ORIGINAL PAPER

Development of an enantioselective membranefrom cellulose acetate propionate/cellulose acetate,for the separation of trans-stilbene oxide

Lucıa Z. Flores-Lopez • Jonathan Caloca •

Eduardo Rogel-Hernandez • Heriberto Espinoza-Gomez

Received: 26 September 2013 / Accepted: 28 March 2014 / Published online: 3 April 2014

� Springer Science+Business Media Dordrecht 2014

Abstract This paper reports the characterization of

new synthesized chiral polymeric membranes, based

on a cellulose acetate propionate polymer. The flux

and permselective properties of the membrane were

studied using 50 % ethanol solution of (R,S)-trans-

stilbene oxide as feed solution. Scanning electron

microscopy revealed the asymmetric structure of these

membranes. The roughness of the surface was mea-

sured by atomic force microscopy. The resolution of

over 97 % enantiomeric excess was achieved when

the enantioselective membrane was prepared with

18 wt% cellulose acetate and 8 wt% cellulose acetate

propionate in the casting solution of dimethyl form-

amide/N-methyl-2-pyrrolidone/acetone, at 20 �C and

55 % humidity, and a water bath at 10 �C for the

gelation of the membrane. The operating pressure and

the feed concentration of the trans-stilbene oxide were

275.57, 345.19, and 413.84 kPa and 2.6 mM,

respectively.

Keywords Enantioselective membrane �Chiral separation � Cellulose acetate propionate

membrane

Introduction

Chirality is a phenomenon which is of great impor-

tance to some biological and chemical processes, and

is a geometric property of non-identity of an object

with its mirror image. It is considered that more than

50 % of worldwide-approved drugs are chiral (Li et al.

1999; Wang et al. 2002, 2005). A stereoisomer that

possesses a chiral property is called an enantiomer.

Enantiomers possess identical physical and chemical

properties in an achiral environment but exhibit those

properties differently in a chiral environment, such as

biological systems (Wang et al. 2003). Generally, only

one enantiomer is useful for desired activities while

the other enantiomer does not perform the activities

but instead often inhibits the desired activities, gen-

erates side effects, or exhibits toxicity (Wang et al.

2002; Yokota et al. 2006).

The pharmaceutical industry is demanding effec-

tive techniques for the preparative separation of

enantiomers to proceed with pharmacological and

toxicological tests with the individual isomers (Ahuja

2000; Stinson 2001; Jacoby 2005; Rouhi 2005). For

this reason, the development of an effective method of

L. Z. Flores-Lopez (&) � J. Caloca

Centro de Graduados e Investigacion del Instituto

Tecnologico de Tijuana, Blvd. Industrial s/n,

C.P. 22520 Tijuana, Baja California, Mexico

e-mail: lzflores@hotmail.com

E. Rogel-Hernandez � H. Espinoza-Gomez

Facultad de Ciencias Quımicas e Ingenierıa, Universidad

Autonoma de Baja California, Calzada Universidad

14418, Parque Industrial Internacional,

C.P. 22390 Tijuana, Baja California, Mexico

123

Cellulose (2014) 21:1987–1995

DOI 10.1007/s10570-014-0252-0

separation of enantiomers has become of major

importance in a number of fields to obtain pure

enantiomers.

There are usually three alternative ways to get

isolated enantiomers: natural source, asymmetric

synthesis, and separation from racemic mixture. There

are many optical resolution methods for racemic

compounds; currently, enantioseparation of racemic

mixtures (Higuchi et al. 2002, 2003; Ward and

Hamburg 2004) is typically performed by column

chromatography (Okamoto et al. 1984), fractional

crystallization (Nohira et al. 1976) microbiological

methods, kinetic enzymatic resolution technology,

asymmetric catalysis, or stereoselective transforma-

tion (Jacques et al. 1981). Unfortunately these con-

ventional methods have many drawbacks. Therefore,

enantioselective membrane separation systems are a

viable option for large-scale application. This is

mainly because membrane-based enantiomeric sepa-

ration has several advantages over traditional meth-

ods, such as low energy and time saving, set-up

simplicity, large processing capacity and the possibil-

ity to be used in continuous mode (Maier et al. 2001;

Ulbricht 2006).

The chiral selector for membrane separations is a

molecule that preferentially interacts with one of the

isomers, and is usually integrated within the mem-

brane. Enantioseparation using membrane can be

divided into liquid and solid membranes (Yoshikawa

et al. 2006). However, for the separation of enantio-

mers, liquid membrane processes have low stability

and a short lifetime when tested under industrial

separation conditions (Kemperman et al. 1996).

Therefore, solid polymer membranes used in enantio-

separation, are mainly based on ultrafiltration mem-

branes (Higuchi et al. 2002; Masawaki et al. 1992) or

polymer-imprinting membranes (Yoshikawa and

Yonetani 2002).

The main difficulty in developing an optical

resolution membrane is that no enantioselective

membrane material has been found for industrial-

scale use. Although some investigations into optical

resolution membranes have been reported since the

1980s, most of the enantiomers separated by these

membranes were amino acids (Krieg et al. 2000;

Thoelen et al. 2001; Romero and Zydney 2002; Kim

et al. 2003; Hadik et al. 2005; Higuchi et al. 2005; Xie

et al. 2008; Hazarika 2008; Wang et al. 2009a; Svang-

Ariyaskul et al. 2009; Xiong et al. 2009). Up till now,

the articles published on enantioselective membranes

have reported percentage of enantiomeric excess (%

e.e.) and flows that may reach 98 % and 44.2 mg/m2h,

respectively (Wang et al. 2009b; Yang et al. 2009;

Zhao et al. 2009; Ma et al. 2011; Tian et al. 2012; Jiang

et al. 2012).

One of the most studied chiral compounds is the

trans-stilbene oxide. This enantiomer is an important

intermediate reagent for the synthesis of chiral com-

pounds (Fig. 1), such as drugs, pesticides, and chiral

selectors.

Historically, cellulose was the first material to be

used as a chromatographic chiral selector due to its

inherent chiral nature and ready availability. Cellulose

acetate (CA) was the first practical material used for

membrane preparation by phase inversion technology

in the 1960s and has also been widely used in reverse-

osmosis, nanofiltration, ultrafiltration, microfiltration,

etc., for environmental engineering, water softening

engineering and other fields.

Cellulose triacetate, therefore, is the first practical

stationary phase of polysaccharide (Okamoto and

Yashima 1998; Kim et al. 2003; Wang et al. 2007).

Various membrane configurations have already been

proposed for separating a large number of chiral

species, including aminoacids, drugs and their deriv-

atives (Kemperman et al. 1996; Higuchi et al. 2002;

Romero and Zydney 2002; Kim et al. 2003). Cellulose

acetate butyrate (CAB) possesses many asymmetric

carbon atoms in its molecular structure unit and for

this reason has been used in the enantioselective

membrane preparation for the resolution of (R,S)-2-

phenyl-1-propanol (Wang et al. 2009a).

Cellulose acetate propionate (CAP) is a cellulose

ester wherein hydroxyl groups of cellulose are substi-

tuted with acetyl and propionyl (Fig. 2); until now,

there have been no reports of its use in the synthesis of

O

Fig. 1 Molecular structure of trans-stilbene oxide

1988 Cellulose (2014) 21:1987–1995

123

enantiomeric membranes. In this study, the CA/CAP

membrane was prepared by the phase inversion

method and the enantioselective separation of racemic

trans-stilbene oxide was investigated.

Methodology

Materials

The CA (acetyl: 40 % content, MW = 30,000), and

cellulose acetate propionate (CAP; acetyl: 2.5 %,

hydroxyl: 2.6 %, propionyl: 45 %, Mn *25,000) were

obtained from Sigma-Aldrich, and trans-stilbene oxide

was obtained from Across (Belgium). All the reagents

were of analytical grade and were used without any

further purification. A mixture of pure water and

ethanol was used as the solvent for the feed solutions.

Enantioselective ultrafiltration of trans-stilbene

oxide

Filtration experiments were carried out in semibach

conditions, using a solvent resistant dead-end stirred

filtration cell (Millipore XFUF07601), holding a flat

sheet membrane with a diameter of 76 mm (Fig. 3).

The effective membrane area was 40 cm2.

The distilled water used in the preparation of the

feed solutions was prefiltered through a 0.2 lm

membrane. The feed solution was prepared as follows:

20 mg of the trans-stilbene oxide was dissolved in

40 mL mixed solvent (ethanol:water, 1:1), and stored

at 10 �C until use.

The filtration system was set up at 20 �C with a

pressure of 275.57, 345.19, or 413.84 kPa and a

rotation speed of 50 rpm. Constant pressure was

applied through nitrogen gas to apply the required

pressure. The operating pressure was controlled by

adjusting the regulator attached to a gas container.

Preparation of membrane and characterization

The membrane was cast by the so-called phase

inversion method from a solution consisting of CA/

CAP, dimethyl formamide (DMF), acetone, and N-

methyl-2-pyrrolidone (NMP). The composition of the

casting solution with different amounts of polymers is

reported in Fig. 4. All the components were stirred at

45 �C until a homogeneous solution was achieved.

The resulting solution was cast on the surface of a

glass plate using a casting knife to control the

thickness of the membrane to 0.18 mm (7 mills)

under conditions of 55 % humidity and a temperature

of 20 �C. After evaporating the membrane on the glass

O O

OR

OR

RO

RO

O O

OR

OR

RO

R

CH3

O

CH3

O

R= H or

n

or

Fig. 2 Molecular structures

of cellulose acetate

propionate

Fig. 3 Schematic representation of ultrafiltration cell for chiral

separation

Cellulose (2014) 21:1987–1995 1989

123

plate for 10 min, the nascent membrane was immersed

in a water coagulation bath at 10 �C for at least

30 min. The membrane was washed in pure water at

10 �C for 24 h to remove the DMF and acetone.

Finally, the membrane was prepared and stored in pure

water until use (Table 1).

Chiral separation through chiral CA/CAP

membrane

The enantioselective ultrafiltration was performed as

follows: 300 mL of the racemic feed solution were

introduced into the cell. The enantioselective ultrafil-

tration is carried out until there was a remaining of

50 mL of enantiomeric solution inside the cell. Then,

the cell was refilled with the racemic feed solution, and

the enantiomeric ultrafiltration process continues. This

procedure was performed throughout the experiment.

For the pressure-driven permeation experiments,

the permeate solution was sampled, the concentration

of trans-stilbene oxide was measured by HPLC and

the flux was calculated. The HPLC system was a

Perkin Elmer Advanced LC Sample Processor

ISS200V, Perkin Elmer UV–vis detector LC295,

Perkin Elmer Series 200 LC Pump. A personal

computer equipped with a Total Chrom Navigator

Series 200 for the LC system was used to process the

chromatographic data. The chiral analysis was per-

formed using a chiral column Kromasil 3-CelluCoat

(4.6 mm i.d. 150 mm) and a mixture of n-hexane/

isopropanol (90/10, v/v) in the mobile phase at 25 �C.

The detection was examined at 229 nm, and the flow

rate of the mobile phase was 1.0 mL/min. The flux of

racemic compounds was measured according to the

following equation:

fluxmg

m2h

� �¼ Q

Atð1Þ

where, Q is the quantity of the solute permeated (mg)

for a given time, t the permeation time (h), and A is the

effective membrane area (m2).

The percentage enantiomeric excess (% e.e.) of

permeates was calculated from the peak areas of their

two enantiomers, R-isomer (AR) and S-isomer (AS).

The equation is as follows:

e:e: ð%Þ ¼ 100AR � AS

AR þ AS

ð2Þ

The separation factors is the peak area ratio of the R

to S-isomer in the permeation as follows:

Separation factor ð%Þ ¼ AR

AS

ð3Þ

All membranes were used only once; however, the

membrane having the best separation factor was used

0

20

40

60

80

100

0

100

200

300

400

500

% e

e

Ave

rage

mem

bran

e ro

ughn

ess

(nm

)

roughness

% ee

0

20

40

60

80

100

0

100

200

300

400

500

% e

e

Ave

rage

mem

bran

e ro

ughn

ess

(nm

)

roughness

% ee

0

20

40

60

80

100

0

100

200

300

400

500

JC1 JC2 JC3

JC4 JC5 JC6

JC7 JC8 JC9

% e

e

Ave

rage

mem

bran

e ro

ughn

ess

(nm

) roughness

% ee

(a)

(b)

(c)

Fig. 4 Effect of the membrane casting composition on the

average membrane roughness and % e.e. a 0 % CAP (constant)

and 18 % CA (JC1), 22 % CA (JC2), 26 % CA (JC3); b 4 %

CAP (constant) and 14 % CA (JC4), 18 % CA (JC5), 22 % CA

(JC6); c 8 % CAP (constant) and 10 % CA (JC7), 14 % CA

(JC8), 18 % CA (JC9)

1990 Cellulose (2014) 21:1987–1995

123

to determine the fouling-recovery membrane

relationship.

Atomic force microscopy (AFM) of CA/CAP

chiral membranes

In order to preserve the original dimensions of the pore

and the porous structure of the membrane, the

remaining water in the membrane was removed by a

‘‘Solvent Exchange’’ process which was carried out in

the following manner (Espinoza-Gomez and Lin

2001). The wet membrane coupon was first soaked

in pure isopropyl alcohol for 30 min; after that, the

membrane coupon was subsequently soaked for

30 min in each isopropyl alcohol/hexane solution

(75:25, 50:50 and 25:75). Finally, the membrane was

soaked in 100 % hexane for 30 min. The hexane

within the membrane was vacuum-dried. Sample

membranes to be examined by SEM were cut out

and fractured in liquid nitrogen. The dried fractured

membrane samples were sputter-coated with gold, and

then the cross-sectional scanning electron micrograph

of each membrane was recorded.

The surface morphologies of the dried membranes

were characterized by contact mode with an Agilent

5100 AFM, equipped with a non-contact/contact head

and a 100 lm scanner, which was operated at a

constant force mode (reference force 5 nN). The wet

membrane coupons were attached to a platinum

sample holder that was mounted on the piezo scanner

of the AFM. AFM images were acquired at a scan rate

of 1.0–2.0 kHz and an information density of

256 9 256 pixels (area 25 lm2). The mean height is

given by the average of the individual height deter-

minations within the selected height profile.

Results and discussion

Effect of the membrane casting solution

composition on the chiral separation of trans-

stilbene oxide

The AFM analysis shows that the membrane surface

roughness depends to a large extent on the composi-

tion of the membrane casting solution, all the JC

membranes exhibited similar surface morphology

consisting of a rough or mottled surface with well-

defined holes or shallow depressions resembling pores

or channels, respectively. According to the results

obtained by AFM, the membrane casting composition

strongly influences the structure of the asymmetric

membrane, these results are consistent with other

authors observations (Espinoza-Gomez and Lin 2001;

Xie et al. 2008; Jiang et al. 2012).

The effect of the membrane casting solution

compositions on the membrane surface roughness

and % e.e., are shown in Fig. 4.

As can be seen in Fig. 4, by increasing the

weight percentage of CA and keeping constant the

concentration of CAP (Fig. 4a), the membrane

surface becomes rougher, for example membrane

JC1 (18 % CA and 0 % CAP) exhibit an elevation

of 120.3 nm, with a 62 % e.e.; while JC3 (26 % CA

and 0 % CAP) the elevation is 489.9 nm, and a

71 % e.e. These increases in roughness and % e.e.

are attributed to the increase in the concentration of

CA in the casting solution, due the CAP concen-

tration remains constant. By another hand, the

membranes JC4 (14 % CA and 4 % CAP), JC5

(18 % CA and 4 % CAP), and JC6 (22 % CA and

4 % CAP), shows a different membrane surface

roughness, but almost the same % e.e. This can be

attributed to the present of the CAP polymer in the

casting membrane solution (Fig. 4b).

The increases in the roughness of the membrane

surface and the % e.e. are also observed when the

content of CAP increases in the casting solution and

CA concentration remains constant, for example JC1

(18 % CA and 0 % CAP) exhibit an elevation of

120.3 nm, with a 62 % e.e.; JC5 (18 % CA and 4 %

CAP) has an elevation of 307.6 nm and a 89 % e.e.,

but for JC9 (18 % CA and 8 % CAP) the elevation is

352.4 nm (Fig. 5). In these cases, there is an increase

in the solids content in the casting solution, but the

increase in the % e.e. are attributed to the increase in

the content of CAP in the casting solution, due the CA

concentrations remains constant.

In both cases, if the content of polymer in the

casting solution increases, the % e.e. of the membrane

also increases. However, the effect of CAP content on

the % e.e. is higher, compared to the effect of CA

content. When the CAP concentration increased from

0 to 8 wt%, the enantioselectivity for the R-enantio-

mer in permeate increased from 60 to 98 %.

The enantiomer separation of trans-stilbene oxide,

using CA/CAP membranes prepared with different

membrane casting solution composition, and filtered

Cellulose (2014) 21:1987–1995 1991

123

through 120 h, can be seen in Fig. 6. It can be seen that

the % e.e. remains practically constant.

Figure 7 shows the solute permeated (Q) and

permeate enantiomeric excess (% e.e.), with a constant

CA content (18 wt%) in the membrane casting

solution. When the CAP concentration increased from

0 to 8 wt%, the solute permeated (Q) through the

membrane decreased. Whereas a high enantioselec-

tivity (97 % for R-enantiomer) was obtained for

8 wt% CAP membrane, a low enantioselectivity was

observed for 0 wt% (60 %). It is possible to observe

that the enantiomeric excess (% e.e.) in permeate, did

not change with the time operation of the filtration

system. The reason is the tighter membrane structure

for a higher CAP concentration.

On another hand, Fig. 8 shows the effect of the

content of CA in the membrane casting solution,

keeping the CAP content constant. The amount of

permeate solute decreases with the increase in the total

solid content in the solution cast membranes. More-

over, the % e.e. in permeate increases with the

increasing percentage of CA in the casting solution.

Figure 9 shows the separation factor for JC7-JC9. As

can be seen in Fig. 9, the separation factors were

higher than others when the CA/CAP concentrations

were 18/8 %. The reason is because in the membrane

with high polymer content, the structure of membrane

became more compact increasing the diffusion selec-

tivity. Whereas, the structure of membrane became

compact, resulting in less flux, also influencing the

diffusion selectivity.

Table 1 Compositions of CA/CAP membrane casting

solutions

Membrane CA wt% CAP wt% Solvent wt%

JC1 18 0 82

JC2 22 0 78

JC3 26 0 74

JC4 14 4 82

JC5 18 4 78

JC6 22 4 74

JC7 10 8 82

JC8 14 8 78

JC9 18 8 74

0

20

40

60

80

100

0

100

200

300

400

500

JC1 JC5 JC9

% e

e

Ave

rage

mem

bran

e ro

ughn

ess

(nm

)roughness

% ee

Fig. 5 Effect of the increase in the concentration of CAP in the

casting solution on the roughness and % e.e. Membrane casting

solution composition: 18 % CA (constant), and 0 % (JC1), 4 %

(JC5), 8 % (JC9)

50

60

70

80

90

100

0 24 48 72 96 120

% e

e

time, h

JC1 JC2 JC4

JC4 JC5 JC6

JC7 JC8 JC9

8 %

CA

P

4% C

AP

0 %

CA

P

Fig. 6 % e.e. in the enantioseparation of trans-stilbene oxide

racemates through CA/CAP membrane prepared at differ-

ent membrane casting compositions. Operating pressure:

275.57 kPa; feed concentration 2.6 mM; T = 25 �C; 50 rpm

0123456789

50

60

70

80

90

100

0 24 48 72 96 120

Q/m

g

% e

e

time, h

% ee JC1 % ee JC5 % ee JC9

Q JC1 Q JC5 Q JC9

Fig. 7 Quantity of the solute permeated (Q) and percentage

enantiomeric excess (% e.e.) in enantioseparation of trans-

stilbene oxide through CA/CAP membranes fabricated with

18 wt% CA and 0, 4, and 8 wt% CAP. Operating pressure,

275.57 kPa; feed concentration 2.6 mM; T = 25 �C; 50 rpm

1992 Cellulose (2014) 21:1987–1995

123

The effect of the polymer content in the membrane

casting solution on the enantioseparation, is because

the polymers used as membranes materials contains a

large amount of chirally active carbons on the

backbone structure, these active carbons could form

a helical structure (Okamoto and Yashima 1998; Xie

et al. 2008). The helical structure will form chirally

active small spaces in its main chain backbone

structure, and their assembly will be possible to form

certain larger chiral spaces in the membrane (Kim

et al. 2003; Xie et al. 2008). By another hand, the steric

fit of the enantiomers in the chiral conformation space

of the membrane results in the chiral recognition, and

of dispersion, dipole–dipole, and hydrogen-bond

interactions with the C=O in the CAP (Okamoto and

Yashima 1998; Kim et al. 2003; Xie et al. 2008).

In order to demonstrate the efficiency of the

process, and taking into account the % e.e., the JC9

membrane was selected to study the fouling-recovery

relationship of the membrane. For this study, the

membrane was used for a period of 120 h, and then it

was removed from the cell, rinsed with deionized

water several times and placed back into the cell by

another 120 h. This procedure was performed on two

occasions. The membrane accumulated a total of

360 h of operation.

The results of this study are shown in Fig. 10. When

the membrane is used for the second time, presents an

initial % e.e. (12 h) of 96.5, and Q of 1.1 mg, at the

end of this session (120 h) the % e.e. is 92 with Q of

4.25 mg. When the membrane is used for the third

time, the initial value of % e.e. (12 h) is 94, while the

Q is 0.9 mg; the final value of the % e.e. is 92 % and Q

of 1.85 mg. The membrane comprises 360 accumu-

lates operating hours, and the % e.e. do not vary

significantly.

Effect of operating pressure on properties

of membrane

JC9 membrane showed the highest % e.e. and

separation factor for the R-enantiomer, and was used

to determine the effect of the filtration pressure in the

system. Figure 11 shows the behavior under different

operation pressures and the effect of the pressure in the

Q and % e.e. through CA/CAP membranes. It could be

seen that there was an increase in flux and a decrease in

percentage enantiomeric excess while operating

pressure increased from 275.57 to 413.84 kPa. This

behavior came from the interaction between the

membrane and the solutes. With an increase in

operating pressure, the movement of solution accel-

erated, leading to decreasing diffusion selectivity and

sorption selectivity. High enantioselectivity and mod-

erate flux were able to be obtained for the 413.84 kPa

0

1

2

3

4

5

6

60

65

70

75

80

85

90

95

100

0 24 48 72 96 120

Q/ m

g

% e

e

time, h

% ee JC7

% ee JC8

% ee JC9

Q JC7

Q JC8

Q JC9

% e

eQ

Fig. 8 Quantity of the solute permeated (Q) and percentage

enantiomeric excess (% e.e.) in enantioseparation of trans-

stilbene oxide through CA/CAP membranes fabricated with

8 wt% CAP and 10, 14, and 18 wt% CA. Operating pressure,

275.57 kPa; feed concentration 2.6 mM; T = 25 �C; 50 rpm

0102030405060708090

0 24 48 72 96 120

Sepa

rati

on f

acto

r (%

)

Time (h)

JC7 JC8 JC9

Fig. 9 The separation factors in the enantioseparation of trans-

stilbene oxide through CA/CAP membranes. Operating

pressure: 275.57 kPa; feed concentration 2.6 mM; T = 25 �C;

50 rpm

0

1

2

3

4

5

6

90

95

100

0 24 48 72 96 120

Q/ m

g

% e

e

time, h

1st 2nd 3rt

% e

eQ

Fig. 10 Efficiency of JC9 membrane after three cycles of

operation of 120 h each one. Operating pressure, 275.57 kPa;

feed concentration 2.6 mM; T = 25 �C; 50 rpm

Cellulose (2014) 21:1987–1995 1993

123

of operating pressure in this study; however the best

pressure was 275.57 kPa (Fig. 11).

Conclusions

The present work reports for the first time the

elaboration of new chiral polymeric membranes,

based on a cellulose acetate propionate polymer. The

enantioseparation of trans-stilbene oxide is possible

through membranes of CA/CAP, using a pressurized

system.

The enantioseparation is due to the large amount of

active asymmetric carbons in the structure of CA and

CAP. The properties of the membrane can be influ-

enced by changing the CA/CAP proportions in the

casting solution and operating pressure. It was

observed that the increase in CA/CAP content in the

membrane casting solution, increases the % e.e., and

the separation factor, but decreases the amount of

solute permeates.

The structure of membrane with high polymer

content became more compact, resulting in less flux

and increasing the diffusion selectivity.

However, the effect of CAP content on the % e.e. is

higher compared to the effect of CA content. The JC9

membrane (18 wt% CA, 8 wt% CAP), showed the

highest enantioselectivity (97 %) for the R-enantiomer

at 120 h of operation, this can be attributed to the

helical structure which is formed with chirally active

carbon atoms possessed by the polymers used in the

synthesis of membranes, and of dispersion, dipole–

dipole, and hydrogen-bond interactions with the C=O.

After three cycles of operation of 120 h each one

(360 h), the efficiency of JC9 membrane was 92 % of

e.e.

The best operating pressure was 275.57 kPa, with a

96 % of e.e. and the solute permeated was 5 mg in

120 h of operation. The enantioselective CA/CAP

membranes are very useful for the chiral separation of

trans-stilbene oxide.

We think this research work provides a small

advance to the area of separation of chiral compounds,

using enantioselective membranes with good chiral

resolution and also having good physical properties.

Acknowledgments We gratefully acknowledge support for

this research work by Direccion General de Educacion Superior

Tecnologica (DGEST Grant 4374.11-P). Also, to Ing. Israel

Gradilla Marti9nez from Centro de Nanociencias y

Nanotecnologıa de la UNAM (CNyN) for SEM analysis, and

M.C. Pedro Navarro-Vega from Centro de Graduados e

Investigacion del Instituto Tecnologico de Tijuana (CGI-ITT)

for AFM analysis.

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0

1

2

3

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0

20

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0 24 48 72 96 120

Q/ m

g

% e

e

time, h

% ee 275.57 kPa% ee 345.19 kPa

% ee 413.84 kPa

Q 275.57 kPaQ 345.19 kPaQ 413.84 kPa

% e

eQ

Fig. 11 Effect of the operation pressure on the Q and % e.e. in

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