Structural and spectroscopic characterization of Al2−x Cr x O3 powders obtained by polymeric precursor method

Al₂O₃ and Al_2−x Cr _x O₃ (x = 0.01, 0.02 and 0.04) powders have been synthesized by the polymeric precursors method. A study of the structural evolution of crystalline phases corresponding to the obtained powders was accomplished through X-Ray Diffraction and UV-vis spectroscopy (reflectance spectra and CIEL^*a^*b^* color data). The obtained results allow to identify the γ-Al₂O₃ to α-Al₂O₃ phase transition. The single-phase α-Al₂O₃ powder was obtained after heat treatment at 1050 °C for 2 h. The results show that the green to red color transition and ruby luminescence lines observed for the powders of Al_2−x Cr _x O₃ are related to the γ to α-Al₂O₃ phase transition and the temperature and time range for such transition depends on the chromium content.     

Thermal and structural investigation of SnO<Subscript>2</Subscript>/Sb<Subscript>2</Subscript>O<Subscript>3</Subscript> obtained by the polymeric precursor method

SnO₂-based materials are used as sensors, catalysts and in electro–optical devices. This work aims to synthesize and characterize the SnO₂/Sb₂O₃-based inorganic pigments, obtained by the polymeric precursor method, also known as Pechini method (based on the metallic citrate polymerization by means of ethylene glycol). The precursors were characterized by thermogravimetry (TG) and differential thermal analysis (DTA). After characterization, the precursors were heat-treated at different temperatures and characterized by X-ray diffraction. According to the TG/DTA curves basically two-step mass loss process was observed: the first one is related to the dehydration of the system; and the second one is representative to the combustion of the organic matter. Increase of the heat treatment temperature from 500 to 600°C and 700°C resulted higher crystallinity of the formed product.     

Chemometric study of liquid water simulations. I. The parameters of the TIP4P model potential

The multivariate chemometric techniques two level factorial design (TLFD) and principal component analysis (PCA) were used to investigate the TIP4P model potential behavior with respect to perturbations on all intermolecular interaction parameters. The effects of these perturbations were calculated for the enthalpy of vaporization, the density, the first maximum of the radial distribution functions of the O-H and O-O pairs, and the second maximum of the radial distribution function of the O-H pair obtained from Monte Carlo simulations of liquid water at 25 degrees C. The principal effects were quantified and rationalized in terms of the pair-wise interaction potential of the TIP4P model. They also corroborate previously published sensitivity analysis results using molecular dynamics and other model potentials. In addition, significant interaction effects between some parameters of the TIP4P model potential were observed and quantified, which hardly could be obtained without such a statistic approach. These interaction effects are very regular and systematic, and their behavior has not been encountered in other chemometric studies and cannot be rationalized in terms of the functional form of the pair-wise potential.     

Oriented attachment mechanism in anisotropic nanocrystals: a "polymerization" approach.

The kinetic model of stepwise polymerization is revisited, with some adaptations for its application to the kinetics of oriented attachment of nanoparticles in colloidal suspensions, which results in the formation of anisotropic particles. A comparison with experimental data reported in the literature shows good agreement with the model and supports comparisons with other systems.     

A simple approach to treat anisotropic elastic collisions in Monte Carlo calculations of the electron energy distribution function in cold plasmas

A simplified method allowing one to treat anisotropic electron heavy species elastic scattering in MonteCarlo models of gas discharges with the proper value for collision frequency is proposed The method is applied to an electric discharge in a Ne · Xe/HCl mixture, and the results are compared with the solution of the two-term expansion of the Boltzmann equation under the same conditions. Methods for reduction of computational time in Monte Carlo codes and the use of the Monte Carlo flux method are also discussed.     

Mechanism of the hydroxycarbonylation of 1-hexene catalyzed by immobilized rhodium complexes of pyridine ligands

Summary In this work, a mechanistic study of the hydroxycarbonylation of 1-hexene to heptanoic acid and the water gas shift reaction (WGSR) catalyzed by the rhodium(I) complexes, [Rh(COD)(amine)₂](PF₆) (COD = 1,5-cyclooctadiene, amine = 4-picoline, 3-picoline, 2-picoline, pyridine, 3,5-lutidine or 2,6-lutidine) immobilized on poly(4-vinylpyridine) in contact with water under CO is discussed. Catalytic cycles for these reactions bearing common Rh-H catalytic species are proposed.     

General characterization of noncommercial microbial lipases in hydrolytic and synthetic reactions

Fourteen noncommercial preparations of microbial lipases were investigated with respect to their catalytic activity for hydrolysis and synthesis of ester bonds. Six of the lipases were derived from microorganisms that have not previously been described as lipase producers, and another four were characterized for the first time. The synthetic reactions were carried out in two solvents of different polarities (n-heptane and acetone) using a series of fatty acids and primary and secondary alcohols with different chain lengths. Under the culture conditions employed, Pseudomonas cepacia produced more active enzyme than the other microorganisms. The lipase preparations produced using Ovadendron sulphureo-ochraceum, Monascus mucoroides, Monascus sp., Fusarium oxysporum, Penicillium chrysogenum, Rhodotorula araucariae, Pseudomonas cepacia, Streptomyces halstedii, and Streptomyces sp. were the most efficient catalysts for hydrolysis at lipid-water interfaces. Enzyme preparations from P. cepacia, Streptomyces sp., S. halstedii, and R. araucariae were good biocatalysts for esterification in the polar medium (acetone). When the lipase preparations with the greatest activity for hydrolytic, reactions were excluded, regression analysis of the data for the hydrolytic and synthetic activities of the remaining lipase preparations yielded high multiple correllation coefficients for these reactions in both n-heptane and acetone (R=0.82 and 0.91, respectively).     

Novel polymer-supported coupling/dehydrating reagents for use in organic synthesis

Two novel dehydrating reagents and, based on a phosphonium anhydride and an oxyphosphonium triflate respectively, were prepared by reaction of the corresponding polymer-supported phosphine oxides with triflic anhydride. Reagent, based on the novel phosphorus heterocycle 1,1,3,3-tetraphenyl-2-oxa-1,3-diphospholanium bis(trifluoromethanesulfonate), was found to be a useful reagent for ester and amide formation. A wide range of coupling/dehydration-type reactions, such as ester, amide, anhydride, peptide, ether and nitrile formation, were performed in high yield using the more readily prepared polymer-supported triphenylphosphine ditriflate, which was easily recovered and re-used several times without loss of efficiency. With primary alcohols, both reagents and provide an alternative to the Mitsunobu reaction, where the use of azodicarboxylates and chromatography to remove the phosphine oxide by-product can be avoided. The use of 4-dimethylaminopyridine allowed the esterification of secondary alcohols with to proceed in high yield but with retention of configuration.