ACID ZEOLITES AS ELECTRON ACCEPTORS - USE OF THIANTHRENE RADICAL-CATION AS A PROBE

Adsorption of thianthrene (Th) within a series of acid zeolites with medium, large, and extra large pore size including novel zeotypes MCM-22 and MCM-41 led as a general process to the generation of Th radical cation (Th-.+). This species was unambiguously characterized by UV/vis diffuse reflectance, EPR, and IR spectroscopies. In the case of Y and beta hosts, dimeric aggregates could be observed. Vapor phase incorporation of Th at 473 K under Ar atmosphere was found to be the most clean and convenient adsorption procedure. Ambient moisture was the main factor responsible for aging of Th-.+ composites. This process was totally reversible. This result suggest that reversible complexation of Th-.+ with H2O is the process ocurring at initial stages. On the other hand, oxygen is not necessary for the formation of Th-.+. The Bronsted and Lewis population of the acid sites were characterized by IR spectroscopy, following the pyridine adsorption-desorption method. Bronsted zeolites prepared by removal of the extraframework Al are active to generate Th-.+. Neutralization of Bronsted sites by Na+ exchange fully deactivated the solids despite of the presence of residual Lewis sites. Th oxidation must occur predominantly on the zeolite Bronsted sites.