Pichler and Schulz

The third class of mechanisms is that originated by Pichler and Schulz in which chain growth is accomplished by direct insertion of an absorbed CO molecule into a carbon-metal bond produced by hydrogenation of a surface carbonyl (89). A complete representation of this mechanism (see... 

Pichler and Schulz were the first to suggest that the initial step in metal-catalyzed CO/H2 reactions might be the migration of catalyst-bound hydride to CO to yield a formyl intermediate [Eq. (1)] (3). The alkyl ligand in many isolable metal carbonyl alkyls can similarly be induced to migrate to coordinated CO (4. ). Thus, Eq. (1) has some precedent in homogeneous chemistry. Other suggestions for the involvement of formyl intermediates have come from Wender (6) and Henrici-Olive and Olive (7). 

Pichler and Schulz [7] suggested a third mechanism, which is worth considering and seeking to verify. According to this suggestion, a carbonyl of a metal in question is formed first, which is followed by the formation of formyl (in later... 

Aikane Aicohoi, Aidehyde SCHEME 2 Fischer-Tropsch mechanism as proposed by Pichler and Schulz (2). 

CO insertion, that is, x = 0, y = 1 This possibility was in particular advocated by Pichler and Schulz (20). They write the reaction sequence leading to chain growth by one C atom as shown in Scheme 1 ... 

The one-carbon addition schemes are typical of processes in which the growing species may be envisioned to be a chemisorbed a olefin but with a finite probability that the double bond may move to position. These schemes may be consistent with the recent mechanism of Pichler and Schulz (17). The one- and two-carbon-addition schemes are typical of the old "alcohol dehydration" mechanism, which incidentally has found no analogy in homogeneous organometallic reactions. Extension of the... 

CO insertion. The CO insertion mechanism, in which CO inserts into R-CH2 directly, was originally proposed by Pichler and Schulz. Hu and Saeys have calculated the activation barrier for the CH2-I-CO coupling step. The barrier for insertion is about 1.87 eV, being too high to... 

H. Pichler, H. Schulz and A.Z. El Deen, Erdoel Kohle Erdgas Petrochem., 29... 

Pichler, H., Schulz, H., Reitemeyer, H. O., and Weit-kamp, J., Erdol, Kohle-Erdgas-Petrochem. (1972), 25,... 

The second proposed mechanism for the Fischer-Tropsch reaction is the Pichler-Schulz mechanism (2,16) (Scheme 2), which is characterized by chain growth by insertion of CO. This mechanism implies that the insertion step and subsequent dehydroxylation are fast compared with the chain-termination reaction. We emphasize that, according to this scenario, CO dissociation does not need to proceed rapidly. 

To initiate chain growth, a "Ci" surface species may be required as in the Sachtler-Biloen mechanism, but the rate of formation of such species may be low. This scenario is comparable to conventional polymerization catalysis, in which initiation is usually the rate-limiting step. Assuming the generation of "Ci" species to be rate determining contrasts the Pichler-Schulz reaction scheme from the Sachtler-Biloen scheme, in which the slow step of the reaction is the termination. Because of the structure sensitivity of the CO dissociation reaction, and also because of the expected structure sensitivity of the chain-growth reaction, the Pichler-Schulz mechanism requires unique sites. The rate of CO insertion and consecutive steps should be fast compared with the rate of CO dissociation. Of course, the rate of termination should be low compared with that of chain growth. 

Pichler et al. (18) first reported deviations from Flory distributions on Co catalysts and suggested that they reflected the higher intrinsic readsorption reactivity of larger olefins, which led to surface chains that continued to reform until they terminated irreversibly to unreactive paraffins. Schulz et al. (42) noted that products begin to leave the reactor with the liquid phase in the same hydrocarbon size range where deviation from Flory kinetics occurred. Later, these authors proposed that the higher solubility of larger... 

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