Ethane, deuterium distribution

A subsequent investigation of HT-propane and D2-propane exchange on Ni powder was carried out. The aim was to determine whether a 1,2-diadsorbed surface complex was formed, as had previously been postulated for ethane.The similarity of activation energies, reaction orders with respect to the two reactants, and the initial deuterium distribution in ethane and propane demonstrated that the mechanism of hydrogen exchange in propane was probably via a 1,2-diadsorbed species, as with ethane. 

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. 

We had previously determined isotopic distribution patterns for alkanes derived from the deuterogenation of several olefins on an amorphous catalyst activated to 300° in hydrogen followed by activation in nitrogen to 470° (52). For reactions at about 60°, the patterns for the alkanes from j)ropylene, 1-butene, cyclopentene, and 1-hexene closely resemble those obtained for hexane from 1-hexene on amorphous catalysts in the present work that for pentane from 2-pentene resembles that for hexane from lower selectivity for alkane-d2. We consider it important that the previous work showed that ethylene led to no ethane containing more than two deuterium atoms. In the previous investigation, the effect of the temperature of... 

An examination of the ethylene-deuterium reaction at the surface of a palladium thimble has been briefly reported 54) the gas diffusing through the thimble during the reaction was found to contain 10% H. Final ethane distributions from this reaction over palladium-silica and palladium-charcoal under high pressure at —78° have been reported 46). 

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.

All of the metals of Group VIII catalyze the hydrogenation of acetylene. This section will be concerned with three types of information which have been reported, namely, kinetics, the distribution of deutero-ethylenes obtained when deuterium is added to acetylene, and the yields of ethylene and ethane obtained as initial products. Kinetic information has been available for some time (73, 75-78), and further reports have been made recently (71, 79 82) the nickel-catalyzed reaction of dideuteroacetylene with hydrogen was reported in 1958 (83)... 

Complete analyses of the various deutero-ethylenes and deutero-ethanes were first obtained by Turkevich et at. (4), using a nickel wire, and this type of information has since been reported by Wilson et al. 5) for a bulk nickel catalyst and by Kemball 6) for a series of evaporated metallic films. In all cases, the ethanes produced range over the complete spectrum from do-ethane to do-ethane, and Kemball showed that it is possible to correlate the nature and the amount of the deutero-ethylenes formed over metallic films with the distribution of deuterium in the ethanes. Although the detailed mechanisms of the exchange and deuteration of ethylene are still the subject of controversy, it is clear that both are closely related and both involve the half-hydrogenated state, i.e., the adsorbed ethyl radical, as an intermediate. The main objective of the present research was to extend such studies to the benzene-deuterium system. 

It can be shown that under certain circumstances, deuterated alkane distributions arising from the interaction of an olefin with deuterium can be described by two disposable parameters (1) an amount of direct addition (D.A.) and (2) a constant a equal to (C2Hs iDx+i)/(C2H6 xDx) in the case of ethane. Of the ethane distributions shown in Tables I and II of the paper by Professor Turkevich and his associates, five can be reproduced. As an example, the first one from Table II is compared below with the calculated distribution obtained using 6.9% D.A. and a = 0.52 the proportions of ethane-do and ethane-di are automatically fixed by non disposable parameters. 

Deuteroethane distributions have been interpreted in terms of a parameter P, which is the quotient of the rate constants for ethyl to ethene and ethyl reverting to ethane. For molybdenum, tantalum, rhodium and palladium films, a single value of P (respectively 0.25, 0.25, 18 and 28) sufficed to reproduce the observed distribution, assuming that a further deuterium atom is acquired at every opportunity. With other metals, however, two simultaneous values of P appeared to operate, one contributing 30 to 50% of the reaction having a high P value (13.5-18) and another having a much lower P value (0.36-2). This analysis has not however been accorded an interpretation in terms of the metals physical properties or of ensemble sizes and structures responsible for each participant. 

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. ... 

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