Hydrogenolysis versus isomerization, selectivity

Fig. 11. Selectivities in hexane conversions versus temperature for hydrogenolysis (Hy), isomerization (ISO), benzene (Be), and methylcyclopentane (MCP) formations (1% Ir on inert Si02).

The temperature dependence of the selectivity for isomerization versus hydrogenolysis depends on the type of catalyst. Thus, over thick platinum film catalysts this selectivity was temperature independent for the reaction of the butanes and neopentane (24). However, in Boudart and Ptak s (122) reaction of neopentane over platinum/carbon the selectivity to isomerization decreased slightly with increasing temperature while Kikuchi et al. (128) found an increased trend for isomerization in the reaction of n-pentane over platinum/silica and platinum/carbon catalysts. 

To provide adequate background for the work to be described next, some further findings by Anderson and Avery may be mentioned. The selectivity for isomerization versus hydrogenolysis (St = 77/77 ) of isobutane on evaporated films of platinum claimed to expose (111) faces predominantly was found to be enhanced by a factor of 5 relative to unoriented films this enhancement was not observed for n-butane (Table VI). Anderson and Avery (47) proposed that a symmetrical triadsorbed species (Diagram 1) is the preferred reaction intermediate for isobutane, such an intermediate not being possible for n-butane. This intermediate fits the triplets of metal atoms on the (111) plane of platinum, suggesting, they believed, a basis for the enhanced efficiency of the (111) plane for the isomerization of isobutane. We note that inspection of rates of isomerization given in the paper of Anderson and Avery shows a factor of only... 

---