We provide a general thermodynamic framework for the understanding of guest-induced structural transitions in hybrid organic-inorganic materials. The method is based on the analysis of experimental adsorption isotherms. It allows the determination of the free energy differences between host structures involved in guest-induced transitions, especially hard to obtain experimentally. We discuss the general case of adsorption in flexible materials and show how a few key quantities, such as pore volumes and adsorption affinities, entirely determine the phenomenology of adsorption, including the occurrence of structural transitions. On the basis of adsorption thermodynamics, we then propose a taxonomy of guest-induced structural phase transitions and the corresponding isotherms. In particular, we derive generic conditions for observing a double structural transition upon adsorption, often resulting in a two-step isotherm. Finally, we show the wide applicability and the robustness of the model through three case studies of topical hybrid organic-inorganic frameworks: the hysteretic hydrogen adsorption in Co(1,4-benzenedipyrazolate), the guest-dependent gate-opening in Cu(4,4'-bipyridine)(2,5-dihydroxybenzoate)2 and the CO2-induced "breathing" of hybrid material MIL-53.

Much attention has recently been focused on a fascinating subclass of metal-organic frameworks that behave in a remarkable stimuli-responsive fashion. These soft porous crystals feature dynamic crystalline frameworks displaying reversible, large-amplitude structural deformations under external physical constraints such as temperature, electric field or gas exposure. The number of reported syntheses of such materials is rapidly growing and they are promising for practical applications, such as gas capture, purification and fluid separation. Herein, we summarize the recently developed thermodynamic tools that can help understand the process of fluid adsorption and fluid mixture coadsorption in these flexible nanoporous materials. These tools, which include both molecular simulation methods and analytical models, can help rationalize experimental results and predict adsorption properties over a wide range of thermodynamic conditions. A particular focus is given on how these methods can guide the experimental exploration of a large number of materials and working conditions (temperature, pressure, composition) to help design efficient processes relying on fluid adsorption in soft porous crystals.

A thermodynamic analysis based on the osmotic ensemble scheme enables the prediction of structural changes occurring in silicalite-1 zeolite upon halocarbon molecule adsorption.

We have adapted a grand ensemble Monte Carlo simulation method to directly compute, for the first time to our knowledge, univalent cation exchange isotherms in zeolites. The computed isotherms for the exchange of sodium in NaY faujasite by lithium, potassium, rubidium, and cesium ions, respectively, are in good agreement with the experimental ones. They display the three main types of behavior observed in zeolites, namely, a monotonous evolution of selectivity throughout the exchange process (Li(+)), a selectivity reversal (K(+)), and an incomplete exchange (Rb(+) and Cs(+)). The initial stage of the cation exchange is shown to be dominated by the hydration energy of the cations in the external aqueous solution. The final part of the process is often dominated by the cation-framework and cation-cation interactions. A crossover between these two regimes explains the frequently observed reversal of selectivity phenomenon. The incomplete exchange observed in the case of Rb(+) and Cs(+) is shown to correspond to a blocked state of the system for highest accessible composition of the aqueous solution. This stable state is shown not to be linked to an inability of the cesium cations to cross the six-ring window in order to penetrate into the smallest cages.

The knowledge of aluminum distribution in zeolites is a difficult task due to limitations in experimental measurements. In the present paper, we propose a new methodology to simultaneously determined aluminum atoms distribution as well as the extraframework cation location in a given experimental structure of the framework and thus allows to compared different synthesis routes. Aluminum mean distribution is obtained over a great number of configurations that are generated during the course of the simulations at finite temperature. The obtained aluminum atom repartition is in agreement with the experimental and model data available. The consequences of aluminum distribution on solid properties such as extraframework Na + cation location has been analyzed and successfully compared with the available information for different zeolite topologies. The proposed methodology can be used as a powerful complementary tool for aluminum location on RX or neutron experimental structure determinations.

Grand Canonical Monte Carlo and molecular dynamics simulations are employed to investigate the influence of water adsorption on the arrangements and the dynamics of the sodium cations in faujasites Na 56 Y and Na 96 X. The water adsorption provokes significant cation redistributions in Na 56 Y, while the partition of the cations among the different crystallographic sites is not affected upon the whole adsorption process in Na 96 X. The first water molecules in Na 56 Y are adsorbed both in the sodalite cage and in the supercage, that is, interacting with cations in SI′ and SIII′, respectively. In contrast, the first water molecules in Na 96 X are located within the supercage interacting with cations in SIII′ only. The cation dynamics are then explored. In Na 56 Y, only very local motion is observed for cations in sites SI′ whatever the water loading. At low and intermediate water loadings, the cations initially in SII and SIII′ present local displacements around their initial sites only whereas they move over much longer distances at high loading. Finally, due to a strong steric repulsion between cations in Na 96 X, the average cation mean square displacement for this system is always smaller than for Na 56 Y. IntroductionMicroporous zeolite materials attract a great deal of attention because of their use in industry for catalysis, phase separation, ionic exchange, and so forth. 1,2 The significant impact of these systems results from their large surface area, the nanoscopic size of their pores, and the large variety of chemical compositions, that is, Si/Al ratio and the nature of the extra-framework cations. From a fundamental point of view, zeolites are model systems that can be used to investigate the effect of nanoconfinement on both thermodynamic and dynamic properties of fluids. 3,4 The substitution of Si with Al atoms in the aluminosilicate zeolites induces a net negative charge on the framework that is compensated by the introduction of extra-framework cations (Li + , Na + , K + , Ca 2+ , Ba 2+ , Mg 2+ , and so forth). Many experimental and theoretical studies have shown that the location of these cations plays a crucial role on the thermodynamics, dynamics, and catalysis of various adsorbates/zeolite systems. 5-10 Because of the hydrophilic character of these materials (arising from the strong electrostatic cation-water interaction), the presence of water molecules in the porosity of aluminosilicate zeolites cannot be avoided in many applications operating at ambient temperature. This byproduct can have a beneficial or detrimental role on the adsorption/separation properties of zeolites depending on the degree of hydration necessary to observe significant cation displacements. As a typical illustration, in the separation of para-and meta-xylene, it is known that 3 wt % of water can improve up to 50% the separation ability of barium faujasite BaX by favoring the displacements of the

We report the observation of an unusual hysteresis loop in the water adsorption−desorption isotherm in NaY in the very low pressure range. Vacuum equilibrium thermodesorption and n-pentane adsorption experiments combined with grand canonical Monte Carlo simulations show that this hysteresis phenomenon is consecutive to the trapping of water in sodalite cages. We propose that the adsorption−desorption of water in NaY occurs as follows. (i) Initial adsorption of a few water molecules on the most accessible cations located on sites II in the supercages. The solvation of these cations makes the sodalite cages accessible to water. (ii) Complete filling of supercages and sodalite cages associated with a migration of nonframework cations from D6R prisms to sodalite cages. (iii) Desorption of water from supercages with water still trapped in the sodalite cages. (iiii) Thermally activated desorption of water from sodalite cages. Thus, according to the hydration procedure used, the residual water can be located either in supercages or in sodalite cages. This provides a way to prepare zeolite samples with the same amount of residual water but with different nonframework cation and water molecule distributions. These two samples would presumably behave differently with regard to selective adsorption of hydrocarbon mixtures, a feature that could be of interest for separation technologies.

No abstract