Influence of anion size and electronic structure on the gas separation performance of ionic liquid/ZIF-8 composites

Microporous and Mesoporous Materials 306 (2020) 110446

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Microporous and Mesoporous Materials
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Influence of anion size and electronic structure on the gas separation
performance of ionic liquid/ZIF-8 composites
Muhammad Zeeshan a, b, Harun Kulak a, b, Safiyye Kavak b, c, H. Mert Polat b, c, Ozce Durak a, b,
Seda Keskin a, b, *, Alper Uzun a, b, d, **
a

Department of Chemical and Biological Engineering, Koç University, Rumelifeneri Yolu, 34450, Sariyer, Istanbul, Turkey
Koç University TÜPRAŞ Energy Center (KUTEM), Koç University, Rumelifeneri Yolu, 34450, Sariyer, Istanbul, Turkey
Department of Materials Science and Engineering, Koc University, Rumelifeneri Yolu, 34450, Sariyer, Istanbul, Turkey
d
Koỗ University Surface Science and Technology Center (KUYTAM), Koỗ University, Rumelifeneri Yolu, 34450, Sariyer, Istanbul, Turkey
b
c

A R T I C L E I N F O

A B S T R A C T

Keywords:
Ionic liquids (ILs)
Metal organic frameworks (MOFs)
Gas adsorption
Porous material
Hybrid composites

We investigated the influences of the changes in the electronic structure and size of the anion of an imidazolium
ionic liquid (IL) on gas adsorption and separation performance of the IL/ZIF-8 (zeolitic imidazolate framework)
composites. We studied four different imidazolium ILs having the same cation, 1-n-butyl-3-methylimidazolium,
[BMIM]ỵ, with anions having structures allowing a systematic comparison of the changes in the electronic
structure and size. To examine the influence of changes in the electronic structure, we considered anions rep­
resenting the fluorination on the anion, methanesulfonate, [MeSO3]À , and trifluoromethanesulfonate,
[CF3SO3]À . To investigate the change in the anion size, methyl sulfate, [MeSO4]À , and octyl sulfate, [OcSO4]À ,
were studied. Characterization of IL/ZIF-8 composites demonstrated successful incorporation of each IL in ZIF-8
without causing any detectable changes in the crystal structure and morphology of ZIF-8. Thermogravimetric
analysis and infrared (IR) spectroscopy indicated the presence of direct interactions between ILs and ZIF-8, which
directly control gas separation performance of the composite. Gas adsorption measurements illustrated that
incorporation of ILs significantly improves the gas separation performance of the pristine ZIF-8. [BMIM]
[MeSO4]/ZIF-8 composite had 3.3- and 1.8-times higher CO2/N2 and CH4/N2 selectivities compared to ZIF-8,
respectively, at 1 bar. When the IL has a fluorinated anion, CO2/CH4 selectivity improved 3-times compared
to its non-fluorinated counterpart. Upon the incorporation of IL with a small anion, IL/ZIF-8 composite showed
higher CO2/N2 and CH4/N2 selectivities compared to the composite having an IL with a bulky anion. These
results will contribute in guiding rational design of IL/MOF composites for different gas separations.

1. Introduction
Excessive combustion of fossil fuels led to a significant increase in
CO2 concentration in the atmosphere. This increase is the main reason
for the climate change and global warming [1–4]. Moreover, purifica­
tion of natural gas is a crucial process because the presence of impurities,
such as CO2, reduces the total calorific value of natural gas and promotes
the corrosion in pipelines and equipment [5]. Among the existing CO2
capture and separation technologies, adsorption-based gas separation
process by nanoporous materials has emerged as an energy- and
cost-effective technology [6,7]. Thus, it is critical to design and syn­
thesize novel microporous materials that have a potential to selectively

capture CO2 from a mixture of gas streams, such as CH4 and N2. Metal
organic frameworks (MOFs), a novel class of porous crystalline mate­
rials, have been recently considered for the capture and separation of
CO2 from gas mixtures containing CH4 and N2 as alternatives to tradi­
tional adsorbents, such as zeolites, alumina, silica gels, carbon molec­
ular sieve, and carbon nanotubes [8–11]. Furthermore, owing to the
ability of changing the metal nodes and linkers, MOFs offer large surface
areas, high pore volumes, variety of pore sizes and shapes, and reason­
able chemical and thermal stabilities [12,13]. Several studies demon­
strated tuning of the physicochemical properties of a parent MOF by
various post-synthesis modification techniques, such as amine func­
tionalization, metal, and ligand exchange, and surface functionalization

* Corresponding author. Department of Chemical and Biological Engineering, Koç University, Rumelifeneri Yolu, 34450, Sariyer, Istanbul, Turkey.
** Corresponding author. Koç University TÜPRAŞ Energy Center (KUTEM), Koç University, Rumelifeneri Yolu, 34450, Sariyer, Istanbul, Turkey.
E-mail addresses: (S. Keskin), (A. Uzun).
/>Received 25 May 2020; Received in revised form 25 June 2020; Accepted 28 June 2020
Available online 2 July 2020
1387-1811/© 2020 The Authors.
Published by Elsevier Inc.
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M. Zeeshan et al.

Microporous and Mesoporous Materials 306 (2020) 110446

[14–17]. Among these approaches, post-synthesis modification of MOFs
by combining them with ionic liquids (ILs) has offered a broad prospect
in tuning gas adsorption and separation performance of a parent MOF
[18–20]. ILs are novel solvents that are composed of cations and anions,
and generally have lower melting points than 100 � C [21]. The unique
properties of ILs, such as low vapor pressure, high thermal stability, and
tunable physicochemical properties, offer a broad potential for various
applications, such as catalysis [22], lubricants [23], electrolytes [24],
sensors [25], and gas adsorption and separation processes [26,27].
Among these applications, post-synthesis modifications of MOFs by
combining them with ILs offer opportunities especially for designing
novel materials having a high performance in CO2 adsorption and sep­
aration because of the high solubility of CO2 in most ILs.
Several studies reported that upon the incorporation of ILs into the
pores of a MOF, gas adsorption and separation performance of the

parent MOF improved significantly [28–37]. For instance,
1-n-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) was
incorporated into copper benzene-1,3,5-tricarboxylate (CuBTC) and
zeolite imidazolate framework, ZIF-8 [28,29]. Results showed im­
provements in gas separation performance of both MOFs. Mohamedali
et al. [30–32] reported impregnation of 1-n-butyl-3-methylimidazolium
acetate ([BMIM][Ac]) and 1-ethyl-3-methylimidazolium acetate
([EMIM][Ac]) into CuBTC, ZIF-8, and MOF-77. Results demonstrated an
improved CO2 adsorption capacity and CO2/N2 separation performance
for each IL/MOF composite. Our group reported that incorporation of
1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6])
and 1-n-butyl-3-methylimidazolium thiocyanate ([BMIM][SCN]) resul­
ted in enhanced CO2/CH4, CO2/N2, and CH4/N2 separation perfor­
mances of ZIF-8 [33,34]. Ma et al. [35] studied incorporation of a
task-specific
IL
1-(3-aminopropyl)-2-butylimidazolium
tris(tri­
fluoromethanesulfonyl)methide ([C3NH2BIM][Tf2N]) into chromium 1,
4-benzenedicarboxylate (NH2-MIL-101(Cr)) and reported an improve­
ment in CO2/N2 selectivity. In a similar study, Ding et al. [36] explored
the incorporation of imidazolium-based poly (ionic liquid)s (polyILs)
into MIL-101 and reported an improvement in both CO2 uptake capacity
and CO2/N2 separation performance. Besides, our group recently
examined six different imidazolium-based ILs by incorporating them
into aluminum 1,4-benzenedicarboxylate (MIL-53(Al)) and reported
increased CO2/CH4 and CO2/N2 selectivities compared to those of
parent MOF [37,38].
Studies discussed above imply that gas adsorption capacity and gas
separation performance of IL/MOF composites are significantly

controlled by the IL‒MOF interactions. Furthermore, these studies also
suggest that anion part of the IL is dominant in controlling the IL‒MOF
and IL‒adsorbate interactions. None of these studies, however, focused
on systematically investigating the structural changes on the individual
components of IL/MOF composites and their consequences on the gas
separation performance of the materials. Such investigations potentially
provide insights on the structure-performance relationships of these
composites and, therefore, they are crucial for the rational design of IL/
MOF composites with a high gas separation performance. To contribute
into this field, in this work, we geared at examining the impact of sys­
tematic changes on the electronic structure and size of the anion of an
imidazolium-type IL on the gas separation performance of the corre­
sponding IL/MOF composite.
We studied four different imidazolium ILs having the same [BMIM]ỵ
cation and different anions (methanesulfonate, [MeSO3] ; tri­
fluoromethanesulfonate, [CF3SO3]À ; methyl sulfate, [MeSO4]À ; octyl
sulfate, [OcSO4]À ) and incorporated them into ZIF-8 at comparable IL
loadings. We chose ZIF-8, as this MOF offers a versatile platform for the
incorporation of ILs [33,34,39]. The composites were prepared by the
post-synthesis modification as before [29] and characterized in detail by
combining different experimental techniques, such as X-ray diffraction
(XRD), Brunauer-Emmett-Teller (BET) analysis, scanning electron mi­
croscopy (SEM), thermogravimetric analysis (TGA), X-ray fluorescence
(XRF) and infrared spectroscopies (IR). Afterward, to examine the gas

adsorption and separation performance of the pristine ZIF-8 and
IL/ZIF-8 composites, volumetric adsorption measurements for CO2, CH4,
and N2 were performed. Results showed that IL/ZIF-8 composite with a
fluorinated anion led to an improved CO2/CH4 separation performance,
whereas incorporation of the IL with a smaller anion into ZIF-8 resulted

in a superior CO2/N2 and CH4/N2 separation performance. Character­
ization data indicated that these changes in separation performance are
directly related with the changes in the interactions between the IL
molecules and the ZIF-8 cage. These results illustrate that the changes in
both the electronic environment and the size of the IL’s anion play a
significant role in determining the interactions and their consequences
on the separation performance. Thus, they provide much needed in­
sights for the rational design of IL/MOF composites having improved gas
separation performances.
2. Materials and methods
2.1. Materials
All the ILs, ZIF-8 (Basolite Z1200, 2-methylimidazole zinc salt), and
acetone were obtained from Sigma–Aldrich and stored in an Ar-filled
glove box (Labconco). CH4 (99.95%), CO2 (99.9%), N2 (99.998%),
and He (99.999%) were purchased from Air Liquide.
2.2. Sample preparation
Pristine ZIF-8 was first activated at 105 � C overnight under vacuum
prior to incorporation of the IL. Each IL/ZIF-8 composite was prepared
by wet impregnation, as previously reported [33]. The IL/ZIF-8 com­
posites with a targeted IL loading of 30 wt% were prepared by dissolving
300 mg of IL in 20 mL of acetone by stirring for 1 h under ambient
conditions. Then 700 mg of dehydrated ZIF-8 powder was added to the
solution and the mixture was stirred at 35 � C in an open atmosphere
allowing the solvent to evaporate itself at a slow pace. After the solvent
was completely evaporated, the resulting IL/ZIF-8 composites were
further dried at 105 � C overnight. The synthesized IL/ZIF-8 composites
were stored in a desiccator.
2.3. X-ray fluorescence (XRF) spectroscopy
The elemental analyses of the IL/ZIF-8 composites were conducted
on a Bruker S8 Tiger spectrometer. The analyses were performed under

He atmosphere and an X-ray tube with 4 kW Rh anode was used to
generate X-rays. SpectraPlus Eval2 V2.2.454 software was used for the
interpretation of obtained data.
2.4. Brunauer-Emmett-Teller (BET) analysis
A Micromeritics ASAP 2020 was utilized to determine the surface
area and pore volume from the N2 adsorption isotherms obtain at À 196
�
C for pristine ZIF-8 and IL/ZIF-8 composites. Prior to each measure­
ment, 150 mg of sample was degassed at 125 � C for 12 h under vacuum.
N2 adsorption isotherm was obtained between the pressure range of
10À 6 and 1 bar. The BET equation and the t-plot method were used to
calculate surface area and pore volume of the samples in the relative
pressure range 0.05–0.65.
2.5. Scanning electron microscopy (SEM)
SEM images of the pristine ZIF-8 and its composites with the ILs were
obtained with a Zeiss Evo LS 15 using an accelerating voltage of 3 kV
under vacuum. The sample surfaces were sputtered with gold prior to
each measurement. The SEM images were obtained at two different
magnifications (100 k� and 25 k� ).
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Microporous and Mesoporous Materials 306 (2020) 110446

2.6. X-ray diffraction (XRD) spectroscopy

error range of XRF measurements are presented in Table 1. Data showed
that each IL/ZIF-8 composite had an IL loading of 25.5 � 1.5 wt%. This

amount matches the amount reported to be the highest IL loading on IL/
ZIF-8 composites that can be achieved before overfilling the pores of ZIF8 to exceed the wetness point [28,29].
Fig. 1 represents the N2 adsorption-desorption isotherms of the
pristine ZIF-8 and the IL/ZIF-8 composites measured at À 196 � C. The
results presented in Fig. 1 showed typical type-I isotherms without any
profound hysteresis loop for pristine ZIF-8 and IL-incorporated com­
posites. These observations suggest that upon the incorporation of ILs,
ZIF-8 retains its microporous feature in each IL/ZIF-8 composite.
BET surface areas and pore volumes obtained from the correspond­
ing N2 isotherms for pristine MOF and IL-incorporated MOF composites
are summarized in Table 2. The data showed that the BET surface areas
and pore volumes of IL/ZIF-8 composites are notably lower than those of
the pristine ZIF-8, as observed in previous reports [33]. This difference
can be attributed to the successful incorporation of IL molecules into
MOF pores, thereby reducing the overall N2 uptake. However, it is
noteworthy that overall N2 uptake is also dependent on the solubility of
N2 in the ILs. Thus, because of the poor solubility of N2 in the IL, espe­
cially at the measurement conditions, some IL molecules located at the
gate openings of the ZIF-8 might block the accessibility of N2 molecules
into the completely/partially available MOF pores [45]. Therefore, we
note that the BET measurements of IL/ZIF-8 composites are not very
reliable even though they consistently present a decrease in surface area
upon the incorporation of IL.
Fig. 2 shows the SEM images of the pristine ZIF-8 and IL/ZIF-8
composites demonstrating the surface morphologies of the materials.
Accordingly, SEM images of IL-incorporated ZIF-8 composites clearly
show that the rhombic dodecahedron structure of pristine ZIF-8 was
preserved upon the incorporation of IL.
To further evaluate the crystal structure of the samples, XRD patterns
of the pristine ZIF-8 and IL/ZIF-8 composites were obtained as shown in

Fig. 3. Results showed that all the characteristics peaks of ZIF-8 were
intact for each IL/MOF composite, thus, it can be inferred that the
crystallinity of ZIF-8 was well-maintained upon the incorporation of ILs.
However, compared to pristine ZIF-8, the intensities of the diffraction
peaks were slightly different in the IL-incorporated MOF composites,
implying the presence of possible changes in the electronic structure
inside the MOF pores or in the crystal orientation.
Next, we investigated the thermal stabilities of the IL/MOF com­
posites. TGA measurements were performed for the pristine MOF, the
bulk ILs, and the IL/MOF composites as shown in Fig. 4. The derivative
onset temperatures (T0 onset) for pristine ZIF-8, bulk ILs, and IL/ZIF-8
composites were determined from the derivative TG curves.
Results presented in Fig. 4 showed a typical one-step decomposition
for ZIF-8 and bulk ILs, whereas a two-step decomposition mechanism
was observed in the TGA curves of IL/ZIF-8 composites. The initial
weight loss between 100 and 150 � C observed on each TGA curve can be
attributed to the evaporation of the moisture content in each sample.
Accordingly, the T0 onset of pristine ZIF-8, bulk [BMIM][MeSO3], [BMIM]
[CF3SO3], [BMIM][MeSO4], and [BMIM][OcSO4] were found as 375,
278, 324, 302, and 252 � C, respectively. Upon incorporation of the ILs
into ZIF-8, thermal stability of each IL/ZIF-8 decreased compared to that
of pristine ZIF-8. Accordingly, T0 onset for [BMIM][MeSO3]/ZIF-8,
[BMIM][CF3SO3]/ZIF-8, [BMIM][MeSO4]/ZIF-8, and [BMIM][OcSO4]/
ZIF-8 composites were found to be 257, 315, 241, and 242 � C, respec­
tively. Thus, these changes in T0 onset values and in the total weight losses
indicate changes in the decomposition mechanisms in the composites,
which confirm the presence of IL‒MOF interactions. These results are
consistent with a comprehensive report on the structural factors deter­
mining the thermal stabilities of the IL/MOF composites, reported pre­
viously [41]. To further identify these IL‒MOF interactions, IR spectra of

the pristine MOF, bulk IL, and the IL/ZIF-8 composite were acquired and
examined in detail. Fig. 5 shows the IR spectra of pristine ZIF-8, bulk IL,
and IL-incorporated ZIF-8 composites in the spectral regions of

XRD pattern of pristine ZIF-8 and IL/ZIF-8 composites were obtained
using a Bruker D8 Advance instrument with Cu-Kα1 radiation (λ ¼
1.5406 Å) operating at a voltage of 30 kV and a current of 10 mA. Each
diffraction pattern was collected in a 2θ range of 5–50� , with a step size
of 0.0204� .
2.7. Thermal gravimetric analysis (TGA)
TGA of the pristine ZIF-8, bulk ILs, and IL/ZIF-8 composites were
performed on a TA Instruments Q500 thermogravimetric analyzer. The
analysis was carried under N2 atmosphere of 40 and 60 mL minÀ 1 for
balance and purge gases, respectively. After taring the pan, approxi­
mately 10 mg of each sample was loaded into a platinum pan and
temperature was increased from room temperature to 100 � C at a ramp
rate of 5 � C minÀ 1. After an isothermal treatment for 8 h at 100 � C,
temperature was further increased at a ramp rate of 2 � C minÀ 1 to 700
�
C. For comparing thermal decomposition temperature of the samples,
the thermal decomposition temperatures, the onset (Tonset) and deriva­
tive onset (T0 onset) temperatures were determined from the thermogra­
vimetric (TG) and derivative TG curves. In this study, derivative onset
temperatures (T0 onset) were considered for comparison analysis, because
onset temperature values (Tonset) generally overestimate the decompo­
sition temperature, as previously reported [40,41].
2.8. Infrared spectroscopy (IR)
IR spectra of the pristine ZIF-8 and IL/ZIF-8 composites were
recorded using a Bruker Vertex 80v FTIR spectrometer averaging 512
scans collected at a spectral resolution of 2 cmÀ 1. Sample was loaded

between two potassium bromide (KBr) windows in an IR cell, and ana­
lyses were performed under vacuum at room temperature to obtain IR
spectra between 650 and 4000 cmÀ 1 in transmission mode. IR bands
deconvolution was performed using Fityk software by employing the
Voigt function [42].
2.9. Conductor-like screening model for realistic solvents (COSMO-RS)
calculations
To predict the CO2, CH4, and N2 solubilities, we used the COSMO­
ThermX software, version C30_160 [43]. The gas solubilities were
calculated in a pressure range of 0.1–1 bar at 25 � C. These calculations
were performed using the TZVP parameterizations, whereas the solu­
bility values were obtained from the activity coefficients.
2.10. Gas adsorption measurements
A High-Pressure Volumetric Analyzer (Micromeritics HPVA II-200)
was used to perform single-component gas adsorption measurements
of samples for CO2, CH4, and N2 gases. For each measurement,
approximately 300 mg of sample was loaded into the sample holder and
degassed overnight at 150 � C under vacuum. After degassing, the system
was purged with He gas three-times to remove the unwanted residual
gases from the previous measurement. Afterward, adsorption isotherm
of CO2, CH4, and N2 gases were obtained in a pressure range of 0.1–1 bar
at 25 � C. Gas adsorption isotherms were fitted to the dual-site Langmuir
(DSL), Langmuir-Freundlich (LF), and dual-Site Langmuir-Freundlich
(DSLF) models using Ideal Adsorbed Solution Theory (IAST)ỵỵ [44],
software to calculate the ideal and mixture CO2/CH4, CO2/N2, and
CH4/N2 selectivities. Fitting parameters for gas adsorption isotherms are
provided in Table S1 of Supporting Information (SI).
3. Results and discussion
The elemental composition of the composites determined within the
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Table 1
Zn and S amount in IL/ZIF-8 composites determined from XRF analysis.
Sample

Zn
Concentration [wt%]

S
Concentration [wt%]

Corresponding IL Loading [wt%]

[BMIM][MeSO3]/ZIF-8
[BMIM][CF3SO3]/ZIF-8
[BMIM][MeSO4]/ZIF-8
[BMIM][OcSO4]/ZIF-8

13.44
14.20
15.91
16.79

2.16
2.05

2.46
1.73

25
27
26
24

Fig. 1. N2 isotherms of pristine ZIF-8 and IL/ZIF-8 composites at À 196 � C.

these interactions were shared with the MOF. However, in the case of the
composite involving the IL with a fluorinated anion, the interionic in­
teractions between the sulfonate groups become stronger as evidenced
by a major blue-shift. This increase in strengthening of the interionic
interaction can be attributed to the presence of highly electronegative
character of fluorine atoms, which probably attracts electrons from the
MOF. Furthermore, we note that these shifts in the IR features of the bulk
ILs upon the incorporation of ILs into ZIF-8 are consistent with a pre­
vious report, where [BMIM][CF3SO3] was incorporated into MIL-53(Al)
[38].
Likewise, to demonstrate the influence of IL’s anion size on the IL‒
MOF interactions, we examined the IR spectra of bulk [BMIM][MeSO4],
[BMIM][OcSO4], and the corresponding IL/ZIF-8 composites in detail.
The peaks at 1009 and 1218 cmÀ 1 correspond to νas (—SO3) and νs
(—SO3) stretching modes of bulk ILs, respectively. The peak at 1009
cmÀ 1 red-shifted to 1003 and 1006 cmÀ 1 in the IR spectra of [BMIM]
[MeSO4]/ZIF-8 and [BMIM][OcSO4]/ZIF-8 composites, respectively.
However, the peak at 1218 cmÀ 1 did not exhibit any shifts. Here, we
infer that the change in the anion size of imidazolium ILs have no sig­
nificant impact on IL‒MOF interactions. Furthermore, to investigate any

evidence of the IL‒MOF interactions in the higher IR region, we
considered ν(C2H) band related to the ring structure of IL’s cation. The
corresponding bands were located at 3109, 3117, 3105, and 3107 cmÀ 1
in the bulk IR spectra of [BMIM][MeSO3], [BMIM][CF3SO3], [BMIM]
[MeSO4], and [BMIM][OcSO4], respectively [46]. These bands exhibi­
ted blue-shifts of 7, 12, 8, and 10 cmÀ 1 in the corresponding IR spec­
trums of IL/ZIF-8 composites, respectively. As the interionic interaction
energies between cation and anion of the bulk IL is probed by the ν(C2H)
band, a major blue-shift in the band position of this feature implies the
weak interactions between the cation and anion of the IL when it is
confined in the MOF cage [47,48]. As most of these shifts were observed
in the IR spectra of IL’s anion, we infer that incorporation of the IL into
MOFs’ pores leads to the direct interactions between IL’s anion and MOF
surface. These shifts in the IR features of bulk IL indicate the possibility
of electron sharing between the IL and MOF, defined the nature of IL‒

Table 2
BET surface area and pore volume of pristine ZIF-8 and IL/ZIF-8 composites.
Sample

SBET [m2gÀ 1]

Vpore [cm3gÀ 1]

ZIF-8
[BMIM][MeSO3]/ZIF-8
[BMIM][CF3SO3]/ZIF-8
[BMIM][MeSO4]/ZIF-8
[BMIM][OcSO4]/ZIF-8

1208
195
362
233
195

0.63
0.09
0.16
0.12
0.08

2800–3200 cmÀ 1 and 650–1800 cmÀ 1.
Appearance of all of the characteristic peaks of each IL in the IR
spectra of the IL/ZIF-8 composites in Fig. 5 further confirms the suc­
cessful incorporation of ILs into framework. To examine the influence of
the fluorination and size change of the IL’s anion on the IL‒MOF in­
teractions, we thoroughly analyzed the changes in the band positions of
the IR features related to the corresponding anion. First, we investigated
the influence of fluorination of IL’s anion on the IL‒MOF interactions,
comparing the data related with [BMIM][MeSO3] and its counterpart
with the fluorinated anion, [BMIM][CF3SO3]. In the lower region of the
IR spectrum of bulk [BMIM][MeSO3], the peaks at 1037 and 1170 cmÀ 1
correspond to asymmetric νas (—SO3) and symmetric νs (—SO3)
stretching modes of the anion of IL, respectively; whereas the corre­
sponding peaks for [BMIM][CF3SO3] were located at 1224 and 1250
cmÀ 1 [37]. In the case of non-fluorinated anion, both νas (—SO3) and νs
(—SO3) modes presented red-shifts of 3 and 4 cmÀ 1 in the IR spectra of
[BMIM][MeSO3]/ZIF-8 composite, respectively; whereas no shifts were
observed in the corresponding IR bands for the composite containing the

fluorinated IL. However, the band at 1154 cmÀ 1 corresponding to νas
(—CF3) in [BMIM][CF3SO3] presented a major blue-shift of 12 cmÀ 1 in
the IR spectra of [BMIM][CF3SO3]/ZIF-8 composite. From these obser­
vations, we infer that incorporation of IL with a non-fluorinated anion,
the interionic interaction between the sulfonate groups of IL’s anion
becomes weaker as indicated by red-shifts, as the electrons involving in
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Fig. 3. XRD patterns of pristine ZIF-8 and IL/ZIF-8 composites.

([BMIM][CF3SO3]) and non-fluorinated ([BMIM][MeSO3]) anions have
similar CO2 uptakes; however, significant differences were observed in
their CH4 and N2 uptakes. CH4 uptake significantly decreased in the case
of the composite having the IL with the fluorinated anion, whereas the
composite having the IL with the non-fluorinated anion was measured to
have a lower N2 uptake. Such differences in gas uptakes could be
attributed to the higher affinity of the fluorinated anion towards CO2
and N2 molecules, which have quadrupole moments, while having a
comparatively weak attraction towards non-polar CH4. Furthermore, it
is well-known that CO2 has a great affinity towards fluorine moieties,
thus the presence of C–F bond in a highly fluorinated anion may act as
Lewis base to interact with acidic carbon atom of CO2. Such interactions
improves the CO2-philicity by providing preferential adsorption sites for
CO2 molecules compared to CH4 [49,50]. We also calculated CO2, CH4,
and N2 solubilities of bulk ILs using COSMO-RS, which is widely used to

estimate the solubilities of various hydrocarbons and gases in ILs
[51–53]. The stronger interactions between CO2 and N2 with the fluo­
rinated anion are in agreement with the gas solubilities estimated by
COSMO-RS calculations, where bulk [BMIM][CF3SO3] has higher CO2
and N2 solubilities compared to the bulk IL having a non-fluorinated
anion ([BMIM][MeSO3]) as shown in Fig. S1. Thus, the lower CH4 up­
take of [BMIM][CF3SO3]/ZIF-8 composite can be attributed to the weak
interactions between the fluorinated anion and CH4, which is also
consistent with the poor solubility of CH4 in the bulk ([BMIM][CF3SO3])
as predicted by COSMO-RS calculations.
In addition to the IL‒MOF surface interactions with the adsorbate
molecules, the size of the IL’s anion significantly influences the
adsorption capacity of the corresponding IL/ZIF-8 composite. The data
showed that incorporation of the IL having a bulky anion ([OcSO4]À )
into ZIF-8 led to the lowest uptakes for each gas compared to the gas
uptakes of [BMIM][MeSO4]/ZIF-8 composite. Here, we note that the gas
solubilities in bulk ILs generally increase with the increase in alkyl chain
length or electronic environment of cation/anion [54,55]. However,
when the anion size of the IL increased, we observed a different trend in
the gas uptakes of the corresponding IL/ZIF-8 composite. This opposite
trend between the bulk ILs’ gas solubilities and their gas uptakes in the
corresponding IL/ZIF-8 composite can be attributed to the change in the
affinity of the corresponding IL towards the adsorbate molecules [38].
Furthermore, we note that when a bulky IL is incorporated into ZIF-8,
less pore volume is available for the adsorbate molecule in the corre­
sponding composite compared to the composite having an IL with a
small anion. Thus, [BMIM][OcSO4]/ZIF-8 composite having an IL with a

Fig. 2. SEM images of (a) ZIF-8, (b) [BMIM][MeSO3]/ZIF-8, (c) [BMIM]
[CF3SO3]/ZIF-8, (d) [BMIM][MeSO4]/ZIF-8, and (e) [BMIM][OcSO4]/ZIF-8 at

magnifications of 100 k� and 25 k�.

MOF interactions in the composites. To examine the influence of these
interactions on the gas adsorption and separation performance of the
materials, single-component gas adsorption isotherms for CO2, CH4, and
N2 were measured in a pressure range of 0.1–1 bar for pristine ZIF-8 and
IL/ZIF-8 composites at 25 � C. The gas adsorption isotherms for pristine
ZIF-8 and IL/ZIF-8 composites are presented in Fig. 6.
As demonstrated in Fig. 6, the gas uptake capacity of each IL/MOF
composite reduced compared to that of pristine ZIF-8. This decrease in
the uptake capacity of the IL-incorporated ZIF-8 composites can be
attributed to the reduced available surface area and pore volume by the
presence of the IL molecules in the cages of the ZIF-8. Furthermore, the
data showed that the IL/MOF composites with ILs having a fluorinated
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Microporous and Mesoporous Materials 306 (2020) 110446

Fig. 4. TGA curves of pristine ZIF-8, bulk ILs, and IL/ZIF-8 composites: (a) [BMIM][MeSO3]/ZIF-8, (b) [BMIM][CF3SO3]/ZIF-8, (c) [BMIM][MeSO4]/ZIF-8, and (d)
[BMIM][OcSO4]/ZIF-8.

bulky anion showed the lowest uptake for each gas compared to [BMIM]
[MeSO4]/ZIF-8 composite.
Finally, to assess the influence of the differences in the gas uptakes on
the gas separation performance of the IL/MOF composites, we fitted the
individual single-component gas adsorption isotherms of pristine ZIF-8
and IL/ZIF-8 composites to dual-site Langmuir, Langmuir-Freundlich,

and dual-site Langmuir-Freundlich models and calculated their ideal
and mixture selectivities. Ideal CO2/CH4, CO2/N2, and CH4/N2 selec­
tivities were calculated from the fitted adsorption isotherms, whereas
mixture selectivities were calculated using the Ideal Adsorption Solution
Theory (IAST) for pristine ZIF-8 and IL/ZIF-8 composites [38]. IAST is
an effective method to predict the gas mixture adsorption data by using
experimentally measured single-component gas adsorption isotherms.
Fig. 7 shows the ideal and mixture (CO2/CH4:50/50, CO2/N2:15/85, and
CH4/N2:50/50) selectivities of IL/ZIF-8 composites normalized by their
corresponding values on the pristine ZIF-8 at the same condition. Thus,
having a normalized value higher than unity for any selectivity value
indicates an improvement in the corresponding selectivity of ZIF-8 upon
the incorporation of IL.
According to Fig. 7(a–c), [BMIM][CF3SO3]/ZIF-8, having an IL with
the fluorinated anion, exhibited 3-times higher ideal selectivity than
that of [BMIM][MeSO3]/ZIF-8 at a low pressure range for CO2/CH4
separation. In addition, IL/ZIF-8 composites with both fluorinated and
non-fluorinated anions showed 2.5- and 2-times higher ideal CO2/CH4
selectivities, respectively, than those of the pristine ZIF-8 at 1 bar.
However, an opposite trend was observed for the ideal CO2/N2 and CH4/
N2 selectivities of the composites. At low pressures, [BMIM][MeSO3]/
ZIF-8 exhibited 3-times higher ideal CO2/N2 selectivity compared to
[BMIM][CF3SO3]/ZIF-8. Furthermore, as the pressure increases, ideal
CO2/N2 selectivity of [BMIM][MeSO3]/ZIF-8 decreased; however, the
selectivity remains 2-times higher compared to that of [BMIM]
[CF3SO3]/ZIF-8 at 1 bar. Similarly, [BMIM][MeSO3]/ZIF-8 having a
non-fluorinated anion exhibited 4.3-times higher ideal CH4/N2 selec­
tivity than that of [BMIM][CF3SO3]/ZIF-8 at 0.01 bar. Here, we
conclude that high electronegativity of the fluorinated anion promotes
the IL interactions with the surface electrons of ZIF-8 cage as discussed

in the IR analysis, where a major blue-shift was observed for νas(—CF3)

band in the IR spectra of [BMIM][CF3SO3]/ZIF-8 composite. Presence of
such strong interactions between the IL and MOF cage favors the pref­
erential adsorption of CO2 compared to CH4, leading to an improvement
in the CO2/CH4 separation performance. On the other hand, because of
the very poor solubility of N2 compared to CO2 and CH4 in a nonfluorinated bulk IL ([BMIM][MeSO3]), the corresponding IL/ZIF-8
composite showed significantly improved CO2/N2 and CH4/N2 separa­
tion performances compared to those of the [BMIM][CF3SO3]/ZIF-8
composite. Furthermore, it is noteworthy here that when ZIF-8 was
incorporated with an IL having either a fluorinated or bulky anion, the
corresponding IL/ZIF-8 composite becomes N2 selective over CH4,
which is the opposite of the separation performance of the pristine ZIF-8.
Thus, we infer that by changing the electronic structure or size of the IL
anion, ZIF-8 separation characteristics can be switched from being CH4
selective to N2 selective in the IL/ZIF-8 composite. This observation
further demonstrates the broad potential of incorporating ILs into MOFs
in tuning the adsorption and separation characteristics of MOF.
Next, we compared the selectivities of [BMIM][MeSO4]/ZIF-8 with
those of [BMIM][OcSO4]/ZIF-8 to investigate the impact of the changes
in the anion size of IL on the corresponding CO2 separation performance
of the composites. At low pressures, [BMIM][OcSO4]/ZIF-8 composite
presented 2-times higher CO2/CH4 separation performance than
[BMIM][MeSO4]/ZIF-8 composite. Moreover, both [BMIM][MeSO4]/
ZIF-8 and [BMIM][OcSO4]/ZIF-8 composites showed approximately
1.5-times higher CO2/CH4 separation performance compared to pristine
ZIF-8 at 1 bar. Furthermore, [BMIM][MeSO4]/ZIF-8 showed 1.5- and
3.3-times higher ideal CO2/N2 selectivity than that of the [BMIM]
[OcSO4]/ZIF-8 composite at 0.01 and 1 bar. Likewise, [BMIM][MeSO4]/
ZIF-8 composite having a small anion showed approximately 1.8-times

higher CH4/N2 separation performance compared to [BMIM][OcSO4]/
ZIF-8 in the whole pressure range (0.01–1 bar). These results demon­
strate that incorporation of IL with a small anion ([MeSO4]À ) into ZIF-8
significantly improved CO2/N2 and CH4/N2 separation performance of
the composite.
Next, we considered another structural change in the anion by
comparing the ILs having sulfite and sulfate groups in their anions to
demonstrate their impact on CO2 separation of the composites.
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Microporous and Mesoporous Materials 306 (2020) 110446

Fig. 5. IR spectra of pristine ZIF-8, bulk IL, and IL-incorporated ZIF-8 composite: (a) [BMIM][MeSO3]/ZIF-8, (b) [BMIM][CF3SO3]/ZIF-8, (c) [BMIM][MeSO4]/ZIF-8,
and (d) [BMIM][OcSO4]/ZIF-8.

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Microporous and Mesoporous Materials 306 (2020) 110446

Fig. 6. Excess adsorption isotherms of (a) CO2, (b) CH4, and (c) N2 in pristine ZIF-8 and IL/ZIF-8 composites at 25 � C.

Accordingly, IL/ZIF-8 composites ([BMIM][MeSO3]/ZIF-8 and [BMIM]
[MeSO4]/ZIF-8) having ILs with sulfite and sulfate groups in their an­
ions, respectively, have almost similar CO2/CH4 and CH4/N2 separation

performances; however, the IL/ZIF-8 composite with an IL having a
sulfate group in its anion presented 2-times higher ideal CO2/N2 selec­
tivity than IL/ZIF-8 composite with sulfite anion at 1 bar.
Furthermore, for a qualitative comparison between the separation
performances of the bulk ILs and their corresponding IL/ZIF-8 com­
posites, CO2/CH4, CO2/N2, and CH4/N2 selectivities of bulk ILs based on
the ratios of the corresponding gas solubilities determined from the
COSMO-RS calculations were estimated as presented in Fig. S2.
Accordingly, fluorination of the anion led to an improvement in CO2/
CH4 separation performance, whereas the corresponding CO2/N2 and
CH4/N2 selectivities were lower than those of composites having the IL
with a non-fluorinated anion. On the other hand, an increase in the
anion size of the IL leads to a decrease in both ideal CO2/CH4 and CO2/
N2 selectivities; however, the ideal CH4/N2 selectivity was improved.
Similarly, the bulk IL with a sulfate anion has higher CO2/CH4 and CO2/
N2 selectivities compared to those of the IL with a sulfite anion. These
trends in CO2/CH4, CO2/N2, and CH4/N2 separation performances of
bulk ILs are consistent with their corresponding IL/ZIF-8 composites.
Gases exist as mixtures in real processes, therefore, we performed
IAST calculations to predict the corresponding mixture selectivities for
CO2/CH4:50/50, CO2/N2:15/85, and CH4/N2:50/50 separations as
presented in Fig. 7(d–f). Accordingly, the highest improvement in CO2/
CH4:50/50 separation was observed for [BMIM][MeSO4]/ZIF-8 fol­
lowed by [BMIM][CF3SO3]/ZIF-8, [BMIM][MeSO3]/ZIF-8, and [BMIM]

[OcSO4]/ZIF-8. At 1 bar, the corresponding mixture selectivities were
3.7-, 2.7-, 2-, and 1.3-times higher than the CO2/CH4 separation per­
formance of pristine ZIF-8, respectively. Likewise, CO2/N2 mixture se­
lectivities of [BMIM][MeSO4]/ZIF-8 and [BMIM][MeSO3]/ZIF-8 were
calculated to be 2.3- and 1.7-times higher than those of pristine ZIF-8 at

1 bar, respectively. Furthermore, at low pressure (0.01 bar), [BMIM]
[MeSO3]/ZIF-8 exhibited 5.5-times higher CH4/N2:50/50 separation
performance compared to its fluorinated counterpart composite
([BMIM][CF3SO3]/ZIF-8). Similarly, a 1.4-times improvement in CH4/
N2:50/50 separation performance was observed in the whole pressure
range of 0.01–1 bar for IL/ZIF-8 composite with small anion ([MeSO4]À )
compared to [BMIM][OcSO4]/ZIF-8. As a result, we can conclude that
IL/MOF composites with a fluorinated anion offer significantly
improved the CO2/CH4 mixture separation performances especially at
low pressures. Whereas the IL/MOF composite having an IL with a nonfluorinated anion led to improvements in CO2/N2:15/85 and CH4/
N2:50/50 separation performances. On the other hand, IL/ZIF-8 com­
posite with a small anion ([MeSO4]À ) demonstrated improvements in
CO2/CH4, CO2/N2, and CH4/N2 mixture selectivities compared to pris­
tine ZIF-8. In contrast, an increase in the anion size improved the CO2/
CH4 mixture selectivity of IL/ZIF-8 composite at low pressures; however,
CO2/N2 and CH4/N2 selectivities of the composite were adversely
affected compared to pristine ZIF-8. Furthermore, IL/ZIF-8 composite
having a sulfate anion ([BMIM][MeSO4]/ZIF-8) showed approximately
1.5-times higher CO2/CH4:50/50 separation performance in the whole
pressure range compared to IL/ZIF-8 composite having an IL with a
sulfite anion ([BMIM][MeSO3]/ZIF-8). Likewise, CO2/N2:15/85 and
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Microporous and Mesoporous Materials 306 (2020) 110446

Fig. 7. Normalized ideal and mixture selectivities of IL/ZIF-8 composites at 25 � C.

CH4/N2:50/50 separation performance of IL/ZIF-8 composite having a
sulfate anion was only 0.6- and 0.4-times of those of the IL/ZIF-8 com­
posite with a sulfite anion at 1 bar. These results suggest that changes in
both electronic environment and the size of the IL’s anion have a sig­
nificant impact on both the ideal and mixture selectivities of IL/ZIF-8
composites.

4. Conclusions
In this study, four different imidazolium ILs having the same cation,
[BMIM]ỵ, and different anions ([MeSO3]À ; [CF3SO3]À ; [MeSO4]À ; and
[OcSO4]À ) were incorporated into ZIF-8 to demonstrate the impact of
changes in the electronic structure and the size of the anion on the gas
adsorption and separation performance of the corresponding IL/ZIF-8
composites. The resultant IL/MOF composites were characterized in
detail by using various techniques. The characterization results of the
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Microporous and Mesoporous Materials 306 (2020) 110446

composites illustrated the successful incorporation of ILs in ZIF-8, while
the crystal structure and morphology of the ZIF-8 were well-maintained.
TGA and IR data confirmed the presence of IL‒MOF interactions that
were accompanied by the changes in the decomposition temperature
and shifts in IR features of bulk ILs in composite samples. Finally, to
investigate the gas adsorption and separation performance, CO2, CH4,
and N2 adsorption isotherms were measured for pristine ZIF-8 and IL/
ZIF-8 composites and their corresponding ideal and mixture selectiv­

ities were determined. Accordingly, [BMIM][CF3SO3]/ZIF-8 and
[BMIM][MeSO4]/ZIF-8 composites exhibited 2.5- and 3.3-times higher
ideal CO2/CH4 and CO2/N2 selectivities compared to pristine ZIF-8 at 1
bar. [BMIM][MeSO3]/ZIF-8 composite showed a 4.3-times higher ideal
CH4/N2 selectivity than that of the pristine ZIF-8 at 0.01 bar, which was
the highest level of improvement among all the IL/ZIF-8 composites
examined in this work. Similarly, CO2/CH4:50/50 mixture selectivities
of [BMIM][CF3SO3]/ZIF-8 and [BMIM][MeSO4]/ZIF-8 were improved
3.7- and 2.7-times compared to those of pristine ZIF-8 at 1 bar, respec­
tively. CO2/N2:15/85 and CH4/N2:50/50 mixture selectivities of
[BMIM][MeSO4]/ZIF-8 improved 2.3- and 1.8-times compared to those
of pristine ZIF-8 at 1 bar, respectively. In summary, we demonstrate that
the ILs with a fluorinated anion significantly improved the CO2/CH4
separation performance, owing to a stronger affinity of C–F bond to­
wards CO2 compared to that of C–H. The poor solubility of N2 in the IL
with a non-fluorinated anion led to improved CO2/N2 and CH4/N2
separation performance of the corresponding IL/ZIF-8 composite. On
the other hand, change in IL anion size did not have a significant impact
on CO2/CH4 separation performance, however, remarkable improve­
ments in the CO2/N2 and CH4/N2 separation performance were
observed for the composites having ILs of small anion. These results
demonstrated that the change in electronic environment and anion size
of ILs alter the IL-MOF interactions, which have significant impacts on
the gas separation performances of the IL/MOF composites.

Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.micromeso.2020.110446.
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Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence

the work reported in this paper.
CRediT authorship contribution statement
Muhammad Zeeshan: Methodology, Investigation, Writing - orig­
inal draft, Writing - review & editing. Harun Kulak: Investigation,
Validation. Safiyye Kavak: Investigation, Validation. H. Mert Polat:
Investigation, Validation. Ozce Durak: Investigation, Validation. Seda
Keskin: Conceptualization, Supervision, Methodology, Writing - orig­
inal draft, Writing - review & editing. Alper Uzun: Conceptualization,
Supervision, Methodology, Writing - original draft, Writing - review &
editing.
Acknowledgments
This work is supported by the Scientific and Technological Research
Council of Turkey (TUBITAK) under 1001-Scientific and Technological
Research Projects Funding Program (Project Number 114R093) and by
Koç University Seed Fund Program. S.K. acknowledges ERC-2017Starting Grant. This study received funding from the European Research
Council (ERC) under the European Union’s Horizon 2020 research and
innovation programme (ERC-2017-Starting Grant, grant agreement no.
756489-COSMOS). M.Z. acknowledges HEC-Pakistan Scholarship. The
authors thank Koç University Surface Science and Technology Center
(KUYTAM) for providing help with the sample characterization. The
authors thank TARLA for the collaborative research support. Support
provided by the Koç University TÜPRAŞ Energy Center (KUTEM) is
gratefully acknowledged.

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