Methane partial oxidation in iron zeolites: theory versus experiment

The conversion of methane to methanol over zeolitic cr-oxygen sites has bee n demonstrated using Fe-ZSM-5. To discriminate between mono- and poly-nucle ar active sites, we prepared the [Fe]-ZEO with iron in the ZEOlite lattice via direct synthesis and Fe, -ZEO, by dispersion of x wt.% iron on the ZEOl ite. 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 t o 4.3 Angstrom 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 a nd presence of a hyperfine magnetic field (HMF) for [Fe]-CIT-5 and Fe-CIT-5 are seen. The quantum mechanics calculations analyze the different environ ments of iron, e,g, the tetrahedral lattice occluded species, the di-nuclea r sites attached to the zeolite, the nano-phase hematite sites. The molecul ar mechanics calculations involve a new molecular mechanics force field, th e universal force field (UFF). alpha -Oxygen can be formed on di-nuclear ir on rites in zeolites by N2O decomposition at elevated temperatures and is d ependent on the zeolite structure utilized. Fe-chabazite (CHA), Fe-mordenit e (MOR) and Fe-CIT-5 (CFI) were found to be less active than Fe-ZSM-5. A ra nge 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 degreesC, activation with N2O at 2 50 degreesC and reaction with methane at 20 degreesC, Extraction of methano l from the catalyst is performed with H2O Structure-activity effects are di scussed. (C) 2001 Elsevier Science B.V, All rights reserved.