The cause of shape selectivity of transalkylation in mordenite

Csicsery, Sigmund M.

doi:10.1016/0021-9517(71)90032-7

Cited by 122 publications

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“…Restricted transition state shape selectivity was first identified by Csicsery [69]. A typical example for this type of shape selectivity is depicted in Fig.…”

Section: Intrinsic Chemical Effects: Restricted Transition State Shapmentioning

confidence: 93%

“…Shape-selective conversion of alcohols over zeolite CaA [2) there must be at least one other type of shape selectivity [68,69]: He studied the acid-catalyzed disproportion at ion of 1-ethyl-2-methylbenzene (to toluene and diethylmethylbenzenes or to ethylbenzene and ethyldimethylbenzenes). Among the trialkylbenzenes, the 1,3,5-isomers predominate if thermodynamic equilibrium is established, and this is indeed observed experimentally if the reaction is conducted over catalysts with sufficiently large pores (i.e., amorphous Si0z-Al 2 0 3 or zeolite HY).…”

Section: Early Observationsmentioning

confidence: 99%

“…In zeolite H-mordenite, however, which possesses a more restricted pore system, the same 1,3,5-trialkylbenzenes are formed only very slowly. Csicsery was able to show that this is not due to a hindered diffusion of these species in the pores of mordenite, but rather, that the intermediates and/or transition states required for the direct formation of the 1,3,5-substituted isomers are too bulky to be formed in the intracrystalline voids of zeolite mordenite [68,69].…”

Section: Early Observationsmentioning

confidence: 99%

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Shape-Selective Catalysis in Zeolites

Weitkamp

Puppe

1999

Catalysis and Zeolites

119

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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“…Restricted transition state shape selectivity was first identified by Csicsery [69]. A typical example for this type of shape selectivity is depicted in Fig.…”

Section: Intrinsic Chemical Effects: Restricted Transition State Shapmentioning

confidence: 93%

Section: Early Observationsmentioning

confidence: 99%

Section: Early Observationsmentioning

confidence: 99%

See 1 more Smart Citation

Shape-Selective Catalysis in Zeolites

Weitkamp

Puppe

1999

Catalysis and Zeolites

119

129

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“…Product shape selectivity [41,49]: When the desorption rate limits the reaction rate, zeolites preferentially yield products that combine a high Gibbs free energy of adsorption with a low Gibbs free energy barrier to desorption. V. Transitions state shape selectivity: Zeolites preferentially form transition states with the lowest Gibbs free energy of formation [43,[50][51][52].…”

Section: Introductionmentioning

confidence: 99%

The selectivity of -hexane hydroconversion on MOR-, MAZ-, and FAU-type zeolites

Calero

Schenk

Dubbeldam

et al. 2004

Journal of Catalysis

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Analyses of a series of published n-hexane hydroisomerization product slates suggest that MAZ-type zeolites yield more dimethylbutane and less methylpentane than either FAU-or MOR-type zeolites. Molecular simulations do not corroborate the traditional view that these selectivity differences are specifically related to the MAZ-, FAU-, or MOR-type zeolite topology. A scrutiny of the literature indicates that reported variation in selectivity relates to a variation in the efficiency of the (de)hydrogenation function relative to the acid function. The FAU-type zeolite catalyst had the most efficient hydrogenation function. The efficiency of the hydrogenation function on the MAZ-type zeolite was low enough to significantly enhance the 2,3-dimethylbutane yield relative to the methylpentane yield, but not low enough to decrease the 2,2-dimethylbutane yield. The efficiency of the hydrogenation function on the MOR-type zeolite was low enough to do both. Only at a sufficiently high n-hexane hydroconversion does the catalyst with the most efficient hydrogenation function exhibit the highest dimethylbutane yield. This new perspective on the reported hexane hydroconversion selectivities suggests that a FAU-type zeolite catalyst with a highly efficient hydrogenation function is best suited for n-hexane hydroisomerization. The FAU topology has the highest porosity which should afford the highest activity without impairing selectivity.  2004 Elsevier Inc. All rights reserved.

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“…While the relatively small pores of the zeolites limit their utilization to the synthesis of small molecules, the well-defined and narrow pore size distribution can be used to induce shape selectivity (4)(5)(6)(7)(8) by (i) allowing only certain reactants to reach the active sites (reactant selectivity) (9), (ii) imposing diffusional constraints on bulkier reaction products (product selectivity) (10,11), or (iii) excluding the formation of certain products due to spatial constraints (restricted transition state selectivity) (12).…”

Section: Introductionmentioning

confidence: 99%

Selectivity Enhancement in Methylamine Synthesis via Postsynthesis Modification of Brønsted Acidic Mordenite

Grundling

Eder‐Mirth

Lercher

1996

Journal of Catalysis

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Methylamine synthesis from methanol and ammonia over parent and modified Brønsted acidic mordenites is studied by in situ infrared spectroscopy and kinetic analysis to elucidate the role of elementary steps for activity and selectivity. In situ infrared spectroscopy reveals that all methylammonium ions are formed in the micropores of these catalysts. The formation of the chemisorbed methylamines, however, is not rate determining. Transient response experiments indicate that the desorption of these methylamines aided by adsorbing ammonia and/or the scavenging of methyl groups with ammonia constitutes the rate determining step. For a given catalyst, the selectivity strongly depends on the methanol conversion and the ammonia to methanol ratio of the feed. Upon modification of mordenite with tetraethoxysilane, the selectivity to the lower methylated amines is strongly enhanced. The transport limitations of the bulkier products, formed in high concentration in the pores, are concluded to cause the enhanced selectivity toward mono-and dimethylamine over the modified catalyst. Since a decrease in activity compared to the parent sample was not observed, it seems that the methyl-scavenging mechanism plays an important role over these catalysts.

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The cause of shape selectivity of transalkylation in mordenite

Cited by 122 publications

References 5 publications

Shape-Selective Catalysis in Zeolites

Shape-Selective Catalysis in Zeolites

The selectivity of -hexane hydroconversion on MOR-, MAZ-, and FAU-type zeolites

Selectivity Enhancement in Methylamine Synthesis via Postsynthesis Modification of Brønsted Acidic Mordenite

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