Unraveling Guest-Induced Structural Transition in a Flexible Metal–Organic Framework for C₄ Alkane/Alkene Separation

The residue from naphtha cracking contains a massive amount of C4 alkane/alkene mixtures and developing a cost-effective and energy-efficient technique for its separation is highly desired. Flexible metal–organic frameworks (MOFs) have attracted considerable attention as potential adsorbents for separation because of their responsive molecular recognition. Nevertheless, interpreting the fundamental mechanism driving their flexibility from a molecular level poses a significant challenge. Herein, we develop a force field and employ molecular simulation to examine the guest-induced structural transition of a flexible MOF (Flex-Cd-MOF-2a). Stepped isotherms are predicted at different response pressures during the adsorption of various C4 hydrocarbons (n-C4, 1-C4H8, and 1,3-C4H6) and agree well with experimental data. Structural transition from a narrow pore (Np) phase to a large pore (Lp) phase is observed and revealed to be driven primarily by the reorientation of organic linkers. C4 species is found to diffuse substantially faster in the Lp phase upon structural transition from the Np phase to the Lp phase. Furthermore, we assess the performance of Flex-Cd-MOF-2a for C4 alkane/alkene separation. From this study, we provide a quantitative understanding of the flexibility of Flex-Cd-MOF-2a and unravel the underlying mechanism in the guest-induced structural transition. These microscopic insights are instrumental in advancing flexible MOFs for practical separation and emerging applications.