Iurii Dovgaliuk

A Composite of Complex and Chemical Hydrides Yields the First Al‐Based Amidoborane with Improved Hydrogen Storage Properties

The first Al-based amidoborane Na[Al(NH2 BH3 )4 ] was obtained through a mechanochemical treatment of the NaAlH4 -4 AB (AB=NH3 BH3 ) composite releasing 4.5 wt % of pure hydrogen. The same amidoborane was also produced upon heating the composite at 70 °C. The crystal structure of Na[Al(NH2 BH3 )4 ], elucidated from synchrotron X-ray powder diffraction and confirmed by DFT calculations, contains the previously unknown tetrahedral ion [Al(NH2 BH3 )4 ](-) , with every NH2 BH3 (-) ligand coordinated to aluminum through nitrogen atoms. Combination of complex and chemical hydrides in the same compound was possible due to both the lower stability of the AlH bonds compared to the BH ones in borohydride, and due to the strong Lewis acidity of Al(3+) . According to the thermogravimetric analysis-differential scanning calorimetry-mass spectrometry (TGA-DSC-MS) studies, Na[Al(NH2 BH3 )4 ] releases in two steps 9 wt % of pure hydrogen. As a result of this decomposition, which was also supported by volumetric studies, the formation of NaBH4 and amorphous product(s) of the surmised composition AlN4 B3 H(0-3.6) were observed. Furthermore, volumetric experiments have also shown that the final residue can reversibly absorb about 27 % of the released hydrogen at 250 °C and p(H2 )=150 bar. Hydrogen re-absorption does not regenerate neither Na[Al(NH2 BH3 )4 ] nor starting materials, NaAlH4 and AB, but rather occurs within amorphous product(s). Detailed studies of the latter one(s) can open an avenue for a new family of reversible hydrogen storage materials. Finally, the NaAlH4 -4 AB composite might become a starting point towards a new series of aluminum-based tetraamidoboranes with improved hydrogen storage properties such as hydrogen storage density, hydrogen purity, and reversibility.

Cooperative Adsorption by Porous Frameworks: Diffraction Experiment and Phenomenological Theory

Materials science of metal open frameworks is a state-of-the-art field for numerous applications, such as gas storage, sensors, and medicine. Two nanoporous frameworks, γ-Mg(BH4 )2 and MIL-91(Ti), with different levels of structural flexibility, were examined with in situ X-ray diffraction guest adsorption-desorption experiments. Both frameworks exhibit a cooperative guest adsorption correlated with a lattice deformation. This cooperativity originates from the long-range interactions between guest molecules, mediated by elastic response of the host porous structure. The observed experimental scenarios are rationalized with a mean field Gorsky-Bragg-Williams (GBW) approach for the lattice-gas Ising model. The adjusted GBW model, in combination with in situ synchrotron powder diffraction, demonstrates an efficient experimental and phenomenological approach to characterize thermodynamics of the adsorption in MOFs not only for the total uptake but also for every specific guest site.

Biporous Metal–Organic Framework with Tunable CO2/CH4 Separation Performance Facilitated by Intrinsic Flexibility

In this work, we report the synthesis of SION-8, a novel metal–organic framework (MOF) based on Ca(II) and a tetracarboxylate ligand TBAPy4– endowed with two chemically distinct types of pores characterized by their hydrophobic and hydrophilic properties. By altering the activation conditions, we gained access to two bulk materials: the fully activated SION-8F and the partially activated SION-8P with exclusively the hydrophobic pores activated. SION-8P shows high affinity for both CO2 (Qst = 28.4 kJ/mol) and CH4 (Qst = 21.4 kJ/mol), while upon full activation, the difference in affinity for CO2 (Qst = 23.4 kJ/mol) and CH4 (Qst = 16.0 kJ/mol) is more pronounced. The intrinsic flexibility of both materials results in complex adsorption behavior and greater adsorption of gas molecules than if the materials were rigid. Their CO2/CH4 separation performance was tested in fixed-bed breakthrough experiments using binary gas mixtures of different compositions and rationalized in terms of molecular interactions. SION-8F showed a 40–160% increase (depending on the temperature and the gas mixture composition probed) of the CO2/CH4 dynamic breakthrough selectivity compared to SION-8P, demonstrating the possibility to rationally tune the separation performance of a single MOF by manipulating the stepwise activation made possible by the MOF’s biporous nature.

Crystal structure of the new superconductor FeSe1−xSx

The discovery of superconductivity in iron-based pnictides and chalcogenides has been at the forefront of interest over the last few years [1,2]. Fe(Se,Te,S) compounds considered as the simplest Fe-based superconductors useful for study correlations between structural, electron, magnetic and superconducting properties. Among these materials FeSe1-xSх is the least studied compound and single crystal X-ray diffraction (XRD) experiments for it was not conducted. FeSe1-xSx (x = 0 – 0.2) single crystals were grown in evacuated quartz ampoules using the AlCl3/KCl flux technique [3] in a temperature gradient (from 400oC to ~50oC) for 45 days. Crystals have a platelike shape with the c axis oriented perpendicular to the crystal plane. Two samples with x = 0.03 and 0.09 were selected for single crystal synchrotron XRD measurements. Both of them were superconducting with Tc = 9.5 and 10.1 K respectively. The XRD data were collected in 90 – 300 K temperature range at the ESRF beamline BM01 using PILATUS@SNBL diffractometer (λ = 0.7458Å, PILATUS2M detector) equipped with Oxford Cryojet cryogenic nitrogen jet system. Complete single crystal XRD measurements were performed for good quality FeSe0.91S0.09 sample at the room temperature. Crystal structure of FeSe0.91S0.09 was refined in sp.gr. P4/n (a= 3.809(1), c= 5.529(1), R=3.5%). It was found that atoms of S and Se statistically occupy 2c position of the structure. Percentage ratio S/Se was defined in the result of the site occupancies refinement. Test XRD room temperature experement for FeSe0.97S0.03 crystal showed that sample was polycrystalline. Polycrystalline low temperature XRD measurements were performed in temperature interval 90300 K. Some additional peaks not corresponding to sp.gr. P4/n were revealed at temperatures below 170 K. The reported study was funded by Ministry of Education and Science of the Russian Federation (Project #14.616.21.0068). [1] Kamihara Y. et al.(2008). J. Am. Chem. Soc. 130, 3296-3297. [2] Hsu F.C. et al. (2008). Proc. Natl. Acad. Sci. U.S.A. 105 14262-14264. [3] Chareev D. et al. (2013). Cryst. Eng. Commun. 15, 1989-1993.