Hydrogenolysis of Alkanes

Supported metal hydrides of early transition metals catalyze the hydrogenolysis of alkanes at relatively low temperatures (50-150 °C) [29,90-92]. Noteworthy are their differences in product selectivities. For example, the hydrogenolysis of propane in the presence of a large excess of H2 gives a 1 1 mixture of ethane and methane in the case of zirconium hydride, a group 4... 

Table 3 Hydrogenolysis of alkanes catalyzed by zirconium hydride supported on silica...

Additionally, grafting molecular entities on surfaces has already allowed to discover several reactions the low temperature hydrogenolysis of alkanes including the depolymerization of polyolefins, the alkane metathesis and the cross-metathesis of methane and alkanes. These two latter reactions can allow higher molecular weight alkanes to be built. 

As a case study, the hydrogenolysis of alkanes over Ir/Si02 will be studied in detail, and the product selectivities at zero contact time for the hydrogenolysis of hexane and cyclohexane are shown in Table 8. 

These surface hydrides are active catalysts for the hydrogenolysis of alkanes at moderate temperatures. Zirconium hydride can catalyze the hydrogenolysis of neopentane, isobutane, butane, and propane at 323 K but cannot catalyze the hydrogenolysis of ethane.259... 

Alkane C-C Bond Activation by Tantalum Hydrides. Low Temperature Catalytic Hydrogenolysis of Alkanes... 

Having set out the properties of tantalum and zirconium hydride toward C-H bond activation of alkanes we now describe the catalytic hydrogenolysis of C-C bonds. It was previously shown in the laboratory that supported-hydrides of group 4 metals, and particularly of zirconium, catalyze the hydrogenolysis of alkanes [21] and even polyethylene [5] into an ultimate composition of methane and ethane. However, to our initial surprise, these zirconium hydrides did not cleave ethane. (=SiO)2Ta-H also catalyzes the hydrogenolysis of acyclic alkanes such as propane, butane, isobutane and neopentane. But, unlike the group 4 metals, it can also cleave ethane [10], Figure 3.7 illustrates this difference of behavior between (=SiO)2Ta(H) and [(=SiO)(4.j,)Zr(H) ], x= or 2). With Ta, propane is completely transformed into methane by successive reactions, while with Zr only equimolar amounts of methane and ethane are obtained. 

Group II The activity drops more than the Ni surface concentration (Fig. 13), i.e., at least about 20 times. However, for several reactions this drop is two or more orders of magnitude. The reactions included in this group are methanation and Fischer-Tropsch synthesis, isomerization, de-hydrocyclization or hydrogenolysis of alkanes, ether formation from alcohols, metathesis of alkylamines, and possibly other reactions. 

The support induced changes in hydrogenolysis reactions of alkanes can be explained to a large extent by support induced changes in the Pt-H bond strength and hydrogen adsorption site on Pt. This can easily explain the well-known compensation effect found in the kinetics of the hydrogenolysis of alkanes catalyzed by supported metal catalysts. 

Pt-Ir Coimpregnation on A1203 / - / - Deuterium exchange reactions and hydrogenolysis of alkanes. 

Hydrogenolysis of alkanes and related reactions on these catalysts have been studied by a number of groups [18, 19, 21-23], Kinetic parameters for ethane, propane, and n-butane on both catalysts are summarized in Table 3. With propane, selectivities to ethane on... 

Silica-supported Ta hydride (=SiO)2Ta-H (93a) presents unusual properties in the activation of alkanes. It catalyzes the metathesis reaction of alkanes to give higher and lower molecular weight alkanes, and the hydrogenolysis of alkanes such as ethane to methane. This hydride also activates the C H bonds of cycloalkanes to form the corresponding surface metal-cycloaUcyl complexes, and catalyses the H/D exchange reaction between CH4 and CD4, prodncing the statistical distribution of methane isotopomers. ... 

Metal alkylidyne fragments are frequently invoked as intermediates in the transformation of hydrocarbons on metal surfaces. These species are usually formulated as triply bridging alkylidynes however, terminal surface alkylidynes may be considered as reactive surface intermediates (30). Evidence for metal carbyne intermediates on Pt—Co bimetallic surfaces was found in a study of the isomerization and hydrogenolysis of alkanes (3]). 

Chabanas M, Vidal V, Coperet C, ThivoUe-Cazat J, Basset J-M (2002) Low-temperature hydrogenolysis of alkanes catalyzed by a silica-supported tantalum hydride complex, and evidence for a mechanistic switch from group IV to group V metal surface hydride complexes. Angew Chem Int Ed 39 1962... 

Some aspects of the particle size, alloying effect, and metal-support interaction in nano-sized supported metal particles are presented for the oxidation of ethylene, the hydrogenolysis of alkanes, and the hydrogenations of unsaturated hydrocarbons and a,j8-unsaturated aldehydes. The influence of these phenomena is highlighted on the... 

Multimetallic formulations are currently in use to improve the properties of metal catalysts, in hydrocarbons processing, Fischer-Tropsch synthesis, and so on [28]. The hydrogenolysis of alkanes is an unwanted reaction in naphta reforming, and has... 

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