Intracrystalline porosity

A number of crystals other than zeolites possess permanent intracrystalline porosity on a molecular scale. These include Dianin s compound, /3-Ni(or Co)(4-methylpyridine)4(NCS)2, and smectites in which the... 

Besides the intracrystalline porosity, the number and strength of the acid sites and the EFAL/FAL ratio, the porosity of the binder/zeolite composite have to be optimized, too. 

The intracrystalline porosity is often taken as the fraction of volume occupied by liquid water evolved on heating and evacuating the zeolite. Typical values are ... 

The intracrystalline porosity may be available in the form of large cavities, linked by large or small shared windows (zeolites X, A, or chabazite), or this porosity may be subdivided into many small cavities joined by small windows e.g., in the nonzeolites, cristobalite and tri-dymite, or in analcite). Table VII illustrates densities, window apertures, and saturation capacities for porous crystals, showing a range in all these... 

The science of micrcporous materials has developed dramatically since intracrystalline porosity was first demonstrated in certain crystals. 

Zeolite H-T catalyzed the ketonization of short-chain carboxylic acids. The formation of anhydrides is a side reaction, occurring on the outer surface of the zeolite ctystals. The propionic and butyric acid molecules seem to have the optimum size for a bimolecular ketonization reaction inside an erionite cavity. The ketonization of carboxylic acids is an example of zeolite specificity in catalysis, illustrating the necessity of strict adaptation of the transition state of the reaction to the intracrystalline porosity of the zeolite. 

This book is a comprehensive and dedicated source of information on clays and related layered materials. Related materials are defined for our purposes as those that share the ability to pillar, i.e., materials in which permanent intracrystalline porosity can be created within the layers. While myriad layered materials exist, they do not all share this pillaring ability (graphite, for example), and therefore are not included in this book. The layered mataials that form the core of this book certainly stand on their own merits in terms of their own particular chemistries, yet their applied technologies tend to be similar, including catalytic applications. 

This handbook serves as a companion volume to Zeolite Science and Technology, which was published in 2003. The scope and philosophy of the two books is much the same because they share the same publisher and coeditors. Because both volumes are strong on the basics and fundamentals, they are handbooks rather than simple monographs on the most recent applications, which tend to become outdated quickly. It has been our intent to focus keenly on the fundamental properties of the materials. Advances made in recent years concerning synthesis, characterization, host-guest chemistry, and modern applications are included. The permanent intracrystalline porosity feature ties these layered materials together for adsorptive and catalytic applications, among others. 

Zeolites are water-bearing tectosilicates, porous on the scale of molecules. As tectosilicates, they are 3-dimensional, A-connected nets of tetrahedral units, TO4, where T is usually A1 or Si. The water content may be large, up to 50 % of the volume of the crystals, but the zeolites feel dry to touch. The water is readily distilled out of the crystals and can as readily be re-imbibed. The intracrystalline porosity takes the form of diverse 1-, 2-, or 3-dimensional networks of channels, or of cavities linked by shared windows. 

It is seen that the intracrystalline MgO induces pore blockage in a fraction of the pore system and alters the porosity as well as D0, and/or r with the latter factors contributing most to the reduced diffusivity. In contrast, the coke modifier appears to affect mainly the surface-to-volume ratio and suggests that the effective surface area, number of available entrance ports, is reduced by two orders of magnitude. 

In a word, the intracrystalline mesopores are developed in the boronation of zeolites p since silicon atoms in the framework are dissolved by base in a large amount. The small atom size and poor stability of boron should be responsible for this dissolution and the modification in porosity. 

In the first, the pores may be an inherent feature of crystalline structures (e.g. zeolites, clay minerals). Such intracrystalline pores are generally of molecular dimensions and result in very regular networks often described as "structural" porosity. 

The finite information of the pore structure comes from the crystal structure analysis. Adsorption measurements also provide data on the pore system, based on the minimal size of molecules that can be excluded from the interior of the zeolites [12,13]. Xe NMR spectroscopy, via the chemical shifts of Xe, provides information about the porosity in zeolites [14,15]. Figure 11.4 demonstrates that in the range of porosity typically found in zeolites, the intracrystalline diffusivities can change by 12 orders of magnitude depending on the pore size and the size and shape of the molecule diffusing through the zeohtes [16]. 

A model of a composite zeolite pellet must thus be represented by a combination of coupled equations for intracrystalline diffusion and macropore diffusion. The diffusion of adsorbate within crystals was discussed in the previous section and intracrystalline diffusion is given by equation (4.17). Macropore diffusion for a spherical pellet of radius Rp, macropore diffusivity Dp and porosity Cp is described by... 

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