Alkanes dissociative adsorption

A promoting effect of Zn on H/D exchange is accounted for by involvement of Zn-alkyl species formed by the alkane dissociative adsorption on Zn-modified zeolite [142,153-156] in this reaction. The formation of Zn-alkyl species presumably occurs on small ZnO clusters existing inside the zeolite pores [153]. 

Room-temperature adsorption on finely divided metal catalysts has been shown in several cases (Ni/Si02 and Pt/Si02) to give rise to dissociative adsorption as alkylidynes and other products. It is therefore very clear that, contrary to often-expressed views, C-H bonds within the alkanes can readily be broken by interaction with metal catalyst surfaces. Methane is a very important feedstock and, although this may be the most resistant to chemisorption, there is clearly much further of interest to be discovered in this area involving interactions of light alkanes with different metals. 

IV. Hydrocarbon Surface Species Derived from the Dissociative Adsorption of Halogen- or Nitrogen-Substituted Alkanes... 

The dissociative adsorption of alkanes on Ir(l 1 0) surface was investigated in a series of studies performed by the Madix group [13, 32, 33]. A study performed by Hamza et al. [32] probed the dynamics of the dissociative chemisorption of methane, ethane, propane, and n-butane on the Ir(l 1 0)-(l x 2) surface. These investigations were complemented by a later study of propane dissociation on the Ir(l 1 0)-(l x 2) surface by Soulen and Madix [13]. Shown in Figs 6 and 7 are plots of S0 vs. E obtained for propane at various surface temperatures [32] and a plot of experimental and theoretical values of S0 for propane (at an incident translational energy of 50kcal/mol) on Ir(l 1 0)-(l x 2) as a function of surface temperature [13]. 

We have presented a mechanism for the hydrogenation of olefins and exchange of alkanes based upon heterolytic dissociative adsorption and its reverse, associative desorption (21). Ignoring the question of whether the acidic sites are Cr or Cr2+, the mechanism is represented... 

The initial step in alkane hydrogenolysis is the dissociative adsorption, or reactive sticking of the alkane. One might suspect that this first step may be the key to the structure sensitivity of this reaction over Ni surfaces. Indeed, the reactive sticking of alkanes has been shown to depend markedly on surface structure... 

In this mechanism the dihydrogen molecule must dissociatively adsorb prior to reacting with the alkene A. This requires two sites (see Figure 2). When the surface reaction takes place to convert the alkene and two hydrogen atoms into one alkane, the two sites are regenerated. Therefore, we need to examine how the dissociative adsorption step is handled and what ramification this has upon the rate expression, assuming all the adsorption-desorption steps are at equilibrium. 

In the transition state of the dissociating alkane molecule, an unoccupied orbital asymmetric with respect to the a symmetric CH bond becomes partiaUy populated during the dissociation process. When parallel to the surface, it interacts with a partially occupied asymmetric d-valence atomic orbital of the transition metal. The increased population of the asymmetric CH orbital will weaken the CH bond and hence lowers the activation energy for bond cleavage. The quantum-chemistry of the surface oxidative addition process is illustrated for dissociative adsorption in Figure 10.38. 

In principle there is a possibility to stabilize bivalent metal cation with a ligand, which transforms bivalent cations to monovalent species, for example, (ZnOH)+. It is reasonable to expect appearance of such species as a result of dissociative adsorption of water and it will be discussed below. However, the most interesting catalytic processes proceed at high temperatures when the above-mentioned species became unstable. It should be noted that all the above considerations are applicable to zeolites with rather high zeolite module Si/Al >5. The theoretical treatment of traps of exchange metal cations in zeolites with lower module requires different approach. The stabilization of bivalent metal cations in zeolites is considered in the Sect. 20.2 of this review which discusses the example of Zn/ZSM-5 zeolites. These zeolites are active in the catalytic process of small alkane dehydrogenation. Thus, we will explore the influence of the stabilization forms of zinc cation on different stages of this catalytic reaction. 

The pathway of H/D exchange for C-n-C alkanes with involvement of Zn-alkyl species is outlined in Scheme 4 (pathway 1). Dissociative adsorption of the deuterated alkane on zinc oxide sites, which precedes the reaction of H/D exchange, leads to Zn-alkyl species and Zn-OD groups. Protonafion of the zinc-alkyl species by the vicinal acidic OH group of the zeolite with synchronous reverse transfer of the deuterium from the Zn-OD to the... 

Many molecules undergo partial oxidation on adsorption and many alkanes and alkenes are believed to yield an adsorbed CHO group on adsorption (Petrii, 1968). These processes usually lead to the complete oxidation of the organic molecule to carbon dioxide and few workers have attempted to halt the reaction at an intermediate stage. Hence, although there are undoubtedly possibilities for using dissociative chemisorption for synthetic reactions, this chapter will not consider these processes further. 

In general, in Part II we apply the same pattern of analysis to the numerous published vibrational spectra derived from the adsorption of alkynes, alkanes, and aromatic hydrocarbons. In addition, we summarize recently obtained spectroscopic results characterizing hydrocarbon species obtained by thermal, photochemical, or electron-bombardment dissociation of halogen- or nitrogen-substituted alkanes on single-crystal metal surfaces. The hydrocarbon surface species so obtained are normally as anticipated from the replacement of the heteroatoms by surface metal atoms. The... 

Hydrogenolysis on a platinum metal surface proceeds through a dissociative alkane adsorption, followed by a carbon-carbon cleavage leading to methylene... 

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