Theoretical Basis of the Activation of C-H Bond

Abstract

Activation of the C-H bond is the first step of the majority of transformations of hydrocarbon molecules and therefore it is the basis of petrochemical industry. Among others, all processes of selective oxidation of hydrocarbons, which are the main route for functionalisation of hydrocarbon molecules, start with activation of the C-H bond. However, in this case, the activation of C-H bonds must be usually repeated several times before the given reactant transforms into the desired product, because reactions of selective oxidation form a network of parallel and consecutive elementary steps of hydrogen abstraction and oxygen addition [1]. As an example. Fig.l shows the network of reactions which may take place when a hydrocarbon molecule interacts at the surface of a solid catalyst. When the solid is a metal, interaction with the surface may result in the abstraction of hydrogen and formation of a surface alkyl, adsorbed hydrogen atoms recombining to desorb. If deuterium is simultaneously introduced, isotopic exchange takes place. The surface alkyl may further react in two directions. Either further hydrogen atoms are abstracted from the α carbon atom of the alkyl group and alkilidyne species are formed, as observed experimentally at the surface of several metals, e.g. Pt [2], or the second hydrogen atom is abstracted from the β carbon atom of the alkyl group and a bridging surface species appears, which desorbs as an olefin molecule. Different reaction networks develop at oxide surfaces. Activation of the C-H bond may result in the formation of a surface alkoxy-species, which is an intermediate of several different reaction pathways. When deuterium is passed over, isotopic exchange is observed similarly as in the case of metals. The alkoxy species may pick up a proton and desorb as an alcohol, the overall process being equivalent to the insertion of oxygen into the terminal C-H bond. The alkoxy species may also lose the second hydrogen atom, either from the α carbon atom with the formation of a precursor of an aldehyde, which may then desorb, or from the β carbon atom, in which case a bridging species is formed. This is again an intermediate of two reaction pathways: either it desorbs as an olefin molecule, or undergoes abstraction of a third hydrogen atom from the γ carbon atom, resulting in the formation of an allyl species, σ-bonded side-on to the surface. This allyl can transform along four different pathways. Activation of the C-H bond at the δ carbon atom results in the formation of a diene. Nucleophilic addition of oxygen to the α carbon atom, accompanied by abstraction of a hydrogen atom leads to the desorption of an unsaturated aldehyde, whereas the nucleophilic addition of oxygen to the γ carbon atom with the ensuing abstraction of hydrogen atom — results in the appearance of a vinyl ketone. Finally, the allyl species bonded side-on may flip and rearrange into an end-on bonded alkyl. Two such alkyl species may recombine to form a diene with the double number of carbon atoms. The role of oxidizing agents in these steps of the reaction sequence is played by cations of the catalyst lattice. After the nucleophilic addition of the lattice oxygen ion, the oxygenated product is desorbed, leaving a vacancy at the catalyst surface.