Methylpentanes hydrogenolysis

TABLE 7.14. Kinetic Parameters for 3-Methylpentane Hydrogenolysis Over Metal Catalysts... 

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)... 

Microwave activation of alkane transformations was studied in detail by Roussy et al., who summarized their results in several papers [2, 28, 29, 79]. Isomerization of hexane, 2-methylpentane, 2-methyl-2-pentene, and hydrogenolysis of methylcydo-pentane have been investigated, and the diversity of possible effects has been specified [2]. The course of 2-methylpentane isomerization on a 0.3% Pt/Al203 catalyst depended on the mode of heating - the distribution of hexane products was different... 

Hydrogenolysis of 2-methylpentane, hexane, and methylcyclopentane has been also studied on tungsten carbide, WC, a highly absorptive catalyst, at 150-350 °C in a flow reactor [80], These reforming reactions were mainly cracking reactions leading to lower molar mass hydrocarbons. At the highest temperature (350 °C) all the carbon-carbon bonds were broken, and only methane was formed. At lower temperatures (150-200 °C) product molecules contained several carbon atoms. 

Gault et al. noticed in their early papers (757) that the product pattern of methylcyclopentane (MCP) hydrogenolysis is sometimes surprisingly similar to that of hexane or methylpentane(s) isomerizations. They suggested that isomerization proceeded via a cyclic, methylcyclopentane-like intermediate. Later it appeared that the similarity was not always found, but an important idea was already born and, more importantly, was brilliantly confirmed by later papers from the laboratory of Gaults. 

Higher hydrocarbon molecules allow study of the unique cracking pattern of metals. These studies are usually carried out at low conversion to observe only primary hydrogenolysis. Nickel exhibits high selectivity to cleave terminal C—C bonds leading to demethylation that is, it cleaves only bonds that involve at least one primary carbon atom. For example, in the transformation of n-hexane, only methane and n-pentane are formed (180°C, Ni-on-silica catalyst, 0.3% conversion), whereas 2-methylpentane and 3-methylpentane yield methane, n-pentane, and isopentane.260 In the transformation of 2-methylpentane, the n-pentane isopentane ratio is close to 2, which corresponds to the statistical value. Under more forcing conditions, successive demethylations lead eventually to methane as the only product. 

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. 

The benzene selectivity in /i-hexane conversion over Pt/KL catalysts increases with conversion, but the selectivity for methylcyclopentane decreases (21). Lane et al. show that the MCP yield passes through a maximum between 40 and 60% conversion, 2-methylpentane and 3-methylpentane peak around 80% conversion, and hydrogenolysis products and benzene increases with -hexane conversion (29). The same reaction product distribution was found for a variety of supports, e.g., AI2O3, Si02, KL, and KY. On Pt/KY, Pt/NaY, and Pt/ALOj the 2MP/3MP ratios... 

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). 

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,... 

Finally, the partially selective Mechanism C in hydrogenolysis of cyclopentanes has a counterpart in dehydrocyclization of methylpentanes and n-hexane. The intervention of this mechanism, involving metallocyclobutane intermediates, is strongly supported by studies of aromatization (see Section V). 

On platinum, the a, -dicarbene mechanism which accounts for the hydrogenolysis of cycloalkanes (Scheme 34) is no longer predominant in the hydrocracking of acyclic alkanes. It has already been emphasized that the internal fission of isopentane and n-pentane is related to the metallocyclobutane bond shift mechanism of isomerization (see Section III, Scheme 29), and that in more complex molecules, the favored rupture of the C-C bonds in a p position to a tertiary carbon atom is best explained by the rupture of an a,a,y-triadsorbed species (see Section III, Scheme 30). The latter scheme can account for the mechanism of hydrocracking of methylpentanes on platinum. Finally, the easy rupture of quaternary-quaternary C-C bonds in... 

In order to discuss the effect of particle size on selectivity we must consider the mechanisms that have been proposed to explain the isomerization of 2-methylpentane to 3-methylpentane and the hydrogenolysis of methylcyclopentane. These matters have been extensively studied (22, 24, 200, 280), and we discuss here only the elementary aspects of the problem. Gault and his group (280) have proposed the following sequence, in which the reactant is 2-methylpentane-2-l3C the labeled carbon is indicated by the black dot. 

---