A CONCURRENT PROTONIC ELECTRONIC CONDUCTION IN DEALUMINATED HY ZEOLITE

This study illustrates the role of zeolite dealumination on the altered electric properties of a protonic HY zeolite. The a.c.-impedance shows a high frequency Cole-Cole Z"(Z') dependence indicative of protonic conduction. The imaginary part-Z" shows a band associated with ionic relaxation; its maximum shifts to higher frequencies above 413 K in a manner useful for estimating the relaxation time and activation energy {17.9 x 10(-10)s and 0.53 eV}. The low frequency real part-Z' shows a band dependence on temperature heading to a maximum at 413 K. Both results could suggest transfer of the protonic conduction from low temperature water assisted to high temperature thermally enhanced mechanisms; their average activation energies are 0.14 and 0.46 eV, respectively. The lower activation energy confirms the role of water in mobilizing the conduction protons as H3O+ ions. At variance with dominant protonic conduction characterizing H-form zeolites, a concomitant dielectric conduction emerges for the present sample. This is shown by a high frequency Cole-Cole epsilon"(epsilon') dependence. The real part-epsilon' shows a band associated with electronic relaxation above 413 K; its dominant time and activation energy are 6.28 x 10(-8)s and 0.37 eV. The contribution of such a dielectric conduction could be explained in terms of strong dipole-dipole interaction at the nest silanols, created as lattice defects which could survive up to 823 K. Strong association of these sites raises the thermal activation to 413 K, rather than at 373 K for nonmodified zeolites.