Use of water as a probe of zeolitic properties: interaction of water with HZSM-5

Abstract

The adsorption of water in ZSM-5 has been used to study the crystal chemistry of the family of HZSM-5 materials. Several of the features discussed are generic in nature, applying to all high-silica zeolites. The strongest interaction of water is with the acidic protons associated with framework aluminum atoms. The quantity of water adsorbed is proportional to the framework aluminum content and this water adsorbs rapidly. Heats of adsorption are shown to be a monotonic function of the number of water molecules adsorbed per H(Al) unit and independent of the framework aluminum content of the zeolite. Weaker adsorption occurs on external surface silanol groups and on centers ascribed to intracrystalline defects created as charge balancing centers for the charged template, tetrapropylammonium (TPA) ions. An adsorption isotherm is presented for external surface adsorption. For a high-silica ZSM-5, adsorption ascribed to defect sites corresponds to four H 2O molecules per unit cell, equal in number to the number of TPA + ions and hence the number of these sites.