Zeolites channel ring size

Table 1. 3.2 Zeolites as Zeolites and zeotypes and their major channel ring size. Catalysts 81... 

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]. 

Molecules with a cross section diameter greater than about 0.60 nm caimot diffuse into the TS-1 channels and are, therefore, not oxidized. This size restriction limits this system essentially to the oxidation of linear molecules and monocyclic aromatic rings with at most, small substituents. Even with these limitations, TS-1, and to a lesser extent TS-2, is an effective catalyst for the selective oxidation of a number of different types of organic compounds. Thirty percent hydrogen peroxide is the most commonly used oxidant. The more bulky alkyl peroxides are not effective because of their inability to diffuse into the zeolite channels to react with the titanium sites. While the oxidation of most primary and secondary alcohols occurs with reasonable ease, methanol is sufficiently inert to oxidation under the common reaction conditions that it is the solvent of choice for most TS-1 catalyzed reactions. 2 26,27... 

The authors used a 16 channel reactor at a pressure of 25 bar and a temperature of 400°C and studied zeolite catalysts with different ring sizes and pore connectivity. A clear correlation between the dealkylation selectivity for ethylbenzene and the zeolite pore volume was observed. Dealkylation selectivity increased with decreasing pore volume. In addition, toluene, which is formed by a bimolecular reaction as a primary product, is also favoured by specific pore systems, with that of zeolite beta and ITQ-23 being the most favourable ones. This study showed, that the combination of high-throughput reactors with properly designed experiments can accelerate the understanding of the catalytic action, in this case of zeolites. 

Zeolite Structure type Code8 Diameter of largest channel (A) Dimensionality Ring size of channels... 

For example, the shape and dimension of channels and/or pores of some common zeolites are being depicted in Fig. 2.1 Ih, i, j [8]. It has been established that Si/Al ratios have considerable effect on other properties (viz., CEC, channel dimension, void volume and specific gravity). Figure 2.1 Ih represents the zeolite, Analcime (ANA)-distorted 8-ring, viewed along [110], which has cubic shaped irregular channels of size (A) 4.2 x 1.6 [8]. 

The best correlation of the observed isomerization selectivities was found in terms of the diameter of the intracrystalline cavity, determined from the known crystal structure (9) of these zeolites, as shown in Figure 2. While faujasite, mordenite and ZSM-4 all have 12-membered ring ports and hence should be similar in their diffusion properties, they differ considerably in the size of their largest intracrystalline cavity both mordenite and ZSM-4 have essentially straight channels, whereas faujasite has a large cavity at the intersection of the three-dimensional channel system. 

CHA (-34, -44, -47), ERI (-17), GIS (-43), LEV (-35), LTA (-42), FAU (-37) and SOD (-20). Also shown is the pore size and saturation water pore volume for each structure type. The structures include the first very large pore molecular sieve, VPl-5, with an 18-ring one-dimensional channel with a free pore opening of 1.25 nm [29], large pore (0.7-0.8nm), intermediate pore (0.6nm), small pore (0.4 nm) and very small pore (0.3 nm) materials. Saturation water pore volumes vary from 0.16 to 0.35cm /g, comparable to the pore volume range observed in zeolites (see Chapter 2 for detailed structures). 

The presence and position of the cations in zeolites is important for several reasons. The cross section of the rings and channels in the structures can be altered by changing the size or the charge (and thus the number) of the cations, and this... 

Possibly the earliest theoretical study of diffusion of aromatics in zeolites was published in 1987 by Nowak et al. (89), who considered diffusion of benzene and toluene in the pores of silicalite and theta-1. Theta-1 (90) has a unidimensional medium-sized pore opening bounded by 10-rings. In this study, only the straight channel of silicalite was considered, making the... 

Na-A may be explained easily as a consequence of the aromatic rings of 97 being unable to enter the pores. Photoreactions are forced to occur at the surface in a two-dimensionally restrictive environment reminiscent of that provided by silica. The absence of selectivity upon irradiation of 97 in the Na-Zeolite-/ or in Na-mordenite follows from the relatively large size of their pores and channels which impose few conformational restrictions on BRs in the two dimensions that affect the ease of c-BR formation. 

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