Synthesis of formaldehyde by direct methane oxidation in microreactor.

Summary Formaldehyde, one of the basic products of chemistry, is industrially synthesized by a multi-step process, in which the energy efficiency is limited. Thus, a synthesis by direct oxidation of methane in gas phase, which could be more advantageous, has been studied experimentally and by kinetic modeling, in the framework of this work. To favor the production of formaldehyde, an intermediate product of methane oxidation, low flow times (< 100 ms) were considered. An annular microreactor (0.5 mm annular space) made of quartz was used, in which the reaction was studied by varying the following operating parameters: temperature (600-1000°C), passage time (20-80 ms), XO2/XCH4 ratio (0.5-15) and added NO2 content (0-0.6%). Without NO2, HCHO selectivities decrease with conversion and the maximum yield without recycling is 2.4% (950°C, 60 ms and XO2/XCH4 = 8). The addition of NO2 decreases the required temperature by 300°C, and increases the best HCHO yield to 9% (700°C, 30 ms and XO2/XCH4 = 7 and 0.5% NO2). At low advancement, the reaction without NO2 can be modeled with the Gri-Mech 3.0 mechanism without any adjustments. For the reaction with NO2, after some corrections and modifications based on a literature review, the mechanism of Zalc et al. (2006) can correctly account for the experimental results. The flow analysis showed that the inter-conversion between NO2 and NO plays an important role in the reaction medium. It continuously forms the reactive OH- radicals, and converts CH3- and CH3O2- radicals to CH3O- radicals.