Redox properties of alkaline and acidic L zeolites as studied by EPR spectroscopy using TCNE and perylene as probe molecules

The oxidizing and reducing character of various exchanged L zeolites has been studied by EPR spectroscopy via the formation of perylene and tetracyanoethylene ion radicals, respectively, and correlated with the acido-basic properties of zeolites. In the series of alkaline L forms the reducing character increases from LiL to CsL in parallel with the calculated framework average negative charge on oxygen atoms (oxygen basicity). An opposite variation is obtained for the oxidizing properties. The exchange of K+ ions by H+, enhancing the acidity of zeolites, decreases the reducing and increases the oxidizing properties. The present work reveals the links which exist between the acidity of zeolites with oxidizing centers on one hand and the basicity with reducing character on the other hand. This observed parallelism between the redox properties and the acido-basicity of zeolites could be of great importance to explain reliably the catalytic behaviour of L zeolites as support. The first attempt has been made concerning the correlations between the changes in the redox properties and the selectivity for the aromatization of n-hexane on Pt alkaline L zeolite and the sulphur resistance of Pt/acidic or basic zeolites.