The effect of crystallite size on pressure amplification in switchable porous solids

Abstract

Negative gas adsorption (NGA) in ordered mesoporous solids is associated with giant contractive structural transitions traversing through metastable states. Here, by systematically downsizing the crystal dimensions of a mesoporous MOF (DUT-49) from several micrometers to less than 200 nm, counterintuitive NGA phenomena are demonstrated to critically depend on the primary crystallite size. Adsorbing probe molecules, such as n-butane or nitrogen, gives insights into size-dependent activation barriers and thermodynamics associated with guest-induced network contraction. Below a critical crystal size, the nitrogen adsorption-induced breathing is completely suppressed as detected using parallelized synchrotron X-ray diffraction–adsorption instrumentation. In contrast, even the smallest particles show NGA in the presence of n-butane, however, associated with a significantly reduced pressure amplification. Consequently, the magnitude of NGA in terms of amount of gas expulsed and pressure amplification can be tuned, potentially paving the way towards innovative concepts for pressure amplification in micro- and macro-system engineering.

Adsorption Contraction Mechanics: Understanding Breathing Energetics in Isoreticular Metal–Organic Frameworks

“…Nitrogen at 77 K was found to be a very sensitive adsorbate only initiating structural transitions for DUT-49 crystals larger than 1 -2 µm. 42 Consequently, adsorption isotherms close to ambient temperature were recorded using n-butane at the respective boiling point of 273 K and also at 295 K and 303 K ( Figure 1 and Figure 2). These conditions were previously found to induce a contraction from DUT-49op to DUT-49cp independent of particle size, which has been well studied by in situ PXRD in parallel to adsorption and derived from multiple steps, hysteresis and spikes in the adsorption isotherms.…”

Pillared-layered metal–organic frameworks for mechanical energy storage applications

“…For instance, the crystal size has shown to modify the hysteresis behavior and corresponding threshold values in exible MOFs in different ways. [93][94][95]…”

Unraveling the thermodynamic conditions for negative gas adsorption in soft porous crystals

“…Finally, focussing now on the response of the type IIb material, we notice the introduction of a new region in between PGA and NB2, which will further be referred to as NGA2 and has a rather large window for methane (25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35) MPa) and a rather small window for carbon dioxide (34)(35). The origin of this extra region is related to two aspects.…”

“…They found that a stiffening of the framework (due to the smaller linker) with respect to DUT-49, in combination with a lower adsorption enthalpy of methane was responsible for the lack of transitions. The same group also observed that the structural transitions for nitrogen in DUT-49 critically depend on the particle size of the single crystals 28 . It was found that the particle size alters the complex structural transformation mechanism and yields multiple structural transformations figuring unprecedented intermediate structures.…”

“…However, we still lack a fundamental understanding of the specific thermodynamic conditions that are required to observe NGA, which is prerequisite for the design and tuning of new state-of-the-art applications. Given the unique nature of NGA in which the material counteracts an external force applied to it, the ability to trigger NGA in a controlled fashion could give rise to an entirely new branch of applications such as gas-releasing rescue systems and a wide array of pneumatic devices 22,[26][27][28] . From the experiments done so far, it is already clear that NGA can occur in certain materials, but it is still unclear whether from a fundamental theoretic point of view, the number of systems in which NGA could be observed can be expanded provided the thermodynamic conditions are right.…”