EFFECT OF THE SYNTHESIS CONDITIONS OF ORGANIC COMPOUNDS FROM CARBON MONOXIDE AND WATER VAPOR ON THE SELECTIVITY OF A COBALT–THORIUM CATALYST

S. I. Smolyaninov, A. V. Kravtsov, V. N. Yatsko

(Presented by the Scientific-Methodological Seminar of the Faculty of Chemical Technology)

In the modern trend 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 the process is carried out—temperature, pressure, ratio between reactants, contact time, etc.—may be no less important in achieving the necessary selectivity of the process; in a number of cases they entirely determine success [1, 2, 3, 4, 10, 18].

Despite the conventionality involved in defining a side reaction, in some organic processes it is possible to distinguish fairly clearly between groups of useful and unwanted reaction products. In 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 [5, 6, 8, 9, 11, 12, 13], the predominant side reactions are: conversion of carbon monoxide by water vapor, the Boudouard reaction, and the methanation reaction. Conditions ensuring minimal occurrence of the above-mentioned reactions have been found more or less definitely. On the other hand, it is of theoretical and practical interest to investigate the parameters that affect the qualitative composition of the target synthesis product, both in terms of enriching it with particular groups of substances and with respect to the maximum yield of individual compounds. No special studies devoted to this question have been published. In the known works, only certain general regularities have been determined for changes in product composition with changes in the temperature and pressure of the process, as well as dependences on the catalyst used.

In synthesis from carbon monoxide and water vapor over a cobalt–thorium catalyst, practically pure gaseous, liquid, and solid aliphatic hydrocarbons of normal structure are formed [5, 6].

The selectivity of the process (we use the concept of integral selectivity [14, 15]) for this synthesis may be defined as the ratio of the amount of carbon monoxide consumed in the formation of a given product to the total amount of carbon monoxide converted.

The synthesis was carried out at atmospheric pressure over a stationary catalyst under isothermal conditions. Carbon monoxide was purified with ascarite and fed to the unit for saturation with water vapor—a saturator-bubbler—from which it passed through a heated gas line into the reactor. The starting feedstock and the products obtained were analyzed on KhT—2M and UKh—1 chromatographs. Individual hydrocarbons up to \(C_{10}\), the total yield of hydrocarbons, and the yields of carbon dioxide and hydrogen were determined. From the data obtained, the total yields of gaseous and higher hydrocarbons were calculated. The following were studied: the effect of temperature in the range 200—240°C, space velocity \((6—5\ \text{h}^{-1})\), the ratio of carbon monoxide to water vapor (from 2 : 1 to 4 : 1), the concentration of carbon monoxide in the initial gas (from 30 to 100%), and the catalyst-bed length within the range from 26 to 200 cm. In all cases, except for specially designed series of experiments, the ratio of carbon monoxide to water vapor was taken as 3 : 1, and inert impurities were absent from the initial synthesis gas.

Effect of Temperature

The degree of influence of temperature on the selectivity of parallel reactions is determined, for the kinetic region, by the difference in their temperature coefficients; in the diffusion region this difference should be minimal because of the insignificant effect of temperature on the rate of diffusion. For some types of parallel reactions, a theoretical analysis of the effect of temperature on selectivity is presented in the work of Elovich [1].

For consecutive reactions, an increase in temperature, for any degree of temperature dependence of the rates of the intermediate stages, should apparently increase the yield of the final reaction products. In reality, these phenomena may proceed in a considerably more complex form, as is also shown in the cited work of Elovich [1].

Figure 1 presents the experimental data obtained by us for the selectivity of formation of certain individual hydrocarbons and their mixtures. It is interesting to note the sharp change in the relative yields of unsaturated and saturated hydrocarbons at synthesis temperatures above 230°C.

The increase in the yield of gaseous hydrocarbons can be explained if it is assumed that the primary act of synthesis is the formation of high-molecular-weight hydrocarbons with their subsequent conversion into low-molecular-weight ones. As is known, a similar reaction mechanism was proposed by Craxford (16). In this case, one must assume a larger temperature coefficient for the rate of the methylene-radical formation reaction than for its polymerization.

Effect of Space Velocity

This factor has a substantial influence on the selectivity of the process for consecutive reactions occurring in the kinetic region. Under conditions in which the rate of the process is determined by diffusion stages, the space velocity may affect selectivity both for parallel and for consecutive reactions in the case of a difference in the orders of the reactions being compared; this is apparently connected not with a change in the contact time proper, but with a change in mass transfer when the flow rate of the reactants is varied.

Figure 2 presents experimental data for a number of synthesis products. The increase in the relative yield of gaseous hydrocarbons with increasing space velocity can be explained from the standpoint of the secondary origin of these substances.

Fig. 1. Dependence of the selectivity of the synthesis process on temperature. Curve 1—ethylene, 2—propylene, 3—ethane, 4—propane, 5—sum of higher hydrocarbons, 6—sum of gaseous hydrocarbons. Space velocity—25 h\(^{-1}\), catalyst-bed length 200 cm.

Fig. 2. Effect of the space velocity of carbon monoxide on the selectivity of synthesis. Curve 1—ethylene, 2—propylene, 3—ethane, 4—propane, 5—sum of higher hydrocarbons, 6—sum of gaseous hydrocarbons. Synthesis temperature—220°C, catalyst-bed length—200 cm.

Similar interrelations may apparently also exist between the formation of saturated and unsaturated hydrocarbons.

Effect of the ratio of carbon monoxide to water vapor

The ratio of the partial pressures of carbon monoxide and water vapor in the synthesis gas has a significant effect both on the course of the reaction and on the stability and service life of the catalyst.

G. Kölbel and co-workers [2] showed that increasing the carbon monoxide content above the theoretical consumption ratio \(3\mathrm{CO} : 1\mathrm{H_2O}\) is of no practical interest because of insufficient utilization of the valuable gaseous component; moreover, this leads to carbon deposition, which affects side reactions and the distribution of products. With an increase in the partial pressure of water vapor, the overall degree of conversion increases, and with it the formation of methane, which has an especially marked effect on lowering the degree of liquid formation.

A noticeable influence of the ratio of carbon monoxide and water vapor on the selectivity of the process is possible primarily when there is a difference in the orders of the reactions being compared during operation in the kinetic region.

Fig. 3. Dependence of the selectivity of the synthesis process on the ratio \(\mathrm{CO} : \mathrm{H_2O}\) in the initial synthesis gas. Curves: 1—ethylene, 2—propylene, 3—ethane, 4—propane, 5—sum of higher hydrocarbons, 6—sum of gaseous hydrocarbons. Space velocity—25 h\(^{-1}\), synthesis temperature—240°C, catalyst-bed length—200 cm.

The experimental data obtained by us (Fig. 3) are of definite interest. If the slight change in the relative yields of gaseous and higher hydrocarbons may apparently be ...

... may be explained by some change in the space velocity of the synthesis gas (in our experiments the space velocity with respect to carbon monoxide was kept constant, irrespective of its ratio to water vapor), then the sharp change in the selectivity of the process with respect to saturated and unsaturated hydrocarbons indicates a more complex dependence of the mechanism of their formation.

Effect of the Composition of the Initial Gas

Synthesis from carbon monoxide and water vapor differs from the well-known Fischer–Tropsch process by the conversion of carbon monoxide even when the synthesis gas contains a very large amount of ballast.

At a constant space velocity with respect to carbon monoxide, an increase in the content of inert impurities in the initial gas will lead to an increase in the linear velocity of the reaction mixture and, consequently, to an increase in the rate of mass transfer. (We used nitrogen as the inert diluent.) This circumstance should probably explain the change in the selectivity of the process with respect to certain reaction products (Fig. 4). However, the relative yields of saturated and unsaturated hydrocarbons show that their changes as a function of the factor under consideration must, in our view, be explained by the difference in the kinetic orders of these reactions.

Fig. 4. Dependence of selectivity on the carbon monoxide content in the initial gas. Curve 1—ethylene, 2—propylene, 3—ethane, 4—propane, 5—sum of higher hydrocarbons, 6—sum of gaseous hydrocarbons. Space velocity with respect to carbon monoxide—12 h\(^{-1}\), synthesis temperature—240°C, catalyst bed length—200 cm.

Thus, under the conditions of the experiments we carried out, a noticeable effect of a number of factors on the selectivity of formation of individual products of synthesis from carbon monoxide and water vapor was found. The obtained

The results give grounds for hoping that changing the external conditions under which the synthesis process is carried out can be used in practice to regulate, within known limits, the composition of the final reaction products.

Conclusions

1. The influence of temperature, contact time, the ratio of the reacting components, and the content of inert impurities in the synthesis gas on the selectivity of formation of certain individual compounds and groups in the process of synthesis from carbon monoxide and water vapor has been shown.

2. A sharp change in selectivity with respect to the formation of saturated and unsaturated hydrocarbons has been established.

3. By changing the external conditions under which the process is carried out, the composition of the final synthesis products can be regulated to a certain degree.

References

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

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

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

4. J. J. Carberry. “Chemical and Process Engineering,” vol. 44, No. 6, 1963, 306–315.

5. H. Kölbel, H. Hammer. “Zeitschrift für Electrochemie,” 64, 1960, 224.

6. N. V. Lavrov, M. A. Samarskaya. Transactions of the Institute of Fossil Fuels, Academy of Sciences of the USSR, vol. XI, 1959, 100–104.

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

8. S. I. Smolyaninov, A. V. Kravtsov, V. I. Odintsov. On certain secondary reactions occurring in the synthesis of organic compounds from carbon monoxide and water vapor. Proceedings of TPI, 1965 (accepted for publication).

9. S. I. Smolyaninov, V. M. Mironov, A. V. Kravtsov. Influence of hydrodynamic conditions on the synthesis of organic compounds from carbon monoxide and water vapor. Chemistry and Technology of Fuels and Oils, No. 8, 1962, 12–17.

10. P. V. Zimakov. Uspekhi Khimii, 28, II, 1959, 1343–1352.

11. G. Kölbel, E. Förverk. Chemistry and Chemical Technology, II, 1957, 122–139.

12. I. F. Bogdanov, A. M. Mosin, M. A. Samarskaya. Transactions of the Institute of Fossil Fuels, Academy of Sciences of the USSR, 1958.

13. G. Kölbel, I. Kuchel, G. Hammer. “Annalen der Chemie,” 6, 1–3, 1960, 8–10.

14. G. Boreskov. Kinetics and Catalysis, III, 4, 1962, 470–480.

15. V. A. Roiter. Introduction to the Kinetics of Chemical Reactions. Publishing House of the Ukrainian Academy of Sciences of the USSR, 1960.

16. G. Storch, N. Golambik, R. Anderson. Synthesis of Hydrocarbons from Carbon Monoxide and Hydrogen. Foreign Literature Publishing House, Moscow, 1954.

17. D. A. Frank-Kamenetskii. Diffusion and Heat Transfer in Chemical Kinetics. Publishing House of the Academy of Sciences of the USSR, 1947.

18. M. Spielman. “A. I. Ch. E. Journal,” 10, No. 4, 1964, 496–501.