Fischer-Tropsch synthesis termination reaction

The Fischer-Tropsch synthesis follows a polymerization mechanism where a Q unit is added to the growing chain. A simplified representation of the reaction network is shown in Fig. 1, where the key points are termination by either H-abstraction to give a-olefins or by hydrogenation to give w-paraffins. 

According to the Sachtler-Biloen mechanism, the Fischer-Tropsch reaction is initiated through CO adsorption followed by CO dissociation. Experimental evidence for the involvement of an oxygen-free intermediate exists it was observed that predeposited C is incorporated into the product during Fischer-Tropsch synthesis when CO was included in the feed gas (3). It is important to distinguish whether during the Fischer-Tropsch s)mthesis CO dissociation is strictly monomolecular or instead involves a reaction with Hads to produce an intermediate "HCO" formyl species that in a subsequent reaction decomposes to "CH" and Oads-Another question is how the rates of CO dissociation, chain growth, and termination depend on the catalyst surface structure. Thus, it is essential to know the relative values of the rate constants for these three elementary reactions. 

Fig. 1. Chain growth and termination and secondary reactions in Fischer-Tropsch synthesis on Co and Ru catalysts.

CO reactants and the H2O product of the synthesis step inhibit many of these secondary reactions. As a result, their rates are often higher near the reactor inlet, near the exit of high conversion reactors, and within transport-limited pellets. On the other hand, larger olefins that are selectively retained within transport-limited pellets preferentially react in secondary steps, whether these merely reverse chain termination or lead to products not usually formed in the FT synthesis. In later sections, we discuss the effects of olefin hydrogenation, oligomerization, and acid-type cracking on the carbon number distribution and on the functionality of Fischer-Tropsch synthesis products. We also show the dramatic effects of CO depletion and of low water concentrations on the rate and selectivity of secondary reactions during FT synthesis. 

Recent studies indicate that a-olefins, the major primary products formed during Fischer-Tropsch synthesis, participate in secondary reactions [12], Chains can terminate either by P-hydrc en abstraction to form an a-olefin or by H-addition to form a paraffin [13,14], Olefins can undergo secondary reactions by subsequent readsorption leading to isomerization or hydrogenation. We observe selectivity relationships that are consistent with Egiebor s proposal that significant secondary hydrogenation reactions can occur on iron catalysts [12],... 

The hypothesis of multiple build-in where a chain can interact with a chain Cj leads one to reflect on the possibility of a chain termination by combination. If reactions were occurring in which termination could occur by simple desorption and also by combination, two peaks would be observed. The second maxima would have a center at approximately twice the value of the flrst, as doubling of the most prevalent adsorbed chain lengths is likely (25). Furthermore, secondary events such as those discussed above or chain transfer could cause the distributions of the two peaks to be different from one another. Thus the fact that secondary reactions during Fischer-Tropsch synthesis occur and that multiple build-in and termination by combination are viable propositions help rationalize distributions that do not follow the Schulz-Flory law and appear with more than a single maximum. 

The readsorption of olefins is an important reaction in the Fischer-Tropsch synthesis that reverses the overall termination and increases the chain growth probability. Thus, readsorption results in heavier products. A larger time delay of the transient for the isotopic responses of olefin as compared with corresponding paraffin with increase in residence time (see Figure 51.10 for an example data on the ethane-ethene pair) is due to olefin readsorption. Transient experiments indicated that 1-olefins are the major candidates for readsorption on the catalyst surface, while the internal and iso-olefins readsorbed to a much less extent. 

Hydroformylation is a precious metal-catalyzed reaction of synthesis gas, a 1 1 mixture of hydrogen and carbon monoxide, and an olefinic organic compound to form aldehydes. The reaction was discovered by Otto Roelen in 1938 in experiments for the Fischer-Tropsch reaction [8]. In Scheme 3, hydroformylation of a terminal olefin is shown in which the addition of carbon monoxide can be conducted at both carbon atoms of the double bond, thus yielding linear (n) and branched (iso) aldehydes. 

We conclude that linear and branched olefins and paraffins can be formed after one surface sojourn by termination of growing surface chains. Therefore, they are primary Fischer-Tropsch products (4,14). a-Olefins readsorb and initiate chains in a secondary reaction. Thus, olefins reenter the primary chain growth process and continue to grow. These chains ultimately terminate as olefins or paraffins, in a step that can resemble a secondary hydrogenation reaction because it leads to the net consumption of olefins and to the net formation of paraffins, but which proceeds via primary FT synthesis pathways. 

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