Charlotte Martineau-Corcos

Hydrolytically stable fluorinated metal-organic frameworks for energy-efficient dehydration

Drying natural gas efficiently Natural gas must be purified before it can be transported. The preparation process also includes a drying step to remove water. Microporous adsorbents such as zeolites are used for this purpose, but they often need to be heated to temperatures up to 250°C to remove the water so that they can be reused. Cadiau et al. describe a fluorinated metal-organic framework containing nickel metal centers that can remove water from gas streams but that can be regenerated by heating to only 105°C. Science, this issue p. 731 A microporous material preferentially adsorbs water over the other components in natural gas and can release it at 105°C. Natural gas must be dehydrated before it can be transported and used, but conventional drying agents such as activated alumina or inorganic molecular sieves require an energy-intensive desiccant-regeneration step. We report a hydrolytically stable fluorinated metal-organic framework, AlFFIVE-1-Ni (KAUST-8), with a periodic array of open metal coordination sites and fluorine moieties within the contracted square-shaped one-dimensional channel. This material selectively removed water vapor from gas streams containing CO2, N2, CH4, and higher hydrocarbons typical of natural gas, as well as selectively removed both H2O and CO2 in N2-containing streams. The complete desorption of the adsorbed water molecules contained by the AlFFIVE-1-Ni sorbent requires relatively moderate temperature (~105°C) and about half the energy input for commonly used desiccants.

A phase transformable ultrastable titanium-carboxylate framework for photoconduction

Porous titanium oxide materials are attractive for energy-related applications. However, many suffer from poor stability and crystallinity. Here we present a robust nanoporous metal–organic framework (MOF), comprising a Ti12O15 oxocluster and a tetracarboxylate ligand, achieved through a scalable synthesis. This material undergoes an unusual irreversible thermally induced phase transformation that generates a highly crystalline porous product with an infinite inorganic moiety of a very high condensation degree. Preliminary photophysical experiments indicate that the product after phase transformation exhibits photoconductive behavior, highlighting the impact of inorganic unit dimensionality on the alteration of physical properties. Introduction of a conductive polymer into its pores leads to a significant increase of the charge separation lifetime under irradiation. Additionally, the inorganic unit of this Ti-MOF can be easily modified via doping with other metal elements. The combined advantages of this compound make it a promising functional scaffold for practical applications.Porous TiO2 materials are attractive for energy-related applications owing to their accessible active sites, but suffer from poor stability. Here the authors synthesize a highly stable and porous metal–organic framework containing polymeric 1D Ti–O subunits, which displays a high condensation degree and high photoconductivity.

27Al-27Al double-quantum single-quantum MAS NMR: Applications to the structural characterization of microporous materials

In this paper, we review and illustrate applications, reported in the literature or used in our group, of 27Al-27Al double-quantum single-quantum (DQ-SQ) MAS NMR experiments for the structural characterization of Al-containing microporous solids, namely zeolites, aluminophosphates and metal-organic frameworks. Information regarding the periodic frameworks or the localization of the various aluminum species in the materials are obtained from the analysis of the two-dimensional NMR spectra, which allows getting local structural details sometimes inaccessible from other characterization technique. An application of 27Al-27Al of the DQ-SQ experiment for the detection of aluminum pairing in zeolite is shown.

Unlocking the observation of different proton populations in fluorinated polymers by solid-state 1H and 19F double resonance NMR spectroscopy

Nafion proton exchange membranes (PEMs) for fuel cell applications are extensively studied and commercially applied, but their unique proton conduction capabilities are still somewhat unexplained. For studying proton dynamics in situ, molecular level spectroscopic techniques have been of limited utility so far. By solid-state 1H and 19F double resonance nuclear magnetic resonance (NMR) spectroscopy using the recently revived multiple contact cross-polarization (MC-CP) pulse sequence along with double-quantum 1H-1H filtering, high resolution proton populations distinct from the dominant water resonance were observed in Nafion for the first time. This methodology quenches signal decay due to spin-lattice relaxation in the rotating frame and enables magnetization transfer between the relatively mobile 1H and 19F spin baths in Nafion. Further studies of these previously unrevealed proton populations will lead to a better understanding of the Nafion proton conduction mechanism and proton exchange processes in general.

Impact of crystalline packing on the mechanochromic luminescence properties of copper based compounds: towards functional coatings

Mechanochromic luminescent materials exhibit a reversible change of the emission wavelength in response to external mechanical forces. These properties are particularly appealing for applications as memory or sensor devices. In order to rationally design and develop such materials, an in-depth understanding of the mechanochromic mechanisms is highly required. In this work, a comparative study of two copper iodide compounds whose difference lies in the subtle modification of the ligands has been conducted. These two clusters present very close crystalline structures but strikingly different optical properties with only one of them exhibiting luminescence mechanochromic properties. Structural and optical characterizations demonstrate that the two clusters endorse different internal stresses due to slight differences in the crystal packing controlled by the nature of the ligands. The release of these constraints upon mechanical solicitation leads to modification of the intramolecular interactions and is at the origin of the observed mechanochromic properties. Additionally, by taking advantage on the sensitive and highly contrasting mechanochromic properties of the clusters, the preparation and characterization of functional coatings on a glass substrate are also presented. An intense signal is observed upon mechanical solicitation which can be erased upon mild thermal treatment.

Luminescence Mechanochromism Induced by Cluster Isomerization

Luminescent mechanochromic materials exhibiting reversible changes of their emissive properties in response to external mechanical forces are currently emerging as an important class of stimuli-responsive materials because of promising technological applications. Here, we report on the luminescence mechanochromic properties of a [Cu4I4(PPh3)4] copper iodide cluster presenting a chair geometry, being an isomer of the most common cubane form. This molecular cluster formulated [Cu4I4(PPh3)4]·2CHCl3 (1) exhibits a highly contrasted emission response to manual grinding, and, interestingly, the optical properties of the ground phase present striking similarities with those of the cubane isomer. In order to understand the underlying mechanism, a comparison with two related compounds has been conducted. The first one is a pseudopolymorph of 1 formulated as [Cu4I4(PPh3)4]·CH2Cl2 (2), which exhibits luminescent mechanochromic properties as well. The other one is also a chair compound but with a slightly different phosphine ligand, namely, [Cu4I4(PPh2C6H4CO2H)4] (3), lacking mechanochromic properties. Structural and optical characterizations of the clusters have been analyzed in light of previous electronic structure calculations. The results suggest an unpreceded mechanochromism phenomenon based on a solid-state chair → cubane isomer conversion. This study shows that polynuclear copper iodide compounds are particularly relevant for the development of luminescent mechanochromic materials.

Maximizing the sensitivity in 13 C cross-polarization magic-angle-spinning solid-state NMR measurements with flip-back pulses

The flip-back pulse combined with cross-polarization magic-angle spinning (CPMAS-FB) has been widely used to reduce the optimal repetition delay, resulting in enhancing sensitivity per unit time. Despite its common use in samples with long 1H T1 relaxation time, the experimental setup of the repetition delay in CPMAS-FB is not obvious. In this article, a simple model is used to derive the optimal repetition delay and expected sensitivity gain through the 1H T1 relaxation time and signal decay during 1H spinlock.

Synthesis, characterisation and application of pyridine-modified chitosan derivatives for the first non-racemic Cu-catalysed Henry reaction

The preparation, characterisation and application of two pyridine-modified chitosan derivatives (C1 and C2) containing Cu(OAc)2 adsorbed as catalysts for the conversion of benzaldehyde into 2-nitro-1-phenylethanol are described. Quantitative solid-state 13C multiple-contact cross-polarization, magic-angle-spinning, nuclear magnetic resonance (MC-CP MAS NMR) measurements confirmed the successful grafting of 2-pyridinecarboxaldehyde and 6-methylpyridine-2-carboxaldehyde to the chitosan backbone and indicated that 47(±2)% of the NH2 groups were grafted for both C1 and C2. The use of C1-Cu(OAc)2 as a catalyst in the nitroaldol reaction led to 96(±1)% conversion and 19(±4)% enantiomeric excess (ee), while the use of C2-Cu(OAc)2 as a catalyst also promoted the nitroaldol reaction, affording almost quantitatively the expected 2-nitro-1-phenylethanol (98(±1)%) with 14.5(±1.5)% ee.

Solid-state NMR tools for the structural characterization of POSiSils: 29Si sensitivity improvement with MC-CP and 2D 29Si-29Si DQ-SQ at natural abundance.

The 1H–29Si multiple‐contact cross polarization (MC‐CP) MAS NMR experiment is evaluated for the class of silicate‐siloxane copolymers called POSiSils, that is, polyoligosiloxysilicones. It proves a reasonably good solution to tackle the challenge of recording quantitative 29Si NMR data in experimental time much reduced compared with single pulse acquisition. In a second time, we report 29Si–29Si MC‐CP double‐quantum single‐quantum (MC‐CP‐DQ‐SQ) NMR experiment, which provides information about the through‐space proximities between all silicon species despite the high degree of heterogeneity of this material. This work furthers the NMR tools for NMR crystallography for inorganic polymers, as it covers flexible polymers with different dimensionalities and long or heterogeneous relaxation characteristics at low 29Si natural abundance.

A Heptanuclear Copper Iodide Nanocluster

Nanoscale molecular clusters are attractive for the design of materials exhibiting original functions and properties. In particular, copper iodide clusters of high nuclearity are well-known for their stimuli-responsive luminescence properties. The synthesis and characterization of an unprecedented copper(I) iodide molecular cluster based on an original heptanuclear inorganic core are reported. This nanometer-size cluster is formulated as [Cu7I7(P(C6H4CF3)3)6(CH3CN)] and its novel structure has been characterized by X-ray diffraction and multinuclear solid-state 63Cu, 31P, 13C, 19F, and 1H NMR spectroscopy. The photoluminescence properties of this cluster have been studied at variable temperature. Density functional theory calculations have been performed on this large molecular structure and allow one to rationalize the observed luminescence properties. This study highlights the crucial role of cuprophilic interactions in molecular copper iodide clusters for exhibiting photoactive properties.