Carbidic intermediates

Many attempts have been made to elucidate the mechanism(s) of this reaction. The original proposal by Fischer and Tropsch is based on carbide intermediates [2]. 

Carbenium ions, 42 115, 143 acid catalysis, 41 336 chemical shift tensors, 42 124-125 fragments in zeolites, 42 92-93 history, 42 116 superacids, 42 117 Carbide catalysts, 34 37 Carbidic carbon, 37 138, 146-147 Carbidic intermediates, 30 189-190, 194 Fischer-Tropsch synthesis, 30 196-197, 206-212... 

Dow Chemical is now a major producer of vinyl chloride. This was initially made by adding HCl to acetylene (the Union Carbide intermediate),... 

Carbides. As might he expecled from its position in the periodic table, carbon forms binary compounds with the metals in which it exhibits a negative valence, and binary compounds with the non-metals in which it exhibits a positive valence. A convenient classification of the binary compounds of carbon is into ionic or salt-like carbides, intermediate carbides, interstitial carbides, and cuvalent binary carbon compounds. 

Older IR-imwstigalions by Kolbel et at. led to the assumption of surface formyl or hydroxycarbene groups [113]. More recent measurements on supported ruthenium under synthesis conditions showed absorptions for mr ecularly adsorbed CO and for formate and CH species. As a result of deuieration experiments, however, the latter were concluded not to be reaction intermediates (113. 106], It was also shown that the production of methane and ethane continued for a significant period after CO had been removed from tlic reaction mixture and after the disappearance of all IR-observable COads species [106J. It was concluded that product formation occurs via carbidic intermediates. /n-situ IR studies at higher pressures (3 bar) revealed formation of CHjf species with absorptions in the 3000 cm region (114). 

Under the low-pressure (and low-a) conditions prevailing in the studies (37, 51), neither carbidic intermediates CH nor growing chains are detectable by IR. 

Surface spectroscopy, as discussed in the previous section, has recently provided evidence that concurrent with the hydrocarbon synthesis a reactive carbidic overlayer develops, and that this reactive overlayer may contain the intermediates operative in the FT synthesis. However, the claims of surface spectroscopy regarding the relevance of carbidic intermediates have had a precedent in one of the very first publications of Fischer and Tropsch... 

In the absence of CO the predominant product of the hydrogenation is methane. The first of the recent proposals regarding the role of carbidic intermediates therefore related specifically to methanation (48, 56). In subsequent studies (36,37,58,61), however, it was found that smaller quantities of higher hydrocarbons, up to butane, are being coproduced (cf. Fig. 11), especially at low temperature, and/or low hydrogen pressures, stimulating the authors to propose the participation of carbidic species in chain growth. 

It thus follows that methanation proceeds via rapid formation of a carbidic intermediate CH which is converted relatively slowly to methane ... 

T he higher hydrocarbons were predominantly straight-chain ones, with a close to linear Schulz-Flory chain length distribution. Therefore, it was concluded that the carbidic intermediates CH, leading to methane [cf. Eq. (43)] were also participating in chain growth. Moreover, the extent of incorporation into the longer chains was, just as with methane, indicative of... 

It follows that the chain growth in FT synthesis occurs via incorporation of carbidic intermediates CH,., originating from CO in a kinetically fast step ... 

The predominant role of carbidic intermediates in chain growth proper does not preclude a role of oxygenated intermediate, for instance, as the precursors of the carbidic ones ... 

Summarizing, we conclude the available evidence to be compatible with the genesis of carbidic intermediates via Pathway (55), that is, via CO dissociation. To the extent that hydrogen overall enhances the formation of surface carbon it is plausible that its role is primarily one of removing after the dissociation step proper (37,48). 

Regarding the role of carbide intermediate layers in diamond nucleation processes, some contrary experimental data have also been reported. 

Experiments have recently been completed by Kummer, DeWitt, and Emmett (75), using C as a tracer in the synthesis on an iron catalyst. The results are inconclusive. If the total catalyst surface is uniformly active in the synthesis, the results show that only a small fraction of the reaction proceeds by way of the carbide. However, if only occasional active patches of the surface are participating in the synthesis, then it is possible to interpret the results as indicating that all of the reaction proceeds by way of the carbide. The carbide intermediate hypothesis for the mechanism of the synthesis on iron catalysts, however, is probably incorrect. Thus, the results of recycle operations at low temperatures on iron catalysts show that alcohols are formed earlier in the synthesis than olefinic hydrocarbons. 

The generation of the catalytically active 14-electron species is an important factor for the improvement of catalyst activity. A true 14-electron complex with no free L ligand has been reported by Piers et al. [24]. This phosphonium-aUcylidene complex 14 was obtained by protonation of ruthenium carbide intermediates and isolated in 87-95% yield (Scheme 6). 

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