The oxidative coupling of methane over BaCO₃/LaOCl catalysts

The performance of LaOCl at 800°C was promoted by BaCO₃ in OCM reaction. When 10 mol% BaCO₃ was added, there was little change in CH₄ conversion, but C₂ selectivity was increased from 37% to 66%. With the increase in BaCO₃ loading, the BaCO₃/LaOCl catalysts decreased in specific surface area. The improvement in C₂ selectivity is partly due to surface area diminution. The addition of BaCO₃ has also caused surface modification of LaOCl. In the absence of BaCO₃, LaOCl in OCM reaction is capable of generating dioxygen species O^2-₂, O^n-₂ and O^-₂, which can cause deep oxidation of CH₄, C₂H₆ and C₂H₄. With the addition of BaCO₃, sites for the activation of oxygen molecules are depleted. The nearby Cl^- ions have the ability of destabilizing the BaCO₃, causing it to decompose at ca. 780°C, a decomposition temperature about 200°C lower than that of pure BaCO₃. With the deprivation of dioxygen species, the CH₄ oxidative dehydrogenation process is enhanced, while deep oxidation processes are suppressed. Compared with the deep oxidation reaction processes, the number of sites required in the H-abstraction process is a lot fewer. Hence, although the specific surface area of LaOCl was reduced with the addition of BaCO₃, the conversion of CH₄ over the BaCO₃/LaOCl catalysts did not drop significantly.