Schulz-Flory product distributions

Figure 2. Schulz-Flory product distribution dependence on the chain growth factor K.

Zein el Deen et al. (30) studied the kinetics of the FTS on sintered oxides of iron and manganese. They observed, too, that the rate is independent of the CO partial pressure. Bub et al. (20) developed empirical expressions for the production rate of CO2 and Ci to C4 hydrocarbons on a Mn/Fe catalyst which could be used to successfully describe the conversion and selectivity in a pilot plant fixed bed reactor (2 cm ID by 80 cm length). If a catalyst like Mn/Fe gives a Schulz-Flory product distribution the hydrocarbon fraction can be calculated from the overall conversion rate and the chain growing probability a... 

In recent years, catalysts have been developed which give a non-Schulz-Flory product distribution in fixed beds. Such catalysts should be tested in slurry phase operation. 

The practical value of the Fischer-H opsch reaction is limited by the unfavorable Schulz-Flory distribution of hydrocarbon products that is indicative of a chain growth polymerization mechanism. In attempts to increase the yields of lower hydrocarbons such as ethylene and propylene (potentially valuable as feedstocks to replace petrochemicals), researchers have used zeolites as supports for the metals in attempts to impose a shape selectivity on the catalysis [114] or to control the performance through particle size effects. [IIS] These attempts have been partially successful, giving unusual distributions of products, such as high yields of C3 [114] or C4 hydrocarbons. [116] However, the catalysts are often unstable because the metal is oxidized or because it migrates out of the zeolite cages to form crystallites, which then give the Schulz-Flory product distribution. 

When determining the product selectivities, all compounds of equal carbon numbers (paraffines, olefins, isomers, and oxygen compounds) were summarized to one product fraction. The chain growth probability was determined by the Anderson-Schulz-Flory (ASF) distribution ... 

The carbon number distribution of Fischer-Tropsch products on both cobalt and iron catalysts can be clearly represented by superposition of two Anderson-Schulz-Flory (ASF) distributions characterized by two chain growth probabilities and the mass or molar fraction of products assigned to one of these distributions.7 10 In particular, this bimodal-type distribution is pronounced for iron catalysts promoted with alkali (e.g., K2C03). Comparing product distributions obtained on alkali-promoted and -unpromoted iron catalysts has shown that the distribution characterized by the lower growth probability a, is not affected by the promoter, while the growth probability a2 and the mass fraction f2 are considerably increased by addition of alkali.9 This is... 

Concurrent with FTS mechanism studies, product distribution models were developed based on the analysis of product composition. Friedel and Anderson28 29 in the 1950s published the Anderson-Schulz-Flory (ASF) distribution model to predict the wide range of products yielded from FTS. The equation is shown as follows ... 

The nickel concentration in the catalyst system is in the range 0.001-0.005 mol% (approx. 10-50 ppm). The oligomerization is carried out in a series of reactors at temperatures of 80-140°C and pressures of 7-14 MPa. The rate of the reaction is controlled by the rate of catalyst addition [19]. A high partial pressure of ethylene is required to obtain good reaction rates and high product linearity [11]. The linear a-alkenes produced are obtained in a Schulz-Flory-type distribution with up to 99% linearity and 96-98% terminal alkenes over the whole range from C4 to C+ (cf. Table 2) [23]. 

A comparative study of the alumina-supported catalysts prepared from [H2FeOs3(CO)i3], [H20s3Rh(acac)(CO)io], and [Rli4(CO)i2] was performed and each catalyst was found to be active in the conversion of CO -I- H2. The major product observed in each experiment was methane and the hydrocarbon products were formed in approximately a Schulz-Flory-Anderson distribution. The heterogeneous [Os3Rh] catalyst was two orders of magnitude more active at 543 K than the [FeOs3] catalyst, but showed a lower selectivity for ether formation. ... 

For the production of higher alcohols from syngas, two kinds of perovskites have been reported in the literature. First, perovskites with noble metals, like LaRhOs, have been studied in the past [23,24] for the ethanol synthesis with a CO/H2 mixture. More recently, LaCoi Cu 03 perovskites have been investigated to explore the opportunity of the Ci-C alcohols synthesis following an Anderson-Schulz-Flory (ASF) distribution [25-32] similar to that obtained by the so-called Co-Cu IFP catalyst [33]. 

The FTS mechanism could be considered a simple polymerization reaction, the monomer being a Ci species derived from carbon monoxide. This polymerization follows an Anderson-Schulz-Flory distribution of molecular weights. This distribution gives a linear plot of the logarithm of yield of product (in moles) versus carbon number. Under the assumptions of this model, the entire product distribution is determined by one parameter, a, the probability of the addition of a carbon atom to a chain (Figure 4-7). ... 

From Table 2 it can also be observed that the selectivity towards different hydrocarbon groups strongly depended on the acid properties of solids. Large amounts of C4 and C6 olefins were obtained for the mesoporous NiMCM-41 and NiMCM-48 catalysts with the lowest acid site concentration. In this case, a near Schulz-Flory-type product distribution (C4>C6>C8>Cio) was observed. The increase in acid site density (for the catalysts NiY, NiMCM-36, NiMCM-22) results in decrease of C 6/C8 ratio. These results are in agreement with the reaction network proposed in Scheme 1. 

The description of the product distribution for an FT reaction can be simplified and described by the use of a single parameter (a value) determined from the Anderson-Schulz-Flory (ASF) plots. The a value (also called the chain growth probability factor) is then used to describe the total product spectrum in terms of carbon number weight fractions during the FT synthesis. In the case... 

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... 

The sensitivity of the product distribution for small changes of these parameters can also be exploited to our advantage the product distribution can be easily adjusted to changes of the demand for certain oligomers, but only within the limits of Schulz-Flory distributions ... 

The production of hydrocarbons using traditional F-T catalysts is governed by chain growth (polymerization) kinetics. The theoretical equation describing the distribution of hydrocarbon products, commonly referred to as the Anderson-Schulz-Flory (ASF) equation, is... 

The termination step for 1-alkene formation is now the reaction of the surface alkenyl with surface H instead of the p-elimination step. Chain branching can proceed by the involvement of allylic intermediates. Since this new mechanism involves different types of reactions to form C2 and C2< hydrocarbons, it is not expected that the amounts of C2 products will lie on the normal curve of the Ander-son-Schulz-Flory distribution. 

Flory polymerization kinetics (4). Henrici-Olive and Olivd (5) proposed the use of the related equation of Schulz (6). Over the last decade the Flory equation has been used frequently to describe product distributions in FT synthesis. The Friedel-Anderson (i) or Flory (4) approaches apply when the rates of propagation and termination are independent of carbon number. We do not attempt here to discuss all previous research on FT product distributions except to say that the literature contains many examples of product distributions that obey Flory kinetics within relative narrow carbon number ranges and many that do not. 

Because in some cases the production of methane does not obey the Schulz-Flory distribution, kj is allowed to differ from kj. The Schulz-Flory constant is defined as a = rn/rn i, for n 3,... 

The minimum value for the rate constant k on ruthenium is calculated from the data published by Dautzenberg et al. for the on iron the data of Vannice are used. For the ruthenium case it is assumed that the methane production meets the Schulz-Flory distribution (kj = k3). The value of the rate constant of propagation, where the reaction is completely determined by this constant is shown for the different models in table II. In the first column of this table minimum k values are shown for ruthenium. The data used fpom the work of Dautzenberg and coworkers were ... 

In a recent communication (250), deviations from Schulz-Flory kinetics were observed for a RuNaY synthesis gas conversion catalyst (see Fig. 24). A comparative catalyst, prepared by impregnating silica with ruthenium, i.e., Ru/SiOz, and tested under the same conditions, yielded a product distribution which gave a good fit to Schulz-Flory kinetics. The sharp decrease in chain growth probability for Cf0 products over RuNaY is perhaps surprising for such a relatively large-pore zeolite. Further studies (251-253) on this system indicated that there was a correlation between the ruthenium particle size in the zeolite and the product distribution. 

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