Evolution of isomorphously substituted iron zeolites during activation: comparison of Fe-beta and Fe-ZSM-5

A number of techniques (ICP-OES, XRD, SEM, ²⁷Al MAS-NMR, N₂ adsorption, NH₃-TPD, FTIR of pyridine adsorbed, HRTEM, UV/vis, and EPR) have been applied to characterize the morphology, structure, porosity, acidity, and forms of iron and aluminum in Fe-beta and Fe-ZSM-5 prepared by hydrothermal synthesis and in their products of activation by calcination and steam treatment. The compositions of the zeolite matrices were very similar, with nominal Si/Al and Si/Fe ratios of 36 and 152, respectively. Removal of the template by calcination leads to extensive dealumination in Fe-beta, while having no influence on the state of aluminum in Fe-ZSM-5. Steam treatment was required to massively create extra-framework Al species in the latter sample. In contrast, the sensitivity of framework iron to migration into non-framework positions during postsynthesis treatments and the nature and distribution of the resulting iron species were remarkably analogous in the two zeolite structures. Catalytic tests with direct N₂O decomposition and N₂O reduction with CO over the steamed iron zeolites confirmed that the microporous matrix intrinsically does not play a decisive role on the activity in these reactions, provided that the resulting forms of iron are similar.