Breaking H—C Bonds Dehydrogenation

BREAKING H-C BONDS DEHYDROGENATION 5.3.1. Alcohols to Aldehydes and Ketones... 

After the first C-H bond is broken, a surface alkyl species is formed. There are at least three possible reactions for this alkyl species that lead to different products. Dehydrogenation products would be formed if the alkyl species reacts by breaking another C—H bond at the 6-position ... 

In these species, a reaction in which a C -H bond is broken would lead to dehydrogenation. However, breaking a C"-H bond or cleaving a C"-C bond would lead to degradation products. The statistical probabilities of these three processes are proportional to the number of these bonds in the species, which are shown in Table III. They show that these species only differ in the relative number of C -H bonds. If the C"-H and the C-C bond react with equal probability, whereas the C -H bond reacts somewhat faster, combustion would be more likely for ethane than for the other alkanes. Since the reaction conditions, especially temperatures, for the data on Mg2V207 and Mg3(V04)2 were similar, this argument would account for the low dehydrogenation selectivity observed on these two oxides. 

The data presented above showed that the oxidative dehydrogenation reactions of the various alkanes share many common features. Thus it is tempting to discuss selectivity for alkane oxidative dehydrogenation with a common scheme. The reaction scheme for ethane oxidation [Eqs. (5)-(7)] provides a useful basis for such a discussion. It shows that the primary reaction of alkane oxidation can take on three different pathways depending on the reaction temperature (Scheme I). The first step in all three pathways is breaking a C—H bond, which is the rate-limiting step. The three pathways are described below. 

In these species, a reaction in which aCp—H bond is broken would lead to dehydrogenation. However, breaking a C"—H bond or cleaving a C —Cp bond would lead to degradation products. The statistical probabilities of these three processes are proportional to the number of these bonds in the species, which are shown in Table XII. They show that these species... 

The chemisorption of over 25 hydrocarbons has been studied by LEED on four different stepped-crystal faces of platinum (5), the Pt(S)-[9(l 11) x (100)], Pt(S)-[6(l 11) x (100)], Pt(S)-[7(lll) x (310)], and Pt(S)-[4(l 11 x (100)] structures. These surface structures are shown in Fig. 7. The chemisorption of hydrocarbons produces carbonaceous deposits with characteristics that depend on the substrate structure, the type of hydrocarbon chemisorbed, the rate of adsorption, and the surface temperature. Thus, in contrast with the chemisorption behavior on low Miller index surfaces, breaking of C-H and C-C bonds can readily take place at stepped surfaces of platinum even at 300 K and at low adsorbate pressures (10 9-10-6 Torr). Hydrocarbons on the [9(100) x (100)] and [6(111) x (100)] crystal faces form mostly ordered, partially dehydrogenated carbonaceous deposits, while disordered carbonaceous layers are formed on the [7(111) x (310)] surface, which has a high concentration of kinks in the steps. The distinctly different chemisorption characteristics of these stepped-platinum surfaces can be explained by... 

The transformation of propane over the acidic sites can occur by various routes, namely by breaking of a C-C bond, resulting in the production of methane and ethylene, or by breaking of two C-H bonds, resulting in propene as a dehydrogenation product The detailed scheme for the transformation of this hydrocarbon is out of the scope of this p rer and will be the subject of a separate publication. 

The chemistry of the multistep reactions of Ti+ with propylene was also examined. Bare Ti+ exhibits an active reactivity toward breaking C-H bonds of the alkene molecules. But, the coordination of ligands on Ti+ was found to dramatically alter its dehydrogenation reactivity. For propylene, the multistep reactions were terminated at the fourth step, whereas the reactions with ethylene molecules were found to proceed far beyond the fourth step. Over 20 ethylene molecules have been... 

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