Zeolite Shape Selectivity

The consecutive formation of o-hydroxybenzophenone (Figure 3) occurred by Fries transposition over phenylbenzoate. In the Fries reaction catalyzed by Lewis-type systems, aimed at the synthesis of hydroxyarylketones starting from aryl esters, the mechanism can be either (i) intermolecular, in which the benzoyl cation acylates phenylbenzoate with formation of benzoylphenylbenzoate, while the Ph-O-AfCL complex generates phenol (in this case, hydroxybenzophenone is a consecutive product of phenylbenzoate transformation), or (ii) intramolecular, in which phenylbenzoate directly transforms into hydroxybenzophenone, or (iii) again intermolecular, in which however the benzoyl cation acylates the Ph-O-AfCL complex, with formation of another complex which then decomposes to yield hydroxybenzophenone (mechanism of monomolecular deacylation-acylation). Mechanisms (i) and (iii) lead preferentially to the formation of p-hydroxybenzophenone (especially at low temperature), while mechanism (ii) to the ortho isomer. In the case of the Bronsted-type catalysis with zeolites, shape-selectivity effects may favor the formation of the para isomer with respect to the ortho one (11,12). 

Selective mono- or dicyclohexylation of naphthalene with cyclohexene (and cyclohexyl bromide) was achieved over HY zeolites.170 Monocyclohexylnaphtha-lenes are formed with 99% selectivity at 80°C. The amount of dicyclohexyl-naphthalenes increases dramatically with increasing temperature. Due to zeolite shape selectivity, mainly the p,p-disubstituted derivatives (2,6- and 2,7-dicyclo-hexylnaphthalene) are formed at 200° C. 

When the hydrogenation function is embedded in the crystal voids of an MFI topology, the formation of trans-isomers is strongly reduced. After partial reduction of soy bean oil with such catalyst from an iodine value of 140 to 80, virtually no trans-isomers are obtained (56). This is the result of pore mouth catalysis combined with zeolite shape selectivity. Due to the bent character of the cts-isomer chains in triglycerides, trans-configured chains preferentially enter the pore mouths for hydrogenation. In this environment, metal-catalyzed cis-trans isomerization is restricted for steric reasons as multiple readsorption is minimal. 

Perez Zurita, M. J., G. Vitale, M. R. de Goldwasser, D. Royas, and J. J. Garcia. 1996. Fe-pillared clays A combination of zeolite shape selectivity and iron activity in the CO hydrogenation reaction. J. Mol. Catal. 107 175-183. 

FIGURE 10.1 Key illustrations of zeolite shape selectivity in chemical reactions. 

Comparative stu(iy of isopropylation and cyclohexylation of naphthalene over zeolites shape selective synthesis of a 2,6-dialkylnaphthalene. 

H2 to aromatic molecules or to high-octane-number gasoline. First, methanol and olefins are produced by the catalytic reactions of CO and H2, as discussed above. Then, using a zeolite shape-selective catalyst that is introduced along with the ruthenium or other metal catalyst in the same reaction chamber, methanol and the olefins are converted to aromatic molecules, cycloparaffins, and paraffins. The mechanism involves the dehydration of methanol to dimethyl ether. The light olefins that also form are alkylated by methanol and by the dimethyl ether [134] to produce higher-molecular-weight olefins and then the final cyclic and aromatic products. 

Hoelderich (1990) has written a thought-provoking review on the shape-selective aspects of zeolite catalysis, and the various features of zeolites have been summarized by Perot and Guisnet (1990). Obviously, shape selectivity can operate only if the reaction occurs within the zeolite pores. Some reactions also take place on the outer surface of the zeolite shape selectivity effects are not manifested in such reactions. 

Zeolites of catalytic utility are microporous. Such materials are permeated by channels and cages of molecular dimensions (513 A). The interior of the zeolite is accessed via pores having various well-defined geometries. As a result, intracrystalline sorption can be hi ly selective depending on sorbate size and shape. This phenomenon is known as "molecular sieving" or "shape selectivity." When eatalytie sites are present in the zeolite, "shape-selective catalysis" [1214] can occur. 

M. Sayed, A. Auroux, J.C. Vedrine et al.. The effect of boron on ZSM-5 zeolite shape selectivity and activity 11. Coincorporation of aluminium and boron in the zeohte lattice. J. Catal. 116,1-10 (1989)... 

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