Li/MgO catalysts used for the oxidative coupling of methane are found to deactivate relatively rapidly in use; this deactivation can be reversed by treating the the catalyst with COP under the reaction conditions and the deactivation can be avoided completely if C 0 2 is added in low concentrations to the reaction mixture.

No abstract

The rate of reaction of methane with oxygen in the presence of a Li-doped MgO catalyst has hern studied as a function of the partial pressures of CH,, O2 and CO, in a well-mixed reaction system which is practically gradientless with respect to gas-phase concentrations. It is concluded that the rate determining step involves reaction of methane adsorbed on the catalyst surface with a da-atomic oxygen species, The adsorption of oxygen is relatively weak. Carbon dioxide acts as a poison for the reaction of methane with oxygen, this probably being caused by competitive adsorption on the sites where oxygen (and possibly also methane) adsorbs.

Active sites are created on the surface of a Li/MgO catalyst used for the selective oxidation of methane by the gradual loss of carbondioxide from surface carbonate species in the presence of oxygen. Decomposition of the carbonat~ species in the absence of oxygen is detrimental to the activity of the cataIy~t. The active sites'created are not stable but disappear either as a result of reaction with SiO2~ fort 0 Li2SiQ Dr by the formation and subsequent loss of the volatile compound LiOH. hug~neral th~adAti~on of water to the gas feed is detrimental to the stability of the catalyst. In the case of Li2CO3 strongly bonded on the surface of Li/MgO catalyst, the decomposition of the carbo~te and thus the initial activity, can be enhanced by the addition of water to the gas feed. The addi~bn-of carbon dioxide to the gas feed results in a poisoning of the catalyst, the degree of this poisoning depending on the activity of the catalyst. The deactivation of the catalyst can be retarded if low concentration of carbon dioxide are added to the reaction mixture. It is possible to improve the stability of the catalyst by periodic reversal of the direction of flow of the gas steam.

A comparison has been made of the behaviour in the oxidative coupling of methane of the oxides of Sm, Dy, Gd, La and Tb with that of a Li/MgO material. All but the Tb407 (which gave total oxidation) were found to give higher yields than the Li/MgO material at temperatures up to approaching 750°C but the Li/MgO system gave better results at higher temperatures. The cubic structure of Sm203 was found to be responsible for its good performance while the monoclinic structure was relatively inactive and unselective. The addition of Na or Ca to cubic Sm203 gives a higher optimum C2 yield than that of unpromoted Sm203. Sm203 and Ca/Sm203 catalysts are more stable than Li/MgO, Li/Sm203 or Na/Sm203. The addition of Li or Na to Sm203 causes the structure to change from cubic to monoclinic; the deactivation of the Na/Sm203 catalysts is caused by a loss of Na coupled with the formation of the monoclinic form of Sm203.