Vinylic initiation of the Fisher-Tropsch reaction over ruthenium on silica catalysts

Vinylic probes were added to a ruthenium-catalysed Fischer-Tropsch reaction in order to study the mechanism of hydrocarbon formation from CO/H2. 13C2 units derived from 13CH2=13CHBr or 13CH2=13CH2 are readily incorporated into the alkene and alkane products formed from 12CO and H2 over a Ru/SiO2 catalyst (1 atm, 180°C). The levels of 13C2 incorporation suggest that the probes are sources of 13C2-labelled initiating units, proposed to be surface vinyls (-CH=CH2). Further evidence for vinyl initiation is given by the increase in hydrocarbon formation rate during vinylic probe addition. This rate increase is not observed when ethyl bromide is added as a probe and shows the significance of the unsaturated initiating unit. 13C4 incorporation in the C4 hydrocarbon products (especially cis-2-butene) is attributed to a facile vinyl dimerisation reaction, as previously observed on rhodium catalysts and in discrete organometallic complexes. Hydrogenolysis of the 13C2-probes was an appreciable side reaction over Ru/SiO2 and resulted in some incorporation of 13C1 units.