Caracterização textural e estrutural de V2O5/TiO2 obtidos via sol-gel: comparação entre secagem convencional e supercrítica

This work describes a modified sol-gel method for the preparation of V2O5/TiO2 catalysts. The samples have been characterized by N2 adsorption at 77K, x-ray diffractometry (XRD) and Fourier Transform Infrared (FT-IR). The surface area increases with the vanadia loading from 24 m² g-1, for pure TiO2, to 87 m² g-1 for 9wt.% of V2O5. The rutile form is predominant for pure TiO2 but became enriched with anatase phase when vanadia loading is increased. No crystalline V2O5 phase was observed in the catalysts diffractograms. Two species of surface vanadium observed by FT-IR spectroscopy a monomeric vanadyl and polymeric vanadates, the vanadyl/vanadate ratio remains practically constant.

Efeito da concentração do catalisador acetilacetonato férrico na cura de poliuretano à base de polibutadieno líquido hidroxilado (PBLH) e diisocianato de isoforona (IPDI)

The reaction between hydroxy-terminated polybutadiene and isophorone diisocyanate constitutes the base of the curing process of the most composite solid propellant used in the propulsion of solid rocket propellant. In this work, differential scanning calorimetry and viscosity measurements were used to evaluate the effect of the ferric acetylacetonate catalyst concentration on the reaction between HTBR and IPDI. These analyses show one exotherm, which shifts to lower temperatures as the catalyst concentration increases. The viscosity analyses show that the increase of temperature causes, at first, a reduction in the mixture viscosity, reaching a minimum range called gelification region (increasing the crosslinking density).

Redução de poluentes de efluentes das indústrias de papel e celulose pela floculação/coagulação e degradação fotoquímica

The abatement of recalcitrant lignin macromolecules from effluents of pulp and paper industry was investigated by combined process. Flocculation and coagulation with aluminum sulfate and natural polyelectrolytes extracted from cactus Cereus peruvianus were used in the first step. After separation of solid residues by filtration, the photochemical methods using TiO2 as catalyst were employed for photocatalytic degradation of lignin compounds from solution. The abatement of lignin compounds after flocculation and coagulation was 46%, and after the overall process, the pollutants reduction observed were 66%. The remaining organic compounds may be removed by any biological treatment.

Síntese de membranas cerâmicas via método sol-gel utilizando TEOS e N,N-dimetilformamida

In this work we obtained microporous and mesoporous silica membranes by sol-gel processing. Tetraethylortosilicate (TEOS) was used as precursor. Nitric acid was used as catalyst. In order to study the affect of N,N-dimethylformamide (NDF) as drying additive, we used a molar ratio TEOS/NDF of 1/3. The performance of N,N-dimethylformamide was evaluated through monolithicity measurements. The structural evolutions occurring during the sol-gel transition and in the interconnected network of the membranes during thermal treatment were monitored by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analyses and nitrogen sorption. We noted that in the presence of N,N-dimethylformamide, polymerization goes through a temporary stabilization of oligomers. The Si-O(H) bonds are stronger and belong to a more cross-linked structure for the N,N-dimethylformamide containing sol. The membranes obtained in the presence of N,N-dimethylformamide have larger surface area and its pore structure is in the range of mesoporous. The membranes obtained without additive have pore structure in the range of microporous.

Síntese, caracterização e estudo das propriedades catalíticas e magnéticas de nanopartículas de Ni dispersas em matriz mesoporosa de SiO2

Nickel nanoparticles supported on amorphous silica ceramic matrix were synthesized by the polymeric precursor method. The nanostructure was characterized by NMR, BET, XRD, SEM, TEM, and flame atomic absorption spectrometry techniques. It was observed a dependence of the crystallite size on the thermal annealing, under a N2 atmosphere. The materials presented a high catalytic activity and selectivity upon the beta-pinene hydrogenation reaction. The magnetic hystereses were also correlated with the morphology of the processed material.

Álcoois quirais: métodos químicos e catalíticos de obtenção por redução assimétrica

The importance of chiral alcohols as starting materials for the production of fine chemicals and as useful chirons for the building of several interesting molecules or natural products is reported. The useful and common methods of asymmetric reduction such as the chemical (with organoboron or organoaluminum reagents) and the catalytic ones (with ruthenium or rhodium complexes) for preparation of chiral alcohols are described; even the newer and much more rare electrocatalytic methods are reported.

Proposição de um reator fotocatalítico para destruição de microrganismos em ambientes interiores

The TiO2/UV photocatalytic reactor was investigated as an alternative system to inactivate airborne microorganisms. The experiments performed in the absence of the catalyst showed that direct photolysis was not efficient to destruct microorganisms, with only 30% of inactivation. Similar inactivation percentage was obtained using TiO2 in absence of UV radiation. The destruction of microorganisms present in a contaminated indoor atmosphere, using the combination of TiO2/UV was very efficient, reaching more than 98% of destruction.

Catalisadores Ni/Al2O3 promovidos com molibdênio para a reação de reforma a vapor de metano

Mo-promoted Ni/Al2O3 catalysts for the methane steam reforming reaction were studied in this work. The Ni/Al2O3 catalysts were prepared by precipitation and molibdenum was added by impregnation up to 2%wt. The solids were tested using a micro-reactor under two H2Ov/C conditions and were characterized by ICP-OES, XRD, N2 adsoption, H2 chemisorption and TPR. NiO and NiAl2O4 phases were observed and the metallic area decreased with the increase of the Mo content. From the catalytic tests high stability was verified for H2Ov/C=4.0. On the other hand, only the catalyst containing 0,05% Mo stayed stable during 30 hours of the test at H2Ov/C=2.0.

Impacto dos catalisadores automotivos no controle da qualidade do ar

Advanced industrialized nations have experienced severe pollution problems over the past forty years, caused mainly by carbon monoxide, hydrocarbons and nitrogen oxide emissions from automobiles. Catalyst technology has played a major part in minimizing these emissions as required by even more restrictive laws. The catalyst has been optimized over the years to meet the requirements of high activity and long life. The oxidation of hydrocarbon and carbon monoxide are in advanced development stage while that of NOx catalysts is far less advanced. In the future, catalyst technology is expected to contribute to overcome the challenges to get a cleaner air.

Síntese do isobutileno e seu emprego em reações de esterificação: propostas de aulas práticas de química orgânica para a graduação

An experiment for the synthesis of isobutylene from tert-butanol dehydratation using oxalic acid as catalyst, followed by preparations of tert-butyl benzoate and tert-butyl cinnamate is described. The synthesis are simple, requiring two periods of 4 hours and are suitable for undergraduate organic chemistry experimental courses.

Estudo microestrutural do catalisador Ni/gama-Al2O3: efeito da adição de CeO2 na reforma do metano com dióxido de carbono

The carbon dioxide reforming of methane was carried out over nickel catalysts supported on the gamma-Al2O3/CeO2 system prepared by wet impregnation. With the increase of the CeO2 weight in the catalyst, a higher stability was observed in the catalytic activity, together with an excellent resistance to carbon deposition and a better Ni dispersion. The catalysts were characterized by means of surface area measurements, TPR, H2 chemisorption, XRD, SEM, EDX, XPS and TEM. An interaction between Ni and CeO2 was observed to the Ni/CeO2 sample after activation in a H2 atmosphere above 300 ºC. Such behavior has a significantly influence on the catalytic activity.

Novos materiais à base de nanofibras de carbono como suporte de catalisador na decomposição da hidrazina

Today satellites propulsion is based on the use of monopropellant and/or bipropellant chemical systems. The maneuvering of satellite is based on the hydrazine decomposition micropropulsors catalyzed by metallic iridium supported on g-alumina. This reaction is a surface reaction and is strongly exothermic and implies that the operation of the micropropulsor is controlled by the mass and heat diffusions. For this reason and for the fact that the propulsor operation is frequently in pulsed regime, the catalyst should support high pressure and temperature variations within a short time period. The performance and the durability of the commercial catalyst are jeopardized by the low thermal conductivity of the alumina. The low thermal conductivity of the alumina support restricts the heat diffusion and leads to the formation of hot spots on the catalyst surface causing the metal sintering and/or fractures of the support, resulting in loss of the activity and catalyst destruction. This work presents the synthesis and characterization of new carbon composite support for the active element iridium, in substitution of the commercial catalysts alumina based support. These supports are constituted of carbon nanofibers (30 to 40 nm diameter) supported on a macroscopic carbon felt. These materials present high thermal conductivity and mechanical resistance, as well as the easiness to be shaped with different macroscopic shapes. The mechanical stability and the performance of the iridium supported on the carbon composite support, evaluated in a laboratory scale test in hydrazine decomposition reaction, are superior compared to the commercial catalyst.

The chemistry involved in the steam treatment of lignocellulosic materials

Pretreatment of lignocellulosic materials is essential for bioconversion because of the various physical and chemical barriers that greatly inhibit their susceptibility to bioprocesses such as hydrolysis and fermentation. The aim of this article is to review some of the most important pretreatment methods developed to date to enhance the conversion of lignocellulosics. Steam explosion, which precludes the treatment of biomass with high-pressure steam under optimal conditions, is presented as the pretreatment method of choice and its mode of action on lignocellulosics is discussed. The optimal pretreatment conditions for a given plant biomass are defined as those in which the best substrate for hydrolysis is obtained with the least amount of soluble sugars lost to side reactions such as dehydration. Therefore, pretreatment optimization results from a compromise between two opposite trends because hemicellulose recovery in acid hydrolysates can only be maximized at lower pretreatment severities, whereas the development of substrate accessibility requires more drastic pretreatment conditions in which sugar losses are inevitable. To account for this heterogeneity, the importance of several process-oriented parameters is discussed in detail, such as the pretreatment temperature, residence time into the steam reactor, use of an acid catalyst, susceptibility of the pretreated biomass to bioconversion, and process design.

Recuperação de metais de catalisadores mássicos e monometálicos

This work presents a study on the dissolution of some commercial monometallic and non-supported deactivated catalysts in HF + H2O2 mixtures (and, eventually, other media) under mild experimental conditions, after a previous oxidation step. The samples were neither crushed nor grinded. The best experimental conditions were dependent on the nature of the support and of the active phase. For example, the Pt/Al2O3 catalyst was dissolved in about 10 minutes, without agitation and heating; however, dissolution of the Pd/Al2O3, Ni/Al2O3, Ni/SiO2, Cu/Al2O3 and V2O5 samples required a temperature of 60 ºC and an agitation of 400 rpm. A careful addition of a NaOH solution allowed a quantitative precipitation of aluminium as criolite (Na3AlF6) or precipitation of Si as Na2SiF6; NaF was obtained as a by-product. As expected, processing of Pd/C, V2O5 and CuO.Cr2O3 samples was relatively simple. Metals recovery from catalysts reached a quantitative level in all samples studied; it is particularly interesting that platinum and palladium could be easily recovered in a single step process, thus separing them from aluminium.

Emprego de óxidos tipo perovskita nas oxidações do propano e CO

Simultaneous oxidation/co-precipitation of an equimolar mixture of La(III) and Co(II) nitrates and La(III) nitrate and Mn(II) chloride afforded a hydroxide gel, which was converted to LaCoO3 and LaMnO3 on calcination at 600 °C. After calcination, the obtained perovskites have been characterised by X-ray diffraction (XRD), X- ray photoelectron spectroscopy (XPS), thermogravimetric analysis (DTA - TGA) and BET specific surface determination. Specific surface areas of perovskites were 12 - 60 m²/g. XRD analysis showed that LaCoO3 and LaMnO3 are simple phase perovskite - type oxides. Traces of LaOCl, in addition to the perovskite were detected in the LaMnO3. The catalytic behavior was examined in the propane and CO oxidation. The LaCoO3 catalyst was more active to CO2 than the LaMnO3 catalyst.