Methane partial oxidation in iron zeolites: theory versus experiment

Abstract

The conversion of methane to methanol over zeolitic α-oxygen sites has been demonstrated using Fe-ZSM-5. To discriminate between mono- and poly-nuclear active sites, we prepared the [Fe]-ZEO with iron in the ZEOlite lattice via direct synthesis and Fe x -ZEO, by dispersion of x wt.% iron on the ZEOlite. Shape-selective formation of nano-clusters of iron oxides with various sizes is realized inside the pore-sizes varying from 10.0 to 8.0 and 6.3 to 4.3 Å of the CFI, MOR, MFI, and CHA zeolites. The Fe–K edge X-ray absorption data were obtained for the Fe-CIT-5, Fe-ZSM-5, Fe-MOR and Fe-CHA zeolites containing iron clusters. In Mossbauer spectroscopy the absence and presence of a hyperfine magnetic field (HMF) for [Fe]-CIT-5 and Fe-CIT-5 are seen. The quantum mechanics calculations analyze the different environments of iron, e.g. the tetrahedral lattice occluded species, the di-nuclear sites attached to the zeolite, the nano-phase hematite sites. The molecular mechanics calculations involve a new molecular mechanics force field, the universal force field (UFF). α-Oxygen can be formed on di-nuclear iron sites in zeolites by N 2O decomposition at elevated temperatures and is dependent on the zeolite structure utilized. Fe-chabazite (CHA), Fe-mordenite (MOR) and Fe-CIT-5 (CFI) were found to be less active than Fe-ZSM-5. A range of preparative and activation conditions were studied preceding methane conversion. Proper activation is essential to maximize catalyst actvity, e.g. pretreatment under vacuum at 800–900°C, activation with N 2O at 250°C and reaction with methane at 20°C. Extraction of methanol from the catalyst is performed with H 2O. Structure–activity effects are discussed.