Zeolites medium-pore, catalyzing isomerization

The styrene oxide isomerization is known to be an easy reaction due to the carbonium stabilization by the aromatic nucleus. In the case of H-ZSM-5, taking into account the respective size of this medium-pore zeolite (5.5A) and the kinetic diameter of the styrene oxide molecule (5.9A), it was assumed that the weak external acidic sites are active enough to catalyze the reaction (ref. 16). If this were the case for all zeolites, no shape-selectivity could be obtained for any epoxide rearrangement. Nevertheless, for large-pore zeolites, the contribution of all the acidic sites cannot be excluded. 

I he recent literature related to selective skeletal isomerization of -butenes catalyzed by medium-pore zeolites and Me-aluminophosphates is reviewed. In the presence of medium-pore molecular sieve catalysts, o-butenes are selectively transformed into isobutylene via a monomolecular mechanism. This is an example of restricted transition state shape selectivity, whereby the space available around the acidic site is restricted, constraining the reaction to proceed mainly through a monomolecular mechanism. Coking of (he ciitalysl that leads to poisoning of (he acidic sites located on the external surfaces and to a decrease in the space around the acidic sites located in the micropores renders the catalyst more selective. 

The literature related to selective skeletal isomerization of n-butenes catalyzed by medium-pore zeolites and Me-aluminophosphates leads to the following conclusions ... 

Meriaudeau and Naccache conclude the volume with a concise description of skeletal isomerization of butenes catalyzed by medium-pore zeolites and molecular sieves. This isomerization is a relatively new industrial process. and it is remarkable how fast a good fundamental understanding of it has developed in a few years the chapter is an account of catalysis by well-defined acidic groups in pores that exert a subtle control over catalyst performance, including selectivity. It is a story that was deeply appreciated by Werner Haag. 

Medium pore aluminophosphate based molecular sieves with the -11, -31 and -41 crystal structures are active and selective catalysts for 1-hexene isomerization, hexane dehydrocyclization and Cg aromatic reactions. With olefin feeds, they promote isomerization with little loss to competing hydride transfer and cracking reactions. With Cg aromatics, they effectively catalyze xylene isomerization and ethylbenzene disproportionation at very low xylene loss. As acid components in bifunctional catalysts, they are selective for paraffin and cycloparaffin isomerization with low cracking activity. In these reactions the medium pore aluminophosphate based sieves are generally less active but significantly more selective than the medium pore zeolites. Similarity with medium pore zeolites is displayed by an outstanding resistance to coke induced deactivation and by a variety of shape selective actions in catalysis. The excellent selectivities observed with medium pore aluminophosphate based sieves is attributed to a unique combination of mild acidity and shape selectivity. Selectivity is also enhanced by the presence of transition metal framework constituents such as cobalt and manganese which may exert a chemical influence on reaction intermediates. 

Even though the synthesis of many medium pore SAPO molecular sieves are well documented, only SAPO-11 has been studied in detail with respect to its shape selectivity and catalytic activity in acid catalyzed reactions. The reaction of m-xylene on zeolites, besides its industrial importance, is abundantly described in literature not only because it provides information on the geometry of the zeolite channels, but also because it is considered as an appropriate reaction to give information on the acidic properties of solid catalysts. Both isomerization and disproportionation are catalyzed by Bronsted acid sites , the disproportionation reactions requiring stronger acid sites than isomerization reactions. Hence SAPO molecular sieves with medium acidity should give better selectivity for m-xylene isomerization than zeolites. 

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