Fragmentary kinetic models of the mechanism of the Fischer-Tropsch synthesis

Abstract

Unlike traditional kinetic models describing the dependence of the concentrations of all the products on the number of carbon atoms and widely used for analysis of the mechanism of the Fischer-Tropsch synthesis, fragmentary models establish a relationship between the concentrations of a limited number of products. They can be constructed if it is possible to obtain a dimensionless function of the concentrations that does not change with change in the concentrations themselves. Such models are based on fragments of the kinetic schemes of the process and are designed both for identification and discrimination among the mechanisms with data obtained in both differential and integral reactors. The general principles for the construction of fragmentary models are described. Examples of identification and discrimination among the various mechanisms from the results of fragmentary modelling using previously obtained experimental data for a cobalt catalyst are presented.Kinetic modelling has recently been used widely to investigate the mechanism of the Fischer-Tropsch (FT) synthesis [1][2][3][4]. In this case, mathematical models describing the dependence of the concentrations of the products on the carbon number have been derived on the basis of hypothetical schemes for the mechanism of the synthesis. As kinetic coefficients they contain the probabilities of the occurrence of reactions with the participation of various intermediates. The initial kinetic schemes usually include reactions leading to the formation of products with numbers of carbon atoms between 1 and ¥. As a rule the mathematical models establish a relationship between the concentrations of all the reaction products.Earlier [5,6] to study the mechanism of the FT synthesis it was proposed to use models constructed from fragments of the kinetic schemes of the mechanism. In the present work the general approach to the construction of such models and examples of their application to identification and discrimination among the various versions of the mechanism are examined. The isolated fragments of the kinetic schemes and the mathematical models based on them will subsequently be called fragmentary models (FM). PRINCIPLES OF THE CONSTRUCTION OF FRAGMENTARY MODELSLet us assume that it is possible from a kinetic scheme containing an arbitrary number of reactions to single out a fragment consisting of several reactions that take place in an adsorbed layer. Let such a fragment consist of m nonequilibrium reactions, the rate of which obeys the Langmuir equation 0040-5760/06/4202-0071

Chain termination mechanism in the Fischer-Tropsch synthesis on a Co/SiO2Zr(IV) catalyst at elevated pressure

“…Thus, the mechanism for paraffin formation through intermediate hydroalkene complexes may be considered identified, while the mechanism for the formation of C 2+ paraffins due to hydrogenation of alkyl intermediates should be subjected to discrimination. In previous work [8], we came to a similar conclusion upon analyzing other fragments of the kinetic scheme. We should also note that we previously arrived at the conclusion that paraffins are formed at elevated pressures largely from hydroalkene complexes [8].…”

“…In previous work [8], we came to a similar conclusion upon analyzing other fragments of the kinetic scheme. We should also note that we previously arrived at the conclusion that paraffins are formed at elevated pressures largely from hydroalkene complexes [8].…”

“…Two parameters calculated in our previous work [8] were proposed for the quantitative analysis of the experimental data in the present work: a 1 is the probability of chain propagation through alkyl intermediates and g is the ratio of the number of four-carbon intermediates formed as the result of a secondary reaction of ethylene to the number of such intermediates obtained from three-carbon alkyl compounds as the result of the addition of methylene groups.…”

“…Let us represent this matrix as the product of two other matrices Since the occupancy indices for the matrix elements are selected such that their algebraic sum is equal to zero and M 0 = 1, then, as shown in our previous work [8], the following relationship is valid:…”

“…However, an alternative variant is possible, in which paraffins, similar to olefins, are obtained through the intermediate formation of hydroalkene complexes [7,8]. The finding that the hydrogenation of the C=C double bond is the most rapid secondary reaction in the FT synthesis at low pressures, when readsorption of olefins on free surface sites is possible, is evidence for this alternative mechanism [3].…”