Skeletal isomerization, metal catalyzed

A wide range of nonacidic metal oxides have been examined as catalysts for aromatization and skeletal isomerization. From a mechanistic point of view, chromium oxide catalysts have been, by far, the most thoroughly studied. Reactions over chromium oxide have been carried out either over the pure oxide, or over a catalyst consisting of chromium oxide supported on a carrier, usually alumina. Depending on its history, the alumina can have an acidic function, so that the catalyst as a whole then has a duel function character. However, in this section, we propose only briefly to outline, for comparison with the metal catalyzed reactions described in previous sections, those reactions where the acidic catalyst function is negligible. 

Two main pathways of metal-catalyzed skeletal rearrangement have been distinguished bond shift mechanism and C5 cyclic isomerization (7, 8). 

Bifunctional catalytic reactions involve a series of catalytic steps over acidic and hydrogenating-dehydrogenating sites with formation of intermediate compounds. Thus n-hexane (hydro)isomerization involves successively n-hexane dehydrogenation in n-hexenes (metal catalyzed), skeletal isomerization of n-hexenes into isohexenes over protonic acid sites followed by the (metal catalyzed) hydrogenation of isohexenes into isohexanes (Figure 1.4). 

Isomerization of n-paraffins in the C5-C6 range, such as those present in the LSR (light straight run) fraction, is industrially carried out to improve the octane number of the gasoline. Skeletal isomerization of n-paraffins is an acid-catalyzed reaction that is thermodynamically favored at lower temperatures. Therefore, acid catalysts with strong acidity have to be used in order to perform the reaction at temperatures as low as possible. The process is carried out in the presence of hydrogen and a bifunctional catalyst, which typically consists of a noble metal (Pt) supported on an acidic carrier. 

The discussion to this point has emphasized kinetics of catalytic reactions on a uniform surface where only one type of active site participates in the reaction. Bifunctional catalysts operate by utilizing two different types of catalytic sites on the same solid. For example, hydrocarbon reforming reactions that are used to upgrade motor fuels are catalyzed by platinum particles supported on acidified alumina. Extensive research revealed that the metallic function of Pt/Al203 catalyzes hydrogenation/dehydrogenation of hydrocarbons, whereas the acidic function of the support facilitates skeletal isomerization of alkenes. The isomerization of n-pentane (N) to isopentane (I) is used to illustrate the kinetic sequence associated with a bifunctional Pt/Al203 catalyst ... 

The skeletal isomerization of tetrabydrodicyclopentadiene into adamantane is an example of a very complex rearrangement diat is commercially carried out over strong Lewis acids with a hydride transfer initiator. The reaction can be catalyzed by rare earth (La, Ce, Y, Nd, Yb) exchanged faujasites (Scheme 1) in a Hj/HCl atmosphere at 25(yX3. Selectivities to adamantane of up to 50% have been reported, when a metal fimction, such as Pt, capable of catalyzing hydrogenation is added [54]. Initially acid catalyzed endo- to exo- isomerization of tetrahydro-dicyclopentadiene takes place and then a series of 1,2 alkyl shifts involving secondary and tertiary carbonium ions leads eventually to adamantane[55]. The possible mechanistic pathways of adamantane formation from tetrahydro-dicyclopentadiene are discussed in detail in ref [56]. 

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]. 

On the other hand, it was proposed that acid catalyzed reactions such as skeletal isomerization of paraffin [2], hydrocracking of hydrocarbons [3] or methanol conversion to hydrocarbon [4] over metal supported acid catalysts were promoted by spillover hydrogen (proton) on the acid catalysts. Hydrogen spillover phenomenon from noble metal to other component at room temperature has been reported in many cases [5]. Recently Masai et al. [6] and Steinberg et al. [7] showed that the physical mixtures of protonated zeolite and R/AI2O3 showed high hydrocracking activities of paraffins and skeletal isomerization to some extent. 

Along with typical acid catalyzed reactions such as cracking, the metal catalyzed reactions such as hydrogenolysis and isomerization could also take place over Pt-HZSM-5 catalysts [4, 5]. An earlier study of us [11] showed, however, that metal catalyzed isomerization over a Pt catalyst on a strong acidic support can manifest themselves under specific experimental conditions at low temperatures (<573 K) and with higher Ho/nH ratios only. While n-hexane underwent aromatization also on HZSM catalyst without Pt [11], the presence of Pt was indispensable for skeletal isomerization. The appearance of methylcyclopentane (MCP) - as a possible intermediate [12] - was crucial to assume that the so-called C5-cyclic metal catalyzed isomerization took place. 

The conversion of n-hexane over various Pt-zeolite catalysts responded strongly to changes in hydrogen pressures [13-15]. Increasing the hydrogen/n-hexane ratio promoted metal catalyzed skeletal isomerization, mainly by the C5-cyclic pathway and shifted also the fragment composition towards vmues typical of metal-catalyzed hydrogenolysis. 

However, new investigations showed the occurrence of skeletal isomerization on various metal films (57) and on supported platinum (52, 55) under conditions (2OO-3OO°C) such that the support (glass or alumina) is catalytically inactive. This clearly shows that the metal itself catalyzes the reaction. 

Because various important industrial organic processes utilize olefins, convenient methods to convert olefins into various products are vital. Transition metal catalysts with proper ligands have proved most useful in controlling the course of these reactions. Transition metal complexes catalyze skeletal isomerization, double bond isomerization, polymerization, and other processes. Insertion of a terminal olefin into a transition metal hydride bond by 1,2-inserfion or... 

Oxides of Cr, Mo, and W are usually used for catalysts as mixed oxides with other oxides such as alumina and silica which are prepared by coprecipitation, impregnation, etc. They are seldom put to practical use as simple oxides. Principal reactions catalyzed by these oxides, unlike those observed for silica-alumina or zeolites, often involve redox-type reaction steps, and during these steps reaction intermediates having covalent carbon-metal bonds are formed. Examples of those reactions are dehydrogeneration, hydrogenation and skeletal isomerization of hydrocarbons, and polymerization of olefms, as well as metathesis of olefins and hydrodesulfurization. Therefore, acid-base properties of catalysts usually play secondary roles in catalysts. 

Certain metal oxides treated with SbFs exhibit superacidic character. The SbFs-treated metal oxides can catalyze skeletal isomerization of saturated hydrocarbons at room temperatures. The catalysts were prepared by repeated exposure of the heat-treated metal oxides to SbFs vapor followed by outgassing to remove excess SbFs. 

The clusters [AuOs3(/A-X)(CO)10(PPh3)] have been attached to phosphine-functionalized silica for X = H or Cl (175,176) or polymer (styrene-divinylbenzene) for X = H (176). On both supports, the immobilized hyd-rido cluster was found to be inactive for alkene hydrogenation and isomerization, whereas the supported Cl-containing species catalyzed alkene hydrogenation. The different behavior was initially incorrectly attributed to different metal framework structures for the two clusters, but, in fact, both species adopt similar butterfly skeletal geometries (12,54). 

Before the introduction of metal-ammonia solutions for the reduction of a,p-unsaturated carbonyl compounds,sodium, sodium amalgam, or zinc in protic media were most commonly employed for this purpose. Some early examples of their use include the conversion of carvone to dihydrocarvone with zinc in acid or alkaline medium, and of cholest-4-en-3-one to cholestanone with sodium in alcohol. These earlier methods are complicated by a variety of side reactions, such as over-reduction, dimerization, skeletal rearrangements, acid- or base-catalyzed isomerizations and aldol condensations, most of which can be significantly minimized by metal-ammonia reduction. 

The current theory of bifunctional catalysis assumes that paraffin isomerization is induced by olefin formation at the metal surface, followed by a typical acid-catalyzed reaction of the olefin at the active centers of the acidic component. Consequently, similar skeletal conversions must be found with olefins and an acid catalyst, and paraffins and a bifunctional catalyst. Our findings substantiate this theory. If these results (Figs. 2 and 3) are put together and compared to the predictions of the carbonium mechanism (Fig. 4), one can see that all the expected structures have been obtained in our experiments. 

The dimerization of ethylene to form a mixture of butene isomers is not particularly useful in the field of commodity chemicals at this time because this mixture of butenes is usually cheaper than ethylene. Selective dimerization of ethylene to 1-butene using a titanium catalyst is practiced, but this chemistry occurs through metallacycles and is described in the next section. The dimerization of propylene by migratory insertion chemistry typically produces the mixture of isomeric olefins shown in Equation 22.32. Four skeletal isomers of the intermediate metal alkyl can arise from the two different directions of M-H insertion, followed by two different inodes of M-R insertion. The dimerization of ethylene is particularly fast when catalyzed by the combination of NiBr(-r) -C3H5)(PCy3) and EtAlCl this dimerization in chlorobenzene at 25 °C occurs witii turnover frequencies up to 60,000 per second. The more selective dimerization of propene to 2,3-dimethylbutene is conducted on an industrial scale with titanium catalysts, again via metallac clic intermediates described in the next section. 

Zeolite MCM-22 in its Br0nsted-acid form has been described in the hterature as a useful catalyst for a variety of acid-catalyzed reactions, such as iso-alkane/olefin alkylation [e.g.40,41],skeletal and double-bond isomerization of olefins [42] and ethylbenzene synthesis via alkylation of benzene with ethylene [43], to name merely a few. Moreover, due to its very large intracrystalline cavities, zeolite MCM-22 has also been demonstrated to be a suitable host material for a variety of catalytically active guests, e.g. transition metal complexes which are useful in selective oxidation [44] or hydrogenation [45] reactions. Due to these interesting properties it seems worthwhile to focus on the synthesis features of MCM-22 (see below). 

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