Synthesis and Physicochemical Properties of Polyamideimide Membranes Containing ZIF-8 and CMS Particles for Potential Gas Separation Applications


 The present study involves the fabrication and characterization of Polyamideimide (PAI) membrane holding ZIF-8 and CMS particles for potential gas separation applications. Zeolitic imidazolate frameworks-8 (ZIF-8) nanocrystals were prepared by a precipitation reaction with Zinc metal cluster and 2-methylimidazole, whereas the carbon molecular sieves (CMS) were synthesized by pyrolysis of polyamideimide (PAI) polymer. ZIF-8 and CMS particles were characterized comprehensively for the functional group, crystallinity, and morphological analyses. The successful formation of ZIF-8 nanocrystals was evident from the rhombic octahedron shape, and EDX confirms the presence of Zn metal cluster and methylimidazole linker. The ZIF-8 and CMS nanoparticles incorporated PAI membranes were prepared using phase inversion technique with varying loading wt.% of 1, 2, and 3%. PAI/ZIF-8 and PAI/CMS membranes cross-sectional morphology confirmed that synthesized nanoparticles were well embedded through the PAI membrane. The PAI/ZIF-8 (3 %) membrane, thin dense skin top layer, and well-defined honeycomb porous substructure were observed. Furthermore, the ZIF-8 and CMS particles incorporation have a beneficial impact on the mechanical properties of PAI at the low loading of nanoparticles. Thus, the inclusion of ZIF-8 and CMS particles in the PAI matrix positively altered the physicochemical properties of the resulting hybrid membranes, which could help them achieve remarkable gas permeance and selectivity.


Introduction
The effective CO 2 separation from other light gases is a crucial environmental challenge that has been extensively studied in recent years. The imminent climate change issues associated with the increasing concentration of anthrophonic CO 2 led to the importance of CO 2 separation (Taheri and Raisi 2021). The hydrogen production using a water shift gas reaction produces CO and CO 2 as a by-product. Furthermore, CO 2 is a component of natural and biogas. So, it is essential for CO 2 capture from gas streams for fuels with improved energy content and reduces the climatic impact of CO 2 gas. Conventional technologies like adsorption, cryogenic distillation, and absorption were high energy-intensive processes, whereas membrane separation proved to be a promising energy-e cient technology for CO 2 separation (Xie et al. 2019).
Membrane gas separations have drawn attention over the years due to it lower cost and low energy requirement. Mixed matrix membranes (MMMs) are a combination of ller nanoparticle dispersed phase and polymers continuous phase (Cheng et al. 2019). The combination of higher permeability and selectivity with desired membrane stability of membranes are essential for gas separation. Although MMMs have a great future in gas separations, vital problems are the uniform dispersion at high ller content and mainly the attachment between heterogeneous phases. The functionalization of the ller surface and the use of additives can overcome both problems (Aroon et al. 2010). Another aspect of interest is the long-term stability of MMMs. In most glassy polymers, the ller inclusion can also help control the polymer relaxation, which results in a decline in the rate of permeation over time (Goh et al. 2011). Robeson, in early 1990, anticipated an upper limit for the working e ciency of polymeric membranes in gas separation (Robeson 2008;Zhang et al. 2013). MMMs can achieve more excellent permeance and selectivity, or both concerning the existing polymeric materials, due to the inclusion of the inorganic llers with their built-in remarkable separation properties (Chung et al. 2007;Ismail et al. 2009).
ZIFs, a subclass of metal-organic frameworks (MOFs), offer diverse structures, higher surface area, and tunable pore size. ZIFs proved to be a promising material for llers in polymeric matrices because of their sorption and separation of gases and vapors properties. ZIFs are proving as potential material to improve the performance of membranes to overcome the Robeson bound. The advantage of incorporating ZIFs into the polymer matrix over non-porous materials is that their organic components enhance the interfacial compatibility with the polymer matrices. ZIF-8 has a sodalite topology with exceptional CO 2 a nity with better mechanical and thermal stability. Ordoñez et al. reported a signi cant enhancement in gas permeability even at higher ller loading (40 wt%) with the absence of interfacial voids because of the unique properties of ZIF-8 (Ordoñez et al. 2010). Dai et al. obtained a 20% increase in selectivity of CO 2 /N 2 using 17 vol % of ZIF-8 in the selective skin layer compared to neat Ultem® membranes ). Nordin et al. investigated the performance of ZIF-8 with 0.5 wt.% loading on at sheet membranes using PSf as membrane matrix (Nordin et al. 2015), and it has found that the CO 2 /CH 4 selectivities were enhanced by 19 % compared to the neat membrane. CMS is derived from the pyrolysis of polymeric precursors having well-de ned microporous structures that could separate gas pairs based on molecular sieving properties and thus exhibit higher gas permeability and selectivity. Koros et al. synthesized two CMS hollow bers using Matrimid-5218 and 6FDA/BPDA-DAM as precursors for CO 2 /CH 4 separation.
Both CMS bers have shown different permeation results under similar conditions. Furthermore, the CMS bers properties can be varied by varying the parameter, namely temperature, thermal soak time, etc. (Vu et al. 2002). MMMs with CMS showed a 19% increase in CO 2 /CH 4 selectivities than pure matrimid polymer lm (Vu et al. 2003a). This shows that CMS particles could be a promising material to enhance the performance of MMMs for gas separation applications. Herein, we report the synthesis of ZIF-8 nanocrystals and CMS particles to fabricate PAI-based MMMs for potential gas separation applications.

Materials
Zinc hydrate crystals were procured from Merck, India. Sigma Aldrich has supplied 2-methylimidazole and n-butylamine. PAI polymer was purchased from Solvay specialty polymer, India. Merck, India also supplied acetone and methanol.

ZIF-8 nanocrystals synthesis
For ZIF-8 nanocrystal synthesis, two solutions, one of 2.469 mmol, Zn (NO 3 ) 2 ·6H 2 O crystals, were dissolved completely in 50 ml methanol under constant magnetic stirring 9.874 mmol, 2-methylimidazole (Hmim) in 50 mL of methanol were prepared separately. Furthermore, the modulator n-butylamine was added to the later solution to control the crystal particle size. The solution containing organic ligand 2methylimidazole (Hmim) was mixed to the other under constant stirring and the solution aged for 24h at 30°C. The resultant white powder was ltered using centrifugation at 10000 rpm, washed with methanol, and dried. The resulting white powder is nely grounded and stored in the desiccator for further characterization (Maya et al. 2015).

Synthesis of CMS particles
The CMS particles was synthesized using PAI as the polymer matrix by solution casting technique. The predetermined PAI polymer was added into 25ml NMP solvent in a Scott bottle under constant magnetic stirring. Then the PAI polymer solution was allowed to stand still for 24 h at 30°C to release the bubbles from the dope solution. Further, the solution was poured onto a glass plate, and then it was cast using a thin-lm applicator with 750 µm thickness. Furthermore, the casted membrane was dried at 30°C overnight for solvent evaporation. The PAI membrane was kept in the tubular furnace for pyrolysis at a speci c temperature pro le (Vu et al. 2002(Vu et al. , 2003b and under an N 2 environment. After pyrolysis, the CMS particles so formed were grounded into ne powder. The con rmation of these CMS particles was done using XRD analysis.

Fabrication of ZIF-8 and CMS nanoparticles incorporated PAI hybrid membranes
A series of ZIF-8 and CMS ller particles incorporated PAI asymmetric composite membranes were fabricated with 1, 2, and 3 wt. % loading using dry phase inversion method. The detailed composition of ZIF-8 and CMS ller dope solution was displayed in Table 1 and Table 2, respectively. The predetermined ZIF-8 and CMS particles were added separately to NMP solvent and stirred for 20 mins under a magnetic stirrer. Furthermore, the complete dispersion of ller particles was ensured by the sonication for 60 min. The ller suspension was then subjected to priming technique to ensure the optimum wetting and make the particle more compatible with the bulk polymer lm to promote the a nity between the ller and the PAI polymer matrices. Initially, 1/10th of the PAI polymers were added, and it was kept under magnetic stirring at 60°C for 3 h. Then, the remaining PAI polymers were mixed and stirred until the complete dissolution. Furthermore, the solution was allowed degassed at room temperature overnight. A prepared dope solution was poured into a at glass plate and cast using a thin-lm applicator with 750 µm thickness. Finally, the casted membranes were allowed to dry to 35°C for 24 h for the complete evaporation of NMP solvent, and the fabricated membranes were stored in a desiccator for further characterization.

Materials and membrane characterization:
The UV Vis absorption spectroscopy was used for the ZIF-8 nanocrystals con rmation. ZIF-8 and CMS llers particles crystallinity was investigated using XRD (Perkin Elmer, USA) at 40 kV and 30 mA in the 2θ scanning range. The surface morphology of ZIF-8 particles, PAI/ZIF-8, and PAI/CMS membranes was investigated using FESEM (JSM-6701F, JOEL). Furthermore, the surface morphology of the ZIF-8 ller particles was also analyzed using a TEM (JEM 2100, JOEL, Japan). The lm elemental analysis to con rm the carbon presence of carbon was conducted by utilizing EDX with a magni cation of 3000× and 15 kV voltage. The functional group analysis of fabricated PAI/ZIF-8 and PAI/CMS MMMs and ZIF-8 particles were con rmed using FTIR (Thermo scienti c Nicolet iS5, USA) in the range of 4000 and 400 cm −1 . The mechanical properties of PAI/ZIF-8 and PAI/CMS membranes were investigated using a tensile strength analyzer (INSTRON 4500, UK) as per the ASTM D882 standards at room temperature. The experiments were performed three times, and the average values were presented. The hydrophilicity and porosity of the membranes were determined using the procedure as reported earlier in the literature (Aditya Kiran et al. 2016).

Results And Discussion
3.1 UV visible spectroscopy and crystallinity of ZIF-8: The UV Visible spectroscopy analysis of ZIF-8 nanocrystals were illustrated in Figure 1. The penetration was proportionate to excitation wavelength, i.e., 325 nm, due to the decrease in absorbance. The presence of an absorption peak at 325 nm con rms the ZIF-8 nanocrystal formation ).
The crystallinity of ZIF-8 nanocrystals was studied using XRD techniques, and their corresponding XRD patterns were illustrated in Figure 2. The instantaneous solid formation was observed with the inclusion of n-butylamine as a modulating ligand, and the ZIF-8 crystals were recovered from the solution after 24 h (Pan et al. 2011). The strong characteristic peaks around 18° con rm the successful formation of ZIF-8 nanocrystals (Cravillon et al. 2011). The average size of particles was noted to be about 18 nm.
3.2 Chemical structure and thermal properties of ZIF-8: The functional group analysis of ZIF-8 nanocrystals was studied using FTIR techniques, and their corresponding spectra were illustrated in Figure 3. Several bands were observed between 400-2000 cm −1 .
The main bands that characterized the ZIF-8 were shown at 1148 and 763 cm −1 , ascribed to C=N and C-N stretching, respectively (Anastasiou et al. 2016). The absorption bands at 2929 and 3137 cm −1 correspond to the aromatic and aliphatic C-H imidazole stretch bands. The peak at 1579 cm −1 corresponds to the C=N stretching, and absorption bands at 420 cm −1 were noted for the Zn-N stretching mode. Besides, the peaks at 1100-1400 cm −1 (C-N stretching), con rming the successful formation of ZIF-8 nanocrystals (Liu et al. 2016). The thermal stability of the ZIF-8 nanocrystals was characterized using TGA analysis and presented in Figure 4. The synthesized ZIF-8 nanocrystals were highly thermally stable, and it is started to decomposition after 520°C due to the 2-methylimidazole, and it can enable to work on high gas separation applications when we incorporate them into the membrane matrix (Sasikumar et al. 2021).

Surface morphology of ZIF-8 nanocrystals
The ZIF-8 has theoretical cavities of 11.6 Å connected by 3.4 Å aperture size, making them suitable materials to be used membranes to enhance the molecular sieving properties (Sorribas et al. 2014). The performance of ZIF-8 nanomaterials depends mainly on their size, shape, and well-de ned porous nature (Cravillon et al. 2011). The surface morphology of ZIF-8 ( Figure 5) displayed nanocrystals with a rhombic dodecahedral shape crystal structure (Pan et al. 2011;Liu et al. 2013). The EDX spectra of ZIF-8 nanocrystal were displayed in Figure 5, and their corresponding elemental composition was also presented in Table 3. The EDX spectra con rm the presence of C, N, O along with Zn. In addition, the ZIF-8 nanocrystals were also investigated using TEM analysis, and it is illustrated in Figure 6. The roughly spherical particles with sharp edges were observed in TEM images with a particle size of <100 nm.

Con rmation of CMS particles
The synthesized CMS lms were nely grounded into powders for use in mixed matrix membrane fabrications. It is desirable to ensure the permeation capability of CMS particles is comparable with the CMS lms. However, it is tough to determine particles permeation properties like CMS lms (Vu et al. 2003b). So the XRD diffractograms were done for CMS lms and powders, as shown in Figure 7. The powder and lm CMS displayed analogous d-spacings and peaks indicating the amid planar dimensions among powder and lm pyrolyzed CMS particles, indicating CMS's successful synthesis (Vu et al. 2002(Vu et al. , 2003b.

Surface morphology of CMS membrane
CMS membranes inherent CO 2 /CH 4 selectivities were 200 and CO 2 permeability of 44 Barrers (Vu et al. 2003b). The signi cant enhancement of about 45% CO 2 /CH 4 selectivities was reported for Matrimid®-CMS mixed matrix membranes. The top surface morphology of CMS was displayed in Figure 8a, and their corresponding EDX spectra were illustrated in Figure 8b. The presence of carbon peaks with the wt.% of 4.40 con rms the formation of CMS polymer lms, whereas the sharp Si peak corresponds to the Si detector used during EDX analysis. Besides, the Oxygen peaks due to the oxygen atmosphere during the EDX.

Morphology of PAI/CMS membranes
The cross-sectional morphology of CMS incorporated PAI membranes were presented in Figure 8. The morphology of PAI/CMS (1%), PAI/CMS (2%), and PAI/CMS (3%) MMMs looks similar with the increase in loading wt% of CMS particles into the PAI matrix. However, the thickness of the top surface layer was found to be different from each other. The PAI precursor has a thin skin dense (1000-2000 Å) layer on a porous core, whereas the membrane cross-section showed a transition from the inner porous cores to the dense outer structure (Vu et al. 2003b). However, the CMS particle's surface incorporated PAI membranes showed a very dense structure without any interfacial defects, indicating the excellent compatibility between the PAI matrix and CMS particles. The presence of CMS particles is expected to offer better permeance of CO 2 , CO 2 /H 2 , and CO 2 /CH 4 selectivities of gas pairs (Vu et al. 2002(Vu et al. , 2003c.

Morphology of PAI/ZIF-8 membranes
The cross-sectional morphology of PAI/ZIF-8 (1%), PAI/ZIF-8 (2%), and PAI/ZIF-8 (3%) membranes were illustrated in Figure 10. The asymmetric structure was observed with a thin dense skin layer followed by a porous sublayer. The thin dense layer indicates the good adhesion of bare ZIF-8 to the PAI matrix (Zhang et al. 2012). The synthesized ZIF-8 nanocrystals have shown good intimate contact with the PAI polymer without any modi cation, and this could be because of the ZIF-8 hydrophobic nature (Zhang et al. 2014).
Interestingly, the PAI/ZIF-8 membrane morphology was quite different compared to PAI/CMS membranes. With the increase in loading wt.% of ZIF-8, a large array of non-ideal crystals from 50 to several microns were observed. The larger size ZIF-8 particles (> 50 nm) were also observed along with the uniformly dispersed ZIF-8 nanocrystals with the increase in loading wt.%. However, the surface of these ZIF-8 large arrays looks smooth ( Figure 10) without aggregation, and approximately no defects were seen in the PAI/ZIF-8 membrane. It is essential to get the uniform dispersion of ZIF-8 nanocrystals within the polymer matrix to achieved desired gas separation results. A better understanding of the formation of larger size ZIF-8 cluster formations and the impact on gas transport characteristics of the MMMs is required to further extend the asymmetric membranes for practical gas separation applications. The incorporation of ZIF-8 in the PAI membranes increases the thermal and chemical stability and gas separation performance of membranes (Keskin and Sholl 2009;Cravillon et al. 2011). Furthermore, the ZIF-8 as a ller nanocrystal would be expected to offer quadrupole moments of CO 2 and the electrostatic eld of ZIF-8, which resulted in enhanced CO 2 /CH 4 selectivities. Also, the ZIF-8 contributes to the increase in solubility of CO 2 due to its electrostatic interaction between the Zn metal and CO 2 molecules (Meshkat et al. 2020).

Functional group analysis PAI/ZIF-8 membranes
The functional group analysis of PAI/ZIF-8 membranes was investigated using FTIR, and their corresponding spectra were presented in Figure 11. The characteristic peaks of PAI polymer were observed at 3506 cm −1 (-NH), 1717 cm −1 (-CO) groups. The FTIR results shown in Figure 11 indicates the presence of aluminosilicates in the PAI/ZIF-8 membranes con rms the inclusion of ZIF-8 nanocrystals.
Besides, the presence of ester, dimer, conjugated cyclic bond, and enols con rm due to the PAI polymer matrix (Rajesh et al. 2012). It can also be noted that with the loading percentage increases, the transmittance value also increases, and this shows us that as the loading concentration of the inorganic ller increases, the permeability of the membranes increases.

Functional group analysis of PAI/CMS membranes
The chemical structure of PAI/CMS membranes was investigated using FTIR techniques, and their corresponding spectra were presented in Figure 12. The characteristic peaks of PAI polymer were observed at 3506 cm −1 attributed to the -NH amide linkage and the peaks at 1653 and 1717 cm −1 correspond to -CO stretching of amide linkage and stretching vibration of the ve-member ring. With the increase in loading wt.% of CMS particles, the transmittance peaks were increased slightly, improving free volume in the membrane matrix. The presence of a carbon bond was observed at 1660 cm −1 (C=O bond) of carbon material, con rming the presence of CMS particles in the membrane matrix. The presence of CMS shows bands at 3228 cm −1 corresponding O-H stretching and the peaks at 2915 cm −1 assigned to C-H stretching. In addition, the 1576 cm −1 peak belongs to the carboxyl group (C=O). Furthermore, the presence of ester, dimer, conjugates cyclic bonds, and enols con rm the PAI polymer backbone and the band at 1000 were due to the O-H bending (El-Sheikh 2016). Furthermore, with the increase in loading wt%, there increase in transmittance %, which is expected to offer increased gas permeability. The CMS nanoparticles have proven to offer good selectivity and permeability. The CMS with rigid and well-de ned pore structures is ideal for high pressure and temperature separation applications like hydrogen puri cation (Lei et al. 2021

Mechanical properties of PAI/ZIF-8 and PAI/CMS membranes
The mechanical strength of the ZIF-8 and CMS incorporated PAI membranes were studied using tensile study with the help of a universal tensile testing machine, and the values were displayed in Table 5. In the case of PAI/ZIF-8 (1 %), the yield ultimate stress and % elongation was found to be 2.58 MPa and 8.25 %, respectively. However, with the increase in loading percentage of ZIF-8 nanocrystal, the ultimate stress and % elongation was found to be decreasing. At 2 wt. % loading of ZIF-8 nanocrystals, the ultimate strain was noted to be 77.69% lower than the PAI/ZIF-8 (1%). The decrease in mechanical stability could be due to the agglomeration of ZIF-8 nanocrystal at a higher ller loading percentage. In the case of PAI/ZIF-8 (3 %), the membrane was very brittles that tensile test couldn't perform due to the aggregation of nanocrystals. Besides, in PAI/CMS (1%), the tensile stress was 5.41 MPa, whereas, in the PAI/CMS (3%) membrane, the stress was comparatively less. This shows that the stability of the membranes could decrease at the higher ller loading due to the aggregation of ller nanoparticles. Comparing the mechanical stability of PAI/ZIF-8 and PAI/CMS membranes shows that 1% PAI/ZIF-8 can endure a strain of 8.25%, whereas 1% PAI/CMS of about 3.72%. This could be due to the excellent adhesion and uniform dispersion of ZIF-8 nanocrystals at low loading weight because of the MOF structure. The increase in ller loading percentage in the membranes decreased the amount of force that the membrane withstands. However, the increase in the ller loading percentage is expected to increase their permeability and selectivity values. Therefore, optimizing the loading wt.% of ller nanoparticles in the membrane matrix is crucial for potential gas separation applications.  The effect of humid conditions during the gas separation measurements plays a crucial role in membranes performance. So, the water holding capacity and porosity determine the effectiveness of the membranes. The results of the tests are given in Table 3. In PAI/CMS membranes, the amount of water absorbed increases with the increase in CMS loading percentage. This is due to the hydrophilic property of the CMS nanoparticles that are present. This enhanced water retention property is expected to offer increased CO 2 permeance of the membranes. The incorporated ZIF-8 and CMS increase the free volume in the membrane matrix, which signi cantly increases the permeability. Therefore, these membranes can withstand fouling for a longer period due to their hydrophilic property. PAI/ZIF-8 membranes also followed a similar trend. Comparing the amount of water absorption of PAI/ZIF-8 and PAI/CMS membranes shows that 1% PAI/ZIF-8 can absorb 0.0146g of water while 1% CMS/PAI can absorb 0.0013g of water. So, compared to the amount of water absorbed, PAI/ZIF-8 membranes are more hydrophilic than PAI/CMS membranes. Also, as the concentration of the ZIF-8 increases, the porosity of the PAI/ZIF-8 membranes increases slightly due to the ZIF-8 structure. This phenomenon increases the permeability of the membranes as the porosity of the PAI/ZIF-8 membranes increases, thereby offering good CO 2 /H 2 gas separation. On the other hand, the porosity of the PAI/CMS membranes keeps on decreasing as the concentration of CMS nanoparticles in the membrane increases. This may be due to the agglomeration caused due to the increase in the concentration of CMS particles which leads to a decrease in the permeability of the CMS/PAI membranes for gas separation. The increase in membrane water absorption capacity is expected to offer better fouling resistance.

Conclusion
A series of ZIF-8 and CMS particles incorporated PAI membranes were fabricated using dry-phase inversion technique for potential gas separation applications. The ZIF-8 and CMS particles were synthesized and characterized using FTIR, XRD, and TGA techniques. The functional group analysis and crystalline study con rming the formation of ZIF-8 nanocrystal. The morphological studies of ZIF-8 support evidence for forming rhombic dodecahedral crystals with a particle size of around 100 nm. Thermal studies of ZIF-8 nanocrystals have also shown thermal stability up to 500°C; proving can be applied for high-temperature gas separation applications, whereas the formation of CMS from the pyrolysis of PAI polymer was con rmed with the help of XRD spectra. Further, the effect of varying loading percentages of ZIF-8 and CMS particles on the PAI membrane matrix physicochemical properties were investigated for possible usage in gas separation applications. Cross-section morphology revealed that the ZIF-8 and CMS particles addition has a dominant effect on the internal and porosity and surface pore structures. A signi cant variation in tensile strength at the low loading (1 wt.%) into the PAI membrane. The experimental results con rmed that the physical and chemical properties of the PAI membrane were altered depending on the loading wt.% of ZIF-8 and CMS. Furthermore, the ZIF-8 nanocrystal is expected to offer an electrostatic eld with the CO 2 quadrupole moment, resulting in increased CO 2 selectivity. From the results, it can be concluded, the fabricated PAI/ZIF-8 and PAI/CMS can be promising membrane materials for gas separation with appropriate permeation, permeability, and selectivity properties.