S.J. Korf

Oxidative coupling of methane over lithium doped magnesium oxide catalysts

Abstract Active sites are created on the surface of a Li/MgO catalyst used for the selective oxidation of methane, by the gradual loss of CO 2 from surface lithium carbonate species in the presence of oxygen. The sites created are not stable but disappear either as a result of reaction with SiO 2 to form Li 2 SiO 3 or by the formation and subsequent loss of the volatile compound LiOH. The deactivation can be reversed, at least partially, by treating the catalyst in CO 2 under reaction conditions; it can be retarded if low concentrations of CO 2 are added to the reaction mixture.

Kinetic and mechanistic aspects of the oxidative coupling of methane over a Li/MgO catalyst

The rate of reaction of methane with oxygen in the presence of a Li-doped MgO catalyst has been studied as a function of the partial pressures of CH4, O2 and CO2 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 di-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.

The selective oxidation of methane to ethane and ethylene over doped and un-doped rare earth oxides

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 Tb4O7 (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 Sm2O3 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 Sm2O3 gives a higher optimum C2 yield than that of unpromoted Sm2O3. Sm2O3 and Ca/Sm2O3 catalysts are more stable than Li/MgO, Li/Sm2O3 or Na/Sm2O3. The addition of Li or Na to Sm2O3 causes the structure to change from cubic to monoclinic; the deactivation of the Na/Sm2O3 catalysts is caused by a loss of Na coupled with the formation of the monoclinic form of Sm2O3.

Lithium chemistry of lithium doped magnesium oxide catalysts used in the oxidative coupling of methane

Active sites are created on the surface of a Li/MgO catalyst used for the selective oxidation of methane by the gradual loss of carbon dioxide from surface carbonate species in the presence of oxygen. Decomposition of the carbonate species in the absence of oxygen is detrimental to the activity of the catalyst. The active sites created are not stable but disappear either as a result of reaction with SiO2 to form Li2SiO3 or by the formation and subsequent loss of the volatile compound LiOH. In general the addition 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 carbonate and thus the initial activity, can be enhanced by the addition of water to the gas feed. The addition 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.

Oxidative coupling of methane over Ba/CaO catalysts: a comparison with Li/MgO

A comparison has been made of the behaviour in the oxidative coupling of methane of a Ba/CaO catalyst with that of a Li/MgO material. Doping of CaO with BaCO3 resulted in a catalyst which is more active at lower reaction temperatures than is BaCO3. The active oxygen entity in the case of Ba/CaO is probably an O2−2 species. Ba/CaO is more stable but less selective than is Li/MgO. The effect of residence time was studied for both Ba/CaO and Li/MgO. The direct oxidation of methyl radicals to give carbon monoxide and carbon dioxide plays a more important role in the case of Ba/CaO than is the case with Li /MgO.

Reaction path of the oxidative coupling of methane over a lithium-doped magnesium oxide catalyst : Factors affecting the Rate of Total Oxidation of Ethane and Ethylene

Experiments using gas mixtures of O2, C2H6 or C2H4 and CH4 or He have been carried out with a Li/MgO catalyst using a well-mixed reaction system which show that the total oxidation products, CO and CO2, are formed predominantly from ethylene, formed in the oxidative coupling of methane. It is therefore concluded that the network of reactions taking place during oxidative coupling of methane over a Li-doped MgO catalyst can be simplified to a serial reaction scheme: CH4→C2H6→C2H4→COx. Additional experiments have shown that the rates of gas-phase oxidation reaction of C2H6 and C2H4 are lowered by the presence of excess CH4 or by alkali metal carbonates.

An investigation of the comparative reactivities of ethane and ethylene in the presence of oxygen over Li/MgO and Ca/Sm2O3 catalysts in relation to the oxidative coupling of methane

In order to examine the importance of the further oxidation of the desired C2 products in the oxidative coupling of methane, ethylene and ethane have been added to the feed (containing methane and oxygen) to a Li/MgO or Ca/Sm2O3 catalyst. The results of these measurements show that neither of these C2 molecules is stable under these conditions with either of the catalysts. Additionally, the rates of the oxidation of ethane and of ethylene alone have been measured using a gradientless reactor for both catalysts as well as for a quartz bed. It was found that the Ca/Sm2O3 material had higher activities for the oxidation of C2H6 and C2H4 (and also of CH4) than had the Li/MgO material. These higher activities result in a lower optimal reaction temperature for the oxidative coupling of methane and are (at least partially) responsible for the lower selectivity to C2 products observed with the Ca/Sm2O3 catalyst compared to that with the Li/MgO catalyst.

A study of the kinetics of the oxidative coupling of methane over a Li/Sn/MgO catalyst

The rate of reaction of methane with oxygen in the presence of a Li/Sn/MgO catalyst has been studied as a function of the partial pressures of CH4, O2 and CO2 using 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 a molecule of CH4 adsorbed on the catalyst surface with an adsorbed di-atomic oxygen species. The kinetics are consistent with a Langmuir-Hinshelwood type mechanism involving competitive adsorption of CH4, O2 and CO2 on a single site. A comparison is made with previously published results for the Li/MgO material.

The Development of Doped Li/MgO Catalyst Systems for the Low-Temperature Oxidative Coupling of Methane

The direct conversion of methane, the predominant component of natural gas, to more-useful chemicals and fuels is a topic that is attracting more and more interest, it having been recognized as one of the most technologically important and challenging problems facing industrialized nations in the latter part of the twentieth century. Although this chapter emphasizes work performed in our group, reference also is made, where appropriate, to parallel work by other groups.

The Oxidative Coupling of Methane: Catalyst Requirements and Process Conditions

Abstract Good methane coupling catalysts are basic materials which cannot provide large amounts of lattice oxygen and which contain electronic or structural defects on the surface. Using high gas velocities in a fixed bed reactor results in improved conversions and selectivities. The improvement in selectivity is explained by a better plug-flow behaviour of the reactor.