Carbenium ions in zeolites

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

The importance of modeling the medium has previously been suggested by other shift calculations for simple cations (97) it would be interesting to demonstrate examples of this for carbenium ions in zeolites. In favorable... 

The principal components of the trityl cation in zeolite HY are <5 = 282 ppm and <5j = 55 ppm. It is instructive to tabulate all of the 13C principal component data measured for free carbenium ions in zeolites as well as for a few carbenium ions characterized in other solid acid media (Table III). The zeolitic species, in addition to the trityl cation (119), are the substituted cyclopentenyl cation 8 (102), the phenylindanyl cation 13, and the methylindanyl cation 12 (113). Values for the rert-butyl cation 2 and methylcyclopentyl cation 17 (prepared on metal halides) (43, 45) are included for comparison. Note that the ordering of isotropic chemical shifts is reasonably consistent with one s intuition from resonance structures i.e., the more delocalized the positive charge, the smaller the isotropic shift. This effect is even more apparent in the magnitudes of the CSA. Since... 

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. 

Stable states of carbenium-ions in zeolites correspond often to covalently bonded alkoxy species. Carbenium ions behave freely only when part of transition states. 

Nicholas and Haw concluded that stable carbenium ions in zeolites are observed by NMR if the parent compound (from which the carbenium ion is obtained by protonation) has a proton affinity of 875 kj mol i or larger [6]. Simulations by quantum methods showed that this statement is more general and that proton transfer from a H-zeolite to a molecule or molecular cluster occurs if its proton afFinity is about that of ammonia (854 kJ mol i) or larger [7]. In the light of Eq. (22.4) this means that proton transfer occurs (AEpf < 0)) if dp(Z) - Eneutral(S) + Eip(SH+) is smaller than 854 kJ mohh Table 22.1 shows proton affinities and indicates in which cases and by which method protonated species have been detected. 

The lifetime of the carbenium ion formed will be limited by transferring a proton back to the zeolite, thus completing the dehydrogenation ofthe hydrocarbon. Hydride abstraction from xylene is assumed to be the initial step in its disproportionation into toluene and trimethylbenzene [9]. The parent compound (7, Fig. 22.9) ofthe carbenium ion formed (6) has such a high proton affinity (1031 kj mohh Table 22.1) that proton transfer back to the zeolite does not occur at all. However, the lifetime of carbenium ions in zeolites is not only limited by proton transfer, but also formation of a C-O bond between the carbenium ion and a framework oxygen atom, yielding an alk-oxide, needs to be considered. In ferrierite (FER) the alkoxide of 6 is found to be 50 to 60 kJ mofi more stable than the carbenium ion [9]. 

Song, W., Nicholas, J.B. and Haw, J.F. (2001). Acid-base chemistry of a carbenium ion in a zeolite under equilibrium conditions verification of a theoretical explanation of carbenium ion stability. J. Am. Chem. Soc. 123, 121-129... 

Estimates of the kinetics of methyl loss from energy-selected CztHg" species have been made by calculation.23 The hydride transfer from alkanes to carbenium ions in the gas phase is calculated to involve a species with a symmetric potential well, which is different from the situation in superacid or zeolite media.24 A correlation between the charge on a carbon and the in-plane tensor component of its 13 C chemical shift has been observed for a number of simple cationic and anionic species.25 High-level calculations... 

Theoretical and Experimental NMR techniques provide powerful tools for the investigation of heterogeneous catalysis. Recent advances in in situ NMR techniques are summarized, as are advances in theoretical methods. The utility of our combined theoretical/experimental approach is illustrated by studies of the pentamethylbenzenium cation and the 1,3-dimethylcyclopentenyl cation in zeolite HZSM-5, acetylene adsorption on MgO, and the isopropyl cation on frozen SbF5. We also discuss the role of the basicity of adsorbates in the formation of stable carbenium ions on zeolites. 

This section describes elementary reaction steps and reaction chemistry of proton activated alkane reactions as understood mainly from studies in superacids. The reaction steps and reaction intermediates are also useful to consider in zeolite catalysis. However there is an important difference. Whereas carbonium-ion and carbenium-ion in superacids are usually stable intermediates, in zeolites they are highly activated states often corresponding to transition states [53]. 

It can be concluded that there are bimolecular reactions in n-hexane cracking, and there is a quick equilibrium between carbenium ions in a hydrocarbon chain. The zeolite catalysts have suitable micro-pore structures and acid sites for intermolecular hydrogen transfer and are favorable to gasoline production. 

Localization of stationary points along the reaction path for reactions taking place inside the zeolite pores is one of the greatest challenges in zeolite modeling. The reactions of hydrocarbons are particularly difficult to model since the hydrocarbon...zeolite interaction can be dominated by the dispersion interaction that is not properly accounted at the DFT level. Only one example is presented here. Clark et al. investigated the role of benzenium-lype carbenium ion in the bimolecular w-xylene disproportionation reaction in zeolite faujasite.163] The benzenium-type carbenium ion 1 was identified in zeolite catalyst for the... 

The sequence is exactly opposite to that of conventional acid catalysis The reactants that are best able to form carbenium ions in solution are the least reactive with zeolite catalysis. The restricted transition state selectivity suppresses cracking of the more highly branched hydrocarbons in the cavities [T25]. 

Microporous solids are not superacidic. As a result, the only unambiguous observations of carbenium ions on zeolites are of strongly stabilised aromatic and cyclic species observed by solid state NMR, such as cyclopentenyl, indanyl and pentamethylbenzenium ions observed during in situ studies of the reaction of methanol (see Sections 8.5.1 and 8.6.1). The nature of the intermediates... 

Direct evidence for toe stabilisation of carbenium ions formed from simple olefins on zeolitic Broensted sites is not extensive. Comparison of bands in toe uv-vis spectra for olefins, and their oligomeric products, sorbed in acid zeolites wito bands for appropriate carbenium ions, in acid solution, provides su[ rt for the existoice of carbenium-ion surface species. A recent pqrer describes a UV-vis study using aromatics as basic probes. In toe presence of strxHig Broensted acids or of Lewis acid/hydrogen halides, ionic complexes wito aromatics may be formed as shown below. 

The charged carbocations produced after the C-H and C-C bond cleavage can be considered protonated alkenes or carbenium ions, in which the hybridization around the positively charged carbon is sp. The carbenium ions subsequently adsorb to form alkoxy intermediates on the zeolite lattice, as illustrated in Fig. 4.5. 

Carbenium and Carboninm Ions in Zeolites and Solid Acids... 

The combination of metal and zeolite in one catalyst system offers attractive possibilities for isomerization reactions, for example in the reforming of naphta. The advantage is caused by the ease at which metals catalyze (de)hydrogenation reactions. Whereas carbenium ions on zeolites form readily by protonation of alkenes, the formation of carbonium ions from alkanes requires a high activation energy (see also Section 6.4.3) Because transition metals possess excellent activity for the dehydrogenation of alkanes (see also Section 3.1.2.3), the isomerization of... 

The possibility of NMR to follow the intramolecular migration of the selective label in adsorbed hydrocarbons gave the researches a chance to provide an evidence for the formation of alkyl carbenium ions as intermediates in reactions on solid acid catalysts, including zeolites. Carbenium ions in superacids exhibit a unique property to scramble the selective... 

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