Propylene/Ethylene Separation in Heptanuclear-Nodes Metal Organic Frameworks via a Bifunctional Auxiliary Ligand Strategy

The methanol-to-olefins (MTO) process is a pivotal technology for producing high-value chemicals like ethylene (C2H4) and propylene (C3H6) from nonpetroleum resources. A major challenge in this process is the energy-intensive purification of ethylene from a complex MTO product stream. To address this, we developed two isostructural metal–organic frameworks (MOFs), Co7-MOF and Ni7-MOF, for efficient MTO gas separation using a bifunctional auxiliary ligand strategy. Initially, a two-dimensional (2D) metal–organic layer (MOL) was constructed from isonicotinic acid and a heptanuclear metal cluster (Co7 or Ni7). The auxiliary ligand, isophthalic acid, was then introduced to serve a dual purpose: it partitions the rhombic pores of the 2D layer into smaller triangular pores and simultaneously pillars the layers into a three-dimensional framework. The successful synthesis and stability of these MOFs were confirmed by powder X-ray diffraction (PXRD) and variable-temperature PXRD. Nitrogen adsorption isotherms revealed their porous nature, with specific surface areas of 680 m2 g–1 for Co7-MOF and 698 m2 g–1 for Ni7-MOF, and a uniform pore size of ∼0.85 nm. Given this suitable pore geometry and charge distribution, we investigated their performance for C3H6/C2H4 separation. At 298 K and 0.01 bar, the C3H6 uptake reached 31.85 cm3 g–1 (43.54 cm3 cm–3) for Co7-MOF and 36.78 cm3 g–1 (51.85 cm3 cm–3) for Ni7-MOF, surpassing most benchmark materials. Breakthrough experiments and simulations demonstrated that both MOFs can effectively produce high-purity C2H4 from a C3H6/C2H4 mixture. CP2K calculations elucidated that the separation mechanism is driven by synergistic host–guest interactions, including van der Waals forces between the framework H atoms and gas molecules, interactions between the oxygen atoms of the metal clusters and the H atoms of the gases, and significant C–H···π interactions. This work presents a strategy of bifunctional ligand insertion for designing advanced adsorbents for challenging gas separations.