NO2-promoted oxidation of methane to formaldehyde at very short residence time. Part I: Experimental results

The direct gas phase oxidation of methane to formaldehyde has been investigated in the presence of NO2 at short residence time (20–80 ms). A quartz annular flow microreactor was used and the effects of various operating parameters (temperature, residence time, XO2/XCH4XO2/XCH4 ratio and NO2 concentration) were examined. Compared with the reaction without NO2, the addition of NO2 decreases the reaction initiation temperature by 300 K and it remarkably improves the CH4 conversion as well as the HCHO yield. High HCHO yields (>4%) were obtained at relatively high XO2/XCH4XO2/XCH4 ratio (3–7) and/or relatively high NO2 concentration (>0.1%). The best experimental yield is 9%, which was obtained at 24.3% of CH4 conversion. By comparison between both systems (CH4/O2 and CH4/O2/NO2), it was concluded that NO2 favors the oxidation pathway of methane, at the expense of the formation of C2 hydrocarbons. Furthermore, much less CO2 was detected in the reaction with NO2 than that without NO2, which can be explained by the lower reaction temperature.

► NO2 is used as a promoter in CH4 partial oxidation to yield HCHO at τ ≈ 20–80 ms. ► The effects of various operating parameters (T, τ  , XO2/XCH4XO2/XCH4 and XNO2XNO2) are studied. ► NO2 lowers the reaction temperature and enhances the CH4 conversion. ► The formation of CO2 and C2 is limited in the reaction with NO2. ► With NO2, fairly good HCHO yields are obtained with a maximum value of 9%.