Hydrogenolysis methylcyclopentane

Relative Bond Rupture Probabilities for Methylcyclopentane Hydrogenolysis on Platinum Catalysts... 

As discussed in Section IV, Barron et al. (55, 61) found the cyclic mechanism of isomerization to be predominant, perhaps the sole route, on a highly dispersed platinum-alumina (0.2% w/w Pt). The cyclic mechanism was shown to be important also over platinum films and supported platinum of moderate dispersion (>100 A). Here, although the product distributions were very different from that found over the dispersed catalyst, the initial product distributions at 300°C were practically identical in the isomerization and in methylcyclopentane hydrogenolysis. At lower temperatures they were somewhat different as they also were at all temperatures on platinum films. It was suggested that, especially on platinum films, a bond-shift isomerization could accompany the cyclic... 

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). 

Pt and on Pt-Ce and Pt-Co this ratio is equal to 0.7 0.1 which means that the methylcyclopentane hydrogenolysis is non selective (19) and that the catalysts are well dispersed. The values are equal "before" and "after" the carbon monoxide reaction on Pt-Ce and Pt-Co. At the opposite, on catalysts with Ni this ratio is higher (3.9 0.9). 

From previous works undertaken in the laboratory we were able to correlate, in the isomerization of 2-methylpentane and in the methylcyclopentane hydrogenolysis, that larger amounts of n-hexane compared with 3-methylpentane are due to the presence of... 

Nevertheless, there are no great differences in the product distribution occur after an oxidation-reduction cycle. On both catalysts the selectivity in isomers is increased after an oxidative reaction. The values of the ratio 3-MP/n-H obtained from the methylcyclopentane hydrogenolysis are not modified by this redox cycle. 

TABLE 7.17. Kinetic Parameters for Methylcyclopentane Hydrogenolysis Over Metal Catalysts... 

If there are differences in the isomer selectivities when the metal is changed, as underlined above, there are also differences in the relative distribution of the isomers when the Pt particle sizes are changed. In Fig. 17.1 is presented the influence of the platinum particle sizes in 2-methylpentane isomerization reactions and methylcyclopentane hydrogenolysis. Notice that ... 

Caspar AB, Dieguez LC. Dispersion stability and methylcyclopentane hydrogenolysis in Pd/A1203 catalysts. Appl Catal A. 2000 201 241. 

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)... 

The values of the adsorption coefficient of hydrogen for both reactions were practically identical (1.9 and 2.1 atm-1). Here, the selectivity of the branched reactions depends on the partial pressure of methylcyclopentane. This difference may be accounted for by assuming that either the cleavage of the C—C bond of methylcyclopentane in the (3-position and in the 7-position with respect to the methyl group does not take place on the same sites of the surface of platinum (or on the sites of the same activity), or that the mechanism of hydrogenolysis is more complex than that ex-... 

Gault and coworkers [ 149] have observed that the distribution of products obtained by hydrogenolysis and isomerization of methylcyclopentane was the same as those obtained with hexane. They proposed two competing mechanisms a selective mechanism implying an a, a, p, j0-tetra-adsorbed species and a non-selective mechanism implying coordinated olefin and bis-carbene intermediates (Scheme 38). 

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 ... 

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. 

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. 

Tantalum hydride(s) also catalyzes the hydrogenolysis of cyclic alkanes (substituted or not) but the reachvity order decreases with the cycle size as cycloheptane > methylcyclohexane > cyclohexane > methylcyclopentane > cyclopentane for the latter no reaction is actually observed (Figure 3.8). Activity decreases with hme and becomes low after 20 h. 

Figure 3.8 Conversion with time in the hydrogenolysis of cycloalkanes (19Torr, 14.5 equiv.) catalyzed by (=SiO)2TaH (3) at 160°C under hydrogen (470Torr) cycloheptane ( ), methylcyclohexane ( ), cyclohexane ( ), methylcyclopentane (A) and cyclopentane (x).

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. 

Fig. 10. Selectivities in hexane conversions versus temperature for benzene formation (Be), hydrogenolysis (Hy), methylcyclopentane formation (MCP), isomerization (ISOM), and dehydrocyclization (Dehy) (9 wt. % Pt on inert Si02).

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. 

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