Site-engineered Zr-based metal–organic frameworks for ultrahigh-performance one-step separation of methanol-to-olefins products

Abstract

The separation of methanol-to-olefins (MTO) products to obtain high-purity ethylene (C₂H₄) and propylene (C₃H₆) is a critical yet challenging task. However, developing adsorbents with high C₃H₆/C₂H₄ selectivity and productivity for one-step C₂H₄ purification and C₃H₆ recovery remains an industrially significant but demanding objective. In this study, a site-engineering strategy was employed to tailor cage size and polarity within a confined pore space. Using post-synthetic modification, the robust Zr-based metal–organic framework (MOF-808) was functionalized with oxygen- and benzene ring-containing ligands, including benzoic acid (Ben), 2-formylbenzoate (For), and 2-acetylbenzoic acid (Ace). Among these, MOF-808-For exhibited a twofold increase in both C₃H₆ adsorption capacity (5.7 mmol g⁻¹) and C₃H₆/C₂H₄ separation efficiency compared to unmodified MOF-808 under identical conditions. Additionally, MOF-808-For enabled the simultaneous recovery of C₃H₆ and C₂H₄ through a one-step adsorption–desorption process. Adsorption thermodynamics and theoretical calculations revealed that C₃H₆ interacted more strongly with the framework than C₂H₄. The optimal cage size and oxygen-binding sites of MOF-808-For facilitated the selective trapping of C₃H₆ via multiple strong C–H⋯C and C–H⋯O hydrogen bonds, resulting in exceptional C₃H₆/C₂H₄ selectivity (11.1) and high C₂H₄ productivity (99.1 L kg⁻¹ at 2 mL min⁻¹ for 1 : 1 C₃H₆/C₂H₄ mixture) among reported MOFs. Furthermore, multi-cycle experiments demonstrated no significant loss in separation performance, confirming the excellent reusability of MOF-808-For for MTO product separation.