Activation of alkanes

Saturated hydrocarbons are much less reactive than alkenes. In the context of catalysis, activation of an alkane basically means making value-added chemicals from alkanes by catalytic processes. At a molecular level, this means making C-C and/or C-H bonds of an alkane sufficiently reactive through the intermediary of a catalyst. 

In heterogeneous catalytic reactions such as cracking and reformation, this is routinely done on a very large scale. In enzymatic catalysis too, selective oxidation of hydrocarbons is well established. In homogeneous catalytic systems, such reactions are rare, but they continue to be of great interest because alkanes such as methane are abundantly available as potential feedstocks. 

In this section, we look at the different mechanisms by which an organometallic complex may activate the C-H bond of an alkane. 

Stmctures 2.66-2.69 are a few representative examples of complexes that have shown promise for alkane activation reactions. Complexes 2.66 and 2.67 are electronically saturated as they both have an electron count of 18. Complex 2.68 has a pincer ligand and an electron count of 16. 

The mechanism of alkane activation by all the three iridium complexes, involves OA. For 2.66 and 2.67, electronic and coordinative unsaturations are generated by subjecting them to photolysis. On photolysis, 2.66 and 2.67 lose hydrogen and one CO, respectively, and generate 16-electron intermediates. As shown by (2.3.5.1) and (2.3.5.2), alkanes can then oxidatively add to the electronically unsaturated intermediates. 

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Microwave activation of alkane transformations was studied in detail by Roussy et al., who summarized their results in several papers [2, 28, 29, 79]. Isomerization of hexane, 2-methylpentane, 2-methyl-2-pentene, and hydrogenolysis of methylcydo-pentane have been investigated, and the diversity of possible effects has been specified [2]. The course of 2-methylpentane isomerization on a 0.3% Pt/Al203 catalyst depended on the mode of heating - the distribution of hexane products was different... 

The activation of alkanes represents a very important field, and a host of reviews and important publications have recently appeared.7,7a 7t... 

Bergman s group showed that low-temperature selective C-H activations of -alkanes could be achieved using cationic-solvated iridium complexes (Equation (2)).10 10a... 

The discussion has focused so far on activation of alkanes, where formation of the a-complex seems to precede oxidative addition. For arenes, formation of the analogous a(cH)-arene complex is thought to occur before oxidative addition to form an aryl hydride. These a-com-plexes have never been observed, presumably because they are unstable with respect to the 71-complexes. Both types of arene complexes are, for the case of benzene, shown in Scheme 25 the a(CH)-arene complex as A and... 

The activation of alkanes on transition metal surfaces is an important step in many catalytic reactions. Hydrogenolysis, steam reforming and isomerization of alkanes all involve alkane dissodation. Thus, much interest exists in the mechanistic and kinetic aspects of alkane dissociation. 

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. 

These Rh complexes have been the subject of intense interest due to their propensity for C-H activation of alkanes (Section 3.3.2.7). The noble gas complexes [CpRh(CO)L] and [Cp Rh(CO)L] (L = Kr, Xe) have also been studied in supercritical fluid solution at room temperature [120]. For both Kr and Xe, the Cp complex is ca. 20-30 times more reactive towards CO than the Cp analogue. Kinetic data and activation parameters indicated an associative mechanism for substitution of Xe by CO, in contrast to Group 7 complexes, [CpM(CO)2Xe] for which evidence supports a dissociative mechanism. 

Activation of Alkanes The selective oxidation of these unreactive hydrocarbons continues to receive attention. Progress in this area is reported by Matsumura, et al, Driscoll, et oL, Banares and Fierro, Erdohelyi, et oL, Owens, et al, and Khouw, et al. 

The CH-activation of alkanes and especially of methane and their catalytic conversion to alcohols is one of the major challenges for chemists. Methane as the major part of natural gas is currently the cheapest source of hydrocarbons and the need for methanol will increase in the near future. Methane conversion to methanol would make a conveniently transportable fuel and also a new carbon source for the chemical industry. 

The dependence of selectivity for dehydrogenation on the conversion of alkane shows that for the more selective catalysts known, the reaction proceeds with a sequential mechanism. The first step of the reaction is the breaking of a C—H bond of the alkane molecule, which is also the rate-limiting step. For these more selective catalysts, alkene is the primary product. Therefore, high selectivities can be obtained at low conversions. However, as the conversion increases, the selectivity decreases because of the secondary reaction of the alkene. The rate constant for the reaction of the alkene on the most selective catalyst is still about the same in magnitude as the rate constant for the activation of alkane. It is larger for the less selective catalysts. Thus the maximum yield of alkene among the catalysts known to date is still less than about 35%. To improve this yield, catalysts that react with alkene less rapidly than with alkane need to be found. 

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