Synthesis and Physical-Chemical Characterization of DEA ... · PDF fileglassy blocks confer a...

Contemporary Engineering Sciences, Vol. 10, 2017, no. 33, 1631 - 1641

HIKARI Ltd, www.m-hikari.com

https://doi.org/10.12988/ces.2017.711185

Synthesis and Physical-Chemical Characterization

of DEA-Sulfonated Styrene-Ethylene-Propylene-

Styrene (SEPS) Membranes for CO2 Separation

Acevedo Morantes María, Tapia Alejandra and Realpe Jiménez Álvaro

Department of Chemical Engineering, Research Group of Modeling of Particles

and Processes, University of Cartagena, Av. Consulado, Cartagena – Colombia

Copyright © 2017 Acevedo Morantes María et al. This article is distributed under the Creative

Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any

medium, provided the original work is properly cited.

Abstract

Styrene-ethylene-propylene-styrene (SEPS) block copolymer membranes for the

separation of CO2 were synthesized and modified by sulfonation in the presence of

diethanolamine (DEA) as a carrier molecule. To establish the influence on the

transportation of gases trough the membrane, the water absorption and the CO2

solution, and the porosity on the membrane permeability, the effect of

physicochemical properties such as water uptake content and porosity were

analyzed. The SEPS-membranes were characterized by Scanning Electron

Microscopy (SEM) to elucidate the effects of modifications on the surface

morphology. The membranes that were sulfonated in the presence of a carrier,

showed the highest values for water absorption and porosity.

Keyword- CO2 separation membranes, sulfonation, diethanolamine, carrier

molecule, permeability, surface morphology

1 Introduction

The world population growth and the global energy demand have been

accelerating in the recent century, along with the dependence on fossil fuels. This

source of supply provides 85-93% of all energy production [1]; process that releases

a large amount of greenhouse gasses, in particular carbon dioxide (CO2), which is

considered one of biggest contributor to global warming [2]. The techniques, used

1632 Acevedo Morantes María et al.

in different industrial sectors for CO2 capture, depend on the steps on which the

strategy occurs: oxy-combustion, pre-combustion or post-combustion [3, 4].

Among different approaches including the post-combustion process, cryogenic

fractionation, adsorption and chemical absorption, membranes are widely known

as promising procedures for CO2 capture [3].

Polymers are the most used materials for membranes because of their versatility

in the separation of gases, this ability depends on the correct balance between

selectivity and permeability. According to those characteristic, polymer membranes

can be classified as non-porous and porous. The permeability and the selectivity of

non-porous membranes are governed by the interactions between the gases through

the permeant and the polymer which is in agreement with the solution-diffusion

model. For porous membranes, the transport mechanism is determined by the

viscous flow and the molecular sieving effect [5]. Non-porous membranes have a

high selectivity, but a non-competitive permeability, while porous membranes

exhibit an elevated permeability, but the efficiency of separation is limited by a

substantial difference in the molecular size of the gas [6]. In order to improve

selectivity in the porous membranes, a facilitated transport mechanism have been

proposed by the incorporation of a “carrier” which interacts reversely only with the

target gas by a complexing reaction [1, 2, 7-9]. Amines are utilized in the chemical

absorption process due to their high reactivity with CO2, this convenient property

allows to consider them as a promising carrier in the membranes for gas separation

[4, 9]. In this work, diethanolamine (DEA) is used as carrier molecule due to its

higher perm-selectivity when compared to other alkanol amines in facilitating the

transport through the membranes [7].

Here, we propose the use of SEPS, a thermoplastic elastomer with good

processability properties, as the main material for membranes synthesis. SEPS has

glassy and rubbery polymer segments, the nature of the flexible chain structure and

high free volume in the rubbery segments allow a good permeability, whereas

glassy blocks confer a mechanical support and such phenomenon is associated with

the selectivity [10]. Hence, the sulfonation of styrenic blocks in the SEPS was

implemented. In addition, the sulfonic group attached on the para-positions of the

phenyl rings, promotes the formation of cavities through the polymer structure,

which increase the permeability.

2 Materials and Methods

2.1 Materials

The main material in the membranes, SEPS is a block copolymer produced

commercially by Kraton G1730, which was kindly supplied by this company. For

the sulfonation, the dichloromethano (DCM) purchased from Panreac was used as

a solvent of SEPS. The sulfonating agent, acetyl sulfate, was prepared by reaction of sulfuric acid (Chemi, 95-97%) and acetic anhydride (J.T. Baker; 99.99%) in DCM.

Synthesis and physical-chemical characterization … 1633

Methanol (Panreac) was employed to terminate the sulfonation reaction. The DEA

reagent (Panreac) and toluene (J.T. Baker; 99.97%) were used as a carrier molecule

and as a solvent, respectively, for casting process.

2.2 Membrane synthesis

The sulfonation process of SEPS was carried out over several steps, which have

been adapted from various procedures used by other authors. Firstly, acetyl sulfide

was prepared by reacting 1:1 mole ratio of acetic anhydride and sulfuric acid, just

prior to the sulfonation of SEPS so as to avoid disturbances on the quality [11, 12].

The required amount of acetic anhydride was added to 100 mL of DCM at 0°C.

After 10 min, sulfuric acid was added carefully. The solution, or sulfonating agent,

was stirred for another 10 min.

The sulfonating agent was added to a solution prepared by mixing 100 mL of

DCM and 5g SEPS under vigorous and continuous stirring during 30 min. The

sulfonation of the polymer was terminated by slow addition of methanol (100 mL)

[13]. Sulfonated SEPS (s-SEPS) was isolated by evaporating DCM at room

temperature and then was washed in deionized water until a neutral pH was reached.

This procedure was carried out rigorously to avoid the deterioration in the final

properties of the s-SEPS due to residual reagents [14]. The initial evaporation of

the solvent was needed for precipitating the polymer due to the moderate affinity

of materials highly sulfonated with methanol or deionized water [14]. Finally, the

polymer was completely dried at 50°C in a vacuum oven.

The porous membranes were fabricated by solution casting method. The dried

polymer was dissolved in 150 mL of toluene at 70°C, and an amount fixed of DEA

was added under strong stirring. The solution was concentrated for 30 minutes and

carefully poured into petri dishes. The membranes were placed at room

temperatures for a week and the residual solvent was evaporated under vacuum at

50°C [13].

2.3 Membrane characterization

Different sulfonation degree were analyzed, this factor was related with the time of

the sulfonation reaction (0, 2 and 3 h). The effect of DEA content (0, 15 and 20%

w/w) on the physical-chemical properties was also evaluated. The membranes were

washed with deionized water and dried under vacuum before each characterization

procedure [15].

1) Water uptake contents: it was measured by immersing samples with

dimensions of 1cm x1cm into deionized water during 24 and 48 h. The excess of

water was dried with an absorbent paper before weighing the immersed samples.

The previously method was replicated 3 times and the water uptake was obtained

by averaging the data calculated by the following relationship:


𝑤 =𝑤𝑤−𝑤𝑑

𝑤𝑑× 100 (1)

Where, 𝑤 is the water uptake content expressed in percentage, 𝑤𝑑 and 𝑤𝑤 are

the weight in grams of membrane samples before and after immersion in water,

respectively [13, 20].

2) Porosity: samples with dimensions of 2cm x 2cm were immersed during 1

h into distilled water at room temperature. The superficial excess of water was

eliminated with filter paper and the samples were placed in an oven at 80°C for 24

h. Samples were weighed in grams before (𝑄𝑜) and after (𝑄1) drying and those

values were replaced in the following equation:

𝑃(%) =𝑄0−𝑄1

𝐴ℎ× 1000 (2)

Where P is the membrane porosity (%), 𝐴 is the membrane surface area (cm2)

and ℎ is the membrane thickness (mm)

3) SEM: The morphology of the membrane surface was investigated using a

JEOL JSM-6490 scanning electron microscope.

4) Permeability and Selectivity: the permeability (PA) of nitrogen and CO2

through the membranes was determined by:

𝑃𝐴 =𝑁𝐴𝑙

(𝑝2−𝑝1) (3)

Where NA is the steady-state gas flux through the film [cm3 (STP) (cm2. s)-

1], l is the film thickness (cm), and p2 and p1 are the feed pressure and permeate

pressure (cmHg), respectively. The ideal selectivity (𝛼𝐶𝑂2 𝑁2⁄ ) for CO2 relative to

N2, was calculated by the ratio of permeabilities of the two components:

𝛼𝐶𝑂2 𝑁2⁄ =𝑃𝐶𝑂2

𝑃𝑁2 (4)

3. Results and discussion

3.1 Water uptake contents

The water absorbed in a membrane can improve the CO2 permeability; therefore,

materials possessing such ability may bring about interesting advantages to the

separation process through membranes. The water uptake content in SEPS

membranes after 48 hours of immersion can be observed in Figure 1.


Figure 1. Water uptake content in SEPS membranes.

In membranes with a sole modification, the water uptake percentage was directly

proportional to the amount of DEA and sulfonation hours. For instance, the sulfonic

groups affect the hydrophilicity of polymer [15] and the membrane absorbed a

greater quantity of water as sulfonation degree increases [16].

3.2 Porosity

The quantity of porous and voids through the membrane is attributed to the

porosity, which can improve the permeability of gases, this property was calculated

and showed in the Figure 2.

Figure 2. Porosity percentage in SEPS membranes.

The sulfonic groups in the para-position of the aromatic ring expand the distance

between polymer chains and promote the formation of empty spaces; hence, the porosity is amplified. As is shown in Figure 2, the porosity increased with the sulfo-

0

15

20

0

10

20

30

0 2

3

03,89 4,34

0,78

25,1423,03

6,28

1,25

23,39

DEA (%)

Wate

r u

pta

ke

con

ten

t (%

)

Sulfonation time (hour)

0

15

20

0

10

20

30

02

3

0,07 2,19 3,93

0,61

9,4013,45

3,43

29,00

8,46

DEA (%)

Poro

sity

(%

)

Sulfonation time (h)


nation time with a sole modification, similar to the trend presented with an

increment of carrier percentage.

3.3 SEM

The SEM images of SEPS modified are depicted in Figures 3, 4 and 5 in order

to evaluate the effect of sulfonation time and DEA percentage on the morphology

of cross-section and surface membrane. Membranes without carrier molecules,

showed in figures 14a, 15a and 16a, did not present perceptible pores on the surface

or cross-section, whereas the presence of imperfections in the surface texture raise

as the sulfonation time increased. An increment in the pore size can be observed in

Figure 3 when the amount of DEA increased. Non-sulfonated membranes with 15%

DEA (0S 15C) shown in Figure 3b, presented pores of diameter around 6.9 μm at

the surface; even so, the vast majority of all pores size was less than 0.5 μm. In the

cross-section of the 0S 15C membrane, the number of pores and their size

diminished, as the distance from de surface of membrane increased. As it seen in

Figure 3c, pores of diameter around 21 and 6.8 μm were observed at the surface

and cross-section of non-sulfonated membranes with 20% DEA (0S 20C),

respectively.

Figure 3. Surface and cross-section SEM micrographs of the non-sulfonated SEPS

membranes with (a) 0% DEA, (b) 15% DEA and (c) 20% DEA

The afore mentioned expansion of pores due to an increment in the amount of

DEA, was also observed in two-hour-sulfonated membranes, the mean diameter of

pores at the surface increased from 2.15 to 7 μm in membranes with 15% and 20%

of DEA, correspondingly, as can be seen in Figures 4b and 4c. In spite of the high

a b c

a b c


number of pores presented at the surface of 2 h sulfonated membrane with 15%

DEA (2S 15C), scarce pores were appreciated in the bulk of membrane. Similarly,

it was also detected for the comportment describe for 0S 15C membrane. In contrast

to the pores size at the surface for 2h-sulfonated membrane with 20% DEA (2S

20C), reduced diameter of pores around 0.6- 2.3 μm were homogeneously

distributed along the bulk.

Figure 4. Surface and cross-section SEM micrographs of the two-hour-sulfonated

SEPS membranes with (a) 0% DEA, (b) 15% DEA and (c) 20% DEA

In figure 5b and 5c, it can be seen that pores for 3h-sulfonated membranes with

15 and 20% DEA (3S 15C and 3S 20C respectively) had a similar size with diameter

around 6.5-2.7 μm, Nonetheless, pores were more connected and flattened in 3S

20C membranes.

The relation between the sulfonation time and the DEA percentage on the

morphology of the membranes, considering the SEM images, allowed us to suggest

that increasing of the sulfonic groups influence the formation of high number of

small pores, whereas an increment in the DEA percentage modified the size and

connection between pores, making them bigger and flattened, according to other

reports [17, 18].

a c b

a b c


Figure 5. Surface and cross-section SEM micrographs of the three-hour-

sulfonated SEPS membranes with (a) 0% DEA, (b) 15% DEA and (c) 20% DEA

3.4 Permeability and Selectivity

Permeation performance for CO2 against N2 for SEPS membranes is located above

the upper limit of Robeson [19] at 3 h sulfonated membrane (α= 42.8) and 2 h

sulfonated membranes at 15% DEA (α= 3.2) (Table 1). The permeance increased

with sulfonation degree of membrane was due to the reduction of pore size (See

Figure 5). The water uptake, distribution and size of pore favored the selectivity

with 2 h sulfonated membrane at 15% DEA, the smaller pores (lower than 2.3 μm) allowed the molecular size separation (dynamic diameter of CO2 and N2 is 0.33

nm and 0.364 nm, respectively [20]). Robeson plots for CO2/N2 is shown in Figure

6.

Table 1. Selectivity of SEPS membranes

Membrane Selectivity Membrane Selectivity Membrane Selectivity

SEPS 0S 0C 0.5 SEPS 2S 0C 2.1 SEPS 3S 0C 42.8



a c b

a b c

http://www.mdpi.com/2227-9717/4/3/29/htm#fig_body_display_processes-04-00029-f004


Figure 6. The Robeson plot relevant to porous polymeric membranes for a

CO2/N2 gas pair showing the data for three-hour-sulfonated SEPS membranes

with 0% DEA (1); two-hour-sulfonated SEPS membranes with 15% DEA (2),

20% (7) and 0% DEA (3); SEPS membrane with 20% DEA (4), 15% DEA (5)

and 0% DEA (6).

4. Conclusions

In this work the physicochemical properties related to selectivity and permeability

of gases that facilitates the transport through membranes, such as water absorption

and porosity, were analyzed. The ability regarding uptaking water and the porosity

had the same direct relation with the amount of DEA or sulfonation time. However,

it was seen a maximum value of DEA content and sulfonation time on water uptake

and porosity properties, respectively, in membranes with both modifications.

The SEM analysis suggests that the amount of sulfonic groups presented in

membranes, which is related with the sulfonation time, can promote the formation

or distribution of pores, while the DEA content increases the size of pores,

increasing the permeability and selectivity of SEPS membrane. The size and

distribution of pores were determinants in the exclusion molecular size between the

carbon dioxide and nitrogen [6].

Acknowledgments. The authors would like to acknowledge the University of

Cartagena for providing the recourses and the space to perform this project with

code No. 093-2015. They are also grateful to Kraton Corporation and

COLCIENCIAS, Colombia for supporting this research work.

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Received: December 3, 2017; Published: December 21, 2017

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