Antonio C. O. Guerra

Oxidative dehydration of glycerol to acrylic acid over vanadium-impregnated zeolite beta

The oxidative dehydration of glycerol to acrylic acid was studied over vanadium-impregnated zeolite Beta. Catalysts were prepared by wet impregnation of ammonium metavanadate over ammonium-exchanged zeolite Beta, followed by air calcination at 823 K. Impregnation reduced the specific surface area, but did not significantly affected the acidity (Bronsted and Lewis) of the zeolites. The catalytic evaluation was carried out in a fixed bed flow reactor using air as the carrier and injecting glycerol by means of a syringe pump. Acrolein was the main product, with acetaldehyde and hydroxy-acetone (acetol) being also formed. Acrylic acid was formed with approximately 25% selectivity at 548 K over the impregnated zeolites. The result can be explained by XPS (X-ray photoelectron spectroscopy) measurements, which indicated a good dispersion of the vanadium inside the pores.

Rearrangement, Nucleophilic Substitution, and Halogen Switch Reactions of Alkyl Halides over NaY Zeolite : Formation of the Bicyclobutonium Cation Inside the Zeolite Cavity

Rearrangement and nucleophilic substitution of cyclopropylcarbinyl bromide over NaY and NaY impregnated with NaCl was observed at room temperature. The first-order kinetics are consistent with ionization to the bicyclobutonium cation, followed by internal return of the bromide anion or nucleophilic attack by impregnated NaCl to form cyclopropylcarbinyl, cyclobutyl, and allylcarbinyl chlorides. The product distribution analysis revealed that neither a purely kinetic distribution, similar to what is found in solution, nor the thermodynamic ratio, which favors the allylcarbinyl halide, was observed. Calculations showed that bicyclobutonium and cyclopropylcarbinyl carbocations are minimal over the zeolite structure, and stabilized by hydrogen bonding with the framework structure. A new process of nucleophilic substitution is reported, namely halogen switch, involving alkyl chlorides and bromides of different structures. The reaction occurs inside the zeolite pores, due to the confinement effects and is an additional proof of carbocation formation on zeolites. The results support the idea that zeolites act as solid solvents, permitting ionization and solvation of ionic species.

Vibrational and electronic spectroscopy of neutral antimony coordination compounds of the 1,3-dithiole-2-thione-4,5-dithiolate (dmit).

The S 1s X-ray absorption near edge structure (XANES) and X-ray photoelectron spectra (XPS) of the neutral complexes [SbL(dmit)] (L = Br or I; dmit =1,3-dithiole-2-thione-4,5-dithiolate) have been measured using tunable synchrotron radiation. The valence shell electronic excitation by ultraviolet-visible (UV-vis) spectroscopy and the infrared vibrational spectra are presented and analyzed. The UV-vis results lead to an assignment of bands at 400 nm as π(Sm) → π*(C═S), where S(m) is the thiolate sulfur. The corresponding S 1s → π*(C═S) transition was identified at 2468.3 eV. Ab initio calculations, within the improved virtual orbital (IVO) method, carried out with the GSCF3 program, were applied to establish a complete and accurate spectral assignment. It has been the first attempt to apply such methodology for dmit coordination compounds, and very consistent results were obtained.

Characterization and Ore Dressing of Bauxite from Brazil

The Bauxite from North Brazil is characterized by a geological profile with different layers. The differences among these layers are percentage of bauxite constituents (gibbsite, kaolinite and hematite). Nowadays, only one layer is used for alumina production by Bayer process, the commercial or crystallized bauxite. This work intended to characterize and to ore dress crystallized bauxite (CB), amorphous crystallized (MB) and nodular bauxite (MNB) from Para to make them usable in the Bayer process. The ore dressing was developed by mechanochemical activation with an alkaline reagent in different concentrations and conditioning time. After ore dressing, the samples were submitted to characterization by X-ray diffraction (XRD), X-ray fluorescence (XFR) and rheological characterization. CB and MB ore dressing products were characterized by an increase of mass ratio Al2O3available/SiO2reactive and it was observed that the total silica of MNB, after ore dressing, decreased around 13%. These results were a strong indication that these bauxites could be used in the Bayer process.