Exploring the Influence of Linker Substitution and Ratios on Cooperative Framework Flexibility through the Mixed-Linker Approach

Abstract

Cooperatively flexible metal−organic frameworks that exhibit step-shaped, or Type V-like, adsorption−desorption profiles can lower requisite pressure−temperature swings and thus energy input, necessary for an array of gas storage, delivery, and separations applications. However, such benefits are lost if the pressure threshold of the adsorption and desorption steps at a given temperature does not match the conditions dictated by the application, such as H 2 storage and delivery or olefin−paraffin separations. Unfortunately, the discovery of cooperatively flexible frameworks remains wholly serendipitous and de novo design remains impossible. Accordingly, there is a great need to further our understanding of flexibility such that we can intuitively derivatize known frameworks and, ultimately, design entirely new ones to meet the requisite conditions of energy-consumptive processes. In this work, we demonstrate that the mixed-linker, or multivariate, approach is a powerful tool for the derivation of a known flexible framework, with variances in linker substitution and ratio giving rise to a family wherein significant changes to the step-shaped adsorption−desorption profiles for multiple adsorbates are observed. Specifically, we report 12 isostructural mixed-linker derivatives of CdIF-13 (sod−Cd(benzimidazolate) 2 ) with six point-modified benzimidazole linkers spontaneously synthesized through prototypical solvothermal conditions. Each is shown by PXRD to exhibit similar reversible flexibility to CdIF-13 and by TGA and DSC to be similarly thermally stable. Isothermal gas adsorption measurements with N 2 at 77 K, CO 2 at 195 K, and propane at 298 K demonstrate the wide-ranging, adsorbate-dependent effects of linker substitution and ratio, including dramatic reductions in the adsorption threshold pressure, evolution of multiple steps, and complete absence of observable adsorption. Aided by prior crystallographic characterization of CdIF-13, the observed trends were analyzed in an attempt to resolve the influence of linker substitution and ratios on structural behavior. In total, this work illustrates how the mixed-linker approach enables the synthesis and study of a large catalog of functionally modified frameworks, which can help identify the variables that influence flexibility and sorption behavior.

Reversible Phase Transformations in a Double-Walled Diamondoid Coordination Network with a Stepped Isotherm for Methane

“…1000 known to be flexible metal–organic materials (FMOMs) as characterized by stimuli-induced structural transformations between low- and high-porosity phases. − Whereas rigid microporous materials typically display Type I (Langmuir) adsorption isotherms, including for several high uptake methane sorbents, − FMOMs can undergo structural phase transformations in response to external stimuli, such as exposure to gases or vapors. , This in turn results in stepped sorption isotherms with a characteristic threshold “gate-opening” pressure ( P GO , Scheme ). − Thanks to this characteristic, FMOMs have emerged as a class of porous materials with potential utility, especially for methane (adsorbed natural gas, ANG) storage. ,, This potential is enhanced by the effects of flexibility, including improved heat management and higher working capacity (Type F–II and F–IV isotherms can offer higher uptake differences between the loading and release pressures than Type I isotherms, Scheme ). …”