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Significance
Metal-organic frameworks (MOFs) are a class of highly porous materials that consist of metal ions or clusters connected by organic ligands. These materials possess remarkable properties, including a large surface area and controllable pore dimensions/structure, which make them promising candidates for various gas separation applications. However, MOFs are obtained as powders and their inherent fragility limits their practical use, as crystals composed of MOFs are prone to breaking or collapsing under mechanical load or stress. To address this issue, researchers have been exploring alternative structures and configurations that can overcome the mechanical deficiencies while retaining the high porosity and selectivity of MOFs.
In a recent study published in the peer-reviewed Chemical Engineering Journal, Yufeng Song and Professor Kamalesh K. Sirkar from the New Jersey Institute of Technology, University Heights, Newark, NJ presented a novel method for utilizing fragile MOF nanoparticles and microparticles in a compact hollow fiber membrane device for gas/vapor adsorption via in situ MOF synthesis. Specifically, they investigated the adsorption of gases and vapors using a particular MOF identified as UiO-66-NH2. For the material of the hollow fiber membranes, Nylon 6 was chosen due to its high inertness, melting point, and tensile strength. The MOFs were synthesized within the pores of the Nylon hollow fiber membranes through a process involving spontaneous wetting via immersion in a reactant solution, and solvothermal synthesis. The resulting hollow fiber membranes, whose pores are filled with MOF nanocrystalss, were then rinsed, dried, and incorporated into a cylindrical module.
The researchers characterized the MOF-filled Nylon 6 hollow fiber membranes using various techniques. Powder X-ray diffraction was employed to analyze the crystal structure, Fourier-transform infrared spectroscopy to identify MOF absorption bands, scanning electron microscopy to observe the structure and packing of the hollow fiber membranes as well as the dispersion of MOF nanocrystals within the porous walls, and N2 isotherm measurements to determine the adsorption properties of the MOF-filled membranes. Scanning electron microscopy images revealed nanocrystals of MOF dispersed within the porous walls, with dimensions of approximately 100 nanometers. The images also showed the presence of void spaces within the membrane walls, indicating minimal pressure decrease during gas flow. These void spaces allow for increased loading of nanocrystals, facilitating additional MOF nanocrystal synthesis within the membrane pores as was demonstrated for flat membranes. Fourier-transform infrared spectra confirmed the successful synthesis of MOF within the membrane pores, despite some overlapping absorption bands with the Nylon membrane.
The researchers investigated the ammonia adsorption characteristics of the packed bed based on the hollow fiber membranes. A gas stream containing 100 parts per million by volume of NH3 was introduced into the exterior side of the hollow fibers in the module; the gas stream had a relative humidity of 50% The breakthrough time, which indicates when NH3 first appears in the outlet gas stream, was significantly extended compared to previous investigations using flat membranes. The breakthrough time per gram of MOF was approximately 3000-4000 minutes, indicating enhanced adsorption performance. Regeneration tests on the MOF-loaded membranes revealed that the efficacy of the regenerated bed was slightly better than the initial sorption run, suggesting that the solvent was not completely removed prior to testing with the virgin adsorbent. The researchers estimated the sorption capacity of the MOF-loaded membranes for NH3 at 50% relative humidity (RH) to be 0.86 mmol/g, increasing to 1.58 mmol/g when considering the complete breakthrough concentration. The presence of MOF microcrystals within the hollow fiber bores extended the breakthrough time, indicating a higher sorption capacity compared to nanocrystals. Introduction of microcrystals in the extra capillary space of the hollow fiber module introduced additional sorption capacity in the device. The microcrystals are locally supported via friction if the device is kept vertical.
Professor Kamalesh K. Sirkar’s study holds several significant implications in the field of gas separation and adsorption. MOFs are known for their high porosity and selectivity, but their fragility has limited their practical applications. The new study presented a novel method to utilize fragile MOF nanoparticles and microparticles in a compact hollow fiber membrane device, effectively addressing the mechanical deficiencies of MOFs. This breakthrough opens up new possibilities for using MOFs in gas separation procedures and other applications requiring mechanical stability. The authors demonstrated that the MOF-filled hollow fiber membranes exhibit significantly prolonged breakthrough times compared to those based on conventional flat membranes. This indicates improved adsorption performance and suggests that the developed configuration offers higher sorption capacities. Such enhanced performance is crucial in various industries where gas purification, air stream cleansing, and toxic gas removal are required. Moreover, they introduced an innovative approach to synthesize MOFs directly within the Nylon hollow fiber membranes using solvothermal synthesis. This in situ synthesis method eliminates the need for separate MOF synthesis and preparation of MOF-binder based pellets and beads by mechanical compression, palletization, extrusion etc. and simplifying the overall process and enhancing the integration of MOFs into membrane devices. This approach can potentially be adapted for other MOF types and membrane materials, expanding the scope of applications for MOFs. Furthermore, the authors’ findings contribute to the understanding of MOF behavior within hollow fiber membranes and can guide future research in optimizing MOF synthesis and membrane design. Increasing the MOF loading is of particular interest.
The outcomes of the new study have practical implications for industries involved in gas separation and purification. The high sorption capacities and improved breakthrough behavior exhibited by the MOF-filled hollow fiber membranes open up possibilities for more efficient and effective gas adsorption and separation processes. This research contributes to advancing technologies related to gas purification, air quality control, and removal of toxic gases, which are crucial in sectors such as environmental protection, healthcare, and manufacturing.
In summary, the researchers demonstrated the applicability of MOFs, particularly UiO-66-NH2, within the innovative configuration of porous hollow fiber membranes for gas adsorption and separation processes. The results exhibited high sorption capacities and improved breakthrough behavior compared to conventional methods. These findings have the potential to drive advancements in gas separation technologies and offer practical solutions for various industries requiring efficient gas adsorption and separation processes.
Reference
Yufeng Song, Kamalesh K. Sirkar. Hollow fiber membrane supported metal organic framework-based packed bed for gas/vapor adsorption. Chemical Engineering Journal, Volume 454, Part 2, 2023, 140228.
