Abstract
Conventional SO
2
scrubbing agents, namely calcium oxide and zeolites, are often used to remove SO
2
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 SO
2
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 SO
2
. Direct mixed gas column breakthrough and/or column desorption experiments revealed an unprecedented SO
2
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 SO
2
at low concentrations ranging from 25 to 500 p.p.m.
Conventional SO
scrubbing agents, namely calcium oxide and zeolites, are often used to remove SO
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 SO
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 SO
. Direct mixed gas column breakthrough and/or column desorption experiments revealed an unprecedented SO
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 SO
at low concentrations ranging from 25 to 500 p.p.m.
Brolly good MOFs: A new series of hydrophobic isoreticular porous Zr oxide dicarboxylate MOFs have been prepared (see picture, Zr blue polyhedra, O red, C black). They have a one‐dimensional pore ...system, a rare combination of Lewis acidity and hydrophobic character, and a higher hydrothermal and mechanical stability than their UiO MOF polymorph counterparts.
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,
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.
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.
Context. The iron Kα line at 6.4 keV provides a valuable spectral diagnostic in several fields of X-ray astronomy. The line often results from the reprocessing of external hard X-rays by a neutral or ...low-ionized medium, but it can also be excited by impacts of low-energy cosmic rays. Aims. This paper aims to provide signatures allowing identification of radiation from low-energy cosmic rays in X-ray spectra showing the 6.4 keV Fe Kα line. Methods. We study in detail the production of nonthermal line and continuum X-rays by interaction of accelerated electrons and ions with a neutral ambient gas. Corresponding models are then applied to XMM-Newton observations of the X-ray emission emanating from the Arches cluster region near the Galactic center. Results. Bright 6.4 keV Fe line structures are observed around the Arches cluster. This emission is very likely produced by cosmic rays. We find that it can result from the bombardment of molecular gas by energetic ions, but probably not by accelerated electrons. Using a model of X-ray production by cosmic-ray ions, we obtain a best-fit metallicity of the ambient medium of 1.7 ± 0.2 times the solar metallicity. A large flux of low-energy cosmic ray ions could be produced in the ongoing supersonic collision between the star cluster and an adjacent molecular cloud. We find that a particle acceleration efficiency in the resulting shock system of a few percent would give enough power in the cosmic rays to explain the luminosity of the nonthermal X-ray emission. Depending on the unknown shape of the kinetic energy distribution of the fast ions above ~1 GeV nucleon-1, the Arches cluster region may be a source of high-energy γ-rays detectable with the Fermi Gamma-ray Space Telescope. Conclusions. At present, the X-ray emission prominent in the 6.4 keV Fe line emanating from the Arches cluster region probably offers the best available signature for a source of low-energy hadronic cosmic rays in the Galaxy.
Single-file diffusion of neo-pentane in the channel-like MIL-47(V) was recently revealed by quasi-elastic neutron scattering experiments. The origin of such unprecedented dynamic behavior in the ...field of metal–organic frameworks (MOFs) has not been elucidated at the atomistic level yet. Here, molecular dynamics simulations are performed to first confirm this single-file diffusion and further unveil the molecular insights into this abnormal diffusion process. Whatever the loading, a subdiffusive regime is highlighted, in good agreement with the single-file diffusion experimentally detected. We show that this subdiffusive regime is uncorrelated with the flexibility of the MIL-47(V) and comes mostly from the pore dimension of the MOF that hinders the crossing of molecules along the channel. Neither translational jumps nor correlation dynamics of the guest molecules present in the same channel and in the neighbor channels were observed. Moreover, the rotational dynamics was also carefully explored, and a relatively homogeneous rotational motion was evidenced along the three directions. While the translational diffusion coefficient decreases when the loading increases, the rotational diffusion coefficient remains constant, which corresponds to a clear deviation to the Stokes–Einstein relation.
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.
The unusual adsorption behavior of CO2 in a nanoporous hybrid metal– organic solid is discussed (see figure). The results indicate that the gas adsorption–desorption step is related to a breathing ...phenomenon. This study also suggests that the main interactions responsible for the breathing phenomenon are strong guest–framework CO2–OH interactions as well as CO2–CO2 interactions along the tunnels present in the structure.
Anisotropic isobaric/isothermal molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) techniques are combined in a hybrid scheme to get an osmotic Monte Carlo approach able to deal with a ...guest-assisted structural transition of a metal organic framework (MOF) porous solid corresponding to a large reversible breathing of its structure. This strategy based on (i) a consideration of a more general expression of the partition functions and (ii) a rigorous homogenization of the MD and MC parts allows us to capture the structural transition of the MIL-53(Cr) MOF-type solid in relation to the CO2 pressure. Further, we show that combining this revisited hybrid osmotic Monte Carlo (HOMC) approach to a newly developed “phase mixture” model, which is based on the existence of a pressure domain where several structural forms of MIL-53(Cr) are present, is an efficient way to accurately predict the adsorption behavior of this solid in the whole range of pressures.
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