Zeolite Inclusion Chemistry

Bein, Thomas

doi:10.1021/bk-1992-0499.ch020

Cited by 19 publications

(5 citation statements)

References 17 publications

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“…(a) X-ray absorption near edge structure (XANES) and (b) Fourier transforms of the k -weighted extended X-ray absorption fine structure (EXAFS) for 1.1 wt % of Pt/NaA samples at various stages in the mercaptosilane-assisted synthesis process (A, as-synthesized Pt/NaA; B, Pt/NaA treated in O 2 and H 2 at 623 K; C, bulk Pt).…”

Section: Resultsmentioning

confidence: 99%

“…The encapsulation of metal or metal oxide clusters within small voids in zeolites or other microporous solids can be used to contain and protect active structures and to select reactants, products, or transition states, thus providing diverse strategies for the design and synthesis of more effective catalysts. − The small and uniform size of channels and voids within crystalline microporous solids allows selective access of molecules to intracrystalline active sites based on size or shape. The placement of clusters within such constrained environments also tends to stabilize small clusters against sintering and poisoning more effectively than less restrictive mesoporous structures.…”

Section: Introductionmentioning

confidence: 99%

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Mercaptosilane-Assisted Synthesis of Metal Clusters within Zeolites and Catalytic Consequences of Encapsulation

2010

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We report here a general synthetic strategy to encapsulate metal clusters within zeolites during their hydrothermal crystallization. Precursors to metal clusters are stabilized against their premature colloidal precipitation as hydroxides during zeolite crystallization using bifunctional (3-mercaptopropyl)trimethoxysilane ligands. Mercapto (-SH) groups in these ligands interact with cationic metal centers, while alkoxysilane moieties form covalent Si-O-Si or Si-O-Al linkages that promote zeolite nucleation around ligated metal precursors. These protocols led to the successful encapsulation of Pt, Pd, Ir, Rh, and Ag clusters within the NaA zeolite, for which small channel apertures (0.41 nm) preclude postsynthesis deposition of metal clusters. Sequential treatments in O(2) and H(2) formed small (approximately 1 nm) clusters with uniform diameter. Titration of exposed atoms with H(2) or O(2) gave metal dispersions that agree well with mean cluster sizes measured from electron microscopy and X-ray absorption spectroscopy, consistent with accessible cluster surfaces free of mercaptosilane residues. NaA micropore apertures restrict access to encapsulated clusters by reactants based on their molecular size. The ratio of the rates of hydrogenation of ethene and isobutene is much higher on clusters encapsulated within NaA than those dispersed on SiO(2), as also found for the relative rates of methanol and isobutanol oxidation. These data confirm the high encapsulation selectivity achieved by these synthetic protocols and the ability of NaA micropores to sieve reactants based on molecular size. Containment within small micropores also protects clusters against thermal sintering and prevents poisoning of active sites by organosulfur species, thus allowing alkene hydrogenation to persist even in the presence of thiophene. The bifunctional nature and remarkable specificity of the mercapto and alkoxysilane functions for metal and zeolite precursors, respectively, render these protocols extendable to diverse metal-zeolite systems useful as shape-selective catalysts in demanding chemical environments.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Mercaptosilane-Assisted Synthesis of Metal Clusters within Zeolites and Catalytic Consequences of Encapsulation

2010

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“…Zeolite MFI (ZSM-5) [162,163] Zeolite Y [10,11,75] Zeolite LTL (L) [163] Zeolite FAU (Faujasita) [10,11] Porous Vycor Glass (PVG) [ the pores of these membranes can be performed chemically or electrochemically. By controlling the duration of polymerization, it is possible to obtain tubes or filaments of conductive polymers.…”

Section: -D Zeolitesmentioning

confidence: 99%

Spectroscopy of Nanostructured Conducting Polymers

Nascimento

Souza

2010

Nanostructured Conductive Polymers

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“…Nesse caso, a zeólita funciona como um "solvente dentro de um solvente", extraindo da fase líquida as moléculas de substrato somente se houver compatibilidade entre as características físico-químicas da zeólita com o substrato 30 . Portanto, além do tamanho de poro apropriado, as afinidades por água (hidrofilicidade) do substrato e da zeólita devem ser coincidentes: um substrato altamente polar não entraria nos poros de uma zeólita Y desaluminada, por exemplo 32 . Esta característica pode ser considerada análoga à do manto protéico das enzimas naturais, que identifica qual o substrato terá acesso ao sítio ativo com seletividade absoluta.…”

Section: Polaridadeunclassified

Modificação de zeólitas para uso em catálise

Luna¹,

Schuchardt²

2001

Quím. Nova

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Recebido em 27/10/00; aceito em 7/3/01 MODIFYING ZEOLITES FOR USE IN CATALYSIS: 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.Keywords: molecular sieves; zeolites; catalysts; heterogeneous catalysis. Quim. Nova, Vol. 24, No. 6, 885-892, 2001. 3 . O termo se aplica a sólidos porosos capazes de adsorver seletivamente moléculas cujo tamanho permite sua entrada dentro dos canais, como ilustrado na Figura 1. Divulgação INTRODUÇÃOAs zeólitas são consideradas os materiais mais promissores na busca da pedra filosofal do químico contemporâneo: o catalisador ideal. O mecanismo desse catalisador onírico funcionaria como uma pinça molecular, imobilizando cada molécu-la de substrato na posição apropriada para romper somente a ligação química necessária a fim de formar o produto esperado com altíssima atividade e seletividade absoluta 7 . ZEÓLITAS E MATERIAIS ANÁLOGOSSegundo a definição clássica, o termo zeólitas abrange somente aluminossilicatos cristalinos hidratados de estrutura aberta, constituída por tetraedros de SiO 4 e AlO 4 ligados entre si pelos áto-mos de oxigênio 5 . A rigor, somente esses materiais podem ser chamados de zeólitas; no entanto, é razoável advogar que o uso desse termo seja estendido para designar estruturas análogas contendo também tetraedros de outros elementos (PO 4 , GaO 4 , etc.). Zeólitas têm sido utilizadas principalmente como adsorventes para purificação de gases e como trocadores iônicos em detergentes, mas se mostram extremamente úteis como catalisadores no refino de petróleo 9 , na petroquímica 6 , e na síntese de produtos orgânicos cujas moléculas possuem diâmetro cinético inferior a 10 Å 10,11 . A síntese de zeólitas com poros de diâmetros maiores para utilização no processamento de moléculas mais volumosas, tí-picas de produtos farmacêuticos e na química fina, tem sido um dos principais desafios enfrentados por pesquisadores na última década 12 . A criação dos aluminofosfatos, uma nova classe de zeólitas, em 1982, abriu caminho para a síntese de estruturas de poros maiores do que até então conhecidas 13 No entanto, somente há três décadas zeólitas, como a estilbita de Cronstedt, começaram a ser empregadas como catalisadores para as mais variadas reações químicas 5 . Um breve histórico dos principais desenvolvimentos da Ciência relacionados à catálise com peneiras moleculares é mostrado na Tabela 1.A partir da aplicação pioneira em 1962 em processos de craqueamento de petróleo, as zeólitas assumiram hoje a posição de catalisadores mais importantes n...

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Zeolite Inclusion Chemistry

Cited by 19 publications

References 17 publications

Mercaptosilane-Assisted Synthesis of Metal Clusters within Zeolites and Catalytic Consequences of Encapsulation

Mercaptosilane-Assisted Synthesis of Metal Clusters within Zeolites and Catalytic Consequences of Encapsulation

Spectroscopy of Nanostructured Conducting Polymers

Modificação de zeólitas para uso em catálise

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