Paulo S. Calefi

The blue phosphor Sr2CeO4 synthesized by Pechini's method

Abstract Blue emitters are very attractive research subjects because of their possible industrial applications as phosphors for field emission displays. Recently, a blue phosphor, Sr 2 CeO 4 , was identified by a combinatorial materials synthesis technique. This material was further synthesized using the chemical coprecipitation method and the decomposition of acetates, carbonates and oxalates in a tedious procedure. In this work, we present the use of Pechini’s method as an alternative for the synthesis of this promising material. When the precursor, (Sr/Ce) polymer citrate–ethylene glycol, was fired at 850°C for 2 h, a mixture of Sr 2 CeO 4 and SrCeO 3 (inactive) was obtained, whereas the amount of Sr 2 CeO 4 was increased by further heating at 1100°C for 2 h in an oxygen atmosphere, as shown by the diffraction pattern. The excitation spectra present two broad bands with maxima at 294 and 344 nm and the emission spectrum has a broad band (half width ∼80 nm) centered at ∼475 nm. The unusually long lifetime of the compound fired at 1100°C of ∼0.04 ms is in good agreement with literature data. In conclusion, the blue phosphor material Sr 2 CeO 4 synthesized at 1100°C in an oxygen flux has comparable or even better emitter properties than that prepared over several days and/or at higher temperatures. The method is very attractive for industrial purposes.

New synthesis strategies for effective functionalization of kaolinite and saponite with silylating agents

Functionalization of Brazilian São Simão kaolinite and Spanish Yunclillos saponite with the alkoxysilanes 3-aminopropyltriethoxysilane and 3-mercaptopropyltrimethoxysilane is reported. The resulting hybrids were characterized by X-ray diffraction, thermal analysis, infrared absorption spectroscopy, and scanning electron microscopy, which demonstrated the effectiveness of the interlamellar grafting process. The X-ray diffractograms revealed incorporation of the alkoxide molecules into the interlayer space of the clays. The displacement of the stretching bands of interlayer hydroxyls in the infrared spectra of the modified kaolinites and the increased intensity of the Mg-OH vibrations in the spectra of the modified saponites confirmed the functionalization of the clays. The thermal behavior of the organoclays confirmed the stability of the hybrids, which was dependent on the clay used for preparation of the materials.

Porphyrin-kaolinite as efficient catalyst for oxidation reactions.

The preparation, characterization, and application in oxidation reactions of new biomimetic catalysts are reported. Brazilian Sao Simao kaolinite clay has been functionalized with [meso-tetrakis(pentafluorophenyl)porphinato]iron(III), Fe(TPFPP). To obtain the functionalized clay, the natural clay was purified by dispersion-sedimentation, expanded by insertion of dimethyl sulfoxide (DMSO), and functionalized with amino groups by substitution of DMSO with ethanolamine. These previous steps allowed clay functionalization with Fe(TPFPP), leading to a layered material with a basal spacing of 10.73 A. Clay functionalization with the porphyrin was confirmed by formation of the secondary amine, as demonstrated by FTIR bands at 3500-3700 cm(-1). UV-vis spectroscopy revealed a red shift in the Soret band of the iron porphyrin in the functionalized material as compared to the parent iron porphyrin catalyst in solution, indicating Fe(III)P --> Fe(II)P reduction. The catalytic performance of the functionalized clay was evaluated in the epoxidation of cyclooctene, with complete selectivity for the epoxide (100% epoxide yield), and ketonization of cyclohexane, cyclohexanone being the major product. The novel catalyst was also evaluated in the Baeyer-Villiger (BV) oxidation of cyclohexanone, with 85% conversion of cyclohexanone in epsilon-caprolactone, with total selectivity to epsilon-caprolactone.

New highly luminescent hybrid materials: terbium pyridine-picolinate covalently grafted on kaolinite.

Luminescent hybrid materials derived from kaolinite appear as promising materials for optical applications due to their specific properties. The spectroscopic behavior of terbium picolinate complexes covalently grafted on kaolinite and the influence of the secondary ligand and thermal treatment on luminescence are reported. The resulting materials were characterized by thermal analysis, element analysis, X-ray diffraction, infrared absorption spectroscopy, and photoluminescence. The thermogravimetric curves indicated an enhancement in the thermal stability up to 300 °C for the lanthanide complexes covalently grafted on kaolinite, with respect to the isolated complexes. The increase in the basal spacing observed by X-ray diffraction confirmed the insertion of the organic ligands into the basal space of kaolinite, involving the formation of a bond between Al-OH and the carboxylate groups, as evidenced by infrared spectroscopy. The luminescent hybrid material exhibited a stronger characteristic emission of Tb(3+) compared to the isolated complex. The excitation spectra displayed a broad band at 277 nm, assigned to a ligand-to-metal charge transfer, while the emission spectra presented bands related to the electronic transitions characteristic of the Tb(3+) ion from the excited state (5)D(4) to the states (7)F(J) (J=5, 4, and 3), with the 4→5 transition having high intensity with green emission.

Hybrid materials prepared by interlayer functionalization of kaolinite with pyridine-carboxylic acids

This paper presents the results of the functionalization of Brazilian São Simão kaolinite with pyridine-2-carboxylic and pyridine-2,6-dicarboxylic acids. The functionalization involved refluxing of the pyridine-carboxylic acid in the presence of kaolinite previously intercalated with dimethyl sulfoxide; both acids effectively displaced dimethyl sulfoxide from the clay interlayer. The resulting materials were characterized by X-ray diffraction, thermal analysis, infrared absorption spectroscopy, and C and N elemental analysis. The X-ray diffractograms revealed the incorporation of the acid molecules into the interlayer space of kaolinite. The thermogravimetric curves of the kaolinite samples functionalized with the pyridine-carboxylic acids indicated that the materials were thermally stable up to 300 degrees C. The displacements of the bands due to interlayer hydroxyls in the infrared absorption spectra also confirmed the functionalization of the kaolinite with the pyridine-carboxylic acids.

Luminescence of a new Tm3+ β-diketonate compound

Abstract This work reports the results for the synthesis, characterization and photophysical properties of a new Tm(ppa) 3 ·2H 2 O complex (ppa=3-phenyl-2,4-pentanedionate). Its characterization was carried out by EDTA titration and TGA analysis, which indicates the presence of the tris-β-diketonate complex with two water molecules completing the metal coordination sphere. The photophysical analyses of the Tm(ppa) 3 ·2H 2 O complex were carried out at room (25°C) and liquid nitrogen (77 K) temperatures. Excitation and absorption spectra showed a broad band centered at ∼335 nm which is ascribed to the complex, since the ppa absorbance maximum is centered at 296 nm. The emission spectra ( λ ex =335 nm) presented the characteristic bands of Tm 3+ due to the 1 G 4 → 3 H 6 (478 nm), 1 G 4 → 3 F 4 (650 nm), 1 G 4 → 3 H 5 (770 nm) and 3 H 4 → 3 H 6 (790 nm) transitions. A pyridine adduct was also prepared but no spectral features could be observed in it. The Gd(ppa) 3 ·2H 2 O was synthesized in order to determine the triplet state of the ppa.

Characterization and spectroscopic studies of Eu3+ and Tb3+ complexes with 2,2′-bipyridine-4,4′-dicarboxylic acid

This work reports on the synthesis of the ligand 2,2′-bipyridine-4,4′-dicarboxylic acid (bpc), the characterization and photophysical properties of Ln2(bpc)3·nH2O (Ln=Eu,Tb,Gd). The compounds were synthesized by the addition of LnCl3 aqueous solution to an aqueous ligand (bpc) solution (pH ∼7.0). The characteristic transitions 5Do→ 7FJ (J=0, 1, 2, 3 and 4) of the Eu3+ and 5D4→7FJ (J=6, 5, 4 and 3) of Tb3+ were observed. The lifetime measurements of the 5D0 and 5D4 excited levels (0.37 and 0.90±0.01 ms) are in agreement with three to four coordinated water molecules. The monoexponential decay curves reveal only one type of site for each ion in the complexes. The complexes were also characterized by thermogravimetric analyses and infrared spectroscopy.

Estudo das condições de estocagem do bagaço de cana-de-açúcar por análise térmica

An evaluation was made of the properties of sugarcane bagasse during the storage process for subsequent burning in a boiler. Samples of bagasse were collected over a period of 150 days from the Caete sugar mill (MG) at various points of the stockpile soon after the sugarcane was pressed to extract its juice, as well as from natural bagasse, dry and damp. Thermal analyses of the samples were carried out and the results indicated that, during the storage of bagasse, dry or damp, the biomass loses up to 32% of its calorific power, due to decomposition of the hemicellulose.

Fenilsilicato dopado com Eu III obtido pelo método sol-gel

In this work, we report the synthesis and the photoluminescence features of a Eu(III)-doped modified silica matrix obtained by the sol-gel method. The matrix was prepared by reaction between tetraethylorthosilicate and phenyltriethoxysilane alkoxide. The hydrolysis occurred using basic catalysis. The solids were treated at 100, 200 and 300 oC during 4 h and the structure was determined by thermogravimetric analysis (TG/DTG), nuclear magnetic resonance (NMR 29Si and 13C), infrared spectroscopy (IR) and photoluminescence (PL). The PL spectra display the Eu(III) lines characteristic of the ion, 5D0 ® 7FJ (J=0, 1, 2, 3, 4), the blue emission as ascribed in the silica matrix. The NMR and TG showed the stability of hybrid silica.

Synthesis and some properties of hybrid gels of titanium oxide containing europium (III)

Abstract In this work, we report the synthesis and properties of TiO 2 and of an organic–inorganic hybrid of TiO 2 where the inorganic network was obtained by hydrolysis (sol–gel process) of the titanium tetraisopropoxide precursor. Both materials were prepared and dried in air. The organic oligomer was synthesized by esterification between ethylene glycol and dipicolinic acid, using Eu 3+ as probe. These materials were studied by luminescence and infrared spectroscopies, thermogravimetric analysis and X-ray diffraction. The results obtained for the hybrid material show the formation of two crystalline polymers. Eu 3+ has been used as structural probe due to its photophysical properties and also as emission center for luminescent materials and the luminescence data indicate that Eu 3+ occupies one kind of site that is isolated from water molecules and OH groups.