Channels, typical, zeolites

The heads of the stopcock molecules are too large to be able to enter the channels. Typical functionalized groups such as bucky balls [34], chelating centers [35], and others can be used. Some examples are given in Fig. 21. Similarly to the labels, heads can be reactive or nonreactive. Reactive heads may have arms , which can interact with each other to form a monolayer polymer or bind to the zeolite external surface. 

The mode of coke deposition is closely related to the pore structure of the zeolite (5-8). Figure 1 shows how coke deposits on typical zeolites. In the case ofZSM-5, coke deposits at intersections of the straight and zigzag channels, and also on the outer surface of the crystal. Whereas, Y type zeolites and mordenites have supercages whose sizes are almost equal to the molecular sizes of aromatic compounds composed of a few benzene rings, and coke is easily formed in the supercages. These differences in the manner of the coke formation reflect on mode of the deactivation... 

A very typical zeolitic organic material is anhydrous cholesterol. Its crystals are penetrated by channels along the three crystallographic directions a, b, and c (Fig. 2.1.45). It is therefore a good reagent for gas-solid and solid-solid reactions. This has already been verified by quantitative reactions with bromine [1] and oxalic add [1]. 

Apohost 2Co 3(18) 4(N03) has a similar pore size (3 x 6 A) and coordina-tively saturated Co centres. Typical zeolite guests such as CH4,02 and N2 can be reversibly bound in the internal channels under high pressure conditions [72]. 

Zeolites are microporous crystalline materials with pores that have about the same size as small molecules like water or n-hexane (pore size is usually 3-12 A). The structure of a zeolite is based on a covalently bonded TO4 tetrahedra in which the tetrahedral atom T is usually Silicium or Aluminum. The very famous Lowenstine rule only allows the existence of zeolites with a Silicium/Aluminum ratio of at least 1. As all corners of a tetrahedrcd have connections to other tetrahedra, a three dimensional pore network of channels and/or cavities is formed. Currently, these are about 100 different zeolite structures [1], several of these Ccin be found in nature. To clarify the topology of a typical zeolite, the pore structure of the zeolite Silicalite [2] is shown in figure 1.1. This zeolite has a three dimensional network of straight and zigzag channels that cross at the intersections. 

Zeolite IZA structure code Typical unit cell composition Si02/Al203 range by synthesis Dimensionality of channel system Pore apertures (nm)... 

In general, zeolites are crystalline aluminosilicates with microporous channels and/or cages in their structures. The first zeolitic minerals were discovered in 1756 by the Swedish mineralogist Cronstedt [3], Upon heating of the minerals, he observed the release of steam from the crystals and called this new class of minerals zeolites (Greek zeos = to boil, lithos = stone). Currently, about 160 different zeolite structure topologies are known [4] and many of them are found in natural zeolites. However, for catalytic applications only a small number of synthetic zeolites are used. Natural zeolites typically have many impurities and are therefore of limited use for catalytic applications. Synthetic zeolites can be obtained with exactly defined compositions, and desired particle sizes and shapes can be obtained by controlling the crystallization process. 

The pores of zeolites can be regarded as extensions of their surfaces zeolites have an external surface, i.e., the surface of the zeolite crystallites, and an internal surface, i.e., the surface of their channels and/or cages. In total, the surface areas of zeolites are remarkably large. One gram of a typical Faujasite zeolite expresses a geometric surface area of about 1100 m2/g (specific surface area). The contribution of the external surface area to this number is almost negligible (about 5 m2 g 1 for 1 pm crystallites), and almost the complete surface area is due to the surface of the micropores. 

Zeolite type Channel system Pore openings (A hydrated form) Typical Sl02 Al203 mole ratio Theoretical ion exchange capacity (meq/g Na form, anhydrous)... 

Zeolite crystal size can be a critical performance parameter in case of reactions with intracrystalline diffusion limitations. Minimizing diffusion limitations is possible through use of nano-zeolites. However, it should be noted that, due to the high ratio of external to internal surface area nano-zeolites may enhance reactions that are catalyzed in the pore mouths relative to reactions for which the transition states are within the zeolite channels. A 1.0 (xm spherical zeolite crystal has an external surface area of approximately 3 m /g, no more than about 1% of the BET surface area typically measured for zeolites. However, if the crystal diameter were to be reduced to 0.1 (xm, then the external surface area becomes closer to about 10% of the BET surface area [41]. For example, the increased 1,2-DMCP 1,3-DMCP ratio observed with decreased crystallite size over bifunctional SAPO-11 catalyst during methylcyclohexane ring contraction was attributed to the increased role of the external surface in promoting non-shape selective reactions [65]. 

Zeolites are crystalline aluminosilicates with porous, framework structures made up of linked [Si04] and [A104] tetrahedra that form channels and cages of discrete size [24]. The framework structures of zeolites bear a net negative charge, which must be balanced by positively charged species, typically alkali or alkaline earth metal cations these cations maybe exchanged for one another under appropriate experimental conditions. Zeolites are capable of... 

In Fig. 12 in Ref 25, fluorescence microscopy images of different dye-loaded zeolite L single crystals are shown. Each line consists of three pictures of the same sample, but with different polarizations of the fluorescence observed. In the first one, the total fluorescence of the crystals is shown, and in the others, the fluorescence with the polarization direction indicated by the arrows is displayed. The zeolite was loaded with the following dyes (A) Py+, (B) PyGY", (C) PyB +, (D) POPOP (see Table 1). Most crystals show a typical sandwich structure with fluorescent dyes at the crystal ends and a dark zone in the middle. This situation can be observed when the diffusion of the dyes in the channels has not yet reached its equilibrium situation. It illustrates nicely how the molecules penetrate the crystals via the two openings on each side of the one-dimensional channels. 

Figure 20 (a) Typical shape of a stopcock molecule located at the end of a zeolite channel, (b) Fluorescent molecules which have already been inserted in zeolite L are modified with an inert head in order to build stopcock molecules with fluorescent tails, (c) Examples of molecules which can be used as stopcocks with a fluorescent head. 

Zeolites are crystalline aluminosilicates whose primary structure is formed by Si04 and A104 tetrahedra sharing the edges . Their tertiary structure forms uniform channels and cavities of molecular dimensions that are repeated along the zeolite lattice. Due to the lower valence of the aluminium relative to silicon, the excess negative charge (one per A1 atom) is balanced by alkali metal cations, mainly Na". An important class of the zeolite family are the faujasites, known as zeolites X and Y, which have the typical composition for the unit cell as follows ... 

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