Ethene, reaction with deuterium

ICI catalyst showed that ethene adsorbed competitively with the ethyne, but the results required two types of site (see Table 9.6) (or two modes of chemisorption of the ethyne) to explain them. Their properties did not however match any of those proposed by Webb. Type X, in the majority, adsorbed both hydrocarbons, but ethene was favoured by a factor of 2200 Type Y adsorbed ethene only, perhaps because of its high concentration. The main source of the ethane was confirmed as ethene, since in the reaction with deuterium the main product was ethane-d2- When the pressure of ethyne was varied in the presence of excess ethene, its rate of removal (and that of formation of dimers) passed through a maximum, while that of ethane formation feU to zero at an ethyne pressure of 2 kPa (see Figure 9.7). The ethane rate was almost independent of the ethene pressure. Extensive work by Borodzinski and colleagues led " to detailed proposals for the identity of two types of site, designated A and E, that were thought to be created as the carbonaceous overlayer developed, and a third type (E ) that may play a role on certain supports. Type A sites, in the majority, were small, so that only ethyne and hydrogen could adsorb on them, the former perhaps as vinylidene (>C=CH2),... 

Scheme 9.2 Mechanism of the reaction of ethene with deuterium over a metal catalyst.

The purpose of this section is to provide an overview of the principal kinetic features of the hydrogenation of ethene and of propene, as providing a framework (or at least part of one) within which discussion of mechanisms must be conducted. Their reactions with hydrogen (and with deuterium) are quite comparable the addition of the methyl group leads to somewhat higher reactivity, due to weaker chemisorption as might be predicted from its lower heat of hydrogenation (Table 7.1). Relative rates for other alkenes will be considered later. The problem of deactivation by carbon deposition has already been mentioned, but quantitative... 

This section is concerned with the reactions of ethene and of propene with deuterium on forms of metal catalyst other than single crystals, which are covered in the next section. A fuller discussion of reaction mechanisms is reserved to Section 7.2.6. 

Figure 7.6. Reaction of ethene with deuterium on nickel wire at 363 K variation of pressures of deuteroethenes with conversion. = C2H3D 2 = C2H2D2 3 = C2HD3 4 = C2D4.

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... 

Rapid exchange of ethene and propene on iron and nickel films appears to proceed in this way. Very detailed studies by Japanese scientists using microwave spectroscopy have identified the structure of propene-di formed in reaction of propene with deuterium over metals of Groups 10 and 11, either supported on silica or as powders. Interpretation of the results is somewhat difficult because although addition and exchange show very similar kinetics, and are therefore thought to have the same intermediates, the locations of the deuterium atom in the propene-di are not entirely as expected by the alkyl reversal mechanism. Except on palladium and platinum, the major initial product was propene-2-di " this could arise if... 

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. ... 

Other metals of Groups 8 to 10 have very different characteristics in respect of reactions of the butenes with hydrogen and deuterium as might be expected from the way they behave in the ethene- (and propene-) deuterium reactions, nickel, palladium, ruthenium, rhodium and osmium are able under some conditions to exhibit much higher values of r,/rfc, so that the butenes are able to achieve their equilibrium concentrations before their hydrogenation is finished. 

Low-energy electron bombardment of monolayers of ethylene on a silver(lll) surface brings about C—H bond fission. The H is absorbed on the surface and the vinyl radical dimerizes to yield buta-1,3-diene. Higher doses of electrons lead to the formation of ethyne Olefins can undergo catalytic oxidation when they are irradiated in the presence of silver catalysts either as silver powder or supported silver metals on anatase titania, sihca and porous glass. Irradiation under these conditions increases the reaction rate. When the surface is coated with ethene and deuterium, partly deuteriated ethene is formed on irradiation, presumably as a result of a vinyl radical reacting with deuterium. UV... 

In the UHV the successful results of the TPR experiments towards ethene hydrogenation on different cluster sizes are further investigated. Vibrational information (IRRAS) in combination with the electronic stfucture (EES) of cluster adsorbates is believed to elucidate the size dependent behavior. In this respect, first isotope experiments with deuterium look promising to elucidate the role of the hydrogen activation in the hydrogenation reaction. In addition, the application of isothermal pMBRS will help to gain kinetic data which will make the results more widely applicable [1 ]. 

Naito et al. studied hydrogenation with use of adsorption measurements, mass spectrometry, and microwave spectroscopy for product analysis. In the room temperature deuteriation of propene, butene, and 1,3-butadiene, the main products were [ H2]-propane, [ H2]-butane, and l,2-[ H2]-but-l-ene, respectively. They showed, using mixtures of H2 and D2, that deuterium was added in the molecular form and at a rate proportional to the partial pressure of D2, as opposed to D surface coverage the reaction rates were zero order in hydrocarbon. They proposed, therefore, in contrast to the model of Dent and Kokes for ethene (but note in this case that reaction rate was 0.5 order in hydrogen pressure and proportional to ethene surface coverage), that hydrogenation proceeded by interaction of adsorbed hydrocarbon with gas-phase D2, that is by an Eley-Rideal mechanism. 

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