Benzene, additional reactions lifetimes

WL(C0)8-02-RAlCl2 (R=Me or Et) catalyses alkene metathesis [activity, L=mesitylene> benzene >(CO)s lifetime, reverse order] in addition to doublebond migrations. The initial products of the reaction of c/j-pent-2-ene with the alkylidene complex (17a) are shown in Equation (18) [(17a) R=Bu (17b)... 

The formation of C02 -CHD has been postulated in order to account for the generation of benzoic acid as a reaction product (see above). We expect that the lifetime of this radical is longer than 50 ps in order to be able to undergo disproportionation to benzoic acid. The observed absorption at A >340 nm may in fact be attributed to the C02 -CHD radical, because of the agreement of its absorption spectrum with that of the structurally related H adduct of benzoic acid showing, both, absorption maxima at around 350 nm. Despite the fact that Ph-CHD phenylcyclohexadienyl radicals also show an absorption with a maximum between 330 and 340 nm, its contribution to the observed spectrum is neglected, because the recombination product, phenylcyclohexadiene, was not found (see above). Consequently, the time-resolved experiments with benzene support both the addition and electron transfer pathways of reaction. 

There are several areas of concern for the industrial practice of the reaction at the current stage of development (131). In addition to the equilibrium limitation of the methane-to-benzene conversion, the intrinsic activity of the Mo/ZSM-5 catalyst is rather low, for example, 7% conversion at Gas Hourly Space Velocity (GHSV) = 800 h (107). The promotion of the catalyst with W, Zr, Ru (132), and Fe (133) resulted in only modest success. Catalyst stability or lifetime should also be improved. Coke formation causes a gradual decrease in methane conversion to benzene with time on stream. Catalyst promotion with Fe or the addition of CO/CO2 in the methane feed improves the stability (133). 

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