Designed Synthesis and Crystallization of Fe-Al-EU-1 Zeolites Containing Framework-Iron

Highly crystalline Fe-Al-EU-1 zeolites were hydrothermally synthesized in a HMBr2-Na2O-Al2O3-SiO2-Fe2O3-H2O system by using hexamethonium bromide (HMBr2) as a template. The physical and chemical properties, and the bonding state of Fe in the zeolite framework for the prepared Fe-Al-EU-1 samples were characterized by a series of techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric-derivative thermogravimetric (TG-DTG) analysis, N-2 adsorption-desorption, solid-state nuclear magnetic resonance (NMR), UV-visible diffuse reflectance spectroscopy (UV-Vis DRS), and X-ray absorption fine structure (XAFS). The results show that with increasing the mass fraction of Fe in the original sol-gel, the unit cell volume of Fe-Al-EU-1 zeolite inflates; the zeolite doped with Fe causes an increase in its surface area (BET) from 272 to 365 m(2) . g(-1) and a reduced amount of template removal, as well as a decrease in decomposition temperature. Adding Fe ions into sol-gel also leads to lowered zeta potential, easily gathered colloidal particles, and increased shape of zeolite. The UV-Vis spectrum shows that there is a characteristic peak at about 220-250 nm with a sharply increased intensity due to the p-d transition of the bonding electrons from the 2p-orbital of 0 atom to the d-orbital of the four-coordinated Fe atom in the framework. Also at around 373 nm, the coordination bonding of four-coordinated Fe atoms and adjacent Si - O groups leads to a d-d charge transition peak with an energy level splitting and increased peak intensity. The XAFS results show that with the crystallization process going on, the pre-edge absorption peak for 1s -> 3d and main absorption peak for 1s -> 4p change significantly. Four-coordinated structural units of iron species formed from the original sol-gel samples are gradually transformed into a tetrahedral coordinated iron-silicon-oxygen skeleton, in which iron-silicon-oxygen ionic structural unit is also transformed into skeleton iron species with a tetrahedral covalent bonding structure.