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.