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...

This paper demonstrates that nanospace engineering of KOH activated carbon is possible by controlling the degree of carbon consumption and metallic potassium intercalation into the carbon lattice during the activation process. High specific surface areas, porosities, sub-nanometer (<1 nm) and supra-nanometer (1-5 nm) pore volumes are quantitatively controlled by a combination of KOH concentration and activation temperature. The process typically leads to a bimodal pore size distribution, with a large, approximately constant number of sub-nanometer pores and a variable number of supra-nanometer pores. We show how to control the number of supra-nanometer pores in a manner not achieved previously by chemical activation. The chemical mechanism underlying this control is studied by following the evolution of elemental composition, specific surface area, porosity, and pore size distribution during KOH activation and preceding H(3)PO(4) activation. The oxygen, nitrogen, and hydrogen contents decrease during successive activation steps, creating a nanoporous carbon network with a porosity and surface area controllable for various applications, including gas storage. The formation of tunable sub-nanometer and supra-nanometer pores is validated by sub-critical nitrogen adsorption. Surface functional groups of KOH activated carbon are studied by microscopic infrared spectroscopy.

A simple laboratory-scale protocol that enables the evaluation of the effect of adsorbed water on CO uptake is proposed. 45 metal-organic frameworks (MOFs) were compared against reference zeolites and active carbons. It is possible to classify materials with different trends in CO uptake with varying amounts of pre-adsorbed water, including cases in which an increase in CO uptake is observed for samples with a given amount of pre-adsorbed water. Comparing loss in CO uptake between "wet" and "dry" samples with the Henry constant calculated from the water adsorption isotherm results in a semi-logarithmic trend for the majority of samples allowing predictions to be made. Outliers from this trend may be of particular interest and an explanation for the behaviour for each of the outliers is proposed. This thus leads to propositions for designing or choosing MOFs for CO capture in applications where humidity is present.

Isotherms and differential enthalpies of adsorption are obtained for nitrogen at ambient temperature on monovalent (Li(+), Na(+), K(+)) and divalent (Ca(2+), Ba(2+), Sr(2+), Mn(2+)) substituted X-faujasite systems by microcalorimetry measurements. These experimental data are compared with those obtained by combining grand canonical Monte Carlo simulations and newly derived force fields for describing the interactions between the extra-framework cations and the adsorbates obtained from a simple model based only on the intrinsic properties of the cations. It is the first time that such good qualitative agreement is reported between experiment and simulation for a series of both monovalent and divalent cations.

The incorporation of coordinatively unsaturated metal sites in microporous metal−organic frameworks (MOFs) has emerged as an important synthetic strategy for the development of potential room-temperature hydrogen storage materials, because the relatively strong, localized interaction of hydrogen with the metal centers induces an increase of the isosteric heat of hydrogen adsorption. Previous modeling studies have shown that these interactions are not adequately modeled when literature force-field parameters are used. Typical results of grand-canonical Monte Carlo (GCMC) simulations exhibit a pronounced underestimation of the hydrogen uptake at low pressures and low temperatures. In this study, it is shown that this shortcoming can be resolved by deriving a new set of potential parameters to represent the metal−dihydrogen interaction from ab initio calculations for molecular model systems. The approach is computationally efficient and could be applied for any coordination environment of the metal center. The present work focuses on three MOFs with unsaturated copper centers. The newly derived Cu−H2 potential model is combined with literature force-field parameters to model the dispersive interactions with other framework atoms. At cryogenic temperatures and pressures up to 1 bar, GCMC simulations using these parameters provide for a massively improved prediction of the hydrogen storage characteristics when compared to parameters from a literature force field. On the other hand, the unmodified literature parameters perform best in predicting the saturation uptake. At room temperature, the effect of the potential modification is much smaller, and the best agreement with experiment is obtained when the localized metal−dihydrogen interaction is not accounted for in the simulations. This indicates that the metal−dihydrogen interaction is too weak to permit a significant adsorption at the metal sites under these conditions. Calculations using an artificially enhanced potential model show that a drastic increase of the interaction strength could boost the hydrogen storage capacity at room temperature, although the attainable uptake remains limited by the number of available metal sites. The implications of these results for the synthesis of new MOFs are critically discussed.

Metal-organic frameworks are widely considered for the separation of chemical mixtures due to their adjustable physical and chemical properties. However, while much effort is currently devoted to developing new adsorbents for a given separation, an ideal scenario would involve a single adsorbent for multiple separations. Porous materials exhibiting framework flexibility offer unique opportunities to tune these properties since the pore size and shape can be controlled by the application of external stimuli. Here, we establish a proof-of-concept for the molecular sieving separation of species with similar sizes (CO 2 /N 2 and CO 2 /CH 4 ), via precise mechanical control of the pore size aperture in a flexible metal-organic framework. Besides its infinite selectivity for the considered gas mixtures, this material shows excellent regeneration capability when releasing the external mechanical constraint. This strategy, combining an external stimulus applied to a structurally compliant adsorbent, offers a promising avenue for addressing some of the most challenging gas separations.

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