Chinese Journal of Chemical Engineering, 19(5) 821—832 (2011)
An Effective Method to Improve the Performance of Fixed Carrier
Membrane via Incorporation of CO2-selective Adsorptive Silica
Nanoparticles*
YU Xingwei (于型伟)1,2,3, WANG Zhi (王志)1,2,3,**, ZHAO Juan (赵娟)1,2,3, YUAN Fang
(袁芳)1,2,3, LI Shichun (李诗纯)1,2,3, WANG Jixiao (王纪孝)1,2,3 and WANG Shichang (王世昌)1,3
1
2
3
Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University,
Tianjin 300072, China
State Key Laboratory of Chemical Engineering, Tianjin University, Tianjin 300072, China
Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300072, China
Abstract Fixed carrier membrane exhibits attractive CO2 permeance and selectivity due to its transport mechanism of reaction selectivity (facilitated transport). However, its performance needs improvement to meet cost targets
for CO2 capture. This study attempts to develop membranes with multiple permselective mechanisms in order to
enhance CO2 separation performance of fixed carrier membrane. In this study, a novel membrane with multiple
permselective mechanisms of solubility selectivity and reaction selectivity was developed by incorporating
CO2-selective adsorptive silica nanoparticles in situ into the tertiary amine containing polyamide membrane formed
by interfacial polymerization (IP). Various techniques were employed to characterize the polyamide and polyamide-silica composite membranes. The TGA result shows that nanocomposite membranes exhibit superior thermal
stability than pure polyamide membranes. In addition, gas permeation experiments show that both nanocomposite
membranes have larger CO2 permeance than pure polyamide membranes. The enhanced CO2/N2 separation performance for nanocomposite membranes is mainly due to the thin film thickness, and multiple permselective
mechanisms of solubility selectivity and reaction selectivity.
Keywords carbon dioxide, fixed carrier, interfacial polymerization, tertiary amine, CO2-selective adsorptive silica
1
INTRODUCTION
The greenhouse effect, caused by excessive
emissions of carbon dioxide (CO2), has become a serious environmental problem in the world. The CO2
capture and storage (CCS) is considered as an effective method for reducing CO2 emissions and has great
promise in the near future from both technological and
economic viewpoints [1, 2]. Lowering the cost for CO2
capture is very important because the initial separation
of CO2 may account for 60%-80% of the total cost of
CO2 sequestration [3, 4]. Compared with conventional
CO2 capture processes such as amine scrubbing,
membrane technology for CO2 capture has been attracting more and more attention due to its low capital
and operating costs, low energy requirement and generally ease of operation [5, 6]. One promising means
for lowering the cost of CO2 capture is to develop
high-performance CO2 separation membranes for CO2
recovery from the flue gas [7].
It is well known that fixed carrier membranes are
very effective in simultaneously improving gas permeability and selectivity due to its transport mechanism of reaction selectivity (facilitated transport).
Most fixed carrier membranes reported in literature
contain amine moieties as the CO2-reactive functional
groups [8-11]. The tertiary amine containing membrane shows a promising future in CO2 capture due to
its larger CO2 sorption capacity [12], higher catalysis
efficiency for CO2 hydration [13], and more stability in
air [14] than primary or secondary amine containing
membrane. Therefore, the study on introducing tertiary amine groups into the polymer backbone may be
more attractive.
The development of thin-film composite (TFC)
membrane is a major breakthrough in the field of membrane science and technology [15, 16]. TFC membrane is
characterized by an ultra-thin selective barrier layer
laminated on a chemically different asymmetric porous
substrate. The selective layer is the key component, which
mainly controls the separation properties of the membrane, while the porous substrate gives the necessary
mechanical strength [16]. Interfacial polymerization
(IP), firstly reported by Morgan and Kwolek [17], has
become a well-established synthesis method for the
preparation of TFC membranes. The IP reaction is
self-inhibiting through a limited supply of reactants to
the already formed layer, resulting in an extremely
thin film down to 50 nm in thickness [18]. Therefore,
interfacially polymerized membranes can offer high
permeance. Since a thin film is formed nearly instantaneously at the interface between the two phases, the
IP process may lack strict requirement for reactant
Received 2011-06-10, accepted 2011-07-21.
* Supported by the National Natural Science Foundation of China (20836006), the National Basic Research Program
(2009CB623405), the Science & Technology Pillar Program of Tianjin (10ZCKFSH01700), the Programme of Introducing
Talents of Discipline to Universities (B06006), and the Cheung Kong Scholar Program for Innovative Teams of the Ministry
of Education (IRT0641).
** To whom correspondence should be addressed. E-mail: [email protected]
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Chin. J. Chem. Eng., Vol. 19, No. 5, October 2011
purity. Further, the IP technique is easy to control and
can be scaled up to industrial scale. Based on the
above consideration, we have developed a novel tertiary amino containing thin film composite membrane
for CO2 capture via the interfacial polymerization
of water-soluble 3,3’-diamino-N-methyldipropylamine
(DNMDAm) and hexane-soluble trimesoyl chloride
(TMC) on the polysulfone (PS) support membrane
[19]. The DNMDAm-TMC/PS membrane shows good
CO2 permeance and CO2/N2 selectivity, but its performance needs to be improved to meet cost targets
for CO2 capture.
Nanocomposite is a viable route to improve
membrane performance. Polymer membranes doped
with inorganic nanoparticles, such as silica particles
[20-25], can enhance gas permeability and selectivity.
Moaddeb and Koros [20] studied the gas transport
properties of thin polyimide membranes in the presence of silica particles and found that the presence of
silica improved the gas separation performance of the
membrane, particularly for O2 and N2. Kim and Lee
[22] investigated the effect of the incorporation of silica particles in poly(amide-6-b-ethylene oxide) (PEBAX) and realized that silica had a significant influence on the membrane morphology, and both the gas
permeability and solubility were increased by 2, 3
times in composite membranes. Merkel et al. [23, 24]
reported that the addition of nano-sized impermeable
particles of commercial fumed silica to poly(4-methyl2-pentyne) (PMP) enhanced the gas permeability and
attributed this behavior to the increase in the free
volumes without introducing large cavities. Recently,
Sadeghi et al. [25] studied the effect of silica particles
on polybenzimidazole (PBI) and found that the incorporation of silica in the membrane resulted in a 4.5
times increase in permeability of CO2 and 20 times
increase in selectivity of CO2/N2. It was pointed out
that the increased CO2 permeability and CO2/N2 selectivity were mainly attributable to the increase in CO2
solubility. Similar opinion could be found in other
work for silica nanoparticles-polymer composite
membranes, such as silica-polyimide [21], silicabrominated poly(phenylene oxide) (BPPO) [26], and
silica-ethylene viny acetate (EVA) [27].
However, to the best of our knowledge, there is
no report on silica-fixed carrier composite membranes.
More importantly, it seems that there is still lacking
effective strategy to improve the performance of fixed
carrier membranes. Only a few studies were focused
on improving the performance of fixed carrier membranes by incorporation of mobile carriers (such as
amino acid salts and potassium hydroxide) in the
membranes [28, 29]. It is expected that if CO2-selective
Table 1
adsorptive silica nanoparticles are incorporated into
the fixed carrier membrane, the silica-fixed carrier
composite membrane will present more attractive
performance for CO2 capture due to its multiple
permselective mechanisms of solubility selectivity and
reaction selectivity.
Herein, we develop a novel nanocomposite fixed
carrier membrane with multiple permselective mechanisms of solubility selectivity and reaction selectivity
for CO2 capture using IP technique. Our goal is to
supply an effective approach to improve the performance of fixed carrier membranes. Thus, CO2-selective
adsorptive silica nanoparticles are incorporated into
tertiary amino containing fixed carrier membranes. In
order to evaluate the effects of nanoparticles on film
formation, two types of silica nanoparticles, LUDOX®
HS-30 and Cabosil TS-530, are dispersed in aqueous
phase and in organic phase during IP process, respectively. The influences of the particles on the membrane structure, thermal stability of membrane and
separation of CO2/N2 mixed gas are explored.
2
2.1
EXPERIMENTAL
Materials
Colloidal silica [LUDOX® HS-30, 30% (by mass)
suspension in water] was provided by Sigma-Aldrich.
Fumed silica (Cabosil TS-530) was kindly supplied by
Cabot Corporation (Tuscola, IL). The fumed silica is
hydrophobic because the surface hydroxyl groups are
replaced with trimethylsilyl groups. Their properties
obtained from the suppliers are given in Table 1.
The polysulfone (PS) ultrafiltration membranes
with an average cut-off molecular mass of 6000 were
supplied by Vontron Technology Co., Ltd. (China).
3,3′-diamino-N-methyldipropylamine
(DNMDAm,
purity ≥98%) purchased from Aladdin Reagent Co.
(China) was used as an active monomer of aqueous
phase. Trimesoyl chloride (TMC, purity >99.5%)
obtained from Qingdao Sanli Chemical Engineering
Technology Co. Ltd. (China) was used as an active
monomer of organic phase. Na2CO3 as the acid
acceptor and hexane as the organic phase solvent were
from Reagent Chemical Engineering Technology
Co. (China). Sodium dodecylsulfonated (SDS) as
surfactant was of chemical grade and obtained from
Shanghai Yingpeng Additive Chemical Engineering
Co. Ltd. (China). All chemicals were used without any
further purification. For the preparation of aqueous
solutions, pure water with conductivity less than 10
μS·cm−1 was used.
Silica properties from the suppliers
Supplier
Description
Density/g·cm−3
Surface area/m2·g−1
LUDOX® HS-30
Aldrich
colloidal silica
1.21
~220
—
TS-530
Cabot Corp.
fumed silica
2.2
225
trimethylsilyl
Surface functional group
Chin. J. Chem. Eng., Vol. 19, No. 5, October 2011
2.2
Nanoparticle characterization
The dry LUDOX silica nanoparticles were obtained by evaporating LUDOX solution in a hot-air
circulating oven at 100 °C for several days until no
solvent (water) was left and the resulting nanoparticles
were stored in vacuum at 100 °C for tests. The surface
areas of the silica nanoparticles were determined in
this work by BET (Tristar 3000), while the surface
functional groups were characterized by Fourier
transform infrared (FTIR, FTS-6000). The sizes of
nanoparticles were estimated by transmission electron
microscopy (TEM, Tecnai G2 F20).
CO2 and N2 with purity higher than 99.99% were
used as adsorptives to characterize the sorption properties of nanoparticles. Single-component adsorption
isotherms of CO2 and N2 on silica nanoparticles were
collected on a typical volumetric apparatus adapted to
high-pressure adsorption studies. The working principle and details of the apparatus were presented previously [30]. The sorption isotherms were obtained by
recording the mass uptake at each gas pressure equilibrium. The equilibrium condition was reached in
about 1 h. The adsorption studies were conducted at
25 °C and up to a pressure of 1.7 MPa.
2.3
Membrane preparation
The membranes were prepared by conventional
interfacial polymerization technique, as described
previously [5]. For the preparation of (DNMDAmTMC)/PS composite membrane, the PS support membrane taped to a glass plate was firstly placed in the
aqueous solution containing 0.0615 mol·L−1 DNMDAm,
0.038 mol·L−1 Na2CO3 and 0.0017 mol·L−1 SDS for 10
min at 25 °C. The DNMDAm saturated PS membrane
was then immersed into the hexane solution of 0.01
mol·L−1 TMC for 3 min at 25 °C. After that, the resulting composite membrane was rinsed with pure
hexane and then heat-treated in a hot-air circulating
oven at 70 °C for 12 min. Finally, the composite
membrane was thoroughly washed with reverse osmosis (RO) deionized water to eliminate excess amine
and byproducts. The resulted membrane was kept for
12 h at room temperature. (LUDOX silica-DNMDAmTMC)/PS nanocomposite membranes were made by
dispersing 0.018%-1.089% (mass/volume ratio)
LUDOX silica nanoparticles in the water-DNMDAm
solution prior to interfacial polymerization, and then
followed the same procedure for (DNMDAm-TMC)/PS
composite membrane. For the preparation of (fumed
silica-DNMDAm-TMC)/PS nanocomposite membranes,
0.018%-1.089% (mass/volume ratio) fumed silica
nanoparticles were dispersed in the hexane-TMC solution prior to interfacial polymerization, and then
followed the same procedure for (DNMDAm-TMC)/PS
composite membrane. Nanoparticles dispersion were
achieved by initially stirring with a magnetic stirrer
for 1 h, followed by ultrasonic treatment at room
823
temperature for 1 h, resulting in clear homogeneous
solution. The membrane series made from LUDOX
silica were designated as “LU series” whereas the
membrane series made from fumed silica were designated as “FS series”.
2.4
Membrane characterization
2.4.1 Chemical composition analysis
X-ray photoelectron spectroscopy (XPS, PHI-1600)
and energy dispersive X-ray (EDX) spectroscopy
(equipped with the SEM, see below) were employed
to determine qualitatively the silica content of membranes. The chemical composition of the membrane
surfaces was characterized by attenuated total reflectance infrared (ATR-FTIR) spectroscopy (FTS-6000).
2.4.2 Morphology analysis
The surfaces and cross-section morphologies of
membranes were observed with scanning electron microscopy (SEM, FEI Nanosem 430). Membrane samples were prepared for cross-section observation by
peeling away the polyester backing fabric gently to
ensure polysulfone and polyamide layers remained
together. Wet fabric free membrane samples were
broken in the liquid nitrogen before being sputtered
with gold. Film thickness was visually characterized
from SEM images using drawing tool in MS Word.
Film thickness is reported as the mean values
(±standard deviations).
2.4.3 Thermal stability measurement
Thermal stability of membranes was examined
using a Pyris thermogravimetric analyzer (TGA,
PerkinElmer). Membrane samples were prepared by
peeling away the polyester backing fabric. The fabric-free polyamide-polysulfone composite material was
kept in dichloromethane overnight to completely dissolve the polysulfone portion and the undissolved portion (polyamide or polyamide nanocomposites) was
separated by filtration and dried for TGA analysis.
About 10 mg of the dry samples were taken and heated
from 30 to 800 °C at a heating rate of 10 °C·min−1.
2.4.4 CO2/N2 separation study
The gas separation performance of the membranes was evaluated by using CO2/N2 mixed gases
[20% (by volume) CO2 and 80% (by volume) N2]. The
procedure and apparatus were the same as those described previously [19]. The effective area of the
membrane located at test cell was 19.26 cm2. The feed
gas pressures ranged from 0.1 to 1.5 MPa, while the
downstream pressure was maintained at atmosphere
pressure. Before contacting the membrane, both feed
gas and sweep gas (H2) were sufficiently humidified
by moisteners. The composition of outlet gases was
analyzed by a gas chromatograph equipped with a
thermal conductivity detector (HP4890, Porapak N).
The fluxes of CO2 ( N CO2 ) and N2 ( N N 2 ) were calculated
from the sweep gas flow rate and its composition. The
permeance (Ri) is defined as the flux (Ni) divided by
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Chin. J. Chem. Eng., Vol. 19, No. 5, October 2011
the partial pressure difference (ΔPi) between the upstream and downstream sides of the membrane, Ri =
Ni/ΔPi, and the selectivity is given by aij = Ri/Rj. The
gas permeance is customarily expressed in the unit of
cm3 (STP)·(cm2·s·cmHg)−1 [1 cm3 (STP)·
(cm2·s·cmHg)−1 = 3.35×10−4 mol·(m2·s·Pa)−1]. At the
beginning of each experiment, the test system was run
at low feed pressure (0.11 or 0.12 MPa) for at least 2 h.
All the data were read after at least 1 h for stabilization under selected conditions. The sweep gas flow
rate and its composition under each condition were the
average of three measurements. All the permeation
experiments were carried out at room temperature.
3
(a) LUDOX® silica
®
○, △: LUDOX silica; ●, ▲: LUDOX silica treated with
hexane solution of 0.01 mol·L−1 TMC for 3 min
®
RESULTS AND DISCUSSION
3.1
Characterization of nanoparticles
3.1.1 BET, FTIR and TEM analysis
The surface areas of the samples listed in Table 1
were determined in this work from a BET instrument.
The surface functional groups and sizes of samples
were ascertained by FTIR and TEM, respectively.
These results are compiled in Table 2. LUDOX® silica
with bigger size (16 nm) has a smaller surface area in
comparison to TS-530. In addition, LUDOX® silica is
covered by hydroxyl groups whereas trimethylsilyl
groups are found on TS-530.
Table 2
(b) TS-530 silica
Related properties measured in this work
for silica samples listed in Table 1
Surface area
①
/m2·g−1
Particle
Surface
②
③
diameter/nm functional group
LUDOX® silica
184
16
hydroxyl
TS-530 silica
228
12
trimethylsilyl
①, ②,
and ③ represent samples measured with BET, TEM and
FTIR, respectively.
3.1.2 Adsorption
The adsorption isotherms of CO2 and N2 on the
dry LUDOX® silica and TS-530 silica were collected
at 298 K, and the results are shown in Fig. 1 (a) and
(b), respectively. In the pressure region from 0.1 to 1.6
MPa, LUDOX® silica exhibits the CO2 adsorption
capacity increasing from 0.25 to 1.22 mmol·g−1, while
TS-530 silica shows the CO2 adsorption capacity from
0.075 to 0.826 mmol·g−1. They both show good CO2
adsorption capacity at high pressure, which fulfill the
requirement [0.4 mmol·g−1 (based on sorbent)] for
economic separation of CO2 from the flue gas [31].
Besides, both of them show preferable adsorption affinity for CO2 compared to N2, which is potentially
applicable for the separation of CO2/N2 mixture.
CO2/N2 adsorption ratios (defined as CO2 adsorption
capacity/N2 adsorption capacity) are summarized in
Fig. 1 (c). The CO2/N2 adsorption ratio of LUDOX®
silica gradually decreases from 39 to 12 as pressure
increases, while CO2/N2 adsorption ratio of TS-530
silica is around 5 without significant change.
■
LUDOX silica;
(c) CO2/N2 adsorption ratio
● treated LUDOX silica; ▲ TS-530 silica
Figure 1 Adsorption isotherms of CO2 and N2 and adsorption ratio of silica samples at 298 K
From Fig. 1 (a) and (b), it should also be pointed
out that TS-530 silica with a larger surface area (see
Table 2) presents higher N2 adsorption capacity than
LUDOX® silica, while larger CO2 adsorption capacity
is obtained on LUDOX® silica. This can be explained
by the fact that hydroxyl ( OH) groups on the surface
of LUDOX® silica (see Table 2) have a good affinity
with CO2. However, during the preparation of (silica-DNMDAm-TMC)/PS
composite
membrane,
OH groups may be consumed by reacting with
COCl groups of TMC, decreasing the adsorption
capacity for CO2. In order to evaluate this effect,
LUDOX® silica samples were treated by immersing
them in the hexane solution of 0.01 mol·L−1 TMC for
3 min to simulate the process for preparing nanocomposite membranes. After that, the samples were dried
at 100 °C for 12 h in a vacuum oven before adsorption
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Chin. J. Chem. Eng., Vol. 19, No. 5, October 2011
Table 3
Sample names of prepared membranes and their silica contents
Membrane sample
SiO2 content in
aqueous phase/g·ml−1
SiO2 content in
organic phase/g·ml−1
SiO2 mass fraction
in membrane from EDX/%
Si atomic percent
in membrane from XPS/%
silica-free membrane
0
0
0
0
LU-1
0.00018
0
0.34
0.5
LU-2
0.00091
0
0.41
0.6
LU-3
0.00363
0
2.34
0.6
LU-4
0.00726
0
3.32
0.8
LU-5
0.01089
0
4.27
1.2
silica-free membrane
0
0
0
0
FS-1
0
0.00018
0.95
4.9
FS-2
0
0.00091
2.89
7.6
FS-3
0
0.00363
4.75
14.6
FS-4
0
0.00726
18.23
17.7
FS-5
0
0.01089
23.83
20.9
measurements. The isotherms of CO2 and N2 on
treated LUDOX® silica were measured and are presented in Fig. 1 (a) for a comparison. It is seen that
treated LUDOX® silica has a lower CO2 adsorption
capacity and an almost coincided N2 adsorption capacity in comparison to untreated silica. Thus, the
CO2/N2 adsorption ratio is a little lower than that of
untreated silica, as shown in Fig. 1 (c). The decrease
of CO2 adsorption capacity is due to the consumption
of OH groups on LUDOX® silica surface, which
suggests that OH groups have important impact on
CO2 adsorption. It should be noted that, in the actual
process of membrane preparation, TMC would mainly
react with DNMDAm to form the film besides the
reaction between TMC and OH group as mentioned
above. The former reaction rate with the order of
102-104 L·mol−1·s−1 in homogeneous solution [17] is
much higher than the latter one. Therefore, the consumption of OH groups on LUDOX® silica surface
is very little during the preparation of (silica-DNMDAmTMC)/PS composite membrane, and its effect on
LUDOX® silica adsorption behavior may be negligible.
3.2
EDX, XPS and ATR-FTIR of membrane
To determine the amount of silica embedded in
the membranes, EDX and XPS analysis were performed on the membrane surface and the results along
with the silica content in aqueous and organic phase
prior to interfacial polymerization are given in Table 3.
Compared with EDX result of silica-free polyamide
membrane sample, silica content is obvious in silica-polyamide nanocomposite membranes. The
amount of silica increases gradually with the addition
of SiO2 in aqueous phase or organic phase. This phenomenon is more pronounced for FS series samples.
The XPS results further show that the atomic concentration of Si in the surface region increases with silica
content in aqueous phase or organic phase, consistent
with EDX results.
The ATR-FTIR spectra of the membranes as well
as silica particles are demonstrated in Fig. 2. The
ATR-FTIR spectra of a composite membrane reveal
bands attributed to both interfacially polymerized skin
layer and the support membrane since the IR beam
penetration depth exceeds the thickness of skin layer.
In Fig. 2 (a), the silica-free membrane and LU series
samples show obvious bands at 1645 cm−1 and 1540
cm−1, which are characteristic of C O (amide I) and
N H (amide II) [32], respectively. These bands are
clearly distinguishable from polysulfone IR bands of
1488 cm−1, 1242 cm−1, and 1151 cm−1, which are due
to CH3 C CH3 stretching, C O C stretching and
C SO2 C symmetric stretching, respectively [33].
Typically, the most intensive peak at 1080 cm−1 representing Si O Si asymmetric stretching in silica
can be seen in the spectra of LU samples. As shown in
Fig. 2 (b), FS series samples containing low silica
content (FS-1 and FS-2) exhibit a similar result to that
of LU samples. However, high silica containing membranes (FS-3, FS-4 and FS-5) show unclear characteristic of polyamide and polysulfone in comparison with
the noticeable band at 1080 cm−1 due to Si O Si
stretching, which suggests that the addition of large
amount of silica in the polymerization process may
alter the surface composition of membranes. Further
an increase in the 1080 cm−1 band intensity of the FS
samples with increasing silica content is observed.
3.3
Membrane morphology
The surface and cross-section morphology of silica-free membrane and silica-polyamide nanocomposite membranes were characterized with SEM. The
representative SEM images of silica-free membrane,
FS and LU series samples are shown in Figs. 3 and
Fig. 4, respectively. In Figs. 3 (a) and 4 (a), the silica-free membrane shows the typical nodular structure
packed by spherical globules on the surface. With the
addition of LUDOX silica in aqueous phase, the surface
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Chin. J. Chem. Eng., Vol. 19, No. 5, October 2011
(a) LUDOX® silica and LU series samples
(b) TS-530 silica and FS series samples
Figure 2 ATR-FTIR of membrane samples and silica
1645: C O (amide I): 1540: N H (amid II); 1488: CH3 C CH3; 1242: C O C; 1151: S SO2 C; 1080: Si O Si
microstructure morphology of the LU membrane is
changed slightly, as shown in Fig. 3 (c), (e), (g), (i)
and (k). Moreover, from the cross-section images (Fig.
3), the thickness of the IP film presents a decrease
trend with the addition of silica. They are 469±24,
457±22, 460±10, 368±25, 336±25, 225±18 nm for Fig.
3 (b), (d), (f), (h), (j) and (l), respectively. Actually, the
film thickness is mainly determined by the kinetics of
film formation, while the addition of silica may have
important effects on the kinetics of film formation
from our viewpoint. After immersion of the aqueous
saturated PS membrane into the TMC solution, a thin
incipient film is instantaneously formed due to the
extremely rapid reaction between amine and acyl
chloride [17]. The DNMDAm molecule has to diffuse
through this layer for further reaction since the reaction
Chin. J. Chem. Eng., Vol. 19, No. 5, October 2011
Figure 3 SEM images of silica-free membrane and LU series samples
(a), (b) silica-free membrane; (c), (d) LU-1; (e), (f) LU-2; (g), (h) LU-3; (i), (j) LU-4; (k), (l) LU-5
Figure 4 SEM images of silica-free membrane and FS series samples
(a), (b) silica-free membrane; (c), (d) FS-1; (e), (f) FS-2; (g), (h) FS-3; (i), (j) FS-4; (k), (l) FS-5
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Chin. J. Chem. Eng., Vol. 19, No. 5, October 2011
region is in the organic phase near the interface [34],
which contributes to the growth of film thickness [35].
For the preparation of LUDOX silica- polyamide/PS
membrane, the diffusion of DNMDAm is more limited as some of the DNMDAm in aqueous solution
may be blocked by silica nanoparticle. In that case, the
effective concentration of DNMDAm in reaction region is decreased, so that a thinner polyamide film is
formed. This is more pronounced as the addition of
silica is increased in aqueous solution [see Fig. 3 (f),
(h), (j) and (l)].
As shown in Fig. 4, with the addition of TS-530
silica in organic phase, the surface morphology of the
FS membrane is changed greatly. Some silica aggregations can be observed on the surface of low silica
containing membranes (FS-1 and FS-2). For high silica containing membranes (FS-3, FS-4 and FS-5), a
thin silica film with some fissures on the surface is
found covering on the surface of the membrane, as
shown in Fig. 4 (g), (i) and (k). Further, the thicknesses of the IP film are 469±24, 340±20, 235±11,
390±15, 336±23, and 277±20 nm for Fig. 4(b), (d), (f),
(h), (j) and (l), respectively. In the preparation of
fumed silica-polyamide/PS membrane, some of silica
may be trapped in the incipient film since silica particles exist in the reaction region. Subsequently, the
diffusion of DNMDAm into TMC solution is also reduced due to the barrier of silica in the incipient film
and reaction region. As a result, the film thickness
shows a decreasing trend [see Fig. 4 (d) and (f)].
However, when the silica content exceeds a certain
value, some silica aggregations will appear on the film
surface due to the favorable interactions with silica in
the incipient film [see Fig. 4 (h), (j) and (l)]. These
silica aggrerations may contribute to the increase of
the film thickness [see Fig. 4 (f) and (h)].
3.4
Membrane thermal stability
Figure 5 shows the thermal behavior of silica-free
(a)
Figure 5
membrane, LU series samples and FS series samples.
The degradation of silica-containing membrane in
term of mass-loss is reduced in comparison to silica-free membrane. For “LU series” membranes, as
shown in Fig. 5 (a), the silica-containing membranes
have a little more mass loss before 300 °C than the
silica-free membrane, which is possibly due to the
removal of residual traces of solvents and water. After
that, the silica-containing membranes have slower rate
of mass loss than the silica-free membrane because of
the good thermal stability of silica. Further, the mass
loss of the samples is decreased with the increase of
silica content. More noticeable decrease trend of the
mass loss with the increase of silica content in the
samples are observed in the “FS series” samples as
shown in Fig. 5 (b). Unlike what occurs for silica-free
and LUDOX silica containing membranes, no more
mass loss is observed below 300 °C for fumed silica-containing membranes with the exception of FS-3
membrane sample. This result may indicate that the
hydrophobic trimethylsilyl groups on fumed silica
surface (see Table 2) facilitate the evaporation of water during heating procedure in the preparation of
fumed silica containing membranes. Smaihi et al. [36]
also observed that the hydrophobic methyl groups
present in polyimide-siloxane copolymers promote the
escape of water during the heat treatment in the polyimide synthesis.
3.5 CO2/N2 separation performance of membranes
Three membrane samples prepared under the
same condition, described in Section 2.3, were tested,
and the CO2/N2 permeance and selectivity of the membrane for a preparation condition were reported as the
mean values (±standard deviations) of the three samples.
Figure 6 shows CO2/N2 permeance and selectivity of the membranes as a function of SiO2 content in
aqueous phase. Compared with silica-free membrane,
(b)
TGA of silica-free membrane and LU series samples (a) and silica-free membrane and FS series samples (b)
Chin. J. Chem. Eng., Vol. 19, No. 5, October 2011
(a) CO2 permeance
(b) N2 permeance
(c) CO2/N2 gas selectivity
Figure 6 CO2/N2 permeance and selectivity of the membranes as a function of SiO2 content in aqueous phase
[CO2/N2 mixed gas: 20% (by volume) CO2 + 80% (by mass) N2]
dashed line: a guide for the eyes; ■ 0.1 MPa; ● 0.5 MPa;
▲ 1.3 MPa
(LUDOX silica-DNMDAm-TMC)/PS nanocomposite
membranes have higher CO2 permeance and N2 permeance. As SiO2 content increases from 0 to 0.00363
g·ml−1, CO2 permeance increases significantly and
reaches the maximum at 0.00363 g·ml−1 SiO2, then
decreases as SiO2 content increases. Meanwhile, N2
permeance shows a quite similar variation with that of
CO2 permeance. In Fig. 6 (c), CO2/N2 selectivity increases gradually and then decreases with the increase
of SiO2 content. CO2 permeance of membrane prepared with 0.00363 g·ml−1 SiO2 in aqueous phase are
2.4, 2.3 and 1.7 times that of pure polyamide membrane at the feed gas pressure of 0.1, 0.5 and 1.3 MPa,
829
respectively [see Fig. 6 (a)]. Meanwhile, the corresponding CO2/N2 selectivity is 1.1, 1.2 and 1.2 times
that of pure polyamide membrane [see Fig. 6 (c)].
Figure 7 shows CO2/N2 permeance and selectivity
of the membranes as a function of SiO2 content in organic phase. The permeance of CO2 and N2 of (fumed
silica-DNMDAm-TMC)/PS nanocomposite membranes
is significantly increased in comparison to (DNMDAmTMC)/PS composite membrane. As SiO2 content increases, the permeance of CO2 and N2 has similar
trend. The comparison of CO2/N2 selectivity indicates
a slightly decrease in (fumed silica-DNMDAmTMC)/PS composite membranes [see Fig. 7 (c)]. CO2
(a) CO2 permeance
(b) N2 permeance
(c) CO2/N2 gas selectivity
Figure 7 CO2/N2 permeance and selectivity of the membranes as a function of SiO2 content in organic phase
[CO2/N2 mixed gas: 20% (by volume) CO2 + 80% (by volume)
N2];
dashed line: a guide for the eyes; ■ 0.1 MPa; ● 0.5 MPa;
▲ 1.3 MPa
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Chin. J. Chem. Eng., Vol. 19, No. 5, October 2011
(a) Silica-free membrane
(b) Silica-containing membrane
Figure 8 Schematic representation of CO2 and N2 transport mechanism in the membranes of silica-free membrane and
silica-containing membrane
permeance of the membrane prepared with 0.00091
g·ml−1 SiO2 in organic phase is 2.1, 2.1 and 1.7 times
that of pure polyamide membrane at the feed gas pressure of 0.1, 0.5 and 1.3 MPa, respectively [see Fig. 7
(a)]. Meanwhile, the corresponding CO2/N2 selectivity
is 0.93, 0.98 and 0.87 times that of pure polyamide
membrane [see Fig. 7 (c)].
The higher CO2 permeance of (silica-DNMDAmTMC)/PS nanocomposite membranes than (DNMDAmTMC)/PS membrane can be attributed to two factors.
Firstly, the nanocomposite membranes have smaller
film thickness than (DNMDAm-TMC)/PS membrane
(see Figs. 3 and 4), which offers smaller diffusion path
for the transport of CO2. Secondly, the nanocomposite
membranes have multiple permselective mechanisms
of solubility selectivity and reaction selectivity, while
the silica-free membrane has only permselective
mechanism of reaction selectivity (see Fig. 8). The
incorporation of CO2-selective adsorptive silica
nanoparticles into the polyamide membrane makes
more CO2 molecules dissolve in the membrane at the
feed side than N2 molecules. Thus, more CO2 molecules
Chin. J. Chem. Eng., Vol. 19, No. 5, October 2011
react with the carrier, and the reaction selectivity is
enhanced. On the other hand, the solubility selectivity
of the membrane will make more CO2 molecules
transport through the membrane than N2 molecules
[see Fig. 8 (b)]. In addition, an optimized range of
silica content for CO2 transportation through the membrane is obtained. If the silica content exceeds this
range, CO2 permeance will be decreased [see Figs. 6
(a) and 7 (a)], which is primarily caused by the lower
CO2 diffusion coefficient in the membrane. When silica content in the membrane is high enough, the
chance that CO2 molecules are blocked by silica particles will be higher, and they will be forced to take a
tortuous course around silica particles to traverse the
membrane (see Fig. 8), increasing the diffusion path
length and thus decreasing the gas permeance, as
Barrer suggested [37]. These phenomena could also be
found in N2 permeance [see Figs. 6 (b) and 7(b)].
4
CONCLUSIONS
(1) A novel membrane with multiple permselective mechanisms of solubility selectivity and reaction
selectivity was successfully developed by incorporating CO2-selective adsorptive silica nanoparticles in
situ into the tertiary amine containing polyamide
membrane formed by interfacial polymerization (IP).
(2) Two types of CO2-selective adsorptive silica
nanoparticles (LUDOX® silica and TS-530 silica) were
incorporated into tertiary amine containing membranes
during the interfacial polymerization process. Gas
adsorption results showed that both LUDOX® silica
and TS-530 silica had good CO2 adsorption capacity
and CO2/N2 adsorption ratio, which could fulfill the
requirement [0.4 mmol·g−1 (based on sorbent)] for
economic separation of CO2 from the flue gas.
(3) The two nanocomposite membranes present
larger CO2 permeance and are more stable thermally
than pure polyamide membranes. The LUDOX silica-polyamide nanocomposite membrane prepared
with 0.00363 g·ml−1 SiO2 in aqueous phase has the
maximum CO2 permeance of 5.94×10−5, 5.13×10−5
and 3.08×10−5 cm3 (STP)·cm−2·s−1·cmHg−1, which are
2.4, 2.3 and 1.7 times that of pure polyamide membrane at the feed gas pressure of 0.1, 0.5 and 1.3 MPa,
respectively. The corresponding CO2/N2 selectivity is
85.4, 85.8, and 82.9, which are 1.1, 1.2 and 1.2 times
that of pure polyamide membrane, respectively. The
TS-530 silica-polyamide nanocomposite membrane
prepared with 0.00091 g·ml−1 SiO2 in organic phase
has the maximum CO2 permeance of 5.28×10−5,
4.66×10−5 and 3.09×10−5 cm3 (STP)·cm−2·s−1cmHg−1,
which are 2.1, 2.1 and 1.7 times that of pure polyamide membrane at the feed gas pressure of 0.1, 0.5 and
1.3 MPa, respectively. The enhanced CO2/N2 separation performance for the nanocomposite membranes
mainly attributes to the thin film thickness, and multiple permselective mechanisms of solubility selectivity
and reaction selectivity.
831
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