Hydrocarbons exchange reaction with deuterium

To understand the electrocatalytic steps, a more detailed characterization of surface species and of the activity of surface sites for prospective catalysts is needed. Some adsorption results only on Pt, discussed earlier, indicate the existence of two differnet catalytic sites that contribute to benzene exchange reactions with deuterium (795,194). Adsorption and reaction on two different catalytic sites is not unknown in heterogeneous, gas phase catalytic reductions of olefinic and cyclic hydrocarbons (560). 

R. V. Culver and F. C. Tompkins Gas Reactions of Carbon P. L. Walker, Jr., Frank Rusinko, Jr., and L. G. Austin The Catalytic Exchange of Hydrocarbons with Deuterium... 

Kaesz et al. have shown that simple, primary amines (e.g. MeNH2) will react with Ru3(CO)12 to form p-acetamido ligands at temperatures as low as -15°C. We find that simple primary, secondary and tertiary alkyl amines will react with Ru3(CO) 2 at temperatures of 70-150°C to undergo catalytic deuterium for hydrogen exchange reactions on the hydrocarbon groups and transalkylation (52). We have found that a... 

Thus, evidence has accumulated in support of hydrogen exchange in benzene by a mechanism involving associatively chemisorbed benzene, and without the necessity to postulate the participation of chemisorbed C Hs. One attractive test of these ideas which, so far as we know, has not been made, would be to repeat, for example, the reaction of para-xylene with deuterium using as catalyst a palladium thimble. This system would allow the exchange reaction to proceed either in the presence of molecular deuterium (both reactants on same side of the thimble) or in the presence of atomic deuterium only (xylene and molecular deuterium on opposite sides of the thimble, so that the hydrocarbon reacts only with chemisorbed atomic deuterium that arrives at the surface after diffusion through the metal). 

The articles by J. R. Anderson, J. H. Sinfelt, and R. B. Moyes and P. B. Wells, on the other hand, deal with a classical field, namely hydrocarbons on metals. The pattern of modem wTork here still very much reflects the important role in the academic studies of deuterium exchange reactions and the mechanisms advanced by pioneers like Horiuti and Polanyi, the Farkas brothers, Rideal, Tw igg, H. S. Taylor, and Turkevich. Using this method, Anderson takes ultrathin metal films with their separated crystallites as idealized models for supported metal catalysts. Sinfelt is concerned with hydrogcnolysis on supported metals and relates the activity to the percentage d character of the metallic bond. Moyes and Wells deal with the modes of chemisorption of benzene, drawing on the results of physical techniques and the ideas of the organometallic chemists in their discussions. 

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 1946, H. S. Taylor ( ) wrote, The year 1933 was a red letter year for the science of surface catalysis since, in that year, the deuterium isotopes became available for catalytic studies. Although it was soon realized that the catalytic exchange of hydrocarbons with deuterium was a field which was worthy of study, progress was limited until the mass spectrometer was adapted as an analytical tool by chemists. The wealth of detail about the nature of the reactions taking place on catalysts which can be obtained by the use of this instrument has resulted in extensive develop-... 

With the techniques available at that time, it was not possible to discover whether a single hydrogen atom is replaced by a deuterium atom or whether more extensive exchange occurs during the lifetime of the hydrocarbon molecule on the surface of the catalyst. This type of information became available only after the mass-spectrometric technique of following exchange reactions was introduced. 

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. 

Multiple Exchange Reactions, An exchange reaction in which more than one deuterium atom is introduced into the hydrocarbon molecule on each interaction of the molecule with the catalyst is classified as a multiple exchange reaction. The existence of such a process may be recognized in three ways, exactly analogous to those described above for simple exchange reactions ... 

In the study of multiple exchange reactions, it is desirable to work with a high ratio of deuterium to hydrocarbon. This minimizes the influence of isotopic dilution of the deuterium on the rate of production of the more highly deuterated species during the early part of the reaction. The use of deuterium containing as small a percentage of hydrogen as possible is also important if true initial distributions of products are to be obtained. 

Evidence that the presence of water is important in these reactions has been obtained by Hansford (82). Pretreatment of the catalyst with a stream of predried air at 500°C resulted in a marked decrease in the rate of cracking. Further, if deuterium oxide was substituted for the water in the catalyst a large percentage of the deuterium exchanged with hydrogen atoms contained in the hydrocarbon undergoing reaction. Hansford also pointed out that effective catalysts for cracking reactions are always prepared from one or more hydrous oxides. 

The exchange reactions of methane with deuterium over several metals (21, 39,146,147,195, 215, 216, 229) exhibited compensation behavior (Table III, G), as has been pointed out previously (3). No such relationship was found in the data available for exchange reactions of C2-C6 hydrocarbons. This is consistent with discussion given above (see Section III, B, 4) in which it was concluded that the differences in the kinetic behavior of the exchange reactions between methane and the other hydrocarbons varied with the individual metals concerned. 

However, there is a lot of discussion on the relations between support material and electronic properties on one hand and the catalytic properties on the other. Thus, the need for a good test reaction is obvious. An important requirement for a proper test reaction is that it should be metal-catalyzed only. Among the possible reactions are the conversion of neopentane , neohexane23 25 and the isotopic exchange of hydrogen with deuterium in hydrocarbons (H/D exchange)26 29. 

Transition metals catalyze a very wide variety of hydrocarbon reactions ranging from hydrogenation of olefins and exchange of paraffins with deuterium at lower temperatures to skeletal rearrangement, cyclization, hydrogenolysis, cracking, and carbiding under more severe conditions. Because of this flexibility in... 

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