Intersecting channel zeolites

Figure 2. Transition state complex in the ethanol + 2-pentanol 8, 2 reaction activated by the proton at the chaimel intersection of H21SM-5 [14]. The zeolite pore structure is represented as a wire-frame section of the intersecting channels produced by the MAPLE V software package. The zeolite proton that activates the 2-pentanol molecule is marked with. ...

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. 

Among the medium-pore sized zeolites, perhaps the most studied are the pentasil zeolites, ZSM-5 and ZSM-11 (Figure 13). These zeolites also have three-dimensional pore structures a major difference between the pentasil pore structures and the faujasites described above is that the pentasil pores do not link cage structures as such. Instead, the pentasils are composed of two intersecting channel systems. For ZSM-5, one system consists of straight channels with a free diameter of about 5.4 x 5.6 A and the other consists of sinusoidal channels with a free diameter of about 5.1 x 5.5 A. For ZSM-11, both are straight channels with dimensions of about 5.3 x 5.4 A. The volume at the intersections of these channels is estimated to be 370 A3 for a free... 

The structure of the high silica zeolite ZSM-5 ty/x = 10 to>501X1) contains a high concentration of live-membered rings and has two intersecting channels. It has become a very important catalyst for petrochemical reactions. 

The high silica/alumina ratio zeolites ZSM-5 and ZSM-11 both contain two intersecting channel systems composed of 10-membered oxygen rings. The channels in these zeolites are elliptical, with a free cross-section of 5.5 x 5.1 for the linear channels, and a cross-section of 5.6 x 5.4 for the sinusoidal channels in ZSM-5. The channel structures of these two zeolites are shown in Figure 1. 

The zeolite crystal is modeled here as a finite, two-dimensional rectangular grid of intersecting channels. The adsorption and the desorption of molecules take place at border sites only according to the characteristics of zeolites, and the diffusion of the sorbed molecules in the channels is modeled as a random walk process. The reaction occurB in sorbed phase. The simulation technique was described elsewhere [2, 3], and the simulation results are calculated as the follows ... 

Y. Zhou, H. Zhu, Z. Chen, M. Chen, Y. Xu, H. Zhang, and D. Zhao, A Large 24-Membered-ring Germanate Zeolite-type Open-framework Structure with Three-dimensional Intersecting Channels. Angew. Chem., Int. Ed., 2001, 40, 2166-2168. 

The key catalyst in the MTG process is zeolite ZSM-5, which catalyzes the conversion of methanol to hydrocarbons. The framework of ZSM-5 has two types of intersecting channels one nearly circular and the other elliptical (ref. 10). The size of the openings exerts a strong influence on product distribution. ZSM-5 s high hydrothermal stability and low coke selectivity are critical for the MTG process to ensure satisfactory catalyst life. The low coke selectivity allows reasonable cycle lengths to be achieved without excessive catalyst requirements. 

The geometry of the pore space may be described in terms of cages, as in zeolites A and X, where the internal free dimensions far exceed the pore window size, or in terms of channels, where the free dimensions are approximately constant and equal to the window size, as in SSZ-24, for example. There may also be combinations of cages and channels within the same structure two different, unconnected pore systems, as in the zeolite MCM-22 or intersecting channel systems (such as observed in ZSM-5 and in TNU-9 (Figure 2.12)). 

The mordenite pore structure (Fig. 3.62) consists of elliptical and noninterconnect-ed channels parallel to the r-axis of the orthorhombic structure. Their openings are limited by twelve-membered rings (0.6 —0.7 nm). ZSM-5 zeolite (Fig. 3.63) shows a unique pore structure that consists of two intersecting channel systems one straight and the other sinusoidal and perpendicular to the former (Fig. 3.63). Both channel systems have ten-membered-ring elliptical openings (ra. 0.55 A in diameter)... 

Medium-pore zeolites have been shown to have excellent restricted transition state selectivity [164,166]. The high resistance toward coke formation on medium-pore zeolites has also been attributed to this type of shape selectivity [21,167-170]. Closely related to the shape selectivity on ZSM-5 is the effect of molecular traffic control [170]. This results from the existence of two types of intersecting channels reactant molecules preferentially enter the catalyst through a given chaimel system while the products diffuse out through the other, so that counter diffusion limitations are avoided. 

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