Abstract
Variation of the crystallite size in flexible porous coordination polymers can significantly influence or even drastically change the flexibility characteristics. The impact of crystal morphology, however, on the dynamic properties of flexible metal-organic frameworks (MOFs) is poorly investigated so far. In the present work, we systematically modulated the particle size of a model gate pressure MOF (DUT-8(Ni), Ni2(2,6-ndc)2(dabco), 2,6-ndc−2,6-naphthalenedicarboxylate, dabco−1,4-diazabicyclo[2.2.2]octane) and investigated the influence of the aspect ratio, length, and width of anisotropically shaped crystals on the gate opening characteristics. DUT-8 is a member of the pillared-layer MOF family, showing reversible structural transition, i.e., upon nitrogen physisorption at 77 K. The framework crystalizes as rod-like shaped crystals in conventional synthesis. To understand which particular crystal surfaces dominate the phenomena observed, crystals similar in size and differing in morphology were involved in a systematic study. The analysis of the data shows that the width of the rods (corresponding to the crystallographic directions along the layer) represents a critical parameter governing the dynamic properties upon adsorption of nitrogen at 77 K. This observation is related to the anisotropy of the channel-like pore system and the nucleation mechanism of the solid-solid phase transition triggered by gas adsorption.
Highlights
Metal-organic frameworks (MOFs) are nowadays well-known as a class of porous coordination polymers which are constructed from organic linkers and metal clusters by modular building principle (Schneemann et al, 2014; Elsaidi et al, 2018)
The influence of the crystal downsizing on the structural transition characteristics in classical materials stimulated by temperature or mechanical pressure was intensively studied in the past, showing pronounced particle size dependence (Li et al, 2016a; Anwar and Zahn, 2017)
We report the influence of crystal size and morphology on gate opening transition in switchable DUT8(Ni), intending to identify the crystal faces significantly affecting the gate opening pressure
Summary
Metal-organic frameworks (MOFs) are nowadays well-known as a class of porous coordination polymers which are constructed from organic linkers and metal clusters by modular building principle (Schneemann et al, 2014; Elsaidi et al, 2018). GRAPHICAL ABSTRACT 1 | Particle size dependent adsorption behavior of DUT-8(Ni) This dynamic nature of flexible MOFs makes them attractive for several potential applications such as sensors, molecular separation, gas storage, drug delivery, and catalysis (Mason et al, 2015; Majewski et al, 2017; Allendorf et al, 2020; Hou et al, 2020; Semrau et al, 2020; Wang et al, 2020). The influence of the crystal downsizing on the structural transition characteristics in classical materials (alloys, organic compounds) stimulated by temperature or mechanical pressure was intensively studied in the past, showing pronounced particle size dependence (Li et al, 2016a; Anwar and Zahn, 2017). The postulated observations could be rationalized, considering the internal pressure and surface energy differences between polymorphs, confinement effects, and nucleation theory (Yang, 1999; Nièpce and Pizzagalli, 2007; Li et al, 2016a; Anwar and Zahn, 2017)
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