From Macro- to Nanoscale: Finite Size Effects on Metal–Organic Framework Switchability

Abstract

Switchable metal-organic frameworks (MOFs) standt 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. MOF Switchability and FunctionalityMOFs are among the most porous systems, with great potential in diverse applications such as energy storage, separation, air purification, and many more [1,2]. 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]. The terms 'flexible MOF', 'soft porous crystals', and 'switchable MOF' are used synonymously [7,8]. An important aspect indicated by the term 'switchability' is the stepwise (first order) character of the structural transition between two phases (bistability). Recently, even multistable systems have emerged, leading to remarkable recognition effects [9,10]. 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. An activation barrier (E A or ΔG ǂ ) governs the kinetics and potential windows for metastable states. Nucleation (for nucleation theory see Glossary and see supplemental information online for additional Glossary terms) of the new solid phase is an important characteristic of the solid-solid phase transition. Nevertheless, the fluid kinetics of adsorption and, potentially, fluid nucleation kinetics may also play a role. HighlightsSwitchable metal-organic frameworks (MOFs) conquer advanced applications in energy storage, sensing, gas separation, catalysis, and biomedicine. They benefit from unique adsorption characteristics and responsive behavior leading to anomalously high separation selectivity and deliverable storage capacity.Nanostructuring provides an effective means to tailor the responsive behavior of switchable MOFs. The characteristic switching pressure d...

Impact of Crystal Size and Morphology on Switchability Characteristics in Pillared-Layer Metal-Organic Framework DUT-8(Ni)

“…Thus, the distribution of nucleation barriers of the individual grains controls the transition pressure spread over a wider pressure range. This is typically reflected by the less-steep adsorption branch in the gating region (Ehrling et al, 2021). The first derivative of the adsorption branch of the nitrogen physisorption isotherm reflects the distribution of transition pressures over the sample and, correspondingly, the distribution of gate opening activation energies (Figure 5, Supplementary Figures 11 and 12, ESI).…”

“…Taking into account the decreasing α max for the small crystallites, the crystal width approaches a certain critical value where the transition is completely suppressed due to the height of the barrier ( G * ), which cannot be overcome at given conditions. Another potential explanation for the observations is based on thermodynamic arguments, most importantly the differences in free energies of the empty cp and op frameworks ( F = F cp -F op ), originating from differences in surface energy and, probably, less important gradients of the adsorbed phase (Ehrling et al, 2021).…”

“…Recently, the impact of the crystal size on functional properties of MOFs has also received considerable attention (Sakata et al, 2013;Zhang et al, 2014;Li et al, 2016b;Saitoh et al, 2016;Tian et al, 2016). It was recognized that, by changing the particle size, the intermolecular interaction between the adsorbent and adsorbate, the catalytic performance of the MOF, and the magnetic properties can be tuned (Hatakeyama et al, 2011;Kiyonaga et al, 2015;Ehrling et al, 2021).…”

“…The findings regarding the particle size-dependent phenomena in flexible MOFs are summarized in a recent publication (Ehrling et al, 2021). However, only a few studies dedicated to the crystal shape and its role on the characteristics of structural transition have been published so far (Watanabe et al, 2017;Zhang et al, 2017).…”

Unraveling the Guest-Induced Switchability in the Metal-Organic Framework DUT-13(Zn)

“…A nearly complete op → cp → op transition was observed in the case of n-butane adsorption at 273 K, while in case of weakly interacting nitrogen molecules a portion of the sample remains in the op phase in the entire pressure range. Apparently, in case of DUT-13 the contraction is crystallite size-dependent, similarly as in a number of other switchable MOFs, [20] which should be investigated more in detail in the future. Methane adsorption at varying temperatures showed a wide hysteresis at the temperatures between 111 K and 140 K. The hysteresis width decreases until it disappears completely at 170 K leading to a reversible isotherm, typical for rigid frameworks.…”