Morphological and structural features of activated Fe-silicalites: a 129Xe-NMR and EPR investigation

Xe-129-NMR has been employed in the characterization of the activation of two iron silicalites, with different iron loadings. The Xe-129 chemical shift recorded as a function of the gas pressure is extremely sensitive to the state of the system and in particular to the hydration and oxidation state of extraframework iron ions. The NMR spectrum is composed of two main lines. The first one (in the range 120-140 ppm) is due to the interaction with the silicalite walls of the channels, while the second one (120-220 ppm) is due to the interaction with iron ions. The intensity and the chemical shift values of this peak markedly depend on the amount of iron in the system and, in particular, on the amount of iron ions dislodged from the framework and on the oxidation state. Thermal activation brings about a progressive dislodgment of iron ions from the framework positions and their partial reduction to Fe(II). Upon activation at 973 K, particularly large oxo-iron moieties are formed that (at least in the case of the higher loaded sample) prevent penetration of Xe atoms in the silicalite channels. The steric hindrance is removed both by further reduction in hydrogen and rehydration of the sample. The description of this complex phenomenology has been possible by comparing Xe-129-NMR data with EPR data (which directly monitor the oxidation state of the systems and the effect of hydration-dehydration) and by performing parallel experiments on an Fe-exchanged ZSM-5 zeolite containing exclusively extraframework iron. The Xe-129-NMR technique proved to be an excellent tool to characterize the systems and, potentially, other iron-containing porous materials.