The influence of carbon dioxide on the catalytic oxidative coupling of methane over A-La2O3 and II-La2O2CO3

Previous work conducted in our laboratory suggested a strong correlation between the presence of oxycarbonate and the ability of lanthanum oxide-based materials to selectively promote the oxidative coupling of methane (OCM). Since A-La[subscript]2O[subscript]3 and II-La[subscript]2O[subscript]2CO[subscript]3 may be prepared from each other by the loss and gain, respectively, of CO[subscript]2 at temperatures ranging from 415 to 970°C (depending on the gas phase environment), the influence of CO[subscript]2 on the OCM process is of great interest;This work found that co-feeding CO[subscript]2 over pure A-La[subscript]2O[subscript]3 and II-La[subscript]2O[subscript]2CO[subscript]3 starting materials depresses catalytic performance by both decreasing the catalysts\u27 ability to activate methane and inhibiting the homogeneous reaction pathway by which ethane dehydrogenates to ethene. Superior catalytic performance was found to depend upon the presence of both A-La[subscript]2O[subscript]3 and II-La[subscript]2O[subscript]2CO[subscript]3;Both A-La[subscript]2O[subscript]3 and II-La[subscript]2O[subscript]2CO[subscript]3 starting materials were found to uptake CO[subscript]2 at typical OCM reaction temperatures via incorporation between their (LaO)[subscript]2[superscript]2+ layers. CO[subscript]2 uptake by II-La[subscript]2O[subscript]2CO[subscript]3 was greater due to its more open structure long the c axis of the unit cell (i.e., perpendicular to the (LaO)[subscript]2[superscript]2+ planes);Isotopic labeling studies revealed that the CO[subscript]2 used in preparing II-La[subscript]2O[subscript]2CO[subscript]3 from A-La[subscript]2O[subscript]3 inserts directly into the oxide layers of A-La[subscript]2O[subscript]3 to give layers of (CO[subscript]3)[superscript]2- anions (all C-O bonds being equivalent). Carbon dioxide evolved during the thermal decomposition of II-La[subscript]2O[subscript]2CO[subscript]3 originates from the same (CO[subscript]3)[superscript]2- layers of the oxycarbonate;During OCM, C atoms originating in II-La[subscript]2O[subscript]2CO[subscript]3 were detected in products CO and CO[subscript]2, but not in the coupling products. Therefore, C[subscript]2H[subscript]6 and C[subscript]2H[subscript]4 are produced only via methane coupling. The O atoms originating in the (CO[subscript]3)[superscript]2- layers of the catalyst were also detected in products CO and CO[subscript]2 during OCM. Furthermore, CO[subscript]2 was concluded to be produced on the catalyst rather than in the gas phase;Oxygen present in II-La[subscript]2O[subscript]2CO[subscript]3 was found to be capable of activating methane. The amounts of coupling products detected following a He/CH[subscript]4 pulse were equivalent to approximately 2% of those observed following a He/CH[subscript]4/O[subscript]2 feed pulse;Finally, a scheme was proposed which explains the catalytic performance of A-La[subscript]2O[subscript]3 and II-La[subscript]2O[subscript]2CO[subscript]3 in terms of their relative basicity