Confinement of Mononuclear Iron Hydroxides within CHA-Type Zeolite Cages for Stable Methane Oxidation Catalysis

Abstract

Mononuclear metal centers offer molecularly defined catalytic active sites but readily aggregate under reaction conditions. Here we show that a simple post-treatment for Fe-containing CHA-type zeolite produces cha-cage-confined mononuclear Fe hydroxides. Acid treatment removes external hematite while preserving the CHA topology. UV–vis and NH3-TPD indicate highly dispersed residual Fe species in the micropores after the acid treatment. Fe K-edge XANES exhibits an enhanced pre-edge peak, and FT/WT-EXAFS shows Fe–O coordination with no significant Fe–Fe scattering; curve fitting gives a short Fe–O distance of 1.87 Å, consistent with four-coordinate Fe3+. In addition, 57Fe Mössbauer spectra are dominated by quadrupole-split absorption from high-spin Fe3+, and the magnetically split components of hematite present in the parent material disappear after acid treatment. XANES spectra simulated for the density functional theory (DFT)-optimized Fe(OH)3(H2O) model reproduce the measured increase in pre-edge intensity relative to hematite. Automated reaction route mapping with periodic DFT identifies Fe(OH)3(H2O) as the most plausible cage-confined species. Such confined mononuclear Fe hydroxides catalyze the oxidation of methane with H2O2, while maintaining both catalytic activity and their local Fe structure during recycling, without detectable Fe leaching. Transition-state calculations reveal a possibility of concerted H atom transfer from CH4 to H2O2 assisted by a proton relay around an Fe(OH)4 intermediate. This work establishes a simple route to robust, cage-confined mononuclear Fe hydroxides that couple molecular-level definition with solid-state stability for the liquid-phase methane oxidation.