Fischer-Tropsch synthesis olefin selectivity

Fischer-Tropsch synthesis products contain also high quantities of n-a-olefins that can be recovered by selective sorption processes with suitable molecular sieves [19]. A large-scale Fischer-Tropsch synthesis plant operates in South Africa [20]. Another plant was started in Indonesia in 1993 [21]. 

Comparing heterogeneous Fischer-Tropsch synthesis with homogeneous olefin hydroformylation can be seen as a source for understanding catalytic principles, particularly because the selectivity is complex and therefore highly informative. Reliable analytical techniques must be readily available. 

Iron-based Fischer-Tropsch synthesis (FTS) catalysts are preferred for synthesis gas with a low H2/CO ratio (e.g., 0.7) because of their excellent activity for the water-gas shift reaction, lower cost, lower methane selectivity, high olefin... 

Of the technological modifications, Fischer-Tropsch synthesis in the liquid phase (slurry process) may be used to produce either gasoline or light alkenes under appropriate conditions249,251 in a very efficient and economical way.267 The slurry reactor conditions appear to establish appropriate redox (reduction-oxidation) conditions throughout the catalyst sample. The favorable surface composition of the catalyst (oxide and carbide phases) suppresses secondary transformations (alkene hydrogenation, isomerization), thus ensuring selective a-olefin formation.268... 

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. 

In summary, cluster-derived catalysts have been widely used in various types of CO-based reactions such as Fischer-Tropsch synthesis, methanol synthesis, hydroformylation, carbonylation, and water-gas shift reactions. The catalytic performances of cluster-derived species are evaluated in terms of higher activities and selectivities for lower olefins and oxygenates in CO hydrogenation, compared with those of metal complexes in solution and conventional metal catalysts (Table XIII). 

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 catalysts derived from supported iron clusters exhibit in Fischer-Tropsch synthesis a high selectivity for propylene. Those catalysts are also selective for the stoechiometric homologation of ethylene to propylene and of propylene to n and iso butenes. The results are explained on the basis of a new mode of C-C bond formation which implies < - olefin coordination to surface methylene fragments or methylene insertion into a metal alkyl bond. 

The catalytic results are given in Table 6 (the conversion for the catalyst heat treated at 873 K was about six times lower than that for DPColO and DPColOSOO and is therfore omitted from the table). The main product formed is methane. The data in Table 6 shows that, at similar conversion, the olefin selectivity increases and the activity remains constant with increasing heat treatment temperature. This demonstrates the important influence of the surface-oxygen functionalities on the product selectivity in the Fischer-Tropsch synthesis. 

In the case of Fe, the thermal decomposition of [HFe3(CO)ii] absorbed on alumina, leads to the formation of very small particles of Fe (10 A) which cannot be obtained by any other route. When these catalysts are used in Fischer—Tropsch synthesis, those supported particles are selective for low molecular weight olefins and the selectivity appears to be much larger than that obtained with conventionally prepared catalysts [29]. 

DB Bukur, X Lang, A Akgerman, Z Feng. Effect of process conditions on olefin selectivity during conventional and supercritical Fischer-Tropsch synthesis. Ind Eng Chem Res 36 2580-2587, 1997. 

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