Channel zigzag

Politecnico di Milano and Ineos Vinyls UK developed a tubular fixed bed reactor comprising a metallic monolith 67,130). The walls were coated with catalytically achve material, and the monolith pieces were loaded lengthwise. Coming, the world leader in ceramic structured supports, developed metallic supports with straight channels, zigzag channels, and wall-flow channels. These metallic supports were produced by extrusion of metal powders, for example, copper, tin, zinc, aluminum, iron, silver, nickel, and mixtures and alloys (131). An alternative method is extmsion of softened bulk metal feed, for example, aluminum, copper, and alloys thereof. The metal surface can be covered with coating such as carbon, carbides, and alumina, for example, by application of a CVD technique (132). 

Figure C2.12.7. Channel system of MFI (top) and MEL (bottom). The linear channels are interconnected by zigzag channels in ZSM-5 while exclusively straight miming channels are present in ZSM-11 - larger internal openings are present at the chaimel intersections - the arrows indicate the pathways for molecular transport tlirough the channel system.

Figure 2.18 Different channel shapes for which flow distributions have been computed, a zigzag (upper left), a sinusoidally curved (upper right) and a converging-diverging channel (bottom).

Monte Carlo simulations and energy minimization procedures of the non-bonding interactions between rigid molecules and fixed zeolite framework provide a reasonable structural picture of DPP occluded in acidic ZSM-5. Molecular simulations carried out for DPB provide evidence of DPB sorption into the void space of zeolites and the preferred locations lay in straight channels in the vicinity of the intersection with the zigzag channel in interaction with H+ cation (figure 1). 

DPB as well as other DPP molecules (t-stilbene, diphenyl-hexatriene) with relatively low ionization potential (7.4-7.8 eV) and low vapor pressure was successfully incorporated in the straight channel of acidic ZSM-5 zeolite. DPP lies in the intersection of straight channel and zigzag channel in the vicinity of proton in close proximity of Al framework atom. The mere exposure of DPP powder to Bronsted acidic ZSM-5 crystallites under dry and inert atmosphere induced a sequence of reactions that takes place during more than 1 year to reach a stable system which is characterized by the molecule in its neutral form adsorbed in the channel zeolite. Spontaneous ionization that is first observed is followed by the radical cation recombination according to two paths. The characterization of this phenomenon shows that the ejected electron is localized near the Al framework atom. The reversibility of the spontaneous ionization is highlighted by the recombination of the radical cation or the electron-hole pair. The availability of the ejected electron shows that ionization does not proceed as a simple oxidation but stands for a real charge separated state. 

In the solid state the molecules pack in a way that zigzag chains -S-I- -S-I- (angular L S—I, approximately linear S- -I-S) are formed.60 The kind of chain topology is similar as that of the Se-I- -Se-L Se-I chains of dureneselenenyl iodide solid (2,3,5,6-Me4C6HSeI). Channels of weak soft-soft interactions... 

Figure 2.21 Cation sites in MFI. Site 1 is in near the intersection of the two ten-ring channel systems. Site 2 is in the straight ten-ring channel. Site 3 is in the zigzag ten-ring channel.

From N2 adsorption isotherms, the surface area (Sbet) of the ASS is 680 m2 g 1 and its pore volume is 0.46 cm3 g, which is to be compared with a pore volume of 0.17 cm3 g 1 for the HZSM-5 samples. The average pore diameter of AAS is 50 A, whereas in ZSM-5 there is an intersecting network of straight and zigzag channels (average diameter 5.5 A), the cavities at the intersections being ca. 9 A in diameter. 

MFI ZSM-5 Nan[AlnSi90 nO192] 16H20 (n < 27) High silica 3D Elliptical 10-rings 5.5 A (mean) None One straight and one zigzag channel... 

Figure 4.11 shows an example of how ZSM-5 is applied as a catalyst for xylene production. The zeolite has two channel types - vertical and horizontal - which form a zigzag 3D connected structure [62,63]. Methanol and toluene react in the presence of the Bronsted acid sites, giving a mixture of xylenes inside the zeolite cages. However, while benzene, toluene, and p-xylene can easily diffuse in and out of the channels, the bulkier m- and o-xylene remain trapped inside the cages, and eventually isomerize (the disproportionation of o-xylene to trimethylbenzene and toluene involves a bulky biaryl transition structure, which does not fit in the zeolite cage). For more information on zeolite studies using computer simulations, see Chapter 6. 

Figure6.4 Simulated structure ofthe MFI-type zeolite Silicalite-1 (O atoms in dark gray and Si atoms light gray), projected on the be plane and showing the zigzag channels. The broken lines indicate the periodic cell boundaries. Alkane molecules (not drawn to scale) are indicated by black circles. Thanks to Dr. Merijn Schenk for the zeolite picture.

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