An Unexpected Catalytic Dehydrogenation

Abstract

DURING the course of an investigation which is being made on the decomposition of amines in the presence of hydrogen over evaporated metal-film catalysts, an unexpected example of selective catalytic action has been observed. The decomposition of cyclohexylamine on evaporated platinum films at 134° C. was expected to produce cyclo-hexane and ammonia according to the equation The reaction was followed by continuous massspectrometric analysis of the reacting gases by a technique described elsewhere¹. As the ions of mass 99 (C₆H₁₁NH₂ ⁺) decreased in number, ions of mass 17 (NH₃ ⁺) appeared but very few ions of mass 84 (C₆H₁₂ ⁺) were observed. A search throughout the mass-range covered by the spectrometer indicated that large quantities of ions of mass 78 were being produced, suggesting that the primary product of the reaction was benzene (C₆H₆) rather than cyclohexane. Now the position of the equilibrium² in the reaction is well over on the side of cyclohexane under the conditions used in this work. The initial reaction mixture consisted of 1.8 mm. cyclohexylamine and 23 mm. hydrogen and, assuming complete conversion according to reaction (1) and the known equilibrium constant for reaction (2), the maximum pressure of benzene to be expected at 134° C. was only about 10⁻³ mm. The identification of the source of the mass 78 ions as benzene was therefore considered tentative until it was found that when most of the cyclo-hexylamine was used up the ions of mass 78 began to disappear again and in turn substantial quantities of ions of mass 84 were formed. This showed that the normal catalytic hydrogenation of benzene to cyclohexane, to be expected on a metal like platinum, was then taking place. The formation of benzene was also confirmed by stopping a typical reaction and identifying the benzene in the reaction mixture by vapour-phase chromatography. The whole course of the reaction is shown in Fig. 1.