Evaluation of amount of framework iron(III) ions of ZSM-5 type ferrisilicate by water vapor adsorption

The solid-state Al-27 MAS NMR is taken to be the best technique to quantify the amount of framework aluminums of zeolite. However, it is very difficult to evaluate accurately the amount of framework metals of metalosilicate because the solid-state MAS NMR is not necessarily effective for all metals in the metalosilicates. It is desirable to develop a new method for quantification of the framework metals. In this paper, we investigated an evaluation of the amount of framework iron(III) ions of ZSM-5 type ferrisilicate by water vapor adsorption. Adsorption isotherms of water vapor were measured on the H-ferrisilicates with various SiO2/Fe2O3 ratios. From the analysis of the adsorption isotherms by the D-R method it was found that the micropore volume W-0 (H2O) accessible to water molecules increases linearly with an increase in the framework iron(III) ions and that the number of water molecules associated with one framework iron(III) ion is on average 5.5. This indicates that water molecules are able to be used to determine the amount of framework iron(III) ions in the H-ferrisilicate structure, Next, based on the above result, the amounts of framework iron(III) ions in H-ferrisilicates treated in air at 873 similar to 1073 K was estimated. It was found that the rate of the removal of iron(III) ions from the ferrisilicate framework by thermal treatment is apparently second order depending on the amount of framework iron(III) ions in the H-ferrisilicate structure.