ARC Study of Thermal Stability of Alkali Metal Alkoxides

Alkali metal alkoxides can be formed by the direct reaction of alkali metals with the corresponding alcohol. Under certain conditions, however, these reactions become dangerous. One of the reasons for the instability build-up in the reaction mixture is related to the electrochemical behaviour of the heterogeneous medium. Another reason is the instability introduced by the simultaneous presence of oxygen and alkali metal atoms in the reagents. Accelerating rate calorimetry is an excellent way to determine safe working conditions for the handling of such compounds. The hazards that are encountered are discussed by means of some examples.This revised version was published online in November 2005 with corrections to the Cover Date.     

Monolithic metal–organic framework MIL‐53(Al)‐polymethacrylate composite column for the reversed‐phase capillary liquid chromatography separation of small aromatics

A monolithic capillary column containing a composite of metal–organic framework MIL‐53(Al) incorporated into hexyl methacrylate‐co‐ethylene dimethacrylate was prepared to enhance the separation of mixtures of small aromatic compounds by using capillary liquid chromatography. The addition of 10 mg/mL MIL‐53(Al) microparticles increased the micropore content in the monolithic matrix and increased the Brunauer–Emmett–Teller surface area from 26.92 to 85.12 m²/g. The presence of 1,4‐benzenedicarboxylate moieties within the structure of MIL‐53(Al) as an organic linker greatly influenced the separation of aromatic mixtures through π–π interactions. High‐resolution separation was obtained for a series of alkylbenzenes (with resolution factors in the range 0.96–1.75) in less than 8 min, with 14 710 plates/m efficiency for propylbenzene, using a binary polar mobile phase of water/acetonitrile in isocratic mode. A reversed‐phase separation mechanism was indicated by the increased retention factor and resolution as the water percentage in the mobile phase increased. A stability study on the composite column showed excellent mechanical stability under various conditions. The higher resolution and faster separation observed at increased temperature indicated an exothermic separation, whereas the negative values for the free energy change of transfer indicated a spontaneous process.     

A thermodynamic analysis of specific interactions in blends of poly(styrene‐co‐N,N‐dimethylacrylamide) and poly(styrene‐co‐acrylic acid). Screening and self‐association effects

In a first step of this contribution, the observed glass transition temperature‐composition behavior of miscible blends of poly(styrene‐co‐N,N‐dimethylacrylamide) (SAD17) containing 17 mol % of N,N‐dimethylacrylamide and poly(styrene‐co‐acrylic acid) (SAA18, SAA27, and SAA32) containing increasing acrylic acid content, are analyzed according to theoretical approaches. Both Kwei and Brostow equations describe well the experimental data though better fits were obtained with the Brostow's approach. The specific interactions involved in these systems are a combination of intra and interassociation hydrogen bonding. The positive deviation from the linear mixing rule of T_g‐composition observed within the SAA18+SAD17 blend system, indicates that interassociation interactions are prevailing. More pronounced intra‐association interactions within the SAA32+SAD17 blend system led to a large negative deviation while a fine balance is established between these two types of interactions within the SAA27+SAD17 blend. A thermodynamic analysis was carried out according to the Painter‐Coleman association model. The miscibility and phase behavior of SAD17+SAA18 and SAD17+SAA27 blends are well predicted. However, this model predicts a partial miscibility of SAD17+SAA32 system. Finally, the fitting parameter free method developed by Coleman to predict the T_g‐composition behavior is applied. This method predicts fairly well the evolution trend of experimental T_gs of the SAA18+SAD17 and SAA27+SAD17 blend systems. However, the compositional dependence of SAA32+SAD17 blend T_g was not predictable by this method. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47:2074–2082, 2009     

Photopyroelectric investigation of binary mixtures of liquid crystals in contact preparations

The photopyroelectric (PPE) method is proposed as a sensitive technique to study a binary mixture of liquid crystals in a contact preparation. The photothermal signal is generated while scanning the contact preparation. The crystal (K)/smectic-A (S _A), smectic-A/nematic (N), and nematic/isotropic (I) interphase boundaries are detected. The displacement of these boundaries due to the variation of the temperature is monitored. Received: 13 Oktober 1998 / Accepted: 7 July 1999 / Published online: 3 December 1999     

Carbon-13 chemical shift assignment of some organophosphorus compounds: 1—Dialkyl and diaryl (alkylamido)phosphates with general formula Y2P(X)NHR

Carbon-13 chemical shifts and the POC, POCC, PNC and PNCC coupling constants of 18 compounds containing the amine moiety, and with the general formula Y₂P(X)NHR [Y=C₂H₅O, C₆H₅O, CH₂O, Y₂=1,2-dioxybenzene; X = O or S; R = H, CH₃, C₂H₅, PhCH₂CH₂, (CH₃)₂CH, C(CH₃)₃, C₆H₁₁, C₆H₅, C₆H₅NH] have been determined. The Y₂P(X) group shows a sterically induced effect on the amine moiety; the ¹³C chemical shift of the Y group is, however, almost unaffected on replacing P(O) by a P(S) group.     

Reactive chemical doping of the Bi2Se3 topological insulator

Using angular resolved photoemission spectroscopy we studied the evolution of the surface electronic structure of the topological insulator Bi(2)Se(3) as a function of water vapor exposure. We find that a surface reaction with water induces a band bending, which shifts the Dirac point deep into the occupied states and creates quantum well states with a strong Rashba-type splitting. The surface is thus not chemically inert, but the topological state remains protected. The band bending is traced back to Se abstraction, leaving positively charged vacancies at the surface. Because of the presence of water vapor, a similar effect takes place when Bi(2)Se(3) crystals are left in vacuum or cleaved in air, which likely explains the aging effect observed in the Bi(2)Se(3) band structure.     

Activated carbon supported Co1.5PW12O40 as efficient catalyst for the production of 1, 2 cyclohexane diol by oxidation of cyclohexene with H2O2 in the presence of CO2

ABSTRACT

Vicinal diols are important building blocks for chemicals and pharmaceuticals. Currently, they are produced from olefins using solvents and harmful oxidants unfavorable from an environmental and economic point of view. This work lies on the synthesis of 1,2 cyclohexane diol from cyclohexene by a green route. To achieve it, a series of Cobalt Keggin heteropolyanion salt (Co_1.5PW₁₂O₄₀) loaded on activated carbon with different contents was prepared, characterized and tested for the synthesis of diol. The effect of various parameters such as reaction temperature, reaction time and CO₂ pressure on the reaction was studied. The effect of reaction temperature in the range 60-80 °C showed that high temperatures favor diol formation while low temperatures favor cyclohexanone and a segmented concave Arrhenius graph was observed. The results of this work showed that oxidation by H₂O₂ in the presence of CO₂ is an efficient oxidant system for the production of 1.2 cyclohexane diol over carbon activated carbon supported Co_1.5PW₁₂O₄₀. Thanks to CO₂ as a soft oxidizing agent, a conversion of 96.9% and a selectivity in 1, 2 cyclohexane diol of 64.2% was obtained. This simple, safe and environmentally method could be an alternative green route for vicinal diols production from alkenes.     

Efficiency of succinylated-olive stone biosorbent on the removal of cadmium ions from aqueous solutions

Chemical functionalization of olive stone wastes with succinate linkers can potentially improve the performance of wastewater treatment technologies via enhanced adsorption and high affinity of the covalently attached succinate groups for heavy metals. In this study, a novel reusable adsorbent material based on agricultural waste has been synthesized by esterifying the lignocellulosic matrix of olive stones with succinic anhydride in toluene under basic conditions. Characterization of the as-prepared material by FTIR and solid-state MAS ¹³C NMR spectroscopies and TGA confirmed that the heterogeneous esterification has proceeded very efficiently to yield the succinylated-olive stone (S–OS). Subsequent alkaline treatment of S–OS with saturated NaHCO₃ aqueous solution led to the resulting sodic material (NaS–OS), which was subjected to batch experiments in order to evaluate its cadmium-removing efficiency from aqueous solutions at realistic concentrations of cadmium found in industrial effluents. The results obtained from the sorption characteristics have revealed that NaS–OS material is highly effective in removing cadmium from aqueous solutions, with a maximum uptake capacity of 200 mg g⁻¹ (1.78 mmol g⁻¹). The Langmuir isotherm model was found to fit adequately the equilibrium isotherm data. Cadmium adsorption occurs rapidly and the adsorption mechanism is a chemical sorption via ionic exchange between the adsorbate and adsorbent. Thermodynamic parameters were also evaluated from the effect of temperature studies. Regenerability of NaS–OS material was ascertained by quantitative desorption of cadmium with 1 M aqueous NaCl and the reusability of the matrix after five repeated cycles led to nearly no attenuation in its performance (less than 2% in the sorption capacity), indicating that repeated use of NaS–OS is quite feasible. Compared to other low-cost adsorbents utilized for the removal of Cd(II) from water/wastewater, NaS–OS shows higher sorption capacity. These results have important implications for the design of low-cost adsorbents based on agricultural wastes.     

Blow-up and symmetry of sign-changing solutions to some critical elliptic equations

In this paper we continue the analysis of the blow-up of low energy sign-changing solutions of semi-linear elliptic equations with critical Sobolev exponent, started in [M. Ben Ayed, K. El Mehdi, F. Pacella, Blow-up and nonexistence of sign-changing solutions to the Brezis-Nirenberg problem in dimension three, Ann. Inst. H. Poincaré Anal. Non Linéaire, in press]. In addition we prove axial symmetry results for the same kind of solutions in a ball.