The burning of fossil fuels to meet the rising energy demand is the primary source of anthropogenic carbon dioxide (CO₂) emissions in the atmosphere, which is a major contributor to greenhouse gas (GHG) emissions and global warming (Habib et al., 2020). The concentration of atmospheric CO₂ has steadily increased during the last century, rising from 280 parts per million in the 1800s to more than 389 parts per million in 2019 and 400 parts per million in 2020 (Al-Rowaili et al., 2021). Adsorption processes based on porous solids (e.g., carbon materials) are becoming a great alternative to real technology based on absorption processes, utilizing basic solutions in terms of cost and environmental impact (Casco et al., 2014). Membrane-based gas separation techniques, on the other hand, provide an economically and technologically viable alternative for CCS (carbon, capture, and storage) due to their high energy efficiency, cheap capital cost, low carbon footprint, and simple and continuous operation mode (Cheng et al., 2019). In comparison to inorganic and molecular sieve materials, polymeric membranes have drawbacks such as lower polymer/particle interface compatibility. As a result, mixed-matrix membranes (MMMs) with a polymer matrix and an inorganic substance as the dispersion phase were developed. To improve polymer separation capabilities, various particles such as metal oxide nanoparticles and inorganic molecular sieves were introduced to the polymer matrix (Dorosti and Alizadehdakhel, 2018).

One of the most significant problems is that the fillers do not have adequate interfacial compatibility with the polymer matrix. MMMs frequently underperform their projected separation performance behavior due to poor adhesion between the polymer matrix and the fillers (Ishaq et al., 2019). Metal organic frameworks (MOFs) are made up of organic linkers, and the metal center functions as a filler in membrane preparation. MOFs have changeable pore/aperture size, high surface area, and specific adsorption affinity. As a result, MOF-based membranes have high potential for gas separation application (Yuan et al., 2017).

ZIFs (zeolitic imidazolate frameworks), a new type of metal organic frameworks (MOFs) with outstanding chemical and thermal stability, are being tested for a variety of applications such as gas adsorption and catalysis, etc. (Qian et al., 2012). The introduction of ZIFs into polymeric membranes improves the efficiency of gas separation processes. The Co-N bond in ZIF-67 is stronger than the Zn-N bond in ZIF-8, and the smaller pore size of ZIF-67 makes it a more suitable material for gas separation application (Gupta and Murthy, 2022). Zhang et al. (2012a) established successful separation output with C₃H₆-C₃H₈ by utilizing a zeolitic imidazolate framework. They found it excellent for C₃H₆/C₃H₈ separation due to effective pore size (3.4 A^o) of ZIF-8 (Zhang et al., 2012b).

When evaluating MOFs for adsorption applications, their behavior in the presence of water is a critical consideration. Water vapor is present in many industrial streams and must be taken into consideration when choosing adsorbents for adsorption separation and purification systems (Burtch et al., 2014).

S. Meshkat et al. prepared a mixed matrix membrane using the solvent evaporation method with different loadings of ZIF-67 up to 5 wt. % with Pebax polymer for CO₂/N₂ gas separation application (Zhang et al., 2012b; Meshkat et al., 2020). N. Gupta et al. studied the effect of different wt.% loadings of ZIF-67 with polysulfone for extraction of NaNO₃ and Na₂SO₄ from synthetic solutions using the phase inversion method (Gupta and Murthy, 2022). S. Feng et al. fabricated a mixed-matrix membrane incorporated with morphological regulated ZIF-67 nanosheets up to 5 wt.% loadings with Pebax using the solvent evaporation method for CO₂/N₂ separation application (Feng et al., 2020). Another scientific report suggested Pebax as a copolymer due to its thermoplastic elastomer property, with PA recognized for mechanical strengthening and PE for flexible gas transport (Barbi et al., 2003; Car et al., 2008). Hägg et al. (Nafisi and Hägg, 2014) reported that the permeability of CH₄, N₂, and CO₂ was elevated by using ZIF-8 nanoparticles in Pebax polymer and fabricated dual sheet MMM. Furthermore, the selectivity parameter for CO₂/NO₂ decreased from 33.8 to 32.3 of the Pebax membrane in comparison with ZIF-8 loaded MMMs. Another study adapted IL@ZIF-8 socked with activated filler in methanol and BMIIm, finally blended with Pebax polymer. The 15 wt% MMM can efficiently raise the selectivity of CO₂-CH₄ and CO₂-N₂ by 92 % and 74%, respectively, whereas CO₂ permeability by 45% (Li et al., 2016). Rodrigue et al. fabricated Pebax-based ZIF-67 and reported a 130% (162 barrer) increase in CO₂ permeability. Another investigation on the fabricated ZIF-67 NS/Pebax reported enhanced CO₂-N₂ selectivity to 73.2 barrer and permeability of CO₂ to 139.4 barrer, whereas DnBMCl Pebax 1,657 with ZIF8 association delivered high selectivities and permeabilities for CO₂/CH₄, CO₂/H₂, and CO₂/N₂ (Jomekian et al., 2017; Meshkat et al., 2020).

In this study, as far as our knowledge, there are no studies on ZIF- 67 based MMM using THF as a solvent. In this work, we synthesized a mixed-matrix membrane based on water stable ZIF-67 and hydrophobic polymer using the solvent evaporation method to check the CO₂ adsorption performance of lower MOFs. In this work, ZIF-67 was made at room temperature, and a ZIF-67-based mixed matrix membrane was developed. The prepared ZIF-67 mixed-matrix membrane is highly stable after exposure to organic solvents and pure water. The CO₂ adsorption isotherms were measured to evaluate the performance prepared membrane.

Polysulfone (PS) average M_w ∼35,000 g/mol, density (1.24 g/ml at 25°C) by Sigma Aldrich and cobalt nitratehexahydrate (Co(NO₃)₂∙6H₂O, 98%),2-methylimidazole (C₄H₆N₂, 99%) were obtained from Sigma Aldrich. All solvents, such as methanol (CH₃OH) and tetrahydrofuran (C₄H₈O), were analytical grade. All chemicals and glassy polymer were used without any processing. Pure gas CO₂ was used for gas sorption analyses.

In a typical synthesis of ZIF-67 with some changes in the previously reported method, 1.436 g of Co (NO₃)₂·6H₂O and 3.244 g 2-MeIm were weighed and separately dissolved in 100 ml of methanol. The metal solution was slowly poured into ligand solution under stirring at room temperature until the solution turned into dark purple in color. Then, stirring was carried out at room temperature for 3 h and the solution was allowed to stand without stirring at room temperature for 24 h. Afterward, the product was collected using centrifugation at 5000 rpm for 15 min. The washing was carried out three times with methanol and one time with chloroform to remove the dissolved impurities. The product was dried at 120°C in a vacuum oven for 3 days (Gupta and Murthy, 2022).

For the preparation of 3% ZIF-67 mixed-matrix membrane, 0.03 g of ZIF-67 was weighed in a 20-ml vial, and then 67 ml of THF was added into the vial. The solution was sonicated for 30 min followed by stirring for 24 h at room temperature. For polymer solution preparation, 0.97 g of polysulfone was taken in a 20-ml vial and then 67 ml of THF added to the vial, allowing it to stir for 24 h at room temperature. Then, the entire polymer solution was added into MOF solution and sonicated for 30 min followed by 24 h stirring at RT. After 24 h, sonication is carried out for 30 min before casting solution. The solution was poured into a glass petri dish and allowed to evaporate at room temperature under an inverted funnel. The inverted funnel was used for controlled evaporation of the solvent.

The results of different characterization techniques, that is, XRD, ATR -FTIR, TGA, and gas sorption analysis of ZIF-67-based mixed-matrix membrane are described below.

In summary, a mixed-matrix membrane with lower MOF loading has been prepared with the solution casting method using THF as the solvent. The fabricated 3 wt.% ZIF-67 membrane showed higher CO₂ uptake than the pure polymer. The prepared ZIF-67/polysulfonemixed matrix membrane was stable, flexible, and free standing. The as-synthesized ZIF-67 was highly stable in pure water and organic solvents. The combination of a hydrophobic polymer and a hydrophobic MOF was proved compatible with each other to fabricate a mixed-matrix membrane. This study provides an opportunity to utilize ZIF-67 MOF as competent material for CO₂ capture from wet flue gases.