Time dependence of the activity and selectivity of a cobalt-containing catalyst in Fischer-Tropsch synthesis

Yakubovich, M. N.; Strizhak, P. E.

doi:10.1007/s11237-006-0068-6

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“…The molecular mass distribution of the hydrocarbons formed in the Fischer-Tropsch synthesis at Co/SiO 2 catalysts, which have been attracting increased interest [35][36][37][38][39][40], differs from the Anderson-Schulz-Flory distribution [7,9,16,20,30]. Such a distribution has a low content of the C 2 and sometimes of the C 3 hydrocarbons and shows an increase in the effective probability of chain growth with increase in its length for intermediates containing more than two or three carbon atoms.…”

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“…For example, relationships were established between the probability of chain growth and the degree of dispersion of the metal [42,43] and the hydrogenating capacity of the catalyst [44]. In [30] it was shown that decrease in the hydrogenating capacity of the catalyst leads to increase of a 1 and w and decrease of g but does not affect a 1 and x. Replacement of part of the cobalt by iron leads to an increase of a 1 and a decrease of x [45].…”

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confidence: 99%

“…Such agreement occurs under conditions where the values of a 1 can differ substantially. Figure 5 shows the weight fractions of propane, propylene, and C 3 hydrocarbons after use of the catalysts for various times (t), calculated from previously published experimental data [30]. These data were obtained at 510 K and 5 MPa.…”

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confidence: 99%

“…The Dependence of the Probabilities of Chain Growth on the Methane Content of the Products on the Length of Use of the Catalyst[30] Note. t is the time passed from the beginning of the investigation.…”

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Kinetic models of the molecular mass distribution of the products of the Fischer-Tropsch synthesis at cobalt catalysts

Yakubovich

Strizhak

2007

Theor Exp Chem

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Kinetic models were used to investigate the Fischer-Tropsch synthesis at increased pressures. The general mechanism proposed for the process at low and increased pressures was based on quantitative analysis of the products obtained with cobalt catalysts at pressures of 2.5-10 MPa using various kinetic models, the previously developed reference point method, and published data.The molecular mass distributions (MMD) of the hydrocarbons formed in the Fischer-Tropsch (FT) synthesis often differ from the Anderson-Schulz-Flory (ASF) distribution [1]. Various schemes have been proposed for the mechanism of the process to explain the deviations from the ASF distribution [2][3][4][5][6][7][8][9]. On the basis of such schemes mathematical models establishing a relationship between the concentrations of the individual products and the carbon number were derived. As kinetic coefficients they contain the probabilities or functions of the probabilities that the reactions occur with the participation of intermediates. They are models of the selectivity, but they make it possible to calculate the absolute rates of all stages of the kinetic scheme, starting from the rate of transformation of CO or H 2 [2]. A whole series of reasons for the deviations from the ASF distribution have been discussed in the literature.The reduced or increased methane content of the products is explained by the fact that the C 1 and C 2+ hydrocarbons are formed at different centers on the surface of the catalyst [10] or as a result of reactions leading to termination of the chain [1]. The deviation from the ASF distribution is sometimes attributed to increase in the probability of growth of the chain of C-C bonds (a) according to Herington scheme [11] with increase in the carbon number [1].The molecular mass distributions of the hydrocarbons often take the form of concave curves with a clearly defined kink [1,12,13]. Anderson approximated such distributions by two straight lines with different slopes in relation to the abscissa axis [13]. The existence of two probabilities of chain growth in the Fischer-Tropsch synthesis (usually a 1 < a 2 ) was thus mentioned first in this paper. Later Huff and Sutterfield [12] proposed an equation describing such distributions. This equation was obtained on the basis of the idea that the experimental molecular mass distributions represent the superimposition of the distributions of the a 1 -and a 2 -hydrocarbons.Initially the existence of two values for the probability of chain growth was explained by the fact that the growth of the C-C bonds takes place at different centers of the catalyst [12, 14]. However, subsequent data indicated that the two values of a arise as a result of simultaneous growth of the chain by two different mechanisms through two types of intermediates [9,[15][16][17][18]. The distributions of the hydrocarbons formed at Fe catalysts usually obey the Huff-Sutterfield equation [1,[11][12][13].(Here, they can often be approximated with sufficient accuracy by two straight lines.) The distributions o...

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: ________ ‡mentioning

confidence: 99%

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Kinetic models of the molecular mass distribution of the products of the Fischer-Tropsch synthesis at cobalt catalysts

Yakubovich

Strizhak

2007

Theor Exp Chem

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“…The catalyst was Co/SiO 2 Zr(IV) containing 5 mass % cobalt and 1 mass % zirconium. The composition of the products formed was given in our previous work [9]. The experimental data required for solving the problem posed are given in Table 1.…”

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

Yakubovich

Strizhak

2008

Theor Exp Chem

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The mechanism for the formation of C 2+ paraffins differs from the mechanism for the formation of methane. While methane is obtained due to hydrogenation of methyl groups, C 2+ hydrocarbons are formed as the result of the conversion of alkyl intermediates into hydroalkene intermediates, which can decompose to give olefins or undergo hydrogenation to give saturated hydrocarbons.The Harrington scheme [1] is the basis for the mechanism of the Fischer-Tropsch (FT) synthesis. According to this scheme, the growth of C-C-bond chains proceeds due to the addition of one-carbon fragments until chain termination.Most of the products on cobalt catalysts are obtained from alkyl intermediates (metal-alkyl complexes) [2, 3]. The increase in chain length is the result of the insertion of methylene groups into the metal-alkyl bond [2]. In accord with current concepts [2, 4, 5], chain termination with olefin formation proceeds as the result of b-elimination of hydrogen from the metal-alkyl intermediates, leading to the formation of intermediate hydroalkene complexes. The mechanism of termination leading to paraffin hydrocarbons remains unclear.Chain termination occurs most likely due to hydrogenation of the metal-carbon bond in the alkyl intermediates containing two or more carbon atoms since the mechanism for the formation of C 2+ paraffins is analogous to the mechanism for the formation of methane from methyl groups [2, 6]. 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]. (At elevated pressures, readsorption does not take place [3]. Thus, we disregard such reactions.)In the present work, we attempted to determine which mechanism is responsible for the formation of C 2+ paraffins from alkyl intermediates on a cobalt-zirconium catalyst at elevated pressures. EXPERIMENTALThe experiments were carried out at 510 K and 5 MPa. The volumetric velocity of the synthesis gas (H 2 : CO = 2.5) was varied from 1.1·10 3 to 1.4·10 3 h -1 . The length of operation of the catalyst (t) was varied. The catalyst was Co/SiO 2 Zr(IV) containing 5 mass % cobalt and 1 mass % zirconium. The composition of the products formed was given in our previous work [9]. The experimental data required for solving the problem posed are given in Table 1. The C 2 and C 4+ hydrocarbons are mainly saturated hydrocarbons (Fig. 1). A significant amount of olefin was noted only for C 3 hydrocarbons. We should note 0040-5760/08/4403-0183

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The Possibilities of Applying QCM Method in Pulsed Mode for Investigating the Adsorption Dynamics of Volatile Compounds on Finely Dispersed Sorbents

Филиппов

2012

Adsorption Science & Technology

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ABSTRACT:In this study, the possibilities of applying quartz crystal microbalance (QCM) method in pulsed mode for investigating the adsorption dynamics of volatile compounds on finely dispersed sorbents is shown. The use of piezoelectric quartz resonator (PQR) sensors with chemical coverings on the base of metal stearates or thiolates, as well as other low-inertia sensors is proposed to evaluate the concentration of gas-phase sorbate current in the flow cell of electronic nose-type sensors. These sensors are placed in the cell in parallel to the PQR with a sorbent under investigation.

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Time dependence of the activity and selectivity of a cobalt-containing catalyst in Fischer-Tropsch synthesis

Cited by 3 publications

References 10 publications

Kinetic models of the molecular mass distribution of the products of the Fischer-Tropsch synthesis at cobalt catalysts

Kinetic models of the molecular mass distribution of the products of the Fischer-Tropsch synthesis at cobalt catalysts

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

The Possibilities of Applying QCM Method in Pulsed Mode for Investigating the Adsorption Dynamics of Volatile Compounds on Finely Dispersed Sorbents

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