The kinetics of hydrogen-deuterium exchange in methane, ethane, and propane have been studied with unsupported α-chromia as catalyst in the temperature range of 598-688 °K. The apparent activation energies for methane, ethane, and propane are 88, 130, and 84 kJ mol-1, respectively, similar to those found on chromia gel. The order of reaction with respect to hydrocarbons and deuterium has been determined as also having the kinetic isotope effect. The main initial products are CH3D and CD4 with methane, C2H4D2 and C2D6 with ethane, and C3H7D and C3D8 with propane. A change in product distribution as a function of temperature, conversion, aging, and oxygen-deuterium treatment has been observed. In agreement with the kinetic data and the effect of oxygen, Cr3+ has been assumed as the active species of the chromia catalyst, whereas Cr2+ is an inactive site on the surface. There is no direct proof that chromium ion in a valence state higher than 3+ plays an important role in the exchange reaction. It was established that dual Cr3+-Cr3+ sites are responsible for the formation of ethane-d2. After a long deuterium treatment the number of dual sites is decreased, the effect being revealed in the decreased rate of exchange and the small amount of ethane-d2. On the contrary, the formation of methane-d4 is interpreted by the formation of CrC bonds by analogy with homogeneous complexes. In the case of propane, the exchange reaction can be adequately interpreted by a π-allyl mechanism.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry