Zeolites persistent carbenium ions

A.4. Persistent Carbenium Ions in Zeolites Characterized by NMR Spectroscopy... 

In early 1993, Haw and co-workers (107) reported in situ studies of allyl alcohol-/-13C on HZSM-5 and CsHX. No persistent carbenium ions were observed, but 1,3 label exchange was observed for the alcohol on the weakly acidic zeolite. We interpreted this as support for a transient allyl cation. The low stability of this cation was invoked to explain the failure to observe this species as a persistent species. Downfield signals observed in that study were attributed to the formation of propanal. Later in 1993, Biaglow, Gorte, and White (BGW) (108) reported similar studies conducted at different loadings and assigned a downfield resonance (variously reported at 216 and 218 ppm by BGW) to the allyl cation in HZSM-5. 

B3LYP/TZVP geometries of all species shown in Scheme 1, followed by MP4(sdtq)/6-311+G single-point energy calculations. The theoretical proton affinities are shown for comparison, the experimental PA for pyridine is 222 kcal/mol. It would appear that olefins that protonate on a zeolite to yield persistent carbenium ions are very basic indeed. 

So far, fewer than 10 types of carbenium ions have been reported to be persistent species formed upon adsorption of olefins or alcohols on acidic zeolites. Instead, surface alkoxy (alkoxide) species with carbenium-ion-like properties are suggested to act, most likely, as catalytic intermediates in reactions catalyzed by acidic zeolites. Various groups have observed that, upon adsorption of olefins or alcohols on acidic zeolites, alkoxy species are formed the observations are based on both in situ and ex situ A MAS NMR spectroscopy (49,50,71-80). 

The allyl cation has never been characterized as a persistent species in solution. If prepared, it would be the smallest carbenium ion universally accepted to have been formed in condensed media (this title has for many years been held by the isopropyl cation). No allyl cation derivatives, e.g., 10, were observed in a 1990 report of a CAVERN study of butadiene on HY and HZSM-5 (104). The same year, Hutchings reported a flow reactor study of allyl alcohol on HZSM-5 (105) and Gorte (106) reported a TPD study of allyl alcohol on the same zeolite. Hutchings and co-workers found that allyl alcohol had two reaction paths, one in which propanal was formed and another that formed hydrocarbons. The latter route was proposed to proceed through an allyl cation intermediate, but no claim for its persistence or spectroscopic observation was made or implied by Hutchings. Gorte... 

We also obtained NMR spectra of the phenylindanyl cation 13 in the large-pore zeolite HY, and a small amount of cation 14 was formed on HZSM-5 by dimerization of a-methylstyrene. The dimethylphenyl carbenium ion 15 was not persistent on any zeolite we examined. This is not surprising if one reads the solution acid literature. 15 cannot be observed in 100% H2S04 stabilizing this cation requires 30% oleum (S03/H2S04) or other superacids (115). HZSM-5 is not a superacid. The observation of the much less stable styryl cation 11 was hailed as a triumph of superacid solution chemistry (116). If the styryl cation, with the phenyl group provid-... 

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... 

Apparently the H-source is elusive to NMR because it is strongly chemisorbed to the solid surface. This implies that secondary carbenium ions do not persist because they are more acidic [12] than the zeolite acid sites. 

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