Controlling the steepness of gate-opening behavior on elastic layer-structured metal-organic framework-11 via solvent-mediated phase transformation

Abstract

Metal-organic frameworks (MOFs), particularly flexible MOFs, have garnered significant attention because of their potential in various applications due to their unique ability to undergo reversible structural transitions in response to guest adsorption, the so-called gate-opening behavior, making their scale-up synthesis increasingly important. This study focuses on the high-concentration synthesis of elastic layer-structured MOF-11 (ELM-11), one of the earliest observed flexible MOFs capable of gate adsorption. We demonstrate that, while low-concentration synthesis yields pre-ELM-11, the precursor to ELM-11, high-concentration conditions favor the formation of a metastable phase of pre-ELM-11. Although both can be transformed into ELM-11 by heating under vacuum and, consequently, exhibit nearly identical adsorption properties, the steepness of the S-shaped curve upon gate opening varies significantly. Our investigation reveals two main points: one is that the metastable phase of pre-ELM-11 can be transformed into its stable phase through extended aging in a solvent-mediated phase transformation process. The other is that the phase of pre-ELM-11 affects the crystallite size and, consequently, the cooperative deformation domain size within the framework structure, directly impacting the steepness of the gate-opening behavior according to the structural transition-type adsorption equation. We also demonstrate that seeding the stable phase during synthesis can expedite the solvent-mediated phase transformation, offering a practical approach for the efficient production of high-quality ELM-11. These findings provide valuable insights into the complex interplay between synthesis conditions, structural properties, and adsorption behavior in flexible MOFs, paving the way for their future industrial applications.