Biblioteca Digital

Materiais Carbonosos Ativados (MCA), tais como Carvões Ativados (CA) e Peneiras Moleculares de Carbono (PMC), são caracterizados por apresentar elevada área superficial específica e um grande volume de poros em sua matriz carbonosa, sendo sua principal propriedade a de adsorver moléculas tanto na fase líquida quanto na gasosa. O Brasil importa em sua totalidade peneiras moleculares produzidas a partir de precursores carbonosos, empregando-os nas mais diversas áreas de ciência e tecnologia e em vários segmentos industriais. O coque de petróleo é um resíduo com alto teor de carbono fixo e baixo teor de cinzas, e em decorrência de sua estrutura praticamente amorfa, é um material de pouco valor comercial, sendo considerado um resíduo problemático, tanto em termos ambientais quanto comerciais. Desta forma, a utilização de coque de petróleo para a produção de CAs e de PMCs torna-se atrativa para a utilização deste resíduo. O presente trabalho teve como objetivo estudar a síntese de carvões ativados através das ativações física e química de coque de petróleo proveniente da unidade de coqueamento retardado da REPLAN - PETROBRÁS, buscando avaliar as influências dos parâmetros de processo na qualidade dos CAs...

Devido a grande importância dos processos industriais de separação de xilenos por adsorção, técnicas de simulação molecular foram usadas para estudar as causas da orto-seletividade de isômeros C8 em peneiras moleculares aluminofosfatadas. Aplicaram-se campos de força aproximados e otimizados do tipo AA no ensemble grande canônico com algoritmos Monte Carlo (GCMC) convencional e dirigido. Foram calculadas isotermas monocomponentes, calores de adsorção a baixa concentração e realizamos uma detalhada análise estrutural para definir sítios de adsorção e posicionamentos moleculares nos poros das peneiras AlPO4-5, AlPO4-8, AlPO4-11 e VPI-5. Obteve-se acordo quantitativo entre as isotermas experimentais e simuladas para o sistema xilenos/AlPO4-5 e um acordo qualitativo para o sistema xilenos/AlPO4-11. A análise estrutural levou em conta as variações no interior dos poros dos aluminofosfatos que ocorre em duas regiões: uma mais larga, denominada região de grades, e outra mais estreita, que corresponde à região de janelas. A orto-seletividade evidenciada experimentalmente é causada pela forte interação xileno-peneira e pela modulação dos canais. Este conjunto de fatores determina o posicionamento das moléculas dentro dos poros. Para o orto-xileno este microambiente provoca um posicionamento face-a-face na região das grades...

Este trabalho consiste no estudo de um processo híbrido para obtenção de peneiras moleculares de carbono (PMC) a partir de madeiras e ou resíduos agrícolas tais como: casca de macadâmia e mesocarpo do coco verde que são insumos de origem renovável. A seguir descrevem-se as seguintes etapas do processo: seleção e conformação da matéria-prima; carbonização e pré-ativação (envolvendo tecnologias convencionais de ativação física ou química) e finalmente a obtenção de PMC em um reator de plasma com catodo oco para ativação. O estudo incluiu o projeto e construção de equipamentos para sua implementação em uma unidade piloto, a realização dos testes operacionais, a apreciação da influência dos parâmetros de processo (monitorados com caracterizações dos precursores em cada etapa) até a obtenção das PMC. Também a avaliação de possíveis aplicações em indústrias energo intensivas (eletro-metal e químicas), realizou-se através de estudo de caso (a remoção de metais pesados nos efluentes industriais), comparando o desempenho das PMC com os de amostras comerciais disponíveis no mercado. Finalmente, para análise da viabilidade técnico-econômico-financeira, foram realizados um levantamento da evolução da exportação e importação de carvão ativado (CA) no Brasil...

Desde meados do século passado as peneiras moleculares compõem uma importante classe de materiais, que revolucionou o processo de craqueamento do petróleo. Propriedades como seletividade geométrica, alta área superficial e capacidade de troca iônica dentre outras, possibilitaram a otimização de vários processos de interesse. No final da década de 80 surgiu uma nova família de peneiras moleculares, formadas a partir da polimerização de fontes de carbono no interior dos canais e cavidades dos zeólitos e subsequente remoção do molde inorgânico por extração ácida ou básica. Esses carbonos porosos bem ordenados foram testados em diversas aplicações e apresentaram resultados promissores como suportes para catalisadores em células a combustível, materiais para a produção de supercapacitores e adsorventes de vitaminas e outras moléculas de importância biológica. O presente trabalho consistiu na preparação e caracterização de peneiras moleculares de carbono a partir da polimerização de acrilonitrila, álcool furfurílico e fenol/formaldeído no interior dos canais dos zeólitos com estrutura FAU e MWW. O conjunto de resultados obtidos indica que nos processos otimizados a polimerização ocorreu apenas no interior da estrutura cristalina dos zeólitos. Os carbonos sintetizados de modo geral apresentam uma estrutura formada por anéis poliaromáticos condensados contendo grupos superficiais oxigenados ou nitrogenados. A acrilonitrila apresentou um comportamento particular quanto à difusão e polimerização no interior da estrutura MWW...

The present invent describes a novel method to produce carbon molecular sieves. The carbon materials are submitted to a thermal treatment in a microwave furnace in order to selectively introduce modifications on the porous and chemical structure of the materials. The modified materials proved to be excellent when used as carbon molecular sieves for gas separations, namely CO2/CH4 and O2/N2.

The modification of activated carbon fibres prepared from a commercial textile acrylic fibre into materials with monolithic shape using phenolic resin as binder was studied. The molecular sieving properties for the gas separations CO2/CH4 and O2/N2 were evaluated from the gas uptake volume and selectivity at 100 s contact time taken from the kinetic adsorption curves of the individual gases. The pseudo-first order rate constant was also determined by the application of the LDF model. The samples produced show high CO2 and O2 rates of adsorption, in the range 3–35 · 10_3 s_1, and in most cases null or very low adsorption of CH4 and N2 which make them very promising samples to use in PSA systems, or similar. Although the selectivity was very high, the adsorption capacity was low in certain cases. However, the gas uptake in two samples reached 23 cm3 g_1 for CO2 and 5 cm3 g_1 for O2, which can be considered very good. The materials were heat-treated using a microwave furnace, which is a novel and more economic method, when compared with conventional furnaces, to improve the molecular sieves properties.

Carbon molecular sieves (CMS) have been made for the first time from PET textile fibres by carbonisation and pore mouth narrowing using CVD of benzene. The diffusion of O2, N2, CO2 and CH4 in these materials, and also in the commercial CMS Takeda 3A, was studied. It was found that the best PET based CMS was obtained after 10 min CVD time and had adsorption capacities and rates of diffusion similar to those of the Takeda 3A.

The use of zeolites and other molecular sieves as catalysts is discussed at an introductory level. The text includes a brief historic background on the use of zeolites in catalysis, and a discussion of some chemical and physical properties of silicalite, aluminosilicate, and aluminophosphate molecular sieves. The strategies currently used to chemically modify zeolites and related materials to produce catalysts with increased activity and selectivity are discussed, including the use of redox molecular sieves for hydrocarbon oxidation and the leaching of the active metals from the support.

Catalysis by solid acids has received much attention due to its importance in petroleum refining and petrochemical processes. Relatively few studies have focused on catalysis by bases and even les on using basic molecular sieves. This paper deals with the potential application of micro and mesoporous molecular sieves in base catalysis reactions. The paper is divided in two parts, the first one dedicated to the design of the catalysts and the second to some relevant examples of catalytic reactions, which find a huge field of applications essentially in the synthesis of fine chemicals. Here, recent developments in catalysis by basic molecular sieves and the perspectives of applications in correlated catalytic processes are described.

Vanadium-containing molecular sieves are redox catalysts and are good candidates as substitutes for oxide-supported V2O5 in a number of reactions. These materials have the advantage of presenting better dispersion of vanadium species, as well as shape-selective properties and controllable acidities. They may be prepared by one-pot synthesis or by post-synthesis methods and a number of techniques such as diffuse reflectance UV-visible spectroscopy, 51V nuclear magnetic resonance and electron paramagnetic resonance, to name but a few, have been used to characterize these materials. In this review, methods of preparation of vanadium-modified molecular sieves, their characterization and applications in catalysis are discussed.

The composite membranes prepared via incorporation of 12.5% of molecular sieves 3A, 4A, 5A and 13X into chitosan/poly(vinyl alcohol) (1:1). The composite membranes were immersed in the cross-linker sulfuric acid in order to acquire high proton conductivity for applications in electrolytes in PEMCF and DMF. The influence of the molecular sieves on the structural, optical, thermal, mechanical, morphologic and protonic conductivity properties and water/methanol swelling degree of membranes were investigated.

This paper describes the production methods and the prospective uses of carbon molecular sieves. The main route to these materials is replication synthesis, where a silica or aluminosilicate molecular sieve is used as template to grow the carbonaceous phase in the voids. These materials may have applications as varied as in separation, adsorption and storage of gases, as electrodes in batteries, and as catalyst supports, all of them highly dependent on the molecular sieve porosity.

The per-O-acetylation of D-ribono-1,4-lactone and representative carbohydrates through the combination of acetic anhydride and molecular sieves under solvent-free conditions is demonstrated. The use of 13X/KCl molecular sieves as the heterogeneous catalyst was found to be more efficient than the excess of pyridine normally employed in the conventional method, giving high yields of the expected peracetylated product after 3 h at 25 ºC or 1 h at 50 ºC. The transformation can be carried out in gram scale and in an open flask. Additionally, the catalyst is readily separated from the reaction medium and can be reutilized without significant loss of activity. This green procedure for acetylation was extended to D-ribonolactone derivatives and natural carbohydrates. To demonstrate the synthetic utility of the method, 2,3,5-tri-O-acetyl-D-ribonolactone was selected as the substrate for the regioselective alcoholysis of acetyl group catalyzed by Candida antarctica lipase B in EtOH to selectively produce 2,3-di-O-acetyl-D-ribonolactone in gram scale.

The aim of this work was to study the feasibility of simultaneous process of high-pressure extraction and fractionation of lemongrass essential oil using molecular sieves. For this purpose, a high-pressure laboratory-scale extraction unit coupled with a column with four different stationary phases for fractionation: ZSM5 zeolite, MCM-41 mesoporous material, alumina and silica was employed. Additionally, the effect of carbon dioxide extraction variables on the global yield and chemical composition of the essential oil was also studied in a temperature range of 293 to 313 K and a pressure range of 100 to 200 bar. The volatile organic compounds of the extracts were identified by a gas chromatograph coupled with a mass spectrometer detector (GC/MS). The results indicated that the extraction process variables and the stationary phase exerted an effect on both the extraction yield and the chemical composition of the extracts.

A study of the synthesis of MCM-41 mesoporous molecular sieves in fluoride media, having no alkaline metal ions, was performed by changing the gel composition and crystallization temperature and time. X-ray diffraction and nitrogen adsorption analyses showed that highly ordered MCM-41 samples were obtained from gels with a NH4OH/SiO2 molar ratio in the 3.25-4.3 range (room temperature synthesis) or in the 4.3-20 range (24 hours at 373 K). During calcination, unit cell shrinkage, caused by high temperature polycondensation of the SiOH groups, was observed for all samples. The samples synthesized at high temperature (373 K) or using low pH gels (7.5) underwent lower unit cell shrinkage than those obtained at room temperature or high pH (9.0), indicating that the former samples had lower SiOH groups content than the latter. These highly-ordered samples showed large surface area (ca. 1100 m²/g) and pore volume (ca. 0.80 cm³/g), also presenting a narrow pore size distribution. Due to higher silicate polycondensation and a thicker pore wall, the samples synthesized at 373 K were more hydrothermally stable than those obtained at room temperature.

A participação do gás natural na geração de energia vem crescendo significativamente nos últimos anos, tanto no Brasil como no mundo. O seu uso na combustão gera menor emissão de poluentes quando comparado aos outros combustíveis fósseis. Isto é muito importante numa época em que países do mundo inteiro concentram esforços para atender a meta do protocolo de Kyoto. Neste sentido o governo brasileiro vem incentivando o uso do gás natural visando atingir, em 2010, uma participação de até 12 % deste combustível na matriz energética do país. Esta Tese teve como objetivo determinar a utilidade das peneiras moleculares como suporte de catalisadores de paládio, na reação de combustão catalítica de metano com fins de geração de energia em baixas temperaturas. Para isso, foram sintetizadas peneiras moleculares micro e mesoporosas do tipo ETS-10, (Si e AI)-MCM-41 e meso-SAPO. As estabilidades térmicas dessas peneiras moleculares indicaram que para a reação de interesse, somente o MCM-41 seria útil, tanto puramente silícico (Si-MCM-41) como com alumínio (AI-MCM-41). Também foram estudados os efeitos causados por aditivos como cério, lantânio e zircônio, introduzidos por impregnação nos suportes antes da impregnação do paládio. A estrutura dos suportes do tipo MCM-41 apresentaram uma boa estabilidade térmica até 900°C e o suporte a base de silicato demonstrou ser mais estável do que o AI-MCM-41. A caracterização realizada permitiu verificar a presença de óxido de paládio (PdO) nos catalisadores. Os catalisadores de ambas as séries Si-MCM-41 e AI-MCM-41 foram ativos na reação de interesse. O paládio suportado na série Si-MCM-41 apresentou maior atividade que quando suportado na série AI-MCM-41. Este fato pode estar relacionado com a diferença no caráter hídrofílico/hidrofóbico do suporte. A acidificação dos suportes antes da impregnação do paládio melhorou performance catalítica enquanto que a presença de aditivos não apresentaram a melhora esperada...

5 pages, 4 figures, 1 table. -- Available online 27 June 2007.; Various microporous carbon molecular sieves are studied as active electrode material for supercapacitors in order to clarify the controversy about the accessibility of the electrolyte to the micropores. Cyclic voltammetry experiments were performed in electrolytes with different ion size. The results showed a clear ion sieving effect when the porosity of the carbon was similar to that of the ions of the electrolyte. Impedance spectroscopy was also useful to evidence diffusion restrictions of the ions into the pores. The results obtained in this study clearly demonstrate that in aqueous media very narrow micropores (0.5 nm) are still capable of forming the electrical double layer. Therefore, the majority of microporous carbons, with wider porosity, are perfectly suitable as active electrode materials for supercapacitors when aqueous electrolytes are used.; This work has been performed with financial support from MICYT (project MAT2004-03480-C02) and FICYT (project IB05-086-C1). V. Ruiz acknowledges a predoctoral research grant from FICYT.; Peer reviewed

[ES] Se han preparado tamices moleculares de carbono (CMS) mediante el dep sito de tomos de carbono, por pir lisis de benceno, sobre la superficie de carbones activados. La pir lisis de benceno a temperaturas comprendidas entre 650-850 ¼C genera el cierre progresivo de los microporos debido a la creaci n de constricciones en la red microporosa que limitan la accesibilidad de determinadas mol culas. El uso de temperaturas superiores a la temperatura de carbonizaci n del precursor (850 ¼C) introduce complicaciones debido a la descomposici n y sinterizaci n parcial del s lido. Flujos bajos de nitr geno (30 mL min-1) con alto contenido en benceno (13 %) producen un dep sito homog neo a lo largo de las paredes y los materiales presentan distribuciones microporosas mas anchas. El dep sito de carbono sobre carbones activados por pir lisis de benceno a temperaturas inferiores a 850 ¼C, utilizando flujos relativamente altos (150 mL min-1) con baja concentraci n de benceno (1 %) genera tamices con vol menes de microporos alrededor de 0,25 cm3 g-1 y anchuras de poro distribuidas en intervalos estrechos: inferiores a 0,33 nm, entre 0,33-0,41 nm y entre 0,41-0,54 nm. El porcentaje de quemado del carb n activado de partida tiene relativamente poca influencia sobre las propiedades del tamiz molecular...

8 pages, 5 figures, 4 tables.-- Available online Dec 20, 2005.; The modification of activated carbon fibres prepared from a commercial textile acrylic fibre into materials with monolithic shape using phenolic resin as binder was studied. The molecular sieving properties for the gas separations CO2/CH4 and O2/N2 were evaluated from the gas uptake volume and selectivity at 100 s contact time taken from the kinetic adsorption curves of the individual gases. The pseudo-first order rate constant was also determined by the application of the LDF model. The samples produced show high CO2 and O2 rates of adsorption, in the range 3–35 × 10−3 s−1, and in most cases null or very low adsorption of CH4 and N2 which make them very promising samples to use in PSA systems, or similar. Although the selectivity was very high, the adsorption capacity was low in certain cases. However, the gas uptake in two samples reached 23 cm3 g−1 for CO2 and 5 cm3 g−1 for O2, which can be considered very good. The materials were heat-treated using a microwave furnace, which is a novel and more economic method, when compared with conventional furnaces, to improve the molecular sieves properties.; The authors are grateful to the Fundação para a Ciência e a Tecnologia (Portugal)...

Lewis acid sites isolated within low-defect, hydrophobic molecular sieves (Sn-Beta-F, Ti-Beta-F) catalyze monosaccharide (glucose–fructose) and disaccharide (lactose–lactulose) aldose–ketose isomerization reactions in liquid water at initial turnover rates (per total metal atom; 373 K) that are, respectively, ∼10–30 and ∼10^3–10^4 factors higher than sites isolated within highly defective, hydrophilic molecular sieves (Ti-Beta-OH) or amorphous co-precipitated oxides (TiO_(2)–SiO_(2)). Glucose-H2/glucose-D2 kinetic isotope effects of ∼2 (at 373 K) for intramolecular C2–C1 hydride shift isomerization to fructose indicate that glucose transport to active sites within Ti-Beta-F or Ti-Beta-OH does not limit turnover rates in liquid water or methanol, in spite of dramatic differences in the volumetric occupation of hydrophobic and hydrophilic void spaces by physisorbed solvent molecules. Glucose isomerization turnover rates (per total Ti; 373 K) in liquid water are first-order in aqueous glucose concentration (at least up to 1.5% (w/w)). The mechanistic interpretation of measured first-order isomerization rate constants indicates that they reflect free energies of kinetically relevant isomerization transition states relative to two bound solvent molecules...