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 wa s possible to simulate the spectra on the assumption that in the studi ed 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 b inding 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 a nd to propose a structure of the 'true' Lewis site.