ACID ZEOLITES AS ELECTRON-ACCEPTORS - GENERATION OF XANTHYLIUM, DIBENZOTROPYLIUM, AND FLUORENYLIUM CATIONS FROM THEIR CORRESPONDING HYDRIDES THROUGH AN ELECTRON-TRANSFER MECHANISM

Adsorption of xanthene (XH) and dibenzo[a,d]cycloheptatriene onto large pore acid zeolites containing Bronsted sites gives rise to the formation of xanthylium (X(+)) and dibenzotropylium (DT+) ions, respectively. In the case of fluorene (FH) using acid Y and beta zeolites, the same adsorption treatment did not allow the detection of the 9-fluorenylium cation (F+), but instead the longer-lived radical cation FH.+ was identified. Interestingly, when FH was adsorbed within acid mordenite, the simultaneous presence of FH.+ and F+ was observed. Detection of FH.+, together with the product distribution of the reaction of XH under electron-transfer conditions, suggests that X(+), DT+, and F+ are formed from the corresponding radical cations. Concerning the nature of the zeolite oxidizing centers, it was established that formation of tricyclic carbenium ions does not take place on the nonacidic Na+ form of zeolites. By contrast, purely Bronsted zeolite HYD-W was found to be active for the generation of X(+) and FH.+. On the other hand, selective neutralization of Bronsted sites with exhaustive Na+ exchange causes the deactivation of a sample in spite of the presence of unaltered residual Lewis sites.