Activation‐Dependent Breathing in a Flexible Metal–Organic Framework and the Effects of Repeated Sorption/Desorption Cycling

Abstract

An on-interpenetrated metal-organic framework with apaddle-wheel secondary building unit has been activated by direct thermal evacuation, guest exchange with av olatile solvent, and supercritical CO 2 drying. Conventional thermal activation yields am ixture of crystalline phases and some amorphous content. Exchange with av olatile solvent prior to vacuum activation produces apure breathing phase with high sorption capacity,s electivity for CO 2 over N 2 and CH 4 ,a nd substantial hysteresis.Supercritical drying can be used to access ag uest-free open phase.P ressure-resolved differential scanning calorimetry was used to confirm and investigate asystematic loss of sorption capacity by the breathing phase as af unction of successive cycles of sorption and desorption. A corresponding loss of sample integrity was not detectable by powder X-ray diffraction analysis.T his may be an important factor to consider in cases where flexible MOFs are earmarked for industrial applications.Scheme 1. Solvothermal synthesis of 1.

Experimental Evidence for Vibrational Entropy as Driving Parameter of Flexibility in the Metal–Organic Framework ZIF-4(Zn)

“…A subclass of MOFs, oftentimes referred to as soft porous crystals (SPCs) or flexible MOFs, 4−9 exhibit structural flexibility with large volume changes when triggered by an external stimulus 10 such as temperature, 11 pressure, 12 light 13 or gas, and guest adsorption. 14 Structural flexibility in SPCs is not only fascinating from a fundamental point of view but also offers striking opportunities in the areas of gas separation 15,16 and sensing, 17 mechanocalorics, 18 and their application as mechanical dampers and shock absorbers. 19 Prototypical examples of SPCs that exhibit a wine-rack type structure motif are MIL-53 (MIL = Mateŕiaux de l'Institut Lavoisier; M(OH)(bdc); M 3+ = Al 3+ , Fe 3+ , Cr 3+ etc., bdc 2− = 1,4-benzenedicarboxylate), 20−22 DUT-8 (DUT = Dresden University of Technology, M 2 (2,6ndc) 2 dabco with M 2+ = Ni 2+ , Zn 2+ , Co 2+ and ndc 2− = 2,6naphthalenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane), 23,24 and the material class of pillared-layered MOFs with the general formula M 2 (fu-bdc) 2 (dabco) (fuMOFs, M = Cu 2+ or Zn 2+ , fu-bdc 2− = 2,5-difunctionalized-1,4-benzenedicarboxylate), 25,26 which exhibit a structural flexibility as a response to gas sorption, temperature variation, and/or hydrostatic pressure.…”

Spiers Memorial Lecture: Coordination networks that switch between nonporous and porous structures: an emerging class of soft porous crystals

“…It should be noted that the activation can also influence the switching pressure and uptake. 28,136 Cost of ingredients and processing is also a consideration that must be addressed when assessing the viability of CN-based products and technologies. Ironically, even though this is not considered by most academic researchers, this is perhaps the one factor that a chemist can control.…”

“…Apart from working capacity and selectivity as discussed above, recyclability and kinetics are two other key metrics that must be evaluated for practical application, yet both remain largely understudied. Despite the strong recyclability demonstrated by Co(bdp) 74 and X-pcu-n-Zn, 146,147 several 3D switching CNs, such as X-dia-1-Ni, 67 DUT-8(Ni) 246 and [Zn 2 (DPT) 2 (bpy)], 136 suffered from degradation in terms of switching pressure and uptake after multiple gas sorption cycles.…”

“…In addition, that switching between closed and open phases usually results in large volume changes in switching CNs may pose challenges to downstream processes such as formulation/shaping. It should be noted that the activation can also influence the switching pressure and uptake 28,136. Cost of ingredients and processing is also a consideration that must be addressed when assessing the viability of CN-based products and technologies.…”

Programmable Multicomponent Self‐Assembly Based on Aromatic Amino Acids

“…Efforts that have been made in recent years regarding multicomponent systems are concentrated largely on the metallosupramolecular coordination chemistry and dynamic covalent chemistry . Transition metal ions could coordinate with different ligands to form multicomponent metal organic frameworks (MOFs), cages, and molecular polyhedras . Alternatively, orthogonal dynamic covalent bonds like BO and CN that feature easy formation upon dehydration have been employed to prepare metal‐free molecular cages and covalent organic frameworks (COFs) .…”