Ethane, metathesis

The mechanism of such a reaction, represented in Scheme 3.9, comes directly from the reverse mechanism of ethane metathesis, each step being considered as microscopically reversible. In a clockwise catalytic cycle ethane metathesis occurs, whereas in the counter-clockwise catalytic cycle propane methane-ol-ysis occurs the case with heavier alkanes is of course more complicated. 

Scheme 2. Possible mechanism for ethane metathesis (productive and degenerate).

Another application for these silica-supported, tantalum polyhydrides [TaH /Si02l was the cross metathesis between ethane and toluene under partial pressures of these reactants in a 97.6 3.7 ratio, respectively, at 250 C (Scheme 2.4) [34]. The resulting aromatic products were typically found to be benzene (0.5%), ethylbenzene (15%), xylene (2.5%), and propylbenzene (3.6%). Moreover, the major products were also found to be the linear alkanes resulting from the competing self-metathesis of ethane, as ethane was used in large excess. Dynamic studies employing a continuous-flow reactor have shown that the formation of ethylbenzene competes at a slower rate than ethane metathesis. 

Ta(CH2Ph)5 to ethane or hydrogen [48]. EXAFS spectroscopy confirmed the existence of a Ta-Ta species, demonstrating the formation of tantalum clusters on silica. This supported that Ta sample catalyzed the metathesis of ethane at high temperature (250 "C) [49]. The initial conversion of ethane metathesis was 24% the molar ratio of methane to propane exceeded 1, and small quantities of butanes were also observed. This catalytic system was also found to convert propane into ethane and butanes [48, 50, 51]. The authors found that the observed catalytic activity was dependent on average cluster size. Increasing the average cluster diameter by 0.2-0.3 nm led to a decrease in activity [51]. 

While alkane metathesis is noteworthy, it affords lower homologues and especially methane, which cannot be used easily as a building block for basic chemicals. The reverse reaction, however, which would incorporate methane, would be much more valuable. Nonetheless, the free energy of this reaction is positive, and it is 8.2 kj/mol at 150 °C, which corresponds to an equihbrium conversion of 13%. On the other hand, thermodynamic calculation predicts that the conversion can be increased to 98% for a methane/propane ratio of 1250. The temperature and the contact time are also important parameters (kinetic), and optimal experimental conditions for a reaction carried in a continuous flow tubiflar reactor are as follows 300 mg of [(= SiO)2Ta - H], 1250/1 methane/propane mixture. Flow =1.5 mL/min, P = 50 bars and T = 250 °C [105]. After 1000 min, the steady state is reached, and 1.88 moles of ethane are produced per mole of propane consmned, which corresponds to a selectivity of 96% selectivity in the cross-metathesis reaction (Fig. 4). The overall reaction provides a route to the direct transformation of methane into more valuable hydrocarbon materials. 

This is a major achievement, mainly due to Basset and his group, in surface organometallic chemistry because it has been thus possible to prepare single site catalysts for various known or new catalytic reactions [53] such as metathesis of olefins [54], polymerization of olefins [55], alkane metathesis [56], coupHng of methane to ethane and hydrogen [57], cleavage of alkanes by methane [58], hydrogenolysis of polyolefins [59] and alkanes [60], direct transformation of ethylene into propylene [61], etc. These topics are considered in detail in subsequent chapters. 

In the presence of an acyclic alkane, 3 catalyzes at moderate temperature (25-200 °C) the metathesis reaction, leading to the formation of heavier and lower homolog alkanes by simultaneous breaking and formation of C-H and C-C bonds. For example, propane is transformed, even at 25 °C into a quasi-equimolar mixture of ethane and butanes (n- and iso-mixture) as well as methane and pentanes, in lower quantities. Lower and heavier homologs are also obtained starting from... 

Alkane metathesis can also formally appear as the breaking and reformation of one C-H and one C-C bonds. Scheme 3.7 shows, for example, the case of ethane where methane and propane are produced ... 

Ethylbenzene can be prepared by cross-metathesis between toluene and ethane, catalyzed by (=SiO)2TaH, according to the equation [42] ... 

As indicated previously, alkane metathesis is a thermodynamically equilibrated reaction. Indeed, for ethane at 150°C, with a AG° = -2kcalmoT, an equilibrium... 

Figure 3.14 Toluene (a) and ethane (b) pressure effect on their TON during their crossed metathesis reaction at 250°C (=SiO)2TaH (3) (400mg) ethane/toluene (bar/Torr) A 10/28 B 25/28 C 10/310 D 25/540.

Reaction between methane and propane requires the right conditions. Firstly, it has a positive free energy of AG° = -1- 2kcalmoT at 150 °C for amethane/propane ratio of 1 but this can be overcome by increasing this ratio, which for a value of 1250 allows 98% propane conversion at 250°C. Secondly, it has to be separated from other reactions catalyzed by tantalum hydride, such as propane hydrogenoly-sis, leading to 1 equiv. of methane and 1 equiv. of ethane, or propane metathesis, leading to 0.5 equiv. of ethane ... 

Alkane metathesis was first reported in 1997 [84]. Acyclic alkanes, with the exception of methane, in contact with a silica supported tantalum hydride ](=SiO)2TaH] were transformed into their lower and higher homologues (for instance, ethane was transformed into methane and propane). Later, the reverse reaction was also reported [85]. Taking into accountthe high electrophilic character ofa tantalum(III) species, two mechanistic hypotheses were then envisaged (i) successive oxidative addition/reductive elimination steps and (ii) o-bond metathesis. Further work has shown that aLkyhdene hydrides are critical intermediates, and that carbon-carbon... 

The reaction follows the stoichiometry which requires 3 equivalents of the anion for each tris(halogenomethyl)ethane, furnishing one equivalent of the cyclopropylcarbinyl complex and one dinuclear complex [CpM(CO) ]2. One possible mechanism is shown in Scheme 2, in which the key step proposed for ring formation is a metathesis involving the metal, the halogen and two carbon atoms. 

The compound (dppe)PtMe(OMe),256 which is prepared by a metathesis reaction involving NaOMe and (dppe)PtMe(Cl) in a mixed benzene/methanol solvent system (dppe = bis(l,2-di-phenylphosphino)ethane), does not react with ethylene or pentene but does react with activated alkenes such as acrylonitrile, methylacrylate and fluoroalkenes. The reaction involving tetrafluoro-ethylene has been shown to give (dppe)PtMe(CF2CF2OMe), providing the first example of an alkene insertion into an M—OR bond.256 Interestingly, no insertion into the Pt—Me bond was observed. 

In this section we shall consider the results recorded in the literature that pertain to the structures of the adsorbed species. Kinetic or catalytic aspects, as could be relevant to hydrogenation, hydrogenolysis, or metathesis processes, will be treated in Part 11. Spectra of the much-investigated alkenes are discussed in detail in Part I. The spectra of the other principal types of hydrocarbon adsorbates, viz. alkynes, alkanes, cycloalkanes, and aromatics, will be analyzed in Part II. Most results are available for the type-molecules ethene, ethyne, ethane, and benzene as well as for the metals, Pt, Pd, Ni, Rh, and Ru. 

At present, Mo, W, Re and Ru complexes are known to catalyse alkene metathesis [7]. This unique reaction, catalysed by transition metal complexes, is impossible to achieve by other means. Later, based on studies of the reactivities of Fischer-type carbene complexes, it was discovered that carbene complexes are the intermediates in alkene metatheses. WClg reacts with EtAlCl2 to afford the diethyltungsten complex 3 by transmetallation, and subsequent elimination of a-hydrogen generates ethane and the carbene complex 4 which is the active catalyst. 

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. ... 

Maury O, Lefort L, Vidal V, Thivolle-Cazat J, Basset J-M (1999) Metathesis of alkanes Evidence for degenerate metathesis of ethane over a silica-supported tantalum hydride prepared by surface organometallic chemistry. Angew Chem Int Ed 38 1952... 

With (=SiO)2TaH, hydrogenolysis of simple alkanes also occurs at rather moderate temperatures but, in contrast with group 4 metals, ethane is also cleaved, which suggests an alternative mechanism to y9-alkyl elimination for C-C bond cleavage [17]. This surprising shift in reactivity implies a completely different mechanism of C-C cleavage which we believe to be closely related to that of alkane metathesis (vide infra). 

The Ta hydride has been shown to catalyze an entirely unexpected and potentially very interesting reaction, the metathesis or disproportionation of simple alkanes, under very mild conditions [19]. For example, when ethane is heated to 150 °C... 

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