Mechanistic Studies of Methylene Chain Propagation in the Fischer–Tropsch Synthesis

Abstract

Probe molecules designed to generate methylene (CH2) were added to rhodium-, ruthenium-, and cobalt-catalysed Fischer–Tropsch reactions to investigate the mechanism of hydrocarbon chain growth during CO hydrogenation. Statistical incorporation of13CH2derived from13CH3NO2or13CH2N2occurred during the hydrogenation of12CO over Co/SiO2catalysts (1 atm, 523 K) to give the isotopically mixed alkenes,13Cx12Cn−xH2nand the alkanes derived from them. These results show that there is complete scrambling of12C and13C labels over cobalt; the levels of13CH2incorporation from the probe are consistent with a process which involves the participation of methylene groups in the chain propagation step. The data are also consistent with the alkenyl cycle proposed earlier for the Fischer–Tropsch reaction. By contrast, the same probe experiments over Rh/SiO2or Rh/Ce/SiO2catalysts gave the isotopically distinct12CnH2n(from12CO hydrogenation) and13CnH2n(from oligomerisation of the13CH2from the13CH3NO2or13CH2N2probes), and there was very little of the isotopically mixed alkenes,13Cx12Cn−xH2nand the alkanes derived from them. Similar reactions over Ru/SiO2exhibited behaviour intermediate between cobalt and rhodium. Significant amounts of substituted amines and nitrile compounds are additionally formed when nitromethane is used as a probe;13C incorporation into the nitrogenous products was observed when13CH3NO2was used as probe. The relative ability of nitromethane to produce N-containing compounds decreases in the order rhodium>ruthenium>cobalt. There was little13C incorporation into the oxygenates (methanol, ethanol, and acetaldehyde) when13CH2N2or13CH3NO2was used as a probe.