MECHANISTIC STUDIES OF METHYLENE CHAIN PROPAGATION IN THE FISCHER-TROPSCH SYNTHESIS

Probe molecules designed to generate methylene (CH2) were added to rho dium-, ruthenium-, and cobalt-catalysed Fischer-Tropsch reactions to i nvestigate the mechanism of hydrocarbon chain growth during CO hydroge nation. Statistical incorporation of (CH2)-C-13 derived from (CH3NO2)- C-13 or (CH2N2)-C-13 occurred during the hydrogenation of(12)CO over C o/SiO2 catalysts (1 atm, 523 K) to give the isotopically mixed alkenes ,C-13(x) (Cn-xH2n)-C-12 and the alkanes derived from them. These resul ts show that there is complete scrambling of C-12 and C-13 labels over cobalt; the levels of (CH2)-C-13 incorporation from the probe are con sistent with a process which involves the participation of methylene g roups 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/SiO2 or Rh/Ce/SiO2 cat alysts gave the isotopically distinct (CnH2n)-C-12 (from (CO)-C-12 hyd rogenation) and (CnH2n)-C-13 (from oligomerisation of the (CH2)-C-13 f rom the (CH3NO2)-C-13 or (CH2N2)-C-13 probes), and there was very litt le of the isotopically mixed alkenes, C-13(x) (Cn-xH2n)-C-12 and the a lkanes derived from them. Similar reactions over Ru/SiO2 exhibited beh aviour intermediate between cobalt and rhodium. Significant amounts of substituted amines and nitrile compounds are additionally formed when nitromethane is used as a probe;C-13 incorporation into the nitrogeno us products was observed when (CH3NO2)-C-13 was used as probe. The rel ative ability of nitromethane to produce N-containing compounds decrea ses in the order rhodium > ruthenium > cobalt. There was little C-13 i ncorporation into the oxygenates (methanol, ethanol, and acetaldehyde) when (CH2N2)-C-13 or (CH3NO2)-C-13 was used as a probe. (C) 1998 Acad emic Press.