PHYSICAL CHEMISTRY

Academician Vikt. I. SPITSYN, I. E. MIKHAILENKO, and G. N. PIROGOVA

EFFECT OF IONIZING RADIATION ON THE CATALYTIC ACTIVITY OF ALUMINUM OXIDE IN THE DEHYDRATION REACTION OF n-DODECYL ALCOHOL

Aluminum oxide is among the most active catalysts for the dehydration of alcohols (¹–⁵). Several points of view exist for explaining the mechanism of this reaction. However, they all reduce to assumptions concerning the formation on the catalyst surface of unstable compounds: in some cases, radicals that can react with neighboring molecules (⁶,⁷); in others, compounds of the ether type (⁴,⁵); and, finally, carbonium ions (¹–³). The latter also explains the phenomenon of isomerization of the double bond in olefins—the products of alcohol dehydration. It is known that the catalytic activity of aluminum oxide in various heterogeneous processes changes after its irradiation. The increase in the catalytic activity of Al₂O₃ observed for the deuterium–hydrogen exchange reaction as a result of the action of slow neutrons, γ-rays, and α-particles is attributed to the formation of a large number of defects in the crystal lattice of Al₂O₃ (⁹,¹⁰).

The isomerization of olefins over an aluminosilicate catalyst decreases (¹¹,¹²), while over silica gel it increases (¹³), if the catalyst has previously been subjected to ionizing radiation.

In a number of examples it has been shown that the introduction of significant amounts of radioactive isotopes into catalysts changes the rate of such heterogeneous processes as the dehydration of alcohols and the isomerization of unsaturated hydrocarbons (¹⁴–¹⁶).

In the present work, the effect of ionizing radiation on the dehydration of n-dodecyl alcohol and on the isomerization of the reaction products over aluminum oxide was studied. The catalyst was used in the form of grains 1–2 mm in size and had the following composition: 99.87% Al₂O₃; 0.11% Fe₂O₃; and traces of SiO₂. The specific surface area of the aluminum oxide was 245 m²/g. n-Dodecanol had \(d_4^{30}\) 0.8286; \(n_D^{30}\) 1.4390; \(MR_D\) calculated 59.14, found 59.19.

Three series of experiments were carried out.

1. Aluminum oxide was exposed to slow neutrons with a flux intensity of \(\sim 0.8 \cdot 10^{13}/\text{cm}^2 \cdot \text{sec}\) and γ-rays in a nuclear reactor, and was then used in the catalytic reaction. The characteristics of the catalysts are presented in Table 1.

Table 1

Irradiation of Al₂O₃ in a nuclear reactor. Neutron flux intensity \(\sim 0.8 \cdot 10^{13}/\text{cm}^2 \cdot \text{sec}\)

1. The dehydration reaction was carried out over a mixture of $\mathrm{Al_2O_3}+\mathrm{CaCl_2}$. The aluminum oxide was impregnated with a $\mathrm{CaCl_2}$ solution, dried, and heated for 2 h at $400^\circ$. Some of the experiments were performed with preparations containing the radioactive isotope $\mathrm{Ca}^{45}$ ($\beta$-emitter, $E_{\max}=0.255$ MeV).

2. The catalyst and dodecanol vapors during the reaction were irradiated with fast electrons with an energy of 800 keV, obtained in an accelerator tube. Data on the catalysts used in this case are given in Table 2.

Fig. 1. Dehydration of n-dodecyl alcohol over $\mathrm{Al_2O_3}$. Horizontal apparatus. 1—initial $\mathrm{Al_2O_3}$; samples irradiated in the reactor: 2—5 days; 3—10 days; 4—30 days.

Fig. 2. Yield of the $\alpha$-isomer upon irradiation of the catalyst in the reactor. 1—initial $\mathrm{Al_2O_3}$; irradiated samples: 2—5 days; 3—10 days.

For the experiments, a flow-type catalytic apparatus was used [7]. When working with $\mathrm{Al_2O_3}$ irradiated in the reactor, dodecyl alcohol was fed at a space velocity of $0.16\ \mathrm{min}^{-1}$; the catalyst weight was 0.5 g. In experiments with introduced $\mathrm{CaCl_2}$, the space velocity was $0.68\ \mathrm{min}^{-1}$ with a catalyst charge of 0.15 g. The degree of conversion of n-dodecanol was determined for the catalyst collected during the second 15 min after the start of the experiment.

Table 2

Dehydration of n-dodecanol over $\mathrm{Al_2O_3}+19.4\%\,\mathrm{CaCl_2}$ under the action of radioactive radiation and external electron irradiation at $350^\circ$

As can be seen from the results obtained (Fig. 1), preliminary irradiation of aluminum oxide with neutrons causes a decrease in its catalytic activity. The greatest negative effect was observed for the preparation that had remained for 30 days in the reactor, despite prolonged holding after irradiation (Table 1). Isomerization of the reaction products as a result of neutron irradiation of the catalyst increases (Fig. 2).

Treatment with slow neutrons does not change the magnitude of the surface area of Al₂O₃. Thus, a sample irradiated for 5 days had a specific surface area of 249 m²/g. Evidently, under the action of slow neutrons the number of active centers in the catalyst decreases, and a kind of “smoothing” of its surface occurs. The longer this treatment is carried out, the more strongly the catalytic activity of Al₂O₃ decreases. The induced radioactivity that arises is undoubtedly too small to exert its own effect. These same causes apparently lead to the fact that electrons

Fig. 3. Dehydration of n-dodecyl alcohol over Al₂O₃ + 19.4% CaCl₂. Vertical apparatus.
1 — Al₂O₃ + 19.4% CaCl₂; 2 — Al₂O₃ + 19.4% CaCl₂ (specific activity 39.1 mCu/g); 3 — Al₂O₃ + 19.4% CaCl₂ under external irradiation with electrons, dose \(10^{20}\) eV/g; 4 — Al₂O₃ + 19.4% CaCl₂ (specific activity 39.1 mCu/g) under external irradiation with electrons, dose \(10^{20}\) eV/g.

Fig. 4. Yield of the α-isomer in the dehydration of n-dodecyl alcohol over Al₂O₃ + 19.4% CaCl₂.
1 — nonradioactive catalyst; 2 — Al₂O₃ + 19.4% CaCl₂ (specific activity 39.1 mCu/g); 3 — nonradioactive catalyst under external irradiation with electrons, dose \(10^{20}\) eV/g; 4 — Al₂O₃ + 19.4% CaCl₂ (specific activity 39.1 mCu/g) under external irradiation with electrons, dose \(10^{20}\) eV/g.

of the double bond, not retained by charges on the catalyst surface, already in the course of the reaction readily migrate toward the middle of the molecule. A decrease in catalytic activity was previously observed by us also in the dehydration of n-decyl alcohol over alumina irradiated in a nuclear reactor \((^{16})\).

In carrying out the dehydration reaction of n-dodecanol over a catalyst of composition Al₂O₃ + CaCl₂, with a molar ratio of 1.00 Al₂O₃ : 0.18 CaCl₂, a decrease in the degree of conversion of the alcohol was noted in comparison with pure alumina. Introduction of a radioactive isotope into the catalyst and irradiation with fast electrons during the reaction increase the rate of the catalytic process (Figs. 3 and 4). The results of experiments with the radioactive catalyst Al₂O₃ + CaCl₂ under the simultaneous action of fast electrons made it possible to decide unambiguously the question of the influence of surface charge on the catalytic process. Upon emission of β-particles, the surf-

the activity of the catalyst \(\mathrm{Al_2O_3 + CaCl_2}\) acquires localized positive charges, which evidently facilitates the initial stage of the reaction—the adsorption of alcohol vapor on alumina. Irradiation of the radioactive catalyst with electrons from outside will lead to neutralization of the positive charges that arise and should consequently reduce the rate of the process under study. Indeed, in Fig. 3, curve 4 lies below curve 2. It should be noted that the action of fast electrons on the nonradioactive catalyst \(\mathrm{Al_2O_3 + 19.4\%\, CaCl_2}\) increases the degree of conversion of \(n\)-dodecanol (curve 3, Fig. 3). Apparently, electron bombardment produces excitation and ionization of atoms on the surface of \(\mathrm{Al_2O_3}\). It is possible that localized charges of different sign arise simultaneously, including those favoring acceleration of the catalytic process.

Isomerization of the reaction products decreases when radioactive or external electron radiation acts on the catalyst and on the vapors of \(n\)-dodecanol (Fig. 4). The yield of \(\alpha\)-dodecene (Fig. 4) proves to be greatest in the case of the radioactive catalyst: a 100% content of the \(\alpha\)-isomer in the olefin mixture is retained up to a temperature of \(420^\circ\). A similar effect was observed in the dehydration of \(n\)-dodecanol over \(\mathrm{MgSO_4}\) (15), but with a lower yield of unsaturated hydrocarbons than over nonradioactive magnesium sulfate. The occurrence of positive charges on the catalyst surface probably hinders the movement of electrons along the chain of carbon atoms, which causes displacement of the double bond.

Irradiation with fast electrons has a less pronounced effect on the catalytic process. Electron bombardment of the radioactive catalyst sharply increases the rate of isomerization, from which it may once again be concluded that positive charges on the catalyst surface play an exceptionally important role in inhibiting the isomerization process.

Thus, the introduction of a radioactive emitter into \(\mathrm{Al_2O_3}\) and the use of external electron irradiation make it possible to influence effectively the rate of dehydration of \(n\)-dodecanol and the composition of the products obtained.

Physical Chemistry Institute
Academy of Sciences of the USSR

Received
6 I 1962

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