Reactivity in hydrogenation

Second, catalytic reactions do not necessarily proceed via the most stable adsorbates. In the ethylene case, hydrogenation of the weakly bound Jt-C2H4 proceeds much faster than that of the more stable di-cr bonded C2H4. In fact, on many metals, ethylene dehydrogenates to the highly stable ethylidyne species, =C-CH3, bound to three metal atoms. This species dominates at low coverages, but is not reactive in hydrogenation. It is therefore sometimes referred to as a spectator species. Hence, weakly bound adsorbates may dominate in catalytic reactions, and to observe them experimentally in situ spectroscopy is necessary. 

The triplet state of carbonyl chromophores frequently shows a high reactivity in hydrogen abstraction reactions (l ). These processes can take place intermolecularly (photoreduction) ( l) or intramolecularly, for example in the Norrish Type II process, reaction 1 (.2,3.). 

The demethanizer, deethanizer, and debutanizer are fractionating columns that separate the lighter and heavier compounds from each other. Traces of triple bonds are removed by catalytic hydrogenation with a palladium catalyst in both the C2 and C3 stream. Cumulated double bonds are also hydrogenated in the C3 fraction. These are more reactive in hydrogenation than ethylene or propylene. The C2 and C3 splitters (Fig. 8.4) are distillation columns that can be as high as 200 ft. The mechanism of cracking was previously discussed in Chapter 7, Section 6. 

The n,7r triplets of a-diketones are quite reactive in hydrogen abstraction reactions.393-395... 

Figure 9.2 Relative reactivities in hydrogen abstraction, (a) In abstraction by a reactive radical the transition state is early and relatively little affected (small AAG ) by changing the structure of R so as to stabilize the radical being formed, (b) In abstraction by a less reactive radical, with a later transition state, structure change in the product is more strongly felt at the transition state (larger A AG ).

We propose that crystals provide a useful model for the active sites of enzymes, to supplement data from the solution state, because enzyme active sites are ordered, and chemically complex. But what happens in enzyme active sites regarding the transition state is of course unclear to us, and issues of reactivity in hydrogen bond networks still await further investigation. 

More important, they are less selective in reactions with tetralin. The presence of sulfur containing compounds in the macerals has a notable influence on their reactivity in hydrogen transfer reactions. 

Several lines of evidence led us to conclude that most of the macerals contain compounds that are reactive in hydrogen transfer reactions, but that these materials are present in different amounts in the different macerals. This feature of the chemistry was examined by the study of the reactions of the... 

The relationship between the structure of olefins and their reactivities in hydrogenation as described above is complicated by the double-bond migration and the cis-trans isomerization that may accompany the hydrogenation. 

While each of the different types of saturation sites has a different reactivity in hydrogenation reactions, in the oxidation of CO or iso-propanol20 each of these sites has essentially the same activity. 

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... 

If the sample is activated in hydrogen at 402° (run 248), the pattern is nearly random some deuterium enters the methyl but only 10-20% of the total. Direct activation in hydrogen or reactivation in hydrogen after activation in helium gave nearly the same results. 

Terminal macro-radicals produced in polyethylene by the Norrish type I reaction exhibit a high reactivity in hydrogen transfer reactions. Hydrogen abstraction from a neighbouring chain... 

The difference in Cs-hydrocarbon concentrations observed for Pt/Ga-silicate and Ga-silicate can be related to the Pt reactivity in hydrogen spillover. Since Cs-hydrocarbons are likely to be Cs-dimer cracking products produced over acidic sites, they can also be involved in subsequent oligomerization reactions over acidic sites, too. Cs-hydrocarbon reactions are more probable than Cs-hydrocarbon reactions, because C3-hydrocarbons are more volatile. However, Cs-unsaturated intermediates are rapidly hydrogenated over Pt/Ga-silicate by hydrogen which was activated on platinum and spills over the acidic sites. Consequently, they did not participate in further conversion and their concentration remained constant. The platinum hydrogenation activity was confirmed by the data on the hydrogen effect on the activity for Pt/Ga-silicate and Ga-silicate. 

Rapid deactivation occurs when n-dodecane is dehydrogenated over platinum-alumina without any diluent or with an inert diluent such as nitrogen. The rate of deactivation is decreased greatly when hydrogen is used as a diluent. However, even with hydrogen dilution, a slow deactivation (accompanied by carbonization of the catalyst) occurs. Eventually it is necessary to regenerate the catalyst by combustion of the coke deposits and reactivation in hydrogen. 

Fig. 17. Size-dependent reactivity of iron, cobalt and nickel clusters. Solid circles connected by the solid lines show reactivity in hydrogen-molecule adsorption. Open triangles connected by the dashed lines show promotion energies of an electron from the highest-occupied molecular orbital to the lowest-unoccupied molecular orbital. The correlation between the promotion energy and the reactivity was found, where the higher the promotion energy, the lower the reactivity. (Adapted from Ref. 30.)...

Alko l radicals and chlorine atoms are highly reactive in hydrogen abstraction leading to the formation of macroalkyl radicals (P), which in turn react with almost zero activation energy with ground state oxygen which is itself a diradical (reaction 2) ... 

The newly formed carbon whiskers are very reactive when there is a thermodynamic potential for gasification (reverse Reaction R6 in Table 5. 2). This was demonstrated in the TGA tests close to equilibrium. In situ electron microscopy [33] has shown how nickel crystals may eat channels through carbon by a reverse whisker growth mechanism. Similar observations have been made for catalytic filters for car exhaust where cerium oxide crystals react with soot deposits [460]. The gum layer is also reactive in hydrogen [205]. 

Primary and secondary peroxy radicals are more reactive in hydrogen abstraction than analogous tertiary radicals [64, 65]. The secondary peroxy radicals can abstract a labile hydrogen atom from another polyethylene molecule to form hydroperoxides species (Reaction 3 from Scheme 2.2), which are the compoxmds with higher contribution to the oxidation cycle, as well as another radical, through which the process can continue. In most polymers, the rate of this step in the chain reaction determines the overall rate of oxidation [66]. Finally, due to the thermal instability of the 0-0 bonds of these species (bond energy = 40 kcal/mol), they readily decompose into hydroxyl (OH ) and alkoxy (RO ) radicals, which ultimately result in the appearance of final... 

Among alcohols, 2-propanol and diphenylmethanol stand out with respect to their reactivity in hydrogen transfer reactions with azoalkanes [45,48]. The abstraction from secondary a-CH groups with two radical-stabilizing substituents, i.e methyl or phenyl, proceeds with quite high rate constants, due to strongly exergonic thermodynamics [184]. 

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