Ethane, exchange with deuterium

Figure 63. Compensation plot of Arrhenius parameters for ethane total exchange with deuterium (see Table 6.2) O foils, films, Pt(l 11) blacks, powders A supported Pt. The enclosed points are fa- Pt and Pd (see text).

Table 6.4 shows Arrhenius parameters, rates and multiplicity factors for higher alkane exchanges with deuterium, mainly on metal films rates are generally somewhat higher than for ethane, and the temperature ranges used correspondingly... 

It is important to understand why this apparent first-order behavior is found for the course of an exchange reaction with time whatever the true kinetics of the reaction. A failure to understand this feature of exchange reactions has sometimes led to unjustifiable statements about the ratedetermining step in such reactions. It is convenient to discuss a specific example—the exchange of ethane with deuterium. Suppose that the only adsorbed species taking part in the reaction are (a) physically adsorbed... 

When ethene reacts with deuterium on a metal catalyst, the products typically consist of (i) deuterated ethenes containing from one to four deuterium atoms, formed by alkene exchange, (ii) deuterated ethanes containing from zero to six deuterium atoms formed by addition and (iii) hydrogen... 

Table XIX presents a selection of the results obtained in a study of the reaction of ethylene with deuterium over rhodium-alumina (31), together with some calculated distributions obtained by the method previously employed. The proportion of deuterated ethylenes in the initial products rises from 30% at —18° to 75% at 110°. In contrast to the behavior of palladium, ethane-dj is the major ethane throughout and hydrogen exchange is significant at all but the lowest temperature studied. The parameters used in the calculations attribute the greatest effect of temperature to the variation of the chance of ethylene desorption, which rises from 25% at —18° to 62% at 110°. The effect of temperature on the chance of alkyl reversal is relatively small. Another resjject in which the reaction over rhodium differs from that over palladium is that the chance of acquisition of deuterium in the hydrogenation steps is higher, and indeed it appears that, as with iridium, molecular deuterium may be substantially responsible for the conversion of ethyl radicals to ethane. E — E, is 3 kcal mole and E, — E, is 4.5 kcal mole. The reaction is first-order in hydrogen and zero in ethylene.

At the same time that our work on ethane hydrogenolysis and cyclohexane dehydrogenation on nickel-copper alloys was published, a paper by Ponec and Sachtler on the reactions of cyclopentane with deuterium appeared (9). These workers reported data on the rates of formation of deuterocyclopen-tanes via exchange, and of CD4 by hydrogenolysis. The exchange reaction occurred at about the same rate (per surface nickel atom) on nickel-copper alloys as on pure nickel, while the rate of formation of CD4 was substantially decreased. 

Apparent Arrhenius Parameters ( , in A), Rates at 423 K Parameters (M) for Exchange of Ethane with Deuterium on Metals of Groups 8 to 10... 

Figure 6.4. Hegarty and Rooney scheme for the exchange of ethane with deuterium." ...

These features are best illustrated by reference to the reaction of ethene with deuterium on various nickel catalysts °° ° and on supported platinum catalysts.On nickel there was seen the stepwise formation of al the deuterated ethenes (Figure 7.6), in consequence of which (hydrogen exchange being minimal) the deuterium number of the ethane rose progressively thus ethane-rfo was the major initial product, but all deuterated ethanes were seen, and the formation of ethane-r/e was more marked towards the end of the reaction, when most of the ethene was ethene-da (Figure 7.7). The stepwise character of the... 

There are few reports of alkene-deuterium reactions on bimetallic catalysts, but those few contain some points of interest. On very dilute solutions of nickel in copper (as foil), the only product of the reaction with ethene was ethene-di it is not clear whether the scarcity of deuterium atoms close to the presumably isolated nickels inhibits ethane formation, so that alkyl reversal is the only option, or whether (as with nickel film, see above) the exchange occurs by dissociative adsorption of the ethene. Problems also arise in the use of bimetallic powders containing copper plus either nickel, palladium or platinum. Activation energies for the exchange of propene were similar to those for the pure metals (33-43 kJ mol ) and rates were faster than for copper, but the distribution of deuterium atoms in the propene-di clearly resembled that shown by copper. It was suggested that the active centre comprised atoms of both kinds. On Cu/ZnO, the reaction of ethene with deuterium gave only ethane-d2. as hydrogens in the hydroxylated zinc oxide surface did not participate by reverse spillover. ... 

Similar results have been obtained for methane 12) and for ethane 19). The values quoted in Table II also illustrate the point that the distribution of deuterium between hydrogen and propane differs from the value expected for a random distribution. With the ratio of pressures used, the expected percentage for the mean deuterium content of the hydrocarbon would be 33.3, which is substantially less than the experimental value of 40.9 %. This type of deviation is also found with other hydrocarbons, but it does not affect the validity of using classical theory for the calculation of the interconversion equilibrium constants in studies of mechanism of exchange reactions. More accurate values for these equilibrium constants are necessary, however, if one is interested in the separation of isotopes by chemical processes. 

We shall now consider in outline a general method devised by Anderson and Kemball (19) for the interpretation of the initial distributions of products obtained in multiple-exchange processes. The method was devised, in the first instance, to apply to the exchange of ethane and deuterium, but it can be extended quite simply to cover other types of molecules. It involves the adoption of a specific mechanism from which calculated distributions are then obtained for comparison with observed distributions. The method will be illustrated for the exchange of ethane. The mechanism adopted is as follows. 

One of the earliest studies of the reaction of C2H4 with D2, in which a full mass spectrometric analysis of the products was performed, used a nickel wire as catalyst [115,116]. Some typical results are shown in Fig. 11. These results showed that ethylene exchange was rapid and the deutero-ethylenes are probably formed in a stepwise process in which only one deuterium atom is introduced during each residence of the ethylene molecule on the surface, that is there is a high probability of ethylene desorption from the surface. From Fig. 11(a) it can also be seen that the major initial products are ethane-d0 and ethane-d,. This is consistent with a mechanism in which hydrogen transfer occurs by the reaction... 

Similar results were obtained with ethylene but not with ethane. Apparently the catalyst tends to produce deuterium atoms and to open up the double bond so that a deuterium atom can attach to one of the carbon atoms. Either exchange or addition may follow. 

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