Alkanes skeletal isomerization

Hosoi et al. reported that Pt/S042--Zr02 persists a high activity for a long period in alkane skeletal isomerization when the reaction is carried out in the... 

The (=Si03)3TiH surface complex was active in the skeletal isomerization of alkanes at temperatures as low as 50°C and this remarkable activity is very promising because this catalyst has the possibility to isomerize light hydrocarbons under very mild conditions.261... 

The rearrangement of platinacyclobutanes to alkene complexes or ylide complexes is shown to involve an initial 1,3-hydride shift (a-elimina-tion), which may be preceded by skeletal isomerization. This isomerization can be used as a model for the bond shift mechanism of isomerization of alkanes by platinum metal, while the a-elimination also suggests a possible new mechanism for alkene polymerisation. New platinacyclobutanes with -CH2 0SC>2Me substituents undergo solvolysis with ring expansion to platinacyclopentane derivatives, the first examples of metallacyclobutane to metallacyclopentane ring expansion. The mechanism, which may also involve preliminary skeletal isomerization, has been elucidated by use of isotopic labelling and kinetic studies. 

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). 

Beta scission of a carbenium ion is an elementary step that is inihated by the weakening of the bond beta to the positive charge, leading to a smaller carbenium ion and an alkene. This elementary step is further discussed in Sections 13.8.1, 13.8.3.1 and 13.8.4 within the context of alkene skeletal isomerization, isobutane-2-butene alkylation and alkane cracking, respectively. 

All these results were interpreted by a free-radical mechanism with the involvement of alkenes, and smaller (C3,C4) and larger (C7,C8) ring intermediates in aromatization. Skeletal isomerization was found to occur through vinyl shift and C3,C4 cyclic intermediates.202 Transition metals with the exception of Fe and Os, as well as Re, Co, and Cu, are active in aromatization of alkanes. Platinum,... 

Mechanism. The proven acidity of the effective alkane isomerization catalysts suggests that carbocations are involved in acid-catalyzed alkane isomerization. Such a mechanism was first proposed by Schmerling and coworkers54 on the basis of the pioneering ideas of Whitmore55 for the skeletal isomerization of alkanes and cycloalkanes in the presence of aluminum chloride and a trace of olefin or other promoter. Subsequently these concepts were used to explain the mechanism of the acid-catalyzed isomerizations in general. 

Low-molecular-weight hydrocarbons (C4 and C5 alkanes) usually undergo isomerization through a simple bond shift. The transformation of [l-l3C]-butane, for instance, yields isobutane via skeletal isomerization and the isotopomer [2-13C]-butane 155... 

The skeletal isomerization of straight-chain paraffins is important for the enhancement of the octane numbers of light petroleum fractions. The isomerization of H-butane to isobutane has attracted much attention because isobutane is a feedstock for alkylation with olefins and MTBE synthesis. It is widely believed that the low-temperature transformation of n-alkanes can be catalyzed only by superacidic sites, and this reaction has often been used to test for the presence of these sites. 

The formation of C6 and C7 acids along with some ketones was reported in the reaction of isopentane, along with methylcyclopentane and cyclohexane with CO in HF-SbF5 at ambient temperatures and atmospheric pressure.406 Yoneda et al.407 have also found that other alkanes can be carboxylated as well with CO in HF-SbF5. Tertiary carbenium ions, which are produced by protolysis of C—H bonds of branched alkanes in HF-SbF5, undergo skeletal isomerization and disproportionation before reacting with CO. Formation of the tertiary carboxylic acids in the... 

Another important and well studied paramagnetic ion in the lattice of oxide semiconductors is Zr3+ in Zr02. Zirconia dioxide is widely used both as a catalyst of different chemical processes, and as a carrier for constructing supported metal-complex catalysts. In the last years, sulfated zirconia attracted significant interest as an active and selective catalyst in skeletal isomerization of normal alkanes at low temperatures, cracking of paraffins, alkylation and acylation of aromatics [42, 53 and Refs therein]. The appropriate experimental data are collected in the following Table 8.2. 

Extensive studies have been described of the reactions of linear alkanes with H2 over EUROPT-l [6, 7, 12, 15, 16]. Table 2 summarizes the kinetic parameters reported for ethane, propane, and -butanc. Skeletal isomerization is of course possible with this last molc-... 

Much work has also been carried out on the reactions in the presence of H2 of branched alkanes, and on the mechanism of their skeletal isomerization [6, 7], The use of 13C-labeled molecules permits alternative reaction pathways to be distinguished. Thus, for example, most of the 3-methylpentane formed from 2-methylpentane has followed the bond-shift route, but most of the n-hexane has resulted from the cyclic mechanism. Labeled molecules also allow mechanisms of aromatization of C7 and Cg alkanes to be followed... 

The acidic salts of heteropolyacids, in which the protons are partially substituted with Cs+, are active for acid-catalyzed reactions such as Friedel Crafts reactions, Ritter-type reactions, and skeletal isomerization of alkanes. The... 

Thermodynamical the skeletal isomerization of alkenes is fiivoured at low tenq>eratures and the rec rocal tenqrerature increases with increating carbon number. The equ rhim concentration of isobutene in the fraction of butenes decreases from ca. 50 % at 200°C to 37% at 500°C [149]. Thus, the convertion of n-butoies into isobutene at these temperatures will be limited by thermodynamic constraints. The skeletal isomerization of the alkenes with more than 4 carbon atoms is a relatively dle reaction step, vdiich is carried out at ca. 290°C over H-Ferrierite [150] or at 340 C over ZSM-5 [151]. This reaction proceeds via the skeletal rearrangement of a carbenium ion yielding a secondary carbenhun ion. The angular reaction meclumism indicates that side product formation can be minimized. Even the skeletal isomerization of C5- and C5-alkanes over Pt-Mordenite, vtiiich is thought to proceed... 

Isomerization of n-paraffin, especially normal pentane to iso-pentane is essential for making high octane gasoline with low aromatics content. Isomerization of lower paraffins has been conducted in the solid catalyzed gas-phase reaction system by using noble metal-supported solid acid under hydrogen atmosphere. The most predominant reaction mechanism for the isomerization of alkane is as follows (1) the dehydrogenation of alkane to alkene on the supported metal (2) proton addition to the alkene to form carbenium ion on the acidic component (3) skeletal isomerization of the carbenium ion on the acidic component (4) deprotonation of the isoraerized carbenium ion to form alkene on the acidic component (5) hydrogenation of the alkene to alkane on the metal [1]. 

Since the early 1960s, superacids have been known to react with saturated hydrocarbons to yield carbocations, even at low temperature [41]. This discovery initiated extensive studies devoted to electrophilic reactions and conversions of saturated hydrocarbons. Thus, the use of superacidic activation of alkanes to their related carbocations allowed the preparation of alkanecarboxylic acids from alkanes themselves with CO. In this respect, Yoneda et al. have found that alkanes can be directly carboxylated with CO in an HF-SbFs superacid system [42]. Tertiary carbenium ions formed by protolysis of C-H bonds of branched alkanes in HF-SbFs undergo skeletal isomerization and disproportionation prior to reacting with CO in the same acid system to form carboxylic acids after hydrolysis (eq. (9)). 

When using tertiary C5 or Ce alkanes, considerable amounts of secondary carboxylic acids are produced by the reaction of CO with secondary alkylcarbenium ions. Such cations are formed as transient intermediates by skeletal isomerization of the initially formed tertiary cations (eq. (10)) [43]. 

Since 1-5 ring closure provides a route for the skeletal isomerization of alkanes, isomerization of substituted benzenes by a cyclic mechanism should also be possible. That was verified by Shephard and Rooney (95), who found that, on 0.5% Pt/Al2O3, interconversion of o-ethyltoluene and n-propyl-benzene accompanied dehydrocyclization to indane (Scheme 80). In these... 

New approach to preparation and investigation of active sites in sulfated zirconia catalysts for skeletal isomerization of alkanes... 

Thus, we have suggested a new approach not only to the generation of active sites in sulfated zirconia-based catalysts for skeletal isomerization of alkanes, but also to investigation of their formation mechanism. The possibility of synthesis of active surface sites by deposition of sulfate ions on crystalline doped zirconia materials with defective cubic structure without changing the bulk properties of the samples opens many new opportunities for investigation of their nature. 

It has been reported by several workers that the addition of platinum to zirconia modified by sulfate ions enhances catalytic activity in the skeletal isomerization of alkanes without deactivation when the reaction is carried out in the presence of hydrogen [5,6]. The high catalytic activity and small deactivation can be explained by both the elimination of the coke by hydrogenation and hydrogenolysis [5], and the formation of Bronsted acid sites from H2 on the catalysts [6]. 

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