Catalytic oxidative coupling of methane to C\\(_2\\) hydrocarbon

In this work the direct conversion of methane to C\\(_2\\) hydrocarbons through the catalytic oxidative coupling reaction (nCH\\(_4\\) + y/2 0\\(_2\\) => C\\(_n\\)H\\(_{4n-2y}\\) + y H\\(_2\\)0) is investigated. The catalytic materials studied can be categorised as single chemical compounds, different loading of Li\\(_2\\)CO\\(_3\\) on MgO, various lithium salts on MgO, Li\\(_2\\)CO\\(_3\\) on various metal oxides, transition metal oxides on MgO and transition metal oxides on Li\\(_2\\)CO\\(_3\\)/Mg0 systems. Most of these materials were prepared by wet aggregation or impregnation methods and precalcined at 900\\(^o\\)C for 10 hours. Atomic absorption spectroscopy was used to determine the metal composition of these materials. Their nature after calcination was studied by SEM and X-ray Micro-Probe Analysis. Surface area measurements and XRD analysis were also performed on most of the catalytic materials. The nature of selected materials under conditions similar to the catalytic studies were monitored by FTTR, ESCA and TGA. Changes occurring on the material during the reaction were simulated and studied in situ by FTIR techniques. A conventional flow reactor operating under atmospheric pressure was used in the activity determinations. The effects of reaction temperature, reactant composition, CO partial pressure in reactant flow and time-on-stream were investigated. The decomposition of azomethane under the conditions of the catalytic studies was also performed in order to determine if similar product distributions can be achieved. The precalcination was found to cause large physical and chemical changes on some materials. Segregation of Li\\(_2\\)CO\\(_3\\) on the Li\\(_2\\)CO\\(_3\\) /Mg0 based catalysts occurred resulting in the enrichment of this component on the surface. Considerable transformation also occurred on these catalysts during the catalytic reaction. Various materials were found to be active for the oxidation of methane and generally a chemically basic material formed a better oxidative coupling catalyst. The surface area of these materials did not determine whether they were active for the oxidative reaction but for an active material, good oxidative coupling behaviour was associated with small surface area. It was noted that materials with a reducible oxidation state favoured the exhaustive oxidation reaction while the addition of Li\\(_2\\)CO\\(_3\\) increased the oxidative coupling properties accompanied by the reduction in available redox sites. An \intrinsic limit\" for the C\\(_2\\) hydrocarbon yield was also observed while the results of azomethane experiments agreed with the proposal of the occurrence of methyl radicals coupling reactions in the gaseous phase."