Alkane hydrogenolysis

Alkane hydrogenolysis, or crackmg, involves the dissociation of a larger alkane molecule to a smaller alkane molecule. For example, ethane hydrogenolysis in tire presence of FI2 yields methane ... 

The initial step in alkane hydrogenolysis is the dissoeiative adsorption, or reaetive stieking of the alkane. One might suspeet that this first step may be the key to the stnieture sensitivity of this reaetion over Ni surfaees. Indeed, the reaetive stieking of alkanes has been shown to depend markedly on surfaee stnieture... 

Note that, while this reaction (as well as alkane hydrogenolysis) involves known elementary steps of molecular organometallic chemistry, it has not been discovered in homogeneous catalysis. One reason might be that highly... 

These differences underline a larger steric hindrance in the coordinahon sphere of Hf metal. Thermolysis experiments prove that (=SiO)Hf(CH2Bu )3 is more stable and more active in alkane hydrogenolysis than (=SiO)Zr(CH2Bu )3. 

Obviously, the alkane hydrogenolysis reaction highlights a clear difference in behavior between supported-hydrides of group 4 metals and (=SiO)2Ta(H),g (3) tantalum hydride. This difference in behavior can further be illustrated as follows ... 

On the one hand, a comparison of initial rates for various alkane hydrogenolysis shows a marked difference between zirconium and tantalum hydrides (Table 3.2). Indeed, the reaction rates for [(=SiO)(4.,<)Zr(H)J, x = 1, 2) are weakly dependent on the nature of the alkanes. They are higher than those obtained... 

I 3 Catalytic Properties of Single Site Catalysts Prepared via Surface Organometallic Chemistry Table 3.2 Comparison of initial rates in alkane hydrogenolysis". 

In a nutshell, cyclic alkane hydrogenolysis leads to acyclic and cyclic alkanes, with the size of the cyclic ones being smaller. However, on the one hand, TONs... 

A study of the stoichiometric cyclopentane reaction over Ta-H has revealed that tantalum hydride very easily achvates cyclopentane, forming the corresponding cyclopentyl derivative. However, the latter is very quickly transformed into a cyclo-pentadienyl compound, as shown by NMR and EXAFS studies. This cyclopenta-dienyl derivative presents no achvity in alkane hydrogenolysis ... 

Indeed, cyclopentane formation during all cyclic alkane hydrogenolysis and cyclopentane transformation into a cyclopentadienyl derivative, inert in hydrogenolysis, explains the rapid deactivation process of this catalyst in the presence of cyclic alkanes. 

For example, maximum activity for alkane hydrogenolysis is to be found in Group 8 (Ru, Os) rather than in Group 9 or 10, because the hydrocarbon intermediates have to be multiply bonded to the surface, and metals in the later Groups have insufficient unpaired electrons for this purpose. Palladium is outstandingly the best metal for hydrogenating alkynes, but is of little use for hydrogenating aromatics. 

The kinetics of alkane hydrogenolysis, that is, the dependence of rate on reactant concentration, have been the subject of numerous studies, and much effort has been devoted to devising rate expressions based on the Langmuir-Hinshelwood formalism to interpret them. Reactions of ethane, propane, and n-butane with H2 on EUROPT-3 and -4 have been carefully studied, with the surfaces in either as clean a state as possible, or deliberately carbided [21, 22] kinetic measurements at different temperature permitted adsorption heats and true activation energies to be obtained. There were two surprises (but like all surprises they were obvious afterwards) ... 

Alkene hydrogenation, alkane hydrogenolysis, and methanation of CO are used as test reactions for evaluating the catalytic activity of cluster-derived metal catalysts. Catalysts derived from noble metal carbonyl precursors such... 

Resasco D, Haller GL (1982) Dispersion effects on alkane hydrogenolysis over rhodium supported on titanium oxide. Stud Surf Sci Catal 11 105... 

Ichikawa M, Rao L, Ito T, Fukuoka A (1989) Ensemble and Ugand effects in selective alkane hydrogenolysis catalysed on well characterized Rhir and RhFe bimetaUic clusters inside NaY zeolite. Faraday Discuss Chem Soc 87 321... 

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