Shape-Selective Catalysis Zeolites

Csicsery S M 1976 Shape-selective catalysis Zeolite Chemistry and Catalysis ACS Monograph vol 171, ed J A Rabo (Washington, DC American Chemical Society) pp 680-713... 

Shape-Selective Catalysis Zeolites Exercises for Chapter 7... 

Zeolites are crystalline aluminosilicates which assemble into well-defined three-dimensional structures comprised of microporous channels that interconnnect cavities which approach molecular scale dimensions ranging from 2 to 12 A. Mesoporous silica materials with pore sizes over 40 nm have also been made. They are relatively easy to synthesize and offer outstanding control over the pore size as well as its three-dimensional pore architecture, which makes them ideal for gas separations and shape-selective catalysis. Zeolites are perhaps the most widely recognized catalytic material. They are found in nature in over 50 distinct mineral forms. In addition, over 140 man-made zeolites have also been synthesized, with a total of 165 different framework structmesl l. 

Ti, B, Ni, Cr, Fe, Co, Mn) has been described, as was the synthesis of nonsiliceous materials such as oxides of W, Fe, Pb, Mo, and Sb [18]. Although these materials do not represent tme zeolites, they are highly interesting materials which are commonly covered in the zeolite literature with great potential for shape-selective catalysis of bulky molecules. 

Anotlier important modification metliod is tire passivation of tire external crystallite surface, which may improve perfonnance in shape selective catalysis (see C2.12.7). Treatment of zeolites witli alkoxysilanes, SiCl or silane, and subsequent hydrolysis or poisoning witli bulky bases, organophosphoms compounds and arylsilanes have been used for tliis purjDose [39]. In some cases, tire improved perfonnance was, however, not related to tire masking of unselective active sites on tire outer surface but ratlier to a narrowing of tire pore diameters due to silica deposits. 

Figure C2.12.10. Different manifestations of shape-selectivity in zeolite catalysis. Reactant selectivity (top), product selectivity (middle) and transition state selectivity (bottom).

Only a very few selected examples have been discussed. The number of processes based on shape-selective catalysis by zeolites is ever increasing, particularly in the field of speciality and fine chemicals and quite a few have been... 

Weitkamp J, Ernst S and Puppe L 1999 Shape-selective catalysis in zeolites Cataiysis and Zeoiites, Fundamentais and Appiications ed J Weitkamp and L Puppe (Berlin Springer) pp 327-76... 

Shape selective catalysis as typically demonstrated by zeolites is of great interest from scientific as well as industrial viewpoint [17], However, the application of zeolites to organic reactions in a liquid-solid system is very limited, because of insufficient acid strength and slow diffusion of reactant molecules in small pores. We reported preliminarily that the microporous Cs salts of H3PW12O40 exhibit shape selectivity in a liquid-solid system [18]. Here we studied in more detail the acidity, micropore structure and catal3rtic activity of the Cs salts and wish to report that the acidic Cs salts exhibit efficient shape selective catalysis toward decomposition of esters, dehydration of alcohol, and alkylation of aromatic compound in liquid-solid system. The results were discussed in relation to the shape selective adsorption and the acidic properties. 

Zeolites possess the remarkable property of exhibiting shape-selective catalysis even when they are X-ray amorphous. Clearly, even though there is no long range order, there is still a degree of structural organization in the aluminosilicate adequate to exert shape-selectivity in the "noncrystalline" regions of the samples. Thanks to HREM we can now understand how this state of affairs arises (17). 

Csicsery, S.M. (1984). Shape-selective catalysis in zeolites. Zeolites 4, 202-213... 

HZSM-5, 5 241 72 192 nitration using, 5 333 shape-selective catalysis, 5 243-244 HZSM5 zeolite. See Zeolite HZSM5 HZSM-11, nitration using, 5 333... 

Derouane, E.G., New aspects of molecular shape-selectivity catalysis by zeolite ZSM-5 Imelik, B. Naccache, C. ... 

Figure 4.20 MTG/MTO reaction path and aromatics distribution with different zeolites as catalysts. (Reprinted from C.D. Chang, W.H. Lang, W.K. Bell, Catalysis in Organic Reactions, Molecular Shape-Selective Catalysis in Zeolites, pp. 93-94. Copyright 1981. With permission from Marcel Dekker.)...

Weitkamp, J., Ernst, S., and Puppe, L. (1999) Shape-selective catalysis in zeolites, in Catalysis and Zeolites, vol. 

The advantages of shape selective catalysis are alreacfy ejq)loited in a number of industrial processes [11-14]. Astonishingly, virtually all these processes rely on a single structural type of catalyst, viz. zeolite ZSM-5 in various modifications, or its titanium containing analogue TS-1 [15]. It is, moreover, noteworthy that many of these processes convert and/or produce mononuclear aromatic compounds. It is not surprising, therefore, that a vast scientific literature exists on shape selective reactions of benzene derivatives in zeolite ZSM-5. 

In this paper, we report on the shape selective isomerization of l methylnaphthalene in suitable zeolite catalysts in which the undesired transalkylation reaction is suppressed. Furthermore, new results concerning the alkylation of 2-methylnaphthalene with methanol are presented in an endeavor to contribute to a critical evaluation of Fraenkel s model. At the same time, the potential of shape selective catalysis for the manufacture of... 

Reglospeclflc functionalization of biphenyl is drawing attention as one of key steps in developing advanced materials such as liquid crystals and liquid crystal polymers [1-5]. Catalysis using zeolites is the most promising way to prepare sterlcally small molecules by differentiating between reactants, products, and/or intermediates according to their size and shape. Sterlc restrictions by zeolites Increase the formation of preferred products and prevent the formation of undesirable products [6]. We describe herein shape selective catalysis of 12-membered zeolites, H-mordenite (HM), HY and HL In the alkylation of biphenyl. 

Three types of shape-selective catalysis are distinguished depending on whether pore size limits the entrance of reactant molecules, the departure of product molecules, or the formation of certain transition states [6]. The suitability of zeolite structure for the catalysis is essential for high shape-selectivity. Alkylation of biphenyl is also explained by sterlc control by pore size and shape of zeolite. HY, HL and HM have different pore structures... 

Zeolites are well known for shape-selective catalysis. Here the shape of the zeolite pores or cavities can control the shape of product. When catalytic reactions take place in channels of zeolites only those products that can be accommodated in the channels advance and emerge. Mobil s ZSM-5 is an example of a shape-selective catalyst. Many more zeolites with different pore sizes or large surface areas are being synthesized, extending the principle of shape-selective catalysis. Such developments are helpful for both existing industrial processes and environmental protection. 

Shape-selective catalysis in zeolite requires that the reactants diffuse inwards to the active sites located at the intracrystalline volume (pores), and that products counterdif-fuse after the reaction. At the active sites, presence of a high local electric field may direct the reaction according to steric requirements to yield specific products. Thus, shape-selectivity may be achieved by virtue of geometric factors, Coulombic field at the active sites and/or difference in diffusion rates. Accordingly, three different kinds of shape-selectivity are distinguished (Dwyer, 1984). If the geometric factors are such that... 

Figure 8.21 Relation between diffusion and shape-selective catalysis by zeolites (a) diffusion coefficients of -alkanes in potassium T-zeolite and (b) product distribution for the cracking of n-tricosane over H-erionite. (Following Gorring, 1973.)...

The dehydrated zeolites exchanged with various cations have been of catalytic interest in many reactions, among which cracking (259) and shape-selective catalysis (260) are most important. Other reactions include oxidation, carbonylation, and related reactions (261) as well as other nonacid catalytic reactions (262). 

Zeolites are the most promising microporous materials for achieving highly shape-selective catalysis because their pores are uniformly distributed and have dimensions allowing both the organic reactants and products to enter, to react, and to leave.1... 

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