Deuterium-saturated hydrocarbon exchange reaction

The last reaction is effectively irreversible under the usual conditions employed to hydrogenate olefins however much information pertinent to this discussion has been obtained by studies of the exchange of saturated hydrocarbons with deuterium (7, 59), a reaction which is initiated through the reversal of reaction (4). 

In most cases the slow step of the reaction is not simply the activation or chemisorption of hydrogen, but involves other chemisorbed species. Thus, the exchange of deuterium with methane and with other saturated hydrocarbons is much slower than with hydrogen and probably proceeds through dissociative adsorption of the hydrocarbon. 

In connection with the interpretation of these trends it should be noted that in some reactions (e.g., ethylene hydrogenation) the activation energy remains substantially constant and the frequency factor changes as the metal is varied, while in other reactions (e.g., deuterium-ammonia exchange) the reverse is the case. In the exchange of deuterium with saturated hydrocarbons, a compensation effect (Cremer, 128) has been noted. The significance of these different patterns is not clear. 

To assess any differences in isotopic exchange of a kanes on microcrystalline and amorphous catalysts, we examined isotopic exchange between deuterium and two saturated hydrocarbons, cyclopentane and hexane. The range of reaction temperatures for cyclopentane was 175-275° and the molar ratio of deuterium to hydrocarbon was 5.6. Catalysts were activated at 400° first in helium and then, after runs on the amorphous catalyst, the catalyst was reactivated in hydrogen to make it microcrystalline. Two series of such runs were made. At 250°, reaction rates on the amorphous catalysts were as follows run 285, 0.40 mmoles per hour per gram of Cr203 run 296, 0.36. On the micro-crystalline catalyst, the exchange rates were, for run 288, 2.0 run 299, 1.1. These rates were computed by... 

One approach to obtain information about the intermediates involved in catalytic hydrogenation is to examine the pattern of introduction of deuterium when saturated hydrocarbons are exposed to catalytic surfaces that are saturated with deuterium. Below are shown the initial products of several such partial exchange reactions. (Continued exposure eventually results in more random distribution of deuterium.) Discuss the pattern revealed by these exchange studies and indicate what information this provides about the absorbed intermediate in hydrogenation reactions. 

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