Muller-Gault mechanism

Muffin-tin scatterers, 34 212 periodic muffin-tin, 34 218 Muller and Schaich formalism, 34 217 Muller-Gault mechanism, 30 17 Multicomponent catalysts, early studies of, 2 81... 

Fig. 2. Mechanisms of bond-shift isomerization on metal catalysts, (a) Anderson-Avery mechanism (63). (b) Muller-Gault mechanism for isomerization of neopentane on Pt (64). (c) McKervey-Rooney-Samman mechanism (69). (d) Muller-Gault mechanism of isomerization of isobutane on Pd (64). (e) Clarke-Rooney mechanism (46). (f) Garin-Gault mechanism (65).

Of the four basic mechanisms, two, the Anderson-Avery mechanism (Scheme 19) and the Garin-Gault mechanism (Scheme 25), should be associated with isomerization and hydrogenolysis two, the Muller-Gault mechanism (Scheme 20) and the Rooney-Samman or the Clarke-Rooney mechanism (Schemes 22 and 28) should promote only isomerization. 

The mechanism of bond-shift isomerization of alkanes is still a subject of controversy. Several suggested mechanisms involved an intermediacy of multiply bonded species [Anderson-Avery (63), Muller-Gault (64), and Garin-Gault (65) mechanisms] (Fig. 2). More recent data (51, 52, 66-68) seem to support the 1,2 bond-shift isomerization involving monoadsorbed alkyl species (69). Figure 2 shows these mechanistic ideas. 

Muller and Gault proposed (57) an adsorbed cyclopropane-like intermediate (Scheme VIIIB). The McKervey-Rooney-Samman mechanism involves a n complex like adsorption of two carbon atoms (88) (Scheme Vine). 

The rearrangement and formation of 2,2-dimethylbutane—with a quaternary carbon atom—is only possible via the above mechanism. Over platinum, 2,3-dimethylbutane (75) gave more benzene over palladium, 2,2-dimethylbutane (97a) gave more benzene. This is the opposite selectivity, as reported by Muller and Gault, for ring expansion of 1,1,3-trimethyl-cyclopentane (57). This may be more evidence that at least two different types of bond shift mechanism can occur. 

Muller, J. M., and Gault, F. G., Presented at the Fourth Int. Congr. Catal., Moscow, 1968. Symposium Kinetics and Mechanism of Complex Catalytic Reactions," Paper 15. 

In a further study, Muller and Gault (94) reported that isomerization of 2,3-dimethylbutane on thick platinum films yielded, as well as the expected bond-shift initial products (2-methylpentane and 2,2-dimethylbutane), substantial amounts of 3-methylpentane, n-hexane, and methylcyclopentane even at 273°C. Clearly, this is another example of a multistep mechanism. On the same basis, isomerization of 2,2-dimethylbutane should give only 3-methylpentane, 2,3-dimethylbutane, and 2-methylpentane as initial products in fact, Muller et al. report that n-hexane, methylcyclopentane, and benzene represented 15% of their initial products at 275°C. Somewhat in contrast to the situation for Pt/Al203, the number of surface reactions before desorption appeared to be no greater than two or three. It turns out that in the formation of 3-methylpentane the distribution was best explained by the succession of a bond shift and cyclic mechanism. This is quite distinct from the formation of n-hexane where two consecutive bond shifts occur. Perhaps in consequence of this difference, they conclude, a marked variation with temperature of the product distributions is observed. 

This reaction mechanism is completely analogous to that proposed by Gault and Muller (77) for the formation of a cyclic intermediate from diadsorbed hydrocarbon ... 

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