Zeolitic imidazolate frameworks (ZIFs), a sub-family of metal-organic frameworks (MOFs), have garnered significant attention for their exceptional characteristics, including high surface area, porous structure, and permeability flexibility [1]. Carbon dioxide is widely acknowledged as a greenhouse gas and the primary contributor to global warming, with its impact intensified by the utilization of fossil fuels [2]. A recent report indicates that CO2 levels have risen by approximately 100 ppm over the past six decades and are projected to increase further due to the persistent reliance on fossil fuels as the primary energy source [3]. Despite the European Union's efforts in CO2 policy, the latest data indicates that CO2 emissions in the region increased by approximately 6.5% in 2021 [4], [5]. Consequently, the reduction of CO2 emissions and the capture/adsorption of CO2 pose global challenges, as the pollution of CO2 can exacerbate greenhouse effects, climate change, and the melting of snow cover [6]. In response, there has been ongoing progress in the development of various CO2 capture and separation systems. Notably, membrane separation (specifically metal-organic frameworks - MOFs) stands out as an efficient technique for CO2 adsorption and separation due to its low energy consumption, ease of industrialization, substantial economic benefits, and high efficiency [7]. MOFs, which are porous crystalline compounds consisting of metal ions connected by organic linkers, have emerged as a rapidly advancing field in chemistry due to their structural and functional tunability [8]. They have also gained recent prominence for CO2 uptake and capture due to their high porosity, thermal and chemical stability, and expansive surface area [9]. Within the realm of MOFs, zeolitic imidazolate frameworks (ZIFs) have particularly captivated researchers due to their outstanding properties, such as their substantial surface area, porous structure, and exceptional permeability characteristics. ZIFs represent a class of porous hybrid materials bearing zeolitic-like structures [10]–[12].
ZIFs are characterized by their tetrahedral frameworks, formed by a divalent metal cation (such as Zn2+ or Co2+) connected to nitrogen atoms in an anionic imidazolate molecule [13]. Specifically, ZIF-8 consists of Zn2+ cations linked with 2-methylimidazole (2-mIm) organic linkers, creating spacious cavities of 1.16 nm interconnected with windows of approximately 0.34 nm [14]. Recently, a ZIF-based filter has been developed for the separation of various flue gases, including CO2, H2, and CH4, benefiting from its superior chemical and thermal stability as well as flexibility [15]. ZIF-8, in particular, has shown potential as a gas storage material, exhibiting a fully reversible adsorption and desorption process without hysteresis, which signifies rapid kinetics [16]. Its porous nature and expansive surface area make it highly suitable for gas storage applications [17]. While ZIF-8 is recognized for its flexibility, it is expected to favor the adsorption of H2 (kinetic diameter of 0.29 nm) over CO2 (0.33 nm) [18].
Additionally, ZIF-8 has demonstrated excellent chemical resistance, remaining unchanged even after immersion in boiling methanol, benzene, water for seven days, and concentrated sodium hydroxide at 100 ºC for 24 hours [19]. The synthesis of ZIF-8 is straightforward and exhibits molecular sieving properties, resulting in outstanding CO2 selectivity and permeability [20]. Among reported ZIFs (including ZIF-7, ZIF-67, ZIF-90, and ZIF-9-67), ZIF-8 has shown the highest permeability (240×10− 7 mol m− 2 s− 1 Pa− 1) and selectivity (~ 7) in the separation of CO2/CH4 [21]. ZIFs are built upon tetrahedral topologies, where 4-coordinated transition metals are connected by imidazolates with an angle of 145°, allowing the synthesis of various ZIFs with zeolite-type tetrahedral topologies [22].
The filter material plays a vital role in effectively capturing and separating CO2 from gas streams. Recently, there has been growing interest in utilizing softwood kraft pulp as a filter material for CO2 capture applications, primarily due to its notable characteristics such as high surface area, porosity, and inherent hydrophobicity. The high surface area of softwood kraft pulp facilitates increased contact between the gas stream and the filter material, thereby enhancing the adsorption capacity for CO2. Additionally, the pulp's porous nature provides a multitude of micro and mesopores, which significantly contribute to the overall efficiency of CO2 trapping. Moreover, the hydrophobic properties of softwood kraft pulp make it resistant to moisture, effectively reducing the risk of moisture-related degradation or filter clogging. This attribute ensures the long-term effectiveness and durability of the filter material in CO2 capture systems.
The exceptional characteristics of softwood kraft pulp make it a promising choice for CO2 capture applications, as it offers an ideal combination of high surface area, porosity, and hydrophobicity. By harnessing these properties, softwood kraft pulp can be utilized effectively as a filter material in CO2 capture processes [23]–[25]. Thus, the objective of the present work is to synthesis and characterization of ZIF-8 and ZCAF for CO2 adsorption. Overall, this study provides valuable insights into the synthesis and characterization of ZIF-8 and highlights its effectiveness as a CO2 capture material in air filters. The findings contribute to the development of sustainable solutions for mitigating CO2 emissions and combating climate change.