Hydrogen--deuterium exchange in saturated hydrocarbons on. cap alpha. -chromia catalyst
The kinetics of hydrogen--deuterium exchange in methane, ethane, and propane have been studied with unsupported ..cap alpha..-chromia as catalyst in the temperature range of 598 to 688/sup 0/K. The apparent activation energies for methane, ethane, and propane are 88, 130, and 84 kJ mol/sup -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 CH/sub 3/D and CD/sub 4/ with methane, C/sub 2/H/sub 4/D/sub 2/ and C/sub 2/D/sub 6/ with ethane, and C/sub 3/H/sub 7/D and C/sub 3/D/sub 8/ 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, Cr/sup 3 +/ has been assumed as the active species of the chromia catalyst, whereas Cr/sup 2 +/ 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 Cr/sup 3 +/ -- Cr/sup 3 +/ sites are responsible for the formation of ethane-d/sub 2/. 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-d/sub 2/. On the contrary, the formation of methane-d/sub 4/ is interpreted by the formation of Cr = C bonds by analogy with homogeneous complexes. In the case of propane, the exchange reaction can be adequately interpreted by a ..pi..-allyl mechanism.
- Research Organization:
- Hungarian Academy of Sciences, Budapest
- OSTI ID:
- 5404542
- Journal Information:
- J. Catal.; (United States), Journal Name: J. Catal.; (United States) Vol. 47:3; ISSN JCTLA
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
400303* -- Organic Chemistry-- Isotope Exchange & Isotope Separation-- (-1987)
ALKANES
CATALYTIC EFFECTS
CHALCOGENIDES
CHEMICAL REACTION KINETICS
CHROMIUM COMPOUNDS
CHROMIUM OXIDES
CRYOGENIC FLUIDS
DEUTERIUM
ETHANE
FLUIDS
HYDROCARBONS
HYDROGEN ISOTOPES
ISOTOPES
ISOTOPIC EXCHANGE
KINETICS
LIGHT NUCLEI
METHANE
NUCLEI
ODD-ODD NUCLEI
ORGANIC COMPOUNDS
OXIDES
OXYGEN COMPOUNDS
PROPANE
REACTION KINETICS
STABLE ISOTOPES
TRANSITION ELEMENT COMPOUNDS