Oxidative coupling of methane ± the transition from reaction to transport control over La 2 O 3 /MgO catalyst Maria Traykova a , Nadka Davidova a , Jeng-Shiang Tsaih b , Alvin H. Weiss b,* a Institute of Kinetics and Catalysis, 1040 So®a, Bulgaria b Worcester Polytechnic Institute, Worcester, MA 01609, USA Received 30 July 1997; received in revised form 15 December 1997; accepted 15 December 1997 Abstract A 10% La 2 O 3 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 La 2 O 3 !2La 3 O 2 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: 2CH 4  1=2O 2 ! C 2 H 6  H 2 O; C 2 H 6  1=2O 2 ! C 2 H 4  H 2 O; 2CH 4  3O 2 ! 2CO  4H 2 O; 2CO 2H 2 O  2CO 2  2H 2 .The equilibrium proceeds so fast that it is not possible to distinguish whether CO or CO 2 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±8258C range. Oxygen conversion is strictly transport controlled at 850±8758C while methane reaction rate remains surface controlled. It is in this oxygen-transport controlled regime at 8758C that our highest yield of 16.2 mol% C 2 was obtained. We have ®tted 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 2228C temperature rise at 8758C gas temperature was observed. # 1998 Elsevier Science B.V. Keywords: Methane oxidative coupling; Catalyst lanthanum oxide/magnesium oxide; Catalyst lanthanum oxide/periclase; Transport control; Transition to transport control; Independent reactions; Kinetics of reactions; Temperature rise on catalyst 1. Introduction The partial combustion of methane to ethane and ethylene is highly favored thermodynamically. There has been recently a great effort to study the process. High selectivity to (or yield of) ethylene and ethane (C 2 ) has been the goal. It has been found that, in the absence of catalyst, there is little or no conversion below 8008C and 1 atm. Higher temperatures and pressures do result in signi®cant non-selective con- versions [1±3]. A review of the best catalysts for oxidative methane coupling is provided in Ref. [4]. The potential for achieving 25% C 2 yield is dis- cussed in Refs. [4±6]. Currently, the highest yields are between 15±20%, usually at low partial pressures of methane (e.g. p CH 4  18  20 kPa) and short contact times. La 2 O 3 catalysts generally give 2.5±7% C 2 yields [7,8]. Applied Catalysis A: General 169 (1998) 237±247 *Corresponding author. 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(98)00009-X