Zeolite channel structure

The unreacted ethanol and the diethylether product retained >98% of from the starting 0-ethanol, indicating that no isotope scrambling occurred. Data in Table 4 demonstrate that was retained in the mixed ether and ethanol attack of the acid-activated 2-pentanol via an axial S 2 rear-attack was the predominant synthesis pathway. Evidently, the shape selectivity induced by the 2 M-5 zeolite channel structure (Figure 2) plays an important role in achieving the remarkably higher configuration inversion... 

A number of interesting features can be seen in Figure 11.21. First, the level of water adsorption at p/p° = 0.90 by Silicalite-I is only about 10% of the capacity available for nitrogen and other small adsorptive molecules (see Table 1 l.S). This is increased to about 18% for HZSM-5, when the Si/Al ratio is reduced to 90. The presence of the hysteresis loops in the capillary condensation range indicates that a high proportion of the water adsorption has occurred within the secondary pore structure or defect structure rather than in the zeolitic channel structure. Similar findings have been reported by Llewellyn et al. (1996). 

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

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. ...

Due to blockage of the main zeolite channels by anions present in the structure, few applications have been reported for these tectosilicates. However, the presence of carbonate anions could make them suitable as antacids. Our group has worked with a series of natural minerals as antacids, such as clays [4], modified carbons [5] and zeolites[6]. In this way, a carbonated cancrinite and its intermediate phase were tested as an antacid with a synthetic gastric juice. 

Specific structure-directing effects of some organic bases or cations When in the procedure BT Pr N is replaced by other organics, various pentasil-type zeolitic precursors are formed. It appears that specific zeolites are formed only when quaternary ammonium salts are used, their nature (structure) being essentially dependent on the length of the alkyl chains pure ZSM-8, ZSM-5 and ZSM-11 are obtained respectively with Et N+, Pr N+ and Bu N cations. TG data indicate that the latter fill nearly completely the zeolitic channel system (Table VIII). 

As a result of steric constraints imposed by the channel structure of ZSM-5, new or improved aromatics conversion processes have emerged. They show greater product selectivities and reaction paths that are shifted significantly from those obtained with constraint-free catalysts. In xylene isomerization, a high selectivity for isomerization versus disproportionation is shown to be related to zeolite structure rather than composition. The disproportionation of toluene to benzene and xylene can be directed to produce para-xylene in high selectivity by proper catalyst modification. The para-xylene selectivity can be quantitatively described in terms of three key catalyst properties, i.e., activity, crystal size, and diffusivity, supporting the diffusion model of para-selectivity. 

Knowing the framework type of a material, the size of molecules that can be adsorbed can be estimated. Kinetic diameters for various molecules [5-9] are given in Table 2.2. Thus neopentane (kinetic diameter of 0.62 nm) is expected to be adsorbed by NaX zeolite (FAU structure type) which has channels defined by 12-... 

One of the most signiflcant variables affecting zeolite adsorption properties is the framework structure. Each framework type (e.g., FAU, LTA, MOR) has its own unique topology, cage type (alpha, beta), channel system (one-, two-, three-dimensional), free apertures, preferred cation locations, preferred water adsorption sites and kinetic pore diameter. Some zeolite characteristics are shown in Table 6.4. More detailed information on zeolite framework structures can be found in Breck s book entitled Zeolite Molecular Sieves [21] and in Chapter 2. 

In the same gel containing octylamine, ZSM-48 also rapidly crystallizes in presence of small amount of Al (Table 4, samples 4 and 5). However, for larger amounts of Al, a poorly crystalline ZSM-48 is obtained after 7 days heating, which for longer crystallization times, completely transforms into ZSM-22, another zeolite involving a linear channel structure (sample 6, Table 4). This iatter is probably formed upon a... 

In small pore zeolites with cage structure, e. g., faujasites, dye molecules encapsulated by in situ synthesis or crystallization inclusion are stable against extraction.1 2 However, these methods fail for MCM-41 due to the channel structure and the wider pore diameter (3 nm) of the host material. Covalent bonding of guests is necessary to obtain diffusion stability. Therefore, anchoring of organic molecules with catalytic functions into MCM-41 by covalent bonding was recently reported by Brunei et al.3... 

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 photochemistry of 6-arylpropiophenones has been examined in Silicalite and other zeolites The channel structure of Silicalite prevents intramolecular quenching by the 6-phenyl group in 6-phenylpropiophenone and leads to a dramatic enhancement of the phosphorescence. 

Crystalline g-phenylpropiophenone is not phosphorescent at room temperature. Similarly, adsorption on silicagel does not lead to significant phosphorescence. It would thus appear that a channel structure is essential. In fact, channel (or pore) dimensions also appear to be quite important for example, when g-phenylpropio-phenone is adsorbed on Mordernite, a zeolite with 9A pores, no luminescence can be detected (15). 

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