Oxygen pathways in oxidative coupling of methane and related processes. Case study: NaWMn/SiO 2 catalyst

Abstract The chemical complexity of processes that accompany the oxygen turnover during the high-temperature selective oxidation processes, such as oxidative coupling of methane (OCM), oxidative dehydrogenation and oxidative cracking of light alkanes is analyzed. Particularly, chemical reactions and phase transitions in the complex oxide OCM catalyst NaWMn/SiO 2 are studied using the X-ray photoelectron spectroscopy, X-ray powder diffraction, and scanning electron microscopy combined with local elemental analysis. It was found that during oxygen desorption crystalline Na 2 WO 4 and Mn 2 O 3 phases disappear, and newly-formed compounds solidify in the form of crystalline MnWO 4 and the glue-like matter. The latter contains sodium and most likely Mn 2+ ions. Its localization on the grain boundaries of the SiO 2 (α-cristobalite) bulbs and morphology indicate that this matter is amorphous and has a high adhesion to the silica surface. After re-oxidation, the phase composition of the sample restores, although the size and location of the restored Na 2 WO 4 and Mn 2 O 3 particles evidence for their formation from the glue-like matter or its molten precursor. It was concluded that the supported components are present during the oxygen turnover in a molten state, in which all chemical and mass-transfer processes are accelerated.