THE STRENGTH OF ONE-ELECTRON ACCEPTOR SITES OF SOME X-ZEOLITES AND Y-ZEOLITES - AN ESR STUDY

In the present study the one-electron acceptor sites of some X and Y zeolites and of a silica-alumina catalyst have been investigated by adsorption of a series of electron donor compounds (anthracene (An), dibenzo-p-dioxin (Dx), phenoxathiin (Phx) and phenothiazine (Pht)) which have different ionization potentials but similar molecular ''diameters''. The donor compounds yielded on the studied surfaces adsorbed cation radicals of which the electron spin resonance (ESR) spectra have been measured. For the zeolite samples, using the above donors, the one-electron acceptor sites could be distinguished according to their oxidizing strength. The ratio between the concentrations of the sites measured with Pht and An as donors is proposed as a parameter for evaluating the strength distribution of oxidizing sites on the studied samples. Some differences between the results obtained on zeolites and on silica-alumina are underlined. The ratio between the concentrations of ''inner'' and ''outer'' surface acceptor sites could be evaluated by using Pht and perylene, the ionization potential values of which are close but the molecular ''diameters'' are different.