stage for membrane transport of Glu (a), Phe (b), and Lys (c).

The relationships give straight lines, passing through origin. Even though nanofiber membrane gives higher flux value than the corresponding usual dense membrane, infinite-stage cascade separation, as anticipated, leads to null throughput as shown in the figure. As demonstrated in the present article, even though the selectivity of a given membrane is not enough, higher permselectivity would be attained by adopting multi-stage cascade membrane separation. As described in the early part of the present article, Introduction, not only permselectivity but also flux (throughput) is important membrane performances. Reasonable but not so high flux could be attained by using nanofiber membrane, instead of usual dense membrane, as membrane for multi-stage cascade membrane separation. Composite or asymmetric membrane, of which thin active layer contributes to separation, can be also applicable alternatives as separation membranes for multi-stage cascade membrane separation. 4. Conclusions Chitin nanofiber membranes were fabricated via electrospray deposition. The nanofiber membrane adsorbed L-isomer from racemic mixture of Glu and Phe in preference to the corresponding antipodes, while D-Lys was preferentially incorporated into the membrane. The adsorption isotherms revealed that all enantiomers studied in the present study were incorporated into the nanofiber membrane without any specific interaction. In membrane transport experiments, D-isomers were preferentially transported through the nanofiber membrane. In order to attain higher permselectivity, a multi-stage cascade separation was applied in the present study. As expected, the permselectivity toward D-enantiomer was enhanced exponentially with the increase in number of stages from one to five stages via three by applying a multi-stage cascade membrane separation. References [1] W.S.W. Ho, K.K. Sirkar (Eds.), Membrane Handbook, Chapman & Hall, New York, 1992. [2] M. Mulder, Basic Principles of Membrane Technology, second ed., Kluwer Academic Publishers, Dordrecht, 1996. [3] R.W Baker, Membrane Technology and Applications, second ed., Wiley, West Sussex, 2004. [4] Y. Sueyoshi, C. Fukushima, M. Yoshikawa, Molecularly imprinted nanofiber membranes from cellulose acetate aimed for chiral separation, J. Membr. Sci. 357 (2010) 90–97.

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