Isomerization methyl shift reaction

When -butenes are used, the initiation produces a secondary carbenium ion/butoxide. This species may isomerize via a methyl shift (Reaction (2)) or accept a hydride from isobutane to form the tertiary butyl cation (Reaction (3)). Isobutylene forms the tertiary cation directly. 

However, the zeolite framework effect on the reaction is not limited only to a stabilization of charged species. We saw already that a transition state from the cluster approach turns to be an inflection point when the zeolite framework contribution is considered. An effect exists also on transition state. In the case of the shift isomerization transition state, it is found an alternative geometry. Before protonated toluene changes its orientation with respect to the deprotonated Brytasted site, the methyl shift reaction step can be achieved (see Figure 12). 

Butene as the feed alkene would thus—after hydride transfer—give 2,2,3-TMP as the primary product. However, with nearly all the examined acids, this isomer has been observed only in very small amounts. Usually the main components of the TMP-fraction are 2,3,3-, 2,3,4-, and 2,2,4-TMP, with the selectivity depending on the catalyst and reaction conditions. Consequently, a fast isomerization of the primary TMP-cation has to occur. Isomerization through hydride- and methyl-shifts is a facile reaction. Although the equilibrium composition is not reached, long residence times favor these rearrangements (47). The isomerization pathways for the TMP isomers are shown schematically in Fig. 3. 

The correlation between selectivity and intracrystalline free space can be readily accounted for in terms of the mechanisms of the reactions involved. The acid-catalyzed xylene isomerization occurs via 1,2-methyl shifts in protonated xylenes (Figure 3). A mechanism via two transalkylation steps as proposed for synthetic faujasite (8) can be ruled out in view of the strictly consecutive nature of the isomerization sequence o m p and the low activity for disproportionation. Disproportionation involves a large diphenylmethane-type intermediate (Figure 4). It is suggested that this intermediate can form readily in the large intracrystalline cavity (diameter. 1.3 nm) of faujasite, but is sterically inhibited in the smaller pores of mordenite and ZSM-4 (d -0.8 nm) and especially of ZSM-5 (d -0.6 nm). Thus, transition state selectivity rather than shape selective diffusion are responsible for the high xylene isomerization selectivity of ZSM-5. 

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

Xylene Isomerization There are several mechanisms by which the three xylene isomers can be interconverted. The one that is of the greatest interest with respect to industrial applications is the so-called monomolecular or direct xylene isomerization route. This reaction is most commonly catalyzed by Bronsted acid sites in zeolitic catalysts. It is believed to occur as a result of individual protonation and methyl shift steps. 

In this part, we will summarize some of our results on the investigation of the toluene intramolecular isomerization pathways." " Both cluster approach and periodic approach methods have been employed which allow giving an illustration of the consequence of the simplistic model in the cluster approach. H-Mordenite (H-MOR) zeolite is used for the periodic calculations. The toluene molecule does not have a problem to fit within the large 12-membered ring channels of this zeolite. Furthermore, the intramolecular transition states do not suffer from steric constraints. It is known that intramolecular aromatics isomerization can proceed via two different reaction pathways (see Figure 7). The first route proceeds through a methyl shift isomerization, whereas the second route involves a dealkylation or disproportionation reaction which results in the formation of a methoxy species and benzene as intermediate. 

We will use now the same method and Mordenite zeolite model as in the previous part, and investigate the isomerization of xylene isomers. As described in the previous part, this reaction can proceed via two alternative routes, viz. a methyl shift isomerization, and disproportionation reactions. Moreover, we observed than in the case of toluene isomerization, the location of toluene with respect to the Br0nsted acidic site for the shift isomerization was of no consequence for the activation energy barrier. We will check these mechanisms for the three xylenes. 

Shape selectivity is not confined to reactions of hydrocarbons in the absence of polar functional groups. MFI type materials have been reported to catalyze the isomerisation of cresols, chlorotoluenes, toluonitriles and toluidines [259]. In the isomerization of aniline derivatives the reaction temperatures have to be relatively mild as under severe reaction temperatures isomerization to methylpyridine would occur [260]. For dimethylanilines it could be shown that only the isomers with the smallest minimum kinetic diameter reacted (reactant selectivity), and that those with a larger kinetic diameter did not form (product selectivity). The isomerization is concluded to occur via a 1,2 methyl shift which is interpreted to indicate transition state selectivity [261]. 

The intermediate of the irreversible isomerization can be isolated. The EMc2 unit is <7 bonded in the educt it is a centre of basic activity, shoeing tendency for reactions with electrophiles. Thus the electron-rich state of the atoms of group 15 elements leads to irreversible isomerization with shifting of one methyl group from the PR3 ligand to arsenic or antimony, respectively. The donor capacity of the transition metal is enlarging this ability. 

With medium-pore zeolites (e.g. ZSM-5) very little disproportionation occurs, because the smaller cavities of ZSM-5 cannot easily accommodate this bulky intermediate. Much more of the product is formed through the isomerization reaction proceeding via a mechanism in which only a single molecule of xylene is involved. In this unimolecular xylene undergoes a 1,2-methyl shift... 

Some exceptions to the general rules occur. Cyclopentene is completely combusted, undoubtedly because of the high reactivity of cyclo-pentadiene. 4,4-Dimethyl-1-pentene is expected to produce an unsaturated aldehyde, but instead 2,3-dimethylpentadiene is the initial product. A methyl shift from a quarternary carbon is apparently easy, permitting formation of a diene instead of the oxygenated compound. 3,3-Dimethyl-l-butene is not expected to react at all under the general rules, but here also a methyl shift occurs so that diene, olefin aldehyde, diene aldehyde, and diene dialdehyde are formed. The reactivity of the latter olefin relative to 1-butene, measured by oxidation of a mixture at low conversion, was 0.21, while that of 4,4-dimethyl-1-pentene was 0.75. These reactivities suggest that isomerization occurs before reaction for 3,3-dimethyl-l-butene, while isomerization probably occurs after the aUyl intermediate is formed in the case of the pentene. 

While it was found by means of isotopic studies than on amorphous silica-alumina the reaction proceed by an intramolecular mechanism (194), in zeolite Y, the distribution of isomers in the trimethylbenzene fraction indicates that some of the isomers could be obtained by a bimolecular mechanism (172,175). In a very recent work (196,197) it has been demonstrated by means of isotopic studies, that on some 12 MR zeolites such as Y, and mordenite, xylenes are isomerized by both uni and bimolecular transalkylation mechanism. The ratio of the uni to bimolecular increases when increasing the Si/Al ratio, and decreases when increasing the reaction temperature, the partial pressure of the feed, and the contact time. Another 12 MR, Beta zeolite, while being able to disproportionate xylene, does not isomerize via the bimolecular mechanism. This was explained by space constraints to accommodate a xylene and a trimethylbenzene as a bimolecular intermediate in the channels of the zeolite. A medium pore zeolite (ZSM-5) does isomerize only through a unimolecular 1,2 methyl-shift mechanism. 

Scheme 6.10.2 Initial steps of the refinery alkylation reaction proton addition to 2-butene, isomerization via methyl shift or hydride transfer with isobutane to form a tert-butyl cation.

The complex reaction sequence shown in equation 34 might provide some rationalization. The formation of the silylcarbene 141 is suggested, based on experimental results from related reactions , but there is no evidence for the formation of 141 nor for a silylene intermediate. Thus, the transformation 137 142 might proceed via a dyotropic rearrangement as well. The facile 1,3-methyl shift in 2-trimethylsilylsilenes which interconverts 142 139 is well known from Wiberg -type silenes . 139 (R = i-Bu) is stable in solution at room temperature over days and isomerizes only slowly to 140 (R = t-Bu) which rapidly dimerizes giving a 1,3-disilacyclobutane . 

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