Oxidative coupling of methane – the transition from reaction to transport control over La2O3/MgO catalyst Original Research Article

A 10% La2O3 supported on fused MgO (periclase) catalyst has been studied in the 0.45–3.04 atm range for methane partial oxidation to ethane plus ethylene. X-ray diffraction showed that this catalyst, operated by a mechanism La2O3→2La3+O2−e−+[O], provides active oxygen and vacancies e− for the surface reaction. Four independent reactions were found that allowed calculation of the rates and approximate kinetics of every species in the reacting system: 2CH4+1/2O2→C2H6+H2O; C2H6+1/2O2→C2H4+H2O; 2CH4+3O2→2CO+4H2O; 2CO+2H2O⇌2CO2+2H2.The equilibrium proceeds so fast that it is not possible to distinguish whether CO or CO2 is the primary product. It was found that oxygen conversion was in a transition regime between surface reaction and mass-transport control in the 700–825°C range. Oxygen conversion is strictly transport controlled at 850–875°C while methane reaction rate remains surface controlled. It is in this oxygen-transport controlled regime at 875°C that our highest yield of 16.2 mol% C2 was obtained. We have fitted kinetics in both transition- and transport-controlled regimes. Finally, it should be noted that catalyst surface temperatures were calculated to be far above than those of the bulk gas temperatures as a consequence of the transport control; as much as 222°C temperature rise at 875°C gas temperature was observed.