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Catalysis and temperature dependence on the formation of ZnO nanoparticles and of zinc acetate derivatives prepared by the sol-gel route

Tokumoto, M. S.; Pulcinelli, S. H.; Santilli, C. V.; Briois, V
Fonte: Amer Chemical Soc Publicador: Amer Chemical Soc
Tipo: Artigo de Revista Científica Formato: 568-574
Português
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The chemical and structural nature of powders prepared from the zinc acetate-derived precursor using the sol-gel route is discussed. The influence of the synthesis temperature and of the hydrolytic catalyst on the structural features of the powder is focused on the basis of X-ray powder diffraction (XRPD) and extended X-ray absorption fine structure (EXAFS) measurements and complemented with density and thermoanalysis (TG-DTA) results. EXAFS and XRPD results show that no-washed nanoparticulate powders are composed of a mixture of ZnO (wurtzite), zinc acetate, and zinc hydroxyacetate. The latter has a layered structure typical of hydroxy double salts (HDS). The main component of no-washed powders is always unreacted zinc acetate solid but the relative amount of the zinc-based compounds depends on the nature of the hydrolytic catalyst, hydrolysis ratio, and of synthesis temperature. According to the proportion of the three zinc-based compounds, three families of powders could be distinguished. The amount of ZnO nanoparticles (1.6 +/- 0.6 nm) decreases as the synthesis temperature increases, as the hydrolysis ratio decreases, or by changing from basic to acid catalysis. This finding suggests that the formation of zinc compounds is controlled by the equilibrium between hydrolysis-condensation and complexation-reprecipitation reactions.

Catalytic routes to convert saccharides to furanic aldehydes; Conversão catalítica de sacarídeos em 2-furaldeído e 5-hidroximetil-2-furaldeído

Antunes, Maria Margarida Feitor Pintão Moreno
Fonte: Universidade de Aveiro Publicador: Universidade de Aveiro
Tipo: Tese de Doutorado
Português
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The conversion of plant biomass-derived carbohydrates (preferably non-edible) into added-value products is envisaged to be at the core of the future biorefineries. Carbohydrates are the most abundant natural organic polymers on Earth. This work deals with the chemical valorisation of plant biomass, focusing on the acid-catalysed conversion of carbohydrates (mono and polysaccharides) to furanic aldehydes, namely 2-furaldehyde (Fur) and 5-hydroxymethyl-2-furaldehyde (Hmf), which are valuable platform chemicals that have the potential to replace a variety of oil derived chemicals and materials. The investigated reaction systems can be divided into two types depending on the solvent used to dissolve the carbohydrates in the reaction medium: water or ionic liquid-based systems. The reaction temperatures were greater than 150 ºC when the solvent was water, and lower than 150 º C in the cases of the ionic liquid-based catalytic systems. As alternatives to liquid acids (typically used in the industrial production of Fur), solid acid catalysts were investigated in these reaction systems. Aiming at the identification of (soluble and insoluble) reaction products, complementary characterisation techniques were used namely, FT-IR spectroscopy...

Comparative study of Al-MCM materials prepared at room temperature with different aluminium sources and by some hydrothermal methods

Ribeiro Carrott, Manuela; Conceição, Filipe; Lopes, J.M.; Carrott, Peter; Bernardes, C.; Rocha, J.; Ribeiro, F. Ramôa
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica Formato: 95816 bytes; application/pdf
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A comparison of the pore structural properties and catalytic activity of MCM-41 containing aluminium, prepared at room temperature with different aluminium sources, is presented. In addition, they are compared with those of MCM-41 obtained by two conventional hydrothermal procedures and room temperature synthesised Al-MCM-48 grades. Al-MCM-41 samples were prepared by direct synthesis at room temperature with tetraethoxysilane (TEOS) and aluminium isopropoxide, aluminium sulfate or sodium aluminate, and by two hydrothermal routes using silica Ludox suspension and sodium aluminate (at 377K) or Cab-O-Sil silica and alumina (at 377K and 423K). The synthesis of MCM-48 grades was also carried out at room temperature with TEOS and aluminium sulfate or nitrate. The materials were characterised by X-ray diffraction, nitrogen adsorption at 77K and solid-state 27Al MAS NMR. The catalytic activity was evaluated in the reaction of double bond position isomerisation of 1-butene to 2-butenes at 373K. It is concluded that the room temperature synthesis method used resulted in the preparation in a short period of time of well structured MCM-41 materials that contained predominantly tetracoordinated aluminium and presented acidic catalytic activity similar to or better than those prepared by the two hydrothermal procedures. Aluminium sulfate is a good alternative to isoproxide in the room temperature synthesis as it resulted in samples with very uniform pore size...

Hydrolysis of oligosaccharides over solid acid catalysts: a review

Vilcocq, Léa; Castilho, Paula C.; Carvalheiro, Florbela; Duarte, Luís C.
Fonte: Wiley Publicador: Wiley
Tipo: Artigo de Revista Científica
Publicado em //2014 Português
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Mild fractionation/pretreatment processes are becoming the most preferred choices for biomass processing within the biorefinery framework. To further explore their advantages, new developments are needed, especially to increase the extent of the hydrolysis of poly- and oligosaccharides. A possible way forward is the use of solid acid catalysts that may overcome many current drawbacks of other common methods. In this Review, the advantages and limitations of the use of heterogeneous catalysis for the main groups of solid acid catalysts (zeolites, resins, carbon materials, clays, silicas, and other oxides) and their relation to the hydrolysis of model soluble disaccharides and soluble poly- and oligosaccharides are presented and discussed. Special attention is given to the hydrolysis of hemicelluloses and hemicellulose-derived saccharides into monosaccharides, the impact on process performance of potential catalyst poisons originating from biomass and biomass hydrolysates (e.g., proteins, mineral ions, etc.). The data clearly point out the need for studying hemicelluloses in natura rather than in model compound solutions that do not retain the relevant factors influencing process performance. Furthermore, the desirable traits that solid acid catalysts must possess for the efficient hemicellulose hydrolysis are also presented and discussed with regard to the design of new catalysts.

The use of polyvinyl alcohol glutaraldehyde as solid-phase in ELISA for plague

Araujo,Aureci Maria; Petribú,André Tavares S; Barbosa,Gustavo Henrique T Sales; Diniz,José Ricardo P; Almeida,Alzira MP de; Azevedo,Walter Mendes; Malagueño,Elizabeth; Carvalho Jr,Luiz B
Fonte: Instituto Oswaldo Cruz, Ministério da Saúde Publicador: Instituto Oswaldo Cruz, Ministério da Saúde
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/04/1996 Português
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Discs of polyvinyl alcohol cross-linked with glutaraldehyde were synthesized under acid catalysis (H2SO4). Then, the antigen F1 purified from Yersinia pestis was covalently linked to this modified polymer. Afterwards, an enzyme-linked immunosorbent assay (ELISA) was established for the diagnosis of plague in rabbit and human. The best conditions for the method were achieved by using 1.3 ¼g of F1 prepared in 0.067 M phosphate buffer, pH 7.2, containing 1 M NaCl (PBS); anti-IgG peroxidase conjugate diluted 6,000 times and as a blocking agent 3% w/v skim milk in PBS. The titration of positive rabbit serum according to this procedure detected antibody concentrations up to 1:12,800 times. The present method, the conventional ELISA and passive haemagglutination assay are compared.

Desafios da acidez na catálise em estado sólido

Moreno,Esteban Lopez; Rajagopal,Krishnaswamy
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/01/2009 Português
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This study explores the similarities between solid and liquid acid catalysts highlighting the advantages and the main challenges of heterogeneous catalytic processes. We describe the main developments in technical procedures like selection of compounds and reaction models involved in: increasing acidity, characterization of solid acidity and in coke formation.

Nonconventional amide bond formation catalysis: programming enzyme specificity with substrate mimetics

Bordusa,F.
Fonte: Associação Brasileira de Divulgação Científica Publicador: Associação Brasileira de Divulgação Científica
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/05/2000 Português
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This article reports on the design and characteristics of substrate mimetics in protease-catalyzed reactions. Firstly, the basis of protease-catalyzed peptide synthesis and the general advantages of substrate mimetics over common acyl donor components are described. The binding behavior of these artificial substrates and the mechanism of catalysis are further discussed on the basis of hydrolysis, acyl transfer, protein-ligand docking, and molecular dynamics studies on the trypsin model. The general validity of the substrate mimetic concept is illustrated by the expansion of this strategy to trypsin-like, glutamic acid-specific, and hydrophobic amino acid-specific proteases. Finally, opportunities for the combination of the substrate mimetic strategy with the chemical solid-phase peptide synthesis and the use of substrate mimetics for non-peptide organic amide synthesis are presented.

Silica functionalized sulfonic acid catalyzed one-pot synthesis of 4,5,8a-triarylhex-ahydropyrimido[4,5-d]pyrimidine-2,7(1H,3 H)-diones under liquid phase catalysis

Gupta,Princy; Kumar,Vineet; Paul,Satya
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/01/2010 Português
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SiO2-SO3H catalyzed one-pot synthesis of 4,5,8a-triarylhexahydropyrimido[4,5-d]pyrimidine-2,7(1H,3 H)-diones (4a-k) is reported under liquid phase catalysis. The catalyst is easily prepared, highly stable, very simple to handle and recycled for five times without loss of significant activity.

Conversion of by-products from the vegetable oil industry into biodiesel and its use in internal combustion engines: a review

Piloto-Rodríguez,R.; Melo,E. A.; Goyos-Pérez,L.; Verhelst,S.
Fonte: Brazilian Society of Chemical Engineering Publicador: Brazilian Society of Chemical Engineering
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/06/2014 Português
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25.98%
Biodiesel produced from by-products and waste materials can be an economical way of reducing traditional oil consumption and environmental problems. The by-products from the vegetable oil refining industry such as soapstock, acid oil and fatty acid distillates are suitable for producing biodiesel. The present work is a survey related to the use of these by-products to obtain biodiesel, covering not only the traditional and most widely used acid/base catalysis, but also solid and enzymatic catalysis. Details of the techniques are presented and compared. The advantages and drawbacks of the different approaches are mentioned and analyzed. The synthesis and use of by-products from the vegetable oil refining industry are covered in this work. The use of the obtained biodiesel in diesel engines is also included, demonstrating the disparity between the number of papers related to biodiesel production and engine performance assessment.

Solid-phase synthesis of chiral 3,4-diazaphospholanes and their application to catalytic asymmetric allylic alkylation

Landis, Clark R.; Clark, Thomas P.
Fonte: National Academy of Sciences Publicador: National Academy of Sciences
Tipo: Artigo de Revista Científica
Português
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Functionalized chiral diazaphospholanes ligate to a variety of transition metals, yielding chiral, catalytically active, metal complexes. Previous work has established that amino acid derivatization of the carboxyl groups of (R,R)-N,N′-phthaloyl-2,3-(2-carboxyphenyl)-phenyl-3,4-diazaphospholane (1) yields phosphines that are excellent ligands for palladium-catalyzed asymmetric allylic alkylation reactions. Alanine functionalization is particularly effective for allylic alkylation of 1,3-dimethylallyl acetate. Standard Merrifield resins and amino acid coupling methods are used to synthesize the bead-attached phosphine having the topology bead-linker-lAla-(R,R)-1-lAla-OMe, as a 1:1 mixture of linkage isomers. Use of this supported phosphine in Pd-catalyzed asymmetric allylic alkylation yields 92% enantiomeric excess, matching prior solution-phase results. A 20-member collection of amino acid-functionalized phosphines on beads with the topology bead-linker-AA2-AA1-1-AA1-AA2 was synthesized by using parallel solid-state methods and screened for efficacy in allylic alkylation. Resulting enantioselectivities indicate that the AA1 position has the strongest effect on the reaction. Catalyst activities can vary widely with the nature of the phosphine ligand and the reaction conditions. Meaningful analysis of intrinsic catalytic activities awaits identification of the structure and abundance of the active catalyst.

SYNTHESIS OF TRIFLUOROMETHYL-IMINES BY SOLID ACID/SUPERACID CATALYZED MICROWAVE ASSISTED APPROACH

Abid, Mohammed; Savolainen, Markku; Landge, Shainaz; Hu, Jinbo; Surya Prakash, G. K.; Olah, George A.; Török, Béla
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em 01/06/2007 Português
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A new solid acid/superacid catalyzed microwave assisted synthesis of trifluoromethyl-imines is described. Various α,α,α-trifluoromethylketones react readily with primary amines to produce the corresponding imines. Two different strategies have been employed; one is the application of microwave irradiation coupled with solvent-free solid acid catalysis. The other method, for highly deactivated substrates includes the use of a pressure vessel at 175 °C temperature, with solid superacid catalysis. Using the solid acid K-10 montmorillonite or the superacidic perfluorinated resinsulfonic acid Nafion-H, a wide variety of trifluoromethylated imines have been synthesized using the above methods. The products have been isolated in good to excellent yields and high selectivities. This new environmentally friendly synthetic methodology provides significantly higher yields than traditional methods during relatively short reaction times for the preparation of the target compounds.

Síntese, caracterização e avaliação catalítica do silicoaluminofosfato SAPO-11 sulfatado

Silva, Edjane Fabiula Buriti da
Fonte: Universidade Federal do Rio Grande do Norte; BR; UFRN; Programa de Pós-Graduação em Química; Físico-Química; Química Publicador: Universidade Federal do Rio Grande do Norte; BR; UFRN; Programa de Pós-Graduação em Química; Físico-Química; Química
Tipo: Dissertação Formato: application/pdf
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Due to environmental restrictions around the world, clean catalytic technology are of fundamental importance in the petrochemical industry and refineries. Creating the face of this a great interest in replacing the liquid acids for solid acids, so as molecular sieves have been extensively studied in reactions involving the acid catalysis to produce chemical substances with a high potential of quality. Being the activity of the catalysts involved in the reaction attributed to the acid character of them involved for the Lewis and Brönsted acid sites. Based on this context, this study aimed to prepare catalysts acids using a molecular sieve silicoalumino-phosphate (SAPO-11) synthesized in hidrotermical conditions and sulphated with sulphuric acid at different concentrations, using to it the method of controlled impregnating. The samples resulting from this process were characterized by x-ray difratometry (DRX), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), thermal analysis (TG-DTG) and determination of total acidity (by n-butilamin adsorption). The results show that the synthesis method used was efficient in the formation of AEL structure of SAPO-11 and when being incorporated the sulfate groups in this structure the acidity of the material was increased...

Desenvolvimento de catalisadores nano e microestruturados para produção de bio-olefinas

Borges, Luciana Diniz
Fonte: Universidade de Brasília Publicador: Universidade de Brasília
Tipo: Tese
Português
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Tese (doutorado)—Universidade de Brasília, Instituto de Química, 2014.; Texto parcialmente liberado pelo autor. Conteúdo restrito: resultados e apêndice.; A produção seletiva de olefinas a partir da desidratação de alcoóis foi alcançada utilizando uma zeólita Y desaluminizada no estado sólido, Y(80)5C. Resultados experimentais obtidos por CG-FID mostraram que uma quantidade reduzida (0,1 g) desse sólido foi capaz de gerar valores de seletividades superiores a 96% para produção de etileno com conversão média de 90% durante 5 dias de reação, mesmo mediante uma alta velocidade espacial de etanol de 14,2 h'1. A recuperação e a reativação da zeólita modificada reduziu o número de sítios ácidos do material de 1,1088 para 0,4850 mmol g1 e. apesar da menor acidez, a Y(80)5C manteve os valores de conversão do etanol PA acima de 86% e os de seletividade para etileno acima de 90%, convertendo quase 20% a mais que a zeólita HY no mesmo período. Estudos com soluções aquosas de etanol em distintas concentrações mostraram que a presença de água no álcool de partida não se comportou como um veneno para o catalisador desaluminizado durante o período reacional investigado. Ao contrário, elevou a 100% os valores de seletividade para a formação da olefina devido á menor proporção de álcool etílico que passava por minuto pela zeólita sem prejuízo nas taxas de conversão. O bioetanol...

Identification of Active and Spectator Sn Sites in Sn-β Following Solid-State Stannation, and Consequences for Lewis Acid Catalysis

Hammond, Ceri; Padovan, Daniele; Al-Nayili, Abbas; Wells, Peter P; Gibson, Emma K; Dimitratos, Nikolaos
Fonte: Blackwell Publishing Ltd Publicador: Blackwell Publishing Ltd
Tipo: Artigo de Revista Científica
Português
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Lewis acidic zeolites are rapidly emerging liquid-phase Lewis acid catalysts. Nevertheless, their inefficient synthesis procedure currently prohibits greater utilization and exploitation of these promising materials. Herein, we demonstrate that SnIV-containing zeolite beta can readily be prepared both selectively and extremely rapidly by solid-state incorporation (SSI) method. Through a combination of spectroscopic (XRD, UV/Vis, X-ray absorption, magic-angle spinning NMR, and diffuse reflectance infrared Fourier transform spectroscopy) studies, we unambiguously demonstrate that site-isolated, isomorphously substituted SnIV sites dominate the Sn population up to a loading of 5 wt % Sn. These sites are identical to those found in conventionally prepared Sn-beta, and result in our SSI material exhibiting identical levels of intrinsic activity (that is, turnover frequency) despite the threefold increase in Sn loading, and the extremely rapid and benign nature of our preparation methodology. We also identify the presence of spectator sites, in the form of SnIV oligomers, at higher levels of Sn loading. The consequences of this mixed population with regards to catalysis (Meerwein–Pondorf–Verley reaction and glucose isomerization) are also identified.

Solid-State Nuclear Magnetic Resonance Studies Delineate the Role of the Protein in Activation of Both Aromatic Rings of Thiamin

Balakrishnan, Anand; Paramasivam, Sivakumar; Chakraborty, Sumit; Polenova, Tatyana; Jordan, Frank
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
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Knowledge of the state of ionization and tautomerization of hetero-aromatic cofactors when enzyme-bound is essential for formulating a detailed stepwise mechanism via proton transfers, the most commonly observed contribution to enzyme catalysis. In the bi-functional coenzyme, thiamin diphosphate (ThDP), both aromatic rings participate in catalysis, the thiazolium ring as an electrophilic covalent catalyst, the 4’-aminopyrimidine as acid-base catalyst involving its 1’,4’-iminopyrimidine tautomeric form. Two of four ionization and tautomeric states of ThDP are well characterized via circular dichroism spectral signatures on several ThDP superfamily members. Yet, the method is incapable of providing information about specific proton locations, which in principle may be accessible via NMR studies. To determine the precise ionization/tautomerization states of ThDP during various stages of the catalytic cycle (exemplified in Scheme 1), we report the first application of solid-state NMR spectroscopy to ThDP enzymes, whose large mass (160,000–250,000 Da) precludes solution NMR approaches. Three de novo synthesized analogs, [C2,C6’-13C2]ThDP, [C2-13C]ThDP and [N4’-15N]ThDP used with three enzymes revealed that: (a) Binding to the enzymes activates both the 4'-aminopyrimidine (via pKa elevation) and the thiazolium rings (pKa suppression)...

Role of dehydration catalyst acid properties on one-step DME synthesis over physical mixtures

Ramos, F. S.; Farias, Andréa Maria Duarte de; Borges, Luiz Eduardo Pizarro; Monteiro, José L. F.; Fraga, Marco André; Sousa-Aguiar, Eduardo Falabella; Appel, Lúcia Gorenstin
Fonte: Instituto Nacional de Tecnologia Publicador: Instituto Nacional de Tecnologia
Tipo: Artigo de Revista Científica
Português
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Catalysis Today 101 (2005) 39-44; The direct synthesis of dimethyl ether (DME) was studied in a continuous high-pressure unit composed basically of a Berty reactor and online gas chromatograph. A commercial methanol synthesis catalyst and some solid-acid catalysts (alumina. HZSM-S. tungsten-zirconia and sulfated-zirconia) were used as physical mixtures. The dehydration catalysts were characterised by pyridine adsorption followed by IR spectroscopy and tested in the methanol dehydration reaction itself. All samples were active regardless of the differences in their acidity and a relationship between catalytic activity for methanol-to-DME and acidity could be envisaged; the activity of a solid acid on this reaction was found to be determined mainly by the number of its more acidic sites. On the one-step DME synthesis from syngas the addition of an acid catalyst to the catalytic system strongly shifted the methanol synthesis reaction, increasing significantly the pass conversion of CO. As a general rule. it could be concluded that the determining rate of DME direct synthesis is determined by the acid properties of the dehydrating catalyst, i.e.• its acid strength and number of acid sites.

The chiral cyclobutane motif in advanced materials and catalysis: organogelators, surfactants, hybrid silicas and metal ligands

Sans Valls, Marta
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 Português
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Les diamines i els diàcids carboxílics en forma cíclica, ambdues estructures substituïdes en les posicions 1,2 de l'anell carbonat, constitueixen versàtils building blocks en química orgànica. Han estat utilitzats en la preparació de diferents derivats des de materials híbrids a lligands en reaccions catalítiques i en auto-ensamblatge de molècules (utilitzades com a plantilles per a nanoestructures inorgàniques), gràcies a la possibilitat de controlar la seva estereoquímica, rigidesa i requeriments estèrics. Així doncs, en la primera part d'aquesta Tesi Doctoral s'ha portat a terme la síntesi enantio- i diastereocontrolada dels quatre estereoisòmers de la ciclobutan-1,2-diamina amb ambdós grups amino protegits ortogonalment. Aquesta protecció assegura una funcionalització regioselectiva dels dos grups amino i en reté la quiralitat en el cas dels estereoisòmers cis que d'altra banda esdevindrien formes meso. La versatilitat d'alguns d'aquests compostos s'ha utilitzat en la preparació dels següents compostos o derivats. Hem de mencionar els organogeladors de baix pes molecular, els quals han mostrat habilitat de gelar toluè entre altres solvents. Aquests organogeladors estan constituïts per estructures ciclobutàniques i han estat comparats amb els organogels formats pels seus anàlegs amb anell ciclohexà. El seu comportament ha estat estudiat mitjançant l'ús de tècniques microscòpiques i per experiments de RMN a temperatura variable per tal de seguir el procés de gelació...

Boron removal and reinsertion studies in ??B???B exchanged HAMS-1B (H-[B]-ZSM-5) borosilicate molecular sieves using solid-state NMR

Hough, Andrea; Routh, Alexander F.; Clarke, Stuart M.; Wiper, Paul V.; Amelse, Jeffrey A.; Mafra, Lu?s
Fonte: Elsevier Publicador: Elsevier
Tipo: Article; accepted version
Português
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This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.jcat.2015.11.006; Novel atomic-level insight in boron removal and reinsertion into the framework of a HAMS-1B (H-[B]-ZSM-5) borosilicate molecular sieve was obtained by a combination of wet chemistry and one-/two-dimensional 11B solid-state NMR (SSNMR) spectroscopy. Uncalcined HAMS-1B shows only tetrahedral boron. However, three boron species are observed in 11B SSNMR spectra of as-prepared and then calcined HAMS-1B: tetrahedral framework boron ([4]Bfr), trigonal framework boron ([3]Bfr), and non-framework trigonal boron ([3]BNF). A picture has emerged as to the origins of these three species. Trigonal boron species are formed via hydrolysis by reaction with the water formed from water release and water formed by oxidation and removal of the template during calcination. The trigonal boron species are readily removed from the framework by slurrying in water or mild acid solutions. Tetrahedral boron remains at a concentration about equal to that in the calcined sieve not slurried, indicating that it is more difficult to remove. The extent of boron removal and reinsertion is pH dependent. We demonstrate that boron is removed to a greater extent at low pH and can be reinserted when pH is increased. Boron reinsertion into the framework is proven by 11B SSNMR on a series of 10B?11B exchanged borosilicate zeolites. We found that when boron is reinserted it enters at higher concentrations (?40% more) as tetrahedral boron...

Heteroatom-Substituted Delaminated Zeolites as Solid Lewis Acid Catalysts

Ouyang, Xiaoying; Hwang, Son-Jong; Xie, Dan; Rea, Thomas; Zones, Stacey I.; Katz, Alexander
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em 01/05/2015 Português
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This manuscript represents a comparative study of Lewis acid catalysis using heteroatom-substituted delaminated zeolites, which are synthesized using an approach that obviates the need for surfactants and sonication during exfoliation. The comparison involves heteroatom substitution into silanol nests of delaminated zeolites consisting of DZ-1 and deboronated UCB-4. Diffuse reflectance ultraviolet (DR-UV) spectroscopy demonstrates framework heteroatom sites, and the Lewis acidity of these sites is confirmed using infrared spectroscopy of adsorbed pyridine. The enhanced catalytic accessibility of these Lewis acid sites is confirmed when performing Baeyer–Villiger oxidation of substituted 2-adamantanones with hydrogen peroxide as the oxidant. Comparison of delaminated Sn-DZ-1 with three-dimensional Sn-Beta for this reaction shows that the delaminated zeolite is more active for bulkier ketone substrates. The role of the two-dimensional crystalline framework of the delaminated zeolite on catalysis is highlighted by comparing delaminated zeolites Sn-DZ-1 with Sn-UCB-4. The former exhibits a significantly higher activity for Baeyer–Villiger oxidation, yet when comparing Ti-DZ-1 with Ti-UCB-4, it is the latter that exhibits a significantly higher activity for olefin epoxidation with organic hydrogen peroxide...

Structure and reactivity of dehydroxylated bronsted acid sites in H-ZSM-5 zeolite: generation of stable organic radical cation and catalytic activity for isobutane conversion

Yun, Jang Ho
Fonte: University of Delaware Publicador: University of Delaware
Tipo: Tese de Doutorado
Português
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Lobo, Raul F.; Zeolites are crystalline aluminosilicate materials that have wide application in industry as solid acid catalyst. Since zeolites have high acidity, high surface area as well as the ability to do shape selectivity, they are used primarily as a solid catalyst in oil refining and petrochemical industries, in processes such as hydrocracking, fluid catalytic cracking (FCC). Bronsted acid sites are described as a hydroxyl group bridged between Al and Si (Al-OH-Si). It is well known that Bronsted acid sites are significant in a number of hydrocarbon processes, such as alkane cracking and isomerization and many other. The Bronsted acid sites of zeolites are decomposed at high temperatures, usually above 600 ̊C. This high temperature condition is commonly found in the fluidized catalytic cracking where catalyst is recycled forth and back between the riser and the regenerator under an oxidative atmosphere. The process of decomposition of hydroxyl group from the initial structure is called dehydroxylation. The dehydroxylation is believed to proceed via a dehydration mechanism of the acid sites. This heterolytic pathway of Bronsted acid site decomposition has been the accepted dehydroxylation path for low-silica zeolites for decades...