Pore Size and Shape Effects in Zeolite Catalysis

Bendoraitis, J. G.; Chester, Arthur W.; Dwyer, Francis G.; Garwood, W. E.

doi:10.1016/s0167-2991(09)60933-2

Cited by 25 publications

(12 citation statements)

References 4 publications

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“…The authors further argued that, under adsorption conditions, the pore openings of the investigated zeolites have a more or less elliptical shape, hence the faster uptake of cyclohexane. Similar conclusions for the shape of the pore cross-sections in zeolites ZSM-5 and ZSM-23 under catalytic conditions were drawn by Bendoraitis et al from catalytic data obtained during dewaxing of distillates [33]. They also concluded that pore sizes determined under catalytic conditions are always larger than computed crystallographic pore sizes, as also observed in adsorption measurements.…”

Section: Molecular Sievingsupporting

confidence: 81%

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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Section: Molecular Sievingsupporting

confidence: 81%

Shape-Selective Catalysis in Zeolites

Weitkamp

Puppe

1999

Catalysis and Zeolites

119

129

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“…Tubular-pore zeolites (e.g., TON-, MTT-, MTW-, GON-, and ZSM-48-type zeolites) are of industrial interest because they absorb and hydroisomerize long linear alkanes (wax), effectively turning them into high-value base oil [31][32][33][34][35][36]. Catalysts based on tubular-pore zeolites have replaced catalysts based on intersecting-pore MFI-type zeolites, which absorb and hydrocrack the long linear alkanes out of the base oil fraction into less-valuable products [32][33][34][35].…”

Section: Introductionmentioning

confidence: 99%

“…Catalysts based on tubular-pore zeolites have replaced catalysts based on intersecting-pore MFI-type zeolites, which absorb and hydrocrack the long linear alkanes out of the base oil fraction into less-valuable products [32][33][34][35]. The higher hydroisomerization selectivity of the nonintersecting tubular-pore zeolites and the higher hydrocracking selectivity of the intersecting-pore MFItype zeolite result from the way in which these zeolites process hydrocracking precursors.…”

Section: Introductionmentioning

confidence: 99%

Shape-selective n-alkane hydroconversion at exterior zeolite surfaces

Maesen

Krishna²,

Baten³

et al. 2008

Journal of Catalysis

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A critical review of the adsorption and catalysis of n-and methylalkanes demonstrates that the interior surface of TON-and MTT-type zeolites dominates both adsorption and catalysis, and that the contribution from the exterior surface is negligible. For both n-and methylalkane isomers, the experimental Henry constants at the interior TON-type zeolite surface are more than an order of magnitude greater than those at the exterior surface. Molecular simulations on exclusively interior TON-type silica surface reproduce the adsorption isotherms of n-and methylalkane isomers remarkably well and suggest that even an isomer as bulky as 2,3-dimethylpentane could have access to the interior TON-type zeolite surface. Only the reference state used in solution thermodynamics affords an equitable comparison between internal and external surface thermodynamics. It indicates that methylalkanes adsorb in a structured fashion at the exterior TON-type zeolite surface when the interior surface is inaccessible. But the entropic penalty for this organized exterior surface "pore mouth" or "key-lock" adsorption is high, so that methylalkanes prefer adsorption at the interior surface when it is accessible. We speculate that CHA-and ERI-type sieves exhibit exterior surface catalysis in long n-alkane conversion, but the database remains too small to allow investigation of the full potential of shape selectivity in exterior zeolite surface catalysis.

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“…Noble metal loaded AELtype silicoaluminophosphate molecular sieves selectively absorb the wax-like, long-chain normal paraffins from an oil feedstock and hydroconvert them selectively into branched paraffins (1)(2)(3). Catalysts based on TON-(4-7) and MTTtype (1,4,(7)(8)(9) zeolites combine a strong affinity for longchain, normal paraffins with a significantly higher selectivity for hydroisomerization than for hydrocracking (1)(2)(3)(4)(5)(6)(7)(8)(9).…”

Section: Introductionmentioning

confidence: 99%

The Shape Selectivity of Paraffin Hydroconversion on TON-, MTT-, and AEL-Type Sieves

Maesen

Schenk

Vlugt

et al. 1999

Journal of Catalysis

140

137

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Based on a comparison between simulated and measured adsorption properties, we demonstrate that both normal and monomethylparaffins are able to fully enter the pores of TON-, MTT-, and AEL-type molecular sieves. This disproves the theory that monomethylparaffins only partially enter these pores and that normal paraffins are predominantly hydroisomerized at the pore mouths of these sieves. Instead, we attribute the high selectivity for paraffins with terminal methyl groups to product shape selectivity, and the low selectivity for paraffins with neighboring methyl groups to transition state selectivity. These traditional shape selectivity concepts explain not only the detailed product distribution of n-heptane hydroconversion, but also that of longer-chain n-paraffins.

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Pore Size and Shape Effects in Zeolite Catalysis

Cited by 25 publications

References 4 publications

Shape-Selective Catalysis in Zeolites

Shape-Selective Catalysis in Zeolites

Shape-selective n-alkane hydroconversion at exterior zeolite surfaces

The Shape Selectivity of Paraffin Hydroconversion on TON-, MTT-, and AEL-Type Sieves

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