CO 2 Selective, Zeolitic Imidazolate Framework-7 Based Polymer Composite Mixed-Matrix Membranes

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Introduction
Natural gas is a paramount energy source of the future and the most environmental friendly non-renewable fossil fuel until now.The composition of natural gas widely depends on the reservoir source but mainly composes of methane (30-90%), ethane, propane, carbon dioxide (CO 2 ), nitrogen (N 2 ) including smaller traces of hydrogen sulphide (H 2 S), helium (He), and water vapour as impurities (Baker and Lokhandwala, 2008).It is essential to remove CO 2 from natural gas before processing due to the formation of carbonic acid by the reaction of CO 2 with water vapor which corrodes the gas transport pipelines (Bhide and Stern, 1993).Presence of CO 2 decreases the heat capacity of natural gas and the concentration of CO 2 over 2 mole% makes the gas transport less viable (Baker, 2001).
Membrane technology receives a significant attention for the separation of CO 2 from natural gas (Adeole et al., 2013) and it competes with conventional technologies like amine scrubbing (Bottoms, 1930) and cryogenic distillation process (Michael et al., 2011).Polymeric membranes are widely used in the membrane-based gas separation process due to their ease of fabrication, low operation cost, and high energy efficiency.Membranes for natural gas processing from cellulose acetate polymer were first commercialized during the 1980s for CO 2 removal (Schell, 1989) but in general polymeric membranes are prone to a trade-off between permeability and selectivity (Robeson, 2008), which means that membranes with high permeance have a low selectivity and vice versa.One of the approaches to overcome such limitation is the incorporation of selective fillers namely metal-organic frameworks (MOF), carbon nanotubes and others into the polymer matrix to develop mixed matrix membranes (MMM) (Rodenas et al., 2015;Li & Peinemann, 2013;Ahmed et al., 2014).Filler properties such as size, chemical structure, and porosity are some of the parameters to be considered for selection.Compatibility between polymer and filler is one of the important parameters for a defect-free membrane fabrication.Proper selection of materials in MMM combines the merits of both filler and polymer to achieve a high gas selectivity and maintain permeability.
One of the widely used filler material in MMM for CO 2 separation is the zeolitic imidazolate framework ZIF-7.ZIFs are a sub-class of metal-organic frameworks (MOF) (Song et al., 2012;Bae et al., 2010;Dai et al., 2012).Xiaochun, Jiepeng, and Xiaoming (2003) discovered ZIF-7 by bridging imidazolate (benzimidazole) linker with a transition metal cluster (Zn 2+ ) to form a 3D structure with sodalite (SOD) framework topology.ZIF-7 is a sixmembered structure with a pore size of 0.3 nm (Bergh et al., 2011), it is hydrophobic in nature.It exhibits excellent chemical and thermal stability (Park et al., 2006) and is stable up to 500°C (Li et al., 2010).In this work, ZIF-7 has been used as a filler material with a CO 2 -philic co-polymer polyacrylonitrile-r-poly (ethylene glycol) methyl ether methacrylate (PAN-r-PEGMA) synthesized in our laboratory (Karunakaran et al., 2017) for the development of mixed matrix membranes.
MMMs with various ZIF-7 loadings (20, 30 and 40%) to the weight of the polymer was developed and their permeation properties were evaluated with single gases of CO 2 and CH 4 permeation on a custom made time lab equipment (Karunakaran et al., 2017).Polyester backed ultrafiltration range polyacrylonitrile (PAN) polymeric membrane was used as a mechanical support for the membranes developed in this work.

Materials and Methods
Commercially available PAN membrane support was kindly provided by GMT Membrantechnik GmbH (Rheinfelden) Germany.Zinc nitrate hexahydrate (Zn (NO 3 ) 2 .6H 2 O, 99%), benzimidazole (bIm, >98%) were purchased from Sigma-Aldrich and Across-organics respectively.Dimethylformamide (DMF), tetrahydrofuran (THF) and methanol solvents were purchased from Sigma-Aldrich and Fisher Scientific respectively.Solvents were used as received without any additional purification.Pure gases of CO 2 and CH 4 (purity 99.995%) were purchased from Air liquid Alkhafrah Industrial gases, Saudi Arabia.

Synthesis of ZIF-7and PAN-r-PEGMA67 Polymer
ZIF-7 particles were synthesized following the procedure reported by Wang et al., 2016.Zn(NO 3 ) 2 .6H 2 O of 6.8 gm was dissolved in 250 ml of DMF and benzimidazole of 6 gm was dissolved in 250 ml of methanol at room temperature individually.Previously dissolved metal and ligand solutions were mixed and stirred together for 6 hours to get a white suspension.The final product of ZIF-7 was obtained by centrifuging and rinsing the white suspension three times with methanol to remove any excess solvents.The ZIF-7 particles were oven dried at 60 o C for 12 hours under vacuum.
Many polymers have been reported for the separation of CO 2. An attractive polymer group for this separation is polyether based polymers, which show high CO 2 selectivities due to the affinity of PEG or PEO with CO 2 .By blending PEG with Pebax 1657 polymer Car, A., Stropnik, C., Yave, W., and Peinemann K. V. ( 2008) reported an enhancement in the gas transport performance.In this work, we have used a laboratory synthesized CO 2 -philic polymer PAN-PEGMA67 which has a high content of PEGMA (950Da) (Karunakaran et al. (2017), scheme 1 shows the structure of the polymer.For the following reasons we chose this polymer to achieve a better gas separation property: (1) Due to the higher presence of PEG units in the polymer, it has higher selectivity of CO 2 /CH 4 when compared with the commercial Pebax1657 (2) the polymer dissolves well in a mixture of ethanol/water (70/30 wt%) which is an environmental-friendly binary mixture for membrane preparation (3) a commercial PAN membrane was used as a porous support material for all the membranes developed in this work.Since PAN-r-PEGMA67 contains PAN as a copolymer we expect a good adhesion between the support and the coating for a better gas separation performance Scheme 1. Structure of PAN-r-PEGMA67 co-polymer

Membrane Preparation
PAN-r-PEGMA67 polymer (2wt %) was dissolved in a mixture of ethanol and water by (70:30)wt % at 75 °C temperature for 4 hours under stirring.The polymer solution was filtered by a steel filter of 32µm pore size to remove any dust and gel particles.The synthesized ZIF-7 particles were dispersed into the polymer solution with different particle loadings.The mixture was sonicated and stirred for 12 hours at room temperature for a complete dispersion.
Particle loadings were calculated as: The PAN support membrane was placed between a square-shaped Teflon plate and a frame set-up with silicone rubber gasket.These are stacked and held tightly by binder clips to avoid any leak.A known volume of the dispersed mixture was poured into the plate and frame set-up and was allowed to dry at room temperature for 24 hours.Dried membranes were carefully removed from the set-up and further dried in an oven at 60°C for 12 hours under vacuum.Mixed matrix membranes with various ZIF-7 loadings (0, 20, 30 and 40%) with respect to the polymer weight were correspondingly denoted as MMM0, MMM20, MMM30 and MMM40 in this article.Membranes developed with 50% loadings were brittle in nature and their gas permeation was not evaluated due to its difficulty in handling.

Characterization
X-ray diffraction measurement (XRD) for the developed samples were carried out in reflection mode with a Bruker D8 Advanced XRD instrument equipped with CuK-alpha (1.54 Aº wavelength, ƛ) with LynxEye 1-d as a detector.Data were collected by step-scan mode in the range of 0.02-degree step size with an interval of 2 sec.
Dense films were made from pristine PAN-r-PEGMA67, MMM20, MMM30 and MMM40 for ATR-FTIR measurements.Spectra of the dense membranes were recorded using Nicolet iS10 (Thermo Scientific) FTIR spectrometer.Data were collected for 16 scans at ambient temperature with a resolution of 4 cm -1 .
Thermal stability of ZIF-7, pristine polymer and MMM samples were measured by a thermogravimetric analyzer (NETZSCH, TG209F1 Iris).Samples were conditioned at 80°C under vacuum before analysis to remove any guest molecules from the pores.All the experiments were carried out at a heating rate of 20 °C/min from room temperature to 1000 °C under a nitrogen atmosphere.
Scanning electron microscopy (SEM) images were performed on a Magellan 400 SEM, FEI/ Thermo scientific equipped with field emission gun with the unicolor mode.A through the lens detector (TLD) and concentric backscatter detector (CBS) were used for surface and cross-sectional imaging.Samples were fractured in liquid nitrogen for the cross-sectional imaging and were sputter-coated to a thickness of 3 nm with iridium to avoid any charging effect.Atomic Force Microscopy (AFM) analysis was performed using an Agilent (Model 5400) microscope for all the samples in this work.Images of 2D and 3D views were taken by tapping mode to avoid any damage to the AFM tip which has a resonant frequency of 60-80 Hz and a spring constant of 3Nm -1 .A transmission electron microscopy (TEM) FEI-Tecnai twin equipped with ORIUS SC1000 camera (Gatan) at 120 kV was used to analyze the ZIF-7 particles shape and size.

Gas Permeation Test
Gas permeation experiments were determined with a constant volume / variable pressure set up at 25°C temperature and 600 mbar as feed pressure for the measured gases.Membrane permeance was calculated by using the following equation: where V is the downstream volume (m 3 ) , R is the ideal gas constant (0.083 L atm K -1 mol -1 ), T is the permeation temperature (K), A is the membrane area (m 2 ), t is the time (s), P F and P 0 are the pressure at feed side (bar) and Pp (t) is permeate pressure.

Characterization of ZIF-7 filler and PAN-r-PEGMA67 MMMs
Figure 1 illustrates the XRD patterns of pure ZIF-7 particle and pristine PAN-r-PEGMA67 polymer.The wide-angle XRD (WXRD) has a broad 2θ peak observed near 18° but due to the amorphous nature of the polymer, no sharp peaks were detected.The structural data of synthesized pure ZIF-7 by powder X-ray diffraction (PXRD) was in good agreement with the reported values (Maythalony et al., 2017;Kang et al., 2013).In Figure 1 the angles of diffracted peaks of the samples MMM-20, 30 and 40 series were shifted to broader angles when compared with virgin ZIF-7 particles.These angle changes correspond to the change of d-spacing in ZIF-7 particles due to their interaction with the polymer.
FTIR (Figure S1) peaks that appeared at 1455 and 777 cm -1 on pure ZIF-7 particle were identified as C=C and C-H bonds which resulted from benzene functional group present in benzimidazole.Figure 2 shows the FTIR spectra of mixed matrix membranes MMM-20, 30 and 40.
Existence of peaks at 1455 cm -1 and 777 cm -1 and the couplet of different peaks in the range of 1200-1800cm -1 and at 700 cm -1 confirm the presence of ZIF-7 and were in good agreement with the literature reports (Tu et al., 2014;Maythalony et al., 2017;Kang et al., 2013)

Thermal Properties of ZIF-7 Composite Mixed-Matrix Membranes (MMMs)
Figure 3 and Figure 4 show the thermogravimetric analysis (TGA) curves of pure ZIF-7 particles, pristine PANr-PEGMA67 polymer and MMM-20, 30 and 40 % particle loaded samples.In Figure 3 a two-step weight loss was observed, an initial and final weight loss of 13.1% and 62.5% at 250°C and 500-600°C respectively.Initial weight loss was due to the evaporation of DMF solvent present in ZIF-7 cages and the final weight loss was due to the decomposition of ZIF-7 to zinc oxide.TGA curves obtained in this work matches well with the earlier reports (Kang et al., 2013).In Figure 4 an initial weight loss in pristine polymer occurred around 150°C due to the presence of moisture content.
Compared with the pristine polymeric membrane (MMM0), the thermal stability of MMM improved after the incorporation of ZIF-7 with an increased 1 st and 2 nd degradation temperatures from 320°C to 380°C and 400 to 450°C.Table 2 shows that the final residue of ZnO increases along with the increase of ZIF-7 loadings in the MMM.However, such residual values of MMM were less when compared with the final residue of pure ZIF-7 particles.

Morphology of ZIF-7/MMMs
The surface morphology of as-prepared ZIF-7 particles were analyzed by FE-SEM (Figure 5a) and the approximate size of the particles was in the range of 40-50 nm.As shown in Figure 5b, particle shape and size were re-confirmed with TEM analysis.
Figure 6 and Figure 7 depicts the SEM images of surface and cross-section of pristine and ZIF-7 loaded MMM respectively.Surface images of various ZIF-7 loaded membranes shows a good distribution of particles with no agglomeration (Figure 6b-6d) which helps to improve the membranes gas separation property with no defect formation.A detailed AFM height profile for the developed MMMs is given in Figure S2.The surface of the neat membrane was found to be smooth but an incremental increase of roughness was observed with the increase of particle loadings on (Figure 8b-8d).The thickness of the selective layer for all MMMs was analyzed by SEM cross-section (Figure 7a-7d) at least at three different locations of each sample and the average thickness value is shown in Table 1.SEM cross-section images confirm that the membrane thickness increases with the higher filler loadings for the MMM samples.

ZIF-7 Loading in Mixed-Matrix Membranes
The determination of accurate particle loading is important to estimate the performance of MMM.In this work, thermogravimetric analysis (TGA) was used to determine the loading percentage of ZIF-7 particles in the MMMs (Zhang et al., 2012).Samples to be tested were annealed until 200 • C in air for 5 h to remove any guest molecules present in the sample.Subsequently, the temperature was increased to 1000 • C with a heating rate of 10 • C/min in air.At 550 • C both the polymer and ZIF-7 particles got decomposed and oxidized leaving zinc oxide as residue.
Percentage loading of ZIF-7 from the residue in mixed-matrix membranes was then back-calculated on Table 2. Calculated loading of ZIF-7 in wt% and the TGA determined zinc mass percentage for each type of mixed-matrix membrane are shown in Table 2.The zinc mass percentage determined by TGA was 25% and the theoretical mass percentage by their molar mass value was calculated as 35%.From the obtained residual values, higher particle loadings had an increased presence of zinc which helps to determine the percentage loading of MMMs theoretically.

Pure Gas Permeation Measurement
Gas permeation was studied for the developed membranes with single gases CO 2 and CH 4 .Table 3 represents the gas permeation results of pristine polymeric membrane MMM0 and ZIF-7 loaded MMM-20, 30 and 40% membranes.Membranes with higher loading of 50% ZIF-7(MMM50) were brittle and they were not used for gas permeation measurement.An increase of ZIF-7 loading leads to an increase of gas selectivity of CO 2 over CH 4 with a decrease in CO 2 permeability.A similar trend was observed by Li et al., 2010 work on ZIF-7 with Pebax1657.Note: 1Barrer=10 -10 cm 3 (STP) cm / (cm 2 s cm Hg).
Feed Pressure: 1 bar, Thickness of membranes were measured from SEM cross-section.
Gas permeance and selectivity of measured samples are summarized in Table 3.The increase of loadings from 20, 30 and 40 wt% caused a decrease of CO 2 permeance whereas the selectivity increased to 68%, 89%, and 99% higher for MMM20, MMM30 and MMM40 compared with the pristine polymeric membrane (MMM0).

Conclusions
In summary, we report a series of MMMs with ZIF-7 particles as fillers and PAN-r-PEGMA67 polymer for CO 2 /CH 4 separation.Structure and morphology of synthesized ZIF-7 particles and developed MMMs were characterized by XRD, FESEM, TEM, FTIR and TGA analysis.Defect-free MMMs were fabricated with 20, 30 and 40 wt% of ZIF-7 loadings.The ZIF particles had a good adhesion with the polymer matrix and no agglomeration was observed.All MMMs, except the brittle MMM50, were measured by constant volume/variable pressure method to evaluate their gas permeance and selectivity with single gases CO 2 and CH 4 .MMM with 40% ZIF-7 loading achieved a high selectivity of 39 for CO 2 /CH 4 and a CO 2 permeance of 4.51E-03 m 3 /m 2 hbar.The increase of ZIF-7 loading in the MMM leads to a small decrease in CO 2 permeance due to the polymer chain rigidification effect.
In conclusion, the resulted ZIF-7/ PAN-r-PEGMA67 MMMs showed promising performance for CO 2 /CH 4 separation and further works will be continued to enhance the permeance along with selectivity of MMMs for CO 2 separation.

Table 1 .
Thickness measurement of membrane selective layer from SEM cross-section

Table 2 .
TGA analysis results of residue and calculated loading of ZIF-7

Table 3 .
Particle loading in mixed matrix membranes and selectivity towards CO 2 /CH 4