Characterization of Lewis-acid sites in zeolites by EPR using the NO molecule as a probe

EPR spectra of NO adsorbed on both Na-ZSM-5 and H-ZSM-5 zeolites have been recorded at 10 and 78 K. Using the experimental parameters, it was possible to simulate the spectra on the assumption that in the studied zeolites, an NO molecule is tightly bound to an Na+ ion, already at 78 K. In the case of a ‘true’ Lewis site in these zeolites, a tight binding of NO at 10 K, but a rotation of the molecule around its z axis at 78 K, was assumed. It was possible to estimate the Al–N distance and to propose a structure of the ‘true’ Lewis site.