Fischer-Tropsch reaction product distribution

The product distribution frcm the Fischer-Tropsch reaction on 5 is shown in Table I. It is similar but not identical to that obtained over other cobalt catalysts (18-21,48, 49). The relatively low amount of methane production (73 mol T when compared with other metals and the abnormally low amount of ethane are typical (6). The distribution of hydrocarbons over other cobalt catalysts has been found to fit the Schulz-Flory equation [indicative of a polymerization-type process (6)]. The Schulz-Flory equation in logarithmic form is... 

Metal molybdates421 and cobalt-thoria-kieselguhr422 also catalyze the formation of hydrocarbons. It is believed, however, that methanol is simply a source of synthesis gas via dissociation and the actual reaction leading to hydrocarbon formation is a Fischer-Tropsch reaction. Alumina is a selective dehydration catalyst, yielding dimethyl ether at 300-350°C, but small quantities of methane and C2 hydrocarbons423 424 are formed above 350°C. Heteropoly acids and salts exhibit high activity in the conversion of methanol and dimethyl ether.425-428 Acidity was found to determine activity,427 130 while hydrocarbon product distribution was affected by several experimental variables.428-432... 

These mixed metal systems have also been tested with the transient method for catalytic activity in the Fischer-Tropsch reaction. We would like to remark here that the nature of the cation, anion, and zeolite are all important factors in the Fischer-Tropsch reactions that we have studied. Further details of these catalytic studies can be found elsewhere (23). We do observe here, however, that some catalysts that are completely reduced to the metallic state are not necessarily the most active catalysts. Also, even though the Mossbauer experiments suggest that 400°C is sufficient for complete reduction, higher activation temperatures can increase the activity and selectivity of these reactions. We have also observed that the cation definitely changes the product distribution and the activity. 

The Fischer-Tropsch reaction consists of an initiation step, a chain-growth step, and a termination step. However, the observed product distribution may reflect secondary reactions such as alkene insertion... 

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. 

Figure 2 shows the effect of method of Fe addition on product distributions. Cu0-Zn0/Ti02 (cat. A) was active for methanol synthesis, but it was not effective for the synthesis of hydrocarbons. This indicates that Cu species alone is not enough to produce hydrocarbons. On the contrary, Fe-based catalysts are known as hydrocarbon synthesis catalysts from CO, that is, Fischer-Tropsch reaction. However, Fe/TiOj catalyst (cat. C) showed poor... 

The reaction products obtained over K/Cu-Zn-Fe oxides catalyst showed Schulz-Flory distribution. This suggests that C-C bonds were formed under the mechanism similar to that in Fischer-Tropsch reaction. 

In a long and detailed paper on CO/H2 and CO2/H2 reactions on polycrystalline Rh, Sexton and Somorjai used a UHV-AES apparatus designed to allow sample scrutiny at low pressure yet to permit high-pressure (700 torr) reactions. They established good correlation of turn-over numbers between their results and results obtained on supported catalysts for the Fischer-Tropsch reaction. AES established that C was present on reactive surfaces yet this C (1 to 2 monolayers) did not influence rates of reaction or product distribution for high-pressure runs. It is however interesting to note that the most important influences on catalysis reported in this paper were found to be subsurface C and O, neither detectable by AES. The reactions studied at 250—300 °C showed that CO/H2 produced mainly Ci but also some C2, C3, and C4 hydrocarbons, whereas CO2/H2 produced CH4 exclusively. 

Transient Isotopic Kinetic Studies of Methanation. - The Fischer-Tropsch reaction results in the formation of a wide distribution of hydrocarbons containing different numbers of carbon atoms. In contrast, the related reaction of methanation of CO/H2 mixtures involves only one product and is easier to study using isotope transient kinetic techniques. The results of the methanation reaction have a direct relevance to the Fischer-Tropsch reaction and are reviewed below. 

Kinetic expressions similar to that of Equation 3 and similar activation energies have been reported for methanation over a cobalt-alumina catalyst (4) and for Fischer-Tropsch reaction over a cobalt-thoria catalyst (5). This similarity, despite appreciably different product distributions in the three cases, argues for a common rate-controlling step in the mechanisms. 

Hall, Kokes, and Emmett (24), who used radioactive tracers to study the Fischer-Tropsch reaction, suggested that besides stepwise growth with single-carbon intermediates, multiple build-in could and probably did occur in the synthesis. They also showed that multiple build-in could not be distinguished from single-carbon stepwise growth below C12-C16 hydrocarbons, and thus the effect of multiple build-in could only be seen if detailed product distributions up to large carbon numbers were obtained. 

Maitlis has reviewed the homogeneous and heterogeneous systems and noted that the former produce mainly oxygenated products, such as alcohols and esters. Most of the recent work on the Fischer-Tropsch reaction has focused on heterogeneous catalysts. The mechanism(s) of the later ate mainly inferred from isotope labeling and product distributions. The observations and mechanistic proposals of the Sheffield group have been summarized by Maitlis. However, there is still some controversy about the heterogeneous pathways. ... 

The Fischer-Tropsch process can be considered as a one-carbon polymerization reaction of a monomer derived from CO. The polymerization affords a distribution of polymer molecular weights that foUows the Anderson-Shulz-Flory model. The distribution is described by a linear relationship between the logarithm of product yield vs carbon number. The objective of much of the development work on the FT synthesis has been to circumvent the theoretical distribution so as to increase the yields of gasoline range hydrocarbons. 

In this chapter a two a selectivity model is proposed that is based on the premise that the total product distribution from an Fe-low-temperature Fischer-Tropsch (LIFT) process is a combination of two separate product spectrums that are produced on two different surfaces of the catalyst. A carbide surface is proposed for the production of hydrocarbons (including n- and iso-paraffins and internal olefins), and an oxide surface is proposed for the production of light hydrocarbons (including n-paraffins, 1-olefins, and oxygenates) and the water-gas shift (WGS) reaction. This model was tested against a number of Fe-catalyzed FT runs with full selectivity data available and with catalyst age up to 1,000 h. In all cases the experimental observations could be justified in terms of the model proposed. 

The readsorption and incorporation of reaction products such as 1-alkenes, alcohols, and aldehydes followed by subsequent chain growth is a remarkable property of Fischer-Tropsch (FT) synthesis. Therefore, a large number of co-feeding experiments are discussed in detail in order to contribute to the elucidation of the reaction mechanism. Great interest was focused on co-feeding CH2N2, which on the catalyst surface dissociates to CH2 and dinitrogen. Furthermore, interest was focused on the selectivity of branched hydrocarbons and on the promoter effect of alkali on product distribution. All these effects are discussed in detail on the basis... 

In Fischer-Tropsch synthesis the readsorption and incorporation of 1-alkenes, alcohols, and aldehydes and their subsequent chain growth play an important role on product distribution. Therefore, it is very useful to study these reactions in the presence of co-fed 13C- or 14 C-labeled compounds in an effort to obtain data helpful to elucidate the reaction mechanism. It has been shown that co-feeding of CF12N2, which dissociates toward CF12 and N2 on the catalyst surface, has led to the sound interpretation that the bimodal carbon number distribution is caused by superposition of two incompatible mechanisms. The distribution characterized by the lower growth probability is assigned to the CH2 insertion mechanism. 

Kuipers, E.W., Scheper, C., Wilson, J.H., Vinkenburg, I.H., and Oosterbeek, H. 1996. Non-ASF product distributions due to secondary reactions during Fischer-Tropsch synthesis. J. Catal. 158 288-300. 

The Fischer-Tropsch synthesis, which may be broadly defined as the reductive polymerization of carbon monoxide, can be schematically represented as shown in Eq. (1). The CHO products in Eq. (1) are any organic molecules containing carbon, hydrogen, and oxygen which are stable under the reaction conditions employed in the synthesis. With most heterogeneous catalysts the primary products of the reaction are straight-chain alkanes, while the secondary products include branched-chain hydrocarbons, alkenes, alcohols, aldehydes, and carboxylic acids. The distribution of the various products depends on both the type of catalyst and the reaction conditions employed (4). 

Ruthenium is known to catalyze a number of reactions, including the Fischer-Tropsch synthesis of hydrocarbons (7) and the polymerization of ethylene (2). The higher metal dispersions and the shape selectivity that a zeolite provides has led to the study of ruthenium containing zeolites as catalytic materials (3). A number of factors affect the product distribution in Fischer-Tropsch chemistry when zeolites containing ruthenium are used as the catalyst, including the location of the metal (4) and the method of introducing ruthenium into the zeolite (3). 

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