PHYSICAL CHEMISTRY

Academician A. A. BALANDIN, O. K. BOGDANOVA, and A. P. SHCHEGLOVA

ON THE FREE ENERGY, HEAT, AND ENTROPY OF ADSORPTIVE DISPLACEMENT OF ALCOHOLS BY WATER FROM THE SURFACE OF AN OXIDE CATALYST

In works \((^{1})\) the kinetics of dehydrogenation of a series of alcohols over a mixed oxide catalyst was investigated. It was shown that the dehydrogenation reaction proceeds on the catalyst without appreciable formation of products of decomposition and dehydration of the alcohols. The relative adsorption coefficients of the reaction products were determined. It was found that the relative adsorption coefficient of hydrogen is practically equal to zero, while the relative adsorption coefficients of aldehydes are a function of temperature and decrease with increasing temperature.

Table 1

Dehydrogenation of binary alcohol—water mixtures

In the present work the kinetics of dehydrogenation of binary mixtures of primary alcohols of normal structure with water over the same oxide catalyst was investigated. The aim of the work was to determine the relative adsorption coefficients of water and to study the dependence of these quantities on the length of the carbon chain of the alcohol. Investigation of the adsorption coefficients is of interest from the point of view of the orientation of molecules on the surface. The following alcohols were taken for the work: n-propyl, n-butyl, and n-hexyl. Before the experiment the alcohols were dried with magnesium methylate \((^{2})\) and fractionated on a column. The alcohols had the following constants: n-propyl alcohol, b.p. 97–97.2°, \(d_4^{20}\) 0.8044, \(n_D^{20}\) 1.3858; n-butyl alcohol, b.p. 117–117.5°, \(d_4^{20}\) 0.8098, \(n_D^{20}\) 1.3992; n-hexyl alcohol, b.p. 155.1–155.8°, \(d_4^{20}\) 0.8196, \(n_D^{20}\) 1.4188.

The initial alcohol—water mixtures were obtained by adding water to a weighed amount of alcohol. In preparing the mixtures, the limited mutual solubility of the components imposed a limit on the choice of concentrations. In the experiments with n-propyl and n-butyl alcohols the mixtures contained 34.2 mol.% water, and in the experiments with n-hexyl alcohol, 17.1 mol.% water. The experiments were carried out by the flow method under the same conditions as in works \((^{1})\). The reaction rate was determined from the evolution of hydrogen, which occurred

at a constant rate; as analysis showed, the gas evolved contained a small amount of CO₂ and unsaturated hydrocarbons, not exceeding 2% (Table 1). The experiments were carried out in the temperature range 300–360° with a mixture flow rate of 1.05 ml in 5 min for mixtures of propyl and butyl alcohols, and 1.4 ml in 5 min for mixtures of hexyl alcohol.

Table 2

Dehydrogenation of binary alcohol–water mixtures. Relative adsorption coefficients of water, calculated by formula (1)

The data obtained are given in Table 1 and arranged in order of increasing temperature. Table 2 gives the values of the relative adsorption coefficients of water, $z_4$, calculated from the experimental data by formula (3):

\[ z_4=\left(\frac{m_0}{m}-z\right)\bigg/\left(\frac{100}{p}-1\right). \tag{1} \]

As can be seen from the data presented, the relative adsorption coefficient of water depends on temperature; as the temperature rises, its value decreases. Comparing the rates of alcohol dehydrogenation at the same temperature, observed with alcohol–water mixtures, with the rates obtained with pure alcohol ($m$ and $m_0$, Table 2), one can see that the addition of water reduces the rate of alcohol dehydrogenation by more than 45% at a temperature of 320°. With increasing temperature the inhibiting effect of water gradually decreases. Thus, one may conclude that water vapors are adsorbed by the catalyst, and the more strongly the lower the temperature. The adsorption coefficients of water on the active centers of the catalyst in the temperature interval studied prove to be 3.5–1.3 times greater for water than for alcohol. Figure 1 shows the values of $z_4$ for the alcohols studied at different temperatu—

Fig. 1. Value of $z_4$ as a function of temperature for mixtures of different alcohols with water: 1 — n-propyl alcohol; 2 — n-butyl alcohol; 3 — n-hexyl alcohol.

... vapors. As is seen from Table 2, the values of the relative adsorption coefficients of water, calculated from the experimental data for n-propyl, n-butyl, and n-hexyl alcohols, have values close to one another. Applying to the data obtained the formula derived earlier \(^{(4,5)}\)

\[ \Delta F = -RT \ln z_4, \tag{2} \]

we find the free energy of the adsorption displacement of alcohol by water from the active surface of the catalyst (see Table 2). In the coordinates \(\lg z_4\)—\(1/T\) (Fig. 2), the experimental points lie on a straight line. From this, the changes in heat content \(\Delta H\) and entropy \(\Delta S\) during the adsorption displacement of alcohols by water vapor from the active surface of the catalyst were calculated (see Table 2).

Fig. 2. Dependence of \(\lg z_4\) on the reciprocal absolute temperature

Since

\[ z_4=\frac{a_{\mathrm{w}}}{a_{\mathrm{sp}_1}}=\frac{a_{\mathrm{w}}}{a_{\mathrm{sp}_2}}, \tag{3} \]

where \(a_{\mathrm{w}}\), \(a_{\mathrm{sp}_1}\), and \(a_{\mathrm{sp}_2}\) are the adsorption-equilibrium constants, respectively, of water, of one alcohol, and of another alcohol, then

\[ \frac{a_{\mathrm{sp}_2}}{a_{\mathrm{sp}_1}}=1 \tag{4} \]

and, consequently, the adsorption coefficients of the alcohols in this case are identical. The equality of the adsorption coefficients, in turn, indicates the identical orientation of the molecules of the alcohols studied with respect to the active centers of the catalyst surface.

On the basis of the experimental data obtained by us, it may be concluded that the reaction of alcohol dehydrogenation on an oxide catalyst is appreciably inhibited by water; moreover, the absolute adsorption coefficients water—alcohol for primary alcohols of normal structure are a function of temperature and do not depend on the length of the carbon chain of the alcohol.

Institute of Organic Chemistry named after N. D. Zelinskii
Academy of Sciences of the USSR

Received
23 VIII 1957

REFERENCES CITED

\(^{1}\) O. K. Bogdanova, A. A. Balandin, A. P. Shcheglova, Izv. AN SSSR, OKhN, 1957, 788, 795.
\(^{2}\) H. Lund, J. Bjerrum, Ber., 64, 210 (1931); 56, 894 (1923).
\(^{3}\) A. A. Balandin, O. K. Bogdanova, A. P. Shcheglova, Izv. AN SSSR, OKhN, 1946, 497.
\(^{4}\) A. A. Balandin, O. K. Bogdanova, A. P. Shcheglova, Izv. AN SSSR, OKhN, 1955, 723.
\(^{5}\) A. A. Balandin, DAN, 63, 33 (1948).