Synergistic effects of highly selective ionic liquid confined in nanocages: Exploiting the three component mixed matrix membranes for CO2 capture

Journal Pre-proof Synergistic effects of highly selective ionic liquid confined in nanocages: Exploiting the three component mixed matrix membranes for CO2 Capture Iqra Yasmeen, Ayesha Ilyas, Zufishan Shamair, Mazhar Amjad Gilani, Sikander Rafiq, Muhammad Roil Bilad, Asim Laeeq Khan

PII:

S0263-8762(20)30008-3

DOI:

https://doi.org/10.1016/j.cherd.2020.01.006

Reference:

CHERD 3957

To appear in:

Chemical Engineering Research and Design

Received Date:

21 July 2019

Revised Date:

9 December 2019

Accepted Date:

3 January 2020

Please cite this article as: Yasmeen I, Ilyas A, Shamair Z, Gilani MA, Rafiq S, Bilad MR, Khan AL, Synergistic effects of highly selective ionic liquid confined in nanocages: Exploiting the three component mixed matrix membranes for CO2 Capture, Chemical Engineering Research and Design (2020), doi: https://doi.org/10.1016/j.cherd.2020.01.006

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Synergistic effects of highly selective ionic liquid confined in nanocages: Exploiting the three component mixed matrix membranes for CO2 Capture

Iqra Yasmeen1, Ayesha Ilyas2, Zufishan Shamair1, Mazhar Amjad Gilani3, Sikander Rafiq1, Muhammad Roil Bilad4, and Asim Laeeq Khan1*,

Department of Chemical Engineering, COMSATS University Islamabad, Lahore Campus,

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1

Pakistan

Center of Surface Chemistry and Catalysis, Faculty of Bioscience Engineering, KU Leuven,

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2

Belgium

Department of Chemistry, COMSATS University Islamabad, Lahore Campus, Pakistan

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Chemical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar,

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3

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* [email protected]

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32610 8 Perak, Malaysia

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Graphical abstract

Research highlights

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Modification of ZIF-67 with highly selective 3-(trimethoxysilyl)propan-1-aminium acetate

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Simultaneous increase in CO2/CH4 and CO2/N2 selectivity and CO2 permeability

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Enhanced CO2 solubility owing to the molecular sieving of ZIF-67 and CO2 selective properties of RTIL

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Abstract

Mixed matrix membranes (MMMs) have shown remarkable progress in the development of

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high-performance membranes for gas separation. These membranes combine the advantageous properties of both organic polymers and highly selective inorganic fillers. Developments are in

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process for synthesis and modification of fillers to promote the separation performance. Ionic liquids (ILs) are an emerging class of solvents having high CO2 solubility. In this study, amine

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based RTIL 3-(trimethoxysilyl) propan-1-aminium acetate [APTMS][Ac] was synthesized. Zeolitic imidazolate framework, ZIF-67, was used as inorganic filler due to its shape selective

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property and appropriate pore size. ZIF-67 was modified by incorporation of RTIL, and subsequently MMMs with Polysulfone (PSf) matrix were prepared. Gas separation data showed that RTIL having –NH2 group as well as acetate ions combined with ZIF-67 nano porous

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structure and large surface area improved the separation performance of membranes. Increase in selectivity of three component MMMs by 84% and 90% was observed ranging from 39.02 to 72.06 and 39 to 74.47 for CO2/CH4 and CO2/N2 respectively in compared with PSf-RTIL blend membranes. RTIL provided facilitation role in gas permeations due to its high CO2 selective

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nature.

Keywords: CO2 separation; ZIF-67; ionic liquid modification; MMMs

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1. Introduction CO2 is one of the primary contributors to global warming leading to rise in global temperature. Over the years there has been global increase in energy consumption, subsequently escalating the CO2 emissions (MacDowell et al., 2010; Songolzadeh et al., 2014). Natural gas sweeting and CO2 removal from post-combustion flue gases are two of the major sources of CO2 emissions. The conventional separation techniques suffer from shortcomings such as high operation cost, intricate operation and environment footprint (Jusoh et al., 2016). Hence, there is a need of an

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alternative method for CO2 removal that are sustainable and cost effective. Membranes based gas separation have definite benefits over conventional techniques such as

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easy processing, less ecological footprint, energy efficiency and so on (Baker, 2002; Jusoh et al., 2016; Zhao et al., 2016). Practically, membrane technology faces two major challenges in CO2

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separation i) low amount of CO2 in the feed stream and ii) high volume of gases in feed stream. This translates into two major perks that membrane based gas separation should have, i) high

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permeability of CO2 and ii) high selectivity for CO2, to compete with the existing technologies (Yang et al., 2008). Unfortunately, membranes are limited by the so-called trade-off between

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permeability and selectivity defined by Robeson upper bound (Basu et al., 2010). Researchers have been conducting research to surpass this upper bound by preparing high permeable and selective membranes. Among the different classes of membranes, mixed matrix membranes

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(MMMs) are the type of membranes which combine incentives of both organic and inorganic materials and have shown potential to surpass the Robeson upper bound. For preparation of MMMs different types of inorganic fillers such as zeolites (Goh et al., 2011), carbon nanotubes (Khan et al., 2012), silica (Khan et al., 2013) and carbon molecular sieves (Vu et al., 2003) have been extensively used. Here again, researchers have to address another

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bottleneck, compatibility issue of inorganic filler in organic membranes, to obtain defect free, permeable and selective membranes. To overcome this issue another category of materials, namely metal organic frameworks (MOFs) (Habib et al., 2019), are used to prepare MMMs as it is comprised of both organic and inorganic counterparts. MOFs have effectively been used to prepare defect free membranes. Recently room temperature ionic liquids (RTILs) have also been used to as an additive to improve the performance of the MMMs. RTILs are compounds comprising of ions with melting points below 100 ℃ (Lei et al., 2017). RTIL’s unique properties 3

such as low volatility (Earle et al., 2006), thermal stability (Anderson et al., 2005) and solubility in wide range of solvents (Wang et al., 2016) make them exceptional candidates as additives in membranes for gas separation. Furthermore, selective RTILs have been reported to have outstanding affinity towards CO2. RTILs are usually incorporated in membranes in many different ways such as: supported ionic liquid membranes (SILMs), polymer-ionic liquid composite membranes, polymer-ionic liquid gel membranes, polymerized ionic liquid membranes, three component MMMs. Hao et al.

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studied gas separation results by incorporating RTIL into ZIF- 8, which was subsequently used for sweetening of natural gas and post combustion CO2 capturing. Their results showed that an

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increase of ZIF-8 by 25% wt. in ionic liquid, tripled the CO2 permeability without significant loss of CO2/N2 and CO2/CH4 selectivity (Hao et al., 2013). Ban and co-workers incorporated

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ZIF-8 in PSf matrix along with RTIL which increased permeability of H2, O2 and CO2 along with improved selectivity of CO2/CH4, O2/N2, H2/CH4, CO2/N2, and H2/N2 gas pairs (Ban et al., 2015). Ahmad et al. studied separation performance of MMMs for CO2/CH4 and CO2/N2 by

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combining RTIL, [emim][Tf2N], with zeolite SAPO-34 particles. Increase in both permeability and selectivity of CO2 was observed (Ahmad et al., 2017). Casado-Coterillo and co-workers

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prepared membranes with RTIL/ZIF-8/Chitosan and RTIL/HKUST-1/Chitosan. The study reported that membranes with ZIF-8 fillers showed better results due to smaller size of filler (Casado-Coterillo et al., 2015). Overall, integration of ionic liquids in membranes have yielded

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improved results than simple membranes.

In this study, a recently reported novel RTIL was employed to form different membranes to investigate its performance in removal of CO2. The reported RTIL has previously been used by our group in preparation of supported ionic liquid membrane (Ilyas et al., 2017) and grafting of

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zeolites 4A particles (Ilyas et al., 2018a). Both of the studies showed unprecedented results for gas separation using this RTIL. The RTIL synthesized was based on (3-amino propyl)trimethoxy silane and acetic acid both having high affinity for CO2. ZIF-67, a cobalt and 2methylimidazolate based ZIF with sodalite topology, was selected as filler in MMMs. ZIF-67 was selected owing to its superior stability and high surface area (1300 m2/g) (Khan et al., 2018). The synthesis of ZIF-67 can be carried out at room temperature within short interval of time thereby making it energy efficient and green synthesis. The as-synthesized ZIF-67 was modified 4

with RTIL to make a ZIF-67/RTIL composite that was then used as filler in MMMs. The modification was carried out with the objective of exploiting the CO2 selective nature. Three different types of membranes were prepared and their results were compared. First, dense membranes were prepared by blending Polysulfone (PSf), selected as polymer matrix, with different concentrations of RTIL. The second class of membranes were prepared by incorporating the as-synthesized ZIF-67 in PSf. And the last class were prepared by embedding RTIL/ZIF-67 composite in the same polymer matrix to form three component mixed matrix

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membrane. All these membranes were tested for CO2/CH4 and CO2/N2 separations under different operating conditions and the results were compared with Robeson upper bound to

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evaluate the true potential of these materials.

2. Experimental

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2.1. Materials

(3-amino propyl)-trimethoxy silane (APTMS), 2-methyl imidazole and polysulfone (PSf) pellets

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were purchased from Acros Organics (Belgium). Acetic acid (Ac) and cobalt nitrate hexahydrate were bought from BDH laboratory supplies (England). Methanol and ethanol were procured

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from Fisher Scientific (United States). Chloroform was purchased from Carlo ERBA (France) reagents. All reagents were analytical grade and were used as received.

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2.2. Ionic liquid preparation

Synthesis of RTIL was carried out according to the procedure reported in our earlier work (Ilyas et al., 2017). For the preparation of [APTMS][Ac], 0.1 mole of (3-aminopropyl) trimethoxysilane was added drop wise in 0.1 mole of acetic acid in two neck flask under cooling conditions. The solution was stirred for 12 h under atmospheric conditions. The mixture was then

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kept in rotary evaporator for drying for 4 h and pale-yellow viscous liquid as product was obtained. 1H NMR (400 MHz, DMSO) δ 4.79 (s, OCH3), 2.77 (m, CH2), 1.74 (s, CH2), 0.62 (broad s, CH2).

2.3. ZIF-67 synthesis

ZIF-67 particles were synthesized according to the procedure mentioned in reported literature (Feng and Carreon, 2015). Two stocks solutions of 0.35 g of cobalt nitrate hexahydrate and 0.66 g of 2-methylimidazole in 11.3g of methanol were prepared. Both solutions were stirred for 2h. 5

The solutions were then mixed and stirred continuously for 12 h at room temperature. Mixture containing purple precipitates was centrifuged for 20 min at 3500 rpm and washed with methanol three times. The slurry obtained was dried at 85°C overnight to obtain ZIF-67 crystals.

2.4. ZIF-67/RTIL composite preparation RTIL incorporated ZIF-67 was prepared by impregnation method. ZIF-67 was dried at 105 ℃ in a vacuum oven for removal of any moisture and volatile impurities. 0.3 g of RTIL was dissolved in 30 ml ethanol and the container was tightly covered with parafilm to prevent loss of solvent.

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Magnetic stirring was done for 2 h at room temperature. 0.7 g of ZIF-67 was added to the

solution and stirring was continued at 30 ℃ for 6 h. After complete evaporation of ethanol, the

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sample was dried in oven at 75 ℃. The final product was labeled as ZIF-67/RTIL, and stored in a desiccator to avoid impurities and humidity.

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2.5. Membrane fabrication 2.5.1. PSf-RTIL blend membranes

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Solution casting method was used for the preparation of polymer/RTIL blend membranes. The polymer was dried for 12 hours to remove any traces of moisture or any other impurities. Then it

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was dissolved in chloroform by stirring at room temperature for 4 h. 10% of total RTIL, calculated to prepare 10% loading membrane, was added to the solution and stirred for another 2 h. The solution was sonicated for 5 min, followed by the addition of 20% of remaining RTIL

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along with stirring for 2 h. Remaining RTIL was added after 5 min sonication. Solution was casted in glass petri dish and covered with inverted funnel. After 24 h of controlled evaporation, the membranes were dried in an oven to obtain blend membranes. 10%, 20% and 30 wt% loading of RTIL in blend membranes were prepared.

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2.5.2. PSf-ZIF-67 based MMMs

Membranes containing 0%, 10%, 20% and 30 wt% loading of ZIF-67 were prepared by adding the stipulated amount of ZIF-67 in half of the total amount of solvent. The polymer was added in the remaining half. Stirring was continued overnight and then 10% of the total polymer solution was added into the filler solution. The solution was stirred for 2 h and sonicated for 5 min. Subsequently, 20% solution of the total polymer was added and stired for 3 h. This step was repeated until the total polymer was added in the filler solution. The obtained solution was poured on flat-bottomed petri dish and covered with an inverted funnel to control the evaporation 6

rate. The prepared membranes were dried following the procedure mentioned in the previous section. 2.5.3. PSf-ZIF-67/RTIL membranes MMMs containing ZIF-67/RTIL composite as filler were prepared by following the same procedure as for ZIF-67 containing MMMs. Different loadings (10%, 20% and 30 wt% of ZIF67/RTIL composite) were prepared according to the said procedure. The fabricated membranes with desired composition are shown in Table 1.

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2.6. Characterizations techniques

RTIL formation and successful incorporation of RTIL in ZIF-67 particles was confirmed by

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Thermo-Nicolet 6700 P FTIR Spectrometer (USA) from 400-4000 cm-1. X-ray diffraction

patterns of composite and pristine ZIF-67 were obtained with powder XRD (Panalytical X’pert

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pro) using Cu-Kα radiation at voltage of 45 kV and 40 mA. Morphology of membranes and filler was examined by TESCAN Vega3 LMU at various voltages. Membrane samples were prepared

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by breaking them under liquid nitrogen and then sputter-coated with gold using Cressington HR 208 (UK) coater. 1H NMR spectra of RTIL was taken from Bruker Avance 400. Viscosity was

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calculated at room temperature using a Ubblehode viscometer. Density of RTIL was determined by 25 cm3 class-A specific gravity bottle and known mass. The mass of RTIL and specific gravity volume gave the density of RTIL.

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2.7. Gas separation performance testing Gas separation results were obtained using in-house built gas permeation setup having capability to measure the pure and mixed gas permeability. TCD gas chromatograph (YL Instruments, South Korea) was used to find out the compositions of permeate. To examine the membrane performance, the membranes were placed on porous metallic membrane holder. The membranes

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were tested in the temperature range of 298 K to 338 K at gas flow rate of 1 L/min (Sierra Instruments Mass Flow Controllers) at constant pressure of 10 bar. Each membrane was tested three times and standard error is also reported. The detailed construction and working is described in an earlier publication (Khan et al., 2010). The gas permeability (P) was calculated by the following equation.

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𝑃=

273×106 760

×

𝐴𝑇[

𝑦𝑖 𝑉 76 ]𝑥 𝑃 14.7 𝑖 2

𝑑𝑃

× [ 𝑑𝑡 ]

(1)

Where, “V” represents the downstream volume (cm3), “A” is the membrane permeation area (cm2), “T” denotes operating temperature (K), “P2” represents the pressure of feed gas (psi), xi and yi refer to mole fractions of component i in upstream and downstream respectively. Transport mechanism in these membranes follows “solution-diffusion model” represented mathematically in Eq 2: 𝑃 = 𝑆∗𝐷

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(2)

Where D stands for diffusivity coefficient, S donates solubility of gas in membrane phase and P

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refers to permeability of gas. To evaluate membrane gas separation performance diffusion was calculated by time lag method and solubility was determined mathematically by Eq 1. Afterward

𝑦𝐴 ⁄𝑦𝐵 𝑥𝐴 ⁄𝑥𝐵

(3)

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𝛼𝐴𝐵

𝑃 = 𝐴⁄𝑃 or 𝐵

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selectivity was estimated by following equation:

𝑦𝐴 and 𝑦𝐵 are concentrations of gas components A and B in permeate phase. 𝑥𝐴 and 𝑥𝐵 are

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concentrations of gas components A and B in feed phase.

3. Results and discussions

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3.1. Characterization of RTIL

A comparison FTIR of Ac, APTMS and RTIL is presented in Fig. 1. Ac was characterized by stretching vibrations of C=O at 1755 cm−1 and 1703 cm−1 and asymmetric CH3 deformation at 1400 cm-1. Broad peak near 3000 cm−1 of O-H also validated the presence of Ac (Colomer,

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2013). In APTMS and RTIL, peaks at 2932 and 2840 are due to stretching vibrations of C-H. CN stretching vibration is represented by peak at 1190. C-N bending vibration is at 1630 is depicted in the spectra as well (Haniffa et al., 2019). RTIL formation was confirmed by absence of O-H stretch near 3000 and appearance of prominent peaks at 1543 and 1395 of symmetric and asymmetric C-O stretching respectively. Asymmetric CH3 deformation at 1400 cm-1 is also visible in RTIL further confirming the synthesis of RTIL. These FTIR results and NMR results stated above are consistent with the earlier reported literature (Ilyas et al., 2017).

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Physical properties of RTIL are presented in Table 2. The viscosity and density of RTIL are key perimeters in deciding the performance of gas separation. Viscosity of the prepared IL is high and density is comparable to water.

3.2. Characterization of ZIF-67/RTIL composite XRD pattern of pristine ZIF-67 and ZIF-67/RTIL composite are shown in Fig. 2 (a) and (b) respectively. Prominent peaks at 10°, 13° and 18° in both fillers match the characteristic peaks of ZIF-67 particles (Feng and Carreon, 2015) which validate that the particles were successfully

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synthesized. The diffractogram of composite shows that the intensity of the characteristic peaks is reduced due to incorporation of RTIL disrupting the crystallinity. It is evident that RTIL has

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not brought any structural changes in the filler and the characteristic peaks of ZIF-67 are still

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visible in the diffractogram.

Morphology of ZIF-67 and ZIF67/RTIL composite was investigated by SEM shown in Fig. 3(a) and 3(b) respectively. Small particles size of ZIF-67 is expected to promote homogenous

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distribution of particles in membrane. Fig. 2(b) shows structure of ZIF-67/RTIL composite particles which seems to be distorted by incorporation of RTIL on ZIF-67. Dispersion of ionic

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liquid somewhat deteriorates crystal structure of ZIF-67 particles. However, as shown by XRD characterization the characteristic peaks depicting crystallinity, though suppressed, were present

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in the RTIL modified samples.

FTIR comparison between ZIF-67, RTIL and ZIF-67/RTIL composite is shown in Fig. 4. In all spectra, bands at 2924 cm-1 represent the stretching bands of C-H aliphatic group. The band at 1648 and 1173 cm-1 are ascribed to N-H bond and stretching vibration of C-N respectively. Small peak at 1540 and 1397 cm-1 were assigned to symmetric and antisymmetric C-O stretches

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respectively. Strong peaks at 991 and 691 cm-1 are associated with =C-H and C-H bending and ring puckering. Moreover, the peaks obtained at 3135 cm-1 were attributed to the stretching of aromatic ring of 2-MIM, respectively. Out of plane bending vibrations of ZIF-67 was attributed at 752 cm-1. Strong and broad peak near 1059 cm-1 is due to Si-OCH3 bonding of ionic liquid. Peaks between 500 to 1700 cm-1 confirms imidazole rings in structure (Khan et al., 2010). In ZIF-67/RTIL composite spectra, all bands of RTIL and ZIF-67 were present confirming physical incorporation of RTIL in pore of ZIF-67 particles. However, the intensity of RTIL peaks is reduced due to the difference in concentration of RTIL as per their loading in ZIF-67. 9

3.3. Membranes Characterization FTIR analysis of pure PSf membrane and PIL, PZ and PZI membranes are presented in Fig. 5. For the sake of simplified comparison, the blend and MMMs membranes at 30% loading were included in the spectra. In all samples, the stretches near 2965 cm-1 correspond to C-H bonds. The peaks at 1905 cm-1 is ascribed to C=C bond. The bands at 1363 cm-1 and 1167 cm-1 corresponds to stretching S=O bonds. A sharp peak at 1484 cm-1 in all membranes shows CH2 and CH3 deformation. All bands of RTIL, ZIF-67 and PSf were reflected in MMMs.

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SEM analysis was also used to investigate the dispersion of filler in the polymer matrix and the effect of RTIL modification. SEM images of ZIF-67 and RTIL modified ZIF-67 membranes are

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presented in Fig. 6. PZ10 is showed in Fig. 6a which clearly illustrates good dispersion. As the loading is increased to PZ20 (Fig. 6b) and PZ30 (Fig. 6c) respectively the roughness of the

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membranes is increased due to increase in concentration of ZIF-67 in the matrix. Effect of RTIL modification is investigated in the SEM images of PZI (Fig. 6 d-f). The modification of RTIL

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resulted in change in morphology of ZIF-67 particles and similar trend could be seen in the membrane formation. Fig. 6f represents PZI10 membranes, showing uniform dispersion of RTIL

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modified filler. The membranes structure is seemingly more uniform and smoother than PZ10 membranes due to the incorporation of RTIL.

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3.4. Gas separation performance

The pure gas permeability of CO2, CH4, N2 and ideal selectivity of CO2/CH4 and CO2/N2 of membranes are shown in Table 3. It was observed that permeability of CO2 increased with increase in filler loading in all MMMs. In case of PIL membranes without ZIF-67, the permeability of pure gas CO2 increased up to 135% from 6.8 to 16 Barrer at 30% RTIL loading

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relative to pure PSF membrane. The increase in permeability is attributed to the affinity of CO2 due to the presence of polar amine groups and silyl bonds in RTIL enhancing solubility of CO2 (Ilyas et al., 2018b). The ideal selectivity of both tested gas pairs (CO2/N2 and CO2/CH4) increased up to 54% and 43% respectively on increasing the concentration of RTIL in the blend membranes. This increase is ascribed to similar reason of increased interaction between CO2 and RTIL. The increase in solubility selectivity eventually leads to higher selectivity.

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In case of PZ membranes, permeability of pure gas CO2 increased by 287% from 6.8 to 26.34 Barrer at 30% loading with respect to PSF membrane. This increase in permeability can be attributed to the increased diffusivity of CO2 through the ZIF-67 particles. The highly porous ZIF-67 structure provides pathways for gas molecules to diffuse through them (Perez et al., 2009). As reported in similar previous reports, the introduction of highly porous filler in glassy polymer provides more free volume for gas molecules due to increase in polymer chain movement which results in increased permeability (Merkel et al., 2003). High permeability can

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also be related to shape selective property of ZIF-67 particles owing to their topology with major cage windows of 0.34 nm which allows CO2 (0.33 nm) to pass easily with exclusion of CH4

(0.38 nm). This unique property of molecular sieving of these fillers further leads to increased

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selectivity. As presented in Table 3, significantly higher selectivity of 67.54 and 69.34 for

CO2/CH4 and CO2/N2 respectively were obtained by incorporation of ZIF-67 particles. This

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major increase in selectivity also confirm defect free synthesis of membranes.

The PZI membranes exhibit an increase in pure gas permeability of CO2 by 228% from 6.8 to

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22.34 Barrer at 30% filler loading. However, by incorporation of RTIL in ZIF-67 the increase in permeability is less as comparison to unmodified ZIF-67. RTIL modified ZIF-67 particles

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enhanced linkages with polymer matrix due to addition of RTIL which deteriorates chain movement (Frisch et al., 2016). Camper theory, stating low molar volume RTIL have greater solubility (Scovazzo, 2009), also explained that increase in CO2 permeability is due to enhanced

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CO2 solubility in RTIL phase as shown in Table 3. The molar volume of [APTMS][Ac] is lower than most of the commercially available RTILs (Palomar et al., 2007). On contrary, PZI membranes showed an increase of 186% in ideal selectivity at 30% filler loading in comparison to pristine PSF membrane. Increase in selectivity was due to the reason that RTIL acts as a selective barrier which allows desirable CO2 molecules to pass while retaining CH4 and N2. As in

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the case of CO2/CH4, the permeability of CO2 in PZI membranes was slightly less than PZ membranes. The decrease is due to impregnation of RTIL in porous filler. The permeability of N2 increased 32% in PZI membranes as compared to PZ membranes which showed a 20% increase in N2 permeability. However, selectivity is increased by 19% and 130% as compared to PZ and PI membranes respectively due to incorporation of RTIL which has better affinity for CO2.

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The performance of the PZ and PZI membranes was better than the PIL membranes. The increase in selectivity for CO2/CH4 and CO2/N2 was greater in PZ and PZI than PI membranes. CO2 permeability was also greater in PZ and PZI membranes. The inclusion of ZIF-67 resulted in better membranes as depicted by gas separation results. In PIL membranes the reason of increase in CO2 permeability and selectivity was RTIL incorporation. PIL membranes behaved like conventional dense membranes and CO2 was only attracted by RTIL as the loading was increased. Addition of ZIF-67 provided pathways for CO2 to pass and molecular sieving effect

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enhanced the selectivity of the PZ membranes. PZI membranes combined the effect of the PIL and PZ membranes by showing better results than both of these membranes. ZIF-67 provided the

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diffusion pathways for CO2 to pass and RTIL contributed in enhancing the solubility of the CO2. Mixed gas (50% CO2/50% CH4) selectivity and permeability of all MMMs at different loadings

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for both gas mixtures are shown in Table 4. The overall trend is very similar to that of pure gas permeation. The comparison of results reveals lower permeability as well as selectivity as compared to pure gas results. This difference is due to the effect of competitive sorption by the

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presence of CH4 and N2 in feed mixture that hinder the diffusion of CO2 molecules through the membranes. The partial pressure of CO2 in the mixture is 5 bar which is significantly less than

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the plasticization pressure of PSF thereby excluding the effect of plasticization. Table 5 shows diffusivity and solubility values of PIL, PZ and PZI membranes. For PIL blend

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membranes, the result shows a consistent drop in diffusivity with increase in RTIL content. This is attributed to the high viscosity of RTIL resulting in decreased mass transfer through the membrane. On the contrary, the high CO2 solubility of PIL membranes is explained by combine effect of methoxy groups present in the cation, along with acetate ions and amine groups both having good ability to absorb molecules of CO2 which results in increase in solubility. The

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incorporation of ZIF-67 particles in PZ membranes leads to increased diffusivity with the increase in filler loading. This is due to high porosity of ZIF-67 particles leading to shorter diffusion pathways through the membrane. Therefore, the enhancement in permeability results of PZ membranes may came from increase in gas diffusivity as shown in Table 5. The solubility of these membranes is low as compared to PIL based blend membranes. This could be due to reason that ZIF-67 has rigid polyhedral structure which provides less sorption sites for gas. PZI membranes exhibited both increase in diffusion and solubility. In this case, solubility coefficient 12

is dominant factor due to the presence of –CO2 philic groups in RTIL incorporated in pores of filler. The slightly reduced diffusion in comparison to unmodified ZIF-67 is due to the reduced pore size of filler due to incorporation of the RTIL in the pores of the ZIF-67 particles. Comparing all the membranes, PZI membranes presented better CO2/CH4 and CO2/N2 selectivity than PIL and PZ membranes. The enhancement in selectivity in PIL membranes is attributed to presence of RTIL in the membranes. RTIL provides better solubility of CO2 due to its intrinsic affinity of the CO2 molecules. In PZ membranes the increase in selectivity is mainly due to the

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molecular sieving abilities of ZIF-67 particles. One noticeable fact is that in both of these types of membranes CH4 permeability is also increased as the loading of RTIL or filler is increased. In

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PZI membranes combined effect of PIL and PZ membranes can be seen. Diffusion and solubility coefficients are both increasing showing effect of both filler and RTIL. But the reason of better

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selectivity results is the reduced increase in CH4 permeability. Due to the incorporation of RTIL in ZIF-67 particles the CH4 molecules could not pass as easily as in PIL or PZ membranes. The inherent affinity of RTIL with CO2 and diffusion pathways in ZIF-67 particles provide CO2

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3.5. Effect of temperature

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superior permeability which gives better selectivity.

The effect of temperature on permeability of CO2 was studied for all the membranes prepared and is demonstrated in Fig. 7. The linear relationship of CO2 permeability with inverse of

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temperature depicts Arrhenius behavior of all membranes with temperature ranging from 298 K to 338 K. This suggests that on increasing the temperature the permeability of CO2 is also increased. The increase in permeability is due to increase in free volume in membranes due to the flexibility of polymeric chains at high temperatures. The higher mobility of polymer chains provides easier passage for gas molecules to pass leading to higher permeability of gasses. The

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increase in temperature results in the increase of diffusion coefficient which results in increase permeability as polymer chains provide vigorous motion (Liu et al., 2005). The PIL membranes show least increase in the permeability with temperature, then comes PZ membranes followed by PZI membranes which show maximum increase. This behavior was predicted as PIL membranes are classic dense membranes and RTIL fused with the polymer matrix does not allow significant movement of polymer chains even at elevated temperature. The increase in permeability in PZ membranes was explained by easy escape of CO2 molecules from the polymer matrix and ZIF-67 13

nanoparticles. PZI membranes exhibit improved permeability then PI and PZ membranes because RTIL incorporated in the ZIF-67 nanoparticles becomes less viscous at high temperature and diffusion increases in the membranes. Arrhenius equation demonstrates gas permeability, operating feed temperature and activation energy of permeation (Ep) and is given as; −𝐸𝑝

𝑃 = 𝑃0 𝑒𝑥𝑝 ( 𝑅𝑇 )

(4)

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where, P is the gas permeability, R is the gas constant, Po is the pre-exponential factor, and T is the absolute feed temperature. Activation energy of all MMMs are shown in Fig. 8. PZ MMMs

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show a consistent decrease in activation energy of CO2 molecules. This decrease is due to

increase in porosity providing lesser resistance for diffusion of gas molecules. Activation energy

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of PIL membranes decreased with increase in RTIL content but showed an increase at 30% concentration of RTIL. The initial decrease can be explained by the increase in solubility of CO2

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in matrix leading to enhanced permeability. However, as the content of RTIL is increased considerably its high viscosity decrease the mass transfer to such an extent that it eventually increases the overall resistance of flow through membranes. PZI shows gradual decrease in

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activation energy. Hence, modification of RTIL on ZIF-67 results in decreased activation energies of membranes associated with an increase in permeation of CO2 molecules.

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3.6. Comparison with Robeson upper bound In order to compare the results obtained in this study, the data was plotted on Robeson’s tradeoff graph for CO2/CH4 and CO2/N2 (Fig. 9). The results for CO2/CH4 gas mixture show presence of ionic liquid in ZIF-67 particles improves selectivity results of PZI membranes due to presence

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of acetate ions and amine groups in RTIL which are CO2 philic groups. Along with these groups, molecular sieving ability and large surface area of ZIF-67 gives comparable selective and intermediate permeability results, balancing overall performance of membranes. PZI-30 membrane and PZ-30 show results that lie beyond the upper bound. These results confirm the exceptional performance of membranes synthesized in this study. In case of CO2/N2 gas mixture the separation performance was also increasing depicting the effect of RTIL and ZIF-67 particles.

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4. Conclusion Incorporation of RTIL in ZIF-67 showed enhanced gas permeation results. SEM images show good dispersion of all types of fillers in membranes. RTIL in PZI membranes have acetate ions, methoxy and amine groups which are highly attract CO2. While large surface area and controlled porosity of ZIF-67 gives significant results. The synergistic effect of [APTMS][Ac] and and ZIF67 resulted in overall improve separation performance as compared to PIL and PZ membranes. Increase in selectivity of PZI MMMs by 84% and 90% was observed ranging from 39.02 to

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72.06 and 39.2 to 74.47 for CO2/CH4 and CO2/N2 respectively as compared to PIL membranes for 30% wt filler loading. Whereas, PZI membranes showed increase in 6.7% and 7.4% ranging

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from 67.54 to 72.06 Barrer and 69.32 to 74.47 Barrer for CO2/CH4 and CO2/N2 gas pairs

respectively at same loading. However, with maximization of various factors such as different

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Declaration of Interest Statement

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ionic liquid and filler choice this idea can be extended to a certain limit.

Acknowledgements

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I hereby declare there is no conflict of interest in the submission of this manuscript.

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Dr. A. L. Khan would like to thank Pakistan Science Foundation (PSF, Pakistan for their grant

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under PSF/Res/P-CIIT/Engg (124).

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Fig. 1 - FTIR spectra of (a) Ac, (b) APTMS and (c) RTIL.

Fig. 2 - XRD diffractogram of (a) neat ZIF-67 (b) ZIF-67/RTIL composite. 19

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Fig. 3 - Image of (a) ZIF-67 and (b) RTIL modified ZIF-67 acquired from SEM.

Fig. 4 - FTIR spectra of (a) ZIF-67, (b) RTIL and (c) ZIF-67/RTIL composite.

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Fig. 5 - FTIR spectra of (a) P, (b) PI30, (c) PZ30 and (d) PZI30.

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Fig. 6 - SEM images of membrane (a) PZ10, (b) PZ20, (c) PZ30, (d) PZI30, (e) PZI20 and (f) PZI10.

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Fig. 7 - Effect of temperature variation on CO2 permeability of (a) PI, (b) PZ and (c) PZI membranes.

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Fig. 8 - Activation energy of CO2 permeability for all membranes at different temperature.

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Fig. 9 - Robeson’s trade off plot for (a) CO2/CH4 and (b) CO2/N2 gas mixture

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Table 1 - Composition of synthesized membranes Composition Pure PSf 10 % IL+90% PSf 20% IL+80% PSf 30% IL+70% PSf 10 % ZIF-67+90% PSf 20 % ZIF-67+80% PSf 30% ZIF-67+70% PSf 10% IL modified ZIF-67+90% PSf 20% IL modified ZIF-67+80% PSf 30% IL modified ZIF-67+70% PSf

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Abbreviations P PIL10 PIL20 PIL30 PZ10 PZ20 PZ30 PZI10 PZI20 PZI30

Table 2 – Physical Properties of RTIL Viscosity (cp) 521.32

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Density (g/cm3) 1.20

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Molecular Weight (g/mol) 281.42

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Membrane PSf PSf-IL-10 PSf-IL-20 PSf-IL-30 PSf-ZIF-67-10 PSf-ZIF-67-20 PSf-ZIF-67-30 PSf-ZIF 67/IL-10 PSf-ZIF 67/IL-20 PSf-ZIF 67/IL-30

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Molar Volume (cm3/mol) 234.51

Table 3 - Pure gas separation results Permeability (Barrer)

Selectivity

Membranes CH4

N2

CO2/CH4

P

6.80 ± 0.34

0.27 ± 0.03

0.25 ± 0.03

25.19 ± 0.88 27.20 ± 0.95

PIL10

10.63 ± 0.53 0.33 ± 0.03

0.32 ± 0.03

32.21 ± 0.81 33.22 ± 0.83

PIL20

14.2 ± 0.71

0.40 ± 0.04

0.40 ± 0.04

35.50 ± 0.89 35.00 ± 0.88

PIL30

16.00 ± 0.80 0.41 ± 0.04

0.41 ± 0.04

39.00 ± 0.98 39.00 ± 0.98

P

6.80 ± 0.34

0.25 ± 0.03

25.19 ± 0.88 27.20 ± 0.95

PZ10

12.56 ± 0.63 0.33 ± 0.03

PZ20

18.00 ± 0.90 0.34 ± 0.03

0.34 ± 0.03

52.94 ± 0.79 53.00 ± 0.80

PZ30

26.34 ± 1.32 0.39 ± 0.04

0.38 ± 0.04

67.54 ± 1.01 69.32 ± 1.04

0.27 ± 0.03

0.25 ± 0.03

25.19 ± 0.88 27.20 ± 0.95

11.12 ± 0.56 0.28 ± 0.03

0.26 ± 0.03

39.71 ± 0.60 45.00 ± 0.68

15.33 ± 0.77 0.29 ± 0.03

0.28 ± 0.03

52.86 ± 0.79 54.75 ± 0.82

22.34 ± 1.12 0.31 ± 0.03

0.30 ± 0.03

72.06 ± 1.08 74.47 ± 1.12

PZI10

PZI30

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6.80 ± 0.34

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0.27 ± 0.03

CO2/N2

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38.06 ± 0.95 36.94 ± 0.92

Table 4 - Mixed gas separation results Permeability Membranes

Selectivity

(Barrer)

Permeability

Selectivity

(Barrer)

CH4

CO2/CH4

P

6.16 ± 0.31

0.26 ± 0.04

PIL10

9.12 ± 0.46

0.31 ± 0.03

PIL20

12.33 ± 0.62 0.38 ± 0.04

32.45 ± 0.97 13.00 ± 0.65 0.39 ± 0.04

PIL30

14.43 ± 0.72 0.40 ± 0.04

36.08 ± 0.90 14.93 ± 0.75 0.40 ± 0.04

P

6.16 ± 0.31

23.69 ± 0.83 6.39 ± 0.32

PZ10

11.45 ± 0.57 0.33 ± 0.03

PZ20

16.78 ± 0.84 0.34 ± 0.03

PZ30

23.50 ± 0.94 0.37 ± 0.04

63.51 ± 0.95 25.34 ± 1.01 0.37 ± 0.04

68.49 ± 0.68

P

6.16 ± 0.31

23.69 ± 0.83 6.39 ± 0.32

0.25 ± 0.03

25.56 ± 0.89

10.30 ± 0.52 0.27 ± 0.03

38.15 ± 0.76 11.45 ± 0.57 0.26 ± 0.03

44.04 ± 0.77

15.00 ± 0.75 0.28 ± 0.03

53.57 ± 1.07 15.12 ± 0.76 0.28 ± 0.03

54.00 ± 0.95

67.07 ± 1.17 20.55 ± 0.92 0.30 ± 0.03

68.50 ± 1.20

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PZI20 PZI30

N2

CO2/N2

23.69 ± 0.83 6.39 ± 0.32

0.25 ± 0.03

25.56 ± 0.89

29.42 ± 0.74 9.55 ± 0.48

0.31 ± 0.03

30.81 ± 0.77

20.12 ± 0.91 0.30 ± 0.03

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33.33 ± 0.83 37.33 ± 0.93 25.56 ± 0.89

34.70 ± 0.87 12.01 ± 0.60 0.33 ± 0.03

36.39 ± 0.91

49.35 ± 0.99 17.12 ± 0.86 0.33 ± 0.03

51.88 ± 0.78

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0.25 ± 0.03

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0.26 ± 0.04

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PZI10

0.26 ± 0.04

CO2

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Table 5 - Diffusion coefficients and solubility coefficients of CO2 of membranes Solubility

(10-8 cm2/s)

(10-2 cm3/cm3 cmHg)

P

3.60 ± 0.18

1.89 ± 0.19

PIL10

3.30 ± 0.17

3.22 ± 0.32

PIL20

3.00 ± 0.15

4.73 ± 0.47

PIL30

2.60 ± 0.13

6.15 ± 0.62

P

3.60 ± 0.18

1.89 ± 0.19

PZ10

5.43 ± 0.27

2.31 ± 0.23

PZ20

6.44 ± 0.32

2.80 ± 0.28

PZ30

7.87 ± 0.39

3.35 ± 0.33

P

3.60 ± 0.18

1.89 ± 0.19

PZI10

3.89 ± 0.19

3.01 ± 0.30

PZI20

4.03 ± 0.20

3.80 ± 0.38

PZI30

4.23 ± 0.21

5.28 ± 0.53

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Diffusion

Membranes

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