OXIDATIVE CONVERSION OF METHANE TO C-2 HYDROCARBONS OVER LI, MN, CD AND ZN PROMOTED MGO CATALYSTS .2. PERFORMANCE OF FRESH AND REOXIDIZED CATALYSTS IN THE ABSENCE OF FREE OXYGEN

Involvement of lattice oxygen in the oxidative coupling of methane over MgO, Li-MgO, Mn-MgO, Li-Mn-MgO, Cd-MgO, Li-Cd-MgO, Zn-MgO and Li-Zn-MgO catalysts at 1073 K has been investigated by carrying out the reaction in the absence of free oxygen as a function of methane pulse number, using a pulse microreactor connected to a gas chromatograph. The influence was also studied of catalyst reoxidation after the pulse experiments on the conversion of methane and C2-selectivity in the pulse reaction. A high C2 selectivity in the methane pulse reaction is observed for the lithium-promoted catalysts. The order of the C2 selectivity observed on the catalysts with and without the lithium promoter was found to be as follows: Li-Zn-MgO > Li-Cd-MgO > Li-Mn-MgO almost-equal-to Li-MgO greater-than-or-equal-to Cd-MgO >> Zn-MgO > Mn-MgO > MgO. Upon reoxidation of the reduced catalyst in pulse experiments, the selectivities in the formation of carbon monoxide, carbon dioxide and C2 hydrocarbons are strongly influenced, due to the formation of active chemisorbed oxygen species and/or restructuring of the catalyst surface.