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.
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