1. Introduction

The escalating global concern over climate change and the imperative to reduce greenhouse gas emissions have intensified research efforts in developing innovative technologies for efficient carbon capture and utilization. Among these, membrane-based gas separation has emerged as a promising avenue, offering a cost-effective and energy-efficient alternative to traditional separation methods. This paper delves into the forefront of this field, exploring the application of Polymer of Intrinsic Microporosity (PIM-1) as a cutting- edge material for CO₂ gas separation membranes[ 1-6].

PIM-1, renowned for its exceptional intrinsic microporosity, stands out as a polymer with unique structural characteristics that lend themselves well to gas separation applications. The intrinsic microporosity of PIM-1 arises from its highly crosslinked and rigid structure, which creates a network of micropores that selectively allows the permeation of certain gas molecules, such as carbon dioxide, while impeding others[ 7-9]. This inherent property positions PIM-1 as a promising candidate for enhancing the performance of gas separation membranes, particularly in the context of CO₂ capture from industrial processes and power plants.

The adoption of PIM-1 in gas separation membrane technology is not only driven by its inherent microporous structure but also by its tunable chemical properties[ 10-13]. Researchers have harnessed the versatility of PIM-1 to tailor its chemical composition, thereby fine-tuning its gas permeability and selectivity[14,15]. This level of customization enables the development of membranes optimized for specific gas separation tasks, making PIM-1 an invaluable tool in the quest for highly efficient and selective CO₂ capture.

In this paper, we aim to provide a comprehensive overview of recent advancements in the utilization of PIM-1 membranes for CO₂ gas separation. The molecular weight of PIM-1 was controlled by adjusting the synthesis condition such as temperature, chemical components and etc., to Fig. out the effect of backbone chain length on the CO₂ separation performance. By examining the synthesis methods, structural properties, and performance characteristics of these membranes, we seek to contribute to the growing body of knowledge that underpins the development of next-generation membrane technologies with a focus on mitigating the impact of anthropogenic carbon emissions on the environment.

2. Materials and Methods

2.1. Materials

For the synthesis of PIM-1, 5,5',6,6'-Tetrahydroxy- 3,3,3',3'-tetramethyl-1,1'-spirobisindane (TTSBI, 96%), tetrafluoroterephthalonitrile (TFTPN, 99%) and anhydrous potassium carbonate (K₂CO₃, 99.99%) were purchased from Sigma Aldrich, was sublimated under vacuum at room temperature before using. Anhydrous toluene (99.8%) and anhydrous N,N-dimethylacetamide (DMAc, 99.8%) from Sigma Aldrich were used as received. The above all chemicals were used without further purification.

2.2. Synthesis of PIM-1 using polycondensation method

TTSBI (10.0 mmol) and TFTPN (10.0 mmol) were combined in a solvent mixture of DMAc and toluene under N₂ atmosphere, with continuous stirring until complete dissolution was achieved. K₂CO₃ (6 g) was then introduced into the solution, and the mixture was stirred under N₂ for 1 hour. Subsequently, the reaction mixture underwent reflux under N₂ at 160°C for 4 hours, with simultaneous stirring. Upon completion of the reaction, the resulting polymer was precipitated in methanol to obtain the solid polymer form. The obtained polymer was redissolved in chloroform, followed by reprecipitation in methanol. After washing the polymer twice with hot deionized (D.I) water, solvent exchange was performed using methanol. The molecular weight was controlled with a different composition of solvent during the synthesis, such as the ratio between DMAc and toluene. The polymer was then dried at 60°C for 24 hours to achieve the final polymer product.

2.3. Fabrication of the PIM-1 membrane

The preparation of H-PIM-1 and L-PIM-1 membranes involved dissolving the polymers in chloroform with vigorous stirring for 6 hours to achieve a 5 wt% solution. Subsequently, the polymer solution was cast onto a glass petri dish and covered directly with aluminum foil. The casting was left to dry at ambient temperature for 24 hours, followed by an additional 24 hours of vacuum oven drying. The resulting membranes were carefully peeled off from the glass dish, resulting in yellowish gas separation membranes. The thickness measurements indicated that H-PIM-1 and L-PIM-1 membranes had thicknesses of 187 μm and 193 μm, respectively.

2.4. Characterization

The morphology of membrane surface were observed using a field-emission scanning electron microscopecold type (FE-SEM, JEOL, JSM6700F, 10 kV). The characterization bands of polymer were recorded using a Fourier transform-infrared spectroscopy (FT-IR, 300E, Shimadzu, Japan) in the attenuated total reflection (ATR) within wavenumber range of 4000-600 cm^-1. Number average molecular weight and polydispersity index (PDI) of PIM-1 were investigated via gel permeation chromatography (GPC, Shimadzu, Japan). The samples were dissolve in chloroform and tested at 30°C. The gas separation performance was measured via Time-lag method with the gas separation apparatus provided from the Airrrane Co. Ltd. using the CO₂ and N₂ pure gases. The ideal CO₂/N₂ selectivity was obtained by dividing the CO₂ permeability with N₂ permeability.

3. Results and Discussion

The synthesis process of intrinsically microporous polymer (PIM-1) is elucidated in Scheme 1, employing the polycondensation method[16]. The chemical structure of the PIM-1 backbone is intentionally designed to be bulky, disrupting molecular chain packing and facilitating the creation of a microporous architecture. The non-linear configuration of the polymer chains contributes to the formation of intrachain pores, establishing a highly permeable pathway for CO₂. This strategic molecular design enhances the overall gas separation performance of PIM-1, making it a promising candidate for applications requiring efficient gas permeation. The PIM-1 was then casted on the petri dish, followed by an evaporation of the solvents to obtain the free-standing PIM-1 membrane. The detailed explanation provided in Scheme 1 highlights the key structural features and synthesis principles crucial for the development of this intrinsically microporous polymer.

The chemical bonds in TFTPN, TTSBI, H-PIM-1, and L-PIM-1 were verified through FT-IR spectroscopy, as depicted in Fig. 1. The monomers displayed distinct vibrational modes corresponding to the aromatic C−H and C≡N bonds for TTSBI and TFTPN, respectively. Following polycondensation, the appearance of characteristic aromatic C=C bonds and C-H vibrational modes confirmed the presence of a bulky backbone in the PIM-1 chain. Notably, the C−O stretching band of TFTPN (989 cm^-1) experienced a blue shift to a higher wavenumber (1013 cm^-1), indicating a significant increase in the strength of the C−O bond, as predicted by Hooke’s Law[17]:

where ν is the wavenumber, k is the force constant, and μ is the reduced mass of the combined atoms. Furthermore, the characteristic bands of the aromatic C −H bond in the PIM-1 backbone appeared at 2951 cm^-1, and the C−F bond in TFTPN completely disappeared during the polycondensation process[18]. These observations provide conclusive evidence that PIM-1 was successfully synthesized, and the monomers were completely eliminated during the washing process.

GPC measurements were conducted to determine the molecular weights (MW) of H-PIM-1 and L-PIM-1. The molecular weight distribution, as indicated by retention time, revealed that H-PIM-1 eluted much faster than L-PIM-1. This disparity can be attributed to the pore penetration of L-PIM-1, providing a longer pathway for its elution. Consequently, it can be inferred that H-PIM-1 and L-PIM-1 were prepared with different molecular weights, with calculated values of 31,428 g mol^-1 and 15,990 g mol^-1, respectively. The molecular weight of H-PIM-1 was nearly double that of L-PIM-1, presenting a significant variation. This distinction is crucial for evaluating the impact of molecular weight on the CO₂ separation performance of the membrane[ 19,20].

Fig. 3 presents FE-SEM images depicting the surfaces of H-PIM-1 and L-PIM-1 membranes, along with corresponding photographic images. In Fig. 3a, the surface of H-PIM-1 appears remarkably even, exhibiting no significant cracks or bumpy morphology. Upon closer inspection in Fig. 3b, a slightly bumpy surface is observed, attributed to the bulky nature of the PIM-1 backbone, essential for constructing micropores. Conversely, Fig. 3c reveals that the L-PIM-1 membrane displays a rougher surface with pronounced bumps, indicative of its lower molecular weight designed to achieve membrane uniformity. Additionally, Fig. 3d illustrates a nanocrystal-like surface on L-PIM-1, highlighting a lack of physical properties conducive to forming a highly organized membrane[ 21]. Notably, the distinct colors of the membranes in Fig. 3e and f contribute further insights. H-PIM-1 exhibits a brownish hue, while L-PIM-1 appears more yellow. These visual differences align with the variations in molecular weight between H-PIM-1 and L-PIM-1, underscoring the influence of molecular weight on membrane properties.

Gas separation performance of H-PIM-1 and L-PIM-1 was investigated through the Time-lag method and described in Fig. 4. The H-PIM-1 achieved the CO₂ permeability of 5137 Barrer and CO₂/N₂ selectivity of 19.4 while the L-PIM-1 scored the CO₂ permeability of 1843 Barrer and CO₂/N₂ selectivity of 16.7. The higher permeability of H-PIM-1 would be attributed to the more developed microporous structure due to the higher chain length from the high MW, supported by the N₂ permeability which is highly affected by the diffusivity of membrane[22]. Moreover, the higher CO₂/N₂ selectivity was obtained in the H-PIM-1 membrane. This result is attributed to the more developed polar groups such as cyano group and ether group compared with those of L-PIM-1[23,24]. Therefore, the higher molecular weight from 15000 to 30000 g mol^-1 resulted in the higher CO₂ separation performance. In this regard, the CO₂ separation membrane composed of polymer of intrinsic microporosity would be highly dependent on the molecular weight, which would provide a more CO₂-philic pathway and highway, simultaneously.

4. Conclusion

In conclusion, this study establishes Polymer of Intrinsic Microporosity (PIM-1) as a promising material for CO₂ gas separation membranes, emphasizing its unique structural characteristics and tunable chemical properties. The controlled synthesis of PIM-1, varying its molecular weight, revealed a substantial influence on CO₂ separation performance. Higher molecular weight (H-PIM-1) membranes demonstrated superior CO₂ permeability and selectivity compared to lower molecular weight counterparts (L-PIM-1), attributed to a more developed microporous structure and enhanced polar groups. Comprehensive characterization, including FT-IR spectra, GPC curves, and FE-SEM images, provided insights into the membrane's composition, molecular weight distribution, and surface morphology. The study's findings underscore the critical role of molecular weight in tailoring PIM-1 membrane properties, contributing to the advancement of next-generation membrane technologies for efficient and selective CO₂ capture−a pivotal step towards mitigating anthropogenic carbon emissions and addressing the pressing global challenge of climate change.