The Alkenyl Mechanism for Fischer‐Tropsch Surface Methylene Polymerisation; the Reactions of Vinylic Probes with CO/H2 over Rhodium Catalyst

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Abstract
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 13C2 into the alkene and alkane (C3–C7) hydrocarbon products occurred when doubly labelled vinyls (13C2H3Br. (13C2H3)4Si, or 13C2H4) were added as molecular probes to the hydrogenation of carbon monoxide over rhodium/ceria/silica catalysts (1 atm, 220°C). There was, by contrast, no significant incorporation of 13C1 into any of the organic products; thus cleavage of the C2 probe did not occur. The degree of 13C2 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 13C2 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 13C4 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 13C incorporation into the oxygenates (methanol, ethanol, acetaldehyde); these products are formed by a different path.

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