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 (C 2 H 4 ) and propylene (C 3 H 6 ) 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), Co 7 -MOF and Ni 7 -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 (Co 7 or Ni 7 ). 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 m 2 g –1 for Co 7 -MOF and 698 m 2 g –1 for Ni 7 -MOF, and a uniform pore size of ∼0.85 nm. Given this suitable pore geometry and charge distribution, we investigated their performance for C 3 H 6 /C 2 H 4 separation. At 298 K and 0.01 bar, the C 3 H 6 uptake reached 31.85 cm 3 g –1 (43.54 cm 3 cm –3 ) for Co 7 -MOF and 36.78 cm 3 g –1 (51.85 cm 3 cm –3 ) for Ni 7 -MOF, surpassing most benchmark materials. Breakthrough experiments and simulations demonstrated that both MOFs can effectively produce high-purity C 2 H 4 from a C 3 H 6 /C 2 H 4 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.