Síntese de 1-indanonas através da reação de acilação de Friedel-Crafts intramolecular utilizando NbCl5 como ácido de Lewis

The intramolecular Friedel-Crafts acylation reaction of 3-arylpropanoic acids to give 1-indanones can be effected in good yields under mild conditions (room temperature) by using niobium pentachloride. Our results indicate that NbCl5 acts both as reagent (to transform carboxylic acids into acyl chlorides) and as catalyst in the Friedel-Crafts cyclization.

Síntese de derivados 4-aril-3,4-di-hidrocumarínicos catalisada por NbCl5

Multicomponent reactions between phenols, β-diesters and benzaldehydes for the synthesis of 4-aryl-3,4-dihydrocoumarin derivatives were carried out under mild conditions (room temperature) and presented moderate yields (38-88%) and reasonable reaction times (2-4 days), using niobium pentachloride as a catalyst.

Preparation and electrochemical characterization of Pt nanoparticles dispersed on niobium oxide

Electrodes consisting of Pt nanoparticles dispersed on thin films of niobium oxide were prepared onto titanium substrates by a sol-gel method. The physical characterization of these electrodes was carried out by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis. The mean size of the Pt particles was found to be 10.7 nm. The general aspects of the electrochemical behavior were studied by cyclic voltammetry in 1 mol L-1 HClO4 aqueous solution. The response of these electrodes in relation to the oxidation of formaldehyde and methanol in acidic media was also studied.

Caracterização e propriedades catalíticas da zeolita HZSM5 modificada com nióbio

HZSM5 zeolite was modified by exchanging proton by niobium (V). Several samples were obtained with various degrees of exchange. Pore volumes and acidity were measured to characterize these exchanged zeolites. Catalytic properties were evaluated with two reaction tests: m-xylene transformation and n-heptane cracking. The introduction of niobium on HZSM5 zeolite decreases the diffusion coefficient of 2-methyl-pentane and increases the zeolite acidity. The sample containing niobium are initially more active in cracking of n-heptane and m-xylene isomerization than HZSM5 alone.

Preparo de óxido de nióbio suportado em alumina por deposição química em fase vapor: caracterização por espectroscopia vibracional e termogravimetria

Alumina supported niobium oxide was prepared by chemical vapor deposition (CVD) of NbCl5. The alumina was calcined and pretreated at differents temperatures in order to vary the density of OH groups on the surface which was determined by thermogravimetric analysis. A good correlation was found between the amount of anchored niobium and the total number of anionic sites (oxide and hydroxyl groups) on the surface of the alumina. The infrared spectra on the OH stretching region indicate that OH groups coordinated to at least one tetrahedral aluminum were more reactive towards NbCl5.

Filmes de óxidos anódicos de nióbio: efeito eletrocrômico e cinética da reação de eletro-intercalação

The aim of this work is to study the electrochromism and the reaction kinetics of lithium electrointercalation in anodic niobium oxide films. The oxide grown in an acid environment by application of an alternating potential shows interference colour (iridescence) and when reduced in lithium perclorate/PC solution, the intercalation of Li+ ions and electrons causes a reversible colour change (electrochromism), characterized here by electrochemical and optical measurements. A model where the reaction kinetics is dominated by diffusion of ionic pairs (Li+, e-) in the oxide film permitted the reproduction of current and absorbance temporal dependence, confirming the relationship between the electrochromic and electrochemical reactions. From the results obtained, a relation was established where the colour change is associated to the reduction of Nb+5 to Nb+4 ions with simultaneous cations injection.

Caracterização de filmes finos de Nb2O5 com propriedades eletrocrômicas

The sols for thin electrochromic coatings of Nb2O5 were obtained by synthesis of the niobium butoxide from BuONa and NbCl5. The ~300nm thick films were deposited by dip-coating technique from the alkoxide solution and calcined at 560ºC in O2 atmosphere during 3 hours. The particles size of niobium oxide (V) powder (~20mm) was obtained from x-ray diffraction using the Scherrer equation. The coatings were characterized by cyclic voltammetry and cronoamperommetry techniques. The spectral variation of the optical transmittance were determined in situ as a function of the cyclical potencial and memory effect. The insertion process of lithium is reversible and change the film color from transparent (T=80%) to dark blue (T=20%).

Estudo eletroquímico do azul de metileno adsorvido sobre sílica gel quimicamente modificada com óxido de nióbio

Methylene blue (AM) was immobilised on surface of the silica gel modified with niobium oxide. This material was incorporated in a carbon paste electrode, which showed a redox couple in a potential of E= -113 mV vs SCE in KCl solution at pH 7.0. The formal potential, in 0.5 mol L-1 KCl at pH 7.0, shifted about 290 mV towards more positive values compared to those observed for AM solubilized in aqueous solution. The dependence on the formal potential with solution pH between 2 and 7 was much lower than those observed for AM solubilized in aqueous solution.

Otimização da preparação de eletrodo de pasta de carbono contendo riboflavina imobilizada em suporte inorgânico

The aim of this work was to optimize the preparation of electrodes with riboflavin (RF) immobilized on a silica surface modified with niobium oxide and carbon paste. Electrode preparation was optimized employing a factorial design consisting of two levels and three factors. The electrochemical properties of immobilized RF were investigated by cyclic voltammetry. The factorial analysis was carried out analysing the current intensity (Ipa). It was possible to optimize the electrode to get the best reversibility in the redox process, i. e. the lowest separation between anodic and cathodic peak potentials and a current ratio close to unity. The concentration of supporting electrolyte has a small effect. The proportion has the highest effect and the interaction factor between proportion and mixture has also a significant effect on the current intensity.

Influência do agente precipitante na preparação do óxido de nióbio (V) hidratado pelo método da precipitação em solução homogênea

This work reports the preparation, characterization and study of the ion exchange behavior of hydrous niobium oxide prepared by a homogeneous precipitation method. The precipitating agent was obtained in aqueous solution by thermal decomposition of urea or ammonium carbonate. The compounds were chemically and physically characterized by X-ray diffractometry, thermal analysis (TG/DTG), surface area measurements and ion exchange behavior with sodium. The materials prepared with ammonium carbonate presented a higher degree of crystallinity and better ion exchange capacity with sodium than materials prepared with urea. In the homogeneous precipitation method, materials were obtained with specific surface area of 123 - 224 m² g-1. A variation of the preparation process produced hydrous niobium oxide with a different degree of hydration and specific surface area. This provided materials with different physico-chemical properties.

Síntese e propriedades catalíticas em reações de oxidação de goethitas contendo nióbio

Nb-substituted goethites have been prepared and characterized by Mössbauer spectroscopy, XRD, SEM and BET surface area measurements. Mössbauer and XRD analyses suggested that Nb replaces Fe3+ in the structure with duplet formation. The insertion of Nb into the goethite structure caused a significant increase in the BET surface area of the material. The prepared alpha-Fe1-xNb xOOH was investigated for the H2O2 decomposition to O2 and for the Fenton reaction to oxidize the dye methylene blue. It was observed that the introduction of Nb in to goethite produced a strong increase in the activity of oxidation of the dye contaminant by H2O2.

Nióbia magnética como adsorvente de contaminantes orgânicos em meio aquoso: efeito da temperatura e do pH

This work describes novel materials based on pure iron oxide and iron oxide/niobia composite to produce a magnetic adsorbent. These materials were prepared with synthetic iron oxide and characterized by powder XRD, SEM, FTIR, TPR and Mössbauer spectroscopy. Results showed that the main iron oxides formed were goethite (aFeOOH) and maghemite (gFe2O3) with small particle size. The iron oxide and iron oxide/niobia composite showed high adsorption ability for organic compounds. The positive enthalpy indicated an endothermic adsorption process suggesting physical adsorption.

Estudo da adsorção de surfactante catiônico na matriz inorgânica fosfato de nióbio hidratado

This work describes the study the adsorption of a cationic surfactant, cetyl trimethyl ammonium bromide (CTAB) in the hydrous niobium phosphate matrix. The matrix was characterized by powder X-ray diffraction (DRX), thermal analysis (TG), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and surface area measurements (BET). The Langmuir and Freundlich isothermal models were used in the CTAB adsorption study. The adsorption process wasn`t favorable for the NbOPO4.nH2O in both studied models.

Utilization of Sn/Nb2O5 composite for the removal of methylene blue

In this work, the oxidation of methylene blue textile dye in the presence of hydrogen peroxide, using niobium oxide impregnated with different proportions of tin (1, 5 and 10% in mass) as catalyst was studied. The materials were characterized by TPR, XPS, XRD and FTIR. The oxidation tests monitored by ESI-MS showed that the composite containing the higher amount of tin was the most efficient in the removal of the dye. The XRD, XPS, and TPR data presented evidence of the formation of the tin-niobium oxide composite containing Sn0 and supported SnO2.

Adsorção de chumbo, cádmio e prata em óxido de nióbio (V) hidratado preparado pelo método da precipitação em solução homogênea

This paper describes the adsorption of heavy metals ions from aqueous solution by hydrous niobium oxide. Three heavy metals were selected for this study: cadmium, lead and silver. Adsorption isotherms were well fitted by Langmuir model. Maximum adsorption capacity (Q0) for Pb2 +, Ag+ and Cd2 + was found to be 452.5, 188.68 and 8.85 mg g-1, respectively.