TEMPO RADICALS AS EPR PROBES TO MONITOR THE ADSORPTION OF DIFFERENT SPECIES INTO X-ZEOLITE

A computer-aided analysis of the electron paramagnetic resonance (EPR) spectra of TEMPO radicals adsorbed in X zeolites is performed under v arious experimental conditions, both in the absence and in the presenc e of various species. In dehydrated NaX zeolites the radical gives ris e to two superimposed signals corresponding to species characterized b y different mobility and polarity and by a different temperature depen dence. Controlled addition of water leads to an increase of mobility u p to 4.2 water molecules per supercage. Under conditions of partial hy dration, the radicals show an increase in the correlation time for mot ion with an increase in the size of the cation in the exchanged zeolit es, whereas an increase in mobility is found upon going from NaX to Na Y samples. These results are analyzed in terms of the variation of the local electric field at the surface. Dipolar broadening is shown to b e responsible for the line shape variations found at TEMPO loadings up to 0.32 molecules per supercage. Heisenberg spin-spin interactions al so become relevant at higher loading, Increasing amounts of 2,2,6,6-te tramethylpiperidine induce a restriction of mobility. This result is i nterpreted in terms of a decrease in the free volume available to radi cals which occupy the zeolite cavities. However, loading of pentane le ads to a similar increase in mobility as found with increasing amounts of water. The environmental polarity, as expected, decreases due to t he replacement of water molecules with pentane. It is proposed that th e zeolite hosts, when saturated with pentane, slow moving radicals whi ch are trapped in the pores.