Boosting membrane carbon capture via multifaceted polyphenol-mediated soldering

The escalating atmospheric CO2 levels pose a significant threat to the global climate, necessitating effective carbon capture and sequestration (CCS) strategies. Membrane-based gas separation offers an energy-efficient CCS solution due to its low cost and ease of operation. However, conventional polymer membranes suffer from a trade-off between permeability and selectivity, limiting their performance. Mixed-matrix membranes (MMMs), combining inorganic fillers with a polymer matrix, offer a potential solution. However, achieving optimal polymer-filler compatibility remains a challenge, especially with highly permeable polymers like polymers of intrinsic microporosity (PIMs). This research addresses this challenge by introducing a novel molecular soldering technique using polyphenols to enhance the interaction between PIM-1 and metal-organic frameworks (MOFs) within the MMM structure. The strategy aims to simultaneously enhance both permeability and selectivity, surpassing the limitations of existing MMMs and providing a more effective approach for CO2 capture.

Existing literature highlights the potential of MMMs in overcoming the permeability-selectivity trade-off in gas separation membranes. However, challenges in interfacial compatibility between the polymer matrix and inorganic fillers persist. Numerous studies focus on improving this compatibility through surface functionalization, morphology control, and ligand exchange. While strong polymer-filler interactions can improve selectivity, they often reduce permeability due to free volume loss. Metal-organic frameworks (MOFs), known for their unique topological structures and potential as MMM fillers, often suffer from poor compatibility with polymer matrices. Hollow MOFs, offering high free volume for gas transport, represent a promising improvement but require effective integration with polymers to fully realize their potential. Previous research on PIM-1-based MMMs has shown improvements in certain aspects of separation performance, but often at the expense of others, highlighting the need for a multifaceted approach like the one presented in this study.

The study employs a multifaceted approach involving the synthesis of hollow ZIF-8 (HZIF-8) MOFs using tannic acid (TA) as a polyphenol-based molecular solder. TA modifies the ZIF-8 surface, creating a rougher texture and introducing hollow structures within the MOF nanocrystals. The modified HZIF-8 particles are then incorporated into a PIM-1 matrix via solution casting to create MMMs. The resulting membranes undergo various characterization techniques to assess their physicochemical properties and gas separation performance. These techniques include scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption-desorption isotherms, powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), zeta potential measurements, solid-state 13C NMR spectroscopy, positron annihilation lifetime spectroscopy (PALS), and nanoindentation. Gas transport properties are evaluated using a constant volume-variable pressure method, employing pure and mixed gases (CO2, N2, CH4). The time-lag method is employed to calculate sorption and diffusion coefficients, helping elucidate the gas transport mechanism. Aging tests are conducted to assess the long-term stability of the membranes. The study also compares the performance of PIM-1/HZIF-8 MMMs with other reported PIM-1/MOF MMMs in the literature, analyzing both permeability and selectivity enhancements.

The use of tannic acid (TA) as a molecular solder dramatically improved the interfacial compatibility between PIM-1 and ZIF-8, effectively eliminating interfacial voids observed in PIM-1/ZIF-8 membranes. The introduction of TA created a hollow structure within the ZIF-8 particles, enhancing permeability. The resultant PIM-1/HZIF-8 MMMs exhibited significantly improved CO2 permeability and selectivity compared to both pristine PIM-1 and PIM-1/ZIF-8 membranes. Specifically, the optimized membrane (H-5-1) achieved a CO2 permeability of 8268 Barrer with CO2/N2 and CO2/CH4 selectivities of 25.1 and 18.7, respectively. This performance surpasses the Robeson upper bound, representing a significant advance in CO2 separation technology. Detailed analysis using a solution-diffusion model revealed that the observed enhancements are primarily due to changes in diffusivity rather than solubility. The polyphenol soldering strategy also significantly suppressed plasticization under high feed pressures and physical aging over time compared to control membranes. The superior performance of the polyphenol-soldered MMMs was further validated by testing with different polymers, demonstrating the general applicability of the method. The improved mechanical properties of the PIM-1/HZIF-8 membranes, also a key finding, are attributed to the enhanced interfacial bonding promoted by the polyphenol soldering.

The results demonstrate that the multifaceted polyphenol-mediated soldering strategy successfully addresses the long-standing challenge of polymer-filler incompatibility in MMMs. By simultaneously enhancing both permeability and selectivity, this strategy surpasses the limitations of traditional MMMs and pushes the boundaries of CO2 separation performance. The synergistic effects of improved interfacial adhesion, increased chain rigidity, and reduced mass transfer resistance contribute to the observed superior performance. The study's findings have significant implications for the development of next-generation carbon capture technologies, enabling more efficient and cost-effective CO2 separation. The general applicability of the polyphenol soldering approach, demonstrated across different polymer matrices, suggests a wide range of potential applications beyond carbon capture.

This study presents a novel and effective strategy for fabricating high-performance MMMs for CO2 capture using polyphenol-mediated soldering. The method’s success lies in its ability to simultaneously enhance permeability and selectivity by creating defect-free interfaces and incorporating hollow MOF structures. The demonstrated improvement in membrane performance, along with the technique’s broad applicability, opens new avenues for designing advanced MMMs for various gas separation applications. Future work could explore different polyphenols and MOF structures to further optimize membrane performance and explore industrial-scale applications.

While the study demonstrates significant performance improvements, potential limitations include the scalability of the synthesis method for large-scale industrial production. Further research is needed to fully optimize the synthesis parameters for different polymers and MOFs to achieve even greater performance enhancements. The long-term stability under various operating conditions could be further investigated.