2- Methylpentane product distribution

Referring first of all to the reactions over 0.2% platinum/alumina (Table V) the major features of the product distributions may be explained by a simple reaction via an adsorbed C5 cyclic intermediate. For instance, if reaction had proceeded entirely by this path, 2-methylpentane-2-13C would have yielded 3-methylpentane labeled 100% in the 3-position (instead of 73.4%) and would have yielded n-hexane labeled 100% in the 2-position (instead of 90.2%). Similarly, 3-methylpentane-2-I3C would have yielded a 2-methylpentane labeled 50% in the methyl substituent (instead of 42.6%), and would have yielded n-hexane labeled 50% in the 1- and 3-positions (instead of 43.8 and 49% respectively). The other expectations are very easily assessed in a similar manner. On the whole, the data of Table V lead to the conclusion that some 80% or so of the reacting hydrocarbon reacts via a simple one step process via an adsorbed C5 cyclic intermediate. The departures from the distribution expected for this simple process are accounted for by the occurrence of bond shift processes. It is necessary to propose that more than one process (adsorbed C6 cyclic intermediate or bond shift) may occur within a single overall residence period on the catalyst Gault s analysis leads to the need for a maximum of three. The number of possible combinations is large, but limitations are imposed by the nature of the observed product distributions. If we designate a bond shift process by B, and passage via an adsorbed Cs cyclic intermediate by C, the required reaction paths are... 

Unlike the behavior over 0.2% platinum/alumina, the main features of the labeled product distributions obtained over 10% platinum/alumina and over platinum film catalysts (Tables VI and VII respectively) cannot be explained in terms of a single dominant reaction pathway via an adsorbed C6 cyclic intermediate. Again, parallel, multiple-step reaction pathways are involved. The results from 2-methylpentane-2-13C have been qualitatively accounted for (84) by the pathways... 

Work with ultrathin and thick fringe-free platinum films has shown that not only does the product distribution change with catalyst structure, but the specific rate of raction (per unit platinum area) changes also (30). The data in Fig. 12 for the reaction of 2-methylpentane and n-hexane show a decrease in the specific rate with increasing particle size. 

The main products formed by the catalytic alkylation of isobutane with ethylene (HC1—AICI3, 25-35°C) are 2,3-dimethylbutane and 2-methylpentane with smaller amounts of ethane and trimethylpentanes.13 Alkylation of isobutane with propylene (HC1—AICI3, — 30°C) yields 2,3- and 2,4-dimethylpentane as the main products and propane and trimethylpentanes as byproducts.14 This is in sharp contrast with product distributions of thermal alkylation that gives mainly 2,2-dimethylbutane (alkylation with ethylene)15 and 2,2-dimethylpentane (alkylation with propylene).16... 

The product distributions in the ring opening of substituted cyclopentanes show striking similarities to those in isomerization of the corresponding alkanes (n-hexane, 2- and 3-methylpentane),301 which led to the formulation of the cyclic mechanism involving adsorbed cyclopentane intermediates to interpret alkane isomerization15,251,252 (see Section 4.3.1). 

This result can only be explained by the /3-scission of the trivalent 4-methylpent-2-yl ion 30 as the initial step in the cracking process. Based on this and on the product distribution versus time profile, a general scheme for the isomerization and cracking process of the methylpentanes has been proposed103,104 (Scheme 5.16). 

It was shown that the SbF5-intercalated graphite efficiently promotes disproportionation of various alkylbenzenes by simple mixing at room temperature (41). The isomerization of methylpentanes was carried out over the catalyst at room temperature, -30, and - I7°C in a continuous flow system a careful study of the kinetically controlled product distribution was performed to obtain information for the reaction path (42, 43). The skeletal rearrangements of l3C- abeled 2-methyl, 3-methylpentane and... 

To illustrate this method of approach, results of three of the experiments by Corolleur et al. are shown in Tables XIII and XIV. In the examples selected here, a 13C-labeled 3-methylpentane and a labeled 2-methylpentane are reacted, in turn, and distribution of 13C is shown for the methylpentane products in Table XIII and for the n-hexane products in Table XIV. Distributions expected for a pure cyclic mechanism (C) and for a methyl shift (T) are indicated. Detailed discussion by the authors of abnormal products (i.e., those not predicted by the single-stage purely carbocyclic mechanism) led to the conclusion that these are formed on a second, less numerous, type of surface site by the action of which a succession of several rearrangements, according to a cyclic or a bond-shift mechanism, takes place. In Table XIV it can be seen that assumption of a simple skeletal rearrangement of the type... 

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. 

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). 

As a model for cracking of alkanes, the reaction of 2-methylpentane (MP) over SbFs-intercalated graphite has been studied in a flow system, the hydrocarbon being diluted in a hydrogen stream. A careful study of the product distribution vs time on stream showed that propane was the initial cracking product whereas isobutane and isopentane (as major cracking products) appear only later. 

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. 

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... 

Figure 5.13. Distribution of cracking products (mass%) of 2-methylpentane (16) versus time on stream at 20°C.104...

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