Dehydrogenation higher alkenes

Higher alkenes can be obtained from thermal cracking of wax, and although a thermodynamic mixture of internal alkenes might have been expected, the wax-cracker product contains a high proportion of 1-aIkenes, the kinetically controlled product. For the cobalt-catalyzed hydroformylation the nature of the alkene mixture is not relevant, but for other derivatizations the isomer composition is pivotal to the quality of the product. Another process involves the catalytic dehydrogenation of alkanes over a platinum catalyst. 

The first step in this preparation, the epoxidation of 1,4,5,8-tetra-hydronaphthalene, exemplifies the well-known selectivity exerted by peracids in their reaction with alkenes possessing double bonds that differ in the degree of alkyl substitution.12 As regards the method of aromatization employed in the conversion of ll-oxatricyclo[4.4.1.01-6]-undeca-3,8-diene to l,6-oxido[10]annulene, the two-step bromination-dehydrobromination sequence is given preference to the one-step DDQ-dehydrogenation, which was advantageously applied in the synthesis of l,6-metliano[10]annulene,2,9 since it affords the product in higher yield and purity. 

Hydrogenation dehydrogenation reactions. The end products of the F-T process are a mixture of higher alkanes and alkenes. The promoter elements could show under F-T conditions some activities for hydrogenation or dehydrogenation reactions leading to a shift in the relative ratio of alkanes to alkenes. 

A commercial Pt-Sn on y-Al203 catalyst showed 2-3 times higher activity in the catalytic dehydrogenation of a mixture of Ci0—CJ2 alkanes to linear monoalkenes when applied in a supercritical phase.332 The strong shift of the equilibrium under supercritical conditions is believed to be due to the high solubility of the product in supercritical fluids or the rapid desorption of alkenes from the catalyst surface. 

In spite of significant fundamental studies and its significant economic potential as an alternate route to alkenes, the oxidative dehydrogenation of alkanes to alkenes is not currently practiced.383 The main reason is that the secondary oxidation of the primary alkene products limits severely alkene yields, which becomes more significant with increasing conversion. This is due mainly to the higher energies of the C—H bonds in the reactant alkanes compared to those of the product alkenes. This leads to the rapid combustion of alkenes, that is, the formation of carbon oxides, at the temperatures required for C—H bond activation in alkanes. 

Unlike higher alkanes, ethane contains only primary C—H bonds, and the dehydrogenation product ethene contains only vinylic C—H bonds. As shown in Table I, these are strong bonds. Thus one would expect that, compared to other alkanes, the activation of ethane would require the highest temperature, but the reaction might be the most selective in terms of the formation of alkene. Indeed, this appears to be the case. 

These electronic properties in turn give rise to some unique catalytic properties for vanadium carbide. Compared to metallic vanadium, vanadium carbide shows an enhancement in the activation of the C-H bond of alkanes and a reduction in the interaction with the C=C bond of alkenes. The surface reactivity of VC/V(110) can be generally described as similar to those of Pt group metals, although the VC/V(110) surface might have an even higher activity towards the activation of C-H bonds. The dehydrogenation of isobutane on VC powder catalysts will be compared to the reactivities of the VC/V(110) model surfaces. 

Alcohols can undergo acid-catalyzed dehydration to give either the corresponding alkenes or the corresponding ethers. The product distribution of the dehydration of alcohols over Nafion-H catalyst shows temperature dependence187 (Table 5.40). Alcohols are thus efficiently dehydrated in the gas phase over Nafion-H under relatively mild conditions with no evidence for any side reactions such as dehydrogenation or decomposition. At higher temperature, olefin formation predominates. 

Heterogeneous catalysts activate C—H bonds at significantly higher temperatures. For example, a Fe/Co modified Mo-supported acidic ZSM-5 zeolite catalyst dehydrogenates methane under non-oxidizing conditions at 700°C to a mixture of Q-C4 alkanes/alkenes and Q-Q2 aromatics such as benzene and naphthalene.140... 

In a base-free medium (dry MeCN), Fe Ch activates HOOH to form a reactive intermediate that oxygenates alkanes, alkenes, and thioethers, and dehydrogenates alcohols and aldehydes. Table 11 summarizes the conversion efficiencies and product distributions for a series of alkene substrates subjected to the Fe Cfi/HOOH/MeCN system. The extent of the Fe Cb-induced monooxygenations is enhanced by higher reaction temperatures and increased concentrations of the reactants (substrate, Fe Cls, and HOOH). For 1-hexene (representative of all of the alkenes), a substantial fraction of the product is the dimer of 1-hexene oxide, a disubstituted dioxane. With other organic substrates (RH), Fe Cb activates HOOH for their monooxygenation the reaction efficiencies and product distributions are summarized in Tables 11(b). In the case of alcohols, ethers, and cyclohexane, a snbstantial fraction of the product is the alkyl chloride, and with aldehydes, for example, PhCHO, the acid chloride represents one-half of the product. In the absence of snbstrate the Fe Cls/MeCN system catalyzes the rapid disproportionation of HOOH to O2 and H2O. 

---