Phase and structural study of clathrate formation in the [Zn(MePy)2(NCS)2]-MePy system (MePy=4-methylpyridine)

A phase diagram of the [Zn(MePy)₂(NCS)₂]-MePy binary system has been studied by DTA and solubility methods. Two compounds melting incongruently (at 63 and 57°C) have been discovered in the system. They have been obtained as separate phases with crystals of different shape (needles and octahedra). Their composition has been determined by analytical methods and verified by X-ray structural analysis: [Zn(MePy)₄(NCS)₂]·0.67 MePy·xH₂O, wherex depends on the synthesis conditions, and [Zn(MePy)₄(NCS)₂]·MePy, respectively.The preliminary X-ray study of the first compound (at –100°C) has shown it to be isostructural with the known clathrates of the common formula [M(MePy)₄(NCS)₂]·0.67MePy·xH₂O, where M=Cu, Mn, Mg andx=0–0.33. The unit cell parameters are as follows:a=27.20(1),c=11.202(4) Å, space group ,Z=9.The X-ray study of [Zn(MePy)₄(NCS)₂]·MePy (at–50°C) has shown it to be analogous to the organic zeolite -phase with the guest MePy molecules located in the channels formed by molecular packing of the [Zn(MePy)₄(NCS)₂]host. The cell is tetragonal, the space groupI4₁/a,a=16.845(6),c=23.496(7) Å,V=6667(4) ų,Z=8,D _calc=1.289 g cm^–3,R=0.074. The zinc cation is surrounded by a slightly irregular octahedron of six nitrogen atoms of the MePy and NCS ligands. The crystallisation field of the host [Zn(MePy)₄(NCS)₂] complex in the temperature range concerned is absent in the phase diagram. It suggests contact stabilization of the [Zn(MePy)₄(NCS)₂] molecule by the guest in the clathrates. Supplementary Data relevant to this paper have been deposited with the British Library at Boston Spa, Wetherby, West Yorkshire, U.K. as Supplementary Publication No. SUP 82166 (15 pages).     

Direct observation of defect levels in InN by soft X-ray absorption spectroscopy

We have used synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to study the electronic structure of nitrogen-related defects in InN(0001). Several defect levels within the band gap or the conduction band of InN were clearly resolved in NEXAFS spectra around the nitrogen K-edge. We attribute the level observed at 0.3 eV below the conduction band minimum (CBM) to interstitial nitrogen, the level at 1.7 eV above the CBM to antisite nitrogen, and a sharp resonance at 3.2 eV above the CBM to molecular nitrogen, in full agreement with theoretical simulations.     

Noncatalytic hydrogenation of decene-1 with hydrogen accumulated in a hybrid carbon nanostructure in nanosized membrane reactors

Studies on the creation of nanosized membrane reactors (NMRs) of a new generation with accumulated hydrogen and a regulated volume of reaction zone were continued at the next stage. Hydrogenation was performed in the pores of ceramic membranes with hydrogen preliminarily adsorbed in mono- and multilayered orientated carbon nanotubes with graphene walls (OCNTGs)—a new hybrid carbon nanostructure formed on the inner pore surface. Quantitative determination of hydrogen adsorption in OCNTGs was performed using TRUMEM ultrafiltration membranes with D _av = 50 and 90 nm and showed that hydrogen adsorption was up to ～1.5% of the mass of OCNTG. The instrumentation and procedure for noncatalytic hydrogenation of decene-1 at 250–350°C using hydrogen accumulated and stored in OCNTG were developed. The conversion of decene-1 into decane was ～0.2–1.8% at hydrogenation temperatures of 250 and 350°C, respectively. The rate constants and activation energy of hydrogenation were determined. The latter was found to be 94.5 kJ/mol, which is much smaller than the values typical for noncatalytic hydrogenations and very close to the values characteristic for catalytic reactions. The quantitative distribution of the reacting compounds in each pore regarded as a nanosized membrane reactor was determined. The activity of hydrogen adsorbed in a 2D carbon nanostructure was evaluated. Possible mechanisms of noncatalytic hydrogenation were discussed.     

X-ray diffraction study of the clathrate [MnPy4(NO3)2)]·2Py and complex [MnPy3(NO3)2] formed in the pyridine (Py)-manganese nitrate system

A clathrate compound [MnPy₄(NO₃)₂]·2Py was synthesized and studied by single-crystal X-ray diffraction analysis (KM-4 diffractometer, CuK_α,ω/2? scan mode,? _max=78°). The unit cell is base-centered orthorhombic, space group Ccca, a=12.097(4), b=15.197(4), c=17.213(4) Å, V=3164(2) ų, Z=4, R=0.072, for 632 reflections and 107 parameters in the least-squares analysis. The host molecule has 222 symmetry. Two monodentate nitrato groups and four pyridine ligands are coordinated to Mn(II) along the symmetry axes (the former via O). The pyridine guest molecules lie in the cavities of the crystal framework. For [MnPy₃(NO₃)₂)], only unit cell parameters were determined: a=12.467(5), b=9.514(2), c=16.383(5) Å,β=108.93(4), V=1838(1) ų space group C2/c or Cc (according to extinctions). The pyridine-manganese nitrate system is shown to be analogous to the previously investigated Co, Ni, Cu, Zn, and Cd systems.     

Research and application of inorganic selective sorbents at Mayak PA

This work has been performed in order to identify selective inorganic sorbents for caesium and strontium. Thin-layer sorbents with nickel ferrocyanide embedded in an inert matrix were found to be the best for caesium. Sorbents including non-stoichiometric manganese dioxide were selected for strontium. Bench tests have been carried out on the purification of desalted water of SNF storage-pool from ¹³⁷Cs, and on the purification of contaminated natural water from ⁹⁰Sr. The facility for synthesizing the ferrocyanide sorbent with the registered mark ‘Seleks-CFN’ has been brought into operation. The sorbent ISM-S seems promising for ⁹⁰Sr decontamination. To cite this article: M.V. Logunov et al., C. R. Chimie 7 (2004).

Résumé

Recherches et applications sur l’utilisation de sorbants spécifiques à Mayak. Des sorbants sélectifs du césium et du strontium ont été identifiés. Pour le césium, le meilleur s’avère être le ferrocyanure de nickel, s’il est utilisé sous forme de couche mince. Pour le strontium, on a sélectionné des composés non stœchiométriques de dioxyde de manganèse. On a mené des tests visant à épurer en césium l’eau des piscines d’entreposage des combustibles nucléaires usagés, et en strontium des eaux contaminées. Une installation industrielle permettant de préparer le sorbant à base de ferrocyanure de marque « Seleks-CFN » a été réalisée. Le sorbant ISM-S semble prometteur pour la décontamination en strontium. Pour citer cet article : M.V. Logunov et al., C. R. Chimie 7 (2004).     

The depth of nanocrystallites of pyrocarbon deposition in pores of ultrafiltration membranes and its influence on their efficiency

Scanning electron microscopy and energy dispersion spectrometry were used to determine the depth of the deposition of pyrocarbon nanocrystals (PNCs) on the pore surface of TRUMEM inorganic ultrafiltration membranes (D _pore = 50 nm) depending on the pressure of the gas subjected to pyrolysis (methane). It was shown that, while the deposition of PNCs occurred over the whole pore length equal to the selective layer thickness (20 μm) at 4.9 kPa methane pressure, an increase in pressure to 40.0 kPa decreased the deposition depth to 5.0 μm. Simultaneously, the λ/D ratio decreased from 94 to 11. At a pressure increased to 100.0 kPa, the depth of PNC deposition was ～1.5 μm (λ/D = 5). Changes in the depth of PNC deposition and the λ/D ratio depending on pressure were similar in character and could be described by exponential dependences. With PNCs deposited ～1.5 nm deep (100 kPa), membranes with pore size 8–17 nm were obtained. Their efficiency, for instance, for ethanol, was 50–75% of that of the initial membranes. The diameter of pores decreased by 3–6 times.     

Revisiting the nature of Cu sites in the activated Cu-SSZ-13 catalyst for SCR reaction

Cu-SSZ-13 is a highly active NH₃-SCR catalyst for the abatement of harmful nitrogen oxides (NO_x, x = 1, 2) from the exhausts of lean-burn engines. The study of Cu-speciation occurring upon thermal dehydration is a key step for the understanding of the enhanced catalytic properties of this material and for identifying the SCR active sites and their redox capability. Herein, we combined FTIR, X-ray absorption (XAS) and emission (XES) spectroscopies with DFT computational analysis to elucidate the nature and location of the most abundant Cu sites in the activated catalyst. Different Cu species have been found to be dominant as a function of the dehydration temperature and conditions. Data analysis revealed that the dehydration process of Cu cations is essentially completed at 250 °C, with the formation of dehydrated [CuOH]⁺ species hosted in close proximity to 1-Al sites in both d6r and 8r units of the SSZ-13 matrix. These species persist at higher temperatures only if a certain amount of O₂ is present in the gas feed, while under inert conditions they undergo virtually total “self-reduction” as a consequence of an OH extra-ligand loss, resulting in bi-coordinated bare Cu⁺ cations. Synchrotron characterization supported by computational analysis allowed an unprecedented quantitative refinement of the local environment and structural parameters of these Cu(ii) and Cu(i) species.     

Electronic structure of RbCl and Rb2ZnCl4in the solid and the liquid state: Analysis of the local fine structure of x-ray absorption

Experimental absorption spectra were obtained near the absorption K edge of Rb in RbCl and Rb₂ZnCl₄ in the solid state and in a melt. Detailed analysis of the electronic structure of RbCl was conducted by means of the total multiple scattering method, and good agreement with the experimental spectrum was obtained. The fine structure of the unoccupied electronic states was determined in the conduction band of the compound studied. The unoccupied p states of rubidium were shown to be hybridized, for the most part, with the Cl p states in the conduction band. It was proved that the RbCl melts can be described in good approximation on the basis of the cluster model with the Debye-Waller parameter σ²=0.3 Å^?2.     

Processes of Thermal Dissociation of Clathrates on the Base of Coordination Compounds

Inclusion compounds are very interesting materials for thermoanalytical investigations. Stable guest molecules are packed in the cages, channels or layers in the host matrix, thermal dissociation reactions are reversible and have the clear-cut stoichiometry. The clathrates with a matrix of coordination compounds (of the type [MA₄X₂]·nA) were synthesized (M=transition metals; A=Py, 4-MePy; X=NO₃, NCS, NCO; n=0.67 or 1 or 2). Four Py or MePy molecules are tightly bound in the coordination sphere by M-N bonds and others are confined in the matrix cavities by van der Waals forces. Processes of thermal dissociation of the clathrates were studied. Q-thermogravimetry (for checking the stable intermediate phases) and the home-made gas-flow reactor (as EGA-device for kinetic studies) were used. The compensation dependence (lgA_i=aE_i+b) was observed for the most of clathrates series, it was explained by the identity of the reaction mechanism. This revised version was published online in August 2006 with corrections to the Cover Date.     

Surface modification of epoxy resin by amphiphilic fluoroorganosiloxane copolymers

Russian Chemical Bulletin - An amphiphilic fluoroorganosiloxane copolymer designed for a surface modification of epoxy resin by fluorine-containing fragments has been synthesized. The copolymer...