Influence of ions and molecules on single crystal zeolite conductivity under in situ conditions

Basic investigations on the conductivity behavior of alumosilicate zeolites were conduced in the gas atmosphere, using impedance spectroscopy under in situ conditions where 'zeolitic' water molecules are still present in the channel and cage system. Natural zeolites of stilbite (STI) and phillipsite (PHI) structure types were used for single crystal investigations, while natural compacted 'ceramic' PHI was used to clarify influences of polycrystallinity on conductivity. At a given water partial pressure, the effect of polar organic molecules on conductivity was investigated under isothermal in situ conditions. It could be shown that the observed interaction processes are mainly adsorptive and that catalytic oxidation of polar gaseous organic compounds is taking place at much higher temperatures, where the zeolite structures are already irreversibly altered and zeolitic water is almost completely desorbed. Dependent on different parameters affecting the zeolite single crystal conductivity behavior, a schematic model is developed to describe the features observed. (C) 2001 Elsevier Science B.V. All rights reserved.