Olefin isomerization over

In view of this situation we thought it worthwhile to investigate further the mechanism of normal olefin isomerization over Na-Y-zeolite. 

Brody, J. F., J. W. Johnson, G. W. McVicker, and J. J. Ziemiak. 1989. Olefin isomerization over an alumina-pillared fluoromica catalyst. Solid State Ionics 32/33 350-3. 

A catalyst used for the u-regioselective hydroformylation of internal olefins has to combine a set of properties, which include high olefin isomerization activity, see reaction b in Scheme 1 outlined for 4-octene. Thus the olefin migratory insertion step into the rhodium hydride bond must be highly reversible, a feature which is undesired in the hydroformylation of 1-alkenes. Additionally, p-hydride elimination should be favoured over migratory insertion of carbon monoxide of the secondary alkyl rhodium, otherwise Ao-aldehydes are formed (reactions a, c). Then, the fast regioselective terminal hydroformylation of the 1-olefin present in a low equilibrium concentration only, will lead to enhanced formation of n-aldehyde (reaction d) as result of a dynamic kinetic control. 

When an alumina catalyst contains a small amount of alkali metal ions, it loses its olefin isomerizing properties, inasmuch as the relatively strong acidic sites of the alumina are neutral. Most of the dehydration reactions are usually performed over such aluminas. Consequently, the sequence of reaction types as discussed for weakly acidic media seem also to apply to dehydration over alumina catalysts. 

The greater basicity of alcohols over olefins is responsible for the fact that dehydration can be performed by weaker acidic sites than are necessary for olefin isomerization. There are, however, also other factors, such as participation of neighboring groups, which may influence the rate of dehydration of alcohols. 

The method of preparation of the alumina has a marked effect on the product distribution as shown in Table VIII (47). Over the pure alumina (P) the olefinic products are nearly equilibrated. The alkali-containing catalysts, however, give kinetically controlled products. The very low activity of these catalysts for olefin isomerization had been ascertained independently. It may, therefore, be concluded that the compo.sition of the olefins produced at 350° is very nearly that of the primary dehydration products. 

Indications of the mechanism of isomerization of saturated hydrocarbons were obtained by Ciapetta (C3), who observed that olefins were isomerized over nickel-silica-alumina catalyst at appreciably lower temperatures than were the corresponding saturated hydrocarbons, suggesting that olefins were intermediates in the reaction. Ciapetta also suggested that the rearrangement of the carbon skeleton took place via a carbonium... 

In Western Europe it is expected that new isomerization capacity may exceed alkylation installations since naphtha availability generally exceeds demand. By selecting isomerization over alkylation the octane number of the gasoline pool may be increased without increasing the volume. Moreover, olefinic charge stock avails for alkylation are considerably smaller in Europe since there are fewer catalytic cracking units per refinery than in the United States and Canada. It is predicted that C5, and to a lesser extent C5/C6 isomerization, will prevail over alkylation in Western Europe until more catalytic cracking units are installed and/or a shift in the demand for naphtha over fuel oil is experienced. 

Despite the presence of sites that strongly chemisorb a variety of molecules, pure silica gel is catalytically inactive for skeletal transformations of hydrocarbons. However, as has recently been emphasized by West et al. (79), only trace amounts of acid-producing impurities such as aluminum need be present in pure silica gel to provide catalytic activity— especially when a facile reaction such as olefin isomerization is used as a test reaction. They found that addition of 0.012% Al to silica gel resulted in a 10,000-fold increase in the rate of hexene-1 isomerization at 100°C over the pure gel. An earlier study by Tamele et al. (22) showed that introduction of 0.01% wt Al in silica gel produces a 40-fold increase in cumene conversion when this hydrocarbon is cracked at 500°C. The more highly acidic solids that are formed when substantial concentrations of metal oxides are incorporated with silica are discussed in following sections. 

An ab initio method has been employed to study the mechanism of the thermal isomerization of buta-1,2-diene to buta-1,3-diene. The results of the study have indicated619 that the transformation proceeds in a stepwise manner via a radical intermediate. Experimental free energies of activation for the bond shift in halocyclooctatetraenes have been reported and analyzed by using ab initio MO calculations.620 The isomerization of hexene using a dihydridorhodium complex in dimethyl sulfoxide has been reported,621 and it has been suggested622 that the Pd(II)-catalysed homogeneous isomerization of hexenes proceeds by way of zr-allylic intermediates. A study has been made623 of alkene isomerization catalysed by the rhodium /-phosphine-tin dichloride dimeric complex, and the double-bond isomerization of olefinic amines over potassium amide loaded on alumina has been described.624... 

The performance of C3-C5, and the Zhan catalyst (structure in Scheme 6) in the SM of methyl 10-undecenoate was compared by Meier et al. [45]. Loadings between 0.1 and 1 mol% were tested at 50 and 70°C, observing higher conversions (over 96%) for the second-generation catalysts, if compared to C3 (67-87%). However, also higher degree of double-bond isomerization was observed in the case of second-generation catalysts (55-90%), if compared to C3 (below 17%). Furthermore, 1,4-benzoquinone was used to suppress olefin isomerization side... 

The Synthol light oil (Cs-Ci ) is highly olefinic and is isomerized over an acidic catalyst to improve the octane rating of the gasoline. The hydrocarbon products from the fixed-bed reactors arc distilled to separate the gasoline and diesel oil. The residue is vacuum-disiilled to produce medium wax (320 500 C) and hard wax (>500 C). Both products arc hydrofined using nickel catalysts to remove olefins and oxygenates. 

The fundamental carbonium ion-type reactions of olefins— including double bond and carbon skeleton isomerization, polymerization, isotopic exchange, and hydrogen transfer—have been reviewed earlier (62). The importance of a thorough understanding of the nature of olefin transformations over zeolite catalysts cannot be underestimated. Probably the most important and frequently recurring pattern is the transfer or redistribution of hydrogen that is observed with olefins over acidic crystalline aluminosilicate catalysts. 

The extent of double bond isomerization also varies with the nature of the catalyst. The degree of isomerization over metal catalysts usually decreases in the order Pd > Ni > Rh > Ru > Os =Ir =Pt.5.6 The extensive double bond isomerization observed with palladium and, to some extent, with nickel catalysts can be attributed to the formation of the adsorbed 7t-allyl species with these catalysts. While double bond isomerization may not be important in a routine alkene hydrogenation, it may influence a selective hydrogenation because the isomerized olefin can have different adsorption characteristics from those of the... 

With such a catalyst available for the olefin isomerization step one may hope to find that mechanical combination with a (de-) hydrogenative catalyst, an over-all paraflin isomerization may be accomplished. 

It is found that Mode E behaves similarly to the zeolite free Pt-Re/Al203 Both catalysts have a relatively high proportion of isomer products which could be formed over the metal surface via a bond-shift mechanism [8]. Isomers are formed by doublebond isomerization and skeletal isomerization reactions at both the acid sites of the alumina support and the metal sites. The later provides a dehydrogenation-hydrogenation function and the acid sites an isomeiization function for the olefins to dehydrogenate from paraffins over the metal function, since it is known that olefin isomerization proceeds much quicker than the respective paraffin isomerization [8]. On the other hand, branched paraffins are less easily cracked than linear ones [10]. Therefore, once isomers are formed over conventional reforming catalysts, they are likely to be the final products. Evidently, the isomerization of paraffin requires the metal function in the bimetallic catalyst, and so does the paraffin aromatization. This can also explain the obseiwed decrease in the isomers and aromatics production with time-on-Hne since it is well- known that coke preferentially deposits on a metal surface first [14]. 

Paraffin isomerization over dual function catalysts based on zeolite Y and mordenite has been reviewedand a reaction mechanism was proposed in which olefin-paraffin equilibrium is established and carbonium ions are formed from both paraffins and olefins. The isomerization of n-hexane and hydrocrack-... 

Among early reported Pd-catalyzed reactions, the Mori-Ban indole synthesis has proven to be very useful for pyrrole annulation. In 1977, based on their success with the nickel-catalyzed synthesis of indole from 2-chloro-A -allylaniline, the group led by Mori and Ban disclosed Pd-catalyzed intramolecular reactions of aryl halides with pendant olefins [111]. Compound 117, easily prepared from 2-bromo-A-acetylaniline and methyl bromocrotonate, was adopted as a cyclization precursor. Treatment of 117 with PdiOAc), (2 mol%), PhjP (4 mol%), and NaHCOj in DMF provided indole 118 via an intramolecular Heck reaction followed by olefin isomerization to afford the fully aromatic product. Although yields fr om the initial report were moderate, they have been greatly improved over the last two decades [112]. 

This reaction probably involves the participation of more than two carbon atoms and the extension of a delocalized n-electron system. The results are readily explained by the unequal stabilities of the two possible intermediates, involving carbene formation on the methyl group and on the alkyl group, respectively (Scheme 74). In the first case, demethylation easily occurs by metallocarbyne formation, while in the second case, stabilization by carbene-olefin isomerization (very fast compared to hydrocracking) prevents dealkylation. However, when the a-carbon atom in the alkyl group cannot dehydrogenate to give a metallocarbene, as for 1-methyl-1-terf-butylcyclohexane, dealkylation prevails over demethylation. 

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