A study is conducted to demonstrate catalytic dehydrogenation of light alkanes on metals and metal oxides. The study provides a complete overview of the materials used to catalyze this reaction, as dehydrogenation for the production of light olefins has become extremely relevant. Relevant factors, such as the specific nature of the active sites, as well as the effect of support, promoters, and reaction feed on catalyst performance and lifetime, are discussed for each catalytic Material. The study compares different catalysts in terms of the reaction mechanism and deactivation pathways and catalytic performance. The duration of the dehydrogenation step depends on the heat content of the catalyst bed, which decreases rapidly due to the endothermic nature of the reaction. Part of the heat required for the reaction is introduced to the reactors by preheating the reaction feed, additional heat being provided by adjacent reactors that are regenerating the coked catalysts.

Catalytic dehydrogenation of light alkanes on metals and metal oxides.

Publisher Summary The skeletal rearrangements of hydrocarbons represent an important class of reactions catalyzed by metal surfaces, that have few counterparts in homogeneous catalysis. The first examples of skeletal rearrangements on metals were reported by the Soviet school of catalysis. A major step in hydrocarbon chemistry is the finding that platinum, unlike palladium and nickel, selectively catalyzes the hydrogenolysis of cyclopentane hydrocarbons. At about 300°C, on the classical Zelinskii platinum-charcoal catalyst, cyclopentane yields n -pentane as sole reaction product, while palladized charcoal is completely inactive and nickel-alumina produces all the possible acyclic hydrocarbons, from methane to pentane. The predominant precursor species in skeletal rearrangements are metallocyclobutanes and metallocarbenes, which can be further dehydrogenated to metallocarbynes, dicarbenes, or carbene-olefin complexes, and react like the analogous species in coordination chemistry. Metallocyclobutane dismutation and the reverse reaction, carbene-olefin addition, are the two major steps in olefin metathesis, and dicarbene recombination also has an analogue in organometallic reactions.

Mechanisms of Skeletal Isomerization of Hydrocarbons on Metals

Publisher Summary Hydrogenolysis reactions of hydrocarbons involve the rupture of carbon–carbon bonds and the formation of carbon–hydrogen bonds. The reactions are exothermic and are catalyzed by various transition metals. This chapter discusses the catalytic hydrogenolysis on metals. At temperatures much lower than are required for hydrogenolysis, the chemisorption of hydrocarbons on metals is accompanied by evolution of hydrogen. Exchange reactions between paraffins and deuterium to yield deuteroparaffins occur at similar temperatures. These results indicate that carbon–hydrogen bonds are activated more readily than carbon–carbon bonds and that dehydrogenative chemisorption of the hydrocarbon is the initial step in hydrogenolysis. The hydrogen deficient surface species formed from the hydrocarbon then undergoes carbon–carbon bond scission, followed by hydrogenation and desorption to complete the reaction. The chapter describes the specificity of metal catalysts for hydrogenolysis reactions. This includes an extensive comparison of the catalytic activities of various metals and an attempt to relate the resulting “activity patterns” to properties of the metallic state.

Specificity in Catalytic Hydrogenolysis by Metals

The subject of catalytic hydrogenolysis is a broad one, referring in general to a class of catalytic reactions involving the rupture of chemical bonds via interaction with hydrogen.

Catalytic Hydrogenolysis over Supported Metals

The direct catalytic conversion of alkanes into aromatics has found potentially important industrial applications. Initially only alkanes with 6 and more carbon atoms in the chain were concerned. Supported platinum catalysts were found active for the aromatization of alkanes; the drawbacks of these catalysts were their deactivation with time on stream and the existence of simultaneous parallel reactions. Much discussion has been published on the aromatization of C6+ alkanes. A bifunctional mechanism which involves both the metal and the acid sites of the support and a monofunctional mechanism involving only the metallic sites operate over, respectively, Pt supported on acidic support and Pt supported on nonacidic support. In the present review the mechanisms proposed for the aromatization of alkanes are described. Over monofunctional Pt catalysts two possible mechanisms prevail: 1,6 ring closure on the Pt surface involving primary and secondary C-H bond rupture, followed by dehydrogenation of the...

J.M. Muller

Papers

The mechanisms of hydrogenolysis and isomerization of hydrocarbons on metalsII. Mechanisms of isomerization of hexanes on platinum catalysts