PHYSICAL CHEMISTRY

Academician A. A. BALANDIN, A. A. TOLSTOPYATOVA, and V. STIZHEVSKII

ON THE CATALYTIC ACTIVITY OF TUNGSTEN PENTOXIDE

The kinetics of the reaction of dehydration of alcohols—ethyl, isopropyl, n-butyl, tert-butyl, cyclohexanol—and of the dehydrogenation of methyl alcohol and tetralin on \(W_2O_5\) under isothermal conditions was studied in this work. The catalyst—blue tungsten oxide \(W_2O_5\)—was obtained from tungstic acid by calcination at \(350—400^\circ\) in a stream of air to \(WO_3\); the latter was reduced to \(W_2O_5\) during the reaction with alcohols at the experimental temperature \(200—300^\circ\). The apparatus was an ordinary flow unit with automatic feeding of the initial liquid substances. Gas was supplied and gaseous products were collected with the aid of an automatic Patrikeev gas meter. The experiments were carried out at low conversions, not more than 30%, in the kinetic region. The gaseous catalyzate was analyzed on a VTI apparatus and by chromatography. In the liquid catalyzate the amount of dissolved unsaturated hydrocarbons was determined by the Kauffman–Halpern method. After careful purification the initial substances had constants coinciding with those in the literature.

Table 1

Apparent activation energies for the dehydration of alcohols on tungsten oxide under isothermal conditions

* A mixture of cyclohexanol with cyclohexene was passed.

In carrying out experiments on \(W_2O_5\) it was observed that the thermal effect of the endothermic reaction of alcohol dehydration strongly affects the results of kinetic measurements. To eliminate this phenomenon, the catalyst, 2 g (2 ml) (2 parts), was diluted with pieces of quartz (3 parts), whose size corresponded to the size of the catalyst grains. The alcohol was diluted with the reaction product—water or the corresponding unsaturated hydrocarbon. Thus, for example, in determining the relative adsorption coefficients of water, three aqueous solutions with molar alcohol contents of 80, 65, and 54% were prepared. Such dilution of the catalyst and alcohol ensured practical isothermicity of the process. By this method, under isothermal conditions, the apparent activation energies of the dehydration of the alcohols ethyl, isopropyl, tertiary butyl, and cyclohexanol were determined, Table 1. A regularity is observed in the values of the apparent activation energies as a function of the structure of the alcohol. Primary alcohols, ethyl and n-butyl, are dehydrated with the same activation energy, equal to \(\sim 30\) kcal/mol. The activation energy of the secondary alcohol (isopropyl) is \(\sim 6\) kcal/mol lower than that of the primary, and the activation energy of tertiary butyl alcohol is \(\sim 6\) kcal/mol lower than that of the secondary. The difference in the activation energies of dehydration of primary, secondary, and tertiary alcohols on \(W_2O_5\) amounts to…

is approximately 6 kcal/mol. The regularity obtained by us is close to that obtained by Adadurov and Krainii [^2], who, in studying the dehydration of alcohols on blue tungsten oxide in a flow system, observed that a methyl group in the α-position lowers the activation energy of alcohol dehydration by 5.5 kcal/mol.

To calculate the true rate constants of the reactions, \(K\), equation (1) was used, following from the general kinetic equation of A. A. Balandin [^3] for monomolecular heterogeneous-catalytic reactions in a flow. For the case of binary mixtures alcohol—reaction product, this equation has the form:

\[ K=(z_2N+z_3A_1)\ln\frac{A_1}{A_1-m}-(z_2+z_3-1)m, \tag{1} \]

where \(A_1\) is the volumetric rate of the alcohol (in ml/min, recalculated to gas at NTP), \(N\) is the total amount of alcohol and reaction products in the initial mixture (in ml/min, recalculated to gas at NTP), \(m\) is the amount of evolved unsaturated hydrocarbons in experiments with binary mixtures (in ml/min); \(z_2\) and \(z_3\) are the relative adsorption coefficients of water and unsaturated hydrocarbons, respectively, i.e., the ratio of the adsorption coefficient of the reaction product to the adsorption coefficient of the initial alcohol; \(z_2\) and \(z_3\) were calculated by formula (2), also following from the general kinetic equation of A. A. Balandin [^3]

\[ z=\left(\frac{m_0}{m}-1\right)\Big/\left(\frac{100}{P}-1\right), \tag{2} \]

where \(m\) is the amount of unsaturated hydrocarbons evolved per unit time in experiments with a binary mixture (alcohol—water, alcohol—unsaturated hydrocarbon); \(m_0\) is the calculated value of \(m\) for the pure alcohol; \(P\) is the molar percentage of alcohol in the binary mixture.

For the calculation of \(z_2\) and \(z_3\) by formula (2), not the experimental but the computed \(m_0\) was substituted. Using the fact that \(z\) is a constant quantity at a given temperature and does not depend on the percentage composition of the mixture, we substitute into formula (2) the value \(P_1\) and \(m_1\), found from experiment, for one mixture and then \(P_2\) and the corresponding \(m_2\) for another mixture and, solving both equations simultaneously with respect to \(m_0\), obtain the following formula for calculation:

\[ m_0=(1-a)\Big/\left(\frac{1}{m_1}-\frac{a}{m_2}\right),\quad \text{where } a=\left(\frac{100}{P_1}-1\right)\Big/\left(\frac{100}{P_2}-1\right). \tag{3} \]

Since the work was carried out with three mixtures of each reaction product with the alcohol, a total of three equations of type (3) could be compiled,

Table 2

Relative adsorption coefficients \(z_2\) and \(z_3\) of the products of alcohol dehydration under isothermal conditions

Table 3

Verification of the applicability of kinetic equation (1) to the reaction of alcohol dehydration

which made it possible to calculate the mean value of \(m_0\) for each temperature. Table 2 presents the found values of the relative adsorption coefficients of the reaction products of the dehydration of isopropyl and \(n\)-butyl alcohols under isothermal conditions. The relative adsorp-

tion coefficients of water, propylene, and butylene in the temperature interval studied do not depend on temperature. The applicability of equation (1) to the reaction of dehydration of alcohols is confirmed by the fact that, when the experimental values of \(m\) obtained at a given temperature for mixtures of different percentage composition at constant catalyst activity were substituted into it, the calculated values of the rate constants of the dehydration reaction proved to be practically identical (Table 3). Table 4

Table 4

Dehydration of alcohols on tungsten oxide under isothermal conditions

gives the main results of kinetic determinations for the dehydration of isopropyl and \(n\)-butyl alcohols. From the data of Table 4 it is seen that the true activation energy exceeds the apparent one by approximately 3 kcal/mol.

The relative adsorption coefficients \(z\) are equilibrium constants of the process of adsorption displacement of the initial substance by the products of its conversion from the catalytically active part of the surface. In the case of dehydration of alcohols, these are processes of displacement of the alcohol by water and the corresponding unsaturated hydrocarbon. Therefore, by the usual thermodynamic formulas it is easy to calculate for this process the change in free energy \(\Delta F\), entropy \(\Delta S\), and heat content \(\Delta H\) (i.e., the heats of adsorption displacement with the opposite sign).

From the independence of the relative adsorption coefficients of water and unsaturated hydrocarbons from temperature, which is observed in our case, it follows that the corresponding values of \(\Delta H\) for them will be equal to zero. Since the heats of adsorption displacement of the alcohol by water are identical (equal to zero), the values of the heats of adsorption of the alcohols themselves must be equal to one another. Hence it follows that both alcohols, different in their structure, during dehydration are oriented toward the catalytically active part of the surface of \(\mathrm{W_2O_5}\) identically, by the common group for each alcohol

\[ \begin{matrix} >C—C<\\ \ \ \ \vert\ \ \ \vert\\ \mathrm{H}\ \ \mathrm{O}— \end{matrix} \]

so that each of these atoms proves to be bound by adsorption forces to the catalytically active surface of \(\mathrm{W_2O_5}\).

Determination of the bond energies of the reacting atoms C, H, and O of molecules with the catalytically active surface of \(\mathrm{W_2O_5}\). The bond energies were determined by the kinetic method of Balandin \((^{4})\). For this purpose, on \(\mathrm{W_2O_5}\) the activation energy of dehydrogenation of tetralin, \(\varepsilon_1 = 26.8\) kcal/mol, was determined in the interval 449—491°, and the activation energy of dehydrogenation of methyl alcohol, \(\varepsilon_2 = 24.4\) kcal/mol, in the interval 380—420°C. For calculation of the bond energies \((^{5,6})\) \(Q_{\mathrm{HK}}\), \(Q_{\mathrm{OK}}\), \(Q_{\mathrm{CK}}\) according to

Table 5

Bond energies of the reacting atoms and molecules with the surface of the catalyst \(\mathrm{W_2O_5}\)

| Name of alcohol | Activation energy of dehydration of alcohols | \multicolumn{3}{c}{Values of bond energies} |
|---|---:|---:|---:|---:|
| Name of alcohol | Activation energy of dehydration of alcohols | \(Q_{\mathrm{HK}}\) | \(Q_{\mathrm{CK}}\) | \(Q_{\mathrm{OK}}\) |
| \(n\)-Butyl | 29.9 | 56.7 | 15.9 | 39.2 |
| Ethyl | 29.4 | 56.4 | 16.3 | 39.5 |
| Isopropyl | 23.7 | 52.6 | 19.4 | 43.3 |
| Cyclohexanol | 21.9 | 51.5 | 21.3 | 44.4 |
| tert-Butyl | 17.8 | 48.7 | 24.0 | 47.3 |

in formulas (4), in addition to the quantities \(\varepsilon_1\) and \(\varepsilon_2\), the values of the activation energies of dehydration \(\varepsilon_3\) of primary, secondary, and tertiary alcohols were also used, Table 1. This made it possible to study the influence of the structure of the dehydrating alcohols on the values of the bond energies (the bond energies between the atoms \(Q_{\mathrm{CH}}, Q_{\mathrm{CO}}, Q_{\mathrm{OH}}\) were taken from Cottrell’s data \((^7)\))

\[ Q_{\mathrm{HK}}=\frac{1}{3}(-\varepsilon_1-2\varepsilon_2+2\varepsilon_3)+62; \]

\[ Q_{\mathrm{CK}}=\frac{1}{3}(-\varepsilon_1+2\varepsilon_2-2\varepsilon_3)+28.5; \tag{4} \]

\[ Q_{\mathrm{OK}}=\frac{1}{3}(3\varepsilon_1-2\varepsilon_2-2\varepsilon_3)+48.6. \]

It turned out that, upon successive replacement by methyl radicals of the hydrogen atoms at the carbon atom bonded to the hydroxyl group of the alcohol, the quantities \(Q_{\mathrm{OK}}\) and \(Q_{\mathrm{CK}}\) increase, while \(Q_{\mathrm{HK}}\) decreases (Table 5: ethyl, isopropyl, tert-butyl alcohols). It should be noted that both hydrogen atoms and the methyl groups replacing them do not participate directly in the reaction, i.e., here there is an influence of remote substituents \((^8)\) on the magnitudes of the bond energies of C, H, and O with the catalyst,

Moscow State University
named after M. V. Lomonosov

Received
17 V 1960

CITED LITERATURE

1. G. D. Galpern, Tr. Inst. nefti, 4, 141 (1954).
2. I. E. Adadurov, P. Ya. Krainii, ZhFKh, 5, 1125 (1934).
3. A. A. Balandin, ZhFKh, 31, 745 (1957).
4. A. A. Balandin, ZhOKh, 16, 793 (1946).
5. A. A. Balandin, A. A. Tolstopyatova, ZhFKh, 30, 1367, 1636 (1956).
6. A. A. Tolstopyatova, A. A. Balandin, Problems of Kinetics and Catalysis, X. Physicochemical Catalysis, USSR Academy of Sciences Press, 1960, p. 351.
7. T. Cottrell, The Strength of Chemical Bonds, IL, 1956.
8. A. A. Balandin, Uch. zap. MGU, issue 175, 97 (1956).