Methylcyclopentane ring opening

TABLE 7.16. Kinetic Parameters for Methylcyclopentane Ring Opening Over Metal Catalysts... 

The alkylcyclopentane (AGP) to aromatics process (ACP ACH Ar) is less efficient than ACH dehydrogenation, owing to the slowness of the first step and to ACP ring opening. Under conditions where cyclohexane is converted to benzene with close to 100% efficiency, only 50—75% of methylcyclopentane may be converted to benzene. 

With methylcyclopentane (166, 153) as with methylcyclobutane (112), there is a leveling effect with increasing temperature so that the ring opening probabilities become more nearly equal. 

The subscripts denote as follows H, hydrogenolysis i, isomerization C5, C5 cyclization Ar, aromatization ol, olefin formation. Ring opening products from methylcyclopentane are given under 5j. 

In helium, all hexanes give benzene (Table IV) (97, 97a). Methylcyclo-pentzne and methylpentanes give similar benzene yields. No saturated isomers are detected, and the ring opening of methylcyclopentane is negligible. 

Cycloheptane is about the largest ring in which the participation of the whole molecule can be expected in catalystic reactions. This molecule exhibits practically all the reactions described for cyclic hydrocarbons. The ring opening of methylcycloheptane is also selective, but total reactivity is much less than that of methylcyclopentane (702) ... 

The isomerization of cycloalkanes over SbF5-intercalated graphite can be achieved at room temperature without the usual ring opening and cracking reactions, which occur at higher temperatures and lower acidities.110 In the presence of excess hydrocarbon after several hours, the thermodynamic equilibrium is reached for the isomers. Interconversion between cyclohexane (20) and methylcyclopentane (21) yields the thermodynamic equilibrium mixture [Eq. (5.46)]. 

Ring opening of methylcyclopentane is selective on metal particles larger than 2 nm and non-selective on smaller particles. Thus, the ratio of 3-methylpentane to n-hexane decreases from 4.6 to 0.4 as the dispersion increases from 4 to 55%. This corresponds to a change in the non-selective cyclic mechanism (NSCM) from 11 to 100% as the dispersion increases. 

The isomerization of a series of cyclic and bicyclic saturated hydrocarbons over SbF,-intercalated graphite was achieved at or below room temperature without the ring opening and cracking reactions, and the thermodynamic equilibrium was reached for the isomers in all cases (39). Interconversion between cyclohexane and methylcyclopentane also yielded the thermodynamic equilibrium mixture. 

FIG. 14. Nonselective ring opening of methylcyclopentane in interconversion ofn-hexane, 2-methylpentane, and 3-methylpentane (62). 

Rates of the losses of methyl and methane from the molecular ion of methylcyclopentane have been determined over the time range 40 ps to microseconds [288]. The loss of ethylene from methylcyclopentane and decompositions of methylcyclohexane were also investigated. With support from I3C labelling, it was suggested that at times shorter than 1 ns, methyl was lost from the intact cyclopentane ion, but that at longer times ring opening preceded the decomposition. 

In the high temperature region consecutive reactions play a role also when the overall conversion is kept low (under 10%) a fraction of isomerization products are formed through a ring opening of the previously produced methylcyclopentane (when hexane is the feed). This indicate that each shortening of the apparent contact time T enhances dehydrocyclization. Contact time T can be varied in a controlled manner by changing the amount of the catalyst and the extent feed. 

In this section we present experimental evidence for a bifunctional alkane isomerization mechanism obtained by selective poisoning of the acidic sites of Pd-NiSMM with pyridine, which was pulse-injected into the liquid hydrocarbon feed stream. The possibility of additional poisoning of the metallic sites was checked by studying the hydrogenation of benzene and the isomerization and ring opening of methylcyclopentane (MCP). 

The methylcyclopentane (MCP) hydrogenolysis has been employed to study the catalytic surface, due to its sensitivity to metal dispersion and electronic structure [10-14], Depending on operation conditions and type of catalyst, it is possible to obtain products from ring opening, cracking, and ring enlargement and aromatization reactions in case of acid sites in the catalyst [11,12,15],... 

Unfortunately we were not able to check the metallic particle size by T.E.M. so we used the test reaction of the methylcyclopentane hydrogenolysis (19). We have shown that the selectivity of the ring opening is correlated to the particle size (20). 

Fig. 17.1 Influence of the platinum dispersion on the relative contribution of the cyclic mechanism (CM) and on the ring opening reaction of methylcyclopentane.

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