Experimental verification of theoretically calculated transition barriers of the reactions in a gaseous selective oxidation of CH4-O2-NO2

The selective oxidation of methane with oxygen to C-1 oxygenates (methanol and formaldehyde) is an important process. However, the precise reaction mechanism in a gaseous chain reaction has not been clarified. Methane activation and the selectivity of C-1 oxygenates (CH3OH, CH2O) in a gaseous selective oxidation of CH4-O-2-NO2 have been examined under atmospheric pressure with both theoretical and experimental approaches. Theoretically calculated transition barrier of hydrogen abstraction from CH4 of the reaction CH4 + NO2 --> CH3 + HNO2 was lower than that of the reaction with O-2, i.e., CH4 + O-2 --> CH3 + O2H. This decrease of the transition barrier was experimentally verified by the linear enhancement of CH4 conversion with NO2 concentration in CH4-O-2-NO2. The experimental varied results of selectivity of C-1 oxygenates on various reaction conditions (NO2 concentration, CH4/O-2 ratio, space velocity) showed the appropriateness of the consideration on the selectivity with the calculated values of transition barriers and rate constants of the selected reaction routes from CH3O to CH3OH and CH2O. After considering the transition barriers and rate constants of each elemental reaction route, we attained ca. 7% yield of C-1 oxygenates.