Version 2 2024-03-15, 19:13Version 2 2024-03-15, 19:13
Version 1 2024-01-09, 14:13Version 1 2024-01-09, 14:13
journal contribution
posted on 2024-03-15, 19:13authored byArijit Halder, C. Michael McGuirk
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 H2 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 N2 at 77 K, CO2 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.