Olefin skeletal isomerization

The specific metal-snpport interaction existing for CNTs or CNFs has been demonstrated in several stndies. A strong interaction between the H3PW12O40 heteropolyacid and the snrface gronps of MWCNTs was reported for the esterification of n-bntanol with acetic acid [371]. For olefin skeletal isomerization on WO3/MWCNT [344], it was shown that on CNTs, by contrast to oxide snpports such as alumina and zirconia, the formation of metallic tungsten is inhibited, thus pointing to the absence of an observable deactivation phenomenon. For dibenzotiophene HDS, TPR data have shown that the active species on Co and... 

Cg olefin skeletal isomerization WO3/MWCNT 17%W03/MWCNT catalyst is significantly more active and much more stable than a commercial 25% W03/Zr02 catalyst. 344... 

A survey of the heterogeneous oxidation of olefins shows quite clearly that the results depend strongly on the structure of the olefin. Skeletal isomerization normally does not occur, and the rates and selectivities vary markedly with the position of the double bond and the nature of the substituent groups. A large amount of experimental evidence has accumulated to show that many oxidations occur by removal of a hydrogen atom at the allylic position, i.e., the H atom in... 

Trombetta, M., Busca, G., Rossini, S., Piccoli, V., andComaro, U. FT-IR studies on light olefin skeletal isomerization catalysis. Part I the interaction of C4 olefins with pure y-alumina. J. Catal 1997,168, 334-348. 

Finocchio, E., Busca, G., Rossini, S., Comaro, U., Piccoli,V., and MigUo, R. FT-IR characterization of sihcated aluminas, active olefin skeletal isomerization catalysts. Catal. Today 1997,33,335-352. 

Slow double-bond shifts and little skeletal isomerization H-transfer is minor and nonselective for tertiary olefins only small amounts of aromatics formed from aliphatics at 932°F (500°C)... 

Rapid double-bond shifts, extensive skeletal isomerization, H-transfer is major and selective for tertiary olefins large amounts of aromatics formed from aliphatics at 932°F (50t) O... 

It was assumed that C—C bond cleavage passes through an elementary step of p-alkyl transfer. The mechanism of hydroisomerization passes also by a p-alkyl transfer step, but in this case the P-H elimination-olefin reinsertion occurs rapidly and a skeletal isomerization also occurs. 

Reactions over chromium oxide catalysts are often carried out without the addition of hydrogen to the reaction mixture, since this addition tends to reduce the catalytic activity. Thus, since chromium oxide is highly active for dehydrogenation, under the usual reaction conditions (temperature >500°C) extensive olefin formation occurs. In the following discussion we shall, in the main, be concerned only with skeletally distinguished products. Information about reaction pathways has been obtained by a study of the reaction product distribution from unlabeled (e.g. 89, 3, 118, 184-186, 38, 187) as well as from 14C-labeled reactants (89, 87, 88, 91-95, 98, 188, 189). The main mechanistic conclusions may be summarized. Although some skeletal isomerization occurs, chromium oxide catalysts are, on the whole, less efficient for skeletal isomerization than are platinum catalysts. Cyclic C5 products are of never more than very minor impor-... 

A mixture of EtsSiH/TFA in dichloromethane reduces 3-methyl-5-a-cholest-2-ene to give the pure equatorial methyl isomeric product, 3/3-methyl-5o -cholestane, in 66% yield (Eq. 79).126 On the other hand, attempts to reduce cholest-5-ene using the same technique yield neither 5a-cholestane nor 5/3-cholestane, but instead an isomeric mixture of rearranged olefins. This result is presumably because of the inability of hydride attack to compete with carbocation skeletal isomerization and elimination.126... 

The reversal of the insertion reaction [Eq. (10)] is not normally observed [in contrast to nickel hydride addition to olefins, Eq. (9)]. An exception is the skeletal isomerization of 1,4-dienes (88, 89). A side reaction—the allylhydrogen transfer reaction [Eq. (5)]—which results in the formation of allylnickel species such as 19 as well as alkanes should also be mentioned. This reaction accounts for the formation of small amounts of alkanes and dienes during the olefin oligomerization reactions (51). 

ISOFIN [Iso olefins] A cataylytic process for making iso-olefins from normal olefins by skeletal isomerization. The principle example converts //-butenes to isobutylene, needed as a feedstock for making methyl /-butyl ether. Developed by BP Oil Company, Mobil Corporation, and MW Kellogg from 1992. 

SKIP [Skeletal isomerization process] A process for converting linear butenes into isobutene. Developed by Texas Olefins in the 1990s and operated by that company in Houston, TX. 

Ipatieff explained the observed skeletal isomerization of olefins by readsorption of the olefins and formation of a cyclopropane intermediate, e.g. ... 

While double bond migration in olefins might arise from base (31) as well as acid catalysis (32), the occurrence of skeletal isomerization under these conditions can be ascribed to acid catalysts. This presumption would attribute acidic properties to the alumina. 

The mechanism of dehydration of alcohols over acidic and non-acidic alumina is the same. In the presence of the acidic alumina, however, readsorption of the dehydrated product can occur, leading to either double bond migration or skeletal isomerization, depending on the strength of the acid sites, the structure of the olefins produced, and the experimental conditions. 

The isomerization of light olefins is usually carried out to convert -butenes to isobutylene [12] with the most frequently studied zeolite for this operation being PER [30]. Lyondell s IsomPlus process uses a PER catalyst to convert -butenes to isobutylene or n-pentenes to isopentene [31]. Processes such as this were in larger demand to generate isobutene before the phaseout of MTBE as a gasoline additive. Since the phaseout, these processes often perform the reverse reaction to convert isobutene to n-butenes which are then used as a metathesis feed [32]. As doublebond isomerization is much easier than skeletal isomerization, most of the catalysts below are at equilibrium ratios of the n-olefins as the skeletal isomerization begins (Table 12.5). 

Olefins, unlike paraffins, do not show significant gains in octane number with skeletal isomerization (see Table 14.2). As a result, olefin isomerization is not a useful octane boosting strategy. However, tertiary olefins (olefins with three alkyl substituents on the double bond), do react fairly readily with olefins to form ethers, which do have good octane numbers-for example, methyl tert-butyl ether (MTBE). 

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