CO2 gas separation using mixed matrix membranes based on polyethersulfone/ MIL-100(Al)
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Open Chemistry 2021; 19: 307–321
Research Article
Witri Wahyu Lestari*, Robiah Al Adawiyah, Moh Ali Khafidhin, Rika Wijiyanti, Nurul Widiastuti,
Desi Suci Handayani
CO2 gas separation using mixed matrix
membranes based on polyethersulfone/
MIL-100(Al)
https://doi.org/10.1515/chem-2021-0033 addition of MIL-100(Al) of 30% (w/w), the permeability
received November 21, 2019; accepted February 9, 2021 of the MMMs for CO2, O2, and N2 gases was enhanced
Abstract: The excessive use of natural gas and other fossil by approximately 16, 26, and 14 times, respectively, as
fuels by the industrial sector leads to the production of compared with a neat PES membrane. The addition of
great quantities of gas pollutants, including CO2, SO2, and MIL-100(Al) to PES increased the thermal stability of
NOx. Consequently, these gases increase the temperature the membranes, reaching 40°C as indicated by thermo-
of the earth, producing global warming. Different strategies gravimetry analysis (TGA). An addition of 20% MIL-
have been developed to help overcome this problem, 100(Al) (w/w) increased membrane selectivity for CO2/O2
including the utilization of separation membrane tech- from 2.67 to 4.49 (approximately 68.5%), and the addition
nology. Mixed matrix membranes (MMMs) are hybrid of 10% MIL-100(Al) increased membrane selectivity for
membranes that combine an organic polymer as a matrix CO2/N2 from 1.01 to 2.12 (approximately 110.1%).
and an inorganic compound as a filler. In this study, Keywords: CO2, MMMs, MIL-100(Al), PES
MMMs were prepared based on polyethersulfone (PES)
and a type of metal–organic framework (MOF), Materials
of Institute Lavoisier (MIL)-100(Al) [Al3O(H2O)2(OH)(BTC)2]
(BTC: benzene 1,3,5-tricarboxylate) using a phase inversion 1 Introduction
method. The influence on the properties of the produced
membranes by addition of 5, 10, 20, and 30% MIL-100(Al) The consumption of energy continues to increase with
(w/w) to the PES was also investigated. Fourier-transform population growth and further technological development.
infrared spectroscopy (FTIR) analysis indicated that no che- The most widely used energy sources are natural gas
mical interactions occurred between PES and MIL-100(Al). (53.3%) and coal (26.3%) [1]. The worldwide consumption
Scanning electron microscope (SEM) images showed agglom- of natural gas has reached 100 trillion ft3 in 2018 and is
eration at PES/MIL-100(Al) 30% (w/w) and that the thickness estimated to increase to 160 trillion ft3 in 2035. In Indonesia,
of the dense layer increased up to 3.70 µm. After the natural gas is the most widely used energy source after
petroleum and coal. Natural gas consists of hydrocarbon
compounds and gas pollutants, such as carbon dioxide
* Corresponding author: Witri Wahyu Lestari, Department of (CO2), nitrogen (N2), sulfur dioxide (SO2), and hydrogen
Chemistry, Faculty of Mathematics and Natural Sciences, sulfide (H2S) [2]. The emission factor of CO2 from various
Universitas Sebelas Maret, Jl. Ir Sutami No. 36A, Kentingan-Jebres, energy sources is very high as compared to other gases [1].
Surakarta, Central Java, 57126, Indonesia, e-mail: witri@mipa.uns. As an acid gas, CO2 is corrosive to gas pipelines and
ac.id, tel: +62-271-663375, fax: +62-271-663375,
reduces heating value. Furthermore, an increase in CO2
Hp: +62-82227833424
Robiah Al Adawiyah, Moh Ali Khafidhin, Desi Suci Handayani: gas in the atmosphere increases the earth’s temperature,
Department of Chemistry, Faculty of Mathematics and Natural leading to climate change and global warming [3]. CO2 pol-
Sciences, Universitas Sebelas Maret, Jl. Ir Sutami No. 36A, lutants also make an impact on human health and contribute
Kentingan-Jebres, Surakarta, Central Java, 57126, Indonesia to asthma and other respiratory diseases that can potentially
Rika Wijiyanti: Master of Applied in Medical Science, National
cause cardiovascular disease and cancer [4]; therefore, a sig-
Intelligence College, Sumur Batu, Babakan Madang, Bogor, 16810,
Indonesia
nificant effort must be made to reduce CO2 emissions.
Nurul Widiastuti: Department of Chemistry, Faculty of Science, In general, CO2 gas emissions can be reduced through
Institut Teknologi Sepuluh Nopember, Surabaya, 60111, Indonesia its absorption, adsorption, cryogenic distillation, and
Open Access. © 2021 Witri Wahyu Lestari et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0
International License.
308 Witri Wahyu Lestari et al.
separation with membranes [5]. CO2 separation by the stability than organic polymer membranes [14]. MMMs
absorption method can be done using monoethanolamine are composite membranes consisting of an organic poly-
[6,7]. The absorption method for CO2 gas separation has mer matrix as a continuous phase and filler particles as
advantages because it requires less energy, has high a dispersion phase [15]. One polymer matrix that has
absorption capacity, and is flexible; however, the use potential in the fabrication of MMMs is polyethersulfone
of this method requires high costs and an appropriate (PES). This polymer is widely used in the manufacture of
solvent [5]. Adsorption methods for CO2 gas separation membranes because it has good mechanical properties,
can use physical or chemical adsorbents. The adsorbents good thermal and chemical stability [16], and high glass
that have been investigated for CO2 separation include transition temperatures (Tg) of up to 225°C [17]. With
Zeolite CaX [8] and Zeolite Ceca 13X [9]. Songolzadeh addition of ethylenediamine (EDA)-TiO2 of 5%, PES-based
et al. stated that, while the adsorption method requires MMMs had the highest selectivity for CO2/CH4 gas separa-
little energy, adsorption capacity is low, the process of tion at 41.42 and CO2 permeability at 10.11 Barrers (4 bars)
regenerating adsorbents is difficult, and further research [18]. In addition, PES/Zeolitic Imidazolate Framework-8
is required to find new adsorbents [5]. Gas separation by (ZIF-8) MMMs have been investigated for H2/CO2 gas
a cryogenic distillation method has been investigated separation, demonstrating a selectivity of 9.3, and H2/N2
by Li et al. and includes technology that is relatively easy gas separation, demonstrating a selectivity of 11.5 [19].
to produce and can be used on an industrial scale. In CO2/CH4 gas selectivity increases from 3.57 to 11.15 with
addition, this method does not use solvents or produce the addition of a 15% carbon molecular sieve in PES mem-
liquid CO2. However, the amount of energy required for branes [20].
cooling and compacting the CO2 at low temperatures is Promising fillers can increase both permeability and
great, which can cause other operational problems [10]. selectivity. So far, the materials that have been most
Gas separation using membrane technology depends on widely used as fillers are porous materials such as silica,
gas pressure. This technology has advantages over other carbon nanotubes, zeolites, and graphite, as well as used
CO2 gas separation techniques for reasons such as its use in the development of metal–organic frameworks (MOFs)
of simple tools, clear processes, high permeability and [15]. MOFs have good porosity, large surface areas, and
selectivity, high thermal and chemical stability, resistance adjustable pore sizes and topology, making them ideal
to plasticization, and low production costs [5]. selections as fillers in MMMs [21]. Research on MOF-based
Membrane technology for gas separation is highly MMMs for CO2 gas separation has been performed in
developed in the industry because it can be easily applied the past, including on loading of 30 wt% [Ti8O8(OH)4-
and is environmentally friendly; therefore, membranes (C6H3–C2O4–NH2)6] also known as NH2-Material of Insti-
have been widely commercialized [11]. Membrane appli- tute Lavoisier (MIL)-125(Ti) on polysulfone (PSf) have
cations have been found for the separation of O2/N2 shown an increase of 320% compared with pristine PSf
CO2/N2, and vapors, air dehydration, and the removal membrane [22], [Al(OH)(O2C–C6H4–CO2)] also known as
of volatile organic compounds from waste [12]. Metal Material of Institute Lavoisier (MIL)-68(Al) modified into
membranes made of platinum or palladium have good PSf showed only small increases in H2 and CO2 permeabil-
performance, but the high cost of these metals greatly ities in the ranges of 11.1–12.4 and 4.7–5.4 Barrer, respec-
influences their selection. Inorganic membranes can be tively [23]. MMMs containing [Zr6O4(OH)4(C8H4O4)6(NH2)6]
used as alternatives because of their better chemical also known as UiO-66-NH2 and Matrimid® polymer [24]
stability and lower fabrication costs; however, their use exhibited not only enhanced mechanical and thermal
requires high temperatures of 200 to 900°C. Organic stabilities, but also CO2 permeability was increased by
polymer membranes increasingly dominate the field because 200% and CO2/N2 ideal selectivity was increased by
they are economical and their performance is quite com- 25%. In addition, NH2-Cu3(BTC)2 (BTC: benzene 1,3,5-tri-
petitive [11]. However, the selectivity and permeability carboxylate) deposited on Pebax (Pebax/sub-NH2-Cu-BTC)
properties of polymer membranes are limited [13]. These showed 303% higher CO2 permeability than neat Pebax due
deficiencies have pushed researchers toward the development to the fine dispersion and the presence of groups with a
of alternative ways of making membranes that are more stable superior CO2-philicity in the framework [25], and incorpora-
and economical and have a high separation performance, tion of MOF-5 (15 wt%) into polyimide (PI) increased the
namely through combinations of membrane materials known permeability toward CO2 gas up to 290% [26].
as mixed matrix membranes (MMMs). The fabrication Trivalent metals, for example, Al3+ ions, are promising
technology for MMMs promises to improve mechanical candidates for the synthesis of MOFs because of their por-
properties, producing better separation capabilities and osity, low density, and high thermal and physicochemical
CO2 gas separation using MMMs based on polyethersulfone/MIL-100(Al) 309
stability [27]. Al-fumarate-based MOFs have been investi- placed in a Teflon linedSS Autoclave and heated at 120°C
gated by Nuhnen et al. as a filler in Matrimid® polymers, for 1 h. The resulting white gel was dried in an oven
showing increased permeability for the separation of CO2 at 80°C for 19 h until a yellow solid was obtained, which
and CH4 gases [28]. One type of MOF is the Material of was then washed with DMF (1 × 30 mL) and ethanol
Institute Lavoisier (MIL). MIL-n types based on Al3+ metal (1 × 30 mL) to remove the remaining ligand. The MIL-100(Al)
ions that have been investigated in gas separation pro- material was activated at 100°C for 2 h and sonicated for 5 h
cesses include MIL-53(Al) and NH2-MIL-53(Al) in polyviny- to reduce particle size (yield: 87.19%).
lidene fluoride [29] and MIL-68(Al) in polysulfone [23].
Another aluminum-based MIL-n is MIL-100(Al). The synth-
esis of an aluminum trimesate-based MIL-100 was first 2.2.2 Preparation of MMMs based on PES/MIL-100(Al)
performed by Volkringer et al. using the hydrothermal
method [30], and MIL-100(Al) has been investigated for The preparation of MMMs was performed with a phase
hydrogen storage with a Pd metal carrier [31], as a catalyst inversion method modified from the study by Dama et al.
in sulfoxidation reactions [32], and for the loading of doxo- [34]. Filler MIL-100(Al) in different weight percentages of
rubicin hydrochloride on MIL-100(Al) gel as an anticancer 5, 10, 20, and 30% (w/w) was added to dimethylaceta-
treatment [33]. The use of MIL-100(Al) filler for separation mide. Each mixture was sonicated for 20–30 min so that
applications has never been done; therefore, this study the MIL-100(Al) was well dispersed in the solution and
aimed at researching the development of MMMs using then stirred for 1 h at room temperature to homogenize it.
MIL-100(Al) with PES polymers, expecting that this would Afterward, PES (35%) (w/w) was added to each mixture
improve the CO2 gas separation abilities of MMMs. and stirred for 24 h at room temperature. Each obtained
dope solution was cast on a glass plate (20 × 15 cm). Next,
each membrane on a glass plate was evaporated for about
20 s and then dipped in a water-filled coagulant tub, and
2 Materials and methods after the membrane was removed from the glass plate, the
membrane was washed with distilled water for 24 h, fol-
lowed by solvent exchange with methanol for 2 h and drying
2.1 Materials
for 48 h at room temperature to evaporate the solvent.
All chemicals were used in analytical grade without
further purification. Aluminum nitrate nonahydrate (99%) 2.2.3 Gas separation test
and benzene-1,3,5 tricarboxylic acid (95%) were commer-
cially provided by Sigma Aldrich, Germany. Ethanol (99%), The resulting membranes were tested for permeability
N,N′-dimethylformamide (DMF, 99.8%), PES (99%), and and selectivity using a single gas (N2, O2, and CO2) flat
N,N′-dimethylacetamide (99%) were supplied by Merck sheet membrane method as reported by Lee et al. [35].
EMSURE, Germany. Nitrogen (UHP 99.995%), oxygen (UHP The measurement was performed at a pressure of 1 bar
99.9%), and carbon dioxide (UHP 99.9%) gases were pur- and room temperature. Each membrane was fabricated
chased from Samator, Indonesia. by cutting it in a circle with an effective diameter of
5.7 cm, placing it on a membrane permeation cell, and
then sealing it before connecting it to the gas stream. The
data were collected by measuring the amount of time
2.2 Methods taken for the gas to reach a volume of 10 mL, with the
volume of gas measured with a bubble flow meter and the
2.2.1 Synthesis of MIL-100(Al) time was measured with a stopwatch. Data retrieval was
done twice. The scheme of the gas permeation test equip-
The MIL-100(Al) was synthesized under the solvothermal ment is shown in Figure 1 [36]. The permeability and
conditions according to a procedure modified from the selectivity of each membrane were calculated based on
literature [33]. Al(NO3)3·9H2O (2.851 g, 7.6 mmol) and tri- equations (1) and (2) [37].
mesic acid (H3BTC: benzene 1,3,5-tricarboxylate) (1.051 g, V
5 mmol) were dissolved in ethanol (36 mL) and stirred Pi Q 273.15 t 273.15 (1)
= =
for 15 min at room temperature. The solution was then l (A × ΔP ) T (A × ΔP ) T
310 Witri Wahyu Lestari et al.
Figure 1: Single gas permeation rig (adapted from Gunawan et al. [36]).
where, 2.3 Characterization of the materials
Pi = gas permeability (GPU l i [1 GPU = 1 × 10−6 cm3
P
X-ray diffraction (XRD) (X-Pert Pan Analytical) was used
(STP)/cm3 s cm Hg] or Barrer [1 Barrer = 1 × 10−10 cm3
to analyze the phase purity and crystallinity of the pre-
(STP)/cm2 s cm Hg])
pared materials. The functional groups of the compounds
l = membrane density (cm)
were observed using Fourier-transform infrared spectro-
V = measured volume (cm3, STP)
scopy (FTIR) (Shimadzu IR Prestige-21). The thermal stabi-
A = membrane area (cm2)
lity of the materials was measured using TG analysis to a
t = required time for gas to pass through the membrane (s)
temperature of 900°C (Hitachi STA 7000) with a heating
ΔP = gas pressure (cm Hg)
rate of 10°C/min under nitrogen flow. The morphology
T = temperature condition during the measurement (K)
and elemental composition of the materials were observed
Pi by a scanning electron microscope-energy dispersion X-ray
αi / j = (2) (SEM–EDX) (FEI Inspect-S50). A surface area analyzer
Pj
(SAA) (Quadrasorb Evo, Quantachrome) was used to
where, monitor the surface area and porosity of the materials.
αi / j = selectivity of the membrane for gas i and gas j
Pi = permeability of gas i (Barrer) Ethical approval: The conducted research is not related to
Pj = permeability of gas j (Barrer) either human or animal use.
CO2 gas separation using MMMs based on polyethersulfone/MIL-100(Al) 311
3 Results and discussion
3.1 Characterization of the materials
3.1.1 X-ray diffraction (XRD)
The characterization of the prepared materials using XRD
was performed to determine the purity and suitability of
the phase. The XRD analyses demonstrated that the peak
of the synthesized MIL-100(Al) corresponds to the stan-
dard pattern for MIL-100(Al) (CCDC No. 789872) but
has lower crystallinity as shown by the broad peak in
Figure 2. This result is in accordance with the study con-
ducted by Xia et al. [38], which stated that the powder Figure 3: FTIR spectra of the synthesized MIL-100(Al) and H3BTC
ligand.
XRD of MIL-100(Al) synthesized through gel formation
shows low crystallinity.
to 2,886–3,650 cm−1, representing the formation of hydrogen
bonding, whereas OH in MIL-100(Al) comes from the
3.1.2 Fourier transform infrared spectroscopy (FTIR) ligand that forms MIL-100(Al) with the formula {Al3O
(OH)(H2O)2[BTC]2 · xH2O}. Al–O bonds are generally
The properties of the synthesized MIL-100(Al) can also be observed at wavenumbers below 700 cm−1. The absorp-
seen in the shift of the absorption peak shown in the FTIR tion peak at 675 cm−1 in the synthesis results shows the
analysis of this material in comparison with the H3BTC Al–O bond that has been coordinated with the carboxyl
ligand (Figure 3). These results are in accordance with the group of the H3BTC ligand. The uptake of the C–O func-
research reported by Xia et al. [38]. tional groups also matched the literature at wave number
The significant shift from 1,721 to 1,673 cm−1 corre- 1,246 cm−1 [39] (Table 1).
sponds to the stretching vibration of the carbonyl group The success of the membrane preparation can be
(C]O) caused by the deprotonation of the C]O–H group seen in a comparison of the FTIR spectra of the neat
by the free ligand that is coordinated with the Al3+ metal PES membrane and that of the PES/MIL-100(Al) mem-
ion in MIL-100(Al). In addition, there is also a shift in the branes (Figure 4). The comparison of the PES FTIR spectra
absorption peak of the OH group from 2,500–3,100 cm−1 with those of the PES/MIL-100(Al) with different percen-
tage additions of MIL-100(Al) shows that there were no
changes to the structure of the PES, which is demon-
strated by the similar FTIR spectra. The addition of MIL-
100(Al) to PES causes the character of the MIL-100(Al) to
be more visible. This is marked by a widening of the
absorption peak of the OH functional groups at 3,447 cm−1
due to overlapping OH groups from the –OH ligand and
Table 1: Comparison of the absorption spectra of the synthesized
MIL-100(Al) with the H3BTC ligand and MIL-100(Al) in the literature
Absorption Wavenumber (cm−1)
H3BTC [38] Synthesized MIL-100(Al)
MIL-100(Al) literature [38]
O–H 2,500–3,100 2,886–3,650 2,500–3,500
C]O stretch 1,721 1,673 1,670–1,729
Figure 2: Diffractogram of synthesized MIL-100(Al) as compared
C–O 1,276 1,246 1,131–1,221
with the standard patterns of CCDC No. 789872 and the H3BTC
Al–O — 675 680–620
ligand (ICSD No. 30245).
312 Witri Wahyu Lestari et al.
3.1.3 Scanning electron microscopy (SEM)
SEM analysis was used in this study to determine the
morphology and particle size of the MIL-100(Al) and
the effect of the addition of MIL-100(Al) to PES on the
density of the MMMs. The morphology of the synthesized
MIL-100(Al) showed an irregular shape (Figure 5) with an
average particle size of 1.69 ± 0.35 µm (see ESI Figures S1).
The morphological appearance of the MIL-100(Al) is in
accordance with a study conducted by Feng et al. [33].
Differences in synthesis methods can affect morphology
due to different solvents that are used, for example, the
MIL-100(Al) synthesized by Volkringer et al. [30] under
Figure 4: FTIR spectra of MIL-100(Al), PES, and MIL-100(Al)/PES hydrothermal conditions using H2O and HNO3 solvents
MMMs with several different percentage additions of MIL-100(Al)
had an octahedral form.
to PES.
The surface morphologies of the resulting MMMs are
shown in Figure 6. The neat PES membrane is almost
H2O molecules in the MIL-100(Al) (formula {Al3O(OH)
defect-free, representing the dense layer, and the surface
(H2O)2[BTC]2 · xH2O}) and overlapping OH from water during
images of MMM up to 20 wt% MIL-100(Al) loading also
coagulation. The PES uptake band is in accordance with
look like a dense membrane. However, the MIL-100(Al)
the study by Mushtaq et al. [40] and is listed in Table 2,
does not appear to be homogenously distributed in the
showing the uptake at 1,149 and 1,241 cm−1, which corre-
MMMs with 5 and 10% additions of MIL-100(Al). This may
sponds to the absorption peak of the S]O group. CSO2C
be occurred since the amount of MIL-100(Al) incorpo-
absorption is indicated at wavenumber 1,324 cm−1. C–O
rated is very small. In contrast, the MIL-100(Al) is well
uptake at 1,237 and 1,100 cm−1 indicates the presence of
distributed at a 20% addition, but at a 30% addition, the
ether groups. The absorption of C]C aromatic is shown in
agglomeration has occurred that might be due to too large
the area of 1,576–1,487 cm−1. The addition of MIL-100(Al) to
loading of MOFs, affecting membrane performance [13],
PES does not produce a new absorption peak, indicating that
and this is shown in Figure 6e. The effect of the addition of
there is no chemical bond between the PES and MIL-100(Al).
MIL-100(Al) can also be seen from the morphologies of
However, non-covalent interactions might have occurred (but
the cross-sections (Figure 7). Each cross-section mor-
could not be observed in FTIR) such as π–π stacking between
phology appears as an asymmetrical structure with a
the aromatic rings, ion–dipol interaction between metal ion
dense top layer where the filler is located and a lower
part of MOF and sulfon and ether group and also dipol–dipol
layer with a cavity formed like fingers, which accords
interaction; therefore, good composite could be formed and
with a previous study conducted by Qadir et al. [41].
reinforced each other. The decrements in intensity of OH
The distribution of the aluminum from MIL-100(Al) in
band in PES/MIL-100(Al) 5% might be due to lower content
the membrane is demonstrated through EDX analysis as
of water during the coagulation process in MMM casting or
shown in Table 3 and Appendix 2.
during MOF activation.
Table 2: Comparison of the absorption peaks of the PES from the
research results with that of the PES from the literature
Absorption Wavenumber (cm−1)
PES MIL-100(Al) PES/
literature [40] MIL-100(Al)
S]O 1,150 and 1,307 — 1,149 and 1,241
CSO2C 1,322 — 1,324
C–O 1,244, 1246 1,237 and 1,100
1,260–1,000
Ar C]C 1,587–1,489 1,372–1,481 1,576–1,487
O–H — 2,886–3,650 3,447
Figure 5: Depiction of the synthesized MIL-100(Al).
CO2 gas separation using MMMs based on polyethersulfone/MIL-100(Al) 313 Figure 6: Surface morphology of (a) PES, (b) PES/MIL-100(Al) 5%, (c) PES/MIL-100(Al) 10%, (d) PES/MIL-100(Al) 20%, and (e) PES/MIL- 100(Al) 30%.
314 Witri Wahyu Lestari et al.
Figure 7: Cross-section of morphological appearance of (a) PES, (b) PES/MIL-100(Al) 5%, (c) PES/MIL-100(Al) 10%, (d) PES/MIL-100(Al)
20%, and (e) PES/MIL-100(Al) 30%.
The addition of MIL-100(Al) to PES can increase the 3.1.4 Thermogravimetric analysis (TGA)
thickness of the dense layer as shown in Figure 8 and
determine the nature of gas permeation. As shown in The thermal stability of the synthesized MIL-100(Al) was
Figure 8, the dense layer thickness increases at higher determined from TGA analysis. Figure 9 shows that there
MIL-100(Al) loading. It can be attributed since the exchange are two instances of mass reduction, occurring at tem-
rate between the coagulant at the outer layer and the solvent peratures below 200°C and at 500–550°C, which is in
is slower due to the high concentrated area of the polymer. agreement with a previous study by Xia et al. [38]. The
It results in the delayed demixing, affecting the formation first mass reduction indicates loss of water molecules of
of the thicker dense layer. approximately 11.09% at temperatures from 26 to 157°C,
CO2 gas separation using MMMs based on polyethersulfone/MIL-100(Al) 315
Table 3: Results of the EDX analyses of the PES/MIL-100(Al) MMMs
Membrane %w/w C %w/w O %w/w Al
PES/MIL-100(Al) 5% 50.62 49.24 0.13
PES/MIL-100(Al) 10% 50.55 49.03 0.42
PES/MIL-100(Al) 20% 50.39 48.10 1.51
PES/MIL-100(Al) 30% 49.56 49.25 1.19
Figure 10: Thermogram of different percentage additions of MIL-
100(Al) to PES.
MMMs by 40°C as compared with neat PES membrane
(Figure 10), which had thermal stability of up to 500°C.
This is because the thermal stability of MIL-100(Al) (600°C)
is higher than that of PES [32]. The presence of filler inside
the PES membrane can act as a barrier, obstructing the
transport of degradation product.
Figure 8: The thickness of PES/MIL-100(Al) MMMs dense layer with
different percentage additions of MIL-100(Al).
3.1.5 Surface area analyzer (SAA)
SAA analysis was used to determine the surface area and
pore chrematistics of the MIL-100(Al) material, including
pore volume, pore radius, and pore size distribution.
The results of the SAA analysis are shown through the
adsorption–desorption isotherm graph in Figure 11.
The adsorption–desorption isotherm graph for MIL-
100(Al), based on the International Union of Pure and
Figure 9: Thermogram of the synthesized MIL-100(Al).
which is possible due to the loss of 6 water molecules.
Two DMF molecules (ca. 21.12%) that were used during
the purification were probably released at temperature
range 162 to 277°C [30,38]. The third mass reduction was
49.99% at temperatures from 276 to 616°C due to the loss of
two BTC ligand molecules. The total final product residue
of 17.80% Al2O3 was obtained at 520°C [30]. The calculation
of the mass reduction is given in ESI Appendix 3.
The addition of MIL-100(Al) to the MMMs to form
PES/MIL-100(Al) increases the thermal stability of the Figure 11: Nitrogen sorption isotherm analysis of MIL-100(Al).
316 Witri Wahyu Lestari et al.
6.00E-04 material (type I) with a BET surface area of 920 m2/g and
a pore volume of 0.535 cm3/g [33]. Figure 12 shows the
5.00E-04
results for the MIL-100(Al) pore size distribution using
the Barrett, Joyner, and Halenda (BJH) method with a pore
dV(r) (cm3/Ȃ/g)
4.00E-04
volume and radius of 0.026 cc/g and 18.437 Å, respectively.
3.00E-04
2.00E-04
1.00E-04 3.2 Gas separation performance of the PES/
0.00E+00
MIL-100(Al) MMMs
0 50 100 150 200
Pore radius (Å)
The gas separation performance by the MMMs can be
Figure 12: Graph of MIL-100(Al) pore distribution.
known from a gas permeability test using a single gas
(CO2, O2, and N2). A gas permeability test was used to
determine the nature of the gas transport and the effect
Applied Chemistry classifications, shows type II, which
of different additions of MIL-100(Al) on the permeability
indicates a nonporous or macropore material character [42].
and selectivity of the membranes. The results for the
Based on an analysis using the Brunauer–Emmett–Teller
gas permeability tests for different additions of MIL-
(BET) method, a surface area of 1.844 m2/g was obtained.
100(Al) to PES are shown in Figure 13, Table 4, and
The synthesized MIL-100(Al) does not have the same char-
Appendix 4.
acter as that shown in the study by Feng et al., where the
Figure 13 shows that CO2 gas permeation was higher
MIL-100(Al) from a gel formation included microporous
than that of N2 or O2. This is because the kinetic diameter
of CO2 (3.20 Å) is smaller than that of O2 (3.46 Å) and N2
(3.64 Å) [42], which can affect gas flow. O2 gas has a
smaller kinetic diameter than N2, but O2 (32) has a greater
molecular weight than N2 (28), and thus molecular weight
can also affect gas flow [44]. The addition of MIL-100(Al)
to PES can increase permeability with a similar pattern of
increase for each gas. Increasing the maximum perme-
ability of the CO2, O2, and N2 gases with the addition of
30% MIL-100(Al) causes the permeability of the CO2 gas to
increase 16 times (1,609%) from 570.03 ± 6.37 Barrer
to 9741.99 ± 1519.91 Barrer, O2 gas to increase 26 times
from 213.89 ± 6.81 Barrer to 5743.87 ± 27.61 Barrer, and N2
gas to increase 14 times from 564.52 ± 5.54 Barrers to
8536.14 ± 744.06 Barrers. The increase in CO2 gas perme-
ability in these PES/MIL-100(Al) MMMs is greater than
Figure 13: Gas permeability graph of CO2, O2, and N2 for different that of Matrimid®-based MMMs and UiO-66-NH2 type
percentage additions of MIL-100 (Al) to PES. MOFs (∼200%) [24].
Table 4: Gas Permeability of the PES/MIL-100(Al) MMMs
Membrane Permeability (Barrer)
N2 O2 CO2
PES neat 564.52 ± 5.54 213.89 ± 6.81 570.03 ± 6.37
PES/MIL-100(Al) 5% 1149.91 ± 63.09 862.13 ± 157.42 1799.51 ± 23.09
PES/MIL-100(Al) 10% 824.62 ± 128.68 448.06 ± 26.78 1749.68 ± 99.05
PES/MIL-100(Al) 20% 1194.20 ± 139.31 492.21 ± 4.99 2210.77 ± 308.46
PES/MIL-100(Al) 30% 8536.14 ± 744.06 5743.87 ± 27.61 9741.99 ± 1519.91CO2 gas separation using MMMs based on polyethersulfone/MIL-100(Al) 317
portion of the filler particles. The MIL-100(Al) particles
have pores of about 1.8 nm, which is microporous and
below the region of Knudsen diffusion. Also, the Knudsen
selectivity values are not in agreement with calculated
selectivity values (Table 5), indicating that the gas trans-
port is not controlled by the Knudsen mechanism. The
pore of MIL-100(Al) is bigger than the gas molecular
diameter, leading to the conclusion that the transport
mechanism is not controlled by molecular sieving. The
transport mechanism can be explained by surface diffu-
sion, with a mechanism of more favorable surface diffu-
sion of fast, lower kinetic diameter gas (CO2), relative to
slow, larger kinetic diameter gas (O2 and N2), followed
by gas diffusion in the MIL pores. CO2 gas has higher
Figure 14: Gas selectivity graph of CO2/O2, CO2/N2, and O2/N2 for polarizability and quadrupole moments (29.11 × 1025/cm3
different percentage additions of MIL-100(Al) to PES. and 4.30 × 1026/esu cm2, respectively) as compared with
N2 (17.403 × 1025/cm3; 1.52 × 1026/esu cm2) or O2 gas
The addition of MIL-100(Al) fillers to PES/MIL-100(Al) (15.812 × 1025/cm3; 0.39 × 1026/esu cm2) [45]. This shows
MMMs can increase CO2/O2 gas selectivity as shown in that CO2 gas can be adsorbed more selectively than N2 or
Figure 14, Table 5, and Appendix 6. An addition of MIL- O2 gas due to the greater presence of CO2 quadrupole
100(Al) of 20% can increase the selectivity for CO2/O2 gas moments that can affect the gas separation selectivity
separation from 2.67 ± 0.04 to 4.49 ± 0.67 (68.5%), which [45]. In addition, after the activation process, the metal
then decreases with an addition of MIL-100(Al) of 30%. ions in MIL-100(Al) do not have coordinatively saturated
This phenomenon is due to the presence of the filler bonds, allowing the CO2 gas to be easily adsorbed. The
agglomeration as shown in Figure 6e and permeability interaction with the absorbed gas shows a strong activity
that is too high. The existence of agglomeration that is to adsorb in the pore walls, thus gas separation occurs
too large will reduce the performance of membrane separa- based on surface diffusion [46].
tion [13]. CO2/N2 gas selectivity was increased by 110.1% to The separation of O2/N2 gas with an addition of 30%
2.16 ± 0.46 as compared to a neat PES membrane (1.01 ± MIL-100(Al) increased selectivity by 97.9% to 0.75 ± 0.18
0.00) with an addition of MIL-100(Al) of 10%. The increase as compared with neat PES (0.38 ± 0.01). However,
in the CO2 gas separation performance is influenced by the O2/N2 performance has lower selectivity than CO2/O2
polarity and high CO2 quadrupole moments as compared or CO2/N2. This is because gas permeation is influenced
with N2 [45], causing the CO2 to be more easily adsorbed, by the molecular weight of O2 (32), which is greater
and so the selectivity for CO2 increases. The increased than that of N2 (28) [44]. As a result, gas permeation
selectivity for CO2/N2 from the obtained MMMs is higher based on Knudsen diffusion, as shown by produced
than that of Matrimid®/UiO-66-NH2 MMMs (∼25%) [24]. O2/N2 selectivity, is close to Knudsen selectivity (0.94).
The gas transport in the membrane is controlled by Knudsen selectivity is calculated based on equation (3)
the solution–diffusion mechanism of PES matrix with a [47].
Table 5: The selectivity of the PES/MIL-100(Al) MMMs
Membrane Selectivity
O2/N2 CO2/N2 CO2/O2
PES neat 0.38 ± 0.01 1.01 ± 0.00 2.67 ± 0.04
PES/MIL-100(Al) 5% 0.75 ± 0.18 1.57 ± 0.11 2.12 ± 0.36
PES/MIL-100(Al) 10% 0.55 ± 0.12 2.16 ± 0.46 3.91 ± 0.01
PES/MIL-100(Al) 20% 0.42 ± 0.05 1.85 ± 0.04 4.49 ± 0.67
PES/MIL-100(Al) 30% 0.68 ± 0.06 1.15 ± 0.28 1.70 ± 0.26
Knudsen selectivity [47] 0.94 0.80 0.85318 Witri Wahyu Lestari et al.
Figure 15: Comparison of the O2/N2 gas separation performance as compared with the literature [48,49].
Mj The successful preparation of the MMMs can be eval-
αi , j = (3)
Mi uated by comparing them with the Robeson upper bound
as a dividing boundary between organic and inorganic
where, αi, j is the selectivity of the membrane in gases i
polymer membranes. The for O2/N2 (Figure 15) and CO2/
and j, Mi is the molecular weight of gas i, and Mj is the
N2 gas separation performance (Figure 16) of the pro-
molecular weight of gas j.
duced MMMs has not been able to reach the upper bound
Figure 16: Comparison of CO2/N2 gas separation performance as compared with the literature [49].CO2 gas separation using MMMs based on polyethersulfone/MIL-100(Al) 319
due to low selectivity, but different additions of MIL- Conflict of interest: Authors state no conflict of interest.
100(Al) can push gas permeation performance toward
the upper bound as compared with neat PES membranes, Data availability statement: All data generated or ana-
so the preparation of the PES/MIL-100(Al) MMMs can be lyzed during this study are included in this published
categorized as successful. article [and its supplementary information files].
4 Conclusion References
The addition of MIL-100(Al) to PES can affect the char- [1] Islam MA, Hasanuzzaman M, Rahim NA, Nahar A,
acteristics of the produced MMMs, including an increase Hosenuzzaman M. Global renewable energy-based electricity
in thermal stability of 40°C and in the thickness of the generation and smart grid system for energy security. Sci
World J. 2014;2014:1–13. doi: 10.1155/2014/197136.
dense membrane layer of up to 3.70 μm. Moreover, the
[2] Kartohardjono S, Alexander K, Larasati A, Sihombing IC. Effect
addition of MIL-100(Al) to PES can improve membrane of feed gas flow rate on CO2 absorption through super
performance for the permeability of CO2, O2, and N2 gases hydrophobic hollow fiber membrane contactor. IOP
by 16, 26, and 14 times, respectively, as compared with Conference series: materials science and engineering, volume
neat PES membranes. Furthermore, an addition of 20% 316, quality in research: international symposium on
materials, metallurgy, and chemical engineering;
MIL-100(Al) can increase the selectivity for CO2/O2 gas
2017 July 24–27, Bali, Indonesia. Bristol: IOP Publishing; 2018.
separation from 2.67 to 4.49. CO2/N2 gas selectivity can p. 1–7.
be increased from 1.01 to 2.12 with an addition of 10% [3] Mustafa J, Farhan M, Hussain M. CO2 separation from flue
MIL-100(Al) as filler. Another strategy for increasing mem- gases using different types of membranes. J Membrane Sci
brane performance is to modify the membrane surface with Technol. 2016;26(2):1–7. doi: 10.4172/2155-9589.1000153.
[4] Perera F. Pollution from fossil-fuel combustion is the leading
the use of a coating material such as polydimethylsiloxane.
environmental threat to global pediatric health and equity:
Solutions exist. Int J Environ Res Public Health.
Acknowledgments: The authors acknowledge Dr. rer. nat. 2017;15(16):1–17. doi: 10.3390/ijerph15010016.
Ubed Sonai Fahruddin Arrozzi from the State University [5] Songolzadeh M, Soleimani M, Ravanchi MT, Songolzadeh R.
of Malang for the nitrogen sorption isotherm measurement Carbon dioxide separation from flue gases: a technological
and Dr. Grandprix T. M. Kadja from Bandung Institute of review emphasizing reduction in greenhouse gas emissions.
Sci World J. 2014;2014:1–34. doi: 10.1155/2014/828131.
Technology for the thermogravimetry measurement. More-
[6] Nugrogho DH, Sari R, Tivandale T. CO2 removal from natural
over, we thank the Ministry of Research, Technology and gas using monoethanolamine (MEA) in packed absorber. J of
Higher Education of the Republic of Indonesia through the Built Environ Technol Eng. 2018;4:29–32.
University Leading Fundamental Research Grant (PDUPT) [7] Kamopas W, Asanakham A, Kiatsiriroat T. Absorption of CO2 in
2021 for funding. biogas with amine solution for biomethane enrichment. J Eng
Technol Sci. 2016;48(2):1–11. doi: 10.5614/
j.eng.technol.sci.2016.48.2.9.
Funding information: The funding of this research was sup-
[8] Oddy S, Poupore J, Tezel FH. Separation of CO2 and CH4 on CaX
ported by the Ministry of Research, Technology and Higher zeolite for use in landfill gas separation. Can J Chem Eng.
Education of the Republic of Indonesia through the University 2013;91:1031–9. doi: 10.1002/cjce.21756.
Leading Fundamental Research Grant (PDUPT) 2021. [9] Mulgundmath VP, Tezel FH, Saatcioglu T, Golden TC.
Adsorption and separation of CO2/N2 and CO2/CH4 by 13X
zeolite. Can J Chem Eng. 2012;90:730–8. doi: 10.1002/
Authors’ contributions: W. W. L. – conceptualization;
cjce.20592.
W. W. L., N. W., D. S. H. – data curation; R. A. A., R. W. – [10] Li X, Li J, Yang B. Design and control of the cryogenic distilla-
formal analysis: W. W. L. – funding acquisition; R. A. A., tion process for purification of synthetic natural gas from
M. A. K. – investigation; R. A. A., M. A. K., R. W. – methanation of coke oven gas. Ind Eng Chem Res.
methodology; W. W. L. – project administration; W. W. L., 2014;53(50):19583–93. doi: 10.1021/ie5024063.
[11] Alqaheem Y, Alomair A, Vinoba M, Pérez A. Polymeric
N. W. – resources; R. A. A., M. A. K. – software; W. W. L, N.
gas-separation membranes for petroleum refining.
W. – supervision; W. W. L, N. W. – validation; R. A. A., M.
Int J Polym Sci. 2017;2017:1–19. doi: 10.1155/2017/4250927.
A. K., R. W. – visualization; R. A. A., W. W. L. – writing – [12] Yuan H, Yu B, Cong H, Peng Q, Yang R, Yang S, et al.
original draft; W. W. L., D. S. H., N. W., R. W. – writing – Modification progress of polymer membranes for gas
review & editing. separation. Rev Adv Mater Sci. 2016;44(3):207–20.320 Witri Wahyu Lestari et al.
[13] Shahid S, Nijmeijer K, Nehache S, Vankelecom I, Deratani A, Biosens Bioelectron. 2017;91:804–10. doi: 10.1016/
Quemener D. MOF-mixed matrix membranes: Precise disper- j.bios.2017.01.059.
sion of MOF particles with better compatibility via a particle [28] Nuhnen A, Dietrich D, Millan S, Janiak C. Role of filler porosity
fusion approach for enhanced gas separation properties. and filler/polymer interface volume in metal-organic frame-
J Membrane Sci. 2015;492:21–31. doi: 10.1016/ work/polymer mixed-matrix membranes for gas separation.
j.memsci.2015.05.015. ACS Appl Mater Interfaces. 2018;10(39):33589–600.
[14] Mohshim DF, Mukhtar HB, Man Z, Nasir R. Latest development doi: 10.1021/acsami.8b12938.
on membrane fabrication for natural gas purification: a review. [29] Feijani EA, Tavasoli A, Mahdavi H. Improving gas separation
J Eng. 2013;2013:1–7. doi: 10.1155/2013/101746. performance of poly(vinylidenefluoride) based mixed matrix
[15] Zhang W, Liu D, Guo X, Huang H, Zhong C. Fabrication of mixed- membranes containing metal organic frameworks by chemical
matrix membranes with MOF-derived porous carbon for CO2 modification. Ind Eng Chem Res. 2015;54(48):12124–34.
separation. AIChE J. 2018;64(9):3400–9. doi: 10.1002/aic.16187. doi: 10.1021/acs.iecr.5b02549.
[16] Abdallah H, Shalaby MS, Shaban AMH. Performance and [30] Volkringer C, Popov D, Loiseau V, Férey G, Burghammer M,
characterization for blend membrane of PES with manganese Riekel C, et al. Synthesis, single-crystal X-ray microdiffraction,
(III) acetylacetonate as metalorganic nanoparticles. Int J Chem and NMR characterizations of the giant pore metal-organic
Eng. 2015;2015:1–9. doi: 10.1155/2015/896486. framework aluminum trimesate MIL-100. Chem Mater.
[17] Alenazi NA, Hussein MA, Alamry KA, Asiri AM. Modified 2009;21(24):5695–7. doi: 10.1021/cm901983a.
polyether-sulfone membrane: a mini review. Des Monomers [31] Zlotea C, Campesi R, Cuevas F, Leroy E, Dibandjo P,
Polym. 2017;20(1):532–46. doi: 10.1080/15685551.2017.1398208. Volkringer C, et al. Pd nanoparticles embedded into a metal-
[18] Mustafa SGEE, Mannan HA, Nasir R, Mohshim DF, Mukhtar H. organic framework: synthesis, structural characteristics, and
Synthesis, characterization, and performance evaluation of hydrogen sorption properties. J Am Chem Soc.
PES/EDA-functionalized TiO2 mixed matrix membranes for 2010;132(9):2991–7. doi: 10.1021/ja9084995.
CO2/CH4 separation. J Appl Polym Sci. 2017;134(39):1–8. [32] Vinu M, Lin WC, Raja DS, Han JL, Lin CH. Microwave-assisted
doi: 10.1002/app.45346. synthesis of nanoporous aluminum-based coordination poly-
[19] Hess SC, Grass RN, Stark WJ. MOF channels within porous mers as catalysts for selective sulfoxidation reaction.
polymer film: flexible, self-supporting ZIF-8 poly(ether Polymers. 2017;9(10):1–11. doi: 10.3390/polym9100498.
sulfone) composite membrane. Chem Mater. [33] Feng Y, Wang C, Ke F, Zang J, Zhu J. MIL-100(Al) gels as an
2016;28(21):7638–44. doi: 10.1021/acs.chemmater.6b02499. excellent platform loaded with doxorubicin hydrochloride for
[20] Farnam M, Mukhtar H, Shariff A. Analysis of the influence of pH-triggered drug release and anticancer effect.
CMS variable percentage on pure PES membrane gas separa- Nanomaterials. 2018;8(6):1–11. doi: 10.3390/nano8060446.
tion performance. Procedia Eng. 2016;148:1206–12. [34] Kobbe-Dama N, Szymczyk A, Tamsa AA, Tchatchueng JB.
doi: 10.1016/j.proeng.2016.06.449. Preparation and characterization of PES-based membranes:
[21] Jeazet HBT, Staudt C, Janiak C. Metal-organic frameworks in Impact of the factors using a central composite an experi-
mixed-matrix membranes for gas separation. Dalton Trans. mental design. Int J of Chemistry and Chem Eng Syst.
2012;41:14003–27. doi: 10.1039/c2dt31550e. 2019;4:5–15.
[22] Guo X, Huang H, Ban Y, Yang Q, Xiao Y, Li Y, et al. Mixed [35] Lee RJ, Jawad ZA, Ahmad AL, Ngo JQ, Chua HB. Improvement of
matrix membranes incorporated with amine-functionalized CO2/N2 separation performance by polymer matrix cellulose
titanium-based metal-organic framework for CO2/CH4 acetate butyrate. IOP conference series: materials science and
separation. J Membrane Sci. 2015;478:130–9. doi: 10.1016/ engineering, volume 206: 29th symposium of Malaysian che-
j.memsci.2015.01.007. mical engineers (SOMChE); 2016 Dec 1–3; Miri, Sarawak,
[23] Seoane B, Sebastian V, Tellez C, Coronas J. Crystallization in Malaysia. Bristol: IOP Publishing; 2017. p. 1–9.
THF: the possibility of one-spot synthesis of mixed matrix [36] Gunawan T, Rahayu RP, Wijiyanti R, Salleh WNW, Widiastuti N.
membranes containing MOF MIL-68(Al). CrystEngComm. P84/Zeolite-Carbon composite mixed matrix membrane for
2013;15:9483–90. doi: 10.1039/C3CE40847G. CO2/CH4 separation. Indones. J. Chem. 2019;19(3):650–9.
[24] Venna SR, Lartey M, Li T, Spore A, Kumar S, Nulwala HB, et al. doi: 10.22146/ijc.35727.
Fabrication of MMMs with improved gas separation properties [37] Zulhairun AK, Ismail AF, Matsuura T, Abullah MS, Mustafa A.
using externally-functionalized MOF particles. J Mater Chem A. Asymmetric mixed matrix membrane incorporating
2015;3:5014–22. doi: 10.1039/c4ta05225k. organically modified clay particle for gas separation.
[25] Ge B, Xu Y, Zhao H, Sun H, Guo Y, Wang W. High performance Chem Eng J. 2014;241:495–503. doi: 10.1016/
gas separation mixed matrix membrane fabricated by j.cej.2013.10.042.
incorporation of functionalized submicrometer-sized [38] Xia W, Zhang X, Xu L, Wang Y, Lin J, Zou R. Facile and eco-
metal-organic framework. Materials. 2018;11:1–16. nomical synthesis of metal-organic framework MIL-100(Al)
doi: 10.3390/ma11081421. gels for high efficiency removal of microcystin-LR. RSC Adv.
[26] Ozen HA, Ozturk B, Obekcan H. Synthesis, characterization 2013;3:11007–13. doi: 10.1039/c3ra40741a.
and gas separation properties of MOF-5 mixed matrix mem- [39] Mahalakshmi G, Balachandran VFT-IR. and FT-Raman spectra,
branes. Nevsehir Bilim ve Teknoloji Dergisi. 2017;6:415–23. normal coordinate analysis and ab initio computations of
doi: 10.17100/nevbiltek.332792. trimesic acid. Spectrochim Acta A. 2014;124:535–47.
[27] Liu CS, Sun CX, Tian JY, Wang ZW, Ji HF, Song YP, et al. Highly doi: 10.1016/j.saa.2014.01.061.
stable aluminum-based metal-organic frameworks as biosen- [40] Mushtaq A, Mukhtar HB, Shariff AM. FTIR study of enhanced
sing platforms for assessment of food safety. polymeric blend membrane with amine. Research Journal ofCO2 gas separation using MMMs based on polyethersulfone/MIL-100(Al) 321
Applied Sciences, Engineering and Technology. 2015 July 1–3; London, UK. Hong Kong: Newswood Limited;
2014;7(9):1811–20. doi: 10.19026/rjaset.7.466. 2015. p. 737–9.
[41] Qadir D, Mukhtar H, Keong LK. Synthesis and characterization [45] Li JR, Kuppler RJ, Zhou HC. Selective gas adsorption and
of polyethersulfone/carbon molecular sieve based mixed separation in metal-organic frameworks. Chem Soc Rev.
matrix membranes for water treatment applications. 2009;38:1477–504. doi: 10.1039/b802426j.
Procedia Eng. 2016;148:588–93. doi: 10.1016/j/ [46] Shimekit B, Mukhtar H. Natural gas purification technologies –
proeng.2016.06.517. major advances for CO2 separation and future directions.
[42] Sotomayor F, Cychosz KA, Thommes M. Characterization of In: Al-Megren H, eds., Advances in natural gas technology.
micro/mesoporous materials by physisorption: concepts and London: IntechOpen; 2012.
case studies. Acc Mater Surf Res. 2018;3(2):34–50. [47] Weigelt F, Georgopanos P, Shishatskiy S, Filiz V, Brinkmam T,
[43] Vinoba M, Bhagiyalakshmi M, Alqaheem Y, Alomair AA, Abetz V. Development and characterization of defect-free
Perez A, Rana MS. Recent progress of filler in mixed matrix Matrimid® mixed-matrix membranes containing activated
membranes for CO2 separation: a review. Sep Purif Technol. carbon particles for gas separation. Polymers.
2017;188:431–50. doi: 10.1016/j.seppur.2017.07.051. 2018;10(51):1–21. doi: 10.3390/polym10010051.
[44] Orakwe IR, Nwogu NC, Kajama M, Shehu H, Okon E, Gobina E. [48] Robeson LM. Correlation of separation factor versus perme-
An initial study of single gas permeation using a ability for polymeric membranes. J Membr Sci,
commercial alumina membrane. In: Ao SI, Gelman L, 1991;62(2):165–85. doi: 10.1016/0376-7388(91)80060-J.
Hukins DWL, Hunter A, Korsunsky AM, eds., Proceedings [49] Robeson LM. The upper bound revisited. J Membr Sci. 2008;
of the 2015 world congress on engineering (WCE 2015), 320(1–2):390–400. doi: 10.1016/j.memsci.2008.04.030.You can also read
