Size exclusion propyne/propylene separation in an ultramicroporous yet hydrophobic metal–organic framework

Abstract

Propyne/propylene separation is important in the petrochemical industry but challenging due to their similar physical properties and close molecular sizes. Herein, we present two isoreticular ultramicroporous Zn(II)-MOFs, Zn₂(ATZ)₂(TPDC) (BUT-305, H₂TPDC = [1,1′:3′,1′′-terphenyl]-4,4′′-dicarboxylic acid, HATZ = 3-amino-1,2,4-triazole) and Zn₂(ATZ)₂(MeTPDC) (BUT-306, H₂MeTPDC = 5′-methyl-[1,1′:3′,1′′-terphenyl]-4,4′′-dicarboxylic acid). The pore aperture of BUT-306 (∼1.6 Å) is smaller than that of BUT-305 due to the presence of extra gate-like methyl groups in the 1D channels of the former. With a narrow and hydrophobic pore aperture, BUT-306 exhibits high hydrophobicity and hydrolytic stability and adsorbs C₃H₄ but excludes C₃H₆ in a wide temperature range. The C₃H₆ and C₃H₄ adsorption capacities of BUT-306 at 298 K and ∼1 bar were 2.4 and 29.6 cm³ g⁻¹, respectively. Dynamic column breakthrough experiments confirmed the high capability of BUT-306 to remove C₃H₄ from the equimolar binary gas mixture of C₃H₄ and C₃H₆. The C₃H₄/C₃H₆ separation performance of BUT-306 was largely retained even when the binary C₃H₄/C₃H₆ gas for breakthrough experiments was pre-saturated with water vapor. In addition, the single-crystal structure of C₃H₄-loaded BUT-306 was determined, which revealed that the adsorbed C₃H₄ molecules were located in the center of channel cavities and interacted with the MOF by multiple weak C^δ−⋯C^δ+ dipole–dipole interactions and C–H⋯п interactions. This work demonstrates the high potential of an ultramicroporous, hydrophobic, and hydrolytically stable MOF in the removal of C₃H₄ from the C₃H₄/C₃H₆ gas mixture by size exclusion adsorption. The structure and gas adsorption studies shed light on the design and synthesis of new adsorbents for the separation of light hydrocarbons.