(594b) The Structure-Property Relationships of Metal-Organic Frameworks for Ethylene/Ethane Separation

Oxidative coupling of methane (OCM) is a promising route to convert natural gas (CH₄) to value-added ethylene. However, the existing OCM catalysts will typically produce a large amount of by-products like ethane that is difficult to separate from ethylene due to their similar physical properties. In industry, these two gases are separated via high energy-consuming cryogenic high-pressure distillation. Therefore, there is significant motivation to develop a simple and effective separation method to afford low energy consumption and high ethylene purity. Metal organic frameworks (MOF) are promising candidates for use in this application due to their high surface areas, large pore volumes, high adsorption capacities and selectivity for specific adsorbates. DOMF and its isostructural family were synthesized to determine the structure-property relationship of metal-organic frameworks for ethylene/ethane separation due to the ability of their framework ligands to possess a myriad of functional groups. The results show that both DMOF and DMOF-TMBDC demonstrate preferential selective adsorption of ethane over ethylene. Since the pore size of DMOF-TMBDC is closer to the kinetic diameter of ethylene and ethane than DMOF, it can reach saturation faster with higher IAST predicted selectivity, indicating the methyl groups facilite the interactions between gas molecules via increasing van der Waals forces. Furthermore, DMOF-TMBDC exhibits much higher ethane uptake in the low-pressure region than DMOF, which is helpful for the ethylene production from OCM reaction where the partial pressure of ethylene and ethane is low. In addition, both heats of adsorption of ethylene and ethane are lower than 40 kJ/mol, excluding chemisorption interactions in the framework. Low heats of adsorption suggest adsorbed gas can be regenerated easily without consuming large amount of energy, which is desirable for efficient purification of ethylene.