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Synthesis of silica xerogels with high surface area using acetic acid as catalyst

Arenas, Leliz Ticona; Simm, Carolina Wildner; Gushikem, Yoshitaka; Dias, Silvio Luis Pereira; Moro, Celso Camilo; Costa, Tania Maria Haas; Benvenutti, Edilson Valmir
Fonte: Universidade Federal do Rio Grande do Sul Publicador: Universidade Federal do Rio Grande do Sul
Tipo: Artigo de Revista Científica Formato: application/pdf
ENG
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489.89312%
Nesse trabalho foi estudada a influência do ácido acético na estrutura de poros e na área superficial de sílicas preparadas pelo método sol-gel. Condições experimentais de síntese, tais como temperatura de policondensação e solventes, também foram estudadas. Isotermas de adsorção de N2 das amostras foram classificadas como do tipo 1, típicas de materiais microporosos, o que explica os altos valores de área superficial obtidos. A adição simultânea dos ácidos acético e clorídrico como catalisadores e de acetona como solvente, bem como o emprego de uma temperatura de policondensação de 20 °C, possibilitaram a preparação de sílicas amorfas com valores de área superficial de até 850 m2 g-1. O alto valor de área superficial dessas amostras pode ser explicado principalmente pela microporosidade e também pelo tamanho nanométrico das partículas.; The influence of acetic acid on the pore structure and surface area of silica prepared by the sol-gel method was investigated. Experimental conditions of synthesis, such as gelation temperature and solvents, were also studied. N2 adsorption isotherms of the samples were type 1, typical of microporous materials, explaining the high surface area values (BET) observed. The simultaneous addition of acetic and hydrochloric acids as catalysts and of acetone as solvent...

Mercury (II) removal from aqueous solutions by microporous materials

Lopes, Cláudia Maria Batista
Fonte: Universidade de Aveiro Publicador: Universidade de Aveiro
Tipo: Tese de Doutorado
ENG
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500.7456%
O Mercúrio é um dos metais pesados mais tóxicos existentes no meio ambiente, é persistente e caracteriza-se por bioamplificar e bioacumular ao longo da cadeia trófica. A poluição com mercúrio é um problema à escala global devido à combinação de emissões naturais e emissões antropogénicas, o que obriga a políticas ambientais mais restritivas sobre a descarga de metais pesados. Consequentemente o desenvolvimento de novos e eficientes materiais e de novas tecnologias para remover mercúrio de efluentes é necessário e urgente. Neste contexto, alguns materiais microporosos provenientes de duas famílias, titanossilicatos e zirconossilicatos, foram investigados com o objectivo de avaliar a sua capacidade para remover iões Hg2+ de soluções aquosas. De um modo geral, quase todos os materiais estudados apresentaram elevadas percentagens de remoção, confirmando que são bons permutadores iónicos e que têm capacidade para serem utilizados como agentes descontaminantes. O titanossilicato ETS-4 foi o material mais estudado devido à sua elevada eficiência de remoção (>98%), aliada à pequena quantidade de massa necessária para atingir essa elevada percentagem de remoção. Com apenas 4 mg⋅dm-3 de ETS-4 foi possível tratar uma solução com uma concentração igual ao valor máximo admissível para descargas de efluentes em cursos de água (50 μg⋅dm-3) e obter água com qualidade para consumo humano (<1.0 μg⋅dm-3)...

Developing novel microporous materials; Desenvolvimento de novos materiais microporosos

Craveiro, Rita Paula Paiva
Fonte: Universidade de Aveiro Publicador: Universidade de Aveiro
Tipo: Dissertação de Mestrado
ENG
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508.70355%
Os materiais microporosos têm elevado interesse científico e industrial, principalmente devido às suas propriedades, tais como catálise selectiva, separação selectiva e troca iónica, directamente relacionadas com a sua estrutura. De modo a desenvolver novos materiais, com novas propriedades, ou para ajustar propriedades de materiais já conhecidos, estudos sistemáticos nesta área são usuais, especialmente em relação aos processos de síntese de materiais microporosos. O principal objectivo deste trabalho é o de sintetizar e caracterizar novos materiais microporosos, partindo do ajuste da síntese do material ETS-4 e de um estudo sistemático de uma composição de fosfato de vanádio. O ETS-4 é um óxido misto sintético conhecido, que contém átomos de Ti em coordenação octaédrica e piramidal quadrada, em conjunto com unidades silicato tetraédricas. A sua estrutura distinta possui cadeias de titânio que se podem comportar como fios quânticos. O ETS-4 também exibe o efeito “portão molecular”, o que significa que a abertura do poro pode ser ajustada consoante a temperatura, melhorando as características de separação do material. Neste trabalho, tentou-se substituir isomorficamente o Fe pelo Ti no ETS-4...

Hybrid organic-inorganic materials: application in oxidative catalysis

Rocha, G M S R O
Fonte: Nova Science Publishers Publicador: Nova Science Publishers
Tipo: Parte de Livro
ENG
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375.92656%
Oxidative catalysis has been and will be playing an important role in the production of large quantities of intermediates. This technology has a great potential for improvement and this has led to a series of better processes, including the development of a great diversity of suitable catalysts. Within these compounds, metal phosphates and phosphonates possessing layered and pillared structures were found to constitute a very good alternative as catalysts to be used in oxidative catalysis. Metal phosphates and phosphonates can be considered as strong inorganic solid acids and much of their catalytic activity has been attributed to the Brønsted acidity of the interlayered hydroxyl groups and to the Lewis acidity of the metal center. Metal phosphates and phosphonates can be obtained at low temperatures, in aqueous media, using soft chemical routes and their preparation is quite accessible if the correct phosphonic acids and phophates are available. These heterogeneous materials are very interesting from the economical as well as from the environmental points of view because of the high yields and short reaction times, easy recovery from the reaction mixtures and the possibility to perform reactions in solvent-free conditions. Due to their particular physical and chemical properties and high versatility...

Metal-organic nanoporous structures with anisotropic photoluminescence and magnetic properties and their use as sensors

Harbuzaru, B. V.; Corma, A.; Rey, F.; Atienzar, P.; Jordá, J. L.; García, H.; Ananias, Duarte; Carlos, Luís Dias; Rocha, João
Fonte: Wiley-VCH Publicador: Wiley-VCH
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
375.92656%
Porous metal–organic framework materials are important owing to their potential applications as sensors. A new family of nanoporous, multifunctional rare-earth metal–organic materials displays emission quantum yields and efficiencies that are the highest reported for solid-state Eu3+ compounds with organic ligands and magnetic properties. The picture shows [Gd,Eu]- and [Tb]-containing materials under UV light and interacting with a magnet.

Experimental Gravimetric Adsorption Equilibrium of n-Alkanes and Alkenes, Carbon Dioxide and Nitrogen in MIL-53(Al) and Zeolite 5A

Camacho, Bárbara Cassiana Rodrigues
Fonte: Faculdade de Ciências e Tecnologia Publicador: Faculdade de Ciências e Tecnologia
Tipo: Dissertação de Mestrado
Publicado em //2014 ENG
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373.60562%
Dissertação para obtenção do Grau de Mestre em Engenharia Química e Bioquímica; The objective of this work was the measuring of adsorption equilibrium, by the gravimetric method. Experimental results are presented for the adsorption equilibrium of the series of n-alkanes, ethylene, nitrogen and carbon dioxide in two microporous materials, the metal-organic framework, MIL-53(Al) and zeolite 5A. Both of them have desirable characteristics for adsorption processes, such as the capture and storage of carbon dioxide, natural gas storage, separation of components of biogas, and separation of olefin/paraffin. The determination of the equilibrium of the pure components (ethane, propane, butane, ethylene, carbon dioxide and nitrogen) covers a wide range of thermodynamic conditions; temperatures between 303.15K and 373.15K, as well as pressure values between 0 and 50 bar. The adsorption equilibrium data were analyzed through the global adjustment for each adsorbate/adsorbent system, using the Sips and Toth models. The isosteric heat was also determined. The experimental data of methane, carbon dioxide and nitrogen were correlated successfully by the potential theory of adsorption collapsing into a single characteristic curve, independent of temperature. This analysis allows the extrapolation of adsorption data for other gases...

Nanoporous materials: pillared clays and regular silicas as an example of synthesis and their porosity characterization by X-ray diffraction

Pires,João
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/01/2014 EN
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Because of their practical applications, porous materials attract the attention of undergraduate students in a way that can be used to teach techniques and concepts in various chemistry disciplines. Porous materials are studied in various chemistry disciplines, including inorganic, organic, and physical chemistry. In this work, the syntheses of a microporous material and a mesoporous material are presented. The porosity of the synthesized materials is characterized by X-ray diffraction analysis. We show that this technique can be used to determine the pore dimensions of the synthesized materials.

Synthesis of silica xerogels with high surface area using acetic acid as catalyst

Arenas,Leliz T.; Simm,Carolina W.; Gushikem,Yoshitaka; Dias,Silvio L. P.; Moro,Celso C.; Costa,Tania M. H.; Benvenutti,Edilson V.
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/01/2007 EN
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489.89312%
The influence of acetic acid on the pore structure and surface area of silica prepared by the sol-gel method was investigated. Experimental conditions of synthesis, such as gelation temperature and solvents, were also studied. N2 adsorption isotherms of the samples were type 1, typical of microporous materials, explaining the high surface area values (BET) observed. The simultaneous addition of acetic and hydrochloric acids as catalysts and of acetone as solvent, together with the use of a gelation temperature of 20 ºC, made it possible to prepare amorphous silica materials with surface area values up to 850 m² g-1. The high surface area value of these samples could be explained by the microporosity and the nanometric size of the particles.

Powdered microporous glasses: changing porosity through aging

Perruso,Carlos Renato; Alcover Neto,Arnaldo; Neumann,Reiner; Nascimento,Regina Sandra Veiga; Fonseca,Marcus Vinicius de Araujo
Fonte: Associação Brasileira de Cerâmica Publicador: Associação Brasileira de Cerâmica
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/10/1998 EN
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Recently, we have reported the production of a microporous high purity silica powder from the acid leaching of glasses with average pore size around 2.5nm and specific surface area of 420m2/g (BET). The employed glasses derived from the melting of two types of waste from the industrial processing of Brazilian oil shale (retorted oil shale and the top fraction of the intermediate layer of Irati Formation). Depending on the proportion of the two wastes employed in the formulation of the glasses, either a silica gel or a powdered one is obtained, after leaching. The acid leaching of those glasses with hydrochloric acid, at 90 °C, was used to produce powdered microporous silica. In the present work it is studied the effect of aging time and temperature on the morphology and structure of the obtained powdered silica. Aging studies were performed in two different media, an acidic (hydrochloric acid) and a basic one (ammonium hydroxide) for different periods of time and temperatures. XRD, SEM/EDX, TEM, specific surface area measurements and DTA/TGA were used to characterize these materials. The results have shown a decrease of specific surface area with increasing time and temperature. Apparently, this behavior may be associated with dissolution and re-precipitation mechanisms.

Polyelectrolyte multilayers : nanofabricated architectures for bio-interface materials

Mendelsohn, Jonas Daniel, 1975-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 135 p.; 14268994 bytes; 14268753 bytes; application/pdf; application/pdf
ENG
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The layer-by-layer process, whereby aqueous solutions of oppositely charged polymers are alternately and repeatedly deposited onto a substrate, has emerged in recent years as a promising approach for creating thin films with nanoscale control of structure, composition, and surface properties. Applications ranging from surface modification to optical and electronic devices have arisen from the versatility of this nanocomposite fabrication technique. The additional ability to assemble into films a wide variety of biological entities, such as enzymes and DNA, has expanded the use of polyelectrolyte multilayers for biosensor and other biomaterials applications. This thesis further explores the rationale of using multilayers as biomaterials, with particularly emphasis on the importance of the underlying molecular architecture. Many of the results presented here concern films assembled from weak polyions, i.e., ones with pH-dependent charge densities, including poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH). Using weak polyions enables the creation of thin films with chemical and structural properties controlled with nanoscale precision by simply adjusting the pH of the polymer solutions. Under certain assembly conditions...

Mechanism of preferential adsorption of SO$_2$ into two microporous paddle wheel frameworks M(bdc)(ted)0.5

Tan, Kui; Canepa, Pieremanuele; Gong, Qihan; Liu, Jian; Johnson, Daniel H.; Dyevoich, Allison; Thallapally, Praveen K.; Thonhauser, Timo; Li, Jing; Chabal, Yves J.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 26/10/2013
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378.92137%
The selective adsorption of a corrosive gas, SO$_2$, into two microporous pillared paddle-wheel frameworks M(bdc)(ted)0.5 [M = Ni, Zn; bdc = 1,4-benzenedicarboxylate; ted = triethylenediamine] is studied by volumetric adsorption measurements and a combination of in-situ infrared spectroscopy and ab initio density functional theory (DFT) calculations. The uptake of SO$_2$ in M(bdc)(ted)0.5 at room temperature is quite significant, 9.97 mol/kg at 1.13 bar. The major adsorbed SO$_2$ molecules contributing to the isotherm measurements are characterized by stretching bands at 1326 and 1144 cm$^{-1}$. Theoretical calculations including van der Waals interactions (based on vdW-DF) suggest that two adsorption configurations are possible for these SO$_2$ molecules. One geometry involves an SO$_2$ molecule bonded through its sulfur atom to the oxygen atom of the paddle-wheel building unit and its two oxygen atoms to the C-H groups of the organic linkers by formation of hydrogen bonds. Such a configuration results in a distortion of the benzene rings, which is consistent with the experimentally observed shift of the ring deformation mode. In the other geometry, SO$_2$ establishes hydrogen bonding with -CH$_2$ group of the ted linker through its two oxygen atoms simultaneously. The vdW-DF-simulated frequency shifts of the SO$_2$ stretching bands in these two configurations are similar and in good agreement with spectroscopically measured values of physisorbed SO$_2$.In addition...

Textural development and hydrogen adsorption of carbon materials from PET waste

Parra Soto, José Bernardo; Ovín Ania, María Concepción; Arenillas de la Puente, Ana; Rubiera González, Fernando; Palacios, José María; Pis Martínez, José Juan
Fonte: Elsevier Publicador: Elsevier
Tipo: Artículo Formato: 21442 bytes; application/pdf
ENG
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10 pages, 7 figures.-- Printed version published on Oct 6, 2004.; Polyethyleneterephthalate (PET) has become one of the major post-consumer plastic wastes. PET products present a problem of considerable concern due to the huge amount of solid waste produced. The disposal of this waste, together with its low bio- and photo−degradability represents a serious challenge for industrial countries all over the world. Pyrolysis could provide an alternative and economically viable route for processing PET waste due to the potential uses of different by-products: energy from the pyrolysis gases (58% yield in this work), recovery of terephthalic acid and other subproducts (20%), and a solid residue (22%), which has shown a high textural development after activation. The pyrolysis of PET waste was performed in a quartz reactor (i.d. 35 mm) under an inert atmosphere. Further activation was carried out at a temperature of 925°C, with a flow rate of 10 ml min−1 of CO2. A series of carbon materials with different burn-off degrees was obtained. Textural characterisation of the samples was carried out by performing N2 adsorption isotherms at −196°C. Changes in the morphological and structural properties of chars were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The carbons obtained were isotropic and highly microporous materials with apparent BET surface areas of up to 2500 m2 g−1. The suitability of the samples for hydrogen storage was studied by performing H2 adsorption measurements in the 0–1 bar pressure range. Adsorption–desorption experiments showed that reversible physisorption takes place in all the samples. The hydrogen adsorption capacities of the activated PET waste compare favourably well with those attained with high-value carbon materials.; Peer reviewed

The Large Electrochemical Capacitance of Microporous Doped Carbon Obtained by Using a Zeolite Template

Ovín Ania, María Concepción; Khomenko, Volodymyr; Raymundo-Piñero, Encarnación; Parra Soto, José Bernardo; Béguin, François
Fonte: John Wiley & Sons Publicador: John Wiley & Sons
Tipo: Artículo Formato: 918459 bytes; application/pdf
ENG
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487.4529%
9 pages, 7 figures.; A novel microporous templated carbon material doped with nitrogen is synthesized by using a two-step nanocasting process using acrylonitrile (AN) and propylene as precursors, and Na-Y zeolite as a scaffold. Liquid-phase impregnation and in situ polymerization of the nitrogenated precursor inside the nanochannels of the inorganic scaffold, followed by gas-phase impregnation with propylene, enables pore-size control and functionality tuning of the resulting carbon material. The material thereby obtained has a narrow pore-size distribution (PSD), within the micropore range, and a large amount of heteroatoms (i.e., oxygen and nitrogen). In addition, the carbon material inherits the ordered structure of the inorganic host. Such features simultaneously present in the carbon result in it being ideal for use as an electrode in a supercapacitor. Although presenting a moderately developed specific surface area (S_BET = 1680 m2 g-1), the templated carbon material displays a large gravimetric capacitance (340 F g-1) in aqueous media because of the combined electrochemical activity of the heteroatoms and the accessible porosity. This material can operate at 1.2 V in an aqueous medium with good cycleability - beyond 10 000 cycles - and is extremely promising for use in the development of high-energy-density supercapacitors.; The authors thank Norit for kindly supplying the activated carbon adsorbent. C.O.A. thanks MEC...

Suitability of plastics wastes and coal-tar pitch as precursors of carbon materials for environmental applications

Díez Díaz-Estébanez, M.ª Antonia; Lorenc-Grabowska, Ewa; Barriocanal Rueda, Carmen; Gryglewicz, Grazyna; Machnikowski, Jacek
Fonte: Conselho Superior de Investigações Científicas Publicador: Conselho Superior de Investigações Científicas
Tipo: Comunicación de congreso Formato: 105454 bytes; application/pdf
ENG
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373.60562%
En: 1st Spanish National Conference on Advances in Materials Recycling and Eco – Energy Madrid, 12-13 November 2009.-- Editors: F. A. López, F. Puertas, F. J. Alguacil and A. Guerrero.-- 4 pages, 2 figures, 2 tables.; Activated carbons were prepared from chars obtained by carbonization of blends of a coal-tar pitch (CTP) and different polymers (1:1 w/w) at 800 ºC and, then, by activation with steam at 50 % burn-off. The polymers selected were: PET, PAN, PS and ABS. The porous structure of the activated carbons was determined from the adsorption of nitrogen at 77 K. All the activated carbons were microporous materials. However, nitrogen adsorption differs substantially from one activated carbon to another. While the polymers are responsible for the textural characteristics that are suitable for adsorption applications, CTP contributes to increasing the char yield without causing any significant loss in its adsorption properties. Among the polymers tested, PET and PAN were found to be the most suitable polymers for blending with CTP.; Peer reviewed

Titanium-Containing Zeolites and Microporous Molecular Sieves as Photovoltaic Solar Cells

Atienzar Corvillo, Pedro; Valencia, Susana; Corma, Avelino; García Gómez, Hermenegildo
Fonte: John Wiley & Sons Publicador: John Wiley & Sons
Tipo: Artículo Formato: 22092 bytes; application/pdf
ENG
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487.4529%
Final edited full-text version of the paper available at: http://dx.doi.org/10.1002/cphc.200700019; Four titanium-containing zeolites and microporous molecular sieves differing on the crystal structure and particle size (Ti/Beta, Ti/Beta-60, TS-1 and ETS-10) are prepared, and their activity for solar cells after incorporating N3 (a commercially available ruthenium polypyridyl dye) is tested. All the zeolites exhibit photovoltaic activity, and the photoresponse is quite independent of the zeolite pore dimensions or particle size. The photoresponse increases with titanium content in the range 1-7% wt. In this way, cells are obtained that have open-circuit voltage Voc=560 mV and maximum short-circuit photocurrent density Isc=100 μA, measured for 1×1 cm2 surfaces with a solar simulator at 1000 W through and AM 1.5 filter. These values are promising and comparable to those obtained for current dye-sensitized titania solar cells.; Financial support by the Spanish Ministry of Science and Education (CTQ 2006–06 875) is gratefully acknowledged. P.A. thanks the Ministry of Science and Education and the Universidad Politécnica for two post-graduate scholarships.; Peer reviewed

Functionalized adsorbent materials using supercritical CO2

López-Aranguren Oliver, Pedro; Rodríguez Viejo, Javier
Fonte: [Barcelona] : Universitat Autònoma de Barcelona, Publicador: [Barcelona] : Universitat Autònoma de Barcelona,
Tipo: Tesis i dissertacions electròniques; info:eu-repo/semantics/doctoralThesis; info:eu-repo/semantics/publishedVersion Formato: application/pdf
Publicado em //2015 ENG
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380.70496%
La silica (SiO2) porosa es uno de los adsorbentes históricamente más utilizado a nivel industrial en multitud de procesos. Sin embargo, el crecimiento acelerado de la demanda de nuevos materiales basados en nanotecnología y de nuevos procesos límpios y sostenibles ha hecho necesario el desarrollo de adsorbentes con nuevas o mejores propiedades fisico-químicas. Una de las opciones más empleadas para modificar la silica porosa es la incorporación de moléculas orgánicas con grupos funcionales dando lugar a materiales híbridos en los que se combinan las propiedades de ambos componentes. En esta tesis doctoral, se ha optado por el uso de dióxido de carbono supercrítico (scCO2) como disolvente para llevar a cabo ésta funcionalización. Estos procesos supercríticos, diseñados como sostenibles, actúan como sustitutos de técnicas convencionales que emplean disolventes orgánicos. En esta tesis se ha trabajado principalmente con matrices de sílica amorfa de poros structuralmente ordenados (MCM-41, 4 nm) y de poros desordenados (silica gel, 4-9 nm), analizándose sólo marginalmente las propiedades conferidas mediante la funcionalización a matrices cristalinas microporosas tipo zeolitas. Como agentes funcionalizantes, se han estudiado alkyl- y aminosilanos (octiltrietoxisilano y (methylamino)propyltrimethoxysilane ) y la aziridina que mediante una reacción de polimerización catalizada por CO2 forma la polietilenimina (PEI) dentro de los poros de la sílica...

Conductivity of Mono- and Divalent Cations in the Microporous Zincosilicate VPI-9

McKeen, John C.; Davis, Mark E.
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em 04/06/2009
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Impedance spectroscopy is used to investigate the long-range ionic conductivity of the microporous, zincosilicate VPI-9 (Si/Zn = 4.0) (International Zeolite Association framework type VNI) containing the alkali cations Li^+, Na^+, K^+, Rb^+, and Cs^+, and the alkaline earth cations Mg^(2+), Ca^(2+), and Sr^(2+). Monovalent cation-exchanged samples Li- and Na-VPI-9 lose X-ray crystallinity upon vacuum dehydration at 450 °C, whereas K-, Rb-, and Cs-VPI-9 remain crystalline and exhibit conductivities of 1.7 × 10^(−4), 3.5 × 10^(−4), and 4.9 × 10^(−4) S/cm, respectively, at 450 °C and activation energies of 0.72, 0.64, and 0.69 eV, respectively, in the temperature range 150−450 °C. Divalent cation-exchanged sample Mg-VPI-9 also loses X-ray crystallinity, but Ca- and Sr-VPI-9 remain crystalline and exhibit conductivities of 2.3 × 10^(−6) S/cm and 7.7 × 10^(−7) S/cm, respectively, at 450 °C, and activation energies of 0.88 and 0.91 eV, respectively, over the temperature range 150−450 °C. When compared to aluminosilicate zeolite X (Si/Al = 1.25) exchanged with the same cations, all crystalline M-VPI-9 materials have greater conductivities than M-X, with the exception of K-X (1.6 × 10^(−3) S/cm at 450 °C), with the greatest differences arising between the divalent exchanged materials. Dense...

An electron diffraction, XRD and lattice dynamical investigation of the average structure and rigid unit mode (RUM) modes of distortion of microporous AlPO4-5

Liu, Yun; Withers, Raymond; Noren, Lasse
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
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387.4529%
Electron diffraction is used to investigate the unit cell and space group symmetry of the average structure of AlPO4-5 as well as the zero frequency rigid unit mode (RUM) modes of distortion thereof. Despite recent reports to the contrary, absolutely no evidence could be found to justify the lowering of the average structure space group symmetry of AlPO4-5 from P6cc. Direct experimental evidence (in the form of a highly structured, characteristic diffuse intensity distribution) is found, however, for the (presumably dynamic) excitation of numerous zero frequency RUM modes of distortion of the ideal AlPO4-5 tetrahedral framework structure. The lattice dynamic program CRUSH is used to confirm the existence of these zero frequency RUM modes of distortion as well as to show that they are an intrinsic, predictable property of the AlPO4-5 tetrahedral framework structure. Six zero frequency RUM modes of distortion are predicted to exist for modulation wave-vectors of the form [hk0]*, 4 for wave-vectors of the form [00]* and a single RUM for wave-vectors falling on a curved (hyperbolic) distribution. These RUM modes of distortion involve tetrahedral rotation about in-plane rotation axes. Their simultaneous (presumably dynamic) excitation is necessary to understand the apparent linear Al-O-P angles in average structure refinements.

Rigid unit modes (RUMs) of distortion, local crystal chemistry and the inherent displacive flexibility of microporous AlPO4-11

Liu, Yun; Withers, Raymond
Fonte: Academic Press Publicador: Academic Press
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
387.4529%
Electron diffraction is used to investigate the average structure of microporous AlPO4-11 as well as the zero-frequency rigid unit mode (RUM) modes of distortion of the ideal AlPO4-11 tetrahedral framework. Direct experimental evidence (in the form of a highly structured, characteristic diffuse intensity distribution) has been found for the (presumably dynamic) excitation of numerous zero-frequency RUM modes of distortion. The lattice dynamic program CRUSH is used to confirm the existence of zero-frequency RUM modes of distortion with wave-vectors falling on the observed diffuse distribution. The simultaneous (presumably dynamic) excitation of such RUM modes of distortion needs to be taken into account in order for the local crystal chemistry of AlPO4-11 to be understood.

Electrochemical cleaning of microporous metallic filters fouled with bovine serum albumin and phosphate under low cross-flow velocities

Webster, Richard; Chilukuri, S; Webster, B J
Fonte: Kluwer Academic Publishers Publicador: Kluwer Academic Publishers
Tipo: Artigo de Revista Científica
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386.12156%
A three-electrode electrochemical cell has been designed to test the cleaning efficiency of applying potentials or currents to 0.2 μm pore size silver membranes fouled with bovine serum albumin and phosphate under low crossflow velocities. It was found t