Activation of Methane to C₂Hydrocarbons over Unpromoted Calcium Oxide Catalysts

Abstract

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 C 2 -hydrocarbon production from natural gas.1,2 To identify suitable catalysts giving high C 2 -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 C 2 selectivity in the reaction. 3Alkali 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, 4 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. [4][5][6][7][8] 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. ExperimentalCaO powder, 30 wt.% CaO/silica, and CaO fully-coated silica particles were prepared to examine as catalysts for the oxidative coupling of methane. CaO powder was prepared by decomposition of calcium nitrate (Ca (NO 3 ) The catalytic tests were performed in a fixed-bed continuous flow reactor operated at atmospheric pressure. The catalyst was loaded over fused silica bed placed in the middle of the reactor which was made of quartz tubing with 0.8 cm inner diameter and 20 cm length. In a typical experiment, 200 mg of catalyst was loaded into the reactor and the feed flow rate of reaction mixture was CH 4 /O 2 /He = 8/2/10 cm 3 / min. The purity of gases was greater than 99.99% and the gases were dehydrated and purified with suitable filters. Gaseous products were analyzed using on-line gas chromatography equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). The methane conversion and product selectivities were typically compared after 1 h time-on-stream. Blank test showed 3-5% conversion of methane to carbon oxides in the temperature range of 600-800 o C. The details of kinetic expe...

Enhanced Photocatalytic Degradation with Sustainable CaO Nanorods Doped with Ce and Cellulose Nanocrystals: In Silico Molecular Docking Studies

“…The catalytic activity of CaO was enhanced by unsaturated surface sites, and photoluminescence spectra are formed by unsaturated sites on the CaO surface . Surface unsaturated sites with the coordination number <5 was exhibited by the lower energy region . In low coordination states, surface absorption bands connected with electronic transitions in surface oxygen ions display low PL energy excitation.…”

“… 54 Surface unsaturated sites with the coordination number <5 was exhibited by the lower energy region. 55 In low coordination states, surface absorption bands connected with electronic transitions in surface oxygen ions display low PL energy excitation. Surface absorption bands are linked with the electronic transitions in surface oxygen ions in the low coordination state, which causes lower excitation energy of PL.…”