EFFECT OF THE CONDITIONS OF SYNTHESIS 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. Yashko

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

In the modern direction of development of organic synthesis, one of the most important problems is the production of very pure individual substances or mixtures homogeneous in their chemical properties. The achieved level of progress in science and technology, characterized by the widespread introduction of catalytic processes, makes it possible to solve the stated tasks by developing catalysts that act with a sufficient degree of selectivity.

However, external conditions under which the process is carried out—temperature, pressure, the ratio between reagents, contact time, etc.—may be no less important in achieving the required selectivity of the process; in a number of cases these conditions 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 rather clearly between groups of useful and undesirable 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 the conversion of carbon monoxide by water vapor, the Boudouard reaction, and the methane-formation reaction. The conditions ensuring minimal occurrence of the above-mentioned reactions have been found more or less definitely. On the other hand, the study of parameters affecting the qualitative composition of the target synthesis product is of theoretical and practical interest, both in terms 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 the known works, only certain general regularities of changes in product composition with changes in the temperature and pressure of the process, as well as dependences on the catalyst used, have been determined.

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 can be defined as the ratio of the amount of carbon monoxide that has gone into the formation of a given product to the total amount of converted carbon monoxide.

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 water-vapor saturation unit—a saturator-bubbler, from which it passed through a heated gas line into the reactor. The feedstock and products obtained were analyzed on KhT-2M and UKh-1 chromatographs. Individual hydrocarbons up to \(C_{10}\), the total hydrocarbon yield, 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 \(200\text{–}240^\circ\mathrm{C}\), space velocity \((6\text{–}5\ \mathrm{h}^{-1})\), 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 length of the catalyst bed within the range from 26 to 200 cm. In all cases, except for specially arranged series of experiments, the ratio of carbon monoxide to water vapor was taken as \(3:1\), and inert impurities in the initial synthesis gas were absent.

Effect of Temperature

The degree to which temperature affects 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 temperature has only a slight effect on the diffusion rate. 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, at 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 was shown in the cited work of Elovich [1].

Figure 1 presents the experimental data we obtained for the selectivity of formation of several individual hydrocarbons and their mixtures. It is interesting to note the abrupt change in the relative yields of unsaturated and saturated hydrocarbons at synthesis temperatures above \(230^\circ\mathrm{C}\).

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, it should be assumed that the temperature coefficient of the rate of formation of the methylene radical is greater than that of its polymerization.

Effect of Space Velocity

This factor has a substantial influence on the selectivity of the process for consecutive reactions proceeding in the kinetic region. Under conditions where the process rate is determined by diffusion stages, the space velocity can affect selectivity both for parallel and for consecutive reactions when the orders of the reactions being compared differ; 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 synthesis selectivity. 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 relationships may apparently 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 considerable 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 feed ratio \(3\mathrm{CO} : 1\mathrm{H_2O}\) is of no practical interest because the valuable gaseous component is used insufficiently; 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 total degree of conversion increases, and with it the formation of methane, which is reflected especially in a decrease in the degree of liquefaction.

A noticeable influence of the carbon monoxide–water vapor ratio on the selectivity of the process is possible primarily when there is a difference in the orders of the reactions being compared as they proceed 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 some interest. If the slight change in the relative yields of gaseous and higher hydrocarbons may apparently be—

mu, is explained by some change in the volumetric rate of synthesis gas (in our experiments the volumetric rate with respect to carbon monoxide was kept constant, irrespective of its ratio to water vapor), then the abrupt 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 in the conversion of carbon monoxide even at a very high ballast content in the synthesis gas.

At a constant volumetric rate 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. (Nitrogen was used by us 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

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. Volumetric rate with respect to carbon monoxide—12 h\(^{-1}\), synthesis temperature—240°C, catalyst-bed length—200 cm.

hydrocarbons show that their changes as a function of the factor under consideration must, as it seems to us, be explained by the difference in the kinetic orders of these reactions.

Thus, under the conditions of the experiments we carried out, a noticeable influence 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 permit the hope that changing the external conditions under which the synthesis process is carried out may 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 has been established in the selectivity for the formation of saturated and unsaturated hydrocarbons.

3. By changing the external conditions under which the process is carried out, it is possible, to a certain extent, to regulate the composition of the final synthesis products.

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