Local Structure of Framework Iron in Fe-Substituted Al-Mordenites by Fe K Edge XAFS

Abstract

The Fe-substituted Al-mordenites (Fe-MOR) with several Fe ratios were synthesized, and the detailed structure was obtained by means of Al K edge XANES and Fe K edge XAFS. From the results of the N2-adsorption isotherm, the Fe-substituted mordenites with an Fe/(Fe + Al) ratio as high as 75 mol % have a high crystallinity and appropriate micropore. The completely substituted mordenite (Fe100MOR) shows low pore volume and mordenite crystallinity. The Al K edge XANES results show the smaller effect of structural change around Al ions by Fe (25 mol %) substitution. Fe K edge XAFS results indicate existence of tetrahedral Fe ions into the mordenite framework in all the Fe ratios. The local structure around the Fe3+ ions is quite different from that around the Fe-modified mordenites. The Fe−O distance increases with an increase in Fe content. This suggests the distortion of the zeolite framework in the local structure of the mordenites. It is concluded that the stable structure around the Fe3+ ions in the Fe-substituted mordenites is brought up to about 75 mol % in the Fe ratio.