Unique and contrasting mixed-metal lithium–calcium and lithium–barium hexamethyldisilazide complexes

Addition of lithium hexamethyldisilazide to calcium or barium bis(hexamethyldisilazide) in THF resulted in the synthesis of two unique but very different mixed-metal complexes: X-ray crystallography shows these to be, respectively, the heterobimetallic complex [Ca{N(SiMe₃)₂}₃Li(THF)] (1), containing two calcium–lithium bridging amide ligands and the remarkable co-crystalline compound [Ba{N(SiMe₃)₂}₂(THF)₃][Li₂{N(SiMe₃)₂}₂(THF)₂] (2).     

Hydrothermal synthesis and structure of three novel open-framework lanthanide sulfate–oxalates

Three novel hybrid open-framework lanthanide sulfate–oxalates, {[NH₄][Ln(H₂O)(SO₄)(C₂O₄)]}_n [Ln = Y (I), La (II), Sm (III)] have been synthesized via hydrothermal reaction, and characterized by single crystal X-ray diffraction, powder X-ray diffraction, infrared spectrophotometry, thermal gravimetric analysis and fluorescence analysis. These three compounds were isostructural, and all crystallized into the monoclinic system with space group of P2₁ / n. In their structures, LnO₈ dodecahedra, SO₄ tetrahedral and C₂O₄ groups are linked to give rise to a three-dimensional open-framework, which contains two kinds of 12-membered ring channel systems running along the a and b axis, respectively.     

Sol–gel synthesis and characterization of lithium aluminate (L–A–H) and lithium aluminosilicate (L–A–S–H) gels

Hydrous lithium aluminosilicate (L–A–S–H) and lithium aluminate (L–A–H) gels are candidate precursors for glass-ceramics and ceramics with potential advantages over conventional processing routes. However, their structure before calcination remained largely unknown, despite the importance of precursor structure on the properties of the resulting materials. In the present study, it is demonstrated that L–A–S–H and L–A–H gels with Li/Al ≤ 1 can be produced via an organic steric entrapment route, while higher Li/Al ratios lead to crystallization of gibbsite or nordstrandite. The composition and the structure of the gels was studied by thermogravimetric analysis, X-ray diffraction, ²⁷Al and ²⁹Si magic-angle spinning nuclear magnetic resonance, and Raman spectroscopy. Aluminium was found to be almost exclusively in six-fold coordination in both the L–A–H and the L–A–S–H gels. Silicon in the L–A–S–H gels was mainly in Q⁴ sites and to a lesser extent in Q³ sites (four-fold coordination with no Si–O–Al bonds). The results thus indicate that silica-rich and aluminium-rich domains formed in these gels.     

Copper salt–crown ether systems as catalysts for the oxidation of isopropyl arenes with tertiary hydroperoxides to peroxides

Symmetrical and unsymmetrical peroxides can be obtained by the oxidation of isopropyl arenes with tertiary hydroperoxides in the presence of copper salt–crown ether as catalysts. The efficiency of the investigated catalytic system enhances markedly in the presence of some alkali metal salts, with a cation diameter corresponding to the inner diameter of the crown ether. The oxidation of isopropyl arenes with hydroperoxides in the presence of a copper salt–crown ether–alkali metal salt catalytic system proved to be a simple and efficient method for the synthesis of ditertiary peroxides. A possible mechanism of this phenomenon is discussed. The results obtained confirm strongly that phase transfer catalysis operates in the investigated reaction.     

‘Green’ synthesis and optical properties of silver–chitosan complexes and nanocomposites

Silver nanoparticles were synthesized in a chitosan biopolymer by an in situ ‘green’ chemical procedure, using d-glucose as the reducing agent. The reaction intermediates (silver–chitosan complexes) as well as the obtained nanocomposites were investigated using transmission electron microscopy, UV–vis, FTIR and photoluminescence spectroscopy. The theoretical analysis of the UV–vis absorption of the Ag–chitosan complexes suggested that the significant contribution to the complex spectrum arises from clusters of silver containing 4–9 atoms. The absorption spectrum of the nanocomposite exhibited a strong surface plasmon resonance band at 406 nm. The photoluminescence behavior of the pure chitosan, the silver–chitosan complexes and the nanocomposites were discussed in terms of morphology and silver weight content.     

In situ sonochemical synthesis of Fe3O4–graphene nanocomposite for lithium rechargeable batteries

In this paper, we have presented experimental results for preparation of Fe₃O₄–graphene nanocomposite that uses an ultrasound assisted method. The graphene oxide (GO) was prepared from graphite powder using modified Hummers–Offeman method. Subsequently, the synthesis of graphene-Fe₃O₄ nanocomposite was carried out by ultrasound assisted co-precipitation of iron (II) and (III) chlorides in the presence of GO. The formation of GO and graphene-Fe₃O₄ nanocomposite was confirmed by X-ray diffraction (XRD), Energy dispersive X-ray (EDX) analysis and Fourier transform-infrared (FTIR) analysis. The particle size of Fe₃O₄ nanoparticles loaded on graphene nanosheets (observed from TEM image) was found to be smaller than 20 nm. The use of ultrasonic irradiations during synthesis of graphene-Fe₃O₄ nanocomposite resulted in uniform loading of Fe₃O₄ nanoparticles on graphene nanosheets. The prepared graphene-Fe₃O₄ nanocomposite material was used for the preparation of anode for lithium ion batteries. The electrochemical performance of the material was tested by cyclic voltammetry (CV) and charge/discharge cycles. It was observed that the capacity of Li-battery when the anode material was made using graphene-Fe₃O₄ nanocomposite showed stable electrochemical performance for around 120 cycles and the battery could repeat stable charge–discharge reaction.     

Chemical-free synthesis of graphene–carbon nanotube hybrid materials for reversible lithium storage in lithium-ion batteries

Graphene–carbon nanotube hybrid materials were successfully prepared through the π–π interaction without using any chemical reagent. We found that the ratio between carbon nanotube and graphene had critical influences on the state in aqueous solution and morphology of hybrid materials. Field emission scanning electron microscope and transmission electron microscope analysis confirmed that graphene nanosheets wrap around individual carbon nanotubes and form a homogeneous three-dimensional hybrid nanostructure. When applied as an anode material in lithium ion batteries, graphene–carbon nanotube hybrid materials demonstrated a high reversible lithium storage capacity, a high Coulombic efficiency and an excellent cyclability.     

Controlled synthesis and optical spectroscopy of lanthanide-doped KLaF₄ nanocrystals

KLaF(4), as a good host matrix for trivalent lanthanide (Ln(3+)) ions to fabricate upconversion (UC) or downconversion (DC) phosphors, has been rarely reported. Herein, monodisperse (～10 nm) cubic-phase Ln(3+)-doped KLaF(4) nanocrystals (NCs) were synthesized via a facile thermal decomposition method. Upon excitation at 980 nm, UC luminescence properties of KLaF(4):Ln(3+)/Yb(3+) (Ln = Tm, Ho, Er) NCs were comprehensively surveyed. Particularly, after coating an inert KLaF(4) shell, the green and red UC luminescence intensity of KLaF(4):Er(3+)/Yb(3+) NCs was enhanced ～35 times, and the corresponding UC lifetimes of (4)S(3/2) and (4)F(9/2) levels of Er(3+) were observed significantly prolonged from 42 and 68 μs in core-only NCs to 87 and 136 μs in core/shell counterparts. Furthermore, intense DC luminescence was also achieved in Ce(3+)/Tb(3+) and Eu(3+) doped KLaF(4) NCs, with absolute quantum yields of 39.8% (Tb(3+)) and 17.3% (Eu(3+)). The luminescence lifetimes of (5)D(0) (Eu(3+)) and (5)D(4) (Tb(3+)) were determined to be 4.2 and 4.7 ms, respectively. Water-soluble Ln(3+)-doped KLaF(4) NCs featuring excellent monodispersion, long luminescence lifetime, and high UC/DC efficiency may have versatile and promising applications as luminescent nano-biolabels.     

Three lanthanide–nitronyl nitroxide complexes: Syntheses,crystal structures and magnetic properties

Three rare earth–nitronyl nitroxide radical complexes [Ln(hfac)₃(NIT-1′-MeBzIm)] (Ln(III) = Gd 1, Tb 2, Dy 3; hfac = hexafluoroacetylacetonate; NIT-1′-MeBzIm = 2-{2′-[(l′-methyl)-benzimidazolyl]}-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) have been successfully prepared and structurally characterized. X-ray crystallographic analysis reveals that all the three complexes are isomorphous. The center lanthanide(III) ions are eight-coordinated to one NIT-1′-MeBzIm and three hexafluoroacetylacetonate ligands. The NIT-1′-MeBzIm acts as a bidentate ligand via its nitrogen atom of the imidazole ring and the oxygen atom of the N–O group. The magnetic properties of complexes 1, 2 and 3 were studied. All of them exhibit ferromagnetic interaction. AC magnetic susceptibility studies show that Tb complex displays frequency-dependent signals at low temperature suggesting that it behaves as SMM.     

Effect of adsorption–desorption of reaction mixture components on ethyl-tert-butyl ether synthesis over commercial sulfonic acid resins

Effect of adsorption properties of commercial sulfonic acid resins of different morphology on the trend of temperature dependence of ethyl-tert-butyl ether (ETBE) yield in a reaction between ethanol and iso-butylene at elevated pressure studied. It has been shown that porous structure of a catalyst affects significantly the adsorption–desorption dynamics of reaction components. Consequently, at low temperatures the yield of ETBE is limited by adsorption–desorption dynamics, whereas at high temperatures it is restricted by thermodynamics. That results in the appearance of a maximum on the experimental dependence of the ETBE yield on temperature.     

Large-scale synthesis of ordered mesoporous carbon fiber and its application as cathode material for lithium–sulfur batteries

A novel type of one-dimensional ordered mesoporous carbon fiber has been prepared via the electrospinning technique by using resol as the carbon source and triblock copolymer Pluronic F127 as the template. Sulfur is then encapsulated in this ordered mesoporous carbon fibers by a simple thermal treatment. The interwoven fibrous nanostructure has favorably mechanical stability and can provide an effective conductive network for sulfur and polysulfides during cycling. The ordered mesopores can also restrain the diffusion of long-chain polysulfides. The resulting ordered mesoporous carbon fiber sulfur (OMCF-S) composite with 63% S exhibits high reversible capacity, good capacity retention and enhanced rate capacity when used as cathode in rechargeable lithium–sulfur batteries. The resulting OMCF-S electrode maintains a stable discharge capacity of 690 mAh/g at 0.3 C, even after 300 cycles.     

Catalytic kinetics of dimethyl ether one-step synthesis over CeO_2–CaO–Pd/HZSM-5 catalyst in sulfur-containing syngas process

CeO_2–CaO–Pd/HZSM-5 catalyst was prepared for the dimethyl ether(DME) one-step synthesis in a continuous fixed-bed micro-reactor from the sulfur-containing syngas. The catalytic stability over hybrid catalyst of CeO_2–CaO–Pd/HZSM-5 was investigated to ensure that the kinetics experimental results were not significantly influenced by induction period and catalytic deactivation. A large number of kinetic data points(40 sets) were obtained over a range of temperature(240–300 °C), pressure(3–4 MPa), gas hourly space velocity(GHSV)(2000–3000 L·kg~(-1)·h~(-1)) and H_2/CO mole ratio(2–3). Kinetic model for the methanol synthesis reaction and the dehydration of methanol were obtained separately according to reaction mechanism and Langmuir–Hinshelwood mechanism. Regression parameters were investigated by the method combining the simplex method and Runge–Kutta method. The model calculations were in appropriate accordance with the experimental data.     

Controlled synthesis of Bi2O3–YSZ composite powders and their sintering behavior for high-performance electrolytes

Bismuth oxide (Bi₂O₃) is a promising additive to decrease the sintering temperature of yttria-stabilized zirconia (YSZ)-based electrolyte for solid oxide fuel cell application. However, Bi₂O₃ tends to grow into large column bars (>50 µm) in a chemical coprecipitation method, which dramatically limits the mixing uniformity of Bi₂O₃ and YSZ, even much worse than that of mechanical mixing. In this study, the reaction temperature was increased from room temperature to 90°C to increase the number of nucleation during the violate reaction between Bi³⁺ solution and YSZ suspension in NaOH. On this basis, the violence of the reaction was further moderated by adding half of NaOH first, then YSZ powders and the other half of an NaOH solution. The size of Bi₂O₃ was further decreased to sub-micrometer and Bi₂O₃ was homogeneously mixed with YSZ particles, even when its addition amount was as large as 20 mol%. These composite powders effectively promoted the sintering behavior of YSZ. The sintering temperature of YSZ was decreased to 900 and 1000°C with 10 and 5 mol% Bi₂O₃ doping, respectively. Increasing the doping ratio induced severe volatilization of Bi₂O₃ and pore formation. Raising the sintering temperature (no more than 1200°C) enhanced the doping effect of Bi₂O₃ into the YSZ lattice but induced instability in the YSZ crystal structure.     

Sol–gel preparation and characterization of nano-crystalline lithium–mica glass–ceramic

The nano-crystalline lithium–mica glass–ceramic with separated crystallite size of 13 nm was prepared using sol–gel technique. In such a process, the structural evolutions and microstructural characteristics of the synthesized samples were investigated through X-ray diffraction, transmission electron microscopy, thermal analysis and Fourier transform infrared spectroscopy. It was found that the crystallite size of the mica obtained from sol–gel method is smaller than the one synthesized via conventional melted method. The XRD results also showed that the crystallization of mica occurred above 675 °C and it could originate from MgF₂ so that the next stage will also be the transformation from mica to norbergite and norbergite to chondrodite. The activation energy of the crystallization and Avrami factor were measured as 376.7 kJ mol^?1 and 2.3, respectively. It is found that the bulk crystallization could be considered as the predominant crystallization mechanism for the glass–ceramic.     

Shuttle inhibitor effect of lithium perchlorate as an electrolyte salt for lithium–sulfur batteries

Lithium perchlorate (LiClO₄), used as an electrolyte salt for lithium–sulfur batteries, has been shown to give rise to the effective inhibition of the chemical polysulfide shuttle and thereby to enhance the coulombic efficiency through the stabilized charge process. In 1,2-dimethoxy ethane (DME)/1,3-dioxolane (DOL) (50:50 by volume), LiClO₄ showed the lowest charge-transfer resistance among the various lithium salts studied and demonstrated the highest coulombic efficiency with an extreme reduction in the polysulfide shuttle. The origin of this behavior is considered to be the rapid formation of a passivation layer on the surface of the lithium metal anode. Hence, as well as being a good electrolyte salt in itself, LiClO₄ is shown to be an effective polysulfide shuttle inhibitor.     

Two new mononuclear tri-spin lanthanide–nitronyl nitroxide radical complexes: Syntheses,structure and magnetic properties

Based on a new nitronyl nitroxide radical, 2-(benzo[b]thiophen-2-yl)-4,4,5,5-tetramethylimidazolin-1-oxyl-3-oxide (NITThioPh), two new mononuclear tri-spin complexes [Ln(hfac)₃(NITThioPh)₂] (Ln = Gd (1), Tb (2); hfac = hexafluoroacetylacetonate) are successfully synthesized and fully characterized. Complexes 1 and 2 are isostructural and crystalize in P2₁/c space group, in which the central metal ions are eight-coordinated in distorted triangular dodecahedron LnO₈ coordination geometries (D_2d symmetry) finished by three bischelate hfac⁻ ligands and two monodentate radicals. Magnetic studies on 1 suggest the coexistence of ferromagnetic nearest-neighbor (NN) coupling between Gd(III) ions and radical spins and antiferromagnetic next-nearest-neighbor (NNN) interaction between intramolecular radical spins. In addition, antiferromagnetic ordering below 1.8 K was detected in the Tb(III) complex.     

Hydrothermal synthesis of a monoclinic VO2 nanotube–graphene hybrid for use as cathode material in lithium ion batteries

The hydrothermal reaction of a mixture of a colloidal dispersion of graphite oxide and ammonium vanadate yielded a hybrid made of graphene and a nanotubular metastable monoclinic polymorph of VO₂, known as VO₂(B). The formation of VO₂(B) nanotubes is accompanied by the reduction of graphite oxide. Initially the partially scrolled graphite oxide layers act as templates for the crystallization of VO₂(B) in the tubular morphology. This is followed by the reduction of graphite oxide to graphene resulting in a hybrid in which VO₂(B) nanotubes are dispersed in graphene. Electron microscopic studies of the hybrid reveal that the VO₂(B) nanotubes are wrapped by and trapped between graphene sheets. The hybrid shows potential to be a high capacity cathode material for lithium ion batteries. It exhibits a high capacity (～450 mAh/g) and cycling stability. The high capacity of the hybrid is attributed to the interaction between the graphene sheets and the VO₂(B) tubes which improves the charge-transfer. The graphene matrix prevents the aggregation of the VO₂(B) nanotubes leading to high cycling stability.     

Facile synthesis of hierarchical MoS₂ microspheres composed of few-layered nanosheets and their lithium storage properties

In this work, we demonstrate an interesting polystyrene microsphere-assisted synthesis of hierarchical MoS(2) spheres composed of ultrathin nanosheets. The as-prepared sample exhibits promising lithium storage properties with improved cyclic capacity retention and rate capability.     

Paal–Knorr synthesis of pyrroles: from conventional to green synthesis

The pyrrole molecular framework is found in a large number of natural and synthetic compounds of great importance. Since functionalized pyrroles are essential for the progress of many branches of science, its synthesis by simple, efficient and eco-friendly routes are particularly attractive in modern organic and bio-organic chemistry. To this end, a number of synthetic methods have been developed, in which the Paal–Knorr pyrrole synthesis stands out to be the easiest route to synthesize pyrroles. In spite of the efficiency, Paal–Knorr synthesis of pyrroles is considered limited by harsh reaction conditions, such as prolonged heating in acid, which may degrade sensitive functionalities in many potential precursors. Through this route almost all dicarbonyls can be converted to their corresponding heterocycles and therefore it is a synthetically valued process. To address the adverse issues this reaction route has undergone numerous modifications recently and today it can be said that this reaction route is a prominent green route for the synthesis of pyrroles. This review is a tour from the evolution and application of this harsh synthetic route to the eco-friendly greener route developed for the synthesis of pyrroles.     

Preparation and chemical properties of π-conjugated poly(1,10-phenanthroline-3,8-diyl)s with crown ether subunits

π-Conjugated polymers consisting of 1,10-phenanthroline units and crown ether subunits (Poly-1, Poly-2, and Poly-3) were prepared by dehalogenation polycondensation of the corresponding dibromo monomers using a zero-valent nickel complex as a condensing agent. They were characterized by elemental analysis, ¹H NMR and UV–Vis spectroscopies, and cyclic voltammetry (CV). They were partly soluble in CHCl₃, and the number average molecular weight of the soluble part of Poly-2, which had 15-crown-5 subunits, was estimated to be 5300. The polymers exhibited UV–Vis peaks at approximately λ_max = 360 nm, which was reasonable. Complexation with [Ru(bpy)₂]²⁺ and alkaline metal ions made the polymer soluble in organic solvents. The complexation of [Ru(bpy)₂]²⁺ to the 1,10-phenanthroline unit proceeded quantitatively, and the [Ru(bpy)₂]²⁺ complexes exhibited CV curves characteristic of [Ru(N-N)₃]²⁺ complexes.