The membrane separation process lacks intrinsic permeation characteristics to compete with other separation technologies like adsorption, sedimentation, coagulation, skimming, and distillation. A mixed matrix membrane (MMM) is one of the strategies to improve the separation characteristics with embedded nanofillers particles. Zeolite imidazolate framework (ZIF) has a new subclass of inorganic–organic hybrid materials that are being introduced as new fillers for incorporation into the polymer matrix for various applications such as oily wastewater separation, wastewater treatment, natural gas dehydration, landfill gas upgrading, and mixed gas separation. In this experimental work, a metal-organic framework called ZIF-8 was synthesized and used as a filler for modification of MMMs and characterized with FTIR and SEM. ZIF-8 nanoparticles up to 5 wt% loading were added to PSF casting solution then the permeation characteristics of MMMs showed an improved result like the pure water flux of the modified membrane at 2.5 bar was increased up to 456.38 L/m2h. In the case of pure gas separation, at 5 wt% ZIF-8 loading in PSF, the pure gas CO2 permeability at 9 bar pressure had increased to 10.54 barrer.
We have studied water and gas permeation characteristics incorporated with ZIF-8 containing mixed matrix membranes.
ZIF-8 was made as a gateway for quick transport of CO2 gas molecules and water molecules through the polymer matrix.
As per the observed results, higher permeability of the MMMs can be possible with higher loading of ZIF-8.
Zeolitic Imidazolate Frameworks-8
Pure water flux
Mixed matrix membranes
Mass flow controller
Plain PSF membrane
1 wt% ZIF-8 mixed matrix membrane
5 wt% ZIF-8 mixed matrix membrane
Pure water flux
Surface area (cm2)
Permeability of gas A
Permeability of gas B
Selectivity of gas
Fourier transform infrared radiation
Thermal gravimetric analysis
Scanning electronic microscope
Water contact analysis
No living person can live without pure water. Freshwater availability on earth is meagre, and demand increases as the world's population grows. Water is also contaminated by industrial and agricultural activities, medications, technological civilization, pesticides, clothing, and worldwide changes (Gnanasekaran & Balaguru 2019). Furthermore, greenhouse and toxic gases produced by the dumping and burning of fossil fuels are increasing environmental damage and global warming. This directly affects an ecosystem's biological cycle, causing skin and respiratory diseases from air pollution. Gas separation is essential, particularly in chemical and petrochemical plants that purify landfill gas, upgrade biogas, and sweeten natural gas. Natural gas has a significant amount of energy that can be utilized for cooking, heating, power, fuel in cars, and chemical feedstock. Crude natural gas includes corrosive gases such as CO2 and H2S; it is corrosive, toxic, and flammable due to this problem, creating corrosion in gas transportation pipelines. Hence, it is necessary to remove the impurity from the natural gas. To address these issues, less use of processing water and fossil fuels is impossible for economic development. Thus, it is essential to get fresh water and capture CO2 to reduce waste emissions to protect the environment. So, it requires cost-effective and environmentally friendly techniques for purifying contaminated water and air (Le et al. 2021).
Water and carbon dioxide purification has been done by equilibrium separation and rates governing process. Equilibrium processes such as skimming, sedimentation, filtration, adsorption, absorption, and cryogenic distillation required solvent or adsorbent for product separation (Vatanpour & Khorshidi 2020). So, it has a costly process for operation and maintenance. Other disadvantages are that an open loop requires considerable space, demands more energy, and is challenging to scale up. However, the rate governing processes like membrane-based separation does not require a solvent for water and gas separation (Lin et al. 2019; Salahshoori et al. 2021). So, there are low operation and maintenance costs, greater process simplicity, closed loop, greater ease of operation, less space, requirement easy scale-up, and greater energy efficiency compared to the equilibrium separation process (Kumar et al. 2018; Nabipour et al. 2020).
In the last few decades, polymeric membranes such as polysulfone, polyamide, cellulose acetate, polyethersulfone, and Pebax have been used because of easy scale-up and high-performance gas separation. Most polymeric membranes have a fundamental problem, such as a limitation between permeability and selectivity (Maghami et al. 2021). So, the polymeric membrane is incorporated with highly porous solid material as filler and fabricated mixed matrix membranes (MMMs) on the geometry base. Many researchers worked on the different types of fillers like zeolites (Susanti 2019), nano-silica (Salahshoori et al. 2021), graphene oxide (Sainath et al. 2021), SiO2 (Yang et al. 2021), and CNTs (Singh et al. 2021). However, due to the surpassing trend of the limit between permeability and selectivity, the previous work has reported metal-organic frameworks (MOFs) having excellent results for water and gas separation performance. For example, incorporating 3 wt% MOF in polyethersulfone MMMs improved the water flux by 121.5 L/m2 h (Zhang et al. 2020). In another work, authors reported that adding 20 wt% ZIF-68 in a Matrimid MMM enhanced the permeability of CO2 by 122% (Essen et al. 2021). Furthermore, 5 wt% ZIF-67/Pebax-1657 membrane has a higher permeability of CO2 compared to the plain Pebax membrane (Meshkat et al. 2019). Whereas on the addition of 20 wt% ZIF-301 with polyimide, the permeability of CO2 was found to be around 899 barrer and the selectivity CO2/CH4 of 29.3 (Wang et al. 2021).
We focused on ZIF-8 as a subclass MOF. MOFs are crystalline organic–inorganic hybrid complexes of bivalent or trivalent metal clusters or ions linked by organic linkers (Nuhnen et al. 2018). The presence of organic linkers is suitable for various strategies to fine pore structure, aperture, and pore polarity. Zeolitic imidazolate frameworks (ZIFs) have high crystalline nature, high surface area, porous structure, high thermal stability and chemical stability (Essen et al. 2021), and high adsorption capacity (Furukawa et al. 2010). On the basis of microporosity, ZIFs were suitable for CO2 gas separation and water purification.
Nevertheless, despite the wide variety in the unravelled structure of ZIFs, they are not used in MMMs to apply the water and gas separation process (Chen et al. 2014; Qian et al. 2020). ZIF-8 is synthesized by 2-methyl imidazolate, linked with Zn2+ metal ions, forming a cub octahedral structure. The presence of 2-methyl imidazolate made the ZIF-8 framework structure dynamic, where the crystallographic size of the pore aperture 3.4A0 (Banerjee et al. 2009). In the case of gas separation, the molecular size of CO2 has 3.3A0 and CH4 of 3.8A0 (Furukawa et al. 2014), which is close to ZIF-8 pore size of 3.4A0 and becomes the flexible structure to diffuse the polar CO2 gas, which was not attributed non-selective flow channels for CH4 gas (Deng et al. 2020). In the case of water, the molecular size is 2.8A0, smaller than the ZIF-8 pore size, making it very easy for water molecules to pass through ZIF-8. So, they have emerged as a promising material for water purification and CO2 gas separation processes (Aframehr et al. 2020). The polymer used as PSF has an inherent material, and high mechanical and chemical stability (Wu et al. 2019).
In the current work, ZIF-8 filler was synthesized and it was utilized for the modification of porous and non-porous mixed matrix PSF membranes. The weight percentage of ZIF-8 varied from 0.5 to 5%. The plain and modified membranes were characterized with the help of FTIR, XRD, water contact angle, and SEM. Finally, the fabricated MMMs were investigated for water and gas permeation studies.
Zinc nitrate hexahydrate, polyethylene glycol (4000), 2-methylimidazole (C4H6N2 > 99%), and polysulfone (average Mw 35,000) were purchased from Sigma-Aldrich. Methanol (HPLC grade) and N-methyl 2-pyrrolidone (98%) were purchased from Loba Chemical.
Synthesis of ZIF-8
Fabrication of membranes
Phase inversion method (for water application)
For MMMs, the desired quantity of a predetermined ZIF-8 was mixed with PSF and PEG4000 homogeneous solution, which was then put in a sonication water bath for an hour. The polymer solution was then cast onto the glass plate before maintaining the air gap thickness of 255 μm. The thin wet film was obtained on the glass plate and put in a water bath for 24 h to remove the solvent. The film was dried at room temperature overnight. The membranes obtained were 15M0, 15M0.5, 15M1, 15M2, 15M3, and 15M5.
Solution casting method (for gas application)
For MMMs, the desired quantity of a predetermined ZIF-8 was mixed with PSF homogeneous solution, which was then put in a sonication water bath for an hour. The resultant solutions were cast onto a glass plate before maintaining the air gap between the digital applicator 255 μm. The thin film was obtained on the glass plate and allowed to dry for 24 h at a temperature of 180 °C in an oven. The membranes obtained were PSF, 1, 3, and 5 wt% ZIF-8/PSF MMMs.
The ATR-FTIR characterization was carried out with the help of an FTIR spectrometer (Perkin Elmer spectrum) over a wave number of 4000–400 cm−1 to study the different functional groups of ZIF-8 modified MMM. Thermogravimetric analyses characterized the thermal decomposition temperature of MMMs by Hitachi STA7200. The crystalline structure MOFs and mix matrix membranes were characterized with the help of an X-ray diffractometer (XRD) Model D8 DISCOVER (Bruker). The morphology of all MMMs was observed with the help of a scanning electron microscope (FE-SEM) Model JSM 7600F (Jeol). The DFT-based investigations were studied using the Gaussian 09 computational package and Goniometer (APEX S/N: ACAMNSC 34, model Acam-D2) was analysed for the measurement of static water contact angles of unmodified and modified water membranes.
Evaluation of water flux through membrane performance
Pure CO2 gas permeability studies
RESULTS AND DISCUSSION
Characterization of membranes
Water contact angle measurement and wettability study
Density functional theory (DFT)
Here, Ecomplex is the energy of the ZIF-8 and gas complex, EZIF-8 is the energy of the ZIF-8, and is the energy of a gas molecule in their optimized geometries. The energy of the complexes includes the basis set superimposition errors (BSSE) by performing counterpoise corrections.
X-ray diffraction (XRD)
Pure water flux (PWF) permeation characteristics studies
Gas permeation studies
Pure gas studies
Mixed gas studies
In this work, we synthesized ZIF-8, which was the primary key to improving the permeability of gas and water flux. ZIF-8 was incorporated into PSF (using phase inversion and solution casting method) and fabricated MMMs (porous and non-porous) in which ZIF-8 made a barrier through which the selective component of molecules diffuse through it. These membranes were characterized with FTIR, TGA, SEM, WCA, DFT, and XRD, to study the functional group of filler, thermal decomposition temperature, cross-sectional morphology, hydrophilicity, complexation energy studies (CO2, CH4), and crystalline structure of the MMMs, respectively. We studied water and gas permeation characteristics based on the ZIF-8 mechanism in MMMs. The water flux of the 15M1, 15M3, and 15M5 membranes was increased by 152, 248, and 456 L/m2h, respectively, compared to the 15M0 (Plain PSF) membranes. We observed a higher water flux of 456 L/m2h at 15M5 membranes. Similarly, in pure gas (CO2) and mixed gas (CO2/CH4) permeation studies, the permeability of CO2 was increased 10.57 and 12.30 barrer, respectively, as compared to plain PSF (7.02 barrer). From experimental results, the CO2 permeability was higher in mixed gas permeation studies and has a novelty for mixed gas studies. In this work, we calculated the complexation energy between ZIF-8 and CO2 gas molecules as −10.30 kJ/mol using DFT studies, indicating that ZIF-8 is selective towards CO2 molecules. From both experimental and theoretical results, we observed that ZIF-8 is an excellent and promising filler for studying the permeation characteristics with MMMs.
We thank Pandit Deendayal Energy University and the Indian Institute of Technology (IIT) Gandhinagar (India) for supporting the central instrumental facilities for materials characterization.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.