Formselektive Katalyse in Zeolithen

Weitkamp, Jens; Ernst, Stefan; Dauns, Harald; Gallei, E.

doi:10.1002/cite.330580803

Cited by 67 publications

(17 citation statements)

References 80 publications

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“…A reduction in the contribution of active sites at the external surface cannot only be achieved by selecting large crystallites, but also by coating the crystallites with an aluminum-free shell having the same structure as the core. This is possible during hydrothermal synthesis by first initiating crystallization of the desired (aluminum-containing) zeolite and 13. The influence of the crystallite size and the reaction temperature on the selectivity to para-xylene in the disproportionation of toluene over HZSM-S-type catalysts [78) then altering the crystallization medium to eliminate the aluminum therein, as demonstrated by Rollmann [120].…”

Section: Variation Of the Crystallite Size And Compositional Zoningmentioning

confidence: 99%

Shape-Selective Catalysis in Zeolites

Weitkamp

Puppe

1999

Catalysis and Zeolites

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110

129

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ScopeAmong the primary tasks of catalysis is the control of the selectivity in chemical reactions. In heterogeneously catalyzed reactions in zeolites and zeolite-related microporous solids, this can, inter alia, be achieved by exploiting the phenomenon of shape-selective catalysis. In a very simplified manner, shape-selective catalysis can be described as the combination of catalysis with the molecular sieve effect. Shape selectivity effects can occur, if the sizes and shapes of reactants, of products, of transition states or of reaction intermediates are similar to the dimensions of the pores and cavities of the zeolite.The first examples of shape-selective catalysis in zeolites were reported more than 35 years ago by Weisz and Frilette from the Mobil laboratories [1,2]. Since those days, research in the field of shape-selective catalysis has expanded both at universities and in industrial laboratories in an impressive manner. This has resulted in numerous examples of shape selectivity effects in the course of catalytic conversions over zeoli tic catalysts, which are nowadays no longer restricted to acid-catalyzed reactions as described in the early days. Rather, there are also examples for shape-selective catalysis on metals, on redox active sites, on basic guests in zeolites, and even for shape-selective photochemistry in a zeoli tic environment. Moreover, from the intensive research done in the field of shapeselective catalysis during the last decades, several large-scale commercial applications in the refining and the petrochemical industries have emerged, demonstrating the usefulness and importance of the phenomenon of shape-selective catalysis.In the past, numerous review papers on shape-selective catalysis, its principles and its commercial applications have been published [e. g., 3 -19]. In view of the vast amount of work that has been done in this field in recent years, it is obvious that a review on shape-selective catalysis cannot be exhaustive. Rather, the discussion has to be restricted to selected topics. In the present contribution, emphasis will be placed on a thorough discussion of the classical and of more recent principles of shape-selective catalysis and on how these principles can be used to rationalize the observed catalytic behavior in selected examples. In addition, it is the aim of the present contribution to discuss the available methods for tailoring the shape-selective properties of zeolite catalysts and for their characterization by catalytic methods. Finally, an attempt will be under-

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Section: Variation Of the Crystallite Size And Compositional Zoningmentioning

confidence: 99%

Shape-Selective Catalysis in Zeolites

Weitkamp

Puppe

1999

Catalysis and Zeolites

Self Cite

110

129

View full text Add to dashboard Cite

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“…[2,21] The impact of methods to change the amount, distribution or nature of existing species on the catalyst behaviour is well documented in scientific literature. [2,6,7,24] In most cases efficiency of dealumination techniques by liquid phase transformation, e.g.…”

Section: Exemplary Methods For Dealumination and Desilicationmentioning

confidence: 99%

“…by use of a (N H 4 ) 2 SiF 6 solution, improves the stability of the framework but imposes the risk of pore blocking by hard removable flouride species with alumina. [2,21] Gas phase dealumination techniques such as hydrothermal treatment or use of volatile SiCl 4 improves efficiency of dealumination processes. Also within these processes structural damages of zeolitic materials occure depending on post-synthetic preparation conditions such as temperature, pressure or duration.…”

Section: Exemplary Methods For Dealumination and Desilicationmentioning

confidence: 99%

“…Alumochlorine-oxo-species which are hard to remove by washing procedures afterwards, is often inevitable. [2,21,22] The impacts of these methods on catalyst samples have been proven by 27 Al-MAS-NMR. However, the applicability of the observed changes to EFSPE and their distribution is very limited.…”

Section: Exemplary Methods For Dealumination and Desilicationmentioning

confidence: 99%

“…[20] Especially Lewis acidic sites are generated during processes such as dehydroxylation during thermal treatment or different postsynthetic dealumination techniques, compare below. [21,22] EFSPE can influence the geometric as well as the chemical properties of H-ZSM-5. Their steric influence may M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT lead to pore blocking and diffusion limitations.…”

Section: Impact and Chemical Nature Of Extra-lattice Speciesmentioning

confidence: 99%

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