Carbenium ion reactions

The chemistry of acid catalyzed reactions is deeply documented and will not be considered here. It is sufficient to say that they are based in carbenium ion reactions, which yield lower molecular weight compounds and branched paraffins (since tertiary carbenium ions are the most stable). The result of the cracking of a paraffin of n-carbon atoms is another paraffin with (n-x) carbon atoms and an olefin of x-carbon atoms. 

Only large-pore zeolites exhibit sufficient activity and selectivity for the alkylation reaction. Chu and Chester (119) found ZSM-5, a typical medium-pore zeolite, to be inactive under typical alkylation conditions. This observation was explained by diffusion limitations in the pores. Corma et al. (126) tested HZSM-5 and HMCM-22 samples at 323 K, finding that the ZSM-5 exhibited a very low activity with a rapid and complete deactivation and produced mainly dimethyl-hexanes and dimethylhexenes. The authors claimed that alkylation takes place mainly at the external surface of the zeolite, whereas dimerization, which is less sterically demanding, proceeds within the pore system. Weitkamp and Jacobs (170) found ZSM-5 and ZSM-11 to be active at temperatures above 423 K. The product distribution was very different from that of a typical alkylate it contained much more cracked products trimethylpentanes were absent and considerable amounts of monomethyl isomers, n-alkanes, and cyclic hydrocarbons were present. This behavior was explained by steric restrictions that prevented the formation of highly branched carbenium ions. Reactions with the less branched or non-branched carbenium ions require higher activation energies, so that higher temperatures are necessary. 

However, due to competitive reaction pathways the reaction of small alkanes yields a quite complex product distribution (Schemes 5.2 and 5.3). Especially hydride transfer to the initially formed carbenium ions [reactions (2) and (3) in Scheme 5.2] makes it difficult to study the initial steps. 

Typical carbenium ion reaction mechanisms are used to explain the general differences in yields obtained for the two zeolite structures. 

Moreau et al.56 obtained unexpected results in the alkylation of naphtalene with 2-propanol over H-Beta in the liquid phase at 200°C. Here a cyclic compound 1 was formed with a selectivity around 40% at 28.5% conversion. When applying HY as the catalyst alkylation to di- and trialkylnaphthalenes was faster but the cyclic compound was not observed. These results illustrate the more confined space within the zeolites Beta channels. The cyclic compound is assumed to be formed through iso-propylation of naphthalene followed by a hydride abstraction giving a carbenium ion, reaction with a propylene and finally ring-closure. 

The migratory aptitude of vanou.s groups is somewhat different to the order found for carbenium ion reactions ... 

Trends in volatile paraffin/olefin ratios and alkyl aromatic yields observed when polyethylene is cracked by aluminosilicate catalysts cannot be correlated with catalyst acidity or pore size variations alone. Instead, product slate differences occur because relative rates of specific carbenium ion reactions are affected by the combined effects of catalyst acidity and pore size. 

The relative rates of carbenium ion reactions in faujasite supercages appear to follow a sequence type A /3-scission > type A alkyl shift > type Bi /3-scission > type B2 /3-scission > type B (PCP) isomerization > type C /3-scission (298). Normal alkanes, therefore, are transformed via the B-type (PCP) isomerization into branched isomers, which undergo /3-scission only after the creation of two or three side chains in the carbon skeleton. 

In contrast with readsorption reactions, which broaden the carbon number distribution of FT synthesis products, cracking reactions narrow such distributions, within the constraints imposed by the random nature of C—C bond cleavage in carbenium ion reactions of large olefins. Cracking of n-paraffins can also occur on intrapellet acid sites, but acid-catalyzed paraffin reactions are much slower than those of corresponding olefins of equal size. As in all secondary reactions, cracking sites are used most efficiently when... 

The results obtained in this study indicate that in Al-ffee H-boralite (BOR 1) only weak BrOnsted acid sites (Si—OH—B) are present. They are active only in cyclohexanol dehydration. Their catalytic activity is, however, relatively low. The insertion of A1 into the framework results in the creation of strong Bronsted acid sites. Most probably they are Si—OH—Al, the same as in zeolites. The IR band which could be characteristic of such Si—OH—Al (at about 3610 cm ) was not seen in the spectrum because of the very low concentration of these hydroxyls. The catalytic activity of Si—OH—Al is much higher that of Si—OH - B. Contrary to Si—OH -B, Si—OH— A1 are active in consecutive reactions of cyclohexene (isomerization and disproportionation). Cyclohexene isomerization (to methylcyclopentenes), a typical carbenium ion reaction is catalysed by strong Brdnsted acid sites even at temperatures as low as 450 K. The same strong Bronsted acid sites catalyse also cyclohexene disproportionation (to cyclohexane, methylcyclopentane and coke). Our earlier... 

The yields of individual volatile hydrocarbons obtained on HZSM5 in the kinetic range of autocatalysis are recorded in Table 1- The first compound to observe is methane, the second is ethene and thereafter the typical compounds to be formed in carbenium ion reactions (propane, i-butane and i-pentane) are the major products. Initial methane formation (on the fresh very active catalyst at... 

Cracking reactions are carried out in order to reduce the molecular size and to produce more valuable transport fuel fractions (gasoline and diesel). Fluid catalytic cracking is acid catalyzed (zeolites) and a complex network of carbenium ion reactions occur leading to size reduction and isomerization (see Chapter 4, Section 4.4). Hydrogenation also takes place in hydrocracking, as well as cracking. 

D-Norsteroids have provided a series of compounds especially appropriate for the examination of carbenium ion reactions of cyclobutanes of defined conformation. ... 

By the use of very fast photodetectors, it has recently become possible to analyse intermediates in the deflagrations and detonations of polyol nitrates. Although data are still limited, it seems as if detonations are initiated by 5 nI departure of nitrate, followed by carbenium ion reactions, whereas deflagrations are initiated by thermal homolysis of the O-N bond. 

Large a-deuterium KIEs were found for carbenium ion reactions and small KIEs for Sn2 reactions, and since the Crl 11(D) stretching vibration becomes stronger as the sp3 hybridized substrate is converted into the sp2 hybridized carbenium ion in an SN1 reaction, an inverse KIE, not the large normal KIEs observed for SN1 reactions, should be observed. Therefore, it was suggested that the magnitude of the KIE was... 

Although this looks very different from an "adsorbed carbenium ion", reactions at the surface of solid acids somewhat resemble those in liquid acids. Kazansky proposes that the cause for this similtuity is that the transition state leading to the alkoxy group is very similar to a carbenium ion. The present paper s objective is to identify some fundamental differences in the catalytic chemistry between liquid and solid acid catalysts. 

In the early eighties, it was found that when some transition metal oxides such as Zr02, Ti02, Sn02, Pc2 3> and HfO were sulfated with either sulfuric acid or ammonium sulfate and were subsequently calcined, a remarkable increase in the surface acidity and in the catalytic activity for carbenium ion reactions occurred 139). 

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