Easy and efficient energy storage is one of the problems treated by numerous researchers today. Hydrophobic nanoporous materials can potentially be used as actuators, but also as molecular springs, dampers, or shock absorbers. [1] In this case, the reversible intrusion of a liquid in nonwetting pores at high pressure, a process subject to hysteresis, is used to store or produce mechanical work. Herein, we show the possibility to store mechanical energy using porous metal-organic framework materials (MOFs) in which their flexibility is used instead of nonwetting properties. Indeed, MOFs have found increasing interest over the past few years in potential applications such as gas separation and storage, [2][3][4] liquidphase separation, [5] or drug delivery.[6]One of the unique properties of some of these materials is their high degree of framework flexibility, which has reached 230 % in the case of the MIL-88 series (MIL stands for Materials from Institut Lavoisier). In most cases, the flexibility of these materials has been induced by adsorption of guest species, [7] which produce various types of flexibility. [8,9] One of the most interesting classes of flexible solids are those of the MIL-53 series. These are metal(III) terephthalates built up from chains of corner sharing metal(III) octahedral (M = Al, Cr, Fe, Ga, In, …) and terephthalate groups that delimit one-dimensional microporous pore system.[2] The Al and Cr forms are found in the large pore (LP) form after thermal removal of the guest species, whereas in the presence of various fluids, a narrow pore (NP) form is observed (MIL-53(Cr) NP; space group C2/c; V % 1020 3 ) before re-expansion to the LP form (MIL-53(Cr) LP; space group Imcm; V % 1490 3 ; see Figure 1). In the case of MIL-53(Al), this reversible flexible character has also been observed as being dependent on the temperature [10] with hysteresis between the cooling and heating processes. The transition from LP to NP occurs on cooling in the 125-150 K range whereas that from NP to LP occurs on heating in the range 325-375 K. The presence of these different crystalline states opens the possibility for phase diagrams to be established.Although previous gas adsorption studies have been carried out under various pressures, [4,5,9] to the best of our knowledge, the response of these materials solely under pressure, without any adsorption effect in pores, has not been reported to date. In the recent study of Moggach et al., ZIF-8 samples were submitted to a high hydrostatic pressure that provokes crystal phase transitions, but with the presence of liquid inside pores.[11] Logically, one would expect that a transition from the LP to the NP phase would occur provided a high enough pressure is applied to the MOF phase in question. One reason for this behavior is that the MIL-53 phases are usually synthesized in the form of micrometersized particles and it is not easy to impose a mechanical stress around the particle in a controlled manner. However, the use of mercury porosimetry permits an isostatic press...

We present a unified thermodynamic description of the breathing transitions between large pore (lp) and narrow pore (np) phases of MIL-53 (Cr) observed during the adsorption of guest molecules and the mechanical compression in the process of mercury porosimetry. By revisiting recent experimental data on mercury intrusion and in situ XRD during CO(2) adsorption, we demonstrate that the magnitude of the adsorption stress exerted inside the pores by guest molecules, which is required for inducing the breathing transition, corresponds to the magnitude of the external pressure applied from the outside that causes the respective transformation between lp and np phases. We show that, when a stimulus is applied to breathing MOFs of MIL-53 type, these materials exhibit small reversible elastic deformations of lp and np phases of the order of 2-4%, while the breathing transition is associated with irreversible plastic deformation that leads to up to ∼40% change of the sample volume and a pronounced hysteresis. These results shed light on the specifics of the structural transformations in MIL-53 (Cr) and other soft porous crystals (SPC).

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Para-disubstituted alkylaromatics such as p-xylene are preferentially adsorbed from an isomer mixture on three isostructural metal-organic frameworks: MIL-125(Ti) ([Ti(8)O(8)(OH)(4)(BDC)(6)]), MIL-125(Ti)-NH(2) ([Ti(8)O(8)(OH)(4)(BDC-NH(2))(6)]), and CAU-1(Al)-NH(2) ([Al(8)(OH)(4)(OCH(3))(8)(BDC-NH(2))(6)]) (BDC = 1,4-benzenedicarboxylate). Their unique structure contains octahedral cages, which can separate molecules on the basis of differences in packing and interaction with the pore walls, as well as smaller tetrahedral cages, which are capable of separating molecules by molecular sieving. These experimental data are in line with predictions by molecular simulations. Additional adsorption and microcalorimetric experiments provide insight in the complementary role of the two cage types in providing the para selectivity.