Programmed Fluorine Binding Sites of MOF Materials Achieve Record High Acetylene/Ethylene Separation

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Fluorine materials refer to materials containing fluorine elements, which are a very important new chemical material widely used in various fields such as machinery, chemicals, textiles, etc.

Efficient capture and removal of trace gases or volatile organic compounds is an important way to address environmental pollution and energy consumption issues. The selective capture of trace acetylene is crucial for obtaining polymer grade ethylene. Compared to traditional industrial separation technologies such as catalytic hydrogenation and solvent extraction, adsorption separation technology based on porous materials will be more energy-efficient and environmentally friendly.

However, in the presence of trace amounts of gas, most physical adsorbents face scientific challenges such as low adsorption capacity or insufficient selectivity for the target gas due to insufficient host guest interactions. Anionic column supported SIFSIX materials have been proven to be one of the most excellent acetylene/ethylene separation materials. This dual site fluorine binding model and effective pore regulation result in it exhibiting the best acetylene/ethylene separation performance currently available. However, the low acetylene adsorption energy of the dual site fluorine binding model (about 40 kJ mol^-1) still greatly limits the acetylene adsorption capacity of the material at low pressure (0.01 bar is 1.5-1.8 mmol g^-1). In addition, most of the SIFSIX materials currently reported have poor chemical stability and are difficult to achieve industrial applications.

Recently, researchers have proposed a new strategy for constructing a multi fluorine binding model in SIFSIX materials, promoting more fluorine atoms to interact with C₂H₂ simultaneously and maximizing C₂H₂ affinity. Based on this, this work designed and constructed a stable third-generation SIFSIX material ZJU-300a using nitrogen containing ligands with four connection points. ZJU-300a exhibits the highest C₂H₂ adsorption capacity and high separation selectivity at 0.01 bar to date.

Due to the successful construction of the multi fluorine binding model, ZJU-300a exhibits strong C₂H₂ adsorption capacity. At 296 K and 0.01 bar, the C₂H₂ adsorption capacity of ZJU-300a is as high as 3.23 mmol g^-1, which is much higher than all SIFSIX materials and other materials with excellent performance reported so far. Meanwhile, ZJU-300a exhibits high separation selectivity for C₂H₂/C₂H₄ mixture, much higher than SIFSIX materials and other reference materials except for SIFSIX-14-Cu-i. Therefore, compared to the first and second generation SIFSIX materials, ZJU-300a exhibits significantly increased C₂H₂ adsorption capacity and maintains relatively high separation selectivity. Compared to other materials with the best performance, ZJU-300a has the highest C₂H₂ adsorption capacity and high separation selectivity at 0.01 bar to date.

Programmed Fluorine Binding Sites of MOF Materials Achieve Record High Acetylene/Ethylene Separation

The X-ray diffraction study of gas carrier single crystal shows that there are mainly three adsorption sites for C₂H₂ molecule in ZJU-300a. SiF₆^2- has constructed a unique multifluorinated binding model that enables C₂H₂ molecules to form 8 C-H • • F hydrogen bonds with 8 F atoms from two adjacent SiF₆^2-, while also forming multiple supramolecular interactions with the hydrogen atoms of the surrounding pyridine ring. In addition, the dense distribution of these three binding sites also promotes the intermolecular interaction of C₂H₂. Therefore, the multi fluorine binding model constructed in ZJU-300a not only provides strong affinity for C₂H₂ adsorption, but also promotes the dense stacking of C₂H₂ molecules, resulting in an ultra-high C₂H₂ adsorption capacity at low pressure.

The actual penetration experiment shows that ZJU-300a can achieve efficient separation of 1/99 C₂H₂/C₂H₄ gas mixtures, with its dynamic separation coefficient (264) and high-purity C₂H₄ production (436.7 mmol g^-1) much higher than all SIFSIX materials and other materials with the best performance. It also proves that when the separation selectivity reaches a very high level, the C₂H₂/C₂H₄ separation performance will be mainly dominated by the low-pressure C₂H₂ adsorption capacity. In addition, unlike other SIFSIX materials, ZJU-300 can still maintain structural stability and has ultra-high chemical stability after being treated in different acid and alkali environments. Penetration experiments have shown that it can maintain excellent separation performance even in acidic environments with 40% relative humidity or 1000 ppm H₂S. Therefore, this material exhibits the highest C₂H₂ adsorption capacity at 0.01 bar, high separation selectivity, and excellent chemical stability, demonstrating good practical industrial application prospects.