OXIDATIVE COUPLING OF METHANE - THE TRANSITION FROM REACTION TO TRANSPORT CONTROL OVER LA2O3 MGO CATALYST/

A 10% La2O3 supported on fused MgO (periclase) catalyst has been studi ed in the 0.45-3.04 atm range for methane partial oxidation to ethane plus ethylene. X-ray diffraction showed that this catalyst, operated b y a mechanism La2O3-->2La(3+)O(2-)e(-)+[0], provides active oxygen and vacancies e(-) for the surface reaction. Four independent reactions w ere found that allowed calculation of the rates and approximate kineti cs of every species in the reacting system: 2CH(4) + 1/20(2) --> C2H6 + H2O; C2H6 + 1/20(2) --> C2H4 + H2O; 2CH(4) + 3O(2) --> 2CO + 4H(2)O; 2CO + 2H(2)O reversible arrow 2CO(2) + 2H(2). The equilibrium proceed s 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 tran sition regime between surface reaction and mass-transport control in t he 700-825 degrees C range. Oxygen conversion is strictly transport co ntrolled at 850-875 degrees C while methane reaction rate remains surf ace controlled. It is in this oxygen-transport controlled regime at 87 5 degrees C that our highest yield of 16.2 mol% C-2 was obtained. We h ave fitted kinetics in both transition- and transport-controlled regim es. Finally, it should be noted that catalyst surface temperatures wer e calculated to be far above than those of the bulk gas temperatures a s a consequence of the transport control; as much as 222 degrees C tem perature rise at 875 degrees C gas temperature was observed. (C) 1998 Elsevier Science B.V.