Sandra Shirley X. Chiaro

Ab Initio Simulation of Changes in Geometry, Electronic Structure, and Gibbs Free Energy Caused by Dehydration of Hydrotalcites Containing Cl− and CO32− Counteranions

This ab initio study was performed to better understand the correlation between intercalated water molecules and layered double hydroxides (LDH), as well as the changes that occur by the dehydration process of Zn-Al hydrotalcite-like compounds containing Cl⁻ and CO₃²⁻ counterions. We have verified that the strong interaction among intercalated water molecules, cointercalated anions, and OH groups from hydroxyl layers is reflected in the thermal stability of these compounds. The Zn(2/3)Al(1/3)(OH)₂Cl(1/3)·2/3H₂O hydrotalcite loses all the intercalated water molecules around 125 °C, while the Zn(2/3)Al(1/3)(OH)₂(CO₃)(1/6)·4/6H₂O compound dehydrates at about 175 °C. These values are in good agreement with experimental data. The interlayer interactions were discussed on the basis of electron density difference analyses. Our calculation shows that the electron density in the interlayer region decreases during the dehydration process, inducing the migration of the Cl⁻ anion and the displacement of the hydroxyl layer from adjacent layers. Changes in these compound structures occur to recover part of the hydrogen bonds broken due to the removal of water molecules. It was observed that the chloride ion had initially a lower Löwdin charge (Cl(-0.43)), which has increased its absolute value (Cl(-0.58)) after the water molecules removal, while the charges on carbonate ions remain invariant, leading to the conclusion that the Cl⁻ anion can be more influenced by the amount of water molecules in the interlayer space than the CO₃²⁻ anion in hydrotalcite-like compounds.

“Imprinting” Catalytically Active Pd Nanoparticles onto Ionic-Liquid-Modified Al2O3 Supports

A new sputtering chamber that allows the constant mixing of the solid support during sputtering by using an electro-magnetic oscillator was developed for the generation of metal nanoparticles (top down approach) uniformly distributed over the solid supports. By using this new chamber, small and welldistributed Pd nanoparticles (Pd-NPs) of 2.6 or 4.3 nm were produced over Al2O3 and new imidazolium ionic liquids covalently supported on Al2O3 by simple sputtering from a Pd foil. The Pd-NPs uniformly distributed over the solid supports display comparable catalytic performances in the hydrogenation of 1,3-cyclohexadiene and 1,3-cyclooctadiene to that achieved by using a catalyst prepared by conventional chemical methods (bottom up approach). Mechanistic and labelling studies show that the hydrogenation of 1,3-cyclohexadiene catalysed by Pd-NPs occurs via meta-stable p-allyl intermediates, characteristic of homogeneous-like catalytically active sites, and disproportionation through the outer-sphere mechanism. The ratio of hydrogenation to disproportionation is dependent on the Pd-NP size, and the disproportionation products are more pronounced with small NPs because of the higher affinity of dienes for this size of particle. Therefore, the surface structural features of small Pd-NPs facilitate the arrangement of diene molecules in the correct geometry for the transfer of H from the donor to the acceptor sites, typical of poly-metallic catalytically active sites. It is proposed that the reaction of 1,3-cyclohexadiene under H2 can be used to probe the homogeneous/ heterogeneous nature of supported metal NPs. It is demonstrated for the first time that highly active and selective nanocatalysts were obtained by using the sputtering-deposition technique (top down approach) and this opens a new window of opportunity for the preparation of size-controlled metal NPs with clean surfaces. Introduction

Supported ionic liquid phase rhodium nanoparticle hydrogenation catalysts

Rh(0) nanoparticles (ca. 4 nm) dispersed in an ionic liquid (1-n-butyl-3-methylimidazolium tetrafluoroborate) were immobilized within a silica network, prepared by the sol-gel method. The effect of the sol-gel catalyst (acid or base) on the encapsulated ionic liquid and Rh(0) content, on the silica morphology and texture, and on the catalyst alkene hydrogenation activity was investigated. The Rh(0) content in the resulting xerogels (ca. 0.1 wt% Rh/SiO(2)) was shown to be independent of the sol-gel process. However, acidic conditions afforded higher contents of encapsulated ionic liquid and xerogels with larger pore diameters, which in turn might be responsible for the higher catalyst activity in hydrogenation of the alkenes.

Comparação estrutural entre amostras de materiais tipo hidrotalcita obtidas a partir de diferentes rotas de síntese

Three samples of hydrotalcite-like materials (HTC) were synthesized and their structural characteristics were compared with two HTCs obtained commercially. Thermal analyses, FT-IR, PXRD and textural analyses were used to investigate the structural differences between commercial and synthetic samples. Particularly, the memory effect was observed at temperature higher than 600 oC. The Rietveld refinements were obtained with expressive accuracy and the statistical parameters of goodness of fit are quite satisfactory. In conclusion, the procedures adopted in synthesis of HTC produced crystalline materials with high surface area materials.

Influence of Metal Oxides Impregnated on Silica-Alumina in the Removal of Sulphur and Nitrogen Compounds from a Hydrotreated Diesel Fuel Stream

The environmental legislation of many countries imposes severe restrictions on the emissions of gaseous pollutants, including NOx and SOx. Efficient alternatives for the removal of nitrogen and sulphur contaminants are required to obtain increasingly cleaner fuels. In this regard, adsorption is economically promising, because it requires less energy than the traditional hydrotreating processes due to mild conditions of temperature and pressure required. The objective of this study was to evaluate the influence of nickel, cerium, molybdenum and cobalt oxides impregnated on silica–alumina in removing nitrogen and sulphur compounds from a hydrotreated diesel. The incorporation of metal oxides increased the density of acid sites and promoted the removal of nitrogen and sulphur compounds, especially the one impregnated with molybdenum oxide. The influence of molybdenum oxide loading was also studied. It was observed that this synthesis parameter affected acid sites density and contaminant removal.