Water-assisted ionic conductance of zeolites with ZSM-5 structure

The electrical conductivity (sigma) of compressed powders of an isostructural series of zeolites with MFI topology(1) was measured using AC impedance spectroscopy. Silicalite-l and ZSM-5 with different Si/Al ratios containing varying numbers of Na+ and/or H+ were studied al 100% relative humidity and immersed in liquid water. In the latter case, sigma is 1-2 orders of magnitude larger than that in saturated water vapour. 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 surface. Both in water and at 100% RH the dependence of sigma on cation content follows a curve with a maximum. A decrease in conductivity at a high concentration of charge carriers is ascribed to an increase in the repulsive interaction between ions in the conductive water layer on the zeolite channel walls. (C) 1997 Elsevier Science B.V.