Solid acids zeolites

There is considerable controversy with regard to the formation of persistent (long-lived) carbenium ions in zeolite solid acids (8). The conventional view has been that stable carbenium and even carbonium ions are readily formed from even weakly basic molecules such a propene. However, there is increasing evidence that the formation of persistent species requires a strongly basic precursor, and is thus only possible in select cases (25). Consequently, we feel that any positive claims of persistent carbenium species in zeolites warrant exceptional proof. 

Theory helps the experimentalists in many ways this volume is on chemical shift calculations, but the other ways in which theoretical chemistry guides NMR studies of catalysis should not be overlooked. Indeed, further theoretical work on two of the cations discussed above has helped us understand why some carbenium ions persist indefinitely in zeolite solid acids as stable species at 298 K, and others do not (25). The three classes of carbenium ions we were most concerned with, the indanyl cation, the dimethylcyclopentenyl cation, and the pentamethylbenzenium cation (Scheme 1), could all be formally generated by protonation of an olefin. We actually synthesized them in the zeolites by other routes, but we suspected that the simplest parent olefins" of these cations must be very basic hydrocarbons, otherwise the carbenium ions might just transfer protons back to the conjugate base site on the zeolite. Experimental values were not available for any of the parent olefins shown below, so we calculated the proton affinities (enthalpies) by first determining the... 

Table 8.3 Industrial processes involving zeolitic solid acids. 

Although zeolitic solid acids have replaced supported mineral acids in many reactions, they cannot be used universally. For some reactions their acidity is too strong and their pore structure is readily blocked (etherification, alkene hydration) for others they are not strong enough acids. An example where zeolites have not been applied successfully is in the alkylation of alkanes with alkenes. Hydrocarbon fuels with high octane numbers are in constant demand for the majority of automobile engines, and branched alkanes of the gasoline fraction possess the required properties. Isoalkanes such as these are currently... 

Kinetics of Zeolitic Solid Acid-Catafyzed AUgrlation of Isobutane with 2-Butene... 

SIMPSON ETAL. Zeolitic Solid Acid-Catalyzed Alkylation of Isobutane 107... 

SIMPSON ET AL. Zeolitic Solid Acid-Catalyzed AOiylatwn of Isobutane 113... 

Alkaloid synthons, synthesis, 191 Alkylation, zeolitic solid acid catalyzed, kinetics, 105-114 Alternative feedstocks and starting materials... 

Alkylation Mechanism with Zeolite Catalysts. The fact that the alkylate produced over a zeolite solid acid is quite similar in terms of product distribution to that obtained with sulfuric acid (Table 8) strongly points toward an alike general reaction mechanism occurring in both types of catalysts. The main catalytic cycles involved in the isobutane/butene alkylation mechanism are illustrated in Figure 6. 

In the first part of this section we have shown for zeolite solid acids that carbenium or carbonium ion intermediates are typically present as transition states or unstable intermediates. The activation energies depend on the deprotonation energy of the zeolite, the stabilization of the charged cationic intermediates by screening effects and by their interaction with the negative charge left on the zeolite lattice. 

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