The electrochemical and chemical oxidation of the defect perovskite SrFeO{sub 2.5} to the cubic perovskite SrFeO{sub 3} at ambient temperature in alkaline electrolyte are shown to be in principle reversible processes with the appearance of intermediate compounds; the oxidation can be described by the formation of line phases SrFeO{sub 2.75} and SrFeO{sub 2.87} and one solid solution range SrFeO{sub 3{minus}x} with 0 {le} x {le} 0.03. The structure of the brownmillerite type parent phase is characterized by the presence of stacking faults in the octahedral and tetrahedral sequence of layers whose concentration increases significantly upon formation of the intermediate phases. Models based on observed microscopic features are discussed in order to explain the unusual phenomenon of room temperature oxygen transport.

Room Temperature Electrochemical Redox Reactions of the Defect Perovskite SrFeO2.5+x

Kinetic study of liquid-phase glycerol hydrogenolysis over Cu/SiO2 catalyst

Crown ethers have proved to be unique cyclic molecules for molecular recognition of suitable substrates by hydrogen bonds, ionic interactions, and/or hydrophobic interactions. The study of interactions involved in complexation of different cations with crown ethers in mixtures of solvents is important for a better understanding of the mechanism of biological transport, molecular recognition, and other analytical applications. This fundamental review summarizes different aspects of cation-crown ether complexes studied by conductometry in diverse solvent systems. A comprehensive literature summary is presented in tabular form for the complexation of cations with different crown ethers and their derivatives and other related crown ethers, highlighting their formation constant and thermodynamic parameters in diverse solvent media. Various factors that influence the stability and selectivity of cation-crown ether complexes have been discussed for thermodynamic consideration.

Conductometric Studies on Cation-Crown Ether Complexes: A Review

The structural, electronic, and magnetic properties of the oxygen-vacancy-ordered mixed-valence ${\mathrm{Sr}}_{4}{\mathrm{Fe}}_{4}{\mathrm{O}}_{11}$ phase have been investigated using spin-polarized electronic-structure total-energy calculations. The optimized structural parameters obtained from accurate total-energy calculations are found to be in very good agreement with low-temperature neutron-diffraction findings. Among the different spin configurations considered for ${\mathrm{Sr}}_{4}{\mathrm{Fe}}_{4}{\mathrm{O}}_{11}$, the $G$-type antiferromagnetic configuration is found to represent the magnetic ground state. The calculations show finite magnetic moments at both Fe sites, and this is against the conclusions arrived at from M\"ossbauer spectroscopy and low-temperature powder neutron-diffraction measurements, but consistent with the results from magnetization measurements. The present study clearly show that one of the magnetic sublattices is frustrated and hence ${\mathrm{Sr}}_{4}{\mathrm{Fe}}_{4}{\mathrm{O}}_{11}$ can be considered as a phase-separated system with one phase in the $G$-type antiferromagnetic state and the other in a spin-glass-like state. Our theoretical results show unambiguously that Fe atoms with the square pyramidal environment have a lower oxidation state than that in the octahedral coordination. However, the presence of a covalent interaction between Fe and the neighboring oxygen atoms makes the actual oxidation state considerably smaller than the formal oxidation state of $3+$ and $4+$ for square-pyramidal and octahedral coordination, respectively. ${\mathrm{Sr}}_{4}{\mathrm{Fe}}_{4}{\mathrm{O}}_{11}$ is found to be semiconducting in the antiferromagnetic ground state.

Spin- and charge-ordering in oxygen-vacancy-ordered mixed-valence Sr 4 Fe 4 O 11

Macrocycles are a class of intrinsically porous organic molecules that can host guest molecules selectively. Owing to their diversified porous characteristics, host–guest/supramolecular feature, unique chemical versatility and tunable chemical functionalities, macrocycles are considered fascinating membrane building blocks for advanced and efficient separations. Continuously growing research explorations on macrocycle-based membranes have recently emerged ranging from preparation tactics to applications; however, their significance for membrane separations is far from being fully recognized and understood. Therefore, it is important to timely and systematically summarize the progress of macrocycle-based membranes and provide genuine insight for future studies in this interesting field. In this review, we first discuss the different types of macrocycles used to produce high-quality membranes and elucidate their characteristics. We then focus on fabrication approaches, including blending, surface (or pore) modification, crosslinking and self-assembly by discussing the merits and limitations of each approach. We also break-down the different applications of macrocycle-based membranes including water treatment, pervaporation and organic solvent nanofiltration and chiral separation. All separation applications are evaluated and analyzed in terms of current efficiency and future prospects. Most importantly, this review highlights the challenges and opportunities for improving these smart membranes in order to scale-up and translate this promising technology directly into the consumer market.

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Engineering membranes with macrocycles for precise molecular separations

Formation of the charge-transfer complexes between calix[4]crowns (1 - 4) and iodine in chloroform solution was studied using UV-vis spectrophotometry. The stability constants and the thermodynamic data of the resulting 1 : 1 complexes were determined and was found to decrease with increasing the size of the crown moiety of the calixcrown. All complexes formed were found to be enthalpy stabilized, and all except the complex of 2 were entropy destabilized.

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A Thermodynamic Study of the Charge Transfer Complexes of Iodine with Different tert-Butylcalix[4]crowns

A conductance study of the interaction between substituted ammonium ions with three crown ethers in aqueous solution has been carried out at different temperatures. The formation constants of the 1 : 1 complexes at various temperatures were determined from the molar conductance-mole ratio data and found to vary in the order 18C6 > 15C5 > 12C4 for the same salt and with the same crown, the formation constants vary in the order (C2H5)3NHCl > (C2H5)4NBr > (CH3)3NPhI.

Conductance study and thermodynamics of some substituted ammonium salts with crown ethers in aqueous solution

The hydrogenation of toluene over perovskites of composition La₀.₈Sr₀.₂Fe₁₋ₓ CoₓO₃₋z (x = 0.0, 0.2, 0.5, 0.8 and 1.0) is reported. The catalyst material has been prepared by the coprecipitation method using maleic acid as the complexing agent. The activity for the gas phase reaction of toluene and hydrogen at temperatures between 50 °C to 300 °C is reported for as-prepared and activated materials. The conversion has been found to be nearly temperature-independent over the as-prepared-per­ ovskites with a maximum of 40% conversion found for a composition La₀.₈Sr₀.₂Fe₀.₅Co₀.₅O₂.₈₆. Activation by an oxygen treatment at 500 °C was successful to enhance the activity of the Co-rich perovskites. All of these catalysts show a complete conversion of toluene at 100 °C, but activity decreases by time due to a reduction of the catalyst material. Again, La₀.₈Sr₀.₂Fe₀.₅Co₀.₅O₂.₈₆ exhibits the slowest loss of activity. A model for its special properties is mainly attributed to the p-type conduction behaviour. The conversion behaviour of the perovskite type catalysts investigated here is compared to the results obtained with noble-metal supported catalysts and it can be concluded, that the perovskites exhibit higher conversion rates, maximum temperatures and higher cracking onset temperatures than these.

Hydrogenation of Toluene over La0.8Sr0.2Fe1-xCoxO3-z Perovskite Catalysts

The kinetics of the liquid-phase hydrogenation of cinnamyl alcohol over Ir/Al2O3 catalyst (d ≈ 7 nm) was investigated in toluene under mild conditions (T = 25–95 °C, hydrogen pressure = 3–8 bar and cinnamyl alcohol concentration = 0.0075–0.375 M). The kinetic results could be successfully modeled based on the assumption that the Langmuir–Hinshelwood surface reaction between competitively adsorbed cinnamyl alcohol and hydrogen is the rate determining step. The model predicts that cinnamyl alcohol requires two metallic sites to adsorb on with an adsorption constant of 1.4 M−1. The apparent activation energy of the reaction was experimentally determined to be ~26 kJ mol−1.

Riyadh Saymeh

Papers

A Thermodynamic Study of the Charge Transfer Complexes of Iodine with Different tert-Butylcalix[4]crowns

Conductance study and thermodynamics of some substituted ammonium salts with crown ethers in aqueous solution

Hydrogenation of Toluene over La0.8Sr0.2Fe1-xCoxO3-z Perovskite Catalysts

Kinetic modeling of the liquid-phase hydrogenation of cinnamyl alcohol over alumina-supported Ir catalysts