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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:
1634 Acevedo Morantes María et al.
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.
Synthesis and physical-chemical characterization … 1635
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)
1636 Acevedo Morantes María et al.
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
Synthesis and physical-chemical characterization … 1637
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
1638 Acevedo Morantes María et al.
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
SEPS 0S 15C 0.6 SEPS 2S 15C 3.2 SEPS 3S 15C 0.0
SEPS 0S 20C 0.8 SEPS 2S 20C 0.2 SEPS 3S 20C 0.0
a c b
a b c
Synthesis and physical-chemical characterization … 1639
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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