EFFECT OF THE NATURE OF THE CATALYST ON THE SELECTIVITY OF THE SYNTHESIS PROCESS FROM CARBON MONOXIDE AND WATER VAPOR

S. I. Smolyaninov, A. V. Kravtsov

(Presented by the scientific-methodological seminar of the Faculty of Chemical Technology)

The nature of the catalyst may prove to be the most effective and, in most cases, the decisive factor determining the magnitude of the selectivity of a chemical process [1, 2].

When considering the expressions:

\[ \zeta = f(E_1; E_2; \psi) \tag{1} \]

\[ \zeta = k_0 \cdot e^{\frac{\Delta E}{RT}} \cdot C_A^{m-n} \tag{2} \]

it may be assumed that, with all other conditions constant, the selectivity of the process on a given catalyst will depend mainly, apparently, on the difference in activation energies and the difference in the kinetic orders of the reactions being compared.

The work carried out was aimed at determining the selectivity of the process of synthesizing organic compounds from carbon monoxide and water vapor with respect to the main reaction—hydrocarbon formation—on two known synthesis catalysts, an industrial cobalt–thorium catalyst and an experimental iron–copper catalyst, under various process conditions.

The synthesis of organic compounds from carbon monoxide and water vapor was carried out under isothermal conditions over a fixed catalyst bed in a flow apparatus at atmospheric pressure.

The selectivity of the indicated catalysts was calculated according to the following expression [3]:

\[ \zeta = \frac{y_{\mathrm{CH}_2}}{y_{\mathrm{H}_2}}, \quad \text{where} \tag{3} \]

\(y_{\mathrm{H}_2};\ y_{\mathrm{CH}_2}\) are the degrees of conversion of carbon monoxide to hydrogen and hydrocarbons.

The experiments were carried out while varying the following parameters: temperature, contact time, the ratio of CO to H\(_2\)O, the content of CO in the initial mixture, and the length of the catalyst bed.

The results of the work show that, under all changes in the external conditions of the process of synthesizing organic compounds from carbon monoxide and water vapor, the character of the change in selectivity with respect to the main reaction—hydrocarbon formation—for the catalysts tested is identical, although their absolute selectivity values under

under identical conditions differ significantly. As a first approximation, this permits the assumption that the mechanism of formation of hydrogen and hydrocarbons on these catalysts is unambiguous.

It has been established experimentally that the selectivity of the cobalt–thorium catalyst, within the tested limits of variation of the external conditions, is much higher than the selectivity of the iron–copper catalyst with respect to the principal synthesis reaction. This sufficiently confirms the assumption made above that not only is the difference between the activation energies of the principal and side synthesis reactions on the cobalt–thorium catalyst greater than the corresponding difference on the iron–copper catalyst ($\Delta E_{\mathrm{Co-Th}} = 34.4 - 27.6 = 6.8$ kcal/mol and $\Delta E_{\mathrm{Fe-Cu}} = 23.8 - 22.8 = 1.0$ kcal/mol, respectively), but also, probably, the difference in the kinetic orders of the principal and side synthesis reactions is greater on the cobalt catalyst than on the iron catalyst. Experimentally it has been confirmed that not only the temperature dependence of the selectivity, determined by the difference in activation energies of the reactions being compared, but also the dependence of selectivity on contact time and on the ratio of carbon monoxide to water vapor in the initial mixture—determined, with the process mechanism preserved, mainly by the difference in the orders of these reactions—is expressed much more strongly for the cobalt–thorium catalyst than for the iron–copper catalyst, while the general direction of its change is preserved.

In conclusion it should be noted that the experimentally determined ratio of selectivities of the catalysts tested: $\Delta E$; $\Delta E_2$—the differences in activation energies of the reactions being compared on Co—Th and Fe—Cu catalysts, respectively; $\Delta m$; $\Delta n$—the differences in reaction orders on these same catalysts,

\[ \frac{\zeta_{\mathrm{Co-Th}}}{\zeta_{\mathrm{Fe-Cu}}} = \kappa_0 \cdot e^{-\frac{\Delta E_1-\Delta E_2}{RT}} \cdot C_A^{\Delta m-\Delta n}, \quad \text{where} \tag{4} \]

for the principal synthesis reaction under different external conditions is equal to six at the same overall degree of conversion of carbon monoxide (Fig. 1). From Fig. 1 and expression (4) it follows that

Fig. 1

when the mechanism of formation of hydrocarbons and hydrogen is preserved over the entire range of tested conditions on both catalysts, the values of the kinetic constants differ in magnitude but are constant in value

comparison, and therefore the process can be described by a single kinetic equation for each catalyst.

When comparing the selectivity of the catalysts tested with respect to the total amount of gaseous hydrocarbons at the same degree of conversion of carbon monoxide via the main reaction, it turns out that the formation of gaseous hydrocarbons on the iron–copper catalyst proceeds much more intensely than on the cobalt–thorium catalyst, which is apparently explained by the lower polymerizing ability of iron in comparison with cobalt.

Conclusions

1. It has been shown that the character of the change in selectivity within the tested range of variation of the external conditions is identical for both catalysts.

2. It has been experimentally determined that the selectivity of the process with respect to the main reaction (hydrocarbon formation) on the Co—Th catalyst under various external conditions is, on average, 6 times higher than on the Fe—Cu catalyst at the same overall degree of conversion of carbon monoxide.

3. The selectivity of the Fe—Cu catalyst with respect to gaseous hydrocarbons is considerably higher than that of the Co—Th catalyst at the same degree of conversion of carbon monoxide via the main synthesis reaction—the formation of hydrocarbons.

Literature

1. Collection, Scientific Foundations of Catalyst Selection. “Nauka” Publishing House, 1966.
2. V. A. Roiter. Kinetics and Catalysis, vol. VIII, no. 5, 1967, 1034–1038.
3. G. K. Boreskov. Kinetics and Catalysis, vol. III, no. 4, 1962, 470–480.

Noted Misprints