Computational Study On Oxidative Coupling Of Methane Catalyzed By Heteroatom Doped Magnesium Oxide（100）

The large scale reserves of natural gas and shale gas are found at home and abroad,and its main component is methane.Efficient conversion of these relatively cheap and environmental friendly natural resources into high value-added chemicals such as ethane and ethylene have high economic benefits.The activation of methane molecule is extremely difficult because of its very strong C-H bond.Therefore,the methane activation remains as one of the most challenging catalytic reactions.The efficient catalyst is key to the efficient conversion of methane.In this paper,density functional theory is used to identify the active sites,reaction paths and reaction mechanisms of oxidative coupling of methane reaction（OCM）catalyzed by heteroatom doped Mg O（100）.The objective of current study is to understand the nature of C-H bond activation and establish the structure-activity relationship which will lay a theoretical foundation for designing OCM catalyst.The various dopants show distinct features in the modulating Mg O（100）catalysts in OCM reaction.The lithium doping creates more defect sites which caused the exposure of（110）and（111）crystal planes.Conversely,the molybdenum dopant did not change the surface structure of Mg O catalyst.Instead of,Mo could improve the reactivities of Mg O（100）which can facilitate gas phase oxygen molecule adsorption.The activated adsorbed oxygen subsequently break C-H bond in methane molecule.Both Li and Mo dopants significantly increase the catalytic performance of Mg O.In this paper,the reaction paths and mechanisms of methane C-H bond activation on Mg O（110）were calculated by density functional theory.When methane and oxygen molecules are coadsorbed on Mg O（110）,the surface Os captures the hydrogen in methane.The C-H bond is homogeneously broken to form the methyl radical.The activation energy is calculated to be 0.70 e V.The site of methane C-H bond heterolytic breaking is surface O-Mg,which requires activation energy of 0.45 e V and the desorption energy of adsorbed methyl group is 0.41 e V.The rate limiting step is C-H bond activation on both pathways.Therefore,it suggested that C-H bond heterolytic breaking is more favorable.The reaction paths and mechanisms of methane C-H bond activation on Mo-Mg O（100）surface were investigated by using DFT calculations.The results show that Mg on the surface of Mg O（100）can adsorb and activate oxygen molecules by the Mo doping.The activated adsorbed oxygen behaves as the active site to break the C-H bond activation of methane.However,when the C-H bond is heterolytic activated,the methyl group adsorbed on the adsorbed oxygen is difficult to desorb,and the methyl group has high desorption energy.The lattice oxygen as the active site is not conducive to the activation of C-H bond.Therefore,The C-H bond activation does not need to overcome a barrier,only needs to surmount the dissociative adsorption energy of C-H bond.