Hydrogenolysis of methylcyclopentane

This is a structure-sensitive reaction (RSS), and hence the intrinsic activity or turnover frequency (TOP) depends on the particle sizes. The hydrogenolysis reaction was also tested with the Pt/Al203 and promoted with Sn. The reaction was carried out at 1 atm and 573 K and a mixture of hydrogenolysis of methylcyclopentane (MCP)/H2 (1 10.5). The selectivity toward n-hexane, 2-methylpentane, and 3-methylpentane was determined for conversions less than 10 % and was presented in Table 3.2 [15]. 

Since this reaction is structure sensitive, the product distribution depends also on the particle sizes. For very small particles, the reaction presented a product distribution of about 25 % of 3-methylpentane, 42 % of 2-methylpentane, and 30 % n-hexane, which is similar to the statistical probability. For bigger particles, the ring opening presented different selectivity, producing 33 % of 3-methylpentane, 67 % of 2-methylpentane, and without any formation of n-hexane. 

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Nevertheless, in another branched reaction, the hydrogenolysis of methylcyclopentane on Pt-AhOs (10% Pt) at 230°C, leading to 2- and 3-methylpentane (n-hexane is not practically formed under the conditions used)... 

Methylcyclopentane is a powerful probe molecule for the study of metal surfaces. The product distribution on platinum depends on the following factors particle size 491 reaction conditions 492-494 carbonaceous residues,492,493,495 and the extent of the interface between the metal and the support.492,493,495 The hydrogenolysis rate of methylcyclopentane depends on the hydrogen pressure.496,497 The rate exhibits a maximal value as a function of hydrogen pressure on EuroPt catalysts.498 The hydrogenolysis of methylcyclopentane has also been studied over Pt-Ru bimetallic catalysts.499... 

Isomer Distributions in Initial Products from Hydrogenolysis of Methylcyclopentane and from Isomerization of Hexanes over Platinum Catalysts ... 

The 13C-labelling experiments allowed one to determine the relative contributions of cyclic and of bond-shift mechanisms in the isomerization and cracking reactions of 2-methylpentane and hydrogenolysis of methylcyclopentane over Pt TiC>2 catalysts prepared by different methods566. 

FIG. 13. Common intermediate for dehydrocyclization and isomerization of n-hexane and hydrogenolysis of methylcyclopentane (61). 

Hydrogenolysis of Methylcyclopentane and Isomerization of Hexanes. Comparison Between Calculated and Observed Distributions a... 

For the intimate (5m particle) composite, such control of the X-function results in the results shown in Fig. 16. The fresh platinum, under these conditions leads to complete hydrogenolysis of methylcyclopentane. Platinum deactivation controls the reaction, optimizing the isomerization products (benzene -p cyclohexane), until the generating step itself becomes insufficient. 

The effects of oxidation - reduction cycles on the activity and selectivity of supported Rh catalysts were investigated using the hydrogenolysis of methylcyclopentane (MCP) as a test reaction. Prom the analysis of catalytic properties and reduction profiles it Is concluded that, on silica-supported catalysts, following the initial oxidation at 400 C successive reduction treatments at increasing temperatures cause a progressive reconstruction of the Rh particles. On y-alumina-supported catalysts the situation is more complex. The interaction of Rh with the support during the Initial oxidation makes a fraction of the Rh Inaccessible to the gas phase. Only after subsequent oxidation - reduction cycles do they behave like the silica-supported catalysts... 

In tlie hydrogenolysis of methylcyclopentane, and the isomerization of hexanes, striking differences in initial product distributions on dispersed (0.2% 14-AI2O3) and concentrated (10% Pt-Al203) catalysts were ascribed to the occurrence of 2.0 and 20.0 nm crystallites, which promoted the cyclic mechanism or the bond shift + cyclic mechanisms, respectively. The possible roles of support acidity or secondary reactions were debated, for example ref. 182, which also lists earlier references. 

Activity and deactivation constants of Pt/AbOa and Pt-Au/AbOs catalysts for the hydrogenolysis of methylcyclopentane at 623 K. 

The cyclic mechanism was demonstrated by comparing the initial product distributions in the hydrogenolysis of methylcyclopentane and in isomerization of methylpentanes and -hexane. For instance, the ratios 3-methyl-pentane/n-hexane, extrapolated to zero conversion, are the same in hydrogenolysis of methylcyclopentane and in isomerization of 2-methylpentane. Since cyclic type isomerization involves first carbon-carbon bond formation and then carbon-carbon bond rupture, one does not expect hydrocracking of alkanes to occur by this mechanism. In contrast, as suggested early on (55), if bond shift isomerization involves first carbon-carbon bond rupture and then carbon-carbon bond recombination, a common intermediate should exist, leading to both the isomerization and the hydrocracking products. 

A more careful study of the hydrogenolysis of methylcyclopentane on two catalysts of extreme dispersion (0.2 and 10% Pt) showed that, in the temperature range 250°-310°C, the product distributions were temperature insensitive on the 0.2% Pt/Al2O3 catalyst, but temperature sensitive on the 10% Pt/AljOj catalyst (86). On the latter, all the observed distributions appeared as combinations of two limiting distributions, one of which includes only methylpentanes and therefore corresponds to a completely selective hydrogenolysis of—CH2—CH2— bonds the other one contains n-hexane, but is different from the one obtained on the 0.2% Pt/Al2O3 catalyst. Platinum films are intermediate between the two types of supported catalysts (86,87. ... 

The first approach to the cyclic mechanism of isomerization was the finding that the interconversion of n-hexane and methylpentanes takes place under the conditions where the nonselective mechanism of hydrogenolysis (Mechanism A) is the only one operating that is, on 0.2% Pt/AljOj (32). The identical product distributions in isomerization of hexanes and hydrogenolysis of methylcyclopentane suggested that both reactions involve a common intermediate with a methylcyclopentane structure. It was then proposed that the species responsible for dehydrocyclization of hexanes are a,j8, -triadsorbed species involving a single metal atom (55) (Scheme 40). 

Indeed the carbene-alkyl insertion mechanism in Scheme 45 neatly explains why the rates of dehydrocyclization of 1, 2, and 3 are so similar. However, since 2-methylhexane also undergoes 1-5 dehydrocyclization, involvement of methylenic carbon atoms and not simply terminal carbon atoms must also be possible. The pathway for the C7-alkanes must be the reverse of nonselective hydrogenolysis of methylcyclopentane (Mechanism A), since it also results in isomerization to 2,4-dimethylpentane and 3-methylhexane, most likely via adsorbed 1,3-dimethylcyclopentane (scheme 46). It is... 

Hence, three different dehydrocyclization mechanisms, corresponding to the three mechanisms of hydrogenolysis of methylcyclopentane, may be characterized ... 

We suggest that the precursor species in Mechanism A is a 1,5-dicarbene, which undergoes recombination to a 7i-adsorbed olefin—the precursor species in nonselective hydrogenolysis of methylcyclopentane 44, 88, 91) (Scheme 47). The metallodicarbenes in Scheme 41 have stable analogs (98), and dicarbene recombination has been suggested as a transient step in organometallic reactions (99-101). The mechanism in Scheme 47 is now... 

On the other hand, the selective dehydrocyclization, which does not allow the formation of secondary-primary C-C bonds, must involve only two methylic carbon atoms in the 1 and 5 positions. Although the reverse reaction (selective hydrogenolysis of methylcyclopentane) could be observed on platinum catalysts of low dispersion at 220°C (86), the selective dehydrocyclization of methylpentanes on these catalysts is detectable only at higher temperatures (280°-300°C), where it competes with another process, ascribed to Mechanism C (33). Fortunately, it was found recently that iridium supported on AI2O3 or SiOj selectively catalyzes at 150°C the cyclic type interconversion of 2-methyl- and 3-methylpentanes (88). n-Hexane under the same conditions yields only cracked products (702) (Scheme 52). Similarly,... 

As already pointed out, the first particle size effect in skeletal rearrangement was found for the hydrogenolysis of methylcyclopentane (55). Nonselective hydrogenolysis takes place on highly dispersed catalysts, with a metal loading smaller than 0.6%, while selective rupture of bisecondary C-C bonds occurs on heavily loaded catalysts (more than 6% platinum on alumina). 

Fig. 11. Particle size effect in hydrogenolysis of methylcyclopentane 3-methylhexane/n-hexane ratio.

To summarize the previous investigations, most of the results indicate a remarkable constancy of the selectivities on all the metal particles down to a size of 10-12 A the ratios of bond shift to hydrogenolysis, and of cyclic mechanism to bond shift, remain constant, the only exception being around 25 A crystallite size, where the change from nonselective to selective hydrogenolysis of methylcyclopentane occurs (Fig. 16). This constancy could mean that all reactions take place on edge atoms. However, in the case of neo-... 

Curves 5-8 are based on the data of Del Angel et al. (282). For the hydrogenolysis of cyclopentane, Rh/Al203 is structure sensitive and Pd/Al203 is essentially structure insensitive. For the hydrogenolysis of methylcyclopentane, Rh/Al203, as already mentioned, gives the unusual result shown by curve 8, which is difficult to explain. Note also the point... 

Fig. 16. Hydrogenolysis of methylcyclopentane. Selectivity and mechanistic effects as a function of fraction exposed FE and particle size d (see Table IX for details of the studies).

Details of the Selectivity Studies Presented in Fig. 16 in the Hydrogenolysis of Methylcyclopentane... 

Fig. 17. Percentage of normal hexane in the six-carbon products as a function of mean particle diameter for the hydrogenolysis of methylcyclopentane over supported Pt catalysts (292) (see Table X for details of the studies).

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