Suppression of the reversible thermal behavior of the layered double hydroxide (LDH) of Mg with Al: stabilization of nanoparticulate oxides

The layered double hydroxide of Mg with Al decomposes below 600 degrees C with the loss of nearly 48% mass, resulting in the formation of an oxide residue having the rock salt structure and nanoparticulate morphology. However, this product reconstructs back into the parent LDH, owing to its compositional and morphological metastability. The oxide can be kinetically stabilized within an amorphous phosphate network built up through an ex situ reaction with a suitable phosphate source such as (NH4)H2PO4. This oxide transforms into a thermodynamically more stable phase with a spinel structure on soaking in an aqueous medium. The oxide residue has a nanoparticulate morphology as revealed by the Scherrer broadening of the Bragg reflections as well as by electron microscopy. This work shows that the hydroxide reconstruction reaction and spinel formation are competing reactions. Suppression of the former catalyzes spinel formation as the excess free energy of the metastable oxide residue is unlocked to promote the diffusion of Mg2+ ions from octahedral to tetrahedral sites, which is the essential precondition to the formation of a normal spinel. This reaction taking place as it does at ambient temperature and in solution helps in the retention of a nanostructured morphology for the spinel. Another way of stabilizing the oxide is by incorporating the thermally stable borate anion into the LDH. This paves the way for an in situ reaction between the cations of the host LDH and the borate guest. The in situ reaction directly leads to the formation of an oxide with a spinel structure.     

Review of synthesis and various biological activities of spiro heterocyclic compounds comprising oxindole and pyrrolidine moities

Spirocyclic compounds isolated from plant and animal origins have important applications in medicinal chemistry. Spiro compounds having cyclic structures fused at a central carbon are of recent interest because of their interesting conformational features and their structural implications for biological systems. Spiro heterocycles like spiropyrrolidines and oxindole moieties have been found to play fundamental roles in biological processes and have exhibited diversified biological activity and pharmacological and therapeutical properties. In view of these facts we decided to present a representative synthetic scheme and various biological activities of the heterocycles like spiropyrrolidines and oxindole moieties derivatives, especially in relation to microbial infections like cancer and tubercular, viral, HIV, bacterial, and fungal infections.     

New polymorphs of alumina

Abstract

We present the results of high-pressure, high-temperature studies on xerogel gamma alumina which is made up of nanocrystalline alumina and an amorphous phase. Not many studies of this type have been reported on xerogel materials. At about 5 GPa and 1400°C, the xerogel alumina transforms into a polymorphic mixture containing α Al₂O₃, B Al₂O₃ and C Al₂O₃ where the last two phases have B Ln₂O₃ and C Ln₂O₃ type structures respectively. Here Ln stands for the rare earths. The xerogel alumina containing 1 wt.% Cr₂O₃ under these conditions transforms into a polymorphic mixture containing Al₂O₃, k' Al₂O₃ and H Al₂O₃ where H Al₂O₃ has the H Ln₂O₃ type structure. For the first time, the observation of rare earth sesquioxide structures of alumina is reported. At about 5 GPa at 27°C, the xerogel aluminal transforms to an amorphous phase. The results are discussed using the free energy diagram for xerogels proposed by R. Roy (J. Amer. Cer. Soc., 52, 344, 1969; 67, 468, 1984).     

Raney Nickel–Catalyzed Hydrogenation of Unsaturated Carboxylic Acids with Sodium Borohydride in Water

A mild, selective, and green method for the reduction of unsaturated carboxylic acids with sodium borohydride–Raney nickel (W6) system in water is reported. This method is practical and safe and avoids use of organic solvents.     

Copper(0) in the Ullmann heterocycle-aryl ether synthesis of 4-phenoxypyridine using multimode microwave heating

The action of nanoparticulate copper catalysts with a mean particle size of 10 nm in the Ullmann ether synthesis is reported using multimode microwave heating and employing stable chloropyridine salts and unactivated phenol, with stabilized copper nanoparticles outperforming other copper catalysts in terms of stability and reusability.     

Optimized AC conductivity correlated to structure,morphology and thermal properties of PVDF/PVA/Nafion composites

Polyvinylidene fluoride (PVDF) and polyvinyl alcohol (PVA) composites were prepared by controlled loading of Nafion (5 to 15 wt%) by solution casting using water and dimethylformamide (DMF) as a solvent. The surface morphology of composite analyzed by atomic force microscopy (AFM) reveals the presence of Nafion ionomers. The increase in interlayer spacing of modified PVDF/PVA polymer system as a function of Nafion was detected by X-ray diffraction (XRD). The major change in Fourier transform infrared (FTIR) spectroscopy confirms the chemical bond C=O stretching around 1,700 cm^?1 due to Nafion. Differential scanning calorimetry (DSC) demonstrates the thermal stability of polymer composites and the decrease in melting temperature (T _m). The optimized AC conductivity (σ) of the prepared composite was evaluated by using an impedance analyzer as a function of temperature (40 to 150 °C) at constant 30-MHz frequency. The highest conductivity of 1.3?×?10^?2 S m^?1 was observed at 80 °C for 10 wt% of Nafion and correlated with structure, morphology and thermal properties of modified PVDF/PVA/Nafion composites. The experimental results may be useful for sensors, fuel cells and battery application domains.     

Scaling of x-ray emission and ion velocity in laser produced Cu plasmas

The x-ray emission from slab targets of copper irradiated by Nd:glass laser (1.054 μm, 5 and 15 ns) at intensities between 10¹² and 10¹¹W/cm² has been studied. The x-ray emissions were monitored with the help of high quantum efficiency x-ray silicon photo diodes and vacuum photo diodes, all covered with aluminium filters of different thickness. The x-ray intensity vs the laser intensity has a scaling factor of (1.2–1.92). The relative x-ray conversion efficiency follows an empirical relationship which is in close agreement with the one reported by Babonneau et al. The ion velocities were monitored using Langmuir probes placed at different angles and radial distances from the target position. The variation of the ion velocity with the laser intensity follows a scaling of the form Φ ^β where β ∼0.22 which is in good agreement with the reported scaling factor values. The results on the x-ray emission from Cu plasma are reported.     

The effect of ‘crystallinity’ and structural disorder on the electrochemical performance of substituted nickel hydroxide electrodes

β_bc-Nickel hydroxide exhibit non-uniform broadening reflections in their PXRD pattern due to the presence of structural disorder. β_bc-Nickel hydroxide electrodes with smaller crystallite size and structural disorder reversibly exchanges 0.9e/Ni. Co/Zn/Ca/Cd-substituted β_bc-nickel hydroxide samples also display non-uniform broadening of reflections in their powder X-ray diffraction patterns with smaller crystallite size and exchanges 0.7–0.8e/Ni. Hydrothermal treatment of β_bc-nickel hydroxide slurry at 170 °C results in an ordering of reflections in their powder X-ray diffraction pattern with an increased crystallite size. Crystalline β-nickel hydroxide electrode reversibly exchanges 0.3–0.4e/Ni. Hydrothermal-treated Co/Zn/Ca/Cd-substituted β_bc-nickel hydroxide slurries at 170 °C display sharp reflections with similar crystallite size and electrochemical activities as that of crystalline β-nickel hydroxide. This clearly demonstrates that partial substitution of Co/Zn/Ca/Cd in the nickel hydroxide matrix does not show any dramatic improvement in their electrochemical activity at 25–30 °C. Structural disordered material with smaller crystallite size delivers electrochemical activity close to theoretical capacity.     

Photolysis of MAg4I5 superionic films

Photolysis studies on MAg₄I₅ films are reported. The electrical conductivities of MAg₄I₅ films have been found to change when exposed to mercury light. This is explained on the basis of a model similar to Mott-Gurney theory of photolysis for silver halides. The formation of silver specks has been confirmed by photomicrographic studies.