Hexahydroxytriphenylene for the synthesis of group 13 MOFs - a new inorganic building unit in a ß-cristobalite type structure.

Two new, microporous MOFs of framework composition ((CH3)2NH2)2[M3O(HHTP)(HHTP˙)], M = Al3+, Ga3+, H6HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene, are described. Electron diffraction combined with molecular simulations show that these compounds crystallize in the ß-cristobalite structure, containing a new type of trinuclear inorganic building unit for MOFs and radical anions.

Fluorinated MOF platform for selective removal and sensing of SO2 from flue gas and air.

Conventional SO2 scrubbing agents, namely calcium oxide and zeolites, are often used to remove SO2 using a strong or irreversible adsorption-based process. However, adsorbents capable of sensing and selectively capturing this toxic molecule in a reversible manner, with in-depth understanding of structure-property relationships, have been rarely explored. Here we report the selective removal and sensing of SO2 using recently unveiled fluorinated metal-organic frameworks (MOFs). Mixed gas adsorption experiments were performed at low concentrations ranging from 250 p.p.m. to 7% of SO2. Direct mixed gas column breakthrough and/or column desorption experiments revealed an unprecedented SO2 affinity for KAUST-7 (NbOFFIVE-1-Ni) and KAUST-8 (AlFFIVE-1-Ni) MOFs. Furthermore, MOF-coated quartz crystal microbalance transducers were used to develop sensors with the ability to detect SO2 at low concentrations ranging from 25 to 500 p.p.m.

Understanding the origins of metal-organic framework/polymer compatibility.

The microscopic interfacial structures for a series of metal-organic framework/polymer composites consisting of the Zr-based UiO-66 coupled with different polymers are systematically explored by applying a computational methodology that integrates density functional theory calculations and force field-based molecular dynamics simulations. These predictions are correlated with experimental findings to unravel the structure-compatibility relationship of the MOF/polymer pairs. The relative contributions of the intermolecular MOF/polymer interactions and the flexibility/rigidity of the polymer with respect to the microscopic structure of the interface are rationalized, and their impact on the compatibility of the two components in the resulting composite is discussed. The most compatible pairs among those investigated involve more flexible polymers, i.e. polyvinylidene fluoride (PVDF) and polyethylene glycol (PEG). These polymers exhibit an enhanced contact surface, due to a better adaptation of their configuration to the MOF surface. In these cases, the irregularities at the MOF surface are filled by the polymer, and even some penetration of the terminal groups of the polymer into the pores of the MOF can be observed. As a result, the affinity between the MOF and the polymer is very high; however, the pores of the MOF may be sterically blocked due to the strong MOF/polymer interactions, as evidenced by UiO-66/PEG composites. In contrast, composites involving polymers that exhibit higher rigidity, such as the polymer of intrinsic microporosity-1 (PIM-1) or polystyrene (PS), present interfacial microvoids that contribute to a decrease in the contact surface between the two components, thus reducing the MOF/polymer affinity.

Thermodynamic insight into stimuli-responsive behaviour of soft porous crystals.

Knowledge of the thermodynamic potential in terms of the independent variables allows to characterize the macroscopic state of the system. However, in practice, it is difficult to access this potential experimentally due to irreversible transitions that occur between equilibrium states. A showcase example of sudden transitions between (meta)stable equilibrium states is observed for soft porous crystals possessing a network with long-range structural order, which can transform between various states upon external stimuli such as pressure, temperature and guest adsorption. Such phase transformations are typically characterized by large volume changes and may be followed experimentally by monitoring the volume change in terms of certain external triggers. Herein, we present a generalized thermodynamic approach to construct the underlying Helmholtz free energy as a function of the state variables that governs the observed behaviour based on microscopic simulations. This concept allows a unique identification of the conditions under which a material becomes flexible.

A Comparison of Barostats for the Mechanical Characterization of Metal-Organic Frameworks.

In this paper, three barostat coupling schemes for pressure control, which are commonly used in molecular dynamics simulations, are critically compared to characterize the rigid MOF-5 and flexible MIL-53(Al) metal-organic frameworks. We investigate the performance of the three barostats, the Berendsen, the Martyna-Tuckerman-Tobias-Klein (MTTK), and the Langevin coupling methods, in reproducing the cell parameters and the pressure versus volume behavior in isothermal-isobaric simulations. A thermodynamic integration method is used to construct the free energy profiles as a function of volume at finite temperature. It is observed that the aforementioned static properties are well-reproduced with the three barostats. However, for static properties depending nonlinearly on the pressure, the Berendsen barostat might give deviating results as it suppresses pressure fluctuations more drastically. Finally, dynamic properties, which are directly related to the fluctuations of the cell, such as the time to transition from the large-pore to the closed-pore phase, cannot be well-reproduced by any of the coupling schemes.

Rationalization of the entrapping of bioactive molecules into a series of functionalized porous zirconium terephthalate MOFs.

The encapsulation of two different bioactive molecules, the cosmetic caffeine and the analgesic and anti-inflammatory ibuprofen, has been evaluated by combining impregnation and advanced characterization experimental tools in a series of microporous rigid zirconium(iv) terephthalates UiO-66 bearing different polar or apolar functional groups (-H, -Br, -NH2, -2OH, -NO2, -Cl, -2CF3, -CH3, -2CH3). It has been first evidenced that these hybrid solids exhibit drug payloads that significantly outperform those obtained using current drug formulations or other conventional porous solids. A quantitative structure-activity relationship strategy has been further conducted with the aim of rationalizing the experimental drug uptakes and further emphasizing the most relevant chemical and structural features that significantly impact their encapsulation performances. Indeed, it appears that the caffeine loading is optimized when the functionalized organic linker both shows a large octanol-water partition coefficient and contains grafted functions with low hydrogen bond acceptor abilities, whereas the ibuprofen entrapping is enhanced when the organic linker contains functional groups with a large solvent surface area and free volume, and to a lesser extent low hydrogen bond acceptor abilities. Moreover, it has been shown that the solvent used as media for the biomolecule impregnation plays a crucial role in the encapsulation performance due to the formation of a competitive adsorption process between the solvent and the active molecule.

Porous metal organic framework nanoparticles to address the challenges related to busulfan encapsulation.

Busulfan is an alkylating agent widely used in chemotherapy, but with severe side effects. Many attempts have been made to entrap busulfan in nanocarriers to avoid liver accumulation and to protect it against rapid degradation in aqueous media. However, poor loadings (≤ 5 wt%) and fast release were generally obtained due to the low affinity of busulfan towards the nanocarriers. Moreover, drug crystallization often occurred during nanoparticle preparation. To circumvent these drawbacks, metal organic framework (MOF) nanoparticles, based on crystalline porous iron (III) carboxylates, have shown an unprecedented loading (up to 25 wt%) of busulfan. This was attributed to the high porosity of nanoMOFs as well as to their hydrophilic-hydrophobic internal microenvironment well adapted to the amphiphilic character of busulfan. NanoMOFs formulations have kept busulfan in molecular form, preventing its crystallization and degradation. Indeed, busulfan was released intact, as proved by the maintenance of its pharmacological activity.

Adsorption of light hydrocarbons in the flexible MIL-53(Cr) and rigid MIL-47(V) metal-organic frameworks: a combination of molecular simulations and microcalorimetry/gravimetry measurements.

The adsorption of short linear alkanes has been explored in the highly flexible MIL-53(Cr) porous metal-organic framework by means of molecular simulations based on configurational bias grand canonical Monte Carlo. The unusual shape of the adsorption isotherms with the existence of steps has been successfully modelled by creating a (narrow pore, large pore) phase mixture domain, the composition of which varies with pressure. A further step consisted of combining our computational approach with several experimental tools including microcalorimetry, gravimetry and in situ X-ray diffraction, to fully characterize the adsorption behaviour of the isostructural MIL-47(V) rigid MOF, i.e. the preferential arrangement of each type of alkane inside the pores and the resulting interaction energy. Finally, relationships are established between the adsorption enthalpies and both alkyl chain length and polarisability of the alkanes that can be further utilised to predict the energetics of the adsorption process for longer alkane chains.

Complex adsorption of short linear alkanes in the flexible metal-organic-framework MIL-53(Fe).

This investigation is based on a combination of experimental tools completed by a computational approach to deeply characterize the unusual adsorption behavior of the flexible MIL-53(Fe) in the presence of short linear alkanes. In contrast to the aluminum or chromium analogues we previously reported, the iron MIL-53 solid, which initially exhibits a closed structure in the dry state, shows more complex adsorption isotherms with multisteps occurring at pressures that depend on the nature of the alkane. This behavior has been attributed to the existence of four discrete pore openings during the whole adsorption process. Molecular simulations coupled with in situ X-ray powder diffraction were able to uncover these various structural states.

Experimental evidence supported by simulations of a very high H2 diffusion in metal organic framework materials.

Quasielastic neutron scattering measurements are combined with molecular dynamics simulations to extract the self-diffusion coefficient of hydrogen in the metal organic frameworks MIL-47(V) and MIL-53(Cr). We find that the diffusivity of hydrogen at low loading is about 2 orders of magnitude higher than in zeolites. Such a high mobility has never been experimentally observed before in any nanoporous materials, although it was predicted in carbon nanotubes. Either 1D or 3D diffusion mechanisms are elucidated depending on the chemical features of the MIL framework.

Charge distribution in metal organic framework materials: transferability to a preliminary molecular simulation study of the CO(2) adsorption in the MIL-53 (Al) system.

Density functional theory calculations have been performed in order to extract the charge distribution in the aluminium-containing MIL-53 structure, to allow further computational studies of adsorption in these materials. Both cluster and periodic methods have been used and the charges calculated for each atom constituting the organic and inorganic part of the material, were discussed. Preliminary grand canonical Monte Carlo simulations, based on a consistent set of potential parameters and this newly derived charge distribution, predicted for enthalpies of adsorption for CO(2) at low coverage in the "large" and "narrow" pore versions of MIL-53 (Al) to be significantly different. These calculated enthalpies reproduced the two distinct ranges of values observed by microcalorimetry on either side of 6 bars quite well. This agreement between experiment and simulation validated our previous assumption, suggesting a structural switching of the hybrid material during the adsorption process. The microscopic mode of interaction between the hybrid porous framework and the CO(2) adsorption was then carefully analysed in both of the MIL-53 (Al) structures.

Use of dielectric relaxation for measurements of surface energy variations during adsorption of water on mordenite.

This paper tries to assess simply and quantitatively the link between classical adsorption theories and dielectric spectroscopy, in order to demonstrate that dielectric spectroscopy can be used as a tool of determination of surface energy variations due to movements of charge carriers at the surface of solids. A simple theory is developed to analyze hops of cations at the surface of mordenite, which are detected by complex impedance spectroscopy during adsorption of water. An energy of extraction of the cation can be determined from measurements and its dependence on the quantity of water molecules adsorbed is shown and qualitatively and quantitatively explained, using relationships developed in order to interpret adsorption phenomena generally. The agreement with other determinations of the adsorption energies and solid surface energy is correct.

Cation migration upon adsorption of methanol in NaY and NaX faujasite systems: a molecular dynamics approach.

Molecular dynamics simulations have been carried out to address the question of cation migration upon adsorption of methanol in NaY and NaX faujasite systems as a function of the loading. For NaY, it has been shown that, at low and intermediate loadings, SII cations can migrate toward the center of the supercage due to strong interactions with the adsorbates, followed by a hopping of SI' from the sodalite cage into the supercage to fill the vacant SII site. A SI' cation can also migrate across the double six ring and takes a SI' vacant position. SI cations mainly remain trapped in their initial sites whatever the loading. At high loading, only limited motions are observed for SII cations due to steric effects induced by the presence of adsorbates within the supercage. For NaX, the SIII' cations which occupy the most accessible adsorption sites are significantly moving upon coordination to the methanol molecules; the extent of this mobility exhibits a maximum for 48 methanol molecules per unit cell before decreasing at higher loadings due to steric hindrance. In addition, the SI' and SII cations remain almost trapped in their initial sites whatever the loading. Indeed, the most probable migration mechanism involves SIII' cation displacements into nearby SIII' sites.

Modeling the concentration dependence of the methanol self-diffusivity in faujasite systems: comparison with the liquid phase.

Molecular dynamics simulations were performed to understand further the concentration dependence of the self-diffusion of methanol in the faujasite zeolite systems. The evolution of the self-diffusivity was investigated as a function of coverage for DAY and NaY systems to study the effect of both the pore confinement and the presence of the extraframework cations within the supercage. It was found that the self-diffusivity decreases with loading for DAY, whereas for NaY it passes through a maximum at intermediate coverage, in agreement with pulse-field gradient NMR and quasi elastic neutron scattering data reported in similar systems. The activation energies of the methanol diffusion corresponding to a combination of both intra- and intercage motions were evaluated as a function of the coverage. The simulated trends are interpreted on the basis of the predominant interactions which take place in both systems. Finally, the preferential arrangement of the adsorbate molecules are provided and compared with those simulated in the liquid phase. For the fully loaded materials, it was seen that the methanol molecules form a one-dimensional hydrogen-bonded chain along the channels in DAY whereas only dimers are present in NaY.

Molecular dynamics simulation of the cation motion upon adsorption of CO2 in Faujasite zeolite systems.

Molecular Dynamics simulations have been carried out in NaX and NaY Faujasite systems to deepen understanding of the cation rearrangement during the CO2 adsorption process suggested by our recent diffusivity measurements. This study is a major contribution since the rearrangement of the cations in Faujasite, the most promising adsorbent for CO2 storage, can represent a significant breakthrough in understanding the adsorption and diffusion processes at the mircroscopic scale. For NaY, it has been shown that at low and intermediate loadings, SII cations can migrate toward the center of the supercage due to strong interactions with the adsorbates, followed by a hopping of SI'cation from the sodalite cage into the supercage to fill the vacant SII site. The SI cations are only displaced at a higher loading, leading to cation de-trapping out of the double six rings into the vacant SI' sites. For NaX, the SIII' cations which occupy the most accessible adsorption sites move significantly upon coordination to the carbon dioxide molecules. The SI' and SII cations remain consistently located in their initial sites whatever the loading. Indeed, the most probable migration mechanism involves SIII' cation displacements into nearby vacant SIII' sites.

Influence of extra-framework cations on the adsorption properties of X-faujasite systems: microcalorimetry and molecular simulations.

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.

Adsorption mechanism of carbon dioxide in faujasites: grand canonical monte carlo simulations and microcalorimetry measurements.

Molecular simulations have been coupled with adsorption microcalorimetry measurements in order to understand more deeply the interactions between carbon dioxide and various types of faujasite surfaces. The modeling studies, based on newly derived interatomic potentials for describing the interactions within the whole system, provide isotherms and evolutions of the differential enthalpy of adsorption as a function of coverage for DAY, NaY, and NaLSX which are in very good accordance with those obtained experimentally. The microscopic mechanism of CO2 adsorption was carefully analyzed, with different behaviors proposed, depending on the energetic characteristics of each faujasite surface, which are consistent with the trends observed for the differential enthalpies of adsorption.

Comparative toxicity and hazards of pesticides to Apis and non-Apis bees. A chemometrical study.

The adverse effects of 158 pesticides to the Honey bee (Apis mellifera), the alfalfa leafcutting bee (Megachile rotundata) and the alkali bee (Nomia melanderi) were compared by means of various linear and non-linear multivariate analyses. A comparison exercise including the bumble bee (Bombus spp.) was also performed from a more restricted set of 32 pesticides. While no difference of sensitivity was found between A. mellifera and Bombus spp., M. rotundata appeared the most susceptible to pesticides followed by N. melanderi.

Structure-toxicity modeling of pesticides to honey bees.

A quantitative structure-activity relationship (QSAR) model was derived for estimating the acute toxicity of pesticides on the honey bee. Chemicals were described by means of autocorrelation descriptors encoding lipophilicity (H), molar refractivity (MR) and the H-bonding acceptor ability (HBA) of the pesticides. A three-layer feedforward neural network trained by the back-propagation algorithm was used as statistical engine for deriving a powerful QSAR model. The root mean square residual (RMSR) values for the training and testing sets were 0.430 and 0.386, respectively. The practical interest of this original model was discussed.