Selective Formation Of Alpha-Fe(Ii) Sites On Fe-Zeolites Through One-Pot Synthesis

alpha-Fe(II) active sites in iron zeolites catalyze N2O decomposition and form highly reactive alpha-O that selectively oxidizes unreactive hydrocarbons, such as methane. How these alpha-Fe(II) sites are formed remains unclear. Here different methods of iron introduction into zeolites are compared to derive the limiting factors of Fe speciation to alpha-Fe(II). Postsynthetic iron introduction procedures on small pore zeolites suffer from limited iron diffusion and dispersion leading to iron oxides. In contrast, by introducing Fe(III) in the hydrothermal synthesis mixture of the zeolite (one-pot synthesis) and the right treatment, crystalline CHA can be prepared with >1.6 wt % Fe, of which >70% is alpha-Fe(II). The effect of iron on the crystallization is investigated, and the intermediate Fe species are tracked using UV-vis-NIR, FT-IR, and Mossbauer spectroscopy. These data are supplemented with online mass spectrometry in each step, with reactivity tests in alpha-O formation and with methanol yields in stoichiometric methane activation at room temperature and pressure. We recover up to 134 mu mol methanol per gram in a single cycle through H2O/CH3CN extraction and 183 mu mol/g through steam desorption, a record yield for iron zeolites. A general scheme is proposed for iron speciation in zeolites through the steps of drying, calcination, and activation. The formation of two cohorts of alpha-Fe(II) is discovered, one before and one after high temperature activation. We propose the latter cohort depends on the reshuffling of aluminum in the zeolite lattice to accommodate thermodynamically favored alpha-Fe(II).