EFFECT OF THE CONDITIONS OF SYNTHESIS OF ORGANIC COMPOUNDS FROM CARBON MONOXIDE AND WATER VAPOR ON THE SELECTIVITY OF AN IRON–COPPER CATALYST

S. I. SMOLYANINOV, A. V. KRAVTSOV

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

In the present direction of development of organic synthesis, one of the most important problems is the preparation of very pure individual substances or mixtures homogeneous in their chemical properties. The level of progress achieved in science and technology, characterized by the widespread introduction of catalytic processes, makes it possible to solve the tasks posed by developing catalysts that act with the required degree of selectivity. However, the external conditions under which a process is carried out—temperature, pressure, ratio between reactants, contact time, etc. [1, 2; 3; 4]—may be of no less importance in attaining the necessary selectivity of the process, and in a number of cases they determine success entirely. The synthesis of organic compounds from carbon monoxide and water vapor, the development of which is receiving attention both in the Soviet Union and abroad [4; 6; 10], is a complex chemical process. Along with the principal reaction of formation of organic compounds, a number of side reactions also proceed: conversion of carbon monoxide by water vapor, the Boudouard reaction, and methane formation. Conditions ensuring minimal occurrence of the above-mentioned side reactions have been found more or less definitely.

On the other hand, it is of theoretical and practical interest to study the parameters that influence the qualitative composition of the target synthesis product, both from the standpoint of enriching it with certain groups of substances and with respect to the maximum yield of individual compounds. No special studies devoted to this question have been published.

In synthesis from carbon monoxide and water vapor over an iron–copper catalyst, gaseous, liquid, and solid aliphatic hydrocarbons of normal structure are formed predominantly [5, 6]. The selectivity of the process, generally defined as the ratio of the rates of the target and side reactions, in the present case may be determined through the ratio of the amount of carbon monoxide consumed in the formation of the given product to the total amount of converted carbon monoxide [11].

Experimental Part

The synthesis from carbon monoxide and water vapor was carried out at atmospheric pressure over a stationary iron–copper catalyst.

under isothermal conditions. Carbon monoxide was obtained by decomposing carbon dioxide over incandescent coal according to the reaction:

\[ \mathrm{CO_2 + C \to 2CO}. \]

The carbon monoxide obtained was purified from unreacted \(\mathrm{CO_2}\) with Ascarite and was fed to the unit for saturation with water vapor—a saturator-bubbler, from which it passed through a heated gas line into the reactor. Analysis of the starting material and of the products obtained was carried out completely on an KhT-2M chromatothermograph and a UKh-1 chromatograph. The experimental data were treated from the standpoint of the effect of various factors on the integral selectivity of the synthesis process.

1. The effect of temperature on the selectivity of the process is manifested because the temperature coefficients of the reactions occurring simultaneously in this process differ. The experimental data obtained by us, presented in Fig. 1, show that the yield of gaseous hydrocarbons increases with increasing synthesis temperature. When the main reaction (hydrocarbon formation) and the side reaction (hydrogen formation) are compared, it turns out that the selectivity of the process with respect to the main reaction increases under all conditions.

2. The effect of the contact time of the feedstock with the catalyst surface has a substantial influence on the selectivity of the process

Fig. 1. Dependence of the selectivity of the synthesis process on temperature. Curve 1—selectivity to ethylene, curve 2—selectivity to propylene, curve 3—selectivity to ethane, curve 4—selectivity to gaseous hydrocarbons, curve 5—selectivity with respect to the hydrogen-formation reaction.

Fig. 2. Dependence of the selectivity of the synthesis process on space velocity. Curve 1—selectivity to ethylene, curve 2—selectivity to propylene, curve 3—selectivity to ethane, curve 4—selectivity to gaseous hydrocarbons, curve 5—selectivity with respect to the hydrogen-formation reaction.

in the case where the orders of the reactions being compared differ [3]. The selectivity with respect to the main reaction increases markedly with increasing contact time, and when comparing the yield of gaseous and higher hydrocarbons in the synthesis from carbon monoxide and water vapor, it turns out, as is evident from Fig. 2, that the dependence of the integral selec-

…of the synthesis process with respect to gaseous hydrocarbons on the contact time is insignificant and has a certain minimum at the middle of the contact times being compared. This leads to the assumption that the reactions forming gaseous and higher hydrocarbons are consecutive and that their orders differ only slightly.

3. Effect of the ratio of carbon monoxide to water vapor, i.e., the mole fraction of carbon monoxide in the starting feed, will affect the selectivity of the process only if the orders of the reactions occurring in the process are different. Our data show that the selectivity of the process with respect to gaseous and higher hydrocarbons changes only slightly when the CO : H₂O ratio is changed from 2 : 1 to 4 : 1.

Increasing the water-vapor content in the initial synthesis gas promotes an increase in the fraction of carbon monoxide, out of the total amount of reacted carbon monoxide, that goes to the water-gas-shift reaction.

4. Increasing the content of inert impurities in the initial synthesis gas should lead to a change in the selectivity of the process when the orders of the reactions being compared differ and because of changes in the conditions of transport of the initial feed to the catalyst surface.

For the present synthesis, dilution of the initial feed with nitrogen led to a decrease in selectivity with respect to the principal hydrocarbon-formation reaction.

The amount of carbon monoxide that went to the formation of gaseous hydrocarbons increased as the dilution of the initial feed with nitrogen increased.

5. Effect of the linear velocity of the starting reactants on the overall indices of the synthesis was studied by us in a previous work [9].

The selectivity of catalyst operation will also depend on the linear velocity of the initial gas, since as the latter decreases diffusion complications will increase [12].

Fig. 3. Dependence of the selectivity of the synthesis process on the fictitious linear velocity. Curve 1—selectivity with respect to ethylene; curve 2—selectivity with respect to propylene; curve 3—selectivity with respect to ethane; curve 4—selectivity with respect to gaseous hydrocarbons; curve 5—selectivity with respect to the water-gas-shift reaction.

As is seen from Fig. 3, the integral selectivity with respect to the water-gas-shift reaction increases as the linear velocity of the synthesis gas decreases, while that for the reaction forming gaseous hydrocarbons decreases. The latter argues in favor of a consecutive mechanism for the formation of gaseous and higher hydrocarbons.

Thus, the investigations carried out have experimentally confirmed that changing the conditions for conducting the chemical process, in particular the synthesis of organic compounds from carbon monoxide and water vapor, can substantially affect the selectivity of the process with respect to the target product.

References

1. Problems of Kinetics and Catalysis. VI. Heterogeneous Catalysis. Publishing House of the Academy of Sciences of the USSR, 375–404, 1949.

2. Catalysis. Questions of Theory and Methods of Investigation. IL Publishing House, Moscow, 551–560, 1955.

3. S. L. Kiperman. Introduction to the Kinetics of Heterogeneous Catalytic Reactions. Nauka, Moscow, 385–444, 1964.

4. P. V. Zimakov. Uspekhi khimii, 28, No. 11, 1343–1352, 1959.

5. H. Kölbel, H. Hammer. Z. für der Bunsges. für physikalische Chemie, Bd. 64, No. 2, 224–234, 1960.

6. I. F. Bogdanov, A. M. Mosin, M. A. Samarskaya. Transactions of IGI, Academy of Sciences of the USSR, Moscow, 1958.

7. D. I. Orochko. Theoretical Foundations of Conducting the Synthesis of Liquid Fuels. Gostoptekhizdat, Moscow, 1951.

8. S. I. Smolyaninov, A. V. Kravtsov, V. M. Mironov. Proceedings of the Tomsk Polytechnic Institute, 1966 (in press).

9. S. I. Smolyaninov, V. M. Mironov, A. V. Kravtsov. Chemistry and Chemical Technology of Fuels and Oils, No. 8, 12–17, 1962.

10. W. Wolski. Przem. chemie, 44, No. 3, 135–138, 1965.

11. G. K. Boreskov. Kinetics and Catalysis. Vol. III, issue 4, pp. 470–480, 1962.

12. V. A. Roiter, S. S. Gaukhman. The influence of macrokinetic factors on the selectivity of catalytic processes. Proceedings of the All-Union Conference on Catalysis. Publishing House of the Academy of Sciences of the Ukrainian SSR, 161–163, 1950.