Group VIII metals isomerization

D. Isomerization with Palladium and Other Group VIII Metals. 38... 

In this connection, a recent article by Phillipson and Wells 44) dealing with the isomerization of butenes is of interest. The Group VIII metals catalyze both the hydrogenation and isomerization of butenes, cobalt... 

Transition metals are used as catalysts for a variety of reactions hydrogenation, hydrogenolysis and isomerization of hydrocarbons (group VIII metals), oxidation of... 

The relative contribution of the two mechanisms to the actual isomerization process depends on the metals and the experimental conditions. Comprehensive studies of the isomerization of n-butenes on Group VIII metals demonstrated179-181 that the Horiuti-Polanyi mechanism, the dissociative mechanism with the involvement of Jt-allyl intermediates, and direct intramolecular hydrogen shift may all contribute to double-bond migration. The Horiuti-Polanyi mechanism and a direct 1,3 sigma-tropic shift without deuterium incorporation may be operative in cis-trans isomerization. 

Oxidation of olefins other than ethylene occurs more slowly and often in lower yields. Propylene is oxidized selectively to acetone,516,519 and normal butenes give methyl ethyl ketone.519 Higher olefins generally produce mixtures as a result of olefin isomerization.520 Aqueous solutions of other Group VIII metal salts, such as Pt(II), Ir(III), Ru(III), and Rh(III) oxidize olefins in an analogous manner to Pd(II), although at significantly lower rates.512... 

Alkynes containing methylene (—CHi—) functionalities a to the triple bond readily undergo thermal C-H activation under conditions necessary to initiate reaction with group VIII metal carbonyls (54). The allenyl clusters obtained isomerize thermally via a 1,2-hydrogen atom shift to afford the thermodynamically more favorable 1,3-dimetalloallyl clusters (Table 1) (49,55). A similar chemistry has been recognized for osmium clusters, albeit under more severe conditions. 

In the work of the author and his associates on bimetallic catalysts comprising various combinations of Group VIII and Group IB metals, it was discovered that the activity of the Group VIII metal for hydrogenolysis reactions of hydrocarbons was decreased markedly by the presence of the Group IB metal (11-15). It was shown that the inhibition of hydrogenolysis leads to improved selectivity for alkane isomerization reactions (11) and for reactions in which saturated hydrocarbons are converted to aromatic hydrocarbons (12,14,15). Interest in bimetallic catalysts increased markedly with the discovery of this selectivity phenomenon. 

In general, inclusion of a Group IB metal with a Group VIII metal markedly decreases the hydrogenolysis activity of the latter but has a much smaller effect on the activity for reactions such as the dehydrogenation and isomerization of hydrocarbons (1,2,5-8). These observations have been supported by the work of other investigators (9-11). 

In all of these studies, the presence of the Group IB metal, copper or gold, with the Group VIII metal, nickel or palladium, led to inhibition of hydrogenol-ysis on the Group VIII metal. As a consequence, selectivities for isomerization of the n -alkanes to branched alkanes, and for aromatization of methylcy-dopentane to benzene, were improved. The studies of the Netherlands workers on nickel-copper alloys, like those conducted in our laboratories on the same system, eliminated the possibility of complications caused by the presence of a carrier and were therefore useful for clarifying the selectivity phenomenon. 

Table 17.1 The isomerization of2-methylpentane 2 C on Group VIII metals, following Scheme 17.3, under PHc=5Torr and Pu2=760Torr.

The rates of olefin hydrogenation and isomerization by Group VIII metal-phosphine complexes are increased by the presence of hydroperoxides and/or oxygen. A similar rate enhancement is observed in the hydroformylation of alkenes catalysed by [RhCl(CO)(PPh3)2]. The addition of small amounts of cyclohexenyl hydroperoxide is considered to effect the unusual transformation of [RhCl(CO)(PPh3)2] to cw-[RhCl(CO)2(PPh3)], which appears to be a very active alkene hydroformylation and isomerization catalyst. Asymmetric induction in hydroformylation reactions has been achieved. ... 

Extremely high regioselectivity has been observed for hydroformylation of fluoro-olefins RfCH=CH2, catalysed by group VIII transition metals. While a Co catalyst gives the normal product 345 on hydroformylation of 344, a Rh catalyst gives mostly the isomeric aldehyde 346470. In another study, hydroformylation of 1-hexene was catalysed by rhodium(I) with concomitant isomerization471. 

Many of the significant reactions of unsaturated hydrocarbons (hydrogenation, isomerization, carbonylation, oxidation, polymerization) are catalyzed heterogeneously by metals in or near Group VIII or homogeneously by salts and complexes of these elements. Those reactions effected in both systems are discussed in terms of probable common intermediates anomalies, where they occur, are ascribed either to the ability of surfaces to form intermediate species which cannot be stabilized by single metal atoms or to the ability of the latter to coordinate simultaneously more than one hydrocarbon molecule. 

To summarize, it may be said that the addition of hydrogen to 1,3-butadiene in gas phase reactions occurs partly by a 1-2-inechanism over all metal catalysts giving 1-butene in the gas phase. 2-Butene is either produced directly by a simultaneous 1-4-addition process as in the cobalt- and palladium-catalyzed reactions, or it is produced indirectly by the isomerization of 1-butene after its initial formation on the surface as is the case with the remaining metals of Group VIII and copper. The fraction of adsorbed olefin which is hydrogenated to w-butane depends upon the manner in which the thermodynamic and mechanistic factors, discussed previously, operate in each particular reaction. 

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