Direct catalytic oxidation of methane to formaldehyde: New investigation opportunities provided by an improved flow circulation method

An improved flow circulation reactor with reaction mixture quenching was used to study the direct oxidation of methane to formaldehyde at 600–770°C, a CH 4 /O 2 volume ratio of 1: 1 in the starting mixture, and gas mixture flow rates of 0.4–2.0 l/h. It was found that, in the presence of a surface containing SiO 2 (quartz reactor walls, the surface of silica packing materials, or the surface of SiO 2 as a catalyst constituent), the process occurred by a heterogeneous-homogeneous mechanism with chain continuation in the volume of a gas phase. The process was controlled by the size and shape of the free reaction volume, the contact surface area, and the residence time of a mixture in the reaction volume. The introduction of typical oxidation catalysts containing, for example, Pt or V 2 O 5 as an active component along with SiO 2 resulted in a decrease in the yield of and selectivity for formaldehyde.