Mechanistic studies on the oxidative coupling of methane

Abstract

The magnitude of the KIE during the catalytic oxidative coupling of methane and its variation with respect to methane-to-oxygen ratio is taken as evidence that under most conditions the formation of CH3 - radicals at the surface occurs at a limiting rate comparable to the incorporation of oxygen into the lattice, i.e., the regeneration of the active center. There is no unique rate-limiting step for the oxidative coupling of methane over both Li+/MgO and Li+-MgO-Cl- catalysts. With N2O as the oxidant, the rate-limiting step shifted from oxygen incorporation into the lattice at pressures used for conventional catalytic reactions to the formation of CH3+ radicals at the surface at much lower pressures. This shift in the rate-limiting step shows that the partial pressures of both methane and oxidant can be factors in modifying the relative rates of the individual steps in the catalytic cycle. The oxidative coupling of CH4 over a Li+/MgO catalyst may be effectively modeled as a coupled heterogeneous-homogeneous reaction. At 700°C, CH4 is activated primarily at the surface, and the resulting CH3- radicals couple in the gas phase. Secondary reactions of CH3 - radicals with the surface are not a major source of COx over this catalyst. The ultimate yield that may be achieved in oxidative coupling at 700°C is determined mainly by the homogeneous oxidation of C2H4 and C2H6. The studies of the origin of carbon oxides during the oxidative coupling of methane over various catalysts showed that under most conditions the rate constant for COx formation from C2H4 oxidation is 3 to 6 times larger than that for COx formation from CH4. The actual rate depends on the partial pressure of the hydrocarbons. At temperatures lower than 700°C, the oxidation of methyl radicals in the gas phase may be an important pathway for COx formation. Over Li+/MgO catalysts, there may exist a non-selective oxygen center that reacts with CH3- to form COx. At the same levels of conversion, the Ba/MgO catalyst is a more selective catalyst than the Sr/La2O3 catalyst, and the rate for COx formation from CH4 is relatively slower. CO2 has a strong poisoning effect on active centers, and it increases the activation energy for the oxidative coupling of methane over Li+/MgO catalysts.