Activation of Methane to C 2 Hydrocarbons over Unpromoted Calcium Oxide Catalysts

The oxidative coupling of methane has received a great deal of attention for the last twenty five years because the reaction has been understood as a promising route for the C2-hydrocarbon production from natural gas. To identify suitable catalysts giving high C2-yield, a lot of catalysts have been examined for the oxidative coupling of methane. It has been reported that high surface basicity of a catalyst is necessary to enhance the C2 selectivity in the reaction. Alkali metal oxides and alkaline earth metal oxides are strong bases so that they have catalytic activities for the oxidative coupling of methane. Alkali metal oxides seem to be unsuitable catalysts for the reaction because they form liquid phase of metal hydroxide in the presence of water vapor under reaction conditions, while alkaline earth metal oxides are suitable catalysts because they are easily dehydrated under reaction conditions. When alkali metal ion such as Li or Na is incorporated into MgO or CaO catalyst, its catalytic activity and selectivity for the oxidative coupling of methane can be enhanced. However, the incorporation of alkali ions into CaO or MgO is not quite favorable. CaO itself is so basic that it has good catalytic activity for the oxidative coupling of methane but deactivated due to the formation of carbon solids or carbonate during the reaction. Nevertheless, CaO is still an attractive one in developing the catalyst for the oxidative coupling of methane because it has additional advantages of low cost and widespread availability. Recently, we have examined a series of CaO-based catalysts for the oxidative coupling of methane and have found that CaO fully-coated silica particles showed appreciable selectivity and durability for the oxidative coupling of methane. This paper reports catalytic behaviors and photoluminescence results of the CaO fully-coated silica particles.