Alkane protonation states

Operando DRIFTS examination of the working zeolite catalysts shows adsorbed hexane but do not support the presence of bound alkoxide/olefin/carbenium ion species. Data substantiate that alkanes may be activated without full transfer of zeolite proton to the alkane, i.e., without generation of any kind of real carbocation as transition state or surface intermediate. 

The three-state RIS model of conformer statistics is used to analyze the 16 independent dipole coupling constants measured in a proton NMR study of n-hexane in a nematic liquid crystal solvent. The orientational ordering of the n-hexane molecule is treated in the context of the modular formulation of the potential of mean torque. This formulation gives an accurate description of alkane solute orientational order and conformer probabilities in the nematic solvent. Consequently, substantially more accurate calculated diplar couplings are obtained, and this is achieved without the need to resort to unconventionally high values of the trans-gauche energy difference E(g) in the RIS model. 

From these experiments, some general conclusions can be drawn concerning the behavior of small alkanes in the strongest HF-SbF5 system. (1) The reversible protonation of the alkanes (i) is very fast in comparison with the ionization step (ii) it takes place on all cr-bonds independently of the subsequent reactivity of the alkane (iii) it involves carbonium ions (transition states), which do not undergo molecular rearrangements. (2) Protonation of an alkane is atypical acid-base reaction and carbon monoxide has no effect on this step. 

It is interesting to compare this transition state in the solid with the one calculated from the HF-SbF5 system. In the liquid superacid, the ionic character is very strong and it is easier to connect the reactivity with the unusual activity of the proton even when solvated by the HF solvent. In contrast, on the solid the theoretical calculated transition state is further away from the carbonium ion type and in line with the much higher temperatures needed to activate the alkane with weaker acids. 

For non-electrophilic strong oxidants, the reaction with an alkane typically follows an outer-sphere ET mechanism. Photoexcited aromatic compounds are among the most powerful outer-sphere oxidants (e.g., the oxidation potential of the excited singlet state of 1,2,4,5-tetracyanobenzene (TCB) is 3.44 V relative to the SCE) [14, 15]. Photoexcited TCB (TCB ) can generate radical cations even from straight-chain alkanes through an SET oxidation. The reaction involves formation of ion-radical pairs between the alkane radical cation and the reduced oxidant (Eq. 5). Proton loss from the radical cation to the solvent (Eq. 6) is followed by aromatic substitution (Eq. 7) to form alkylaromatic compounds. 

If a hydrogen atom is abstracted from an alkane by an alkyl radical, both the initial and final state of the reaction involve neutral species and it is only the transition state where some limited charge separation can be assumed. In the case of a homolytic O—H bond fission, however, the initial state possesses a certain polarity and possible changes in polarity during the reaction depend on both the lifetime of the transition state and the nature of the attacking radical. If the unpaired electron is localized mainly on oxygen in the reactant radical, the polarity of the final state will be close to that of the initial state and any solvent effect will primarily depend on the solvation of the transition state. Solvent effects can then be expected since the electron and proton transfers are not synchronous. 

As stated above, the aromatization of short alkanes is carried out in presence of bifunctional catalysts, in where the dehydrogenating function is given by the metal component (Ga, Zn, Pt) and the H-ZSM-5 zeolite carries the acid sites. Although there is still some uncertainty concerning the initial activation of the alkane, probably both the metal and the zeolite acid sites are involved in this step. Metal sites can dehydrogenate the alkane to give the corresponding alkene, which can then be protonated on the Bronsted acid sites of the H-ZSM-5 zeolite to produce the carbocation. 

More recently, it has been proposed (78, 79) that on zeolite catalysts the reaction can also start by protonation of a C-C bond by the acid site of the zeolite forming a pentacoordinated carbonium ion transition state. This can then eliminate H2 or a short alkane molecule leaving an adsorbed carbenium ion on the zeolite (protolytic cracking), as shown below ... 

The dehydrogenation reaction proceeds through the simultaneous elimination of the zeolitic proton and a hydride ion from the alkane molecule, giving rise to a transition state which resembles a carbenium ion plus an almost neutral H2 molecule to be formed. For the linear alkanes, the TS decomposes into an H2 molecule and the carbenium ion correspondent alkoxide. However, for the isobutane molecule the reaction follows a different path, the TS producing isobutene and H2. Most certainly the olefin elimination is flavored to the alkoxide formation due to steric effects as the t-butyl cation approaches the zeolite framework. The same mechanism is expected to be operative for other branched alkanes. 

A turning point in the revival of interest in strong acid chemistry was a publication in 1968 in which ionization of the C-H bonds of the extraordi-norily unreactive "lower poroffins" methane and ethane In HS03F-Sbp5 at 50°C was reported (14). The propiosed mechanism (Scheme 2) proposes the existence, in super acid solution, of protonated alkanes or pentacoordinoted ions, at least as possible transition states, and attempts quite logically to draw a parallelism between the presence of such species in solution chemistry... 

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]. 

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