TO DEVELOP NEW CATALYST FOR THE OXIDATIVE COUPLING OF METHANE TO C2 PRODUCTS, PREFERABLY WITH A SELECTIVITY FOR ETHYLENE.
Factors influencing the behaviour of various catalyst systems for the oxidative coupling of methane were studied. Systems included (lead oxide/aluminium oxide, lithium/magnesium oxide), various rare earth oxides and barium carbonate/calcium oxide. Particular attention was given to the lithium/magnesium oxide system.
It was shown that the active phase in such catalysts is derived from lithium carbonate species, and that the lifetime of such systems can be prolonged by adding carbon dioxide to the feed to the reactor. Considerable improvements in yield can be achieved by paying attention to the reactor construction and reaction conditions. It has also been shown that further improvements in the lithium/magnesium oxide catalyst system can be achieved by adding various oxides to the formulation, the most promising of these being tin oxide and cobalt oxide. The lithium/tin/magnesium oxide system is very stable and can be used at relatively low temperatures.
Detailed investigation of the reaction network over the different catalyst systems has shown that the sequence: methane to ethane to ethene to oxides of carbon is predominant over the lithium/magnesium oxide and doped lithium/magnesium oxide systems. A parallel route to oxides of carbon (ie directly from methane) is important for many of the other catalyst systems, and is responsible for lower selectivities over these systems.
It is concluded that more attention should be given to developing stable low temperature catalysts. Under these conditions, there is a higher chance of obtaining the high carbon compounds (C2) yields needed for a commercial application of oxidative methane coupling.
Research has been carried out into the development of a very promising series of 3-component catalyst based on the lithium magnesium oxide system. Work has taken place in the following areas:
general aspects of the methane coupling reaction over lithium magnesium oxide catalysts;
the stability of lithium magnesium oxide catalysts;
the influence of carbon dioxide and water on the oxidative coupling of methane over lithium magnesium oxide;
the influence of periodic reversal of the direction of flow of the gas stream;
a reaction model for the lithium magnesium oxide catalyst system;
the effect of the concentration of oxygen in the gas feed;
the effect of back mixing;
addition of ethylene and ethane to a gas mixture containing methane and oxygen;
determination of the reaction path;
the effect of additives on lithium doped magnesium oxide catalysts;
the effect of content of the added oxide in lithium doped magnesium oxide catalysts;
the oxidation of ethane and ethylene over lithium doped magnesium oxide catalysts;
the function of the promoter in the lithium doped magnesium oxide catalyst system;
the stability of lithium doped magnesium oxide catalysts;
kinetics of the oxidative coupling of methane over a lithium tin magnesium oxide and a lithium magnesium oxide catalyst.
It was possible to improve the activity and stability of lithium magnesium oxide catalysts by the addition of small amounts of various oxides. For a number of these oxides this had little or no influence on the selectivity of hydrocarbons with 2-carbon chains of the resultant catalyst. A comparison of the various systems showed that lithium tin magnesium oxide is a promising catalyst system for the oxidative coupling of methane. More research is needed to elucidate the role of tin. Other 3 component systems should also be examined.
VALUABLE C2 HYDROCARBONS CAN BE OBTAINED BY OXIDATIVE COUPLING OF METHANE. CATALYSTS OF THE LI2O/MGO TYPE ARE KNOWN TO GIVE FAIRLY GOOD CONVERSION RATES AND SELECTIVITIES. SINCE IT IS NOT COMPLETELY CLEAR WHETHER THE ACTIVITY LIES WITH THE LI ION OR WITH SITES CONSISTING OF BOTH LI+ AND MG2+ SPECIES, A SERIES OF CATALYSTS IN WHICH LI2O IS COUPLED WITH DIFFERENT OXIDE SUPPORTS RANGING FROM BASIC TO ACIDIC WILL BE MADE AND TESTED. COMBINATIONS OF MATERIALS CONTAINING NA, K AND ALKALINE EARTHS WILL BE EXAMINED, AS WELL AS NON-OXIDE MATERIALS WITH BASIC PROPERTIES, LIKE SULPHIDES, SULPHATES AND PHOSPHATES. FINALLY, A NUMBER OF RARE-EARTH OXIDES KNOWN TO POSSESS VERY HIGH SELECTIVITIES WILL BE CONSIDERED, EITHER ALONE OR IN COMBINATION WITH OTHER ELEMENTS. FULL CHARACTERIZATION OF THE CATALYSTS WILL BE CARRIED OUT WITH THE AVAILABLE TECHNIQUES. IT IS POSSIBLE THAT CATALYSTS WITH LOW SPECIFIC SURFACE WILL PERFORM BETTER. SINCE AN IMPORTANT STEP IN THE OXIDATIVE COUPLING OF METHANE IS BELIEVED TO BE THE GAS-PHASE COMBINATION OF CH3 RADICALS FORMED AT THE SURFACE OF THE CATALYST, AN INCREASE OF PRESSURE SHOULD FAVOUR THIS SECOND-ORDER REACTION WITH RESPECT TO THE TOTAL OXIDATION. HIGH-PRESSURE TESTS WILL THEREFORE BE PERFORMED ON THE MOST PROMISING CATALYST. FUNDAMENTAL STUDIES WILL BE CARRIED OUT WITH THE AIMED OF IDENTIFYING THE SPECIES PRESENT AT AND NEAR THE CATALYST SURFACE UNDER REACTION CONDITIONS. THE FT-IR TECHNIQUE WITH SPECIAL CELLS WILL BE USED. MASS-SPECTROMETRIC STUDIES OF EXCHANGE REACTIONS OF METHANE WITH D2 UNDER OXIDATION CONDITIONS WILL ALSO FURNISH INFORMATION ON THE NATURE OF THE SURFACE SPECIES AND THEIR REACTIVITIES. EXPERIMENTS WITH 18O WILL REVEAL THE EXTENT TO WHICH THE REDUCTION AND RE-OXIDATION OF THE SURFACE IS OF IMPORTANCE FOR THE VARIOUS TYPES OF CATALYST. KINETIC STUDIES WITH THE MOST INTERESTING CATALYSTS WILL BE CARRIED OUT OVER A WIDE RANGE OF CONDITIONS.