Origin of kinetic isotope effects during the oxidative coupling of methane over a Li[sup +]/MgO catalyst
- Texas A M Univ., College Station (United States)
Kinetic isotope effects (KIEs) during the oxidative coupling of methane over a Li[sup +]/MgO catalyst at 700[degrees]C have been determined using several different experimental methods and by a reaction model that includes both heterogeneous and homogeneous reactions. By maintaining a constant partial pressure of methane of 190 Torr and changing the partial pressure of oxygen, a variation in the H/D KIE, based on differing rates of CH[sub 4] and CD[sub 4] conversion, was observed. In addition, a similar variation in KIE was found from the isotopic distribution of H and D in the ethane product when CH[sub 4] and CD[sub 4] were co-fed into the reactor. The decrease in KIE with increasing methane-to-oxygen reactant ratio is evidence that a unique rate-limiting step does not exist; rather, the rates of methane activation and of oxygen incorporation, as individual steps, are comparable. Other factors, such as the absolute partial pressures of the reagents may become even more important than the methane-to-oxygen ratio under some conditions. When N[sub 2]O was used as the oxidant instead of O[sub 2] and normal catalytic conditions were employed, the observed KIE was unity, even though the N[sub 2]O was present in excess. In this case, a unique rate-limiting step was operative, viz., the incorporation of oxygen into the lattice. The KIE was also determined separately for the production of methyl radicals, but these experiments were carried out at much lower reagent partial pressures (<1 Torr). With O[sub 2] as the oxidant, KIEs of 1.5 [+-] 0.2 were observed, although the methane-to-oxygen ratios were large (>40). This result confirms that there is a KIE associated with the activation of methane on the surface but that this is not necessarily a unique rate-limiting step. With N[sub 2]O as the oxidant, it was possible to vary the methane-to-nitrous oxide ratio over a large range (from 0.29 to 44). 29 refs., 4 figs., 5 tabs.
- OSTI ID:
- 6395672
- Journal Information:
- Journal of Physical Chemistry; (United States), Vol. 97:1; ISSN 0022-3654
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
02 PETROLEUM
DEUTERIUM
ISOTOPE EFFECTS
LITHIUM COMPOUNDS
CATALYTIC EFFECTS
MAGNESIUM OXIDES
METHANE
OXIDATION
CATALYSIS
CATALYSTS
CHEMICAL REACTION KINETICS
ETHANE
METHYL RADICALS
NITROUS OXIDE
OXYGEN
PARTIAL PRESSURE
QUANTITY RATIO
ALKALI METAL COMPOUNDS
ALKALINE EARTH METAL COMPOUNDS
ALKANES
ALKYL RADICALS
CHALCOGENIDES
CHEMICAL REACTIONS
ELEMENTS
HYDROCARBONS
HYDROGEN ISOTOPES
ISOTOPES
KINETICS
LIGHT NUCLEI
MAGNESIUM COMPOUNDS
NITROGEN COMPOUNDS
NITROGEN OXIDES
NONMETALS
NUCLEI
ODD-ODD NUCLEI
ORGANIC COMPOUNDS
OXIDES
OXYGEN COMPOUNDS
RADICALS
REACTION KINETICS
STABLE ISOTOPES
400201* - Chemical & Physicochemical Properties
020400 - Petroleum- Processing