The role of external surface activity in the effectiveness of zeolites

Farcasiu, M.; Degnan, Thomas F.

doi:10.1021/ie00073a010

Cited by 33 publications

(23 citation statements)

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“…The plateau corresponds to the expected quantity of THA adsorbed on the external surface sites, in good agreement with earlier computed geometric external surface of zeolite Y. [67] In the case of this particular LZY-64, its geometric surface evaluated assuming spherical crystallites (500 nm diameter) and no Al zoning, as described elsewhere, [58,71] amounts to 10 m 2 /g, i. e. the external acid sites contribute to about 1.3 % of the total acidity, i. e. 45 μmol/g of zeolite.…”

Section: External Acidity Of the Fau Structure: Quest For The Right Msupporting

confidence: 90%

Probing the Brønsted Acidity of the External Surface of Faujasite‐Type Zeolites

et al. 2020

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We outline two methodologies to selectively characterize the Brønsted acidity of the external surface of FAU‐type zeolites by IR and NMR spectroscopy of adsorbed basic probe molecules. The challenge and goal are to develop reliable and quantitative IR and NMR methodologies to investigate the accessibility of acidic sites in the large pore FAU‐type zeolite Y and its mesoporous derivatives often referred to as ultra‐stable Y (USY). The accessibility of their Brønsted acid sites to probe molecules (n‐alkylamines, n‐alkylpyridines, n‐alkylphosphine‐ and phenylphosphine‐oxides) of different molecular sizes is quantitatively monitored either by IR or 31P NMR spectroscopy. It is now possible, for the first time to quantitatively discriminate between the Brønsted acidity located in the microporosity and on the external surface of large pore zeolites. For instance, the number of external acid sites on a Y (LZY‐64) zeolite represents 2 % of its total acid sites while that of a USY (CBV760) represents 4 % while the latter has a much lower framework Si/Al ratio.

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Section: External Acidity Of the Fau Structure: Quest For The Right Msupporting

confidence: 90%

Probing the Brønsted Acidity of the External Surface of Faujasite‐Type Zeolites

et al. 2020

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“…This correlation is visualized in Fig. 5.5 together with a very similar correlation for zeolites of the faujasite structure, which has been derived by Farcasiu and Degnan [48] based on the known crystallite density and again under the assumption of a homo- It can be seen from Fig. 5.5 that the calculated fraction of T-atoms on the external surface is very similar for both equations.…”

Section: Effect Of the Crystallite Sizesupporting

confidence: 77%

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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“…Zeolite crystallites are, however, finite‐dimensional objects. The structure and acidity of the so‐called “external surface” is expected to be different from that of the internal surface (within micropores), and its role is expected to become more important with small crystallite sizes (in particular below 100 nm), which is an attractive field of research for the conversion of bulky reactants from petroleum or renewables . For small‐pore zeolites (e.g., ZSM‐22) and long alkanes, the direct implication of acid sites located at the external surface was invoked and called “pore‐mouth catalysis” .…”

Section: Introductionmentioning

confidence: 99%

Ab Initio Simulation of the Acid Sites at the External Surface of Zeolite Beta

2017

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The acidity at the external surface of protonic zeolites, because of the finite size of crystallites, has been questioned strongly for decades. We used density functional theory (DFT) calculations to propose atomistic models for the external surface of zeolite Beta, which show that bridging Si−(OH)−Al groups still exist at the pore mouth in what we call open micropores (pores that emerge at the external surface). However, at the outermost surface (no emerging micropores), water molecules adsorbed on Al atoms [Al−(H2O)] prevail. The local structure of those surface Al atoms depends on the temperature and water partial pressure. A detailed vibrational study of adsorbed CO helps in the assignment of the different sites and reveals a generalized vibrational Stark effect. Proton‐transfer ability was quantified by the adsorption of isobutene. Carbenium ions appear to be stabilized on the bridging Si−(OH)−Al groups located on the open micropores of the external surface in a similar way as in the bulk of zeolite Beta. By contrast, the outermost surface is not able to stabilize carbenium ions and promotes the existence of alkoxides. This work brings new atomic‐scale insights into the concept of pore‐mouth catalysis and provides the molecular architecture of potential active sites located in open micropores.

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The role of external surface activity in the effectiveness of zeolites

Cited by 33 publications

References 0 publications

Probing the Brønsted Acidity of the External Surface of Faujasite‐Type Zeolites

Probing the Brønsted Acidity of the External Surface of Faujasite‐Type Zeolites

Shape-Selective Catalysis in Zeolites

Ab Initio Simulation of the Acid Sites at the External Surface of Zeolite Beta

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