Methane cross-metathesis

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. 

Fig. 4 Methane-propane cross-metathesis catalyzed by tantalum 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. 

Methane-Propane Cross-Metathesis ( Alkane Methane-olysis )... 

Figure 3.15 Propane crossed metathesis reaction with methane under SObar at 250°C, catalyzed by (=SiO)2TaH (3) (300mg, d = l.SmLmin-, CH4 C3Hg = 1250 1).

Scheme 36 (a) H/D exchange, (b) alkane hydrogenolysis, (c) alkane metathesis, (d) cross-metathesis of toluene/ethane, (e) cross-metathesis of propane/methane. 

This reaction can be exploited to carry out the cross-metathesis of toluene and ethane to give ethylbenzene and xylenes (Scheme 36(d)) or that of methane and propane to yield two ethanes (Scheme 36(0)). ... 

Cross metathesis between two different alkanes represent one of the most difficult challenges in organic chemistry [53]. In 2001, Basset et al. first demonstrated the possibilities of sigma bond metathesis between two different alkanes [55]. In 2004, this same group has reported the cross metathesis between ethane and toluene [81] and methane and propane [82]. Silica-supported tantalum hydride catalyst [(=SiO)2TaH] [(=SiO)2TaH3] was employed for cross-metathesis reaction between toluene and ethane at 250°C. Under static condition, it produced mainly ethyl benzene and xylenes as major product along with propane and methane (Scheme 24). 

Scheme 25 Possible mechanism of cross metathesis of toluene and methane with silica-supported tantalum tris-hydride...

Scheme 26 Cross metathesis of methane and propane in the presence of silica-supp

Scheme 27 Possible mechanism for cross metathesis of methane and propane with silica-supported tantalum hydride...

The cross metathesis between propane and methane to transform methane into higher alkanes has also been reported [35]. Mechanistic studies performed on a mixture of propane and C-labeled methane (1/1250) in a batch reactor afforded isotopomers of ethane (unlabeled, mono-, and dilabeled), confirming the incorporation of the labeled methane (>85%). Moreover, metathesis of the C-monolabeled ethane catalyzed by [TaH, /Si02] in a batch reactor was examined. The results revealed that the degenerative and productive alkane metathesis processes concomitantly occur the alkane product distribution afforded a 1/1 mixture of unlabeled and monolabeled methane [36]. Further... 

R.B.A. Pardy, G.J. Sunley - Cross-Metathesis of Propane and Methane ... 

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