Highly Efficient Propyne/Propylene Separation in a "Flexible-Robust" and Hydrolytically Stable Cu(II)-MOF

Propyne/propylene separation is important in the petrochemical industry but challenging due to their similar physical properties and close molecular sizes. Metal-organic frameworks (MOFs) are a class of promising adsorbents for light hydrocarbon separations. Among them, the so-called "flexible-robust" MOFs combine the advantages of flexibility and rigidity in structure and could show enhanced gas separation selectivity as well as improved gas uptake at low pressure. Interpenetrated MOFs offer a platform to explore the "flexible-robust" feature of MOFs based on their subnetwork displacement in the process of gas adsorption. Herein, we present two hydrolytically stable MOFs (BUT-308 and BUT-309) with interpenetrated structures and fascinating propyne/propylene separation performance. BUT-308 is composed of interpenetrated 2D Cu(BDC-NH₂)BPB layers (H₂BDC-NH₂ = 2-aminobenzene-1,4-dicarboxylic acid; BPB = 1,4-bis(4-pyridyl)benzene), while BUT-309 consists of twofold interpenetrated 3D pillared-layer Cu₂(BDC-NH₂)₂(BPB-CF₃) nets (BPB-CF₃ = 2-trifluoromethyl-1,4-bis(4-pyridyl)benzene). Gas adsorption measurements showed that BUT-309 was a "flexible-robust" adsorbent with multistep adsorption isotherms for C₃H₄ rather than C₃H₆ at a wide temperature range. The guest-dependent pore-opening behavior endows BUT-309 with high potential in the C₃H₄/C₃H₆ separation. The C₃H₄ adsorption measurements of BUT-309 at 273-323 K showed that the lowering of the temperature induced the pore-opening action at lower pressure. Column breakthrough experiments further confirmed the capability of BUT-309 for the efficient removal of C₃H₄ from a C₃H₄/C₃H₆ binary gas, and the C₃H₆ processing capacity at 273 K (15.7 cm³ g^-1) was higher than that at 298 K (35.2 cm³ g^-1). This work shows a rare example of "flexible-robust" MOFs and demonstrated its high potential for C₃H₄/C₃H₆ separation.