Abstract
Switchable metal–organic frameworks (MOFs) stand out for potential applications in energy storage, separation, sensing, and catalysis. The understanding of MOF switchability mechanisms has progressed significantly over the past two decades. Nanostructuring is essential for the integration of such materials into thin films, hierarchical composites, and membranes and for biological applications. However, downsizing below critical dimensions causes dramatic changes in the dynamic behavior and responsiveness towards external stimuli. We discuss the most important experimental findings and derive general guidelines and hypotheses of relevance for the impact of crystal size on switchability. Understanding nanostructure thermodynamics and implications for the tailoring of dynamic porous systems requires an interdisciplinary approach, advanced physical characterization techniques, and new modeling strategies to cover a wider range of time and length scales.
Highlights
Information on Adsorption IsothermsGas uptake as a function of relative gas pressure p/p0 (p0 = saturation pressure) at constant temperature is a characteristic signature of pore size and pore volume in materials, called the ‘adsorption isotherm’
The switchability of the porous solid adds new degrees of freedom in the system resulting in novel adsorption isotherms in which multiple solid-phase transformations are highly coupled to the adsorption process and fluid-phase transitions leading to characteristic hysteresis even for microporous materials [8]
We start with a general discussion of solid-phase thermodynamic and kinetic effects, emphasizing that factors affecting switchability are system dependent and that the magnitude of the energetics of each of the aspects discussed in the following may differ depending on the composition of the solid and the stimulus present
Summary
From Macro- to Nanoscale: Finite Size Effects on Metal–Organic Framework Switchability. An outstanding feature, compared with other, traditional porous materials, is their ability to transform (switch) between different phases with well-defined crystalline structures triggered by external stimuli, often by guest inclusion [3,4] The latter leads, in some cases, to important improvements in gas separation or storage due to ultrahigh selectivity [5] or high deliverable capacity [6]. Advancement of computational and analytical tools is crucial to achieve a deeper understanding of surface, interfacial, and finite size effects These phase transitions induced by external stimuli (e.g., changes in temperature, external pressure, gas pressure, vapor pressure, or electromagnetic radiation) are associated with a latent heat of transformation, L, governing the energetics of the bulk phase transition.
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