Iodine as a visible probe for the evaluation of zeolite donor strength

The visible absorption band of iodine adsorbed on zeolite blue-shifted with increasing the electropositivity of the countercation and the aluminum content in the framework. This phenomenon was attributed to the increase in the donor strength of the zeolite framework based on the analogous spectral shift of iodine in solution. In support of this, a negative linear correlation was observed between the measured visible bands of iodine adsorbed on various zeolites and their calculated Sanderson's intermediate electronegativities. However, the simultaneous change in the electrostatic field strength within the zeolite pores, as a result of the change in the number and the size of the cation, has to be taken into account in order to interpret the overall spectral shifts more precisely. The iodine band sensitively blue-shifted with decreasing the moisture content in the framework but red-shifted with the loss of NH3 from NH4+-exchanged zeolites. The visible band of iodine also progressively red-shifted with increasing the adsorbed amount, presumably due to the nature of iodine to deplete electron density from the framework. The framework structure also affected the spectral shift of the visible iodine band. The overall results established that iodine can be used as a novel molecular probe for the quantitative evaluation of the zeolite donor strength.