THE ALKENYL MECHANISM FOR FISCHER-TROPSCH SURFACE METHYLENE POLYMERIZATION - THE REACTIONS OF VINYLIC PROBES WITH CO/H-2 OVER RHODIUM CATALYSTS

Results consistent with the participation of vinyls in the initiation and of alkenyl species in the propagation steps of the Fischer-Tropsch reaction are reported. Substantial incorporation of C-13(2) into the alkene and alkane (C-3-C-7) hydrocarbon products occurred when doubly labelled vinyls ((C2H3Br)-C-13, ((C2H3)-C-13)(4)Si, or (C2H4)-C-13) were added as molecular probes to the hydrogenation of carbon monoxide over rhodium/ceria/silica catalysts (1 atm, 220 degrees C). There was, by contrast, no significant incorporation of C-13(1) into any of the organic products; thus cleavage of the C-2 probe did not occur. The degree of C-13(2) incorporation decreased with increasing molecular mass of the hydrocarbon; this indicates that the probe molecule initiated but did not propagate. A mathematical model based on polymerisation of surface methylenes initiated by a vinyl, propagated by alkenyls and terminated by reaction with a surface hydrogen or by coupling has been developed to explain the C-13(2) incorporation data. Under the conditions of the experiments, the relative ability of the probes to initiate is: vinyl bromide (60%) > tetravinylsilane (30%) > ethene (15%), Substantial formation of C-13(4) products also occurred when vinyl bromide or tetravinylsilane were used as probes; this arises from a dimerisation of the vinyl on the surface, a process which has been modelled in homogeneous systems and also by other workers in studies on single crystal surfaces, There was no significant C-13 incorporation into the oxygenates (methanol, ethanol, acetaldehyde); these products are formed by a different path.