WATER-ASSISTED IONIC CONDUCTANCE OF ZEOLITES WITH ZSM-5 STRUCTURE

The electrical conductivity (sigma) of compressed powders of an isostr uctural series of zeolites with MFI topology(1) was measured using AC impedance spectroscopy. Silicalite-l and ZSM-5 with different Si/Al ra tios containing varying numbers of Na+ and/or H+ were studied al 100% relative humidity and immersed in liquid water. In the latter case, si gma is 1-2 orders of magnitude larger than that in saturated water vap our. The value of sigma for silicalite immersed in water was found to be higher than that of water before and after the test. It was assumed that this is due to enhanced water dissociation on the silicalite sur face. Both in water and at 100% RH the dependence of sigma on cation c ontent follows a curve with a maximum. A decrease in conductivity at a high concentration of charge carriers is ascribed to an increase in t he repulsive interaction between ions in the conductive water layer on the zeolite channel walls. (C) 1997 Elsevier Science B.V.