Study of iron/mordenite catalysts by Mössbauer and ferromagnetic resonance spectroscopy for carbon monoxide hydrogenation

Abstract

Iron was loaded on mordenites with varying cation type and degree of dealumination, and their catalytic properties were characterized by means of Mössbauer and ferromagnetic resonance spectroscopy and carbon monoxide hydrogenation as a probe reaction. When iron was impregnated on mordenites, it was found that about 1.2 wt.-% of iron existed as ion-exchanged cationic iron in the oxide or hydroxide form. Iron oxide particles formed after calcination at 450°C showed a wide distribution in their size, ranging from cluster-sized superparamagnetic oxide to large particles of α-Fe2O3. After reduction at 450°C, iron oxide particles were sintered to be reduced to bimodally distributed metallic iron particles. Small iron particles generated on hydrogen-form mordenites are believed to be stabilized by some defect sites generated by dealumination of mordenite. On sodium-mordenite, extremely small and uniform iron clusters were generated because free sodium ions, which had been detached from the framework by previous ion exchange with impregnated iron ions, promoted reduction of ion-exchanged cationic irons. The result was applied to reduce ion-exchanged iron (FeNaM10) by injecting a high concentration of sodium ions into the pores of mordenite and about 27% of the iron was able to be reduced upon treatment in hydrogen gas at 450°C. These iron clusters showed lower activity but higher selectivity toward alkenes in the carbon monoxide hydrogenation reaction than conventional impregnated catalysts.