Ethylene catalytic cycles, elementary step

In the foregoing it has been discus.sed how a metal can dissociate H2. Fig. 3.6 explains the principle of catalysis with an example of the hydrogenation of ethylene, for which dissociative chemisorption of hydrogen is an elementary step in the catalytic cycle. The adsorption of alkenes, on the other hand, is non-dissociative. 

The [Os3(CO)io( t-H)( t-OSi)]surface catalyzes the isomerization and hydrogenation of olefins. When the hydrogenation of ethylene is carried out at 90 °C the trinuclear framework of the initial cluster remains intact in all the proposed elementary steps of the catalytic cycle [133]. However, at higher reaction temperatures the stability of the [Os3(CO)io( t-H)( t-OSi)]sujface depends on the nature of the reactant molecule. It is moderately active in the isomerization of 1-butene at 115 °C but decomposes under reaction conditions to form surface oxidized osmium species that have a higher activity [134]. 

Hydrogenation is an important industrial reaction that often requires the presence of a heterogeneous catalyst to achieve commercial yields. Ethylene, C2H4, is the smallest olefin that can be used to investigate the addition of hydrogen atoms to a carbon-carbon double bond. Even though many experiments and theoretical studies have been carried out on this simple system, the reaction is still not completely understood. Microkinetic analysis provides insights into the relevant elementary steps in the catalytic cycle. 

This chapter is organized as follows. In the following section the tentative catalytic cycle proposed by Wilke and co-workers is outlined, followed, in the next section, by a short description of the computational approach employed and the catalyst model chosen. The structural and energetic aspects of all critical elementary steps of the complete catalytic cycle are presented after that. Then we propose a theoretically verified, refined catalytic reaction cycle, and follow that with the elucidation of the product distribution between linear and cyclic Cjo-olefins. Finally, the catalytic reaction courses of the [Ni°]-catalyzed co-oligomerization of 1,3-butadiene and ethylene and of the cyclooligomerization of 1,3-butadiene are compared. 

Scheme 3 Condensed Gibbs free-energy profile (kcal mol ) of the complete catalytic cycle of the co-oligomerization of 1,3-butadiene and ethylene catalyzed by zerovalent bare nickel complexes affording linear and cyclic Cio-olefins, focused on viable routes for individual elementary steps. The favorable [Ni (ri -frans-butadiene)2(ethylene)] isomer of the active catalyst species lb was chosen as reference. Activation barriers for individual steps are given relative to the favorable stereoisomer of the respective precursor (given in italics)...

The activity of complex [lT2(CH3CN)(H)3(p-H)(P Pr3)2(p-Pz)2] as a catalyst for the hydrogenation of diphenylacetylene and ethylene contrasts with its inactivity when employed in the hydrogenation of A -benzylideneaniline. However, when transformed into its protonated derivative, for example, [lr2(CH3CN)(H)2(H2) ( 4-H)(P Pr3)2(p-Pz)2]BF4 by reaction with HBF4, the new complex becomes a very active catalyst for C=N hydrogenation [111]. The catalytic cycle involves fast elementary steps of hydride and proton transfer according to an ionic outer sphere mechanism that takes place at one of the iridium centers of the binuclear complex (Scheme 27). 

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